Abstract:
A process for manufacturing a wafer from a layer of material such as silicon and having a multiplicity of MEMS devices such as mirrors with gimbals formed thereon is disclosed. The features of the devices on the wafer as well as the boundaries which separate individual devices are defined by lines having a constant width so as to avoid microloading effects. Waste areas of the layer of material which are greater than the constant line width are removed as breakout pieces during the release process.

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 60/342,248, filed Dec. 21, 2001, which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This unit relates generally to apparatus and methods for manufacturing MEMS (micro-electromechanical systems) by forming a multiplicity of such devices on a silicon wafer. More specifically, the invention relates to a process for manufacturing a multiplicity of very flat (e.g. having a radius of curvature greater than 40 meters) two-axis silicon elliptical mirrors used for optical switching and other light beam steering applications by minimizing the microloading effect during plasma etching. 
     BACKGROUND OF THE INVENTION 
     MEMS devices are becoming more and more available and common. However, these devices are extremely small compared to regular machines, but still very large when compared to the individual circuits or components and features found on IC&#39;s and other electronic chips. Some MEMS devices such as the digital micromirror device arrays produced by Texas Instruments are made significantly smaller than most other types of MEMS devices, but are also very large compared to components on an IC or other chips and use existing geometry and patterning techniques common for the productions of semiconductor circuits. For example, small analog MEMS devices used for optical switching of transmitted data streams may have a size in the millimeter range, whereas the mirrors on micromirror arrays used for display devices are typically between about 15-20 microns on a side. Thus, it is seen that MEMS devices are not comfortably compared with either full-size machines or devices (they are much smaller) or a true array of micro devices such as IC&#39;s, memory chips, and the like (they are much larger). 
     Known processes for fabricating MEMS type mirrors include micro-machining polysilicon, using SOI (silicon on insulator) wafers. Mirrors produced by this process however, are typically very thin and consequently maintaining the desired flatness is very difficult. 
     SUMMARY OF THE INVENTION 
     The present invention provides a process for manufacturing a plurality of MEMS devices on a first layer of material, such as for example, a thin wafer of silicon typically having a thickness of about 115 μm. The process comprises attaching the thin silicon wafer to a carrier or backing wafer and then defining boundaries and features for each individual device of said plurality of devices with a constant line width. 
     After defining or placing the lines which define the features of the individual devices and the boundary or separation lines between individual devices, the wafer while attached to the backing wafer is etched such that both the separation lines and lines defining features on the device are etched through the selected thickness. The use of a constant line width helps eliminate over-etching or under-etching due to the phenomenon called microloading. Microloading is the differential etch rate between wide lines and narrow lines (wide lines etch faster) in a plasma reactor. 
     The devices are then separated from the backing layer. It should also be noted that the wafer used to manufacture the mirrors or the devices could be silicon or another suitable material. Further, the wafer may also undergo other processing steps before and after the etching process. For example, electronics, sensors or other mechanical features can be created by standard IC or MEMS fabrication processes before the etching step, and a reflective coating such as gold may be applied before the devices are separated from the backing layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned features as well as other features of the present invention will be more clearly understood from consideration of the following description in connection with the accompanying drawings in which: 
     FIGS. 1A through 1H illustrate the various steps of the manufacturing process of this invention; 
     FIG. 2A illustrates a wafer such as silicon having a multiplicity of mirrors; 
     FIG. 2B shows a top view and a side view of a fixture for catching the individual mirror devices upon release; 
     FIG. 2C is a cross-section of the apparatus used to release the silicon devices from the backing wafer used by the manufacturing process of this invention; 
     FIG. 2D is an enlarged view of a pocket on the fixture of FIG.  2 B and also shows an individual mirror device caught by the fixture of FIG. 2B; 
     FIGS. 3A and 3B illustrate a method of removing wafer waste areas greater than about 50 μm; and 
     FIGS. 4A and 4B illustrate how device features having a separation less than a narrow line width may be formed by a first method. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention discloses a process for manufacturing a two-axis analog micromirror MEMS device fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 115 μm. The die layout consists of an oval mirror, normally 3.8 mm×3.2 mm supported on a gimbal frame by two silicon torsional hinges. The gimbal frame is attached to the die frame by another orthogonal set of torsional hinges. According to a preferred embodiment, the mirror die (i.e. each individual device) is fabricated by using a deep resistive ion etch (DRIE) of the 115 μm thick silicon wafer in a specialized ICP (Inductively Coupled Plasma) reactor. 
     The process flow for manufacturing two-axis analog mirror devices wherein the individual dies, elements or devices are diced or separated by the same through the wafer etch that forms the features of the mirror is illustrated in FIGS. 1A-1H. As shown in FIG. 1A, a 115 μm thick wafer  10  is bonded to a carrier or backing wafer  12  (see FIG.  1 B). Optional alignment marks  14  may then be etched into the thin wafer material or other suitable material using a resist layer  16  along with photolithography and plasma etching as shown in FIG.  1 C. After the plasma etch, the resist  16  used to form the optional alignment marks  14  is then stripped as shown in FIG.  1 D. The features of the micromirror or MEMS devices and the device boundaries or lines separating individual devices are then patterned with photolithography as indicated by line gaps  18  and  20  respectively patterned in a second resist layer  22  as is well known by those skilled in the art. This arrangement is shown in FIG.  1 E. It will also be appreciated that wafer  10  may also undergo various other semiconductor processing steps before a second etching step discussed below. For example, electronic components, sensors or additional mechanical features can be created on wafer  10  by various IC or MEMS fabrication processes. 
