Laser direct exposure of photodefinable polymer masks using shaped-beam optics

In this paper we explore laser direct-write photolithography and ablation processes to finely pattern polymer resists, enabling the creation of densely patterned circuit substrates. For the exposure experiments, a UV laser drilling system (Electro Scientific Industries Model 5330) with highly attenuated output, was used to directly expose a photodefinable resist, followed by chemical developing. To improve the exposure process, special beam shaping optics were utilized which convert the normal Gaussian beam output to a "top hat" flat exposure, ideal for even exposure of the photoresist across the beam aperture. To further improve the exposure process, a square-shaped aperture was used to compensate for the spatial average fluence variation across the scanned, imaged line. For the ablation experiments, the square shaped beam was used at higher fluence to directly ablate the resist without subsequent developing. Laser direct patterning is presented as an alternative to mask-imaging photolithography to produce fine-line traces (50 microns or smaller) for flex circuits, circuit boards, and other interconnect substrates.