Beam parameters and characterization technique

Nuclear Instruments and Methods in Physics Research B 85 (1994) 716-721 North-Holland

Beam parameters

and characterization

I. Krafcsik ap*, L. Farkas R. Poggiani ’ a Research Institute for Materials

a, A. Zimmer

NIMI B

Beam Interactions with Materials 8 Atoms

technique

a, C. Rossi b, A. Fabris b, G. Gorini

‘,

Science, P.O. Box 49, H-1525 Budapest, Hungary b INFN - Sezione di Trieste, Padriciano 99, 34012 Trieste, Italy ’ INFN - Sezione di Piss, via Livornese 582 /a, 56010 S. Pier0 a Grade, Piss, Italy

A review of the electrical and optical beam parameters is given. We describe our computer controlled, four slit beam characterization technique called computer controlled beam diagnostic CCBD system. The CCBD system can be operated in beam profile and emittance modes. The data collection is finished with appearance on the screen of the corresponding three-dimensional diagram: the current density map of x; x’ and y; y’ emittance maps. As a result of software manipulation further beam characteristics like current contours, emittance patterns, emittance as a function of beam fraction etc. can be deduced. The setup consists of four motorized, water cooled slits with variable slit width allowing to handle beams from PA up to hundreds of mA.

1. Beam quality parameters The Child-Langmuir law describes the current density .I that can be extracted by an electric field under space charge limited conditions [2,4]: J = 4/9[ to(2q/mi)1’*U3/2d2],

(1)

where q is the ion charge (q = Ke, with e the electron charge), mi is the ion mass, U is the applied voltage causing the electric field and d is the extraction gap width. The same in a more useful form: J = 1.72(tc/A)1’21J3’2/d2,

(2) with J [mA/cm2], K is the ion charge state, A is the ion mass [amu], U [kVl and d [cm]. The validity of these equations has two limitations. First is the space-charge limited condition, which means that more ions are produced than are extracted. It is very difficult to satisfy the other condition: the plasma of a practical ion source, i.e. the emitting surface, must be exactly plane. Eq. (1) gives rise to other two definitions: The perveance of an ion gun is determined as

P = IU-3/2(

A/K)“‘,

and the normalized, determined as

(3)

or proton

I, = 1( A/K)~‘~. *

Corresponding 1695 653.

equivalent

current

is

(4)

author, phone

016%583X/94/$07.00

+361 1695 653, fax +361

Besides the above electrical parameters, the following definitions characterize the optical qualities of particle beams: r beam radius, (Y divergence half-angle of the trajectories, E emittance, B brightness. First of all, one should clarify whether the entire beam is considered for a given parameter, or limited parts only determined by reasonable border values, cutting off for example the weak beam halo. In this case the threshold beam density must be specified, where the cutoff occurred. According to the other approach in contrary to the density threshold, the cutoff criterion is a purely geometrical condition (not an electrical one), which can be either a divergence limit, or a maximum radius, or an acceptance. The acceptance of a beam line is the largest emittance a beam can have while still passing through the line without any losses. By strict definition the beam emittance is related to the pattern that the beam particles occupy within the six-dimensional phase space. In many practical cases the three coordinate pairs within the entire phase space are completely decoupled, and the longitudinal projection of the actual pattern does not have any meaning for quasistationary beams. In this context only the two remaining transverse projections are of importance. Assuming that the transverse motions are slow compared to the velocity in the beam direction, and

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717

I. Krafcsik et al. /Nucl. Instr. and Meth. in Phys.Res. B 85 (1994) 716-721 and

-Ednns = 4(;;zx’2-

(4

(b)

