Abstract:
The present invention is directed to an image sensor comprising a thin film transistor on a transparent substrate, the first interlayer film covering the thin film transistor, a photodiode as a photodetector on the first interlayer film and the second interlayer film on the photodiode and the first interlayer film, where the first and the second interlayer films are made of different materials and at least a contact hole to the element consisting of polysilicon in the thin film transistor is formed after removing the second interlayer film around the area where contact holes are to be formed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an image sensor where a thin film transistor (hereinafter, referred to as “TFT”) such as a switching element and a photodiode (hereinafter, referred to as “PD”) as a photodetector are formed on a single substrate. 
     2. Description of the Related Art 
     In an image scanner or facsimile, an image sensor is incorporated for detecting a reflected light from an original surface by which a light is irradiated on an original. An image sensor generally has a configuration in which pixels are disposed in line, each of which consists of a PD made of amorphous silicon (a-Si) as a photodetector and an analogue switch, i.e., a polysilicon TFT. FIG. 3 is a schematic plan view of a pixel in such an image sensor. As shown in the figure, a single TFT is formed to one PD. The PD is connected to a bias line  41  via the TFT, while a gate is connected to a scanning circuit (not shown). Charge generated in each PD is temporally stored in its junction capacitance, and is chronologically read as an electric signal via a signal readout line  42  by driving a switching device by the TFT, at a rate of several hundred kHz to several hundred MHz. In such a TFT driving type of image sensor, reading can be conducted with a single driving IC owing to TFT operation, and thus the number of driving ICs for driving an image sensor may be reduced. 
     In such an image sensor in which TFTs and PDs are formed in sequence, it is necessary to form the PDs with a thickness of at least one-micron meter for sufficient sensitivity. An insulating film covering the PDs, therefore, must have a thickness of at least 200 nm in the light of its coverage. 
     Conventionally, such an image sensor has been manufactured by, for example, the procedure shown in FIGS.  5 ( a ) to ( k ). The cross section in the description below is one taken on line A-A′ in FIG.  3 . 
     Specifically, on a transparent insulating substrate is deposited a polysilicon film  1  with a thickness of 50 to 100 nm by an appropriate technique such as CVD; the film is patterned according to a TFT channel shape by photolithography; and on the film is deposited a gate oxide film  2  with a thickness of 50 to 100 nm (FIG.  5 ( a )). Alternatively, a polysilicon film may be formed by crystallizing an a-Si by laser annealing, which has been separately formed by CVD. 
     Then, on the film is deposited a layered structure consisting of a polysilicon or metal film and a silicide film as a gate electrode  3  with a thickness of about 100 to 300 nm, which is then similarly patterned (FIG.  5 ( b )). 
     Then, ion doping is conducted for forming source-drain region  4  while phosphorous (P) and boron (B) ions are introduced at given doses for n- and p-types, respectively (FIG.  5 ( c )). 
     Subsequently, an SiO 2  film as first interlayer film  5  is deposited by CVD with a thickness of 200 to 500 nm, covering the whole surface (FIG.  5 ( d )). 
     On first interlayer film  5  is formed lower electrode  6  in a PD, made of a metal such as Cr with a thickness of 100 nm, and it is then patterned into a desired shape (FIG.  5 ( e )). 
     On the surface is formed by CVD a p-i-n type of a-Si layer  7  with a thickness of 1 μm consisting of n-, i- and p-layers from the bottom. On the layer are sequentially formed ITO layer  8  as a transparent electrode with a thickness of 100 nm and barrier metal layer  9  such as tungsten silicide with a thickness of 50 to 100 nm (FIG.  5 ( f )), and then barrier metal layer  9 , ITO layer  8  and a-Si layer  7  are patterned by photolithography into the shape of PD  10  (FIG.  5 ( g )). 
     On the surface is formed an Si 3 N 4  film as a second interlayer film  11  by CVD with a thickness of about 200 to 500 nm (FIG.  5 ( h )). As described above, since a-Si layer  7  in PD  10  has a thickness of about 1 μm, it is necessary to form the insulating film covering the PD with a thickness of 200 nm in the light of its coverage. 
