U.S. patent application number 15/615691 was filed with the patent office on 2018-12-20 for control of directionality in atomic layer etching.
The applicant listed for this patent is Lam Research Corporation. Invention is credited to Andreas Fischer, Richard Janek, Thorsten Lill.
Application Number | 20180366343 15/615691 |
Document ID | / |
Family ID | 63246986 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180366343 |
Kind Code |
A9 |
Fischer; Andreas ; et
al. |
December 20, 2018 |
CONTROL OF DIRECTIONALITY IN ATOMIC LAYER ETCHING
Abstract
A method for performing atomic layer etching (ALE) on a
substrate is provided, including the following operations:
performing a surface modification operation on a substrate surface,
the surface modification operation configured to convert at least
one monolayer of the substrate surface to a modified layer, wherein
a bias voltage is applied during the surface modification
operation, the bias voltage configured to control a depth of the
substrate surface that is converted by the surface modification
operation; performing a removal operation on the substrate surface,
the removal operation configured to remove at least a portion of
the modified layer from the substrate surface, wherein removing the
portion of the modified layer is effected via a ligand exchange
reaction that is configured to volatilize the portion of the
modified layer. A plasma treatment can be performed to remove
residues from the substrate surface following the removal
operation.
Inventors: |
Fischer; Andreas; (Castro
Valley, CA) ; Lill; Thorsten; (Santa Clara, CA)
; Janek; Richard; (Oakland, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lam Research Corporation |
Fremont |
CA |
US |
|
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20180247832 A1 |
August 30, 2018 |
|
|
Family ID: |
63246986 |
Appl. No.: |
15/615691 |
Filed: |
June 6, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15423486 |
Feb 2, 2017 |
|
|
|
15615691 |
|
|
|
|
62464360 |
Feb 27, 2017 |
|
|
|
62291392 |
Feb 4, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 21/32138 20130101;
H01L 21/326 20130101; H01L 21/225 20130101; H01L 21/0206 20130101;
H01L 21/67 20130101; H01L 21/32136 20130101; H01L 21/02068
20130101; H01J 37/32862 20130101; H01L 21/31122 20130101; H01J
37/32174 20130101; H01J 37/32357 20130101 |
International
Class: |
H01L 21/3213 20060101
H01L021/3213; H01L 21/225 20060101 H01L021/225; H01L 21/326
20060101 H01L021/326; H01L 21/02 20060101 H01L021/02; H01L 21/311
20060101 H01L021/311 |
Claims
1. A method for performing atomic layer etching (ALE) on a
substrate, comprising: performing a surface modification operation
on a substrate surface, the surface modification operation
configured to convert at least one monolayer of the substrate
surface to a modified layer, wherein a bias voltage is applied
during the surface modification operation, the bias voltage
configured to control a depth of the substrate surface that is
converted by the surface modification operation; performing a
removal operation on the substrate surface, the removal operation
configured to remove at least a portion of the modified layer from
the substrate surface, wherein removing the portion of the modified
layer is effected via a ligand exchange reaction that is configured
to volatilize the portion of the modified layer.
2. The method of claim 1, wherein the surface modification
operation is configured to diffuse ions into the substrate surface
to the depth as controlled by the bias voltage.
3. The method of claim 1, wherein the bias voltage is configured to
have a magnitude and a time duration during the surface
modification operation to achieve the depth of the substrate
surface that is converted by the surface modification
operation.
4. The method of claim 1, wherein the depth is defined by one or
more monolayers of the substrate.
5. The method of claim 1, wherein the bias voltage is configured to
shift the surface modification operation from being primarily
isotropic to being primarily anisotropic, depending on a magnitude
of the bias voltage.
6. The method of claim 1, wherein the bias voltage is applied
during part of the surface modification operation, the part during
which the bias voltage is applied to increase an amount of the
depth in a vertical direction that increases anisotropy of the ALE,
and a portion during which the bias voltage is not applied to
increase the depth in a non-vertical direction that increases
isotropy of the ALE.
7. The method of claim 1, further comprising: performing, following
the removal operation, a plasma treatment on the substrate surface,
the plasma treatment configured to remove residues generated by the
removal operation and/or the surface modification operation from
the substrate surface, wherein the residues are volatilized by the
plasma treatment.
8. The method of claim 7, wherein the removal operation is
configured to remove less than an entire portion of the modified
layer from the substrate surface; and, the method further
comprising: repeating the removal operation and the plasma
treatment until the entire portion of the modified layer is removed
from the substrate surface.
9. The method of claim 8, further comprising: repeating the surface
modification operation, the removal operation and the plasma
treatment until a predefined thickness has been etched from the
substrate surface.
10. The method of claim 1, wherein the bias voltage is in the range
of approximately 20 to 100 V.
11. The method of claim 1, wherein performing the surface
modification operation includes exposing the substrate surface to a
fluorine-containing plasma, wherein the exposure to the
fluorine-containing plasma is configured to convert the at least
one monolayer of the substrate surface to a fluoride species.
12. The method of claim 11, wherein the substrate surface includes
a metal, metal oxide, metal nitride, metal phosphide, metal
sulfide, metal arsenide, or metal compound; wherein the exposure to
the fluorine-containing plasma forms a metal fluoride.
13. The method of claim 11, wherein exposing the substrate surface
to the fluorine-containing plasma includes introducing a
fluorine-containing gas into a chamber in which the substrate is
disposed, and igniting a plasma.
14. The method of claim 13, wherein the exposure to the
fluorine-containing plasma is performed at a chamber pressure of
about 10 to 500 mTorr, for a duration of less than about 15
seconds.
15. The method of claim 11, wherein performing the removal
operation includes exposing the substrate surface to tin-(II)
acetylacetonate (Sn(acac).sub.2) vapor, the exposure to the
Sn(acac).sub.2 vapor configured to exchange acetylacetonate (acac)
ligands for fluorine atoms in the modified layer.
16. The method of claim 15, wherein exposing the substrate surface
to the Sn(acac).sub.2 includes introducing the Sn(acac).sub.2 as a
vapor into a chamber in which the substrate is disposed.
17. The method of claim 16, wherein the exposure to the
Sn(acac).sub.2 is performed for a duration of about 1 to 30
seconds.
18. The method of claim 7, wherein performing the plasma treatment
includes exposing the substrate surface to a hydrogen plasma, the
exposure to the hydrogen plasma configured to volatilize tin, tin
fluoride or tin oxide residues on the substrate surface.
19. The method of claim 18, wherein exposing the substrate surface
to the hydrogen plasma includes introducing a hydrogen gas into a
chamber in which the substrate is disposed, and igniting a
plasma.
20. The method of claim 19, wherein the exposure to the hydrogen
plasma is performed for a duration of about 1 to 30 seconds.
21. The method of claim 1, wherein the surface modification
operation is performed in a first chamber; wherein the removal
operation is performed in a second chamber.
22. A method for performing atomic layer etching (ALE) on a
substrate, comprising: performing a surface modification operation
on a substrate surface, the surface modification operation
including exposing the substrate surface to a first plasma that
converts at least one monolayer of the substrate surface to a
modified layer, wherein a bias voltage is applied during the
surface modification operation, the bias voltage being configured
to control a depth of the substrate surface that is converted by
the surface modification operation, wherein the bias voltage is
configured to accelerate ions from the first plasma towards the
substrate surface without substantially etching the substrate
surface; performing a removal operation on the substrate surface,
the removal operation including removing at least a portion of the
modified layer from the substrate surface, wherein removing the
portion of the modified layer is effected via a ligand exchange
reaction that is configured to volatilize the portion of the
modified layer; performing a clean operation on the substrate
surface, the clean operation including removing residues generated
by the removal operation from the substrate surface, the clean
operation further including exposing the substrate surface to a
second plasma, wherein the residues are volatilized by the exposure
to the second plasma.
Description
CLAIM OF PRIORITY
[0001] This application claims priority to U.S. Provisional
Application No. 62/464,360, filed Feb. 27, 2017, entitled "CONTROL
OF DIRECTIONALITY IN ATOMIC LAYER ETCHING," the disclosure of which
is incorporated by reference.
FIELD OF THE INVENTION
[0002] Implementations of the present disclosure relate to atomic
layer etching (ALE), and more specifically to control of
directionality in atomic layer etching.
