title: Primate city

text: Some global cities are considered national or regional primate cities. An example of a global city that is as a primate city is Istanbul in Turkey. Istanbul serves as the primate city of Turkey due to the unmatched economic, political, cultural, and educational influence that the city possesses in comparison to other Turkish cities such as the capital Ankara, İzmir, or Bursa. Another example is London in the United Kingdom. However, not all regions or countries will even possess a primate city. The United States has never had a primate city on a national level due to the decentralized nature of the country, and because the second-largest city of the country, Los Angeles, is not far behind in population and GDP, from the largest city of the country, New York City. The metropolitan area of New York City has 21 million residents and Los Angeles has 16 million residents. Mexico City, Paris, Cairo, Jakarta, and Seoul have been described as primate cities in their respective countries.


title: Primate city

text: Sub-national divisions can also have primate cities. For instance, New York City is New York State's primate city because its population is 32 times bigger than the state's second-largest city of Buffalo. New York City has 44% of the population and has 65% of the GDP of New York State. China does not have a primate city at a national level, but a number of provincial capitals are disproportionately larger than other urban areas in the respective province. For example Henan, Hubei and Sichuan have provincial capitals (Zhengzhou, Wuhan, and Chengdu respectively) that are significantly larger than the second-largest city despite these provinces having the population of a large European country.


title: Primate city

text: Bangkok, the capital of Thailand, has been called "the most primate city on Earth" when it was 40 times larger than the second-largest city of that time, Nakhon Ratchasima, in the year 2000. As of 2022, Bangkok is nearly nine times larger than Thailand's current second-largest city of Chiang Mai. Taking the concept from his examination of the primate city during the 2010 Thai political protests and applying it to the role that primate cities play if they are national capitals, researcher Jack Fong noted that when primate cities like Bangkok function as national capitals, they are inherently vulnerable to insurrection by the military and the dispossessed. He cites the fact that most primate cities serving as national capitals contain major headquarters for the country. Thus, logistically, it is rather "efficient" for national targets to be contested since they are all in one major urban environment.


title: Primate city

text: The metropolitan area of the city of Moscow, the capital of Russia, is almost four times the size of the metropolitan area of the next largest city, Saint Petersburg, and plays a unique and uncontested role of the cultural and political center of the country. It can therefore be considered to be a primate city.


title: Primate city

text: Primate cities need not be capital cities: governments may attempt to establish a new planned capital city to challenge the primacy of the largest city and provide more balanced growth, for example in Tanzania, Dar es Salaam can still be considered a primate city although the capital was moved to Dodoma in 1996. A non-capital primate city may also emerge organically: for example, the existing city of Wellington was chosen to be New Zealand's capital in 1865, although Auckland commands a greater share of the population and economy.


title: Primate city

text: For the Philippines, figures are for Metro Manila and Metro Cebu. Manila is the national capital, which is within Metro Manila, a region. Meanwhile, Cebu City is the capital city of the province of Cebu, with Metro Cebu being its main urban center. Metro Manila is within Mega Manila, the megapolis that has a population of around 25 million.


title: Primate city

text: Although Belize does not have a primate city, Belize City is more than twice the size of San Ignacio, the country's second-largest city/urban area. It is also the cultural and economic centre of Belize. The capital is Belmopan, third-largest in the country.


title: Primate city

text: Australia does not have a primate city, but at the state level, each of the capital cities of the states and territories act as the primate city of that state or territory.


title: RollerCoaster Tycoon

text: RollerCoaster Tycoon is a series of simulation video games about building and managing an amusement park. Each game in the series challenges players with open-ended amusement park management and development, and allowing players to construct and customize their own unique roller coasters and other thrill rides.


title: RollerCoaster Tycoon

text: The first game was created by Scottish programmer Chris Sawyer, with assistance from various leading figures from the real-world roller coaster and theme park industry. The rest of the series contains three other main games, expansion packs, a number of ports, and a mobile installment. A refresh of the series, "RollerCoaster Tycoon World", was released in November 2016, and followed up by "RollerCoaster Tycoon Adventures" for Switch in 2018 and PC in 2019.


title: RollerCoaster Tycoon

text: Licensing for the series is currently held by Atari SA, who renewed their deal with Chris Sawyer on October 11, 2022 for ten more years.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon" was released for Microsoft Windows on 22 March 1999. "RollerCoaster Tycoon" was later ported to the Xbox game console in 2003. The game was developed by Chris Sawyer and published by Hasbro Interactive under the MicroProse brand.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon" received two expansion packs: "Added Attractions" (released in North America as "Corkscrew Follies") in 1999, and "Loopy Landscapes" in 2000. Two special editions were released: "RollerCoaster Tycoon Gold"/"Totally RollerCoaster" in 2002, which contained the original game, "Corkscrew Follies", and "Loopy Landscapes"; and "RollerCoaster Tycoon Deluxe" in 2003, which contained the content in "Gold" plus more designs for the different customizable rides.


title: RollerCoaster Tycoon

text: A port for the Xbox was released on 23 March 2003, handled by Frontier Developments and published by Infogrames Interactive. This version featured both expansion packs as well.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon 2" was released on October 15th 2002. The game was developed by Chris Sawyer and published by Infogrames Interactive.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon 2" has two official expansion packs: "Wacky Worlds" and "Time Twister", both released in 2003 and had no involvement from Chris Sawyer, instead being handled by Frontier Developments. The "Combo Park Pack" edition contains the original game and the "Wacky Worlds" expansion. The "Triple Thrill Pack" contains the original game and both expansions.


title: RollerCoaster Tycoon

text: In April 2014, an open-source project, known as OpenRCT2, was launched to enhance the gameplay of RollerCoaster Tycoon 2, including fixing bugs and allowing the game to run natively on macOS, Linux and modern Windows. The game is completely re-written in the C++ programming language, but still relies on assets from the original game. OpenRCT2 reduces limitations in the original, and adds other completely new features such as multiplayer.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon 3" was released on November 2nd 2004 in North America. Although the core features of "RollerCoaster Tycoon 3" are based on the previous games, Chris Sawyer, the developer of the first two games, acted only as a consultant. The game was developed by Frontier Developments instead, and published and advertised by Atari Interactive featuring a completely different structure.


title: RollerCoaster Tycoon

text: Two expansion packs for "RollerCoaster Tycoon 3" were released - "Soaked!" and "Wild!". A bundle, "RollerCoaster Tycoon 3 Gold", was also released, including the original game and the "Soaked!" expansion pack; this was followed by "RollerCoaster Tycoon 3 Platinum" (Deluxe for the EU version of the game), including both expansion packs and the original game. "RollerCoaster Tycoon 3 Platinum" was released for Windows and Mac OS X.


title: RollerCoaster Tycoon

text: A remastered version of the game titled "RollerCoaster Tycoon 3 Complete Edition", developed and published by Frontier Foundry, was released for Windows, macOS, and Nintendo Switch, featuring widescreen support and revamped controls to accommodate for Switch features.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon World" was developed by Nvizzio Creations for Atari Interactive and RCTO Productions and released on 16 November 2016. The installment is different from previous games in that players build coasters with a spline-based system. It also introduced a new "Architect mode" and "safety-rating" options when building coasters. The game is also the first to incorporate the Steam Workshop.


title: RollerCoaster Tycoon

text: The game received largely negative reviews, both from critics and fans of the franchise, particularly compared to "Planet Coaster", which was released the day after and received largely positive reviews.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon 3D" was released on October 16th 2012. It was developed by n-Space for the Nintendo 3DS. While using many assets and engine content from "Rollercoaster Tycoon 3", this game reverted to an isometric view and, due to the limitations of the Nintendo 3DS, removed features such as additional scenery and pools.


title: RollerCoaster Tycoon

text: "RollerCoaster Tycoon 4 Mobile" was released on April 10th 2014. The game was initially available for iPhone, iPad and iPod touch devices. The game was later released for Android devices (OS 4.0.3 and higher) on October 18th 2014. The game was developed by On5 Games without Chris Sawyer's input (other than licensing). The game returns to the isometric view used in the first two games. The game is built on the freemium model with social media integration.


title: RollerCoaster Tycoon

text: This installment was universally panned amongst fans and critics alike. It was criticized due to Chris Sawyer's absence in the making of the game and the heavy use of microtransactions and wait times.


title: RollerCoaster Tycoon

text: Released in August 2015, "RollerCoaster Tycoon 3: Mobile Edition" is an iOS version of "RollerCoaster Tycoon 3" developed and published by Frontier and is a faithful port of the original game (tutorial mode, original 18 scenarios for career, and a sandbox).


title: RollerCoaster Tycoon

text: On December 22nd 2016 a modified port of both "RollerCoaster Tycoon" and "RollerCoaster Tycoon 2" was released for iOS and Android, featuring a single price point for the majority of content from both of the original games. It also features three in-app purchases for Expansion Pack content, based on the two expansion packs from "RollerCoaster Tycoon 2" and an editor pack that allows users to create custom scenarios.


title: RollerCoaster Tycoon

text: First released for iOS devices in December 2016, "RollerCoaster Tycoon Touch" is a free-to-play mobile port of "RollerCoaster Tycoon World". Like "RollerCoaster Tycoon 4", it contains in-app purchases and wait times, but expands upon the social media integration. An Android version was released in April 2017. In October 2017, items based on the Barbie doll brand were added to the game. On January 28th items based on Shaun The Sheep were added to the game.


title: RollerCoaster Tycoon

text: An on-rails shooter based on the RollerCoaster Tycoon franchise, named RollerCoaster Tycoon Joyride, was released in 2018. The game was poorly received.


title: RollerCoaster Tycoon

text: In January 2018, Atari Game Partners announced it was seeking equity crowdfunding via the StartEngine platform in order to develop a new game in the series. Titled "RollerCoaster Tycoon Adventures", it is an adaption of "RollerCoaster Tycoon Touch" and was released for the Nintendo Switch in Europe on November 29th 2018, and in North America on December 13th. The game was also released on Microsoft Windows via the Epic Games Store on March 19th 2019.


title: RollerCoaster Tycoon

text: RollerCoaster Tycoon Puzzle (originally known as RollerCoaster Tycoon Story) is a free-to-play entry in the series developed by Graphite Lab. Atari released the game in January 2020 for iOS and Android mobile devices.


title: RollerCoaster Tycoon

text: The free-to-play title is based on the tile-matching genre, in which the tiles to match move each turn on rollercoaster tracks within each level. Completing levels helps the player to restore a run-down theme park as part of the game's narrative.


