Study On Residual Stress Of Thin Films In The Characterization Method Of Ultrasound Surface Wave

Laser-induced ultrasonic surface wave technology has the advantages of fast measurement,accurate measurement and no damage to the material.Therefore,it has been widely used in the field of integrated circuits for measuring the Young's modulus,adhesion,Poisson's ratio and other mechanical properties of the thin film material.In previous studies,the ideal model was usually used to obtain the theoretical dispersion curve without considering the effect of material residual stress on the measurement results.In the process of IC manufacturing and processing,the residual stress will inevitably be introduced into the material,and the residual stress is an important performance parameter of the thin film material,which cannot be neglected in the field of IC.Therefore,it is of great importance to discuss the influence of film residual stress on the mechanical properties testing by laser-induced ultrasonic surface wave technology and the non-destructive testing of the film residual stress.In this paper,the effective elastic constants?EECs?including residual stress are introduced into the ideal theoretical model,and the theoretical calculation model of semi-infinite with residual stress and the theoretical calculation model of the film/substrate structure with residual stress are established.In combination with two numerical examples,the effect of residual stress on the Young's modulus of silica and silicon dioxide films testing by ultrasonic surface wave is calculated and analyzed in detail.The scope of application of ideal theoretical model in two cases is also be calculated.The results show that the influence of residual stress can be ignored on the Young's modulus of the silica material testing by using the ideal theoretical calculation model when the residual compressive stress is less than 900 MPa for the silica bulk material.For the SiO₂ thin film/Si substrate structure,the range of the ideal theoretical calculation model of the ultrasonic surface wave propagating along the Si[110]crystal orientation in the SiO₂ thin film/Si substrate structure is higher than that in the silicon Si[100].Taking the 1000 nm silicon dioxide film as an example,when the ultrasonic surface wave propagates along the silicon Si[110]crystal orientation,the condition of ideal theoretical calculation model's application is that the residual compressive stress is less than 508 MPa while the ultrasonic surface wave propagates along the Si[100]crystal the condition of ideal theoretical calculation model's application is that the residual compressive stress is less than 373MPa.At the end of the paper,a series of samples of silica film with different film thicknesses were tested by laser-induced ultrasonic surface wave technique.The results of the test were compared with those testing by the curvature method.The experimental results show that the residual stress in the silicon dioxide film sample is residual compressive stress,and as the film thickness increases,the residual compressive stress value gradually decreases.Among them,the residual compressive stress of the silicon dioxide film with the film thickness of 1000 nm is the smallest,while the compressive residual stress of the silicon dioxide film with the thickness of300 nm is the largest,which agrees well with the experimental results measured by the curvature method.The results show that the correctness of measuring residual stress by laser-induced surface acoustic wave technology.