Abstract:
A wire bonding method and apparatus, in which after the tail of a wire extends out of a capillary, the capillary moves to a measurement position above a tail length measuring member; the capillary descends so that the end of the tail contacts the tail length measuring member; a position of the capillary or a distance by which the capillary is lowered at the time that electrical continuity is established with the tail length measuring member is detected; and the tail length is calculated based upon a height level of the tail length measuring member, the position of the capillary before being lowered for measuring the tail, and the position of the capillary when the wire contacts the tail length measuring member, or upon the height of the capillary above the tail length measuring member before lowering the capillary for measuring the tail, and a distance the capillary is lowered.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wire bonding method and apparatus for connecting a first bonding point and a second bonding point with a wire, and more particularly to a tail setting method and apparatus in such a wire bonding method and apparatus. 
     2. Prior Art 
     Various types of wire bonding methods have been proposed. FIG. 5 shows the most common method. 
     In FIG. 5, first, in step (a), a ball  2   a  is formed by the discharge of an electric torch  3  on a wire  2  that extends out of the lower end of the capillary  1 . Afterward, the electric torch  3  is moved in the direction indicated by arrow. Next, in step (b), the capillary  1  is moved to a point above the first bonding point  4   a  of a semiconductor chip  4 . Then, in step (c), the capillary  1  is lowered, and the ball  2   a  on the tip end of the wire  2  is bonded to the first bonding point  4   a.    
     Afterward, in step (d), the capillary  1  is raised. Then, in step (e), the capillary  1  is moved to a point above the second bonding point  5   a  of a lead  5 . Next, in step (f), the capillary  1  is lowered, and the wire  2  is bonded to the second bonding point  5   a.  Subsequently, after the capillary  1  has been raised to a fixed position, a damper  6  is closed, and the capillary  1  and damper  6  are raised together so that the wire  2  is cut from the root portion of the second bonding point  5   a  in step (g), thus causing a tail  2   b  to be formed at the lower end of the capillary  1 . 
     As a result, one wire connection is completed. 
     Japanese Patent Application Laid-Open (Kokai) Nos. S57-87143 and H1-26531 disclose wire bonding methods of the type described above. In Japanese Patent Application Laid-Open (Kokai) No. S57-87143, the capillary is moved along an upwardly rounded arc trajectory at the uppermost point of the movement of the capillary between the first bonding point and the second bonding point. In Japanese Patent Application Laid-Open (Kokai) No. H1-26531, after the bonding to the first bonding point, the capillary is moved above the first bonding point and toward the second bonding point along an arc trajectory, and then bonding is made to the second bonding point. 
     The above-described operation in which the wire  2  is cut from the root portion of the second bonding point  5   a  is accomplished as a result of the damper  6  that is closed at an intermediate point during the upward movement of the capillary  1  and is raised together with the capillary  1 . Accordingly, if there is a variation in the amount of opening of the damper  6  depending on the respective dampers  6  attached to the wire bonding apparatus, there will be a time discrepancy at which the damper  6  is closed to clamp or hold the wire  2 , even if the timing at which the clamping or holding of the wire  2  is initiated for the purpose of cutting the wire  2  as described above is the same. Consequently, the length of the tail  2   b  will vary. This will be described in more detail with reference to FIG.  6 . 
     In FIG. 6, the diameter of the wire  2  is, for example, 30 μm, and the opening and closing control per 1 μm of movement of the damper  6  is accomplished by output control at, for example, 0.025 ms. In addition, in FIG. 6, amount of opening of a certain clamper  6 A is 80 μm, and the amount of opening of another damper  6 B is 60 μm. The amount of closing in a case where the damper  6 A whose amount of opening is 80 μm holds a wire  2  that has a diameter of 30 μm is (80−30)=50 μm. Accordingly, the clamping time TA is as follows: TA=50 μm×0.025 ms/μm=1.25 ms. The amount of closing in a case where the damper  6 B whose amount of opening is 60 μm holds a wire  2  that has a diameter of 30 μm is (60−30)=30 μm; accordingly, the clamping time TB in this case is as follows: TB=30 μm×0.025 ms/μm=0.75 ms. In other words, the damper  6 B holds the wire  2  earlier than the damper  6 A by a time of Tb=(1.25−0.75)=0.5 ms. 
