US 20080096313 A1
A projection system, a spatial light modulator, and a method for forming a MEMS device is disclosed. The spatial light modulator can have two substrates bonded together with one of the substrates comprising a micromirror array. The two substrates can be bonded at the wafer level after depositing a getter material and/or solid or liquid lubricant on one or both of the wafers. The wafers can be bonded together hermetically if desired, and the pressure between the two substrates can be below atmosphere.
106. A method for making a micromirror array, comprising:
providing a first wafer;
providing a second wafer;
forming circuitry and a plurality of electrodes on the first wafer to form a circuitry and electrode array;
forming a plurality of deflectable micromirror elements on either the first or second wafer to form a micromirror array;
before or after forming the plurality of deflectable micromirror elements and circuitry and electrode array, forming a trench or cavity proximate to the micromirror array and/or circuitry and electrode array;
depositing a stiction reducing agent and/or a getter into the trench or cavity;
aligning the first and second wafers;
bonding the first and second wafers together to form a wafer assembly; and
separating the wafer assembly into individual wafer assembly dies.
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124. A method for making a MEMS device, comprising:
forming a plurality of MEMS elements on a first wafer;
providing a second wafer;
hermetically bonding and sealing the first and second wafers together as a substrate assembly; and
singulating the hermetic substrate assembly into multiple hermetically sealed substrate assembly dies, each comprising a MEMS element.
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bonding a substrate assembly die onto a third substrate that does not fully encapsulate the substrate assembly to form an exposed and packaged substrate assembly.
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This application is a continuation-in-part of U.S. patent application Ser. No. 10/005,308 to Patel et al. filed Dec. 3, 2001, which claims priority from U.S. provisional application 60/254,043 to Patel et al. filed Dec. 7, 2000 and U.S. provisional application 60/276,222 to Patel et al. filed Mar. 15, 2001, each of the above incorporated herein by reference.
The present invention is in the field of MEMS, and in particular in the field of methods for making micro electromechanical devices on a wafer. The subject matter of the present invention is related to manufacturing of multiple MEMS devices on a wafer, releasing the MEMS structures by removing a sacrificial material, bonding the wafer to another wafer, singulating the wafer assembly, and packaging each wafer assembly portion with one or more MEMS devices thereon, without damaging the MEMS microstructures thereon. A wide variety of micro-electromechanical devices (MEMS) devices can be made in accordance with the embodiments herein, including accelerometers, DC relay and RF switches, optical cross connects and optical switches, microlenses, reflectors and beam splitters, filters, oscillators and antenna system components, variable capacitors and inductors, switched banks of filters, resonant comb-drives and resonant beams, and micromirror arrays for direct view and projection displays.
Processes for microfabricating a MEMS device such as a movable micromirror and mirror array are disclosed in U.S. Pat. Nos. 5,835,256 and 6,046,840 both to Huibers, the subject matter of each being incorporated herein by reference. A similar process for forming MEMS movable elements (e.g. mirrors) on a wafer substrate (e.g. a light transmissive substrate or a substrate comprising CMOS or other circuitry) is illustrated in FIGS. 1 to 4. By “light transmissive”, it is meant that the material will be transmissive to light at least in operation of the device (The material could temporarily have a light blocking layer on it to improve the ability to handle the substrate during manufacture, or a partial light blocking layer for decreasing light scatter during use. Regardless, a portion of the substrate, for visible light applications, is preferably transmissive to visible light during use so that light can pass into the device, be reflected by the mirrors, and pass back out of the device. Of course, not all embodiments will use a light transmissive substrate). By “wafer” it is meant any substrate on which multiple microstructures or microstructure arrays are to be formed and which allows for being divided into dies, each die having one or more microstructures thereon. Though not in every situation, often each die is one device or product to be packaged and sold separately. Forming multiple “products” or dies on a larger substrate or wafer allows for lower and faster manufacturing costs as compared to forming each die separately. Of course the wafers can be any size or shape, though it is preferred that the wafers be the conventional round or substantially round wafers (e.g. 4″, 6″ or 12″ in diameter) so as to allow for manufacture in a standard foundry.
