Voltage contrast at 6 K in a scanning electron microscope

Voltage contrast at 6 K in a scanning electron microscope H. Sadorf University of Karlsruhe, Institut f/Jr Elektrotechnische Grundlagen der Informatik, Hertzstrasse 16, D-7500 Karlsruhe 21, FRG Received 8 April 1987; revised 24 June 1987 An arrangement for voltage contrast measurements in a scanning electron microscope (SEM) of integrated circuits and devices at any low temperatures has been conceived and tested experimentally. The arrangement comprises a low temperature specimen stage and an integrated combination of secondary electron analyser and local cryopump (SEAC). Even millivolt signals of switching Josephson junctions have been measured with the electron probe. Detrimental effects of condensation or electron beam contamination at 6 K have been practically eliminated.

Keywords: electronics; low temperature studies; instrumentation

Electron beam testing by voltage contrast represents an important tool to localize and characterize failures inside integrated circuits. It has been reviewed extensively in the past few years ~-3. As far as integrated circuits based on the Josephson effect4 are concerned, the electron probe has been used successfully as a heating and phonon source. An overview of these techniques has been given by Huebener L6. Recently, the technique has become image-forming; inhomogeneities in Josephson junction arrays can be displayed 7. Table 1 gives a comparative short overview of the requirements to be met by an el~tron beam testing machine for Josephson integrated circuits and for semiconductor circuits. The testing of semiconductor devices demands the suppression of the local field effect8 by magnetostatic 9'a° or electrostatic 1~ fields. Owing to the small signal amplitudes in the millivolt range, the local field effect error is veiled by noise in electron beam testing of integrated circuits with Josephson junctions. Instead, the operation of superconducting quantum interference devices (SQUIDs) may be severely hampered by magnetic fields.

Principle of operation Electron beam testing using the voltage contrast method is based on the fact that the secondary electron (SE) energy distribution is shifted linearly with specimen voltage. Changes in specimen voltage lead to a change in SE current. Keeping the SE current constant by a SE energy analyser and a feedback loop, quantitative measurements of the specimen voltage are possible. As the signal amplitudes of Josephson junctions are in the millivolt range, the noise bandwidth of the electron beam detection system has to be limited. Flicker noise in the SE current does not allow the application of a simple low-pass filter. A band-pass filter of small bandwidth is necessary. A lock-in amplifier can be used with a sufficient narrow bandwidth around the signal frequency 11 13. In this Paper, the detection of amplitude modulated signals, the so called frequency detection either by a lock-in amplifier 1~-13 or by a spectrum analyser is demonstrated to be suitable for very small voltages at electrodes, e.g. in superconducting integrated circuits. The theoretical voltage resolution, Vmln,mainly determined by the shot noise of the SE current can be estimated according to Gopinath's formula 14

Table I Comparison of the requirements to be met by an electron beam tester for Josephson and semiconductor integrated circuits

Requirement Voltage resolution Temperature Magnetic fields Linearity of the voltage measurement

Josephson integrated circuits

Semiconductor integrated circuits

millivolts 4-80 K(?) Careful shielding Uncritical (the voltage resolution is of the order of the signal amplitudes)

Tens of millivolts 4-300 K No shielding 1% [electrostatic (1 kV mm 1) and/or magnetic fields (1 O0 mT) are employed]

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Cryogenics 1987 Vol 27 November

