U.S. patent application number 11/670797 was filed with the patent office on 2007-06-07 for method and apparatus for annealing copper films.
Invention is credited to B. Michelle Chen, Robin Cheung, Yezdi Dordi, Ratson Morad, Ho Seon Shin.
Application Number | 20070128869 11/670797 |
Document ID | / |
Family ID | 30003733 |
Filed Date | 2007-06-07 |
United States Patent
Application |
20070128869 |
Kind Code |
A1 |
Chen; B. Michelle ; et
al. |
June 7, 2007 |
METHOD AND APPARATUS FOR ANNEALING COPPER FILMS
Abstract
A method and apparatus for annealing copper. The method
comprises forming a copper layer by electroplating on a substrate
in an integrated processing system and annealing the copper layer
in a chamber inside the integrated processing system.
Inventors: |
Chen; B. Michelle; (San
Jose, CA) ; Shin; Ho Seon; (Mountain View, CA)
; Dordi; Yezdi; (Palo Alto, CA) ; Morad;
Ratson; (Palo Alto, CA) ; Cheung; Robin;
(Cupertino, CA) |
Correspondence
Address: |
PATTERSON & SHERIDAN, LLP
3040 POST OAK BOULEVARD, SUITE 1500
HOUSTON
TX
77056
US
|
Family ID: |
30003733 |
Appl. No.: |
11/670797 |
Filed: |
February 2, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10611589 |
Jun 30, 2003 |
7192494 |
|
|
11670797 |
Feb 2, 2007 |
|
|
|
09513734 |
Feb 18, 2000 |
|
|
|
10611589 |
Jun 30, 2003 |
|
|
|
09396007 |
Sep 15, 1999 |
6276072 |
|
|
09513734 |
Feb 18, 2000 |
|
|
|
09263126 |
Mar 5, 1999 |
6136163 |
|
|
09513734 |
Feb 18, 2000 |
|
|
|
Current U.S.
Class: |
438/687 ;
257/E21.582; 257/E21.583 |
Current CPC
Class: |
H01L 21/67098 20130101;
H01L 21/76838 20130101; H01L 21/76886 20130101; C25D 7/123
20130101; C25D 5/48 20130101; C22F 1/08 20130101; H01L 21/67109
20130101; H01L 21/6723 20130101; C22F 1/02 20130101; H01L 21/67103
20130101; H01L 21/7684 20130101; C25D 5/50 20130101 |
Class at
Publication: |
438/687 |
International
Class: |
H01L 21/44 20060101
H01L021/44 |
Claims
1. A method for annealing a copper layer, comprising: forming the
copper layer on a substrate by electroplating in a first chamber of
an integrated processing system; rinsing the substrate in a
cleaning station of the integrated processing system; treating the
copper layer in a gas environment in a second chamber of the
integrated processing system, wherein the gas environment comprises
nitrogen (N.sub.2) and hydrogen (H.sub.2); and bringing the
substrate in proximity to a cooling plate to cool the substrate to
a temperature below about 100.degree. C.
2. The method of claim 1, wherein the hydrogen is present at a
concentration of less than about 4% in the gas environment.
3. The method of claim 1, wherein the copper layer is treated for a
time duration less than about 5 minutes.
4. The method of claim 3, wherein the time duration is about 30
seconds to about 2 minutes.
5. The method of claim 1, wherein the copper layer is treated at a
temperature of between about 150 degrees Celsius to about 250
degrees Celsius.
6. The method of claim 1, wherein the gas environment comprises
less than about 100 parts per million of oxygen.
7. The method of claim 1, wherein the gas environment comprises a
pressure of 760 Torr.
8. The method of claim 1, wherein the substrate is cooled to a
temperature below about 80.degree. C.
9. The method of claim 1, wherein the substrate is cooled to a
temperature below about 50.degree. C.
10. The method of claim 1, wherein the substrate is cooled before
additional processing.
11. A method for annealing a copper layer, comprising: forming the
copper layer on a substrate by electroplating in a first chamber of
an integrated processing system; rinsing the substrate in a
cleaning station of the integrated processing system; treating the
copper layer in a gas environment in a second chamber of the
integrated processing system, wherein the gas environment comprises
hydrogen (H.sub.2) and a gas selected from the group consisting of
nitrogen (N.sub.2), argon (Ar), and helium (He); and bringing the
substrate in proximity to a cooling plate to cool the substrate to
a temperature below about 100.degree. C.
