Abstract:
A bonding wire for a semiconductor device contains high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling element III having a boiling point lower than a melting point of the base metal and soluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of the base metal and insoluble in Pd, or high purity Pd or Pd alloy as a base metal, and 5-10000 atppm of low boiling point element III and low boiling point element IV, the low boiling point element III having a boiling point lower than a melting point of the base metal and being soluble in Pd, the low boiling point element IV having a boiling point lower than a melting point of the base metal and being insoluble in Pd, the low boiling elements III and IV being present in a concentration so that (content of the low boiling point element III)/25 + (content of the low boiling element IV)/5≧1≧ (content of the low boiling element III)/10000 + (content of the low boiling element IV)/500. The low boiling point elements III and IV may be replaced by low boiling point element I soluble in Au and low boiling point element II soluble in Au, respectively.

Description:
This application is a continuation of application No. 08/171,698, filed Dec. 22, 1993, now abandoned, which is a divisional of application Ser. No. 07/708,204, filed May 31, 1991, now U.S. Pat. No. 5,298,219. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a bonding wire for a semiconductor device adapted to be used for connection between a chip electrode of the semiconductor device and an external lead, and more particularly to such a bonding wire suitable for a ball bonding process. 
     It is known that a chip electrode of a semiconductor device is connected through a bonding wire such as Au wire to an external lead by melting a tip of the Au wire depending from an end of a capillary by means of an electrical torch to form a ball at the tip of the Au wire, pressing the ball against the chip electrode to bond together, then leading the Au wire to the external lead so as to form a loop, bonding the loop to the external lead, and finally cutting the Au wire. 
     In such a conventional bonding wire for a semiconductor device, however, a neck portion formed immediately above the ball is influenced by heat upon formation of the ball to relax a stress accumulated in the wire. Accordingly, a mechanical strength of the neck portion becomes lower than that of the base wire not influenced by heat. As the result, the neck portion is ruptured, or wire falling or wire sagging is generated during bonding work. Furthermore, in temperature cycle life test of products, a stress is generated by heat expansion and contraction due to repeated temperature change, and the stress is concentrated at the neck portion, causing a problem that the rupture of the neck portion is easily generated. 
     Meanwhile, in recent years, high-density mounting of LSI has accompanied a marked tendency to provide a multiplicity of pins. In this circumstance, it is demanded to reduce a diameter of the bonding wire and thereby reduce a bonding pitch. 
     However, as the neck portion of the aforementioned bonding wire is easy to rupture, it is impossible to reduce the diameter of the bonding wire, and the above demand cannot accordingly be met. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the present invention to make the strength of the neck portion equal to or more than that of the base wire. 
     According to a first aspect of the present invention to achieve the above object, there is provided a bonding wire for a semiconductor device, containing high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au, or there is provided a bonding wire for a semiconductor device, containing high purity Au or Au alloy as a base metal and 500 atppm of low boiling point element II having a boiling point lower than a melting point of said base metal and insoluble in Au, or there is provided a bonding wire for a semiconductor device, containing high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of the low boiling point element I)/25+ (content of the low boiling point element II/5≧1≧ (content of the low boiling point element I)/10000 + (content of the low boiling point element II)/500. 
     The aforementioned high purity Au contains 99.99% or more of Au and a residual amount of unavoidable impurities. 
     The aforementioned Au alloy contains the high purity Au and at least one of Pd, Ag (20 at % or less), Pt (10 at % or less), Rh (2 at % or less), Os, Ru (1 at %), Be, Ca, Ge, Y, La, Mg, Zr, Ga, In, Mo, Re, Cu, Fe (1 atppm-8 at %). The use of this Au alloy improves the mechanical strength of the base metal itself at ordinary temperatures and high temperatures to thereby enable high-speed bonding, and also prevents enlargement of crystal grain in the neck portion upon formation of the ball. 
     The low boiling point element I having a boiling point lower than the melting point of the base metal and soluble in Au contains at least one of Zn, Cd, Hg, Te, etc., while the low boiling point element II having the boiling point lower than the melting point of the base metal and insoluble in Au contains at least one of P, S, As, Se, Rb, Cs, etc. 
     According to a second aspect of the present invention achieving the above object, there is provided a bonding wire for a semiconductor device, containing high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd, or there is provided a bonding wire for a semiconductor device, containing high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of the base metal and insoluble in Pd, or there is provided a bonding wire for a semiconductor device, containing high purity Pd or Pd alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd and low boiling point element IV having a boiling point lower than the melting point of the base metal and insoluble in Pd under the condition of (content of the low boiling point element III)/25 + (content of the low boiling point element IV)/5≧1≧ (content of the low boiling point element III)/10000 + (content of the low boiling point element IV)/500. 
     The aforementioned high purity Pd contains 99.9% or more of Pd and a residual amount of unavoidable impurities. 
     The aforementioned Pd alloy contains the high purity Pd and at least one of Au (10 at % or less), Ag (5 at % or less), Pt (20 at % or less), Rh (8 at % or less), Ru, Os, Ir (1 at % or less), Cu, Mo, Fe, Ti (1 atppm-5 at %). The use of this Pd alloy improves a mechanical strength of the base metal itself at ordinary temperatures and high temperatures to thereby enable high-speed bonding, and also prevents enlargement of crystal grain in the neck portion upon formation of the ball. 
     The low boiling point element III having the boiling point lower than the melting point of the base metal and soluble in Pd contains at least one of Zn, Cd, Hg, Li, Mg, Te, Yb, etc., while the low boiling point element IV having the boiling point lower than the melting point of the base metal and insoluble in Pd contains at least one of P, S, As, Tl, etc. 
     The content of each component of the bonding wire for the semiconductor device according to the present invention is limited for the following reasons. 
