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High-k insulators pose instability challenges

Posted: 03 Apr 2003 ?? ?Print Version ?Bookmark and Share

Keywords:imec? high-k materials? ic? semiconductor?

The high-k gate dielectric materials being contemplated by the semiconductor industry to reduce leakage current bring formidable stability and reliability challenges that may not be easily resolved, said a senior researcher at IMEC (Interuniversity Microelectronics Center).

Speaking at the International Reliability Physics Symposium (IRPS), Robin Degraeve said the high-k materials exhibit "significant and immediate shifts in threshold voltage" when voltages are applied, due to charge trapping in the high-k insulation layer.

The shifts are so large, measuring as high as 80mV to 100mV under the pulsed conditions typical during integrated circuit operation, that there is no obvious way to design around them, he said in an interview. The shifts are smaller, in the range of 40 mV, under dc conditions.

An IC operating with a 1V supply will typically have a threshold voltage in the range of 400mV. A shift of 80mV to 100mV in the threshold voltage would present enormous challenges to circuit designers. The shifts would make analog circuit design, which is sensitive to very small changes in threshold voltage, particularly challenging, he said.

"This is an intrinsic property of high-k materials, and there are no indications about how to solve these issues. The question for designers is, 'Can you live with it?'" Degraeve said.

The voltage instability issue is "very difficult to get rid of. The charge trapping effect is extremely fast, and it is immediately reversible," he said. When a voltage is no longer applied, the trapped electrons flow out of the insulator, unlike silicon dioxide which tends to build up trapped charges gradually over a period of years.

The IMEC team has tested many combinations of high-k insulators, he said, and all exhibit charge trapping properties. The most promising material, from a process stability point of view, is hafnium oxide, he said.

Degraeve leads a research team that is considered one of the best in high-k research. IMEC has pioneered the use of atomic layer deposition techniques to carefully build the high-k insulation layer.

The reliability symposium officially opens with about 400 reliability experts participating. Degraeve presented his team's research at a pre-conference tutorial on high-k reliability issues.

- David Lammers

EE Times

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