Influence of Surface Preprocessing on 4H-SiC Wafer Slicing by Using Ultrafast Laser

At a Glance

Metadata	Details
Publication Date	2022-12-22
Journal	Crystals
Authors	Hanwen Wang, Chen Qiu, Yongping Yao, Linlin Che, Baitao Zhang
Institutions	Shandong University, State Key Laboratory of Crystal Materials
Citations	22
Analysis	Full AI Review Included

Technical Documentation & Analysis: Precision Surface Preprocessing for Advanced Wafer Slicing

Source Paper: Wang et al., Influence of Surface Preprocessing on 4H-SiC Wafer Slicing by Using Ultrafast Laser. Crystals 2023, 13, 15.

Executive Summary

This research validates the critical role of surface quality in achieving high-efficiency, low-damage wafer separation using ultrafast laser stealth dicing (KABRA method). The findings directly support 6CCVD’s focus on ultra-precision polished materials for advanced engineering applications.

Core Achievement: Demonstrated that reducing 4H-SiC surface roughness (Ra) significantly improves the quality and efficiency of femtosecond laser slicing.
Surface Quality Impact: Ultra-smooth surfaces (Ra ≈ 0.5 nm via CMP) minimized diffuse reflection and laser ablation damage, preventing surface carbonization and blackening.
Process Stability: Good surface quality promoted the stable and uniform formation of the internal modified layer, which is essential for successful kerf-free separation.
Efficiency Gain: Optimal surface preprocessing reduced the required tensile force for wafer stripping by 58% (from 450 N down to 189 N).
Methodology: Utilized a 1030 nm, 500 fs femtosecond laser to induce internal modification, followed by mechanical tensile testing for separation.
Material Relevance: The requirement for Ra < 1 nm surfaces aligns perfectly with 6CCVD’s SCD and PCD polishing capabilities, enabling the extension of this technique to high-value diamond materials.

Technical Specifications

Parameter	Value	Unit	Context
Material Studied	4H-SiC	N/A	High-purity, wide-bandgap semiconductor
Sample Dimensions	10 x 10 x 1	mm	R&D sample size
Laser Wavelength	1030	nm	Selected for low SiC absorption
Pulse Width	500	fs	Ultrafast processing to avoid thermal effects
Maximum Laser Energy	10	µJ	Energy per pulse
Peak Power Density	9.9 x 10¹⁸	W/cm²	Required for multi-photon ionization
Lowest Surface Roughness (Sample 4)	0.5	nm	Achieved via Chemical Mechanical Polishing (CMP)
Highest Surface Roughness (Sample 1)	500	nm	After diamond wire cutting
Tensile Force (Sample 4, Ra 0.5 nm)	189	N	Minimum force required for stripping
Tensile Force (Sample 3, Ra 20 nm)	450	N	Force required for stripping of mechanically polished sample
SiC Absorption Rate @ 1030 nm	0.5	%	Lowest absorption point, minimizing surface heating
Cleavage Height Difference	20 to 40	µm	Observed in samples with uneven modified layers
Modified Layer FWHM (XRD)	223.5	arcsec	Indicates significant crystal damage/amorphous state

Key Methodologies

The experiment focused on correlating initial surface roughness with the quality of the laser-induced modified layer and the subsequent mechanical stripping force.

Material Preparation: Four 10 x 10 x 1 mm 4H-SiC samples were prepared with controlled surface roughnesses (Ra: 500 nm, 250 nm, 20 nm, 0.5 nm) using standard industrial polishing techniques (wire cutting, polishing, mechanical polishing, and CMP).
Laser System Setup: A femtosecond laser system was configured with a 1030 nm wavelength, 500 fs pulse width, and a repetition frequency ranging from 1 to 100 kHz. The beam was focused through a 20x objective lens to achieve a peak power density of 9.9 x 10¹⁸ W/cm².
Internal Modification: The laser was scanned across the samples to form an internal modified layer (stealth dicing trace). Samples with rough surfaces (500 nm, 250 nm) failed to form a stable modified layer due to excessive surface absorption and ablation.
Mechanical Separation: Samples that successfully formed a modified layer (Ra 20 nm and 0.5 nm) were fixed to a universal tensile testing machine, and tensile force was applied perpendicular to the surface to induce separation along the modified layer.
Post-Processing Analysis: Characterization techniques including AFM, LEXT (confocal microscopy), X-ray Double Crystal Diffraction (XRD), and Raman spectroscopy were used to analyze surface damage, modified layer uniformity, crystal quality (FWHM), and elemental changes (amorphous carbon/silicon formation).

