A scientific study of the problems
of digital engineering for space flight systems,
with a view to their practical solution.
Synopsis
Joseph B. Bernstein, Wei Zhang, and Carl H. Nicholas
Materials and Nuclear Engineering Department
University of Maryland at College Park
2100 Marie Mount Hall, College Park, MD 20742-7531
phone:(301)405-0357, fax:(301)314-9601
joey@eng.umd.edu
Abstract
Solid metallic connections have been successfully formed between two standard levels of metalization using a focussed IR laser. This new process of laser formed connections has been used to link continuous chains and with resistances of less than 0.8ohm per connection. A commercial laser repair system used extensively by the memory industry was employed to perform approximately 50,000 individual links without failure. The electromigration resistance is comparable to standard metal interconnect. This technology has the potential to replace laser fuse cutting techniques to implement repair schemes and it can be used to program wiring in MCM-D or wafer scale integration applications implemented on silicon substrates. Furthermore, because it is an additive process, it lends itself to redundancy for higher yield and reliability.
Table of Contents
List of Figures
Figure 1. FIB cross-section of a 2um wide metal line after 0.33uJ of laser energy showing ejected metal at the surface and a void remaining in the line.
Figure 2. Schematic of the laser spot and cross-sectional image of a segment from two link points in the the vertical link chain, showing the laser spot and cross-section plane.
Figure 3A. Yield and B. Average resistance per link for chains of 1320 links as a function of laser energy and spot size.
Figure 4. FIB cross-section of a vertical link between the 4um wide metal 1 and two sides of the hole in metal 2.
Figure 5. Result of an open circuit from an accelerated electromigration test.
Figure 6. Schematic of another vertical link implementation with increased metal 1 under the metal 2 link annulus.
Figure 7. Schematic of a 20um metal pitch interconnect layout with two vertical links per crossing.
Conclusions
The mechanism of laser induced fracture and metal flow has been used to form metallic connections between standard lines of aluminum based metalization. This process can be used in laser repair or to program cells in a circuit. The connections do not breach the passivation and can be applied to any standard multi-level metalization process. Connections can be made between any two levels of metal processed by standard silicon fabrication techniques. It is also scalable to the minimum dimensions of metal line lithography and the smallest laser spot that can be focussed through an opening of the upper level metal.
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