U.S. patent application number 11/627026 was filed with the patent office on 2007-05-31 for apparatus and method for detecting an endpoint in a vapor phase etch.
This patent application is currently assigned to Texas Instruments Incorporated. Invention is credited to Douglas B. MacDonald, Niles K. MacDonald, Satyadev R. Patel, Gregory P. Schaadt, Hongqin Shi.
Application Number | 20070119814 11/627026 |
Document ID | / |
Family ID | 32068709 |
Filed Date | 2007-05-31 |
United States Patent
Application |
20070119814 |
Kind Code |
A1 |
Patel; Satyadev R. ; et
al. |
May 31, 2007 |
APPARATUS AND METHOD FOR DETECTING AN ENDPOINT IN A VAPOR PHASE
ETCH
Abstract
Processes for the removal of a layer or region from a workpiece
material by contact with a process gas in the manufacture of a
microstructure are enhanced by the ability to accurately determine
the endpoint of the removal step. A vapor phase etchant is used to
remove a material that has been deposited on a substrate, with or
without other deposited structure thereon. By creating an impedance
at the exit of an etching chamber (or downstream thereof), as the
vapor phase etchant passes from the etching chamber, a gaseous
product of the etching reaction is monitored; and the endpoint of
the removal process can be determined. The vapor phase etching
process can be flow through, a combination of flow through and
pulse, or recirculated back to the etching chamber
Inventors: |
Patel; Satyadev R.; (Elk
Grove, CA) ; Schaadt; Gregory P.; (Santa Clara,
CA) ; MacDonald; Douglas B.; (Los Gatos, CA) ;
MacDonald; Niles K.; (San Jose, CA) ; Shi;
Hongqin; (San Jose, CA) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Assignee: |
Texas Instruments
Incorporated
Dallas
TX
|
Family ID: |
32068709 |
Appl. No.: |
11/627026 |
Filed: |
January 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10269149 |
Oct 11, 2002 |
7189332 |
|
|
11627026 |
Jan 25, 2007 |
|
|
|
09954864 |
Sep 17, 2001 |
6942811 |
|
|
10269149 |
Oct 11, 2002 |
|
|
|
Current U.S.
Class: |
216/59 ;
156/345.24; 216/58; 438/706 |
Current CPC
Class: |
H01J 37/32935 20130101;
B81C 1/00587 20130101; B81C 2201/0132 20130101; B81C 1/00476
20130101; B81C 99/0065 20130101; B81C 2201/0138 20130101; H01J
37/32963 20130101; H01J 37/32449 20130101 |
Class at
Publication: |
216/059 ;
438/706; 216/058; 156/345.24 |
International
Class: |
C03C 25/68 20060101
C03C025/68; G01L 21/30 20060101 G01L021/30; H01L 21/306 20060101
H01L021/306; H01L 21/461 20060101 H01L021/461; B44C 1/22 20060101
B44C001/22 |
Claims
1-100. (canceled)
101. An apparatus for etching a sample, said apparatus comprising:
(a) a source of etchant gas; (b) an etching chamber in
communication with said source of etchant gas; (c) a recirculation
loop passing through said etching chamber; (d) a pump disposed
within said recirculation loop for recirculating etchant gas along
said recirculation loop; and (e) a gas analyzer for analyzing gas
components within the recirculation loop.
102. The apparatus in accordance with claim 101 which said source
of etchant gas comprises a source chamber.
103. The apparatus in accordance with claim 102 further comprising
an expansion chamber communicating with said source chamber and
with a gas source for a gas other than said etchant gas, said
expansion chamber arranged for mixing gas from said source chamber
with gas from said gas source.
104. The apparatus in accordance with claim 103 which said
expansion chamber is in communication with said recirculation
loop.
105. The apparatus in accordance with claim 101 further comprising
a filter disposed within said recirculation loop, said filter being
one that removes a member selected from the group consisting of
byproducts or effluent from gases flowing through said
recirculation loop, or particulates.
106. The apparatus in accordance with claim 101 in which said pump
is a dry pump.
107. The apparatus in accordance with claim 106 in which said dry
pump has no wet seals and adds no gas to said recirculation
loop.
108. The apparatus in accordance with claim 107 in which said dry
pump is a bellows pump.
109. The apparatus in accordance with claim 108 in which said
bellows pump comprises a housing with bellows-type wall sections
enclosing a hollow interior, and at least one partition disposed to
divide said hollow interior into a plurality of sections.
110. The apparatus in accordance with claim 101 in which said pump
is constructed to circulate etchant gas substantially continuously
within said recirculation loop.
111. The apparatus in accordance with claim 103 in which said pump
is defined as a first pump and said apparatus further comprises a
second pump arranged to draw gases from a member selected from the
group consisting of said expansion chamber, said source chamber,
and said recirculation loop.
112. The apparatus in accordance with claim 103 further comprising
gas flow spreading means in said source chamber for diverting
incoming gas.
113. The apparatus in accordance with claim 112 in which said gas
flow spreading means is a baffle.
114. The apparatus in accordance with claim 112 in which said gas
flow spreading means is a perforated plate.
115. The apparatus in accordance with claim 101, further comprising
an energy source and/or electric field source at the etching
chamber for forming a plasma therein.
116. The apparatus in accordance with claim 102 in which said
source of etchant gas further comprises fluoride crystals retained
within said source chamber.
117. The apparatus in accordance with claim 116 in which said
fluoride crystals are xenon difluoride crystals.
118. The apparatus in accordance with claim 103 in which said gas
source for a gas other than said etchant gas comprises a source of
a gas with molar averaged molecular weight less than or equal to
that of N2.
119. The apparatus in accordance with claim 118 in which said gas
other than said etchant gas is a member selected from the group
consisting of Ar, Ne, He and N2.
120. The apparatus in accordance with claim 103 in which said gas
source for a gas other than said etchant gas comprises a plurality
of gas sources, the gases from which, when mixed, yield a gaseous
mixture with molar averaged molecular weight less than or equal to
that of N2.
121. The apparatus in accordance with claim 120 in which said
plurality of gas sources are sources of two or more members
selected from the group consisting of Ar, Ne, He and N2.
122. A method, comprising: a) providing a sample to be etched in a
chamber; b) providing an etchant to the chamber, capable of etching
the sample; c) providing no or substantially no impedance to gas
exiting the etching chamber; d) monitoring a partial pressure of an
etch product; and e) repeating steps a) to d) except providing an
increased impedance each time steps a) to d) are repeated, until an
impedance is reached that allows for determining an endpoint based
on monitoring the partial pressure of the etch product.
123. The method of claim 122, wherein an impedance is reached that
results in a partial pressure that begins to decrease at or near a
time that the endpoint of the etch is reached.
124. The method of claim 122, wherein an endpoint of the etch
corresponds to a point where all of the material has been
removed.
125. The method of claim 122, wherein the material is silicon.
126. An apparatus comprising: an etching chamber; a source of an
etchant; a gas recirculation loop for recirculating the etchant
repeatedly through the etching chamber; and a gas analyzer within
the etching chamber or within the gas flow line downstream of the
etching chamber.
127. The apparatus of claim 126, wherein the gas analyzer is a
spectrometer.
128. The apparatus of claim 127, wherein the spectrometer is
capable of detecting levels of fluoride etchants or fluoride
etching products.
129. The apparatus of claim 126, wherein the gas analyzer is a
residual gas analyzer.
130. The apparatus of claim 126, further comprising a semiconductor
or visible light transmissive wafer held on a wafer chuck in the
etching chamber.
131-135. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] This U.S. patent application is a divisional patent
application of co-pending U.S. patent application Ser. No.
10/269,149 to Patel et al filed Oct. 10, 2002, which is a
continuation in part of U.S. patent application Ser. No. 09/954,864
to Patel et al., filed Sep. 17, 2001. This application is related
to U.S. patent application Ser. No. 09/427,841, to Patel et al.,
filed Oct. 26, 1999, and U.S. patent application Ser. No.
09/649,599 to Patel et al, filed Aug. 28, 2000, the subject matter
of each being incorporated herein by reference.
[0002] End point detection in plasma etching reactions is known in
the art. However, end point detection by monitoring gases with a
gas analyzer in a non-plasma system has not been available till
now, particularly in a flow-through or recirculation etch system.
Though other types of end point detection methods have been used in
etch systems (optical monitoring, electrical monitoring, etc.),
such methods can be difficult to set up and inaccurate.
[0003] The present invention is in the area of the manufacture of
MEMS (microelectromechanical systems) as well as semiconductor
devices, or any other devices that require removal of a material in
accordance with the present invention. In particular, this
invention addresses gas-phase etching procedures, with particular
emphasis on detection of the endpoint in an etching process. The
invention is also directed to apparatus useful for etching and
detecting the endpoint of the etching reaction. "MEMS",
"microelectromechanical" and "micromechanical" are used
interchangeably throughout this application and each may or may not
have an electrical component in addition to the microstructure
component. The end point detected can be a point in an etch process
where all of the material that is capable of reacting with the
etchant gas has been removed and there is no more of the material
remaining on the substrate or exposed to the etchant gas.
[0004] The use of etchants for removing sacrificial layers or
regions in a multilayer structure without removal of an adjacent
layer or region is a common step in the manufacture of
semiconductor and MEMS devices. The MEMS devices of the present
invention can be devices for inertial measurement, pressure
sensing, thermal measurement, micro-fluidics, optics, and
radio-frequency communications, with specific examples including
optical switches, micromirror arrays for projection displays,
accelerometers, variable capacitors and DC or RF switches. If a
semiconductor device is etched, it can be any device that is made
of or has thereon a material that is to be removed with a gas phase
chemical etchant.
[0005] The success of an etch step in the manufacture of
microstructures is improved not only due to the selectivity of the
etchant, but also due to the ability to accurately determine the
endpoint of the etching process. Isotropic etching is of particular
interest in processes where the purpose of the etch is to remove a
sacrificial layer that is intervening between functional layers or
between a functional layer and a substrate. Gas phase etchants,
particularly in the absence of plasma, are desirable for
isotropically removing a sacrificial layer.
