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Electron-optical components for E-beam testing
Microeleclxonic Engineering 12 (1990) 189-204 Elsevier Science Publishers B.V.
ELECTRON-OPTICAL
COMPONENTS
189
FOR E - B E A M T E S T I N G
Erich PLIES
R e s e a r c h Laboratories, Siemens AG O t t o - H a h n - R i n g 6, D-8000 M ~ n c h e n 83, Federal R e p u b l i c of G e r m a n y
The e l e c t r o n - o p t i c a l components of an e - b e a m tester comprise the e l e c t r o n gun, the beam b l a n k i n g system, the lenses and the s e c o n d a r y e l e c t r o n analyzer. For each component, a concise r e v i e w of the various e x i s t i n g types is p r e s e n t e d and a c o m p a r i s o n made. For e - b e a m testing of future VLSI ICs (with i n t e r c o n n e c t i o n s < 0.75 ~m), significant i m p r o v e m e n t s need be made m e r e l y in the analyzer.
i. I N T R O D U C T I O N
Up to the m i d d l e of the eighties, m o d i f i e d scanning e l e c t r o n m i c r o s c o p e s (SEMs) w e r e used for e l e c t r o n - b e a m (e-beam) testing of i n t e g r a t e d circuits (ICs). The e l e c t r o n - o p t i c a l m o d i f i c a t i o n s of the SEM u s u a l l y c o m p r i s e d a b e a m - b l a n k i n g system b e t w e e n the anode and the first m a g n e t i c c o n d e n s e r lens and a simple r e t a r d i n g - f i e l d spectrometer between the m a g n e t i c o b j e c t i v e lens and the IC. Today, d e d i c a t e d e - b e a m testers (EBTs) are available. The elect r o n - o p t i c a l column of such an EBT must be o p t i m i z e d with respect to p r i m a r y e l e c t r o n (PE) probe forming (small d i a m e t e r of 0.i 0.3 ~m and high current > 1 nA for low beam v o l t a g e ~ 1 kV) and PE pulse g e n e r a t i o n (I0 ps for GaAs devices and = 200 ps for MOS ICs). A d d i t i o n a l l y , the c o l l e c t i o n of the s e c o n d a r y electrons (SEs) m u s t be o p t i m i z e d to achieve precise v o l t a g e m e a s u r e m e n t s and a n e g l i g i b l e influence of the local fields of the IC. The low e n e r g y of the PEs is n e c e s s a r y to avoid damage, c h a r g i n g and loading of the IC. O p t i m i z e d l o w - v o l t a g e optics are also of interest for other applications, e.g. w a f e r inspection, surface science or b i o c h e m i s t r y . Fig. 1 i l l u s t r a t e s the e l e c t r o n - o p t i c a l column of a d e d i c a t e d EBT [32, 138-140]. Part a) shows a schematic cross section through the column. The column consists - in p r i n c i p l e like every d e d i c a t e d EBT - of the following e l e c t r o n - o p t i c a l components: e l e c t r o n gun, b e a m - b l a n k i n g system, condenser lenses and a c o m p o u n d s p e c t r o m e t e r o b j e c t i v e lens w h i c h acts as an o b j e c t i v e for the PEs and a s p e c t r o m e t e r for the SEs. Part b) shows the axial d i s t r i b u t i o n of the p o t e n t i a l ~(z) = ~(0,0,z) r e f e r r e d to the cathode and the magnetic flux d e n s i t y B(z)=Bz(0,0,z), w h i c h are among the factors a f f e c t i n g the e l e c t r o n - o p t i c a l p e r f o r m a n c e data (cardinal elements, image aberrations). The kinetic energy of the axial primary e l e c t r o n s is e~(z). The stage is, of course, grounded, and the cathode has the p o t e n t i a l - U ~ with respect to this g r o u n d connection, w h e r e Up~ is the beam voltage of the p r i m a r y electrons at
0167-9317/90/$3.50 © 1990, Elsevier Science Publishers B.V.
190
E. Plies / Electron-optical components for e-beam testing
the IC. Part c) of Fig. 1 shows the l i g h t - o p t i c a l a n a l o g of the column and the p r i n c i p l e of p r i m a r y - e l e c t r o n (PE) focusing, with the e q u i v a l e n t e l e m e n t s of e l e c t r o s t a t i c o r i g i n (concave and convex lenses and p l a n e p a r a l l e l plates) shown dark and the equivalent lenses for the m a g n e t i c lens fields shown bright. The p r i m a r y and s e c o n d a r y electron-optical performance data of this d e d i c a t e d EBT is s u m m a r i z e d in Tables 1 and 2, r e s p e c t i v e l y , t o g e t h e r with the p e r f o r m a n c e of a m o d i f i e d SEM [168]. The c l e a r l y superior performance data of the d e d i c a t e d EBT w e r e a t t a i n e d by i m p r o v i n g the i n d i v i d u a l e l e c t r o n - o p t i c a l c o m p o n e n t s and the customized low-voltage column concept. In the following, we will p r e s e n t a c o n c i s e r e v i e w of the i n d i v i d u a l e l e c t r o n - o p t i c a l components of an EBT, discuss various instrument types and b r i e f l y touch upon i m p r o v e m e n t s still r e q u i r e d in the future. In so doing, we will r e f e r b a c k to Figure 1 and Tables 1 and 2. In this paper, o n l y EBTs for ICs and not for c o n d u c t o r n e t w o r k s or c i r c u i t packages are considered. In the d e t a i l e d reviews [6,7] a s p e c t s are also discussed.
a)
of
e-beam
testing,
b)
electron-optical
c)
Triode electron gun
UEx
Beam blanking deflector
~- UL-~ Immersion condenser lens
Magnetic condenser lens
® Compound spectrometer objective lens
UpE
.-
.. -,. ".
FIGURE 1 Schematic representation of the e l e c t r o n - o p t i c a l column of a d e d i c a t e d EBT (ICT 9010); b) D i s t r i b u t i o n s of the axial (cathode related) p o t e n t i a l ~(z) and the axial m a g n e t i c flux d e n s i t y B(z) along the optical z-axis. UEx = extraction voltage for the p r i m a r y electrons in the t r i o d e gun, UL = lens v o l t a g e of the i m m e r s i o n c o n d e n s e r lens, UE = extraction voltage for the secondary electrons and UpE = final b e a m v o l t a g e of the p r i m a r y electrons; c) L i g h t - o p t i c a l counterpart of the e l e c t r o n - o p t i c a l column and p r i m a r y - e l e c t r o n ray path. E q u i v a l e n t lenses of m a g n e t i c origin are in light print, w h e r e a s e q u i v a l e n t s of e l e c t r o s t a t i c origin (concave and convex lenses and plane p a r a l l e l plates) are shown dark. a)
E. Plies / Electron-optical components for e-beam testing
Cathode Condenser system Final PE-energy
Modified SEM
Dedicated EBT
w
LaB6
purely magnetic
immersion lens
2.5
1.0
Brightness for final PE energy [A cm2 sr 1]
1.2 • 104
3.2 • 104
Minimum PE-pulse width
1
0.15
purely magnetic
combined electrostatic - magnetic
11
2
[keV]
Ins]
Objective lens
191
TABLE 1 C o m p a r i s o n of the p r i m a r y - e l e c t r o n o p t i c a l p e r f o r m a n c e of two electron-beam testers - a modif i e d S E M [168] (based on a C a m b r i d g e I n s t r u m e n t s S150) a n d a d e d i c a t e d E B T [32, 81, 1 3 5 - 1 4 0 ] (ICT 9010). TABLE 2 C o m p a r i s o n of t h e s e c o n d a r y e l e c t r o n o p t i c a l p e r f o r m a n c e of t w o t y p e s of e l e c t r o n b e a m t e s t ers - m o d i f i e d S E M a n d a d e d i c a t e d EBT.
Working distance
Modified SEM
[ram]
Spherical aberration constant [ram]
70
10
Axial chromatic aberration constant
20
4
0.02
0.02
0.4
0.12
5
2.5
[mm]
Final aperture [rad] PE-probe diameter [~m]
PE-probe current [nA]
Type of spectrometer Location of spectrometer Detector Spectrometer constant
Dedicated EBT
planar retarding field
semispherical retarding field
post-lens
through-the-lens
one EverhartThornley
two EverhartThornley
5 • 10-8 V ~
"
5 • 10.9 V
Cross-talk
between 1,5 pm
< 40 %
< 3%
interconnection lines
2. C A T H O D E S
AND
ELECTRON
GUNS
For high voltage resolution and/or short measuring t i m e s of v o l tage waveforms, a m a x i m u m p o s s i b l e s p e c i m e n c u r r e n t m u s t be realized within a specified probe diameter. D u e to t h e i r l o w b r i g h t ness, e l e c t r o n g u n s w i t h t u n g s t e n c a t h o d e s are out of the q u e s tion. D u e to t h e i r l o w w o r k f u n c t i o n , L a B 6 s i n g l e - c r y s t a l cathodes s u p p l y a h i g h e r b r i g h t n e s s . T h e y h a v e b e e n i n t e n s i v e l y s t u d i e d and t e s t e d in t h e past, see [8-16], for e x a m p l e , a n d h a v e b e e n on the m a r k e t f o r a n u m b e r of years. In t h e o r y , the beam brightness is p r o p o r t i o n a l to the b e a m v o l tage. In t h e l o w - v o l t a g e r a n g e (< 2 kV), h o w e v e r , the b e a m b r i g h t n e s s d e c r e a s e s m o r e s t e e p l y if a t r i o d e e l e c t r o n g u n is used. T h i s is a s c r i b e d to s a t u r a t i o n e f f e c t s [17] a n d to t h e B o e r s c h e f f e c t [ 1 7 2 - 1 8 3 ] of the e l e c t r o n s in the gun. With a tetrode electron gun, this loss is a l r e a d y s m a l l e r [13], but an e v e n b e t t e r s o l u t i o n is the c o m b i n a t i o n , s h o w n in Fig. i, of a triode gun with a retarding immersion condenser lens [81,94,138,139]. B e f o r e the i m m e r s i o n c o n d e n s e r lens, the PEs h a v e an e n e r g y e U ~ x = 5 k e y and are t h e n r e t a r d e d b y e U = to e U p E > 500 eV. A b r i g h t n e s s of 3.2 104 A c m - 2 s r -z for U p E = 1 k V w a s t h u s o b t a i n e d for a ( 1 0 0 ) - o r i e n t e d LaB6 c a t h o d e w i t h a t e m p e r a t u r e of 1775 K [ 1 3 8 , 1 3 9 ] . The question naturally a r i s e s w h e t h e r it w o u l d n o t e m p l o y an e l e c t r o n g u n w i t h a f i e l d - e m i s s i o n cathode
be b e t t e r to f r o m the out-
192
E. Plies / Electron-optical components for e-beam testing
set. Papers [18-27] deal, inter alia, w i t h a c o m p a r i s o n of LaB6 with field emission, and papers [19,22-24,26] even treat this subject under the a d d i t i o n a l s t a n d p o i n t of e - b e a m testing. This comp a r i s o n is not simple, because, for instance, the a b e r r a t i o n s of the o b j e c t i v e lens have a very s i g n i f i c a n t influence. In brief, it can be said that for a probe d i a m e t e r of less than 0.05 to 0.i ~m, the thermal field e m i s s i o n supplies a g r e a t e r s p e c i m e n current at the same probe d i a m e t e r than the LaB6 cathode. The a u t h o r agrees with L. Reimer, who points out on p. 31 of [4] that for p r o b e d i a m e t e r s ~ 0.i ~m, more s p e c i m e n c u r r e n t is supplied by the field e m i s s i o n and for probe d i a m e t e r s > 0.I ~m by the LaB6 cathode. This is also c o n f i r m e d by a c o m p a r i s o n of e x i s t i n g d e d i c a t e d EBTs. Indeed, Hitachi Ltd. (Ti-W thermal field emission) and ICT GmbH (LAB6) g u a r a n t e e [31,32] the same probe p e r f o r m a n c e , n a m e l y 0.i ~m, 1 nA and 1 kV. For an EBT with LaB6 cathode, the c r o s s - o v e r is imaged in r e d u c e d form, so that as a rule only the a b e r r a t i o n s of the o b j e c t i v e lens c o n t r i b u t e to inf l u e n c i n g the probe diameter. This is not the case for a system w i t h a f i e l d - e m i s s i o n cathode, so that the use of e l e c t r o n guns w i t h a s u p e r p o s e d m a g n e t i c lens field [ 2 3 , 2 6 , 2 8 - 3 0 , 3 3 - 3 8 ] brings good results. The d e d i c a t e d EBT from Hitachi Ltd. a l r e a d y possesses an e l e c t r o n gun of this kind [133, 134]. But it still remains open w h e t h e r the f i e l d - e m i s s i o n cathode will p r o v i d e the only m e a n i n g f u l solution for the e - b e a m testing of ICs with i n t e r c o n n e c t i o n line widths < 0.5 ~m (i. e. probe d i a m e t e r < 0.i ~m). This is b e c a u s e very p r o m i s i n g silicon a v a l a n c h e cathodes [39-41] are being developed. Their chopping s y s t e m could be d i r e c t l y i n t e g r a t e d for g e n e r a t i n g short e l e c t r o n pulses. Also p r o m i s i n g are l a s e r - t r i g g e r e d p h o t o c a t h o d e s [42-48], w h i c h not only p o s s e s s a very high brightness, but can also supply very short e l e c t r o n pulses, e.g. 4 ps [45] and have a l r e a d y been succ e s s f u l l y used by P.May et al. [45-47] for e - b e a m testing. In the case of t r a n s m i s s i o n p h o t o c a t h o d e s , it should also be p o s s i b l e to attain effective photoelectron source d i a m e t e r s < 5 ~m, thus e a s i l y o b t a i n i n g probe d i a m e t e r s < 0.i ~m. Using l a s e r - t r i g g e r e d p h o t o e l e c t r o n s reduces f l e x i b i l i t y w i t h r e s p e c t to s y n c h r o n i z a t i o n with the e l e c t r i c a l d r i v i n g of the IC. The e m i s s i o n of thermal e l e c t r o n s by laser pulse i r r a d i a t i o n is also p o s s i b l e [49]. With r e s p e c t to further details r e g a r d i n g e m i s s i o n p r o p e r t i e s and e l e c t r o n gun design, the reader is r e f e r r e d to C h a p t e r IX of [i].
