Learn about every aspect of photoresist (2Basic components and classification of photoresist )
2. Basic components and classification of photoresist
The production of photoresist is not only for general demand, but also for specific demand. It is adjusted according to the wavelength of different light and exposure source. In addition, photoresist, with a specific characteristic of heat flow, is prepared by a specific method and combined with a specific surface. These properties are determined by the type, quantity and mixing process of different chemical components of photoresist.
There are four basic components of photoresist, including polymer, solvent, photosensitive agent and additive.
Components of photoresist
Polymer: When polymer is exposed to lithography machine, its structure changes from soluble to polymeric. The polymer is a special photosensitive and energy-sensitive polymer, generally composed of a group of large and heavy molecules, including carbon, hydrogen and oxygen. Plastics are typical polymers.
Solvent: It is the largest component of photoresist, used to dilute the photoresist and keep the photoresist in liquid. Then, photoresist coats on the wafer surface to form a thin layer by rotation. The solvent used for negative photoresist is a fragrant xylene, while that used for positive photoresist is ethyl ethoxyacetate or dimethoxyacetaldehyde.
Photosensitive agent: Control and adjust the chemical reaction of photoresist during exposure.
Photosensitizer: It is added to the photoresist to limit the spectral range of reactive light or to limit the reactive light to a specific wavelength.
Additives: Different types of additives and photoresist are mixed together to achieve specific results, such as adding dyes in negative photoresist to absorb and control light, and adding anti-dissolving agent in positive photoresist.
Polymers of negative photoresist change from non-polymer state to polymer state after exposure. In fact, these polymers form a cross-linked substance, which is anti-etching substance. Therefore, the production of negative photoresist is carried out under the condition of yellow light in order to prevent accidental exposure. Negative photoresist is the earliest use of photoresist, with good adhesion, good blocking effect and fast photosensitivity. However, it will deform and expand when developing, which limits its resolution, so generally negative photoresist is only used in the field of large online width.
The basic polymer of positive photoresist is phenol-formaldehyde polymer (Novolak resin). In photoresist, the polymer is relatively insoluble. After exposure with appropriate light energy, the photoresist will turn into a soluble state, known as photolysis reaction. Then, the dissolution part will be removed by solvent in the development. Anti- dissolving additives may be added to the positive photoresist to prevent the unexposed part from dissolving during development.
Positive photoresist generally has the characteristics of high resolution, good step coverage and good contrast. At the same time, it also has the problems of poor adhesion, poor etching resistance and high cost.
Negative photoresist is divided into cyclized rubber system negative photoresist and chemical amplified negative photoresist (the main resins and their working principles are different.); positive photoresist includes traditional positive photoresist (DNQ-Novolac system) and chemical amplified photoresist (CAR).
In practical applications, the performance of photoresist needs to meet various requirements. Generally speaking, the performance requirements of photoresist include the following points:
Resolution: The smallest pattern or spacing produced by the photoresist layer is usually referred to as the photoresist resolution. The resolution of a particular photoresist refers to the resolution of a particular process, which includes the exposure source and the development process. Changing other process parameters will change the inherent resolution of the photoresist.
Generally speaking, the thinner the line width, the thinner the photoresist film is. Therefore, the photoresist film must be thick enough to resist etching and ensure that there is no vacuum. The choice of photoresist is a trade-off between the two objectives. Aspect ratio is usually used to measure resolution and thickness of photoresist. As positive photoresist has a higher aspect ratio than negative photoresist, it is more suitable for large-scale integrated circuits.
Bonding ability: Photoresist must adhere well to the wafer surface, otherwise the etched pattern will be distorted. The bonding ability of photoresist for different surface is different. Many processes of photoresist are designed to increase the bonding ability. Negative photoresist usually has stronger bonding ability than positive photoresist.
Exposure speed and sensitivity: The faster the photoresist reacts, the faster the processing speed will be. The sensitivity of photoresist is related to the total energy required to cause polymerization or photolysis, when energy is related to the specific wavelength of the exposure source. Ultraviolet, visible light, radio and X-ray are electromagnetic radiation. The shorter the wavelength, the higher the energy will be. Therefore, exposure source energy sorted from large to small is X-ray, extreme ultraviolet, deep ultraviolet, ultraviolet and visible light, respectively.
Width of process capacity: Internal deviation may occur in every step of the process. Some photoresist have a larger margin of process variation and a wider process range. The wider the process range, the more likely it is to meet the required size specifications on the wafer surface.
Pinhole: The pinhole is a very small hole in the photoresist layer. Pinhole allows etchants to penetrate the photoresist layer, and then small holes are etched in the wafer surface. Pinholes are caused not only by particulate contaminants in the environment in the coating process, but also by holes in the photoresist layer. The thinner the photoresist layer, the more pinholes there are. Because the positive photoresist has a higher aspect ratio, it is generally suitable to make a thicker photoresist film.
Step coverage: There are many layers on the wafer surface before lithography. With the development of wafer process technology, more layers are obtained on the wafer surface. In order to make photoresist block etching, it must have enough film thickness on the previous layer.
Thermal flow: The lithography process includes soft baking and hard baking. Because photoresist is similar to plastic material, becoming soft or even flowing in baking and then affecting the final pattern size, photoresist must maintain its properties and structure in baking.
Particle and pollution level: Like other arts and crafts, photoresist must meet strict standards in contents of particle, sodium, trace metal impurities, water and so on.
In addition, there are many other factors to consider in the production process. For instance, the bright field (exposure area) in the mask is vulnerable to glass cracks and dirt. Using negative photoresist, there will be small holes in bright field, while it’s not easy to appear pinholes in dark field. So for pattern with smaller hole area, positive photoresist is the only choice. Besides, in the process of removing photoresist, it is also found that it is easier to remove positive than negative photoresist.
Comparison between positive photoresist and negative photoresist