Fundamentals of optical beam testing

Microelectronic Elsevier

Engineering

Fundamentals

24 (1994) 327-339

321

of optical beam testing

D. Jager, G. David, and W. von Wendorff* Sonderforschungsbereich 254, FG Optoelektronik Universitat Duisburg KommandantenstraBe 60 D-47057 Duisburg, Germany * Now with Motorola GmbH, Mtinchen In this paper, an overview is given on the fundamental concepts of optical beam testing. Optoelectronic and electrooptic correlation and sampling techniques for on-wafer and incircuit measurements are particularly reviewed. For that purpose optoelectronic switches as well as electro-optical modulators are discussed and the respective measurement set-ups for optical testing are described. Recent experimental results are also presented including wave propagation effects on coplanar waveguides, spatially resolved probing of MMICs and RFdevices and two-dimensional field distributions in MMICs. 1. INTRODUCTION In the past, a growing interest has been paid to laser beam testing methods because of their outstanding significance for contactless on-wafer and in-circuit probing facilities [ 11. Special emphasis is further laid upon ultrafast techniques in the picosecond and millimetre wave range well beyond the capabilities of conventional electronic measurement systems. This is because the shortest pulses which are available to the experimentalists today are provided in the optical domain. As a result, ultrashort optical pulses are now sucessfully applied to measure picosecond and even femtosecond events in electronics, especially in monolithic microwave integrated circuits (MMICs) [l-3]. Optical beam testing methods are based upon two principles. (i) The tirst is the physical interaction between the optical and the electrical signal to be measured, leading to a modulation of either the electrical or the optical wave. As a consequence, an optoelectronic or an electrooptic modulation takes place, respectively, and the corresponding electrical [4,5] or optical [6] output signals have to be analyzed in order to get the desired information on the input electrical signal. (ii) Correlation and sampling techniques are additionally applied [l-6] to obtain the dynamics of the circuit or device under test (DUT) in the time or frequency domain as well known from usual electronic measurement techniques. Because optical pulses with FWHM of less than 100 fs are readily available a temporal resolution better than 1 ps [6,7] or a measurement bandwidth exceeding 1 THz [8-lo] can be achieved. In this paper, an overview is given on the fundamental concepts of optoelectronic and electrooptic correlation and sampling techniques for contactless on-wafer and in-circuit measurements. The basic experimental set-ups are described and recent results on wave propagation effects and field distributions in microwave devices and MMICs are presented. 0167-9317/94/$07.00

0 1994 - Elsevier Science B.V. All rights reserved.

328 2. LASER

D. Jiiger et al. I Fundamentuls of optical beam testing

BEAM TESTING

In Fig. 1 the physical background of contactless optical beam testing is sketched. Accordingly, the aim is to probe the electrical input signal by an optical beam in a suitable sensor element as shown.

li

optical beam

electrical signal

I I

I I

a Figure 1. Interaction between optical and electrical signal in laser beam testing For that purpose, some physical interactions are necessary, where two situations can be distinguished: (i) The optical signal modulates the electrical signal, i. e. the sensor element itself has the properties of an optoelectronic converter or, equivalently, of a photodetector. As a result, the electrical output signal has to be analyzed. (ii) The unknown electrical signal modulates the optical beam and the sensor element has to be an electrooptical converter/modulator. In this case the optical output signal is measured in order to get the desired information on the electrical input in Fig. 1. The element shown in Fig. 1 can basically be an external sensor immersed into the electrical field to be measured, which may be the stray field of a device or a circuit under Test (DUT) or the electric field radiated from an antenna, for example. Note, however, that in the case of an electrooptical sensor element no electrical wiring is needed and the optical beam can easily be guided by a fiber. On the other hand, the element of Fig. 1 can be the DUT itself or any node in a circuit where the electrical potential has to be measured. In the latter case the interaction region has to exhibit photodetector or electrooptical modulator characteristics as dicussed in the previous chapter. The optical beam consists of ultrashort pulses with a repetition frequency fu_ When the unknown electrical signal is synchronized to fu optoelectronic or electrooptic sampling can be applied. In the time domain the electrical input signal can be temporally resolved provided that the optical pulse width is sufficiently small. A sampling oscilloscope results when the repetition frequeny of the unknown electrical input signal is set to an exact multiple of fu plus a small frequency offset, eg. 0. l- 1 kHz. In the frequency domain a sinusoidal electrical input signal is used and the frequency offset with respect to f, can directly be used to emulate a network analyzer or a vector voltmeter. Note that a sinusoidal modulation of the optical beam instead of the pulse modulation can also be used.

