U.S. patent application number 10/282621 was filed with the patent office on 2003-07-31 for feram capacitor stack etch.
Invention is credited to Celii, Francis G., Summerfelt, Scott R., Thakre, Mahesh.
Application Number | 20030143853 10/282621 |
Document ID | / |
Family ID | 26961561 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030143853 |
Kind Code |
A1 |
Celii, Francis G. ; et
al. |
July 31, 2003 |
FeRAM capacitor stack etch
Abstract
The present invention is directed to a method of forming an
FeRAM integrated circuit, which includes performing a capacitor
stack etch to define the FeRAM capacitor. The method comprises
etching a PZT ferroelectric layer with a high temperature BCl.sub.3
etch which provides substantial selectivity with respect to the
hard mask. Alternatively, the PZT ferroelectric layer is etch using
a low temperature fluorine component etch chemistry such as
CHF.sub.3 to provide a non-vertical PZT sidewall profile. Such a
profile prevents conductive material associated with a subsequent
bottom electrode layer etch from depositing on the PZT sidewall,
thereby preventing leakage or a "shorting out" of the resulting
FeRAM capacitor.
Inventors: |
Celii, Francis G.; (Dallas,
TX) ; Summerfelt, Scott R.; (Garland, TX) ;
Thakre, Mahesh; (Dallas, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
26961561 |
Appl. No.: |
10/282621 |
Filed: |
October 29, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60353535 |
Jan 31, 2002 |
|
|
|
Current U.S.
Class: |
438/694 ;
257/E21.009; 257/E21.253; 257/E21.272; 257/E21.314; 257/E21.664;
257/E27.104 |
Current CPC
Class: |
H01L 21/31122 20130101;
H01L 28/55 20130101; H01L 27/11502 20130101; H01L 27/11507
20130101; H01L 21/31691 20130101; H01L 21/32139 20130101 |
Class at
Publication: |
438/694 |
International
Class: |
H01L 021/311 |
Claims
What is claimed is:
1. A method of etching a capacitor stack associated with a
ferroelectric memory cell, comprising: forming a bottom electrode
layer, a PZT ferroelectric layer, a top electrode layer, and a hard
mask layer over a substrate; patterning the hard mask layer;
patterning the top electrode layer in accordance with the patterned
hard mask; patterning the PZT ferroelectric layer using a BCl.sub.3
etch at a substantially high temperature in accordance with the
patterned hard mask; and patterning the bottom electrode layer in
accordance with the patterned hard mask.
2. The method of claim 1, wherein patterning the PZT ferroelectric
layer comprises using a BCl.sub.3 etch at a temperature of at least
150.degree. C.
3. The method of claim 2, wherein patterning the PZT ferroelectric
layer comprises using a BCl.sub.3 etch at a temperature of about
350.degree. C., wherein the patterning of the PZT layer is
substantially selective with respect to the patterned hard
mask.
4. The method of claim 3, further comprising adding Ar to the
BCl.sub.3 etch of the PZT ferroelectric layer, wherein a ratio of
BCl.sub.3 to Ar comprises about 1:1.
5. The method of claim 2, further comprising adding Ar to the
BCl.sub.3 etch of the PZT ferroelectric layer, wherein a ratio of
BCl.sub.3 to Ar comprises about 20% Ar or more and about 30% Ar or
less.
6. A method of forming a capacitor stack in a ferroelectric memory
cell, comprising: forming a bottom electrode layer, a PZT
ferroelectric layer, a top electrode layer, and a hard mask layer
over a substrate; patterning the hard mask layer; patterning the
top electrode layer using a Cl.sub.2+O.sub.2 or a Cl.sub.2+CO etch
in accordance with the patterned hard mask; patterning the PZT
ferroelectric layer using a BCl.sub.3+Ar etch at a temperature of
about 150.degree. C. or more in accordance with the patterned hard
mask; and patterning the bottom electrode layer using a
Cl.sub.2+O.sub.2 or a Cl.sub.2+CO etch in accordance with the
patterned hard mask.
7. The method of claim 6, wherein patterning the top electrode
layer, the PZT ferroelectric layer and the bottom electrode layer
is performed at a temperature of about 350.degree. C. or more.
8. The method of claim 6, wherein the hard mask layer comprises
TiAlN, and wherein an oxygen content in the Cl.sub.2+O.sub.2 or the
Cl.sub.2+CO etch of the top and bottom electrode layers is at least
about 5%, thereby providing a substantial etch selectivity of the
capacitor stack with respect to the patterned TiAlN hard mask.
9. The method of claim 8, wherein the temperature of the
BCl.sub.3+Ar PZT ferroelectric layer etch is about 350.degree. C.,
thereby providing a substantial etch selectivity of the PZT
ferroelectric layer with respect to the patterned TiAlN hard
mask.
10. The method of claim 9, wherein a ratio of BCl.sub.3 to Ar in
the PZT ferroelectric layer etch comprises about 1:1.
11. The method of claim 6, wherein a ratio of BCl.sub.3 to Ar in
the PZT ferroelectric layer etch comprises about 20% Ar or more and
about 30% Ar or less.
12. A method of forming a capacitor stack in a ferroelectric memory
cell, comprising: forming an iridium bottom electrode layer, a PZT
ferroelectric layer, an iridium top electrode layer, and a TiAlN
hard mask layer over a substrate; patterning the TiAlN hard mask
layer using a BCl.sub.3 etch; patterning the iridium top electrode
layer using a Cl.sub.2+O.sub.2 or a Cl.sub.2+CO etch in accordance
with the patterned hard mask, wherein an oxygen content in the
iridium top electrode layer etch is at least about 5%, thereby
providing a substantial etch selectivity with respect to the TiAlN
hard mask; patterning the PZT ferroelectric layer using a
BCl.sub.3+Ar etch at a temperature of about 150.degree. C. or more
in accordance with the patterned hard mask, wherein the temperature
of about 150.degree. C. or more provides for an etch of the PZT
ferroelectric dielectric layer that is substantially selective with
respect to the TiAlN hard mask; and patterning the bottom electrode
layer using a Cl.sub.2+O.sub.2 or a Cl.sub.2+CO etch in accordance
with the patterned hard mask, wherein an oxygen content in the
iridium bottom electrode layer etch is at least about 5%, thereby
providing a substantial etch selectivity with respect to the TiAlN
hard mask.
13. The method of claim 12, wherein patterning the PZT
ferroelectric layer comprises using the BCl.sub.3+Ar etch at a
temperature of at about 350.degree. C.
14. The method of claim 13, wherein a ratio of BCl.sub.3 to Ar
comprises about 1:1.
15. The method of claim 12, wherein the temperature of the PZT
ferroelectric layer etch is about 150.degree. C., and wherein a
ratio of BCl.sub.3 to Ar comprises about 20% Ar or more and about
30% Ar or less.
16. A method of etching a capacitor stack associated with a
ferroelectric memory cell, comprising: forming a bottom electrode
layer, a PZT ferroelectric layer, a top electrode layer, and a hard
mask layer over a substrate; patterning the hard mask layer;
patterning the top electrode layer in accordance with the patterned
hard mask; patterning the PZT ferroelectric layer, wherein a
resulting PZT ferroelectric sidewall edge has a profile having an
angle of less than about 88 degrees; and patterning the bottom
electrode layer in accordance with the patterned hard mask, wherein
the PZT profile angle of less than about 88 degrees causes a
re-deposition rate of bottom electrode material on the PZT sidewall
edge during the bottom electrode layer patterning to be less than a
removal rate thereof due to ion impingement, thereby preventing
bottom electrode material from forming on the PZT ferroelectric
layer sidewall during the capacitor stack etch.
17. The method of claim 16, wherein patterning the PZT
ferroelectric layer comprises etching using a fluorine
gas+Cl.sub.2+an oxidizer at a relatively low temperature.
18. The method of claim 17, wherein the temperature of the PZT
ferroelectric layer etch is about 60.degree. C.
19. The method of claim 17, wherein etching the PZT ferroelectric
layer comprises using a CHF.sub.3+Cl.sub.2+O.sub.2+N.sub.2 at a
temperature of about 60.degree. C.
20. The method of claim 16, wherein patterning the top and bottom
electrode layers comprise etching with a Cl.sub.2+O.sub.2 or a
Cl.sub.2+CO at substantially high temperature, and wherein
patterning the PZT ferroelectric layer comprises etching using a
fluorine gas+Cl.sub.2+an oxidizer at a relatively low
temperature.
21. The method of claim 20, wherein the substantially high
temperature comprises about 350.degree. C., and the relatively low
temperature comprises about 60.degree. C.
22. The method of claim 16, wherein the sidewall edge profile angle
of the PZT ferroelectric layer is about 80 degrees or more.
Description
RELATED APPLICATION
[0001] This application claims priority to Serial No. 60/353,535
filed Jan. 31, 2002, which is entitled "FeRAM Capacitor Stack
Etch".
FIELD OF THE INVENTION
[0002] The present invention relates generally to the field of
integrated circuit processing, and more particularly relates to an
FeRAM structure and a method of manufacture thereof having a
capacitor stack etch which effectively etches the ferroelectric
dielectric layer without degradation thereof.
BACKGROUND OF THE INVENTION
[0003] Several trends exist, today, in the semiconductor device
fabrication industry and the electronics industry. Devices are
continuously getting smaller and smaller and requiring less and
less power. A reason for this is that more personal devices are
being fabricated which are very small and portable, thereby relying
on a small battery as its supply source. For example, cellular
phones, personal computing devices, and personal sound systems are
devices that are in great demand in the consumer market. In
addition to being smaller and more portable, personal devices are
requiring more computational power and on-chip memory. In light of
all these trends, there is a need in the industry to provide a
computational device that has a fair amount of memory and logic
functions integrated onto the same semiconductor chip. Preferably,
this memory will be configured such that if the battery dies, the
contents of the memory will be retained. Such a memory device that
retains its contents while a signal is not continuously applied to
it is called a non-volatile memory. Examples of conventional
non-volatile memory include: electrically erasable, programmable
read only memory ("EEPROM") and FLASH EEPROM.
[0004] A ferroelectric memory (FeRAM) is a non-volatile memory that
utilizes a ferroelectric material, such as SBT or PZT, as the
capacitor dielectric situated between a bottom electrode and a top
electrode. Both read and write operations are performed for a
FeRAM. The memory size and memory architecture affect the read and
write access times of a FeRAM. Table 1 illustrates the differences
between different memory types.
1TABLE 1 FeRAM Property SRAM Flash DRAM (Demo) Voltage >0.5 V
Read >1 V 3.3 V >0.5 V Write (12 V) (.+-.6 V) Special
Transistors NO YES YES NO (High Voltage) (Low Leakage) Write Time
<10 ns 100 ms <30 ns 60 ns Write Endurance >10.sup.15
<10.sup.5 >10.sup.15 >10.sup.13 Read Time (single/ <10
ns <30 ns <30 ns/ 60 ns multi bit) <2 ns Read Endurance
>10.sup.15 >10.sup.15 >10.sup.15 >10.sup.13 Added Mask
for 0 .about.6-8 .about.6-8 .about.3 embedded Cell Size
(F.about.metal .about.80 F.sup.2 .about.8 F.sup.2 .about.8 F.sup.2
.about.18 F.sup.2 pitch/2) Architecture NDRO NDRO DRO DRO Non
volatile NO YES NO YES Storage I Q Q P
[0005] The non-volatility of an FeRAM is due to the bi-stable
characteristic of the ferroelectric memory cell. Two types of
memory cells are used, a single capacitor memory cell and a dual
capacitor memory cell. The single capacitor memory cell (referred
to as a 1T/1C or 1C memory cell) requires less silicon area
(thereby increasing the potential density of the memory array), but
is less immune to noise and process variations. Additionally, a 1C
cell requires a voltage reference for determining a stored memory
state. The dual capacitor memory cell (referred to as a 2T/2C or 2C
memory cell) requires more silicon area, and it stores
complementary signals allowing differential sampling of the stored
information. The 2C memory cell is more stable than a 1C memory
cell.
[0006] As illustrated in prior art FIG. 1, a 1T/1C FeRAM cell 10
includes one transistor 12 and one ferroelectric storage capacitor
14. A bottom electrode of the storage capacitor 14 is connected to
a drain terminal 15 of the transistor 12. The 1T/1C cell 10 is read
from by applying a signal to the gate 16 of the transistor (word
line WL)(e.g., the Y signal), thereby connecting the bottom
electrode of the capacitor 14 to the source of the transistor (the
bit line BL) 18. A pulse signal is then applied to the top
electrode contact (the plate line or drive line DL) 20. The
potential on the bit line 18 of the transistor 12 is, therefore,
the capacitor charge divided by the bit line capacitance. Since the
capacitor charge is dependent upon the bi-stable polarization state
of the ferroelectric material, the bit line potential can have two
distinct values. A sense amplifier (not shown) is connected to the
bit line 18 and detects the voltage associated with a logic value
of either 1 or 0. Frequently the sense amplifier reference voltage
is a ferroelectric or non-ferroelectric capacitor connected to
another bit line that is not being read. In this manner, the memory
cell data is retrieved.
[0007] A characteristic of the shown ferroelectric memory cell is
that a read operation is destructive. The data in a memory cell is
then rewritten back to the memory cell after the read operation is
completed. If the polarization of the ferroelectric is switched,
the read operation is destructive and the sense amplifier must
rewrite (onto that cell) the correct polarization value as the bit
just read from the cell. This is similar to the operation of a
DRAM. The one difference from a DRAM is that a ferroelectric memory
cell will retain its state until it is interrogated, thereby
eliminating the need of refresh.
[0008] As illustrated, for example, in prior art FIG. 2, a 2T/2C
memory cell 30 in a memory array couples to a bit line 32 and an
inverse of the bit line ("bit line-bar") 34 that is common to many
other memory types (for example, static random access memories).
Memory cells of a memory block are formed in memory rows and memory
columns. The dual capacitor ferroelectric memory cell comprises two
transistors 36 and 38 and two ferroelectric capacitors 40 and 42,
respectively. The first transistor 36 couples between the bit line
32 and a first capacitor 40, and the second transistor 38 couples
between the bit line-bar 34 and the second capacitor 42. The first
and second capacitors 40 and 42 have a common terminal or plate
(the drive line DL) 44 to which a signal is applied for polarizing
the capacitors.
[0009] In a write operation, the first and second transistors 36
and 38 of the dual capacitor ferroelectric memory cell 30 are
enabled (e.g., via their respective word line 46) to couple the
capacitors 40 and 42 to the complementary logic levels on the bit
line 32 and the bar-bar line 34 corresponding to a logic state to
be stored in memory. The common terminal 44 of the capacitors is
pulsed during a write operation to polarize the dual capacitor
memory cell 30 to one of the two logic states.
[0010] In a read operation, the first and second transistors 36 and
38 of the dual capacitor memory cell 30 are enabled via the word
line 46 to couple the information stored on the first and second
capacitors 40 and 42 to the bar 32 and the bit line-bar line 34,
respectively. A differential signal (not shown) is thus generated
across the bit line 32 and the bit line-bar line 34 by the dual
capacitor memory cell 30. The differential signal is sensed by a
sense amplifier (not shown) that provides a signal corresponding to
the logic level stored in memory.
