Precision Layered Stealth Dicing of SiC Wafers by Ultrafast Lasers

At a Glance

Metadata	Details
Publication Date	2022-06-26
Journal	Micromachines
Authors	Bo Yang, Heng Wang, Sheng Peng, Qiang Cao
Institutions	Wuhan University
Citations	31
Analysis	Full AI Review Included

Precision Layered Stealth Dicing (PLSD) of SiC Wafers by Ultrafast Lasers

Executive Summary

This research proposes and validates a Precision Layered Stealth Dicing (PLSD) method using ultrafast lasers to separate hard Silicon Carbide (SiC) wafers, overcoming the limitations of traditional diamond blade dicing (debris, mechanical stress).

Core Achievement: Successful separation of 508 µm thick semi-insulated 4H-SiC wafers, achieving high-quality kerfs with a surface average roughness (Sa) of approximately 1 µm.
Methodology: PLSD utilizes 20 internal modified layers created by an ultrafast picosecond laser, scanning from bottom to top with linear power attenuation (100% to 62%) to ensure uniform energy deposition across the wafer depth.
Damage Mitigation: Low pulse energy (45 µJ) and skipping layers near the surfaces minimized surface ablation, approaching true “stealth” dicing.
Pulse Width Optimization: Picosecond lasers (5 ps) were required to induce a sufficiently large internal modified region for separation, whereas femtosecond lasers were insufficient.
Anisotropy Quantification: The quality of the diced cross section is highly dependent on crystal orientation. Dicing along the primary cleavage plane {10-10} resulted in 20% lower roughness (Sa = 0.894 µm) compared to the secondary cleavage plane {11-20} (Sa = 1.126 µm).
Industrial Advantage: PLSD is a non-contact, dry process that eliminates debris and mechanical stress, offering high precision and narrow kerf width, thereby increasing chip density and wafer utilization.

Technical Specifications

Parameter	Value	Unit	Context
Wafer Material	4H-SiC (Semi-insulated)	N/A	Experimental object
Wafer Thickness	508	µm	Total thickness
Laser Wavelength	1028	nm	Center wavelength used
Laser Pulse Width	5	ps	Standard experimental setting
Laser Repetition Frequency	200	kHz	Standard experimental setting
Laser Average Power	9	W	Standard experimental setting
Laser Pulse Energy (E₀)	45	µJ	Standard experimental setting
Objective Lens NA	0.7	N/A	Numerical Aperture of 100× lens
Scanning Speed	10	mm/s	Processing rate
PLSD Layer Spacing	20	µm	Vertical distance between modified layers
Active Scanning Layers	20	N/A	Total layers processed (4 layers skipped near surfaces)
Power Attenuation	100% to 62%	%	Linear gradient from bottom to top
Kerf Roughness (Sa) - {10-10}	0.894	µm	Primary cleavage plane
Kerf Roughness (Sa) - {11-20}	1.126	µm	Secondary cleavage plane
Fracture Toughness - {10-10}	1.4	MPa m^1/2	Theoretical calculation
Fracture Toughness - {11-20}	1.8	MPa m^1/2	Theoretical calculation
SiC Transmittance (1028 nm)	65.8	%	Required for internal focusing

Key Methodologies

The Precision Layered Stealth Dicing (PLSD) method was executed using the following steps and parameters:

Material and Setup: A 508 µm thick semi-insulated 4H-SiC wafer was used. The ultrafast laser (PHAROS, 5 ps pulse width) was focused inside the wafer through a 100× objective lens (NA 0.7).
Layered Scanning Strategy: The wafer was scanned from the bottom surface towards the top surface. The vertical spacing between successive modified layers was set to 20 µm.
Energy Compensation (Attenuation): To counteract the loss of laser intensity due to absorption at increasing depth, the laser power was linearly attenuated from 100% (at the bottom layer) down to 62% (at the top layer) using a motorized attenuator.
Surface Protection: Four layers (1 near the bottom, 3 near the top) were intentionally skipped to prevent laser ablation and thermal damage on the wafer surfaces. This resulted in 20 active modified layers.
Scanning Parameters: The laser focus was moved along the preset path at a scanning speed of 10 mm/s, ensuring high pulse spot overlap to create a dense modified layer.
Crystal Orientation Testing: Dicing was performed parallel and perpendicular to the primary flat (along <11-20> and <10-10> directions) to evaluate anisotropic effects on kerf quality.
Wafer Separation: After laser processing, external tensile force (e.g., tape expansion) was applied to the wafer to separate the chips along the stress-induced microcracks within the modified layers.

Commercial Applications

The PLSD technology is critical for advancing the manufacturing of high-performance SiC devices, particularly where high precision, minimal damage, and high wafer utilization are paramount.

High-Power Electronics: Essential for dicing SiC power devices (MOSFETs, diodes) used in high-voltage, high-frequency applications, including:
- Electric Vehicles (EVs) and charging infrastructure.
- Smart Grid and power conversion systems.
- Rail transit systems.
Advanced IC Manufacturing: Provides a superior dicing solution for hard-brittle materials, enabling:
- Increased chip density on a single wafer due to narrow kerf widths.
- Elimination of debris contamination and mechanical stress, improving chip yield and reliability.
5G and Radar Systems: Supports the production of SiC components requiring high-efficiency and severe-environment adaptability for advanced communications and detection systems.
Micro/Nano-Fabrication: The precision and non-contact nature of ultrafast laser processing are valuable for general micro-nanomachining of wide-bandgap semiconductors.

View Original Abstract

With the intrinsic material advantages, silicon carbide (SiC) power devices can operate at high voltage, high switching frequency, and high temperature. However, for SiC wafers with high hardness (Mohs hardness of 9.5), the diamond blade dicing suffers from problems such as debris contaminants and unnecessary thermal damage. In this work, a precision layered stealth dicing (PLSD) method by ultrafast lasers is proposed to separate the semi-insulated 4H-SiC wafer with a thickness of 508 μm. The laser power attenuates linearly from 100% to 62% in a gradient of 2% layer by layer from the bottom to the top of the wafer. A cross section with a roughness of about 1 μm was successfully achieved. We have analyzed the effects of laser pulse energy, pulse width, and crystal orientation of the SiC wafer. The anisotropy of the SiC wafer results in various qualities of PLSD cross sections, with the roughness of the crystal plane {10−10} being 20% lower than that of the crystal plane {11−20}.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi13071011

References

2017 - Review of Silicon Carbide Power Devices and Their Applications [Crossref]
2018 - Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges [Crossref]
2021 - Machining of SiC ceramic matrix composites: A review [Crossref]
2011 - SiC versus Si—Evaluation of Potentials for Performance Improvement of Inverter and DC-DC Converter Systems by SiC Power Semiconductors [Crossref]
2011 - Ultra-precision dicing and wire sawing of silicon carbide (SiC) [Crossref]
2020 - Metallic glass coating for improving diamond dicing performance [Crossref]
2015 - Thermal Laser Separation—A Novel Dicing Technology Fulfilling the Demands of Volume Manufacturing of 4H-SiC Devices [Crossref]
2009 - Picosecond pulsed laser ablation and micromachining of 4H-SiC wafers [Crossref]
2015 - Comparison of Different Novel Chip Separation Methods for 4H-SiC [Crossref]
2007 - Advanced Dicing Technology for Semiconductor Wafer—Stealth Dicing [Crossref]