[IEEE 1998 International Conference on Ion Implantation Technology. Proceedings. Ion Implantation...

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Radiation Emission from Ion Implanters when Implanting Hydrogen and Deuterium Kourosh Saadatmand, Edward McIntyre, Steve Roberge, Zhimin Wan, and Kevin Wenzel Eaton Semiconductor Equipment Operation 108 Cherry Hill Dr. Beverly, MA 0 19 1 5 e-mail:[email protected] Robert Rathmell and Jerry Dykstra Eaton Semiconductor Equipment Operation 10000 Spectrum Drive Austin, TX 78717 Abstract - An increasing interest in proton and deuterium implants has been expressed among the semiconductor device fabricators. The possible production of neutrons and high energy gamma rays during such implants necessitated an extensive radiation study of implanters. A series of experiments to study neutron and gamma production with beams of H, D, B, P and As on common implanter materials such as C, Al, Si and the dopants themselves have been performed. Based on our experimental data, and the underlying physical reactions, we concluded that neutron generation is negligible in operation of B from BF3 feed materials and from P or As from hydride feed materials. Operation of the implanter with feed materials composed of enhanced amounts of deuterium at any energy will result in significant neutron generation. Proton beams below certain energies can be used safely, with common implanter materials as targets, while maintaining acceptable radiation levels. At higher energies the risk of activation and gamma ray production increases, depending on the material used for the target and beam line components. I. INTRODUCTION Interest in the use of hydrogen and deuterium as implant species has recently been increased. These implants appear to have process advantages in damage engineering and interface state control processes. Hydrogen is also commonly used to reduce backgating and improve substrate isolation in Gallium Arsenide wafer processes. However, hydrogen and deuterium can react with materials used in the beam line, residues of other dopant species, or process wafers to produce harmful neutron and gamma radiation. This possible danger has prompted Eaton to investigate the safety implications of accelerating hydrogen and deuterium in ion implanters. This paper will summarize Eaton's experimental work to date, test results, and latest recommendations regarding the use of these materials. 11. NEUTRON EXPERIMENTS The most likely reactions for producing neutrons in ion implanters are: '3C(d,n)'4N and '2C(d,n)'3N. Graphite is used in ion implanters for beam line components, strike plates, and Faraday cups. I3C comprises 1% of natural carbon, and deuterium naturally comprises approximately 0.015% of hydrogen. The "C(~JI)'~N reaction has a threshold energy of 328 keV. A. 90 keV hydride and hydrogen experiments A Health Physics neutron detector, Model REM 500 which measures background dose rate to be approximately 15 premkr, was placed approximately 27 cin from a graphite strike plate. Phosphine PH3 source material was used to produce a 90 keV hydrogen beam dxected onto the strike plate. This hydrogen beam consisted of 4 mA of ions with amu=l (H+), 2.2 mA of ions with amu=2 ( H2+ and D+) and 100 pA of ions with amu=3 (H3+ and €ID+). This calculates to approximately 0.6 pA of 90 keV D+ beam striking the graphite target. The beam was allowed to impact the strike plate for approximately two hours. The measured neutron dose rate was not distinguishable from background. The strike plate experiment was repeated using hydrogen as a source material (with Argon as a co-gas). Again, for calculated D+ beam current of 1.3 PA, the measured neutron dose rate was not distinguishable from background. Thus, at maximum extraction energy (90 keV), acceleration of natural hydrogen or hydrogen containing compounds does not constitute a risk of neutron generation. B. Acceleration of deuterium A similar experiment to the hydride and hydrogen acceleration from the ion source was conducted using pure deuterium gas as a source material. A 1.5 mA, 90 keV D+ beam was directed onto a graphite target with the neutron detector placed 27 cm away. An averaged neutron dose rate of approximately 200 mrem/hr was recorded over duration of the run, approximately 6 minutes. The I3C(d,n)'4N reaction, by itself, cannot explain the neutron dose rates observed due to its small cross section. It 0-7803-4538-X/99/$10.00 Q 1999 IEEE 292

Transcript of [IEEE 1998 International Conference on Ion Implantation Technology. Proceedings. Ion Implantation...