[0011] A memory cell of a ferroelectric memory is limited to a
finite number of read and write operations before the memory cell
becomes unreliable. The number of operations that can be performed
on a FeRAM memory is known as the endurance of a memory. The
endurance, is an important factor in many applications that require
a nonvolatile memory. Other factors such as memory size, memory
speed, and power dissipation also play a role in determining if a
ferroelectric memory is viable in the memory market.
SUMMARY OF THE INVENTION
[0012] In essence, the instant invention relates to the fabrication
of an FeRAM device which is either a stand-alone device or one
which is integrated onto a semiconductor chip which includes many
other device types. Several requirements either presently exist or
may become requirements for the integration of FeRAM with other
device types. One such requirement involves utilizing, as much as
possible, the conventional front end and back end processing
techniques used for fabricating the various logic and analog
devices on the chip to fabricate this chip which will include FeRAM
devices. In other words, it is beneficial to utilize as much of the
process flow for fabricating these standard logic devices (in
addition to I/O devices and potentially analog devices) as
possible, so as not to greatly disturb the process flow (and thus
increase the process cost and complexity) merely to integrate the
FeRAM devices onto the chip.
[0013] The following discussion is based on the concept of creating
the ferroelectric capacitors in a FeRAM process module that occurs
between the front end module (defined to end with the formation of
W contacts) and the back end process module (mostly metallization).
Other locations of the FeRAM process module have also been
proposed. For example, if the FeRAM process module is placed over
the first layer of metallization then a capacitor over bar
structure can be created with the advantage that a larger capacitor
can be created. One disadvantage of the approach is that either
Metal-1 or a local interconnect must be compatible with FeRAM
process temperature (W for example) or the FeRAM process
temperature must be lowered to be compatible with standard
metallization (Al.about.450 C., Cu-Low-K.about.400 C.). This
location has some advantages for commodity memory purposes, but has
cost disadvantages for embedded memory applications. Another
proposed location for the FeRAM process module is near the end of
the back end process flow. The principal advantage of this approach
is that it keeps new contaminants in the FeRAM module (Pb, Bi, Zr,
Ir, Ru, or Pt) out of more production tools. This solution is most
practical if the assumption is that all of the equipment used after
deposition of the first FeRAM film must be dedicated and cannot be
shared. This solution has the drawback of requiring FeRAM process
temperatures compatible with standard metallization plus wiring of
the FeRAM capacitor to transistor and other needs of metallization
are not compatible with a minimum FeRAM cell size.
[0014] The requirements for the other locations will have many of
the same concerns, but some requirements will be different.
[0015] The FeRAM process module must therefore be compatible with
front-end process flow including the use of W contacts (currently
standard in most logic flows) as the bottom contact of the
capacitor. The FeRAM thermal budget must also be low enough so that
it does not impact the front-end structures such as the low
resistance structures (such as tungsten plugs and silicided
source/drains and gates) required by most logic devices. In
addition, transistors and other front-end devices such as diodes
are sensitive to contamination and the FeRAM process module cannot
contaminate these devices either directly (diffusion in chip) or
indirectly (cross contamination through shared equipment). The
FeRAM devices and process module must also be compatible with a
standard back end process flow. Therefore the FeRAM process module
must have minimum degradation of logic metallization resistance and
parasitic capacitance between metal and transistor. In addition,
the FeRAM devices must not be degraded by the back end process flow
with minimal, if any, modification. This is a significant challenge
since ferroelectric capacitors have been shown to be sensitive to
hydrogen degradation and most logic back end process flows use
hydrogen/deuterium in many of the processes (SiO.sub.2,
Si.sub.3N.sub.4, and CVD W deposition, SiO.sub.2 via etch, and
forming gas anneals).
[0016] Commercial success of FeRAM also requires minimization of
embedded memory cost. Total memory cost is primarily dependent on
cell size, periphery ratio size, impact of yield, and additional
process costs associated with memory. In order to have a cost
advantage per bit compared to standard embedded memories such as
embedded DRAM and Flash it is necessary to have cell sizes that are
not much larger than these competing technologies. Some of the
methods discussed in this patent to minimize cell size is to make
the process flow less sensitive to lithography misalignment, have
the capacitor directly over the contact, and using a single mask
for the capacitor stack etch.
[0017] In accordance with one aspect of the present invention, a
method of forming an FeRAM capacitor is provided in which the
etching of the ferroelectric capacitor stack is greatly improved.
The method comprises an etch of the capacitor stack using a
patterning hard mask, for example, a TiAlN hard mask. An etch of
the PZT ferroelectric layer during the capacitor stack etch
comprises a BCl.sub.3 etch at a substantially high temperature, for
example, about 150.degree. C. or more (e.g., 350.degree. C.).
Surprisingly, the BCl.sub.3 PZT etch at a relatively high
temperature is substantially selective with respect to the
overlying patterned hard mask, thereby providing a quality etched
PZT film without substantial hard mask erosion, thereby resulting
in good critical dimension control of the capacitor stack.
[0018] In accordance with another aspect of the present invention,
the capacitor stack comprises iridium top and bottom electrode
layers, and a PZT ferroelectric layer disposed between the top and
bottom electrode layers. A nitride hard mask, for example, TiAlN,
is formed and patterned over the capacitor stack layers. A
Cl.sub.2+O.sub.2 or a Cl.sub.2+CO etch is employed to pattern the
top electrode layer, wherein the oxygen content therein helps
provide a substantial etch selectivity with respect to the hard
mask. The PZT layer is then etched with a BCl.sub.3 etch at a
temperature of at least about 150.degree. C. Unexpectedly, the high
temperature BCl.sub.3 etch provides good selectivity with respect
to the hard mask despite the fact that no oxygen is provided during
such etch, thereby providing for a high quality etched PZT film
without substantial erosion of the hard mask. Accordingly, a
minimal capacitor stack critical dimension is maintained. The
bottom electrode is then etched in a manner similar to that of the
top electrode layer.
[0019] In accordance with another aspect of the present invention,
a capacitor stack etch is disclosed in which a sidewall profile of
the PZT ferroelectric layer is made non-vertical. Use of a sloped
or non-vertical PZT sidewall profile is not obvious because
typically vertical or closely vertical sidewalls are desired to
minimize the critical dimension of the capacitor. The PZT sidewall
profile is made non-vertical (e.g., less than about 88 degrees) in
order to facilitate ion impingement thereon during the subsequent
etch of the bottom electrode layer. The ion impingement (e.g.,
chlorine ions) on the sloped PZT sidewall during the bottom
electrode layer etch ensures that re-deposition of conductive
bottom electrode material onto the PZT sidewall does not occur by
having the removal rate thereof be greater than the deposition rate
due to re-sputtering. Accordingly, after the bottom electrode etch,
no bottom electrode material resides on the PZT sloped sidewall,
thereby preventing leakage or a shorting out of the FeRAM
capacitor.
[0020] In accordance with still another aspect of the present
invention, a capacitor stack etch having a sloped PZT sidewall
profile comprises etching the PZT layer with a
fluorine+Cl.sub.2+oxidizer etch chemistry at a low temperature, for
example, about 60.degree. C. The low temperature causes the
sidewall profile of the PZT to not be vertical (a non-anisotropic
etch). In addition, the low PZT etch temperature surprisingly
eliminates gaps or voids in the PZT ferroelectric layer that
occurred with fluorine containing etch chemistries at high
temperatures. In one particular example, the PZT etch comprises a
CHF.sub.3+Cl.sub.2+O.sub.2+N.sub.2 at a temperature of about
60.degree. C., resulting in a PZT sidewall profile of less than
about 88 degrees. A PZT sidewall angle of less than 88 degrees is
sufficient to ensure no net deposition of conductive material
thereon during the subsequent patterning of the underlying bottom
electrode layer.
[0021] To the accomplishment of the foregoing and related ends, the
invention comprises the features hereinafter fully described and
particularly pointed out in the claims. The following description
and the annexed drawings set forth in detail certain illustrative
aspects and implementations of the invention. These are indicative,
however, of but a few of the various ways in which the principles
of the invention may be employed. Other objects, advantages and
novel features of the invention will become apparent from the
following detailed description of the invention when considered in
conjunction with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a prior art schematic diagram illustrating an
exemplary 1T/1C FeRAM memory cell;
[0023] FIG. 2 is a prior schematic diagram illustrating an
exemplary 2T/2C FeRAM memory cell;
[0024] FIG. 3 is a fragmentary cross-sectional view of a partially
fabricated device containing FeRAM capacitors and transistors
associated therewith fabricated in accordance with one exemplary
aspect of the present invention;
[0025] FIG. 4 is a flow chart diagram illustrating a method of
forming an FeRAM capacitor in accordance with another exemplary
aspect of the present invention;
[0026] FIGS. 5 and 6 are fragmentary cross-sectional views of two
neighboring FeRAM capacitor stacks having a bottom electrode
diffusion barrier layer etched and a result thereof;
[0027] FIG. 7 is a schematic diagram illustrating an apparatus for
forming a PZT ferroelectric film in accordance with the present
invention;
[0028] FIGS. 8-11 are graphs illustrating various performance
characteristics of a PZT ferroelectric film formed in accordance
with the present invention;
[0029] FIGS. 12 and 13 are fragmentary cross section diagrams
illustrating how an etch of the bottom electrode diffusion barrier
layer causes a rounding of the hard mask layer which may cause a
contamination of a top electrode layer;
[0030] FIG. 14 is a flow chart diagram illustrating a method of
forming a multi-layer hard mask layer in accordance with the
present invention;
[0031] FIGS. 15a and 15b are fragmentary cross section diagrams
illustrating steps in etching the bottom electrode diffusion
barrier layer using a multi-layer hard mask according to the
present invention;
[0032] FIG. 16 is a flow chart diagram illustrating a method of
forming an FeRAM capacitor wherein a sidewall diffusion barrier
layer is deposited and selectively patterned prior to the
patterning of the bottom electrode diffusion barrier layer
according to the present invention;
[0033] FIGS. 17 and 18 are fragmentary cross section diagrams
illustrating steps in depositing and selectively patterning a
sidewall diffusion barrier layer prior to patterning a bottom
electrode diffusion barrier layer according to the present
invention;
[0034] FIG. 19 is a fragmentary cross section diagram illustrating
an etch of the bottom electrode diffusion barrier layer after the
deposition and patterning of the sidewall diffusion barrier layer
according to the present invention; and
[0035] FIGS. 20 and 21 are fragmentary cross section diagrams
illustrating how identification of aluminum oxide "ears" are
employed to ascertain whether the sidewall diffusion barrier layer
is sufficiently thick on sidewalls of FeRAM capacitors stacks
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The present invention will now be described with respect to
the accompanying drawings in which like numbered elements represent
like parts. While the following description of the instant
invention revolves around the integration of the FeRAM devices with
logic devices and other devices which can be found on a digital
signal processor, microprocessor, smart card, microcomputer,
microcontroller or system on a chip, the instant invention can be
used to fabricate stand-alone FeRAM devices or FeRAM devices
integrated into a semiconductor chip which has many other device
types. In particular, the improved performance of the FeRAM device
of the instant invention compared to standard semiconductor
memories appears to make FeRAM the memory of choice for any
handheld device which requires low power and large degree of device
integration.
[0037] The figures provided herewith and the accompanying
description of the figures are provided for illustrative purposes.
One of ordinary skill in the art should realize, based on the
instant description, other implementations and methods for
fabricating the devices and structures illustrated in the figures
and in the following description. For example, while shallow trench
isolation structures ("STI") are illustrated, any conventional
isolation structures may be used, such as field oxidation regions
(also known as LOCOS regions) or implanted regions. In addition,
while structure 102 is preferably a single-crystal silicon
substrate that is doped to be n-type or p-type structure 102 (FIG.
3) may be formed by fabricating an epitaxial silicon layer on a
single-crystal silicon substrate.
[0038] In accordance with the present invention, a plurality of
methods are disclosed which decrease a reduction of an iridium
oxide bottom electrode during a subsequent formation of a
ferroelectric dielectric layer in an FeRAM capacitor. By decreasing
a reduction of the iridium oxide bottom electrode, a fatigue
resistance of the FeRAM cell is improved substantially over the
prior art.
[0039] Referring initially to FIG. 3, an exemplary, fragmentary
cross section of a semiconductor device 100 is provided in which
two devices are illustrated. A first device 103 represents a
partially fabricated version of am FeRAM cell in accordance with
the present invention, and a second device 105 represents any
high-voltage transistor, low-voltage transistor, high-speed logic
transistor, I/O transistor, analog transistor, or any other device
which may be included in a digital signal processor,
microprocessor, microcomputer, microcontroller or any other
semiconductor device. Except for the specific cell structure
provided in the device 103, the structures utilized therein may be
the same as the device structures of the device 105 (except for
some possible variations in the transistors due to the-different
device types that device 105 may be).
[0040] Basically, gate structures 106 include a gate dielectric
(for example, comprising silicon dioxide, an oxynitride, a silicon
nitride, BST, PZT, a silicate, any other high-k material, or any
combination or stack thereof), a gate electrode (for example,
comprising polycrystalline silicon doped either p-type or n-type
with a silicide formed on top, or a metal such as titanium,
tungsten, TiN, tantalum, TaN or other type metal). The gate
structures 106 further comprise sidewall insulators (for example,
comprising an oxide, a nitride, an oxynitride, or a combination or
stack thereof). In general, the generic terms oxide, nitride and
oxynitride refer to silicon oxide, silicon nitride and silicon
oxy-nitride. The term "oxide" may, in general, include doped oxides
as well, such as boron and/or phosphorous-doped silicon oxide.
Source/drain regions 108 may be formed via, for example,
implantation using conventional dopants and processing conditions.
Lightly doped drain extensions 109 as well as pocket implants may
also be utilized. In addition, the source/drain regions 108 may be
silicided (for example, with titanium, cobalt, nickel, tungsten or
other conventional silicide material).
[0041] A dielectric layer 112 is formed over the entire substrate
102 and is patterned and etched so as to form openings for contacts
to the substrate and gate structures 106 to be formed (see, e.g.,
step 202 of FIG. 4). These openings are filled subsequently with
one or more conductive materials, such as a plug 114 (for example,
comprising a metal such as tungsten, molybdenum, titanium, titanium
nitride, tantalum nitride, or a metal silicide such as Ti, Ni or
Co, copper or doped polysilicon). A liner/barrier layer 116 may or
may not be formed between the plug 114 and dielectric 112. Such a
liner/barrier layer 116 is illustrated in FIG. 3 and comprises, for
example, Ti, TiN, TaSiN, Ta, TaN, TiSiN, a stack thereof, or any
other conventional liner/barrier material. Preferably, the contacts
are formed so as to land on the silicided regions of the
source/drain regions and gate structures.
[0042] The dielectric layer 112 comprises, for example, SiO.sub.2
(doped or undoped with preferable dopants such as boron or
phosphorous) possibly with a layer of hydrogen or deuterium
containing silicon nitride next to the gate. After deposition of
the diffusion barrier 116 it is likely that the barrier will be
planarized for improved lithography of overlying layers using a
process such as chemical mechanical polishing (CMP). In addition,
an added diffusion barrier/etch stop (not shown) may be included
near the top surface of layer 112 such as AlO.sub.X, AlN,
Si.sub.3N.sub.4, TiO.sub.2, ZrO.sub.2, or TaO.sub.X that would be
deposited after the planarization process. This diffusion barrier
is particularly useful if damascene processes are used to create
the via or metallization to the contact. The formation of the plug
114 will require etching through this optional barrier/etch
stop.