D. Jiiger et al. I Fundamentals of optical beam testing

3. OPTOELECTRONIC

329

TESTING

The optoelectronic sampling and correlation technique has been pioneered by Auston in I975 [4]. The principle is based on the properties of the optoelectronic switch (OES) which is an ultrafast photodetector. The underlying concept is that of a photoconductor where the metallization is that of a microstrip line or a coplanar waveguide with a gap in the strip or center conductor (Figs. 2(a) and (b)) or a gap providing a coupling to a shunt transmission line (Fig. 2(c)).

a)

b)

Figure 2. Sketch of an optoelectronic shunt connection

c) switch. (a) microstrip; (b) coplanar configuration and (c)

Such devices are capable of generating pica- or femtosecond electrical transients propagating on the output line. The high-speed response is usually achieved by ion bombardement or implantation in order to reduce the free-carrier lifetime or by using materials with already large densities of naturally occuring defects acting as ultrafast trapping or recombination centers [4,5,11]. Optoelectronic switches as shown in Fig. 2 are lumped microwave photoconductive detectors defining the spatial position of the nodes to be measured. In contrast, it has been shown that coplanar strip lines on semiconductors can be used as a ‘sliding -contact’, i. e. as a distributed detector [7,9,10] where the position of optical illumination can be moved arbitrarily along the line (see Fig. 3). optical signal

n

-I-

optical signal

Figure 3. Travelling-wave

photodetector as a distributed OES

330

D. Jiiger et ~11.I Fundamentals

of optical beum test1n.g

Recently it has been shown that radiation damage is not necessary to achieve ultrafast response. Instead it has been found that subpicosecond pulses can be generated using a material with long carrier-lifetimes and a suitable illumination of the metallic contacts [12]. Similar results have been obtained from travelling-wave MSM photodetectors when the optical pulses were incident from the back [ 13-151. Fig. 4 shows the principle of optoelectronic sampling in the time domain. An optical pulse incidentat the OES (a) at time t generates an electrical picosecond pulse to examine the DUT. The output signal is then analyzed at a second OES (b) used as a sampling gate which is optically operated with a time delay of At. Note that in order to avoid undesired reflections a shunt OES (Fig. 2 (b)) should be used as the sampling gate.

I I t+At

Figure 4. Optoelectronic

sampling

The optical beam in a configuration as shown in Fig. 3 can be scanned in two dimensions. When the optical beam induced current (OBIC) or voltage (OBIV) is measured, spatially resolved signals can be obtained similar to measurements using electro-optic sampling (see below). In contrast, the optoelectronic samling technique of Fig. 4 cannot give local informations. 4. OPTOELECTRONIC

SAMPLING OSCILLOSCOPE

Fig. 5 represents an experimental setup which can be used as an optoelectronic sampling oscilloscope. It consists of the following parts: A CPM-laser provides fs-pulses which areobserved by an autocorrelator setup. A beamspitter is used to separate the optical beam for pulse generation in the left OES from a synchronized pulse train for the sampling OES on the rhs. The optical delay is introduced by a mirror which is mounted on a computer controlled translation stage. Choppers modulate the optical pulses and the difference signal drives the lock-in amplifier to be connected to a recording unit, eg. an oscilloscope.

D. Jiiger et al. I Fundamentals

331

of optical beam testing

delay with motor

Figure 5. Optoelectronic sampling oscilloscope, analog/digital converter, DUT: device under test 5. EXPERIMENTAL

PD: photodiode,

BS: beamsplitter,

A/D:

RESULTS

Optoelectronic sampling using the sliding contact method has been applied to study the propagation characteristics of a coplanar waveguide. The Cr:GaAs substrate was bombarded by 300 keV protons at a dose of 1015cm-2. A CPM laser with a FWHM of 50 fs and G= 80 MHz has been used to generate an electrical signal on the line. The pulse has been sampled after propagating a distance Ax. In Fig. 6 the results are plotted. As can be seen, the pulse is distorted during propagation as a result of dissipation and dispersion. A fast Fourier transformation yields the spectra of attenuation and the effective dielectric constant in Fig. 7, the experimental bandwidth being as large as 1.2 THz. Clearly, E r,eff increases from the quasistatic value of 6.9 for frequencies c 100 GHz to 8.7 at 1.2 THz. The measurements are in accordance with experiments by Frankel et al [ 161 carried out in the frequency range up to 900 GHz. 140