DESCRIPTION OF THE RELATED ART
[0003] Conventional techniques of etching material on semiconductor
substrates with fine-tuned control over the uniformity and etch
rate are limited. For example, reactive ion etch is conventionally
used to etch materials on a semiconductor substrate during
semiconductor processing and etch rates of materials etched using
reactive ion etch are controlled by modulating radio frequency
plasma power and chemistry selection. Typically, a wafer plasma
sheath forms at the top of the substrate, and thus ions from the
plasma are typically accelerated onto the wafer surface to etch the
substrate. However, as technology nodes progress to atomic-scale
devices, control of etch processes with atomic-scale fidelity will
be required.
SUMMARY
[0004] In accordance with some implementations, a method for
performing atomic layer etching (ALE) on a substrate is provided,
including the following operations: performing a surface
modification operation on a substrate surface, the surface
modification operation configured to convert at least one monolayer
of the substrate surface to a modified layer, wherein a bias
voltage is applied during the surface modification operation, the
bias voltage configured to control a depth of the substrate surface
that is converted by the surface modification operation; performing
a removal operation on the substrate surface, the removal operation
configured to remove at least a portion of the modified layer from
the substrate surface, wherein removing the portion of the modified
layer is effected via a ligand exchange reaction that is configured
to volatilize the portion of the modified layer.
[0005] In some implementations, the surface modification operation
is configured to diffuse ions into the substrate surface to the
depth as controlled by the bias voltage.
[0006] In some implementations, the bias voltage is configured to
have a magnitude and a time duration during the surface
modification operation to achieve the depth of the substrate
surface that is converted by the surface modification
operation.
[0007] In some implementations, the depth is defined by one or more
monolayers of the substrate.
[0008] In some implementations, the bias voltage is configured to
shift the surface modification operation from being primarily
isotropic to being primarily anisotropic, depending on a magnitude
of the bias voltage.
[0009] In some implementations, the bias voltage is applied during
part of the surface modification operation, the part during which
the bias voltage is applied to increase an amount of the depth in a
vertical direction that increases anisotropy of the ALE, and a
portion during which the bias voltage is not applied to increase
the depth in a non-vertical direction that increases isotropy of
the ALE.
[0010] In some implementations, the method further includes:
performing, following the removal operation, a plasma treatment on
the substrate surface, the plasma treatment configured to remove
residues generated by the removal operation and/or the surface
modification operation from the substrate surface, wherein the
residues are volatilized by the plasma treatment.
[0011] In some implementations, the removal operation is configured
to remove less than an entire portion of the modified layer from
the substrate surface; and, the method further comprising:
repeating the removal operation and the plasma treatment until the
entire portion of the modified layer is removed from the substrate
surface.
[0012] In some implementations, the method further includes:
repeating the surface modification operation, the removal operation
and the plasma treatment until a predefined thickness has been
etched from the substrate surface.
[0013] In some implementations, the bias voltage is in the range of
approximately 20 to 100 V.
[0014] In some implementations, performing the surface modification
operation includes exposing the substrate surface to a
fluorine-containing plasma, wherein the exposure to the
fluorine-containing plasma is configured to convert the at least
one monolayer of the substrate surface to a fluoride species.
[0015] In some implementations, the substrate surface includes a
metal, metal oxide, metal nitride, metal phosphide, metal sulfide,
metal arsenide, or metal compound; wherein the exposure to the
fluorine-containing plasma forms a metal fluoride.
[0016] In some implementations, exposing the substrate surface to
the fluorine-containing plasma includes introducing a
fluorine-containing gas into a chamber in which the substrate is
disposed, and igniting a plasma.
[0017] In some implementations, the exposure to the
fluorine-containing plasma is performed at a chamber pressure of
about 10 to 500 mTorr, for a duration of less than about 15
seconds.
[0018] In some implementations, performing the removal operation
includes exposing the substrate surface to tin-(II) acetylacetonate
(Sn(acac).sub.2) vapor, the exposure to the Sn(acac).sub.2 vapor
configured to exchange acetylacetonate (acac) ligands for fluorine
atoms in the modified layer.
[0019] In some implementations, exposing the substrate surface to
the Sn(acac).sub.2 includes introducing the Sn(acac).sub.2 as a
vapor into a chamber in which the substrate is disposed.
[0020] In some implementations, the exposure to the Sn(acac).sub.2
is performed for a duration of about 1 to 30 seconds.
[0021] In some implementations, performing the plasma treatment
includes exposing the substrate surface to a hydrogen plasma, the
exposure to the hydrogen plasma configured to volatilize tin, tin
fluoride or tin oxide residues on the substrate surface.
[0022] In some implementations, exposing the substrate surface to
the hydrogen plasma includes introducing a hydrogen gas into a
chamber in which the substrate is disposed, and igniting a
plasma.
[0023] In some implementations, the exposure to the hydrogen plasma
is performed for a duration of about 1 to 30 seconds.
[0024] In some implementations, the surface modification operation
is performed in a first chamber; wherein the removal operation is
performed in a second chamber.
[0025] In some implementations, a method for performing atomic
layer etching (ALE) on a substrate is provided, including the
following operations: performing a surface modification operation
on a substrate surface, the surface modification operation
including exposing the substrate surface to a first plasma that
converts at least one monolayer of the substrate surface to a
modified layer, wherein a bias voltage is applied during the
surface modification operation, the bias voltage being configured
to control a depth of the substrate surface that is converted by
the surface modification operation, wherein the bias voltage is
configured to accelerate ions from the first plasma towards the
substrate surface without substantially etching the substrate
surface; performing a removal operation on the substrate surface,
the removal operation including removing at least a portion of the
modified layer from the substrate surface, wherein removing the
portion of the modified layer is effected via a ligand exchange
reaction that is configured to volatilize the portion of the
modified layer; performing a clean operation on the substrate
surface, the clean operation including removing residues generated
by the removal operation from the substrate surface, the clean
operation further including exposing the substrate surface to a
second plasma, wherein the residues are volatilized by the exposure
to the second plasma.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 illustrates various classifications of ALE, in
accordance with implementations of the disclosure.
[0027] FIG. 2 illustrates the chemical reaction of an ALE process
for etching Al.sub.2O.sub.3, in accordance with implementations of
the disclosure.
[0028] FIG. 3 illustrates an ALE process for etching
Al.sub.2O.sub.3, in accordance with implementations of the
disclosure.
[0029] FIGS. 4A-4C illustrate performance of ALE process operations
in a plasma treatment chamber and a vapor treatment chamber, in
accordance with implementations of the disclosure.
[0030] FIG. 5 illustrates a process flow diagram for a method
performed in accordance with disclosed implementations.
[0031] FIG. 6 illustrates a graph showing Al Oxide thickness and Al
oxy-fluoride thickness under various conditions, using ARXPS
characterization of the surface following single fluorination, in
accordance with implementations of the disclosure.
[0032] FIG. 7 illustrates surface fluorination depth using
zero-bias plasma, in accordance with implementations of the
disclosure.
[0033] FIGS. 8A-E are STEM images of a cross section of a
fluorinated film, in accordance with implementations of the
disclosure.
[0034] FIGS. 9A and 9B illustrate film loss following a 30 second
fluorine plasma exposure versus a 300 second fluorine plasma
exposure, respectively, in accordance with implementations of the
disclosure.
[0035] FIG. 10 is a graph showing SE characterization of film loss,
demonstrating the self-limiting nature of the fluorination and
Sn(acac)2 exposure, in accordance with implementations of the
disclosure.
[0036] FIG. 11 is a graph illustrating calculated fluorination
depth as a function of fluorine ion energy, in accordance with
implementations of the disclosure.
[0037] FIG. 12 illustrates a method for performing ALE using
multiple ligand exchange and plasma cleaning operations per single
surface modification operation, in accordance with implementations
of the disclosure.
[0038] FIG. 13A conceptually illustrates a cross section of a
substrate surface feature, and performance of an anisotropic ALE
process performed thereon, in accordance with implementations of
the disclosure.
[0039] FIG. 13B conceptually illustrates a cross section of a
substrate surface feature, and performance of an isotropic ALE
process performed thereon, in accordance with implementations of
the disclosure.
[0040] FIGS. 14A-D illustrate a process for providing increased
anisotropy through deposition of a passivation layer, in accordance
with implementations of the disclosure.