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A second layer 20 (the “hinge” layer) is deposited as can be seen in
As also seen in
In the embodiment illustrated in
In one embodiment, the reinforcing layer is removed in the area of the hinge, followed by depositing the hinge layer and patterning both reinforcing and hinge layer together. This joint patterning of the reinforcing layer and hinge layer can be done with the same etchant (e.g. if the two layers are of the same material) or consecutively with different etchants. The reinforcing and hinge layers can be etched with a chlorine chemistry or a fluorine (or other halide) chemistry (e.g. a plasma/RIE etch with F2, CF4, CHF3, C3F8, CH2F2, C2F6, SF6, etc. or more likely combinations of the above or with additional gases, such as CF4/H2, SF6/Cl2, or gases using more than one etching species such as CF2Cl2, all possibly with one or more optional inert diluents). Of course, if different materials are used for the reinforcing layer and the hinge layer, then a different etchant can be employed for etching each layer. Alternatively, the reflective layer can be deposited before the first (reinforcing) and/or second (hinge) layer. Whether deposited prior to the hinge material or prior to both the hinge material and the reinforcing material, it is preferable that the metal be patterned (e.g. removed in the hinge area) prior to depositing and patterning the hinge material.
It should also be noted that materials and method mentioned above are examples only, as many other method and materials could be used. For example, the Sandia SUMMiT process (using polysilicon for structural layers) or the Cronos MUMPS process (also polysilicon for structural layers) could be used in the present invention. Also, a MOSIS process (AMI ABN—1.5 um CMOS process) could be adapted for the present invention, as could a MUSiC process (using polycrystalline SiC for the structural layers) as disclosed, for example, in Mehregany et al., Thin Solid Films, v. 355-356, pp. 518-524, 1999. Also, the sacrificial layer and etchant disclosed herein are exemplary only. For example, a silicon dioxide sacrificial layer could be used and removed with HF (or HF/HCl), or a silicon sacrificial could be removed with ClF3 or BrF3. Also a PSG sacrificial layer could be removed with buffered HF, or an organic sacrificial such as polyimide could be removed in a dry plasma oxygen release step. Of course the etchant and sacrificial material should be selected depending upon the structural material to be used. Also, though PVD and CVD are referred to above, other thin film deposition methods could be used for depositing the layers, including spin-on, sputtering, anodization, oxidation, electroplating and evaporation.
After forming the microstructures as in FIGS. 1 to 4 on the first wafer, it is preferably to remove the sacrificial layer so as to release the microstructures (in this case micromirrors). This release can be performed at the die level, though it is preferred to perform the release at the wafer level.
Also, though the hinge of each mirror can be formed in the same plane as the mirror element (and/or formed as part of the same deposition step) as set forth above, they can also be formed separated from and parallel to the mirror element in a different plane and as part of a separate processing step. This superimposed type of hinge is disclosed in
The second or “lower” substrate (the backplane) die contains a large array of electrodes on a top metal layer of the die. Each electrode electrostatically controls one pixel (one micromirror on the upper optically transmissive substrate) of the microdisplay. The voltage on each electrode on the surface of the backplane determines whether its corresponding microdisplay pixel is optically ‘on’ or ‘off,’ forming a visible image on the microdisplay. Details of the backplane and methods for producing a pulse-width-modulated grayscale or color image are disclosed in U.S. patent application Ser. No. 09/564,069 to Richards, the subject matter of which is incorporated herein by reference.
The display pixels themselves, in a preferred embodiment, are binary, always either fully ‘on’ or fully ‘off,’ and so the backplane design is purely digital. Though the micromirrors could be operated in analog mode, no analog capability is necessary. For ease of system design, the backplane's I/O and control logic preferably run at a voltage compatible with standard logic levels, e.g. 5V or 3.3V. To maximize the voltage available to drive the pixels, the backplane's array circuitry may run from a separate supply, preferably at a higher voltage.
One embodiment of the backplane can be fabricated in a foundry 5V logic process. The mirror electrodes can run at 0-5V or as high above 5V as reliability allows. The backplane could also be fabricated in a higher-voltage process such as a foundry Flash memory process using that process's high-voltage devices. The backplane could also be constructed in a high-voltage process with larger-geometry transistors capable of operating at 12V or more. A higher voltage backplane can produce an electrode voltage swing significantly higher than the 5-7V that the lower voltage backplane provides, and thus actuate the pixels more robustly.
In digital mode, it is possible to set each electrode to either state (on/off), and have that state persist until the state of the electrode is written again. A RAM-like structure, with one bit per pixel is one architecture that accomplishes this. One example is an SRAM-based pixel cell. Alternate well-known storage elements such as latches or DRAM (pass transistor plus capacitor) are also possible. If a dynamic storage element (e.g. a DRAM-like cell) is used, it is desirable that it be shielded from incident light that might otherwise cause leakage.
