Abstract:
A semiconductor device is provided which comprises a thin film transistor (TFT) comprising a gate electrode formed on an insulating substrate, a gate insulating film formed on the gate electrode, and a pair of electrodes having a semiconductor layer and an ohmic contact layer therebetween; and a gate wiring connected to the gate electrode, and a signal wiring connected to one of the pair of electrodes, wherein the gate wiring and the signal wiring are arranged in superposition in the film thickness direction with an interlayer insulating layer therebetween to have a plurality of crossings with each other and the interlayer insulating layer has a plurality of steps overstriding a lower wiring at the crossings. A radiation detection device and a radiation detection system that have the semiconductor device are also provided.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device, and more particularly to a liquid crystal panel, a radiation detection device and a radiation detection system that use TFTs. 
     2. Related Background Art 
     Currently, the tendency of enlargement of the picture area of panels using thin film transistors (TFTs) has been accelerated. The tendency results from the development in manufacturing technique for liquid crystal panels using TFTs and the increase in applicability of area sensors having photoelectric conversion elements to various fields (for example, X-ray imaging apparatuses). Further, with the tendency of enlargement of the picture area, the pixel pitch becomes finer. 
     FIG. 5 is a schematic view showing an example of a photoelectric conversion device using a TFT matrix panel. The device consists of gate lines  53  for driving TFTS, photoelectric conversion elements  55  each consisting of a PIN-type diode, bias lines  52 , signal lines  51  and TFT sections  57 . 
     Carriers as generated by light incidence to the photoelectric conversion element  55  are stored, and when reading out the stored carriers, the gate line  53  is biased to effect readout. At this time, the driving speed of TFTs is limited by the resistance of the gate line  53 . Especially, with photoelectric conversion devices, there is a problem that increase of wiring resistance increases the sensor noise, in addition to limiting the response speed. This is attributable to the floating capacitance of a crossing of the gate line  53  with the bias line  52  or the signal line  51 . 
     Additionally, as the pixel pitch of the panels becomes finer, the aperture ratio per pixel becomes small. The reason is that in order to accomplish the finer pixel pitch while optimizing the performance of TFT as the switching element as well as the wiring resistance, it becomes necessary to reduce the electrode area of the aperture section. This will decrease the transmittance of the backlight of liquid crystal panels using TFTs to lower the luminance of the liquid crystal display. Furthermore, in an imaging device such as a photoelectric conversion device, the area of a light receiving section will be reduced to result in lowering of the sensitivity. 
     Therefore, in order to solve the two technical problems, it is considered that the film thickness of each wiring is increased to reduce the wiring resistance and to increase the aperture area. However, merely increasing the film thickness of the gate line  53  will deteriorate the coverage of an interlayer insulating film of a crossing of the gate line with other wirings to make it difficult to maintain the dielectric strength. Further, there is a possibility that the reduced thickness of the insulating film may increase the floating capacitance. 
     On the other hand, when the thickness of the insulating film is increased in order to thicken the wiring to reduce its resistance and to maintain the dielectric strength of the crossing, the driving capability of the TFTs will be lowered. Therefore, when the driving speed is to be increased, the driving voltage needs to be raised, which necessitates further increase of the dielectric strength. 
     Thus, the enlargement of the panels needs to reduce the resistance of a lower wiring such as the gate line, that is, a wiring located nearest to the substrate. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to increase the metal film thickness of a lower metal wiring and to reduce the wiring resistance, thereby assuring the coverage of an interlayer insulating film formed between the lower metal wiring and an upper metal wiring to secure the reliability. 
     According to a first aspect of the present invention, there is provided a semiconductor device comprising: 
     a thin film transistor (TFT) comprising a gate electrode formed on an insulating substrate, a gate insulating film formed on the gate electrode, and a pair of electrodes having a semiconductor layer and an ohmic contact layer therebetween; and 
     a gate wiring connected to the gate electrode, and a signal wiring connected to one of the pair of electrodes, 
     wherein the gate wiring and the signal wiring are arranged in superposition in the film thickness direction with an interlayer insulating layer therebetween to have a plurality of crossings with each other and the interlayer insulating layer has a plurality of steps overstriding a lower wiring at the crossings. 