[0041] FIG. 15 illustrates a cluster tool 1500, in accordance with
implementations of the disclosure.
[0042] FIG. 16 illustrates an example etching chamber or apparatus,
in accordance with implementations of the disclosure.
[0043] FIG. 17 shows a control module for controlling the systems
described above, in accordance with implementations of the
disclosure.
DETAILED DESCRIPTION
[0044] In the following description, numerous specific details are
set forth to provide a thorough understanding of the presented
implementations. The disclosed implementations may be practiced
without some or all of these specific details. In other instances,
well-known process operations have not been described in detail to
not unnecessarily obscure the disclosed implementations. While the
disclosed implementations will be described in conjunction with the
specific implementations, it will be understood that it is not
intended to limit the disclosed implementations.
[0045] Provided herein are methods of controlling directionality of
atomic layer etching (ALE) of metal oxides (such as aluminum oxide
(Al.sub.2O.sub.3)) via a ligand exchange mechanism involving a
fluorine-containing plasma and a tin-containing etchant. Methods
described herein involve modifying a surface of the material to be
etched using a fluorine-containing plasma and exposing the modified
surface to tin-(II) acetylacetonate (Sn(acac).sub.2) vapor to
remove the material in a self-limiting manner. A ligand exchange
reaction is sustained in a vapor deposition chamber with
Sn(acac).sub.2 vapor without plasma.
[0046] Atomic layer etching (ALE) is one approach for atomic scale
control of etching behavior. ALE is a type of cycling process. ALE
is a technique that removes thin layers of material using
sequential self-limiting reactions. Generally, ALE may be performed
using any suitable technique. Examples of atomic layer etch
techniques are described in U.S. Pat. No. 8,883,028, issued on Nov.
11, 2014; and U.S. Pat. No. 8,808,561, issued on Aug. 19, 2014,
which are herein incorporated by reference for purposes of
describing example atomic layer etch and etching techniques. In
various implementations, ALE may be performed with plasma, or may
be performed thermally.
[0047] An ALE process sequence can be described as follows, in
accordance with implementations of the disclosure. Initially, a
portion of a surface of a substrate is in an unmodified state. The
outermost layer (or surface layer) of molecules/atoms of the
substrate surface are exposed for the ALE process. A surface
conversion/modification operation is performed to convert the
surface layer of the substrate to a functionalized state. For
example, the surface layer is modified by exposure to a surface
conversion reactant, which may adsorb or chemisorb on the surface.
The surface conversion reactant can include molecules or low energy
radicals in various implementations, which react with the surface
layer atoms to effect the surface conversion operation. The
resulting surface layer includes a functionalized outermost layer
of molecules to enable subsequent ALE steps. In some
implementations, the operation is self-limiting, and only (or
substantially only) the outermost layer of the substrate surface
will undergo conversion. In some implementations, the specific
depth of the conversion is controlled, at least in part via
application of a bias voltage which also affects directionality of
the conversion, as described in further detail below. In some
implementations, this surface conversion entails conversion of the
surface species to a halide. In some implementations, following the
(self-limiting) surface conversion, the chamber is purged to remove
any reaction byproducts or excess surface conversion reactant.
[0048] Following the surface conversion operation, then a ligand
exchange reaction/operation is performed. The modified surface
layer of the substrate is exposed to a ligand containing reactant,
which effects a ligand exchange reaction wherein the ligand
containing reactant adsorbs on the substrate surface and transfers
its ligands to the converted surface species which were formed
during the earlier surface conversion/modification operation. The
ligands bond with the modified surface layer of molecules/atoms,
forming a reaction product including ligand substituted surface
species, which can be released.
[0049] Desorption drives removal of the outermost layer of surface
species (the reaction product following the ligand exchange
operation) from the substrate surface. In some implementations, the
release can be achieved by the application of thermal energy, which
can be applied simultaneously with the exposure to the ligand
containing reactant or in a separate step (e.g. by heating the
chuck/chamber, lamp heating, etc.).
[0050] The concept of an "ALE cycle" is relevant to the discussion
of various implementations herein. Generally, an ALE cycle is the
minimum set of operations used to perform an etch process one time,
such as etching a monolayer or a predefined thickness of the outer
layer of the substrate. The result of one cycle is that at least
some of a film layer on a substrate surface is etched. Typically,
an ALE cycle includes a modification operation to form a reactive
layer, followed by a removal operation to remove or etch, in whole
or in part, only this reactive layer. Modification may be performed
by using a chemisorption mechanism, deposition mechanism, top layer
conversion mechanism, or extraction mechanism. The cycle may
include certain ancillary operations such as sweeping one of the
reactants or byproducts. Generally, a cycle contains one instance
of a unique sequence of operations.
[0051] As an example, a method for an ALE cycle may include the
following operations: (i) delivery of a reactant gas, (ii) optional
purging of the reactant gas from the chamber, (iii) delivery of a
removal gas and an optional plasma, and (iv) optional purging of
the chamber. Further description and examples of ALE are described
in U.S. patent application Ser. No. 14/696,254, filed on Apr. 24,
2015 and titled "INTEGRATING ATOMIC SCALE PROCESSES: ALD (ATOMIC
LAYER DEPOSITION) AND ALE (ATOMIC LAYER ETCH)," which is
incorporated herein by reference for purposes of describing atomic
layer etch processes.
[0052] Disclosed implementations result in highly controlled
etching methods with a high degree of uniformity. Disclosed
implementations may be used to perform isotropic etching of various
materials and may also be modified to perform anisotropic etching
by applying a bias at a bias voltage between about 20 V.sub.b and
about 80 V.sub.b, such as at about 50 V.sub.b.
[0053] ALE may be done by a surface modification operation (e.g.,
chemisorption by reactive chemistry on a substrate surface)
followed by a removal operation. Such operations may be repeated
for a certain number of cycles. During ALE, the reactive chemistry
and the removal chemistry are delivered separately to the
substrate.
[0054] Isotropic atomic layer etching (ALE) of Al.sub.2O.sub.3 has
been demonstrated via a ligand exchange method utilizing a fluorine
plasma for the surface modification step and tin-(II)
acetylacetonate (Sn(acac).sub.2) vapor for the non-plasma removal
step. In implementations wherein the steps are performed in the
absence of any directional energy to the wafer, such as provided
via ion bias, the overall etch process is isotropic. However, in
accordance with implementations of the present disclosure,
anisotropy can be introduced in a controlled way to an isotropic
baseline process through the controlled application of a bias
voltage.
[0055] In various implementations, processes are performed in
suitable process equipment/chambers (e.g. Kiyo for fluorination,
and ICS for vapor treatment, both of which are manufactured by Lam
Research Corporation).
[0056] Atomic layer etching of Al.sub.2O.sub.3 using sequential
plasma fluorination and self-limiting thermal reactions with
tin(II)-acetylacetonate (Sn(acac).sub.2) has been demonstrated. One
approach for performing ALE of Al.sub.2O.sub.3 is to perform a
spatial ALE process wherein the wafer (with AL.sub.2O.sub.3 top
layer) is cycled between a plasma treatment chamber (for performing
fluorination) and a vapor treatment chamber (for performing removal
of AlF.sub.3 with Sn(acac).sub.2 vapor) without breaking vacuum.
Another approach for performing ALE is to perform both the plasma
treatment and the vapor treatment in a single chamber, so that the
wafer does not need to be moved between different chambers.
[0057] FIG. 1 illustrates various classifications of ALE, in
accordance with implementations of the disclosure. Broadly
speaking, in a generic ALE process, a modification operation is
performed, followed by a removal operation. The purpose of the
modification operation is to weaken the surface layer without
actually etching it. One technique for modifying the surface is via
chemisorption, which is self-limiting by Langmuir kinetics. Another
way to modify the surface for ALE is via deposition. In this case,
the deposition is not necessarily self-limited unless it is ALD.
Even so, the removal step can be limited by reactant availability.
A third way for performing the surface modification is via a
conversion reaction. One example of a conversion is halogenation of
the top layer. This is a diffusion-limited process and can be
performed via a plasma, bath, or other methods.
[0058] Depending on the specifics in the removal step, e.g. ion
assisted or via ligand exchange, it is possible to obtain
directional or isotropic ALE.
