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	State Key Laboratory of Crystal Materials, Shandong University
Citations	22
Analysis	Full AI Review Included

Executive Summary

This research investigates the critical role of surface preprocessing (roughness) in optimizing 4H-SiC wafer slicing using ultrafast femtosecond laser technology, aiming to overcome the limitations of traditional diamond wire cutting (high loss, long processing time).

Core Achievement: Demonstrated that reducing surface roughness significantly improves the quality and efficiency of internal laser modification and subsequent wafer stripping.
Roughness Impact: Rough surfaces (500 nm) caused diffuse reflection, leading to surface ablation, carbonization, and failure to form a stable internal modified layer, preventing successful stripping.
Optimal Result: The sample processed by Chemical Mechanical Polishing (CMP), achieving a roughness of 0.5 nm, showed no surface ablation and formed a stable, uniform modified layer.
Efficiency Gain: The required tensile force for separation was drastically reduced from 450 N (20 nm roughness) to just 189 N (0.5 nm roughness), indicating a more uniform stress distribution in the modified layer.
Structural Analysis: X-ray diffraction and Raman spectroscopy confirmed that the laser modification transforms the 4H-SiC crystal into an amorphous state (amorphous carbon and silicon), facilitating controlled cleavage.
Mechanism Confirmation: EDS analysis confirmed that the modification reaction occurs internally, proving the stealth dicing mechanism and preventing oxygen contamination.

Technical Specifications

Parameter	Value	Unit	Context
Material Type	4H-SiC	N/A	Third-generation wide-bandgap semiconductor
Wafer Production Method	Physical Vapor Transmission (PVT)	N/A	Method used to grow the SiC ingot
Laser Wavelength	1030	nm	Chosen wavelength for lowest SiC absorption
Absorption Rate (1030 nm)	~0.5	%	Absorption rate of 4H-SiC at operating wavelength
Pulse Width	500	fs	Ultrafast laser setting
Repetition Frequency	1 to 100	kHz	Laser operation range
Spot Radius	10	µm	Focused beam size
Maximum Laser Energy	10	µJ	Per pulse energy
Peak Power Density	9.9 x 10¹⁸	W/cm²	Focused intensity at the modification plane
Processing Speed	100-200	mm/s	Processing platform moving speed
Roughness (Wire Cut)	500	nm	Sample 1 (unprocessed)
Roughness (CMP)	0.5	nm	Sample 4 (best surface quality)
Separation Force (0.5 nm R_a)	189	N	Required tensile force for Sample 4 stripping
Separation Force (20 nm R_a)	450	N	Required tensile force for Sample 3 stripping
Cleavage Height Difference	20-40	µm	Observed height variation in Sample 3 stripping interface
Modified Layer Transformation	Amorphous C and Amorphous Si	N/A	Phase change confirmed by Raman spectroscopy

Key Methodologies

The experiment utilized a four-step process involving material preparation, laser modification, mechanical separation, and comprehensive characterization.

Sample Preparation:
- High-purity 4H-SiC wafers (10 x 10 x 1 mm) were prepared using the Physical Vapor Transmission (PVD) method.
- Four samples were subjected to different surface preprocessing methods to achieve varying roughness (R_a):
  - Sample 1: Wire cutting (500 nm).
  - Sample 2: Polishing (250 nm).
  - Sample 3: Mechanical polishing (20 nm).
  - Sample 4: Chemical Mechanical Polishing (CMP) (0.5 nm).
Laser Modification (Stealth Dicing):
- A femtosecond laser system (1030 nm wavelength, 500 fs pulse width) was used.
- The laser beam was focused through a 20x objective lens to achieve a peak power density of 9.9 x 10¹⁸ W/cm².
- The laser energy was focused internally to create a modified layer (amorphous phase) below the surface, utilizing the low absorption rate (0.5%) of SiC at 1030 nm.
Wafer Separation:
- Samples were fixed using epoxy resin adhesive in an electronic universal tensile machine.
- Tensile force was applied perpendicular to the surface direction to strip the wafer along the modified layer.
Characterization and Analysis:
- Morphology: Atomic Force Microscopy (AFM) measured surface roughness. Optical microscopy (LEXT) and Scanning Electron Microscopy (SEM) observed surface damage and cross-sectional views of the modified layer and stripping interface.
- Structural Integrity: Raman spectrometry (532 nm excitation) and X-ray Double Crystal Diffraction (XRD) rocking curves were used to analyze crystal damage (FWHM broadening) and phase transformation (amorphous carbon/silicon formation).
- Elemental Composition: Energy Dispersive Spectroscopy (EDS) confirmed the presence of Si and C elements on the stripped surface, verifying internal bond-breaking without external contamination (O element).

Commercial Applications

This optimized ultrafast laser slicing technique offers significant advantages over traditional mechanical methods, leading to higher yield, reduced kerf loss, and lower processing costs for SiC wafers.

Power Electronics: Enables high-throughput manufacturing of SiC substrates for high-voltage and high-current devices used in electric vehicles (EVs), charging infrastructure, and smart grid applications.
High-Frequency RF Devices: Provides high-quality, damage-free substrates necessary for 5G/6G base stations and radar systems where SiC is utilized for its superior high-frequency performance.
Semiconductor Manufacturing: Directly applicable to wafer fabrication processes, replacing diamond wire cutting to reduce material waste (kerf loss) and decrease overall processing time (e.g., reducing 6-inch ingot slicing time from 100 hours).
High-Temperature/Harsh Environment Sensors: Improves the quality of SiC wafers used in sensors designed for extreme conditions, leveraging SiC’s high thermal conductivity and breakdown field.
Advanced Wafer Processing: The methodology is relevant for other wide-bandgap materials (like GaN) where internal laser modification is used for separation and dicing.

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

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