EP1000505A2

EP1000505A2 - Solid-state imaging device

Info

Publication number: EP1000505A2
Application number: EP99917036A
Authority: EP
Inventors: Jan T. J. Bosiers; Dick De Greef; Frans F. Vledder
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 1998-05-26
Filing date: 1999-05-14
Publication date: 2000-05-17
Also published as: KR20010022156A; JP2002517136A; WO1999062245A3; WO1999062245A2

Abstract

It is known to use sub-sampling in large solid-state imaging devices, for example photocameras, for the purpose of picture display in the viewfinder of the camera. If use is made of a mosaic color filter, a number of lines of different colors must be read each time during sub-sampling in order to preserve the color information. In the case of interlaced mode display, this may give rise to distortion of the picture. The invention provides a way of sub-sampling in which the picture can be displayed in a manner which is substantially or entirely free of distortion, and in which the color information is preserved.

Description

Solid-state imaging device.

The invention relates to a solid-state imaging device comprising a matπx, referred to as imaging matπx, of charge-storage sites arranged in rows and columns in which, duπng an integration peπod, a field of charge packets generated in dependence upon the local intensity of light captured, via a color filter, at a photosensitive surface can be stored, and read-out means by means of which a field of charge packets can be read row by row, the color filter used being such that, within a common column, charge packets m two successive rows correspond to two different colors

The imaging device may be of the type commonly referred to m the literature as "CMOS Imager" The imaging device of this type includes a system of intersections in which the cells or pixels are formed by a photodiode, said cells or pixels being coupled via a MOS switch to bit lines, the word lines used to select the rows being connected to the gates of the MOS switches. Most of the imaging devices, however, are of the charge-coupled-device type. Such a charge-coupled imaging device as well as a method of operating this device are descπbed in patent application WO 98/17051, in the name of the current applicant, which application is laid open to public inspection on 23 Apπl 1998, beaπng publication number WO 98/17051. In said document, a descπption is given of a charge-coupled imaging device compnsmg a first matπx, refeπed to as imaging matπx, of charge-storage sites arranged in rows and columns, and, coupled thereto, a second matπx of charge-storage sites arranged in rows and columns, which is a memory matπx in which charge packets of a first field are temporaπly stored, while a second field of charge packets is generated in the imaging matπx, the number of columns in the memory matπx corresponding to the number of columns in the imaging matπx, and the number of rows in the memory matπx being smaller than the number of rows in the imaging matπx, so that upon transport of a field of charge packets from the imaging matπx to the memory matπx, only a limited number of rows of the entire field is stored in the memory matπx, and the remaining lines are dumped via a dra region. Although the following descπption will specifically relate to a charge-coupled imaging device, it should be kept in mind that the invention is not limited thereto and can also suitably be used in the above-descπbed CMOS imager. The invention is of importance, in particular, to imaging devices having a large number, for example several millions, of picture elements (hereinafter referred to as pixels), which are used, for example, for digital photography For direct picture display, for example on a LCD screen m the viewfinder of the camera, it is often desirable to use a memory matπx in addition to the imaging matπx. On account of the large number of pixels, it is not very well possible to display all pixels in the viewfinder duπng operation. For this reason, it is generally not necessary to provide the device with a memory matπx having an equally large number of storage sites as the imaging matπx. In addition, it is desirable to employ a smaller memory matπx m order to preclude the dimensions of the device from becoming too large As descπbed m the above-menuoned application, a much smaller memory is sufficient, so that the overall dimensions of the imaging device can be kept within reasonable limits Since the memory can hold only a part of the pattern of charge packets generated in the imaging matπx, only a part of the rows is stored in the memory matπx while the rest of the rows are not read out (for example in the case of the CMOS imager or CCD imager of the interline type) or are dumped In the literature, this method is referred to as "sub-sampling" As descπbed hereinabove, it is possible, for example, to dram off the redundant charge packets via the substrate The manner in which the unused lines are drained is not relevant to the invention in question, other ways of draining can also be used in combination with the invention

The above-mentioned application also descπbes the possibility of sub-sampling if the surface of the imaging device is provided with a color filter, whereby two successive rows in the imaging section contain different color information By way of example, a descπption is given of an imaging device including a filter of the following composition row 1: R G R G R G row 2: G B G B G B row 3- R G R G R G row 4- G B G B G B etc. In this case, the pattern of the color filter recurs every other row In order to preserve the color information duπng sub-sampling (and hence be able to display the imaged picture in color m the viewfinder of the camera) it has already been proposed in the above-cited application to store each time two successive rows of the imaged picture in the memory matπx, then dump a number of lines and subsequently store two successive lines again In practice, due to picture distortion, the reproduction quality is often found to be indifferent or at least not as good as one would wish it to be. It is an object of the invention to provide, inter alia, a charge-coupled imaging device of the type descπbed in the opening paragraph, in which the above-mentioned distortion upon displaying a sub-sampled picture is at least substantially overcome.

