Abstract:
A semiconductor device that generates a desired internal power supply by using, as a reference potential, a potential obtained by adjusting a preset standard potential, the semiconductor device comprises; a reference potential selection circuit selecting the reference potential on the basis of digital data from among a plurality of potentials of different levels which are obtained by dividing a power supply voltage, and outputting the reference potential instead of the standard potential; a first decision circuit deciding bits of the digital data; a second decision circuit deciding the bits of the digital data, separately from the first decision circuit; and a data transfer circuit transferring to the reference potential selection circuit the digital data which is decided by either one of the first and second decision circuits.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2003-340131, filed on Sep. 30, 2003, the entire contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor device and a method for testing the same. 
   2. Related Background Art 
   The actuation of a semiconductor memory (for example, to read out therefrom or write therein data) requires various potentials. To supply all the potentials from outside, many kinds of external power supplies will be needed. This inevitably calls for a large and complex system that effects the operation of the semiconductor memory. Besides, the necessity for mounting many external power supply terminals on the semiconductor chip increases the chip area and the package size accordingly. 
   To cope with this, it is customary in the art to supply power from a single power source to the semiconductor chip and generate therein potentials necessary for its operation (which potentials will hereinafter be referred to also as internal potentials). The internal potentials of various levels are each generated based on a potential (hereinafter referred to also as a reference potential) obtained by dividing the potential of the external power supply. 
   Generally, the reference potential may sometimes deviate from a design value due to stresses applied to the semiconductor wafer during the semiconductor manufacturing process.  FIG. 14  is a diagram showing a conventional semiconductor device test procedure. Semiconductor elements formed on the semiconductor wafers in the semiconductor manufacturing process are tested for each die in a D/S (Die/Sort) step. At this time, the reference potential is also measured (S 1 ). If a deviation of the reference potential from the design value is found, the reference potential is trimmed to be close to the design value (S 2 ). This is done by physically cutting a wiring or wirings on the semiconductor wafer through laser irradiation. A description is given below of a conventional reference potential generator for trimming the reference potential. 
     FIG. 15  is a block diagram of a conventional reference potential generator  500  that generates a reference potential VBGR. Data decision circuits  540 - 0  to  540 - 2 , which form part of the reference potential generator  500 , are each configured as shown in  FIG. 16 . Each data decision circuit  540  has a fuse  541 . Depending on whether or not the fuse  541  in step S 2 , the data decision circuits  540 - 0  to  540 - 2  output high- or low-level 1-bit data as signals SELECT 0  to SELECT 2 . 
   Referring back to  FIG. 15 , data transfer circuits  530 - 0  to  530 - 2  respectively transfer the signals SELECT 0  to SELECT 2  to a decode circuit  520 . The data transfer circuits  530 - 0  to  530 - 2  are also capable of transferring test mode signals TMFUSEDIS to the decode circuit  520  instead of sending thereto the signals SELECT 0  to SELECT 2 . 
     FIG. 17  shows the configuration of the decode circuit  520 , which receives the signals SELECT 0  to SELECT 2  or TMFUSEDIS as 3-bit digital data composed of signals PRETMBGR 0  to PRETMBGR 2 . Based on the digital data it receives, the decode circuit  520  makes any one of signals TMBGR 0  to TMBGR 4  high-level and sends it to a reference potential selection circuit  510 .  FIG. 5  shows the relationships between the signals PRETMBGR 0  to PRETMBGR 2  and the signals TMBGR 0  to TMBGR 4 . For example, the decode circuit  520  makes the signal TMBGR 1  high-level by making the signal PRETMBGR 0  low-level and the signals PRETMBGR 1  and PRETMBGR 2  high-level. 
     FIG. 18  shows the configuration of the reference potential selection circuit  510 , which divides the power supply voltage by resistors R 1  and R 2  to generate a plurality of different potentials. Based on that one of the signals TMBGR 0  to TMBGR 4  which is sent thereto, the reference potential selection circuit  510  selects any one of potentials BGR to BGR 4 , and outputs the selected potential as the reference potential VBGR. For example, when the signal TMBGR 1  is high-level, a switch SW 1  operates, and the reference potential selection circuit  510  outputs the potential BGR 1  as the reference potential VBGR. 
   In the test mode, the decode circuit  520  receives the test mode signals TMFUSEDIS as digital data, and outputs the signals TMBGR 0  to TMBGR 4  based on the test mode signals. The reference potential selection circuit  510  responds to the signals TMBGR 0  to TMBGR 4  to output a preset default potential (hereinafter referred to as a standard potential) as the reference potential VBGR. For example, when the potential BGR 2  is the standard potential, the signal TMFUSEDIS is preset so that the reference potential selection circuit  510  selects the potential BGR 2 . 
   Referring back to  FIG. 14 , trimming of the reference potential VBGR in step S 2  is followed by a final semiconductor test of the semiconductor wafer (S 3 ), which is thereafter divided into individual semiconductor chips and packaged in an assembling step (S 4 ) and in a packaging step (S 5 ), respectively. Following this, the semiconductor chips undergo a reliability test (S 6 ) and a packaging final test (S 7 ), and are shipped as products. 
   One possible cause for a semiconductor chip to fail the reliable test (S 6 ) is a deviation of the reference potential VBGR from a design value. The reason for this is that the reference potential VBGR, though adjusted to the design value in step S 2 , shifts again due to stresses applied to the chip in the reliability test. Since no trimming is possible in the reliable test (S 6 ), however, the semiconductor chip decided as a reject is discarded. 
