Plasma-Treated PDMS
Wrinkle-Free Metal Films

Surface Modification | Stress Mismatch Reduction | Clean Patterning

        🎯 Project Type: Process Development & Failure Analysis

        🧪 Key Techniques: O₂ Plasma Treatment (RIE) | E-beam Evaporation | AZ nLof 2020 Negative Resist | Lift-Off

        🎭 Role: Sole process developer - Designed experiment, optimized plasma treatment, characterized results

        🔬 Key Finding: Plasma treatment before metal deposition creates silicate-like surface layer with higher Young's modulus, reducing thermal/mechanical mismatch and eliminating wrinkle formation

Figure 1: Smooth Au/Cr hybrid deposition via e-beam evaporation with cracks present after pretreating PDMS with strong plasma treatment (failed trial)

Wrinkle-Free Device — Figure 2: Wrinkle-free, crack-free Au/Cr hybrid deposition after optimizing the plasma treatment of the PDMS pre-deposition

Patterned Wafer — Figure 3: Patterned wrinkle-free 100um metal features design 1

Clean Patterns — Figure 4: Patterned wrinkle-free 100um metal features design 2

Microscope Inspection — Figure 5: Patterned wrinkle-free 200um metal features design 2

Final Device — Figure 6: Patterned wrinkle-free 100um metal features design 1

Project Overview

This project investigates the effect of O₂ plasma treatment on PDMS surfaces prior to metal deposition to suppress wrinkle formation in thin metal films. Unlike the tunable wrinkling project where thermal mismatch was intentionally used to create wrinkles, here plasma treatment modifies the PDMS surface properties—creating a stiffer silicate-like layer—that reduces the thermal and mechanical mismatch between the elastomer and the deposited Cr/Au film, resulting in smooth, wrinkle-free, crack-free metal patterns.

Problem Statement

Initial trials of metal deposition on untreated PDMS resulted in:

Wrinkle formation that interfered with device performance
Poor pattern fidelity during subsequent photolithography

Hypothesis

        💡 Hypothesis: O₂ plasma treatment of PDMS before metal deposition will create a silicate-like surface layer with higher Young's modulus than bulk PDMS. This stiffened surface layer will reduce the effective thermal and mechanical mismatch between the substrate and metal film, eliminating wrinkle and crack formation.
      

Before vs. After Plasma Treatment

❌ Without Plasma Treatment

Wrinkles
Poor pattern fidelity
Lift-off failures

✅ With Plasma Treatment

Smooth, wrinkle-free surface
No cracking
Clean pattern transfer
Successful lift-off

Modified Fabrication Process

Step 1: Substrate Preparation

Silicon wafer cleaned with acetone (120 sec), IPA (60 sec), DI water rinse, and SRD.

Step 2: PDMS Deposition & Curing

PDMS (10:1 ratio) spin-coated at 200 rpm → 1000 rpm. Cured at 80°C for 12 hours.

Step 3: CRITICAL STEP O₂ Plasma Treatment (Before Metal Deposition)

MARCH RIE System: O₂ plasma treatment performed on PDMS surface before E-beam evaporation.

Plasma Parameters (approximate):

Power: 100W
Pressure: 400 mTorr
O₂ Flow Rate: 60 sccm
Duration: 60-120 seconds

Surface Modification Mechanism:

O₂ plasma converts surface Si-CH₃ groups to Si-O-Si (silicate-like network)
Creates a thin, stiff "skin" layer with higher Young's modulus
Introduces polar functional groups (-OH, -COOH) for better metal adhesion
Reduces thermal expansion mismatch between PDMS and metal

Step 4: Metal Deposition (E-beam Evaporation)

Adhesion Layer: Chromium (150Å at 2 Å/s)
Conductor Layer: Gold (1000Å at 1.2 Å/s)
Measured thickness: Cr = 30.5 nm, Au = 115.7 nm
Note: Plasma treatment performed immediately before metal deposition (within 30 minutes) to maximize surface activation effect.