[0006] Of potential relevance to certain embodiments of this
invention is the prior art relating to particular etchant gases.
Prominent among the etchants that are used for the removal of
sacrificial layers or regions in both isotropic and anisotropic
etching procedures are noble gas fluorides and halogen fluorides.
These materials, used in the gas phase, selectively etch silicon
relative to other materials such as silicon-containing compounds,
non-silicon elements, and compounds of non-silicon elements.
Descriptions of how these materials are used in etching procedures
appear in co-pending U.S. patent application Ser. Nos. 09/427,841
and 09/649,569 to Patel et al. and in portions of the present
specification that follow.
[0007] The method of the present invention is useful for detecting
an endpoint in methods for producing deflectable MEMS elements
(deflectable by electrostatic or other means) which, if coated
(before or after gas phase processing) with a reflective layer, can
act as an actuatable micromirror. Arrays of such micromirrors can
be provided for direct view or projection display systems (e.g.
projection television or computer monitors). If the micromirrors
are provided alone or in an array and of a size of preferably 100
micrometers or more (preferably 500 micrometers or more), the
mirror can be useful for steering light beams, such as in an
optical switch. The present invention is also adaptable to
detecting an endpoint in methods for etching microfabricated
devices other than MEMS devices (e.g. semiconductor based devices,
carbon nanotubes on glass, etc.)
SUMMARY OF THE INVENTION
[0008] The present invention provides improvements in the apparatus
and methods used for the etching of layers or areas, and in
particular, for determining an end of the etching reaction. In one
embodiment of the invention, a method for etching a sample
comprises: providing a sample to be etched in a chamber; providing
a vapor phase etchant to the chamber to etch the sample, the vapor
phase etchant capable of etching the sample in a non-energized
state; monitoring the gas from the etching chamber; and determining
the end point of the etch based on the monitoring of the gas from
the etching chamber.
[0009] Another example of the invention is a method for etching a
sample, comprising: providing a sample to be etched to an etching
chamber; passing a gas phase etchant through the etching chamber;
impeding the gas flow out of the etching chamber, wherein the
impedance is less than infinity but greater than 0; analyzing the
gas from the etching chamber and determining an end of the
etch.
[0010] A further aspect of the invention is an etching method, that
comprises etching a material from a sample with a gas phase
etchant; monitoring one or more gas components from the etching
reaction substantially in the absence of plasma and determining the
endpoint of the etching reaction based on the monitoring of the one
or more gas components.
[0011] Another embodiment of the invention is a method for etching
a material, comprising: performing an etch on a material on a
substrate by providing an etchant so as to chemically but not
physically etch the material on the substrate; monitoring an etch
product of the material being etched; and determining an endpoint
of the etch of the material based on the monitoring of the etch
product.
[0012] Still another example of the invention is a method,
comprising: a) providing a sample to be etched in a chamber; b)
providing an etchant to the chamber, capable of etching the sample;
c) providing no or substantially no impedance to gas exiting the
etching chamber; d) monitoring a partial pressure of an etch
product; repeating steps a) to d) except providing an increased
impedance each time steps a) to d) are repeated, until an impedance
is reached that allows for determining an endpoint based on
monitoring the partial pressure of the etch product.
[0013] Another part of the invention is an apparatus. The apparatus
comprises an etching chamber; a source of a vapor phase spontaneous
chemical etchant; a gas flow line for recirculating the etchant; a
gas analyzer connected to the etching chamber or to the gas flow
line downstream of the etching chamber, though preferably upstream
of any impedance in the gas out flow line from the etching
chamber.
[0014] Another example of the apparatus of the invention comprises:
an etching chamber; a source of etchant capable of being in fluid
communication with an entrance aperture in the etching chamber; an
exit flow line connected to an exit aperture in the etching
chamber; and an impedance valve within the exit flow line for
providing an impedance to the gas flow out of the etching
chamber.
[0015] Yet another apparatus in accordance with the present
invention is an apparatus comprising: an etching chamber; a holder
for holding a sample to be etched; a source of gas phase etchant
for supplying a gas phase etchant to the etching chamber, wherein
the gas phase etchant is a fluoride compound capable of etching a
sample in a non-energized state; and a gas analyzer for analyzing
gas components from the etching of the sample.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a diagram showing an example of a system for
etching and stopping the etch in accordance with the present
invention;
[0017] FIG. 2 is a diagram of a second example of a system for
etching and stopping the etch in accordance with the present
invention;
[0018] FIG. 3A is a side elevation view of one example of a
reciprocating pump for use in one embodiment of the invention;
[0019] FIG. 3B is a pump flow diagram of the reciprocating pump of
FIG. 3A together with associated flow lines and shutoff valves;
[0020] FIG. 4 is a process flow diagram for the apparatus of FIG.
2;
[0021] FIG. 5 is a graph of the partial pressure (ion current in a
residual gas analyzer) of different etching products vs. time in
the invention;
[0022] FIG. 6A is a graph of the partial pressure of SiF3 vs.
time;
[0023] FIG. 6B is a graph of the data of FIG. 6A back averaged over
40 previous data points;
[0024] FIG. 7A is a graph of the derivative taken from the data of
FIG. 6B;
[0025] FIG. 7B is a graph of the data of FIG. 7A back averaged over
40 previous data points; and
[0026] FIG. 8 is a graph of the partial pressure (ion current in a
residual gas analyzer) of different etching products vs. time in a
prior art method and apparatus.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0027] While this invention is susceptible to a variety of
constructions, arrangements, flow schemes, and embodiments in
general, the novel features that characterize the invention are
best understood by first reviewing a typical process flow
arrangement in which the various aspects of this invention might be
used.
[0028] As can be seen in FIG. 1, an apparatus is provided for
etching a sample that includes a source chamber 11 containing a
source of chemical etchant, maintained at a particular temperature
and pressure for maintaining the etchant source in a solid or
liquid state (e.g. solid state for XeF2 crystals, liquid state for
BrF3, etc.). An expansion chamber 12 is in fluid communication with
source chamber 11 and has any suitable size (e.g. a volumetric
capacity of 29 cubic inches (0.46 liter)) to receive etchant gas
from the source chamber 11, with a shutoff valve 13 joining these
two chambers. An etch chamber 15 is provided in fluid communication
with expansion chamber 12 and has any suitable size (e.g.
volumetric capacity of 12 cubic inches (0.18 liter)) to contain the
sample microstructure to be etched. It is preferred that the etch
chamber be smaller than the expansion chamber. The etch chamber 15
is connected to the expansion chamber 12 via a shutoff valve 85.
Also included in the apparatus is a first gas source 19 in fluid
communication with the expansion chamber 12 via a further shutoff
valve 21, a second gas source 20 in fluid communication with the
expansion chamber through a separate shutoff valve 22, a vacuum
pump 23 and associated shutoff valves 24, 25 to control the
evacuation of the chambers.
[0029] Also shown in FIG. 1 are a third gas source 29 serving as a
pump ballast with an associated shutoff valve 30 to prevent
backstreaming from the pump 23, and needle valves 32, 33, 31 to set
the gas flow rates through the various lines and to permit fine
adjustments to the pressures in the chambers. Also shown, as will
be discussed in more depth below, are gas analyzer 1 and valves 3
and 5 on opposite sides of the analyzer. The expansion chamber 12
and the etch chamber 15 can both be maintained at a particular
temperature, while different gases are placed in the first and
second gas sources for the various etching processes. It should be
noted that a single gas source could be used in place of gas
sources 19 and 20.
[0030] The general procedure followed in these experiments began
with the evacuation of both the expansion chamber 12 and the etch
chamber 15, followed by venting both chambers to atmospheric
pressure with gas from the first gas source 19 by opening the two
shutoff valves 21, 85, between this gas source and the two
chambers. The sample was then placed in the etch chamber 15 (with
the shutoff valves 21, 85 open during the sample insertion) which
was then sealed, and both the expansion chamber 12 and the etch
chamber 15 were evacuated. All valves were then closed.
[0031] The connecting valve 85 between the expansion chamber 12 and
the etch chamber 15 was opened, and the shutoff valve 21 at the
outlet of the first gas source 19 was opened briefly to allow the
gas from the first gas source to enter the expansion and etch
chambers. The shutoff valve 21 is then closed. The connecting valve
85 is then closed, and the expansion chamber 12 is evacuated and
isolated. The supply valve 13 from the etchant source chamber 11 is
then opened to allow etchant gas to enter the expansion chamber
(due to the higher temperature of the expansion chamber). The
supply valve 13 is then closed, outlet valve 25 is opened, and the
needle valve 33 is opened slightly to lower the etchant pressure in
the expansion. Both the outlet valve 25 and the needle valve 33 are
then closed. The shutoff valve 22 at the second gas source 20 is
then opened and with the assistance of the needle valve 32, gas
from the second gas source is bled into the expansion. At this
point the expansion chamber 12 contains the etchant gas plus gas
from the second gas source 20, while the etch chamber 15 contains
gas from the first gas source.
[0032] With pump 23 on, the connecting valve 85 between the
expansion chamber 12 and the etch chamber 15 is then opened, and
valves 3 and 5 are opened on both sides of gas analyzer 1, to allow
the gas mixture from the expansion chamber to enter the etch
chamber and flow through the etch chamber and gas analyzer, thereby
beginning the etch process. As will be discussed further below, the
etch process is continued until an end point is detected via the
gas analyzer.
[0033] Many alternatives to the process scheme described above can
be used. Additional gas sources and chambers, for example, can be
utilized. For example, depending upon the diluent(s) used (gas
sources 19 and 20), a plurality of diluent sources (N2, Ar, He,
etc.) can be connected to the expansion chamber and/or to the
recirculation loop for bleeding the system after an etch. The air
distribution system within the etching chamber can also be varied,
for example by including U-shaped or cone-shaped baffles, or by
using additional perforated plates and/or baffles.
