U.S. patent application number 09/753512 was filed with the patent office on 2001-09-13 for method of fabrication of a torsional micro-mechanical mirror system.
This patent application is currently assigned to THE MICROOPTICAL CORPORATION. Invention is credited to Aquilino, Paul Daniel, McClelland, Robert William, Rensing, Noa More, Spitzer, Mark Bradley, Zavracky, Paul Martin.
Application Number | 20010021058 09/753512 |
Document ID | / |
Family ID | 26736800 |
Filed Date | 2001-09-13 |
United States Patent
Application |
20010021058 |
Kind Code |
A1 |
McClelland, Robert William ;
et al. |
September 13, 2001 |
Method of fabrication of a torsional micro-mechanical mirror
system
Abstract
A torsional micro-mechanical mirror system includes a mirror
assembly rotatably supported by a torsional mirror support assembly
for rotational movement over and within a cavity in a base. The
cavity is sized sufficiently to allow unimpeded rotation of the
mirror assembly. The mirror assembly includes a support structure
for supporting a reflective layer. The support structure is
coplanar with and formed from the same wafer as the base. The
torsional mirror support assembly includes at least one torsion
spring formed of an electroplated metal. An actuator assembly is
operative to apply a driving force to torsionally drive the
torsional mirror support assembly, whereby torsional motion of the
torsional mirror support assembly causes rotational motion of the
mirror assembly. In another embodiment, a magnetic actuator
assembly is provided to drive the mirror assembly. Other actuator
assemblies are operative to push on the mirror assembly or provide
electrodes spaced across the gap between the mirror assembly and
the base. A process for fabricating the torsional micro-mirror is
provided. The torsional micro-mirror is useful in various
applications such as in biaxial scanner or video display
systems.
Inventors: |
McClelland, Robert William;
(Norwell, MA) ; Rensing, Noa More; (West Newton,
MA) ; Spitzer, Mark Bradley; (Sharon, MA) ;
Aquilino, Paul Daniel; (Canton, MA) ; Zavracky, Paul
Martin; (Norwood, MA) |
Correspondence
Address: |
WEINGARTEN, SCHURGIN, GAGNEBIN
& HAYES, LLP
TEN POST OFFICE SQUARE
BOSTON
MA
02109
US
|
Assignee: |
THE MICROOPTICAL
CORPORATION
|
Family ID: |
26736800 |
Appl. No.: |
09/753512 |
Filed: |
January 3, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09753512 |
Jan 3, 2001 |
|
|
|
09138367 |
Aug 26, 1998 |
|
|
|
6201629 |
|
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60057700 |
Aug 27, 1997 |
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Current U.S.
Class: |
359/223.1 ;
359/224.1; 359/247 |
Current CPC
Class: |
G02B 26/085 20130101;
G02B 26/0841 20130101 |
Class at
Publication: |
359/223 ;
359/224; 359/247 |
International
Class: |
G02B 026/08; G02F
001/03; G02F 001/07 |
Goverment Interests
[0002] The invention was made with Government support under
Contract No. DAAK60-96-C-3018 awarded by the Soldier Systems
Command of the United States Army. The Government has certain
rights in the invention.
Claims
We claim:
1. A process for fabricating a torsional micro-mechanical mirror
system, comprising: providing a wafer substrate of a substrate
material; providing electrical contact pads and anchors for
torsional spring structures on one surface of the wafer; forming
the torsional spring structures on the anchors; removing a portion
of the substrate material to define a mirror support structure
separated by a gap from surrounding substrate material; and
providing a mirror on the mirror support structure.
2. The process of claim 1, wherein the step of forming the
torsional spring structures comprises: applying a release layer to
the one surface of the wafer, the release layer patterned with
holes to expose the anchors; depositing a layer of a conducting
material over the release layer to form a portion of the torsional
spring structures; applying and patterning a photoresist layer to
form a mask having exposed regions configured to allow deposition
of material for the torsional spring structures; depositing a
material to the exposed regions to form the torsional spring
structures; and removing the release layer and mask.
3. The process of claim 1, further comprising etching a back
surface of the wafer to form a membrane for the mirror support
structure.
4. The process of claim 1, wherein the step of providing the mirror
comprises providing a metal or dielectric coating to the mirror
support structure through a mask.
5. The process of claim 1, wherein the step of providing the mirror
comprises bonding a mirror to the mirror support structure.
6. The process of claim 1, wherein the step of providing the mirror
comprises bonding a mirror base and a reflecting layer to the
mirror support structure.
7. The process of claim 1, wherein the step of providing the mirror
comprises bonding a curved mirror to the mirror support
structure.
8. The process of claim 1, further comprising applying a stress
compensation material to the mirror support structure.
9. The process of claim 1, further comprising forming a stiffening
pattern in a back side of the wafer.
10. The process of claim 1, wherein the step of providing a wafer
comprises providing a polished wafer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119(e) of U.S. Provisional Application No. 60/057,700, filed on
Aug. 27, 1997, and under 35 U.S.C. .sctn. 120 of U.S. application
Ser. No. 09/138,367 filed on Aug. 26, 1998, the disclosures of
which are incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0003] Micro-electromechanical system (MEMS) mirrors (or
micro-mirrors) have been evolving for approximately two decades as
part of the drive toward integration of optical and electronic
systems, for a range of uses including miniature scanners, optical
switches, and video display systems. These structures consist of
movable mirrors fabricated by micro-electronic processing
techniques on wafer substrates (for example silicon, glass, or
gallium arsenide). The torsional micro-mirror typically comprises a
mirror and spring assembly suspended over a cavity formed in or on
a base. The mirrors are electrically conductive, as is at least one
region behind the mirror, affixed to the stationary base, so that
an electric field can be formed between the mirror and the base.
This field is used to move the mirror with respect to the base. An
alternative comprises the use of magnetic materials and magnetic
fields to move the mirrors.
[0004] Typically the mirror surface consists of either the wafer
itself or a deposited layer (metal, semiconductor, or insulator),
and generally in the prior art the springs and mirror are formed
from the same material (but not in all cases). The mirror and
torsion springs are separated from the base by an etch process,
resulting in the formation of a cavity between the mirror and
base.
[0005] For display or image acquisition applications, the goal is
to develop compact systems with rapid frame rates (at least 60 Hz)
and high resolution, consisting of between 200 and 2000 miniature
pixels per line. For scanning system designs in this range, the
mirrors should be large (in the range of 200 .mu.m.times.200 .mu.m
to 2 mm.times.2 mm), fast (in the range of between 3 kHz and 60 kHz
for resonant devices), and scan a photon beam through a large angle
7 to 40 degrees).
[0006] In optical systems that contain very small elements,
diffraction by the smallest element may introduce diffraction
broadening and deleteriously increase the final pixel size.
Enlarging the limiting element reduces this broadening and
militates for larger mirrors. However, as mechanical systems get
larger (for example, increasing the size of a torsional mirror),
they tend to be characterized by greater mass and consequently
lower resonant frequency; this resonant frequency sets the scanning
speed of the system. A frequency in the range of 5 to 50 kHz is
desirable. Prior art mirror designs have been limited by the
difficulty inherent in obtaining a high resonant frequency with a
large mirror size, free from diffraction broadening effects. In
prior art cases in which the mirror mass is made very low to obtain
high resonant frequency, the resultant reduction in stiffness of
the mirror is a limiting factor in the quality of the reflected
image. This problem is exacerbated by the possibility of heating of
the mirror by light absorbed in the mirror. Such heating militates
for a thick mirror capable of conducting the heat away from the
source.