Page 1: [IEEE 1998 International Conference on Ion Implantation Technology. Proceedings. Ion Implantation Technology - 98 - Kyoto, Japan (22-26 June 1998)] 1998 International Conference on

Radiation Emission from Ion Implanters when Implanting Hydrogen and Deuterium

Kourosh Saadatmand, Edward McIntyre, Steve Roberge, Zhimin Wan, and Kevin Wenzel Eaton Semiconductor Equipment Operation

108 Cherry Hill Dr. Beverly, MA 0 19 1 5

e-mail: [email protected]

Robert Rathmell and Jerry Dykstra Eaton Semiconductor Equipment Operation

10000 Spectrum Drive Austin, TX 78717

Abstract - An increasing interest in proton and deuterium implants has been expressed among the semiconductor device fabricators. The possible production of neutrons and high energy gamma rays during such implants necessitated an extensive radiation study of implanters. A series of experiments to study neutron and gamma production with beams of H, D, B, P and As on common implanter materials such as C, Al, Si and the dopants themselves have been performed. Based on our experimental data, and the underlying physical reactions, we concluded that neutron generation is negligible in operation of B from BF3 feed materials and from P or As from hydride feed materials. Operation of the implanter with feed materials composed of enhanced amounts of deuterium at any energy will result in significant neutron generation. Proton beams below certain energies can be used safely, with common implanter materials as targets, while maintaining acceptable radiation levels. At higher energies the risk of activation and gamma ray production increases, depending on the material used for the target and beam line components.

I. INTRODUCTION Interest in the use of hydrogen and deuterium as

implant species has recently been increased. These implants appear to have process advantages in damage engineering and interface state control processes. Hydrogen is also commonly used to reduce backgating and improve substrate isolation in Gallium Arsenide wafer processes. However, hydrogen and deuterium can react with materials used in the beam line, residues of other dopant species, or process wafers to produce harmful neutron and gamma radiation. This possible danger has prompted Eaton to investigate the safety implications of accelerating hydrogen and deuterium in ion implanters. This paper will summarize Eaton's experimental work to date, test results, and latest recommendations regarding the use of these materials.

11. NEUTRON EXPERIMENTS The most likely reactions for producing neutrons in

ion implanters are: '3C(d,n)'4N and '2C(d,n)'3N. Graphite is

used in ion implanters for beam line components, strike plates, and Faraday cups. I3C comprises 1% of natural carbon, and deuterium naturally comprises approximately 0.015% of hydrogen. The "C(~JI) '~N reaction has a threshold energy of 328 keV.

A. 90 keV hydride and hydrogen experiments A Health Physics neutron detector, Model REM 500

which measures background dose rate to be approximately 15 premkr, was placed approximately 27 cin from a graphite strike plate. Phosphine PH3 source material was used to produce a 90 keV hydrogen beam dxected onto the strike plate. This hydrogen beam consisted of 4 mA of ions with amu=l (H+), 2.2 mA of ions with amu=2 ( H2+ and D+) and 100 pA of ions with amu=3 (H3+ and €ID+). This calculates to approximately 0.6 pA of 90 keV D+ beam striking the graphite target. The beam was allowed to impact the strike plate for approximately two hours. The measured neutron dose rate was not distinguishable from background.

The strike plate experiment was repeated using hydrogen as a source material (with Argon as a co-gas). Again, for calculated D+ beam current of 1.3 PA, the measured neutron dose rate was not distinguishable from background. Thus, at maximum extraction energy (90 keV), acceleration of natural hydrogen or hydrogen containing compounds does not constitute a risk of neutron generation.