     According to a second aspect of the present invention, there is provided a semiconductor device comprising: 
     a thin film transistor (TFT) comprising a gate electrode formed on an insulating substrate, a gate insulating film formed on the gate electrode, and a pair of electrodes having a semiconductor layer and an ohmic contact layer therebetween; and 
     a gate wiring connected to the gate electrode, and a signal wiring connected to one of the pair of electrodes, 
     wherein the gate wiring and the signal wiring are arranged in superposition in the film thickness direction with an interlayer insulating layer therebetween to have a plurality of crossings with each other, the interlayer insulating layer has a plurality of steps overstriding a lower wiring at the crossings, and the film thickness of the lower wiring is smaller at the crossings than at a portion thereof not crossing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing a pattern of a TFT matrix panel according to a first embodiment of the present invention; 
     FIG. 2 is a sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a schematic view showing a pattern of a TFT matrix panel according to a second embodiment of the present invention; 
     FIG. 4 is a sectional view taken along line  4 — 4  of FIG. 3; 
     FIG. 5 is a schematic view showing an example of a pattern for a TFT matrix panel; 
     FIG. 6 is a schematic view showing the application of the TFT matrix panel according to the present invention to a direct-type radiation detection device; 
     FIG. 7 is a schematic view showing an image when the semiconductor device according to the present invention is applied to a radiation detection system; and 
     FIG. 8 is a schematic view showing another image when the semiconductor device according to the present invention is applied to a radiation detection system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments of the present invention will be described with reference to the drawings. 
     (Embodiment 1) 
     FIG. 1 is a schematic view showing a pattern of a photoelectric conversion device using a TFT matrix panel as the semiconductor device according to Embodiment 1 of the present invention. In this embodiment, the film thickness of a gate line at crossings is smaller than that of a portion not crossing. 
     The photoelectric conversion device of FIG. 1 consists of signal lines  11 , bias lines  12 , gate lower wirings  13 , gate upper wirings  14 , photoelectric conversion elements  15 , lower electrodes  16 , and TFT sections  17 . The gate lower wiring  13  and the corresponding gate upper wiring  14  form a gate line. The TFTs  17  arranged in a matrix pattern are driven by a bias applied by a driver (not shown) to the gate line  13 . In this embodiment, each photoelectric conversion element  15  has a PIN structure consisting of p-type a-Si (amorphous silicon), i-type a-Si, and n-type a-Si and carriers generated by light incidence on the photoelectric conversion elements  15  are read out from the lower electrodes  16 . At this time, the bias lines  12  connected to a common electrode driver are each maintained at a constant potential. 
     Now, crossings between wirings will be described. For the purpose of description, a crossing of the gate line with the signal line  11  is described herein. FIG. 2 shows a sectional view taken along line  2 — 2  of FIG.  1 . In this embodiment, the gate line  13  is a wiring arranged nearest to a substrate of wirings formed on the substrate. In the figure, reference numeral  21  denotes an insulating substrate,  13  denotes the gate lower wiring,  14  denotes the gate upper wiring,  24  denotes an insulating film,  25  denotes an i-type semiconductor layer,  26  denotes an n + -type semiconductor layer,  11  denotes the signal line,  27  denotes a first protective film, and  28  denotes a second protective film. 
     Here, a method of fabricating the TFT panel according to this embodiment is described below. 
     1. An Al film is formed entirely on a surface of the substrate and then patterned through photolithography or the like in a pattern corresponding to the pattern of the gate lower wiring  13  to form the gate lower wiring  13 . 
     2. The insulating layer  24  is formed of SiN in a thickness of 2000 to 4000 Å, then the i-type semiconductor layer  25  is formed of a-Si in a thickness of 400 to 3000 Å and the n + -type semiconductor layer  26  is further formed of a-Si in a thickness of 300 to 2000 Å by a continuous film forming process. 
