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

Technical Analysis and Documentation: Precision Layered Stealth Dicing of SiC Wafers

Executive Summary

This technical analysis focuses on the successful application of Precision Layered Stealth Dicing (PLSD) using ultrafast lasers to separate hard-brittle 4H-SiC wafers. The findings directly inform the requirements for advanced micro-machining of wide-bandgap materials, a core competency of 6CCVD.

Core Achievement: Successful separation of 508 µm thick semi-insulated 4H-SiC wafers using non-contact, dry ultrafast laser PLSD.
Quality Metric: Achieved a cross-section surface average roughness (Sa) as low as 0.894 µm, demonstrating high-quality kerfs superior to traditional diamond blade dicing.
Methodology: Utilized a 5 ps pulse width picosecond laser, scanning 20 internal layers with 20 µm vertical spacing, and applying a linear power attenuation (100% to 62%) for uniform energy deposition.
Material Anisotropy: Confirmed the critical role of crystal orientation; the primary cleavage plane {10-10} yielded 20% lower roughness than the secondary plane {11-20}.
Laser Selection: Picosecond pulse widths were found to be optimal, inducing a sufficiently large internal modified region for separation, unlike femtosecond pulses which resulted in insufficient modification size.
Market Relevance: The PLSD technique offers a high-precision, low-damage alternative for dicing hard-brittle materials (Mohs 9.5), paving the way for high-density integration of SiC devices.
6CCVD Value Proposition: The extreme precision and low roughness requirements demonstrated in this SiC research are standard capabilities for 6CCVD’s Single Crystal Diamond (SCD) products, which offer superior hardness (Mohs 10) and polishing quality (Ra < 1 nm).

Technical Specifications

Parameter	Value	Unit	Context
Wafer Material	4H-SiC (Semi-Insulated)	N/A	Target material for dicing
Wafer Thickness	508	µm	Total thickness separated
Optimal Surface Roughness (Sa)	0.894	µm	Achieved on {10-10} crystal plane
Secondary Surface Roughness (Sa)	1.126	µm	Achieved on {11-20} crystal plane
Roughness Difference	20	%	Lower roughness on {10-10} vs. {11-20}
Laser Wavelength	1028	nm	Center wavelength used (NIR)
Laser Pulse Width	5	ps	Optimal setting for effective separation
Repetition Frequency	200	kHz	Laser operational parameter
Pulse Energy (E₀)	45	µJ	Used in PLSD experiments
Numerical Aperture (NA)	0.7	N/A	Objective lens specification (100x)
Vertical Layer Spacing	20	µm	Spacing between modified layers
Total Scanning Layers	20	N/A	Actual layers processed (out of 24 planned)
Power Attenuation Gradient	100% to 62%	%	Linear attenuation from bottom to top
Scanning Speed	10	mm/s	Processing feed rate
SiC Mohs Hardness	9.5	N/A	High hardness driving need for laser dicing

Key Methodologies

The Precision Layered Stealth Dicing (PLSD) method relies on precise control of laser parameters and focus depth to induce internal modification without surface ablation.

Material Characterization: The 4H-SiC wafer demonstrated high transmittance (>60%) at the laser wavelength (1028 nm), confirming suitability for internal focusing (Stealth Dicing).
Laser Focusing: An ultrafast laser (5 ps pulse width, 45 µJ pulse energy) was tightly focused 80 µm below the surface using a 100x objective (NA 0.7) to create a modified volume element.
Layered Processing: The 508 µm wafer was diced using 20 active layers, with the laser focus moving up 20 µm after each layer, starting near the bottom surface.
Energy Compensation: A motorized attenuator was used to linearly reduce the laser power from 100% (bottom layer) to 62% (top layer) in a 2% gradient per layer. This compensated for transmitted laser intensity differences at varying depths, ensuring uniform energy deposition.
Crystal Orientation Control: Scanning was performed along specific crystal directions (<11-20>), with results analyzed for the {10-10} and {11-20} cleavage planes.
Wafer Separation: After internal modification, external tensile force was applied to the wafer to separate the chips along the modified layers, leveraging the induced internal stress and microcracks.

6CCVD Solutions & Capabilities

The research highlights the critical need for materials that can withstand and be precisely processed by ultrafast lasers, requiring exceptional material purity, crystal quality, and surface finish. 6CCVD’s MPCVD diamond materials are ideally suited to replicate and extend this high-precision processing into applications demanding the ultimate in hardness and thermal management.

Research Requirement (SiC PLSD)	6CCVD Diamond Solution	6CCVD Capability Match
Processing Hard-Brittle Materials	Single Crystal Diamond (SCD)	SCD is the hardest known material (Mohs 10). 6CCVD provides SCD wafers with superior mechanical and thermal properties for extreme environments.
Achieving Low Surface Roughness	Ultra-Precision Polishing (SCD)	The paper achieved Sa < 1 µm. 6CCVD guarantees SCD polishing to Ra < 1 nm, representing a 1000x improvement in surface quality for critical interfaces.
Ultrafast Laser Transparency (1028 nm)	Optical Grade SCD	High-purity, low-defect SCD is highly transparent across the NIR spectrum (1028 nm), ensuring minimal absorption and high laser damage threshold for internal modification techniques.
Custom Wafer Thickness	SCD/PCD Custom Substrates	The paper used 508 µm SiC. 6CCVD offers SCD and PCD wafers in thicknesses from 0.1 µm up to 500 µm, and robust substrates up to 10 mm thick.
Crystal Orientation Control	Electronic Grade SCD	6CCVD provides SCD wafers grown with precise crystallographic orientation (e.g., <100>, <110>, <111>) to optimize anisotropic mechanical and electronic properties, crucial for replicating the orientation-dependent dicing results observed in SiC.
Custom Dicing/Metalization	Custom Engineering Services	If the SiC devices required specific kerf geometries or metal contacts (e.g., Ti/Pt/Au), 6CCVD offers in-house metalization (Au, Pt, Pd, Ti, W, Cu) and laser cutting services for diamond wafers up to 125 mm.

Applicable Materials

To replicate or extend this research into high-power, high-frequency applications where diamond’s superior thermal conductivity is required, 6CCVD recommends:

Optical Grade SCD: For applications requiring high transparency in the Near-Infrared (NIR) spectrum (1028 nm) used by the ultrafast laser, ensuring maximum energy delivery for internal modification.
Electronic Grade SCD: For high-power device substrates where crystal purity, low defect density, and precise orientation control are paramount for optimal device performance and mechanical cleavage.

Engineering Support

6CCVD’s in-house PhD engineering team specializes in the material science and advanced processing of hard-brittle wide-bandgap materials. We offer consultation services to assist researchers and engineers in selecting the optimal diamond material specifications (thickness, orientation, surface finish) for similar Ultrafast Laser Micro-Machining and Stealth Dicing projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. Global shipping (DDU default, DDP available) is provided for all custom orders.

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]