Fabrication of Multiscale 1-Octadecene Monolayer Patterned Arrays Based on a Chemomechanical Method

At a Glance

Metadata	Details
Publication Date	2022-05-30
Journal	Processes
Authors	Liqiu Shi, Feng Yu, Zhouming Hang
Institutions	Zhejiang University of Water Resource and Electric Power
Analysis	Full AI Review Included

Executive Summary

This research details a highly controllable and efficient chemomechanical method (CM) for fabricating multiscale (micro/nano) patterned 1-octadecene Self-Assembled Monolayers (SAMs) on silicon substrates.

Core Achievement: Successful one-step patterning and functionalization of hydrogen-terminated Si surfaces using a diamond tip in 1-octadecene solution.
Mechanism: The CM process combines “top-down” mechanical scribing with “bottom-up” chemical self-assembly. Scribing breaks Si-H bonds, generating Si free radicals that covalently bond (Si-C) with 1-octadecene molecules.
Chemical Verification (XPS): Analysis confirmed successful grafting: the C1s peak significantly increased (from 18.0 AT% to 50.0 AT%), while the Si2p and O1s peaks decreased, confirming the replacement of Si-O bonds with Si-C bonds.
Wettability Control: The surface hydrophilicity was significantly reduced. The water contact angle increased from 25° (bare Si) and 77° (Si-H surface) to 102° after SAM formation.
Pattern Fidelity: AFM confirmed the formation of regular, clear rectangular arrays (10 µm x 2 µm) covered by a homogeneous lattice structure in the scribed areas.
Nanofriction Performance: The SAM surface exhibited low surface activity and adhesion energy, resulting in a friction force that changed slowly with increasing normal load, demonstrating stable tribological characteristics.
Application Potential: The method is fast and convenient, enabling the creation of specific three-dimensional structures suitable for hydrophobic fences and masks in wet etching.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	(100) n-boron	N/A	Silicon wafer
Substrate Resistivity	0.001~0.004	Ω·cm	Silicon wafer
Monolayer Material	1-Octadecene	N/A	Analytical reagent
Pattern Dimensions (Rectangle)	10 x 2	µm	Length x Width
Array Spacing	2	µm	Between patterned rectangles
Scribing Depth (Target)	~10	nm	Controlled depth of diamond tip
AFM Scanning Rate	2.0	Hz	Used for morphology and friction analysis
Water Contact Angle (Bare Si)	25	°	Before pretreatment
Water Contact Angle (Si-H Surface)	77	°	After hydrogen termination
Water Contact Angle (1-Octadecene SAM)	102	°	After self-assembly (Hydrophobic)
C1s Content (After Assembly)	50.0	AT%	XPS analysis (Significant increase)
O1s Content (After Assembly)	27.6	AT%	XPS analysis (Decrease due to Si-O bond breaking)
Si-C Bond Energy (XPS)	100.375	eV	Si2p peak attribution after assembly
Si-O Bond Energy (XPS)	103.375	eV	Si2p peak attribution before assembly

Key Methodologies

The fabrication process involves rigorous silicon pretreatment to achieve a hydrogen-terminated surface, followed by controlled chemomechanical scribing in solution.

1. Silicon Pretreatment (Hydrogen Termination)

Initial Cleaning: Silicon wafer ultrasonically cleaned sequentially with acetone, ethanol, and ultra-pure water (5 min each).
Oxide Etch (1): Etched in 5% HF solution for 5 min.
Heating/Cleaning: Heated in a mixture of water, concentrated hydrochloric acid, and hydrogen peroxide (4:1:1 volume ratio) for 20 min.
Oxygen Removal: Nitrogen gas injected into a semiconducting pure 40% NH₄F solution for 20 min.
Oxide Etch (2): Substrate placed in NH₄F solution for 10 min to ensure complete oxide layer removal.
Final Rinse: Rinsed thoroughly with ultra-pure water to yield the hydrogen-terminated silicon surface (Si-H).

2. Chemomechanical Self-Assembly

System Setup: A controllable self-assembly micromachining system was used, featuring a 3D high-precision stage (PI), CCD, three-way dynamometer (KISTLER), and a diamond tool.
Environment Control: The entire system was maintained in a nitrogen airtight environment.
Immersion: The Si-H sample was immersed in a pool of 1-octadecene solution.
Scribing Parameters: Prewritten programs drove the 3D stage to scratch the surface using the diamond tool.
- Pattern: Rectangular arrays (10 µm length, 2 µm width).
- Spacing: 2 µm.
- Depth: Controlled at approximately 10 nm.
Chemical Reaction: Mechanical stress from the diamond tip breaks the surface Si-H bonds, creating highly reactive silicon free radicals. These radicals immediately react with the unsaturated C=C double bonds of the surrounding 1-octadecene molecules, forming stable Si-C covalent bonds and resulting in a patterned SAM.

Commercial Applications

This chemomechanical patterning technique offers significant advantages in precision, speed, and stability for various engineering fields.

Micro/Nanoelectronics Fabrication:
- Creating highly controllable, localized functionalization patterns on silicon wafers for advanced device architectures.
- Realizing cross-scale micro/nano monolayer arrays for next-generation integrated circuits and sensors.
Surface Protection and Stability:
- Grafting stable alkyl chains (Si-C bonds) onto silicon surfaces, providing superior resistance to oxidation, hot acid, water, and air compared to native Si-H surfaces.
Microfluidics and Lab-on-a-Chip Devices:
- Using the patterned SAMs (WCA 102°) as hydrophobic fences to precisely define and control liquid flow paths within microchannels.
Wet Etching and Lithography:
- Utilizing the chemically stable SAM patterns as masks in wet etching processes, enabling high-resolution feature transfer onto the silicon substrate.
Tribology and MEMS/NEMS:
- Modifying the friction characteristics of micro-electromechanical systems (MEMS) components by creating surfaces with low adhesion energy and stable friction coefficients under varying loads.

View Original Abstract

A controlled and self-assembled micromachining system was built to fabricate a mico/nanoscale monolayer patterned array on a silicon surface using a diamond tip. The process was as follows: (1) we preprocessed a silicon wafer to obtain a hydrogen-terminated silicon surface; (2) we scratched three rectangular arrays of 10 μm × 3 μm with a spacing of 2 μm on the silicon surface with a diamond tip in 1-octadecene solution; the Si-H bonds were broken, and silicon free radicals were formed; (3) the 1-octadecene molecules were connected with silicon atoms based on Si-C covalent bonds, and the 1-octadecene nano monolayer was self-assembled on the patterned arrays of the silicon surface. Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Sessile water contact angles were used to detect and characterize the self-assembled monolayers (SAMs). The XPS results showed that the Si2p peak and the O1s peak were significantly decreased after self-assembly; however, the C1s peak was successively significantly increased. Sessile water contact angles showed that the hydrophilicity was weakened after the formation of 1-octenecene SAMs on the silicon substrate. The nanofriction of the sample was measured with AFM. The change in nanofriction also demonstrated that the SAMs were formed in accordance with the patterned array. We demonstrated that, by using this method, self-assembled multiscale structures on silicon substrate can be formed quickly and conveniently.

Tech Support

Original Source

DOI: https://doi.org/10.3390/pr10061090

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