Abstract:
The invention provides a microfabrication process which may be used to manufacture a MEMS device. The process comprises depositing one or a stack of layers on a base layer, said one layer or an uppermost layer in said stack of layers being a sacrificial layer; patterning said one or a stack of layers to provide at least one aperture therethrough through which said base layer is exposed; depositing a photosensitive layer over said one or a stack of layers; and passing light through said at least one aperture to expose said photosensitive layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/941,042, filed Sep. 14, 2004, now published as U.S. Patent Publication No. 2005/0142684, which is a continuation of U.S. patent application Ser. No. 10/074,562, filed Feb. 12, 2002, now issued as U.S. Pat. No. 6,794,119, the disclosures of each of which are hereby incorporated by reference in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to microfabrication. In particular, it relates to the microfabrication of a structure for a Microelectromechanical Systems (MEMS) device.  
       BACKGROUND  
       [0003]     Microfabrication techniques used to fabricate MEMS devices generally involve the deposition of one or more layers on a substrate and the subsequent patterning of the layers to produce useful structures. One technique for patterning a layer involves the use of photolithography. With photolithography a photographic definition of a desired pattern on a photo or optical mask is used to impart the pattern onto a surface of the layer. When manufacturing a MEMS device usually several masking steps are required, each masking step adding to the cost of the device. Accordingly, it is desirable to reduce the number of masking steps required during fabrication of a MEMS device.  
       SUMMARY OF THE INVENTION  
       [0004]     According to one aspect of the invention there is provided a microfabrication process comprising depositing a first layer on a substrate; patterning the first layer; depositing a second layer on the first layer; and patterning the second layer using the first layer as a photomask.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  shows a three dimensional drawing of a part of a MEMS device which may be manufactured using the microfabrication process of the present invention; and  
         [0006]     FIGS.  2  to  10  show various stages in the manufacture of the MEMS device of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0007]     Aspects of the present invention will now be described with reference to FIGS.  2  to  10  of the drawings which show the stages during fabrication of a MEMS device such as a Visible Spectrum Modulator Array described in U.S. Pat. No. 5,835,255 or an Interferometric Modulater (IMOD) described in U.S. Pat. No. 6,040,937. Naturally, describing the present invention with reference to stages in the manufacture of a Visible Spectrum Modulator Array or an IMOD is intended to enhance understanding of the present invention and it is to be understood that the present invention may used in the manufacture of other MEMS devices. Thus, the description of the present invention with reference to the manufacture of a Visible Spectrum Modulator Array or an IMOD is intended to be non-limiting.  
         [0008]      FIG. 1  of the drawings shows an example of a part of a Visible Spectrum Modulator Array  10  which may be fabricated in accordance with techniques described herein. Referring to  FIG. 1 , an antenna array is fabricated on one-half of a microfabricated interferometric cavity which transmits and reflects certain portions of incident electromagnetic radiation depending on (a) the dimensions of the cavity itself and (b) the frequency of response of dielectric mirrors in the cavities. In  FIG. 1 , the array  10  is shown to include two cavities  12 ,  14  fabricated on a transparent substrate  16 . A layer  18 , the primary mirror/conductor may comprise a combination of one or more films of metals, oxides, semiconductors, and transparent conductors. Insulating supports  20  hold up a second transparent conducting membrane  22 . Each array element has an antenna array  24  formed on the membrane  22 . The two structures  22 ,  24 , together comprise the secondary mirror/conductor. Conversely, the antenna array may be fabricated as part of the primary mirror/conductor. Secondary, mirror/conductor  22 / 24  forms a flexible membrane, fabricated such that it is under tensile stress and thus parallel to the substrate, in an undriven state.  
         [0009]     Because layers  22  and  24  are parallel, radiation which enters any of the cavities from above or below the array can undergo multiple reflections within the cavity, resulting in optical interference. Depending on the dimensions of the antenna array, the interference will determine its reflective and/or transmissive characteristics. Changing one of the dimensions, in this case the cavity height (i.e. the spacing between the inner walls of layers  18 ,  22 ), will alter the optical characteristics. The change in height is achieved by applying a voltage across the two layers of the cavity, which due to electrostatic forces, causes layer  22  to collapse. Cavity  12  is shown collapsed (7 volts applied), while cavity  14  is shown uncollapsed (0 volts applied).  
