Manufacturing Bits: Oct. 12

MoSi2 pellicles for EUV
Hanyang University has presented a paper that describes a novel molybdenum disilicide (MoSi2) pellicle membrane for use in extreme ultraviolet (EUV) lithography.

With a 28nm thickness, a MoSi2 membrane has demonstrated a 89.33% transmittance for EUV lithography. The pellicle technology is still in R&D. MoSi2, which is a silicide of molybdenum, is a refractory ceramic used in heating elements. It has a melting point 2030 °C, while also being electrically conductive.

Nonetheless, used in the wafer fabrication process, a pellicle is a thin membrane that covers a photomask. The pellicle is like a dust cover, preventing particles from landing on the mask.

Pellicles are especially critical for EUV. TSMC and Samsung are in production with EUV lithography at 7nm and 5nm. Using 13.5nm wavelengths, EUV is a next-generation lithography technology that patterns tiny features on a chip in a fab.

Before chips are produced in a fab, chipmakers require a component called a photomask. For this, a device maker designs a chip, which is then translated into a file format. Using various equipment in a photomask facility, the file format is transformed into a mask, which is basically a master template for an IC design.

In the fab, the mask and wafer are placed in a lithography scanner. The scanner then projects light through the mask onto the wafer, creating patterns on the wafer.

During the EUV process as well as other steps, chipmakers would like to use an EUV pellicle, which prevents particles from landing on the mask. There are several reasons why pellicles are needed. Let’s say a particle lands on the mask or the mask has a defect. During the EUV lithographic process, the irregularities might get printed on the wafer, which in turn can impact yield or kill a die.

That’s where pellicles fit in. ASML is developing polysilicon-based EUV pellicles, but they are not quite ready. Others are developing EUV pellicles in R&D.

As a result, TSMC has decided to move into EUV production without a pellicle. Chipmakers can make do without pellicles, but in the process flow, they must clean the EUV mask often to get rid of the particles on the mask. Then, a vendor must inspect the mask more often to ensure there are no defects on the structure.

To mitigate the problem, TSMC has developed a new dry-clean technology for photomasks used in EUV.

Ultimately, though, the industry wants a pellicle. MoSi2 is one of several options. To make a pellicle based on this material, a silicon nitride material is first deposited on a surface, according to Hanyang University in a video presentation. Then, the material is wet etched and thinned. A MoSi2 material is sputtered on the surface. The surfaced is etch and cleaned, according to researchers.

The pellicle membrane was fabricated. “EUV reflectance was measured through a EUV coherent scattering microscope, and thermal stability was evaluated using a 355nm laser source emulating EUV exposure conditions,” said Ha Neul Kim, a researcher from Hanyang University, in the paper.

The pellicle demonstrated a low reflectance of 0.003%. “As a result, it was confirmed that MoSi2 pellicles showed sufficiently lower reflectance and comparable thermal durability under a 300-watt EUV matched source power,” Kim said. Further study is now underway to improve emissivity and mechanical stability.

Carbon nanotube pellicles
Imec has disclosed the latest results with the development of its carbon nanotube (CNT) pellicles for EUV lithography.

Multiple CNT-based pellicles from Imec were mounted on reticles and exposed in ASML’s NXE:3300 EUV scanner with good results. Imec’s CNT pellicles are still in R&D.

A full-field CNT-based pellicle Source: Imec

Imec is developing full-field CNT-based pellicles. CNTs are thin carbon sheets rolled into tubes. Since 2015, Imec has been working with selected CNT suppliers, such as Canatu Oy, Lintec of America and the Nano-Science & Technology Center.

The goal is to develop membranes that meet the EUV pellicle targets for properties like transmittance, thermal durability, permeability, and strength and to enable the good imaging results.

In Imec’s latest results, the tested pellicles had a single-pass EUV transmission up to 97%. The impact on imaging was found to be low and correctable based on critical dimension (CD), dose, and transmission measurements.“Imec has leveraged partners in the semiconductor industry, materials companies and fundamental research to develop an innovative EUV pellicle design with potential to survive scanner powers beyond 600 watts,” said Emily Gallagher, principal member of technical staff at Imec. “We have seen progress in carbon nanotube membrane development in the past year and, based on strong collaborations with our partners, are confident it will result in a high-performance pellicle solution in the near future.”

Besides pellicles, the industry is working on other technologies for EUV. For example, A team of researchers recently presented a new paper on the tradeoffs of photomask absorber materials for high-NA EUV lithography.