Fabrication of Through-glass Vias (TGV) based 3D microstructures in glass substrate by a lithography-free process for MEMS applications

Fabrication of Through-glass Vias (TGV) based 3D microstructures in glass substrate by a lithography-free process is presented. Through-holes having an opening size of ∼ 300–500 µm and spiral-shaped embedded redistribution lines of ∼ 290 µm width and ∼ 50 µm depth were directly created in a 400 μm thick glass substrate by the electrochemical discharge machining. A 20 nm thick conformal seed layer was deposited by electroless nickel deposition, which can be a cost-effective alternative to vacuum-based physical vapor deposition techniques. The surface morphology of the electroless layer was characterized using an atomic force microscope and field emission scanning electron microscopy. Scotch-tape test showed a good adhesion of the seed layer with the glass substrate. Copper electrodeposition was performed to increase the thickness, after which thermal annealing was carried out to release the residual stress in the film. The adhesion of electrodeposited copper significantly improved after thermal annealing at 240 °C for 30 min. Electrodeposition is performed again to increase the thickness to the desired level. The current–voltage characterizations show good connectivity, and the average electrical resistance of a 3-turn square-shaped 3D microstructures is 2.41 Ω. Other spirals and square-shaped designs are also fabricated to prove the flexible capability of this ‘lithography-free’ technique.