U.S. patent application number 13/019351 was filed with the patent office on 2011-05-26 for apparatus and a method for controlling the depth of etching during alternating plasma etching of semiconductor substrates.
This patent application is currently assigned to TEGAL CORPORATION. Invention is credited to Nicolas Launay, Michel Puech.
Application Number | 20110120648 13/019351 |
Document ID | / |
Family ID | 34978971 |
Filed Date | 2011-05-26 |
United States Patent
Application |
20110120648 |
Kind Code |
A1 |
Puech; Michel ; et
al. |
May 26, 2011 |
APPARATUS AND A METHOD FOR CONTROLLING THE DEPTH OF ETCHING DURING
ALTERNATING PLASMA ETCHING OF SEMICONDUCTOR SUBSTRATES
Abstract
The present invention provides apparatus for controlling the
operation of plasma etching a semiconductor substrate by an
alternating etching method, the apparatus comprising: a process
chamber (1) in which said substrate (2) is processed, means for
generating a plasma (6); at least one first window (7) formed in a
first wall (8) of said chamber (1) facing the surface (2a) to be
etched of said substrate (2); at least one second window (10)
formed in a second wall (11) of said chamber (1) lying in a plane
different from said first wall (8); first means (18) coupled to
said second window (10) to detect a light signal (17) relating to a
selected wavelength emitted by said plasma (6); means (13, 15) for
emitting a monochromatic light signal (14) through said first
window (7) towards said surface (2a) in a direction (9)
substantially perpendicular to said surface (2a) in such a manner
that said incident signal (14a) is reflected on said surface (2a);
second means (16) for detecting said reflected signal (14b); and
transformation means (19) coupled to said first means (18) and to
said second means (16) to transform the signal detected by said
second means (16) as a function of the signal detected by said
first means (18).
Inventors: |
Puech; Michel; (Metz-Tessy,
FR) ; Launay; Nicolas; (Annecy, FR) |
Assignee: |
TEGAL CORPORATION
Petaluma
CA
|
Family ID: |
34978971 |
Appl. No.: |
13/019351 |
Filed: |
February 2, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11319506 |
Dec 29, 2005 |
7892980 |
|
|
13019351 |
|
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Current U.S.
Class: |
156/345.26 ;
156/345.24 |
Current CPC
Class: |
H01J 37/32935 20130101;
H01L 21/3065 20130101; H01L 21/30655 20130101 |
Class at
Publication: |
156/345.26 ;
156/345.24 |
International
Class: |
C23F 1/08 20060101
C23F001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 31, 2004 |
FR |
0453276 |
Claims
1. A plasma etching reactor for etching a substrate by an
alternating etching process, which comprises an etching portion and
a deposition portion, the reactor comprising: a process chamber; a
plasma source to form a plasma in the process chamber; a first
detector for receiving a reflected signal reflected from the
substrate which is representative of the alternating etching
process; a second detector for detecting an etch signal indicative
of the etching portion of the alternating etching process; and a
controller coupled to the first and second detectors, the
controller receiving simultaneously the reflected signal and the
etch signal and using the etch signal to transform the reflected
signal as a function of the etch signal.
2. A reactor as in claim 1 further comprising a light source for
emitting a light signal towards the substrate surface in a
direction substantially perpendicular to the surface, wherein the
light signal is reflected from the substrate surface to be detected
by the first detector.
3. A reactor as in claim 1, wherein the second detector detects at
least one of a light signal relating to a selected wavelength
emitted by the plasma, and a signal associated with the presence in
the plasma of a species stemming from the reaction of an etching
gas with said substrate.
4. A reactor as in claim 1, wherein the controller transforms the
reflected signal, which is representative of the alternating
etching process, in order to obtain a curve representative of
etching steps alone.
5. A reactor as in claim 1, wherein the transformed reflected
signal provides information regarding at least one of an etch rate
and an end of the alternating etching process.
