Abstract:
A semiconductor radiation detector device, comprising a bulk layer ( 103 ) of semiconductor material, and on the first surface of the bulk layer ( 303 ) in the following order: a modified internal gate layer ( 104 ) of semiconductor material of second conductivity type, a barrier layer ( 305 ) of semiconductor material of first conductivity type and pixel dopings ( 131, 132, 133 ) of semiconductor material of the second conductivity type, adapted to be coupled to at least one pixel voltage in order to create pixels corresponding to pixel dopings, characterized in that the device comprises a first contact of first conductivity type and said pixel voltage is defined as the potential difference between the pixel doping and the first contact.

Description:
FIELD OF THE INVENTION 
     The invention is related to semiconductor radiation detectors and particularly to a semiconductor radiation detector having a modified internal gate. 
     BACKGROUND OF THE INVENTION 
     Radiation is converted to electron hole pairs in semiconductor material. In semiconductor radiation detectors the electron hole pairs are separated by an electric field. The measured charge type is referred to as signal charge and the opposite charge type is referred to as secondary charge. 
     The patent applications WO 2006/018470 A1, WO 2006/018477 A1, PCT/FI2006/000009 and PCT/FI2006/000058 which are included herein by reference disclose a semiconductor radiation detector having a modified internal gate (MIG). The MIG detector presented in the patent applications WO 2006/018470 A1, WO 2006/018477 A1 is optimized for the detection of low energy X-rays and for particle and near infra-red radiation in case the semiconductor material is silicon. The MIG detector presented in the last two applications is optimized for the detection of visible light in low light level circumstances in case the semiconductor material is silicon. The MIG detectors are comprised of a bulk layer, of a MIG layer of the second conductivity type on top of the bulk layer, of a barrier layer of the first conductivity type on top of the MIG layer and of pixel dopings of the second conductivity type on top of the barrier layer. There may be also channel stop dopings of the first conductivity type on top of the barrier layer. The MIG detectors enable non-destructive reading of the signal charge, isolation between the signal charges and the surface generated charges and it has a low capacitance. For these reasons the MIG structure provides the best possible detection sensitivity of all semiconductor radiation detectors. 
     The problem associated with the MIG detectors is, however, a low dynamic range. This is due to the low full well capacity of the MIG. The row reset or the rolling shutter mechanism provides an equal integration time for each pixel which improves the image quality especially if a short integration time is used. The MIG detectors of PCT/FI2006/000009 and PCT/FI2006/000058 comprise an additional clear contact (e.g. 1334 in PCT/FI2006/000058) of the second conductivity type, which can be used also as an anti blooming drain, and a clear gate (e.g. 1343 in PCT/FI2006/000058) controlling the flow of signal charges from the MIG layer to the clear contact. This arrangement enables row reset since the clear gates of a row of pixels can be interconnected and a reset signal can be provided to the interconnected row of gates. The afore mentioned arrangement enables actually individual reset of pixels since the pixel doping (e.g. 1333 in PCT/FI2006/000058) closest to the clear gate can function also as an additional clear gate beside being a drain. This requires that these pixel dopings are connected for instance row wise in the pixel matrix and that the clear gates are connected column wise in the pixel matrix. The problem with the individual reset arrangement is that a large current may run between the drain and the clear contact during the reset operation which enhances the power consumption of the device. 
     A problem of the row reset related especially to still images is that images of fast moving objects are blurred since the start and the end points of the integration period are different in different rows although the integration time is the same. 
     BRIEF DESCRIPTION OF THE INVENTION 
     An objective of the invention is to provide a MIG detector with an improved dynamic range. Another objective of the invention is to provide a global electronic shutter for the MIG detector presented in PCT/FI2006/000009 and PCT/FI2006/000058 enabling the start and end times of the integration period to be the same for all pixels of the detector. Another objective of the invention is to provide row reset for the MIG detector presented in WO 2006/018470 A1 and WO 2006/018477 A1. Yet another objective of the invention is to provide for the MIG detector presented in PCT/FI2006/000009 and PCT/FI2006/000058 individual reset for the pixels with reduced power consumption. 
