Experimental Study on the Role of Bond Elasticity and Wafer Toughness in Back Grinding of Single-Crystal Wafers

At a Glance

Metadata	Details
Publication Date	2025-10-25
Journal	Materials
Authors	Joong-Cheul Yun, Dae‐Soon Lim
Institutions	Korea University
Analysis	Full AI Review Included

Technical Analysis and Documentation: Diamond Grinding Optimization

Executive Summary

This study provides critical insights into optimizing the back-grinding process for ultra-hard semiconductor wafers, particularly Silicon Carbide (SiC), by quantifying the interplay between the grinding wheel’s Elastic Bond Modulus (Eb) and the wafer’s Fracture Toughness (K_IC).

Predictive Modeling: A validated log-linear empirical model accurately predicts diamond protrusion height (h_p) based on the wheel’s Eb and the wafer’s K_IC, enabling precise tool design initialization.
Material Toughness Correlation: For high-toughness materials like 4H-SiC (K_IC ≈ 2.64 MPa·m^0.5), a higher Eb (up to 131 GPa) is required to ensure sufficient diamond protrusion and effective force transmission, shifting the removal mode toward fracture-based efficiency.
Optimal SiC Performance: The optimal grinding window for SiC was identified at Eb = 122.07 GPa (BGW4), achieving a high Material Removal Rate (MRR) exceeding 740 µm/h while eliminating observable Subsurface Damage (SSD).
Surface Quality Control: Surface Roughness (Ra) was found to increase linearly with h_p, demonstrating the necessity of precise h_p control to maintain quality (optimal Ra < 0.64 µm for SiC).
Industrial Relevance: The methodology shortens process development time across diverse hard materials (Si, GaP, Sapphire, SiC) and provides a framework for designing high-throughput diamond tools for next-generation power electronics substrates.

Technical Specifications

The following table summarizes the key material properties and performance metrics, focusing on the optimal results achieved for 4H-SiC, the most demanding material tested.

Parameter	Value	Unit	Context
Wafer Material Tested	4H-SiC	N/A	Si-Face (0001) orientation
SiC Hardness (Vickers)	3445 ± 189	Kg/mm²	Highest hardness tested
SiC Fracture Toughness (K_IC)	2.64 ± 0.019	MPa·m^0.5	Highest toughness tested
Optimal Bond Elastic Modulus (Eb)	122.07 ± 4.93	GPa	BGW4, minimized SSD for SiC
Diamond Grit Size (Average)	51.2	µm	Fixed parameter (≈D54)
Diamond Content	12.5	Volume%	Fixed parameter
Optimal MRR (SiC)	753.87 to 794.94	µm/h	High-throughput back grinding
Optimal Volumetric MRR (SiC)	100	mm³/min	Equivalent rate
Optimal Surface Roughness (Ra)	0.627 to 0.638	µm	Achieved with BGW4 on SiC
Grinding Load (SiC, Optimal)	45.92 to 49.67	N	Stable, low load condition
Subsurface Damage (SSD)	None observed	N/A	Verified via cross-sectional TEM
Debris Size (SiC)	1 to 5	µm	Finer debris due to high K_IC
Eb Range Tested	95.24 to 131.38	GPa	Spanning five Back-Grinding Wheels (BGWs)

Key Methodologies

The experimental design focused on isolating the effect of bond stiffness by systematically varying the Cobalt (Co) content in the metal matrix while holding abrasive parameters constant.

Wafer Selection and Characterization:
- Four single-crystal wafers (Si, GaP, Sapphire, 4H-SiC) were selected, focusing on orientations prone to brittle fracture (e.g., SiC Si-face (0001)).
- Mechanical properties (Hardness, Eb) were measured using a micro-Vickers tester.
- Fracture Toughness (K_IC) was assessed using the Lawn and Evans nanoindentation technique.
Diamond Wheel Fabrication (BGWs):
- Five BGWs were fabricated using a Cu-Sn matrix bond.
- Cobalt (Co) content was varied from 10 wt% (BGW1) to 50 wt% (BGW5) to control the Elastic Bond Modulus (Eb).
- Eb ranged from 95.24 GPa (BGW1) to 131.38 GPa (BGW5).
Abrasive Parameters (Fixed):
- Diamond Size: Average 51.2 µm (MBG-660 #325/400).
- Diamond Content: 12.5 Volume%.
- Relative Density: Held constant at approximately 90%.
Grinding Process Parameters:
- Equipment: INSEMITEC IVG-3030 back-grinding machine.
- Material Removal: 200 µm thickness removed per test.
- Wheel Peripheral Speed: 23.5 m/s.
- Wafer Rotation Speed: 1.6 m/s.
- Feed Rate: 0.5 µm/s.
Performance Evaluation:
- Diamond Protrusion Height (h_p): Measured using a confocal microscope (KEYENCE VK-910K).
- Surface Roughness (Ra): Measured using an optical 3D surface profiler (Wyko NT3300).
- Subsurface Damage (SSD): Evaluated via cross-sectional TEM analysis on SiC wafers.
- Debris Size: Analyzed via FE-SEM (Axia ChemiSEM) after drying the grinding slurry.

6CCVD Solutions & Capabilities

This research confirms that the efficient processing of ultra-hard materials like SiC requires highly controlled diamond tooling, where the mechanical properties of the bond and the abrasive material must be precisely matched to the substrate’s fracture toughness. 6CCVD provides the foundational diamond materials and advanced processing required to support and extend this high-throughput grinding research.