[0043] Formation of metal structures that are situated above the
contacts is considered to be part of the back end processes. Other
than the specific FeRAM process module, the back end process steps
may be those standard in the semiconductor industry. The
metallization may be, for example, either Al or Cu based. The Al is
preferably etched while the Cu is preferably used in a damascene
approach. However, etching Cu and Al formed in a damascene process
is also possible. According to one example, aluminum metallization
will preferably have CVD tungsten plugs or Al plugs, and the Al
will preferably be Cu-doped for improved electromigration
resistance. Metal diffusion barriers for Al may include, for
example, TiN and/or Ti. Copper metallization may have, for example,
Cu or W plugs with either Ti, TiN, TiSiN, Ta, tantalum nitride,
and/or TaSiN diffusion barriers.
[0044] A thin dielectric layer (not shown) may be formed between
each of the interlevel dielectric (ILD) layers (layers 112, 134 and
160). If formed, this thin dielectric comprises, for example,
silicon nitride, silicon carbide, SiCNO or a silicon oxide (for
example, a high-density plasma oxide). In addition, interlevel
dielectric layers 112, 134, and 160 may comprise, for example, an
oxide, FSG, PSG, BPSG, PETEOS, HDP oxide, a silicon nitride,
silicon oxynitride, silicon carbide, silicon carbo-oxy-nitride, a
low dielectric constant material (for example, SiLK, porous SiLK,
teflon, low-K polymer (possibly porous), aerogel, xerogel, BLACK
DIAMOND, HSQ, or any other porous glass material), or a combination
or stack thereof.
[0045] The interconnects and the metal lines preferably comprise
the same material. Plugs 136 and 150 and conductors 144 and 164
comprise a metal material (for example, copper, aluminum, titanium,
TiN, tungsten, tungsten nitride, or any combination or stack
thereof). A barrier/liner may be formed between the plug and the
respective interlevel dielectric layer. If formed, the
barrier/liner layer (shown as layers 138 and 148 and liners 142,
146, 162 and 166) comprises, for example, Ti, TiN, W, tungsten
nitride, Ta, tantalum nitride, any conventional barrier/liner
layer, or any combination or stack thereof). The interlayer
dielectric and plug material should be compatible with the FeRAM
thermal budget. With existing technology (i.e., one that
incorporates a W plug and SiO.sub.2 ILD), the FeRAM thermal budget
should be less than approximately 600 or 650 C., however, the
present invention is not limited thereto. If the ILD is modified to
include a low dielectric constant ("low K") layer, the FeRAM
thermal budget may need to be reduced further. The preferred
interlayer dielectric 112 is therefore a material that can
withstand a thermal budget in excess of 600 C., such as silicon
oxide (doped and/or undoped), silicon nitride, and/or silicon
oxy-nitride.
[0046] Level 127 is added so as to accommodate the FeRAM cells
(FeRAM process module). This FeRAM process module allows the
creation of ferroelectric or high dielectric constant capacitors to
be easily added with maximum thermal budget for the new process
module yet not impact the thermal budget of backend process. In
particular, this level allows FeRAM devices with capacitor under
bit line configuration compatible with a high-density memory.
However, it is possible, if planarity is not a necessity, to form
the FeRAM devices while not forming layer 127 in region 105. Hence,
the FeRAM portion 103 would be taller than the region 105 by the
height of layer 127.
[0047] Initially, a further discussion of FIG. 3 will be provided
to appreciate the structure of an FeRAM cell and an exemplary
integration position of such a cell within a semiconductor
fabrication process. Subsequently, a flow chart and a number of
fragmentary cross section diagrams will be provided to illustrate
an exemplary process for fabricating such an FeRAM cell in order to
provide a context in which the present invention may reside. In
conjunction therewith, the present invention will be described and
illustrated in greater detail. It should be understood, however,
that although the present invention will be shown and described in
conjunction with one exemplary context, the invention is applicable
to other fabrication methodologies, structures and materials, and
such alternatives are contemplated as falling within the scope of
the present invention.
[0048] An FeRAM capacitor, as illustrated in FIG. 3 at reference
numeral 125, resides above the interlayer dielectric 112, and
comprises several layers. The FeRAM capacitor 125 of FIG. 3
comprises an electrically conductive barrier layer 122 upon which a
conductive bottom capacitor electrode 124 resides (hereinafter, the
terms conductive and insulative are employed to indicate
electrically conductive and electrically insulative, respectively,
unless indicated otherwise). A capacitor dielectric layer 126, a
ferroelectric material, is formed over the bottom electrode 124,
and is covered by, for example, a conductive multi-layer top
electrode 128, 130. A top portion of the FeRAM capacitor 125
comprises a hard mask layer 132 that, as will be discussed in
greater detail later, may be employed to facilitate the capacitor
stack etch. The capacitor stack is then covered by a multi-layer
sidewall diffusion barrier 118, 120.
[0049] An exemplary method of forming an FeRAM capacitor in
accordance with the present invention which is similar in many
respects to the capacitor 125 of FIG. 3 will now be discussed in
conjunction with FIGS. 4, wherein a method 200 of forming an FeRAM
capacitor is disclosed. At 202, the interlevel dielectric 112 is
formed and conductive contacts, for example, tungsten (W) contacts
114 are formed therein with a barrier layer 116 (e.g., TiN)
disposed therebetween to avoid oxidation of the tungsten contacts.
Formation of the interlayer dielectric 112 and the contacts 114 may
be formed by various means and any such process is contemplated as
falling within the scope of the present invention. Then the FeRAM
capacitor(s) are formed over the interlayer dielectric 112 and the
contacts 114, as illustrated in FIG. 3.
[0050] According to one aspect of the present invention, once the
interlayer dielectric 112 and the tungsten contacts 114 are formed,
the FeRAM capacitor formation process begins. According to one
prior art fabrication technique, a bottom electrode was formed
directly over the interlayer dielectric, however, the bottom
electrode material, for example, an iridium or an iridium/iridium
oxide multi-layer would not provide a sufficient diffusion barrier
(e.g., oxygen or hydrogen) during subsequent processing. That is,
during the subsequent formation of the ferroelectric dielectric,
for example, oxygen may diffuse through the bottom electrode and
cause the tungsten to oxidize, thus increasing disadvantageously a
resistance between the contact and the FeRAM capacitor. One
solution to the above problem is to increase a thickness of the
bottom electrode. Since a bottom electrode material acts in some
fashion as a diffusion barrier, by increasing the thickness
thereof, less diffusion would pass therethrough. Such a solution,
however, disadvantageously increases the thickness of the FeRAM
capacitor that preferably is as thin as possible.
[0051] In order to overcome the above disadvantages, another prior
art solution provided a dedicated bottom electrode diffusion
barrier layer. Such a barrier layer is formed over the interlayer
dielectric and the tungsten contact prior to the formation of the
bottom electrode. Such a barrier is electrically conductive and
serves to provide an effective diffusion barrier without having to
increase a thickness of the bottom electrode. Since the diffusion
barrier is a more efficient barrier than the bottom electrode
material(s), even though the additional barrier is employed, the
net thickness is less than would otherwise be required if the
bottom electrode material was increased to provide an equivalent
barrier performance. The prior art bottom electrode diffusion
barrier material was TiAlN, and was formed via physical vapor
deposition.
[0052] The prior art TiAlN diffusion barrier provides an effective
barrier for diffusion of materials such as oxygen and hydrogen. A
problem has been discovered by the inventors of the present
invention that the TiAlN material is a source of integration
problems during subsequent processing of the FeRAM capacitor. More
particularly, it was discovered that during subsequent processing
of the FeRAM capacitor stack (via etching), an etching of the TiAlN
barrier layer between neighboring FeRAM capacitor cells caused a
substantial undercutting of the TiAlN beneath the capacitor stack
and such phenomena negatively contributed to poor step coverage of
a subsequently formed electrically insulating FeRAM sidewall
diffusion barrier. Consequently, the overall diffusion barrier
(top, bottom and sidewalls) of the FeRAM capacitor is compromised.
In order to fully appreciate the problem associated therewith, a
discussion of the FeRAM capacitor stack etch will be discussed in
conjunction with FIGS. 5-6.
[0053] After the TiAlN bottom electrode diffusion barrier layer 122
is formed, the bottom electrode layer 124, the ferroelectric
dielectric layer 126, the top electrode layer 128, 130, and a hard
mask layer 132 are deposited and subsequently etched using the hard
mask to self-align one or more FeRAM capacitor stacks 220, as
illustrated in FIG. 5. The TiAlN bottom electrode diffusion barrier
122 also needs to be etched because TiAlN is electrically
conductive and without further etching, the barrier layer 122 would
short the neighboring capacitors together.
[0054] An etch of the exposed TiAlN barrier 122 between neighboring
FeRAM capacitor stacks is performed typically using a dry, chlorine
based etch and ideally would be anisotropic. However, the inventors
have noted that the chlorine etch has a significant chemical
component that causes the etch to be relatively isotropic,
resulting in an undercutting of the FeRAM stacks, as illustrated in
FIG. 6. Although such an undercut would not appear to negatively
impact the capacitor performance, it has been discovered that such
undercutting negatively impacts the step coverage of a subsequently
deposited sidewall diffusion barrier (not shown). Consequently, the
TiAlN barrier layer 122, due to integration problems, results in
degradation in the subsequently formed sidewall diffusion barrier,
thereby negatively impacting the capacitor performance.
[0055] The inventors of the present invention have overcome the
difficulties of the problem highlighted above by replacing the
TiAlN barrier layer with a TiAlON barrier layer at 204 of FIG. 4.
Such a replacement, however, is not a mere substitution of another
material because one of ordinary skill in the art would not be
motivated to add oxygen because such an addition increases the
electrical resistance of the resulting layer. As discussed
previously, it is desirable to keep the resistance of the barrier
layer as low as possible to thereby reduce the resistance between
the bottom electrode of the FeRAM capacitor and the underlying
transistor through the tungsten contact. The inventors of the
present invention, however, discovered that by adding a small
amount of oxygen, one could obtain a substantial reduction in the
isotropy of the chlorine etch without substantially increasing a
resistivity of the barrier layer, that is, increasing the
resistivity above a predetermined level.
[0056] According to one exemplary aspect of the present invention,
the composition of the TiAlON is tuned to provide sufficient
aluminum therein for adequate oxidation resistance (of the
underlying tungsten contact(s)) and enough oxygen to prevent
undercutting during an etch thereof, yet concurrently maintaining
the resistivity thereof low enough to prevent any appreciable
degradation of the electrical performance of the circuit. In one
example, the aluminum composition is at least about 20 cation atom
%, and less than about 50 cation atom %; and the oxygen composition
is at least about 5 anion atom %, and less than about 50 anion atom
%. In another example, the aluminum composition is at least about
35 cation atom %, and less than about 45 cation atom %; and the
oxygen composition is at least about 10 anion atom %, and less than
about 20 anion atom %.
[0057] In accordance with one exemplary aspect of the present
invention, it is desirable to have a barrier resistivity of about
4300 .mu..OMEGA.-cm or less. Therefore in tailoring the TiAlON
bottom electrode diffusion barrier film, it is desired to keep the
amount of oxygen therein sufficiently low such that the resistivity
does not exceed substantially the 4300 .mu..OMEGA.-cm figure. For
example, with a TiAlON content of about Ti.apprxeq.0.5,
Al.apprxeq.0.4, O.apprxeq.0.1, and N.apprxeq.0.9, a film
resistivity of about 1800 .mu..OMEGA.-cm was obtained. Such a
TiAlON film is formed, for example, via a PVD or sputter deposition
process using an Endura TTN.TM. chamber with a heater temperature
of 400 C., Ar heater 15 sccm, Ar chamber 56 sccm, O.sub.2 chamber 9
sccm, cryo pump. In addition, with such a barrier, experimental
tests indicate that the etch rate is about 150 nm/min using a
BCl.sub.3+Cl.sub.2 chemistry and is about 15 nm/min with solely
Cl.sub.2, compared to a TiAlN etch rate of about 300 nm/min.
Consequently, the TiAlON barrier of the present invention provides
a substantial reduction in etch isotropy with an acceptable
increase in resistivity.
[0058] In addition, another alternative method of forming the
TiAlON film is via MOCVD using TiAlN (creating TiAICON). The
composition is tuned in order to obtain enough Al for oxidation
resistance and enough oxygen for etch undercut yet keeping the
resistance low enough so as not to degrade the electrical
performance.
[0059] The inventors of the present invention also ascertained that
the CMP process used to planarize the interlayer dielectric 112 and
the tungsten contact 114 generates seams in the tungsten contact.
Most seams are about 200-400 Angstroms in diameter and are
adequately filled by the subsequently formed barrier layer, for
example, TiAlN. However, a small percentage of such seams are
substantially larger, for example, about 600-800 Angstroms in
diameter. With such larger seams, the conventional barrier layer,
formed via physical vapor deposition (PVD), does not adequately
fill the seam due to necking, for example. Such poor seam coverage
may undesirably increase the contact resistance between the
underlying transistor and the FeRAM capacitor.
[0060] In accordance with one aspect of the present invention, the
large seams in the tungsten contact 114 are addressed by making the
diffusion barrier layer a multi-layer, with a first layer of TiN
formed over the interlayer dielectric 112 and the tungsten contact
114 via chemical vapor deposition (CVD). Due to the CVD process,
the TiN exhibits exemplary step coverage, thus filling in a portion
of any large seams in the tungsten contacts. For example, a TiN
layer of about 100 Angstroms reduces a seam having a diameter of
about 600 Angstroms to about 400 Angstroms. Since a CVD TiN process
is employed as the contact barrier for tungsten contacts, the
well-developed and characterized process may be employed readily
for such a first layer barrier. The second layer of the multi-layer
barrier is then formed over the CVD TiN, for example, a TiAlN film
or a TiAlON film for the reasons highlighted above. Because the TiN
layer has reduced the size of any large seams, the second layer
(formed via PVD, for example) easily fills the reduced size seam,
and accordingly any potential increase in resistivity due to the
seam is substantially eliminated.
[0061] Therefore in accordance with one aspect of the present
invention, the bottom electrode diffusion barrier comprises a
bi-layer process. For example, first a CVD TiN film is formed
having a thickness of about 40 nm, followed by a PVD TiAlN film of
about 30 nm. In accordance with one exemplary aspect of the present
invention, the TiAlN have at least about 30% aluminum, and less
than about 60% . Alternatively, the TiAlN has at least about 40%
aluminum, and less than about 50% in order to exhibit improved
oxidation resistance.
[0062] In the above example, TiAlN is discussed as one form of
diffusion barrier in conjunction with the initial TiN layer.
