U.S. patent application number 11/611864 was filed with the patent office on 2007-04-19 for optically coupled sealed-cavity resonator and process.
Invention is credited to David W. Burns.
Application Number | 20070086502 11/611864 |
Document ID | / |
Family ID | 36578654 |
Filed Date | 2007-04-19 |
United States Patent
Application |
20070086502 |
Kind Code |
A1 |
Burns; David W. |
April 19, 2007 |
Optically Coupled Sealed-Cavity Resonator and Process
Abstract
A process to form a laterally offset photodiode for an optically
coupled resonator includes implanting a semiconductor substrate to
form the laterally offset photodiode adjacent to the resonator. The
resonator masks the implanting underneath the resonator when the
semiconductor substrate is implanted. Also disclosed is an
optically coupled resonator, a process for fabricating an optically
coupled resonator, and a device including an optically coupled
resonator having a laterally offset photodiode.
Inventors: |
Burns; David W.; (San Jose,
CA) |
Correspondence
Address: |
DAVID W. BURNS
15770 RICA VISTA WAY
SAN JOSE
CA
95127
US
|
Family ID: |
36578654 |
Appl. No.: |
11/611864 |
Filed: |
December 16, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10905036 |
Dec 12, 2004 |
7176048 |
|
|
11611864 |
Dec 16, 2006 |
|
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Current U.S.
Class: |
372/92 |
Current CPC
Class: |
G01P 15/093 20130101;
G01P 15/097 20130101; G01L 1/103 20130101; G01L 9/002 20130101 |
Class at
Publication: |
372/092 |
International
Class: |
H01S 3/08 20060101
H01S003/08 |
Claims
1. An optically coupled resonator, comprising: a laterally offset
photodiode adjacent to the resonator.
2. The optically coupled resonator of claim 1, wherein the
resonator is driven by an electric field generated between the
laterally offset photodiode and the resonator when incident light
strikes the photodiode.
3. The optically coupled resonator of claim 1, wherein a vibration
of the resonator is sensed with a reflected light, the reflected
light reflected from at least one surface of the resonator.
4. A device including an optically coupled resonator, comprising: a
laterally offset photodiode adjacent to the resonator, wherein the
resonator is driven by an electric field generated between the
laterally offset photodiode and the resonator when incident light
strikes the photodiode.
5. The device of claim 4 selected from the group consisting of a
strain sensor, a pressure sensor, an accelerometer, an angular rate
sensor, a temperature sensor, a chemical sensor, a biological
sensor, an explosives detector, a radiation detector, a
radio-frequency filter, a voltage-controlled oscillator, a
mechanical oscillator, and a resonant device.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a divisional application and claims the
priority benefits of currently pending U.S. application Ser. No.
10/905,036, filed 12 Dec. 2004, which is incorporated herein by
reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates generally to semiconductor devices,
and more specifically to optically coupled resonators for sensors,
filters and oscillators.
BACKGROUND OF THE INVENTION
[0003] Resonant sensors are used in the precision pressure
measurement field because of their high stability, high sensitivity
and low-temperature coefficients. Resonant sensors can be
constructed of primarily silicon-based materials using standard
processes of the semiconductor industry including thin-film
deposition, etching, doping and lithography. While resonant
pressure sensors are generally more complex than piezoresistive
pressure sensors, their stability and accuracy are less dependent
on electronic signal processing circuitry than are comparable
piezoresistive sensors and capacitive sensors. Currently available
resonant sensors have resonators comprised of a single material
such as quartz, single crystal silicon or deposited polysilicon
films. Very high precision resonators have been made from well-cut
quartz.
[0004] The vibrating micromechanical body or resonator of a
resonant pressure sensor provides a frequency as output data, the
frequency depending upon a stress such as pressure that modifies
the natural resonant vibrational frequency of the resonator. A load
applied to the sensor structure strains the resonator causing a
resonant frequency shift of the resonator. The frequency output of
the resonator provides a measure of the magnitude of the mechanical
load applied to the sensor structure, and as a result, pressure can
be measured as a consequence of the frequency shift. Currently
available resonant pressure sensors interface with analog and/or
digital electronics to measure pressure.
