Photolithography

Photolithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a substrate. Often crystalline silicon in the form of a wafer is used as a choice of substrate, although there are several other options including, but not limited to, glass, sapphire, and metal. Photolithography (also referred to as "microlithography" or "nanolithography") bears a similarity to the conventional lithography used in printing and shares some of the fundamental principles of photographic processes.

Related Topics:
Semiconductor device fabrication - Photomask - Substrate - Silicon - Wafer - Glass - Sapphire - Nanolithography - Lithography - Printing

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Photolithography involves a combination of:

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

• substrate preparation
• chemical deposition (eg evaporation, sputtering, plasma enhanced chemical vapour deposition)
• photoresist spinning
• soft-baking
• exposure
• developing
• hard-baking
• etching

and various other chemical treatments (thinning agents, edge-bead removal etc.) in repeated steps on an initially flat substrate.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A part of a typical silicon lithography procedure would begin by depositing a layer of conductive metal several nanometers thick on the substrate.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

A layer of photoresist -- a chemical that hardens when exposed to light (often ultraviolet) -- is applied on top of the metal layer.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

The photoresist is selectively "hardened" by illuminating it in specific places.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

For this purpose a transparent plate with patterns printed on it, called a photomask or shadowmask, is used together with an illumination source to shine light on specific parts of the photoresist.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Some photoresists work well under broadband ultraviolet light, whereas others are designed to be sensitive at specific frequencies to ultraviolet light, it is also possible to use other types of resist that are sensitive to X-Rays and others that are sensitive to electron-beam exposure.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Generally most types of photoresist will be available as either "positive" or "negative". With positive resists the area that you can see (masked) on the photomask is the area that you will see upon developing of the photoresist. With negative resists it is the inverse, so any area that is exposed will remain, whilst any areas that is not exposed will be developed.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

After developing, the resist is usually hard-baked before subjecting to a chemical etching stage which will remove the metal underneath.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Finally, the hardened photoresist is etched using a different chemical treatment, and all that remains is a layer of metal in the same shape as the mask (or the inverse if negative resist has been used).

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Lithography is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In a complex integrated circuit, (for example, CMOS) a wafer will go through the photolithographic area up to 50 times. For Thin-Film-Transistor (TFT) processing many fewer photolithographical processes are usually required.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

 
Photolithography: Technology ►