Alternatively, a TiAlON layer as discussed above may be utilized in
conjunction with the TiN; and other diffusion barrier layers may be
employed, for example, TaSiN, TiSiN, TaAlN, Ti, TiN, Ta, TaN, HfN,
ZrN, HfAlN, CrN, TaAlN, CrAlN. The preferred deposition technique
for these barrier layers is a reactive sputter deposition using
Ar+N.sub.2 or Ar+NH.sub.3. Other deposition techniques that might
be used include CVD or plasma enhanced CVD. In any event, it is
preferred to use materials that have a slower oxidation rate than
TiN.
[0063] In addition to the discussion above regarding the diffusion
barrier layer, it is desirable to perform a clean operation of the
interlayer dielectric 112 and the tungsten contact 114 prior to the
deposition of the barrier layer. For example, one option is to
sputter clean with Ar prior to the deposition of the barrier layer.
It is further preferred that this pre-clean occur without a vacuum
break prior to the deposition of the barrier.
[0064] The next layer is the oxygen stable bottom electrode 124, as
illustrated at 206 of FIG. 4. This layer needs to be stable during
the subsequent deposition of the ferroelectric and can strongly
impact the properties of the ferroelectric capacitor. For example,
with a PZT ferroelectric the reliability is improved with oxide
electrodes. The electrode experiences the thermal budget and
oxidizing conditions of the ferroelectric deposition and possibly
anneal. Therefore the bottom electrode preferably is stable in
oxygen and does not form insulating layers as a result of such
oxygen. It is also advantageous that the bottom electrode at least
partially impedes the oxidation and reaction of the underlying
diffusion barrier.
[0065] In addition, the electrode preferably maintains a relatively
low contact resistance. A list of possible materials includes Pt,
Pd, PdOx, IrPt alloys, Au, Ru, RuO.sub.x, (Ba,Sr,Pb)RuO3,
(Sr,Ba,Pb)IrO3, Rh, RhO.sub.x, LaSrCoO.sub.3, (Ba,Sr)RuO.sub.3,
LaNiO.sub.3.
[0066] The bottom electrode 124 of the capacitor is formed either
with or without the barrier layer 122 so as to make electrical
connection with the underlying contact structure 114. Preferably,
the bottom electrode 124 is around 30-100 nm thick, is stable in
oxygen and is comprised of a noble metal or conductive oxide such
as iridium, iridium oxide, Pt, Pd, PdOx, Au, Ru, RuO.sub.x, Rh,
RhO.sub.x, LaSrCoO.sub.3, (Ba,Sr)RuO.sub.3, LaNiO.sub.3 or any
stack or combination thereof. For any electrode using noble metals
it is advantageous from a cost and ease of integration standpoint
to use layer as thin as possible.
[0067] For a PZT dielectric, it is preferred to have an oxide
electrode such as IrOx in contact therewith at the top and/or
bottom electrode. In addition, it is preferred to have a noble
metal between the conductive oxide electrode and either the top or
bottom diffusion barrier. The noble metal prevents oxidation of the
diffusion barrier and the resulting formation of an insulating
layer that increases undesirably the contact resistance. The
preferred thickness of this noble metal layer is 10-30 nm. Thus in
one aspect of the invention, the bottom electrode 124 comprises an
Ir/IrO.sub.2 sandwich.
[0068] The preferred bottom electrode for PZT is either 50 nm Ir
deposited by sputter deposition for Ir (Ar) and/or reactive sputter
deposition (Ar+O.sub.2) for IrOx. A second preferred electrode
stack uses 10 nm Ir on 30 nm IrOx on 20 nm Ir (one potential
thickness is shown). A third preferred embodiment is IrOx
(.about.30-40 nm) on Ir (20-30 nm). The preferred deposition
technique for these layers is sputter or reactive sputter
deposition or chemical vapor deposition.
[0069] The capacitor dielectric 126 is formed on the bottom
electrode 124, as illustrated at 208 of FIG. 4. Preferably, the
capacitor dielectric is less than 150 nm thick (more preferably
less than 100 nm thick-most preferably less than 50 nm thick) and
is comprised of a ferroelectric material, such as Pb(Zr,Ti)O.sub.3
PZT (lead zirconate titanate), doped PZT with donors (Nb, La, Ta)
acceptors (Mn, Co, Fe, Ni, Al) and/or both, PZT doped and alloyed
with SrTiO3, BaTiO3 or CaTiO3, strontium bismuth tantalate (SBT)
and other layered perovskites such as strontium bismuth niobate
tantalate (SBNT) or bismuth titanate, BaTiO3, PbTiO3,Bi2TiO3
etc.
[0070] PZT is a desirable choice for the capacitor dielectric
because it has the highest polarization and the lowest processing
temperature of the aforementioned materials. Thin PZT (<100 nm)
is extremely advantageous in making integration more simple (less
material to etch) and less expensive (less material to deposit
therefore less precursor). Because PZT has the largest switched
polarization, it is also possible to minimize capacitor area using
such material.
[0071] The preferred deposition technique for these dielectrics is
metal organic chemical vapor deposition (MOCVD). MOCVD is preferred
especially for thin films (<100 nm). MOCVD also permits the film
thickness to be scaled without significant degradation of switched
polarization and coercive field, yielding PZT films with a low
operating voltage and large polarization values. In addition, the
reliability of the MOCVD PZT film is better than that generally
obtained using other deposition techniques, particularly with
respect to imprint/retention.
[0072] Specifically, in one example, MOCVD PZT ferroelectric films
are grown in an Applied Materials 200 mm MOCVD Giga-Cap.TM. chamber
integrated with a liquid delivery system and vaporizer installed on
a Centura.TM. mainframe, as illustrated in FIG. 7. The baseline PZT
film deposition parameters are described in Table PZT1 provided
herein below, and the preferred metalorganic precursors are
detailed in Table PZT2, also provided infra. Use of the pre-mixed
precursor "cocktails" described in Table PZT2 provides enhanced
repeatability and throughput relative to the use of elemental
precursor solutions. The precise composition and molarity of the
starting precursor solutions can be varied as needed. Use of
pre-mixed "cocktails" permits hardware simplification since only a
single vaporizer is needed. Earlier processes employed multiple
elemental precursors and required two vaporizers rather than the
simplified configuration shown in FIG. 7.
[0073] The preferred process sequence 208 proceeds as follows.
After the wafers are delivered to the CVD chamber, the wafer sits
on the lift pins above the wafer heater for about 30 sec (shorter
times are also possible, for example, .about.5 sec to 30 sec). This
allows the wafer to pre-heat, avoiding thermal shock, which causes
the wafer to break. Next, the wafer is lowered onto the wafer
heater and the temperature is allowed to stabilize for about 60
sec. During these first two steps, the metalorganic precursors are
sent directly to the vaporizer bypass line, bypassing the process
chamber. PZT deposition begins once the precursor flow is diverted
into the CVD chamber by opening the chamber valve and closing the
bypass valve. The process parameters shown in Table PZT1 provide a
deposition rate of approximately 160 .ANG./min. PZT deposition ends
when the precursors are sent back to the vaporizer bypass line.
Following deposition, the wafer remains in the chamber for 5 sec
with the throttle valve open to evacuate the chamber before opening
the slit valve between chambers for removal.
2TABLE PZT1 Preferred process parameters for depositing PZT thin
films by MOCVD. Heater Temperature 640 to 650.degree. C. Wafer
Temperature .about.600 to 609.degree. C. Pre-Deposition Time on
Chuck 30 sec on-pins/60 sec on-heater He B Carrier Flow Through 250
sccm Vaporizer #1 He B Carrier Flow Through 250 sccm Vaporizer #2
Oxygen Flow 1000 sccm Ar Purge Flow 250 sccm Ar Push Gas Pressure
on Precursor 60 psi Ampoules Vaporizer Temperature 190.degree. C.
Jackets/Lid/Feedthrough 190.degree. C. Temperatures Showerhead to
Heater Spacing 350 mils Chamber Pressure 4 Torr Precursor #1 Flow
(PZTG-2103) 65 to 82 mg/min Precursor #2 Flow (PZTG-2104) 118 to
135 mg/min Total Precursor Flow 200 mg/min Pb/(Zr + Ti) (in liquid)
1.00 to 1.14 Zr/(Zr + Ti) (in liquid) 0.40 Deposition Rate 160
.ANG./min Substrate Ir (100 nm)/Si.sub.3N.sub.4/SiO.sub.2/Si and
IrO.sub.x (50 nm)/Ir (50 nm)/Si.sub.3N.sub.4/SiO.sub.2/ Si
[0074]
3TABLE PZT2 Metalorganic precursors employed for CVD PZT
deposition. Precursors were purchased from Advanced Technology
Materials (ATMI). Pb Zr Ti Conc Conc Conc Ampoule Precursor (Molar)
(Molar) (Molar) #1 Pb(thd).sub.2-pmdeta, Zr(O- 0.090 0.090 0.135
iPr).sub.2(thd).sub.2, and Ti(O- iPr).sub.2(thd).sub.2 in an
octane-based solvent system (ATMI Product #: PZTG- 2103) #2
Pb(thd).sub.2-pmdeta + 0.205 0.045 0.066 Zr(O-iPr).sub.2(thd).sub.2
+ Ti(O-iPr).sub.2(thd).sub.2 in octane-based solvent system (ATMI
Product #: PZTG-2104)
[0075] In accordance with one example, the dependence of film
Pb/(Zr+Ti).sub.film ratio and the Pb/(Zr+Ti).sub.gas ratio in the
gas phase is illustrated in FIG. 8. The process described in Table
PZT1 provides the characteristic plateau region in which the Pb
composition in the film is self-correcting. Generally, the plateau
region, an example of which is shown in FIG. 8, is the processing
regime that provides the optimum PZT properties. Using the
preferred process parameters in Table PZT1, the self-correcting
plateau is observed between Pb/(Zr+Ti).sub.gas ratios of
approximately .about.0.8 and 1.3 for a wafer temperature of about
600.degree. C. (heater set point temperature of about 640.degree.
C.). Within this composition region, stoichiometric, single-phase,
(111)-textured PZT thin films are obtained. The optimum physical
and electrical properties are observed within the more narrow range
of Pb/(Zr+Ti).sub.gas=1.00 to 1.14, with a preferred ratio of
1.07.
[0076] In addition to the preferred process described above, lower
temperature processes may also be used for the PZT deposition step.
Reducing the overall thermal budget simplifies capacitor
integration, and depositing the PZT film is typically the highest
temperature step. FIG. 9 shows the effect of reduced temperature on
the size of the self-correcting Pb composition regime for the
standard process conditions given in Table PZT1. It is evident in
FIG. 9 that the size of the self-correcting plateau decreases
significantly as the temperature is reduced. The PZT films also
become increasingly rough at low temperature. For a wafer
temperature of about 587.degree. C. (heater set point of about
620.degree. C.), the self-correcting behavior is no longer observed
when the standard process conditions are used. As mentioned above,
a self-correcting plateau is needed in order to obtain high quality
PZT films and a robust process.
[0077] To reduce the deposition temperature and maintain a large
self-correcting plateau, the process pressure is reduced from about
4 Torr to about 2 Torr and N.sub.2O is added to the conventional
O.sub.2 oxidizer gas stream (total oxidizer flow is kept constant).
As shown in FIG. 10, for a wafer heater temperature of about
630.degree. C., reduced pressure combined with N.sub.2O+O.sub.2 gas
flow significantly increases the size of self-correcting plateau
region relative to the standard process. A similar effect is shown
in FIG. 11 for a wafer heater temperature of 620.degree. C. A range
of pressures (1 to 10 Torr) and N.sub.2O additions (10% to 100%)
may be used for the low temperature deposition processes. Two
exemplary processes are described in Tables PZT3 and PZT4, and
these processes provide the results shown in FIGS. 10 and 11,
respectively. Using this approach, the wafer heater temperature can
be reduced from about 640.degree. C. to about 620.degree. C., with
a corresponding reduction in wafer temperature from about
600.degree. C. to about 575.degree. C.
4TABLE PZT3 Low temperature CVD PZT process #1. Heater Temperature
630.degree. C. Wafer Temperature .about.586.degree. C.
Pre-Deposition Time on Chuck 30 sec on-pins/60 sec on-heater He B
Carrier Flow Through 250 sccm Vaporizer #1 He B Carrier Flow
Through 250 sccm Vaporizer #2 Oxygen Flow 500 sccm N.sub.2O Flow
500 sccm Ar Purge Flow 250 sccm Ar Push Gas Pressure on Precursor
60 psi Ampoules Vaporizer Temperature 190.degree. C.
Jackets/Lid/Feedthrough 190.degree. C. Temperatures Showerhead to
Heater Spacing 350 mils Chamber Pressure 2 Torr Precursor #1 Flow
(PZTG-2103) 82 to 115 mg/min Precursor #2 Flow (PZTG-2104) 85 to
118 mg/min Total Precursor Flow 200 mg/min Pb/(Zr + Ti) (in liquid)
0.79 to 1.00 Zr/(Zr + Ti) (in liquid) 0.40 Deposition Rate
.about.107 .ANG./min Substrate Ir (100
nm)/Si.sub.3N.sub.4/SiO.sub.2/Si and IrO.sub.x (50 nm)/Ir (50
nm)/Si.sub.3N.sub.4/SiO.sub.2/ Si
[0078]
5TABLE PZT4 Low temperature CVD PZT process #2. Heater Temperature
620.degree. C. Wafer Temperature .about.575.degree. C.
Pre-Deposition Time on Chuck 30 sec on-pins/60 sec on-heater He B
Carrier Flow Through 250 sccm Vaporizer #1 He B Carrier Flow
Through 250 sccm Vaporizer #2 Oxygen Flow 250 sccm N.sub.2O Flow
750 sccm Ar Purge Flow 250 sccm Ar Push Gas Pressure on Precursor
60 psi Ampoules Vaporizer Temperature 190.degree. C.
Jackets/Lid/Feedthrough 190.degree. C. Temperatures Showerhead to
Heater Spacing 350 mils Chamber Pressure 2 Torr Precursor #1 Flow
(PZTG-2103) 82 to 115 mg/min Precursor #2 Flow (PZTG-2104) 85 to
118 mg/min Total Precursor Flow 200 mg/min Pb/(Zr + Ti) (in liquid)
0.79 to 1.00 Zr/(Zr + Ti) (in liquid) 0.40 Deposition Rate
.about.96 .ANG./min Substrate Ir (100
nm)/Si.sub.3N.sub.4/SiO.sub.2/Si and IrO.sub.x (50 nm)/Ir (50
nm)/Si.sub.3N.sub.4/SiO.sub.2/ Si
[0079] The top electrode 128, 130 is formed on the capacitor
dielectric 126, as illustrated at 210 of FIG. 4. In this embodiment
of the instant invention, the top electrode is comprised of two
layers 128, 130, however, the top electrode can be implemented in
just one layer. Preferably, the layer next to the PZT dielectric is
comprised of iridium oxide (preferably less than 100 nm thick--more
preferably less than 50 nm thick and even more preferably less than
30 nm thick). Preferably, the layer between the conductive oxide
and top electrode diffusion barrier/hard mask is comprised of
iridium (preferably less than 100 nm thick--more preferably less
than 50 nm thick and even more preferably less than 20 nm
thick).