3. E L E C T R O N
BEAM P U L S I N G
As a l r e a d y m e n t i o n e d in the previous chapter, e l e c t r o n pulses can be g e n e r a t e d even w i t h o u t an e-beam chopping system in the column. However, all c o m m e r c i a l l y a v a i l a b l e d e d i c a t e d EBTs and most modified SEMs p o s s e s s such an e - b e a m chopping system. A d i s t i n c t i o n should be m a d e here b e t w e e n two methods of g e n e r a t i n g short PE pulses, namely beam intensity modulation and beam d e f l e c t i o n across a c h o p p i n g aperture.
E. Plies / Electron-optical components for e-beam testing
193
3.1 Beam i n t e n s i t y m o d u l a t i o n
W e h n e l t m o d u l a t i o n has been used by various authors [50-54]. But this m e t h o d has not found general acceptance, b e c a u s e the electrical pulse has to be p r o d u c e d at high v o l t a g e and the c a p a c i t a n c e to be c h a r g e d and d i s c h a r g e d is too high [6]. In [55], an intensity m o d u l a t i o n w i t h a filter lens (special Einzel lens) was suggested. E s t i m a t e s showed that, due to t i m e - o f - f l i g h t effects, no pulses < 300 ps can be g e n e r a t e d with a chopper of this kind, even if the filter lens is c o n f i g u r e d as a triplate strip line. M. T r o y o n [56] has taken the g r a t e d the filter lens in the systems [55,56] is that the and both have a m o n o c h r o m a t o r
s u g g e s t i o n in [55] further and intee l e c t r o n gun. The a d v a n t a g e of these probe d i a m e t e r d e g r a d a t i o n is small effect.
Using the space focusing action of a k l y s t r o n cavity lens, L.C. O l d f i e l d [57] a t t a i n e d pulse widths of several ps at a repetition rate of 9 GHz. The phase focusing action of a k l y s t r o n cav i t y b u n c h e r was used by T. H o s o k a w a et al. [58], and H. H~bner and E. R 6 h m [60] used m i c r o w a v e field e m i s s i o n to g e n e r a t e 1 ps pulses at 14 GHz.
3.2 Beam d e f l e c t i o n
3.2.1 Plate c a p a c i t o r s D e f l e c t i o n across a chopping aperture by plate c a p a c i t o r s was used in [61-67]. Using the blanking system described in [66], M. B r u n n e r et al. [67] have r e p o r t e d 15 ps e l e c t r o n pulses, and in the m e a n t i m e have a l r e a d y attained 7 ps pulses with this relativ e l y simple and flexible b l a n k i n g system. Since the time resolution is l i m i t e d not only by the electron pulse w i d t h but also by other effects, e.g. the t r a n s i t i o n time effect of the SEs and the t i m i n g jitter of the control electronics, there seems to be no reason for the use of more complex d e f l e c t o r designs for the ebeam t e s t i n g of ICs. Probe b l u r r i n g due to energy spread of the PEs caused by the blanking p r o c e s s in the plate capacitor [68] could not be c o n f i r m e d as a r e l e v a n t degradation.
3.2.2 T r a v e l i n g wave structures and reentrant
cavity structures
T h r o u g h - t y p e t r a v e l i n g wave deflectors have also been used for ebeam t e s t i n g [69-74] and pulses < i0 ps have been r e p o r t e d [70,71]. The t r a n s v e r s e reentrant cavity s t r u c t u r e has a still h i g h e r d e f l e c t i o n s e n s i t i v i t y than the t r a v e l i n g w a v e deflectors. T. H o s o k a w a et al. [58] used such a t r a n s v e r s e r e e n t r a n t cavity in combination with an additional longitudinal reentrant cavity (klystron c a v i t y buncher). They g e n e r a t e d pulses of 0.2 ps and detected them. In terms of PE energy or pulse r e p e t i t i o n frequency, t r a v e l i n g wave and r e e n t r a n t cavity d e f l e c t o r s are not as flexible as the plate capacitor. Y.D. C h e r n o u s o v and I.V. S h e b o l a e v [59] s p e c i f y an u l t r a - h i g h frequency r e s o n a t o r - d e f l e c t o r w h o s e resonator f r e q u e n c y can be v a r i e d m e c h a n i c a l l y to some extent.
194
3.2.3
E. Plies / Electron-optical components for e-beam testing Multiple deflector
systems
Several authors [75-78] have r e p o r t e d on c o n f i g u r a t i o n s in which one plate c a p a c i t o r is placed before and other one after a PE cross-over, and the c h o p p i n g a p e r t u r e is located in the c r o s s - o v e r plane. T i m e - o f - f l i g h t effects are c o m p e n s a t e d to a c e r t a i n extent if the signal of the second d e f l e c t o r is delayed. J.T.L. Thong et al. [79] report on a new type of b e a m p u l s i n g stem w i t h a m u l t i - e l e c t r o d e d e f l e c t i o n element, w h i c h permits g e n e r a t i o n of pulses < 15 ps at r e p e t i t i o n rates of 1-50 GHz.
sythe
For a m o r e e x t e n s i v e c o m p a r i s o n of the various c h o p p i n g systems, the r e a d e r is r e f e r r e d to the reviews [6,7,63,80]. In [7], the various d r i v i n g m e t h o d s and types of a r r a n g e m e n t s are p r e s e n t e d in detail.
4. C O N D E N S E R AND O B J E C T I V E
LENSES
The c o n d e n s e r lenses and the o b j e c t i v e lens of all m o d i f i e d SEMs and of most d e d i c a t e d EBTs are p u r e l y magnetic. The d e s i g n of purely m a g n e t i c lenses is at the state of the art and their properties, i n c l u d i n g those of single p o l e - p i e c e lenses [84], are known [3,82,83,85]. The c o m m e r c i a l EBT from Sentry S c h l u m b e r g e r [157, 158] and the l a b o r a t o r y setup by L. D u b b e l d a m and P. Kruit [159162] use a v a r i a b l e - a x i s i m m e r s i o n lens (VAIL), w h i c h has a l r e a d y been s u c c e s s f u l l y used for a number of years in e l e c t r o n beam lit h o g r a p h y [86-88] and r e p r e s e n t s a clever s u p e r p o s i t i o n of the mag n e t i c lens and d e f l e c t i o n field. A c o n d e n s e r lens of the d e d i c a t e d EBT s c h e m a t i c a l l y shown in Fig.l is a c o m b i n e d e l e c t r o s t a t i c - m a g n e t i c lens [81,138]. In the compound s p e c t r o m e t e r o b j e c t i v e lens too, the e l e c t r o s t a t i c spectrom e t e r fields ( ~ / ~ @ 0 in Fig, ib) must be taken into account when c a l c u l a t i n g the PE focusing and the a b e r r a t i o n s [94]. This also applies to a n u m b e r of other in-lens or t h r o u g h - l e n s analyzers. If a m a g n e t i c d e f l e c t i o n element is added in the o b j e c t i v e lens, as in Fig. i, then we obtain a c o m b i n e d focusing and deflection s y s t e m w i t h s p a t i a l l y s u p e r i m p o s e d fields w h i c h is no longer q u i t e so simple to calculate. However, e x i s t i n g theories and programs [89-94] a l l o w the n u m e r i c a l c a l c u l a t i o n of c o m p o u n d systems from any s u p e r p o s e d e l e c t r o s t a t i c and m a g n e t i c lens and d e f l e c t i o n fields.