B. Acceleration of deuterium A similar experiment to the hydride and hydrogen

acceleration from the ion source was conducted using pure deuterium gas as a source material. A 1.5 mA, 90 keV D+ beam was directed onto a graphite target with the neutron detector placed 27 cm away. An averaged neutron dose rate of approximately 200 mrem/hr was recorded over duration of the run, approximately 6 minutes.

The I3C(d,n)' 4N reaction, by itself, cannot explain the neutron dose rates observed due to its small cross section. It

0-7803-4538-X/99/$10.00 Q 1999 IEEE 292

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is believed that the graphite strike plate (and, by extension, all other graphite components exposed to deuterium) absorbs and is implanted with deuterium and, in essence, creates a deuterium target for the incoming beam. Calculations show that less than 0.05% concentration of deuterium in the graphite would be required to produce the measured neutron radiation from the well known D ( ~ , I I ) ~ H ~ reaction.

To determine the energy dependence of the neutron dose rate resulting from a deuterium beam, an additional experiment was conducted in which the energy of the beam was varied from 90 keV down to 15 keV. The beam current was kept constant at 1 mA D+, except for the 15 keV case, where beam current fell to 0.6 mA D+. The Health Physics neutron detector was placed -27 cm from the target. The results are shown in figure 1.

C e

6 "0 1000 I I 1

8 0.1 ! ,

-I I I

points) are also shown in this figure. At the implanter enclosure, the neutron dose rate from one pA of D+ beam falls below Eaton specification for total radiation dose rate (O.O6mrem/hr, 15 cm from machine surface for properly tuned beams) for beam energies below 300 keV. These results indicate that the potential for significant neutron generation exists, particularly in high energy H2+ and D+ ion implantation.

4 e 1000 0

I I I I El 0.01 -1 I I I I

I 1 E 0 200 400 600 800 1000

Beam Energy (keV)

Fig. 1. The energy dependence of the neutron dose rate resulting from low energy deuterium beam.

Even at low energies (- 20 kev), neutron production €rom acceleration of pure deuterium within an ion implanter is likely to produce dose rates at the implanter enclosure which exceed Occupational Safety and Health Administration Permissible Exposure Limits for radiation exposure ( 5 rein / year) [ 11.

C. Acceleration of H2+ to High Energies up to 1 MeV In order to determine the neutron production rates

from the acceleration of H2 to high energies (up to 1 MeV), the Health Physics neutron detector was placed approximately 17 cm from the flag faraday at the resolving aperture of a NV-GSD-HE ion implanter. An H2+ beam (containing natural abundance D+ beam) was accelerated up to 1 MeV. It is believed that the "C(d,n)13N reaction dominates in production of neutrons under these conditions. At various energies, the neutron production rate was measured and the results (circular data points) are shown in figure 2. The measured dose rates (radiation levels) extrapolated by inverse square of the distance for the nearest point outside the enclosure of the NV-GSD-HE (square data

Fig. 2. The energy dependence of the neutron dose rate resulted from high energy deuterium beam.

111. GAMMA RAY EXPERIMENTS

Two y producing reactions that could take place in an ion implanter, when protons are accelerated are: "B(p,y)"C and 12C(p,y)'3N. In "B(p,y)"C reaction, y rays are produced by the impact of a proton into "B which is a common dopant and, therefore, significant contaminant of the graphite Faraday cups. A quantitative measure of dose rate for "B(p,y)"C reaction is not universal since the y ray yield is strongly dependent on target condition. The IIB@J)'~C reaction has a resonance at 163 keV, producing characteristic peaks of gamma radiation at 4.44 MeV, 11.52 MeV and 15.96 MeV [2]. The I2C@,y)l3N reaction has a resonance at 457 keV, however literature reports that this reaction becomes significant at about 400 keV[3,4]. This reaction produces 1.94 MeV gamma rays, however, it also activates the carbon within the target, forming I3N which then undergoes a positron decay to "C. This positron decay results in production of a 0.51MeV gamma ray. I3N has approximately a 10 minute half life.

