Patent Document

GOVERNMENT FUNDING 
       [0001]    This invention was made with Government support under contract No. 70NANB3H3029, awarded by the National Institute of Standards and Technology (NIST). The Government has certain rights in this invention. 
     
    
     BACKGROUND 
       [0002]    This disclosure relates to sensor arrays and, in particular, to flexible sensor arrays. 
         [0003]    During the deposition of image sensors on amorphous silicon thin film transistor (TFT) backplanes, a metal layer known as the contact metal/mushroom metal is used to contact a positive (p doped)-intrinsic-negative (n doped) (PIN) sensor to a TFT. In the case of flexible sensor arrays, this layer can cause undesired effects due to film stress and creation of electrical faults by contact with other metal layers. 
         [0004]    The contact metal/mushroom metal is part of the connection between the TFT and the sensor. A via through an encapsulation layer over the TFT can connect the contact metal/mushroom metal to a TFT contact. The contact metal/mushroom metal layer can be disposed over the TFT. The mushroom metal shadows the TFT active region from light to minimize leakage current. However, this adds an additional layer between the TFT and the PIN sensor. 
         [0005]    In the formation of a sensory array, after a TFT is encapsulated, an opening is formed in the encapsulation layer exposing a contact of the TFT. The mushroom metal is deposited over the encapsulation layer and contacts the TFT contact through the via. The n, i, and p layers of the PIN sensor are deposited on the mushroom metal. Accordingly, the creation of the connection between the TFT and the PIN sensor requires deposition and/or patterning processes separate from the TFT fabrication process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cross-sectional view of a transistor and sensor in a sensor array according to an embodiment. 
           [0007]      FIGS. 2-9  are cross-sectional views illustrating a process of forming a transistor and sensor in a sensor array according to an embodiment. 
           [0008]      FIG. 10  is a cross-sectional view of a transistor and sensor in a sensor array according to another embodiment. 
           [0009]      FIGS. 11-14  are cross-sectional views illustrating a process of forming a transistor and sensor in a sensor array according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Embodiments will be described with reference to the drawings. Embodiments include a sensor array having the source/drain (s/d) layer in contact with the sensor media. Accordingly, the pixel contact metal or mushroom metal can be eliminated by adding an ohmic contact layer to the s/d metal. As a result, the number of layers is reduced, not only reducing the complexity of production, but also reduces the probability of defects, increasing yield. 
         [0011]      FIG. 1  is a cross-sectional view of a transistor and sensor in a sensor array according to an embodiment. The sensor array includes a substrate  14 . The substrate can be glass, plastic, or any insulator coated rigid or flexible material. A gate contact  16  is formed on the substrate  14 . A gate insulator  12  separates a semiconductor island  10  from the gate contact  16 . 
         [0012]    Two contact structures  19  and  25  are disposed on the semiconductor island  10 . The first contact structure  19  includes contact layer  18 , a conductive layer  20 , and a doped semiconductor layer  22 . The second contact structure  25  includes a contact layer  24 , a conductive layer  26 , and a first sensor semiconductor layer  28 . 
         [0013]    In an embodiment, the contact layers  18  and  24  can be n+ type semiconductor layers. Accordingly, the contact layers  18  can form ohmic contacts between the conductive layers  20  and  26  and the semiconductor island  10 . An ohmic contact is a contact to a semiconductor that has substantially linear current-voltage characteristics. As a result, the conductive layers  20  and  26  can form source/drain contacts for the TFT. 
         [0014]    An encapsulation layer  30  covers the contact structures  19  and  25 , and the semiconductor island  10 . An opening  33  in the encapsulation layer exposes the first sensor semiconductor layer  28 . A second sensor semiconductor layer  32  is formed in the opening  33  and extends over the encapsulation layer  30 . A third semiconductor layer  34  is formed on the second sensor semiconductor layer  32 . 
         [0015]    In an embodiment, the first sensor semiconductor layer  28  can be an n+ type semiconductor layer. The second sensor semiconductor layer  32  can be an intrinsic semiconductor layer. The third semiconductor layer  34  can be a p-type semiconductor layer. Accordingly, the first, second, and third semiconductor layers  28 ,  32 , and  34  together form a PIN sensor. The n+ type semiconductor of the PIN sensor is directly connected to a conductive layer  26  forming a contact of a TFT. As a result, a mushroom metal layer is not needed to make the connection. 
         [0016]      FIGS. 2-9  are cross-sectional views illustrating a process of forming a transistor and sensor in a sensor array according to an embodiment.  FIG. 2  illustrates a substrate  14 . In  FIG. 3 , a gate contact  16  is deposited on the substrate  14 . A gate insulator  12  is deposited over the gate contact  16 . A first semiconductor layer  36  is deposited over the gate insulator  12 . The first semiconductor layer  36  can be formed from an intrinsic semiconductor. 
