Science and Technology
`
`edited by
`
`James R. Sheets
`
`Bruce W. Smith
`
`—-
`
`1
`
`-—
`
`ZE|SS’l127
`
`

`
`
`
`Bruce W. Smith
`Rochester Institute of "I’ecImology
`Rochester, New York
`
`
`
`Sciefice and Tec:hn010gy
`
`edited by
`
`James R. Sheets
`HewIeft~Packczrd laboratories
`Palo Alto, Calfiornin
`
`NEW YORK - BASEL BEKKER
`
`MARCEL DEKKER, INC.
`
`

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`
`Sheais, James R.
`Microlithography: science and teclmoiogy 1 James R. Sheets, Bruce W’. Smith.
`p.
`cm.
`Includes bibliographim! refiarenccs and index.
`ISBN 0-8247-9953-4 (aik. paper)
`I, Microlithography. 2. Integrated cireuits—Masks. 3, Meta} oxide semiconductors,
`Comp1ementmy—~—Design and construction.
`4. Manufacturing processes
`1. Smith,
`Bruce W. U. Titie.
`TK7836.S46 H398
`62I.38l5‘3l-~dc21
`
`9846713
`CI?
`
`
`
`

`
`
`
`1. System Overview of Optical Stoppers and Scanners
`Michael S. Hibbs
`1.
`Introduction
`2. The Lithographic Exposure System
`3.‘ Variations on a Theme
`4. Lithographic Light Sources
`5. Optical Considerations
`6. Latent Image Formation
`7. The Resist Image
`8. Alignment and Overlay
`9. Mechanical Considerations
`
`Conte nts
`
`iii
`
`1
`
`1
`2
`5
`15
`26
`30
`37
`43
`55
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`72
`'76
`
`83
`87
`101
`106
`
`‘I6
`
`
`
`ztron
`robe
`mi-
`st of
`l the
`
`‘tears
`lmith
`
`Preface
`
`PART I: Exposure Systems
`
`Chapter
`
`10. Temperature and Environmental Control
`11. Mask Issues
`
`12. Control of the Lithographic Exposure System
`13. Optical Enhancement Techniques
`14. Lithographic Tricks
`References
`
`

`
`
`
`1
`
`
`
`System Qverview of Gptical
`$ieppers and Scanners
`
`Michael S. Hibbs
`
`IBM .MicraeIec!rom'c Division
`Essex Jzmcrioxz, Vermont
`
`
`
`
`
`V
`
`5
`
`‘
`
`I
`
`V Inrrnooucrlom
`1 _
`':Mir:rolitl1ogr‘aphy is a,n1anofacturing process for producing highly accurate, mi-
`croscopic, iwo—dimensi.onal patterns in a photosensitive resist material. These pat~
`terns are repilcas of a master pattern on a durable phoiomask, typically made of
`a thin patterned. layer of clironiinm on a transparent glass p1aie.At the end of the
`iithographio process, the photoresist is used to create a useful structure in the de-
`vice that is being built. For example, trenches can be etched into an insulator, or
`.a’ uniform coating of metal. can beetcheci to leave a network of eiecnicai wiring
`onrhe surface of 21 semiconductor chip. Microlithography is used at every stage
`~_ofUthe semicondiictor rnannfacturing process. An advanced memory chip can
`have 20 orrnore masking levels, and approximately one tliirrl of the total cost of
`semiconductor manufacture can be attributed to microlithograpliic processing.
`The progress of microlithography has been measured by the ovensmaller sizes
`of .the images that can be printed. There is a strong economic incentive for im~
`_ (‘proving lens resolution. A decrease in minimum image size by a factor of 2 leads
`j ma factor of 4 increase in the number of circuits zimt can be built on a given area
`of vtlie semiconductor chip, as well as significant increases in switching speeds. Ii:
`‘inns been traditional to define a decrease in minimurn image size by nfacior of
`‘N ~1N§ as a new litliograpiiic generation. These lithographic generations are roughly
`1 coincident with generations of dynamic rancloni-access memory (DRAM) chips,
`‘which are defined by an increase in memory storage by a factor of 4. Table l.
`1
`
`