   In the D/S step  12 , trimming is carried out (S 2 ) on the assumption that the resistors R 1  and R 2  in  FIG. 18  have their design values. Accordingly, the reference potential VBGR does not always become close to the design value, but in some cases it further deviates from the design value. 
   In the prior art, since the reference potential VBGR measuring step (S 1 ) and the fuse blowing step ( 2 ) are separate from each other, an exact value of the trimmed reference potential VBGR cannot be known prior to the fuse blowing step. 
   To obviate the above-mentioned defects of the prior art, there is a demand for a semiconductor device and its testing method that permits re-trimming or readjustment of the reference potential in the reliability test for each semiconductor chip. 
   Also, there is a demand for a semiconductor device and its testing method that permits the selection of a reference potential closest to its design value in the test of semiconductor elements for each die and in the adjustment of the reference potential. 
   SUMMARY OF THE INVENTION 
   A semiconductor device according to an embodiment of the invention that generates a desired internal power supply by using, as a reference potential, a potential obtained by adjusting a preset standard potential, the semiconductor device comprises a reference potential selection circuit selecting said reference potential on the basis of digital data from among a plurality of potentials of different levels which are obtained by dividing a power supply voltage, and outputting said reference potential in place of said standard potential; a first decision circuit deciding bits of said digital data; a second decision circuit deciding the bits of said digital data, separately from said first decision circuit; and a data transfer circuit transferring to said reference potential selection circuit said digital data which is decided by either one of said first and second decision circuits. 
   A method for testing a semiconductor device according to an embodiment of the invention that includes: a reference potential selection circuit selecting a reference potential on the basis of digital data from among a plurality of potentials of different levels which are obtained by dividing a power supply voltage, and outputting said reference potential in place of a standard potential preset by default so as to generate a desired internal power supply; a first decision circuit deciding the value of said digital data; a second decision circuit deciding the value of said digital data separately from said first decision circuit; a test data input portion tentatively inputting various pieces of digital data of different values from output; and a data transfer circuit transferring digital data which is fed thereto from any one of said first decision circuit, said second decision circuit and said test data input portion to said reference potential selection circuit; the method comprising:
         transferring said various pieces of digital data of different levels from said test data input portion to said reference potential selection circuit by said data transfer circuit, and measuring reference potentials on the basis of said various pieces of digital data of different levels, respectively; and setting first digital data in said first decision circuit, said first digital data generating first one of said reference potentials which is optimum for the generation of said desired internal power supply.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a reference potential generator  100  according to a first embodiment of the present invention; 
       FIG. 2  is a circuit diagram showing the configuration of a data decision circuit  40   a - 0 ; 
       FIG. 3  is a timing chart showing the operation of a data decision circuit  40   a - 0 ; 
       FIG. 4  is a circuit diagram showing the configuration of a data selection circuit  50 ; 
       FIG. 5  is a table showing the relationships between signals PRETMBGR 0  to PRETMBGR 2  and signals TMBGR 0  to TMBGR 4 ; 
       FIG. 6  is a circuit diagram of a data transfer circuit  30 - 0 ; 
       FIG. 7  is a conceptual diagram of testing a semiconductor device which comprises the reference potential generator  100 ; 
       FIG. 8  is a flowchart of the procedure for testing the semiconductor device which comprises the reference potential generator  100 ; 
       FIG. 9  is a block diagram of a reference potential generator  200  according to a second embodiment of the present invention; 
       FIG. 10  is a circuit diagram of a standard data selection circuit  52 ; 
       FIG. 11  is a circuit diagram of a standard data transfer circuit  34 ; 
       FIG. 12  is a circuit diagram of a data selection circuit  54 ; 
       FIG. 13  is a circuit diagram of a data transfer circuit  32 - 0 ; 
       FIG. 14  is a diagram showing a conventional semiconductor device test procedure; 
       FIG. 15  is a block diagram of a conventional reference potential generator  500  that generates a reference potential VBGR; 
       FIG. 16  is a circuit diagram showing the configuration of a data decision circuit  540 - 0  to  540 - 2 ; 
       FIG. 17  shows the configuration of a decode circuit  520 ; and 
       FIG. 18  shows the configuration of the reference potential selection circuit  510 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention are described below with reference to the accompanying drawings. However, the invention is not limited by the embodiments. 
   A reference potential generator according to the present invention includes a plurality of data decision circuits, and is capable of supplying a reference potential selection circuit with digital data decided by one of the data decision circuits in place of digital data decided by the other. This permits readjustment of the reference potential in the reliability test. The reference potential generator has a test data input unit, and is capable of temporarily transferring digital data of various values to the reference potential selection circuit in place of the digital data decided by the data decision circuits. This ensures adjustment of the reference potential to bring it closer to the design value. 
   (First Embodiment) 
     FIG. 1  is a block diagram of a reference potential generator  100  according to a first embodiment of the present invention. The reference potential generator  100  includes: a reference potential selector circuit  10 ; decode circuit  20 ; data transfer circuits  30 - 0  to  30 - 2 ; data decision circuits  40   a - 0  to  40   a - 2 ; data decision circuits  40   b - 0  to  40   b - 2 ; an input data selection circuit  50 ; and a standard data decision circuit  60 . The reference potential selection circuit  10  and the decode circuit  20  may have the same configurations as shown in  FIGS. 18 and 17 , respectively. 
