A PRECISION MEASUREMENT OF ABSOLUTE IONIC MASSES AND ENERGIES OF FIELD EMITTED IONS

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A PRECISION MEASUREMENT OF ABSOLUTE IONIC MASSES AND ENERGIES OF FIELD

EMITTED IONS

T. Tsong, S. Mclane, Y. Liou

To cite this version:

T. Tsong, S. Mclane, Y. Liou. A PRECISION MEASUREMENT OF ABSOLUTE IONIC MASSES

AND ENERGIES OF FIELD EMITTED IONS. Journal de Physique Colloques, 1984, 45 (C9), pp.C9-

71-C9-75. �10.1051/jphyscol:1984913�. �jpa-00224391�

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A PRECISION MEASUREMENT OF ABSOLUTE IONIC MASSES AND ENERGIES OF FIELD EMITTED IONS*

T.T. Tsong, S.B. McLane and Y. Liou

Physics Department, The Pennsylvia State University, University Park, PennsyZvania 16802, U.S.A.

Resume

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Des methodes ont dt6 mises au point pour mesurer la masse et l'bnergie absolues d'ions Qmis par effet de champ avec une precision meilleure que 0,0005 uma et 0,5 eV en util'isant la sonde 5 atomes 5 temps de vol putsee par laser.

Abstract

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Methods have been devised to measure the absolute mass and energy of field emitted ions to an accuracy better than 0.0005 amu and 0.5 eV using the pulsed-laser time-of-flight atom-probe.

I

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METHODS FOR ABSOLUTE MASS AND ENERGY MEASUREMENT

It is now possible to measure the absolute mass and energy of field emitted ions with high precision using the pulsed-laser time-of-fligh atom probe /I/. For this purpose we recognize the fact that the maximum kinetic energy of field emitted n+ ions is

(neVo

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^{A E ~ )}where Vo is the emitter voltage and

A E ~

is the critical energy deficit of the ions which is given by

When the emitter is a semiconductor, the work function @ should be replaced by the electron affinity Ea. To determine accurately the absolute mass and kinetic energy of the field emitted ions, one measures the onset flight time to of the flight time distribution of the field emitted ions, i.e., the flight time of the most energetic ions taking the shortest flight path to reach the ion detector. If the ions are not subjected electric field in the flight tube, then the mass-to-charge ratio is related

where C = 2e/,t2 is the flight-path constant, 6 is the trigger time-delay constant, and R is the shortest flight path of the ions. 6 accounts for the fact that both the trigger and the ion signals take time to generate and to transmit through the connec- ting cables. C and 6 can be determined with great accuracy by measuring the onset flight times of pulsed-laser field desorbed gas ions at different emitter voltages, and then making a linear plot of to vs [~/n (Vo

-

A~~/nefl'/~. According to

the slope of the linear plot is 1 / f i and its intercept is

-

6.

The problem now reduces to determine accurately C and 6. For this purpose it is neces- sary to have an electronic timer of good time resolution, a dc power supply of good stability and a high precision measurement of the emitter voltage. We use a LeCroy 4208 TDC of better than 1 ns time resolution, a DVM of 5 1/2 digits and a dc power supply of 0.001 % stability. The entire flight path is carefully shielded from an electric field, and every precaution, such as the tip positioning and the flight- path length change due to thermal expansion by a room temperature change, has to be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984913

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JOURNAL DE PHYSIQUE

X Y Z Micro-

Laser pulse initiation

J-7

Pulses

Triggers

LeCra/ 4208

TDC, Control. sage Monitor Mini-Com~.

R

Oscilloscope

FET Robe 6 Rearnp.

s

Adjualable Einzel Lens

Fig. 1. The p u l s e d - l a s e r t i m e - o f - f l i g h t atom-probe, now equipped w i t h a 1 n s r e s o l u - t i o n e l e c t r o n i c t i m e r and a d a t a processor.

c a r e f u l l y c o n s i d e r e d ( F i g . 1 ) . We f i n d t h a t

t o

i s r e p r o d u c i b l e t o 5 1 n s o u t of a t o t a l f l i g h t time of "20 000 n s o v e r a time p e r i o d of s e v e r a l days o r l o n g e r ( F i g . 2 ) . Accurate v a l u e s of C and 6 a r e determined from a l i n e a r p l o t of eq. ( 3 ) using t h e measured v a l u e s of

t o

f o r He', N2+ and Ar+ taken a t v a r i o u s v o l t a g e s . T h i s s e t of d a t a a r e l i s t e d i n Table I t o g e t h e r w i t h o t h e r s e t s of d a t a . For our p u l s e d - l a s e r atom-probe with a f l i g h t - t u b e of 4.2 m, t h e b e s t f i t v a l u e s o b t a i n e d a t a s p e c i f i c t i p p o s i t i o n i s g i v e n by

I t i s important t o recognize h e r e t h a t t h e method p r e s e n t e d h e r e i s based o n l y on t h e a b s o l u t e mass of He+, N2+ and Ar+, v a l u e s of which a r e d e r i v e d from a s t a n d a r d i s o - t o p e t a b l e . A l l o t h e r parameters, such a s time and v o l t a g e , o n l y a high p r e c i s i o n i s r e q u i r e d i n t h e measurement.

