U.S. patent application number 09/772459 was filed with the patent office on 2001-09-13 for encapsulated mems band-pass filter for integrated circuits.
Invention is credited to Speidell, James L., Ziegler, James F..
Application Number | 20010020878 09/772459 |
Document ID | / |
Family ID | 23483004 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010020878 |
Kind Code |
A1 |
Speidell, James L. ; et
al. |
September 13, 2001 |
Encapsulated mems band-pass filter for integrated circuits
Abstract
Integrated circuit fabrication technique for constructing novel
MEMS devices, specifically band-pass filter resonators, in a manner
compatible with current integrated circuit processing, and
completely encapsulated to optimize performance and eliminate
environmental corrosion. The final devices may be constructed of
single-crystal silicon, eliminating the mechanical problems
associated with using polycrystalline or amorphous materials.
However, other materials may be used for the resonator. The final
MEMS device lies below the substrate surface, enabling further
processing of the integrated circuit, without 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: |
Speidell, James L.;
(Poughquag, NY) ; Ziegler, James F.; (Yorktown
Heights, NY) |
Correspondence
Address: |
Richard L. Catania, Scully, Scott, Murphy
& Presser
400 Garden City Plaza
Garden City
NY
11530
US
|
Family ID: |
23483004 |
Appl. No.: |
09/772459 |
Filed: |
January 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09772459 |
Jan 30, 2001 |
|
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09375942 |
Aug 17, 1999 |
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Current U.S.
Class: |
333/197 ;
333/198; 333/199 |
Current CPC
Class: |
H03H 9/1057 20130101;
H03H 3/0073 20130101; H03H 2009/02511 20130101; H03H 9/462
20130101; H03H 9/02409 20130101; H03H 3/0072 20130101 |
Class at
Publication: |
333/197 ;
333/198; 333/199 |
International
Class: |
H03H 009/24 |
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 encapsulated
resonator device of single crystal silicon formed on a
silicon-on-insulator (SOI) substrate comprising the steps of: a)
forming a top silicon layer, and an intermediate layer of SiO.sub.2
on said substrate; b) depositing a layer of SiO.sub.2 on the top
silicon layer; c) opening a first mask and creating a conductive
structure in said top silicon layer, said conducting structure
having dimensions of a resonator device to be formed; d) opening a
second mask and depositing a metal layer on top of said layer of
SiO.sub.2 on the surface silicon layer in substantial alignment
with said conductive structure and having dimensions corresponding
to said resonator device; e) opening a third mask transverse to
said conductive structure and said metal layer in said silicon
surface layer to define a length of said resonator device, and
etching first and second trenches at respective first and second
sides of said metal layer and conductive structure down through the
top SiO.sub.2 layer, the top silicon layer and, the intermediate
layer of SiO.sub.2; f) opening a fourth mask corresponding in area
to said third mask and forming a well structure by etching down to
remove the SiO.sub.2 and the intermediate layer of SiO.sub.2 above
and below said conductive structure; and, g) depositing SiO.sub.2
onto the structure commensurate in thickness to that of said well
structure etched in step f) for filling said well on both said
first and second sides of said conductive structure and said metal
layer, wherein said conductive structure forms said encapsulated
resonator device lying entirely within a vacuum in alignment with
said top metal layer and operates by capacitively coupling an input
signal provided at said metal layer to said resonator device.
2. The method as claimed in claim 1, wherein step e) further
includes the step of selectively etching the top silicon layer to
narrow the width of said conductive structure relative to said
metal layer.
3. The method according to claim 1, wherein step c) of creating
said conductive layer in said top silicon layer includes
implementing ion implantation technique.
4. The method according to claim 1, wherein said first mask opening
for creating said conductive structure in said step c) includes
first and second end contact areas in said top silicon layer.
5. The method according to claim 4, wherein a first contact area is
connected with an output connector for propagating a filtered
signal from said resonator device.
6. The method according to claim 4, a second contact area is
connected with a connector for receiving a stimulus for changing a
modulus of elasticity of said conductive structure for altering a
band-pass frequency characteristic of said resonator device.