1 mm

20 0 0

0.5

1

1.5

2

2.5

3

t (us)

Figure 6. Pulse propagation along a coplanar waveguide on Cr:GaAs implanted with H+-ions

332

D. Jiiger et ul.

I Fundamentals of optical beam testing

72s cu

I

I

z

& ‘a

z*8-

z

7 *3 : .S 1 ;;i Z e, Z ao I”,.l,.‘.l,.‘.‘,‘,.I”,,l..,,‘; 0.2 0.4 0.6 0.8 0 frequency (THz)

1

1.2

t /’ /’ 8 ,/‘Z ,.,’ ___..e S 07- __-A .g M tin Q6 ‘,.‘.‘,,‘i’...,i”““.‘.l,‘~ 0 0.2 0.4 0.6 0.8 frequency (THz)

(a)

, ,’

,’

1

1.2

(b)

Figure 7. Attenuation waveguide of Fig. 6

coefficient

6. ELECTROOPTIC

TESTING

(a) and effective

dielectric

constant (b) of the coplanar

The concept of electrooptic measurement techniques is based upon the modulation of an optical beam propagating through an electrooptic active medium exposed to the electrical field to be measured [6,17-191. The underlying physical effects are the electrically induced change of the optical losses or the optical index of refraction. Note, however, that in the latter case an intensity modulation can only be obtained when optical interference is utilized or a polarization change is detected by an analyzer. Fig. 8 shows the basic set-ups when the Pockels effect is used. The optical input beam is assumed to be circularly polarized. When

Laser

Figure 8. Optical intensity modulator passing the crystal an elliptical polarization is generated due to the electrical field E. The output polarizer converts this modulation into a variation of the intensity which can be measured. As mentioned above, direct and indirect probing is possible, as schematically illustrated in Fig. 9. Fig. 9 (a) shows the configuration of backside probing of a coplanar structure. In this case the substrate material has to be electrooptically active. Besides the usually applied Pockels effect in GaAs or InP the following mechanisms can also be utilized: Franz Keldysh effect, band filling, quantum confined Stark effect, thermooptic properties and the plasma

D. Jiiger et al. I Fundamentals

of optical beam testing

333

GaAssubstrate

\

laser beam

(b)

(a)

Figure 9. Direct (a) and indirect (b) electrooptic probing of a coplanar structure effect. It should, however, be pointed out that the material itself determines whether a mechanism is available or not while taking into account that optical pulses with the desired photon energy are needed. Fig. 9 (b) elucidates the set-up for indirect probing. In this case an external electrooptical sensor material (eg. LiTa03) or modulator (eg. nin MQW microresonator [20] ) is applied and the optical set-up is realized according to the respective physical mechanism. When using LiTa03 and the Pockels effect, the optical polarization is changed and the arrangement of Fig. 8 has to be realized. 7. ELECTROOPTIC

NETWORK

ANALYZER

A fully computer controlled electrooptic network analyser system for two-dimensional field distribution analysis in MMICs has been realized. The configuration is sketched in the block diagram of Fig. 10: (i) The optical part consists of a mode-locked Nd:YAG laser (1064 nm) providing pulses with a FWI-IM of 92 ps (observed with the detector PD and the sampling oscilloscope) to be compressed to about 4-5 ps in a fiber compressor. The polarizer together with the quarter-wave plate provide a circular polarization. The pulses are incident from the back into the substrate of the MMIC. The microscope objective gives a focus with a diameter of about 4 urn. The reflected signal is analyzed and detected with the photodetector PD. (ii)Mechanically the DUT is moved by a translation stage with respect to the optical beam. A two-dimensional scan across an area of 1 mm x 1 mm and detection at 10,000 points is carried out in about 1 hour. (iii) Electrically the MMIC is driven by a microwave source and terminated by a matched load. The synthesizer is phase locked to the modelock generator of the laser and to the spectrum analyzer measuring the output signal of the photodetector PD. The set-up to Fig. 10 is used to study the two-dimensional distribution of electrical field amplitudes in GaAs-MMICs. The qualitative results include also topography contrast and informations about interface layers and surfaces because the fundamental reflectivity at different spatial positions depends on the optical properties of the interfaces, where interference can additionally give some influence. Basically, this interference can enhance the sensitivity because of resonance effects when the structure is that of a Fabry Perot interferometer (cf. Fig. 9(a)).