[0005] In a conventional piezoresistive pressure sensor,
deformations of a silicon diaphragm with applied pressure cause
shifts in a Wheatstone bridge fabricated from single-crystal
piezoresistors in the diaphragm resulting in a voltage output
indicating the amount of pressure applied to the sensor. The output
voltage from the Wheatstone bridge requires an analog-to-digital
(A/D) conversion to be used in digital systems. An example of a
surface-micromachined absolute pressure sensor has a pressure
diaphragm formed from a deposited thin film of polysilicon with an
integral vacuum cavity reference directly under the diaphragm and
dielectrically isolated polysilicon piezoresistors, as described in
"Sealed cavity semiconductor pressure transducers and method of
producing the same," U.S. Pat. No. 4,744,863, Guckel et al., issued
May 17, 1988.
[0006] The widespread use and continuing trend toward digital
information and control systems, together with the need for more
accurate and higher-pressure instrumentation, have prompted the
development of digital pressure transducers capable of precision
measurements in pressure ranges up to about 250 MPa. One exemplary
high-precision digital pressure sensor operates on the principle of
changing the resonant frequency of load-sensitive quartz crystals
with pressure-induced stress. Frequency signals from the quartz
crystals are counted and linearized through microprocessor-based
electronics to provide two-way communication and control in digital
formats. The aforementioned quartz crystal pressure transducers
have a resolution as good as a few parts per billion and have been
used to determine the performance of high precision, primary
standard dead-weight testers.
[0007] One example of a surface-micromachined resonant sensor has a
resonant strain gage formed from a deposited thin film of
polysilicon with an integral vacuum cavity surrounding the
resonator. Several patents providing background to such resonant
sensors include "Dielectrically isolated resonant microsensors,"
U.S. Pat. No. 5,417,115, Burns, issued May 23, 1995; "Static
pressure compensation of resonant integrated microbeam sensors,"
U.S. Pat. No. 5,458,000, Burns et al., issued Oct. 17, 1995;
"Cantilevered microbeam temperature sensor;" U.S. Pat. No.
5,511,427, Burns, issued Apr. 30, 1996; "Method for making a thin
film resonant microbeam absolute;" U.S. Pat. No. 5,747,705 Herb et
al., issued May 5, 1998; and "Thin film resonant microbeam absolute
pressure sensor," U.S. Pat. No. 5,808,210 Herb et al., issued Sep.
15, 1998.
[0008] Another example of a resonant pressure sensor, which is
fabricated from single-crystal silicon, is disclosed in
"Semiconductor pressure sensor and its manufacturing method,"
Watanabe et al., U.S. Pat. No. 5,880,509 issued Mar. 9, 1999. The
sensor comprises a single-crystal silicon substrate, a closed
air-gap chamber, a measured diaphragm made by epitaxial growth, and
a strain detection element incorporated in the measuring
diaphragm.
[0009] The operation of a resonant pressure sensor requires a
resonator to be excited into vibrational motion and detection of
this motion. Forces and moments are applied that bend, twist,
elongate or contract the resonator. Various methods for excitation
and detection of resonant sensors have been proposed including
thermal excitation with piezoresistive detection; electrostatic
excitation with capacitive detection; Lorentz force excitation with
magnetic flux detection; piezoelectric excitation with
piezoelectric detection, and optical excitation with optical
detection. In an exemplary method, resonant microbeams are driven
and sensed by a single multimode optical fiber using a
strain-sensitive oscillator, as described in "Fiber-optic vibration
sensor based on frequency modulation of light-excited oscillators,"
U.S. Pat. No. 6,246,638, Zook et al., issued Jun. 12, 2001. A
suggested method for driving and sensing a resonant sensor by using
modulated and unmodulated light from multiple light sources is
described in "Multi-wavelength optical drive/sense readout for
resonant microstructures," U.S. Pat. No. 5,844,236, Wilson, issued
Dec. 1, 1998. A photodetector detects the filtered reflected light
to determine the resonant frequency of the resonator.
[0010] Resonators may be hermetically sealed in an evacuated cavity
or enclosure to provide separation from the surrounding
environment, eliminating effects such as air damping of the
resonator and mass loading on the resonator body.
[0011] Micro-electrical-mechanical systems (MEMS) researchers are
working on producing precision resonant pressure sensors having
increased noise immunity, intrinsic safety, and long line-driving
capability. It is desirable that a pressure sensor can operate in
the harsh conditions associated with, for example, turbine engines,
high-speed combustors, and other aerospace and industrial
applications.
