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New Layer Transfer Technique Creates Many Thin Layers from a Single Gallium Nitride Wafer

For years, investigations were carried out on the impact brought about by a simple technique, for removing thin layers from otherwise rigid, thick semiconductor crystals, on the semiconductor industry.

With this prolonged analysis, integrated circuits produced on thin layers prove to be promising for developments such as lightweight stackability, improved thermal characteristics and a greater degree of flexibility compared to substrates that are conventionally thick.

This image shows a thick bulk gallium nitride (GaN) crystal wafer (2 inches in diameter) with a GaN film in the foreground fabricated by controlled spalling (its film thickness is ~20 microns or 1/5th the thickness of a sheet of paper. CREDIT: Bedell/IBM Research

In a significant advance, a group of Researchers from IBM successfully applied their newly developed new “controlled spalling” layer transfer technique to gallium nitride (GaN) crystals, a common semiconductor material and then developed a pathway for producing several layers from a single substrate.

In the Journal of Applied Physics, from AIP Publishing, the research group reported that controlled spalling can be employed for producing thin layers from thick GaN crystals without leading to any crystalline damage.  The new technique also makes it possible to measure basic physical properties of the material system, such as fracture toughness and strain-induced optical effects, which are otherwise difficult to measure.

Single-crystal GaN wafers are very expensive with just one 2″ wafer costing thousands of dollars, thus having increased layers means attaining more value out of each wafer. Performance benefits for power electronics are provided by thinner layers, since it provides lower electrical resistance and heat is easier to remove.

Our approach to thin film removal is intriguing because it’s based on fracture. First, we first deposit a nickel layer onto the surface of the material we want to remove. This nickel layer is under tensile strength -- think drumhead. Then we simply roll a layer of tape onto the nickel, hold the substrate down so it can’t move, and then peel the tape off. When we do this, the stressed nickel layer creates a crack in the underlying material that goes down into the substrate and then travels parallel to the surface.

Stephen W. Bedell, Research staff member at IBM Research and Co-Author of the paper

Their technique boils down to just peeling off the tape, nickel layer and then a thin layer of the substrate material attached to the nickel.

“A good analogy of how remarkable this process is can be made with a pane of glass,” Bedell said. “We’re breaking the glass in the long direction, so instead of a bunch of broken glass shards, we’re left with two full sheets of glass. We can control how much of the surface is removed by adjusting the thickness of the nickel layer. Because the entire process is done at room temperature, we can even do this on finished circuits and devices, rendering them flexible.”

The reasearc done by the group is remarkably significant for a number of reasons. For starters, it is almost considered to be the simplest method ideal for transferring thin layers from thick substrates. It could also be the only layer transfer method that is materially agnostic.

We’ve already demonstrated the transfer of silicon, germanium, gallium arsenide, gallium nitride/sapphire, and even amorphous materials like glass, and it can be applied at nearly any time in the fabrication flow, from starting materials to partially or fully finished circuits.

Stephen W. Bedell, Research staff member at IBM Research and Co-Author of the paper

Turning a parlor trick into a reliable process, working to ensure that this approach would be a consistent technique for crack-free transfer, led to surprises along the way.

Bedell added, “The basic mechanism of substrate spalling fracture started out as a materials science problem,” he said. “It was known that metallic film deposition would often lead to cracking of the underlying substrate, which is considered a bad thing. But we found that this was a metastable phenomenon, meaning that we could deposit a thick enough layer to crack the substrate, but thin enough so that it didn’t crack on its own -- it just needed a crack to get started.”

The next discovery of the group was how to make the crack initiation consistent and reliable. Chemical etching, mechanical, thermal and laser are a few ways that help to generate a crack, however, despite the existence of these techniques, Bedell points out that the simplest way is to terminate the thickness of the nickel layer in an extremely abrupt manner close to the edge of the substrate.

“This creates a large stress discontinuity at the edge of the nickel film so that once the tape is applied, a small pull on the tape consistently initiates the crack in that region,” he said.

Gallium nitride is considered to be a vital material in everyday lives even though it may not be obvious. It is indeed the fundamental material used to fabricate blue, and presently white, LEDs (for which the 2014 Nobel Prize in physics was granted) and also for high-power, high-voltage electronics. Gallium nitride could also be ideal for use in inherent biocompatibility, which when blended with control spalling may allow implantable sensors or ultrathin bioelectronics.

“Controlled spalling has already been used to create extremely lightweight, high-efficiency GaAs-based solar cells for aerospace applications and flexible state-of-the-art circuits,” Bedell said.

The research group is currently collaborating with research partners in order to fabricate high-voltage GaN devices using this approach.

We’ve also had great interaction with many of the GaN technology leaders through the Department of Energy’s ARPA-E SWITCHES program and hope to use controlled spalling to enable novel devices through future partnerships.

Stephen W. Bedell, Research staff member at IBM Research and Co-Author of the paper

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