AgilentEvaluation of MOSCapacitor Oxide C-VCharacteristics Using theAgilent 4294A

Application Note 4294-3

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1 IntroductionAs a result of extremely highintegration of logic LSIs, MOSFETs with gate lengths of 0.1 µmor less have been producedrecently. A consequence of thisminiaturization has been theneed for very thin gate oxide layers. In the past, C-V measure-ments using the Agilent 4284Aprecision LCR meter have beenperformed up to 1 MHz, howeverfor gate oxide thicknesses of less than 2 nm, traditional C-Vmeasurements cannot be useddue to large leakage currents.

The Agilent 4294A is a state-of-the-art precision impedance analyzer, providing the broadestimpedance coverage and

expanding the measurement frequency range up to 110 MHz.Furthermore, combining the4294A with the CascadeMicrotech probe station hasmade it possible to achieve highly accurate and repeatablemeasurements, even for MOSFETs with very thin gate oxidelayers.

In this product note, the break-through technologies of theAgilent Technologies 4294A are discussed. Integration of apractical measurement system,suitable for ultra-thin gate oxideevaluation, also is explained.Several problems that may occurduring actual device evaluationsthen are outlined. Finally,detailed solutions for theseproblems are discussed.

2 The differencesbetween the 4284Aand the 4294AThis section describes the functionality of the 4294A andprovides comparisons to the4284A. Table 1 summarizes the functionality differencesbetween the 4284A and the4294A.

Function 4284A 4294A

Frequency range 20 Hz to 1 MHz 40 Hz to 110 MHz

Sweep function No: Point measurement only Yes: Frequency (linear sweep/log sweep)DC bias (voltage/current)AC signal level (voltage/current)

Constant voltage and No Yes (auto level control function)current DC bias function

List sweep function Yes: Point measurement only Yes: Sweep measurement

Display function Numeric display Graphic display

Internal programming function No: External PC is required Yes: Internal IBASIC programming function (standard)

Extension cable 1 m/2 m/4 m 1 m/2 m (with phase compensation function)

Data transfer interfaces GPIB GPIB, LAN

Grounded device measurement No Yes: 42941A (impedance probe)

Other Touchstone format support (firmware rev. 1.1 or later)

As shown in this table, the 4294A covers not only a wider measurement frequency range than the 4284A, but also is equipped with various analysis functions for the evaluation of ultra-thin gate oxides.

Table 1. Comparison of 4284A and 4294A

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3 Breakthrough technology: Impedance probe useThe 4294A achieves the broadestimpedance coverage at high frequencies, using the Four-Terminal Pair configuration(4TP) with the Auto-Balancing-Bridge (ABB) method.Furthermore, grounded devicemeasurement is realized withthe 4TP and ABB measurementmethod and is kept by using the4294A together with the 42941Aimpedance probe. The 42941Aprovides not only easy connec-tion to a probe station, but alsostable measurement results athigh frequencies. In this section,the measurement principle ofthe 42941A impedance probeand the connection with a probestation is explained.

Figure 1. 42941A impedance probe

3.1 Measurement principle of the 42941Aimpedance probeThe measurement technology ofthe 42941A is different from the41941A (previous I-V method).The 42941A achieves an expan-sion of both the frequency range and the impedance measurement range.

There are two main technicaldifferences in the design of this advanced probe due to thedistinctions between the previousI-V method and the new I-Vmethod.

1. With the new method, a nearly ideal current meter is used, without the need for a transformer. This enables accurate measurement of small currents, which enhances the ability to measure high-impedance.

2. Since the transformer is not employed in the advanced I-V method, the operational frequency range of the 42941Aprobe is not dependent on the frequency response of the transformer. This is of particular importance in the lower frequency range, where the 41941A was limited to a minimum of 10 kHz. Using theadvanced method the 42941A can be used from 40 Hz.

Both the impedance measure-ment and the measurementranges of the 42941A comparedto the 41941A have been drasti-cally improved due to above-mentioned technologies. The42941A’s advanced I-V probingmethodology makes ultra-thingate oxide evaluation possible.

