12 The Industrial Physicist

Ion implanters are essential to modern integrated-cir-cuit (IC) manufacturing. Doping or otherwise modify-

ing silicon and other semiconductor wafers relies on thetechnology, which involves generating anion beam and steering it into the sub-strate so that the ions come to restbeneath the surface. Ions may be allowedto travel through a beam line at the ener-gy at which they were extracted from asource material, or they can be accelerat-ed or decelerated by dc or radio-frequen-cy (RF) electric fields.

Semiconductor processors today useion implantation for almost all doping in silicon ICs. Themost commonly implanted species are arsenic, phospho-rus, boron, boron difluoride, indium, antimony, germa-nium, silicon, nitrogen, hydrogen, and helium. Implant-

ing goes back to the 19th century, and has been continu-ally refined ever since. Physicist Robert Van de Graaff ofthe Massachusetts Institute of Technology and PrincetonUniversity helped pioneer accelerator construction, andthe high-voltage technology that emerged from this effortwas instrumental in building High Voltage EngineeringCorp. (HVEC) in the late 1940s and 1950s. HVECserved as an incubator for the technology essential tobuilding the first commercial ion implanters and theindividuals who pioneered the field.

William Shockley first recognized the potential of ionimplantation for doping semiconductor materials, andhis 1954 patent application demonstrates a remarkableunderstanding of the relevant process issues long beforeimplantation entered mass production. However, thepatent expired in 1974, just as the commercial ion-implantation market began taking off. So although

Shockley demonstrated visionary insight, hispatent earned few royalties.

Ion-implantation equipment and applica-tions gradually came together in the 1960s.Experience gained in building researchaccelerators improved hardware reliabilityand generated new techniques for purifyingand transporting ion beams. Theoristsrefined the hypothesis of ion stopping,which enabled the precise placement of ionsbased on the energy and angle of implanta-tion, and experimenters determined thathigh-temperature postimplant annealingcould repair implantation-induced crystaldamage. Initially, these anneals were done ata temperature of 500 to 700° C, but afterseveral years, semiconductor processorsfound that the optimum annealing tempera-ture ranged from 900 to 1,100° C. After theresolution of process integration issues, ionimplantation rapidly displaced thermal dif-fusion of deposited dopants as the dominantmethod of semiconductor doping because itwas more precise, reliable, and repeatable.

IC manufacturers, especially IBM andWestern Electric, designed and built many ofthe early ion implanters, almost exclusivelyfor in-house use. But in the early 1970s, themarket for commercial ion implanters beganopening as start-up companies tapped thetechnology spun off from HVEC and the

Ion Implantation inSilicon Technologyby Leonard Rubin and John PoateFEATURE

JUNE/JULY 2003 © American Institute of Physics

Withoutimplanters,

today’s integratedcircuits would be

impossible

CO

NC

ENT

RAT

ION

(ato

ms/

cm3 )

DEPTH (µm)

4 keV

1 MeV

2 MeV

3 MeV

0 1 2 3 4

1021

1020

1019

1018

1017

1016

1015

Figure 1. Some of the most commonly implanted species highlightedon the periodic table, along with typical concentration-versus-depthtraces for various implant energies.

technology developed by IC manufacturers, whobecame their customers.

Some memory circuits now sell for less than 20nanodollars/transistor. Today, implanters and other fab-rication hardware must meet aggressive productivity tar-gets to achieve this minuscule cost. A large wafer fabri-cator may process up to 50,000 wafers/month, witheach wafer requiring 20 to 30 implants. This outputrequires the use of about 20 implanters, each with thecapacity to implant more than 200 wafers/h. In practice,maximum implanter throughput typically ranges from250 to 270 wafers/h, including placing the wafers intoand removing them from sealed cassettes used by auto-mated material-handling systems. This throughput isachieved for wafer sizes of 150, 200, and 300 mm.Depending on the configuration of the beam line and theend station (the wafer-processing chamber), an implanteroccupies an area of 16 to 28 m2. Thus, fabrication spaceposes almost as significant a barrier as capital costagainst compensating for poor throughput by installingadditional implanters.

