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-A P-AST ION BE-AM P-ATTERN GENER-ATOR.
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus, prof. dr. J.M. Dirken,
in het openbaar te verdedigen
ten overstaan van een commissie door het College van Dekanen daartoe aangewezen, op 16 februari 1988 te 16.00 uur door
Hendrik Nicolaas Slingerland,
geboren te Soestdijk, natuurkundig ingenieur.
Dit proefschrift is goedgekeurd door de promotor,
STELLINGEN
behorende b i j h e t p r o e f s c h r i f t
A FAST ION BEAM PATTERN GENERATOR
van
B.N. Slingerland
Delft 16 februari 1988
1 Het is zinloos om bij veldemissiebronnen en Liquid Metal Ion Sources over helderheid te spreken, aangezien er bij de vet van behoud van helderheid vanuit gegaan wordt dat de elektronen resp. ionen geen onderlinge interaktie hebben.
2 Het is niet in de haak om de Langmuir grens op te geven als de maximaal bereikbare helderheid van een bestaande bron, vooral niet als een veel lagere waardes wordt gemeten.
(onder andere: K.D. van der Mast, A laser heated schottky emission gun for electron microscopy, doctoral thesis (1975)).
3 De door Cleaver voorgestelde in-lens deflector is, alhoewel goed in theorie, onpraktisch.
(J.R.A. Cleaver, In-lens deflection for scanning ion beam systems, Optik 75 (2) (1987), pp. 75-81).
4 De afbeelding van een puntbron met een Gaussische energie verdeling in een chromatisch begrensd systeem zal, in tegenstelling tot wat algemeen wordt aangenomen, niet een Gaussische verdeling hebben.
5 Het gebruik van supergeleiders in electronen microscopen zal zich niet beperken tot stroomvoerende delen, maar ook het gebruik van deze materi alen als afscherming inhouden.
6 De conclusies gedaan door Richard Dawkins zijn voor meerderlei uitleg vatbaar, of zoals Samuel Butler al eens gezegd heeft: 'A chicken is an egg's way of making another egg'.
(R.Dawkins, The selfish gene (1976), Oxford University Press) 7 Het probleem met Artificiële Intelligentie (AI) is dat hierbij het
begrip intelligent streng in regels moet worden omschreven, maar dat iets niet meer als intelligent zal worden ervaren als het in regels te vatten is.
8 Het is onvermijdelijk dat vrouwen vroeger bij intelligentie testen gemiddeld minder intelligent leken dan mannen.
(A. Binet en T. Simon, Sur la necessite d'etablir un diagnostic scientifique des etats inferieurs de 1'intelligence, L'annee psychologique 11 (1905), p. 191)
(L.M. Terman, The measurement of intelligence, Boston (1916), pp. 6-7)
9 De s t a p met de g r o o t s t e f i n a n c i ë l e gevolgen in een mensenleven i s n i e t , z o a l s een n a t i o n a l e b a n k o r g a n i s a t i e b e w e e r t , de aanschaf van een h u i s , maar h e t ontvangen van k i n d e r e n .
10 Verandering in h e t onderwijs i s h e t s t o k p a a r d j e van v r i j w e l a l l e n e d e r l a n d s e m i n i s t e r s van onderwijs en wetenschappen. De r e s u l t e r e n d e veranderingen z i j n e c h t e r lang n i e t a l t i j d t e n goede.
11 Een u i t e r s t e houdbaarheidsdatum d i e n t n i e t a l l e e n op producten t o e g e p a s t t e worden, maar ook op de termen 'NIEUW!' en 'VERNIEUWD!' van de b e g e l e i d e n d e r e c l a m e s .
About the author.
Hendrik Nicolaas Slingerland (Rik) was born September 2 6t n, 1955 in
Soest, the Netherlands.
After finishing highschool (Atheneum) in 1974 he studied at the Delft University of Technology, department of Applied Physics.
The last two years of this study he studied in the research group Elec tron Optics, headed by professor J.B. Le Poole.
In his last year he studied the construction of electrostatic high vol tage lenses.
He recieved his Ingenieur degree in October 1982.
From 1982 until June 1987 he was a Ph.D. student at the Delft University in the same group, although the group changed its name into Particle Optics and professor K.D. van der Mast succeeded professor Le Poole. Since August 1s t, 1987, he works in the Development Group Electron
Optics for Electron Microscopy at Philips Eindhoven.
He published several papers concerning his Ph.D. work, which also re sulted in a patent.
Contents.
General:
Samenvatting 6 Summary 7
1 Introduction 9 2 Design goal and project history 22
Theory:
3 Ion optics 25 4 Optimization of an ion probe 34
5 Correction of Transverse Chromatic Aberration 42
6 Sources 58
Module design and results:
7 Design philosophy 69 8 Liquid Metal Ion Source and source optics 74
9 Mass filter 80 10 Beam shaping optics 87
11 Beam blanker and scan deflectors 99
12 Results and conclusions 104
13 Suggestions 107
Appendices:
Al High voltage insulation 109 A2 Deflector aberrations 119 A3 "An achromatic mass filter employing permanent
magnets for the Delft Ion Beam Pattern Generator" 129
A4 Computer controls 159 A5 Sample multi-tasking program 163
Samenvatting.
De laatste jaren is er een toenemende belangstelling voor de toepassing van gefocusseerde ionen bundels. De toepassingsgebieden liggen in de halfgeleider fabricage.
In dit proefschrift wordt de ontwikkeling beschreven van een ionen bundel patroon generator teneinde patronen op halfgeleiders over te brengen. Aangezien de gebruikte ionenbron een grote energiespreiding vertoond t.o.v. een elektronenbron, is er speciale zorg besteed aan het miniraalizeren van de fouten die door deze energiespreiding optreden.
In dit kader worden beschreven:
de ontwikkeling van een theorie over de optimalizatie van chromatisch gelimiteerde systemen, met als conclusie dat 'shaped beam' voordeliger is dan het afbeelden van de bron,
de ontwikkeling en realizatie van een geoptimalizeerde ionen bron module de ontwikkeling en realizatie van een nieuw type achromatisch massa filter, met gebruikmaking van permanente magneten,
de ontwikkeling van een theorie over de compensatie van chromatische afbuigfouten.
Verder wordt een overzicht gegeven van het de ontwikkelde apparaat, dat als doelstelling heeft:
een maximale ionen energie van 150 keV (voor enkelvoudig geladen ionen) een bundelkantscherpte van 10 nm
een bundelgrootte van 50 tot 1000 nm
een stroomdichtheid bij het preparaat van 10 A/cm2 bij gebruik van een
gallium ionenbron
een minimale belichtingstijd van 0.1 j/s
Dit onderzoek is gedaan binnen het onderzoekskader van de stichting FOM (stichting voor Fundamenteel Onderzoek der Materie) en STW (Stichting voor de Technische Wetenschappen) aan de Technische Universiteit Delft, terwijl de ionenbron beschikbaar is gesteld door Dubilier Scientific.
Su—ary.
In the last few years interest in the use and applications of focussed ion beams has grown. The usage of this technique is in the field of semiconductor manifactoring.
In this thesis describes the development of an ion beam pattern genera tor in order to transfer patterns to semiconductors. Because the ion source used has a much larger energy spread than an electron source, special care is given to the minimization of aberrations introduced by the energy spread.
Described are:
The development of a theory concerning the optimization of chromatically limited systems, concluding that a shaped beam system has a higher per formance than a Gaussian imaging system.
The development and realization of an optimized ion source module. The development and realization of an achromatic mass filter, employing permanent magnets.
The development of a theory concerning the compenzation of chromatic deflection aberrations.
