U.S. patent application number 13/568002 was filed with the patent office on 2012-11-29 for method and apparatus for growing thin oxide films on silicon while minimizing impact on existing structures.
This patent application is currently assigned to MATTSON TECHNOLOGY, INC.. Invention is credited to Yaozhi HU, Wilfried LERCH, Zsolt NENYEI, Bruce W. PEUSE, Georg ROTERS, Stephen E. SAVAS, Ashok SINHA, Sing-Pin TAY, Paul Janis TIMANS, Guangcai XING.
Application Number | 20120298039 13/568002 |
Document ID | / |
Family ID | 41478827 |
Filed Date | 2012-11-29 |
United States Patent
Application |
20120298039 |
Kind Code |
A1 |
PEUSE; Bruce W. ; et
al. |
November 29, 2012 |
METHOD AND APPARATUS FOR GROWING THIN OXIDE FILMS ON SILICON WHILE
MINIMIZING IMPACT ON EXISTING STRUCTURES
Abstract
Plasma assisted low temperature radical oxidation is described.
The oxidation is selective to metals or metal oxides that may be
present in addition to the silicon being oxidized. Selectivity is
achieved by proper selection of process parameters, mainly the
ratio of H2 to O2 gas. The process window may be enlarged by
injecting H2O steam into the plasma, thereby enabling oxidation of
silicon in the presence of TiN and W, at relatively low
temperatures. Selective oxidation is improved by the use of an
apparatus having remote plasma and flowing radicals onto the
substrate, but blocking ions from reaching the substrate.
Inventors: |
PEUSE; Bruce W.; (San
Carlos, CA) ; HU; Yaozhi; (San Jose, CA) ;
TIMANS; Paul Janis; (Mountain View, CA) ; XING;
Guangcai; (Fremont, CA) ; LERCH; Wilfried;
(Dornstadt, DE) ; TAY; Sing-Pin; (Fremont, CA)
; SAVAS; Stephen E.; (Fremont, CA) ; ROTERS;
Georg; (Duelmen, DE) ; NENYEI; Zsolt;
(Blaustein, DE) ; SINHA; Ashok; (Los Altos Hills,
CA) |
Assignee: |
MATTSON TECHNOLOGY, INC.
Fremont
CA
|
Family ID: |
41478827 |
Appl. No.: |
13/568002 |
Filed: |
August 6, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12334425 |
Dec 12, 2008 |
8236706 |
|
|
13568002 |
|
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|
Current U.S.
Class: |
118/723I ;
118/723R |
Current CPC
Class: |
H01L 21/0223 20130101;
H01L 21/0234 20130101; H01L 21/02323 20130101; H01L 21/68771
20130101; H01L 21/67248 20130101; H01L 21/02238 20130101; H01L
21/31654 20130101; H01L 21/67115 20130101 |
Class at
Publication: |
118/723.I ;
118/723.R |
International
Class: |
C23C 16/50 20060101
C23C016/50; C23C 16/505 20060101 C23C016/505 |
Claims
1. An apparatus for selective oxidation of silicon on a substrate
in the presence of other materials, comprising: a vacuum enclosure;
a substrate pedestal situated within the enclosure; a plasma
generation region; a conduit enabling radical flow from the plasma
generation region to the vacuum enclosure; a gas source coupled to
the plasma generation region; and a steam source coupled to the
plasma generation region.
2. The apparatus of claim 1, further comprising a baffle situated
between the vacuum enclosure and the plasma generation region, the
baffle substantially preventing ions from reaching a substrate
positioned on the substrate pedestal.
3. The apparatus of claim 1, further comprising a UV source
configured for illuminating a substrate positioned on the substrate
pedestal.
4. The apparatus of claim 3, wherein the UV source comprises an
optical baffle situated between the vacuum enclosure and the plasma
generation region, so as to control the amount of illumination from
the plasma that reaches the substrate.
5. An apparatus for selective oxidation of silicon on a substrate
in the presence of other materials, comprising: a vacuum enclosure;
a substrate pedestal situated within the enclosure; a plasma
generation region, the plasma generation region comprising an RF
transparent wall, an electrostatic shield provided about the wall,
and an inductive coil provided about the shield; a conduit enabling
radical flow from the plasma generation region to the vacuum
enclosure; a gas source coupled to the plasma generation
region.
6. The apparatus of claim 5, further comprising a steam source
coupled to the plasma generation region.
7. The apparatus of claim 5, further comprising a baffle situated
between the vacuum enclosure and the plasma generation region, the
baffle preventing ions from reaching a substrate positioned on the
substrate pedestal.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This is a Divisional Application of U.S. patent application
Ser. No. 12/334,425, filed on Dec. 12, 2008, the disclosure of
which is incorporated herein in its entirety.