         [0017]    As used in this disclosure, depositing, deposition, or the like, can refer to any technique of applying materials. For example, deposition can include chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE), or the like. In another example, depositing can include depositing by printing. For example, ink-jet printing can be used for depositing. Moreover, deposition is not limited to depositing a uniform layer. In contrast, deposition can include the patterned application of materials such as pattern-wise printing of materials through inkjet printing. 
         [0018]    In  FIG. 4 , a second semiconductor layer  38 , a conductive layer  40 , and a third semiconductor layer  42  are deposited on the first semiconductor layer  36 . As the conductive layer  40  can be patterned into source/drain contacts for a TFT, the conductive layer  40  can be referred to as a source/drain contact layer. In an embodiment, the second and third semiconductor layers  38  and  42  can be formed from n+ type semiconductors. The second semiconductor layer  38  can be selected to provide an ohmic contact between the conductive layer  40  and the first semiconductor layer  36 . The third semiconductor layer  42  can, but need not be the same material as the second semiconductor layer  38 . For example, the third semiconductor layer  42  can be selected for a semiconductor in a PIN sensor, while the second semiconductor layer  38  can be selected to optimize the ohmic contact to conductive layer  40 . 
         [0019]    In an embodiment, a thickness of the third semiconductor layer  42  can be larger than a desired thickness of a corresponding portion of the first sensor semiconductor layer  28 . 
         [0020]    In  FIG. 5 , the second semiconductor layer  38 , the conductive layer  40 , and the third semiconductor layer  42  are patterned, forming contact structures  45  and  47 . Accordingly, second semiconductor layer  38 , conductive layer  40 , and third semiconductor layer  42  are patterned into the contact layers  18  and  24 , first and second conductive layers  20  and  26 , and patterned third semiconductor layers  44  and  46 . In an embodiment, due to the patterning of the conductive layer  40  and the third semiconductor layer  42 , cross-sections of the conductive layer  26  and the patterned third semiconductor layer  46  can have substantially the same shape. 
         [0021]    In  FIG. 6 , the first semiconductor layer  36  is patterned to form the semiconductor island  10 . In this embodiment, the patterned third semiconductor layers  44  and  46  were not protected from the process used in patterning the first semiconductor layer  36 . Accordingly, the patterned third semiconductor layers  44  and  46  can be etched and reduced in thickness. 
         [0022]    As the thickness of the third semiconductor layer  42  was greater than the desired thickness of the first sensor semiconductor layer  28 , when the first semiconductor layer  36  is patterned, the patterned third semiconductor layer  46  can be reduced in thickness to form the first sensor semiconductor layer  28 . Similarly, the patterned third semiconductor layer  44  can be reduced in thickness to form the semiconductor layer  22 . 
         [0023]    In  FIG. 7 , the structure is encapsulated. Encapsulation layer  30  is deposited over the, first sensor semiconductor layer  28 , the semiconductor layer  22 , the semiconductor island  10 , and the gate insulator  12 . The encapsulation layer  30  and the dielectric can, but need not be formed from the same material. 
         [0024]    In  FIG. 180 , an opening  48  is formed in the encapsulation layer  30 . The opening  48  exposes the first sensor semiconductor layer  28 . In  FIG. 9 , the second sensor semiconductor layer  32  is deposited over the encapsulation layer  30 . The second sensor semiconductor layer  32  can extend into the opening  48 . As a result, the second sensor semiconductor layer  32  can contact the first sensor semiconductor layer  28 . 
         [0025]    Referring back to  FIG. 1 , the third semiconductor layer  34  can be deposited on the second sensor semiconductor layer  32 . As a result, a PIN sensor has been formed with the third semiconductor layer  34 , the second sensor semiconductor layer  32 , and the first sensor semiconductor layer  28 . 
         [0026]    Since the first sensor semiconductor layer  28  was in direct contact to the conductive contact  26 , the PIN sensor is in direct contact to the conductive contact  26 . A transparent top contact layer (not shown) can be deposited over the sensor array. As described above, there is not a mushroom metal layer between the TFT and the PIN sensor. Accordingly, fewer layers are deposited, not only reducing cost and complexity, but also increasing reliability. 
         [0027]    Although a particular structure of for a TFT has been described above, other structures can be used, but still having a direct connection between a semiconductor layer of a sensor and a source/drain contact of the TFT. For example, the TFT could be formed with a top gate structure. The semiconductor layer of the sensor can be formed on a source/drain contact and exposed by a via to the other semiconductor layers of the sensor. Accordingly, even with alternative transistor structures, the mushroom metal layer need not be included and the associated process for forming the mushroom metal layer need not be performed. 
