E-Beam Evaporation and Lift-Off

Cr/Au deposition parameters

Why high vacuum. Lowering pressure reduces collisions between metal atoms and other molecules, increasing the mean free path (MFP), the average distance a particle travels before colliding with another. That gives highly directional, line-of-sight deposition, which lift-off needs. Non-directional deposition can coat sidewalls, leaving metal "wings" or preventing lift-off entirely.

Lower pressure → longer MFP → fewer collisions → more directional deposition

Why Cr first. Au adheres poorly to many oxide surfaces, so a thin Cr layer forms a strong interface between the substrate and the Au film. The slow 0.2 Å/s rate gives precise thickness control, stable initial nucleation, and better film quality for this interface-critical layer.

Why Au faster. The higher 1 Å/s rate keeps throughput reasonable and deposition stable, and cuts processing time for the much thicker Au layer.

How e-beam evaporation works

1. Electron emission. A filament inside the e-beam source is heated until electrons escape (thermionic emission).
2. Acceleration. The filament is held at a large negative potential relative to the grounded source, accelerating the electrons.
3. Magnetic steering. A magnetic field bends the beam toward the metal source.
4. Impact. Energetic electrons strike the source and their kinetic energy becomes heat, making the spot extremely hot.
5. Melting and evaporation. The metal goes from solid to liquid to vapor.
6. Beam sweep. The beam is swept across the source instead of fixed on one point. This spreads the heat, keeps the molten region controlled, stabilizes evaporation, and avoids hitting the crucible wall.
7. Atoms leave the source. Once the metal's vapor pressure is high enough, atoms escape the liquid surface and form a vapor cloud.
8. Ballistic transport. With very few background gas molecules at such low pressure, atoms travel in straight lines.
9. Nucleation. Atoms reach the wafer and adsorb, forming small islands that grow.
10. Continuous film. As more metal arrives, the islands merge into a continuous thin film.

Running the Temescal

1. Prepare the lift-off pattern with the LOR5B / AZ 1518 bilayer.
2. Load the sample. Vent and open the chamber, mount the sample on the holder, check the O-ring, and check the metal sources.
3. Verify the Cr and Au source pockets.
4. Close the chamber and pump down: atmosphere → rough pumping → high-vacuum pumping → 10⁻⁶ Torr → keep pumping to at least 4 × 10⁻⁷ Torr.
5. Program the deposition controller with the thickness and rate.
6. Select the Cr pocket.
7. Enable e-beam power. Filament → electrons → HV acceleration → magnetic deflection → Cr.
8. Ramp the Cr source. The beam heats the Cr gradually, avoiding dumping too much power into a cold source. The Temescal uses programmed rise and soak steps before deposition.
9. Keep the shutter closed until the Cr rate stabilizes.
10. Open the shutter and deposit Cr. The quartz crystal monitor tracks rate and thickness.
11. Close the shutter.
12. Switch to Au and repeat steps 8–11.
13. Shut down the beam and let the source, crucible, and chamber cool.
14. Vent and remove the wafer.

Lift-off

1. Place the wafer in heated Remover PG, which dissolves the resist under the unwanted metal.
2. The metal on top of the resist lifts away.
3. Sonication adds mechanical agitation to remove leftover metal and resist fragments.

Tool anatomy: Temescal BJD-1800