Step 5: Photolithography (Negative Resist)

AZ nLof 2020 Negative Resist: 500 rpm → 3000 rpm, 35 sec
Soft Bake: 70 sec at 110°C
UV Exposure: 80 mJ/cm² (OAI 800 Mask Aligner)
Post-Exposure Bake: 70 sec at 110°C
Development: AZ 300 MIF, 60 sec

Step 6: Metal Etch & Lift-Off

Gold Etchant: Remove exposed gold
Chromium Etchant: Remove exposed chromium
Remover PG Soak: 4 hours for clean lift-off

Proposed Mechanism: Why Plasma Treatment Suppresses Wrinkling

Property	Untreated PDMS	Plasma-Treated PDMS Surface
Surface Chemistry	Si-CH₃ (hydrophobic)	Si-O-Si (silicate-like, hydrophilic)
Young's Modulus	~1-3 MPa (bulk)	Higher (stiff "skin" layer)
Thermal Expansion Coefficient	~310 ppm/°C	Reduced (closer to metal)
Surface Energy	~20 mJ/m² (low)	>70 mJ/m² (high)
Metal Adhesion	Poor	Excellent

Why This Matters:

Reduced thermal mismatch: The stiffer plasma-treated layer better matches the thermal expansion of the metal film
Stress distribution: The stiff surface layer distributes thermal stress more uniformly
Improved adhesion: Polar functional groups (-OH, -COOH) form stronger bonds with deposited metal
No wrinkle nucleation: Without compressive stress concentration, wrinkles do not form

Results Summary

Parameter	Untreated PDMS	Plasma-Treated PDMS
Wrinkle Formation	Present	Eliminated
Metal Cracking	Present	Eliminated
Pattern Fidelity	Poor	Clean/Sharp
Lift-Off Success	Inconsistent	Reliable
Surface Roughness	High (wrinkled)	Low (smooth)

Equipment & Tools Used

Spin Coater (Headway Research)
Despatch Oven (PDMS curing)
MARCH RIE (O₂ Plasma Treatment)
CHA E-beam Evaporator (Cr/Au deposition)
OAI 800 Mask Aligner (UV exposure)
Hotplates (soft/post-exposure bake)
Develop Deck (AZ 300 MIF)
Wet Deck (Gold/Chromium etch)
Solvent Hood (Remover PG lift-off)
Olympus Optical Microscope (inspection)
Alpha Step Profilometer (thickness)

Materials Used

Substrate: Silicon wafer
Elastomer: PDMS (Sylgard 184, 10:1 ratio)
Plasma Gas: Oxygen (O₂)
Negative Resist: AZ nLof 2020
Adhesion Layer: Chromium (150Å)
Conductor: Gold (1000Å)
Developer: AZ 300 MIF
Etchants: Gold etchant, Chromium etchant
Lift-Off Solvent: Remover PG

Downloads

Key Insights & Learnings

Plasma treatment timing is critical: Metal deposition must occur within 30 minutes of plasma treatment (surface activation decays over time)
Stiffness gradient matters: The stiff plasma-treated surface layer acts as a buffer zone between soft PDMS and metal
Process control: Plasma parameters (power, time, pressure) must be carefully controlled to avoid over-treatment (which can cause surface cracking)
Failure analysis approach: Systematic troubleshooting identified plasma treatment as the key variable controlling wrinkle formation

Skills Acquired

O₂ Plasma Treatment (RIE) Surface Activation Stress Mismatch Engineering Failure Analysis PDMS Surface Modification E-beam Evaporation AZ nLof 2020 Processing Lift-Off on Flexible Substrates Process Optimization Microscope Inspection

Relationship to Tunable Wrinkling Project

This project represents the controlled suppression of wrinkle formation, complementing the tunable wrinkling project where wrinkles were intentionally created:

Aspect	Tunable Wrinkling Project	This Project (Plasma-Treated)
Objective	Create controlled wrinkles	Eliminate wrinkles
Plasma Treatment	After metal deposition (descum)	Before metal deposition
PDMS Surface State	Untreated (hydrophobic)	Plasma-treated (hydrophilic, stiffened)
Result	Wrinkled metal films	Smooth, flat metal films
Application	Stretchable electronics, tunable optics	High-fidelity patterning, sensors

Conclusion

This project successfully demonstrated that O₂ plasma treatment of PDMS surfaces prior to metal deposition effectively suppresses wrinkle and crack formation in thin Cr/Au films. The plasma treatment creates a stiff, silicate-like surface layer with higher Young's modulus and reduced thermal expansion mismatch with the deposited metal. Key findings include: (1) untreated PDMS results in cracked, wrinkled metal films; (2) plasma treatment before E-beam evaporation eliminates wrinkles and cracking; (3) the modified surface enables clean photolithography patterning and reliable lift-off; (4) plasma treatment timing is critical (deposition within 30 minutes). This process is essential for applications requiring smooth, flat metal patterns on PDMS for flexible sensors and electronics.

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