An overview is presented of the development of an instrument, with the following goals:
maximum ion energy of 150 keV (for singly charged ions) a probe edge sharpness of 10 nm.
a probe size of 50 to 1000 nm.
a probe current density of 10 A/cm2, using a gallium ion source. a minimum illumination time of 0.1 (is.
This work is done at the Delft University of Technology and is supported by the Foundation for Fundamental Research on Matter (stichting FOM) and The Netherlands Technological Foundation (STW), while the ion source is made available by Dubilier Scientific.
IHTRODUCTIOH.
History.
The semiconductor industry started the production of Integrated Circuits or IC's in the 1960's. Since then the minimum feature size has decreased from more than 10 /im to 1 /im, which is now (1987) considered state of the art.
Especially the very high density memory chips and recently the charge-coupled devices (CCD's) employ such small feature sizes. An example is the CCD shown in figure 1.1 with lines of 0.7 /im linewidth and a pitch of 1 /im.
Small feature sizes not only result in more components per cm2, but also in higher speed and/or lower dissipation per component. Therefore seve ral projects have been started with the object to push the minimum fea ture size for production below one //m.
Figure 1.1: Charge Coupled Device (CCD) showing 0.7 /im linewidth and 1 /im pitch (courtesy of Philips NatLab).
Basic techniques.
Basically lithography (from "Lithos' * stone and 'Grapho' = writing) is a technique which uses stones to transfer a pattern (for instance a drawing) to a substrate (such as paper).
In semiconductor industry the 'stone' is certainly not a stone anymore, while the 'paper' is much more stonelike than in the original meaning. Here lithography is the word used for the transfer of a pattern (which can even be an accumulation of data on a computertape) to a semiconduc tor, or more precisely: a layer covering the semiconductor.
In most cases this layer is a plastic like film, the resist, which is sensitive to irradiation. This irradiation is done by exposure to pho tons (light, UV, X-ray), electrons or ions of any species. During the
Introduction. subsequent de velopment a pattern of holes is foiled in the resist. Parts of the wafer are thus exposed to the next step in the process. which can be an Figure 1.2: cross section of a Dynamic Random Access oxidation, im-Memory (DRAM) memory cell. plantation, me
tallization or such like. A cross-section of a memory cell is depicted in figure 1.2, showing the result of all the process steps.
Lithography using 'light'.
At the moment almost all production machines for lithography use 'light' in the blue, the UV and deep UV band. Transfer of the pattern to the resist is achieved by using the imaging properties of a lens system, more or less like a slide projector, or by the illumination of a shadow
mask, as shown in figure 1.3.
Mostly the monochromatic light of a gas discharge or a laser is used in order to make the (chromatic) aberrations of the optics as small as possible while focusing as much light as possible on the target. However, the use of monochromatic light makes the problems related to diffraction severe. Some machines therefore use several spectral lines or rock the beam during exposure. Already the wave length of the light used is one of the limiting factors for the resolution of the pat terns to be transferred.
On the other hand to decrease the wavelength further than approximately 300 nm asks for a com plete new approach. Not only are high powered light sources not readily available (with the exception of excimer lasers), but there are not many materi als available for the optical elements.
Another problem is that the large acceptance angle, used to focus as much light as possible, limits the depth of field. Because the topography of the wafer is approximately one micron, as is the resist thickness, the flatness of the wafer and the thickness of the resist presents a limit to the numerical aperture
y///////////A
Figure 1.3: Wafer illumina tion by means of a shadowmask. lilIntroduction.
of lens systems. This results in a limited acceptance angle of the sour ce, thus limiting the amount of light available at the target and a larger diffraction aberration.
One of the promising techniques of the moment is deep-UV lithography using excimer lasers. These lasers, with output powers up to 50 W conti nuously, have the advantage to operate with low spatial coherence. They are essentially speckle free, a problem which occurs with most other
(UV) lasers, while the available wavelengths are in the range of 351 nm (XeF) to 157 nm ( F2) .
Therefore deep-UV lithography is capable of operating in the sub-micron region (as long as diffraction limited). Rapid progress is made in the development of both the lasers and the optical materials to be used. For further information on this technique see e.g. Elliott [1] and Goodall
[21.
Another promising technique is X-ray lithography, which uses the X-rays emitted by e.g. a synchrotron to illuminate a shadow mask. The shadow mask, a thin membrane of a low Z material such as SiC covered with a high Z-pattern of for instance gold, is positioned between the X-ray source and the wafer.
The use of superconducting magnets makes it possible to build very com pact synchrotrons with a diameter of only a few meters which can illumi nate in the order of 10 wafers simultaneously.
Some major problems still to be solved are the mechanical stability of the shadow masks and changes of the mechanical and optical properties of those masks due to absorption of X-rays. Much work is done at the Fraun-hofer Institute fuer Mikrostrukturtechnik in Berlin, headed by Heuber-ger. For further information on this technique see e.g. Heuberger [3).
Limitation of lithography using 'light'.
Summarizing it seems likely that both X-ray and deep-UV lithography are able to reproduce patterns with minimum feature sizes of approximately 0.25 //m. For production both techniques thus offer enough leeway for the next 10 years.
Whether X-ray lithography will be used in the future is still a point of discussion. The problem is the very small gap in final resolution be tween lithography using e.g. excimer lasers and X-ray lithography. The advantage of the light technique is that it can be phased in gradually. X-ray lithography on the other hand is a novel technique which has to be introduced as a whole.
For best performance the mask has the same dimensions as the image. This will be clear for a shadow projection, but also lens systems exhibit best resolution if the magnification is - 1 . Therefore none of the tech niques mentioned before is suited to make the masks. Heuberger [3]
pro-Introduction.
poses to solve this by making special copies from a mother mask, much like the scheme used in the record industry. But even then the mother masks have to be made by another technique.
The influence of the price of these masks is not too large, because they can be used over and over again, until damaged.
Also the repair of the (expensive) masks is interesting.
The only available technique available for economical production of these masks is (at the moment) electron beam (E-beam) lithography. A limitation of techniques using masks is that, because of the price of
the masks, the minimum economical production will be in the order of at least hundreds of wafers with identical chips. If the duplication scheme of Heuberger is used the economical minimum will even be thousands of wafers. Therefore the mask machines will only be used for large volume production, producing tens of thousands of identical chips.
The demands for low volume production will be met by other techniques like E-beam lithography, or mixed techniques.
Lithography using electrons.
Another approach for the irradiation of resist is the use of energetic particles, such as electrons, to illuminate the resist. The wavelength of electrons is quite small, as already shown by de Broglie. Experimental proof has over and over been given by electron micro scopes.
E-beam lithography has one large draw back when compared to lithography using light: the low throughput. Most E-beam lithographic systems use a scanning beam 'see figure 1.4), which means that data is transferred serially instead of the paral lel transfer used by projection systems.
Still E-beam lithography is the only method commer cially available at the moment for sub-micron work. It is used for the mask making of the X-ray expe riments and for the few sub micron structures used now. Furthermore, because the information is transferred by electrical signals, it is an extre mely flexible technique. Therefore it can be used
for very small volume production. An example is the IBM's EL 3, used in the QTAT (Quick Turn Around Time) production line (see Moore [4)).
Some examples of state of the art Electron Beam Pattern Generators (EBPG's) are that of HP (5) and the AEBLE 150 of Perkin-Elmer [6).
Figure 1.4: E-beam writer using a
scanning beam.
As stated before a severe problem for EBPG's is the serial transfer of data. Of course it is possible to make those machines faster and faster
1-Introduction.
by using more elaborate electron optical systems, sources etc., but for instruments imaging the source on the target it will always be a margi nal improvement. On the other hand the parallel transfer of data could improve speed with orders of magnitude.
