It is imperative to increase SPL writing speeds if we are to pattern larger areas in reasonable times. We have identified a resist material that is suitable for high speed, nanometer scale lithography using the AFM. The material is siloxene, a Si-O polymer with attached methyl groups, commonly known as Spin-on-Glass (SOG). SOG is a material commonly used in fabricating silicon integrated circuits. It is generally used as a planarizing layer to provide smooth surfaces for photolithography. We have found that SOG can be easily patterned with electrons emanating from a scanning tip.
Patterns in SOG written using the AFM scanning at 1 mm/s.
SOG Exposure Chemistry. The siloxene type of SOG consists of a methyl (CH3) group and silanol groups (Si-OH) bonded to the Si atoms in the Si-O backbone. We speculate that when SOG films are exposed with electrons the methyl group is decomposed, thereby decreasing the organic content. We believe that the generated dangling silicon bonds are filled with the silanol group. The wet etch rate of an SOG film in buffered oxide etch (BOE) decreases exponentially with increasing organic content. Thus we can achieve a large etch selectively between exposed and unexposed regions, making SOG an effective resist material for SPL.
Exposure of SOG with the AFM. In this experiment, we used methylsiloxene SOG (ACCUGLASS-111, Allied Signal Inc.) with 11 percent organic content. The sample substrate was a 2.5 _-cm n-type Si (100) wafer. We mounted the sample in the AFM (Autoprobe CP, Park Scientific Instruments) operating in contact mode in air. We used a silicon cantilever with a force constant of 2.8 N/m and a 10 nm nominal tip radius of curvature. A positive voltage of 70-100 V was applied to the sample. The measured current ranged from 0.5 to 3.0 nA. After the current has settled to the desired value, lines were written over the sample across a 90 x 90 µm area at a predetermined scan speed. The scan speed was varied from 180 µm/s to 3024 µm/s. Larger scan areas and higher scan speeds were limited by the response of the piezo scanner.
Pattern Transfer. After exposure, the sample was etched in 100:1 BOE for 15 seconds, rinsed in deionized water for 2 min, blown dry with nitrogen, and transferred back to the AFM for imaging. No latent image appeared if the sample was scanned with the AFM immediately after exposure and before development. This verifies that there is no change in topography during exposure. The unexposed region etched at a rate of 3.6 Å/sec and the etch selectivity was 20:1 between the exposed and unexposed regions.
High Resolution at High Speeds. We have written patterns with linewidths of 100 nm in SOG with scan speeds as high as 3 mm/sec.