`
`
`
`2
`
`Hfbbs
`
`Overview of Optical Steppers r
`
`Table 1 Seven Lithographic and DRAM Generations
`
`4096
`l024
`256
`64
`I6
`4
`l
`DRAM storage (mcgabits)
`
`Minimum image size (pm) 0.13 1.00 0.70 0.50 0.35 0.25 0.18
`
`
`
`
`
`
`
`l:_,___..,_l
`
`[3:11]
`
`shows the correspondence of lithographic and DRAM generations. About half of
`the 4x increase per generation in DRAM capacity is due to the reduced litho-
`graphic image size, and the remaining increase is accomplished by advances in
`design techniques and by increasing the physical dimensions of the DRAM. His-
`torically, there have been about 3 years between.1ithograplu‘c generations. with
`leadingazdge manufacturing at 0.35 pm starting in 1995.
`
`2 THE l_l'THlI3§fiAPHlC EXPOSURE SYSTEM
`
`At the heart of the microlithographic process is the exposure system. This
`complex piece of machinery projects the image of a desired photomaslc pattern
`onto the surface of the semiconductor device being fabricated on a silicon
`wafer. The image is captured in El thin layer of a resist material and trans- ‘
`formed into a pcmiarient part of the device by a series of chemical etch or de~
`position processes. The accuracy with which the pattern must be formed is
`astonishing: lines smaller than a micron must be produced with dimensional
`tolerances of a few tens of nanometers, and the pattern must be aligned with
`underlying layers of patterns to better than one fourth of the minimum line
`width, All of these tolerances must be met throughout an exposure field of sev~
`eral square centimeters. A litltographlc exposure system filling an enclosure the
`size of a small office and costing several million dollars is used to meet these
`severe requirements.
`4
`An exposure system for optical microlithography cotisists of three parts: a
`‘lithographic lens, an illumination system, and :1 wafer positioning system. A
`typical’ exposure system will be described in detail, followed by an expanded
`description of the many possible variations on the typical design.
`
`2.1 The Lithographic Projection Lens
`
`The lithographic lens is a physically large, compound lens. It is made up of 10
`to 20 simple lens elements, mounted in a massive, rigid barrel. The total as
`sembly can weigh more than 100 ‘pounds. The large number of elements is,
`needed to correct optical aberrations to a very high degree over a 30-min cir-
`cular field of exposure. The lens is designed to produce an optical image of a
`photomask, reduced by a dernagnification of 5X. A silicon wafer, containing
`hundreds of partially fabricated integrated circuits,
`is exposed to this image
`The image is captured by a layer of photosensitive resist, and this latent image
`
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`
`Optical layout oft-1
`Figure '1
`was designed in 1985. More reset
`more complex
`
`will eventually be chemical];
`Every aspect of the lens dcsig
`duce the smallest possible in’
`only by fundamental cliffractl
`wavefront aberration at every
`of the optical wavelengthl Ti‘
`plaoarity by more than about
`the maximum transverse geo‘
`pm. The lens is corrected fr
`wavelengths centered on the i
`
`2.2 The lllumination S
`
`The illumination source for
`
`mercury arc larnp. An elliptic
`desired wavelengths are remo
`ing 365~nm light is sent throu
`and is then projected through
`
`

