Abstract:
A defect detection system and related method take advantage of multilevel detection technique for detecting defects on an integrated circuit. The defect detection system utilizes an analog-to-digital converter for converting an analog sensing signal into an output code having a plurality of bits. The defect detection methods include an open test method and a short test method. The open and short test methods both include a calibrating method and a testing method individually. The calibrating method functions to determine a preset reference voltage for the analog-to-digital converter based on a predetermined code. The testing method makes use of the preset reference voltage and the predetermined code for generating the output code having a plurality of bits. The output code is then utilized to determine whether or not there are open or short defects on the integrated circuit and to classify the defects.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates in general to a defect detection system and method, and more particularly, to a defect detection system and method capable of outputting a multilevel signal for identifying defects. 
   2. Description of the Prior Art 
   During the fabrication process, a wafer receives a number of doping, layering, patterning, and metallization steps. Each of these steps must meet exacting physical requirements. However, all steps have some variation from perfect calibration, thereby resulting in some variation on the wafer surface, especially while bringing up a new semiconductor fabrication process. Accordingly, a variety of yield monitors are required to determine the health of the process and to find systematic problems. 
   Several known conduction line structures, such as poly-crystal lines, diffusion lines, metal lines, and N-type or P-type well lines are subject to being detected for determining whether there are undesirable defects existing. Furthermore, there are electrical-isolated routes, each of which may comprise two adjacent and electrical-isolated conduction lines, subjected to being detected for determining whether there are undesirable defects existing. Once detected, these defects are analyzed in a process called failure analysis. During failure analysis, valuable information regarding problems with fabrication materials, process recipes, ambient air, personnel, and machines can be discovered. Therefore, detection of defects on an integrated circuit is critical to high yields and process control. 
   Please refer to  FIG. 1 , which is a schematic diagram showing a prior art defect detection system  100  for identifying defects in an integrated circuit. The defect detection system  100  comprises a sense amplifier  110 , a pull-down transistor  115  having a gate furnished with an enable signal Venable, selection transistors  121  and  122 , and a selection circuit  190 . The gates of the selection transistors  121  and  122  are both electrically connected to the selection circuit  190  for performing a selecting process so as to select a test route  150  as a selected test route for testing. A test input terminal  140  is coupled to the selection transistor  122  for inputting an input voltage Vin. 
   The sense amplifier  110  has a first terminal for receiving an analog sensing signal Sanalog, a second terminal for receiving a reference voltage Vref, and an output terminal for outputting an output signal Sout. In general, the first terminal of the sense amplifier  110  and the test input terminal  140  are connected to two pluralities of selection transistors having all the gates connected to the selection circuit  190  respectively for selecting a certain test route from a plurality of test routes to be tested as the selected test route for testing. 
   However, the output signal Sout outputted from the sense amplifier  110  is fundamentally a two-level signal similar to one-bit signal in a digital system, which can be utilized only for identifying whether or not the selected test route has defects. Accordingly, operational flexibility of the defect detection system  100  is quite limited. That is to say, the prior art defect detection system  100  cannot meet requirements when multilevel output signals are preferred under some test situations. 
   SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the present invention, a defect detection system having multilevel output capability is provided for higher operational flexibility. The defect detection system comprises an analog-to-digital converter, a test input terminal, and a pull-down device. The analog-to-digital converter has an input terminal coupled to a first terminal of a selected test route, a reference input terminal, and an output port. The test input terminal is coupled to a second terminal of the selected test route. The pull-down device is coupled between the input terminal of the analog-to-digital converter and ground. 