title: RollerCoaster Tycoon

text: The player is given control over an amusement park and is tasked with reaching particular goals, such as improving the park's value, attracting more guests, or maintaining the park rating. Some scenarios in the game provide an empty plot of land and allow the player to build a park from scratch, while others provide a ready-built park which usually suffers from deterioration, bad planning, or underdevelopment. The player must wisely invest the limited amount of money provided.


title: RollerCoaster Tycoon

text: Most scenarios require that the goals be achieved by a specific in-game date, or else the scenario is not 'complete'. Completion of scenarios is a prerequisite for unlocking further scenarios in the first "RollerCoaster Tycoon" game. In "RollerCoaster Tycoon 2", all the scenarios are available for play and the player can complete them in any order they choose.


title: RollerCoaster Tycoon

text: The player is responsible for building out the park such as modifying terrain, constructing footpaths, adding decorative elements, installing food/drink stalls and other facilities, and building rides and attractions. Many of the rides that can be built are roller coasters or variations on that, such as log flumes, water slides and go-kart tracks. The player can build these out with hills, drops, curves, and other 'special' track pieces (such as loops, corkscrews and helixes), limited only by cost and the geography of the park and other nearby attractions. There are also stationary rides, such as Ferris wheels, merry-go-rounds, and bumper cars, most of which only contain single ride 'piece' and are very limited in terms of variation. Rides are ranked on scales of excitement, intensity, and nausea, all which influence which park guests will ride those attractions and how they will behave afterward. The player can set the prices for park admission rides and guest amenities, although care must be taken so that guests will not think prices are too high. The player is also responsible for hiring park staff to maintain the rides, keep the park clean, enforce security, and entertain guests. Players may also invest in 'research', which unlocks new rides and improvements as time goes on, though it costs money to continue research. Research in a particular category is disabled when all attractions in that category are researched.


title: RollerCoaster Tycoon

text: The guests, who are integral to the gameplay, are treated as separate entities which can each have particular characteristics and be tracked by the player around the park. The game keeps track of how much money they have, what they are carrying, their thoughts, and what their current needs are (thirst, hunger, etc.). Each guest also has some unique features such as their preferred ride intensity, and their nausea tolerance. Some scenarios are even biased towards a specific guest demographic and require the player to take this into account in designing the park. In "RollerCoaster Tycoon 3", the player can create their own guest groups to visit their parks.


title: RollerCoaster Tycoon

text: Sequels have continually upgraded the number of rides and amount of customization available to the player.


title: RollerCoaster Tycoon

text: "Planet Coaster", which has been called a spiritual successor to "RollerCoaster Tycoon 3", was developed and published by Frontier Developments for Microsoft Windows and was released worldwide on November 17th 2016. Frontier Developments had previously worked in the amusement park construction and management genre with the Xbox port of "RollerCoaster Tycoon", "RollerCoaster Tycoon 3", "Thrillville", "Thrillville: Off the Rails", and "Zoo Tycoon".


title: RollerCoaster Tycoon

text: "OpenRCT2" (2014) is an open-source version of "RollerCoaster Tycoon 2" which adds some additional features and fixes numerous bugs.


title: RollerCoaster Tycoon

text: Several user-created rollercoasters received media attention after footage of them were posted on various imageboards and social media.


title: RollerCoaster Tycoon

text: The "RollerCoaster Tycoon" series spawned a popular Internet meme based on a series of posts made in 2012 on 4chan featuring a slow, ghost train track ride called "Mr. Bones' Wild Ride" that took four in-game years to complete, leading the passengers in the game to express "I want to get off Mr. Bones Wild Ride". At the ride's conclusion, the passengers were then greeted by a skeleton prop with a top hat, with a sign stating "The ride never ends". Both phrases have become popular memes.


title: RollerCoaster Tycoon

text: The franchise has also spawned a board game and a pinball machine by Stern, both released in 2002, and a series of gamebooks released in 2002 and 2003. In 2010, it was reported that Sony Pictures Animation had acquired the rights to develop a film adaptation of the series.


title: Extreme ultraviolet lithography

text: Extreme ultraviolet lithography (also known as EUV or EUVL) is an optical lithography technology used in steppers, machines that make integrated circuits (ICs) for computers and other electronic devices. It uses a range of extreme ultraviolet (EUV) wavelengths, roughly spanning a 2% FWHM bandwidth about 13.5 nm, to produce a pattern by exposing reflective photomask to UV light which gets reflected onto a substrate covered by photoresist. It is widely applied in semiconductor device fabrication process.


title: Extreme ultraviolet lithography

text: As of 2022, ASML Holding is the only company who produces and sells EUV systems for chip production, targeting 5 nm and 3 nm. At the 2019 International Electron Devices Meeting (IEDM), TSMC reported use of EUV for 5 nm in contact, via, metal line, and cut layers, where the cuts can be applied to fins, gates or metal lines. At IEDM 2020, TSMC reported their 5 nm minimum metal pitch to be reduced 30% from that of 7 nm, which was 40 nm. Samsung's 5 nm is lithographically the same design rule as 7 nm, with a minimum metal pitch of 36 nm.


title: Extreme ultraviolet lithography

text: In the 1960s, visible light was used for IC-production, with wavelengths as small as 435 nm (mercury "g line"). Later UV light was used, with wavelength of at first 365nm (mercury "i line"), then excimer wavelengths first of 248 nm (krypton fluoride laser) and then 193 nm (argon fluoride laser), which was called deep UV. The next step, going even smaller, was dubbed Extreme UV or EUV. The EUV technology was considered impossible by many. EUV is absorbed by glass and even air, so instead of using lenses, as before, to focus the beams of light, mirrors in a vacuum would be needed and a reliable production of EUV was also problematic. The then leading producers of steppers, Japanese companies Canon and Nikon gave up trying. And some even predicted the end of Moore's law.


title: Extreme ultraviolet lithography

text: But in 2018 Dutch company ASML finally succeeded after 2 decades of research. This led MIT Technology Review to name it 'the machine that saved Moore's law'. The first prototype in 2006 produced one wafer in 23 hours. As of 2022, a stepper produces 200 wafers per hour. The stepper uses Zeiss lenses, which that company calls "the most precise mirrors in the world" and are produced by locating imperfections and then knocking off individual molecules.


title: Extreme ultraviolet lithography

text: This made the once small company ASML the world leader in the production of steppers and monopolist in this cutting edge technology and resulted in a record turnover of 18.6 billion € in 2021, dwarfing their competitors Canon and Nikon. Because it is such a key technology for development in many fields, the United States even pressured Dutch authorities to not sell these machines to China. ASML has followed the guidelines of Dutch export controls and until further notice, will have no authority to ship the machines to China.


title: Extreme ultraviolet lithography

text: EUV photomasks work by reflecting light, which is achieved by using multiple alternating layers of molybdenum and silicon. This is in contrast to conventional photomasks which work by blocking light using a single chromium layer on a quartz substrate. An EUV mask consists of 40 alternating silicon and molybdenum layers; this multilayer acts to reflect the extreme ultraviolet light through Bragg diffraction; the reflectance is a strong function of incident angle and wavelength, with longer wavelengths reflecting more near normal incidence and shorter wavelengths reflecting more away from normal incidence. The pattern is defined in a tantalum-based absorbing layer over the multilayer. The multilayer may be protected by a thin ruthenium layer.


title: Extreme ultraviolet lithography

text: Blank photomasks are mainly made by two companies: AGC Inc. and Hoya Corporation. A blank photomask is covered with photoresist, which is then baked (solidified) in an oven, and later exposed to laser light, using maskless lithography. The exposed photoresist is developed (removed) and the unprotected areas are etched. The remaining photoresist is then removed. Masks are then inspected and later repaired using an electron beam. Etching must be done to a very specific depth thus making etching difficult when compared with conventional photomask manufacturing.


title: Extreme ultraviolet lithography

text: The tool consists of a laser-driven tin (Sn) plasma light source, reflective optics comprising multilayer mirrors, contained within a hydrogen gas ambient. The hydrogen is used for keeping the EUV collector mirror in the source free of Sn deposition.


title: Extreme ultraviolet lithography

text: EUVL is a significant departure from the deep ultraviolet lithography standard. All matter absorbs EUV radiation. Hence, EUV lithography requires a vacuum. All optical elements, including the photomask, must use defect-free molybdenum/silicon (Mo/Si) multilayers (consisting of 40 Mo/Si bilayers) that act to reflect light by means of interlayer interference; any one of these mirrors absorb around 30% of the incident light.


title: Extreme ultraviolet lithography

text: Current EUVL systems contain at least two condenser multilayer mirrors, six projection multilayer mirrors and a multilayer object (mask). Since the mirrors absorb 96% of the EUV light, the ideal EUV source needs to be much brighter than its predecessors. EUV source development has focused on plasmas generated by laser or discharge pulses. The mirror responsible for collecting the light is directly exposed to the plasma and is vulnerable to damage from high-energy ions and other debris such as tin droplets, which require the costly collector mirror to be replaced every year.


title: Extreme ultraviolet lithography

text: The required utility resources are significantly larger for EUV compared to 193 nm immersion, even with two exposures using the latter. Hynix reported at the 2009 EUV Symposium that the wall plug efficiency was ~0.02% for EUV, i.e., to get 200-watts at intermediate focus for 100 wafers-per-hour, one would require 1-megawatt of input power, compared to 165-kilowatts for an ArF immersion scanner, and that even at the same throughput, the footprint of the EUV scanner was ~3x the footprint of an ArF immersion scanner, resulting in productivity loss. Additionally, to confine ion debris, a superconducting magnet may be required.


title: Extreme ultraviolet lithography

text: DUV vs. EUV Tool Energy Consumption (measured 2020): EUV tools consume at least 10x more energy than immersion tools.


title: Extreme ultraviolet lithography

text: The following table summarizes key differences between EUV systems in development and ArF immersion systems which are widely used in production today:


title: Extreme ultraviolet lithography

text: The different degrees of resolution among the 0.33 NA tools are due to the different illumination options. Despite the potential of the optics to reach sub-20 nm resolution, secondary electrons in resist practically limit the resolution to around 20 nm (more on this below).


title: Extreme ultraviolet lithography

text: Neutral atoms or condensed matter cannot emit EUV radiation. Ionization must precede EUV emission in matter. The thermal production of multicharged positive ions is only possible in a hot dense plasma, which itself strongly absorbs EUV. As of 2016, the established EUV light source is a laser-pulsed tin plasma. The ions absorb the EUV light they emit, and are easily neutralized by electrons in the plasma to lower charge states which produce light mainly at other, unusable wavelengths, which results in a much reduced efficiency of light generation for lithography at higher plasma power density.