     3. Then, the gate wiring, and the insulating layer  24 , the i-type semiconductor layer  25 , and the n + -type semiconductor layer  26  of the photoelectric conversion element are patterned through photolithography and removed in the pattern by the RIE method. 
     4. The photoelectric conversion element and the TFT section are masked and the signal line  11  and the gate upper wiring  14  are formed. 
     5. The TFT section  17  and the wiring are masked and the PIN-type photoelectric conversion element  15  is formed. 
     6. A prescribed pattern is formed through photolithography, then the n + -film, the a-Si film and the SiN film are simultaneously etched by the RIE method to effect element isolation, and thereafter the protective film  27  made of SiN is formed as a passivation film in a thickness of 3000 to 15000 Å. The bias line  12  is formed of Al or the like in a thickness of about 3000 to 10000 Å, and finally the protective film  28  is formed of an organic material such as polyimide in a thickness of 2 to 10 μm. 
     The above mentioned steps completes the TFT panel according to this embodiment. 
     Incidentally, the gate line is made of a tantalum or titanium film of 400 to 3000 Å in thickness and the signal line and the bias line are made of an aluminum film of 500 to 20000 Å in thickness, respectively. 
     Further, the PIN-type photoelectric conversion element  15  may be made of, for example, a p-type semiconductor layer and an n-type semiconductor layer each having a film thickness of 400 Å to 1500 Å and an i-type semiconductor layer having a film thickness of 4000 Å to 15000 Å. 
     In this embodiment, the signal line  11  and the gate upper wiring  14  can be simultaneously formed by the same process, which simplifies the process. Incidentally, the film configuration at the crossings between the wirings may be the same as that for the semiconductor layers of the TFT sections  17 , so that the TFT sections and at least the crossings may be fabricated through the same process and only the channels of the TFT sections may be formed through dry etching or the like. Therefore, there is no need to provide isolation between the wirings at the crossings using additional steps, so that the crossings can be fabricated through a simple process. Incidentally, the insulating layer  24 , the i-type layer  25 , and the n + -type layer  26  may be formed not only at the crossings but also entirely below the signal line  11 . If a desired dielectric strength can be maintained only by the insulating film  24  in relation with the gate line or the like at the crossings, an additional process may be carried out to remove the other semiconductor layers. Although the crossings between the gate line and the signal line has been described above, it should be noted that the crossings with the bias line  12  may be similarly configured. 
     The gate line consisting of the gate lower wiring  13  and the gate upper wiring  14  is provided as a single layer to have a reduced film thickness at the crossing between the signal line  11  and the bias line  12  as compared with the other portion as shown in FIG. 1, and there is no overstriding of the insulating film  24  with regard to the gate line consisting of the gate lower wiring  13  and the gate upper wiring  14  so that the dielectric strength is ensured without increasing the film thickness of the insulating layer  24 . 
     Furthermore, improvement in the coverage and dielectric strength of the interlayer insulating film provided between the gate line and the signal line or the bias line as well as the reliability can be ensured. On the other hand, since the two metal layers are provided at the portion other than the crossings to increase the film thickness thus lowering the wiring resistance, the noise is reduced and a higher driving speed is attained. In addition, if a breaking occurs in the gate lower wiring  13  of the gate line and there exists the gate upper wiring  14  in that region, the upper wiring functions as a redundant wiring to maintain electrical conduction, thereby reducing the fraction defective. 
     In this embodiment, although the gate upper wiring  14  has a single layer configuration, it is needless to say that a multi-layer configuration may be adopted. Further, the semiconductor device according to this embodiment can be used as a radiation detection device by providing a wavelength converter such as GOS (gadolinium oxysulphide phosphor) or CsI on an upper surface of the panel. 
     (Embodiment 2) 
     FIG. 3 is a view showing a pattern of a photoelectric conversion device using a TFT matrix panel according to Embodiment 2. In the figure, some elements performing similar functions to those shown in FIG. 1 have the same reference numerals as those in FIG.  1  and are not described here in detail. In this embodiment, MIS-type capacitors are used as photoelectric conversion elements  35 . After carriers are generated by light incidence on the photoelectric conversion elements  35 , a bias is applied by a driver to the signal lines  11  and the carriers are read out from electrodes  36 . Further, a common electrode driver connected to the signal lines  11  can be driven to remove the stored carriers, thus refreshing the photoelectric conversion elements  35 . 
