Photolithography is a microfabrication technique used for addition/removal of material to/from a thin film or bulk material, following a detailed geometric pattern.
Cleaning: The wafer is cleaned usually via the RCA procedure.
Preparation: The wafer is heated to drive off moisture.
A liquid or gaseous “adhesion promoter” such as Bis(trimethylsilyl)amine (aka HMDS) is applied to promote adhesion of the photoresist to the wafer.
(The surface layer of SiO_2 reacts with HDMS to form tri-methylated SiO_2, a highly water repellant layer which keeps out the aqueous developer)
Photoresist: A photoresist, or resist, is adhered to the sample via spin coating. Spinning runs from 1200 to 4800 rpm for 30 to 60 seconds to create a 0.5 – 2.5 µm thick coat with variation of 5 – 10 nm. Aspect ratio of resist after development is limited to 4:1. Resist-coated wafer is baked at 90 – 100 ºC for 30 – 60 seconds.
Exposure: The photoresist is exposed to a patter of light, causing a chemical change such that some of the resist can be removed by the developer solution.
- Positive photoresist becomes soluble in the developer when exposed to light. (most common)
- Negative photoresist becomes insoluble in the developer when exposed to light. (less common)
- Maskless lithography projects a precise beam onto the photoresist without use of a mask. Uncommon in commercial processes. Used in Very-large-scale integration lithography (this covers only one or an array of die, not a whole wafer).
- Mask Lithography uses a photomask, as a negative is used in photography, to produce a pattern of light and shadows on the photoresist.
- A proximity printer puts a small gap “d” between the photomask and wafer. (resolution is √(d•λ) )
- A contact printer puts a photomask in direct contact with a wafer and is exposed to uniform light. This is liable to damage the resist and wafer, and thus is uncommon.
(for both printers the mask covers the entire wafer and simultaneously patterns every die)
The post-exposure bake (PEB) is done to reduce standing wave phenomena. For deep ultraviolet lithography chemically amplified resist (CAR) chemistry is used, in which much of the “exposure” reaction takes place during PEB (and is thus sensitive to PEB time etc).
The develope chemistry is delived on a spinner. Developers used to contain NaOH, but now are metal-ion-free and contain TMAH.
The wafer is hard-baked at 120 – 180 ºC for 20 to 30 minutes. This solidifies the remaining photoresist to make a more durable protecting layer for ion implantation, wet chemical etching, or plasma etching.
Etching: a liquid (“wet”) or plasma (“dry”) chemical agent removes the uppermost layer of the substrate in areas not protected by photoresist.
- Dry etching techniques can be made anisotropic (used for semiconductor fabrication to avoid undercutting resist)
- Wet etching is generally isotropic (used for microelectromechanical systems where structures must be released from underlying layer)
Photoresist Removal: A liquid “resist stripper” is used to chemically alter resist so that it no longer adheres to the wafer. Oxidation of the resist via oxygen containing plasma can also be used.