Semiconductor die for plastic encapsulation having an adhesion promoter

A semiconductor die for plastic encapsulation having an adhesion promoter selectively disposed on an outer surface enabling better adhesion between the semiconductor die and a plastic encapsulation. The improved adhesion allows for less relative motion between the semiconductor die and the plastic encapsulation. The reduction of relative motion significantly decreases the delamination progression throughout the semiconductor device and allows for an increased semiconductor device lifetime.

BACKGROUND OF THE INVENTION 
This invention relates, in general, to semiconductor devices, and more 
particularly to a semiconductor die for plastic encapsulation which 
includes an adhesion promoter. 
A semiconductor device in which this invention will typically be used 
includes a silicon semiconductor die having metal interconnect lines which 
are covered by passivation glass. The die is mounted on the flag of a 
leadframe and the die and flag are then encapsulated in plastic at a high 
temperature. The expansion coefficient of the plastic encapsulant is much 
larger than that of the silicon die and, therefore, the plastic 
encapsulant cannot fully contract as it cools. In large packages, 
deleterious results of this thermal expansion mismatch are especially 
evident during temperature cycling tests where the temperature extremes 
often range between -65 and 150 degrees centigrade. 
When the plastic encapsulant contracts, large magnitudes of stress act on 
the silicon semiconductor die. The stress is highest at the edges and 
corners of the die. The stress causes the plastic encapsulation to crack 
adjacent to the corner of the semiconductor die. This allows for relative 
motion between the plastic encapsulant and the semiconductor die which 
causes the passivation glass of the semiconductor die to crack and break, 
further causing delamination, especially at the high stress corners. It is 
common for this delamination to travel through the metal interconnect 
lines and shear them into separate plates. This results in a semiconductor 
device having a decreased lifetime. 
Prior attempts at solving the delamination problem have included voiding 
the die corners of the semiconductor die of circuitry, interconnects and 
wire bonds. Although this does not stop the delamination, it increases the 
lifetime of the semiconductor device because the operational circuitry is 
further away from the corner regions and is not affected by the initial 
delamination. The present invention improves adhesion between the 
semiconductor die and the plastic encapsulation thereby reducing the 
relative motion between the plastic encapsulant and the semiconductor die 
to prohibit delamination and increase semiconductor device lifetime. 
SUMMARY OF THE INVENTION 
Accordingly, it an object of the present invention to provide a 
semiconductor die for plastic encapsulation which will result in an 
increased semiconductor device lifetime. 
Another object of this invention is to provide a semiconductor die for 
plastic encapsulation having improved adhesion between the semiconductor 
die and the plastic encapsulation. 
It is an additional object of the present invention to provide a 
semiconductor die for plastic encapsulation that reduces the delamination 
of the passivation glass and the metal system of the semiconductor die. 
The foregoing and other objects and advantages are achieved in the present 
invention by the selective application of an adhesion promoter to the 
outer surface of a semiconductor die. The adhesion promoter adheres the 
semiconductor die to the plastic encapsulation and reduces the amount of 
relative motion caused by cracks in the plastic encapsulation. The reduced 
amount of relative motion between the semiconductor die and the plastic 
encapsulation reduces the inherent delamination problem. 
A more complete understanding of the present invention can be attained by 
considering the following detailed description in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a highly enlarged top view of a plastic encapsulated 
semiconductor device of the type in which the present invention will be 
used. The device includes a leadframe 10 having a flag 12. A semiconductor 
die 14 is mounted on flag 12 of leadframe 10. A portion of leadframe 10 
which includes flag 12 is encapsulated in a plastic encapsulation 16 which 
has been partially cut away in this figure. 
FIG. 2 shows a highly enlarged top view of an outer surface 18 of 
semiconductor die 14. Semiconductor die 14 includes an operational 
circuitry area 20 which contains all the operational circuitry of 
semiconductor die 14. The operational circuitry includes a plurality of 
bond pads 22 and interconnect lines 24. Both bond pads 22 and interconnect 
lines 24 are comprised of metal. One skilled in the art will recognize 
that many well known metals may be employed. Outer metallization lines 28 
reside in certain peripheral regions of semiconductor die 14. Outer 
metallization lines 28 isolate operational circuitry area 20 from the 
remainder of semiconductor die 14. Semiconductor die 1 further includes 
circuit area corners 26 which are void of any operational circuitry and 
die corners 27. Circuit area corners 26 provide longer device lifetime by 
being void of operational circuitry because delamination there does not 
effect the operational circuitry of semiconductor die 14. 