   The data decision circuits  40   a - 0  to  40   a - 2  are used in the D/S step in  FIG. 7 , whereas the data decision circuits  40   b - 0  to  40   b - 2  are used in the reliability test. The standard data decision circuit  60  is used when a standard potential is selected in the reliability test. 
   The reference potential generator  100  has an input part that temporarily inputs signals TMFUSESEL 0  to TMFUSESEL 2  and a signal TMFUSEDIS from outside in the test mode. These signals are used in the test mode, but on power-off, they are stopped. 
     FIG. 2  is a circuit diagram showing the configuration of the data decision circuit  40   a - 0 . The data decision circuits  40   a - 1  to  40   a - 2  and  40   b - 0  to  40   b - 2  have the same configuration as that of the data decision circuit  40   a - 0 . 
     FIG. 3  is a timing chart showing the operation of the data decision circuit  40   a - 0 . The other data decision circuits  40   a - 1  to  40   a - 2  and  40   b - 0  to  40   b - 2  operate in the same manner as does the data decision circuit  40   a - 0 . With reference to  FIGS. 2 and 3 , the configuration and the operation of the data decision circuit  40   a - 0  are described below. 
   The data decision circuit  40   a - 0  has a fuse E-Fuse, which can electrically be treated unlike a fuse called an L-Fuse which is cut by a laser. The fuse E-Fuse is nonconducting when untreated and conducts when treated. 
   A description is given first of the operation of the data decision circuit  40   a - 0  when the fuse E-Fuse is not blown, that is, when it is nonconducting without being electrically treated. In  FIG. 3 , the operation is indicated by “E-Fuse not Blow.” 
   On power-up at time t1, gate potentials FPUP and FPUN both are in low. At this time, since a transistor Trp 10  turns ON and a transistor Trn 10  turns OFF, the potential at a node N 10  goes high by the power supply voltage VDD. 
   At time t2, the gate potential FPUP goes high. As a result, the transistor Trn 10  turns OFF, but since the potential at the node N 10  remains high and is latched in an inverter circuit In 10 . 
   At time t3, the gate potential FPUN goes high, turning ON the transistor Trn 10 , but since the fuse E-Fuse is nonconducting, the potential at the node N 10  remains high. Accordingly, a transistor Trn 11  turns ON. 
   At time t4, the gate potential FPUN goes low, turning OFF the transistor Trn 10 , but since the potential at the node N 10  is still latched in the inverter circuit In 10 , the transistor Trn 11  remains ON. 
   From time t1 to t4, gate potentials FPUPd and FPUNd are both held low. Hence, transistors Trp 12  and Trn 12  are in the ON state and in the OFF state, respectively. Accordingly, the potential at a node N 11  is high at the beginning. 
   At time t5, the gate potential FPUPd goes high, turning OFF the transistor Trp 12 , but the potential at the node N 11  is still latched in an inverter circuit In 11 . 
   At time t6, the gate potential FPUNd goes low, turning ON the transistor Trn 12 . Since at this time the transistor Trn 11  is ON, the potential at the node N 11  goes low. As a result, a signal SELECT goes high. 
   At time t7, the gate potential FPUNd goes low, turning OFF the transistor Trn 12 , but since the potential at the node N 11  is latched as “low” in the inverter circuit In 11 , the signal SELECT remains high. 
   A description is given of the operation of the data decision circuit  40   a - 0  which is effected when the fuse E-Fuse is blown, that is when it is electrically treated and hence is conducting. In  FIG. 3 , the operation is indicated by “E-Fuse Blow.” The operation from time t1 to t2 is the same as in the case where the fuse E-Fuse is nonconducting. 
   When the gate potential FPUN goes high at time t3, the transistor Trn 10  turns ON. Since the fuse E-fuse is conducting at this time, the potential at the node N 10  goes low, turning OFF the transistor Trn 11 . 
   At time t4, the gate potential FPUN goes low, turning OFF the transistor Trn 10 . At this time, the transistor Trp 10  is also OFF. Accordingly, the potential at the node N 10  is latched as being “low” in the inverter circuit In 10 . Hence, the transistor Trn 11  remains OFF. 
   At time t5, the gate potential FPUPd goes high, turning OFF the transistor Trp 12 , but the potential at the node N 11  is latched as being “high” in the inverter circuit In 11 . 
   At time t6 the gate potential FPUNd goes high, turning ON the transistor Trn 11 . Since the transistor Trn 11  is OFF at this time, the potential at the node N 11  remains high, making the signal SELECT low. 
   At time t7 the gate potential FPUNd goes low, turning OFF the transistor Trn 12 , but the potential at the node N 11  is still latched as being “high” in the inverter circuit In 11 . This causes the signal SELECT to remain low. 
   As described above, the data decision circuit  40   a - 0  outputs a high-level signal as the signal SELECT when the fuse E-Fuse is nonconducting, and outputs a low-level signal as the signal SELECT when the fuse E-Fuse is conducting. Accordingly, the data decision circuit  40   a - 0  changes the potential of the signal SELECT, depending on the fuse E-Fuse is conducting or nonconducting, and decides a bit value based on the potential of the signal SELECT. Incidentally, the electrical treatment of the fuse is also called trimming. 
   The data decision circuits  40   a - 0  to  40   a - 2  output 1-bit signals SELECTA 0  to SELECTA 2 , respectively, and the data decision circuits  40   b - 0  to  40   b - 2  also output 1-bit signals SELECTB 0  to SELECTB 2 , respectively. 