BACKGROUND
[0002] Oxidation of silicon is a fundamental technology to CMOS
fabrication, dating back to the inception of the integrated
circuit. The most common methods for oxidation of silicon rely on
thermal processes in ambient of O2, H2O/H2, H2O/O2, O2/H2 or
combinations thereof. The hardware used to provide the silicon
oxidation process in the IC manufacturing are batch thermal
furnaces and RTP. In conventional oxidation systems and processes,
high temperature (above 700.degree. C.) is required to provide the
activation energy for the oxide growth on silicon or poly-silicon.
At temperatures below 700.degree. C., insufficient oxide growth
occurs for practical consideration.
[0003] Advanced integrated circuit fabrication requires a number of
process steps where thin films of silicon oxide are grown on
silicon or polysilicon structures. For some applications, the
oxidation process must be selective, such that other materials
including tungsten are not oxidized. These critical oxidation steps
are used for DRAM and FLASH memory and logic devices. Currently
thermal processing in either an ambient of O2, H2O/H2, or H2O/O2,
at high temperature (>700.degree. C.) is used to perform this
oxidation processes. This is typically done with an RTP system such
as an ATMOS.RTM. system available from Mattson Thermal Products
GmbH Dornstadt, Germany. Another single wafer alternative has
offered a thermal `radical oxidation` by thermal processing in a
low pressure H2/O2 ambient. As device dimensions continue to
shrink, a number of serious limitations in the afore-mentioned
methods for growing these oxide films have begun to appear. The
current processes all require high temperatures in excess of
700.degree. C. and more typically on the order of 900.degree. C.
The high temperatures are necessary to obtain the oxide growth rate
to make the process practical and in some cases are required for
oxide quality. Many of the next generation devices will undergo
serious damage at the point in the process flow where the oxide
growth is required, if exposed to the combination of high
temperature and an oxidizing environment.
[0004] At the current state of the art, the various problems facing
oxidation include the following examples. For FLASH Poly sidewall
oxidation the tunnel oxidation encroachment limits operating
temperature to below 700.degree. C. Also, dopant diffusion limits
operating temperature to 750.degree. C. For shallow trench
isolation (STI) liner oxidation requires conformal oxidation to
reduce stress and leakage.
[0005] Plasma oxidation as well as UV photon-enhanced oxidation
have been described in a number of technical journals and papers.
This topic has been an area of research at universities as well.
Presently, the leading edge IC manufacturers carry out the most
research in this area. Recently, various equipment suppliers have
tested hardware in the field that provides various plasma oxidation
capabilities.
SUMMARY
[0006] The following summary of the invention is included in order
to provide a basic understanding of some aspects and features of
the invention. This summary is not an extensive overview of the
invention and as such it is not intended to particularly identify
key or critical elements of the invention or to delineate the scope
of the invention. Its sole purpose is to present some concepts of
the invention in a simplified form as a prelude to the more
detailed description that is presented below.
[0007] This invention describes apparatus and methods for
selectively or non-selectively oxidizing silicon, poly-silicon or
other semiconductor materials on semiconductor wafers over a range
of temperatures from 700.degree. C. down to room temperature. The
oxidation is performed with the use of a type of plasma source that
produces from a feed gas or gas mixture various reactive species,
including but not limited to H, O and or OH radicals and ions, in
such proportions that it selectively does not oxidize specified
other materials exposed on the wafer surface that are also exposed
to the process. These processes are controllable within a
substantial window of conditions of process gas flow, pressure and
plasma source power, that do not expose the work piece to
contamination and are suitable for manufacturing of semiconductor
devices. Other embodiments include injection of H2O steam as one
constituent of the gas mixture fed into the plasma.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The accompanying drawings, which are incorporated in and
constitute a part of this specification, exemplify embodiments of
the present invention and, together with the description, serve to
explain and illustrate principles of the invention. The drawings
are intended to illustrate various features of the illustrated
embodiments in a diagrammatic manner. The drawings are not intended
to depict every feature of actual embodiments nor relative
dimensions of the depicted elements, and are not necessarily drawn
to scale.
[0009] Various other objects, features and attendant advantages of
the present invention will become fully appreciated as the same
become better understood when considered in conjunction with the
accompanying detailed description, the appended claims, and the
accompanying drawings, in which:
[0010] FIG. 1 illustrates a plasma reactor according to an
embodiment of the invention.
[0011] FIG. 2 illustrates an apparatus enabling steam co-injected
plasma source, according to an embodiment of the invention.
[0012] FIG. 3 illustrates an apparatus according to another
embodiment of the invention.
[0013] FIG. 4 is a plot of results from investigation of plasma
assisted oxidation of silicon with respect to tungsten.
[0014] FIG. 5 illustrates results obtained for tungsten plasma
oxidation.
[0015] FIG. 6 illustrates the results for selective silicon
oxidation in the presence of tungsten using 75% H2 in H2+O2
plasma.
[0016] FIGS. 7A and 7C illustrate plasma oxidation sheet resistance
results for TiN, while FIGS. 7B and 7D illustrate steam plasma
oxidation sheet resistance results for TiN.
[0017] FIG. 8 illustrates selective oxidation process window for
TiN in steam plasma, while FIG. 9 illustrates selective oxidation
process window for W in steam plasma.
