Lift-off is a method of patterning a target material (typical a metal) using a sacrificial layer (typically photoresist) to define the pattern. First, the sacrificial layer is applied and patterned; then the target material is deposited on top. The final step is the removal of the sacrificial material which lifts off the overlying target material.
|Equipment||Solvent Bench 14,Solvent Bench 84, Solvent Bench 83|
The main benefit of lift-off is the ability to pattern a wide variety of materials, especially inert and hard to etch metals. It also avoids damage and compatibility issues that can arise with etching. The limitations include difficult lift-off in the spacing between lines, edge effects, and redeposition of material onto the sample. All of these can be mitigated with proper preparation and processing.
Solvent Bench 14
- This bench is used for 4" and 6" silicon and glass wafers
- 2 ultrasonic heated baths
Solvent Bench 83
- This bench is used for pieces, 4" and 6" silicon and glass wafers
- no ultrasonic baths
Solvent Bench 84
- This bench is used for pieces, long-term soaking, and exotic materials
- 3 ultrasonic heated baths
Method of operation
The most common type of lift off is using photoresist as the sacrificial layer and dissolving it in a solvent. This Lifts off the overlying target material leaving the desired pattern behind.
Lift-off results can be improved by heating the resist in a solvent which causes the resist to swell and break any connection to the patterns. Ultrasonic agitation can help break up the metal and remove any connections between the metal on the resist and the remaining patterns.
For good liftoff you need a discontinuity in the film. Ideally this happens at the interface between the substrate and the bottom of the photoresist. Otherwise you will still have some of your sidewalls attached to the final metal. This will give you a raggedy edge and could lead to shorts. You can improve the quality of your liftoff by keeping the following information in mind while designing your masks and process.
Ratio between photoresist and deposited metal thickness
- The metal must be thinner than the thickness of the resist.
- Positive resists, ratios larger than 5:1 are preferable.
- Negative and LOR resists can have a smaller ratio
Angle of photoresist/formation of discontinuous films
The photoresist (PR) profile has a major impact on the quality of your lift-off. To resolve the smallest gaps and have the cleanest edges you need to have a PR pattern that when the metal is deposited it will create a discontinuous metal film so the solvent that is dissolving the PR can cleanly remove the unwanted metal. This is achieved by having either a negative profile (an overhang of the top of the resist relative to the bottom) or a step profile. In the LNF there are two options for creating these profiles:Image reversal and Lift-off resist.
|Positive Profile||Negative/Image Revesal Profile||LOR Profile|
Metal deposition technique
- If photoresist is used as the lift-off material, it requires that the metal deposition is completed with the substrate held at near room temperature (<100C) otherwise the sacrificial layer will be compromised.
- Line of sight deposition
- If sample perpendicular to source minimal sidewall coating.
- This makes it ideal for liftoff as it is easier to create discontinuous films.
- Preferred technique for liftoff
- Not all materials can be evaporated
- Sputter deposition:
- Higher conformity because of random angle deposition
- Requires negative or step profile for good liftoff
Metal Adhesion to the substrate
Metal adhesion problems during liftoff are often traced back to scum or photoresist left on the substrate after Lithography. This can be taken care of with an descum O2 plasma after the photoresist develop step.
- LNF Tech Talk for Lift-Off is Coming Soon!
- MicroChemicals Lift-off websiteː https://www.microchemicals.com/technical_information/lift_off_photoresist.pdf
- LNF wiki page about LORː http://lnf-wiki.eecs.umich.edu/