(4

(4

Fig. 1. Two-dimensional emittance pattern in four realistic cases. The corresponding beams are divergent (a), convergent (b), parallel (cl and focused (d). nonrelativistic can substitute

conditions are fulfilled throughout, one the transverse linear momenta [l-4]

and m dy/dt

m dx/dt

by the tangent x’= dx/dz

(5)

values

and y’= dy/dz

(6)

of the divergence angles for all individual trajectories. Thus, the commonly used two-dimensional emittance definitions regard the patterns that the trajectories independently occupy in the (x; x’) and (y; y’l planes. As a gross classification, a beam is divergent if its emittance pattern mostly extends from the first to the third quadrant of the coordinate plane, convergent if its major extension runs from the second into the fourth one, roughly parallel if it is extended along the positional coordinates (x or y), and it has a focus if the emittance pattern runs along the angle coordinates (x’ or y’) as it is shown in Fig. 1. To evaluate an emittance different conventions are in use. The first is to take directly the area occupied by the emittance pattern and express its value in mm mrad. According to the other convention an emittance is defined as the area of the emittance pattern divided by r. The reason of this second convention is to deduce directly the extension of the second semi-axis when the emittance and the extension of the first semi-axis are numerically known, because quite frequently the emittance pattern is of elliptical shape. In order to avoid misunderstandings which of the conventions are in use, in this latter case the IT is written to the emittance value as a distinct factor, practically giving it the meaning of a measuring unit. This gives the understanding that T times the numeric value of the emittance results in the emittance pattern area size. According to this the units to measure emittances are, for example r mm mrad. The statistical (rms) emittances [2,4] are defined as the second moments of the distributions representing the beam in the given two-dimensional subspaces of the phase space, or commonly their equivalents in the angle/position planes, as explained above. Two definitions are commonly used for the quantitative expression of the rms emittance, and they differ by a factor of 4 for the same pattern: EmIs=

27

(

_

(qy2,

(7)

(XX~)~)l’z.

(8)

Definition (8) has the advantage that the 4rms emittance exactly covers the real emittance pattern for the Kapchinskij-Vladimirskij distribution. These expressions also regard the actual beam fraction contained within the actual emittance pattern. It is considered to be a useful algorithm to plot the beam fraction against the emittance, thus allowing to make a distinction between the weak beam halo, where the current density is low, and the beam core with higher density [2]. The emittance of a beam will shrink if the beam is further accelerated, because for a given transverse velocity the longitudinal velocity has increased. To get rid of this effect, normalized emittance is defined as follows: l, = Pre,

(9)

where E, is the normalized emittance /3 = v/c = 1.46 is the relativistic parameter and y = x 10-3(K~/~)1’2 (1 - /?2)-i’2. Brightness [l-4] of an ion beam is defined simply as the beam current over the product of the two transverse emittance values: B =Z/(E*E~). The second

(10) widely used brightness

definition

B = 2Z/( 5~~~~5).

is: (11)

Brightness values according to Eq. (11) are smaller by a factor of .rr2/2 = 5 than those calculated according to Eq. (10) with identical beam parameters. Of course in any brightness determination the current as well as the emittance values have to be taken for identical beam fractions. For the sake of completeness two more conventions are introduced that will greatly facilitate the comparison of brightness values for different experimental conditions. Whenever the normalized emittances are taken for the calculation the resulting brightness (called) emittance-normalized, is defined as B, n = Z/(e,,,e,,y).

(12)

The Child-Langmuir equation (1) suggests the comparison of brightness values for different ion species by using the normalized currents Z,,, according to definition (4), and the absolute emittances. Therefore, such a brightness is defined as &

=Zn/(excy)?

and is called

(13)

current-normalized brightness. XII. ACCELERATORS/BEAMS

I. Krafcsik et al. /Nucl.

718

Instr. and Meth. in Phys. Res. B 85 (1994) 716-721

Fig. 2. Basic concept of the double slit principle.

