U.S. patent application number 09/216375 was filed with the patent office on 2001-08-23 for multilevel mechanical grating device.
Invention is credited to KOWARZ, MAREK W., KRUSCHWITZ, BRIAN E..
Application Number | 20010015850 09/216375 |
Document ID | / |
Family ID | 22806812 |
Filed Date | 2001-08-23 |
United States Patent
Application |
20010015850 |
Kind Code |
A1 |
KOWARZ, MAREK W. ; et
al. |
August 23, 2001 |
MULTILEVEL MECHANICAL GRATING DEVICE
Abstract
A mechanical grating device for improving the diffraction
efficiency. The mechanical grating device is built on a base having
a surface. Above the base a spacer layer, having an upper surface,
is provided, and a longitudinal channel is formed in said spacer
layer, said channel having a first and second opposing side wall
and a bottom. A plurality of spaced apart deformable ribbon
elements are disposed parallel to each other. The deformable
elements are organized in groups of N elements wherein N is greater
than 2. When the device is actuated each of said groups forms a
pattern of discrete levels wherein the pattern has n levels wherein
n is greater than 2.
Inventors: |
KOWARZ, MAREK W.;
(ROCHESTER, NY) ; KRUSCHWITZ, BRIAN E.;
(ROCHESTER, NY) |
Correspondence
Address: |
PATENT LEGAL STAFF
EASTMAN KODAK COMPANY
343 STATE STREET
ROCHESTER
NY
14650-2201
US
|
Family ID: |
22806812 |
Appl. No.: |
09/216375 |
Filed: |
December 18, 1998 |
Current U.S.
Class: |
359/572 |
Current CPC
Class: |
G02B 26/0808
20130101 |
Class at
Publication: |
359/572 |
International
Class: |
G02B 005/18 |
Claims
What is claimed is:
1. A mechanical grating device comprising: a base having a surface;
a spacer layer, having an upper surface, is provided above the
base, and a longitudinal channel is formed in said spacer layer,
said channel having a first and second opposing side wall and a
bottom; a plurality of spaced apart deformable ribbon elements
disposed parallel to each other and spanning the channel, said
deformable ribbon elements defining a top and a bottom surface and
are fixed to the upper surface of the spacer layer on each side of
the channel, said deformable elements are organized in groups of N
elements wherein N is greater than 2; and each of said groups forms
a pattern of discrete levels in an actuated state wherein the
pattern has n levels wherein n is greater than 2.
2. The mechanical grating device as recited in claim 1 has a
plurality of standoffs provided, and according to the longitudinal
direction of the device at least N-2 standoffs are associated with
each group.
3. The mechanical grating device as recited in claim 2 wherein the
standoffs are formed on the bottom of the channel.
4. The mechanical grating device as recited in claim 2 wherein the
standoffs are formed on the bottom surface of the ribbon
elements.
5. The mechanical grating device as recited in claim 2 wherein
according to the width of said device each standoff is divided into
a plurality of individual elements of equal height.
6. The mechanical grating device as recited in claim 1 wherein in
the actuated state the levels of adjacent ribbon elements in each
group are separated by 6 2 N + p 2 .
7. The mechanical grating device as recited in claim 6 wherein in
the actuated state the levels of successive ribbon elements in each
group are reduced by a constant amount with respect to the bottom
of the channel, and thereby representing a staircase of equal
steps.
8. The mechanical grating device as recited in claim 1 wherein said
side walls are substantially vertically disposed with respect to
the bottom.
9. The mechanical grating device as recited in claim 1 wherein said
channel has a constant cross section along the entire length of the
device.
10. The mechanical grating device as recited in claim 1 wherein a
reflective layer is provided on the top surface of the ribbon
elements.
11. An electro-mechanical grating device comprising: a base having
a surface; a spacer layer, having an upper surface, is provided
above the base, and a longitudinal channel is formed in said spacer
layer, said channel having a first and second opposing side wall
and a bottom; a first conductive layer being provided below the
bottom of the channel; a plurality of spaced apart deformable
ribbon elements disposed parallel to each other and spanning the
channel, said deformable ribbon elements defining a top and a
bottom surface and are fixed to the upper surface of the spacer
layer on each side of the channel, said deformable elements are
organized in groups of N elements wherein N is greater than 2; each
of said groups forms a pattern of discrete levels in an actuated
state wherein the pattern has n levels wherein n is greater than 2;
and a second conductive layer being part of each actuable ribbon
element.
