Abstract:
A semiconductor device carries out a test utilizing contact with a probe needle without being affected by narrowing of the pitch at which output pads are arranged. The device is equipped with test circuits provided between a plurality of output buffers via which signals are output and output pads corresponding thereto. The test circuit includes output switches caused to sequentially make connections by a controller in test and interpad switches involved in making connections of the output pads with a test pad by the controller in test. In test, probe needles are brought into contact with the test pad. The output pads are not used in test, and can be arranged at a narrowed pitch. Thus, the chip area can be reduced and are therefore so that the pitch for the output pads can be narrowed and the chip area can be decreased.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to semiconductor devices, and more particularly, to a semiconductor device suitably applicable to an integrated circuit for driving a liquid crystal panel.  
           [0003]    2. Description of the Related Art  
           [0004]    Integrated circuit chips of manufactured semiconductor devices are tested in various ways. One of the tests is a function test that confirms whether an expected signal is available at an output terminal in response to a given signal applied to an input terminal. Generally, in the function test, connections with all pads used on the chip are made in a certain way.  
           [0005]    [0005]FIG. 7 shows a conventional manner of testing semiconductor devices. Referring to FIG. 7, a plurality of pads  102  are formed around a circuit formation surface of a semiconductor chip  101 . The pads  102  are connected to all terminals used as inputs, output and power supply of circuits formed on the semiconductor chip  101 .  
           [0006]    The function test of the semiconductor chip  101  is carried out in such a manner that probe needles  103  connected to a test device are contacted to all the pads  102  used. That is, input signals that are output from the test device are input to the pads  102  of the given input terminals of the semiconductor chip  101  via the probe needles  103 , and the resultant signals that are output to the given output terminals are sent to the test device via the probe needles  103 .  
           [0007]    The number of pads  102  on the semiconductor chip  101  increases as the integration progresses. For example, a recent integrated circuit for driving a liquid crystal panel has output terminals as many as 384 outputs. Thus, the pitch of the pads  102  is narrowed and the pitch is now as narrow as 50 μm.  
           [0008]    Recently, an increased number of terminals are required as the number of pixels increases due to progress to higher precision of the liquid crystal panel. It is estimated that the integrated circuit for driving the liquid crystal panel further progresses from the 384 outputs and has 480 or 512 outputs. The conventional pad pitch needs an increased chip area and raises the production cost. Therefore, there has been considerable activity in narrowing the pad pitch to thus reduce the chip area so that an increased number of outputs are realized at a low cost. The recent assembly technique goes toward a pad pitch as narrows as 45 μm and further 35 μm.  
           [0009]    However, a new problem will arise from the narrowing of the pad pitch. More particularly, a difficulty in contacting pads with the probe needles will be encountered. It will become difficult to correctly make contact the pads with the probe needles due to the narrowing of the pad pitch. The adjacent pads may frequently be short-circuited. Further, it may be difficult to make an adjustment for cancellation of the difference in contact pressure among the pads due to the difference in height so as to have a uniform constant contact pressure on each pad because each of the all pads is contacted with the respective probe needle. The factors mentioned above will reduce the yield in mass production.  
         SUMMARY OF THE INVENTION  
         [0010]    Taking into consideration the above, an object of the present invention is to provide a semiconductor device that can be tested using probe needles without being affected by narrowing of the pad arrangement pitch.  
           [0011]    To accomplish the above object, there is provided a semiconductor device in which a plurality of output circuits and output pads corresponding to output terminals of the output circuit are arranged, said semiconductor device comprising: output switches provided in series between the output terminals of the output circuits and the output pads corresponding thereto; a test pad used in test; interpad switches provided between the output pads adjacent to each other and between the test pad and the output pad adjacent to the test pad; and controller controlling the output switches and the interpad switches.  
