One of the factor that degrade the quality of SiC power devices is defects on SiC wafers. The defects often appear during polishing or thin-film fabricating process, besides defects originally existing on the wafer. The area adjoining the defects has local stress, which often creates new defects and/or often affects the performance of semiconductor devices. To fabricate highly reliable semiconductor devices, evaluation of invisible stress distribution around the defects as well as visible defects itself is needed. The shift of the peak position of SiC on the Raman spectra depends on the stress. Raman micro-spectroscopy, by which the peak shift can be seen, is a suitable technique to evaluate stress distribution on the wafer.

Stress distribution imaging around defects on SiC wafers by Raman micro-spectroscopy

The figure above depicts stress distribution on a ground SiC substrate surface. In this evaluation, the peak shift of 6H-SiC at 789 cm-1 (FTO(2/6)E2) was analyzed. The linearity constant between the peak shift and stress is -185 MPa/cm-1 under an assumption of isotropic biaxial stress fields [1]. By comparing the optical image and the Raman image, tensile stress was found on the defects, which looks dark on the optical image, and compressive stress was found on the area among the defects.

Laser wavelength532 nm
Objective lensx100 (NA=0.90)
Grating2400 gr/mm
The number of spactra32,000 (400×80)
Measurment time27 min.

Stress distribution imaging along the depth direction to the SiC wafer surface

Raman micro-spectroscopy can be applied to stress distribution imaging along the depth (z-) direction due to the resolution along the z-direction. Figures (1)-(3) shows stress distribution along the z-direction at each step of processing SiC wafers: (1) before grinding, (2) after grinding, (3) after polishing to remove defects that appeared after grinding. Tensile stress was found on the defects, and compressive stress was found on the area among the defects.

The figure below shows stress profiles along the z axis before and after polishing. The peak shift profiles on the dashed line shown in the magnified figures (2) and (3) are also depicted. After grinding, the stress exists up to 4 μm of the depth (2)after grinding, whereas smaller stress exists up to 2 μm of the depth (3)after grinding and polishing. The x-y and z- stress profiling can be an effective way to evaluate defects on SiC wafers in detail. To create stress distribution images, high wavenumber resolution, high spacial resolution, and high speed of imaging are required. Our laser Raman microscope, RAMANtouch/RAMANforce, fully satisfies these requirements.

Laser wavelength532 nm
Objective lensx100 (NA=0.90)
Grating2400 gr/mm
The number of spectra32,000 (400×80)
Measurement time27 min.

Changes in SiC Raman spectra induced by stress

The peak of Raman spectra of crystals shifts due to strains caused by stress. The peak shifts to the lower wavenumber under tensile stress and shifts to the higher wavenumber under compressive stress, compared to the peak position under no stress. Since the shift of the peak is proportional to the stress intensity, we can estimate the intensity of the stress.

In the measurements shown above, the carbon surfaces (000-1) of 6H-SiC wafers were chosen, and the stress was evaluated using the shift of the peak at 789 cm-1 (FTO(2/6)E2) under an assumption of isotropic biaxial stress fields and -185 MPa/cm-1 of the linearity constant. Here, we note that the linearity constant between the peak shift and the stress depends on polytypes or plane orientation [2,3]. For example, Sugiyama et al. [1] reported that the constant for the peak of 4H-SiC at 776cm-1 (FTO(2/4) E2) is -510 MPa/cm-1 in the case of c surface (0001), and -480 MPa/cm-1 in the case of a surface (11-20). By using proper peaks and its linearity constants, stress distributions can be evaluated reasonably.

References

[1] Correlation of Stress in Silicon Carbide Crystal and Frequency Shift in Micro-Raman Spectroscopy,
N. Sugiyama et al., MRS Proc, 1693 (2014)

[2] “Raman Investigation of SiC Polytypes”, 
S. Nakashima and H. Harima, phys. stat. sol. (a), 162, 39 (1997)

[3] “Raman Scattering from Electronic Excitations in n-Type Silicon Carbide”,
P. J. Colwell and M. V. Klein, Phys. Rev. B, 6, 498 (1972)

※These samples were provided by EL-Seed Corp.