Lithography involves 3 sequential steps:
- Application photoresist (resist), which is a photosensitive emulsion layer.
- Optical exposure to print an image of the mask onto the resist.
- Immersion in an aqueous developer solution to dissolve the exposed resist and render visible the talent image.
1) Optical Lithography:
The mask itself consists of a patterned chromium (most common) emulsion, or iron oxide layer an a transparent fused-quartz.
The pattern layout is generated using a CAD tool and transferred into the opaque layer at a specialized mask- making facility as electron beam or laser-beam writing.
- A complete micro fabrication process normally involves several lithographic operations with different masks.
- Positive photoresists is an organic resign material containing a sensitizer. It is spin- coated on the wafer with a typical thickness between 0.5 µm and 10 µm.
- The sensitizer prevents the dissolution of uneposed resist during immersion in the developer solution.
- The exact opposite process happens in negative resists in which exposed areas dissolve in the developer.
Optical exposure can be accomplished in one of 3 different models.
a) Contact b) Proximity c) Projection.
- In contact lithography, the mask touches the wafer. This normally shortens the life of the mask and leaves undesired photoresist residue on the wafer and the mask.
- In proximity made, the mask is brought to within 25 to 50 µm of the resist surface, while.
c) Projection:Projection lithography projects
- an image of the mask onto the wafer through complex optics.
Resolution is defined as the minimum feature the optical system can resolve which has limitations for micro aching applications.
- For proximity system, it is limited by Fresnel diffraction a minimum of about 5µm & in contact systems it is approximately 1 to 2 µm.
- For projection systems, it is given by 0.5 ⅹ where is wavelength (400nm) & NA is numerical aperture of the optics( 0.25 for steppers used in MEMS).
- Resolution in projection lithography is better than 1 µm.
- Depth of focus is a more severe constraint on lithography, especially in use of light to expose thick resist or accommodate geometrical height variations across the wafer.
- Depth of focus for contract and proximity system is poor and also limited by Fresnel Diffraction.
- In Projection systems, the image plane can be moved by adjusting the focus settings, but once it is fixed the depth of focus about that plane is limited to . which is limited to few microns.
∴ Projection lithography is a superiar approach but an optical projection system cn cost more than proximity or contact systems.
Projection optical lithography has been the mainstream technology in the semiconductor industry for more than two decades. Despite prognostications to the contrary, it has been able to keep up with the expectations of Moore's Law and the semiconductor industry's roadmap1. In fact, it has enabled the shrinking of critical dimensions to the sub-100 nm region, and current trends will keep it on course to at least 45 nm, i.e. to at least the year 2010. It is plausible to expect that new inventions will keep optical lithography as the dominant process well into the next decade.