12. The electro-mechanical grating device as recited in claim 11
has a plurality of standoffs provided, and according to the
longitudinal direction of the device at least N-2 standoffs are
associated with each group.
13. The electro-mechanical grating device as recited in claim 12
wherein the standoffs are formed on the bottom of the channel.
14. The electro-mechanical grating device as recited in claim 12
wherein the standoffs are formed on the bottom surface of the
ribbon elements.
15. The electro-mechanical grating device as recited in claim 12
wherein according to the width of said device each standoff is
divided into a plurality of individual elements of equal
height.
16. The electro-mechanical grating device as recited in claim 11
wherein in the actuated state the levels of adjacent ribbon
elements in each group are separated by 7 2 N + p 2 .
17. The electro-mechanical grating device as recited in claim 16
wherein in the actuated state the levels of successive ribbon
elements in each group with respect to the bottom of the channel
are reduced by a constant amount, thereby representing a staircase
of equal steps.
18. The electro-mechanical grating device as recited in claim 11
wherein said side walls are substantially vertically disposed with
respect to the bottom.
19. The electro-mechanical grating device as recited in claim 11
wherein said channel has a constant cross section along the entire
length of the device.
20. The electro-mechanical grating device as recited in claim 11
wherein a reflective layer is provided on the top surface of the
ribbon elements.
21. The electro-mechanical grating device as recited in claim 11
comprises a driving means for applying a voltage between the first
and the second conductive layer to actuate the ribbon elements.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to U.S. Ser. No. 09/______ (EK Docket No.
78,657), filed concurrently, entitled "A Mechanical Grating
Device," and to U.S. Serial No. 09/______ (EK Docket No. 78,593),
also filede concurrently, entitled, "Method for Producing Co-planar
Surface Structures."
FIELD OF THE INVENTION
[0002] This invention relates to the field of modulation of an
incident light beam by the use of a mechanical grating device. More
particularly, this invention discloses a multilevel mechanical
grating device which has a significant improvement in the output of
the diffracted light beam by approximating a continuous blaze
grating with m discrete levels.
BACKGROUND OF THE INVENTION
[0003] Electro-mechanical spatial light modulators have been
designed for a variety of applications, including image processing,
display, optical computing and printing. Optical beam processing
for printing with deformable mirrors has been described by L. J.
Hornbeck; see U.S. Pat. No. 4,596,992, issued Jun. 24, 1986,
entitled "Linear Spatial Light Modulator and Printer". A device for
optical beam modulation using cantilever mechanical beams has also
been disclosed; see U.S. Pat. No. 4,492,435, issued Jan. 8, 1995 to
Banton et al., entitled "Multiple Array Full Width
Electro-mechanical Modulator," and U.S. Pat. No. 5,661,593, issued
Aug. 26, 1997, to C. D. Engle entitled "Linear Electrostatic
Modulator". Other applications of electro-mechanical gratings
include wavelength division multiplexing and spectrometers; see
U.S. Pat. No. 5,757,536, issued May 26, 1998, to Ricco et al.,
entitled "Electrically-Programmable Diffraction Grating".
[0004] Electro-mechanical gratings are well known in the patent
literature; see U.S. Pat. No. 4,011,009, issued Mar. 8, 1977 to
Lama et al., entitled "Reflection Diffraction Grating Having a
Controllable Blaze Angle", and U.S. Pat. No. 5,115,344, issued May
19, 1992 to J. E. Jaskie, entitled "Tunable Diffraction Grating".
More recently, Bloom et al. described an apparatus and method of
fabrication for a device for optical beam modulation, known to one
skilled in the art as a grating-light valve (GLV); see U.S. Pat.
No. 5,311,360, issued May 10, 1994, entitled "Method and Apparatus
for Modulating a Light Beam". This device was later described by
Bloom et al. with changes in the structure that included: 1)
patterned raised areas beneath the ribbons to minimize contact area
to obviate stiction between the ribbon and substrate; 2) an
alternative device design in which the spacing between ribbons was
decreased and alternate ribbons were actuated to produce good
contrast; 3) solid supports to fix alternate ribbons; and 4) an
alternative device design that produced a blazed grating by
rotation of suspended surfaces; see U.S. Pat. No. 5,459,610, issued
Oct. 17, 1995, to Bloom et al., entitled "Deformable Grating
Apparatus for Modulating a Light Beam and Including Means for
Obviating Stiction Between Grating Elements and Underlying
Substrate," and U.S. Pat. No. 5,808,797, issued Sep. 15, 1998 to
Bloom et al., entitled "Method and Apparatus for Modulating a Light
Beam." Bloom et al. also presented a method for fabricating the
device; see U.S. Pat. No. 5,677,783, issued Oct. 14, 1997, entitled
"Method of Making a Deformable Grating Apparatus for Modulating a
Light Beam and Including Means for Obviating Stiction Between
Grating Elements and Underlying Substrate".