12. The method of claim 11, wherein the hydrogen is present at a
concentration of less than about 4% in the gas environment.
13. The method of claim 11, wherein the copper layer is treated for
a time duration less than about 5 minutes.
14. The method of claim 13, wherein the time duration is about 30
seconds to about 2 minutes.
15. The method of claim 11, wherein the copper layer is treated at
a temperature of between about 150 degrees Celsius to about 250
degrees Celsius.
16. The method of claim 11, wherein the gas environment comprises
less than about 100 parts per million of oxygen.
17. The method of claim 11, wherein the gas environment comprises a
pressure of 760 Torr.
18. The method of claim 11, wherein the substrate is cooled to a
temperature below about 80.degree. C.
19. The method of claim 11, wherein the substrate is cooled to a
temperature below about 50.degree. C.
20. The method of claim 11, wherein the substrate is cooled before
additional processing.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of co-pending U.S. patent
application Ser. No. 10/611,589 (APPM/001717.D2), filed Jun. 30,
2003, which application is a divisional of U.S. patent application
Ser. No. 09/513,734 (APPM/001717.P2), filed Feb. 18, 2000, which is
a continuation-in-part of commonly-assigned U.S. patent application
Ser. No. 09/396,007 (APPM/001717.P1), filed on Sep. 15, 1999, now
U.S. Pat. No. 6,276,072, and of U.S. patent application Ser. No.
09/263,126 (APPM/003421), filed on Mar. 5, 1999, now U.S. Pat. No.
6,136,163, all of which are herein incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a method and apparatus for metal
processing and, more particularly, to a method and apparatus for
depositing and annealing metal films.
[0004] 2. Background of the Related Art
[0005] Copper has gained increasing popularity as a metal
interconnect in advanced integrated circuit fabrication. Copper can
be deposited using electrochemical deposition from electrolytes
such as copper sulfate or from electroless processes. Typically,
electrolytes also contain carriers and additives to achieve certain
desired characteristics in electroplated films. Some copper films,
e.g., those deposited from electrolytes containing organic
additives, exhibit "self-annealing" or re-crystallization behavior.
For example, abnormal grain growth may occur in the as-deposited
film such that film properties such as resistivity, stress and
hardness may be adversely affected. The rate of grain growth may
depend on the electroplating recipe, electrolyte types, as well as
the organic additive concentrations.
[0006] These continual changes in microstructure at room
temperature may lead to formation of stress-induced voids, or
affect subsequent chemical mechanical polishing (CMP) behavior
because of varying polishing rates for the film. Therefore, thermal
annealing is usually performed on the as-deposited copper film to
stabilize the film by promoting grain growth prior to subsequent
processing.
[0007] Typically, copper films are annealed in a high temperature
furnace or using rapid thermal anneal processing, both of which
require relatively expensive and complex equipments. Furnace anneal
of electroplated copper films, for example, is a batch process that
is performed at an elevated temperature of typically about
400.degree. C., either under a vacuum or in a nitrogen environment
for at least about 30 minutes, which is a rather high thermal
budget, time-consuming and costly process.
[0008] Therefore, there is a need for a method and apparatus for
annealing copper that would allow film stabilization to be
performed at a relatively low operating temperature in a simple gas
environment with wide process windows, along with high throughput
and relatively low cost.
SUMMARY OF THE INVENTION
[0009] The present invention provides a method and apparatus for
annealing copper by forming a copper layer on a substrate in an
integrated processing system, and then treating the copper layer in
an annealing gas environment.
[0010] In one embodiment of the invention, the annealing process is
performed in-situ. The gas environment comprises a gas selected
from nitrogen, argon, helium, or other inert gases. Annealing is
performed at a temperature between about 100 and about 500.degree.
