Lithography/Etch

The fabrication of circuits on silicon wafers requires that several different layers, each with a different pattern, be deposited on the surface one at a time, and that doping of the active regions be done in very controlled amounts over tiny regions of precise areas.  The various patterns used in depositing layers and doping regions on the substrate are defined by a process called lithography.  

Simply put, the lithography process generally consists of the following steps.  A layer of photoresist (PR) material is first spin-coated on the surface of the wafer. The resist layer is then selectively exposed to radiation such as ultraviolet light, electrons, or xrays, with the exposed areas defined by the exposure tool, mask, or computer data. 

After exposure, the PR layer is subjected to development which destroys unwanted areas of the PR layer, exposing the corresponding areas of the underlying layer.  Depending on the resist type, the development stage may destroy either the exposed or unexposed areas.    The areas with no resist material left on top of them are then subjected to additive or subtractive processes, allowing the selective deposition or removal of material on the substrate.

During development, the unwanted areas in the PR are dissolved by the developer.  In the case wherein the exposed areas become soluble in the developer, a positive image of the mask pattern is produced on the resist.  Such a resist is therefore called a positive photoresist.  Negative photoresist layers result in negative images of the mask pattern, wherein the exposed areas are made less soluble in the developer.  Wafer fabrication may employ both positive and negative photoresists, although positive resists are preferred because they offer higher resolution capabilities.

Photoresist materials consist of three components: 1) a matrix material (also known as resin), which provides body for the photoresist; 2) the inhibitor (also referred to as sensitizer), which is the photoactive ingredient; and 3) the solvent, which keeps the resist liquid until it is applied to the substrate.

Etching is the process of removing regions of the underlying material that are no longer protected by photoresist after development.  The rate at which the etching process occurs is known as the etch rate.  The etching process is said to be isotropic if it proceeds in all directions at the same rate.  If it proceeds in only one direction, then it is completely anisotropic. 

Since etching processes generally fall between being completely isotropic and completely anisotropic, an etching process needs to be described in terms of its level of isotropy.  Wet etching, or etching with the use of chemicals, is generally isotropic.  On the other hand, dry etching processes that employ reactive plasmas are generally anisotropic.

Reactive plasma etching involves the removal of surface material not protected by lithographic masks using chemically active species. These species are usually oxidizing and reducing agents produced from process gases that have been ionized and fragmentized by a glow discharge.  The species react with the exposed surface material, removing them from the substrate while forming volatile byproducts in the process.  

See also:  Resist Processing;  Electron Lithography; Optical Lithography; Wet Etching;

Pattern Transfer Defects;  Die Delayering; The Lift-Off Process;  Masks and Reticles

Fig. 3.  Examples of Etching Systems