A digitally controlled scanning microprobe for protons and heavy ions

NUCLEAR

INSTRUMENTS

AND

METHODS

157 ( 1 9 7 8 )

55-63

;

(~)

NORTH-HOLLAND

PUBLISHING

CO.

A DIGITALLY CONTROLLED SCANNING MICROPROBE FOR PROTONS AND HEAVY IONS G. BONANI, M. SUTER ~, H. JUNG, Ch. S T O L L E R and W. WI3LFLI

Laboratoriurn .fiir Kernphysik, Eidg. Technische Hochschule, H6nggerberg, 8093 Ziirich, Switzerland Received 27 J u n e 1978 A microprobe facility has been installed at t h e E T H - E N T a n d e m accelerator w h i c h is capable to focus protons and heavy ions with a m a s s - e n e r g y product P_< 70 a m u . M e V / Z 2 to a spot size of 1 0 - 3 0 / z m 2. By m e a n s of a digitally controlled electrostatic s c a n n i n g s y s t e m a proton b e a m o f 3 MeV can be deflected in c o m p u t e r controlled patterns over a target area of 1.5 x 1.5 m m 2 . T h e s y s t e m can be u s e d either for highly sensitive trace e l e m e n t analysis or for the deep implantation of heavy ions.

1. Introduction The detection limit of elements which can be analysed with proton induced X-ray emission (PIXE) is smaller by two to three orders of magnitude in comparison with that of electron induced X-ray emission. In the last few years this method has been developed to a standard procedure in trace element analysis at the ppm-level~). It has been demonstrated that the PIXE method is especially well suited to analyze samples weighing 1 ltg or less or to determine the local distribution of the trace elements in selected areas of a sample. For such investigations a finely focussed proton beam and an appropriate scanning system is needed. In several papers, it has been shown, that a proton beam with an energy of a few MeV can be focussed into a spot size of a few microns by magnetic quadrupole lenses2-6). Up to now mainly mechanically driven scanning systems have been used. In this paper a microprobe for protons and heavy ions with a digitally controlled electrostatic scanning system is described. The system is installed at the ETH EN-Tandem accelerator and consists of two conventional magnetic quadrupole doublet lenses and two pairs of electrostatic deflection plates placed between the second doublet and the target. With this system protons and heavy ions having a mass-energy product up to 70 a m u . M e V / Z 2 (Z=degree of ionisation) can be focussed to a spot size of 10-30/zm and deflected over an area of 1.5 x 1.5 mm 2. The performance of the computer controlled scanning and data handling system is illustrated by a few examples. Present address: Oak Ridge National Lab., Post Office Box )~i, Oak Ridge, T e n n e s s e e 37830, U.S.A.

2. Experimental arrangement The microprobe is placed on the 90°-beam line of the Tandem accelerator. The positions of the two quadrupole doublets and the beam profiles (defined by the emittance of the accelerator and the acceptance of the lens system) are shown in fig. 1. As the divergence of the beam is larger in the X- than in the Y-direction, the polarity of the first singlet was chosen to provide the first focussing in the horizontal plane. The object aperture (MBL) is placed directly behind the stabilizing slits (SS) of the Tandem behind the 90° analyzing magnet. It consists of two pairs of highly polished crossed tungsten bars providing a square opening of 1 mm side length. This aperture can be set (and removed) pneumatically by remote control. An intermediate image of this aperture is produced by the first quadrupole doublet at the position BV1. It is reduced by the second doublet at the position of the target to a square image whose side length is determined by the dimension of the square aperture MBL and the total magnification of the two lenses. The main features of our system have been calculated by means of the program TRANSPORTT). The results are listed in table 1. The total magnification is 0.01 in both directions. Therefore, with an aperture of 1 mm side length, a spot size of 1 0 x l 0 / z m 2 is expected. The properties of some other systems have been calculated by Heck 5) for a final spot size of 3 x 3/zm 2 and a maximum image aberration of less than 1/zm. These results as well as the corresponding values of our system (2nd lens only), are also given in table 1. The large linear dimension of our system causes some problems. For instance a proton beam of 3 MeV is deflected by the earth's magnetic field

56

G. B O N A N I

et al.