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Figure 2. Circuit diagram of the 42941A and the 41941A

Figure 3. Auto-Balancing-Bridge method

4. Breakthrough technology: ABBmethod with 4TPIn this section, the measurementbasics of the ABB method withthe 4TP are explained. Thebreakthrough technologies thatenable the frequency rangeexpansion of the 4294A also arediscussed. By understandingthese advanced measurementtechnologies in the 4294A,knowledge of the actual probesystem installation will makesense.

4.1 Basic measurement theory The 4294A provides the bestmeasurement accuracy and thebroadest impedance coverageusing the 4TP configuration with the ABB method. (Refer toFigure 4.) Figure 3 shows thecircuit diagram of the ABBmethod. In this circuit, the volt-age at the virtual ground pointis maintained close to the zerovoltage; and the current (I1),which flows through the DUT,goes to the range resistor (Rr).The DUT impedance (Z) is calculated using a voltagemeasurement (V1) at the highterminal and a current measure-ment (I2), which is given as thevoltage measurement (V2) at the range resistor.

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Figure 4. Four-Terminal Pair cable configuration

In order to improve measurementaccuracy, the 4TP configurationis employed by the 4294A. The4TP configuration can reducethe effects of lead impedancebecause the signal path and the voltage sensing cables areindependent. In addition to this,the configuration also solves themutual coupling problem becauseit uses coaxial cables to isolatethe voltage sensing cables fromthe signal current path.

As shown in Figure 4, at the Hcand Lc terminals, the currentsof the inner and outer (shield)conductors are at the same leveland flowing in the oppositedirection. Therefore, the magneticflux generated by the inner andouter conductor (shield) canceleach other. At the Hp and Lpterminals, only the difference of voltage between the inner and outer conductor (shield) isdetected. The generated electro-motive force due to the externalmagnetic flux at the inner conductor and outer conductortherefore can be eliminated.Consequently, it is possible tomeasure a wide impedancerange without the effect of measurement path length becausethe magnetic flux generatedfrom the measurement cablesand sensitivity to the externalmagnetic flux are drasticallyreduced by using the 4TP configuration.

To ensure this measurement performance, the outer conductorof the measurement cable (shield)should not be connected toground. In other words, thewhole system should be floatingfrom the actual ground level. Ifthe virtual ground of the ABBwere connected to the actualground, the ABB circuit wouldnot operate correctly. This is akey point of the cabling of the4294A to the probe station,which will be discussed later in this product note.

For more details about themeasurement theory, refer to Impedance MeasurementHandbook, 2nd Edition(Publication number 5950-3000).

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Figure 5. Cable terminated Auto-Balancing-Bridge method

frequencies, and is utilized above 15 MHz without cable extension. It is also valid above 5 MHz when a 1 m or 2 m extension cable is used. When a 42941A (impedance probe) is used with the 4294A,the cable terminated ABB method also is effective.

2. The bridge circuit is stabilizedby using phase compensation technology. In order to improve the bridge balance stability at high frequencies, the 4294A has a special circuitcircuit designed to compensatesate for the null-loop characteristics, which can vary with frequency and cablelength. The null-loop circuit is located at the low terminal, as shown in Figure 3, and adjusts the feedback loop to maintain a zero voltage at the virtual ground.

In the null-loop circuit, shown inFigure 6, the signal source (Eφ)is located in front of the vectorgenerator, and the vector volt-meter (Vφ) is located after theintegrated circuit. When connecting both low terminals,Lp and Lc, together, the outputsignal from the signal source(Eφ) can flow clockwise throughthe null-loop circuit, and ismeasured at the vector voltmeter(Vφ) in order to characterize the whole null-loop circuit. Byperforming this phase compen-sation, the ABB circuit acquiresthe capability to balance both athigh frequencies, and when longextension cables are used.

4.2 Frequency extension of the 4294AThe 4294A measurement fre-quency range has been extendedto 110 MHz. In order to achievethe frequency extension, the4294A uses several breakthroughtechnologies described below.