334

D. Jiiger el al. I Fundamenrals

qf opricol beam lesling

(

Figure 10. Electrooptic network analyzer, PD: analog/digital converter, DUT: device under test 8. EXPERIMENTAL

photodiode,

BS:

microscope objective

beamsplitter,

A/D:

RESULTS

In a first experiment to be discussed, the standing wave patterns along coplanar transmission lines are measured. From a simulation the transmission line parameters, propagation constant and characteristic impedance, have been determined [21]. When the spectrum analyzer in Fig. 10 is set to an harmonic signal, nonlinear effects can also be detected as has been shown for the second harmonic generated on a Schottky contact transmission line [22]. In Fig. 11 the measured standing wave patterns along a coplanar transmission line are plotted when a GaAs probe tip is located above the structure [23]. This external probe emulates the situation of indirect optical beam testing (Fig. 9 (b)), and the experimental results reveal clearly the influence of this load. From a simulation it is found that the load can be described by a capacitance which amounts to 7.1 fF in the present case.

D. J@er

et al. I Fundamentals

335

of optical beam testing

-96 2 % -z EL ‘Z

-98

-100

I

-1o2o

3I

6’

9’

12 I

15 I

18

y-position (mm) Figure 11. Standing wave pattern on a coplanar waveguide with a GaAs probe tip at y= 6.5 mm From the field distributions in Fig. 12 it is obvious that the wave travelling along the drain transmission line of the TWA experiences a maximum and a minimum at 5 GHz, but at 9 GHz the signal propagates at constant amplitude along the line connecting the three stages (note that mirror images are obtained due to the backside probing configuration).

(b) Figure 12. Field distribution on the drain line of MMIC 1-12 GHz TWA [24]. (a) 5 GHz and (b) 9 GHz (contour plots, 10,000 data points, scanned area: 2800x1090 pm), the TWA and the scanned area (white rectangle) is shown in (c)

D. ./tiger ef al. I Fundamentals

336

qfop~icui beam testing

Fig. 13 presents the results of measurements in a GaAs-MESFET (a) with a structure as given in (b). Obviously, several dark and light regions are detected which not always coincide with the metallization pattern. Hence the observed field distribution provides additional informations on the electrical behaviour of a FET in the microwave region.

(a>

(b)

Figure 13. Spatially resolved field distribution (a) of a FET with a metallization as shown in (b). f = 3 GHz (contour plot, 10,000 data points, scanned area: 470x110 p). The meas ured area of (a) is given by the dotted line in (b)

(4

(b)

Figure 14. Field distribution generated from a coplanar antenna at 0.15 GHz (a) and 18 GHz (b),( contour plot, 10,000 data points, scanned area: 605x5 10 pm) Fig. 14 shows the results of the field distribution close to a short coplanar line with open ends driven by RF-signals via a coplanar probe. As can be seen, at 150 MHz a clear pattern resembling standing waves is obtained. However, the wavelength with roughly 10 - 30 pm (depending on the orientation) is very small. Because the interference vanishes at GHz frequencies (see Fig. 13 (b)) it is concluded that the pattern in Fig. 14 (a) is caused by acoustical waves on the surface of GaAs.

D. Jiiger et al. I Fundamentals

of optical beam testing

337

9. DISCUSSION AND CONCLUSION In summary, two optical beam testing methods have been described where a laser beam is used for contactless on-wafer and in-circuit probing of electrical fields in MMICs. It should, however, be mentioned that other optical techniques have also been applied in the past. For example, interesting information can be obtained from optical beam induced currents (OBIC) [13] and the detection of recombination radiation and photoluminescence. In any of these cases a spatial resolution in the l.trn-range can be achieved but a temporal resolution in the picosecond range has not been demonstrated up to now although sampling techniques are again feasible. At present, it is generally accepted that optoelectronic or electrooptic sampling provides several advantages as compared to other methods working in the same frequency range. In Table 1 optical beam testing methods are compared with conventional techniques. It is obvious that the main advantages are in the extremely high bandwidths and the contactless testing principle where, however, the external probe in indirect electrooptical sampling behaves similar to a contact with a very critical positioning when air gaps have to be adjusted within pm accuracy. With the exception of OBIC experiments two-dimensional scans cannot be carried out with optoelectronic methods. Instead the switches have to be integrated monolithically - with the problem of reproducibility - and therefore only limited information can be obtained from this method [25]. Because of the spatial adjustment of external probes in indirect electrooptic sampling the time to get a two-dimensional scan can become extremely large. Moreover, the direct electrooptic sampling is best when regarding the perturbation of the DUT due to measurement and with respect to the spatial resolution. The measurement Table 1. The main optical beam testing methods