~

X(mm)

e

/

Lens 1

c

ns 2

III

z

ot

---...d 1

Y(mm) I 0

' I

' 2

3'

-Z(m)

Fig. 1. Schematic view of the microbeam facility. The two X, Y beam profiles shown to the right of BV1 are those defined by the emittance of the accelerator (outer profile) and by the acceptance of the 2nd quadrupole doublet tens (inner profile). (BPM = beam scanner, SS = stabilizing slits, MBL = object aperture, SB = moveable high precision horizontal and vertical apertures, BV = beam viewer).

TABLE 1 Lens systems calculated for 3 MeV protons, x o , (Y0) = one-half of the horizontal (vertical) beam dimension (at the indicated location); % , ~ 0 ) = one-half of the horizontal (vertical) beam divergence; / actual length of pole face; s = actual separation between quadrupole singlets; /31, f12, f13, /34: quadruplet parameters indicating relative magnetic field strengths and directions of the quadrupole singlets. For comparison of the different systems, the same final spot size and image aberrations <_ 1 ~ m are assumed (lower part). System

Distance object (cm)

Distance image (cm)

Magnification factors Mx My

Max. acceptance parameters +-Xo -+Yo -+ % ±/30 (#m) (mrad)

Phace space volume/4 n ( a m 2 mrad 2)

Image size ~ m 2)

ETH 1st doublet l 25.4 cm s - 12.4 cm ETH 2nd doublet

607

42

0.29

0.044

500

500

2.900

1.500

1.1 × 106

l = 23.5 cm s = 23.2 cm

877.7

48.3

0.035

0.227

145

22

0.514

1.628

2670

877.7

48.3

0.035

0.227

43

6.6

0.240

0.427

29

3× 3

266

15.4

0.434

0.034

3.46

43.6

1.17

0.47

83

3×3

169

10

0.21

0.038

7

40

0.70

0.39

76

3×3

350

21

0.18

0.18

8.5

0.62

0.4

18

3×3

10× 10

ETH 2nd doublet only 1 - 23.5 cm s = 23.2 cm Karlsruhe doublet 5) / = 10 cm s = 28 cm Heidelberg doublet 3) / - 4 cm s-4cm Harwell quadruplet 2)

/31-- --f14' --/32=/~3 / = 18.05 cm s - 4.2 cm

8.5

D I G I T A L L Y CONTROLLED

by almost 2 cm along the path from the objective aperture to the target. To prevent this it is necessary not only to shield the beam line magnetically, but also to add correction coils in order to bring the beam as accurately as possible onto the magnetic axis of both lenses. In our case this is accomplished with three Helmholtz coils, one mounted immediately after the object aperture and the others between the two lenses. To control the position of the beam, moveable high precision horizontal and vertical apertures (SB1 and SB4) are arranged in front of each lens, which permit the measurement of the difference of the beam currents impinging on opposite sides of each aperture. For this adjustment highly sensitive differential amplifiers are usedS). The beam spot on the target can be deflected in the X- and Y-directions by two pairs of electrostatic deflection plates. The Y-plates are placed 25 mm behind the second lens and have a length of 112 mm, a width of 20 mm and are 10 mm apart. After a separation of 15 mm the X-plates follow which are dimensioned such (length: 191 mm, width: 20ram, separation: 10 mm) that at the position of the target the deflection is equal in both direction for the same deflection voltage. In order to reduce multiple scattering of the ions along the long flight path, which is responsible for a halo around the beam focus, a good vacuum is required. This is achieved by means of 5 turbo-molecular pumps and several liquid nitrogen cooling traps placed along the beam line. In addition, the last part of the beam tube and the vacuum chamber are manufactured from stainless steel material and, whenever possible, provided with metallic seals. Up to SB1 (see fig. 1) the vacuum is better than 10 6 and, between SB1 and the target, better than 3 × 10 7 mbar. The vacuum chamber allows the placement of X-ray and electron detectors at angles of 90 ° , as well as at 135 ° with respect to the beam axis. The latter position is especially favorable for the trace element analysis, because secondary electron bremsstrahlung, which essentially determines the concentration sensitivity of the PIXE method9), is about a factor 2 smaller under this angle than at 90 °. An optical microscope can be added which allows the observation of the beam spot on a quartz plate placed at the target position. The digitally controlled deflection system, interfaced with a computerized data handling system described in the next section, simplifies consider-

