VLSI optimized modulator

Abstract: An electro-optic modulator device, such as a multi-gate light valve for an electro-­ optic line printer, wherein the different elements are uniquely combined in a package utilizing VLSI principles. A specially designed integrated circuit package (140) is utilized to enable the bonding of one or two electro-optical devices (110, 112) as a single unit. As each electro-optical device has its chip pads at one end of the device, special modifications (175, 177) need to be made to join the pads to the external connectors. The electro-optical crystal, which may be mounted on a glass plate, is bonded to the integrated circuit chip. Sonic bonding or the two layer metal technique may be used for the crystal to chip bonding.

Description: This invention relates to an electro-optic modulator device, such as a multi-gate light valve for an electro-optic line printer. VLSI techniques are utilized to construct an operating electro-optic modulator device in an integrated circuit package. It has been shown that an electro-optic element having a plurality of individually addressable electrodes can be used as a multi-gate light valve for line printing. See, for example, US-A-4,396,252, "Proximity Coupled Electro-Optic Devices; US-A-4,367,925 "Integrated Electronics for Proximity Coupled Electro-Optic Devices; and US-A-4,380,373, "Conformable Proximity Coupled Electro-Optic Devices." Also see "Laser Printing with the Linear TIR Spatial Light Modulator," Sprague, R.A.; Turner, W.D.; Johnson, R.V., Proceedings of the SPIE; April 19, 1983; 396: 44-49. These references also cite other prior art which is relevant. These references all disclose, in various improvements of the other, attempts to create a multi-gate light valve which could be used to perform line printing on a photosensitive recording medium, such as a xerographic photoreceptor which is exposed in an image configuration as it advances in a cross line direction relative to the light valve. More particularly, to carry out the exposure process, a sheet-like collimated light beam is transmitted through the electro-optic element of the light valve, either along its optical axis for straight through transmission or at a slight angle relative to that axis for total internal reflection. Successive sets of digital bits or analog signal samples (data samples), which represent respective collections of picture elements or pixels for successive lines of the image, are sequentially applied to the electrodes. As a result, localized electronic bulk or fringe fields are created within the electro-optic element in the immediate vicinity of any electrodes to which non-reference level data samples are applied. These fields, in turn, cause localized variations in the refractive index of the electro-optic element within an interactive region (i.e., a light beam illuminated region of the electro-optic element which is subject to being penetrated by the electric fields). Thus, the phase front or polarization of the light beam is modulated in accordance with the data samples applied to the electrodes as the light beam passes through the interaction region. Schlieren readout optics may be used to convert a phase front modulated light beam into a light beam having a correspondingly modulated intensity profile. That is, the modulated light beam is converted into a correspondingly modulated intensity profile to image or project the light beam onto the recording medium. A previous attempt at a reduction to practice of this technology resulted in Figures 3 and 4 of US-A-4,380,373, noted above. In those figures, the light valve 12 includes an electro-optic element 17 and a plurality of individually addressable electrodes 18a - 18i. For a total internal reflection (TIR) mode of operation, the electro-optic element 17 is a suitable cut crystal of, say, Li NbO₃, lithium niobate, having an optically polished reflecting surface 21 which is integral with and disposed between optically polished input and output faces 22 and 23, respectively. The electrodes 18a - 18i are ultimately coupled to the electro-optic element 17 adjacent the reflecting surface 21 and are distributed across essentially the full width thereof If the electrodes are aligned parallel to the optical axis of the electro-optic element 17, the light valve 12 will produce a diffraction pattern which is symmetrical along the zero order diffraction component. If, on the other hand, the electrodes are of the Bragg angle relative to the optical axis of the electro-optic element 17, the light valve 12 will produce an asymmetrical diffraction pattern. This embodiment described in the above referenced patents utilizes an integrated circuit 41 which is resiliently biased against the reflective surface 21 of the electro-optic element 17, thereby causing the electrodes 18a - 18i to generally conform to the contour of the mating surface 21. As in other proximity coupled electro-optic devices, there is a clamp or similar bonding agent, which is schematically represented by arrows 44 and 45, for maintaining the electrode bearing surface of the integrated circuit 41 in pressure contact with the reflective surface 21. Additionally, there is a resilient pressure pad 51 or the like for applying a more or less uniform bias to the electrode bearing portion of the integrated circuit 41 so that it flexes as required to cause the electrodes 18a - 18i to closely conform to the contour of the reflective surface 21. The improved conformance of the electrodes 18a - 18i to the mating or reflective surface 21 of the electro-optic element 17 increases the uniformity with