C., for a time duration of less than about 5 minutes. In another
embodiment, the annealing gas further comprises a
hydrogen-containing gas, e.g., hydrogen. In another aspect of the
invention, the annealing gas environment is controlled so that the
concentration of an oxidizing gas, e.g., oxygen, is less than about
100 parts per million.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The teachings of the present invention can be readily
understood by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0012] FIG. 1 depicts a schematic side elevational view of an
apparatus that is suitable for practicing the present
invention;
[0013] FIG. 2 depicts a top plan view of an integrated processing
system comprising the apparatus of FIG. 1;
[0014] FIG. 3 depicts a process sequence illustrating the method
steps of metal deposition and annealing;
[0015] FIGS. 4a-c depict cross-sectional views of a substrate
undergoing various stages of metal processing. FIG. 4a depicts a
substrate with an insulating layer and a barrier layer prior to
copper deposition. FIG. 4b depicts the substrate with a seed layer
and a layer of electroplated copper. FIG. 4c depicts the layer of
electroplated copper under an annealing gas environment;
[0016] FIG. 5 depicts a plot of the sheet resistance change of a
copper layer as a function of anneal temperature for different
annealing times;
[0017] FIG. 6 depicts a plot of the reflectivity change of a copper
layer as a function of anneal temperature for different annealing
times;
[0018] FIG. 7 depicts the sheet resistance and reflectivity changes
as a function of hydrogen content in an annealing gas environment;
and
[0019] FIG. 8 depicts a plot of the chemical mechanical polishing
rate as a function of anneal temperature for different annealing
times.
[0020] To facilitate understanding, identical reference numerals
have been used, where possible, to designate identical elements
that are common to the figures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0021] The method of the present invention performs annealing of a
copper layer by exposing the copper layer to an annealing gas
environment at an elevated temperature.
[0022] In one embodiment of the invention, the annealing is
performed in-situ--i.e., within the same apparatus as that used for
depositing the copper layer. The annealing gas environment
comprises a gas selected from nitrogen (N.sub.2), argon (Ar) or
helium (He), or other inert gases. Annealing is performed at a
temperature between about 100 and about 500.degree. C. for a time
duration less than about 5 minutes. In another embodiment, the
annealing gas environment further comprises hydrogen (H.sub.2),
preferably a mixture of less than about 5% of H.sub.2 in N.sub.2 or
other inert gas. In another aspect of the invention, the annealing
gas environment preferably has an oxygen concentration of less than
about 100 parts per million (ppm), more preferably less than about
30 ppm. By exposing the copper layer to the annealing gas
environment within a short time, e.g., less than about five
minutes, at an annealing temperature between about 100 and about
500.degree. C., the microstructure of the copper layer can be
stabilized and a reduced film resistivity and/or enhanced
reflectivity of the copper layer can be achieved.
[0023] Apparatus
[0024] FIG. 1 is a schematic representation of an apparatus 100
that is suitable for practicing the present invention. The
apparatus 100 comprises a process chamber 102 and a controller 180
connected to various hardware components (e.g., wafer handling
robot 170, isolation valve 172 and mass flow controller 174, among
others).
[0025] A detailed description of the chamber 102 has been disclosed
in commonly-assigned U.S. patent application, entitled "Method and
Apparatus for Heating and Cooling Substrates," Ser. No. 09/396,007,
filed on Sep. 15, 1999, and is incorporated herein by reference. A
brief description of the apparatus 100 is given below.
[0026] The apparatus 100 allows for rapid heating and cooling of a
substrate within a single chamber 102, which comprises a heating
mechanism, a cooling mechanism and a transfer mechanism to transfer
a substrate 190 between the heating and the cooling mechanisms. As
shown in the embodiment of FIG. 1, the heating mechanism comprises
a heated substrate support 104 having a resistive heating element
106, and the cooling mechanism comprises a cooling fluid source 176
connected to a cooling plate 108 disposed at a distance apart from
the heated substrate support 104. The transfer mechanism is, for
example, a wafer lift hoop 110 having a plurality of fingers 112,
which is used to transfer a substrate from a position proximate the
heated substrate support 104 to a position proximate the cooling
plate 108. A vacuum pump 178 and an isolation valve 172 are
connected to an outlet 122 of the chamber 102 for evacuation and
control of gas flow out of the chamber 102.
[0027] To perform copper annealing, the substrate 190 is placed on
the heated substrate support 104, which is preheated to a
temperature between about 100.degree. C. and about 500.degree. C. A
gas source 120 allows an annealing gas mixture to enter the chamber
102 via the gas inlet 124 and the mass flow controller 174. The
substrate 190 having a deposited copper layer is then heated under
the annealing gas environment for a sufficiently long time to
obtain the desired film characteristics. For example, the copper
layer may be annealed to achieve a desirable grain growth
condition, a reduction in sheet resistance, or an increase in film
reflectivity.