`
`Overview of Optical Stoppers and Scanners
`
`C::::l
`
`Optical layout of a smali—field, experimental lithographic lens. This lens
`Figure 1
`was designed in 1985. More recent lenses used in commemiol microlithography are even
`more complex.
`
`will eventually be chemically developed to leave the desired resist pattern.
`Every aépect of the ‘lens design has extrenieiy tight tolerances. in order to pro-
`duce the smallest. possible images, the resolution of the lens must be limited
`only ‘by_ ‘fundamental diffraction effects. In "practice, this means that the total
`wavefront aberration at every point in the exposuife field must. be less than ill!)
`of the optical wavelength. The focal plane of the lens must not deviate from
`planarityisy more than about 0.1 turn over the entire usable exposure field, _and
`the ‘maximum transverse geometrical distortion must be less than about 0.05
`tun. The lens is corrected for chromatic aberration over :1 narrow range of
`.\vave'lengths centered on the illumination wavelengtli of 365 nm.
`
`l
`
`i
`
`j 2.2 The Illumination" Subsystem
`Tire illumination source for the exposure system is a 1000~W, high~pressure
`mercury arc lampt An elliptical mirror is used to collect this light, and the un-
`desired wavelengths are removed with multilayer dielectric filters. The remain;
`'
`ififg 365-nm light is sent through a series of relay optics and uoiformizing op‘ti¢§i
`arid“ is then projected through the pliotomask, Nonunifomiity of the illumination 4
`
`r
`
`

`
`
`
`
`
`
`
`Hibbs
`
`
`
`Figure 2 A rather simple, experimental illurninator. Laser light is randomized in a
`light tunnel (a), then projected through a series of five lenses and two folding mirrors
`onion photomttsk (1)). This illuminator was designed to be used with the lithographic
`lens in Fig.
`l.
`
`intensity at the photomask must be less than about 1%. The light continues
`through the photomaslc to form an image of the effective illumination source in
`the entrance pupil of the lithographic lens. The fraction of the pupil filled by
`the illumination source's image determines the degree of coherence in the litho~
`graphic lens’s image formation, The light traversing the entire chain of illumi-
`nator and lithographic lens optics forms an image with an intensity of a few
`hundred mW/cmi. A fast shutter within the illurninator assembly exposes the
`photoresist to the image for a few tenths of a second. The integrated energy of
`each exposure must be repeatable to within 1%. Although the tolerances of the
`illuminator are not as tigiitns those of the lithographic lens, its optical quality
`must he surprisingly high. Severe aberrations in the illumination optics‘ will pro-
`duce a variety of problems in the final image, even if there are no aberrations
`in the lithographic lens.
`
`2.3 The Wafer Positioning Subsystern
`
`The wafer positioning system is one of the most precise mechanicai systems
`used in any technology today. A silicon tvefct‘, typically 150 to 200 mm in di-
`
`Ovmriew of Optical Sleppers .
`
`nmeter, may contain lOO or rr
`chip in its turn must be physit
`lithographic lens and held in :
`ing the exposure. To expose ti
`held by a vacuum chuck on 2
`determined by laser interferoi
`stage has less than 1 second to
`tle to within the alignment tol
`quence of stepping from one
`to be called a stepwanckrepeat
`per." Prior to exposure, the pa
`rately as possible with an ant
`standardized ‘alignment marks
`levels of lithography, The pool
`rlety of optical detection techr
`can be used, but at minimum
`its ;r- and Mranslation error
`image. The positioning systeir
`tolerance before each exposur
`The stepper must also autc
`sition this surface at the Corr
`
`stepper lens within a tolerance
`over a large exposure field, it
`along two orthogonal axes. T
`that the focus tolerance will
`
`ously, so the automated focus
`the wafer.
`During the entire process
`exposing, and unloading, spec
`per that can expose 60 wafer
`twice as fast as a stepper that
`
`3 VARiATlOl\!S ON A
`
`The typical stepper outlined ir
`ment for semiconductor mico
`
`of other styles of equipment l
`alions were the historical pre
`still in use today, earning thei
`density seniiconductor desigr
`cializeri niches in the lithogr
`someday become the new Sifl
`
`
`
`

