Topic Review: Application of Raman Spectroscopy Characterization in Micro/Nano-Machining

Abstract

The defects and subsurface damages induced by crystal growth and micro/nano-machining have a significant impact on the functional performance of machined products. Raman spectroscopy is an efficient, powerful, and non-destructive testing method to characterize these defects and subsurface damages. This paper aims to review the fundamentals and applications of Raman spectroscopy on the characterization of defects and subsurface damages in micro/nano-machining. Firstly, the principle and several critical parameters (such as penetration depth, laser spot size, and so on) involved in the Raman characterization are introduced. Then, the mechanism of Raman spectroscopy for detection of defects and subsurface damages is discussed. The Raman spectroscopy characterization of semiconductor materials' stacking faults, phase transformation, and residual stress in micro/nano-machining is discussed in detail. Identification and characterization of phase transformation and stacking faults for Si and SiC is feasible using the information of new Raman bands. Based on the Raman band position shift and Raman intensity ratio, Raman spectroscopy can be used to quantitatively calculate the residual stress and the thickness of the subsurface damage layer of semiconductor materials. The Tip-Enhanced Raman Spectroscopy (TERS) technique is helpful to dramatically enhance the Raman scattering signal at weak damages and it is considered as a promising research field.

Keywords: Raman spectroscopy; TERS; micro/nano-machining; phase transformation; residual stress.

Figure 1

Diagram of the Rayleigh and Raman scattering processes: (a) Rayleigh scattering, (b) Stokes Raman scattering and (c) Anti-Stokes Raman scattering.

Figure 2

The typical information of Raman spectra and corresponding material information.

Figure 3

Raman spectra of single crystal 6H-SiC for different spectral resolution induced by: (a) different laser wavelengths: 532 nm (1.712 cm⁻¹), 638 nm (1.023 cm⁻¹); and (b) different grating’s groove densities: 1800 gr/mm (1.712 cm⁻¹), 2400 gr/mm (1.1378 cm⁻¹). (spot size ≈ 1 μm).

Figure 4

Stacking faults in semiconductors: (a) intrinsic stacking fault; and (b) extrinsic stacking fault.

Figure 5

(a) Raman spectra of a 3C-SiC at the back scattering geometry using (001) face, (b) Comparison of the TO band, the dashed line and solid line are spectra measured using the (110) face and (001) face, respectively. Reproduced with permission from Elsevier [61]. (spot size ≈ 1.32 μm, 588 nm laser wavelength).

Figure 6

Raman spectra of 6H-SiC (0001) crystals and 4H-SiC (0001) crystals with different stacking faults densities. Reproduced with permission from Springer Nature [47]. The samples with different densities of stacking faults were cut from 4H- and 6H-SiC ingots and the stacking faults densities were evaluated by etch-pit-density measurements. (spot size ≈ 1.32 μm, 588 nm laser wavelength).

Figure 7

Raman spectra of surface regions machined under different depths of cut: (a) 5 nm and (b) 120 nm (c) transmission electron microscopy (TEM) micrographs at depths of cut of 120 nm. Reproduced with permission from Elsevier [52]. (laser spot size was 1 μm, the laser wavelength was 532 nm).

Figure 8

Raman spectra of the silicon: (a) before machining; (b) after machining in brittle mode; and (c) after machining in ductile mode. Reproduced with permission from Springer Nature [71]. (performed with the 488 nm exciting light, the spot size was sufficiently small compared with the dimensions of the machined surface).

Figure 9

Raman intensities of (a) crystalline phase (c-Si) and (b) amorphous phase (a-Si) with respect to undeformed chip thickness during single point diamond machined Si substrates. Reproduced with permission from AIP Publishing [51]. (laser spot size: 1 μm, the laser wavelength: 514.5 nm).

Figure 10

Theoretical relationship and experimental results of Raman intensity ratio and amorphous layer thickness induced by micro/nano-machining. Reproduced with permission from Elsevier [52].

Figure 11

Raman spectra of c-Si samples which were diamond-scribed along (110) [001] under different scribing speeds: (a) 5 mm/min, (b) 1 mm/min. Reproduced with permission from Elsevier [74]. (the black square in the SEM image indicates the analyzed points).

Figure 12

(1) Three typical load–displacement curves in the nano-indentation of Si (loading rate of 3 mN/s and maximum load of 50 mN), (2) Corresponding Raman spectra of the nano-indentations: (a) metastable phases, (b) a mixture of a-Si and metastable phases, (c) amorphous Si. Reproduced with permission from AIP Publishing [76]. (spot size ≈ 1 μm, 514.5 nm Ar⁺ laser).

Figure 13

The machined surface and corresponding typical Raman spectra of single crystal 6H-SiC under different machined surface finishes: (a) standard finish, (b) grit blast, (c) rotary ground, and (d) mirror finish. Reproduced with permission from John Wiley and Sons [80]. (40 μm × 40 μm areas of the machined surfaces were scanned point by point with the individual points spaced 2 μm apart, the spot size was smaller than 1 μm).

Figure 14

Raman spectra of 6H-SiC samples at different pressures. Reproduced with permission from American Physical Society [88]. The pressure was applied and measured by a diamond anvil cell device and ruby fluorescence, respectively. (spot size ≈ 5 μm, 514.5 nm Ar⁺ laser).

Figure 15

Residual stresses of silicon carbide and unreacted silicon around the Sylramic fibers measured by micro-Raman spectroscopy in a 30 μm × 30 μm map. Reproduced with permission from Elsevier [101]. (30 μm × 30 μm area of the machined surfaces was scanned point by point with the individual points spaced 1 μm apart, the spot size was smaller than 1 μm, 514.5 nm Ar⁺ laser).

Figure 16

Schematic of a Tip-Enhanced Raman Spectroscopy device which combines an AFM/SPM Tip and a Laser Excitation. Reproduced with permission from Springer Nature [103].

Figure 17

Schematic of TERS microscopy for thin strained Si layer. Reproduced with permission from John Wiley and Sons [105].

Figure 18

TERS spectra (=with tip, red solid), far-field Raman spectra (=without tip, blue solid) and the near-field spectra (=subtracted, black solid) of 30nm strained Si layer. Reproduced with permission from John Wiley and Sons [105].