   The standard data decision circuit  60  outputs an inverted potential of the output potential shown in  FIG. 2 . To perform this, the standard data decision circuit  60  has a configuration that includes an inverter additionally placed in the output unit shown in  FIG. 2  or that excludes one of the existing inverters in the output unit shown in  FIG. 2 . The standard data decision circuit  60  is identical in construction with that of  FIG. 2  except the above. 
   In this embodiment, when the reference potential generator  100  outputs the standard potential as the reference potential VBGR, the signals PRETMBGR 0  to PRETMBGR 2 , which are sent to the decode circuit  20 , are set so that they all have the same bit values “111” (see  FIG. 5 ). In this instance, the standard data decision circuit  60  sends signals SELDISABLEn of the same bit value to the data transfer circuits  30 - 0  to  30 - 2 . Accordingly, this embodiment requires only one standard data decision circuit  60 . 
   Referring back to  FIG. 1 , the test signals TMFUSESEL 0  to TMFUSESEL 2  are input from outside in the test mode. Since the test signals TMFUSESEL 0  to TMFUSESEL 2  may be changed variously, digital data also may have various values. Based on such various pieces of digital data, any of the signals TMBGR 0  to TMBGR 4  can be selected. 
   In this embodiment, while not in the test mode, the test signals TMFUSESEL 0  to TMFUSESEL 2  are all in the low state “000”. AT this time, the data selection circuit  50  deselects the test signals TMFUSESEL 0  to TMFUSESEL 2 . 
     FIG. 4  is a circuit diagram showing the configuration of the data selection circuit  50 . The data selection circuit  50  is configured to select any one of the digital data output from the data decision circuits  40   a - 0  to  40   a - 2 , the digital data output from the data decision circuits  40   b - 0  to  40   b - 2 , and digital data composed of the test signals TMFUSESEL 0  to TMFUSESEL 2 . 
   For example, when the signals SELECTB 0  to SELECTB 2  are all high and the signals TMFUSESEL 0  to TMFUSESEL 2  are all low, the data selection circuit  50  makes both of signals DISABLEA and DISABLEB high. In this case, the data selection circuit  50  selects the digital data output from the data decision circuits  40   a - 0  to  40   a - 2 . Because any one of the signals SELECTB 0  to SELECTB 2  being low means that any one of the data decision circuits  40   b - 0  to  40   b - 2  is not trimmed (see  FIG. 2 ), and the signals TMFUSESEL 0  to TMFUSESEL 2  being all low means that the current mode is not the test mode. 
   When any one of the signals SELECTB 0  to SELECTB 2  is low and the signals TMFUSESEL 0  to TMFUSESEL 2  are all low, the data selection circuit  50  makes the signal DISABLEA low, and the signal DISABLEB high. In this instance, the data selection circuit  50  selects the digital data output from the data decision circuits  40   b - 0  to  40   b - 2 . Because any one of the signals SELECTB 0  to SELECTB 2  being low means that any one of the data decision circuits  40   b - 0  to  40   b - 2  is trimmed, and the signals TMFUSESEL 0  to TMFUSESEL 2  being all low means that the current mode is not the test mode. 
   When any one of the signals TMFUSESEL 0  to TMFUSESEL 2  is high, the data selection circuit  50  makes both of the signals DISABLEA and DISABLEB low irrespective of the states of the signals SELECTB 0  to SELECTB 2 . In this instance, the data selection circuit  50  selects the digital data composed of the signals TMFUSESEL 0  to TMFUSESEL 2 . The reason for this is that any one of the signals TMFUSESEL 0  to TMFUSESEL 2  is high means the test mode. 
   As described above, the data selection circuit  50  is capable of selecting any one of the digital data output from the data decision circuits  40   a - 0  to  40   a - 2 , the digital data output from the data decision circuits  40   b - 0  to  40   b - 2 , and the digital data composed of the test signals TMFUSESEL 0  to TMFUSESEL 2 . 
     FIG. 5  is a table showing the relationships between the signals PRETMBGR 0  to PRETMBGR 2  and the signals TMBGR 0  to TMBGR 4 . This embodiment is set so that the signal TMBGR 2  generates the standard potential. 
   When the standard potential is used as the reference potential VBGR in the test mode, it is necessary to input, separately of the test signals TMFUSESEL 0  to TMFUSESEL 2 , a standard test signal TMFUSEIS that makes all of the signals PRETMBGR 0  to PRETMNBGR 2  high “111.” The reason for this is that in the case of making all of the signals PRETMBGR high by the test signals TMFUSESEL 0  to TMFUSESEL 2 , all the test signals need to be low “000,” which causes the data selection circuit  50  to deselect the test signals TMFUSESEL 0  to TMFUSESEL 2 . Accordingly, the test mode requires the standard test signal TMFUSEDIS that is used to output the standard potential. 
   As in the test mode, the data decision circuits  40   b - 0  to  40   b - 2  cannot be set to use the standard potential as the reference potential VBGR, either. The reason for this is that in the case of using the standard potential as the reference one VBGR, all of the signals SELECTB 0  to SELECTB 2  need to be high “111,” which causes the data selection circuit  50  to deselect the signals SELECTB 0  to SELECTB 2 . Accordingly, the standard data decision circuit  60  is required to set the standard potential as the reference potential VBGR. 
     FIG. 6  is a circuit diagram of the data transfer circuit  30 - 0 . The other data transfer circuits  30 - 1  to  30 - 2  are common in construction to the data transfer circuit  30 - 0 . It should be noted here that the data transfer circuits  30 - 0  to  30 - 2  each input thereto and output therefrom different data. 