`
`
`
`
`
`
`
` ()verview of Optical Stepper: m2c2Scamtr1rs
`5
`ameter, may contain N10 or more semiconductor devices, called “chips? Each
`chip in its turn must be physically aligned to the image being projected by the
`lithographic lens and held in alignment with a tolerance of about 100 nm dur-
`ing the exposure. To expose all the chips on a wafer sequentially, the wafer is
`held. by a vacuum chuck on an ultrztprecision x-y stage. The stage position is
`determined by laser intcrferornetry to an accuracy of better than 20 nm. The
`stage has less than 1 second to move between successive exposure sites and set.
`no to within the alignment tolerance before the next exposure begins. This se~
`quencc oi‘ stepping from one exposure to the next has led this type of system
`to be called a step-and-repeat lithographic system, or more informally a “step
`per." Prior to exposure, the position of the wafer must be determined as accu-
`rately as possible with an automatic alignment system. This system looks for
`standardized alignment marks that were printed on the wafer during previous
`levels of lithogtaphyt The position of these marks is determined by one of 3 va~
`riety of optical. detection techniques. A number of different alignment strategies
`can he used, but at mininntm the within-plane rotation error of the wafer and
`its .r- and yhanslation errors must be determined relative to the projected
`image. The positioning system must reduce these errors to within the alignment
`tolerance before each exposure begins.
`The stepper must also nutornatically detect the surface of the resist and po~
`sition this surface» at the correct height to match the exact focal plane of the
`stopper lens within a tolerance of about 200 nm. In order to meet this tolerance
`over a large exposure field, it is also necessary to detect and correct tilt errors
`along two orthogonal axes. The wafer surface is not flat enough to guarantee
`that the focus tolerance will be satisfied everywhere on the wafer simultane-
`ously, so the automated focus procedure is repeated at every exposure site on
`the wafer.
`During the entire process of loading a wafer, aligning, stepping, focusing,
`exposing, and unloading, speed of the process is. of utmost importance. A step.-
`pcr that can expose 60 wafers in an hour can pay back its huge capital cost
`twice as fast as a stepper tiianican manage only 30 wafers perhour.
`
`
`
`
`3 VARIATIONS on A THEME
`The typical stepper outlined in the previous section has been the standard equip-
`ment for semiconductor niicrolithograpity for the past I0 years. But a number
`of other styles of equipment have been used as W6”! Some of these other vari-
`ations werc the historical predecessors of today’s stoppers. l.\»iany of them are
`still in use today, earning their keep by providing low-cost lithography for low-
`density semiconductor designs. Other variations on the basic design fill spe-
`cialized niches in the lithography market or represent new designs that may
`
`someday become the new standard,
`
`
`
`

