Petroff, Pierre

Washburn, Jack.

eScholarship - University of California

Lawrence Berkeley National LaboratoryRecent WorkTitleLIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONSPermalinkhttps://escholarship.org/uc/item/85v3t220AuthorsPetroff, PierreWashburn, Jack.Publication Date1967-05-01eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaUCRL-17347 Rev. University of California Ernest O. Radiation Lawrence Laboratory LIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONS Pierre Petroff and Jack Washburn May 1967 o R~CE/Vt=-'Ai' '~~c:.:.D~ ________ ------... RAD 1.,/IW-lC tAftON LAB JUL 2 ri 1 TWO-WEEK LOAN COpy ooct::::iY A This is a Ubrar~ Circulating Cop~ S SE' which ma~ be borrowed for two weeks. for a personal retention cop~, call Tech. Info. Division, Ext. 5545 DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. Submitted to Review of Scientific Instruments UNIVERS ITY OF CALIFORNIA Lawrence Radiation Laborator y Berkeley) California AEC Contract No . W-7405- eng- 48 LIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONS Pierre Petroff and J ack Washburn May 1967 UCRL- 17347- Rev . Preprint -iii-LIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONS Pierre Petroff and Jack Washburn UCRL-17347 Rev. Inorganic Materials Research Division, Lawrence Radiation Laboratory, Department of Mineral Technology, College of Engineering, University of California, Berkeley, California ABSTRACT A field ion microscope is described which allows Ifin situ" irrad-iation of a metal specimen at a temperature close to ,oK. It has been used with the .88" Cyclotron at Berkeley which provides 10 MeV protons. The design permits lffield off" irradiation of the specimen, as well as "in situ" temperature cycling, Qualitative observations on the surface damage of iridium upon "field off" and "field on" irradiation are given. -1- UCRL-17347 Rev . I. INTRODUCTION Because of its high resolution, the field ion microscopel enables direct observation of atomic .structure at the surfacE; of a metal. Experiments by MUller, 2 Brandon and Wald,3 and Sinha and Mulley~' have proved its usefulness in analyzing the effects of radiation dam8.ge irl metals. The liquid helium field ion microscope described in this paper was built primarily for direct observation of the defects produced by irradiation and the changes that accompany subsequent heating. It allows irradiation of the specimen inside the field ion microscope at 5°K and can be used with a variety of radiation sources. For the pre-sent eA~eriments the instrument was set at the end of the beam pipe of the 88" cyclotron of the Berkeley Lawrence Radiation Laboratory . 10 MeV protons were used as the bombarding particles. II. DESCRIPTION OF THE INSTRUMENT The microscope chamber (Figo 1) is composed of a large stainless steel dewar (A) holding liquid nitrogen and surrounding the liquid helium cold finger (B). The stainless steel cold finger is surrounded by a silver-plated copper heat shield (C) to minimize the evaporation rate of liquid heliwn. The cold finger is composed of a liquid helium tank encased by a vacuum jacket (D) which is attached to the thermal shield through two thin wall stainless steel tubes (E). An alumina specimen holder (F) in which the specimen is clamped with a copper block was used to insure proper cooling of the specimen as well as electric insulation of the microscope chamber. Figure 2 shows a detailed view of the specimen holder and of the high voltage lead which is fed into the microscope chamber through a glass-Kovar tube mounted on a 1-1/2" -2-UCRL-l T54 7 -Rev. flange. Good thermal contact between the copper block at JJ· o~°i( I:lnd trw alumina specimen holder is insured through an indium- galliwfi alloy coating on the copper threads and a deposit of gallium vaporized on the threaded part of the specimen holder. The high voltage line is made of a 0 . 