E-beam evaporation vs sputtering for lift-off metallisation

Lift-off is the simplest way to pattern metal in a research cleanroom: expose and develop resist, deposit the film over everything, then dissolve the resist so the metal on top of it floats away. Whether that last step works cleanly depends less on the chemistry than on how the metal arrived.

E-beam evaporation and sputtering both put down good films, but they deliver atoms to the wafer in very different ways, and that difference decides whether your lift-off gives sharp electrodes or torn edges and "ears" along every line. If you are choosing between a used e-beam evaporator and a sputter system for lift-off work, this is the part of the decision that matters most.

Why lift-off needs a broken film

Lift-off only works if the metal on the resist and the metal on the substrate are not connected. The solvent has to reach the resist through a gap at the edge of each feature. If the deposited film coats the resist sidewall, it forms a continuous skin from the top of the resist down to the substrate. The solvent then has nowhere to get in, or it gets in and the metal tears along a random line instead of the pattern edge.

So the goal is a discontinuous film: metal on top of the resist, metal on the substrate, and nothing on the sidewall in between. Two things help. The first is a resist profile with an undercut or a re-entrant sidewall, so the top edge of the resist shadows the base. The second is a deposition that arrives at the wafer in straight lines from a small source, so the shadow stays sharp. E-beam evaporation provides the second almost by default. Sputtering mostly does not.

How e-beam evaporation deposits

In an e-beam evaporator the source is a small molten spot in a crucible, heated by a focused electron beam. The chamber runs at high vacuum, typically in the 10⁻⁶ to 10⁻⁷ mbar range during deposition. At that pressure the mean free path of the evaporated atoms is far longer than the distance from source to wafer, so atoms travel in straight lines without colliding with gas molecules.

The result is a highly directional flux from what is close to a point source. Placed far enough away and facing the source, the wafer sees metal arriving almost perpendicular to its surface. The undercut in the resist casts a clean shadow, the sidewall stays bare and lift-off in acetone or a commercial remover is usually quick.

A few practical points follow from this geometry:

• Throw distance matters. A longer source-to-substrate distance means more directional arrival and better lift-off, at the cost of lower rate and more material on the chamber walls.
• Planetary fixtures help uniformity but change the angle. A planetary or dome holder tilts and rotates wafers to even out thickness. If the wafer is not facing the source, the arrival angle shifts across the run, which can put metal on one side of each resist sidewall. Lift-off domes designed so the wafers face the source are the usual answer.
• Radiant heat reaches the resist. The molten source radiates heat, and high-melting metals at high rates can soften or reflow resist. Lower rates, shutter pauses or a cooled fixture help on thick films.
• The beam produces X-rays. Electrons hitting the crucible generate X-rays, which can damage gate oxides on MOS structures. For most research metallisation this is not an issue, but it is worth knowing if you deposit on finished devices.

Platforms like the Temescal FC-2000 were built around this process: a multi-pocket e-beam source, a large box chamber and fixtures sized for batch lift-off.

How sputtering deposits

Sputtering works differently. A plasma, usually argon, runs at a pressure in the 10⁻³ to 10⁻² mbar range. Ions from the plasma strike a target and knock atoms off it. At that pressure the sputtered atoms collide with gas atoms on the way to the wafer, so they arrive from a wide range of angles, and the target itself is a large area source rather than a point.

That is exactly what you want for step coverage over topography, and exactly what you do not want for lift-off. Sputtered metal coats resist sidewalls. Even with an undercut profile, some material reaches the base of the sidewall, and the film can bridge from the resist top to the substrate.

Sputtering also brings energy to the substrate. Ion bombardment, secondary electrons and radiant heat from the target can harden the resist surface, which makes it harder to strip, and can cross-link it enough that the remover struggles.

When sputtering can still work for lift-off

Sputtering is not ruled out. Many labs that own only a sputter system do lift-off routinely, but they adapt the process around it:

• A deep undercut. A bilayer stack with a lift-off resist under an imaging resist gives a pronounced undercut that sputtered metal has trouble reaching. Image-reversal and negative resists designed for lift-off also give re-entrant profiles.
• Thin films. Lift-off with sputtering is more forgiving for a few tens of nanometres than for several hundred.
• Lower pressure and longer throw. Running at the low end of the stable pressure range and increasing the target-to-substrate distance makes the flux more directional, at the cost of rate.
• Ultrasonic or heated strip. A more aggressive strip can break a thin sidewall film, though it may also lift fine features you wanted to keep.

A combined tool such as the Kurt J. Lesker PVD 75, which can be configured for sputtering, thermal or e-beam evaporation depending on configuration, lets a lab use evaporation for lift-off and sputtering for blanket films and alloys in the same chamber family.

Choosing between them

For a lab whose main metal work is lift-off patterning of contacts, electrodes and alignment marks, an e-beam evaporator is usually the more reliable choice. The table sets out the trade-offs that come up most often.

If you also need dielectric films, alloys such as TiW, or films with good coverage over steps, a sputter system is the better second tool. Many teaching cleanrooms end up with both, and use each for what it does well.

What to check on a used tool for lift-off

If you are buying a used e-beam evaporator for lift-off, inspect the parts that decide directional deposition and repeatable rates:

• Base pressure and pump-down time. A tired cryopump or diffusion pump extends cycle time and raises the background pressure during deposition.
• E-beam gun and sweep. Check the filament assembly, the beam sweep controller and the pocket indexer. Poor sweep leads to crucible damage and spitting, which leaves particles in your lift-off pattern.
• Crucible liners and pocket wear. Cracked or alloyed pockets contaminate films.
• Quartz crystal monitor. Confirm the sensor head, feedthrough and controller work, and that tooling factors are recorded.
• Fixtures. Ask whether the tool has a lift-off dome or only a flat or planetary fixture, and whether the dome geometry matches the source position.
• Shields and chamber coating. Heavy build-up flakes onto wafers; a proper chamber refurbishment strips and bead-blasts the shields before the tool goes back into use.

For a sputter system intended for lift-off, the key checks are the range of stable plasma pressure, the available target-to-substrate distance and whether the substrate holder is cooled.

What we can do

We inspect and refurbish e-beam evaporators and sputter systems for research and pilot-line use, and we can configure fixtures and sources around a lift-off process. If you are deciding between platforms, send us your metals, film thicknesses and resist process and we will suggest which tool fits. See the Temescal FC-2000 for a typical lift-off evaporator, or read how we approach chamber refurbishment.