Abstract:
A monitored burn-in test method includes: subjecting an element set, including elements, to a writing process for writing data into each of the elements, the elements requiring a refresh process; subjecting the element set to the refresh process after the writing process; and interrupting the refresh process for a selected one or ones of the elements, when instructions for readout of data are supplied to the selected one or ones during the refresh process, and subjecting the selected one or ones to a readout process in accordance with the instructions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuing application, filed under 35 U.S.C. §111(a), of International Application PCT/JP2008/051581, filed on Jan. 31, 2008, the contents of which are incorporated herein by reference. International Application PCT/JP2008/051581 is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-023319, filed on Feb. 1, 2007, Japanese Patent Application No. 2007-026584, filed on Feb. 6, 2007, and Japanese Patent Application No. 2007-073432, Mar. 20, 2007, the entire contents of which are also incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiments discussed herein are related to a monitored burn-in test apparatus. 
       BACKGROUND 
       [0003]    A so-called monitored burn-in test is performed prior to shipment of semiconductor devices such as static random access memories (SRAM), for example. A plurality of semiconductor devices as test objects to be subjected to the monitored burn-in test are set on a burn-in board. The semiconductor devices are heated by heaters, for example. The temperatures of the semiconductor devices are kept high such as at 100 degrees Celsius, for example. Simultaneously, the semiconductor devices are driven to operate. Voltage of a level higher than usual is applied to the semiconductor devices. The operation of the semiconductor devices is monitored in this condition. 
         [0004]    The monitored burn-in test includes (1) a writing/reading stage, (2) a burn-in stage and (3) a reading stage. (1) The writing/reading stage is first performed. A data writing process and a data reading process are conducted. The written data and the read data are compared with each other. Next, (2) the burn-in stage is performed. A data writing process is continued for a long period of time. Subsequently, (3) the reading stage is performed. The writing process and the readout process are performed. The written data and the read data are compared with each other in the same manner as in the writing/reading stage. 
         [0005]    Memories such as a synchronous dynamic random access memory (SDRAM) and a dynamic random access memory (DRAM) require a refresh process, for example. In the case where such memories are subjected to a monitored burn-in test, a time duration from writing operation of data to reading operation of data in the excess of a so-called refresh cycle causes the written data to be lost. Accordingly, the writing process and the readout process are continuously performed to each memory. It takes a considerably long time to apply the writing process and the readout process to all the memories. It is thus quite troublesome to perform the monitored burn-in test on the memories requiring the refresh process. 
         [0006]    The monitored burn-in test is performed on all the semiconductor devices of the same type en bloc. All the semiconductor devices need to be kept at a uniform temperature. Temperature sensors are attached to the semiconductor devices one by one for controlling the temperature. Temperature measuring units are connected to the temperature sensors one by one. The temperature measuring units determine the temperatures measured by the temperature sensors, respectively. A controller circuit refers to the determined temperatures to control the temperatures of the heaters. A monitored temperature testing apparatus of this type requires the same number of the temperature measuring units as that of the temperature sensors. This results in an increase in the production cost of the monitored temperature testing apparatus. 
         [0007]    The heaters are utilized to heat the test objects. The individual heater includes a cylindrical metallic tube, for example, as disclosed in Japanese Patent No. 3425825, for example. A heat-generating object is inserted in the metallic tube. The bottom surface of the metallic tube is urged against the test object so that the test object is heated. However, the bottom surface of the metallic tube is designed to have a predetermined area. If the test object, which receives the bottom surface of the metallic tube, has a large size, the bottom surface of the heater cannot contact with the test object over a sufficient area, for example. The heater lacks versatility. 
       Patent Publication 1: JP Patent Application Laid-open No. 5-36793 
     Patent Publication 2: JP Patent Application Laid-open No. 2005-156172 
     Patent Publication 3: JP Patent Application Laid-open No. 2005-252225 
     Patent Publication 4: JP Patent Application Laid-open No. 10-320974 
     Patent Publication 5: JP Patent No. 3425825 
     Patent Publication 6: JP Patent Application Laid-open No. 2001-167600 
     Patent Publication 7: JP Patent Application Laid-open No. 4-17349 
     Patent Publication 8: JP Patent Application Laid-open No. 2001-184896 
     SUMMARY 
       [0008]    According to an aspect of the present invention, there is provided a monitored burn-in test method comprising: subjecting an element set, including elements, to a writing process for writing data into each of the elements, the elements requiring a refresh process; subjecting the element set to the refresh process after the writing process; and interrupting the refresh process for a selected one or ones of the elements, when instructions for readout of data are supplied to the selected one or ones during the refresh process, and subjecting the selected one or ones to a readout process in accordance with the instructions. 