`
`6
`
`Hibbs
`
`Overview of Optical Snappers
`
`1
`
`3.2 X-ray Proximity Li
`
`A more modern variation of o
`graphy. The diffructive effect:
`very short wavelengths of the
`responding to 3 t-1<eV x-ray
`Et factor of 300 relative to opt
`resolution by a factor of aho
`the best resolution of any litl
`back from large~scale manufar
`dies {Z}. The electron synchr
`and must support a very higl
`able. Since a single electron s
`a dozen wafer aligners or mo
`tion on an entire manufaeturi
`Each x-ray mask alignmer
`absorption and scattering of I
`and masks. to and from the e
`The most challenging fean
`of producing the lx membrar
`making infrastructure in the 5
`5x rectuction masks, considt
`are needed to produce the rut
`ductions in line‘ width toleran
`
`proximity lithography has 1)
`support of national governn
`continued progress of optical
`imity lithography in the role
`ably continue to be the c
`difficulties at image dimensit
`may become the dominant it
`taken by one of the other ex
`the stage of laboratory Lresea
`
`3.3
`
`1>< Scanners
`
`in the 19703, optical proxirr
`scanning lithography [3]. C)
`mask through a lens system
`as that used by at proximity
`enough to cover the entire V»
`masks are no longer damage
`surface. It would be difficult
`
`
`
`3.1 Optical Contact Printing and Proximity Printing
`
`The earliest exposure systems were Contact printers and proximity printers. In
`these systems, 21 chrome-on-glass mask is held in close proximity or in actual
`contact with a photoresist-covered wafer, The resist is exposed through the back
`side of the mask by a flood exposure source. The mask pattern covers the en-
`tire wafer and is necessarily designed with a magnification of ix. Alignment is
`accomplished by an operator manipulating a mechanical stage to superimpose
`two previously printed alignment marks on the wafer with corresponding align-
`ment marks on the mask. Alignment of the two pairs of marks is verified by
`the operator through a spliotield microscope that can view opposite sides of the
`wafer simultaneously. The wafer and mask can be aligned with respect to rota-
`tion and displacement on two orthogonal axes.
`Contact printing provides higher resolution than proximity printing but at the
`cost of enormous wear and tear on the musket No matter how scrupulous the
`attention to cleanliness, particles of dirt eventually are ground into the surfaces
`of the wafer and the mask during the exposure. A frequent source oficontami~
`nation is fragments of photoresist that adhere to the surface of the musk when
`it makes contact with the wafer. Masks have to be cleaned frequently and fi-
`nally replaced as they wear out. This technology is not used in mainstream
`semiconductor manldactnre todayr
`Proximity printing is much more kind to the masks but in many ways is a
`more demanding technology [1]. The proximity gap has to be as small as p03»
`sible to avoid loss of resolution from optical diffraction. The resolution limit for
`a proximity printer is proportional to «I731: where )\I is the exposure wavelength
`and d is the proximity gap. When optical or neaouitraviolet exposure wave
`lengths are used, the minimum image sizes that can be practically achieved are
`around 2 or 3 pm. This limits optical proximity printing to the most unde~
`mending applications of semiconductor lithography.
`
`
`
`Figure 3 In optical proximity printing light is blocked from the photosensitive resist
`luyer by chromium patterns on a photornaslc. The gap between the mask and the resist
`must be as small as possible to minimize diffractive blurring at the edges. of the patterns‘
`
`
`
`

`
`
`
`
`A more modem variation of optical proximity printing is .x—ray proximity litho»
`gr-aphy. The diifractivc effects that ‘limit resolution are greatly reduced by the
`very short wavelengths of the x~rays used, typically around 1.0 to 1.5 nm, con
`responding to a l—l‘<e\/ X-ray energy. This represents a wavelength decrease of
`a factor of 300 relative to optical proximity lithography, or an improvement in
`resolution by a factor of about 15. X-ray proximity lithogrephyis capable of
`the best resolution of any lithographic technology today, but it has been held
`back from large-scale manufacturing by a variety of technical and financial hur-
`dles [2]. The electron synchrotron used as’ the x~ray source is very expensive
`and must support a very high volume of wafer production to make it afford
`able, Since a single electron synchrotron will act as the illumination source for
`a dozen wafer aligners or more, a failure of the synchrotron could halt produc-
`tion on an entire manufacturing line.
`Each x~ray mask alignment system requires a helium atmosphere to prevent
`absorption and scattering of the x-rays. This complicates the transfer of wafers
`and masks to and from the exposure system.
`The most challenging feature of x-ray proximity lithography is the difficulty
`of producing the lx membrane mask to the required tolerances. Since the mask»
`making infrastructure in the semiconductor industry is ‘largely geared to 4X and
`5X reduction masks, considerable improvements in tnzislomalcirig technology
`are needed to produce the much smaller features. on a IX mask. Proportional rc-
`ductions in line width tolerance and placement tolerance are also needed. X~ray
`proximity lithography has been under development for many years with the
`support. of national governments and large semiconductor corporations, The
`continued progress of optical reduction lithography has always kept x-ray prox-
`imity lithography in the role of a ‘‘next-generation.’’ technology. This will proh-
`ably continue to be the case until optical
`lithography runs into serious
`diffictilties at image dimensions between 0.15 and 0.10 pm’. At that time, x-rays
`may become the dominant microlithogrnphic technology, or they may be over»
`taken by one of the other experimental lithographic techniques that are now at
`the stage of laboratory research projects.
`
`Overview of Optical Stepper: and Scanners
`
`3.2 X—ray Proximity Lithography
`
`’l>< Scanners
` 3.3
`In the 19705, optical proximity printing was replaced by the newly developed
`scanning lithography [3]. Optical scanners are able to project the image of a
`mask through a lens system onto the surface of a wafer. The mask is the same
`as that used by ti proximity printer: il lx chrome-on-glass pattern that is large
`enough to cover the entire wafer. But the use of it projection system means that
`masks are no longer damaged by accidental or deliberate Contact with the wafer
`surface. It would be difficult to design a lens capable of projecting micromscalc
`
`__11__
`
`