2. Experimental setup The basic concept of our computer controlled beam diagnostic system is based on the four slit beam characterization technique [l] illustrated in Fig. 2. The setup consists of four motorized slits and a Faraday cup to measure the necessary parameters for calculation and plot of diagrams characterizing a beam in a maximum area of 50 X 50 mm. All slits are water cooled to be able to handle currents up to 100 mA. The slit widths are adjustable in a range of 0.1-3 mm. The slits are driven by step motors via linear actuators, providing a 3.2 km resolution. The whole measurement including the process control, the data collection and data processing is controlled by an IBM PC AT. The following two basic beam ch~acterization procedure can be carried out by running different parts of the same programs: Beam profile - giving the current density as a function of coordinates. In this case the X and Y’ slits in pair are used for position definition (see Fig. 3). Emittance - giving the current density as a function of coordinate and beam divergence. In this case the X and X’ or the Y and Y’ slits in pair are in operation for the measurement of x; x’ or y; y’ emittances. The X and Y slits define the position within the beam while the beam divergence is deduced from the relative position of the X and X’ or Y and Y’ slits (see Fig. 4 for x; X’ emittances). During the beam profile program first the total current is measured, when all slits are moved out. This

Fig. 3. X and Y’ slits in operation, when plotting the beam profile.

is followed by calibration, as a result of which the beam position is located within the 50 x 50 mm window by determining the beam edges defined as the coordinates of the programmed promil value (l/l~) of the total current. Current values are measured as function of coordinates within a rectangular scan area defined by the beam edge positions. The scan resolution in x-di-

Fig. 4. X and X’ slits in operation when collecting data for x; x ’ emittance.

I. Krafcsik et al. / Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 716-721

rection can be programmed from 2 to 100 paths while in y’ direction it is fixed at 100. In case of CALIB~TION OFF, beam edge coordinates of the previous measurement are automatically used for defining the scan area, unless they are changed by the operator before START MEASUREMENT. The emittance program follows the same sequences as in the case of the beam profile to determine the beam edges. Then the beam center position is determined as the mean value of the beam edge positions. Having set the X slit in this center position, the X’ center edges are found in the 50 mm wide window, which are defined as the programmed promil value of the maximum current measured during this last run. The X’ beam center XL is determined as the mean value of the X’ center edges, and the beam width xbw as the difference between the two beam edges. During the data collection the X slits step within a scan distance defined by the two X beam edges with the programmable scan resolution in a range from 2 to 100 paths. 100 current values are measured at each X step, as a function of x’ coordinates between the two scan border coordinates (xi and xi> defined by the

File

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programmed value of beam divergence distance (L), as follows: x; = [(x;+xbw/2)

-nAx]

+DI+

(D) and slit

(14)

and x;=[(x;+x,/~)-nAhx]-DL,

(15)

where IZ is the serial number of the X slit’s step, and Ax is the length of it. In case of CALIBRATION OFF, the beam center coordinate and the beam width of the previous measurement are automatically used for defining the X scan area, unless they are changed by the operator before START MEASUREMENT. The X’ beam center position is determined at the beginning of each measurement with a short calibration. When the data collection is completed in both parts of the program three-dimensional figures (the emittance map or the current density map) appear on the screen. The two-dimensional patterns can be drawn by programming the constant current value defined in two ways, first as a percentage of the maximum current density (ABSOLUTE), second as a percentage of the

Haintenance

REIUlY

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1991.

etrittance

pattern ZOOH FteDra&

Rdd Clear

Enittance

versus

bean fraction 1 ZOON

ReDrau

,.,...1’ __~,,,_((,,,._......

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..““’

.

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Enittance:

1.0 abs%

89.8 beati color: 21.09 U~nn~nrad 93.8 beati color:

.O.O abs% 84.7 bean% color: Enittance: 155.41 lf.nn.nrad 5.0 abG% 73.0 bean% color: Enittartce: LOO.21 U~rm~nt-ad ‘5,O abs% 31.3 bean% color: Enittance: 31.53 H+n-wad

Fig. 5. Results of an emittance measurement with evaluation of the emittance patterns and the emittance versus beam fraction. XII. ACCELERATORS/BEAMS

I. Kkzfcsik

720 File

et al. /Nucl.