         [0009]    The object and advantages of the embodiments will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the embodiments, as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view schematically depicting a monitored burn-in test apparatus according to an embodiment of the present invention; 
           [0011]      FIG. 2  is an enlarged sectional view schematically depicting a burn-in board and a monitored temperature test apparatus; 
           [0012]      FIG. 3  is an enlarged partial plan view schematically depicting a monitored temperature test apparatus according to a specific example of the present invention; 
           [0013]      FIG. 4  is a sectional view taken along the line  4 - 4  in  FIG. 3 ; 
           [0014]      FIG. 5  is an enlarged partial plan view schematically depicting the monitored temperature test apparatus; 
           [0015]      FIG. 6  is an enlarged sectional view schematically depicting a heater; 
           [0016]      FIG. 7  is a block diagram schematically depicting a control system of the monitored burn-in test apparatus; 
           [0017]      FIG. 8  is a view depicting a writing command; 
           [0018]      FIG. 9  is a view depicting a readout command; 
           [0019]      FIG. 10  is a view depicting a refresh command; 
           [0020]      FIG. 11  a view depicting a refresh cancellation command; 
           [0021]      FIG. 12  is a graph schematically depicting the stages of a monitored burn-in test; 
           [0022]      FIG. 13  is a flow chart schematically depicting the flow of the monitored burn-in test; 
           [0023]      FIG. 14  is a view depicting that a writing process is applied to all the elements; 
           [0024]      FIG. 15  is a view depicting that a refresh process is applied to all the elements; 
           [0025]      FIG. 16  is a view depicting that a readout process is applied to element set  1  while the refresh process is applied to element sets  2 - 10 ; 
           [0026]      FIG. 17  is a view depicting that the readout process is applied to element set  2  while the refresh process is applied to element sets  1  and  3 - 10 ; 
           [0027]      FIG. 18  is a view depicting that the readout process is applied to one of the element sets while the refresh process is applied to the other element sets; 
           [0028]      FIG. 19  is a block diagram schematically depicting a control system of the monitored temperature test apparatus according to a specific example of the present invention; 
           [0029]      FIG. 20  is a block diagram schematically depicting a control system of the monitored temperature test apparatus according to another specific example of the present invention; 
           [0030]      FIG. 21  is an enlarged partial sectional view schematically depicting a heating jig; 
           [0031]      FIG. 22  is a perspective view schematically depicting the heating jig; 
           [0032]      FIG. 23  is a perspective view schematically depicting the heating jig; 
           [0033]      FIG. 24  is a perspective view schematically depicting the heating jig; 
           [0034]      FIG. 25  is a side view schematically depicting that the heating jig contacts with the element at a first contact surface; 
           [0035]      FIG. 26  is a side view schematically depicting that the heating jig contacts with the element at a second contact surface; and 
           [0036]      FIG. 27  is a side view schematically depicting that the heating jig contacts with the element at a third contact surface. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0037]    Description will be made below on the embodiment of the present invention with reference to the attached drawings. 
         [0038]      FIG. 1  schematically depicts a monitored burn-in test apparatus  11  according to an embodiment. The monitored burn-in test apparatus  11  includes a burn-in board  12 . The burn-in board  12  includes a board body  13  made of resin, for example. A printed wiring board  14  is fixed on the board body  13 . The contour of the printed wiring board  14  is defined inside the contour of the board body  13 . Sockets  15  are mounted on the surface of the printed wiring board  14 . The sockets  15  are arranged in four rows and four columns, for example. 
         [0039]    Elements  16  as test objects, to be subjected to a monitored burn-in test, are set in the sockets  15 , respectively. All the elements  16  are semiconductor devices of the same type. The elements  16  include memory chips, such as synchronous dynamic random access memory (SDRAM) chips, for example. Such memory chips require a refresh process, for example. A connector  17  is mounted on the board body  13  at a position off the printed wiring board  14 . The elements  16  are connected to the connector  17  via wiring patterns, not depicted, formed on the printed wiring board  14 . The connector  17  is connected to a controller circuit for a monitored burn-in test, which will be described later. 
         [0040]    A monitored temperature test apparatus  21  is located above the burn-in board  12 . The monitored temperature test apparatus  21  includes a substrate  22  made of resin, for example. The contour of the substrate  22  is identical to that of the board body  13  of the burn-in board  12 . Four support posts  23  are located between the substrate  22  and the board body  13 . The support posts  23  are located at the four corners of the board body  13 . The support posts  23  serve to space the back surface of the substrate  22  from the front surface of the board body  13  at a predetermined interval. The substrate  22  and the board body  13  are coupled to each other with the supports posts  23 . 
         [0041]    Heaters  25  are supported in the substrate  22 , for example. The heaters  25  are arranged in four rows and four columns, for example. The individual heater  25  is formed in the shape of a column, for example. Four parallel fixation plates  26  are fixed to the substrate  22  for supporting the heaters  25 . The heaters  25  stand upright from the front and back surfaces of the substrate  22 . The heaters  25  are related to the aforementioned sockets  15  one by one. The positions of the heaters  25  on the substrate  22  correspond to and reflect the positions of the sockets  15  on the board body  13 , respectively. In this manner, the lower ends of the heaters  25  are received on the elements  16  in the sockets  15 , respectively. The structure of the heaters  25  will be described later in detail. 
         [0042]    A power supply wiring  27  and a ground wiring  28  are connected to the individual heater  25 . The power supply wiring  27  and the ground wiring  28  are connected to electrically-conductive pads  29  on the substrate  22 , respectively. The electrically-conductive pads  29  are formed on the substrate  22  at positions off the fixation plate  26 . A connector  31  is mounted on the substrate  22 . A power supply cable, not depicted, is connected to the connector  31 . The power supply cable is connected to a power supply. The electrically-conductive pads  29  are connected to the connector  31  through an electrically-conductive pattern. In this manner, electric power is supplied to the heaters  25 . 
