Effect of Deposition Voltage on Tensile Properties of Single Crystal Silicon Microstructure Fully Coated by Plasma CVD DLC Film

At a Glance

Metadata	Details
Publication Date	2017-01-01
Journal	The Proceedings of Mechanical Engineering Congress Japan
Authors	Wenlei Zhang, Akio Uesugi, Yoshikazu Hirai, Toshiyuki Tsuchiya, Osamu Tabata
Institutions	Aichi Institute of Technology, Kyoto University
Analysis	Full AI Review Included

Technical Analysis and Commercial Solutions: High-Strength MPCVD Diamond Coatings

6CCVD analyzes the research on the effect of plasma CVD bias voltage on the mechanical properties and reliability of Diamond-Like Carbon (DLC) coated silicon microstructures. This research is highly relevant to applications requiring high mechanical reliability and extreme hardness, aligning directly with 6CCVD’s expertise in customized, high-performance MPCVD (Microwave Plasma Chemical Vapor Deposition) diamond materials.

Executive Summary

The following points summarize the methodology and critical findings regarding the mechanical enhancement of silicon microstructures via DLC coating:

Objective: Investigate the influence of substrate bias voltage (ranging from -200 V to -600 V) during Plasma CVD of 150 nm DLC films on the resulting film structure and the tensile strength of underlying silicon MEMS specimens.
Structural Modification: Increased negative bias voltage effectively modified the DLC film structure, resulting in a decrease in sp² phase content and hydrogen content, and a corresponding increase in sp³ (diamond) phase content.
Mechanical Enhancement: The structural modification led to a significant increase in film hardness (up to 10.53 GPa) and internal residual compressive stress (up to 1.88 GPa).
Optimal Strength: The DLC coating resulted in a 13.2% to 29.6% improvement in the average tensile strength of the silicon microstructure compared to the bare silicon baseline.
Peak Performance: The maximum average tensile strength recorded was 3.94 GPa, achieved using a -400 V deposition bias.
Improved Reliability: Increasing the bias voltage substantially increased the Weibull modulus (m), indicating a crucial reduction in the deviation (scatter) of strength and thus enhancing the mechanical reliability of the microstructures.
Mechanism: The high compressive residual stress induced by the DLC coating is hypothesized to suppress the initiation and propagation of micro-cracks in both the silicon substrate and the DLC film itself.

Technical Specifications

Parameter	Value	Unit	Context
DLC Film Thickness	150	nm	Uniform coating on Si microstructures
Si Specimen Length	120	µm	Design dimension
Si Specimen Width	4	µm	Design dimension
Si Specimen Thickness	5	µm	Design dimension
Si Crystal Orientation	(100)	N/A	Surface plane of SOI wafer
Tensile Axis Direction	<110>	N/A	Test orientation
Test Temperature	RT	°C	Room Temperature, atmospheric environment
Loading Rate	0.75	µm/s	Piezo stage displacement speed
Hardness (Max)	10.53	GPa	Achieved at -600 V bias
Residual Stress (Max)	1.88	GPa	Compressive stress at -600 V bias
Tensile Strength (Bare Si)	3.04	GPa	Baseline average strength
Tensile Strength (Max)	3.94	GPa	Achieved at -400 V bias (29.6% increase)
Weibull Modulus (Bare Si)	8.81	N/A	Reliability indicator
Weibull Modulus (Max)	18.42	N/A	Achieved at -600 V bias (Improved reliability)

| DLC Film Properties vs. Bias Voltage (Table 1 Data) | | :--- | :--- | | Bias (V) | I_D/I_G | m/I_G (x 10^-6) | Hardness (GPa) | Residual Stress (-GPa) | | -200 | 1.08 | 0.78 | 6.36 | 0.47 | | -300 | 1.03 | 0.34 | 7.44 | 1.00 | | -400 | 0.87 | 0.22 | 8.09 | 1.13 | | -500 | 0.84 | 0.17 | 9.56 | 1.42 | | -600 | 0.82 | 0.12 | 10.53 | 1.88 |

Key Methodologies

The experiment utilized Plasma Enhanced Chemical Vapor Deposition (PECVD) coupled with detailed material characterization and statistical mechanical testing.

