U.S. patent application number 10/000825 was filed with the patent office on 2002-10-10 for substrate temperature control in an ald reactor.
This patent application is currently assigned to Angstron Systems, Inc.. Invention is credited to Babcoke, Jason E., Brown, Jeffrey A., Chiang, Tony P., Leeser, Karl F..
Application Number | 20020144786 10/000825 |
Document ID | / |
Family ID | 34084299 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020144786 |
Kind Code |
A1 |
Chiang, Tony P. ; et
al. |
October 10, 2002 |
Substrate temperature control in an ALD reactor
Abstract
A deposition system includes a process chamber for conducting an
ALD process to deposit layers on a substrate. An electrostatic
chuck (ESC) retains the substrate. A backside gas increases thermal
coupling between the substrate and the ESC. The ESC is cooled via a
coolant flowing through a coolant plate and heated via a resistive
heater. Various arrangements are disclosed.
Inventors: |
Chiang, Tony P.; (Santa
Clara, CA) ; Leeser, Karl F.; (Sunnyvale, CA)
; Brown, Jeffrey A.; (San Francisco, CA) ;
Babcoke, Jason E.; (Menlo Park, CA) |
Correspondence
Address: |
SKJERVEN MORRILL LLP
25 METRO DRIVE
SUITE 700
SAN JOSE
CA
95110
US
|
Assignee: |
Angstron Systems, Inc.
|
Family ID: |
34084299 |
Appl. No.: |
10/000825 |
Filed: |
October 24, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10000825 |
Oct 24, 2001 |
|
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09902080 |
Jul 9, 2001 |
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60281628 |
Apr 5, 2001 |
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Current U.S.
Class: |
156/345.51 ;
118/723E; 118/728; 156/345.52; 156/345.53 |
Current CPC
Class: |
C23C 16/45544 20130101;
C23C 16/18 20130101; C23C 16/45561 20130101; C23C 16/34 20130101;
H01J 37/3244 20130101; C23C 16/4411 20130101; C23C 16/08 20130101;
H01L 21/76814 20130101; H01L 21/76838 20130101; C23C 16/45565
20130101; H01J 37/32862 20130101; C23C 16/4557 20130101; C23C
16/0227 20130101; C23C 16/45525 20130101; H01L 21/28562 20130101;
C23C 16/45557 20130101; C23C 16/4586 20130101; H01J 37/32449
20130101; C23C 16/4486 20130101; C23C 16/515 20130101; H01L
21/76843 20130101; C23C 16/45527 20130101; Y10T 279/23 20150115;
C23C 16/45536 20130101; C23C 16/4412 20130101 |
Class at
Publication: |
156/345.51 ;
118/728; 118/723.00E; 156/345.53; 156/345.52 |
International
Class: |
C23C 016/00; C23F
001/00 |
Claims
What is claimed is:
1. A system for controlling the temperature of a substrate in a
process chamber comprising: an electrostatic chuck assembly for
retaining a substrate on said chuck assembly by electrostatic
attraction, said substrate, when retained by said chuck assembly,
forming a backside gas volume bounded by a backside surface of said
substrate; and a gas inlet leading to said backside gas volume for
supplying a backside gas to said backside gas volume for providing
a thermal transfer between said chuck assembly and said backside
surface of said substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 09/902,080, entitled "Variable Gas Conductance
Control For A Process Chamber," filed Jul. 9, 2001. The present
application also claims priority from Provisional Application
Serial No. 60/281,628, entitled "A Reactor For Atomic Layer
Deposition," filed Apr. 5, 2001, incorporated herein by
reference.
[0002] This application is also related to the following co-pending
applications, which are incorporated herein by reference:
[0003] U.S. application Ser. No. 09/812,352, entitled "System And
Method For Modulated Ion-Induced Atomic Layer Deposition
(MII-ALD)," filed Mar. 19, 2001.
[0004] U.S. application Ser. No. 09/812,486, entitled "Continuous
Method For Depositing A Film By Modulated Ion-Induced Atomic Layer
Deposition (MII-ALD)," filed Mar. 19, 2001.
[0005] U.S. application Ser. No. 09/812,285, entitled "Sequential
Method For Depositing A Film By Modulated Ion-Induced Atomic Layer
Deposition (MII-ALD)," filed Mar. 19, 2001.
[0006] U.S. application Ser. No. 09/854,092, entitled "Method And
Apparatus for Improved Temperature Control In Atomic Layer
Deposition," filed May 10, 2001.
[0007] U.S. Provisional Application Serial No. 60/255,812, entitled
"Method For Integrated In-Situ Cleaning And Subsequent Atomic Layer
Deposition Within A Single Processing Chamber," filed Dec. 15,
2000.
FIELD OF THE INVENTION
[0008] The present invention relates to advanced thin film
deposition apparatus and methods used in semiconductor processing
and related technologies.
BACKGROUND
[0009] As integrated circuit (IC) dimensions shrink, the ability to
deposit conformal thin film layers with excellent step coverage at
low deposition temperatures is becoming increasingly important.
Thin film layers are used, for example, as MOSFET gate dielectrics,
DRAM capacitor dielectrics, adhesion promoting layers, diffusion
barrier layers, and seed layers for subsequent deposition steps.
Low temperature processing is desired, for example, to prevent
unwanted diffusion of shallow junctions, to better control certain
reactions, and to prevent degradation of previously deposited
materials and their interfaces.
[0010] The need for conformal thin film layers with excellent step
coverage is especially important for high aspect ratio trenches and
vias, such as those used in metallization layers of semiconductor
chips. For example, copper interconnect technology requires a
continuous thin film barrier layer and a continuous thin film
copper seed layer to coat the surfaces of trenches and vias
patterned in an insulating dielectric prior to filling the features
with copper by electrochemical deposition (ECD or
electroplating).
[0011] A highly conformal, continuous barrier layer is required to
prevent copper diffusion into the adjacent semiconductor (i.e.,
silicon) material or dielectric. The barrier layer also often acts
as an adhesion layer to promote adhesion between the dielectric and
the copper seed layer. Low dielectric constant (i.e., low-k)
dielectrics are typically used to reduce inter- and intra-line
capacitance and cross-talk, but often suffer from poorer adhesion
and lower thermal stability than traditional oxide dielectrics,
making the choice of a suitable adhesion layer more critical. A
non-conformal barrier layer, or one with poor step coverage or
discontinuous step coverage, can lead to copper diffusion and
current leakage between adjacent metal lines or to delamination at
either the barrier-to-dielectric or barrier-to-seed layer
interfaces, both of which adversely affect product lifetime and
performance. The barrier layer should also be uniformly thin, to
most accurately transfer the underlying trench and via sidewall
profile to the subsequent seed layer, and have a low film
resistivity (e.g., .rho.<500 .mu..OMEGA.-cm) to lessen its
impact on the overall conductance of the copper interconnect
structures.
[0012] A highly conformal, uniformly thin, continuous seed layer
with low defect density is required to prevent void formation in
the copper wires. The seed layer carries the plating current and
acts as a nucleation layer. Voids can form from discontinuities or
other defects in the seed layer, or they can form from pinch-off
due to gross overhang of the seed layer at the top of features,
both trenches and vias. Voids adversely impact the resistance,
electromigration, and reliability of the copper lines, which
ultimately affects the product lifetime and performance.
[0013] Traditional thin film deposition techniques, for example,
physical vapor deposition (PVD) and chemical vapor deposition
(CVD), are increasingly unable to meet the requirements of advanced
thin films. PVD, such as sputtering, has been used for depositing
conductive thin films at low cost and at relatively low substrate
temperature. Unfortunately, PVD is inherently a line of sight
process, resulting in poor step coverage in high aspect ratio
trenches and vias. Advances in PVD technology to address this issue
have resulted in high cost, complexity, and reliability issues. CVD
processes can be tailored to provide conformal films with improved
step coverage. Unfortunately, CVD processes often require high
processing temperatures, result in the incorporation of high
impurity concentrations, and have poor precursor (or reactant)
utilization efficiency, leading to a high cost of ownership.
[0014] Atomic layer deposition (ALD), or atomic layer chemical
vapor deposition (AL-CVD), is an alternative to traditional CVD
methods to deposit very thin films. ALD has several advantages over
PVD and traditional CVD. ALD can be performed at comparatively
lower temperatures (which is compatible with the industry's trend
toward lower temperatures), has high precursor utilization
efficiency, can produce conformal thin film layers (i.e., 100% step
coverage is theoretically possible), can control film thickness on
an atomic scale, and can be used to "nano-engineer" complex thin
films.
[0015] A typical ALD process differs significantly from traditional
CVD processes. In a typical CVD process, two or more reactant gases
are mixed together in the deposition chamber where either they
react in the gas phase and deposit on the substrate surface, or
they react on the substrate surface directly. Deposition by CVD
occurs for a specified length of time, based on the desired
thickness of the deposited film. Since this specified time is a
function of the flux of reactants into the chamber, the required
time may vary from chamber to chamber.
[0016] In a typical ALD process deposition cycle, each reactant gas
is introduced sequentially into the chamber, so that no gas phase
intermixing occurs. A monolayer of a first reactant is physi- or
chemisorbed onto the substrate surface. Excess first reactant is
pumped out, possibly with the aid of an inert purge gas. A second
reactant is introduced to the deposition chamber and reacts with
the first reactant to form a monolayer of the desired thin film via
a self-limiting surface reaction. The self-limiting reaction halts
once the initially adsorbed first reactant fully reacts with the
second reactant. Excess second reactant is pumped out, again
possibly with the aid of an inert purge gas. A desired film
thickness is obtained by repeating the deposition cycle as
necessary. The film thickness can be controlled to atomic layer
(i.e., angstrom scale) accuracy by simply counting the number of
deposition cycles.
[0017] Physisorbed precursors are only weakly attached to the
substrate. Chemisorption results in a stronger, more desirable
bond. Chemisorption occurs when adsorbed precursor molecules
chemically react with active surface sites. Generally,
chemisorption involves cleaving a weakly bonded ligand (a portion
of the precursor) from the precursor, leaving an unsatisfied bond
available for reaction with an active surface site.
[0018] The substrate material can influence chemisorption. In
current dual damascene copper interconnect structures, a barrier
layer such as tantalum (Ta) or tantalum nitride (TaN) must often
simultaneously cover silicon dioxide (SiO.sub.2), low-k
dielectrics, nitride etch stops, and any underlying metals such as
copper. Materials often exhibit different chemical behavior,
especially oxides versus metals. In addition, surface cleanliness
is important for proper chemisorption, since impurities can occupy
surface bonding sites. Incomplete chemisorption can lead to porous
films, incomplete step coverage, poor adhesion between the
deposited films and the underlying substrate, and low film
density.
[0019] The ALD process temperature must be selected carefully so
that the first reactant is sufficiently adsorbed (e.g.,
chemisorbed) on the substrate surface, and the deposition reaction
occurs with adequate growth rate and film purity. A temperature
that is too high can result in desorption or decomposition (causing
impurity incorporation) of the first reactant. A temperature that
is too low may result in incomplete chemisorption of the first
precursor, a slow or incomplete deposition reaction, no deposition
reaction, or poor film quality (e.g., high resistivity, low
density, poor adhesion, and/or high impurity content).
[0020] Traditional ALD processes have several disadvantages. First,
since the process is entirely thermal, selection of an appropriate
process temperature is often confined to a narrow temperature
window. Second, the small temperature window limits the selection
of available precursors. Third, metal precursors that fit the
temperature window are often halides (e.g., compounds that include
chlorine, flourine, or bromine), which are corrosive and can create
reliability issues in metal interconnects. Fourth, either gaseous
hydrogen (H.sub.2) or elemental zinc (Zn) is often used as the
second reactant to act as a reducing agent to bring a metal
compound in the first reactant to the desired oxidation state of
the final film. Unfortunately, H.sub.2 is an inefficient reducing
agent due to its chemical stability, and Zn has a low volatility
and is generally incompatible with IC manufacturing. Thus, although
conventional ALD reactors are suitable for elevated-temperature
ALD, they limit the advancement of ALD processing technology.
[0021] Plasma-enhanced ALD, also called radical enhanced atomic
layer deposition (REALD), was proposed to address the temperature
limitations of traditional thermal ALD. For example, in U.S. Pat.
No. 5,916,365, the second reactant passes through a radio frequency
(RF) glow discharge, or plasma, to dissociate the second reactant
and to form reactive radical species to drive deposition reactions
at lower process temperatures. More information on plasma-enhanced
ALD is included in "Plasma-enhanced atomic layer deposition of Ta
and Ti for interconnect diffusion barriers," by S. M. Rossnagel, et
al., Journal of Vacuum Science and Technology B 18(4) July/August
2000 pp. 2016-2020.
[0022] Plasma enhanced ALD, however, still has several
disadvantages. First, it remains a thermal process similar to
traditional ALD since the substrate temperature provides the
required activation energy, and therefore the primary control, for
the deposition reaction. Second, although processing at lower
temperatures is feasible, higher temperatures must still be used to
generate reasonable growth rates for acceptable throughput. Such
temperatures are still too high for some films of interest in IC
manufacturing, particularly polymer-based low-k dielectrics that
are stable up to temperatures of only 200.degree. C. or less.
Third, metal precursors, particularly for tantalum (Ta), often
still contain chlorine as well as oxygen impurities, which results
in low density or porous films with poor barrier behavior and
chemical instability. Fourth, the plasma enhanced ALD process, like
the conventional sequential ALD process described above, is
fundamentally slow since it includes at least two reactant gases
and at least two purge or evacuation steps, which can take up to
several minutes with conventional valve and chamber technology.
[0023] Conventional ALD reactors, including plasma enhanced ALD
reactors, include a vertically-translatable pedestal to achieve a
small process volume, which is important for ALD. A small volume is
more easily and quickly evacuated (e.g., of excess reactants) than
a large volume, enabling fast switching of process gases. Also,
less precursor is needed for complete chemisorption during
deposition. For example, the reactors of U.S. Pat. No. 6,174,377
and European Patent No. 1,052,309 A2 feature a reduced process
volume located above a larger substrate transfer volume. In
practice, a typical transfer sequence includes transporting a
substrate into the transfer volume and placing it on top of a
moveable pedestal. The pedestal is then elevated vertically to form
the bottom of the process volume and thereby move the substrate
into the process volume. Thus, the moveable pedestal has at least a
vertical translational and possibly a second rotational degree of
freedom (for high temperature process uniformity).