The perception of a grayscale or full-color image will be produced by modulating pixels rapidly on and off, for example according to the method in the above-mentioned U.S. patent application Ser. No. 09/564,069 to Richards. In order to support this, it is preferable that the backplane allows the array to be written in random-access fashion, though finer granularity than a row-at-a-time is generally not necessary.
It is desirable to minimize power consumption, primarily for thermal reasons. Decreasing electrical power dissipation will increase the optical/thermal power budget, allowing the microdisplay to tolerate the heat of more powerful lamps. Also, depending upon the way the microdisplay is assembled (wafer-to-wafer join+offset saw), it may be preferable for all I/O pads to be on one side of the die. To minimize the cost of the finished device it is desirable to minimize pin count. For example, multiplexing row address or other infrequently-used control signals onto the data bus can eliminate separate pins for these functions with a negligible throughput penalty (a few percent, e.g. one clock cycle for address information per row of data is acceptable). A data bus, a clock, and a small number of control signals (5 or less) are all that is necessary.
In use, the die can be illuminated with a 200 W or more arc lamp. The thermal and photo-carrier effects of this may result in special layout efforts to make the metal layers as ‘opaque’ as possible over the active circuitry to reflect incident optical energy and minimize photocarrier and thermal effects. An on-chip PN diode could be included for measuring the temperature of the die.
In one embodiment the resolution is XGA, 1024×768 pixels, though other resolutions are possible. A pixel pitch of from 5 to 24 um is preferred (e.g. 14 um). The size of the electrode array itself is determined by the pixel pitch and resolution. A 14 um XGA device's pixel array will therefore be 14.336×0.752 mm.
After the upper and lower substrates (wafers) are finished being processed (e.g. circuitry/electrodes on lower wafer, micromirrors on upper wafer), the upper and lower wafers are joined together. This joining of the two substrates allows micromirrors on one substrate to be positioned proximate to electrodes on the other substrate. This arrangement is illustrated in
The method for the assembly of the wafers and separation of the wafer assembly into individual dies and is similar in some ways to the method for assembly of a liquid crystal device as disclosed in U.S. Pat. No. 5,963,289 to Stefanov et al, “Asymmetrical Scribe and Separation Method of Manufacturing Liquid Crystal Devices on Silicon Wafers”, which is hereby incorporated by reference. Many bonding methods are possible such as adhesive bonding (e.g. epoxy, silicone, low K material or other adhesive—described further herein), anodic bonding, compression bonding (e.g. with gold or indium) metal eutectic bonding, solder bonding, fusion bonding, or other wafer bonding processes known in the art. Whether the upper and lower wafer are made of the same or different materials (silicon, glass, dielectric, multilayer wafer, etc.), they can first be inspected (step 30 in the flow chart of
The mirrors are preferably released at this point (step 34). Releasing immediately prior to the application of epoxy or bonding is preferable (except for an optional stiction treatment between release and bonding). For silicon sacrificial layers, the release can be in an atmosphere of xenon difluoride and an optional diluent (e.g. nitrogen and/or helium). Of course, other etchants could be used, including interhalogens such as bromine trifluoride and bromine trichloride. The release is preferably a spontaneous chemical etch which does not require plasma or other external energy to etch the silicon sacrificial layer(s). After etching, the remainder of the device is treated for stiction (step 36) by applying an anti-stiction layer (e.g. a self assembled monolayer). The layer is preferably formed by placing the device in a liquid or gas silane, preferably a halosilane, and most preferably a chlorosilane. Of course, many different silanes are known in the art for their ability to provide anti-stiction for MEMS structures, including the various trichlorsilanes set forth in “Self Assembled Monolayers as Anti-Stiction Coatings for MEMS: Characteristics and Recent Developments”, Maboudian et al., as well as other unfluorinated (or partially or fully fluorinated) alkyl trichlorosilanes, preferably those with a carbon chain of at least 10 carbons, and preferably partially or fully fluorinated. (Tridecafluoro-1,1,2,2-tetrahydro-octyl)trichlorosilane available from Gelest, Inc. is one example. Other trichlorosilanes (preferably fluorinated) such as those with phenyl or other organic groups having a ring structure are also possible. Various vapor phase lubricants for use in the present invention are set forth in U.S. Pat. Nos. 6,004,912, 6,251,842, and 5,822,170, each incorporated herein by reference.