SCANNING MICROPROBE

57

ably the study of the properties of the microprobe. For instance, to set up the microprobe an electroplated copper grid with a known mesh width is scanned and the lenses are adjusted until the mesh is reproduced in the X-ray emission image. In addition, a quantitative determination of the resolution is usually made by means of a linear scan perpendicular across a sharp, highly polished copper edge. The Cu-K~ intensity distribution from such a scan in the horizontal plane is displayed in fig. 2. The resolution is obtained by dif106

I

I

I

I

I

Nx

105

'oeeooo*ooooooe$$go oo o e

104

o

d

8

~T 103

o

•

,,.r LIJ

F~=3" 10-6m bar

..../....'.;.......~

e°OOoooOO o OOoOOOOOO~

(i. 102 F-Z CO

P=3"IO m bor

o i01

I0 o x

I0 3 12

I

I

I

I

l

l

l

l

I

I

I

I

I

I

I

I

Z~Nx/Z~X

I0

6 42

/ -

O~"'d'"'20

iL=..L

40

....

60

I~.===L==.

80

X(/.zm)

Fig, 2. Upper part: Linear scan in the X-direction perpendicular across a sharp, highly polished copper edge. The X-scale was calibrated by means of a test grid with a known mesh width (see insertion in the upper part). The Cu K~ intensity is displayed for two different pressures between SB3 and the target (see fig. 1). It can be seen that the effect of the beam halo increases by a factor 2 as the ambient tube pressure is increased by a factor 10. Lower part: Spatial resolution of the proton beam, obtained by differentiation of the Cu K~ intensity distribution across a sharp edge. The full width at half maximum is 11 ~m in the horizontal plane. The curve is drawn only to guide the eye.

58

G. B O N A N I et al.

ferentiation of this distribution. The full width at half maximum is 11 #m, This is very close to the expected value since the calculated spherical aberrations are _+4/zm. In the Y-direction a resolution of only 26 #m has been achieved, in agreement with the calculated spherical aberrations of _+25/zm. With an analyzed beam of 960 nA measured at the position of the object aperture a value of 400pA has been obtained on the target. The ratio of these two values is 2400 which is in reasonable agreement with the ratio determined by the phase space volume of the Tandem ( , ~ 6 × 1 0 6 p m 2 mrad 2) and the theoretical phase space volume of our system given in table 1. The linear scan mode can be used also to investigate the beam halo (fig. 2). From the measured X-ray intensity distribution obtained from a linear scan across a highly polished copper edge the ratio of the current density in the beam halo and that in the focussed beam can be deduced. The difference in the irradiated target areas must be taken into account properly. From fig. 2 one can show that the current density ratio is of the order of 10 -8 if the pressure is kept below 10 6 mbar and if the antiscattering slits, denoted by SB2 and SB3 in fig. 1, are adjusted carfully to the inner beam profile. DEFLECTION PLATES y X

TARGET

3. Scanning and data handling system In the present case, the scanning system is controlled by a digital circuit. This has the advantage that the area which is scanned and the number of steps per scanning line can easily be adapted to specific problems. In addition the X-ray intensity can easily be stored as a function of the beam position in a digital storage device (computer, magnetic tape). The block diagram of the electronics is shown in fig. 3. The X- and Y-coordinates of any selected beam position are stored to two 8-bit registers. This binary information is converted into analog signals. These signals regulate the high voltage supplies of the deflection plates, which direct the proton beam to the desired position. Deflection voltages up to _+1000 V are applied. For monitoring purposes, the deflection voltage is also used to move the beam spot on the screen of an oscilloscope. The scanning pattern is produced by a control unit, which determines the sequence of the scanning points by changing the content of the X- and Y-registers. It is possible to operate this control unit locally or by an on-line computer (PDP 15). When operating the control unit in the local mode, a line by line scanning pattern is used, whereby one line is scanned from left to right and the following line in the opposite direction. In order to facilitate this pattern, the X- and Y-registers can function as forward and backward counters. A clock determines the scanning rate. Each clock pulse increases or decreases the content of the Xcounter by one. At the output of the digital-analog converter (DAC) a step shaped signal is generated (fig. 4). Since the band-width of the high voltage amplifier is limited, the form of the output signal is smoothed and the requested voltage is reached about 200 ~s after the command. In order ux

I

L~

o)

b)

CAMAC / I ADC IINTE~FACE IfINTER# ACE '

PDP-15COMPUTERI

Fig. 3. Blockdiagram of the scanning system. (AMP = amplifier, DAC = digital to analog converter, ADC = analog to digital converter, OSC = oscilloscope, PUR = pile-up rejection, CFC = current to frequency converter, CM = current meter, MCA = multichannel analyzer). The fast discriminator signal of the X-ray detector (BCS) controls the brightness of the variable-persistance oscilloscope. The dead-time signal (DTS) is used to correct the clock rate.

