HAL Id: jpa-00226829
https://hal.archives-ouvertes.fr/jpa-00226829
Submitted on 1 Jan 1987
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
SINGLY - AND DOUBLY-CHARGED ION
FORMATION IN LIQUID-METAL-ION SOURCES
T. Ishitani, K. Umemura, Y. Kawanami
To cite this version:
T. Ishitani, K. Umemura, Y. Kawanami. SINGLY - AND DOUBLY-CHARGED ION FORMATION
IN LIQUID-METAL-ION SOURCES. Journal de Physique Colloques, 1987, 48 (C6), pp.C6-159-C6-
164. �10.1051/jphyscol:1987626�. �jpa-00226829�
SINGLY- AND DOUBLY-CHARGED ION FORMATION IN LIQUID-METAL-ION SOURCES
T. Ishitani, K. Umemura and Y. Kawanami
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185, Japan
(Abstract)
-
The process of singly- and doubly-charged ion formation i nliquid-metal-ion sources is studied using Cu-P. Pt-P qnd Pt-P-Sb alloys. Strong matrix effects a r e observed on t h e intensity r a t i o of P++ t o P+ ions and their energy widths, AE. This effect is explained by an increase i n electric field, F, of a s l i t t l e a s 10 % from F = 27 t o 30 V/nm, which is estimated from the post-ionization model. Experimental data on AE agree with the prediction t h a t a higher F brings about l e s s space-charge effect on t h e emitted ions, resulting in a narrower AE. The origins of energy t a i l s observed on t h e M + energy distribution curves a r e discussed.
1. Introduction
Liquid-metal-ion sources (LMISs) have attracted increasing interest due t o their potential advantages for focused-ion-beam technology. The variety of LMIS has increased a s a r e s u l t of t h e use of alloy rather than pure elements.
However, many aspects concerning ion formation i n MISS a r e not yet f u l l y understood.
This paper is a sequel t o recent papers by t h e authors.'-3 In t h e
development of a phosphorus P) LMIS using Cu-P and Pt-P base alloys, a strong matrix effect is observed on the intensity r a t i o of PC+ t o P+. The source characteristics of M+ and
ions
a r e discussed adding other new data from the viewpoint of their formation processes.2. Experiments
The mass and energy analyses for t h e LMISs were performed using a double
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987626
JOURNAL DE PHYSIQUE
focussing mass spectrometer in which t h e energy resolution was set t o 1.1 eV. 1 The source was a movable needle type L M I S ~ using C U ~ ~ P ~ S , PtQP2% and PtGg P1q Sb15 alloys. The former two a l l o y s presumably suffered t h e
preferential thermal-evaporation of t h e P element t o r a i s e their melting points, which reduced t h e i r source lifetime t o only about 30 hours. The Pt-P-Sb alloy exhibited a source lifetime of over 200 hours. In t h e energy analysis, ions were detected i n a pulse counting mode and recorded through a logarithmic ratemeter i n a dynamic range a s wide a s 3 - 5 orders of magnitude.
3. Mass and Energy Analyses
The charge-state of the emitted ions a r e discussed i n relation t o field strength, F, of t h e emitter-apex. The ion energy distribution (hereafter
referred t o a s ED) is then analyzed with respect t o energy width and t h e energy t a i l s observed on ED curves.
3.1 Charge-States of Emitted Ions
A strong matrix e f f e c t has been observed on the intensity r a t i o of P++ t o P+ f o r LMISs using Cu-P and Pt-P alloys? The P++/P+ values a r e plotted a s a function of It f o r these and Pt-P-Sb a l l o y s i n Fig. 1. Typical P++/P+ values a t I t = 20 U A a r e about 2 and 2 x
lo-'
f o r Pt-P base and Cu-P alloys, respectively.Dixon et al.4 suggested t h a t t h e F v a l u e could be estimated by comparing observed M++/M+ values with calculated post-ionization (hereafter referred t o a s PI) p r ~ b a b i l i t i e s . ~ The applicability of PI t o LMIS have been f u r t h e r discussed i n Refs. [1,6,71.
