This technical note is a simple introduction to microlithography, that is the widespreadly used technique for micropatterning planar surfaces, mostly applied before an etching process. A detailed description of the microlithografic process is not the aim of this report. However, a very short introduction is given below.

All details can be found in the huge amount of available literature. An excellent review is contained in the classical book: W.M.Moreau, "Semiconductor Lithography", Plenum (1988).

The processes for the fabrication of planar structures on different substrates (metals, semiconductors, and dielectrics, as crystals, polycrystals or amorphous) can be divided into two groups. One is based on an "additive" procedure, the other on a "subtractive" procedure. In both cases the same surface structure can be fabricated. Let us assume that an aluminium strip has to be made on a silicon substrate. With the "subtractive" technique the strip is obtained from an aluminium film previously deposited on the whole substrate and then removed (subtracted) from the area not to be covered with the strip. In such case, the protective "mask" layer must cover the aluminium layer in the areas not to be removed, and patterning is made after depositing the aluminium film. If the "additive" technique is used instead, aluminium is placed on the substrate surface only in a selected area, while the remaining areas are prevented from coming into contact with the aluminium film. This is obtained by means of the masking layer, which is patterned before adding aluminium.

Hence, the subtractive technique is based on the possibility of "etching" the deposited film to spatially limit its area. With the "additive" process it is necessary to "lift-off" the deposit in selected areas. In common practice, the terms "etching" and "lift-off" are used with direct reference to the corresponding patterning procedure. The choice between one technique and the other depends on a number of elements, such as the type of mask layer, its thickness, and the technique used for its deposition.

Whichever the patterning technique is - etching or lift-off - the shaping of the desired surface structure is obtained through a "mask layer". In most cases, such layer is photoresist, that is a polymeric film sensitive to visible or ultraviolet light. When exposed to light (directly, as it occurs with laser writing, or through a chromium mask) its solubility in a development bath is modified. "Positive" resist becomes soluble - and then is removed - only in the areas exposed to light. "Negative" resist becomes insoluble only where exposed.

Note that the word "mask" is often indifferently used either with the meaning of masking layer, that is the resist film, or to indicate a "photo-mask" (usually made of a metal film deposited on a transparent substrate) used for transferring the pattern it contains onto the final substrate. Usually the actual meaning of the word "mask" can be immediately deduced from the written context.

By selecting the resist "polarity" as well as the type of surface processing - etching or lift-off - the designer can optimise the microfabrication process, in terms of time, cost, resolution, environmental impact, etc.

As an example, a typical microlithographic process flow used in the fabrication of thin film microwave devices is given below.
- Deposition of positive resist on the substrate (typically alumina or PTFE coated with a number of metal layers and a final gold plating).
- Drying in oven, for removing the thinner from the resist layer.
- Exposure through a chomium mask or by laser direct writing.
- Development in alcaline solution.
- Wet or plasma etching of the metal layers.
- Resist stripping.