[0024] Typical ALD reactors have significant disadvantages. First,
conventional ALD reactors suffer from complex pedestal
requirements, since the numerous facilities (e.g., heater power
lines, temperature monitor lines, and coolant channels) must be
connected to and housed within a pedestal that moves. Second, in
the case of plasma enhanced ALD, the efficiency of radical delivery
for deposition of conductive thin films is significantly decreased
in downstream configurations in which the radical generating plasma
is contained in a separate vessel remote from the main process
chamber (see U.S. Pat. No. 5,916,365). Both gas phase and wall
recombinations reduce the flux of useful radicals to the substrate.
In the case of atomic hydrogen (H), recombination results in
diatomic H.sub.2, a far less effective reducing agent. Other
disadvantages of known ALD reactors exist.
[0025] Accordingly, improved ALD reactors are desirable to make ALD
better suited for commercial IC manufacturing. Desirable
characteristics of such reactors might include higher throughput,
improved deposited film characteristics, better temperature control
for narrow process temperature windows, and wider processing
windows (e.g., in particular with respect to process temperature
and reactant species).
SUMMARY
[0026] A deposition system in accordance with one embodiment of the
present invention includes a process chamber for conducting an ALD
process to deposit layers on a substrate. An electrostatic chuck
(ESC) retains the substrate. A backside gas increases thermal
coupling between the substrate and the ESC. The ESC is cooled via a
coolant flowing through a coolant plate and heated via a resistive
heater.
[0027] These and other aspects and features of the disclosed
embodiments will be better understood in view of the following
detailed description of the exemplary embodiments and the drawings
thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic diagram of a novel ALD reactor.
[0029] FIG. 2 shows various embodiments of the shield and shadow
ring overlap region of FIG. 1.
[0030] FIG. 3 is a schematic diagram showing top introduction of
gas into the process chamber of the ALD reactor of FIG. 1.
[0031] FIG. 4 is (a) a schematic diagram and (b) a plan view
schematic diagram showing side introduction of gas into the process
chamber of the ALD reactor of FIG. 1.
[0032] FIG. 5 is (a) a schematic diagram and (b) a plan view
schematic diagram showing both top and side introduction of gas
into the process chamber of the ALD reactor of FIG. 1.
[0033] FIG. 6 is a schematic diagram of a control system for the
pedestal of FIG. 1.
[0034] FIG. 7 is a schematic diagram of a circuit for electrical
biasing of the electrostatic chuck of FIG. 1.
[0035] FIG. 8 is a front-side perspective view of a novel ALD
reactor.
[0036] FIG. 9 is a back-side perspective view of the ALD reactor of
FIG. 8.
[0037] FIG. 10 is a back-side perspective view, from below, of the
ALD reactor of FIG. 8.
[0038] FIG. 11 is a front-side cutaway perspective view of the ALD
reactor of FIG. 8.
[0039] FIG. 12 is a front-side cutaway perspective view of the ALD
reactor of FIG. 8.
[0040] FIG. 13 is a cross-sectional view of a chamber portion of
the ALD reactor along line 13-13 of FIG. 8.
[0041] FIG. 14 is a detailed cross-sectional view of the right side
of the chamber portion of FIG. 13 showing a load shield
position.
[0042] FIG. 15 is a detailed cross-sectional view of the right side
of the chamber portion of FIG. 13 showing a low conductance process
shield position.
[0043] FIG. 16 is a detailed cross-sectional view of the right side
of the chamber portion of FIG. 13 showing a high conductance
process shield position.
[0044] FIG. 17 is a detailed cross-sectional view of the right side
of the chamber portion of FIG. 13 showing a purge shield
position.
[0045] FIG. 18 is a schematic diagram of a valve system for gas
delivery in the ALD reactor of FIG. 8.
[0046] FIG. 19 is a schematic diagram of a valve system for gas
delivery in the ALD reactor of FIG. 8.
[0047] FIG. 20 is a schematic diagram of a valve system for gas
delivery in the ALD reactor of FIG. 8.
[0048] FIG. 21 is a schematic diagram of a valve system for gas
delivery in the ALD reactor of FIG. 8.
[0049] FIG. 22 is a schematic diagram of a valve system for gas
delivery in the ALD reactor of FIG. 8.
[0050] FIG. 23 is a perspective cross-section of two embodiments of
a showerhead for gas distribution.
[0051] FIG. 24 is a perspective cross-section of an embodiment of a
shield assembly for the ALD reactor of FIG. 8.
[0052] FIG. 25 is a perspective cross-section of an embodiment of a
shield assembly for the ALD reactor of FIG. 8.
[0053] FIG. 26 is a perspective cross-section of an embodiment of a
shield assembly for the ALD reactor of FIG. 8.
[0054] FIG. 27 is a cutaway perspective view of an embodiment of an
electrostatic chuck assembly for the ALD reactor of FIG. 8.
[0055] FIG. 28 is a schematic diagram of a control system for the
electrostatic chuck assembly of FIG. 27 of the ALD reactor of FIG.
8.
[0056] FIG. 29 is a schematic diagram of a control system including
an alternative energy source for the electrostatic chuck assembly
of FIG. 27 of the ALD reactor of FIG. 8.
[0057] FIG. 30 is a perspective view of an embodiment of a portion
of an electrostatic chuck assembly for the ALD reactor of FIG.
8.
[0058] FIG. 31 is a schematic diagram of a circuit for electrical
biasing of the electrostatic chuck of the ALD reactor of FIG.
8.
[0059] FIG. 32 is a schematic diagram of a circuit for electrical
biasing of the electrostatic chuck of the ALD reactor of FIG.
8.
[0060] FIG. 33 is a schematic diagram of a circuit for electrical
biasing of the electrostatic chuck of the ALD reactor of FIG.
8.
[0061] FIG. 34 is a schematic illustration of a conventional ALD
process.
[0062] FIG. 35 is a schematic illustration of a novel ALD
process.
[0063] FIG. 36 shows timing diagrams for (a) a typical prior art
ALD process and (b) a novel ALD process.
[0064] FIG. 37 shows timing diagrams for an alternative embodiment
of a novel ALD process.
[0065] FIG. 38 shows timing diagrams for an alternative embodiment
of a novel ALD process.
[0066] FIG. 39 is a schematic illustration of a novel chemisorption
technique for ALD processes.
[0067] FIG. 40 is a schematic diagram of a circuit for electrical
biasing of the electrostatic chuck of the ALD reactor of FIG. 8 for
improved chemisorption.
[0068] In the drawings, like or similar features are typically
labeled with the same reference numbers.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0069] Basic ALD Reactor Design
[0070] FIG. 1 is a schematic diagram of a novel ALD reactor 2.
Reactor 2 includes a stationary pedestal 4, which may include an
electrostatic chuck (ESC) 6 on top of which a substrate 8 rests.
Substrate 8 is usually a semiconductor wafer (e.g., silicon), but
may be a metallized glass substrate or other substrate. A chamber
lid 10 and ESC 6 define the top and bottom boundaries,
respectively, of a process chamber 12. The surrounding wall of
chamber 12 is defined by a moveable shield 14, which is attached to
a plurality of shield support legs 16. The volume of process
chamber 12 is smaller than prior art batch reactors, but may be
similar in size to prior art single wafer systems. The
configuration of reactor 2, however, provides an overall volume of
reactor 2 that can be smaller than that of prior art reactors,
while providing the small volume of process chamber 12.
[0071] The small volume of process chamber 12 achieves the
advantages of small process volumes discussed above, including
quick evacuation, fast switching of process gases, and less
precursor required for complete chemisorption. The volume of
process chamber 12 cannot be made arbitrarily small, however, since
substrate 8 must still be transferred into, and out of, process
chamber 12.
[0072] In FIG. 1, the fixed position of pedestal 4, including its
supporting hardware, simplifies overall design of reactor 2,
allowing ease of use and maintenance as well as improved
performance. In comparison to massive moveable pedestals in prior
art reactors, shield 14 includes less associated hardware and is
much lighter, which allows precision positioning of shield 14 to
adjust the conductance of, and facilitate pumping of, chamber 12
with rapid response.
[0073] A chamber body 18 surrounds shield 14, chamber lid 10, and
pedestal 4 (including ESC 6), defining an annular pumping channel
20 exterior to shield 14. During processing, shield 14 separates
process chamber 12, at low pressure, from annular pumping channel
20, which is maintained at a lower pressure than the chamber to
maintain a clean background ambient in reactor 2. The volume of
chamber 12 is coupled to annular pumping channel 20 via a shield
conductance upper path 22 and a shield conductance lower path 24.
Upper path 22 and lower path 24 are each defined by portions of
shield 14 and corresponding features of stationary components of
reactor 2. In the embodiment shown in FIG. 1, upper path 22,
typically a variable low leakage path during processing, is bounded
by an inner wall of shield 14 and chamber lid 10. Lower path 24, a
variable high leakage path through a shield and shadow ring overlap
region 26, is bounded by a portion of shield 14 and a shadow ring
28. Shadow ring 28 is actually separate from ESC 6 and is shown in
greater detail in subsequent figures.
[0074] The structures of shield 14 and shadow ring 28 may vary to
provide different conductances of lower path 24 as shown in FIG. 2,
which shows various embodiments of the shield and shadow ring
overlap region 26 of FIG. 1. The conductance of a flow path is
related to the length of the restriction as well as the physical
dimensions of the path. For example, a shorter path with a large
cross-sectional area has a higher conductance. For the embodiments
shown in FIG. 2, the structural configurations of shield 14 and
shadow ring 28 result in a highest conductance path 30, a second
highest conductance path 32, a third highest conductance path 34,
and a lowest conductance path 36. Practitioners in the art will
appreciate that many other embodiments of shield and shadow ring
overlap region 26 are possible.
[0075] Various shield positions are employed throughout a novel ALD
process. Raising shield 14 to its highest position (along with
shadow ring 28) allows for introduction or removal of substrate 8.
Dropping shield 14 to its lowest position allows rapid evacuation
of chamber 12 via upper path 22 by exposure to the vacuum of
annular pumping region 20. Shield 14 is positioned at intermediate
positions during processing depending on gas delivery and
conductance requirements.
[0076] The motion of shield 14 can be used to precisely control the
spatial relationship between shield 14 and shadow ring 28, thereby
providing a tunable conductance for chamber 12 primarily via lower
path 24. This allows quick, precise control of the pressure in
chamber 12, even during processing, which is not possible in prior
art methods that employ a moveable pedestal since vertical motion
of substrate 8 is undesirable during processing. The tunable
conductance also allows quick, precise control of the residence
time of gases introduced to chamber 12 for multiple flow rates, and
it allows minimal waste of process gases.
[0077] Basic Gas Introduction to an ALD Reactor
[0078] Reactor 2 of FIG. 1 supports gas introduction through
multiple points, including top introduction, side introduction, or
a combination of both top and side introductions.
[0079] FIG. 3 is a schematic diagram showing top introduction of
gas into process chamber 12 of ALD reactor 2 of FIG. 1. A top mount
feed (not shown) has a single introduction point (or multiple
introduction points) with an optional added device (not shown),
such as a showerhead and/or a baffle, to ensure that a top
introduction flow distribution 38 is uniform over the substrate.
The added device includes at least one passage, and may include
many. The added device may also include intermediate passages to
regulate gas distribution and velocity.
[0080] FIG. 4 is (a) a schematic diagram and (b) a plan view
schematic diagram showing side introduction of gas into process
chamber 12 of ALD reactor 2 of FIG. 1. Gas is introduced from a gas
channel 40 in shield 14 into process chamber 12 through orifices in
an inner wall of shield 14. Gas is introduced in a symmetric
geometry around substrate 8 designed to ensure that a side
introduction flow distribution 42 is even. In addition, the plane
of the gas introduction may be adjusted vertically relative to
substrate 8 before or during gas introduction, which can be used to
optimize flow distribution 42.
[0081] FIG. 5 is (a) a schematic diagram and (b) a plan view
schematic diagram showing both top and side introduction of gas
into process chamber 12 of ALD reactor 2 of FIG. 1. The gases for
novel ALD processes, including precursor and purge gases, can be
introduced through the same introduction path or separate paths as
desired for optimal performance and layer quality.
[0082] Basic Electrostatic Chuck Assembly Design for an ALD
Reactor
[0083] Reactor 2 of FIG. 1 can be used in a deposition process
where the activation energy for the surface reaction is provided by
ions created in a plasma above the substrate. Thus, atomic layer
deposition can be ion-induced, rather than thermally induced. This
allows deposition at much lower temperatures than conventional ALD
systems. Given the sufficiently low process temperatures, pedestal
4 may include an electrostatic chuck (ESC) 6 for improved
temperature control and improved radio frequency (RF) power
coupling.
[0084] Additional detail of ion-induced atomic layer deposition may
be found in the following related applications. U.S. application
Ser. No. 09/812,352, entitled "System And Method For Modulated
Ion-Induced Atomic Layer Deposition (MII-ALD)," filed Mar. 19,
2001, assigned to the present assignee and incorporated herein by
reference. U.S. application Ser. No. 09/812,486, entitled
"Continuous Method For Depositing A Film By Modulated Ion-Induced
Atomic Layer Deposition (MIT-ALD)," filed Mar. 19, 2001, assigned
to the present assignee and incorporated herein by reference. U.S.
application Ser. No. 09/812,285, entitled "Sequential Method For
Depositing A Film By Modulated Ion-Induced Atomic Layer Deposition
(MI-ALD)," filed Mar. 19, 2001, assigned to the present assignee
and incorporated herein by reference.
[0085] FIG. 6 is a schematic diagram of a control system 44 for
pedestal 4 of FIG. 1. Substrate 8 rests on an annular sealing lip
46 defining a backside gas volume 48 between substrate 8 and a top
surface 50 of ESC 6 of pedestal 4. The backside gas flows from a
backside gas source 52 along a backside gas line 54, through a
backside gas passageway 56 in ESC 6, and into gas volume 48. The
backside gas improves the thermal communication between substrate 8
and ESC 6 by providing a medium for thermal energy transfer between
substrate 8 and ESC 6. A means of flow control, such as a pressure
controller 58, maintains the backside gas at a constant pressure,
thus ensuring a uniform substrate temperature.
