U.S. patent application number 09/752571 was filed with the patent office on 2001-08-23 for process for fabricating single crystal resonant devices that are compatible with integrated circuit processing.
Invention is credited to Ziegler, James F..
Application Number | 20010016367 09/752571 |
Document ID | / |
Family ID | 23482995 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010016367 |
Kind Code |
A1 |
Ziegler, James F. |
August 23, 2001 |
Process for fabricating single crystal resonant devices that are
compatible with integrated circuit processing
Abstract
This invention describes fabrication procedures to construct
MEMS devices, specifically band-pass filter resonators, in a manner
compatible with current integrated circuit processing. The final
devices are constructed of single-crystal silicon, eliminating the
mechanical problems associated with using polycrystalline silicon
or amorphous silicon. The final MEMS device lies below the silicon
surface, allowing further processing of the integrated circuit,
without any protruding structures. The MEMS device is about the
size of a SRAM cell, and may be easily incorporated into existing
integrated circuit chips. The natural frequency of the device may
be altered with post-processing or electronically controlled using
voltages and currents compatible with integrated circuits.
Inventors: |
Ziegler, James F.; (Yorktown
Heights, NY) |
Correspondence
Address: |
SCULLY, SCOTT, MURPHY & PRESSER
400 Garden City Plaza
Garden City
NY
11530
US
|
Family ID: |
23482995 |
Appl. No.: |
09/752571 |
Filed: |
December 28, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09752571 |
Dec 28, 2000 |
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09375940 |
Aug 17, 1999 |
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6238946 |
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Current U.S.
Class: |
438/50 ; 438/149;
438/52; 438/53 |
Current CPC
Class: |
H03H 9/2457 20130101;
B81C 1/00246 20130101; H03H 3/0072 20130101; B81B 2201/0271
20130101; H03H 9/2463 20130101; B81C 2203/0735 20130101 |
Class at
Publication: |
438/50 ; 438/52;
438/53; 438/149 |
International
Class: |
H01L 021/00; H01L
021/84 |
Claims
Having thus described our invention, what we claim as new, and
desire to secure by Letters Patent is:
1. A method for constructing an integrated circuit resonator device
of single crystal silicon formed on a silicon-on-insulator (SOI)
substrate comprising the steps of: a) forming a surface silicon
layer, and an intermediate layer of SiO.sub.2 on said substrate; b)
opening a first mask area at said surface layer and creating a
first conductive structure in said substrate below said
intermediate layer, said first conductive structure having
dimensions corresponding to said first mask opening; c) opening a
second mask area at said surface layer having dimensions of a
resonator device to be formed across an area corresponding to the
first conductive structure, and creating a second conductive
structure in said surface silicon layer corresponding to said
resonator device; d) opening a third mask area comprising first and
second sub-areas in said silicon surface layer abutting a
respective first and opposite edge of said conductive structure
forming said resonator device, and a third sub-area hole spaced
apart from said other two holes, said first, second and third
sub-areas having dimensions spatially limited by said first mask
opening; e) etching down through said surface silicon layer at each
said first and second sub-areas forming a hole to expose said
intermediate layer, and etching through the said surface silicon
and intermediate layer at said third sub-area forming a hole to
expose said first conductive structure; f) depositing a conducting
metal at said formed hole at said third sub-area, to enable input
of signals to said first conductive structure; and, g) etching down
through said intermediate layer at each said formed hole at said
first and second sub-areas for removing said intermediate layer at
each side and underneath said second conductive structure and
exposing said first conductive structure, wherein said second
conductive structure forming said resonator device lies entirely at
or below the silicon layer surface and operates by capacitively
coupling an input signal at said first conductive structure to said
resonator.
2. The method according to claim 1, wherein step b) of creating a
first conductive layer in said substrate includes implementing ion
implantation technique.
3. The method according to claim 1, wherein said step c) of opening
said second mask area includes opening resonator structure
comprising first and second end contact areas in said silicon
surface at opposite sides of said first opening and an resonator
area connecting said end areas formed therebetween.
4. The method according to claim 1, wherein step c) of creating a
second conductive layer in said surface silicon layer corresponding
to said resonator structure includes implementing ion implantation
technique.
5. The method according to claim 1, wherein said first mask, second
mask, and third mask opening steps b)-d) includes implementing
photolithography techniques.
6. The method according to claim 1, further including the step of
changing a density of said second conductive structure for altering
a band-pass frequency characteristic of said resonator device.
