Method of fabricating full width scanning or imaging arrays from subunits

Abstract: A method for fabricating a coplanar full width scanning array (40) from a plurality of relatively short scanning subunits (20) for reading and writing images. The subunits are fixedly mounted in an end-to-end relationship on a flat structural member (30) with the subunit surfaces containing the scanning elements (12) all being coplanar even though at least some of the subunits have varying thickness. This is accomplished by forming from a photopatternable thick film layer (14) one or more keys (18, 38) on the subunit surface having the scanning elements and associated circuitry and positioning the keys into keyways (28, 39) produced from a photo­ patternable thick film layer (23) on a flat surface of an alignment fixture (24). A conformal adhesive (36) bonds the structural member to the assembled subunits to form the full width scanning array.

Description: This invention relates to full width scanning arrays, and more particularly to a method for fabricating coplanar full width scanning arrays from a plurality of relatively short scanning subunits. It is well known in the raster scanning industry to assemble pagewidth raster input scanning (RIS) and raster output scanning (ROS) bars from relatively short RIS/ROS subunits placed end-to-end. Once assembled, the pagewidth RIS/ROS bars or scanning arrays have the requisite length and number of image processing elements to scan an entire line of information at once with a high image resolution. The subunits have either image reading arrays which comprise a succession of image sensing elements to convert the image line into electrical signals or pixels or image writing arrays which comprise a succession of light producing or other elements employed to produce images in response to an image signal or pixel input. The prior art has failed to provide a means for fabricating a pagewidth scanning or imaging array from subunits which has adequate precise alignment tolerance in X, Y, and Θ space and which is commercially (i.e. economically) feasible. Further, the prior art has failed to provide such a pagewidth scanning array when an additional requirement of uniformity in the Y space direction is necessary. By Y space direction, it is meant coplanarity of the surfaces of the subunits making up the pagewidth scanning array. The prior art solutions put forward to overcome this inability to provide cost effective pagewidth scanning arrays include optical and electrical arrangements for overlapping several short arrays and abutting short arrays together end-to-end. However, none of these attempts have met with any great degree of success. For example, in the case of abutting smaller arrays together, losses and distortions of the pagewidth image often occurs because of the inability to achieve exact alignment of the smaller arrays with respect to each other. Another important problem with simply abutting chips or subunits is that chip or subunit width errors accumulate over the length of the pagewidth array. IBM Technical Disclosure Bulletin, Vol. 23, No. 5, October 1980 to B.D. Martin et al, entitled "Chip Protective Coating", discloses a chip or wafer having solder balls on one surface thereon. This surface and the balls are coated with a polyimide layer, so that the polyimide around each solder ball serves to envelope it. A shaving operation removes the tops of the balls including the polyimide layer thereover and produces flats for electrical testing or connection. U.S. patent 4,645,688 to Makino et a discloses a composition for coating material of a semiconductor which consists of principal elements of a semiconductor memory element, an encapsulating layer containing an inorganic material and an intermediate layer of a protective coating material. U.S. patents 4,690,391; 4,712,018; and 4,735,671 to Stoffel et al disclose a method for fabricating long full width scanning arrays. Smaller arrays are assembled in abutting end-to-end relationship by an aligning tool having predisposed pin-like projections insertable in locating grooves in a surface of the smaller arrays. Vacuum ports in the aligning tool surface to draw the smaller arrays into tight face-to-face contact with the tool. A suitable base is then affixed to the aligned small arrays and the aligning tool withdrawn leaving a full width scanning array composed of a row of end-to-end abutted smaller arrays. It is an object of the present invention to provide a cost effective method of fabricating a full pagewidth scanning or imaging array from a plurality of relatively short scanning or imaging subunits and concurrently to provide the required alignment precision,even though the subunits have different thicknesses. The present invention provides a method of fabricating a scanning/imaging array from a plurality of subunits each having scanning/imaging elements on a front surface thereof, comprising the steps of: (a) forming at least one projecting structure on the front surface of each subunit; (b) providing an alignment fixture comprising a substrate having a flat surface and a patterned layer on the surface, the patterned layer having a plurality of accurately-located recesses of a shape complementary to that of the subunit projecting structures; the depth of the recesses being less than the height of the subunit projecting structures in order to space the scanning or imaging elements from the alignment fixture on subsequent insertion of said projecting structures into said recesses and the recesses being open at one end and closed at the other for precision alignment of the subunits on the alignment fixture; (c) inserting the subunit projecting structures into the alignment fixture recesses, so that one end of the projecting structures contacts the closed end of the