Ion beam trimming of aluminium nitride thin film with silicon dioxide mask

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Sähkötekniikan korkeakoulu | Master's thesis
Photonics and Nanotechnology
Degree programme
Master’s Programme in Electronics and Nanotechnology (TS2013)
The main goal of this work was to develop a process for creating etched-back recesses in AlN thin film at which the location is lithographically defined or applying film thickness correction by ion beam trimming in lithographically defined areas with thickness accuracy in the order of less than +/-1 nm. A second goal was to fabricate vertical wall into AlN masking with hard masks, which is difficult by other methods such as ICP-RIE or wet etching using chemical solutions. This process development focuses on the process steps after AlN deposition sputtered by the magnetron sputtering technique. In this development, PECVD SiO2 and sputtered Mo were used to block AlN being trimmed by ion beam as hard masks. Two issues were observed from the process designed initially. The first issue related to the residue formation was observed at the dry-stripping step, and it was solved by replacing descum with ashing. The second issue causing the poor process repeatability occurred at the wet-stripping step. The technique replacement from Posistrip to acetone effectively solved the poor process repeatability issue with less than 0.2 nm standard deviation. As a result, nearly vertical sidewalls with a slope angle of 85 have been achieved by improved process flow. Also, less than 0.5 nm of standard deviation was consumed by applied process steps excluding the ion beam trimming step in three wafers on average. During the ion beam trimming step, less etching (up to a 3 nm decrease) was observed. To investigate factors that caused the reduction, three hypotheses have been established, and have been verified experimentally. The mask material has an influence on the etching rate. When the trimming was done on AlN thin film without SiO2 mask, the etching rate was increased at the same rate as non-mask trimming. Additionally, the trimming was done with the same parameters using Mo mask, and the etching rate was increased averagely up to 30 - 31 nm. The most persuasive speculation on the etching rate difference depending on the materials is the mask charging effect caused by not fully neutralized ions.
Soldano, Caterina
Thesis advisor
Salo, Tomi
Pensala, Tuomas
Ion beam trimming, microfabrication, nanofabrication, process development, MEMS, aluminium nitride