[0005] The GLV device can have reflective coatings added to the top
surface of the ribbons to improve the diffraction efficiency and
lifetime of the GLV device. Preferred methods of fabrication use
silicon wafers as the substrate materials requiring the device to
operate in reflection for the wavelengths of interest. An increase
in reflectivity is important to reduce damage of the top surface of
the ribbons and avoid mechanical effects that might be attributed
to a significant increase in the temperature of the device due to
light absorption.
[0006] For GLV devices, the positions and heights of the ribbons
have been symmetric in design. One drawback to this design is an
inability to isolate the optical intensity into a single optical
beam. This relatively poor optical efficiency is primarily due to
the symmetry of the actuated device, which produces pairs of equal
intensity optical beams. Each period of the improved grating must
include more than two ribbons and create an asymmetric pattern of
the ribbon heights. By creating an asymmetric pattern for the
heights of the ribbons, the intensity distribution of the
diffracted optical beams is asymmetric and can produce a primary
beam with a higher optical intensity. Furthermore, by adjusting the
asymmetry of the pattern of ribbon positions and heights, the
intensity distribution of the diffracted optical beams can be
altered. In this way, the device can be used to switch between
various diffracted optical beams.
SUMMARY OF THE INVENTION
[0007] It is an object of the present invention to provide a
mechanical grating device wherein the diffraction efficiency of a
blazed grating is accomplished.
[0008] The object is achieved by a mechanical grating device
comprising:
[0009] a base having a surface;
[0010] a spacer layer having an upper surface, is provided above
the base, and a longitudinal channel is formed in said spacer
layer, said channel having a first and second opposing side wall
and a bottom;
[0011] a plurality of spaced apart deformable ribbon elements
disposed parallel to each other and spanning the channel, said
deformable ribbon elements defining a top and a bottom surface and
are fixed to the upper surface of the spacer layer on each side of
the channel, said deformable elements are organized in groups of N
elements wherein N is greater than 2; and
[0012] each of said groups forms a pattern of discrete levels in an
actuated state wherein the pattern has n levels wherein n is
greater than 2.
[0013] It is a further object of the present invention to provide
an electro-mechanical grating device wherein the diffraction
efficiency of a blazed grating is accomplished.
[0014] The object is achieved by an electro-mechanical grating
device comprising:
[0015] a base having a surface;
[0016] a spacer layer, having an upper surface, is provided above
the base, and a longitudinal channel is formed in said spacer
layer, said channel having a first and second opposing side wall
and a bottom;
[0017] a first conductive layer being provided below the bottom of
the channel;
[0018] a plurality of spaced apart deformable ribbon elements
disposed parallel to each other and spanning the channel, said
deformable ribbon elements defining a top and a bottom surface and
are fixed to the upper surface of the spacer layer on each side of
the channel, said deformable elements are organized in groups of N
elements wherein N is greater than 2;
[0019] each of said groups forms a pattern of discrete levels in an
actuated state wherein the pattern has n levels wherein n is
greater than 2; and
[0020] a second conductive layer being part of each actuable ribbon
element.
[0021] An advantage of the mechanical grating device of the
invention is that the position of the ribbons across the area of
the substrate and the periodic sequence of the ribbon heights can
be used to improve the diffraction efficiency of the optical beam.
This invention presents a periodic sequence of ribbon heights that
resembles a blazed grating with discrete levels and is predicted to
significantly increase the optical diffraction efficiency. The
multi-level mechanical grating device can be fabricated using
methods that are compatible with the microelectronics industry. The
device is more reliable and more appropriate for printing
applications than other blazed mechanical and/or electro-mechanical
gratings in the patent literature. Further advantageous effects of
the present invention are disclosed in the dependent claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] The subject matter of the invention is described with
reference to the embodiments shown in the drawings.
