"Lift-off" is a simple, easy method for patterning films that are deposited. A pattern is defined on a substrate using photoresist. A film, usually metallic, is blanket-deposited all over the substrate, covering the photoresist and areas in which the photoresist has been cleared. During the actual lifting-off, the photoresist under the film is removed with solvent, taking the film with it, and leaving only the film which was deposited directly on the substrate.
Depending on the type of lift-off process used, patterns can be defined with extremely high fidelity and for very fine geometries. Lift-off, for example, is the process of choice for patterning e-beam written metal lines. Because film sticks only where photoresist is cleared, the defect modes are opposite what one might expect for etching films (for example, particles lead to opens, scratches lead to shorts in metal lift-off.)
Any deposited film can be lifted-off, provided:
There are three basic ways in which lift-off can be performed: 1. Standard photoresist processing; 2. LOL 2000 processing; 3. Surface-modified photoresist processing. The method you choose will depend on your process requirements. For more info about lift-off processes, check out chapter 6.1.7 of Micromachined Transducers Sourcebook by Greg Kovacs. A good website (which also lists primary references) is the Swedish Nanometer Lab.
STANDARD PHOTORESIST PROCESSING:
This is the easiest method, because it involves only one mask step and the photolithography is completely standard. The main disadvantage of this method is that film is deposited on the sidewall of the photoresist and will generally continue to adhere to the substrate following resist removal. This sidewall may peel off in subsequent processing, resulting in particulates and shorts, or it may flop over and interfere with etches or depositions that follow.
Prior to film deposition, particularly for sputtering or evaporation processes, a post-develop bake is recommended. This will drive off excess solvent so that there will be less outgassing during the film deposition. However, bake should not too long or at too high a temperature, otherwise resist will reflow slightly.
The film should be deposited as usual. Lift-off can be accomplished by immersing in acetone. The length of time for lift-off will depend on the film quality (generally, the higher the film quality, the more impermeable it is and the longer it will take to lift-off.) Depending on how robust the film and substrate are, sidewalls from deposited film can be removed using a gentle swipe of a clean-room swab or a directed stream of acetone from a squeeze bottle. As a rule, keep the substrate immersed in acetone until all the film has been lifted-off and there are no traces of film particulates -- once particles dry on the substrate, they are notoriously difficult to remove.
a.) Substrate with AZ3612 resist.
b.) Following expose and develop.
c.) Following film deposition.
d.) Following lift-off.
e.) Following mechanical "scrub".
*Modification of Standard Photoresist Processing (from Yahong Yao, useful if feature size is not critical):
LOL2000 DUAL LAYER RESIST PROCESSING:
LOL2000 is an inert, non-UV-sensitive polymer, which can be etched with most standard developers. The LOL2000 is first spun on the wafer and baked, then standard photoresist, such as AZ3612, is spun on and baked. The AZ3612 is exposed as usual, and then the substrate is developed. The standard LDD26W developer will clear the exposed AZ3612 areas, but will also "etch" away the LOL2000, leading to undercutting of the photoresist. It is this overhang that prevents sidewall deposition of film. The resist can be lifted-off using acetone or a resist removal solvent (such as Microposit 1165). Acetone will not completely remove LOL2000, so additional polymer removal measures should be taken. Because the overhang profile is caused by undercutting the resist, care must be taken to avoid completely undercutting (and lifting off) very narrow geometries.
a.) Substrate with underlying LOL2000 layer (purple) and overlying layer of AZ3612 (brown.)
b.) Area of AZ3612 resist exposed to UV (light purple).
c.) Following develop in standard developer. The developer will etch the underlying LOL2000.
d.) Following directional sputter film deposition.
e.) Following lift-off and clean.
The procedure for LOL2000 lift-off processing is as follows:
SURFACE-MODIFIED RESIST PROCESSING:
The top surface of the photoresist can be chemically modified to develop at a slower rate than the underlying resist. The result is something similar to the LOL2000 process, where there is undercutting of the top layer. This can be accomplished by soaking in chlorobenzene (the traditional method -- for more info, check out the Berkeley Microlab website on the Chlorobenzene process or the SNL process) or toluene. Basically, the standard photoresist procedure is used, but the wafer is soaked in chlorobenzene (for 10 minutes, typically) or toluene (for 1 minute, usually) either prior to UV-expose or just after, but prior to develop. In either case, the wafer is blown dry and baked briefly. Following standard develop, the result should be an undercut profile (similar to figure c., above.)
Stanford Nanofabrication Facility
Last Modified 08/29/2003