Fabrication and characterization of gold plated edges on thin substrates for measurement of e-beam diameter

Microelectronic Engineering 14 (1991) 183-196 Elsevier

183

Fabrication and characterization of gold plated edges on thin substrates for measurement of e-beam diameter M. Gentili, L. Grella, L. Luciani, M. Baciocchi, E. Di Fabrizio Istituto di Elettronica dello Stato Solido-- CNR, Via Cineto Romano 42, 1-00156 Roma, Italy

B. A. Wallman Leica Cambridge, Clifton Road CB1 3QH Cambridge, UK Received November 30, 1990 Accepted March 26, 1991

Abstract. Gold plated edges were fabricated on various substrates and were used as targets for precise electron beam diameter measurement and for system calibration in a commercial Cambridge Instruments e-beam lithography machine. Use of 4 ~m square gold plated marker (0.85 p.m thick), fabricated on 2 p.m thick silicon nitride membrane, was found to enhance significantly the marker-to-substrate signal ratio, in comparison to commonly used lifted-off marker, and thus has improved the reliability of beam diameter measurement accuracy at low beam current for a 40 kV beam voltage. In a statistical sampling of about 2000 measurements, an accuracy of 5 nm (1 sigma value) has been obtained for 38 nm beam diameter by using this gold plated edge on silicon nitride membrane. Further, a Monte Carlo program, which models the effects of electron scattering taking place in target and substrate was used to increase the understanding of the experimental results. Simulated contrast behaviour was found to agree well with the experimental data for all three types of marker used in this study. Finally, the influence of the geometrical shape of the marker edge on beam diameter measurement was studied. By using an appropriate analytical function it was possible to take into account the effect of not perfectly vertical edge on small beam diameter measurements.

Key words. Electron beam diameter measurement, Monte Carlo simulation, electron scattering, electroplating.

0167-9317/91/$03.50

© 1991--Elsevier Science Publishers, B.V.

184

M. Gentili et al. / Gold plated edges on thin substrates Massimo Gentili was born in Italy in 1959. He received his degree in Solid

State Physics from the University of Rome in 1984. He joined the Solid State Electronics Institute of CNR (CNR-IESS) in 1985, working in the field of e-beam and X-ray lithography. His interests include X-ray mask making, lithography simulation and patterning and fabrication on I I I - V compounds. He is the head of the microlithography group at CNR-IESS.

Luca Grella was born in Rome, Italy in 1961. He joined the CNR-IESS

Institute in Rome in 1988 where he has been working on the simulation of high-resolution electron beam scattering processes. He received his degree in Electron Physics from the University of Rome in 1990. He is responsible for the lithography simulation research at the microlithography group of CNR-IESS Institute.

Laura Luciani was born in Italy in 1962. She received her degree in Physics

from the University of Rome in 1988. Her past activities dealt with optical and thermal properties of semiconductors. Now she is working as researcher in the fields of e-beam and X-ray lithography at the microlithography group of CNR-IESS Institute.

Marco Baciocchi was born in Rome, Italy in 1961. He received his degree

in Electron Physics from the University of Rome in 1987. His past activities dealt with the electronic noise in materials and devices, particularly 1/f noise. Recently he joined the microlithography group at CNR-IESS Institute, where he is involved in metrology and structure analysis.

M. Gentili et al. / Gold plated edges on thin sub~trates

185

Enzo Di Fabrizio was born in Italy in 1961. He received his degree in

Physics from the University of Rome in 1987. His past activities dealt with the study of magnetic properties of alloys like FesoCos0 by means of neutron scattering. Moreover he worked in the field of systems with hydrogen bonds such as H20 and alcohols using Raman spectroscopy. Recently he joined the microlithography group of CNR-IESS Institute, where he is involved in metrology and structure analysis.

Bernard Wallman gained a Higher National Diploma in Electrical Engineering from Cambridge College of Arts and Technology at the completion of an industry-based apprenticeship with Cambridge Instruments. He worked in engineering design on a wide range of scientific products, specialising in digital system design and computer control. In 1972, as Project Manager, he lead the design of the first commercially supplied "EBMF" e-beam lithography systems, of which 70 units have been installed. Since 1980 Mr Wallman has held a variety of marketing and application posts and is currently Technical Marketing Manager for Leica Cambridge Ltd for EBML/EBPG systems.