[0018] FIG. 10 illustrates an example of rotating baffles for
variable UV illumination.
[0019] FIG. 11 illustrates the effects of adding O2 to the steam,
while FIG. 12 illustrates the effect of adding H2 to the steam.
[0020] FIG. 13 illustrates an embodiment wherein the plasma is
generated in a remote chamber 132, using, e.g., microwave source
152 (although other sources may be used), and utilizing a conduit
119 to enable drift of plasma species towards the wafer 110, which
resides in vacuum processing chamber 105.
[0021] FIG. 14 illustrates the temperature effects on O2+H2 plasma
oxidation at a high temperature regime.
DETAILED DESCRIPTION
1. Embodiments of Apparatus for Performing PALTROX
[0022] The invention provides methods and apparatus for performing
plasma assisted low temperature radical oxidation, hereinafter
referred to as PALTROX. According to an embodiment of the
invention, the apparatus that could be used to perform the low
temperature plasma enhanced processes is based on the Suprema.RTM.
system available from Mattson Technology of Fremont, Calif. A
description of the base Suprema system can be found in other
documents and a drawing of the system as modified according to an
embodiment of the invention is shown in FIG. 1.
[0023] The system 100 has a main processing chamber body 105,
enabling processing of two wafers 110 simultaneously. A throttle
valve 160 is capable of controlling the gas pressure in the process
area. The pressure range can be controlled over a range from 50
mTorr to 5 Torr but usually in the range of 100 mTorr to 1 Torr. A
susceptor heater 115 placed under each substrate 110 maintains the
substrate at a constant temperature during processing. The
susceptor control temperature may be set at values ranging from
room temperature up to 400.degree. C.
[0024] Plasma is ignited and maintained by an inductively coupled
plasma source (ICP) 125 employing an electrostatic shield 165 to
reduce capacitive coupling between antenna and plasma. The source
load is connected to automatic matching network 150 with an RF
power generator 155 that, in this example, operates at 13.6 MHz.
The source generates plasma inside a quartz cylinder 130 mounted
directly above the wafer 110. Process gases, whose composition and
flow rates are controlled by flow controllers not shown in the
figure, enter the source from a gas port 145, through a gas
diffuser 140 situated in the center of a top plate of the source,
and via showerhead 135. Plasma is maintained below the showerhead
135 and plasma species exit at the bottom through a grid or baffle
structure 120 that separate(s) the source volume from the process
chamber 105 containing the susceptor and wafer. The RF power can be
adjusted from 0 to 3.0 kW, but higher powers up to 10 kW are also
available.
[0025] The ICP source incorporates a slotted electrostatic shield
165 between the source coil and the walls of the quartz confinement
chamber. The electrostatic shield serves to reduce the capacitive
coupling between the plasma and the source that lowers the RF
displacement current, which would otherwise flow through the plasma
and into the grounded components of the process module and the
wafer. This feature serves to reduce the flux of charged particles
onto the surface of the wafer.
[0026] The grid or baffle structure 120, which may include single
or multiple grids or baffles, serves as a device to facilitate in
the recombination of ions with electrons, thus limiting the charged
particles that enter the process area. In this example the grid
structure 120 is made of quartz, but other materials may also work
including Al2O3, AlN, Y2O3, YAG or a suitable metal coated with one
of these materials. In some cases a conducting material may be used
such as aluminum.
[0027] The grid structure 120 also functions as a secondary
showerhead and aids in providing a uniform flux of radicals above
the wafer 110. The arrangement and sizes of holes can be used to
adjust the flow distribution over the wafer surface. Further, the
grid may also be designed so as to serve as a heat and UV radiation
shield. In such configuration, its function as a heat shield
greatly reduces the IR thermal radiation from the wafer and
pedestal, in the event such are heated for the process, that would
cause the plasma source walls to be hotter than if heated only by
the plasma. Furthermore, it may also be designed as a radiation
shield which reduces the UV from the plasma source that is incident
on the substrate being processed. Control of such UV radiation is
important to controlling the rate of oxidation and uniformity
thereof, both for silicon and other materials on the surface of the
substrate.
[0028] A gas panel (not shown) containing a set of mass flow
controllers, regulators and valves provides the prescribed mixtures
of gases to the ICP source gas port 145 as well as providing purge
gases. Gases provided include but are not limited to O2, O3, H2,
D2, N2, Ar, He, Kr, NH3, H2O or D2O.
[0029] A machine controller (not shown) operates the system and
executes a predetermined set of sequential process steps as in a
process recipe. The machine controller also automatically loads and
unloads substrates for continuous operation.
[0030] According to another embodiment, the process module would
include these additional attributes. A susceptor heater made of
ceramic materials capable of operation in excess of 700.degree. C.
The ceramic material is robust to either strong reducing or
oxidizing environments. The ceramic material may be AlN, although
other materials or coatings could be considered. The susceptor has
a capability that electrical contact is made with the back of the
wafer. The electrical contact is made in such a way that a DC bias
can be applied to the backside of the wafer. The purpose of this
biasing capability is to place the substrate at a positive
potential to enhance the diffusion of the negatively charged
radicals through the oxide towards the oxide silicon interface and
thus provide enhanced oxide growth.