6. A plasma etching reactor for etching a substrate by an
alternating etching process, which comprises an etching portion and
a deposition portion, the reactor comprising: a process chamber; a
plasma source to form a plasma in the process chamber; a
monochromatic laser for sending a photon beam to the substrate in
such a manner that the photon beam is reflected on the substrate
surface; a first detector for detecting the reflected signal which
is representative of the alternating etching process; a second
detector for detecting an emission signal associated with the
presence in the plasma of a species stemming from the reaction of
an etching gas with the substrate; and a controller coupled to the
first and second detectors, the controller receiving simultaneously
the reflected signal and the emission signal and using the emission
signal to extract from the reflected signal a signal representative
of the etching portion within the alternating etching process.
7. A reactor as in claim 6, wherein the second detector detects
plasma species of the type SiF.sub.x or CF.sub.x.
8. A reactor as in claim 6, wherein the controller extracting from
said reflected signal those portions of the signal that correspond
to the presence of said species in order to obtain a curve
representative of etching steps alone.
9. A reactor as in claim 6, wherein extracting a signal comprises
retrieving a portion of the signal associated with the time that
the second signal shows the etching process.
10. A reactor as in claim 6, wherein the extracted signal provides
information regarding at least one of an etch rate and an end of
the etching process.
11. A plasma etching reactor for etching a substrate by an
alternating etching process, which comprises an etching portion
using an etching gas and a deposition portion, the reactor
comprising: a process chamber; a plasma source to form a plasma in
the process chamber; an interferometer for sending and receiving an
interferometer signal reflected from the substrate which is
representative of the alternating etching process; an emission
spectrometer for receiving an emission signal associated with the
presence in the plasma of a species stemming from the reaction of
the etching gas with the substrate; and a controller coupled to the
interferometer and the emission spectrometer, the controller
receiving simultaneously the interferometer signal and the emission
signal and using the emission signal to extract from the
interferometer signal a signal representative of the etching
portion within the alternating etching process, wherein extracting
a signal comprises retrieving a portion of the interferometer
signal associated with the time that the interferometer signal
shows the etching process.
12. A reactor as in claim 11 wherein the process chamber comprises
a first window formed in a first wall of the chamber facing the
substrate surface to be etched, and wherein the interferometer is
coupled to the first window to send and receive the interferometer
signal.
13. A reactor as in claim 12 wherein the process chamber comprises
a second window formed in a second wall of the chamber lying in a
plane different from the first wall, and wherein the emission
spectrometer is coupled to the second window to detect the emission
signal.
14. A reactor as in claim 13 wherein the plane of the second window
is substantially perpendicular to the plane of the first
window.
15. A reactor as in claim 11 wherein the interferometer comprises
at least one of a monochromatic laser, a helium-neon laser, a laser
diode, and a semireflecting mirror.
16. A reactor as in claim 11 wherein the emission spectrometer
detects at least one of a light signal relating to a selected
wavelength emitted by the plasma, and a signal associated with the
presence in the plasma of a species stemming from the reaction of
the etching gas with the substrate.
17. A reactor as in claim 11 wherein the emission spectrometer
detects plasma species of the type SiF.sub.x or CF.sub.x.
18. A reactor as in claim 11 further comprising an etching gas
source, and means for controlling the etching flow rate to govern
the introduction of etching gas into the plasma source; a
passivation gas source, and means for controlling the passivation
flow rate for governing the introduction of passivation gas into
the plasma source; and a control device adapted to cause the
etching gas flow rate control means and the passivation gas flow
rate control means to operate in alternation.
19. A reactor as in claim 11 wherein the controller extracting from
the reflected signal those portions of the emission signal in order
to obtain a curve representative of etching steps alone.
20. A reactor as in claim 11 wherein the extracted signal provides
information regarding at least one of an etch rate and an end of
the etching process.
Description
[0001] This is a Divisional Application of U.S. patent application
Ser. No. 11/319,506, filed Dec. 29, 2005, entitled "APPARATUS AND A
METHOD FOR CONTROLLING THE DEPTH OF ETCHING DURING ALTERNATING
PLASMA ETCHING OF SEMICONDUCTOR SUBSTRATES" (Attorney Docket No.
TEGL-01255US0) and which claims priority to French Patent No. FR
0453276 filed Dec. 31, 2004. All of the aforementioned patent and
patent applications are incorporated herein by reference in their
entireties.
