Materials modification with accelerators

1232

Nuclear

Instruments

and Methods

in Physics Research

B56/57

(1991) 1232-1235 North-Holland

Materials modification with accelerators S.V. Nablo Energy Sciences Inc., 42 Industrial Way, Wilmington, MA 01887, USA

Electron initiated bulk crosslinking of polyolefins and polyvinylchloride for shrink tubing, wire sheathing, and food packaging have been well established commercial processes for decades and will be briefly reviewed. Over the past decade, a number of surface curing applications involving radiation initiated grafting and addition polymerization of materials have been commercialized. These applications continue to be driven by emission control regulations and the unique product properties achievable, and utilize selfshielded accelerators in the 100-300 kV range. The development of high energy ion implanters, generating currents over 1 nn4 at several MeV, has been motivated by the deep implantation needs of sub-micron device developments. High current implanters offering powers of 10 kW and above will be discussed for the important silicon on insulator (SOI) and separation by implanting oxygen (SIMOX) use. Other non-semiconductor applications are also reviewed.

1. Introduction The application particle cation

beams in the

of the “directed has

past

processors,

the

apply

beams

these

enjoyed decade

ability

to

energy”

increasing [l].

In

the

manipulate

to the product

available

industrial case and

in

appli-

of electron efficiently

at atmospheric

pres-

sure has added great flexibility in their use. Their selfshielded nature (for V,I 500 kV) and good optical design provide compact, power efficient (2 60%) energy sources for an increasing range of curing processes: some 600 units are now in industrial use with growth in those energy intensive industries beset with tightening pollution control regulations, particularly for graphics (printing) and food packaging [2]. High speed product handling so that the product can be easily introduced to the processor in-line with other converting operations has become an increasingly important part of the (accelerator) system design, in particular for elimination of any ozone/NO, contamination in the work area with high speed sheet or web products. On the other hand, however, applications of ion implanters, because of the need for in vacua processing, have involved batch processing with the special cleanroom product handling required by semiconductor devices. With these accelerators, the product handling becomes an even more integral part of the accelerator system, and a good deal more sophisticated product (wafer) handling has been developed for the implanter field. For electron processors, the window isolation of the accelerator vacuum system from the “ambient” process zone provides for great flexibility in its use, even for its remote articulation about a complex surface. The ion implanter requires good vacuum technique in 0168-583X/91/$03.50

0 1991 - Elsevier Science Publishers

the introduction and treatment of the product and lends itself well to the precise ion optical control required for high resolution semiconductor modification. This paper will review some of the newer, higher visibility applications of accelerated particle beams for the modification of materials, always with the continuous/batch distinction of the electron/ion beam techniques.

2. Surface graft modification of matter [3] One of the major advantages of the new class of processors is their ability to deliver energy to a surface with good spectral quality. This feature is illustrated in the experimental curves of fig. 1, recorded on a low energy Electrocurtair? processor modified for surface treatment. As shown in the 100 kV curve, the half-dose point of the electron spectrum is 55 g/m* in matter, so

B.V. (North-Holland)

70 6050 40 30 20 10 OO

25

50

75

100

125

150

175

200

flange(ghw) Fig. 1. Low voltage

depth-dose

profiles.

225

250

: 5

S. V. Nablo / Ma~e~a~s~#dijica~ion with accelerators

02 Permeability (cc/m%+3 hrs)

10

12

14

Coating Thickness (microns) Fig.

2. Dry a_ permeability as a function of coating thickness

(125 pm LDPE).

that a practical penetration 55 ym in unit density material can be realized with no energy delivered beyond 100 g/m’. These energy sources can provide a well-defined depth of treatment in matter while eliminating any radiation effects at a depth 50% beyond the working range 141. This surface treatment technique has been used for the graft modification of polyolefinic materials to impart functional surface properties desirable for food packaging [S]. In this case the grafted material consists of hydrolyzed vinyl benzyl amine s&me, wherein the vinyl groups are copolymerized to the olefinic structure, and the resulting Si-O-Si structure imparts good oxygen barrier and superb resistance to aroma and flavor constituents of foodstuffs. An example of the Oz barrier performance of film modified with this process is shown in fig. 2 on a typicai 125 km low density polyethylene substrate. The highly polar nature of the silane structure offers a high surface tension (> 50 dyn/‘cm) for easy wetting by inks or coatings, or by other functional, copolymer&able coatings. SEM examination of the surface of the grafted silane coating reveals its high gloss and flatness 161. This priming technique has been used to provide tie or adhesion promoting coatings, particularly on inorganic substrates [7]. This has been particularly effective for meeting the rigorous requirements of electron cured coatings on metals, where the free radical polymerized monomer-polymer structure is stressed due to shrinkage upon curing, at the metal surface. No thermal ~nealing occurs here due to the room temperature nature of the process. For example, most coating curing takes place at under 5 Mrads (< 12 cal/g of coating) so that only modest temperature elevation of the coating/substrate system takes place upon curing. For example, the process developed by Nippon Steel 181 for its Folio sheet (white coated steel) process utilizes an electron curable tie coat as a primer, which is wet, trapped under the much thicker topcoat. Adhesion appropriate for the requirements of metal forming and

