Method of unsticking components of micro-mechanical devices

A method of unsticking contacting elements (11, 17) of a micro-mechanical device (30). The device is exposed to either a low surface tension liquid with a surfactant (32) or to a supercritical fluid (62) so as to avoid damage to fragile components of the device (30). The exposure conditions are controlled so as to provide optimum results without damage to the device.

TECHNICAL FIELD OF THE INVENTION 
This invention relates to micro-mechanical devices, and more particularly, 
to micro-mechanical devices having contacting elements and of treating 
such devices after their contacting elements have become stuck together. 
BACKGROUND OF THE INVENTION 
A recent development in the field of electro-mechanics has been the 
miniaturization of various mechanical devices. Typical of such devices are 
tiny motors, gears, levers, and valves. These "micro-mechanical" devices 
are manufactured using integrated circuit techniques, often together with 
electrical control circuitry. Common applications include accelerometers, 
pressure sensors, and actuators. As another example, micro-mirrors can be 
configured for use in spatial light modulators. 
Reliability has been difficult to achieve with micro-mechanical devices. A 
common reliability problem is sticking, which occurs when the device has a 
moving element that contacts another surface of the device. The moving 
element may become stuck against that surface, causing the device to cease 
to operate properly. A related problem is that repeated contact between 
elements can cause their surfaces to wear. 
Because of the tiny dimensions of micro-mechanical devices, the cause of 
sticking and the effects of various attempts to solve the sticking problem 
are difficult to discern. A discussion of causes of "stiction" is set out 
in an article entitled "The Effect of Release-Etch Processing on Surface 
Microstructure Stiction" by Alley, Cuan, Howe, and Komvopoulos in 
Proceedings of the IEEE Solid State Sensor and Actuator Workshop (1992) 
pp. 202-207. Various causes of stiction that are discussed are solid 
bridging, liquid bridging, Van de Waals forces, electrostatic forces, and 
surface roughness. 
Many attempts to solve the sticking problem have been directed to 
overcoming assumed adhesion forces by coating contacting surfaces with 
various substances, such as lubricants, during fabrication. However, these 
techniques are preventative rather than curative, and do not solve the 
problem of unsticking contacting elements once they have become stuck. 
Supercritical fluids have been recognized as a means for drying 
micro-mechanical devices which have been rinsed in liquids, as part of 
their fabrication, resulting in stuck elements. A discussion of 
ameliorating liquid bridging by critical point (SCF) drying is set out in 
an article entitled "Supercritical Carbon Dioxide Drying of 
Microstructures" by Mulhern, Soane, and Howe in 7th International 
Conference on Solid State Sensors and Actuators (1993) pp. 296-299. 
However, these techniques are limited to post-rinse drying during 
fabrication. 
SUMMARY OF THE INVENTION 
One aspect of the invention is a method of unsticking of contacting 
elements of a micro-mechanical device. The device is placed in a liquid 
container and its contacting elements are immersed in a low surface 
tension liquid containing a surfactant. Then, the container is enclosed, 
and the device is dried in a manner that substantially preserves pressure 
equilibrium between the low surface tension liquid and its vapor phase. 
The low surface tension liquid may be a fluorocarbon liquid, such as a 
perfluorinated alkane. Alternatively, instead of being immersed in a low 
surface tension liquid, the contacting elements may be exposed to a 
supercritical fluid, which has zero surface tension. 
A technical advantage of the invention is that it unsticks stuck components 
without damaging them. Various embodiments of the invention remove residue 
that might have contributed to the sticking, and thus prevent the sticking 
from recurring. The method can be performed on the micro-mechanical device 
as a finishing step near the end of its fabrication process, or it can be 
performed after the device is finished and has been in operation.

DETAILED DESCRIPTION OF THE INVENTION 
For purpose of example, the following description is in terms of a 
particular type of micro-mechanical device, a "digital micro-mirror 
device" (DMD), sometimes also referred to as a "deformable mirror device". 
A DMD is a type of spatial light modulator. The DMD has one or more tiny 
mirrors, which are selectively deflectable so as to reflect or not reflect 
light to a desired location. The method of the invention cures sticking of 
the mirrors to a "landing pad", which they touch when they fully deflect. 
One application of DMDs is for forming images, where the DMD has an array 
of hundreds or thousands of deflectable mirrors. Each mirror selectively 
reflects light to an image plane. The images formed by the DMD can be used 
in display systems or for non-impact printing applications. Other 
applications of DMD's are possible that do not involve image formation, 
such as optical steering, optical switching, and accelerometers. In some 
of these applications, the "mirror" need not be reflective. Also, in some 
applications, the DMD is operated in an analog rather than digital mode. 
In general, the term "DMD" is used herein to include any type of 
micro-mechanical device having at least one hinge-mounted element that 
contacts a landing surface in response to an applied voltage or other 
force such as that caused by acceleration in the case of an accelerometer. 