Specimen Fabrication:
- Single Crystal Silicon (SCS) tensile test chips were fabricated from SOI (Silicon-on-Insulator) wafers using MEMS processing techniques.
- Design dimensions were 120 µm length, 4 µm width, and 5 µm thickness.
PECVD Process (ACV-1060 System):
- Substrate Cleaning: Argon (Ar) ion plasma cleaning was performed for 60 seconds using a -400 V bias voltage to prepare the Si surface.
- Adhesion Layer (SiC Gradient): A thin SiC interlayer was deposited for 20 seconds using Tetramethylsilane (TMS) at a flow rate of 30 sccm to improve adhesion between Si and the subsequent DLC layer.
- DLC Deposition: DLC films (150 nm thick) were deposited for 90 seconds using Acetylene gas (150 sccm).
- Bias Variation: Substrate bias voltage was systematically varied across five conditions: -200 V, -300 V, -400 V, -500 V, and -600 V.
Material Characterization:
- Composition: Raman spectroscopy (488 nm wavelength) was used to determine film composition (sp² vs. sp³ ratio, I_D/I_G) and hydrogen content (using the slope of the baseline, m/I_G).
- Hardness: Measured using nanoindentation (0.2 mN load).
- Residual Stress: Calculated using the Stoney equation based on substrate curvature measurement.
Mechanical Testing:
- A static charge type uniaxial tensile testing machine was used.
- Twenty specimens were tested for each bias condition.
- Tensile strength and statistical reliability (Weibull modulus, m) were calculated from the failure data.

6CCVD Solutions & Capabilities

This research successfully demonstrates how specialized coatings can significantly enhance the mechanical strength and reliability of microstructures. 6CCVD excels in providing CVD diamond materials that offer intrinsic properties superior to DLC, often required for MEMS/NEMS, high-power electronics, and demanding mechanical applications.

Applicable Materials for High-Reliability Structural Applications

For research seeking the highest possible material hardness, thermal stability, and mechanical reliability, 6CCVD recommends transitioning from DLC to true MPCVD diamond materials:

Ultra-High Purity Single Crystal Diamond (SCD): Offers the highest intrinsic hardness (Vickers Hardness: 80-100 GPa, compared to 6-10 GPa for DLC) and lowest defect density. Ideal for high-stress optical windows, ultimate micro-abrasive tools, or high-performance electronic substrates where reliability is paramount.
High-Quality Polycrystalline Diamond (PCD): Excellent for large-area mechanical reinforcement or heat spreader applications. 6CCVD offers high-integrity PCD wafers up to 125 mm in diameter, capable of thicknesses matching or exceeding the study’s substrate (up to 500 µm).
Boron-Doped Diamond (BDD): Highly relevant if the research requires integrated electrodes or sensors alongside mechanical strength, as BDD offers metal-like conductivity with diamond’s hardness.

Customization Potential & Engineering Services

6CCVD’s specialized fabrication and processing services are perfectly suited to replicate or advance the experimental methodology described in this paper:

Research Requirement	6CCVD Capability	Benefit to Client
Microstructure Coating	Custom Thickness: SCD/PCD films available from 0.1 µm to 500 µm.	Provides capability to study the effect of diamond thickness (beyond 150 nm) on residual stress and tensile enhancement.
MEMS Dimensions	Precision Laser Cutting: High-accuracy machining for micro-scale geometries.	Allows precise replication of the 120 µm x 4 µm microstructures or scaling to unique MEMS/NEMS device sizes.
Material Interconnection	Custom Metalization: In-house deposition of Ti, Pt, Au, Cu, Pd, W films.	Necessary for future integration of electronics (strain gauges, heating elements) onto the reinforced structures.
Surface Finish	Advanced Polishing: Polishing down to Ra < 1 nm (SCD) or Ra < 5 nm (PCD).	Crucial for minimizing surface defects, which are often the initiation points for failure in tensile tests (critical for high Weibull modulus).
Statistical Reliability	Expert Material Selection: Support for structural applications requiring low failure probability.	Our in-house PhD team provides consultation to select the diamond material best suited to maximize hardness, internal stress management, and Weibull modulus for specific high-reliability projects.

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View Original Abstract

This paper reports on the effect of deposition bias voltage on tensile properties of single crystal silicon microstructure fully coated by plasma CVD diamond like carbon (DLC) film. DLC film with the thickness of 150 nm was uniformly coated on silicon microstructure of 120 μm long, 4μm wide and 5 μm thick. The result shows that by increasing of bias voltage, sp2 phase and hydrogen content decreased, while sp3 phase increased. The average tensile strength of DLC coated structure shows 13.2-29.6% higher values compared to that of the bare silicon structure and the -400 V bias voltage gave the highest strength of 3.94 GPa. Moreover, the Weibull modulus increased with higher bias voltage, which indicates smaller deviations in strength.

Tech Support

Original Source

DOI: https://doi.org/10.1299/jsmemecj.2017.j2210103