[0012] Because of limitations in the use of electrically
powered/electrical output sensors in high-noise environments,
hazardous areas, and some medical applications, it is advantageous
to combine the precision of resonator-based sensors with the total
optical isolation of fiber-optic technology. One method of
optically powering a resonant low-pressure sensor is described in
"Optically powered resonant integrated microstructure pressure
sensor," Youngner, U.S. Pat. No. 6,710,355, issued Mar. 23,
2004.
[0013] In the pressure-sensor industry and other sensor application
areas, the need exists for precise and stable sensors having low
hysteresis, remote access, electromagnetic interference (EMI)
immunity, and increased safety in harsh, volatile, or explosive
environments with the elimination of voltage and electronic
circuitry at the sensor element. Additional features that are
desirable for resonant sensors include a simplified fabrication
process, integral vacuum sealing, reduction or elimination of
stiction and snap-down or pull-in problems associated with the
resonator, accurate positioning of the drive and sense electrodes,
alignment of the phase between the drive frequency and resonator
movement over a wide frequency range, high signal-to-noise ratio of
the detected signal, simplification of the optical interface to the
resonator, and the opportunity for relatively easy integration of
the sensor with more complex fabrication processes such as
complementary metal-oxide-semiconductor (CMOS) and bipolar
complementary metal-oxide-semiconductor (BiCMOS) processes.
SUMMARY OF THE INVENTION
[0014] One aspect of the invention is a process for fabricating an
optically coupled resonator. A semiconductor substrate is provided.
A lower sacrificial layer is formed on the semiconductor substrate.
A structural layer is formed on the lower sacrificial layer. The
structural layer is patterned and etched to form a resonator body
having a resonator sidewall and a resonator upper surface. An upper
sacrificial layer is formed on at least the resonator sidewall and
the resonator upper surface. The semiconductor substrate is doped
to form a laterally offset photodiode. A portion of the
semiconductor substrate near the resonator body is exposed. A
resonator shell is formed around the resonator body. A portion or
more of the lower sacrificial layer and the upper sacrificial layer
are etched to form a resonator cavity around the resonator body. A
sealing layer is applied to seal the resonator cavity.
[0015] Another aspect of the invention is a process to form a
laterally offset photodiode for an optically coupled resonator. A
semiconductor substrate is implanted to form the laterally offset
photodiode adjacent to the resonator. The resonator masks the
implanting underneath the resonator when the semiconductor
substrate is implanted.
[0016] Another aspect of the invention is an optically coupled
resonator including a laterally offset photodiode adjacent to the
resonator.
[0017] Another aspect of the invention is a device including an
optically coupled resonator. A laterally offset photodiode is
adjacent to the resonator. The resonator is driven by an electric
field generated between the laterally offset photodiode and the
resonator when incident light strikes the photodiode.
[0018] Other aspects, features and attendant advantages of the
present invention will become more apparent and readily appreciated
by the detailed description given below in conjunction with the
accompanying drawings. The drawings should not be taken to limit
the invention to the specific embodiments, but are for explanation
and understanding and are not necessarily drawn to scale. The
detailed description and drawings are merely illustrative of the
invention rather than limiting, the scope of the invention being
defined by the appended claims and equivalents thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] Various embodiments of the present invention are illustrated
by the accompanying figures, wherein:
[0020] FIG. 1 illustrates a process for fabricating an optically
coupled resonator, in accordance with one embodiment of the current
invention;
[0021] FIG. 2 is a cross-sectional view of an optically coupled
sealed-cavity resonator, in accordance with one embodiment of the
current invention;
[0022] FIG. 3 illustrates a process for fabricating an optically
coupled resonator, in accordance with another embodiment of the
current invention;
[0023] FIG. 4 is a cross-sectional view of an optically coupled
sealed-cavity resonator, in accordance with another embodiment of
the current invention;
[0024] FIG. 5 is a cross-sectional view of an optically coupled
sealed-cavity resonator, in accordance with another embodiment of
the current invention; and
[0025] FIG. 6 is a flow diagram of a process for fabricating an
optically coupled resonator, in accordance with another embodiment
of the current invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] FIG. 1 illustrates a process for fabricating an optically
coupled resonator, in accordance with one embodiment of the present
invention. For the description of this figure and the figures that
follow, similarly numbered objects correspond to similar elements.
