Mechanical and Electrical Properties of Free‐standing Polycrystal Diamond Membranes
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-06-28 |
| Journal | Advanced Science |
| Authors | Chenyu Wang, Dmitry Shinyavskiy, L. J. Suter, Zubaida Altikriti, Q. X. Jia |
| Institutions | University at Buffalo, State University of New York, State University of New York |
| Citations | 1 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study demonstrates a novel, scalable approach for synthesizing free-standing and transferable polycrystalline diamond membranes (PCDm), enabling access to the smoother bottom surface for advanced device integration.
- Novelty and Transferability: PCDm cantilevers (3.8 µm thick) were fabricated using MPECVD growth on Si/SiO2, followed by ICP-RIE etching, HF release, and micro-transfer printing onto SU-8/Si substrates. This method allows for the creation of both Top-Surface-Up (TSU) and Bottom-Surface-Up (BSU) configurations.
- Superior Mechanical Stability: All PCDm geometries exhibited consistently high elastic modulus (E), ranging from 911 GPa to 1014 GPa, approaching the stiffness of single-crystal diamond (≈1130 GPa).
- Surface Morphology: The BSU surface (the original growth interface) was significantly smoother (roughness < 90 nm) and had smaller grains (≈500 nm) compared to the TSU surface (roughness > 800 nm, grains ≈1200 nm).
- Bandgap Tuning: TSU-PCDm showed a lower bandgap (3.31 eV, XPS) attributed to boron incorporation near the top surface. BSU-PCDm showed a higher bandgap (3.98 eV, XPS) due to increased hydrogen content.
- Strain-Dependent Conductivity: TSU-PCDm exhibited a decrease in sheet resistance under strain, linked to structural disorder variations in sp2 carbon content. In contrast, BSU-PCDm maintained stable sheet resistance under the same strain conditions.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| PCDm Thickness (t) | 3.8 | µm | Measured via surface profilometer |
| Cantilever Lengths | 1200, 2000 | µm | Short and Long beams |
| Cantilever Widths | 150, 300 | µm | Short and Long beams |
| Elastic Modulus (E) | |||
| TSU-PCDm (Long) | 1014 | GPa | Highest measured stiffness |
| TSU-PCDm (Short) | 993 | GPa | |
| BSU-PCDm (Long) | 911 | GPa | Lowest measured stiffness |
| BSU-PCDm (Short) | 956 | GPa | |
| Surface Morphology | |||
| TSU Average Grain Size | ≈1200 | nm | Top surface (rougher) |
| BSU Average Grain Size | ≈500 | nm | Bottom surface (smoother) |
| Electrical/Optical Properties | |||
| TSU Bandgap (XPS) | 3.31 | eV | Lower bandgap due to boron incorporation |
| BSU Bandgap (XPS) | 3.98 | eV | Higher bandgap due to hydrogen content |
| TSU Sheet Resistance (Flat) | 60.19 | Ω/sq | Higher resistance, strain-dependent |
| BSU Sheet Resistance (Flat) | 49.7 | Ω/sq | Lower resistance, strain-stable |
| Strain Range Tested | -0.8 to 1.3 | % | Compressive to tensile strain |
| TSU sp2/sp3 Ratio (Raman) | 0.004 | - | Lower sp2 content |
| BSU sp2/sp3 Ratio (Raman) | 0.008 | - | Higher sp2 content |
Key Methodologies
Section titled “Key Methodologies”The free-standing PCDm cantilevers were fabricated using a multi-step process involving MPECVD growth, microfabrication patterning, and chemical release.
-
Diamond Film Growth (MPECVD):
- Substrate: (100) Si/SiO2 wafers, scratch-seeded with 0-2 µm diamond powder to enhance nucleation density.
- Gases: CH4 (8 sccm), H2 (385 sccm), B2H6 (7.5 sccm). (Boron doping was used.)
- Conditions: Pressure 60 Torr, forward power 7 kW.
- Temperature/Duration: 936 °C for 6 hours and 45 minutes.
- Result: Boron-doped diamond film with 3.8 µm thickness.
