SENSOR ARRAYS

Abstract: Apparatus and method of fabricating a sensor array (10) from a silicon substrate (11) having a row of photosites (21) to provide a sensor array chip with uniformly smooth and angled ends designed to permit the chip to be lock butted end-to-end with other like chips, to form a full-length scanning array, in which the substrate is etched for separation along the planes at the desired boundaries of the chip, the etched boundaries delimiting the chip ends running between selected pairs of photosites to provide whole photosites right up to the ends of the chip, so as to avoid any gap or interruption between the photosites at the junctions with other chips. In a second embodiment, the chip ends are formed with complementary irregular shapes to enhance locating and butting of the chips with one another, and in a third embodiment, grooves are etched between each of the photosites in the row to ensure photosite uniformity and prevent cross talk between photosites.

Description: The invention relates to image sensor arrays and method of fabrication, and more particularly, to an image sensor array chip fabricated to permit the array to be assembled with other like clips to form a longer array without distortion or loss of image at the chip junctions. Image sensor arrays for scanning document images, such as charge-coupled-devices (CCD's), typically have a row or linear array of photosites (that is, photosensor sites) together with suitable supporting circuitry integrated on to a substrate. Usually, an array of this type scans an image line-by-line across the document width while the document is moved in synchronism therewith in a direction parallel to the document length. The image resolution of an array of this type is proportional to the ratio of the length of scan and the number of array photosites. Because of the difficulty in economically designing and fabricating arrays with a large number of photosites on one chip, image resolution for the typical commercial array available today is relatively low. While resolution may be improved electronically by interpolating extra image signals or pixels, or by using several sensor arrays and electrically interlacing the arrays with one another so as to switch in succession from one array to the next as scanning across the line progresses, electronic manipulations of this type are costly. Further, single or multiple array combinations of the type described usually require a more complex and expensive optical system to ensure that the array or arrays accurately scan the image line without loss or distortion. A single array equal in size to the width of the document to be scanned, yet with a very large packing of photosites to ensure the high resolution is needed but not available currently in the art. One concept that has been suggested is to form a single long or full width array by butting several small chips together. Since photosites can be closely packed on smaller chips without substantial and costly reduction in yield rates, a full width array having the large number of photosites needed to achieve high resolution can be achieved in this fashion. At the same time, optical requirements are greatly simplified. However, the inability to make the chips with ends that are smooth and straight enough to permit the chips to be butted together as well as the inability to separate a failed chip from the other chips for replacement or repair without damage to the photosites of the neighboring chips has heretofore effectively precluded this approach. EP-A-0 115 946 discloses a process for assembling a plurality of smaller photosensor array chips in exact end-to-end relation with one another to form a longer uninterrupted array, in which aligning V-shaped groves are formed in the body of the chips for self-aligning engagement with pins on an alignment tool. IBM Technical Disclosure Bulletin, volume 6, number 2, November 1979, 'Long arrays of silicon nozzles', W Crooks et al , page 2469, discloses a process for aligning a short series of ink jet nozzles to produce a longer array, in which accurate abutting faces are formed by anisotropic etching along the (111) planes. The present invention seeks to address and rectify the above by providing a sensor array chip capable of being physically abutted with other like chips accurately and without loss of image quality to form a longer composite array, the chip being fabricated in accordance with the method according to claim 1. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is an enlarged front view in cross-section of an image sensor array; Figure 2 is an isometric view of an image sensor array fabricated on (110) silicon in accordance with the invention, to provide an image sensor chip having angled and uniformly-smooth chips ends with unimpaired photosites extending up to the chip ends; Figure 3 is an isometric view illustrating image sensor chips of the type shown in Figure 1 butted together end-to-end to form a longer continuous array; Figures 4a and 4b are isometric views illustrating different interlocking configurations for the chip ends to facilitate butting of the chip ends together, and Figure 5 is an isometric view of the array shown in Figure 1 in which isolation grooves are formed between each photosite in the photosite row to ensure substantially equal-sized photosites. In Figure 1 of the drawings, there is shown an image sensor array or chip having a base or substrate 11 of silicon with a row of photosites 21 along one surface thereof. Base 11 for example may comprise p-type silicon, while photosites 21 may comprise n-type photodiodes. A separation area 22 is provided between the photosites 21 to isolate the photosites from one another to prevent