5. A N A L Y Z E R S AND D E T E C T O R S
The SE e n e r g y a n a l y z e r is the key e l e c t r o n - o p t i c a l c o m p o n e n t of any EBT. All p r e v i o u s l y used analyzers are r e t a r d i n g - f i e l d energy devices, w h i c h i n t e g r a t e over the SE s p e c t r u m or parts of it. Under ideal conditions, the d e t e c t e d SE current depends only on the p o t e n t i a l d i f f e r e n c e b e t w e e n the m e a s u r i n g point in the IC and the r e t a r d i n g - f i e l d grid. To linearize the q u a n t i t a t i v e v o l t a g e m e a s u r e m e n t , the e n e r g y a n a l y z e r is u s u a l l y o p e r a t e d in a feedback loop. The f o l l o w i n g e l e c t r o n - o p t i c a l requirements are made on today's analyzers:
E. Plies / Electron-optical components for e-beam testing
-
the SEs must be detected, their angle of emission,
as
far as
possible,
independently
195
of
- the d e t e c t i o n must, as far as possible, be i n d e p e n d e n t of existing microfields (local fields) of the IC or the a n a l y z e r must reduce these m i c r o f i e l d s as far as possible, -
the a n a l y z e r must not have a n e g a t i v e effect on the p e r f o r m a n c e of the p r i m a r y e l e c t r o n probe, e.g. give rise to u n w a n t e d def l e c t i o n or astigmatism,
- b e y o n d t h i s , t h e a n a l y z e r must be d e s i g n e d and c o n f i g u r e d so that it does not,in p r i n c i p l e , l i m i t the p e r f o r m a n c e of the PE probe. Up to 1984, e x c l u s i v e use was made of p o s t - l e n s analyzers. Starting w i t h the first in-lens analyzer by E. Menzel [129], a series of in-lens and t h r o u g h - l e n s analyzers were still p r o p o s e d and constructed. In the meantime, all d e d i c a t e d EBTs are e q u i p p e d with a n a l y z e r s of this kind, because they a l l o w both s u p e r i o r PE and SE p e r f o r m a n c e data to be attained, see Tables 1 and 2. However, this d o u b l e p e r f o r m a n c e gain was traded off a g a i n s t a more complex e l e c t r o n optical system in every d e d i c a t e d EBT. In the following, let us i n i t i a l l y present an o v e r v i e w of the various p o s t - l e n s analyzers and then discuss the v a r i o u s in-lens and t h r o u g h - l e n s analyzers. In v i e w of the ongoing increase of component i n t e g r a t i o n w i t h ever smaller i n t e r c o n n e c t i o n d i m e n s i o n s and thus i n c r e a s i n g field strength of the m i c r o f i e l d s at the surface of the ICs, we will at the c o n c l u s i o n of this chapter b r i e f l y discuss the q u e s t i o n of w h e t h e r r e t a r d i n g - f i e l d a n a l y z e r s do in fact r e p r e s e n t the best solution for e l e c t r o n - b e a m m e a s u r e m e n t technology.
5.1
Post-lens
analyzers
R e t a r d i n g - f i e l d analyzers have a l r e a d y been used in other sectors prior to e - b e a m testing of ICs and are still used in other a p p l i c a t i o n s today, see [95-108]. A p p l i c a t i o n sectors w e r e or are SE e m i s s i o n studies [96,106], Auger e l e c t r o n s p e c t r o s c o p y [97], e l e c t r o n s p e c t r o s c o p y for chemical analysis [98, i01] and M ~ b a u e r s p e c t r o s c o p y [108]. C o m p a r e d with d i s p e r s i v e e n e r g y analyzers, ret a r d i n g - f i e l d devices have a high l u m i n o s i t y but a low e n e r g y resolution. The latter results from the w i d t h of the cut-off curve of the r e t a r d i n g - f i e l d analyzer [95,119], w h i c h has a high-pass c h a r a c t e r i s t i c . This smeared cut-off c h a r a c t e r i s t i c of the retard i n g - f i e l d a n a l y z e r produces an effect on the form of the d e t e c t e d integral SE s p e c t r u m (S-curve). The slope of the S-curve d e c r e a s e s with increasing field strength of the r e t a r d i n g field, with g r o w i n g m e s h w i d t h of the r e t a r d i n g - f i e l d grid, and the S-curve is flatter for a planar r e t a r d i n g field than for a spherical one [95,99,100,119]. In e l e c t r o n - b e a m m e a s u r e m e n t technology, a flatter S - c u r v e means a w o r s e voltage r e s o l u t i o n (greater s p e c t r o m e t e r constant) [119]. The r e t a r d i n g - f i e l d analyzers used by A. G o p i n a t h et al. [109-111] for e - b e a m testing had h e m i s p h e r i c a l grids and were c o n s t r u c t e d s i m i l a r l y to the a n a l y z e r s used for the SE e m i s s i o n studies, e.g. [96]. They s u f f e r e d from a weak e x t r a c t i o n field, w h i c h was increased by Y. Goto et al. [112] by means of an a d d i t i o n a l control
196
E. Plies / Electron-optical components for e-beam testing
grid. J.M. H a n n a h [113] a n d L.J. B a l k et al. [114] u s e d c y l i n d r i cal d e f l e c t o r analyzers which have a band-pass characteristic and suffer from a long working distance (65 m m and 22 mm, r e s p e c t i v e l y ) . T h e a n a l y z e r d e s i g n e d by H.P. F e u e r b a u m [115] w a s o p t i m i z e d w i t h r e s p e c t to h e i g h t and e x t r a c t i o n f i e l d s t r e n g t h a n d is still w i d e l y used. T h e r e t a r d i n g field spectrometer by G.S. P l o w s [116] was t h e f i r s t c o m m e r c i a l l y available system (Lintech Instruments Ltd.). It is a t t r a c t i v e d u e to its r o t a t i o n a l symmetry and was evaluated in [117,118]. A l l p r e v i o u s l y m e n t i o n e d post-lens analyzers [ 1 0 9 - 1 1 6 ] w e r e c o m p a r e d in d e t a i l in [119] a n d / o r [120]. E. P l i e s [121] i n v e n t e d an a n a l y z e r in w h i c h a p l a n e e x t r a c t i o n field and a hemispherical r e t a r d i n g f i e l d are c o m b i n e d so t h a t the virtual source of the SEs coincides with the center of the hemispherical retarding field. A n a n a l y z e r of this t y p e w a s first constructed by K. N a k a m a e et al. [122-124]. D u e to its h i g h extraction in c o n j u n c t i o n w i t h the a n g l e - i n d e p e n d e n t detection of the SEs, a c c o r d i n g to [122-125] this a n a l y z e r p r o v i d e s a steeper S-curve, an i m p r o v e d spectrometer constant and a smaller local f i e l d e f f e c t [126] t h a n the F e u e r b a u m a n a l y z e r w i t h a p l a n e r e t a r d i n g field. In the local f i e l d e f f e c t (LFE), K. N a k a m a e et al. [126] m a k e a d i s t i n c t i o n b e t w e e n b e t w e e n two e f f e c t s or types. LFE I is c h a r a c t e r i z e d as the e f f e c t by w h i c h s l o w SEs c a n n o t o v e r c o m e a retarding local microfield a b o v e the i n t e r c o n n e c t i o n . However, the t r a j e c t o r i e s of SEs w h i c h are n o t r e f l e c t e d f r o m the m i c r o field can nevertheless still be a f f e c t e d by it, w h i c h a l s o leads to an e r r o r in the q u a n t i t a t i v e voltage measurement. This effect is d e s i g n a t e d as L F E II.
5.2
Through-lens
analyzers
The great disadvantage of a post-lens analyzer is the large working distance b e t w e e n the o b j e c t i v e lens a n d the IC. T h i s nec e s s a r i l y m a k e s the a x i a l a b e r r a t i o n s of the o b j e c t i v e lens relat i v e l y high, see T a b l e I, a n d an e l e c t r o n p r o b e w i t h small d i a m e ter and high current c a n n o t be g e n e r a t e d . This drawback can be avoided by m e a n s of i n - l e n s and t h r o u g h - l e n s analyzers. In the following, w e i n t e n d l a r g e l y to u s e the d e s i g n a t i o n through-lens a n a l y z e r q u i t e a p a r t f r o m the o r i g i n a l t e r m u s e d by the i n v e n t o r s for t h e i r a n a l y z e r . F o r in m o s t cases, the d e t e c t o r i t s e l f is pos i t i o n e d a b o v e the o b j e c t i v e lens. T h e first a n a l y z e r of this type was invented b y E. M e n z e l [129,130]. It is a t t r a c t i v e b e c a u s e of its s i m p l i c i t y , b u t has n o t b e c o m e p o p u l a r b e c a u s e it is s e n s i t i v e to L F E II.
5.2.1
Through-lens
analyzers
with
hemispherical
retarding
fields
The commercial dedicated EBTs from Hitachi [132-134], ICT [135140] a n d C a m b r i d g e I n s t r u m e n t s [141] m a k e u s e of t h r o u g h - l e n s analyzers with hemispherical grids. A c c o r d i n g to [121], in the c a s e of ICT's through-lens analyzer, the p r i n c i p l e of the p o s t - l e n s analyzer with a reduced LFE described in s e c t i o n 5.1 was m o d i f i e d so t h a t the c e n t e r of the s p h e r i c a l r e t a r d i n g f i e l d n o w c o i n c i d e s w i t h an SE i m a g e of the m e a s u r i n g p o i n t [135]. H i t a c h i a p p e a r e d to u s e the s a m e p r i n c i p l e , a l t h o u g h no d i r e c t r e f e r e n c e w a s m a d e to t h i s in [ 1 3 2 - 1 3 4 ] , a n d t h e s e 3 p a p e r s a l s o s p e c i f y v a r i o u s n u m b e r s of g r i d s and different voltage values at the grids. The m a i n difference between the two a n a l y z e r s lies in t h e i r PE f o c u s i n g .
E. Plies / Electron-optical components for e-beam testing
197
H i t a c h i uses a d o u b l e lens as the objective, so that the PE beam is t e l e c e n t r i c at the point of the h e m i s p h e r i c a l grids. This gives rise to a s o m e w h a t larger center hole in the spherical grids, so that it may h a p p e n that a higher p r o p o r t i o n of the p a r a x i a l l y e m i t t e d SEs cannot be d e t e c t e d c o r r e c t l y or even at all. In the case of the ICT analyzer, a high e x t r a c t i o n field as well as a r e l a t i v e l y high r e t a r d i n g field is used. To e l i m i n a t e lens effects of the axial r e t a r d i n g - f i e l d grid mesh with r e s p e c t to the PE beam from the outset, therefore, a PE i n t e r m e d i a t e image was placed in the hemispherical retarding field (see Fig. ic). The high e x t r a c t i o n field (ikV/mm) in the ICT analyzer g r e a t l y reduces LFE I but m a k e s the v o l t a g e m e a s u r e m e n t s on p a s s i v a t e d or buried i n t e r c o n n e c t i o n s d i f f i c u l t to perform, due to charging. D e v i a t i n g from the first d e s i g n [135,138], J. Frosien [140] therefore i n t r o d u c e d an a d d i t i o n a l electrode d i r e c t l y above the IC, w h i c h w e a k e n s the e x t r a c t i o n field there and permits good m e a s u r e m e n t s on p a s s i v a t e d ICs [140]. This a d d i t i o n a l e l e c t r o d e ensures that the f o c u s i n g p r i n c i p l e of the SEs is not changed. No further d e t a i l s are known about the t h r o u g h - l e n s a n a l y z e r from Cambridge Instruments [141], apart from the fact that it is, i n c l u d i n g the scintillator, entirely symmetrical about the PE beam. In contrast, the systems from Hitachi and ICT each have two d e t e c t o r s a r r a n g e d o p p o s i t e each other. T.T. Tang et al. [131] s p e c i f y a t h r o u g h - l e n s a n a l y z e r which is very similar to the ICT device, but can work e f f e c t i v e l y with plane grids. This is possible due to its configuration, since the PEs have a high beam v o l t a g e of i0 kV. The a s s o c i a t e d high lens e x c i t a t i o n means that the SEs ( e x t r a c t i o n v o l t a g e 820V) are i n i t i a l l y focused and then collimated.