01 cm. diameter inconel lead whi ch is effectively thermally anchore d on an al~~ina block at 77°K and then connected to the specimen hol der. The ultra-high vacuum was obta i ned t hrough an oil diffusion pumping system and a vac ion pump . Two bakeabl e valves a l lovling to switch fro:n the vac ion pump which was used duri ng ultra -high vacuum experiments to the oil diffusion pumping sy st em vlhi ch was used for pumping the imaging gas. The entire system is bakeable to 300°C vlhich allows a vacuum of 10-8 Torr to be attained in 4 to 6 hours pumping time) and a vacuum of 10-9 Torr with the liquid nitrogen and the liquid heli~~ filled in the microscope. For a precooled cold finger) the l iquid helium consumption was found to be strongly dependent on the gas pressure in the microscope chamber as shown in Table I . Table I . Liquid Helium Consumption Helium Gas Pressure 500 cC/hour lx -4 10 Torr 800 cC/hour 1 X 10-3 Torr 1000 cC/hour 3 x 10-3 Torr 1500 cC/hour .5 X 10-3 Torr -3-UCRL-173L~7-Rev. At the attachment of the field ion microscope to the cyclotron beam pipe, a 0.01 em. thick aluminum foil was inserted in the c0nnect-ing flange (G), thus providing isolation of the vacuum system of the microscope from the cyclotron vacuum system. The temperature of the specimen was measured with a resistance thermometer made of a 4 cm. long loop of annealed platinum wire. This wire (99.999% pure) was mounted between two copper attachment points which were screwed and glued into the specimen holder (Fig. 1). The wire was then spot welded in its middle to the specimen. The current and potential leads were fed into the microscope chamber in the same way as the high voltage lead. They were also thermally anchored to alumina blocks at 77~K before being spot welded to the platinum loop. Prior to use inside the microscope the resistance thermometer mounted on the specimen holder was calibrated between 4°K and 273°K using standard resistivity measurement methods. With a well pre-cooled cold finger filled ·with liquid helium the specimen temperature , as measured with this method, was 5~K. Cooling of the specimen from liquid nitrogen to liquid helium temperature5,6 led to the expected improvement in image resolut ion and qualityo Fig~es 3(a) and (b ) show the same iridium surface at o ° 77 K and 5 K. Both pictures were recorded under the same imaGine voltage and imaging gas pressure in order to observe only the tem-perature effects on the image features . "In situ" temperature cycling between , .. oK and temperatures T ~ l4°K was obtained by letting the liquid helium evaporate and the c'old finger warm up to the desired temper ature. The temperature increase -4-UCRL-17347-Rev. of the cold finger was measured with a CU-Cu constantan thermocouple held in contact with the copper block a t the bottom of the cold f inger (Fig. 2). The specimen temperature was deduced from these measurements by using the results of a calibration run in which both the specimen and the copper block temperatures had been measured simultaneously. All irradiation and temperature cycling experiments were performe d with the vacuum inside the microscope chamber and with the specimen holder stripped of the resistance thermometer (Fig. 2). III. QUALITATIVE OBSERVATIONS The radiation damage resulting from 10 MeV protons was observed by comparing the specimen surface before and after irradiation and after successive field evaporation~ "The direct comparison method requires that no changes other than those induced by the bombarding particle s occur at the surface, i.e., there should not be any contamination or corrosion of the surface during the irradiation time. Such a condition is easily achieved so long as the high electrostatic imaging field i s continuously applied to the specimen, since it ionizes all gas impurities before they reach the specimen surface. This technique has been success -fully used by Brandon and Wald,2 Sinha and MU~ler3 in their investiga-ti~ns of radiation damageq However, the imaging electrostatic field introduces up to 10% dilatation in the specimen lattice7 and this may affect considerably the nature of the damage at the surface and even in the bulk of the specimen. The present system, because of the ultra high vacuum and because the cyclotron could provide a short pulse at a high flux density, allows "field off" irradiation, i.e., no imaging field is applied and the lattice is not distorted during the bombardment time. -5-UCRL-17347 -Rev. Prior to irradiation, the contamination and corr0sion rates of the specimen surface were observed as a function of time when the imaging field was not applied o For a vacuum of 10-9 Torr in the microscope the first changes of the specimen surface appeared after 8 minutes under "field off" conditions. rigure 4 (a,b) shows the surface damage produced on an iridium specimen by 10 MeV protons at liquid helium temperature, with the "field offll. The total