[0007] The scanning angle through which the mirror moves determines
the number of distinguishable pixels in a display or imaging
system. Therefore, a large scanning angle is desirable. Generally
in the prior art the scan angle is limited by the presence of
electrodes that interfere with mirror motion (but not in all
cases).
[0008] Electrostatic actuation is the most common method used to
drive micro-mirrors. In order to produce a force, a voltage is
generated between two electrodes, usually the plates of a parallel
plate capacitor, one of which is stationary and the other of which
is attached to the mirror as described previously. By making the
mirror an electrical conductor, the mirror itself can be made to
serve as one of the plates. The force generated for a given voltage
depends on the plate area and on the gap between the plates, which
may change as the mirror position changes. For torsional mirrors,
the important drive parameter is the torque, and the effective
torque on the structure is also proportional to the distance
between the resultant force and the axis of rotation of the mirror.
Thus, a large driving force can be achieved using large capacitor
plates and small gaps; by applying the force at a distance from the
rotation axis, a large torque may be obtained.
[0009] In many prior art designs the criteria for a large
deflection angle range tend to be in conflict with the criteria for
large driving forces. The deflection angle is limited by the
presence of surfaces behind the mirror. An example of a limiting
surface would be the bottom of a cavity in the base etched beneath
the mirror, or some other substrate on which the mirror is mounted.
The maximum angle is achieved when the mirror contacts this
backplane, so the small separation between the mirror and the
backplane needed for generating adequate electrostatic deflection
force limits the maximum angle. Accordingly, in prior art designs
in which the mirror is used as one of the drive electrodes and the
other electrode is on the backplane, increasing the gap reduces the
force or torque obtained at a given voltage. Some prior art designs
use electrodes that are offset from the main mirror body and which
are connected through actuator linkages, allowing the backplane to
be moved further away or even eliminated entirely. Typically,
though, these electrodes have smaller active areas and shorter
moment arms, which tend to reduce the effective forces and torques
as well. Additionally, if as the mirror moves, the gap between the
drive electrodes narrows, then the gap still may be a limiting
factor for the range of motion of the structure.
[0010] A second set of design problems arises in the selection of
the mirror. Prior art designs and processes do not permit the
mirror to be made from very low mass material without also
sacrificing structural rigidity. One of the process limitations is
the use of the same material for torsion spring and mirror mass, or
the same set of patterning steps for spring and mirror mass. The
selection of mirror materials with a view toward the elastic or
fatigue properties of the springs restricts the suitability of the
material with respect to mirror mass rigidity, and also limits the
optical performance of the mirrors.
[0011] In 1980, Peterson disclosed a silicon torsional
micromachined mirror (U.S. Pat. No. 4,317,611; K. E. Peterson,
"Silicon torsional scanning mirror," IBM J. Res. Dev., 24(5), 1980,
pp. 631-637). Both the mirror and torsion elements were patterned
in a thin (134 microns) silicon wafer and retained the full
thickness of the wafer. The structure was then bonded to a glass
substrate, over a shallow well to allow room for the mirror motion.
Actuation of the device was electrostatic. The mirror body was used
as one electrode and the other electrodes were placed at the bottom
of the well under the mirror. A narrow ridge in the well under the
axis of rotation of the mirror was used to eliminate transverse
motion of the structure. The manufacturing process for this device
was relatively simple, requiring a single patterning step for the
silicon and two patterning steps for the glass substrate. Its
resonance frequency was about 15 kHz, and at resonance the angular
displacement reached about 1.degree.. The limitations of this
device are related to the depth of the well. A 2 mm mirror touches
the bottom of a 12.5 .mu.m well at a displacement of 0.7.degree.
(1.40.degree. total motion). Increasing the well depth to increase
the range of motion is not necessarily desirable, because it
proportionally reduces the torque achieved for a given voltage.
[0012] Nelson (U.S. Pat. No. 5,233,456), Baker et al (U.S. Pat. No.
5,567,334), Hornbeck (U.S. Pat. No. 5,552,924), and Tregilgas (U.S.
Pat. Nos. 5,583,688 and 5,600,383) have developed and patented a
series of torsional mirror designs and improvements for use in
deformable mirror device (DMD) displays. These mirrors are
fabricated by surface micromachining, consisting of a series of
patterned layers supported by an undisturbed substrate. The DMD
display uses an individual mirror at each pixel. The mirrors are
therefore designed to be very small, to be operated in a bi-stable
mode, and to maximize the packing fraction on the surface of the
display. To minimize the gaps between the reflecting surfaces of
adjacent mirrors, the support structure and drive components are
fabricated in underlying layers, requiring a complicated multi-step
deposition and patterning process. As with the Peterson mirror, the
Hornbeck mirror is designed to serve as one of the deflection
electrodes, and the others are placed behind the mirror. Owing to
the small size of the mirrors (about 20 .mu.m.times.20 .mu.m), high
deflection angles are attainable with reasonably small gaps. These
mirrors are designed for driving at low frequencies, and for
significant dwell at a given angle (on or off), rather than for
continuous motion, although the early development included mirrors
designed for resonant operation (U.S. Pat. No. 5,233,456). A
scanned display or imager requires, however, a large mirror, and
the difficulties with scaling up torsional mirrors that are driven
electrostatically with plates mounted behind the mirrors prevent
the Hornbeck mirrors from being easily modified for use in scanning
display applications.
[0013] Toshiyoshi describes a silicon torsion mirror for use as a
fiber optic switch (H. Toshiyoshi and H. Fujita, "Electrostatic
micro torsion mirrors for an optical switch matrix, " J.
Microelectromechanical Systems, 5 (4), 1996, pp. 231-237). The
Toshiyoshi mirror is a relatively large device (400 .mu.m on a side
and 30 .mu.m thick), which rotates about an axis close to one edge
of the mirror. The mirror is defined by etching the silicon wafer
from the front, and the excess wafer material is etched from the
back of the wafer. It is thus suspended over a cavity in the wafer,
supported by very thin (0.3 .mu.m) metal torsion rods. The
structure is then bonded onto another substrate, on which
electrodes have been plated. Toshiyoshi has demonstrated separation
of the mechanical properties of the springs and mirror by using
silicon for the mirror mass, and metal for the springs. Actuation
is electrostatic, by placing a voltage between the mirror body and
the electrodes of the lower substrate. The range of motion is
limited by the mirror hitting the glass substrate, at about
30.degree.. In order to obtain the maximum deflection at an applied
voltage of 80 volts, the stiffness of the torsion members must be
very low, achieved by making them very thin. This also limits the
resonant frequency of the structure to 75 Hz, making the approach
unsuitable for a scanned display or scanned imager. Thus Toshiyoshi
has not shown how the separation of the mechanical properties of
the spring and mirror can be used to attain a high resonant
frequency and high angular displacement.
[0014] Dhuler of the MCNC has disclosed a mirror wherein the mirror
body is formed from the silicon substrate, while the supports and
actuators are fabricated above the mirror plane using surface
micromachined polycrystalline silicon layers (V. J. Dhuler, "A
novel two axis actuator for high speed large angular rotation,"
Conference Record of "Transducers '97," 1997). The mirror body is
first defined using ion implantation of boron as an etch stop, and
then by removal of the excess Si wafer from the back of the mirror.
