U.S. patent application number 10/423185 was filed with the patent office on 2004-11-18 for low-temperature etching environment.
Invention is credited to Dutton, David, Seaward, Karen L..
Application Number | 20040226911 10/423185 |
Document ID | / |
Family ID | 33415864 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040226911 |
Kind Code |
A1 |
Dutton, David ; et
al. |
November 18, 2004 |
Low-temperature etching environment
Abstract
A low-temperature etching environment comprising a halogen and
an inert gas in a ratio that does not induce the formation of an
etch-limiting surface reaction layer during etching in the
low-temperature etching environment. The surface temperature of a
material being etched in the low-temperature environment is below
that which would melt a photoresist material that has not been
treated to increase its glass-reflow temperature.
Inventors: |
Dutton, David; (San Jose,
CA) ; Seaward, Karen L.; (Palo Alto, CA) |
Correspondence
Address: |
AGILENT TECHNOLOGIES, INC.
Legal Department, DL429
Intellectual Property Administration
P.O. Box 7599
Loveland
CO
80537-0599
US
|
Family ID: |
33415864 |
Appl. No.: |
10/423185 |
Filed: |
April 24, 2003 |
Current U.S.
Class: |
216/58 ;
156/345.29; 257/E21.222 |
Current CPC
Class: |
H01L 21/30621 20130101;
C23F 4/00 20130101 |
Class at
Publication: |
216/058 ;
156/345.29 |
International
Class: |
C23F 001/00 |
Claims
We claim:
1. A low-temperature etching environment, comprising: a halogen;
and an inert gas; wherein said halogen and said inert gas are
provided in a ratio that does not induce the formation of an
etch-limiting surface reaction layer during etching in said
low-temperature etching environment.
2. The low-temperature etching environment of claim 1, wherein the
temperature of a surface of a material being etched in said
low-temperature etching environment below that which would melt a
photoresist material that has not been treated to increase its
glass-reflow temperature.
3. The low-temperature etching environment of claim 1, wherein said
halogen is selected from fluorine, chlorine, bromine, iodine and
compounds comprising fluorine, chlorine, bromine, or iodine.
4. The low-temperature etching environment of claim 1, wherein said
inert gas is selected from helium, neon, argon, krypton, and
xenon.
5. The low-temperature etching environment of claim 1, wherein said
halogen comprises chlorine.
6. The low-temperature etching environment of claim 1, wherein said
inert gas comprises argon.
7. The low-temperature etching environment of claim 1, wherein said
ratio of said halogen and said inert gas is achieved by a flow rate
of said halogen less than 20 percent of the combined flow rate of
said halogen and said inert gas.
8. A method of etching a surface reaction layer limited material,
the method comprising: a) receiving said surface reaction layer
limited material in a low-temperature etching environment
comprising: a halogen and an inert gas in a ratio that does not
induce the formation of an etch-limiting surface reaction layer
during etching in said low-temperature etching environment; and b)
etching said surface reaction layer limited material within said
low-temperature etching environment.
9. The method of claim 8, wherein said surface reaction layer
limited material comprises a semiconductor.
10. The method of claim 8, wherein said surface reaction layer
limited material comprises indium.
11. The method of claim 8, wherein said surface reaction layer
limited material comprises copper.
12. The method of claim 8, wherein said halogen comprises
chlorine.
13. The method of claim 8, wherein said inert gas comprises
argon.
14. The method of claim 8, wherein the temperature of said
low-temperature etching environment is below that which would melt
a photoresist material that has not been treated to increase its
glass-reflow temperature.
15. A method for inhibiting formation of a surface reaction layer
formed in a low-temperature etching environment, said method
comprising: introducing an inert gas into said low-temperature
etching environment; introducing a halogen into said
low-temperature etching environment in a ratio to said inert gas
that does not induce the formation of an etch-limiting surface
reaction layer during etching in said low-temperature etching
environment.
16. The method of claim 15, wherein the temperature of said
low-temperature etching environment is below that which would melt
a photoresist material that has not been treated to increase its
glass-reflow temperature.
17. The method of claim 15, wherein said halogen is selected from
fluorine, chlorine, bromine, iodine and compounds comprising
fluorine, chlorine, bromine, or iodine.
