Some applications of accelerated ions in industry

CLE S

Nuclear Physics A538 (1992) 533c-540c North-Holland, Amsterdam

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APPLICATIONS

F ACCELERATED IONS

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Peter H. Rose Baton Corporation Beverly, Massachusetts 01915 U.S.A. T

UCTI N

The basic understanding ofnature that comes from the current pure nuclear and particle research now encompasses phenomena not likely to find industrial applications for decades to come . However, it is quite otherwise with the technological physics which supports the developmentof new and advanced experimental apparatus. This physics can move rapidly into the industrial realm once it isrecognized and understood . Coupling the new knowledge with an industrial need is not, unfortunately, so easy because together with the understanding ofthe technical aspects of the problems, it is necessary to consider what profits it will bring to the commercial enterprise . In spite of the obstacles to commercialization, there have been many successful applications of nuclear physics and the tools of nuclear physics. Among these the accelerator is perhaps the preeminent example. There are now a large number of accelerators built every year for use in industry including the medical industry and in order to narrow the discussion a selection is made of only a few of these accelerator applications . Several types of heavy ion accelerators are used in the semiconductor industry and cyclotrons are extensively employed to produce short-lived radio isotopes for medical research, diagnosis and treatments . The range of energies over which ions can be successfully applied is enormous (Figure 1) especially if the recent medical applications ofvery energetic heavy ions is included. HIGH FLUX DENSITY ION BEAM PROCESSES

1e r 0 11, X rd

9a

10

10 14

10

12

10

ACCELERATORS FOR RESEARCH

90

10

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Beam Energy eV

Figure ]-The energy andflux range of accelerator applications . 0375-9474/92/$05 .00,5 1992 - Elsevier Science Publishers B.V. All rights reserved .

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ose / Some applications of accelerated ions in industry

he huge semiconductor industry would not exist were it not possible to dope regions of the semiconductor material with receptor or donor materials so as to form junctions. Control ofthe quantity and distribution of the dopant is important and, as early as 1957, OhIs realized that accelerated boron, phosphorous or arsenic ions could be used to dope a wafer in an accurate manner. Unfortunately, as is well known, the implanted layer becomes amorphous even at low doses _1012 ions/cm2 because of the damage caused by the penetrating particles and it was not until it was realized theelectrical activity and crystallinity could be restored by high temperature annealing (c 11M1200"C) that implantation could be regarded as a practical process. Even so, market surveys made around 1970 revealed that, in the eyes of the device manufacturing engineers, implantation would never compete successfully with furnace diffusion. Unfamiliarity the with the new processwas onefactor, but increased complexity ofanion implanteras compared with a diffusion furnace was equally important. The existence ofhigh voltages, radiation and the aeed to move wafers in and out of the vacuum were complications they wished to avoid, if possible . Fortunately, at about the same time, a new semiconductor company called Mostek, ignoring the conventional wisdom incorporated a low dose boron implant as a way of precisely adjusting switching thresholds. From this small beginning, implantation rapidly displaced diffusion in all process steps including those using the highest doping levels up to 1011 ions/cm' with the result that annual sales of ion implantation equipment is now about $400M. The ions most commonly used are boron, phosphorous and arsenic at energies between 5 and 200 keV. Higher energies are not generally required because the active device layers arevery shallow anddepth profiles like those shown in the example ofFigure 2 are accepta'Tile for most device fabrication. Recently, there has been some interest in energies up to 3 or 4 MeV used to implant retrograde doping profiles . For this purpose, special Unacs and tandem accelerators have been developed, but it is not yet clear that high energy implantation will be a mainstream process.

17

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Z 0 I-Cr

W 0 U

0

0 .2

M DEPTH

M aB (p m)

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Figure 2-depth profile of boron implanted into silicon.