7. The method according to claim 1, wherein said step c) further
includes step of changing a density of said conductive structure of
said resonator device by 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 1, wherein said step c) further
includes step of changing a density of said conductive structure of
said resonator device by 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, wherein said step c) further
includes step of changing a density of said conductive structure of
said resonator device by ion implanting atoms for changing an
internal bonding structure of said 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 performing
subsequent planarizing and metallization steps to said integrated
circuit encapsulated resonator device.
12. The method according to claim 1, wherein said top silicon layer
is a single-crystal silicon material.
13. The method according to claim 1, including substituting for
step a) the steps of: opening an area on a substrate and etching a
trench in an area having an area larger than an area of each said
first-fourth masks; depositing a lower release layer of SiO.sub.2
in said trench; depositing one or more layers of material to be
used as said resonator device on top of said lower release layer in
said trench; and, depositing an upper release layer of SiO.sub.2 on
top said one or more layers of silicon material in said trench.
14. The method according to claim 13, wherein a material used as
said resonator device includes poly-crystalline silicon.
15. The method according to claim 13, wherein a material used as
said resonator device includes amorphous silicon.
16. An encapsulated integrated circuit bandpass filter device
comprising: a) a substrate including a vacuum gap formed beneath a
surface metal layer for conducting an input signal to said device,
said metal layer lying along a linear dimension; and, b) a
conductive resonator structure having first and second contact ends
and a middle portion exposed completely within said vacuum gap
along said linear dimension, one of said first and second for
propagating an output signal from said device, whereby an input
signal is capacitively coupled from said metal layer to said
conductive resonator structure to enable vibration of said
resonator device within said gap in a vertical direction at a
desired frequency of vibration.
17. The device as claimed in claim 16, wherein said metal layer is
of a defined width, said conductive resonator structure having a
width narrower than the defined width of said metal layer along
said gap.
18. The device as claimed in claim 16, wherein said second
conductive contact end is 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.
19. The device as claimed in claim 16, wherein said conductive
resonator structure comprises single crystal silicon material.
20. The device as claimed in claim 16, wherein said conductive
resonator structure comprises poly-crystalline silicon
material.
21. The device as claimed in claim 16, wherein said conductive
resonator structure comprises amorphous silicon.
22. The device as claimed in claim 16, wherein said conductive
resonator structure is multi-layered.
23. The device as claimed in claim 16, 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.
24. An encapsulated integrated circuit bandpass filter device
comprising: a) a substrate including a vacuum gap formed beneath a
surface metal layer for conducting an input signal to said device,
said metal layer lying along a linear dimension; and, b) a
conductive resonator structure having first and second contact ends
and a middle portion exposed completely within said vacuum gap and
lying transverse to said linear dimension, one of said first and
second for propagating an output signal from said device, whereby
an input signal is capacitively coupled from said metal layer to
said conductive resonator structure to enable vibration of said
resonator device within said gap in a vertical direction at a
desired frequency of vibration.
25. The device as claimed in claim 24, wherein said second
conductive contact end is 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.
26. The device as claimed in claim 24, wherein said conductive
resonator structure comprises single crystal silicon material.
27. The device as claimed in claim 24, wherein said conductive
resonator structure comprises poly-crystalline silicon
material.
28. The device as claimed in claim 24, wherein said conductive
resonator structure comprises amorphous silicon.
29. The device as claimed in claim 24, wherein said conductive
resonator structure is multi-layered.
30. The device as claimed in claim 24, 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.
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 band-pass filters, and particularly
a process for manufacturing integrated circuit (IC) filters using
micro electro-mechanical system (MEMS) technology that involves the
encapsulation of a single-crystal MEMS band-pass filter in a vacuum
environment, at or below the substrate surface using standard
processing, compatible with other standard integrated circuit
devices, and, in a manner that eliminates the need for precision
lithography.
[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, MEMS 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:
[0006] 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).
[0007] 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", 1998 IEEE 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 micro-mechanical
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 morphous
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, references entitled "Micromachined Devices for Wireless
Communications", "Surface Micromachining for Microelectromechanical
Systems", both detail the problems 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.