     The objectives of the invention are achieved by a semiconductor radiation detector of claim  1 , comprising a bulk layer of semiconductor material, and on the first surface of the bulk layer in the following order: a modified internal gate layer of semiconductor material of second conductivity type, a barrier layer of semiconductor material of first conductivity type and pixel dopings of semiconductor material of the second conductivity type, adapted to be coupled to at least one pixel voltage in order to create pixels corresponding to pixel dopings, characterised in that the device comprises a first conductivity type first contact said pixel voltage being defined as a potential difference between the pixel doping and the first contact. 
     The dynamic range of the MIG detector can be improved by interconnecting a clear contact which functions also as an anti blooming drain to an additional electronic circuitry. The signal charge is first read for instance with a double MIGFET (MIG Field Effect Transistor) presented in WO 2006/018470 A1, WO 2006/018477 A1, PCT/FI2006/000009 and PCT/FI2006/000058. After this the signal charge is transferred to the clear contact by applying a signal to a clear gate and the charge is read again using the additional electronic circuitry. If the measured amount of signal charge exceeds a certain limit the result obtained from the additional electronic circuitry is used and if the signal charge does not exceed the limit the result is obtained from the double MIGFET. 
     The global electronic shutter can be provided for the MIG detector by introducing on the semiconductor chip a separate light to signal charge conversion area, which is not covered by an opaque layer. From the light to signal charge conversion area the signal charges are guided to the MIG layer where the flow of the signal charges can be controlled by two global shutter gates. The first global shutter gate controls the flow of the signal charges to e.g. a double MIGFET and the second global shutter gate controls the flow of the signal charge to a clear contact. During the signal charge integration period the first global shutter gate is open and the second global shutter gate is closed. On the other hand, during the signal charge read out period the first global shutter gate is closed and the second shutter gate is open. In this fashion the start and the end of the signal charge integration period can be the same for all pixels in the pixel matrix. 
     The rolling shutter mechanism can be introduced to the MIG detector presented in the patent applications WO 2006/018470 A1 and WO 2006/018477 A1 by providing for each pixel individual channel stop dopings which are separated from the channel stop dopings of the neighbouring pixels. In this manner the pixels can be reset row wise. The separation of the channel stop dopings can be obtained for instance by providing a separation doping of the second conductivity type between the individual channel stop dopings. When a proper reverse bias is applied between the channel stop dopings and the separation dopings the channel stop dopings of adjacent pixels can be at different potentials during reset, which enables row reset. Another possibility to provide row reset for the MIG detector presented in the patent applications WO 2006/018470 A1 and WO 2006/018477 A1 is to isolate the pixels completely from each others using for instance SOI (Silicon On Insulator) technology. 
     The individual reset of the pixels in the MIG detectors presented by the patent applications in PCT/FI2006/000009 and PCT/FI2006/000058 can be performed by using instead of a single MOS (Metal Oxide Semiconductor) clear gate two separate MOS clear gates. The first of the MOS clear gates is connected row wise to first clear gates in the pixel matrix and the second clear gate is connected column wise to the second clear gates in the pixel matrix. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an embodiment of the invention, where the clear contact is connected to an additional read out circuitry in order to improve the dynamic range, 
         FIG. 2  illustrates an embodiment of the invention, where two global shutter gates are added to the structure of  FIG. 1  in order to enable the start and the end times of an integration period to be the same for each pixel in the pixel matrix, 
         FIG. 3  illustrates a cross-section of the device presented in  FIG. 1 , 
         FIG. 4  illustrates a cross-section of the device presented in  FIG. 2 , 
         FIG. 5  illustrates an embodiment of the invention, where the channel stop dopings are separated from the channel stop dopings of the neighbouring pixels, 
         FIG. 6A  illustrates a cross-section of the device presented in  FIG. 5 , 
         FIG. 6B  illustrates an alternative cross-section of the device presented in  FIG. 5 , 
         FIG. 7A  illustrates electron potentials of the device presented in  FIG. 6A  during integration, 
