High-efficiency planarization method combining mechanical polishing and atmospheric-pressure plasma etching for hard-to-machine semiconductor substrates

At a Glance

Metadata	Details
Publication Date	2016-01-01
Journal	Mechanical Engineering Journal
Authors	Yasuhisa Sano, Kousuke Shiozawa, Toshiro Doi, Syuhei KUROKAWA, Hideo Aida
Institutions	Kyushu University, The University of Osaka
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Efficiency Planarization for Hard-to-Machine Substrates

This document analyzes the research on combining Chemical Mechanical Polishing (CMP) and Atmospheric-Pressure Plasma Etching (P-CVM) for planarizing hard-to-machine semiconductors, focusing on how 6CCVD’s advanced MPCVD diamond materials and processing capabilities can support and extend this technology.

Executive Summary

Core Challenge Addressed: Developing a high-efficiency planarization preprocessing technique for hard-to-machine wide-band-gap semiconductors, including Silicon Carbide (SiC), Gallium Nitride (GaN), and Diamond.
Novel Methodology: Introduction of a combined CMP/P-CVM process where mechanical polishing selectively introduces a damaged layer on convex surface peaks, which is then efficiently and preferentially removed by atmospheric-pressure plasma etching.
Key Performance Achievement: The combined CMP/P-CVM process demonstrated a seven times greater decreasing rate of the Peak-to-Valley (PV) surface value compared to mechanical polishing alone.
Planarization Success: Successfully reduced the PV value of a 4H-SiC mesa structure from approximately 6 µm to less than 0.5 µm after 320 repetitions.
Surface Quality: Achieved a final surface roughness (rms) of 1.69 nm, suitable for subsequent final finishing processes.
Relevance to 6CCVD: This technique is directly applicable to the high-volume, high-precision planarization of 6CCVD’s Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) substrates, reducing overall wafering cost and time.

Technical Specifications

Parameter	Value	Unit	Context
Target Substrate	4H-SiC (0001) 2° off	N/A	Substrate with fabricated mesa structure
Initial Peak-to-Valley (PV) Value	~6	µm	Before combined processing
Final Peak-to-Valley (PV) Value	< 0.5	µm	After 320 repetitions
PV Decreasing Rate Improvement	7	times greater	Compared to mechanical polishing (CMP) alone
Final Surface Roughness (rms)	1.69	nm	Measured after combined CMP/P-CVM process
Plasma Frequency	13.56	MHz	RF power supply for P-CVM
RF Power	50	W	Atmospheric-pressure plasma generation
Electrode-Specimen Gap	300	µm	Plasma etching distance
Reactive Gas Mixture	He:SF₆ = 99.4:0.6	Ratio	Used for SiC etching
Gas Flow Rate	1.5	l/min	Optimized to minimize N₂ contamination
Mechanical Polishing Abrasive	Diamond film (#8000)	N/A	Fixed abrasive used for damage introduction
Mechanical Polishing Pressure (Initial)	30	kPa	Repetitions 1-200
Mechanical Polishing Pressure (Final)	60	kPa	Repetitions 201-320 (Increased for faster rate)
Mechanical Polishing Time per Cycle	5	sec	Short duration for controlled damage
P-CVM Etching Time per Cycle	15	sec	Short duration for efficient removal

Key Methodologies

The planarization process relies on a precise, two-step, repetitive cycle designed to leverage the selective etching properties of plasma on mechanically damaged material.