In order to bond the two wafers together, spacers are mixed into sealant material (step 38). Spacers in the form of spheres or rods are typically dispensed and dispersed between the wafers to provide cell gap control and uniformity and space for mirror deflection. Spacers can be dispensed in the gasket area of the display and therefore mixed into the gasket seal material prior to seal dispensing. This is achieved through normal agitated mixing processes. The final target for the gap between the upper and lower wafers is preferably from 1 to 10 um, though other gaps are possible depending upon the MEMS device being formed. This of course depends upon the type of MEMS structure being encapsulated and whether it was surface or bulk micromachined. The spheres or rods can be made of glass or plastic, preferably an elastically deforming material. Alternatively, spacer pillars can be fabricated on at least one of the substrates. In one embodiment, pillars/spacers are provided only at the side of the array. In another embodiment, pillars/spacers can be fabricated in the array itself. Other bonding agents with or without spacers could be used, including anodic bonding or metal compression bonding with a patterned eutectic or metal.
A gasket seal material can then be dispensed (step 40) on the bottom substrate in a desired pattern, usually in one of two industry standard methods including automated controlled liquid dispensing through a syringe and printing (screen, offset, or roller). When using a syringe, it is moved along X-Y coordinates relative to the parts. The syringe tip is constrained to be just above the part with the gasket material forced through the needle by positive pressure. Positive pressure is provided either by a mechanical plunger forced by a gear driven configuration and/or by an air piston and/or pressed through the use of an auger. This dispensing method provides the highest resolution and process control but provides less throughput.
Then, the two wafers are aligned (step 42)—preferably to within 1 micron. Alignment of the opposing electrodes or active viewing areas requires registration of substrate fiducials on opposite substrates. This task is usually accomplished with the aid of video cameras with lens magnification. The machines range in complexity from manual to fully automated with pattern recognition capability. Whatever the level of sophistication, they accomplish the following process: 1. Dispense a very small amount of a UV curable adhesive at locations near the perimeter and off of all functional devices in the array; 2. Align the fiducials of the opposing substrates within the equipment capability; and 3. Press substrates and UV tack for fixing the wafer to wafer alignment through the remaining bonding process (e.g., curing of the internal epoxy).
The final cell gap can be set by pressing (step 44) the previously tacked laminates in a UV or thermal press. In a UV press, a common procedure would have the substrates loaded into a press where at least one or both of the press platens are quartz, in order to allow UV radiation from a UV lamp to pass unabated to the gasket seal epoxy. Exposure time and flux rates are process parameters determined by the equipment and adhesive materials. Thermally cured epoxies require that the top and bottom platens of a thermal press be heated. The force that can be generated between the press platens is typically many pounds. With thermally cured epoxies, after the initial press the arrays are typically transferred to a stacked press fixture where they can continue to be pressed and post-cured for 4-8 hours.
Once the wafers have been bonded together to form a wafer assembly, the assembly can be separated into individual dies (step 46). Silicon substrate and glass scribes are placed on the respective substrates in an offset relationship at least along one direction. The units are then separated, resulting in each unit having a bond pad ledge on one side and a glass electrical contact ledge on an opposite side. The parts may be separated from the array by any of the following methods. The order in which the array (glass first) substrate is scribed is important when conventional solid state cameras are used for viewing and alignment in a scribe machine. This constraint exists unless special infrared viewing cameras are installed which make the silicon transparent and therefore permits viewing of front surface metal fiducials. The scribe tool is aligned with the scribe fiducials and processed. The resultant scribe lines in the glass are used as reference marks to align the silicon substrate scribe lanes. These scribe lanes may be coincident with the glass substrate scribes or uniformly offset. The parts are then separated from the array by venting the scribes on both substrates. Automatic breaking is done by commercially available guillotine or fulcrum breaking machines. The parts can also be separated by hand.
Separation may also by done by glass scribing and partial sawing of the silicon substrate. Sawing requires an additional step at gasket dispense. Sawing is done in the presence of a high-pressure jet of water. Moisture must not be allowed in the area of the fill port or damage of the MEMS structures could occur. Therefore, at gasket dispense, an additional gasket bead must be dispensed around the perimeter of the wafer. The end of each scribe/saw lane must be initially left open, to let air vent during the align and press processes. After the array has been pressed and the gasket material cured, the vents are then closed using either the gasket or end-seal material. The glass is then aligned and scribed as described above. Sawing of the wafer is done from the backside of the silicon where the saw streets are aligned relative to the glass scribe lanes described above. The wafer is then sawed to a depth of 50%-90% of its thickness. The parts are then separated as described above.