   The present invention further provides a method for performing an open test for analyzing an open in a test route of an integrated circuit. The method for performing the open test comprises a calibrating method and a testing method. The calibrating method of the open test comprises: connecting a pull-down resistor between the input terminal of an analog-to-digital converter and ground, connecting an open calibration test route between a test input terminal and the input terminal of the analog-to-digital converter, applying an input voltage to the test input terminal, applying a reference voltage to the reference input terminal of the analog-to-digital converter, and adjusting the reference voltage until an output code of the analog-to-digital converter is equal to a predetermined code and setting the adjusted reference voltage as a preset reference voltage. The testing method of the open test comprises: connecting the pull-down resistor between the input terminal of the analog-to-digital converter and ground, connecting a test route between a test input terminal and the input terminal of the analog-to-digital converter, applying the input voltage to the test input terminal, applying the preset reference voltage determined by the calibrating method of the open test to the reference input terminal of the analog-to-digital converter, and determining whether or not the test route is open according to an input code of the analog-to-digital converter. 
   Furthermore, the present invention provides a method for performing a short test for analyzing a short in a test route of an integrated circuit. The method for performing the short test comprises a calibrating method and a testing method. The calibrating method of the short test comprises: connecting a pull-down resistor between the input terminal of an analog-to-digital converter and ground, connecting a short calibration test route between a test input terminal and the input terminal of the analog-to-digital converter, applying an input voltage to the test input terminal, applying a reference voltage to the reference input terminal of the analog-to-digital converter, and adjusting the reference voltage until an output code of the analog-to-digital converter is equal to a predetermined code and setting the adjusted reference voltage as a preset reference voltage. The testing method of the short test comprises: connecting the pull-down resistor between the input terminal of the analog-to-digital converter and ground, connecting a test route between a test input terminal and the input terminal of the analog-to-digital converter, applying an input voltage to the test input terminal, applying the preset reference voltage determined by the calibrating method of the short test to the reference input terminal of the analog-to-digital converter, and determining whether or not the test route is shorted according to an output code of the analog-to-digital converter. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing a prior art defect detection system for identifying defects in an integrated circuit. 
       FIG. 2  is a schematic diagram showing a defect detection system for identifying defects in an integrated circuit in accordance with an embodiment of the present invention. 
       FIG. 3  is a schematic diagram showing the structure of the analog-to-digital converter in  FIG. 2  in accordance with an embodiment of the present invention. 
       FIG. 4  is a schematic diagram showing a defect detection system for identifying defects in an integrated circuit in accordance with another embodiment of the present invention. 
       FIG. 5  is a schematic structure for performing a calibration mode operation of an open test for analyzing an open in a test route of an integrated circuit. 
       FIG. 6  is a flowchart depicting a method for performing the calibration mode operation of the open test according to the present invention. 
       FIG. 7  is a schematic structure for performing a test mode operation of the open test so as to analyze an open in a test route of an integrated circuit. 
       FIG. 8  is a flowchart depicting a method for performing the test mode operation of the open test according to the present invention. 
       FIG. 9  is a schematic structure for performing a calibration mode operation of a short test for analyzing a short in a test route of an integrated circuit. 
       FIG. 10  is a flowchart depicting a method for performing the calibration mode operation of the short test according to the present invention. 
       FIG. 11  is a schematic structure for performing a test mode operation of the short test so as to analyze a short in a test route of an integrated circuit. 
       FIG. 12  is a flowchart depicting a method for performing the test mode operation of the short test according to the present invention. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2 , which is a schematic diagram showing a defect detection system  200  for identifying defects in an integrated circuit in accordance with an embodiment of the present invention. The defect detection system  200  comprises an analog-to-digital converter (ADC)  210 , a pull-down device  215 , a first selection transistor  221 , a second selection transistor  222 , and a selection circuit  290 . The defect detection system  200  can be incorporated in a wafer or a chip to be tested. 