crosstalk. Referring now to Figures 2 and 3 of the drawings, the image sensor array chip of the present invention, designated generally by the numeral 10, has a base 11 of (110) type silicon. As will be understood by those skilled in the art, (110) silicon has four (111) planes that intersect the (110) surface 14 of base 11 at 90°. Chip 10 is fabricated from a larger (110) silicon substrate or wafer 8 on which a plurality of chips 10 is formed. Chips 10 are fabricated with one or more rows 25 of photosites 21 and may have other supporting circuits and components integrated thereon. Orientation-dependent etching is used to delineate the top and bottom edges 15, 16 precisely and the array ends 17, 18 of chip 10 during fabrication. Later, the chips are separated from the substrate 8 and each other along the etched lines. However, since the (111) planes are not normal to each other but instead intersect at an acute angle of 75.53°, ends 17, 18 of chip 10 are not perpendicular to the top and bottom edges 15, 16 of chip 10 but instead are at an acute angle of 75.53° with respect thereto. As a result, chip 10, following orientation etching and separation from the wafer, is in the shape of a parallelogram. The row 25 of photosites 21, which may for example be anisotropically etched, have boundaries parallel to the top and bottom edges 15, 16 and the ends 17, 18 of chips 10, so that photosites 21 are similarly in the shape of a parallelogram. In order not to impair or damage the photosites adjoining the chip ends 17, 18, ends 17, 18 are etched along a line 26 in the separation area 22 between adjoining photosites at the point where separation is to take place. While a single row 25 of photosites 21 is shown and described, plural rows of photosites may be used. While photosites 21 are preferably formed prior to the etching of the chip 10, the photosites may be formed after chip 10 has been separated from the (110) silicon wafer. Other supporting circuits and components of the sensor array such as shift registers 31, 32, are preferably made integral with chip 10. Components such as shift registers 31, 32 may have a conventional rectangular shape as shown or alternatively may be in the form of a parallelogram similar to that of chip 10 and photosites 21. Chips 10, with photosites 21 of row 25 extending to the opposite sides 17, 18 of the chip base 11, are intended to be butted together to form a composite full-width array 40. Advantageously, the number of chips 10 joined together would be sufficient to form a longer composite array 40 having an overall operating scan length equal to or slightly greater than the width of the longest document to be scanned in that circumstance, array 40 would be termed a full-width or contact-type array. As will be understood, however, the overall length of the composite array 40 may be greater or less than the width of the largest document, in which case suitable optical means (not shown) would be employed to correlate the array operating scan length with that of the document. Referring particularly to Figure 3, during assembly of chips 10 together to form composite array 40, the chips are placed on a substrate 23 with the photosite rows 25 of each chip aligned with the other to avoid any image offset or loss along the scan line at the juncture of the chips with one another. Following alignment, the chips are abutted together, with end 17 of the first chip in mating engagement with the end 18 of the second chip, and the end 17 of the second chip in mating engagement with the end 18 of the third chip, and so on. A suitable adhesive (not shown) may be used to retain the chips in assembled relation with one another on substrate 23. Referring to Figures 4a and 4b, where like numbers refer to like parts, to facilitate locating and butting of the chip 10 with other like chips to form a longer composite array 40, the chip ends, 17, 18 may be formed with complementary interlocking geometrical shapes. In Figure 4a, end 17 of base 11 of chip 10 is etched to provide a central recess or cutout 50, while the opposite end 18 has a complementary projection 53. Cutout 50 and projection 53 each have a width w and depth d that are substantially equal to one another to ensure a tight interlocking fit between the end 17 or 18 of one chip with the complementary end 18 or 17 of a second chip. Matching cutout 50 and projection 53 have a width w sufficient to span across the width of the photosites 21 to prevent any intrusion into, or impairment of, the photosites adjoining the chip ends 17, 18. In the arrangement shown in Figure 4b, cutout 50′ extends along chip end 17 to one edge 15 of the base 11, with the complementary projection 53′ at the other end 18 similarly extending to edge 15 of base 11. The width w and depth d of cutout 50′ and projection 53′ are substantially equal to one another to ensure a tight fit, with the width w of both cutout 50′ and projection 53′ being of sufficient dimension to span across the width of photosites 21. In the embodiment shown in Figure 5, where like numerals refer to like parts, generally U-shaped grooves 65 are etched along the boundary lines between each photosite 21 of the photosite row 25. Grooves 65 serve to help isolate photosites 21 from one another, thereby eliminating or reducing crosstalk between photosites, improving mean transfer function (MTF), and ensuring that all the photosites that comprise photosite row 25 are substantially equal in size. To facilitate handling and assembly of chips 10 with one another, alignment grooves may be formed in the surface 14 of base 11 of sensor array 10. Such grooves, however, as will be understood by those skilled in the art, would in this application be U-shaped in configuration.