1233

zero t-bending has been achieved using this “wet” techniques. The use of these highly crosslinked, addition polymerized topcoats is now common in a wide variety of surface protection applications. In particular, the use of low viscosity impregnants for wood-grain printed paper has provided a composite polymer/paper product ]9], used in the d~ration/finis~ng of particle board. This process is now practiced worldwide and offers physical surface characteristics, such as scratch, abrasion, bum and stain resistance comparable to the much more costly high pressure laminates, traditionally used for panel finishing. The protection of soft substrates such as fiberglass reinforced gypsum tiles and roofing tiles has been cornrnerci~~~ for many years in Japan with great energy savings. There are strong incentives to these applications other than product aesthetics. Consider the elimination of the need to bulk heat a product which may be one thousand times the thickness and thermal capacity of the coating to be cured. For the electron polymerized system, energy need be invested only in the coating and the substrate interface. A similar technique to that described for the impregnation of paper has been developed [lo] for controlling the conductivity of packaging films. This system utilized a low viscosity electron curable vehicle as the carrier for a long chain quatemary a~o~urn salt. When coated on the host film and electron treated, the vehicle is polymerized, accompanied by a phase separation of the salt, which then migrates through the film. Since solubility and diffusi~ty are temperature dependent, this is often used to accelerate the conductivity improvement of a coated film where migration of the quaternary salt is accelerated during a coated film thermoforming cycle, for example. For example, Keough has reported that in 750 pm PET-G, a virgin surface resistivity of 3.8 x lOI $‘&,&Iwas reduced to 1.3 x 10”’ Q/a after thermoforming. This technique for permanent conductivity control of polymers will find applications beyond this EDP (Electrical Dissipative Packaging) application for charge-sensitive semiconductors, where surface resistivities below 10” @/cm are required. This brief review of the most successful of commercial electron modification of materials has emphasized surface properties and bulk film properties where, indeed, most of the active chemistry R&D is currently focussedd. This does not blush the fact that real progress continues in buIk property modification with high energy (0.5-5 MeV) electrons where the replacement or augmentation of thermal processes is sought. An important appli~tion is the upgrading of elastomer properties (notably for tire manufacture) as shown in fig. 3. In this case, the prevulcanization of SBR is used in stabi~~tion of the reinforcing cords in the composite ply before thermal vulcanization of the laid-up, multi-ply tire carcass is accomplished [ll]. Here, the modification XVII. RADIATION PROCESSING

1234

S. V. Nablo / Materials modification with accelerators

Electron

Vulcanized

Bleed Reinforcing

‘\ SE

depth of treatment

results in a very significant (15%) reduction in ply weight at comparable performance. The reader will be familiar with electron beam modifications of resist in microlithography for producing microscopic surface patterns in the manufacture of semiconductors. These are bulk processes in vacua, utilizing electron energies in the 10’s of kV range and will not be reviewed here. Modem in-line electron selfshielded processors or single gap accelerators are capable of speeds to 1400 Mrad m/mm. The compactness of these machines and their good power conversion efficiencies, coupled with the high energy efficiency of the free-radical addition polymerization chemistry they initiate, ensures an attractive future for these nonpolluting processes.

3. High energy ion implantation (Si) By 1984, research activities in high energy, deep implants, using conventional positive ion accelerators, pointed the way for many promising applications [12]. The need for energies above 200 keV motivated the development of high current accelerators capable of a MV so that these processes could be reduced to commercial practice. Eaton Corporation initiated the development of a radio frequency linear accelerator at that time leading to a design capable of energies of 500-2000 keV with singly charged ions. Using the modular two gap structure shown in fig. 4, particles Z are injected through the grounded envelope surrounding the rf excited cylindrical electrode C. If the ion spends half an rf period in the field-free electrode, it

of composite

schematic

Tack

Preserved

(ref. [13]).

products.

will be accelerated across both gaps of G, and Gz. Fig. 4 from ref. [13] illustrates the wide particle velocity acceptance of such a structure. Even though the antimony 121 ion is much slower than the boron 11 ion, and requires more than a half period to transit C, both particles acquire about the same energy gain. These multicavity implanters can provide 200 mm wafer throughputs of 135/h at lOi ions/cm2. The major application of these accelerators is for the deep, very high concentration oxygen doping of silicon, typically at levels to 2 X 10i8/cm2, so that a very thin, flat SO1 (silicon on insulator) structure is formed deep in the structure. In this way, the insulating barrier, a few thousand A thick, isolates the active surface layer from the bulk silicon, so that SEU’s (single event upsets) caused by natural or man-made radiation are much less likely; i.e. a smaller charge collection volume is available to the active device when, say, an alpha particle passes through the device. Other advantages offered by the SIMOX technique are: operating speed increase due to precise control of the active surface layer, high temperature operation through reduced leakage by the buried insulating layer and high yield.

