Spatial light modulator

A spatial light modulator includes a reflector which is electrostatically deflectable out of a normal position, whereat a supporting beam is unstressed, into a deflected position, whereat a portion of the mirror contacts a portion of a landing electrode at the same electrical potential as the reflector. An inorganic layer or solid lubricant is deposited on the contacting portions. After the modulator is operated for a period of time, the tendency of the reflector to stick or adhere to the landing electrode is diminished or eliminated by the layer so that the reflector is returned to its normal position without any reset signal or with a reset signal having a reasonably low value. Preferred materials for the layer are SiC, AlN or SiO.sub.2.

BACKGROUND OF THE INVENTION 
The present invention relates to an improved spatial light modulator 
("SLM"), and, more particularly, to an SLM of the digital micromirror 
device ("DMD") variety having improved operating characteristics. 
SLM's are transducers that modulate incident light in a spatial pattern 
pursuant to an electrical or other input. The incident light may be 
modulated in phase, intensity, polarization or direction. SLM's of the 
deformable mirror class include micromechanical arrays of electronically 
addressable mirror elements or pixels which are selectively movable or 
deformable. Each mirror element is movable in response to an electrical 
input to an integrated addressing circuit formed monolithically with the 
addressable mirror elements in a common substrate. Incident light is 
modulated in direction and/or phase by reflection from each element. 
As set forth in greater detail in commonly assigned U.S. Pat. No. 
5,061,049, deformable mirror SLM's are often referred to as DMD's (for 
"Deformable Mirror Device" or "Digital Micromirror Device"). There are 
three general categories of deformable mirror SLM's: elastomeric, membrane 
and beam. The latter category includes torsion beam DMD's, cantilever beam 
DMD's and flexure beam DMD's. 
Each movable mirror element of all three beam types of DMD's includes a 
relatively thick metal reflector supported in a normal, undeflected 
position by an integral relatively thin metal beam. In the normal 
position, the reflector is spaced from a substrate-supported, underlying 
control electrode which may have a voltage selectively impressed thereon 
by the addressing circuit. 
When the control electrode carries an appropriate voltage, the reflector is 
electrostatically attracted thereto and moves or is deflected out of the 
normal position toward the control electrode and the substrate. Such 
movement or deflection of the reflector causes deformation of its 
supporting beam thereby storing therein potential energy which tends to 
return the reflector to its normal position when the control electrode is 
de-energized. As a practical matter may be, and often is, insufficient to 
return the reflector to the normal position. This necessitates the 
application of one of a variety of reset signals or voltages between the 
control electrode and the reflector to achieve this end. 
The deformation of a cantilever beam comprises bending about an axis normal 
to the beam's axis; that of a torsion beam comprises deformation by 
twisting about an axis parallel to the beam's axis; that of a flexure 
beam, which is a relatively long cantilever beam connected to the 
reflector by a relatively short torsion beam, comprises both types of 
deformation, permitting the reflector to move in piston-like fashion. 
Thus, the movement or deflection of the reflector of a cantilever or 
torsion beam DMD is rotational, with some parts of the reflector rotating 
toward the substrate and other parts rotating away from the substrate if 
the axis of rotation is other than at an edge or terminus of the 
reflector. The movement or deflection of the reflector of a flexure beam 
DMD maintains all points on the reflector generally parallel with the 
substrate. 
When the reflector of a beam DMD is operated in binary fashion by its 
addressing circuit, it occupies one of two positions, the first being the 
normal position which is set by the undeformed beam, the second position 
being a deflected position. In one of the positions, the reflector 
reflects incident light to a selected site, such as a viewing screen, the 
drum of a xerographic printer or other photoreceptor. In the other 
position, incident light is not reflected to the photoreceptor. 
A typical DMD includes an array of numerous pixels, the reflectors of each 
of which are selectively positioned to reflect or not reflect light to a 
desired site. 
