Strong alkaline etching using SiO2/CrAu mask for Si-based wafer level packaging

The deep etching basin on silicon wafer was fabricated by the etchant of 52% NaOH (saturated) solution and 10 mL sopropyl alcohol (IPA) using SiO2, CrAu and SiO2/CrAu thin films as the mask. The corrosion-resistance properties of the three masks were compared at the different etching time. The three masks maintained the compact surface in one minute. After 3 minutes, the SiO2 mask completely fell off. After 5 minutes, the CrAu mask began to fall off and the small holes appeared. After 10 minutes, the pores in the SiO2 and CrAu masks increased, which seriously affected the structure of the etching basin. The mask of the SiO2/CrAu composite layer did not detach after 15 mins etching which showed the strong corrosion-resistance. In addition, strong alkaliin etching using the SiO2/CrAu and SiO2/CrPt masks heated at 90°C in the above ethant was compared. It showed that the lateral/depth etching ratio was 0.82:1 and 1.17: 1 under two kinds of masks respectively. SiO2/CrAu mask after 15 minutes etching showed smooth and firm surface, while some pits appeared at the surface of SiO2/CrPt due to the larger stress and inferior adhesion. The etching rate of silicon under the different concentration and temperature was studied. The rate was up to 15 μm/ min in 52% saturated NaOH etchant at 100°C. The experimental results provide the effective mask for wafer level packaging of uncooled infrared focal plane detectors.