     As shown in FIG. 1F, and as mentioned above, the mirror features and the boundaries between devices formed by gap or line patterns  18  and  20  respectively are then etched completely through the wafer  10  as indicated by reference numbers  24  and  26 . Although other etching methods may be acceptable, a particularly effective etching process is the use of a special ICP reactor along with the Bosch process described in U.S. Pat. No. 5,498,312. It is important to note at this point that according to the method of manufacturing of this invention, at the same time the mirror features as indicated at etch line  24  are etched completely through the wafer  10 , the line patterns or etches such as indicated at etch  26  used to separate the individual dies or mirrors are also etched completely through the wafer  10 . After the etching process, the second photo resist layer  22  is stripped away, and as shown in FIG. 1G, the multiplicity of devices formed in wafer  10 , and still bonded to the carrier or backing wafer  12  are given a reflective coat  28 , such as gold for example. Finally, the mirror die or individual mirrors are released from the carrier wafer  12  as shown in FIG.  1 H. This is accomplished by placing the combination carrier or backing wafer  12  and the devices formed in wafer  10  in a solvent bath to dissolve the agent that was used to bond the carrier wafer  12  and wafer  10  together. The bonding agent is typically a photo resist. Therefore, according to one embodiment, the solvent for separating the backing wafer  12  from the wafer  10  is acetone. 
     Referring now to FIGS. 2A,  2 B,  2 C and  2 D, there is shown the wafer  10  with the individual mirrors etched therein, a fixture for catching the individual dies or mirrors after they are released from the backing or carrier wafer  12  (top and side view shown in FIG.  2 B), and a cross-sectional view of the solvent bath with the wafer  10  and fixture of  2 B in place as used during the release process (FIG.  2 C). As shown in FIG. 2A, the embodiment illustrates 178 individual mirrors or dies etched into the wafer  10 . Likewise, fixture  30  shows an equal number or 178 pockets such as pocket  32  more clearly seen in the broken out or blown up illustration of FIG. 2D, which catch the individual mirrors or dies, such as mirror and gimbal structure  34 , after they are released. The mirrors formed in wafer  10  are aligned on the fixture  30  of FIG. 2B so that each individual mirror is over a pocket  32  that catches the mirror after release. As shown in FIG. 2C, the mirrors formed in wafer  10  are loaded upside down in the fixture so that gravity will pull the individual mirrors down into an aligned pocket  32  as they are released from the carrier wafer  12 . 
     Once the mirrors formed in wafer  10  are properly located and aligned over fixture  30 , the combination wafer structure and fixture  30  are placed in tank  36  having a solvent  38 , such as acetone, for a time period sufficient to dissolve the bonding agent. As an example, if the bonding agent is a photo resist and the solvent is acetone, then a soak of about two hours should provide a clean release. The fixture with the released mirrors is then moved to a hot IPA (Isopropyl Alcohol) bath of about 50° C. for about five minutes. The fixture  30  and mirrors are then removed from the IPA bath and air dried. 
     Since the individual mirrors are very difficult to ship (if shipping is to occur), the individual dies because they are fragile and cannot readily be shipped using the accepted methods for shipping electronic die or chips, such as gel-track trays or chip trays, the bonded combination of devices formed in wafer  10  and backing wafer  12  is shipped as a unit. Thus, it may also be appropriate to transfer the release process to the assembly vendor. This means that yield data on the mirror dies may not be available until final testing of the assembled mirrors. 
     The present invention relates to individual mirror devices formed on a wafer using processing steps some of which have similarity to steps used in manufacturing IC&#39;s and other semiconductor devices. Referring again to the process discussed with respect to FIGS. 1A through 1H, it is noted that the described process follows “mask” guidelines which requires all features on the wafer, or each individual device, to be created by etching trenches having a constant width, such as for example only, 10 μm. This rule or guideline is followed because of “microloading” which occurs with plasma etching. As discussed above, microloading results because lines of different widths etch at different rates, and more specifically, “wide” lines etch at a faster rate than “thin” lines. Thus, to provide consistency in etching of features, a standard rule is that all lines including features and separating lines are to be etched by lines having a constant width (such as for example 10 μm). Consequently, as shown in FIGS. 3A and 3B, if an area  40  (FIG.  3 B), that is larger than 10 μm is to be removed, the process of FIGS. 1A through 1H will require etching 10 μm lines  42  around the area to be removed so as to leave a break-away area or portion  44  as shown in FIG.  3 A. For example in FIGS. 3A and 3B, the area to be removed is 50 μm. This break-away area or portion  44  is then removed after the release process. The break-away area or portion  44  will typically simply fall away after the individual dies or mirrors are removed from the backing wafer  12 . As mentioned above, care should be taken to assure the break-away areas or portions  44  are completely removed and do not stick to one of the mirror devices as this would cause a failure. FIG. 4A illustrates the gimbal support structure  46  and a mirror  48  attached to the gimbal support structure  46  by a torsion hinge  50 , as well as a blow up view of an alignment stop  52  (there may be more than one stop) between structure  46  and mirror  48  as formed by this process. This illustration shows how all etch lines may be limited to a minimum of 10 μm, yet some parts of the structure may be divided by a spacing less than 10 μm. 
     It will also be appreciated by those skilled in the art that although silicon is often preferred for such processes, other suitable materials such as, but not limited to, gallium arsenide, quartz and silicon carbide may also be used. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.