ON-WAFER MILLIMETER-WAVE NETWORK ANALYSIS

FOR DEVICE AND CIRCUIT DESIGN

James Tabuchi, Brian Hughes and Julio Perdomo

Hewlett-Packard CompanyNetwork Measurements DivisionMicrowave Technology Division

Santa Rosa, CA 95403

ABSTRACT

A broadband, millimeter-wave on-wafernetwork analyzer system is necessary for thecharacterization and design of millimeter­wave devices and circuits. The HP 85109B45 MHz to 62.5 GHz network analyzer was'used to determine the fT and fmax of a

0.25}Lm MODFET. The S-Parameters werealso used to model an equivalent circuit of theMODFET. A broadband 0.5 to 50 GHztravelling wave amplifier [I] was measured todetermine the cutoff frequency and thepotential for out-of-band oscillations.Finally, a criteria was established for theselection of a broadband millimeter-wavenetwork analyzer.

INTRODUCTION

In recent years improvements in semi­conductor processes have resulted in theability to produce FETs with gate lengths assmall as O.I}Lm[2]. This, together withimproved material systems, such asAlGaAsjInGaAs and AlInAsjInGaAs[3][4],has provided the capability to produceMODFETs with fTS as high as 250 GHz[5].

Until recently the only broadband networkanalyzer systems available to measure thesedevices and circuits were 26.5 or 40 GHzsystems. Measurements were made on thesesystems and the data was extrapolated topredict performance at higher frequencies.Since' these extrapolations were made overlarge frequency spans, large uncertaintiesresulted. In order to reduce these uncer­tainties, it is necessary to measure as high in

frequency and reduce the extrapolation span.It is evident that as technology pushes devicesand circuits to higher frequencies, thenetwork analyzer systems must also evolve.

CRITICAL REQUIREMENTS OF ABROADBAND ON-WAFER NETWORK

ANALYZER SYSTEM

The most important criteria of abroadband on-wafer network analyzer systemis measurement bandwidth. For devicemeasurements, it is desirable to have thewidest bandwidth possible. For circuitmeasurements the bandwidth should not onlybe adequate to measure the device under testin its operating bandwidth, but should alsoextend higher in frequency for guardbandmeasurements. Broadband on-wafer networkanalyzers are currently limited to 62.5 GHzand broadband wafer probes are limited to 65GHz. V-Band network analyzer systems andwafer probes are available for measurementsfrom 50 to 75 GHz [6], but are narrowbanded.

In addition to measurement bandwidth,another very important characteristic toconsider in selecting an on-wafer networkanalyzer system is accuracy. Since modellingaccuracy is derived from measurement data, itis necessary to use measurement data of thehighest accuracy. The accuracy of themeasurement system will be determined bythe raw performance of the system and theaccuracy of the calibration. It is important tonote that there is a trade-off betweenmeasurement bandwidth and accuracy. Thebest trade-off should be sought in the

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Another very important characteristic to

consider is calibration stability. The accuracy

of the measurements depends not only on the

accuracy of the calibration, but on the amount

of system drift between calibration and

measurements. It is important, therefore, to

choose a dependable system that minimizes

the effects of drift.A third important characteristic of a

broadband network analyzer is high dynamic

range. It is often thought that if high gain

devices are to be measured, high dynamic

range is unimportant. This is not true,

however, because accurate device modelling

also requires accurate measurements of the

high loss, reverse isolation SI2 term. The Sl2

term is used to determine Cgd, which is then

used for calculating the f max and gain of the

FET.

521 log ~G

REF -0.5 dBe.2 dB/

Figure 2: Measurements of a 15 ps coplanar

transmission line using the OSLT and LRM

calibration techniques reveals large uncer­

tainties at high frequencies when using the

OSLT technique.

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Figure 1: A summary of the three most

common on-wafer calibration techniques.

selection of a broadband network anal 'zer

system.For versatility, the network analyzer

should support at least the three most common

on-wafer calibration techniques: Open-Short­

Load-Thru (OSLT), Thru-Reflect-Line (TRL)

and Line-Reflect-Match (LRM). The

selection of the appropriate technique will

depend upon the availability of standards, the

wafer probe configuration (fixed vs. moveable

probes), and the measurement frequency

range (see Figure I).

As can be seen in Figure 2, the OSLT

technique provides good measurement results

at low frequencies, but is limited at higher

frequencies by the parasitics of the open and

the short standards. On the other hand, the

TRL technique is adequate at the upper

frequencies, but is limited below 1 GHz

because the line standards become longer than

the maximum probe separation. The LRM

calibration technique combines the advantages

of TRL and OSLT without their disadvantages

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•Cgd Uncertainty" {GAmaxUncertainty( f T Uncertainty

LOW HIGHDYNAMI C RAf\GE DYNAMI C RANGE

(NO AVGl (1024 AVGI

S12 Signal

Noise floor

SIN Ratio

S12 Uncertainty

-18.75 dB

-39.32 dB20.57 dB

+1- 0.80 dB

-18.75 dB

-49.32 dB

30.57 dB+1- 0.24 dB

t-P851098

45 MHz to 62.5 GHzOn-wafer Network Analyzer

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Figure 3: A comparison of S12 uncertaintywith low and high dynamic range.

