Backmetal drain terminal with low stress and thermal resistance

Details Backmetal drain terminal with low stress and thermal resistance Backmetal drain terminal with low stress and thermal resistance

1999-06-24

2001-04-03

Loke, Steven (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

Field effect device

Having insulated electrode

C257S761000, C257S762000, C257S763000

Reexamination Certificate

active

06211550

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor device and, more particularly, to a backmetal drain terminal for a device on a thin silicon wafer.
BACKGROUND OF THE INVENTION
The manufacture of a semiconductor power device typically entails metallization of the backside of the silicon wafer to form a drain terminal. To maximize device performance, such backmetal films or film stacks must have low ohmic contact resistance and high thermal conductivity. To reduce the contribution of the substrate to overall operational resistance and to meet advanced packaging requirements, wafers for power devices are typically back thinned, frequently to less than 14 mils, which requires that the backmetal films on these thin wafers also have low tensile or compressive stress. Furthermore, device packaging considerations frequently require that the total combined thickness of the die and backmetal film stack be decreased and that the backmetal be solderable to allow solder mounting of the die on a lead frame.
Solderable backmetal stacks known in the art is typically comprise some combination of Ag, Al, Au, Cr, Ni, or Ti films, which all have varying stresses and thicknesses. The additive stress characteristics of unpatterned metal films have a significant effect on the overall stress induced on a wafer, causing considerable concave deformation, or warpage, of the wafer, and consequent wafer breakage in subsequent process operations. The effect of the backmetal stress on wafer bow increases in severity with increasing diameter and decreasing thickness of the wafer, trends that are ongoing in power device manufacturing.
There is a need for backmetal drain terminals with low resistance, high thermal conductivity, low tensile or compressive stress, and ready solderability to a frame. The present invention meets this need.
SUMMARY OF THE INVENTION
A semiconductor device includes a source region and a gate disposed at the upper surface of a silicon substrate, which includes a drain region for the device. On the lower surface of the substrate is disposed a backmetal drain terminal comprising a stack that includes a first layer of tantalum and an outermost second layer of copper.


REFERENCES:
patent: 4985740 (1991-01-01), Shenai et al.
patent: 61-220344 (1986-09-01), None

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