The supports and drive electrodes are offset from the top surface
of the substrate by posts, which define the gap between the drive
capacitor plates. Thus the mirror is free to rotate unhindered by
the bottom surface of a well, while the drive torque, being applied
by actuators, is not limited by a requirement for a large capacitor
gap. While it represents a significant advance in the state of the
art, this device suffers from certain flaws which the current
invention resolves.
[0015] In the MCNC process the mirror body thickness is limited by
the boron implantation process, which has limited penetration
depth; the disclosed mirror was 4 .mu.m thick. The stiffness of the
mirror is limited by both its size and thickness, so larger mirrors
need to be thicker to avoid deformation of the mirror surface in
use. For scanning applications, flexure in the mirror leads to
uncertainty in the pixel size and location and distortion of the
pixel shape. The implantation process also introduces stress into
the mirror body, causing deformation of the reflective surface. The
supports and actuators of the MCNC device are formed in a
multi-step process and, as they are non-conducting, require the
separate deposition and patterning of electrodes.
[0016] Kiang describes a 200 .mu.m.times.250 .mu.m mirror that has
a frequency of 15 kHz and maximum displacement of 15.degree. (M. H.
Kiang, "Surface micromachined electrostatic comb driven scanning
micromirrors for barcode applications," 9th Annual Workshop on
Micro Electro-Mechanical Systems, 1996, San Diego, Calif., pp.
192-197). This mirror is made of deposited and patterned surface
layers, and before using it must be first rotated out of the plane
of the substrate using a comb drive and locked into position using
complicated hinges. This approach obviates the problem of forming a
cavity behind the mirror. However, the use of surface micromachined
layers means that the structural rigidity of the micro-mirror
cannot be controlled (because the thickness is limited to thin (a
few microns) layers). The mirror motion is obtained by
electrostatic drive applied by an actuator linked to one edge of
the mirror. The motion of the mirror is restricted by the actuation
mechanism.
[0017] Other torsional micromirrors are mentioned in the literature
(M. Fischer, "Electrostatically deflectable polysilicon torsional
mirrors." Sensors and Actuators, 44(1), 1996, pp. 372-274; E.
Mattsson, "Surface micromachined scanning mirrors," 22d European
Solid State Device Research Conference, Sep. 14-17, 1992, vol. 19,
pp. 199-204). Most are small (less than 100 .mu.m on a side) and
have very small displacements, not suitable for scanning
applications. The exceptions tend to be complicated to fabricate or
actuate and suffer from the same shortcomings as the mirrors
described above.
[0018] Magnetically actuated cantilevered MEMS mirrors have been
disclosed by Miller et al. of the California Institute of
Technology (R. Miller, G. Burr, Y. C. Tai and D. Psaltis, "A
Magnetically Actuated MEMS Scanning Mirror," Proceedings of the
SPIE, Miniaturized Systems With Micro-Optics and Micromachining,
vol. 2687, pp. 47-52, Jan. 1996; R. Miller and Y. C. Tai,
"Micromachined electromagnetic scanning mirrors," Optical
Engineering, vol. 36, no. 5, May 1997). Judy and Muller of the
University of California at Berkeley disclosed magnetically
actuated cantilevered structures which may be used to support
mirrors (Jack W. Judy and Richard S. Muller, "Magnetic
microactuation of polysilicon flexure structures," Journal of
Microelectromechanical Systems, 4(4), Dec. 1995, pp. 162-169). In
both cases, the moving structures are supported by cantilever beams
along one edge. They are coated with a magnetic material, and upon
the application of a magnetic field at an angle to the mirror
surface, the mirror rotates in the direction of the field, bending
the cantilevers. Miller has also disclosed a similar mirror which
uses a small coil fabricated on the moving structure to provide it
with magnetic moment. In Miller's mirror, the springs are formed
out of the original silicon wafer, and in Judy's mirror the springs
are fabricated out of a polysilicon layer deposited for the
purpose. The conduction path for the magnetic coil device is
provided by a separate NiFe contact.
SUMMARY OF THE INVENTION
[0019] The invention relates to micro-machined
optical-electromechanical systems (MOEMS), and, more particularly,
to resonant and non-resonant torsional micro-mirrors and their
method of fabrication.
[0020] The principal embodiment of the present invention comprises
a mirror assembly rotatably supported over a cavity in a substrate
or base. A torsional mirror support assembly is provided comprising
torsional suspension springs and force pads attached to the springs
and to the base. Actuation of the mirror is achieved by torsionally
driving the springs via the force pads, thereby causing rotation of
the mirror assembly. The upper surface of the mirror assembly may
be coplanar with the surface of the base. For the case in which the
micro-mirror is formed from a silicon wafer, both the base surface
and the mirror surface may be formed from the original silicon
wafer surface (coated by metal) so that if a polished wafer is
used, a high quality mirror is easily formed. The mirror support
structure is suspended above a cavity in the base by micromachined
torsional springs. The mirror is separated from the base by etching
away the wafer material from between the mirror support structure
and the base. The mirror support structure is provided with a low
mass stiffener, and the springs are provided with electrostatic
deflection plates, so that the actuation force is applied directly
to the springs.
[0021] In an alternative embodiment, magnetic actuation of the
mirror assembly is provided. The mirror assembly includes a
magnetic material thereon to provide a permanent or temporary
magnetic moment. A magnetic actuator assembly is operative in
conjunction with the magnetic material on the mirror to
rotationally drive the mirror. The magnetic material can cover all
or a portion of a surface of the mirror assembly. The magnetic
material can be applied to a surface of the mirror assembly in a
pattern preselected to improve the magnetic and mechanical
performance of the system, such as to minimize moment of inertia
and lowering of resonant frequency. Alternatively, the magnetic
material can comprise a conduction coil formed on a surface of the
mirror assembly, whereby a magnetic moment is formed when current
is established within the conduction coil. The magnetic material
can be formed along an edge of the mirror assembly, with an
electromagnet disposed out of the plane of the mirror assembly.
[0022] The advantages of this invention over the prior art lie in
the simplicity of manufacture, the size and performance of the
mirror attainable in this design, and the accessible range of
motion. The mirror can be made nearly as large as the starting
wafer substrate (however, the sizes contemplated for the preferred
embodiment are typically in the range of 50 .mu.m.times.50 .mu.m to
3 mm.times.3 mm). The resonant frequency depends on the mirror
size; for a 600 .mu.m square mirror, resonant frequencies of over
20 kHz have been demonstrated, and with minor design changes,
frequencies appropriate for scanning at frequencies above 30 kHz
may be achieved. In one embodiment, the drive mechanism is
electrostatic. However, several aspects of the invention lend
themselves well to magnetic actuation. Because the mirror itself is
supported over a cavity in the substrate, large angular
displacement of the mirror and its supporting structure can be
achieved while maintaining a small gap between the plates of the
drive capacitor formed at the supporting springs. The fabrication
of the mirror is relatively simple. The thickness of the mirror is
easily controlled and may be adjusted to tune the resonant
frequency or change the stiffness of the mirror. The surface of the
mirror may be metallized for greater reflectance, or shaped to give
it optical power.
