U.S. patent application number 09/295100 was filed with the patent office on 2001-11-22 for method of anisotropic etching of substrates.
Invention is credited to FELDBAUM, MICHAEL, PANDHUMSOPORN, TAMARAK, PUECH, MICHEL, YU, KEVIN.
Application Number | 20010044213 09/295100 |
Document ID | / |
Family ID | 23136218 |
Filed Date | 2001-11-22 |
United States Patent
Application |
20010044213 |
Kind Code |
A1 |
PANDHUMSOPORN, TAMARAK ; et
al. |
November 22, 2001 |
METHOD OF ANISOTROPIC ETCHING OF SUBSTRATES
Abstract
A method of plasma etching of silicon that utilizes the plasma
to provide laterally defined recess structures through a mask. The
method is based on the variation of the plasma parameters to
provide a well-controlled anisotropic etch, while achieving a very
high etch rate, and a high selectivity with respect to a mask. A
mixed gas is introduced into the vacuum chamber after the chamber
is evacuated, and plasma is generated within the chamber. The
substrate's surface is exposed to the plasma. Power sources are
used for formation of the plasma discharge. An integrated control
system is used to modulate the plasma discharge power and substrate
polarization voltage levels.
Inventors: |
PANDHUMSOPORN, TAMARAK;
(FREMONT, CA) ; YU, KEVIN; (SAN JOSE, CA) ;
FELDBAUM, MICHAEL; (FREMONT, CA) ; PUECH, MICHEL;
(ANNCY CEDEX, FR) |
Correspondence
Address: |
SUGHRUE MION ZINN MACPEAK & SEAS PLLC
2100 PENNSYLVANIA AVENUE NW
WASHINGTON
DC
200373213
|
Family ID: |
23136218 |
Appl. No.: |
09/295100 |
Filed: |
April 21, 1999 |
Current U.S.
Class: |
438/712 ;
438/700; 438/701; 438/706; 438/707; 438/714; 438/719; 438/734;
438/735 |
Current CPC
Class: |
H01L 21/30655
20130101 |
Class at
Publication: |
438/712 ;
438/700; 438/701; 438/707; 438/706; 438/714; 438/719; 438/734;
438/735 |
International
Class: |
H01L 021/311 |
Claims
What is claimed:
1. An anisotropic etching process for etching a silicon substrate
comprising the steps of: introducing a mixed gas containing etchant
and passivating gases into a chamber with the silicon substrate to
be etched; etching said substrate in the presence of said mixed gas
by controlling excitation power and substrate bias during a first
period of time to remove material from the surface of said
substrate and thereby provide an exposed surface; passivating said
substrate in the presence of said mixed gas by controlling
excitation power and substrate bias during a second period of time
to cover said exposed surface with a polymer layer, wherein said
mixed gas is the same during said etching and passivating steps;
and repeating said etching step.
2. The etching process of claim 1, further comprising the step of
repeating said passivating step
3. The etching process of claim 1, wherein said etching step
includes the steps of applying RF power to an inductive coupler
disposed in said chamber in the range of 100 to 800 W and applying
a voltage to said substrate to produce a substrate bias in the
range of 50 to 300 eV.
4. The etching process of claim 3, wherein said passivating step
includes the steps of applying RF power to an inductive coupler
disposed in said chamber in the range of 1000 to 3000 W and
applying a voltage to said substrate to produce a substrate bias in
the range of 0 to 25 eV.
5. The etching process of claim 3, wherein said etching step is
carried out for a period ranging from 10 to 100 seconds.
6. The etching process of claim 4, wherein said etching step is
carried out for a period ranging from 10 to 100 seconds.
7. The etching process of claim 4, wherein said passivating step is
carried out for a period ranging from 0.5 to 3 seconds.
8. The etching process of claim 5, wherein said passivating step is
carried out for a period ranging from 0.5 to 3 seconds.
9. The etching process of claim 1, wherein said introducing step
includes the step of introducing SF.sub.6, C.sub.4F.sub.8, and
CHF.sub.3 into the chamber.
