Material Removal on Hydrogen-Terminated Diamond Surface via AFM Tip-Based Local Anodic Oxidation

At a Glance

Metadata	Details
Publication Date	2025-08-26
Journal	Micromachines
Authors	Jinyan Tang, Zhongliang Cao, Zhongwei Li, Yuan-Liu Chen
Institutions	Zhejiang University
Analysis	Full AI Review Included

Executive Summary

This study introduces a novel and efficient method for material removal and surface modification on hydrogen-terminated (H-terminated) diamond using Atomic Force Microscope (AFM) tip-based Local Anodic Oxidation (LAO).

Novel Fabrication Regime: A new regime of material removal is achieved using high bias voltages (> -5 V), resulting in the detachment of the thin conductive surface layer.
Large-Area Removal: Material removal occurs over an area significantly larger than the AFM tip size. A depressed structure diameter of 340 nm was achieved using a tip with a radius of less than 30 nm.
Material Softening: The hardness of the diamond material surrounding the oxidation zone is substantially reduced, enabling subsequent mechanical scratching using a standard silicon tip (which is normally impossible on pristine diamond).
Mechanism: The process relies on localized heating and high current density caused by the large bias voltage. This induces oxidation (H replacement by O), leading to graphitization or amorphization, which weakens the bond between the conductive layer and the substrate.
Controllable Etch Depth: The removed layer thickness remained nearly constant at approximately 0.5 nm, consistent with the depth of the thin conductive H-terminated layer.
Scalability Potential: By connecting adjacent oxidized spots, efficient large-area material removal (e.g., 1.5 µm x 1 µm) was demonstrated, offering a pathway for enhanced diamond machinability and polishing.

Technical Specifications

Parameter	Value	Unit	Context
Sample Size	5 x 5 x 0.5	mm³	Single-crystal diamond, (100) plane
Initial Roughness (Ra)	~1	nm	After polishing
AFM Tip Radius	Less than 30	nm	Conductive Si tip with Pt coating
AFM Operation Mode	Contact	N/A	During oxidation
AFM Imaging Mode	Tapping	N/A	For post-oxidation topography
Normal Force (LAO)	40	nN	Applied during oxidation
Normal Force (Scratching)	200	nN	Used for testing hardness reduction
Bias Voltage Range (Protrusions)	-3 to -4	V	Low bias regime (artifact formation)
Bias Voltage Range (Removal)	-5 to -10	V	High bias regime (material removal)
Oxidation Duration (Spot)	1 to 4	s	Used for testing voltage/time dependence
Removed Layer Thickness	~0.5	nm	Consistent depth of the etched layer (Sample II)
Depressed Structure Diameter	340	nm	Achieved at -9 V, 1 s (vs. <30 nm tip radius)
Phase-Shifted Area Diameter	1.9	µm	Area surrounding the removal zone (Sample II, -9 V)
Nano-Groove Depth (Center)	1.0	nm	Scratch depth in the softened center of oxidized area
Nano-Groove Depth (Outer Edge)	0.5	nm	Scratch depth toward the outer region of oxidized area
Continuous Scan Speed	100	nm/s	Used for line oxidation experiments
Raman Peak	1332	cm^-1	Corresponds to sp³ bonding structure of diamond

Key Methodologies

The experimental process involved three main stages: sample preparation, mechanical control testing, and AFM tip-based local anodic oxidation (LAO).

1. Sample Preparation (H-Termination)

Initial Polishing: Diamond samples (5 x 5 x 0.5 mm³, (100) plane) were polished to an initial roughness of approximately Ra 1 nm.
Acid Cleaning: Samples were rinsed in an acidic solution (H₂SO₄ and KNO₃) at 300 °C for 30 minutes to remove surface impurities.
Plasma Etching (H-Termination): Samples were treated in a microwave-plasma-enhanced Chemical Vapor Deposition (CVD) chamber to achieve hydrogen termination.

Parameter	Chamber Cleaning	Sample I Recipe	Sample II Recipe
Temperature	850 °C	800 °C	800 °C
Gas Pressure	80 Torr	80 Torr	80 Torr
Microwave Power	2500 W	3000 W	2500 W
Etching Time	90 min	60 min	180 min
Gas Flow	1000 sccm	1000 sccm	100 sccm

Storage: Samples were transported under nitrogen packaging to preserve the unstable hydrogen termination.

2. Mechanical Control and Setup

Control Test: Mechanical scratching was performed on Sample I using a 500 nN normal force (256 passes, 200 nm/s velocity) to confirm that mechanical force alone does not affect the surface.
LAO Setup: A commercial AFM (Bruker Dimension Icon) was used in contact mode. Conductive silicon tips (Pt coated, R < 30 nm) served as the cathode (negative bias), and the sample was grounded (anode).
Force Reduction: The normal force during LAO experiments was reduced to 40 nN to minimize tip wear.

3. Local Anodic Oxidation (LAO) and Analysis

Low Bias Regime (Protrusion Study): Bias voltages from -1 V to -4 V were applied. Protrusions (0.2-0.5 nm high) were observed, attributed to absorbed water layer artifacts or increased electrostatic force.
High Bias Regime (Material Removal): Bias voltages from -5 V to -10 V were applied, resulting in large-area depressed structures.
Hardness Testing: Nano-grooves were fabricated on the oxidized area using the same silicon tip and a 200 nN normal force. The decreasing groove depth from the center (1 nm) to the outer edge (0.5 nm) confirmed a hardness gradient.
Large-Area Fabrication: Spot oxidations (-6 V, 3 s, 500 nm spacing) were connected in a 2 x 3 array to demonstrate efficient removal of a 1.5 µm x 1 µm surface layer.
Imaging: Post-oxidation topography and phase shifts were imaged using AFM tapping mode with the same tip used for oxidation.

Commercial Applications

This AFM tip-based LAO technique offers a precise, mask-less method for nanostructuring and surface modification of diamond, addressing the critical challenge of diamond machinability for advanced electronic and optical devices.

Industry/Application	Relevance to LAO Technique
Semiconductor Fabrication	Enables the formation of selective high-resistance regions at the micrometer scale, crucial for fabricating diamond metal-oxide-semiconductor field-effect transistors (MOFETs) and other microdevices based on H-terminated diamond conductivity.
Diamond Polishing/Planarization	The ability to efficiently remove the thin, conductive surface layer (0.5 nm) over large areas by connecting oxidized spots provides a potential method for ultra-smooth diamond surface polishing and defect removal.
Quantum Sensing	Diamond is essential for nitrogen-vacancy (NV) centers. Precise, localized surface modification is necessary for integrating NV centers into functional quantum sensors and optoelectronic devices.
High-Power Electronics	Diamond’s wide band gap and superior thermal conductivity make it ideal for next-generation high-power devices. The LAO method allows for the precise patterning required for device integration.
Nanofabrication	Offers a flexible, mask-less lithography technique for creating nanostructures on hard materials, overcoming the limitations of conventional etching processes on diamond.

View Original Abstract

Diamond is a promising next-generation semiconductor material, offering a wider band gap, higher electron mobility, and superior thermal conductivity compared with silicon. However, its exceptional hardness makes it challenging to fabricate. In this study, we demonstrate a novel approach to realize material removal on hydrogen-terminated diamond surfaces by atomic force microscope (AFM) tip-based local anodic oxidation. By adjusting both the applied voltage and hydrogen plasma etching parameters, the material is removed over an area larger than the AFM tip size. Notably, the hardness of the material surrounding the removal zone is significantly reduced, enabling it to be scratched with a silicon tip. These findings open a promising pathway for improving the machinability of diamonds in future device applications.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi16090981

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