Abstract:
A micromachined membrane particle filter is formed by making holes in a silicon and coating over the holes with Parylene.

Description:
This application is a divisional of U.S. application Ser. No. 09/187,475, filed Nov. 6, 1998 (pending). 
     This application claims the benefit of the U.S. Provisional Application No. 60/064,807, filed on Nov. 7, 1997, which is incorporated herein by reference. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
     The U.S. Government may have certain rights in this invention pursuant to Grant No. N66001-96-C-8632 awarded by the U.S. Navy. 
    
    
     BACKGROUND 
     A mechanical filter can be used to remove, filter, or collect particles. This filtering and collection of particles can be used for sampling of particles, chemical detection, and/or biological cell analysis. 
     It is known to make such filters using micromachining techniques to form small features in a silicon wafer. For example, this has been described in C. J. M. Van Rijin and M. C. Elwenspoek, “Micro Filtration Membrane Sieve with Silicon Micro Machining for Industrial and Biomedical Applications,” Proceedings of IEEE Micro Electro Mechanical Systems Workshop (MEMS&#39;95), pp. 83-87, 1995 and G. Kittilsland and G. Stemme, “A Submicron Particle Filter in Silicon,” Sensors and Actuators, A: Physical, Vol. 23, pp. 904-907, 1990. However, the present disclosure describes a different way of developing filters which has certain improved characteristics. 
     One problem with prior micromachined filters is their overall strength. 
     SUMMARY 
     A membrane particle filter is described which uses micromachining technologies. The filters are fabricated using a substrate membrane that is perforated with holes. The holes can have different shapes, different dimensions, and different opening factors. Preferred shapes include circular, hexagonal, and rectangular, with dimensions ranging from 6-13 μm. 
     In a preferred mode, a layer of Parylene material is uniformly coated on the filters and on the inner surfaces of the holes in order to increase the overall strength of the filter. 
     Another important feature is the amount of power which is necessary to provide the desired pressure drop across the filter. Proper control of the opening size allows determining various tradeoffs, including the energy and power necessary to form the desired pressure drop. Another feature of this disclosure is that the sizes of the openings can be more specifically controlled by deposition of Parylene material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will now be described in detail with reference to the accompanying drawings, wherein: 
     FIGS. 1A-1F show a membrane filter fabrication process; 
     FIG. 2 shows different results of different filtering regions; 
     FIGS. 3A-3D show various fabricated membrane filters and their openings; and 
     FIGS. 4 and 5 show different pressure drops as functions of the flow rate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The fabrication process for the preferred mode filter is shown in FIGS. 1A-1F. The process begins in FIG. 1A by depositing a layer of low stress LPCVD silicon nitride  100  and  102  (“SiN”) at 850° C. on a silicon substrate  104 . The layer  100  is preferably between a ½ and 1 μm micron thick. The material is deposited with an SIH 2 Cl 2 :.NH 3  gas flow ratio of 4 to 1. The SiN layers  100 ,  102  are deposited on both sides of silicon substrate  104 . 
     FIG. 1B shows patterning the SiN layer  102  on the backside of the silicon substrate  104 . First, the desired areas to be protected are covered with photoresist layer  110 . This is followed by dry etching the SiN  102  to form a pattern. 
     FIG. 1C shows the anisotropic etching step which uses an anisotropic etchant such as KOH. This leaves a window  120  left in the wafer, with only a thin area of the silicon wafer  122  remaining. The thin wafer area can be between 20 and 100 μm thick. The overlying silicon nitride  100  remains unchanged during this step. 
     FIG. 1D shows using RIE to provide patterns  130  in the SiN layer  100 . The pattern includes holes  132  into the silicon nitride layer  100 . The holes can be of any desired size and shape as described above. 
     This is followed by placing the wafer into another anisotropic solution (for example, KOH) to remove the remaining silicon layer  120 . This frees the membrane to form the unsupported membrane shown in FIG.  1 E. The membrane includes unsupported silicon nitride portion  140 , defining holes therebetween. These holes, however, would have low structural integrity. 
     Finally, a layer of Parylene is deposited over the entire wafer. Parylene is available from Specialty Coating Systems, Inc., 5707 West Minnesota Street, Indianapolis, Ind. 46241. The Parylene uniformly covers the entire wafer surface, forming Parylene overlayers  150 ,  152 . Each of the holes therefore includes a Parylene overlayer on each of its surfaces: top surface, bottom surface, and all sides. 
     The layer of Parylene  150  serves three main purposes. The Parylene overlayer greatly improves the strength of the membrane filter by providing a reinforcement to the filter. Uniform deposition of Parylene also allows changing the hole size. Different hole sizes can be obtained from the same basic filter skeleton. Control of the thickness of the Parylene layer can be used to obtain these different hole sizes. For example, a 10 μm opening can be changed to a 6 μm opening by depositing 2 μm of Parylene on the entire device, forming two, 2 μm barriers at two ends of the hole. Hence, the same basic filter can have different holes sizes by changing the thickness of the Parylene layer. 
     Parylene is biocompatible, making the filter suitable for biological applications. 
     This fabrication process can be used for various membrane filters. The preferred hole shapes include circular, hexagonal, and/or rectangular. Filters as large as 8×8 square millimeters can be fabricated. The opening area ratio increases as the hole size increases. The hole size also defines the filtering threshold—the minimum size of the particles that can be blocked by the filter. 
     For example, a filter with a 10.6 μm diameter hole has an opening area ratio of approximately 12½%. Hexagonal holes can provide higher opening area ratios, but cause higher stress concentration in the membrane. This effectively reduces the strength of the filter. Rectangular holes can provide a large range of opening area ratio without changing the filtering threshold. One dimension of the rectangular holes must be kept constant. 
     FIG. 3A shows a top view of a circular hole. FIG. 3B shows the rectangular hole, while FIGS. 3C and 3D respectively show the hexagonal holes for these filters. 
     FIG. 2 shows different characteristics for the filters. The far left side of the figure shows a top view of 8 millimeter×8 millimeter area. This is defined into different non-filtering regions and filtering regions. 
     TABLE 1 shows how the final Parylene coating layer increases the strength of the filters. Burst pressure of various membrane filters was tested using differential pneumatic pressure across the filter membrane. The first pressure of filter  6  in TABLE 1 was more than 4 times higher when coated with 2.69 microns of Parylene. 
     
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Filter Strength 
               
               
                 (Filter #6, β = 43.6%) 
               
             
          
           
               
                   
                 Parylene Thickness (μm) 
                 Burst Pressure (PSI) 
               
               
                   
                   
               
             
          
           
               
                   
                 0 
                 0.9 
               
               
                   
                 1.38 
                 1.9 
               
               
                   
                 2.69 
                 4.2 
               
               
                   
                   
               
             
          
         
       
     
     The fluid dynamic performance of the membrane filters was also tested, and the results are shown in FIGS. 4 and 5. Each of the pressure drops is a function of flow rate per unit area was fitted with a second order polynomial function. The power to sustain a desired flow rate is calculated by multiplying the pressure drop by the volumetric flow rate. Hence, these features show how the pressure drop across the microfilter and power requirement increase dramatically as the opening factor increases. 
     Although only a few embodiments have been disclosed in detail above, many modifications are possible in the preferred embodiment without undue experimentation.