10. The etching process of claim 1, wherein a gas pressure in said
chamber is in the range of 0.1 to 10 Pa.
11. The etching process of claim 1, further comprising the step of
varying the pressure in said chamber.
12. The etching process of claim 1, wherein during said introducing
step said mixed gas is introduced at a constant rate during said
etching steps and said passivating step.
13. The etching process of claim 1, wherein said excitation power
during the etching steps is lower than the excitation power during
said passivating step.
14. The etching process of claim 1, wherein said substrate bias
during said etching steps is lower that said substrate bias during
said passivating step.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a method of anisotropic plasma
etching of substrates preferably defined with an etching mask in
which the etch rate and selectivity is increased. The method can be
well implemented for manufacturing microelectromechanical system
(MEMS), as well as microelectronic devices.
[0003] 2. Background of the Related Art
[0004] Anisotropic plasma etching, particularly for single crystal
silicon, can work independent of crystal orientation of the
substrate or doping level. This method also applies to doped or
undoped polysilicon. Preferred fields of applications are MEMS
technology, where structures have a high aspect ratio, i.e., a high
structural height to width ratio. Other examples include surface
wave technology, where narrow grooves and vertical walls are etched
to produce actuators, surface wave filters, delay lines, etc.
Additional microelectronics applications include storage cells,
insulation, collector contacts, etc.
[0005] The Reactive Ion Etching (RIE) processes which are commonly
used for anisotropic silicon etch employ relatively high energy
ions (.gtoreq.100 eV) and reactive halogens, such as fluorine,
chlorine or bromine, which are used directly in the plasma or are
released from corresponding compounds, like CF.sub.4, CF.sub.3Br,
C.sub.2F.sub.6, CCI.sub.4, CHCI.sub.3. The resulting ion
bombardment of the etching ground, i.e., the area to be etched,
initiates the reaction of the radicals with the silicon to be
depleted. The etching of the sidewalls is minimal due to the
directionality of the ions.
[0006] Problems occur when, to increase the speed of silicon
removal (i.e., the etch rate), one tries to enhance the plasma
density by increasing the power coupled to the plasma discharge.
This can be accomplished by either increasing the power of the
source for the plasma discharge or by increasing the value of the
polarization voltage applied to the substrate. However, as the
power is increased more hot ions are produced and the direction of
ion movement becomes more random. The results is that more ions and
radicals are depleted by the walls of the trenches, with the
inevitable loss of anisotropy of the etch. To overcome this
problem, one must reduce the etch rate, resulting in the loss of
the throughput.
[0007] An additional problem encountered is mask degradation. As
etch rate is reduced, etch time, and therefore mask exposure time,
are increased, leading to more rapid mask degradation, i.e.,
reduced selectivity.
[0008] U.S. Pat. No. 5,501,893 discloses an etching method that
includes alternating etching and polymerizing steps where the
purpose of the polymerizing step is to provide a polymer layer on
the surfaces that were exposed in the previous etching step to form
a temporary etch stop. Thus the side walls are protected from
etching during the etching steps. However, the gas mixtures
introduced during the etching and polymerizing steps are different
such that different gas mixtures are cycled during the respective
etching and polymerizing steps. In the etching step the gas mixture
includes SF.sub.2 and Ar and in the polymerizing step the gas
mixture includes CHF.sub.3 and Ar.
[0009] The problem with cycling different gases is that the time
ratio of the etch deposition cycle depends on the speed of the gas
mixtures and varies from point to point, affecting the uniformity.
Also, the time for species to arrive at the bottom of the etched
trench varies drastically for trenches having different sized
openings. Also, this method typically requires more complex
hardware and controls to introduce the two different gas mixtures
in cycles.
[0010] It is desirable to make the plasma as cold as possible with
coexisting polymer-producing unsaturated monomers and fluorine,
bromine or iodine radicals. The energy level sufficient for
ionization is different for each gas. In certain cases, the
activation energy for polymer-producing gases (C.sub.4F.sub.8,
CHF.sub.3) is two times higher than for radical producing gases
(SF.sub.6). The object of the invention is to establish a method
for enhancing the treatment of the silicon surface being etched, by
using the differences in the energies of activation of the reactive
gases to arrive at optimal conditions for both etching and
passivation, and alternating those conditions at a high rate to
produce a high aspect ratio, and high selectivity etch process.