Uy

'

I I

I

--t


-

:

-

J

Fig. 4. (a) Voltages on the X and Y deflection plates (fig. 3) with the Control Unit in local mode (see text). (b) Scanning pattern resulting from the voltage pattern shown in (a).

DIGITALLY

CONTROLLED

to preserve the spacial resolution, the data acquisition is interrupted during this time. The interruption time can be selected and adapted to the desired accuracy. Whenever a given maximum or minimum is reached in the X-counter the counting direction is changed and at the same time the content in the Y-counter is increased (or decreased) by one. The next line is therefore running in the opposite direction. Any of the following numbers of steps per line (and number of lines) can be selected: 32, 64, 128, 256. The X- and Y-counters can be operated singly if only one particular line is to be measured. The control unit can be connected by a CAMAC-interface with the on-line computer. In that case any number of lines and points per line can be chosen (up to 256). The upper and lower limits are stored in a 256-bit memory-chip, which has been loaded from the computer. In addition the Xand Y-registers can be set directly by the computer. Therefore it is possible to program any other scanning pattern. In order to change the size of the area to be analysed, the data for the X- and Y-deflection can be multiplied by factors of 2 with a bit-shift unit before they are transferred to the 12-bit digital-analog converter (DAC). In addition the gains of the amplifiers are variable. The base line voltages which determine the origin of the scan pattern can be set with potentiometers. For a quantiative comparison of the composition in different areas, it is important to correct for all dead time effects. Also fluctuations of the beam inlensity must be taken into account. In order to include these effects the clock is generated in the following manner: The measured beam current is amplified and then converted into a pulse rate which is proportional to the current. This pulse rate is of the order of 100kHz to 1MHz. The pulses are gated out whenever the data aquisition system is busy and not able to accept data or when the deflection amplifiers have not settled to the correct output voltage. The dead-time corrected pulse rate is divided by scalers to the desired clock frequency, which is used to trigger the scanning sequence. The clock rate which can be applied to the scanning system has an upper limit of about 1 kHz, due to the response time of the deflection amplifiers. f h e information of the beam position and the energy of each event can be transferred by a fast

SCANNING

MICROPROBE

59

interface to the computer. This interface is designed for multiparameter measurements with up to 4 ADCs. Instead of using the ADCs the digital data of the X- and Y-coordinates can be interfaced directly. It is also possible to store a part of the data in the memory of a multichannel analyzer (MCA), making it possible to operate the microprobe independently of the computer. The 8-bit registers for the X- and Y-coordinates allow a local resolution of 256 channels in each direction. The energy signal can be converted into a maximum of 4096 channels. But, in order to get a reasonable number of counts per channel, the number of scanning steps as well as the number of energy channels must be reduced. In addition data reduction is necessary due to the limitation storage capacity. Experience has shown that the following procedures are useful: 1) In order to get a rough survey of a specimen a 64 x 64 channel scan can be taken without energy analysis. In this mode up to 20 000 cps can be handled. The spectrum can be stored in the core memory of the computer or in the memory of the multichannel analyzer. 2) To determine the local distribution of the main elements, a program is developed, which stores simultaneously the local intensities of 4 selected energy windows. A local resolution of 32 × 32 is reached if the spectra are stored in the core memory. A better resolution is possible if disc storage is applied, but in this case the acceptable count rate is limited to about 2000 events/s. 3) For measurements of trace elements, the analysis is usually reduced to a line scan or selected point measurements. In the line scan mode the complete energy spectrum is stored for each scanning point on the disc of the computer. In this case a maximum count rate of 1000 can be accepted.