I t is found t h a t a presumable cause of t h e strong matrix effect on P++/P+
value is an increase i n F of a s l i t t l e a s 10 % from F = 27 t o 30 V/nm. These values a r e estimated from t h e PI model. A stronger F seems t o be required t o form a Taylor cone f o r source materials with higher surface tension,
r.
The7 values of the Pt-P base and Cu-P a l l o y s are, unfortunately, not known, but the former value is assumed t o be larger. This is expected from t h e
r
values of 1800, 1285 and 367 dyn/cm f o r pure elements of Pt, Cu and Sb, respectively, a t t h e i r melting points. 83.2 Energy Distribution
Typical ED curves of Mnf ions (n = 1 and 2) from a Pt-P-Sb alloy LMIS a t 1% = 6 P A a r e shown i n Fig. 2, including t h e sb3+. Figures 3 (a) and (b)
show t h e ED curves a t various 1.t values f o r P+ and P + + ions, respectively.
Fig.1 Intensity r a t i o of pf + and P + a s a function of 1t f o r Cu-P, Pt-P, and Pt-P-Sb alloys.
"
10
-
60 40 20 0 -20 -40 -60 Relative Ion E n e r g y , E/n CeV)
10
60 40 20 0 -20 -40 -60 R e l a t ~ v e Ion E n e r g y , E/n CeV)
Fig.2 Energy d i s t r i b u t i o n s of M ~ + ions (r, = 1 and 2) from Pt-P-Sb a l l o y a t It = 6 fiA; (a) Pt, (b) P and
(c) Sb (including n = 31
JOURNAL DE PHYSIQUE
Here, the energy axis for Mnf ions is scaled a s the energy per unit charge, .E/n, and its zero point (i.e. t h e energy corresponding t o the emitter potential)
is estimated.
(a) Energy Width
The experimental energy width (FWHM) per unit charge for Mnf ions (n = 2 and 3). AE(M*)/~, supports the general relationship
hE(Mnf)/n 5 AE(M+) < AE(Mnt),
except f o r sbn+ ions, where t h e observed A E ( s ~ ~ + ) / ~ values a t It = 6 D A a r e 31, 11 and 9 e V f o r n = 1, 2 and 3, respectively. This relationship is reasonable when multi-charged ions a r e formed from t h e evaporated ions through the PI process.3 The break in the general relationship for sbn+ ions is
brought on by t h e appearance of small shoulders on t h e low-energy ED curves, which probably correspond t o t h e field ipnization (FI) process a s discussed later.
Next, the AE(pn+)/n values a r e plotted a s a function of I t in Fig. 4.
It. is found that t h e aE(pn+)/n values f o r Cu-P alloy a r e larger than those for t h e Pt-P base alloys i n t h e region of I t > 30 1rA. The AE can be s p l i t into two components; A E O due t o energy broadening from t h e i n i t i a l mechanism of io,n formation and a E b due t o t h a t in t h e beam caused by space-charge effect;' i.e. (AEl 2 = (AEO? + (AEb)*. The AE difference among
alloys can be qualitatively explained by t h e expectation that a stronger F r e s u l t s in a smaller AE b
.
(b) Energy Tails of ED Curves
It is observed in Fig. 2 t h a t a l l M f ED curves have long low-energy t a i l s increasing with It. ~ i x o n ' observed t h e energy t a i l also for Au LMIS and explained it in terms of a resonant charge transfer process [i-e., fast) + M(s1ow) + M(fast1 + ~+(slowll, which occurrs in a vapor cloud close t o the emitting region. He derived t h e energy t a i l distribution a s
dN(V)/dV = (& no r, q/V, exp(-n, r, qV/Vo ),
where, Vo is t h e voltage applied t o the emitter, r, t h e radius of t h e
emitting site, n o t h e gas density on t h e emitting surface, q t h e cross section for resonant charge exchange reaction, and io t h e ion current of a l l energies.
