The development and growth of semiconductor industry has been following Moor’s low in recent 50 years, and it is fair to say that the continuous shrinking of transistors in microchips has been mainly driven by the development of photolithography technology^[1,2,3]. Since the appearance of the first G-line (436nm wavelength) step-and-repeat system, namely wafer stepper, in late 1970s, this exposure system quickly has become mainstream and dramatically promoted the capability and efficiency of photolithography^[4,5]. Due to the Rayleigh diffraction effect, smaller wavelength of light has to been adopted in order to get a reduced feature size with high resolution through a lens system, which makes a semiconductor device smaller. After jumping onto the fast lane of step-and-repeat system, the light source adopted in photolithography has been continuing switching from excimer laser for i-line (365nm)^[6], to krypton fluoride (KrF) excimer laser for 248nm deep ultraviolet^[7] and to argon fluoride (ArF) for 193nm wavelength in order to get even smaller feature size^[8,9]. On this track, 157nm wavelength of F₂ excimer laser was supposed to be produced to reach smaller half pitch than 65nm^[10,11]. However, wavelength is not the only factor to improve the resolution of smaller pitches. Numerical aperture, NA as the Acronym, plays another role by the Rayleigh criterion:

R (smallest feature size) = k1 * λ / NA

Where λ is the wavelength of the light used, NA is the numerical aperture of the system’s lens and k1 is known as the resolution factor and accounts for all other process variables. Furthermore, since NA=n*sinα, where α is the light incident angle and n is the refractive index of the medium surrounding the lens, and liquid has a larger n than air, an immersion ArF scanner was developed to reach a higher resolution for smaller features, in which immersion fluid (mainly water) is well controlled to stay between last lens and wafer surface in order to get higher NA^[12] (Figure 1). The first commercial tool with this innovative technology was built and shipped by ASML in 2006 which started to dominate the market since then. The cost-effective implementation of immersion ArF scanner in the industry killed further development of tools with 157nm wavelength.

The implementation of immersion ArF scanner, enhanced by RET (Resolution Enhancement Technology) and multi-patterning technologies enabled the industry to reach 10nm/7nm FinFET tech node with acceptable cost, efficiency and die yield. But what is next? To resolve lines (half-pitch) smaller than 7nm, quadruple patterning or even more masks need to be used only for one functional layer, which will dramatically increase the cost and process variation. The industry has to follow Rayleigh criterion again to push the exposure wavelength to the limit.

The industrialization of EUV lithography (EUVL) has been turned on since the turn of the 21st century and even earlier. Almost all key players from the industry and academic institutes have been involved in EUV light source development, Bragg mirrors with multi-coated layer, reflection system in vacuum, reflection reticle design and material development, EUV photoresist synthesis and optimization, contamination and defects reduction, DTCO (Design Technology Co-Optimization) and EDA tool development, as well as many other related engineering fields^[13]. Most of the accomplishments of research and development have been concentrated to the ASML NXE EUVL module, which began its journey of High Volume Manufacture (HVM)^[1,14] since 2017. Modern EUV scanner is equipped with a high power EUV light source which can generate 13.5nm wavelength radiation at a high power up to 250W (above 400W light source is also under development), a full refection mirror system in a vacuum chamber which prevent high adsorption on EUV optical path and maximize reflectivity, as well as redesigned reflection mirror and reticle system with 0.33 NA from Zeiss SMT to enable up to 140 wph (Wafer Per Hour) throughput. Figure 2 shows visual structures of a deep UV immersion module TWINSCAN NXT:1980i and a latest TWINSCAN NXE-3400B EUV module which is clearly distinguishable from the deep UV module due to the light reflection system. With the help of EUV lithography system, together with economic affordable multi-patterning technologies semiconductor technology node can be pushed further to beyond 3nm, and extend Moore’s Law to next decades (Figure 3). In the following chapter, main sub-modules of a EUV scanner will be briefly reviewed.

Unlike the direct deep UV generation from excimer lasers, extreme UV light with 13.5nm wavelength can only be produced by excitation and relaxation of relatively inner electrons of atoms (e.g. Sn²⁰⁺), which could only be realized by atom ionization to form hot dense plasma^[15,16,17]. In a commercial ASML EUV system, high power CO₂ laser with 40~60kW has been applied to bombard liquid tin (Sn) droplet to generate dense plasma at high temperature around the droplet, where EUV radiation takes place^[18,19]. EUV photons are collected by a multilayer coated collector in an ellipsoidal shape (Figure 4A) and conducted to the reflection optics in a scanner. As a standalone component of ASML EUV scanner, the LPP (laser produced plasma) EUV light source is composed of high power laser generator, laser beam delivery system and EUV source vessel, which is one of the most distinguish parts of a EUV system compared with deep UV scanners^[15] . Due to the high gas absorption of 13.5nm wavelength UV light, the EUV vessel has to be kept in high vacuum which may suffer the system with particle contamination in contrast with deep UV chamber filled by clean nitrogen. In addition, high energy laser beam will evaporate liquid tin and produce tiny microparticles of tin vapor. These tin deposit contamination in the vacuum chamber was one of the biggest challenges in EUV system^[20]. ASML is implementing hydrogen buffer gas with pressure about 100 Pa around the collector mirror to decelerate tin ions and reduce atomic tin deposition by a chemical reaction: Sn (s) + 4H (g) --> SnH₄ (g), these SnH₄ gas product can be pumped away from the chamber, as shown in Figure 4^[21].

