Abstract:
A method for forming a screen suitable for screen printing a pattern of small closely spaced features onto a substrate is provided. The method includes the steps of providing a fine mesh screen and then forming a patterning layer on the mesh using a photosensitive emulsion. A mask or phototool is used to pattern the patterning layer. During the patterning process, open areas of the mask are aligned with the openings on the mesh using a microscope or other vision device to align the mask with the mesh. During a screen printing process, the open areas of the patterning layer will thus not be obstructed or split into smaller openings by the screen wires. The patterning layer can be patterned using laser energy directed through the mask or using UV exposure followed by development with a suitable developer.

Description:
This invention was made with Government support under Contract No. DABT63-93-C-0025 awarded by Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to screen printing and more particularly to an improved method and apparatus for screen printing patterns with small closely spaced features. The method of the invention is particularly suited to the fabrication of screen printed electronic devices such as microelectronic components of field emission displays. 
     BACKGROUND OF THE INVENTION 
     In the electronics industry screen printing is used to make various components. For example, microelectronic packages called hybrids, or multi-chip modules, utilize circuits and electrical devices made by screen printing. Conductive, resistive and dielectric patterns of a circuit can be formed by screen printing onto a rigid substrate such as a circuit board. Screen printing is also used in the fabrication of field emission displays (FEDs) for flat panel displays. With a FED, the normal topography used to make a hybrid can be multi level due to the gap which is required between the anode and cathode components of the display. This places some additional demands on the screen printing process. 
     Screen printing for microelectronics is similar to the method used to make t-shirts and printed panels for industrial equipment but at the high end of the technology. A typical screen printing process for a multi level hybrid would be to print a conductive layer, dry the layer and fire. The substrate would then be processed with the next layer, usually a dielectric composition. After dry and fire another layer of conductor would be fired. 
     With screen printing, a screen is used to deposit a thick-film paste, or other printing material, onto a substrate (e.g., polyimide circuit board, silicon baseplate). Different techniques are used to transfer the desired pattern from a mask containing artwork to the screen. 
     To produce a screen, a stainless steel or monofilament polyester screen mesh is stretched and attached to a metal frame. A negative pattern must then be generated on the mesh so that the printing material can be forced through the screen to produce a positive pattern for the substrate. A photosensitive emulsion can be used to make the negative pattern on the screen. There are three methods for the application of the emulsion to the mesh: direct, indirect and indirect-direct. 
     In the direct emulsion method, the emulsion is initially applied to the mesh in a viscous state and then dried. After drying, the emulsion is exposed through a mask using UV light, and then developed under a water jet to form a patterning layer on the mesh. For a negative acting photosensitive emulsion, the exposed regions of the emulsion are polymerized and the mesh is sealed in these regions. Conversely, the unexposed regions of the emulsion are washed away and form open areas on the mesh. For a positive acting photosensitive emulsion, the exposed portions are removed and the unexposed portions of the emulsion are left. 
     For screen printing the pattern defined by the patterning layer onto a substrate, the substrate is secured to a support platform within a screen printer. The screen is mounted within the screen printer, parallel to the substrate but spaced apart from the substrate with a slight gap. The printing material is then applied to the screen and a squeegee (e.g., rubber blade) is moved across the screen at a constant rate. The squeegee forces the printing material through the open areas of the screen and prints the pattern defined by the patterning layer onto the substrate. 
     For printing small closely spaced features, fine mesh screens are preferred. The screen mesh count refers to the number of screen openings per linear inch. The width of a screen opening is related to the mesh count and to the diameter of the screen wire by the formula 
     
         W.sub.o= (1-DM)/M 
    
     where 
     W o  is the width of the screen opening in inches 
     D is the diameter of the screen wire in inches 
     M is the mesh count 
     By way of example a commercially available 400 mesh screen has a wire diameter of about 0.75 mil (19.05 μm). A 400 mesh screen is formed with square openings that have a width of about 1.75 mil (44.45 μm). 
     One problem with the screen printing of patterns having small closely spaced features, is that the resolution and spacing of the features of the pattern can be adversely affected by the screen wires. In particular, with a pattern having feature sizes approximately equal to the size of the openings in the mesh, the resolution and spacing of the features can be distorted by the screen wires. 