         [0043]    As depicted in  FIG. 2 , the individual support post  23  is made of a hollow pipe. The interval between the substrate  22  and the board body  13  is adjusted by adjusting the length of the hollow pipe. The screw shaft of a bolt  32  is received in the support post  23 . The bolt  32  penetrates through the substrate  22  and the board body  13 . The head of the bolt  32  is received on the front surface of the substrate  22 . A nut  33  is engaged with the screw shaft of the bolt  32  on the back surface of the board body  13 . In this manner, the substrate  22  and the board body  13  are coupled to each other. Screws  34  are utilized to fix the fixation plate  26  to the substrate  22 . 
         [0044]    As depicted in  FIG. 3 , pairs of attachment plates  35  are coupled to the fixation plate  26  on the front surface of the fixation plate  26 . The individual heater  25  is sandwiched between the inner ends of the attachment plates  35 ,  35 . A recess  36  is defined in the inner end of the individual attachment plate  35 . The end surface of the attachment plate  35  along the recess  36  contacts with the outer peripheral surface of the heater  25 . The edge of the recess  36  extends along an arc of a predetermined curvature. The radius of curvature of the edge coincides with the radius of the heater  25 . In this manner, the attachment plates  35 ,  35  support the heater  25 . The heater  25  is received in a through hole  37  formed in the fixation plate  26 . A predetermined gap is formed between the outer peripheral surface of the heater  25  and the wall surface of the through hole  37 . 
         [0045]    A screw  38  is utilized to couple the individual attachment plate  35  to the fixation plate  26 . The screw shaft of the screw  38  is received in a slit  39  formed in the attachment plate  35 . The slit  39  extends on an imaginary straight line connecting the electrically-conductive pads  29 ,  29  to each other. The screw  38  is screwed in the fixation plate  26 . Referring also to  FIG. 4 , the head of the screw  38  is received on the surface of the attachment plate  35 . Rectangular openings  41 , for example, are formed in the substrate  22 . The rectangular openings  41  are assigned to the heaters  25 , respectively. The fixation plate  26  closes the openings  41 . The heaters  25  and the screw shafts of the screws  38  are received in the openings  41 , respectively. 
         [0046]    The attachment plates  35  are allowed to slide on the front surface of the fixation plate  26  on the aforementioned imaginary straight line. The combination of the screws  38  and the corresponding slits  39  serves to guide the sliding movement of the attachment plates  35 . In this manner, as depicted in  FIG. 5 , for example, the attachment plates  35  can be positioned at outward positions, which are distanced from the heater  25 . Here, the diameter of the through hole  37  of the fixation plate  26  is set larger than that of the heater  25 . Therefore, when the attachment plates  35  are positioned at the outward positions, the vertical movement of the heater  35  is accepted in the direction of the longitudinal axis of the heater  25 . 
         [0047]    As depicted in  FIG. 6 , the individual heater  25  includes a cylindrical casing  42 . The cylindrical casing  42  may be made of a metallic material such as aluminum, for example. A heat-generating body  43  is located in the cylindrical casing  42 . The heat-generating body  43  may be a heating wire, for example. The aforementioned power supply and ground wirings  27 ,  28  are connected to the heat-generating body  43 . The heat-generating body  43  generates heat in response to electric power supplied through the power supply and ground wirings  27 ,  28 . The temperature of the heat-generating body  43  is determined depending on the amount of the electric power supplied to the heat-generating body  43 . 
         [0048]    A temperature sensor  44  is incorporated in the cylindrical casing  42  of the heater  25 . The temperature sensor  44  is located along the bottom plate of the cylindrical casing  42 , for example. Wirings  45  are connected to the temperature sensor  44 . The wirings  45  are also connected to the substrate  22 . The lower end or bottom plate of the cylindrical casing  42  of the heater  25  contacts with the element  16 , as described above. The temperature sensor  44  thus detects the temperature of the element  16 . The detected temperature is output to the outside from the substrate  22 . 
         [0049]    As depicted in  FIG. 7 , the elements  16 , fifty of them, are mounted on the burn-in board  12 , for example. The fifty elements  16  are arranged in five rows and ten columns, for example. The elements  16  are set in the sockets  15  on the burn-in board  12 , respectively. The elements  16  are SDRAMs. Here, the individual elements  16  are labeled with identifiers from “Element  1 ” to “Element  50 ”. One element set is established on the burn-in board  12  based on the five elements  16  of each column. Since the fifty elements  16  are located on the burn-in board  12 , ten element sets, namely the first to tenth element sets, are established on the burn-in board  12 . Each element set contains the five elements  16 . Alternatively, it should be noted that one element set may be established based on the ten elements  16  of each row, for example. 
         [0050]    A controller circuit, namely a controller  46 , is connected to the connector  17  of the burn-in board  12 . The controller  46  operates based on a software program held in a flash memory, not depicted, for example. The controller  46  is connected to a CLK signal generating section  47 , a CKE signal generating section  48 , an address data generating section  49 , an RAS signal generating section  51 , a CAS signal generating section  52 , a WE signal generating section  53  and a test data generating section  54 . The controller  46  is configured to control the output of the signals and data generated in the generating sections  47 - 54 . 
