Abstract:
The invention relates to large-dimension linear image sensors operating by relative translation between the image and the sensor and consisting of a plurality of linear arrays abutting in a staggered fashion. In order to improve the design of the sensor, according to the invention the arrays are mounted on packages whose upper surface has an elongate rectangular shape provided on two opposite sides of the rectangle with two extensions substantially covered by the ends of the array, two adjacent packages bearing against each other along a respective extension of each of them. The staggered arrangement prevents the blind zones, photosensitive points being present in the extensions, and the offset between the two rows of staggered chips is practically no greater than the width of the chips.

Description:
FIELD OF THE INVENTION 
   The invention relates to the manufacture of large-dimension image sensors operating by relative translation between the image and the sensor. They are therefore linear sensors, that is to say sensors comprising one or more lines of photosensitive detectors and intended, during the relative translation between the image and the sensor, to reconstruct an overall image in two dimensions, namely one dimension in the direction of the line of photosensitive detectors and one dimension in the translation direction perpendicular to this line. 
   It will be understood that this definition includes linear array sensors with a plurality of lines, and in particular linear sensors of the TDI (Time Delay Integration) type which may comprise a plurality of lines, or even several tens of lines, intended to improve the signal/noise ratio of the detection by successive observation of a same image line by a plurality of detector lines. 
   BACKGROUND OF THE INVENTION 
   For particular applications, for example observation of the earth from satellites, it is desirable to produce linear sensors of great length, more than that which can be produced on a single silicon chip, and for this purpose a plurality of chips may be aligned end-to-end in the direction of their length. 
     FIG. 1  represents a top view of a row, on a printed circuit board  10 , of a plurality of packages B 1 , B 2 , B 3  . . . each carrying a respective semiconductor chip P 1 , P 2 , P 3  . . . in the form of an array, each chip being electrically connected to its package by wires welded between connection terminals of the chip and connection terminals of the package. 
   The drawback of this layout is the presence of blind zones: even if the photosensitive points practically reach the edge of the chip and even if the chip practically reaches the edge of the package, the junction zones at the boundary between two chips will not contain photosensitive points (or pixels in image detection). In fact, the pixels will scarcely reach closer than about fifty micrometers from the edge of the chip, and the chip can only reach the edge of the package with a tolerance of the same order. For pixels having a pitch of about 10 micrometers, about ten pixels risk being absent in each abutment zone of adjacent packages. This is not acceptable because the overall image reconstructed in the course of the relative displacement of the sensor will comprise black columns in the translation direction. 
     FIG. 2  represents a lateral section of a solution which has already been attempted in order to resolve this drawback: the photographic optics project the image to be detected not directly onto the photosensitive sensor but onto the plane entry face of a set of optical fiber bundles F 1 , F 2 , F 3  . . . ; the fiber bundles F 1 , F 2 , F 3  are juxtaposed and leave no gap between them on the side of their entry face; each bundle is deformed by tightening the side turned toward the chip, however, so that its exit face occupies only the useful length of the linear array. The blind zones are thus eliminated. This solution is very expensive, in particular because of the extreme precision which is required in the juxtaposition of the fiber bundles and in the deformation of these bundles so that their exit occupies exactly the active photosensitive length of each array. 
     FIG. 3  represents a more practical solution which may be envisaged: the chips abut, although in a staggered fashion; they are therefore not aligned, or more exactly they are divided into two groups of aligned chips and the two groups form two parallel rows, each chip of the second group being placed between two separated chips of the first group but offset out of the row of chips of the first group. The distance between two chips of a row is less than the length of the package of a chip. The packages therefore bear against one another at the end, along an edge parallel to the row direction; the ends of the adjacent chips comprise detection zone portions in mutual overlap so that there is absolutely no dead zone. The separation of the row axes of the two groups is equal to the width L of the package. It is perfectly known and, when photographing in translation, taking into account the translation speed V, an image line is reconstructed by combining the information provided by two adjacent chips not at the same time but with a time interval L/V, which compensates for the fact that the chips do not see the same image line at the same time. 
   This solution suffers from drawbacks associated with the need to reconstruct the final image, with risks of distortion and instability which are commensurately more problematic as the distance between the two chip rows is greater. The distance is moreover equal to the width of the package, that is to say several centimeters. 
