TECHNICAL FIELD
[0001] The present invention relates to raised microstructures for silicon based devices.
BACKGROUND OF THE INVENTION
[0002] The use of silicon based capacitive transducers as microphones is well known in the
art. Typically, such microphones consist of four elements: a fixed backplate; a highly
compliant, moveable diaphragm (which together form the two plates of a variable air-gap
capacitor); a voltage bias source and a buffer.
[0003] The batch fabrication of acoustic transducers using similar processes as those known
from the integrated circuit technology offers interesting features with regard to
production cost, repeatability and size reduction. Futhermore, the technology offers
the unique possibility of constructing a single transducer having a wide bandwidth
of operation with a uniform high sensitivity. This provides for a transducer that,
with or no modification, can be used in such diverse applications as communications,
audio, and ultrasonic ranging, imaging and motion detection systems.
[0004] The key to achieve wide bandwidth and high sensitivity lies in creating a structure
having a small and extremely sensitive diaphragm. Designs have previously been suggested
in U.S. Patent No. 5,146,435 to Bernstein, and in U.S. Patent No. 5,452,268 to Bernstein.
In these structures, the diaphragm is suspended on a number of very flexible movable
springs. However, the implementation of the springs leads to an inherent problem of
controlling the acoustic leakage in the structure, which in turn affects the low frequency
roll-off of the transducer. Another approach is to suspend the diaphragm in a single
point, which also provides an extremely sensitive structure. See U.S. Patent No. 5,490,220
to Loeppert. Unfortunately, in this case the properties of the diaphragm material
become critical, especially the intrinsic stress gradient which causes a free film
to curl. Eventually, this leads to a similar problem for this structure concerning
the reproducibility of the low frequency roll-off of the transducer.
[0005] The two mechanical elements, the backplate and diaphragm, are typically formed on
a single silicon substrate using a combination of surface and bulk micromachining
well known in the art. One of these two elements is generally formed to be planar
with the surface of the supporting silicon wafer. The other element, while itself
generally planar, is supported several microns above the first element by posts or
sidewalls, hence the term "raised microstructure."
[0006] In general, the positioning of the two elements with respect to each other affects
the performance of the entire device. Intrinsic stresses in the thin films comprising
the raised microstructure cause the structure to deflect out of the design position.
In a microphone in particular, variations in the gap between the diaphragm and backplate
affect the microphone sensitivity, noise, and over pressure response.
[0007] Many other factors also affect the manufacture, structure, composition and overall
design of the microphone. Such problems are more fully discussed and addressed in
U.S. Patent No. 5,408,73 to Berggvist; U.S. Patent No. 5,490, 220 to Loeppert, and
U.S. Patent No. 5,870,482 to Loeppert.
[0008] In the specific example of the design of a microphone backplate as a raised microstructure,
the goal is to create a stiff element at a precise position relative to the diaphragm.
One method to achieve this is to form the backplate using a silicon nitride thin film
deposited over a shaped silicon oxide sacrificial layer which serves to establish
the desired separation. This sacrificial layer is later removed through well known
etch processes, leaving the raised backplate. Intrinsic tensile stress in the silicon
nitride backplate will cause it to deflect out of position. Compressive stress is
always avoided as it causes the structure to buckle.
[0009] FIG. 12 depicts one such raised microstructure 110 of the prior art. After the oxide
is removed leaving the raised microstructure 110, an intrinsic tension will be present
within the plate 112. This tension T results from the manufacturing process as well
as from the difference between the coefficient of expansion of the material of the
raised microstructure 110 and the supporting wafer 116. As shown, the tension T is
directed radially outwards. The tension T intrinsic in the plate 112 will result in
a moment as shown by arrow M about the base 118 of sidewall 114. This moment M results
in a tendency of the plate 112 to deflect towards the wafer 116 in the direction of
arrow D. This deflection of plate 112 results in a negative effect on the sensitivity
and performance of the microphone.
[0010] A number of undesirable means to negate the effects of this intrinsic tension within
a thin-film raised microstructure are known in the prior art. Among them are that
the composition of the thin film can be adjusted by making it silicon rich to reduce
its intrinsic stress levels. However, this technique has its disadvantages. It results
in making the thin film less etch resistant to HF acid, increasing the difficulty
and expense of manufacture. An additional solution known in the prior art would be
to increase the thickness of the sidewall supporting the raised backplate thereby
increasing the sidewall's ability to resist the intrinsic tendency of the thin film
to deflect. While this sounds acceptable from a geometry point of view, manufacture
of a thick sidewall when the raised microstructure is made using thin film deposition
is impractical.