BACKGROUND
[0002] The present invention relates to the field of micromachining
semiconductor substrates to make components for
microelectromechanical systems (MEMS) or for microoptical
electromechanical systems (MOEMS). The invention relates more
particularly to controlling the depth of etching during
micromachining of silicon by plasma using the alternating etching
technique, and in particular the invention relates to the apparatus
and to the method used.
[0003] Integrated circuits are made in the bulk of semiconductor
material wafers. Lines are reproduced on the surface of the wafer
in a grid pattern so that the individual integrated circuits, known
as "chips", can easily be separated from one another. Once
treatment of the wafer has been finished, it is cut up along the
lines in order to separate the chips.
[0004] Most applications using components etched on silicon
substrates require the etched pattern to be of very great
precision, in particular in terms of depth. It is therefore
necessary to control depth very precisely in order to determine
without ambiguity when the etching operation has come to an end.
The most widespread method for detecting the end of the etching
operation are methods that rely on optical techniques.
[0005] One method consists in sending a light beam of uniform
frequency, preferably a laser beam, onto a substrate for etching
that comprises two distinct layers having different indices of
refraction. The beam is reflected therefrom and picked up by a
detector. A sudden change in the detected light intensity, due to
the change in the index of refraction on passing from one layer to
another marks the end of etching.
[0006] Proposals have thus been made to use a method based on laser
interferometry. In that method, a monochromatic beam generated by a
laser is directed substantially perpendicularly onto a
semitransparent layer for etching. The partially-reflected beam is
picked up by a suitable photodetector. The beams coming from
reflection on the interface between said layer and the substrate
interfere so as to give a characteristic sinusoidal curve. The
interference phenomenon is controlled by the relationship
d=.lamda./2n, where d is the thickness of the layer to be etched,
is the wavelength of the beam, and n is the index of refraction in
the propagation medium (n=1 in a vacuum). The flattening in the
curve indicates that the semitransparent layer has been consumed in
full, and thus marks the end of the etching operation.
[0007] Another document describes a device for controlling the
operation of plasma etching a semiconductor substrate. The
treatment chamber has two windows coupled to a spectrometer for
observing the plasma. Each spectrometer delivers a signal based on
the wavelength of a selected species in the radiation of the
light-emitting discharge. The first window, observing in a plane
parallel to the surface of the substrate, provides a signal
relating to variation in intensity during etching at a selected
wavelength. The second window, observing in a plane normal to the
surface of the substrate, gives a signal containing information
relating to the variation in the intensity of the selected
wavelength and to the variation in the reflectivity of the layer of
SiO2 which is redeposited in continuous manner during the
treatment. By interferometry, a signal is obtained relating to the
variation over time in the intensity of the surface reflectivity of
the wafer, and thus relating to the thickness of the SiO.sub.2
layer which depends thereon directly. Means enable the depth of the
etching and the thickness of the SiO.sub.2 layer to be deduced
therefrom.
[0008] Micromachining silicon using a plasma, also known as deep
reactive ion etching (DRIE), commonly makes use of an alternating
etching technique which is characterized by alternating steps of
removal and of deposition which follow one another very quickly.
That method is described in particular in document U.S. Pat. No.
5,501, 893. The technique consists in hiding the silicon substrate
in part by means of a mask, and in subjecting the hidden substrate
to an alternating succession of etching steps using an etching gas
plasma and of passivation steps using a passivation gas plasma.
During each etching step, the etching gas plasma such as sulfur
hexafluoride SF.sub.6 makes cavities in those zones of the
substrate that are not hidden by the mask. During each passivation
step, the passivation gas plasma, such as a fluorocarbon gas, e.g.
C.sub.4F.sub.8, deposits a protecting polymer film on the wall of
the cavity. Each of the etching and passivation steps has a
duration that is very short, a few seconds, and the passivation
prevents the etching gas plasma from etching the side wall of the
cavity during the subsequent etching step. As a result, etching
takes place selectively in the bottom of the cavity after the
etching gas plasma has removed the film of protective polymer at
the bottom of the cavity. Thus, in spite of the isotropic nature of
the way in which silicon is etched by a plasma of an etching gas,
such as a fluorine containing gas, the etching of the silicon that
is obtained is, in fact, practically anisotropic, fast, and
selective.