     For example, features that are about the size of the screen openings (e.g., 1.75 mils in diameter for a 400 mesh screen) require open areas in the patterning layer that are approximately the same size as the screen openings. If a 0.75 mil (19.05 μm) screen wire intersects an open area of the patterning layer, then the open area is either partially blocked or split into two smaller openings by the screen wire. For a complex pattern with many features these intersections can occur at many places. In general, the interference of the screen wires with the open areas of the patterning layer, distorts some of the features that are transferred onto the substrate. Because of this distortion, conventional screen printing processes cannot be used to successfully print feature sizes that are less than about 4 mils (101.6 μm) in size and spacing. 
     Another limitation of screens formed for screen printing occurs during the development of the photosensitive emulsion which forms the patterning layer. During a development step, the unexposed material for a negative emulsion, or the exposed material for a positive emulsion, must be cleared from the screen. Clearing out this material is complicated by the presence of the screen wires. Consequently, if the material is not completely cleared, pattern defects can occur. 
     OBJECTS OF THE INVENTION 
     Because of the above limitations in screen printing, it is an object of the present invention to provide an improved method for making screens for screen printing and an improved method of screen printing capable of printing small closely spaced features. 
     It is another object of the present invention to provide an improved method for making screens for screen printing in which a patterning layer formed on the mesh includes open areas that align with the screen openings. 
     It is yet another object of the present invention to provide an improved method for making a screen using a laser to pattern and clear a photosensitive emulsion from a mesh. 
     It is yet another object of the present invention to provide an improved method of screen printing that is low cost, simple and compatible with conventional screen printing equipment. 
     Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention an improved method for forming a screen for screen printing and an improved method for screen printing using the screen are provided. The screen includes a fine mesh (e.g., 80-500 mesh) and a patterning layer formed on the mesh. The patterning layer includes open areas that are formed in alignment with corresponding screen openings. In an illustrative embodiment, the patterning layer is formed of a photosensitive emulsion that is initially applied to the mesh and then hardened by exposure to a UV source. The photosensitive emulsion is then patterned using a laser directed through a mask. The laser ablates and clears the photosensitive emulsion from the mesh to form the open areas of the pattern. Alternately in place of laser patterning, a standard UV exposure and solvent development process can be used to expose and develop the photosensitive emulsion. 
     In either case, prior to patterning of the photosensitive emulsion, the mask is aligned with the mesh so that open areas of the patterning layer will be formed in alignment with corresponding screen openings. With the open areas of the patterning layer in alignment with the screen openings, the screen wires do not intersect the open areas. This improves the printed pattern because features defined by the open areas of the patterning layer are not blocked or split by the screen wires. Alignment of the mask and mesh can be effected using a microscope or other vision system used in conjunction with an exposure and alignment tool to manipulate the mask or mesh as required. 
     The method requires the mesh to be selected with a pitch or frequency for the screen openings that matches the pitch or frequency for the pattern features. In practice not all of the interference between the screen wires and the pattern openings can be eliminated. This is due in part to defects introduced during manufacture of the mesh. One such defect occurs in the repeatability of the screen pattern across different areas of the mesh. For example, the screen wires in one area may be evenly spaced and parallel to one another in that area but not with respect to wires in an adjoining area. The screen frequency can interfere with the pattern frequency to such an extent that a beat frequency will result causing whole areas of the pattern to be blocked. Because of this problem larger mesh sizes (e.g., 80-230) are preferred for some patterns. In addition, a simple algorithm can be used to ascertain an acceptable phase match between the mesh and pattern frequency as an aid in selecting a mesh for a particular pattern. 