7. The method according to claim 6, wherein said step of changing a
density of said second conductive structure forming said resonator
device includes ion implanting neutral light atoms for lowering
material density of said resonator device and increasing a
band-pass frequency characteristic of said resonator device.
8. The method according to claim 6, wherein said step of changing a
density of said second conductive structure forming said resonator
device includes ion implanting neutral heavy atoms for increasing
material density of said resonator device and decreasing a
band-pass frequency characteristic of said resonator device.
9. The method according to claim 1, further including the step of
implementing ion implantation technique for changing an internal
bonding structure of said second conductive structure for altering
a band-pass frequency characteristic of said resonator device.
10. The method according to claim 1, further including the step of
implementing thermal oxidation and etching techniques for
decreasing thickness of said resonator device and decreasing a
band-pass frequency characteristic of said resonator device.
11. The method according to claim 1, further including the step of
implementing epitaxial silicon growth techniques for increasing
thickness of said resonator device and increasing a band-pass
frequency characteristic of said resonator device.
12. The method according to claim 1, further including the step of
depositing a surface layer for increasing thickness of said
resonator device and increasing a band-pass frequency
characteristic of said resonator device.
13. The method according to claim 1, further including performing
subsequent planarizing and metallization steps to said integrated
circuit resonator device.
14. The method according to claim 1, further including the step of
ion implanting silicon atoms to enable conversion of said resonator
device to one of polycrystalline or amorphous silicon to widen a
band-pass frequency characteristic of said integrated circuit
resonator device.
15. An integrated circuit bandpass filter device comprising: a) a
substrate including a top surface silicon layer, and an
intermediate layer of SiO.sub.2 formed thereon; b) a conductive
contact formed at said surface silicon layer for receiving an input
electrical signal; c) an open well structure formed in said silicon
surface, said well having a bottom surface conductive layer
connecting said input contact; and, d) a resonator structure formed
at said surface silicon layer and lying across said well structure,
wherein an input signal is capacitively coupled from said bottom
surface conductive layer to said resonator structure to enable
vibration of said resonator device in a vertical direction at a
desired frequency of vibration.
16. The device as claimed in claim 15, wherein said resonator
structure includes a first conductive contact formed at a surface
at one side of said open well structure for further propagating
input signals at said desired frequency.
17. The device as claimed in claim 15, wherein said resonator
structure includes a second conductive contact formed at a surface
at an opposite side of said open well structure for enabling
injection of electrical stimulus for heating said resonator
structure and altering its frequency of vibration.
18. The device as claimed in claim 15, wherein said resonator
structure comprises a material for altering a band-pass frequency
of vibration, said material comprising one selected from a group
including: carbon atoms, germanium, boron, and, arsenic.
19. An integrated circuit bandpass filter device comprising: a) a
substrate including a top surface silicon layer, and an
intermediate layer of SiO.sub.2 formed thereon; b) an open well
structure formed in said silicon surface; c) a conductive contact
formed at said surface silicon layer near one end of said open well
structure layer for receiving an input electrical signal, said
contact including contact portion at said surface silicon layer
extending over said open well structure; and, d) a resonator
structure formed at said surface silicon layer and lying across
said well structure in proximity with said extending contact
portion, wherein an input signal is capacitively coupled from said
extending contact portion to said resonator structure to enable
vibration of said resonator device in a horizontal direction at a
desired frequency of vibration.
20. The device as claimed in claim 19 wherein said resonator
structure includes a first conductive contact formed at a surface
at one side of said open well structure for further propagating
input signals at said desired frequency.
21. The device as claimed in claim 19, wherein said resonator
structure includes a second conductive contact formed at a surface
at an opposite side of said open well structure for enabling
injection of electrical stimulus for heating said resonator
structure and altering its frequency of vibration.
22. The device as claimed in claim 19, wherein said resonator
structure comprises a material for altering a band-pass frequency
of vibration, said material comprising one selected from a group
including: carbon atoms, germanium, boron, and, arsenic.