4. Metal surface modification The development of high current implanters for the semiconductor industry has provided suitable sources for the emerging markets of surface property modification on non-semiconductors. The ability to achieve novel surface alloys not possible by other techniques under room temperature conditions has opened up several materials modification applications of a rather specialized nature. The basic physics of the ion-substrate interactions involved in implantation have been reviewed in the literature [14]. The reader is referred to Hirvonen’s recent review [15] of the subject of surface modification by implantation. 4.1. Surface

accelerator

Green

Rubber

Fig. 3. Controlled

Fig. 4. Multi-gap

Cord

Cord

implantation

(161

Many surface modification processes involve implantation of nitrogen, usually for lifetime increase of tools.

S. K Nablo / Materials modification with accelerators

Doses are typically in 2-6 x 1017 ions/cm’. The treatment of Ti/6A1/4V alloy prostheses has also been a well publicized application of this art [X5]; in this application the implantation forms titanium nitrides which improve the surface hardness. This process has been commercialized at Spire Corporation [15] for some time. A dual process involving a high dose Ti implant followed by C produces an amorphous ternary Ti/C,/Fe surface alloy of improved wear resistance. Ta implantation offers improved wear resistance under heavy loads, for gear tooth surfaces for example. Yttrium offers oxidation and wear resistance at levels < 10*6/cm2, while a dual Cr-C implant greatly improves the lifetime of fiber extrusion spinnerettes, which suffer erosive (che~cal/mech~cal) wear. Many other potential applications in ceramics and polymers are reviewed by Hirvonen. 4.2. Ion beam enhanced deposition (IBED) This process utilizes ion bomb~~ent augmented vapor deposition of metals. Low energies are utilized (1 keV) to enhance film adhesion, density and morphology, and to reduce internal stress in the films. Harper et al. [17] have reviewed this technique. Film stoichiometry can be controlled precisely, for example, as in silicon nitride coatings, to control refractive index in depth for thin film infrared filters of heretofore unattainable performance.

5. Conclusion The utilization of accelerator-assisted materials modification continues to be an exciting area for new process development. The precise control offered by these energy sources and their high power efficiencies will ensure expanded industrial use. It is their unique ability to implement these changes which will continue to underly their commercial success.

Acknowledgements

1235

and of James K. Hirvonen of Spire Corporation, Bedford, MA in providing data on current implantation developments is gratefully acknowledged.

References PI Radiation Processing: State of the Art, eds. J.G. Leemhorst and A. Miller, Proc. 7th Int. Mtg. Rad. Proc., Noordwijkerhout, Nd; Rad. Phys. Chem. 35, nos. 1-6 (1990); Pergamon, NY. PI Proc. RadTech ‘90, Radiation Curing, vol. 1-2, March 25-29, (1990) Chicago, IL (RadTech Intemational NA, Northbrook, IL 60062). t31 V.I. Stannett, Radiat. Phys. Chem. 18 (1981) 215-222. [41 I.J. Rangwalla, K.E. Williams and S.V. Nablo, Nucl. Instr. and Meth. B40/41 (1989) 1146. 1.7 S.V. Nablo, I.J. Rangwalla and J.E. Wyman, Radiation Curing of Polymeric Materials, eds. C.E. Hoyle and J.F. Kinstle, chap. 36, 535, ACS Symposium Series 417, ACS, Washington, DC (1990). WI J.E. Wyman, US Patent 4,803,126, Feb. 7 (1989). 171 K. Koshiishi et al., Proc. RadTech ‘90, ibid. ref. [2] (1990) 478. PI S. Fujioka, J. Fujikawa, N. Veno and A. Okamoto, Rad. Phys. Chem. 18 (1981) 865. 191J.M. Bosch, J.D. LeFors and T. Hetzel, RadTech ‘88 Conf. Rad. Curing, New Orleans, April (1988) (RadTech International NA, Northbrook, IL 60062). ml A.H. Keough, US 4,623,594, Nov. 18 (1986) and US 4,933,233, June 12 (1990). [Ill J.D. Hunt and G. Alliger, Rad. Phys. Chem. 14 (1979) 39. 1121 M.A. Guerra, in press Solid State Technol. (1990). u31 H.F. Glavish. D. Bemhardt, P. Boisseau, B. Libby, G. Simcox and A.S. Deuholm, Nucl. lnstr. and Meth. B21 (1987) 264. 1141 ST. Picraux, Arm. Rev. Mat. Sci. 14 (1984) 335. WI J.K. Hirvonen, Ann. Rev. Mat. Sci. 19 (1989) 401. [I61 Surface Alloying By Ion, Electron and Laser Beams, eds. L.E. Rehn, ST. Picraux and H. Wiedersich (Am. Sot. Met., Metals Park, OH, 1987). iI71 J.M. Harper et al., in: Ion Bombardment Modification of Surfaces, eds. 0. Aucielo and R. Kelly (Elsevier, NY, 1984) pp. 127-162.

The assistance of A. Stuart Denholm of the Ion Beam Systems Division of Eaton Cotpt Beverly, MA

XVII. RADIATION PROCESSING