Abstract:
A method of detecting thin film transistor (TFT) defects in a TFT-liquid crystal display (LCD) panel, includes, in part, applying a stress bias to the TFTs disposed on the panel; and detecting a change in electrical characteristics of the TFTs. The change in the electrical characteristics of the TFTs may be detected using a voltage imaging optical system or an electron beam. The panel temperature may be varied while the bias stress is being applied. The change in the electrical characteristics is optionally detected across an array of the TFTs.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to testing of thin film transistor (TFT) arrays, and more particularly to testing the functionality and reliability of such arrays. 
         [0002]    Thin film transistor liquid crystal displays (TFT-LCD) for, e.g., television applications require brighter backlight for better image quality.  FIG. 1  is a cross-sectional view of a TFT-LCD module assembly. The stack includes a polarizer layer  14  and optical film  12 , followed by the TFT panel  10  above which liquid crystal layer  16  is formed, and then the backlight  20 . Color filter  22  and polarizer  14  are disposed above liquid crystal layer  16 . Brighter backlight increases the temperature of TFT-LCD during operation, thus resulting in an increase in the TFT-LCD off current I off . For good TFTs, the variation in I off  as a function of temperature is relatively small, and does not affect the TFT-LCD image quality. However, In the case of defective TFTs, the off-current variation with temperature is large enough to deteriorate the TFT-LCD image quality during operation. 
         [0003]      FIG. 2  is a cross-sectional view of a typical amorphous silicon (a-Si) TFT, which are typically N-channel enhancement type field effect transistors. Metal gate  40  is patterned first on a glass plate, followed by plasma enhanced CVD (chemical vapor deposition) deposition of a gate insulator dielectric material  42 , such as silicon nitride (SiN), and layers of amorphous silicon semiconductor (a-Si)  44  and n+ a-Si  46 . Source metal layer  48  and drain metal layer  50  are then patterned. Next, a passivation layer  52  is deposited over the whole structure. The n+ a-Si layer  46  acts as a low resistance ohmic contact for electrons to maximize the ON current. It also blocks injection of holes into the intrinsic layer to minimize the leakage current in the OFF state. 
         [0004]    TFTs in flat panel displays operate as switches. If the gate voltage exceeds the threshold voltage, and a voltage is applied across the source and drain terminals, current flows from the source to drain. Gate layer  40  and a-Si layer  44  act as parallel plates of a capacitor between which dielectric SiN layer  42  is disposed. 
         [0005]    Amorphous silicon is not very stable and its properties can be modified when exposed to strong illumination or injection of charge carriers. Over time, the interface between the a-Si layer  44  and SiN dielectric layer  42  can accumulate charge during normal operation of the TFT, thereby causing a shift over time of the threshold of the a-Si TFT. Under normal operating conditions, the threshold voltage shift during the ON-times is of the opposite polarity to that occurring during the OFF-times. Therefore, the shifts partially cancel one another. Furthermore, as long as the TFT drive can overcome this shift or variation, operation is not compromised. 
         [0006]      FIG. 4A  is an energy band diagram for an ideal amorphous semiconductor. For amorphous semiconductors, intrinsic localized states separated by the gap between the conduction band and valence band are established near the band edges. However, impurities, such as defects or dangling bonds within the amorphous material, populate the band gap with localized defect states, as shown in  FIG. 4B . The localized defect states result in mobility of charges at nonzero temperatures due to thermally assisted tunneling between localized states. Thus, unlike normal semiconductors, the activation energy in amorphous semiconductors such as a-Si is related to the mobility gap rather than an energy gap. 
         [0007]    The source-to-drain current ISD of a TFT is related to the density of states by the following expression: 
         [0000]    
       
         
           
             
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                 I 
                 SD 
               
             
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               [ 
               
                 A 
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                       E 
                       C 
                     
