U.S. patent application number 09/182955 was filed with the patent office on 2001-06-07 for method for in-situ, post deposition surface passivation of a chemical vapor deposited film.
Invention is credited to CHANG, MEI, SRINIVAS, RAMANUJAPURAM, WU, LI.
Application Number | 20010003015 09/182955 |
Document ID | / |
Family ID | 22670782 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010003015 |
Kind Code |
A1 |
CHANG, MEI ; et al. |
June 7, 2001 |
METHOD FOR IN-SITU, POST DEPOSITION SURFACE PASSIVATION OF A
CHEMICAL VAPOR DEPOSITED FILM
Abstract
Method for passivating a layer of titanium that has been
deposited on a substrate in a reaction chamber to coat the titanium
thereby reducing the likelihood of contamination by byproducts of
the deposition process or ambient oxygen or similar reactants. The
method includes adding a flow of hydrogen and a flow of nitrogen to
the chamber. The flows of hydrogen and nitrogen are approximately
800 sccm and continue for approximately 10-30 seconds respectively.
The method may further comprise the step of forming a nitrogen
plasma in the chamber for approximately 10 seconds wherein such
case the flows of hydrogen and nitrogen continue for approximately
8 seconds respectively. The plasma is formed by applying RF power
to an electrode located within said chamber or by a remote plasma
source and channeled to said reactor chamber. Alternately, the
passivation layer may be formed just by using a nitrogen plama
alone for approximately 10-30 seconds at the same RF power level.
The plasma in either case may further comprise hydrogen and argon
and the layer of titanium has been deposited by CVD.
Inventors: |
CHANG, MEI; (SARATOGA,
CA) ; SRINIVAS, RAMANUJAPURAM; (SAN JOSE, CA)
; WU, LI; (FREMONT, CA) |
Correspondence
Address: |
APPLIED MATERIALS
P O BOX 450A
SANTA CLARA
CA
95052
|
Family ID: |
22670782 |
Appl. No.: |
09/182955 |
Filed: |
October 29, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09182955 |
Oct 29, 1998 |
|
|
|
08982872 |
Dec 2, 1997 |
|
|
|
Current U.S.
Class: |
427/569 ;
257/E21.17; 427/255.394; 427/535; 427/585 |
Current CPC
Class: |
C23C 16/14 20130101;
H01L 21/321 20130101; H01L 23/53223 20130101; H01L 21/28556
20130101; H01L 21/76856 20130101; H01L 21/76843 20130101; C23C
16/0245 20130101; H01L 2924/0002 20130101; H01L 2924/0002 20130101;
H01L 2924/00 20130101 |
Class at
Publication: |
427/569 ;
427/585; 427/535; 427/255.394 |
International
Class: |
C23C 016/00 |
Claims
What is claimed is:
1. A method for passivating a layer of titanium that has been
deposited on a substrate in a reaction chamber, comprising the
steps of: (A) adding a flow of hydrogen and a flow of nitrogen to
the chamber.
2. The method of claim 1, wherein said flows of hydrogen and
nitrogen are each approximately 800 sccm.
3. The method of claim 1, wherein said flows of hydrogen and
nitrogen each continue for approximately 10-30 seconds.
4. The method of claim 1 wherein said layer of titanium has been
deposited by CVD.
5. The method of claim 1 further comprising the step of: (B)
forming a plasma in the chamber.
6. The method of claim 5, wherein said plasma is a nitrogen
plasma.
7. The method of claim 5, wherein said flows of hydrogen and
nitrogen are each approximately 800 sccm.
8. The method of claim 5, wherein said flows of hydrogen and
nitrogen each continue for approximately 8 seconds.
9. The method of claim 5, wherein said plasma continues for
approximately 10 seconds.
10. The method of claim 5, wherein said plasma is formed by
applying RF power to an electrode located within said chamber.
11. The method of claim 5 wherein said plasma is formed in a remote
plasma source and channeled to said reactor chamber.
12. A method for passivating a layer of titanium that has been
deposited on a substrate in a reaction chamber, comprising the step
of: (A) forming a plasma in the chamber.
13. The method of claim 12 wherein said plasma is a nitrogen
plasma.
14. The method of claim 12 wherein said plasma continues for
approximately 10-30 seconds.
15. The method of claim 12 wherein said plasma is generated by
applying an RF power to the chamber of approximately 600 W.
16. The method of claim 12 wherein said plasma is formed by
applying the RF power to an electrode located within said
chamber.
17. The method of claim 15 wherein said plasma is formed in a
remote plasma source and channeled to said reactor chamber.
18. The method of claim 12 wherein the plasma further comprises
hydrogen and argon.
19. The method of claim 12 wherein said layer of titanium has been
deposited by CVD.
20. In a semiconductor wafer processing system comprising a reactor
chamber for processing a semiconductor wafer onto which a layer of
titanium has been deposited and a processor for controlling the
operation of said reactor chamber, a processor readable medium
containing a program that, when executed by said processor, causes
said reactor chamber to passivate said layer of titanium by
performing the following step: adding a flow of nitrogen and a flow
of hydrogen to said reactor chamber in the presence of said
semiconductor wafer.
21. The processor readable medium of claim 20 further containing a
program that, when executed by said processor, causes the reactor
chamber to passivate said layer of titanium by performing the
following step: forming a plasma in said reactor chamber in the
presence of said semiconductor wafer.
22. The processor readable medium of claim 21 wherein said plasma
is a nitrogen plasma.
23. The processor readable medium of claim 20 wherein said flow of
nitrogen and hydrogen continues for approximately 10-30 seconds.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 08/982,872, filed Dec. 2, 1997.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to the fabrication of
integrated circuits and the deposition of film layers over a
semiconductor substrate. More specifically, the present invention
relates to an improved chemical vapor deposition method and
apparatus for depositing and treating a titanium layer having
improved sheet resistance uniformity and excellent bottom coverage
at contacts.
[0004] 2. Description of the Background Art
[0005] One of the primary steps in fabricating modern semiconductor
devices is forming various layers, including dielectric layers and
metal layers, on a semiconductor substrate. As is well known, these
layers can be deposited by chemical vapor deposition (CVD) or
physical vapor deposition (PVD) among other methods. In a
conventional thermal CVD process, reactive gases are supplied to
the substrate surface where heat-induced chemical reactions take
place to produce a desired film. In a conventional plasma CVD
process, a controlled plasma is formed to decompose and/or energize
reactive species to produce the desired film. In general, reaction
rates in thermal and plasma processes may be controlled by
controlling one or more of the following: temperature, pressure,
plasma density, reactant gas flow rate, power frequency, power
levels, chamber physical geometry, and others.
[0006] Increasingly stringent requirements for fabricating these
high integration devices are needed and conventional substrate
processing systems are becoming inadequate to meet these
requirements. Additionally, as device designs evolve, more advanced
capabilities are required in substrate processing systems used to
deposit films having the properties that are required to implement
these devices. For example, the use of titanium is increasingly
being incorporated into integrated circuit fabrication processes.
Titanium has many desirable characteristics for use in a
semiconductor device. Titanium can act as a diffusion barrier
between, for example, a gold bonding pad and a semiconductor, to
prevent migration of one atomic species into the next. Also,
titanium can be used to improve the adhesion between two layers,
such as between silicon and aluminum. Further, use of titanium,
which forms titanium silicide (TiSi.sub.x) when alloyed with
silicon, can enable, for example, formation of ohmic contacts. One
common type of deposition system used for depositing such a
titanium film is a titanium sputtering or physical vapor deposition
(PVD) system. Such sputtering systems are often inadequate,
however, for forming devices with higher processing and
manufacturing requirements. Specifically, sputtering may damage
previously deposited layers and structures in such devices creating
performance and/or yield problems. Also, titanium sputtering
systems may be unable to deposit uniform conformal layers in high
aspect ratio gaps because of shadowing effects that occur with
sputtering.
[0007] In contrast to sputtering systems, a plasma-enhanced
chemical vapor deposition (PECVD) system may be more suitable for
forming a titanium film on substrates with high aspect ratio gaps.
As is well known, a plasma, which is a mixture of ions and gas
molecules, may be formed by applying energy, such as radio
frequency (RF) energy, to a process gas in the deposition chamber
under the appropriate conditions, for example, chamber pressure,
temperature, RF power, and others. The plasma reaches a threshold
density to form a self-sustaining condition, known as forming a
glow discharge (often referred to as "striking" or "igniting" the
plasma). This RF energy raises the energy state of molecules in the
process gas and forms ionic species from the molecules. Both the
energized molecules and ionic species are typically more reactive
than the process gas, and hence more likely to form the desired
film. Advantageously, the plasma also enhances the mobility of
reactive species across the surface of the substrate as the
titanium film forms, and results in films exhibiting good gap
filling capability.