To my knowledge only two such systems are under serious development at the moment: the Electron Image Projector and the Electron Proximity Printer.
The Electron Image Projector, shown in figure 1.5, employs a
photo-riLufl
• . ■ ■
M I .
i reel
■ M ' .on c o l l n
Figure 1.5: Schematic drawing of an electron image projector.
cathode emitting electrons when irradiated with <UV) light (see Ward [7]). These electrons in turn are focused by a magnetic field onto a wafer. Magnification is approximately unity. Prototypes have been made
imaging a 5" wafer at once. The possibility of a 'step and repeat* version, imaging only one chip at a time and then stepping the wafer to the next position, is reported too. The last method has better potenti alities for correction of wafer distortions etc.
The conversion of photons to electrons is the key to a large improvement in opening angle: the electron beam has a much smaller opening angle the wafer than a light beam with a comparable energy contents, thereby gi ving a much larger depth of field.
The Electron Image Projector lacks the flexibility of the normal E-beam writers, because the information is not transferred by electrical sig nals, but by means of a mask on the cathode. A change of the pattern
Introduction.
implies a change of this mask, which is, for small volume production, costly.
Resolutions under 0.1 fim have been demonstrated.
However a production instrument based on this principle has not been built yet and a major effort will be required to develop such an instru ment.
The Electron Proximity Printer has been proposed by Bohlen (8] and is essentially the same as proximity printing used in light optics. Of course the mask has to be transparent for electrons instead of for pho tons. This causes a problem because the stencil mask needs to be self supporting too.
A way to work at least partially parallel is the 'shaped beam' concept, in which the pattern is sub divided into more elementary patterns (mostly rec tangles), which are imaged (serially) on the wafer
(see figure 1.6).
The imaging with this 'shaped beam' enables a much higher throughput when compared to the imaging of the source onto the wafer, as is done in normal EBPG's.
Some instruments already use this strategy 'see e.g. Moore [4]). Work on this subject is done in several groups, like the group headed by van der Mast at Delft [9].
Still another approach is the multi-beam approach, as proposed by Le Poole and investigated by Roelofs
[10], Here several beamlets, which can be turned on and off separately, are imaged onto the target. By scanning this multi-probe raster over the target. the whole image field can be irradiated with a pattern. The practical problems for this type of machine are so high that it has never passed the stage of a feasibility study.
Figure 1.6: Shaped beam EBPG.
Limitations of electron lithography.
One of the basic problems of electrons, whether using a scanned beam or transferring whole patterns at once, is the interaction of the electrons with the resist.
Not only does a transfer of energy to the resist (resulting in the exposure of the resist) occur, but also the original momentum of the electrons is changed: the electrons are scattered. This effect is par tially responsible for the proximity effect, together with the produc tion of secondary electrons etc..
Introduction.
The consequence is that the area which is illuminated is not the same as the pattern which was to be transferred. Although this problem can part ly be compensated e.g. by correction of the dosage used or the use of higher voltages and is no longer a limiting factor, it poses a problem.
Limitations of the semiconductors.
Although sometimes the impression is made that the only limiting factor in the semiconductor industry is the production of the structures which form the components, also the semiconductor themselves might impose a limit. With decreasing dimensions the electrical field strengths in the semiconductor will get higher and higher until breakdown occurs.
Furthermore the fringe effects become more dominant. An example of the last is the depletion layer of FETs (Field Effect Transistors), shown in figure 1.7. First of all the FET is not completely in the 'off' state, due to the 'thickness' of the structures, resulting in a small leakage. Second the field strength in the FET is locally very high, which can result in the formation of electron-hole pairs or even breakdown of the device.
Although until now these problems are always solved, the research in vestments necessary are becoming very high. Also some effects 'e.g. quantum effects) can spoil the working of the now well known components, thereby making the use of other components necessary.
On the other hand the effects discovered in sub-micron structures, such
Figure 1.7a: Fet with small lateral Figure 1.7b: Fet's equipotential
dimensions. planes.
as quantum effects (Josephson junctions) in room temperature supercon ductors could be the base of a completely new field of components. Also opto-electronics, employing both light and electrical signals on
Introduction.
one chip, interacting with each other, can lead to a different type of components (not necessarily Boolean either).
Therefore the research in the sub-100 nm range will probably increase. As stated before photons (including X-rays) will not be useful m this
regime.
Focused^Ion Bea»s.
The use of sub-micron Focused Ion Beams (FIBs) is a relatively novel technique. In the last few years several instruments are marketed which focus ions of e.g. gallium, gold, silicon or semiconductor dopants into a spot with a (Full Width Half Maximum) diameter less than 100 nm at
beam voltages of 30 kV or more. Current densities of more than 1 A/cm2 have been obtained (using gallium). A typical FIB instrument is shown in figure 1.8.
Although most FIBs are presently in use in laboratories, semi-commercial use of FIBs in production is expected soon for mask repair. Any sub-micron technique using masks will probably have ion beam milling as its companion for mask repair.
Furthermore the use of SIMS (Secondary Ion Mass Spectroscopy) is a tech nique which is becoming more and more popular, partly because of the availability of FIBs.
Possible usages of FIBs.
FIBs can be used for:
Micro machining for e.g. mask repair.
Deposition of material for e.g. mask repair. Lithography with negligible proximity effect.
Direct implantation for e.g. custom made circuits or EHF (Extra High Frequency) devices.
Three dimensional semiconductor structures for very fast (digital) circuits or opto-electronic circuits.
Surface modifications for e.g. micromechanics.
Surface analysis techniques and depth profiling (SIMS).
Micromachining:
Because the mass of the ions in a FIB is comparable with the mass of the atoms in a substrate, the incoming ions will sputter away atoms. When the so-called sputter yield is larger than one (for non-volatile ions). more atoms are removed then deposited. When the ions are of a volatile
species, this will of course always be the case. In practice the sputter yield is dependent on ion energy, incoming ion angle and combination of ion/substrate material. It is quite easy to remove material from a
Introduction.
LIQUID METAL FIELD EMISSION SOURCE
Figure 1.8: typical Focused Ion Beam machine: the IBL-100 (courtesy of V.G. Semicon).
Introduction.
strate, although the re-deposition of the sputtered material can disturb processing.
This is the most promising way to remove excess material from a mask due to process imperfections during the mask making (resulting from e.g. dust).
Deposit ion:
Material can be deposited from the ion beam. This is the same as sputte ring with a sputter yield lower than one with a non-volatile ion spe cies. This is a rather difficult process. Nevertheless it might be at tractive, especially in those cases that the beam not only consists of ions, but also contains charged clusters.
Another approach is the ion assisted deposition from a gas, or more precisely: from a material adsorbed on the substrate. Hereby it is pos sible to deposit in the order of ten atoms per incoming ion.
This is the complement of the above mentioned micro machining and to gether these techniques can repair every fault in masks, as long as the mask is not damaged by the beam.
Lithography:
One of the main problems of E-beam lithography, the proximity effect, is negligible in FIB because of the larger mass of the ions. Therefoie it might be interesting to use a Ion-beam lithographer instead of an E-beam. In most cases the current in an ion beam is lower than that of an electron beam. This disadvantage is partly compensated by the greater sensitivity of the resist for ions.
Direct implantation:
The ions of the beam can penetrate the substrate directly. If the beam consists of semiconductor dopants these dopants will thus dope the mate rial. Because the range of the ions in the substrate is quite well defined 'within approximately */- 10 X) and is a function of the ion
energy, it is possible to create very steep dopant gradients in the semiconductors. This approach would eliminate many steps in the manufac turing of a semiconductor device.