which the fringe fields 28 are coupled into the electro-optic element 17, thereby increasing the spatial uniformity of the electro-optic response of the light valve 12. According to the present invention, there is provided a multi-gate electro-optic modulator constructed in an integrated circuit package comprising an integrated circuit package, at least one integrated circuit means including a plurality of electrically conductive first electrodes in spaced apart relationship in a predetermined pattern and a plurality of electric lead connections connected to lead connections of said package, an electro-optic element comprising a layer of optically transparent electro-optically responsive crystal containing a plurality of electrically conductive second electrodes which are adapted to contact the first electrodes on said integrated circuit means so as to allow electric fields to be produced into said electro-optically responsive crystal, and a layer of resilient compressed material for pressing the first electrodes to the integrated circuit means and the second electrodes of the electro-optic element substantially uniformly into contact, characterised in that the integrated circuit package comprises a flat upper surface and a hollow area opening into said flat surface, with said lead connections of said package emanating from within said hollow area of said integrated circuit package, the integrated circuit means is mounted in the hollow area, and is aligned in a predetermined aspect within said integrated circuit package, the electro-optic element is mounted on said flat surface, and partially overlaps the edge of said hollow area of said package, said layer of resilient material is compressed between the bottom of the hollow area of said package and the bottom of said integrated circuit means, and a layer of glass or similar protection means is mounted on the top surface of said electro-optic element for protection of said electro-optic element. In the modulator of the invention, the different elements shown in the prior art are uniquely combined in a package utilizing VLSl principles. A specially designed integrated circuit package is utilized to enable the bonding of one or two electro-optic devices as a single unit. As each electro-optic device has its chip pads at one end of the device, special modifications need to be made to join the pads to the external connectors. The electro-optic crystal, which may be mounted on a glass plate, is bonded to the integrated circuit chip. Sonic bonding or the two layer metal technique may be used for the crystal to chip bonding. For a more complete understanding of the invention, reference may be made to the following detailed description of the invention in conjunction with the drawings wherein: Figures 1A, 1B, and 1C are top, front, and side views of a two chip electro-optic modulator constructed in accordance with one embodiment of the principles of the present invention, Figure 2 is a top view of the package design for a single chip electro-optic modulator, Figure 3 is a top view of the package design for a two chip electro-optic modulator, Figure 4 is an isometric view of the package design for a two chip electro-optic modulator, Figure 5 is an enlarged view of the TAB cable used in the modulator set forth in Figure 1. Figures 6A and 6B are top and side views of the crystal,chips and glass backing sub assembly used in the modulator set forth in Figure 1. Figures 7A,7B,and 7C are top and side views of another embodiment of the present invention showing the placement and use of a substrate. Figures 1A, 1B, and 1C are top, front, and side views, respectively, of the present invention and show the actual construction of one embodiment of the electro-optical VLSI optimized modulator in accordance with this invention. These figures show a typical ceramic package 140 with standard input/output pins 150. Chips 110 and 112 are seen as mounted inside the ceramic package, but Figure 1A shows them to be offset and overlapping. This is to provide for continuous scanning on the output medium without seam interruptions. Otherwise, the output copy would have a blank band vertically on the page. The offset aspect is necessary due to the fact that manufacture of the integrated circuit chips will not approach a satisfactory output level yield. So, instead of using one output integrated circuit, two chips are utilized. The chips 110 and 112 are shown in place with either wire bonds 175 or TAB 177 linking the connections 90 (Figure 2) on the integrated circuits 110 and 112 to the input/output pins 150 of the integrated circuit package 140. Each lead on the integrated circuit package 140 would be connected to one of the input/output pins 150. Integrated circuit chips 110 and 112 would be mounted in the package and the leads on their pads would be connected, as set forth above, to the individual pads in the integrated circuit package 140. The crystal 100, glass backing 180 and aligned integrated circuits 110 and 112 are shown to be mounted directly upon the ceramic integrated circuit package 140. Glue areas 182 are shown, which bond the crystal to the top of the integrated circuit package 140. The crystal 100, glass backing 180 and aligned integrated circuits 110 and 112 are normally provided as a sub-assembly as shown in figures 6A and 6B. Inside the ceramic integrated circuit package 140, would be placed an elastomer material 190. This elastomer provides a means to apply pressure to ensure uniform contact between the electrodes of the integrated circuit chips 110 and 112 to the crystal 100 in a direct manner. Without the