基于硅各向同性腐蚀工艺的三维曲面壳体微谐振器制备技术

Fabrication of microscale axial symmetric three-dimensional curved shell resonators based on silicon isotropic wet etching

轴对称3D曲面壳体谐振器具有自身对称性好、耐冲击能力强、可靠性高的特点,是应用最为广泛的谐振器之一,但因制备工艺涉及薄壳结构的曲面成形过程,属于MEMS3D工艺,很难进行批量化制备。基于硅各向同性湿法技术对多晶硅3D曲面壳体谐振器的批量化制备工艺进行了实验研究。设计了脱模法制备多晶硅3D曲面壳体结构的工艺流程,实验优选体积比为15∶10∶75的醋酸、氢氟酸、硝酸的混合液各向同性腐蚀制备硅半球腔模具,利用自制的水浴设备和水平度可调夹具,对刻蚀液温度和扩散速度进行了有效控制,减弱了反应放热和硅基片放置不水平对硅半球腔圆度与粗糙度的影响。制备了直径为0.8~1.3mm的多晶硅3D曲面壳体结构,测试对称性最好优于0.4%,多晶硅3D曲面壳体结构的表面粗糙度优于1nm。在0.2Pa的压强下,激光多普勒测试得到曲面壳体谐振器的谐振频率为28kHz,Q值为14365,调节驱动电压和偏置电压可实现谐振器四波腹谐振模态的模式匹配,基频差趋于零。

Axial symmetric curved shell resonators are among the most-widely used resonators because of their high symmetry,robustness,and reliability. However,the fabrication of this kind of resonator is based on newly developed three-dimensional (3D) microelectromechanical system technologies,making it difficult to fabricate. Polysilicon shell resonator is suitable for batch fabrication owing to its compatibility with the materials and fabrication technologies used in the IC industry,which is of high value and should be studied thoroughly. A 3D polysilicon shell resonator with high symmetry was produced using silicon cave as its mold,and the fabrication process was designed and tested experimentally. The cave was formed by isotropic etching of silicon,using a mixture of HF/HNO 3/CH 3CHOOH with bulk ratio15∶10∶75 as the etchant. The temperature of the etchant was controlled using a water bath to ensure that the speed of the etching does not change abruptly. The level of the wafer was maintained by a clamp with which the wafer s position could be adjusted freely. The abovementioned measures ensured that the silicon caves had good properties,especially the symmetry and roughness. Then,3D polysilicon shells with diameters ranging from 0.8 to1.3 mm were fabricated,with the smallest roundness less than 0.4%. The roughness of the shell was less than1 nm. The resonant properties of the fabricated shells were investigated using noncontact characterization methods facilitated by a laser Doppler vibrometer. A mechanical quality ( Q ) of14 365 at 28 kHz was obtained in vacuum with a pressure of 0.2 Pa. By adjusting the bias voltage and actuated voltage,the mode-matching situation was achieved,and the relative frequency mismatch between the two degenerate four-node wineglass modes was reduced to zero.