Laser-formed Vertical Metallic Link and Potential Implementation
                             in Digital Logic Integration

Wei Zhang, Xiaoyan Xie and Joseph B. Bernstein
Reliability Engineering Program
Material and Nuclear Engineering Department
University of Maryland
2100 Marie Mount Hall
College Park, MD 20742

ABSTRACT

In this work, we present a novel metallic link formed vertically between
two metal layers by using IR laser pulses. Its inherit advantages, like
hermetic and radiation hard, find it many potential implementations in
digital logic integration.

From the theoretical point of view, this paper discusses the link
scalability, an integration density sensitive factor. The basic technology
is shown to be scaleable down to one micron metal pitch based on the
commercially available laser system. For small size links, the limited
lower metal volume could arise reliability concerns, so more erect link is
preferred since it is shorter and consumes less lower metal. For this
purpose, thermal/Stress coupled simulations (have been conducted) to
optimize the laser parameters and the link structures.  With fixed lower
metal width and M1-M2 space, laser power density distribution, which is
controlled by both laser spot size and energy, is related to the stress
distribution around the metals. This indicates that the dielectric
cracking trajectory or the metallic link length can be controlled by
adjusting laser parameters. 

An experimental method is also proposed to optimize the structural and
laser parameters by using a PROM-based mixed-signal circuit. The core of
this method is a four-terminal resistance-measuring element that consists
of a vertical link and two n-type MOSFET transistors. The electrical
resistance of any individual link in a 16k-array, which is structured with
different links and could certainly be processed under different laser
parameters, can beobtained by addressing specific word and bit lines. A
test chip has been designed and verified using Cadence (TM). Layout and
simulated logic will be presented. 

Finally, a possible implementation of the technology in LPGA is discussed. 
We will show some results proving the robustness of the connections and
high-density applications where unique circuits need to be produced
quickly in field-ready robust designs.