4. Applications So far the microprobe facility has been used to investigate the composition and local distributions of trace elements in various samples, such as aerosols, biological materials, microinclusions in mica and various other minerals. The implantation of heavy ions in selected pattern has also been tested using 20 MeV Fe 5+-ions. To illustrate the performance of the digitally controlled deflection system three applications of the two-dimensional as well

60

G. B O N A N I et al.

Fig. 5. (a) Enlargement of a mica specimen with two monazite inclusions (bright spots). The photograph was taken with an optical microscope. The size of the smaller inclusion is approximately 50 × 80 u m 2 . 5

iiii!iiii!!i! iiiiii!i!!ii!ii!ii!iiii

Fig. 5. (c) Two-dimensional X-ray intensity distribution around the smaller inclusion shown in fig. 5a. The figure is 32 times magnified in comparison to fig. 5b. The measured intensities were compiled with a computer controlled imaging equipment having 256 brightness levels. The inclusion is represented by a m i n i m u m in the X-ray intensity (see text).

::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : ::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

mm 4-

i!iiiiiiii!!!iiiiiiiiiiiiiiii!!!iiiiiiiiii iil i i! ii

::" gig i !glgiiiiggggSgg$$gg$$gg[ig[[ggggggigg!!!!iiiiiiiiii

Th Th Th

~V °V~V

:i: i;:iligigili!iiii! iiiiiiiiiiii!g iiiiii ii!!i?:::::::::::::::::::::

iiiiiiiiiilliiiiiiiiiiiiiiiiiiiiiiiiiiiiiii!iiiiiiii iii i l iiiiiiiiiiiiiiii!iiiiiiiiiiiililiiiiiiiiiiiii!iiii(iiii!i

LoC~L2

Pb Sr Y

~n, v5

,P

~'~Nd

~

fO 4

n Ce

n

p~

. . . . . . . . . . :: .... :'-:,::,':'::,':-':::l:"|::l:;:|llI;::::::::::::: ::: : : : f ; ; ' ; ; ; l ; ; ; , ; , : : ; ; l ; ; I ; ; . . , l , , , . , , , , , . . , ................

103 .i .i!!!".!!~:~.".!!!~,i~ . . . . . . . . . . , , , , , , , .r~;:~:::r'-:.-,..-...-:::::::::::::::::: ....... ::::::::::::::::::::::::::::::::::: !!~!~iii£::;~::~:::~::::::::::.: ..................... : : : .:.:.:. :. .: .:.:.:. :. .: .:.:.:. :. .: .:.:.:. :. .: .:.:.:. :. : : : :

~ 10z

::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

0

............... 0

I'":' I

I 2

I 3

........ 4

mm F i g . 5. (b) P r o t o n - i n d u c e d X - r a y i m a g e o f t h e w h o l e m i c a speci m e n s h o w n i n f i g . 5a. T h e s p a t i a l r e s o l u t i o n is d e t e r m i n e d by

the digital resolution of the scanning system and not by the beam. Therefore, the two inclusions are not apparent in this figure. Fig. 6. Upper part: 3 MeV proton-induced X-ray spectrum from the center of the smaller monazite inclusion shown in figs. 5a and c. The spectrum was measured with a 30 mm 2 × 3 mm Si(Li) detector. An absorber of 600/~m Aluminiurn was used to prevent pile-up effects from the strong U and Th L lines. Lower part: Enlargement of the X-ray energy region between U and Th L lines and rare-earth K lines. Only X-ray lines corresponding to small amounts of Ag and Sn could be detected.

z o ~)

2' 1 • I0 [UL 1211FT'ThL

i

l

I

i

i

i

r~ ~al,2

I

i ~,~'t _

i0 L

21 20

l

J 22

I

I i I 24 26 X - RAY ENERGY (keY)

n,°',~

J

] 28

_

I 30

DIGITALLY CONTROLLED S C A N N I N G MICROPROBE

as the one-dimensional scan mode are discussed more in detail. The trace element composition of monazite inclusions is of some interest because of the recently reported existence of primordial super-heavy elements in this type of mineral6'l°-12). An enlargement of a small mica specimen with two typical inclusions is shown in fig. 5a. Typically, these inclusions have diameters of the orders of 10-30/~m. The problem here is not only to localize the inclusion of interest, but also to adjust the beam with high precision to the center of the sampie,. Our system permits the solution of these problems in the following way: First, the beam is focussed to the ultimate resolution and its deflection is calibrated by means of a test grid as described in section 2. The test grid is then replaced by the specimen and by measuring the X-rays emitted by the mica an image of the whole probe is produced (fig. 5b). It should be noted, that in this case the spatial resolution is not determined by the beam but by the digital resolution of the scanning system. A comparison of fig. 5a and 5b gives the coordinates of the inclusion, to which the beam is now adjusted. Starting from this new position a much smaller area is scanned until the inclusion can be