18. The method of claim 15, wherein said inert gas is selected from
helium, neon, argon, krypton, and xenon.
19. The method of claim 15, wherein said halogen comprises
chlorine.
20. The method of claim 15, wherein said inert gas comprises
argon.
21. The method of claim 15, wherein said ratio of said halogen to
said inert gas is achieved by a flow rate of said halogen less than
20 percent of the combined flow rate of said halogen and said inert
gas.
22. The method of claim 15, wherein said low-temperature etching
environment comprises plasma etching of sufficient ion density to
physically minimize the surface reaction layer as it is being
formed.
Description
TECHNICAL FIELD
[0001] Embodiments of the present invention relate to the field of
forming microelectronic devices. Specifically, embodiments of the
present invention relate to a low-temperature etching environment
comprising a halogen and an inert gas.
Background Art
[0002] Conventional high density plasma etching of materials such
as indium comprising compounds is subject to a tradeoff between
etch rate and compatibility with standard photoresist masks and
process steps. It is possible to achieve a high etch rate if the
surface temperature of the material being etched is high enough.
Unfortunately, to achieve a high etch rate with conventional
techniques, the surface temperature of the material being etched
must be greater than the glass-reflow point of standard photoresist
materials. The glass-reflow point of standard photoresist material
is about 110-120 degrees Celsius. The high temperature can be
achieved by either raising the temperature of the substrate of the
material being etched or by allowing the high-density plasma source
to heat the surface of the material being etched. Consequently and
undesirably, non-standard photoresists and/or process steps must be
used when etching in the high temperatures conventionally needed
for a high etch rate.
[0003] FIG. 1 is a graph 100 of a surface temperature versus time
curve 110 and a etch rate versus time curve 120. Graph 100
illustrates the effect that surface temperature has on etch rate in
one conventional etching system. FIG. 1 shows the surface
temperature dependent etch rate for indium phosphide (InP) when 100
watts (W) of microwave power were used. The conventional etch
environment corresponding to FIG. 1 further includes a
chlorine/argon (Cl.sub.2/Ar) etchant with a flow rate of ten
standard cubic centimeters per minute (sccm) for both the chlorine
and the argon. Thus, the chlorine flow rate is 50 percent the
combined flow rate of chlorine and argon. The pressure in the etch
environment was 0.27 pascals (Pa).
[0004] Referring again to FIG. 1, the surface temperature versus
time curve 110 shows that the surface temperature rose
significantly during the etching and after about four minutes
approached 150 degrees Celsius. The etch rate is about 100
nanometers per minute (nm/min) when the surface temperature is
below 100 degrees Celsius. Etch rates as high as 2.4 micrometers
per minute (.mu.m/min) are shown for higher surface temperatures.
However, the surface temperature must be close to 150 degrees
Celsius for the etch rate to be significantly greater than 100
nm/min. More precisely, the etch rate is approximately 200 nm/min
when the temperature is approximately 140 degrees Celsius, and the
etch rate is approximately 450 nm/min when the temperature 150
degrees Celsius. Thus, in this conventional etch process, the
surface temperature of the material being etched must be well above
the glass-reflow point of standard photoresist materials to achieve
a high etch rate.
[0005] While etching at a higher surface temperature achieves a
higher etch rate, such higher surface temperatures are incompatible
with standard photoresist masks and processes and render the
photoresist mask difficult to remove. Hence, to etch in a high
temperature environment, a hard mask such as silicon dioxide
(SiO.sub.2) or a silicon-nitrogen-hydrogen compound (SiNH.sub.x) is
conventionally used. Alternatively, a standard photoresist mask
having a low glass-reflow temperature can be pre-processed such
that it will not melt at the higher surface temperature that is
conventionally needed for a high etch rate.
[0006] When migrating to a new technology that etches a new
material it is desirable to continue to use the same process steps
that were used when etching a material for a previous technology.
However, using a hard mask or pre-processing a standard mask is
incompatible with the process steps of the previous technology.
Moreover, removing a hard mask is more difficult than removing a
standard photoresist mask and the changes to a standard photoresist
due to the pre-processing cause the photoresist mask to be
difficult to remove after etching.