[0031] In one embodiment, the walls of the process chamber are
lined with a material that minimizes contamination. Liners that
would serve this purpose can be made of a number of materials
including quartz, Al2O3, AlN, Y2O3, YAG to list some. The liners
can also be made of a suitable metal such as aluminum or stainless
steel and either anodized or coated with one of the aforementioned
materials.
[0032] As noted above, the grid structure that separates the wafer
environment from the plasma source environment may also serve as a
heat shield to protect the plasma source and all of its components
from excessive heating. More than one grid can be installed to
improve the shielding effect without adversely affecting the
delivery of radicals to the wafer surface since the hole
distribution and size can be suitably chosen in each grid
separately, and for round grids their relative positioning varied
to permit more or less UV radiation from the source to pass
through. Various materials can be used to fabricate the grid,
although quartz has been found to be optimal.
[0033] According to yet another embodiment of this invention, an
apparatus that could be used to perform the PALTROX processes is
based on a modified version of the Low Pressure Anneal (LPA) module
developed by Mattson Technology. The apparatus 300 is illustrated
in FIG. 3, comprising a vacuum anneal process module 305 with the
ICP source 325 on the topside. Of course, another type of plasma
source could be used with this system by simply exchanging the ICP
source. The wafer 310 is heated directly from the backside with a
set of tungsten halogen lamps 370. A thick quartz window 374 allows
transmission of the lamp radiation to heat the wafer. A control
system 376 compares the output of the wafer temperature measurement
system with a set point temperature to obtain close loop control of
the temperature.
[0034] Some specific attributes of the apparatus of FIG. 3, as
adopted to perform the PALTROX process, would include the
following. The oxidation is performed in a vacuum process module
that is capable of controlling the pressure and flow of gases into
the process area 305. The pressure range can be controlled over a
range from 50 mTorr to 760 Torr but usually in the range of 100
mTorr to 10 Torr. The pressure near the higher end (760 Torr) is
typical of post-oxidation anneal, but is not limited to anneal
because high pressure plasma generation may also be employed.
Energy is provided by a lamp heating system comprising a set of
tungsten halogen lamps 370, a reflector lamp holder 372 that
supports the lamps and provides cooling, a lamp power supply (not
shown) and control system to control the lamp power (not shown).
The chamber has a window 374 mounted in the bottom of the chamber
that allows direct radiation from the lamp system to heat the wafer
and wafer support structures. A temperature control system 376
monitors the temperature of the wafer and controls the lamp power
supply system so that the wafer can be ramped to a desired
temperature and held at the desired temperature. Wafer temperature
ramp up rates are on the order of 50.degree. C./s and cool down
rates are about 25.degree. C./s, on average. In this example, a
guard ring structure 378 is provided to improve the heating
uniformity. The wafer and guard ring are supported by a quartz
support 380. Also, in this example, a rotation mechanism 382 is
provided in order to rotate the wafer during processing to achieve
better uniformity.
[0035] The wafer is placed on a wafer rotation system that rotates
the wafer during processing to enable more uniform heating. The
topside of the process module system consists of an ICP plasma
source similar to the one already described for generating radicals
for the purpose of performing plasma oxidation. A gas panel
containing a set of mass flow controllers, regulators and valves,
provides the prescribed mixtures of gases to the ICP source gas
injector as well as providing purge gases. Gases provided include
but are not limited to O2, H2, D2, N2, He, Ar, Kr, NH3, H2O or D2O.
This system allows wafers to be heated to temperatures up to
600.degree. C. to form the `low temperature` plasma oxide. Higher
process temperatures, up to 1200.degree. C. are also available.
[0036] Subsequent anneal steps in a non-oxidizing ambient can be
performed in the same chamber as part of the same process sequence.
The annealing temperature could be up to 1200.degree. C. This
feature has an important advantage over the prior embodiments for
processes requiring an anneal step to follow. Plasma-assisted
processing or subsequent annealing can be performed with
temperature-time cycles that are "soak" processes or "spike
processes". Alternative heat sources, such as heating with
arc-lamps, lasers, RF or microwave energy, or streams of energetic
particles, can also be employed.
[0037] Other means of producing the radical species can be used.
The production of the radical species required for plasma oxidation
from O2, H2, D2, N2, He, Ar, Kr, NH3, H2O or D2O gas can be
accomplished by other means than those listed above. The following
is a list of some of means that radical species can be produced.
