Laser Graphitization for SiC Ohmic Contacts
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Solution Overview
Problem
Wide band gap semiconductor devices face limitations in forming ohmic contacts with low resistance due to high temperature processing requirements, which restricts design freedom and requires sealed ampoules, and existing methods like metal silicide formation and substrate doping are inefficient and fragile, especially when thinning wafers for reduced electrical resistance.
Innovation Solution
Laser pulses are used to directly interact with silicon carbide semiconductor crystals to form ohmic contacts after topside features are completed, creating a polycrystalline carbon-rich graphitic layer with narrow band gap properties, eliminating the need for metal silicides and wafer carriers, and allowing thinning without high temperature processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high temperature anneal/diffusion process is used to form ohmic contacts, then low barrier resistance is achieved, but temperature sensitive functional surface features are damaged and process order is restricted
Solution Approach 1:
The laser beam is focused to a specific spot on the semiconductor surface, heating only the localized contact area to form the ohmic contact while leaving the rest of the wafer and its temperature sensitive features unaffected. This localized heating resolves the contradiction by achieving the necessary high temperature for ohmic contact formation only where needed.
Solution Approach 2:
The conventional thermal field (furnace heating) is replaced with a focused laser beam (optical field) that can be precisely positioned and controlled. This substitution allows ohmic contact formation without subjecting the entire wafer to high temperatures, thereby protecting temperature sensitive features.
2Reliability
If wafer is thinned to reduce electrical resistance, then substrate conductivity is improved, but wafer becomes excessively fragile and warpage increases
Solution Approach 1:
Ohmic contacts are formed on the back surface of the wafer before the wafer is thinned. This preliminary action ensures that the electrical contacts are already in place and can support the thinned wafer structure, preventing excessive fragility and warpage issues that would occur if thinning were performed first.
3Reliability
If conventional ohmic contact formation is performed before topside features, then low resistance contacts are achieved, but design freedom and process flexibility are greatly restricted
Solution Approach 1:
The replacement of furnace-based thermal processing with laser beam processing enables ohmic contact formation at any stage of wafer fabrication. The laser can be applied locally and on-demand, allowing flexible process sequencing and design freedom that was not possible with conventional high temperature furnace processes.
4Reliability
If sealed ampoules are used for laser doping, then high concentration dopant diffusion is achieved, but process complexity and equipment requirements increase
Solution Approach 1:
The complex sealed ampoule system is replaced by directly introducing the dopant gas into the laser processing chamber. The laser beam creates a melted surface that directly absorbs the dopant gas without requiring sealed containers, thereby achieving the same high concentration doping effect with much simpler equipment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method achieves low resistance ohmic contacts (1x10^-5 Ohm-cm²) without compromising the structural integrity of thinned wafers and enables flexible processing steps, including after wafer thinning, without the need for sealed ampoules or high temperature processes.
Implementation Method 1
Laser pulses are used to directly interact with silicon carbide semiconductor crystals to form ohmic contacts
Implementation Method 2
creating a polycrystalline carbon-rich graphitic layer
Implementation Method 3
field emission tunneling ohmic contacts are sought where the band structure of the silicon carbide very near the surface is altered in such a way that electrons can tunnel through a thin electrical barrier
Implementation Method 4
In the traditional case of forming a metal silicide in a solid-state diffusion furnace, silicon atoms diffuse and alloy with the metal atoms
Implementation Method 5
Another method of forming a tunneling ohmic contact to silicon carbide is by highly doping the semiconductor region near the surface
Data Source
Figure 1A~1C
Figure 2A~2C
Figure 2D~2E
AI summary
Systems and methods for semiconductor wafer processing include irradiating a surface of a semiconductor wafer with a laser beam of sufficient energy to alter a band gap of semiconductor material thereby melting a portion of the wafer to generate a graphitic layer area. A metal layer is then depositing on the surface to create ohmic contacts at the area that where melted by the laser.