Palladium bonding wire for semiconductor device

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.gtoreq.1.gtoreq. (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.

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.gtoreq.1.gtoreq. (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.gtoreq.1.gtoreq. (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 .gtoreq.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.ltoreq.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.gtoreq.1.gtoreq. (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.gtoreq.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.ltoreq.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.gtoreq.1.gtoreq. (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.