[0023] In the process disclosed here, the mirror support structure
is formed from the wafer substrate. The excess substrate material
(if any) is first removed from the back of the mirror support
structure by patterned etching, thus defining its thickness, mass,
stiffness and thermal conductivity, while the mirror surface
geometry is defined by patterned etching from the front. Using the
substrate material to form the mirror support structure has many
advantages. The wafers are in general available highly polished and
extremely flat, giving good specular reflections (for example, Si
and GaAs wafers intended for integrated circuit production are flat
and specular). The reflectance of such wafers can be easily made to
exceed 90% by metallizing the surface, for example with a thin
layer of aluminum. Such a layer can be sufficiently thin (less than
0.5 .mu.m) so as not to introduce undesirable topological features
to the mirror surface. This is an advantage of the current
invention over mirrors formed by surface micromachining, for
example by electrodeposition of metal or CVD polysilicon, which are
generally rough and so require a separate polishing step.
[0024] Silicon is a good choice for the substrate because the
mechanical properties of single crystal silicon are nearly ideal
for micro-mirror applications. Silicon is light, strong, and stiff,
yielding rigid mirrors with low moments of inertia. The process
disclosed here, applied to silicon, can yield a wide range of
mirror thicknesses, and even allows for engineered structures that
may be used for the construction of stiffer yet lighter mirror
supports. The fabrication process for the current invention is
relatively simple, requiring only a limited number of steps and
mask levels. The springs in the current invention are conducting
and serve as the top electrode, eliminating one fabrication layer.
Finally, this invention uses an electrodeposited metal layer which
makes possible magnetically actuated designs, by choosing a
magnetic material (such as nickel or permalloy) for the metal.
[0025] Accordingly, the present invention relocates the driving
force, either electric or magnetic, to sites that do not interfere
to the same degree with mirror motion. Also, the present invention
provides a suitably large mirror while maintaining a high resonant
frequency (low mass), adequate stiffness, and adequate thermal
conductivity. A mirror of this invention overcomes the problem of
obtaining high mirror mass and structural rigidity, while also
attaining the desired elastic constants in the springs. The mirror
also overcomes the problem of attaining mirrors with the desired
optical properties, including optical power.
DESCRIPTION OF THE DRAWINGS
[0026] The invention will be more fully understood from the
following detailed description taken in conjunction with the
accompanying drawings in which:
[0027] FIG. 1 is a perspective illustration of a torsional
micro-mirror system of the present invention;
[0028] FIG. 2A is a plan view of a torsional micro-mirror system of
the present invention;
[0029] FIG. 2B is a cross-sectional view of the system of FIG. 2A
taken along centerline 7;
[0030] FIG. 3 is a scanning electron micrograph of a micro-mirror
system of the present invention;
[0031] FIG. 4A illustrates a plan view of a further spring
embodiment;
[0032] FIG. 4B is a cross-sectional side view of the spring
embodiment of FIG. 4A;
[0033] FIG. 5 is a scanning electron micrograph of a spring
embodiment;
[0034] FIG. 6A is a plan view of a further embodiment of a
torsional micro-mirror system of the present invention;
[0035] FIG. 6B is a side view of the system of FIG. 6A;
[0036] FIG. 7 is a plan view of a further embodiment of a torsional
micro-mirror of the present invention;
[0037] FIG. 8A is a schematic side view of an embodiment of a
torsional micro-mirror system incorporating a magnetic actuation
assembly;
[0038] FIG. 8B is a schematic isometric view of a further
embodiment of a magnetically actuated system;
[0039] FIG. 8C is a schematic isometric view of a further
embodiment of a magnetically actuated system;
[0040] FIG. 8D is a schematic plan view of a further embodiment of
a magnetically actuated system;
[0041] FIG. 8E is a schematic isometric view of a further
embodiment of a magnetically actuated system;
[0042] FIG. 9 is a schematic plan view of a further embodiment of
an actuator assembly for a torsional micro-mirror system of the
present invention;
[0043] FIG. 10A is a schematic plan view of a further actuator
assembly;
[0044] FIG. 10B is an image of a torsional micro-mirror such as
that in FIG. 10;
[0045] FIG. 11 is a plan view of a further embodiment incorporating
cantilevered springs to actuate torsional motion;
[0046] FIG. 12 is a schematic plan view of a multi-axis torsional
micro-mirror according to the present invention;
[0047] FIG. 13A is a schematic cross-sectional view of a further
embodiment of a multi-axis torsional micro-mirror having wire-bond
wire jumpers according to the present invention;
[0048] FIG. 13B is a schematic plan view of a multi-axis
micro-mirror of FIG. 13A having integrally fabricated contact
structures;
[0049] FIG. 13C is a schematic view of a multi-layered torsional
spring containing multiple electrical paths to the mirror;
[0050] FIG. 14A is a schematic cross-sectional view of a torsional
micro-mirror incorporating a damping material surrounding the
springs;
[0051] FIG. 14B is a schematic isometric view of a torsional
micro-mirror incorporating damping material at several positions
along the moving edge of the mirror;
[0052] FIG. 14C is a schematic cross-sectional view of a torsional
micro-mirror with a damping coating on the springs;
[0053] FIG. 14D is a schematic cross-sectional view of a torsional
micro-mirror with high damping layers within the springs;
[0054] FIG. 15A is an image of a biaxial micro-mirror with
wire-bond wire jumpers and damping with vacuum grease;
[0055] FIG. 15B is an image of the biaxial micro-mirror of FIG. 15A
illustrating a magnetic foil laminated to the back to provide a
magnetic moment;
[0056] FIG. 16 is a schematic isometric view of a cantilevered
micro-mirror according to the present invention;
[0057] FIG. 17 is schematic cross-sectional view of a micro-mirror
incorporating optical sensing;
[0058] FIG. 18A is a schematic plan view of a torsional spring
[0059] FIG. 18B is a schematic plan view of a further embodiment of
a torsional spring;
[0060] FIG. 18C is a schematic plan view of a further embodiment of
a torsional spring;
[0061] FIG. 19 is a schematic plan view of a micro-mirror with
tapered supports;
[0062] FIG. 20 is a schematic plan view of a micro-mirror with
springs having necked down regions;
[0063] FIGS. 21A-21G are schematic cross-sectional views
illustrating a fabrication process for a micro-mirror according to
the present invention;
[0064] FIG. 22A is a schematic side view illustrating the step in
the fabrication process of providing a mirror;
[0065] FIG. 22B is a schematic side view illustrating the step in
the fabrication process of providing a mirror with an adhesive or
other attachment layer, support layer, and a reflecting layer;
[0066] FIG. 22C is a schematic side view illustrating the step in
the fabrication process of providing a mirror with an adhesive or
other attachment layer and a support layer and a curved reflecting
layer;
[0067] FIG. 22D is a schematic side view illustrating the step in
the fabrication process of providing a stress compensating layer;
and
[0068] FIGS. 23A-23G are schematic side views illustrating the
fabrication process of providing a backside patterning.
DETAILED DESCRIPTION OF THE INVENTION
[0069] FIG. 1 shows a perspective illustration of a torsional
micro-mirror system. Support posts 1 are mounted to a base 2. A
mirror 3 and mirror support structure 4 are provided between the
posts and are suspended by torsional springs 5. A cavity 6 formed
in the base 2 below and around the mirror support structure 4 is
provided to facilitate the rotation of the mirror. The deeper the
cavity 6, the greater the rotation angle of the mirror. In some
embodiments of the invention, the cavity extends entirely through
the base so that the base does not limit the rotation at all. It
should also be noted that although the mirror shown in FIG. 1 is
rectangular, the mirror support structure and mirror surface may be
any practical shape including round, ovoid, or octagonal, and may
be selected to reduce the mass of the mirror support structure,
while still yielding a satisfactory mirror area.