1

1

optoelectronic sampling

/*-I

,

bandwidth

++

contactless

+

++

measurement time for one- and twodim. scans

__

+

+

++ I

invasiveness standard MMIC

__

spatial resolution

(only with OBIC)

accuracy

0

D. Jiiger et al.

338

i Fundamentals

of optical

beam testing

accuracy is still a problem, because quantitative results are still lacking due to the underlying physical principle. Optoelectronic sampling requires MMICs with integrated OES and direct electrooptic sampling is only possible with MMICs on transparent substrates and with free surfaces. These drawbacks are still a problem because MMICs have to be prepared in a suitable manner so that optical beam testing becomes possible. Several trends can be discovered in the area of the above optical techniques. First of all, there is the need to use a cheap and small laser (diode) instead of the giant and powerconsuming Nd:YAG or CPM lasers. Second, fiber technology could simplify the optical waveguiding and fiber bundles could be used for providing a large number of parallel contacts. Third, investigations on external probe tips with enhanced sensitivity and using absorptive effects should lead to improvements in indirect electrooptic sampling. Note that specially doped polymers can become excellent electrooptical and waveguiding materials. Fourth, quantitative measurements are required. Simultaneously, the physical interpretation of the results and the comparison with numerical simulations of devices and circuits have to be improved. ACKNOWLEDGEMENT The authors would like to thank the Deutsche Forschungsgemeinschaft for fmancial support. The MMICs were fabricated in the Institute of HalbleitertechniW Halbleitertechnologie, Universitat Duisburg. Numerous other researchers have significantly contributed to the work of this paper. Among them are W. Mertin and F. Taenzler (Institute of Werkstoffe der Elektrotechnik, Universitat Duisburg, Dr. R. Tempel (Institut fur Mobil- und Satellitenfunktechnik, Kamp-Lintfort) and Dr. R.M. Bertenburg (Institute of Halbleitertechnik/ Halbleitertechnologie, Universitat Duisburg). REFERENCES 1. 2. 3. 4. 5.

6. 7.

8.

T.T. Lee, T. Smith, H.G. Huang, E. Chauchard, and C.H. Lee, “Optical techniques for on-wafer measurements of MMICs”, Microwave Journ. 91, (1990>, 91-102 D.M. Bloom, K.J. Weingarten, and M.J.W. Rodwell, “Probing the limits of traditional MMIC test equipement”, Microwaves & RF 74, (1987), 101-106 D.M. Bloom, K.J. Weingarten, and M.J.W. Rodwell, “Electrooptic sampling measures MMICs with polarized light”, Microwaves & RF 74, (1987), 74-90 D.H. Auston, “Picosecond optoelectronic switching and gating in silicon”, Appl. Phys. Lett. 26, (1975) 101 D.H. Auston, A.M. Johnson, P.R. Smith, J.C. Bean, “Picosecond optoelectronic detection sampling, and correleation measurements in amorphous semiconductors”, Appl. Phys. Lett. 37, (1980), 371 J.A. Valdmanis and G. Mourou, “Subpicosecond electrooptic sampling: Principles and applications”, IEEE J. Quantum Electron., vol. QE-22, (1986), 69-78 M.B. Ketchen, D. Grischkowsky, T.C. Chen, C.-C. Chi, I.N. Dulling III, N.J. Halas, J.M. Halbout, J.A. Kash, G.P. Li, “Generation of subpicosecond electrical pulses on coplanar transmission lines”, Appl. Phys. Lett. 48, (1986), 75 1. J. A. Valdmanis, “1 THz-bandwidth prober for high-speed devices and integrated circuits”, Electron. Lett. 23, (1987), 1308-l 310

D. Jiiger et al. I Fundamentals of optical beam testing

9.

10. 11. 12.

13.

14.

15.

16.

17. 18. 19.

20.

21.

