The Nikon eReview: Double Patterning Single Etch Solutions from JSR

The increasing cost to operate a wafer fabrication facility, combined with the uncertainty of the next-generation lithography technology, places more focus on extending current equipment capabilities. Lithographers are continually being asked to improve resolution, process windows, and defectivity, as well as develop new integration solutions to enable next-generation technology nodes. The ability to address these challenges by migrating to shorter wavelengths requires large capital expenses in tooling, masks, and process development efforts; therefore, alternative methods for shrinking such items as critical dimensions (CD) are needed. Over the past few years, many materials, equipment, and chip manufacturers have developed innovative double patterning techniques to create pitch splitting lithography. These techniques vary between double and single etch patterning processes, each offering their own challenges and advantages. In general, single etch processes are preferred as they help simplify a complex process, save valuable throughput, and reduce capital spending.

Pitch splitting lithography offers many benefits, and the goal of developing a process by which the wafer does not leave the track before creating the pitch splitting is still being pursued by lithographers today. Such a technique requires the ability to coat one photoresist over another. This introduces a series of concerns that lithographers must take into consideration—creating dual sets of imaging requirements. Additionally, this approach poses a technical challenge to the traditional linear behavior of acid generation of the photoresist, and therefore may require some treatment of the resist layer upon first exposure.

One elegant approach to the challenges of double patterning single etch solutions comes in the form of chemical freeze. Development of a freeze process, also known as litho-freeze-litho-etch (LFLE), requires photoresists and freeze materials having special properties. The first process is achieved using conventional lithography at a one line, three space pitch. The first photoresist is then frozen by the application of a chemical freeze agent that is coated, baked, and then developed away. During this process it is critical for the end result that the first photoresist is resistant to the second photoresist processing, including solvent, exposure, and developer resistance. Once the first resist is frozen, the second resist can be applied using conventional lithography.

Figure 1. Process flow of JSR FZX freeze technology for double patterning single etch solutions.

In order to make the first photoresist resistant to the second photoresist processing, there is a need to cross link or cure the first resist. Many companies have proposed various techniques such as ultraviolet (UV) cure, thermal/exposure cure, or chemical cure. Studies by JSR have indicated that the chemical freeze approach is the most straight forward in terms of utilizing the existing photoresist technology of today. This allows the user to capitalize on the advanced process windows, low mask error enhancement factor (MEEF), minimized line width roughness (LWR), and high resolution of existing photoresists currently on the market. Additionally, the chemical freeze process utilizes existing lithography tracks since it uses a standard coat bowl, bake plate, and developer unit.

Figure 2. JSR advanced immersion resist and FZX freeze process used to achieve 32 nm and 40 nm images with the Nikon NSR-S610C (NA = 1.30) immersion scanner.

Chemical freeze approaches do have their own set of materials challenges though—mainly potential CD variation during the first photoresist freeze, as well as the possibility for interaction between the second resist and the frozen first resist. Many unique material solutions are being implemented with the goal of zero or minimal CD growth of the first resist, along with making the first photoresist completely inert to the second photoresist upon freezing.

Originally, chemical freeze for double patterning single etch solutions was predominantly used for bright field line/space applications. This leant itself well to two-dimensional logic applications and other line-centric lithography layers. However, more recent work has shown that the chemical freeze application is also relevant to trench applications and other traditional dark field applications when using creative photomask solutions. Additionally, the use of chemical freeze for contact hole applications utilizing a cross line approach has enabled extremely low k₁ imaging on traditionally challenging contact and via layers.

Figure 3. JSR immersion resist and FZX freeze process used to accomplish 2x1 spaces (80 nm x 40 nm) with the NSR-S610C.

JSR’s chemical shrink materials, the FZX series, have been developed to provide solutions in the double patterning single etch arena. FZX freeze materials, coupled with advanced JSR immersion photoresists, provide solutions for 30 nm and below half-pitch applications. Additionally, etch work published at SPIE Advanced Lithography 2008 showed excellent image transfer using materials that were processed with freeze materials.

Double patterning is the most likely candidate for the 32 nm half-pitch. Requirements on the lithography process are severe, but within reach. As with any technology, the key to double patterning is balancing cost of ownership with process complexity. Resist freeze is an elegant way of reducing both.