Optically coupled resonator 10 is fabricated on semiconductor
substrate 20 such as an n-type single-crystal silicon wafer, a
silicon-on-insulator wafer, a handle wafer of a
silicon-on-insulator wafer, or another suitable substrate. As
illustrated in FIG. 1a, lower sacrificial layer 30 is formed on
semiconductor substrate 20 and comprises a thermally grown layer of
silicon dioxide also referred to as thermally grown oxide, a
deposited layer of silicon dioxide also referred to as a deposited
oxide, a buried oxide layer of a silicon-on-insulator (SOI) wafer,
or a layer of another suitable sacrificial material. The deposited
oxide may be doped while being deposited, or the oxide may be
deposited as a laminate or multi-layer film. The thickness of lower
sacrificial layer 30 is selected, in part, to allow resonator 10 to
move without striking or sticking to semiconductor substrate 20
after sacrificial layer 30 is sacrificially etched and removed as
described below.
[0027] Structural layer 40 such as amorphous silicon,
polycrystalline silicon, epi-poly, epitaxial silicon,
single-crystal silicon, or a combination thereof is deposited or
otherwise formed on lower sacrificial layer 30. For example,
structural layer 40 may comprise the active silicon layer of an SOI
wafer, formed in one embodiment by bonding a second silicon wafer
to an oxidized semiconductor substrate 20, and grinding and
polishing the bonded wafer to the desired thickness, as is known in
the art. Structural layer 40 may be deposited or otherwise formed
in a substantially undoped or doped condition. Additional dopant
may be added, such as with ion implantation or a dopant diffusion
process, to dope structural layer 40. In the process flow
illustrated, semiconductor substrate 20 and structural layer 40 are
doped n-type, although p-type dopants may be used for either.
Structural layer 40 is deposited or otherwise formed to a desired
thickness of resonator 10 between a few tenths of micrometers to a
few micrometers or more. Structural layer 40 is patterned and
etched to form resonator body 12 having resonator sidewall 14 and
resonator upper surface 16 using, for example, photoresist,
photolithography, and wet or dry etching techniques as are used in
the semiconductor processing industry.
[0028] As illustrated in FIG. 1b, upper sacrificial layer 50 is
formed on at least resonator sidewall 14 and resonator upper
surface 16. Similar to lower sacrificial layer 30, upper
sacrificial layer 50 may comprise a thermally grown oxide, a
deposited oxide, a doped oxide such as a borosilicate glass, a
phosphosilicate glass, a borophosphosilicate glass, or other
suitable sacrificial material. The thickness of upper sacrificial
layer 50 is selected, in part, to allow resonator 10 to move after
upper sacrificial layer 50 is sacrificially etched and removed as
described below.
[0029] To form laterally offset photodiode 24, semiconductor
substrate 20 is locally implanted or otherwise doped with implant
species 22 such as a p-type dopant (as shown) or an n-type dopant.
In one embodiment, resonator body 12 and portions of upper
sacrificial layer 50 deposited on resonator sidewall 14 locally
mask the implanting. Use of resonator body 12 as an implant mask
allows laterally offset photodiode 24 to be located in close
proximity to resonator 10, positioned laterally off to the side and
in a different plane than resonator body 12. Upper sacrificial
layer 50 deposited on resonator sidewall 14 allows finer control of
the position of laterally offset photodiode 24 in semiconductor
substrate 20.
[0030] As illustrated in FIG. 1c, a portion of semiconductor
substrate 20 near resonator body 12 is exposed. Portions of upper
sacrificial layer 50, structural layer 40, and lower sacrificial
layer 30 are patterned and etched to expose semiconductor substrate
20 in desired regions. Optionally, exposed portions of
semiconductor substrate 20 may be implanted, such as with an n-type
dopant (as shown) or a p-type dopant.
[0031] As illustrated in FIG. 1d, shell layer 60 is deposited or
otherwise formed on upper sacrificial layer 50 and exposed portions
of semiconductor substrate 20 to form resonator shell 64 around
resonator body 12. Resonator shell 64 is formed around resonator
body 12, for example, by depositing shell layer 60 on at least
resonator body 12 and exposed portions of semiconductor substrate
20, and patterning and etching shell layer 60 to form resonator
shell 64 around resonator body 12 with shell layer 60 attached to
semiconductor substrate 20. In the example shown, vertical etch
channels 66 are formed when resonator shell 64 is patterned and
etched. Shell layer 60 such as amorphous silicon, polycrystalline
silicon, epi-poly, epitaxial silicon, single-crystal silicon, or a
combination thereof may be doped during deposition or subsequent to
deposition. The thickness of shell layer 60 is selected, in part,
to provide protection for resonator 10 and to allow vibrations of
resonator 10 in vacuum without significant distortion of resonator
shell 64. Shell layer 60 may be doped or undoped.