Three dimensional semiconductor structures:
At the moment all lithography is two dimensional, that is: the compo nents are located in a plane. It might be possible however to place the components not only side by side, but also on top of each other. The direct implantation mentioned above would be the tool to achieve this. Another possibility is the creation of deep holes and trenches, which can be used for e.g. capacitors or more intricate interconnections. Although the first option is rather futuristic, the latter is already in use.
Introduction. Surface modifications:
Not only the electrical properties of a material change when implanted with dopants, but also the mechanical properties like wear behavior. Therefore the name 'surface modifications' is used here to include all other uses in which the surface (as opposed to bulk) material is chan ged.
SIMS:
SIMS i s an a l r e a d y w e l l e s t a b l i s h e d a n a l y s i s t e c h n i q u e . The combination of SIMS with a s p u t t e r i n g t o o l such a s the FIB e n a b l e s the t h r e e dimen s i o n a l a n a l y s i s of s t r u c t u r e s , such a s semiconductors. S e v e r a l i n s t r u ments a r e a l r e a d y marketed which not o n l y g a t h e r t h e i n f o r m a t i o n , but a l s o s t o r e i t in computer memory in order to show the d i s t r i b u t i o n of s e v e r a l m a t e r i a l s i n 3-D images. This i s not only an improvement of ordinary SIMS, but e n a b l e s c o m p l e t e l y new s t u d i e s .
Duoplasmatron source Mask Accelerating lens Stigmator Final lens Substrate
Figure 1.9: Ion Projection Lithography Machine (courtesy of IMS Austria).
Types of FIBs.
As s t a t e d before no FIB i n s t r u m e n t s a r e used in a p r o d u c t i o n e n v i r o n m e n t y e t . The biggest problem to overcome a t the moment i s , a s i t i s with focused e l e c t r o n b e a m s , t h e throughput.
Only one company (IMS) m a r k e t s an i n s t r u m e n t (shown s c h e m a t i c a l l y in f i g u r e 1.9, w h i c h t r a n s f e r s data p a r a l l e l by imaging (demagnified) a mask i r r a d i a t e d by i o n s . Some problems to be e n c o u n t e r e d a r e t h e transparency of the mask, the mechanical s t a b i l i t y of the mask and the s p u t t e r i n g of t h e m a s k . A l t h o u g h i t i s a good approach, I doubt whether the d i f f i c u l t i e s encoun t e r e d i n such an i n s t r u ment w i l l make i t a t t r a c t i v e when compared to e.g E-beam l i t h o g r a p h e r s .
Introduction.
For further information on this machine see Stengl 111).
The multi-beam approach is, at the moment, not feasible because it is still impossible to construct bright multiple ion sources with high place accuracy and matching brightness, although the machine of IMS can be used as a limited multi-beam machine.
Therefore in all FIBs (but one) data is transferred serially.
At the moment the machine described here is the onliest shaped beam machine under development, although a proposal for another instrument has been made by Orloff 112].
Furthermore a FIB can comprise one ion species or several different species. Especially for direct implantation the beam has to be very pure, while for micromachining the type of ions is largely indifferent, as long as the sputter yield is high.
The beam purity can be guaranteed by the source or by a filter following the source. The last method might be preferable from the viewpoint of flexibility, but it also adds to the complexity of the instrument.
Current status of Focused Ion Beam machines.
At the moment interest in FIB machines is growing. Instruments are mar keted by several companies. Especially the instruments for mask repair are developing rapidly. Furthermore the sputtering/SIMS instruments are a commercially attractive analysis tool.
Much of the near future developments of FIB machines will be dependent on the need for mask repair. This will probably be acute within 5 years. For other purposes: FIB has some unique aspects, but the throughput will probably limit its usage in the near future.
References.
[1] D.J. Elliott and B.P. Piwczyk, Electronic materials surface proces sing with excimer lasers. Proceedings of the Hicrocircuit Enginee
ring 86 Conference ed. H.W. Lehmann and Ch. Bleiker, North-Holland
(1986), pp. 435-445.
[2] F. Goodall, R.A. Lawes and P.H. Sharp, Excimer lasers as deep UV sources for photolithographic system. Proceedings of the Hicrocir
cuit Engineering 86 Conference ed. H.W. Lehmann and Ch. Bleiker,
North-Holland (1986), pp. 445-453.
(3] A. Heuberger, X-ray lithography. Proceedings of the Hicrocircuit Engineering 86 Conference ed. H.W. Lehmann and Ch. Bleiker.
North-Holland (1986), pp. 3-39.
[4] R.D. Moore, EL systems: high throughput electron beam lithography tools, Solid State Technology, September 1983 pp. 127-132.
[5] J. Kelly, T. Groves and H.P. Kuo, A high-current, high speed elec tron beam lithography column, J. Vac. Sci. Technol. 19(4) (Nov/Dec
1981), pp. 936-940.
[6] L. H. Veneklasen, A high speed EBL column designed to minimize beam interactions, J. Vac. Sci. Technol. B 3 (1) (Jan/Feb 1985), pp.
185-189.
(7] R. Ward, A.R. Franklin, I.H. Lewin, P.A. Gould and M.J. Plummer, A 1:1 electron stepper. Proceedings of the 29tn symposium on elec tron, ion and photon beams 1985,J. Vac. Sci. Technol. B 4 (!)
(Jan/Feb 1986), pp. 89-93.
(8] H. Bohlen, J. Greschner, J. Keyser, W. Kulcke and P. Nemiz, Elec tron-beam proximity printing - A new high-speed lithography method
for submicron structures,IBM J. Res. Develop.26 (1982), pp.
568-579.
[9] K.D. van der Mast, F.J. Pijper and J.E. Barth, A flexible beam shaper. Proceedings of the Microcircuit Engineering 86 Conference
ed. H.W. Lehmann and Ch. Bleiker, North-Holland (1986), pp. 115— 122.
[10] B.J.G.M. Roelofs and J.E. Barth, Feasibility of multi-beam electron lithography, Microelectronics Engineering 2 (1984), pp. 259-280.
[11] G. Stengl, H. Loeschner, W. Maurer and P. Wolf, Ion projection lithography machine IPLM-01: a new tool for sub-0.5-micron modifi cation of materials. Proceedings of the 29th symposium on electron, ion and photon beams 1985, J. Vac. Sci. Technol. B 4 (1) (Jan/Feb
1986), pp. 194-200.
[12] J. Orloff and P. Sudraud, Design of a 100 kV high resolution focu
sed ion beam column with a liquid metal ion source. Proceedings of the Microcircuit Engineering 85 Conference ed. K.D. van der Mast
Design goals and project history.
DESIGH GOALS AMD PROJECT HISTORY.
Project history.
This project «as started in 1982 in the group of Particle Optics at the Delft University of Technology. At that time the group was headed by prof. Le Poole, who was succeeded by prof, van der Mast in 1983. The project is supported by the Foundation for Fundamental Research of Mat ter (Stichting FOM) and the Netherlands Technological Foundation (STW), while an ion source has been made available by Dubilier.
People devoted to this project up to 1987 are one Ph.D. student (H.N. Slingerland), several undergraduate students (J.P. Adriaanse, J.H. Boh-lander, F. Gehring. J. Hercules and W. Quist) and (since 1986) one tech nician (E. van Straten). Additional help was given by the staff of the group Particle Optics and the facilities of the Department of Applied Physics of the Delft University.
This project is presently continued by L.J. Vijgen.
Design goals.
The main goal of this project as stated in 1984 was to make an Ion Beam Pattern Generator (IBPG's) and/or direct implanter with much better resolution than available using Electron Beam Pattern Generators
(EBPG's), while maximizing throughput. The aim for the achievable line-width was set to be well below 100 nm.
As mentioned in the introduction E-beam machines are hindered by the proximity effect. The proximity effect is partly caused by the scatter ing of the electrons in the resist. The electrons will thereby also (partly) illuminate areas not meant to be illuminated.
