Achieving micron-scale plasticity and theoretical strength in Silicon

At a Glance

Metadata	Details
Publication Date	2020-05-29
Journal	Nature Communications
Authors	Ming Chen, László Pethö, Alla S. Sologubenko, Huan Ma, Johann Michler
Institutions	Swiss Federal Laboratories for Materials Science and Technology, ETH Zurich
Citations	79
Analysis	Full AI Review Included

Technical Analysis: Achieving Theoretical Strength and Micron-Scale Plasticity in Diamond-Structured Materials

Source Paper: Chen et al. (2020), Achieving micron-scale plasticity and theoretical strength in Silicon (Nature Communications).

Executive Summary

This study details a breakthrough in surface engineering of diamond-cubic materials, demonstrating that eliminating surface damage via advanced lithographic processing achieves ultrahigh elastic strain limits and near-theoretical strength in silicon. These findings have critical implications for the reliability and performance enhancement of diamond-based microelectromechanical systems (MEMS) and electronic components.

Ultrahigh Strength Achievement: Lithographically-etched Si pillars achieved critical resolved shear stress (TCRSS) of approximately 4 GPa, approaching the ideal theoretical strength (6.8 GPa DFT prediction).
Enhanced Plasticity Scale: Enhanced plasticity was extended into the micron-scale (up to 3.5 µm diameter), an order of magnitude larger than previously observed in samples prepared using Focused Ion Beam (FIB) machining.
Surface Quality is Critical: Superior surface quality (damage-free, near-pristine) achieved through advanced RIE etching and multi-step wet cleaning/oxidation was the key enabler, promoting reliable random surface dislocation nucleation.
Mechanistic Insight: The transition in deformation mechanisms—from full dislocations (preferred at nanoscale/high stress) to partial dislocations (dominating at micron scale/lower stress)—was observed at ambient temperature, a finding relevant to all diamond-cubic/zincblende materials.
FIB Damage Mitigation: The study confirms that the amorphous surface layer and defects induced by FIB milling severely restrict dislocation nucleation, leading to premature brittle failure compared to clean lithographically processed structures.
Application Driver: This research validates a robust surface engineering pathway for fabricating high-strength, high-reliability SCD and PCD structures for next-generation micro-devices and strain-engineered functional components.

Technical Specifications

Extraction of key performance and mechanical parameters from the Si microcompression study.

Parameter	Value	Unit	Context
Critical Resolved Shear Stress (TCRSS)	~4	GPa	Lithographic Si pillars, ambient temperature, (100) orientation
Theoretical Strength (DFT)	6.8	GPa	Density Functional Theory prediction for Si
Strength Comparison (Litho vs. FIB)	50-85	%	Lithographic pillars exhibited 50-85% higher strength
Maximum Plastic Diameter (Litho)	3.5	µm	Pillars exhibited measurable plastic deformation
Maximum Plastic Strain Achieved	0.3	%	Engineering plastic strain observed at micro-scale (3.5 µm D)
Elastic Strain Limit (Max Uniaxial)	0.09	N/A	Highest limit achieved, demonstrating ultrahigh elasticity
Strain Rate (Microcompression)	5 × 10^-4	s^-1	Constant intrinsic displacement control rate
Size Effect Exponent (n, Litho)	0.081	N/A	Indication of relatively weak size effect for clean surfaces
Pillar Crystalline Orientation	(100)	N/A	Used for primary microcompression tests
Dislocation Transition Trigger	Size / Stress	N/A	Transition from full to partial dislocations observed at ambient T

Key Methodologies

The core value of the lithographic processing pathway lies in the rigorous multi-step surface preparation designed to produce a near-pristine, damage-free surface, crucial for surface-mediated dislocation nucleation.

Material and Orientation: Single-crystal Si wafers (100 mm diameter, (100) orientation, P type/Boron-doped) were utilized.
Patterning:
- Large Pillars (D > 2 µm): Direct UV laser writing (VPG 200/MLA 150) on photoresist.
- Small Pillars (D ≤ 1 µm): High-resolution Electron-beam (E-beam) writing (Vistec EBPG5000) on E-beam sensitive resist.
Reactive Ion Etching (RIE) Processing:
- Deep Etching (D > 2 µm): Alternating Bosch process cycles of SF₆ (etch) and C₄F₈ (passivation), resulting in slightly scalloped sidewalls.
- Small Pillar Etching (D ≤ 2 µm): Simultaneous SF₆ and C₄F₈ gas mixture, avoiding the Bosch process scalloping and leading to straighter sidewalls.
Critical Surface Cleaning Sequence: (Designed to remove RIE residues and structural damage):
- Residue Removal: Standardized solution of ammonium hydroxide and hydrogen peroxide to remove fluorocarbon residues.
- Contamination Removal: Mixture of sulfuric acid and hydrogen peroxide to remove ionic contaminants.
- Thermal Oxidation (Damage Consumption): Wet atmospheric thermal oxidation was performed to grow an oxide layer (up to 2 µm for large pillars) that consumed the damaged surface Si and incorporated residual contaminants.
- Final Etch: Immersion in Hydrofluoric Acid (HF) bath to strip the sacrificial oxide layer, yielding a near-pristine, smooth, damage-free Si surface.
Mechanical Testing: Microcompression performed in situ in a SEM using a diamond flat punch tip under a constant strain rate of 5 × 10^-4 s^-1.