SUMMARY OF THE INVENTION
[0011] The object of the present invention is accomplished by
providing a method of anisotropic plasma etching of substrates
(typically silicon) comprising the following steps:
[0012] a) placing the substrate with the surface to be selectively
etched on an electrode connected to an electromagnet power
source;
[0013] b) introducing mixed gases consisting of an etching gas
(SF.sub.6) and a passivation gas (CHF.sub.3, C.sub.4F.sub.8, etc.)
into the processing chamber;
[0014] c) exciting the mixed gases with lower power (100-800 W)
electromagnetic radiation sufficient to produce a plasma containing
ions and radicals for etching;
[0015] d) concurrent with step (c), applying high polarizing
voltage (50-500 eV) to the substrate via its electromagnet power
source to produce a highly anisotropic etch;
[0016] e) exciting the mixed gases with high power (1000-3000 W)
electromagnetic radiation to produce in the plasma unsaturated
monomers for protective polymer coating formation;
[0017] f) concurrent with step (e), applying low polarizing voltage
(0-25 eV) to the substrate to form a conformal polymer coating on
the exposed side walls of the surfaces being etched; and
[0018] alternating steps c) and d) with steps e) and f) to achieve
an anisotropic etch with a high etch rate and selectivity than is
currently being achieved using other methodologies.
[0019] The method used in this invention enables the substrate to
be etched without using helium gas as a cooling medium. This is
because lower power is used to excite the etching gas resulting in
less heat being generated during the process.
[0020] A further advantage of this invention is that a constant
flow of mixed gas is injected into the process chamber during
processing, resulting in a process that is more stable and
repeatable.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The invention is described in detail below in conjunction
with the attached drawings in which:
[0022] FIG. 1 is a schematical view showing the etching device for
use in the present invention; and
[0023] FIGS. 2(a) to 2(c) are views showing the process steps of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0024] The following is a detailed description of the invention
with reference to FIGS. 1 and 2(a) to 2(c). Referring to FIG. 1,
the device includes an etching chamber 10. Provided within the
chamber 10 is a substrate holder 12 for holding substrate 14 as
well as an inductive coupler 16 provided near the top of the
chamber 10. The substrate holder 12 is an electrode which is
electrically connected to generator 18, including a power supply 19
and a matching network 21, for polarizing the substrate 14. Located
at the top of the chamber is an inlet line 20 for introducing
process gases into the chamber. The process gases are stored in gas
tanks 22, 24 and 26. The flow rate of the process gases into the
chamber 10 is controlled by a control valves 28. The plasma
stimulation is provided by inductive coupler 16 powered by an RF
power source 30 and an associated matching network 32. Pressure
within the chamber 10 is controlled by mechanical pump 34,
turbo-molecular pump 36 and throttle valve 38, in the conventional
manner. It is of course understood that the invention is not
limited to this particular device.
[0025] The following is a description of the process of etching the
substrate with reference to FIGS. 2(a) to 2(c). The substrate 14,
including a silicon substrate 40 and an etching mask 40 that
exposes the regions of the silicon substrate 40 that are intended
to be anisotropically etched, is placed on the substrate
holder/electrode 12 and subjected to the first etching step. In
this step, a mixture of gases containing etchant and passivating
gases (e.g., SF.sub.6, C.sub.4F.sub.8, and CHF.sub.3) with a
certain flow rate, preferably in the range of 100 to 400 sccm and
pressure in the range of 0.1 to 10 Pa, is introduced into the
chamber 10. The plasma is stimulated by applying a relatively low
RF power, preferably in the range of 100 to 800 W, from the power
source 30. At the same time, a polarization potential is provided
by the substrate generator 18 to produce an electrical field of a
relatively high value in the range of 50 to 500 eV, and preferably
80-300 eV. The low RF power applied for plasma stimulation provides
the directional etch with an extremely high rate. Specifically, the
plasma is "cold" due to the low energy applied so that the
directionality of the ions can be controlled. Also, the high
potential of the substrate 14 provides strong acceleration of the
ions toward the etched surface. Both factors provide excellent
directionality of the ions, resulting in a high anisotropic etch.
