1. What is photoresist

Photoresist, also known as photoresist and photoresist, is a kind of photochemically sensitive etch-resistant thin film organic material. It is also a key chemical material in micromachining technology. It is usually composed of three main components: sensitive resin, sensitizer and solvent.

2. Types of photoresist

Photoresist can be divided into three types based on the chemical structure of the photosensitive resin.

(1) Photopolymerization, the use of alene monomer, under the action of light to generate free radicals, free radicals further lead to monomer polymerization, and finally polymer, has the characteristics of forming positive.

(2) photodecomposition, the use of materials containing azoquinone compounds, after light, photodecomposition reaction will occur, from oil soluble into water soluble, can be made into positive glue.

(3) photocrosslinking type, using polyvinyl alcohol laurate as photosensitive materials, under the action of light, the double bond in the molecule is opened, and cross-linking between chains occurs, forming an insoluble network structure, and plays an anti-corrosion role, which is a typical negative photoresist.

3. Production steps of photoresist

(1) Prepare the substrate: Before applying the photoresist, the silicon wafer is usually treated by dehydration and baking to evaporate the water on the surface of the silicon wafer, It is coated with a compound used to increase the adhesion of the photoresist to the silicon wafer (hexa-methyl-disilazane (HMDS) and tri-methyl-silyl-diethyl-amime (TM) SDEA)).

(2) Coated with photoresist: the silicon wafer is placed on a flat metal tray. The tray is connected with the vacuum tube through small holes, and the silicon wafer is sucked on the tray, so that the silicon wafer can rotate with the tray. The coating process is generally divided into three steps: 1. Spray the photoresist solution on the surface of the silicon sheet; 2. 2. Accelerate the rotation of the tray (silicon wafer) until the required rotation speed is reached; 3. After reaching the desired rotation speed, keep rotating for a certain amount of time. Because the photoresist on the surface of the silicon wafer moves to the periphery of the silicon wafer by the centrifugal force during rotation, the glue coating can also be called the glue dumping. After gluing, the photoresist remaining on the surface of silicon wafer is less than 1% of the original.

(3) Soft drying: also known as pre-drying. In the liquid photoresist, the solvent component accounts for 65%-85%. Although the liquid photoresist has become a solid film after gluing, there is still 10%-30% solvent, which is easy to stain the dust. By drying at a higher temperature, the solvent can be volatilized from the photoresist (the solvent content is reduced to about 5% after pre-drying), thus reducing the contamination of dust. At the same time, the stress of the film formed by high-speed rotation can be reduced, so as to improve the adhesion of the photoresist. In the pre-drying process, due to solvent volatilization, the thickness of the photoresist will also be reduced, generally by about 10%-20%.

(4) Exposure: During the exposure process, photochemical reaction occurs in the photoresist photosensitive agent, so that the sensitive region of the positive adhesive and the non-sensitive region of the negative adhesive can be dissolved in the developer. The photochemical reaction of the photosensitive agent DQ in the positive photoresist changes into vinyl ketone, which is further hydrolyzed to Indene-Carboxylic Acid (ICA). The solubility of the carboxylic acid to the alkaline solvent is about 100 times higher than that of the unsensitized photoresist, and it also promotes the dissolution of the phenolic resin. Therefore, the transfer of mask pattern can be carried out by using the different solubility of photoresist and unphotoresist to alkaline solvent.

(5) Development: after development, the sensitive region of the positive adhesive and the non-sensitive region of the negative adhesive are dissolved in the developer solution. After exposure, the potential graphics in the photoresist layer will appear after development, forming three dimensional graphics on the photoresist. In order to improve the resolution, almost every photoresist has a special developer to ensure high quality development results.

(6) hard drying: also known as hard film. After development, the silicon wafer also goes through a high temperature treatment process, the main role is to remove the remaining solvent in the photoresist, enhance the adhesion of the photoresist to the silicon wafer surface, and improve the corrosion resistance and protection ability of the photoresist in the process of etching and ion implantation. At this temperature, the photoresist will soften and form a molten state similar to that of vitreous body at high temperature. This will smooth the surface of the photoresist under the action of surface tension, and thus reduce the defects in the photoresist layer (such as pinholes), thereby correcting the edge profile of the photoresist pattern.

(7) etching or ion implantation

(8) gluing: after etching or ion injection, there is no longer a need for photoresist as a protective layer, it can be removed, called gluing, divided into wet gluing and dry gluing, wet gluing and organic solvent gluing and inorganic solvent gluing. Organic solvent degumming is mainly to dissolve photoresist in organic solvent and remove; Inorganic solvent degumming is the use of photoresist itself is also the characteristics of organic matter, through some inorganic solvent, the photoresist carbon element oxidation into carbon dioxide and remove it; Dry gumming is done by stripping the photoresist using plasma.