`
`8
`
`Hibbs
`
`Overview of Optical Stappmzs
`
`images onto an entire 4« to 6-inch wafer in a single field of view. But a clever
`design hy the Perl-tin-Elmer Corporation allows wafers of this size to be printed
`by simultaneously scanning the mask and wafer through a lens field shaped like
`a narrow are. The lens design takes advantage of the fact that most lens aber-
`rations are functions of the radial position within the field of view. A lens with
`an extremely large circular field can he designed, with aberrations corrected
`only at a single radius within this field. An aperture limits the exposure field to
`a narrow are centered on this radius. Because the projector operates at 1>< m21g~
`nifioation, a rather simple mechanical system can scan the wafer and mask si~
`inultaneously through the object and image fields of the lens,
`Resolution of the projection optics is determined by the wavelength and nu-
`merical aperture using R.ayleigh’s formula,
`I
`
`.
`
`k, .2.NA
`
`where D is the minimum dimension that can be printed, 9» is the exposure wave-
`length, and NA is the numerical aperture of the projection lens. The propor~
`tionality constant k1 is a dimensionless number in an approximate range from
`0.6 to 0.8. The numerical aperture of the i’erkin—Elmer scanner is about 0.17,,
`
`
`
`l>< mask into an arc»
`Figure 4 A scanning exposure system projects the image of rt
`shaped slit, The wafer and mask are simultaneously scanned across the field aperture
`(shaded area) until the entire wafer is exposed.
`
`and its illumination source t
`around 400 rim. The Rayleigl
`what smaller than 2 ttm for t‘.
`The l>< scanners are still
`throughout the world‘ Resoh
`pm by using a deep~u1trnviote
`advanced lithography is hem
`described in the example at
`still retained by a 1x Scttnne
`semiconductor devices, such
`this large field size, but in r
`lithography toward stoppers
`
`3.4 Reduction Stepp
`
`Steppers were first commert
`used with a field size just 3
`ships, The fields are expose:
`an accurate .r~y stage betwe
`siderabiy greater than with
`
`
`
`A stepper oinp
`Figure 5
`The 4X or 5>< mask. remains
`sure field is shown as the sh.
`moves the wafer to the post
`is small enough, two or mt?‘
`
`
`
`...t-.w'-_‘
`
`“wt-'l¢te~\_y«fi<nnu«M~<<(
`
`{L
`
`v
`
`»
`
`__ 12 -_
`
`

`
`C)pgfVi€lV of Optical Stepper: and Scrmners
`
`and its illumination source contains it broad band of wavelengths centered
`around 400 nm. The Rayleigh formula predicts a minimum image size some-
`what smaller than 2 tun for this system.
`The ix scanners are still
`in common use for semicontiuctor lithography
`throughout the world. Resolution of ‘these systems can be pushed to nearly 1
`ton by using a. deep-ultraviolet light source at 250—nm wtwelength. But the most
`advanced lithography is being done by reduction projectors, similar to the one
`described in the example at the beginning of this chapter. The one advantage
`stiil retained by a ix scanner is the immense size of the scanned field. Some
`semiconductor devices’, such as tivodimensiormi video detector arrays, require
`this large field size, but in most cases the need for sinalljsr iinages has driven
`lithography toward steppers or the newer step~and-scan technology.
`
`3.4 Reduction Stsppers
`
`Steppers were first oontmerclalized in the early l980s [4]. A projectrion lens is
`used with a field size just large enough to expose one or two semicondtxctor
`chips. The fields are exposed sequentially, with the wafer being repositioned by
`an accurate x-y stage between exposures. The time to expose a wafer is con«
`siderahly greater than with a scanner, but there are some great advantages to
`
`Figure 5 A stepper empioys reduction optics and exposes only one chip at 21 time.
`The 4>< or fix mask remains stationary with respect to the lens, whose maximum expo-
`sure field is shown as the shaded area. After each chip is exposed, a higlvprecision stage
`moves the ‘wafer to the position where the next exposure will occur. If the chip pattern
`is smail enough, two or more chips may be printed in each exposure.
`
`