           [0012]    According to another aspect of the present invention, there is provided a liquid crystal driver device equipped with a plurality of drive circuits for driving pixels of a liquid crystal panel and a plurality of output pads provided so as to correspond to output terminals of the drive circuits. The liquid crystal driver device includes: a test pad used in test; and a test circuit including output switches disconnecting the output terminals of the drive circuits and the output pads corresponding thereto in test, interpad switches connecting all the output pads and the test pad in test, and a controller sequentially making connections via the output switches in test.  
           [0013]    The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a diagram of the principal structure of a semiconductor device of the present invention;  
         [0015]    [0015]FIG. 2 is a diagram of a part of the structure of a test circuit according to a first embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a waveform diagram of signals observed in the circuit shown in FIG. 2;  
         [0017]    [0017]FIG. 4 is a diagram of a part of the structure of a test circuit according to a second embodiment of the present invention;  
         [0018]    [0018]FIG. 5 is a conceptual diagram of pad formation surface of an integrated circuit for a data driver;  
         [0019]    [0019]FIG. 6 is a view showing how the integrated circuit of the data driver is tested; and  
         [0020]    [0020]FIG. 7 is a view of a conventional manner of testing a semiconductor device. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    First, the outline of the present invention is described with reference to the accompanying drawings.  
         [0022]    [0022]FIG. 1 is a diagram showing the principle of the semiconductor device of the present invention.  
         [0023]    The semiconductor device of the present invention is equipped with a test circuit  1  located at the stage following an output buffer that outputs a plurality of output signals. The test circuit  1  has output buffers  2   1, 2   2 , . . . ,  2   n , output pads  3   1 ,  3   2 , . . . ,  3   n , output switches  4   1 ,  4   2 , . . . ,  4   n , a single test pad  5 , interpad (pad-to-pad) switches  6   1 ,  6   2 , . . . ,  6   n , and a controller  7 . The output buffers  2   1 ,  2   2 , . . . ,  2   n  form respective output circuits. The output pads  3   1 ,  3   2 , . . . ,  3   n , are connected in series between the output pads  3   1 ,  3   2 , . . . ,  3   n , and the output buffers  2   1 ,  2   2 , . . . ,  2   n . The interpad switches  6   1 ,  6   2 , . . . ,  6   n , are provided between the adjacent pads  3   1 ,  3   2 , . . . ,  3   n  and between the output pad  3   n  and the test pad  5 . The controller  7  controls the output switches  4   1 ,  4   2 , . . . ,  4   n  and the interpad switches  6   1 ,  6   2 , . . . ,  6   n .  
         [0024]    In the semiconductor device with the above-mentioned test circuit  1 , the function test is carried out as follows. On the signal input side, the probe pads are brought into contact with all the pads of the input terminals used in the test, and the test signals are input thereto. On the signal output side, only the test pad  5  is brought into contact with the probe needle, and all the output signals available at the output pads  3   1 ,  3   2 , . . . ,  3   n  are detected via the test pad  5 .  
         [0025]    In the function test, the controller  7  of the test circuit turn OFF all the output switches  4   1 ,  4   2 , . . . ,  4   n , and simultaneously, turns ON all the interpad switches  6   1 ,  6   2 , . . . ,  6   n .  
         [0026]    Nest, the controller  7  sequentially turns ON one of the output switches  4   1 ,  4   2 , . . . ,  4   n . More particularly, the controller  7  initially turns ON only the output switch  4   1 . Thus, the output of the output buffer  2   1  is electrically connected to the test pad  5  via the output switch  4   1  and all the interpad switches  6   1 ,  6   2 , . . . ,  6   n . Then, the output signal of the output buffer  2   1  is output to the test pad  5 . Next, the first output switch  4   1  is turned OFF and only the second output switch  4   2  is turned ON. This connects the output of the output buffer  2   2  to the test pad  5  via the output switch  4   2  and the interpad switch  6   2 , . . . ,  6   n . Then, the output signal of the output buffer  2   2  is output to the test pad  5 . In the above manner, one of the output switches  4   1 ,  4   2 , . . . ,  4   n  is sequentially turned ON, so that the output signals of the output buffers  2   1 ,  2   2 , . . . ,  2   n  can be sequentially output to the test pad  5  one by one. Then, the output signal available at the test pad  5  is monitored via the single probe needle, so that the outputs of all the output buffers  2   1 ,  2   2 , . . . ,  2   n , can be tested.  