[0086] Substrate temperature is modulated by heating or cooling ESC
6. A temperature sensor 60 is coupled via a sensor connection 62 to
a temperature monitor 64. A temperature controller 66 controls a
heater power supply 68 applied via an electrical connection 70 to a
resistive heater 72 embedded in ESC 6. A coolant temperature and
flow controller 74, as is widely known, controls the coolant from a
coolant supply 76 as it flows in a plurality of coolant channels 78
in pedestal 4.
[0087] ESC 6 includes at least a first electrode 80 and a second
electrode 82 embedded in a dielectric material. FIG. 7 is a
schematic diagram of a circuit 84 for electrical biasing of
electrostatic chuck 6 of pedestal 4 of FIG. 1. First electrode 80
and second electrode 82 are biased with different DC potentials to
provide the "chucking" action that holds substrate 8 (FIG. 1) to
ESC 6 prior to plasma ignition and during deposition. The biasing
scheme of FIG. 7 allows establishment of the electrostatic
attraction (i.e., "chucking") at low biases that would be
insufficient to generate enough electrostatic attraction with a
conventional monopolar chuck. In FIG. 7, one terminal of a DC power
supply 86 is coupled via a first inductor 88 to first electrode 80.
The other terminal of DC power supply 86 is coupled via a second
inductor 90 to second electrode 82. Inductors 88 and 90 serve as RF
filters.
[0088] RF power (e.g., at 13.56 MHz) is also supplied
simultaneously to both first electrode 80 and second electrode 82
using an RF generator 92 coupled to a ground terminal 94. A first
capacitor 96 and a second capacitor 98 are respectively coupled
between RF generator 92 and first electrode 80 and second electrode
82. Capacitors 96 and 98 serve as DC filters to block the DC
voltage from power supply 86. Circuit 84 allows improved coupling
of RF power to substrate 8 during processing due to the close
proximity (e.g., 0.6 mm-2 mm spacing) of substrate 8 to first
electrode 80 and second electrode 82 embedded in ESC 6.
[0089] Since substrate 8 is in such close proximity to first and
second electrodes 80 and 82, the transmission efficiency of RF
power through the intervening dielectric of ESC 6 is higher than in
conventional reactors where RF power is applied to electrodes at a
greater distance from the substrate. Thus, less power is needed to
achieve sufficient RF power coupling to substrate 8 in novel ALD
reactor 2 (FIG. 1), and the same power to generate the bias on
substrate 8 can also be used to create a plasma above substrate 8
at very low powers (e.g., <600 W, and typically <150 W).
[0090] ALD Reactor Detail
[0091] FIG. 8, FIG. 9, FIG. 10, FIG. 11, and FIG. 12 show external
views and internal cutaway views of a novel ALD reactor 100. FIG. 8
is a front-side perspective view of reactor 100. FIG. 9 is a
back-side perspective view of reactor 100. FIG. 10 is a back-side
perspective view, from below, of reactor 100. FIG. 11 is a
front-side cutaway perspective view of reactor 100. FIG. 12 is
another front-side cutaway perspective view of reactor 100.
[0092] Referring to FIG. 8, a substrate 8 (FIG. 12) is transferred
into or out of a process chamber 12 (FIG. 11 and FIG. 12) of
reactor 100 through a substrate entry slot 102 in a slit valve 104.
Substrate 8 is loaded onto or unloaded from the pedestal (e.g., an
electrostatic chuck assembly 106 as seen in FIG. 11 and FIG. 12) by
a plurality of lift pins 108. In the load or unload position, the
tips of lift pins 108 extend through orifices in an electrostatic
chuck (ESC) 6 to hold substrate 8 above the top surface of ESC 6.
In the process position, the tips of lift pins 108 retract below
the top surface of ESC 6 allowing contact between substrate 8 and
ESC 6 (FIG. 11 and FIG. 12).
[0093] Referring to FIG. 11 and FIG. 12, lift pins 108 extend
downward from process chamber 12 in the interior of reactor 100
through an electrostatic chuck assembly 106 (including ESC 6, a
cooling plate 110, and a baseplate 112) to the exterior under-side
of reactor 100. Each of lift pins 108 is attached to a lift pin
spider 114 to coordinate their motion. Vertical translation of lift
pin spider 114 is accomplished with an off-axis lift pin actuator
116 (e.g., a pneumatic cylinder), which controls motion of a tie
rod 118 that is coupled to lift pin spider 114 by a spherical joint
120 as seen in FIG. 10. Spherical joint 120 transmits lifting
forces to lift pin spider 114 but no moments.
[0094] Referring to FIG. 11, to facilitate substrate transfer, a
moveable shield 14, must be in a load position. Shield 14 is raised
or lowered using a linear motor 122, which moves a linear motor
output rod 124 attached to a shield lift spider 126 by a collet
clamp 128 (best seen in FIG. 10). Each one of a plurality of shield
support legs 16 (FIG. 11) extends through a shield support leg seal
130 and is coupled between shield lift spider 126 and shield 14.
The axis of linear motor 122 is aligned with the axis of process
chamber 12 resulting in no net moments on shield lift spider 126.
Lift pin spider 114 rides a portion of linear motor output rod 124,
coaxial with output rod 124 and shield lift spider 126. Lift pin
spider 114, however, is unaffected by movement of rod 124, and this
arrangement results in no net moments on lift pins 108.
[0095] As mentioned above, linear motor 122 provides actuation of
shield 14. This is in contrast to conventional moveable pedestals
wherein slower stepper motors are used for actuation. Conventional
rotational stepper motors use lead screws (possibly in conjunction
with a gear train), which are slow but capable of moving heavy
masses, to effect movement of the heavy pedestal. Linear motor 122
does not use a gear train, but instead directly drives the load.
Linear motor 122 includes a plurality of alternating magnets to
effect motion of output rod 124.
[0096] Linear motor 122 can be a commercially available linear
motor and typically includes a sleeve having a coil and a moveable
rod enclosing the series of alternating magnets. The movement of
the rod through the sleeve is precisely controlled, using a Hall
Effect magnetic sensor, by a signal applied to the coil. In one
embodiment, pulses applied to the coil precisely control the
position of the rod with respect to the sleeve, as is well known.
Since shield 14 is a light weight compared to conventional heavy
pedestals, linear motor 122 provides high performance positioning,
with response times on the order of milliseconds. Linear motor 122
thus provides a quicker response and more accurate shield
positioning than is achievable with conventional stepper or servo
motors used to actuate the pedestal of conventional ALD
reactors.
[0097] Referring to FIG. 11, a pump, such as a turbomolecular pump
132, maintains a background ambient pressure as low as a few
microtorr or less in an annular pumping channel 20 surrounding
shield 14. Pump 132 is attached to reactor 100 at an angle such
that a circular pump throat 134 is fully exposed to a narrow
pumping slot 136 aft of process chamber 12, maximizing the
conductance between them. In this manner, pump 132 with a diameter,
d, has maximum exposure to pumping slot 136 of height, h (where
h<d), with minimum restriction between pump 132 and chamber 12
(see also FIG. 13 discussed below). For specific processing
applications, a pumping speed restrictor 138 can be inserted at
pump throat 134 to restrict the conductance as needed. In some
embodiments, a pressure controlling throttle valve (e.g., a
butterfly valve) can be used instead of, or in conjunction with,
restrictor 138. Pressure in pumping slot 136 and annular pumping
channel 20 is monitored by a pump pressure sensor 140 mounted on
the top surface of reactor 100.
[0098] Process chamber 12 is bounded on top by a chamber lid 10.
Pressure in process chamber 12 of reactor 100 may be on the order
of a few microtorr up to several torr. The pressure of chamber 12
is monitored by a fast chamber pressure sensor 142 and a precision
chamber pressure sensor 144, both of which are mounted on an upper
peripheral flange of chamber lid 10 (FIG. 8). The temperature of
chamber lid 10 is controlled by fluid flowing in a plurality of lid
cooling/heating channels 146 (FIG. 11). One possible path of gas
introduction to process chamber 12 is through a showerhead
three-way valve 148 mounted centrally on chamber lid 10. Another
possible method of gas introduction to process chamber 12 is
through a shield gas channel 40.
[0099] RF power is transferred to electrodes in ESC 6 via an RF
conductor 150 shielded within an RF insulator tube 152. A gas
medium (commonly referred to as a backside gas) is provided via a
backside gas valve 154 to ESC 6 to improve the thermal coupling
between ESC 6 and substrate 8. During processing, an optional
shadow ring 28 rests on a portion of ESC 6 fully surrounding a
peripheral edge of substrate 8.
[0100] FIG. 13 is a cross-sectional view of a chamber portion 156
of ALD reactor 100 along line 13-13 of FIG. 8. Substrate entry slot
102 is shown on the left hand side extending through a chamber body
18. Pumping slot 136, of height h, is shown on the right hand side
extending through chamber body 18 to pump throat 134, of diameter
d. The temperature of chamber body 18 is controlled by fluid
flowing in a chamber cooling/heating channel 158.
[0101] Chamber lid 10 rests atop chamber body 18. A vacuum seal, to
maintain low pressure in the interior of reactor 100, is maintained
through the use of an upper O-ring 160 between chamber lid 10 and
chamber body 18. Laterally spaced from O-ring 160 between chamber
lid 10 and chamber body 18 is an upper RF gasket 162, forming an RF
shield. The temperature of chamber lid 10 is controlled by fluid
flowing in lid cooling/heating channels 146. Alternatively, the
temperature of chamber lid 10 may be controlled by an electric or
resistive heater or other cooling/heating means.
[0102] The pressure in process chamber 12 is monitored, in part, by
fast chamber pressure sensor 142, which is mounted on an upper
peripheral flange of chamber lid 10. Pressure sensor 142 monitors
the pressure in a pressure tap volume 164, which is coupled to
process chamber 12 by a pressure sensor orifice 166. This
arrangement allows exposure of pressure sensor 142 to the pressure
of chamber 12, while preventing plasma and other process
chemistries from reaching, and possibly damaging, pressure sensor
142.
[0103] Gases can be introduced into process chamber 12 through a
showerhead gas feed inlet 168, which leads to a plenum 170 above a
showerhead 172 attached to a lower surface of chamber lid 10.
Showerhead 172 includes a showerhead lip 174 and a plurality of
showerhead gas orifices 176, which are used to distribute gas
evenly into process chamber 12.
[0104] Substrate 8 rests on an upper surface of an ESC assembly
106, which includes in part, ESC 6, cooling plate 110, and
baseplate 112. The vertical spacing between the upper surface of
ESC assembly 106 and showerhead 172 may be 0.3 inches to 1 inch,
typically less than 0.6 inches. Backside gas passageway 56 is shown
centrally located in and extending through ESC 6. ESC 6, which
includes the largest portion of the upper surface on which
substrate 8 rests, is held in contact with cooling plate 110 using
a clamp ring 178, which overlaps a surrounding flange at the base
of ESC 6. A plurality of clamp ring fasteners 180, each extending
through clamp ring 178 into cooling plate 110, secure the
connection between ESC 6 and cooling plate 110. A process kit 182
fully surrounds clamp ring 178 and electrically hides clamp ring
fasteners 180 from ESC 6 and substrate 8. For a more detailed view
of clamp ring 178, fasteners 180, and process kit 182, see FIG. 16,
discussed below.
[0105] The temperature of cooling plate 110 is controlled using
fluid flowing in a plurality of coolant channels 78 as shown in
FIG. 13. An upper surface of cooling plate 110 is patterned to
create a plurality of thermal breaks 184, or gaps, between ESC 6
and cooling plate 110. Thermal breaks 184 increase the temperature
difference between ESC 6 and cooling plate 110. This allows the
temperature of ESC 6 to rise substantially higher than the
temperature of baseplate 112, which stays relatively cool. For a
more detailed view of thermal breaks 184, see FIG. 27, discussed
below.
[0106] As shown in FIG. 13, a lower surface of cooling plate 110 is
attached to an upper surface of baseplate 112. The upper surface of
baseplate 112 forms the lower walls of coolant channels 78 in
cooling plate 110. A vacuum seal, to maintain low pressure in the
interior of reactor 100, is maintained through the use of an O-ring
186 between baseplate 112 and chamber body 18. Laterally spaced
from O-ring 186 between baseplate 112 and chamber body 18 is an RF
gasket 188.
[0107] One of the plurality of lift pins 108 is shown in retracted
process position, with the tip of lift pin 108 below the top
surface of ESC 6. Lift pin 108 extends through a lift pin seal 190,
which maintains the low pressure in the interior of reactor 100. A
lift pin bushing 192 reduces friction during vertical translation
of lift pin 108 through aligned orifices in baseplate 112, cooling
plate 110, and ESC 6.
[0108] In FIG. 13, shield 14 is shown in an intermediate process
position. Process chamber 12 is thus bounded on the top by
showerhead 172, on the bottom largely by ESC 6, and on the sides by
shield 14 to confine a plasma 194. Shield 14 includes shield gas
channel 40 and is attached to each shield support leg 16 using a
shield cap 196. Each shield support leg 16 extends through shield
support leg seal 130, which maintains the low pressure in the
interior of reactor 100. A plurality of shield support leg bushings
198 reduce friction during vertical translation of shield support
legs 16 through orifices in baseplate 112.
[0109] A shadow ring hook 200 is attached to a lower portion of
shield cap 196. Shadow ring hook 200 is shown interdigitated with
shadow ring 28, which fully surrounds a peripheral edge of ESC
assembly 106 and rests on a process kit bevel 202 of process kit
182. Shadow ring 28 protects the underlying portions of ESC
assembly 106 during deposition onto substrate 8. Shadow ring 28
also defines the circumferential region near the edge of substrate
8 where deposition is masked. Shadow ring 28 also plays a role in
defining the chamber conductance. For a more detailed view of
process kit bevel 202, see FIG. 16, discussed below.
[0110] In FIG. 13, two leakage paths modulate gas flow between
process chamber 12 and annular pumping channel 20, which is largely
bounded by chamber body 18, chamber lid 10, and ESC assembly 106.