   With reference to  FIG. 6 , the configuration and the operation of the data transfer circuit  30 - 0  are described below. The broken-line box A indicates a configuration related to the data decision circuit  40   a - 0 , the broken-line box B indicates a configuration related to the data decision circuit  40   b - 0 , and the broken-line box C indicates a configuration related to the test mode. 
   In the broken-line box A, the data transfer circuit  30 - 0  receives the signal SELECTA 0  from the data decision circuit  40   a - 0 . The signal SELECTA 0  is sent via a transistor Trnp 15  to a node N 15 . The power supply voltage VDD is applied via a transistor Trp 15  to the node N 15 . Accordingly, the potential of a signal ASELECTA 0  at the node N 15  is either the potential of the signal SELECTA 0  or the potential of the power supply voltage VDD (always high). 
   The signal DISABLEA is the data output from the data selection circuit  50  (see  FIG. 4 ). The signal TMFUSEDIS is used to output the standard potential as the reference potential VBGR in the test mode. The signal TMFUSEDIS is high only in the test mode, and is low in the other modes. 
   When the data selection circuit selects the data decision circuits  40   a - 0  to  40   a - 2 , the signal DISABLEA is high and the signal TMFUSEDIS is low. Accordingly, the transistor Trnp 15  turns ON and the transistor Trp 15  turns OFF. As a result, the signal SELECTA 0  is sent to the node N 15 , where it becomes the above-mentioned signal ASELECTA 0 . 
   When the data selection circuit  50  does not select the data decision circuits  40   a - 0  to  40   a - 2 , or in the test mode, the signal DISABLEA is low or the signal TMFUSEDIS is high. Accordingly, the transistor Trnp 15  turns OFF and the transistor Trp 15  turns ON. As a result, the power supply voltage VDD is applied to the node N 15  to generate the signal ASELECTA 0 . That is, in this case, the signal ASELECTA 0  is always high. 
   In the broken-line box B, the data transfer circuit  30 - 0  receives the signal SELECTB 0  from the data decision circuit  40   b - 0 . The signal SELECTB 0  is applied via a transistor Trnp 16  to a node N 16 . The power supply voltage VDD is applied via a transistor Trp 16  to the node N 16 . Accordingly, the potential of a signal ASELECTB 0  at the node N 16  is either the potential of the signal SELECTB 0  or the potential of the power supply voltage VDD. 
   When the data selection circuit  50  selects the data decision circuits  40   b - 0  to  40   b - 2 , the signal DISABLEB is high and the signal TMFUSEDIS is low. Accordingly, the transistor Trnp 16  turns ON and the transistor Trp 16  turns OFF. As a result, the signal SELECTB 0  is sent to the node N 16 , where it becomes the above-mentioned signal ASELECTB 0 . 
   On the contrary, when the data selection circuit  50  does not select the data decision circuits  40   b - 0  to  40   b - 2 , or in the test mode, the signal DISABLEB is low or the signal TMFUSEDIS is high. Accordingly, the transistor Trnp 16  turns OFF and the transistor Trp 16  turns ON. As a result, the power supply voltage VDD is applied to the node N 16  to generate the signal ASELECTB 0 . That is, in this case, the signal ASELECTB 0  is always high. 
   In the broken-line box C, the data transfer circuit  30 - 0  receives an external test signal TMFUSESEL 0 , which is always low except in the test mode and at the time of selecting the standard potential. A NAND gate G 1  is supplied with a signal bTMFUSESEL 0  that is an inverted version of the test signal TMFUSESEL 0 . That is, the signal bTMFUSESEL 0  is always high except in the test mode and at the time of selecting the standard potential. 
   As described above, signals from two deselected ones of the data decision circuits  40   a - 0  to  40   a - 2  (broken-line box A), the data decision circuit  40   b - 0  to  40   b - 2  (broken-line box B) and the test mode (broken-line box C) are always high. The NAND gate G 1  sends an inverted version of the signal from the selected box to a NAND gate G 2 . The NAND gate G 2  is supplied with the inverted signal and the signal SELDISABLE from the standard data decision circuit  60 . When the standard data decision circuit  60  is not selected, the signal SELDISABLE is always high. Accordingly, the signal from the selected one of the data decision circuit  40   a - 0  to  40   a - 2  (broken-line box A), the data decision circuits  40   b - 0  to  40   b - 2  (broken-line box B) and the test mode (broken-line box C) is output as the signal PRETMBGR 0 . 
   When the standard potential is selected, none of the data decision circuits  40   a - 0  to  40   a - 2  (broken-line box A), the data decision circuits  40   b - 0  to  40   b - 2  (broken-line box B) and the test mode (broken-line box) is selected, and the signal SEDISABLE from the standard data decision circuit  60  is output. At this time, the signals ASELECTA 0  and ASELECTB 0  are both high and the signal bTMFUSESEL 0  is low. As a result, the output from the NAND gate G 1  becomes always high, outputting the potential of the signal SEDISABLE as the signal PRETMBGR 0 . 
   As described above, the data transfer circuit  30 - 0  transfers any one of the signals SELECTA 0 , SELECTB 0  and bTMFUSESEL 0  or SELDISABLE as the signal PRETMBGR 0 . 
     FIG. 7  is a conceptual diagram of testing a semiconductor device equipped with the reference potential generator  100 .  FIG. 8  is a flowchart of the procedure for testing the semiconductor device equipped with the reference potential generator  100 . In a front-end of the semiconductor manufacturing, semiconductor elements are formed on the semiconductor wafer. 
