Abstract:
With a probe-test method and a prober for examining certain electric characteristics of an object of examination, a main chuck is adapted to be driven to move in the X-, Y-, Z- and 0-directions in order to bring the object into contact with the probes of the prober and then the shaft of the support of the main chuck is warped under the contact pressure applied by the probes to tilt the main chuck. The position where each of the probes contacts the corresponding one of the electrodes on the object is displaced (moved) in the X-, Y- and Z-directions by the tilt. The displacement is predicted by an operation unit and the main chuck is moved in the X-, Y- and Z-directions to correct the displacement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-036361, filed Feb. 15, 2000, the entire contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to a needle load measuring method, a needle load setting method and a needle load detecting mechanism, particularly, to a method and a mechanism for measuring on the real time basis the needle load applied to a wafer chuck by a probe in the inspecting step and a needle load setting method for setting an appropriate needle load.  
           [0003]    A probe apparatus is widely used for inspecting the electrical characteristics of electric circuits formed on a to-be-inspected object, e.g., a wafer. As shown in FIG. 4, the conventional probe apparatus comprises a wafer chuck  1  on which a wafer W is placed, an X-stage  2  for supporting the wafer chuck  1 , a Y-stage  3  for supporting the X-stage  2 , and a base table  4  for supporting these X-stage  2  and Y-stage  3 . When the electrical characteristics of the wafer W are inspected, the wafer chuck  1  is moved in the X- and Y-directions via the X-stage  2  and the Y-stage  3  and is also moved in a vertical direction by a vertical driving mechanism, e.g., member  31  shown in FIG. 1. The X-stage  2  performs a reciprocating movement on the Y-stage  3  along a rail  6  extending in an X-direction via a driving mechanism  5  in the X-direction. On the other hand, the Y-stage  3  performs a reciprocating movement on the base table  4  along a rail  8  extending in a Y-direction via a driving mechanism  7  in the Y-direction. The driving mechanism  5  in the X-direction comprises a motor  5 A (not shown), and a ball screw  5 B. Likewise, the driving mechanism  7  in the Y-direction comprises a motor  7 A and a ball screw  7 B. These ball screws  5 B and  7 B are engaged with the X-stage  2  and the Y-stage  3 , respectively, so as to move these X- and Y-stages  2  and  3 . The wafer chuck  1  is moved in the Z-direction via the vertical driving mechanism so as to bring the wafer W placed on the wafer chuck  1  into an electrical contact with a plurality of probes  9 A of a probe card  9  arranged above the wafer chuck  1 . The electrical characteristics of IC chips formed on the wafer W are inspected by the plural probes  9 A. During the inspection, the overdriving amount of the wafer chuck  1  is controlled at a predetermined value so as to permit the wafer W to be brought into an electrical contact with the probes  9 A.  
           [0004]    It is desirable for the overdriving amount to be defined for each probe card  9  in conformity with, for example, the characteristics of the probes  9 A. By setting the overdriving amount on the basis of the defined value, the needle load in the inspecting step can be set at a predetermined value. Reference numerals  10 A and  10 B shown in FIG. 4 denote aligning mechanisms for aligning the position of the wafer W with the position of the probe card  9 .  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    In the conventional inspecting apparatus, the overdriving amount once set is maintained constant until the inspection is finished. A head plate  19 B and an adapter ring  19 C, to which the probe card is mounted, are considered to be thermally deformed under the influence of, for example, the heat generated from the wafer chuck during the inspection. By the thermal deformation, the magnitude of the needle load is changed over the entire region or a part of the wafer chuck. At the place where the needle load has been diminished, the contact between the wafer and the probes is rendered poor. On the other hand, at the place where the needle load has been increased, the probe card is likely to be damaged.  
           [0006]    An object of the present invention is to overcome the above-noted problems inherent in the prior art.  
           [0007]    Another object of the present invention is to monitor on the real time basis the contact state (needle load) between a to-be-inspected object, i.e., a wafer, and the probe.  
           [0008]    Another object of the present invention is to prevent a damage done to, for example, a probe card.  
           [0009]    Further, still another object of the present invention is to provide an improved needle load measuring method, an improved needle load setting method and an improved needle load inspecting mechanism.  