The leakage occurs due to differing pressures between process
chamber 12 and annular pumping channel 20. A shield conductance
upper path 22 is bounded on one side by an inner upper surface of
shield 14, and on the other side by outer surfaces of chamber lid
10 and showerhead 172. A shield conductance lower path 24 is
bounded on one side by surfaces of a lower portion of shield 14,
shield cap 196, and shadow ring hook 200, and on the other side by
surfaces of shadow ring 28. Upper path 22 leads from process
chamber 12 to an upper portion 204 of annular pumping channel 20,
while lower path 24 leads from process chamber 12 to a lower
portion 206 of annular pumping channel 20.
[0111] Shield 14 can be vertically translated by either raising it
into upper portion 204 of annular pumping channel 20 or lowering it
into lower portion 206 of annular pumping channel 20. As shield 14
is translated, the conductances of upper path 22 and lower path 24
are changed. The variations in conductance can be controlled to
vary the pressure in process chamber 12 in a controlled manner as
needed for various steps in an atomic layer deposition process
sequence.
[0112] Shield Operation
[0113] Unlike in conventional ALD reactors, reactor 2 includes a
stationary pedestal 4 (see FIG. 1). For example, reactor 100 of
FIG. 12 includes ESC assembly 106. Transfer of substrate 8 into
process chamber 12 of reactor 100 is facilitated through the use of
moveable shield 14, which also plays a significant role during
processing.
[0114] Various shield positions are employed throughout the ALD
process. FIG. 14, FIG. 15, FIG. 16, and FIG. 17 show detailed
cross-sectional views of the right side of chamber portion 156 of
FIG. 13, showing shield 14 in a substrate load shield position 208
(FIG. 14), a low conductance process shield position 210 (FIG. 15),
a high conductance process shield position 212 (FIG. 16), and a
purge shield position 214 (FIG. 17).
[0115] In load shield position 208 of FIG. 14, shield support legs
16 are raised by linear motor 122 (FIG. 8). When shield 14 is
raised above a certain point, shadow ring hook 200 contacts shadow
ring 28 and lifts it as well. Shield 14 and shadow ring 28 are then
raised together. Shield 14 enters upper portion 204 of annular
pumping channel 20. Shield 14 and shadow ring 28 can be raised
until shadow ring 28 contacts showerhead lip 174, which prevents
shadow ring 28 from contacting showerhead 172.
[0116] Load shield position 208 thus allows loading (or unloading)
of substrate 8 into (or out of) process chamber 12 via substrate
entry slot 102 (FIG. 13). For example, to load substrate 8 into
process chamber 12, a substrate blade or paddle (not shown) carries
substrate 8 into process chamber 12. Lift pins 108 are raised by
lift pin actuator 116 (FIG. 10) to contact substrate 8 and lift it
off the top surface of the blade. The blade is then retracted out
of chamber 12 through entry slot 102. Lift pins 108 are retracted
past the top surface of ESC 6 allowing substrate 8 to rest on ESC 6
as shown in FIG. 14. A similar process is followed to unload
substrate 8 from chamber 12.
[0117] In an alternative embodiment, shadow ring 28 is not used,
and shield 14 forms variable conduction paths with other surfaces
that may be fixed or moveable. In some embodiments, it is possible
that the load position may be achieved by lowering shield 14
sufficiently so that substrate 8 may pass over the top edge of
shield 14.
[0118] Once substrate 8 has been loaded into process chamber 12,
shield 14 is lowered by linear motor 122 (FIG. 8) for processing.
The low conductance process shield position 210 shown in FIG. 15,
shows the positions of shield 14 and shadow ring 28 at the moment
that shadow ring 28 contacts process kit 182. An angled shadow ring
seat 216 of shadow ring 28 rests on process kit bevel 202 of
process kit 182. This is the only point of contact between shadow
ring 28 and process kit 182. Air gaps separate shadow ring 28 and
process kit 182 away from each edge of process kit bevel 202. The
airgaps between shadow ring 28 and process kit 182 allow for
differential thermal expansion of shadow ring 28 and process kit
182 during processing. The angle of process kit bevel 202 helps
center shadow ring 28, through interaction with the angle of shadow
ring seat 216, so that the edge of substrate 8 is shadowed
uniformly by a shadow ring edge 218 of shadow ring 28.
[0119] Lowering shield 14 into process position creates shield
conductance upper path 22 and shield conductance lower path 24, as
described with respect to FIG. 13 above. While it is possible to
reduce the conductance of lower path 24 to zero (FIG. 15), during
deposition upper path 22 generally forms a low conductance leakage
path, while lower path 24 generally forms a higher conductance
leakage path (FIG. 16).
[0120] By changing the relative position of shield 14 to shadow
ring 28, the conductance out of chamber 12 can be modulated. This
modulation, in turn, alters the pressure of chamber 12. The high
conductance process shield position 212 shown in FIG. 16, shows the
positions of shield 14 and shadow ring 28 at an intermediate step
of an ALD process. Lower path 24 includes several distinct regions:
a plurality (three in this embodiment) of fixed conductance regions
220 (fixed gaps between shadow ring hook 200 and shadow ring 28)
interspersed with a plurality (two in this embodiment) of variable
conductance regions 222 (variable gaps). The volumes of fixed
conductance regions 220 and variable conductance regions 222 can be
precisely controlled (by precise positioning of shield 14 by linear
motor 122) to adjust the conductance of lower path 24, and
therefore the pressure of chamber 12, as needed during the
process.
[0121] In purge shield position 214 of FIG. 17, shield support legs
16 are lowered by linear motor 122 (FIG. 8). Shield 14 and shadow
ring hook 200 are lowered into lower portion 206 of annular pumping
channel 20. Shadow ring 28 remains seated on process kit 182. Both
shield conductance upper path 22 and shield conductance lower path
24 become high conductance paths. Purge shield position 214 allows
quick evacuation of the gases in process chamber 12 into annular
pumping channel 20 due to the high conductances created and the
lower pressure of annular pumping channel 20 compared to chamber
12.
[0122] As mentioned above, linear motor 122 (FIG. 8) provides
actuation of shield 14. This allows quick and accurate variation of
the conductance of shield conductance upper and lower paths 22 and
24. This translates into quick and accurate variation of the
pressure in process chamber 12 for given gas flows into process
chamber 12.
[0123] In some embodiments, a throttle valve (i.e., a butterfly
valve, a variable position gate valve, a pendulum valve, etc.)
positioned at pump throat 134 (FIG. 13) can also be used in
conjunction with moveable shield 14 to effect quick pressure
changes in process chamber 12 by modulating the maximum pumping
speed of pump 132 (FIG. 12). The throttle valve augments the
pressure range achievable in process chamber 12, providing a
"coarse adjustment" of the pressure in process chamber 12, while
shield 14 provides a "fine adjustment" of the pressure.
[0124] Showerhead and Shield Design for Gas Introduction and
Temperature Control
[0125] The novel hardware for ALD reactor 100 (FIG. 11) supports
the introduction of gases into process chamber 12 through multiple
points. The primary introduction point is through the top of
reactor 100, in particular, through showerhead three-way valve 148
(mounted on chamber lid 10) and showerhead 172 (best seen in FIG.
13). Gases may also be introduced into chamber 12 through shield
14, which may be additionally configured for temperature
control.
[0126] FIG. 18 is a schematic diagram of a novel valve system 224
for gas delivery in ALD reactor 100 of FIG. 8. This embodiment
delivers a single precursor and a purge gas to process chamber 12,
either separately or in a mixed proportion. The purge gas is used
to purge the chamber and as the gas source to strike a plasma. A
carrier gas for the precursor flows from a first gas source 226,
and the purge gas flows from a second gas source 228.
[0127] When either the carrier gas or the purge gas is not flowing
to chamber 12, it is diverted by a first three-way valve 230 and a
purge three-way valve 232, respectively, through a pump bypass gas
line 234 to a vacuum pump 236. Utilization of vacuum pump 236
allows the carrier and purge gases to flow in steady state
conditions even when they are not flowing to chamber 12. This
avoids disturbances in the gas flows caused by the long settling
times of gas sources that are switched on and off.
[0128] A showerhead three-way valve 148 controls access to a
chamber gas line 238, which leads to process chamber 12. Three-way
valve 148, located centrally on chamber lid 10 as seen in FIG. 11,
provides at least two distinct advantages. First, gases introduced
to chamber 12 can be switched rapidly with minimal loss or delay.
Second, gases are isolated from each other outside of chamber 12,
resulting in no cross-contamination of reactants.
[0129] A first on/off valve 240 is coupled between first ends of a
second on/off valve 242 and a third on/off valve 244. The opposite
ends of second and third on/off valves 242 and 244 are each coupled
to a first precursor source 246. First on/off valve 240 is also
coupled between first three-way valve 230 and showerhead three-way
valve 148 via a gas line 248 and a gas line 250, respectively.
Precursor source 246 can be isolated by closing on/off valves 242
and 244. This may be done, for example, to change precursor source
246. In this case, on/off valve 240 may be closed, or opened to
allow carrier gas to flow through three-way valves 230 and 148 into
chamber 12. During deposition, first on/off valve 240 is normally
closed, and second and third on/off valves 242 and 244 are normally
open.
[0130] Three-way valves 230, 232, and 148 are switched
synchronously to deliver either precursor or purge gas to chamber
12. When delivering precursor, purge three-way valve 232 is
switched to flow the purge gas to vacuum pump 236, and showerhead
three-way valve 148 is switched to the precursor side.
Simultaneously, three-way valve 230 is switched to allow carrier
gas to flow from first gas source 226 through gas line 248 and
on/off valve 242 into precursor source 246. The carrier gas picks
up precursor in precursor source 246, typically by bubbling through
a liquid source. The carrier gas, now including precursor, flows
through on/off valve 244, through gas line 250, through showerhead
three-way valve 148, through chamber gas line 238, and into chamber
12.
[0131] When delivering purge gas, first three-way valve 230 is
switched to flow the carrier gas to vacuum pump 236. Purge
three-way valve 232 and showerhead three-way valve 148 are switched
to allow purge gas to flow from second gas source 228 through a gas
line 252 and chamber gas line 238 into chamber 12.
[0132] Valve system 224 keeps gas line 248 charged with carrier
gas, gas line 250 charged with carrier plus precursor, and gas line
252 charged with purge gas. This allows fast switching between gas
sources by significantly reducing the gas delivery time to chamber
12. Valve system 224 also minimizes waste of gases since gas lines
do not need to be flushed between deposition steps. Furthermore,
any gas bursts from transient pressure spikes upon gas switching,
due to the charged gas lines, would only help the initial stages of
chemisorption or surface reaction.
[0133] Practitioners will appreciate that alternative embodiments
of valve systems for gas delivery to reactor 100 are possible. In
the embodiment shown in FIG. 18, two separate gas sources are shown
providing the carrier gas and the purge gas, which may be different
gases. It is possible, however, that in some embodiments the same
gas used as the purge gas may be used as the carrier gas for the
precursor. In this case, separate gas sources may be used as shown
in FIG. 18, or first gas source 226 may be used singly in a valve
system 254, which has many similar components to valve system 224
of FIG. 18, as shown schematically in FIG. 19. Valve system 254 can
be simplified by replacing three-way valve 230 with a T-junction
256 as shown schematically in FIG. 20 for a valve system 258, which
has many similar components to valve system 224 of FIG. 18. As in
valve system 224 of FIG. 18, showerhead three-way valves 148 in
valve system 254 (FIG. 19) and valve system 258 (FIG. 20) control
the flow of purge gas or carrier-plus-precursor gas to chamber 12.
As shown in valve system 254 (FIG. 19) and valve system 258 (FIG.
20), pump 236 may not be used in some embodiments.
[0134] In some embodiments, gas delivery of multiple precursors may
be desirable. Two embodiments of multiple precursor delivery are
shown in the schematic diagrams of a valve system 260 in FIG. 21
and a valve system 262 in FIG. 22. Valve systems 260 (FIG. 21) and
262 (FIG. 22) each have many similar components to valve system 224
of FIG. 18. Valve systems 260 (FIG. 21) and 262 (FIG. 22) are shown
configured for two precursor sources, but may be further adapted
for additional precursor sources. In each of valve systems 260
(FIG. 21) and 262 (FIG. 22), a second three-way valve 264 controls
the flow of carrier gas to a second precursor source 266. A fourth
on/off valve 268, a fifth on/off valve 270, and a sixth on/off
valve 272 are coupled similarly to, and operate similarly to,
valves 240, 242, and 244, respectively, to control the flow of
carrier gas through second precursor source 266. A gas line 274,
similar to gas line 248, is coupled between three-way valve 264 and
on/off valve 270.
[0135] In FIG. 21, valve system 260 further includes a third gas
source 276 in addition to first and second gas sources 226 and 228
of valve system 224 of FIG. 18. A third three-way valve 278,
coupled to on/off valve 272 via a gas line 280, controls delivery
of the second precursor to showerhead three-way valve 148 via a gas
line 282. A fourth three-way valve 284 controls delivery of the
purge gas via gas line 252 and a gas line 286 to three-way valve
278, which directs the purge gas to showerhead three-way valve 148
as needed via gas line 282.
[0136] In FIG. 22, valve system 262 is shown configured to use gas
source 226 for both the purge and carrier gases. The carrier gas is
delivered from gas source 226 to three-way valve 264 via a gas line
288. The purge gas is delivered to the second terminal of a third
three-way valve 278 (and similar valves of any additional precursor
sources) via gas line 252. The third terminal of three-way valve
278 is coupled to the second terminal of showerhead three-way valve
148 via gas line 282. Three-way valve 278 thus controls delivery of
the second precursor and the purge gas to showerhead three-way
valve 148.
[0137] Other modifications may be made for alternative embodiments
of the valve systems of FIGS. 18, 19, 20, 21, and 22. The functions
of showerhead three-way valve 148 may be accomplished instead with
an equivalent network of on/off valves (similar to valves 240, 242,
and 244) and fittings. Metering valves may be added to branches to
regulate the flow for specific branches. Pressure sensors may be
added to branches and coupled with the valve actuation to introduce
known amounts of reactant. Valve timing may be manipulated to
deliver "charged" volumes of gas to process chamber 12. The
traditional valves may be replaced with advanced designs such as
micro-electromechanical (MEM) based valves or valve networks. The
entire valve system can be heated to prevent condensation of
reactants in the network.