   The second selection transistor  222  has a gate, a first terminal electrically connected to a test input terminal  240  for receiving an input voltage Vin, and a second terminal electrically connected to a first terminal of a test route  250 . Similarly, the first selection transistor  221  has a gate, a first terminal electrically connected to a second terminal of the test route  250 , and a second terminal. The first and second selection transistors  221  and  222  can be NMOS transistors or PMOS transistors. The test route  250  can be a conduction line or an electrical-isolated route on a wafer or a chip to be tested. The conduction line means that the first and second terminals of the test route  250  are electrically connected normally. The electrical-isolated route means that the first and second terminals of the test route  250  are electrical-isolated normally. Please note that in the description hereinafter when the first terminal of a certain transistor functions as a source terminal, the second terminal of the certain transistor will function as a drain terminal. On the contrary, when the first terminal of a certain transistor functions as a drain terminal, the second terminal of the certain transistor will function as a source terminal. The selection circuit  290  provides a plurality of selection signals furnished to the gates of the first and second selection transistors  221  and  222 . In other words, the gates of the first and second selection transistors  221  and  222  are both electrically connected to the selection circuit  290  for performing a selecting process so as to select the test route  250  as a selected test route for the testing. 
   The pull-down device  215  has a first terminal electrically connected to the second terminal of the first selection transistor  221  and a second terminal coupled to ground. The pull-down device  215  can be a pull-down resistor having a predetermined resistance. Alternatively, the pull-down device  215  can be an NMOS transistor having a first terminal electrically connected to the second terminal of the first selection transistor  221 , a second terminal coupled to ground, and a gate furnished with an enable signal. 
   The analog-to-digital converter  210  has an input terminal electrically connected to the second terminal of the first selection transistor  221  for receiving an analog sensing signal Sanalog, a reference input terminal for receiving a reference voltage Vref, and an output port for outputting an output code Sdigital having a plurality of bits such as an n-bit output code. 
   Please refer to  FIG. 3 , which is a schematic diagram showing the structure of the analog-to-digital converter  210  in accordance with an embodiment of the present invention. The analog-to digital converter  210  comprises a plurality of resistors  310 , a plurality of comparators  330 , and an encoder  360 . Each comparator has a first input terminal coupled to the input terminal of the analog-to-digital converter  210  for receiving the analog sensing signal Sanalog, a second input terminal, and an output terminal. The first and second input terminals of one comparator can be a positive input terminal and a negative input terminal of the comparator respectively. The plurality of resistors  310  are electrically connected in series and are coupled between the reference input terminal of the analog-to-digital converter  210  and ground. Each connection node between adjacent resistors of the plurality of resistors  310  is electrically connected to the second input terminal of one corresponding comparator. The encoder  360  has a plurality of input terminals and a plurality of output terminals. In one embodiment, the encoder  360  may have 7 input terminals indexed from 1 to 7 and 3 output terminals as shown in  FIG. 3 . The index number from low to high may correspond to low-to-high significant bits of corresponding input terminals of the encoder  360 . Each input terminal of the encoder  360  is electrically connected to the output terminal of one corresponding comparator. The plurality of terminals of the encoder  360  are coupled to the output port of the analog-to-digital converter  210  for outputting the output code Sdigital having a plurality of bits such as a 3-bit output code. The encoder  360  can be a priority encoder for selecting an input terminal with highest index and having a high logic level in positive logic or a low logic level in negative logic as the input terminal to be encoded. 
   In the embodiment shown in  FIG. 2 , the input terminal of the analog-to-digital converter  210  and the test input terminal  240  are not limited to be electrically connected to the first and second selection transistors  221  and  222  respectively. As a preferred alternative in another embodiment shown in  FIG. 4 , the input terminal of the analog-to-digital converter  210  can be electrically connected to a plurality of first selection transistors  225 , and the test input terminal  240  can be electrically connected to a plurality of second selection transistors  226 . Furthermore, all the gates of the two pluralities of first and second selection transistors are electrically connected to the selection circuit  290  so as to perform a selection operation for selecting a certain test route from a plurality of test routes  255  to be tested as the selected test route for each testing. 
   However, taking advantage of several pluralities of selection transistors in conjunction with a selection circuit for sequentially selecting a certain test route from a plurality of test routes on a wafer or a chip to be tested for each testing is a well-known technology and, for the sake of brevity, further detailed description for test route selection architecture are omitted. In summary, the scope of the present invention is not limited thereto the embodiment shown in  FIG. 2 , and any defect detection system taking advantage of test route selection architecture together with an analog-to-digital converter for generating a test output code having a plurality of bits is within the spirit and scope of the invention. 