recesses, to form an array in which the front surfaces of the subunits are coplanar; (d) coating a conformal adhesive on a surface of a structural member; (e) placing the adhesive-coated surface of the structural member in contact with the rear surfaces of the subunits; and (f) removing the structural member, with the subunits bonded thereto, from the alignment fixture. In one embodiment the method comprises: (a) forming a plurality of sets of relatively short, linear scanning or imaging elements and associated circuitry for each set of elements on a planar substrate; (b) depositing and patterning a thick film, photopatternable layer on the surface of the planar substrate having the sets of scanning or imaging elements to form one or more thick film projecting structures for each set of elements thereon; (c) dicing the planar substrate to produce a plurality of subunits each containing a set of scanning or imaging elements, associated circuitry, and thick film projecting structures; (d) providing a pagewidth alignment fixture comprising a substrate having a flat surface and a patterned thick film photopatternable layer on the flat surface, the patterned thick film layer having a plurality of accurately located recesses which have shapes complementary to the projecting structures of the subunits, the recesses being open at one end and closed at the other for precision alignment of the subunits on the alignment fixture; (e) inverting the subunits and inserting the subunit projecting structures into the alignment fixture recesses, so that one end of the projecting structures contact the closed end of the recesses; (f) coating a conformal adhesive on a surface of the structural member; (g) placing the conformal adhesive on the structural member in contact with the surface of the subunits residing upside down with the subunit projecting structures residing in the recesses of the alignment fixture; (h) removing the structural member with the accurately positioned scanning subunits bonded thereto, the surfaces of the subunits being precisely colinearly fixed to form a pagewidth scanning or imaging array in which all of the surfaces of the subunits are coplanar even though the thicknesses of the subunits vary. Except for their thickness, the subunits are generally identical. In one embodiment of the invention, a plurality of scanning element arrays, together with their associated circuitry, are formed on a planar substrate, such as a silicon wafer. The scanning element arrays and associated circuitry are coated with a thick film photopatternable layer which is patterned to form one or more shapes, preferably two, per scanning element array which are subsequently to be used as projecting structures, and then the wafer is diced to produce the subunits. As the planar substrates may vary slightly in thickness, so do the subunits from different substrates or wafers. By using a flat alignment fixture containing a patterned, thick film photopatternable layer to provide recesses open at one end to accommodate the shapes of the projecting structures on the subunits, they are precisely aligned on the alignment fixture. The one or more shapes or projecting structures on each subunit provide three basic functions; viz., they accurately align the subunits end-to-end, they act as spacer to protect the scanning elements and associated circuitry by preventing contact with the alignment fixture, and finally they equally space the subunit surface from the alignment fixture, regardless of subunit thickness. When non-accumulation subunit width errors over the pagewidth array are a concern, the subunit widths are slightly undersized so that adjacent subunits do not touch, but the projecting structures and recesses maintain the alignment and coplanarity of the pagewidth array. A conformal adhesive is applied to the surface of a flat structural member, and the structural member is placed on the assembled subunits while they are still in the alignment fixture, with the conformal adhesive sandwiched therebetween. After the adhesive is cured, the structural member containing the precision aligned subunits are removed from the alignment fixture. The subunit surfaces containing the scanning elements in each full width scanning array are all coplanar. By way of example only, embodiments of the invention will now be described in conjunction with the accompanying drawings, wherein like parts have the same index numerals. Figure 1 is a schematic isometric view of a wafer having a plurality of small scanning array subunits thereon. Figure 1A is an enlarged isometric view of two scanning array subunits obtained from the wafer of Figure 1. Figure 2 is a partially shown schematic isometric view of an aligning fixture with the scanning array subunits of Figure 1A inverted for insertion therein. Figure 2A is a partially shown schematic isometric view similar to Figure 2 but showing an alternative configuration. Figure 3 is a schematic front elevation view of the assembled scanning array subunits mounted on the aligning fixture with a mounting structural member bonded to the scanning array subunits. Figure 4 is a schematic front elevation view of the assembled pagewidth scanning array of Figure 3 after removal from the aligning fixture. In Figure 1, a planar substrate 10 is shown, such as, for example, a silicon wafer having a plurality of sets or linear arrays of scanning, imaging, or sensing elements 12 and associated control circuitry 16 formed thereon. The control circuitry 16 is not shown in Fig. 1 but is shown in Fig. 1A described below. A thick film photopatternable layer 14, partially shown in Figure 1, such as polyimide, is deposited and patterned to form, for example, pairs of accurately located and dimensioned projecting structures 18 (also described below with reference to