     The low boiling point element having the boiling point lower than the melting point of the base metal is vaporized and scattered from the molten ball upon formation thereof. However, the low boiling point element cannot be vaporized from the neck portion, but it tends to be vaporized in the neck portion to generate a stress. Above all, the low boiling point element I soluble in Au is hard to escape from Au. Accordingly, if the content of the low boiling point element I is less than 25 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element I is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element I not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element I needs to be set in the range of 25-10000 atppm. 
     On the other hand, the low boiling point element II insoluble in Au is easy to escape from Au. Accordingly, if the content of the low boiling point element II is less than 5 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element II is more than 500 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element II not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element II needs to be set in the range of 5-500 atppm. 
     Under the condition of (content of the low boiling point element I)/25 + (content of the low boiling point element II)/5 ≧1, if a lower limit of the total content of the low boiling point element I soluble in Au and the low boiling point element II insoluble in Au is less than 5 atppm, a satisfactory characteristic cannot be obtained. Therefore, the lower limit of the total content of both the elements I and II under the above condition needs to be set to 5 atppm. 
     Further, under the condition of (content of the low boiling point element I)/10000 + (content of the low boiling point element II)/500≦1, if an upper limit of the total content of the low boiling point element I soluble in Au and the low boiling point element II insoluble in Au is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element I and II not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. Therefore, the upper limit of the total content of both elements I and II needs to be set to 10000 atppm. 
     Consequently, the bonding wire of the present invention contains high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element 1 having a boiling point lower than a melting point of said base metal and soluble in Au, or contains high purity Au or Au alloy as a base metal and 5-500 atppm of low boiling point element II having a boiling point lower than a melting point of the base metal and insoluble in Au, or contains high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of said low boiling point element I)/25 + (content of said low boiling point element II)/5≧1≧ (content of said low boiling point element I)/10000 + (content of said low boiling point element II)/500. With this constitution, the low boiling point element in the ball is vaporized and scattered upon formation of the ball to thereby prevent gas absorption peculiar to metal and obtain a ball satisfactory for bonding. On the other hand, the low boiling point element in the neck portion cannot be vaporized from the surface thereof, but it tends to be vaporized in the neck portion to generate a stress. Accordingly, a rupture strength of the neck portion after bonding is improved as compared with that of the base wire generating no stress. 
     The low boiling point element having a boiling point lower than the melting point of the base metal is vaporized and scattered from the molten ball upon formation thereof. However, the low boiling point element cannot be vaporized from the neck portion, but it tends to be vaporized in the neck portion to generate a stress. Above all, the low boiling point element III soluble in Pd is hard to escape from Pd. Accordingly, if the content of the low boiling point element III is less than 25 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element III is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element III not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element III needs to be set in the range of 25-10000 atppm. 
     On the other hand, the low boiling point element IV insoluble in Pd is easy to escape from Pd. Accordingly, if the content of the low boiling point element IV is less than 6 atppm, a satisfactory characteristic cannot be obtained. In contrast, if the content of the low boiling point element IV is more than 500 atppm, embrittlement of the base wire appears to cause a difficulty of wire drawing. Moreover, an amount of the element IV not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. For the above reasons, the content of the low boiling point element IV needs to be set in the range of 5-500 atppm. 
     Under the condition of (content of said low boiling point element III)/25 + (content of said low boiling point element IV)/5≧1, if a lower limit of the total content of the low boiling point element III soluble in Pd and the low boiling point element IV insoluble in Pd is less than 5 atppm, a satisfactory characteristic cannot be obtained. Therefore, the lower limit of the total content of both the elements III and IV under the above condition needs to be set to 5 atppm. 
     Further, under the condition of (content of the low boiling point element III)/10000 + (content of the low boiling point element IV)/500≦1, if an upper limit of the total content of the low boiling point element III soluble in Pd and the low boiling point element IV insoluble in Pd is more than 10000 atppm, embrittlement of the base wire appears to cause difficulty in wire drawing. Moreover, an amount of the element III and IV not scattered from the ball but residing in the ball upon formation of the ball becomes large to excessively harden the ball, causing chip cracking upon bonding. Therefore, the upper limit of the total content of both the elements III and IV needs to be set to 10000 atppm. 
     Consequently, the bonding wire of the present invention contains high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling point element III having a boiling point lower than a melting point of said base metal and soluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of said base metal and insoluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element III having a boiling point lower than a melting point of said base metal and soluble in Pd and low boiling point element IV having a boiling point lower than the melting point of the base metal and insoluble in Pd under the condition of (content of the low boiling point element III)/25 + (content of the low boiling point element IV)/5≧1≧ (content of the low boiling point element III)/10000 + (content of the low boiling point element IV)/500. With this constitution, the low boiling point element in the ball is vaporized and scattered upon formation of the ball to thereby prevent gas absorption peculiar to metal and obtain the ball satisfactory for bonding. On the other hand, the low boiling point element in the neck portion cannot be vaporized from the surface thereof, but it tends to be vaporized in the neck portion to generate a stress. Accordingly, a rupture strength of the neck portion after bonding is improved as compared with that of the base wire generating no stress. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     There will now be described some preferred embodiments of the present invention. 
     In a first preferred embodiment employing high purity Au or Au alloy as the base metal, various samples were prepared in the following manner. As the base metal were used high purity Au having a purity of 99.999%, Au alloy containing the high purity Au and 20 at % of Pd, Au alloy containing the high purity Au and 10 atppm of Be, Au alloy containing the high purity Au and 0.5 at % of Mo, and Au alloy containing the high purity Au and 8 at % of Cu. Then, at least one of Zn, Cd, Hg, P and Rb was added to the high purity Au and the Au alloys prepared above, and these mixtures were molten to be cast. The castings thus obtained were then subjected to a grooved roll, during which they were annealed. Then, they were subjected to wire drawing, thereby forming base wires each having a diameter of 30 μm. Thereafter, further stress relieving was sufficiently conducted to finally obtain the samples. 