   The D/S step (S 10 ) begins with inputting the test signals TMFUSESEL 0  to TMFUSESEL 2  from outside (S 12 ). By changing or modifying the test signals TMFUSESEL 0  to TMFUSESEL 2 , it is possible to obtain pieces of digital data of various values. The next step is to measure reference potentials VBGR based on the pieces of digital data of various values (S 14 ). In the case of using the standard potential to conduct the test, the standard test signal TMFUSEDIS is input from outside. 
   The next step is to specify the digital data for generating the optimum reference potential VBGR closest to the design value (S 16 ). The data decision circuits  40   a - 0  to  40   a - 2  are trimmed to output the optimum digital data (S 18 ). The trimming can be achieved electrically without using a laser, and hence it can be done in the D/S step. Thereafter the semiconductor elements on the semiconductor wafer are tested for each die. 
   For example, when it is found that TMBGR 1  in  FIG. 5  is the optimum, the data decision circuits  40   a - 0  to  40   a - 2  are so trimmed as to output “011.” When the optimum reference potential VBGR closest to the design value is the standard potential, the data decision circuits  40   a - 0  to  40   a - 2  are not trimmed. In this instance, the standard potential is output as the reference potential VBGR based on the digital data that the standard data decision circuit  60  outputs. 
   Redundancy (S 20 ) and a wafer final test (S 30 ) are carried out next. In this case, since the data decision circuits  40   a - 0  to  40   a - 2  are already trimmed, circuits (for example, memory circuit and so on) other than the reference potential generator  100  are trimmed in the redundancy step (S 20 ). In the wafer final test (S 30 ), the results of trimming in the redundancy step (S 20 ) are tested. Accordingly, the redundancy step (S 20 ) and the wafer final test step (S 30 ) are essentially unnecessary for the reference potential generator  100 . 
   Thereafter, in the assembling step, the semiconductor wafer is divided into individual semiconductor chips, which are each packaged (S 40 ). This is followed by a packaging test (S 50 ). 
   The reliability test (S 60 ) is then conducted. Semiconductor chips decided as defective in the reliability test includes those rejected by reason of variations of the reference potential VBGR. In this case, the external test signals TMFUSESEL 0  to TMFUSESEL 2  are input (S 62 ). By changing or modifying the test signals TMFUSESEL 0  to TMFUSESEL 2 , pieces of digital data of various values. The next step is to measure reference potentials VBGR based on the pieces of digital data of various values (S 64 ). In the case of using the standard potential to conduct the test, the standard test signal TMFUSEIS is input from outside. 
   The next step is to specify the digital data for generating the optimum reference potential VBGR closest to the design value (S 66 ). The data decision circuits  40   b - 0  to  40   b - 2  are trimmed to output the optimum digital data (S 68 ). The trimming can be achieved electrically without using a laser, and hence it can be done in the reliability test step. 
   For example, when it is found at the time of the reliability test that TMBGR 0  is optimum although TMBGR 1  was selected in the D/S step, the data decision circuits  40   b - 0  to  40   b - 2  are so trimmed as to output “101.” When the optimum reference potential VBGR closest to the design value is the standard potential, the data decision circuits  40   b - 0  to  40   b - 2  are not trimmed. In this instance, the standard potential is output as the reference potential VBGR based on the digital data that the standard data decision circuit  60  outputs. 
   Further, the semiconductor chips undergo a packaging final test (S 70 ). Thereafter the semiconductor chips are shipped as products. The semiconductor chips decided as non-defective in the reliability test of step S 60  undergo the packaging final test in step S 70  without going through steps S 62  to S 68 . 
   In this embodiment, the data selection circuit  50  is capable of selecting the test signals TMFUSESEL 0  to TMFUSESEL 2 . This allows the reference potential generator  100  to operate in the test mode in the D/S step (S 10 ) and to input various pieces of digital data from external. By this, it is possible to measure the actual reference potential VBGR corresponding to each piece of digital data. As a result, the data decision circuits  40   a - 0  to  40   a - 2  can be so trimmed as to output optimum digital data in the D/S step. 
   According to this embodiment, the data decision circuits  40   ab - 0  to  40   b - 2  are each equipped with the electrically treatable fuse E-Fuse. Accordingly, the data decision circuits  40   b - 0  to  40   b - 2  can be trimmed in the reliability test (S 60 ). This trimming permits correction of a shift or deviation of the reference voltage VBGR due to stresses applied to the semiconductor chip in the assembling step (S 40 ) or in the packaging test (S 50 ). As a result, it is to recover the semiconductor chips rejected as defective in the reliability test (S 60 ). 
   In the reliability test (S 60 ), too, the data selection circuit  50  is capable of selecting the test signals TMFUSESEL 0  to TMFUSESEL 2 . This allows the reference potential generator  100  to operate in the test mode in the D/S step (S 10 ) to input various pieces of digital data from external. By this, it is possible to measure the actual reference potential VBGR corresponding to each piece of digital data. As a result, the data decision circuits  40   b - 0  to  40   b - 2  can be so trimmed as to output optimum digital data in the reliability test. 
   The data decision circuits  40   a - 0  to  40   a - 2  are each provided with the electrically treatable fuse E-Fuse. Accordingly, in the D/S step (S 10 ) it is possible to perform trimming of the data decision circuits  40   a - 0  to  40   a - 2  as well as the electrical test including the measurement of the reference potential VBGR. 