     Then, contact holes  12  are formed, reaching source-drain region  4  for the TFT, gate electrode  3 , lower electrode  6  in the PD and barrier film  9 , the upper layer in the PD (FIG.  5 ( i )). Subsequently, a metal  13  such as Al is deposited to a thickness of 500 to 1000 nm, which is then etched into a desired interconnection shape (FIG.  5 ( j )). Finally, on the surface is deposited an Si 3 N 4  film or organic film such as polyimide as a passivation film  14  with a thickness of 1 μm or less, to form an image sensor as shown in FIG.  5 ( k ). 
     When forming the contact holes  12  in the step shown in FIG.  5 ( i ), the second interlayer film  11  and the first interlayer  5  are sequentially etched. It is necessary to strictly control etching conditions for forming such deep contact holes. Furthermore, the base layer is inevitably damaged because it is extremely difficult to control an etching endpoint. In particular, during forming a contact hole to a polysilicon layer such as the source-drain region  4 , etching damage to the polysilicon may significantly deteriorate device properties. During removing the first interlayer film  5  on the TFT, the PD may be over-etched, which may affect metal films such as the lower electrode  6  in the PD and the barrier film  9  to some extent. 
     Thus, the interconnections in the TFT and the PD may be separately formed for avoiding forming such a deep contact hole. FIGS. 6 (( a ) to ( g )) is a process cross-section showing the procedure. 
     First, a TFT is formed and then the first interlayer film  5  is deposited as described above (FIG.  6 ( a )). Then, the interconnection layer  13   a  a for the TFT is formed (FIG.  6 ( b )), and on the interconnection layer  13   a  is formed the third interlayer film  15  such as a silicon oxide film with a thickness of about 200 to 500 nm. 
     Then, the lower electrode  6  (FIG.  6 ( d )) for the PD and the PD  10  (FIG.  6 ( e )) are formed as described above. Subsequently, the second interlayer film  11  is formed, covering the whole surface as described above (FIG.  6 ( f )), and finally the interconnection layer  13   b  for the PD is formed (FIG.  6 ( g )). 
     Thus, interconnection layers for the TFT and the PD may be separately formed for avoiding damage to the base layer during forming contact holes. It, however, requires two interconnection-forming steps, leading to a complicated process. 
     SUMMARY OF THE INVENTION 
     An objective of this invention is to provide an uncomplicated process for manufacturing an image sensor where damage to a base layer is minimized during forming a contact hole, as well as an image sensor structure prepared thereby. 
     This invention which can solve the above problems provides an image sensor comprising a thin film transistor on a transparent substrate, the first interlayer film covering the thin film transistor, a photodiode as a photodetector on the first interlayer film and the second interlayer film on the photodiode and the first interlayer film, where the first and the second interlayer films are made of different materials and at least a contact hole to the element consisting of polysilicon in the thin film transistor is formed after removing the second interlayer film around the area where contact holes are to be formed. 
     In particular, it is preferable that the contact hole formed by removing the second interlayer film is for a source-drain region or the source-drain region and a gate electrode, and the second interlayer film is removed over the whole surface of the thin film transistor or over the whole surface of the thin film transistor and an area sufficient to expose a part of the lower electrode in the photodiode. It is more preferable that the first and the second interlayer films are a silicon oxide film and a silicon nitride film, respectively. 
     This invention also provides a process for manufacturing an image sensor, comprising the steps of (1) forming a thin film transistor on a transparent substrate, (2) forming the first interlayer film covering the thin film transistor, (3) forming a photodiode as a photodetector on the first interlayer film, (4) forming the second interlayer film made of a different material from that for the first interlayer film, on the photodiode and the first interlayer film, (5) removing the second interlayer film at least around the area where are to be formed contact holes to the elements consisting of polysilicon in the thin film transistor, (6) forming contact holes in the first interlayer film exposed in the area where the contact holes are to be formed, after removing the second interlayer film and (7) forming interconnection layers for the thin film transistor and the photodiode. 
     In particular, a preferable aspect of this invention is the process described above wherein a contact hole for connecting an interconnection layer to the second interlayer film is formed while forming a contact hole in the first interlayer film exposed in the area where the second interlayer film has been removed for forming the contact hole, and wherein the first and the second interlayer films are adjusted in their thickness according to the etching rates for the first and the second interlayer films by the etchant used during forming the contact holes to the first and the second interlayer films. 
     Another preferable aspect of this invention is the process described above wherein a contact hole for connecting an interconnection layer to the second interlayer film while removing the second interlayer film around the area where are to be formed contact holes to the elements consisting of polysilicon in the thin film transistor. 