[0070] FIG. 2A shows a plan view of one preferred embodiment, and
FIG. 2B shows the cross section of the system shown in FIG. 2A,
taken along centerline 7. The invention consists of a mirror
surface 3, parallel in this case with the surface of base 2 and
suspended above an opening 6 in the base by two torsional springs
5. The springs 5 are collinear and aligned with the mirror support
structure 4 centerline 7, and are offset from the substrate and
mirror surface by posts 1. The mirror surface 3 may be formed by
vapor deposition of a metal such as Al onto mirror support
structure 4. Metallized regions 8 on the base surface, under the
spring 5 and offset laterally from the centerline 7 form one plate
of the electrostatic drive capacitor, and the spring 5 serves as
the other plate. Electrical contact to the drive capacitor plates
is made by microfabricated conduction lines, and isolation between
the conductors and the base is achieved by an oxide layer 13 formed
on the surface of the substrate. The posts 1 offsetting the springs
5 from the substrate surface set the size of the capacitor gap.
[0071] A preferred embodiment of the invention is designed to be
operated at the resonance frequency of the device, which depends on
the geometry and thickness of the mirror and the shape and material
of the supports, as will be discussed later. In brief, necked down
regions 9 reduce the spring constant of the springs 5 and reduce
the driving force required for actuation. The reduced spring
constant also reduces the resonant frequency. The extent of motion
in response to a given drive voltage is determined by the stiffness
of the springs 5, including necked down regions 9, and the size of
the gap 10 between springs 5 and plates 8, the area of the plates
8, and the quality factor Q of the resonant structure. The maximum
displacement is limited by the angle the torsional spring can twist
through before the edge of the spring contacts the base. A plan
view scanning electron micrograph of a mirror system formed in this
way is shown in FIG. 3.
[0072] There are several key advantages of this invention over the
prior art. First, this invention uses electrostatic actuation,
applied to the springs 5. FIG. 4A and B illustrate one spring
design, in which it can be seen that electrostatic force pads 11,
12, placed on an oxide 13 upon base 2 (not behind the mirror), are
connected to pads 14 and 15. FIG. 4B shows a cross sectional view
along line 16, and FIG. 5 shows a scanning electron micrograph of a
spring system formed in this way. Spring 5, (metallic in this
embodiment), is connected to pad 17. An electrostatic driving force
can be applied between the spring 5 and the fixed force plate 12 by
biasing pad 15 (in electrical contact with 12 through wire trace
18) with respect to pad 17 (in contact with spring 5 through trace
19), and similarly, a force can be applied between fixed force
plate 11 and spring 5 by biasing pad 14 with respect to pad 17.
Alternately biasing the pads with a periodic potential at or near
the resonant frequency of the system will excite resonant motion of
the mirror. Alternatively, the resonant motion may be excited by
biasing only one plate and in such a case the other plate may be
used for sensing the motion of the mirror by measuring the
capacitance changes as the gap changes. For example, an AC bias may
be applied between spring 5 and plate 11 at or near the resonant
frequency to induce motion, and a DC bias voltage applied between
plate 12 and spring 5. The changes in capacitance between spring 5
and plate 12 owing to the motion of the spring 5 may be sensed by
monitoring the current at pad 12. On a given mirror support
structure, one or both of the springs 5 may be biased and or
sensed.
[0073] Note that in the embodiments shown herein the force pads are
not behind the mirror, thus the mirror motion is not limited by the
problem of the mirror contacting the force pad. The spring itself
may contact the base, which will ultimately limit the range of
motion of the mirror; however, the spring and force pads may be
designed to yield a much greater range of motion, as we will show
in the various preferred embodiments. Additionally, the fixed force
plates 11, 12 may be designed so as to prevent electrical contact
between the spring 5 and the fixed plates 11, 12, for example by
coating the plates with a dielectric or by limiting the width of
the plates so that the distance from the centerline to the plate
edge is less than the product of one half the spring width and the
cosine of the maximum deflection angle.
[0074] For certain applications, it may be desirable to modify the
design by bonding external mirrors to the mirror support structure
in a separate step. A mirror may be physically bonded to mirror
support structure 4 (FIG. 2). In a different design (FIG. 6) the
spring 5 also serves as the mirror support structure, and extends
across the cavity 6. The mirror is bonded directly to this spring.
These external mirrors 20 could be made of materials that are not
convenient to include in the fabrication process, or could be made
of mirrors having optical power. The approach shown in FIG. 6 has
the additional and desirable feature that the cavity in the base
wafer may be formed by etching only from the front side of the
wafer.
[0075] Another embodiment of the invention uses a different
electrostatic capacitor design for the drive, and is shown in FIG.
7. A pair of electrodes 21 is formed along the edges of the mirror
parallel to the rotation axis. A second pair of electrodes 22 is
formed on the base along the edges of the cavity nearest the mirror
electrode. The force resulting from an applied voltage is
proportional to the capacitance between the electrodes, which is
inversely proportional to the gap between them. In this
configuration no gap is required between the torsional springs 5
and the base 2, so the fabrication of the posts may be eliminated
if desired. The length of the freely rotating portion of the
springs 5 may be controlled by enlarging the width of the cavity 6
so as to increase the distance 23 between the mirror support
structure 4 and the base 2. The distance 23 may differ from the
distance 24 between electrodes 21 and 22.
[0076] A variation of the design involves magnetic actuation of the
structure (FIGS. 8A though 8D), again without restricting the
freedom of motion of the mirror. In this case, magnetic material 25
is applied to the mirror support structure (FIG. 8A) partially or
totally covering the structure surface in order to give it a
magnetic moment. The driving field is provided by an external
electromagnet 26 which exerts a torque on the magnetic structure,
causing the mirror support to rotate. The electromagnet may be
placed at a sufficient distance from the mirror to allow free
rotation. However, if only limited motion is required, the magnetic
coil may be placed closer to the mirror, thus reducing the coil
current necessary to achieve actuation.
[0077] The magnetic material may be patterned to improve the
magnetic and mechanical performance of the device. Specifically
(FIG. 8B) the material may be confined near the axis of rotation so
as to minimize its moment of inertia and avoid lowering the
resonant frequency, and may be shaped into one or several elongated
structures 27 so that the preferred magnetic axis is perpendicular
to the rotation axis.
[0078] The magnetic material may either have a permanent magnetic
moment or temporarily acquire a moment upon the establishment of an
external magnetic field, in which case the torque is due to the
shape anisotropy of the magnetic structure. The electrodeposited
layer comprising the springs may be chosen to be of a magnetic
material, for example (but not necessarily) permalloy, in which
case the magnetic structure may be formed in the same operation as
the fabrication of the springs, or incorporated into the design of
the springs themselves. Alternatively, two different materials may
be used for the springs and magnetic structures, which may be
formed in separate steps. Alternately, the magnetic material may be
separately deposited or laminated onto the structure, in which case
the magnetic material need not be formed by or even be compatible
with standard fabrication processes, allowing the use of, for
example, ceramic ferromagnets.
[0079] Another approach comprises patterning of a conduction coil
28 (FIG. 8C) onto the mirror support structure, which creates a
magnetic moment when current is established. If multiple turns are
used, a bridging structure 29 may be fabricated to connect the
center of the coil to the electric leads. The magnetic moment
established when current flows in the coil interacts with the field
of a small permanent magnet 30 to rotate the mirror support
structure to the desired angle.