ASML subsidiary Cymer and Japan-based Company Gigaphoton are long-term rivals, both are struggling to win the battle to develop high power, high conversion efficiency light sources for EUV lithography applications^[22].

The following paragraph describe main parts of a typical LPP system in a market-available ASML NXE 3x00 EUV system.

Due to the strong absorption of 13.5nm EUV to any gases, the whole optical path must be fully reflective and enclosed in a vacuum environment, which is different from that of a deep UV scanner system. Moreover, in order to further reduce absorption and boost reflectivity, metal-based reflection mirrors is coated with alternating molybdenum and silicon (MoSi) layers with several nanometer in thickness (called Bragg mirrors), similar as the collector in EUV light source vessel. High reflectance is achieved with careful control of mirror quality, layer thicknesses, multilayer materials, interface quality, and surface termination. After several years of research and development, reflectance and film properties are relatively stable and can satisfy the requirements of an advanced EUV lithography system. The reproducibility of the reflectance peak was characterized to be as small as 0.2 percent^[24]. The coating uniformity has been improved to be better than 0.5 percent across a 150 mm diameter substrate^[25].

Reflection optics is also critical to next generation EUV scanner with high NA (> 0.5). High NA system will improve the wafer throughput to above 160 wph with dramatically reduced multi-pattern masks (number of LE), as shown in Figure 9. However, to reach high NA, reflection mirrors have to be redesigned to be asphere and the size of the mirror will be dramatically enlarged, accompany with EUV power over 400W^[26]. Besides, anamorphic half filed reticle and exposure system has been introduced with 8x Y-magnification instead of traditional 4x magnification in both X and Y direction (Figure 9). Accordingly mask stage and wafer stage acceleration have to be adjusted to support twice number of scans/shots on wafer. Figure 10 illustrates mask comparison between 0.33 NA and 0.5 NA and wafer shot difference.

High NA EUV will benefit wafer throughput and reduce mask layers, but at a cost of super high EUV light power, very large mirror, asymmetry and complicate reticle and wafer stage, as well as a special wafer cooling system. ASML future EXE module is supposed to be equipped with high NA optics^[26,27,28] .

In the EUVL reflective optical system, a reflective reticle is crucial to imaging quality and lithography process window on wafer. Similar as multiple coating on reflective mirrors in the vacuum system, 40 pairs of Si/Mo layers are deposited on the substrate of an EUV mask in order to get maximum reflection, which are capped by a thin film of Ruthenium (Ru) as thermal emission and reinforcing layer[^25,29]. On top of the thermal emission layer, absorber layers will be patterned to form “clear” or “dark” features. Material selection of absorber is critical to maximize absorption and minimize reflection. Shadowing effect and mask 3D effects are also related to absorber layer thickness and side-wall profile^[30]. Moreover, UV, thermal and mechanical stability of the reticle also need to be considered.

Stochastic effect of photo resist is another major concern for EUVL application^[40,41,42]. Due to limited light source power, short wavelength and high photon energy of EUV light, Auger electrons or 2^nd electrons will be generated after photon absorption by the resist, resulting in uncontrolled free electron path and photo acid spread after electronic relaxation, which is considered to be the main root cause of line edge roughness and CD variation^[43,44]. Interdisciplinary teams worldwide have been working on understanding fundamental mechanism of photo chemical reaction, new material development, and process control.

Gupta team from UCLA focus on LER analysis and impact on FEoL and BEoL of advanced FinFET technologies, LER mathematic model and LER impact on device reliability have been investigated^[45]. Resist half-pitch (HP) resolution, dose sensitivity and their relationship (Figure 16) to LER have been evaluated by Paul Scherrer Institut (PSI) to get a comprehensive solution^[46]. It was found that amount of photons, quencher and acid rank top three LER contributor, LER could be reduced by optimizing multi-trigger component ratio and quencher amount of the resist chemicals^[47,48].

