Laser Grinding of Single-Crystal Silicon Wafer for Surface Finishing and Electrical Properties

At a Glance

Metadata	Details
Publication Date	2021-03-04
Journal	Micromachines
Authors	Xinxin Li, Yimeng Wang, Yingchun Guan
Institutions	University of Nottingham Ningbo China, Beihang University
Citations	6
Analysis	Full AI Review Included

Executive Summary

This study reports a highly efficient, non-contact laser grinding method utilizing a nanosecond pulsed laser to improve the surface quality and electrical properties of diamond-sawn single-crystal silicon (Si) wafers.

Surface Finishing Achievement: The process successfully reduced the arithmetic mean surface roughness (Ra) by over 81%, dropping from an initial 400 nm (as-received) to 75 nm (laser-grinded).
Damage Layer Removal: The laser ablation mechanism efficiently removed the mechanically induced amorphous SiO₂ layer and surface pollutants (Carbon and Oxygen content decreased significantly).
Crystallinity Recovery: Laser melting and subsequent rapid cooling resulted in bottom-up epitaxial regrowth, transforming damaged polycrystalline silicon into the desired single-crystal structure, confirmed by Raman spectroscopy.
Mechanism: Grinding involves two steps: direct laser removal of scratches/oxides via plasma shock wave, followed by laser melting and redistribution of molten silicon via surface tension and Marangoni effect.
Electrical Improvement: The elimination of the polycrystalline silicon phase led to a reduction in electrical resistivity from 0.572 Ω·cm (as-received) to 0.417 Ω·cm (laser-grinded).
Process Advantage: Provides a facile, high-efficiency method for surface finishing Si wafers, offering a potential cost-saving alternative to conventional post-processing steps like chemo-mechanical polishing (CMP).

Technical Specifications

Parameter	Value	Unit	Context
Starting Wafer Material	Single-crystal Si	N/A	Diamond-sawn, 4-inch diameter
Wafer Thickness	525	µm	Starting material
Wafer Orientation	<100>	N/A	Starting material
Boron Doping Concentration	10¹⁶	cm^-3	Starting material
Laser Wavelength	1064	nm	Nanosecond pulsed laser source
Laser Pulse Width	220	ns	Nanosecond pulsed laser source
Optimal Laser Power (Set Point)	65	%	Used for grinding
Optimal Scanning Speed	3000	mm/s	Used for grinding
Laser Frequency	100	kHz	Used for grinding
Scanning Interval	20	µm	Used for grinding
Optimal Laser Intensity	6.75 x 10⁶	W/cm²	Calculated intensity
Initial Surface Roughness (Ra)	400	nm	As-received surface
Final Surface Roughness (Ra)	75	nm	Laser-grinded surface (81% reduction)
Initial Resistivity (ρ)	0.572	Ω·cm	As-received wafer
Final Resistivity (ρ)	0.417	Ω·cm	Laser-grinded wafer
Initial SiO₂ Peak (XRD)	32.985, 61.75	° (2θ)	As-received surface
Single-Crystal Si Peak (Raman)	520.016	cm^-1	Laser-grinded surface

Key Methodologies

Material Selection: Single-crystal silicon wafers (525 µm thick, <100> orientation, Boron doped) were used as received after diamond sawing, exhibiting significant surface damage (Ra = 400 nm).
Laser Grinding Setup: A 1064 nm nanosecond pulsed laser (220 ns pulse width) was focused using a two-mirror galvanometric scanner and an F-theta objective lens, achieving a 35 µm focal beam diameter.
Process Optimization: The optimal grinding parameters were determined via Design of Experiment (DoE) analysis using an orthogonal table (L₆₄(8⁹)). The reported optimal recipe was 65% power, 3000 mm/s scanning speed, 100 kHz frequency, and 20 µm scanning interval.
Environmental Control: All grinding experiments were conducted within an Argon (Ar) shield environment to minimize atmospheric contamination and prevent re-oxidation of the silicon surface.
Surface Characterization: Surface topography and roughness (Ra, Rz) were evaluated using a 3D laser scanning confocal microscope (LSCM) and surface profilometers.
Chemical Analysis: Elemental distribution and composition (Si, C, O) were analyzed using Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). Chemical bonding states and oxide layer identification (SiO₂) were confirmed via X-ray Photoelectron Spectroscopy (XPS).
Microstructure Analysis: Crystallinity and phase evolution were tracked using X-ray Diffraction (XRD) and laser micro-Raman spectroscopy (532 nm laser), focusing on the transformation of polycrystalline Si to single-crystal Si.
Electrical Testing: Room-temperature resistivity (ρ) and I-V characteristic curves were measured using the four-probe method to assess the impact of laser grinding on electrical properties.

Commercial Applications

Semiconductor Wafer Fabrication: Direct application in the post-slicing process (lapping, grinding) to rapidly remove subsurface damage (SSD) and achieve high-quality surface finishing required for microelectronic devices.
Cost Reduction in Wafer Processing: Serving as a highly efficient, non-contact alternative to traditional mechanical grinding and reducing the reliance on expensive, time-consuming chemo-mechanical polishing (CMP) steps.
High-Performance Photovoltaics: Producing damage-free silicon substrates necessary for high-efficiency solar cells, where surface defects significantly impact performance.
MEMS/NEMS Manufacturing: Providing ultra-smooth, defect-free silicon surfaces essential for the reliability and function of micro- and nano-electromechanical systems.
Advanced Material Recovery: Potential use in laser recovery processes to repair damaged or defective semiconductor materials, extending wafer lifetime and yield.

View Original Abstract

In this paper, we first report the laser grinding method for a single-crystal silicon wafer machined by diamond sawing. 3D laser scanning confocal microscope (LSCM), X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), laser micro-Raman spectroscopy were utilized to characterize the surface quality of laser-grinded Si. Results show that SiO2 layer derived from mechanical machining process has been efficiently removed after laser grinding. Surface roughness Ra has been reduced from original 400 nm to 75 nm. No obvious damages such as micro-cracks or micro-holes have been observed at the laser-grinded surface. In addition, laser grinding causes little effect on the resistivity of single-crystal silicon wafer. The insights obtained in this study provide a facile method for laser grinding silicon wafer to realize highly efficient grinding on demand.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi12030262

References

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