1. The 4294A uses an innovative technology called the Cable-terminated Auto-Balancing-Bridge method. As shown in Figure 5, the measurement path is terminated by the characteristic impedance of the cable (R0 = 50 ohm) at high frequencies. This termination solves the standing wave problem at high frequencies so that the measurement signal can be precisely conveyed, independ-ent of the frequency or the measurement path length. Hence, this technology provideshighly accurate impedance measurements for higher

Figure 6. Bridge circuit stability improvement by phase compensation

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Contact method Instruments Measurement method Features

Probing directly Best 4294A+42941A Advanced I-V method • Setup is easyabove the wafer (Refer to the • Stable measurement is

section 5.1.1) achieved at high frequencies

Good 4294A 4TP method • Measurement frequency is(Refer to the higher than probing throughsection 5.2.1) the chuck configuration

• Setup is complicated

Probing through Not 4294A 3T method • Setup is comparatively easythe chuck Recommended (Refer to the • Measurement frequency

section 6.1) is limited about 10 MHz

5. Measurement system installation(Probing directlyabove the wafer)As shown in Table 2, severalmeasurement methods are available.

This section initially introducesthe required products for theprobe system, which are neededto evaluate ultra-thin gate oxidesby probing directly above thewafer. Next, measurement issuessuch as system configurationand calibration are discussed.Finally, special considerationsand the resolutions devised todeal with these issues will beexplained.

Probing directly above the waferis simpler than probing throughthe chuck, and it makes it possi-ble to extend the measurementfrequency easily. While the42941A is, at present, the bestsolution for obtaining suchmeasurements, the measurementfrequency is limited 10 MHz* orless when probing the waferthrough the chuck. Therefore,probing directly above the waferis strongly recommended.

Table 2. Measurement system

* Note: The measurement frequency range changes due to elements in the measurement environment such as cable, a probe station, or other items.

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5.1 System configurationusing the 42941A

5.1.1 Required products for42941A systemThe following items are neces-sary to set up the probe systemusing the 42941A.

1. Agilent 4294A precision impedance analyzer with 42941A impedance probe (Table 3).

The cable, which connects the42941A to the ACP probe, isnot a manufactured productso it needs to be constructedby the user. The length of thiscable should be as short aspossible, because it can be anerror source for the entiresystem.

2. Cascade Microtech probe station, ACP probe head, and impedance standard substrate(Table 4).

5.1.2 Special considerationswhen connecting the 42941Ato a probe stationFigure 7 illustrates a cable connection when probing directlyabove the wafer.

A cable (SMA (m) to SMA (m)) is used to connect the 42941Aand the ACP probe. This cableshould be as short as possiblesince residual inductance of thecable may cause measurementerror at high frequencies. Figure 7. Cabling for the probing directly above wafer

Note: Cascade Microtech products listed in Table 4 need to be purchased from Cascade Microtech.

Table 3. Agilent products required for 42941A system

Item Description Remarks

4294A Precision impedance analyzer

42941A Impedance probe

SMA (m) to SMA (m) cable (1 each) Semi-rigid cable is recommended

Items Description Remarks

Probe Summit 11000 or 12000 seriesstation S300 series

Probe ACP series Frequency range: DC to 40 GHz

Table 4. Cascade Microtech products required for 4294A system

SMA(m) to SMA(m) cable

Cascade providesACP probe head

Agilent provides 42941A

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5.1.3 Compensation procedures for the 42941AFor accurate measurements, it isvery important to perform com-pensation properly.

1. Phase compensationAs shown in Figure 6, the phase compensation function is available on the 4294A.

• Push the [Cal] button and choose [PROBE] in the [Adapter] menu.

• Choose [PHASE COMP] in the [SETUP] menu and then perform the phase compensation. The phase compensation should be performed with nothing connected to the 3.5-mm port. When the phase compensation data measurement is completed, the softkey labelchanges to PHASE COMP[DONE].

• After the phase compensa-tion sequence, push the [DONE] button. In the 4294A’s operation manual, the open, short, and load measurement is also mentioned. However, this is not necessary since open/short/load compensa-tion will be performed at the tip of the ACP probe.

2. Open/short/load compensationFigure 8 shows the effect of open/short/load compensationdata, which compares the measurement data with and without open/short/load compensation. The open/short/load compensationcan drastically improve the measurement accuracy and stability at high frequencies. It also is possible to improve measurement accuracy when non-standard-length test leads or external circuits are used.