[0008] 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 anisotropic 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.
[0009] A further limitation of all MEMS band-pass structures is
that they are formed above the silicon surface (see references
1-9), making 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 destroyed by either of the above planarization processes.
[0010] Finally, most MEMS band-pass filter structures have been
"open" structures, i.e. on top of the substrate and operating in
air. References 1)-3) report that operating resonators in air adds
significant friction to the system, reducing the device
efficiency.
[0011] Prior 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.
[0012] 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. Pat. 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.
[0013] It would thus be highly desirable to construct an IC MEMS
filter device in a manner consistent with current integrated
circuit fabrication techniques that avoids completely or reduces
significantly all of the above-described limitations.
[0014] Furthermore, it would be highly desirable to construct an IC
MEMS filter device in a manner consistent with current integrated
circuit fabrication techniques including features for removing
problems of environmental contamination and also the deleterious
effects of air on the vibrating resonator.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide an
improved IC MEMS resonator device of a construction that lends
itself to manufacture in accordance with current IC manufacturing
techniques and that overcomes the fundamental weaknesses as
outlined above.
[0016] Particularly, according to one aspect of the invention,
there is provided a fabrication technique for constructing novel
MEMS devices, specifically band-pass filter resonators, in a manner
compatible with current integrated circuit processing, and
completely encapsulated to optimize performance and eliminate
environmental corrosion. The final devices may be constructed of
single-crystal silicon, eliminating the mechanical problems
associated with using polycrystalline or amorphous materials.
However, according to another aspect of the invention, other
materials such as polycrystalline or amorphous silicon may be used
for the resonator. The final MEMS device lies at or below the
substrate surface, enabling further processing of the integrated
circuit, without protruding structures.
[0017] Advantageously, the MEMS device of the invention 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] 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:
[0019] FIG. 1 is a schematic diagram of a conventional MEMS
bandpass filter device of a suspended resonator design.
[0020] FIGS. 2(a)-2(c) illustrate respective top, side and end
views of an encapsulated MEMS bandpass filter 100 having a
suspended resonator with a third contact for possible electrical
frequency tuning.
[0021] FIGS. 3(a)-3(c) illustrate respective top, side and end
views of an encapsulated MEMS bandpass filter 110 having a
suspended resonator without a third contact for electrical
tuning.
[0022] FIGS. 4(a)-4(c) illustrate respective side, top and end
views of an encapsulated MEMS bandpass filter 200 with the
resonator transverse to the input conductor, with a third contact
for possible electrical frequency tuning.
[0023] FIGS. 5(a)-5(k) illustrate the various steps and masks used
in construction of the encapsulated MEMS bandpass filter device,
and also depicts intermediate structures during the fabrication
process.
[0024] FIGS. 6(a) and 6(b) illustrate the initial cross-section of
the layers used for the fabrication of poly crystalline resonators
and multi-layer bonded resonator.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0025] FIGS. 2(a), 2(b) and 2(c) illustrate respective top, side,
and edge views of the novel encapsulated resonator 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 100
includes a conducting wire input or contact terminal 101 that is
suspended over a vacuum cavity 103 containing a conducting
resonator 102 which is constructed at or below a primary surface
104 of the device. As shown, the resonator is connected to output
wire or contact terminal 105. It should be understood that the
input (101) and output (105) terminals of this device may be
reversed, without changing its operating characteristics, but for
simplicity the input and output contacts will be discussed as shown
in FIG. 2(b). In operation, an alternating signal on the input 101
causes an image charge to form on the resonator 102, attracting it
and deflecting it upwards. 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 capacitively coupled AC signal. This meshing of
the electrical and mechanical vibrations selectively isolates and
transmits desired frequencies for further signal amplification and
manipulation.
[0026] As shown in the end view of the device in FIG. 2(c), the
single-crystal resonator 102 suspended in the vacuum cavity 103
under input conductor 101. The side view of FIG. 2(b) shows the
same resonator 102 clamped at both ends, under the input conductor.