These means can be used in place of or in conjunction with the
means described above. A plasma source may be employed, that uses
an electron cyclotron resonance, microwaves, inductive coupling, or
plasma jet, as the primary means to generate a plasma in gas to
which the wafer is exposed. The plasma source may be located in the
process chamber so that it generates the plasma on or near the
wafer surface, or it may be located up stream from the wafer so
that wafers are not exposed to the direct plasma from the source
but rather just the gas issuing from the source. In the case of
plasma jet, such a source may operate at or near atmospheric
pressure. Since the jet is usually concentrated into a cross
section area of a few centimeters squared, multiple sources may be
required in the process chamber to form plasma just above or in
contact with the wafer surface. In the case of a single jet located
upstream from the wafer, it could produce radicals that would flow
into a chamber volume at the same or lower pressure than the plasma
jet source. Radicals can also be generated as by-products of
chemical reactions, or decomposition of an unstable species. For
example, they can be generated from ozone, or from a flame arising
from a chemical reaction. Such flow of gas species to the wafer may
be shaped or distributed by grids or baffles that are interposed
between plasma source and wafer.
[0038] Also, Photo excitation of gas molecules and atoms can be
done with the use of Far Ultra-Violet (UV) spectrum (200 nm-122 nm,
6.2-10.2 eV) or Middle UV (300 nm-200 nm, 4.13-6.2 eV). Various
sources can be used including excimer lamps, mercury lamps and
various laser sources (excimer, quadrupled pulsed YAG, etc.) to
name a few. The plasma produced in the process chamber by the
plasma source can also provide suitable UV radiation for this
purpose. One type of such source would produce short wavelength
radiation in the vacuum UV region of the spectrum. A wafer is
exposed to this radiation in the gas environment. This process may
be carried out at any pressure.
[0039] According to an embodiment of the invention, a silicon
plasma oxidation process is performed in an apparatus enabling
steam injection into the plasma. The chamber has provisions for
injecting various gases, e.g., H2, O2, Ar, etc. to sustain plasma,
and in addition, provisions for injecting steam into the plasma. An
example of such an apparatus is illustrated in FIG. 2. In FIG. 2,
plasma reactor 200 is similar to that illustrated in FIG. 1,
although it should be appreciated that other plasma reactors may be
used. In FIG. 2, 205 indicates plasma precursor gas supply, while
215 indicates steam supply. Steam is generated at steamer 225,
which generates steam from DI water supply 230. Steam may be
generated by, e.g., a pyrogenic or catalytic generator or by
boiling or bubbling of high purity water. Steamer 225 may be, for
example, RASIRC steamer available from RASIRC of San Diego, Calif.
The flow of steam into the reactor is controlled by valve 240. By
controlling the steam flow and gas flow, one can control the ratio
of the gas species to steam. Gas flow may be provided using a
plurality of delivery channels, each comprising a precursor gas
source 252, a mass flow controller 254, and a shut off valve 256.
In one example, gas delivery includes O2 and H2, each of which can
be dialed from zero flow up to a desired ratio, so that steam may
be mixed with O2 only, with H2 only, or with both H2 and O2.
[0040] The apparatus of FIG. 3 may also be adapted to enable
enabling steam injection into the plasma. Process gasses are
provided to the plasma from gas sources 352, mass controllers 354
and shut off valves 356. Steam is generated at steamer 325, which
generates steam from DI water supply 330. The flow of steam into
the reactor is controlled by valve 340.
2. Embodiments of Processes for Performing PALTROX--Plasma
[0041] A thin oxide film can be grown on a silicon substrate by
using, for example, either of the processing reactors described
above. According to various methods described below, the process is
performed by properly controlling species of O, H and OH in the
plasma. Notably, various variables have been investigated to find
the conditions that provide the widest process window for various
structures. Selective oxidation of silicon in the presence of TiN
or tungsten has been investigated, among others.
[0042] According to aspects of the invention, the oxidation process
is improved by controlling and intentionally limiting the exposure
of the substrate to ions produced in the plasma, so as to minimize
damage or degradation of the oxide film. A limitation of
conventional plasma chambers is that energetic species from the
plasma reach the surface of the wafer. These energetic or charged
species can cause lower quality of oxide, with poor electrical
characteristics such as increased trapped charge defects in the
grown oxide. Trapped charge defects lead to breakdown of the
dielectric at lower field strength. Further, plasma also produces
some vacuum UV radiation which may have harmful effects on the
properties of the grown oxide.
[0043] According to further aspects of the invention, O, H, and OH
(or OD) radicals flow from the plasma and pass proximate to the
substrate, thereby enabling, among others, oxidation at low
temperatures. Additionally, this arrangement enables improved
selective oxidation wherein oxide is grown on silicon or
polysilicon while not oxidizing adjacent exposed materials, such as
tungsten or other materials such as TiN, TaN or WN. In one example,
the distribution of the species from the plasma across the wafer is
controlled using a grid or baffle structure, such as, e.g., baffle
120 shown in FIG. 1. Alternatively, a remote plasma source may be
used where the plasma is maintained away from the substrate and the
radicals are directed to flow over the substrate while preventing
ions from reaching the substrate.
[0044] The oxidation process utilizing O, H, and OH radicals and
minimizing ions (e.g., O.sup.-, O.sup.+, H, H.sup.+, OH.sup.+,
OH.sup.-, OD.sup.+, OD.sup.-) enables formation of conformal
oxides. Prior art techniques that expose the substrates to the
plasma ions result in inferior oxides which do not conform well to
the underlayer. It is believed that the reasons for the
non-conformity may be surface excitation processes or local
charging effects caused by ion bombardment of the substrate.