Sample Preparation (Mesa Structure Fabrication):
- A thin Aluminum film shadow mask was deposited onto a 4H-SiC substrate (10 mm x 10 mm) via vacuum vapor deposition.
- The exposed SiC was etched using atmospheric-pressure plasma (He/SF₆ mixture) to create a micrometer-order mesa structure (initial PV ~6 µm).
- The Aluminum mask was removed via acid cleaning.
Plasma Generation Optimization (P-CVM):
- The plasma region was purged with Helium gas to minimize environmental air contamination (Nitrogen peak).
- Optimal conditions were determined to be a 300 µm gap between the electrode and sample, and a 1.5 l/min gas flow rate.
- Plasma was generated using 50 W RF power (13.56 MHz).
Mechanical Polishing (CMP) Cycle:
- A fixed diamond abrasive film (#8000) was used to ensure that only the convex (peak) parts of the surface were attacked, preventing damage to the concave areas.
- A short process time (5 sec) and slow rotation speed (5 rpm) were selected to prevent edge chipping and control damage thickness.
- Processing pressure was initially 30 kPa, later increased to 60 kPa (repetitions 201-320) to compensate for decreased contact area and accelerate planarization.
Plasma Etching (P-CVM) Cycle:
- The sample was immediately transferred to the P-CVM part (no intermediate cleaning step).
- The atmospheric-pressure plasma (He:SF₆) preferentially and efficiently removed the crystallographically damaged layer introduced by the mechanical polishing step.
- Process time was 15 sec per cycle.
Evaluation: The combined cycle was repeated 320 times, demonstrating rapid planarization and a final RMS roughness of 1.69 nm.

6CCVD Solutions & Capabilities

This research validates a critical high-throughput planarization method essential for manufacturing next-generation wide-band-gap semiconductor devices. 6CCVD is uniquely positioned to supply the high-quality MPCVD diamond substrates required to replicate and advance this technology.

Research Requirement	6CCVD Solution & Capability	Technical Advantage
Hard-to-Machine Substrates (Diamond)	Optical Grade SCD & High-Purity PCD	The paper explicitly targets diamond planarization. 6CCVD supplies high-purity MPCVD diamond, offering superior thermal management and electrical properties for high-power applications.
Large-Area Planarization	Custom Dimensions up to 125 mm	While the research used 10 mm x 10 mm SiC, 6CCVD offers Polycrystalline Diamond (PCD) plates/wafers up to 125 mm, enabling scale-up for industrial wafer processing.
Final Surface Quality Requirement	Ultra-Low Roughness Polishing	The combined process serves as a bulk planarization step. 6CCVD provides the necessary final finish: SCD polishing to Ra < 1 nm and inch-size PCD polishing to Ra < 5 nm, ensuring device-ready surfaces.
Controlled Damage Layer Introduction	Precision Thickness Control (0.1 µm - 500 µm)	6CCVD supplies SCD and PCD materials with highly controlled thickness tolerances, crucial for ensuring consistent mechanical response and predictable damaged layer formation during the CMP step.
Custom Masking and Contact Fabrication	In-House Metalization Services	The fabrication process requires thin film deposition (e.g., Aluminum mask). 6CCVD offers custom metalization (Au, Pt, Pd, Ti, W, Cu) services, supporting complex lithography and device integration steps.
Process Optimization & Material Selection	Expert Engineering Support	6CCVD’s in-house PhD team specializes in diamond material science and surface preparation, ready to assist engineers and scientists in optimizing material selection (SCD vs. PCD vs. BDD) and process parameters for similar High-Efficiency Planarization projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We offer global shipping (DDU default, DDP available) to ensure rapid delivery of your specialized diamond substrates.

View Original Abstract

A high-efficiency planarization technique for preprocessing before final polishing is needed for hard-to-machine wide-band-gap semiconductors, such as silicon carbide (SiC), gallium nitride, and diamond. We proposed a novel planarization method that combines chemical mechanical polishing (CMP) and atmospheric-pressure plasma etching (plasma chemical vaporization machining [P-CVM]) and developed a prototype of the basic type CMP/P-CVM combined processing system. This prototype has a mechanical polishing part for introducing a damaged layer on the convex part of the sample surface and a P-CVM part for efficient etching of the damaged layer. Process conditions for plasma generation were determined in order to minimize the optical emission intensity ratio of nitrogen to helium because nitrogen comes from circumstance air and should not exist in the plasma region. Process conditions for mechanical polishing were determined in order to efficiently generate a damaged layer only on the convex part of the sample surface. The combined process was performed using a SiC substrate on which the mesa structure was fabricated as a sample. As a result, we found that the convex parts of the mesa structure were preferentially removed and the surface of the sample was planarized. We also found that the decreasing rate of the peak-to-valley value of the mesa structure obtained by CMP/P-CVM combined processing was approximately seven times greater than that during mechanical polishing.

Tech Support

Original Source

DOI: https://doi.org/10.1299/mej.15-00527