Figure 1A). Mutually perpendicular lines 13 and 15 in Figure 1 delineate the dicing lines for cutting the substrate 10 into a plurality of relatively small scanning array subunits 20. Referring to Figure 1A, there are shown two scanning sensor array chips or subunits 20 for use in fabricating a full width scanning array. As will appear, a plurality of such relatively short scanning array subunits 20 are assembled on a base structural member 30 (shown in Figures 3 and 4) in aligned end-to-end relation to form a full pagewidth long composite scanning array assembly 40, also shown in Figures 3 and 4. Preferably, the length of the full width array is made equal to the maximum size in the scan direction "Z" of the image to be processed. The composite, full width scanning array may be formed from either a series of image read arrays (i.e., Charge Coupled Devices, photodiodes, etc.) to provide a composite read array for scanning document originals and converting the document image to electrical signals or pixels, or a series of image write arrays (i.e., Light Emitting Diodes laser diodes, magnetic heads, or other printing heads, such as ink jet printheads) to provide a composite write array for writing images on a suitable imaging member or recording medium, such as, for example, a photoconductor for a xerographic copying system or paper for an ink jet printer in accordance with an image signal or pixel input. Scanning array subunit 20 includes a generally rectangular base 11, which is preferably silicon and obtained by dicing a wafer 10 such as that shown in Figure 1 along dicing lines 13, 15. Each subunit 20 has a plurality of sensors 12 arranged in a linear row or array 17 in one surface 19 thereof. Sensor row 17 is parallel to side edge 21 of the subunit base 11. Cooperating control circuitry 16, which may include logic gates and a shift register (not shown), can also be integrated onto the subunit base 11 for controlling operation of the sensors or scanning elements 12. Sensors or scanning elements 12 may, for example, comprise photodiodes adapted to convert image rays impinging thereupon to electrical signals or pixels in the case of a read array or LEDs selectively operated in response to an image signal input to produce image rays corresponding to the image represented by the image signals for exposing an imaging member (not shown) or for pulsing bubble generating resistors in a thermal ink jet printer (not shown). To permit scanning array subunit 20 to be joined in abutting relation with other like subunits (as seen in Figures 3 and 4), the row 17 of scanning elements or sensors 12 extends to the ends 22 of the base 11. To enable scanning array subunit 20 to be accurately and exactly aligned in end-to-end abutment with other like subunits, as will be explained more fully later, predetermined subunit aligning formations or projecting structures 18, comprising, for example, a pair of longitudinally extending parallel strips 18 of photopatternable thick film polymer material, such as polyimide, patterned from thick film layer 14, are provided on the surface 19 of subunit base 11. In the embodiment shown in Figure 1A, the parallel strips 18 extend in the Z direction or scanning direction and are substantially perpendicular to the side edge 21 and parallel to ends 22 of subunit base 11. The pair of parallel strips 18 have predetermined dimensions of width W and length L, and are located on subunit surface 19 a distance "a" from subunit side edge 21 and a distance "b" from subunit ends 22. The strips 18 are spaced apart by distance D. The thick film layer 14 and therefore the thickness of the projecting structures 18 may have a thickness between 0.5 and 2 mils (20 to 80µm). Referring to Figure 2, an aligning fixture 24 is shown which comprises a flat substrate 25 containing on surface 26 thereof a patterned thick film layer 23 of photopatternable material such as polyimide to define recesses 28 dimensioned to accept accurately the parallel strips 18 of subunits 20. The patterned thick film layer 23 has a thickness less than the thickness of the subunit strips 18. By using a high tolerance dicing machine, the adjacent scanning elements 12 on adjacent subunits may be assembled to within 1 to 2 micrometers of each other in the aligning fixture and thus provide a high resolution full width scanning array 40 after assembly. Therefore, the slightly undersized, adjacent subunits do not necessarily touch each other, so that subunit width errors or out of tolerance variations do not accumulate over the length of the fully assembled, pagewidth array. In order to use the aligning fixture 24, the subunits must be inverted and the strips or projecting structures 18, shown in dashed line in Figure 2, inserted into the recesses 28 as indicated by arrows 29. With the subunits inverted or flipped, the sensors and associated control circuitry are spaced from the aligning fixture surface 26 and thus are protected from harm during the assembly process. During assembly of the subunits 20 into a full width scanning array 40, the ends 27 of the parallel strips or projecting structures 18 of subunit 20 are tightly brought into contact with the recess bottoms 31. The accurately formed recess sidewalls 32 cooperate with the pairs of subunit strips 18 to center the subunits, thereby accurately locating each subunit 20 in X, Z, and Θ space, while the uniform thickness of the subunit strips 18 provide coplanarity of the subunit surface 19, referred to as the Y direction according to the coordinates depicted in Figures 1A and 2. An alternate embodiment is shown in Figure 2A. In this embodiment, the projecting structures 38 are patterned in a triangular shape and the recesses 39 have a complementary shape to receive the projecting structures of the subunit and accurately locate