Applicable Materials for Replication and Extension

To replicate or extend the high-performance grinding demonstrated in this study, 6CCVD offers materials essential for both the grinding tool and the advanced substrates themselves:

High-Purity Polycrystalline Diamond (PCD): Ideal for manufacturing the abrasive grains used in the BGWs. We can supply PCD material tailored for specific grit sizing (e.g., the 51.2 µm range used here) and concentration (12.5 vol%) requirements for metal-bonded tools.
Optical Grade Single Crystal Diamond (SCD): While the paper focuses on grinding, the resulting SiC wafers are often used for high-power devices requiring superior thermal management. 6CCVD provides SCD plates up to 500 µm thick, which are critical for advanced heat spreader applications derived from SiC thinning processes.
Custom Substrates: We offer SCD and PCD substrates up to 10mm thick, suitable for use in specialized grinding fixtures or as reference materials for advanced mechanical testing (e.g., K_IC evaluation via nanoindentation).

Customization Potential for Advanced Tooling

The study emphasizes the need for precise control over diamond protrusion and surface quality (Ra < 0.64 µm). 6CCVD’s advanced processing capabilities ensure that researchers and engineers can achieve superior results beyond the grinding stage:

Requirement from Research	6CCVD Capability	Technical Advantage
Wafer Size (4-inch)	Custom Dimensions up to 125mm	Supports scaling of grinding processes to larger, next-generation PCD wafers.
Ultra-Low Surface Damage	Polishing Ra < 1nm (SCD), < 5nm (PCD)	Guarantees post-grinding surface quality far superior to the 0.63 µm Ra achieved in the study, crucial for minimizing SSD and maximizing device yield.
Tool/Substrate Integration	Custom Metalization Services	Internal capability for depositing Au, Pt, Pd, Ti, W, and Cu. Essential for creating robust diamond-to-bond interfaces or for subsequent device fabrication steps on thinned wafers.
Precise Thickness Control	SCD/PCD Thickness 0.1µm - 500µm	Provides the necessary precision for manufacturing thin diamond layers used in high-frequency or thermal management applications derived from the thinned SiC wafers.

Engineering Support

The development of the log-linear h_p model (linking Eb and K_IC) demonstrates the complexity of optimizing grinding processes for High-Toughness Semiconductor Wafers. 6CCVD’s in-house team of PhD material scientists specializes in the mechanical and thermal properties of CVD diamond.

We offer expert consultation to assist engineers in:

Material Selection: Determining the optimal diamond grade (SCD vs. PCD) and morphology for specific abrasive applications based on target K_IC values.
Process Initialization: Utilizing the principles of the Eb-K_IC model to define initial feed/infeed parameters for coarse and fine removal passes, minimizing the need for extensive Design of Experiments (DoE).
Quality Assurance: Providing certified SCD and PCD materials with guaranteed surface roughness and crystallographic orientation control, ensuring consistency in subsequent processing steps.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. Global shipping (DDU default, DDP available) ensures rapid delivery of critical materials worldwide.

View Original Abstract

Grinding semiconductor wafers with high hardness, such as SiC, remains a significant challenge due to the need to maximize material removal rates while minimizing subsurface damage. In the back-grinding process, two key parameters—the elastic modulus (Eb) of the grinding wheel bond and the fracture toughness (KIC) of the wafer—play a critical role in governing the behavior of diamond and the extent of wafer damage. This study systematically investigated the effect of Eb and KIC on diamond protrusion height (hp), surface roughness (Ra), grinding forces, and the morphology of generated debris. The study encompassed four wafer types—Si, GaP, sapphire, and ground SiC—using five Back-Grinding Wheels (BGWs), with Eb ranging from 95.24 to 131.38 GPa. A log-linear empirical relationship linking ℎₚ to Eb and KIC was derived and experimentally verified, demonstrating high predictive accuracy across all wafer-wheel combinations. Surface roughness (Ra) was measured in the range of 0.486 − 1.118𝜇m, debris size ranged from 1.41 to 14.74𝜇m, and the material removal rate, expressed as a thickness rate, varied from 555 to 1546𝜇m/h (equivalent to 75−209 mm³/min using an effective processed area of 81.07 cm²). For SiC, increasing the bond modulus from 95.24 to 131.38 GPa raised the average hp from 9.0 to 1.2 um; the removal rate peaked at 122.07 GPa, where subsurface damage (SSD) was minimized, defining a practical grindability window. These findings offer practical guidance for selecting grinding wheel bond compositions and configuring process parameters. In particular, applying a higher Eb is recommended for harder wafers to ensure sufficient diamond protrusion, while an appropriate dressing must be employed to prevent adverse effects from excessive stiffness. By balancing removal rate, surface quality, and subsurface damage constraints, the results support industrial process development. Furthermore, the protrusion model proposed in this study serves as a valuable framework for optimizing bond design and grinding conditions for both current and next-generation semiconductor wafers.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma18214890

References

2022 - High Thermal Conductivity in Wafer Scale Cubic Silicon Carbide Crystals [Crossref]
2019 - Hardness and Mechanical Anisotropy of Hexagonal SiC Single Crystal Polytypes [Crossref]
2013 - An Overview of Recent Advances in Chemical Mechanical Polishing (CMP) of Sapphire Substrates [Crossref]
2017 - Removal Mechanism of Sapphire Substrates (0001, 112¯0 and 101¯0) in Mechanical Planarization Machining [Crossref]
2000 - Technology Roadmaps for Compound Semiconductors [Crossref]
2018 - Unified Theory of the Direct or Indirect Bandgap Nature of Conventional Semiconductors [Crossref]
2024 - Subsurface Damage in Sapphire Ultra-Precision Grinding [Crossref]
2018 - Subsurface Damage in Grinding of Brittle Materials Considering Machining Parameters and Spindle Dynamics [Crossref]