title: Extreme ultraviolet lithography

text: The throughput is tied to the source power, divided by the dose. A higher dose requires a slower stage motion (lower throughput) if pulse power cannot be increased.


title: Extreme ultraviolet lithography

text: EUV collector reflectivity degrades ~0.1-0.3% per billion 50 kHz pulses (~10% in ~2 weeks), leading to loss of uptime and throughput, while even for the first few billion pulses (within one day), there is still 20% (+/-10%) fluctuation. This could be due to the accumulating Sn residue mentioned above which is not completely cleaned off. On the other hand, conventional immersion lithography tools for double patterning provide consistent output for up to a year.


title: Extreme ultraviolet lithography

text: Recently, the NXE:3400B illuminator features a smaller pupil fill ratio (PFR) down to 20% without transmission loss. PFR is maximized and greater than 0.2 around a metal pitch of 45 nm.


title: Extreme ultraviolet lithography

text: Due to the use of EUV mirrors which also absorb EUV light, only a small fraction of the source light is finally available at the wafer. There are 4 mirrors used for the illumination optics, and 6 mirrors for the projection optics. The EUV mask or reticle is itself an additional mirror. With 11 reflections, only ~ 2% of the EUV source light is available at the wafer.


title: Extreme ultraviolet lithography

text: The EUV light source limits tool uptime besides throughput. In a two-week period, for example, over seven hours downtime may be scheduled, while total actual downtime including unscheduled issues could easily exceed a day. A dose error over 2% warrants tool downtime.


title: Extreme ultraviolet lithography

text: While state-of-the-art 193 nm ArF excimer lasers offer intensities of 200 W/cm, lasers for producing EUV-generating plasmas need to be much more intense, on the order of 10 W/cm. A state-of-the-art ArF immersion lithography 120 W light source requires no more than 40 kW while EUV sources are targeted to exceed 40 kW.


title: Extreme ultraviolet lithography

text: The power target for EUV lithography is at least 250 W, while for other conventional lithography sources, it is much less. For example, immersion lithography light sources target 90 W, dry ArF sources 45 W, and KrF sources 40 W. High-NA EUV sources are expected to require at least 500 W.


title: Extreme ultraviolet lithography

text: A fundamental aspect of EUVL tools, resulting from the use of reflective optics, is the off-axis illumination (at an angle of 6 degrees, in different direction at different positions within the illumination slit) on a multilayer mask. This leads to shadowing effects resulting in asymmetry in the diffraction pattern that degrade pattern fidelity in various ways as described below. For example, one side (behind the shadow) would appear brighter than the other (within the shadow).


title: Extreme ultraviolet lithography

text: The behavior of light rays within the plane of reflection (affecting horizontal lines) is different from the behavior of light rays out of the plane of reflection (affecting vertical lines). Most conspicuously, identically sized horizontal and vertical lines on the EUV mask are printed at different sizes on the wafer.


title: Extreme ultraviolet lithography

text: The combination of the off-axis asymmetry and the mask shadowing effect leads to a fundamental inability of two identical features even in close proximity to be in focus simultaneously. One of EUVL's key issues is the asymmetry between the top and bottom line of a pair of horizontal lines (the so-called "two-bar"). Some ways to partly compensate are the use of assist features as well as asymmetric illumination.


title: Extreme ultraviolet lithography

text: An extension of the two-bar case to a grating consisting of many horizontal lines shows similar sensitivity to defocus. It is manifest in the critical dimension (CD) difference between the top and bottom edge lines of the set of 11 horizontal lines.


title: Extreme ultraviolet lithography

text: Polarization by reflection also leads to partial polarization of EUV light, which favors imaging of lines perpendicular to the plane of the reflections.


title: Extreme ultraviolet lithography

text: The EUV mask absorber, due to partial transmission, generates a phase difference between the 0th and 1st diffraction orders of a line-space pattern, resulting in image shifts (at a given illumination angle) as well as changes in peak intensity (leading to linewidth changes) which are further enhanced due to defocus. Ultimately, this results in different positions of best focus for different pitches and different illumination angles. Generally, the image shift is balanced out due to illumination source points being paired (each on opposite sides of the optical axis). However, the separate images are superposed and the resulting image contrast is degraded when the individual source image shifts are large enough. The phase difference ultimately also determines the best focus position.


title: Extreme ultraviolet lithography

text: The multilayer is also responsible for image shifting due to phase shifts from diffracted light within the multilayer itself. This is inevitable due to light passing twice through the mask pattern.


title: Extreme ultraviolet lithography

text: The use of reflection causes wafer exposure position to be extremely sensitive to the reticle flatness and the reticle clamp. Reticle clamp cleanliness is therefore required to be maintained. Small (mrad-scale) deviations in mask flatness in the local slope, coupled with wafer defocus. More significantly, mask defocus has been found to result in large overlay errors. In particular, for a 10 nm node metal 1 layer (including 48 nm, 64 nm, 70 nm pitches, isolated, and power lines), the uncorrectable pattern placement error was 1 nm for 40 nm mask z-position shift. This is a global pattern shift of the layer with respect to previously defined layers. However, features at different locations will also shift differently due to different local deviations from mask flatness, e.g., from defects buried under the multilayer. It can be estimated that the contribution of mask non-flatness to overlay error is roughly 1/40 times the peak-to-valley thickness variation. With the blank peak-to-valley spec of 50 nm, ~1.25 nm image placement error is possible. Blank thickness variations up to 80 nm also contribute, which lead to up to 2 nm image shift.


title: Extreme ultraviolet lithography

text: The off-axis illumination of the reticle is also the cause of non-telecentricity in wafer defocus, which consumes most of the 1.4 nm overlay budget of the NXE:3400 EUV scanner even for design rules as loose as 100 nm pitch. The worst uncorrectable pattern placement error for a 24 nm line was about 1.1 nm, relative to an adjacent 72 nm power line, per 80 nm wafer focus position shift at a single slit position; when across-slit performance is included, the worst error is over 1.5 nm in the wafer defocus window In 2017, an actinic microscope mimicking a 0.33 NA EUV lithography system with 0.2/0.9 quasar 45 illumination showed that an 80 nm pitch contact array shifted -0.6 to 1.0 nm while a 56 nm pitch contact array shifted -1.7 to 1.0 nm relative to a horizontal reference line, within a +/- 50 nm defocus window.


title: Extreme ultraviolet lithography

text: Wafer defocus also leads to image placement errors due to deviations from local mask flatness. If the local slope is indicated by an angle α, the image is projected to be shifted in a 4x projection tool by 8 α x (DOF/2) = 4 α DOF, where DOF is the depth of focus. For a depth of focus of 100 nm, a small local deviation from flatness of 2.5 mrad (0.14°) can lead to a pattern shift of 1 nm.


title: Extreme ultraviolet lithography

text: The direction of illumination is also highly dependent on slit position, essentially rotated azimuthally. Nanya Technology and Synopsys found that horizontal


title: Extreme ultraviolet lithography

text: vs. vertical bias changed across slit with dipole illumination. The rotating plane of incidence (azimuthal range within -25° to 25°) is confirmed in the SHARP actinic review microscope at CXRO which mimics the optics for EUV projection lithography systems. The reason for this is a mirror is used to transform straight rectangular fields into arc-shaped fields. In order to preserve a fixed plane of incidence, the reflection from the previous mirror would be from a different angle with the surface for a different slit position; this causes non-uniformity of reflectivity. To preserve uniformity, rotational symmetry with a rotating plane of incidence is used. More generally, so-called "ring-field" systems reduce aberrations by relying on the rotational symmetry of an arc-shaped field derived from an off-axis annulus. This is preferred, as reflective systems must use off-axis paths, which aggravate aberrations. Hence identical die patterns within different halves of the arc-shaped slit would require different OPC. This renders them uninspectable by die-to-die comparison, as they are no longer truly identical dies. For pitches requiring dipole, quadrupole, or hexapole illumination, the rotation also causes mismatch with the same pattern layout at a different slit position, i.e., edge vs. center. Even with annular or circular illumination, the rotational symmetry is destroyed by the angle-dependent multilayer reflectance described above. Although the azimuthal angle range is +/- ~20° (NXE3400 field data indicate 18.2°) on 0.33 NA scanners, at 7 nm design rules (36-40 nm pitch), the tolerance for illumination can be +/-15°, or even less. Annular illumination nonuniformity and asymmetry also significantly impact the imaging.


title: Extreme ultraviolet lithography

text: The larger incident angle for pitch-dependent dipole illumination trend across slit does not affect horizontal line shadowing so much, but vertical line shadowing does increase going from center to edge. In addition, higher NA systems may offer limited relief from shadowing, as they target tighet pitches.


title: Extreme ultraviolet lithography

text: The slit position dependence is particularly difficult for the tilted patterns encountered in DRAM. Besides the more complicated effects due to shadowing and pupil rotation, tilted edges are converted to stair shape, which may be distorted by OPC. In fact, the 32 nm pitch DRAM by EUV will lengthen up to at least 9F cell area, where F=active area half-pitch (traditionally, it had been 6F). With a 2-D self-aligned double patterning active area cut, the cell area is still lower at 8.9F.


title: Extreme ultraviolet lithography

text: Aberrations, originating from deviations of optical surfaces from subatomic (<0.1 nm) specifications as well as thermal deformations and possibly including polarized reflectance effects, are also dependent on slit position, as will be further discussed below, with regard to source-mask optimization (SMO). The thermally induced aberrations are expected to exhibit differences among different positions across the slit, corresponding to different field positions, as each position encounters different parts of the deformed mirrors. Ironically, the use of substrate materials with high thermal and mechanical stability make it more difficult to compensate wavefront errors


title: Extreme ultraviolet lithography

text: In combination with the range of wavelengths, the rotated plane of incidence aggravates the already severe stochastic impact on EUV imaging.