                     - 
                     
                       E 
                       F 
                     
                     - 
                     
                       q 
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         [0000]    where A is a constant, E C  is the conduction energy, E F  is Fermi energy, Ψ S  is density of states, q is charge of electron, k is Boltzmann&#39;s constant, and T is temperature in Kelvin.  FIG. 5  is an energy band diagram of the metal-insulator-semiconductor (MIS) structure, shown in  FIG. 3 . 
         [0008]    With no voltages applied and at room temperature, the source-to-drain current ISD (I OFF ) of the TFT has a small but nonzero value. As temperature increases, I SD  rises, as illustrated in  FIG. 6 . In some TFT-LCD panel applications, such as televisions, in which the TFTs are illuminated and therefore heated by backlights, current I off  normally remains sufficiently low. 
         [0009]    During the processing of a TFT, a-Si is deposited through plasma enhanced chemical vapor deposition (PECVD) of silane or similar materials and methods. The resulting a-Si film is left with dangling bonds when the silicon-to-silicon bonds are broken. The dangling bonds are defects within the amorphous semiconductor layer and contribute to a nonzero density of states within the band gap, thereby resulting in the mobility of charges (off current). To minimize the density of states due to dangling bonds, the a-Si is hydrogenated. Typically for TFTs, a-Si:H film contains approximately 10 to 20% hydrogen. 
         [0010]    During processing, however, the Si:H bond can be inadvertently broken. For example, during ion bombardment of the a-Si:H film, high energy ions can break the Si:H bond, leaving dangling bonds that lead to an increase in the density of states, and higher I off . Generation of high energy ions during processing can be due to poor or incorrect process parameters, and may result in a global plate (panel) effect rather than in a single, stand-alone TFT defect. In other words, a whole area of a panel rather than a single isolated TFT may have poor quality a-Si:H film. 
         [0011]    A good TFT has a lower density of states in the band gap of a-Si:H and SiNx film, whereas a defective TFT has a higher density of states in the band gap of a-Si:H and SiNx film. As the temperature increases, the charges which are trapped in the band gap transport to the conduction band and contribute to TFT off current. Therefore, a defective TFT will have a larger I off  at higher temperature (See  FIG. 6 ). 
         [0012]    Before the introduction of high illumination backlights for TFT-LCD televisions, the defects described above did not result in failed pixels, and the threshold voltage shifts due to turning the TFTs on and off canceled one another. Recently, the TFT-LCD panel manufacturers have noticed at module assembly that the powerful (and therefore heating) backlights cause such defects and adversely affect the yield. This type of defect cannot be repaired, but detecting it sufficiently early in the fabrication process is important to enable feedback and correction to the fabrication operational parameters to minimize loss. 
         [0013]    One known method of detecting these defects takes advantage of the dependency of doff on temperature. Off current is measured while heat is applied to a TFT-LCD plate or panel that has been assembled into a module. In practice, however, such a method is difficult to implement at the high throughput rates required by the TFT-LCD manufacturers. Sampling is an acceptable technique, and currently manufacturers test fully assembled modules after the array is fabricated and after many of the assembly steps are completed. The main drawbacks associated with heating full panels and measuring I off  are (a) the time required to heat the panels and (b) the complexity of the apparatus needed to accommodate the large-sized panels, which may be two meters long, and two meters wide. 
         [0014]    A need continues to exist for a method and apparatus that detects this type of TFT defect during array testing of LCD plates and well before the process steps in which plates are divided into panels and assembled into modules. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    A method of detecting thin film transistor (TFT) defects in a TFT-liquid crystal display (LCD) panel, includes, in part, applying a stress bias to the TFTs disposed on the panel; and detecting a change in electrical characteristics of the TFTs. The change in the electrical characteristics of the TFTs may be detected using a voltage imaging optical system or an electron beam. 
         [0016]    In some embodiments, the panel temperature is varied while the bias stress is being applied. The panel may be heated or cooled while the bias stress is being applied. In some embodiments, the change in the electrical characteristics is detected across an array of the TFTs. 
         [0017]    The defect detection may be applied at the TFT fabrication level to screen defective plates prior to assembly into modules. The defect detection is performed at an early stage in the process and thus reduces the overall costs. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a cross sectional view of a flat panel display (FPD) assembly, as known in the prior art. 
           [0019]      FIG. 2  is a cross sectional view of an amorphous silicon (a-Si) thin film transistor (TFT), as known in the prior art. 
           [0020]      FIG. 3  shows the formation of the conductive channel and current flow in the TFT of  FIG. 2 , as known in the prior art. 
           [0021]      FIG. 4A  is an energy band diagram of an ideal amorphous semiconductor, as known in the prior art. 
           [0022]      FIG. 4B  is an energy band diagram of a typical amorphous semiconductor, as known in the prior art. 
           [0023]      FIG. 5  is an energy band diagram of an MIS (metal-insulator-semiconductor), as known in the prior art. 
           [0024]      FIG. 6  shows a number of plots of drain-to-source currents of TFTs as a function of inverse temperature, as known in the prior art. 
           [0025]      FIG. 7A  is an energy band diagram of an MIS device prior to the application of an electric bias. 
           [0026]      FIG. 7B  is an energy band diagram of the MIS device of  FIG. 7A  after the application of an electric bias causing charges to be trapped in the band gap. 