The effect of dynamic range on the accu­racy of the S12 term can be simulated byusing measurement averaging. As can be seenin Figure 3. increasing the averaging increasesthe dynamic range. which yields almost afour-fold decrease in uncertainty.

HP 85109B NETWORK ANALYZERSYSTEM OVERVIEW

The HP 85109B is a network analyzer sys­tem specifically designed for on-wafermeasurements of devices and circuits from 45MHz to 62.5 GHz. This system satisfies all ofthe above criteria for successful high fre­quency measurements. Because the HP85109B system measures higher in frequencythan previous systems. it reduces uncertain­ties resulting from extrapolations of lowerfrequency data.

The system is based on the high perfor­mance HP 851 OC and family of test sets andsources (see Figure 4). The system uses anHP 8517A test set for measurements from 45MHz to 50 GHz and an HP U85104A K09 U­Band test set for measurements from 40 GHzto 62.5 GHz.

3

Figure 4: HP 85109B system diagram.

The key to the HP 85109B system is acomponent that combines the ports of the twotest sets. This test port combiner takes themicrowave and millimeter wave inputs. sendsthese signals through a frequency selectivecoupling structure and provides a singlebroadband output port. Each output port isthen connected to a Cascade MicrotechWPH-405 (DC-65GHz) probe through a1.85mm connector cable.

Control software is provided with the HP85~09B system to change frequency bands,gUIde the user through a calibration sequence,control one of several possible bias suppliesand display a single broadband trace to theHP 8510C.

Calibrations are performed on-wafer byusing either the Open-Short-Load-Thru(OSLT). Thru-Reflect-Line (TRL) or Line­Reflect-Match (LRM) calibration techniques.Hewlett-Packard and Cascade Microtech haveco-developed the LRM technique and haveconcentrated on making the on-waferimpedance standard substrates compatiblewith the HP 8510.

To verify the calibration stability of theHP 85109B system. an LRM calibration wasperformed and measurements were made on a0.25#Lm HEMT FET 15 minutes. 6 hours and22 hours after calibration. Figure 5 showsminimal variation throughout the 22 hourperiod. Temperature during the evaluationperiod was maintained to within approxi­mately +-2 degree Celsius.

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21 8 128 211K =

The four S-Parameters were then used tocalculate the stability factor K, the H21current gain, and the maximum available gainGAmax versus log frequency, using theequations shown below [9].

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Figure 5: Sil measurements of a HEMT FETmade 15 minutes, 6 hours and 22 hours aftercalibration.

H21 = _

(1-8 11) (1+822)+812 8 21

and

GArnax = (K- VK2_1 ),

APPLICATIONS IN THE MODELLINGOF A O.2SJLm MODFET

for K > 1.One very important application of broad­

band millimeter test data is for the character­ization and modelling of discrete FETs. Inthis example, the S-Parameters of a 0.25JLmgate length by 122JLm gate width MODFETwere measured from 45 MHz to 62.5 GHz.The unmatched MODFET showed S21 gain of8.9 dB at 45 MHz, with greater than OdB upto 61 GHz, as shown in figure 9 on thefollowing page.

Gate Drain

The H21 plot can be extrapolated to

determine that the fT is approximately 58

GHz. It can also be seen that the FET is notunconditionally stable until about 33 GHz,where the K-factor equals unity. At thispoint the maximum available power gain cannow begin to be calculated. When theGAmax curve has a slope of -20 dB/decadethe curve can be extrapolated to determinefmax' which is 98 GHz for this FET. Thecurve, in this case, approaches a slope of-20dB/decade only at frequencies above 40

GHz. It can be clearly seen that measurementsfrom a 62.5 GHz network analyzer system arecritical for fitting the GAmax curveaccurately.

Source

Figure 6:Layout of a 0.25JLm T-gate MODFEl

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Figure 7: Figures of merit - H21, K-factorand Gma of a 0.25~m MODFET.

rrequenc:y (GHz)

The S-Parameter data was also used toextract the small signal model parameters ofthe MODFET. Results of the parameterextraction are shown below.

Figure 9: A comparison of measured versusmodelled S-parameters.

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Figure 8: Extracted small signal modelparameters.

After determining the model parameters,the four S-Parameters were simulated toverify that the model parameters were accu­rate. Figure 9 shows the very close agreementbetween measured and modeled S-Parameters.

