(Invited) Anisotropoic Strain Evaluation in the Finite Si Area by Surface Plasmon Enhanced Raman Spectroscopy

Introduction

The strain in the channel of a transistor can enhance carrier mobility. The strain state is complicated because the channel is affected by the various components. Raman spectroscopy has been performed for the strain evaluation in Si owing to high-strain sensitivity and high-spatial resolution [1]. However, it is difficult to evaluate the complicated strain state using conventional Raman. In this study, Raman spectroscopy using z-polar light excitation was performed in order to evaluate anisotropic biaxial strain states in the finite Si area.

 

Experiment

Two types of strained Si substrates, strained Si on insulator (SSOI) as shown by the TEM image in the inset of Fig. 1, and strained-Si followed by SiGe on insulator (SGOI) as shown in the inset of Fig. 2, were used as the samples. The Si substrate with the line-and-space pattered SiN film was also used for the strain evaluation in the finite Si area. The space widths were varied from 5.0 to 0.6 mm.

The excitation light component perpendicular to the surface, i.e., z-polarization, were achieved by oil immersion Raman with high-NA objective lens or surface enhanced Raman spectroscopy (SERS) with Ag particle on the sample surface. The z-polar light excitation allows the excitation of the transverse optical (TO) phonon as well as longitudinal optical (LO) phonon [2]. The relationship between the Raman wavenumber shifts Dw_LO and Dw_TO and biaxial stresses were expressed by the following equation (1) (The coordinate is defined: x: [110], y: [1(_)10], and z: [001]) [3].

In the experiment, the 532 nm laser and the oil-immersion lens were used. The numerical aperture and refractive index of oil were 1.4 and 1.5, respectively.

 

Results and Discussion

Figure 1 shows the oil-immersion Raman spectra from SSOI in the LO and TO active conditions. Although the peak positions originating in the Si substrate are the same, the peak positions originating in the strained Si layer are different each other. This result shows the excitation of the TO phonon. Figure 2 shows the Raman spectra from the strained Si on SGOI with SERS (w/ Ag) and the oil-imersion (w/o Ag) conditions. It should be noticed that the intensity ratio of the strained Si peak around 515 cm^-1 to the Si substrate peak at 520 cm^-1 increased in the SERS condition. Furthermore, the line width of the strained Si peak in the SERS conditions was broadened. This was caused by the strong excitation of the TO phonon owing to the depolarization. The electrical field component ratios were calculated by finite-difference time-domain method [4]. From the calculation, the value of the z-polarization was the highest for the SERS conditions among the SERS, oil-immersion, and dry conditions, as shown in Table 1. Figure 3(a) shows the wavenumber shifts of TO and LO phonons from the Si substrate with the L&S pattered SiN film as a function of the space width obtained by SERS. Using the above equation, the biaxial stresses s_xx and s_yy were calculated, as shown in Fig. 3(b). s_xx perpendicular to the pattern increased with the decrease in the space width, while s_yy parallel to the pattern remained in the low value. The biaxial stress states were able to be evaluated efficiently by using the SERS technique.

 

Acknowledgement

This study was partially supported by the Semiconductor Technology Academic Research Center (STARC), the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research B (No. 24360125), and the Japan Science and Technology Agency through the Adaptable and Seamless Technology transfer Program (A-STEP) through target-driven R&D.

References

[1] A. Ogura et al., Jpn. J. Appl. Phys. 45, 3007 (2006).

[2] H. Hashiguchi et al., Appl. Phys. Lett. 17, 172101 (2012).

[3] D. Kosemura et al., 51, 02BA03 (2012).

[4] D. Kosemura et al., to be published in J. Raman Spectrosc. (2014).