[0028] After annealing, the substrate 190 is optionally cooled to a
desirable temperature, e.g., below about 100.degree. C., preferably
below about 80.degree. C., and most preferably below about
50.degree. C., within the chamber 102. This can be accomplished,
for example, by bringing the substrate 190 in close proximity to
the cooling plate 108 using the wafer lift hoop 110. For example,
the cooling plate 108 may be maintained at a temperature of about 5
to about 25.degree. C. by a cooling fluid supplied from the cooling
fluid source 176.
[0029] As illustrated in FIG. 1, the chamber 102 is also coupled to
a controller 180, which controls the chamber 102 for implementing
the annealing method of the present invention. Illustratively, the
controller 180 comprises a general purpose computer or a central
processing unit (CPU) 182, support circuitry 184, and memories 186
containing associated control software. The controller 180 is
responsible for automated control of the numerous steps required
for wafer processing--such as wafer transport, gas flow control,
temperature control, chamber evacuation, and so on. Bi-directional
communications between the controller 180 and the various
components of the apparatus 100 are handled through numerous signal
cables collectively referred to as signal buses 188, some of which
are illustrated in FIG. 1.
[0030] In general, the annealing chamber 102 may be used as a stand
alone system for thermal annealing or wafer cooling. Alternatively,
the chamber 102 may be used as part of a cluster tool or an
integrated processing system having multiple process chambers
associated therewith. As illustrated in FIG. 2, for example, an
integrated processing system 200 comprises several process chambers
such as metal deposition chambers 202a, 202b, 202c and 202d,
cleaning stations 204a and 204b, two annealing chambers 102 and a
loading station 206. A robot 208 is provided for wafer transfer and
handling. Details of an integrated processing system have been
disclosed in commonly assigned U.S. patent application Ser. No.
09/263,126, entitled "Apparatus for Electrochemical Deposition of
Copper Metallization with the Capability of In-Situ Thermal
Annealing," filed on Mar. 5, 1999, which is herein incorporated by
reference. An Electra Cu.TM. Integrated ECP system is one example
of such an integrated processing system 200, and is commercially
available from Applied Materials, Inc., of Santa Clara, Calif.
[0031] During integrated processing, a copper layer is formed on a
substrate 190 in one of the processing chambers 202a-d using
electroplating or other deposition techniques. After suitable
cleaning processes inside the cleaning station 204 or 206, the
substrate 190 having the deposited copper layer is transferred to
the annealing chamber 102 by a transfer mechanism such as a robot
170. Thus, the integrated processing system allows in-situ
annealing of the copper layer--i.e., annealing the deposited copper
layer without removing the substrate from the system. One advantage
of such in-situ processing is that the time delay between the
cleaning and annealing steps can be kept relatively short, e.g., to
about a few seconds. Therefore, undesirable oxidation of the
deposited copper layer can be minimized. A controller (not shown)
is also used to control the operation of the integrated processing
system 200 in a manner similar to that previously described for the
annealing chamber 102.
[0032] Process
[0033] FIG. 3 depicts a process sequence 300 for annealing a copper
layer according to one embodiment of the invention. This process
sequence 300, for example, may be implemented by the integrated
system 200 when the controller (not shown) executes a software
program embodying the appropriate program code. In step 301, a
copper layer is formed on a substrate using a copper deposition
technique--e.g., electroplating or an electroless process. The
invention will be described with reference to a copper film formed
by electroplating. It should be understood, however, that the
invention can also be used to anneal copper films formed by other
deposition techniques. Characteristics of the copper layer may vary
with the copper deposition process and thus affect process
conditions to be employed, for example, in subsequent annealing of
the copper layer. After the copper deposition step 301, the
substrate undergoes appropriate cleaning in step 303, e.g., through
a de-ionized water rinse and dry procedure.
[0034] According to the present invention, the copper layer is then
annealed in step 305 in an annealing gas environment at an elevated
temperature, e.g., between about 100 and about 500 C. In one
embodiment, the annealing gas environment comprises a gas selected
from nitrogen, argon and helium. In general, these and other inert
gases may be used either singly or in combinations to form the
annealing gas environment. In another embodiment, the annealing gas
environment further comprises a hydrogen-containing gas, preferably
hydrogen (H.sub.2). Alternatively, other hydrogen-containing gases,
e.g., ammonia (NH.sub.3), may also be used. A total gas flow of up
to about 50 standard liters per minute (slm) and a pressure of up
to about 1000 Torr may be used. In general, the process window for
the operating pressure is relatively wide--e.g., in one embodiment,
about 760 to about 1000 Torr may be used. In addition, annealing
may also be performed under a reduced pressure condition. The
annealing step 305 results in a decrease in sheet resistance of the
copper layer. Additionally, reflectivity of the copper layer may
also be increased through the annealing step 305.