     In order to obtain a length of the tail  2   b  (tail length) of 360 μm, it is sufficient in the case of the damper  6 A to initiate the clamping operation at a time of 1.25 ms prior to the time that the capillary  1  reaches 360 μm during the rise of the capillary  1  from the step (f) in FIG.  5 . However, if the clamping operation is similarly initiated 1.25 ms in advance for the damper  6 B, the position (timing) at which the wire  2  is held or clamped by the damper  6 B will be advanced by Tb=0.5 ms, so that the tail length is shortened. For example, in a case where the capillary  1  performs a constant-speed operation at a speed of 72 μm/ms (7.2 μm/pulse), the damper  6 B is shortened by a length of 72 (μm/ms)×0.5 ms=36 μm. In other words, assuming that the tail length LA of the damper  6 A is 360 μm, the tail length LB of the damper  6 B is 360−36=324 μm. 
     In cases where the damper  6  is, for instance, replaced, it is necessary to adjust the closing timing of the damper  6  ( 6 A or  6 B) in the process from the step (f) to step (g) of FIG. 5 in accordance with the amount of opening of the damper  6  ( 6 A or  6 B). Conventionally, the following two methods have been used for this. In the first method, the capillary  1  is stopped in a position in which the capillary  1  has been raised by an amount equal to the tail length; then, after the damper  6  is closed, the capillary  1  is again raised. In the second method, the speed at which the damper  6  is raised is slowed beginning at a point immediately prior to a tail position of a specified length, so that the amount of variation in the tail length is reduced. 
     In the above-described methods, a stopping operation or low-speed operation is performed. Accordingly, the bonding cycle is slowed, and the productivity drops. Furthermore, in the above-described description, the speed at which the capillary  1  is raised is assumed to be 72 μm/ms; in actuality, however, this speed is set at 360 to 720 μm/ms in order to increase the productivity, so that it is desirable that the tail length be stable even under such high-speed conditions. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a wire bonding method and apparatus which eliminates differences between individual dampers and obtains a stable tail length without lowering the speed for raising a capillary. 
     The above object is accomplished by unique steps taken in a wire bonding method in which a wire is bonded to a second bonding point, a capillary is raised by a specified amount, then a damper is closed, and said capillary and said damper are raised together so that the wire is cut from the root portion of the second bonding point, thus causing a tail of the wire used for forming a ball to extend from the lower end of the capillary; and in the present invention, 
     the capillary is moved to a measurement position above a tail length measuring member after the tail has been extended, 
     the capillary is then lowered so that the tip end of the tail contacts the tail length measuring member, and the position of the capillary or the distance by which the capillary has been lowered, at the time that electrical continuity is established with the tail length measuring member, is detected, and 
     the tail length is calculated based upon: 
     (a) the height level of the tail length measuring member, the position of the capillary prior to the lowering of said capillary for the purpose of tail measurement, and the position of the capillary at the time that the wire contacts the tail length measuring member, or from 
     (b) the height of the capillary above the tail length measuring member prior to the lowering movement of the capillary for the purpose of tail measurement, and the distance by which the capillary is lowered. 
     In the above, the tail length measuring member is an electric torch which is used to form a ball on the tail, or a semiconductor chip. 
     Furthermore, in the present invention, the tail length is calculated by the abovedescribed method, and the clamp timing of the damper is corrected on the basis of the results of this calculation. 
     Furthermore, the correction of the clamp timing of the damper is performed according to: 
     the speed at which the capillary is raised at the time that the capillary is raised following bonding to the second bonding point, and 
     the difference between the measured tail length and a reference tail length. 
     The above object is accomplished by a unique structure for a wire bonding apparatus in which a wire is bonded to a second bonding point, a capillary is raised by a specified amount, then a damper is closed, and said capillary and said damper are raised together so that the wire is cut from the root portion of the second bonding point, thus causing a tail of the wire used for forming a ball to extend from the lower end of the capillary; and the unique structure of the present invention comprises: 
     a micro-voltage application circuit which applies a micro-voltage to the wire, 
     a contact detection section which outputs a detection signal when the capillary from which the tail has been extended is moved to a measurement position above a tail length measuring member and is then lowered so that the tip end of the tail contacts the tail length measuring member, and 
     a computer which calculates the tail length based upon: 
     (a) the height level of the tail length measuring member, the position of the capillary prior to the lowering of said capillary for the purpose of tail measurement, and the position of the capillary at the time that the wire contacts the tail length measuring member, or 
     (b) the height of the capillary above the tail length measuring member prior to the lowering movement of the capillary for the purpose of tail measurement, and the distance by which the capillary is lowered. 