[0080] In particular it is advantageous for Pb based ferroelectrics
to have a conductive oxide top electrode such as IrO.sub.x,
RuO.sub.x, RhO.sub.x, PdO.sub.x, PtO.sub.x, AgO.sub.x,
(Ba,Sr)RuO.sub.3, LaSrCoO.sub.3, LaNiO.sub.3,
YBa.sub.2Cu.sub.3O.sub.7-X rather than a noble metal in order to
minimize degradation due to many opposite state write/read
operations (fatigue). Many of the Bi ferroelectrics such as SBT can
also use noble metal electrodes such as Pt, Pd, Au, Ag, Ir, Rh, and
Ru and still retain good fatigue characteristics.
[0081] If the top electrode is an oxide it is advantageous to have
a noble metal layer above it in order to maintain low contact
resistance between the top metal contact and oxide. For example, it
is possible that a TiN in contact with IrOx might form TiO2 during
subsequent thermal processes that is insulating. For any electrode
using an expensive noble metal such as Pt, Ru, Pd, or Ir it is
advantageous from a cost and integration standpoint to use as thin
of layer as possible.
[0082] For PZT electrodes (electrodes bounding a PZT dielectric),
the preferred top electrode stack is .about.20 nm Ir (130)
deposited by PVD in Ar on .about.30 nm IrOx (128) deposited by
reactive PVD in Ar+O.sub.2 on top of the PZT 126. IrOx is preferred
to be deposited .about.500.degree. C. in gas mixtures between 30%
and 50% O2 with the rest oxygen with a relatively low sputter power
and hence slow deposition rate (preferred to be less than 20
nm/min).
[0083] Preferably, the entire capacitor stack (220 of FIG. 5) is
patterned and etched at one time (preferably using a different
etchant for some of the layers), but each layer or grouping of
layers alternatively may be etched prior to the formation of the
subsequent layer or layers. If multiple layers or all of the layers
are etched simultaneously, then a hard mask layer (e.g., 132 of
FIG. 3) is formed over the stack at 212 of FIG. 4. Preferably, the
hard mask is thick enough and comprised of a material so as to
retain its integrity during the etch process.
[0084] It is preferred that the capacitor be completely enclosed by
conductive top and bottom diffusion barriers and an insulating
sidewall diffusion barrier. In some integration approaches it is
preferred to have a conductive diffusion barrier remain on top of
the capacitor after etching the entire capacitor. In all cases it
is preferred to have a capacitor as short as possible for ease of
integration.
[0085] Prior art hard masks 132 are composed of a single layer of
material, typically a TiN layer or a TiAlN layer, which in some
cases has been left on top of the FeRAM capacitor stack after
definition thereof for subsequent use as a hydrogen barrier.
Unfortunately, since a bottom electrode diffusion barrier layer 122
may also be made of the same material, a substantial portion of the
hard mask 132 will be removed during the etch of the bottom
electrode diffusion barrier layer 122, as illustrated in FIGS. 12
and 13.
[0086] Note that in FIG. 12, the hard mask 132 and the bottom
electrode diffusion barrier 122 are the same material (e.g.,
TiAlN). Since the barrier 122 is electrically conductive, the
barrier must be removed in order to electrically isolate
neighboring FeRAM cells from one another. As the barrier layer 122
is etched, the hard mask 132 etches at the same rate, thereby
causing a substantial reduction of the hard mask thickness and a
substantial rounding at the corners 131, as illustrated in FIG. 13.
Such corner rounding can lead to an exposure of the top electrode
130 (e.g., an iridium layer) at the corners 131 that is undesirable
for several reasons. First, such exposure may cause the top
electrode 130 to contaminate production tools during subsequent
processing; in addition, since the exposed electrode 130 is not an
effective hydrogen diffusion barrier, the ferroelectric dielectric
material 126 may experience hydrogen contamination (for reasons
described in greater detail below) that may degrade substantially
the FeRAM performance.
[0087] One prior art solution is to further increase the thickness
of the hard mask 132 to compensate for such loss during etch of the
bottom electrode diffusion barrier 122. Such a solution, however,
undesirably increases the overall height of the FeRAM capacitor. In
addition, through experiments it has been found that even with such
compensation, a side portion of the underlying top electrode 130
(e.g., an iridium electrode) may still be subsequently exposed.
Although a subsequent insulative barrier (e.g., an AlOx film, not
shown) will be deposited over the capacitor stack, the insulative
barrier is relatively thin and is intended as a sidewall barrier,
and thus does not cover the exposed top electrode well after
patterning thereof. Since the exposed portion of the top electrode
130 is not an effective barrier, the FeRAM is then negatively
susceptible to hydrogen exposure, thereby degrading the performance
thereof as described above.
[0088] In accordance with one aspect of the present invention, a
multi-layer hard mask is disclosed in which a hard masking layer
overlies an etch stop layer. The etch stop layer is substantially
more selective than the overlying masking layer with respect to an
etch employed to remove the bottom electrode diffusion barrier
layer. Therefore during an etch of the capacitor stack, an etch of
the bottom electrode diffusion barrier layer results in a
substantially complete removal of the hard masking layer. However,
due to the substantial selectivity (e.g., 10:1 or more) of the etch
stop layer with respect to the overlying masking layer and bottom
electrode diffusion barrier 122, the etch stop layer completely
protects the underlying top electrode, thereby preventing exposure
thereof. In addition, in accordance with another aspect of the
present invention, the etch stop layer is electrically conductive
and serves as a diffusion barrier, thereby eliminating a need for
another diffusion barrier layer and reducing an overall height of
the FeRAM capacitor stack.
[0089] In accordance with one aspect of the present invention, a
method 212 (e g., of FIG. 4) of forming an FeRAM using a
multi-layer hard mask is illustrated in FIG. 14, and designated at
reference numeral 212. Initially, the capacitor stack layers are
formed at 204, 206, 208 and 210, as discussed previously, and
comprise, for example, the bottom electrode layer(s) 124, the
ferroelectric dielectric layer 126, and the top electrode layer(s)
128, 130. Subsequently, a multi-layer hard mask 299 is formed
thereover at 300, for example, by forming an etch stop layer 302 at
304 of FIG. 14 followed by a masking layer 306 at 308, as
illustrated in FIG. 15a. In accordance with one aspect of the
present invention, the etch stop layer 302 comprises an
electrically conductive material that has a substantially slower
etch rate than the overlying masking layer 306 with respect to an
etch chemistry employed to etch the bottom electrode diffusion
barrier layer 122. For example, for a TiAlN masking layer 306, the
etch stop layer 302 comprises a TiAlON layer with a substantial
amount of oxygen therein, and the benefits of such an etch stop
layer 302 will be apparent in the subsequent discussion of the
method 212.
[0090] The method 212 then continues at 310, wherein a substantial
portion of the capacitor stack is etched by patterning the
multi-layer hard mask 299 and using the multi-layer hard mask to
etch the underlying layers in the stack, for example, etching the
top electrode layer(s) 128, 130, the ferroelectric layer 126, and
the bottom electrode layer(s) 124. Note that the bottom electrode
diffusion barrier layer 122 has still not been etched, and an etch
of such layer must proceed since the layer is electrically
conductive and thus the exposed portion must be removed in order to
electrically isolate neighboring FeRAM capacitor stacks.
[0091] In accordance with the present invention, the bottom
electrode diffusion barrier layer 122 has a composition
substantially the same as the masking layer 306, for example,
TiAlN. Therefore, as the method 212 proceeds to 312, an etch of the
bottom electrode diffusion barrier layer 122 results in a
substantial or complete removal of the masking layer 306 on top of
the capacitor stack, as illustrated in FIG. 15b. In accordance with
one exemplary aspect of the present invention, the masking layer
306 is sufficiently thin that an overetch of the barrier layer 122
results in substantially all of the masking layer 306 being
removed, thereby exposing the underlying etch stop layer 302.
[0092] As discussed above, an etch rate of the etch stop layer 302
is substantially less than an etch rate of the overlying masking
layer 306 during the patterning of the bottom electrode diffusion
barrier layer 122. In the above manner, a substantial overetch of
the barrier layer 122 may be performed without an exposure of the
top electrode layer 130 (e.g., an iridium layer). In addition, by
completely removing the overlying masking layer 306 off of the
capacitor stack, the total height of the stack is reduced, which
advantageously aids in subsequent integration steps.
[0093] In accordance with one exemplary aspect of the present
invention, an etch of the TiAlN barrier layer 122 and the TiAlN
masking layer 306 is performed using a chlorine etch chemistry.
With such etch chemistry, an etch selectivity between the masking
layer 306 and the underlying TiAlON etch stop layer 302 is about
10:1 or more. Therefore when etching the barrier layer 122, once
the etch removes the masking layer 306 and reaches the etch stop
layer 302 on top of the capacitor stack, the etch slows down
substantially, thereby allowing a substantial overetch of the
barrier layer 122. In addition, since the etch slows down
substantially, the etch stop layer 302 completely protects the
underlying top electrode layer 130, thereby preventing an exposure
of a side portion 131 thereof, as illustrated in FIG. 15b. Thus,
the etch stop layer 302 is all that remains of the multi-layer hard
mask after the patterning of the bottom electrode diffusion barrier
layer 122.
[0094] Therefore, in accordance with one aspect of the present
invention, a multi-layer hard mask 299 and a method 212 of forming
an FeRAM using such a multi-layer hard mask is disclosed. In such a
solution, the hard mask 299 is composed of two or more layers (302,
306), wherein the top layer 306, the masking layer, acts as the
hard mask and is removed as part of the capacitor stack etch
process. The second, underlying layer 302, the etch stop layer,
acts as the top electrode diffusion barrier and an etch stop during
the capacitor stack etch. A third, optional layer (not shown) is an
additional diffusion barrier, and may be located between the etch
stop layer 302 and the underlying top electrode 130.
[0095] As discussed above, the top layer, the masking layer 306,
may be the same material as the bottom electrode diffusion barrier
layer, for example, TiAlN, however, such a solution is not required
by the present invention. Rather, as long as the etch rate of the
masking layer 306 is approximately the same rate as the bottom
electrode diffusion barrier layer 122, any material may be employed
and is contemplated as falling within the scope of the present
invention. For example, one may use a conductive nitride, carbide
or carbo nitride as the masking layer.
[0096] Similarly, although TiAlON is disclosed as one exemplary
etch stop layer, it should be understood that any material that
exhibits a high selectivity with respect to the bottom electrode
diffusion barrier layer 122 (and the masking layer 306) may be
employed and is contemplated by the present invention. For example,
an oxygen doped material may be employed as the etch stop. An oxide
is a good etch stop using the appropriate etch chemistries. It has
been noted that Cl.sub.2+BCl.sub.3 chemistries with quickly etch
both materials yet Cl.sub.2 or Cl.sub.2+Ar chemistries only slowly
etch the oxy-nitride films. Although adding oxygen to these
materials will in general increase the resistivity, there is in
general a window where the resistance is acceptable and where a
large change in the etch characteristics can be obtained. An
estimate of the maximum resistivity for the etch stop layer 302 is
about 10 m.OMEGA.-cm.
[0097] In one example of the present invention, the multi-layer
hard mask 299 is about 50 to 500 nm thick (more preferably around
100 to 300 nm thick--most preferably around 200 nm thick). The hard
mask 299 thickness is controlled by the etch process and the
relative etch rates of the various materials, the thicknesses of
the etched layers, the amount of overetch required, and the desired
remaining etch stop layer thickness after etching all of the
layers. Thinner ferro stack layers can use thinner hard masks. The
hard mask 299 may or may not be removed after the etching of the
capacitor stack. If the hard mask 299 is not removed, then it is
preferable to form the hard mask of a conductive material. However,
a non-conductive or semiconductive material may be used, but the
interconnection to the top electrode of the capacitor should
preferably be formed through this hard mask so as to make direct
connection to the top electrode.
[0098] In accordance with an alternative aspect of the present
invention, a single layer, pure oxide hard mask etch stop that is
insulating may be employed, but the etch process must then be
adjusted in order to ensure that this layer is completely
removed.
[0099] In accordance with one exemplary aspect of the present
invention, the etch stop layer 302 also serves as the top electrode
diffusion barrier layer and comprises TiAlON having a film
thickness of about 20 nm or more and about 50 nm or less. In
addition, the TiAlON has a composition of oxygen that maintains the
resistivity of the layer to about 10 m.OMEGA.-cm or less.
Alternatively, etch stop layer and the barrier layer may comprise
separate layers, for example, a TiAlON etch stop layer having a
thickness of about 15 nm or more and about 40 nm or less, with the
etch stop layer overlying a TiAlN diffusion barrier layer (not
shown) having a thickness of about 20 nm or more and about 40 nm or
less.
[0100] In the preferred embodiment shown in FIG. 15a, the hard mask
299 comprises a TiAlN masking layer 306 having a thickness of about
200 nm on top of a TiAlON etch stop layer 302 having a thickness of
about 30 nm and a composition of about O 0.6, N 0.4. In addition, a
separate barrier layer (not shown) may comprise a TiAlN layer
having a thickness of about 30 nm.
[0101] In alternative preferred embodiment, the hard mask 299 is
further simplified to two layers of TiAlN layer of about 200 nm
thick on top of a hard mask etch stop layer of TiAlON (composition
(O 0.6 N 0.4)) and about 40 nm thick where the TiAlON layer acts
both as hard mask etch stop and also as conductive hydrogen
diffusion barrier.
[0102] In another alternative aspect of the present invention, one
may employ TiN and TiON as the masking layer and the etch stop
layer/barrier layer, respectively. These materials are
advantageously because the materials are relatively simple to
deposit, have a low resistivity. In particular, the resistivity of
TiO is not much higher than TiN and because of the higher oxygen it
creates an excellent etch stop.
[0103] After the contact formation several different deposition
steps have been described. In particular, the formation of the
bottom diffusion barrier 122, bottom electrode 124, ferroelectric
126, top electrode 128, 130 and hard mask 132, 299. It is likely
that all or nearly all of these pieces of equipment will be
considered potentially contaminated by ferroelectric elements.
Therefore these pieces of equipment may be considered dedicated.
The wafers will most likely have a reasonable, if not a high
contamination level on the backside of the wafers. The next process
step after hard mask deposition is typically lithography. It is
likely that processing wafers with backside contamination through
this tool will contaminate the tool and hence result in
contamination of clean wafers processed through this tool with
FeRAM contaminants on their backside. Therefore it is preferred to
clean the backsides of the FeRAM wafers in order to share the
lithography equipment and allow clean wafers to be processed
through the lithography equipment and not have any FeRAM
contamination.
[0104] The clean process depends on the backside contamination
elements and their contamination levels. Assuming the preferred
approach (PVD barrier, hard mask, bottom electrode, top electrode
and MOCVD PZT) there will be low levels of Ir on the backside, but
continuous films assuming the MOCVD process does not have edge
exclusion. Therefore for this type of wafer contamination the
preferred backside wafer clean process is a wet etch process that
etches the back, edges and small region on the front side of the
wafer near the edge. The etch process is somewhat dependent on the
materials present on the backside of the wafer (for example if it
is Si, SiO.sub.2 or Si.sub.3N.sub.4). As discussed earlier it is
preferred to have SiN present on the wafer backside because very
aggressive chemicals can be used to etch any ferro contamination
while minimizing the amount of SiN etched. An example of an
aggressive chemical is hot 80.degree. C. concentrated HF or a bath
chemistry that combines hot aqua regia (HCl+HNO.sub.3) or SC.sub.2
(HCl+H.sub.2O.sub.2) with HF. Instead of using peroxide dissolved
ozone can be used and can be even more effective. The SiN also has
a slow diffusion rate of these materials and hence only a small
amount of SiN needs to be removed in order to have surfaces so
clean that there are no detectable levels of ferro elements.