         FIG. 7B  illustrates electron potentials of the device presented in  FIG. 6A  during reset, 
         FIG. 8A  illustrates an embodiment of the invention where the pixels are isolated completely from each others using SOI technology, 
         FIG. 8B  illustrates another embodiment of the invention where the pixels are isolated completely from each others using SOI technology, 
         FIG. 8C  illustrates yet another embodiment of the invention where the pixels are isolated completely from each others using SOI technology, 
         FIG. 8D  illustrates still yet another embodiment of the invention where the pixels are isolated completely from each others using SOI technology, 
         FIG. 9  illustrates a MIG detector where the signal charge is detected using a floating gate arrangement, 
         FIG. 10A  illustrates a MIG detector having a clear configuration formed of two overlapping dopings, 
         FIG. 10B  illustrates a cross-section of the device presented in  FIG. 10A , 
         FIG. 10C  illustrates a cross-section of the device presented in  FIG. 10A , 
         FIG. 11  illustrates a double MIGBJT configuration, 
         FIG. 12  illustrates an alternative double MIGFET configuration, 
         FIG. 13  illustrates an alternative double MIGBJT configuration, 
         FIG. 14  illustrates an alternative double MIGFET configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an embodiment of the semiconductor radiation detector according to the invention. The detector comprises a light to signal charge conversion area  144 ′ which is not covered by an opaque layer. The light to signal charge conversion area further includes a buried doping  106  of the second conductivity which is connected to a MIG layer  104  on the edge of the light to signal charge conversion area. The signal charges created in the light to signal charge conversion area flow from the buried doping  106  to the MIG layer  104 . Between a channel stop doping  121  of the first conductivity type and a pixel doping  131  of the second conductivity type is a surface doping  171  of the second type, which is already introduced in the patent applications PCT/FI2006/000009 and PCT/FI2006/000058. The surface doping  171  is during operation preferably fully depleted. The pixel dopings  131  and  133  of the second conductivity type are preferably drain dopings and the pixel doping  132  of the second conductivity type is preferably a source doping of a double MIGFET. Between the source and drain dopings are the gates  141  and  142  of the double MIGFET. The clear gate  143  controls the flow of the signal charges from the MIG layer  104  to the clear contact  134  of the second conductivity type through a second type doping  193  situated between the MIG layer and the clear contact. The doping  193  is already introduced in PCT/FI2006/000009 and PCT/FI2006/000058 and it is preferably formed of the same implant which is used to form the buried doping  106 . 
     The source  132  is connected to a constant current source D. The amount of signal charge in the MIG can be deduced from the voltage V 1 . The full well capacity of the MIG is, however, limited and thus the clear contact  134  is connected to an additional electronic circuitry  150 . The signal charge is first read by the double MIGFET multiple times after which it is transferred to the clear contact. Then the signal charge is read again using the additional electronic circuitry. If the amount of signal charge in one or both measurements exceeds a certain limit the measurement result obtained by the additional electronic circuitry is used and if the amount of signal charge is lower than this limit the measurement result obtained by the double MIGFET is used. 
     The additional electronic circuitry  150  presented in  FIG. 1  represents one possible configuration. This configuration is comprised of three transistors A, B and C which belong to one pixel and of a current source E. The potential V 2  is used to determine the amount of signal charge transferred to the clear contact. The transistor A is used to measure the signal charge, the transistor B is a selection transistor and the transistor C is a reset transistor. The potentials V 3  and V 4  are constant. It is important to note that any kind of electronic circuitry applied for measuring the amount of signal charge can be used as the additional electronic circuitry, i.e. the additional electronic circuitry  150  in  FIG. 1  comprising the three transistors A, B and C is just an example. For instance one can use an electronic circuitry having a linear response region and a logarithmic response region. In this fashion the dynamic range of the MIG detector can be improved enormously. One can also use controlled double sampling technique to reduce the read out noise in the electronic circuitry  150 . 