Ions on the other hand will not show large scattering due to their larger mass.
On of the other causes for proximity effect is the generation of secon dary electrons. This will happen using ions, but the maximum energy an electron can pick up from an ion is relatively low.
Throughput for a lithographer can be defined as the surface area proces sed per unit of time. As already shown by Liebl [1], Liquid Metal Ion Sources 'LMISses) are a prime candidate for instruments using submicron probe sizes.
A further increase of the current density of the probe, when compared to existing FIB machines proved to be possible by improving the source optics.
Throughput can not only be increased by decreasing aberrations etc., but also by optimizing the probe form using the so-called shaped beam
Design goals and project history.
proach. Because of simplicity a fixed shaped beam approach is investi gated first.
The ion penetration range in resist or wafer materials is very limited. High energies have to be used to illuminate a resist layer from top to bottom. A problem using these high energy ions is that very high volta ges will occur in the instrument. Therefore a trade off has to be made between the high ion energy and the insulation problems occurring.
Using a source which emits several ion species, as is normally the case, a mass filter has to be included. Because of the energy spread of the source used, an achromatic mass filter has been developed.
After a feasibility study the design goals as stated in table 2.1 seemed possible. The underlying studies and a more thorough investigation of the design goals are described in the following chapters.
Table 2.1: target specifications
Shaped beam technology
Minimum edge sharpness 10 nm
Shape size 50-1000 nm, continuously variable Ion energy (for singly charged ions) 30-180 keV, continuously variable Probe current 0-10 nA, continuously variable Minimum illumination time 0.1 fis
Achromatic mass filter resolution 1/40
Detector Secondary electron detector Secondary ion detector Writing field 0.1 mm square
Writing field resolution 5 nm
The machine uses UHV (ultra high vacuum) technology and will accomodate 4" wafers.
It has to be stressed that not all of the above mentioned values can be obtained simultaneously. The probe voltage will influence the maximum probe current density etc..
To give an impression of what 10 nm resolution means: if a map of a town of Delft (approximately a 5 km square) is drawn on a 5 mm square chip (most memory chips nowadays are even larger), the 10 nm resolution on the chip would correspond with a size of only 1 cm. A spot size of 50 nm would correspond with 5 cm.
Design goals and project history.
thus reveal details of 2.5 m with an edge resolution corresponding to 50 cm. Every car on the roads would be visible.
This illustrates the complexity of chips and machines employing such resolutions if, or rather when, they become available.
References.
(1] H. Liebl, Ion optics of ion microprobe instruments. Vacuum 33 (9)
(1983), pp. 525-531.
Ion opties. ION OPTICS.
In the optical system used here only three types of optical elements, besides the source, are used: lenses, dipoles and quadrupoles. The op tics of these elements can be described in different fashions, each with certain advantages and disadvantages.
The theory used here is known as geometrical optics. Only paraxial rays are taken into account. This means that approximations as sin a = a and cos a = 1 are valid. A short description of the principles is given.
Brightness.
One of the important parameters of a system is the brightness, especial ly of the source. The importance of brightness is that it tells what current densities can be expected at each plane and each cross-over in an optical column (assuming perfect optics). Aberrations in the follow ing optics can only diminish the brightness. If the optics of a probe forming system show aberrations in a certain plane which can not be corrected for, it might be useful to define an effective brightness for this plane, which can be treated as 'the' brightness for the following imaging system.
Because several definitions for brightness exist, it is explained first which definition is used here. The brightness B of a certain point of a source is here, in analogy with the brightness as used in light optics, defined as:
B - dl/dS dfi
l-l-l
with dl the current, dS an infinitesimal small part of the emitting surface of the source and dfi
an infinitesimal small emit-tance angle of the source (see figure 3.1).
This brightness can be a func tion of both S (which part of the source) and fi (under which emittance angle). Normally dS and dfi are chosen such, that the brightness is as high as possible.
Simple geometry shows that for any plane dS and dfi can be interchanged by dS* and dfi*, respectively the surface area described by dfi at that plane
SOURCE
Figure 3.1: angles and surfaces used for the definition of brightness.
Ion opties.
and the acceptance solid angle under which dS is seen from that plane. Of course dl/dS can be exchanged by a current density J.
When the current is emitted (or accepted) in a small cone, this formula can be written as:
B = J/(n a2) [-2-]
with a the beam half angle.
A cause for misunderstanding is often that the definition given here is inversely proportional to the 'refractive index'. Very often authors fail in giving the beam energy at which the brightness is measured. This can be prevented by using the reduced brightness Br defined as:
Br - J/(JI a2 V) I-3-]
with J the current density. V the beam voltage and a the beam half
angle.
From Liouville's theorem it can be deduced that, for non interacting particles, the reduced brightness Br is conserved. In practice Br is an
upper limit for the current measured in a probe with a certain acceptan ce angle, because in practice the probe is blurred by aberrations, vi brations etc. For a more detailed analysis of brightness see e.g. Wilson [11 and Grivet 12).
Lenses.
For ions electrostatic 'lenses' are used. Similarities with electron optics are large.
The reason why magnetic lenses are not used -as is the case with almost all electron optical systems- is that for a comparable behavior, the magnetic field necessary varies proportional to the mass squared. Bar
ring the use of superconducting magnets the lens action of normal magne tic lenses is only weak for ions. The use of magnetic lenses would also introduce problems when using a beam with ions of different masses. For an ideal lens the deflection 0 of a ray entering the lens parallel at a distance r from the center is given by:
P -
a,r1-4-1
with a, the strength of the le.is. The focal length of the lens, f, is the inverse of a,. All lens action can be derived by the use of two planes, the principal planes 'see figure 3.2). The principal planes lone foi a parallel incoming bean, and one parallel exiting beam) do not ne cessarily coincide. If the distance between these planes is large
pared to the focal length), the lens is said to be thick, otherwise it is considered to be a thin lens.
focal principal focal plane plane(s) plane
focal principal focal plane planes plane
Figure 3.2a: thin lens with principal planes.
Figure 3.2b: thick lens with principal planes
Spherical aberration.
In practice no lens is ideal. First of all P can be expanded in a power series of r:
■ »ir a3rJ + a5ra
I-5-]
The even terms are zero because of rotational symmetry. The other terms are related to spherical aberration. In practice r is small and there fore only the term a3r3 is of importance. Assuming a parallel beam en tering a lens centered on the axis (see figure 3.3), the coefficient of spherical aberration, Cs, is defined as
cs = Pi
[-6-1
with a the opening angle at the image side. Therefore Cs = a$ f*.
It has to be stressed that, because of the beam caustic. ps is not the minimum beam radius, but the beam radius in the focal plane! The minimum beam radius is four times smaller.
Third order lens aberrations.
Spherical aberration is only one of several third order lens aberra tions. The others, sometimes also called field aberrations occur if off-axis points are imaged.
Ion opties.
system these aberrations are deduced quite easily.
A beamlet in the lens can be characterized by
h = ï * t = y ?y • r
e,-with r the radius of the beam in the lens and y the height of the beam in the lens (see figure 3.4). ey and er are the unit vectors in the y and r direction respectively.
t-7-]
Figure 3.3: centered beam entering a lens
Y
'U-Figure 3.4: vectors used for the definition of third order aberrations.
In general we can write for the error in the deflection angle dp:
dP_ = <X + r) dA 1-8-1
with A the lens strength. dA equals:
dA = a} (y^r)'(y^r)=a3{X"y.*y/r*r-v->-r'r} (-9-1
(aj is left out because it presents the correct lens action) and thus
dp = a3{y3ey+y2r[2(ey-e, >^y*er l«-yr; I2(ey ••?,-)ej-*ey ]+r3er} 1-10-]
The term a 3 y3ey is the distortion. As it is not dependant on the beams radius, it does not give a blurring but only a misplacement. Therefore it is, for a probe forming system, an easily corrected aberration.