Because a potential difference must exist between the reflector and the 
control electrode to deflect the reflector, it is undesirable for these 
two elements to engage. Engagement of a deflected reflector and its 
control electrode effects current flow therethrough which may weld them 
together and/or cause the thinner beam to melt or fuse. In either event 
the functionality of the involved pixel is destroyed. In response to the 
foregoing problem, a landing electrode may be associated with each 
reflector. Typically, in the case of a cantilever- or torsion-beam DMD, 
the landing electrode resides on the substrate at a greater distance from 
the rotational axis than the control electrode, both distances being taken 
parallel to the reflector in its normal position. In a flexure-beam DMD, 
the top of the landing electrode is elevated above the top of the control 
electrode. In view of the foregoing, the deflected reflector ultimately 
engages the landing electrode, but not the control electrode. To prevent 
damage to reflector, the landing electrode is maintained at the same 
potential as the reflector. Again, see commonly assigned U.S. Pat. No. 
5,061,049. 
Notwithstanding the use of a landing electrode, it has been found that a 
deflected reflector will sometimes stick or adhere to its landing 
electrode. Such sticking or adherence prevents the energy stored in the 
deformed beam, or reasonable forces applied to the reflector in other 
ways, from returning or "resetting" the reflector to its normal position 
after the control electrode is deenergized. It has been postulated that 
such sticking is caused, inter alia, by (a) welding (b)intermolecular 
attraction between the reflector and the landing electrode or (c) high 
surface energy substances or other particulate, liquid or gaseous 
contaminants sorbed or deposited on the surface of the landing electrode 
and/or on the portion of the reflector which contacts the landing 
electrode. Substances which impart high surface energy to the 
reflector-landing electrode interface include water vapor and other 
ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, hydrogen, 
nitrogen) and gases and organic components resulting from or left behind 
following production of the DMD, including gases produced by outgassing 
from UV-cured adhesives which mount a protective cover to the DMD. Such a 
protective cover and other DMD "packages" are disclosed in commonly 
assigned U.S. patent application Ser. No. 033,687, filed Mar. 16, 1993. 
Sticking of the reflector to the landing electrode has been overcome by 
applying selected numbers, durations, shapes and magnitudes of voltage 
pulses (the previously noted "reset signals") to the control electrode. 
One type of reset signal attempts to further attract toward the landing 
electrode a reflector which already engages the landing electrode. This 
further attraction stores additional potential energy in the already 
deformed beam. When the control electrode is de-energized, the potential 
energy stored in the beam is now able to unstick the reflector from the 
landing electrode and return the reflector to its normal position. A 
variant reset signal comprises a train of pulses applied to the control 
electrode to induce a resonant mechanical wave in a reflector already 
engaging a landing electrode. De-energizing the control electrode as a 
portion of the reflector is deformed away from the landing electrode 
unsticks the reflector. For more details concerning the foregoing and 
other unsticking techniques, see commonly assigned U.S. Pat. No. 
5,096,279. 
Sticking or adherence of the reflector and the landing electrode may be 
reduced by appropriate liquid lubricants. Moreover, in commonly assigned 
U.S. Pat. No. 5,331,454, there are disclosed techniques for passivating 
the portion of the landing electrode engaged by the deformed reflector 
and/or the portion of the deformed reflector which engages the landing 
electrode so that sticking or adherence therebetween is reduced or 
eliminated. Passivation may be effected by lowering the surface energy of 
the landing electrode and/or the reflector--or otherwise preventing 
sticking or adhering. Passivation may be, in turn, effected by chemically 
vapor-depositing on the engageable surfaces of interest a monolayer of a 
long-chain aliphatic halogenated polar compound, such as a 
perfluochemical, examples of which are perfluoroalkyl acid, 
perfluorodecanoic acid (PFDA), perfluoropolyether (PFPE) and 
polytetrafluoroethylene (Teflon). 
The polar compound perfluoroalkyl acid comprises a chain having an F.sub.3 
C molecule at a first end, a COOH molecule at the second end and 
intermediate CF.sub.2 molecules. The COOH end becomes firmly attached to 
surfaces of the DMD--following pretreatment, if necessary, to achieve 
same--to present very low surface energy F.sub.3 C and CF.sub.2 molecules 
for engagement. The other materials function similarly. 
The application of the foregoing a compounds to at least that portion of 
the landing electrode which is engaged by a deformed reflector has 
resulted in an amelioration of the sticking or adhesion problem. 