Applications Among semiconductor-processing techniques, ion

implantation is nearly unique in that process parameters,such as concentration and depth of the desired dopant,are specified directly in the equipment settings forimplant dose and energy, respectively (Figure 1). This dif-fers from chemical vapor deposition, in which desiredparameters such as film thickness and densityare complex func-tions of the tun-able-equipmentsettings, whichinclude tempera-ture and gas-flowrate. The numberof implants need-ed to complete anIC has increasedas the complexityof the chips hasgrown. Whereasprocessing a sim-ple n-type metaloxide semiconduc-tor during the1970s may have

required 6 to 8 implants, a modern complementary-metal-oxide-semiconductor (CMOS) IC with embeddedmemory may contain up to 35 implants.

The technique’s applications require doses and ener-gies spanning several orders of magnitude. Mostimplants fall within one of the boxes in Figure 2. Theboundaries of each box are approximate; individualprocesses vary because of differences in design trade-offs. Energy requirements for many applications havefallen with increased device scaling. A shallower dopantprofile helps keep aspect ratios roughly constant as later-al device dimensions shrink. As energies drop, ion dosesusually, but not always, decline as well. The width of thestatistical distribution of the implanted ions decreaseswith energy, and this reduces the dose required to pro-duce a given peak dopant concentration. The result is

13 The Industrial Physicist

Bonded wafer splittingfor silicon on insulator

(H, He)

1017

1016

1015

1014

1013

1012

10110.1 1 10 100 1,000 10,000

ENERGY (keV)

Polysilicondoping (As, B)

Source–draincontact

(As, BF2, B)

DO

SE(a

tom

s/cm

2)

Source–drainextension

(As, BF2, B)

Preamorphization(Ge, Si)

Bipolar buriedsubcollector (P, As)

Latch-up/electrostatic dischargeprotection

(B)

CMOS retrograde

wells(P, B, As)

Channel engineering(As, BF2, P, B, In, Sb)

Thresholdvoltage adjust

(As, BF2, B, P, In)

Antipunch-through

(As, B, In, Sb)

Noise isolationwells (P, B)

Charge-coupleddevice wells (B)

Dual-slit extractionelectrode

Electron confinementbeam guide

Analyzer magnet edge focusing

Electron confinement beam tunnel

Flag Faraday

Selectableresolvinghousing

Decelerationgroundextention

Plasmaelectronflood(xenon)

Wafer

Figure 2. Dose andenergy require-ments of majorimplantation appli-cations (speciesshown roughly inorder of decreasingusage).

Figure 3.Schematic of theelectron confine-ment technologynecessary totransport severalmilliamperes ofbeam at energiesbelow 10 keV in amodern high-cur-rent beam line.

14 The Industrial Physicist

the sloping lines in Figure 2. Implantation is actuallyextremely inefficient at modifying material composition.The highest ion dose implanted with an economicalthroughput is about 1016/cm2, yet this corresponds tobut 20 atomic layers. Only the extreme sensitivity ofsemiconductor conductivity to dopant concentrationmakes ion implantation practical.

Ion energy requirements vary from less than 1 keV tomore than 3,000 keV. Accelerating ions to higher ener-gies requires a longer beam line, yet low-energy beamsare difficult to transport intact over longer distancesbecause the beam cross section expands to a point whereit can no longer travel down the beam tube. This funda-

mental physics makes it nearly impossible to construct abeam line capable of all required ion energies. Figure 2indicates that the largest magnitude in required dosesoccurs in the middle of the energy range. Because dose isessentially the beam’s charge multiplied by the implanta-tion time, available beam currents in the 5- to 200-keVrange must vary by at least 4 orders of magnitude to per-form all required implants efficiently. This level is diffi-cult to reproduce repeatedly in a single ion-source/beam-line configuration.