`
`I 0
`
`Hilibs
`
`Overview of Optical Steppe.
`
`stepper lithograplty. The stepper lens can be made with a considerably higher
`numerical aperture than is practical for the full-wafer scanner lenses. The ear-
`liest stoppers had numerical apertures of 0,28, yielding a resolution of about
`1.25 pm at an exposure wavelength of 436 nm (the mercury g line). Another
`key advantage of stoppers is their ability to use a reduction lens. The damagin-
`fieation factor of 4x to 10>»: provides considerable relief in the minimum fea-
`ture size and dimensional tolerances that are required on the mask.
`The resolution of stoppers has improved considerably since their first intro-
`duction. The numerical aperture of lithographic lens designs has gradually in»
`creased, so that today values above 0.50 are commonly available, At the same
`time, there have been incremental changes in the exposure wavelength. in the
`mid-19803 there was a shift from the g-line (4313 run) to i~llne (365 um) wave~
`length for 1eadi‘ng—edge lithography. More recently steppers are being designed
`to use deep-ultrtwiolet wavelengths around 248 nm. This combination of higher
`numerical aperture and shorter wavelength allows a resolution of 0.5 pm to be
`routinely achieved and 0.35~}tm resolution to be produced in the most advanced
`production lines of 1995. Future extensions to numerical. apertures greater than
`0.60, coupled with recent advances in lithographic enhancement techniques
`such as phaseushifting masks and off‘-axis illumination, give lithographers great
`confidence that 0.25-um resolution wiil be a practical reality in the very near
`future. Even the 0.l.8~prn lithographic generation is being targeted in semicon-
`ductor development laboratories around the world, with the expectation that a
`combination of enhancement techniques and even shorter wavelengths (around
`193 run) will achieve this target before the year 2000.
`
`3.5
`
`1>< Stoppers
`
`Although the main development of lithography over the past decade has been
`with the use of reduction stoppers, a few other notable lithographic techniques
`have been used. The Ultratech Stepper Corporation developed 2: stepper with lx
`magnification. using 21 particularly simple and elegant lens design. This lens de-
`sign has been adapted to numerical apertures from 0.35 to 0.70 and wavelengths
`from 436 rim to 193 nm. The requirernent for a 1x mask has prevented the gen-
`eral acceptance of this technology for the most critical levels of lithography, but
`it is an economical alternative for the less demanding tnasltfiing levels {S},
`
`3.6 Step~and~Scan
`
`As lithographic image sizes evolve to smalier and smaller dimensions, the size
`of the semiconductor chip has been gradually increasing. DRAM chips are usu-
`ally designed as rectangles, with a 2:1 le'ngth~to-width ratio. A typical
`i6~
`megabit DRAM has dimensions slightly less than 10 X 20 mm, and the linear
`dimensions tend to increase by 15 to 20% each generationi Two adjacent
`
`DRAM chips form a Squan
`to 30 mm in diameter, Lo,
`have a square aspect ratio a
`bitter! requirements of highs
`an enormous challenge for 1
`case the demands on field 3
`veloped lithographic exposy
`scan,” in which a reduction
`field onto a portion of a we
`where the scanning process
`narrow slit, as in the older :
`whose height is the diamete
`only by the size of the mes
`Step-and—scan technolog
`of the stage motion. Where
`move the wafer rapidly to a
`during exposure. the step-as
`wafer simultaneously, hold
`nanorneters continuously dx
`
`
`
`Figure 6 A step-and-scan
`ner. The dashed outline repress
`slit~shaped exposure field ape
`across the field aperture. At lit
`where the scanning process is
`patterns.
`
`
`
`__ 14 __
`
`