[0080] Yet another approach to magnetic actuation uses a reluctance
circuit approach (FIG. 8D). One or more external electromagnets 31
are mounted either slightly displaced above the mirror structure or
at an angle to it. Upon the establishment of a current in the coil,
the magnetic material 32 is pulled towards the center of the coil,
forming part of the electromagnet core. In this case, additional
magnetic material 33 may be deposited on the base 2 for the
attachment of the external coils 31 and used to define the shape of
the applied magnetic field. The electromagnet may be fabricated on
the wafer using standard techniques.
[0081] A fifth approach to magnetic actuation uses a permanent
magnet attached to the mirror structure to provide a magnetic
moment normal to the mirror surface. See FIG. 8E. In this
configuration a magnetic field applied parallel to the mirror
surface and orthogonal to the axis of rotation exerts a torque on
the permanent magnet causing the mirror to rotate. The field could
be produced, for example, by a pair of electromagnet coils placed
on either side of the mirror along an axis orthogonal to the
rotation axis, with the coil axis parallel to the surface of the
mirror.
[0082] Yet another embodiment of the invention suspends the mirror
about an axis of rotation 34 displaced from its centerline 7, as
shown in FIG. 9, allowing greater linear displacement on one side
of the axis of rotation than the other. In this embodiment a large
movable capacitor plate 35 is arranged to push on the springs 5.
The large fixed plate 36 is biased with respect to plate 35,
thereby generating a large force which is transferred to the
springs. The force of plate 35 may also be transferred directly to
the mirror by an arm 37 as shown in FIG. 10A, which may or may not
be attached to the mirror. The drive of the mechanical actuator may
be electrostatic, magnetic, or piezoelectric. An image of a
scanning electron micrograph of such a device is shown in FIG.
10B.
[0083] Alternatively, one or both springs may be split into two
elements 99 along the axis of rotation 7 (FIG. 11) in order to
provide additional electrical paths onto the mirror support
assembly. In this embodiment, rotational compliance of the springs
may be replaced by bending or twisting of the support posts 100 at
the mirror support platform. This has the further advantage that
the rotational motion of the mirror support platform can result
from cantilevered bending of the springs. At the fixed base end,
the springs may be made as wide as desired to increase the
electrostatic force. Applying a voltage between the springs and the
capacitor plates on the fixed base, alternately on each side of the
centerline 7, causes the springs to bend and push down on the
support platform. By attaching the springs to the platform near the
axis of rotation, large angular displacement can be achieved for
very small vertical motion. This embodiment improves on a device
such as described by Dhuler in several important aspects. First no
bearing or additional element is required for supporting the moving
platform, simplifying the fabrication and eliminating wear
associated with surfaces moving against each other. Second, the
springs and supports described in this invention may serve as
electrical paths onto the mirror support platform.
[0084] Video images require sweeping in two orthogonal directions,
but the second sweep direction need not move faster than the frame
rate, ranging from 30-180 Hz. To obtain images, two separate
mirrors could be used, rotating about orthogonal axes, or a single
reflecting surface could be made to scan both directions. A single
mirror that scans two orthogonal directions is achieved either by
mounting the current invention on a scanning platform, or modifying
the design so the reflecting surface is supported within a
gimballed frame, and made to scan in both directions. The actuating
mechanism for the two directions could be direct or indirect
electric or magnetic force, or any combination thereof.
[0085] A multi-axial micro-mirror may be formed using the designs
and processes described herein. The actuation mechanism for the two
directions is direct or indirect electric or magnetic force, or any
combination thereof. FIG. 12 illustrates one possible multi-axial
design: a first pair of springs 38 is used for rotation of the
mirror support structure 4 along one axis, and a second pair of
springs 39 is used for rotation along a second axis which in this
case is perpendicular to the first axis. The first pair 38 joins
the base 40 to a movable support frame 41; this support frame 41 is
connected to the mirror support structure 4 by the second pair 39.
(Other pairs and additional movable supports may be added that can
be designed to operate at resonance or in a bi-stable mode with the
advantage of providing aiming or alignment of the micro-mirror
system. Other pairs may also be useful in distortion correction.)
The deflection voltage is supplied to the pair 38 through bias
applied to pads 42 and 43, and through bias applied through pads 44
and 45. The deflection voltage for pair 39 is supplied through pair
38 by traces 46 and 47. Thus, by relatively biasing the two springs
in pair 38, bias to the pair 39 may be attained, without addition
of further conduction paths to the moving parts.
[0086] If separate electrical contacts are to be provided for the
inner mirror of a gimballed structure, they may be combined with
the mechanical support of the outer mirror support frame, or may be
run through separate structures bridging the gap between the inner
and outer support structures, as shown in FIG. 13. These separate
structures may or may not contribute to the mechanical properties
of the system. The structures may either be added as part of the
packaging process, such as the addition of wire-bond wire jumpers
48 in FIG. 13A, or may be fabricated along with the rest of the
structure. An example of integrally fabricated contact structures
which minimally impact the mechanical properties is shown in FIG.
13B. Soft serpentine springs 49 bridge the gap, and connect through
traces 50 to the inner springs 39. The outer springs 38 are
separately connected to the drive pads 11 and 12 by traces 52 and
53 respectively. Multilayered spring supports may also be used to
increase the number of separate electrical paths to the inner
mirror. FIG. 13C shows such a spring; an insulating layer 54
separates the upper conducting layer 55 from the lower conducting
layer 56. Each conducting layer connects to a separate pair of
traces (not shown).
[0087] It may be desirable for the inner and outer support
structures to have different damping characteristics. For example,
in a scanning display, the line scanning structure may be
resonantly driven and benefit by low damping (high Q), while the
frame scanning structure is driven linearly at low frequency and
benefits from high damping for uniform motion. Thus, the device can
be operated in a vacuum package to minimize the air damping of the
fast mirror, with specific damping means provided for the slow
scanning structure. For example, a damping material 57 may be
applied to the slow moving structure or the springs (FIG. 14A).
This damping material may consist of a liquid, gel, or soft
semi-solid surrounding the springs, with or without an enclosure to
confine it. Many materials are suitable for this purpose, including
vacuum grease such as Dow Corning DC 976 or Apiezon N type, RTV
silicone, or spin-on polymers used in the fabrication process such
as polyimide or photoresist (for example Shipley AZ1308). One
embodiment is the application of a drop of DC 976 57 to the
substrate so that it encloses the springs (FIG. 14A). Optionally,
the damping material 98 may instead be applied along the moving
edge away from the spring (FIG. 14B), so that it bridges the gap
between the moving support element and the fixed base of the
device. The damping material may be applied anywhere along the gap
between the moving component and the fixed component, or between
two components moving at different velocities, for example, the
mirror platform and the gimballed outer frame of a biaxial
embodiment. An alternative is to coat the springs 38 with a high
damping coating such as photoresist 58 (FIG. 14C). Another
alternative is to enclose a high damping material within the
springs, for example by making a multilayer structure incorporating
high strength layers 59 surrounding high damping layers 60. Damping
may also be provided by mechanically attaching damping devices
(dashpots) between the moving structure and the base.