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3 85 (1994)

716-721

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RECIDY

Heasurenent nade: Non Nw 25 11:50:48 1991 Total current: 59.2 ph Density distribution

Density nap (front uieu)

I zclon

k zoon ReDrac Md Cleai

Nuneric data 91,6 bean% color: 97.5 bean% color: 38,i bean% Wx;i 0.8 bean% 99.3 bean% color i 95.6 bee& color: 7.3 beat&! color: ,O abr% 22.6 bean% color: I ,O absk ,O a&% ,O abs% ,O abs% ,O a&% ,O abs% ,O abs%

Fig. 6. Results of a beam profile measurement with evaluation of the density distribution contours.

total current (BEAM FRACTION). The emittance versus beam fraction relation is drawn by a menu-driven program command.

3. E~e~mental

results

‘I’be capability of the CCBD system is illustrated by the experimental results in Figs. 5 and 6. The ion beam was provided by a hollow cathode ion source and focused by an electrostatic einzel lens. A y; y ’ emittance measurement is shown in Fig. 5, with an x resolution of 25. The three-dimensional emittance map in the upper left box appears right after the measurement, while the emittance contours in the upper right box with their numeric data in the lower right box and also the emittance versus beam fraction in the lower left box are the results of further evaluation. The emittance patterns clearly show the fact that the beam was convergent, and the growing aberrations for higher beam fractions. A beam profile measurement is shown in Fig. 6. The three-dimensional current density map in the upper left quadrant appears immediately after measurement, while the current contours in the upper right quadrant with their numeric data in the lower right

quadrant are the results of further software manipulation. The beam profile and the emittance modes of the CCBD system provide quantitative information about beam handling elements for beam line construction. Recently attempts have been made to equip the ion implanters with a beam profiler [6-8f to control the actual status of the beam line. Sharp edges in the profile, that are found in Fig. 6 too, effect the dose uniformity [S] and the charge neutralization with an electron flood gun [8].

Acknowledgements

The authors would like to thank G. Torelli, J. Gyulai and G. Zimmer for their support and encouragement during this work, and furthermore G. Eisler and A. 1116sfor their technical cont~bution.

References [I] W.B. Thompson, I. Honjo and N. Turner, Proc. 4th Int. Conf. on Ion Implantation: Equipment and Techniques. Berchtesgaden, Germany, 1982 (Springer-Verlag, 1983) p. 86.

I. Krafcsik et al. / Nucl. Instr. and Meth. in Phys. Res. B 85 (1994) 716-721 [2] R. Keller, Ion Extraction, in: The Physics and Technology of Ion Sources, ed. I.G. Brown (Wiley, 1989) p. 23. [3] A.J.Y. Holmes, Beam Transport, in: The Physics and Technology of Ion Sources, ed. I.G. Brown (Wiley, 1989) p. 53. [4] J.D. Lawson, The Physics of Charged Particle Beams, (Clarendon, Oxford, 1977). [5] I. Krafcsik, L. Farkas, A. Zimmer, C. Rossi, A. Fabris, G. Gorini and R. Pogiani, Proc. 9th Int. Conf. on Ion Implantation Technology, Gainesville, FL, USA, 1992 (Elsevier, Amsterdam, 1993) p. 483.

721

(61 V. Benveniste, P. Kellerman and J. Schussler, Proc. 9th Int. Conf. on Ion Implantation Technology, Gainesville, FL, USA, 1992 (Elsevier, Amsterdam, 1993) p. 437. [7] W.J. Szajnowski, Proc. 9th Int. Conf. on Ion Implantation Technology, Gainesville, FL, USA, 1992 (Elsevier, Amsterdam, 1993) p. 441. [8] M. Naito et al., Proc. 9th Int. Conf. on Ion Implantation Technology, Gainesville, FL, USA, 1992 (Elsevier, Amsterdam, 1993) p. 385.

XII. ACCELERATORS/BEAMS