[0007] Thus, one problem with conventional etching methods is that
to achieve a high etch rate the surface temperature of the material
being etched must be undesirably high. A further problem is the
difficulty realized in incorporating the photoresist into original
processing steps when migrating to a new material being etched. A
still further problem is the difficulty in removing a pre-processed
photoresist once it has been processed to withstand the high
temperature conventionally needed for a high etch rate.
Alternatively, the surface temperature of the material being etched
can be kept below the glass-reflow point of the photoresist mask;
however, conventional low-temperature etching methods have a very
low etch rate.
DISCLOSURE OF THE INVENTION
[0008] The present invention pertains to a low-temperature etching
environment. An embodiment in accordance with the present invention
provides a low-temperature etching environment comprising a halogen
and an inert gas in a ratio that does not induce the formation of
an etch-limiting surface reaction layer during etching in the
low-temperature etching environment. The surface temperature of a
material being etched in the low-temperature environment is below
that which would melt a photoresist material that has not been
treated to increase its glass-reflow temperature.
[0009] Another embodiment in accordance with the present invention
is a method of etching a surface reaction layer limited material.
The method comprises receiving the surface reaction layer limited
material in a low-temperature etching environment that comprises a
halogen and an inert gas in a ratio that does not induce the
formation of an etch-limiting surface reaction layer during etching
in the low-temperature etching environment. The method also
comprises etching the surface reaction layer limited material
within the low-temperature etching environment.
[0010] Various embodiments in accordance with the present invention
achieve a high etch rate with a low surface temperature of the
material being etched. Embodiments do not require inconvenient
pre-processing steps to increase the glass-reflow temperature of
the photoresist. Embodiments do not require the use of a
process-incompatible hard photoresist mask. Embodiments allow the
use of a standard photoresist with a low glass-reflow temperature.
Thus, embodiments provide for easy removal of the photoresist
mask.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and
form a part of this specification, illustrate embodiments of the
invention and, together with the description, serve to explain the
principles of the invention:
[0012] FIG. 1 is graph illustrating the effect of temperature on
the rate of etching an indium-comprising material.
[0013] FIG. 2 is an exemplary device for producing a
low-temperature etching environment in accordance with embodiments
of the present invention.
[0014] FIG. 3A is a graph illustrating etch rates achieved in
accordance with embodiments of the present invention.
[0015] FIG. 3B is a graph illustrating etch rates achieved in
accordance with embodiments of the present invention.
[0016] FIG. 4A is a graph illustrating resist selectivity achieved
in accordance with embodiments of the present invention.
[0017] FIG. 4B is a graph illustrating resist selectivity achieved
in accordance with embodiments of the present invention.
[0018] FIG. 5 is a flowchart illustrating a process of etching a
surface reaction layer limited material, according to an embodiment
of the present invention.
[0019] FIG. 6 is a flowchart illustrating a process of inhibiting
formation of a surface reaction layer in a low-temperature etching
environment, according to an embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] During etching of some materials, a surface reaction layer
forms on the surface of the material being etched. The surface
reaction layer has a low volatility at low temperatures such that
it accumulates to a thickness that limits the etch rate. For
purposes of the present invention, the term "low-temperature" means
a temperature below which would melt a standard photoresist mask
that has not been specially treated to withstand higher
temperatures. For example, standard photoresist material has a
glass-reflow point of about 110-120 degrees Celsius. The surface
reaction layer is believed to accumulate to a thickness typically
on the order of nanometers, which is sufficient to limit the etch
rate.
[0021] For example, when etching a material comprising InP, the low
etch rate at low temperatures is believed to be due to the
formation of a surface reaction layer of a compound of an indium
and chlorine (InCl.sub.x) layer on the InP surface. Conventionally,
to achieve a high etch rate, the surface of the material being
etched is raised to a high temperature. The higher etch rates at
higher temperatures are believed to be due to the increased
volatility of InCl.sub.x at the higher temperatures. Hence, higher
temperatures reduce the thickness of the surface reaction layer.
This surface reaction layer is also referred to in the art as a
selvedge layer.
