Abstract:
A laser-based drilling technique provides a microporous filter having very small holes with known diameters and locations. One embodiment of the technique entails using a laser beam with one or more uniform spot sizes to form each hole. The laser beam ablates material depthwise for corresponding known distances into a substrate to form a desired number of hole steps in each hole. Another embodiment of the technique entails using an imprint patterning toolfoil to stamp in the substrate depressions of specified diameters and distances that correspond to the hole steps. In both embodiments, a laser beam of Gaussian shape removes the last portion of material to form a very small diameter final hole step.

Description:
RELATED APPLICATIONS  
       [0001]     This application is a division of U.S. patent application Ser. No. 10/931,440, filed Aug. 31, 2004, which claims benefit of U.S. Provisional Patent Application Ser. No. 60/512,007, filed Oct. 15, 2003, and U.S. Provisional Patent Application Ser. No. 60/542,626, filed Feb. 6, 2004. 
     
    
     TECHNICAL FIELD  
       [0002]     This disclosure relates to microporous filters and, in particular, to a microporous filter having small diameter holes of reliable sizes and in known locations.  
       BACKGROUND INFORMATION  
       [0003]     Microporous filters are currently made of inherently slightly porous materials such as woven cotton fibers, paper, and woven synthetic fabric. Such filters find applications in the manufacture of pharmaceutical drugs; in industrial fuel cells; and in separating body fluids, chemical particles, and different materials for analysis. The sizes and locations of the holes forming the filter pores vary with the filter material structure.  
         [0004]     What is needed is a microporous filter formed of very small, predictable diameter holes placed in known locations and therefore arranged in a known population density.  
       SUMMARY OF THE DISCLOSURE  
       [0005]     Preferred embodiments of a laser-based drilling technique entail forming in a substrate an array of stepped holes, each of which having a very small, predictable final diameter in a known location. The array includes a final hole step, which is formed by a laser of an ultraviolet (UV) wavelength, which is shorter than 400 nm. The remaining hole step or steps of the array are formed by use of a laser or an imprint patterning technique. The final hole step diameter and population density of the holes define the porosity of the microporous filter formed from the membrane.  
         [0006]     In a first preferred embodiment, a UV laser emitting either 355 nm or 266 nm light ablates material from, to form a hole through, a polymer-based, flexible membrane, such as polyimide, polycarbonate, or polytetrafluoroethylene (PTFE). The UV laser ablates and therefore breaks the chemical bonds of the organic material to form holes of final or exit diameters of between about 1.0 μm and about 5.0 μm in a membrane material of between about 50 μm and about 250 μm in thickness. (This compares to 20 μm-100 μm holes formed in 200 μm thick organic packaging materials.) The holes are formed in steps of decreasing diameters depthwise through the thickness of the membrane to give a desired aspect ratio to reduce plasma and debris effects that would inhibit or prevent formation of a large aspect ratio, small diameter hole. A large aspect ratio hole is one in which the ratio of its length to width is greater than 5:1. This technique is accomplished by changing the spot size of the laser beam as it ablates the target material depthwise and allows the escape of plasma gases and debris produced during the ablation process. Gases and debris trapped at the bottom of a large aspect ratio hole interferes with the process of drilling a small diameter final hole step.  
         [0007]     Stepped holes are advantageous because they cause a reduced drop in pressure that enables passage of material of the desired size through the final, smallest diameter hole.  
         [0008]     In a second preferred embodiment, an imprint patterning toolfoil, which is a sheet of metal with an array of protruding features, is pushed into the flexible membrane to form in it an array of depressions. The UV laser forms the final hole step through the bottom of each of multiple depressions in the array. Imprint patterning opens up the region around the intended hole location and thereby permits the escape of gases and debris. This allows the formation of a small aspect ratio final hole step.  
         [0009]     The central axes of the stepped holes need not be perpendicular to the upper and lower major surfaces of the membrane. Angled holes may be advantageous to enable filtering particles composed of helical molecular structures of different rotational senses.  
         [0010]     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is an enlarged fragmentary cross sectional view of a microporous filter having a stepped hole formed with its central axis disposed perpendicular to the upper and lower major surfaces of a flexible polymeric membrane.  
         [0012]      FIG. 2  is an enlarged fragmentary cross sectional view of an alternative microporous filter having a stepped hole formed with its central axis inclined at a nonperpendicular tilt angle relative to the upper and lower major surfaces of a flexible polymeric membrane.  
         [0013]      FIGS. 3 and 4  are enlarged fragmentary views of toolfoils containing patterns of cylindrical protrusions having, respectively, uniform diameters and lengthwise sections of different diameters. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]      FIG. 1  shows a cross sectional view of a microporous filter  10  formed of a flexible polymeric membrane  12  having an upper major surface  14  and a lower major surface  16  that are generally parallel and define between them a membrane thickness  18 . Polymeric membrane  12  is preferably formed of polyimide, polycarbonate, PTFE, or other organic membrane material. The porosity of filter  10  is accomplished by formation of a number of stepped holes  30  (only one hole shown in  FIG. 1 ) passing in a depthwise direction through membrane thickness  18  to form the filter pores. Preferred embodiments of filter  10  are fabricated with holes  30  formed with two or more hole steps. The following is a description of a preferred hole  30  formed with three hole steps of progressively decreasing sizes, i.e., cross sectional areas measured parallel to upper and lower major surfaces  14  and  16 . Because in preferred embodiments holes  30  can be of either circular or elliptical shape in cross section, for the sake of convenience, a hole size is referred to herein by its major axis dimension.  
