Spin Coater System
Applying uniform thin films to flat substrates can be achieved by spin coating procedure. An excess amount of a solution is applied to a substrate (manually: using a syringe, or automatically: with a dispense unit and dispense nozzles in the lid of the spin coater). The substrate is then rotated at high speed (e.g. up to 10.000 rpm) in order to spread the fluid by centrifugal force. A machine used for spin coating is called a spin coater, spin processor or simply spinner.
Rotation is continued while the fluid spins off the edges of the substrate, until the desired thickness of the film is achieved. The applied solvent is usually volatile, and simultaneously evaporates. Next to the material characteristics of the photoresist used (solid content and viscosity) the final film thickness is defined by the rotational speed. Keeping the speed constant during the film formation as well as from wafer to wafer is essential for a homogenous layer and a reproducible process.
There are four different stages of Spin Coating Process, and those are as follows
- Deposition of the coating fluid onto the wafer or substrate- This can be done by using a nozzle and pouring the coating solution or by spraying it onto the surface. A substantial excess of coating solution is usually applied compared to the amount that is required.
- Acceleration of the substrate up to its final, desired, rotation speed.
- Spinning of the substrate at a constant rate; fluid viscous forces dominate- the fluid thinning behavior.
- Spinning of the substrate at a constant rate; solvent evaporation- dominates the coating thinning behavior.
Applications of Spin Coater
Semiconductor Industry- Spin coating is widely used in the semiconductor industry, as one of the applications of thin films, creating thin films with thicknesses below 10 nm of even high quality thickness. It is used intensively in photolithography, to deposit layers of photoresist about 1 micrometre thick. Photoresist is typically spun at 1000 to 4000 revolutions per minute for 30 to 60 seconds.
Transparent Titanium dioxide Deposition- Owing to the low values of thickness which can be achieved using spin coating methods, this method is often also employed in the fabrication of transparent titanium dioxide thin films on quartz or glass substrates.
Polymer Coating- Although many polymers with a wide range of weights may be spin coated, one of the easiest and most typical polymers to spin coat is poly(methyl methacrylate), commonly known as PMMA, with a moderate molecular weight (e.g. ~120,000), dissolved in a solvent with some moderate polarity. The solvent 1, 2, 3-trichloropropane was once a traditional solvent commonly used in spin coating, but due to toxicity issues cyclohexanone is generally preferred and produces coatings of comparable quality. More polar solvents such as N,N-dimethylformamide (DMF) are also commonly used. Typically between 10 and 30% (w/w) polymer is dissolved in the solvent. Both the choice of solvent and especially the molecular weight of the polymer significantly affect the viscosity of the solution and thus the thickness of the resultant coating. Many other polymers, such as polystyrene or more polar polymers such as polysulfones or polyetherimides, may also be spin-coated.
Once spin coating is complete, the plate is typically placed quickly onto a hot plate (heated up to somewhere around 100 ºC) for several seconds or minutes to initially evaporate solvent and solidify the coating. The slide is than baked-out for several hours, or typically overnight, in an oven or vacuum oven, at a temperature high enough to sufficiently remove the remaining solvent.
A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry.
The process begins by coating a substrate with a light-sensitive organic material. A patterned mask is then applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is then applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened (either polymerized or cross-linked) by light, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.
On the basis of exposure to light, Photo Resist may be classified into two main categories
- Positive Photoresist- A positive photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble to the photoresist developer.
- Negative Photoresist- A negative photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer. Based on the chemical structure of photoresists, they can be classified into three types:
Photopolymeric Photoresist- It is a type of photoresist, usually allyl monomer, which could generate free radical when exposed to light, then, initiates the photo polymerization of monomer to produce a polymer. Photopolymeric photoresists are usually used for negative photoresist, e.g. methyl methacrylate.
Photodecomposing photoresist- It is a type of photoresist that generates hydrophilic products under light. Photodecomposing photoresists are usually used for positive photoresist. A typical example is azide quinone, e.g. diazonaphthaquinone (DQ).
Photocrosslinking photoresist- It is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are usually used for negative photoresist.