23. A method for constructing an integrated circuit resonator
device of single crystal silicon formed on a silicon-on-insulator
(SOI) substrate comprising the steps of: a) forming a top surface
silicon layer, and an intermediate layer of SiO.sub.2 on said
substrate; b) opening a first mask area at said surface layer
having dimensions of a resonator device to be formed, and creating
a conductive structure in said surface silicon layer corresponding
to said resonator device; c) opening a second mask area comprising
first and second sub-areas in said silicon surface layer abutting a
respective first and opposite edge of said conductive structure
forming said resonator device; d) etching down through said surface
silicon layer and said intermediate layer at said first and second
sub-areas for removing said intermediate layer at each side and
underneath said conductive structure to form a well structure; and
e) forming a contact by depositing a metal layer at a bottom
surface of said well structure beneath said conductive structure,
wherein said device lies entirely at or below the silicon layer
surface and operates by capacitively coupling an input signal at
said first conductive structure to said resonator.
24. The method according to claim 23, further including the step of
changing a density of said second conductive structure for altering
a band-pass frequency characteristic of said resonator device.
25. The method according to claim 24, wherein said step of changing
a density of said second conductive structure forming said
resonator device includes ion implanting neutral light atoms for
lowering material density of said resonator device and increasing a
band-pass frequency characteristic of said resonator device.
26. The method according to claim 24, wherein said step of changing
a density of said second conductive structure forming said
resonator device includes ion implanting neutral heavy atoms for
increasing material density of said resonator device and decreasing
a band-pass frequency characteristic of said resonator device.
27. The method according to claim 23, further including the step of
implementing ion implantation technique for changing an internal
bonding structure of said second conductive structure for altering
a band-pass frequency characteristic of said resonator device.
28. The method according to claim 23, further including the step of
implementing thermal oxidation and etching techniques for
decreasing thickness of said resonator device and decreasing a
band-pass frequency characteristic of said resonator device.
29. The method according to claim 23, further including the step of
implementing epitaxial silicon growth techniques for increasing
thickness of said resonator device and increasing a band-pass
frequency characteristic of said resonator device.
30. The method according to claim 23, further including the step of
depositing a surface layer for increasing thickness of said
resonator device and increasing a band-pass frequency
characteristic of said resonator device.
31. The method according to claim 23, further including performing
subsequent planarizing and metallization steps to said integrated
circuit resonator device.
32. The method according to claim 23, further including the step of
ion implanting silicon atoms to enable conversion of said resonator
device to one of polycrystalline or amorphous silicon to widen a
band-pass frequency characteristic of said integrated circuit
resonator device.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is directed to systems and methods for
fabricating integrated circuit resonant devices, and particularly a
process for manufacturing integrated circuit (IC) band-pass filters
using micro electromechanical system (MEMS) technology on single
crystal silicon-on-insulator (SOI) wafers in a manner consistent
with current integrated circuit fabrication techniques.
[0003] 2. Discussion of the Prior Art
[0004] Micro Electro-Mechanical Systems (MEMS) technology is
currently implemented for the fabrication of narrow bandpass
filters (high-Q filters) for various UHF and IF communication
circuits. These filters use the natural vibrational frequency of
micro-resonators to transmit signals at very precise frequencies
while attenuating signals and noise at other frequencies. FIG. 1
illustrates a conventional MEMS bandpass filter device 10 which
comprises a semi-conductive resonator structure 11, e.g., made of
polycrystalline or amorphous material, suspended over a planar
conductive input structure 12, which is extended to a contact 13.
An alternating electrical signal on the 12 input will cause an
image charge to form on the resonator 11, attracting it and
deflecting it downwards. If the alternating signal frequency is
similar to the natural mechanical vibrational frequency of the
resonator, the resonator may vibrate, enhancing the image charge
and increasing the transmitted AC signal. The meshing of the
electrical and mechanical vibrations selectively isolates and
transmits desired frequencies for further signal amplification and
manipulation. It is understood that the input and output terminals
of this device may be reversed, without changing its operating
characteristics.
[0005] Typically, resonator filter devices 10 are fabricated by
standard integrated circuit masking/deposition/etching processes.
Details regarding the manufacture and structure of MEMS band-pass
filters may be found in the following references: 1) C. T. -C.
Nguyen, L. P. B. Katehi and G. M. Rebeiz "Micromachined Devices for
Wireless Communications", Proc. IEEE, 86, 1756-1768; 2) J. M.
Bustillo, R. T. Howe and R. S. Muller "Surface Micromachining for
Microelectromechanical Systems", Proc. IEEE, 86, 1552-1574 (1998);
3) C. T. -C. Nguyen, "High-Q Micromechanical Oscillators and
Filters for Communications", IEEE Intl. Symp. Circ. Sys., 2825-2828
(1997); 4) G. T. A. Kovacs, N. I. Maluf and K. E. Petersen, "Bulk
Micromachining of Silicon", Proc. IEEE 86, 1536-1551 (1998); 5) K.
