Novel patterning technology for the LTCC based packaging of an optical encoder

Abstract

Powder blasting technology is proposed in this thesis as a new structuring tool for Low Temperature Co-fired Ceramic (LTCC). The process, consisting of mechanical abrasion through high speed particles, is mostly used on brittle material but was successfully adapted for the patterning of microstructures onto the fragile green tape substrate, through the manufacturing of novel stencil masks. These masks are based on high resolution patterned nickel sheet produced using UV-LIGA process or laser cutting coated with a thin layer of photopolymer which prevents efficiently the metal sheet deformations under particles bombardment. The magnetic properties of the metal allowed magnetic clamping to be used to maintain the mask down onto the substrate. The etching rate of the metal was shown to be low enough at a pressure of 50 psi (344kPa) at a distance nozzle-substrate (N-S) of 20mm and 50mm so that the mask could be re-used several times and ensured good pattern transfer quality from the mask to the substrate. The process was systematically characterised on DuPont 951 P2 (~165μm thick) green tapes. The erosion of the green tape ceramic was then characterised with the micro-patterned electroplated masks. It showed that the powder blasted structures had U shape walls and verticality of the walls closed to 90o can be obtained with increasing the number of passes. The structures have smooth edges and do not have any melting parts. Smoother structures were obtained with distance nozzle-substrate of 50mm favouring lower under etching of about 15-20μm at the expense of a three times increase in process duration. Vias as small as 62μm in entry diameter and 20μm exit diameter were produced along with beams 25μm top width and 54μm bottom width were produced. Following the green tape characterisation, a LTCC package for an optical encoder featuring 16 layers with the glass cavity was manufactured. 45x45mm nickel masks coated with LF55gn flexopolymer were produced featuring stacking pins, fiducials, cavities and circular apertures ranging from 100μm to 400μm diameters for interconnections. Each mask was powder blasted at 50 psi for a flow rate of about 0.1g/s, a distance N-S of 20mm and a speed of 5mm/s. The optical encoder was successfully attached on the package and tested.