[0011] The object of the present invention is to solve these and other problems.
SUMMARY OF THE INVENTION
[0012] One aspect of the present invention results from a realization that a diaphragm has
the highest mechanical sensitivity if it is free to move in its own plane. Furthermore,
if the diaphragm is resting on a support ring attached to the perforated member, a
tight acoustical seal can be achieved leading to a well controlled frequency roll-off
of the transducer. Additionally, if a suspension method is chosen such that the suspension
only allows the diaphragm to move in its own plane and does not take part in the deflection
of the diaphragm to an incident sound pressure wave, complete decoupling from the
perforated member can be achieved which reduces the sensitivity to external stresses
on the transducer.
[0013] The present invention consists in a raised microstructure for use in a silicon based
device, the raised microstructure comprising:
a generally planar thin-film;
a sidewall supporting the film;
wherein the sidewall has at least one rib formed therein.
[0014] The rib may be of generally arcuate, triangular or rectangular cross section.
BRIEF DESCRIPTION OF THE DRAWINGS:
[0015]
FIG. 1 is an enlarged schematic cross-sectional view taken along the line 1-1 in FIG.
2 of an acoustic transducer with clamped suspension in accordance with the present
invention;
FIG. 2 is a top plan view, partially in phantom, of the acoustic transducer of FIG.
1;
FIG. 3 is a cross-sectional perspective view of the acoustic transducer of FIG. 2
taken along line 3-3 of FIG. 2;
FIG. 4 is an enlarged partial top view, partially in phantom, of an acoustic transducer
similar to FIG. 2 wherein the perforated member includes an optionally shaped attach
perimeter;
FIG. 5 is an enlarged schematic cross-sectional view taking along the plane 5-5 in
FIG. 6 of an acoustic transducer with high compliance spring suspension in accordance
with the present invention;
FIG. 6 is a top plan view, partially in phantom, of the acoustic transducer of FIG.
5;
FIG. 7 is a cross-sectional perspective view of the acoustic transducer of FIG. 6
taken along plane 7-7;
FIG. 8 is a greatly enlarged partial top view, partially in phantom, of an acoustic
transducer similar to FIG. 5 wherein the perforated member includes an optionally
shaped attach perimeter;
FIG. 9 is an electrical circuit for the detection of the change of the microphone
capacitance whilst maintaining a constant electrical charge on the microphone;
FIG. 10 is an electrical circuit for the detection of the change of the microphone
capacitance while maintaining a constant electrical potential on the microphone;
FIG. 11 is a cross-sectional perspective view of the acoustic transducer of FIG. 4;
FIG. 12 is a cross sectional schematic of a raised microstructure known in the prior
art;
FIG. 13 is a cross sectional perspective view of a raised microstructure embodying
the present invention;
FIG. 14 is a cross section of the raised microstructure of FIG. 13; and
FIG. 15 is a plan view of FIG. 13, taken along line 11-11.
DETAILED DESCRIPTION OF THE INVENTION:
[0016] While this invention is susceptible of embodiment in many different forms, there
is shown in the drawings and will herein be described in detail preferred embodiments
of the invention with the understanding that the present disclosure is to be considered
as an exemplification of the principles of the invention and is not intended to limit
the broad aspect of the invention to the embodiments illustrated.
[0017] Referring now to the drawings, and particularly to FIGS. 1-3, an acoustic transducer
in accordance with the present invention is disclosed. The acoustic transducer 10
includes a conductive diaphragm 12 and a perforated member 40 supported by a substrate
30 and separated by an air gap 20. A very narrow air gap or width 22 exists between
the diaphragm 12 and substrate 30 allowing the diaphragm to move freely in its plane,
thereby relieving any intrinsic stress in the diaphragm material and decoupling the
diaphragm from the substrate. A number of small indentations 13 are made in the diaphragm
to prevent stiction in the narrow gap between the diaphragm and substrate. The lateral
motion of the diaphragm 12 is restricted by a support structure 41 in the perforated
member 40, which also serves to maintain the proper initial spacing between diaphragm
and perforated member. The support structure 41 may either be a continuous ring or
a plurality of bumps. If the support structure 41 is a continuous ring, then diaphragm
12 resting on the support structure 41 forms tight acoustical seal, leading to a well
controlled low frequency roll-off of the transducer. If the support structure 41 is
a plurality of bumps, then the acoustical seal can be formed either by limiting the
spacing between the bumps, by the narrow air gap 22, or a combination thereof.