Alternatively, both the glass and silicon substrates may be partially sawed prior to part separation. With the same gasket seal configuration, vent and seal processes as described above, saw lanes are aligned to fiducials on the glass substrates. The glass is sawed to a depth between 50% and 95% of its thickness. The silicon substrate is sawed and the parts separated as described above.
For an illustrated example of the above, reference is made to
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Reflective deflectable elements (e.g., mirrors 48, 48 a, 48 b and 48 c), each corresponding to a respective pixel cell 54, 54 a, 54 b and 54 c, are attached to the lower surface 14 of the optically transmissive substrate 52 in an undeflected position. Thus, mirrors 48, 48 a, 48 b and 48 c are visible through optically transmissive substrate 52 in
The optically transmissive substrate 52 is made of materials which can withstand subsequent processing temperatures. The optically transmissive substrate 52 may be, for example, a 4 inch quartz wafer 500 microns thick. Such quartz wafers are widely available from, for example, Hoya Corporation U.S.A. at 960 Rincon Circle, San Jose, Calif. 95131. Or, the substrate can be glass such as Corning 1737 or Corning Eagle2000 or other suitable optically transmissive substrate. In a preferred embodiment, the substrate is transmissive to visible light, and can be display grade glass.
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The method for forming micromirrors as set forth above is but one example of many methods for forming many different MEMS devices (whether with or without an electrical component), in accordance with the present invention. Though the electrical component of the final MEMS device is formed on a separate wafer than the micromirrors in the above example, it is also possible to form the circuitry and micromechanical structures monolithically on the same substrate. The method for forming the MEMS structures could be similar to that described in
It is also possible to bond multiple substrates (smaller than a single wafer) to another wafer. In the embodiment illustrated in
The MEMS wafers could be made of any suitable material, depending upon the final application for the devices, including silicon, glass, quartz, alumina, GaAs, etc. Silicon wafers can typically be processed to include circuitry. For an optical MEMS application (e.g. micromirrors for optical switching or for displays), the “top” wafer of
Then, the two wafers are aligned, bonded, cured (e.g. with UV light or heat depending upon the type of adhesive used) and singulated as set forth above.
Preferably, the light transmissive substrate bonded to the semiconductor substrate (or other substrate having circuitry and electrodes thereon) hermetically seals the MEMS elements from the surrounding environment, and preferably at a pressure lower than the surrounding environment. In order to achieve the lower pressure, the two substrates (dies or wafers) are bonded together at subatmospheric pressure and hermetically sealed (followed by wafer singulation if performed at the wafer level). Or, the two substrates could be bonded at ambient pressure, though not hermetically, followed by a second seal at a lower pressure that hermetically seals off the interior of the bonded substrates from the surrounding environment. In one example of such a method, the two substrates are first bonded with an adhesive (e.g. epoxy or silicone) followed by soldering (this can be application of a solder or solder reflow) or other hermetic seal (e.g. glass frit seal) if the adhesive is insufficiently hermetic. It is also possible to perform both seals at subatmospheric pressure, or perform such a double seal at ambient pressure if it is not desired to have a lower pressure within the area between the two substrates. In addition, it may be desirable, whether at lower pressure or not, to seal the two substrates with a gas other than air, such as an inert gas or gases (nitrogen, helium, etc.), or moisture can be added if an anti-stiction agent would better perform if some moisture is present in the package. This packaging can also be desirable for a monolithic MEMS device where both the circuitry and MEMS elements are on the same substrate, as well as where the MEMS elements are formed on a substrate different from the circuitry.
If an anti-stiction agent is deposited on the MEMS elements after release, but before bonding the substrates together, it may be desirable to protect the areas that will be required for bonding (e.g. if an adhesive is used—protecting those areas where the adhesive will be applied), followed by application of the anti-stiction agent, in turn followed by removing the protecting agent or film, followed in turn by application of the bonding agent. Such a film could be applied in strips or rings that correspond to the location of the later applied bonding agent, and could be a photoresist or an inorganic thin film applied by CVD or sputtering. In the alternative, the MEMS elements could be released, followed by application of an anti-stiction agent. Then, the applied anti-stiction agent (e.g. a self assembled monolayer formed from chlorosilane or alkoxysilane precursors) is removed in the areas where the bonding agent will be applied (in any pattern—though preferably circumferentially around the micromirror array if the MEMS elements are micromirrors in an array). Removal can be accomplished by laser ablation (preferably focusing laser above the substrate), particle beam, application of a stripping chemical (e.g. acetone) or even mechanical removal (scoring with a hard or soft object or with a polishing wheel).
