"Lift-off" is a simple, easy method for patterning deposited films and films which are difficult to dry etch. A pattern is defined on a substrate using photoresist and standard photolithography. 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 remains only where the photoresist has been cleared. The defect modes are opposite what one might expect for etching films since the defects may be in the underlaying photoresist layer (for example, particles lead to opens, scratches lead to shorts in metal lift-off.)
Any deposited film can be lifted-off, provided:
- During film deposition, the substrate does not reach temperatures high enough to burn the photoresist.
- For electron-beam evaporation of metals through Innotec, the substrate temperature does not usually rise above 90C, unless evaporating thick films (> 1000A) of refractory metals such as Molybdenum, Tungsten, Tantalum or Platinum.
- For Atomic Layer Deposition of oxide, the substrate temperature for Shipley 3612 should be kept to less than 110C
- The film quality is not absolutely critical. Photoresist will outgas very slightly in vacuum systems, which may adversely affect the quality of the deposited film.
- Adhesion of the deposited film on the substrate is very good.
- For metallic films of non-reactive metals (Au, Pt, Pd), an adhesive film of a reactive metal such as Titanium or Chromium is used. The thickness of such a film should be > 20A for good adhesion. Even with noble metals, the presence of this film is not absolutely critical. With careful processing of resist profiles or the use of LOL-2000, it is possible to get patterns without the adhesion layer. Nevertheless, the adhesion layer greatly improves the yield of liftoff processing, especially when using sonication at the wbsolvent.
- The film can be easily wetted by the solvent.
- The film is thin enough and/or grainy enough to allow solvent to seep underneath.
- The thickness of the film being lifted off should be preferably kept to be less than 1/3rd of the total photoresist thickness.
- The film is not elastic and is thin and/or brittle enough to tear along adhesion lines.
Lift-off can be performed in two ways:
1. Standard photoresist processing
2. LOL (Lift-off Layer) 2000 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.
This is the easiest method, because it involves only one lithography step and the lithography 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 Shipley 3612 resist.
b.) Following expose and develop.
c.) Following film deposition.
d.) Following lift-off.
e.) Following mechanical "scrub".
The photoresist profile is critical for single layer liftoff. The use of the ASML i-line stepper with i-line photoresist (955) is recommended as the resist profile is more vertical than the resist profiles obtained by using the Karlsuss contact mask aligners.
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 is spun on and baked. The photoresist is exposed as usual, and then the substrate is developed. Standard developer will clear the exposed photoresist areas, but will also "etch" away the LOL2000, leading to undercutting of the photoresist. It is this undercut 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. The extent of the undercutting of the LOL2000 is controlled by the LOL2000 bake procedure prior to spinning the photoresist. The higher the bake temperature (up to ~200C), the less undercutting you should observe.
a.) Substrate with underlying LOL2000 layer (purple) and overlying layer of photoresist (brown.)
b.) Area of photoresist resist exposed to UV (light purple).
c.) Following develop in standard
d.) Following film deposition (Metals: evaporation (Innotec) or sputtering (Metallica), Oxide: Low Temperature Atomic Layer Deposition).
e.) Following lift-off and clean.
The procedure for LOL2000 lift-off processing is as follows:
- Make sure to obtain and use dedicated LOL2000 boats. These are labeled and stored near the Litho cassettes. Please do not use standard Litho cassettes for handling LOL2000 coated wafers -- this polymer will stain and contaminate the cassettes (it is not removed with Acetone).
- Singe for 30 minutes at 150°C (unless your wafers came out of a furnace or deposition system within the previous one hour).
- Prime with HMDS before LOL2000 (not absolutely necessary, but results are consistently better.)
- Apply LOL2000 on the Headway ONLY at 3000 rpm's for 60 seconds (=2000Å). There is a spin speed chart in the notebook in litho (range 1300Å to 3500Å)
- Bake for 5 minutes on a 150°C or 170°C hotplate (for large features (> 10 µm), or 30 minutes oven bake. There is a chart on the effects of bake temp and time on dissolution rates. If the LOL2000 undercut is excessive increase the temperature of the LOL2000 bake or go in to the white-oven at 180-190C for 30 to 45 minutes. For critical features (< 1 µm lines and spaces), a 30 to 45 minute bake should be done at 240C in the white-oven.
- Coat with photoresist, you may use the SVG coater. Shipley 3612 or 955 i-line resist will work fine. If your devices have topography, use at least 1.6 microns of photoresist to ensure coverage. However, with a thicker photoresist, the develop time is longer and the LOL-2000 undercut may be too much, causing thin photoresist lines to liftoff during the development.
- Bake for 60 seconds on a 90°C hotplate.
- Expose with standard exposure times for the photoresist used.
- Bake (Post-Exposure-Bake) with standard bake times for the photoresist used
- Develop for 60 seconds (Shipley 3612) or 44 seconds (955 i-line resist) in developer; you may use the SVG developer track.
- The develop time is critical for small photoresist lines. The develop time can be reduced to 45 seconds for Shipley 3612 or 30 seconds for 955 i-line resist to control the LOL-2000 undercut.
- Very short development time may result in incomplete photoresist development or too little undercut resulting in the same problems as single layer liftoff.
- Bake 60 seconds on a 110°C hotplate.
- Lift-off can be done using either:
- Acetone. However, Acetone alone does not completely remove LOL2000 polymer. Additional clean should be done using O2 plasma ash, or photoresist developer, or Microposit Remover 1165. You can use the SVG developer tracks to remove the LOL-2000 after the photoresist has been removed using Acetone. For oxide liftoff, it may be necessary to do 1 min of Ultrasonic agitation in acetone at the wbsolvent before removal of the LOL-2000.
- Ultrasonic immersion in Microposit Remover 1165 at
50°C. This should be done at the solvent bench ultrasonic bath.
Microposit waste should be collected locally in the labeled carboy at
this wet bench, according to chemical waste handling procedures.
Carefully monitor the temperature, as the flash point of Microposit 1165
is around 85°C.
- If you choose to use PRS1000 resist strippers to do the
final clean, make sure your substrate is completely dry before immersing
in these solutions (trace moisture will result in highly corrosive
conditions, which may etch your metal, especially Aluminum.) PRS1000 can only be used at the wbflexcorr, not wbsolvent.