Photolithography is an important technique which is used to transfer intricate patterns onto substrates in semiconductor manufacturing. This allows integrated circuits and other microfabricated components to be manufactured accurately and easily. To achieve this, it is important to harness the power of photoresists, the photosensitive materials that can form a patterned layer on a substrate. This process can involve either positive or negative photoresists and whilst they both serve the same fundamental purposes; they can differ in the way they behave and how they are applied.
In this article, we look at the differences between positive and negative photoresists and the ways in which their roles can differ to better understand how they can be used most effectively.
What is a photoresist?
Photoresists are light-sensitive polymers which are used in the process of photolithography. They help to create a patterned coating that sits on a substrate, such as a silicon wafer. This is designed to create a protective layer that helps to dictate where the material is added, etched or removed. Most photoresists will consist of a polymer base, a photosensitive compound, and a solvent. When UV light is applied to the photoresist, its properties are altered, and this enables a selective removal of certain regions.
Positive photoresists
The key factor when it comes to positive photoresists is that they can become more soluble in the developer solution when they are exposed to UV light. This means that the exposed regions can be dissolved and washed away, leaving the desired pattern behind.
This is because the energy from the UV light breaks the molecular bonds of the polymer when it hits a positive photoresist. This enables it to be transformed into a more soluble form and the exposed areas of the photoresists can then be removed during the development process. Any remaining photoresist will then serve as a mask for the subsequent processes that take place, such as etching or deposition.
The key characteristics of positive photoresists include their light sensitivity which makes them soluble and their pattern transferrable. Positive photoresists are renowned for their ability to produce fine, intricate patterns with high resolution and are generally applied as thin layers, so they are ideal for applications that need high levels of precision.
Advantages of positive photoresists
Positive photoresists can define very small features, so they are ideally suited for the fabrication of complex microstructures. They can also achieve very fine pattern details and so they tend to be preferred for any advanced lithography processes that are used in semiconductor fabrication. However, positive photoresists tend to be the more expensive option due to their superior performance and they are also typically less robust, so they are not always suitable for processes that require a high edge resistance.
Negative photoresists
In direct contrast to their positive counterparts, negative photoresists become less soluble when exposed to light. When exposed to UV light, the polymer chains in a negative photoresist tend to cross-link so that the exposed regions are more resistant to the developer solution. This means that the exposed portions are what remain on the substrate and the unexposed regions are washed away and the pattern is therefore revealed. This means negative photoresists are preferred for applications where thicker coatings are needed.
Negative photoresists usually have a lower resolution than positive photoresists, so they are less suitable for highly detailed patterns. They also need to be applied in thicker layers, so they are more suitable for certain types of applications such as the fabrication of printed circuit boards and microelectromechanical systems (MEMS).
One of the biggest advantages of a negative photoresist is the fact that it tends to be more durable and resistant to the etching process. It is also a much more cost-effective option, so it can be very attractive for less demanding applications. The fact that negative resists need to be applied in thicker layers has also proved to be very useful in applications that require substantial material build-up.
It is worth noting that negative resists do not offer the same level of fine detail as positive resists, so it can lead to less precise patterning in complex microstructures. The lower resolution of negative photoresists also means that they are not ideal for creating extremely small features.