         [0027]    A description will now be given of an embodiment of the present invention applied to an integrated circuit for driving the liquid crystal panel.  
         [0028]    [0028]FIG. 2 is a circuit diagram that partially illustrates a structure of the test circuit according to the first embodiment of the present invention, and FIG. 3 is a waveform diagram of signals observed in the circuit shown in FIG. 2.  
         [0029]    An integrated circuit called a source driver or data driver, and another integrated circuit called a gate driver are connected to the liquid crystal panel. The circuit shown in FIG. 2 is a part of the data driver. The final stage of the data driver is an output circuit that supplies each pixel of the liquid crystal panel with an image voltage. The output circuit is composed of a plurality of operational amplifiers  10   1 ,  10   2 , . . . provided to the respective pixels. The output terminals of the operational amplifiers  10   1 ,  10   2 , . . . , are connected to output pads  12   1 ,  12   2 , . . . via transfer gates  11   1 ,  11   2 , . . . Each of the transfer gates  11   1 ,  11   2 , . . . is made up of a P-channel MOS transistor and an N-channel MOS transistor. Each transfer gate functions as a switch that operates as follows. Each transfer gate is turned OFF when a high-level voltage is applied to the gate terminal of the P-channel MOS transistor, and a low-level voltage is applied to the gate terminal of the N-channel MOS transistor. Each transfer gate is turned ON when the low-level voltage is applied to the gate terminal of the P-channel MOS transistor and the high-level voltage is applied to the gate terminal of the N-channel MOS transistor.  
         [0030]    The gate terminals of the transfer gates  11   1 ,  11   2 , . . . on the N-channel side are connected to non-inverting output terminals of flip-flops  13   1 ,  13   2 , . . . , and the gate terminals thereof on the P-channel side are connected to inverting output terminals. A data input terminal (D) of the flip-flop  13   1 , is connected to the controller  14 , and the non-inverting output terminal thereof is connected to a data input terminal of the next flip-flop  13   2 . Similarly, the non-inverting output terminal of the flip-flop  13   2  is connected to the data input terminal of the next flip-flop. In the above-mentioned manner, the plurality of flip-flops  13   1 ,  13   2 , . . . are cascaded. Clock input terminals (CLK) and a reset input terminal (R) of the flip-flops  13   1 ,  13   2 , . . . are connected to a clock line  15  and a reset line  16  both connected to the controller  14 .  
         [0031]    Transfer gates  18   1 ,  18   2 , . . . that have switching functions are connected between the adjacent output pads  12   1 ,  12   2 , . . . and the output pad arranged at the final stage of the output circuit and a test pad  17 . Each of the transfer gates is made up of a P-channel MOS transistor and an N-channel MOS transistor. The gate terminals of the transfer gates  18   1 ,  18   2 , . . . on the N-channel side are connected to a test line  19  on which a non-inverting test signal travels, and gate terminals thereof on the P-channel side are connected to a test line  20  on which an inverting test signal travels.  
         [0032]    A description will now be given of an operation of the test circuit with reference to FIG. 3.  
         [0033]    It is assumed that gradation voltage signals A and F that have levels corresponding to an image signal applied for test use are available at the output terminals of the operational amplifiers  10   1 ,  10   2 , . . . First, the controller  14  outputs the reset signal to the reset line  16  to thereby reset all the flip-flops  13   1 ,  13   2 , . . . and to turn OFF all the transfer gates  11   1 ,  11   2 , . . . , so that all the outputs of the operational amplifiers  10   1 ,  10   2 , . . . are in the high-impedance state. Next, the controller  14  outputs a high-level voltage C and a low-level voltage to the test lines  19  and  20 , respectively, so that all the transfer gates  18   1 ,  18   2 , . . . are in the ON state.  