[0059] One use case for ALE is in addressing a problem known as the
"four-color challenge." Broadly speaking, the four-color challenge
poses the problem of removing one specific color out of four
without corner rounding, wherein each color represents a different
material.
[0060] By way of example, isotropic ALE could enable etching of a
single "color" via a ligand exchange involving transmetalation.
[0061] An unstable reaction by-product or a non-existent ligand
exchange mechanism would prevent etching of the other three
"colors" thereby providing selectivity to the etched color.
[0062] Table I below provides examples of ligand exchange
pre-cursors, including Sn(acac).sub.2, Al(CH.sub.3).sub.3,
AlCl(CH.sub.3).sub.2, SiCl.sub.4, and the amount of material
removed per cycle, as demonstrated with reference to Y. Lee, C.
Huffman, S. M. George, "Selectivity in Thermal Atomic Layer Etching
Using Sequential, Self-Limiting Fluorination and Ligand-Exchange
Reactions", Chem. Mater., 2016, 28 (21), pp 7657-7665. As shown,
etch selectivity is also achievable depending upon the particular
pre-cursor utilized.
TABLE-US-00001 TABLE I Modification Removal Molecule Molecule
Etching No Etching HF Sn(acac).sub.2 Al.sub.2O.sub.3 (0.23 .ANG.)
SiO.sub.2 ZrO.sub.2 (0.14 .ANG.) SiN HfO.sub.2 (0.06 .ANG.) TiN
Al(CH.sub.3).sub.3 Al.sub.2O.sub.3 (0.45 .ANG.) SiO.sub.2 (TMA)
HfO.sub.2 (0.10 .ANG.) SiN TiN ZrO.sub.2 Al(CH.sub.3).sub.2Cl
ZrO.sub.2 (0.96 .ANG.) SiO.sub.2 (DMAC) HfO.sub.2 (0.77 .ANG.) SiN
Al.sub.2O.sub.3 (0.32 .ANG.) TiN SiCl.sub.4 ZrO.sub.2 (0.14 .ANG.)
SiO.sub.2 HfO.sub.2 (0.05 .ANG.) SiN TiN Al.sub.2O.sub.3
[0063] FIG. 2 illustrates the chemical reaction of a removal step
of an ALE process for etching Al.sub.2O.sub.3, in accordance with
implementations of the disclosure. As has been noted, the surface
portion of Al.sub.2O.sub.3 is first converted to AlF.sub.3 by
performing a surface modification/conversion step. Then, as shown,
tin(II)-acetylacetonate (vapor) is provided to react with the
aluminum(III)-fluoride (solid), to yield
tin(II)-fluoro-acetylacetonate and aluminum(III)-acetylacetonate,
both of which are volatile at the chosen process temperature. As
the reaction products are volatile, they can be removed from the
surface and evacuated from the chamber.
[0064] FIG. 3 illustrates an ALE process for etching
Al.sub.2O.sub.3, in accordance with implementations of the
disclosure. Initially, a substrate with an Al.sub.2O.sub.3 surface
is situated in a plasma treatment chamber. Then the Al.sub.2O.sub.3
surface is fluorinated, by way of example, utilizing a 0-bias ICP
plasma (i.e. no RF power applied to the wafer pedestal). After
completion, the substrate is moved, without breaking vacuum, into a
vapor treatment chamber in which the Sn(acac).sub.2-based ligand
exchange reaction with the fluorinated surface takes place. A final
chamber pump-out step completes the first ALE cycle after which the
wafer can be shuttled back to the plasma treatment chamber for the
next ALE cycle. Alternatively, all cycles can be performed in a
single chamber.
[0065] FIGS. 4A-4C illustrate performance of ALE process operations
in a plasma treatment chamber 400 and a vapor treatment chamber
410, in accordance with implementations of the disclosure. With
reference to FIG. 4A, the substrate 402 is shown atop a substrate
holder 404 in a plasma treatment chamber 400. Following initiation
of process gas flow through a feed gas shower head 408 and warm-up,
an inductively coupled plasma (ICP) is generated by applying power
to the ICP coil 406. In various implementations, fluorine plasma
type 1 or 2 or other types can be used for fluorination. It will be
appreciated that a fluorine plasma can be generated from various
flurorine-containing precursors, such as CF.sub.4, NF.sub.3,
SF.sub.6, CHF.sub.3, C.sub.2H.sub.2F.sub.4, F.sub.2, SiF.sub.4,
etc. In some implementations, the substrate holder 404 is heated to
a temperature of about 100.degree. C. In some implementations, the
fluorination operation is performed at a pressure of approximately
20 mTorr. Following the plasma exposure, a pump-out is performed to
remove process gases from the chamber.
[0066] With reference to FIG. 4B, after completion of the
fluorination operations in the plasma treatment chamber 400, the
substrate 402 is moved, without breaking vacuum, to a vapor
treatment chamber 410 for performance of a ligand exchange
operation. Following a warm-up, a vapor treatment is applied by
flowing a vapor over the substrate as it is disposed atop the
substrate holder 412. For example, Sn(acac)2 vapor can be generated
by a vaporizer 414 flowed through a heated vapor line 416 and
distributed over the substrate via a vapor nozzle plate 418. The
vapor treatment does not entail generation of a plasma. In some
implementations, Sn(acac)2 vapor for ligand exchange is applied. In
some implementations, the substrate holder 412 is heated to a
temperature of approximately 200.degree. C. In some
implementations, the chamber pressure is maintained at
approximately 20 mTorr to 120 mTorr. In some implementations, the
vapor treatment is applied for approximately one second to
approximately 15 seconds. Following application of the vapor
treatment, a pump-out is performed to remove process gases from the
vapor treatment chamber 410.
[0067] With reference to FIG. 4C, after completion of the ligand
exchange operation, the substrate 402 is moved, without breaking
vacuum, to a plasma treatment chamber, which may be the same plasma
treatment chamber 400 as that utilized for the fluorination
operation, or a different plasma treatment chamber. A hydrogen
plasma treatment is performed in order to remove residual tin from
the substrate surface. H.sub.2 gas is flowed through the feed gas
shower head 408 and power is applied to the ICP coil 406 to
generate the H.sub.2 plasma. In some implementations, 500 W ICP
power is applied. In some implementations, the substrate holder 404
is heated to a temperature of approximately 100.degree. C. In some
implementations, the hydrogen plasma treatment is performed at a
chamber pressure of approximately 20 mTorr. In some
implementations, the hydrogen plasma treatment is performed for a
duration of approximately 5 to 45 seconds. Following the plasma
exposure, a pump-out is performed to remove process gases from the
plasma treatment chamber.
[0068] Though in the illustrated implementation, separate chambers
for plasma treatment and vapor treatment have been shown, it will
be appreciated by those skilled in the art that in other
implementations, a single chamber can be used for plasma and vapor
treatments. Such a system can have appropriate valves to enable
switching between different process gases (e.g. individual valves
controlling the introduction of each process gas into the chamber).
Purge or pump-out operations can be performed following each of the
fluorination, vapor treatment, and hydrogen plasma treatments.
[0069] A process flow diagram for a method performed in accordance
with disclosed implementations is provided in FIG. 5. During
operations 501-507, an inert gas such as an argon gas may be
continuously flowed in the background as a carrier gas.
[0070] In operation 501, a substrate including a material to be
etched is exposed to a fluorine-containing plasma to modify the
surface of the substrate.
[0071] The fluorine-containing plasma may be generated by
introducing a fluorine-containing gas and igniting a plasma. For
example, in some implementations, the fluorine-containing gas may
be carbon tetrafluoride (CF.sub.4), nitrogen trifluoride
(NF.sub.3), sulfur hexafluoride (SF.sub.6), fluorine (F.sub.2), or
any fluorine-containing gas. In various implementations, CF.sub.4
may be introduced with O.sub.2 to generate an abundance of fluorine
ions in the plasma to etch the substrate. In some implementations,
about 35% of the total flow of gases to the chamber to generate the
fluorine-containing plasma is O.sub.2 gas. Other
fluorine-containing gases that include carbon may be used in some
implementations when introduced with another gas to inhibit the
formation of a carbide. For example, other fluorine-containing
gases may have the formula C.sub.xH.sub.yF.sub.z, where x may be
any integer greater than or equal to 1, y may be any integer
greater than or equal to 0, and z may be any integer greater than
or equal to 1. Examples include fluoroform (CHF.sub.3) and
difluoromethane (CH.sub.2F.sub.2). In some implementations, the
fluorine-containing gas may be generated by vaporizing a
fluorine-containing liquid.
