Slanting Edges of Etched Oxide

Slanting Edges of Etched Oxide

Material Name: Silicon
Record No.: 55
Primary Chemical Element in Material: Wafer
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Wet etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: A 0.004-mm oxide layer was grown thermally on Si (front and back) by using a wet oxidation method. The hole pattern was transferred to the wafer using the photolithography process described previously.
Wet etching of an oxide resulted in an undesirable oxide profile at the hole edges, as shown in Fig. 1. The AZ5214TM photoresist (Clariant Corp., AZ Electronic Materials) was ultimately used as the mask. The exposed oxide was etched in a buffered oxide etch solution.
Several different photoresists were tried as masks for oxide etching, including AZ5214 (Clariant Corp.), SU8 2025 (MicroChem Corp.), PR1-4000A (Futurrex, Inc.), and SU8 2010 (MicroChem Corp.). AZ5214 and PR1-4000 A, being positive photoresists, had poor RIE selectivity and developed poor sidewall profiles. These two materials were not used as oxide etch masks. SU8 2025 photoresist is widely used for MEMS applications because of its robust nature. This photoresist develops with vertical profiles and has high selectivity to oxide. Better results were obtained using SU8 2025 in RIE, but other problems arose. SU8 2025 is a highly viscous photoresist and often formed bubbles when applied to the wafer. These bubbles had to be removed by using a pipette. We finally used SU8 2010, which has a much lower viscosity than SU8 2025. It was easier to remove the bubbles from this photoresist while maintaining high oxide selectivity during the RIE process.
Reference: Ankita Verma, Joshah Jennings, Ryan D. Johnson, Marc H. Weber, and Kelvin G. Lynn, Fabrication of 3D charged particle trap using through-silicon vias etched by deep reactive ion etching, J. Vac. Sci. Technol. B 31, 032001 (2013); doi: 10.1116/1.4799662.

Figure 1: Slanting edges of etched oxide.