P.H. Rose / Some applications of accelerated ions in industry

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Essential to the origin ofion implantation were the low energy accelerators and isotope separators developed in the early days of nuclear physics. Equally important was the understanding of the penetration phenomena that occurs when lightand heavy ions pass throughmatter. Had this body ofknowledge not existed together with the physicists expert in the technology, it is arguable that the device manufacturers would not by themselves have developed the machinery for ion implantation. The performance of integrated circuits would in that case be considerably inferior to what is now available . Itis interesting todiscuss a recentexample ofion implantation in which sufficient ions are implanted to create a new material. Active devices occupy the top 2 to 3 microns ofa waferandthe materialbelow this can beregarded as infinitely deep conducting medium. Stray electrical fields can couple circuits together in unwanted modes (e.g. latch up) and ionizing particles create charges which can trigger circuits . It is, therefore, attractive to consider providing an insulating or isolating layer below the active region. One way of doing this is to form a layer ofSiO2 by implanting a massive dose of oxygen (1018 ions/cm2) below the silicon surface (see Figure 3). Watanube and TOW [ 1 ] were the first to research thispossibility, but it remained an academic exercise until acommercially built 200 keV, 100mA oxygen implanterwas developed in 1986 . As a result ofthe availability of this implanter, there is growing use of wafers modified by this process now called 'Separation by Implanted Oxygen' (SIMOX) [2]. The segregation ofthe SiO21ayers from the crystalline silicon both in front of the SiO2 layer and behind it is remarkable and no longer resembles a conventional implant distribution . The high flux of oxygen ions through the crystalline surface layers of the silicon wafer would quickly render it amorphous were it not that the waferis maintained at a temperature of 600°C during implantation. Figure 3 shows the SiO2 distribution after a post implant high temperature (c 1200°C) anneal which causes almost complete segregation of the oxygen into the SiO2 layer. The temperature of the wafer during implantation and the post implantation annealing cycle are critically important in reducing dislocations and other defects. Defect densities as low as 103 defects/cm2 have been achieved whichis satisfactory fordevice fabrication and, if necessary, an epitaxial layer can be grown on top of the implanted surface layer toprovide a better quality and thicker substrate . 0.8 0.6

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0.4

0.0 DEPTH BELOW SURFACE (microns)

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Figure 3-Profile ofthe implanted silicon dioxide layer as determined by reflection spectrometry. The temperature was held at 600°C during implantation and followed by a high temperature anneal .

ose / Sonne applications ofaccelerated ions in industry

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s a second example of the use ofions in the semiconductor industry it is interesting to consider the possibility for using them in lithography . Compared with electrons, ions are rapidly stopped in solid materials and exhibit very little straggling as is evident from the Monte Carlo calculations shown in Figure 4. Wafer patterning is the most important step in semiconductor manufacture and is usually carried outusing monochromatic lightfrom a mercury arc lamp toproject theimage of a mask onto a wafer surface with a reduction lens. The minimum feature sizes of the device on a wafer have been going down steadily and are now at about 0.5grn and it looks as though optical techniques car. bee extended to just below 0.2pm using 1)UV light sources and phase shiftingmasks. Ten years ago,this progress inopticswas notanticipated and a significantnumber of x-ray programs were initiated to develop intense sources of collimated x-rays (e.g. Superconducting Storage Rings) and 1:1 proximity masks with line structures on them of absorbing materials . Because of the recent progress in optics and difficulties in making masks for 1:1 x-ray lithography, the window forapplication ofthis technique maybe closing . Onthe other hand, ion beam projection lithography [3] is a high resolution process which has demonstrated capability down to 9.01gm, and has the advantage that demagnification is possible between the mask and the wafer in a way which is closely analogous to the optical stepper (see Figure 5). A very important advantage of charged particle beam technology is that the exposure field can be controlled in position and shape by electric or magnetic fields with arapid time response. Figure 6 shows a control scheme which is now being tested in the apparatus of Figure 5. The eight lets are generated by apertures in the stencil mask and are located at the corners ofthe field toe imaged. After acceleration and focusing, each beamlet passes through a slot in a reference plate shown in the figure and can be used to measure and adjust the projected image in X, Y, Mx, y and M on the wafer with a response time of about one millisecond . X

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2

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L. Karaplperis1981

4L T 40 * IONS

ELECTRONS

Figure onte Carlo simulations ofproton and electron scattering in a thin layer of A resist on a silicon substrate.

P.H. Rose l Some applications of accelerated ions in industry

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Figure 5--drawing ofan ion projection stepper as described by G. Stengel &c H. Glavish in U.S. patent 4,985,634 .

SECONDARY ELECTRONS DETECTED 1N AN ARRAYOF CHANNELTRONS

REFERENCE PLATE WITH SLOTS SCAN PLATES ALIGNMENT - BEAMLETS FIELD BEAM

Figure 6-Array ofeight beamlets accompanying thefield beam which are individually scanned across the reference plate ofFigure 5 to control the size and position of the projected imagefteld.

