Chenming Hu: SOI Can Empower New Transistors to 10nm and beyond

The following is a special guest post by Dr. Chenming Hu, TSMC Distinguished Professor at UC Berkeley. He and his team published seminal papers on FinFETs (1999) and UTB-SOI (2000). This post first appeared as part of the Advanced Substrate News special edition on FD-SOI industrialization. 

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The good, old MOSFET is nearing its limits. Scaling issues and dopant-induced variations are leading to high leakage (Ioff) and supply voltage (Vdd),  resulting in excessive  power consumption and design costs. While these challenges have been increasing over time, they’ve finally gotten painful enough that the industry is ready to embrace new transistor structures.

The essence of the problem is that the leakage current does not flow along the Si-oxide interface, but nanometers below the interface  when the gate lengths (Lg) becomes very small. That leakage path is physically far from the gate even if the oxide were infinitely thin. The gate cannot shut off the leakage as if the oxide were nanometers thick. Essentially the MOSFET becomes a resistor. Ioff and variations got worse and worse with Lg reductions.

The solution is new MOSFET structures, in which there is no Si far (more than nanometers) from the gate(s). In other words, the transistor body must be ultra thin. Body doping becomes optional.

Both FinFETs and FD-SOI devices are ultra-thin-body transistors. As such, compared to traditional planar bulk CMOS, they both provide:

• Higher speed and lower leakage
• Lower supply voltage (Vdd) and power consumption
• Further scaling and lower cost
• Better sub-threshold swing and scaling
• No random dopant fluctuation (RDF), less variability
• Better mobility, especially for future sub-threshold design

FinFET

The FinFET body is a thin fin and the thin body is controlled from three sides instead of just the top.

FinFET is easy to scale because leakage is well suppressed if the fin thickness is equal to or less than Lg. Thin fins can be made with the same gate patterning/etching tools.

While our original FinFET work was on SOI wafers, a few years later (2003), Samsung presented a way to manufacture them on bulk substrates. There is an advantage to continued use of  bulk substrates; however, FinFET on bulk requires heavy implant below the fin to suppress leakage and that requires tradeoffs with FinFET performance.

When built on SOI, the FinFET does not suffer from leakage below the fin. Building FinFETs on SOI also confers certain advantages in simplifying manufacturing. The choice will be made by performance and comparisons.

Planar FD-SOI

Planar FD-SOI requires SOI wafers with a very, very thin top layer of silicon.  When we first invented the concept in 2000, the availability of such SOI substrates was the major obstacle. The final silicon layer thickness had to be about a quarter to a third of the gate length.

However, Soitec has surmounted the wafer challenge and with that, commercial production can now become a reality.

The FD-SOI approach can save the fabs and designers significant investment. Existing chip designs and associated IP can be ported with minimum effort, starting today at the 28nm node.

While FinFETs have a larger Ion, FD-SOI has a good back-gate bias option, which make it particularly interesting for low-power applications.

Conclusion

This is a very exciting time for the industry. Although it may seem that the industry is splitting into FinFETs and FD-SOI camps, both approaches use body thickness as the new scaling parameter, and can use undoped body for high performance chips without RDF. Both allow MOSFETs to be scaled beyond traditional MOSFET’s limit. And both can derive substantial benefits from SOI wafers. Real choice is good news because competition will bring the best out of both new transistor technologies.

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