   The data decision circuits  40   a - 0  to  40   a - 2  may be of such a configuration as shown in  FIG. 16 . In such an instance, a laser trimming step is needed separately of the D/S step. On the other hand, the data decision circuits  40   b - 0  to  40   b - 2  each have the configuration shown in  FIG. 2 , and hence they can be re-trimmed in the reliability test (S 60 ) to output optimum digital data. 
   While in this embodiment, the digital data has been described as being 3-bit data, it may also be of 2 or 1 bit, or 4 or more bits. In this case, the data decision circuits  40   a ,  40   b  and the data transfer circuits  30  are respectively provided by a number equal to that of bits of the digital data used. The number of the test signals TMFUSESEL to be input from outside is also equal to the number of bits. 
   For example, when the digital data is 8-bit, the reference potential generator  100  needs only to be provided with data decision circuits  40   a - 0  to  40   a - 7 , data decision circuits  40   b - 0  to  40   b - 7  and data transfer circuits  30 - 0  to  30 - 7 . In the test mode test signals TMFUSESEL 0  to TMFUSESEL 7  are input from outside. 
   In the above the signals PRETMBGR 0  to PRETMBGR 2  are set at “111,” but this value can properly be changed. This can be done by changing the settings of the signals TMFUSESEDIS and the standard data decision circuit  60  to conform with the signals PRETMBGR 0  to PRETMBGR 2  which generate the standard potential. 
   (Second Embodiment) 
     FIG. 9  is a block diagram of a reference potential generator  200  according to a second embodiment of the present invention. The reference potential generator  200  does not have the decode circuit  20 , and the signals TMBGR 0  to TMBGR 4  are transferred directly to the reference potential selection circuit  10  from a data transfer circuits  32 - 0  to  32 - 3  and a standard data transfer circuit  34 . Like parts corresponding to those in the first embodiment are designated by like reference numerals. In this embodiment, the signals TMBGR 0  to TMBGR 4  are used as digital data. When the signal TMBGR 2  is high, the standard potential is output as the reference potential VBGR. 
   The data decision circuits  40   a , the data decision circuits  40   b  and the data transfer circuits  32  are respectively provided by a number equal to that having subtracted the reference potential from the number of signals TMBGR, that is, equal to a number having subtracted by one from the number of bits forming the digital data. In this embodiment, the reference potential generator  200  includes: data decision circuits  40   a - 0 ,  40   a - 1 ,  40   a - 3  and  40   a - 4  (hereinafter referred to also as data decision circuits  40   a - 0  to  40   a - 4 ); data decision circuits  40   b - 0 ,  40   b - 1 ,  40   b - 3  and  40   b - 4  (hereinafter referred to also as data decision circuits  40   b - 0  to  40   b - 4 ); and data transfer circuits  32 - 0 ,  32 - 1 ,  32 - 3  and  32 - 4  (hereinafter referred to also as data transfer circuits  32 - 0  to  32 - 4 ). 
   A data selection circuit  52  and the standard data transfer circuit  34  are used to select the standard potential as the reference potential VBGR. 
     FIG. 10  is a circuit diagram of the standard data selection circuit  52 . The standard data selection circuit  52  outputs a high-level signal when any one of signals TMBGR 0 , TMBGR 1 , TMBGR 3  and TMBGR 4  (hereinafter referred to also as signals TMBGR 0  to 4) is high. When the signals TMBGR 0  to 4 are all low, the standard data selection circuit  52  outputs a low-level signal. That is, the standard data selection circuit  52  outputs the high-level signal as the signal DISABLE in the case of deselecting the signal TMBGR 2 , and outputs the low-level signal as the signal DISABLE in the case of selecting the signal TMBGR 2 . 
     FIG. 11  is a circuit diagram of the standard data transfer circuit  34 . The standard data transfer circuit  34  responds to the signal DISABLE from the standard data selection circuit  52  to select (high) or deselect (low) the signal TMBGR 2 . A signal SELECT 2  is high. Accordingly, the standard data transfer circuit  34  deselects (low) the signal TMBGR 2  when the signal DISABLE is high, and selects (high) the signal TMBGR 2  when the signal DISABLE is low. In this way, the standard data selection circuit  52  and the standard data transfer circuit  34  select the signal TMBGR 2 . As a result, the standard potential is output as the reference potential VBGR. 
     FIG. 12  is a circuit diagram of a data selection circuit  54 . The data selection circuit  54  outputs the signal DISABLEA for deselecting the data decision circuits  40   a - 0  to  40   a - 4  when the data decision circuits  40   b - 0  to  40   b - 4  are selected. 
   The data selection circuit  54  outputs a high-level signal when the data decision circuits  40   a - 0  to  40   a - 4  are selected in the D/S step. Thereafter, when the data decision circuits  40   b - 0  to  40   b - 4  are selected in the reliability test, any one of signals SELECTB 0 ,  1 ,  3  and  4  becomes low. As a result, the data selection circuit  54  outputs a low-level signal as the signal DISABLEA. 
     FIG. 13  is a circuit diagram of the data transfer circuit  32 - 0 . Since the data transfer circuits  32 - 1 ,  32 - 2  and  32 - 3  have the same configuration as that of the data transfer circuit  32 - 0 , no description is repeated in connection with them. The broken-line box A 2  indicates a configuration related to the data decision circuit  40   a - 0 , the broken-line box B 2  indicates a configuration related to the data decision circuit  40   b - 0 , and the broken-line box C indicates a configuration related to the test mode. 