In the case of probing above thewafer, the open/short/load com-pensation can be performed byusing the Impedance StandardSubstrate (ISS).

For compensation, the 4294Ahas two modes. One is the COMPPOINT: FIXED mode, whichmeasures compensation data atpre-specified frequency pointsand determines the compensa-tion's affect at other frequencypoints by using interpolation.The other mode is the COMPPOINT: USER, which measurescompensation data at the fre-quency points that are actuallyset up. In this case, you canmake accurate measurements,but compensation needs to beperformed again when the meas-urement frequency is changed.In other words, if measurementparameters are changed fre-quently, COMP POINT: FIXEDmode is recommended and ifhighly accurate measurementsare required, COMP POINT:USER mode is recommended.

Figure 8. The effect of the open/short/load compensation

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Perform the open/short/loadcompensation by using the ISSat the tip of ACP probe. Beforeperforming compensation, calibration kit values need to be entered in the 4294A. Thisenables you to make more accurate compensation.

• Push the [Cal] button and choose the [FIXED] or [USER] mode from the [COMPOINT] menu.

• Push the [Cal] button again and choose the [DEFINE VALUE] from the [FIXTURE COMPEN] menu.

• In the case of ACP probe, the value of calibration kit is indicated in the box of probe head. The value of [OPEN CAP(C)], [SHORT INDUCT(L)], and [LOAD INDUCT(L)] values corresponding to the ACP probe need to be entered.

• Go to the [FIXTURE COMPEN] menu and begin performing the [OPEN], [SHORT], and [LOAD]compensation using the ISS.

• When performing the open compensation, ACP probe needs to be floating from the chuck.

• Perform short compensation by connecting both probes to the short on the ISS.

• Perform load compensation byconnecting both probes to the load on the ISS.

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Items Description Remarks

Probe station Summit 11000 or 12000 series

S300 series

Probe DCP-100 series or DCP-HTR series Frequency range: DC to 100 MHz

Probe type: Kelvin/non-Kelvin

Cable Tri-axial cables (4 each) These items are not required when BNC-

Part number104-330-LC SSMC cable (P/N: 105-540) is used

BNC-SSMC cables Part number 105-540 Connect to the DCP probe directly

Guard cable Part number 123-625

Standard for calibration Impedance standard substrate (ISS) Choose the appropriate ISS for the probe used

5.2 System configurationusing the 4294A

5.2.1 Required products for4294A systemThe following items are necessary to set up the 4TP configuration system when probing directly above the wafer.

1. Agilent 4294A precision impedance analyzer with 16048G or H test leads (Table 5).

2. Cascade Microtech probe station, probe head, and impedance standard substrate (Table 6).

Item Description Remarks

4294A Precision impedance analyzer

16048G or H Test leads, BNC (1 m or 2 m)

BNC (m) to BNC (m) adapters (4 each)Part number 1250-0216Tri-axial BNC(m) to BNC(f) adapters (4 each) Part number 1250-2650

Note: Cascade Microtech products listed in Table 6 need to be purchased from Cascade Microtech.

Table 5. Agilent products required for 4294A system

These items are not required when Cascade Microtech’sBNC-SSMC cable (Part number 105-540)is used

Table 6. Cascade Microtech products required for 4294A system

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not only an unbalancedbridge, but also inaccuratemeasurement results. It isalso important to note thatwafer evaluation may beincorrect at certain measure-ment frequencies and envi-ronmental conditions eventhough the guard cable (Partnumber 123-625) is used. Insuch a case, connect bothguards with a shorter cable to reduce the residual imped-ance and extend the system’soperation frequency, asshown in Figure 10. This figure shows a measurementresult of a short on the ISSwithout the open/short/loadcompensation. It comparesthe guard cable (Part number123-625) with the shorterguard cable (about 2 cm).

5.2.2 Special considerationswhen connecting the 4294A to a probe stationFigure 9 illustrates the cableconnection when probing directly above the wafer.

The following points are indicatedas special considerations for theactual probe system configuration.

1. Test leads and adaptersThe BNC-SSMC cable (Partnumber 105-504), which isprovided by CascadeMicrotech, is recommendedfor connecting to the probestation. With this cable, the4294A and DCP series probecan be connected directly.