[0138] FIG. 23 is a perspective cross-section of two embodiments of
a showerhead 172 for gas distribution. Showerhead 172 is designed
to have a larger diameter, and thus a larger area, than substrate 8
and ESC 6 (FIG. 13). Showerhead 172 includes a plurality of
mounting holes 290 used to facilitate attachment of showerhead 172
to chamber lid 10 with a plurality of fasteners (see FIG. 13).
Showerhead 172 also includes a plurality of pressure sensor
orifices 166, one for each pressure sensor used to sense the
pressure in process chamber 12. For example, fast chamber pressure
sensor 142 and precision chamber pressure sensor 144 (FIG. 8) would
each require a pressure sensor orifice 166 in showerhead 172.
Showerhead 172 also includes showerhead lip 174 peripherally around
the edge of showerhead 172 used to prevent shadow ring 28 from
hitting showerhead 172.
[0139] Showerhead 172 also includes a cavity 292 centrally located
in an upper surface of showerhead 172 as shown in FIG. 23 (a).
Cavity 292 forms plenum 170 (FIG. 13) upon attachment of showerhead
172 to chamber lid 10. A plurality of showerhead gas orifices 176
are arranged within cavity 292 in a pattern designed for a
particular gas flow distribution. The diameter of cavity 292 is
designed to be larger than the diameter of substrate 8 (FIG. 13).
In the embodiment of FIG. 23 (b), showerhead 172 includes a cavity
294 that is similar to cavity 292 of FIG. 23 (a), but cavity 294
has a diameter designed to be smaller than the diameter of
substrate 8. Practitioners will appreciate that a number of
different diff-using devices may be used to tailor the
directionality of the gas flows as needed.
[0140] As mentioned above, gas may also be introduced into process
chamber 12 through shield 14. This allows cylindrical gas
introduction around the volume of process chamber 12 as discussed
above with reference to FIG. 4. FIG. 24 is a perspective
cross-section of an embodiment of a shield assembly 296, including
a shield gas channel 40, for ALD reactor 100 of FIG. 8. A plurality
of shield support legs 16 attach to shield cap 196, which is
attached to the base of shield 14. Most of shield support legs 16
are solid. Gas is introduced into shield 14, through at least one
hollow shield support leg 298, which extends through shield cap 196
into shield gas channel 40 in shield 14.
[0141] Shield gas channel 40 is annular and runs completely around
the base of shield 14. Shield gas channel 40 is a high conductance
channel that allows introduced gas to distribute evenly around
shield gas channel 40 of shield 14 before introduction into process
chamber 12 (FIG. 13). Gas is introduced to chamber 12 through a
plurality of gas flow orifices 300, which are evenly spaced along
shield gas channel 40 and extend through an inner wall of shield 14
into process chamber 12. The gas introduction path of shield
assembly 296 is designed to ensure uniform gas flow around
substrate 8 as discussed with reference to FIG. 4.
[0142] Introduction of gas through shield 14 allows tremendous
flexibility in designing ALD processes. In some embodiments, the
same gas introduced through showerhead 172 can be simultaneously
introduced through shield 14 to provide improved coverage in
process chamber 12 and on substrate 8 (FIG. 13). Alternatively, in
some embodiments, one gas can be introduced through showerhead 172
while a different gas is introduced through shield 14, allowing
improved gas isolation and quicker cycling of the gases.
[0143] Movement of shield 14, either before or during the gas flow,
allows gas to be introduced at different planes within process
chamber 12, parallel to the plane of substrate 8. The shield motion
can be used to optimize the gas flow distribution of a particular
ALD process.
[0144] As discussed previously, another role of shield 14 is to
confine plasma 194 during processing (FIG. 13), which can result in
heating of shield 14. To maintain the shield at an acceptable
process temperature, a cooling/heating channel can be incorporated
in the shield design. This also helps prevent deposition on shield
14.
[0145] FIG. 25 is a perspective cross-section of an embodiment of a
shield assembly 302, including a shield cooling/heating channel
304, for ALD reactor 100 of FIG. 8. Shield assembly 302 includes
some shield support legs 16, which are solid, attached to shield
cap 196 at the base of shield 14. Similar to shield assembly 296 of
FIG. 24, which includes gas channel 40, a cooling or heating fluid
flows up into shield 14 through at least one hollow shield support
leg 306, which extends through shield cap 196 into cooling/heating
channel 304 in shield 14. Shield cooling/heating channel 304 is
annular and runs about two-thirds of the way around the base of
shield 14. The cooling or heating fluid flows down, out of shield
14, through at least one other hollow shield support leg (not
shown), which is similar to hollow shield support leg 306.
[0146] Cooling or heating of shield 14 using a fluid flowing in
cooling/heating channel 304 also allows improved control of the
temperature of gases introduced into process chamber 12 through
shield 14. FIG. 26 is a perspective cross-section of an embodiment
of a shield assembly 308, including both shield gas channel 40 and
shield cooling/heating channel 304, for ALD reactor 100 of FIG. 8.
In the embodiment shown in FIG. 26, gas channel 40 is located above
cooling/heating channel 304. Hollow shield support leg 306 extends
through shield cap 196 into cooling/heating channel 304 to allow
fluid flow. Hollow shield support leg 298 extends through shield
cap 196 and cooling/heating channel 304 into gas channel 40 to
allow gas introduction from shield 14 into process chamber 12 via
gas flow orifices 300.
[0147] Practitioners will appreciate that shield assembly 308 could
include alternative arrangements of gas channel 40 and
cooling/heating channel 304, including multiple gas channels 40
and/or multiple cooling/heating channels 304.
[0148] Design of particular shield assembly embodiments is
extremely flexible, and reactor 100 is designed to facilitate
removal, replacement, and use of various shield assemblies. This
allows the easy introduction of a shield assembly that might
include gas delivery and cooling/heating (i.e., shield assembly
308), or only one of these (i.e., shield assemblies 296 or 302), or
neither gas delivery nor cooling/heating, depending on the
requirements of the customer and the process.
[0149] Electrostatic Chuck Assembly Design
[0150] ALD processes in the disclosed embodiments are ion-induced
(see, for example, application Ser. No. 09/812,352, application
Ser. No. 09/812,486, and application Ser. No. 09/812,285,
referenced above), rather than thermally induced, through use of
plasma 194 generated in process chamber 12 (FIG. 11 and FIG. 13).
This allows deposition at lower temperatures than in conventional
ALD systems, allowing replacement of conventional heated susceptors
with an electrostatic chuck (ESC) assembly 106 to retain substrate
8. ESC assembly 106 may be further designed for improved
temperature control and improved radio frequency (RF) power
coupling.
[0151] FIG. 27A is a cutaway perspective view of an embodiment of
an electrostatic chuck assembly 106 for ALD reactor 100 of FIG. 8.
ESC assembly 106 includes in part, an electrostatic chuck (ESC) 6,
a cooling plate 110, and a baseplate 112. Cooling plate 110 and
baseplate 112 can be shaped as annuli with overlapping central
orifices that together define an access port 310, which provides
access to a central region of the underside of ESC 6.
[0152] Substrate 8 rests on an annular sealing lip 46, peripherally
surrounding a top surface 50 of ESC 6. Annular sealing lip 46 holds
substrate 8 above surface 50 defining a backside gas volume 48
bounded by surface 50, sealing lip 46, and the backside of
substrate 8.
[0153] A backside gas is provided to gas volume 48 through a
backside gas entry 312 to a backside gas valve 154. Gas valve 154
is located on the exterior underside of reactor 100 at the outer
edge of baseplate 112 to provide easy access (FIG. 8 and FIG. 11).
The backside gas flows along a backside gas line 54, which runs
radially inward along a lower surface of baseplate 112. Gas line 54
curves upward through access port 310 and is attached to the center
of the bottom surface of ESC 6 using a backside gas line flange
314. The backside gas flows through a backside gas passageway 56
centrally located in and extending through ESC 6 to gas volume 48.
A backside gas line seal 316 inside flange 314 maintains the
pressure of gas volume 48. The backside gas plays an important role
in the temperature control of substrate 8.
[0154] Electrostatic chucks are usually made of a dielectric
material (e.g., aluminum nitride AIN, or polyimide). ESC 6 may be
designed to have its bulk material effects dominated by the
Johnson-Rahbek (JR) effect rather than a coulombic effect, since
the JR effect provides a stronger, more efficient electrostatic
attraction. A JR ESC typically has a bulk resistivity between
10.sup.8 and 10.sup.12 .OMEGA.-cm, while a coulombic ESC generally
has a bulk resistivity greater than 10.sup.13 .OMEGA.-cm.
[0155] Embedded in the dielectric material of ESC 6, close to top
surface 50, are at least two electrodes. A first electrode 80 and a
second electrode 82 are shaped as concentric annular plates made of
a conductive material, for example, tungsten or molybdenum. First
electrode 80 is biased using a first electrode terminal 318, which
is coupled to first electrode 80 and extends down through ESC 6
into access port 310. Second electrode 82 is biased using a
separate second electrode terminal (not shown). A DC "chucking"
voltage is applied to both first electrode 80 and second electrode
82 to create an electrostatic attraction between substrate 8 and
top surface 50 of ESC 6 to retain substrate 8 during processing.
Simultaneously, RF bias power is coupled to each electrode 80 and
82 as well. The RF bias power provides the power for plasma and
hence ion generation during modulated ion induced atomic layer
deposition.
[0156] In addition to generating a plasma, the RF bias power also
induces a slight negative potential (e.g., a DC offset voltage
typically -10 V to -80 V at.ltoreq.150 W RF power and 0.1-1 Torr
pressure) on substrate 8. The magnitude of the potential should be
.ltoreq.150 V. The induced voltage defines the ion energy of the
positively charged ions in the plasma and attracts the positively
charged ions toward the surface of substrate 8. The positively
charged ions impinge on the wafer, driving the deposition reaction
and improving the density of the deposited film.
[0157] A resistive heater 72 is also embedded in ESC 6. Resistive
heater 72 is shaped as at least one coil or ribbon that winds
throughout ESC 6 in a plane located about midway between electrodes
80 and 82 and the bottom of ESC 6. Heater 72 is controlled via at
least one resistive heater terminal 320 coupled to heater 72.
Terminal 320 extends down through ESC 6 into access port 310. Thus,
ESC 6 is basically a dielectric substrate support with an embedded
heater 72 and embedded electrodes 80 and 82 for DC biasing and RF
power coupling.
[0158] ESC 6 is held in contact with cooling plate 110 using an
annular clamp ring 178, which overlaps a clamp land 322 of a
surrounding flange at the base of ESC 6. An ESC O-ring 324 creates
a vacuum seal between ESC 6 and cooling plate 110. A plurality of
clamp ring fasteners 180, each extending through clamp ring 178
into cooling plate 110, secure the connection between ESC 6 and
cooling plate 110. A process kit 182, having an annular elbow
shape, fully surrounds clamp ring 178 covering a top surface and a
side surface of clamp ring 178. Process kit 182 includes a process
kit bevel 202 used for centering a shadow ring 28 (FIG. 15) on
process kit 182. Process kit 182 may be made of a dielectric
material (e.g., aluminum oxide, aluminum nitride, or hard-anodized
aluminum) to electrically isolate clamp ring fasteners 180 from ESC
6 and substrate 8. Process kit 182 also protects clamp ring 178 and
fasteners 180 from process gases, facilitating cleaning of reactor
100 (FIG. 12).
[0159] Cooling plate 110 can be made (e.g., machined) from a
variety of thermally conductive materials, for example, aluminum or
stainless steel. An upper surface of cooling plate 110 is patterned
to create a plurality of small area contacts 326 and a plurality of
thermal breaks 184. Contacts 326, which have the form of ridges,
contact the bottom surface of ESC 6. Thermal breaks 184 are gaps
between ESC 6 and cooling plate 110, which increase the temperature
difference between ESC 6 and cooling plate 110. The temperature of
cooling plate 110 can be controlled using a fluid (e.g., water)
flowing in a plurality of coolant channels 78. Coolant channels 78
are designed to allow the fluid to flow in a largely circular
manner at various diameters of cooling plate 110.
[0160] A lower surface of cooling plate 110 is attached to an upper
surface of baseplate 112. The upper surface of baseplate 112 forms
the lower walls of coolant channels 78 in cooling plate 110.
Baseplate 112, which may be made of aluminum, provides structural
support for ESC assembly 106. Thermal breaks 184 of cooling plate
110 allow maintenance of a significant temperature difference
between top surface 50 (which may be near 300.degree. C.) of ESC 6
and a bottom surface of baseplate 112 (which is exposed to air and
may be less than 50.degree. C.).
[0161] One of a plurality of lift pins 108, which facilitate
loading and unloading of substrate 8, is shown in retracted process
position, with the tip of lift pin 108 below top surface 50 of ESC
6. Each lift pin 108 extends through a lift pin orifice 328, which
includes a plurality of aligned orifices in baseplate 112, cooling
plate 110, and ESC 6.
[0162] Alternative embodiments of ESC assembly 106 are possible.
For example, in some embodiments, at least one peripheral ring of
holes can be used to introduce the backside gas, rather than just a
centrally located hole, as discussed in more detail below. In
addition, in some embodiments, ESC 6 can be replaced with a
conventional susceptor to facilitate ALD processes at higher
temperatures.
[0163] FIG. 27B illustrates interdigitated electrodes 79 and 83,
and FIG. 27C illustrates D-shaped electrodes 85 and 87, that may be
used instead of the concentric annular plate electrodes 80 and 82
in FIG. 27A. Electrodes 85 and 87 may be solid or have an opening,
such as shown by dashed lines. Practitioners will appreciate that
various other embodiments of the electrodes are possible.
[0164] In one embodiment, the showerhead 172 (FIG. 23) is not
grounded but is coupled to another RF source in a manner similar to
the RF source coupling to the ESC electrodes in FIG. 7. The phase
difference between the RF power applied to showerhead 172 and the
RF power coupled to electrodes 80 and 82 in the ESC controls ion
density and energy, with a difference of 180.degree. creating the
maximum ion density and energy. In another embodiment, the two RF
sources have different frequencies.