[0032] As illustrated in FIG. 1e, lower sacrificial layer 30 and
upper sacrificial layer 50 are sacrificially etched to form
resonator cavity 62 between resonator shell 64 and resonator body
12, and between resonator body 12 and semiconductor substrate 20.
Lower sacrificial layer 30 and upper sacrificial layer 50 may be
etched through one or more vertical or horizontal etch channels 66
in resonator shell 64 or between resonator shell 64 and
semiconductor substrate 20 to form resonator cavity 62 using, for
example, a liquid sacrificial etchant such as hydrofluoric
acid.
[0033] To seal resonator cavity 62 in a vacuum or at another
predetermined level such as atmospheric pressure, sealing layer 70
is applied over resonator shell 64 to seal etch channels 66. For
example, resonator cavity 62 is sealed with sealing layer 70 when
etch channels 66 used to etch and form resonator cavity 62 are
filled, capped off, or otherwise plugged. Sealing layer 70 includes
a material such as amorphous silicon, polysilicon, epi-poly,
epitaxial silicon, silicon dioxide, silicon nitride, aluminum,
gold, chromium, copper, nickel, palladium, tungsten, titanium,
platinum, photoresist, an insulating film, a semiconducting film, a
metal film, a metal alloy, a polymeric film, or a combination
thereof. Sealing layer 70 may be deposited under vacuum conditions
such as existing in a sputtering process, a low-pressure plasma
deposition process or a low-pressure chemical vapor deposition
process to obtain the desired vacuum level and to seal off
resonator cavity 62.
[0034] Variations in the order and sequence of process steps and
unit processes that are used may be varied from those detailed
above, such as the exchange of p-type dopants for n-type dopants
and vice-versa, adjustments to the dopant species, and
modifications to the dopant concentrations. Other substrates may be
used such as silicon carbide substrates and insulative substrates.
Other materials such as silicon carbide may also be used for the
resonator. The substrates may be augmented with additional
epitaxial layers and deposited thin films, other mechanical
structures such as pressure-sensitive diaphragms, active electronic
devices such as transistors, integrated circuits and optoelectronic
devices, and optical waveguides without loss of generality in the
described processing. Other masking materials than photoresist may
be used for patterning and etching the desired features. Other
processing steps such as annealing or driving an implanted dopant
are not detailed herein for brevity. Additional masking steps and
unit processes (not shown) may be used to form, for example, one or
more photodiodes around or under resonator body 12, or to form
other structures and features. The relative dimensions of the
resonator, shell, gaps and substrate may also be varied from those
shown. Although a resonator attached to the substrate at one or
both ends is implied by the illustrations, other variations of
resonator designs such as those of balanced resonators or multiple
resonators or combinations of cantilevered and doubly supported
resonators may be equally incorporated. The laterally offset
photodiode, although shown in close proximity to resonator body 12,
may be positioned further away from resonator 10 and be
electrically connected to a laterally offset electrode that is
proximate to resonator body 12. In one example, the photodiode may
be positioned in resonator shell 64. In some cases, resonator shell
64 may be omitted from resonator 10.
[0035] FIG. 2 shows a cross-sectional view of an optically coupled
sealed-cavity resonator, in accordance with one embodiment of the
present invention. Optically coupled resonator 10 includes
resonator body 12 with resonator sidewalls 14 and laterally offset
photodiode 24 adjacent to resonator 10. Laterally offset photodiode
24, in one example, is formed in semiconductor substrate 20
adjacent to resonator 10 by implanting semiconductor substrate 20
while using resonator body 12 of resonator 10 to locally mask the
implanting.
[0036] To optically excite resonator 10 and to determine one or
more resonant frequencies of resonator 10, electric field 26 is
generated between laterally offset photodiode 24 and resonator body
12 of resonator 10 when incident light 80 strikes photodiode 24.
One side of laterally offset photodiode 24 is electrically
connected to resonator body 12, and electric field 26 results when
a photovoltage is generated from incident light 80 striking
photodiode 24. Electric field 26 or a component thereof provides
driving force 28 to excite and actuate resonator 10.
