Daniel Fan

EUV resists towards 11nm half-pitch

As extreme ultraviolet lithography (EUVL) prepares for its insertion into the high-volume manufacturing phase, many challenges still remain to be addressed. Among several issues, development of EUV resists with tight specifications of sensitivity (dose), resolution (HP) and line-edge roughness (LER) is required. Chemically-amplified resists (CARs) have been the major paradigm in the development of EUV resists, although several alternatives, such as molecular resists and inorganic resists, are also under development. Here we present a comparative study of the performance of CARs using the PSI’s EUV interference lithography tool, which can achieve patterning down to 7 nm HP. Also the current status of EUV resist availability towards 11 nm HP technology nodes is discussed. We show resolution down to 12 nm HP with CARs. Nevertheless, for patterning below 18 nm HP, the resolution is achieved at the expanse of sensitivity and LER. The global trend of decreasing sensitivity with increasing LER is valid across the different resists. This trade-off between resolution, LER, and sensitivity (i.e. RLS trade-off) is mainly dominated by the acid diffusion blur and remains a challenge. In addition, pattern collapse becomes a significant problem with increasing resolution. This can be partly overcome by the reducing the resist thickness, which leads to an increase in LER. Therefore, a new trade-off between pattern-collapse limited resolution and LER emerges. These two trade-offs make the progress in EUV resist development increasingly difficult.

Photolithography reaches 6 nm half-pitch using extreme ultraviolet light

Abstract. Extreme ultraviolet interference lithography records the interference pattern of two diffracted, coherent light beams, where the pattern resolution is half the diffraction grating resolution. The fabrication of diffraction grating masks by e-beam lithography is restricted by the electron proximity effect and pattern transfer limitations into diffraction efficient materials. By patterning HSQ lines at a relaxed pitch to avoid the electron proximity effect, depositing conformal iridium via atomic layer deposition, followed by ion milling the top and bottom iridium and HSQ removal, we fabricated iridium diffraction gratings at double the line spacing of the original HSQ lines. Line/space patterns of 6-nm half-pitch patterns were achieved using these masks, marking a new record resolution in photolithography.

Nanolithography using Bessel Beams of Extreme Ultraviolet Wavelength

Bessel beams are nondiffracting light beams with large depth-of-focus and self-healing properties, making them suitable as a serial beam writing tool over surfaces with arbitrary topography. This property breaks the inherent resolution vs. depth-of-focus tradeoff of photolithography. One approach for their formation is to use circularly symmetric diffraction gratings. Such a ring grating was designed and fabricated for the extreme ultraviolet (EUV) wavelength of 13.5 nm, a candidate wavelength for future industrial lithography. Exposure of the aerial images showed that a Bessel beam with an approximately 1 mm long z-invariant central core of 223 nm diameter had been achieved, in good agreement with theory. Arbitrary patterns were written using the Bessel spot, demonstrating possible future application of Bessel beams for serial beam writing. Lithographic marks of ~30 nm size were also observed using a high resolution Bessel beam.

Facile fabrication of high-resolution extreme ultraviolet interference lithography grating masks using footing strategy during electron beam writing

The authors demonstrate a method for facile fabrication of transmission grating masks used in extreme ultraviolet interference lithography (EUV-IL) based on a footing strategy during electron beam writing of mask gratings. By modifying the electron beam lithography pattern and dose distribution, a thin footing is generated between the mask grating lines. This thin footing layer effectively prohibits subsequent metal deposition using a standard electroplating setup in the grating region, while outside the grating region metal is electrodeposited freely, absorbing EUV radiation and serving as an EUV-IL mask grating central photon stop layer. The strategy the authors present greatly simplifies the mask fabrication process and increases the yield, as the crucial central stop region can be fabricated directly and without further overlay exposure steps either using electron beam or photolithography.