   One possible solution for limiting this drawback is represented in  FIG. 4 : the chips are all placed at the edge of the package, all the wires connecting the chip to the package being relocated to one side of the chip. The chips are still staggered, and the signal which they provide therefore has to be processed in order to carry out reconstruction, taking into account the offset of the row axis of the chips, although this offset is now practically only the width of the chip and no longer the width of the package. 
   Unfortunately, this solution is expensive because it requires a particular chip design, having connection terminals on only one side, and it may furthermore require the use of two different components depending on whether the chip belongs to one row or the other because the chips are alternately reversed; reversal, however, is not necessarily compatible with the operation of the chip. This is the case in particular for multilinear arrays operating in TDI (Time Delay Integration) mode, or multispectral imaging arrays having a plurality of lines of pixels corresponding to different colors or ranges of the spectrum. This is because reversal of the chip entails signal processing problems in both cases. This solution is therefore difficult to implement. 
   SUMMARY OF THE INVENTION 
   In order to further improve the staggered abutment, with mutual overlap of the photosensitive zones in order to avoid blind zones, the present invention principally relates to a sensor whose package, instead of being simply rectangular as is conventionally the case, comprises two small lateral extensions substantially with the same width as the chip and with a length substantially equal to the mutual overlap desired between the chips. These extensions extend in the length direction of the chip. The chip practically entirely covers these extensions (which do not contain connection terminals). The extensions of two packages are adjacent (in physical contact or almost in physical contact) via their side parallel to the row of chips. There is no connection terminal on the substrate in the region of the lateral extensions. 
   There is therefore a staggered abutment, with an inter-axial distance equal to the width of the chips or nearly so, without having the need to design a special component (this is because the terminals can lie on both sides of the chip) and without having the need for two different components, one for each of the rows, as is the case in the arrangement of  FIG. 4 . 
   The invention therefore relates to an image sensor comprising a plurality of linear image detection arrays associated in order to form a linear image sensor with a length greater than that of each array, characterized in that the arrays are mounted on packages whose upper surface has an elongate rectangular shape provided on two opposite short sides of the rectangle with two extensions substantially covered by the ends of the array, the packages being mounted in a staggered fashion and two packages being adjacent via a respective extension of each of them. These extensions have a width smaller than that of the short sides of the rectangle. There are no connection terminals on the packages, so that they can all be identical and juxtaposed without reversal. 
   A variant may nevertheless be envisaged in which the chips protrude from a rectangular package without extensions of this rectangular package supporting them. This variant is less favorable because it subjects the chips to stresses which are not always compatible with severe environmental conditions, particularly from the point of view of shocks and vibrations; on the other hand, the grounding of the chip on its entire rear face is inferior, as is the thermal dissipation. 
   In order to take account of these two possibilities according to the invention, the following general definition of the invention has been formulated: a large-dimension image sensor operating by relative translation of the image with respect to the sensor, comprising a plurality of individual monolithic chips each capable of detecting a linear image portion perpendicular to the translation direction, the individual chips being arranged in two groups, the chips of one group being aligned in order to detect aligned image portions and the chips of the other group being aligned in order to detect other image portions which are aligned but offset with respect to the chips of the first group in the relative translation direction, the chips of the second group being arranged in a staggered fashion with respect to the chips of the first group so that all the chips of the first group lie beside at least one chip of the second group, the chips each being mounted on a package, connection wires being connected between connection terminals of the package and connection terminals on the upper surface of the chip, characterized in that the upper surface of the package, on which the chip is mounted, comprises a principal zone the shape of which is an elongate rectangle whose length is less than the length of the chip and whose width is greater than that of the chip, all of the connection terminals lying inside this rectangle along the chip, all the chips having two ends protruding from the rectangle, a protruding end of a chip being adjacent, via a side parallel to the row of chips, to a side likewise parallel to the row of chips of the protruding edge of another chip. The packages are all identical and juxtaposed without reversal. 
   The chip end therefore protrudes according to the invention either into emptiness or on an extension of the package extending the rectangular zone in the general direction of the length of the rectangle. 