[0008] One known CVD method of depositing titanium films includes
forming a plasma from a process gas that includes a TiCl.sub.4
source gas and a hydrogen (H.sub.2) reactant gas in a standard
PECVD process. Such TiCl.sub.4/H.sub.2 PECVD processes result in
the deposition of a titanium film that has good via-fill,
uniformity and contact resistance properties making the film
appropriate for use in the fabrication of many different
commercially available integrated circuits. However, the high
energy and temperatures associated with PECVD also increases the
reaction rate of contaminants such as carbon and oxygen at the
wafer surface during the deposition process. Additionally, surface
titanium may react (oxidize) with ambient oxygen as a wafer is
transferred between process chambers. As such, the deposited
titanium may contain impurities which alter (increase) the
resistance of the deposited layer and render devices constructed
therefrom defective or inoperable.
[0009] Therefore, there is a need in the art for a suitable method
of treating deposited layers such as titanium so as to protect them
from contaminants during the fabrication process.
SUMMARY OF THE INVENTION
[0010] The present invention provides an improved CVD deposition
and treatment process for titanium films. According to the method
of the present invention, a passivating layer for a titanium layer
that has been deposited on a substrate in a reaction chamber is
formed by adding a flow of hydrogen and a flow of nitrogen to the
chamber. The flows of hydrogen and nitrogen are approximately 800
sccm and continue for approximately 10-30 seconds respectively. The
method may further comprise the step of forming a nitrogen plasma
in the chamber for approximately 10 seconds wherein such case the
flows of hydrogen and nitrogen continue for approximately 8 seconds
respectively. The plasma is formed by applying RF power to an
electrode located within said chamber or by a remote plasma source
and channeled to said reactor chamber. Alternately, the passivation
layer may be formed just by using a nitrogen plama alone for
approximately 10-30 seconds at the same RF power level. The plasma
in either case may further comprise hydrogen and argon and the
layer of titanium has been deposited by CVD.
[0011] Additionally, in a semiconductor wafer processing system
comprising a reactor chamber for processing a semiconductor wafer
onto which a layer of titanium has been deposited and a processor
for controlling the operation of said reactor chamber, a processor
readable medium containing a program that, when executed by said
processor, causes said reactor chamber to passivate said layer of
titanium by adding a flow of nitrogen and a flow of hydrogen to
said reactor chamber in the presence of said semiconductor wafer.
The processor readable medium further contains a program that, when
executed by said processor, causes the reactor chamber to passivate
said layer of titanium by forming a nitrogen plasma in said reactor
chamber in the presence of said semiconductor wafer wherein said
flow of nitrogen and hydrogen continues for approximately 10-30
seconds.
[0012] In accordance with the method of the present invention, a
passivation layer is formed over the titanium layer, for example a
titanium nitride layer. The passivation layer coats the titanium
thereby reducing the likelihood of contamination by byproducts of
the deposition process or ambient oxygen or similar reactants that
may otherwise attack and alter the stability of the resultant
deposited film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The teachings of the present invention can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0014] FIG. 1A is a vertical, cross-sectional view of one
embodiment of a simplified plasma enhanced chemical vapor
deposition system according to the present invention;
[0015] FIG. 1B shows a simplified cross-sectional view of ceramic
pedestal 36 shown in FIG. 1A according to one embodiment of the
present invention;
[0016] FIG. 1C is a simplified cross-sectional view of deposition
chamber 30 shown in FIG. 1A, according to an embodiment of the
present invention;
[0017] FIG. 1D shows an interface between a user and a processor
that can control the deposition system of the present
invention;
[0018] FIG. 1E shows a simplified, partially-sectioned perspective
view of the flow of gas across a wafer and into the exhaust system
according to one embodiment of the present invention;
[0019] FIG. 1F shows an illustrative block diagram of the
hierarchical control structure of the system control software,
according to an embodiment of the present invention;
[0020] FIG. 2A shows a simplified cross-sectional view of an
exemplary contact structure in which a titanium layer deposited
according to the present invention may be employed;
[0021] FIG. 2B is a simplified cross-sectional view illustrating
the formation of a defect in the contact structure of FIG. 2A;
[0022] FIG. 3 is a flowchart of a process sequence used to deposit
a titanium layer according to the currently preferred embodiment of
the method of the present invention;
[0023] FIG. 4 is a graph showing measured reflected power as a
function of time and deposition length during a chamber clean
step;
[0024] FIGS. 5A and 5B are film thickness measurements illustrating
experimental results of the present invention;
[0025] FIG. 6 depicts a table (Table 1) of data for practicing a
preferred CVD of titanium process of the subject invention and
[0026] FIG. 7 depicts a table (Table 2) of data for practicing a
preferred CVD of titanium clean process of the subject
invention.
[0027] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0028] I. Introduction
[0029] The present invention allows for deposition of an improved
titanium film by pretreating a substrate in which the titanium film
is to be deposited over with a pretreatment plasma step. The
present inventors discovered that such a plasma pretreatment step
is particularly useful when the deposited titanium layer is used as
part of a multilayer stack (e.g., a titanium/titanium nitride
stack) used to make an ohmic contact to a semiconductor substrate
in a contact area etched through a dielectric layer such as a
silicon oxide layer. The plasma pretreatment step etches any
residual dielectric material left in the contact area of the
substrate and cleans the contact area prior to deposition of the
titanium layer. After completion of the plasma pretreatment step,
the titanium layer can be deposited by introducing a
titanium-containing source gas while maintaining the previously
formed plasma. Titanium layers deposited by the method of the
present invention are suitable for use in the fabrication of
integrated circuits having feature sizes of 0.35 to 0.11 microns or
less. Also, the present invention can be used to deposit titanium
films in CVD chambers of conventional design using readily
available gases. Finally, after depositing titanium, a
post-deposition passivation step is conducted. The passivation step
forms a protective layer over the titanium to reduce contamination
of the deposited film.
[0030] II. Exemplary CVD Chamber
[0031] FIG. 1A illustrates one embodiment of a simplified, parallel
plate chemical vapor deposition (CVD) system 10 in which the
titanium layer according to the present invention may be deposited.
CVD system 10 includes a reactor chamber 30, which receives gases
from a gas delivery system 89 via gas lines 92A-C (other lines may
be present but not shown). A vacuum system 88 is used to maintain a
specified pressure in the chamber and removes gaseous byproducts
and spent gases from the chamber. A power supply 5 provides power
to the chamber to form a plasma from the deposition gas during
titanium deposition and from the chamber cleaning gas during a
chamber clean operation. In a preferred embodiment of the
invention, the power supply 5 is capable of providing AC power in
the range of approximately 300 Khz--2.5 Ghz and preferably in the
radio-frequency range of approximately 300-450 Khz. A heat exchange
system 6 employs a liquid heat transfer medium, such as water or a
water-glycol mixture, to remove heat from the reactor chamber and
keep certain portions of the chamber suitably cool in order to
maintain a chamber temperature for stable process temperatures, or
to heat portions of the chamber, if necessary. A processor 85
controls the operation of the chamber and sub-systems according to
instructions stored in a memory 86 via control lines 3, 3A-D (only
some of which are shown).
[0032] Gas delivery system 89 includes gas supply panel 90 and gas
or liquid sources 91A-C (additional sources may be added if
desired), containing gases or liquids that may vary depending on
the desired processes used for a particular application. Liquid
sources may be held at temperature much greater than room
temperature to minimize source temperature variations due to
changes in the room temperature. Gas supply panel 90 has a mixing
system which receives the deposition process and carrier gases (or
vaporized liquids) from the sources 91A-C for mixing and sending to
a central gas inlet 44 in a gas feed cover plate 45 via supply
lines 92A-C. Liquid sources may be heated to provide a vapor at a
pressure above the chamber operating pressure, or a carrier gas,
such as He, Ar, or N.sub.2 may be bubbled through the liquid (or
heated liquid) to produce a vapor. Generally, the supply line for
each of the process gases includes a shut-off valve (not shown)
that can be used to automatically or manually shut off the flow of
process gas, and a mass flow controller (not shown) that measures
the flow of gas or liquid through the supply lines. When toxic
gases (for example, ozone or halogenated gas) are used in the
process, the several shut-off valves may be positioned on each gas
supply line in conventional configurations. The rate at which the
deposition and carrier gases including, for example, titanium
tetrachloride (TiCl.sub.4) vapor, hydrogen (H.sub.2), helium (He),
argon (Ar), and nitrogen (N.sub.2) and/or other dopant or reactant
sources, are supplied to the reaction chamber 30 is also controlled
by liquid or gas mass flow controllers (MFCs) (not shown) and/or by
valves (not shown). In preferred embodiments, a gas mixing system
(not shown) includes a liquid injection system for vaporizing
reactant liquids (e.g., TiCl.sub.4). A liquid injection system is
preferred as it provides greater control of the volume of reactant
liquid introduced into the gas mixing system compared to
bubbler-type sources. The vaporized gases are then mixed in the gas
panel with a carrier gas, such as helium, before being delivered to
the supply line. Of course, it is recognized that other compounds
may be used as deposition sources.