     The method of the invention, broadly stated, includes the steps of: providing a mesh having screen openings of a predetermined size and pitch; selecting a pattern having features spaced apart by an integral multiple of the pitch of the mesh; forming a mask with solid areas and open areas corresponding to the pattern; depositing a photosensitive emulsion on the mesh; aligning the mask with the mesh so that the open areas of the mask align with the screen openings; hardening the photosensitive emulsion by exposure to a UV source; and then patterning the photosensitive emulsion using a laser directed through the mask. The method of the invention forms a screen that can be used for screen printing a substrate using conventional screen printing processes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a screen printing apparatus for screen printing a pattern with small closely spaced features using a screen formed in accordance with the invention; 
     FIG. 2 is a plan view of a portion of a screen formed in accordance with the invention; 
     FIG. 3 is an enlarged plan view of a portion of a mesh suitable for forming a screen in accordance with the invention; 
     FIG. 4 is an enlarged cross sectional view of the mesh taken along section line 4--4 of FIG. 3; 
     FIG. 5 is a schematic cross sectional view showing a patterning step during formation of the screen; 
     FIG. 6 is a graph that plots the total number of holes per square inch (left hand y-axis) available for a pattern of holes 1 mil. in diameter on a 11.67 mil.×8.75 mil spacing along with the number of pattern repeats (right hand y-axis) plotted against a mesh range (x-axis); and 
     FIGS. 7A and 7B are schematic drawings used in an algorithm for picking screens suitable for a particular pattern. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a screen printing apparatus 10 suitable for screen printing in accordance with the invention is shown. The screen printing apparatus 10 includes a support platform 12 on which a substrate 14 has been mounted. A predetermined pattern will be screen printed onto the substrate 14. In an illustrative embodiment, the substrate 14 is a baseplate or other component of a field emission display and the pattern is a pattern of glue dots. A screen 16 is stretched across a support frame 18 mounted within the screen printing apparatus 10. In addition, a squeegee 20 is mounted within the screen printing apparatus 10 for movement across the screen 16 in the direction indicated by arrow 22. The squeegee 20 is adapted to force a viscous printing material 24, such as a paste or glue, through the screen 16 and onto the substrate 14. 
     With reference to FIG. 2, the screen 16 includes a fine mesh 26 and a patterning layer 28 formed thereon. As will be more fully explained, the patterning layer 28 is formed of a patterned photosensitive polymer and includes a pattern of open areas 30 that align with the screen openings in the mesh 26. During the printing process, the viscous printing material 24 (FIG. 1) is placed on the patterning layer 28 and forced by the squeegee 20 (FIG. 1) through the open areas 30 of the patterning layer 28, through the mesh 26 and onto the substrate 14. The open areas 30 are formed in a repetitive pattern having a pitch P1 in one direction and a pitch P2 in an orthogonal direction. As used herein the term pitch also refers to the frequency of the pattern. 
     With reference to FIG. 3, the mesh 26 includes screen wires 32 interwoven to form screen openings 34. The screen openings 34 have a generally square outer peripheral configuration. The mesh 26 is a fine mesh screen (e.g., 80 to 500 mesh) formed of a material such as stainless steel or a monofilament polyester. This type of fine mesh screen is commercially available from manufacturers such as Rigsby Screen &amp; Stencil, Inc., Torrance, Calif.; Utz Engineering, San Marcos, Calif.; and Micro-Screen, South Bend, Ind. 
     As shown in FIG. 4, the screen wires 32 are formed with an outside diameter &#34;OD&#34;. In addition, the screen openings 34 are formed with a width on each side of &#34;s&#34; (FIG. 3). Table 1 lists the wire mesh specifications for standard mesh count screens. 
     
                       TABLE 1______________________________________WIRE MESH SPECIFICATIONSMesh      Wire           Mesh      OpenMtl Count Diameter (OD)  Opening (s)                              Area+/-3%     +/-.1 mil      +/-.1 mil %______________________________________SS     80     3.7            8.8     49.6SS     80     2.0            10.5    70.6SS     105    3.0            6.5     46.9SS     150    2.6            4.1     37.2SS     165    2.0            4.1     44.9SS     200    2.1            2.9     33.6SS     200    1.6            3.4     46.2SS     230    1.4            2.9     46.0SS     230    1.1            3.2     55.0SS     250    1.6            2.5     36.0SS     270    1.4            2.3     38.0SS     280    1.2            2.4     44.1SS     325    1.1            2.0     41.3SS     325    .9             2.2     50.1SS     400    1.0            1.5     36.0SS     400    .75            1.75    49.0SS     500    .8             1.2     36.0______________________________________ 
    
     As also shown in FIG. 4, a pitch &#34;P&#34; or frequency of the mesh 26 is the spacing from center to center of the screen wires 32 (or openings 34). A thickness of the mesh 26 is equal to 2× OD The pitch &#34;P&#34; is equal to the diameter &#34;OD&#34; of the screen wires 32 and the width &#34;s&#34; for the screen openings 34 (P=OD+s). 