[0045] According to another aspect of the invention, it is believed
that upon reaching the surface of the substrate, some of the O and
OH radical cause a negatively charged oxygen ion to be formed
within the silicon oxide. Therefore, as shown in broken line in
FIG. 1, a DC bias is applied to the wafer to generate an electric
field across the oxide so as to attract the negative ions.
Consequently, the flux of the oxygen ions through the oxide film to
the oxide-silicon interface is enhanced, resulting in higher growth
rates. With application of a DC bias to the substrate the flux of
ionic species of oxygen, radicals can be controlled. This also
helps to sustain an electric field across the oxide layer that is
being grown, which causes mobile negative oxygen ions to diffuse
through the growing film and sustain its growth.
[0046] A plasma chamber, such as that shown in FIG. 1, has been
used to investigate plasma assisted oxidation of silicon or
poly-silicon and to investigate selective oxidation. More
specifically, the effect of various variables on the plasma
assisted oxidation process has been performed. Among the variables
investigated are gas pressure in the process chamber, power to the
RF plasma source, power to the RF bias, substrate temperature, and
relative flow of O2 and/or H2.
[0047] FIG. 4 is a plot of results from investigation of plasma
assisted oxidation of silicon along with that of tungsten. The
vertical axis indicates the oxide layer thickness, while the
horizontal axis shows the partial flow of H2 in the O2+H2 gas
mixture. The initial native oxide film thickness was 12-13 .ANG..
For this particular run the substrate temperature is held at
300.degree. C., the pressure is held at 500 mT, the source power is
2500 W and each run was performed for 200 seconds. FIG. 4
illustrates that at flow of about 60% H2 concentration, silicon
oxidation is at maximum, while perhaps a minimal amount of tungsten
oxidation occurs. From FIG. 4, the polynomial curve fit to the
measured W loss seems to indicate zero W oxidation or tungsten
oxide reduction at or above 60% H2 concentration. Consequently, it
is seen that for effective selective oxidation of silicon in the
presence of tungsten, the concentration of H2 gas must be above
about 60%. On the other hand, for a concentration of H2 gas above
85%, the oxide growth may be of lower quality and at some point
would actually stop. Therefore, for effective selective oxidation
of silicon in the presence of tungsten, the concentration of H2 gas
should be maintained to below 95%, and in some cases even below
85%. From that, it is concluded that for oxidizing silicon in the
presence of tungsten, especially in plasma without steam, the
concentration of H2 gas by flow rate should be maintained at
between about 60-95%, and, sometimes to between 60-85%.
[0048] FIG. 5 illustrates results obtained for tungsten plasma
oxidation performed at 300.degree. C. for various times with two
different ambient conditions while the power is constant,
demonstrating the selective and the non-selective (metal oxidation)
process. In FIG. 5, square data points indicate processing with H2
to O2 flow ratio of 3.0, (H2 fraction of total=75%), while
triangular data points indicate processing with no H2 flow. As can
be seen, oxidation of tungsten increased with processing time when
no H2 was present, while no oxidation of tungsten occurred at H2/O2
ratio of 75%, which is consistent with the results shown in FIGS. 3
and 4.
[0049] FIG. 6 illustrates the results for selective silicon
oxidation in the presence of tungsten using 75% H2 in O2 plasma. As
can be seen, while silicon oxidation progressed with time, no
tungsten oxidation occurred for the entire duration. Therefore,
effective plasma oxidation of silicon selective to tungsten can be
performed with H2/O2 ratio of 75%.
3. Embodiments of Processes for Performing PALTROX Using Plasmas
Injected with Gas Mixtures Including Steam.
[0050] The following describes improved processes for performing
selective oxidization of silicon in the presence of one or multiple
metals and metal nitrides. It has been reported that Ti/TiN/WN
stack has been used in experiments as a barrier between Poly-Si and
W gate. The barrier Ti/TiN/WN, or any one of its components, may be
present with the W gate and hence the need to find the common
process window for these barrier materials and W is clear. In
general, the process windows for avoiding oxidation of such metals
or silicides do not overlap and in some cases no overlap may exist.
The inventors have discovered a silicon plasma oxidation process
selective to TiN and W using a steam co-injected with H2 gas into a
plasma source having a common process window between room
temperature and 400.degree. C. This was not expected based on our
previous tests of gas mixtures of hydrogen and oxygen gases without
steam. In one set of process conditions at 400.degree. C., the
window between 50% to 90% H2 in steam has been found to selectively
oxidize silicon in the presence of both TiN and W.