elastomer, the slot in the package would have to be highly accurate, as would the bottom surfaces of integrated circuits 110 and 112. Thus, the elastomer 190 serves to ensure uniform contact of chip to crystal while allowing package tolerances to be relaxed. To protect the wire bonds 175 or TAB 177, an encapsulent 130 would be placed on and around the interconnection areas and the edges of crystal 100. This encapsulent could be any low mobile ion integrated circuit encapsulent readily available in the marketplace. Figure 2 shows a top view of a VLSI package containing only one integrated circuit 110 mounted in the ceramic package 140. In the blow-up of Figure 2, the one end of the integrated circuit 110 containing the input and output connections 90 is clearly seen. The other end could have input and output connections also, but would not be needed. Having all the connecting points on one end is advantageous in that with the leads at one end of the integrated circuit 110, more room on the chip is left for the electrodes. Shown also in Figure 2 are the spacing dimensions for the integrated circuit connecting sockets. The lead pads are 25 µm in size on 25 µm centers. Figure 3 is the top view of a VLSI package containing two integrated circuits 110 and 112. Here, however, the integrated circuits are offset slightly and are overlapping so that the last electrode of integrated circuit 110 and the first electrode of integrated circuit 112 are only separated by 10 microns so as to ensure a continuous scanned light beam at the photosensitive surface to eliminate any vertical striping down an output page. The pads for the electrode connecting points would be at the left or outward edge of integrated circuit 110, and the right or outward edge of integrated circuit 112. Figure 4 of the present invention discloses an isometric view of the VLSI electro-optic modulator package shown schematically in Figure 3. Mounted in the ceramic package 140 are the connecting pads which would be individually connected to the pads on the integrated circuits. The connecting points on pad 160 would be connected to the complementary connecting points 90 on the pads on integrated circuit 112. Similarly, the connecting points on pad 162 would be connected to the complementary connecting points on the pad on integrated circuit 110. The method of connecting the pads on the integrated circuit package 140 to the complementary points on the integrated circuits would be accomplished by most of the known integrated circuit manufacturing techniques. Figure 5 shows an enlarged view of the Tape Automated Bonding (TAB) interconnection cable. The use of a bumped tape is performed by first interconnecting the TAB cable to the chip so that all chip pads are interconnected to the cable 177. The chips are then aligned and glued to the glass/crystal assembly and glued in place with the glue, as set forth above. This subassembly is then placed into the VLSI package 180 (see Figure 1B) and pressed against the elastomer 190 until the proper pressure is achieved. The crystal/glass assembly is then bonded to the top of the VLSI package with glue 182. The outer connections of the TAB cable 177 are then bonded to the interconnection pads on the VLSI package 140. An encapsulent 130 is then used to seal the pad area around the TAB cable to seal the VLSI device. Figures 6A and 6B show how the electro-optic crystal 100 would be glued to a glass backing 180 which provides protection and maintains the flatness of the electro-optic crystal 100. The integrated circuits 110 and 112 are aligned to a set of corresponding electrodes 91 located on the crystal surface and should be as intimate as possible with the crystal 100 so as to allow the fringe fields established by the electrodes 91 to affect as much as possible of the lithium niobate, or other, crystal. The integrated circuits once aligned are glued 185 in place to ensure that the integrated circuit electrodes maintain alignment with the crystal electrodes 91. The glue 185 used could be any quick curing glue with a low mobile ion content readily available in the marketplace. The wire bonds or TAB (Tape Automated Bonding) 177 are shown which would connect the electrical pads on the integrated circuits to the electrical pads on the integrated circuit package 140. Yet another improvement to the present invention could utilize a VLSI package wherein the bottom plate of the package is non-existent. Thus, the LSI chip or chips 110, 112 are mounted on a suitably flat substrate 200 as shown in Figure 7A and 7B. Then the crystal 100 could be aligned and pressed onto the chips as discussed above in relation to the other embodiments, Figure 7C. In the above embodiments, the crystal/integrated circuits would have been pre-aligned and then inserted into an integrated circuit package on a layer of elastomer to ensure contact of all electrodes 91. Here, however, the integrated circuits/substrate is inserted into the hollow package (Figure 7B) so just the edges thereof contact the edges of the open bottom area of the VLSI package. The crystal 100 is then aligned to the integrated circuits and pressed against them until all crystal and integrated circuit electrodes make contact. The encapsulent 130 is allowed to flow into the package ensuring permanent contact remains between the integrated circuits and the crystal after assembly. While the invention has been described with reference to specific embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the invention. In addition, many modifications may be made without departing from the essential teachings of the invention.