There are many alternatives to the method of the present invention. In order to bond the two wafers, epoxy can be applied to the one or both of the upper and lower wafers. In a preferred embodiment, epoxy is applied to both the circumference of the wafer and completely or substantially surrounding each die/array on the wafer. Spacers can be mixed in the epoxy so as to cause a predetermined amount of separation between the wafers after bonding. Such spacers hold together the upper and lower wafers in spaced-apart relation to each other. The spacers act to hold the upper and lower wafers together and at the same time create a space in which the movable mirror elements can move. Alternatively, the spacer layer could comprise walls or protrusions that are micro-fabricated. Or, one or more wafers could be bonded between the upper and lower wafers and have portions removed (e.g. by etching) in areas corresponding to each mirror array (thereby providing space for deflection of the movable elements in the array). The portions removed in such intermediate wafers could be removed prior to alignment and bonding between the upper and lower wafers, or, the wafer(s) could be etched once bonded to either the upper or lower wafer. If the spacers are micro-fabricated spacers, they can be formed on the lower wafer, followed by the dispensing of an epoxy, polymer, or other adhesive (e.g. a multi-part epoxy, or a heat or UV-cured adhesive) adjacent to the micro-fabricated spacers. The adhesive and spacers need not be co-located, but could be deposited in different areas on the lower substrate wafer. Alternative to glue, a compression bond material could be used that would allow for adhesion of the upper and lower wafers. Spacers micro-fabricated on the lower wafer (or the upper wafer) and could be made of polyimide, SU-8 photo-resist.
Instead of microfabrication, the spacers could be balls or rods of a predetermined size that are within the adhesive when the adhesive is placed on the lower wafer. Spacers provided within the adhesive can be made of glass or plastic, or even metal so long as the spacers do not interfere with the electrostatic actuation of the movable element in the upper wafer. Regardless of the type of spacer and method for making and adhering the spacers to the wafers, the spacers are preferably from 1 to 250 microns, the size in large part depending upon the size of the movable mirror elements and the desired angle of deflection. Whether the mirror arrays are for a projection display device or for optical switching, the spacer size in the direction orthogonal to the plane of the upper and lower wafers is more preferably from 1 to 100 microns, with some applications benefiting from a size in the range of from 1 to 20 microns, or even less than 10 microns.
Regardless of whether the microstructures and circuitry are formed on the same wafer or on different wafers, when the microstructures are released by removal of the sacrificial layer, a sticking force reducing agent can be applied to the microstructures (micromirrors, microrelays, etc) on the wafer to reduce adhesion forces upon contact of the microstructures with another layer or structure on the same or opposing substrate. Though such adhesion reducing agents are known, in the present invention the agent is preferably applied to the wafer before wafer bonding (or after wafer bonding but before singulation), rather than to the singulated die or package for the die. Various adhesion reducing agents, including various trichlorosilanes, and other silanes and siloxanes as known in the art for reducing stiction for micro electromechanical devices, as mentioned elsewhere herein.
Also, a getter or molecular scavenger can be applied to the wafer prior to wafer bonding as mentioned above. The getter can be a moisture, hydrogen, particle or other getter. The getter(s) is applied to the wafer around the released MEMS structures (or around, along or adjacent an array of such structures, e.g. in the case of a micromirror array), of course preferably not being in contact with the released structures. If a moisture getter is used, a metal oxide or zeolite can be the material utilized for absorbing and binding water (e.g. StayDry SD800, StayDry SD1000, StayDry HiCap2000—each from Cookson Electronics). Or, a combination getter could be used, such as a moisture and particle getter (StayDry GA2000-2) or a hydrogen and moisture getter (StayDry H2-3000). The getter can be applied to either wafer, and if adhesive bonding is the bonding method, the getter can be applied adjacent the epoxy beads or strips, preferably between the epoxy and the microstructures, and can be applied before or after application of the adhesive (preferably before any adhesive is applied to the wafer(s). In one embodiment, a getter (or getters if more than one type of getter is used) are provided in a trench or other cavity formed in either (or both) substrates. For example, a trench extending along one or more sides of a micromirror array (or around the entire periphery of the array) could be formed prior to depositing the sacrificial layer and thin films (or at the end before or after release of the micromirrors). Such a trench (or cavity) could be formed in a silicon substrate (e.g. with circuitry and electrodes thereon if formed as the dual substrate approach set forth above, or circuitry, electrodes and micromirrors thereon if formed monolithically). Or such a trench or cavity for the getter(s) could be formed in the glass substrate. It is also possible to form a trench or cavity in both substrates with the same or different getters deposited therein.