[0072] In some implementations, the substrate is not patterned. In
other implementations, the substrate may be patterned. The
substrate may include a transistor structure which may include an
additional gate layer such as a blocking oxide or an etch stop
layer. For example, the substrate may include an aluminum oxide
layer over a fin of a FinFET transistor. In some implementations,
the substrate may include a 3D NAND structure with a metal oxide
etch stop layer at the bottom of trenches formed in the structure
such that the metal oxide etch stop layer is the material to be
etched. In various implementations, features on the substrate may
have an aspect ratio between about 1.5:1 and about 5:1. In some
implementations, features may have aspect ratios up to about
10:1.
[0073] The plasma in operation 501 may be generated in situ or may
be a remote plasma. In many implementations, the plasma is
generated in situ to generate an inductively coupled plasma.
[0074] However, in other implementations, a capacitively coupled
plasma (CCP) can be employed. In such implementations, the CCP
reactor can be configured to enable a low-bias mode, to provide for
isotropic ALE. For example, such a CCP reactor may employ an RF
electrode on top of the reactor, a substrate holder configured to
have a floating ground, and run at a relatively high RF frequency
setting, e.g. 60 MHz.
[0075] In various implementations, the substrate includes a metal
oxide, metal nitride, metal phosphide, metal sulfide, metal
arsenide, pure metal or any other metal compound layer to be
etched. Examples include aluminum oxide (Al.sub.2O.sub.3) and
hafnium oxide. Note that in many implementations,
silicon-containing material (e.g., silicon oxide, silicon nitride,
silicon carbide, silicon, etc.) may not be etched using disclosed
implementations, which contributes to achieving etch selectivity
particularly when etching a material such as a sacrificial gate
oxide layer over a fin on a FinFET transistor structure. Although
it will be understood that disclosed implementations may be used to
etch various materials, FIG. 5 will be described with respect to
etching aluminum oxide.
[0076] In some implementations, operation 501 may be performed
without applying a RF bias to allow isotropic modification of the
substrate surface. Note that although some disclosed
implementations may be used to perform isotropic etch, in other
implementations, an anisotropic etching process may also be
performed by applying a bias during operation 501. The method
described herein with respect to FIG. 5 can thus be configured for
isotropically or anisotropically etching aluminum oxide.
[0077] Without being bound by a particular theory, during operation
501, a metal oxide surface such as an aluminum oxide surface, may
be fluorinated by the fluorine-containing plasma, isotropically or
anisotropically, to modify the surface of the aluminum oxide to
form aluminum fluoride (e.g., AlF.sub.3). One or a few monolayers
of the aluminum oxide surface may be modified to form aluminum
fluoride. The modification operation may be limited by the depth of
diffusion/penetration of fluorine ions. Under the influence of a
bias, the penetration/diffusion depth of the fluorine ions becomes
deeper (along the bias axis) and also more anisotropic. The
substrate may be exposed to the fluorine-containing plasma at a
chamber pressure between about 10 mTorr and about 100 mTorr, such
as at about 20 mTorr for a duration less than about 15 seconds but
greater than 0 seconds.
[0078] In another implementation, a thermal fluorination operation
is performed, as opposed to the plasma-driven process described
above. That is, the substrate is exposed to a fluorine-containing
gas (e.g. NF.sub.3, etc.) at a sufficient temperature to induce
surface fluorination, without the need for generating a plasma.
[0079] Note that in some implementations, after performing
operation 501, the chamber housing the substrate may not be purged.
In some implementations, the substrate may be purged.
[0080] In operation 503, the substrate is exposed to tin-(II)
acetylacetonate (Sn(acac).sub.2) vapor. In various implementations,
Sn(acac).sub.2 may be vaporized in an external vaporizer prior to
delivering the vapor to the substrate.
[0081] Without being bound by a particular theory, it is believed
that when the modified AlF.sub.3 surface is exposed to
Sn(acac).sub.2 vapor, a ligand exchange reaction occurs such that
one acac ligand on Sn(acac).sub.2 replaces one fluorine atom on a
AlF.sub.3 molecule, forming AlF.sub.2(acac). Additional
Sn(acac).sub.2 and/or Sn(acac) may then react with AlF.sub.2(acac)
again twice to replace the second and third fluorine atoms with
(acac), resulting in Al(acac).sub.3. It is believed that as the
acac ligands are substituted for the fluorine atoms, the
Al(acac).sub.x species becomes increasingly volatile, enabling it
to be etched from the substrate. The reaction is self-limiting, and
it is believed that some tin, tin fluoride, tin oxide, and
Sn(acac).sub.2 may begin to build up on the surface of the material
to be etched, thus blocking further etching of any modified
underlayers of AlF.sub.3.
[0082] In some implementations, operations 501 and 503 may be
performed in the same chamber. In such implementations, a rapid
temperature change between the ligand exchange step and the H.sub.2
plasma flash is achieved, as the ligand exchange reaction needs to
be above approximately 190 C, whereas the H.sub.2 plasma flash must
be below approximately 150 C or else etching of the Al.sub.2O.sub.3
with the H.sub.2 plasma will occur. In operation 503, the plasma is
turned off and the fluorine-containing gas flow may be turned off
prior to turning on the vapor flow. In some implementations, the
chamber is not purged prior to operation 503.
[0083] In some implementations, operations 501 and 503 may be
performed in separate chambers of the same apparatus. An apparatus
having multiple chambers for performing ALE operations can be
provided, in accordance with implementations of the disclosure. In
various implementations, the substrate may be shuttled or moved
between a first chamber for exposing to a fluorine-containing
plasma in operation 501 to a second chamber for exposing to
Sn(acac).sub.2 vapor in operation 503. In some implementations, the
second chamber is a vapor deposition chamber. In some
implementations, the second chamber is a modified chamber that does
not include a plasma source. Note that movement or shuttling of the
substrate between chambers may be performed without breaking
vacuum.
[0084] In alternative implementations, the substrate may be exposed
to another chemical in vapor phase that is selective to the metal
fluoride but does not react with the metal oxide. The chemical may
include one or more ligands that, when reacted with a metal
fluoride, generates a volatile compound including the metal bonded
to the ligand (e.g. Sn(acac).sub.2).
[0085] In some implementations, operation 503 may be performed for
a duration of about 1 second with the temperature of the wafer
holder or pedestal holding the wafer set to a temperature of about
200.degree. C. In various implementations, the chamber pressure at
the end of the exposure to the Sn(acac).sub.2 vapor may be about 20
mTorr.
[0086] In operation 505, the substrate may be exposed to a plasma
treatment (e.g. a hydrogen plasma). Without being bound by a
particular theory, it is believed that operation 505 is performed
to volatilize tin, tin fluoride or tin oxide buildup on the surface
of the substrate, which can accumulate from performing operation
503. Exposing the substrate to hydrogen may form tin hydrates which
are volatile at the chosen substrate temperature, which may then be
pumped from the processing chamber. The substrate may be exposed to
the plasma treatment for a duration greater than 0 seconds and less
than 5 seconds. The duration of plasma exposure may depend on the
amount of tin on the surface. For example, in some implementations,
the amount of tin may be determined by evaluating tin lines in an
emission spectrum. In some implementations, the plasma may be
turned off when the tin lines in an emission spectrum disappear. In
some implementations, the substrate is exposed to the plasma for
about 5 seconds. In some implementations, the substrate is exposed
to the plasma for a duration greater than about 5 seconds. In
various implementations, the plasma treatment may include
introducing a hydrogen gas and igniting a plasma. Operation 505 may
be performed in the same chamber as in operation 501 and/or 503.
Note that although operation 505 may be performed by exposing the
substrate to hydrogen plasma, in some implementations a different
chemistry may be used to remove tin or tin oxide buildup on the
surface of the material to be etched. For example, in some
implementations, ammonia (NH.sub.3) plasma may be used.