   In the embodiment with a rectangular package having two extensions lying under the chip protrusions, the dimension of these extensions, in length in the direction of the row and in width in the translation direction of the image, is preferably only just greater than the length and width of the protruding chip part which covers this extension. Only a positioning tolerance margin is provided, for example of the order of 50 to 100 micrometers, is provided around the chip in order to take into account risks of poorly positioning the chip on the package. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other characteristics and advantages of the invention will become apparent on reading the following detailed description, which is given with reference to the appended drawings in which: 
       FIGS. 1 to 4 , already described, represent various possibilities of mounting linear arrays in order to constitute a large-dimension sensor; 
       FIG. 5  represents a first possibility of the abutment of arrays according to the invention, having a package provided with two extensions and a chip covering these extensions; 
       FIG. 6  represents a package according to the invention in more detail; 
       FIG. 7  represents an alternative embodiment with a rectangular package without extension and with a protruding chip. 
   

   DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 5  shows the linear sensor according to the invention, with a plurality of linear chips abutting in a staggered fashion with partial overlap between the ends of the chips. The staggered chips form two parallel rows, the axes of which are straight lines D 1  and D 2  parallel to the translation direction Ox of the image with respect to the sensor. 
   The first row comprises the odd rank linear chips P 1 , P 3 , etc., each carried by a respective package B 1 , B 3 , and the line passing linearly through the centre of the chips is the straight line D 1 . The second row comprises the even rank chips P 2 , P 4  carried by the respective packages B 2 , B 4  etc. and their central line is the straight line D 2 . 
   The straight lines D 1  and D 2  are separated by a distance L which is equal to or slightly greater (for example by 100 to 200 micrometers) than the width of the chips (the width counted along the direction Ox, for example equal to a few millimeters). This width is much less than that of the packages which support them (one or several centimeters), so that the spacing L of the straight lines D 1  and D 2  is much less than that which exists in  FIG. 3 . The chips are all identical and mounted identically on their package, in contrast to the case of  FIG. 4 . 
   The packages, all of which are identical, have an elongate overall shape since the chips are linear arrays (one line or several lines), and they therefore have two long sides parallel to the straight lines D 1  and D 2  and two short sides at the ends of these long sides. The short sides each comprise an extension or “lug” which extends the package in the direction of the straight lines D 1  and D 2 . The chip mounted on a package essentially covers the upper surface of these extensions, although a slight margin (50 to 100 micrometers) may nevertheless be provided on all the edges of the extension so that the package protrudes beyond the surface of the chip in the region of the extension. 
   The staggered packages bear against one another via their extensions: the packages are in contact along edges parallel to the straight lines D 1  and D 2 . On the other hand, the extension of a package may bear against a short side of the package wherever there is no extension. 
   The packages are therefore positioned very precisely with respect to one another so long as the dimensions (width and length) of the extensions are precisely machined. 
   Since the width of the extensions (in the translation direction Ox) is only just greater than the width of the chip, the connections between the chip and the package are produced exclusively in the principal zone of the upper surface of the package, and not in the extensions: there is no connection terminal on the upper surface of the package in the extensions. 
   In other words, the upper surface of the package has an elongate rectangular shape with two long sides and two short sides, and an extension protruding from the rectangle on each short side, and the connection terminals all lie inside the rectangle which constitutes the principal zone of the package, the extension constituting an auxiliary zone. 
   The staggered chips mutually overlap in the region of the extensions, that is to say the chip comprises useful photosensitive points in the region of the extensions, and the photosensitive points of an odd rank chip see a given image line slightly before the photosensitive points of an adjacent even rank chip see the same image line. If the translation speed is V and if the distance between the straight lines D 1  and D 2  is L, then the even rank line will see the same image line after a time L/V. In order to obtain an image in two dimensions during the image reconstruction, care will be taken to associate the signals provided at a time t by the odd rank chips with the signals provided by the even rank chips at time t+L/V. 
   The photosensitive points which are in mutual overlap make it possible to avoid having any blind zone in the line of arrays of photosensitive points: most of the points of an image line are seen by a single array, but the points are seen twice in the overlap zone, once at time t and once at time t+L/V; simple image processing makes it possible to give a single value to these points, which value may be the one read by one chip or the other chip, or the average of the two values read, optionally weighted as a function of the position of the point on the array. 