[0022] The temperature to which the surface is heated to achieve a
high etch rate will melt a photoresist material that has not been
treated to increase its glass-reflow temperature. Embodiments in
accordance with the present invention inhibit the formation of the
surface reaction layer by providing a halogen and an inert gas at a
ratio that inhibits the formation of the surface reaction layer. As
a result, the etch rate is higher than that obtained using
conventional etching techniques in a low-temperature
environment.
[0023] Embodiments according to the present invention are suited to
etching any material whose etching is limited by the formation of a
surface reaction layer. For purposes of the present application,
the term "surface reaction layer limited material" includes any
material for which etching is adversely affected by the formation
of a surface reaction layer. InP is used herein as an example of
one such surface reaction layer limited material. However, the
present invention is not limited solely to the etching of
indium-phosphide (InP). Furthermore, chlorine is used as an example
of one etchant that contributes to the formation of the surface
reaction layer. However, the present invention is not limited to
chlorine being the contributor to the surface reaction layer.
[0024] Embodiments according to the present invention provide a
low-temperature etching environment comprising a halogen and an
inert gas. The etching environment comprising the halogen and the
inert gas inhibits the formation of an etch-limiting surface
reaction layer during etching in the low-temperature etching
environment. FIG. 2 is a side sectional view of an exemplary device
200 for creating such a low-temperature etching environment. The
exemplary device 200 is an inductively-coupled plasma reactor.
However, the present invention is not limited to
inductively-coupled plasma reactors. The exemplary device 200
comprises a power supply 202 coupled to an induction coil 204. Also
included is a dielectric window 205. The exemplary device 200 also
has a wafer chuck 208 coupled to a bias power supply 210.
Furthermore, a wafer 212 is shown on the wafer chuck 208. The
exemplary device 200 generates the low-temperature etching
environment comprising a halogen and an inert gas (halogen and
inert gas not depicted), which is introduced into and passed
through the etching environment. The present invention is well
suited to other plasma etch environments than the
inductively-coupled plasma environment of FIG. 2. Such systems
require a sufficiently high plasma density to physically minimize
the surface reaction layer as it is being formed.
[0025] Embodiments according to the present invention introduce a
halogen and an inert gas into the etching environment in a ratio
that inhibits formation of an etch-limiting surface reaction layer.
The inert gas serves as a diluent that reduces the concentration of
the halogen. FIG. 3A is a graph 300 illustrating etch rate achieved
in accordance with embodiments of the present invention. Results
are shown for etching four different materials, each comprising
indium. The graph 300 shows an InP etch rate curve 310, an
indium-gallium-arsenide (InGaAs) etch rate curve 320, an
indium-aluminum-arsenide (InAlAs) etch rate curve 330, and an
aluminum-indium-gallium-arsenide (AlInGaAs) etch rate curve 340.
The results show that the etch rate for each material is a function
of the concentration of the halogen to the inert gas. In this
example, chlorine and argon were used for the halogen and the inert
gas, respectively. The etch rates were achieved under the following
conditions. The process pressure was 0.27 Pa; the
inductively-coupled power (ICP) was 1000 W; the reactive ion
etching (RIE) power, which is also known as wafer chuck power, was
55 W; the chuck temperature was -5 Celsius; the total gas flow was
15 sccm; and the etch time was three minutes.
[0026] Each curve (310, 320, 330, 340) in FIG. 3A shows an initial
increase in etch rate when the chlorine concentration increases
above zero percent. The etch rate for each curve peaks when the
chlorine concentration is below 20 percent. The chlorine
concentration is defined as the flow rate of chlorine divided by
the combined flow rate of chlorine and argon. The data points of
the various curves between about 20 percent and 40 percent chlorine
concentration show that the etch rate decreases for each material
as the chlorine concentration is further increased. The data points
of the various curves between 60-80 percent chlorine concentration
show a slight increase in etch rate compared to a chlorine
concentration of about 40 percent. However, the etch rates for the
various curves at this chlorine concentration are well below the
peak etch rates which occur at a chlorine concentration of less
than 20 percent.