This equation predicts t h a t a t a i l r i s e s slightly towards lower energies with the slope of -nor, q/V in the log [dN(V)/dVl vs. V graph (corresponding t o t h e present ED graph). The increase in n o and ro with I t is expected t o make the t a i l slope steeper. This expectaion, however, disagrees with t h e experimental P+ ED curves a s shown i n Fig. 3.
Another possible cause of t h e low-energy t a i l is t h e contribution of FI process. If FI is occuring a s well a s field evaporation, a two-peak structure
I , , L I S
6 0 40 20 i) ' -20 ' -40 ' Relative Ion Energy, E CeV)
Fig.3 Energy distributions of P ions from Pt-P-Sb alloy a t various values; fa) P+ and (b) P f +
Fig.4 Energy width per u n i t charge, AE(P"+)/~, as a function of It
Total lon current, It ( P A ) for Cu-P, Pt-P and Pt-P-Sb alloys.
JOURNAL DE PHYSIQUE
would be expected in t h e ED curve.? The small and broad shoulders observed on the low-energy ED curves f o r ptn+ and sbn+ ions In = 1 and 2) in Fig. 3
probably correspond t o the FI process.
Rather f l a t t a i l s a r e also observed on t h e high-energy side of the M + ED curves. In the case of the high-energy t a i l , it is noteworthy t h a t t h e M2+
peak intensities a r e larger than t h e M + ones and the t a i l s of the M + ions rise by 2.5 - 3 orders of magnitude below t h e peaks of the M 2 + ED curves.
This suggests that t h e follwing two processes occured sequentially or simultaneouesly,
X + -+
x2+
(PI process)x2+(fast) + slow) -t xC[fast) + slow).
The measured 'X ions finally have an energy surplus corresponding t o half of the energy accerelated during
x2+,
where momentum transfer is assumed t o be negligible in t h e charge exchange process. A more detailed discussion w i l l be performed elsewhere.4. Conclusion
Mass and energy analyses of LMISs using Cu-P and Pt-P base alloys were carried out and t h e r e s u l t s were discussed from the viewpoint of ion formation.
The strong matrix effect observed on P+ +/P+ value for Cu-P and Pt-P base alloys was explained by an increase i n F of a s l i t t l e a s 10 % from F = 27 t o 30 V/nm, a s estimated from the PI model. The matrix effect on t h e AE(P+) value is quantitatively explained also by the increase in F.
The experimental &E(Mn+)/n values support the general relationship, A E ( M ~ + ) / ~
s
AE(M+)<
AE(M"+Xexcept for sbn+ ions, some of which a r e formed by the FI process. The FI process presumably also contributes t h e low-energy t a i l s of t h e M + ED curves.
References
1 T.Ishitani. K.Umemura and Y.Kawanami: J. Appl. Phys. 61 (1987) 748.
2 K-Umemura, Y.Madokoro, S.Shukuri and T-Ishitani: in Proc. 18th Symp. Ion Implantation and Submicron Fabrication (1987, Rikagaku Kenkyusho, Wako-Shi Saitama) p.97.
3 T.Ishitani, K.Umemura and T.Aida: J. Vac. Sci. Technol. (October, 19871 in press.
4 A.Dixson, C.Colliex. P.Sudraud, J. Van de Walle: Surface Sci. 108 (1981) L424.
5 D.R.Kingham: Surface Sci.
116
(1982) 273.6 D.R.Kingham: Appl. Phys. a 1 (1983) 161.
7 L.W.Swanson and D.R.Kingham: Appl. Phys.
A41
(1986) 223.8 C.J.Smithells: Metals Reference Book (Butterworth, London & Boston, 1976).
9 A.J.Dixson: J. Phys. D (Appl. Phys.)