flux during the 1 minute pulse, as measured on 14 the dummy target, was about 10 protons spread over a 111 diameter circular area. Changes in the surface layer of the specimen due to the 1 proton bombardment were identified by using an image comparator. The changes consisted mostly of surface vacancies, the partial evaporation of atomic layers on several planes, and the appearance of a few inter-stitials. Figure 5 (a) and (b) shows the surface damage produced on an iridium bicrystal at liquid nitrogen temperature with 10 MeV protonso This specimen was irradiated IIfield on" with a flux of about 1014 pro-tons spread over a :til diameter circle. In this case the surface damage was more extensive than in the IIfield off" irradiated specimen. Further experiments are required before it can be definitely con-eluded that the lattice distortion due to the imaging field is the cause of this more extensive damage. The annealing characteristics of radiation induced defects have been studied for the annealing stages 1, 2, 3, and the results will be presented in a later paper. -6-UCRL-173 47-Rev. ACKNOWLEDGEMENTS The authors are grateful to Dr. S. Ranganathan for helpful dis-cussions; they also wish to t~ank Durai N. Raghavan ahd the staff of the Berkeley Lawrence Radiation Laboratory 88 11 Cyclotron for their help with the irradiation experiments. This work was carried out under the auspices of the United States Atomic Energy Commission through the InQrganic Materials Research Division of the Lawrence Radiation Laboratory. -7-UCRL-17347 -Rev. REFERENCES 1. E. W. MUller, Advances in Electronics and Electron Physics (1960). 2. E. W. MUller, Proc. 4th Intern. Syrnp. Reactivity of Solids, Amsterdam (1960), Ed. Boer (Elsevier Pub. Co., New York, 1960). 3. D. Go Brandon and M. Wald, Phil. Mag. ~ 1035 (1961). 4. M. K. Sinha and E. W. MUller, J. Appl. Phys. ~ 4, 1256 (1964). 5. E. W. MUller, S. Makamura, S. B. McLane, and O. Wishikawa, Trans. 3rd Intern. Vac. Congr. 1965, Vol. 2, (:Perg~on Press" New Yo*k, '1966)? p. 431& 6. M. J. A. Bardo, J. M. Galligan and J. Sadofski, J. Sci. Inst. ~ 607 (1966). 7. E. W. ~uller, Sur. Sci., ~ 484-494 (1964). Beam a Specimen -8-Liqu i d nitrogen I-i--++--C u,u*,---H- 0 B Imaging gas .--------. / To cold trap and oil diffusion pump vacuum system UCRL-1731.n-Rev . Vac ion pump XBL675 - 3060 Fi g . 1 Schemat ic d iagram of microscope chamber and experime ntal se t up. Alumina block - 9- UCRL-17341-Rev. L-.--+J----el ~~s..1L===:JLcopper block High voltage line Specimen Opening for radiation beam XBL675-3059 Fig . 2 Detail ed view of specimen holder and hi gh vol tage lead. (Cros s section thr ough plane ~ shown in Fig . 1). Fig. 3 -10- UCRL-17347-Rev. XBB 674-1897 Iridium single crystal (a) at 77°K) (b ) same surface at 5°K. (Picture taken with the same imaging voltage a nd imaging gas pressure as Fig . 3a). -11- UCRL-17347-Rev. XBB 674-2080 Fig. 4 Iridium single crystal before and after "field off 'I irradiation with 10 MeV protons at a temperature below lOoK. (a) before, (b) after irradiation. Dotted atoms in (a) have been ejected from the surface during irradiation and are missing in (b). Extensively damaged regions are indicated by arrows. Dotted atoms in (b ) are surface interstitials and are missing in (a). Fig. 5 -12- UCRL-17347 -Rev. XBB 674-2081 Iridium. bicrystal before (a) and after (b) IIfield on" irradiation at li~uid nitrogen temperature. Missing atoms, damaged regions and interstitials identified as in Fig. 4. This report was prepared as an account of Government sponsored work. Neither the United States, nor the Com-mlSSlon, nor an y person acting on behalf of the Commission: A. Make s any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, appa-ratus, method, or process disclosed in this report may not infringe privately owned rights; or 8. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any infor-mation, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any employee or contractor of the Com-mission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor. 

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LIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONS

A LIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONS

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A LIQUID HELIUM FIELD-ION MICROSCOPE FOR RADIATION DAMAGE INVESTIGATIONS

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