Manufacturing Bits: Oct. 6

High-NA EUV mask materials
A team of researchers have presented a new paper on the tradeoffs of photomask absorber materials for high-NA extreme ultraviolet (EUV) lithography.

In the paper, researchers concluded that the industry will likely require an alternative mask absorber stack for high-numerical aperture (high-NA) EUV lithography. Fraunhofer, Imec, ASML and Zeiss contributed to the work.

The paper appears in the SPIE Digital Library as well as the current issue of the Journal of Micro/Nanolithography, MEMS, and MOEMS.

Nonetheless, EUV lithography is a next-generation technology. Used in advanced fabs, EUV involves the use of a giant and expensive lithography scanner, which patterns tiny features on chips.

Today, EUV lithography is in production at the 7nm and 5nm logic nodes at Samsung and TSMC. Chipmakers are using ASML’s EUV scanner, called the NXE:3400C. Using a 13.5nm wavelength, the 0.33 NA system has 13nm resolutions.

Chipmakers are patterning the tiny chip features using an EUV-based single patterning approach, which creates patterns with a single lithographic exposure. Chipmakers would like to extend EUV single patterning as far as possible, because it’s a straightforward process.

Beyond a certain pitch, though, chipmakers may have to implement double-patterning EUV, which is more complex and expensive.

That’s why chipmakers are pushing for a new technology called high-NA EUV lithography. Targeted for 3nm and beyond, high-NA EUV enables chipmakers to continue with the simpler single-patterning approach.

But high-NA EUV also involves the development of a completely new lithographic system. Still in R&D, ASML’s new high-NA EUV system features a complex 0.55 NA lens capable of 8nm resolutions. The 0.55 NA tool is targeted for the 3nm node in 2023, but it will likely appear at a later node, such as 2nm. The mammoth-size tool is complex and expensive.

If or when the industry moves into the high-NA EUV era, end-users will require new types of resists and photomasks.

Both are critical and EUV masks are complex. An EUV mask consists of 40 to 50 alternating layers of silicon and molybdenum on a substrate, resulting in a multi-layer stack that is 250nm to 350nm thick. A ruthenium capping layer is deposited on the multi-layer stack, followed by a tantalum absorber.

EUV masks work for today’s 0.33 NA EUV scanners. The industry is mainly using binary EUV masks. But high-NA EUV may require different mask types with new and complex materials. Advanced EUV-based binary masks and phase-shift masks are in R&D.

“Next-generation extreme ultraviolet (EUV) systems with numerical apertures of 0.55 have the potential to provide sub-8-nm half-pitch resolution. The increased importance of stochastic effects at smaller feature sizes places further demands on scanner and mask to provide high contrast images,” according to the paper from Fraunhofer, Imec, ASML and Zeiss.

The absorber is a critical part of the high-NA mask puzzle, according to researchers in the paper. But there are several complex considerations here.

“Pushing the anamorphic NA = 0.55 EUV projection optics to k1 values below 0.4 and to its ultimate resolution limit will require an alternative mask absorber stack,” according to the paper from Fraunhofer, Imec, ASML and Zeiss. “Simulations of various use cases and material options indicate two main types of solutions: high k materials (k ≥ 0.05), especially for vertical lines spaces and for vertically oriented long vias and low refractive index materials (n ≈ 0.9) to provide phase shift mask solutions for the other 2D use cases. It is important to find materials with the indicated range of optical properties. From a modeling perspective, the specific numbers of n and k are less important, but a thickness optimization of the absorber stack is required to obtain the best performance.”

Researchers employed mask diffraction and imaging simulation to understand the impact of the EUV mask absorber and to identify the most appropriate optical parameters for high- NA EUV imaging. Andreas Erdmann (Fraunhofer), Hazem S. Mesilhy (Fraunhofer), Peter Evanschitzky (Fraunhofer), Vicky Philipsen (Imec), Frank J. Timmermans (ASML) and Markus Bauer (Zeiss) are the authors of the paper.

Focused electron beam-induced processing
The Georgia Institute of Technology and Pusan National University have developed a direct-write electron-beam technology that enables high-resolution patterns on two-dimensional layers of graphene oxide.

The technology, called focused electron beam-induced processing (FEBIP), has demonstrated the ability to both etch and deposit high-resolution nanoscale patterns in structures.

This is done by varying the energy and dose of the focused electron beams. In one application, a high-energy electron beam in the system hit a surface, enabling nanoscale features. Then, in another application, the technology enables etched structures in surfaces, which could be filled with metals.

The technology is suitable for depositing materials such as metals and semiconductors. For this work, researchers demonstrated the technology with graphene oxide. These types of 2D materials are difficult to pattern using traditional techniques.

“Focused electron beam-induced processing (FEBIP) enables a material chemistry/site-specific, high-resolution multimode atomic scale processing and provides unprecedented opportunities for ‘direct-write’, single-step surface patterning of 2D nanomaterials with an in-situ imaging capability,” said Songkil Kim from the School of Mechanical Engineering, at Pusan National University, in ACS Applied Materials & Interface, a technology journal. Others contributed to the work.

“It allows for realizing a rapid multiscale/multimode approach, ranging from an atomic scale manipulation (e.g., via targeted defect introduction as an active site) to a large-area surface modification on nano- and microscales, including patterned doping and material removal/deposition with 2D (in-plane)/three-dimensional (3D) (out-of-plane) control,” Kim said.

“By timing and tuning the energy of the electron beam, we can activate interaction of the beam with oxygen in the graphene oxide to do etching, or interaction with hydrocarbons on the surface to create carbon deposition,” added Andrei Fedorov, professor and the Rae S. and Frank H. Neely Chair in the George W. Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “With atomic-scale control, we can produce complicated patterns using direct write-remove processes. Quantum systems require precise control on an atomic scale, and this could enable a host of potential applications.

“We are demonstrating structures that would otherwise be impossible to produce,” Fedorov said. “We want to enable the exploitation of new capabilities in areas such as quantum devices. This technique could be an imagination enabler for interesting new physics coming our way with graphene and other interesting materials.”