[0037] Vibrations or displacements of resonator 10 may be sensed,
for example, with reflected light 82 that is reflected from at
least one surface of resonator 10 such as resonator upper surface
16. Incident light 80 partially transmits through resonator shell
64 to allow reflections of incident light 80 from upper surface 16
of resonator body 12. Reflected light 82 is intensity modulated in
correspondence with vibrations or displacements of resonator body
12 within resonator shell 64. Portions of incident light 80 may
reflect one or more times in the gap between resonator shell 64 and
resonator body 12 before being transmitted back through resonator
shell 64 or continuing towards semiconductor substrate 20.
Similarly, portions of incident light 80 may reflect one or more
times in the gap between resonator body 12 and semiconductor
substrate 20 before being transmitted back through resonator body
12 and resonator shell 64 or into semiconductor substrate 20.
Similarly, portions of incident light 80 may reflect one or more
times between surfaces of sealing layer 70, surfaces of resonator
shell 64, or between surfaces of resonator body 12. Each reflection
of incident light 80 may constructively or destructively interfere
with other reflection portions of incident light 80 to enhance or
diminish the intensity of reflected light 82. The absorption of
incident light 80 in sealing layer 70, resonator shell 64,
resonator body 12 or semiconductor substrate 20 reduces the
intensity of reflected light 82, and the wavelengths of incident
light 80 are chosen to allow suitable transmission of incident
light 80 and reflected light 82.
[0038] Optically coupled resonator 10 and laterally offset
photodiode 24 adjacent to resonator body 12 of resonator 10 may be
may be included in devices such as a strain sensor, a pressure
sensor, an accelerometer, an angular rate sensor, a temperature
sensor, a chemical sensor, a biological sensor, an explosives
detector, a radiation detector, a radio-frequency filter, a
voltage-controlled oscillator, a mechanical oscillator, or a
resonant device.
[0039] Although incident light 80 and reflected light 82 are shown
as perpendicular to resonator shell 64 and in line with resonator
body 12, it will be appreciated that other design variations exist,
such as the use of angled incident and angled reflected light, or
the use of light impinging from below through a suitably
transmissive substrate. Incident light 80 may be comprised of a
single wavelength of light or of multiple wavelengths of light, and
may cover a narrow or broad range of wavelengths. In one example,
incident light 80 of a single wavelength is partially reflected
from a surface of resonator body 12. In another example, incident
light 80 comprises two wavelengths from two light sources, one of
which is used to generate the photovoltage while the other is used
to determine vibrations or displacements of resonator body 12. In
an alternative configuration, vibrations or displacements of
resonator body 12 are detected with a second laterally offset
photodiode, and the resulting electrical signal is processed by
on-chip or off-chip electronics. In another alternative
configuration, laterally offset photodiode 24 is configured in a
self-resonant mode wherein lateral displacements of resonator body
12 diminish the intensity of incident light 80 striking the
photodiode, which in turn reduce driving force 28 and allow
resonator 10 to return towards an equilibrium position. The driving
cycle is then repeated.
[0040] FIG. 3 illustrates a process for fabricating an optically
coupled resonator, in accordance with another embodiment of the
present invention. Optically coupled resonator 10 is fabricated on
semiconductor substrate 20 such as a single-crystal silicon wafer
or a silicon-on-insulator wafer. As illustrated in FIG. 3a, lower
sacrificial layer 30 such as a thermally grown oxide or a deposited
oxide is formed on semiconductor substrate 20. Structural layer 40
such as amorphous silicon, polycrystalline silicon, epi-poly,
epitaxial silicon, single-crystal silicon, or a combination thereof
is deposited or otherwise formed on lower sacrificial layer 30.
Structural layer 40 may be deposited in an undoped or doped
condition. Additional dopant may be added to structural layer 40
after deposition, such as with ion implantation or a dopant
diffusion process. Structural layer 40 is patterned and etched to
form resonator body 12 having resonator sidewall 14 and resonator
upper surface 16 using, for example, a suitable photomask and
photoresist process.
[0041] As illustrated in FIG. 3b, upper sacrificial layer 50 such
as a thermally grown oxide or a deposited oxide is deposited on at
least resonator sidewall 14 and resonator upper surface 16.
Semiconductor substrate 20 is implanted with a p-type dopant 22 to
form laterally offset photodiode 24, using resonator body 12 and
upper sacrificial layer 50 deposited on resonator sidewall 14 to
locally mask the implanting.
[0042] As illustrated in FIG. 3c, portions of upper sacrificial
layer 50 and lower sacrificial layer 30 are patterned and etched by
using patterned photoresist 54 to expose a portion of semiconductor
substrate 20 near resonator body 12. The exposed portion of
semiconductor substrate 20 may be implanted with an n-type dopant
32 to junction isolate laterally offset photodiodes 24.