title: Extreme ultraviolet lithography

text: Unlike DUV lithography sources, based on excimer lasers, EUV plasma sources produce light across a broad range of wavelengths. Though the EUV spectrum is not completely monochromatic, nor even as spectrally pure as DUV laser sources, the working wavelength has generally been taken to be 13.5 nm. In actuality, the reflected power is distributed mostly in the 13.3-13.7 nm range. The bandwidth of EUV light reflected by a multilayer mirror used for EUV lithography is over +/-2% (>270 pm); the phase changes due to wavelength changes at a given illumination angle may be calculated


title: Extreme ultraviolet lithography

text: and compared to the aberration budget. Wavelength dependence of reflectance also affects the apodization, or illumination distribution across the pupil (for different angles); different wavelengths effectively 'see' different illuminations as they are reflected differently by the multilayer of the mask. This effective source illumination tilt can lead to large image shifts due to defocus. Conversely, the peak reflected wavelength varies across the pupil due to different incident angles. This is aggravated when the angles span a wide radius, e.g., annular illumination. The peak reflectance wavelength increases for smaller incident angles. Aperiodic multilayers have been proposed to reduce the sensitivity at the cost of lower reflectivity but are too sensitive to random fluctuations of layer thicknesses, such as from thickness control imprecision or interdiffusion. In particular, defocused dense lines at pitches up to twice the minimum resolvable pitch suffer wavelength-dependent edge shifts.


title: Extreme ultraviolet lithography

text: A narrower bandwidth would increase sensitivity to mask absorber and buffer thickness on the 1 nm scale.


title: Extreme ultraviolet lithography

text: Flare is the presence of background light originating from scattering off of surface features which are not resolved by the light. In EUV systems, this light can be EUV or out-of-band (OoB) light that is also produced by the EUV source. The OoB light adds the complication of affecting the resist exposure in ways other than accounted for by the EUV exposure. OoB light exposure may be alleviated by a layer coated above the resist, as well as 'black border' features on the EUV mask. However, the layer coating inevitably absorbs EUV light, and the black border adds EUV mask processing cost.


title: Extreme ultraviolet lithography

text: A key challenge for EUV is the counter-scaling behavior of the line tip-to-tip (T2T) distance as half-pitch (hp) is scaled down. This is in part due to lower image contrast for the binary masks used in EUV lithography, which is not encountered with the use of phase shift masks in immersion lithography. The rounding of the corners of the line end leads to line end shortening, and this is worse for binary masks. The use of phase-shift masks in EUV lithography has been studied but encounters difficulties from phase control in thin layers as well as the bandwidth of the EUV light itself. More conventionally, optical proximity correction (OPC) is used to address the corner rounding and line-end shortening. In spite of this, it has been shown that the tip-to-tip resolution and the line tip printability are traded off against each other, being effectively CDs of opposite polarity.


title: Extreme ultraviolet lithography

text: In unidirectional metal layers, tip-to-tip spacing is one of the more severe issues for single exposure patterning. For the 40 nm pitch vertical lines, an 18 nm nominal tip-to-tip drawn gap resulted in an actual tip-to-tip distance of 29 nm with OPC (optical proximity correction), while for 32 nm pitch horizontal lines, the tip-to-tip distance with a 14 nm nominal gap went to 31 nm with OPC. These actual tip-to-tip distances define a lower limit of the half-pitch of the metal running in the direction perpendicular to the tip. In this case, the lower limit is around 30 nm. With further optimization of the illumination (discussed in the section on source-mask optimization), the lower limit can be further reduced to around 25 nm.


title: Extreme ultraviolet lithography

text: For larger pitches, where conventional illumination can be used, the line tip-to-tip distance is generally larger. For the 24 nm half-pitch lines, with a 20 nm nominally drawn gap, the distance was actually 45 nm, while for 32 nm half-pitch lines, the same nominal gap resulted in a tip-to-tip distance of 34 nm. With OPC, these become 39 nm and 28 nm for 24 nm half-pitch and 32 nm half-pitch, respectively.


title: Extreme ultraviolet lithography

text: Assist features are often used to help balance asymmetry from non-telecentricity at different slit positions, due to different illumination angles, starting at the 7 nm node, where the pitch is ~ 41 nm for a wavelength ~13.5 nm and NA=0.33, corresponding to k1 ~ 0.5. However, the asymmetry is reduced but not eliminated, since the assist features mainly enhance the highest spatial frequencies, whereas intermediate spatial frequencies, which also affect feature focus and position, are not much affected. The coupling between the primary image and the self images is too strong for the asymmetry to be eliminated by assist features; only asymmetric illumination can achieve this. Assist features may also get in the way of access to power/ground rails. Power rails are expected to be wider, which also limits the effectiveness of using assist features, by constraining the local pitch. Local pitches between 1x and 2x the minimum pitch forbid assist feature placement, as there is simply no room to preserve the local pitch symmetry. In fact, for the application to the two-bar asymmetry case, the optimum assist feature placement may be less than or exceed the two-bar pitch. Depending on the parameter to be optimized (process window area, depth of focus, exposure latitude), the optimum assist feature configuration can be very different, e.g., pitch between assist feature and bar being different from two-bar pitch, symmetric or asymmetric, etc..


title: Extreme ultraviolet lithography

text: At pitches smaller than 58 nm, there is a tradeoff between depth of focus enhancement and contrast loss by assist feature placement. Generally, there is still a focus-exposure tradeoff as the dose window is constrained by the need to have the assist features not print accidentally.


title: Extreme ultraviolet lithography

text: An additional concern comes from shot noise; sub-resolution assist features (SRAFs) cause the required dose to be lower, so as not to print the assist features accidentally. This results in fewer photons defining smaller features (see discussion in section on shot noise).


title: Extreme ultraviolet lithography

text: As SRAFs are smaller features than primary features and are not supposed to receive doses high enough to print, they are more susceptible to stochastic dose variations causing printing errors. This is particularly prohibitive for EUV, where even when the primary feature is printed at 80 mJ/cm, the SRAF suffers from stochastic printing.


title: Extreme ultraviolet lithography

text: Due to the effects of non-telecentricity, standard illumination pupil shapes, such as disc or annular, are not sufficient to be used for feature sizes of ~20 nm or below (10 nm node and beyond). Instead certain parts of the pupil (often over 50%) must be asymmetrically excluded. The parts to be excluded depend on the pattern. In particular, the densest allowed lines need to be aligned along one direction and prefer a dipole shape. For this situation, double exposure lithography would be required for 2D patterns, due to the presence of both X- and Y-oriented patterns, each requiring its own 1D pattern mask and dipole orientation. There may be 200–400 illuminating points, each contributing its weight of the dose to balance the overall image through focus. Thus the shot noise effect (to be discussed later) critically affects the image position through focus, in a large population of features.


title: Extreme ultraviolet lithography

text: Double or multiple patterning would also be required if a pattern consists of sub-patterns which require significantly different optimized illuminations, due to different pitches, orientations, shapes, and sizes.


title: Extreme ultraviolet lithography

text: Largely due to the slit shape, and the presence of residual aberrations, the effectiveness of SMO varies across slit position. At each slit position, there are different aberrations and different azimuthal angles of incidence leading to different shadowing. Consequently, there could be uncorrected variations across slit for aberration-sensitive features, which may not be obviously seen with regular line-space patterns. At each slit position, although optical proximity correction (OPC), including the assist features mentioned above, may also be applied to address the aberrations, they also feedback into the illumination specification, since the benefits differ for different illumination conditions. This would necessitate the use of different source-mask combinations at each slit position, i.e., multiple mask exposures per layer.


title: Extreme ultraviolet lithography

text: The above-mentioned chromatic aberrations, due to mask-induced apodization, also lead to inconsistent source-mask optimizations for different wavelengths.


title: Extreme ultraviolet lithography

text: The best focus for a given feature size varies as a strong function of pitch, polarity, and orientation under a given illumination. At 36 nm pitch, horizontal and vertical darkfield features have more than 30 nm difference of focus. The 34 nm pitch and 48 nm pitch features have the largest difference of best focus regardless of feature type. In the 48-64 nm pitch range, the best focus position shifts roughly linearly as a function of pitch, by as much as 10-20 nm. For the 34-48 nm pitch range, the best focus position shifts roughly linearly in the opposite direction as a function of pitch. This can be correlated with the phase difference between the zero and first diffraction orders. Assist features, if they can fit within the pitch, were found not to reduce this tendency much, for a range of intermediate pitches, or even worsened it for the case of 18-27 nm and quasar illumination. 50 nm contact holes on 100 nm and 150 pitches had best focus positions separated by roughly 25 nm; smaller features are expected to be worse. Contact holes in the 48-100 nm pitch range showed a 37 nm best focus range. The best focus position vs. pitch is also dependent on resist. Critical layers often contain lines at one minimum pitch of one polarity, e.g., darkfield trenches, in one orientation, e.g., vertical, mixed with spaces of the other polarity of the other orientation. This often magnifies the best focus differences, and challenges the tip-to-tip and tip-to-line imaging.


title: Extreme ultraviolet lithography

text: A consequence of SMO and shifting focus windows has been the reduction of pupil fill. In other words, the optimum illumination is necessarily an optimized overlap of the preferred illuminations for the various patterns that need to be considered. This leads to lower pupil fill providing better results.


title: Extreme ultraviolet lithography

text: A commonly touted advantage of EUV has been the relative ease of lithography, as indicated by the ratio of feature size to the wavelength multiplied by the numerical aperture, also known as the k1 ratio. An 18 nm metal linewidth has a k1 of 0.44 for 13.5 nm wavelength, 0.33 NA, for example. For the k1 approaching 0.5, some weak resolution enhancement including attenuated phase shift masks has been used as essential to production with the ArF laser wavelength (193 nm), whereas this resolution enhancement is not available for EUV. In particular, 3D mask effects including scattering at the absorber edges distort the desired phase profile. Also, the phase profile is effectively derived from the plane wave spectrum reflected from the multilayer through the absorber rather than the incident plane wave. Without absorbers, near-field distortion also occurs at an etched multilayer sidewall due to the oblique incidence illumination; some light traverses only a limited number of bilayers near the sidewall. Additionally, the different polarizations (TE and TM) have different phase shifts.


title: Extreme ultraviolet lithography

text: EUV light generates photoelectrons upon absorption by matter. These photoelectrons in turn generate secondary electrons, which slow down before engaging in chemical reactions. At sufficient doses 40 eV electrons are known to penetrate 180 nm thick resist leading to development. At a dose of 160 μC/cm, corresponding to 15 mJ/cm EUV dose assuming one electron/photon, 30 eV electrons removed 7 nm of PMMA resist after standard development. For a higher 30 eV dose of 380 μC/cm, equivalent to 36 mJ/cm at one electron/photon, 10.4 nm of PMMA resist are removed. These indicate the distances the electrons can travel in resist, regardless of direction.


title: Extreme ultraviolet lithography

text: In the most recent measurement of the significant impact of secondary electrons on resolution, it was found that 93 eV photoelectrons (from a gold underlayer) had a 1/e attenuation length of 28 nm in resist. The electron number attenuation was measured from the fraction of electrons captured in an electrical current from the resist. This indicates 37% of the released electrons still migrate beyond 28 nm from the exposure release point.