The invention is useful for other types of micro-mechanical devices that 
have contacting elements. Like the DMD's mirrors, other micro-mechanical 
devices may have tiny rotors, levers, or other moving parts that contact 
with another surface and become stuck. 
FIGS. 1 and 2 illustrate a single mirror element 10 of a DMD. In FIG. 1, 
the mirror element 10 is in a flat (undeflected) state, whereas in FIG. 2, 
the mirror element 10 is deflected. As stated above, various DMD 
applications may use such mirror elements 10 singly or in arrays. 
The mirror element 10 of FIGS. 1 and 2 is known as a "torsion beam" 
element. Other types of mirror elements 10 can be fabricated, including 
cantilever beam types and flexure beam types. Various DMD types are 
described in U.S. Pat. No. 4,662,746, entitled "Spatial Light Modulator 
and Method"; U.S. Pat. No. 4,956,610, entitled "Spatial Light Modulator"; 
U.S. Pat. No. 5,061,049 entitled "Spatial Light Modulator and Method"; 
U.S. Pat. No. 5,083,857 entitled "Multi-level Deformable Mirror Device"; 
and U.S. patent Ser. No. 08/097,824. Each of these patents is assigned to 
Texas Instruments Incorporated and each is incorporated herein by 
reference. 
In operation for image display applications, a light source illuminates the 
surface of the DMD. A lens system may be used to shape the light to 
approximately the size of the array of mirror elements 10 and to direct 
this light toward them. Each mirror element 10 has a tilting mirror 11 
supported by torsion hinges 12 attached to support posts 13. These support 
posts 13 are formed on and extend away from the substrate 15. The mirrors 
11 are positioned over a control circuit 14, which is comprised of address 
and memory circuitry and fabricated on the substrate 15. 
Voltages based on data in the memory cells of control circuit 14 are 
applied to two address electrodes 16, which are located under opposing 
corners of mirror 11. Electrostatic forces between the mirrors 11 and 
their address electrodes 16 are produced by selective application of 
voltages to the address electrodes 16. The electrostatic force causes each 
mirror 11 to tilt either about +10 degrees (on) or about -10 degrees 
(off), thereby modulating the light incident on the surface of the DMD. 
Light reflected from the "on" mirrors 11 is directed to an image plane, 
via display optics. Light from the "off" mirrors is reflected away from 
the image plane. The resulting pattern forms an image. The proportion of 
time during each image frame that a mirror 11 is in the "on" position 
determines shades of grey. Color can be added by means of a color wheel or 
by a three-DMD setup. 
In effect, the mirror 11 and its address electrodes 16 form capacitors. 
When appropriate voltages are applied to the mirror 11 and to the address 
electrodes 16, the resulting electrostatic force (attracting or repelling) 
causes the mirror 11 to tilt toward the landing electrode 17 associated 
with the attracting address electrode 16. The mirror 11 tilts until its 
edge contacts the landing electrode 17. 
Once the electrostatic force between the address electrodes 16 and the 
mirror 11 is removed, the energy stored in the hinge 12 provides a 
restoring force to return the mirror 11 to its undeflected position. 
Appropriate voltages may be applied to the mirror 11 or address electrodes 
16 to aid in returning the mirror 11 to its undeflected position. However, 
if a mirror 11 has become stuck, these forces may not be sufficient to 
overcome the sticking force. 
FIGS. 3-5 illustrate a method of treating a micro-mechanical device whose 
contacting elements are stuck, using a low surface tension liquid in 
accordance with the invention. FIG. 6 illustrates an alternative to the 
step illustrated in FIG. 3, using a supercritical fluid rather than a low 
surface tension liquid. 
As stated above, for purposes of example, a DMD micro-mechanical device is 
illustrated, but the same process may be performed on any micro-mechanical 
device. The method is performed on a device that is at least substantially 
finished. The method can be performed as a "back-end" fabrication step on 
a device whose micro-mechanical components have become stuck during a 
prior fabrication step. For example, the method may be performed during 
fabrication of a wafer that is to be eventually separated into chips, each 
having a DMD array. This method is well suited for volume production of 
this type, and is easily integrated into the process flow for making DMDs 
or other micro-mechanical devices. Alternately, the method can be 
performed as a maintenance treatment on a device that has already been in 
operation for some time. 
In FIG. 3, a DMD 30 has been at least substantially fabricated, including 
the landing electrodes 17, address electrodes 16, and mirrors 11 of each 
mirror element 10. Typically, the fabrication of these components involves 
the use of a temporary spacer material, which has been removed in FIG. 3. 
As indicated, at least some of the mirror elements 10 have become stuck in 
a deflected position. 
