In chip-scale rubidium atomic clock

the Rb—87 filter is required to be loaded in micro-cavity

so present a new technology for fabrication of the high-quality micro-cavity. In order to gain smooth side surfaces

i. e. Si（ 111） plane

of micro-cavity and to avoid undercutting at the concave corner

ultrasonic etching and concave corner compensation techniques are used. First

silicon（ 100） wafer is wet etched in the pure KOH etching solution with ultrasonic and stirring

and the roughness of Si { 111 } wet-etched surface is measured by laser scanning confocal microscope（ LSCM）. On this basis

a strip mask compensation method for concave corner is adopted for micro-cavity etching. The results show that the etched Si{ 111} surface roughness can achieve 0. 117 μm（ RMS） in mass fraction 30 % KOH etching solution at 80 ℃ with ultrasonic frequency of 59 kHz and power of 160 W

and when the strip length is 1 200 μm

smooth micro-cavity can be obtained.