           [0027]      FIG. 7C  is an energy band diagram of the MIS device of  FIG. 7A  after the application of an electric bias causing states to be created in the band gap 
           [0028]      FIG. 8  shows the dependence of TFT threshold voltage shift on bias stress time and bias stress voltage. 
           [0029]      FIG. 9  show various plots of the drain-to-source current as a function of gate-to-source voltage for a good and a defective TFT before and after application of a bias stress. 
           [0030]      FIG. 10  is a flowchart of steps taken to detect defects related to the a-Si:H layer in TFTs, in accordance with one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    In accordance with the present invention, to detect defects in a TFT panel, an electric bias is applied to the TFT panel for a known time period. The applied electric bias induces charge trapping in the SiNx film and/or state creation in the a-Si:H film, thus giving rise to the TFT threshold voltage shift. The shift in the threshold voltage results in the variation of the TFT I OFF  current. The amount of the threshold voltage shift (ΔV T ) depends on the applied bias voltage, the duration of the bias, as well as the initial density of state in the films. 
         [0032]      FIG. 7A  is an energy band diagram of an MIS device prior to the application of an electric bias.  FIG. 7B  is an energy band diagram of the MIS device of  FIG. 7A  after the application of an electric bias causing charges to be trapped in the band gap.  FIG. 7C  is an energy band diagram of the MIS device of  FIG. 7A  after the application of an electric bias causing states to be created in the band gap. 
         [0033]      FIG. 8  shows the dependence of TFT threshold voltage shift on the bias stress time and bias stress voltage. As seen from  FIG. 8 , the longer the stress time or the greater the bias voltage VGB, the greater is the amount of the threshold voltage shift AVT. 
         [0034]    Plot  100  of  FIG. 9  shows the drain-to-source current as a function of gate-to-source voltage for both a good and a defective TFT before application of a bias stress. Plot  102  of  FIG. 9  shows the drain-to-source current as a function of gate-to-source voltage for a good TFT after application of a bias stress. Plot  104  of  FIG. 9  shows the drain-to-source current as a function of gate-to-source voltage for a defective TFT after the application of a bias stress. As seen from  FIG. 9 , for each gate-to-source voltage, the shift in current—caused by the shift in the threshold voltage—is greater for a defective TFT than a good TFT. 
         [0035]    Thus, in accordance with the present invention, to detect defects related to the a-Si:H layer in TFTs, an electric bias stress is applied for a time sufficient to increase the defect&#39;s density of states. The increase in the defect&#39;s density of states causes a corresponding shift in the threshold voltage and the I off  of the device. The stressed plate or panel with shifted threshold voltage can then be electrically tested using standard TFT array testers, such as the Array Checker manufactured by Photon Dynamics, Inc., located at 5970 Optical Court, San Jose, Calif. 95138, which uses a voltage imaging optical system (VIOS) technology. Other electrical array testers, such as those using electron beam technology or any other means to measure threshold voltage shift, may also be used. 
         [0036]      FIG. 10  is a flowchart of steps taken to detect defects related to the a-Si:H layer in TFTs in accordance with one embodiment of the present invention. Electric (voltage) bias stress is applied to the panel under test  202 . The voltage level and the duration of the bias is selected by the user. The application of the electric bias test ends at  204 . The bias stress causes defective panels to have shifted threshold voltage shift. Next, a pixel electric test using a tester, such as the Array Checker, manufactured by Photon Dynamics, Inc., is performed to measure voltage changes. The defect threshold is set either prior or after the application of the stress test  208 . The bias stress causes defective panels to have shifted threshold voltage shift which is detectable by the VIOS. Following the defect extraction  210 , the worthiness of panel based on degree of defectiveness is determined  212 . 
         [0037]    In some embodiments, the user adjustable stress voltage may be ±50 volts, and the user adjustable stress time may vary between 1000 to 2000 seconds. The stress may be applied on a sample of panels in the fabrication flow or on every panel. 
         [0038]    In some embodiments, the bias stress time may be reduced if accompanied by a temperature change in the panel. As such, the plate under test may be warmed or cooled simultaneously with the application of the voltage stress. Alternatively, the plate under test may be warmed or cooled either before or after the application of the voltage stress. 
         [0039]    As long as the temperature of the a-Si:H film remains below the a-Si:H deposition temperature of approximately, e.g., 250 to 350° C., the TFTs (both good and defective) are not further damaged. Elevating the TFT temperature to, for example, 50° C. in combination with the stress test may be sufficient to reveal the defects. 
         [0040]    TFTs stressed by the application of the heat relax back to their normal (good or defective) condition after the heat source is removed. Thus, heating may be required as the voltage testing is in progress. This arrangement may have a drawback if the voltage testing method has a dependency on temperature. 
         [0041]    TFTs stressed by the application of a bias voltage relax back to their normal (good or defective) condition after the bias voltage is removed. Typical relaxation time may be several hours, and usually less than a day. Thus, a bias voltage may be applied to a plate at a different location from the array tester machine. The plate may subsequently be placed into the array tester for testing within a short period of time (less than a few hours). This may be helpful to keep the utilization of the array tester high. 
         [0042]    The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. Other additions, subtractions or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.