     This invention can provide an image sensor structure with reduced damage to the base polysilicon without complicating the manufacturing process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 (( a ) to ( k )) is a process cross-section illustrating a process for manufacturing an image sensor according to an embodiment of this invention. 
     FIGS.  2 (( a ) to ( d )) is a process cross-section illustrating a process for manufacturing an image sensor according to another embodiment of this invention. 
     FIG. 3 is a schematic plan view of a pixel in an image sensor. 
     FIGS.  4 (( a ) to ( d )) is a process cross-section illustrating a process for manufacturing an image sensor according to a further embodiment of this invention. 
     FIGS.  5 (( a ) to ( k )) is a process cross-section illustrating a process for manufacturing an image sensor according to the prior art. 
     FIGS.  6 (( a ) to ( g )) is a process cross-section illustrating another process for manufacturing an image sensor according to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of this invention will be specifically described with reference to the process cross-section in FIGS.  1 (( a ) to ( k )). 
     On a transparent substrate is deposited a polysilicon film  1  with a thickness of 50 to 100 nm by an appropriate technique such as CVD; the film is patterned according to a TFT channel shape by photolithography; and on the film is deposited a gate oxide film  2  with a thickness of 50 to 100 nm (FIG.  1 ( a )). 
     Then, on the film is deposited a layered structure consisting of a polysilicon or metal film and a silicide film as a gate electrode  3  with a thickness of about 100 to 300 nm, which is then patterned as described above (FIG.  1 ( b )). 
     Then, ion doping is conducted for forming a source-drain region  4  while phosphorous (P) and boron (B) ions are introduced at given doses for n- and p-types, respectively (FIG.  1 ( c )) Subsequently, an SiO 2  film as the first interlayer film is deposited by CVD with a thickness of 200 to 500 nm, covering the whole surface (FIG.  1 ( d )). 
     On the first interlayer film  5  is formed a lower electrode  6  in a PD, made of a metal such as Cr with a thickness of 100 nm, and it is then patterned into a desired shape (FIG.  1 ( e )). 
     On the surface is formed by CVD a p-i-n type of a-Si layer  7  with a thickness of 1 μm consisting of n-, i- and p-layers from the bottom. On the layer are sequentially formed an ITO layer  8  as a transparent electrode with a thickness of 100nm and a barrier metal layer  9  such as tungsten silicide with a thickness of 50 to 100 nm (FIG.  1 ( f )), and then the barrier metal layer  9 , the ITO layer  8  and the a-Si layer  7  are patterned by photolithography into the shape of the PD  10  (FIG.  1 ( g )). 
     On the surface is formed an Si 3 N 4  film as the second interlayer film  11  by CVD with a thickness of about 200 to 500 nm (FIG.  1 ( h )). Since the a-Si layer  7  in the PD has a thickness of about 1 μm, it is necessary to form the second interlayer film  11  with a thickness of 200 nm or more for adequate coverage. 
     Then, the second interlayer film on the TFT is etched off (FIG.  1 ( i )). During the etching, for removing only the second interlayer film  11 , a nitride film, without damaging the first interlayer film  5 , an oxide film, etchants for the dry etching are CF 4  and O 2 . The etchant mixture ratio may be optimized to increase the selection ratio between the oxide and the nitride films, to selectively remove the second interlayer film  11 , a nitride film, without damaging the first interlayer film  5 , an oxide film. 
     Then, dry etching is conducted using CF 4  and H 2  as etchants in an appropriate mixture ratio to form contact holes  12  to the source-drain region in the TFT, the gate electrode and the lower and the upper electrodes in the PD, i.e., the contact holes in the first and the second interlayer films are simultaneously formed (FIG.  1 ( j )). It is preferable that the etching rate between the oxide and the nitride films is determined and that the thickness ratio between the combination of the first interlayer and the gate insulating films and the second interlayer film is adjusted to be the same as the above etching rate ratio before deposition. For example, when the etching rate ratio between the oxide and the nitride films is 4:5 and the second interlayer film  11 , a nitride film, has a thickness of 500 nm, the first interlayer film  5 , an oxide film, and the gate oxide film  2  may be formed with a total thickness of 400 nm. Subsequently, a metal such as Al is deposited with a thickness of 500 to 1000 nm, which is then etched into a desired interconnection shape (FIG.  1 ( k )). Finally,on the surface is deposited an Si 3 N 4 film or organic film such as polyimide as a passivation film  14  with a thickness of 1 μm or less, to form an image sensor. 