           [0010]    According to a first aspect of the present invention, there is provided a method of measuring the needle load applied by a plurality of probes to a wafer chuck in inspecting the electrical characteristics of a to-be-inspected object by using a probe apparatus, comprising the steps of:  
           [0011]    overdriving a wafer chuck having a to-be-inspected object mounted thereon by using a wafer chuck lift mechanism so as to bring the to-be-inspected object into contact with a plurality of probes of the probe apparatus; and  
           [0012]    measuring the sinking amount of the wafer chuck caused by the needle load applied by the plural probes to the wafer chuck via the to-be-inspected object when the to-be-inspected object is brought into contact with the plural probes;  
           [0013]    wherein the needle load corresponding to the measured sinking amount is obtained on the basis of the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck caused by the needle load.  
           [0014]    According to a second aspect of the present invention, there is provided a method of setting a needle load applied by a plurality of probes to a to-be-inspected object at a predetermined value in inspecting the electrical characteristics of the to-be-inspected object by overdriving a wafer chuck having the to-be-inspected object mounted thereon in a probe apparatus so as to bring the to-be-inspected object into an electrical contact with the plural probes, comprising the step of:  
           [0015]    detecting the sinking amount of the wafer chuck caused by the needle load applied by the plural probes in the process of overdriving the wafer chuck;  
           [0016]    wherein the needle load applied to the to-be-inspected object mounted on the wafer chuck is set at a predetermined value by setting the sinking amount detected in the step at a sinking amount corresponding to a predetermined needle load on the basis of the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck caused by the needle load.  
           [0017]    In the method according to each of the first and second aspects of the present invention described above, it is desirable for the sinking amount of the wafer chuck to be measured at a plurality of points of the wafer chuck.  
           [0018]    In the method according to each of the first and second aspects of the present invention described above, it is desirable for the wafer chuck having the to-be-inspected object mounted thereon to be overdriven by a lift mechanism comprising a ball screw, a driving mechanism for rotating the ball screw and a nut member.  
           [0019]    In the method according to each of the first and second aspects of the present invention described above, it is desirable for the step of measuring the sinking amount to comprise the sub-steps of:  
           [0020]    detecting a pseudo overdriving amount of the wafer chuck performed by the lift mechanism, the pseudo overdriving amount being the overdriving amount of the wafer chuck performed by the lift mechanism when the needle load is not applied to the wafer chuck;  
           [0021]    detecting the actual overdriving amount of the wafer chuck performed by the lift mechanism, the actual overdriving amount being the actual overdriving amount of the wafer chuck performed by the lift mechanism when the needle load is applied to the wafer chuck; and  
           [0022]    obtaining the sinking amount of the wafer chuck from the difference between the pseudo overdriving amount and the actual overdriving amount.  
           [0023]    In the method described above, it is desirable for the step of detecting the pseudo overdriving amount to comprise the process of detecting the pseudo overdriving amount on the basis of the amount of rotation of the driving mechanism for rotating the ball screw measured by a rotary encoder.  
           [0024]    In the method described above, it is desirable for the step of detecting the actual overdriving amount to comprise the process of detecting the actual overdriving amount of the wafer chuck by a linear encoder.  
           [0025]    According to a third aspect of the present invention, there is provided a probe apparatus for inspecting the electrical characteristics of a to-be-inspected object, comprising:  
           [0026]    a wafer chuck for mounting a to-be-inspected object thereon;  
           [0027]    a lift mechanism for vertically moving the wafer chuck;  
           [0028]    a plurality of probes brought into an electrical contact with a plurality of electrodes of the to-be-inspected object mounted on the wafer chuck overdriven by the lift mechanism;  
           [0029]    a measuring mechanism for measuring the sinking amount of the wafer chuck caused by the needle load applied by the plural probes to the to-be-inspected object when the wafer chuck is overdriven to bring the to-be-inspected object mounted on the wafer chuck into contact with the plural probes; and  
           [0030]    a needle load detecting mechanism for obtaining the needle load corresponding to the measured sinking amount on the basis of the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck.  
           [0031]    In the measuring mechanism of the probe apparatus described above, it is desirable for the sinking amount of the wafer chuck to be measured at a plurality of points of the wafer chuck.  
           [0032]    It is desirable for the lift mechanism of the probe apparatus described above to comprise a ball screw, a driving mechanism for rotating the ball screw and a nut member.  