As further shown, all conductors are electrically isolated from the
substrate by an insulating layer 108, except where a contact
purposely is made. The resonator 102 is made electrically
conductive by a buried implantation layer 106, which also connects
it to the output conductor 105. With resonator dimensions of 1
mm.times.1 mm, 0.3 mm thickness, for example, the resonant
frequency may range from about 500 MHz - 800 MHz, with a first
harmonic in the band of 3-5 GHz, using standard theory of vibrating
bar frequencies when clamped at both ends (see Ref. 11).
[0027] Additionally shown in FIGS. 2(a)-2(c) is a third contact 107
which is electrically connected to the resonator 102 by the same
buried conducting layer 106. The insulating layer 108 prevents
electrical contact between the buried layer and input conductor
101. Conductor 107 may be used for tuning the resonator frequency
either by applying DC bias relative to the input contact 101 which
will apply internal stress to the resonator, or by applying DC bias
relative to the output contact 105 which will cause a current to
flow through the resonator, increasing its temperature. Both types
of bias change the modulus of elasticity of the resonator,
resulting in a change of its fundamental natural vibrational
frequency.
[0028] That is, 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, pp. 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.
[0029] A variation of the device 100 is illustrated in FIGS.
3(a)-3(c) which show respective top, side and end views of an
encapsulated resonator device structure 110 according to a second
embodiment of the invention. In this embodiment, the resonator
device 110 includes an input contact 111 which spans cavity 113
that contains resonator 112 which straddles the cavity and is free
to vibrate. The resonator is electrically connected by a deep
implant layer 116 to an output pad 115, which propagates the final
filtered signal. The input contact 111 is capacitively coupled to
the resonator 112, so that the input signal will cause the
resonator to vibrate. The resonator will have a natural frequency
of vibration, based on its dimensions and material, and signals
with this frequency (or its harmonics) will be preferentially
propagated through the resonator to the output terminal 115. This
variation does not use a separate tuning contact, and any real-time
tuning (after processing is completed) must be done by a DC voltage
bias between the input and output conductors, applying stress to
the resonator.
[0030] The devices illustrated in FIGS. 2 and 3, both show the
resonator perpendicular to the input conductor. A variation of
these structures is now described with respect to FIG. 4(a)-4(c)
which illustrate a resonator device structure 200 according to a
third embodiment of the invention.
[0031] In FIG. 4 (a), there is illustrated an encapsulated MEMS
bandpass filter device 200 in which the resonator 202 lies at
90.degree. with respect to the structure described and shown in
FIG. 2. Thus, input conductor 201 covers the buried cavity 203 in
which the MEMS resonator 202 is suspended. The resonator is made
conducting by a deep implantation 206 which electrically connects
it to an output conductor 205, and also to a tuning conductor 207.
The device 200 has the advantage over the structure illustrated in
FIG. 2 in that there is less area overlap, and hence less
capacitive coupling between the input conductor 201 and the buried
conducting layer 206. However, this structure may require more
steps to fabricate, and hence, more costly to manufacture.
[0032] In accordance with the invention, the process used to
fabricate each of the encapsulated MEMS resonator bandpass filter
devices may utilize 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. Further, as will be described in greater detail
hereinbelow in view of FIGS. 6(a) and 6(b), the devices may be
constructed by evacuating a trench in the surface of a silicon
wafer, and depositing layers of material corresponding to those
illustrated in view of FIG. 5(a). The fabrication of the resonators
would be similar to that described for SOI starting materials,
except the final resonator will be polycrystalline or amorphous
silicon.
[0033] Typically, SOI wafers may be 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.
[0034] FIGS. 5(a)-5(k) illustrate the process steps in
manufacturing a SOI encapsulated MEMS device, e.g., having the
resonator structure shown in FIG. 2(a).
[0035] 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
SiO.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 purposes of illustration. Next, as shown in FIG. 5(a)
in a second step, a top layer 232 of SiO.sub.2 is deposited on the
wafer having a thickness of about 30 nm, for example (not shown).
Then, in a third step, a thick photoresist layer (not shown) is
applied, and implementing photolithographic technique, a structure
225 as shown in FIG. 5(b), is opened which represents a first mask.