[0009] The methods of using optical techniques to detect the end of
the etching operation in prior art methods are not suitable for use
with the alternating etching method since the information provided
is disturbed by the alternation, and is therefore unusable.
[0010] The problem posed by the present invention is to improve the
apparatus and the method for alternating etching of silicon by a
succession of etching steps and of passivation steps in such a
manner as to provide improved control over the depth to which a
semiconductor substrate is etched.
[0011] An object of the invention is to provide apparatus for
controlling the end of a plasma etching operation on a substrate
once the desired depth has been reached.
[0012] Another object of the invention is to provide a method that
makes it possible in certain and accurate manner to determine the
end of the etching operation.
[0013] The present invention provides apparatus for controlling the
operation of plasma etching a semiconductor substrate by an
alternating etching method, the apparatus comprising: [0014] a
process chamber in which said substrate is processed; [0015] means
for generating a plasma; [0016] at least one first window formed in
a first wall of said chamber facing the surface to be etched of
said substrate; [0017] at least one second window formed in a
second wall of said chamber lying in a plane different from said
first wall; [0018] first means coupled to said second window to
detect a light signal relating to a selected wavelength emitted by
said plasma; [0019] means for emitting a monochromatic light signal
through said first window towards said surface in a direction
substantially perpendicular to said surface in such a manner that
said incident signal is reflected on said surface; [0020] second
means for detecting said reflected signal; and [0021]
transformation means coupled to said first means and to said second
means to transform the signal detected by said second means as a
function of the signal detected by said first means.
[0022] In a first embodiment of the invention, the emitter means
comprise a helium-neon laser generating a monochromatic signal
having a wavelength of 632.8 nanometers (nm). The emitter means
preferably also comprise a semireflecting mirror. The mirror
enables the beam emitted by the laser towards the substrate to be
reflected and allows the beam reflected by the substrate towards
the detector to pass through.
[0023] In a second embodiment, the second detector means comprise
an interferometer.
[0024] In a third embodiment, the first detector means comprise an
emission spectrometer.
[0025] In a fourth embodiment, the plane of the second window is
substantially perpendicular to the plane of the first window.
[0026] The invention also provides a method of controlling the
operation of plasma etching the surface of a semiconductor
substrate by the alternating etching method using the above
apparatus, the method comprising the following steps: [0027]
generating a monochromatic signal; [0028] sending said signal to
the substrate in a direction substantially perpendicular to the
surface to be etched; [0029] detecting said signal reflected on
said substrate; [0030] detecting a signal associated with the
presence in the plasma of a species stemming from the reaction of
the etching gas with said substrate; and [0031] extracting from
said reflected signal those portions of the signal that corresponds
to the presence of said species in order to obtain a curve
representative of etching steps alone.
[0032] In an implementation of the invention, the substrate is of
silicon and the species whose presence is detected during the
material-removal step is a species of the SiF.sub.x type, such as
SiF.sub.4, for example.
[0033] In another implementation of the invention, the substrate is
of silicon and the species whose presence is detected during the
material-removal step is a species of the CF.sub.x type, such as
CF.sub.2, for example.
[0034] The present invention has the advantage of making it
possible to obtain a signal which corresponds solely to those
periods during which etching is actually taking place, and which
consequently provides information that is directly usable.
[0035] Other characteristics and advantages of the present
invention appear on reading the following description of an
embodiment that is naturally given by way of non-limiting
illustration, and from the accompanying drawing, in which:
[0036] FIG. 1 is a diagram of etching apparatus in which the method
of the present invention is implemented; and
[0037] FIG. 2 is a graph plotting the reconstituted signal as a
function of time.
[0038] In the embodiment shown in FIG. 1, the installation for
micromachining semiconductor substrates comprises a sealed
enclosure 1 shaped to receive and contain a semiconductor substrate
2 for etching.
[0039] In conventional manner, the substrate 2, e.g. a silicon
wafer, is secured on a support that is electrically biased by bias
means 4 to a potential that is negative relative to ground. Vacuum
generator means 5, connected to the process chamber 1 and
comprising for example a primary pump and a secondary pump, serve
to create and maintain a suitable vacuum inside the enclosure 1.