1. I n t r o d u c t i o n

O n e of the foremost demands in high-resolution pattern writing by e - b e a m lithography is the precise control of b e a m diameter ( B D ) during the patterning process, as even a small B D change can severely affect the delineated pattern linewidth in the resist. This linewidth fluctuation as a result of B D change during the e - b e a m writing process b e c o m e s a more severe p r o b l e m in high-sensitivity resists [1]. Thus m e a s u r e m e n t and control of B D has b e c o m e an essential exercise in any e - b e a m lithography tool. There are several methods to measure b e a m diameter. Rishton et al. have m e a s u r e d the b e a m diameter by sweeping the electron b e a m across a "knife e d g e " target and by measuring the transmitted electron flux in a Faraday cup [2]. This technique is effective in some particular hardware and it is not always feasible to incorporate such an arrangement in a commercial e - b e a m lithography ( E B L ) machine for daily use. O u r E B M F - 6 machine is e q u i p p e d with an annular channel plate electron multiplier, which is sensitive to a wide range of electron energies [3], and has built-in hardware and software for B D m e a s u r e m e n t diagnosis. In such an E B L machine, B D m e a s u r e m e n t is normally accomplished by using lifted-off gold on bulk silicon marker as target. In this p a p e r we describe the application of gold plated edge markers on different substrate as target for B D m e a s u r e m e n t in our commercial e - b e a m lithography machine. Gold plated edge markers on bulk silicon nitride membrane (2 ~m thick) were fabricated at I E S S - C N R as described in Section 2.

186

M. Gentili et al. / Gold plated edges on thin substrates

Experimental and theoretical investigations have been performed in order to characterize the effect of beam energy (in the 20-40 kV range), beam current and marker type on the detected signal. In this study, three different kinds of markers were used: the standard gold lifted-off marker on bulk silicon, which was provided by Cambridge Instruments, the gold plated marker on bulk silicon, and gold plated marker on silicon nitride membrane. A Monte Carlo simulation program was used to analyze the experimental results. The marker signal was computed at different values of beam voltage by the Monte Carlo program and was compared to the experimental one. Finally, as a further investigation to the work done by Rishton et al. [2] and Chisholm [4], the influence of the marker geometrical shape on signal response is also analyzed in this paper. This effect is taken into account to obtain the correct BD value, when using a small beam diameter or a non-vertical edge marker.

2. Marker fabrication

The substrates used for marker fabrication were bulk silicon and Si3N4 m e m b r a n e (2 Ixm thick). The substrate surface was first coated by evaporation of chromium (100 ~ thick) and gold (200 A thick), to ensure a conductive layer for the subsequent electroplating deposition purpose. A layer of 1 Ixm thick P M M A resist was spun on the substrate. The marker pattern (4 Ixm square area) was exposed using an accelerating voltage of 40 kV with a dosage of about 450 p,C/cm 2. The vertical resist profile was achieved using a dilute developer, M I B K : I P A in the ratio 1:3, for a total development time of 6 min. Finally, using a gold cyanide bath solution, the gold transferred resist image of thickness 0.85 Ixm was obtained. Fabricated gold plated marker on silicon membrane substrate is shown in Fig. 1.

3. Measurement of signal contrast

Signal contrast for lifted-off marker, gold plated marker on bulk silicon and on silicon nitride m e m b r a n e was experimentally measured using beam energies in the range 20-40 kV. Reflected electrons from the marker and the substrate, as the beam scans across the edge with a step of 10 nm, were collected by an annular channel plate detector. Experimentally obtained signal profiles for these three types of markers at 20, 30 and 40 kV are shown in Figs. 2(a), (b) and (c) respectively. Figure 3 compares the signal profiles of the plated marker on silicon nitride m e m b r a n e at 20, 30 and 40 kV. The beam probe current was kept at 1 n A in all cases and the signal values are in arbitrary units of the 8-bit (maximum of 255 units) analog-to-digital converter used for level digitisation. The signal contrast for each type of marker was calculated using the expression:

M. Gentili et al. / Gold plated edges on thin substrates

187

Fig. 1. SEM picture of 0.85 I*m thick gold plated marker fabricated on silicon nitride membrane. C = (Smax -- Smin) (Sma x -~- Smin)

where C is the marker signal contrast, Smin is the background level (where the signal is flat and is not affected by the shadowing effect of the marker edge [5]) and Smax is the marker signal level. Table 1 summarizes the experimentally obtained contrast for these three kinds of markers. From Table 1, it is evident that the signal contrast for standard lifted:off gold marker is decreasing with increasing accelerating voltage. However, the contrast for gold plated marker increases with increasing accelerating voltage, and maximum contrast is obtained at 40 kV for gold plated marker on thin silicon nitride membrane. The contrast reduction with increasing beam voltage for lifted-off gold marker is mainly due to the low number of reflected electrons that are collected by the detector, as lifted-off gold (normally 1000-1500/~ thick) becomes transparent to high-energy electrons. On the other hand, 0.85 p,m thick gold plated edge markers remain opaque to these high-energy electrons and, consequently, large numbers of reflected electrons are detected even at 40 kV accelerating voltage (see Fig. 2(c) and 3). Thus, reliable and consistent BD measurements can be accomplished by the use of these gold plated edge markers. For a better understanding of these measurements, we computed the reflected electron signal using a Monte Carlo simulation program. This program is based on the screened Rutherford single scattering elastic cross-section and Bethe energy loss theory. Secondary electrons are not considered [5]. In a