M. Lakin, G. R. Kline and K. T. McCarron, "Development of Miniature
Filters for Wireless Applications", IEEE Trans. Microwave Theory
and Tech., 43, 2933-2939 (1995); and, 6) A. R. Brown,
"Micromachined Micropackaged Filter Banks", IEEE Microwave and
Guided Wave Lett.,8, 158-160 (1998).
[0006] The reference 7) N. Cleland and M. L. Roukes, "Fabrication
of High Frequency Nanometer Scale Mechanical Resonators from Bulk
Si Crystals", Appl. Phys. Lett, 69, 2653-2655 (1996) describes the
advantages of using single crystal resonators as band-pass filters.
The references 8) C. T. -C. Nguyen, "Frequency-Selective MEMS for
Miniaturized Communication Devices", 1098 TEEE Aerospace Conf.
Proc., 1, 445-460 (1998) and 9) R. A. Syms, "Electrothermal
Frequency Tuning of Folded and Coupled Vibrating Micromechanical
Resonators, J. MicroElectroMechanical Sys., 7, 164-171 (1998) both
discuss the effects of heat on the stability of micromechanical
band-pass filters. Of particular relevance as noted in these
references is the acknowledgment that the existing processes for
making MEMS bandpass filters have serious drawbacks. For instance,
as most resonators are made of polycrystalline or amorphous
materials to simplify fabrication, there is exhibited an increase
in mechanical energy dissipation which softens the natural
frequency of oscillation, as noted in above-mentioned references
1)-3) . Etching polycrystalline materials does not allow for device
features smaller than the polycrystalline grain size, which creates
rough surfaces and prevents precise mechanical characteristics. For
example, above-mentioned references 1) and 2) both detail the
problems encountered when polycrystalline material is used in MEMS
resonators. Additionally, in reference 7), there is described the
construction of resonators made of single-crystal silicon including
a description of an attempt to use complex dry-etch techniques to
obtain single-crystal resonators. The reference reports such
resonator structures having scalloped edges, which reduces the
precision of the final mechanical performance to that of
polycrystalline structures. That is, their etch-process produced
surface roughness that was similar to that of polycrystalline
materials.
[0007] Other attempts to use single-crystal silicon have been
reviewed in reference 4), however, these attempts were made to
eliminate the poor device performance when polycrystalline
materials were used for construction. Most used an isotropic etches
to undercut single-crystal silicon surfaces and construct
resonators (and other structures). In all cases, the structures
were quite large, in part to minimize the effects of surface
roughness and non-parallel surfaces on the device performance.
Since the devices were very large, they were useful only for
low-frequency applications (below 100 MHz) , which is of limited
usefulness as a communication frequency filter in the commercial
band of 300-6000 MHz. A further limitation of all MEMS band-pass
structures is that they are formed above the silicon surface (see
references 1-9). This makes the structures incompatible with
standard integrated circuit fabrication, since it prevents
"planarization". After the devices of an integrated circuit have
been fabricated, the wafer enters its final processing which is
called "metallization" and "planarization". Before this step, all
the devices on the wafer are isolated, and for integration they
must be connected together with metal wires. In modern devices, the
wiring is done as a series of layers, each containing wiring in
certain directions (i.e., metallization). After each layer is
deposited, the wafer surface is smoothed, i.e., is planarized so
that subsequent layers of wiring may be deposited on a smooth
surface. Planarization is typically done by chemical-mechanical
polishing (CMP processing) or by melting a thin layer of glass over
the surface. If there is a micro-mechanical device protruding up
above the surface, it would be immediately destroyed by either of
the above planarization processes.
[0008] Additional prior art patented devices such as described in
U.S. Pat. No. 3,634,787 (1972) , U.S. Pat. No. 3,983,477 (1976) and
U.S. Pat. No. 4,232,265A (1980) describe similar mechanical
resonatored structures, but which are incompatible with integrated
circuit processing.
[0009] For instance, U.S. Pat. No. 3,634,787 describes an
electro-mechanical resonator band-pass filter device having a
mechanical component consisting of a support being a unitary body
of semiconductor material and having a piezoelectric field effect
transducer therein. Thus, its electrical operation relies upon the
piezoelectrical effect. U.S. Pat. No. 3,983,477 describes a
ferromagnetic element tuned oscillator located close to a
high-voltage current carrying conductor, however, as such, its
electrical operation relies on the ferromagnetic effect. U.S. Pat.
