Physical Vapor Deposition (PVD)
Sputtering, evaporation, and depositing metal films
Sputtering
Sputtering
Sputtering is the dominant PVD technique in semiconductor manufacturing. It physically ejects atoms from a solid target material using energetic ion bombardment:
- A plasma is created from an inert gas (usually argon)
- Argon ions are accelerated toward the target (a disk of the desired material)
- Ions knock atoms off the target surface (like billiard balls)
- Ejected atoms travel across the vacuum chamber and deposit on the wafer
Common sputtered films in semiconductor fabs:
- TaN/Ta: Barrier/liner for copper interconnects
- Copper seed: Thin copper layer for electroplating
- Titanium/TiN: Contact barriers and hard masks
- Aluminum: Bond pads and some interconnect layers
Key Concept: Magnetron Sputtering
Modern PVD tools use magnetron sputtering — magnets behind the target confine the plasma close to the target surface, dramatically increasing deposition rate and efficiency. Ionized PVD (iPVD) goes further, ionizing the sputtered atoms for better directionality.
Evaporation
Evaporation
Evaporation is the other PVD family. Instead of bombarding a target with ions, the source material is simply heated until atoms boil off and travel ballistically to the wafer in a high vacuum (10⁻⁶–10⁻⁸ Torr):
- Thermal evaporation: A resistively heated boat (W, Mo) melts and vaporizes the source. Simple and cheap, but limited to low-melting metals (Al, Au, Ag, Cu).
- E-beam evaporation: A focused electron beam (5–10 keV) heats a small spot in a water-cooled crucible. Can evaporate refractory metals (Ti, Pt, W) and oxides.
Compared with sputtering, evaporation gives:
- Higher purity — no working gas, so films contain very little background contamination
- Lower step coverage — the source is point-like and atoms travel in straight lines, so vertical sidewalls are barely coated
- Easy lift-off compatibility — the directional flux leaves a clean break between resist and film, making evaporation popular in MEMS, photonics, and III-V research labs
Key Concept: Where Evaporation Survives
Mainstream silicon CMOS fabs use sputtering almost exclusively because they need conformality and uniformity over 300 mm. Evaporation remains the workhorse for compound-semiconductor research, MEMS contacts, optical coatings, and lift-off patterning where directionality is actually a feature.
Knowledge Check
Knowledge Check
1 / 2What gas is typically used to create plasma in sputtering?