           [0033]    It is desirable for the measuring mechanism of the probe apparatus for measuring the sinking amount to comprise:  
           [0034]    a first lift amount detecting mechanism for detecting a pseudo lift amount of the wafer chuck performed by the lift mechanism, the first lift amount detecting mechanism detecting a pseudo overdriving amount that the lift mechanism may overdrive the wafer chuck when the needle load is not applied to the wafer chuck;  
           [0035]    a second lift amount detecting mechanism for detecting the actual overdriving amount of the wafer chuck performed by the lift mechanism, the second lift amount detecting mechanism detecting the actual overdriving amount of the wafer chuck performed by the lift mechanism when the needle load is applied to the wafer chuck;  
           [0036]    a calculating mechanism for obtaining the sinking amount of the wafer chuck from the difference between the pseudo overdriving amount detected by the first lift amount detecting mechanism and the actual overdriving amount detected by the second lift amount detecting mechanism;  
           [0037]    a storing device for storing the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck caused by the needle load; and  
           [0038]    a needle load detecting mechanism for obtaining the needle load corresponding to the sinking amount of the wafer chuck obtained by the calculating mechanism on the basis of the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck caused by the needle load.  
           [0039]    In the probe apparatus of the present invention, it is desirable for the measuring mechanism of the sinking amount to comprise:  
           [0040]    a first lift amount detecting mechanism for detecting a pseudo lift amount of the wafer chuck performed by the lift mechanism, the first lift amount detecting mechanism detecting a pseudo overdriving amount that the lift mechanism may overdrive the wafer chuck when the needle load is not applied to the wafer chuck;  
           [0041]    a second lift amount detecting mechanism for detecting the actual overdriving amount of the wafer chuck performed by the lift mechanism, the second lift amount detecting mechanism detecting the actual overdriving amount of the wafer chuck performed by the lift mechanism when the needle load is applied to the wafer chuck;  
           [0042]    a calculating mechanism for obtaining the sinking amount of the wafer chuck from the difference between the pseudo overdriving amount detected by the first lift amount detecting mechanism and the actual overdriving amount detected by the second lift amount detecting mechanism;  
           [0043]    a storing device for storing the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck caused by the needle load; and  
           [0044]    a needle load detecting mechanism for obtaining the needle load corresponding to the sinking amount of the wafer chuck obtained by the calculating mechanism on the basis of the data showing the relationship between the needle load applied to the wafer chuck and the sinking amount of the wafer chuck caused by the needle load.  
           [0045]    Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.  
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0046]    The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.  
         [0047]    [0047]FIG. 1 is a cross sectional view showing a main portion of an inspecting apparatus according to one embodiment of the present invention;  
         [0048]    [0048]FIG. 2 is for describing the operation of the inspecting apparatus shown in FIG. 1;  
         [0049]    [0049]FIG. 3 is a graph showing the relationship between the needle load and the sinking amount of a ball screw in the inspecting apparatus shown in FIG. 1;  
         [0050]    [0050]FIG. 4 is an oblique view showing a main portion of a conventional inspecting apparatus; and  
         [0051]    [0051]FIG. 5 is a cross sectional view showing a main portion of the inspecting apparatus according to another embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0052]    The present invention will now be described with reference to the embodiment shown in FIGS.  1  to  3 . As shown in FIG. 1, the inspecting apparatus in this embodiment comprises a wafer chuck  11  on which a wafer W is placed, a lift mechanism  31  of the wafer chuck  11  including a ball screw  12 , a nut section  13  and a motor  14 , an X-stage  15  supporting these members, and a control device  16  for controlling a driving mechanism such as the motor  14 . A hole  15 A is formed in substantially the center of the X-stage  15 . The motor  14  can be arranged within the hole  15 A. It is also possible for the motor  14  to be arranged on the X-stage  15 .  