61

seen in the X-ray image (fig. 5c). In this picture the inclusion is represented by a minimum in the X-ray intensity because the Ks-line of iron has been selected to produce the image. This element is abundant in mica, but is almost missing in the inclusion. A last small adjustment then brings the beam to the center of the probe. The result of a point analysis made at this position is shown in fig. 6. The measuring time was about 6 h. During this period the position of the beam remained stable within the accuracy of the measurement, which is determined by the spatial resolution of beam. In many cases it is of interest not only to measure the elemental composition at a given position but also to determine its local distribution. For instance, it is known that the distribution of trace elements in minerals segregated into phases contains information about the temperature during its formation. Such a mineral is perthite, consisting of a matrix of potassiumfeldspar (KAISi3Os) and a lamellar structure of microinclusion of sodiumfeldspar (=albite, NaA1Si308). A fluorescence emission picture, obtained with a scanning electron microscope, is shown in fig. 7. The task was to determine the spatial distribution of the trace elements along a line perpendicular to the two crystal

Fig. 7. Fluorescence emission picture of perthite (potassiumfeldspar with a lamellar structure of microinclusions of sodiumfeldspar), obtained with a scanning electron microscope. The black line (left) indicates the area scanned by the proton beam and reflects the fact that the proton beam has destroyed the luminescence properties of the crystal.

62

o. BONANI et al.

qo5

i

i

i

f

i

i

[

104

K [q

Mn~ .%•

~.',

f lO

~

Y(#

p

Rb r~

Fe z z <

i

• = ,,

Ga

Fq

..;'.

Pb

Zn

•: /

i

;.

•~

Sr J

i

D

::

:"

,~: . .'. ~:"

v,

1o2

D

I

x~2

o

I

w 28

f

•

ee

I

J

I

1

L

I

I

I

I

~D

2

4

6

8

I0

12

14

16

18

a: 14

i

ee

.

eee "e

~ 2t I0°

I

eeee

z z

tOI

I

• e e

,e

•

.e

.'e .e

e•

ee

X - RAY ENERGY (keY) •

~

2O

7 ,

K (%)

"~ /

IOO

12 8

I

200

I

I

30o

X(p.m)

Fig. 9. Upper part: Two-dimensional intensity distribution of the characteristic K~ X-ray line of iron implantations. The implantation was made with the microbeam facility into a carbon matrix along two vertical lines (length 2 7 0 # m , 125 lzm apart). The projection of the intensity distribution perpendicular to the two lines is shown in the lower part. The full width at half maximum is considerably larger ( - 1 0 0 / t i n ) than the original spatial resolution of the Fe-beam (301zm). Beam instabilities have been found to be smaller than the spot size. The observed effect may indicate an unexpectedly strong migration of the implanted ions.

4 0 8OO

8O

I

6O 40 2O 0 9O 60 30 0 ~

0

ioo

200

300 ~ m 400

0

tO0

200

300

~Lm 400

Fig. 8. Upper part: X-ray spectrum of the matrix phase of Perthite, measured with a 80 m m 2 x 5 m m Si(Li) detector. An absorber of 3001tin Be was used to reduce the low-energy Xrays. Lower part Local distribution of the main element pot a s s i u m and of five different trace elements, measured along a line perpendicular to the two crystal phases of perthite (see fig. 7). The error bars indicated are statistical only; the absolute error in the concentration may be as large as 30%.

