The Relationship Between Infrared Absorption And Compressive Stress Of The Cubic Boron Nitride Films

Cubic boron nitride (cBN) thin films show significant and potential technological applications in cutting tools, electronics and optical devices, etc., because cBN possesses excellent physical and chemical properties, such as ultrahigh hardness only inferior to diamond, inertness against oxidation at high temperature, no reaction with iron group metals, as well as the possibility of n- and p- type dopablity. The high-energy ion bombardment, typically used for cBN films synthesis by vapor deposition methods so far, generates high compressive stress, and makes cBN films ready to delaminate from different substrates in a short time, especially for films thicker than several hundreds nano-meters. At the same time, the formation of sp² bonded phases of amorphous BN (aBN) and turbostratic BN (tBN) are unavoidable before cBN nucleation and growth. They are hindrances for cBN epitaxy and its application. In fact, the development of a simple method to evaluate the sp². bonded phases in a cBN film is still a challenge.In this paper, we use Fourier transform infrared (FT-IR) spectroscopy to analyse content of phases and compressive stress respectively. We found that the compressive stress changes IR peaks' positions and suppresses the intensity of IR peak of sp² -bonded phases. By compensating the intensity reduction according to the compressive stress, the thickness of initial sp²-bonded BN layer can be determined more accurately. Meanwhile, in-situ IR measurement was carried out during film delaminating. It was found that the apparent IR absorption peak area near 1380 cm^-1 and 1073 cm^-1 attributed to the B-N stretching vibration of sp²-bonded BN and the transverse optical phonon of cBN, respectively, increased up to 195% and 175% of the original peak area after film delamination induced compressive stress relaxation. The increase of IR absorption of sp² -bonded BN was found to be non-linear and hysteretic to film delamination, which suggests that the relaxation of the turbostratic BN (tBN) layer from the compressed condition is also hysteretic to film delamination. A model which describes the process of cBN film delamination was established.Theoretical investigation of the relationship between compressive stress and infrared absorption was performed by studying the probability of the infrared-exciting phonon function and density of phonon states. According to this, a qualitative physical explanation of the relationship was represnted.