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Voltage contrast at 6 K. H. Sadorf The quality factor should be zero for the oversimplified case that all secondary electrons have exactly the same energy or that the detected SE current, Is~:d, drops abruptly to zero at a critical retarding field electrode voltage, V~, or that the logarithmic derivative s = d(lnlsEd)/dV~ is s = co. The quality factor, Cu, is inversely proportional to s. A near realistic energy distribution yields a finite slope s or Cu = 1.4 x 10 -8 V(As) 1/2 for a clean and plain layer. Thus, the following parameters of the secondary electron analyser (SEA)system for a pulsed PE beam: Ipt~= 0.5 nA, /pm ~-- 20 ps, rpm = 4 ns = 1/250 MHz, Af = 16 m H z and n = I, correspond to a theoretical voltage resolution Vm~, = 1.1 mV. It is sufficiently small to detect transients in the order of the gap voltage of niobium/lead alloy Josephson junctions at T = 4.2 K, 2A/e ~ 2.7 mV t 5. With an unpulsed PE beam (tpm/Tpm = 1) and the same set of parameters, the theoretical voltage resolution, Vmi,, is 0.08 mV. In a dedicated system for voltage contrast an improvement can be achieved by an increase of the primary electron current up to 100 nA. In this Paper only unpuised beams have been used.

Since the SEs are very sensitive to surface charges, especially the low velocity regions of the SEA have to be shielded against condensation. Instead of a warm Everhart Thornley detector close to the SEA, an electron accelerator 2~ is installed between the SEA and the SE detector. The electron accelerator consists of four dynodes. In Figure 1, the first dynode (7) is shown on the right. During cool-down the last two dynodes are at lower temperatures than the first two, since electrons with a kinetic energy of > 1.2 keV are much less sensitive to charges on condensation layers. The acceleration electrode(5) is expected to act as local cryopump between retarding field electrode (2b) and specimen (8). Retarding field electrode and electron deflector (2a) are made of one piece of gold coated Cryoperm 10 to reduce heat input and to shield the specimen against the magnetic fields of the final lens and of the scanning coils. With the exception of the top electrode (2a) of the SEA and the last dynode ( + l . 6 k V ) , all electron optical parts of the secondary electron analyser and local cryopump (SEAC) are surrounded by cold surfaces (~4.2 K).

Low temperature stage Cooling facility

As the specimen has to be moved underneath the SEAC to analyse different parts of the specimen and as the space

A specimen in a scanning electron microscope (SEM) can be cooled down to liquid helium temperature either with a bath cryostat~6'~ 7 or a continuous flow cryostat ~8,~ 9. The bath cryostat may have a quarter of the coolant consumption of the flow cryostat (for example, 0.5 dm 3 h compared to 2 d m 3 h ~). A continuous flow cryostat is superior in cooling speed ( ~ 15 min) and temperature range (4.2 300 K versus 1.5M.2 K). Since a wide temperature range has been required, a commercial continuous flow cryostat (Janis, Supertran) has been chosen. To reduce the heat input by temperature radiation, a liquid nitrogen radiation shield has been installed surrounding the whole cold finger near 4 K. The liquid nitrogen shield consists of Cryoperm 10, a magneticaly shielding material with especially high It~ at low temperatures (y~ > 10 000).

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Figure 1 Cross-section of the secondary electron analyser and cryopump (SEAC) and the specimen holder. 1, Pole piece of the final lens; 2a, electron deflector; 2b, retarding field electrode at the retarding field electrode voltage Vr; 3, SEAC holder (copper); 4, isolator; 5, extraction field electrode grid (EFEG); 6, radiation and condensation shields; 7, first dynode; 8, chip; 9, specimen copper block; 10, carbon resistor; 11, set of copper wires for thermal anchoring of the specimen copper block; 12, printed circuit board; 13, x, y, stage

Cryogenics 1987 Vol 27 November

653

Voltage contrast at 6 K." H. Sadorf

inside the liquid nitrogen shield is confined by the SEAC and the continuous flow cryostat, the x, y stage of the specimen must be miniaturized [(13) in Figure 1]. It can be moved from outside the vacuum by non-magnetic NiCr wires allowing for an excellent thermal isolation. The specimen holder is a copper block (9), mounted thermally decoupled to the stage, but thermally anchored to the cold end of the cryostat by a set of copper wires (11). In addition, the copper wires reduce specimen vibrations. In principle, the copper wires could also be replaced by a pair of bellows and tubes, allowing direct cooling of the specimen by the cryogen. The specimen, e.g., a Si-chip, is glued on the copper block (9), which is soldered onto a printed circuit board (12). There are electric connections via indium bonds between the specimen and the printed circuit board; nonmagnetic plug-in connectors are glued to the cryostat. Two carbon resistors (10) are patched inside the specimen copper block. One resistor is used as a temperature sensor, the other is necessary for rapid heating of the specimen. The weight of the specimen copper block is only 2 g, corresponding to a thermal capacity of 0.3 mJ K - 1 at T = 5 K. The time to exchange mechanically a specimen on the low temperature stage requires only a few minutes, disregarding the heating, cooling and pumping time.