[0023] FIG. 1 is an illustration of diffraction from a binary
reflective grating;
[0024] FIG. 2 is an illustration of diffraction from a blazed
reflective grating;
[0025] FIG. 3 is an illustration of a multi-level diffraction
grating to approximate a blazed grating;
[0026] FIG. 4 is a perspective, partially cut-away view of the
multilevel mechanical grating device of the present invention;
[0027] FIG. 5 is a top view of the multilevel mechanical grating
device of the present invention;
[0028] FIG. 6 is a cross-sectional view along plane A-A indicated
in FIG. 5 to illustrate the layered structure of one embodiment of
the invention;
[0029] FIG. 7 is a cross-sectional view along plane B-B indicated
in FIG. 5 of the three level mechanical grating device wherein no
force is applied to the deformable ribbons;
[0030] FIG. 8 is a cross-sectional view along plane B-B indicated
in FIG. 5 of the three level mechanical grating device wherein
force is applied to the deformable ribbons;
[0031] FIG. 9 is a cross-sectional view along plane B-B indicated
in FIG. 5 of the four level mechanical grating device wherein no
force is applied to the deformable ribbons;
[0032] FIG. 10 is a cross-sectional view along plane B-B indicated
in FIG. 5 of the four level mechanical grating device wherein force
is applied to the deformable ribbons; and
[0033] FIG. 11 shows the effect of gap width on diffraction
efficiency of a two-, three- and four-level grating light
valve.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Periodic corrugations on optical surfaces (i.e. diffraction
gratings) are well known to perturb the directionality of incident
light. Collimated light incident in air upon a grating is
diffracted into a number of different orders, as described by the
grating equation, 1 sin m = sin 0 + m , ( 1 )
[0035] where .lambda. is the wavelength of the light and m is an
integer denoting the diffracted order. FIG. 1 illustrates a
reflective grating 10 having an optical beam 12 incident on the
grating 10 at an angle.theta..sub.0 11 with respect to an
orthogonal axis O-O of the reflective grating 10. The grating
surface is defined to have a period .LAMBDA. 13, which defines the
angles of diffraction according to the relation presented in
Equation 1. A diffracted beam 16 corresponding to diffraction order
m exits the grating 10 at an angle .theta..sub.m 15.
[0036] The diffraction grating 10 pictured in FIG. 1 is a binary
grating where the grating profile is a square wave. The duty cycle
is defined as the ratio of the width of the groove L.sub.1 14 to
the grating period .LAMBDA. 13. A binary phase grating will have
the maximum diffraction efficiency when the duty cycle is equal to
0.5 and R, the reflectivity, is equal to 1.0.
[0037] For uniform reflectivity and 0.5 duty cycle, the relation
presented in Equation 2 is appropriate for the calculation of the
theoretical diffraction efficiency, within the accuracy of scalar
diffraction theory. 2 m = R cos 2 ( ( q m d - m / 2 ) ) sin 2 ( m /
2 ) ( m / 2 ) 2 , ( 2 )
[0038] where q.sub.m is a geometrical factor, 3 q m = cos 0 + cos m
= 1 + 1 - ( m / ) 2 for normal incidence ( 0 = 0 ) . ( 3 )
[0039] For normally incident illumination, the maximum efficiency
in the first (m=1) order occurs when the grating depth
d=.lambda./4. Such a grating has equal diffraction efficiencies
into the +1 and -1 orders of approximately 40% for the gratings of
interest (.lambda./.LAMBDA..ltoreq.- 0.5), while the remaining
light is diffracted into higher odd orders (i.e. .+-.3, .+-.5,
etc.).
[0040] For applications requiring a high optical throughput, the
grating is desired to diffract with a very high efficiency into a
single optical beam. It is well known to one skilled in the art
that this is best accomplished by shaping the grating profile with
a blaze, see C. Palmer, ed., Diffraction Grating Handbook, 2.sup.nd
ed., (Milton Roy Instruments, Rochester, N.Y., 1993). FIG. 2
illustrates the continuous blazed grating profile 20 with a beam 22
incident on the plane of the grating surface 24 to produce
diffracted beams 26a, 26b, 26c, 26d, 26e, 26f associated with the
non-zero orders of diffraction. By proper design of the grating
profile the intensity of the beam in the +1 diffracted order 26d is
maximized.
[0041] The preferred methods of fabricating a grating device do not
allow the grating profile illustrated in FIG. 2. FIG. 3 is an
illustration of the grating profile that would be produced using
microelectronic fabrication techniques to approximate the blaze
with discrete steps wherein each step represents a separate level.