As can be seen from the above, the method of the present invention comprises making a MEMS device, e.g. a spatial light modulator, by providing a first wafer, providing a second wafer, forming circuitry and a plurality of electrodes on the first wafer, forming a plurality of deflectable elements on either the first or second wafer, aligning the first and second wafers, bonding the first and second wafers together to form a wafer assembly, separating the wafer assembly into individual dies, and packaging the individual dies. Each die can comprise an array of deflectable reflective elements. The reflective elements correspond to pixels in a direct-view or projection display. The number of reflective elements in each die is from 6,000 to about 6 million, depending upon the resolution of the display.
In the method of the invention, the first wafer is preferably glass, borosilicate, tempered glass, quartz or sapphire, or can be a light transmissive wafer of another material. The second wafer can be a dielectric or semiconductor wafer, e.g. GaAs or silicon. As noted above, the first and second wafers are bonded together with an adhesive (though metal or anodic bonding are also possible, depending upon the MEMS structure and the type of micromachining.
The releasing can be performed by providing any suitable etchant, including an etchant selected from an interhalogen, a noble gas fluoride, a vapor phase acid, or a gas solvent. And, the releasing is preferably followed by a stiction treatment (e.g. a silane, such as a chlorosilane). Also, a getter can be applied to the wafer before or after the adhesion reducing agent is applied, and before or after an adhesive is applied (if an adhesive bonding method is chosen). Preferably the time from releasing to bonding is less than 12 hours, and preferably less than 6 hours.
The invention need not be limited to a direct-view or projection display. The invention is applicable to many different types of MEMS devices, including pressure and acceleration sensors, MEMS switches or other MEMS devices formed and released on a wafer. The invention also need not be limited to forming the releasable MEMS elements on one wafer and circuitry on another wafer. If both MEMS and circuitry are formed monolithically on the same wafer, a second wafer (glass, silicon or other material) can be attached at the wafer lever following release of the MEMS devices but prior to dividing the wafers into individual dies. This can be particularly useful if the MEMS devices are micromirrors, due to the fragility of such elements.
Though the invention is directed to any MEMS device, specific mirrors and methods for projection displays or optical switching could be used with the present invention, such as those mirrors and methods set forth in U.S. Pat. No. 5,835,256 to Huibers issued Nov. 10, 1998; U.S. Pat. No. 6,046,840 to Huibers issued Apr. 4, 2000; U.S. patent application Ser. Nos. 09/767,632 to True et al. filed Jan. 22, 2001; 09/564,069 to Richards filed May 3, 2000; 09/617,149 to Huibers et al. filed Jul. 17, 2000; 09/631,536 to Huibers et al. filed Aug. 3, 2000; 09/626,780 to Huibers filed Jul. 27, 2000; 60/293,092 to Patel et al. filed May 22, 2001; 09/637,479 to Huibers et al. filed Aug. 11, 2000; and 60/231,041 to Huibers filed Sep. 8, 2000. If the MEMS device is a mirror, the particular mirror shapes disclosed in U.S. patent application Ser. No. 09/732,445 to Ilkov et al. filed Dec. 7, 2000 could be used. Also, the MEMS device need not be a micromirror, but could instead be any MEMS device, including those disclosed in the above applications and in application 60/240,552 to Huibers filed Dec. 13, 2000. In addition, the sacrificial materials, and methods for removing them, could be those disclosed in U.S. patent application 60/298,529 to Reid et al. filed Jun. 15, 2001. Lastly, assembly and packaging of the MEMS device could be such as disclosed in U.S. patent application 60/276,222 filed Mar. 15, 2001. Each of these patents and applications is incorporated herein by reference.
The invention has been described in terms of specific embodiments. Nevertheless, persons familiar with the field will appreciate that many variations exist in light of the embodiments described herein.