     The contents of the additional elements in the samples are shown in Table 1. In Table 1, Nos. 1-54 of the samples correspond to the preferred embodiment, and Nos. 55 and 56 of the samples correspond to comparisons departing from the composition range according to the present invention. 
     
                       TABLE 1______________________________________SAMPLE      ADDITIONAL ELEMENT (at ppm)No.         Zn       Cd      Hg     P    Rb______________________________________PREFERREDEMBODIMENT 1          20 2          25 3          50 4          500 5          5000 6          10000 7          11000 8                   20 9                   2510                   5011                   50012                   500013                   1000014                   1100015                           2016                           2517                           5018                           50019                           500020                           1000021                           1100022                                  323                                  524                                  5025                                  50026                                  60027                                       328                                       529                                       5030                                       50031                                       60032          10       1033          100      10034          1000     100035          5000     500036          6000     600037          10       10      1038          100      100     10039          1000     1000    100040          5000     5000    500041                                  2    342                                  5    543                                  50   5044                                  100  10045                                  250  25046                                  250  30047          10                      248          10       10             249                   10             2    250          10                      250  25051          1000     1000           200  20052          1000     1000    10     200  20053          2000     2000    2000   50   5054          3000     3000    3000   50   50COMPARISON55          Au (99.99)56          Cu (99.9999)______________________________________ 
    
     By using the above samples, a pull test was carried out given times (n=40), and measurements were carried out for a pull strength, number of times of rupture of the base wire rather than the neck portion, condition of workability, and existence of chip crack. The result of the measurements for the high purity Au and each Au alloy is shown in Table 2. 
     
                                           TABLE 2__________________________________________________________________________   Au                        Au + Pd (20 at %)   PULL  C MODE              PULL  C MODESAMPLE  STRENGTH         RUPTURE               WORKA-                     CHIP    STRENGTH                                   RUPTURE                                          WORKA-                                                CHIPNo.     (g)   (n = 40)               BILITY                     CRACK   (g)   (n = 40)                                          BILITY                                                CRACK__________________________________________________________________________PREFERREDEMBODIMENT 1      12.0   4    GOOD  UNOBSERVED                             21.2   6     GOOD  UNOBSERVED 2      12.1  22    &#34;     &#34;       21.3  21     &#34;     &#34; 3      12.2  21    &#34;     &#34;       21.3  25     &#34;     &#34; 4      12.8  30    &#34;     &#34;       21.5  28     &#34;     &#34; 5      13.0  28    &#34;     &#34;       21.6  30     &#34;     &#34; 6      13.4  30    &#34;     &#34;       21.6  33     &#34;     &#34; 7      13.6  31    BAD   &#34;       21.7  32     BAD   OBSERVED 8      12.1   7    GOOD  &#34;       21.2   6     GOOD  UNOBSERVED 9      12.2  21    &#34;     &#34;       21.3  22     &#34;     &#34;10      12.4  23    &#34;     &#34;       21.3  22     &#34;     &#34;11      12.8  35    &#34;     &#34;       21.5  35     &#34;     &#34;12      13.3  36    &#34;     &#34;       21.7  38     &#34;     &#34;13      13.5  34    &#34;     &#34;       21.7  30     &#34;     &#34;14      13.6  35    BAD   &#34;       21.8  31     BAD   OBSERVED15      12.0   9    GOOD  &#34;       21.1   8     GOOD  UNOBSERVED16      12.1  26    &#34;     &#34;       21.2  24     &#34;     &#34;17      12.8  33    &#34;     &#34;       21.2  36     &#34;     &#34;18      13.4  40    &#34;     &#34;       21.5  38     &#34;     &#34;19      13.5  38    &#34;     &#34;       21.7  36     &#34;     &#34;20      13.7  35    &#34;     &#34;       21.7  34     &#34;     &#34;21      13.7  36    BAD   UNOBSERVED                             21.8  37     BAD   OBSERVED22      12.0  13    GOOD  &#34;       21.0  12     GOOD  UNOBSERVED23      12.0  22    &#34;     &#34;       21.0  20     &#34;     &#34;24      12.2  33    &#34;     &#34;       21.1  35     &#34;     &#34;25      12.5  37    &#34;     &#34;       21.4  37     &#34;     &#34;26      12.7  38    BAD   &#34;       21.5  34     BAD   &#34;27      12.0  12    GOOD  &#34;       21.1  12     GOOD  &#34;28      12.0  22    &#34;     &#34;       21.1  25     &#34;     &#34;29      12.3  35    &#34;     &#34;       21.2  36     &#34;     &#34;30      12.4  37    &#34;     &#34;       21.8  38     &#34;     &#34;31      12.5  38    BAD   &#34;       21.8  38     BAD   &#34;32      12.1   7    GOOD  &#34;       21.2   7     GOOD  &#34;33      12.3  33    &#34;     &#34;       21.4  35     &#34;     &#34;34      13.1  37    &#34;     &#34;       21.6  37     &#34;     &#34;35      13.5  35    &#34;     &#34;       21.7  33     &#34;     &#34;36      13.6  36    BAD   &#34;       21.8  34     BAD   OBSERVED37      12.5  28    GOOD  &#34;       21.2  30     GOOD  UNOBSERVED38      12.7  36    &#34;     &#34;       21.3  34     &#34;     &#34;39      13.0  30    &#34;     &#34;       21.8  29     &#34;     &#34;40      13.8  35    BAD   &#34;       21.8  30     BAD   OBSERVED41      12.0  23    GOOD  &#34;       21.1  27     GOOD  UNOBSERVED42      12.1  29    &#34;     &#34;       21.1  31     &#34;     &#34;43      12.4  28    &#34;     &#34;       21.4  35     &#34;     &#34;44      12.4  36    &#34;     &#34;       21.4  38     &#34;     &#34;45      12.5  37    &#34;     &#34;       21.8  36     &#34;     &#34;46      12.4  35    BAD   UNOBSERVED                             21.8  34     BAD   UNOBSERVED47      12.0  12    GOOD  &#34;       21.1  12     GOOD  &#34;48      12.1  23    &#34;     &#34;       21.2  21     &#34;     &#34;49      12.1  27    &#34;     &#34;       21.2  25     &#34;     &#34;50      12.5  36    BAD   &#34;       21.6  33     BAD   &#34;51      13.2  36    GOOD  &#34;       21.6  38     GOOD  &#34;52      13.3  35    BAD   &#34;       21.6  34     BAD   &#34;53      13.5  28    GOOD  &#34;       21.8  35     GOOD  &#34;54      13.7  29    BAD   &#34;       21.8  34     BAD   OBSERVEDCOMPARISON    PULL STRENGTH (g)55       1256       21__________________________________________________________________________    Au + Be (10 at ppm)       Au + Mo (0.5 at %)   PULL  C MODE              PULL  C MODESAMPLE  STRENGTH         RUPTURE               WORKA-                     CHIP    STRENGTH                                   RUPTURE                                          WORKA-                                                CHIPNo.     (g)   (n = 40)               BILITY                     CRACK   (g)   (n = 40)                                          BILITY                                                CRACK__________________________________________________________________________PREFERREDEMBODIMENT 1      12.2   6    GOOD  UNOBSERVED                             13.0   7     GOOD  UNOBSERVED 2      12.2  20    &#34;     &#34;       13.1  22     &#34;     &#34; 3      12.3  22    &#34;     &#34;       13.1  24     &#34;     &#34; 4      12.9  33    &#34;     &#34;       13.5  27     &#34;     &#34; 5      13.3  33    &#34;     &#34;       13.6  29     &#34;     &#34; 6      13.6  34    &#34;     &#34;       13.8  29     &#34;     &#34; 7      13.7  35    BAD   &#34;       13.8  32     BAD   OBSERVED 8      12.2   6    GOOD  &#34;       13.1   8     GOOD  UNOBSERVED 9      12.2  21    &#34;     &#34;       13.2  23     &#34;     &#34;10      12.4  25    &#34;     &#34;       13.2  26     &#34;     &#34;11      13.0  35    &#34;     &#34;       13.3  30     &#34;     &#34;12      13.5  37    &#34;     &#34;       13.5  33     &#34;     &#34;13      13.7  36    &#34;     &#34;       13.6  31     &#34;     &#34;14      13.7  35    BAD   &#34;       13.8  34     BAD   OBSERVED15      12.3   8    GOOD  &#34;       13.3  11     GOOD  UNOBSERVED16      12.4  27    &#34;     &#34;       13.3  29     &#34;     &#34;17      12.5  34    &#34;     &#34;       13.4  30     &#34;     &#34;18      13.1  39    &#34;     &#34;       13.5  35     &#34;     &#34;19      13.4  39    &#34;     &#34;       13.7  36     &#34;     &#34;20      13.7  36    &#34;     &#34;       13.7  35     &#34;     &#34;21      13.7  35    BAD   OBSERVED                             13.8  35     BAD   OBSERVED22      12.2  12    GOOD  UNOBSERVED                             13.2  14     GOOD  UNOBSERVED23      12.2  24    &#34;     &#34;       13.2  25     &#34;     &#34;24      12.7  36    &#34;     &#34;       13.5  39     &#34;     &#34;25      13.3  40    &#34;     &#34;       13.7  39     &#34;     &#34;26      13.4  38    BAD   &#34;       13.7  38     BAD   &#34;27      12.3  11    GOOD  &#34;       13.1  13     GOOD  &#34;28      12.3  21    &#34;     &#34;       13.1  23     &#34;     &#34;29      13.0  28    &#34;     &#34;       13.2  37     &#34;     &#34;30      13.5  29    &#34;     &#34;       13.6  36     &#34;     &#34;31      13.5  30    BAD   &#34;       13.6  36     BAD   &#34;32      12.7   6    GOOD  &#34;       13.1   7     GOOD  &#34;33      12.9  37    &#34;     &#34;       13.2  33     &#34;     &#34;34      13.4  38    &#34;     &#34;       13.4  35     &#34;     &#34;35      13.7  35    &#34;     &#34;       13.6  22     &#34;     &#34;36      13.7  35    BAD   &#34;       13.8  36     BAD   OBSERVED37      12.3  25    GOOD  &#34;       13.4  30     GOOD  UNOBSERVED38      12.6  28    &#34;     &#34;       13.5  34     &#34;     &#34;39      13.3  30    &#34;     &#34;       13.7  37     &#34;     &#34;40      13.8  32    BAD   &#34;       13.7  35     BAD   OBSERVED41      12.2  25    GOOD  &#34;       13.1  27     GOOD  UNOBSERVED42      12.2  27    &#34;     &#34;       13.3  28     &#34;     &#34;43      12.6  34    &#34;     &#34;       13.4  38     &#34;     &#34;44      12.9  39    &#34;     &#34;       13.2  36     &#34;     &#34;45      13.5  33    &#34;     &#34;       13.6  35     &#34;     &#34;46      13.5  33    BAD   UNOBSERVED                             13.6  37     BAD   UNOBSERVED47      12.2  12    GOOD  &#34;       13.1  14     GOOD  &#34;48      12.3  25    &#34;     &#34;       13.0  27     &#34;     &#34;49      12.3  29    &#34;     &#34;       13.1  31     &#34;     &#34;50      13.5  35    BAD   &#34;       13.4  37     BAD   &#34;51      13.5  39    GOOD  &#34;       13.6  36     GOOD  &#34;52      13.6  36    BAD   &#34;       13.5  33     BAD   &#34;53      13.6  34    GOOD  &#34;       13.7  38     GOOD  &#34;54      13.7  37    BAD   &#34;       13.8  35     BAD   OBSERVED    PULL STRENGTHCOMPARISON    (g)        C MODE RUPTURE (n = 40)55       12         256       21         2__________________________________________________________________________                              Au + Cu (8 at %)                              PULL   C MODE                     SAMPLE   STRENGTH                                     RUPTURE                                           WORKA-                                                 CHIP                     No.      (g)    (n = 40)                                           BILITY                                                 CRACK__________________________________________________________________________                     PREFERRED                     EMBODIMENT                      1       21.9    5    GOOD  UNOBSERVED                      2       22.0   21    &#34;     &#34;                      3       22.0   26    &#34;     &#34;                      4       22.2   32    &#34;     &#34;                      5       22.3   36    &#34;     &#34;                      6       22.4   35    &#34;     &#34;                      7       22.5   33    BAD   OBSERVED                      8       22.0    4    GOOD  UNOBSERVED                      9       22.1   22    &#34;     &#34;                     10       22.1   23    &#34;     &#34;                     11       22.2   34    &#34;     &#34;                     12       22.4   36    &#34;     &#34;                     13       22.4   36    &#34;     &#34;                     14       22.5   32    BAD   OBSERVED                     15       22.0    9    GOOD  UNOBSERVED                     16       22.0   27    &#34;     &#34;                     17       22.1   31    &#34;     &#34;                     18       22.3   38    &#34;     &#34;                     19       22.4   36    &#34;     &#34;                     20       22.5   37    &#34;     &#34;                     21       22.5   33    BAD   OBSERVED                     22       22.1   10    GOOD  UNOBSERVED                     23       22.1   33    &#34;     &#34;                     24       22.2   40    &#34;     &#34;                     25       22.5   39    &#34;     &#34;                     26       22.5   39    BAD   &#34;                     27       22.1    9    GOOD  &#34;                     28       22.1   30    &#34;     &#34;                     29       22.2   38    &#34;     &#34;                     30       22.4   35    &#34;     &#34;                     31       22.4   35    BAD   &#34;                     32       22.1    5    GOOD  &#34;                     33       22.1   34    &#34;     &#34;                     34       22.3   35    &#34;     &#34;                     35       22.4   36    &#34;     &#34;                     36       22.5   34    BAD   OBSERVED                     37       22.2   23    GOOD  UNOBSERVED                     38       22.3   30    &#34;     &#34;                     39       22.3   33    &#34;     &#34;                     40       22.3   31    BAD   OBSERVED                     41       22.1   30    GOOD  UNOBSERVED                     42       22.2   34    &#34;     &#34;                     43       22.3   38    &#34;     &#34;                     44       22.1   35    &#34;     &#34;                     45       22.4   33    &#34;     &#34;                     46       22.4   35    BAD   UNOBSERVED                     47       22.1   10    GOOD  &#34;                     48       22.0   33    &#34;     &#34;                     49       22.1   32    &#34;     &#34;                     50       22.3   32    BAD   &#34;                     51       22.4   35    GOOD  &#34;                     52       22.3   36    BAD   &#34;                     53       22.4   38    GOOD  &#34;                     54       22.5   38    BAD   OBSERVED                     COMPARISON                              C MODE RUPTURE (n = 40)                     55       2                     56       2__________________________________________________________________________ 
    
     As apparent from Table 2, it is appreciated that the number of times of C mode rupture in the pull test by using the samples according to the present invention is larger than that by using the samples in the comparisons departing from the composition range according to the present invention, and that the neck portion is stronger than the other portion of the base wire. Accordingly, it is understood that the above-mentioned composition range according to the present invention is optimum. 
     In a second preferred embodiment employing high purity Pd or Pd alloy as the base metal, various samples were prepared in the following manner. As the base metal were used high purity Pd having a purity of 99.95%, Pd alloy containing the high purity Pd and 10 at % Au, Pd alloy containing the high purity Pd and 5 atppm of Ag, Pd alloy containing the high purity Pd and 0.5 at % of Mo, and Pd alloy containing the high purity Pd and 6 at % of Cu. Then, at least one of Zn, Cd, Hg, P and S was added to the high purity Pd and the Pd alloys prepared above, and these mixtures were molten to be cast. The castings thus obtained were then subjected to a grooved roil, during which they were annealed. Then, they were subjected to wire drawing, thereby forming base wires each having a diameter of 30 μm. Thereafter, further stress relieving was sufficiently conducted to finally obtain the samples. 
     The contents of the additional elements in the samples are shown in Table 3. In Table 3, Nos. 1-54 of the samples correspond to the preferred embodiment, and Nos. 55 and 56 of the samples correspond to comparisons departing from the composition range according to the present invention. 
