A parametric study of ICP-RIE etching on a lithium niobate substrate

Z-cut LiNbO₃ single crystal wafers were etched by the inductively coupled plasma reactive ion etching (ICP-RIE) technique by using the boron trichloride (BCl₃)/ Argon (Ar) mixture gases. Effects of the ICP power and RF power ranged from 100 W to 700 W of the ICP-RIE system were studied on the etching rate, surface roughness, and corresponding DC bias under different working pressures ranged from 10 mTorr to 50 mTorr, respectively. Besides, photoresist, Cr(20%)/Ni(80%) alloy and Ni thin films were used as the etching mask, and the selective ratios of the three etching masks were also compared. The surface roughness of the etched LiNbO₃ substrate was increased when the ICP power and RF power were increased. The etching rate of the LiNbO₃ substrate was increased with increasing the ICP power and RF power. It is noted that the etching rate was greater than 100 nm/ min when the working pressure was controlled at 30 mTorr. The selective ratios of the photoresist Cr/Ni and the nickel were calculated to be approximately 0.4, 7, 9, respectively. Under suitable processing parameters of ICP-RIE, the surface roughness less than 40 nm, structure depth greater than 3 μm, and sidewall angle is estimated to be 120° of the LiNbO₃ substrate can be obtained within 28 min, which etching rate is greater than 117 nm/ min.