[0018] The conducting diaphragm 12 is electrically insulated from the substrate 30 by a
dielectric layer 31. A conducting electrode 42 is attached to the non-conductive perforated
member 40. The perforated member contains a number of openings 21 through which a
sacrificial layer (not shown) between the diaphragm and perforated member is etched
during fabrication to form the air gap 20 and which later serve to reduce the acoustic
damping of the air in the air gap to provide sufficient bandwidth of the transducer.
A number of openings are also made in the diaphragm
12 and the perforated member
40 to form a leakage path
14 which together with the compliance of the back chamber (not shown), on which the
transducer will be mounted, forms a high-pass filter resulting in a roll-off frequency
low enough not to impede the acoustic function of the transducer and high enough to
remove the influence of barometric pressure variations. The openings
14 are defined by photo lithographic methods and can therefore be tightly controlled,
leading to a well defined low frequency behavior of the transducer. The attachment
of the perforated member
40 along the perimeter
43 can be varied to reduce the curvature of the perforated member due to intrinsic internal
bending moments. The perimeter can be a continuous curved surface (FIGS. 1-3) or discontinuous,
such as corrugated (FIG. 4). A discontinuous perimeter
43 provides additional rigidity of the perforated member
40 thereby reducing the curvature due to intrinsic bending moments in the perforated
member
40.
[0019] Turning to FIGS. 5-7, an alternative embodiment of an acoustic transducer in accordance
with the present invention is depicted. The transducer
50 includes a conductive diaphragm
12 and a perforated member
40 supported by a substrate
30 and separated by an air gap
20. The diaphragm
12 is attached to the substrate through a number of springs
11, which serve to mechanically decouple the diaphragm from the substrate, thereby relieving
any intrinsic stress in the diaphragm. Moreover, the diaphragm is released for stress
in the substrate and device package.
[0020] The lateral motion of the diaphragm
12 is restricted by a support structure
41 in the perforated member
40, which also serves to maintain the proper initial spacing between diaphragm and perforated
member
40. The support structure
41 may either be a continuous ring or a plurality of bumps. If the support structure
41 is a continuous ring, then diaphragm
12 resting on the support structure
41 forms tight acoustical seal, leading to a well controlled low frequency roll-off
of the transducer. If the support structure
41 is a plurality of bumps, then the acoustical seal can be formed by limiting the spacing
between the bumps, or by providing a sufficiently long path around the diaphragm and
through the perforations
21.
[0021] The conducting diaphragm
12 is electrically insulated from the substrate
30 by a dielectric layer
31. A conducting electrode
42 is attached to the non-conductive perforated member
40. The perforated member contains a number of openings
21 through which a sacrificial layer (not shown) between the diaphragm
12 and the perforated member is etched during fabrication to form the air gap
20 and which later serves to reduce the acoustic damping of the air in the air gap to
provide sufficient bandwidth of the transducer. A number of openings are made in the
support structure
41 to form a leakage path
14 (FIG. 6) which together with the compliance of the back chamber (not shown) on which
the transducer can be mounted forms a high-pass filter resulting in a roll-off frequency
low enough not to impede the acoustic function of the transducer and high enough to
remove the influence of barometric pressure variations. The openings
14 are preferably defined by photo lithographic methods and can therefore be tightly
controlled, leading to a well defined low frequency behavior of the transducer. The
attachment of the perforated member along the perimeter
43 can be varied to reduce the curvature of the perforated member due to intrinsic internal
bending moments. The perimeter
43 can be smooth (FIGS. 5-7) or corrugated (FIGS. 8 and 11). A corrugated perimeter
provides additional rigidity of the perforated member thereby reducing the curvature
due to intrinsic bending moments in the perforated member.
[0022] In operation, an electrical potential is applied between the conductive diaphragm
12 and the electrode
42 on the perforated member. The electrical potential and associated charging of the
conductors produces an electrostatic attraction force between the diaphragm and the
perforated member. As a result, the free diaphragm
12 moves toward the perforated member
40 until it rests upon the support structure
41, which sets the initial operating point of the transducer with a well defined air
gap
20 and acoustic leakage through path
14. When subjected to acoustical energy, a pressure difference appears across the diaphragm
12 causing it to deflect towards or away from the perforated member
40. The deflection of the diaphragm
12 causes a change of the electrical field, and consequently capacitance, between the
diaphragm
12 and the perforated member
40. As a result the electrical capacitance of the transducer is modulated by the acoustical
energy.