   The length of the extensions may correspond to the length necessary in order to place several tens of photosensitive points on the extension. With an array several centimeters long and photosensitive points with a pitch of 10 micrometers, for example, 1 millimeter extensions may be provided. In an example of a spatial imaging sensor, the length of the package may be 5 centimeters, the width of the package may been from 1 to 2 cm, and the width of the chip may be 2 or 3 millimeters. 
     FIG. 6  shows in more detail a top view of an even rank package B 2  abutting with an odd rank package B 3 . 
   The overall shape of the principal part of the upper surface of the package is an elongate rectangle, with two long sides  20  and  22  and two short sides  24  and  26 , and the short sides are provided with two extensions  30  and  32 , respectively on the left and right of the package, which extend parallel to the elongation direction of the rectangle. 
   The chip P 2 , in the form of a linear array, is electrically connected to the package B 2  by connection wires joining terminals  28  of the chip to terminals  34  of the package. It is exclusively on the rectangular principal surface of the package, delimited by the long sides  20  and  22  and the short sides  24  and  26 , that the electrical connection terminals  34  are located. There are no wire connection terminals on the extensions  30  and  32  which, as can be seen, are almost entirely covered by the chip. If absolutely necessary, it is feasible for there to be terminals  28  on the chip in the region of the extension, these wires leading to terminals  34  of the package in the principal zone. 
   The packages B 2  and B 3  abut via their respective extensions, the right extension  32  in the case of the package B 2  and the left extension  30 ′ in the case of the package B 3 : the left end of the extension  30 ′ of the package B 3  bears on the right short side  26  of the package B 2  (or almost bears on it, if it is preferable that there should not completely be mechanical contact at this position); likewise, the extension  32  of the package B 2  bears (or almost bears) against the left short side of the package B 3 ; furthermore, in order to define the spacing between the lines of even rank and odd rank chips, the extensions  32  and  30 ′ bear (or almost bear) against each other in the direction Ox via their lateral edges, which are perpendicular to the translation direction (edges parallel to the elongation direction of the arrays) and which face each other when the packages are put in place staggered. 
   The spacing L between the elongation axes of the arrays of photosensitive points of the two rows of chips is equal to the width of the extension, the chip being placed at the centre of the extension so that all the chip+package assemblies are identical. The width of the extension is equal to the width of the chip, to which may be added a positioning tolerance margin of a few tens of micrometers (preferably from about 50 micrometers to 200 micrometers, for example 50 or 100 micrometers) all around the chip. The margin at the end of the extension may be greater, so long as there are a sufficient number of photosensitive points in the extension in order to ensure overlap of the images taken by two adjacent chips. 
     FIG. 6  represents the zone Z in which the photosensitive points  40  and  40 ′, belonging respectively to the chips P 2  and P 3 , are in mutual overlap, that is to say they can see (albeit at different times) the same points of an image line translating in the direction Ox perpendicular to the lines of photosensitive points. 
   As mentioned above, each chip in the form of an elongate array may comprise one or several lines of photosensitive points  40 . In particular, there are a plurality of lines in TDI type arrays. 
     FIG. 7  represents an alternative embodiment which is less beneficial than that of  FIG. 6  owing to the greater sensitivity of the layout to shocks and vibrations and owing to the inferior thermal and electrical performances of the sensors. There is no lateral extension on the package in this variant, and the package is exactly rectangular with two long sides  20  and  22  and two short sides  24  and  26 , although the chip protrudes laterally on the right and left of the package so that it is the chip itself which constitutes the extension used for abutment. 
   The staggered packages are abutted by making the chips touch via their protruding lateral edges. The end of one chip touches (or almost touches) a short side of the adjacent package, and a protruding lateral edge of the chip touches (or almost touches) a protruding lateral edge of the adjacent chip. The spacing between the axes of the line of even rank chips and the line of odd rank chips is in this case equal to the width of the chips. The image overlap is obtained via the protruding parts of the adjacent chips, which see the same points of an image line. 
   The invention is more particularly applicable to multilinear arrays (several lines of pixels) operating in TDI (Time Delay Integration) mode, in which the signals observed by a line are added to the signals observed by the preceding line in synchronism with the translation of the array; it is also particularly applicable to multispectral arrays in which several lines of pixels are provided, each line corresponding to a given color or spectral band; the invention is more generally applicable to high-resolution imaging, in particular for spatial imaging.