[0051] FIG. 7A illustrates plasma oxidation change of sheet
resistance results for TiN at 400.degree. C., which are favorable
conditions for silicon oxidation. As can be seen, the TiN layer is
oxidized for all conditions, except for very high H2 content. Such
a high H2 content is not desirable for silicon oxidation, in the
presence of tungsten. Conversely, as can also be seen from FIG. 7A,
if the oxidation is performed at the H2/O2 ratio favorable for
selectivity to tungsten, i.e., 60-85%, the TiN layer would be
oxidized. Which may be detrimental to the performance of the
device.
[0052] FIG. 7C illustrates plasma oxidation sheet resistance
results for TiN at 25.degree. C. It is seen that lowering the
temperature reduces the oxidation of TiN and may enable oxidation
of silicon at the H2/O2 ratio favorable to Tungsten. However, the
quality of oxide formed at low temperature is low, which adversely
affects the properties of the oxide layer. It is therefore more
effective to perform the oxidation at higher temperatures.
[0053] FIG. 7B illustrates steam plasma oxidation sheet resistance
results for TiN at 400.degree. C., which are favorable conditions
for silicon oxidation. As can be seen, the presence of steam in the
plasma drastically enlarges the process window for selective
oxidation in the presence of TiN. FIG. 7D illustrates steam plasma
oxidation sheet resistance results for TiN at 25.degree. C., which
also shows an enlarged process window. As can be seen, utilizing
the 60-85% H2/O2 ratio causes no oxidation of TiN.
[0054] FIG. 8 illustrates selective oxidation process window for
TiN in steam plasma, while FIG. 9 illustrates selective oxidation
process window for W in steam plasma. Comparison of these two plots
shows that at 400.degree. C., which is a beneficial temperature for
silicon oxidation, the process windows overlap when the amount of
H2 in the H.sub.2O steam is between about 50-90%. Note also that
FIG. 9 shows that the process window for tungsten at 400.degree. C.
has been dramatically increased due to the presence of steam. In
fact, when steam plasma is used O2, rather than H2 may be added to
the steam.
[0055] FIG. 11 illustrates the effects of adding O2 to the steam,
while FIG. 12 illustrates the effect of adding H2 to the steam.
While FIG. 11 suggests that adding O2 to the H2O steam is
preferable, this may not be optimal when other metals or metal
nitrides are present, since adding oxygen may cause oxidation of
those metals and nitrides. On the other hand, FIG. 12 illustrates
that adding H2 does not materially change the oxidation rate of
silicon, which is better for selective oxidation as shown in FIGS.
8 and 9.
5. Embodiments of Processes for UV Oxidation Enhancement
[0056] The following is a discussion of improving growth rate of
low temperature silicon oxides in presence of metals and metal
nitrides while avoiding oxidation thereof by use of UV radiation of
the substrate. Selective, radical-based, low temperature oxide
growth on silicon, that does not also oxidize any of the exposed
metals or metal nitrides, generally has low growth rates and small
process window when using hydrogen/oxygen gas mixtures. One reason
for low growth rate is the insufficiency of activating reactions
necessary for the growth of the silicon oxide occurring at the
interface between the already grown oxide and the silicon beneath
it. One such growth-rate enhancement method, ion bombardment, has
the undesirable side effect of causing damage to both silicon oxide
and to exposed metals/nitrides. We propose that UV activation can
be used to accelerate the growth rate of the silicon oxide while
not damaging the formed layer. We have found that the use of
mixtures including water vapor as well as hydrogen and oxygen along
with UV illumination of the substrate greatly improves growth rate
of silicon oxide on silicon while substantially increasing the
process window for avoiding oxidation to any of the exposed metals
or metallic nitrides. Since the degree of enhancement of the oxide
growth rate will vary depending on the particular application or
device integration scheme employed, having a variable degree of UV
enhancement may prove beneficial for IC fabrication equipment used
in mass production.
[0057] To be sure, the use of UV radiation in the context of plasma
oxidation has been reported in the literature. However, prior
studies utilized UV radiation of the plasma to enhance molecular
dissociation in the plasma itself. In this work we disclose that UV
irradiation of the substrate enhances the growth mechanism at the
oxide-silicon interface, possibly by activating the formation of
negative oxygen ions in the surface oxide. This is entirely
independent of UV illumination of the gas phase species to enhance
dissociation.
[0058] UV radiation enhancement can be efficiently done in
different ways. One natural way is to use the UV generated in the
plasma source that is employed for generation of radicals from feed
gases. The difficulty lies in making such UV intensity variable
independently of the gas mixture or source power. One such way of
varying UV without varying mixture or source power involves varying
the transparency of the baffles 120. This can be done by, e.g., the
use of a pair of partially open or partially transparent baffles or
grids 120A and 120B (FIG. 10) with hole patterns whose degree of
overlap can be controlled by rotation of one of them as shown by
arrow A. Another way is to use a separate source of UV radiation to
illuminate the wafer.
[0059] FIG. 13 illustrates an embodiment wherein the plasma is
generated in a remote chamber 132, using, e.g., microwave source
152 (although other sources may be used), and utilizing a conduit
119 to enable drift of plasma species towards the wafer 110, which
resides in vacuum processing chamber 105. In this embodiment, UV
transparent windows 121 are provided to enable UV radiation from UV
sources 117 to illuminate the wafer 110. This may enhance the
probability of Si--Si bond breakage, thus increasing the available
Si bonding sites to form Si--O.