5.2.2
Magnetic-collimating
analyzers
It is k n o w n from plasma physics that a "magnetic mirror" or "magnetic bottle" can confine charged p a r t i c l e s [148, 149]. This p r i n c i p l e was first used, in an inverse sense, with the specimen in the "neck" of the bottle, by P. Kruit and F.H. Read [150] to build a m a g n e t i c field p a r a l l e l i z e r for a p h o t o e l e c t r o n spectrometer. S.C.J. G a r t h et al. [151] were the first to build an analyzer for e - b e a m testing w h i c h m a g n e t i c a l l y c o l l i m a t e s the SEs in this manner. For this purpose, they i n i t i a l l y [151-153] fitted a single p o l e - p i e c e lens as per T. M u l v e y [84] under the IC. In a later p a p e r [154], S.C.J. Garth made a critical study of the purely m a g n e t i c e x t r a c t i o n of the SEs. A s i g n i f i c a n t d i s a d v a n t a g e he m e n t i o n s is the high LFE I. An additional e l e c t r o s t a t i c e x t r a c t i o n to reduce the LFE I would prevent the a d i a b a t i c SE motion, w h i c h is p r e c i s e l y what allows this p r o c e d u r e to record a low LFE II. S.C.J. G a r t h t h e r e f o r e suggests a virtual i m m e r s i o n lens spectrom e t e r [155,156] (in w h i c h the single pole piece under the IC is obviated) in his paper [154] for the first time. In this clever technique, the SEs are i n i t i a l l y e x t r a c t e d e l e c t r o s t a t i c a l l y and then r e t a r d e d again in the field m a x i m u m of a m o d i f i e d p i n - h o l e lens. Finally, the SEs are c o l l i m a t e d in the a d i a b a t i c a l l y declining m a g n e t i c field of the second lens half. In the a n a l y z e r of the commercial EBT from S e n t r y S c h l u m b e r g e r [157,158] the SEs are also m a g n e t i c a l l y collimated. For this purpose, a V A I L [86-88] is used and the a n a l y z e r p o s s e s s e s a plane r e t a r d i n g field. P. Kruit and L. D u b b e l d a m have s u g g e s t e d and i m p l e m e n t e d a c o m b i n a t i o n of a VAIL and a t r o c h o i d a l a n a l y z e r with
E. Plies / Electron-optical components for e-beam testing
198
two e n e r g y c h a n n e l s [159,160]. Unfortunately, this p r o m i s i n g sugg e s t i o n has n o t e n t i r e l y f u l f i l l e d the e x p e c t a t i o n s , as a c c o r d i n g to [162], it has h i t h e r t o p r o v e d i m p o s s i b l e to a c h i e v e g o o d performance d a t a for the PE a n d SE o p t i c s simultaneously. A small chromatic aberration of the V A I L is c o u p l e d w i t h a r e l a t i v e l y h i g h e n e r g y e r r o r (0.5 eV) of the t r o c h o i d a l a n a l y z e r , w h e r e a s a small e n e r g y e r r o r g i v e s r i s e to a r e l a t i v e l y high aberration coeffic i e n t (i0 mm) of the a x i a l c h r o m a t i c a b e r r a t i o n . Nevertheless, the multichannel p r o c e d u r e p r o p o s e d by L. D u b b e l d a m a n d P. K r u i t [160] with a dispersive a n a l y z e r c o u l d be a s t e p in the r i g h t d i r e c t i o n . S - c u r v e s or s p e c t r o m e t e r constants can be u s e d for c o m p a r i n g the various through-lens analyzers. However, a much more rigorous comparative t e s t is the m e a s u r e m e n t of k n o w n s i g n a l s , w h i c h are i n j e c t e d i n t o a t e s t c h i p or a p a s s i v e IC w i t h i n t e r c o n n e c t i o n s of maximum fineness. It c a n t h e n be s e e n h o w w e l l the l e v e l is m e a s u r e d a n d h o w h i g h the s e n s i t i v i t y is w i t h r e s p e c t to l o c a l fields. S i n c e t h e a u t h o r was i n v o l v e d in t h e d e v e l o p m e n t of the e l e c t r o n optical column of a commercial EBT, he intends to make no comparative evaluation at t h i s point, b u t m e r e l y to r e f e r to the LFE measurements in [ 1 3 2 , 1 3 3 , 1 3 8 , 1 4 0 , 1 5 2 , 1 5 6 - 1 5 8 , 1 7 1 ] . In his own comparison, the reader should take into consideration the additional conditions of these LFE measurements such as interconnection w i d t h , s p a c i n g to the n e x t i n t e r c o n n e c t i o n a n d the d e g r e e of i n t e r f e r e n c e the s i g n a l s h a v e on t h e n e i g h b o r i n g interconnections. Signals p r e s e n t on n e i g h b o r i n g interconnections can g i v e r i s e to c r o s s t a l k b y m e a n s of L F E II.
5.3
The
detector
unit
W h e n the SEs h a v e t r a v e r s e d the r e t a r d i n g - f i e l d grid, they must s t i l l - for c e r t a i n t y p e s of a n a l y z e r s - (with a p l a n a r or s p h e r i cal r e t a r d i n g f i e l d g r i d n o r m a l to the o p t i c a l axis) be g u i d e d to o n e or two E v e r h a r t - T h o r n l e y detectors. Beam guidance s y s t e m s designed specially for this p u r p o s e w h i c h d e s e r v e m e n t i o n are the m o d i f i e d d o u b l e s t i g m a t o r by S.C.J. G a r t h et al. [ 1 5 3 , 1 5 5 ] , the 0o r d e r W i e n f i l t e r b y T. O t a k a et al. [164] a n d the i s t - o r d e r W i e n filter (crossed quadrupole fields) b y J. Zach a n d H. R o s e [165]. The l a t t e r w a s c o n s t r u c t e d a n d t e s t e d b y R. S c h m i d a n d M. B r u n n e r [166,167]. W e a l s o w i s h to d r a w the r e a d e r ' s attention to the s i m p l e b u t f a s c i n a t i n g c o a x i a l t u b e s y s t e m by N. T a m u r a [163], in w h i c h t h e PE b e a m is t o t a l l y s h i e l d e d f r o m the s c i n t i l l a t o r field. Systems with a coaxially arranged multichannel plate, e.g. [124], b y p a s s the b e a m g u i d a n c e p r o b l e m m e n t i o n e d above. In the a n a l y z e r d e s i g n e d b y G.S. P l o w s [116], w h i c h is t o t a l l y r o t a t i o n a l l y symmet r i c a l - as far as the s c i n t i l l a t o r - the t o p o l o g i c a l conversion p r o b l e m is s h i f t e d to the o p t i c a l w a v e g u i d e system. T h e s a m e is t r u e for the a n a l y z e r b y A.R. D i n n i s [ 1 4 2 - 1 4 5 ] , w h i c h u s e s a nonr o t a t i n g f i n a l lens a n d w h o s e r a d i a l m a g n e t i c f i e l d e x t r a c t i o n is i m p r e s s i v e f r o m an e l e c t r o n - o p t i c a l standpoint.
5.4
Future
trends
E. W o l f g a n g et al. [171] h a v e s h o w n t h a t e - b e a m t e s t i n g can be p e r f o r m e d on 0.75 ~m i n t e r c o n n e c t i o n lines u s i n g ICT's c o m m e r c i a l e-beam tester. B u t the s t r u c t u r a l reduction in m i c r o e l e c t r o n i c s has c o n t i n u e d u n a b a t e d , so t h a t the i n f l u e n c e of the local fields
E. Plies / Electron-optical components for e-beam testing
199
has increased. B. Wolf et al. [146] d e s c r i b e a c o m p e n s a t i o n m e t h o d for the LFE by c o m b i n i n g a w e a k e l e c t r o s t a t i c e x t r a c t i o n field with a s u i t a b l e o b j e c t i v e lens field, through w h i c h a part of the h i g h - e n e r g y SEs can pass only under the i n f l u e n c e of the local field. The c o m p e n s a t i o n is u n f o r t u n a t e l y p e r f o r m e d at the cost of m e a s u r i n g time. In addition, the model taken as a basis for the local field was not very realistic, as the authors t h e m s e l v e s admit, so their c a l c u l a t i o n s did not agree well with existing m e a s u r e m e n t s on 1 ~m interconnections. N e v e r t h e l e s s , this a p p r o a c h is w o r t h i n v e s t i g a t i n g further. In v i e w of the problems a s s o c i a t e d with the LFEs, the use of a l t e r n a t i v e analyzers, i.e. n o n - r e t a r d i n g - f i e l d analyzers, should be more e n e r g e t i c a l l y looked at. An example of an a l t e r n a t i v e of this kind is the time-dispersive detector suggested by A. K h u r s h e e d and A.R. Dinnis [147]. The use of e n e r g y - d i s p e r s i v e multichannel analyzers, as suggested by P. Kruit and L.D. D u b b e l d a m [159, 160], for example, should also be further pursued. The evaluations of energy-dispersive analyzers by E. M e n z e l and E. K u b a l e k [119] on the one hand and those in [159,160] on the other hand are contradictory. The author feels that the e v a l u a t i o n of the e n e r g y - d i s p e r s i v e a n a l y z e r in [119] led to results w h i c h w e r e too poor (due to the d e r i v a t i o n of the measuring error from the energy d i s t r i b u t i o n m a x i m u m and i n t e g r a t i o n over all e m i s s i o n angles) and those in [159,160] w e r e too good (because the local fields were ignored). In principle, m o r e inform a t i o n is o b t a i n e d by d e t e c t i n g the SE s p e c t r u m than by det e r m i n i n g its integral alone, which is what the r e t a r d i n g - f i e l d s p e c t r o m e t e r does. This also applies to the d e g r a d e d spectrum. In an a n a l y z e r w i t h several energy channels, the u n f a l s i f i e d SE spectrum can c e r t a i n l y be c o r r e l a t e d w i t h respect to a d e g r a d e d and shifted s p e c t r u m and thus a q u a n t i t a t i v e v o l t a g e m e a s u r e m e n t can be performed. However, such an analyzer must be r e q u i r e d to operate d i s p e r s i v e l y in an o p e n - l o o p technique [169] and in integrating mode in a feedback loop. Analyzers w h i c h could do this after some m o d i f i c a t i o n are in p r i n c i p l e a l r e a d y known [113, 114, 127, 128, 170]. But they have other drawbacks, so there is still work to be done here.
6. C O N C L U S I O N S
The a n a l y z e r for q u a n t i t a t i v e voltage m e a s u r e m e n t is the only electron-optical component for w h i c h real e l e c t r o n - o p t i c a l research w o r k must still be done to permit chip v e r i f i c a t i o n with the aid of e - b e a m testing for future ICs (from the 64 MB DRAM and beyond). A n o t h e r i m p o r t a n t e l e c t r o n - o p t i c a l p r o b l e m is the Boersch effect [172-183], w h i c h in the future must be taken even more s t r o n g l y into a c c o u n t in defining the overall beam path of the l o w - v o l t a g e column.
ACKNOWLEDGEMENTS
The a u t h o r wishes to thank Dr.M. Guntersdorfer, Prof. H.-J. Pfleiderer and Dr.E. W o l f g a n g for their general support, R. M i c h e l l for the t r a n s l a t i o n and Mrs. U. K r i e b i t z s c h for typing the final m a n u s c r i p t .