[0087] In some implementations, operation 505 may be performed in a
separate chamber. For example, in some implementations, the
substrate may be moved or shuttled to the first station/chamber
where operation 501 was performed, or may be moved or shuttled to a
third station/chamber to perform operation 505. Note that movement
or shuttling of the substrate between chambers may be performed
without breaking vacuum.
[0088] In operation 507, it is determined whether the amount etched
is sufficient to achieve the desired amount to be etched. If the
desired remaining thickness has not yet been achieved, operations
501-505 may be optionally repeated. Note that in some
implementations, operation 505 may only be performed every n cycles
of performing operations 501 and 503, where n is an integer greater
than or equal to 1. Where n is 1, operation 505 is performed in
every cycle. In various implementations, operation 505 is performed
in every cycle. In another example, operation 505 may be performed
every 2 cycles of performing operations 501 and 503 (where n is 2)
such that the following operations may be performed to etch a
substrate: (1) exposure to fluorine-containing plasma, (2) exposure
to Sn(acac).sub.2 vapor, (3) exposure to fluorine-containing
plasma, (4) exposure to Sn(acac).sub.2 vapor, (5) exposure to
hydrogen plasma, and (6) repeat (1)-(5).
[0089] In accordance with some implementations of the disclosure,
isotropic atomic layer etch utilizes a low-bias plasma during the
modification step. When etching metal oxides such as
Al.sub.2O.sub.3, this involves a zero-bias fluorine plasma to form
aluminum-fluoride at the surface of the oxide film. This step is
self-limiting to a few monolayers as the data described below
indicate.
[0090] During the following vapor removal step, Sn(acac).sub.2
reacts with the fluorinated top surface of the film via the ligand
exchange mechanism and etches away the fluorinated layer. As the
vapor treatment brings no directional energy such as ion energy
from a plasma sheath to the wafer, the vapor step etches the metal
fluoride isotropically. The overall sequence of reactions can be
summarized in the following way: (1) Create a fluorinated shallow
surface layer of .about.1.5 nm in a low bias fluorine plasma. The
plasma may be based on CF.sub.4 or NF.sub.3, for example. (2)
Without plasma, apply Sn(acac).sub.2 vapor while the substrate is
heated to an elevated temperature (for example, 200 C) to perform a
ligand exchange reaction between the fluorine and the acac ligands.
(3) Pump away volatile reaction by-products. (4) Apply a brief
hydrogen plasma flash to the surface of the substrate to remove
non-volatile tin by-products from the substrate surface. (5) Return
to step (1) and repeat.
[0091] Anisotropy can be introduced to the etch process in a
controlled fashion by turning on plasma bias during the plasma
fluorination step in a controlled manner. Data have been obtained
showing that the depth of fluorination can be controlled via bias
energy. Fluorine ions will advance deeper into the metal oxide film
before they can be stopped if their incipient ion energy acquired
during the acceleration in the plasma sheath is greater.
[0092] To better understand fluorination, Al.sub.2O.sub.3 films
were exposed to various fluorination conditions after which a set
of characterization techniques were employed to understand changes
to the film. Angle resolved x-ray photoelectron spectroscopy
(ARXPS) was used to measure fluorination depth and total material
loss.
[0093] FIG. 6 illustrates a graph showing aluminum oxide thickness
and aluminum oxy-fluoride thickness under various conditions, using
ARXPS characterization of the surface following single
fluorination, in accordance with implementations of the disclosure.
As indicated, fluorination depth was shown to be dependent on
plasma density and ion energy. However, cathode bias (influencing
ion energy) demonstrated the highest impact on fluorination depth,
to a significantly greater extent than plasma density.
[0094] FIG. 7 illustrates surface fluorination depth using
zero-bias plasma, in accordance with implementations of the
disclosure. The fluorination depth after plasma treatment was
probed via depth-resolved XPS. In agreement with the results shown
at FIG. 6, the fluorination depth was found to be limited down to
approximately 1.5 nm. More specifically, a sample having a 100
angstrom surface thickness of Al.sub.2O.sub.3 over a silicon
dioxide (thermal oxide) layer (1000 angstrom thickness) over a
silicon substrate was profiled. In the illustrated graph, measured
atomic percentages for the elements aluminum, oxygen, and fluorine
as a function of sputter time (seconds) are shown. Measurements are
shown both before and after the application of a fluorination
plasma under a zero bias condition.
[0095] Before the application of the fluorination plasma, the
atomic percentage of oxygen is shown by curve 700; the atomic
percentage of aluminum is shown by curve 702; and the atomic
percentage of fluorine is shown by curve 704. As indicated, the
atomic percentage of aluminum drops off at around 200 seconds,
which corresponds to the complete sputtering of the 100 angstrom
(10 nm) thickness of the aluminum oxide layer. Thus, approximately
1 nm of thickness is sputtered every 20 seconds. The atomic
percentage of oxygen, shown by curve 700, increases after about 200
seconds, as the sputter reaches the silicon dioxide layer. The
atomic percentage of fluorine, shown by curve 704, is zero
throughout, as the fluorination plasma has yet to be applied.
[0096] After the application of the fluorination plasma, the atomic
percentage of oxygen is shown by curve 706; the atomic percentage
of aluminum is shown by curve 708; and the atomic percentage of
fluorine is shown by curve 710. As can be seen, the atomic
percentage of fluorine drops to near zero within about 30 seconds
of sputter time, which corresponds to a depth of about 15 angstroms
(1.5 nanometers). Thus, with zero bias, the fluorination plasma
achieved a fluorine diffusion depth of about 15 angstroms.
[0097] FIGS. 8A-E are STEM images of a cross section of a
fluorinated film, in accordance with implementations of the
disclosure. FIG. 8A shows the pre-fluorination cross-section,
including a surface layer of aluminum oxide having a thickness of
about 11 nanometers (nm), over a layer of thermal silicon dioxide.
FIG. 8B shows the result of fluorination without bias. FIG. 8D
shows a close-up view of a portion of this result.
[0098] FIG. 8C shows the result of fluorination with a 100V bias.
FIG. 8E shows a close-up view of a portion of this result.
[0099] As indicated by these images, the modified depth (indicated
by darker grey) was 5.7 nm without bias, and 6.7 nm with 100V bias.
The fluorinated depth (indicated by light grey top) was 1.4 nm
without bias, and 2.5 nm with 100V bias. The `c` lattice constant
of Al.sub.2O.sub.3 is .about.1.3 nm. As can be seen from the
images, the fluorinated depth has increased as a result of the
application of a 100V bias.
[0100] FIGS. 9A and 9B illustrate film loss following a 30 second
fluorine plasma exposure versus a 300 second fluorine plasma
exposure, respectively, in accordance with implementations of the
disclosure. As shown, the 300 second fluorine plasma exposure did
not produce additional film loss of significance beyond that of the
30 second fluorine plasma exposure.
[0101] FIG. 10 is a graph showing Spectral Ellipsometry (SE)
characterization of film loss, demonstrating the self-limiting
nature of the fluorination and Sn(acac).sub.2 exposure, in
accordance with implementations of the disclosure.
[0102] As shown, the zero-bias fluorination process is
self-limiting. The .about.5 A loss is likely due to refractive
index change during fluorination.
[0103] Additionally, the results show that the material removed in
one ALE cycle may weakly depend on Sn(acac).sub.2 application time.
However, there is some material left on the surface which limits
the ability to etch to the full extent of the fluorination depth
(.about.15 A).
[0104] As noted, the fluorinated depth increases with the
application of a bias. Accordingly, during the Sn(acac).sub.2 vapor
step more metal fluoride will be removed from the film having a
bias applied during the fluorination step (e.g. 100V) than in the
zero-bias case. As the fluorination depth only increases on
surfaces parallel to the plasma sheath edge but not on those
perpendicular to it, an anisotropy in the following removal step
can be achieved. That is, more material will be removed from
horizontal than from vertical surfaces.
[0105] FIG. 11 is a graph illustrating calculated fluorination
depth as a function of fluorine ion energy, in accordance with
implementations of the disclosure. As shown, increased ion energy
results in increased fluorination depth. For example, to achieve
greater than 1 nm fluorination depth would require ion energy
greater than 100 eV. Thus, the amount of anisotropy of the ALE
process increases with increasing ion energy during the
fluorination step.