The grating profile 30 is a multilevel step grating that
approximates a continuous blazed grating profile 32 having a width
L.sub.2 34 and a height of separation h.sub.2 38. Equation 4 is the
scalar diffraction theory expression for the efficiency of
diffraction. The number of discrete steps N within this expression
defines the grating profile 30. For the GLV device, the value of
the integer is selected based on the period of the grating profile
and the selected width of the ribbon L.sub.2 34. The value of
L.sub.2 34 is chosen to achieve the required diffraction
efficiency, but is limited to a minimum by the available
fabrication methods. The value for the height h.sub.2 38 is
optimized for maximum intensity in the +1 diffracted 4 optical beam
according to the expression h 2 = / 2 N + p 2 where p is 0 or a
positive integer .
[0042] The diffraction efficiency .eta..sub.m into the m.sup.th
order for a grating with N steps tuned to the +1 order is predicted
via scalar theory to be, 5 m = R N 2 l = 0 N - 1 l N ( q m - 2 m )
2 sin 2 ( m / N ) ( m / N ) 2 . ( 4 )
[0043] As an example of using these relationships, Table 1 shows
the diffraction efficiency into the -3 through +3 orders for
gratings with differing discrete steps N and R (reflectivity) equal
to 1.0. With the addition of a third discrete step, the grating
profile becomes asymmetric and the intensity in the +1 diffracted
beams 26d is increased by 70% over the power obtained for a grating
profile having a square wave profile, N=2. The improvement in
diffraction efficiency increases with an increasing number of step
levels N.
1 TABLE 1 N .eta..sub.-3 .eta..sub.-2 .eta..sub.-1 .eta..sub.0
.eta..sub.1 .eta..sub.2 .eta..sub.3 2 0.045 0 0.405 0 0.405 0 0.045
3 0 0.171 0 0 0.684 0 0 4 0.090 0 0 0 0.811 0 0 5 0 0 0 0 0.875 0
0
[0044] For the application of the device described here to printing
by photosensitive media or thermal sensitive methods, the
efficiency should be maximized to allow faster rates of printing
while reducing the power requirements of the optical sources
providing the incident illumination. For display and other
applications, increased efficiency is also advantageous. Ideally,
the continuous blaze grating profile could be used to maximize the
efficiency of a single diffracted order. Because of the fabrication
methods chosen, the alternative of using multiple step levels is
desirable. FIG. 3 illustrates a grating profile that can be
produced using the standard fabrication processes of
microelectronic devices.
[0045] Referring now to FIG. 4 which illustrates a perspective,
partially cut-away view of the multilevel mechanical grating device
100 of the present invention. The multilevel mechanical grating
device 100 disclosed therein can form at least three different
levels. The mechanically deformable structures of the device 100
are formed on top of a base 50. The present embodiment as shown in
FIG. 4 discloses a device 100 that can be operated by the
application of an electrostatic force. Because the actuation force
of the multilevel mechanical grating device 100 is electrostatic,
the base 50 comprises several layers of different materials. The
base 50 comprises a substrate 52 chosen from the materials glass
and silicon, which is covered by a bottom conductive layer 56. In
this embodiment the thin bottom conductive layer 56 is necessary
since it acts as an electrode for applying the voltage to actuate
the mechanical grating device 100. The thin bottom conductive layer
56 is covered by a protective layer 58. The bottom conductive layer
56 is selected from the group consisting of aluminum, titanium,
gold, silver, tungsten, silicon alloys and indium tin oxide. Above
the protective layer 58 a standoff layer 60 is formed which is
followed by a spacer layer 65. On top of the spacer layer 65, a
ribbon layer 70 is formed which is covered by a reflective layer or
layers 78. The thickness and tensile stress of the ribbon layer 70
are chosen to optimize performance by influencing the electrostatic
or mechanic force required for actuation and the returning force,
which affects the speed, resonance frequency, and voltage
requirements of the multilevel mechanical grating device 100. In
the present embodiment the reflective layer 78 also has to include
a conductor in order to provide electrodes for the actuation of the
multilevel mechanical grating device 100. The electrodes are
patterned from the reflective and conductive layer 78.