Objects do not easily, if at all, stick or adhere to low surface energy 
surfaces. Further, sticking or adherence of substances to low energy 
surfaces should not occur or be minimized since such surfaces should be 
resistant to sorption thereonto of the above-discussed 
high-surface-energy-imparting substances, such as water vapor. Indeed, 
DMD's on which an anti-stick monolayer, lubricant or other appropriate 
substance has been deposited may initially exhibit little if any 
reflector-electrode adherence. This is evidenced by the low magnitudes of 
reset signals and/or by the proper functioning over time of all or a 
maximal number of reflectors. 
After the DMDs are operated for some time, however, two effects have been 
noted. First higher magnitudes of reset signals may be required to return 
the reflectors to their normal positions. Second, at a given reset voltage 
less than all or a maximal number of the reflectors may return to their 
normal positions. The same two effects have been noted when protective, 
lighttransparent covers are mounted to DMD's with adhesives, such as 
UV-cured epoxies. The above effects have also been noted as worsening 
after extended operation of DMDs. The foregoing suggests that substances 
deposited or outgassed from the ambient, from adhesives or from the DMD 
itself are somehow adhering to, becoming incorporated into or otherwise 
adversely affecting the low surface energy anti-stick deposit or 
lubricant, possibly due to defects or discontinuities in the long chains, 
monolayers or other structure thereof. 
Elimination of the sticking phenomenon is an object of the present 
invention. 
SUMMARY OF THE INVENTION 
The above and other objects are achieved by fabricating an improved DMD. 
Generally, the DMD includes a movable mirror element having a normal 
position set by a deformable beam in its undeformed state. The mirror also 
has a deflected position in which the beam is deformed and a portion of 
the mirror element engages a portion of a stationary member, or landing 
electrode, which is typically at the same electric potential as the 
reflector. 
Deformation of the beam stores energy therein which tends to return the 
mirror element to the normal position. The mirror element is selectively 
electrostatically attractable into its deflected position. 
In the improved DMD, a deposit of an inorganic passivant resides on one or 
both of the engageable portions of the stationary member and the mirror 
element. The passivant is SiC, AIN or SiO.sub.2 and preferably has a 
thickness between about 0.6 nm and about 5 nm thick. The passivant may be 
deposited by RF magnetron sputtering or other convenient method. The 
deposit functions as an anti-stick passivant even though its surface 
energy is higher than that of perfluorochemicals, such as PFDA and Teflon, 
which are used as similar passivants. Indeed, over time, the passivants of 
the present invention have been observed to function better as anti-stick 
passivants than many perfluorochemical passivants. The deposited 
passivants of the present invention are mechanically strong, scratch 
resistant, and thermally and chemically stable.

DETAILED DESCRIPTION 
Referring first to FIG. 1, there are shown two adjacent, individual DMD's 
10, which may be of the type shown in commonly assigned U.S. Pat. No. 
5,061,049 to Hornbeck and U.S. Pat. No. 3,600,798 to Lee. The DMD's 10 may 
also be similar to those shown in U.S. Pat. No. 4,356,730 to Cade, U.S. 
Pat. No. 4,229,732 to Hartstein et al, U.S. Pat. No.3,896,338 to Nathanson 
et al, and U.S. Pat. No. 3,886,310 to Guldberg et al. The above types of 
DMD's 10 may be used in systems such as those shown in commonly assigned 
U.S. Pat. No. 5,101,236 to Nelson et al, 5,079,544 to DeMond et al, U.S. 
Pat. No. 5,041,851 to Nelson, and U.S. Pat. No. 4,728,185 to Thomas. In 
the following Description, the DMD's 10 are described as operating in a 
bistable or digital mode, although they may be operated in other modes, 
such as tristable or analog. 
As generally depicted in FIG. 1, each DMD 10 includes a relatively thick 
and massive, metal or metallic light-reflective, movable or deflectable 
mirror element 12 and associated addressing circuits 14 for selectively 
deflecting the mirror elements 12. Methods of monolithically forming the 
mirror elements 12 and the addressing circuits 14 in and on a common 
substrate 16 are set forth in the above-noted patents. Typically, each 
mirror element 12 deflects by moving or rotating up and down on one or 
more relatively thin, integral supporting beams or hinges 18. Although 
FIG. 1 illustrates a single cantilever beam 18, the mirror element 12 may 
be supported by one or more torsion beams or flexure beams, as discussed 
earlier. 