         [0034]    Then, the controller  14  outputs a clock signal to the clock line  15 . The first flip-flop  13   1 , latches high-level data output to the controller  14  via the data input terminal in synchronism with the clock signal, and outputs data B at the high level and data at the low level to the non-inverting and inverting output terminals, respectively. Thus, the transfer gate  11   1  is turned ON, and the gradation voltage signal A of the operational amplifier  10   1  is output to the output pad  12   1 . The gradation voltage signal A is output, as an output signal E, to the test pad  17  via all the transfer gates  18   1 ,  18   2 , . . .  
         [0035]    During that time, the data that is being output to the flip-flop  13   1 , from the controller  14  is switched to the low level. The flip-flop  13   1 , latches data at the low level in synchronism with the next clock signal, and sets data B of the non-inverting output terminal to the low level, setting data of the inverting output terminal to the high level. Simultaneously, the second flip-flop  13   2  latches the data at the high level being output to the non-inverting output terminal of the first flip-flop  13   1 , and outputs data D at the high level to the non-inverting output terminal, outputting data at the low level to the inverting output terminal. Thus, the transfer gate  11   1  is turned OFF, and cuts off the gradation voltage signal A of the operational amplifier  10   1 . Simultaneously, the transfer gate  11   2  is switched to ON, and outputs a gradation voltage signal F of the operational amplifier  10   2  to the output pad  12   2 . The gradation voltage signal F is output, as an output signal E, to the test pad  17  via the transfer gates  18   2 , . . .  
         [0036]    Hereinafter, similarly, the third flip-flop and the remaining flip-flop sequentially latch the output of the previous stage, so that the third transfer gate and the remaining transfer gates are sequentially turned ON. Thus, the outputs of the operational amplifiers are sequentially output to the test pad  17  one by one. This makes it possible to test all the outputs of the output circuit of the data driver by merely bringing the probe needle to only the test pad  17  without being short-circuited.  
         [0037]    [0037]FIG. 4 is a circuit diagram that partially shows a structure of the test circuit according to a second embodiment of the present invention. The test circuit utilizes a part of the circuit that forms the data driver as a transfer gate that cuts off the operational amplifier that is not to be measured. More particularly, a data driver that drives a liquid crystal panel into which a liquid crystal and a TFT (Thin Film Transistor) are combined a positive-polarity system, a negative-polarity system and a polarity reversing circuit because such a data driver is required to alternately output the gradation voltage positive to the common voltage and the gradation voltage negative thereto. The polarity reversing circuit is utilized as a switch that cuts off the output of the operational amplifier that is not to be measured.  
         [0038]    In FIG. 4, an operational amplifier  30  which outputs a gradation voltage of the positive polarity and an operational amplifier  31  which outputs a gradation voltage of the negative polarity are paired, and a plurality of such pairs are provided. The output terminals of the pairs of operational amplifiers are connected to output pads  32   1 ,  32   2 ,  32   3 ,  32   4 ,  32   5 ,  32   6 , . . . via the polarity reversing circuits. Each of the polarity reversing circuits is made up of four transfer gates  33 ,  34 ,  35  and  36 , each of which transfer gates is made up of a P-channel MOS transistor and an N-channel MOS transistor. The output terminals of the operational amplifiers  30  are connected to odd-numbered output pads  32   1 ,  32   3 ,  32   5 , . . . via the transfer gates  33 , and are connected to even-numbered output pads  32   2 ,  32   4 ,  32   6 , . . . via the transfer gates  35 . The output terminals of the operational amplifiers  31  are connected to the odd-numbered output pads  32   1 ,  32   3 ,  32   5 , via the transfer gates  34 , and are connected to even-numbered output pads  32   2 ,  32   4 ,  32   6 , . . . via the transfer gates  36 .  