Claims: A method of fabricating a linear photosensor array chip (10) for assembly with other like chips to provide a longer composite scanning array, comprising the steps of: a) forming at least one linear row of photosensors (21) on a (110) silicon substrate (11), the longitudinal axis of the or each linear row of photosensors (21) being formed parallel to a first (111) plane of the substrate (11), with the boundaries between photosensors parallel to a second (111) plane of the silicon substrate, whereby the boundaries between photosensors are at an acute angle to the said longitudinal axis of the photosensor row; b) orientation-dependent etching the substrate along said second (111) plane, at predetermined spaced points along the sensor row, to delimit the opposing ends (17, 18) of said chip, the etching being along a line coincident with the boundary between the photosensors, whereby the chip ends are at an acute angle to the said longitudinal axis of the photosensor row, and the photosensors at each end of the chip are unimpaired, and c) separating the chip from the substrate at the point where the chip ends are delimited, whereby to provide a chip having a generally-parallelogram shape, with the chip ends extending at an acute angle to the longitudinal axis of the photosensor row to facilitate abutting and aligning of the chip in end-to-end relation with other like chips to form a longer composite scanning array. The method according to claim 1, including the step of isolating the photosensors from one another by etching grooves (22) between adjacent photosensors. The method according to claim 1 or 2, including the step of fabricating at least one shift register (31, 32) on the substrate, the longitudinal axis of the shift register being parallel to the longitudinal axis of the photosensor row, with the ends of the shift register being perpendicular to the longitudinal axis of the photosensor row. The method according to any preceding claim, including the steps of; a) orientation-dependent etching one of the chip ends to provide an irregularly-shaped surface, and b) orientation-dependent etching the other of the chip ends to provide an irregularly-shaped surface complementary to the shape of the surface of the one end, whereby on end-to-end butting of the chip with the other like chips, the irregularly-shaped surfaces at each end mate with the corresponding complementary irregularly shaped surfaces of the abutted chips to interlock the chips together in aligned relation. The method according to any of claims 1 to 3, including the steps of: a) orientation-dependent etching one of the chip ends to provide a generally-rectangular recess (50) in the surface of the one end; and, b) orientation-dependent etching the other end of the chip to provide a generally-rectangular projection (53) in the surface of the other end complementary with the surface of the said one end, whereby on butting of the chip with other like chips, the ends of the chips engage with each other. The method of fabricating a full-width photosensor array (10) from several smaller arrays, comprising the steps of: a) forming at least one linear row of photosensors (21) on a (110) silicon substrate (11), the longitudinal axis of the or each linear row of photosensors (21) being formed parallel to a first (111) plane of the substrate (11), with the boundary between each successive photosensor being parallel to a second (111) plane of the substrate, whereby the photosensor boundaries are at an acute angle to the said longitudinal axis of the photosensor row; b) orientation-dependent etching the substrate along the said second (111) plane at predetermined spaced points along the photosensor row to define the opposite ends of a series of smaller arrays, the etching being done along a line coincident with the photosensor boundary, whereby the ends of the smaller arrays parallel to the photosensor boundaries, with the last photosensors in each photosensor row extending up to an array end; c) separating the smaller arrays from the substrate at the ends to provide a plurality of smaller arrays, each of the smaller arrays having the shape of a parallelogram with ends extending at an acute angle to the longitudinal axis of the photosensor row, and d) assembling a selected number of the smaller arrays together in end-to-end relationship with one another, so that the photosensor row of each of the smaller arrays is aligned with the photosensor row of the other smaller arrays to form a full-width array. The method according to claim 6, including the step of: orientation-dependent etching the substrate along the said substrate (111) planes, to define the top and bottom sides of the smaller arrays. A photosensor array (10) adapted for assembly with other like arrays to provide a longer array, comprising; a) a (110) silicon chip (11); b) at least one row of photosensors (21) on the said chip (11), the longitudinal axis of the or each row of photosensors (21) being formed parallel to a first (111) plane of the chip (11), the boundaries of the photosensors being parallel to the (111) planes of the chip, whereby the photosensors are generally in the shape of a parallelogram; c) the opposing ends of the chip being uniformly smooth and parallel to the a second one of chip (111) planes, whereby the ends of the chip extend at an acute angle to the said longitudinal axis of the row of photosensors, so that the chip has a generally parallelogram shape complementary to the photosensors, and d) each end of the chip being formed along a line passing along the boundary between adjoining photosensors, so that the photosensors at the border of each end of the chip are unimpaired and the same size as the other photosensors in the row of photosensors, whereby the chip may be assembled with other like chips by bringing the chip ends into abutting engagement with each other, with the photosensor row of the chip being aligned with the photosensor rows of the adjoining chips.

Inventors: ARAGHI, MEHDI NAJM, TANDON, JAGDISH CHAND
Applicants: XEROX CORPORATION
Application year: 1986
Main classification: H01L25
IPC codes: H01L27, H04N1, H01L21