M. Gentili et al. / Gold plated edges on thin substrates

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sampling of a large number of trajectories (about 20,000), accurate statistical information of electron scattering throughout the sample is obtained. Monte Carlo results are summarized in Table 2 and show that electron reflection, in the case of lifted-off gold marker (of 1000 A thickness), decreases from 43% to 25% for a change of accelerating voltage from 20 kV to 40 kV; on the other hand, in the case of gold plated marker on membrane, the amount of reflected electrons from gold is almost constant and the background component from the thin substrate decreases considerably when the beam voltage is increased (see Table 2). The balance of electrons reflected from the marker and the substrate yields the experimental signal detected by the channel plate. Figures 4 and 5

Table 1 Experimentally obtained contrast for the three types of marker as a function of accelerating voltage Marker type Accelerating voltages

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Au on Si bulk

Au on Si3N4

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190

M. Gentili et al. / Gold plated edges on thin substrates

Table 2 Percentage of reflected electrons (both from marker and from substrate) EHT

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Marker 2

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compare the MC simulated reflected electron signal to the experimental one in the case of gold plated marker on silicon at 20 and 40 kV.

4. Statistical analysis of BD measurement The BD measurement was performed using the standard Cambridge Instruments BD measure routine (QSYS V.9.01). This routine uses the BD definition of Full Width at Half Maximum (FWHM) [6]"

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M. Gentili et al. / Gold plated edges on thin substrates

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e l e c t r o - o p t i c s characteristics of the m a c h i n e c o l u m n and with the e x p e c t e d statistical accuracy o f the m e a s u r e m e n t routine. Finally, the m e m b r a n e target was u s e d to characterize the o p e r a t i o n a l brightness o f the e l e c t r o n s o u r c e u s e d in our s y s t e m by taking B D m e a s u r e m e n t s over a w i d e range o f b e a m currents. T h e s e data are p l o t t e d in Fig. 7 as B D square

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M. Gentili et al. / Gold plated edges on thin substrates

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vs probe current I; from this plot, the system aberrations, noise disk diameter BDo and the source brightness/3 values w e r e obtained [7]. Their values at 40 kV accelerating voltage were found to be: BDo ~ 43 nm, /3 w_4.15 x 10 6 m cm-2sr -1.

5. Description of gold plated edge signal by an analytical function In this section, we investigate the effect of the marker edge on the reliability of BD measurements on non-vertical edges. We developed a simplified model to analyze the influence of relevant parameters on the signal response. In this model we have assumed that the reflected electron signal, detected by sweeping a marker edge with a gaussian beam, may be described as a convolution of three distinct terms: a gaussian term representing the beam, the target transfer function and the detector transfer function. Assuming the detector transfer function as a constant in the scan direction, the signal profile can be estimated according to: (2)

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194

M. Gentili et al. / G o l d plated edges on thin substrates

that edge reflectivity in the scan direction is represented by eqn. (4), which also represents with a good approximation the target transfer function. For perfectly vertical edge markers, b e a m diameters can be c o m p u t e d from signal data by (1); otherwise, if the m a r k e r edge is not vertical, the result of expression (1) exceeds the actual BD value. Thus the influence of the m a r k e r edge slope on BD m e a s u r e m e n t s has been examined by means of function (2). This study has been p e r f o r m e d by using expression (2) to fit the experimental signal profile, by using the edge half width m and the gaussian half width w as parameters of the m i n i m u m square program [8]. F r o m the parameter_w, the b e a m diameter value BD is d e t e r m i n e d by the simple relation: BD = "~/2w, for the following two markers: (a) gold plated m a r k e r with vertical edge (when m = 0); (b) gold plated m a r k e r with non-vertical edge. In the first case, besides BD evaluation, we verified the edge verticality by means of the value of m. The experimental signal profile and its fit are shown in Fig. 9(a). Fit parameters and X 2 values, corresponding to a forty-data sample, are shown in Table 3. It is important to point out that: • m v a l u e s , as expected, were found to be very close to zero (vertical condition): • BD value is equal to 66.5 nm, in a g r e e m e n t with the experimental value obtained by eqn. (1). In the second case, for non vertical m a r k e r edge, first the m value was measured by SEM, and subsequently the fit procedure was applied. The experimental signal profile and related fit are shown in Fig. 9(b). Fit parameters values are summarized in Table 3. The m value resulting from the fit program is in good a g r e e m e n t with SEM m e a s u r e m e n t s within the experimental error and a corrected BD value has been obtained. This result confirm the effectiveness of the model to obtain corrected BD values, even when the m a r k e r signal response is affected by the edge slope. Thus, this analytical m e t h o d has shown its advantage to determine very small BD values, since the tolerance on the edge verticality is strictly related to the width of the gaussian beam. In quantitative evaluation, we found that a negligible error in BD m e a s u r e m e n t s (~<1%) occurs, when the value of m is less than BD/2. Since it is very difficult to meet this condition when we consider very small e-beams, the analytical m e t h o d becomes useful, in this case~ to avoid a considerable error in BD determination.