The term a3y2«"[2l.ey-er)ey*er) can be divided into two:
a3y2r. the field curvature and a3y*r(ey-ex), the astigmatism. The field curvature is corrected by re-focusing, while astigmatism can be correc
ted by a quadrupole.
Ion opties.
The term a3yr2[2(ey-er)er+ey] is the coma, which is not easily correct
ed.
The last term. r3er, is the spherical aberration as already dealt with
earlier.
The constant 83 used here equals: a3 = Cs A' = Cs
/f-as can be seen if formula [-6-] and the spherical aberration term in formula [-10-1.
If the incoming beam is not parallel (so M * 0) a different coefficient
CS"(M) has to be used. It can easily be shown that, for thin lenses,
CS*(M) = Cs ( 1 + M )* [-11-1
Chroaatic aberration.
Each coefficient an as used in [-5-] is also a function of the energy of
the particles entering a lens, as is the place of the principal planes. When a parallel beam changes its energy by an amount of AU, the resul ting chromatic aberration disk at the image is described by:
Pc = Cc a AU/U [-12-]
with AU the HWHM (Half Width Half Maximum) energy spread of the beam. The coefficient of chromatic aberration of a lens with focal distance f at energy U is thus defined as
Cc - A Zf U/AU [-13-]
In practice a negative energy shift will result in the same probe diame ter as a positive energy shift. Therefore the aberration disk caused by a beam with a (FWHM) energy spread AU is only half that given by formula
[-12-1.
At a certain magnification M, CC"(M) is for thin lenses, given by:
CC*(M) = Cc (1 ♦ M )2 (-14-J
If a beam is entering the lens off center, a differential aberration, Transverse Chromatic Aberration (TCA) results. In the thin lens
approxi-Ion opties.
raation:
Ax = Cc p AU/U [-15-]
sometimes also called the chromatic magnification aberration. This re sult can also be written as
Ax/x = (Cc/f) AU/U [-16-]
Diffraction.
The Airy disk of a ray passing a circular aperture is given by:
Pd = 0.6 X / a [-17-]
with \ the wave length of the ions, and the factor 0.6 due to the
Ray-leigh criterium. The wave length X can be found by:
X = 2.86 (U M)-'" 10"' ' m [-173-]
with U the beam potential and M its mass in AMU.
For ion probes diffraction will almost always be negligible.
Add11 ion of aberrations for probe forming systeas.
The addition of aberrations must be treated carefully. The problem is that some aberrations are coherent -and must thus be added lmearly-while others are not.
An example in which linear addition has to be used is e.g. with the addition of chromatic aberration disks in an image plane, due to differ ent lenses in the system.
Assuming non-coherent aberrations which each give rise to Gaussian dis tributions in a certain image plane, the total cross-over diameter can be calculated by quadratric addition. Also other (quasi) Gaussian ef fects, like disks due to vibrations or source sizes, can be added in this way.
Although more precise formulae can be used instead of only linear and quadratic addition (see for example Harte [3] and Crewe [4]), results using this method are in most cases adequate.
The electrostatic deflector.
For the deflection of an ion beam an electric field can be used. ideal deflector has a homogeneous field perpendicular to the axis. In first approximation the deflection angle fl is given by:
The
tan fl - EdL/ 2VD [-18-]
with Ed the field strength in the deflector, L the length of the deflec tor and vb the beam voltage (see figure 3.5).
In most cases it is necessary to deflect in two directions. This can be done by combining two deflectors, as shown in figure 3.6.
Figure 3.5: Electrostatic deflec tor for deflection in one direc tion only.
Figure 3.6: Electrostatic deflec tor for deflection in any direc tion.
Because the field of a deflector is generated by (at least) two elec trodes, the deflector is also known as a dipole.
The electrostatic deflector will, even with a perfectly homogeneous field, show aberrations, e.g. a focusing effect in one direction (see also appendix 2: deflector aberrations).
Above that even more aberrations will occur due to a non homogeneous field, caused by e.g. pole misalignment.
The effects of the latter aberrations can be easiest described by think ing of the physical deflector as a superposition of a (perfect) dipole, quadrupole, hexapole etc..
These effects can be partially cancelled, as discussed by Kramer [5], For further information on deflector aberrations see e.g. Pierce [6]. The main aberrations of a deflector are normally astigmatism and distor tion.
The deflection is a function of the beam energy also. Therefore a beam with an energy spread will show Transverse Chromatic Aberration (TCA).
Ion opties.
For small deflection angles (tan 0 = 0 ) the effect of the TCA is given by
M - EdL/ 2AVb [-19-]
In a plane following the deflector, where the beam is deflected over a distance x, the effect of TCA is thus approximately
Ax/x - AVb/Vb I-20-]
The magnetic deflector..
A magnetic field can also be used for the deflection of an ion beam. Assuming again a completely homogeneous field in a plane perpendicular to the axis, the ions will follow a circular path perpendicular to the field, due to the Lorentz force.
Inside the deflector, the radius of this resulting circle is:
r = (2 U m/q>"VB [-21-]
with U the ions energy, m its mass, q its charge and B the magnetic field.
Assuming that the radius of curvature is much larger than the length L of the deflector, the deflection angle is thus:
0 - (L B>/<2 U m/q)"3 [-22-]
Of course magnetic deflectors also exhibit aberrations, similar to elec trostatic deflectors,
It can be seen that this type of deflector not only exhibits an energy dependency, but also a mass dispersion:
Afl/0 = AU/2U * Am/2m [-23-]
Because of this mass dispersion and the high magnetic field necessary for a deflection over a significant angle, this type of deflector will not normally be used in ion optics.
An exception is the use of these deflectors in mass filters, where the mass dispersion of these deflectors is used to separate different masses
(see chapter 9 and appendix A31.
Quadrupoles.
A perfect quadrupole focuses in one plane and defocuses in the perpendi cular plane. The field lines in a quadrupole are drawn in figure 3.7. The resulting forces are proportional to the distance r from the axis and will bend the beam towards the axis in one plane, while deflecting it from the axis in the perpendicular plane. A line focus it thus formed when a parallel beam enters a quadrupole.
Although quadrupoles can be com bined to form a round lens, the most important application in probe forming instruments is as a corrector of astigmatism. Astig matism of round lenses can be corrected by a quadrupole with the proper strength and orienta tion.
Jn order to give a quadrupole its proper orientation, two 45° rota ted quadrupoles can be used. Nor mally these two quadrupoles are realized as an eight pole device.
Figure 3.7: field lines in a quadrupole
Misalignment of the poles will lead to a superimposed dipole, hexapole etc., but normally these effects are small enough to be neglected.
References.
II] R.G. Wilson and G.R. Brewer, Ion beams with applications to ion implantation, John Wiley 6. Sons, New York, 1973, pp. 253-255.
[2] P. Grivet, Electron Optics, Pergamon Press, 1965, pp. 70-73.
[3] K.J. Harte, Theory of aberration mixing in electron-optical systems, J. Vac. Sci. Technol. 10 (1973) pp. 1098-1101.
[4] A.V.Crewe, Optimization of small electron probes, Ultramioroscopy
23 (1987), pp. 159-168.
[51 J. Kramer, Production of homogeneous fields, Brit. J. Appl. Phys.
18 (1967), pp. 1815-1818.
[6] J.R. Pierce, Theory and design of electron beams, D. van Nostrand Company, 1949, pp. 41-44.
Optimization of an ion probe.