phases. The black line in this figure indicates the area scanned by the proton beam, which was positioned by producing first a 2-dimensional X-ray image of the area of interest, similar to fig. 5b. Here the K-lines of potassium were used. It should be noted that the black line on fig. 7 reflects the fact that the proton beam has destroyed the luminiscence properties of the crystal and does not indicate any carbon contamination. This can be checked, if, in addition, the secondary-electron image of the sample is taken with the scanning electron microprobe. The X-ray spectrum observed in the matrix region is displayed in the upper part of fig. 8, whereas in the lower part the local dis-

tribution of the main element potassium and five different trace elements are compiled. As can be seen, the behaviour of the trace elements varies considerably. An interpretation of this result is given elsewhere~3). A combined application of the one- and two-dimensional linear scan mode is illustrated by the third example. Here Fe s+-ions were accelerated to an energy of 20 MeV and focussed to a spot size of 30 # m diameter. These ions were then implanted into a carbon matrix along two vertical lines 125 lzm apart, using the linear scan mode. A total of about 5 × 10 ]3 atoms were deposited in the two areas determined by the amplitude of the deflection (270/~m) and the beam resolution. The implantation depth was 5/~m. The result of this implantation was checked afterwards with the proton-microprobe by measuring the two-dimensional intensity distribution of the characteristic iron K~ X-ray line. The result of this analysis is displayed in fig. 9. It is interesting to note that the full width at half maximum of the X-ray intensity

63

DIGITALLY CONTROLLED SCANNING MICROPROBE

measured along a cut perpendicular to the two lines (see lower part of fig. 9) is considerably larger (100/~m) than the original spatial resolution of the Fe-beam. Beam instabilities have been found to be smaller than the spot size and cannot be responsible for the enlarged profile size. The observed effect may therefore indicate an unexpectedly strong migration of the implanted ions, a result which certainly deserves further attention.

5. Summary It has been demonstrated that protons and heavy ions can be focussed to a spot size of about 10/lm using conventional quadrupole doublet lenses. The beam currents on the target are sufficient to perform trace element analysis of /~g probes at the ppm-level, a sensitivity which is about 2-3 orders of magnitude higher than that of the electron microprobes. It is also possible to implant heavy ions in computer controlled patterns. The described electronic scanning and data handling system simplifies considerably the operation of" the whole microprobe facility. We thank Dr. D. Heck for useful discussions and for furnishing us his ion optical program, Dr. H.-U. Nissen for preparing the mineral specimens and Mr. R. Wessicken for making the electronmicroscope pictures. Part of this work was sup-

ported by the Swiss National Scientific Research.

Foundation

for

References 1) Proc. Int. Conf. on PIXE and its analytical applications, Lund, Sweden 1976, Nucl. Instr. and Meth. 142 (1977) 1. 2) j. A. Cookson and F. D. Pilling, Harwell Report, AERE-R 6300 (1970) and Thin Solid Films 19 (1973) 381. 3) R. Nobeling, K. Traxel, F. Bosch, Y. Civeleko~lu, B. Martin, B. Povh and D. Schwalm, Nucl. Instr. and Meth. 142 (1977) 49. 4) D. Brune, U. Lindh and J. Lorenzen, Nucl. Instr. and Meth. 142 (1977) 51. 5) D. Heck, Kernforschungszentrum Karlsruhe, KFK Report 2379 (1976) p. 108. 6) T. A. Cahill, N. R. Fletcher, L. R. Medsker, J. W. Nelson, H. Kaufmann and R. G. Flocchini, Phys. Rev. CI7 (1978) 1183. 7) K. L. Brown, F. Rothacker, D. C. Carey and Ch. Iselin, SLAC Report no 91 (1970) and CERN Report 73-16 (1973). 8) H. Jung, G. Bonani, M Suter and W. W~lfli, Annual Report, Lab. f. Kernphysik, Eidg. Technische Hochschule, Z[irich (1976) p. 266. 9) F. Folkmann, Ion beam surfi~ce layer analysis, vol. 2 (1976) p. 695. 10) R. V. Gentry, T. A. Cahfll, N. R. Fletcher, H. C. Kaufmann, L. R. Medsker, J. W. Nelson and R. G. Flocchini, Phys. Rev. Lett. 37 (1976) 11. I1) F. Bosch, A. E. Govesy, W. Kr6tschmer, B. Martin, B. Povh, R. Nobeling, K. Traxel and D. Schwalm, Phys. Rev. Lett. 37 (1976) 1515 and Z. Physik A280 (1977) 39. 12) W. W61fli, J. Lang, G. Bonani, M. Surer, Ch. Stoller and H.-U. Nissen, J. Phys. G, Nucl. Phys. 3 (1977) L33. 13) H.-U. Nissen, G. Bonani, Ch. Stoller and W. WOlfli, to be published.