Market segmentsConsequently, the commercial ion-implanter market

long ago evolved into three segments. The red, black,and blue regions of Figure 2 indicate high-current, medi-um-current, and high-energy applications, respectively.

As the name suggests, high-current implanters producethe highest beam currents, up to 25 mA (Figure 3). Forhigh-dose applications, the greater the beam current, thefaster the implantation, which means the output of morewafers per hour. Implanter makers have invested a greatdeal of effort in maximizing beam current, especially at thelowest energies, where Child’s law limits the flux of ionsextractable from a source. Although high-current implanterscan produce beams in the 10-µA range, source instabilitiesmake these beams unsuitable for low-dose applications.The short beam line of these implanters allows an energyrange from <1 keV up to 100 to 200 keV.

Medium-current implanters are designed for maxi-mum dose uniformity and repeatability.Their beam currents are in the range of1 µA to 5 mA, at energies of 5 to ~600 keV.The wafer-processing end stations canimplant ions at angles up to 60° from theperpendicular to the wafer surface. This isessential for certain applications, such asanti-punchthrough implants, for example,in which dopants must be implanted par-tially underneath a previously formed gatestructure. The lower operational cost ofmedium-current implanters when used forlower-dose applications and their ability todo high-tilt implants distinguish them fromhigh-current implanters.

Last, only high-energy implanters cangenerate megaelectron volt ion beams.Commercial high-energy implanters pro-duce beam currents for singly-charged ionsup to ~1 mA. Energies for multiply-chargedions can be up to ~4,000 keV, with beamcurrents of ~50 µA. High-energy implanters

can produce beams down to 10 keV, making them suit-able for many medium-current applications as well. Thisadditional functionality justifies the capital cost of thesemachines. High-energy implanters using both RF linearacceleration (Figure 4) and dc acceleration are usedwidely today in semiconductor manufacturing.

A modern ion implanter costs about $2–5 million,depending on the model and the wafer size it processes. Ofthe three classes of implanters, the high-current machineshave traditionally been the biggest market in terms of rev-enue and unit volume. Revenue is increasing faster thanunit volume because implanters have become more expen-sive. However, both revenue and volume are subject to thesevere boom-and-bust cycles that have affected the entiresemiconductor capital-equipment industry in the pastdecade, a pattern that will likely continue.

Ten radio-frequencyresonators

(80 kV @ 13.56 MHz)

Twelve quadrupolelenses (20 kV dc)

Mass- analyzing

magnet(70°)

Ten radio-frequencyelectrodes

Figure 4. Schematicof a radio-frequency

linear acceleratorused in a high-ener-

gy ion implanter.

15 The Industrial Physicist

Process requirementsThe tiny transistor dimensions that allow the fabrica-

tion of microprocessors with clock speeds exceeding3 GHz necessitate strict doping accuracy. For the mostsensitive devices, the implanted dose must be as uniformas possible. Typically, a 3-standard-deviation (σ) varia-tion of 1.5% is the acceptable upper limit. This unifor-mity must be consistently achieved across wafers as largeas 300 mm in diameter. Wafer-to-wafer and lot-to-lotrepeatability is equally crucial. The energy of the ionbeam should not exceed a 3σ variation of 3.0% across allwafers. The angle of the ion beam to the wafer must alsobe carefully controlled to prevent variations in dopantposition at the edge of device features. Variations indopant depth are also a concern. Implant angle controlwith a 3σ variation of ~1.0° is usually enough to createreproducible device electrical characteristics.