OPTIHIZATIOH OF CURREHT IN AH IOH PROBE.*
Liebl 11] showed the importance of chromatic aberrations on the spot size of systems employing a LMIS. At some probe currents spherical aber-rations and geometrical source size are negligible compared to chromatic aberrations. For such a chromatically limited probe the instrument is easily optimized.
This optimization is described for both source imaging (resulting in a Gaussian beam profile), and a shaped beam system, imaging a shaping aperture on the target. This last approach is made in many state-of-the-art electron beam lithographers.
A comparison between these two approaches is made and a shaped beam system turns out to be much more efficient in terms of throughput ipro cessed area per second) than a Gaussian system.
Optical elements of interest.
A LMIS is a source with very small optical dimensions (45 nm reported by Komuro [2], 30 nm reported by Wagner [31) and a considerable energy spread (4.5 eV FWHM reported by Swanson for a gallium source at an angular intensity of 15 fiA/sr ( 4 ) ; most other source materials have higher energy spreads).
Because of the relatively large energy spread and the small geometric dimensions of a LMIS, systems using a LMIS are limited by a mixture of either ihromatical or spherical aberrations and the virtual source size.
A Focused Ion Beam machine can comprise many lenses. In order to deter mine which lenses are causing aberrations, one has to determine magnifi cation and acceptance angle of each lens, combine those with the aberra tion coefficients, and refer them to a common plane (e.g. the last image plane) in order to compare the contributions of the lenses.
In general however aberrations of the source extraction optics and the projector lens of such a system are dominant.
This is caused by the fact that in the extraction optics the source is magnified, while at the same lime the acceptance angle is reduced, both
by the magnification and by the acceleration which is in most cases taking place here (from the extraction potential to a beam transport potential).
In most cases the projector lens demagnifies (thereby demagnifying all
Part of this chapter has already been published. See H.N. Slingerland, Optimization of a chromatically limited ion microprobe. Microelectronic Engineering 2 (4) (1984). pp. 219-226.
Optimization of an ion probe.
previous aberration disks), thus making the aberrations of all but the source extraction optics negligible.
However in each particular case this has to be verified, but normally it is a safe assumption to state that only two lenses are of importance.
Current and current density.
For a given target, with a certain sensitivity for ions (due to the resist, implantation dope or the thickness to be sputtered away), the processed area is proportional to the number of particles arriving at the target per second. Therefore throughput is proportional to the cur rent on the target.
The exposure time is inversely proportional to the current density on the target.
Gaussian bean imagine.
The symbols, which are used in the following formulae are described in the figures 4.1 and 4.3a.
The current in the beam is considered constant and will be given by:
Ip = I Q H P'
(-1-1
with Ip the probe current, IQ the angular intensity of the source and p
the source acceptance angle.
Source
Extraction Optic» !&> U = U, chromatic aberr cceft Cc
Intermediate Optics
Pro/ectorlens (<- U-Up chromatic aberr. coelt.:CCj
Target
Source
Extraction Optics I" U r u, chromatic aberr. coeft.:Cc
Intermediate Optics
Beamshaper
Intermediate Optics
Projeetortens " U=Up chromatic aberr coett.:Cc
Last x-over Image ot beamshaper Target
Figure 4.1: symbols used for a Gaussian beam imaging system.
Figure 4.2: symbols used for a shaped beam imaging system.
Optimization of an ion probe.
The radius of the probe will be given by the addition of the two (aber ration) disks of source extraction optics and the projector lens (see figure 4.1).
Pp = <<M pg) ' t (M P d ♦ pc 2)2 + (M ps l •• ps2)?t"J ["-"I
with pg the virtual source size. pci the radius of the chromatic aberra tion disk of the source optics, referred to the source side. pcj the chromatic aberration disk of the projector lens, psi the radius of the spherical aberration disk of the source optics, referred to the source Side. ps2 the spherical aberration disk of the projector lens and M the magnification from source to probe. Linear addition is used here for the Chromatic and spherical aberration disks because these are coherent isee also Or loff [5)1.
The aberration disks are described by:
Pel - cc l (i A U / US l-)a-l
Pc2 ■ CC2 " A U / UP l-3b-]
Psl = * Cs l P3 l-3c-l
Ps2 = * Cs 2 a3 [-3d-]
with All the beams (HWHM) energy spread, Us the voltage of the source and Up the voltage of the probe. The defined radii for the spherical aberra
tion are slightly different with those defined in chapter 3, because here the minimum beam radii are taken instead of the beamradii in the focal plane.
Figure 4.3a: beam width defini- Figure A . 3b: beam width and sharpness tion for a Gaussian beam system. definition for a shaped beam system.
Optimization of an ion probe.
The relation between the opening angle a (referred to the source side) and the source emittance angle 0 (referred to the source side) is given by
a = U3/M)(Us/Up)w [-4-]
Because the current is the desired variable, a will be eliminated. This results in a rather complicated formula, wherein p_ is given by
Pp - ( M M Pg2 + (Cs l 03/ 4 )2 * (Cci p A U / US)2 ] +
[ 2 Cci Cc 2 AU2/(UsUp) (Us/Up)'* P2 ] ♦
M -2[ ( Cc 2 P AU/Up (Us/Up)1")2 * Cs ] Cs 2 (Üg/Up.)* P6 / 8 ] ♦
M_ 6[(CS2 (Ug/Up)*|»3 / 4 l2 >* l-5"l
Optimizing M is given by 3p/3M = 0 and 32p/3M2 > 0. The result is not
particularly useful, unless both the spherical aberrations and the vir tual source size can be neglected when compared to the chromatic aberra tion disks. Under this assumption it is possible to find expressions for current and current density for both Gaussian beam systems (imaging the source at the target) and shaped beam systems (imaging a shaper at the target).
For a chromatically limited system the optimum magnification can be derived from formula I-5-):
M = (Cc 2/Cc l)'* ( Us/ Up) * [-6-]
This is the optimum magnification of a Gaussian beam system, and is therefore an important system constant.
Gaussian beam syste».
In order to find an expression of Ip as a function of pp for a chromati
cally limited system, pp is written as
Pp = M * Pel * Pc2 I"7"]
and thus at optimum magnification
Pp = 2 * M * pc l = 2 * pc 2 [-8-]
P can be written as
Optimization of an ion probe.
Together with formulae [-4-1 and 1-8-):
P ■ <PP "s*" Up*)/<2 [Cc l Cc»]* AU) I-10-]
Therefore the current is given by
Ip = (IT ln PP ? Up f Us UpJ * ) / W Cci CC2 A U2) [-11-1
and the current density by
Jp ■ (If) Up [t!s Dpi*)/(4 Cc l Cc 2 All*) 1-12-1
Shaped_spot.
To obtain a shaped spot one can image a shaping aperture on the target without imaging the source on the target. This is the so-called Koehler illumination as described in many light optical books 'see e.g. [6]). This illumination is used in most slide projectois: not the source is focused on the screen, but the slide. For a better understanding figure 4.2 might serve. Symbols used in this paragraph are described in the fi gures 4.2 and 4 . 3D. The shaped spot used here is assumed to be square with a side dimension s.