[0088] FIG. 15A is a micrograph image of a biaxial scanning display
device. The inner mirror support platform 4 is the line scanner,
driven by an electrostatic resonant drive. The oxide has been
removed from its surface and it has been coated with aluminum to
form the mirror reflective surface. The electrical ground lead
contact is made through the outer frame supports 38 and the drive
voltage contacts are made by wire-bond wire jumpers 48. The outer
frame is the frame scanning direction and is driven magnetically to
give a slow linear sweep and a fast retrace. Harmonic oscillations
in the motion are damped by the application of approximately 0.005
microliters of DC 976 vacuum grease 57 to dampen the slow axis
springs 38. The magnetic moment is provided by magnetic foil 25
laminated to the back of the structure, covering half the area and
placed orthogonal to the axis of rotation (FIG. 15B). This device
is then mounted on a small electromagnet and packaged in
vacuum.
[0089] In a preferred embodiment of a device as shown in FIG. 15,
the mirror support platform is approximately 1 mm.times.1 mm and
has a resonant frequency ranging from 7 to 15 kHz. The resonant
frequency of the outer frame is 150 to 700 Hz. The outer frame is
driven at 60 Hz. The device is packaged in vacuum above an
electromagnetic coil, preferably in a T05 package containing
optics.
[0090] In another embodiment of the invention, shown in FIG. 16,
the mirror support structure 92 is connected to the base 40 along
one edge 94 by one or several metal cantilever springs 93, allowing
the mirror support structure to rotate out of the plane of the
wafer. The stiffness of the support springs 93 depends on the total
aggregate width, but this width may be distributed among as many
supports as desired (five in FIG. 16), each providing a separate
electrical path onto the mirror support structure. Other devices,
for example a torsional MEMS mirror such as described herein, a
CMOS circuit, for example the drive circuit, or both, may be
fabricated on or grafted onto the platform 92. The number of
possible electrical contacts is limited only by the length of the
edge 94 and the minimum practical width and separation of the
springs 93. This embodiment is an improvement to the mirror
disclosed in the literature by Miller, in that the support springs
are conducting and may provide multiple distinct electrical
contacts to elements on the mirror support platform. In addition,
the platform in the present invention may be formed of the original
wafer, which may be a single crystal semiconductor, and may have a
desired thickness up to the full thickness of the original wafer.
Thus, it may comprise any desired CMOS circuit or device, which may
be fabricated on the substrate prior to the release of the
cantilevered platform.
[0091] The motion of the platform 92 may be used either to set the
angle 95 between the platform 92 (and any devices carried upon it)
and the base 40 (and any other devices attached to it), for example
for the purpose of optical alignment, or to sweep the angle, for
example for a display. The cantilever may be moved into place
mechanically, for example during the fabrication or packaging of
the device, and locked into place, to fix the angle. Alternatively,
magnetic material may be applied to the front, back, or both
surfaces of the platform and a magnetic field may be applied to it
during operation to rotate it to the desired angle. In this case, a
DC magnetic field bias may be applied to set the center angle of
motion, and an AC field superimposed on it to sweep the angle.
Either the AC or DC component may be zero. In another variation,
mechanical elements such as levers may be provided to set the angle
during operation.
[0092] Sensors may be added to the mirror to detect its position
and the extent of the motion and provide feedback for the drive
electronics. One sensor design consists of capacitors similar to
but separate from the drive pads, with detection of the current
changing as a function of mirror position as previously described.
If magnetic material is present on the moving structure, a magnetic
sensor, for example a pickup coil, may be placed in close proximity
to detect the mirror position. Optical sensing may also be
utilized, as shown in FIG. 17. A light source 61 provides a focused
beam 62 which is reflected off the mirror 3 and detected by a
detector 63. As the mirror rotates, the intensity of light incident
on the detector 63 changes and can be correlated with mirror angle.
The angle of incidence 64 can be chosen so the sensing light does
not interfere with the display illumination, or an infrared source
may be used. Alternatively, the detection may be made from the back
of the mirror structure by reflecting the light off the back of the
mirror support structure 4. Optionally, the mirror position may be
determined by sensing the light incident on it for display
illumination. Detection may also be made by allowing invisible
radiation such as infrared light to pass through a specifically
designed mirror surface coating to a detector mounted on the back
of or underneath the device.
[0093] Support and Spring Design
[0094] For electrostatic actuation, the force is proportional to
the square of the applied voltage and inversely proportional to the
square of the gap 12 (FIG. 2) between the drive capacitor plates.
Making the gap smaller or the spring wider reduces the maximum
angle of rotation but increases the applied torque, while making
the gap 12 larger or the support narrower allows greater motion but
reduces the torque resulting from a given applied voltage. At the
maximum displacement, at least some portion of the spring 5 touches
the base 2. The present invention is designed so that the mirror
itself is free to rotate to any degree within the elastic limits of
the springs, and the displacement is limited by the rotation of the
springs in relation to the base. For a fixed gap height g, if at
any given point z along the length of the spring its width is w(z),
then the maximum rotation allowed at that point is
approximately
sin (.phi.)=2g/w(z).
[0095] The angle of rotation varies along the length of the spring,
from approximately zero at the connection to the support post to
the maximum at the mirror support structure, and the spring shape
can be designed to take advantage of this, for example by making
the spring wider near the fixed end and narrower at the mirror
support structure, allowing a greater angle of rotation while still
allowing for a large plate area and thus allowing for application
of a larger torque than would be obtained in a spring of uniform
cross section.
[0096] Several possible spring designs are shown in FIG. 18. The
stiffness of each design can be calculated from standard mechanical
expressions, for example as found in Mark's Handbook of Mechanical
Engineering. T. Baumeister, editor in chief, Mark's Standard
Handbook for Mechanical Engineers, 8th ed., McGraw-Hill Book
Company, New York, 1978, Section 5. For the uniform cross section
spring shown in FIG. 18A the torsional stiffness K.sub.t is given
by 1 K t = G 3.5 l ( b 3 h 3 b 2 + h 2 ) 1
[0097] where G is the shear modulus, l, b, and h are the length,
width, and depth of the member respectively, and the numerical
constant depends on the aspect ratio of the cross section. For
non-uniform cross section springs, the reciprocals of the stiffness
of individual elements add to give the reciprocal total stiffness:
2 K total - 1 = i K i - 1 2
[0098] For example, introducing a necked down region, as in FIG.
18B, reduces the stiffness to: 3 K total = K 1 K 2 K 1 + K 2 ,
3
[0099] since the wide part 65 of the spring 5 and the necked down
part 66 of the spring 5 have different lengths and widths, but the
thickness of the electrodeposited layer and the material properties
are the same for both. This type of design makes the device easier
to actuate, but also reduces its resonance frequency. For a tapered
support as shown in FIG. 18C, the stiffness is given by: 4 K t = Gh
3 ( b max - b min ) 3.5 l ln ( b max / b min ) , 5
[0100] where the width varies from b.sub.max at the substrate
support post to b.sub.min near the mirror.
[0101] For the case of a mirror support structure in which the
rotation axis coincides with the center of mass, the resonant
frequency depends on the moment of inertia of the mirror element,
J.sub.t, 5 J t = L 3 Wt 12 , 6
[0102] where .rho. is the density of the mirror support structure,
L is its length, W is its width (parallel to the axis of rotation),
and t its thickness. The resonance frequency is then: 6 f = 1 2 K t
J t , 7
[0103] For a given limiting angle and resonant frequency, the
tapered design (FIG. 18C) allows the gap g to be reduced by a
factor of: 7 g tapered g uniform = ln - 1 ' , 8 where = b max b min
9
[0104] over the gap for the uniform cross section design (FIG.
18A). For aspect ratios .xi. less than or approximately equal to 3,
the limiting angle is imposed by contact between the spring and
base at the narrow end of the spring. FIG. 19 illustrates a
micro-mirror with tapered supports.