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se / Some applications o accelerated ions in industry

t is inte sting to speculate why a technique capable of being extended well beyond the is ofthe gigabit memory ( .15gm) has been comparatively neglected when so much require effort t as been spent on x-ray lithography. Perhaps the most important reasons are that experiments with electron projection 4] have not come close to the calculated performance probably cause of difficulties with lens fabrication. Magnetic and electrostatic lenses are not ®gurable' in e sense e optical lenses are, for example, the field in an electrostatic lens is determined y solutions to Laplace's equation and it has proved impossible to reduce the aberrations of a single lens to acceptable value. However it is possible to compensate for distortions by having o or more lenses (shown in the example of Figure 7) where, by having the am cross the axis between the lenses, chromatic and some geometric aberrations can be made to cancel . f the experiments now being carried out at Ion Microfabrication Systems, b ienna, Austria are successful, then a new and very important application for ion beams will have en found.

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Figure 7-Schematic showing the possibility of compensating the geometric and chromatic aberration y having a crossover between two lenses .

P.H . Rose / Some applications of accelerated ions in industry POSIT ON EMISSION TO

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OG AP

Cyclotrons and Linacs have been valuable tools in the medical industry for many years and are used to produce radio isotopes for clinical research and patient treatments. In consequence, a number of companies have come iato existence to produce compact cyclotrons which can be operated safely in a hospitalenvironment. The worldwide market for such machines is estimated in the neighborhood of 15-20 machines a year which extrapolates to a sales potential of $1 annually. The machines are generally sold as a system with automated chemical processing to quickly provide the user with chemicals tagged with the short-lived isotopes. The same companies often find it advantageous to become involved in the manufacture oflarger cyclotrons used in nuclear research. About one half of medical cyclotrons now being built will be used to supply isotopes for Position Emission Tomography (PET). The advantage that makes this technique important is the addition of spacial resolution to radio nuclide techniques greatly expanding the range and number of possible experiments . (See, for example, the review papers of Brownell, et. al.[5l .) The two annihilation gamma rays from the positron decay emerge almost back-to-back except for a small angular deviation which depends on the material in which the annihilation eventoccurs. This deviation is notofsignificance in PET but the range of the positrons prior to assimilation does degrade the resolution to about 2mm. Realization ofthis resolution with good detection sensitivity has bee: made possible by using the Bismuth Germanate detectors developed primarily for nuclearphysics. Table 1 lists some oft' e isotope often used in medical research together with typical proton energies and target materials .

Nuclide

Half Life Minimum

Decay Particle/MeV

Reaction

Reaction Energy-MeV

C-11 N-13 0-15 F-18 F-18 1-123 1-123

20.3m 9.97m 2.07m 109.7m 109.7m 13.3h 13.3h

0+/0.98 0+/1 .19 0+/1.70 0+/.635 ß+/.635 y/.159 y/.159

'4N (p, a)"C 160 (p, (X) 13N 14N (d, n) 110 110 (p, 011F 20Ne (d, (x)"F '24Xe (p . 2p)121 1271 (p, p5n)"

10-30 10-30 10-30 10-30 10 30 70

Table l -Commonly Used Radio Nuclides A fascinating extension to positron emission tomography, made by Alonso, et. al.[6] using the Bevalac, is the injection of very high energy (200 to 300 MeV/nucleon)' 9Ne ions from- a fragmentation reaction of 2°Ne into patients. The deeply implanted neon decays giving the exact location ofthe implanted region in a patient which can sometimes be very important if successful radiation treatment is to be carried out. This work may well lead to the use of very high energy heavy ion acceleration for radiation therapy .

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asanabe and A. Tooi, Japan Journal of Applied Physics, 5, (1966j 737 [21 . Izumi, Nuclear Instruments Methodology Physics Research, B21, (1987), 124 [31 halupka et al, 14th Symposium Sources and Ion Assi Japan [il M.

[41 H. Koops, CO, "Fine Line Lithography", Ed R. Newman, North-Holland Publishing Co., rownell, C.A. Burnham, C.W. Steams, D.A. Chaser, A.L. Brownell and M.R. almer, Int. J. Imaging Syst. and Tech ., Vol. 1, (1989), 207

161 IR. Alonso, A. Chatterjee, C.A. Tobias, I.E.E.E . Trans. Nuclear Science, Vol. NS-26, No. 3, (1979), 3003