[0060] The addition of a noble gas, such as Ar, Kr, Xe, etc., to a
plasma can also be used for the generation of short wavelength
radiation in the DUV and VUV portions of the spectrum from the
resulting ionization and excitation of the noble gas atoms. The
exposure of the process wafer surface to the resulting DUV and VUV
radiation together with the radical oxidation species may result in
further benefits.
6. Embodiments of Steam Plasma Oxidation
[0061] Silicon nitride is difficult to oxidize by conventional
thermal oxidation in O2 or H2O, usually requiring a very high
temperature and long time. It is known to oxidize in atomic oxygen,
including species generated by an oxygen plasma. The use of O2+H2
mixtures activated by a plasma may also allow oxidation of silicon
nitride, while also changing the rate and/or the relative rates of
oxidation of silicon nitride and other materials. Furthermore, the
use of steam in the plasma (on its own or in combination with H2 or
O2, or with other gases such as noble gases) can be optimized for
controlling the degree of oxidation or the relative rates of
oxidation of two materials. For example, steam may be used to
enhance or retard the rate of oxidation of silicon nitride relative
to that of silicon or another material.
[0062] A steam plasma, either on its own or in combination with H2
or O2, or with other gases such as noble gases, can be used to
oxidize a wide variety of materials. It may also allow control over
the relative rates of oxidation of semiconductor materials that may
be present on a wafer. Here the steam may enhance or retard the
relative rates of oxidation of any two (or more) materials. For
example these materials can be those used for a channel or a
source/drain region of a transistor. Materials that can be oxidized
by the steam-bearing plasma may be pure or they may be alloys, and
they may be doped with different atomic species that affect their
electrical conductivity or their lattice spacing. Examples include
the following: [0063] a. Oxidation of regions of silicon doped with
atoms of another species, such as B, P, As, Sb, In, Ge, C, Sn, S,
Se; [0064] b. Oxidation of SiGe alloys; [0065] c. Oxidation of
materials used to form new types of transistors: for example Ge;
GaAs; InGaAs; alloys comprising In, Ga, P, As; GaN; InGaN; alloys
comprising group III elements and group V elements, SiC and
carbon-based semiconductor devices; [0066] d. Oxidation to
passivate surfaces of the materials described above; [0067] e.
Oxidation to form oxide films; [0068] f. Oxidation to form regions
that are subsequently removed by etching [0069] g. Oxidation that
directly etches the material by forming a volatile species. For
example carbon-based devices structures can be etched by forming
CO2 or CO gas.
[0070] The use of steam provides new opportunities to control the
topography of device features, such as trenches. In some cases
isotropic oxidation may be desired, with uniform oxidation of
surfaces with different orientations, such as sidewalls of a
trench. In other cases non-uniform oxidation may be desired. By
using steam, or a combination of steam and another gas, we can
alter the degree of isotropy in the plasma oxidation process.
[0071] Plasma oxidation of silicon generally obeys an Arrhenius
like dependence with temperature, but with a much lower activation
energy than thermal oxidation owing to the presence of oxygen
radicals formed in the plasma. In this work, the oxide growth rate
in non-steam oxidation process is enhanced as the hydrogen
concentration is increased from zero to about 25% to 50% above
which level the growth rate begins to turn over and decrease as
hydrogen concentration approaches 100%. For process temperatures
between room temperature and 300.degree. C., the oxidation growth
rate monotonically increases with higher temperature at each
concentration level of H2.
[0072] In this work a new type of behavior has been discovered
where the oxide growth rate does not continue to monotonically
increase with temperature. FIG. 14 illustrates the temperature
effects on O2+H2 plasma oxidation at a high temperature regime. The
oxidation process was carried out at three different temperatures,
300.degree. C., 400.degree. C. and 500.degree. C. As shown in FIG.
14, at each of these temperatures three different oxide growth rate
regimes exist. The first regime, labeled zone 1, for low H2
concentration, the oxide growth increases; in the second, labeled
zone 2, for intermediate H2 concentration, the growth rate remains
relatively constant; and in the third regime, labeled zone 3, the
oxide growth rate actually decreases with H2 concentration and with
temperature. This novel result can be important for process control
where it is necessary to vary the level of hydrogen to affect one
specific process, while maintaining a constant silicon oxide
growth.
[0073] The present invention has been described in relation to
particular examples, which are intended in all respects to be
illustrative rather than restrictive. Those skilled in the art will
appreciate that many different combinations of hardware, software,
and firmware will be suitable for practicing the present invention.
Moreover, other implementations of the invention will be apparent
to those skilled in the art from consideration of the specification
and practice of the invention disclosed herein. Various aspects
and/or components of the described embodiments may be used singly
or in any combination in the server arts. It is intended that the
specification and examples be considered as exemplary only, with a
true scope and spirit of the invention being indicated by the
following claims.
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