Furthermore, the 4TP config-uration is terminated whenusing a probe positioner forC-V measurement, becausethe outer (shield) and theinner conductor are connect-ed together in the probe posi-tioner. As a result, residualimpedance, which exists inthe cable between the probepositioner and the probeitself, has a negative effect onmeasurement results.Therefore, in order to keepthe 4TP configuration veryclose to the tip of probes, it isrecommended to use theBNC-SSMC cable.

When connecting through theconnecting plate of the probestation, the Agilent 16048G orH (1 m or 2 m) cable is rec-ommended. The characteris-tics of these cables arecarefully evaluated by AgilentTechnologies and measure-ment accuracy is defined atthe tips of these cables. Whenthese cables are used with theprobe station, BNC to tri-axial

BNC adapters (Part number1250-2650) are required. Asshown in Figure 9, the outershield and the inner shield(guard) of these adapters are not connected together, so the 4TP configuration ismaintained to the probe positioner. Using this configu-ration, however, measurementfrequency is limited to 60MHz* due to the frequencyresponse of these adapters.

2. Guard connectionAs shown in Figure 9, theguards of the low and the highterminal should be connectedtogether using the guard cable (Part number 123-625)provided by CascadeMicrotech. It is desirable thatthe 4TP configuration bemaintained very close to the tips of the probes in order to obtain high accuracy.If a guard is improperly connected, the current pathbetween the inner and theouter (shield) conductor isnot formed and it may cause

Figure 9. Configuration for probing directly above the wafer

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From this measurementresult, it can be inferred thatthe residual inductance of theguard cable can’t be ignoredat high frequencies. Usually,such a residual inductancecan be removed to a certainextent by performing compensation. However, whenresidual inductance is large,as shown in Figure 10, com-pensation does not work welldue to the variations in theresidual inductance value. Asa result of this, not only doesthe measurement accuracydegrade. but the measurementalso becomes very unstable.In addition, a resonancecaused by the inductance ofthe guard cable and the straycapacitance between thechuck and the actual ground,or the top deck and the chuck,can degrade measurementaccuracy as well (Figure 11).Consequently, the length ofthe guard cable should becarefully considered.

3. Exchange from 4TP configurationWhen using a probe from a company other than CascadeMicrotech, the length betweenthe 4TP termination pointand the tip of the probeshould be less than 10 cm. If the length is longer, thebridge may be unbalanced.Please note that cable exten-sion using a non-4TP configu-ration has a negative effect onmeasurement results due tothe obvious symptoms ofresidual impedance andmutual inductance.

Figure 11. The variety of factors that influence measurements results

Figure 10. The guard cable’s effect on the measurement

Impe

denc

e [o

hm]

1000

100

10

1

0.11.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09

Short (ISS) measurement

Frequency [Hz]

Cascade Guard Cable Short Guard Cable

100 E (160 nH) 100MHz

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5.2.3 CompensationProcedures for the4294AFor accurate measurements, it is very important to performcompensation properly. This isespecially true when the 4294Auses extension cables andadapters for connecting to theprobe station.

1. Phase compensationAs shown in Figure 6, thephase compensation functionis available on the 4294A.After setup is complete, phasecompensation should be performed.

• Push the [Cal] button and in the [Adapter] menu choose the extension cable length [4TP 1M] or [4TP 2M], whichever is closest to the total cable length.

• Connect the two low terminals, Lp and Lc, together as shown in Figure 6. When using the DCP probe (non-Kelvin type) from Cascade Microtech, the probe shouldbe floating from the chuck because both terminals already have been connectedtogether in the probe. Whenusing the Kelvin probe, Lp and Lc terminals should be connected together by usingthe short on the ISS, which is provided from Cascade Microtech.

• Choose [PHASE COMP] in the [SETUP] menu. After the phase compensation sequence, push the [DONE]button. In the 4294A’s operations manual, the loadmeasurement is mentioned,however its use is not necessary since load compensation will be performed later.

2. Open/short/load compensationIn the case of probing above the wafer, the open/short/loadcompensation can be performedby using the ISS.

• Push the [Cal] button and choose the [FIXED] or [USER] mode from the [COMPOINT] menu.

