HighSpeed Nonrefrigerated Isolation Traps for Ultra HighVacuum SystemsManfred A. Biondi Citation: Review of Scientific Instruments 30, 831 (1959); doi: 10.1063/1.1716767 View online: http://dx.doi.org/10.1063/1.1716767 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/30/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in High−vacuum trapping experiments on the astron Phys. Fluids 18, 96 (1975); 10.1063/1.861000 Barium Absorption Pumps for HighVacuum Systems Rev. Sci. Instrum. 28, 889 (1957); 10.1063/1.1715756 HighSpeed FlexibleBlade Stirrer and Stirrer Seal for HighVacuum Use Rev. Sci. Instrum. 27, 1080 (1956); 10.1063/1.1715460 HighVacuum Jet System for Condensation Pumps Rev. Sci. Instrum. 26, 303 (1955); 10.1063/1.1771285 HIGHSPEED HIGHVACUUM DIFFUSION PUMPS Rev. Sci. Instrum. 3, 482 (1932); 10.1063/1.1748965

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NOTES 831

QSPARK GAP

'\ , , PARABOLIC ~ : \ MIRROR I ' I '~I \

WINDOWS

I' I -! • I ", I +, \ ,I I, ,

" I " ~' " t' ~ PLANE MIRROR I

SHOCK TUBE

POLAROID \-. - ~ - - - : BEAM SPLITTER FILM I I

_. 45!-j I I

• rl '5

CONVENTIONAL FILM

FIG. 1. Schematic diagram of the improved spark shadowgraph system.

Fig. 1, which allows simultaneous records to be obtained on conventional and Polaroid films. The use of this tech­nique has resulted in a significant saving in experimental time, as well as in an over-all improvement in the quality of the results.

* This work was carried out under Defence Research Board Project No. D89-16-01-09. Permission of the Defence Research Board to publish this note is gratefully acknowledged.

High-Speed N onrefrigerated Isolation Traps for Ultra High-Vacuum Systems*

MANFRED A. BIONDI

Westinghouse Research Laboratories, Pittsburgh 35, Pennsylvania

(Received June 3, 1959)

T HE isolation of an ultrahigh vacuum system from the backstreaming products of the diffusion and fore

pumps has been accomplished, in general, either by the use of liquid nitrogen cooled (cold) traps1 or unrefrigerated traps consisting of rolls of corrugated copper foil.2.3 While properly designed cold traps often remain effective so long as they are continuously refrigerated, the copper foil traps must be periodically reprocessed by bakeout in order to restore their effectiveness. For example, one of the con­ventional 6-in. long copper traps2 (conductance ",0.4 liter/sec) used with a small glass vacuum system2 ,4 is effective for approximately three weeks following its acti­vation by high-temperature bakeout. Attempts to scale this trap to higher conductances for use with larger metal diffusion pumps (e.g., Consolidated Electrodynamics Cor­poration MCF-300, having a pumping speed of 300 liter/sec) have failed in that the trap provides isolation

for less than a day, after which the pressure in the ultra­high-vacuum system rises from the 10-10 mm Hg region to the 10-7 mm Hg region. The present note describes a new type of nonrefrigerated isolation trap which provides a much higher conductance and also remains effective for much longer periods of time than the copper trap.

The mechansim of operation of the unrefrigerated traps is incompletely understood; however, studies of the opera­tion of copper traps have been carried out by Lange5 and by Carmichael and Lange.6 It appears that one needs an active material which holds the backstreaming impurities when they collide with the surface7 and prevents migration along the surface of the trap.

The artificial zeolite and activated alumina appeared to be promising materials8 for a nonrefrigerated trap, since their porous structure should provide a very long path to retard the migration of impurities and they might hold impurity molecules on contact. They are suitable for ultrahigh vacuum purposes since they are reported to be stable structures at elevated temperatures (",600°C for zeolite, > lO00°C for alumina), permitting high-tempera­ture bakeout. Also, it has been found that, following bake­out, the degassing rate from these materials is small, in spite of their large internal volume.

Figure 1 (a) illustrates a trap design suitable for small laboratory ultrahigh vacuum systems4 (volume'" 1 liter, internal surface""' 104 cm2, conductance of leads ",0.3 liter/ sec per meter of length). Such low-speed systems are often evacuated by multistage glass or metal oil diffusion pumps having pumping speeds of the order of 10 liter/sec. The dimensions are chosen to give the trap substantially

-to II=====~ -to ultrahigh vacuum system

012 ~

em scale

C.E.C. 2 stage

glass diffusion

pump

(a)

pumps

artificial zeolite

or activated alumina

r--------------, I I I I

I I I I I I I

Bayard - Alpert I ionization I

gauge I I I L ________________ J

bakeout oven

(b)

FIG. 1. (a) Nonrefrigerated isolation trap for small, laboratory ultrahigh vacuum systems. (b) Schematic diagram of the vacuum system used to test the isolation traps. The area inside the dotted line was baked at 450°C before each test.

