Innovative Soak-Lift Processing at the FBH Enables Delicate Structures

At the Ferdinand-Braun-Institut (FBH) a specific lift-off process has been developed which allows the institute to lift-off extremely delicate gate structures at highest quality. In the future, this process will be used to develop innovative microwave components that are capable to operate at ever increasing frequencies. Main functional elements of each microwave transistor are the gates, which are long, narrow metal stripes at the surface of the semiconductor. They are among the most delicate structures being produced at the FBH. Gate structures of 500 nm down to 250 nm have been sufficient so far. However, latest developments of innovative microwave components suitable for highest frequencies require stripe widths far below 250 nm at a length of 125 µm. A new electron lithography tool has been recently put into operation at FBH, which enables to expose resists at structural sizes smaller than 50 nm. To realize these tiny structures on actual devices, further process steps are needed as for example metallization steps.

A crucial step to finally form the device structure is the lift-off process. During this step, the resist is removed by means of special solvents that also take off the additional metal. In today’s standard industry lift processors, a solvent is sprayed under constant pressure on the surface of the wafer. Soaking and lifting are accomplished in one step. Thus, delicate gate structures could easily be torn off or damaged by smallest metal pieces, leading to a malfunction of the device. The most delicate structures were hence lifted manually in a wet dip process.

In order to increase reproducibility and throughput, the institute has recently taken a new lift processor into operation that allows FBH to lift delicate structures very gently. This soak-lift processor separates soaking from lifting. After soaking the wafer with the resist in a soak chamber, a lift step in a spray and rinse chamber follows. Compared to standard lift processors, the chamber has several nozzles with different spray characteristics, allowing the FBH to tailor lift-off process conditions very precisely to the delicate structures. With this process, the first 250 nm gate structures were successfully lifted at a very high quality (see pictures). The development of this gentle process has now been finished and FBH just recently started to transform and to qualify the innovative lift process into its existing products and developments.

FBH research: 13.10.2010