     
                       TABLE 3______________________________________SAMPLE      ADDITIONAL ELEMENT (at ppm)No.         Zn       Cd      Hg     P    S______________________________________PREFERREDEMBODIMENT 1          20 2          25 3          50 4          500 5          5000 6          10000 7          11000 8                   20 9                   2510                   5011                   50012                   500013                   1000014                   1100015                           2016                           2517                           5018                           50019                           500020                           1000021                           1100022                                  323                                  524                                  5025                                  50026                                  60027                                       328                                       529                                       5030                                       50031                                       60032          10       1033          100      10034          1000     100035          5000     500036          6000     600037          10       10      1038          100      100     10039          1000     1000    100040          5000     5000    500041                                  2    342                                  5    543                                  50   5044                                  100  10045                                  250  25046                                  250  30047          10                      248          10       10             249                   10             2    250          10                      250  25051          1000     1000           200  20052          1000     1000    10     200  20053          2000     2000    2000   50   5054          3000     3000    3000   50   50COMPARISON55          Au (99.99)56          Cu (99.9999)______________________________________ 
    
     By using the above samples, a pull test was carried out given times (n=40), and measurements were carried out for a pull strength, number of times of rupture of the base wire rather than the neck portion, condition of workability, and existence of chip crack. The result of the measurements for the high purity Pd and each Pd alloy is shown in Table 4. 
     
                                           TABLE 4__________________________________________________________________________   Pd                        Pd + Au (10 at %)   PULL  C MODE              PULL  C MODESAMPLE  STRENGTH         RUPTURE               WORKA-                     CHIP    STRENGTH                                   RUPTURE                                          WORKA-                                                CHIPNo.     (g)   (n = 40)               BILITY                     CRACK   (g)   (n = 40)                                          BILITY                                                CRACK__________________________________________________________________________PREFERREDEMBODIMENT 1      19.0   4    GOOD  UNOBSERVED                             21.3   7     GOOD  UNOBSERVED 2      19.0  22    &#34;     &#34;       21.3  25     &#34;     &#34; 3      19.2  27    &#34;     &#34;       21.4  29     &#34;     &#34; 4      19.7  35    &#34;     &#34;       21.5  32     &#34;     &#34; 5      19.9  33    &#34;     &#34;       21.6  31     &#34;     &#34; 6      20.1  35    &#34;     &#34;       21.7  34     &#34;     &#34; 7      20.2  30    BAD   &#34;       21.7  35     BAD   OBSERVED 8      19.0   5    GOOD  &#34;       21.3   7     GOOD  UNOBSERVED 9      19.1  21    &#34;     &#34;       21.3  23     &#34;     &#34;10      19.2  27    &#34;     &#34;       21.4  25     &#34;     &#34;11      19.7  36    &#34;     &#34;       21.5  32     &#34;     &#34;12      20.0  35    &#34;     &#34;       21.5  35     &#34;     &#34;13      20.1  32    &#34;     &#34;       21.6  33     &#34;     &#34;14      20.3  31    BAD   &#34;       21.6  35     BAD   OBSERVED15      19.1   4    GOOD  &#34;       21.2   7     GOOD  UNOBSERVED16      19.1  23    &#34;     &#34;       21.3  27     &#34;     &#34;17      19.2  30    &#34;     &#34;       21.4  33     &#34;     &#34;18      19.5  38    &#34;     &#34;       21.6  39     &#34;     &#34;19      19.9  37    &#34;     &#34;       21.6  39     &#34;     &#34;20      20.2  38    &#34;     &#34;       21.7  36     &#34;     &#34;21      20.3  33    BAD   UNOBSERVED                             21.7  35     BAD   OBSERVED22      19.0  12    GOOD  &#34;       21.3  10     GOOD  UNOBSERVED23      19.1  21    &#34;     &#34;       21.3  21     &#34;     &#34;24      19.3  33    &#34;     &#34;       21.4  29     &#34;     &#34;25      19.6  37    &#34;     &#34;       21.5  35     &#34;     &#34;26      19.7  35    BAD   &#34;       21.6  35     BAD   &#34;27      19.0  10    GOOD  &#34;       21.3  10     GOOD  &#34;28      19.2  22    &#34;     &#34;       21.3  20     &#34;     &#34;29      19.3  30    &#34;     &#34;       21.3  27     &#34;     &#34;30      19.6  36    &#34;     &#34;       21.4  32     &#34;     &#34;31      19.7  37    BAD   &#34;       21.5  36     BAD   &#34;32      19.1   6    GOOD  &#34;       21.3   3     GOOD  &#34;33      19.2  22    &#34;     &#34;       21.4  23     &#34;     &#34;34      19.3  28    &#34;     &#34;       21.5  29     &#34;     &#34;35      20.1  35    &#34;     &#34;       21.6  32     &#34;     &#34;36      20.3  36    BAD   &#34;       21.6  34     BAD   OBSERVED37      19.2  27    GOOD  &#34;       21.4  28     GOOD  UNOBSERVED38      19.5  38    &#34;     &#34;       21.5  37     &#34;     &#34;39      19.8  37    &#34;     &#34;       21.5  37     &#34;     &#34;40      20.4  33    BAD   &#34;       21.6  36     BAD   OBSERVED41      19.1  23    GOOD  &#34;       21.3  21     GOOD  UNOBSERVED42      19.3  29    &#34;     &#34;       21.3  32     &#34;     &#34;43      19.4  38    &#34;     &#34;       21.3  37     &#34;     &#34;44      19.5  37    &#34;     &#34;       21.3  35     &#34;     &#34;45      19.6  36    &#34;     &#34;       21.5  34     &#34;     &#34;46      19.7  36    BAD   UNOBSERVED                             21.5  35     BAD   UNOBSERVED47      19.1   6    GOOD  &#34;       21.3   7     GOOD  &#34;48      19.2  20    &#34;     &#34;       21.3  22     &#34;     &#34;49      19.2  24    &#34;     &#34;       21.4  26     &#34;     &#34;50      19.6  37    BAD   &#34;       21.5  37     BAD   &#34;51      19.7  36    GOOD  &#34;       21.6  35     GOOD  &#34;52      19.8  38    BAD   &#34;       21.6  37     BAD   &#34;53      20.1  38    GOOD  &#34;       21.6  38     GOOD  &#34;54      20.2  37    BAD   &#34;       21.7  38     BAD   OBSERVEDCOMPARISON    PULL STRENGTH (g)55       1256       21__________________________________________________________________________   Pd + Ag (5 at %)          Pd + Mo (0.5 at %)   PULL  C MODE              PULL  C MODESAMPLE  STRENGTH         RUPTURE               WORKA-                     CHIP    STRENGTH                                   RUPTURE                                          WORKA-                                                CHIPNo.     (g)   (n = 40)               BILITY                     CRACK   (g)   (n = 40)                                          BILITY                                                CRACK__________________________________________________________________________PREFERREDEMBODIMENT 1      21.5   6    GOOD  UNOBSERVED                             20.0   8     GOOD  UNOBSERVED 2      21.5  22    &#34;     &#34;       20.1  26     &#34;     &#34; 3      21.5  30    &#34;     &#34;       20.1  29     &#34;     &#34; 4      21.6  33    &#34;     &#34;       20.2  32     &#34;     &#34; 5      21.7  34    &#34;     &#34;       20.5  34     &#34;     &#34; 6      21.7  34    &#34;     &#34;       20.6  31     &#34;     &#34; 7      21.8  32    BAD   OBSERVED                             20.7  31     BAD   OBSERVED 8      21.5   6    GOOD  UNOBSERVED                             20.0   8     GOOD  UNOBSERVED 9      21.5  21    &#34;     &#34;       20.0  25     &#34;     &#34;10      21.6  27    &#34;     &#34;       20.1  34     &#34;     &#34;11      21.6  33    &#34;     &#34;       20.2  37     &#34;     &#34;12      21.7  37    &#34;     &#34;       20.6  36     &#34;     &#34;13      21.7  34    &#34;     &#34;       20.6  33     &#34;     &#34;14      21.8  36    BAD   OBSERVED                             20.7  32     BAD   OBSERVED15      21.4   5    GOOD  UNOBSERVED                             20.0  10     GOOD  UNOBSERVED16      21.5  22    &#34;     &#34;       20.0  26     &#34;     &#34;17      21.6  28    &#34;     &#34;       20.2  34     &#34;     &#34;18      21.7  36    &#34;     &#34;       20.3  37     &#34;     &#34;19      21.7  37    &#34;     &#34;       20.5  36     &#34;     &#34;20      21.7  34    &#34;     &#34;       20.6  33     &#34;     &#34;21      21.8  31    BAD   OBSERVED                             20.7  35     BAD   OBSERVED22      21.5   9    GOOD  UNOBSERVED                             20.0  13     GOOD  UNOBSERVED23      21.5  20    &#34;     &#34;       20.1  24     &#34;     &#34;24      21.5  31    &#34;     &#34;       20.2  37     &#34;     &#34;25      21.5  36    &#34;     &#34;       20.4  38     &#34;     &#34;26      21.6  33    BAD   &#34;       20.4  36     BAD   &#34;27      21.5  12    GOOD  &#34;       20.0  10     GOOD  &#34;28      21.5  21    &#34;     &#34;       20.0  22     &#34;     &#34;29      21.5  28    &#34;     &#34;       20.1  30     &#34;     &#34;30      21.6  31    &#34;     &#34;       20.3  36     &#34;     &#34;31      21.6  35    BAD   &#34;       20.4  38     BAD   &#34;32      21.5   3    GOOD  &#34;       20.0   5     GOOD  &#34;33      21.5  24    &#34;     &#34;       20.1  25     &#34;     &#34;34      21.6  30    &#34;     &#34;       20.4  33     &#34;     &#34;35      21.7  33    &#34;     &#34;       20.6  37     &#34;     &#34;36      21.8  35    BAD   OBSERVED                             20.7  35     BAD   OBSERVED37      21.5  30    GOOD  UNOBSERVED                             20.1  31     GOOD  UNOBSERVED38      21.6  35    &#34;     &#34;       20.1  39     &#34;     &#34;39      21.6  36    &#34;     &#34;       20.3  38     &#34;     &#34;40      21.8  33    BAD   OBSERVED                             20.7  38     BAD   OBSERVED41      21.5  22    GOOD  UNOBSERVED                             20.1  25     GOOD  UNOBSERVED42      21.5  28    &#34;     &#34;       20.0  33     &#34;     &#34;43      21.5  36    &#34;     &#34;       20.2  38     &#34;     &#34;44      21.5  38    &#34;     &#34;       20.2  38     &#34;     &#34;45      21.5  37    &#34;     &#34;       20.4  37     &#34;     &#34;46      21.5  36    BAD   UNOBSERVED                             20.3  37     BAD   UNOBSERVED47      21.5   7    GOOD  &#34;       20.1   9     GOOD  &#34;48      21.5  21    &#34;     &#34;       20.1  24     &#34;     &#34;49      21.5  25    &#34;     &#34;       20.1  28     &#34;     &#34;50      21.5  36    BAD   &#34;       20.3  33     BAD   &#34;51      21.6  36    GOOD  &#34;       20.4  37     GOOD  &#34;52      21.6  38    BAD   &#34;       20.4  38     BAD53      21.7  39    GOOD  &#34;       20.5  39     GOOD  &#34;54      21.7  37    BAD   OBSERVED                             20.6  39     BAD   OBSERVED    PULL STRENGTHCOMPARISON    (g)        C MODE RUPTURE (n = 40)55       12         256       21         2__________________________________________________________________________                              Pd + Cu (5 at %)                              PULL   C MODE                     SAMPLE   STRENGTH                                     RUPTURE                                           WORKA-                                                 CHIP                     No.      (g)    (n = 40)                                           BILITY                                                 CRACK__________________________________________________________________________                     PREFERRED                     EMBODIMENT                      1       22.0    7    GOOD  UNOBSERVED                      2       22.0   23    &#34;     &#34;                      3       22.1   27    &#34;     &#34;                      4       22.1   31    &#34;     &#34;                      5       22.2   33    &#34;     &#34;                      6       22.4   35    &#34;     &#34;                      7       22.4   32    BAD   OBSERVED                      8       22.0    4    GOOD  UNOBSERVED                      9       22.1   23    &#34;     &#34;                     10       22.1   29    &#34;     &#34;                     11       22.2   29    &#34;     &#34;                     12       22.3   34    &#34;     &#34;                     13       22.4   33    &#34;     &#34;                     14       22.4   35    BAD   OBSERVED                     15       22.0    6    GOOD  UNOBSERVED                     16       22.1   22    &#34;     &#34;                     17       22.1   30    &#34;     &#34;                     18       22.2   38    &#34;     &#34;                     19       22.2   35    &#34;     &#34;                     20       22.4   34    &#34;     &#34;                     21       22.4   36    BAD   OBSERVED                     22       22.0   11    GOOD  UNOBSERVED                     23       22.0   23    &#34;     &#34;                     24       22.0   32    &#34;     &#34;                     25       22.1   35    &#34;     &#34;                     26       22.1   33    BAD   &#34;                     27       22.0    9    GOOD  &#34;                     28       22.0   21    &#34;     &#34;                     29       22.0   27    &#34;     &#34;                     30       22.1   32    &#34;     &#34;                     31       22.1   35    BAD   &#34;                     32       22.0    4    GOOD  &#34;                     33       22.0   22    &#34;     &#34;                     34       22.1   30    &#34;     &#34;                     35       22.4   35    &#34;     &#34;                     36       22.4   33    BAD   OBSERVED                     37       22.1   32    GOOD  UNOBSERVED                     38       22.1   38    &#34;     &#34;                     39       22.1   37    &#34;     &#34;                     40       22.4   35    BAD   OBSERVED                     41       22.0   24    GOOD  UNOBSERVED                     42       22.0   30    &#34;     &#34;                     43       22.0   35    &#34;     &#34;                     44       22.0   36    &#34;     &#34;                     45       22.1   37    &#34;     &#34;                     46       22.1   37    BAD   UNOBSERVED                     47       22.0    6    GOOD  &#34;                     48       22.0   23    &#34;     &#34;                     49       22.0   27    &#34;     &#34;                     50       22.1   37    BAD   &#34;                     51       22.1   37    GOOD  &#34;                     52       22.1   38    BAD   &#34;                     53       22.2   38    GOOD  &#34;                     54       22.3   39    BAD   OBSERVED                     COMPARISON                              C MODE RUPTURE (n = 40)                     55       2                     56       2__________________________________________________________________________ 
    
     As apparent from Table 4, it is appreciated that the number of times of C mode rupture in the pull test by using the samples according to the present invention is larger than that by using the samples in comparisons departing from the composition range according to the present invention, and that the neck portion is stronger than the other portion of the base wire. Accordingly, it is understood that the above-mentioned composition range according to the present invention is optimum. 
     In summary, the present invention has the following advantage. 
     (1) The bonding wire according to the first aspect of the present invention contains high purity Au or Au alloy as a base metal and 25-10000 atppm of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au, or contains high purity Au or Au alloy as a base metal and 5-500 atppm of low boiling point element II having a boiling point lower than a melting point of said base metal and insoluble in Au, or contains high purity Au or Au alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element I having a boiling point lower than a melting point of the base metal and soluble in Au and low boiling point element II having a boiling point lower than the melting point of the base metal and insoluble in Au under the condition of (content of the low boiling point element I)/25 + (content of the low boiling point element II)/5≧1≧ (content of the low boiling point element I)/10000 + (content of the low boiling point element II)/500. Further, the bonding wire according to the second aspect of the present invention contains high purity Pd or Pd alloy as a base metal and 25-10000 atppm of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-500 atppm of low boiling point element IV having a boiling point lower than a melting point of the base metal and insoluble in Pd, or contains high purity Pd or Pd alloy as a base metal and 5-10000 atppm of a mixture of low boiling point element III having a boiling point lower than a melting point of the base metal and soluble in Pd and low boiling point element IV having a boiling point lower than the melting point of the base metal and insoluble in Pd under the condition of (contempt of the low boiling point element 111)/25 + (content of the low boiling point element IV)/5≧1≧ (content of the low boiling point element III)/10000 + (contempt of the low boiling point element IV)/500. With this constitution, the low boiling point element in the ball is vaporized and scattered upon formation of the ball to thereby prevent gas absorption peculiar to metal and obtain a ball satisfactory for bonding. On the other hand, the low boiling point element in the neck portion cannot be vaporized from the surface thereof, but it tends to be vaporized in the neck portion to generate a stress. Accordingly, a rupture strength of the neck portion after bonding is improved as compared with that of the base wire generating no stress. Accordingly, the strength of the neck portion can be made equal to or more than that of the base wire. 
     In comparison with the prior art wherein the strength of the neck portion becomes smaller than that of the base wire because of influence of heat upon formation of the ball, the bonding wire of the present invention does not generate a rupture of the neck portion, wire falling or wire sagging during the bonding work. Furthermore, in the temperature cycle life test of products, a stress generated by a repeated temperature change is dispensed to be absorbed in the whole of the base wire. Accordingly, the rupture of the neck portion where the rupture of the bonding wire is most frequently generated can be extremely reduced to thereby improve reliability. 
     (2) Since the rupture of the neck portion is hard to occur, the bonding wire can be greatly reduced in diameter to thereby enable reduction in bonding pitch and realize high-density mounting of LSI.