Metamaterial Fabrication Process Guide

Fabrication Overview

Metamaterial fabrication requires precise control at the nanoscale to achieve the desired electromagnetic response. This guide covers the complete fabrication workflow from substrate preparation to final characterization, with detailed parameters for each process step.

Substrate Preparation - Cleaning and surface treatment

Resist Coating - Spin coating of e-beam resist

E-beam Lithography - Pattern writing and development

Metal Deposition - Evaporation or sputtering

Lift-off Process - Pattern transfer

Post-processing - Annealing and protection

1. Electron Beam Lithography

Substrate Preparation

Materials: Silicon, Quartz, or Glass substrates

• Ultrasonic cleaning in acetone (5 min)
• Ultrasonic cleaning in IPA (5 min)
• DI water rinse and N₂ blow dry
• Piranha clean (H₂SO₄:H₂O₂ = 3:1) at 120°C for 10 min
• DI water rinse (5 cycles)
• Dehydration bake at 180°C for 5 min

For better adhesion, use HMDS vapor priming at 150°C for 30 seconds

Resist Coating

Resist Type	Spin Speed	Thickness	Bake Temp	Bake Time
PMMA 950K A4	4000 rpm	200 nm	180°C	90 sec
PMMA 950K A2	6000 rpm	100 nm	180°C	90 sec
ZEP520A	5000 rpm	300 nm	180°C	120 sec
HSQ (6%)	4000 rpm	100 nm	150°C	120 sec

Allow samples to cool for at least 2 minutes after baking before loading into e-beam system

E-beam Writing Parameters

Feature Size	Beam Current	Dose (PMMA)	Dose (HSQ)	Step Size
< 50 nm	100 pA	400 μC/cm²	2000 μC/cm²	2 nm
50-100 nm	300 pA	350 μC/cm²	1500 μC/cm²	5 nm
100-500 nm	1 nA	300 μC/cm²	1000 μC/cm²	10 nm
> 500 nm	5 nA	250 μC/cm²	800 μC/cm²	20 nm

Proximity Effect Correction:

• Use dose modulation for dense patterns
• Apply 10-20% dose reduction for inner features
• Consider using BEAMER or similar software for complex patterns

Development Process

For PMMA:

• Developer: MIBK:IPA (1:3) at 21°C
• Development time: 60 seconds with gentle agitation
• Stopper: IPA for 30 seconds
• Rinse: DI water and N₂ blow dry

For HSQ:

• Developer: 25% TMAH or MIF-319
• Development time: 60 seconds
• Rinse: DI water (3× for 30 seconds)
• Critical point drying recommended for high aspect ratio features

Optional oxygen plasma descum (10W, 30 seconds) to remove residual resist

2. Metal Deposition

Electron Beam Evaporation

Material	Base Pressure	Rate	Thickness	Adhesion Layer
Gold (Au)	< 5×10⁻⁷ Torr	0.5-1 Å/s	30-50 nm	Ti/Cr (3-5 nm)
Silver (Ag)	< 5×10⁻⁷ Torr	1-2 Å/s	30-50 nm	Ti (3 nm)
Aluminum (Al)	< 1×10⁻⁶ Torr	1-3 Å/s	20-40 nm	Not required
Copper (Cu)	< 5×10⁻⁷ Torr	1-2 Å/s	30-50 nm	Ti/Cr (5 nm)

Sample rotation during deposition improves uniformity but may affect lift-off quality

Sputtering (Alternative Method)

DC Magnetron Sputtering Parameters:

• Base pressure: < 5×10⁻⁷ Torr
• Ar pressure: 3-5 mTorr
• Power: 50-100 W (DC)
• Deposition rate: 0.2-0.5 nm/s
• Substrate temperature: Room temperature

Sputtering provides better step coverage but may cause resist heating issues

3. Lift-off Process

Standard Lift-off Procedure

For PMMA-based processes:

1. Immerse in acetone at 50°C
2. Ultrasonic agitation (low power, 1-2 min intervals)
3. Total time: 30 min to 2 hours depending on pattern density
4. Transfer to fresh acetone for final cleaning
5. IPA rinse (30 seconds)
6. DI water rinse and N₂ blow dry

For difficult lift-off:

• Use heated NMP (80°C) or Remover PG
• Gentle pipette flushing near pattern edges
• Extended soak time (overnight if necessary)

Never use high-power ultrasonication directly - it can damage delicate features

4. Alternative Fabrication Techniques

Focused Ion Beam (FIB) Milling

For 3D metamaterial structures and post-processing:

• Ion source: Ga⁺ at 30 kV
• Beam current: 10-100 pA for fine features
• Milling rate: ~0.3 μm³/nC for Si
• Pattern generator: Stream file with dwell time control

Nanoimprint Lithography

For large-area metamaterial fabrication:

• Thermal NIL: 140-180°C, 40 bar, 5-10 min
• UV-NIL: 365 nm exposure, 10-20 mW/cm², 60 sec
• Resist: mr-I 7020E or similar
• Anti-sticking layer on mold: Fluorinated SAM

Two-Photon Polymerization

For 3D metamaterial structures:

• Laser: 780 nm femtosecond, 100 fs pulses
• Objective: 100× oil immersion, NA = 1.4
• Photoresist: IP-S or IP-Dip
• Writing speed: 10-100 μm/s
• Layer spacing: 200-500 nm

5. Quality Control & Characterization

Inspection Methods

Scanning Electron Microscopy (SEM):

• Acceleration voltage: 1-5 kV (to minimize charging)
• Working distance: 5-10 mm
• Magnification: 10,000× - 100,000×
• Conductive coating: 2-3 nm Au/Pd if needed

Atomic Force Microscopy (AFM):

• Mode: Tapping mode for delicate structures
• Tip: Si cantilever, k = 40 N/m
• Scan rate: 0.5-1 Hz
• Resolution: < 1 nm vertical, < 10 nm lateral

Critical Dimension Measurements

Parameter	Target	Tolerance	Measurement Method
Line width	100 nm	±5 nm	SEM cross-section
Gap size	50 nm	±3 nm	High-res SEM
Film thickness	30 nm	±2 nm	AFM/Ellipsometry
Periodicity	400 nm	±10 nm	SEM/FFT analysis

6. Common Issues & Solutions

Pattern Distortion

Problem: Features appear stretched or compressed

Solutions:

• Recalibrate e-beam write field
• Check for stage drift during writing
• Verify beam stigmation and focus
• Use smaller write fields for better accuracy

Poor Adhesion

Problem: Metal peeling during or after lift-off

Solutions:

• Increase adhesion layer thickness
• Improve substrate cleaning protocol
• Use oxygen plasma treatment before deposition
• Reduce deposition rate

Incomplete Lift-off

Problem: Metal fence or wings around features

Solutions:

• Use bilayer resist process
• Increase resist thickness
• Ensure proper undercut profile
• Optimize development time

Required Equipment

Raith EBPG5000+ E-beam Writer Kurt J. Lesker PVD75 Evaporator AJA Orion Sputter System Zeiss Supra 55VP SEM Bruker Dimension Icon AFM Laurell WS-650 Spin Coater YES-310TA Vapor Prime Oven Branson CPX2800 Ultrasonicator Class 100 Cleanroom Fume Hoods for Wet Chemistry