Thermal Evaporation System Guide
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Thermal Evaporation System — How It Works & How to Choose Components
Thermal evaporation is one of the most widely used thin-film deposition techniques, essential in:
- Semiconductor device fabrication
- Metal contact deposition
- Optical coatings
- Research thin films
- Perovskite and organic electronics
- Device prototyping
A well-designed evaporation system extends lifetime, improves film quality, and reduces contamination.
This guide explains the physics, required components, and how to choose the right configuration.
How Thermal Evaporation Works
- A metal source (Au, Al, Ti, Cr…) is placed in a crucible or boat.
- Electrical heating raises the temperature until the metal evaporates.
- Under high vacuum (10⁻⁵–10⁻⁶ Torr), atoms travel in straight lines.
- Atoms condense on the cooler substrate, forming a thin & uniform film.
Why vacuum is essential:
- Prevents oxidation
- Prevents scattering
- Ensures directional transport
- Increases deposition rate
- Reduces contamination
Required Vacuum Level
| Film Quality | Required Vacuum |
|---|---|
| Standard metal coating | 10⁻⁵ Torr |
| Oxidation-sensitive metals | 10⁻⁶ Torr |
| Ultra-clean films | 10⁻⁷ Torr |
Rule:
Use Turbo + Rotary pump for any evaporator.
Key Components of a Thermal Evaporation System
A complete system includes:
✔ 1. Vacuum Chamber
- Stainless steel
- KF/CF ports
- With quartz or sapphire windows (optional)
- Optional substrate rotation
✔ 2. Evaporation Source (Crucible / Boat / Coil)
Common choices:
| Material | Use |
|---|---|
| Tungsten boat | Al, Ag, Cu, etc. |
| Molybdenum boat | Low contamination metals |
| Graphite crucible | High-melting materials |
| Al₂O₃ crucible | Oxide source materials |
| BN crucible | Ultra-clean, high-temp |
✔ 3. Power Supply
Typically 100–500 A depending on metal.
Key parameters:
- Max current
- Stability
- Fast ramp control
- Overcurrent protection
✔ 4. Feedthrough (Electrical)
High-current feedthrough required:
- 50–500 A
- Water-cooling optional
- KF or CF mounted
✔ 5. Substrate Holder
Options:
- Fixed
- Heated (100–300°C)
- Rotating
- Mask holders (shadow mask, photomask)
✔ 6. Thickness Monitor (Quartz Crystal Microbalance, QCM)
QCM allows:
- Real-time thickness measurement
- Deposition rate control
- Higher reproducibility
Choosing Crucibles & Boats (Very Important)
| Source | Best Crucible Material |
|---|---|
| Au, Al, Ag | Tungsten boat |
| Ti, Cr | Molybdenum boat |
| Oxides | Alumina or BN crucible |
| High-temp metals | Graphite crucible |
| Organic materials | Al₂O₃ or low-temp boats |
Deposition Distance & Geometry
Recommended distance:
➡ 10–25 cm from source to substrate.
Effects:
- Too close → rough films, particle generation
- Too far → slow deposition, non-uniformity
Use line-of-sight geometry for best uniformity.
Common Problems & Solutions
Problem: Particles on film
Cause: Overheated boat
Fix: Lower power, preheat gently.
Problem: Non-uniform thickness
Fix: Increase distance or add substrate rotation.
Problem: Contamination
Fix: Use BN or Mo boats; improve vacuum.
Problem: Film peeling
Fix: Pre-clean substrate; use adhesion layer (e.g., Ti under Au).
Frequently Asked Questions
Q1. What vacuum level is required for thermal evaporation?
10⁻⁵–10⁻⁶ Torr.
Q2. Which crucible material should I choose?
W boats for metals, Mo for reactive materials, BN for ultra-clean films.
Q3. Why is a quartz crystal monitor needed?
It enables real-time rate & thickness control.
Q4. Can thermal evaporation deposit oxides?
Yes—with BN or alumina crucibles.
Q5. What pump system is recommended?
Turbo + rotary backing pump.