[0035] After annealing, the substrate may be subjected to a cooling
step 307, e.g., for about 30 seconds, to cool the substrate to a
temperature below about 100 C, preferably below about 80 C, and
most preferably below about 50 C, before additional processing.
Using the chamber 102, for example, annealing and cooling of the
substrate can be performed in a single chamber within the
integrated processing system. In general, the cooling step 307
serves several purposes, such as preventing oxidation of the copper
layer when the substrate is exposed to ambient air, and providing a
suitable temperature for wafer handling and reliable system
operation. It has been found that there is no noticeable oxidation
in a copper layer treated under the annealing gas environment of
the present invention when the substrate is exposed to ambient air
at below about 100 C.
[0036] FIGS. 4a-c illustrate schematic cross-sectional views of a
substrate structure 450 undergoing various stages of processing
according to the process sequence 300. FIG. 4a shows an exemplary
substrate structure 450 prior to copper deposition, comprising a
patterned insulating layer 404 formed on an underlying layer 402.
Depending on whether the substrate structure 450 is a contact, via
or trench, the underlying layer 402 may comprise, for example,
silicon, polysilicon, silicide, copper, tungsten or aluminum, among
others. The insulating layer 404 may be an oxide layer that has
been patterned by conventional lithographic and etching techniques
to form a contact, via or trench 406. A barrier layer 408
comprising a conducting material (e.g., titanium, titanium nitride,
tantalum, or tantalum nitride, among others) is formed over the
insulating layer 404 and inside the via 406 using conventional
techniques such as physical vapor deposition (PVD) or CVD. The
barrier layer 408 has a typical thickness up to about 500 .ANG.,
and preferably about 250 .ANG..
[0037] When electroplated copper is to be used to form the metal
interconnect, a relatively thin seed layer of metal 410, preferably
copper, is vapor deposited over the barrier layer 408, as shown in
FIG. 4b. Typically, the seed layer of metal 410 has a thickness of
up to about 3000 .ANG., e.g., about 2000 .ANG., and may be
deposited using IMP (ionized metal plasma) physical sputtering. In
one example, a bulk layer of copper 412 having a thickness up to
about 2 .mu.m, is then formed over the seed layer 410 by
electroplating, for example, using the electroplating system. The
electroplated copper layer 412 can be used to fill contacts,
trenches or vias having widths of about 0.25 .mu.m or smaller, or
aspect ratios of at least about 2:1.
[0038] According to the present invention, the deposited copper
layer 412 is then subjected to an annealing step under an annealing
gas environment 414, as shown in FIG. 4c. The annealing gas
environment 414 comprises a gas selected from N.sub.2, Ar or He,
among others. These and other inert gases may also be used singly
or in combinations to form the annealing gas environment 414. In
another embodiment, the annealing gas environment 414 further
comprises a hydrogen-containing gas such as hydrogen (H.sub.2).
Other hydrogen-containing gases, e.g., ammonia (NH.sub.3) may also
be used. In one embodiment, H.sub.2 is present at a concentration
of less than about 5%, preferably between about 0.5 to about 4%. In
principle, a higher concentration of H.sub.2 (i.e., over 5%) may be
used for effective treatment of the deposited copper layer, but it
is not necessary. A low H.sub.2 concentration mixture is preferred
because of the reduced manufacturing cost. Thus, there is an
incentive to provide an annealing mixture with as low a H.sub.2
concentration as possible. For example, in one embodiment, a
mixture of H.sub.2 and nitrogen (N.sub.2) is used with a H.sub.2
concentration of less than about 4%. Aside from N.sub.2, inert
gases such as argon (Ar), helium (He), among others, may also be
used. A total gas flow of up to about 50 slm and a pressure of up
to about 1000 Torr, e.g., between about 760 to about 1000 Torr, may
be used.