No. 4,232,265A describes a device for converting the intensity of a
magnetic or an electromagnetic field into an electric signal
wherein movable elements are made as ferromagnetic plates.
Likewise, its electrical operation relies upon the ferromagnetic
effect. U.S. Patent No. 5,594,331 describes a self-excitation
circuitry connected to a resonator to process induced variable
frequency voltage signals in a resonant pass band and is of
exemplary use as a power line sensor. Likewise, U.S. Pat. No.
5,696,491 describes a microelectromechanical resonating resonator
which responds to physical phenomenon by generating an induced
variable frequency voltage signal corresponding to the physical
phenomenon and thus, does not lend itself to manufacture by current
integrated circuit fabrication technology.
[0010] It would thus be highly desirable to construct an IC MEMS
band-pass filter device in a manner consistent with current
integrated circuit fabrication techniques that avoids completely or
reduces significantly all of the above-described limitations.
SUMMARY OF THE INVENTION
[0011] It is an object of Lhe present invention to provide an
improved IC MEMS resonator band-pass filter device of a
construction that lends itself to manufacture in accordance with
current IC manufacturing techniques and that overcomes the
fundamental weaknesses as outlined in the above-mentioned
references.
[0012] Particularly, according to one aspect of the invention,
there is provided a resonatored MEMS bandpass filter device that is
constructed of single-crystal silicon, eliminating the mechanical
problems associated with using polycrystalline or amorphous
materials. The final MEMS device lies below the silicon surface,
allowing further processing of the integrated circuit, without any
protruding structures. The MEMS device is about the size of a SRAM
cell, and may be easily incorporated into existing integrated
circuit chips. The natural frequency of the device may be altered
with post-processing, or electronically controlled using voltages
and currents compatible with integrated circuits.
[0013] According to another aspect of the invention, there is
provided a novel resonatored MEMS bandpass filter device
fabrication technique for constructing such MEMS devices in a
manner compatible with current integrated circuit processing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Further features, aspects and advantages of the apparatus
and methods of the present invention will become better understood
with regard to the following description, appended claims, and
accompanying drawings where:
[0015] FIG. 1 is a schematic diagram of a conventional MEMS
bandpass filter device of a suspended resonator design.
[0016] FIG. 2(a) is a schematic isometric diagram of a MEMS
bandpass filter fabricated with a buried planar input contact
according to a first embodiment of the invention.
[0017] FIG. 2(b) is a side view of this same device.
[0018] FIGS. 3(a) and 3(b) are schematic isometric and side view
diagrams of a MEMS bandpass filter fabricated with the input
contact in a sunken well according to a second embodiment of the
invention. FIGS. 4(a) and 4(b) are schematic diagrams of a MEMS
bandpass filter fabricated with the input contact causing
horizontal oscillation of the resonator according to a third
embodiment of the invention.
[0019] FIGS. 5(a)-5(k) illustrate the various masks used in
construction of the device, and also depicts intermediate
structures during the fabrication process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] FIGS. 2(a) and 2(b) illustrate respective isometric and side
views of a novel resonatored MEMS bandpass filter device 100
manufactured according to a first embodiment of the invention. As
shown in FIGS. 2(a) and 2(b), the device is fabricated to have an
input contact 103 for diverting a received electrical signal
downwards through connection 106 to a bottom planar contact 102. A
well 108 is created in the silicon surface, and a resonator 101
straddles this well and is free to vibrate. The resonator 101 is
electrically connected to an output pad 105, which propagates the
final filtered signal. The input contact 103 is capacitively
coupled to the resonator 101, so that the input signal will cause
the resonator to vibrate-in the vertical direction as indicated by
the arrow A in FIG. 2 (b) . The resonator has a natural frequency
of vibration, based on its dimensions and material, and signals of
this frequency (or its harmonics) are preferentially propagated
through the resonator to the output terminal 105. The natural
frequency of the device may be tuned by heating the resonator, and
changing its elastic constant. This may be accomplished by
fabrication of a pad 104 which functions to enable a current to be
sent through the resonator to pad 105 and consequently heat up the
resonator.
[0021] As described in the reference to H. J. McSkimin, J. Appl.