[0027] Conventionally, a high chlorine concentration is used based
on the belief that a higher chlorine concentration will provide a
higher etch rate. This belief is based on analysis of the portion
of the curves with greater than 40 percent chlorine concentration
without an awareness of the portion of the curves with a chlorine
concentration below 20 percent. Moreover, using a low chlorine
concentration to achieve a high etch rate is considered
counterintuitive. In fact, a high chlorine concentration results in
excessive buildup of a surface reaction layer that inhibits the
etch rate. The present invention uses a low chlorine concentration,
which inhibits the formation of a surface reaction layer.
[0028] The surface reaction layer, if it exists at all, does not
limit the etch rate when the chlorine concentration is low. For
example, referring to the data points in FIG. 3A in which the
chlorine concentration is less the 20 percent, the etch rate
improves as the chlorine concentration increases. This indicates
that if increasing the chlorine concentration forms a surface
reaction layer, the negative impact of such increase on the etch
rate is less than the positive impact on etch rate of increasing
concentration of chlorine. Thus, this is not an example of an
etch-limiting surface reaction layer. However, at a higher chlorine
concentration, the surface reaction layer limits the etch rate. For
example, the etch rate of InP is about 275 nm/min when the chlorine
concentration is about 10 percent. However, the InP etch rate is
only about 110 nm/min when the chlorine concentration is at about
35 percent. Similar results are indicated in FIG. 3A for the other
materials. It is believed that with the higher chlorine
concentration, the negative impact on etch rate of the surface
reaction layer is greater than the positive impact on etch rate of
increased chlorine concentration. For purposes of the present
application, an "etch-limiting surface reaction layer," is a
surface reaction layer that limits the etch rate. Embodiments
according to the present invention provide a low-temperature
etching environment in which the halogen and the inert gas are in a
ratio that does not induce the formation of an etch-limiting
surface reaction layer during etching.
[0029] FIG. 3A also shows that the selectivity between the various
materials being etched is a function of the chlorine concentration
in the etching environment. For example, the ratio of the InP etch
rate to the indium aluminum arsenide (InAlAs) etch rate is greater
than one at low chlorine concentrations. At a chlorine
concentration of about seven percent, the etch rate of InP is about
280 nm/min, whereas the etch rate of InAlAs is about 220 nm/min.
Thus, the InP/InAlAs etch rate ratio is about 1.3 at about a seven
percent chlorine concentration. However, at higher chlorine
concentration, the etch ratio favors InAlAs. For example, at a
chlorine concentration of about 68 percent, the etch rate of InP is
about 120 nm/min, whereas the etch rate for InAlAs is about 180
nm/min. This provides a selectivity of InP to InAlAs of only about
0.7. The favorable etch rate of InP to InAlAs at a low chlorine
concentration, achieved in the present invention, is desirable and
difficult or impossible to achieve with conventional methods. The
favorable etch ratio of InP/InAlAs at low temperature makes it
easier to stop the etch at the InAlAs layer when etching a stack
comprising InP on top of InAlAs. Thus, embodiments according to the
present invention are able to achieve a favorable etching
selectivity between various indium-comprising compounds. Such a
favorable etching selectivity is not limited to indium comprising
compounds.
[0030] FIG. 3B shows a graph 350 illustrating etch rate achieved in
accordance with embodiments of the present invention using an
electron cyclotron resonance (ECR) system. The results are similar
to the inductively-coupled plasma (ICP) results shown in FIG. 3A.
Results in FIG. 3B are shown for etching two different materials,
each comprising indium. The graph 350 shows an InP etch rate curve
360 and an InAlAs etch rate curve 370. The etch rate of InP is
about 375 nm/min at a chlorine concentration of about seven
percent. The etch rates drops below 200 nm/min when the chlorine
concentration is about 40 percent. The etch rate of InAlAs is about
440 nm/min at a chlorine concentration of about seven percent. The
etch rates drops below 300 nm/min when the chlorine concentration
is about 40 percent. Thus, it is believed that a buildup of an
etch-limiting surface reaction layer limits the etch rate at higher
chlorine concentrations.