     With reference to FIG. 5, the formation of the patterning layer 28 (FIG. 2) is illustrated. The patterning layer 28 is formed by depositing a photosensitive emulsion 36 on the mesh 26. Suitable compounds for the photosensitive emulsion include photosensitive polymers such as polyvinyl alcohol and polyvinyl acetate. Advantageously, for subsequent alignment processes, these photosensitive materials will generally be translucent or transparent to light. 
     The photosensitive emulsion 36 is deposited onto the mesh 26 by spin deposition or other suitable deposition process to a thickness of about 1.5 to 20 mils. Following deposition, the photosensitive emulsion 36 can be softbaked by heating. Alternately, meshes precoated with a desired photosensitive formulation to a desired thickness can be purchased from the screen manufacturers previously identified. 
     In an illustrative embodiment, the photosensitive emulsion 36 is patterned using UV hardening followed by laser clearing of the emulsion 36. Alternately the emulsion 36 can be patterned and developed using UV exposure followed by development with a solvent such as water. In either case, a mask 38 (FIG. 5) formed of a thin flexible material, such as &#34;mylar&#34;, is used to pattern the photosensitive emulsion 36. 
     With a laser development process the photosensitive emulsion 36 is initially hardened by exposure to a UV source. For example a photosensitive emulsion 36 formed in a negative tone can be hardened by exposure to intense ultra-violet light without any type of a mask. After hardening the emulsion 36, the mask 38 is placed in close proximity to the photosensitive emulsion 36 using a suitable apparatus such as an alignment and exposure tool. The mask 38 contains solid areas 42 and open areas 44 that form a negative (or alternately a positive) of the pattern that is ultimately transferred to the substrate 14 (FIG. 1). Such a mask 38 is typically referred to in the art as a phototool and contains the artwork for the screen printed pattern. The mask 38 can be made by techniques that are known in the art. Typically the artwork is performed and then reduced in size using a step and repeat process. The mask 38 can also be made larger than the ultimate pattern to be printed by placing a reduction lens 39 between the mask 38 and the emulsion 36. 
     Prior to patterning the photosensitive emulsion 36, the open areas 44 in the mask 38 are aligned with the screen openings 34. Alignment is effected such that the screen openings 34 and open areas 44 of the mask align along alignment axis 46. Alignment can be accomplished using a microscope, or other viewing device, to look through the open areas 44 of the mask 38 and through the photosensitive emulsion 36 to the screen openings 34. Because the photosensitive emulsion 36 is formed of a transparent or translucent material, it is possible to see through this material to the mesh 26. Using the viewing device and suitable tools (e.g., exposure and alignment tool), the location of the mask 38 is adjusted so that the open areas 44 on the mask are aligned with the screen openings 34. 
     With the open areas 44 of the mask 38 aligned with the screen openings 34, a laser light 40 is directed through the mask 38 to the emulsion 36. The laser light 40 can be focused onto the surface of the emulsion using suitable lenses associated with the laser. Depending on the emulsion 36, an excimer or CO 2  laser can be used to ablate and clear the emulsion 36 from the mesh 26. A laser can be operated at an energy level that couples well to the photosensitive polymers typically used for forming a patterning layer. In addition, with a laser, a uniform energy density can be maintained across a relatively large area (e.g., 1cm×1cm). The laser light 40 introduces localized heating and causes the photosensitive emulsion 36 to decompose and ablate in areas corresponding to the open areas 44 on the mask 38. This clears the photosensitive emulsion 36 from the mesh 26 in these areas. As the material is ablated the focus of the laser light 40 can be changed to maintain localized heating at the surface of the photosensitive emulsion 36. At the completion of the laser ablation process, and as clearly shown in FIG. 2, the patterning layer 28 is formed on the mesh 26 with open areas 30 of the patterning layer 28 aligned with the openings 34 in the mesh 26. 
     The pattern to be screen printed must be selected such that the locations of the open areas 42 on the mask 38 coincide as much as possible with the locations of the screen openings 34. In a simplified case, this can be accomplished by locating the features of the pattern to be screen printed, with a pitch (P1 or P2) for the features that is an integral multiple (i.e., whole multiple) of the pitch P of the screen openings 34. As an example, for a 400 mesh screen having a pitch P of 2.5 mils, the features for the pattern can be spaced with a pitch (P1 and P2) of 2.5 mils, 5.0 mils, 7.5 mils, 10.0 mils etc. 