[0106] As noted above, the ligand exchange operation may not fully
consume the fluorinated portion of the substrate in a single
operation, as it may be self-limited due to residue build-up, and a
hydrogen plasma may be applied to remove the residue. Therefore, in
some implementations, ligand exchange and hydrogen plasma
operations can be repeated multiple times per each fluorination
operation.
[0107] FIG. 12 illustrates a method for performing ALE using
multiple ligand exchange and plasma cleaning operations per single
surface modification operation, in accordance with implementations
of the disclosure. The illustrated method is described with
reference to fluorine-containing plasma for surface modification,
Sn(acac).sub.2 for ligand exchange, and hydrogen plasma for residue
removal. However, in various implementations, the method can be
applied for any other set of specific chemistries for surface
modification, ligand exchange, and residue removal.
[0108] At method operation 1201, using a fluorine plasma, a
fluorinated surface layer is created, having a depth that is
controlled by the magnitude of the bias voltage during the fluorine
plasma exposure. In some implementations, the plasma may be
generated using CF.sub.4 or NF.sub.3. It will be appreciated that
the application of a bias voltage will not only increase the depth
of fluorination, but also the anisotropy of the overall etch
process, as the fluorination depth is increased through the
application of the bias in a directional manner (normal to the
substrate/wafer plane).
[0109] At method operation 1203, without plasma, Sn(acac).sub.2
vapor is applied while the substrate is heated to an elevated
temperature (for example, 200 C) to perform a ligand exchange
reaction between the fluorine and the acac ligands. It is noted
that a single Sn(acac).sub.2 vapor application may only perform
ligand exchange with the top layers of the fluorinated film, and
therefore may not completely consume the entire fluorinated film.
This may especially be true in the case where the fluorinated depth
has been increased through the application of a bias voltage (as
compared to a zero-bias fluorination plasma). Following the
Sn(acac).sub.2 vapor application, volatile reaction by-products are
pumped away. However, as noted, there may be a build-up of
(non-volatile) tin-containing residues that remain on the surface,
and which prevent further ligand-exchange reaction (and subsequent
removal of material) from occurring during the Sn(acac).sub.2 vapor
application.
[0110] Therefore, at method operation 1205, a brief hydrogen plasma
flash is applied to the surface of the substrate to remove
non-volatile tin by-products from the substrate surface.
[0111] As noted, the vapor application occurring at method
operation 1203 may not have consumed the entire fluorinated surface
layer. Hence, at method operation 1207, it is determined whether
the fluorinated layer has been consumed by the ligand exchange. If
not, then the method returns to operation 1203, to repeat the
Sn(acac)2 and hydrogen plasma exposures until the fluorinated layer
has been consumed. It will be appreciated that the number of cycles
of method operations 1203 and 1205 required to entirely consume the
fluorinated layer may be experimentally predetermined.
Consequently, determining whether the fluorinated layer has been
consumed at method operation 1207 may be defined by determining
whether the predetermined number of cycles has been performed.
[0112] When the entire fluorinated layer has been consumed, or if a
predetermined number of cycles necessary to fully consume the
fluorinated layer has been performed, then at operation 1209, it is
determined whether the film has been etched to the desired
thickness. If not, then the method returns to operation 1201 to
perform the surface fluorination.
[0113] The process (including method operation 1201, 1203, 1205,
and 1207) is repeated until the film has been etched to the desired
thickness, or until a predetermined number of cycles has been
completed so as to achieve the desired thickness.
[0114] It will be appreciated that the foregoing process is faster
than conventional ALE processes due to the performance of a single
fluorination operation for multiple cycles of the Sn(acac).sub.2
exposure and hydrogen plasma exposure, as opposed to performing the
fluorination operation with each instance of the Sn(acac).sub.2 and
hydrogen plasma exposures. This can increase throughput for the ALE
process. Furthermore, by reducing the number of fluorination
operations, it is possible to preserve selectivity to a mask (e.g.
silicon oxide mask) that may be present on the substrate surface,
and which may be susceptible to degradation through multiple
fluorination operations. By contrast, the ligand exchange is
selective and does not affect the mask.
[0115] FIG. 13A conceptually illustrates a cross section of a
substrate surface feature 1300, and performance of an anisotropic
ALE process performed thereon, in accordance with implementations
of the disclosure. The top surface of the substrate may include a
mask 1302 to prevent other portions of the substrate from being
etched. In the illustrated implementation, an isotropic ALE process
is performed by performing ALE using a surface modification
mechanism with zero bias. By performing the surface modification
with zero bias, then the effect of the surface modification will be
isotropic, producing conversion of available surface species to
approximately uniform depth in an omnidirectional manner. As noted
above, the surface modification may be diffusion limited to produce
the depth of surface modification.
[0116] Because the surface modification has been isotropically
performed, then that portion which has been modified is available
for removal by the subsequent removal operation (e.g. via a ligand
exchange mechanism). The result is an ALE process that is isotropic
due to the lack of bias being applied during the surface
modification operation. As noted above, in some implementations, a
single surface modification may penetrate to a depth that is
greater than that which can be removed by a single removal
operation; and thus in some implementations, multiple removal and
plasma clean operations are performed in succession in order to
fully remove the entire portion that has been modified by the
single surface modification operation.
[0117] The initial surfaces 1304 of the feature 1300 are shown, and
successive cycles of the zero bias etch process isotropically
deepen the surfaces 1304 of the feature 1300. The resulting
surfaces of the feature 1300 following successive etch cycles are
respectively shown by the surfaces 1306, 1308, 1310, and 1312. For
example, following one etch cycle, the feature 1300 is
isotropically etched so as to have surfaces 1306; following a
second etch cyle, the feature 1300 is isotropically etched so as to
have surfaces 1308; etc.
[0118] FIG. 13B conceptually illustrates a cross section of a
substrate surface feature, and performance of an isotropic ALE
process performed thereon, in accordance with implementations of
the disclosure. The implementation of FIG. 13B is similar to that
of FIG. 13A, except that during the surface modification operation,
a bias voltage is applied, which introduces a degree of
directionality to the surface modification operation. With
increased bias power, the ions will be driven deeper in the
downward vertical direction (orthogonal to the plane of the
substrate surface) than the horizontal direction. The result is
that surface modification will occur to greater depths in the
downward vertical direction, and to reduced depths in the
horizontal direction, as compared to a zero-bias surface
modification. Then as the amount of material that has been
converted by the surface modification operation determines that
which is available for removal by the removal operation, then the
result will be an anisotropic ALE exhibiting greater etch rate
along the downward vertical direction and reduced etch rate along
the horizontal direction, as compared to the zero-bias case.
[0119] With continued reference to FIG. 13B, the initial surfaces
1304 of the feature 1300 are again shown. However, in contrast to
the zero bias etch, successive cycles of the biased etch process
anisotropically deepen the surfaces 1304 of the feature 1300. The
resulting surfaces of the feature 1300 following successive
anisotropic etch cycles are respectively shown by the surfaces
1314, 1316, 1318, and 1320. For example, following one etch cycle,
the feature 1300 is anisotropically etched so as to have surfaces
1314; following a second etch cycle, the feature 1300 is
anisotropically etched so as to have surfaces 1316; etc.
Additionally, in some implementations, the vertical surfaces can be
coated with a protective polymer such that they are excluded from
the ALE cycle all together. In this manner, one can avoid etching
the vertical surfaces completely.
[0120] In sum, during the modification step in an ALE process, the
depth of the modified layer (along the direction of the bias flux,
or generally orthogonal to the substrate plane) can be controlled
via the bias voltage applied during that step. The depth
modification is generally limited to those surfaces parallel to the
plasma sheath edge. Because of this, anisotropy can be introduced
in a controlled way by controlling the amount of bias applied
during plasma fluorination. That is, the relative etch rates in the
vertical direction (orthogonal to the substrate plane) versus the
horizontal direction (parallel to the substrate plane) can be
tuned, with the ratio of vertical to horizontal etch increasing
with increased bias power.
[0121] It will be appreciated that the bias power can be tuned for
particular applications and ALE chemistries, and that there may be
trade-offs related to the bias power. For example, as bias power is
increased (e.g. by controlling bias voltage), ion implantation may
occur to greater depths, producing greater surface modification
depth, and increased anisotropy. However, as bias power is
increased, more energy is imparted to the ions, which may also
produce film loss due to reactive-ion etching and/or sputtering.