         [0053]    The ball screw  12  is joined to the motor  14  and extends upward through the hole  15 A so as to be engaged with the nut member  13 . The nut member  13  is moved upward or downward in accordance with rotation of the ball screw  12  in the clockwise direction and the counterclockwise direction. The nut member  13  is mounted to the lower end of a hollow Z-shaft  17  extending downward from the center in the lower surface of the wafer chuck  11 . The ball screw  12  engaged with the nut member  13  is arranged within the Z-shaft  17 . The wafer chuck  11  moves upward or downward via the ball screw  12 , the nut member  13  and the Z-shaft  17  in accordance with rotation of the motor  14  in the clockwise direction and the counterclockwise direction. The Z-shaft  17  extending downward from the wafer chuck  11  is movable upward and downward in the vertical direction within a Z-shaft  18  mounted to the X-stage  15 .  
         [0054]    As shown in FIG. 1, a probe card  19  having a plurality of probes  19 A is arranged above the wafer chuck  11 . The wafer W is brought into an electrical contact with the probe  19 A if the wafer chuck  11  is overdriven above the X-stage  15  by a lift mechanism  31 . The electrical characteristics of an IC chip formed in the wafer W are inspected by a tester connected to the probe  19 A.  
         [0055]    As shown in FIG. 1, a rotary sensor  20  is mounted to the motor  14 . The rotary sensor  20 , e.g., a rotary encoder, detects the lift amount, i.e., a pseudo overdriving amount, that must have been overdriven by the wafer chuck  11 . It is possible to employ a rotary encoder or a resolver as the rotary sensor. The rotary sensor is hereinafter referred to as the “rotary encoder”. A linear scale  21  is arranged on the X-stage  15 . Also, a linear sensor  22  is mounted to the wafer chuck  11 . The graduation  21 A of the linear scale  21  is read by the linear sensor  22  and the linear scale  21  so as to detect the actual overdriving amount of the wafer chuck  11 . The linear scale  21  and the linear sensor  22  are collectively referred to herein later as “a linear encoder  24 ”.  
         [0056]    When the wafer chuck  11  is brought into an electrical contact with a plurality of probes  19 A by the overdriving, the wafer chuck  11  is caused to sink slightly by the needle load from the plural probes  19 A. To be more specific, as shown in FIG. 1, the wafer chuck  11  is moved upward in the Z-direction by the lift mechanism including the motor  14 , the ball screw  12  and the nut  13  so as to be brought into contact with the plural probes  19 A. Further, if the wafer chuck  11  is overdriven, a needle load is applied from the plural probes  19 A to the wafer W. Since the nut member  13  fixed to the lower end of the Z-shaft  17  is engaged with the ball screw  12 , the ball screw  12  receives the needle load through the nut member  13 . In this step, a compression force is generated between the ball screw  12  and the nut member  13  so as to elastically deform the nut member  13 . As a result, the wafer chuck  11  is displaced vertically downward, as schematically shown in FIG. 2. To be more specific, the wafer chuck  11  is overdriven from the position denoted by a thick line to a position denoted by a dot-and-dash line (pseudo overdriving amount of L′), as shown in FIG. 2. However, since the nut member  13  is elastically deformed by the needle load, the wafer chuck  11  is caused to sink from the position denoted by the dot-and-dash line to a position denoted by a thin line. As a result, the actual overdriving amount L detected by the linear encoder  24  is made smaller by a sinking amount δ than the pseudo overdriving amount L′ detected by the rotary encoder  20 . In other words, the difference δ is generated between the pseudo overdriving amount L′ and the actual overdriving amount L. For preventing the linear encoder  24  from receiving the influence given by the change in temperature, it is effective to allow a fluid for maintaining the temperature such as the air to flow through the linear encoder. In the present invention, the apparatus for measuring the actual overdriving amount is not limited to the apparatus shown in FIG. 2. It is possible to employ any apparatus that permits accurately measuring the actual overdriving amount of the wafer chuck  11 .  