     The cut line  181  corresponds to the cross-section presented by  FIG. 3 . Between the isolator layer  307  and the buried doping  106  is a first conductivity type extension  320  of the channel stop doping. It should be noted that contact vias are etched through the isolator layer  307 . On top of a second isolator layer  308  is an opaque layer  144 . Above the MIG layer is the barrier layer  305  and below the MIG layer is the bulk layer  303 . Between the drain  133  and the clear contact  134  is an optional surface doping  371 . In  FIG. 3  the enhancements of the MIG layer dopings  392  of the second conductivity type extend from a location below the edge of the source doping to a location that is under the channel of the MIGFET, i.e. the enhancement of the MIG layer doping does not extend to a location that is below the edge of the drain doping. 
       FIG. 2  illustrates another embodiment of the invention. The second global shutter gate  247  controls the flow of signal charges from the MIG layer  104  to an additional clear contact  235  of the second conductivity type through a doping  293  of the second conductivity type. The first global shutter gate controls the flow of signal charges from the MIG layer  104  to the enhancements of the MIG layer doping  392  of the double MIGFET. During the signal charge integration period the first global shutter gate is open and the second global shutter gate is closed. On the other hand, during the signal charge read out period the first global shutter gate is closed and the second shutter gate is open. In this fashion the start and the end of the signal charge integration period can be the same for all pixels in the pixel matrix. 
     In the detector of  FIG. 2  the MOS clear gate is split into two separate clear gates, i.e. there is a first clear gate  244  and a second clear gate  245 . The first of the MOS clear gates is connected row wise to first clear gates in the pixel matrix and the second clear gate is connected column wise to the second clear gates in the pixel matrix. The power consumption of the individual pixel reset is very small in this configuration. The cut line  281  corresponds to the cross-section presented by  FIG. 4 . 
       FIG. 5  illustrates another embodiment of the invention enabling row reset for the MIG detectors described in the patent applications WO 2006/018470 A1 and WO 2006/018477 A1. The cut line  581  corresponds either to the cross-section presented in  FIG. 6A  or to the cross-section presented in  FIG. 6B . The separation doping  536  of the second conductivity type separates the channel stop dopings  621 ,  622  and  623  of different pixels. The second conductivity type surface dopings  671 ,  672  and  673  belong to different pixels and they are preferably fully depleted during operation. In the device of  FIG. 6B  the MIG layer  104  has gaps  691 . In the MIG detectors of  FIGS. 1 and 2  the blooming current is collected by the clear contacts and in the MIG detector of  FIG. 5  the blooming current is collected by the drain dopings  131 ,  133 . One should note that part of the separation doping can also be replaced by a MOS structure. In this case the MOS gate is preferably connected to the separation doping. 
     Electron potential energy curves on the cut lines  681 ,  682  and  686  are presented in  FIGS. 7A and 7B .  FIG. 7A  corresponds to the signal charge integration period and  FIG. 7B  corresponds to an occasion during the row reset operation. In  FIGS. 7A and 7B  the first conductivity type is n type and the second conductivity type is p type. It is important to note that the first conductivity type can also be p type and the second conductivity type n type. On the back side of the device is a conductive layer  702 . The electron potential energy curve  716  on the cut line  686  penetrating the separation doping  536  is almost the same than the electron potential energy curves  711  and  712  on the cut lines  681  and  682  penetrating the channel stop dopings during the integration period. The separation doping is connected to an electron potential −V R1  and the channel stop dopings to an electron potential −V CS  during the signal charge integration period. It is important to note that the channel stops and the separation doping are reverse biased with respect to each others. During the row reset the reverse biasing between the separation doping and the channel stop dopings is first enhanced by connecting the separation doping to an electron potential −V R2 . After this the reverse biasing between one channel stop doping ( 621 ) and the separation doping  536  is enhanced even more by connecting the channel stop doping  621  to a clear potential −V C . As a result the signal charges in the enhancements of the MIG layer dopings  392  will be collected by the source and drain dopings  132 ,  131  and  133  in the pixels where the channel stop doping is connected to the clear potential. After this the channel stop doping  621  is connected back to electron potential −V CS . In the end of the read out period the separation doping is connected back to the electron potential −V R1 . 