[0043] As illustrated in FIG. 3d, shell layer 60 such as amorphous
silicon, polycrystalline silicon, epi-poly, epitaxial silicon,
single-crystal silicon, or a combination thereof is deposited or
otherwise formed on upper sacrificial layer 50 above resonator body
12 and exposed portions of semiconductor substrate 20. Shell layer
60 may be doped during or after deposition. Shell layer 60 is
patterned and etched to form resonator shell 64 around resonator
body 12. Vertical etch channels 66 may also be formed when shell
layer 60 is patterned and etched.
[0044] As illustrated in FIG. 3e, a portion or more of lower
sacrificial layer 30 and upper sacrificial layer 50 are etched and
selectively removed to form resonator cavity 62 between resonator
shell 64 and resonator body 12 and between resonator body 12 and
semiconductor substrate 20. Lower sacrificial layer 30 and upper
sacrificial layer 50 may be etched through one or more vertical or
horizontal etch channels 66 in resonator shell 64 or between
resonator shell 64 and semiconductor substrate 20 to form resonator
cavity 62.
[0045] To seal resonator cavity 62, sealing layer 70 is applied to
fill, cap off, or otherwise plug one or more etch channels 66.
Suitable materials for sealing layer 70 include a material such as
polysilicon, epi-poly, epitaxial silicon, silicon dioxide, silicon
nitride, aluminum, gold, chromium, copper, nickel, palladium,
tungsten, titanium, platinum, photoresist, an insulating film, a
semiconducting film, a metal film, a metal alloy, a polymeric film,
or a combination thereof. During the sealing process, a vacuum or
other predetermined pressure may be obtained and retained in
resonator cavity 62 after etch channels 66 are sealed.
[0046] FIG. 4 is a cross-sectional view of an optically coupled
sealed-cavity resonator, in accordance with another embodiment of
the present invention. Optically coupled resonator 10 includes
resonator body 12 with one or more resonator sidewalls 14.
Laterally offset photodiode 24 in semiconductor substrate 20
adjacent to resonator 10 is formed, for example, by implanting
semiconductor substrate 20 with resonator 10 locally masking the
implanting.
[0047] Resonator 10 is driven by electric field 26 or a driving
component thereof that is generated between laterally offset
photodiode 24 and resonator body 12 of resonator 10 when incident
light 80 strikes photodiode 24. Vibrations or displacements of
resonator 10 may be sensed with reflected light 82 that is
reflected from a surface of resonator body 12 such as resonator
upper surface 16.
[0048] FIG. 5 shows a cross-sectional view of an optically coupled
sealed-cavity resonator, in accordance with another embodiment of
the present invention. Optically coupled resonator 10, including
resonator body 12 with resonator sidewalls 14, vibrates in a plane
parallel to a surface of semiconductor substrate 20. Laterally
offset photodiode 24, which is formed in semiconductor substrate 20
adjacent to resonator 10, allows resonator 10 to be driven with
incident light 80 that strikes laterally offset photodiode 24.
Laterally offset photodiode 24 may be formed, for example, by
implanting semiconductor substrate 20 with resonator 10 locally
masking the implanting, or by locally implanting semiconductor
substrate 20 after or prior to forming structural layer 40.
Electric field 26 is generated between laterally offset photodiode
24 and resonator body 12 of resonator 10 when incident light 80
strikes photodiode 24. Electric field 26 or a driving component
thereof provides driving force 28 for driving resonator 10, and
selective modulation of incident light 80 allows resonator 10 to be
driven at or near a resonant frequency of resonator 10. Vibrational
modes and frequencies of resonator 10 are determined in part by the
cross-sectional dimensions and end conditions of resonator body 12,
end conditions such as whether resonator body 12 is attached at one
or both ends (not shown).
[0049] Vibrations or displacements of resonator 10 may be sensed
with reflected light 82 reflected from resonator upper surface 16
of resonator 10 or from variations in the gap between resonator
sidewalls 14 and resonator shell 64. Reflected light 82 may be used
to determine one or more resonant frequencies of resonator 10.
Analysis of the detected resonant frequencies allows determination
of an applied stimulus such as mechanical strain that is exerted on
resonator body 12 by a suitable microstructure, microstructures
such as a pressure-sensing diaphragm, a flexure of an
accelerometer, or a bending of the substrate attached thereto.
