A compact helium monitor

InrcrnaritmaZ Jatmtal vf Mass Spcctrumcrr_~and lutt Physics. Ii ( 1975) 31 I -317 ..Q Ekvier ScientificPublishing Company, Amsterdam - Printed in The Ntttheriands

A COMPACT

HELIUM

MONITOR

D_ L_ SWIXGLER Dirisiatt of Chemical Physics, CSIRO. P-0. l?tw I60, Chtyton. Vicforiu. 3168 (AttsfraIitt) (Rcccivcd

3 January 1975)

A miniature dimensions 44 mm weighs 23 kg The up to an operating Ien@h

mass spectrometer is described. The analyser structure has x 28 mm x 12 mm and, with the vacuum chamber and magnet, sensitivity to helium is 1 - IO-* A torr- ’ and it is maintained pressure of 5 - IO-* torr- A combination of a short ion path

and an ekzctrostatic energy selector reduces the backsound

current to a

ne&ible level_ The compact arrangement is achieved by the use of an ion source in which the ions traverse trochoidal paths, and an ion-enerzy-sensitive collector at the faces of a 90= magnetic prism.

increasing

use is being made of very smail mass spectrometers

permanently

attached to high-vacuum systems for leak detection purposes. The most important features desired for such mass spectrometers are: (1) light weight and small size; (2) operation up to a relatively high pressure - 1 - IO- 3 torr (0.1333 Pa); and (3) the detection sensitivity should be a maximum for helium and a minimum for all other gases High-pressure operation requires that the analyser path length should be short and that the ion energy be as high as possible in order that the loss from the ion beam by scattering is minimized_ In the most commonIy employed analyser forms, ion anaIysis occurs over 60-, 90” or 180” magnetic paths_ In each of these forms, about the same magnetic flux and weigbt of magnet are required for instruments of similar size, as shown in Fig- I_ The fargest radius of curvature and the highest ion energy then occur for the 180” case_ It should be noted, however, that for a given magnet pole area the largest radius of curvature occurs when the sector forms are used, but an extended instrument results. Very small instruments employing sector fields are subject to severe aberration problems [I 1.

C-OktPAcf

MOSKOR

In the compact monigor described in this paper an attempt has been made to obtain the srndlest possible instrument with the largest possible radius of cuwature_ This has been achieved by employing an important property possessed by the 90’ maeecic prism, namely, that for ions incident at one face and detected at the other, this form of prism yields a smaller image than that from any other form The oniy strict requirement is that the incident ion beam is non-divergentThis property of the 90’ maeetic prism is illustrated in Fie 2 Ions injected at A, at right angks to the face M, describe paths with constant radius R, as do ions inJected at A,_ The image width at face MB is then 2 = &(+._~=)Q where zdis the entranaz beam width_ For ions injected at a s;ralI angIe fz+ the aberration in the exit pIant is 2&c_ For the optimum ion transmission in the compact monitor the image width is made just Iess than the mass dispersion at mass 4_

Fig- 2 1~ rormaion in a 90’ magnetic prism- This image has minimum dimensions for an incidat beam trrfiidr is normal to face &is and monochromatic in energy-

323 IOX SOURCE

The use of this property has required the development of a broad aperture, non-divergent ion source- In the conventional ion source, ions formed in a smali volume are *passed through a narrow aperture so that they diverse as they enter the magnetic analyser region of the mass spectrometer_ A source of this type, however, cannot be used in the compact monitor where the ion divergence would produce an unacceptable aberration in the exit plane. The source in the compact monitor uses an electrostatic field produced by penetration of the field from an adjacent focusing electrode, and part of the magnetic flux provided by the main anaI_vser magnet_ The field in the ion source was estimated using a conducting-sheet field plotter and the equipotentials are shown in Fig. 3_ An approximate representation was made by assuming that the 0.5 “/;;equipotential curve was made up of four straight lines and that uniform fields existed at right angles to these lines. The four field strengths chosen were O_16,0_50,0_$3 and I _16 V mm- ’ and computer calculations of trajectories were made [2] for each with injection an&s of OXI, 0.4I-I. 0_6ff and O_Sfi radians to each straight line. These trajectories are shown in Fig. 3 for an ion energy uxresponding to 500 K (0.05 eV). These trajectories are continued smoothly through the mesh of the focus electrode to join on to arcs of radius 1445 (VM)*!B cm where M = 4 u, B = 0.25 T, and V is the acceieraring potential in vok The ion-beam divergence is criticalI_v controlied by the focusing-electrode potential, which sharply sets the performance of the instrument by arranging the