[0023] A method to detect the modulation of capacitance is shown in FIG. 9. In the detection
circuit
100, the transducer
102 is connected to a DC voltage source
101 and a unity-gain amplifier
104 with very high input impedance. A bias resistor
103 ties the DC potential of the amplifier input to ground whereby the DC potential "Vbias"
is applied across the transducer. Assuming in this circuit a constant electrical charge
on the transducer, a change of transducer capacitance results in a change of electrical
potential across the transducer, which is measured by the unity-gain amplifier.
[0024] Another method to detect the modulation of capacitance is shown in FIG. 10. In the
detection circuit
200, the transducer
202 is connected to a DC voltage source
201 and a charge amplifier configuration
205 with a feedback resistor
203 and capacitor
204. The feedback resistor ensures DC stability of the circuit and maintains the DC level
of the input of the amplifier, whereby the DC potential "Vbias-Vb" is applied across
the transducer. Assuming in this circuit a constant potential across the transducer,
due to the virtual ground principle of the amplifier, a change of capacitance causes
a change of charge on the transducer and consequently on the input side of the feedback
capacitor leading to an offset between the negative and positive input on the amplifier.
The amplifier supplies a mirror charge on output side of the feedback capacitor to
remove the offset, resulting in a change of output voltage "Vout." The charge gain
in this circuit is set by the ratio between the initial transducer capacitance and
the capacitance of the feedback capacitor. An advantage of this detection circuit
is that the virtual ground principle of the amplifier eliminates any parasitic capacitance
to electrical ground in the transducer, which otherwise attenuate the effect of the
dynamic change of the microphone capacitance. However, care should be taken to reduce
parasitic capacitances to minimize the of gain of any noise on the signal "Vb" and
the inherent amplifier noise.
[0025] An embodiment of the raised microstructure
110 of the present invention is shown in FIGS. 13 and 14. The raised microstructure
110 comprises a generally circular thin-film plate or backplate
112 supported by a sidewall
114.
[0026] The raised microstructure
110 is comprised of a thin film plate
112 of silicon nitride deposited on top of a sacrificial silicon oxide layer on a silicon
wafer
116 using deposition and etching techniques readily and commonly known to those of ordinary
skill in the relevant arts. The sacrificial silicon oxide layer has already been removed
from the figure for clarity. The sidewall
114 of the raised microstructure
110 is attached at its base
118 to the silicon wafer
116 and attached at its opposite end to the plate
112. The sidewall
114 is generally perpendicular to plate
112, but it is noted other angles may be utilized between the sidewall
114 and the plate
112.
[0027] FIG. 15 shows a plan view of the assembly of FIG. 13 with a surface of the sidewall
114 of the present invention shown in phantom. It can be seen that the sidewall
114 of the present invention as shown in FIGS. 13-15 is ribbed, forming a plurality of
periodic ridges
120 and grooves
122. In the preferred embodiment, the ridges
120 and grooves
122 are parallel and equally spaced, forming a corrugated structure. Furthermore, the
preferred embodiment utilizes ridges
120 and grooves
122 of a squared cross section. The effect of corrugating the side wall in this manner
is to create segments
124 of the sidewall
114 that are radial, as is the intrinsic tension
T of the plate
112. By making portions of the sidewall
114 radial, as is the tension
T, the sidewall
114 is stiffened. It has been found that the sidewall
114 of the prior art, which is tangential to plate
112, is easily bent as compared to the radial segments
124 of the present invention.
[0028] Other geometries than that shown in FIGS. 13-15 of the corrugations or ridges
120 and grooves
122 can be imagined and used effectively to increase the sidewall's
114 ability to resist moment
M and the geometry depicted in the FIGS. 13-15 is not intended to limit the scope of
the present invention.
[0029] For example, a generally annular geometry, generally triangular geometry or any combination
or variation of these geometries or others could be utilized for the ridges
122 and grooves
124.
[0030] In the preferred embodiment, the corrugations are radial and hence the sidewalls
114 are parallel to the tension in the backplate
112. Furthermore, the sacrificial material is etched in such a way that the sidewalls
114 are sloped with respect to the substrate to allow good step coverage as the thin
film backplate
112 is deposited.
[0031] While the specific embodiments have been illustrated and described, numerous modifications
come to mind without significantly departing from the spirit of the invention and
the scope of protection is only limited by the scope of the accompanying Claims.