The DMD 30 is placed in a container 31 suitable for holding liquids, and 
immersed in a low surface tension liquid 32 that contains a surface active 
agent (surfactant). Examples of suitable low surface tension liquids 31 
are fluorocarbon liquids, especially perfluorinated alkanes. One such 
suitable liquid is "FC-77", a mixture of perfluorinated alkanes, 
commercially available from 3M Corporation. An example of a suitable 
liquid with a surfactant is a 0.2% non-ionic surfactant in 
perfluoro-n-methyl morpholine. A commercially available liquid that 
contains a perfluorinated alkane with a surfactant, known as "PF-5052" and 
available from 3M Corporation, has been used successfully. 
FIG. 4 illustrates a drying step, performed after the immersion step of 
FIG. 3. In the example of FIG. 4, the drying is a vacuum drying. A cover 
41 has been placed over the container 31, or container 31 is otherwise 
enclosed, so as to retain the vapors 32' from liquid 32 as the liquid 32 
dries, instead of exposing the device 30 directly to a vacuum. The vacuum 
is controlled so that as the liquid is drawn from the container 31, the 
liquid 32 is in pressure equilibrium with its vapor 32'. 
FIG. 5 illustrates a third step, which may be performed after the immersion 
and drying steps of FIGS. 3 and 4. In FIG. 5, the device 30 is cleaned to 
remove any residue of surfactant that may remain on its surfaces. The 
cleaning step of FIG. 5 may be accomplished in a number of different ways. 
The step illustrated is accomplished by exposing the device 30 to a 
supercritical fluid 51, with or without an additive, such that any residue 
is solubilized. For example, a supercritical carbon dioxide/acetone binary 
fluid has been experimentally found satisfactory. 
As indicated in FIG. 5, because of the gas-like diffusion behavior of a 
supercritical fluid, the DMD 30 is placed in a container 52 having baffled 
entry and exit ports 52a. These ports 52a eliminate any damage to the 
mirror elements 10 that might occur as a result of gas turbulence when the 
supercritical fluid 52 enters and exits. An example of a suitable baffle 
52a is densely packed beads. However, various other diffusive material 
could be used. The remainder of container 52 is made from a material 
suitable for containing high pressure fluid, such as stainless steel. 
Another suitable treatment to remove surfactant residue would be a thermal 
treatment. This would be the case for certain surfactants that become 
volatile at temperatures greater than ambient temperatures, but less than 
a temperature that might cause damage to the device. The heating would be 
accompanied by drying to help remove the residue. 
The cleaning step of FIG. 5 is amenable to alterations in the physically 
and chemical processing parameters so as to permit selective extraction of 
residues. For example, in the case of a DMD, residues other than that from 
photoresist may result in a greater need for extraction. 
FIG. 6 illustrates an alternative means of accomplishing the immersion step 
of FIG. 3. In FIG. 6, the DMD 30 is exposed to a supercritical fluid 62, 
which may or may not contain a surfactant. As compared to the 
supercritical fluid drying discussed in the Background, the exposure is to 
a DMD 30 that is "dry" in the sense that it has not been rinsed or 
otherwise immersed in liquid immediately prior to the supercritical fluid 
exposure. The sticking that the invention eliminates could come from 
causes other than by being immersed in a liquid. An example of a suitable 
supercritical fluid 62 is carbon dioxide. The supercritical fluid could be 
a binary fluid, such as one composed of carbon dioxide and ethanol or of 
carbon dioxide and acetone. An example of a suitable supercritical fluid 
62 with a surfactant is one comprised of carbon dioxide and a non-ionic 
surfactant such as that contained in PF-5052. As described above in 
connection with FIG. 5, a special container 52 is used for entry and exit 
of the supercritical fluid. 
Certain conditions of pressure, temperature, and fluid flow may be 
determined for optimum results. Experimental testing with DMD devices 30 
has indicated that a carbon dioxide supercritical fluid 62 at a pressure 
of 500 atmospheres within a range of 35 degrees to 80 degrees centigrade, 
at a flow rate of 400 sccm will provide good results. Exposure times were 
of 30 minutes. For the same supercritical fluid, results were less 
satisfactory for exposure time of 2 hours at a flow rate of 22 liters per 
hour at 80 degrees centigrade and 500 atmospheres. After exposure for an 
appropriate time, the pressure release is controlled such that evacuation 
of the fluid to ambient pressure occurs over time, thereby avoiding damage 
to the device. 
If the supercritical fluid 62 contains a surfactant, the DMD 30 may be 
further treated, as described above in connection with FIG. 5, to remove 
any residue of the surfactant. 
The above-described method results in unsticking the mirror elements 10 of 
a DMD 30, or, more generally, the contacting elements of a 
micro-mechanical device. In contrast, supercritical fluids may also be 
used as part of a cleaning and treating process to prevent sticking of 
contacting elements of a micromechanical device. 
OTHER EMBODIMENTS 
Although the invention has been described with reference to specific 
embodiments, this description is not meant to be construed in a limiting 
sense. Various modifications of the disclosed embodiments, as well as 
alternative embodiments, will be apparent to persons skilled in the art. 
It is, therefore, contemplated that the appended claims will cover all 
modifications that fall within the true scope of the invention.