The effective source brightness is an appropriate parameter because not the geometric source surface is important for the brightness, but the total aberration disk of the source extraction optics. If the chromatic aberration is dominant, then the "source surface' in the definition of brightness is defined as the aberration disk of the extraction optics referred to the source side and the effective source brightness is thus given by:
Be f f = (n p2 I0)/(US - pc l 2 * 02) [-13-1
- 'Ip.2 U g W U p Cc l 2 A U2) [-14-1
Assuming a square shaped spot with sides length s. and the edge sharp ness As is negligible with respect to the spot size s, then the bright ness of the source as seen from the probe will be the same as the brightness of the first source cross-over, so
Ip ■ Be f f Up s2 H a2 [-15-1
Substitution of formula [-15-1 in [-14-1 results in
Ip2 - ( ln 2 Us Up s2 H a2} / { Cc l 2 AU2> [-16-]
Optimization of an ion probe. Taking the square root and substituting a with
a = (As Up)/(Cc 2 AU) [-17-]
gives
lp = <<ln Up [Us Up]'" »*)/(Cci Cc 2 AD2)} * (s As) [-18-]
and
Jp - <(In Up |US D p ] * l*)/(Cci Cc 2 AUJ)> * (As/s) [-19-]
If the same calculations are made for a rectangular shaped spot, than s has to be replaced by (sx Sy)"2, the spot sizes in the x and y direction
respectively.
Comparing Gaussian bean and shaped bean.
Normally the edge sharpness of a structure on a chip is less than a fourth of the minimum linewidth on the chip. Therefore the edge sharp ness is a constant. When scanning a (round) Gaussian beam over a (straight) knife edge the result (transmitted probe current) is the error function. The Gaussian's Full Width at Half Maximum corresponds with the 12-88% values of the error function (see also figure 4.3). Defining the edge sharpness As of a (rectangular) shaped beam as the 12-88% values gives thus a good means to compare a Gaussian beam and a shaped beam, assuming As << s (edge sharpness much less than the shaped beams size).
Using formula [-11-] and [-18-] to compare the throughput results in:
Is/Ig = U/-JO * s/As [-20-]
with Is the current of the shaped spot, Ig the current of the Gaussian
spot, s the dimension of the shaped spot and As the edge sharpness of the shaped spot.
Using formula [-12-] and [-19-] to compare the illumination time results in:
Js/Jg = (4 -Js) * As/s [-21-]
Therefore the result is that the throughput (which is proportional to lp) of a shaped beam system is higher than that of a Gaussian system, but illumination time (which is inversely proportional to Jp) is longer
Optimization of an ion probe.
Optical qualities.
For both the gun with optics and the final lens a quality factor can be derived, assuming both are chromatically limited. When looking at formu
lae 1-11-1, 1-12-], [-18-] or [-19-1, one can divide it in a part which is only due to the quality of the source optics, a part due to the final lens and an 'overall' part.
Taking e.g. formula [-11-1 the 'gun quality' is the part depending on source and source optics, thus:
Qg = Hfj üs'°>/{Cci AU2} I-22-1
Note that the In. of the source is also dependent on the source voltage and thus incorporated into the gun quality. Normally In. will be almost proportional to that voltage.
The 'lens quality' of the final lens is given by
Q j = Up*2 / Cc 2 1-2 3-1
When comparing systems and'or guns for a chromatically limited system these qualities enable an objective and easy way of comparing them.
Conclusions.
Shaped beam imaging is also in chromatically limited ion n.icroprobe systems superior to Gaussian imaging. Furthermore it proves to be impor
tant to make the chromatic aberration coefficients of both extraction optics and objective lens as sr,;all as possible. Formulae are given tu determine the spot current and current density in both Gaussian and shaped spot systems. The optimum magnification for Gaussian beam imaging is derived, assuming that the chromatic aberrations dominate.
References.
(1| H. Liebl, Ion optics of ion microprobe instruments. Vacuum 33 (9>
(1983), pp. S2S-531.
|21 M. Komuro, T. Kanayama, H. Hiroshima, and H. Tanoue, Measurement of virtual cross-over in liquid gallium ion source, Appl. Phys. Lett.
42 (10) (1983). pp. 908-910.
[31 A. Wagner. Applications of focused ion beams. Nucl. Instr. and
Meth. in Phys. Hes. 218 (1983), pp. 355-362.
Optimization of an ion probe.
[4) L.W. Swanson, G.A. Schwind, A.E. Bell, and J.E. Brady, Emission characteristics of gallium and bismuth liquid metal field ion sour
ces, J. Vac. Sei. Technol. 16 (6) (1979), pp. 1864-1867.
[51 J. Orloff, On addition of spherical and chromatical aberration of a pair of electron lenses, Optik 63 (4) (1983), pp. 369-372.
[6] Max Born and Emil Wolf, Principles of optics, Pergamon Press (1980), 6t h ed.
Correction of the TCA of a deflector.
CORRECTIOB OP THE TRABSVERSE CHROMATIC ABERRATION (TCA).
As pointed out in chapter 3, an electrostatic deflector will often show TCA as the main aberration. When deflecting a beam with energy U and
(FWHM) energy spread AU over a distance x it results in a TCA of:
Ax/x - AU/U 1-1-1
When using ion beams from a LMIS with their inherent large energy spread, this aberration will limit the deflection field of a deflector. Also topography on the target can result in place inaccuracy if the beam does not land (almost) perpendicular to the target. Therefore perpendi cular landing has to be used for high accuracy positioning.
One post-lens deflection system and two pre-lens deflection systems capable of perpendicular landing have been studied.
A comparison is made with the in-lens deflection system as described by Cleaver [11.
Target topography.
Any topography on the target will be translated to a misplacement of the beam if the beam is not impinging perpendicular to the target. How se rious this will be is of course dependant on the topography of the target and the landing angle of the beam.
Many lithographic processes, like oxidations, result in a wafer topo graphy of up to 1 j/m. If a beam deflection accuracy of less than 10 nn is to be achieved, the maximum deviation of a perpendicular landing angle is plus or minus 10 mrad. Assuming post-lens deflection and a minimum scan field of 1 mm squared (corresponding with a maximum de
flection of 0.7 m m ) , the working distance of the deflector would then be 7 cm, which is in conflict with the demand of small working distance after the last lens, necessary to obtain small lens aberrations.
Any TCA corrected system for use in a lithography machine, meeting the demands as stated for this machine, has to include perpendicular landing of the beam on the target.
Deflector placement.
In principle the deflector can be placed after a lens (post-lens de flection). before a lens (pre-lens deflection) or in a lens (in-lens deflection), as shown in figure 5.1.
Using a post-lens deflector the post-lens distance has to be as small as possible, because otherwise the aberrations of the last lens are larger then necessary, due to the large working distance.
In-lens deflection (although the deflector is placed in a field free
region) has been studied by e.g. Cleaver [1]. His conclusion is that in-lens deflection is well suited for electrostatic ion optical in-lenses. Although it is true that in theory no objections exist, its usage will be limited because the deflection voltages have to be superimposed on the lens voltage itself, which is in the order of 100 kV or more.
Normally deflection frequencies range from a DC shift of the image to a highest frequency in excess of 16 kHz (TV scan rate). Because of the DC shift AC coupling can not be used. Therefore it will be necessary to add a high accuracy deflection voltage to the lens voltage itself, which is rather complicated.
Furthermore the in-lens system does not include a perpendicular landing of the beam onto the target.
On the other hand pre-lens deflection will, especially if the beam lands perpendicular to the target, introduce several lens aberrations, because the beam enters the lens off-axis. Obviously these aberrations have to be small.
<
H]
1
1
W///////,
Figure 5.1a: Figure 5.1D:
post-lens deflection, pre-lens deflection.
Figure 5.1c: in-lens deflection.
Dynamic aberration correction.
Many of the lens and deflector aberrations specific for deflection can be corrected. This includes lens distortion, astigmatism and field cur vature. However, it is not attractive to correct these aberrations for every position of the beam: it is enough to give an overall correction for all positions within a field. Within such a subfield residual aber rations exist, but for small fields these are negligible (see appendix A2).
This aberration correction is called dynamic aberration correction.
Because the deflector itself employs subfields too, (see chapter 11) these can be made to coincide with the aberration subfields.