Thus, in some implementations, bias power is tuned to provide a
desired depth of surface modification, while also substantially
avoiding film loss or tolerating an acceptable level of film loss
for the given ALE application. One can control the bias to a level
below the sputter threshold of the material that is being etched.
That way, premature film loss can be minimized.
[0122] In some implementations, the degree of anisotropy can be
increased via a polymerizing plasma step prior to fluorination to
deposit a polymer liner inside the structure that is to be etched.
This liner can be opened before or during the first part of the
fluorination step on the bottom surface only (e.g. by photoresist,
photolithography, and ion etch) but would remain intact for
sidewalls, thereby protecting them.
[0123] FIGS. 14A-D illustrate a process for providing increased
anisotropy through deposition of a passivation layer, in accordance
with implementations of the disclosure. FIG. 14A conceptually
illustrates a cross section of a substrate surface feature 1400. A
passivation layer 1402 is deposited in the feature, as shown at
FIG. 14B. The passivation layer is a protective liner that protects
the underlying surface from being etched during a subsequent ALE
process. In various implementations, the passivation layer can
consist of a polymer material, and inorganic material, or any other
material capable of protecting feature surfaces from being etched
during a subsequent ALE process. Furthermore, the passivation layer
can be deposited by any suitable technique, including without
limitation, CVD, ALD, etc.
[0124] At FIG. 14C, the bottom of the passivation layer 1402 is
opened, exposing the underlying substrate material for etching. At
FIG. 14D, an anisotropic ALE process is carried out (using a bias
voltage during the surface modification step). As shown, the bottom
of the feature is etched, while the feature's sidewalls 1401 are
protected by the passivation layer. In some implementations, the
passivation layer 1402 is not etched by the ALE process. Whereas in
other implementations, the passivation layer 1402 is configured to
be partially or fully etched by the ALE process. In such
implementations, the passivation layer 1402 acts to prevent or
delay the onset of etching of the feature's sidewalls 1401,
providing for increased anisotropy of the overall process.
[0125] Various implementations described herein may be performed in
a plasma etch chamber such as the Kiyo, available from Lam Research
Corporation in Fremont, Calif. In various implementations, a
substrate may be shuttled between an etching chamber and a vapor
chamber without breaking vacuum.
[0126] Disclosed implementations may be performed in any suitable
chamber or apparatus, such as the Kiyo.RTM. or Flex, both available
from Lam Research Corporation of Fremont, Calif. In some
implementations, disclosed implementations may be performed in a
cluster tool, which may contain one or more stations. FIG. 15
illustrates a cluster tool 1500, in accordance with implementations
of the disclosure. In various implementations, one station 1501 may
include a module for etching while another station 1503 includes a
module for exposing to vapor (e.g., a vapor chamber). In some
implementations, a third station 1505 includes a module for
exposing to a plasma.
[0127] In some implementations, an inductively coupled plasma (ICP)
reactor may be used. Such ICP reactors have also been described in
U.S. Patent Application Publication No. 2014/0170853, filed Dec.
10, 2013, and titled "IMAGE REVERSAL WITH AHM GAP FILL FOR MULTIPLE
PATTERNING," hereby incorporated by reference for the purpose of
describing a suitable ICP reactor for implementation of the
techniques described herein. Although ICP reactors are described
herein, in some implementations, it should be understood that
capacitively coupled plasma reactors may also be used. With
reference to FIG. 16, an example etching chamber or apparatus may
include a chamber 1601 having a showerhead or nozzle 1603 for
distributing fluorine-containing gases (1605), hydrogen gas (1607),
or Sn(acac).sub.2 vapor (1609) or other chemistries to the chamber
1601, chamber walls 1611, a chuck 1613 for holding a substrate or
wafer 1615 to be processed which may include electrostatic
electrodes for chucking and dechucking a wafer. The chuck 1613 is
heated for thermal control, enabling heating of the substrate 1615.
The chuck 1613 may be electrically charged using an RF power supply
1617 to provide a bias voltage in accordance with implementations
of the disclosure (e.g. at a voltage in the range of approximately
20 to 200V, 13.56 Mhz). An RF power supply 1619 is configured to
supply power (e.g. in the range of approximately 100 W to 3 kW, at
13.56 Mhz) to a coil 1621 to generate a plasma, and gas flow inlets
for inletting gases as described herein. Though an ICP chamber is
shown, in other implementations, a CCP chamber can be utilized. In
various implementations, the chamber walls 1611 may be
fluorine-resistant. For example, the chamber walls 1611 may be
coated with silicon-containing material (such as silicon or silicon
oxide) or carbon-containing material (such as diamond) or
combinations thereof such that fluorine-containing gases and/or
plasma may not etch the chamber walls 1611. Modification chemistry
gases for chemisorption (such as fluorine-containing gases for
generating fluorine-containing plasma) and/or vapor exposure (such
as Sn(acac).sub.2) may be flowed to the chamber 1601. In some
implementations, a hydrogen gas 1607 may be flowed to the chamber
to generate a hydrogen plasma for removing tin, tin fluoride or tin
oxide residues. In some implementations, the chamber walls are
heated to support wall cleaning efficiency with a hydrogen plasma.
In some implementations, an apparatus may include more than one
chamber, each of which may be used to etch, deposit, or process
substrates. The chamber or apparatus may include a system
controller 1623 for controlling some or all of the operations of
the chamber or apparatus such as modulating the chamber pressure,
inert gas flow, plasma power, plasma frequency, reactive gas flow
(e.g., fluorine-containing gas, Sn(acac).sub.2 vapor, etc.); bias
power, temperature, vacuum settings; and other process
conditions.
[0128] FIG. 17 shows a control module 1700 for controlling the
systems described above, in accordance with implementations of the
disclosure. For instance, the control module 1700 may include a
processor, memory and one or more interfaces. The control module
1700 may be employed to control devices in the system based in part
on sensed values. For example only, the control module 1700 may
control one or more of valves 1702, filter heaters 1704, pumps
1706, and other devices 1708 based on the sensed values and other
control parameters. The control module 1700 receives the sensed
values from, for example only, pressure manometers 1710, flow
meters 1712, temperature sensors 1714, and/or other sensors 1716.
The control module 1700 may also be employed to control process
conditions during reactant delivery and plasma processing. The
control module 1700 will typically include one or more memory
devices and one or more processors.
[0129] The control module 1700 may control activities of the
reactant delivery system and plasma processing apparatus. The
control module 1700 executes computer programs including sets of
instructions for controlling process timing, delivery system
temperature, pressure differentials across the filters, valve
positions, mixture of gases, chamber pressure, chamber temperature,
wafer temperature, RF power levels, wafer ESC or pedestal position,
and other parameters of a particular process. The control module
1700 may also monitor the pressure differential and automatically
switch vapor reactant delivery from one or more paths to one or
more other paths. Other computer programs stored on memory devices
associated with the control module 1700 may be employed in some
implementations.
[0130] Typically there will be a user interface associated with the
control module 1700. The user interface may include a display 1718
(e.g. a display screen and/or graphical software displays of the
apparatus and/or process conditions), and user input devices 1720
such as pointing devices, keyboards, touch screens, microphones,
etc.
[0131] Computer programs for controlling delivery of reactant,
plasma processing and other processes in a process sequence can be
written in any conventional computer readable programming language:
for example, assembly language, C, C++, Pascal, Fortran or others.
Compiled object code or script is executed by the processor to
perform the tasks identified in the program.
[0132] The control module parameters relate to process conditions
such as, for example, filter pressure differentials, process gas
composition and flow rates, temperature, pressure, plasma
conditions such as RF power levels and the low frequency RF
frequency, cooling gas pressure, and chamber wall temperature.
[0133] The system software may be designed or configured in many
different ways. For example, various chamber component subroutines
or control objects may be written to control operation of the
chamber components necessary to carry out the inventive deposition
processes. Examples of programs or sections of programs for this
purpose include substrate positioning code, process gas control
code, pressure control code, heater control code, and plasma
control code.
[0134] Although the foregoing implementations have been described
in some detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the disclosed implementations. It should be
noted that there are many alternative ways of implementing the
processes, systems, and apparatus of the present implementations.
Accordingly, the present implementations are to be considered as
illustrative and not restrictive, and the implementations are not
to be limited to the details given herein.
* * * * *