[0034] A specific alternative to the embodiment of FIG. 1 is
illustrated in FIG. 2. FIG. 2 represents such a process flow
arrangement in which the process is an etching process having a
detectable end point. The etchant gas originates in a source
chamber 11. An example of an etchant gas that is conveniently
evaporated from a liquid is bromine trifluoride, whereas an example
of an etchant gas that is conveniently sublimated from solid
crystals is xenon difluoride. Effective results can be achieved by
maintaining the crystals under 40 degrees C. (e.g. at a temperature
of 28.5.degree. C.). (Xenon difluoride is only one of several
etchant gases that can be used. Examples of other gases are
presented below.) The sublimation pressure of xenon difluoride
crystals at 28.5.degree. C. is 5-11 mbar (4-8 torr). An expansion
chamber 12 receives xenon difluoride gas from the crystals in the
source chamber(s) 11, and a shutoff valve 13 is positioned between
the source and expansion chambers. The sample 14 to be etched is
placed in an etch chamber 15 (which contains a baffle 16 a
perforated plate 17), and a reciprocating pump 18 that is
positioned between the expansion chamber 12 and the etch chamber
15. (The reciprocating pump and its valves are shown in more detail
in a FIGS. 3a and 3b and described below.) Also illustrated in FIG.
2, and will be discussed further below, is a gas analyzer 1 with
valves 3 and 5 that control the flow of gas from the etching
chamber through the gas analyzer.
[0035] Also shown are two individual gas sources 19, 20 supplying
the expansion chamber 12 through shutoff valves 21, 22, a vacuum
pump 23 and associated shutoff valves 24, 25, 26, 27, 28 to control
the evacuation of the chambers, a third gas source 29 serving as a
pump ballast with an associated shutoff valve 30 to prevent
backstreaming from the pump 23, and manually operated needle valves
31, 32, 33, 34, 35, 83 to set the gas flow rates through the
various lines and to permit fine adjustments to the pressures in
the chambers. When xenon difluoride is used, the expansion chamber
12 and the etch chamber 15 are typically maintained at around room
temperature (e.g. 25.0.degree. C.). However, the expansion chamber
and etch chamber could also be heated (e.g. to between 25 and 40
degrees C.), though this would likely be performed in conjunction
with directly cooling the sample being processed, as will be
discussed below. A recirculation line 36 permits gas to flow
continuously through the etch chamber 15 in a circulation loop that
communicates (via valves 26, 27, and 34, 35) with the expansion
chamber 12 and reenters the etch chamber 15 by way of the
reciprocating pump 18. Valve 85 permits gas transfer between
expansion chamber 12 and etch chamber 15 via a portion of the
recirculation line 36 without traversing recirculation pump 18.
Valve 86 in path 40 permits introduction of etchant gas into the
expansion chamber 12 to replenish the etchant mixture during the
etching process. The valves are preferably corrosive gas resistant
bellows-sealed valves, preferably of aluminum or stainless steel
with corrosive resistant O-rings for all seals (e.g. Kalrez.TM. or
Chemraz.TM.). The needle valves are also preferably corrosion
resistant, and preferably all stainless steel. A filter 39 could be
placed in the recirculation line 36 to remove etch byproducts from
the recirculation flow (though preferably not the product(s) being
monitored for end point detection), thereby reducing the degree of
dilution of the etchant gas in the flow. The filter can also serve
to reduce the volume of effluents from the process.
[0036] The etch chamber 15 can be of any shape or dimensions, but
the most favorable results will be achieved when the internal
dimensions and shape of the chamber are those that will promote
even and steady flow with no vortices or dead volumes in the
chamber interior. A preferred configuration for the etch chamber is
a circular or shallow cylindrical chamber, with a process gas inlet
port at the center of the top of the chamber, plus a support in the
center of the chamber near the bottom for the sample, and an exit
port in the bottom wall or in a side wall below the sample support.
The baffle 16 is placed directly below the entry port. The distance
from the port to the upper surface of the baffle is not critical to
this invention and may vary, although in preferred embodiments of
the invention the distance is within the range of from about 0.1 cm
to about 6.0 cm, and most preferably from about 0.5 cm to about 3.0
cm. Although its shape is not shown in FIG. 2, the baffle is
preferably circular or otherwise shaped to deflect the gas stream
radially over a 360.degree. range. The perforated plate 17 is wider
than the baffle 16 and preferably transmits all gas flow towards
the sample. A preferred configuration for the perforated plate is
one that matches the geometry of the sample; thus, for a circular
sample the perforated plate is preferably circular as well.
[0037] FIGS. 3a and 3b are diagrams of an example of a
reciprocating pump 18 that can be used in the practice of this
invention. The design shown in these diagrams can be varied in
numerous ways, such as by increasing the number of chambers to
three or more, or by arranging a series of such pumps in parallel.
The following discussion is directed to the particular design shown
in these diagrams.
[0038] The side elevation view of FIG. 3a shows the pump housing
41, which consists of two stationary end walls 42, 43 joined by
bellows-type side walls 44, 45. The bellows-type side walls 44, 45
are so-called because they are either pleated like an accordion or
otherwise constructed to permit bellows-type expansion and
contraction. The end walls 42, 43 and the bellows-type side walls
44, 45 together fully enclose the interior of the pump except for
inlet/outlet ports on each side wall. The arrangement of these
ports is shown in the pump flow diagram of FIG. 3b , the left side
wall 42 having one inlet/outlet port 46, and the right side wall 43
likewise having one inlet/outlet port 48. Remotely controlled
shutoff valves 51, 52, 53, 54 are placed on the external lines
leading to or from each inlet/outlet port. For fail-safe operation,
shutoff valves 51, 54 are normally open and shutoff valves 52, 53
are normally closed.
[0039] The movable partition 60 shown in FIG. 3a divides the pump
interior into two chambers 61, 62, the partition and its
connections to the remaining portions of the housing being
fluid-impermeable so that the two chambers are completely separate
with no fluid communication between them. The partition 60 joins
the bellows-type side walls 44, 45 and moves in the two directions
indicated by the two-headed arrow 63. The movement is driven by a
suitable drive mechanism (not shown) capable of reciprocating
movement. Many such drive mechanisms are known to those skilled in
the art and can be used. In the view shown in FIG. 3a, movement of
the partition to the left causes the left chamber 61 to contract
and the right chamber 62 to expand. With the pump shutoff valves
appropriately positioned, i.e., valves 52 and 53 open and valves 51
and 54 closed, the contracting left chamber 61 will discharge its
contents through its inlet/outlet port 46 while the expanding right
chamber 62 will draw gas in through its inlet/outlet port 48. Once
the partition 60 has reached the end of its leftward travel, it
changes direction and travels to the right and the shutoff valves
are switched appropriately, causing the expanded right chamber 62
to contract and discharge its contents through its inlet/outlet
port 48 while the contracted left chamber 61 expands and draws
fresh gas in through its inlet/outlet port 46. In this manner, the
pump as a whole produces a gas flow in a substantially continuous
manner, the discharge coming alternately from the two chambers. A
controller 64 governs the direction and range of motion, and the
speed and cycle time of the partition 60, and coordinates the
partition movement with the opening and closing of the shutoff
valves 51, 52, 53, and 54. Conventional controller circuitry and
components can be used.
[0040] The pump for recirculating the process gas as shown, and
others within the scope of this invention, has no sliding or
abrading parts or lubricant that come into contact with the process
gas. Alternative pumps that meet this criteria are possible,
including pumps with expandable balloon chambers, pumps with
concentric pistons connected by an elastic sealing gasket, or
peristaltic pumps. The pump materials, including the bellows-type
walls, can thus be made of materials that are resistant or
impervious to corrosion from the etchant gas. One example of such a
material, useful for operating temperatures below 50.degree. C., is
stainless steel. Others are aluminum, Inconel, and Monel. Still
others will be readily apparent to those experienced in handling
these gases. While the capacity and dimensions of the pump and its
chambers may vary, a presently preferred embodiment is one in which
the change in volume of each chamber upon the movement of the
partition across its full range is approximately from 0.05 to 4.2
L, though preferably from 0.1 to 1.5 L, with one example being 0.5
L. Larger chamber sizes (e.g. 5 to 20 L) are possible, which, if
combined with a slower pumping speed, can benefit from lower wear
on the pump. At a partition speed of one cycle every two seconds,
the pump rate (for 0.5 L) will be 30 L/min. Different combinations
of pump sizes and pump speeds are possible, though the preferred
pump volume per time is between 7 and 150 L/min, with a preferred
range of from 30 to 90 L/min.
[0041] The pump described above can be lined with a suitable lining
to further reduce particulate contamination of the process gas
mixture during etching. Pumps that are not of the bellows type can
also be used. The preferred pumps are those that are resistant to
corrosion by the process gas mixture and those that are designed to
avoid introducing particulate or liquid material into the process
gas mixture. Dry pumps, i.e., those that do not add exogenous purge
or ballast gas into the process gas mixture, are preferred.
Alternatively, the process gas could be circulated by temperature
cycling (with large variations in the heating and cooling of the
recirculation path).
[0042] The following is a generalized description of the etching
process and its chemical parameters in relation to FIG. 2. This
description is included to show the context in which the features
described above are most likely to be used.
[0043] The apparatus and methods of this invention can be used in
etching processes that are known in the art and in the literature.
These processes include the use of dry-etch gases in general,
including C12, HBr, HF, CC12F2 and others. Preferred etchant gases,
particularly for etching silicon, are gaseous halides (e.g.
fluorides) such as noble gas fluorides, gaseous halogen fluorides,
or combinations of gases within these groups (again, preferably
without energizing the gas, other than heating to cause
vaporization or sublimation). The noble gases are helium, neon,
argon, krypton, xenon and radon, and among these the preferred
fluorides are fluorides of krypton and xenon, with xenon fluorides
the most preferred. Common fluorides of these elements are krypton
difluoride, xenon difluoride, xenon tetrafluoride, and xenon
hexafluoride. The most commonly used noble gas fluoride in silicon
etch procedures is xenon difluoride. Halogen fluorides include
bromine fluoride, bromine trifluoride, bromine pentafluoride,
chlorine fluoride, chlorine trifluoride, chlorine pentafluoride,
iodine pentafluoride and iodine heptafluoride. Preferred among
these are bromine trifluoride, bromine trichloride, and iodine
pentafluoride, with bromine trifluoride and chlorine trifluoride
the more preferred. Combinations of bromine trifluoride and xenon
difluoride are also of interest. The etch process is generally
performed at a pressure below atmospheric. It is preferred that the
etchants described herein be used in the gaseous state (e.g.
non-plasma) or otherwise in the absence of added energy (except
heat to aid sublimation or vaporization of the starting etchant gas
or liquid), and in the absence of electric fields, UV light or
other electromagnetic energy, or other added fields or energy
sources which would energize the etchant gas beyond it's normal
energy as a gas at a particular temperature.
