A scientific study of the problems
of digital engineering for space flight systems,
with a view to their practical solution.
Laser Energy Limitation for Buried Metal Cuts
Synopsis
Joseph B. Bernstein, Yijia Hua and Wei Zhang
Materials and Nuclear Engineering Department
2100 Marie Mount Hall
College Park, MD 20742-7531
joey@eng.umd.edu
Abstract
Redundancy by laser cutting of polysilicon fuses has been used by the memory industry for many years. As the levels of metalization layers increases, it becomes more difficult and expensive to delete buried polysilicon lines. Ideally, metal fuses will be cut exclusively. However, to achieve reliable metal line cutting, a wide process window has to be found that can cut metal lines buried beneath the passivation layers. The upper energy limit has previously been thought to result from excess laser energy absorbed by the substrate. We show that another failure mode exists at energies far below the threshold to cause substrate damage directly. The same laser pulse which ejects the passivation and removes the metal is also likely to crack the dielectric material below the metal. Molten metal then fills the crack and maintains an electrical short circuit, preventing the line from being disconnected.
Table of Contents
List of Figures
Figure 1. Schematic of the FIB cross-sections taken across the metal lines that have been hit with an elliptical laser spot.
Figure 2. 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 3. FIB cross-section of a 2um wide metal line after 0.875uJ of laser energy representing a threshold of clean cutting.
Figure 4. FIB cross-section of a line after 1.6uJ of laser energy where the metal-filled lower corner cracks are evident
Conclusion
The energy processing window of laser induced cutting is discussed for the first time by consideration of a new failure mode. The high end of a cut processing window based on damage to the substrate is not determined by the laser wavelength, but rather by the fracture dynamics of the metal-dielectric system. We have shown that the upper energy limit is determined by the kinetics of the laser-metal interaction, independent of the wavelength. Vertical linking to metal positioned above the crack trajectory should result in a more robust process, while the energy window for cutting may be less favorable.
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