     An alternative way to perform row reset in the MIG detector described in WO 2006/018470 A1 and WO 2006/018477 A1 is represented in  FIGS. 8A ,  8 B,  8 C and  8 D. In the devices of  FIGS. 8A-8D  the pixels are completely isolated from each others by isolator layers  809  and  827 . The devices in  FIGS. 8A-8D  are made on SOI wafers where a SOI isolator layer  809  is situated between a thick SOI substrate  810  and a thin SOI semiconductor layer. The thin SOI semiconductor layer comprises the bulk layer, the MIG layer, the barrier layer and the pixel dopings. The pixels are isolated from each others by isolator trenches  827 . The isolator trenches are processed by etching trenches to the thin SOI semiconductor layer and by filling the trenches with isolator material. In  FIG. 8A  the channel stop doping  821 , the extension of the channel stop doping  820  and an optional doping  824  are all of the first conductivity type. The opaque layer  844  shielding the source from light is preferably added to front illuminated devices. One should note that the devices in  FIGS. 8A and 8D  can be made back illuminated by etching the thick SOI wafer away. The device in  FIG. 8A  differs from the device in  FIG. 8B  in the aspect that the former device has a buried doping  106  and the latter one has not. 
     In the devices  8 C and  8 D the bulk layer  303  and the MIG layer  304  are both of second conductivity type. At the interface of the bulk layer  303  and the SOI isolator layer there is a conductive layer described in WO 2006/018470 A1, WO 2006/018477 A1, FI20040996 and WO 2006/005803 A1. Such a conductive layer  702  is drawn in  FIGS. 7A and 7B . The conductive layer can for instance be made of a two dimensional (2D) charge gas layer. Such a 2D charge gas layer can be a result of fixed charges in the SOI isolator layer  809  or it can be made by applying a suitable bias to the thick SOI substrate  810 . The device of  FIG. 8D  differs from the device of  FIG. 8C  in the aspect that a preferably opaque and conducting material  828  like metal is added to the trench isolation structure. This conducting material can be used to bias the thick SOI substrate and to shield the pixels from scattered light. 
       FIG. 9  illustrates an embodiment of the invention where the signal charge in the MIG layer  104  is read nondestructively using a floating gate configuration. The floating gate configuration comprises a floating pixel doping  932  of the second conductivity type which is connected to a read out circuitry  950 . The read out circuitry  950  can be for instance similar than the additional electronic circuitry  150 . The pixel dopings  931  and  933  are used to transfer the signal charge back and forth between the local enhancements of the MIG layer dopings  392  situated below the pixel dopings  932  and  933 . The pixel doping  935  is a clear gate and the doping  134  is a clear contact. It is important to note that the clear contact  134  can be optionally connected by a conductor  951  to the read out circuitry  950 . In this manner the dynamic range of the detector can be improved Without connecting a second electronic circuitry  150  to the clear contact. The signal charge is first measured by the floating gate configuration and after that the signal charge is transferred to the clear contact where it can be read again. If the measured amount of the signal charge exceeds a certain limit the result of the latter measurement is chosen. If the measured amount of the signal charge does not exceed this limit the result of the former measurement is chosen. 
       FIG. 10A  describes an embodiment of the invention where the cut lines  1081  and  1082  correspond to the cross-sections presented by  FIGS. 10B and 10C . In FIG.  10 B the area between the two brackets of the form { is the second type doping  1093  connecting the MIG layer  104  of the second conductivity type and the clear contact  134  of the second conductivity type. In this case the MIG layer  104 , the barrier layer  305  and the doping  1093  are formed of two overlapping dopings of the second and the first conductivity type. 
     The MOSFET is not the only possible transistor to be used in conjunction with the MIG. In  FIG. 11  MOSFETs (Metal Oxide Semiconductor Field Effect Transistor) are replaced by BJTs (Bipolar Junction Transistor) forming a double MIGBJT. The pixel dopings  1131  and  1132  of the second conductivity type are the base dopings  10 , and the dopings  1151  and  1152  of the first conductivity type are the emitter dopings of the MIGBJTs. The channel stop doping  121  of the first conductivity type acts as the collector of the MIGBJTs collecting the charges of the first conductivity type emitted by the emitters. In addition to the three conventional nodes of a BJT there is a fourth node in the MIGBJT, i.e. the MIG. 