         [0028]    In an embodiment, referring to  FIG. 9 , the mask used to pattern the via  48  can be used to pattern the deposition of the second sensor semiconductor layer  32  and the third sensor semiconductor layer  34 . A transparent conductive layer can be deposited over the patterned p-type semiconductor  34 . Accordingly, a bias can be provided to each PIN sensor. Furthermore, this transparent conductive layer can serve as a protective layer. In an embodiment, the third semiconductor layer  42  can be patterned into the first sensor semiconductor layer  28 , while either none or both of the sensor semiconductor layers  32  and  34  can be patterned. 
         [0029]    In an embodiment, the encapsulation layer  30  can be increased in thickness due to a desired thickness of the first sensor semiconductor layer  28 , described above. Although this increased thickness of the encapsulation layer  30  could result in cracking or other defects, the thickness of the first sensor semiconductor layer  28  and any other layers within an opening  33  in the encapsulation layer  30  can be adjusted to reduce the probability of defects. 
         [0030]      FIG. 10  is a cross-sectional view of a transistor and sensor in a sensor array according to another embodiment. The sensor array of  FIG. 2  is similar to the sensor array of  FIG. 1 . However, the semiconductor layer  22  of  FIG. 1  is not present. In contrast, the encapsulation layer  30  is formed directly on the conductive layer  20 . 
         [0031]      FIGS. 11-14  are cross-sectional views illustrating a process of forming a transistor and sensor in a sensor array according to another embodiment.  FIG. 11  is similar to  FIG. 4  in that a second semiconductor layer  38  and a conductive layer  40  are deposited on the first semiconductor layer  36 . However, the third semiconductor layer  50  can be deposited without the thickness described in  FIG. 4 . The third semiconductor layer  50  can have a thickness substantially equal to a desired thickness for the PIN sensor. Although this thickness has been described as less than the thickness described with reference to  FIG. 4 , the thickness can be as desired, including the thickness described with reference to  FIG. 4 . 
         [0032]    Similar to  FIG. 5 , in  FIG. 12 , the second semiconductor layer  38 , conductive layer  40 , and third semiconductor layer  50  can be patterned to form contact structures  53  and  55 . First sensor semiconductor layer  28  is disposed on conductive contact  26 . Semiconductor layer  52  is disposed on conductive contact  20 . 
         [0033]    In  FIG. 13 , a protective layer  54  is applied to the first sensor semiconductor layer  28 . However, it is not applied to the semiconductor  52 . Note that the protective layer  54  can be part of or in addition to any mask or other protective layer used in the patterning the semiconductor layer  36 . 
         [0034]    In  FIG. 14 , the semiconductor layer  36  is patterned to form the semiconductor island  10 . The protective layer  54  can be removed. Since it was not covered by a protective layer, the semiconductor layer  52  may be removed. However, since it was protected by the protective layer  54 , the first sensor semiconductor layer  28  can remain. The resulting structure can be processed as described above beginning with  FIG. 7 . That is, the structure can be encapsulated with encapsulation layer  30 , an opening  48  formed, and the like. Accordingly, a resulting structure as shown by the example in  FIG. 10  can be formed. 
         [0035]    The structures described above can be coupled to a storage capacitor to store any charge from the PIN sensor. Such a capacitor and other circuitry present in the sensory array can be included, but have been omitted for clarity. 
         [0036]    In an embodiment, the intrinsic semiconductor layer of the PIN sensor can block light that would otherwise be incident on the semiconductor island. As the transmission through the intrinsic semiconductor layer can be greater than zero, some leakage current may be induced in the TFT. However, an intrinsic semiconductor layer about 1 um thick can absorb light energy that is greater than about 2.1 eV. Accordingly, the amount and/or energy of the light that is incident on the semiconductor island, and consequently any induced leakage current, can be minimized without the need of the mushroom metal layer. 
         [0037]    In an embodiment, the sensor array can be used for x-ray imager applications. In such applications, photon energy can be generated from excited phosphors over the sensor area. The phosphors can block incident visible irradiation. Accordingly, such radiation is prevented from being incident on the semiconductor island. 
         [0038]    Another embodiment includes an article of machine readable code embodied on a machine readable medium that when executed, causes the machine to perform any of the above described operations. As used here, a machine is any device that can execute code. Microprocessors, programmable logic devices, multiprocessor systems, digital signal processors, personal computers, or the like are all examples of such a machine. 
         [0039]    Although examples have been given with reference to a pixel circuit, the above structures and techniques can be used in any thin film transistor circuit. For example, the above structures and techniques can be used for any p-i-n sensor thin film transistor backplane. 
         [0040]    Although particular examples of deposition, patterning, and the like have been given, any combination of deposition techniques can be used. For example, some or all of the layers can be printed rather than or in combination with other deposition, masking, etching, or other techniques. 
         [0041]    Although particular doping of semiconductor layers have been given, any combination of semiconductor and conductive layers can be used. 
         [0042]    Although particular embodiments have been described, it will be appreciated that the principles of the invention are not limited to those embodiments. Variations and modifications may be made without departing from the principles of the invention as set forth in the following claims.

Technology Category: h