[0031] Enhanced photoresist mask selectivity is another benefit of
embodiments according to the present invention. FIG. 4A shows a
graph 400 of selectivity ratio between the material being etched
and the photoresist mask under the same conditions as the data
shown in FIG. 3A. FIG. 4A shows an InP/photoresist selectivity
curve 410, an InGaAs/photoresist selectivity curve 420, an
InAlAs/photoresist selectivity curve 430, and an
AlInGaAs/photoresist selectivity curve 440. Referring to FIG. 4A,
the resist selectivity for all curves 410, 420, 430, 440 is high at
low chlorine concentrations relative to high chlorine
concentrations. For example, when the chlorine concentration is
below 20 percent, the resist selectivity is between two and eight
for the various materials being etched. Furthermore, the
selectivity ratio of InP/photoresist peaks at greater than five and
the selectivity ratio for the other materials to photoresist is
greater than four over most of the range below 20 percent chlorine.
When the chlorine concentration is at about 40 percent or higher,
the resist selectivity is much lower than the resist selectivity at
a low chlorine concentration. Thus, embodiments according to the
present invention provide a high etch selectivity between the
material being etched and a photoresist mask when using a
low-temperature etching environment in which the halogen and the
inert gas are in a ratio that does not induce the formation of an
etch-limiting surface reaction layer during etching.
[0032] The increased etch selectivity of a semiconductor with
respect to the photoresist is because the lower concentration of
chlorine reduces the etch rate of the photoresist, as well as
increasing the etch rate of the semiconductor as seen in FIG. 3A.
The increased etch selectivity of the semiconductor with respect to
the photoresist allows thinner photoresist masks to be used.
Moreover, the increased etch selectivity facilitates incorporating
into standard process flows the fabrication of devices that are
otherwise difficult to fabricate using standard process steps. For
example, heterojunction bipolar transistors are difficult to
fabricate using standard process steps due to the formation of the
surface reaction layer. However, by utilizing embodiments in
accordance with the present invention, heterojunction bipolar
transistors can be fabricated using standard process methods.
[0033] FIG. 4B shows a graph 450 of the selectivity ratio between
the material being etched and the photoresist mask for the ECR
under the same conditions as shown in FIG. 3B. The results are
similar to the ICP results shown in FIG. 4A. FIG. 4B shows an
InP/photoresist selectivity curve 460 and an InAlAs/photoresist
selectivity curve 470. Referring to FIG. 4B, the resist selectivity
for curves 460 and 470 is high at low chlorine concentrations
relative to the resist selectivity at higher chlorine
concentrations. For example, when the chlorine concentration is
below 10 percent, the resist selectivity for InP is 2.5 or greater.
For InAlAs, the resist selectivity is about four or higher for
chlorine concentrations below 10 percent. When the chlorine
concentration is at about 40 percent, the resist selectivity is
below one for InP and about one for InAlAs.
[0034] An embodiment of the present invention is a method of
etching a material for which etching is limited by a surface
reaction layer. Referring to process 500 of FIG. 5, block 510
comprises receiving, in a low-temperature etching environment, a
material for which etching in a low-temperature etching environment
is limited by a surface reaction layer. The low-temperature etching
environment comprises a halogen and an inert gas. The halogen and
the inert gas are provided in a ratio that does not induce the
formation of an etch-limiting surface reaction layer during etching
in the low-temperature etching environment.
[0035] In block 520, the process comprises etching the surface
reaction layer limited material within the low-temperature etching
environment.
[0036] The material for which etching is limited by a surface
reaction layer is not limited to any category of materials. In one
embodiment, copper is the material that is surface reaction layer
limited. In another embodiment, a semiconductor is the material
that is surface reaction layer limited. For example, the material
is indium phosphide in one embodiment.
[0037] Yet another embodiment of the present invention is a method
for reducing a surface reaction layer formed in a low-temperature
etching environment. Referring to process 600 of FIG. 6, block 610
comprises introducing an inert gas into the low-temperature etching
environment.
[0038] Block 620 comprises introducing a halogen into the
low-temperature etching environment. The inert gas and the halogen
are introduced in a ratio that inhibit the formation of an
etch-limiting surface reaction layer during etching in the
low-temperature etching environment. The temperature of the
low-temperature environment is below that which would melt a
photoresist material that has not been treated to increase its
glass-reflow temperature.
[0039] While the present invention has been described in particular
embodiments, it should be appreciated that the present invention
should not be construed as limited by such embodiments, but rather
construed according to the below claims.
* * * * *