         [0057]    The relationship between the needle load and the sinking amount δ has been analyzed. Specifically, the sinking amount δ has been detected by using the rotary encoder  20  and the linear encoder  24  every time the load applied to the wafer chuck  11  has been changed. It has been clarified that there is a relationship as shown in, for example, FIG. 3 between the needle load and the sinking amount δ. It is possible to store the particular relationship in a memory section  16 A of the control device  16  in the form of a numerical formula or a table. A difference (sinking amount δ) between the pseudo overdriving amount detected by the rotary encoder  20  and the actual overdriving amount detected by the linear encoder  24  is obtained in an arithmetic processing section  16 B of the control device  16 . The needle load corresponding to the sinking amount δ is obtained on the basis of the relationship shown in FIG. 3 between the needle load and the sinking amount δ and the needle amount thus obtained is displayed in a display apparatus  23 . As a result, it is possible to grasp the needle load applied to the wafer chuck  11  on the real time basis. By contraries, it is possible to set a desired needle load from the sinking amount δ on the basis of the relationship shown in FIG. 3. For example, when a needle load of 25 kg·f is set, it is necessary to set the sinking amount δ at 10 μm. The needle load can be set at a predetermined value by controlling the lift mechanism  31  to achieve the sinking amount δ. The relationship between the needle load and the sinking amount δ is caused to change by the diameter of the screw, lead, the number of windings of the screw groove of the nut member, the ball diameter, etc. It is desirable for the relationship between the needle load and the sinking amount δ to be obtained for each inspecting apparatus when the inspecting apparatus is assembled. It is also desirable to set in advance the upper limit and the lower limit of an allowable range of the needle load in, for example, the display device  23 . In this case, when the needle load has exceeded or is likely to exceed the allowable range, a comparator section  16 C judges the particular situation so as to rotate the motor  14  in the opposite direction or stop the rotation of the motor  14  so as to prevent the needle load from exceeding the allowable range. As a result, it is possible to prevent in advance the probe card  19  from being damaged by an overload.  
         [0058]    The detecting accuracy of the sinking amount δ is dependent on the resolution of the driving section of the motor  14  and on the resolution of the linear encoder  24 . For example, if the resolution of each of the driving section of the motor  14  and the linear encoder  24  is assumed to be 0.1 μm, the resolution of the needle load is 0.25 kg·f. Recently, the resolution of the linear encoder  24  has been improved to about 8×10 −5  μm. Therefore, the resolution of the needle load can be increased to 0.2 g·f by using a servo motor provided with a rotary encoder of a high resolution as the motor  14 .  
         [0059]    The method of measuring the needle load in this embodiment will now be described. In the first step, the wafer W is placed on the wafer chuck  11  and the position of the wafer W is aligned with the position of the probe card  19  by an aligning mechanism. The wafer W is moved in the X- and Y-directions so as to be brought back to the original position. In this position, the wafer chuck  11  is moved upward by the lift mechanism  31 . After the wafer chuck  11  is moved upward so as to bring the wafer W into contact with the probes  19 A, the wafer chuck  11  is further overdriven. The rotary encoder  20  detects the pseudo overdriving amount L′ caused by the motor driving and, then, the detected value is supplied to the arithmetic calculating section  16 B of the control device  16 . The linear encoder  24  permits the linear sensor  22  to detect the actual overdriving amount L. The detected value is supplied to the arithmetic calculating section  16 B of the control device  16 . The sinking amount δ is obtained in the arithmetic calculating section  16 B on the basis of the detected values L, L′ and the needle load corresponding to the sinking amount δ is obtained on the basis of the relationship shown in FIG. 3. In this fashion, the needle load can be grasped on the real time basis. Also, it is possible to know the distribution state of the needle load in the inspecting step by sequentially recording the needle load in the memory section  16 A. When the probe card  19  is thermally deformed during the inspection so as to cause the needle load to exceed the allowable range, the motor  14  is rotated in the opposite direction via the comparator section  16 C, making it possible to perform the subsequent inspection under the state that the needle load is corrected to fall within the allowable range. It follows that the probe card is prevented in advance from being damaged.  