     In  FIG. 20  the MOSFETs are replaced by junction field effect transistors (JFET) where the gate dopings are replaced by MOS gates  1241  and  1242 . The pixel dopings  1231  and  1232  act as the source, drain and channel dopings. 
     In all of the transistors comprising the MIG that have been introduced until now the signal charges in the MIG decreases the effective channel or base width.  FIGS. 13 and 14  illustrate transistors comprising the MIG where the signal charges in the MIG increase the effective channel or base width. In  FIG. 13  the second conductivity type pixel dopings  1331  and  1332  act as collector dopings and the dopings  1351  and  1352  of the first conductivity type act as base dopings. The emitters  1361  and  1362  of the second conductivity type are formed for instance of polycrystalline semiconductor material like of polycrystalline silicon. In  FIG. 14  the pixel doping  1406  is a continuous layer encapsulating the channel stop doping  1421 . Inside the pixel doping  1406  of first conductivity type there are also the source and drain dopings  1451 ,  1452 ,  1453  and  1454  of two MOSFETs. The conductors  1441  and  1442  are the gates of the two MOSFETs. 
     It is important to note that in operation a reverse bias is connected between pixel dopings of the second conductivity type and a first conductivity type first contact which can be either the channel stop doping or a contact to the bulk layer. This first contact collects the secondary charges created in the semiconductor detector. It is also important to note that the doped region ( 193 ,  1093 ) between the MIG layer and the clear contact can be replaced by a local reduction of the barrier layer net doping or by a trench just like presented in PCT/FI2006/000009 and PCT/FI2006/000058. The MIGFET gate insulator  307  is preferably thick and it is made of a low k material. The insulator layer  307  can be for instance made by first growing a layer of 10 nm of silicon dioxide on top which is deposited a 190 nm thick layer of low k isolator material. 
     It should also be noted that the dynamic range of the MIG detector can be improved by observing an image with several different integration times. As an example one can first use an integration time of 40 ms after which the signal charge is measured and reset. Next one can observe the image with 4 ms and 0.4 ms integration times. In this manner the dynamic range is improved by 100 times. The signal charge is preferably measured multiple times after the first integration time but only one time after the second and third integration time. The problem of this approach is, however, that the images of fast moving objects are blurred. One can also multiply the signal charges while the signal charges are transferred back and forth between the local enhancements of MIG layer dopings in a double MIG detector by the impact ionization process. This requires, however, that high enough voltages are applied to the MIG detector. 
     One can use instead of the double transistor pixels single transistor or multiple transistor pixels. Instead of the MOSFET, JFET and the BJT one can use any unipolar or bipolar transistor in the pixels. The source of a FET or the emitter of a bipolar transistor may be floating and it may be connected to a capacitor. The pixels may be surrounded by preferably ring shaped guard structures which are formed of MOS structures or of dopings in order to increase the pixel area. The dopings of the invention may also be tailored in any possible way using implants having different masks, different energies, different doses, different angles and different conductivity types. In some cases the dopings may also be replaced with suitable metal contacts, i.e. with Ohmic or Schottky type contacts. The semiconductor material is preferably silicon but any other semiconductor material may be used. The semiconductor material could for example be Germanium. The contact openings through the isolator layer  1307  and the contacts to the different dopings are not shown. The MIG detector can be front or back illuminated. One can use anti reflection coatings, scintillator coatings, colour filters or micro lenses in both front and back illuminated MIG detectors. 
     There may also be read and selection electronics on the detector chip. A device that includes a detector according to an embodiment of the invention may also include other semiconductor chips, some of which may have bonded connections to the pixels of the detector. This enables building very compact structures that include detection, amplification, reading and in some cases even storage in a very small space, like an MCM (multi-chip module).