6CCVD Solutions & Capabilities

The findings regarding ultra-high strength, micron-scale plasticity, and the critical role of surface quality in diamond-cubic structures are directly applicable to optimizing MPCVD diamond materials. 6CCVD provides the specialized SCD and PCD necessary to replicate and extend this research for high-performance applications.

The key limitation identified in the paper for achieving theoretical strength (FIB-induced amorphous surface layers) is overcome by 6CCVD’s superior material quality and advanced polishing capabilities, which ensure a pristine, high-integrity surface analogous to the lithographically-etched Si.

Applicable Materials & Specifications

Requirement/Application	6CCVD Material Recommendation	Relevant Specification
High-Strength Microstructures (MEMS/Sensors)	Optical Grade Single Crystal Diamond (SCD)	Highest purity; orientation control (e.g., (100) for analogous Si studies); minimal defects for superior mechanical integrity.
Strain Engineering/Electronic Devices	Thin Film Single Crystal Diamond (SCD)	Thickness control from 0.1 µm up to 500 µm, allowing precise control over structural dimensions and elastic strain limits.
High-Performance Structural Substrates	MPCVD Polycrystalline Diamond (PCD)	Plates/wafers available up to 125 mm diameter; essential for large-scale production of high-strength components.
Electrochemical/Sensing Applications	Boron-Doped Diamond (BDD)	Provides the necessary electronic characteristics for integrated functional components requiring structural robustness.

Customization Potential for Replication & Extension

To fully leverage the mechanical insights from this research—especially the critical dependency on surface state—6CCVD offers bespoke services that meet the rigorous demands of micro-scale structural testing and device fabrication:

Pristine Surface Quality: 6CCVD guarantees ultra-smooth SCD surfaces with roughness Ra < 1 nm and large-area PCD surfaces with Ra < 5 nm. This level of polish eliminates the mechanical instability caused by surface defects and amorphous layers (as observed in the FIB samples).
Custom Dimensions and Etching Alignment: We provide MPCVD wafers in standard sizes up to 125 mm (PCD) and custom-cut dimensions. Our precision laser cutting services enable the pre-structuring of diamond material to fit specific micro-pillar or micro-device geometry requirements, ensuring crystal orientation alignment (e.g., (100)) is maintained relative to the compression axis.
Integrated Device Prototyping: Replication of next-generation devices often requires complex electrical interfaces. 6CCVD offers extensive in-house metalization services including deposition of Au, Pt, Pd, Ti, W, and Cu, providing stable, high-quality contacts critical for functional components.

Engineering Support & Logistics

6CCVD’s in-house PhD engineering team specializes in the material science of diamond-cubic structures and can assist researchers in selecting the optimal MPCVD diamond product (SCD, PCD, BDD) for projects requiring extreme mechanical robustness and high-strength MEMS/micro-device fabrication.

We ensure global material availability with robust shipping logistics (DDU default, DDP available upon request).

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Abstract As the backbone material of the information age, silicon is extensively used as a functional semiconductor and structural material in microelectronics and microsystems. At ambient temperature, the brittleness of Si limits its mechanical application in devices. Here, we demonstrate that Si processed by modern lithography procedures exhibits an ultrahigh elastic strain limit, near ideal strength (shear strength ~4 GPa) and plastic deformation at the micron-scale, one order of magnitude larger than samples made using focused ion beams, due to superior surface quality. This extended elastic regime enables enhanced functional properties by allowing higher elastic strains to modify the band structure. Further, the micron-scale plasticity of Si allows the investigation of the intrinsic size effects and dislocation behavior in diamond-structured materials. This reveals a transition in deformation mechanisms from full to partial dislocations upon increasing specimen size at ambient temperature. This study demonstrates a surface engineering pathway for fabrication of more robust Si-based structures.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41467-020-16384-5