Undercut wells 20 are defined between columnar members 22, which may 
comprise residual photoresist remaining on the substrate 16 after 
functioning as a portion of a etching, deposition, and/or implantation 
mask during the formation of the DMD 10. Each beam 18 is supported by one 
member 22. Each well 20 accommodates the deflection of its associated 
mirror element 12 by permitting it to move toward the substrate 16, as 
shown at the left in FIG. 1, from an undeflected position, shown to the 
right in FIG. 1. Deflection of each mirror element 12 is effected by the 
attractive electrostatic force exerted thereon by an electric field 
resulting from a potential applied to an associated control electrode 24 
in its well 20 and on the substrate 16. The potential is selectively 
applied to the control electrode 24 by its addressing circuit 14. 
When a beam 18 is undeformed, it sets the normal position of its mirror 
element 12, as shown at the right in FIG. 1. Light along a path 26 which 
is incident on the device 10 when a mirror element 12 is in its normal 
position is reflected thereby along a path, denoted at 28, to a first 
site, generally indicated at 30. An angle 32 is defined between the paths 
28 and 30. 
When an addressing circuit 14 applies an appropriate potential to its 
control electrode 24, its mirror element 12 is electrostatically attracted 
out of its normal position toward the control electrode 24 and the 
substrate 16. The mirror element 12 accordingly moves or deflects until it 
engages a landing electrode 34, as shown at the left in FIG. 1, and 
resides in its deflected position. The use of the landing electrode 34 is 
recommended by the aforenoted '279 patent. Specifically, the landing 
electrode 34 serves as a mechanical stop for the mirror element 12, thus 
setting the deflected position thereof. Further, the engagement of the 
landing electrode 34 and the mirror element 12 prevents the mirror element 
12 from engaging the control electrode 24. Because of the potential 
difference between the mirror element 12 and the control electrode 24, 
such engagement would result in current flow through the mirror element 
12. Current flow of this type is likely to weld the mirror element 12 to 
the control electrode 24 and/or to fuse or melt the relatively thin beam 
18. 
In the deflected position of the mirror element 12, the incident light on 
the path 26 is reflected along a path 36 to a second site 38. An angle 40 
is defined between the paths 26 and 36. In the present example, the angle 
32 is smaller than the angle 40. 
The first site 30 may be occupied by a utilization device, such as a 
viewing screen or a photosensitive drum of a xerographic printing 
apparatus. The light 36 directed to the second site 38 may be absorbed or 
otherwise prevented from reaching the first site 30. The roles of the 
sites 30 and 38 may, of course, be reversed. In the foregoing way, the 
incident light 26 is modulated by the DMD's 10 so that it selectively 
either reaches or does not reach whichever site 30 or 38 contains the 
utilization device. 
FIG. 2 generally depicts an area array 42 of the DMD's 10 shown in FIG. 1. 
FIG. 3 depicts a linear array 44 of the DMD's 10 shown in Figure 1. In 
FIG. 3, the incident light 26 is emitted from a suitable source 46 and is 
reflected along either the path 28 or the path 36. The path 28 directs the 
reflected light through a lens 48 to the surface of a photosensitive drum 
50 of a xerographic printing apparatus (not shown). The reflected light 
traversing the path 36 does not reach the drum 50 and may be directed onto 
a "light sink" whereat it is absorbed or otherwise prevented from reaching 
the drum 50 or otherwise affecting the light traversing the paths 26 and 
28. 
When the mirror element 12 is in its deflected position and engages its 
landing electrode 34, its beam 18 is deformed and, accordingly, stores 
energy therein which tends to return the mirror element 12 to its normal 
position. The return of the mirror element 12 to its normal position is 
often aided or achieved by the application of a reset voltage or signal 
between the mirror element 12 and the control electrode 24. 
In theory, when the control electrode 24 is de-energized by the addressing 
circuit 14, the stored energy will return the mirror element 12 to the 
normal position. As discussed in commonly assigned U.S. Pat. No. 
