Laser drilling of micro-hole arrays in tantalum

X ray collimator optics for space application require an array of high aspect ratio holes of 60:1 with a minimal tantalum (Ta) thickness of >= 2 mm and a very high open area fraction (hole versus wall fraction) of 70% to achieve high collimator efficiency. Each collimator with a drilled area of 110 mm x 70 mm contains several million holes and need a fast drilling process. Laser percussion drilling was performed using an IR pulsed disk laser in a 1 and 2 mm thick Ta plate. A tightly spaced hexagonal closed packed pattern was used to maximize open area fraction with hole-to-hole spacing as small as 80 mu m. However, this resulted in a high concentration of debris and a thick recast layer on the remaining walls between the holes. Different process gases were investigated to minimize debris formation and reduce the recast layer thickness. Ramping of pulse energy during the drill cycle was investigated to minimize the adhesion between the substrate and recast layer. Chemical etching was used to remove the debris and recast from the top surface and the inside of the laser-drilled holes. Hole cross sections showed that a high aspect ratio was achieved with a hole diameter of about empty set50 mu m in 2 mm thick Ta. To achieve the shortest drilling time of 200 ms per hole, the process parameters were optimized and a hybrid nozzle, with both horizontal and vertical gas flow, was developed and implemented