• Go to the [FIXTURE COMPEN] menu and begin performing the [OPEN], [SHORT], and [LOAD]compensation using the ISS. When performing the open compensation, each probe should be kept at the same distance of the DUT’s electrode pitch. This technique is particularly effective when measuring small capacitances.

• When performing the open

compensation, probes need to be floating from the chuck. If using the Kelvin probes, both probes should contact the short standard on the ISS.

• Perform short compensa-tion by connecting both probes to the short on the ISS.

• Perform load compensation by connecting both probes to the load on the ISS.

• Standard DUT values, which are used in compensation, can be set in the [DEFINE VALUE] menu.

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6. Measurement system installation(probing through thechuck)In this section, the requiredproducts for the probe systemand the details about the systemconfiguration are explained.Compensation techniques of theinstrument and special consider-ations also will be discussed.

6.1 System configuration The following items are necessaryto set up the probe system whenprobing the wafer through thechuck.

1. Agilent 4294A precision impedance analyzer with BNCcables (Table 7)

2. Cascade Microtech probe station, probe head, and ISS (Table 8)

6.2 Special considerationswhen connecting the4294A to the probe stationFigure 12 illustrates the conceptof cable connection when probingthrough the chuck.

1. Test leads When probing through thechuck, the 16048G or H testlead cannot be used with theprobe station because thehigh and the low terminalsare apart from each other, sogeneral-purpose BNC cablesare required.

2. Guard connectionAs for the connection cablesand the adapters, the sameproducts can be used forprobing directly above thewafer. To perform the meas-urement, a three-terminalconfiguration is recommendedfor probing a wafer throughthe chuck. (Refer to Figure 12.)

3. Connection to the chuckThe stray capacitance, whichis generated by leakage currentbetween the chuck and theactual ground, has a negativeeffect on measurementresults. As shown in Figure 13,to reduce the measurementerrors due to leakage current,the high terminal should beconnected to the chuck. Thisconfiguration eliminatesmeasurement error becauseonly the current flowingthrough the DUT is measuredby the 4294A.

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Item Description Remarks

Probe station Summit 11000 or 12000 series

Probe DCP-100 series or DCP-HTR series Frequency range: DC to 100 MHzProbe type: Kelvin/non-Kelvin

Cable Tri-axial cables (2 each) Cable connection for the probesPart number 104-330-LC

Standard for Impedance standard substrate (ISS) Choose the appropriate ISS for the probe usedcalibration

Adapter Tri-axial BNC (m) to BNC (f) adapters (4 each) Part number 1250-2650

Figure 12. Cabling when probing through the chuck

Table 8. Cascade Microtech products required for 4294A system

Note: Cascade Microtech products listed in Table 8 need to be purchased from Cascade Microtech.

Item Description Remarks

4294A Precision impedance analyzer

BNC (m) to BNC (m) cables (4 each) Cable connection for the chuck Part number 8120-1839 or 8120-1840 (61 cm/122 cm)

Table 7. Agilent products required for 4294A system

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Figure 13. Cable connection to the chuck

6.3 Compensation procedures for the 4294AAlthough performingopen/short/load compensationproperly is crucial to the meas-urement, compensation whenprobing through the chuck getsextremely difficult. A usefulcompensation method isdescribed below.

1. Phase compensation Phase compensation shouldbe performed in the samemanner as probing above thewafer. If a non-Kelvin probe is used, the probe should befloating from the chuck, sinceboth low terminals (Lp andLc) are already connectedtogether in the CascadeMicrotech probe. If a Kelvinprobe is used, both low (Lpand Lc) terminals should beshorted by using the shortstandard on the ISS.

2. Open/short/load compensation The ISS cannot be used forthe compensation of the wafer measurement throughthe chuck. Therefore, it isvery difficult to perform anaccurate compensation whencompared to the probingabove the wafer configuration.The following procedure is one example of theopen/short/load compensa-tion through the chuck.

• Open compensation should be performed by floating the probe from the chuck.

• Short compensation should be performed by connectingthe probe directly to the chuck.