[0033] The heat exchange system 6 delivers coolant to various
components of the chamber 30 to cool these components during the
high temperature processing. The heat exchange system 6 acts to
decrease the temperature of these chamber components in order to
minimize undesired deposition onto these components due to the high
temperature processes. The heat exchange system 6 includes
connections (not shown) that supply cooling water through a coolant
manifold (not shown) for delivering coolant to the gas distribution
system, including the faceplate 40, (discussed below). A waterflow
detector detects the waterflow from a heat exchanger (not shown) to
enclosure assembly.
[0034] A resistively-heated pedestal 32 supports a wafer 36 in a
wafer pocket 34. As shown in FIG. 1B, which is a simplified
cross-sectional view of pedestal 32, pedestal 32 includes an
embedded electrode 22, such as an embedded molybdenum mesh, and a
heating element 33, such as an embedded molybdenum wire coil.
Pedestal 32 is preferably made from aluminum nitride in order to
withstand high processing temperatures and is preferably diffusion
bonded to a ceramic support stem 26 that is secured to a water
cooled aluminum shaft 28 (not shown in FIG. 1B, but shown in FIG.
1C) that engages a lift motor. The ceramic support stem 26 and the
aluminum shaft 28 have a central passage that is occupied by a
nickel rod 25 that grounds electrode 22. The central passage is
maintained at atmospheric pressure to avoid corrosive attacks at
the metal-to-metal connections.
[0035] Ceramic pedestal 32 is manufactured to provide uniform
capacitance by embedding RF electrode 22 at a uniform depth below
the surface of the substrate holder. RF electrode 22 is preferably
positioned at a minimum depth, which depends on the ceramic
material, to provide maximum capacitance while avoiding cracking or
flaking of the thin ceramic layer which covers the RF electrode 22.
In one embodiment, RF electrode 22 is embedded about 40 mil beneath
the upper surface of pedestal 32. Further details of ceramic
pedestal 32 are set forth in commonly assigned U.S. patent
application Ser. No. 08/980,520, filed on Dec. 1, 1997, entitled
"Mixed Frequency CVD Process And Apparatus," having Sebastien
Raoux, Mandar Mudholkar, William N. Taylor, Mark Fodor, Judy Huang,
David Silvetti, David Cheung, Kevin Fairbairn listed as
co-inventors, which is hereby incorporated by reference in its
entirety.
[0036] Pedestal 32 may be moved vertically between a processing
position (shown in FIG. 1C) and a lower loading position (not
shown) using a self-adjusting mechanism, which is described in
detail in commonly assigned U.S. patent application No. 08/738,240,
filed on Oct. 25, 1996, and entitled "Self-Aligning Lift
Mechanism," the disclosure of which is herein incorporated by
reference. Referring to FIG. 1C, lift pins 38 (only two of which
are shown) are slidable within pedestal 32 but are kept from
falling out by conical heads on their upper ends. The lower ends of
the lift pins 38 may be engaged with a vertically movable lifting
ring 39 and thus can be lifted above the pedestal's surface. With
the pedestal 32 in the lower loading position (slightly lower than
a slit valve 56), a robot blade (not shown) in cooperation with the
lift pins and the lifting ring transfers the wafer 36 in and out of
the chamber 30 through the slit valve 56, which can be
vacuum-sealed to prevent the flow of gas into or out of the chamber
through the slit valve 56. The lift pins 38 raise an inserted wafer
(not shown) off the robot blade, and then the pedestal rises to
raise the wafer off the lift pins onto the wafer pocket on the
upper surface of the pedestal. A suitable robotic transfer assembly
is described in commonly assigned U.S. Pat. No. 4,951,601 to
Maydan, the complete disclosure of which is incorporated herein by
reference.
[0037] The pedestal 32 then further raises the wafer 36 into the
processing position, which is in close proximity to a gas
distribution faceplate (hereinafter "showerhead") 40, which
includes a large number of holes or passageways 42 for jetting the
process gas into the process zone 58. The process gas is injected
into the chamber 30 through central gas inlet 44 in gas-feed cover
plate 45 to a first disk-shaped manifold 48 and from thence through
passageways 50 in a baffle plate (or blocker plate) 52 to a second
disk-shaped manifold 54.
[0038] As indicated by the arrows, the process gas jets from holes
42 in showerhead 40 into processing zone 58 (also referred to as
the "deposition zone") between the showerhead and the pedestal, so
as to react at the surface of the wafer 36. The process gas
byproducts then flow radially outward across the edge of wafer 36
and across a flow restrictor ring 46 (described in more detail
below), which is disposed on the upper periphery of pedestal 32
when the pedestal is in the processing position. From thence, the
process gas flows through a choke aperture 50 formed between the
top of flow restrictor ring 46 and the bottom of an annular
isolator 53 into pumping channel 60. Upon entering pumping channel
60, the exhaust gas is routed around the perimeter of the process
chamber, to be evacuated by the vacuum pump 82. Pumping channel 60
is connected through exhaust aperture 74 to pumping plenum 76.
Exhaust aperture 74 restricts the flow between the pumping channel
and the pumping plenum. A valve 78 gates the exhaust through
exhaust vent 80 to vacuum pump 82. The system controller (not shown
in this view) controls a throttle valve 83 according to a pressure
control program stored in memory (not shown) which compares a
measured signal from a pressure sensor (not shown), such as a
manometer, against a desired value which is stored in memory or
generated according to the control program.
[0039] The sides of annular pumping channel 60 generally are
defined by ceramic ring 64, a chamber lid liner 70, a chamber wall
liner 72, and annular isolator 53. FIG. 1E is a simplified,
partially-sectioned, perspective view of pedestal 32, flow
restrictor ring 46, liners 70 and 72, isolator 53, ceramic ring 64,
and pumping channel 60. This figure shows the flow of processing
gas out of nozzles 42 in showerhead 40 towards wafer 36, then
radially outward flow 84 over wafer 36. Thereafter, the gas flow is
deflected upwardly over the top of restrictor ring 46 into pumping
channel 60. In pumping channel 60, the gas flows along
circumferential path 86 towards the vacuum pump.
[0040] Pumping channel 60 and its components are designed to
minimize the effects of unwanted film deposition by directing the
process gas and byproducts into the exhaust system. The exhaust
flow form "dead zones" where little gas movement occurs. These dead
zones approximate a purge gas blanket in that they displace
reactive gases in that area and reduce unwanted depositions. Also,
purge gas (e.g., argon) is introduced from gas nozzles (not shown)
to blanket critical areas, such as ceramic parts and the heater
edge and backside to further reduce unwanted deposition on those
areas.
[0041] Unwanted deposition on the pedestal and other parts of the
chamber is minimized in other ways. Specifically, flow restrictor
ring 46 minimizes gas flow beyond the pedestal to the bottom of the
chamber. In accordance with embodiments of the present invention,
deposition of titanium using TiCl.sub.4 (as described in further
detail below) has flow rates significantly higher than conventional
methods used in conventional deposition systems for forming other
titanium films. In a preferred embodiment suitable for titanium
deposition, flow restrictor ring 46 is made of fused silica because
this material has relatively low thermal conductivity and because
it is not electrically conductive. In another embodiment, the flow
restrictor ring may be made of titanium for a deposition process
for a titanium-containing layer because the ring material will not
contaminate the deposited layer.
[0042] In various embodiments, the restrictor ring covers portions
of the top and edge of the pedestal, so that any undesired films
deposit on the ring, instead of on the pedestal or on the bottom of
the chamber. Advantageously, the flow restrictor ring minimizes the
risk of unwanted deposition (with its associated problems) that
might otherwise occur with this higher flow rate. Chamber lid 66
may be easily removed for cleaning, enabling access to the
relatively inexpensive restrictor ring, which may then be lifted
out and thoroughly cleaned using chemical and/or mechanical
processes.
[0043] Referring again to FIG. 1A, flow restrictor ring 46 is
supported by pedestal 32 during processing, as mentioned above.
When the pedestal is lowered for wafer unloading and loading, the
restrictor ring sits on ceramic ring 64 in ledge 69. As the
pedestal supporting the next wafer is raised into processing
position, it picks up the flow restrictor ring. At the pressures
used in the chamber for the titanium processes according to
embodiments of the invention, gravity is sufficient to hold both
the wafer (disposed in the wafer pocket) and the restrictor ring on
the pedestal.
[0044] Motors and optical sensors (not shown) are used to move and
determine the position of movable mechanical assemblies such as
throttle valve 83 and pedestal 32. Bellows (not shown) attached to
the bottom of pedestal 32 and chamber body 76 form a movable
gas-tight seal around the pedestal. The pedestal lift system,
motors, gate valve, plasma system, including an optional remote
plasma system 4 (which may be used to provide chamber clean
capability using a remote plasma formed using, for example, a
microwave source), and other system components are controlled by
processor 85 over control lines 3 and 3A-D, of which only some are
shown.
