Sheng-Chi Chin

Sub-20nm node photomask cleaning enhanced by controlling zeta potential

As semiconductor manufacturing advances to sub-20nm node (i.e., minimum feature pitch) technology, specifications with respect to ‘dust’ on photomasks are becoming ever more stringent. Photomask cleanliness is essential to high-quality lithography, and flaws seriously affect manufacturing cycle time and productivity. For example, detecting a single newly introduced particle following pellicle (protective cover) mounting results in a loss of one to two days for demounting, repair, and cleaning. A major challenge in developing sub-20nm node mask cleaning processes is removing extremely fine particles (<50nm) from the mask surface. In the past, we have attempted to adjust the nozzle flow and spray force (physical force), but improvement is limited and the method damages the mask pattern. We have also tried increasing the concentration of the cleaning chemical (chemical force), which degrades the photomask film. Here, we describe an alternative mask-cleaning method based on chemical force induced by so-called zeta potential (ZP). Extremely fine contaminating particles derive mostly from the deionized water used in chip fabrication and from cleaning-tool piping. Whether a particle adsorbs onto a mask surface in chemical solution depends on the balance between van der Waals and electrostatic interactions. Van der Waals interactions increase as two surfaces get closer to each other. In contrast, electrostatic interactions result from the electrical potential, or ZP, between the mask and particle surfaces. The behavior of ZP is determined by chemical properties such as pH, electrolyte concentration, and surface energy. As shown in Figure 1, the chemical force between particles and the mask surface changes in different ZP environments.1, 2 Figure 1. Zeta potential (ZP) induces chemical force on both the particle and mask quartz (Qz) surfaces. (a) Positive ZP: attraction between particles and the quartz surface. (b) Negative ZP: repulsion between particles and the quartz surface.

Global CD uniformity improvement in mask manufacturing for advanced lithography

The control of global critical dimension uniformity (GCDU) across the entire mask becomes an important factor for the high-end masks quality. Three major proceses induce GCDU error before after-developing inspection (ADI) including the E-Beam writing, baking, and developing processes. Due to the charging effect, the fogging effect, the vacuum effect and other not-well-known effects, the E-Beam writing process suffers from some consistent GCDU errors. Specifically, the chemical amplified resist (CAR) induces the GCDU error from improper baking. This phenomenon becomes worse with negative CARs. The developing process is also a source of the GCDU error usually appears radially. This paper reports the results of the study of the impact of the global CD uniformity on mask to wafer images. It also proposes solutions to achieve better masks.