Fig- 3. A diagmm of the ion source of the compatct monitor showing tion and the trochafdd ion trzsjectaria Y far as the source aperture_

the equipotmtial

distribu-

cusp I~tions

so that a p&mlfel beam cmeqw from the source aperture_ Experiment showed that the most intense hcam was slightly diucr@ng and IXCUISCof the fringin_e magnetic field the ion courcc was set back a further 12” from the 90” entry condition_ It should bc noted that the ckctrostutic pattern shown in Fi_r- 3 is only correct for the central a&l plane of the sourcc~ Ionr formcci off this plane arc ;mccdcraLcd in:o complex trajectories but are contained and cventudly reach the source aperture_ Further. space charge broadens the cusp locations shown on the equipotenthl zarface of Fie 3_ In spite of tha;c effects, this ion source is highly scnsitin: with cfficicnt ion cxtrxtionFor txample, the calculated ion current for nitrogen at 10’ ’ torr with an electron path length of I cm. an ionization crosssection of 2-5 - 10’ Ib cm- ’ and an ckctron current of 5 - 10e4 A is 4-J - 10BS A, while the current measured on an ;luGliury cfcctrodc pkcd just outside the source

was L - IU - fi pwrespoIl~lng whole instrument

the sensitivity

10 a scnsrrivrly 01 L -

LO mass-sekcLed

helium

IV

_ A LOIT

ions is I

‘)_

L-or

the

- 10e4 A torr- I_

The anrti>ser strucLurc shown sch~mtlticalfy in Fis 4 consisls of a flangemounted stainkxteel arm which carries the ion collector xzmbl_v, an additional

Fis_ 4- sctKsn;ltic sxmbiy

for eht helium mcnitor.

Fig_ 5_ The ;m;llyser SWJCIU~C of the compz~ct helium moniror-

arm for the ion source7 and a beam-Iimiting stop_ Both the collector and source members are electrically insulated and positioned by ruby baIIs in conjunction with appropriate sandwiching plates_ These are the focus electrode and the ion repeIIer plate_ The filament button is formed by fusion of &ES to tungsten posts which support the O-l-mm thoriated tunpten fiIament. The analyser magnet is an alnico-V. c-form permanent magnet with pole faces 3 cm’, and it produces a magnetic intensity of OXT_ Figure 5 is a photograph of the anaIyser structure and the complete head is shown in Fig. 6_

BACKGROUXD

REDLXI-IOS

When a simple ion-collector plate is used in the monitor, a backsround current is detected which cannot be defocused by variation in the ion energy- This current is typically 5 - IO- ‘a A. equivalent to a helium pressure of 5 - IO-’ ’ torr at a system pressure of 5 - IO-” torr. BacQround reduction proves to be a most important feature in the monitor design and some insight into the origins and nature of the unwanted ions is required for it to be effkctive!y controlled.

326

- ; ,_..

‘i

-.

.

Fig- 6- A photogaph of ~hc compact aurn cIumk 2nd magna_

The backgound satisfy the condition A&t&

=

rhc

helium moniror

arises when ions of apparent

mass AfN

and eneqy

VN

Af,,&‘s

where bft, is the mass of the helium ion and V, the ion-source potential_ With this ions which have their velocities directed so that they condition satisfied, unwanted can traverse the same analyar path as the helium ions at-e detected, the only distin_euishing future of the unwanted ions being their reduced energy_ Processes @ding to the formation of ions with modified mass and energy include: (I ) unimokcuhtr decompositions (metastable transition) [3], and (2) coliision-induced dissociations and chare exchange [4]_ in the helium monitor, two methods arc employed for background reduction. The ion source is essentially ttuned for helium ions, the cusp locations for other ions not being suitable for an lemerge_ntpanIle beam. In addition, an enery-filter grid ekctrode carrying a potential of OX V, is positioned between earthed plates immediately before the ion collector, In this way the background ion current is reduced below IO- I6 A at a system pressure of 5 - IO-* torr_

327 ACRSOWLEDGEMENT!S

The author wishes valuable discussions-

1 2 3 4

to thank

Drs. A. J. C. Nicholson

and V. W. M&en

D. L. Swingkr. Yucrtrrm.22 (1972) 359. D_ L. Swingkr. Vizrurwr. 21 (1971) IX_ J_ A_ Hippk, R_ E Fox and E V_ Condon. P&rs_ tier_. 69 (1946) X7_ L_ Kmrin and W. hicGowm. futzizariun Phrtzumctzuin Gusts, Vol. I, Nonh-Holland, dam, 1962, p- 33-

for

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