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Advanced Vacuum Systems for Analytical Electron Microscopy Scanning Microscopy Volume 1 Number 3 Article 7 6-27-1987 Advanced Vacuum Systems for Analytical Electron Microscopy G. S. Venuti Venuti Associates Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Life Sciences Commons Recommended Citation Venuti, G. S. (1987) "Advanced Vacuum Systems for Analytical Electron Microscopy," Scanning Microscopy: Vol. 1 : No. 3 , Article 7. Available at: https://digitalcommons.usu.edu/microscopy/vol1/iss3/7 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. Scanning Microscopy, Vol. 1, No. 3, 1987 (Pages 939-942) 0891-7035/87$3.00+.00 Scanning Microscopy International, Chicago (AMF O'Hare), IL 60666 USA ADVANCED VACUUM SYSTEMS FOR ANALYTICAL ELECTRON MICROSCOPY G.S. Venuti Venuti Associates P.O.Box 29265 San Diego, CA 92129 Phone No.: 619 484 7182 (Received for publication March 27, 1987, and in revised form June 27, 1987) Abstract Introduction Recent technological advancements such as A high-energy beam of electrons used for significantly improved power supply stability and electron microscopy and microanalysis can have polepiece design, as well as increased accelerating significant impact on the specimen. Beam damage is voltage all contribute to the primary objective of the usually more severe in organic and biological speci­ scanning electron microscope (SEM): higher resolu ­ mens ( 8); it has been reported by numerous investi­ tion. Similarly, the advent of analytical electron gators that this damage ranges from loss of organic microscopy (AE M) has also expanded the scope of material (6), and removal of volatile elements to applications to include energy-dispersive spectro ­ significant hydrocarbon contamination (2,10) and metry, wavelength-dispersive spectrometry, and etching. Hydrocarbon contamination can, by absorb­ electron probe analysis. Specimen contamination and ing radiation, decrease X-ray intensities , thereby damage has become increasingly problematic as SEMs degrading analysis. This effect is much more notice ­ are used as analytical instruments. In an effort to able during light element analysis . minimize contamination and etching, and thereby While specimen cooling (4,7,12) has been shown benefit from these advances, cleaner and more to reduce mass loss considerably , specimen cooling efficient vacuum systems are required. Such benefits itself can create problems. If the microscope's were realized when, for the first time, a Cameca vacuum system is not generally free of hydrocarbon microprobe's diffusion pump was replaced by a low contaminates, the cooled specimen can then act as a vibration cryogenic vacuum pump, resulting in faster contamination trap, causing further complications. It pump down and lower ultimate vacuum pressures. is advantageous , therefore , to not only reduce the pressure (14) in the spe c imen c hamber but also to insure that the vacuum system itself is not a source of contamination. This paper describes the achievement of excellent vacuum levels, high pumping speeds, reduction of the water vapor partial pressure (11) and, most importantly, the total elimination of vacuum system - induced contamination (1) through the replacement of the original vacuum pump with an Air Products Low Vibration Cryopump. Original Vacuum System A Cameca Camebax - MBX combined scanning electron microscope (SEM) and microprobe, consisting of three Cameca wave - length dispersive (WDS) spec­ trometers and a single energy-dispersive spectrometer (EDS), was chosen for modification. The SEM (see Figure 1) had been designed and installed with a fully automated vacuum system which included all the necessary safety and control devices. The vacuum system consisted of a 700 liter/sec diffusion pump, a Key Words: Cryogenic vacuum pump, high -vacuum water-cooled baffle, a liquid nitrogen trap, a vacuum tee hnology, contamination reduction, analytical ballast tank and two 12m3 /h mechanical vacuum electron microscopy, vibration isolation, ultimate pumps. vacuum pressure. The elimination of all vacuum system-induced contamination, meaningful pressure reductions and a substantial lowering of the present partial pressure of water vapor would significantly improve the microprobe's analytical capabilities. A table of the advantages and the disadvantages of major vacuum 939 G.S. Venuti systems is shown in Figure 2. Of these systems, Vacuum System Testing and Modification only the cryopump is totally contaminant free; it also reduces substantially the water vapor partial pressure Prior to the modification, the diffusion pump (6) because of its high intrinsic pumping speed for system was evaluated. Testing determined that the water vapor, approaching the theoretical limit. existing primary vacuum gauge, chosen by Camec a and located directly on top of the pumping stack, was correctly indicating and recording specimen SAMPLE AIRLOCK SPECTROMETERS7 chamber pressures. The maximum obtainable vacuum levels using the diffusion pump were evaluated and are shown in Figure 3, which shows baseline pressure readings at the primary vacuum gauge over several days of continuous pumping with and without LN2 in the primary trap. The final series of tests involved pump down cycles from atmosphere to high vacuum, obtaining normal operating conditions of 5x10-6 torr with and without LN2 in the primary trap. These test results are shown in Figure 4. 