title: Extreme ultraviolet lithography

text: The more highly absorbing resist removes more light in the top of the resist, leaving less for the bottom of the resist. The larger absorption leads to larger, more significant differences between the absorbed doses at the top and the bottom of the resist.


title: Extreme ultraviolet lithography

text: In other words, the less absorbing the resist, the more vertically uniform the absorption. Conventionally, photoresists are made as transparent as possible to strive for this vertical uniformity, which enables straighter resist profiles. On the other hand, for EUV, this conflicts with the goal of increasing absorption for more sensitivity at current EUV power levels. Shot noise is another concern, to be explained further below.


title: Extreme ultraviolet lithography

text: A study by the College of Nanoscale Science and Engineering (CNSE) presented at the 2013 EUVL Workshop indicated that, as a measure of EUV photoelectron and secondary electron blur, 50–100 eV electrons easily penetrated beyond 15 nm of resist thickness (PMMA or commercial resist), indicating more than 30 nm range of resist affected centered on the EUV point of absorption, for doses exceeding 200–300 uC/cm. This can be compared with the image contrast degradation reported for sub-40 nm pitches later in 2015.


title: Extreme ultraviolet lithography

text: The process of electron penetration through a resist is essentially a stochastic process; there is a finite probability that resist exposure by released electrons can occur quite far from the point of photon absorption. Increasing the dose increases the number of far-reaching electrons, resulting in more extended resist loss. A leading EUV chemically amplified resist exposed to 80 eV electrons at a dose up to 80 uc/cm showed up to 7.5 nm resist thickness loss. For an open-source resist exposed near 200 uC/cm by 80 eV electrons, the resist thickness lost after post-exposure bake and development was around 13 nm, while doubling the dose resulted in increasing the loss to 15 nm. On the other hand, for doses >500 uC/cm, the resist begins to thicken due to crosslinking.


title: Extreme ultraviolet lithography

text: The degree of photoelectron emission from the layer underlying the EUV photoresist has been shown to affect the depth of focus. Unfortunately, hardmask layers tend to increase photoelectron emission, degrading the depth of focus.


title: Extreme ultraviolet lithography

text: The generation of photoelectrons in the photoresist itself is a fundamental part of the image formation, and is marked by a random spread which increases with dose.


title: Extreme ultraviolet lithography

text: The randomness of the number of secondary electrons is itself a source of stochastic behavior in EUV resist images. The scale length of electron blur itself has a distribution.


title: Extreme ultraviolet lithography

text: The electron blur is also affected by total internal reflection from the top surface of the resist film.


title: Extreme ultraviolet lithography

text: Direct photocurrent measurements have been used to obtain secondary electron yields in response to EUV radiation. Simulations roughly calibrated to this yield show that the secondary electron blur increases with dose. This is associated with fewer trapping sites as dose is increased and the reactions depleting the trapping sites proceed.


title: Extreme ultraviolet lithography

text: Low-energy electron-induced events also increase in number at a given distance from the photon absorption site, as dose is increased. This has been confirmed by resist thickness loss measurements as a function of low-energy electron dose. The dose-dependent spread of secondary electrons was also known before from electron beam lithography.


title: Extreme ultraviolet lithography

text: The increased secondary electron blur with increased dose makes control of stochastic defects more difficult.


title: Extreme ultraviolet lithography

text: Due to the production of secondary electrons of various energies, the charge of the resist may locally fluctuate. An EUV exposure with less blur leads to more pronounced charge differences at the feature edge, which can lead to larger electric fields. Such large electric fields have been observed to lead to dielectric breakdown. The trapping of secondary electrons leads to a reduction of secondary electrons emitted from the film; however, the trap sites may themselves be depleted, resulting in effectively extended secondary electron blur for larger doses. Electron trapping is predicted to occur as part of polaronic behavior, which confines the electron's final energy deposition to trap site locations. The polaron size can be quite large in resists, e.g., 46 nm in PMMA.


title: Extreme ultraviolet lithography

text: Due to the high efficiency of absorption of EUV by photoresists, heating and outgassing become primary concerns. One well-known issue is contamination deposition on the resist from ambient or outgassed hydrocarbons, which results from EUV- or electron-driven reactions. Organic photoresists outgas hydrocarbons while metal oxide photoresists outgas water and oxygen and metal (in a hydrogen ambient); the last is uncleanable. The carbon contamination is known to affect multilayer reflectivity while the oxygen is particularly harmful for the ruthenium capping layers on the EUV multilayer optics.


title: Extreme ultraviolet lithography

text: Atomic hydrogen in the tool chambers is used to clean tin and carbon which deposit on the EUV optical surfaces.


title: Extreme ultraviolet lithography

text: The reaction with tin in the light source or resist or on an optical surface to form volatile SnH4 proceeds via the reaction:


title: Extreme ultraviolet lithography

text: The SnH can reach the coatings of other EUV optical surfaces, where it redeposits Sn via the reaction:


title: Extreme ultraviolet lithography

text: The redeposited Sn might be subsequently removed by atomic hydrogen exposure. However, overall, the tin cleaning efficiency is less than 0.01%, due to both redeposition and hydrogen desorption, leading to formation of hydrogen molecules at the expense of atomic hydrogen.


title: Extreme ultraviolet lithography

text: Hydrogen also reacts with metal-containing compounds to reduce them to metal, and diffuses through the silicon and molybdenum in the multilayer, eventually causing blistering. Capping layers that mitigate hydrogen-related damage often reduce reflectivity to well below 70%. Capping layers are known to be permeable to ambient gases including oxygen and hydrogen, as well as susceptible to the hydrogen-induced blistering defects. Hydrogen may also react with the capping layer, resulting in its removal.


title: Extreme ultraviolet lithography

text: Hydrogen also reacts with resists to etch or decompose them. Besides photoresist, hydrogen plasmas can also etch silicon, albeit very slowly.


title: Extreme ultraviolet lithography

text: To help mitigate the above effects, the latest EUV tool introduced in 2017, the NXE:3400B, features a membrane that separates the wafer from the projection optics of the tool, protecting the latter from outgassing from the resist on the wafer. The membrane contains layers which absorb DUV and IR radiation, and transmits 85-90% of the incident EUV radiation. There is of course, accumulated contamination from wafer outgassing as well as particles in general (although the latter are out of focus, they may still obstruct light).


title: Extreme ultraviolet lithography

text: Reducing defects on extreme ultraviolet (EUV) masks is currently one of the most critical issues to be addressed for commercialization of EUV lithography. Defects can be buried underneath or within the multilayer stack or be on top of the multilayer stack. Mesas or protrusions form on the sputtering targets used for multilayer deposition, which may fall off as particles during the multilayer deposition. In fact, defects of atomic scale height (0.3–0.5 nm) with 100 nm FWHM can still be printable by exhibiting 10% CD impact. IBM and Toppan reported at Photomask Japan 2015 that smaller defects, e.g., 50 nm size, can have 10% CD impact even with 0.6 nm height, yet remain undetectable.


title: Extreme ultraviolet lithography

text: Furthermore, the edge of a phase defect will further reduce reflectivity by more than 10% if its deviation from flatness exceeds 3 degrees, due to the deviation from the target angle of incidence of 84 degrees with respect to the surface. Even if the defect height is shallow, the edge still deforms the overlying multilayer, producing an extended region where the multilayer is sloped. The more abrupt the deformation, the narrower the defect edge extension, the greater the loss in reflectivity.


title: Extreme ultraviolet lithography

text: EUV mask defect repair is also more complicated due to the across-slit illumination variation mentioned above. Due to the varying shadowing sensitivity across the slit, the repair deposition height must be controlled very carefully, being different at different positions across the EUV mask illumination slit.


title: Extreme ultraviolet lithography

text: GlobalFoundries and Lawrence Berkeley Labs carried out a Monte Carlo study to simulate the effects of intermixing between the molybdenum (Mo) and silicon (Si) layers in the multilayer that is used to reflect EUV light from the EUV mask. The results indicated high sensitivity to the atomic-scale variations of layer thickness. Such variations could not be detected by wide-area reflectivity measurements, but would be significant on the scale of the critical dimension (CD). The local variation of reflectivity could be on the order of 10% for a few nm standard deviation.


title: Extreme ultraviolet lithography

text: Multiple EUV pulses at less than 10 mJ/cm could accumulate damage to a Ru-capped Mo/Si multilayer mirror optic element. The angle of incidence was 16° or 0.28 rads, which is within the range of angles for a 0.33 NA optical system.


title: Extreme ultraviolet lithography

text: Production EUV tools need a pellicle to protect the mask from contamination. Pellicles are normally expected to protect the mask from particles during transport, entry into or exit from the exposure chamber, as well as the exposure itself. Without pellicles, particle adders would reduce yield, which has not been an issue for conventional optical lithography with 193 nm light and pellicles. However, for EUV, the feasibility of pellicle use is severely challenged, due to the required thinness of the shielding films to prevent excessive EUV absorption. Particle contamination would be prohibitive if pellicles were not stable above 200 W, i.e., the targeted power for manufacturing.


title: Extreme ultraviolet lithography

text: Heating of the EUV mask pellicle (film temperature up to 750 K for 80 W incident power) is a significant concern, due to the resulting deformation and transmission decrease. ASML developed a 70 nm thick polysilicon pellicle membrane, which allows EUV transmission of 82%; however, less than half of the membranes survived expected EUV power levels. SiNx pellicle membranes also failed at 82 W equivalent EUV source power levels. At target 250 W levels, the pellicle is expected to reach 686 degrees Celsius, well over the melting point of aluminum. Alternative materials need to allow sufficient transmission as well as maintain mechanical and thermal stability. However, graphite, graphene or other carbon nanomaterials (nanosheets, nanotubes) are damaged by EUV due to the release of electrons and also too easily etched in the hydrogen cleaning plasma expected to be deployed in EUV scanners. Hydrogen plasmas can also etch silicon as well. A coating helps improve hydrogen resistance, but this reduces transmission and/or emissivity, and may also affect mechanical stability (e.g., bulging).


title: Extreme ultraviolet lithography

text: Wrinkles on pellicles can cause CD nonuniformity due to uneven absorption; this is worse for smaller wrinkles and more coherent illumination, i.e., lower pupil fill.


title: Extreme ultraviolet lithography

text: In the absence of pellicles, EUV mask cleanliness would have to be checked before actual product wafers are exposed, using wafers specially prepared for defect inspection. These wafers are inspected after printing for repeating defects indicating a dirty mask; if any are found, the mask must be cleaned and another set of inspection wafers are exposed, repeating the flow until the mask is clean. Any affected product wafers must be reworked.