         [0051]    The CLK (clock) signal generating section  47  generates a CLK signal. The CLK signal represents an operation reference clock. The CKE (clock enable) signal generating section  48  generates a CKE signal. The CKE signal specifies whether or not a refresh process is effected. The fresh process will be described later in detail. The address data generating section  49  generates address data. The address data specifies the address for a cell or cells within the individual element  16 . The RAS (row address strobe) signal generating section  51  generates a RAS signal. The CAS (column address strobe) signal generating section  52  generates a CAS signal. The RAS signal and the CAS signal specify a timing for obtaining the address data. A WE (write enable) signal generating section  53  generates a WE signal. The WE signal specifies whether or not a writing process is effected. The test data generating section  54  generates test data. 
         [0052]    One common wiring pattern is connected to all the elements  16  of each row on the burn-in board  12 . The common wiring pattern is connected to one terminal of the connector  17 . The common wiring pattern is connected to a CLK terminal, an address terminal, a RAS terminal, a CAS terminal, a WE terminal and an input/output terminal, which are formed in the individual element  16 . In this manner, “Element  1 ” to “Element  10 ” of the first row are configured to receive the common CLK signal, address data, RAS signal, CAS signal, WE signal and test data, for example. Likewise, “Element  11 ” to “Element  20 ” of the second row, “Element  21 ” to “Element  30 ” of the third row, and . . . are configured to receive the common signals and data, respectively. 
         [0053]    A distinct wiring pattern is individually connected to the individual element  16  on the burn-in board  12 . The distinct wiring pattern is connected to one terminal of the connector  17 . The distinct wiring pattern is connected to a CKE terminal formed in the individual element  16 . In this manner, a CKE signal is separately input into the individual element  16 . In other words, different CKE signals can be input into “Element  1 ” to “Element  50 ”, respectively. The controller  46  controls such CKE signals. It should be noted that distinct wiring patterns cannot be formed on the board body  13  for the aforementioned CLK signal, address data, RAS signal, CAS signal, WE signal and test data because the standard regulates the number of the pins in the connector  17 . 
         [0054]    The signals output from the signal generating sections  47 - 53  under the control of the controller  46  serve to establish various kinds of commands. As depicted in  FIG. 8 , when the WE signal is set at “0” at the time of the rise of the CLK signal, a writing command is established. As depicted in  FIG. 9 , when the WE signal is set at “1” at the time of the rise of the CLK signal, a readout command is established. As depicted in  FIG. 10 , when the CKE signal is set at “0”, a refresh command is established. While the CKE signal is kept at “0”, a self refresh process is continued for a selected one or ones of “Element  1 ” to “Element  50 ”. As depicted in  FIG. 11 , when the CKE signal is set at “1”, a refresh cancellation command is established. 
         [0055]    Next, description will be made on a so-called monitored burn-in test. “Element  1 ” to “Element  50 ” are set in the sockets  15  of the burn-in board  12 , respectively. As depicted in  FIG. 12 , a writing/reading stage (W/R stage) is first performed. At the writing/reading stage, “Element  1 ” to “Element  50 ” are heated in response to the heat generated by the heaters  25 . The temperatures of “Element  1 ” to “Element  50 ” are kept at 70 degrees Celsius approximately. At step S 1  of  FIG. 13 , the controller  46  establishes a common writing command to “Element  1 ” to “Element  50 ” belonging to all the first to tenth element sets. The writing command is broadcast into all of “Element  1 ” to “Element  50 ” through the common wiring patterns and the distinct wiring patterns. As a result, test data output from the test data generating section  54  is written into all of “Element  1 ” to “Element  50 ” en bloc, as depicted in  FIG. 14 . “Element  1 ” to “Element  50 ” receive the address data at a timing determined by the RAS signal and the CAS signal. In this manner, test data is written into a predetermined cell at step S 2 . 
         [0056]    The controller  46  establishes a common refresh command for all of “Element  1 ” to “Element  50 ” at step S 3 . The refresh command is input into all of “Element  1 ” to “Element  50 ” through the common wiring patterns and the distinct wiring patterns. As a result, “Element  1 ” to “Element  50 ” start being subjected to a self refresh process at step S 4 , as depicted in  FIG. 15 . The self refresh process serves to hold the written test data in “Element  1 ” to “Element  50 ”. The controller  46  generates the aforementioned refresh cancellation command at step S 5 . Since the CKE signal can separately be input into each of “Element  1 ” to “Element  50 ” as described above, the CKE signal set at “1” is input only into “Element  1 ”, “Element  11 ”, “Element  21 ”, “Element  31 ” and “Element  41 ” of the first element set. As a result, the self refresh process is interrupted for the elements  16  belonging to the first element set. 
         [0057]    The controller  46  establishes a common readout command for all of “Element  1 ” to “Element  50 ” at step S 7 . The readout command is broadcast into all of “Element  1 ” to “Element  50 ” through the common wiring patterns and the distinct wiring patterns. As a result, test data is simultaneously read out from “Element  1 ”, “Element  11 ”, “Element  21 ”, “Element  31 ” and “Element  41 ” of the first element set at step S 8 . As depicted in  FIG. 16 , since the self refresh process is continued for the second to tenth element sets other than the first element set, the readout command is not input into the second to tenth element sets. Data is output from “Element  1 ”, “Element  11 ”, “Element  21 ”, “Element  31 ” and “Element  41 ” at step S 9 . The controller  46  compares the readout data with the written test data at step S 10 . The controller  46  determines whether or not the test objects pass the test based on whether or not the readout data coincides with the written test data. The controller  46  then establishes the refresh command for the first element set based on the control of the CKE signal at step S 11  in the same manner as described above. The self refresh process is restarted for the elements  16  belonging to the first element set at step S 12 . 
