Determination of Temperature-Dependent Stress in SiC MOSFETs by Raman Spectroscopy Ryuichi Sugie 1 and Tomoyuki Uchida 1 1 Toray Research Center Inc. 3-3-7, Sonoyama, Otsu, Shiga 520-8567, Japan Phone: +81-77-533-8609 E-mail: Ryuichi_Sugie@trc.toray.co.jp Abstract A procedure to determine the tempera- ture-dependent stress in silicon carbide (SiC) power de- vices was developed using Raman spectroscopy. By ap- plying this method to SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) with discrete pack- ages, we observed an upward shift of the folded trans- verse optic (FTO) mode with E 2 symmetry near the in- terface between the SiC chip and solder. The upward shift, which corresponds to the compressive in-plane stress, increased as temperature decreased. The main cause of the compressive stress was explained by the dif- ference of coefficients of thermal expansion (CTE) be- tween SiC and metals. 1. Introduction Silicon carbide (SiC) is one of the wide-bandgap semi- conductors and has many advantages suitable for power device application [1–3]. SiC power devices can be used at a wide range of operating temperature. Residual stresses at extreme environments may cause unintended effects by producing defects, cracks, or delamination. Low stress die-attach materials have intensively studied to improve reliability of the SiC power devices. However, experimental stress measurements for power devices are not straightfor- ward because power devices consist of a wide variety of materials and have complex structures. Raman spectroscopy is one of the most powerful tech- niques to measure local residual stress in semiconductor devices [4–7]. However, most studies have been conducted at room temperature because temperature change also shifts the Raman lines. Precise measurements of the peak shift and temperature control of the sample are needed to determine the temperature-dependent stress. In this work, we investi- gated fundamental behavior of the Raman lines in 4H-SiC and determined the temperature-dependent stress in SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) based on the basic experiments. 2. Experiment The SiC devices used in this study were commercially available 1200 V SiC MOSFETs with TO-247 discrete package. We mechanically cut the package and polished the cross section. A 457.9 nm line of an argon ion laser was used as an exciting light. We used a Jobin Yvon U1000 double monochromator and a charge coupled device (CCD) detector. A high-purity semi-insulating 4H-SiC wafer was used as a stress-free reference. A Linkam THMS600 tem- perature-controlled heating and cooling stage was used for the temperature-dependent measurements. We performed Raman measurements at 233, 296, and 413 K. The tempera- ture of the sample was controlled within 0.5 K. 3. Results and Discussion Figure 1 shows the Raman spectra of 4H-SiC crystal at various temperatures taken at a backscattering geometry from the (11-20) face. In this geometry, the folded transverse optic (FTO) modes with E 2 , A 1 , and E 1 symmetries are ob- servable [8]. These modes shift toward lower frequency with increasing temperature. The temperature dependence of the peak frequency of the E 2 mode is shown in Fig. 2. Accord- ing to theoretical calculations derived by Balkanski et al., the frequency shift is represented by the following equation [9]: . 3 / , 3 3 / , 3 1 2 / , 2 1 ) ( 0 2 0 0 T n T n D T n C T (1) The values C and D are the anharmonic constants for three- and four-phonon processes, respectively. The function n(T, ) is the Bose-Einstein distribution function written as 1 / exp / 1 , T k T n B . (2) The solid curve in Fig. 2 is a theoretical fit using eq. (1). The solid curve reproduces the experimental results very well. We can derive the shift due to the stress at certain tempera- ture by subtracting the shift caused by temperature variation using eq. (1). Fig. 1 Raman spectra of 4H-SiC crystal at various temperatures. Intensity ( a. u. ) 800 780 760 Raman shift (cm -1 ) 296 K 473 K 573 K E 2 A 1 (TO) E 1 (TO) PS-14-04 Extended Abstracts of the 2017 International Conference on Solid State Devices and Materials, Sendai, 2017, pp1063-1064 - 1063 -