     In the above structure, the tail length measuring member is an electric torch which is used to form a ball on the tail, or a semiconductor chip. 
     In addition, the computer calculates the tail length and corrects the clamp timing of the damper on the basis of the results of this calculation. 
     Furthermore, the correction of the clamp timing of the damper by the computer is performed according to: 
     the speed at which the capillary is raised at the time that the capillary is raised following bonding to the second bonding point, and 
     the difference between the measured tail length and a reference tail length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram which illustrates one embodiment of the wire bonding apparatus of the present invention; 
     FIG. 2 shows the steps according to one embodiment of the tail length measurement method of the present invention; 
     FIG. 3 shows the steps according to another embodiment of the tail length measurement method of the present invention; 
     FIG. 4 is an explanatory diagram showing the correction of the clamp initiation timing of the clamper; 
     FIG. 5 shows steps of prior art wire bonding method; and 
     FIG. 6 is a diagram which illustrates the change in the tail length according to the amount of opening of the clamper. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One embodiment of the present invention will be described with reference to FIGS. 1 through 3. Furthermore, elements which are the same as in those in FIGS. 5 and 6, or which correspond to those in FIGS. 5 and 6, will be labeled with the same reference numerals. 
     As shown in FIG. 1, a micro-voltage application circuit  11  is connected to a damper  6  via a switch  10 , and this micro-voltage application circuit  11  is connected to the operational control part of a computer  13  via a contact detection section  12 . An electric torch  3  is connected to an electric torch power supply device  15  via a switch  14 , and this electric torch power supply device  15  is connected to the computer  13 . 
     Accordingly, so as to adjust the closing timing of the damper  6 , after the tail  2   b  is formed by steps (a) through (g) of FIG. 5, the capillary  1  is moved in the X and Y directions and the vertical (Z) direction, so that the capillary  1  is positioned in the ball formation position Z 1  above the electric torch  3  as in step (a) of FIG.  2 . The ball formation position Z 1  of the capillary  1  and the electric torch level Z 2  are stored beforehand in the memory of the computer  13 . Next, the switch  10  is switched to the micro-voltage application circuit  11 , and the switch  14  is switched to ground, so that the micro-voltage of the micro-voltage application circuit  11  is applied to the wire  2  via the damper  6 . 
     When a tail length measurement start button (not shown) is pressed in this state, the capillary  1  is lowered one pulse at a time in accordance with a command from the computer  13 . Then, when the tip end of the tail  2   b  contacts the electric torch  3  in step (b) of FIG. 2, the micro-voltage application circuit  11  is connected to ground via the damper  6 , wire  2 , electric torch  3  and switch  14 ; accordingly, the contact detection section  12  detects the fact that the tip end of the tail  2   b  has contacted the electric torch  3 . The lowering of the capillary  1  is stopped by the detection signal of this contact detection section  12 , and the distance Hi by which the capillary  1  has been lowered is stored in the memory of the computer  13 . Then, the operation memory (not shown) of the computer  13  calculates the tail length L according to Equation 1. 
     
       
           L=| ( Z   2 − Z   1 )|− H   1   Equation 1: 
       
     
     FIG. 3 illustrates another embodiment of the present invention. 
     In the embodiment shown in FIG. 2, the electric torch  3  is to measure the tail length. The embodiment in FIG. 3 uses the bonding surface  4   b  of the semiconductor chip  4  or bonding surface  5   b  of the lead  5  shown in FIG. 5 to measure the tail length. In this case, the bonding surface  4   b  or  5   b  is grounded. Furthermore, the bonding level Z 3  of the bonding surface  4   b  or  5   b  is stored beforehand in the memory of the computer  13 . 