         [0058]    The controller  46  determines whether or not any other element set exists at step S 13 . Here, since the second to tenth element sets have not been subjected to the readout process, the process proceeds to step S 14 . The processes of steps S 5  to S 12  are repeated for the second element set at step S 14 . As depicted in  FIG. 17 , data is read out from “Element  2 ”, “Element  12 ”, “Element  22 ”, “Element  32 ” and “Element  42 ” after the self refresh process has been interrupted. After comparison of the readout data to the written test data, the self refresh process is restarted for the elements  16  belonging to the second element set. In this manner, as depicted in  FIG. 18 , the processes of the aforementioned steps S 5  to S 12  are repeated for each of the third to tenth element sets. Upon completion of the W/R stage for all the element sets, the monitored burn-in test proceeds to a burn-in stage. 
         [0059]    At the burn-in stage, as depicted in  FIG. 12 , the temperatures of “Element  1 ”, “Element  11 ”, “Element  21 ”, “Element  31 ” and “Element  41 ” are kept at 100 degrees Celsius approximately by the heaters  25 . The controller  46  establishes the common refresh command for “Element  1 ” to “Element  50 ” of all the first to tenth element sets again at step S 15 . The refresh command is input into all of “Element  1 ” to “Element  50 ”. As a result, the self refresh process is continued for all of “Element  1 ” to “Element  50 ” at step S 16 . The test data is held in all of “Element  1 ” to “Element  50 ”. The self refresh process is continued for 24 hours, for example. In this manner, a so-called dynamic burn-in process is effected. Upon completion of the burn-in stage, the monitored burn-in test proceeds to a readout stage (an R stage). 
         [0060]    At the R stage, as depicted in  FIG. 12 , the temperatures of “Element  1 ” to “Element  50 ” are kept at 70 degrees Celsius approximately by the heaters  25 . The controller  46  establishes the common refresh command for “Element  1 ” to “Element  50 ” of all the first to tenth element sets again at step S 17 . The refresh command is input into all of “Element  1 ” to “Element  50 ”. As a result, the self refresh process is continued for “Element  1 ” to “Element  50 ”. The test data is held in all of “Element  1 ” to “Element  50 ”. The controller  46  establishes the refresh cancellation command at step S 19 . The refresh cancellation command is input only into the elements  16  belonging to the first element set based on the control of the CKE signal in the same manner as described above. As a result, the self refresh process is canceled for the element of the first element set at step S 20 . 
         [0061]    The controller  46  establishes the common readout command for all of “Element  1 ” to “Element  50 ” at step  21  in the same manner as at the aforementioned W/R stage. The readout command is input into all of “Element  1 ” to “Element  50 ”. Data is read from “Element  1 ”, “Element  11 ”, “Element  21 ”, “Element  31 ” and “Element  41 ” at step S 22 . Since the self refresh process is continued for the elements  16  belonging to the second to tenth element sets other than the first element set, the readout command is not input into the elements  16  of the second to tenth element sets. The data is output at step S 23 . The controller  46  compares the readout data with the written test data at step S 23 . The controller  46  determines whether or not the test objects pass the test depending on whether or not the readout data coincides with the written test data. The controller  46  then establishes the refresh command for the elements  16  belonging to the first element set at step S 24 . The self refresh process is restarted for the elements  16  of the first element set at step S 25 . 
         [0062]    The controller  46  determines whether or not any other element set exists at step S 27 . Here, since the second to tenth element sets have not been subjected to the readout process, the process proceeds to step S 28 . The processes of steps S 19  to S 26  are repeated for the second element set at step S 28 . After the self refresh process has been interrupted, data is read out from “Element  2 ”, “Element  12 ”, “Element  22 ”, “Element  32 ” and “Element  42 ” belonging to the second element set in the same manner as described above. After comparison of the readout data with the written test data, the self refresh process is restarted for the elements  16  belonging to the second element set. The processes of the aforementioned steps S 19  to S 26  are repeated for each of the third to tenth element sets. Upon completion of the R stage for all the element sets, the monitored burn-in test is completed. 
         [0063]    In the monitored burn-in test apparatus  11 , after the test data is simultaneously written into “Element  1 ” to “Element  50 ” en bloc, the self refresh process is effected on all of “Element  1 ” to “Element  50 ”. The self refresh process is interrupted only for the elements  16  belonging to a selected one of the element set for the readout process. Upon completion of the readout process, the self refresh process is restarted for the elements  16  belonging to the selected element set. The self refresh process is continued to for the element  16  belonging to the element sets other than the selected element set. As a result, the test data is reliably held in all of “Element  1 ” to “Element  50 ”. Since the test data is held, it is not necessary to effect the writing process to “Element  1 ” to “Element  50 ” more than once. Therefore, the W/R stage, the burn-in stage and the R stage are performed in series by one monitored burn-in test apparatus  11 . The monitored burn-in test can be efficiently performed. 
