Low-cost photomask fabrication using laser ablation

• Nitrocellulose lacquer local ablation provides fast, cheap and versatile photomask.
• A 150 °C lacquer postbake improves photomask quality.
• Resolution (10 μm) is limited by lacquer side heating and by laser beam diffraction.
• Coplanar tunable microwave device is carried out from such a photomask.

A specific fabrication process has been developed to produce low-cost photomasks using standard consumer products. This process is based on an indirect route, using a numerically controlled excimer laser (KrF) etching technique. It begins with RF-sputtering deposition of a titanium film (200 nm-thick) on float glass substrate. Then a nitrocellulose lacquer, serving as a self-developing resist, is spin-coated on titanium. It is ablated with partially overlapping laser impacts and undergoes self-combustion under laser beam. Proper heat treatments are applied before and after laser ablation to circumvent smearing and adhesion problems. Finally the titanium layer is wet etched through the openings in the resist by dilute hydrofluoric acid. Both nitrocellulose lacquer characterization and laser parameters are discussed. The resolution achieved here (10 μm, limited by diffraction effects of the laser optics and by side heating of the resist due to the titanium film) is suitable for the fabrication of microwave devices.To confirm the suitability of the proposed process, several photomasks have been fabricated to pattern coplanar tunable stub resonators on ferroelectric KTa0.5Nb0.5O3 thin films. The dielectric permittivity of the KTa0.5Nb0.5O3 material is controlled by DC biasing. A 18% tunability of the stub resonance frequency has been measured in X-band (close to 10 GHz), validating thereby the whole patterning process.