   In the broken-line box A 2 , the data transfer circuit  32 - 0  inputs the signal SELECTA 0  from the data decision circuit  40   a - 0 . The signal SELECTA 0  is applied via a transistor Trnp 17  to a node N 17 . The power supply voltage VDD is applied via a transistor Trp 17  to the node N 17 . Accordingly, the potential of the signal ASELECTA 0  at the node N 17  is either the potential of the signal SELECTA 0  or the potential (high) of the power supply voltage VDD. 
   A signal SELDISABLE 0  is data output from a standard data decision circuit  60  (see  FIG. 9 ). When the standard data decision circuit  60  is not used, signals SELDISABLE 0  to SELDISABLE  4  are all high, whereas when the standard data decision circuit  60  is used, the signals SELDISABLE 0  to SELDISABLE 4  are all low. The signal TMFUSEDIS is used to output the standard potential as the reference potential VBGR in the test mode. The signal TMFUSEDIS is high only in the test mode and low in the other modes. The signal DISABLEA is fed from the data selection circuit  54 , and it is high when the data decision circuits  40   a - 0  to  40   a - 4  are selected, and low when the data decision circuits  40   a - 0  to  40   b - 4  are selected. 
   When the data decision circuits  40   a - 0 ,  40   a - 1  and  40   a - 3  are selected, the signals SELDISABLE 0  and DISABLEA are high, but the signal TMFUSEDIS is low. Accordingly the transistor Trnp 17  turns ON, whereas the transistor Trp 17  turns OFF. As a result, the signal SELECTA 0  is sent to the node N 17  to form the signal ASELECTA 0 . 
   When the standard data decision circuit  60  is selected, or the data decision circuits  40   b - 0  to  40   b - 4  are selected, or in the test mode, the signal SELDISALE 0  is low, the signal DISABLEA is low, or the signal TMFUSEDIS is high. Accordingly, the transistor Trnp 17  turns OFF and the transistor Trp 17  urns ON. As a result, the potential of the power supply voltage VDD is sent to the node N 17  to form the signal ASELECTA 0 . AT this time, the signal ASELECTA 0  becomes high. 
   In the broken-line box B 2 , the data transfer circuit  32 - 0  inputs the signal SELECTB 0  from the data decision circuit  40   b - 0 . The signal SELECTB 0  is applied via a transistor Trnp 18  to a node N 18 . The power supply voltage VDD is applied via a transistor Trp 18  to the node N 18 . Hence, the potential of the signal ASELECTB 0  at the node N 18  is either the potential of the signal SELECTB 0  or the potential (high) of the power supply voltage VDD. 
   When the data decision circuits  40   b - 0  to  40   b - 3  are selected, the signal SELDISABLE 0  is high and the signal TMFUSEDIS is low. Accordingly, the transistor Trnp 18  turns ON and the transistor Trp 18  turns OFF. As a result, the signal SELECTB 0  is sent to the node N 18  to form the signal ASELECTB 0 . Since in this case the signal DISABLEA is low, the data decision circuits  40   a - 0  to  40   a - 4  are deselected. 
   On the contrary, when the standard data decision circuit  60  is selected, or in the test mode, the signal SELDISABLE 0  is low, or the signal TMFUSEDIS is high. Accordingly, the transistor Trnp 18  turns OFF and the transistor Trp 18  turns ON, through which the potential of the power supply voltage VDD is sent to the node N 18  to form the signal ASELECTB 0 . In this case, the signal ASELECTB 0  is always high. 
   In the broken-line box C, the data transfer circuit  32 - 0  inputs the external test signal TMFUSESEL 0 , which is always low except in the test mode. 
   In the test mode, high-level signals are always applied to two inputs of a NAND gate G 10  irrespective of the state of the data decision circuits  40   a - 0  and  40   b - 0 . Accordingly, a NAND gate G 11  is supplied with a high-level signal from the NAND gate G 10 , and outputs the signal TMFUSESL 0  as the signal TMBGR 0 . 
   In the case of outputting the standard potential as the reference potential VBGR, the potential (high) of the power supply voltage VDD is sent to both inputs of the NAND gate G 1 . The signal TMFUSESEL 0  is low. Hence, the signal TMBGR 0  is output as a low-level signal from the NAND gate G 11 . 
   “The signal TMBGR 0  is low” means the signal TMBGR 0  is not selected. Accordingly, by making all of the signals TMBGR 0  to TMBGR 4 , except TMBGR 2 , low by the data transfer circuits  32 - 0  to  32 - 4 , the standard potential can be selected as the reference potential VBGR. In this instance, the data selection circuit  53  and the standard data transfer circuit  34  select the signal TMBGR 2  as described above. As a result, the reference potential VBGR becomes the standard potential. 
   When the standard data decision circuit  60  is not selected, or not in the test mode, the signal TMFUSESEL 0  is low. Accordingly, data from any one of the data decision circuit  40   a - 0  to  40   a - 3  or  40   b - 0  to  40   b - 3  is output as the signal TMBGR 0  from the NAND gate G 11 . 
   Since the test procedure of the reference potential generator  200  is the same as that in the first embodiment described previously with reference to  FIGS. 7 and 8 , no description is repeated. 
   While in the second embodiment the digital data is 5-bit data, it is not limited specifically thereto but the number of bits may be of 4 or smaller or more than 6. In this case, the data decision circuits  40   a , the data decision circuits  40   b  and the data transfer circuits  30  are respectively provided by the number smaller by one than the number of bits used. Similarly, the number of external test signal TMFUSESEL is also set as above. 
   This embodiment produces the same effects as obtainable with the first embodiment. This embodiment requires no decode circuit.