   Please refer to  FIG. 5  together with  FIG. 6 .  FIG. 5  is a schematic structure for performing a calibration mode operation of an open test for analyzing an open in a test route of an integrated circuit.  FIG. 6  is a flowchart depicting a method for performing the calibration mode operation of the open test according to the present invention. The structure for performing the calibration operation of the open test comprises an analog-to-digital converter (ADC)  510 , a pull-down resistor  515 , and an open calibration test route  550 . The calibrating method of the open test comprises the following steps: 
   Step S 610 : connect the pull-down resistor  515  between the input terminal of the analog-to-digital converter  510  and ground; 
   Step S 620 : connect the open calibration test route  550  between a test input terminal  540  and the input terminal of the analog-to-digital converter  510 ; 
   Step S 630 : apply an input voltage Vin to the test input terminal  540 ; 
   Step S 640 : apply a reference voltage Vref to the reference input terminal of the analog-to-digital converter  510 ; and 
   Step S 650 : adjust the reference voltage Vref until an n-bit output code Sdigital of the analog-to-digital converter  510  is equal to a predetermined code and set the adjusted reference voltage as a preset reference voltage. 
   In the calibrating method of the open test described above, the operating sequence between the steps S 610  and S 620  is optional. That is, the step S 610  can be performed before or after the step S 620 . Similarly, the operating sequence between the steps S 630  and S 640  is also optional. The pull-down resistor  515  can be replaced by a pull-down transistor having a gate furnished with an enable signal. 
   In one preferred embodiment, the open calibration test route  550  is selected to be a test route having route structure same as that of a test route waiting to be tested on a wafer or a chip to be tested. Moreover, the n-bit output code of the analog-to-digital converter  510  is a 3-bit code and the predetermined code is “100”, which is a median-value code of the 3-bit code. 
   Please refer to  FIG. 7  in conjunction with  FIG. 8 .  FIG. 7  is a schematic structure for performing a test mode operation of the open test so as to analyze an open in a test route of an integrated circuit.  FIG. 8  is a flowchart depicting a method for performing the test mode operation of the open test according to the present invention. The structure for performing the testing operation of the open test comprises the analog-to-digital converter (ADC)  510 , the pull-down resistor  515 , and a test route  580 . The test route  580  of the integrated circuit to be test can be a poly-crystal conduction line, a diffusion conduction line, a metal conduction line, or a P-type or N-type well conduction line. The testing method of the open test comprises the following steps: 
   Step S 810 : connect the pull-down resistor  515  between the input terminal of the analog-to-digital converter  510  and ground; 
   Step S 820 : connect the test route  580  between a test input terminal  541  and the input terminal of the analog-to-digital converter  510 ; 
   Step S 830 : apply an input voltage Vin, same as the input voltage used in the calibration method of the open test, to the test input terminal  541 ; 
   Step S 840 : apply the preset reference voltage Vref determined by the calibration mode operation of the open test to the reference input terminal of the analog-to-digital converter  510 ; 
   Step S 850 : detect whether or not the output code of the analog-to-digital converter  510  is less than the predetermined code, if the output code of the analog-to-digital converter  510  is less than the predetermined code of the open test, then go to step S 860 , else go to step S 870 ; 
   Step S 860 : determine that the test route  580  is open; and 
   Step S 870 : determine that the test route  580  is electrically connected normally. 
   In the testing method of the open test described above, the operating sequence between the steps S 810  and S 820  is optional, and the operating sequence between the steps S 830  and S 840  is also optional. 