[0165] Temperature Control of Electrostatic Chuck Assembly
[0166] Temperature control of ESC assembly 106 (FIG. 27A) is
important for high quality atomic layer deposition. A uniform
temperature across a substrate 8 resting on annular sealing lip 46
of ESC 6 promotes uniform chemisorption of precursors. If the
temperature of substrate 8 is too high, decomposition or desorption
of precursors may occur. If the temperature of substrate 8 is too
low, either or both of the chemisorption and the deposition
reactions will be impeded.
[0167] FIG. 28 is a schematic diagram of a control system 330 for
electrostatic chuck (ESC) assembly 106 (FIG. 27A) of ALD reactor
100 of FIG. 8. Control system 330 may also be applied to various
embodiments of pedestal 4 of ALD reactor 2 of FIG. 1. Control
system 330 is an embodiment of control system 44 of FIG. 6, as
discussed previously.
[0168] Control system 330 is used to establish and maintain a
uniform temperature across substrate 8. As shown in FIG. 28,
substrate 8 rests on an annular sealing lip 46 defining a backside
gas volume 48 between substrate 8 and top surface 50 of ESC 6. A
backside gas (e.g., Ar, He, etc.) is usually chosen from among the
species in chamber 12 to prevent contamination in the deposited
film. The backside gas flows from a backside gas source 52 along a
backside gas line 54, through a backside gas passageway 56 in ESC
6, and into gas volume 48.
[0169] The backside gas improves the thermal contact between
substrate 8 and ESC 6, by providing a medium for thermal energy
transfer between substrate 8 and ESC 6. Heat transfer improves with
increasing backside gas pressure, up to a saturation limit. Ranges
for backside gas pressures are 3-20 torr, and typical ranges are
6-10 torr for good thermal conductivity and temperature uniformity
across the substrate. Using the disclosed embodiments, a
temperature uniformity across the substrate may be
.ltoreq.5.degree. C. Above a backside gas pressure of 5 torr, a
uniformity of .ltoreq.15.degree. C. is typically achieved. A
pressure controller 58 maintains the backside gas at a constant
pressure, thus ensuring constant heat transfer and uniform
substrate temperature. In practice, annular sealing lip 46 may take
the form of several islands scattered across top surface 50 of ESC
6. This introduces a leak rate of the backside gas that must be
taken into account. The amount of direct contact between the chuck
and the substrate can be virtually any amount, such as between
15-50%.
[0170] The temperature of substrate 8 is modulated by heating or
cooling ESC 6. A temperature sensor 60 (e.g., a thermocouple or
optical infrared sensor) is coupled via a sensor connection 62 to a
temperature monitor 64 in a closed loop feedback control circuit
332. A temperature setpoint signal is also provided to monitor 64
via a setpoint electrical connection 334. A temperature controller
66 creates a signal that is amplified through a power amplifier or
modulator 336 and applied via an electrical connection 70 to a
resistive heater terminal 320 (FIG. 27A), which is coupled to a
resistive heater 72 embedded in ESC 6. A coolant temperature and
flow controller 74, as is widely known, controls the fluid from a
coolant supply 76 as it flows in a plurality of coolant channels 78
in pedestal 4 (or in ESC assembly 106 in FIG. 12 and FIG. 13).
[0171] Control system 330 is designed to control the temperature of
substrate 8, by heating and/or cooling, for a wide range of power
and temperature. Temperature control can be accomplished by various
techniques, including regulating the backside gas pressure, heating
ESC 6 directly with resistive heater 72, or regulating the
temperature and/or flow of fluid in coolant channels 78. The
temperature of substrate 8 can thus be periodically or continuously
varied during the deposition process to meet different process
demands. Additional information regarding temperature control in
atomic layer deposition may be found in related U.S. application
Ser. No. 09/854,092, entitled "Method And Apparatus For Improved
Temperature Control In Atomic Layer Deposition," filed May 10,
2001.
[0172] Alternative embodiments of control system 330 of FIG. 28 are
possible. For example, the temperature control system of circuit
332 may have various embodiments. In addition, temperature sensor
60 may have various embodiments. Temperature sensor 60 may be a
thermocouple that measures the temperature of ESC 6. Temperature
sensor 60 may be a pyrometer device that optically measures the
temperature of the backside of substrate 8. Or, temperature sensor
60 could take other equivalent forms.
[0173] In some embodiments of control system 330 of FIG. 28, an
alternative energy source may be included as another option to
control the temperature of substrate 8. FIG. 29 is a schematic
diagram of a control system 338, including an alternative energy
source 340, for pedestal 4 of reactor 2 (FIG. 1) or for ESC
assembly 106 (FIG. 27A) of ALD reactor 100 (FIG. 8). Control system
338 is similar to control system 44 (FIG. 6) and control system 330
(FIG. 28), as discussed previously. Alternative energy source 340
is located outside of pedestal 4 (or ESC assembly 106) near the top
of chamber 12 and may include radiation from lamps, a plasma, or
another source. Alternative energy source 340 could be controlled,
for example, by regulating the power to the lamps or plasma.
Alternative energy source 340 could be used alone, or in
conjunction with one or more of resistive heater 72, the fluid in
coolant channels 78, or the pressure of the backside gas in gas
volume 48.
[0174] In some embodiments, an additional cooling source may be
added to control system 330 of FIG. 28 to improve the cooling
capacity and/or performance. The additional cooling source could be
a refrigeration system, a heat pipe, a refrigerated liquid or gas
coolant system, or other equivalent system.
[0175] In some embodiments of control system 330 of FIG. 28, the
backside gas may be introduced to gas volume 48 through multiple
orifices rather than just a centrally located orifice. FIG. 30 is a
perspective view of an embodiment of a portion 342 of an ESC
assembly 106 (FIG. 27A) for ALD reactor 100 of FIG. 8. ESC 6
includes a central orifice 344 as well as a peripheral ring of
orifices 346 located near the periphery of substrate 8. Various
embodiments of ESC 6 may include either or both of orifice 344 and
orifices 346. Orifices 346 result in improved pressure uniformity
between substrate 8 and ESC 6, which results in improved
temperature uniformity across substrate 8. An additional peripheral
ring of orifices (not shown) can be added outside of orifices 346
to ensure a constant pressure gradient at the edge of substrate 8.
The additional ring of orifices would also serve as an edge purge
to prevent reactive gases from entering gas volume 48 (FIG. 28) and
causing deposition on the backside of substrate 8.
[0176] In some embodiments of control system 330 of FIG. 28,
pressure controller 58 may be replaced by, for example, a flow
regulator such as a metering valve or mass flow controller. In
still other embodiments, an actuation valve can be added between
pressure controller 58 and backside gas volume 48 to isolate
pressure controller 58 and gas source 52 from process chamber 12
during a substrate transfer. This valve may additionally be used to
stop the flow of backside gas to reduce its pressure, allowing the
substrate to "de-chuck" without "popping" (shifting) when
electrodes 80 and 82 in ESC 6 are de-powered. This valve may
additionally be used in conjunction with a pump to more quickly
reduce the backside gas pressure before "de-chucking" substrate
8.
[0177] Practitioners will appreciate that various other embodiments
of control system 330 and its various constituents are
possible.
[0178] Electrical Biasing and Plasma Generation Using Electrostatic
Chuck Assembly
[0179] FIG. 31 is a schematic diagram of a circuit 348 for
electrical biasing of electrostatic chuck (ESC) 6 of ESC assembly
106 (FIG. 27A) of ALD reactor 100 of FIG. 8. Circuit 348 may also
be applied to various embodiments of ESC 6 of pedestal 4 of ALD
reactor 2 of FIG. 1. Circuit 348 is an alternative embodiment to
circuit 84 of FIG. 7, as discussed previously.
[0180] As shown in FIG. 31, ESC 6 includes at least a first
electrode 80 and a second electrode 82. One possible embodiment of
the electrode geometry of first and second electrodes 80 and 82
(shown schematically in FIG. 31) is shown in FIG. 27A, where first
and second electrodes 80 and 82 are shown as concentric annular
plates. A double D (i.e., mirror imaged) configuration or
interdigitated configuration for electrodes 80 and 82 can also be
used, as previously mentioned. In FIG. 31, first and second
electrodes 80 and 82 are each biased with a DC voltage. RF bias
power is also coupled to both electrodes 80 and 82. Embedding
electrodes 80 and 82 in ESC 6 allows improved RF power coupling to
substrate 8 with maximum uniformity and minimal power loss,
compared to applying RF power to cooling plate 110 (or baseplate
112) upon which ESC 6 sits (FIG. 27A). This is because electrodes
80 and 82 in ESC 6 are close to substrate 8, while cooling plate
110 (and baseplate 112) are comparatively far from substrate 8.
[0181] First electrode 80 and second electrode 82 are biased with
different DC potentials to provide the "chucking" action that holds
substrate 8 to ESC 6 prior to plasma ignition and during
deposition. As shown in FIG. 31, first electrode 80 is coupled via
a serial coupling of a first inductor 88 and a first load resistor
350 to one terminal of a DC power supply 86. Second electrode 82 is
coupled via a serial coupling of a second inductor 90 and a second
load resistor 352 to the other terminal of DC power supply 86.
[0182] A third capacitor 354 is coupled between one terminal of
inductor 88 and a ground terminal 94. A fourth capacitor 356 is
coupled between the other terminal of inductor 88 and ground
terminal 94. A fifth capacitor 358 is coupled between one terminal
of inductor 90 and ground terminal 94. A sixth capacitor 360 is
coupled between the other terminal of inductor 90 and ground
terminal 94. Inductor 88 and capacitors 354 and 356 together form
an RF trap circuit 362, which filters RF from the DC bias.
Similarly, inductor 90 and capacitors 358 and 360 together form
another RF trap circuit 362.
[0183] RF power is also supplied to both first electrode 80 and
second electrode 82 using an RF generator 92 with one terminal
coupled to ground terminal 94. A third inductor 364 is coupled
between the other terminal of RF generator 92 and one terminal of a
first variable capacitor 366. The other terminal of variable
capacitor 366 is coupled to one terminal of a first capacitor 96
and to one terminal of a second capacitor 98. The other terminal of
capacitor 96 is coupled to first electrode 80. The other terminal
of capacitor 98 is coupled to second electrode 82. A second
variable capacitor 368 is coupled across the terminals of RF
generator 92, between one terminal of inductor 364 and ground
terminal 94. Inductor 364 and capacitors 366 and 368 together form
an RF impedance matching circuit 370, which minimizes the reflected
power to RF generator 92.
[0184] Circuit 348 of FIG. 31 allows simultaneous application of a
DC "chucking" voltage and of an RF power for plasma generation
during processing. The same RF power is used to create plasma 194
above substrate 8 (FIG. 13) and to generate a negative, induced DC
bias on substrate 8. RF power can be used since the breakdown
voltage required to generate plasma 194 using RF power is far lower
than in the DC case (e.g., 100 V vs. 300-400 V) for a given Paschen
curve of pressure-distance product (P.times.d). In addition, a
stable DC bias can be induced using RF power. Of course, it is
possible to generate plasma 194 using a high DC voltage instead of
RF power, with appropriate modifications to the biasing hardware
(see, for example, the discussion of FIG. 40 below).
[0185] In FIG. 31, coupling RF power to electrodes 80 and 82 allows
a uniform potential to build across substrate 8 while employing low
RF powers, for example, 50 W to 150 W, which is less than the 350 W
to 600 W required in conventional plasma reactors. The frequency of
the RF bias power can be 400 kHz, 13.56 MHz, or higher (e.g., 60
MHz, 200 MHz). The low frequency, however, can lead to a broad ion
energy distribution with high energy tails which may cause
excessive sputtering. The higher frequencies (e.g., 13.56 MHz or
greater) lead to tighter ion energy distributions with lower mean
ion energies, which is favorable for modulated ion-induced ALD
deposition processes. The more uniform ion energy distribution
occurs because the bias polarity switches before ions can impinge
on substrate 8, such that the ions see a time-averaged
potential.
[0186] In conventional plasma reactors, RF power is applied to the
top boundary of the process chamber, usually a showerhead. This
causes sputtering of the top boundary, which is a major source of
impurity incorporation (typically aluminum or nickel) and/or
particulate incorporation in conventionally deposited films. The
sputtering also transfers kinetic energy to the reactor structure,
heating it considerably and requiring active cooling of the reactor
structure.
[0187] In the present embodiments, RF power is applied to
electrodes 80 and 82 (FIG. 31) embedded in ESC 6 of ESC assembly
106 of ALD reactor 100 (FIG. 12), rather than to showerhead 172
(FIG. 13). This minimizes sputtering of showerhead 172 and allows
better control of the bias induced on substrate 8. It also avoids
excessive heating of chamber lid 10, minimizing any cooling
requirements.
[0188] Referring to FIG. 13, showerhead 172 and shield 14 are
grounded so that the higher plasma sheath voltage drop is localized
mostly on substrate 8 where deposition takes place. This is because
the voltage ratio V.sub.hot/V.sub.cold is proportional to the
respective electrode areas according to
(A.sub.cold/A.sub.hot).sup.n, where n is greater than one.
V.sub.hot is the plasma sheath voltage drop at the powered, or
"hot," electrode, that is, ESC 6 of ESC assembly 106. V.sub.cold is
the voltage drop at the non-powered, or "cold," electrode, that is,
showerhead 172 and shield 14. The combined areas of showerhead 172
and shield 14 can be jointly considered as the area of the cold
electrode. This is because the small volume of process chamber 12
results in a showerhead 172 to ESC 6 spacing that is small
(nominally 0.3 to 0.6 inches) so that the powered electrode can
"see" showerhead 172 and shield 14 as a single ground reference.
Taken together, these combined areas are larger than the area of
substrate 8, or the area of the hot electrode. Thus, for this
reactor, A.sub.cold/A.sub.hot>1.