Operational circuitry area 20 is covered by a passivation glass 30 which is 
partially removed in this figure. In this embodiment, passivation glass 30 
is silicon dioxide which is doped with phosphorous. Passivation glass 30 
keeps moisture and impurities away from operational circuitry area 20 with 
the exception of bond pads 2 which remain uncovered. This reduces 
corrosion and increases device lifetime. 
A frequent problem in semiconductor devices of this type is that plastic 
encapsulation 16 delaminates from semiconductor die 14. It is common that 
certain interconnect lines 24 residing in the peripheral regions of 
operational circuitry area 20 are destroyed as a result of this 
delamination. The delamination results because the expansion coefficients 
of plastic encapsulation 16 and semiconductor die 14, which is commonly 
made of silicon, differ. Therefore, when plastic encapsulation 16 
contracts during cooling, it cracks adjacent to die corners 27 of 
semiconductor die 14. This crack allows for relative motion between 
plastic encapsulation 16 and semiconductor die 14. In turn, this relative 
motion causes delamination to occur. 
Relative motion between semiconductor die 14 and plastic encapsulation 16 
is decreased by selectively adhering semiconductor die 44 to plastic 
encapsulation 16. Because the relative motion is reduced, the delamination 
does not occur or is substantially impeded. The stress caused by the 
differing expansion coefficients may be relieved by crack propagation 
through the bulk of plastic encapsulation 16 in the region below 
semiconductor die 14. This increases semiconductor device lifetime because 
failure of the device is not related to the breakup of the passivation 
glass-metal system, but rather to cracking of plastic encapsulation 16 to 
an outer package surface. One skilled in the art will understand that 
although cracking of plastic encapsulation 16 is not desired, it is a 
preferred alternative to passivation glass-metal system breakup. 
In one embodiment of the present invention, passivation glass 30 is not 
disposed on circuit area corners 26 of semiconductor die 14. An adhesion 
promoter is then selectively disposed on circuit area corners 26. In a 
preferred embodiment, circuit area corners 26 have an aluminum surface 
which is chemically treated with an adhesion promoter Such an embodiment 
would require only one mask change in the process sequence and a chemical 
treatment of circuit area corners 26. One skilled in the art will 
understand that many chemical treatments may promote adhesion. 
Another embodiment of the present invention includes selectively disposing 
an adhesion promoter on passivation glass 30 in the areas where it covers 
circuit area corners 26. The layer of adhesion promoter may also extend to 
cover outer metallization lines 28 or the entire first surface 18 of 
semiconductor die 14. An adhesion promoter such as polyimide or epoxy 
which adheres well to both plastic encapsulation 16 and passivation glass 
30 is used. The application of the adhesion promoter is done in wafer 
form. This requires an additional masking step in the process sequence as 
well as leaving bond pads 22 uncovered (which is current practice with 
glass passivation). 
A further embodiment of the present invention would include entirely 
coating outer surface 18 of semiconductor die 14 with a hard organic 
adhesion promoter such as polyimide or epoxy. This is done following wire 
bonding. In this embodiment, the coating is only partially cured prior to 
encapsulating the device in plastic in order to promote adhesion between 
plastic encapsulation 16 and semiconductor die 14 via chemical bonding 
between the adhesion promoter and plastic encapsulation 16. It should be 
understood the adhesion promoter will become completely cured during the 
encapsulation process. 
Thus it is apparent that there has been provided, in accordance with the 
invention, an improved semiconductor device and die which meet the objects 
and advantages set forth above. While specific embodiments of the present 
invention have been shown and described, further modifications and 
improvements will occur to those skilled in the art. It is desired that it 
be understood, therefore, that this invention is not limited to the 
particular form shown and it is intended in the appended claims to cover 
all modifications which do not depart from the spirit and scope of this 
invention.