Microelectronic Engineering 61–62(2002)11–24
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Advanced optical lithography development,from UV to EUV
Bernard Fay
Nikon Rearch Corporation of America ,1399Shoreway Road ,Belmont ,CA 94002,USA
Abstract
This paper will review the development of advanced optical lithography starting from the beginning of the wafer stepper era in the early 1980s and projecting into the future.The evolution of optical lithography technology,from the first g-line wafer steppers to the current 248nm and 193nm scanners will be highlighted.For illustration of each specific type of optical tool,examples will be taken mainly from the Nikon product line.The demi of optical or photon-imaging lithography has been predicted countless times,starting many device generations ago,as the ratio of printed linewidth over wavelength steadily decread.But the combination of shrinking photon wavelength,increasing optic numerical aperture and more recently the growing u of resolution enhancement techniques has m
anaged to continuously reinvigorate optical lithography.Today,the future of optical lithography is as bright as ever.An outline of upcoming optical lithography developments at 157nm and at 13.5nm (EUV)wavelengths will be prented.©2002Elvier Science B.V .All rights rerved.
Keywords :Optical lithography;Wafer steppers;Scanners
1.Introduction
Optical lithography has been the driving force behind the miniaturization of integrated circuits,since the first ICs were produced at Fairchild and at Texas Instruments in the early 1960s.From the ont,optical lithography has always managed to keep pace with Moore’s law,including its recent acceleration.To keep pace with the shrinking feature size,a steady stream of improvements have been introduced time after time,and have enabled optical lithography to hold off the challenges of competing lithography technologies.
In the very early days,lithography technology was limited to 13contact printing using disposable masks.Then proximity printing was introduced to reduce mask wear.The feature sizes were of the order of a few microns at that time.Then 13projection was introduced,using a 13full field projection optics between the mask and the wafer and achieving a resolution of the order of 2–3microns.
In parallel with feature size reduction,wafer size was increasing also at a steady pace,starting from the initial 1-and 2-inch diameter.When the wafer size reached 3to 4inches,and the resolution 0167-9317/02/$–e front matter
©2002Elvier Science B.V .All rights rerved.
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12B.Fay/Microelectronic Engineering61–62(2002)11–24
decread below2–3microns,13scanning projection systems introduced by Perkin-Elmer became the tools of choice for veral years,achieving resolution in the3to1.5micron range.However,as the wafer size continued to grow,towards5and6inches,13scanning systems became limited by overlay errors,primarily caud by wafer distortion.A new advance was needed.
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In the late1970s,there had been veral attempts at converting reduction step and repeat cameras ud for mask-making into wafer step and repeat exposure systems.Thefirst commercial wafer stepper was introduced by GCA Corporation at the Microcircuit Engineering conference,held in Paris in1977,25years ago.
This date marks the beginning of advanced optical lithography,which I will define as Photon Reduction Imaging Lithography(PRIL).With this definition,advanced optical lithography en-compass Extreme Ultra Violet Lithography(EUVL).Advanced optical lithography which has enabled the stunning growth of the miconductor industry since the late1970s,will in all likelihood continue to support veral more device generations as the photon wavelength continues to shrink from the current state of the art193nm to the next generations of157nm and EUVL systems. Reduction imaging which was introduced with thefirst wafer stepper is a key attribute of advanced optical lithography becau it relaxes the dimensional requirements on the mask(CD control and pattern placement accuracy)by a factor equal to the reduction ratio(103to43).
The evolution of advanced lithography(or Photon Reduction Imaging Lithography)tool develop-ment from thefirst wafer stepper to the future EUVL reduction scanners will be reviewed in this paper.It is convenient to consider three generations of exposure tools.
1.Stepper systems(from1977to prent).
2.Scanner systems(from1985to prent).
3.Future systems(from2003on).
For thefirst two generations,the emphasis of the paper will be on the evolution of the system optics and of the k1process factor.For the third generation,the emphasis will be on the challenges and critical issues facing advanced optical lithography at157nm and at13nm wavelengths.
2.Stepper systems(from1977to prent)
2.1.g-line steppers
GCA was thefirst company to announce a production-worthy reduction wafer stepper,the GCA 4800DSW,with a lar interferometer-controlled wafer stage,auto focus and two-point wafer alignment.Thefirst public introduction was made at the European Microcircuit Engineering conference held in Paris,France in September1977.The GCA4800was equipped with a g-line Zeiss lens,0.28NA,103reduction ratio,with afield size of10310mm,and offered a throughput of60 4-inch wafers per hour.An optional53lens with largerfield and lower resolution was available.The GCA4800quickly became a commercial success for the production of devices down to1.25micron. Shipments started in the cond half of1978,and by the end of1978,13units had been delivered. It was around that time that Nikon,which was then an established camera and precision optics company,decided to enter the optical lithography market.Nikon introduced itsfirst wafer stepper,the
B.Fay/Microelectronic Engineering61–62(2002)11–2413 NSR-1010G in1980.This was a g-line stepper,with103reduction ratio,0.35NA,a10310mm field and featuring a resolution of1micron.
The stepper market grew very rapidly in the early1980s.There were many companies worldwide offering g-line wafer steppers(GCA,Nikon,Canon,TRE,Perkin-Elmer/Censor,ASML,Optimetrix). The inten competition between all of the companies fueled the rapid development of advanced stepper technology.
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The evolutionary development of advanced lithography lens occurred on many fronts simul-taneously.
•Numerical aperture increa:from0.28to0.35to0.40to0.48to0.53to0.60to0.63.•Imagefield size increa:from10310mm to15315mm to20320mm to22322mm.•Reduction ratio decrea:from103to53(becau of larger imagefield).