[0044] The etch preferably utilizes an etchant gas capable of
spontaneous chemical etching of the sacrificial material at room
temperature, preferably isotropic etching that chemically (and not
physically) removes the sacrificial material. In a preferred
embodiment, the etchant is capable at room temperature of reacting
with the sacrificial material and where the reaction product(s) is
a gaseous component that is released from the sacrificial material
surface due to being in a gaseous state. No UV or visible light or
other electromagnetic radiation or electric fields are needed, or
any energy that would energize the gas molecules to physically
bombard and physically remove the sacrificial material. Though the
etch could be performed with the application of heat or the
presence of light from the room surrounding the etch chamber, the
etchant is capable of spontaneously etching the sacrificial
material at room temperature without any applied heat, visible, UV
or other electromagnetic radiation, ultrasonic energy, electric or
magnetic fields, etc. The etchant is preferably not broken down
into atoms, radicals and/or ions by an rf glow discharge, the
etchant is transported by diffusion to the surface of the material
being etched (in addition to pumping--e.g. by recirculating the
etchant repeatedly through the etching chamber), the etchant is
adsorbed on the surface, a chemical reaction occurs between the
etchant and the material being etched with the formation of a
volatile product, and the product is desorbed from the surface and
diffuses into the bulk of the gas and eventually exits the etching
chamber. The apparatus, therefore, can be without a source of RF or
microwave energy, though it is possible to run the process of the
invention in a plasma apparatus without energizing the etchant to
form a plasma.
[0045] Taking as an example BrCl3, a molecule of BrCl3 could
hypothetically be placed next to a silicon molecule bound to other
silicon molecules in crystalline silicon, polysilicon or in an
amorphous silicon matrix. The bond energies of the Cl atoms to the
Br atoms are sufficiently weak, and the bond energy of the silicon
atom to other silicon atoms is sufficiently weak, and the
attraction forces between Si and Cl are sufficiently strong, that
without a physical bombardment of the BrCl3 on the silicon, Cl will
disassociate from Br and bond to Si to form various products such
as SiCl, SiCl2, SiCl3, SiCl4, etc., which etch products are a gas a
room temperature and dissipate from the silicon surface, thus
removing sacrificial silicon material. The same process occurs with
XeF2, BrF3 and the other vapor phase spontaneous chemical
etchants.
[0046] Such chemical etching and apparatus for performing such
chemical etching are disclosed in U.S. patent application Ser. No.
09/427,841 to Patel et al. filed Oct. 26, 1999, in U.S. patent
application Ser. No. 09/649,569 to Patel at al. filed Aug. 28,
2000, mentioned previously, and in U.S. Patent Application
60/293,092 to Patel et al. filed May 22, 2001 incorporated herein
by reference. Preferred etchants for the etch are gas phase
fluoride etchants that, except for the optional application of
temperature, are not energized. Examples include gaseous acid
etchants (HF, HCl, HI etc.), noble gas halides such as XeF2, XeF6,
KrF2, KrF4 and KrF6, and interhalogens such as IF5, BrCl3, BrF3,
IF7 and ClF3. It is also possible to use fluorine gas, though
handling of fluorine gas makes this a less desirable option. The
etch may comprise additional gas components such as N2 or an inert
gas (Ar, Xe, He, etc.). In the etching process, except for optional
heating, the gas is not energized and chemically etches the
sacrificial material isotropically. In this way, the sacrificial
material is removed and the micromechanical structure is released.
In one aspect of such an embodiment, BrF3 or XeF2 are provided in a
chamber with diluent (e.g. N2 and He). An initial plasma etch,
preferably in a separate etching apparatus, can be performed prior
to etching as set forth above. This sequential etch is set forth
further in U.S. Patent Application 60/293,092 to Patel et al. filed
May 22, 2001, the subject matter of which is incorporated herein by
reference.
[0047] While the source chamber 11 can be a single chamber, the
arrangement shown in FIG. 4 is an optional one in which the source
chamber is actually a pair of chambers 11a and 11b arranged in
series. The first of these chambers 11a contains the source
material primarily in its condensed form, i.e., either as crystals
to be sublimated or liquid to be vaporized. The second chamber 11b
receives the source material gas evolved by sublimation from the
crystals or by vaporization from the liquid in the first chamber
11a. To maintain these phases, the two chambers 11a and 11b will
preferably be maintained at different temperatures (preferably at
least 5 degrees C. difference), the former 11a at the lower
temperature to keep the material in its condensed form (solid
crystals or liquid) and the latter 11b at the higher temperature
(and possibly a higher pressure as well) to keep the material in
the vapor form (and to avoid the problem of condensation) at a
pressure sufficiently high to allow it to be supplied to the
succeeding chambers at the pressures needed in those chambers. In
one embodiment, the two chambers are held at temperatures above
room temperature, with chamber 11b held at a temperature from 2 to
24 degrees C. (preferably around 5 to 10 degrees C.) higher than
the temperature of chamber 11a. Such a two-chamber embodiment could
likewise be utilized in a system such as that illustrated in FIG.
1. Chambers 11a and 11b could also be arranged in parallel. Also
shown in FIG. 4 are the expansion chamber 12, the etching chamber
15, and pumps 18 and 88.
[0048] The terms "sample" and "microstructure" are used herein to
denote the article from which a material is sought to be removed or
to which a material is to be added, whether the material forms a
layer among a plurality of layers, layers among a plurality of
layers or a region adjacent to other regions in the same plane. The
"sample" may thus be a single mirror element and its associated
layers of other materials, a test pattern, a die, a device, a
wafer, a portion of a wafer, or any article from which a portion is
to be removed or added. The invention is particularly suitable for
processes where the size of the portion of material that is being
added or removed is less than 5 mm in at least one of its
dimensions, preferably less than 1 mm, more preferably less than
500 .mu.m, and most preferably less than 100 .mu.m. The invention
is also well suited to adding or removing material (e.g., in the
form of holes or layers) in a submicron environment (e.g. in the
range of 10 nm to less than 1000 nm) (as sometimes needed, for
example, in MEMS and MOEMS).
[0049] In the system depicted in the drawings, as well as other
systems within the scope of this invention, a single charge of
etchant can be placed in the source chamber or the expansion
chamber, then released (with or without diluents) into the
recirculation loop. Additional etchant can be introduced to
replenish the circulating stream as the amount of etchant in the
recirculation loop decreases over time. Other variations of the
process will be apparent to those skilled in the art.
[0050] When the material to be removed by etching is silicon,
certain etching processes supply the etchant gas as a mixture of
gases of which one component is the etchant gas itself (or a
mixture of etchant gases) while other components are non-etchant
diluents. Whether the gas consists entirely of etchant gas(es) or
contains non-etchant components as well, the rate of the etch
reaction may vary with the partial pressure of the etchant gas. The
partial pressure may vary, but in most applications, particularly
those in which silicon is being etched, best results will be
obtained with the etchant gas at a partial pressure of at least
about 0.1 mbar (0.075 torr), preferably at least 1 Torr, more
preferably within the range of from about 1 to 760 Torr, and most
preferably from about 50 to 600 Torr. These pressure ranges are
particularly applicable to xenon difluoride etching.
[0051] In certain processes, non-etchant gas additive(s) are
included to increase the selectivity of the silicon etch. Preferred
additives for this purpose are non-halogen-containing gases. A
single such additive or a mixture of such additives can be used. In
preferred embodiments of this invention, the additives are those
whose molar-averaged formula weight (expressed in daltons or grams
per mole) is less than the formula weight of molecular nitrogen,
preferably about 25 or less, still more preferably within the range
of from about 4 to about 25, still more preferably within the range
of from about 4 to about 20, and most preferably within the range
of from about 4 to about 10. If a single additive species is used,
the "molar-averaged formula weight" is the actual formula weight of
that species, whereas if two or more additive species are used in
the same gas mixture, the molar-averaged formula weight is the
average of the formula weights of each species in the mixture
(exclusive of the noble gas fluoride) calculated on the basis of
the relative molar amounts (approximately equal to the partial
pressures) of each species. In terms of thermal conductivity,
preferred additives are those whose thermal conductivity at 300 K
(26.9.degree. C.) and atmospheric pressure ranges from about 10
mW/(m K) (i.e., milliwatts per meter per degree Kelvin) to about
200 mW/(m K), and most preferably from about 140 mW/(m K) to about
190 mW/(m K). In general, the higher the thermal conductivity of
the additive, the greater the improvement in selectivity. Examples
of additives suitable for use in this invention are nitrogen (N2,
thermal conductivity at 300 K: 26 mW/(m K)), argon (Ar, thermal
conductivity at 300 K: 18 mW/(m K)), helium (He, thermal
conductivity at 300 K: 160 mW/(m K)), neon (Ne, thermal
conductivity at 300 K: 50 mW/(m K)), and mixtures of two or more of
these gases. Preferred additive gases are helium, neon, mixtures of
helium and neon, or mixtures of one or both of these with one or
more non-etchant gases of higher formula weight such as nitrogen
and argon. Particularly preferred additives are helium and mixtures
of helium with either nitrogen or argon.
[0052] The degree of selectivity improvement may vary with molar
ratio of the additive to the etchant gas. Here again, the molar
ratio is approximately equal to the ratio of the partial pressures,
and in this case the term "molar ratio" denotes the ratio of the
total number of moles of the additive gas (which may be more than
one species) divided by the total number of moles of the etchant
gas (which may also be more than one species). In most cases, best
results will be obtained with a molar ratio of additive to etchant
that is less than about 500:1, preferably within the range of from
about 1:1 to about 500:1, preferably from about 10:1 to about
200:1, and most preferably from about 20:1 to about 150:1. In one
example, the ratio is set at 125:1.