5,096,279, the stored energy alone may be insufficient to return the 
mirror 12 to the normal position. For example, either or both portions of 
the mirror element 12 and the landing electrode 34 which are engaged 
during deflection of the former may become intermetallically welded or 
otherwise stick or adhere due to their possessing high surface energy. 
Such high surface energy may result from the inherent material 
characteristics of the mirror 12 and the electrode 24 or from substances 
or contaminants deposited or sorbed onto the engaged portions. Simple 
de-energization of the control electrode 24 may, accordingly, not result 
in the mirror element 12 returning to its normal position if the mirror 
element 12 and the landing electrode 28 stick or adhere. The '279 patent 
describes a technique for applying special reset signals to the control 
electrode 28 which overcome the sticking or adhering together of the 
mirror element 12 and the landing electrode 34. 
The present invention relates to a technique for passivating a DMD by 
depositing an inorganic passivant, such as a low surface energy inorganic 
coating or solid lubricant on one or both of the engageable portions of 
the mirror element 12 and the landing electrode 34. The inorganic 
passivant discourages the aforenoted sticking or adherence problem, while 
exhibiting higher scratch resistance, mechanical strength, and chemical 
and thermal stability than organic passivants of the type discussed 
earlier. 
It has been found that notwithstanding the implementation of techniques of 
the type disclosed in the '279 patent, mirror elements 12 and landing 
electrodes 34 of DMD's 10 stick or adhere together, especially after an 
extended period of use. It is postulated that the low surface energies of 
the deposited organic materials degrade over time or that low surface 
energy of a deposited passivant is not the only determinant of 
adherence-prevention. This latter postulate follows from the fact that 
deposits of the passivants of the present invention have higher surface 
energies than deposits of many organic DMD passivants, yet result in 
better DMD operation over time. 
As discussed more fully below, prior DMDs passivated with various 
materials, such as the organic materials discussed above, have initially 
properly operated, but have exhibited degraded operation after the passage 
of time. Such degraded operation is characterized by an increasing number 
of mirror elements which will not reset when a previously used reset 
voltage is used--a higher reset voltage being required. Stated 
differently, over time, in order to reset all of a maximal number of the 
reflectors, the absolute value of the reset voltage or signal must be 
increased. Increasing or high sticking of the mirrors 12 to their landing 
electrodes 34 is, therefore, characterized by increasing or high absolute 
values of reset signals or voltages. It is desirable to be able to utilize 
reset voltages which have reasonable magnitudes and, if reset voltages do 
not remain constantly low from the onset of operation of the DMD, for such 
reset voltages to stabilize over time at reasonably low values. The 
present invention is intended to prevent sticking and/or to limit the 
force of such sticking so that the mirrors 12 can be reliably returned to 
their normal positions during operation of the DMDs 10. 
According to the present invention, chemically and mechanically stable 
inorganic passivant or solid lubricant coatings 100 such as SiC, AIN or 
SiO.sub.2 are deposited on the engageable portions of the mirror elements 
12 and the related landing electrodes 34. Such deposition is preferably 
achieved by RF magnetron sputtering. These passivant coatings 100 prevent 
or limit to reasonable values sticking or adherence of the mirror elements 
12 and their landing electrodes 34, and, for reasons not fully 
understood--but which are possibly related to the coatings 100 preventing 
metal-metal bonds or metal-fluorine-metal bonds and/or to the high 
mechanical strength and thermal and chemical stability of the passivants 
100--they appear to offer surface reasonably long lived passivation. 
Interestingly, the surface energy of the passivants 100 of the present 
invention is somewhat higher than the surface energies of organic 
passivants such as PFDA, PFDE and Teflon. 