     However, because of the previously discussed difficulties in forming a screen with a perfectly consistent pattern more sophisticated analytical methods can be used to select a mesh size for the mesh 26 that will produce the least amount of interference with a desired pattern. 
     One such method is illustrated with reference to FIGS. 6, 7A and 7B. In this example, a desired pattern to be printed on a substrate includes holes 30A (FIG. 7A). The holes 30A correspond to the open areas 30 previously described. The holes 30A are approximately 1 mil. in diameter on a 11.67 mil. (P1)×8.75 mil. (P2) pitch. In FIG. 7A, the position of a hole 30A is depicted with respect to a centerline 48 which represents the center of the screen openings 34. A left hand boundary represented by line 50 is spaced from the center line 48 by a distance equal to -1/2 the wire diameter (φ). A right hand boundary represented by line 52 is spaced from the center line 48 by a distance equal to +1/2 diameter (φ). In FIG. 7B, dimension &#34;x&#34; represents the horizontal offset of the hole 30A within the screen opening 34. Dimension &#34;y&#34; represents the vertical offset of the hole 30A within the screen opening 34. The following algorithm can be used to pick a suitable mesh size for a particular pattern. A screen is picked by how many holes 30A partially or totally align with the screen openings. 
     
         ______________________________________1.    Psuedo Code Set hole spacing Set wire spacing Set wire diameter2.    Set Loop Parameters H.sub.Row = # holes in a row in unit area H.sub.Col = # holes in a column in unit area3.    Test For Intersection FLAG Row [H.sub.Row ] BOOLEAN FLAG Col [H.sub.Col ] BOOLEAN Wire Row [ ] FLOAT Wire Col [ ] FLOAT For (i = 1 to H.sub.Row) Position = i x hole spacing - Hor for (K = 1 to N.sub.Row) Center = k x wire spacing Hor + Hor Offset If (((Position &lt; (Center + 1/2 wire .0.)) and ((Position &gt; Center - 1/2 wire .0.))) FLAG Row [i] = FALSE Column Code is identical4.    Count the Tags For (i = 1 to Row)for (j = 1 to col)if (FLAG Col (j) and FLAG Row (i) = TRUE)Count = Count + 1______________________________________ 
    
     For the above specified pattern, and as shown in FIG. 6, good frequency matches between the mesh and pattern occur with an 80 mesh screen and with a 230 mesh screen. An 80 mesh screen formed with 2.0 mil. wire and 10.5 mil. screen openings will include 5355 openings/in 2 . A 230 mesh screen formed with 1.4 mil. wire and 2.9 mil. screen openings will include 4116 openings/in 2 . A 230 mesh screen formed with 1.1 mil. wire and 3.2 mil. open areas will include 4788 openings/in 2 . 
     In general, fine mesh screens with a larger mesh size (e.g., 80 mesh) will include a larger total hole area than a very fine mesh screen (e.g., 400 mesh). For some patterns, a larger mesh screen is thus less likely to form a beat frequency wherein the frequency of the screen openings interferes with the frequency of the pattern openings. A pattern formed on a larger mesh screen is thus less likely to be completely blocked or misaligned due to non alignment of the screen wires in different areas of the screen. 
     Alternately in place of a laser ablation process to form the patterning layer 28, a UV exposure and development process can be used. In this case the photosensitive emulsion 36 in FIG. 5 is exposed by directing a UV light through the mask 38. The UV light would take the place of the laser light 40. The photosensitive emulsion 36 would then be developed using a suitable solvent such as water to form the patterning layer 28 (FIG. 2). For a negative acting photosensitive emulsion 36 (FIG. 5), the exposed portions of the emulsion are polymerized. Conversely, the unexposed portions of the photosensitive emulsion 36 are removed by the water jet to form the open areas 30 (FIG. 2). 
     One application for the method of the invention is in the fabrication of flat panel displays and field emitter devices (FEDs) for flat panel displays. By way of example, substrates formed as a silicon or glass member are used in the construction of baseplates for flat panel displays. Such a flat panel display includes spacer elements that are used to separate the baseplate from a display screen. The spacers are secured to the baseplate with an adhesive. Using the method of the invention, the features printed on the substrate 14 can be adhesive dots used for securing spacers of the flat panel display to the baseplate. 
     Thus the invention provides an improved method for forming a screen for screen printing and an improved method for screen printing. Although the method of the invention has been described with reference to certain preferred embodiments, as will be apparent to those skilled in the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.