[0053] The temperature at which the etch process is conducted can
likewise vary, although the partial pressure of the etchant gas
will vary with temperature. The optimal temperature may depend on
the choice of etchant gas, gaseous additive or both. In general,
and particularly for procedures using xenon difluoride as the
etchant gas, suitable temperatures will range from about
-60.degree. C. to about 120.degree. C., preferably from about
-20.degree. C. to about 80.degree. C., and most preferably from
about 0.degree. C. to about 60.degree. C. For xenon difluoride, the
silicon etch rate is inversely proportional to the temperature over
the range of -230.degree. C. to 60.degree. C. The improvement in
selectivity can thus be further increased by lowering the etch
process temperature.
[0054] The flow parameters will be selected such that the time
during which the sample will be exposed to the etchant gas will be
sufficient to remove all or substantially all of the silicon. By
"silicon" it is meant any type of silicon, including amorphous
silicon, single crystal silicon and polysilicon, which silicon can
have up to 40at % or more (typically from 5 to 25 at %) hydrogen
depending upon the deposition technique, as well as impurities that
can result from the target or atmosphere. The expression
"substantially all of the silicon" is used herein to denote any
amount sufficient to permit the finished product to function
essentially as effectively as if all of the silicon had been
removed. An example of the structures to which this invention will
be applied is that depicted in U.S. Pat. No. 5,835,256, in which a
silicon nitride layer is deposited over a polysilicon layer, and
the silicon nitride layer is patterned to leave vias that define
the lateral edges of the mirror elements. Access to the sacrificial
polysilicon layer is through the vias, and the etching process
removes the polysilicon below the vias by etching in the vertical
direction (i.e., normal to the planes of the layers) while also
removing the polysilicon underneath the silicon nitride by etching
in the lateral direction (parallel to the planes of the layers).
The process is also effective for etching silicon relative to
multiple non-silicon layers. Also, it should be noted that the
silicon can contain impurities, and in particular may contain a
large amount of hydrogen (e.g. up to 25 at % or more) depending
upon the deposition method used.
[0055] The process design shown in FIG. 2 is but one of many
designs in which endpoint detection in accordance with the present
invention can be achieved. The FIG. 2 design itself can be used
with many different combinations and sequences of valve openings
and closings. One such sequence is as follows: [0056] 1. Solid or
liquid etchant material (such as XeF2) is placed in the source
chamber(s) 11. [0057] 2. A sample 14 is placed in the etch chamber
15. [0058] 3. The expansion chamber 12 and the etch chamber 15 are
each evacuated. [0059] 4. The expansion chamber 12 and the etch
chamber 15 are preconditioned by a) flooding one or both of the
chambers with an inert gas (such as N2, for example), b)
implementing a temperature ramp (e.g. consisting of raising the
temperature of one or both of the chambers for fixed time followed
by cooling the temperature of one or both chambers after step 5 and
finishing with raising the temperature of one or both chambers
after step 15), or c) both flooding and implementing temperature
ramp. The sample temperature can be ramped to match or differ from
than the chamber temperature ramp. [0060] 5. Both the expansion
chamber 12 and the etch chamber 15 are then evacuated. [0061] 6.
The expansion chamber 12 and the etch chamber 15 are then filled
with one or more diluents from the individual gas sources 19, 20.
[0062] 7. The expansion chamber 12 is then evacuated. [0063] 8. The
expansion chamber 12 is then filled with XeF2 gas from the source
chamber(s) 11 (generated by sublimation from the XeF2 crystals in
the source chamber). [0064] 9. XeF2 gas is then pumped out of the
expansion chamber 12 by the vacuum pump 23 to lower the XeF2 gas
pressure in the expansion chamber to the desired XeF2 process
pressure to be used for etching the sample. [0065] 10. One or more
diluent gases from the gas sources 19, 20 are then added to the
expansion chamber 12. [0066] 11. All valves are then closed except
the manual needle valves. [0067] 12. The recirculation pump 18 is
then activated to start a flow of diluent gas through the etch
chamber 15. Also valves 3 and 5 are opened to allow part of the
recirculating gas to flow through the gas analyzer. [0068] 13. The
shutoff valves 26, 27 on the XeF2 recirculation loop are then
opened to cause XeF2 gas to enter the recirculation loop 36. [0069]
14. Recirculation of the XeF2 gas through the etch chamber is
continued until an endpoint to the etch is determined via the gas
analyzer. [0070] 15. Both the expansion chamber 12 and the etch
chamber 15 are then evacuated. [0071] 16. The expansion chamber 12
and the etch chamber 15 are post-conditioned by a) flooding one or
both of the chambers with an inert gas, b) increasing the
temperature of one or both of the chambers, c) pumping out one or
both of the chambers, or d) following a time ordered sequence of
one or more of flooding/heating/evacuating. [0072] 17. The finished
sample is then removed from the etch chamber.
[0073] This procedure can be varied without detriment to the
product quality. Steps 12 and 13, for example, can be performed in
reverse order. Other variations will be apparent to those skilled
in the art.
EXAMPLE
[0074] For etching a 6'' glass substrate with MEMS devices, typical
apparatus and process parameters include: double source chamber
design with 11a at 28 C, 11b at 31 C and intermediate connector
piece at 35 C. Expansion chamber 12 and etch chamber 15 at 23 C. In
step 6 above, both chambers 12 and 15 are filled with a mixture of
45 T Nitrogen (N2) and 450 T Helium (He); total gas pressure is 495
T. In step 8, the chamber 12 is filled with XeF2 gas above 4.2 T.
In step 9, the XeF2 gas in chamber 12 is reduced to 4 T for use in
the process. In step 10, chamber 12 receives 47 T of Nitrogen (N2)
and 470 T of Helium (He); total gas pressure in chamber 12 at the
end of step 10 is 521 T.
Endpoint Detection:
[0075] As can be seen in Toda R., Minami K., and Esashi M., "Thin
Beam Bulk Micromachining Based on RIE and Xenon Difluoride Silicon
Etching", Transducers '97, IEEE, pp. 671-3, Fourier Transform
spectroscopy is used to monitor the etching of silicon by xenon
difluoride. The process is run in pulse mode where the etchant gas
enters the etching chamber at the beginning of the etch, and the
etching chamber is evacuated only at the end of the etch. There is
a slow build up of SiF4 in the chamber which gradually forms a
plateau as the etch nears completion. With such an arrangement it
is very difficult to determine where along the plateau is the
proper endpoint. As stated in the reference, it is considered that
the reaction between XeF2 and silicon is mostly finished within 30
seconds after the SiF4 absorption peak is nearly saturated.
Attempting to pinpoint an endpoint on the plateau of the curve of
SiF4 is more of a guess than an actual calculated endpoint
determination.
[0076] Another XeF2 etching method is a flow through system where
an unimpeded gas flows out of the etching chamber at substantially
the same rate as etchant flows into the etching chamber. Such a
system is disclosed in EP 0878824 to Surface Technology Systems. If
a gas analyzer were to be placed at or downstream from the etching
chamber for analyzing etching products from the etching reaction,
due to a lack of impeding the gas flow out of the etching chamber
in accordance with the invention, only noise would be detected by
such a hypothetical arrangement (see FIG. 9).
[0077] In accordance with the present invention, whether the etch
system is a flow through continuous (or at least partially
continuous) system, or a recirculation etch system, a gas analyzer
is provided that is capable of accurately detecting an end point of
the etching reaction. Whether the gas flow is recirculated or
vented to ambient, it is desirable that the gas flow out of the
etching chamber in a vented to ambient system, or the gas flow out
of a recirculation loop in a recirculation system, be impeded to a
degree so as to allow for a build up of etching reaction product in
the etching chamber or recirculation loop (that includes the
etching chamber). The impedance can be any impedance as long as it
is greater than 0 (as in the flow through system mentioned above)
and less than infinite (as in the pulse system mentioned above).
The flow can be continuous or partially continuous (stop-start),
though a pure pulse mode--filling the etching chamber with etchant
and venting the etching chamber only after the etch process is
complete--is not desirable for detecting endpoint in accordance
with the invention).
[0078] Taking FIG. 1 as an example, after an etchant and diluent
are mixed in the expansion chamber, they are passed into etching
chamber 14 by opening valve 15. By running pump 21 and opening
valves 3 and 5, the etchant and diluent can be passed through the
etching chamber during the etch process. The movement of
etchant/diluent through the etching chamber can be continuous or
stop-start as long as gas from the etching chamber is passed out of
the etching chamber throughout the etch. Whether continuous or
semi-continuous, the average impedance will preferably not be
infinite (a closed-off chamber during the etch) and will be greater
than 0 (when there is no build up of etching products in the etch
chamber).
[0079] Or, referring to FIG. 2, while etchant and etching products
circulate from etching chamber 15 via recirculation line 36 and
pump 18 back through the etching chamber, an impedance is created
from this recirculation loop so that gas does not freely flow out
of the recirculation loop, though there is a small amount of gas
that flows out of the recirculation loop and into the etching
chamber 12 during the etch process. In this way gas flow is impeded
(less than an infinite impedance but greater than a 0 impedance) so
that etching products will initially build up in the recirculation
line, but then decrease once the material being etched has been
removed and the end point of the etching reaction has been reached.
The gas analyzer 1 will bleed off a very small amount of gas from
the recirculation loop and allow for monitoring of the etching
products.
[0080] The gas analyzer can be any suitable analyzer that is
capable of detecting etch products such as gaseous SiFx molecules
in a gas stream. Residual Gas Analyzers (RGA's) are available from
AMETEK, Anglo Scientific, Ferran Scientific, Hiden Analytical, VG
Gas Analysis Systems and Stanford Research Systems. Depending upon
the etch product, many gas analysis systems could be used,
including UV and visible spectrometers, Raman Spectrometers, NMR
Spectrometers, Mass Spectrometers, Infrared and Fourier Transform
Infrared, or Atomic Spectrometers.
