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.
- It is used as a protective masking layer for the etching of deep structures and can also be used as a template for the electroplating of metal microstructures.
- Coating of thick resist is achieved either by multiple spin- coating applications or by spinning special viscous resist solutions at slower speeds (up to 100µm).
- With increasing resist thickness, maintaining thickness control and uniformity becomes difficult.
- Exposing resist thicker than 5µm degrades the minimum resolvable features size due to limit depth of focus of the expose tool.
- The maximum aspect ratio (ratio of resist thickness to minimum features dimension) is approximate 3. i.e. minimum achievable feature size is larger than one third of the resist thickness.
Topographical Height Variations:
Topography on the surface of the wafer, such as deep cavities and trenches, are common in MEMS and pose challenges to both resist. Spinning and imaging.
- For cavities deeper than about 10 µm, thinning of the resist at corners and accumulation inside the cavity create problems with exposure and with learing insufficient resist thickness during etches.
- Exposing a pattern on a surface with height variations in excess of 10 µm is also a difficult task because of the limited depth of focus.
- Contact and proximity tools are not suitable for this task unless a significant loss of resolution is tolerable.
- Where the no. of height level is limited (<3) projection lithography can be used to perform Exposure with focus adjustment at each of these height levels but it is costly.
Double sided Lithography:
Often, lithographic patterns on both sides of a wafer need to be aligned with respect to each other with high accuracy. E.g. the fabrication of commercial pressure sensor entails forming on the front side of the wafer piezoresistive sense elements that are aligned to the edges of a cavity on the back side of the wafer.
- Different, methods of front-to-back side alignment, also known as doubly sided alignment, have been commercially available tools.
- Wafer polished on both sides should be used to minimize light scattering during lithography.