Stress relaxation-induced phase transformation in chromium nitride films

Abstract

Chromium nitride (CrN) films have been widely used as tribological coatings and also in the electronics industry, because of their high hardness (Hv 1090) and low resistivity (640 μ -cm) [1]. Although the oxidation of CrN films has been investigated extensively, the phase transformation of CrN to Cr2N has seldom been studied [2–6]. Lai and Wu [2] stated that the Cr2N phase was generated in CrN films annealed at 1150 ◦C in N2 and vacuum (pO2 = 1.3 × 10−3 Pa) conditions. Heau et al. [3] indicated that Cr2N was present in CrN films annealed above 327 ◦C in vacuum (pO2 = 10−3 Pa). Almer et al. [4] reported that CrN transformed to Cr2N between 450 ◦C and 550 ◦C in Ar, and interpreted the phase transformation as tending toward the equilibrium CrN-Cr2N phase fraction within the nitrogen-deficient CrN phase. Hsieh et al. [5] showed that the Cr2N phase appeared in CrN films over temperatures from 500 ◦C to 800 ◦C. The authors’ previous work [6] proposed that a non-thermodynamic factor governs the phase transformation of CrN in a low temperature range. This work determines the residual stresses of the films at different temperatures and then correlates these values to phase transformation. More experimental evidence is also presented to validate the proposed mechanism of phase transformation at low temperatures. (100) p-type Si wafers (Toshiba Ceramics Co., Ltd.) were used as substrates. CrN films were deposited directly onto the substrates by cathodic arc plasma deposition. Before deposition, the chamber was pumped down to a pressure of 6.7 × 10−3 Pa. The bias and the current of the substrate were maintained at −150 V and 60 A, respectively, under a pN2 of 3 Pa during deposition. The deposition time was 30 min and the corresponding thickness of the films was about 1 μm. After deposition, the films were annealed at temperatures between 450 ◦C and 900 ◦C under a reducing atmosphere of N2/H2 = 9 for 2 h, in a gas-tight tube furnace equipped with an oxygen sensor (15% CaOdoped ZrO2) that was used to monitor and ensure a low oxygen level in the flowing gas. Changes in the crystal structure of the films after annealing, were examined by X-ray diffraction (MacScience MXP3, λCu,Kα = 0.154 nm) operated at 40 kV and 30 mA. The collection interval was 0.02 ◦ (2θ mode) and the scanning rate was 5 ◦/min. The residual stress was measured by the scanning laser curvature