     Although the second interlayer film is removed over the whole surface of the TFT in the above embodiment, it may be adequate to remove the area having a larger radius than that of the contact hole to the TFT by about the thickness of the insulating film to be removed (0.3 to 0.5 μm). Alternatively, the second interlayer film  11  may be removed to expose a part of the lower electrode  6  in the PD. Exposing the lower electrode  6  and then connecting the lower electrode in the PD to the drain region via the interconnection layer  13  can eliminate the step of forming a contact hole to the lower electrode  6 . 
     In another embodiment of this invention, the second interlayer film may be removed while forming a contact hole to the second interlayer film; a contact hole may be selectively formed to the first interlayer film exposed in the area where the second interlayer film has been removed; and the interconnections may be simultaneously formed in the contact holes formed in the first and the second interlayer films. Specifically, in the substrate for which the steps to formation of the second interlayer film  11  have been completed as described above (FIG.  2 ( a )), a contact hole  12   a  to the PD is formed in the second interlayer film  11  by dry-etching using CF 4  and O 2  as etchants as described above while removing the second interlayer film  11  over the TFT, as illustrated in FIG.  2 ( b ). 
     Then, as illustrated in FIG.  2 ( c ), dry etching is conducted while covering the areas except for the area where a contact hole  12   b  is to be formed, with a mask, to form the contact hole  12   b  to the first interlayer film. It is preferable to use CF 4  and H 2  as etchants in such a mixture ratio that a selection ratio between the oxide film and polysilicon is increased, for further minimizing damage to the base polysilicon. Thus, forming the contact holes to the first and the second interlayer films  5  and  11  in separate steps may eliminate the necessity for adjusting a thickness for both films as described in the first embodiment, allowing a design to be more flexible. 
     Finally, as illustrated in FIG.  2 ( d ), a metal such as Al is deposited to a thickness of 500 to 1000 nm, which is then etched into a desired shape for an interconnection  13 , and a passivation film is formed to give an image sensor. 
     When the gate electrode is formed with a metal, the second interlayer film may removed in the area where a contact to the source-drain is to be formed while forming a contact hole to the metal electrode in the PD; a contact hole may be formed to the gate electrode; and then a contact hole to the source-drain may be formed in the first interlayer film exposed in the area where the second interlayer film has been removed. Specifically, in the substrate for which the steps to formation of the second interlayer film  11  have been completed as described above, a contact hole  12   a  to the PD is formed in the second interlayer film  11  by dry-etching using CF 4  and O 2  as etchants as described above while removing the second interlayer film  11  over the area where the contact hole to the source-drain in the TFT is to be formed, as illustrated in FIG.  4 ( a ). 
     Then, covering the remaining areas with a mask such as a photoresist, the second and the first interlayer films  11 , are sequentially dry-etched using CF 4  and O 2  as etchants for forming a contact  12   b  to the gate electrode (FIG.  4 ( b )). Then, dry etching is conducted while covering the areas except for the area where a contact hole  12   c  is to be formed, with a mask to form the contact hole  12   c  to the source-drain (FIG.  4 ( c )). It is preferable to use CF 4  and H 2  as etchants in such a mixture ratio that a selection ratio between the oxide film and polysilicon is increased, for further minimizing damage to the base polysilicon. There has been described a case that the contact hole to the gate electrode is formed before forming the contact hole to the source-drain. Instead, the contact hole to the source-drain can be formed before forming the contact hole to the gate electrode. 
     Finally, as illustrated in FIG.  4 ( d ), a metal such as Al is deposited to a thickness of 500 to 1000 nm, which is then etched into a desired shape for an interconnection  13 , and a passivation film is formed to give an image sensor. 
     There has been described a PD for a p-i-n junction amorphous silicon. This invention is, however, not limited to the particular type, and a configuration comprising a Schottky junction to i-a-Si may be employed. Furthermore, although there has been described only a TFT as a switching element to each pixel, this invention is not limited to the particular TFT. Thus, this invention may be applied to any type of TFT such as a resetting TFT and a driving circuit TFT as long as it can be covered by a layered structure consisting of the first and the second interlayer films during its manufacturing process.