To achieve this, a solid-state imaging device of the type descπbed in the opening paragraph is characteπzed in accordance with the invention in that means are available by means of which, for the purpose of reading out, a part of the rows are selected, while the other rows remain unread, and, if a first row is read, at least two succeeding rows are not read and the next row to be read contains color information which differs from that of the above-mentioned first row that has been read, and, in two successive fields, rows of the second field which are to be read, coπespond to positions in the imaging matπx which are situated between positions of successive rows of the first field, which have been read

The invention is, inter alia, based on the recognition that the above-descπbed distortion upon displaying the picture is connected with the interlacing mode for scanning the display screen, in which in a first field the odd lines are wπtten, and in a second field, the even lines are wπtten, or conversely Due to interlacing, two successive rows of the imaging matπx are not displayed as two successive rows or lines but as two lines which are separated by at least a third line which coπesponds to a row in the imaging matπx which is not situated between said two rows and hence is not displayed at the proper location on the display screen By, in accordance with the invention, not reading out successive rows of the imaging matπx (or not stoπng successive rows in the memory matπx in the case of a charge-coupled imaging device), but instead reading out (or stoπng in the case of a charge-coupled imaging device) rows which are farther apart, it is possible, as will be explained hereinafter by means of an example, to display a sub-sampled picture practically without distortion, while retaining the color information An important embodiment of a device in accordance with the invention is characteπzed in that the device constitutes a charge-coupled device compnsmg a first matπx, referred to as imaging matπx, of charge-storage sites aπanged in rows and columns, and, coupled thereto, a second matπx of charge-storage sites arranged in rows and columns, which second matπx constitutes a memory matπx in which charge packets of a first field are temporaπly stored, while a second field of charge packets is generated the imaging matπx, the number of columns in the memory matπx corresponding to the number of columns in the imaging matπx, and the number of rows m the memory matπx being smaller than the number of rows in the imaging matπx, so that upon transport of a field of charge packets from the imaging matπx to the memory matπx only a limited number of rows of the entire field is stored m the memory matπx, and the remaining lines are dumped via a dram region, means being present by means of which charge packets are transported from the first field and the second field of the imaging matπx to the memory matπx, in such a manner that when a first row is stored in the memory matπx at least two succeeding rows are dumped and the next row that is stored includes color information which differs from that of the above-mentioned first row stored in the memory matπx, rows of the second field which are stored in the memory matπx corresponding to positions in the imaging matπx which are situated between positions of successive rows of the first field which are stored in the memory matπx

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments descπbed hereinafter.

In the drawings:

Fig. 1 is a diagrammatic plan view of a charge-coupled device in accordance with the invention,

Fig. 2 shows a part of the color filter used in the device shown in Fig 1. Fig. 1 diagrammatically shows a charge-coupled imaging device as descπbed in the above-mentioned patent application PCT/TB 97/01201, the contents of which is considered to be incorporated in the current application by way of reference. The device compπses an imaging matπx 1 of charge-storage sites arranged in rows 2 and columns 3 The drawing shows a few rows (in continuous lines) and a few columns (in broken lines) The device may be of the field-transfer type m which the imaging elements are formed by charge-storage sites of the charge-transport ducts themselves, as descπbed in the above-mentioned application. The device may alternatively be of the interline type, in which the matπx 1 includes, as is known, columns of separate photosensitive diodes alternated with vertical charge-transport ducts screened against radiation The surface of the imaging matπx is provided with a color filter 4, not shown m Fig. 1, a part of which is shown in Fig. 2. The filter is embodied so as to be a matπx which coincides with the imaging matπx 1. The letters R, G and B represent the colors red, green and blue, respectively. As shown m Fig. 2, the color pattern is repeated every alternate line, so that, in the imaging matπx 1, different color information is formed duπng an integration peπod in successive lines or rows The device includes a second matπx 5, referred to as memory matπx, which is adjacent to the imaging matπx 1 and in which charge packets of a first field, generated in the imaging matπx, are stored for further processing, such as display on a display screen m the viewfinder of a camera while a second field is being formed in the imaging matπx. The matπx 5 ends m a hoπzontal read-out register 9 The block 10 represents voltage means by means of which the proper voltages are applied to the gates and, if necessary, regions in the semiconductor body. As indicated in the drawing, the memory is smaller than the imaging matπx, which can be attributed to the fact that the number of rows in the memory matπx is smaller than in the imaging matπx. The number of columns in both matπces is the same. Since it is not possible to store all lines of a field formed in the imaging matπx in the memory matπx, duπng the charge transport only a part of the information stored in the imaging matπx is stored in the memory, while the rest is dumped via a drain. The way in which redundant lines are dumped is descπbed in greater detail in the above-mentioned patent application PCT/TB 97/01201, in the name of the current applicant. In the method descπbed therein, the drain is formed by the substrate situated below the charge-transport ducts, to which substrate a sufficiently high voltage is applied. Excess charge is dumped at the transition between the imaging matπx and the memory matπx, for example by activating the gates of the imaging matπx and, simultaneously, not activating in the gates of the memory matπx. It is noted that the invention is not limited to this way of draining charge. Other methods, such as the one descπbed in European patent application EP-A 0 720 388, can also be applied within the scope of the invention