         [0064]    The writing command, the readout command, the refresh command and the refresh cancellation command are established to apply the writing process for the test data, to apply the readout process for the test data, to start the self refresh process, and to cancel the self refresh process, respectively. These commands are generated based on conventional CLK signal, RAS signal, CAS signal and WE signal. Therefore, addition of particular terminals to “Element  1 ” to “Element  50 ” is not required. This results in avoidance of a reduction in the access speed to “Element  1 ” to “Element  50 ”. The monitored burn-in test can be performed on “Element  1 ” to “Element  50 ” of a conventional type. Moreover, a particular circuitry is not required for establishing the commands. The structure of the monitored burn-in test apparatus  11  can be simplified. The versatility of the monitored burn-in test apparatus  11  is improved. 
         [0065]      FIG. 19  is a block diagram depicting a control system of the monitored temperature test apparatus  21 . As depicted in  FIG. 19 , the temperature sensors  44   a - 44   p  are grouped into a first temperature sensor set and a second temperature sensor set. The first temperature sensor set includes the temperature sensors  44   b ,  44   d ,  44   e ,  44   g ,  44   j ,  44   l ,  44   m ,  44   o . The second temperature sensor set includes  44   a ,  44   c ,  44   f ,  44   h ,  44   i ,  44   k ,  44   n ,  44   p , the remainder of the temperature sensors  44   a - 44   p . The temperature sensors  44  of the first temperature sensor set are selected from the temperature sensors  44  of each row. Likewise, the temperature sensors  44  of the second temperature sensor set are selected from the temperature sensors of each column. The number of the temperature sensors  44  of the first temperature sensor set is common to each row and each column. The temperature sensors  44  of the first and second temperature sensor sets are arranged in each row. The number of the temperature sensors  44  of the first temperature sensor set is equal to the number of the temperature sensors  44  of the second temperature sensor set in each row. The number of the temperature sensors  44  of the first temperature sensor set is equal to the number of the temperature sensors  44  of the second temperature sensor set in each column. 
         [0066]    First temperature measuring units  71   a - 71   d  are assigned to the rows, respectively. The first temperature measuring units  71   a - 71   d  are arranged in this sequence from the first row. The temperature sensors  44   b ,  44   d  of the first temperature sensor set are in parallel connected to the first temperature measuring unit  71   a , assigned to the first row, through a wiring pattern  72 . The wiring pattern  72  is formed on the substrate  22 . Switches  73  are inserted in the wiring pattern  72 . The switches  73  are assigned to the temperature sensors  44   b ,  44   d , respectively. The temperature sensors  44   b ,  44   d  are selectively connected to the first temperature measuring unit  71   a  through the operation of the switches  73 . The first temperature measuring unit  71   a  detects the temperature of the semiconductor device based on the connected temperature sensors  44 . 
         [0067]    Likewise, the temperature sensors  44   e ,  44   g  of the first temperature sensor set are in parallel connected to the first temperature measuring unit  71   b  assigned to the second row. The temperature sensors  44   j ,  44   l  of the first temperature sensor set are in parallel connected to the first temperature measuring unit  71   c  assigned to the third row. The temperature sensors  44   m ,  44   o  of the first temperature sensor set are in parallel connected to the first temperature measuring unit  71   d  assigned to the fourth row. The switches  73  are inserted in each of the wiring patterns  72  in the same manner as in the first temperature measuring unit  71   a . The switches  73  are assigned to the temperature sensors  44 , respectively. The temperature sensors  44  are switched through the operation of the switches  73 . 
         [0068]    Second temperature measuring units  74   a - 74   d  are assigned to the columns, respectively. The second temperature measuring units  74   a - 74   d  are arranged in this sequence from the first column. The temperature sensors  44   a ,  44   i  of the second temperature sensor set, other than those of the first temperature sensor set, are in parallel connected to the second temperature measuring unit  74   a , assigned to the first column, through a wiring pattern  75 . The wiring pattern  75  is formed on the substrate  22 . Switches  76  are inserted in the wiring pattern  75 . The switches  76  are assigned to the temperature sensors  44   a ,  44   i , respectively. The temperature sensors  44   a ,  44   i  are selectively connected to the second temperature measuring unit  74   a  through the operation of the switches  76 . The second temperature measuring unit  74   a  detects the temperature of the semiconductor device based on the connected temperature sensor  44 . 
         [0069]    Likewise, the temperature sensors  44   f ,  44   n  of the second temperature sensor set are in parallel connected to the second temperature measuring unit  74   b  assigned to the second column. The temperature sensors  44   c ,  44   k  of the second temperature sensor set are in parallel connected to the second temperature measuring unit  74   c  assigned to the third column. The temperature sensors  44   h ,  44   p  of the second temperature sensor set are I parallel connected to the second temperature measuring unit  74   d  assigned to the fourth column. The switches  76  are inserted in each of the wiring patterns  75  in the same manner as in the aforementioned second temperature measuring unit  74   a . The switches  76  are assigned to the temperature sensors  44 , respectively. The switches  76  are utilized to switch the temperature sensors  44 . 