E. Plies / Electron-optical componenL9 for e-beam testing
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ELECTRON-OPTICAL
COMPONENTS
189
FOR E - B E A M T E S T I N G
Erich PLIES
R e s e a r c h Laboratories, Siemens AG O t t o - H a h n - R i n g 6, D-8000 M ~ n c h e n 83, Federal R e p u b l i c of G e r m a n y
The e l e c t r o n - o p t i c a l components of an e - b e a m tester comprise the e l e c t r o n gun, the beam b l a n k i n g system, the lenses and the s e c o n d a r y e l e c t r o n analyzer. For each component, a concise r e v i e w of the various e x i s t i n g types is p r e s e n t e d and a c o m p a r i s o n made. For e - b e a m testing of future VLSI ICs (with i n t e r c o n n e c t i o n s < 0.75 ~m), significant i m p r o v e m e n t s need be made m e r e l y in the analyzer.
i. I N T R O D U C T I O N
Up to the m i d d l e of the eighties, m o d i f i e d scanning e l e c t r o n m i c r o s c o p e s (SEMs) w e r e used for e l e c t r o n - b e a m (e-beam) testing of i n t e g r a t e d circuits (ICs). The e l e c t r o n - o p t i c a l m o d i f i c a t i o n s of the SEM u s u a l l y c o m p r i s e d a b e a m - b l a n k i n g system b e t w e e n the anode and the first m a g n e t i c c o n d e n s e r lens and a simple r e t a r d i n g - f i e l d spectrometer between the m a g n e t i c o b j e c t i v e lens and the IC. Today, d e d i c a t e d e - b e a m testers (EBTs) are available. The elect r o n - o p t i c a l column of such an EBT must be o p t i m i z e d with respect to p r i m a r y e l e c t r o n (PE) probe forming (small d i a m e t e r of 0.i 0.3 ~m and high current > 1 nA for low beam v o l t a g e ~ 1 kV) and PE pulse g e n e r a t i o n (I0 ps for GaAs devices and = 200 ps for MOS ICs). A d d i t i o n a l l y , the c o l l e c t i o n of the s e c o n d a r y electrons (SEs) m u s t be o p t i m i z e d to achieve precise v o l t a g e m e a s u r e m e n t s and a n e g l i g i b l e influence of the local fields of the IC. The low e n e r g y of the PEs is n e c e s s a r y to avoid damage, c h a r g i n g and loading of the IC. O p t i m i z e d l o w - v o l t a g e optics are also of interest for other applications, e.g. w a f e r inspection, surface science or b i o c h e m i s t r y . Fig. 1 i l l u s t r a t e s the e l e c t r o n - o p t i c a l column of a d e d i c a t e d EBT [32, 138-140]. Part a) shows a schematic cross section through the column. The column consists - in p r i n c i p l e like every d e d i c a t e d EBT - of the following e l e c t r o n - o p t i c a l components: e l e c t r o n gun, b e a m - b l a n k i n g system, condenser lenses and a c o m p o u n d s p e c t r o m e t e r o b j e c t i v e lens w h i c h acts as an o b j e c t i v e for the PEs and a s p e c t r o m e t e r for the SEs. Part b) shows the axial d i s t r i b u t i o n of the p o t e n t i a l ~(z) = ~(0,0,z) r e f e r r e d to the cathode and the magnetic flux d e n s i t y B(z)=Bz(0,0,z), w h i c h are among the factors a f f e c t i n g the e l e c t r o n - o p t i c a l p e r f o r m a n c e data (cardinal elements, image aberrations). The kinetic energy of the axial primary e l e c t r o n s is e~(z). The stage is, of course, grounded, and the cathode has the p o t e n t i a l - U ~ with respect to this g r o u n d connection, w h e r e Up~ is the beam voltage of the p r i m a r y electrons at
0167-9317/90/$3.50 © 1990, Elsevier Science Publishers B.V.
190
E. Plies / Electron-optical components for e-beam testing
the IC. Part c) of Fig. 1 shows the l i g h t - o p t i c a l a n a l o g of the column and the p r i n c i p l e of p r i m a r y - e l e c t r o n (PE) focusing, with the e q u i v a l e n t e l e m e n t s of e l e c t r o s t a t i c o r i g i n (concave and convex lenses and p l a n e p a r a l l e l plates) shown dark and the equivalent lenses for the m a g n e t i c lens fields shown bright. The p r i m a r y and s e c o n d a r y electron-optical performance data of this d e d i c a t e d EBT is s u m m a r i z e d in Tables 1 and 2, r e s p e c t i v e l y , t o g e t h e r with the p e r f o r m a n c e of a m o d i f i e d SEM [168]. The c l e a r l y superior performance data of the d e d i c a t e d EBT w e r e a t t a i n e d by i m p r o v i n g the i n d i v i d u a l e l e c t r o n - o p t i c a l c o m p o n e n t s and the customized low-voltage column concept. In the following, we will p r e s e n t a c o n c i s e r e v i e w of the i n d i v i d u a l e l e c t r o n - o p t i c a l components of an EBT, discuss various instrument types and b r i e f l y touch upon i m p r o v e m e n t s still r e q u i r e d in the future. In so doing, we will r e f e r b a c k to Figure 1 and Tables 1 and 2. In this paper, o n l y EBTs for ICs and not for c o n d u c t o r n e t w o r k s or c i r c u i t packages are considered. In the d e t a i l e d reviews [6,7] a s p e c t s are also discussed.
a)
of
e-beam
testing,
b)
electron-optical
c)
Triode electron gun
UEx
Beam blanking deflector
~- UL-~ Immersion condenser lens
Magnetic condenser lens
® Compound spectrometer objective lens
UpE
.-
.. -,. ".
FIGURE 1 Schematic representation of the e l e c t r o n - o p t i c a l column of a d e d i c a t e d EBT (ICT 9010); b) D i s t r i b u t i o n s of the axial (cathode related) p o t e n t i a l ~(z) and the axial m a g n e t i c flux d e n s i t y B(z) along the optical z-axis. UEx = extraction voltage for the p r i m a r y electrons in the t r i o d e gun, UL = lens v o l t a g e of the i m m e r s i o n c o n d e n s e r lens, UE = extraction voltage for the secondary electrons and UpE = final b e a m v o l t a g e of the p r i m a r y electrons; c) L i g h t - o p t i c a l counterpart of the e l e c t r o n - o p t i c a l column and p r i m a r y - e l e c t r o n ray path. E q u i v a l e n t lenses of m a g n e t i c origin are in light print, w h e r e a s e q u i v a l e n t s of e l e c t r o s t a t i c origin (concave and convex lenses and plane p a r a l l e l plates) are shown dark. a)
E. Plies / Electron-optical components for e-beam testing
Cathode Condenser system Final PE-energy
Modified SEM
Dedicated EBT
w
LaB6
purely magnetic
immersion lens
2.5
1.0
Brightness for final PE energy [A cm2 sr 1]
1.2 • 104
3.2 • 104
Minimum PE-pulse width
1
0.15
purely magnetic
combined electrostatic - magnetic
11
2
[keV]
Ins]
Objective lens
191
TABLE 1 C o m p a r i s o n of the p r i m a r y - e l e c t r o n o p t i c a l p e r f o r m a n c e of two electron-beam testers - a modif i e d S E M [168] (based on a C a m b r i d g e I n s t r u m e n t s S150) a n d a d e d i c a t e d E B T [32, 81, 1 3 5 - 1 4 0 ] (ICT 9010). TABLE 2 C o m p a r i s o n of t h e s e c o n d a r y e l e c t r o n o p t i c a l p e r f o r m a n c e of t w o t y p e s of e l e c t r o n b e a m t e s t ers - m o d i f i e d S E M a n d a d e d i c a t e d EBT.
Working distance
Modified SEM
[ram]
Spherical aberration constant [ram]
70
10
Axial chromatic aberration constant
20
4
0.02
0.02
0.4
0.12
5
2.5
[mm]
Final aperture [rad] PE-probe diameter [~m]
PE-probe current [nA]
Type of spectrometer Location of spectrometer Detector Spectrometer constant
Dedicated EBT
planar retarding field
semispherical retarding field
post-lens
through-the-lens
one EverhartThornley
two EverhartThornley
5 • 10-8 V ~
"
5 • 10.9 V
Cross-talk
between 1,5 pm
< 40 %
< 3%
interconnection lines
2. C A T H O D E S
AND
ELECTRON
GUNS
For high voltage resolution and/or short measuring t i m e s of v o l tage waveforms, a m a x i m u m p o s s i b l e s p e c i m e n c u r r e n t m u s t be realized within a specified probe diameter. D u e to t h e i r l o w b r i g h t ness, e l e c t r o n g u n s w i t h t u n g s t e n c a t h o d e s are out of the q u e s tion. D u e to t h e i r l o w w o r k f u n c t i o n , L a B 6 s i n g l e - c r y s t a l cathodes s u p p l y a h i g h e r b r i g h t n e s s . T h e y h a v e b e e n i n t e n s i v e l y s t u d i e d and t e s t e d in t h e past, see [8-16], for e x a m p l e , a n d h a v e b e e n on the m a r k e t f o r a n u m b e r of years. In t h e o r y , the beam brightness is p r o p o r t i o n a l to the b e a m v o l tage. In t h e l o w - v o l t a g e r a n g e (< 2 kV), h o w e v e r , the b e a m b r i g h t n e s s d e c r e a s e s m o r e s t e e p l y if a t r i o d e e l e c t r o n g u n is used. T h i s is a s c r i b e d to s a t u r a t i o n e f f e c t s [17] a n d to t h e B o e r s c h e f f e c t [ 1 7 2 - 1 8 3 ] of the e l e c t r o n s in the gun. With a tetrode electron gun, this loss is a l r e a d y s m a l l e r [13], but an e v e n b e t t e r s o l u t i o n is the c o m b i n a t i o n , s h o w n in Fig. i, of a triode gun with a retarding immersion condenser lens [81,94,138,139]. B e f o r e the i m m e r s i o n c o n d e n s e r lens, the PEs h a v e an e n e r g y e U ~ x = 5 k e y and are t h e n r e t a r d e d b y e U = to e U p E > 500 eV. A b r i g h t n e s s of 3.2 104 A c m - 2 s r -z for U p E = 1 k V w a s t h u s o b t a i n e d for a ( 1 0 0 ) - o r i e n t e d LaB6 c a t h o d e w i t h a t e m p e r a t u r e of 1775 K [ 1 3 8 , 1 3 9 ] . The question naturally a r i s e s w h e t h e r it w o u l d n o t e m p l o y an e l e c t r o n g u n w i t h a f i e l d - e m i s s i o n cathode
be b e t t e r to f r o m the out-
192
E. Plies / Electron-optical components for e-beam testing
set. Papers [18-27] deal, inter alia, w i t h a c o m p a r i s o n of LaB6 with field emission, and papers [19,22-24,26] even treat this subject under the a d d i t i o n a l s t a n d p o i n t of e - b e a m testing. This comp a r i s o n is not simple, because, for instance, the a b e r r a t i o n s of the o b j e c t i v e lens have a very s i g n i f i c a n t influence. In brief, it can be said that for a probe d i a m e t e r of less than 0.05 to 0.i ~m, the thermal field e m i s s i o n supplies a g r e a t e r s p e c i m e n current at the same probe d i a m e t e r than the LaB6 cathode. The a u t h o r agrees with L. Reimer, who points out on p. 31 of [4] that for p r o b e d i a m e t e r s ~ 0.i ~m, more s p e c i m e n c u r r e n t is supplied by the field e m i s s i o n and for probe d i a m e t e r s > 0.I ~m by the LaB6 cathode. This is also c o n f i r m e d by a c o m p a r i s o n of e x i s t i n g d e d i c a t e d EBTs. Indeed, Hitachi Ltd. (Ti-W thermal field emission) and ICT GmbH (LAB6) g u a r a n t e e [31,32] the same probe p e r f o r m a n c e , n a m e l y 0.i ~m, 1 nA and 1 kV. For an EBT with LaB6 cathode, the c r o s s - o v e r is imaged in r e d u c e d form, so that as a rule only the a b e r r a t i o n s of the o b j e c t i v e lens c o n t r i b u t e to inf l u e n c i n g the probe diameter. This is not the case for a system w i t h a f i e l d - e m i s s i o n cathode, so that the use of e l e c t r o n guns w i t h a s u p e r p o s e d m a g n e t i c lens field [ 2 3 , 2 6 , 2 8 - 3 0 , 3 3 - 3 8 ] brings good results. The d e d i c a t e d EBT from Hitachi Ltd. a l r e a d y possesses an e l e c t r o n gun of this kind [133, 134]. But it still remains open w h e t h e r the f i e l d - e m i s s i o n cathode will p r o v i d e the only m e a n i n g f u l solution for the e - b e a m testing of ICs with i n t e r c o n n e c t i o n line widths < 0.5 ~m (i. e. probe d i a m e t e r < 0.i ~m). This is b e c a u s e very p r o m i s i n g silicon a v a l a n c h e cathodes [39-41] are being developed. Their chopping s y s t e m could be d i r e c t l y i n t e g r a t e d for g e n e r a t i n g short e l e c t r o n pulses. Also p r o m i s i n g are l a s e r - t r i g g e r e d p h o t o c a t h o d e s [42-48], w h i c h not only p o s s e s s a very high brightness, but can also supply very short e l e c t r o n pulses, e.g. 4 ps [45] and have a l r e a d y been succ e s s f u l l y used by P.May et al. [45-47] for e - b e a m testing. In the case of t r a n s m i s s i o n p h o t o c a t h o d e s , it should also be p o s s i b l e to attain effective photoelectron source d i a m e t e r s < 5 ~m, thus e a s i l y o b t a i n i n g probe d i a m e t e r s < 0.i ~m. Using l a s e r - t r i g g e r e d p h o t o e l e c t r o n s reduces f l e x i b i l i t y w i t h r e s p e c t to s y n c h r o n i z a t i o n with the e l e c t r i c a l d r i v i n g of the IC. The e m i s s i o n of thermal e l e c t r o n s by laser pulse i r r a d i a t i o n is also p o s s i b l e [49]. With r e s p e c t to further details r e g a r d i n g e m i s s i o n p r o p e r t i e s and e l e c t r o n gun design, the reader is r e f e r r e d to C h a p t e r IX of [i].