         [0039]    A terminal of the controller  37  via which a polarity switching signal POL is connected to a switching control line  38 , which is connected to first input terminals of NAND gates  39 . The output terminals of the NAND gates  39  are connected to the gate terminals of the transfer gates  33  and  36  on the P-channel side and input terminals of inverters (NOT gates)  40 . The output terminals of the inverters  40  are connected to the gate terminals of the transfer gates  33  and  36  on the N-channel side. The switching control line  38  is connected to the first input terminals of the NAND gates  42  via the inverters  41 . The output terminals of the NAND gates  42  are connected to the gate terminals of the transfer gates  34  and  35  on the P-channel side and the input terminals of the inverters  43 . The output terminals of the inverters  43  are connected to the gate terminals of the transfer gates  34  and  35  on the N-channel side.  
         [0040]    The controller  37  has a data output terminal, a clock signal output terminal and a reset signal output terminal, these terminals being connected to flip-flops  44 . The flip-flops  44  are cascaded so that the non-inverting output terminals thereof are connected to data input terminals of the next-stage flip-flops  44 . The inverting output terminals of the flip-flops  44  are connected to the first input terminals of the NAND gates  45 . The second input terminals of the NAND gates  45  are connected to a test line  46  via which the non-inverting test signal from the controller  37  is transferred. The output terminals of the NAND gates  45  are connected to the second input terminals of the NAND gates  39  and  42 .  
         [0041]    Transfer gates  47  are connected between the odd-numbered output parts  32   1 ,  32   3 ,  32   5 , . . . and the gate terminals thereof on the N-channel side are connected to a test line  48  via which the non-inverting test signal from the controller  37  is output. The gate terminals of the transfer gates  47  on the P-channel side are connected to a test line  49  via which the inverting signal from the controller  37  is transferred. The transfer gate  47  of the final stage is connected to a test pad  50 .  
         [0042]    An operation of the test circuit in the data driver is described.  
         [0043]    The controller  37  resets all the flip-flops  44 . At that time, the controller  37  outputs a low-level voltage to the test lines  46 ,  48  and  49  and the switching control line  38 . Thus, the high-level voltages are output via the output terminals of the NAND gates  45  and  39 , and the low-level voltages are output via the output terminals of the NAND gates  42 . Thus, the transfer gates  33  and  36  are OFF, while the transfer gates  34  and  35  are ON.  
         [0044]    When the controller  37  outputs the test signal that is at the high level, the low-level voltages are output via the output terminals of all the NAND gates  45 , and the high-level voltages are output via the output terminals of the NAND gates  39  and  42 . Thus, all the transfer gates  33 ,  34 ,  35  and  36  of the polarity reversing circuit are OFF, and all the transfer gates  47  connected to the odd-numbered output pads  32   1 ,  32   3 ,  32   5  and the test pad  50  are ON.  
         [0045]    Next, when the first flip-flop  44  latches high-level data that is output from the controller  37  in synchronism with the clock signal, the low-level voltage is output via the inverting output terminal thereof. Simultaneously, the controller  37  outputs the polarity switching signal POL at the high level. This causes the transfer gates  33  and  36  of the polarity reversing circuit to be ON while causing the transfer gates  34  and  35  thereof to be OFF. Thus, the output of the operational amplifier that outputs the gradation voltage of the positive polarity are connected to the test pad  50  via the transfer gates  33  and  47 , so that the gradation voltage of the positive polarity can be output to the test pad  50 .  
         [0046]    Then, when the controller  37  outputs the polarity reversing signal POL of the low level, the states of the output terminals of the NAND gates  39  and  42  are reversed. Therefore, in turn, the transfer gates  33  and  36  of the polarity reversing circuit are OFF, while the transfer gates  34  and  35  are ON. Thus, the output of the operational amplifier  31  that outputs the gradation voltage of the negative polarity is connected to the test pad  50  via the transfer gates  34  and  47 , so that the gradation voltage of the negative polarity can be output to the test pad  50 .  