•Wavelength decrea:from436nm(g-line)to365nm(i-line)to248nm(KrF).
On the system hardware side,wafer stages of higher precision and higher speed were being continuously introduced,improved automatic alignment systems and incread automation were being continuously developed and incorporated into production tools.
Fig.1reprents the history of wavelength change at Nikon,as measured by the number of steppers of each wavelength sold yearly.
Sales of g-line steppers show afirst peak in1984,just before the1985recession year,then recovered and peaked again in1988.Sales of g-line tools started to fall off in1989as i-line tool sales were ramping up.Sales of i-line tools peaked in1995,and decread in1996as sales of KrF systems were ramping up.
2.2.i-line steppers逐鹿群雄
Thefirst i-line steppers were introduced in the mid-1980s.The initial i-line lens were affected by lens heating problems due to the high absorption at365nm in some optical glass.But the
Fig.1.Yearly sales volume of Nikon steppers by wavelength.
14B.Fay/Microelectronic Engineering61–62(2002)11–24
problems were quickly solved,and in the late1980s,thefirst wavelength decrea,from436nm g-line to
365nm i-line was implemented in production.
An important milestone of i-line lithography was the publication of printing results by Nikon at SPIE1990using an experimental smallfield(535mm),very high NA(0.65),103i-line lens, showing excellent image quality of0.35micron den lines with large DOF,corresponding to a k1 factor value of0.63[1].This result clearly demonstrated the potential of i-line lithography for sub-half micron lithography,showing that advanced optical lithography could be ud to print excellent patterns with dimensions slightly less than the exposure wavelength.
During the15years from1980to1995,the complexity and size of projection lens incread considerably.This can be en readily from the picture of Fig.2showing thefirst Nikon103g-line stepper lens of1980with10310mmfield size next to the53,0.63NA i-line lens of1995with 22322mmfield size.
2.3.248nm steppers薪资结构表
After365nm,the next wavelength was also a mercury lamp line,at248nm.This wavelength was first utilized in the last generation13reflective scanners from Perkin-Elmer.
Thefirst reduction248nm step and repeat system was developed at Bell Labs.In1986,Bell Labs reported at SPIE thefirst results obtained from a248nm wafer stepper consisting of a GCA4800 DSW stepper body equipped with an all quartz lens designed by Bell Labs and built by Tropel[2]. The lens had an NA of0.38and afield size of14mm to20mm diameter.Becau the lens was made of a single material,chromatic correction usually achieved by mixing different glass materials was not possible,so the light source linewidth had to be narrowed considerably.This dictated the choice for light source of a248nm KrF excimer lar,with a line narrowed bandwidth of5pm.This tool achieved a resolution of0.5to0.4micron.
By1990,all lithography suppliers offered KrF steppers,but the u of KrF steppers in production
Fig.2.Evolution of stepper lens,from1980to1995.
B.Fay/Microelectronic Engineering61–62(2002)11–2415 was delayed until around1996,mainly becau new,improved i-line steppers keptfilling the new equipment slots becau of their lower operating costs and higher process stability.
2.4.The k1factor运动前喝黑咖啡
专升本报考It is appropriate to focus now on the k1factor,and on its evolution during the last20years. The k1factor is the process-dependent coefficient of the resolution criterion for a diffraction limited lens,by which the half-pitch of a printed line and space pattern is given in terms of the NA and the wavelength l by the Rayleigh equation:
R5k1l/NA.(1) So in order to decrea the printed feature size,there are only three options:increa the NA of the lens,decrea the wavelength or decrea the process factor k1.
For many years,until the early1990s there was a sacred rule in manufacturing stating that for a lithography process to be manufacturable with high yield,the k1factor could not be less than0.8.For development though,a k1factor of0.6was acceptable.
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At the SPIE1990conference,Lin,of IBM,delivered a paper entitled‘Methods to print optical images at l
ow k1factor’in which he challenged the belief that the k1factor of0.8was an immovable constant[3].He correctly identified three main caus for the high k1value requirement.
1.Imperfections in the imaging system:stray light,vibration,lens aberrations.
2.Diffraction effects which exist even for perfect imaging systems and which can be reduced by
optimizing partial coherence and applying proximity effect correction.
3.Imperfect conditions at the recording he photoresist and the substrate(causing
reflection and standing waves).减肥咖啡对身体有什么危害
In addition,as we now know there are veral additional ways to reduce diffraction effects and work with lower k1factor by acting on the illumination system(off-axis illumination),on the mask (pha shift mask)and more recently by using double exposure methods combined with pha shift masks.
From the early1980s onward,advanced optical lithography has managed to repel successfully all challenges from Next Generation Lithographies,primarily from X-ray lithography.The primary tactic ud to extend optical lithography was to increa the NA up to the practical limit,then switch to a sh
orter wavelength.However,starting in around1995,lowering the k1process factor became an acceptable way to extend advanced optical lithography and in the process,to prerve and augment all the learning accumulated in optical lithography manufacturing.Process optimization,incread u of modeling and simulation,incread planarization and incread system characterization became esntial tools for controlling low k1factor lithography process in a production environment. Thefirst instance of k1factor reduction occurred when in the early1990s,miconductor manufacturers were ready to ramp up production of half micron devices for the16M DRAM generation.Fig.3is a plot of the relationship of Eq.(1)showing NA vs.k1factor for a resolution of half micron or500nm and for three possible exposure wavelengths:436,365and248nm.
From Fig.3,we e that with a k1factor of0.8,g-line steppers needed a lens with NA of0.7,