[0105] It is preferred to perform pattern and etch the capacitor
stack with only one lithography step. This is not only cheaper but
also allows the cell size to be smaller by eliminating misalignment
tolerances that are necessary if more than one lithography step is
used.
[0106] It is preferred that patterning process is by lithography
and the type of lithography is dependent on the desired size of the
feature size plus the misalignment to underlying layer (CONT in
this example). In this particular example, the lithography was
performed using DUV (248 nm) lithography process using organic BARC
and relatively thin resist. Larger features could use i-line or
even g-line features while even smaller features would require even
smaller wavelengths 193 nm or 157 nm. Instead of an organic BARC an
inorganic BARC could also be used and this might be incorporated
into the hard mask etch process. The resist thickness is chosen
such that it can hold up during the hard mask etch process. In this
example the resist was 510 nm thick while the hard mask was 260 nm
thick, the BARC thickness was 60 nm and 248 nm lithography was used
to pattern capacitors .about.250-600 nm in size with a gap spacing
of 180 to 300 nm narrow space between the capacitors. Wider spaces
were also printed depending on cell size and shape.
[0107] As mentioned before the preferred approach is to use a hard
mask with multiple etch processes. These etch process can be
modified by using elevated temperatures in order to achieve even
steeper sidewall slopes and therefore less CD growth. In general it
is preferred to minimize CD growth and this can be achieved by
having steeper etch profile or by having thinner layers.
[0108] The first step is etching the hard mask and any BARC at 212
of FIG. 4 (inorganic or organic) that might be used followed by
clean/ash processes. In this preferred embodiment the BARC/hard
mask is preferably etched in the same chamber using a one or more
step etch recipe. In general, the preferred etch system is has a
high density plasma. In this example, two steps are used. The BARC
etch consisted of Cl.sub.2+O.sub.2 at low pressures (3 mTorr) at
temperatures low enough not to damage the resist (60.degree. C.)
with high density plasma (300 W RF source power) and small bias (50
W RF on the chuck). An example of a tool of this type is the AMAT
DPS etch tool. After a short gas purge step the multi-layer Ti
containing hard mask stack is etched in Cl.sub.2+BCl.sub.3 gas
chemistry (BCl.sub.3 substantially helps etching of O containing
compounds present as crust on surface or as in this example O
containing layers) at same temperature as BARC etch (60.degree. C.)
at low pressure (5 mTorr) and high plasma density (1400 W source
power) and still low bias (100 W RF on chuck). The exact process is
adjusted to minimize CD growth, maximize uniformity and
repeatability of etch process while trying to minimize hard mask
thickness.
[0109] After the hard mask is etched it is necessary to ash the
resist and if the resist process leaves a residue, a clean is
sometimes used (either before or after the ash) in order to remove
this residue. The ash process can be performed either in a separate
chamber/process tool, in situ in the hard mask etch tool or in situ
in the ferro stack etch chamber. The clean can either be wet or dry
and might be something as simple as DI dunk/rinse or possibly use
more active chemicals such as solvent (EKC for example). The goal
of the clean is to remove particles and residue. In this specific
example, an O.sub.2 ash step (alternatively O.sub.2+H.sub.2O or
even fluorine containing compounds could be used) at elevated
temperatures in order to enhance the ash rate (250.degree. C.)
using high plasma density, but low energy and little if any
substrate bias (AMAT ASP chamber, for example). In order to try to
remove the need for a wet clean process a more aggressive ash
process can be used such as adding fluorine compounds such as
CF.sub.4 to the ash process and adding a small amount of substrate
bias.
[0110] In general it is preferred to etch as many layers in the
same chamber as possible. Since the top electrode and bottom
electrode are typically hard to etch materials and usually very
similar materials there is a strong preference to etch this ferro
stack (top electrode, ferroelectric and bottom electrode) in the
same etch chamber. The following discussion assumes that one etch
chamber is used, to etch this ferro stack at 212 of FIG. 4 because
that is preferred, but it is also possible to use multiple chambers
which has the advantage that each chamber can be optimized for each
process.
[0111] The top electrode 128, 130 is etched after patterning the
hard mask. In order to etch noble metal top electrodes (preferably
Ir/IrOx) a high temperature etch process is used because this is
typically needed in order to achieve a chemical etch. The chemical
etch has the advantage that steeper sidewall profiles can be
achieved with usually less CD growth and better etch selectivity to
other layers such as the hard mask. One exemplary etch chemistry
for these noble metal electrodes is Cl.sub.2+O.sub.2 or
Cl.sub.2+CO. Other gas additives such as N.sub.2 or Ar can also be
added although Ar in particular is-usually not a good choice
because it only etches by physical mechanisms and not chemical. An
exemplary process is sensitive to electrode and hard mask material.
In this exemplary embodiment the top electrode etch is
Cl.sub.2+O.sub.2+N.sub.2. For Ti containing hard masks the oxygen
content of the top electrode etch preferably is >5-10% in order
to achieve a very low hard mask etch rate (e.g., good selectivity
to the hard mask). The optimum etch process uses a high density
plasma (1200 W, for example) and an intermediate substrate bias
(300 W chuck) at intermediate pressures (10 mTorr) and elevated
temperatures (350-400.degree. C.). This type of process results in
etch rates in the range of about 80 nm/min.
[0112] In general there is a tradeoff between physical etching
using a larger substrate bias (lower pressure or higher chuck
power) with the advantage of a faster etch rate and chemical
etching which in general has the advantage of a steeper capacitor
profile, less CD growth and better selectivity to the hard mask.
The gas composition can be optimized to provide the maximum
chemical etch rate or the maximum ratio of chemical etch rate to
physical etch rate. This difference can be estimated by measuring
etch rate as a function of temperature and physical etch rate is
temperature independent while the chemical etch rate varies with
temperature. A rough estimate of this maximum chemical etch
component based on gas composition (varies depending on details of
process) is roughly 30-80% O.sub.2 or CO for Cl.sub.2+O.sub.2 and
Cl.sub.2+CO, respectively. Another important aspect of the etch
process is the time between cleans.
[0113] One reason to maximize the chemical nature of the etch
process is in reducing CD growth. When using these Ti containing
hard masks and O containing etch chemistries it has been determined
that Ti etched from the edges of the hard mask by physical etching
can redeposit on the sides of neighboring capacitors resulting in
deposition of oxygen containing material that causes CD growth of
the capacitor. The physical etching causes the formation of facets
on the sides of the capacitor. In general, larger facet formation
results in larger CD growth and larger CD growth is correlated to
larger facet formation.
[0114] Etching these materials in general results in re-deposition
of noble electrode and other low vapor pressure materials on the
walls of the etch chamber. These layers can cause problems such as
particle generation or variation in the etch process. Therefore the
time between cleans indicates the number of wafers that can be
etched before one of these problems becomes severe enough that the
chamber has to be cleaned and conditioned for further etching. It
has been found that this time between cleans can be impacted by the
choice of etch chemistry plus many other factors including
preconditioning, choice of wall materials and temperature plus
details of the physical design of the etch tool itself for
example.
[0115] Etching the ferroelectric 126 requires choosing an etch
process that is again compatible with the choice of hard mask. For
a PZT ferroelectric material, multiple etch approaches will be
discussed.
[0116] The first exemplary approach uses a modification of the hard
mask etch based on a Cl.sub.2+Fluorine gas+oxidizer (O.sub.2 or CO
for example) with Ar or N.sub.2 possibly added as well. For high
temperature etching the fluorine gas prevents chemical undercut of
the PZT. It turns out that fluorine gas at an elevated substrate
temperature (350.degree. C., for example) can chemically etch at
least part of the PZT. Using fluorine containing gases that also
contain hydrogen helps reduce this effect, for example
CF.sub.4>CHF.sub.3>CH.sub.2F.sub.2>CH.sub.3F- . An
exemplary etch condition using this etch approach is
Cl.sub.2+O.sub.2+CH.sub.2F.sub.2 (75/35/12) at high chuck
temperature (350.degree. C.-400.degree. C.) at medium pressure (10
mTorr) at high density plasma (1200 W) and large substrate bias
(450 W RF on chuck). These types of processes result in etch rates
in the range of 70 nm/min. If the ferroelectric film is rough a
large overetch is in general needed.
[0117] In analyzing the above PZT etch that contain fluorine
components, the inventors of the present invention discovered
problems with the etched PZT layer. In particular, it was noted
that various defects were identified in the PZT layer, for example,
void or gaps therein, in some cases as large as 1 .mu.m or more. In
addition, some spacing or delamination could periodically be
identified between the PZT layer and the underlying bottom
electrode layer. Such defects are undesirable because they may
result in degraded polarization properties, for example, the switch
polarization. It is believed that the cause of such film
degradation is the fluorine component(s) within the PZT etch
chemistry. More particularly, it is believed that fluorine is
selectively chemically attacking elemental portions of the PZT
within the film as opposed merely to its exposed sidewall edges of
the stack. For example, from a thermodynamics point of view, when
fluorine is exposed to zirconium oxide, it reacts to form zirconium
fluoride and oxygen since that result is energetically
preferred.
[0118] Therefore the inventors of the present invention determined
that the fluorine component of the PZT etch should be eliminated to
avoid the PZT degradation highlighted above. In accordance with one
aspect of the present invention, the PZT portion of the capacitor
stack etch employs BCl.sub.3 as opposed to an etch chemistry
containing a fluorine component. The use of BCl.sub.3 as an etchant
for the PZT is not an obvious substitution from the prior art etch
chemistry for at least the following reasons. Initially, BCl.sub.3
does not appear viable because such an etch provides poor
selectivity with respect to the hard mask (e.g., a TiAlN hard
mask), thereby resulting in substantial hard mask erosion. One
conventional way to improve the selectivity of a layer with respect
to the hard mask is to add oxygen, however, BCl.sub.3 is extremely
reactive; so much so that the boron and chlorine may disassociate
due to the oxygen and negatively form gas phase particles.
Therefore initially one of ordinary skill in the art would not be
motivated to use a BCl.sub.3 etch chemistry to etch the PZT.
[0119] The inventors of the present invention discovered that at
low chuck temperatures, if you lower the bias, the hard mask
erosion due to the BCl.sub.3 slows, but the PZT etch rate also
slows. Surprisingly, however, it was discovered that while the low
bias was maintained, if the chuck temperature was increased, the
PZT etch rate increased substantially without any substantial
change in the hard mask etch rate. Therefore at a generally low
bias of about 100-150 W, a high temperature BCl.sub.3 etch will
provide an effective etch of PZT with acceptable selectivity to the
hard mask without any introduction of oxygen. Generally a
temperature of about 150.degree. C. or more is acceptable and
preferably a temperature of about 350.degree. C. is employed.
[0120] In addition, it was found that at lower temperatures, for
example, about 150.degree. C., a higher bias was needed to etch the
PZT (e.g., about 250 W), however, the lower the temperature, the
poorer the selectivity is to the hard mask. As the temperature was
increased, the bias could be lowered and higher selectivities were
obtained. For example, at a temperature of about 350.degree. C., a
bias of about 100-150 W was employed and excellent selectivity to
the hard mask was obtained.
[0121] Further, it was found by the inventors of the present
invention that adding Ar to the BCl.sub.3 during the PZT etch could
be employed to an advantage. For example, at lower temperatures
(e.g., around 150.degree. C.), only enough Ar to generate and
maintain the plasma is used. For example, the ratio of Ar to
BCl.sub.3 in such an example is about 20-30% Ar. At higher
temperatures, for example, about 350.degree. C., a ratio of about
50/50 facilitates hard mask selectivity.
[0122] Therefore according to one aspect of the present invention,
a PZT etch approach uses a BCl.sub.3 etch chemistry which may
further include Ar. The BCl.sub.3 etch chemistry is effective in
etching oxide materials because the B reacts with the oxygen in the
oxide forming BOx which is etched by Cl. The metals in the oxide
can then easily react with Cl.sub.2 forming volatile compounds. For
Ti containing hard masks, the optimum hard mask for purposes of
selectivity is a nitride (e.g., TiAlN) because the B reacts with
the nitride to form a BN layer which slows the etch rate.
[0123] One issue with using this etch chemistry with oxygen
containing chemistries in preceding or subsequent etch steps is to
ensure that there is a purge step between these steps in order to
prevent the oxygen from those etch chemistries from reacting with
the BCl.sub.3. Because of the reactive nature of this etch gas, low
substrate biases can be used (100-200 W RF on the chuck) along with
possibility of higher pressures (10-30 mTorr). Etch rates of 50-100
nm/min are achievable with these type of conditions. These low
biases also result in less physical etching of the hard mask
although if a surface layer of oxide is on the Ti hard mask due to
reaction from the oxygen in the top electrode etch it will be
removed.
[0124] As discussed above, use of a BCl.sub.3 etch for the PZT etch
results in a capacitor stack etch of, for example, Ir/PZT/Ir using
Cl.sub.2+O.sub.2+N.sub.2 for the top/bottom electrodes and
BCl.sub.3+Ar for the PZT with each etch performed at a relatively
high temperature (e.g., about 350.degree. C.). The above etch
methodology allows for a quality PZT layer without any substantial
gap or void degradation. Upon evaluating the resulting etched FeRAM
capacitor stack, however, it was noticed that conductive material
in some cases would tend to form on side edges of the ferroelectric
PZT layer. It is believed that such conductive material comprises
etched iridium that re-deposits on the sidewall of the PZT during
the etch of the iridium bottom electrode layer. Further, upon
analysis, it was appreciated by the inventors of the present
invention that such re-deposition of iridium on the PZT sidewall
edges occurs because of the substantially vertical PZT
sidewall.
[0125] The BCl.sub.3 PZT etch results in a substantially vertical
sidewall having an angle of about 89 degrees or more. Generally,
such a vertical profile is considered highly desirable in order to
minimize the critical dimension (CD) of the FeRAM capacitor. It was
discovered, however, that with such a steep profile, when iridium
from the etched bottom electrode layer re-deposits on the PZT
sidewall, less ion impingement occurs thereon. Therefore due to the
steep PZT sidewall profile caused by the BCl.sub.3 PZT etch,
re-deposited iridium does not tend to be removed by ion impingement
during the bottom electrode etch. Since the re-deposited iridium is
electrically conductive, such re-deposition disadvantageously
results in capacitor leakage and in some cases a "shorting out" of
the FeRAM capacitor.
[0126] It was discovered by the inventors of the present invention
that by lowering the etch temperature during the PZT etch, a
slightly sloped or non-vertical PZT sidewall edge profile may be
obtained. At low etch temperatures (e.g., about 60.degree. C.),
however, BCl.sub.3 does not etch well because of poor selectivity
to the hard mask, and because the etch is extremely slow, and in
some cases may completely stop.