Phys., 24, 988 (1953), and Yu. A. Burenkov and S. P. Nikanorov,
Sov. Phys. Sol. State, 16, 963 (1974) the elastic constant of
single crystal silicon varies with temperature. Further as
described in the reference H. Guckel, Tech. Digest, IEEE
Solid-State Sensor and Actuator Workshop, June, 1988, 96-99, the
elastic constant of polycrystalline silicon varies with
temperature. In accordance with these references, the heating of
silicon by 100.degree. C. will change its elastic constant by about
0.9%, which may modify the resonator natural frequency by about
0.4%. For a 1 GHz natural frequency, this provides a tuning band of
4 MHz by controlling the resonator temperature. Measurements of
such frequency changes may be made in accordance with conventional
techniques (see above-mentioned references 8 and 9). In accordance
with the invention, the thermal properties are used to tune the
device, and improve its performance and flexibility.
[0022] FIGS. 3(a) and 3(b) illustrate respective isometric and side
views of a novel resonatored MEMS bandpass filter device 110
manufactured according to a second embodiment of the invention. In
FIG. 2(a), above, the resonatored MEMS bandpass filter device 100
was of a construction in which the input contact was connected to
the lower contact plane with a conductive via. In FIG. 3(a), a
metal contact 117 is dropped down from the surface to the bottom of
the well 108 holding the resonator 111. The output signal pad 105
and tuning pad 104 are similar to those shown in FIG. 2(a). Again,
as in FIG. 2(b), the resonator vibrates in the vertical direction
as indicated by the arrow B in FIG. 3(b).
[0023] FIGS. 4(a) and 4(b) illustrate respective isometric and side
views of a novel resonatored MEMS bandpass filter device 120
manufactured according to a third embodiment of the invention. In
the embodiment illustrated in FIG. 4(a), input contact 129 and
input contact extension 130 are formed in the same plane of the
resonator 121, thus, eliminating the need to make a contact plane
below the resonator, as is needed for the designs shown in FIGS.
2(a), 2(b) and FIGS. 3(a) and 3 (b). Here, the resonator 121
vibrates horizontally rather than vertically as depicted by the
arrow C in FIG. 4 (b). This design is the simplest of the three
variations to fabricate, however mechanical performance is reduced
because of the edge surface roughness of the resonator in the
direction of vibration. In designs of FIGS. 2(a) and 3(a), the
resonator vibrates perpendicular to the surface of the substrate,
and the top and bottom surfaces are as smooth as the SOI process
can produce (normally<20 nm) However, the resonator design of
FIG. 4(a) requires these surfaces to be defined by
photolithography, which currently limits the roughness of edge
definition to about 100 nm.
[0024] In accordance with the invention, the process used to
fabricate each of the MEMS resonator bandpass filter devices
utilizes silicon on insulator (SOI) substrates as the starting
material. This material consists of a silicon wafer with a thin
layers of SiO.sub.2 and single crystal silicon on its surface (the
silicon is the outmost layer). Such wafers are commercially
available and are made using a variety of techniques. It is
understood that the processes described herein are also applicable
to silicon wafers only partially covered with SOI material. These
wafers are constructed using the widely known SIMOX process
(Separation by IMplanted OXygen) wherein only small areas of the
surface are converted by using masks to form isolated areas of SOI
material.
[0025] Typically, SOI wafers are constructed with the topmost
single crystal silicon being about 200 nm thick, the SiO2 being 400
nm thick, and the substrate being several hundred microns thick.
Other layer thickness of SOI substrates are available, and all are
compatible with the processes described herein.
[0026] FIGS. 5(a)-5(k) illustrate the process steps in
manufacturing a SOI MEMS device, e.g., the resonator structure 100
shown in FIG. 2(a).