Table 3 Fit p a r a m e t e r s and X 2 values c o r r e s p o n d i n g to a sample of 40 data

Marker

BD (nm)

m (nm)

X2

a b

66.8 -+ 2.0 65.1 ÷ 2.0

9.0 + 4.0 51.0 -+ 2.0

28.7 28.3

195

M. Gentili et al. / Gold plated edges on thin substrates

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6. Conclusions

In this paper, we described the suitability of gold plated markers, fabricated by E B L and electroplating process, for BD measurement and system calibration in Cambridge Instruments EBMF-6 machine. These gold plated edges, on bulk silicon and silicon nitride membrane substrates, are straight and opaque targets that were found to enhance the signal contrast and to reduce BD measurement disperson to about 13% of its average value. As gold plated markers on membrane have shown an increasing contrast with the beam voltage, they will be extremely suitable for calibration of high-voltage E B L machines. By Monte Carlo simulation it has been shown that the high signal contrast resulting from the membrane target is mainly due to the electron transmission through the thin substrate, while in the case of the commonly used lifted-

196

M. Gentili et al. / Gold plated edges on thin substrates

off target, the transmission through the metal layer reduces the marker-tobackground signal ratio. BD m e a s u r e m e n t s for different values of beam current using plated edges on the thin substrate were found to be in good agreement with theoretical calculations for the system electron optical column. Also, we have presented an analytical model to describe target signal response behaviour. It was possible to evaluate, by this analytical model, the influence of the m a r k e r geometrical shape on the signal response. By means of a fit procedure a good agreement has been obtained for BD values calculated using a non-vertical edge. Since the range of tolerance on edge verticality is related to BD value (m < B D / 2 ) , this analytical model will also be useful when small B D values have to be measured. Otherwise, in the investigated range of BD values, our gold plated edges were considered as vertical, opaque edges.

7. Acknowledgements The authors wish to thank Dr. Paolo de Gasperis for his continuous encouragements, Mr. L. Mastrogiacomo, G. Petrocco, L. Scopa and P. Musumeci for their valuable technical support and Mr. R. K u m a r for his critical revision of the paper.

References [1] M. G. Rosenfield, J. J. Bucchignano, S. A. Rishton, D. P. Kern, L. M. Kettel, W. W. Molzen, F. J. Hohn, R. Viswanathan and J. M. Warlaumont, Submicron electron beam lithography using a beam size comparable to the linewidth control tolerance, J. Vac. Sci. Technol. B 5(1) (1987) 114. [2] S. A. Rishton, S. P. Beaumont and C. D. W. Wilkinson, Measurements of the profile of finely focused electron beams in a scanning electron microscope, J. Phys. E: Sci. Instrum. 17 (1984) 296. [3] M. J. Penberth and B. A. Wallman, Low profile collector system, J. Vac. Sci. Technol. 16(6) (1979) 1719. [4] T. Chisholm, Spot-size measurement in an electron-beam pattern generator, J. Vac. Sci. Technol. B 6(6) (1988) 2066. [5] G. R. Brewer (ed.), Electron Beam Technology in Microelectronic Fabrication, Academic Press, New York, 1980, pp. 230-232. [6] D. A. Van Leeuwen, R. J. M. van Vucht, J. Romijn and E. van der Drift, Tungsten calibration mark for electron beam pattern generators, Microelectron. Eng. 9 (1989) 247. [7] M. E. Jones and C. Dix, Electron beam lithography in telecommunication device fabrication, Part 1, Br. Telecom. Technol. J. 7(1) (1989). [8] F. James, CERN computing and data processing school, Pertisau, Austria, 1972.