`
`Overview of Optical Stepperxr and Scanners
`
`11
`
`DRAM chips form a square that fits into‘ a circular lens field that must be 28
`to 30 mm in diameter. Logic circuits. such as microprocessor ohips, usually
`i1flVt1! ti square aspect ratio and put similar demands on the field size. The com
`bined requirements of higher numerical aperture and larger field size have been
`an enormous challenge for lithographic lens design and fabrication. One way to
`ease the dent;
`cls on field size is to return to scanning technology. Recently de-
`veloped lithographlc exposure equipment employs a technique called “step-and
`scan,” in which a reduction lens is used to scan the image of a large exposure
`field onto a portion ofs wafer [6], The wafer is then moved to it new position
`where the scanning process is repeated. The lens field is required only to be a
`narrow slit, as in the older full—tvafer scanners. This allows. a scanned exposure
`whose height is the diameter of the static lens field and whose length is limited
`only by the size of the mask and the travel of the maslopositloriing stage.
`Step-and-scan technology puts great demands on the mechanical tolerances
`of the stage motion. Whereas a traditional step«and-repeat system has only to
`move the wafer rapidly to a new position and hold it accurately in one position
`during exposure, the step—and»scan mechanism has to move both the mask and
`wafer simultaneously, holding the positional tolerances within a few tens of
`nanometers continuously during the scan. Since the slep~and~scan technique is
`
`Figure 6 A‘stcp-mid-scan system combines the operations of a stepper and n scram-
`ner. The dashed outline represents the maximum scanned region. The shaded area is the
`slit~shaped exposure field aperture. The wafer and mask are simultaneously sertnned
`across the field aperture. At the end of the scam, the wafer is stepped to it new position,
`where the scanning process is repeated. In this example, the 4x mask contains two chip
`patterns.
`
`

`
`1;;
`
`Hibbs
`
`Overwew of Optical Slepperd y
`
`Various tricks have been
`systems (often called direct—\
`may improve the speed of r
`patterned areas of the circuit
`ing has been introduced \.\
`variable-sized rectangular ole
`lelism is achieved with elect
`stencil mask with zr small re
`cil mask consists of a thin, st
`terial where the toast: prrttem
`whenever there is no suitable
`absorbing pattern, as with e
`masks have difficulty with 1
`which break the continuity o
`usually designed as two cot
`tially to produce the desired
`together from a library of th
`hearrt strategy to fill in parts
`But even the fastest direct
`
`systems. Very rarely, direct-\
`graphy on lowwolume. high
`used occasionally for early
`when the parallelexposure e
`been developed yet. But the i
`been for mask making. In tr
`ous an issue. It can make <
`
`mask. In any case, there is u
`out using a seriahvriting sy
`on the prior existence of it n
`
`3.8 Soft X-Ray Proje
`Lithography
`
`The prospect of exploiting
`has recently stirred excltemr
`with photon energies around
`he considered either the lo‘
`
`wavelength limit of the ex
`with good may reflectivity
`ray astronomy community. '.
`ueuts of an all-reflective lit
`
`activity. A diffraotlon—lirnite
`
`used for reduction lithography, the mask must scan at a much different speed
`than the wafer and possibly in the opposite direction. All of the step-and~scon
`equipment designed so far has used a 4>< reduction ratio. This allows the very
`large scanned field to be aecontmodated on a smaller mask than a 5x reduction
`ratio would permit. it also allows a very accurate digital comparison of the po-
`sitional data from the wafer stage and mask stage interferometers.
`The first step-and~scan exposure system was developed by the Perkin-Elmer
`Corporation, using an arc—shaped exposure slit. The projection lens had a nu-
`merical aperture of 0.35 and was designed to use a broadband light source cen-
`tered at a wavelength of 248 nm. The advantage of the fixed—rad