     Now, a crossing of the signal line  11  and a gate line  33  is described. FIG. 4 is a sectional view taken along line  4 — 4  of FIG.  3 . In this embodiment, the gate line  33  is a wiring arranged nearest to a glass substrate  41  of wirings formed on the substrate. A method of fabricating the TFT matrix panel according to this embodiment is described below. 
     1. After a metal layer made of Cr is deposited entirely on a surface of the glass substrate  41  in a thickness of 1000 to 5000 Å by sputtering, the gate line  33 , a gate electrode of the TFT, and a lower electrode  36  of the MIS-type photoelectric conversion element are formed through photolithography. 
     2. As shown in FIG. 4, a step (or level difference) A at the crossing between wirings is formed by etching through photolithography. 
     3. Next, a step B is formed by half-etching through photolithography with a mask other than a mask used for the formation of the step A, specifically etching in a thickness of 500 to 2500 Å, which is about half the entire thickness of the deposited film forming the gate line  33 . 
     4. Using a plasma CVD method, an SiN insulating layer  44  is formed in a thickness of 2000 to 4000 Å entirely on the surface of the substrate  41 , then an i-type semiconductor layer  45  of a-Si is formed in a thickness of 3000 to 12000 Å, and an n + -type semiconductor layer  46  is formed in a thickness of 300 to 2000 Å by a continuous process. Since the insulating layer  44 , the semiconductor layer  45 , and the n + -layer  46  may be used commonly to the TFT section  17  and the MIS-type photoelectric conversion element  35 , they may be fabricated through the same process. 
     5. The insulating layer  44 , the semiconductor layer  45 , and the n + -layer  46  formed on the gate line  33  are removed, and then a contact hole is formed to connect a TFT source or drain electrode and the lower electrode  36  of the MIS-type photoelectric conversion element to each other. This process is accomplished by a RIE method after a predetermined pattern is formed through photolithography. 
     6. After the signal line  11  and the bias line  12  are formed of Al in a thickness of approximately 5000 to 20000 Å and TFT source/drain electrodes  18  are formed, a channel of the TFT section  17  is subjected to the RIE method to etch the n + -type film by approximately 500 Å and the a-Si film by approximately 200 Å. 
     7. A prescribed pattern is formed through photolithography, and the n + -type film, the a-Si film, and the SiN film are simultaneously etched by the RIE method to effect element isolation, and thereafter a protective film  43  made of SiN is formed as a passivation film in a thickness of 3000 to 15000 Å. Then, a polyimide protective film  47  is coated on the passivation film in a thickness of 2 to 10 μm by use of a spinner. The use of polymide which is photosensitive makes it possible to remove the electrode connected to the IC by photolithography. 
     The TFT matrix panel is fabricated by the above-mentioned process. In this embodiment, all of the insulating layer  44 , the semiconductor layer  45 , and the n + -layer  46  are left below the signal line  11 . This is because the element isolation has been effected after the formation of the signal line  11 , but these layers may be removed by changing the sequence of the steps. Further, all of the insulating layer  44 , the semiconductor layer  45 , and the n +  layer  46  are left at the crossing. This is because the semiconductor layer  45  and the n +  layer  46  can be considered as insulators as long as the device operates in a usual manner. Therefore, the dielectric strength can be maintained without carrying out extra steps to remove the semiconductor layers of the crossing. 
     Referring to FIG. 4, since the gate line has the step (referred to “overstriding step”) at the crossing of the gate line with the signal line or the bias line, the thickness (level difference) of the insulating film necessary to overstride the gate line is at most ½ of the maximum film thickness of the gate line, which provides better coverage at a corner portion or the like as compared with the case where no-steps are provided and ensure the dielectric strength of the insulating film. Although in this embodiment the single step is provided, it is needless to say that a plurality of steps (level differences) may be provided. This embodiment makes it possible to increase the film thickness of the gate line to reduce its wiring resistance, to maintain the dielectric strength at the crossings with the other wirings and to reduce the floating capacitance. 