title: Extreme ultraviolet lithography

text: TSMC reported starting limited use of its own pellicle in 2019 and continuing to expand afterwards, and Samsung is planning pellicle introduction in 2022.


title: Extreme ultraviolet lithography

text: As discussed above, with regard to contamination removal, hydrogen used in recent EUV systems can penetrate into the EUV mask layers. Once trapped, bulge defects were produced. These are essentially the blister defects which arise after a sufficient number of EUV mask exposures in the hydrogen environment.


title: Extreme ultraviolet lithography

text: EUV lithography is particularly sensitive to stochastic effects. In a large population of features printed by EUV, although the overwhelming majority are resolved, some suffer complete failure to print, e.g. missing holes or bridging lines. A known significant contribution to this effect is the dose used to print. This is related to shot noise, to be discussed further below. Due to the stochastic variations in arriving photon numbers, some areas designated to print actually fail to reach the threshold to print, leaving unexposed defect regions. The lower part of the resist layer is more susceptible to underexposure, due to absorption in the upper part. Some areas may be overexposed, leading to excessive resist loss or crosslinking. The probability of stochastic failure increases exponentially as feature size decreases, and for the same feature size, increasing distance between features also significantly increases the probability. Line cuts which are misshapen are a significant issue due to potential arcing and shorting. Yield requires detection of stochastic failures down to below 1e-12.


title: Extreme ultraviolet lithography

text: The tendency to stochastic defects is worse when the image consists of photons from different patterns, such as from a large-area pattern or from defocus over a large pupil fill.


title: Extreme ultraviolet lithography

text: Multiple failure modes may exist for the same population. For example, besides bridging of trenches, the lines separating the trenches may be broken. This can be attributed to stochastic resist loss, from secondary electrons. The randomness of the number of secondary electrons is itself a source of stochastic behavior in EUV resist images.


title: Extreme ultraviolet lithography

text: The coexistence of stochastically underexposed and overexposed defect regions leads to a loss of dose window at a certain post-etch defect level between the low-dose and high-dose patterning cliffs. Hence, the resolution benefit from shorter wavelength is lost.


title: Extreme ultraviolet lithography

text: The resist underlayer also plays an important role. This could be due to the secondary electrons generated by the underlayer. Secondary electrons may remove over 10 nm of resist from the exposed edge.


title: Extreme ultraviolet lithography

text: The defect level is on the order of 1K/mm. In 2020, Samsung reported that 5 nm layouts had risks for process defects and had started implementing automated check and fixing.


title: Extreme ultraviolet lithography

text: Photon shot noise can be linked to stochastic defects through the presence of dose-dependent blur (modeled as Gaussian).


title: Extreme ultraviolet lithography

text: The photon shot noise is removed to some degree by blurring factors such as secondary electrons or acids in chemically amplified resists, but when too significant the blur also reduces the image contrast at the edge. Since the blur is less than the feature size, the feature's total enclosed area can still have fluctuations. It is worse for EUV than DUV wavelengths due to lower photon density at commonly used doses, as well as smaller feature sizes being represented by smaller pixels.


title: Extreme ultraviolet lithography

text: Even with blur taken into account, the final distribution of chemical species is still stochastic. Higher dose helps reduce the stochasticity but leads to larger blur, which can spread the extent of the stochastic fluctuations.


title: Extreme ultraviolet lithography

text: Another approach to dealing with stochastic issues is to use directional etching to smooth sidewall edges.


title: Extreme ultraviolet lithography

text: The resolution of EUV lithography for the future faces challenges in maintaining throughput, i.e., how many wafers are processed by an EUV tool per day. These challenges arise from smaller fields, additional mirrors, and shot noise. In order to maintain throughput, the power at intermediate focus (IF) must be continually increased.


title: Extreme ultraviolet lithography

text: With the natural Poisson distribution due to the random arrival and absorption times of the photons, there is an expected natural dose (photon number) variation of at least several percent 3 sigma, making the exposure process susceptible to stochastic variations. The dose variation leads to a variation of the feature edge position, effectively becoming a blur component. Unlike the hard resolution limit imposed by diffraction, shot noise imposes a softer limit, with the main guideline being the ITRS line width roughness (LWR) spec of 8% (3s) of linewidth. Increasing the dose will reduce the shot noise, but this also requires higher source power.


title: Extreme ultraviolet lithography

text: A 10 nm wide, 10 nm long assist feature region, at a target non-printing dose of 15 mJ/cm, with 10% absorption, is defined by just over 100 photons, which leads to a 6s noise of 59%, corresponding to a stochastic dose range of 6 to 24 mJ/cm, which could affect the printability.


title: Extreme ultraviolet lithography

text: A 2017 study by Intel showed that for semi-isolated vias (whose Airy disk can be approximated by a Gaussian), the sensitivity of CD to dose was particularly strong, strong enough that a reduction of dose could nonlinearly lead to failure to print the via.


title: Extreme ultraviolet lithography

text: The two issues of shot noise and EUV-released electrons point out two constraining factors: 1) keeping dose high enough to reduce shot noise to tolerable levels, but also 2) avoiding too high a dose due to the increased contribution of EUV-released photoelectrons and secondary electrons to the resist exposure process, increasing the edge blur and thereby limiting the resolution. Aside from the resolution impact, higher dose also increases outgassing and limits throughput, and crosslinking occurs at very high dose levels. For chemically amplified resists, higher dose exposure also increases line edge roughness due to acid generator decomposition.


title: Extreme ultraviolet lithography

text: Even with higher absorption at the same dose, EUV has a larger shot noise concern than the ArF (193 nm) wavelength, mainly because it is applied to thinner resists.


title: Extreme ultraviolet lithography

text: As can be seen above, at the target incident dose levels, significantly fewer EUV photons are absorbed in EUV resists compared to ArF photons in ArF resists. Despite greater transparency of the resist, the incident photon flux is about 14 times larger (193/13.5) for the same energy dose per unit area. Quantum yield makes the resist response comparable for chemically amplified resists for both ArF and EUV wavelengths, but ArF resists also allow larger blur, enabling further smoothing. The resist thickness is limited by transparency as well as resist collapse and resist strip considerations.


title: Extreme ultraviolet lithography

text: Due to stochastic considerations, the IRDS 2022 Lithography roadmap now acknowledges increasing doses for smaller feature sizes.


title: Extreme ultraviolet lithography

text: For pitches less than half-wavelength divided by numerical aperture, dipole illumination is necessary. This illumination fills at most a leaf-shaped area at the edge of the pupil. However, due to 3D effects in the EUV mask, smaller pitches require even smaller portions of this leaf shape. Below 20% of the pupil, the throughput and dose stability begin to suffer.


title: Extreme ultraviolet lithography

text: EUV is anticipated to use double patterning at around 34 nm pitch with 0.33 NA. This resolution is equivalent to '1Y' for DRAM. In 2020, ASML reported that 5 nm M0 layer (30 nm minimum pitch) required double patterning.


title: Extreme ultraviolet lithography

text: In H2 2018, TSMC confirmed that its 5 nm EUV scheme still used multi-patterning, also indicating that mask count did not decrease from its 7 nm node, which used extensive DUV multi-patterning, to its 5 nm node, which used extensive EUV. EDA vendors also indicated the continued use of multi-patterning flows. While Samsung introduced its own 7 nm process with EUV single patterning, it encountered severe photon shot noise causing excessive line roughness, which required higher dose, resulting in lower throughput. TSMC's 5 nm node uses even tighter design rules. Samsung indicated smaller dimensions would have more severe shot noise.


title: Extreme ultraviolet lithography

text: In Intel's complementary lithography scheme at 20 nm half-pitch, EUV would be used only in a second line-cutting exposure after a first 193 nm line-printing exposure.


title: Extreme ultraviolet lithography

text: Multiple exposures would also be expected where two or more patterns in the same layer, e.g., different pitches or widths, must use different optimized source pupil shapes. For example, when considering a staggered bar array of 64 nm vertical pitch, changing the horizontal pitch from 64 nm to 90 nm changes the optimized illumination significantly. Source-mask optimization that is based on line-space gratings and tip-to-tip gratings only does not entail improvements for all parts of a logic pattern, e.g., a dense trench with a gap on one side.


title: Extreme ultraviolet lithography

text: In 2020, ASML reported that for the 3 nm node, center-to-center contact/via spacings of 40 nm or less would require double or triple patterning for some contact/via arrangements.


title: Extreme ultraviolet lithography

text: For the 24-36 nm metal pitch, it was found that using EUV as a (second) cutting exposure had a significantly wider process window than as a complete single exposure for the metal layer.


title: Extreme ultraviolet lithography

text: Multiple exposures of the same mask are also expected for defect management without pellicles, limiting productivity similarly to multiple patterning.


title: Extreme ultraviolet lithography

text: Self-aligned litho-etch-litho-etch (SALELE) is a hybrid SADP/LELE technique whose implementation has started in 7 nm and continued use in 5 nm.


title: Extreme ultraviolet lithography

text: Self-aligned litho-etch-litho-etch (SALELE) has become an accepted form of double patterning to be used with EUV, starting at the 5 nm node, where some layers are 28 nm pitch.


title: Extreme ultraviolet lithography

text: A return to extended generations of single patterning would be possible with higher numerical aperture (NA) tools. An NA of 0.45 could require retuning of a few percent. Increasing demagnification could avoid this retuning, but the reduced field size severely affects large patterns (one die per 26 mm × 33 mm field) such as the many-core multi-billion transistor 14 nm Xeon chips. by requiring field stitching of two mask exposures.


title: Extreme ultraviolet lithography

text: In 2015, ASML disclosed details of its anamorphic next-generation EUV scanner, with an NA of 0.55. The demagnification is increased from 4x to 8x only in one direction (in the plane of incidence). However, the 0.55 NA has a much smaller depth of focus than immersion lithography. Also, an anamorphic 0.52 NA tool has been found to exhibit too much CD and placement variability for 5 nm node single exposure and multi-patterning cutting.


title: Extreme ultraviolet lithography

text: Depth of focus being reduced by increasing NA is also a concern, especially in comparison with multi-patterning exposures using 193 nm immersion lithography:


title: Extreme ultraviolet lithography

text: High-NA EUV tools also suffer from obscuration, which can cause errors in the imaging of certain patterns.


title: Extreme ultraviolet lithography

text: A much shorter wavelength (~6.7 nm) would be beyond EUV, and is often referred to as BEUV (beyond extreme ultraviolet). With current technology, BEUV wavelengths would have worse shot noise effects without ensuring sufficient dose.