[0189] In addition, by applying RF power to ESC 6 via electrodes 80
and 82 (FIG. 31), a low RF power can be used to simultaneously
generate plasma 194 (FIG. 13) and to keep the energy of the
impinging ions from plasma 194 low and controlled. The ion energy
is given by
E=e.vertline.V.sub.p.vertline.+e.vertline.V.sub.bias.vertline.,
where V.sub.p is the plasma potential and V.sub.bias is the bias
voltage induced on substrate 8. The ion energy should be
.ltoreq.150 eV, and preferably between 10-80 eV, to drive the
deposition reaction. The magnitude Of V.sub.bias should be
.ltoreq.150V, and preferably V.sub.bias should be between -10 and
-80V, to prevent sputtering of the deposited layer. The magnitude
of V.sub.p is typically 10-30V.
[0190] The induced bias voltage is controlled by the applied RF
power. The induced bias voltage increases with increasing RF power
and decreases with decreasing RF power. Increasing the RF power
also generally increases the number of ions generated.
[0191] Controlling the RF power also controls the density of ions
in the plasma. Higher RF powers are required for larger substrate
diameters. The preferred power density is .ltoreq.0.5 W/cm.sup.2,
which equates to approximately .ltoreq.150 W for a 200 mm
substrate. Power densities .gtoreq.3 W/cm.sup.2 (greater than about
1000 W for a 200 mm diameter substrate) may lead to undesired
sputtering of the deposited film.
[0192] Referring to FIG. 13, cooling plate 110 and baseplate 112
are grounded. Therefore, each clamp ring fastener 180 is also
grounded. Process kit 182, which is made of an insulating material,
electrically shields fasteners 180 so that plasma 194 is not
affected by the ground voltage of fasteners 180.
[0193] Plasma 194 can be controlled in a variety of ways. For
example, plasma 194 can be controlled by varying the applied RF
power. In some alternative embodiments of circuits for electrical
biasing of ESC 6 of ALD reactor 100 (FIG. 12 and FIG. 13), a switch
may be included, for example, in RF impedance matching circuit 370
or with RF generator 92 (FIG. 31). FIG. 32 is a schematic diagram
of a circuit 372, including an RF match switch 374 in RF impedance
matching circuit 370, for electrical biasing of ESC 6. FIG. 33 is a
schematic diagram of a circuit 376, including an RF supply switch
378 in an RF power supply 380 (which also includes RF generator
92), for electrical biasing of ESC 6. Circuit 372 (FIG. 32) and
circuit 376 (FIG. 33) are similar to circuit 348 (FIG. 31), except
for switches 374 and 378. Switches 374 and 378 can be opened to
isolate RF generator 92, or switches 374 and 378 can be closed to
apply RF power to electrodes 80 and 82. Switches 374 and 378 enable
a plasma response time in the 100 ms time range.
[0194] Plasma 194 (FIG. 13) can also be controlled by varying gas
pressure while using, for example, circuit 348 of FIG. 31 with an
RF power constantly applied to electrodes 80 and 82. Referring to
FIG. 15, FIG. 16, and FIG. 17, as discussed previously, shield 14
forms a shield conductance upper path 22 with showerhead 172 and
chamber lid 10. Shield 14 also forms a shield conductance lower
path 24 with shadow ring 28. The conductances of upper and lower
paths 22 and 24 are varied by precision movement of shield 14 by
linear motor 122 (FIG. 8).
[0195] The conductances of upper and lower paths 22 and 24 directly
affect the pressure in process chamber 12 and can be used to vary
that pressure. For example, a high pressure (i.e., relative to the
pressure of annular pumping channel 20) can be established in
chamber 12 using a low conductance process shield position 210 as
shown in FIG. 15. High pressure will strike plasma 194 (FIG. 13)
given a favorable ambient in chamber 12. A low pressure can be
established in chamber 12 using a purge shield position 214, as
shown in FIG. 17, to expose chamber 12 to annular pumping channel
20. Low pressure will effectively terminate plasma 194 since not
enough gas phase collisions will occur to sustain plasma 194.
Applying RF power to electrodes 80 and 82 at pressures that will
not strike or sustain plasma 194 will cause 100% reflection of the
output power from RF generator 92 (FIG. 31). Thus, RF generator 92
should be capable of absorbing this power without detrimental
effects.
[0196] Plasma 194 (FIG. 13) can also be controlled by a combination
of varying gas pressure and applied RF power. For example, plasma
194 may be ignited by a high pressure and favorable ambient in
chamber 12. Plasma 194 may be terminated by a switch, such as
switch 374 in circuit 372 of FIG. 32 or switch 378 in circuit 376
of FIG. 33.
[0197] Practitioners will appreciate that various other embodiments
of circuit 348 of FIG. 31 and its various constituents, for
electrical biasing of ESC 6, are possible. For example, multiple RF
sources may be utilized.
[0198] ALD Processes: Background and Novel Processes
[0199] FIG. 34 is a schematic illustration of a conventional ALD
process. In a typical ALD cycle, which usually includes four steps,
each precursor (or reactant) is introduced sequentially into the
chamber, so that no gas phase intermixing occurs. First, a first
gaseous precursor 382 (labeled Ax) is introduced into the
deposition chamber, and a monolayer of the reactant is chemisorbed
(or physisorbed) onto the surface of a substrate 8 forming a
chemisorbed precursor A 384 as shown in FIG. 34 (a). A free ligand
x 386 is created by the chemisorption of precursor Ax 382. Second,
excess gaseous precursor Ax 382 and ligands x 386 are pumped out,
possibly with the aid of an inert purge gas, leaving the monolayer
of chemisorbed precursor A 384 on substrate 8 as shown in FIG. 34
(b).
[0200] Third, a second gaseous precursor 388 (labeled By) is
introduced into the deposition chamber. Precursor By 388 reacts
with chemisorbed precursor A 384 on substrate 8 as shown in FIG.
34(c) in a self-limiting surface reaction. The self-limiting
reaction halts once initially adsorbed precursor A 384 fully reacts
with precursor By 388. Fourth, excess gaseous precursor By 388 and
any reaction by-products are pumped out, again possibly with the
aid of an inert purge gas, leaving behind an AB monolayer 390 of
the desired thin film as shown in FIG. 34(d). A desired film
thickness is obtained by repeating the deposition cycle as
necessary. The film thickness can be controlled to atomic layer
(i.e., angstrom scale) accuracy by simply counting the number of
deposition cycles.
[0201] ALD processes, however, are slower than traditional
deposition techniques such as CVD and PVD. In order to improve
throughput, shorter deposition cycles are desirable. One way to
shorten the deposition cycle is to shorten the durations of the
individual precursor and pump/purge steps. The individual pulse
lengths, however, cannot be arbitrarily decreased. The first
precursor pulse must be long enough to form an adsorbed layer of
the first precursor on the substrate. The second precursor pulse
must be long enough to allow complete reaction between the first
and second precursors. The pump/purge pulses in between the
precursor pulses must be long enough so that gas phase intermixing
of the precursors does not occur. Gas phase intermixing can lead to
gas phase reactions and/or particle formation, each of which can
cause quality and reliability problems in the deposited film.
[0202] FIG. 35 is a schematic illustration of a novel ALD process.
One deposition cycle includes two steps, rather than four, which
improves process throughput and repeatability. In the base process,
a substrate 8 is maintained at a precise temperature that promotes
chemisorption rather than decomposition.
[0203] In the first step, a gaseous precursor 392 is introduced
into the process chamber. Gaseous precursor 392 includes the
desired thin film species (P) bonded with a plurality of ligands
(L). Species P may be a single element (e.g., Ti, W, Ta, Cu) or a
compound (e.g., TiN.sub.x, TaN.sub.x, or WN.sub.x). In the novel
ALD process, a molecule of gaseous precursor 392 interacts with a
surface bond 394 to form a chemisorbed precursor 396 via a chemical
bonding process that may create a plurality of free ligands 398 as
shown in FIG. 35(a). As a result of the first step, a monolayer of
chemisorbed precursor 396 is formed on substrate 8 as shown in FIG.
35(b).
[0204] In the second step, an inert purge gas is introduced into
the process chamber to purge excess gaseous precursor 392. The
purge gas may include, for example, argon (Ar), diatomic hydrogen
(H.sub.2), and other optional species such as helium (He). RF power
is applied (e.g., using a computer synchronized switch) during this
second step to generate a plasma 194 in the process chamber, or the
plasma is struck by an increased gas pressure under constant RF
power. As shown in FIG. 35(c), plasma 194 includes a plurality of
energetic ions 400 (e.g., Ar.sup.+ ions) and a plurality of
reactive atoms 402 (e.g., H atoms). Some of reactive atoms 402 may
actually be ions.
[0205] Ions 400 and atoms 402 impinge on the surface of substrate
8. Energetic ions 400 transfer energy to substrate 8, allowing
reactive atoms 402 to react with chemisorbed precursor 396 and to
strip away unwanted ligands (which form a plurality of volatile
ligands 404) in a self-cleaning process. Reactive atoms 402, in
conjunction with energetic ions 400, may thus be considered to act
as a "second" precursor. When the plasma power is terminated, a
monolayer 406, usually about one atomic layer of the desired
species P, is left on substrate 8 as shown in FIG. 35(d). This
two-step deposition cycle can be repeated as needed until the
desired film thickness is achieved. The film thickness deposited
per cycle depends on the deposited material. Typical film
thicknesses range from 10-150 .ANG..
[0206] Typical precursors for tantalum (Ta) compounds include PDEAT
[pentakis(diethylamido)tantalum], PEMAT
[pentakis(ethylmethylamido)tantal- um], TaBr.sub.5, TaCl.sub.5, and
TBTDET [t-butylimino tris(diethylamino)tantalum]. Typical
precursors for titanium (Ti) compounds include TiCl.sub.4, TDMAT
[tetrakis(dimethylamido)titanium], and TDEAT
[tetrakis(diethylamino)titanium]. Typical precursors for copper
(Cu) compounds include CuCl and Cupraselect.RTM.
[(trimethylvinylsilyl)he- xafluoroacetylacetonato copper I].
Typical precursors for tungsten (W) compounds include W(CO).sub.6
and WF.sub.6. In contrast to conventional ALD processes,
organometallic precursors can be used in novel ALD processes.
[0207] The purge pulse includes gas, or gases, that are inert
(e.g., argon, hydrogen, and/or helium) to prevent gas phase
reactions with gaseous precursor 392. Additionally, the purge pulse
can include the same gas, or gases, needed to form energetic ions
400 (e.g., Ar.sup.+ ions) and reactive atoms 402 (e.g., H atoms).
This minimizes the gas switching necessary for novel ALD processes.
Acting together, reactive atoms 402 react with chemisorbed
precursor 396, while energetic ions 400 provide the energy needed
to drive the surface reaction. Thus, novel ALD processes can occur
at lower temperatures (e.g., T<300.degree. C.) than conventional
ALD processes (e.g., T.about.400-500.degree. C.). This is
especially important for substrates that already include low
thermal stability materials, such as low-k dielectrics.
[0208] Since the activation energy for the surface reaction is
provided by energetic ions 400 created in plasma 194 above
substrate 8, the reaction will not generally occur without the
energy provided by ion bombardment because the process temperature
is kept below the temperature required for thermal activation.
Thus, novel atomic layer deposition processes are ion-induced,
rather than thermally induced. The deposition reaction is
controlled by modulation of the energy of energetic ions 400, by
modulation of the fluxes of energetic ions 400 and reactive atoms
402 impinging on substrate 8, or by modulation of both energy and
fluxes. The energy (e.g., 10 eV to 100 eV) of energetic ions 400
should be high enough to drive the surface reaction, but low enough
to prevent significant sputtering of substrate 8.
[0209] Timing diagrams for (a) a typical prior art ALD process and
(b) a novel ALD process are compared in FIG. 36. FIG. 36(a) shows
that one deposition cycle in a conventional ALD process includes a
first precursor pulse 408, a purge/pump pulse 410, a second
precursor pulse 412, and another purge/pump pulse 410. Each pulse
is followed by a delay 414, which has a duration that is usually
non-zero. Delays 414, during which only pumping occurs and no gases
flow, are additional insurance against gas phase intermixing of
first precursor pulse 408 and second precursor pulse 412. Delays
414 also provide time to switch gases with conventional valve
systems.
[0210] The durations of first and second precursor pulses 408 and
412 may be between 200 ms and 15 sec. The duration of purge/pump
pulses 410 may be 5-15 sec. The durations of delays 414 may be 200
ms to 5 sec. This results in deposition cycles from 11 sec to 75
sec. Thus, a 50 cycle deposition process could take over one
hour.
[0211] FIG. 36(b) shows two deposition cycles in the novel ALD
process. One deposition cycle includes a first precursor pulse 416
and a purge gas pulse 418. Each pulse is followed by a delay 420.
The elapsed time of one deposition cycle is significantly shorter
in accordance with the novel process when compared to conventional
ALD processes, thereby increasing process throughput.
[0212] Process throughput can be further increased if delays 420
have zero length. Zero-length delays can be accomplished using
three-way valves (in particular showerhead three-way valve 148 of
FIG. 8) or a similar configuration of on/off valves and fittings,
which allow fast gas switching. Delays 420 of zero length are
further facilitated in novel ALD processes by effective use of
purge gas pulse 418, which may include a mixture of more than one
gas. For example, the purge gas may include the "second" precursor
source gas(es) (i.e., as shown in FIG. 35(c), reactive atoms 402,
acting in conjunction with energetic ions 400, created during purge
gas pulse 418). Additionally, the carrier gas for the first
precursor (i.e., flowing during first precursor pulse 416) may be
one of the source gases of the "second" precursor.
[0213] Practitioners will appreciate that alternative embodiments
of novel ALD processes are possible. For example, in some
embodiments, multiple precursors for compound thin films might be
employed. In other embodiments, the deposition cycle of FIG. 36(b)
might begin with a purge gas pulse 418, including a plasma, used as
an in-situ clean to remove carbon-containing residues, native
oxides, or other impurities. In these embodiments, reactive atoms
402 (e.g., H atoms in FIG. 35(c)) react with carbon and oxygen to
form volatile species (e.g., CH.sub.x and OH.sub.x species).
Energetic ions 400 (e.g., Ar.sup.+ and/or He.sup.+ ions in FIG.
35(c)) improve dissociation (e.g., of H.sub.2) and add a physical
clean (e.g., via sputtering by Ar.sup.+ ions generated in the
plasma). In still other embodiments, reactive atoms 402 may not be
needed and plasma 194 may not include reactive atoms 402.