22. 23.

24.

25.

339

D.R. Grischkowsky, M.B. Ketchen, C. Chi, I.N. Duling III, N.J. Halas, J. Halbout und P.G. May, “Capcitance free generation and detection of subpicosecond electrical pulses on coplanar transmission lines”, IEEE J. Quantum Electron., vol. QE-24,(1988), 221-225 D.R. Grischkowsky, I.N. Duling III, J.C. Chen, and C. Chi, “Electromagnetic shock waves from transmission lines”, Phys. Rev. Lett. 59, (1987), 1663-1666 D.H. Auston, “Picosecond photodetectors: Physical properties and applications”, in “Picosecond optoelectronic devices”, ed. Chi H. Lee, Academic Press, New York, 1984 D. Krokel, D. R. Grischkowsky, and M:B: Ketchen, “Subpicosecond electrical pulse generation using photoconductive switches with long carrier lifetimes”, Appl. Phys. Lett. 54, (1989), 1046-1047 W.C. von Wendorff, M. Welters, and D. Jager, “Optoelectronic switching in travellingwave MSM photodetectors using photon energies below bandgap”, Electron. Lett. 26, (1990), 1874-1875 F. Buchali, I. Gyuro, F. Scheffer, W. Prost, M. Block, W. von Wendorff, G. Heymann, F.J. Tegude, P. Speier, and D. Jager, “Interdigitated electrode and coplanar waveguide InAlAs/InGaAs MSM photodetectors grown by LP MOVPE, Proc. 4th IPRM Conference, Newport, USA, April 1992,569-599 R. Kremer, G. David, S. Redlich, M. Dragoman, F. Buchali, F.J. Tegude,and D. Jager, “A high speed MSM-travelling-wave photodetector for InP-based MMICs”, Proc. of MIOP’93, May 1993, Sindelfingen, 27 l-275 M.Y. Frankel, S. Gupta, J.A. Valdmanis, and G.A. Mourou, “Terahertz attenuation and dispersion characteristics of coplanar transmission lines”, IEEE Trans. Microwave Theory Tech., vol. MTT-39, (1991), 910-916 B.H. Kolner and D.M. Bloom, “Electrooptic sampling in GaAs integrated circuits”, IEEE J. Quantum Electron., vol. QE-22, (1986), 79-93 K.J. Weingarten, M.J.W. Rodwell, and D. Bloom, “Picosecond optical sampling of GaAs integrated circuits”, IEEE J. Quantum Electron., vol. QE-24, (1988), 198-2202 J.M Wiesenfeld and R.K. Jain, “Direct Optical Probing of Integrated Circuits and High Speed devices”, in: Semiconductors and Semimetals, vol. 28, 221-334, R.B. Marcus, ed., Academic Press, Inc., 1990 S. Zumkley, G. Wingen, J. L. Oudar, J.C. Michel, R. Planel, and D. Jager, “Vertical n-in-multiple-quantum-well electrooptical modulators for high-speed applications”, IEEE Phot. Lett. 5, (1993), 178-180 G. David, S. Redlich, W. von Wendorff, and D. Jager, “Electra-optic probing of coplanar transmission lines and optoelectronic microwave devices up to 40 GHz”, Proc. of the MIOP’93, May 25-27, Sindelfingen, Germany, 1993,492-496 W. von Wendorff, M. Stopka, and D. Jager, “Electra-optic sampling of nonlinear effects in Schottky coplanar lines”, Microelectron. Eng., 16 (1992), 305 - 312 W. von Wendorff, G. David, U. Dursun, and D. Jager, “Frequency-domain characterisation of a coplanar waveguide up to 40 GHz by electro-optic probing”, IEEE Lasers and Electra-Optics Society 1992, Annual Meeting, November 16-19, Boston, MA, USA, 192-193 G. David, S. Redlich, W. Mertin, R.M. Bertenburg, S. KoBlowski, F.J. Tegude, and D. Jager, “Two-dimensional direct electro-optic field mapping in a monolithic integrated GaAs amplifier”, accepted at 23rd EUMC, September 1993, Madrid, Spain S. L. Huang, E. A. Chauchard, C.H. Lee, T.T. Lee, H.-L.A. Hung, and T. Joseph, “Onwafer testing of MMIC with monolithiocally integrated photoconductive switches”, 1992 MTT-S Int. Microwave Symp. Dig., (1992), 661-664