[0045] The processor 85 executes system control software, which is
a computer program stored in the memory 86 coupled to the processor
85. Preferably, the memory 86 may be a hard disk drive, but of
course the memory 86 may be other kinds of memory. In addition to a
hard disk drive (e.g., memory 86), the CVD apparatus 10 in a
specific embodiment includes a floppy disk drive and a card rack.
The processor 85 operates under the control of the system control
software, which includes sets of instructions that dictate the
timing, mixture of gases, gas flow, chamber pressure, chamber
temperature, RF power levels, heater pedestal position, heater
temperature and other parameters of a particular process. Other
computer programs such as those stored on other memory including,
for example, a floppy disk or other computer program product
inserted in a disk drive or other appropriate drive, may also be
used to operate processor 85. System control software will be
discussed in detail below. The card rack contains a single-board
computer, analog and digital input/output boards, interface boards
and stepper motor controller boards. Various parts of CVD apparatus
10 conform to the Versa Modular European (VME) standard which
defines board, card cage, and connector dimensions and types. The
VME standard also defines the bus structure having a 16-bit data
bus and 24-bit address bus.
[0046] The interface between a user and the processor 85 is via a
CRT monitor 93a and a light pen 93b, shown in FIG. 1D which is a
simplified diagram of the system monitor used with CVD apparatus
10, illustrated as one of the chambers in a multichamber system.
CVD apparatus 10 is preferably attached to a mainframe unit 95
which contains and provides electrical, plumbing and other support
functions for the apparatus 10. Exemplary mainframe units
compatible with the illustrative embodiment of CVD apparatus 10 are
currently commercially available as the Precision 5000J, the
Centura 5200J and the Endura 5500J systems from Applied Materials,
Inc. of Santa Clara, Calif. The multichamber system has the
capability to transfer a wafer between its chambers without
breaking the vacuum and without having to expose the wafer to
moisture or other contaminants outside the multichamber system. An
advantage of the multichamber system is that different chambers in
the multichamber system may be used for different purposes in the
entire process. For example, in a preferred embodiment of the
present invention, one chamber is used for CVD deposition of a
titanium film and another is used for CVD deposition of a titanium
nitride film. In this manner, deposition of a titanium/titanium
nitride stack, which is commonly used in the formation of contact
structures as discussed with respect to FIG. 2A below, may proceed
uninterrupted within the multichamber system, thereby preventing
contamination of wafers that often occurs when transferring wafers
between various separate individual chambers (not in a multichamber
system) for different parts of the titanium/titanium nitride stack
process.
[0047] In the preferred embodiment two monitors 93a are used, one
mounted in the clean room wall for the operators and the other
behind the wall for the service technicians. Both monitors 93a
simultaneously display the same information, but only one light pen
93b is enabled. The light pen 93b detects light emitted by CRT
display with a light sensor in the tip of the pen. To select a
particular screen or function, the operator touches a designated
area of the display screen and pushes the button on the pen 93b.
The touched area changes its highlighted color, or a new menu or
screen is displayed, confirming communication between the light pen
and the display screen. Of course, other devices, such as a
keyboard, mouse, or other pointing or communication device, may be
used instead of or in addition to light pen 93b to allow the user
to communicate with processor 85.
[0048] The processes for depositing the film and for dry cleaning
the chamber is implemented using a computer program product that is
executed by processor 85 (FIG. 1A). The computer program code can
be written in any conventional computer readable programming
language such as, for example, 68000 assembly language, C, C++,
Pascal, Fortran, or other language. Suitable program code is
entered into a single file, or multiple files, using a conventional
text editor and is stored or embodied in a computer-usable medium,
such as a memory system of the computer. If the entered code text
is in a high-level language, the code is compiled, and the
resultant compiler code is then linked with an object code of
precompiled Windows library routines. To execute the linked
compiled object code, the system user invokes the object code,
causing the computer system to load the code in memory, from which
the CPU reads and executes the code to perform the tasks identified
in the program.
[0049] FIG. 1F is an illustrative block diagram of the hierarchical
control structure of the system control software, computer program
160, according to a specific embodiment. Using a light pen
interface, a user enters a process set number and process chamber
number into a process selector subroutine 161 in response to menus
or screens displayed on the CRT monitor. The process sets, which
are predetermined sets of process parameters necessary to carry out
specified processes, are identified by predefined set numbers.
Process selector subroutine 161 identifies (i) the desired process
chamber, and (ii) the desired set of process parameters needed to
operate the process chamber for performing the desired process. The
process parameters for performing a specific process relate to
process conditions such as, for example, process gas composition
and flow rates, temperature, pressure, plasma conditions such as
high- and low-frequency RF power levels and the high-frequency and
low-frequency RF frequencies, (and in addition, microwave generator
power levels for embodiments equipped with remote microwave plasma
systems) cooling gas pressure, and chamber wall temperature.
Process selector subroutine 161 controls what type of process
(deposition, wafer cleaning, chamber cleaning, chamber gettering,
reflowing) is performed at a certain time in chamber 30. In some
embodiments, there may be more than one process selector
subroutine. The process parameters are provided to the user in the
form of a recipe and may be entered utilizing the light pen/CRT
monitor interface.
[0050] The signals for monitoring the process are provided by the
analog input board and digital input board of the system
controller, and the signals for controlling the process are output
on the analog output board and digital output board of CVD system
10.
[0051] A process sequencer subroutine 162 comprises program code
for accepting the identified process chamber and set of process
parameters from process selector subroutine 161, and for
controlling operation of the various process chambers. Multiple
users can enter process set numbers and process chamber numbers, or
a single user can enter multiple process set numbers and process
chamber numbers, so sequencer subroutine 162 operates to schedule
the selected processes in the desired sequence. Preferably,
sequencer subroutine 162 includes program code to perform the steps
of (i) monitoring the operation of the process chambers to
determine if the chambers are being used, (ii) determining what
processes are being carried out in the chambers being used, and
(iii) executing the desired process based on availability of a
process chamber and the type of process to be carried out.
Conventional methods of monitoring the process chambers can be
used, such as polling. When scheduling which process is to be
executed, sequencer subroutine 162 can be designed to take into
consideration the present condition of the process chamber being
used in comparison with the desired process conditions for a
selected process, or the "age" of each particular user-entered
request, or any other relevant factor a system programmer desires
to include for determining scheduling priorities.
[0052] Once sequencer subroutine 162 determines which process
chamber and process set combination is going to be executed next,
the sequencer subroutine 162 initiates execution of the process set
by passing the particular process set parameters to a chamber
manager subroutine 163a-c which controls multiple processing tasks
in a process chamber 30 according to the process set determined by
sequencer subroutine 162. For example, the chamber manager
subroutine 163b comprises program code for controlling CVD
operations in process chamber 30. Chamber manager subroutine 163b
also controls execution of various chamber component subroutines
which control operation of the chamber components necessary to
carry out the selected process set. Examples of chamber component
subroutines are substrate positioning subroutine 164, process gas
control subroutine 165, pressure control subroutine 166, heater
control subroutine 167, and plasma control subroutine 168.
Depending on the specific configuration of the CVD chamber, some
embodiments include all of the above subroutines, while other
embodiments may include only some of the subroutines. Those having
ordinary skill in the art would readily recognize that other
chamber control subroutines can be included depending on what
processes are to be performed in process chamber 30. In operation,
chamber manager subroutine 163b selectively schedules or calls the
process component subroutines in accordance with the particular
process set being executed. Chamber manager subroutine 163b
schedules the process component subroutines much like sequencer
subroutine 162 schedules which process chamber 30 and process set
are to be executed next. Typically, chamber manager subroutine 163b
includes steps of monitoring the various chamber components,
determining which components need to be operated based on the
process parameters for the process set to be executed, and
initiating execution of a chamber component subroutine responsive
to the monitoring and determining steps.
[0053] Operation of particular chamber component subroutines shown
in FIG. 1F will now be described with reference to FIG. 1A.
Substrate positioning subroutine 164 comprises program code for
controlling chamber components that are used to load the substrate
onto pedestal 32 and, optionally, to lift the substrate to a
desired height in chamber 30 to control the spacing between the
substrate and showerhead 40. When a substrate is loaded into
process chamber 30, heater assembly 33 is lowered to receive the
substrate in wafer pocket 34, and then is raised to the desired
height. In operation, substrate positioning subroutine 164 controls
movement of pedestal 32 in response to process set parameters
related to the support height that are transferred from chamber
manager subroutine 163b.