6CCVD Solutions & Capabilities

The research highlights that the success of advanced laser processing techniques hinges on the quality of the starting material surface. 6CCVD specializes in producing MPCVD diamond materials (SCD and PCD) with the ultra-low roughness required to replicate and extend this high-precision slicing methodology to the most demanding wide-bandgap applications.

Applicable Materials for Replication and Extension

Application Focus	6CCVD Material Recommendation	Key Material Property
High-Precision Slicing/Dicing	Optical Grade Single Crystal Diamond (SCD)	Ra < 1 nm polishing capability ensures minimal laser energy loss and stable modified layer formation, enabling kerf-free separation of high-value SCD.
Large-Area Wafer Processing	Polycrystalline Diamond (PCD) Plates	Custom dimensions up to 125 mm diameter (inch-size wafers) with guaranteed polishing quality (Ra < 5 nm).
High-Power Device Substrates	High-Purity SCD Substrates	Superior thermal conductivity (up to 2200 W/mK) compared to SiC, requiring precision slicing for integration into next-generation power modules.
Electrochemical/Sensor Applications	Boron-Doped Diamond (BDD)	Can be supplied with ultra-smooth surfaces for applications requiring both electrical conductivity and high surface integrity.

Customization Potential for Advanced Processing

The success of femtosecond laser processing relies on tightly controlled material specifications. 6CCVD offers comprehensive customization services to meet these stringent requirements:

Ultra-Precision Polishing: We guarantee SCD surfaces with roughness Ra < 1 nm and large-area PCD surfaces with Ra < 5 nm. This directly addresses the critical need for Ra ≈ 0.5 nm surfaces identified in the research for stable laser modification.
Custom Dimensions and Thickness: While the paper used 1 mm thick samples, 6CCVD can supply SCD wafers from 0.1 µm up to 500 µm thick, and PCD plates up to 125 mm in diameter and 500 µm thick, supporting both thin-film and bulk applications.
Advanced Metalization: For researchers integrating sliced wafers into devices, 6CCVD offers in-house metalization services, including deposition of Ti, Pt, Au, Pd, W, and Cu layers, ensuring robust electrical contacts and bonding interfaces.
Substrate Supply: We provide thick diamond substrates (up to 10 mm) suitable for use as high-power heat spreaders or as starting material for deep laser processing research.

Engineering Support

6CCVD’s in-house PhD material science team provides expert consultation to optimize material selection and preparation for complex processes. We can assist researchers in adapting the ultrafast laser slicing methodology to diamond materials, ensuring optimal surface quality and crystal orientation for stealth dicing and high-power electronics packaging projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

The physical properties of silicon carbide (SiC) are excellent as a third-generation semiconductor. Nevertheless, diamond wire cutting has many drawbacks, including high loss, long cutting time and prolonged processing time. The study of 4H-SiC wafer slicing by using an ultrafast laser is hopeful for solving these problems. In this work, the 4H-SiC samples with different surface roughness were processed by laser slicing. Findings revealed that good surface quality could reduce the damage to the wafer surface during laser slicing, reduce cleavage, and improve the flatness and uniformity of the modified layer. Thus, preprocessing on 4H-SiC can significantly improve the quality and efficiency of laser slicing.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst13010015

References

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2015 - Comparison of Different Novel Chip Separation Methods for 4H-SiC [Crossref]
2006 - Internal modified-layer formation mechanism into silicon with nanosecond laser
2022 - Process mechanism of ultrafast laser multi-focal-scribing for ultrafine and efficient stealth dicing of SiC wafers [Crossref]
2020 - Monitoring method for femtosecond laser modification of silicon carbide via acoustic emission techniques [Crossref]
2007 - Theoretical models and qualitative interpretation of fs laser material processing [Crossref]
2005 - Energy transport and material removal in wide bandgap materials by a femtosecond laser pulse [Crossref]
2008 - Ultrafast laser micromachining of 3C-SiC thin films for MEMS device fabrication [Crossref]