     The semiconductor device according to this embodiment can also be used as a radiation detection device by providing a wavelength converter such as GOS (gadolinium oxysulphide phosphor) or CsI on an upper surface of the panel. 
     In addition, an MIS-type capacitor is used as a photoelectric conversion element in this embodiment and a PIN-type diode is used in Embodiment 1 by way of example. Of course, the two types of elements may be replaced by each other and other types of elements may be also used as the photoelectric conversion element. Moreover, the configuration according to this embodiment may be used simultaneously with the configuration according to Embodiment 1 to provide a more preferred TFT panel. 
     (Embodiment 3) 
     In this embodiment, a radiation direct conversion element and a TFT panel used for a radiation detection device are described below. FIG. 6 is a schematic view showing the principle of this embodiment. When a radiation is incident on a direct conversion detector such as of GdTe, a-Se, PbI 2 , etc. fixed with a constant bias, electron-hole pairs are generated, and electrons and holes travel in accordance with an electric field and are stored in capacitors as connected to the detector. Thereafter, they are sequentially transferred to a readout circuit by TFTs. If the capacitor is the MIS-type capacitor used in Embodiment 2, the TFT matrix panel of Embodiment 2 can be used as such. Using the TFT panel according to the present invention makes it possible to reduce the signal noise and to make large the storing capacitors to store a larger amount of charge, so that it can be preferably used for a direct radiation detection device. 
     (Embodiment 4) 
     FIG. 7 is a schematic view showing the configuration of a radiation detection system. As shown in FIG. 7, in a radiation diagnostic system according to this embodiment, a radiation  6060  generated in a radiation tube  6050  pass through the chest part  6062  of a patient or subject  6061  and is then incident on a photoelectric conversion layer  6040  as installed on an upper part of a photoelectric conversion device. 
     The incident radiation contain an internal information of the body of the patient or subject  6061 . In correspondence to the incidence of the radiation, the photoelectric conversion layer generates electrons and holes to provide an electrical information. The information is converted to a digital signal, which is picture-processed in a picture processor  6070  to be observed on a display  6080  in a control room. 
     In addition, the information can be transferred to a remote location through a transmission means such as a telephone line  6090  and can be displayed on a display  6081  in a doctor room or the like at a different location or can be stored in a storage means such as optical disks to permit diagnosis by a doctor at a remote location. Further, the information can be recorded on a film  6110  by a film processor  6100 . 
     Incidentally, although this embodiment has been described with reference to the application of the photoelectric conversion device to the radiation detection system, it should be appreciated that the term “radiation” used herein refers to X-rays, α-rays, β-rays, γ-rays, or the like, and that the photoelectric conversion device of the present invention can apply to a radiation imaging system such as a non-destructive examination system or the like. 
     FIG. 8 is a view for explaining the above-mentioned radiation detection system in more detail and shows a radiation detection system having a radiation detection device using a photoelectric conversion element panel according to the present invention incorporated therein. This system includes a radiation detection device used for stand-up type X-ray imaging, an operation panel for operating the device, and a control station for controlling data storage and radiation detection sensors. The radiation detection device is formed by depositing or bonding a phosphor layer made of CsI or GOS, which converts a radiation into visible light, to the photoelectric conversion element panel. This makes it possible that an X-ray with linear directivity as emitted by an X-ray source passes through the body of a person who stands in front of the radiation detector, the passing X-ray is converted by the detector into visible light, which is then photoelectrically converted to enable imaging of an X-ray transmission distribution for a body region. This device can be used for the field of roentgen imaging as one of decubitus-type which is attached to a bed or of handy-type. Moreover, the device can be also used as a non-destructive examination system by causing a radiation to pass through an object other than human bodies. Although this embodiment has been described with reference to an indirect radiation detection device which detects a visible light converted from a radiation by a wavelength converter, it is needless to say that a direct detection device as described for Embodiment 3 can also be used.