[0039] In another embodiment, the annealing gas environment 414 is
also controlled to contain at most a low level of an oxidizing gas
such as oxygen (O.sub.2), in order to avoid oxidation of the copper
layer 412. If the annealing gas environment contains only nitrogen
or an inert gas, but does not contain H.sub.2, then the level of
O.sub.2 is preferably controlled to be less than about 30 ppm,
preferably less than about 10 ppm, and most preferably, less than
about 5 ppm. On the other hand, if H.sub.2 is present in the
annealing gas environment 414, the level of O.sub.2 that can be
tolerated may be higher, e.g., less than about 100 ppm, due to the
reducing effect of H.sub.2 which minimizes oxidation of the copper
layer. It is preferable that the O.sub.2 level be controlled to
less than about 30 ppm, more preferably less than about 10 ppm, and
most preferably, less than about 5 ppm.
[0040] In general, the copper layer 412 is annealed for a time
duration of less than about 5 minutes, at a temperature of between
about 100 to about 500 C. The specific annealing time may depend on
the nature and thickness of the as-deposited copper layer 412 and
the temperature of the substrate structure 450. For example,
previous self-annealing studies conducted at room temperature show
that to achieve stabilization of a 1 micron copper film, annealing
has to be performed for a sufficiently long time to result in a
sheet resistance change (i.e., decrease) of about 18-20%. From a
manufacturing point of view, a shorter annealing time is preferable
because it contributes to a higher process throughput. However, the
optimal choice will depend on a proper balance of other process
considerations--e.g., for certain applications, thermal budget
concerns may suggest the use of a lower temperature along with a
slightly longer treatment time. Thus, according to one embodiment
of the invention, annealing is performed for a time duration
between about 30 seconds to about 2 minutes, at a temperature
between about 150 to about 400 C. In one preferred embodiment, the
annealing time is about 30 seconds at a temperature of about 250
C.
[0041] Sheet Resistance
[0042] FIG. 5 shows a plot of the percentage of sheet resistance
(Rs) decrease of a metal film stack as a function of the annealing
temperature for various annealing times. The metal film stack
comprises a 1 micron layer of electroplated copper formed upon a
2000 .ANG. copper seed layer (formed by IMP PVD deposition) over a
250 .ANG. thick tantalum nitride (TaN) film. For example, when the
film is annealed at a temperature of about 150 C, a decrease in
sheet resistance of about 8% is achieved after 30 seconds of
annealing. After 60 seconds of annealing, the sheet resistance is
reduced by about 17% and by about 20% after about 120 seconds of
anneal.
[0043] The data in FIG. 5 suggests that film stabilization and
reduction in sheet resistance can be achieved rapidly--e.g., in
less than about 2 minutes, if annealing is performed at a
temperature greater than about 150 C. It is believed that the sheet
resistance may be improved by annealing at a temperature between
about 100 C and about 500 C, and preferably, between about 150 C
and 400 C, for a time duration between about 30 sec. to about 120
sec. For higher annealing temperatures, e.g., at about 200 C or
higher, a shorter annealing time such as 15 sec. may suffice. A
copper layer annealed according to the embodiments of the invention
can achieve a resistivity of below about 1.8 .mu.ohm-cm. For
example, based on an extended run of about 2000 wafers, it is found
that the annealed copper layer has a resistivity approaching the
theoretical bulk resistivity of copper, which is about 1.7
.mu.ohm-cm. Furthermore, the wafer to wafer sheet resistance
uniformity is also improved to about 0.6%, compared to about 1.7%
obtained for as-deposited films.
[0044] Reflectivity
[0045] Aside from sheet resistance, the reflectivity of the
electroplated layer is also another factor for evaluating the
annealing process. In general, the reflectivity of a copper layer
may be affected by the copper grain size (affecting surface
roughness) and the composition of the copper surface. FIG. 6
illustrates the percent reflectivity change for a film stack as a
function of anneal temperature for different annealing times. The
reflectivity is measured at a wavelength of 480 nm for a film stack
comprising a 10,000 .ANG. electroplated copper layer over a 2000
.ANG. sputtered copper film that has been deposited upon a 250
.ANG. of tantalum nitride (TaN) over a 10,000 .ANG. silicon oxide
layer. In this embodiment, the annealing gas environment comprises
a mixture of about 4% H.sub.2 in N.sub.2. Preferably, the annealing
gas has an oxygen content of less than about 30 ppm, more
preferably less than about 10 ppm, and most preferably less than
about 5 ppm.