         [0070]    A controller circuit, namely a controller  77 , is connected to the first temperature measuring units  71   a - 71   d  and the second temperature measuring units  74   a - 74   d . The controller  77  is configured to control the operation of the first temperature measuring units  71   a - 71   d , the second temperature measuring units  74   a - 74   d  and the heaters  25  in accordance with a predetermined software program. The software program may be held in a memory  78 , for example. A monitored temperature test, which will be described later, is performed in accordance with the software program. Data for performing the temperature test may also be held in the memory  78 . 
         [0071]    The controller  77  notifies the first temperature measuring units  71   a - 71   d  of a selected one or ones of the switches  73  for the connection. Likewise, the controller  77  notifies the second temperature measuring units  74   a - 74   d  of a selected one or ones of the switches  76  for the connection. The first temperature measuring units  71   a - 71   d  and the second temperature measuring units  74   a - 74   d  obtains the temperatures of the connected temperature sensors  44 . The controller  77  specifies the amount of electric power for each of the heaters  25  in accordance with the detected temperature. The controller  77  may refers to relationships between the amount of electric power and temperature held in the memory  78  for such specification. 
         [0072]    Next, description will be made on the operation of the monitored temperature test apparatus  21 . The controller  77  executes a predetermined software program. Electric power of a predetermined amount is supplied to the heaters  25  in response to the instructions of the controller  77 . The heaters  25  generate heat. The temperatures of the elements  16  rise. Simultaneously, the controller  77  notifies the first temperature measuring units  71   a - 71   d  and the second temperature measuring units  74   a - 74   d  of a selected one or ones of the switches  73 ,  76  for the connection. Either one of the switches  73  is connected in each row. Either one of the switches  76  is connected in each column. In this manner, each of the first and second temperature measuring units  71   a - 71   d  and  74   a - 74   d  is connected to either one of the temperature sensors  44  of the related row and column. 
         [0073]    The heat generated by the heaters  25  is utilized to set the temperatures of the elements  16  within a predetermined temperature range. Such a temperature range is set higher than 98 degrees Celsius but lower than 102 degrees Celsius, for example. The connected temperature sensors  44  detect the temperatures of the elements  16 , respectively. A measuring process is performed for the first time. The detected temperatures are output to the controller  77 . The controller  77  determines whether or not the detected temperatures are out of the predetermined temperature range. If the detected temperature is 102 degrees Celsius or higher, for example, the amount of the electric power supplied to the related heater  25  is reduced. If the detected temperature is 98 degrees Celsius or lower, for example, the amount of the electric power supplied to the related heater  25  is increased. 
         [0074]    The switches  73 ,  76  are switched in each row and each column. The remaining switches  73 ,  76  are connected. The remaining temperature sensors  44  are connected to the first temperature measuring units  71   a - 71   d  and second temperature measuring units  74   a - 74   d , respectively. The connected temperature sensors  44  detect the temperatures of the elements  16 , respectively, in the same manner as described above. The measuring process is performed for the second time. The controller  77  determines whether or not the detected temperatures are out of the predetermined temperature range. The amount of the electric power supplied to the related heater  25  is adjusted in accordance with the detected temperature. In this manner, the temperatures of all the elements  16  are kept uniform within the predetermined temperature range. 
         [0075]    Electric power is supplied to the elements  16  from a power source via the burn-in board  12 . Voltage of a level higher than a usual level is applied to the elements  16 . The elements  16  are driven to operate. The operation of the elements  16  is examined. It is checked whether or not defective products exist. The monitored temperature test apparatus  21  is removed from the burn-in board  12 . The burn-in test is completed. 
         [0076]    In the monitored temperature test apparatus  21 , each of the first temperature measuring units  71   a - 71   d  can selectively be connected to the temperature sensors  44  of the related row. Likewise, each of the second temperature measuring units  74   a - 74   d  can separately be connected to the temperature sensors  44  of the related column. The first temperature measuring units  71  are respectively assigned to the rows and the second temperature measuring units  74  are respectively assigned to the columns for detection of the temperatures of all the elements  16 . The number of the temperature measuring units can be significantly reduced as compared with the case where the temperature measuring units are connected to all the temperature sensors  44  one by one. The production cost of the monitored temperature test apparatus  21  is significantly reduced. 
         [0077]    As depicted in  FIG. 20 , the temperature sensors  44  may be arranged in ten rows and five columns. In this case, the elements  16  are likewise arranged in ten rows and five columns on the burn-in board  12 . First temperature measuring units  71   e - 71   j  are provided for the added rows, respectively. A second measuring unit  74   e  are provided for the added column. The switches  72 ,  75  are inserted in the wiring patterns  72 ,  75  for the temperature sensors  44 , respectively, in the same manner as described above. The switches  73 ,  76  are configured to identify the temperature sensors  44  for the measurement. In this manner, the temperatures of all the elements  16  can be detected by performing the measurement for four times based on the operation of the switches  73 ,  76 . The number of the temperature measuring units can be reduced in the same manner as described above. The production cost of the monitored temperature test apparatus  21  can be reduced. 
         [0078]    As depicted in  FIG. 21 , a heating jig  81  may be attached to the lower end of the individual heater  25 . The heating jig  81  has a predetermined contact surface received on the surface of the element  16 . The heating jig  81  is made out of a block. The block is made of a metallic material having a high thermal conductivity, such as copper or aluminum. The heating jig  81  has contact surfaces having various areas to contact with the element  16 , as describe later in detail. The heat of the heater  25  is transferred to the element  16  via the heating jig  81 . 
