Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-12-21
2003-06-24
Dang, Hung Xuan (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S292000, C359S295000, C359S234000, C359S223100
Reexamination Certificate
active
06583921
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to spatial light modulators and more specifically to a Digital Micromirror Device (DMD) spatial light modulator utilizing a new noncontacting, edge-coupled hidden hinge geometry.
2. Description of the Related Art
FIG. 1
shows a conventional hidden hinge Digital Micromirror Device (DMD) broken into it's three metal layers of construction, all of which is built on top of a rather standard SRAM memory cell.
FIG. 1
a
shows the first metal layer
200
above the memory cell which consists of yoke address pads
1
, a bias/reset bus
2
, landing sites
3
, and via connections
13
to the SRAM memory cell below (not shown).
FIG. 1
b
shows the second layer
300
of metal structures consisting of the mirror address electrodes
4
, electrode support posts
5
, torsion hinge
6
, hinge support posts
7
, yoke
8
, and the yoke landing tips
9
. This second metal layer
300
assembly sits on top of the first metal layer
200
, being supported by means of the electrode support posts
5
and hinge support post
7
. Finally,
FIG. 1
c
shows the third layer of metal
400
that consists of the highly reflective mirror
10
and it's support post
11
. As before, the mirror assembly
400
sits on top of the second metal layer
300
, being supported by the mirror support post
11
sitting in the middle of the yoke
8
. In operation, a bias voltage is applied to the bias/reset bus
2
that is integral to the yoke assembly
8
by means of the hinge support posts
7
. The yoke address pads
1
and mirror address electrodes
4
are then pulsed to establish an electric field between the address pads and the mirror assembly that generates an electrostatic force causing the yoke/mirror assembly to tilt in one direction or the other depending on the binary state of the underlying memory cell. As illustrated, although the yoke and mirror assemblies rotate together, electrostatic forces are established in two areas
12
(shown as cross hatched areas); i.e., between the yoke address pad
1
on the first level and the yoke
8
on the second level, as well as between the mirror address electrodes
4
on the second level and the mirror
10
on the third level. The yoke
8
rotates until its two landing tips
9
contacts the landing sites
3
on the lower metal layer
200
. The angle of rotation is a function of the yoke geometry and the height of the second metal
300
layer above the lower metal layer
200
. The long, thin, narrow torsion hinges
6
, which supports the yoke
8
and mirror
10
from the hinge support posts
7
, have a torque applied to them allowing the thicker yoke
8
to remain flat. Finally, a reset pulse can be applied to the bias/reset bus
2
to lift off and free the mirror/yoke assembly from the landing sites
3
.
FIG. 2
shows a three-dimensional build-up of a conventional DMD's four layers, including the SRAM memory, which was mentioned above. These consist of the CMOS SRAM memory layer
100
, the address and landing pad layer
200
, the yoke and hinge layer
300
, and the mirror layer
400
. It can be seen from the figure that this conventional DMD device is symmetrical about a diagonal axis running parallel with the hinge, so that in operation the mirror assembly will tilt in the positive or negative direction depending on the binary state (“0” or “1”) of the SRAM memory cell
14
. The geometry of a typical DMD is such that the mirror will tilt on the order of ±100.
FIG. 3
is a 3-D cutaway view of an array of conventional hidden hinge DMD pixels showing three of the mirrors and the underlying structure for other pixels. Included in the view are the following: yoke address pad
1
, bias/reset bus
2
, yoke landing sites
3
, mirror address electrode
4
, electrode support post
5
, torsion hinge
6
, hinge support posts
7
, yoke
8
, yoke landing tips
9
, reflective mirror
10
, mirror support post
11
, vias
13
to SRAM memory cell
14
. The square mirrors tilt on the order of ±10° and are highly reflective to visual light in the color spectrum from 400 to 650 nanometers. The gaps between the mirrors are typically <1 micron in width.
FIG. 4
illustrates two DMD cells, with their mirrors
10
shown transparent so as to expose a view of the underlying structure. One mirror is shown rotated −10° and the other is shown rotated +10°, representing a “0” and “1” binary state, respectively in the underlying memory cells
14
. This figure clearly shows how the yoke
8
, with attached mirror
10
, rotates on the torsion hinge
6
until the yoke landing tips
9
come in contact (lands) with the underlying landing pad sites
3
. It is this mechanical contact between the yoke landing tips
9
and the landing pad sites
3
that is of particular relevance to this invention. A problem with conventional DMD's is that of “sticking mirrors”, where the landing tips are slow in lifting off the pad, effecting the response of the device, or in some cases become permanently stuck to the landing pads. There appear to be several sources of this sticking problem, some of which include moisture in the package, landing tips scrubbing into the metal landing pads, and outgassing of the epoxy sealants used in the manufacturing process for mounting the devices in their packages and mounting the optical glass cover on the packages. This “sticking” problem has been addressed by applying a lubrication or passivation layer to the metal surfaces to make them “slick” and also through the use of resonant reset methods to pump energy into the pixel to help break it free from the constraining surface contact. More recently, “spring-tips” have been added to the tips of the mirrors to help overcome this sticking problem. In addition, gettering material is often added to absorb moisture within the package. Although quite effective, these solutions still have the concern of long-term degradation of the passivant, which could drive the technology to a requirement for hermetic packages and complex process steps prior to window attachment. This would add additional expense, complexity, and difficulty in delivering the product.
It is therefore desirable to implement a DMD that will rotate reliably and predictably to a given angle, consistent across the length of the device or an array of pixels, without physically contacting the memory substrate surface below and as a result to avoid all the difficulties of breaking that contact. Eliminating the stiction problem would allow more predictable performance of the mirror array, and eliminate the most frequent cause of device failure; i.e., stuck bright mirrors. The lack of contact would also provide more immunity to particulates on the first electrode level, allow special dark metal light absorbing layers, and enable the use of conventional CMOS electrical passivation layers like SiO
2
. The invention disclosed herein addresses this need.
Representative prior conventional structures of the general type are shown in U.S. Pat. No. 5,535,047 to Hornbeck, and in publications (1) “Digital Light Processing™ for High-Brightness, High-Resolution Applications,” by Larry J. Hornbeck, Electronic Imaging, EI'97, Projection Displays III, Co-Sponsored by IS&T and SPIE, Feb. 10-12 1997, san Jose, Calif., and (2) “Digital Light Processing and MEMS: Timely Convergence for a Bright Future,” Larry J. Hornbeck, Micromachining and Microfabrication '95, Part of SPIE's Thematic Applied Science and Engineering Series, Oct. 23-24 1995, Austin, Tex.
SUMMARY OF THE INVENTION
A new DMD device and method for non-contacting edge-coupled hidden hinge geometry is disclosed. This approach requires no physical contact between the mirror or underlying yoke and landing pads at the surface of the CMOS substrate. As a result, this eliminates the problem of “sticking” mirrors in conventional devices and significantly reduces the requirements for delicate passivation coatings and costly hermetic packages.
This method uses a more or less conventional DMD structure which sti
Brady III Wade James
Brill Charles A.
Dang Hung Xuan
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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