title: Sucralfate

text: Sucralfate, sold under various brand names, is a medication used to treat stomach ulcers, gastroesophageal reflux disease (GERD), radiation proctitis, and stomach inflammation and to prevent stress ulcers. Its usefulness in people infected by "H. pylori" is limited. It is used by mouth (for upper GIT ulcers) and rectally (for radiation proctitis).


title: Sucralfate

text: Common side effects include constipation. Serious side effects may include bezoar formation and encephalopathy. Use appears to be safe in pregnancy and breastfeeding. How it works is unclear but is believed to involve binding to the ulcer and protecting it from further damage.


title: Sucralfate

text: Sucralfate was approved for medical use in the United States in 1981. It is available as a generic medication. In 2020, it was the 182nd most commonly prescribed medication in the United States, with more than 2million prescriptions.


title: Sucralfate

text: Sucralfate is used for the treatment of active duodenal ulcers not related to the use of nonsteroidal anti-inflammatory drugs (NSAIDs), as the mechanism behind these ulcers is due to acid oversecretion. It is not FDA approved for gastric ulcers, but is widely used because of evidence of efficacy. The use for sucralfate in peptic ulcer disease has diminished recently, but it is still the preferred agent for stress ulcer prevention.


title: Sucralfate

text: The most common side effect seen is constipation (2-3%). Less commonly reported side effects (<0.5%) include flatulence, headache, hypophosphatemia, xerostomia (dry mouth), and bezoar formation.


title: Sucralfate

text: Use of this drug is not recommended for people with chronic kidney failure, as it might cause aluminium accumulation and toxicity.


title: Sucralfate

text: A few well-controlled studies have been carried out investigating the safety and efficacy of sucralfate in children and pregnant women (Pregnancy Category B).


title: Sucralfate

text: Sucralfate is a locally acting substance that in an acidic environment (pH < 4) reacts with hydrochloric acid in the stomach to form a cross-linking, viscous, paste-like material capable of acting as an acid buffer for as long as 6 to 8 hours after a single dose. It also attaches to proteins on the surface of ulcers, such as albumin and fibrinogen, to form stable insoluble complexes. These complexes serve as protective barriers at the ulcer surface, preventing further damage from acid, pepsin, and bile. In addition, sucralfate prevents back diffusion of hydrogen ions, and adsorbs both pepsin and bile acids.


title: Sucralfate

text: It has been thought that sucralfate also stimulates the production of prostaglandin E, epidermal growth factors (EGF), bFGF, and gastric mucus.


title: Sucralfate

text: Brand names include Carafate in U.S.A., Sucramal in Italy, Sucrafil, Sufrate, Sucralpro, Sucralcoat, Pepsigard, Sucral, Hapifate, Sucralpro in India, Sutra or Musin in parts of South-East Asia, Sulcrate in Canada, Discral (sucralfato) in México, Ulsanic in South Africa and Israel, Andapsin in Sweden and Antepsin. Sucracell in India.


title: Playboy Club

text: The Playboy Club was initially a chain of nightclubs and resorts owned and operated by Playboy Enterprises. The first Playboy Club opened in Chicago in 1960. Each club generally featured a Living Room, a Playmate Bar, a Dining Room, and a Club Room. Members and their guests were served food and drinks by Playboy Bunnies, some of whom were featured in "Playboy" magazine. The clubs offered name entertainers and comedians in the Club Rooms, and local musicians and the occasional close-up magician in the Living Rooms. Starting with the London and Jamaica club locations, the Playboy Club became international in scope.


title: Playboy Club

text: In 1991, the club chain became defunct. Thereafter, on October 6, 2006, a Playboy Club was opened in Las Vegas at the Palms Casino Resort, and in 2010 clubs were opened as well in Macau and Cancun. In time, the Las Vegas club closed on June 4, 2012, the Macao club closed in 2013, and the Cancun club closed in 2014. In May 2014 the Commerce Casino in Los Angeles opened a Playboy-themed lounge consisting of gaming tables and Playboy Bunny cocktail waitresses. In September 2018 a Playboy Club was opened in Midtown Manhattan but permanently closed in November 2019 after just over one year in operation.


title: Playboy Club

text: The first club opened at 116 E. Walton Street in downtown Chicago, Illinois, United States, on February 29, 1960. It relocated to Clark and Armitage in Lincoln Park in 1980 and closed in June 1986.


title: Berlin Tempelhof Airport

text: Berlin Tempelhof Airport () was one of the first airports in Berlin, Germany. Situated in the south-central Berlin borough of Tempelhof-Schöneberg, the airport ceased operating in 2008 amid controversy, leaving Tegel and Schönefeld as the two main airports serving the city for another twelve years until both were replaced by Berlin Brandenburg Airport in 2020.


title: Berlin Tempelhof Airport

text: Tempelhof was designated as an airport by the Reich Ministry of Transport on 8 October 1923. The old terminal was originally constructed in 1927. In anticipation of increasing air traffic, the Nazi government began an enormous reconstruction in the mid-1930s. While it was occasionally cited as the world's oldest operating commercial airport, the title was disputed by several other airports, and is no longer an issue since its closure.


title: Berlin Tempelhof Airport

text: Tempelhof was one of Europe's three iconic pre-World War II airports, the others being London's now defunct Croydon Airport and the old Paris–Le Bourget Airport. It acquired a further iconic status as the centre of the Berlin Airlift of 1948–49. One of the airport's most distinctive features is its huge, canopy-style roof extending over the apron, able to accommodate most contemporary airliners in the 1950s, 1960s and early 1970s, protecting passengers from the elements. Tempelhof Airport's main building was once among the twenty largest buildings on earth, but it also formerly contained the world's smallest duty-free shop.


title: Berlin Tempelhof Airport

text: Tempelhof Airport closed all operations on 30 October 2008, despite the efforts of some protesters to prevent the closure. A non-binding referendum was held on 27 April 2008 against the impending closure but failed due to low voter turnout. The former airfield has subsequently been used as a recreational space known as "Tempelhofer Feld". In September 2015, it was announced that Tempelhof would also become an emergency refugee camp.


title: Berlin Tempelhof Airport

text: Tempelhof was often called the "City Airport". In its later years, it mostly had commuter flights to other parts of Germany and neighbouring countries; but it had in the past received long-haul, wide-bodied airliners, such as the Boeing 747, the Lockheed L-1011 Tristar and the Lockheed C-5A Galaxy. The first of these three first appeared at Tempelhof on 18 September 1976, when Pan American World Airways (Pan Am) flew in Boeing 747SP "Clipper Great Republic" to participate in the static exhibition of contemporary military, non-combat and civil aircraft at the annual "Day of Open House" of the United States Air Force (USAF) at the airport.


title: Berlin Tempelhof Airport

text: The Galaxy had its first appearance at Tempelhof on 17 September 1971, when an aircraft of the USAF's 436th Military Airlift Wing flew in from Dover Air Force Base in Delaware, United States, to participate in that year's "Day of Open House" static exhibition. These events respectively marked the debut at Tempelhof of the largest aircraft in commercial airline service at the time and the then-largest aircraft overall.


title: Berlin Tempelhof Airport

text: Tempelhof, compared to Brandenburg Airport and Tegel Airport, wasn't a particularly large airport. The layout of the airport was relatively simple - two parallel runways oriented east–west (09/27 L/R), with a single oval taxiway around the airport, with the main terminal on the north-western segment of the airport. The runways are not comparatively long, either - versus Brandenburg's 3600m and 4000m runways which can easily handle intercontinental airliners like the Boeing 747, Tempelhof has two runways at 2094m (09R/27L) and 1840m (09L/27R), which can, at most, handle Boeing 757 and Airbus A320-size aircraft.


title: Berlin Tempelhof Airport

text: Other possible uses for Tempelhof have been discussed, and many people are trying to keep the airport buildings preserved. In September 2015, in the midst of the 2015 European migrant crisis, it was announced by the Berlin state government that Tempelhof would become an 'emergency refugee shelter', holding at least 1,200 people in two former hangars.


title: Berlin Tempelhof Airport

text: The site of the airport was originally Knights Templar land in medieval Berlin, and from this beginning came the name "Tempelhof". Later, the site was used as a parade field by the Prussian army from 1720 to the start of World War I. In 1909, French aviator Armand Zipfel made the first flight demonstration in Tempelhof, followed by Orville Wright later that same year. Tempelhof was first officially designated as an airport on 8 October 1923. Deutsche Luft Hansa was founded in Tempelhof on 6 January 1926.


title: Berlin Tempelhof Airport

text: The old terminal, originally constructed in 1927, became the world's first with an underground railway. The station has since been renamed "Paradestraße", because the rebuilding of the airport in the 1930s required the airport access to be moved to a major intersection with a station now called Platz der Luftbrücke after the Berlin Airlift.


title: Berlin Tempelhof Airport

text: As part of Albert Speer's plan for the reconstruction of Berlin during the Nazi era, Prof. Ernst Sagebiel was ordered to replace the old terminal with a new terminal building in 1934. The airport halls and the adjoining buildings, intended to become the gateway to Europe and a symbol of Hitler's "world capital" Germania, are still known as one of the largest built entities worldwide, and have been described by British architect Sir Norman Foster as "the mother of all airports".


title: Berlin Tempelhof Airport

text: With its façades of shell limestone, the terminal building, built between 1936 and 1941, forms a quadrant. Arriving passengers walked through customs controls to the reception hall. Tempelhof was served by the U6 U-Bahn line along Mehringdamm and up Friedrichstraße (Platz der Luftbrücke station).


title: Berlin Tempelhof Airport

text: Zentralflughafen Tempelhof-Berlin had the advantage of a central location just minutes from the Berlin city centre and quickly became one of the world's busiest airports. Tempelhof saw its greatest pre-war days during 1938–1939, when up to 52 foreign and 40 domestic flights arrived and departed daily from the old terminal while the new one was still under construction.


title: Berlin Tempelhof Airport

text: The new air terminal was designed as headquarters for Deutsche Luft Hansa (moved in 1938), the German national airline at that time. As a forerunner of today's modern airports, the building was designed with many unique features, including giant arc-shaped aircraft hangars. Although under construction for more than ten years, it was never finished because of World War II. For passengers and freight, the 1927-built terminal stayed in use until 24 April 1945.


title: Berlin Tempelhof Airport

text: The building complex was designed to resemble an eagle in flight with semicircular hangars forming the bird's spread wings. A hangar roof was to have been laid in tiers to form a stadium for spectators at air and ground demonstrations. Norman Foster called Tempelhof "one of the really great buildings of the modern age".


