Thermal compression wafer bonding of tungsten applied to fabrication of small-period tungsten woodpile structures

In this paper, we report on the thermal compression bonding of tungsten at very high temperatures and pressures, and the realization of a 3D tungsten woodpile structure using this method. The structure is fabricated by holographic patterning followed by dry etching of tungsten-on-Si (for the base) and tungsten-on-oxide-on-Si (for subsequent layers). The patterned layers are then wafer-bonded together at a pressure of about 50 MPa and a temperature about 700 °C for several hours. The substrate with the oxide-on-Si is then removed by a combination of lapping (for the bulk of the Si substrate), selective dry etching of Si with SF₆ (for the remaining few microns of Si), and buffered oxide etch for selective removal of the oxide, leaving the two layers of tungsten firmly bonded. The process is repeated for additional layers. Bonding strength for the tungsten layers is between 2 and 9 MPa, depending on bonding conditions. This is a potential pathway for manufacturable fabrication of three-dimensional small-period (~ 500 nm) tungsten woodpile structures, which may have particular applications for high-efficacy incandescent filaments.