         [0015]     Preferred hole  30  has an overall length of about 100 μm, which is defined by membrane thickness  18 . A typical membrane thickness  18  and therefore hole length ranges between 50 μm and 250 μm. Hole  30  is formed with an entrance hole step  32  having a width  34  of about 40 μm and a depth  36  of about 70 μm, an intermediate hole step  38  having a width  40  of about 15 μm and a depth  42  of about 25 μm, and an exit hole step  44  having a width  46  of between about 1 μm and about 5 μm and a depth  48  of about 5 μm. Hole  30  has a central axis  50  to which hole steps  32  and  38  need not be axially aligned, depending on their respective widths  34  and  40  and concomitant need to span width  46  of hole step  44 .  
         [0016]      FIG. 2  shows two angled holes  30 ′, which are the same as hole  30  with the exception that the central axes  50 ′ of holes  30 ′ are inclined at nonperpendicular angles relative to upper and lower major surfaces  14  and  16 .  
         [0017]     The use of a laser beam is a first preferred method of forming holes  30 .  FIG. 1  shows a laser  60  emitting a beam  62  that propagates along a propagation path that is collinear with central axis  50 . Laser  60  preferably emits ultraviolet (UV) light, which represents light of wavelengths shorter than 400 nm, with 355 nm and 266 nm being preferred. A programmable lens system (not shown) optically associated with laser  60  accomplishes setting the spot size of beam  62  to establish the major axis dimensions of hole steps  32 ,  38 , and  44 . A power level controller (not shown) adjusts the power of beam  62  to a level that is appropriate to the sizes of the hole steps being formed, the power used to form hole step  38  being less than that used to form hole step  32 . A beam  62  of uniform shape is preferably used to form hole steps  32  and  38 , and a beam  62  of Gaussian shape is preferably used to form hole step  44 .  
         [0018]     The capability of providing beam  62  of the desired shapes, spot sizes, and power levels to form hole  30  exists in currently available equipment. For example, hole steps  32  and  38  can be formed by a laser beam produced by a Model 5330 Via Drilling System, and hole step  44  can be formed by a laser beam produced by a Model 4420 Micromachining System, both of which are manufactured by Electro Scientific Industries, Inc., Portland, Oreg., which is the assignee of this patent application. The Model 5330 produces a UV laser beam of uniform shape, and the Model 4420 produces a UV laser beam of Gaussian shape with a very small spot size.  
       EXAMPLE  
       [0019]     An array of through holes, each of which having two hole steps, was formed in a 200 μm thick polycarbonate membrane as follows. A 355 nm laser output propagating through a 2× beam expander formed for each hole in the polycarbonate membrane a circular first hole step having a 50 μm diameter and a 180 μm-190 μm depth. The laser beam had a uniform power profile with a 220 mW level at 2 kHz Q-switch rate. A workpiece positioner operating at a 60 mm/sec scan speed moved the laser beam relative to the membrane to repetitively, sequentially scan the hole locations. During the sequential scanning process, the laser beam removed from the hole locations depth-wise portions of membrane material to partly form the first hole steps. The sequential partial removal of portions of membrane material allowed the plasma gases created during the hole step drilling process to escape and thereby ensure formation of high-quality holes. Several iterations of the scanning process sequence were carried out to complete formation of the first hole steps. Skilled persons will appreciate that laser processing parameters can be selected to achieve complete formation of a hole step without return trips to a partly drilled hole step.  
         [0020]     The 355 nm laser output propagating through a 20× Gaussian lens formed through the bottom surface of the first hole step of each hole in the array an exit hole step having 5 μm diameter and a 10 μm-20 μm depth. An exit hole step was formed at each hole location by consecutive application of a pulsed laser beam to effect a hole punching operation. Ten pulses of either a 600 mW or a 950 mW Gaussian-shaped laser beam pulsed at 10 kHz formed in the array of holes exit hole steps of repeatable high quality.  
         [0021]     The use of an imprint patterning toolfoil in combination with a laser beam is a second preferred method of forming holes  30 .  FIG. 3  is an enlarged fragmentary view of a metal toolfoil  80  containing a pattern formed by a regular array of nominally identical cylindrical protrusions  82  mutually spaced apart by a predetermined distance  84 . Protrusions  82  form hole steps in membrane  12  in accordance with an imprint patterning technique. This is accomplished by positioning toolfoil  80  and membrane  12  in a conventional laminating press (not shown) and operating it to urge protrusions  82  into upper major surface  14  and thereby stamp complementary depressions in membrane  12 . Protrusions  82  are of specified diameters  86  and lengths  88  that correspond to, respectively, the major axis (diameter) dimension and depth of the hole step. In  FIG. 1 , the depressions correspond to either of hole steps  32  or hole steps  38 . Laser beam  62  of Gaussian shape is preferably used to form the exit hole step, such as hole step  44  in  FIG. 1 .  
         [0022]     Although protrusions  82  of  FIG. 3  are of uniform diameters,  FIG. 4  shows protrusions  90  configured to have lengthwise sections of different major axis dimensions or diameters can be used to form in one laminating cycle multiple hole steps in each hole of membrane  12 . Because multiple stepped holes of decreasing major axis dimensions are used in part to prevent plasma effects stemming from use of laser  60 , the use of imprint patterning eliminates the need for multiple-step depression or hole formation before laser ablation of the exit hole step.  
         [0023]     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. For example, polymeric membrane  12  can be composed of two laminated sheets in which an upper sheet is perforated with larger diameter hole steps and a lower sheet is perforated with smaller diameter, laser-drilled exit hole steps. The scope of the present invention should, therefore, be determined only by the following claims.