Characterization of deep wet etching of glass

This paper presents a characterization of wet etching of glass in HF-based solutions with a focus on etching rate, masking layers and quality of the generated surface. The first important factor that affects the deep wet etching process is the glass composition. The presence of oxides such as CaO, MgO or Al2O3 that give insoluble products after reaction with HF can generate rough surface and modify the etching rate. A second factor that influences especially the etch rate is the annealing process (560°C / 6 hours in N2 environment). For annealed glass samples an increase of the etch rate with 50-60% was achieved. Another important factor is the concentration of the HF solution. For deep wet etching of Pyrex glass in hydrofluoric acid solution, different masking layers such as Cr/Au, PECVD amorphous silicon, LPCVD polysilicon and silicon carbide are analyzed. Detailed studies show that the stress in the masking layer is a critical factor for deep wet etching of glass. A low value of compressive stress is recommended. High value of tensile stress in the masking layer (200-300 MPa) can be an important factor in the generation of the pinholes. Another factor is the surface hydrophilicity. A hydrophobic surface of the masking layer will prevent the etching solution from flowing through the deposition defects (micro/nano channels or cracks) and the generation of pinholes is reduced. The stress gradient in the masking layer can also be an important factor in generation of the notching defects on the edges. Using these considerations a special multilayer masks Cr/Au/Photoresist (AZ7220) and amorphous silicon/silicon carbide/Photoresist were fabricated for deep wet etching of a 500 μm and 1mm-thick respectively Pyrex glass wafers. In both cases the etching was performed through wafer. From our knowledge these are the best results reported in the literature. The quality of the generated surface is another important factor in the fabrication process. We notice that the roughness of generated surface can be significantly improved by adding HCl in HF solution (the optimal ratio between HF (49%) and HCl (37%) was 10/1).

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