[0054] Process gas control subroutine 165 has program code for
controlling process gas composition and flow rates. Process gas
control subroutine 165 controls the open/close position of the
safety shut-off valves, and also ramps up/down the mass flow
controllers to obtain the desired gas flow rate. Process gas
control subroutine 165 is invoked by the chamber manager subroutine
163b, as are all chamber component subroutines, and receives
subroutine process parameters related to the desired gas flow rates
from the chamber manager. Typically, process gas control subroutine
165 operates by opening the gas supply lines and repeatedly (i)
reading the necessary mass flow controllers, (ii) comparing the
readings to the desired flow rates received from chamber manager
subroutine 163b, and (iii) adjusting the flow rates of the gas
supply lines as necessary. Furthermore, process gas control
subroutine 163 includes steps for monitoring the gas flow rates for
unsafe rates, and activating the safety shut-off valves when an
unsafe condition is detected. Process gas control subroutine 165
also controls the gas composition and flow rates for clean gases as
well as for deposition gases, depending on the desired process
(clean or deposition or other) that is selected. Alternative
embodiments could have more than one process gas control
subroutine, each subroutine controlling a specific type of process
or specific sets of gas lines.
[0055] In some processes, an inert gas such as nitrogen or argon is
flowed into the chamber to stabilize the pressure in the chamber
before reactive process gases are introduced. For these processes,
process gas control subroutine 165 is programmed to include steps
for flowing the inert gas into the chamber for an amount of time
necessary to stabilize the pressure in the chamber, and then the
steps described above would be carried out. Additionally, when a
process gas is to be vaporized from a liquid precursor, for example
TiCl.sub.4, process gas control subroutine 165 would be written to
include steps for bubbling a delivery gas, such as helium, through
the liquid precursor in a bubbler assembly, or introducing a
carrier gas, such as helium, to a liquid injection system. When a
bubbler is used for this type of process, process gas control
subroutine 165 regulates the flow of the delivery gas, the pressure
in the bubbler, and the bubbler temperature in order to obtain the
desired process gas flow rates. As discussed above, the desired
process gas flow rates are transferred to process gas control
subroutine 165 as process parameters. Furthermore, process gas
control subroutine 165 includes steps for obtaining the necessary
delivery gas flow rate, bubbler pressure, and bubbler temperature
for the desired process gas flow rate by accessing a stored table
containing the necessary values for a given process gas flow rate.
Once the necessary values are obtained, the delivery gas flow rate,
bubbler pressure and bubbler temperature are monitored, compared to
the necessary values and adjusted accordingly.
[0056] The pressure control subroutine 166 comprises program code
for controlling the pressure in the chamber 30 by regulating the
aperture size of the throttle valve in the exhaust system of the
chamber. The aperture size of the throttle valve is set to control
the chamber pressure at a desired level in relation to the total
process gas flow, the size of the process chamber, and the pumping
set-point pressure for the exhaust system. When pressure control
subroutine 166 is invoked, the desired or target pressure level is
received as a parameter from chamber manager subroutine 163b. The
pressure control subroutine 166 measures the pressure in chamber 30
by reading one or more conventional pressure manometers connected
to the chamber, compares the measure value(s) to the target
pressure, obtains proportional, integral, and differential (PID)
values corresponding to the target pressure from a stored pressure
table, and adjusts the throttle valve according to the PID values
obtained from the pressure table. Alternatively, pressure control
subroutine 166 can be written to open or close the throttle valve
to a particular aperture size to regulate the pumping capacity in
chamber 30 to the desired level.
[0057] Heater control subroutine 167 comprises program code for
controlling the temperature of the heater coil 33 used to
resistively heat pedestal 32 (and any substrate thereon). The
heater control subroutine is also invoked by the chamber manager
subroutine and receives a target, or set-point, temperature
parameter. The heater control subroutine measures the temperature
by measuring voltage output of a thermocouple located in pedestal
32, comparing the measured temperature to the set-point
temperature, and increasing or decreasing current applied to the
heating unit to obtain the set-point temperature. The temperature
is obtained from the measured voltage by looking up the
corresponding temperature in a stored conversion table, or by
calculating the temperature using a fourth-order polynomial. When
an embedded loop is used to heat pedestal 32, heater control
subroutine 167 gradually controls a ramp up/down of current applied
to the loop. Additionally, a built-in fail-safe mode can be
included to detect process safety compliance, and can shut down
operation of the heating unit if the process chamber 30 is not
properly set up. An alternative method of heater control which may
be used utilizes a ramp control algorithm, which is described in
the co-pending and commonly-assigned U.S. patent application No.
08/746,657, entitled "Systems and Methods for Controlling the
Temperature of a Vapor Deposition Apparatus," listing Jonathan
Frankel as inventor, filed on Nov. 13, 1996, the disclosure of
which is hereby incorporated by reference.
[0058] A plasma control subroutine 168 comprises program code for
setting low- and high-frequency RF power levels applied to the
process electrodes in chamber 30 and heater assembly 32, and for
setting the low RF frequency employed. Like the previously
described chamber component subroutines, plasma control subroutine
168 is invoked by chamber manager subroutine 163b. For embodiments
including a remote plasma generator 4, plasma control subroutine
168 would also include program code for controlling the remote
plasma generator.
[0059] Further details of the above-described CVD system are set
forth in commonly assigned U.S. patent application Ser. No.
08/918,706, filed on Aug. 22, 1997, and entitled "A High
Temperature, High Deposition Rate Process and Apparatus for
Depositing Titanium Layers," which is hereby incorporated by
reference in its entirety. The above reactor description is,
however, mainly for illustrative purposes, and other plasma CVD
equipment such as electron resonance (ECR) plasma CVD devices,
induction coupled RF high density plasma CVD devices, or the like
may be employed. Additionally, variations of the above described
system such as variations in pedestal design, heater design,
pumping channel design, location of RF power connections and others
are possible. The method for forming a titanium layer according to
the present invention is not limited to any specific CVD
apparatus.
[0060] III. An Improved CVD Titanium Process
[0061] The method of the present invention may be employed to
deposit improved titanium films in a substrate processing chamber,
such as the exemplary CVD chamber described above. As described
above, titanium films have a number of uses in the fabrication of
modern-day integrated circuits. One of the primary uses for such
titanium films is as an initial adhesion layer in a
titanium/titanium nitride stack that is part of a contact
structure. Such a contact structure is shown in FIG. 2A, which is a
cross-sectional view of an exemplary contact structure in which
embodiments of the present invention may be employed.
[0062] As seen in FIG. 2A an oxide layer 200 (e.g., an SiO.sub.x
film), is deposited to a thickness of about 1 Fm over a substrate
205 having a surface of crystalline silicon or polysilicon. Oxide
layer 200 may act as a pre-metal dielectric or as inter-level
dielectric in an integrated circuit. In order to provide electrical
contact between levels a contact hole 210 is etched through oxide
layer 200 and filled with a metal such as aluminum.
[0063] In many advanced integrated circuits, contact hole 210 is
narrow, often less than about 0.35 Fm wide, and has an aspect ratio
of about 6:1 or greater. Filling such a hole is difficult, but a
somewhat standard process has been developed in which hole 210 is
first conformally coated with a titanium layer 215. Titanium (Ti)
layer 215 is then conformally coated with a titanium nitride (TiN)
layer 220. Thereafter, an aluminum layer 225 is deposited, often by
physical vapor deposition, to fill the contact hole 225 and to
provide electrical interconnection lines on the upper level.
Titanium layer 215 provides a glue layer to both the underlying
silicon and the oxide on the sidewalls. Also, it can be silicided
with the underlying silicon to form an ohmic contact. The TiN layer
220 bonds well to the Ti layer 215, and the aluminum layer 225 wets
well to the TiN so that the aluminum can better fill contact hole
210 without forming an included void. Also, TiN layer 220 acts as a
diffusion barrier that prevents aluminum 225 from migrating into
silicon 205 and affecting its conductivity.
[0064] To properly fulfill its purpose, titanium layer 215 must
have excellent bottom coverage, low resistivity, uniform
resistivity and uniform deposition thickness both across the entire
bottom of the contact and across the entire wafer (center to edge)
among other characteristics. Also, it is preferred that titanium
layer 215 deposit uniformly along the bottom of contact 210, but
not deposit at all along the sidewalls. Preventing titanium
deposition on the sidewalls prevents the phenomenon known as
"silicon creep" where silicon from the contact area reacts with
titanium in the sidewall and is transported from the contact bottom
up into the sidewall. Titanium layers deposited according to the
method of the present invention meet all the above characteristics
and exhibit markedly improved bottom coverage and sheet resistance
uniformity as compared to prior art titanium deposition processes.
These improvements are achieved through the incorporation of novel
and unique steps that precede and follow the primary titanium bulk
deposition step.