[0050] FIG. 6 is a flow diagram of a process for fabricating an
optically coupled resonator, in accordance with another embodiment
of the present invention.
[0051] A semiconductor substrate is provided, as seen at block 200.
The semiconductor substrate includes, for example, a single-crystal
silicon wafer, a silicon-on-insulator wafer, a handle wafer of a
silicon-on-insulator wafer, or other suitable substrate such as a
silicon carbide wafer. A lower sacrificial layer such as a
thermally grown oxide, a deposited oxide, a doped oxide, or a
combination thereof is formed on the semiconductor substrate. A
structural layer such as amorphous silicon, polycrystalline
silicon, epi-poly, epitaxial silicon, single-crystal silicon, or a
combination thereof is deposited or otherwise formed on the lower
sacrificial layer. The structural layer may be doped. The
structural layer is patterned and etched using, for example,
conventional semiconductor processing techniques to form a
resonator body having resonator sidewalls and a resonator upper
surface.
[0052] An upper sacrificial layer is formed on the resonator
sidewall and the resonator upper surface, as seen at block 202. The
upper sacrificial layer such as a thermally grown oxide, a
deposited oxide, a doped oxide, or a combination thereof is
deposited or otherwise formed on at least the resonator sidewalls
and the resonator upper surface.
[0053] A laterally offset photodiode is formed, as seen at block
204. In one example, the semiconductor substrate is implanted to
form a laterally offset photodiode in the substrate prior to
forming the lower sacrificial layer. In another example, the
photodiode is formed prior to forming the structural layer. In
another example, the photodiode is formed prior to depositing or
otherwise forming the upper sacrificial layer. In another example,
the photodiode is formed by ion implantation through the upper
sacrificial layer after the upper sacrificial layer is deposited or
otherwise formed. In another example, the resonator body and the
upper sacrificial layer deposited on the resonator sidewall locally
mask the implanting in a self-aligned configuration. In another
example, the photodiode is formed after the upper sacrificial layer
is patterned and etched. In another example, the photodiode is
formed after portions of the semiconductor substrate are
exposed.
[0054] A portion of the semiconductor substrate near the resonator
body is exposed, as seen at block 206. In one example, one or more
portions of the semiconductor substrate are exposed when one or
more portions of the upper sacrificial layer, the structural layer,
and the lower sacrificial layer near the resonator body are
patterned and etched. In another example, one or more portions of
the semiconductor substrate are exposed when one or more portions
of the upper sacrificial layer and the lower sacrificial layer are
patterned and etched. The exposed portions of the semiconductor
substrate may then be implanted or otherwise doped.
[0055] To cover the resonator, a resonator shell is formed around
the sidewalls and upper surface of the resonator body. In one
example, the resonator shell around the resonator body is formed by
depositing a shell layer on at least a portion of the resonator
body and the exposed portion of the semiconductor substrate, and
then patterning and etching the shell layer. The shell layer may be
doped during or after deposition.
[0056] A portion or more of the sacrificial layers are etched, as
seen at block 208. As the sacrificial layers are removed, a
resonator cavity is formed between the resonator shell and the
resonator body and between the resonator body and the semiconductor
substrate. The lower sacrificial layer and upper sacrificial layer
are etched, for example, through one or more etch channels in or
under the resonator shell to form the resonator cavity.
[0057] A sealing layer is deposited or otherwise applied to seal
the resonator cavity by plugging the etch channels. The sealing
layer may include a material such as amorphous silicon,
polysilicon, epi-poly, epitaxial silicon, silicon dioxide, silicon
nitride, aluminum, gold, chromium, copper, nickel, palladium,
tungsten, titanium, platinum, photoresist, an insulating film, a
semiconducting film, a metal film, a metal alloy, a polymeric film,
or a combination thereof.
[0058] In another embodiment of the invention, a laterally offset
photodiode is formed for an optically coupled resonator by
implanting the semiconductor substrate to form one or more
laterally offset photodiodes adjacent to the resonator. The
resonator masks the implanting underneath the resonator when the
semiconductor substrate is implanted.
[0059] While the embodiments of the invention disclosed herein are
presently considered to be preferred, various changes and
modifications can be made without departing from the spirit and
scope of the invention. The scope of the invention is indicated in
the appended claims, and all changes that come within the meaning
and range of equivalents are embraced herein.
* * * * *