Coolant consumption and cooling speed The basic continuous flow cryostat has a specified liquid helium consumption, Qene =0.7 dm 3 h -1, in vertical position. Here, the Janis Supertran has to be used in its disadvantageous horizontal position with a larger helium consumption. Its original passive radiation shield is replaced by a liquid nitrogen shield. Most parts of the SEAC and stage are gold coated. In this way the liquid helium consumption is limited to 2.2 dm 3 h-~ at 4.2 K (see Table 2). The thermal coupling of the specimen to the cold end of the cryostat is different to that of the SEAC. The cooling speed of the SEAC is larger than that of the specimen. The residual gases in the vacuum above the specimen surface condense onto the SEAC and not onto the surface of the device under test. A typical cool-down cycle is shown in Table 2. Cooling with helium is usually started 2 h after the shield has been filled with liquid nitrogen. To demonstrate the good thermal coupling of specimens on the cold stage of the SEM, the temperature sensitive energy gap voltage of a Josephson junction is measured. Figure 2 shows the I-V characteristic of a Nb/PblnAu Josephson junction in parallel to an inTable 2 Typical cool-down procedure. TSEACis the temperature of the secondary electron analyser and local cryopump (SEAC), Tsp is the specimen temperature and OLHe is the cryogen consumption Time (min)

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300 >80 12.9 6.6 6.0 6.0 6.0

300 >80 19.6 7.8 4.3 4.2 4.2

0.0 0.6 1.2 1.9 2.6 5.0 7.2

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tegrated resistor of 19 ~ mounted to the cold stage. The energy gap voltage is 2.7 mV. As this energy gap voltage has been measured with the same chip immersed in liquid helium, the junction temperatures must be 4.2 K in both cases. Hence, good thermal coupling between cold finger and specimen is achieved.

Condensation and contamination To investigate the more or less unknown influence of contamination 22 and condensation on the SE yield 23'24 at low temperatures, the first set-up to detect voltages on superconducting devices comprised a SEA simply mounted on the outer, liquid nitrogen cooled radiation shield. Contamination responsible for l / f noise in the SE current and for a reduction in SE yield may be disregarded in a first approximation step if the electron beam hits the surface for a sufficiently short time. The contrast of the SE image at the SEM CRT always changed during cooldown at temperatures below 40K. The condensation leads to a negative charge on oxide surfaces of the chip at a PE energy of ~ 1 keV (see Figure 3) corresponding to a SE yield of < 1, degrading the voltage contrast. However, no surface charging appears at PE energies sufficiently high to penetrate the condensation and oxide layers 15. Unfortunately, high PE energies are unsuitable for electron beam testing of low temperature devices owing to local heating and larger noise generated via backscattered electrons. All the following experiments have been performed with the proposed SEAC. Contamination causes I / f noise in the SE current and a reduction in the SE yield. If the electron beam is switched on soon after reaching superconductivity, a contamination of the chip surface can be observed. However, if the SEAC thus worked for 15 m i n as a cryopump at liquid helium temperature an electron beam induced contamination of the chip surface can not be seen any more (see Figure 4). No change in contrast either by condensation or contamination could be observed even after 3 h at temperatures of ~ 4.2 K. This has been achieved at a pressure of the order of 1 mPa in a standard turbomolecular pumped specimen chamber of the SEM.

Voltage contrast at 6 K. H. Sadorf

Figure 4 SEM micrograph of a crossline Josephson junction. Both electrodes are not covered by an oxide layer. The broader upper electrode consists of a PblnAu alloy. The encircled area has been under electron bombardment of 1 keV energy for over 1 h. Neither contamination nor condensation can be observed at liquid helium temperature

Figure 3 (a) Secondary electron (SE) image of a chip with integrated Josephson junctions through the extraction field electrode grid (EFEG). (b) Sketch of the image showing: 1, EFEG shadow ofthe previous position; 2, PblnAu; 3, SiO2; 4, EFEG; 5, Nb. The specimen had a local condensation layer obtained from a previous position with respect to the EFEG when a temporary air leakage occurred. In part (a) the oxide surfaces of the specimen shadowed by the EFEG are free from the condensation layer; they are charged positive by the low energy electron beam (1 keV) and appear as black areas (white arrows). Condensation layers on the oxide are charged negative and correspond to white areas (black arrows)

V o l t a g e contrast at 6 K As the voltages appearing along the surface of Josephson integrated circuits are usually in the millivolt range, the generation of a voltage contrast picture of the whole chip surface takes too long. Therefore, only a few circuit nodes of interest can be selected to measure the local voltages, The electron probe has to be positioned on a certain spot. The method of frequency detection 11 13 is chosen to check whether a Josephson junction is in the zero voltage state or in the voltage state. A different Josephson junction than that shown in Figure 2 has been used throughout the following measurements. The energy gap voltage of the Josephson junction has been determined by a four point measurement at the pads of the chip from outside the SEM. The measured voltage, 2A/e = 1.9 mV, corresponds to a junction temperature of about ~ 6 K. Current pulses of an amplitude just exceeding the maximum Josephson current of the junction ( l o = 0 . 1 2 m A ) are used to switch the junction between the voltage and the zero voltage state at a repetition frequency, .li- The amplitudes of the voltage pulses on both junction electrodes have been measured via the pads from outside the SEM: Vju = 2.1 mV on the upper electrode, Vjj = 0.2 mV on the lower electrode. The electron probe experiments have been carried out

at a PE energy of 1 keV and a probe current of < 1 nA. With these parameters the electron beam would increase the temperature at the impinging area of the specimen by < 1 K, if its total power is dissipated inside the specimen film 5 The electron beam detection system used comprises a SEAC, a SE detector and feedback loop. Presently the feedback loop limits real-time measurements to signal frequencies of < 10kHz. The measured logarithmic derivative, s, in the optimum working point of the SEAC is approximately the same at 300 and 4.2 K. A value s ..... = 0.2 V ~ could be achieved for Au. The measured voltages at a lock-in amplifier output are plotted ~ersus time in Figures 5a and b. The upper trace corresponds to an electron beam impinging on the upper junction electrode, the lower one on the ground electrode. As the rectangular voltage pulses at the junction with a frequency ~ = 1.7 kHz has been phase modulated with a frequency of ~ 5 mHz, the sign of the in-phase output of the lock-in amplifier changes periodically. The electron probe yields a specimen voltage amplitude of ~ l m V , corresponding to the expected peak-to-peak value of 2mV (see Figure 5a). The measurements have been performed with a lock-in amplifier time constant of 10 s at a 12 dB per octave roll-off corresponding to a noise bandwidth of 12.5 mHz. Noise and signal can be visualized simultaneously in replacing the lock-in amplifier by a spectrum analyser. The effective noise voltage was V~ff~0.3mV for 16.6mHz bandwidth of the spectrum analyser at a specimen temperature of ~ 6 K (see Figures 6 and 7). The spectral line amplitude at J i = 1 7 . 2 H z in Figure 6a corresponds to the junction switched into the voltage state (2.1 mV). In Figure 6b, the gate current has been reduced below the maximum Josephson current of the junction, so that the junction stays in the voltage state and no line exceeding the noise could be detected at fi. Next, the electron beam was placed on the lower grounded electrode outside the junction area. As expected, no spectral line appeared at fj, even if the junction switched into the voltage state. The residual signal amplitude (0.2 mV) at the position of the electron beam on the lower electrode was outside the resolution range of the

Cryogenics 1987 Vol 27 November 655

Voltage contrast at 6 K. H. Sadorf

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F i g u r e 6 Detection of 2 mV at 6 K. Fourier spectrum of the voltage at the retarding field electrode, Vr. The bandwidth of the spectrum analyser was 16.6 mHz. The signal amplitudes of the voltage pulses on the Josephson junction were: (a) 2.1 mV; (b) O.2mV (/G < / 0 ) ; (c) 0.2 mV (electron beam on lower electrode) ; (d) 1.5 inV. The signal frequency was 17.2 Hz. The upper electrode yielded a signal-to-noise ratio of > 2 in (a) and (d)

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Cryogenics 1987 Vol 27 November

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The proposed set-up designed for voltage contrast can also be used for local heating 5'6. In contrast to the local heating method, the proposed voltage contrast set-up enables us to measure voltages inside complex integrated circuits, if the voltages and currents are not accessible at the pads of the chip. Moreover, short transients at the points of interest undistorted by a transfer through many devices to the pads could be investigated. In addition to testing Josephson 2s'29 or superconducting devices, the SEAC can be used to test semiconductor devices, for example, at liquid nitrogen temperature. Quite recently, the impact of semiconductor VLSI operation at low temperatures (4.2 77 K) has been emphasized 3°.

Acknowledgements

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The author is indebted to Professor W. Jutzi for initiating this work, for encouragements and for valuable hints. The preparation of test chips by Drs R. Herwig and M. Neuhaus, and by G. Janzer, A. Stassen and H.-J. Werround, the advice during some of the measurements of D. Drung and H. A. Kratz, and the careful fabrication of the mechanical parts by R. Hennhoefer and J. Schoner are greatly appreciated.

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References Figure 7 Detection of l V + 2 mV at 6 K. Fourier spectrum of the voltage on the retarding field electrode, Vr. The bandwidth of the spectrum analyser was 16.6 mHz. The amplitudes of the voltage pulses with an offset of 1 V were: (a) 2 mV; (b) 0.2 mV. The signal frequency was 17.2 Hz

electron probe. Rectangular pulses with 1.5 mV amplitude yielded the spectrum shown in Figure 6d. Up to now, the junction's lower electrode has been at a very low voltage Vjj ~ 0.2 inV. If the junction is biased by a large d.c. voltage of, for example, Vjj ~ 1 V, the voltage change AVj, ~ 2 mV owing to a switching from the zero to the voltage state can also be detected by the SEAC (see

Figure 7).

Conclusions An electron beam tester for low temperature devices has been proposed where the function of spectrometer and cryopump have been integrated. After 15 rain ofcryopumping, neither condensation nor electron beam contamination could be observed inspite of a turbomolecular pumped specimen chamber vacuum of ~ 1 mPa at room temperature. The proposed principle of an integrated spectrometer and cryopump for contamination and condensation reduction may have an important impact on, for example, the quality of Auger electron spectroscopy and photon stimulated exoelectron emission 27. The detection of modulated voltages in the millivolt range with an unblanked electron beam has been demonstrated at 6 K. With a pulsed electron beam the measurement of periodic millivolt transients should be feasible with a time resolution of 20 p s 26 according to Gopinath's formula, if very long integration times for sufficient signalto-noise ratios can be tolerated.

1 Plies, E. and Otto J. Voltage measurements inside integrated circuits using mechanical and electron probes Scannhlg Electron Microscopy (19853 4 1491 1500 2 Fazekas, P., Fox, F., Papp, A., Widulla, F. and Wolfgang, E. Electron beam measurements in practice Scanning Electron Microscopy (19833 4 1595 1604 3 Feuerbaum, H.P. Electron beam testing: methods and applications Scanning (19833 5 14 24 4 Josephson, B.D. Possible new effects in superconductive tunneling Phys Lett (19623 I 251-253 5 tluebener, R.P. Applications of low-temperature scanning electron microscopy Rep Prog Phys (1984) 47 175 220 6 Huebener, R.P. Applications of low-temperature scanning electron microscopy Scanning Eh,ctron Microscopy (19843 3 1053 1063 7 Bosch, 3, Gross, R. and Huebener, R.P. lnhomogeneities in arrays of Josephson junctions: Their imaging by low-temperature scanning electron microscopy Appl Phys Lett (1985) 47 1004 1006 8 Ura, K., Fujioka, H. and Yokobayashi, T. Calculation of local field effect on voltage contrast Electron Microscopy (1980) I 330 331 9 Garth, S.C.J. and Nixon, W. C. Magnetic field extraction of secondary electrons for accurate integrated circuit voltage measurement J Vac Sci 1~,chno1(1986) B4 217 220 10 Plies, E. and K61zer, J. A new objective lens with in-lens spectrometer for electron beam testing Proc Xlth lnt ('ongr Eh'ctron Microscopy Kyoto, Japan (1986) 11 Menzel, E. and Kubalek, E. Secondary electron detection systems for quantitative voltage measurements Scanning (1983) 5 151 171 t2 Dyukov, V.G., Kolomeytsev, M.I. and Petrov, V.I. Nonstroboscopic dynamic voltage contrast in the SEM Proc Microcircuit Eng Academic Press, London, UK (1983)501 508 13 Brust, H. D. and Fox, F. Frequency tracing and mapping in theory and practice Microeh'ctron Eng (19853 2 299 323 14 (;opinath, A. Estimate of minimum measurable voltage in the SEM ,I Phy~ E(1977)!0911-913 15 Iterwig, R. PhD 7hesis University Karlsruhc, Karlsruhe. FRG (1984) 16 Sloehr, P. L. and ttuebener, R.P. Liquid helium stage in a scanning electron microscope Cryogenics (1979) 19 472~473 17 Seifert, H. Liquid helium cooled sample stage for scanning electron microscopy ('ryogenics (1982) 22 657 660 18 Mueller, B. and Ileiden, C. A continuous flow gas cooled object stage for a scanning electron microscope Proc Eighth lnl Cryogenic

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Voltage contrast at 6 K. H. Sadorf Eng Con['(1980) 8 378-382 19 Heide, H.G. Design and operation of cold stages Uhramicroscopy (1982) 10 125-154 20 Kanaya, K. and Kawakatsu, H. Secondary electron emission due to primary and backscattered electrons J Phys D (1972) 5 1727-1742 21 Sadorf, H. A secondary electron analyzer for voltage measurements Proc Microcircuit Eng 84 Academic Press, London, UK (1985) 461-468 22 Echlin, P. Contamination in the scanning electron microscope Scanning Eh,ctron Microscopy (1975) 679-686 23 Seiler, H. Secondary electron emission in the scanning electron microscope J Appl Phys (1983) 54 R1-R18 24 Seller, H. Electron spectroscopy in the scanning electron microscope Ultramicroscopy (1985) 17 1 8

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Pensak, L. Conductivity induced by electron bombardment in thin insulating films Phys Rev (1949) 75(3) 472-478 26 Sadnrf, H. and Kratz, H.A. Plug-in fast electron beam chopping system Rev Sci Instrum (1985) 56 567-571 27 Owaki, S., Katagiri, K., Okada, T., Nakahara, S. and Sughara, K. Measurements of photostimulated electron emission from superconducting metals at 9 300 K Jpn Appl Phys (1985) 24(4) I18 121 28 Jutzi, W. Applications ofthe Josephson technology Adv SolidState Phys (1981) 20 403-432 29 Weber, S. How Hypres uses its ICs to build 70-GHz scopes Electronics (1987) 49 53 30 Gaensslen, F. H. and Jaeger, R. C. Special issue on low-temperature semiconductor electronics IEEE Trans Electron Devices (1987) ED34(1)