By means of Table I, a descπption is given of the operation of the device

R 1 1 1 1 1 1 1 B 2 2 2 2 2 ® R 3 ® 1 3 ® 1 3 ® 1 3 B 4 © 3 4 4 4 4 4 R 5 5 5 5 © 2 5 © 2 B 6 6 6 © 3 6 © 3 6 R 7 7 ® 2 7 7 7 7 B 8 8 ® 8 © 4 8 © 4 R 9 9 9 9 9 9 9 B 10 10 10 10 10 10 10 R 11 ©5 11 ©5 11 ©5 11 B 12 ©7 12 12 12 12 12 R 13 13 13 13 ©6 13 ©6 B 14 14 14 14 14 14 14 R 15 15 ©6 15 15 15 15 B 16 16 ©8 ©7 16 ©7 16 R 17 17 17 17 17 17 17 B 18 18 18 18 ©8 18 ©8 R 19 19 19 19 19 ©9 19 B 20 20 20 20 20 20 20 R 21 21 21 21 21 21 ® 10 B 22 22 22 22 22 @ 11 22 R 23 23 23 23 23 23 23

B 24 24 24 24 24 24 © 12 field 1 field 2 field 1 field 2 field 1 field 2

960 4/16)960 4/16) <960 6/22x960

= 240 = 240 = 262

(a) (b) (c) ( ) table 1 In the following description, it has been assumed that the imaging matrix includes 960 lines or rows. In column (a) only the first 24 lines of said 960 lines are indicated. The symbols R and G in the first column (a) indicate whether in the relevant row the color red or the color green occurs in the filter. The next column in (a) gives the number of the relevant row or line in the imaging matrix. In part (b) of Table I, the most obvious way of sub-sampling is indicated for two successive fields. Here, it is assumed that the capacity of the memory matrix is large enough to store 240 lines, which means one in four lines of the imaging matrix. In the Table, the encircled (underlined) digits indicate the number of the lines that will be stored. In this method, to preserve the color information, each time two succeeding lines are stored in the memory matrix, whereafter a number of subsequent lines are removed. For example, in the example shown in (b) in Table I, the lines 3 and 4 of field 1 are stored and subsequently the lines 11 and 12. The intermediate lines 5-10 are drained. The lines of field 2 that are stored and read out are situated between the selected lines of field 1; in the example, the lines 7 and 8 and the pair 15, 16 are stored. It can be readily ascertained that, in the cuπent system, out of every 16 lines, 4 lines are stored in the memory. Consequently, if an imaging matrix has 960 lines, then a memory of 240 lines is sufficient. If the picture is displayed by means of a display system in which the display screen is written in accordance with an interlaced mode, then, as indicated in the Table, line 7 of field 2 is written as line 2 on the display screen, while, for example, line 4 in field 1 appears as line 3 on the display screen, i.e. after line 7 of field 2. Table 1(c) indicates the way in which in a device in accordance with the invention this distortion can be, at least substantially, precluded. Instead of storing two succeeding lines in the memory each time, now lines with different color information are successively stored in the memory, which lines are farther apart. In the example shown in Table 1(c), for example the lines 3 and 6 are stored, in field 1, while the intermediate lines 4 and 5 are dumped. The following lines to be stored of field 1 are the lines 11(R) and 16(B); the intermediate lines are all dumped. The lines 3, 6, 11 and 16 of the first field, which are to be stored, correspond to the lines 1, 3, 5, 7 etc., of the display screen. Of the second field, the lines 5, 8, 13, 18 etc., are stored in the memory while the intermediate lines are dumped. The lines to be stored of the second field correspond to the lines, respectively, 2, 4, 6, etc., of the display screen. As shown in Table I, these lines are situated, in the coπect order in conformity with the lines on the display screen, between the lines of the first field, so that the above- described distortion is precluded.