         [0060]    Another embodiment of the present invention will now be described with reference to FIG. 5. The embodiment of the present invention shown in FIG. 5 differs from the mechanism shown in FIG. 1 in that, in FIG. 5, a plurality of lift mechanisms are employed for vertically moving the wafer chuck  11 . To be more specific, the wafer chuck  11  on which the wafer W is placed is moved in the vertical direction by a plurality of, e.g., two or three, lift mechanisms  31  each comprising the ball screw  12 , the nut member  13  and the motor  14  in the embodiment shown in FIG. 5. The control device for controlling the driving mechanism such as the motor  14  is substantially equal in construction to the control device shown in FIG. 1 and, thus, is not shown in FIG. 5. In this embodiment, the motor  14  is arranged in the lateral position of the ball screw  12 , and the driving force of the motor  14  is transmitted to the ball screw  12  by a driving force transmitting mechanism  35 . The motor  14  may be arranged within the hole  15 A formed in substantially the center of the X-stage  15 , as shown in FIG. 1. The ball screw  12  rotated by the motor  14  is engaged with the nut member  13 . The nut member  13  is moved upward or downward along the ball screw  12  by the rotation of the ball screw  12  in the forward and backward directions. The nut member  13  is mounted to the lower end of a hollow cylinder  17 ′ extending downward from the center in the lower surface of the wafer chuck  11 . The ball screw  12  engaged with the nut member  13  is arranged within the cylinder  17 ′. The wafer chuck  11  is moved upward or downward via the ball screw  12 , the nut member  13  and the cylinder  17 ′ in accordance with rotation of the motor  14  in the forward and backward directions. The Z-shaft  17  extending downward from the wafer chuck  11  is movable in the vertical direction along a Z-shaft guide  33  mounted to the X-stage  15 . A reference numeral  34  shown in FIG. 5 denotes a substrate to which the Z-shaft guide  33  is fixed, and a reference numeral  32  denotes a roller bearing (steel balls) of the Z-shaft guide.  
         [0061]    In the embodiment shown in FIG. 5, a plurality of lift mechanisms are employed. Each lift mechanism includes a mechanism for measuring the pseudo overdriving amount and a mechanism for measuring the actual overdriving amount. It is possible to determine appropriately the number of such lift mechanisms, as required.  
         [0062]    In the apparatus of the embodiment shown in FIG. 5, the sinking amount δ can be detected at a plurality of points of the wafer chuck. It is possible to obtain the needle load applied to each point of the wafer chuck on the basis of the detected sinking amount δ. Further, the needle load applied to the wafer chuck can be obtained by summing up the needle loads at these plural points.  
         [0063]    As described above, according to the embodiment of the present invention shown in FIG. 5, it is possible to obtain in advance the relationship between the needle load applied to the wafer chuck  11  and the sinking amount δ of the wafer chuck  11  caused by the needle load. Since the method in this embodiment comprises the step of bringing the to-be-inspected object into contact with a plurality of probes so as to obtain the sinking amount of the wafer chuck and the step of obtaining the needle load corresponding to the sinking amount, it is possible to monitor the needle load during the probe inspection on the real time basis. As a result, even if the probe card  19  is deformed by, for example, heat, it is possible to prevent the probe card  19  from being damaged.  
         [0064]    It was customary in the past to set the overdriving amount for each probe card for inspecting the wafer W. In this embodiment, however, the relationship between the needle load and the sinking amount δ is obtained in advance and is displayed on the display device  23 , making it possible to confirm the needle load on the display screen. It follows that the probe inspection can be performed while observing the present needle load. For example, when the inspection is performed with a needle load of 25 kg·f, the needle load is supplied through, for example, a key board to permit the control device  16  to perform control such that the difference (sinking amount δ) between the pseudo overdriving amount of the rotary encoder  20  and the actual control amount of the linear encoder becomes 10 μm. Therefore, it is possible to set simply and accurately the needle load of 25 kg·f regardless of the kind of the probe card  19  used. It follows that it is possible to set the needle load of the probe card  19  at an accurate value prior to the inspection of the wafer W. Where the needle load deviates or is likely to deviate from the allowable range, the needle load is corrected to fall within the allowable range so as to carry out a predetermined inspection without fail.  
         [0065]    The present invention is not limited at all to the embodiments described above. The basic idea of the present invention is to utilize the sinking phenomenon of the wafer chuck  11  caused by the needle load. The present invention covers the needle load measuring method, the needle load setting method, and the needle load detecting mechanism based on the basic idea given above. For example, where a stepping motor is used as the motor  14 , the sinking amount δ (needle load) can be obtained by using the driving pulse of the stepping motor and the linear encoder  24  without using the rotary encoder  20 .  
         [0066]    According to the present invention, it is possible to monitor the contact state (needle load) between the to-be-inspected object and the probes on the real time basis so as to prevent the probe card, etc. from being damaged and to prevent a poor contact state between the to-be-inspected object and the probes.  
         [0067]    According to the present invention, provided are a needle load setting method in which an appropriate needle load can be set prior to the inspection of the to-be-inspected object and a needle load detecting mechanism.  
         [0068]    Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.