Designers of modern implanters have largely achievedthe desired dose, energy, and angle accuracy. The greatestchallenge remaining is to increase the productivity of thebeam current at energies below 10 keV. Device scaling hasgreatly increased the demand for implants in this energyrange. Designers also need to reduce implanter-inducedcontamination as much as possible. Atoms of previouslyimplanted dopants can be sputtered onto the wafer sur-face (cross-contamination), or ions of the right speciesbut wrong energy or charge state can be implanted (ener-gy contamination). Particles can be deposited onto thewafer surface either by ion-beam transport or duringwafer handling. Even particles as small as 120 nm maycause yield losses. Finally, metallic contaminants can bedeposited onto the wafer surface, usually from sputteringof beam-line components, or worse, implanted into thewafer (energetic metallic contamination). Modern devicesare so sensitive to these problems that many customersdemand levels of particle and metallic contaminationbelow those detected by metrology equipment.

Modern end stations implant either one wafer at atime or a small batch of wafers—typically 13 or 17—mounted on a rapidly rotating disk. Only single-waferend stations are capable of high-tilt implants because ofthe mechanical complications of supplying water andother necessities to a rotating disk. However, the abilityto implant multiple wafers simultaneously gives batchend stations a productivity advantage. Consequently,both end stations have found successful commercialniches. Nearly all medium-current implanters dope asingle wafer at a time, and most high-current and high-energy implanters process wafers in batches.

Makers have shipped more than 6,000 ion implantersworldwide since 1980. Assuming that about 4,000 of

them remain in service, all ions implanted worldwide atany given time represent a mass transfer of only about5 mg/s. Two companies located 25 km apart on Boston’sNorth Shore, Varian and Axcelis, manufacture roughly70% of all ion implanters (Table 1). This siting is notentirely coincidental, because many of the same peoplefounded and/or helped build the two companies, Varianin 1971 and Axcelis in 1978. It is unlikely that a signifi-cant amount of ion-implanter design will migrate to newareas because of the machines’ complexity. The expertiserequired to design a new beam line is not easily duplicat-ed, which makes ion implanters the Swiss analog watchesof the semiconductor industry. They are an elegant, com-plex product designed and made mostly by a small num-ber of skilled craftspeople in a small corner of the world.

Further readingDearnaley, G.; Freeman, J. H.; Nelson, R. S.; Stephen,

J. Ion Implantation; American Elsevier Publishing Co.:New York, 1973; 802 pp.

Rimini, E. Ion Implantation: Basics to Device Fabrication;Kluwer Academic Publishers: Boston, 1995; 393 pp.

Rubin, L.; Morris, W. High-Energy Ion Implanters andApplications Take Off. Semiconductor International 1997,20, 77–85.

Ryssel, H.; Ruge, I. Ion Implantation; John Wiley andSons: New York, 1986; 350 pp.

Ziegler, J. F., Ed. Ion Implantation: Science and Technology;Ion Implant Technology Co.: Edgewater, MD, 2000; 687 pp.

Chason, E.; et al. Ion beams in silicon processing andcharacterization. J. App. Phys. 1997, 81 (10), 6513–6561.

Current, M. I. Ion implantation for silicon device man-ufacturing: A vacuum perspective. J. Vac. Sci. Tech. A:Vacuum, Surfaces, and Films 1996, 14 (3), 1115–1123. W

Leonard Rubin (leonard.rubin@axcelis.com) is a seniorscientist and John Poate (john.poate@axcelis.com) isvice president and chief technology officer at AxcelisTechnologies in Beverly, Massachusetts.

B I O G R A P H Y

Company Location Web site

Axcelis Technologies Beverly, MA www.axcelis.com

Varian Semiconductor Equipment Associates Gloucester, MA www.vsea.com

Applied Materials Implant Division Horsham, U.K. www.amat.com

Sumitomo Easton Nova (joint venture with Axcelis) Tokyo, Japan

Nissin Electric Kyoto, Japan www.nissin.co.jp/e/

Ibis Technology Danvers, MA www.ibis.com

Table 1: Major manufacturers of commercial ion implantation equip-ment in order of decreasing market share.