SiC films 100 were deposited on DMDs, including the engageable portions of 
the mirror elements 12 and landing electrodes 34 by RF magnetron 
sputtering at room temperature. The thickness of the sputtered films was 
measured nominally at 1.7 nm, but thicknesses ranging from about 0.5 nm to 
about 5.0 nm give beneficial results. Referring to FIG. 4, the operation 
of a DMD 10 immediately following passivation with SiC is represented by 
the curve 200. The curve 200 is depicted on a graph in which the abscissa 
represents reset voltage and the ordinate represents the percentage of 
properly operating DMD's in an array thereof, with 100% representing all 
of the DMDs 10 in the array. Curve 200 of FIG. 4 illustrates that at the 
onset of operation of SiC-passivated DMDs 10, slightly more than half will 
reset without the application of a reset voltage and that with increasing 
absolute magnitude of reset voltage 100% of the DMDs 10 reset when the 
reset voltage is about -15 volts. As shown by curve 205, all DMDs 10 in an 
array passivated with PFDA appear to reset without a reset voltage at the 
onset of operation. As shown by curve 210, practically none of the DMDs 10 
passivated with Teflon-AF reset without the application of a reset 
voltage, but 100% thereof reset when the reset voltage reaches about -16.5 
volts. Thus, for reset voltages up to a value of about -15 volts and at 
the onset of operation, SiC (curve 200)is a better passivant than 
Teflon-AF curve 210) and a worse passivant than PFDA (curve 205), even 
though the surface energy of the SiC deposit 100 is higher than that of 
either of the organic materials. 
FIG. 5 is similar to FIG. 4. It depicts the curve 200 and a curve 215 
representing reset voltage versus percent of DMDs reset after the DMD 
array has been continuously operated for 20 hours. About 83% of the DMDs 
10 now reset without a reset voltage and 100% resetting is achieved with a 
lower reset voltage of -12 volts. 
FIG. 6 is a graph in which the abscissa is operating time in hours and the 
ordinate is the percentage of DMDs 10 which properly operate, that is, 
which reset after moving out of the normal position. Curves 220a and 220b 
represent the operation of two arrays of DMDs 10 passivated with SiC 100, 
while curves 225a and 225b represent the operation of two arrays of DMDs 
10 passivated with PFDA, which at the onset of operation appeared to be a 
better passivant than SiC 100. FIG. 6 shows that following about 150 hours 
of operation, an equal or greater percentage of SiC-passivated DMDs (220a 
and 220b) properly operate than DMDs passivated with PFDA (225a and 225b). 
FIG. 7 illustrates that after 150 hours of continuous operation, the reset 
voltages required to reset 100% of SiC-passivated DMDs (230a and 230b)is 
lower than the reset voltage required to reset PFDA-passivated DMDs (235a 
and 235b). This relative comparison obtains for operating periods of at 
least nearly 850 hours. 
FIG. 8 is similar to FIG. 4, but the curves represent the operation of DMDs 
passivated with AIN. Curves 240a and 240b represent the reset voltages 
necessary to reset varying percentages of DMDs 10 passivated with 0.6 nm 
of AIN. Curves 245a, 245b and 250a, 250b represent the reset voltages 
necessary to reset varying percentages of DMDs passivated with 1.8 nm and 
3 nm, respectively, of AIN. 100% of the DMDs are reset at voltages between 
about -11 volts and -17.5 volts. FIG. 9 is similar to FIG. 8 and 
represents the operation of DMDs 10 passivated with SiO.sub.2 at 
thicknesses of 1 nm (curves 255a and 255b) and 5nm (curves 260a and 260b). 
AIN functions as an anti-stick passivant at thicknesses of about 0.6 nm to 
about 7.5 nm; so, too, does SiO.sub.2 at thicknesses of about 0.5 nm to 
about 20 nm. 
Thus, the reliability of DMDs 10 which are operated for extended times 
after being passivated with SiC, AIN or SiO.sub.2 equals or surpasses that 
of DMDs 10 which are not passivated or which are passivated with certain 
organic materials such as polyfluorochemicals. Additionally, when the 
inorganic passivants 100 are deposited on the beams 18 and mirror elements 
12 of the DMD 10, the mechanical strength of these elements may be 
increased. This same relatively higher mechanical strength, and the higher 
scratch resistance and mechanical strength of the inorganic passivants of 
the present invention, along with their higher thermal and chemical 
stability, recommends the passivating materials of the present invention. 
Those skilled in the art will appreciate that the foregoing description 
sets forth only preferred embodiments of the present invention and that 
various modifications and additions may be made thereto without departing 
from the spirit and scope of the present invention.