Superconductivity in planarised nanocrystalline diamond films

At a Glance

Metadata	Details
Publication Date	2017-03-23
Journal	Science and Technology of Advanced Materials
Authors	Georgina M. Klemencic, Soumen Mandal, Jessica M. Werrell, S. R. Giblin, Oliver A. Williams
Institutions	Cardiff University
Citations	15
Analysis	Full AI Review Included

Superconductivity in Planarised Nanocrystalline Diamond Films: A 6CCVD Technical Analysis

Focus: Superconducting Boron-Doped Nanocrystalline Diamond (B-NCD) for High-Frequency Nanoelectromechanical Systems (NEMS).

Executive Summary

This research validates Chemical Mechanical Polishing (CMP) as a critical, non-destructive planarization technique for superconducting Boron-Doped Nanocrystalline Diamond (B-NCD) films grown by Microwave Plasma CVD (MPCVD).

Roughness Reduction: Surface roughness (RMS) was drastically reduced from 44.0 nm (as-grown) to an ultra-smooth 1.5 nm after 14 hours of CMP, eliminating the surface limitations inherent in columnar growth.
Preserved Superconductivity: Crucially, the superconducting transition temperature ($T_c$) of 4.2 K was fully retained, demonstrating that the chemo-mechanical process does not introduce damaging subsurface defects or significantly alter boron distribution.
High-Quality NEMS Fabrication: Achieving an ultra-smooth surface (< 2 nm RMS) while maintaining superconductivity enables the fabrication of high-fidelity superconducting NEMS devices with minimized intrinsic energy losses due to surface scattering.
MPCVD Material Validation: The B-NCD film was grown using standard MPCVD techniques, confirming the scalability and reproducibility of the base material for industrial quantum technology applications.
6CCVD Advantage: 6CCVD specializes in heavily Boron-Doped PCD (B-NCD) and offers custom polishing services that meet or exceed the roughness requirements demonstrated, accelerating research translation into commercial devices.

Technical Specifications

The following hard data points were extracted from the MPCVD growth process and the subsequent CMP planarization results:

Parameter	Value	Unit	Context
Initial RMS Roughness	44.0	nm	As-grown 520 nm B-NCD film
Final RMS Roughness	1.5	nm	Achieved after 14 hours of CMP
Roughness Goal	< 2	nm	Target for high-quality NEMS
Superconducting Transition Temp (T_c)	4.2	K	Maintained after polishing
Film Thickness (B-NCD)	520	nm (0.52 µm)	Determined by pyrometric interferometry
Substrate Diameter	2	inches (51 mm)	Standard (100) Silicon wafer
Buffer Layer	500	nm	Silicon Dioxide (SiO₂)
Methane Concentration (CH₄)	3	%	In H₂ gas mix
Boron/Carbon Ratio (B/C)	12,800	ppm	Heavy doping using Trimethyl-Boron
Growth Temperature	~720	°C	Measured in situ
Reactor Pressure	40	Torr	MPCVD chamber conditions
Microwave Power	3.5	kW	Seki AX6500 Series
Polishing Down-Load Force	2 (13.8)	psi (kPa)	CMP parameter
Slurry Feed Rate	40	ml min^-1	CMP parameter (Alkaline Colloidal Silica)
Nucleation Site Density	> 10¹¹	cm^-2	Nanodiamond colloid seeding

Key Methodologies

The experimental success hinges on two tightly controlled processes: the MPCVD growth of heavily doped NCD and the specialized CMP planarization.

1. B-NCD Film Growth (MPCVD Recipe)

Seeding: Substrates were seeded by ultrasonic agitation in a monodisperse aqueous colloid of nanodiamond particles to achieve extremely uniform nucleation density (> 10¹¹ cm^-2).
Substrate Stack: 500 µm thick, 2” diameter (100) silicon wafers were used, incorporating a 500 nm thick SiO₂ buffer layer.
Gas Phase: A dilute gas mixture of 3% methane (CH₄) and 12,800 ppm trimethyl-boron (for doping) in hydrogen (H₂) was employed.
Deposition Parameters: The reactor was held at 40 Torr pressure and powered by 3.5 kW of microwave energy, resulting in an in situ substrate temperature of approximately 720 °C.
Post-Growth: Films were cooled down in a hydrogen plasma to ensure surface quality.