[0081] When gas outflow is impeded from the etching chamber as
above, gas components monitored in a gas analyzer, particularly
gaseous etching products, increase and then decrease. As can be
seen in FIG. 5, etching products SiF3, SiF and SiF4 increase in
amount (ion current in a residual gas analyzer--RGA) up to a point
around 2000 seconds, which is the end point of the reaction. After
2000 seconds, the etching product amounts that are detected in the
RGA decrease. The increase in the initial curve is not found in a
flow through system (as can be seen in FIG. 9) and the decrease at
the end point is not found in pulse systems (as can be seen in FIG.
3 of the R. Toda reference mentioned above).
[0082] Taking SiF3 as an example, as can be seen in FIG. 6A, the
data from the RGA forms a rising then falling curve as also
illustrated in FIG. 5. If these data are back averaged (e.g. with
the previous 40 data points, a smoother curve results as shown in
FIG. 6B. Because the average is an average with previously acquired
data, this averaging can take place in real time. The new averaged
data of FIG. 6B can be used to take a derivative (the rate of
change of the etching product), which is the data shown in FIG. 7A.
This data can also be back averaged (over 40 data points) to result
in the curve shown in FIG. 7B. It is also possible to further
process the curves of FIG. 6B and/or FIG. 7B with additional curve
smoothing techniques as known in the art.
[0083] An accurate endpoint can be determined visually by a system
operator monitoring the curves of one or more etching products on a
computer monitor or print-out, or preferably, the end point is
automatically determined based on the data from the gas analyzer.
In a preferred embodiment, the end point is flagged (audio signal
or visual alert). The endpoint can be determined in a number of
ways. As can be seen in FIG. 6B, the RGA output increases and then
decreases at a time around 2000 sec (2000 sec is arbitrary and
depends upon the amount of sacrificial material being etched, the
etchant concentration, process temperature and pressure, etc.). A
software program can be used to look for a peak value from the gas
analyzer (corresponding roughly to the datum at time 2000 sec.) or
to look for a decrease (or average decrease over time)--also taking
place at around 2000 sec. in the example in FIG. 6B. In one method
of the invention, the endpoint is detected after the signal from
the gas analyzer decreases for 3/4 of all data points in a 25 to 40
point range. In another way of determining the end point, the
back-averaged data of FIG. 6B is again averaged over, e.g. 10 data
points or more, consecutively along the curve, and when the average
of any group of 10 (or more) data points is lower than the previous
10 point average, the end point is flagged.
[0084] As seen in FIGS. 7A and 7B, the derivative of the data in
FIG. 6B can be taken (FIG. 7A) and then back averaged (FIG. 7B).
Because graph 7B indicates the rate of change of the data of the
gas analyzer, similar to the discussion above with respect to FIG.
6B, when the rate of change passes across point 0 (again at time
2000 sec. in FIG. 7B) this indicates that the rate of change of the
detected etch product is no longer increasing and is, in fact
decreasing. Crossing from positive to negative values in FIG. 7B
can be monitored and flagged as the end point of the etching
reaction.
[0085] At the determined end point, if the method is being run in
real time, in a preferred embodiment, the etch process is
stopped--the bleeding of etchant into the expansion chamber (or
etching chamber if there is no expansion chamber) is stopped, and
any etchant and etch products are vented out of the etching chamber
with an inert gas (e.g. N2, Ne or Ar). It is also possible, upon
determination of the end point as above, to allow the etching
reaction to proceed for a predetermined period of time T (e.g. 20
to 100 seconds), in order to allow for slight over-etching in the
etch process. The stopping of the etch process upon end point
determination can be made manually or automatically.
[0086] Sacrificial silicon layers that can be removed using the
apparatus and method of this invention may be layers of crystalline
silicon, amorphous silicon, partially crystalline silicon,
crystalline silicon of multiple crystal sizes, polysilicon in
general, and silicon doped with such dopants as arsenic, phosphorus
or boron. Amorphous silicon and polysilicon are of particular
interest, although the relative crystalline vs. amorphous character
of polysilicon will vary considerably with the deposition
conditions, the presence or absence of dopants and impurities, and
the degree of annealing. Preferably the silicon sacrificial
material is removed at a relatively slow rate--preferably less than
1/3 um/min.
[0087] Silicon can be preferentially removed relative to
non-silicon materials by the method and apparatus of this
invention. The term "non-silicon material" denotes any material
that contains neither amorphous nor crystalline silicon in any of
the forms described in the preceding paragraph. Non-silicon
materials thus include silicon-containing compounds in which
elemental silicon is bonded to another element, as well as
non-silicon elements and compounds of non-silicon elements.
Examples of such non-silicon materials are titanium, gold,
tungsten, aluminum, and compounds of these metals, as well as
silicon carbide, silicon nitride, photoresists, polyimides, and
silicon oxides. Silicon nitride and silicon oxide are of particular
interest in view of their use in the structures disclosed in U.S.
Pat. No. 5,835,256. Two or more different non-silicon materials may
be present in a single structure, and selectivity of the silicon
etch relative to all such non-silicon materials will be
improved.
[0088] When the present invention is applied to the mirror
structures disclosed in U.S. Pat. No. 5,835,256, to remove silicon
layers from those structures, the thickness and lateral dimensions
of the layers may vary. The silicon portion will generally however
be a layer having a thickness of from about 200 nm to about 5,000
nm, preferably from about 250 nm to about 3,000 nm, and most
preferably from about 300 nm to about 1,000 nm. Similarly, the
non-silicon portion will generally be a layer with a thickness of
from about 10 nm to about 500 nm, preferably from about 20 nm to
about 200 nm, and most preferably from about 30 nm to about 200 nm.
The lateral distance that the etching process must extend under the
typical silicon nitride mirror element in the structures of U.S.
Pat. No. 5,835,256 in order to remove all of the underlying
polysilicon (this distance typically being half the shortest
lateral dimension of the mirror when the etching front travels
inward from opposing edges) may range from a submicron distance to
about 500 microns, preferably from about 3 microns to about 30
microns, and most preferably from about 5 microns to about 15
microns.
[0089] While much of the foregoing description is directed to
applications of the present invention to etching processes, the
invention, and particularly its recirculation aspect, is applicable
in general to processes for adding or removing layers within a
device, particularly layers that have microscopic dimensions.
Examples of such processes are passivation and etching of
semiconductor devices and MEMS/MOEMS devices, lithography (screen
printing, for example), thin-film deposition (chemical vapor
deposition e.g. application of self-assembled monolayers and
spluttering, for example), electroplating (blanket and
template-delimited electroplating of metals, for example), and
directed deposition (as used in electroplating, stereolithography,
laser-driven chemical vapor deposition, screen printing, and
transfer printing, for example). Further examples are plasma
etching, reaction-ion enhanced etching, deep reactive ion etching,
wet chemical etching, electron discharge machining, bonding (such
as fusion bonding, anodic bonding, and the application of
adhesives), surface modification (such as wet chemical modification
and plasma modification), and annealing (such as thermal or laser
annealing). The process gases in each case will be readily apparent
to those skilled in the respective arts. The present invention is
also useful in processes where at least one of the components of
the process gas is corrosive to metal in the presence of water
vapor. Corrosive components can be used for adding or removing
material in a microscopic device without picking up impurities that
will lower the product yield and that might damage the pump used in
the recirculation loop.
[0090] Further variations within the scope of the present invention
are as follows. The recirculation loop 36 of FIG. 2 can be expanded
to include the source chamber(s) 11. A valve arrangement can be
incorporated into the design that allows the operator to choose
between placing the source chamber within the recirculation loop
and isolating the source chamber from the recirculation loop.
Similarly, diluent gas can be added to the recirculation loop
simultaneously with the process gas, and an appropriate valve
arrangement can be incorporated that would provide the operator
with the option to do so. Appropriate valve arrangements can also
provide the option of extending the recirculation loop 36 through
the etch chamber 15 only or through both the etch chamber 15 and
the expansion chamber 12.
[0091] As also noted above, a filter 39 can be placed in the
recirculation loop 36 to filter out at least one of the byproducts
or effluents produced by the reactions occurring in the etch
chamber 15, though preferably not the etching product that is
monitored for end point detection. This improvement may be applied
to an etching or deposition process with or without energetically
enhancing the process gas. A selective filter that allows the
process gas to pass would be preferred. Of course, the filter can
be a basic particulate filter as well. Again, these are only
examples. Other variations and modifications will be readily
apparent to those skilled in the art. For example, the end point
calculations can take into account not only the data from the gas
analyzer, but also additional data if collected, such as data from
previously run samples, change in sample weight, optical monitoring
of the samples, etc. Use of neural networks for endpoint detection
are disclosed in, for example, Liamanond, S., Si, J., Yean-Ling
Tseng, "Production data based optimal etch time control design for
a reactive ion etching", IEEE Trans. on Semiconductor Manufact.,
2/99, vol 12, no. 1, p. 139-47, where neural networks are used to
model the functional relationship between an end point detection
signal from an RIE process, as well as various in situ
measurements, and the resulting film thickness remaining.
[0092] The end point detection of the present invention can be
achieved with a wide variety of etch rates, though in a preferred
embodiment an etch rate is selected that is slower than in the
prior art. In one embodiment of the invention, the etch rate is
less than 30 um/hr, and preferably less than 25 um/hr. Slower etch
rates can achieve better selectivity in the present invention, and
etch rates as low as 10 um/hr or less, or even 7.2 um/hr or less
can be performed for even greater improvements in selectivity.
Though total process time is impacted, etch rates as low as about 3
um/hr or less, 2 um/hr or less, or even 1 um/hr or less are within
the scope of the invention. Of course within all ranges above, the
etch rate is greater than 0.
[0093] Reducing the etch rate can be achieved by altering one or
more of the etch parameters known to effect etch rate (e.g. etchant
concentration, pressure, temperature, etc.). It is not as important
which parameter(s) is used to achieve the low etch rate as long as
the etch depth per time is within the low ranges as set forth
herein. Selectivity, depending upon the etch rate, can be 500:1
(relative to a "non silicon" material, such as a silicon
compound--e.g. silicon nitride or silicon oxide), 1000:1, 2000:1 or
even 10,000:1 or higher depending upon the etch rate and the
non-silicon material.