[0046] In general, the reflectivity of the Cu layer increases with
increasing annealing time and temperature. For example, at an
annealing temperature of about 200 C, the film reflectivity
improves--i.e., has a positive reflectivity change, after annealing
for about 60 seconds. When the temperature is increased to about
250 C, the electroplated copper layer achieves a maximum of about
15% improvement in reflectivity after being annealed for about 30
seconds. However, no additional improvement is obtained even at a
longer annealing time of about 60 seconds. At about 300 C or
higher, the anneal time has little impact on reflectivity, and only
about 15 seconds of annealing is needed to achieve the maximum
reflectivity change of about 15%.
[0047] Thus, in order to improve reflectivity of a copper layer
according to one embodiment of the present invention, copper
annealing is preferably performed at a temperature of at least
about 200 C, and preferably at least about 250 C. A copper layer
annealed according to the embodiments of the invention can achieve
a reflectivity of about 1.35 times that of a silicon reference,
exceeding the typical customer requirement of about 1.2. In
addition, it is found that the annealed copper layer can retain a
high reflectivity without degradation after being exposed to
ambient air environment.
[0048] Effect of H.sub.2 Content
[0049] The annealing effect is further investigated as a function
of the hydrogen content in the chamber environment. This is
illustrated in FIG. 7, which shows the changes in sheet resistance
and film reflectivity as a function of H.sub.2 concentration up to
about 4%. At an annealing temperature of about 300 C, the sheet
resistance decrease of about 18-20% is achieved, independent of the
H.sub.2 concentration. This suggests that the sheet resistance
decrease in this case is primarily a temperature-dependent
effect.
[0050] On the other hand, the reflectivity change depends on the
H.sub.2 content--e.g., at a H.sub.2 concentration of about 0.5% or
higher, a 15% improvement in film reflectivity is achieved. It is
believed that reflectivity is increased partly because H.sub.2 gas
is effective in minimizing oxide formation on the surface of the
copper layer. It is possible that, by adjusting other process
parameters such as temperature, pressure, annealing time and gas
environment, a lower H.sub.2 concentration, e.g., below about 0.5%,
may also be effective in improving reflectivity of the copper
layer.
[0051] Annealing according to the embodiments of the invention
result in an increase of the grain size in the electroplated
copper, as well as a decrease in film hardness. The final copper
grain sizes achieved according to embodiments of the invention are
comparable to those obtained from a conventional furnace anneal.
Furthermore, comparable grain size and sheet resistance results can
be achieved using in-situ annealing either at about 250 C for 30
seconds or at about 350 C for 60 seconds. Annealing according to
certain embodiments of the invention, e.g., at temperatures greater
than about 200 C, also result in a decrease of film hardness by
about 50% (e.g., to about 150 Hv), compared to the as-deposited
value of about 300 Hv.
[0052] After in-situ annealing, the copper layer 412 may be cooled
to a temperature below about 100 C, preferably below about 80 C,
and most preferably below about 50 C, prior to subsequent
processing such as chemical mechanical polishing (CMP). Since the
annealed copper layer 412 has larger grains and reduced film
hardness, the CMP removal rate of the annealed copper is increased
compared to that of non-annealed copper. FIG. 8 illustrates results
of the CMP removal rate for an electroplated copper layer annealed
according to embodiments of the invention at different in-situ
anneal temperatures. Although the CMP removal rate increases after
annealing for about 30 to about 120 seconds at about 100.degree.
C., stabilization of the CMP removal rate is preferably achieved by
annealing at temperatures greater than about 200 C. For example,
the CMP removal rate increases by nearly 42%, or greater than about
40%, for in-situ annealing at about 200 to about 400 C. There is
relatively little fluctuation in the CMP removal rate for annealing
at temperatures above 200 C. Furthermore, the CMP removal rate of
the annealed copper remains constant with time because of the
stabilization of the copper layer 412 according to the embodiments
of the invention. By preventing oxidation of the copper layer 412,
contamination of the slurry during CMP can also be avoided. Thus,
CMP can be performed with improved process reproducibility.
[0053] In general, annealing according to embodiments of the
invention results in copper layers with improved characteristics
such as microstructure stability, enhanced reflectivity and reduced
resistivity. Typically, the resistivity of the fully annealed
copper layer approaches that of the bulk resistivity of copper.
Thus, the invention is an attractive alternative to conventional
furnace or RTP annealing techniques, because it provides an
annealing method with wide process margins at relatively low cost
and high throughput.
[0054] Although several preferred embodiments which incorporate the
teachings of the present invention have been shown and described in
detail, those skilled in the art can readily devise many other
varied embodiments that still incorporate these teachings.
* * * * *