   Please refer to  FIG. 9  together with  FIG. 10 .  FIG. 9  is a schematic structure for performing a calibration mode operation of a short test for analyzing a short in a test route of an integrated circuit.  FIG. 10  is a flowchart depicting a method for performing the calibration mode operation of the short test according to the present invention. The structure for performing the calibration operation of the short test comprises an analog-to-digital converter (ADC)  910 , a pull-down resistor  915 , and a short calibration test route  950 . The calibrating method of the short test comprises the following steps: 
   Step S 1010 : connect the pull-down resistor  915  between the input terminal of the analog-to-digital converter  910  and ground; 
   Step S 1020 : connect the short calibration test route  950  between a test input terminal  940  and the input terminal of the analog-to-digital converter  910 ; 
   Step S 1030 : apply an input voltage Vin to the test input terminal  940 ; 
   Step S 1040 : apply a reference voltage Vref to the reference input terminal of the analog-to-digital converter  910 ; and 
   Step S 1050 : adjust the reference voltage Vref until an n-bit output code Sdigital of the analog-to-digital converter  910  is equal to a predetermined code and set the adjusted reference voltage as a preset reference voltage. 
   In the calibrating method of the short test described above, the operating sequence between the steps S 1010  and S 1020  is optional, and the operating sequence between the steps S 1030  and S 1040  is also optional. The pull-down resistor  915  can be replaced by a pull-down transistor having a gate furnished with an enable signal. 
   In one preferred embodiment, the short calibration test route  950  is selected to be a test route having route structure same as that of a test route waiting to be tested on a wafer or a chip to be tested. Moreover, the n-bit output code of the analog-to-digital converter  910  is a 3-bit code and the predetermined code is “000” or “001”, which are lowest and second-lowest values of the 3-bit code for taking noise effect into consideration. 
   Please refer to  FIG. 11  in conjunction with  FIG. 12 .  FIG. 11  is a schematic structure for performing a test mode operation of the short test so as to analyze a short in a test route of an integrated circuit.  FIG. 12  is a flowchart depicting a method for performing the test mode operation of the short test according to the present invention. The structure for performing the testing operation of the short test comprises the analog-to-digital converter (ADC)  910 , the pull-down resistor  915 , and a test route  980 . The test route  980  of the integrated circuit to be test is an electrical-isolated route, which can be a combination of two adjacent and electrical-isolated conduction lines with each conduction line being a poly-crystal conduction line, a diffusion conduction line, a metal conduction line, or a P-type or N-type well conduction line. The testing method of the short test comprises the following steps: 
   Step S 1210 : connect the pull-down resistor  915  between the input terminal of the analog-to-digital converter  910  and ground; 
   Step S 1220 : connect the test route  980  between a test input terminal  941  and the input terminal of the analog-to-digital converter  910 ; 
   Step S 1230 : apply an input voltage Vin, same as the input voltage used in the calibration method of the short test, to the test input terminal  941 ; 
   Step S 1240 : apply the preset reference voltage Vref determined by the calibration mode operation of the short test to the reference input terminal of the analog-to-digital converter  910 ; 
   Step S 1250 : detect whether or not the output code of the analog-to-digital converter  910  is greater than the predetermined code, if the output code of the analog-to-digital converter  910  is greater than the predetermined code of the short test, then go to step S 1260 , else go to step S 1270 ; 
   Step S 1260 : determine that the test route  980  is shorted; and 
   Step S 1270 : determine that the test route  980  is electrical-isolated normally. 
   In the testing method of the short test described above, the operating sequence between the steps S 1210  and S 1220  is optional, and the operating sequence between the steps S 1230  and S 1240  is also optional. 
   In addition, the step S 1260  of the testing method of the short test may further comprises performing a short classification based on a code difference between the output code of the analog-to-digital converter  910  and the predetermined code, which means that a shorting situation occurred to the test route  980  is more serious as the code difference is higher. In other words, the conductivity between the test input terminal  941  and the input terminal of the analog-to-digital converter  910  is higher with a higher code difference between the output code of the analog-to-digital converter  910  and the predetermined code. Accordingly, the short classification can be performed based on the code difference. 
   To sum up, the defect detection system and related methods provide multilevel output signals to identify open or short defects in an integrated circuit for achieving higher operational flexibility. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.