A. Acceleration of a Proton Beam (H+) in a High EnergV Implantel; NV-GSD-HE

A 7.6 cm by 7.6 cm Sodium Iodide detector with multi channel analysis capability was located approximately 25 cm from the flag faraday assembly and used to collect gamma ray energy spectra. A number of y ray spectra were collected as the proton beam energy was varied. The

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spectrum for a 400 keV proton beam impacting the boron contaminated Faraday cup is shown in figure 3.

100000 - 0.511 MeV

10 - -

I 501 1001 1501 2001 2501 3001 3501

Channel number (function of energy)

Fig. 3. The 400 keV gamma ray spectrum.

This spectrum indicate the presence of two main gamma radiation producing reactions, mentioned above, occurring at the faraday cup. For all proton energies below 400 keV, the 0.51 1 MeV peak returned to background levels upon termination of beam impact. For proton energes of 400 keV and above, with beam off, the degradation of the peak was timed, verifying a half life of approximately 10 minutes. The gamma ray spectrum for 90 keV proton beam resembled that of the background spectrum, while the spectrum for 840 keV beam greatly resembles that of the 400 keV spectrum. The gamma ray spectrum for 200 keV is similar to 400 keV spectrum except for the absence of 1.94 MeV peak due to ”C. Measurements taken outside the machine enclosure during 840 keV beam (143 pA of H+) operation using a Geiger-Mueller detector indicated several areas where the dose rate was O.lmrem/hr. While these dose rates are not likely to result in significant health effects under occupational exposure conditions they do exceed the Eaton specification of 0.06 mredhr.

B. Medium Current, NV-6200A, Implanter In these experiments the flag faraday was replaced

with a boron nitride target and protons were implanted at various energies up to 200 keV into the target. Boron Nitride is considered to be an appropriate “worst case” simulation of boron contamination in graphite. Geiger- Mueller detectors (Ludlum Model 3 with 44-38 probe) were placed in two locations relative to the target. One was placed approximately 15 cm from the target, and another was placed 103 cm from the target, immediately inside the end station adjacent to the operator location. The results are shown in Table I. Based on the experimental results, dose rates (radiation levels) were extrapolated by inverse square of the distance for various locations outside the NV-6200A implanter. The worst case for potential exposure was at a location immediately at the junction of the end station

enclosure and the beam line enclosure. Total radiation dose rate at 200 keV, 1mA H+ beam at this location may exceed the Eaton specification of 0.06 mremhr. These calculations take into account the production of high energy gamma rays due to boron reactions as well as “nominal” implanter x-rays generated by bremsstrahlung from accelerated electrons striking beamline construction materials [ 5 ] .

TABLE I Measured Radiation Dose rate for various Energy Proton Beams (NV-6200A)

Outside Immediately Target Faraday Inside End

Station

0.150 0.005 0.150 0.020 0.150 0.010

200 0.150 0.010

C. High current, GSD- 160, Implanter Experiments were also conducted (using Ludlum

model 12s scintillation detector) while implanting hydrogen @I+) in a GSD-160. Radiation for 19.5 mA of 175 keV H+ into a graphite flag (only 100 hrs old), yielded radiation of less than 2 mredhr, 30 cm from the flag and a large fraction of this may have been bremsstrahlung since radiation outside the enclosure (99 cm from the flag Faraday) dropped to 6 prem/hr with the door (lead shielding) closed. For energies below 150 keV, the detected radiation fell below Eaton total radiation specification (0.06 mrem’hr).

D. Activation of Carbon in Wafer Photoresist Because activation of 12C to 13N could result in the

activation of the photoresist coating on process wafers, Eaton conducted two implantation experiments to vellfy that the I2C(p,y)l3N activation reaction observed at energies of 400 keV and greater does not occur within the typical energy ranges of medium and high current implanters.