If the way of sub-sampling indicated in Table 1(c) is employed, then, in the case of an imaging matrix of 960 lines, a memory of 240 lines, in conformity with the NTSC standard, is sufficient. Table 1(d) gives a way of sub-sampling 288 lines, in conformity with the PAL standard, of the 960 lines in the imaging matrix. In field 1, alternately the R and B lines 3, 6, 11, 14, 17, 20 etc., are stored, which correspond respectively to the lines 1, 3, 5, 7 etc., of the display screen. In the second field, alternately R and B lines are selected, which are situated between the lines of the first field, for example the lines 5, 8, 13, 16, 19, 22 etc., which correspond to the even lines 2, 4, 6, 8, etc., on the display screen. As shown in the Table, the selected lines of the second field are indicated on the display screen in the proper sequence relative to the indicated lines of the first field, while retaining the color information. As is further shown in the Table, the number of lines stored is 6/20*960 = 288, in conformity with the PAL standard.

Fig. 3 schematically shows a solid-state imaging device of the CMOS type without a memory matrix, which imaging device is known per se. Said imaging device includes rows indicated by Ml, M2, M3 etc., and columns indicated by Cl, C2, C3, etc. The imaging device further includes a mosaic color filter of, for example, the same composition as in the preceding example, the lines Ml, M3, M5, etc., coπesponding, for example, to the lines "R", that is the lines 1, 3, 5 in Table I, and the lines M2, M4, M6 etc., corresponding to the lines "B" in Table I. The elements in an imaging element include a diode D which is coupled to one of the main electrode areas of a MOS switch T. The anode of the diode D is connected to a reference voltage. The cathode of the diode D can be connected, via the switch T, to one of the (vertical) bit lines 6 upon reading and resetting the imaging element. The rows are addressed via word lines 7 which are connected to the gates of the transistors T of a row. The word lines 7 are connected to addressing means 8 by means of which a voltage is applied to a selected line 7, thereby causing the switches or transistors T of the relevant line 7 to be closed, and enabling the charge condition of the diodes D to be determined via the bit lines 6. The manner in which, in general, an imaging device of this type is operated is assumed to be known. In this imaging device, the above-described principle of sub-sampling can be employed, in the manner indicated in Table I, with this difference that the encircled numbers are not stored in a memory, but instead read out, and that the intermediate lines, instead of being dumped, are not selected during reading. It will be obvious that the invention is not limited to the examples given herein, and that within the scope of the invention many variations are possible to those skilled in the art. For example, a color filter other than the filter described can be employed, for example a filter of the composition: Cy Ye Cy Ye

Mg Gr Mg Gr

Cy Ye Cy Ye

Mg Gr Mg Gr The invention can also be used in combination with a color filter in which repetition occurs every three, or more, lines. It is also possible to use a charge-coupled imaging device of the "interline" type.

Claims

CLAIMS:

1. A solid-state imaging device comprising a matrix, referred to as imaging matrix, of charge-storage sites arranged in rows and columns in which, during an integration period, a field of charge packets generated in dependence upon the local intensity of light captured, via a color filter, at a photosensitive surface can be stored, and read-out means by means of which a field of charge packets can be read row by row, the color filter used being such that, within a common column, charge packets in two successive rows co╧Çespond to two different colors, characterized in that means are available by means of which, for the purpose of reading out, a part of the rows are selected, while the other rows remain unread, and, if a first row is read, at least two succeeding rows are not read and the next row to be read contains color information which differs from that of the above-mentioned first row that has been read, and, in two successive fields, rows of the second field which are to be read, co╧Çespond to positions in the imaging matrix which are situated between positions of successive rows of the first field, which have been read.

2. A solid-state imaging device as claimed in claim 1, characterized in that the device constitutes a charge-coupled device comprising a first matrix, refe╧Çed to as imaging matrix, of charge-storage sites arranged in rows and columns, and, coupled thereto, a second matrix of charge-storage sites arranged in rows and columns, which second matrix constitutes a memory matrix in which charge packets of a first field are temporarily stored, while a second field of charge packets is generated in the imaging matrix, the number of columns in the memory matrix corresponding to the number of columns in the imaging matrix, and the number of rows in the memory matrix being smaller than the number of rows in the imaging matrix, so that upon transport of a field of charge packets from the imaging matrix to the memory matrix only a limited number of rows of the entire field is stored in the memory matrix, and the remaining lines are dumped via a drain region, means being present by means of which charge packets are transported from the first field and the second field of the imaging matrix to the memory matrix, in such a manner that when a first row is stored in the memory matrix at least two succeeding rows are dumped and the next row that is stored includes color information which differs from that of the above-mentioned first row stored in the memory matrix, rows of the second field which are stored in the memory matrix corresponding to positions in the imaging matrix which are situated between positions of successive rows of the first field which are stored in the memory matrix.

3. A solid-state imaging device as claimed in claim 1 or 2, characterized in that the color filter is of the type:

RGRGRG

GBGBGB RGRGRG

G B G B G B, where R, G, B represent the colors red, green and blue, respectively.