         [0047]    The above-mentioned operation after the test signal is output is performed so that the output status of the flip-flop  44  is serially changed in synchronism with the clock signal. Thus, it is possible to output the gradation voltages of the positive and negative polarities to the test pad  50 .  
         [0048]    [0048]FIG. 5 is a conceptual view of a pad formation surface of an integrated circuit for the data driver.  
         [0049]    An integrated circuit  51  has a pad arrangement in which pads for inputting and outputting are arranged along the sides of the shape thereof. In the example shown in FIG. 5, input pads  52  and a test pad  53  are arranged along a side of the integrated circuit  51 , while output pads  54  are arranged along the remaining three sides. At the time of testing, the input pads  52  and the test pad  53  to which probe needles  55  are to be contacted are arranged at a pitch approximately equal to the conventional pitch so that no problem will be encountered at the time of contacting the probe needles  55 . In contrast, the output pads  54  are arranged at a narrower pitch because the output pads  54  are not brought into contact with the probe needles  55 .  
         [0050]    In the conceptual example, the output signals that are output to all the output pads  54  are tested by the single test pad  53 . However, for a data driver with 384 outputs, for example, all the outputs cannot be efficiently tested using only the single test pad  53 . In practice, the output pads  54  are divided into some groups for each of which groups the single test pad  53  is provided. Preferably, when 384 output pads  54  are provided, the single output pad  54  is provided for the  48  output pads. In total, eight test pads  53  are provided for the 384 output pads  54 , and are arranged in the same line as the input pads  52 . The function test is simultaneously carried out for every group, so that the time necessary to carry out the function test can be reduced.  
         [0051]    In the example illustrated, one side of the integrated circuit  51  is occupied by the input pads  52  and the test pad  53 . Alternatively, part of the side may be used to dispose the output pads  54 .  
         [0052]    [0052]FIG. 6 is a view that explains how the integrated circuit for the data driver is tested.  
         [0053]    For the integrated circuit for the data driver with multiple outputs, conventionally, the probe needles are contacted to the input and output pads along the four sides thereof. In contrast, the input pads and the test pad are arranged along the same side. Therefore, two integrated circuits can be simultaneously tested with the conventional test device.  
         [0054]    A plurality of integrated circuits  512  are arranged side by side and are transported. In the test positions, every the integrated circuits  51  are fixed in given positions every two circuits, and probe needles  55  arranged in two lines for the input pads  52  and the test pads  53  of the integrated circuit can be contacted and detached simultaneously.  
         [0055]    In test, the probe needles  55  are brought into contact with a small number of input pads  52  and the test pad  53 . Thus, it is possible to easily adjust the contract pressure and achieve stable contacts. Further, two integrated circuits  51  are simultaneously tested, so that the time necessary for positioning the probe needles and the test time can be reduced.  
         [0056]    As described above, according to the present invention, the voltages that appear on the output pads can be sequentially output to the single test pad. The test can be carried out using the test pad rather than the output pads, it is possible to reduce the pitch without being restricted by the pitch at which the output pads are arranged. Such narrowing the pitch contributes to reducing the chip area and the cost.  
         [0057]    Further, according to the present invention, the test can be carried out with a number of contacts with the input pads and test pad, so that the contact pressure with which the probe needles are contracted can easily be adjusted and sure contacts can be made.  
         [0058]    Furthermore, according to the present invention, the input pads used in the test and the test pad are arranged in line, so that the probe needles can be positioned with a reduced time. In addition, two adjacent integrated circuits can be tested simultaneously, so that the test can be carried out with a reduced time and the cost can be reduced.  
         [0059]    The foregoing is considered as illustrate only of the principles of the present invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and applications shown and described, and accordingly, all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.