Fabrication of Low-Stress SiN[sub x]H[sub y] Membranes Deposited by PECVD

Abstract

SiN x films deposited on silicon in a plasma-enhanced chemical vapor deposition ͑PECVD͒ reactor from a mixture gas of SiH 4 and NH 3 are investigated as an alternative method for low-stress membrane fabrication. We verified that the stress in the silicon nitride films decreased as a function of deposition pressure. A low tensile stress in the film of 170 MPa was obtained at a pressure of 750 mTorr and the refractive index of the film was 1.8. After KOH wet etching from the back side of the silicon substrate, a flat square ͑5 ϫ 5 mm͒ membrane with thickness of 500 nm could be successfully fabricated. The chemical composition of the film was analyzed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, with lower deposition pressures producing less Si-H bonding.Silicon nitride ͑SiN x ͒ has a wide range of applications in the microelectronic technology, including as an insulator and passivation layer on Si 1 and as a masking material for KOH etching. 2 Silicon nitride has been used in bulk micromachining, as its mechanical strength allows the fabrication of large membranes. 3 In surface micromachining, silicon nitride has been applied to devices such as interferometers, 4 where both the mechanical and the optical properties of the material are important. Most of these applications need membranes with a flat surface. This can be achieved by controlling the initial mechanical stress in films of silicon nitride, which must be tensile, but with a value sufficiently low to avoid membrane cracking after silicon substrate etching. Silicon nitride is the most common dielectric membrane material because it can be deposited with a low tensile stress by various methods and it has a very low etch rate in silicon wet anisotropic etchants. 5 The low stress state is often achieved by using low-pressure chemical vapor deposition ͑LPCVD͒ ͑Ͼ750°C͒ silicon-rich nitride films. 6,7 Low-temperature fabrication ͑Ͻ300°C͒ has many advantages over traditional, higher temperature ͑Ͼ750°C͒ deposition because a wider range of applications is made possible. Plasma-enhanced chemical vapor deposition ͑PECVD͒ is a common method of low-temperature fabrication of these films ͑Ͻ300°C͒. In this work, we succeeded in fabricating low-stress ͑Ͻ300 MPa͒ silicon nitride PECVD films, enabling realization of freestanding 5 ϫ 5 mm membranes. ExperimentalSiN x films were deposited in a radio frequency ͑rf, 13.56 MHz͒ PECVD parallel-plate reactor ͑Unaxis-Nextral D200͒ from a mixture of SiH 4 ͑20%͒, NH 3 , N 2 , and He. Those parameters which affect the stress of silicon nitride film are rf power, working pressure, flow ratio of SiH 4 /NH 3 , and temperature. In our experiments, the rf power was varied from 50 to 200 W. The ratio of silane ͑20%͒ to ammonia was varied from 0.125 to 0.625. The working pressure was varied from 600 to 800 mTorr, and the deposition temperature was varied from 160 to 280°C.Square membranes were fabricated. First the silicon nitride membrane was deposited on both sides of a cleaned p-type Si͗100͘ wafer. Squar...

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