The substrate 2 is oriented in the chamber 1 in such a manner as to
cause its surface 2a for working to be visible. Facing the surface
2a for machining, there are means for generating a plasma that is
directed towards the surface 2a for machining The installation
includes means for selectively injecting gases into the chamber 1,
in particular etching gases and passivation gases. During the
etching step, SF.sub.6 is introduced as the etching gas, for
example, the plasma 6 contains electrically neutral active atoms
such as atoms of fluorine, which propagate in all directions, and
ions such as SF.sub.5.sup.+ which are attracted to the negatively
biased substrate 2 and which attack the silicon. During the
passivation steps, a fluorocarbon gas such as CHF.sub.3,
C.sub.2F.sub.6, C.sub.2F.sub.4, or C.sub.4F.sub.8 is injected which
causes a protective polymer film to be formed over the entire
etched surface.
[0040] In the present invention, the enclosure 1 includes a first
quartz window 7 placed over a first wall 8 facing the surface to be
etched 2a so as to observe the surface to be etched 2a along an
axis 9 which is substantially perpendicular thereto, and a second
window 10 disposed in a second wall 11, in this case perpendicular
to the wall 8, in such a manner as to observe the plasma 6 along an
axis 12, in this case substantially parallel to the axis to be
etched 2a.
[0041] According to the invention, the installation further
comprises means 13 for generating a monochromatic optical signal,
in particular a laser diode or preferably a helium-neon laser, for
example. The light signal 14 emitted by the laser 13 is directed by
means of a semireflecting mirror 15 towards the surface 2a of the
substrate 2 through the window 7. The incident signal 14a is
reflected on the surface 2a that is being etched, and the reflected
signal 14b which is practically totally reflected returns
substantially along the same path in the opposite direction. After
passing through the window 7, and then through the semireflecting
mirror 15, the reflected signal 14b is directed to detector means
16 such as an interferometer by means of an optical fiber, for
example. The laser signal is emitted and detected throughout the
entire duration of the operation of treating the substrate with
alternating etching.
[0042] According to the invention, the installation also includes
means for observing the plasma 6. The light signal 17 coming from
observation of the plasma 6 through the window 10 is directed by
means of an optical fiber, for example, to an emission spectrometer
18 which analyses the signal 17 in order to identify the presence
of species coming from the reaction of the etching gas with the
substrate 2, and in particular the presence of SiF.sub.4 in the
event of SiF.sub.6 being reacted with silicon.
[0043] Signal transformation means 19, such as a computer, extract
from the signal received by the detector 16 periods during which
the spectrometer 18 detects the presence of species coming from the
reaction of the etching gas with the substrate 2, and in particular
SiF.sub.4. This produces a reconstituted laser interferometer
signal 20 as shown in FIG. 2 and which corresponds solely to the
periods during which the substrate is being etched, to the
exclusion of passivation periods 21 and possibly also depassivation
periods.
[0044] In FIG. 2, the reconstituted signal 20 is plotted as
variation as a function in time in the ratio I.sub.0/I.sub.r where
I.sub.0 is the intensity of the signal 12 emitted by the laser, and
I.sub.r is the intensity of the reflected signal 14b received by
the detector 16. The distance d is a function of the wavelength
.lamda. of the laser and of the index of refraction n of the
propagation medium (in this case a vacuum, so n=1), where
d=.lamda./2n and represents the thickness of the laser to be
etched. The computer 19 analyzes the signal 20 to determine the end
of the etching operation.
[0045] For example, if the substrate includes a layer of silicon Si
placed over a fine buried layer of SiO.sub.2, and if the etching
operation consists in totally etching the surface layer of silicon
Si and stopping at the surface of the SiO.sub.2 layer, then the
signal 20 will flatten when the Si/SiO.sub.2 interface is
reached.
[0046] If the silicon substrate is uniform and if the etching
operation consists in etching the silicon to a determined depth, it
is then possible from the signal 20 to estimate the depth that has
been etched and thus the almost instantaneous speed of etching V,
by measuring the time t that elapses between two extrema, in
application of the relationship:
V.sub.i=d/t=.lamda./2nt
[0047] The present invention is not restricted to the embodiments
described explicitly above, but naturally including any variants
and generalizations that are within the competence of the person
skilled in the art.
* * * * *