Multi-stacked Aligned Wafer Bonding for Wide-bandwidth AR treated silicon optics

The objective of this Phase I proposal is to develop multi-stacked wafer bonding techniques for wide-bandwidth anti-reflection (AR) treated silicon optics at terahertz (THz) frequencies. This process can enable high layer-count structures resulting in thick and large needed for the very wide-bandwidth AR treatment. At the end of the Phase I, the goal is to achieve lt;1% reflectance over a prototype of 4-layers AR structures by stacking with precision alignment and bonding techniques that Cactus Materials, Inc. has developed. Phase II of the project is to develop a complete wide-bandwidth AR treatment for silicon optics applicable for vacuum windows and it can be used in the future for powered optics by integration with a gradient-index lens architecture (GRIN) using wafer bonding, circumventing the challenge of AR-treating a curved surface. Transmission (T) and reflectance (R) on bonded wafers are expected to be 100% and lt;1% respectively. A precision alignment of lt;2 micron between wafers will be employed using automated lithographically defined alignment marks. To meet lt;1% reflectance, the bonding interface needs to be defect free, void free, chemicals and moisture free. In addition, bonding strength needs to be close to silicon bulk strength and withstand any vibration or stress as well as hold up as vacuum windows, so under deflection of 1.5-6 mm (depending on the diameters). For example, vibrational stress of a launch could damage the stacked Si lenses. A detailed testing and modeling will be incorporated to ensure the optics are robust enough for space platform. If successful, this technology development will be stepping stone towards making a high-performance, larger diameter, and thicker AR treated silicon optics.