From powerful research platform for industrial EUV photoresist development, to world record resolution by photolithography: EUV interference lithography at the Paul Scherrer Institute

Extreme ultraviolet interference lithography (EUV-IL, λ = 13.5 nm) has been shown to be a powerful technique not only for academic, but also for industrial research and development of EUV materials due to its relative simplicity yet record high-resolution patterning capabilities. With EUV-IL, it is possible to pattern high-resolution periodic images to create highly ordered nanostructures that are difficult or time consuming to pattern by electron beam lithography (EBL) yet interesting for a wide range of applications such as catalysis, electronic and photonic devices, and fundamental materials analysis, among others. Here, we will show state-of the-art research performed using the EUV-IL tool at the Swiss Light Source (SLS) synchrotron facility in the Paul Scherrer Institute (PSI). For example, using a grating period doubling method, a diffraction mask capable of patterning a world record in photolithography of 6 nm half-pitch (HP), was produced. In addition to the description of the method, we will give a few examples of applications of the technique. Well-ordered arrays of suspended silicon nanowires down to 6.5 nm linewidths have been fabricated and are to be studied as field effect transistors (FETs) or biosensors, for instance. EUV achromatic Talbot lithography (ATL), another interference scheme that utilizes a single grating, was shown to yield well-defined nanoparticles over large-areas with high uniformity presenting great opportunities in the field of nanocatalysis. EUV-IL is in addition, playing a key role in the future introduction of EUV lithography into high volume manufacturing (HVM) of semiconductor devices for the 7 and 5 nm logic node (16 nm and 13 nm HP, respectively) and beyond while the availability of commercial EUV-tools is still very much limited for research.

Photolithography reaches 6 nm half-pitch using EUV light

EUV interference lithography records the interference pattern of two diffracted, coherent light beams, where the pattern resolution is half the diffraction grating resolution. The fabrication of diffraction grating masks by e-beam lithography is restricted by the electron proximity effect and pattern transfer limitations into diffraction efficient materials. By patterning HSQ lines at a relaxed pitch to avoid the electron proximity effect, depositing conformal iridium via atomic layer deposition, followed by ion milling the top and bottom iridium and HSQ removal, we fabricated iridium diffraction gratings at double the line spacing of the original HSQ lines. 6 nm half-pitch patterns were achieved using these masks marking a new record resolution in photolithography.

Ultra-dense silicon nanowires using extreme ultraviolet interference lithography

Patterning of ultra-dense, large-area lines down to 11 nm half-pitch using extreme ultraviolet (EUV) interference lithography with two types of inorganic photoresist is shown. The resist patterns are transferred using plasma etching into silicon (Si) for both types of resist. 14 nm half-pitch silicon nanowires with 1:1 aspect ratio and square cross-sectional profile using a hafnium oxide based resist was achieved. For a silicon oxide based resist, the etching selectivity was shown to be critical, and a variety of etching strategies to overcome this deficiency are discussed.

Extreme-UV interference lithography at the Paul Scherrer Institute

Extreme-UV interference lithography (EUV-IL), at a wavelength of 13.5nm, has proved to be a powerful technique due both to its relative simplicity and record-high-resolution patterning capabilities. In an EUV-IL setup, a mask with transmissiondiffraction gratings is illuminated by a spatially coherent beam of EUV light from an undulator synchrotron source. Periodic images are then created by the interference of two or more diffracted coherent beams (see Figure 1). The sinusoidal aerial image produced by two-beam IL has a period that is half of the original mask grating period when first-order diffracted beams are used. Consequently, this method provides a demagnification of grating patterns that are written by electron beam lithography (EBL). Moreover, versatile periodic patterns and quasi-periodic patterns can be obtained by using multiple beams and by controlling their phases.1, 2 The ultimate resolution (i.e., half-pitch, HP) that can be achieved with EUV-IL, however, is limited by light diffraction. Although it is therefore theoretically possible to resolve features to below 4nm with EUV-IL,3 the achievable resolution is limited by the grating resolution and by other factors. In addition to high resolution and throughput, EUV-IL offers many other advantages, such as achromaticity, insensitivity to misalignment, and infinite depth of focus, making this lithography technique extremely useful.4 For example, with EUV-IL, it is possible to pattern high-resolution periodic images to create highly ordered and dense nanostructures. Such structures can be difficult or time-consuming to pattern by EBL, but are interesting for a wide range of applications, such as nanocatalysis,5 Figure 1. Schematic diagram of an extreme-UV interference lithography (EUV-IL) setup, in which first-order diffraction is used to create an aerial image on a resist, by interference. , 1, 2: Diffraction angles. m0;1;2: Diffraction orders. (Adapted from Mojarad et al., 2015.7)