TRAVELLING WAVE AMPLIFIERCHARACTERIZATION

A second example of the need for abroadband network analyzer system like theHP 85109B system is in the characterizationof a broadband MMIC amplifier. In this case,the performance of a 0.5 to 50 GHz GaAsdistributed amplifier was measured. Thisamplifier uses six cascoded 0.25~m gate lengthby 44JLm gate width MODFETs.

The HP 85109B system was used to mea­sure the in- and out-of-band characteristicsof the MMIC amplifier. As shown in Figure10, the amplifier has flat gain (+/-ldB)throughout the design bandwidth. It was alsoimportant to measure the out-of-band gain todetermine the amount of margin between theupper end of the designed bandwidth and theactual cutoff frequency. This differenceprovided a feel for how sensitive theperformance was to process variations.

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Figure 10: S21 gain measurement of the 0.5 to50 GHz travelling wave amplifier.

Next. the amplifier was measured todetermine its sensitivity to gate bias changes.The gate bias was first set to Vgs=-0.6v toachieve maximum gain. The bias was thenchanged to Vgs=-0.85v. Figure 11 shows thatthe amplifier maintained the same general S21shape up to approximately 55 GHz. changingby only approximately 2 dB. Above 55 GHzthe amplifier is very sensitive to changes inbias. changing S21 by more than 7 dB. Thismeasurement has determined that theamplifier is insensitive to bias variationswithin the design bandwidth. However. if theamplifier is to be used above 55 GHz. careshould be used to maintain an accurate gatevoltage.

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Figure 11: Sensitivity of gain due to a changein gate bias.

The final measurement was made todetermine areas of potential oscillation.Oscillations can occur if the output of theamplifier is not well matched to its load. Forexample, in a typical application. a low passfilter is connected to the output of theamplifier to reduce harmonics. However.since the filter has poor match in its rejectband. oscillations can occur due to the re­reflections off of the output of the amplifier.The condition for oscillation to occur is asfollows:

S22amp x Sllfilter >

Figure 12 shows that the output reflection.S22amp was less than 8.3 dB in the designbandwidth. up to 50 GHz. However. athigher frequencies S22amp approaches 0 dBand there is a potential for oscillation tooccur. These oscillations could appear in theoperating band as a result of mixing with theintended signal.

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Figure 12: Output reflections reveal potentialoscillation problems.

With this useful information, the designereliminated this problem on the next designiteration. Measurements outside the designband of this amplifier has provided usefulfeedback of the accuracy of the circuit modeland highlighted potential problems with thedesign.

CONCLUSIONS

As technology has pushed the device andcircuit capabilities to higher frequencies, ithas become necessary for network analyzersystems to evolve to be able to fully charac­terize these components. This paper haspresented a selection criteria for broadbandon-wafer network analyzer systems, namelythe need for broad bandwidth, calibrationaccuracy and versatility, measurement stabil­ity, and high dynamic range. The HP85109B network analyzer system has beenapplied to characterize a 0.25JLm MODFETand a 0.5 to 50 GHz travelling waveamplifier.

7

REFERENCES

1. J. Perdomo, M. Mierzwinski, H. Kondoh,C. Li and T. Taylor, "A Monolithic 0.5 to 50GHz MODFET Distributed Amplifier with 6dB Gain", GaAs IC Symposium 1989Technical Digest, p 91.

2. P.C. Chao, M.S. Shur, R.C. Tiberio,K.H.G. Duh, P.M Smith, J.M. Gallingall, P.Ho and A.A. Jabra, IEEE TRans ElectronDevice, vol. ED-36, pp 461-471, March 1989.

3. L.D. Nguyen, P.J. Tasker, D.C. Radulescuand L.F. Eastman, IEEE Trans ElectronDevice, vol. ED-36, pp 2243-2248.

4. U. K. Mishra, A.S. Brown and S.E.Rosenbaum, IEDM Technical Digest., Dec.1988, pp 89-92.

5. A.J. Tessmer, P.C. Chao, K.H.G. Duh, P.Ho, M.Y. Kao, S.M.J. Liu, P.M. Smith, J.M.Ballingall, A.A. Jabra and T.H. Liu, IEEECornell Conference on Advanced Concepts inHigh Speed Semiconductors, Device andCircuits, Ithaca NY, pp 56-63, Aug. 1989.

6. E. Godshalk, " A V-Band Wafer Probe",1990 ARFTG Technical Digest., November,1990.

7. J. Barr IV, T. Burcham, A. Davidson, E.Strid, "Advancements in On-wafer ProbingCalibration Techniques", presented at theHewlett-Packard RF & Microwave Sympo­sium, 1990.

8. S. Lautzenhiser, A. Davidson, K. Jones,"Improve Accuracy of On-wafer Tests ViaLRM Calibration", Microwaves & RF, pp105-109, January 1990.

9. M. Kumar, R. Goyal and T.H. Chen,"MMIC Design Considerations and AmplifierDesign" in Monolithic Microwave IntegratedCircuits: Technology and Design, ArtechHouse, Inc., 1989.

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