the subunits relative to each other and the aligning fixture. Other shapes (not shown) could be used for the projecting structures and recesses, of course, such as circles, diamonds, etc. to assemble the subunits. The aligning fixture could have several rows of recesses (not shown) which would enable fabrication of a number of two dimensional arrangements of scanning, imaging, or printing subunits into any desired configuration for higher density and higher resolution devices. As shown in Figures 3 and 4, all the scanning array subunits 20 are assembled in the aligning fixture 24, which are required to produce a full width scanning array 40. A conformal adhesive 36 such as Epotek H20-E®, is applied to surface 35 of structural member 30 which forms part of the full width scanning array. The structural member provides the strength to the full width array to prevent cracking or other structural failure modes of the abutted assembly of subunits 20. The structural member is inverted and the conformal adhesive 36 pressed against the bottom surfaces of the subunits, opposite the front surfaces 19 containing the sensors, circuitry, and pair of aligning strips 18. For sake of clarity, only three subunits are shown, although actually a large number may be required depending on the overall subunit width "t" (refer to Figure 1A); in the preferred embodiment, t is between 5 and 7 mm ± 2 micrometers, but may extend from 1 to 10 mm. After the adhesive 36 is cured, the structural member 30 with the subunits 20 adhered thereto is removed from the aligning fixture 24 leaving the unitary composite full width array 40 shown in Figure 4. Other shapes, combinations, and locations for array aligning formations may be used instead of cooperating thick film elongated projecting structures and mating recesses shown and described herein, including a single projecting structure instead of a pair. Also, the projecting structures and recesses may be fabricated by electroplating or high precision machining techniques instead of photopatterning a photosensitive thick film layer.

Claims: A method of fabricating a scanning/imaging array (40) from a plurality of subunits (20) each having scanning/imaging elements (17) on a front surface thereof, comprising the steps of: (a) forming at least one projecting structure (18, 38) on the front surface of each subunit; (b) providing an alignment fixture (24) comprising a substrate (25) having a flat surface and a patterned layer (23) on the surface, the patterned layer having a plurality of accurately-located recesses (28, 39) of a shape complementary to that of the subunit projecting structures; the depth of the recesses (28, 39) being less than the height of the subunit projecting structures (18, 38) in order to space the scanning or imaging elements (17) from the alignment fixture (24) on subsequent insertion of said projecting structures into said recesses and the recesses being open at one end and closed at the other for precision alignment of the subunits on the alignment fixture; (c) inserting the subunit projecting structures into the alignment fixture recesses, so that one end of the projecting structures (18, 38) contacts the closed end of the recesses (28, 39), to form an array in which the front surfaces of the subunits are coplanar; (d) coating a conformal adhesive (36) on a surface (35) of a structural member (30); (e) placing the adhesive-coated surface of the structural member in contact with the rear surfaces of the subunits; and (f) removing the structural member, with the subunits bonded thereto, from the alignment fixture. A method as claimed in claim 1, in which the subunit projecting structures are formed by depositing and patterning a thick film, photopatternable layer (14) on the front surface of the subunits. A method as claimed in claim 1 or claim 2, in which the patterned layer on the surface of the alignment fixture is a patterned thick film layer. A method as claimed in any one of the preceding claims, wherein projecting structures in the form of two parallel strips are formed on each subunit and the complementary recesses on the alignment fixture are in the form of a pair of parallel slots. A method as claimed in any one of claims 1 to 3, wherein the projecting structure shapes in step (a) and recess shapes in step (b) are triangular. A method as claimed in claim 2 and claim 3, wherein the thick film, photopatternable layers of the subunits and alignment fixture are polyimide, and the thicknesses of the thick film layers are in the range 20 to 80µm, and wherein the spacing between recesses of the alignment fixture is such that the subunits aligned thereon have a clearance of 1 to 2µm between adjacent subunits. A method as claimed in any one of the preceding claims, wherein the alignment fixture has more than one row of recesses for two dimensional array configurations. A method as claimed in any one of the preceding claims, in which a plurality of subunits, each with a set of scanning/imaging elements and at least one projecting structure on the front surface thereof, is produced by: (i) forming a plurality of sets of linear scanning/imaging elements on a surface of a planar substrate; (ii) depositing a thick film, photopatternable layer on the said surface of the substrate and patterning the layer to form, for each set of scanning/imaging element, at least one projecting structure; and (iii) dicing the substrate to produce a plurality of subunits, each containing a set of scanning/imaging elements and at least one projecting structure.

Inventors: ATKINSON DIANE, BURKE CATHIE J, CAMPANELLI MICHAEL R, DRAKE DONALD J, ATKINSON, DIANE, BURKE, CATHIE J., CAMPANELLI, MICHAEL R., DRAKE, DONALD J.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1990
Main classification: H01L31
IPC codes: H01L27, H04N1, H01L25