2. Chemical Mechanical Polishing (CMP Recipe)

Polishing System: Logitech Tribo system utilized a soft polyurethane impregnated polyester felt pad.
Slurry: An alkaline colloidal silica polishing slurry (Logitech SF-1) was used. The chemo-mechanical action minimizes fracture damage common in traditional mechanical polishing.
Pad Conditioning: Prior to polishing, the felt pad was conditioned for 30 minutes using diamond grit embedded in a chuck to optimize texture, ensuring the grit did not contact the wafer surface.
Force Control: A light down-load force of 2 psi (13.8 kPa) was applied, alongside a 20 psi (138 kPa) pneumatic back pressure for wafer bow correction.
Process Duration: Total polishing time was 14 hours, conducted intermittently to measure resistance and roughness decrease.
Cleaning: After each step, the film underwent an SC-1 clean for 20 minutes to remove residual slurry and debris.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-specification diamond materials required to replicate or advance this foundational research into quantum nanomechanics. Our capabilities directly address the material and processing demands outlined in the paper.

Applicable Materials

The successful replication of this study requires highly controllable, heavily doped Polycrystalline Diamond (PCD) films.

Requirement from Paper	6CCVD Material Solution	Custom Specification Match
Superconducting Material	Boron-Doped Diamond (BDD)	Custom B/C ratios up to 20,000 ppm are available for optimizing $T_c$.
Nanocrystalline Film Structure	Polycrystalline Diamond (PCD)	Controlled grain size growth recipes for specific NEMS frequencies and mechanical quality factors.
Thin Film Thickness	PCD 0.1 µm - 500 µm	The required 520 nm (0.52 µm) thickness is a standard, repeatable process thickness for 6CCVD.

Customization Potential for Scaling

This research used 2-inch wafers. 6CCVD enables rapid scaling for commercial and advanced engineering requirements:

Large Area Films: We provide custom PCD and BDD plates/wafers up to 125 mm in diameter, exceeding the 2” limitation of the research paper and facilitating higher throughput wafer-scale NEMS fabrication.
Ultra-Low Roughness Polishing: While the paper used external CMP, 6CCVD’s internal planarization capabilities for PCD already achieve RMS roughness < 5 nm (for inch-sized wafers) or highly specific CMP protocols, reducing the extensive post-processing time demonstrated in the research.
Integration Support: If the resulting NEMS devices require electrode integration, 6CCVD offers in-house custom metalization services (Au, Pt, Pd, Ti, W, Cu) crucial for SQUID integration and quantum measurement systems described in the paper.

Engineering Support

The use of highly smooth, superconducting B-NCD is pivotal for advancing quantum nanomechanical devices, where surface roughness contributes directly to energy loss mechanisms.

Our in-house PhD engineering team provides authoritative consultation on material selection, doping optimization, and surface preparation protocols to ensure optimal mechanical and electrical performance for Quantum Nanomechanics and Superconducting NEMS projects. We ensure the delivered material meets the stringent surface finish requirements (e.g., Ra < 2 nm) necessary for low-dissipation applications.

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

View Original Abstract

Chemical vapour deposition (CVD) grown boron-doped nanocrystalline diamond (B-NCD) is an attractive material for the fabrication of high frequency superconducting nanoelectromechanical systems (NEMS) due to its high Young’s modulus. The as-grown films have a surface roughness that increases with film thickness due to the columnar growth mechanism. To reduce intrinsic losses in B-NCD NEMS it is crucial to correct for this surface roughness by polishing. In this paper, in contrast to conventional polishing, it is demonstrated that the root-mean-square (RMS) roughness of a 520 nm thick B-NCD film can be reduced by chemical mechanical polishing (CMP) from 44.0 nm to 1.5 nm in 14 hours without damaging the sample or introducing significant changes to the superconducting transition temperature, [Formula: see text], thus enabling the use of B-NCD films in the fabrication of high quality superconducting NEMS.

Tech Support

Original Source

DOI: https://doi.org/10.1080/14686996.2017.1286223