[0046] The spacer layer 65 has a longitudinal channel 67 formed
therein that extends along the longitudinal direction L-L of the
multilevel mechanical gating device 100. The longitudinal channel
67 comprises a first and second side wall 67a and 67b and a bottom
67c. The channel 67 is open on top and covered by a first and
second set of deformable ribbon elements 72a and 72b. Each
deformable ribbon element 72a and 72b spans the channel 67 and is
secured to the surface of the spacer layer 65 on either side of the
channel 67. The bottom 67c of the channel 67 is covered by the
protective layer 58. As mentioned above, the ribbon layer 70 is
covered by the reflective layer 78. The reflective layer 78
(conductive) is patterned such that there are first and second
conducting regions 78a and 78b, which form comb-like structures
arranged on the surface of the multilevel mechanical grating device
100 in an interdigitated manner. The first and second conductive
region 78a and 78b are mechanically and electrically isolated from
one another. According to the pattern of the reflective layer 78,
the ribbon layer 70 is patterned to form the first and the second
set of deformable ribbon elements 72a and 72b spanning the channel
67. The deformable ribbon elements 72a and 72b are grouped
according to the longitudinal direction L-L of the channel 67. In
the case of the three level mechanical grating device (embodiment
as disclosed in FIG. 4) three deformable ribbon elements belong to
one group. Each group comprises one deformable ribbon element from
the second set 72b and two deformable ribbon elements from the
first set 72a.
[0047] In the embodiment shown in FIG. 4, a plurality of standoffs
61 is positioned on the bottom 67c of the channel 67. The standoffs
61 are patterned from the standoff layer such that a group of
standoffs 61 is associated with the deformable ribbon elements 72a
and 72b of each group. In the embodiment shown here, the group of
standoffs 61 is associated with the second ribbon element
72.sub.L3a.sub.2of each group (valid for three ribbon elements per
group). As shown in FIG. 7, each group comprises a first, second
and third ribbon element 72.sub.L3a.sub.1, 72.sub.L3a.sub.2, and
72.sub.L3b.sub.1. The standoffs 61 may also be patterned in the
form of a single bar.
[0048] A top view of the multilevel mechanical grating device 100
with three levels is illustrated in FIG. 5, which also shows two
planes perpendicular to the view illustrated. View Plane A-A is a
side view of the multilevel mechanical grating device 100 and
depicts the view shown in FIG. 6. View Plane B-B is a side view of
the device and depicts the view shown in FIG. 7. Note that a device
with four or more levels (four or more deformable ribbon elements
per group) is a straightforward extension of the principles
illustrated in FIGS. 5, 6 and 7.
[0049] The mechanical grating device 100 as shown in FIG. 5, is a
device which can be actuated by the application of an electrostatic
force. It is clear that a person skilled in the art can imagine
other ways for actuating the grating device, for example thermal
actuation, piezoelectric actuation or any combination. In the
embodiment shown in FIG. 5, a first and a second electrically
conducting region 78a and 78b are formed on the surface of the
mechanical grating device 100. The first and the second
electrically conducting region 78a and 78b are electrically and
mechanically isolated from each other to allow the application of
different voltages to the first and second sets of deformable
ribbon elements 72a and 72b. The first conducting region 78a
applies the voltage to the first set of deformable ribbon elements
72a and the second conducting region 78b provides the voltage to
the second set of deformable ribbon elements 72b. The second
conducting region 78b is in contact with the bottom conductive
layer 56 (see FIG. 6) designated at the base 50 through at least
one etched opening 74 filled with the thick conducting layer 76.
For operation of the device, the electrostatic force is produced by
a voltage difference between the bottom conductive layer 56 and the
conducting layer 78 atop the ribbon layer 70. Ideally the
conducting layer 78 is highly reflective to maximize the optical
diffraction efficiency when operating the device. The connection
with the bottom conductive layer 56 is carried out by an
interconnect 75. The thin bottom conductive layer 56 is formed
below the bottom 67c of the channel 67. From the view of FIG. 5,
regions of the spacer layer 65 and protective layer 58 are visible
because of patterning of first and second conductive region 78a and
78b to achieve electrical and mechanical isolation of the
deformable ribbon elements 72a and 72b.
[0050] The device presented here is a GLV that incorporates
multiple levels, which means more than two, to discretely
approximate a blazed grating. FIGS. 7 and 8 illustrate this concept
with three levels, and FIGS. 9 and 10 illustrate the concept with
four levels.
