3D Structuring of Silicon using Grayscale Technology

Inhalt:
This paper reports on the use of gray-scale technology for structuring of true 3D shapes into crystalline silicon (c-Si). For this purpose gray-scale lithography and an optimized dry anisotropic etching; namely, reactive ion etching (RIE) process are combined. The primary requirement for an accurate transfer of the 3D photoresist pattern into the c-Si wafer using this technique is to have an equivalent etch rate in the photoresist masking material and Si. This work investigates the influence of RIE operating parameters; namely, the etchant gas flow rate and the RF power on the etch rate of photoresist and Si respectively. A special etchant gas mixture of SF6 and CHF3 was used to attain relative anisotropic etch behavior. A suitable process window was found at SF6 3sccm, CHF3 35sccm, RF power of 100W, 10deg C and pressure of 15mTorr, the etch rate of AZ4562 photoresist and Si was found to be similar (selectivity of 1:1 was achieved). Different height square pillars in the form of a triangle were exposed by grayscale lithography using Heidelberg Instrument DWL66FS in photoresist and it was etched subsequently in RIE using the optimized parameters for 1:1 selectivity. The transferred structures in Si after RIE are compared with the structures in photoresist and they are found to be in good agreement corresponding to the structure dimensions with maximum height difference of 200nm with relative accuracy of pattern transfer of 5%.