         [0079]    As depicted in  FIG. 22 , the heating jig  81  includes a first block  82  and a second block  83 . The first and second blocks  82 ,  83  are formed in a prismatic shape. The first block  82  and the second block  83  are formed integral with each other at their side surfaces. The first block  82  stands upright along a first axis  X 1    perpendicular to an imaginary plane. The second block  83  extends along a second axis  X 2    parallel to the imaginary plane. The half of the first block  82  and the half of the second block  83  in combination define a prismatic shape extending along a third axis  X 3    perpendicular to the first axis  X 1    and the second axis  X 2   . The first axis  X 1    the second axis  X 2    and the third axis  X 3    are set parallel to the y-axis, z-axis and x-axis of a three-dimensional coordinate system, respectively. 
         [0080]    A first insertion hole  84  is formed in one end surface of the first block  82  to extend along the first axis X 1 . The first insertion hole  84  is a bottomed hole. A protrusion  85  is formed on the other end surface of the first block  82 . The protrusion  85  is formed in the shape of a prism, for example. A second insertion hole  86  is formed in one end surface of the second block  83  to extend along the second axis  X 2   . The second insertion hole  86  is a bottomed hole. Referring also to  FIG. 23 , a third insertion hole  87  is formed in the side surface of the first block  82  to extend along the third axis  X 3   . The third insertion hole  87  is a bottomed hole. The third insertion hole  87  is connected to the first insertion hole  84  and the second insertion hole  86 . The diameters of the first, second and third insertion holes  84 ,  86 ,  87  are set sufficiently large to accept insertion of the heater  25 . 
         [0081]    A first contact surface  88  is defined in the top surface of the protrusion  85 . The first contact surface  88  intersects the first axis  X 1   . Likewise, a second contact surface  89  is defined in the other end surface of the second block  83 . The other end surface of the second block  83  is an end surface opposite to the end surface with the second insertion hole  86 . The second contact surface  89  is set perpendicular to the second axis  X 2   . Referring also to  FIG. 24 , a third contact surface  91  is defined in the side surface of the second block  83 . The third contact surface  91  intersects the third axis  X 3   . The areas of the first contact surface  88 , the second contact surface  89  and the third contact surface  91  are different from one another. Here, the area of the second contact surface  89  may be set larger than the area of the first contact surface  88 , while the area of the third contact surface  91  may be set larger than the area of the second contact surface  88 . 
         [0082]    For the use of the heating jig  81 , one contact surface, whose area is suitable to the area of the surface of the element  16 , is selected from the first, second and third contact surfaces  88 ,  89 ,  91 . As depicted in  FIG. 25 , if the area of the surface of the element  16  is smaller than the area of the lower end surface of the heater  25 , for example, the first contact surface  88  is selected. In this case, the heater  25  is inserted in the first insertion hole  84 . The lower end of the heater  25  is received on the bottom of the first insertion hole  84 , namely a bottom wall  92  of the first block  82 . For realizing an efficient heat transfer, the thickness of the bottom wall  92  of the first block  82  is reduced appropriately in view of the strength. The heating jig  81  is urged against the element  16 . The heat of the heater  25  is transferred to the element  16  via the first contact surface  88  of the protrusion  85 . 
         [0083]    If the area of the surface of the element  16  is larger than that of the lower end surface of the heater  25 , for example, the second contact surface  89  is selected, as depicted in  FIG. 26 . In this case, the heater  25  is inserted in the second insertion hole  86 . The thickness of a bottom wall  93  of the second block  83  is reduced in the same manner as described above. The heat of the heater  25  is transferred to the element  16  via the second contact surface  89 . If the area of the surface of the element  16  is much larger than that of the lower end surface of the heater  25 , the third contact surface  91  is selected, as depicted in  FIG. 27 . The heater  25  is inserted in the third insertion hole  87 . The thickness of a side wall  94  of the second block  83  is reduced in the same manner as described above. The heat of the heater  25  is transferred to the element  16  via the third contact surface  91 . 
         [0084]    In the heating jig  81 , since the areas of the contact surfaces  88 ,  89 ,  91  are different from one another, the heater  25  may be inserted in one of the insertion holes  84 ,  86 ,  87  in accordance with the size of the element  16 . The contact surfaces  88 ,  89 ,  91  can contact with the surface of the element  16  with efficiency. The heat of the heater  25  is transferred to the element  16  with efficiency irrespective of the area of the lower end surface of the heater  25 . The heater  25  serves to heat the elements  16  of various sizes with efficiency. The monitored temperature test apparatus  21 , namely the monitored burn-in test apparatus  11 , is usable for monitored burn-in tests for the elements  16  of various sizes. The versatility of the monitored burn-in test apparatus  11  is improved. 
         [0085]    It should be noted that a thermally-conductive body such as a thermally-conductive grease or compound may be utilized to fill a space inside the insertion holes  84 ,  86 ,  87  outside the outer peripheral surface of the heater  25 , for example. The thermally-conductive body serves to reduce thermal resistance between the heater  25  and the heating jig  81 . As a result, the heat of the heater  25  can be transferred to the heating jig  81 , namely the element  16 , with a higher efficiency. 
         [0086]    All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concept contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.