[0051] In FIG. 7, the surface 53a of the substrate is shown with
pedestals or lines as standoffs 61 designed with specific heights
as defined by the relationship between the height h.sub.2 34 and
the number of ribbons N per group. For this case, the value of N is
three for the group which represents one period .LAMBDA.. The first
ribbon element of each group is designated 72.sub.L3a.sub.1, the
second ribbon element of each group is designated 72.sub.L3a.sub.2
and the third ribbon element of each group is designated
72.sub.L3b.sub.1. The first and second ribbon element
72.sub.L3a.sub.1 and 72.sub.L3a.sub.2 of each group are contacted
by the first conductive region 78a or, in other words, the first
and second ribbon elements 72.sub.L3a.sub.1 and 72.sub.L3a.sub.2 of
each group belong to the first set of deformable ribbon elements
72a. The third ribbon element 72.sub.L3b.sub.1 of each group is
contacted by the second conductive region 78b or the third ribbon
element 72.sub.L3b.sub.1 of each group belongs to the second set of
deformable ribbon elements 72b. The height of the intermediate
level is defined by standoff 61 which is associated with the second
ribbon element 72.sub.L3a.sub.2 of each group. In the unactuated
state (no applied force) all the ribbon elements 72a and 72b are
coplanar, defining a first top level 64b and a first bottom level
64a. The unactuated multilevel mechanical grating device 100 acts
like a mirror and an incident light beam 90, having a wave-length
.lambda., is reflected into the 0.sup.th order. The reflected light
beam in the 0.sup.th order is designated 92a. In the actuated state
(FIG. 8) the deformable ribbon elements 72a of the first set are
subjected to a deformation which draws the ribbon elements into the
channel 67. The ribbon elements 72b of the second set are not
subjected to any deformation. Therefore every third ribbon element
72.sub.L3b.sub.1 of each group remains in the unactuated state
thereby defining the first top level 64b and the first bottom level
64a. The second ribbon element of each group abuts against the
standoff 61, thereby defining a first intermediate top level 54b.
The first element 72.sub.L3a.sub.1 of each group is moved to the
bottom of the channel 67, defined by surface 53a, thereby defining
a bottom top level 53b. Each top level 64b, 54b and 53b is spaced
by .lambda./2N above the surface 53a to maximize the efficiency of
diffraction into the +1 order. The diffracted beam is designated
92b.
[0052] Although the ribbons in each group are actuated to different
depths, each does not have to be independently addressed by the
driver circuitry. The presence of standoffs to define the height
54a enables the device to operate as designed with all moving
ribbons receiving the same voltage and initial electrostatic force.
Thus, only two independent voltage levels are required to operate a
device with improved efficiency, ground voltage and operating
voltage. This is equivalent to the requirement of the device
designs of prior art.
[0053] In FIGS. 9 and 10, in which N=4, the lower standoff height
61 is .lambda./8 and the upper standoff height 62 is .lambda./4.
The total depth of the channel should be (1-1/N).lambda./2. For
this case, the value of N is four for a group which represents one
period .LAMBDA.. The first ribbon element of each group is
designated 72.sub.L4a.sub.1, the second ribbon element of each
group is designated 72.sub.L4a.sub.2, the third ribbon element of
each group is designated 72.sub.L4a.sub.3 and the fourth ribbon
element of each group is designated 72.sub.L4b.sub.1. The heights
of the intermediate levels are defined by standoffs 61 which are
associated with the second and third ribbon element
72.sub.L4a.sub.2 and 72.sub.L4a.sub.3 of each group. The standoff
61 associated with the second ribbon element 72.sub.L4a.sub.2
defines a surface 54a. The standoff 61 associated with the third
ribbon element 72a.sub.3 defines a surface 55a. In the unactuated
state (no applied force) all the ribbon elements 72a and 72b are
coplanar, defining a first top level 64b and a first bottom level
64a. The unactuated multilevel mechanical grating device 100 acts
like a mirror and an incident light beam 90, having a wavelength
.lambda., is reflected into the 0.sup.th order. The reflected light
beam in the 0.sup.th order is designated 92a. In the actuated state
(FIG. 10) the deformable ribbon elements 72a of the first set are
subjected to a deformation which draws the ribbon elements into the
channel 67. The ribbon elements 72b of the second set are not
subjected to any deformation. Therefore every forth ribbon element
72.sub.L4b.sub.1 of each group remains in the unactuated state
thereby defining the first top level 64b and the first bottom level
64a. The third ribbon element 72.sub.L4a.sub.3 of each group abuts
against the standoff 61, defining the surface 55a, thereby defining
a first intermediate top level 55b. The second ribbon element
72.sub.L4a.sub.2 of each group abuts against the standoff 61,
defining the surface 54a, thereby defining a second intermediate
top level 55b. The first element 72.sub.L4a.sub.1 of each group is
moved to the bottom of the channel 67, defined by surface 53a,
thereby defining a bottom top level 53b. Each top level 64b, 55b,
54b and 53b is spaced by .lambda./2N above the surface 53a to
maximize the efficiency of diffraction into the +1 order. The
diffracted beam is designated 92b.