[0094] The selectivity of the etch can be further improved by use
of diluents with the gas phase chemical etchant. The etch
selectivity is increased by using as the etching medium a gas
mixture containing the etchant gas(es) and one or more of certain
additional but non-etchant gaseous components. While the inclusion
of non-etchant gaseous additives causes prolongation of the etch
time, those additives whose molar-averaged formula weight is below
that of nitrogen gas prolong the etch time to a much lesser extent
than do those whose molar-averaged formula weight is equal to or
greater than that of molecular nitrogen, while still achieving the
same improvement in selectivity. The improvement in selectivity is
achievable independently of the partial pressure of the etchant gas
in the gas mixture. Likewise, the limitation on the increase in
etch time when the averaged formula weight of the additive gas is
less than that of molecular nitrogen is achievable independently of
the partial pressure of the etchant gas in the gas mixture. Both
the increase in selectivity and the limitation on the etch time are
sufficiently great that significant improvements in uniformity,
yield, and product reliability are achieved.
[0095] These discoveries offer numerous advantages, for example in
overetching, i.e., etching purposely done to a degree beyond that
which is strictly required for removal of the sacrificial silicon.
Since the high selectivity allows one to overetch without
introducing nonuniformity across the mirror array, this invention
permits the use of overetching as a convenient and effective means
of assuring complete removal of the sacrificial silicon while still
preserving the integrity of the mirror elements. The invention thus
eases the requirement for precise end point detection, i.e.,
precise detection of the point at which the last of the sacrificial
silicon is removed. Another advantage stems from the dilution
effect of the additive gas. Dilution improves the circulation of
the gaseous mixture through the system by adding to the mass that
flows through the recirculation system or agitator when such pieces
of equipment are present. Also, the presence of the additive gas
helps reduce high local concentrations of the etchant at the sample
surface. Each of these factors improves microstructure uniformity
and yield.
[0096] This aspect of the invention is of particular interest in
etching processes that are not performed in a plasma environment,
i.e., etching processes performed without the use of externally
imposed energy such as ultraviolet light or high frequency
electromagnetic energy to excite the gases into a plasma state. The
invention is also of particular interest in isotropic etching
processes, notably those in which the silicon and the non-silicon
portions (as defined below) of the microstructure are overlapping
layers, coextensive or otherwise, or nonoverlapping layers, sharing
a common boundary or separated but still simultaneously exposed to
the etchant gas. The invention is particularly useful in structures
in which the silicon is a layer positioned underneath a layer of
the non-silicon material such that removal of the silicon by
etching requires lateral access through vias in the non-silicon
layer. The invention is also of particular interest in the
manufacture of reflective spatial light modulators of the type
described in U.S. Pat. No. 5,835,256, in which the mirror elements
are formed of silicon nitride or silicon dioxide and the underlying
sacrificial layer serving as the support to be removed by etching
is polysilicon.
[0097] As mentioned previously, etching processes addressed by this
invention are those in which the etchant is one or more gaseous
noble gas fluorides, one or more gaseous halogen fluorides, or
combinations of gaseous noble gas fluorides and halogen fluorides.
The noble gases are helium, neon, argon, krypton, xenon and radon,
and among these the preferred fluorides are fluorides of krypton
and xenon, with xenon fluorides the most preferred. Common
fluorides of these elements are krypton difluoride, xenon
difluoride, xenon tetrafluoride, and xenon hexafluoride. The most
commonly used noble gas fluoride in silicon etch procedures is
xenon difluoride. Halogen fluorides include bromine fluoride,
bromine trifluoride, bromine pentafluoride, chlorine fluoride,
chlorine trifluoride, chlorine pentafluoride, iodine pentafluoride
and iodine heptafluoride. Preferred among these are bromine
trifluoride, bromine trichloride, and iodine pentafluoride, with
bromine trifluoride and chlorine trifluoride the more preferred.
Combinations of bromine trifluoride and xenon difluoride are also
of interest.
[0098] The gas mixture is preferably contacted with the sample at a
pressure below atmospheric pressure. The term "sample" is used
herein to denote the article from which the sacrificial silicon is
sought to be removed in a selective manner relative to other
materials which may reside in separate layers or regions of the
article. The "sample" may thus be a single mirror element and its
associated layers of other materials, a test pattern, a die, a
device, a wafer, a portion of a wafer, or any article containing
sacrificial silicon. While the rate of the etching reaction may
vary with the partial pressure of the etchant gas, the partial
pressure is generally not critical to the invention and may vary.
In most applications, best results will be obtained with the
etchant gas at a partial pressure of at least about 0.1 mbar (0.075
torr), preferably at least about 0.3 mbar (0.225 torr), more
preferably within the range of from about 0.3 mbar (0.225 torr) to
about 30 mbar (22.5 torr), and most preferably from about 1 mbar
(0.75 torr) to about 15 mbar (11.25 torr). These pressure ranges
are particularly applicable to xenon difluoride.
[0099] The gaseous additive that is included in the gas mixture to
increase the selectivity of the silicon etch is a gas that is not
itself active as an etching agent, and preferably a
non-halogen-containing gas. The additive may be a single species or
a mixture of species. In preferred embodiments of this invention,
the additives are those whose molar-averaged formula weight
(expressed in daltons or grams per mole) is less than the formula
weight of molecular nitrogen, preferably about 25 or less, still
more preferably within the range of from about 4 to about 25, still
more preferably within the range of from about 4 to about 20, and
most preferably within the range of from about 4 to about 10. If a
single additive species is used, the "molar-averaged formula
weight" is the actual formula weight of that species, whereas if
two or more additive species are used in the same gas mixture, the
molar-averaged formula weight is the average of the formula weights
of each species in the mixture (exclusive of the noble gas
fluoride) calculated on the basis of the relative molar amounts
(approximately equal to the partial pressures) of each species. In
terms of thermal conductivity, preferred additives are those whose
thermal conductivity at 300 K (26.9.degree. C.) and atmospheric
pressure ranges from about 10 mW/(m K) (i.e., milliwatts per meter
per degree Kelvin) to about 200 mW/(m K), and most preferably from
about 140 mW/(m K) to about 190 mW/(m K). In general, the higher
the thermal conductivity of the additive, the greater the
improvement in selectivity. Examples of additives suitable for use
in this invention are nitrogen (N.sub.2, formula weight: 28;
thermal conductivity at 300 K: 26 mW/(m K)), argon (Ar, formula
weight: 40; thermal conductivity at 300 K: 18 mW/(m K)), helium
(He, formula weight: 4; thermal conductivity at 300 K: 160 mW/(m
K)), neon (Ne, formula weight: 20; thermal conductivity at 300 K:
50 mW/(m K)), and mixtures of two or more of these gases. For
embodiments in which the molar-averaged formula weight is below
that of molecular nitrogen, the preferred additive gas is helium,
neon, mixtures of helium and neon, or mixtures of one or both with
one or more of higher formula weight non-etchant gases such as
nitrogen and argon. Particularly preferred additives are helium and
mixtures of helium with either nitrogen or argon.
[0100] The degree of selectivity improvement may vary with molar
ratio of the additive to the etchant gas, but this ratio is
generally not critical to the utility of this invention. Here
again, the molar ratio is approximately equal to the ratio of the
partial pressures, and in this case the term "molar ratio" denotes
the ratio of the total number of moles of the additive gas (which
may be more than one species) divided by the total number of moles
of the etchant gas (which may also be more than one species). In
most cases, best results will be obtained with a molar ratio of
additive to etchant that is less than about 500:1, preferably
within the range of from about 1:1 to about 500:1, preferably from
about 10:1 to about 200:1, and most preferably from about 20:1 to
about 100:1.
[0101] The temperature at which the etch process is conducted is
likewise not critical to this invention. The temperature does
however affect the partial pressure of the etchant gas and the
optimal temperature may depend on the choice of etchant gas,
gaseous additive or both. In general, and particularly for
procedures using xenon difluoride as the etchant gas, suitable
temperatures will range from about -60.degree. C. to about
120.degree. C., preferably from about -20.degree. C. to about
80.degree. C., and most preferably from about 0.degree. C. to about
60.degree. C. For xenon difluoride, the silicon etch rate is
inversely proportional to the temperature over the range of
-230.degree. C. to 60.degree. C. The improvement in selectivity can
thus be further increased by lowering the etch process
temperature.
[0102] The duration of the exposure of the sample to the gas
mixture in the practice of this invention will be the amount of
time sufficient to remove all of the silicon or substantially all,
i.e., any amount sufficient to permit the microstructure to
function essentially as effectively as if all of the silicon had
been removed. An advantage of the high selectivity achieved with
this invention is that it permits overetching of the silicon
without significant loss of the non-silicon material. The time
required for the etching process will vary with the amount of
silicon to be removed and the dimensions and geometry of the
silicon layer, and is not critical to this invention. In most
cases, best results will be achieved with an exposure time lasting
from about 30 seconds to about 30 minutes, preferably from about 1
minute to about 10 minutes. An example of the structures to which
this invention will be applied is that depicted in U.S. Pat. No.
5,835,256, in which a silicon nitride layer is deposited over a
polysilicon layer, and the silicon nitride layer is patterned to
leave vias that define the lateral edges of the mirror elements.
Access to the sacrificial polysilicon layer is through the vias,
and the etching process removes the polysilicon below the vias by
etching in the vertical direction (i.e., normal to the planes of
the layers) while also removing the polysilicon underneath the
silicon nitride by etching in the lateral direction (parallel to
the planes of the layers).
[0103] In certain procedures within the scope of this invention,
the manner and the order in which the gases in the gas mixture are
combined may have an effect on the quality of the finished product.
Variations may thus be introduced in the order of combining the
etchant gas with the non-etchant diluent or whether this is done in
portions, or, when two or more non-etchant diluents are used, the
decision to combine one diluent with the etchant gas before adding
the other diluent, or which diluent or subcombination is the first
to contact the sample. Such variations may affect parameters of the
process such as the diffusion time, the reaction rate at the
surface of the sample, and the rate of removal of reaction products
from the surface.