-
Patterning and Etching:
- A Cr (100 nm) / Ni (300 nm) bi-layered metal stack was deposited via e-beam evaporation to serve as a hard mask.
- Photolithography was used to define cantilever geometries (150 µm x 1200 µm and 300 µm x 2000 µm).
- Inductively Coupled Plasma - Reactive Ion Etching (ICP-RIE) was performed to selectively remove unmasked PCD portions.
-
Membrane Release and Transfer:
- The remaining metal mask was removed using a metal etchant.
- The sample was immersed in 49% hydrofluoric acid (HF) to dissolve the underlying SiO2 layer, releasing the cantilever-shaped PCDm.
- Transfer Printing: Released PCDm cantilevers were carefully transferred onto an SU-8 coated Si wafer using micro-transfer printing.
- Orientation Control: Selective flipping during transfer created both Top-Surface-Up (TSU) and Bottom-Surface-Up (BSU) cantilevers for comparative analysis.
-
Characterization Techniques:
- Mechanical: Atomic Force Microscopy (AFM) with a diamond-coated tip was used to measure force plots and calculate the elastic modulus (E) via cantilever bending tests.
- Structural/Compositional: X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and AFM (for roughness/grain size).
- Optical/Bandgap: X-ray Photoelectron Spectroscopy (XPS) and UV-vis absorption spectroscopy.
- Electrical/Strain: Transmission Line Method (TLM) patterns were fabricated on the PCDm, and I-V characteristics were measured under controlled compressive (-0.8%) and tensile (1.3%) strain conditions.
Commercial Applications
Section titled “Commercial Applications”The development of free-standing, transferable PCDm, leveraging its superior mechanical, electrical, and chemical properties, opens avenues for high-performance devices in several sectors:
- Advanced Microelectromechanical Systems (MEMS):
- Utilizing the ultra-high elastic modulus (E > 900 GPa) and mechanical stability for high-frequency resonators, robust pressure sensors, and accelerometers.
- High-Performance Power Electronics:
- Leveraging the wide bandgap and excellent charge transport characteristics for cost-effective, scalable power devices, potentially outperforming SiC and GaN.
- Bioelectronic and Chemical Sensors:
- Using the chemical inertness, biocompatibility, and ease of reshaping via microfabrication for active or conductive materials in implantable or diagnostic sensors.
- Ultraviolet (UV) Photodetectors:
- Exploiting the wide bandgap for high-performance, solar-blind UV detection, benefiting from the material’s scalability.
- Thermal Management:
- Integration into heat sinks and thermal spreaders, capitalizing on PCD’s exceptional thermal conductivity (up to 1800 W/mK).
View Original Abstract
Abstract In this study, we demonstrate a novel approach for synthesizing free‐standing and transferable polycrystalline diamond membranes (PCDm) to overcome these constraints, thus enabling a much wider spectrum of applications. Two types of PCDm cantilevers —Top‐Surface‐Up (TSU) and Bottom‐Surface‐Up (BSU) are fabricated, each with two different sets of dimensions: 150 µm (width) × 1200 µm (length) and 300 µm (width) × 2000 µm (length). Their mechanical and electrical properties are systematically investigated. Atomic Force Microscopy (AFM) analysis revealed that TSU‐PCDm has a higher elastic modulus than BSU‐PCDm, attributed to differences in grain size and defect distribution. Despite these differences, all PCDms in our work exhibit consistently high modulus values with minimal mechanical degradation across various cantilever geometries. Bandgap measurements using X‐ray Photoelectron Spectroscopy (XPS) and UV-vis absorption spectroscopy indicated a lower bandgap for TSU‐PCDm due to boron incorporation, while BSU‐PCDm exhibited a higher bandgap due to increased hydrogen content. Electrical characterization showed that the sheet resistance of TSU‐PCDm decreases under strain, whereas BSU‐PCDm maintains stable resistance. These findings unveil the material properties of PCDm and their potential usage for myriad diamond‐based electronic applications.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2015 - Traditional Machining Processes: Research Advances