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832 NOTES

100

FIG. 2. P.erformance of an 8-in. diameter, nonrefrigerated trap over a 300 hter/sec speed C.E.C. MCF-300 oil diffusion pump with water baffle.

greater conductance than the tubing leading to and from the trap (in this case l4-mm diam Pyrex). The lower part of the trap and the tray are loaded with i-in. diam artificial zeolite or activated alumina pellets.

The trap was tested using the experimental arrangement shown in Fig. 1 (b). The system was baked for 8 hr at 450°C. The ion gauge was outgassed following bakeout and then operated continuously at lO-ma filament emission. With zeolite the pressure in the gauge fell to ,,-,3XlO-10

mm Hg approximately one day later9 and remained at this value for 75 days (the duration of the test). With alumina the pressure fell to <lXHr10 mm Hg (the x-ray limit of the ion gauge) and remained there for the duration of the test (100 days). A 6-in. long copper trap,3 when tested in this fashion, remains effective for an average of "-'20 days, following which the pressure in the ion gauge rises to "-' 10-7 mm Hg in a few days.

A major advantage of these new nonrefrigerated traps is that they provide effective isolation of large ultrahigh­vacuum systems from backstreaming products of large metal pumps. An 8-in. diam trap consisting of zeolite loaded trays and baffles (see inset of Fig. 2) was mounted directly above a C.E.C. MCF-300 diffusion pump with water-cooled baffle. Following bakeout of the trap and system at 430°C for 8 hr, the pressure on the ultrahigh vacuum system side of the trap followed the course shown in Fig. 2. The scatter in the data points is the result of room temperature fluctuations from day to day. The large conductance trap maintained the pressure below 10-9

mm Hg for approximately 70 days, in contrast to large copper foil traps which remained effective for less than one day.

Thus, the present nonrefrigerated traps offer advantages over the copper foil traps in that they provide a much

larger pumping conductance together with effective isola­tion for very much longer periods of time.

The author wishes to thank W. J. Lange and J. H. Carmichael for their interest and helpful discussions of this work.

* This work was supported by the U. S. Atomic Energy Commis­sion contract AT(30-1)-2176.

1 See, for example, S. Dushman, Scientific Foundations of Vacuum Technique (John Wiley & Sons, Inc., New York, 1949).

2 D. Alpert, Rev. Sci. Instr. 24, 1004 (1953). 3 J. H. Carmichael and D. Alpert (to be published). 4 These systems consist largely of glass tubing, all-metal valves and

pressure guages, etc. They attain ~ressures in the range 10-10_10 ..... mm Hg following bakeout at ,....,450 C for 8 hr. The pressure of back­streaming products on the pump side of the trap is of the order of 10-7 mm Hg.

5 W. J. Lange, "Ultrahigh vacuum techniques," Westinghouse Re­search Report 403FF220-R1, (unpublished).

6 J. H. Carmichael and W. J. Lange, Transactions of the Fifth National Symposium on Vacuum Technology (to be published).

7 The phenomenon does not seem to be simple adsorption. Studies by Lange (private communication), indicate that the surface of the copper trap holds what would normally amount to very many monolayers.

8 Zeolite is an alkali metal aluminosilicate. It is similar to natural clay, and when the water of hydration is removed, the remaining porous structure exhibits sufficiently low vapor pressure for ultrahigh­vacuum applications. The artificial zeolite used in this trap is the 13X type manufactured by the Linde Company. The activated alumina provides a similar structure and is manufactured by the Aluminum Company of America.

9 Tests were also carried out on a similar trap on another vacuum system. Leaks required reprocessing by bakeout three times. In this case the pressure fell to less than 1 X 10-10 mm Hg (the x-ray limit of the gauge) shortly after the final bakeout.

Improved Method of Etching by Ion Bombardment*

T. K. BIERLEIN AND B. MASTEL

General Electric Company, Hanford Atomic Products Operation, Richland, Washington

(Received May 4, 1959)

ION bombardment as a method for etching metals, cermets, and refractories has been shown to produce

excellent surfaces for both optical and electron microscope examinations,I-B A modification of the basic etching pro­cedure, namely, the incorporation of VHF excitation to increase ionization and permit etching of specimens at pressures as low as 1 }J. Hg, has led to the development of an improved etching procedure for routine laboratory use.

Etching by ion bombardment involves selective sputter­ing of atoms from the surface of a cathodic specimen. Normally, sputtering or etching is relatively slow, but it can be accelerated by the use of high ion current densities achieved by increasing the voltage, increasing the gas pressure, or by reducing the area of the specimen and cathode with insulating shields. As indicated by Penning and Moubis, the incorporation of a magnetic field in the chamber will increase the path of the electrons and con-

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