[0105] Another design is shown in FIG. 20, in which the spring 5
has two necked down regions 67, 68, which facilitate a high
rotation angle. The necked down regions are separated by a wider
region 69 which comprises the moving force plate 70 section of
spring 5. The force plate 70 of spring 5 is attracted alternately
to plates 11 and 12 and can rotate through an angle .phi. given by
the distance 67 (denoted W.sub.67) and the gap (g), by the
equation: sin (.phi.)=2g/w.sub.67. The mirror support structure 4
can continue to rotate through an additional angle which is limited
by the elastic limits of section 68 of spring 5. In general, for
rotations of section 67 less than .phi. and equal to .alpha., the
total rotation .gamma. is given by:
.gamma.=.alpha.w.sub.68/w.sub.67.
[0106] By selecting the relative lengths of the necked down
regions, large angular displacements .gamma. are possible with only
a small movement a in the force plate 70 integral to spring 5.
[0107] Fabrication
[0108] FIG. 21 shows the fabrication process. A polished wafer 71,
preferably Si, is first coated on both sides with a material 72 on
the front and 73 on the back that is resistant to etches of the
wafer material. For silicon, this material may be silicon nitride,
silicon dioxide, or other films known in the art. For the case of
silicon dioxide to be formed on Si, the wafer may for example be
oxidized to form a surface layer of silicon dioxide 72, 73 on both
sides of the wafer, or the wafer may be coated by chemical vapor
deposition, or by other means. After application of coating 72, 73,
the wafer 71 and coatings 72,73 are then patterned on both sides
with registered alignment marks and etched to define the marks in
the crystal. These marks, formed on both sides of the wafer, permit
registration of features on the front and back (registration marks
are not shown in FIG. 21).
[0109] Metal films, for example of chromium, gold, and
titanium/tungsten alloy, are deposited on the front coated surface
72, and are patterned and etched to form pads 74 that provide the
electrical contacts and anchors for the mechanical structures. The
coating 73 is patterned and etched to act as a mask for wafer
etching. The back of the wafer is then etched to form a membrane
with surface 75 having thickness in the range of 20 .mu.m to 200
.mu.m. A typical thickness is 60 .mu.m. The coating 72 on the front
surface is then patterned and etched to form groove openings 76 in
the coating which will serve later in the process as an etch mask
for the separation of the mirror support structures 4 from the base
2. The initial coatings may also include or serve as the final
mirror surface.
[0110] A release layer 77 of photoresist or other material is
applied to the front surface and patterned with holes 78 to expose
the metal anchors 74. After heat treatment, thin (0.05 .mu.m to 0.5
.mu.m) layers of a metal or sequence of metals such as chromium,
gold and titanium/tungsten alloy 79 are deposited on the front
surface. Photoresist is then applied and patterned to form a mask
80 for the electrodeposited structures. A metal layer 81, which may
be nickel, is deposited by electroplating on to the exposed regions
82 of metal layers 74 and 79. The thickness of metal layer 81 is in
the range of 0.5 .mu.m to 10 .mu.m; layer 81 constitutes the spring
5 in the plan views described earlier. The mask 80 and release
layer 77 are removed by dissolving the layers in solvents or
preferential etches. This process also removes sections of
intermediate metal layers 83 (of metal layer 79) that are not
reinforced by the electroplating.
[0111] The wafer is diced, and the mirror support structure 4 is
separated from the surrounding base 2 by etching both from the
front, through the grooves 76 defined in the etch masks 72 and 73,
and from the back by etching surface 75, resulting in the formation
of cavity 6 surrounding the mirror support structure 4. The mirror
support structure 4 is thus joined to the base solely by the metal
torsional springs 5. The final thickness of the mirror support
structure 4 depends on the duration of the two etch steps and can
be selected to yield structures with thickness in the range of 10
to 200 .mu.m or more (as large as the wafer thickness if needed).
Typical final support structure thickness is 30 .mu.m.
[0112] The final step comprises addition of a mirror, either by
providing a metal or dielectric coating 84 (FIG. 22A) of the mirror
support structure through a mask (for example with evaporated or
sputtered aluminum with thickness in the range of 500 to 2500
angstrom), or by bonding a finished mirror to the support
structure. FIG. 22B shows a mirror bonded to the mirror support
structure, comprising an adhesive or other attachment layer 85, a
glass or other substance 86 as is used in the art of mirror
formation to support a reflecting layer, and a flat mirror
(reflecting) layer 87. This invention additionally comprises the
capability to use a mirror comprising a curved surface formed in
material 86 and coated with a reflecting layer 88, as shown in FIG.
22C. This surface on material 86 may be either convex or concave,
or aspherical. Additionally, the layers 87 or 88 may comprise a
binary optical element of a diffractive or refractive nature, or a
holographic element.
[0113] Any of the coatings used in the fabrication process (for
example the silicon oxide layer or the optical coatings) may have a
high degree of internal stress. If the mirror support structure is
sufficiently thin, this stress could induce curvature in the
structure. This curvature may be useful, for example for shaping
the optical surface, or may be undesirable. In the latter case, if
it is not possible or desirable to remove the stressed film,
additional layers 89 of equal stress may be deposited on the back
face of the structure to compensate for the stress and restore the
surface flatness (FIG. 22D). Alternately, the compensating layer
may be applied to the front of the support structure, under the
mirror, in which case the compensating layer stress would be equal
and opposite to the stress already present. If the reflectance of
the compensation material is sufficient (e.g. if chromium is used),
it may serve as the reflective surface as well.
[0114] The mirror thickness is determined by the duration of the
etch used to remove the excess material from the back. The choice
of thickness results from a tradeoff between mechanical stiffness,
moment of inertia, and thermal conductivity. Thicker mirrors are
stiffer, and so deform less in use, but the additional mass results
in a higher moment of inertia which lowers the resonant frequency.
By adding backside patterning of the mirror support structure to
the fabrication process as shown in FIG. 23, an engineered mirror
support structure can be made, so as to yield a support which is
stiff yet low in mass. For example, by adding a patterned mask 90
to the back etching, support ribs could be fabricated 91 on the
back of the mirror, perpendicular to the rotation axis, adding
stiffness but minimal weight. Alternately a corrugated, honeycomb,
or other structure may be formed by etching a pattern of wells into
the back of the mirror support structure. Another method that can
be applied when the substrate is a single crystal (such as Si) is
to pattern the back surface with an array of square openings
aligned to the crystal axis, followed by etching in a selective
etch (such as potassium hydroxide etching of Si) which etches {111}
planes much more slowly than other planes of the crystal. The
etching process self terminates when the {111} planes are fully
exposed, resulting in a pyramidal etch pit of highly controlled
depth. In this way a corrugated structure with a precise resultant
mass is formed in the mirror support structure, without the need
for a buried etch stop layer.
[0115] Structures having a magnetic moment, for use in magnetic
actuation, may be electroplated in the same manner as the springs.
If the material used for the springs is magnetic, they may be
deposited at the same time as the springs. If two different
materials are to be used, the magnetic structures may be
electroplated in a separate step using a similar processing
sequence to that described above. A third option for applying
magnetic material to the structure consists of attaching a
preformed magnetic element to the completed device by means of an
adhesive such as cyanoacrylate adhesive or Crystal Bond wax.
[0116] The invention is not to be limited by what has been
particularly shown and described, except as indicated by the
appended claims.
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