         [0010]     In fabricating the array  10 , it is desirable that insulating supports  20  are well defined in regions where contact is made between insulating supports  20  and layers  18 ,  22 . The present invention is especially useful in manufacturing such a support.  FIGS. 2 through 10  show various stages in the manufacture of a MEMS device having supports such as the supports  20 . Referring to  FIG. 2  of the drawings, reference numeral  100  indicates a substrate  100 . The substrate  100  may be of many different materials each being transparent to ultraviolet light. Examples of these materials include plastic, mylar, or quartz. The material must be able to support an optically smooth, though not necessarily flat, finish. A preferred material would likely be glass, which would be both transmissive and reflective operation in the visible range.  
         [0011]     Various layers are deposited on substrate  100  to define a stack. In particular, the substrate  100  is coated with a sacrificial layer  102  using standard techniques such as a physical vapor deposition (PVD) method such as sputtering or e-beam evaporation. Other possible methods include chemical vapor deposition and molecular beam epitaxy.  
         [0012]     In  FIG. 2 , the sacrificial layer is a single layer. However, in other embodiments of the invention layer  102  may be a stack of layers with an uppermost sacrificial layer.  
         [0013]      FIG. 3  of the drawings shows a stage in the manufacture of the MEMS device wherein the sacrificial layer  102  has been patterned so as to define longitudinally spaced grooves  104 . A standard procedure is used to pattern sacrificial layer  102  which includes exposing layer  102  through an appropriate mask and developing to produce the pattern.  
         [0014]     In  FIG. 4  of the drawings, a photosensitive polymeric material in the form of a negative-acting-photosensitive material which could be a negative photoresist has been spun onto sacrificial layer  102  with a thickness that is larger than the total height of the film stack defined by layers  100  and  102 . Thereafter, the negative-acting-photosensitive material is exposed to ultraviolet light through substrate  100  and developed using conventional techniques. Because the longitudinal grooves  104  are the only means by which the negative-acting-photosensitive material is exposed, the negative-acting-photosensitive material over the stack is dissolved during a subsequent development process, leaving only longitudinal ridges  106  of negative-acting-photosensitive material disposed in grooves  104 . Thus, it will be appreciated that by first patterning the sacrificial layer  102  and then exposing the negative-acting-photosensitive material through substrate  100  through longitudinal grooves  104  in the sacrificial layer  102 , the sacrificial layer  102  acts as a photomask thereby allowing the negative-acting-photosensitive material to be lithographically patterned without the need for an additional masking step. In  FIG. 5  of the drawings, a structural layer  108  has been deposited on the stack and the sacrificial layer  102  has been removed, thus the layer  108  is supported by ridges  106 . It will be appreciated that by using different photomasks it will be possible to fabricate support structures of any desired geometry. Thus instead of ridges, in other embodiments pillars or posts may be formed. The layer  108  is highly conductive and reflective and will typically contain aluminum and nickel.  
         [0015]      FIG. 6  of the drawings shows a subsequent stage in the manufacture of the MEMS device wherein the layer  108  is patterned into transversely extending strips.  FIG. 7  of the drawings shows the film stack with an oxide spacer layer  110  deposited on layer  108 .  FIG. 8  of the drawings shows a stage in the manufacture of the MEMS device in which the oxide spacer layer  110  has been patterned.  FIG. 9  of the drawings shows a stage in the manufacture of the MEMS device in which a sealing film  112  is being applied with a pressure adhesive  114  over the entire structure to protect the structure from damage due to mechanical shock loading and to prevent particulates from interfering with the operation of the IMOD structures. The sealing film  112  could be of a variety of materials such as thin metal films or polymeric films which have been coated with a metal or oxide film to provide hermeticity. Finally,  FIG. 10  shows the structure after it has been purged with XeF 2  gas to remove the remains of sacrificial layer  102 . The edges of the structure are then sealed.  
         [0016]     In other embodiments, instead of oxide layer  110  another layer of negative-acting-photosensitive material may be spun over oxide layer  110  and exposed through substrate  100  and using the techniques described above a further set of support structures may be formed. These support structures will provide support for other layers. It will be appreciated that the process may be repeated to build a MEMS structure having multiple layers or “floors” stacked one on top of the other wherein the floors are vertically spaced by support structures fabricated in accordance with the above techniques. One advantage of the present invention is that it provides a microfabrication technique which allows a mechanical support between two layers in an MEMS device to be precisely defined. This allows a clean, well-defined mechanical contact between the support and other structures within the MEMS device.  
         [0017]     Further, the present invention uses a patterned layer on a substrate as a photomask to pattern another layer, thereby saving on a masking step.  
         [0018]     Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that the various modification and changes can be made to these embodiments without departing from the broader spirit of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than in a restrictive sense.