Explain the process of photolithography in detail.
1 Answer

Photolithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer. The steps involved in the photo-lithography process are wafer cleaning barrier layer, formation photo-resist applications soft baking, mask alignment, exposure and development and hard baking.

Wafer cleaning, Barrier formation and photo-resist application :

In the first step, the wafers are chemically cleaned to remove particulate matter on the surface as well as any traces of organic, ionic and metallic impurities.

After cleaning, silicon dioxide, which serves as a barrier layer is deposited on the surface of the wafer.

After the formation of the $Sio_2$ layer, photo resist is applied to the surface of the wafer.

High speed centrifugal whirling of silicon wafers is the standard method for applying photo-resist coatings in MEMS manufacturing.

This technique known as "spin coating" produces a thin uniform layer of photo-resist on the wafer, surface.

Positive and Negative photoresist :

There are two types of photo-resist : Positive and Negative.For positive resist the resist is exposed with UV light wherever the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material.

Negative resists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized and more difficult to dissolve. Therefore the negative resist remains on the surface. Wherever it is exposed and the developer solution removes only the unexposed portions masks used for negative photo-resist, therefore contain the inverse of the pattern to be transferred.

Soft - Baking :

Soft - baking is the step during which almost all of the solvents are removed from the photo-resist coating. Soft baking plays a very critical role in photo imaging. The photoresist coatings become photosensitive, or imageable only after soft baking oversoft - baking will degrade the photo-sensitivity of resists by either reducing the developer solubility or actually destroying a portion of sensitizer.

Under soft-baking will prevent light from reaching the sensitizer. Positive resists are incompletely exposed if considerable solvent remains in the coating. This under soft-baked positive resists then readily attacked by the developer in both exposed and unexposed areas causing less etching resistance.

Mask alignment and exposure :

One of the most important steps in the photo-lithography process is mask alignment. A mask or "photo mask" is a square glass plate with a patterned emulsion of metal film on one side. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern once the mask has been accurately aligned with the pattern on the wafers surface, The photo-resist is exposed through the pattern on the mask with a high intensity UV light.

There are 3 primary exposure methods:

Contact, proximity and projection.

Development :

At low exposure energies the negative resist remains completely insoluble in the developer solution. As the exposure is increased above a threshold energy move of the resist film remains after development. At exposures two or three times the threshold energy, very little of the resist film is dissolved. For positive resist, the resist solubility in its developer is finite even at zero exposure energy. The solubility gradually increases until, at some threshold, it becomes completely soluble.

Hard - Baking:

Hard - Baking is the final step in the photolithography process. This step is necessary in order to harden the photo-resist and improve adhesion of photo-resist to the wafer surface.

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