[0104] The sacrificial silicon layers to which this invention is
applicable may be crystalline silicon, amorphous silicon, partially
crystalline silicon, crystalline silicon of multiple crystal sizes,
polysilicon in general, and silicon doped with such dopants as
arsenic, phosphorus or boron. Polysilicon is of particular
interest, although the relative crystalline vs. amorphous character
of polysilicon will vary considerably with the deposition
conditions, the presence or absence of dopants and impurities, and
the degree of annealing.
[0105] The term "non-silicon" as used herein denotes any material
that does not contain either amorphous or crystalline silicon in
any of the forms described in the preceding paragraph. The term
thus includes silicon-containing compounds in which elemental
silicon is bonded to another element, as well as non-silicon
elements and compounds of non-silicon elements. Examples of such
non-silicon materials are titanium, gold, tungsten, aluminum, and
compounds of these metals, as well as silicon carbide, silicon
nitride, and silicon oxides. Silicon nitride and silicon oxide are
of particular interest in view of their use in the structures
disclosed in U.S. Pat. No. 5,835,256. Two or more different
non-silicon materials may be present in a single structure, and
selectivity of the silicon etch relative to all such non-silicon
materials will be improved.
[0106] The thickness and lateral dimensions of the layers are also
noncritical to the improvement in selectivity achieved by this
invention. In most cases, the silicon portion will be a layer
having a thickness of from about 200 nm to about 5,000 nm,
preferably from about 250 nm to about 3,000 nm, and most preferably
from about 300 nm to about 1,000 nm. Similarly, in most cases the
non-silicon portion will be a layer with a thickness of from about
10 nm to about 500 nm, preferably from about 20 nm to about 200 nm,
and most preferably from about 30 nm to about 200 nm. The lateral
distance that the etching process must extend under the typical
silicon nitride mirror element in the structures of U.S. Pat. No.
5,835,256 in order to remove all of the underlying polysilicon
(this distance typically being half the shortest lateral dimension
of the mirror when the etching front travels inward from opposing
edges) may range from a submicron distance to about 100 microns,
preferably from about 3 microns to about 30 microns, and most
preferably from about 5 microns to about 15 microns.
[0107] The sample being etched comprises a layered structure formed
on a quartz plate measuring 11.3 mm.times.15.6 mm. The first layer
was a continuous polysilicon layer deposited directly on one side
of the quartz, and the second layer was patterned silicon nitride
deposited directly over the polysilicon layer. The polysilicon
layer measured 9.2 mm.times.12.3 mm in lateral dimensions and was
centered on the quartz surface, thereby leaving border regions
along all four sides, and had a thickness of 0.5 micron. The
silicon nitride layer was 249 nm (0.249 micron) in thickness and
was coextensive with the quartz plate, thereby extending over both
the underlying polysilicon layer and the border regions where no
polysilicon had been deposited. The silicon nitride layer was
patterned to form an array of square mirrors measuring 12 microns
on each side with each pair of adjacent mirrors separated by a via
0.8 micron in width to expose the underlying polysilicon.
Measurements of the thickness of the silicon nitride layer to
assess the selectivity of the polysilicon etch were performed at
four locations in the border regions, close to the four comers of
the quartz plate, these locations being spaced apart from the edge
of the polysilicon layer by distances greater than 300 microns.
This distance was chosen to assure, for purposes of uniformity,
that the measurement locations experienced no temperature rise from
the exothermic polysilicon etch reaction, since the thermally
insulating nature of silicon nitride precluded any such temperature
rise at locations beyond approximately 100 microns from the edge of
the polysilicon layer.
[0108] The time required for full removal of the polysilicon layer
was determined by visual observation, as indicated above. Of
course, the time needed for full removal of the sacrificial layer
could also be performed in accordance with the end point detection
methods (monitoring gas reaction products) as set forth previously
herein. The thickness of the silicon nitride at the measurement
locations was determined both before and after the polysilicon
etching by a common industry method of thin-film measurement using
the reflectance of the film (as used in the NanoSpec Thin Film
Measurement System of Nanometrics, Inc., Sunnyvale, Calif., USA,
and in the Advanced Thin Film Measurement Systems of Filmetrics,
Inc., San Diego, Calif., USA). Measurements were performed on two
or three samples for each experiment, and the results averaged. The
results are listed in the table below, which include as the first
experiment a control run with xenon difluoride alone and no
additive.
[0109] Experimental Results TABLE-US-00001 Experimental Results Gas
in the Gas in the Time required Si.sub.3N.sub.4 thickness loss
(Initial Experiment No. of 1.sup.st Gas 2.sup.nd Gas for removal of
Thickness 249 nm during No. Samples Source Source Polysilicon
Polisilicon removal) I 3 None None 65 sec 11-13 nm II 3 N.sub.2
N.sub.2 610 sec 2-3 nm III 2 Ar Ar 590 sec 2-3 nm IV 2 He He 250
sec 2-3 nm
With the etching of the underlying polysilicon layer in the lateral
direction, the etching distance of the polysilicon was one-half the
width of each mirror element, or 0.5.times.12 microns=6,000 nm. The
results in the table indicate that the selectivity of the etch of
polysilicon relative to silicon nitride rose from approximately
500:1 (6,000 nm: 11 nm) with the xenon difluoride-only etch medium
in Experiment I to approximately 2,000:1 (6,000 nm: 3 nm) with the
addition of each of the additive gases in Experiments II, III and
IV, and that the increase in etch time of the polysilicon when the
additive was helium (Experiment IV) was well under half the
attendant increases when the additives were nitrogen and argon,
both of which had formula weights exceeding 25. These diluents are
but examples, and any diluent or combination of diluents can be
used, though preferably as long as the etch rate is within the low
etch rate ranges of the invention.
[0110] As can be seen from the above various etch rates can be used
to remove the sacrificial layer, including an etch rate can be 27.7
um/hr (0.5 um etched in 65 sec.) such as if no diluent is used, or
lower (e.g. 25 or 20 um/hr or less). For example, an etch rate of
7.2 um/hr (0.5 um etched in 250 sec.) can be achieved with a helium
diluent. Even lower etch rates are achieved in an argon diluent or
in a nitrogen diluent (0.5 um etched in 590 or 610 sec=3 um/hr).
Other diluents and mixtures of diluents can be used, though it is
preferred that the etch rate be 10 um/hr or less, 3 um/hr or less,
or even 2 um/hr or 1 um/hr or less.
[0111] In a further embodiment of the invention, a MEMS device is
formed where a sacrificial layer (or layers) is deposited on a
substrate. During or after deposition the sacrificial layer, the
sacrificial material is doped with a dopant. The doping can occur
during deposition of the sacrificial material, such as feeding a
dopant into the process gas during a chemical vapor deposition of
the sacrificial material. Or, the sacrificial material can first be
deposited, followed by implanting the sacrificial layer with the
dopant (e.g. phosphorous, arsenic, boron or other semiconductor
dopant). In a preferred embodiment of the invention, the
sacrificial layer is silicon (e.g. amorphous silicon,
polycrystalline silicon), the material that is not to be etched is
a non-silicon material, such as a metal (Al, Ti, Au, etc.) or metal
compound (e.g. a nitride of titanium, aluminum tantalum-silicon,
tungsten or an oxide of aluminum, silicon, tantalum, titanium, etc.
or a metal carbide), where the silicon sacrificial is doped with
the dopant. The dopant can be any dopant (e.g. borane, arsine or
phosphine), though preferably one that improves the selectivity of
the etch. Possible dopants if the sacrificial material is silicon,
include PH3, P2H5, B2H5, BCl3, etc. The dopant can be implanted in
accordance with standard semiconductor manufacturing implanting
methods, or mixed into the process gas while depositing the
sacrificial material, e.g. in accordance with such doping methods
used in making solar cells. Other doping methods (ion transfer via
thermal anneal, etc.) could also be used. The dopant can be used to
dope only a top portion of the sacrificial layer, or the dopant can
be made to be present throughout the sacrificial material. Doping
can be at 10.sup.10 to 10.sup.18 ions/cm.sup.3, such as around
10.sup.14 ions/cm.sup.3. Implantation can be performed at an energy
of 10 to 70 keV, preferably from 20 to 40 keV. Other implantation
densities and energies could also be used.
[0112] In the present invention, the silicon can be polysilicon as
set forth above, or amorphous silicon deposited by LPCVD or PECVD,
or sputtering, or other materials and techniques as set forth in
U.S. patent application Ser. No. 09/617,149 to Huibers et al. filed
Jul. 17, 2000, U.S. patent application Ser. No. 09/631,536 to
Huibers et al. filed Aug. 3, 2000, U.S. patent application Ser. No.
09/767,632 to True et al., filed Jan. 22, 2001 and/or Ser. No.
09/637,479 to Huibers filed Aug. 11, 2000, each incorporated herein
by reference. Other micromechanical structural materials and
methods can be used other than those set forth above, such as those
materials set forth in U.S. Patent Application 60/293,092 to Patel
et al. filed May 22, 2001, U.S. Patent Application 60/254,043 to
Patel et al. filed Dec. 7, 2000, U.S. patent application Ser. No.
09/910,537 to Reid filed Jul. 20, 2001, and U.S. Patent Application
60/300,533 to Reid filed Jun. 22, 2001, each incorporated herein by
reference.
[0113] Though the apparatus and process disclosed herein are for
etching a material from any work piece (semiconductor device, MEMS
device, device to be cleaned of silicon residue, etc.), in one
embodiment the material being removed is a sacrificial layer in a
MEMS fabrication process. A specific example of a MEMS device that
could be made in accordance with the invention is a micromirror
array such as disclosed in U.S. Pat. Nos. 5,835,256 and 6,046,840
to Huibers et al. The MEMS device, of course, could be any device,
including movable mirror elements for optical switching such as
disclosed in U.S. patent application Ser. No. 09/617,149 to Huibers
et al. filed Jul. 17, 2000. Each of the above patents and
applications are incorporated herein by reference.
[0114] The foregoing description and examples are offered primarily
for purposes of illustration. It will be readily apparent to those
skilled in the art that numerous modifications and variations
beyond those described herein can be made while still falling
within the spirit and scope of the invention.
* * * * *