Cyclohexasilane as a Novel Source for SiC Power Electronics

Silicon carbide (SiC) has gained increased attention from both advanced materials developers and the investment community. But as is the case with most emerging technologies, there’s tremendous room for market adoption and utilization in next-generation electronics. Many in the semiconductor space know that SiC wafers are more complex than traditional silicon wafers, but its properties are better suited for industrial applications, specifically power electronics.

The manufacturing pivot to SiC has already begun, most recently reflected by STMicroelectronics, a multinational electronics and semiconductor manufacturer, purchasing $120 million of advanced 150-mm silicon-carbide wafers to address the demand ramp-up for SiC power devices.

As SiC becomes more prevalent in today’s global materials market, a number of barriers still remain when it comes to mass commercialization and the outright replacement of more traditional forms of silicon. This article provides a technical overview of SiC, its upside for the high-growth power electronics industry, and the role of cost-effective and commercially viable silicon precursors like cyclohexasilane (CHS) in creating these next-generation technologies.

The Potential of Silicon Carbide

SiC is an emerging material for a variety of applications, including high-power electronics and high-temperature sensors, due to its outstanding physical and chemical properties. The large bandgap, 2.3 to 3.4 eV, and high breakdown voltage allows SiC to be utilized in commercial scale diodes and transistors operating from 600 to 1200 V. Furthermore, the thermal stability of SiC enables high-temperature sensors, often in excess of 500°C, to be readily used.

High-performance SiC devices have been limited by the inability to grow SiC films on semiconductor wafers with low defect densities as well as the additional challenges with adhesion of the SiC layer on the substrate. As SiC can exist in more than 200 poly types, control of the material’s morphology is critical to its performance.

Among those types, α-SiC (e.g., 4H-SiC, 6H-SiC) and β-SiC (3C-SiC) are the most well-known. In high-power applications, β-SiC is most capable of developing a process that delivers high-quality thin films well-suited for large-scale deposition, significantly lowering costs compared to α-SiC.

In addition, SiC-on-Si wafers can be made using conventional semiconductor fabrication techniques, unlike manufacturing of bulk SiC, which is difficult due to its relative chemical inertness.

Challenges to Wide-Scale SiC Adoption for Si Wafers   

Wide-scale adoption of β-SiC as an alternative material for power electronics presents several challenges:

• A suitable silicon precursor to replace silane (SiH₄) or other organosilicons doesn’t exist commercially.
• A material that could offer a wide range of deposition temperatures, allowing for a variety of alternative deposition substrates, such as glass or even polymer materials, would significantly expand the application scope of β-SiC as a power electronics material.
• The chemistry of traditional β-SiC films is complicated by deformation, grain boundaries, and surface reactions at the SiC-Si interface. This causes current leakages and plastic deformation of the material.

Creating a Cost-Effective Si Precursor with Cyclohexasilane

The need exists for a silicon precursor that’s not only cost-effective, but can be chemically functionalized to develop advanced materials. Cyclohexasilane, Si₆H₁₂, also known as CHS, is one such silicon precursor. It’s a liquid at room temperature, allowing for easier storage and handling, and it offers a moderate boiling point, 80°C at 15 torr.

CHS has long been considered as the preeminent silicon precursor for numerous applications, including semiconductor devices. It can create thin films of β-SiC on a variety of substrates under mild conditions such as conventional CVD, but also LPCVD, APCVD, and laser-assisted CVD.

The growth rate of β-SiC using CHS is an order of magnitude faster compared to other silicon precursors. Its crystalline quality should rival materials grown by molecular-beam epitaxy.

In addition to these advantages, CHS allows for facile p-doping of materials into the β-SiC films, such as aluminum. And due to the methods of deposition and the deposition conditions amenable to a reagent like CHS, continuous growth with the suppression of unintentional secondary deposits may be readily achieved. Moreover, it may be possible to use CHS to achieve solution growth to deliver high-structural-quality SiC with significantly decreased capital and operational costs.

To achieve widespread commercialization—especially within high-growth sectors such as automotive, defense, and aerospace—SiC manufacturers need to continue to make incremental improvements to their process, including the incorporation of more advanced silicon precursors.

CHS can help power electronics manufacturers overcome the limitations in SiC semiconductor wafer development, providing a more suitable semiconductor that simplifies the process from transportation to storage and deposition. With the ability to deposit more silicon in thin films on a variety of substrates, this advanced material technology will unlock tremendous potential in our mobile, consumer, and industrial electronics.

Dr. Ramez Elgammal is Vice President of Technology at The Coretec Group.