[0127] The inventors of the present invention therefore appreciated
that a PZT etch that generates an intentionally sloped or
non-vertical PZT sidewall profile provides for a resulting stack
without substantial leakage or potential shorted capacitor
problems. Introducing an intentional sloped PZT sidewall profile in
the capacitor stack is counter-intuitive because generally attempts
are made to achieve purely vertical sidewalls to minimize the
capacitor critical dimension. According to one aspect of the
present invention, a sloped PZT profile having a sidewall angle of
about 88 degrees or less is generated using a low temperature etch
process (e.g., about 60.degree. C.) using a fluorine
gas+Cl.sub.2+an oxidizer, for example. Therefore in accordance with
the present example, the capacitor stack process comprises a high
temperature etch for the top electrode, a low temperature etch for
the PZT, and a high temperature etch for the bottom electrode. Such
a process can be employed in a single etch tool using two separate
chambers associated therewith, wherein the top and bottom electrode
etches are performed in one of the chambers at the high temperature
(e.g., about 350.degree. C.), and transferred to and from a second
chamber in which the PZT layer etch is performed at a lower
temperature (e.g., 60.degree. C.).
[0128] As discussed above, the BCl.sub.3 PZT etch does not work
well at low temperatures, however, the inventors of the present
invention discovered that by using a fluorine gas+Cl.sub.2+an
oxidizer with Ar or N.sub.2 at low temperatures, a sloped PZT
profile having an angle of about 88 degrees or less could be
achieved. More particularly, in one example,
CHF.sub.3+Cl.sub.2+O.sub.2+N.sub.2 was used at a temperature of
about 60.degree. C. on chuck. Unexpectedly, unlike at high
temperatures (as discussed earlier), such an etch chemistry does
not cause voids or gaps in the PZT and thus a sloped PZT sidewall
profile is obtained without PZT film degradation.
[0129] In accordance with one aspect of the present invention, a
PZT sloped profile has an angle of about 80 degrees or more and
about 88 degrees or less. In the above manner, the slope is
sufficiently angled to allow ion impingement (e.g., chlorine ions)
to remove re-deposited iridium on the PZT sidewall (during bottom
electrode etch) at a faster rate than the deposition. Concurrently,
by preventing the angle from becoming too small, critical dimension
growth is minimized.
[0130] After removal of the PZT layer, the bottom electrode layer
is patterned. Etching the bottom electrode 124 typically uses an
etch process very similar to the top electrode etch process if the
electrode materials are similar as is the situation in this
preferred embodiment. Note that with a Ti hard mask the bottom
electrode etch will effectively stop on the bottom electrode
diffusion barrier 122 (TiAlON, TiAlN or TiON in this example). This
is an advantage of this approach because it allows large overetch
to be used without the formation of undesirable recesses or
notching next to ferro structures because of overetch.
[0131] After etching the bottom electrode 124, one option is to
perform an oxygen anneal in order to replace any oxygen that might
have been removed by the etch process. One way to perform this
anneal is an in-situ O.sub.2 plasma (1000 W) with minimal (25 W
chuck) at 20 mTorr in the high-T etch chamber or possibly run the
wafers through a standard ash process (250.degree. C.). The ash
process could be O.sub.2 or possibly O.sub.2+H.sub.2O
combination.
[0132] In general, the various etch steps are end pointed using
optical emission spectroscopy, substrate bias changes, or some
other technique such as RGA or optical techniques that look at the
wafer surface. Depending on the details of the etch process,
roughness of the layers being etched and the shapes and especially
spaces in the structures being etched it is typically necessary to
set larger overetch times than might be expected based on endpoint
traces. This is because the etch rate in narrow spaces is slower
than in tight spaces plus rough films like frequently found with
MOCVD PZT need more etch time in order to clear. Another reason to
increase the overetch time is to remove "feet" that are present for
some processes at the bottom of the ferro stack.
[0133] The etch process is a dirty process and hence it is likely
that the etch tool and the front side, edge and backside of the
wafers will have FeRAM contamination or have etch residues with
FeRAM contamination. It is therefore desirable to clean the front
side of the wafer and chemically remove etch residues and possibly
remove a thin layer of damaged PZT. This post capacitor etch wet
clean can with some etch conditions and chemistries, be as simple
as a DI water clean (tank soak with or without megasonic followed
by a spin rinse dry) or the tank etch might be acid based in order
to improve the clean or remove more damage. One exemplary acid
solution might be similar to SC1 or SC2 (possibly without peroxide
but maybe with ozone), for example (NH.sub.4F+O.sub.3+H.sub.2O or
NH.sub.4F+H.sub.2O.sub.2+HCl+H.sub.2O: 1/1/1/300) in order to also
assist in particle removal plus metal contamination removal). The
backside and edges of the wafer are likely to be significantly
contaminated by re-deposition of FeRAM elements. The contamination
is preferably removed prior to process in a shared tool. One method
to remove the backside chemistry is to use a specialized tool such
as a backside clean tool (e.g., as made by SEZ). Even hard to etch
materials such as Ir can be removed if they are sub monolayer
coverage by undercutting the etching of the material on the
backside. Processes as described previously can be used here.
[0134] One option is to anneal the ferroelectric stack in order to
remove etch damage. The preferred thermal budget of this anneal is
.about.600.degree. C. for 2 min. One option for this anneal is to
perform the anneal with a Pb overpressure in order to prevent Pb
loss that might damage the ferroelectric. Methods to achieve this
Pb overpressure include furnace anneal with the capacitor facing Pb
compound such as PbO or maybe PbTiO3 or even PZT. Another method is
to anneal while flowing very low amounts of Pb metalorganic with
oxygen such that the Pb forms PbO but does not deposit as such on
the wafer because its deposition rate is slower than its
evaporation rate.
[0135] As shown in the method of FIG. 4, once the post-etch clean
250 has been completed, an insulating sidewall diffusion barrier
(typically AlOx) is deposited at 260 in order to protect the FeRAM
capacitor from hydrogen contamination, and also to protect other
structures from lead contamination if the ferroelectric dielectric
is PZT. In prior art solutions, the sidewall diffusion barrier was
formed over the capacitor stack after the bottom electrode
diffusion barrier 122 was etched to electrically isolate
neighboring capacitor stacks from one another. The inventors of the
present invention discovered that such a solution was undesirable
since some sidewall diffusion barrier layer materials (e.g., AlOx)
are not substantially selective with respect to the underlying
interlayer dielectric 112, typically SiO.sub.2. Therefore, in prior
art methods, when subsequently etching the sidewall diffusion
barrier, the underlying SiO.sub.2 layer would also be attacked. In
addition, if either the hard mask (e.g., the etch stop layer 302)
or the bottom electrode diffusion barrier layer comprise TiAlON,
for example, and have sufficient oxygen content therein, the AlOx
etch would also attack such layers and in some cases compromise the
top/bottom barrier capability of such layers. Accordingly, the
inventors of the present invention, appreciating the problems of
the prior art, disclose a method of fabricating an FeRAM by forming
the sidewall diffusion barrier layer over the capacitor stack
before the etching of the bottom electrode diffusion barrier layer.
Such a method results in several advantages could be achieved, as
will be discussed in greater detail below.
[0136] Turning to FIG. 16, a method 260 of forming the sidewall
diffusion barrier layer in accordance with one aspect of the
invention is provided. At 262, a sidewall diffusion barrier layer
300, for example, AlOx is formed over the capacitor stack 301 and
the exposed portion 304 of the bottom electrode diffusion barrier
layer 122, as illustrated in FIG. 17. In one example, the sidewall
diffusion barrier 300 comprises AlOx having a thickness of about 8
nm or more and about 120 nm or less. More preferably, the AlOx
barrier layer 300 has a thickness of about 10 nm or more and about
20 nm or less. Alternatively, other materials such as
Ta.sub.2O.sub.5, AlN, TiO.sub.2, ZrO.sub.2, HfO.sub.2, or any stack
or combination thereof may be employed and are contemplated as
falling within the scope of the present invention.
[0137] In one exemplary aspect of the present invention, the
sidewall barrier 300 comprises a multi-layer with two possible
materials, the first material being AlOx or one of the materials
highlighted above, and the second layer comprising SiN or AlN. The
sidewall diffusion barrier 300 primarily needs to prevent reaction
between the PZT ferroelectric layer 126 and the interlayer
dielectric (ILD) 112, 134. Another use as envisioned here is as
part of the hydrogen diffusion barrier. It is currently planned to
have complete protection of the capacitor to hydrogen during
subsequent processing by the use of hydrogen diffusion barriers on
all sides. For example, the TiAlN or TiAlON used as part of the
hard mask 302, 306 and as the bottom electrode diffusion barrier
122 are conductive hydrogen barriers (since the capacitor will need
to make electrical contact to and from the top/bottom electrodes)
while the AlOx is an insulating hydrogen diffusion barrier (in
order to prevent the top and bottom electrodes from shorting out
the capacitor). For the exemplary process described herein, the
AlOx is used as a Pb and H diffusion barrier while the
Si.sub.3N.sub.4 that is deposited later on in the flow is used as a
contact etch stop. In the subsequent text, AlOx will be used,
however, it should be understood that other sidewall diffusion
barrier materials may instead be used.
[0138] The primary reason for deposition at this point (prior to
etching the bottom electrode diffusion barrier) is to make the AlOx
etch process more simple. Other alternatives include, for example:
AlOx etch back after PZT deposition which is helpful for physical
bottom electrode etch processes, AlOx etch back after etching
bottom electrode diffusion barrier which has advantages in limiting
CD growth of this layer, but is a harder etch to develop, and no
AlOx etch back but instead the AlOx is etched as the last part of
the via etch process. In the subsequent discussions it is also
assumed that a via etch stop layer (typically SiN but might
possibly be SiC, for example) will be deposited prior to the
deposition of interlayer dielectric.
[0139] In accordance with one exemplary aspect of the present
invention, a process for depositing the AlOx sidewall diffusion
barrier layer is chemical vapor deposition (CVD) (e.g., MOCVD, CVD
or atomic layer deposition), but other approaches such as sputter
deposition can also be used. The primary advantage of the CVD
approaches is better step coverage along the sidewalls 308 of the
capacitor stack 302, which is desirable to ensure an effective
sidewall barrier 300. Since what is important is the thickness
after etch-back of the AlOx on the sides 308 of the capacitor, a
better step coverage dramatically reduces the planar thickness on
top 310 of the capacitor stack and over the exposed portion 304 of
the bottom electrode diffusion barrier layer 122 that needs to be
deposited and etched. PVD deposition of the sidewall diffusion
barrier 300 will work and one such exemplary deposition process is
deposition using a pure Al target in an Ar+O.sub.2 gas using a
pulsed DC or an RF power supply.
[0140] Returning to FIG. 16, the sidewall diffusion barrier 300
residing on the top 310 of the capacitor stack 301 and over the
exposed portions 304 of the bottom electrode diffusion barrier 122
(between neighboring stacks) is etched at 264, as illustrated in
FIG. 18. The AlOx etch back process needs to remove the AlOx from
planar surfaces 305, 310, but not from the sidewalls 308 of the
capacitors. It is therefore important to minimize the overetch of
the AlOx, yet it is still necessary that the AlOx clears over the
diffusion barrier layer 122 between neighboring capacitor stacks in
order for a complete subsequent etching of the bottom electrode
diffusion barrier layer 122, in order to prevent neighboring
capacitors from shorting out each other (since the bottom electrode
diffusion barrier layer is electrically conductive).
[0141] A conformal AlOx deposition process makes this much easier
to achieve especially for a high aspect ratio ((>1) capacitor
height to capacitor-to-capacitor space). According to one exemplary
aspect of the present invention, the etch chemistry for etching the
AlOx sidewall diffusion barrier is BCl.sub.3+Ar. The BCl.sub.3 is
effective in etching the AlOx with a good selectivity to the
underlying nitride hard mask 306 on top 310 of the capacitor stack
(e.g., TiAlN) and nitride bottom electrode diffusion barrier 122
(e.g., TiAlON with small oxygen content) between 304 the
neighboring capacitor stacks. The Ar may be added (as in the above
example) to the etch chemistry because the resulting surface (of a
top portion of the hard mask and the bottom electrode diffusion
barrier) is smoother, but one disadvantage is that it etches AlOx
on the sloped sides 308 of the capacitor; that is, the etch is less
anisotropic. An exemplary etch process uses a high density plasma
etch tool such as a AMAT DPS at near room temperature
(.about.60.degree. C.) with .about.50% Ar at an intermediate gas
pressure (.about.10 mTorr) 750 W remote plasma power and low bias
(e.g., 150 W RF on chuck). The etch rate is 50 nm/min under this
type of etch process.
[0142] This type of etch process would potentially cause problems
if the etch back was performed after etching the TiAlN bottom
electrode diffusion barrier because it quickly etches SiO.sub.2
because this is also an oxide. However, in accordance with the
present invention, since the AlOx sidewall diffusion barrier etch
is performed prior to the etch of the bottom electrode diffusion
barrier 122, no such problem occurs.
[0143] In addition, the BCl.sub.3 etch is substantially selective
with respect to the underlying nitride layers (the hard mask 306
and the bottom electrode diffusion barrier layers 122). The
aluminum-oxygen bonds in the AlOx layer are extremely strong,
however, the boron in the BCl.sub.3 reacts with oxygen to break the
aluminum-oxygen bonds. The chlorine in the BCl.sub.3 then reacts
with the aluminum to remove the AlOx. After the AlOx is removed,
the boron in the BCl.sub.3 reacts with nitrogen in the underlying
nitrides to form boron nitride, which slows down subsequent
etching. Therefore, one can perform a substantial overetch of the
AlOx sidewall diffusion barrier layer without substantially
impacting the nitrides underneath (e.g., TiAlN or TiAlON (low
content O) masking layer 306 or bottom electrode diffusion barrier
122). This is particularly helpful with regard to the bottom
electrode diffusion barrier layer 122 since it allows the AlOx to
be completely removed thereover, thus ensuring that all of the
underlying barrier 122 is exposed for removal in a subsequent etch
process, and thus ensuring that the neighboring capacitor stacks
301 are electrically isolated from one another.
[0144] Using the same chamber the bottom electrode diffusion
barrier is now etched at 266 of FIG. 16, as illustrated in FIG. 19.
The chemistry, in one example, is changed to Cl.sub.2+Ar which
effectively etches TiAlN or TiAlON (low oxygen) (masking layer 306
and barrier 122), but has good etch selectivity to AlOx on the
sidewalls 308 and to the TiAlON (high O) hard mask etch stop layer
302. The Ar etch gas component in one exemplary aspect of the
present invention, is added to help achieve a smooth post etch
surface although the disadvantage is an increase in the AlOx etch
rate on the sidewalls of the capacitor stack(s). An example etch
process uses a high density plasma etch tool such as a AMAT DPS at
near room temperature (.about.60.degree. C.) with .about.50% Ar at
an intermediate gas pressure (.about.10 mTorr) 1000 W remote plasma
power and low bias (e.g, 100 W RF on chuck). The etch rate is 100
nm/min under this type of etch process.