• One of the ideas of per-forming load compensation is to use a 50 W Surface Mount Device (SMD) resistoras the standard DUT to substitute for the load on the ISS. In this case, an insulator should be insertedunder the one of the electrodes of the SMD in order to isolate the electrodefrom the chuck. Standard DUT values, which are usedin compensation, can be setat the [DEFINE VALUE]menu.

If further operation informationis required, please refer to theprevious section.

When connecting low terminals to chuck sideFlowing current of DUT branches off to the stray capacitance and it will cause ameasurement error

When connecting to high terminalsto chuck sideLeak current due to the stray capacitancedoes NOT effect the measurement currentof DUT

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The two-frequency technique is convenient for extracting theparameters from the three-parameter model. This techniqueis used to calculate the capaci-tance values of three-parametermode using the following equa-tion. In this equation, Cp and D values are obtained at two different frequencies.

As long as the difference of the impedance values at the two different points is large,accurate capacitance values can be obtained. However, if thedifference is not wide enough,the obtained parameters may be inaccurate. Therefore, it is important to choose twoappropriate frequency points.*

7.2 Minimum phasedetection methodThe minimum phase detectionmethod is a new technique toobtain the parameters of thethree-parameter model from thefrequency characteristics of the|Z|-q measurement. As shown in Figure 15, the capacitancevalues (C) can be calculated byfrequency (f0), phase (q0), andimpedance (|Z0|) values at the minimum phase point. Aconstant DC bias voltage shouldbe applied during the |Z|-qmeasurement. In order to obtaina precise capacitance value, it is desirable to measure capaci-tance at the frequency where the dissipation factor (D) is thesmallest. In other words, theloss of device becomes smallestat the minimum phase point.Eventually, the C-V characteris-tics plot can be obtained fromseveral fixed DC bias |Z|-q frequency sweeps.

Figure 14. Two-frequency technique

7. Actual measurementsIn this section, the evaluation of actual ultra-thin gate oxidesand some cautioning factorsrelating to the measurement are discussed.

7.1 Two-frequency techniqueAn equivalent circuit of the cur-rent two-parameter model, suchas a parallel capacitance (Cp)and a parallel resistance (Rp), as shown in Figure 14, is notenough to express the actualultra-thin gate oxide behaviorwhen leakage current increases.It has already been noted thatthe three-parameter equivalentcircuit model, as shown inFigure 14, is used as the modelfor ultra-thin gate oxides.

*For information on choosing frequency points, refer to reference paper 8.

Figure 15. Minimum phase detection method

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Figure 16. Calculation results using IBASIC

The equation for the minimumphase detection method is as follows.

The 4294A is equipped with theInstrument BASIC (IBASIC) programming function and sothe calculation of the minimumphase detection method can be accomplished within theinstrument itself. Hence, anexternal PC is not required for

calculation, although it is needwhen using the 4284A. Figure 16shows the calculation results of each parameter, which werecalculated with IBASIC usingthe minimum phase detectionmethod, and displayed on the4294A.

Figure 17 shows the measuredand corrected capacitance for an ultra-thin gate oxide usingthe minimum phase detectionapproach presented above. Asshown in this figure, the mini-mum phase detection method isvalid to obtain accurate C-Vmeasurement results.

7.3 Another two-frequencytechnique When a minimum phase pointdoes not appear due to measure-ment frequency limitations, thecapacitance values cannot beobtained by the minimum phasedetection method. In this case,the two-frequency techniquepresented above, or the use of atwo-frequency technique usingthe |Z|-q measurement results,is convenient. However, the latermethod may not be able to getthe precise capacitance valuewhen dissipation factors (D) arelarge at both frequency points.

Figure 17. C-V characteristics for ultra-thin gate oxide

Calculating capacitance from |Z|-q measuredby several fixed DC bias and plot it

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3. It is very convenient to use the list sweep function whenmeasuring the |Z|-q charac-teristics under various DCvoltage bias conditions. Thelist sweep function enablesdifferent measurement setupsin a single sweep by dividingthe sweep range in to segments.The measurement setup,including frequency range,number of points, bandwidth,DC bias and so on can be setdifferently for each segment.This will drastically reducemeasurement time because asingle sweep covers several DCvoltage bias measurements.