[0054] As discussed previously, the optical efficiency of the
device can theoretically be increased by up to 70% for a 3-level
grating or 102% for a 4-level grating, assuming ideal reflectors
and ignoring effects from inter-ribbon gaps. Note that, while more
levels yields higher diffraction efficiencies in the ideal grating,
the presence of gaps between ribbons degrades the performance of 3-
and 4-level gratings relative to that of 2-level gratings.
Furthermore, the additional levels will increase the number of
processing steps required to create the standoffs 61. FIG. 11 shows
a plot of the theoretical diffraction efficiency of the
1.sup.st-order beam as a function of the percent ratio of gap width
L.sub.G to the ribbon width L.sub.R, within the accuracy of scalar
diffraction theory. In practice, with an optimized device, the
ratio L.sub.G/L.sub.R can be between 10% and 30% and the
corresponding 3- and 4-level gratings still provide a significant
improvement in diffraction efficiency. Thus, the ideal number of
ribbons per period, N, is probably either three or four, depending
on the minimum feasible size of the gaps between the ribbons and
the allowed pixel width.
[0055] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
PARTS LIST
[0056] 10 reflective grating
[0057] 11 angle .theta..sub.0
[0058] 12 optical beam
[0059] 13 period .LAMBDA.
[0060] 14 width of the groove
[0061] 15 angle .theta..sub.m
[0062] 16 diffracted beam
[0063] 20 blazed grating
[0064] 22 incident beam
[0065] 24 grating surface
[0066] 26a to 26f diffracted beams
[0067] 30 grating profile
[0068] 32 continuous blazed grating profile
[0069] 34 width L.sub.2
[0070] 38 a height of separation h.sub.2
[0071] 50 base
[0072] 50a top surface of base
[0073] 52 substrate
[0074] 53 surface of the base
[0075] 53a surface
[0076] 53b
[0077] 54a top surface of standoffs
[0078] 54b second intermediate top level
[0079] 55a surface
[0080] 55b first intermediate top level
[0081] 56 bottom conductive layer
[0082] 58 protective layer
[0083] 60 standoff layer
[0084] 61 standoff
[0085] 64a first bottom level
[0086] 64b first top level
[0087] 65 spacer layer
[0088] 66 sacrificial layer
[0089] 67 channel
[0090] 67a first side wall of the channel
[0091] 67b second side wall of the channel
[0092] 67c bottom of the channel
[0093] 70 ribbon layer
[0094] 70a bottom surface of the coplanar ribbon elements
[0095] 70b top surface of the coplanar ribbon elements
[0096] 72a first set of deformable ribbon elements
[0097] 72b second set of deformable ribbon elements
[0098] 72.sub.L3a.sub.1 first element of each group of three
[0099] 72.sub.L3a.sub.2 second element of each group of three
[0100] 72.sub.L3b.sub.1 third element of each group of three
[0101] 72.sub.L4a.sub.1 first element of each group of four
[0102] 72.sub.L4a.sub.2 second element of each group of four
[0103] 72.sub.L4a.sub.3 third element of each group of four
[0104] 72.sub.L4b.sub.1 fourth ribbon element of each group of
four
[0105] 74 opening
[0106] 75 interconnect
[0107] 76 thick conductor
[0108] 78a first conducting region
[0109] 78b second conducting region
[0110] 92b diffracted beam
[0111] 100 multilevel mechanical grating device
[0112] L longitudinal direction
[0113] N number of discrete steps
[0114] d grating depth
[0115] m order
[0116] n number of levels
[0117] .eta..sub.m diffraction efficiency
[0118] A-A view plane
[0119] B-B view plane
[0120] L-L longitudinal direction of the device
[0121] O-O orthogonal axis
* * * * *