title: Berlin Tempelhof Airport

text: Fearing Allied bombing of airports, all German civil aviation was halted on 2 September 1939, but gradually restarted from 1 November. However, the 1927-built terminal remained closed to all civil aviation, and all civilian aircraft movements to and from Berlin were transferred to an airfield in Rangsdorf until 7 March 1940, when the 1927 terminal was reopened and civil aviation continued until 24 April 1945.


title: Berlin Tempelhof Airport

text: From January 1940 until early 1944, Weser Flugzeugbau assembled Junkers Ju 87 "Stuka" dive bombers; thereafter, it assembled Focke-Wulf Fw 190 fighter planes in the still unfurnished main hall and hangars 3 to 7 of the new terminal, which were supplied by a railway and trucks via a connecting tunnel. Hangars 1 and 2 were not used to assemble aircraft as these were already used by Luft Hansa for its own planes. Aircraft parts were brought in from all over the city while complete aircraft engines were trucked to Tempelhof. Once the airframes were complete and the engines had been installed, the finished aircraft were flown out. The Luftwaffe did not use Tempelhof as a military airfield during World War II, except for occasional emergency landings by fighter aircraft.


title: Berlin Tempelhof Airport

text: On 21 April 1945, Deutsche Luft Hansa operated its last scheduled flights, and over the coming days laid on additional non-scheduled flights from Johannisthal Air Field which stopped over at Tempelhof to take on freight en route to Travemünde and Munich, where Luft Hansa had relocated its headquarters. Two days later, on 23 April, the airline's last-ever flight to depart Tempelhof left for Madrid, but was later shot down over Southern Germany.


title: Berlin Tempelhof Airport

text: Tempelhof's German commander, "Oberst" Rudolf Böttger, refused to carry out orders to blow up the base, choosing instead to kill himself. Soviet forces took Tempelhof in the Battle of Berlin on 28 and 29 April 1945 in the closing days of the war in Europe. Soviet forces combed through the old and the new terminal searching for treasures, hidden places and documents, opening all rooms.


title: Berlin Tempelhof Airport

text: During their search, they blew up the fortified entrance to a three-level bomb shelter for celluloid films of the Hansa Luftbild GmbH, a Luft Hansa subsidiary specialising in aerial photography. The explosion immediately ignited the celluloid, turning the film shelter under the northern office wing of the new terminal into a furnace and making it impossible to enter for several weeks. The raging inferno led the Soviet commander to order the lower levels to be flooded with water. With no functioning water supply in war-torn Berlin, this was only possible because the new terminal, which had suffered only slight war damage, had its own electricity and groundwater utility with underground reservoirs under the northerly forecourt of the new terminal close to the film shelter.


title: Berlin Tempelhof Airport

text: On 8 May 1945, Western Allied and German signatories of the German Surrender in Berlin and their entourage landed at Tempelhof airport. At the beginning of May, Weser Flugzeugbau opened a workshop in hangar 7 to repair streetcars. In the following weeks, Berliners raided all unguarded parts of the opened buildings searching for food or anything else useful in bartering in the black market. In accordance with the Yalta agreements, Zentralflughafen Berlin-Tempelhof was turned over to the United States Army 2nd Armored Division on 2 July 1945 by the Soviet Union as part of the American occupation sector of Berlin. This agreement was later formalised by the August 1945 Potsdam Agreement, which formally divided Berlin into four occupation sectors.


title: Berlin Tempelhof Airport

text: The 852nd Engineer Aviation Battalion arrived at Tempelhof (Code Number R-95) on 10 July 1945 and conducted the original repairs in the new terminal. After the Allied Control Council had agreed upon West Berlin Air Corridors under control of the Berlin Air Safety Center, these opened in February 1946, enabling civil aviation at Tempelhof to restart.


title: Berlin Tempelhof Airport

text: On 20 June 1948, Soviet authorities, claiming technical difficulties, halted all traffic by land and by water into or out of the western-controlled sectors of Berlin. The only remaining access routes into the city were three -wide air corridors across the Soviet Zone of Occupation. Faced with the choice of abandoning the city or attempting to supply its inhabitants with the necessities of life by air, the Western Powers chose the latter course.


title: Berlin Tempelhof Airport

text: "Operation Vittles", as the airlift was unofficially named, began on 26 June when USAF Douglas C-47 Skytrains carried 80 tons of food into Tempelhof, far less than the estimated 4,500 tons of food, coal and other essential supplies needed daily to maintain a minimum level of existence. But this force was soon augmented by United States Navy and Royal Air Force cargo aircraft, as well as British European Airways (BEA) and many of Britain's fledgling wholly privately owned, independent airlines.


title: Berlin Tempelhof Airport

text: The last included Freddie Laker's Air Charter, Eagle Aviation and Skyways. On 15 October 1948, to promote increased safety and cooperation between the separate US and British airlift efforts, the Allies created a unified command – the Combined Airlift Task Force under Maj. Gen. William H. Tunner, USAF – at Tempelhof. To facilitate the command and control, as well as the unloading of aircraft, the USAF 53d Troop Carrier Squadron was temporarily assigned to Tempelhof.


title: Berlin Tempelhof Airport

text: The grass runways usual in Germany until then could not cope with the great demand, and a subsequently built runway containing perforated steel matting began to crumble under the weight of the USAF's C-54 Skymasters. Hence, American engineers built a new runway at Tempelhof between July and September 1948 and another between September and October 1948 to accommodate the expanding requirements of the airlift. The old airport terminal of 1927 was demolished in 1948 in order to create additional space for unloading more planes. The last airlift transport touched down at Tempelhof on 30 September 1949.


title: Berlin Tempelhof Airport

text: Tempelhof also became famous as the location of Operation Little Vittles: the dropping of candy to children living near the airport. The original "Candy Bomber", Gail Halvorsen noticed children lingering near the fence line of the airport and wanted to share something with them. He eventually started dropping candy by parachute just before landing. His efforts were expanded by other pilots and eventually became a part of legend in the city of Berlin.


title: Berlin Tempelhof Airport

text: As the Cold War intensified in the late 1950s and 1960s, access problems to West Berlin, both by land and air, continued to cause tension. Throughout the Cold War years, Tempelhof was the main terminal for American military transport aircraft accessing West Berlin. In 1969 one of the pilots during the Berlin Airlift, and the original "Candy Bomber", Gail Halvorsen, returned to Berlin as the commander of Tempelhof airbase.


title: Berlin Tempelhof Airport

text: With the fall of the Berlin Wall and the reunification of Germany, the presence of American forces in Berlin ended. The USAF 7350th Air Base Group at Tempelhof was inactivated in June 1993. In July 1994, with President Clinton in attendance, the British, French, and American air and land forces in Berlin were deactivated in a ceremony on the Four Ring Parade field at McNair Barracks. The Western Allies returned a united city of Berlin to the unified German government. The U.S. Army closed its Berlin Army Aviation Detachment at TCA in August 1994, ending a 49-year American military presence in Berlin.


title: Berlin Tempelhof Airport

text: In 1951 the US Army stationed an aviation element of the 6. Infantry Brigade-(6th Infantry Regiment?) with three Hiller OH-23A Raven helicopters at Tempelhof. Over the years it became known as the Berlin Brigade Aviation Detachment (BBDE Avn.Det.). The Hiller OH-23A was soon replaced by Bell OH-13 Sioux. Further helicopters stationed over the years where Sikorsky H-19 Chikasaw (1958–1964), Sikorsky H-34 Choctaw (1962–1964), Bell UH-1B (1964–1971)and finally Bell UH-1H (May 1971– August 1994).


title: Berlin Tempelhof Airport

text: Fixed wing aircraft stationed at Tempelhof were Cessna O-1 Bird Dog (1965–1975), De Havilland Canada U-6 Beaver (1968 – January 1980), Cessna O-2A (1975–1979), Pilatus UV-20A Chiricahua (1979–1991), Beechcraft U-8D Seminole(1960s), Beechcraft U-21 (1970s–1986 and 1991–1994), as well as Beechcraft C-12C (1986–1991).


title: Berlin Tempelhof Airport

text: American Overseas Airlines (AOA), at the time the overseas division of American Airlines, inaugurated the first commercial air link serving Tempelhof after the war with a flight from New York via Shannon, Amsterdam and Frankfurt on 18 May 1946. This was followed by AOA's inauguration of West Berlin's first dedicated domestic air link between Tempelhof and Frankfurt's Rhein-Main Airport on 1 March 1948.


title: Berlin Tempelhof Airport

text: AOA was the only commercial operator at Tempelhof to maintain its full flying programme for the entire duration of the Berlin Blockade (26 June 1948 – 12 May 1949).


title: Berlin Tempelhof Airport

text: Following the end of the Berlin Blockade, AOA launched additional dedicated scheduled services linking Tempelhof with Hamburg Fuhlsbüttel and Düsseldorf Lohausen from 6 March and 1 June 1950 respectively.


title: Berlin Tempelhof Airport

text: On 25 September 1950, Pan Am acquired AOA from American Airlines. This merger resulted in Pan Am establishing a growing presence at Tempelhof. (In addition to continuing AOA's original, multistop Berlin – New York route and dedicated internal German services connecting Berlin with Frankfurt, Hamburg and Düsseldorf, between 1955 and 1959, Pan Am commenced regular, year-round scheduled services to Cologne, Stuttgart, Hanover, Munich and Nuremberg from Tempelhof.) Pan Am's initial equipment for its new Berlin operation were unpressurised, 60-seat Douglas DC-4s, widely available at the time due to the large number of war-surplus C-54 Skymasters.


title: Berlin Tempelhof Airport

text: 1950 was also the year Air France joined Pan Am at Tempelhof. Air France resumed operations to Tempelhof following their cessation during the war years.


title: Berlin Tempelhof Airport

text: This was furthermore the time Allied restrictions on the carriage of local civilians on commercial airline services from/to West Berlin were lifted. It entailed transferring responsibility for processing all commercial flights to West Berlin's city government, including the operation and maintenance of associated passenger, cargo and mail handling facilities. These changes gave a major boost to West Berlin's fledgeling post-war scheduled air services.


title: Berlin Tempelhof Airport

text: On 8 July 1951, BEA transferred its operations from Gatow to Tempelhof, thus concentrating all West Berlin air services at Berlin's iconic city centre airport. BEA's move to Tempelhof resulted in a significant increase in passenger numbers, as well as an increase in its Berlin-based fleet to six Douglas DC-3s.