3. E L E C T R O N
BEAM P U L S I N G
As a l r e a d y m e n t i o n e d in the previous chapter, e l e c t r o n pulses can be g e n e r a t e d even w i t h o u t an e-beam chopping system in the column. However, all c o m m e r c i a l l y a v a i l a b l e d e d i c a t e d EBTs and most modified SEMs p o s s e s s such an e - b e a m chopping system. A d i s t i n c t i o n should be m a d e here b e t w e e n two methods of g e n e r a t i n g short PE pulses, namely beam intensity modulation and beam d e f l e c t i o n across a c h o p p i n g aperture.
E. Plies / Electron-optical components for e-beam testing
193
3.1 Beam i n t e n s i t y m o d u l a t i o n
W e h n e l t m o d u l a t i o n has been used by various authors [50-54]. But this m e t h o d has not found general acceptance, b e c a u s e the electrical pulse has to be p r o d u c e d at high v o l t a g e and the c a p a c i t a n c e to be c h a r g e d and d i s c h a r g e d is too high [6]. In [55], an intensity m o d u l a t i o n w i t h a filter lens (special Einzel lens) was suggested. E s t i m a t e s showed that, due to t i m e - o f - f l i g h t effects, no pulses < 300 ps can be g e n e r a t e d with a chopper of this kind, even if the filter lens is c o n f i g u r e d as a triplate strip line. M. T r o y o n [56] has taken the g r a t e d the filter lens in the systems [55,56] is that the and both have a m o n o c h r o m a t o r
s u g g e s t i o n in [55] further and intee l e c t r o n gun. The a d v a n t a g e of these probe d i a m e t e r d e g r a d a t i o n is small effect.
Using the space focusing action of a k l y s t r o n cavity lens, L.C. O l d f i e l d [57] a t t a i n e d pulse widths of several ps at a repetition rate of 9 GHz. The phase focusing action of a k l y s t r o n cav i t y b u n c h e r was used by T. H o s o k a w a et al. [58], and H. H~bner and E. R 6 h m [60] used m i c r o w a v e field e m i s s i o n to g e n e r a t e 1 ps pulses at 14 GHz.
3.2 Beam d e f l e c t i o n
3.2.1 Plate c a p a c i t o r s D e f l e c t i o n across a chopping aperture by plate c a p a c i t o r s was used in [61-67]. Using the blanking system described in [66], M. B r u n n e r et al. [67] have r e p o r t e d 15 ps e l e c t r o n pulses, and in the m e a n t i m e have a l r e a d y attained 7 ps pulses with this relativ e l y simple and flexible b l a n k i n g system. Since the time resolution is l i m i t e d not only by the electron pulse w i d t h but also by other effects, e.g. the t r a n s i t i o n time effect of the SEs and the t i m i n g jitter of the control electronics, there seems to be no reason for the use of more complex d e f l e c t o r designs for the ebeam t e s t i n g of ICs. Probe b l u r r i n g due to energy spread of the PEs caused by the blanking p r o c e s s in the plate capacitor [68] could not be c o n f i r m e d as a r e l e v a n t degradation.
3.2.2 T r a v e l i n g wave structures and reentrant
cavity structures
T h r o u g h - t y p e t r a v e l i n g wave deflectors have also been used for ebeam t e s t i n g [69-74] and pulses < i0 ps have been r e p o r t e d [70,71]. The t r a n s v e r s e reentrant cavity s t r u c t u r e has a still h i g h e r d e f l e c t i o n s e n s i t i v i t y than the t r a v e l i n g w a v e deflectors. T. H o s o k a w a et al. [58] used such a t r a n s v e r s e r e e n t r a n t cavity in combination with an additional longitudinal reentrant cavity (klystron c a v i t y buncher). They g e n e r a t e d pulses of 0.2 ps and detected them. In terms of PE energy or pulse r e p e t i t i o n frequency, t r a v e l i n g wave and r e e n t r a n t cavity d e f l e c t o r s are not as flexible as the plate capacitor. Y.D. C h e r n o u s o v and I.V. S h e b o l a e v [59] s p e c i f y an u l t r a - h i g h frequency r e s o n a t o r - d e f l e c t o r w h o s e resonator f r e q u e n c y can be v a r i e d m e c h a n i c a l l y to some extent.
194
3.2.3
E. Plies / Electron-optical components for e-beam testing Multiple deflector
systems
Several authors [75-78] have r e p o r t e d on c o n f i g u r a t i o n s in which one plate c a p a c i t o r is placed before and other one after a PE cross-over, and the c h o p p i n g a p e r t u r e is located in the c r o s s - o v e r plane. T i m e - o f - f l i g h t effects are c o m p e n s a t e d to a c e r t a i n extent if the signal of the second d e f l e c t o r is delayed. J.T.L. Thong et al. [79] report on a new type of b e a m p u l s i n g stem w i t h a m u l t i - e l e c t r o d e d e f l e c t i o n element, w h i c h permits g e n e r a t i o n of pulses < 15 ps at r e p e t i t i o n rates of 1-50 GHz.
sythe
For a m o r e e x t e n s i v e c o m p a r i s o n of the various c h o p p i n g systems, the r e a d e r is r e f e r r e d to the reviews [6,7,63,80]. In [7], the various d r i v i n g m e t h o d s and types of a r r a n g e m e n t s are p r e s e n t e d in detail.
4. C O N D E N S E R AND O B J E C T I V E
LENSES
The c o n d e n s e r lenses and the o b j e c t i v e lens of all m o d i f i e d SEMs and of most d e d i c a t e d EBTs are p u r e l y magnetic. The d e s i g n of purely m a g n e t i c lenses is at the state of the art and their properties, i n c l u d i n g those of single p o l e - p i e c e lenses [84], are known [3,82,83,85]. The c o m m e r c i a l EBT from Sentry S c h l u m b e r g e r [157, 158] and the l a b o r a t o r y setup by L. D u b b e l d a m and P. Kruit [159162] use a v a r i a b l e - a x i s i m m e r s i o n lens (VAIL), w h i c h has a l r e a d y been s u c c e s s f u l l y used for a number of years in e l e c t r o n beam lit h o g r a p h y [86-88] and r e p r e s e n t s a clever s u p e r p o s i t i o n of the mag n e t i c lens and d e f l e c t i o n field. A c o n d e n s e r lens of the d e d i c a t e d EBT s c h e m a t i c a l l y shown in Fig.l is a c o m b i n e d e l e c t r o s t a t i c - m a g n e t i c lens [81,138]. In the compound s p e c t r o m e t e r o b j e c t i v e lens too, the e l e c t r o s t a t i c spectrom e t e r fields ( ~ / ~ @ 0 in Fig, ib) must be taken into account when c a l c u l a t i n g the PE focusing and the a b e r r a t i o n s [94]. This also applies to a n u m b e r of other in-lens or t h r o u g h - l e n s analyzers. If a m a g n e t i c d e f l e c t i o n element is added in the o b j e c t i v e lens, as in Fig. i, then we obtain a c o m b i n e d focusing and deflection s y s t e m w i t h s p a t i a l l y s u p e r i m p o s e d fields w h i c h is no longer q u i t e so simple to calculate. However, e x i s t i n g theories and programs [89-94] a l l o w the n u m e r i c a l c a l c u l a t i o n of c o m p o u n d systems from any s u p e r p o s e d e l e c t r o s t a t i c and m a g n e t i c lens and d e f l e c t i o n fields.