[0065] One of these steps is a novel and unique plasma treatment
step that is performed before the titanium deposition step. In this
plasma treatment step, the wafer is subjected to a relatively brief
(e.g., between 5 and 60 seconds in preferred embodiments) plasma
formed from a process gas of H.sub.2 and Ar. In this manner, a
small fraction of the top surface of the wafer is etched away prior
to the deposition step. The inventors have found that this etching
step is particularly beneficial in (1) removing any oxidation
(SiO.sub.x) that has grown in the contact area of the wafer after
the formation of contact hole 210 and (2) further etching any
silicon oxide from layer 200 that was unintentionally left in
contact hole 210 after the hole formation (etching) step. The
formation of oxidation between 10-50 .ANG. thick is very common if
the wafer is exposed to the ambient for any appreciable length of
time prior to contact formation. Also, the inventors have noticed
that many commercial fabrication processes do not completely etch
away layer 200 and instead leave a thin unetched silicon oxide
layer over the contact area. Such a layer is shown in FIG. 2B as
layer 230 and may be between 100-250 .ANG. thick or more depending
on the process.
[0066] Depending on the thickness of this unetched layer 230 or any
oxidation build-up on the wafer, the layer may prevent electrical
contact from being made to the underlying substrate surface when
titanium layer 215 is deposited without the benefit of the present
invention thereby resulting in a part failure that reduces the
overall yield of the fabrication process. In other instances, layer
230 or the oxidation build-up is of a thickness that allows
electrical contact to be made to the underlying silicon at an
increased resistance level. Because of such, the manufactured
device may not meet the manufacturer's performance requirements. In
either of these cases, the pretreatment step of the present
invention can be used to etch away all or a portion of the
remaining layer 230 or the oxidation build-up thus enabling
improved electrical contact to substrate 200. Further details of
this aspect of the present invention are described below with
respect to FIG. 3.
[0067] FIG. 3 is a flowchart detailing the steps undertaken to
deposit a titanium film according to a preferred embodiment of the
present invention. It is to be understood that the steps shown in
FIG. 3 represent a preferred process only and that other
embodiments of the invention may either ship some of the disclosed
steps altogether or alter the format or sequence of the steps. As
shown in FIG. 3, before deposition of the titanium layer can begin
a wafer is loaded into chamber 30 (step 300) and processor 85 sets
the current wafer count (N--used for cleaning purposes as discussed
below) to 1 (step 305). After the wafer is loaded into the chamber,
it is moved to a processing position where pedestal 32 is generally
between 250 and 500 mils from gas distribution showerhead 40. In
one specific and preferred embodiment, pedestal 32 is positioned
329 mils from showerhead 40. During this wafer positioning step,
the chamber is pressurized with a non-corrosive gas, such as argon,
above the pressure at which deposition will occur. The argon fills
voids or hollow spaces within the chamber, particularly the
interior of the heater pedestal, so that it will then outgas as the
chamber pressure is subsequently reduced to the deposition pressure
(5.0 torr in a specific embodiment). In this manner, step 310
minimizes the intrusion of process gases that may corrode or
oxidize parts of the heater pedestal or chamber. The Ar
pressurizing gas is flowed as an upper Ar flow through showerhead
40 and as a lower Ar flow from a point beneath wafer 36. Preferably
the chamber pressure is set to between about 5-90 torr during this
step.
[0068] Also during step 310, the pedestal temperature is set to
between 15.degree. C. of the actual process temperature. The
process may be performed at any temperature between about
400-750.degree. C., but preferably the pedestal temperature is set
to between about 630-700.degree. C. (corresponding to a wafer
temperature of about 535-635.degree. C.) and most preferably at
about 680.degree. C. (corresponding to a wafer temperature of about
605.degree. C.) in a specific embodiment. In one specific
embodiment, the temperature is initially set to about 690.degree.
C. (10.degree. C. higher than the process temperature) in step 310
because the process gases will cool the heater and wafer when they
start flowing. Initially heating the wafer above the process
temperature results in a shorter wafer cycle time and reduces
thermal shock to the heater arising from the thermal gradient
between the heater element and the surface of the heater that
conventionally arises when heater power is increased to bring the
heater back up to processing temperature after the gas flows
start.
[0069] About 10 seconds after the initiation of step 310, the
temperature is reduced to the actual process temperature (which is
then preferably maintained throughout the entire deposition
process), a reactant gas (preferably H.sub.2) flow is turned ON at
an initial flow rate and the upper argon flow rate is increased
(step 315). The reactant gas lowers the energy required for the
decomposition of the source gas (introduced later) to form the
desired film and also reduces the corrosivity of the deposition
byproducts by converting some of the chlorine to hydrogen chloride
(HCl) rather than leaving it as Cl or Cl.sub.2. The flow of gases
is then further increased 2 seconds later in step 320 and again 3
seconds later in step 325. The flow rate of the gases is stepped up
in increments (or, alternatively, ramped) during steps 310 to 325
from an initial to a final flow rate in order to reduce the thermal
shock to the heater; the final flow rate of the gases is fairly
high and would unduly cool the wafer if turned on all at once. This
stepped or ramped onset of gas is particularly important with gases
such as helium or hydrogen, as these gases exhibit high thermal
transfer characteristics.
[0070] The next step, step 325, is the plasma pretreatment step
discussed above. In the plasma pretreatment step, low frequency
(e.g., 300-450 KHZ, most preferably 350 KHz) RF energy is applied
to showerhead 40 to form a plasma from the H.sub.2 and argon
process gas. As discussed above, this plasma either entirely or
partially etches away any thin oxidation layer that has either
grown on substrate 200 after the formation of contact hole 210 or
any layer 230 that was left unetched within contact hole 210, thus
enabling an improved electrical contact to substrate 200. It is
believed that this etching process can be represented by the basic
chemical reaction of: SiO.sub.2+H.sub.2.fwdarw.SiH.sub.4+H.sub.2O
where the silane (SiH.sub.4) and water (H.sub.2O) are both
exhausted from the chamber. Of course it is believed that other
intermediate reactions take place and that the exhausted compounds
also contain ions and other molecules from these intermediate
reactions.
[0071] It is possible to use other gases, referred to as
pretreatment gases, in step 320 to etch away oxidation build-up or
left over silicon oxide. The pretreatment gas should exhibit a high
etch selectivity between silicon oxide and the silicon substrate so
that it can etch the oxidation or left over oxide without damaging
the silicon contact area. Other pretreatment gases that can be used
in step 320 include ammonia (NH.sub.3) and various halogen species
that are known to etch silicon oxide. Fluorine-containing gases
(e.g., CHF.sub.3, CF.sub.4, C.sub.2F.sub.6, BF.sub.3, NH.sub.3 and
the like) are the believed to be the most preferred halogen species
while iodine-containing sources are believed to be the least
preferred because most iodine sources are solids at room
temperature and are difficult to work with. Also,
bromine-containing species are generally preferred over
chlorine-containing species in that the bromine gases are believed
to have less of an effect on the subsequent deposition process. Any
of the pretreatment gases can be, and preferably are, mixed with a
carrier or another inert gas to help stabilize the plasma and the
resulting etching process.
[0072] The TiCl.sub.4 (source gas) and helium flows are also
initiated during step 325. Instead of introducing these flows into
chamber 30 at this time, however, these flows are diverted directly
to the foreline. Diverting the flows in this manner, particularly
the flow of TiCl.sub.4, allows the flows to stabilize before
deposition begins thus improving the uniformity of processing
conditions among the various titanium deposition steps in a
multi-wafer deposition sequence (e.g., a 2000 wafer run).
Optionally, the TiCl.sub.4 and helium flows can be started shortly
after the initiation of the plasma as part of a separate step 330.
In either case it is preferred that the TiCl.sub.4 flow be
stabilized for between at least 6-8 seconds prior to deposition
step 335.
[0073] In deposition step 335, the TiCl.sub.4 and helium gas flows
are redirected to flow into chamber 30 along with the argon and
H.sub.2 flows, and the plasma is maintained by continuing to apply
RF power to showerhead 40. TiCl.sub.4 is in liquid form and is
vaporized using a liquid injection system such as the gas panel
precision liquid injection system (GPLIS) manufactured by STEC
Corporation before being mixed with the helium carrier gas. As
shown in Table 1 below, in the currently preferred embodiment, the
ratio of H.sub.2 to TiCl.sub.4 is 106:1. This ratio is can be
calculated by converting the TiCl.sub.4 mgm flow rate given in the
table to its equivalent sccm rate as can be done by a person of
ordinary skill in the art. In this instance, TiCl.sub.4 is
introduced at a rate of 400 mg/m, which is equivalent to a gaseous
flow rate of 47.23 sccm.
[0074] Deposition step 335 is maintained as long as required to
deposit a film of selected thickness. Because of the high
deposition temperature, increased gas flow rates and other factors,
the titanium film of the present invention is deposited at
deposition rates of at least 100 .ANG./minute and up to about 400
.ANG./minute or higher. Thus, the overall time of step 335 is
generally lower than that required by prior art processes, which in
turn leads to increased wafer throughput.
