Sunil Srinivasan

Quartz etch process to improve etch depth linearity and uniformity using Mask Etcher IV

Alternating Aperture Phase Shift masks (alt-APSM) are being increasingly used to meet present day lithography requirements by providing increased resolution. The quartz dry etch is a critical step in the manufacture of these photomasks. Etch depth linearity, phase uniformity and minimum etched surface roughness are critical factors. To achieve this, etched quartz structures need to have good selectivity to resist / chrome, vertical sidewalls and good etch depth uniformity over the mask area. Using the Mask Etcher IV at Unaxis USA, a series of experiments were performed to study and identify the trends in quartz etching for photomasks. Etch depth uniformity was measured using an n&k1700RT and etch depth linearity from feature sizes ~0.4 micron to ~1.4 micron was measured using an AFM. Cross sections of the ~0.6 micron structure were obtained using a SEM to check for profile and any evidence of micro trenching. After several set-up experiments, an optimized process to minimize etch depth linearity and improve etch depth uniformity was obtained and is presented here.

Quartz etch process for alternating aperture phase shift masks (alt-APSM)

The quartz dry etch is a critical step in the manufacture of Alternating Aperture Phase Shift masks (alt-APSM). In order to maintain uniform phase shift across the mask, the etch depth uniformity has to be strictly controlled. Both the radial and linear components of non-uniformity have to be minimized. The Mask Etcher IV developed at Unaxis USA reduces both the components of non-uniformity using unique hardware adjustments. Using a fluorocarbon based chemistry, etch depth variations between different feature sizes is also minimized. With good etch depth linearity, phase shift does not vary with feature size. To achieve this, etched quartz structures need to have good selectivity to resist / chrome and vertical sidewalls. Etch depth uniformity was measured using an n&k1700 RT and etch depth linearity was measured using an AFM. Etched quartz structure morphologies are observed using a SEM. After preliminary screening experiments, an optimized hardware suite and process conditions that produce good etch depth uniformity, linearity and quartz profiles with vertical sidewalls and minimum microtrenching is determined.