Ultra-Precision Replication Technology for Fabricating Spiral-Structure Metamaterial

At a Glance

Metadata	Details
Publication Date	2020-08-28
Journal	Frontiers in Physics
Authors	Weiguo Zhang, Guodong Zhu, Xiaoqiang Zhu, Chunlei Du
Institutions	Chongqing Institute of Green and Intelligent Technology, Chongqing University
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research presents an ultra-precision replication technology for fabricating Spiral-Structure Metamaterial (SSM), crucial for terahertz (THz) optics applications. The method combines diamond-based ultra-precision turning for mold creation with subsequent compression molding of Polyethylene (PE).

Core Value Proposition: Achieved high-efficiency, low-cost bulk production of SSM elements, overcoming the material and depth limitations of traditional microlithography (etching).
High Precision: The final PE SSM product demonstrated a surface roughness (Ra) of 3.1 nm and a step height error of only 0.63% relative to the designed 30 µm wavelength.
Efficiency: The molding replication process for a 50 mm diameter SSM element was completed in less than 10 minutes, enabling high-throughput manufacturing.
Mold Optimization: The Angular Feeding Method for diamond turning proved superior for mold fabrication, yielding a surface roughness of 4.1 nm and a pattern surface error (PV) of 700 nm, significantly better than the Radial Feeding Method.
Repeatability: The molding process showed excellent stability, with step height repeatability confirmed at 0.25% variation over 50 production batches.

Technical Specifications

Parameter	Value	Unit	Context
Target Wavelength (λ)	30	µm	Designed SSM function (THz range)
Target Total Step Depth	55.56	µm	Required depth difference for SSM
Final PE SSM Step Height	55.37	µm	Measured result after molding
Step Height Error (vs. λ)	0.63	%	Error relative to designed wavelength (30 µm)
Surface Roughness (Ra)	3.1	nm	Final PE SSM product quality
Mold Surface Roughness (Ra)	4.1	nm	Achieved using Angular Feeding Method
Mold Material	7075	Aluminum	Substrate for diamond turning
Product Material	Polyethylene (PE)	N/A	Refractive index (n) = 1.54 (THz)
Molding Repeatability	0.25	%	Step height variation over 50 batches
Molding Pressure	0.22	MPa	Applied during heat preservation
Molding Temperature (T_max)	130	°C	Maximum temperature reached

Key Methodologies

The fabrication relies on a two-stage process: ultra-precision mold fabrication and subsequent compression molding replication.

1. Diamond Ultra-Precision Turning (Mold Fabrication)

Substrate: 7075 Aluminum was used due to its excellent machinability with diamond cutters.
Tool Path Algorithm: The Angular Feeding Method was selected for its superior accuracy and efficiency for the specific SSM geometry (50 mm diameter).
Turning Scheme (Angular Feeding):
- One-step roughing (40 µm depth).
- One-step semi-finishing (10 µm depth).
- One-step finishing (5 µm depth).
Process Parameters: Turning speed was 20 rpm; feed speed ranged from 2 mm/min (finishing) to 10 mm/min (roughing).
Result: Mold processing time was 20 minutes, achieving Ra of 4.1 nm.

2. Compression Molding (PE SSM Replication)

Fixture Design: A specialized fixture was used to fix the mold and PE substrate, preventing lateral sliding and ensuring uniform vertical pressure transmission.
Anti-Adhesion Layer: The aluminum mold surface was coated with a thin, fluorinated polymer layer using the steam plating coating method to facilitate clean demolding.
Temperature and Pressure Cycle:
1. Assembly: Mold and PE substrate assembled in the fixture.
2. Preheating: Temperature increased gradually to 130 °C (maintaining 1 min hold per increment).
3. Molding: 0.22 MPa pressure applied for 5 minutes at 130 °C.
4. Cooling: Refrigerated to below 40 °C at a controlled rate of 20 °C/min.
Demolding: Elements were cooled to room temperature, and separation was performed vertically using mechanical devices to minimize shear stress and damage.

Commercial Applications

This ultra-precision replication technology is highly relevant for the industrialization of complex micro-optics, particularly in the terahertz regime.

Terahertz (THz) Optics: Enables the mass production of high-precision THz components, including:
- THz Lenses and Phase Plates.
- THz Beam Splitter Prisms.
- THz Plane Mirrors.
Advanced Communication: Fabrication of SSM elements that modulate electromagnetic plane waves into spiral waves, critical for controlling the orbital angular momentum (OAM) state in quantum optics and communication systems.
Sensing and Detection: Provides high-quality, low-cost phase plates necessary for advanced THz sensing and imaging systems.
Manufacturing Efficiency: The high-throughput, low-cost replication process makes previously expensive, lab-scale THz components viable for commercial and industrial applications.

View Original Abstract

Spiral-structure metamaterial (SSM) is of great importance, however, there are fewer methods to fabricate SSM due to limitations of material particularity and working accuracy. In this paper, a systematic scheme for fabricating SSM is proposed by employing the metal mold making with diamond-based ultra-precision turning technique and then molding replication method. By studying the path planning algorithm of the turning, molding error law, and a technique of how to compensate for the error, a solution for SSM is consequently formed. Our experimental results show a satisfying SSM with a surface roughness under 5 nm and a surface shape error under 0.63% of the designed wavelength (30 um). Moreover, this SMM element is processed within 10 min, with low cost materials and processes. Based on these advantages, our SSM processing scheme shows a remarkable potential in precise fabricating phase plates and industrialized application of terahertz metamaterial in the future.

Tech Support

Original Source

DOI: https://doi.org/10.3389/fphy.2020.00267

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