Chapter 3 : Manufacturing Process Top Down
Top down method begins with a pattern generated on a large scale, then reduced to Nano scale.
By nature, top down self-assembly is not cheap but quick to manufacture.
Lithography is a process for printing using a metal plate with a completely smooth surface.
The basic steps in the lithographic process are-
Topics covered in this snack-sized chapter:
The first step in the process is the preparation of the substrate.
This step usually involves cleaning the surface through various solvent and plasma cleaning procedures.
The surface may also be treated using a prime step, in which it is exposed to a reagent, such as hexamethyldisilizane (HMDS), that modifies the surface chemistry of the substrate to improve the adhesion of the photoresist to the surface.
In the next step, a material known as a photoresist is applied.
The photoresist is a material that undergoes a change in its physical and/or chemical properties upon exposure to radiation that allows for the formation of a relief pattern in the material.
The resist, in the form of a solution in an organic solvent, is spin coated onto the substrate and then soft baked (SB) on a hot plate or in an oven to remove residual casting solvent.
The desired product from this step is a uniform, glassy, polymeric thin film that is generally on the order of 1 mm thick.
The resist coated substrate is then exposed pattern wise to radiation, typically ultraviolet light.
The exposure tool uses a mask to selectively expose the desired pattern into photoresist.
The resist is then treated with a developing solvent, by immersion or some other technique that selectively dissolves either the exposed or unexposed areas depending on the nature of the resist.
The areas of the substrate that are no longer covered by photoresist can then be subjected to further processing such as plasma etching or ion implantation.
The remaining resist patterns serve as a barrier that protects the underlying substrate from these processes.
Once the substrate is processed, the photoresist layer is stripped and the entire process is repeated for each layer of the device.
Nanolithography is a term used to describe a number of techniques for creating incredibly small structures.
The sizes involved are on the order of tens of Nano meters.
One common method of nanolithography, used particularly in the creation of microchips, is known as photolithography.
One more technology that allows for smaller sizes than photolithography is electron-beam lithography.
Photolithography is a parallel method of nanolithography in which the entire surface is drawn on in a single moment.
The steps involved are wafer cleaning, photoresist application, soft baking, mask alignment and exposure, and development.
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 SiO2
layer, photoresist is applied to the surface of the wafer.
High-speed centrifugal whirling of silicon wafers is the standard method for applying photoresist coatings in IC manufacturing.
This technique, known as Spin Coating, produces a thin uniform layer of photoresist on the wafer surface.
There are two types of photoresist:
For positive resists, 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.
In other words, "whatever shows, goes.” The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer.
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 photoresists, therefore, contain the inverse (or photographic "negative") of the pattern to be transferred.
Soft-baking is the step during which almost all of the solvents are removed from the photoresist coating.
Soft-baking plays a very critical role in photo-imaging. The photoresist coatings become photosensitive, or imageable, only after soft baking.
Over soft-baking will degrade the photosensitivity of resists by either
reducing the developer solubility or actually destroying a portion of the sensitizer.
Undersoft-baking will prevent light from reaching the sensitizer.
Positive resists are incompletely exposed if considerable solvent remains in the coating.
This undersoft-baked positive resists is then readily attacked by the developer in both exposed and unexposed areas, causing less etching resistance.
One of the most important steps in the photolithography 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 wafer's surface, the photoresist is exposed through the pattern on the mask with a high intensity ultraviolet light.
There are three primary exposure methods: contact, proximity, and projection. They are shown in the figure below:
One of the last steps in the photolithographic process is development.
The figure below shows response curves for negative and positive resist after exposure and development.
Hard-baking is the final step in the photolithographic process.
This step is necessary in order to harden the photoresist and improve adhesion of the photoresist to the wafer surface.
The typical steps that are involved in photolithography processing are shown in the schematic diagram below:
A technology that allows for smaller sizes than photolithography is known as electron-beam lithography.
A technique that employs a focused beam of electrons for extremely precise patterning.
The pattern is written directly onto the electron-sensitive resist. (No mask is used).
More precise than photolithography.