760 a: ~ 1 o-3 I- ROUGHING ~- 10-4 PUMP 1 ::::, (/) WITHOUT LN2 fil10 -s fL_ a: ....._-....._ - Fig. 1. Original Cameca SEM a. WITH LN2 .._, - _._ - -- _ -- __ 1 o-s .__...,___ _._,_ __.__ .,___...L.._ _,L _ __,j Low Vibration Cryopumps Turbomolecular Pumps 10 20 30 40 50 60 70 TIME, HRS. Advantages Advantages - Requires no continuou s fore­ - No LN2 req u ired Fig. 3. Diffusion Pump Vacuum Levels pumping - Fas! initial s tartup - No LN2 req uired - Hori zontal o r vertica l inst allation - Highest pumping speeds for all gases Disadvantages - Contamina tion fre e - High throughput - Lowe r pum p ing speeds than cryopump fror waler and - Pumps dangerous ga ses 760 - No moving part s exposed lo hydrocarbo n s vacu um chamb er - Potential for oil backslreaming from - Field maintainab le rough ing pump or bearings 100 - Cas i co mparabl e with lurbo pump - Requires contin uou s forepumping - Low maintenance intervals - Freq uent bearing maint enan ce (10 ,000 hr ) - Cas i comparable wi th cryopump - Potential self -destruction ii parti c le 10 Disadvantages falls into tu r bin e blades - Vibration mu st be consid ered - Regene ration requ ired - Splinter shield recommend ed bul - Long er initial slarlup limes does decrease nel pumping speed s a: 1 - Vertical installation only - Not co ntam 1ination free a: 0 Diffusion Pumps Ion Pumps 1-_10 -1 w a: Advantages Advantages ::::, - Low initial cos t - Requires no co ntinu ous fore ~10 -2 I - No vibration pumping w - No LN2 requir ed a: I Disadvantages - No vibratio r1 Cl. 1 o-3 - Contamina rnon free I - Requ ires LN2 lo prevent I contamination and to pump Disadvanta ·ges condensables 10-4 I - Restricted lo vertical mounting - Low thr oughput - Requires con tinuous forepumping - Gas re-emission, memory effects I - Lower pumping speeds lhan - High cos! WITH LN 10-5 l _ --. 2 cryopump - Difficult lo start ....._ - Potential for catastrophic failure - Strong mag ,netic field --­ - Oil creep problem can be - Electri cal isolation required for ---------◄ significant some appli c ation s 1 o-s---~-~---'---'----'--~---' - Nol contamination free - Extremely h,eavy 0 2 4 6 8 10 12 14 - Nol field ma,intainable MINUTES Fig. 2. Pumping Systems Connparison Fig. 4. Initial Diffusion Pump Vacuum Levels 940 Advanced Vacuum Systems for AEM After the entire microprobe was cleaned using section of soft tissue. In: Microbeam Analysis in standard ultra-high vacuum procedures (3,5,9), a low Biology, C Lechene, R Warner (eds.), Academic Press, vibration cryopump, with a pumping speed of 680 New York, 185-202. liter/sec for N2, was installed in place of the diffusion pump. In replacing the diffusion pump with the cyropump, the LN2 (15) trap and the water baffle were no longer required, as these items were used on the original vacuum system to prevent oil back streaming from the diffusion pump. It is important to note that their use in preventing backstreaming had the adverse effect of reducing the diffusion pump's pumping speed by approximately 40 %• Figure 5 shows the modified system. Vacuum System Data The performance of the modified vacuum system is shown in Figure 6. This graph compares an average pump down cycle of the diffusion pump system with that of the low vibration cryopump. After 70 h, the final recorded vacuum levels were 5 x 10-6 for the diffusion pump, with the LN2 baffle and water trap, and 1 x 10-7 for the cryopump. Though clearly the superior vacuum source, the cryopump was not availab le for use on vibration­ ,HELIUM SUPPLY sensitive equipment such as the microprobe until the l - ~,-HELIUMI RETURN development of the Air Products Low Vibration r-1-1-JCRYOPUMP Cryopump. Its successful use on SEMs is well L_j COMPRESSOR documented ( 13). In this installation and numerous MECHANICAL others, including dual pumped transmission electron VACUUM microscopes, no detectable vibrational interference PUMP has been recorded using this c ryopump. Fig. 5. Modified Cameca SEM Conclusions Successful installations of the Low Vibration Cryopump in the disciplines of electron beam micros­ 760 copy and microlithography now include Hitachi SEM Models S570 and S800, and TEM Models H600 and 100 S800. JEOL installations include TEM Models lOOCX, 200CX, and 1200EX, and SEM Model 35C. ISi Micro­ scopes SE M installations include Models S S-40 and 10 IC-130. Microlithography, Ion Beam, and X-ray Litho­ graphy all use and benefit from cryopumping.
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