[0214] Additional information regarding in-situ cleaning in atomic
layer deposition may be found in related U.S. Provisional
Application Serial No. 60/255,812, entitled "Method For Integrated
In-Situ Cleaning And Subsequent Atomic Layer Deposition Within A
Single Processing Chamber," filed Dec. 15, 2000.
[0215] Alternative Novel ALD Processes
[0216] The novel ALD process described previously may be modified
to further increase performance. Alternative novel ALD processes
may address faster purging of precursors, rapid changes in the
conductance of the process chamber, state-based changes from one
step to the next, self-synchronization of the process steps, and/or
various plasma generation and termination options. Such
alternatives can be used to further decrease the length of a
deposition cycle, thereby increasing throughput.
[0217] For example, in some novel ALD process embodiments, it is
desirable to quickly purge a gaseous precursor 392 from the process
chamber after formation of a monolayer of chemisorbed precursor 396
on substrate 8 (FIG. 35(b)). This can be accomplished using the
in-process tunable conductance achieved by shield 14 (FIG. 13),
which can be moved during the deposition cycle. Referring to FIG.
15, FIG. 16, and FIG. 17, as discussed previously, shield 14 forms
shield conductance upper path 22 with showerhead 172 and chamber
lid 10. Shield 14 also forms shield conductance lower path 24 with
shadow ring 28. The conductances of upper and lower paths 22 and 24
are varied by precision movement of shield 14 by linear motor 122
(FIG. 8).
[0218] It is possible, therefore, to rapidly increase the chamber
conductance by lowering shield 14 after exposing substrate 8 to
gaseous precursor 392. For example, a purge shield position 214 may
be used (FIG. 17). Lowering shield 14 opens up shield conductance
upper and lower paths 22 and 24 to annular pumping channel 20. The
low pressure of pumping channel 20 will hasten removal of excess
gaseous precursor 392, and by-products such as free ligands 398
(FIG. 35(b)), from process chamber 12. Simultaneously, the purge
gas (e.g., Ar, H.sub.2, and/or He) is flowed to assist in purging
excess gaseous precursor 392 and by-products from chamber 12.
Lowering shield 14 also leads to a drop in the pressure in chamber
12 through exposure of chamber 12 to annular pumping channel 20.
Shield 14 can then be moved back up, for example, to a position
similar to shield position 212 of FIG. 16, to decrease the
conductance and raise the pressure in chamber 12 (assuming constant
gas flow) in order to strike plasma 194 (FIG. 35(c)).
[0219] In particular, plasma 194 can be generated while using, for
example, circuit 348 of FIG. 31. Application of RF power may be
synchronized (e.g., by computer control) with the position of
shield 14 (FIGS. 15-17) to generate plasma 194 in chamber 12 (FIG.
13). Alternatively, if RF bias power is constantly applied to
electrodes 80 and 82 using circuit 348 (FIG. 31), high pressure
(i.e., relative to the pressure of annular pumping channel 20) in
process chamber 12 can be used to trigger plasma 194 (FIG. 13). Low
pressure (i.e., near the pressure of annular pumping channel 20)
will effectively terminate plasma 194 since not enough collisions
will occur to sustain plasma 194.
[0220] FIG. 37 shows timing diagrams for an alternative ALD process
embodiment, as discussed above. FIG. 37(a) shows two deposition
cycles including a first precursor pulse 416 followed by a purge
gas pulse 418 with zero length delays after each pulse. FIG. 37(b)
shows the corresponding chamber conductance. Each one of a
plurality of low conductance periods 422 (corresponding to raised
shield positions) is separated from another by one of a plurality
of high conductance periods 424 (corresponding to lowered shield
positions). High conductance periods 424 occur at the beginning and
end of each purge gas pulse 418 to assist in purging chamber 12
(FIG. 13) of resident gases.
[0221] FIG. 37(c) shows the corresponding pressure in chamber 12
(FIG. 13). A low conductance period 422 results in a high pressure
period 426. A high conductance period 424 results in a low pressure
period 428. FIG. 37(c) also shows a plurality of "plasma on"
periods 430 and a plurality of "plasma off" periods 432. Plasma on
periods 430 occur during each high pressure period 426 during purge
gas pulses 418. As discussed, the RF power to generate plasma 194
(FIG. 13) may be synchronized with the shield position.
Alternatively, the plasma can be ignited by high pressure (in the
presence of the purge gas) and terminated by low pressure, while RF
bias power is constantly supplied to electrodes 80 and 82 embedded
in ESC 6 (FIG. 31).
[0222] Conventional ALD hardware and processes rely on the precise
timing of the individual precursor pulses 408 and 412 and
purge/pump pulses 410 (FIG. 36(a)) to decrease the deposition cycle
length and ensure proper process performance. These time-based
processes rely on several assumptions including that steady state
conditions exist, that all ALD reactors behave similarly, and that
all gases and processes are "on time."
[0223] In contrast, some novel ALD process embodiments can use a
state-based approach, rather than a time-based approach, to
synchronize the individual pulses. This can provide
self-synchronization of the individual pulses for improved process
speed, control, and reliability. Instead of introducing a next gas
pulse (with a fixed duration) a predetermined time after the
introduction of the previous fixed duration gas pulse, subsequent
gas pulses can be triggered based upon a change in the pressure
state of process chamber 12 (FIG. 13). This can be accomplished
using a pressure switch mounted in chamber body 18 capable of
sensing changes in the pressure of process chamber 12. The pressure
can be modulated via the in-process tunable conductance, achieved
by a shield 14 that can be moved during the deposition cycle, as
described previously.
[0224] FIG. 38 shows timing diagrams for another alternative
embodiment of a novel ALD process. The ALD process of FIG. 38 is
similar to the ALD process of FIG. 37, but it has an alternate
plasma termination technique. Accordingly, to avoid redundancy, the
discussion focuses on differences in the embodiments.
[0225] In the ALD process of FIG. 38, shield 14 is lowered only
after each precursor pulse 416 to assist in purging excess gaseous
precursor 392 and free ligands 398 from chamber 12 (see also FIG.
17 and FIG. 35(b)). The number of high conductance periods 424 in
FIG. 38(b), corresponding to low pressure periods 428 in FIG.
38(c), is reduced. Thus, a low conductance period 434 in FIG. 38(b)
(corresponding to a high pressure period 436 in FIG. 38(c)) extends
from purge gas pulse 418 into the following precursor pulse 416 in
FIG. 38(a). In this embodiment, the plasma is ignited by, or
synchronized with, the high pressure in chamber 12 (FIG. 13).
Plasma on periods 430 occur during each high pressure period 436
during purge gas pulses 418. Plasma 194 (FIG. 13) is terminated for
subsequent plasma off periods 432 (during precursor pulses 416) by
a means other than pressure change, which may include, for example,
disconnecting the RF power using a switch or setting the RF output
power to zero. A switch could be located, for example, in RF
impedance matching circuit 370 or in RF power supply 380 (FIG. 32
and FIG. 33). Actuation of such a switch would be synchronized with
the deposition steps by, for example, a computer.
[0226] Novel Chemisorption Technique for ALD Processes
[0227] The chemisorption of a gaseous precursor (e.g., precursor
392 in FIG. 35(a)) onto a substrate 8 may be improved by biasing
substrate 8 during first precursor pulse 416 (FIG. 36(b)). As
discussed previously with reference to FIG. 35(a), when a molecule
of gaseous precursor 392 arrives at substrate 8, which is heated, a
weakly bonded ligand will cleave off of the molecule, forming free
ligand 398. This actually leaves the precursor molecule with a net
charge (either positive or negative). An opposite-polarity, low DC
bias (e.g., .vertline.50V.vertline.<.vert-
line.V.sub.bias.vertline.<0V) applied to substrate 8 will
attract the charged precursor molecule to substrate 8 and orient it
so that the desired atom is bonded to substrate 8 to form
chemisorbed precursor 396. The lowest possible bias (e.g.,
.vertline.10V.vertline.<.vertline.V.su- b.bias.vertline.<0V)
that generates a moment on the charged precursor molecule is
desirable to correctly orient the charged precursor molecule with
minimal charging of substrate 8.
[0228] This novel chemisorption technique for ALD processes
promotes uniform and complete (i.e., saturated) chemisorption with
a specified orientation on dielectric and metallic surfaces so that
high quality, reproducible layer-by-layer growth can be achieved
using ALD. The novel chemisorption technique is particularly
effective for the first few precursor monolayers, where, in the
absence of this technique, precursor molecules may chemisorb with a
random orientation. This method is also particularly effective in
the case of organometallic precursors such as those mentioned
previously.
[0229] FIG. 39 is a schematic illustration of the novel
chemisorption technique for ALD processes to deposit thin films,
for example, for copper interconnect technology. Two thin films
used in copper interconnect technology are a barrier/adhesion layer
and a copper seed layer. FIG. 39(a) illustrates chemisorption of
TaN, a typical barrier/adhesion layer material. In the case of a
precursor TBTDET 438, the Bu.sup.t ligand may cleave. A now
negatively charged precursor 440 then orients with a negatively
charged nitrogen 442 (e.g., the N.sup.-1) toward substrate 8, which
is positively biased, for chemisorption. If an NEt.sub.2 ligand is
cleaved instead, then the Ta becomes positively charged and a
negative bias applied to substrate 8 would orient the Ta toward
substrate 8 for chemisorption.
[0230] FIG. 39(b) illustrates chemisorption of Cupraselect.RTM.
(CuhfacTMVS), a typical copper seed layer material. In the case of
a precursor CuhfacTMVS 444, the TMVS ligand is cleaved. A now
positively charged precursor 446 then orients with a positively
charged copper 448 (e.g., the Cu.sup.+1) toward substrate 8, which
is negatively biased, for chemisorption.
[0231] In some embodiments, the novel chemisorption technique may
include an in-situ clean prior to introduction of the first
precursor to promote high quality film deposition. As discussed
above in reference to FIG. 36(b), a purge gas pulse 418 (e.g.,
including Ar, H.sub.2 and/or He) can be used as an in-situ clean to
remove carbon-containing residues, native oxides, or other
impurities (see, for example, application Ser. No. 60/255,812,
referenced above). Removing native oxides from metal layers is
especially important for low resistance and good mechanical
adhesion of the film to substrate 8 (FIG. 39). H atoms can react
with carbon and oxygen to form volatile species (e.g., CH.sub.x and
OH.sub.x species). Ar.sup.+ or He.sup.+ ions improve dissociation
(e.g., of H.sub.2) and add a physical clean (e.g., via sputtering
by Ar.sup.+ ions generated in the plasma). The gas ratios can be
tailored to alter the physical versus chemical components of the
in-situ clean.
[0232] FIG. 40 is a schematic diagram of a circuit 450 for
electrical biasing of ESC 6 of ALD reactor 100 (FIG. 12) for the
novel chemisorption technique described above. The use of ESC 6
helps provide a uniform bias to substrate 8 (FIG. 39). Circuit 450
of FIG. 40 is similar to circuit 372 of FIG. 32 and circuit 376 of
FIG. 33. Accordingly, to avoid redundancy, the discussion will
focus on differences between circuit 450 and circuits 372 and
376.
[0233] In FIG. 40, with the RF power from RF generator 92 decoupled
by opening an RF power switch 452, a first DC power supply 454 and
a second DC power supply 456, which are serially coupled matching
supplies, perform the function of DC power supply 86 in FIGS. 32
and 33 to maintain the potential difference between electrodes 80
and 82. This potential difference provides the "chucking" action
that holds substrate 8 (FIG. 39) to ESC 6. Serially coupled between
the common node (labeled A) of DC power supplies 454 and 456 and a
ground terminal 458 are a current suppression resistor 460, a DC
power switch 462, and a DC reference voltage source 464. Ground
terminal 458 may be the same ground reference as ground terminal
94.
[0234] With DC power switch 462 closed, the reference voltage of
electrodes 80 and 82 (and therefore of substrate 8 during
chemisorption as shown in FIG. 39) is established by DC reference
voltage source 464. Current suppression resistor 460 limits the
current from DC reference voltage source 464. DC reference voltage
source 464 is capable of providing a positive or negative voltage,
as needed for biasing substrate 8 (FIG. 39). The voltage level
provided by DC reference voltage source 464 may additionally reduce
the time required to chemisorb a complete monolayer. This may allow
a reduction in the duration of first precursor pulse 416 (FIG.
36(b)) and/or a reduction in the precursor partial pressure during
first precursor pulse 416.
[0235] Once chemisorption is complete, DC power switch 462 is
opened to isolate voltage source 464 and to electrically float
first and second DC power supplies 454 and 456. RF power switch 452
is closed to reconnect RF generator 92. The remainder of the ALD
process continues as described previously.
[0236] In some embodiments of ALD processes, it is possible to use
a circuit similar to circuit 450 of FIG. 40 to generate plasma 194
above substrate 8 (FIG. 13) by biasing ESC 6 using a high DC
voltage (e.g., 500 V or higher). In this case, RF generator 92, RF
impedance matching circuit 370, and capacitors 96 and 98 would not
be used. DC reference voltage source 464 would supply at least two
distinct voltages, or switch 462 would alternate between two
distinct voltage sources. The first voltage would be a low DC
voltage coupled to electrodes 80 and 82 during plasma off periods
432 (FIG. 37). The low DC voltage might be zero volts, or a
non-zero low voltage used to orient precursor molecules for
improved chemisorption as discussed above. The second voltage would
be a high DC voltage coupled to electrodes 80 and 82 during plasma
on periods 430 (FIG. 37) to generate plasma 194.
[0237] The novel ALD reactor is particularly suitable for thin film
deposition, such as barrier layer and seed layer deposition, but
the teachings herein can be applied to many other types of reactors
and many other types of thin films (e.g., low-k dielectrics, gate
dielectrics, optical films, etc.). The foregoing embodiments of the
ALD reactor, and all its constituent parts, as well as the ALD
processes disclosed herein are intended to be illustrative and not
limiting of the broad principles of this invention. Many additional
embodiments will be apparent to persons skilled in the art. The
present invention includes all that fits within the literal and
equitable scope of the appended claims.
* * * * *