Abstract:
An anti stiction structure for cantilever formation technique. In one embodiment, the polymer cantilever is prevented from sticking to the substrate by at amortized stick layer on the substrate during formation that is later removed as a sacrificial layer. In another embodiment, the cantilever includes downwardly extending legs.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority from provisional application No. 60/235,384, filed Sep. 25, 2000. 
     
    
     
       BACKGROUND OF INVENTION  
         [0002]    Integrated circuit materials and processing technologies enable forming multiple different kinds of MEMS sensors and actuators. MEMS fabrication may leverage off established processing technologies which are used to form semiconductor materials and structures.  
           [0003]    Importantly, this may also allow integration of micromechanical devices on the same chip that holds the electronics.  
           [0004]    Polymer materials may be used as part of the MEMS structural materials, to obtain certain advantages. Biocompatibility of certain polymers may allow use of such structures in the biotechnology industry. Such polymers may also be formed at lower temperatures then other semiconductor devices, enabling operation over a lower temperature range.  
           [0005]    In a polymer surface micromachined process, a specified polymer, such as Parylene, may be used as a structural layer. A sacrificial photoresist layer may hold Parylene in its desired location. Acetone may be used as a releasing agent. Although Parylene structures have been fabricated in this way, it may be difficult to form freestanding devices at a sufficiently small-scale.  
         SUMMARY OF INVENTION  
         [0006]    The present invention teaches anti stick technology to be used in MEMS formation. The anti stick technology may prevent a polymer cantilever from sticking to the substrate. In one embodiment, the anti stick technology includes a sacrificial layer that prevents sticking. In another embodiment, either one or a number of legs extend from the polymer cantilever to the substrate and are freed after formation. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0007]    These and other aspects will now be described in detail with reference to the accompanying drawings, wherein:  
         [0008]    FIGS.  1 A- 1 C show an embodiment using legs to prevents sticking;  
         [0009]    FIGS.  2 A- 2 C show using a special sacrificial anti stick layer; and  
         [0010]    FIGS.  3 A- 3 C show a composite embodiment. 
     