This first mask outlines the deep implant layer 106 which connects
the tuning contact 107 to the resonator 102, makes the resonator
conducting, and finally connects the resonator to the output
contact 105 (FIG. 2(a)). Thus, in a fourth step not shown,
phosphorus ions are implanted, for example, at 95 keV to a dose of
10.sup.15/cm.sup.2 through the mask 225 opening. This will create
an n+ layer in the substrate, just below the SiO.sub.2 layer, and
spatially limited by the mask 225 of FIG. 5(b). Because the silicon
layer is p-type, this n-type conducting layer will be electrically
isolated from the silicon layer.
[0036] Then, the photoresist applied for fabricating opening
structure 225 is removed in a fifth step. It may be convenient to
anneal the wafer at this point to remove the radiation damage from
the implants, and to activate the phosphorus. A typical anneal
would be in forming gas at about 950.degree. C. for about 30
minutes.
[0037] Next, in a sixth step, a new photoresist coating is applied
and a photolithographic technique is implemented to open a
structure 230 as shown in FIG. 5(c), which represents a second
mask. This mask is used to create the metal layer above the
resonator, and be the self-alignment structure for the band-pass
filter. That is, the resonator will be constructed just below this
layer, and its width will be defined by the width of this conductor
(although its final width will be slightly smaller than the width
of this metal layer). The positioning of this mask is shown
relative to the implant mask 225 indicated by the dotted lines in
FIG. 5(c). Next, in a seventh step, a layer of Molybdenum (Mo)
metal, or like equivalent, is deposited to a thickness of about 100
nm according to the pattern defined second mask area. Then, the
photoresist is removed, lifting off the Mo except for the area 230
exposed by the pattern of FIG. 5(c). At this point, in an eighth
step, the structure may be annealed to make the Mo metal adhere to
the underlying SiO . Typically this anneal may take place in
forming gas at 400 C. for 30 minutes.
[0038] Then, in a ninth step, a new photoresist coating is applied
and a photolithographic technique utilized to open a rectangular
shape 235 shown in FIG. 5(d), which represents a third mask. The
positioning of this third mask 235 is shown relative to the implant
mask 225 indicated by the dotted lines as shown in FIG. 5(b). Also
shown in FIG. 5(d) is the cross-sectional plane, labeled by dashed
line A - - - A, which is the reference plane for the
cross-sectional views shown in FIGS. 5(e)-5(i), as will be
described in greater detail herein.
[0039] The opening in the photoresist as depicted by the third mask
of FIG. 5(d) applied in the previous step, will be used to etch
through the various layers, and create a resonator structure below
the conductor. The resonator width will be determined by the width
of the Mo metal, FIG. 5(c), and hence will automatically have the
same basic shape (a self-aligned structure). After application of
the photoresist, the structure will have the cross-section shown in
FIG. 5(e), taken through the reference plane, A - - - A, as
indicated in FIG. 5(d). This cross-sectional view illustrates the
photoresist layer 240, labeled "PR", and the central metal strip
250 defined by the third mask 235. The substrate contains the
following layers (with approximate thicknesses are shown in
parenthesis): SiO.sub.2 (30 nm), Si (200 nm) , SiO.sub.2 (400 nm)
and the substrate silicon.
[0040] Then, in a tenth step, a reactive ion etching (RIE)
technique is utilized to etch through the two rectangular areas
236a, 236b exposed on either side of the Mo metal by the third mask
235 (FIG. 5 (e)) . Typical RIE gases for this etch step is
CF.sub.4+O.sub.2 (10%), which plasma will etch down trenches 242,
244 through the surface SiO.sub.2 (30 nm) and the underlying Si
layer (200 nm) and partially into the lower SiO layer. This etching
step will result in the creation of a structure having a
cross-section as depicted in FIG. 5(f) which illustrates the extent
of each etched trench 242, 244 formed through the upper two
substrate layers 232, 202, and partially into the third layer
212.
[0041] The next step (step 11) requires a removal of the
photoresist layer 240 as shown in FIGS. 5(e) and 5(f). Then, a
twelfth step is implemented to selectively etch the Si layer under
the Mo metal, so that the final resonator will be slightly narrower
than the Mo metal. A liquid etch such as
Ethylene-Dimene-PyroCatehcol Pyrozine (EPPW) may be used which has
an etch rate of Si more than 100:1 than that of SiO.sub.2. The
portions of silicon 255a,b under the Mo are etched back about 100
nm as shown in the resultant structure depicted in FIG. 5(g).
[0042] Then, in a thirteenth step, as shown in the cross-sectional
view of FIG. 5(h), a photoresist layer 260 is applied and a
photolithographic technique implemented to open a fourth mask
structure 265. This fourth mask 265 is similar to the third mask
structure 235 as shown in top-view in FIG. 5(d), except that it
also covers the Mo layer 250 in the middle of the structure.
[0043] In the next step (step 14), an etchant such as buffered HF
is used to etch down through the SiO.sub.2 layers in an amount
sufficient to fully remove the SiO.sub.2 layers under and over the
resonator 102. The result of this step is to leave a "well"
structure 270 as shown in cross-section in FIG. 5(h), with the
resonator 102 separated from the metal conductor 250 by the
thickness of the SiO.sub.2 layer 232 deposited in second step
(SiO.sub.2 application shown in FIG. 5(a)). The resonator 102 is
now mechanically "free" with the removal of the SiO.sub.2 layers
above 271 and below 272 the resonator.
[0044] As a final step 15, SiO.sub.2 is deposited in a vacuum
process onto the structure, preferably commensurate in thickness to
that as the well structure 270 etched in the previous step. Thus,
as shown in FIG. 5(i), this results in the refill of the well 270
with SiO.sub.2 layers 280 of approximately 730 nm in thickness, for
example, on both sides of the conductor 250 and the resonator 102.
Then, the layer of photoresist 260 is removed which will lift-off
all the SiO.sub.2 except which has refilled the etched well,
leaving a structure as illustrated in FIG. 5(i). The resonator is
now fully encapsulated in a vacuum.
[0045] Except for connection wiring to other circuit elements, the
basic band-pass filter structure 100 illustrated in FIG. 2(a) is
completed. The input signal is carried in the Mo conductor
deposited in the seventh step. This signal is capacitively coupled
to the MEMS resonator 102 constructed in step 14. This signal is
then carried through the buried conductor 106 constructed by ion
implantation in the fourth step to an output connector (not
shown).
[0046] A third contact may make contact to the buried implant layer
(see FIG. 2(a) and may be used for frequency tuning of the
band-pass filter. A small current, for example, of about 10 mA,
will raise the temperature of the resonator about 150.degree. C.,
changing the resonator natural vibrational frequency and allowing
the band-pass filter to be tuned. This contact may also be used to
apply DC bias to pre-stress the resonator, changing its natural
vibrational frequency.
[0047] As mentioned herein, the natural frequency of the resonator
structure may be altered by ion implantation into the resonator at
any stage of its construction. Such an implant may be done using
the same mask 225 as described with respect to FIG. 5(b), above,
and may follow the phosphorus implant process step 4. 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 )
[0048] 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:
[0049] 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.
[0050] 2) Implantation of neutral heavy atoms such as germanium
which raises the resonator material density, and lowers the natural
frequency of vibration; and,
[0051] 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.
[0052] 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
[0053] 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.
[0054] 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.
[0055] 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 polycrystalline or amorphous silicon.
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.
[0056] The above process steps illustrated with respect to FIGS.
5(a)-5(i) are for fabricating a silicon crystal encapsulated MEMS
resonator. However, the process described may also be used for
resonators made of other materials. Polycrystalline resonators may
be fabricated at low temperatures, and the mechanical limitations
of using such materials may be tolerated for some applications.