[0027] As shown in the cross-sectional view of FIG. 5(a), a clean
p-type SOI wafer 200 is provided, having a surface silicon layer
202, an intermediate layer of SiO.sub.2 212 on the substrate
silicon 222. For purposes of discussion, it is assumed that the
surface silicon layer 202 is about 200 nm thick, the intermediate
Si0.sub.2 layer 212 is about 400 nm thick and, the silicon
substrate 222 is p-type silicon, of nominal 10 .OMEGA.-cm
resistivity. It is understood that none of these thickness
specifications are critical to the device construction, and are
used only for illustration. Next, as shown in FIG. 5(b) , a thick
photoresist layer 223 is applied to the silicon surface, and
implementing photolithography, a long rectangle 225 is opened, that
is, for example, about 4 mm.times.1 mm in size. Then, as shown in
the cross-sectional view of FIG. 5(c), phosphorus ions are
implanted, for example, at 440 keV to a dose of 10.sup.15/cm.sup.2
through the opening 225 to create an n.sup.- layer 224 in the
substrate 222, just below the SiO.sub.2 layer 212, and spatially
limited by the mask 223. The n.sup.+ phosphorus layer 224 forms the
buried conductive layer 224 of the resultant resonator bandpass
filter device. Then, as indicated in FIG. 5(d), the old photoresist
layer 223 (FIG. 5(c)) is removed, and a new photoresist coating is
applied so that a second opening 235 may be created using
photolithography. This second opening 235 corresponds to the
resonator 226 and its electrical contacts, 227 and 228 and is
related to the prior opening 225 as illustrated by the dotted-line
rectangle. Next, as illustrated in FIG. 5(e), boron ions are
implanted at 15 keV to a dose of 10.sup.15/cm.sup.2 through the
opening 235 to create a p.sup.+ layer in the silicon layer 202
where the resonator 226 is to be constructed. At this point, the
old photoresist is removed. Furthermore, at this point, the wafer
may be annealed to remove any radiation damage from the implants,
and to activate the B (boron) and P (phosphorus) impurities. A
typical anneal process may be implemented in forming gas at
950.degree. C. for 30 minutes.
[0028] The next step requires the application of a new photoresist
coating so that a photolithography technique may be used to open
three rectangles 230, 231 and 232 at the surface as illustrated in
FIG. 5(f). These three holes fit inside the opened rectangle 225.
The relationship of the three holes to the resonator is such that,
in a subsequent etch process performed through the surface silicon
202 exposed by presence of the three holes 230-232, the resonator
Boron implant region 226 is sandwiched between two holes 231 and
232 at the silicon surface layer 202 such as shown in FIG. 5(g) . A
liquid silicon etch such as Ethylene-Dimene-PyroCatehcol Pyrozine
(EPPW) may be used, however, according to a preferred embodiment, a
reactive ion silicon etch (RIE) using CF.sub.4.div.O.sub.2 (10%) is
used because it will leave more abrupt edges. The structure after
this step is illustrated in FIG. 5(g) , which shows the
relationship of the three holes to the resonator 216 and the buried
conductive layer 224.
[0029] Next, as shown in FIG. 5(h), the old photoresist is removed,
and a new photoresist coating is applied so that a photolithography
technique may be used to open a rectangle 233 that is substantially
aligned with the original rectangle 230 (see FIG. 5(f)). Further,
an etch process is performed to etch through opening 233, removing
the Si0.sub.2 layer using an etchant such as buffered HF, down to
the phosphorus implant layer 224.
[0030] As shown in FIG. 5(i), a conducting metal, typically Ti (50
nm thick) followed by Al (550 nm thick) is deposited on the wafer
to form the metal contact 234. Specifically, the prior photoresist
layer is removed which enables all of the Ti and Al to be removed
from the wafer except for that portion which was deposited within
the hole 233 etched in the prior step. Thus, the hole 233 is filled
with metal 234, enabling electrical contact from the surface 201 to
the buried phosphorus implant layer 224.
[0031] Next, as shown in FIG. 5(j), a new photoresist coating is
applied so that a photolithography technique may be used to open
two rectangles, substantially aligned with the remaining two prior
fabricated rectangular openings 231 and 232 (see FIG. 5(f)).
[0032] Finally, as indicated in FIG. 5(k), an etching process is
performed to etch down through holes 231 and 232, through the
SiO.sub.2 layer, utilizing an etchant, e.g., buffered HF.
Preferably, the etching continues until the Si0.sub.2 under the
resonator 226 (between the two open rectangles, 231 and 232) is
fully removed, leaving a resonator structure as shown in the
cross-sectional view of FIG. 5(k). Except for connection to other
circuit elements, the basic band-pass filter structure 100 of FIG.
2(a) is completed.
[0033] In operation, as shown in FIG. 5(k), an input signal is
conducted down the metal layer 234 to the deep contact 224.
Specifically, the input is the reach-through contact 234, which
transmits the signal to the buried phosphorus layer 224. This layer
is n-type (phosphorus doped silicon) and has junction isolation
from the p-type substrate 222. Layer 224 capacitively couples the
input signal to the resonator 226, and enables the resonator to
vibrate at its natural mechanical frequencies, filtering signals
which will transmitted to the output electrical pad 228.