[0145] It is common that the thickest part of the masking layer 306
is thicker than the bottom electrode diffusion barrier. Therefore
the etch time and endpoint traces may be adjusted for this. This
situation will therefore show up on the endpoint trace and also
will result in reasonably large overetch on the W/TiN and SiO.sub.2
under the capacitors. Fortunately the etch chemistry does not
attack these materials very fast except for the TiN, and the Ar
keeps this attack to a minimum because the TiN is only present as a
thin diffusion barrier between the W contacts and SiO.sub.2
dielectric. One problem that has been observed to occur during this
etch was the undercut of the bottom electrode diffusion barrier 122
if it was TiAlN. By the addition of a small amount of oxygen during
the TiAlN deposition process (as discussed surpa), the undercut of
the TiAlON barrier 122 was for practical purposes eliminated.
Another issue with this etch is that a reasonable amount of
overetch is necessary in order to completely remove the TiAlN from
above the hard mask etch stop layer 302. The TiAlN layer, when not
completely removed, is very rough and will be difficult to make
contact to during via formation because of SiN step coverage on the
rough TiAlN.
[0146] In the discussion of the capacitor stack etch, the sidewall
diffusion barrier etch and the bottom electrode diffusion barrier
etch above, the accompanying figures illustrate such etches as
ideally anisotropic, resulting in generally vertical sidewall
profiles. Such sidewall illustrations, however, do not occur in
typical processing, but are provided for ease of illustration and
to illustrate other features of the present invention with
simplicity and clarity. Instead, the above etches are not ideally
anisotropic, and the actual capacitor stack cross section does not
exhibit perfectly vertical sidewalls, but rather exhibits generally
sloped stack sidewalls, as illustrated in FIG. 20. In FIG. 20, the
capacitor stack (hard mask 302, 306, top electrode layer 128, 130,
ferroelectric dielectric layer 126, and bottom electrode layer 124)
has been etched with the bottom electrode diffusion barrier layer
122 remaining and the patterned sidewall diffusion barrier 300 on
the sidewalls 308 of the stack.
[0147] As discussed above, it is desirable to remove the sidewall
diffusion barrier layer 300 off of the top 310 of the capacitor
stack and in the areas 304 between the stacks, however, it is
desirable for the sidewall diffusion barrier 300 to remain on the
stack sidewalls 308 in order to protect the ferroelectric
dielectric 126 in the stack from hydrogen contamination. Because
the capacitor stack is not perfectly vertical and since the etch
thereof has a chemical component, the sidewall diffusion barrier
layer 300 on the sidewalls 308 does get etched to some degree, and
often it is desirable to analyze the capacitor stack after the
sidewall barrier etch to ensure that the barrier 300 still covers
the capacitor stack sidewalls 308. Because the remaining sidewall
layer 300 may be relatively thin (e.g., about 150-200 Angstroms),
analyzing the sidewall layer 300 is difficult, for example,
requiring an expensive and laborious TEM (transmission electron
microscopy) analysis.
[0148] The inventors of the present invention discovered that if
the thickness of the masking layer 306 is sufficiently thick,
during the capacitor stack etch, although rounding will be
experienced at the corners 400 thereof, the sidewall diffusion
barrier layer 300 will overlie such corners 400 upon its
deposition, as illustrated in FIG. 20. Subsequently, during the
etch of the masking layer 306 portion of the hard mask (which will
typically be done concurrently with an etch of the bottom electrode
diffusion barrier layer 122), a portion 402 of the sidewall
diffusion barrier layer 300 overlying a corner portion 404 of the
masking layer 306 will protect such portion 404 of the masking
layer 306 from being etched, thereby resulting in "ears" 406 being
formed on top of the hard mask etch stop layer 302, as illustrated
in FIG. 21. Note, however, if a thickness of the sidewall diffusion
barrier layer 300 is not sufficiently thick, no portion 402 will
overlie the masking layer 306, and consequently no "ears" will
form.
[0149] Therefore by evaluating a capacitor stack after the etching
of the masking layer 306, identification of the "ears" 404 will
allow one to quickly ascertain that the sidewall diffusion layer
300 is adequately covering the capacitor stack sidewalls 308. The
"ears" 406 are easy to detect (by a standard or tilt scanning
electron microscope (SEM), for example), and their presence
indicates that the sidewall barrier 300 (e.g., AlOx) is above the
height of the hard mask etch stop diffusion barrier 302. A lack of
"ears" 406, although not indicative of a lack of sidewall barrier
coverage, does indicate that the sidewall barrier may not be
sufficiently thick, and may warrant further analysis or a process
change.
[0150] Therefore the thicknesses of the hard mask, AlOx deposition
and etch back process and bottom electrode diffusion barrier etch
back process are all adjusted to ensure that ears are observed
because this easy to detect feature proves that AlOx protection is
successfully in place.
[0151] Therefore in accordance with one aspect of the present
invention, a method of ascertaining whether a sidewall diffusion
barrier is sufficiently thick after patterning thereof is
disclosed.
[0152] It is necessary to protect the ferroelectric material from
hydrogen used by the rest of the semiconductor process. Many of the
standard semiconductor processing steps contain hydrogen, for
example, SiN deposition, HDP SiO.sub.2 deposition using silane, CVD
W deposition, forming gas anneal plus many others. Another problem
is that only a few materials are hydrogen barriers, for example,
SiO.sub.2 is not. Hydrogen barrier materials include many nitrides
such as TiN, TiAlN, AlN and SiN and AlOx. Ferroelectric electrode
materials such as Ir or Pt are not effective barriers. Pt in
particular is known to catalyze reaction of H.sub.2 to H that
appears to be much worse for ferroelectric properties.
[0153] Therefore in order to have complete hydrogen protection it
is necessary to have complete protection from hydrogen (top, bottom
and sides). In this disclosure the TiAlN or TiAlON is used as a
hydrogen barrier on the top and bottom while AlOx is used on the
sides of the capacitor. Complete protection requires not having any
small gaps or seams between the various layers. Additional
protection comes from the SiN except that the SiN deposition step
frequently contains hydrogen so a hydrogen barrier is typically
needed before this process as well.
[0154] Although the AlOx etch-back is the primary approach of this
disclosure there are other alternatives that also achieves these
goals. The alternative approach the TiAlON bottom electrode
diffusion barrier is etched immediately after etching the bottom
electrode. This etch can either be performed at high T or low T
although it is preferred that this be done at low T
(<200.degree. C.). The preferred etch chemistry for this
utilizes a short BCl.sub.3 containing (may contain Ar and/or
Cl.sub.2 as well) etch step followed by Cl.sub.2+Ar etch as
described previously. After post stack etch clean the AlOx followed
immediately by deposition of the interlayer dielectric that may or
may not include separate etch stop layer (typically SiN or SiC).
One possibility is that the AlOx can be used as an etch stop by
appropriate changes in the via etch chemistry.
[0155] The last step of the via etch is now etching AlOx. The
preferred etch chemistry for this process is still BCl.sub.3+Ar and
it is desirable to minimize the amount of overetch that is needed.
If a hydrogen diffusion barrier is next deposited in the via such
as TaN or TiN as is typical in standard semiconductor processing
then it might be possible to simplify the ferroelectric stack. By
using this approach it is possible to remove the top and bottom
electrode diffusion barrier or else to use materials that are not
hydrogen barriers. The reason is that the hydrogen protection from
the top comes from the AlOx plus the via barrier. These two layers
also protect the capacitor from additional hydrogen from the bottom
although it might be necessary to adjust the previous processes to
prevent them from supplying hydrogen later on that without a bottom
electrode hydrogen barrier degrade the capacitor properties.
[0156] It is recommended that after this etch step that the wafers
be cleaned. Since no ferroelectric elements are deliberately
exposed at this point, the primary purpose of this wet clean is to
remove halogens (Cl) left on the surface from the etch which
otherwise might cause corrosion problems and also to remove any
particles left on the surface. The suggested chemistry for this
process is therefore a DI H.sub.2O or a dilute SC1 type of
chemistry. For particle removal the use of a megasonic or other
such tool can also be used to promote particle removal.
[0157] At the beginning of the AlOx deposition process the front
side of the wafer has exposed FeRAM elements. The AlOx deposition
process may or may not result in contamination of the tool (defined
to be addition of FeRAM contaminants on subsequent wafers at levels
above care about level (.about.10.sup.10 atoms/cm.sup.2). If the
AlOx deposition process on FeRAM wafers does not result in
contamination then it is preferred to wet clean the backside of the
wafer prior to depositing this sidewall diffusion barrier. If the
AlOx deposition process on FeRAM wafers does result in tool
contamination then instead of performing a backside wafer clean the
clean after stack etch, the backside clean can be done after AlOx
etch back/bottom electrode diffusion barrier etch step. One option
is to anneal the capacitor at this point. One reason to do this
anneal so soon is that if a low-K layer is chosen then it might not
be compatible with the preferred PZT damage recovery anneal
condition (600.degree. C. 2 min).
[0158] The preferred next step is the deposition of a via etch stop
material. Example films are silicon nitride, silicon carbide,
(SiCNO) or an silicon oxide (preferably a high-density plasma
oxide). This etch stop layer is needed primarily because the via
etch is reaching two different via depths. The etch stop makes
simple means in order to etch two via depths without the large
overetch on the thinner via causing a problem. The other advantage
the etch stop has is that the SiN etch can be tuned to be selective
to underlying materials (W, TiN and SiO.sub.2). This prevents
forming narrow grooves when a via is misaligned to the underlying
contact. Forming these grooves is a particular reliability problem
by making a thin spot in the via diffusion barrier especially for
Cu vias. The etch stop thickness is chosen based on the via etch
process.
[0159] Another advantage of having the etch stop is that it
prevents misaligned vias that fall off the capacitor from shorting
the capacitor. In a similar manner it allows vias placed close to
the capacitor to be misaligned and still not short the capacitor.
This behavior allows the FRAM bit cell size to be reduced by
allowing a reduced via to capacitor space than if the etch stop was
not present. The typical deposition process for the SiN would be
PECVD using SiH.sub.4 and N.sub.2 or NH.sub.3. It is preferred to
use SiH.sub.4 and N.sub.2 in order to create a low H SiN barrier.
If there is a worry that the AlOx sidewall protects all of the PZT,
but not the electrode then it is recommended that a no hydrogen SiN
process be used such as PECVD using SiCl.sub.4 and N.sub.2 or
alternatively a very low T SiN process be used such as SiH.sub.4
and N.sub.2 using special low T PECVD such as ECR SiN. Another no
hydrogen option is a PVD SiN barrier.
[0160] The preferred next layer is deposition of gap fill
dielectric such as HDP SiO.sub.2. With the SiN deposited the gaps
between capacitor are very narrow and the aspect ratio is quite
large. Therefore a gap fill dielectric is recommended. Another
example of a gap fill dielectric is spin on glass. Although not
desirable small voids between narrow capacitor space might be
tolerated but voids where stacked vias for BL connection will be
made will cause problems.
[0161] On top of this layer is the primary interlevel dielectric,
as illustrated at 270 of FIG. 4, and possible material choices are
Sio.sub.2, FSG, PSG, BPSG, PETEOS, HDP oxide, a silicon nitride,
silicon oxynitride, silicon carbide, silicon carbo-oxy-nitride , a
low dielectric constant material (preferably SiLK, porous SiLK,
teflon, low-K polymer (possibly porous), aerogel, xerogel, BLACK
DIAMOND, HSQ, or any other porous glass material), or a combination
or stack thereof.
[0162] After the deposition of the interlayer dielectric it is
preferred to planarize the dielectric. The preferred method is
chemical mechanical planarization (CMP) although other methods such
as spin on dielectrics or deposition and etch back. CMP is
preferred as it results in global and not just local planarization.
The CMP planarization will thin the dielectric to a workable
thickness above the capacitor. Preferred values are between 300 nm
to 500 nm. The surface after CMP needs to be as planar as
possible.
[0163] Vias now need to be formed to make electrical connection to
the top electrode and to contacts. Standard semiconductor
processing techniques are used for this process. The via etch needs
to etch through all of the dielectrics to the etch stop without
etching through the etch stop because the vias have two different
heights. Next the etch stop needs to be etched without etching a
significant amount of the underlying material (W, TiN and SiO.sub.2
at the contacts) and hard mask etch stop layer (TiAlON or TiON) on
top of the capacitors under a few preferred embodiments. SiN etch
processes with these characteristics have been developed and the
etch selectivity to TiAlON has been documented to be very good.
This result is useful since the SiN etch rate on the capacitor is
expected to faster and the SiN is also expected to be thinner
compared to at the contacts which is deeper.
[0164] After the via etch, it is recommended that an anneal of the
instant invention is performed, as illustrated at 280 of FIG. 4, so
as to remove damage introduced by the capacitor stack processing
(such as the ferroelectric material etch, encapsulation, and
contact etch) into the capacitor dielectric and to improve the
electrical properties of these features. If this anneal is not done
at this point (i.e. if the anneal is done with the PZT stack
exposed on its sidewalls), then it may result in the loss of Pb
near the perimeter of each capacitor. This loss in Pb in the PZT
film will result in the degradation of the electrical properties of
small capacitors (capacitors with large perimeter to area ratios)
after the capacitor integration.
[0165] The anneal of the instant invention is, preferably,
performed after the interlevel dielectric is formed and the via
holes patterned and etch, but prior to the filling of the vias with
the conductive material. The anneal conditions, for example, are:
around 400 to 800.degree. C. (more preferably around 500 to
700.degree. C.--most preferably around 600.degree. C.) for a
duration of around 30 seconds to 5 minutes (more preferably for
around 1 to 4 minutes--most preferably around 2 minutes) in an
inert gas (N.sub.2, Ar) atmosphere or vacuum. After this anneal is
performed, the via diffusion barrier (liner) and conductor are
formed using standard semiconductor processing techniques. The
conductor is either W with TiN diffusion barrier or more preferably
Cu with TaN, TaSiN, Ta, TiN, WN, or TiSiN diffusion barrier
deposited by enhanced sputter deposition or more preferably CVD.
The Cu is deposited by first depositing a Cu seed by enhanced
sputter deposition or CVD preferably followed by Cu electroplating
in order to fill the via. A standard semiconductor processing
approach after the deposition of the metal in the via is to remove
the metal on the top surface by etch back (W) or CMP (W and
Cu).
[0166] Another alternative is that metal layer above the via is
formed using dual damascene process along with the via. The
disadvantage of this approach is substantial increase in process
complexity.
[0167] Although the invention has been shown and described with
respect to a certain aspect or various aspects, it is obvious that
equivalent alterations and modifications will occur to others
skilled in the art upon the reading and understanding of this
specification and the annexed drawings. In particular regard to the
various functions performed by the above described components
(assemblies, devices, circuits, etc.), the terms (including a
reference to a "means") used to describe such components are
intended to correspond, unless otherwise indicated, to any
component which performs the specified function of the described
component (i.e., that is functionally equivalent), even though not
structurally equivalent to the disclosed structure which performs
the function in the herein illustrated exemplary embodiments of the
invention. In addition, while a particular feature of the invention
may have been disclosed with respect to only one of several aspects
of the invention, such feature may be combined with one or more
other features of the other aspects as may be desired and
advantageous for any given or particular application. Furthermore,
to the extent that the term "includes" is used in either the
detailed description or the claims, such term is intended to be
inclusive in a manner similar to the term "comprising."
* * * * *