    
     DETAILED DESCRIPTION  
       [0011]    The present inventors have found that the polymers used in this kind of process, such as Parylene, have a relatively small Young&#39;s modulus, e.g., around 4 Gpa. These materials may be fairly soft and pliable. After the sacrificial photoresist has dissolved, the surface tension of the acetone material may pull the relatively soft and pliable structural layer down toward the substrate. This may cause a so called stiction effect, in which the formed structure may stick to the substrate. This effect is a well-known problem in surface micromachining technology.  
         [0012]    An attempt to solve this problem has used supercritical CO drying to bypass the air-liquid interface and thus eliminate surface tension. However, the current inventors have found limited success with such structures and techniques. Theoretical calculations have predicted a limit of 84 microns for the longest cantilevered element that could be formed in this way.  
         [0013]    The present application, and specifically the embodiments disclosed herein, define anti stiction techniques, used for fabricating relatively large freestanding polymer MEMS structures. The techniques described in this application may be formed based on gas phase etching such as bromine trifluoride and xenon difluoride etching of certain sacrificial materials. The sacrificial materials used may include silicon and titanium. The disclosed bromine trifluoride and xenon difluoride etching are technologies that provide relatively high etching selectivity of some silicon and titanium as compared with other polymer materials such as the Parylene and the photoresist. Gas phase etching may produce the advantage of avoiding the air-liquid interface force which may be otherwise involved in the final releasing process.  
         [0014]    An embodiment shown in FIGS.  1 A- 1 C. The polymer structural layer, which may be Parylene, may be formed with structural posts therein to avoid the stiction effect.  
         [0015]    [0015]FIG. 1A shows an initial operation of preparing a substrate  100  which may be any material, e.g., silicon. The substrate may have first areas  110 ,  111 ,  112  formed thereon. These areas may be patterned using any desired techniques, such as using sacrificial photoresist.  
         [0016]    Photoresist areas  115 ,  116 ,  117  are formed to define the cavity area(s) underlying the cantilever  130 . In this embodiment, the separations between the cavity parts are formed as  122 ,  123 . For example, the area  122  is an area where no photoresist is formed. This may be located between two of the operative areas  115 ,  116 .  
         [0017]    The overall structure is covered with polymer layer  125 , which may be a Parylene layer. The polymer coats all of the exposed areas. This includes the areas  122  and  123 , where the polymer will actually touch the substrate  100 . Polymer also coats the photoresist areas  115 ,  116 ,  117 . FIG. 1B illustrate the result of using acetone to dissolve the photoresist. All the areas of photoresist such as  115  are removed by the acetone, leaving open spaces where the photoresist was previously located. This leaves the polymer structure  130  generally of the shape of an expanding cantilever having structural posts, effectively forming posts  131 ,  132  at specified locations along the length.  
         [0018]    During the sacrificial photoresist dissolution, these legs act as posts, holding the polymer structure above the substrate, and preventing that structure from sticking to the substrate  100 .  
         [0019]    After the photoresist areas have been removed as shown in FIG. 1B, the device may be dried. A short gas phase etching using BrF 3  may then be applied, to remove substrate material  135  from areas under the posts  131 . An undercut hole edge by BrF 3  may be around 35 microns in diameter and 4.4 microns in depth. This etching frees the polymer structure and enables it to move.  
         [0020]    The final structure shown in FIG. 1C is therefore freed, with the freed areas being allowed to move freely.  
         [0021]    An alternative embodiment is shown in FIGS.  2 A- 2 C, which uses an anti stick layer. FIG. 2A shows initial operations of fabrication of the polymer structure with a composite sacrificial layer. The substrate  200  may be silicon as in previous embodiments. This substrate is covered over the area that will be under the cantilever, with an anti stiction layer  205 . The anti stiction layer may be amorphous silicon, or titanium. Either of these materials can be evaporated or sputtered at low temperatures. This anti stiction layer  205  may be a sacrificial layer. However, thick layers of this material may not be practical because of possible increased deposition time.  
         [0022]    Hence, this may use a composite sacrificial layer, with a first portion of the layer  205  being an anti stiction layer  205 , and a second portion  210  being formed of conventional photoresist.  
         [0023]    As in the above embodiments, the sacrificial layers  210 / 205  are coated with a layer of polymer  215 , which may be Parylene.  
         [0024]    In FIG. 2B, the photoresist is dissolved away with acetone, leaving the second sacrificial layer  205 . When the device is dried, the polymer structure  215  may stick down towards the substrate as shown in FIG. 2B.  
         [0025]    In FIG. 2C, a short gas phase etching using BrF 3  and XeF 3  may operate to remove the second anti stiction layer  205 . This can operate to free the polymer cantilever. FIG. 2C shows how the final polymer structure  215  may be freed from the substrate.  
         [0026]    While the above the embodiment has disclosed a composite layer of sacrificial material, it should be understood that a single layer of sacrificial material  205  may be used especially when only a thin cavity  216  under the cantilever is desired.  
         [0027]    A composite embodiment is shown in FIGS.  3 A- 3 C. In this embodiment, both technologies, that is both the posts, and the anti stick layer are combined. FIG. 3A shows fabrication of a polymer structure  300  with posts  302 . Anti stick portions are located at least in portions under the posts. As an alternative, these portions may be located under the entire cantilever area.  
         [0028]    Each post area is thus in contact with sacrificial layer  304 . This sacrificial layer can be sacrificial amorphous silicon or titanium, or some other material, as above.  
         [0029]    As in the FIG. 1 embodiment, FIG. 3B shows etching in acetone to remove the photoresist. This leaves the polymer structure  300  touching against the anti stick layer  304  at the area of the posts. In FIG. 3C, the anti stick layer  304  may be removed thus freeing the structures. The removal may use gas phase etching as described above.  
         [0030]    The above technique has been used to form many freestanding structures of polymers. The specific polymers that are used may include Parylene. It has been found that this system may allow production of cantilevers, e.g., with about 150 microns between posts. Any beams that have widths larger than 100 microns may show stiction at the edges of the beams. Therefore, a maximum distance between the center of the post to the edge of the beam may be 75 microns.  
         [0031]    Although only a few embodiments have been disclosed in detail above, other modifications are possible. All such modifications are intended to be encompassed within the following claims, in which: