Delay measurement of experimental 2.5μm Josephson current injection logic (CIL)T. R. Gheewala Citation: Applied Physics Letters 34, 670 (1979); doi: 10.1063/1.90632 View online: http://dx.doi.org/10.1063/1.90632 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/34/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Turnon delay analysis of currentinjection Josephson logic circuits J. Appl. Phys. 57, 5028 (1985); 10.1063/1.335279 Logic delays of 5μm resistor coupled Josephson logic Appl. Phys. Lett. 41, 886 (1982); 10.1063/1.93686 Logic delays of 5μm currentswitched Josephson gates Appl. Phys. Lett. 40, 739 (1982); 10.1063/1.93211 A novel current injection Josephson logic gate with high gain Appl. Phys. Lett. 39, 653 (1981); 10.1063/1.92808 Josephson logic circuits based on nonlinear current injection in interferometer devices Appl. Phys. Lett. 33, 781 (1978); 10.1063/1.90503

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Dr. H. Takanashi, Dr. Y. Fukukawa, and Dr. T. Misugi of Fujitsu Laboratories, Ltd. for the encouragement. The au­thors also wish to thank Dr. S. Yamakoshi, Dr. H. Imai, and H. Ishikawa for their helpful discussions, and Dr. K. Shima, S. Ohsaka, K. Segi, M. Morimoto, and N. Takagi for sample fabrication and discussion.

'R.L. Hartman and A.R. Hartman. App!. Phys. Lett. 23.147 (1973). 'H. Kressel. M. Ettenberg. and 1. Ladany. App!. Phy. Lett. 32. 305 (1978). 'S. Ritchie, R.F. Godfrey. B. Wakefield and D.H. Newman. 1. App!. Phys. 49.3127 (1978).

'R.L. Hartman and R.W. Dixon. App!. Phys. Lett. 26. 239 (1975). 'H. Ishikawa. T. Fujiwara. K. Fujiwara. M. Morimoto. and M. Taku­sagawa. 1. App!. Phys. (to be published).

Delay measurement of experimental 2.5-/-Lm Josephson current injection logic (ell)

T. R. G heewala

IBM Thomas J. Watson Research Center, Yorktown Heights, New York 10598 (Received 4 December 1978; accepted for publication 27 February 1979)

Switching speeds of experimental Josephson interferometer circuits are measured on chains of current injection logic (CIL) gates fabricated using 2.5-J..Lm minimum features. Logic delay of 32 ps per gate is measured for a four-input OR gate with a fan-out of 2. The average power dissipation of the four-input OR gate is 4 J..L Wand the corresponding power-delay product is 1.28 X 10 16 J. For four-input AND gates the measured logic delay is 60 ps, the power dissipation is 6 J..L W, and the power-delay product is 3.6 X 10 I" J. The results are found to be in excellent agreement with estimates based on computer simulations.

PACS numbers: 74.50. + r, 85.25. + k, 85.40. - e

Current injection logic' (CIL) is a new family ofJoseph­son logic circuits which combine electromagnetic coupling and novel nonlinear current injection to achieve faster switching speeds, wider margins, and higher fan-in/fan-out capability than the Josephson interferometer logic previous­ly described.' It retains other properties of Josephson logic such as terminated output lines, extremely low power levels, and the requirement for an ac power supply.

input injection AND gate with larger current levels than the device of Fig. I (b). It is used to form a four-input AND gate as described below.

The experimentaI2.5-,um CIL circuits are comprised of the following three interferometer devices which are shown in Fig. 1:

(a) The device shown in Fig. la is a three-junction 1 : 2 : 1 damped interferometer3

•4 with two electromagneti­

cally coupled inputs, A and B. The interferometer is normal­ly biased in the superconducting region by a gate current I g ,

and it switches into a voltage state when either one or both of the two inputs are activated, thus providing a two-input OR function. Additionally, it provides isolation and serial fan­out capability. The threshold curve of the interferometer is also shown.

(b) The two-junction asymmetric injection gate of Fig. l(b) provides a two-input AND function by nonlinear sum­mation' of directly coupled inputs A and B. The critical cur­rents and inductances are chosen so as to provide a nonlinear threshold curve which has the characteristics of a near-ideal AND gate shown. The current required to switch the injec­tion gate when A and B are both applied simultaneously is about one-third the current needed when only one input is applied.

(c) The device of Fig. 1 (c) is also a direct coupled two-

Two examples of CIL circuits are shown in Fig. 2. A four-input OR gate [Fig. 2(a)] is made up of two two-input OR gates, Q, and Q" interconnected via resistors Re' For example, when input A is applied, it switches gate Q, which

4IO~ .. 8 I,

4>O/2L Io.lb

'~. A ,.,

(b) ~ ~Rl 410~" 11].... A·a

\ I - Ib 40

'l4 ' A A·a ®

(c) Rl

SYMBOL DEV1CE THRESHOLD CURVE

FIG. 1. Symbol, equivalent circuit, and threshold curves ofCIL devices: (a) three-junction I : 2 : 1 interferometer; (b) small nonlinear injection AND gate; (e) large nonlinear injection AND gate. (/" = 50 {lA, L = 6 pH. L, = 11 pH. L, = 7 pH, R, = 12 fl. R, = 20 fl, R" = 3.5 fl.)

670 App!. Phys. Lett. 34(10). 15 May 1979 0003-6951/79/100670-03$00.50 © 1979 American Institute of Physics 670

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(a)

(b)

(AI+ A2).(BI+ B2)

• (CI+C 2)'(01+02)

---lRLI

FIG. 2. Equivalent circuits of two elL gates (a) four-input OR; (b) four­input AND. (Rc = 0.3n, R" = 10 n, Rs. = 5 n, R L , = R L• = 12 n, Rp

= 60n).

in turn injects current into gate Q2 (Rc<R L), causing it to switch. The combined gate currents I g , + I g, are delivered to two parallel loads, R L , and R L , The parallel fan-out feature adds to the serial fan-out capability of the interferometer logic and reduces the fan-out delay by one-half. The second example, Fig. 2(b), illustrates a four-input AND gate with a two-way OR gate at each input for a total fan-in of 8. The output currents of the OR gate are added nonlinearly first in the small injection gate and then in the large injection gate to form the four-input AND gate. As before, the output is available at two parallel output lines to reduce the fan-out delay.

0.2 C ~ ~" 0.1

(a)

• EXPERIMENT - THEORY

(c)

(!50 /'A/DIV)

(b)

FIG. 3. (a) SEM of a large nonlinear injection AND gate; (b) measured nonlinear threshold curve; (c) comparison between measured and theoreti­cal threshold curves.

671 Appl. Phys. Lett., Vol. 34, NO.1 0, 15 May 1979

(a)

32psIGATE

(b)

FIG. 4. elL logic delay measurement: (a) a chain of 10 four-input OR gates; (b) delay measurement for the chain shown in (a).

These circuits were fabricated using a lead alloy pro­cess5 similar to that described in Ref. 5 with 2.5-,um mini­mum feature size. The interferometer inductances were formed using a stepped structure4 over a superconducting ground plane. A SEM of a large injection AND gate is shown in Fig. 3(a). Its experimental nonlinear threshold curve is shown in Fig. 3(b), which for the first time verifies the nonlinear current injection threshold curve. Excellent agreement between the measured and the theoretical thresh­old curves is obtained, as indicated by Fig. 3(c). Although the critical currents and inductances of the experimental de­vice differ significantly from the nominal design values of Fig. l(b) due to process variation, the threshold curve main­tains the near-ideal AND gatelike characteristics.

A photomicrograph of a delay chain consisting of 10

~

~

J 50

4-0R

(a) 0.5

\ ~

~

J 100

4 -AND

05

(b)

06 07 08

\ I

06 07 D 8

Ig /41 0

I MEASUREMENTS

CRITICAL CURRENTS

09 10

I MEASUREMENTS

CRITICAL CURRENTS

09 1.0

FIG. 5. A comparison between the delay measurements and computer sim­ulations for (a) four-input OR: (b) four-input AND. The shaded areas indi­cate a ± 10% variation in 10 ,

T.R. Gheewala 671

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four-input OR gates (fan-out of2), start, and output devices is shown in Fig. 4(a). When input Vp is OFF the signal propa­gates through the 10 stages off our-input OR gates and then arrives at the output. On the other hand, when Vp is ON the start signal arrives at the output directly, bypassing the 10 gates. The difference in the arrival time of the output pulse in the above two cases corresponds to the total propagation delay through the 10 four-input OR gates. Figure 4(b) shows an experimental result where a total delay of 320 ps is mea­sured for the chain of 10 four-input OR gates. Similar mea­surements on chains offour-input AND gates (fan-out of 2) yield a minimum delay of 60 ps. The delay measurements for the four-input OR and four-input AND gates are compared to computer simulations in Fig. 5 as a function of the gate current. The simulations allow for a ± 10% variation in the critical current 1o, as indicated by dotted lines. A capacitance value of 0.17 pF per smallest junction was established for the simulations by measuring the internal resonances of the in­terferometers. For the above experiments, the average power dissipation of the four-input OR and AND gates is 4 and 6 pW, respectively, assuming a 70% duty cycle.

In conclusion, experimental results on 2.5-pm Joseph­son current injection logic are presented. The phenomena of nonlinear current injection is verified, and switching delays of 32 and 60 ps are measured for four-input OR and four­input AND gates, respectively.

I would like to thank A. Warnecke, J. Brown, S. Klepner, S. Neely, R. Patt, T. Ross, and J. Stewart for fabri­cation of the samples, and J. Cole, R. Dunne, and D. Piazza for their technical assistance.

IT. Gheewala, App!. Phys. Lett. 33, 783 (1978). 'M. Klein and DJ. Herrell, IEEEJ. Solid-State Circuits SC-l3, 577 (1978). 'H.H. Zappe and B.S. Landeman, J. App!. Phys. 49, 344 (1978). 'L. Geppert, J. Greiner, D. Herrell, and S. Klepner, IEEE Trans. Magn. MAG·lS, 412 (1979).

'J.H. Greiner, S. Basavaiah, and I. Ames, J. Vac. Sci. Techno!. 11,81 (1974).

Measurements of refractive-index dispersion by optical waveguiding

Keiji Tanaka

Department of Engineering Science, Faculty of Engineering, Hokkaido University, Sapporo 060, Japan (Received 28 December 1978; accepted for publication 27 February 1979)

A technique for measuring the refractive-index dispersion in light-guiding thin films by means of the prism-coupling method is reported. A collimated polychromatic light beam is employed, and the wavelength dependence of synchronous angles is determined spectroscopically. A representative experiment performed on an amorphous specimen reveals its usefulness for investigating the detailed dispersion characteristics of thin films with accuracies better than 0.001.

PACS numbers: 78.65. - s, 42.82. + n, 78.20.Dj, 07.65. - b

Although a variety of techniques have been applied to measure optical properties of thin films, the prism-coupling method,' or moline spectroscopy, has substantial advantages for determining the refractive index nf , as well as its aniso­tropy, and film thickness W simultaneously with good accu­racies. Several papers have demonstrated that this method can be applied successfully to the characterization of thin films of various kinds. 2

-7

In this method, a high-refractive-index prism is placed near the thin film, and the angles 8m 's on the prism for which guided waves can be excited synchronously are measured. Data reduction consists of two processes as follows:

(1)

The effective refractive index N m' where m is a mode num­ber, is obtained from 8m with the relation

Nm = np sin [sin-'(sin8m /n p) + 8p ), (2)

where np and 8p are the refractive index and the angle be-

tween the base and the incident face of the prism, respective­ly. With several N m 's (at least two), the film parameters nf and W can be determined by means of the characteristic equation of guided waves':

(3)

In Eq. (3), nc and n, are refractive indices of cover and sub­strate, respectively, and (T characterizes polarization of the guided wave. The wave number k of the light is k = 21T/A. = w/c.

The method is simple, and its accuracy is comparable to those of ellipsometric methods. 8 However, it has been thought, unfortunately, that this method is practical only when cw-Iaser sources are available,' thereby its use for in­vestigating the detailed characteristics of the refractive-in­dex dispersion has been limited. Here, we report for the first time a technique to study the dispersion by means of the prism-coupling method with a polychromatic light source.

672 Appl. Phys. Lett. 34(10), 15 May 1979 0003-6951/79/100672-03$00.50 © 1979 American Institute of Physics 672

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