Metal-contained photoresist, chemical amplified resists (CARs), photo acids (PAs) together with their backbone polymers have been investigated, material quantum yield and absorption coefficient have been measured and correlated to electron mean free path within resist, providing a theoretical guide for photoresist selection. Resist chemical processes after EUV radiation are widely investigated by teams from Berkeley lab, Molecular Foundry, LBNL, Columbia Hill Technical Consulting and IBM Almaden Research Center^[49,50]. Numeric model of imaging mechanism for sol-gel prepared organometallic polymer resists have been created by Inpria Corp., stochastic diffusion of secondary electrons has been simulated with changing dose^[51]. Innovative Inorganic-organic composite photoresist with a metal oxide core and polymer ligands has been synthesized as a non-CAR resist which is supposed to have smaller LWR (3σ <3nm)^[52,53,54]. Moreover, lithography process control and optimization are also essential to LER reduction. Toshiro Itani and Takahiro Kozawa from Japan have conducted experiments to improve line edge roughness by alternating development, bake, rinse solution as well as resist top and bottom coating^[55].

Outgassing is another issue during resist exposure, hydrogen-contained molecules can contaminate the vacuum chamber, mirrors and reticles. ASML has developed a Dynamic Gas Lock (DGL) membrane to prevent outgassing spread and suppress DUV and IR during the wafer exposure^[1] .

In this paper, we had a broad but not specialized overview of extreme ultraviolet lithography technology and its application in mainstream semiconductor wafer manufacturing. Major EUVL modules include light source and vessel, reflection mirror system under vacuum, reflective reticle and aerial inspection system. Multilayer material fabrication and photoresist characterization are also critical in terms of EUV patterning performance, defectivity and edge roughness control. As EUVL scanner is an integrated system, final wafer printing quality is determined by every component especially the weakest ones. Thanks to worldwide collaboration and big investments in recent years, critical issues related to light source, substrate material and process, multilayer defects, mask absorber and defects, as well as resist edge roughness have been overcome in an ASML EUV scanner system, and wafer HVM using these EUV tools are already realized in those leading fabs worldwide.

Major progresses and challenges that are related to wafer throughput and pattern printing quality are summarized below.

250W light power boosts wafer throughput to 140wph, which makes ASML EUV scanner ready for HMV. Next generation EUV source with further increased EUV energy power, improved conversion efficiency and better dose control is under development, not only to raise wafer throughput but also to further minimize LER which is closely related to dose on wafer. It was found 40mJ/cm² dose (compared with 20mJ/cm² from current module) can dramatically reduce LER to smaller than 3nm. 400W EUV source will be implemented in next module of ASML NXE scanner soon. High power EUV is also critical to next generation high NA scanner.

Impurities, thickness inhomogeneity of multilayer mirrors and reticles can change EUV light intensity or phase which will generate defects on wafer. Reticle surface roughness also contributes to LER. Besides superb quality fabrication of multilayer mirrors and reticles, the control of contamination and particles in vacuum chamber is also a big concern since there is no clean pressure gap protection in the reflection chamber and high energy natural of EUV. With the introduction of pellicles and DGL membrane, chamber defects have been reduced to a very low level. In order to detect not only surface impurities but also light phase changes due to internal defects, AIMS inspection system has been developed by ZEISS together with SEMATECH. This tool could check wafer aerial image and detect potential pattern failure on wafer.

Compared with deep UV lithography, line edge roughness is much worse for EUVL due to relatively low dose and secondary electron diffusion (stochastic effect). New photo resists such as metal-oxide with polymer ligands are being developed, lithography process flow has been optimized, and reticle flatness is well-controlled, all efforts have successfully reduced the LER to an acceptable level for an ASML EUV system. Further investigation is trying to reduce LER to below 3nm for 3σ from current 3.5nm range from inorganic resist and 5nm range from CAR.

Due to absorber thickness and the chief-ray angle of incident EUV light onto the mask, horizontal and vertical features show different imaging behavior and critical dimension change on wafer, this is called shadowing effect. Mask 3D is also related to reticle blank flatness and bow, multilayer EUV reflectivity, and absorber properties. These effects will decrease the margin of process window with a reduced or shifted depth of focus (DOF). By introducing phase control SMO and OPC correction, orientation-dependent mask CD bias could be compensated. High-k absorbers are also under development to help further reduce shadowing effect. In next generation of high-NA EUV scanner, anamorphic half filed reticle will be introduced to overcome x-y asymmetry and improve throughput^[56], but with the cost of design, OPC and mask complexity.

EUVL has achieved great progress since 2015, most of breakthroughs take place after the formation of EUV industry alliances among big player like ASML, INTEL, TSMC and SUMSANG. Industry requirements combined with strong financial support finally drive this technology into HVM. As the only EUVL scanner commercial provider, ASML is going to sell over 30 EUV modules in 2019. The market is ramping up and this revolutionary technology will lead the industry to the next decade^[57] (Figure 18).