Claims: A multi-gate electro-optic modulator constructed in an integrated circuit package comprising: an integrated circuit package (140), at least one integrated circuit means (110) including a plurality of electrically conductive first electrodes in spaced apart relationship in a predetermined pattern and a plurality of electric lead connections (90) connected to lead connections (177, 175) of said package, an electro-optic element (100) comprising a layer of optically transparent electro-optically responsive crystal containing a plurality of electrically conductive second electrodes (91) which are adapted to contact the first electrodes on said integrated circuit means so as to allow electric fields to be produced into said electro-optically responsive crystal, and a layer of resilient compressed material (190) for pressing the first electrodes of the integrated circuit means and the second electrodes of the electro-optic element substantially uniformly into contact, characterised in that the integrated circuit package (140) comprises a flat upper surface and a hollow area opening into said flat surface, with said lead connections (177, 175) of said package emanating from within said hollow area of said integrated circuit package, the integrated circuit means (110) is mounted in the hollow area, and is aligned in a predetermined aspect within said integrated circuit package, the electro-optic element (100) is mounted on said flat surface, and partially overlaps the edge of said hollow area of said package, said layer of resilient material (190) is compressed between the bottom of the hollow area of said package (140) and the bottom of said integrated circuit means (110), and a layer of glass (180) or similar protection means is mounted on the top surface of said electro-optic element for protection of said electro-optic element. The multi-gate electro-optic modulator as set forth in Claim 1, further including encapsulant means (130) being placed on and around said lead connections (175) of said package and the edges of said electro-optic element (100). The multi-gate electro-optic modulator as set forth in Claim 1 or Claim 2, wherein the surface of said electro-optically responsive crystal comprising said second electrodes is intimately bonded to the surface of said integrated circuit means (110) containing said electrically conductive first electrodes, and said glass or other protection means (180) is intimately bonded to said crystal layer. The multi-gate electro-optic modulator as set forth in any one of Claims 1 to 3, wherein said plurality of electric lead connections are at one end of said integrated circuit means. The multi-gate electro-optic modulator as set forth in any one of Claims 1 to 4, further including a second integrated circuit means (112) mounted in the hollow area opening of said integrated circuit package, said first and second integrated circuit means (110, 112) being mounted essentially longitudinally in said integrated circuit package, such that said integrated circuit means are almost end to end from one another, but offset slightly from the center of said integrated circuit package. The multi-gate electro-optic modulator as set forth in Claim 5, wherein the ends of said first and second integrated circuit means (110, 112) overlap somewhat in their offset positions in said integrated circuit package. The multi-gate electro-optic modulator as set forth in Claim 6, wherein the electrical lead connections on said first and second integrated circuit means are at opposite ends of said integrated circuit package.

Inventors: CAMPBELL HUGH P, IBRAHIM ABD-EL-FATTAH ALI, CAMPBELL, HUGH P., IBRAHIM,ABD-EL-FATTAH ALI
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1989
Main classification: G02F1
IPC codes: B41J2, H01L21, G02F1, H01L31