Specifically, the signal propagates through the buried layer 224
until it is under the resonator 226. An image charge is induced in
the resonator, and it will mechanically distort towards the buried
layer. For electrical signals in resonance with the natural
mechanical frequencies of the structure, the resonator will vibrate
and capacitively propagate the signal through the P.sup.+ doped
layer to the output contact 228. As shown in FIG. 5(d), a second
contact 227 is placed at the other end of the resonator 226, which
may be used for frequency tuning. For example, a small current,
e.g., of about 10 mA, injected at second contact 227, will raise
the temperature of the resonator to about 150.degree. C., changing
the resonator natural vibrational frequency and allowing the
band-pass filter to be tuned.
[0034] In accordance with the principles of the invention described
herein, similar procedures may be used to construct the variations
on the above MEMS resonator device, such as shown in FIGS. 3 and 4.
It should be apparent that manufacture of the resonator device
structure 110 of FIG. 3(a) is the same but, does not require the
phosphorus implant steps as depicted in FIGS. 5(b) and 5(c) above,
nor, the reach-through etch and metallization steps as depicted in
FIGS. 5(h)-5(i). Rather, the final bottom contact 117 is formed by
depositing a metal layer using a technique such as electroplating
to cover the bottom of the well 108 beneath the resonator.
[0035] Additionally, as mentioned, the MEMS resonator device 120 of
FIG. 4(a) vibrates parallel to the wafer surface, and innovates in
the inclusion of the single-crystal silicon resonator constructed
in accordance with the processes described above.
[0036] Furthermore, as mentioned, the natural frequency of the
resonator structures described herein may be altered by ion
implantation into the resonator. Such an implant may be done using
the same mask as described with respect to FIG. 5(d), above, and
may follow the boron implant process step depicted in FIG. 5(e).
Such ion implantation may be used to alter the resonator elastic
constant in two ways: (1) by changing the density of the material,
or (2) by changing the internal bonding structure of the material.
The general formula which describes the natural fundamental
frequency of a resonator beam supported at both ends is derived in
the reference entitled "Vibration and Sound", e.g., Chapter IV "The
Vibration of Bars", by P. M. Morse, McGraw Hill Book Co., New York
(1948), the contents of which are incorporated herein by reference,
and set forth in equation (1) as follows: 1 Fundamental Frequency =
K T L 2 Y ( 1 )
[0037] where K is a constant, T is the beam thickness, L is the
beam length, Y is the elastic constant of the beam material, and
.rho. is the beam material density. Examples of processes which may
be used to alter the resonator frequency (after subsequent
annealing) include the following:
[0038] 1) Ion implantation of neutral light atoms such as carbon
will, after anneal, maintain the same single-crystal structure of
the resonator but lowers the resonator density, and hence raises
its natural frequency of vibration. It is understood that neutral
atoms are those which are chemically similar to silicon, and may be
directly incorporated into the silicon crystal lattice.
[0039] 2) Implantation of neutral heavy atoms such as germanium
which raises the resonator material density, and lowers the natural
frequency of vibration; and,
[0040] 3) Implantation of dopant substitutional atoms such as B, As
or P will change the local bonding of the silicon, and also effect
the elastic constant of the resonator.
[0041] The resonator frequency may also be lowered by reducing the
thickness of the resonator. This may be simply done by oxidizing
and then etching the silicon prior to any processing, and reducing
the thickness of the surface silicon
[0042] The resonator frequency may also be raised by increasing the
thickness of the resonator. This may be done by growing epitaxial
silicon on the wafer prior to any other processing.
[0043] The resonator frequency may be also raised by the deposition
of any material upon the resonator structure to increase its
thickness. However, any material other than single-crystal silicon
will degrade the device performance by introducing internal
friction losses.
[0044] The width of the band-pass filter may be too narrow for some
applications. This frequency width may be increased (widened) by
ion implantation of the resonator surface with silicon atoms,
partially converting it to polycrstalline or amorphous silicon.
[0045] However, as noted above, internal friction from such
materials reduces the device efficiency and also widens the
band-pass by distorting the natural vibrational frequency.
[0046] While the invention has been particularly shown and
described with respect to illustrative and preformed embodiments
thereof, it will be understood by those skilled in the art that the
foregoing and other changes in form and details may be made therein
without departing from the spirit and scope of the invention which
should be limited only by the scope of the appended claims.
* * * * *