[0075] After the completion of deposition step 335, the H.sub.2,
TiCl.sub.4 and helium flows are turned OFF, the RF power is
dramatically reduced, and the upper argon flow is dramatically
reduced (step 340) in order to loosen any large particles that may
have formed on the chamber during the deposition step. Next about 3
seconds later, RF power is switched OFF and the titanium layer may
be passivated. The titanium layer is passivated by forming a thin
layer of titanium nitride at the titanium layer's surface such that
impurities such as carbon and oxygen cannot be absorbed into the
titanium. Such impurities can alter the resistance of the titanium
layer and form an unsuitable surface for titanium nitride barrier
layer deposition. Passivation may be accomplished by adding a flow
of H.sub.2 and a flow of N.sub.2 to the argon flow as a passivation
step 345 and/or forming a nitrogen plasma in a step 350.
Preferably, both steps 345 and 350 are performed. When done in this
manner, step 345 helps stabilize the chamber before post-deposition
plasma treatment step 350 and purge TiCl.sub.4 residue from the
chamber. Also, the nitrogen reacts with the surface titanium to
begin to form a thin layer of titanium nitride.
[0076] After step 345, the titanium layer is further passivated at
step 350 by applying RF energy to the H.sub.2/N.sub.2/Ar
passivation gases in the chamber to form a plasma. The passivation
plasma may alternatively be formed in a remote plasma source and
channeled to the chamber. The ionized nitrogen in the passivation
plasma reacts with the surface of the titanium layer to complete
the formation of a thin layer of titanium nitride during
approximately 10 seconds of exposure. To form the plasma in the
chamber, RF power is generally applied to the showerhead 40.
However, the RF power may be applied to the pedestal electrode 22
or to both the pedestal electrode 22 and the showerhead 40. In the
preferred embodiment where both steps 345 and 350 are employed,
step 345 lasts for about 8 seconds. In other embodiments where only
step 345 or only step 350 are used, the steps may be employed
longer, e.g., between about 10-30 seconds.
[0077] After step 350 a second plasma purge step 355 is performed
to further loosen any large particles that may be present in the
chamber. Plasma purge step 355 is similar to plasma purge step 340,
except that N.sub.2 and H.sub.2 flows are maintained in step 355 in
addition to the argon flows. Finally, in step 360, all gas flows
are shut off and the chamber is evacuated and then the wafer is
unloaded from the chamber (step 365). Since the wafer has been, in
general, passivated, the wafer can be exposed to the air without
the titanium layer detrimentally absorbing impurities such as
oxygen and carbon. As such, even long term exposure to air, e.g.,
days of exposure, does not degrade the properties of the titanium
layer. Furthermore, the titanium nitride passivation layer provides
a "clean" surface upon which subsequent processing can deposit a
titanium nitride barrier layer. After the wafer is removed, the
temperature is preset to about 680.degree. C. (step 405) before the
next wafer is loaded (step 410) and processor 85 increases the
wafer count (step 415).
[0078] In addition to the plasma purge clean steps 340 and 355
performed after each wafer deposition, a dry clean process (which
is done without opening the chamber lid) is performed periodically
on the chamber after a certain number of wafer deposition processes
to further avoid wafer contamination. According to the present
invention, there is no wafer (e.g., a dummy wafer) in the chamber
during this clean process. The dry clean process is generally run
between every "X" wafers, preferably between every 2-300 wafers.
The dry clean may be performed, for example, between every 3-5
wafers in a specific embodiment. It is desired to keep the dry
clean process efficient, so that it does not significantly affect
the total system wafer output. A preferred dry cleaning process in
accordance with a specific embodiment is described in further
detail below.
[0079] Referring again to FIG. 3, if X (where X=3, for example)
wafers have been processed (step 370), the chamber is due for a dry
clean. First, the heater is moved further away from the showerhead
to a distance of about 650 mil (step 375), and maintained at the
processing temperature of 680.degree. C. At this time, N.sub.2 or
similar nonreactive gas is flowed into the chamber and the chamber
is maintained at a cleaning pressure ranging between about 0.1-10
torr, preferably less than about 5 torr, and about 0.6 torr in a
specific embodiment. This minimizes heat flow from the heater to
the showerhead, thus cooling the showerhead relative to the
heater.
[0080] Three seconds after step 375, chlorine gas (Cl.sub.2) is
flowed into the chamber at a rate of about 250 sccm, and the
pedestal is raised to 600 mil from the showerhead 40 (step 380).
Next, two seconds later, a plasma is struck at a power of about 400
watts (step 385). This condition is held for a set period of time
to allow for the chlorine species to react with unwanted deposits
and etch the deposits from the chamber components. Unwanted
deposits from the deposition processes are generally thickest over
the hottest exposed portions of the chamber, i.e., the top surfaces
of the heater that were not covered by a wafer or not shielded by a
flow restrictor ring. By moving the heater away from the
showerhead, the conditions given above ensure sufficient cleaning
of all chamber components without overetching any of those
components, especially the showerhead.
[0081] The length of step 390 depends on the amount of deposition
build-up within chamber 30, which in turn depends on how many
wafers are processed between dry clean operations and the length of
the deposition process (i.e., thickness of the titanium film
deposited over wafer 36) among other factors. In one specific
embodiment, step 390 lasts for 15 seconds. Alternatively, the
length of step 390 can be determined using a cleaning endpoint
technique. Such techniques are well known and include optical
endpoint detection methods and pressure-based endpoint detection
methods. Optical endpoint detection requires a quartz or similarly
opaque window in the wall of chamber 30 for proper operation and is
less preferred in some embodiments because such a window is
susceptible to titanium deposition that interferes with proper
endpoint detection. Similarly, known pressure-based endpoint
detection methods are also less than ideal because such
pressure-based endpoint detection methods must be individually
calibrated to each chamber 30 to properly and precisely identify
the end of clean step 390.
[0082] Accordingly, the present inventors have developed a new
endpoint detection scheme for step 390 that is based on measured
reflected RF power. This endpoint detection scheme measures power
reflected from chamber 30 onto the power supply lines for RF power
supply 5 (FIG. 1A) throughout the entire clean step 390. At the
beginning of clean step 390, the reflected power increases as
deposits are etched from the chamber walls. This increase in
reflected power represents the increased density of the cleaning
plasma as it incorporates ionic species and energetic molecules
from the etched-away titanium deposits. As more and more deposited
material is etched from the chamber walls, the measured reflected
power reaches a peak before it starts to fall. These observations
are evident in FIG. 4, which is a graph showing measured reflected
power throughout clean step 390 as a function of time and the
length of titanium deposition step 335. The data shown in FIG. 4,
represents an embodiment where X=1, that is, where chamber 30 was
subjected to a dry clean process after a single wafer is
processed.
[0083] The chamber clean process is completed when the measured
reflected power decreases at a de minimis rate or below. For
example in one embodiment, step 390 is stopped 10 seconds after the
rate of decrease of the measured reflected power declines to 0
Watts/second. In another embodiment step 390 is stopped when the
rate of decrease of measured reflected power reaches 2 Watts/second
or less.
[0084] After the plasma clean, the chlorine gas is turned off and
the plasma power is switched OFF (step 390). N.sub.2 flow is
maintained to purge the chamber for about 3 seconds. The pedestal
is then returned to about 650 mil spacing (step 395) and the bottom
argon flow is increased for 10 seconds to further purge the
chamber. Finally, the chamber is pumped out for about 5 seconds
(step 400). Of course, it is recognized that "wet cleans" or
preventive maintenance cleanings (occurring between every hundreds
to thousands of processed wafers) may be performed by opening the
chamber lid to manually clean various parts of the chamber.
[0085] Performing the periodic dry clean process between wafer
depositions minimizes the frequency of these wet clean preventive
maintenance, which are often very time consuming, and provides a
cleaner chamber, which in turn is believed to increase the
efficiency of the deposition process and contribute to faster
deposition rate. Further, employing the periodic dry clean process
improves the repeatability of the titanium deposition process over
an extended wafer run. That is, during an extended wafer run of,
for example, 2000 wafers, the properties of the deposited titanium
layers over the first dozen wafers are much more similar to the
properties of the deposited layers over the last dozen wafers as
compared to an extended wafer run in which such periodic dry
cleaning was not employed.
[0086] The present inventors also discovered that liquid TiCl.sub.4
left in the gas line after the TiCl.sub.4 flow is stopped (step
340) interferes with process repeatability. That is, when the
TiCl.sub.4 flow is stopped in deposition step 340 by shutting off
the appropriate flow control valve connected to the line, some
residual TiCl.sub.4 liquid remains in the line. The present
inventors have found that the amount of this residual liquid varies
from one deposition process to the next and that the residual
TiCl.sub.4 can cause deposition instabilities and otherwise
adversely affect the deposition process. For example, because the
amount of residual TiCl.sub.4 varies, the amount of TiCl.sub.4 that
is flowed into the chamber for any two individual substrates in an
extended wafer run may be different, which in turn can result in
more or less deposition on a particular substrate. Also, the
residual TiCl.sub.4 may react with moisture present on a new
substrate as it is transferred into the chamber to form TiO.sub.2
and create unwanted particles. Finally, the residual TiCl.sub.4 may
leak into the chamber between wafer deposition steps and coat
portions of the chamber or chamber components thereby changing the
color of the coated portion, which also changes the emissivity of
that portion of the chamber or component. A change in the
emissivity of a surface may undesirably change the temperature or
other properties of the surface.