In a NV-6200A, a 4” photoresist wafer was implanted with a proton beam at an energy of 190 keV, 20 pA beam current for approximately 20 minutes. As the wafer exited the implanter approximately 5 seconds ‘ post implant, the wafer was measured using a Geiger-Mueller detector. No discernible reading above background was noted.

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In a GSD-160, a full batch of 17 - 6 ' photoresist coated wafers were implanted with H+ to a 10l6 cm-2 dose at 175 keV. 16 mA H+ current. This recipe is considered to be a realistic worst case production simulation of photoresist coated wafers in terms of providing a high dose implant while maintaining the integrity of the photoresist. The wafers were surveyed using the Ludlum 12s detector before and after implant. The implant lasted approximately 11 minutes. Wafer handling time post implant was approximately 3 minutes. Readings before and after implant indicated background radiation only.

These results indicate that the I2C(p,y)l3N reaction is not a concern for processing photoresist coated wafers in medium and high current implanters.

D. Other Possible Risks of Proton Implantation There are other possible target materials that undergo

reactions with proton beams that may impact the production of radiation within ion implanters. These include but are not limited to [6]:

0

0

0

0

0

IgF(p,a y)I6O with a resonance at 224 keV 24Mg(p,y)25Al(p+)25 Mg with a resonance at 226 keV 27Al(p,y)28Si with a resonance at 226 keV 'Be(p,y)'% with a resonance at 330 keV I5N(p,y)l6O with a resonance at 360 keV I5N(p, a y)I6O with a resonance at 360 keV

Some reactions above 300 keV are listed because their resonance width may allow reactions to start at lower energies. These materials are all present in implanters either as implant species, materials of construction or purge gas and therefore should be considered as potential radiation sources when evaluating proton implanting processes. It must be noted that for energies below 250 keV, the llB(~.y)'~C reaction is expected to have a larger radiation yield than any of the other reaction mentioned above.

IV. CONCLUSION These experimental results clearly show that the use

of deuterium or deuterium enriched materials is likely to result in the production of significant and dangerous levels of neutron radiation. In addition, subjecting an AMU 2 beam to firther acceleration in LINAC beamlines will increase the risk of significant neutron production. This risk significantly increases at energies of 300 keV and above. Use of hydrogen containing source materials such as hydrides do not present a hazard related to neutron production from naturally occurring deuterium within the hydrogen components of these materials.

Implant of Proton (H+) beams may result in the production of high energy gamma rays and activation of

carbon depending on beam energy and dopant history. Implantation of proton beams up to energies of 150 keV does not appear to present any increase in radiation exposure. Implants ranging in energy from 150 keV to 250 keV should be evaluated for potential reactions and tests conducted using appropriate radiation monitoring equipment sensitive to high energy gamma rays. Eaton strongly recommends against performing proton implants at energies above 250 keV. As energy increases beyond this level, the variety and intensity of nuclear reactions from protons impacting numerous target species are likely to increase significantly

ACKNOWLEDGMENT K. Saadatmand thanks Jiong Chen, Jon Merrill, John

Reynolds, Steve Quinn, and Kurt Whaley for their assistance in data collection and implanters set-up.

REFERENCES 29 CFR 1910.96. HANDBOOK ON NUCLEAR ACTIVATION CROSS SECTIONS, Technical reports series No. 156, Intemational Atomic Energy Agency, Vienna, 1974., p. 460 E J.D. Seagrave, Phys. Rev. 84 ( 1 9 5 1 ) ~ . 1219. S.E. Hunt and W.M. Jones, Phys. Rev. 89 (1953), p. 1283 fF. C.J. Maletskos, W.R. Ghen, ION IMPLANTATION SCIENCE AND TECHNOLOGY, 1988, p. 441 to 447. HANDBOOK ON NUCLEAR ACTIVATION CROSS SECTIONS, Technical reports series No. 156, International Atomic Energy Agency, Vienna, 1974., p. 460 ff

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