5. A N A L Y Z E R S AND D E T E C T O R S
The SE e n e r g y a n a l y z e r is the key e l e c t r o n - o p t i c a l c o m p o n e n t of any EBT. All p r e v i o u s l y used analyzers are r e t a r d i n g - f i e l d energy devices, w h i c h i n t e g r a t e over the SE s p e c t r u m or parts of it. Under ideal conditions, the d e t e c t e d SE current depends only on the p o t e n t i a l d i f f e r e n c e b e t w e e n the m e a s u r i n g point in the IC and the r e t a r d i n g - f i e l d grid. To linearize the q u a n t i t a t i v e v o l t a g e m e a s u r e m e n t , the e n e r g y a n a l y z e r is u s u a l l y o p e r a t e d in a feedback loop. The f o l l o w i n g e l e c t r o n - o p t i c a l requirements are made on today's analyzers:
E. Plies / Electron-optical components for e-beam testing
-
the SEs must be detected, their angle of emission,
as
far as
possible,
independently
195
of
- the d e t e c t i o n must, as far as possible, be i n d e p e n d e n t of existing microfields (local fields) of the IC or the a n a l y z e r must reduce these m i c r o f i e l d s as far as possible, -
the a n a l y z e r must not have a n e g a t i v e effect on the p e r f o r m a n c e of the p r i m a r y e l e c t r o n probe, e.g. give rise to u n w a n t e d def l e c t i o n or astigmatism,
- b e y o n d t h i s , t h e a n a l y z e r must be d e s i g n e d and c o n f i g u r e d so that it does not,in p r i n c i p l e , l i m i t the p e r f o r m a n c e of the PE probe. Up to 1984, e x c l u s i v e use was made of p o s t - l e n s analyzers. Starting w i t h the first in-lens analyzer by E. Menzel [129], a series of in-lens and t h r o u g h - l e n s analyzers were still p r o p o s e d and constructed. In the meantime, all d e d i c a t e d EBTs are e q u i p p e d with a n a l y z e r s of this kind, because they a l l o w both s u p e r i o r PE and SE p e r f o r m a n c e data to be attained, see Tables 1 and 2. However, this d o u b l e p e r f o r m a n c e gain was traded off a g a i n s t a more complex e l e c t r o n optical system in every d e d i c a t e d EBT. In the following, let us i n i t i a l l y present an o v e r v i e w of the various p o s t - l e n s analyzers and then discuss the v a r i o u s in-lens and t h r o u g h - l e n s analyzers. In v i e w of the ongoing increase of component i n t e g r a t i o n w i t h ever smaller i n t e r c o n n e c t i o n d i m e n s i o n s and thus i n c r e a s i n g field strength of the m i c r o f i e l d s at the surface of the ICs, we will at the c o n c l u s i o n of this chapter b r i e f l y discuss the q u e s t i o n of w h e t h e r r e t a r d i n g - f i e l d a n a l y z e r s do in fact r e p r e s e n t the best solution for e l e c t r o n - b e a m m e a s u r e m e n t technology.
5.1
Post-lens
analyzers
R e t a r d i n g - f i e l d analyzers have a l r e a d y been used in other sectors prior to e - b e a m testing of ICs and are still used in other a p p l i c a t i o n s today, see [95-108]. A p p l i c a t i o n sectors w e r e or are SE e m i s s i o n studies [96,106], Auger e l e c t r o n s p e c t r o s c o p y [97], e l e c t r o n s p e c t r o s c o p y for chemical analysis [98, i01] and M ~ b a u e r s p e c t r o s c o p y [108]. C o m p a r e d with d i s p e r s i v e e n e r g y analyzers, ret a r d i n g - f i e l d devices have a high l u m i n o s i t y but a low e n e r g y resolution. The latter results from the w i d t h of the cut-off curve of the r e t a r d i n g - f i e l d analyzer [95,119], w h i c h has a high-pass c h a r a c t e r i s t i c . This smeared cut-off c h a r a c t e r i s t i c of the retard i n g - f i e l d a n a l y z e r produces an effect on the form of the d e t e c t e d integral SE s p e c t r u m (S-curve). The slope of the S-curve d e c r e a s e s with increasing field strength of the r e t a r d i n g field, with g r o w i n g m e s h w i d t h of the r e t a r d i n g - f i e l d grid, and the S-curve is flatter for a planar r e t a r d i n g field than for a spherical one [95,99,100,119]. In e l e c t r o n - b e a m m e a s u r e m e n t technology, a flatter S - c u r v e means a w o r s e voltage r e s o l u t i o n (greater s p e c t r o m e t e r constant) [119]. The r e t a r d i n g - f i e l d analyzers used by A. G o p i n a t h et al. [109-111] for e - b e a m testing had h e m i s p h e r i c a l grids and were c o n s t r u c t e d s i m i l a r l y to the a n a l y z e r s used for the SE e m i s s i o n studies, e.g. [96]. They s u f f e r e d from a weak e x t r a c t i o n field, w h i c h was increased by Y. Goto et al. [112] by means of an a d d i t i o n a l control
196
E. Plies / Electron-optical components for e-beam testing
grid. J.M. H a n n a h [113] a n d L.J. B a l k et al. [114] u s e d c y l i n d r i cal d e f l e c t o r analyzers which have a band-pass characteristic and suffer from a long working distance (65 m m and 22 mm, r e s p e c t i v e l y ) . T h e a n a l y z e r d e s i g n e d by H.P. F e u e r b a u m [115] w a s o p t i m i z e d w i t h r e s p e c t to h e i g h t and e x t r a c t i o n f i e l d s t r e n g t h a n d is still w i d e l y used. T h e r e t a r d i n g field spectrometer by G.S. P l o w s [116] was t h e f i r s t c o m m e r c i a l l y available system (Lintech Instruments Ltd.). It is a t t r a c t i v e d u e to its r o t a t i o n a l symmetry and was evaluated in [117,118]. A l l p r e v i o u s l y m e n t i o n e d post-lens analyzers [ 1 0 9 - 1 1 6 ] w e r e c o m p a r e d in d e t a i l in [119] a n d / o r [120]. E. P l i e s [121] i n v e n t e d an a n a l y z e r in w h i c h a p l a n e e x t r a c t i o n field and a hemispherical r e t a r d i n g f i e l d are c o m b i n e d so t h a t the virtual source of the SEs coincides with the center of the hemispherical retarding field. A n a n a l y z e r of this t y p e w a s first constructed by K. N a k a m a e et al. [122-124]. D u e to its h i g h extraction in c o n j u n c t i o n w i t h the a n g l e - i n d e p e n d e n t detection of the SEs, a c c o r d i n g to [122-125] this a n a l y z e r p r o v i d e s a steeper S-curve, an i m p r o v e d spectrometer constant and a smaller local f i e l d e f f e c t [126] t h a n the F e u e r b a u m a n a l y z e r w i t h a p l a n e r e t a r d i n g field. In the local f i e l d e f f e c t (LFE), K. N a k a m a e et al. [126] m a k e a d i s t i n c t i o n b e t w e e n b e t w e e n two e f f e c t s or types. LFE I is c h a r a c t e r i z e d as the e f f e c t by w h i c h s l o w SEs c a n n o t o v e r c o m e a retarding local microfield a b o v e the i n t e r c o n n e c t i o n . However, the t r a j e c t o r i e s of SEs w h i c h are n o t r e f l e c t e d f r o m the m i c r o field can nevertheless still be a f f e c t e d by it, w h i c h a l s o leads to an e r r o r in the q u a n t i t a t i v e voltage measurement. This effect is d e s i g n a t e d as L F E II.
5.2
Through-lens
analyzers
The great disadvantage of a post-lens analyzer is the large working distance b e t w e e n the o b j e c t i v e lens a n d the IC. T h i s nec e s s a r i l y m a k e s the a x i a l a b e r r a t i o n s of the o b j e c t i v e lens relat i v e l y high, see T a b l e I, a n d an e l e c t r o n p r o b e w i t h small d i a m e ter and high current c a n n o t be g e n e r a t e d . This drawback can be avoided by m e a n s of i n - l e n s and t h r o u g h - l e n s analyzers. In the following, w e i n t e n d l a r g e l y to u s e the d e s i g n a t i o n through-lens a n a l y z e r q u i t e a p a r t f r o m the o r i g i n a l t e r m u s e d by the i n v e n t o r s for t h e i r a n a l y z e r . F o r in m o s t cases, the d e t e c t o r i t s e l f is pos i t i o n e d a b o v e the o b j e c t i v e lens. T h e first a n a l y z e r of this type was invented b y E. M e n z e l [129,130]. It is a t t r a c t i v e b e c a u s e of its s i m p l i c i t y , b u t has n o t b e c o m e p o p u l a r b e c a u s e it is s e n s i t i v e to L F E II.
5.2.1
Through-lens
analyzers
with
hemispherical
retarding
fields
The commercial dedicated EBTs from Hitachi [132-134], ICT [135140] a n d C a m b r i d g e I n s t r u m e n t s [141] m a k e u s e of t h r o u g h - l e n s analyzers with hemispherical grids. A c c o r d i n g to [121], in the c a s e of ICT's through-lens analyzer, the p r i n c i p l e of the p o s t - l e n s analyzer with a reduced LFE described in s e c t i o n 5.1 was m o d i f i e d so t h a t the c e n t e r of the s p h e r i c a l r e t a r d i n g f i e l d n o w c o i n c i d e s w i t h an SE i m a g e of the m e a s u r i n g p o i n t [135]. H i t a c h i a p p e a r e d to u s e the s a m e p r i n c i p l e , a l t h o u g h no d i r e c t r e f e r e n c e w a s m a d e to t h i s in [ 1 3 2 - 1 3 4 ] , a n d t h e s e 3 p a p e r s a l s o s p e c i f y v a r i o u s n u m b e r s of g r i d s and different voltage values at the grids. The m a i n difference between the two a n a l y z e r s lies in t h e i r PE f o c u s i n g .
E. Plies / Electron-optical components for e-beam testing
197
H i t a c h i uses a d o u b l e lens as the objective, so that the PE beam is t e l e c e n t r i c at the point of the h e m i s p h e r i c a l grids. This gives rise to a s o m e w h a t larger center hole in the spherical grids, so that it may h a p p e n that a higher p r o p o r t i o n of the p a r a x i a l l y e m i t t e d SEs cannot be d e t e c t e d c o r r e c t l y or even at all. In the case of the ICT analyzer, a high e x t r a c t i o n field as well as a r e l a t i v e l y high r e t a r d i n g field is used. To e l i m i n a t e lens effects of the axial r e t a r d i n g - f i e l d grid mesh with r e s p e c t to the PE beam from the outset, therefore, a PE i n t e r m e d i a t e image was placed in the hemispherical retarding field (see Fig. ic). The high e x t r a c t i o n field (ikV/mm) in the ICT analyzer g r e a t l y reduces LFE I but m a k e s the v o l t a g e m e a s u r e m e n t s on p a s s i v a t e d or buried i n t e r c o n n e c t i o n s d i f f i c u l t to perform, due to charging. D e v i a t i n g from the first d e s i g n [135,138], J. Frosien [140] therefore i n t r o d u c e d an a d d i t i o n a l electrode d i r e c t l y above the IC, w h i c h w e a k e n s the e x t r a c t i o n field there and permits good m e a s u r e m e n t s on p a s s i v a t e d ICs [140]. This a d d i t i o n a l e l e c t r o d e ensures that the f o c u s i n g p r i n c i p l e of the SEs is not changed. No further d e t a i l s are known about the t h r o u g h - l e n s a n a l y z e r from Cambridge Instruments [141], apart from the fact that it is, i n c l u d i n g the scintillator, entirely symmetrical about the PE beam. In contrast, the systems from Hitachi and ICT each have two d e t e c t o r s a r r a n g e d o p p o s i t e each other. T.T. Tang et al. [131] s p e c i f y a t h r o u g h - l e n s a n a l y z e r which is very similar to the ICT device, but can work e f f e c t i v e l y with plane grids. This is possible due to its configuration, since the PEs have a high beam v o l t a g e of i0 kV. The a s s o c i a t e d high lens e x c i t a t i o n means that the SEs ( e x t r a c t i o n v o l t a g e 820V) are i n i t i a l l y focused and then collimated.