[0087] In order to combat the ill-effects of this residual
TiCl.sub.4, the inventors have devised a novel and unique step that
dries the TiCl.sub.4 gas line by flowing helium or another inert
gas source (a gas that does not react with the residual TiCl.sub.4)
through the line during the dry clean process. For example, in each
of steps 375 through 395, a 500 sccm flow of helium can be
introduced into the TiCl.sub.4 line to dry out and purge the
residual TiCl.sub.4 from the line. In this manner, the method of
the present invention ensures that the gas line is in a
reproducible state prior to the deposition on every wafer. Also,
after purging the TiCl.sub.4 line, the flowed helium is routed to
the deposition chamber where it helps stabilize the dry cleaning
plasma. The helium flow is routed through the TiCl.sub.4 line
through the use of appropriate valves and flow controllers as would
be understood by a person of ordinary skill in the art.
[0088] Gas flow rates, pressure levels and other information
according to the currently preferred embodiment of the present
invention described with respect to FIG. 3 above are set forth in
Table 1 (deposition process) and Table 2 (cleaning process) in
FIGS. 6 and 7 respectively. The gas introduction rates set forth in
Tables 1 and 2 are based on utilizing the process shown in FIG. 3
in a resistively heated TixZ CVD chamber manufactured by Applied
Materials that is outfitted for 8-inch wafers. As a person of
ordinary skill in the art would understand, the actual rates at
which gases are introduced in other embodiments will vary if other
chambers of different design and/or volume are employed.
[0089] While the deposition conditions and flow rates shown in
Table 1 and 2 above represent the flow rates employed in the
currently preferred embodiment of the present invention, it is
recognized that other deposition conditions and other flow rates
can be used. For example, with respect to the rate at which the
source and reactant gases are introduced during the deposition
stage, the inventors discovered that the ratio of H.sub.2 to
TiCl.sub.4 should be between about 64:1 and 2034:1. The preferred
ratios depend in part on other deposition conditions including
deposition temperature, pressure, pedestal spacing, RF power level
and other factors. The present inventors have discovered, however,
that the ratios set forth above can be used to deposit titanium
films of good quality over preferred deposition conditions
including a heater temperature range of at least 630-700.degree. C.
and a deposition pressure range of at least 1-10 torr. In certain
specific tests, titanium films of good quality were deposited at a
64:1 H .sub.2/TiCl.sub.4 ratio using an H.sub.2 flow of 3000 sccm
with a TiCl.sub.4 flow rate of 400 mg/m (equivalent to 47.23 sccm)
and at a 2034:1 H.sub.2/TiCl.sub.4 ratio using an H.sub.2 flow rate
of 12,000 sccm and a TiCl.sub.4 flow rate as low at 50 mg/m
(equivalent to 5.9 sccm). At H.sub.2/TiCl.sub.4 flow ratios of less
than 64:1, the reaction becomes hydrogen starved and becomes
unstable, while at flow ratios of greater than 2034:1, the
deposited films start to exhibit unacceptably poor bottom coverage
in the contact and it becomes difficult to manage the exhaust.
[0090] IV. Test Results and Measurements
[0091] To show the effectiveness of the present invention,
experiments were performed depositing titanium layers with and
without the benefits of the method of the present invention. The
experiments were performed in a resistively-heated TixZ chamber
manufactured by Applied Materials. The TixZ chamber was outfitted
for 200-mm wafers and was situated in a Centura multichamber
substrate processing system also manufactured by Applied
Materials.
[0092] In one of these sets of experiments, various pretreatment
steps (step 325) were performed prior to the titanium deposition
step to a wafer having a silicon oxide layer deposited thereon. The
first of these pretreatment steps formed a plasma from a Cl.sub.2
(125 sccm), N.sub.2 (500 sccm) and Ar (200 sccm) process gas. The
plasma was formed using an RF power level of 400 W and was
maintained for between 40 and 100 seconds in different tests. Test
results showed this step etched the silicon oxide layer at a rate
of 1.1 C/sec., but that the etching was not very uniform and was
rather uncontrolled and tenacious etching away silicon in addition
to the silicon oxide.
[0093] Further tests showed that chlorine from the Cl.sub.2 plasma
pretreatment step interfered with the subsequent titanium
deposition step. Specifically, it is believed that residual
chlorine was responsible for slowing down the deposition rate of
the titanium film in step 335. It was also determined that the
resulting titanium layer was less uniform than titanium layers
deposited without a Cl.sub.2 plasma pretreatment step.
[0094] The inventors also tested the plasma pretreatment step using
H.sub.2 according to the currently preferred embodiment of the
present invention. The results of these tests indicated that the
H.sub.2 (12 slm) and Ar (5500 sccm) plasma (RF power 900 W)
uniformly etched the silicon oxide at a rate of about 0.8 C/sec.
Also, the etch process was relatively gentle in that it did not
show any signs of damaging the silicon. FIGS. 5A and 5B show
illustrate the etch uniformity using this treatment. FIG. 5A shows
the thickness of the silicon oxide layer deposited over the wafer
before results of a wafer before being subjected to the plasma
pretreatment step of the present invention. The measurements were
made using a Rudolph Focus Ellipsometer as well known to those of
skill in the art and they show that prior to the pretreatment step,
the oxide layer had a thickness of 132" 15.61 C. FIG. 5B,
represents the thickness of the oxide layer immediately after a 90
second pretreatment step. In FIG. 5B, the oxide layer has a
thickness of 58" 16.7 C. As evident from a comparison of FIG. 5A to
5B, the thickness variation of the oxide layer in FIG. 5B is almost
identical to the variation shown in FIG. 5A. Thus, it is clear from
this comparison that the etch of this step 325 was very
uniform.
[0095] Also, the present inventors measured the resistivity of
titanium layers deposited according to present invention and
titanium layers deposited according to a similar process, but
without the plasma pretreatment step and without a standard HF dip
step, which would normally is commonly used by semiconductor
manufacturers to remove oxidation prior to titanium deposition. The
results of these tests showed that for a 300 .ANG. titanium layer,
the resistivity of the layer was between 0.5 and 1.0 S/G higher for
the titanium films not processed with the plasma pretreatment step
as compared to those processed with the plasma pretreatment
step.
[0096] These results prove that the plasma pretreatment step of the
present invention can be successfully used to etch away unwanted
oxidation on a silicon substrate prior to the deposition of a
titanium layer. As previously mentioned, such oxidation is
routinely built-up on substrates and would previously require a
separate processing step such as a dip in an HF solution to etch
away the build-up prior to transferring the substrate to a separate
chamber for deposition of the titanium film. Such an HF dip step
requires that the wafer be dried afterwards and then immediately
transferred to the deposition chamber before further oxidation
occurs. This process is cumbersome, time consuming and inherently
less reliable than the process of the present invention.
[0097] Other tests showed that the process of the present invention
does not deposit any titanium on the sidewalls of a contact hole
such as hole 210 (FIG. 2A) while resulting in bottom coverage of
greater than 300%. A film exhibiting bottom coverage of 300% has
300 .ANG. of titanium silicide formed at the bottom of the contact
when a 100 .ANG. titanium layer is deposited within the
contact.
[0098] The parameters listed in the above process and experiments
should not be limiting to the claims as described herein. One of
ordinary skill in the art can modify the process described above by
using chemicals, chamber parameters, and conditions other than
those described with respect to the preferred embodiments. As such,
the above description is illustrative and not restrictive and the
present invention is applicable to depositing titanium films in a
variety of different deposition and cleaning processes. For
example, the dry cleaning process can employ remote plasma system 4
to dissociate Cl.sub.2 gas molecules and/or other gases. Similarly,
remote microwave plasma system 4 can be used to dissociate titanium
and other process gas molecules during the deposition process and
the dissociated ions can be channeled to chamber 30. The present
invention can be used with different cleaning sources including
F.sub.2, ClF.sub.3 and others, and the techniques of the present
invention can be employed with different titanium sources, for
example, TiI.sub.4 (a solid) and any other titanium halide
compound. Also, plasma pretreatment step 325 can be used to heat
the wafer and stabilize temperature uniformity across the wafer
prior to the deposition step. And other gases, for example, N.sub.2
above or NH.sub.3, can be used to passivate the titanium layer in
steps 345 and 350. The scope of the invention should, therefore, be
determined not with reference to the above description, but instead
should be determined with reference to the appended claims along
with their full scope of equivalents.
[0099] Although various embodiments which incorporate the teachings
of the present invention have been shown and described in detail
herein, those skilled in the art can readily devise many other
varied embodiments that still incorporate these teachings.
* * * * *