Multi-layer resonators may be fabricated using mixed materials, in
which the mixed properties are superior to those of any individual
material. For example, diamond-like-carbon (DLC) may be chosen to
make a resonator with a stiff elastic constant (and hence a large
natural vibrational frequency), and combine this with a metal to
obtain good electrical conduction in the resonator. The DLC has an
elastic constant of about 1500 Gpa (GigaPascals), in contrast to
about 200 Gpa for single crystal silicon. Hence, the natural
vibrational frequency will increase by the square root of the
difference, i.e., about 2.7 times larger for the DLC material.
Although DLC may be deposited with doping to improve its electrical
conductivity, a thin bonded metal layer may be required to provide
the necessary conductivity to the resonator without compromising on
the elastic constant of the DLC as it is not currently known about
how such doping will affect its elastic constant.
[0057] For a polycrystalline resonator, or a multi-layered
resonator, the fabrication process requires several additional
steps at the beginning of the fabrication cycle. These process
steps are identified as steps 301-305 hereinbelow, so they will not
be confused with steps 1-15 for the fabrication of single crystal
silicon, described above. Once these steps are completed, the rest
of the MEMS fabrication follows the outline of steps 2-15 (above).
These new steps will create a filled trench as illustrated in
cross-section in FIGS. 6(a) (polycrystalline resonator) and 6(b)
(multi-layer resonator).
[0058] FIG. 6(a) illustrates a trench 350 etched into the
substrate. This trench is slightly larger than the final MEMS
device, but its surface area is not critical. Successive
depositions will lay down layers of the lower release layer 351,
the polycrystalline material for the resonator 352 and the upper
release layer 353.
[0059] When the MEMS device is completed, the layers 351 and 353
will be absent, and the resonator will be formed of the material
352.
[0060] FIG. 6(b) illustrates a trench 350 etched into the
substrate. This trench is slightly larger than the final MEMS
device, but its surface area is not critical. Successive
depositions will lay down layers of the lower release layer 351,
several layers which will make up the resonator 354, 355, 356, and
the upper release layer 353. When the MEMS device is completed, the
layers 351 and 353 will be absent, and the resonator will be formed
of the materials 354, 355 and 356.
[0061] According to this embodiment of the invention, typical
process steps include:
[0062] A step 300 which utilizes photoresist to open an area on the
substrate surface slightly larger than the masks shown in FIGS.
5(b)-5(d). Etch a trench into the silicon, for example using
reactive ion etching (RIE). For example, etching a rectangle about
4 mm.times.4 mm in area about 1 mm deep.
[0063] A next step 301 which requires the deposition of a "lower
release layer" which will be later etched away. For example, a
layer of SiO.sub.2, of 670 nm thick, for example, is deposited
using evaporation or sputter-deposition.
[0064] A next step 302 requires the deposition of the material
which will be used as the resonator. For example, a layer of Si,
300 nm thick, may be deposited using evaporation or
sputter-deposition. Alternatively, the resonator material may be
made of several layers to obtain special material properties.
[0065] Next, a step 303 for depositing an "upper release layer"
which will be later etched away. For example, a layer of SiO.sub.2,
30 nm thick, may be deposited using evaporation or
sputter-deposition.
[0066] Finally at step 304, the photoresist is removed, releasing
the deposited materials everywhere except within the box etched in
step 301 above.
[0067] After completing initial steps 301-304, the cross-section of
the device will be as illustrated in depicted in FIGS. 6(a) and
FIG. 6(b) for both the polysilicon resonator and the DLC/metal
resonator.
[0068] The processing of this MEMS device may then proceed as
herein described for the single-crystal silicon resonator, process
steps 2-15. Masks 1-3 (FIGS. 5(b)-5(d)) will be placed within the
area fabricated above in step 301. Only two process steps need to
be changed. The deep etch described in step 10, FIG. 5(f), must be
able to etch through the resonator material deposited in step 302.
Also, the side-etch used in step 12, FIG. 5(g), must be an etch
which is selective in etching the resonator material, while having
little etching effect on the other materials in the trench. Hence,
the suggested EPPW etchant may need to be changed to an etchant
material specific for the resonator material chosen. Other than
these two changes, the process of the device construction is
similar to that described for making a single-crystal silicon MEMS
device, discussed in process steps 2-15 herein.
[0069] 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.
* * * * *