5.2.2
Magnetic-collimating
analyzers
It is k n o w n from plasma physics that a "magnetic mirror" or "magnetic bottle" can confine charged p a r t i c l e s [148, 149]. This p r i n c i p l e was first used, in an inverse sense, with the specimen in the "neck" of the bottle, by P. Kruit and F.H. Read [150] to build a m a g n e t i c field p a r a l l e l i z e r for a p h o t o e l e c t r o n spectrometer. S.C.J. G a r t h et al. [151] were the first to build an analyzer for e - b e a m testing w h i c h m a g n e t i c a l l y c o l l i m a t e s the SEs in this manner. For this purpose, they i n i t i a l l y [151-153] fitted a single p o l e - p i e c e lens as per T. M u l v e y [84] under the IC. In a later p a p e r [154], S.C.J. Garth made a critical study of the purely m a g n e t i c e x t r a c t i o n of the SEs. A s i g n i f i c a n t d i s a d v a n t a g e he m e n t i o n s is the high LFE I. An additional e l e c t r o s t a t i c e x t r a c t i o n to reduce the LFE I would prevent the a d i a b a t i c SE motion, w h i c h is p r e c i s e l y what allows this p r o c e d u r e to record a low LFE II. S.C.J. G a r t h t h e r e f o r e suggests a virtual i m m e r s i o n lens spectrom e t e r [155,156] (in w h i c h the single pole piece under the IC is obviated) in his paper [154] for the first time. In this clever technique, the SEs are i n i t i a l l y e x t r a c t e d e l e c t r o s t a t i c a l l y and then r e t a r d e d again in the field m a x i m u m of a m o d i f i e d p i n - h o l e lens. Finally, the SEs are c o l l i m a t e d in the a d i a b a t i c a l l y declining m a g n e t i c field of the second lens half. In the a n a l y z e r of the commercial EBT from S e n t r y S c h l u m b e r g e r [157,158] the SEs are also m a g n e t i c a l l y collimated. For this purpose, a V A I L [86-88] is used and the a n a l y z e r p o s s e s s e s a plane r e t a r d i n g field. P. Kruit and L. D u b b e l d a m have s u g g e s t e d and i m p l e m e n t e d a c o m b i n a t i o n of a VAIL and a t r o c h o i d a l a n a l y z e r with
E. Plies / Electron-optical components for e-beam testing
198
two e n e r g y c h a n n e l s [159,160]. Unfortunately, this p r o m i s i n g sugg e s t i o n has n o t e n t i r e l y f u l f i l l e d the e x p e c t a t i o n s , as a c c o r d i n g to [162], it has h i t h e r t o p r o v e d i m p o s s i b l e to a c h i e v e g o o d performance d a t a for the PE a n d SE o p t i c s simultaneously. A small chromatic aberration of the V A I L is c o u p l e d w i t h a r e l a t i v e l y h i g h e n e r g y e r r o r (0.5 eV) of the t r o c h o i d a l a n a l y z e r , w h e r e a s a small e n e r g y e r r o r g i v e s r i s e to a r e l a t i v e l y high aberration coeffic i e n t (i0 mm) of the a x i a l c h r o m a t i c a b e r r a t i o n . Nevertheless, the multichannel p r o c e d u r e p r o p o s e d by L. D u b b e l d a m a n d P. K r u i t [160] with a dispersive a n a l y z e r c o u l d be a s t e p in the r i g h t d i r e c t i o n . S - c u r v e s or s p e c t r o m e t e r constants can be u s e d for c o m p a r i n g the various through-lens analyzers. However, a much more rigorous comparative t e s t is the m e a s u r e m e n t of k n o w n s i g n a l s , w h i c h are i n j e c t e d i n t o a t e s t c h i p or a p a s s i v e IC w i t h i n t e r c o n n e c t i o n s of maximum fineness. It c a n t h e n be s e e n h o w w e l l the l e v e l is m e a s u r e d a n d h o w h i g h the s e n s i t i v i t y is w i t h r e s p e c t to l o c a l fields. S i n c e t h e a u t h o r was i n v o l v e d in t h e d e v e l o p m e n t of the e l e c t r o n optical column of a commercial EBT, he intends to make no comparative evaluation at t h i s point, b u t m e r e l y to r e f e r to the LFE measurements in [ 1 3 2 , 1 3 3 , 1 3 8 , 1 4 0 , 1 5 2 , 1 5 6 - 1 5 8 , 1 7 1 ] . In his own comparison, the reader should take into consideration the additional conditions of these LFE measurements such as interconnection w i d t h , s p a c i n g to the n e x t i n t e r c o n n e c t i o n a n d the d e g r e e of i n t e r f e r e n c e the s i g n a l s h a v e on t h e n e i g h b o r i n g interconnections. Signals p r e s e n t on n e i g h b o r i n g interconnections can g i v e r i s e to c r o s s t a l k b y m e a n s of L F E II.
5.3
The
detector
unit
W h e n the SEs h a v e t r a v e r s e d the r e t a r d i n g - f i e l d grid, they must s t i l l - for c e r t a i n t y p e s of a n a l y z e r s - (with a p l a n a r or s p h e r i cal r e t a r d i n g f i e l d g r i d n o r m a l to the o p t i c a l axis) be g u i d e d to o n e or two E v e r h a r t - T h o r n l e y detectors. Beam guidance s y s t e m s designed specially for this p u r p o s e w h i c h d e s e r v e m e n t i o n are the m o d i f i e d d o u b l e s t i g m a t o r by S.C.J. G a r t h et al. [ 1 5 3 , 1 5 5 ] , the 0o r d e r W i e n f i l t e r b y T. O t a k a et al. [164] a n d the i s t - o r d e r W i e n filter (crossed quadrupole fields) b y J. Zach a n d H. R o s e [165]. The l a t t e r w a s c o n s t r u c t e d a n d t e s t e d b y R. S c h m i d a n d M. B r u n n e r [166,167]. W e a l s o w i s h to d r a w the r e a d e r ' s attention to the s i m p l e b u t f a s c i n a t i n g c o a x i a l t u b e s y s t e m by N. T a m u r a [163], in w h i c h t h e PE b e a m is t o t a l l y s h i e l d e d f r o m the s c i n t i l l a t o r field. Systems with a coaxially arranged multichannel plate, e.g. [124], b y p a s s the b e a m g u i d a n c e p r o b l e m m e n t i o n e d above. In the a n a l y z e r d e s i g n e d b y G.S. P l o w s [116], w h i c h is t o t a l l y r o t a t i o n a l l y symmet r i c a l - as far as the s c i n t i l l a t o r - the t o p o l o g i c a l conversion p r o b l e m is s h i f t e d to the o p t i c a l w a v e g u i d e system. T h e s a m e is t r u e for the a n a l y z e r b y A.R. D i n n i s [ 1 4 2 - 1 4 5 ] , w h i c h u s e s a nonr o t a t i n g f i n a l lens a n d w h o s e r a d i a l m a g n e t i c f i e l d e x t r a c t i o n is i m p r e s s i v e f r o m an e l e c t r o n - o p t i c a l standpoint.
5.4
Future
trends
E. W o l f g a n g et al. [171] h a v e s h o w n t h a t e - b e a m t e s t i n g can be p e r f o r m e d on 0.75 ~m i n t e r c o n n e c t i o n lines u s i n g ICT's c o m m e r c i a l e-beam tester. B u t the s t r u c t u r a l reduction in m i c r o e l e c t r o n i c s has c o n t i n u e d u n a b a t e d , so t h a t the i n f l u e n c e of the local fields
E. Plies / Electron-optical components for e-beam testing
199
has increased. B. Wolf et al. [146] d e s c r i b e a c o m p e n s a t i o n m e t h o d for the LFE by c o m b i n i n g a w e a k e l e c t r o s t a t i c e x t r a c t i o n field with a s u i t a b l e o b j e c t i v e lens field, through w h i c h a part of the h i g h - e n e r g y SEs can pass only under the i n f l u e n c e of the local field. The c o m p e n s a t i o n is u n f o r t u n a t e l y p e r f o r m e d at the cost of m e a s u r i n g time. In addition, the model taken as a basis for the local field was not very realistic, as the authors t h e m s e l v e s admit, so their c a l c u l a t i o n s did not agree well with existing m e a s u r e m e n t s on 1 ~m interconnections. N e v e r t h e l e s s , this a p p r o a c h is w o r t h i n v e s t i g a t i n g further. In v i e w of the problems a s s o c i a t e d with the LFEs, the use of a l t e r n a t i v e analyzers, i.e. n o n - r e t a r d i n g - f i e l d analyzers, should be more e n e r g e t i c a l l y looked at. An example of an a l t e r n a t i v e of this kind is the time-dispersive detector suggested by A. K h u r s h e e d and A.R. Dinnis [147]. The use of e n e r g y - d i s p e r s i v e multichannel analyzers, as suggested by P. Kruit and L.D. D u b b e l d a m [159, 160], for example, should also be further pursued. The evaluations of energy-dispersive analyzers by E. M e n z e l and E. K u b a l e k [119] on the one hand and those in [159,160] on the other hand are contradictory. The author feels that the e v a l u a t i o n of the e n e r g y - d i s p e r s i v e a n a l y z e r in [119] led to results w h i c h w e r e too poor (due to the d e r i v a t i o n of the measuring error from the energy d i s t r i b u t i o n m a x i m u m and i n t e g r a t i o n over all e m i s s i o n angles) and those in [159,160] w e r e too good (because the local fields were ignored). In principle, m o r e inform a t i o n is o b t a i n e d by d e t e c t i n g the SE s p e c t r u m than by det e r m i n i n g its integral alone, which is what the r e t a r d i n g - f i e l d s p e c t r o m e t e r does. This also applies to the d e g r a d e d spectrum. In an a n a l y z e r w i t h several energy channels, the u n f a l s i f i e d SE spectrum can c e r t a i n l y be c o r r e l a t e d w i t h respect to a d e g r a d e d and shifted s p e c t r u m and thus a q u a n t i t a t i v e v o l t a g e m e a s u r e m e n t can be performed. However, such an analyzer must be r e q u i r e d to operate d i s p e r s i v e l y in an o p e n - l o o p technique [169] and in integrating mode in a feedback loop. Analyzers w h i c h could do this after some m o d i f i c a t i o n are in p r i n c i p l e a l r e a d y known [113, 114, 127, 128, 170]. But they have other drawbacks, so there is still work to be done here.
6. C O N C L U S I O N S
The a n a l y z e r for q u a n t i t a t i v e voltage m e a s u r e m e n t is the only electron-optical component for w h i c h real e l e c t r o n - o p t i c a l research w o r k must still be done to permit chip v e r i f i c a t i o n with the aid of e - b e a m testing for future ICs (from the 64 MB DRAM and beyond). A n o t h e r i m p o r t a n t e l e c t r o n - o p t i c a l p r o b l e m is the Boersch effect [172-183], w h i c h in the future must be taken even more s t r o n g l y into a c c o u n t in defining the overall beam path of the l o w - v o l t a g e column.
ACKNOWLEDGEMENTS
The a u t h o r wishes to thank Dr.M. Guntersdorfer, Prof. H.-J. Pfleiderer and Dr.E. W o l f g a n g for their general support, R. M i c h e l l for the t r a n s l a t i o n and Mrs. U. K r i e b i t z s c h for typing the final m a n u s c r i p t .
E. Plies / Electron-optical componenL9 for e-beam testing
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