     In step (a) of FIG. 3, the capillary  1  is positioned in the ball formation position Z 1  above the electric torch  3  in the same manner as in the embodiment of FIG.  2 . Next, the switch  10  is switched to the micro-voltage application circuit  11 , so that the micro-voltage of the micro-voltage application circuit  11  is applied to the wire  2  via the damper  6 . When the tail length measurement start button (not shown) is pressed, the capillary  1  is lowered one pulse at a time in accordance with a command from the computer  13 . Then, when the tip end of the tail  2   b  contacts the bonding surface  4   b  or  5   b  in step (b) of FIG. 3, the micro-voltage application circuit  11  is connected to ground via the damper  6 , wire  2  and bonding surface  4   b  or  5   b.  Accordingly, the contact detection section  12  detects the fact that the tip end of the tail  2   b  has contacted the bonding surface  4   b  or  5   b.  The lowering of the capillary  1  is stopped by the detection signal of this contact detection section  12 , and the distance H 2  by which the capillary  1  has been lowered is stored in the memory of the computer  13 . Then, the operation memory of the computer  13  calculates the tail length L according to Equation 2: 
     
       
           L=| ( Z   3 − Z   1 )− H   2   Equation 2: 
       
     
     When the length L of the tail  2   b  is measured by the above-described methods, if the measured length is not the reference (specific) tail length, the timing (position) at which the wire  2  is clamped by the damper  6  is automatically corrected by the computer  13 , so that the tail length is controlled to the reference tail length. The method used to accomplish this will be described next. 
     The speed at which the capillary  1  is raised when the capillary  1  is raised in steps (f) and (g) of FIG. 5 is inputted beforehand into the computer  13 , and operational control is performed by the operational control part of the computer  13  with the position obtained by adding the difference of the tail length from the reference tail length to the reference wire clamping position as a target, so that this is made the position at which the wire  2  is actually clamped. 
     As shown in FIG. 4, the tail length LA (μm) in the case of the damper  6 A is the reference tail length; and in the case of the damper  6 A, the difference of the tail length determined by measuring the tail length by the above-described method is designated as ΔL (μm). The speed at which the capillary  1  is raised is designated as S (μm/ms). In the case of the damper  6 B, the clamping by the damper  6 B can be initiated at a point that is delayed (higher) by an amount equal to ΔL (μm). Accordingly, the clamp initiation correction time ΔT is expressed as ΔT=ΔL÷S. More specifically, the computer  13  compares the tail length measured by the above-described method with the reference tail length and corrects the timing at which the clamping by the damper  6 B is initiated. As a result, the reference tail length is obtained. 
     The above-described procedure will be described in terms of concrete numerical values. In this description, the length of the tail  2   b  is measured by the above-described method illustrated in FIGS. 1 through 3; as shown in FIG. 6, the reference tail length of 360 μm is obtained at the design amount of opening of 80 μm in the case of the damper  6 A; and a tail length of 324 μm is obtained in the case of the damper  6 B. Thus, in order to obtain the reference tail length of 360 μm in the case of the damper  6 B as well, the position (timing) at which the wire  2  is clamped by the damper  6  is corrected by the method described below. 
     Here, the amount by which the capillary  1  is raised per pulse as a result of the constant-speed operation of the capillary  1  in which S=72 μm/ms, i.e., the amount P, is 7.2 μm. The tail length in the case of the damper  6 B is shorter than the reference tail length LA of 360 μm by ΔL=36 μm as described above; accordingly, it is sufficient if holding or clamping is initiated at a position that is later (higher) than the reference clamp initiation timing for a tail length LA of 360 μm by an amount equal to ΔL=36 μm. This clamp initiation correction time ΔT can be expressed as ΔT=ΔL÷S=36 μm÷72 μm/ms=0.5 ms. In other words, it is sufficient if the clamping is initiated 0.5 ms later than the reference clamp initiation timing. 
     In the above-described method, the speed at which the capillary  1  is raised and the closing speed at which the clamper  6  is closed are constant, thus the system has linear characteristics. In the actual movement, however, there is also a non-linear component, so that there may be cases in which a complete correction is not accomplished. In such cases, it is desirable to add a correction for the difference ΔL (μm) of the tail length determined by measuring the tail length by the above-described method, and to adjust the calculation formula of the computer  13  on the basis of data determined by experiment. 
     As seen from the above, according to the present invention, the capillary is moved to a measurement position above a tail length measuring member after the tail has been extended; the capillary is then lowered so that the tip end of the tail contacts the tail length measuring member, and the position of the capillary or the distance by which the capillary has been lowered at the time that electrical continuity is established with the tail length measuring member is detected; and then the tail length is calculated: from the height level of the tail length measuring member, the position of the capillary prior to the lowering of said capillary for the purpose of tail measurement, and the position of the capillary at the time that the wire contacts the tail length measuring member; or from the height of the capillary above the tail length measuring member prior to the lowering movement of the capillary for the purpose of tail measurement, and the distance by which the capillary is lowered. Accordingly, differences between individual dampers can be eliminated, and a stable tail length can be obtained without lowering the speed at which the capillary is raised.