Voltage controls etching process

Electronics Times, May 21, 2001

Scientists in Sweden have developed a photoelectrochemical etching process which can produce structures with diameters as small as 15nm.

The technique involves using electron beam lithography to define the circuit pattern and then etching in a dilute hydrofluoric acid solution to reduce the size. Robert Juhasz and Jan Linnros of the Royal Institute of Technology in Stockholm have published their work in Applied Physics Letters.

"We have a technique which is compatible with cleanroom processing," said Linnros. "We transfer a pattern on metal to oxide, take away the metal and then go into the cleanroom to start etching."

The etching process involves exposing a small section of the sample surface to the etching solution. This creates pillars in the structure of 50 to 100nm in diameter. A 5mW 632.8nm HeNe laser creates free electrons and holes, and a voltage is established between the backside of the sample and a Pt electrode. By varying the voltage, Linnros and Juhasz managed to direct the etching to different points on the pillar. A positive potential focuses the etching at the top of the pillars and a zero or negative potential concentrates the process at the bottom.

"Silicon does not react in a hydrofluoric electrolyte," said Linnros. "You have to drive a current and create holes, which we do with the laser beam. The issue is how to control the etching and do it on a very small scale. The key is to control the voltage, the laser beam intensity and the etching solution."

As well as reducing the pillar diameter to 15nm, the etching process also allows for sharper edges, producing vertical pillars.

Linnros expects the technique could be used to produce electron emitting tips and also to create devices for single electron circuits.

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