I1

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MEASUREMENT OF IONIC MASSES AND CRITICAL ENERGY DEFICIT OF FIELD EMITTED IONS With t h e v a l u e s of C and 6 now being a c c u r a t e l y determined, t h e i o n i c masses of f i e l d e m i t t e d i o n s can be determined w i t h good accuracy from t h e i r o n s e t f l i g h t t i m e s i f t h e i r c r i t i c a l energy d e f i c i t s a r e known a c c u r a t e l y . O r i f t h e i r i o n i c masses a r e known a c c u r a t e l y , t h e n t h e i r c r i t i c a l energy d e f i c i t s can b e determined with good accuracy. We have measured t h e o n s e t f l i g h t times of a l a r g e number of i o n i c spe- c i e s , ranging from g a s i o n s , m e t a l i o n s , semiconductor i o n s , c l u s t e r i o n s t o r a r e complex i o n s such a s ~ h ~ e ~ + and D ~ + e t c . I n Table I , some of t h e d a t a a r e l i s t e d . I o n i c masses a r e c a l c u l a t e d from eq. ( 2 ) by assuming t h e v a l i d i t y of eq. (1) f o r t h e c r i t i c a l energy d e f i c i t . I t i s found t h a t t h e v a l u e s determined from t h i s experi- ment a g r e e w i t h t h e s t a n d a r d t a b l e v a l u e s t o w i t h i n 0.0005 amu f o r l i g h t i o n s and t o w i t h i n 0.005 amu f o r heavy i o n s .

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M = 28.0056 amu AE, = 11.08

eV

t,

= 23978

ns

FLIGHT

TIME

(ns)

Fig. 2. Flight time distribution of N ~ + taken at different runs over a two-day period. The onset flight times are identical in all runs.

The values of critical energy deficit listed in Table I are derived from eq. ( 2 ) by using the value of ionic masses calculated from the standard isotope mass table. The experimental data agree with eq. (1) to within f 0.3 eV for gas ions, and to

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⁵^{1 eV}

for multiply charged metal ions. Thus the validity of eq. (1) is quantitatively established for all gas and metal ions.

Finally we would like to point out here that the precision of the measurements can be further improved by using an electronic timer of better resolution and laser pulses of shorter pulse width. It is possible to imprwe the precision by a factor of 10 to 20 by simply using an electronic timer of 50 ps resolution and a laser unit of 1 ps pulse width. Both of these units are now commercially available. With such an im- provement, the pulsed-laser ToF atom-probe can then be considered a high precision instrument.

REFERENCE

*!Chis work was supported by DOE and NSF.

1. T. T.'Tsong, .S. B. McLane and Y. Liou, Rev. Sci. Instrum. August 1984.

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JOURNAL DE PHYSIQUE

T a b l e I. I o n i c m a s s e s c a l c u l a t e d f r m e x p e r i m e n t a l t0(4-m f l i g h t t u b e ) ( 6 = 26.4 n s , C = 0.01082835 mu/ p s Z / k ~ )

Measured Measured

I o n i c S p e c i e s V

to

^{I o n}^{M a s s} ^&c/n

Mass, h ~ ~ / n (kV) ( u s ) ( m u ) ( e V )

~ e + 7.0 7.250 4.00168 1 9 . 5

4.002054 amu 7.5 7.003 4.00214

20.1 eV 8.0 6.779 4.00188

Av. 4 . 0 0 1 9 0 ~ 0 . 0 0 0 1 9 AM = - ( 0 . 0 0 0 1 5 ~ 0 . 0 0 0 1 9 )

~ 2 + 2.8 1 1 . 5 2 3 4.02836 1 1 . 5

4.027655 amu 3.0 11.130 4.02842 11.6

1 1 . 0 eV 3.3 10.609 4.02840 1 1 . 6

3.7 1 0 . 0 1 5 4.02771 11.1

4.2 9.398 4.02882 1 2 . 2

4.5 9.078 4.02914 1 2 - 7

5.8 7.989 4.02730 1 0 . 5

6.4 7.604 4.02798 11.6

6.7 7.430 4.02698 9.9

7.0 7.268 4.02674 9 . 5

Av. 4.02799?0.00075 11.2f1.0

AM = +(O.00033f0.00075)

~ 2 + 4.5 23.978 28.00807

28.005599 amu 5.0 22.743 28.00718

11.1 eV 6.0 20.753 28 -00106

7.0 19.207 27.99512

AV. 28.00286f 0.00522 AM = -(0.00274+0.00522)

~ r + 2.8 36.352 39.9623

39.961834 amu 3.0 35.112 39.9584

1 1 . 3 eV 3.3 33.474 39.9656

4.5 28.648 39.9642

5.0 27.170 39.9551

6.0 24.794 39.9495

7.0 22.953 39.9609 11.2

Av. 39.9594+0.00520 11.0f0.7

A M = -(0.00237f0.00520)

~ h ~ + 6 . 1 27.910 51.4854 11.6

51.452201 amu 8 . 7 23.357 51.4461 9.9

10.9 eV 9.2 22.713 51.4509 10.7

1 0 . 0 21.785 51.4583 1 2 . 1

Av. 51.4534+0.0052 11.1+0.8

A M = +(0.0013+0.0052)

182w3+ 12.0 21.591 60.6465 14.6

60.6489 amu

183w3+ 1 2 . 0 21.652 60.9889 1 6 . 3

60.9828 amu

184w3+ 12.0 21.710 61.3160 1 4 . 9

61.3166 amu

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Ionic species l7 to Mass, AEc/n (kV) . (1-1s)

Ion Mass ( a m )

AEc/n (eV)