The etched portion 44 of the silicon substrate 40 is shown in FIG.
2(a).
[0026] After a certain period of time, e.g., 10 to 100 sec, the
power source 30 switches to a relatively high power, preferably in
the range of 1000 to 3000 W, to create high energy excitation of
the plasma and the polarization potential developed by the
substrate generator is reduced to a relatively low, or even zero,
value (i.e. 0-25 eV). High energy excitation of the plasma creates
a condition that results in the formation of a polymer layer 46.
Further, isotropic movement of species causes the thickness of the
polymer layer 46 on the bottom of the trench to be the same as the
thickness on the sidewalls, as shown in FIG. 2(b). During this
time, the low polarization on the substrate 14 prevents any
etching. After a certain time, e.g., 0.5 to 3 sec, the protective
polymer layer 46 of a predetermined thickness is formed on the
trench walls, as shown in FIG. 2(b). This protective polymer layer
46 prevents erosion of the trench walls that were formed in the
previous etching steps during the subsequent etching step,
performed in the manner discussed above.
[0027] Specifically, during the next etching step the previously
etched sidewalls remain protected by the polymer layer 46, while
the polymer layer on the bottom of the trench is rapidly stripped
by the anisotropic bombardment of the ions that are accelerated by
the high polarizing potential, allowing the silicon at the base of
the trench to be further etched, as identified by reference numeral
48 in FIG. 2(c). The etching and passivating steps are alternately
repeated until the trench reaches the required depth.
[0028] There are several advantages of this process when compared
to the conventional process discussed above. In particular, the
continuous introduction of a constant mixture of gases according to
the present invention results in a more even distribution of
species. This provides better uniformity across the substrate. In
contrast, in the conventional case, where gases are cycled, the
time ratio of the etch/deposition cycle depends on the speed of the
gases and varies from point to point, affecting the uniformity.
[0029] Further, when using the cycled gas method, the time for the
species to arrive at the bottom of the etched trench varies
drastically for trenches with different sized openings. This
increases the variation in the depths achieved for trenches with
different aspect ratios (Aspect Ratio Dependent Etch). In the
present case where the gas mixture is constant, the gas composition
will be uniform for trenches with different size openings. This
also improves the uniformity (microloading effect).
[0030] Further, the method of the present invention provides very
high selectivity to the mask material, due to the combination of
low power used during the etch and low polarization voltage used
during deposition.
[0031] Since numerous changes can be made without departing from
the spirit of the invention, it is intended that all matter
contained in the foregoing description and shown in the
accompanying drawings shall be interpreted as illustrative and not
in a limiting sense. For example, the invention is not limited to
the list of etchant and passivation gases discussed above.
Additional silicon etching gases include CF.sub.4, NF.sub.3, NF3HF,
HBr, CCL.sub.4, CF.sub.2Cl.sub.2, CFCl.sub.3, Br.sub.2, Cl.sub.2,
12, HCl, CIF.sub.3 and BCl.sub.3. Also, additional passivation
gases include CH.sub.4, CH.sub.2F.sub.2, H.sub.2, C.sub.2H.sub.4,
C.sub.3H.sub.8, CH.sub.3Br, C.sub.2F.sub.6, C.sub.2F.sub.4, and
C.sub.3F.sub.6.
[0032] In addition, the etch or passivation effect may be achieved
by alternating the pressure in the chamber. Specifically, the
polymer formation during passivation is effected by the pressure.
The higher the pressure the stronger the polymer formation and
hence the greater the degree of passivation. In contrast, the lower
the pressure the weaker the polymer formation and hence the lower
the degree of passivation. Similarly, an increase in pressure
during etching increases the etching rate, while a decrease in
pressure decreases the etching rate.
* * * * *