7.5 Beyond the 110 MHzmeasurementIt may not be possible to measure the phase minimumpoint beyond the measurementfrequency of 110 MHz, becausethe minimum phase point isshifted to higher frequencieswhen the gate oxide thicknessbecomes thin and leakage currents increase.

The equation, which becomes a phase minimum, can beexpressed with an equivalentthree-parameter circuit model in the following form.

Since leakage currents increaseexponentially against the gateoxide thickness, the parallelresistance (Rp) decreases by oneorder of magnitude when thegate oxide thickness decreasesby one-half. When leakage currents become very large andRp becomes smaller than Rs (Rp << Rs), the above equationcan be expressed as follows.

In this case, the Rp valuebecomes the dominant factor.The capacitance value (C) of thegate oxide has minimal influencebecause it will only double evenif the thickness becomes one-half. As a result, the value of ω0increases and the minimumphase point shifts to the higherfrequencies. In this case, the use of the Agilent E4991A RFimpedance/material analyzerwith Cascade Microtech’s ACPprobe is recommended. Thisconfiguration can measureimpedance characteristics up to3 GHz. (For more details, referto Agilent E4991A ApplicationNote 1369-3, AccurateImpedance Measurement withCascade Microtech ProbeSystem, (Publication number5988-3279EN).)

7.4 Cautions for actualmeasurements 1. When a DC bias voltage is

applied to the DUT, leakagecurrent from the DUT shouldbe carefully considered sinceit will cause the DC bias volt-age to drop. In the case of the4284A, DC bias source imped-ance is 100 W so if 1 mA cur-rent leaks, the DC bias voltagelevel drops 100 mV and hencethe actual applied DC voltagewill be different from the ini-tial conditions. On the otherhand, the 4294A is equippedwith the DC Bias Auto LevelControl (ALC) function, whichis based on the feedback looptechnique and accuratelymaintains the applied DCvoltage bias. Therefore, a con-stant DC voltage bias can beapplied to the DUT regardlessof the leakage current.

2. It is recommended that the DC current measurementrange (1 mA/10 mA/100 mA)be set properly in order toprevent the 4294A from a DCcurrent overload conditiondue to the leakage current.

21

However, some of devices thatdo not have a minimum phasepoint within 100 MHz cannot bemeasured by simply increasingthe measurement frequencyrange. If the value of Rpbecomes equal or less to thevalue of Rs, the phase becomesconstant over an entire frequen-cy range. In other words, it isdifficult to measure capacitancevalue precisely because itbecomes the condition of meas-uring a device which has a highdissipation factor. The only wayto solve this problem is toredesign the interconnect andcontact pad structures. Byredesigning these structures at the device design phase, itbecomes possible to reduce the value of Rs.

Figure 18. Balancing circuit

GoalOptimGoal1

GOALOptimOptim1OptimType=RandomErrorForm=MML1MaxIters=100P=2DesiredError=0.0StatusLevel=4SetBestValues=yesSeed=5SaveSolns=yesSaveOptimVars=noSaveGoals=noGoalName[1]="OptimGoal1"

SRC1Vac=polar(1,0)Freq=freq

V_AC

SRC2Vac=polar(1,180)Freq=freq

V_AC

S1PSNP2File="C-V-2.s1p

R

R1

R=3.460740e+001 Ohm opt{ 20 Ohm to 50 Ohm }

PRC1R=2.100285e+001 kOhm opt{ 5 kOhm to 25kOhm }

C=1.649386e+002 pF opt{ 140 pF to 170 pF }

ACAC1Start=40 HzStop=110 MHzStep=

AC

NOMINAL OPTIMIZATION

Expr="mag(Vnode)"SimInstanceName="AC1"Min=0Max=0.001Weight=RangeVar[1]="freq"RangeMin[1]=40 HzRangeMax[1]=110 MHz

Equivalent circuit modelMeasured data

component

180 degree out of phase and the same signal level to each source Optimizing node A voltage to be

close to zero

A

8. Equivalent circuitmodeling using ADSIn this section, the modelingmethod for a more complexdevice using the Agilent EEsofAdvanced Design System (ADS)is introduced.

1. Save the measured data in touchstone format and importit into the ADS using the dataitem component, which allowsmeasurement data to be usedin the simulation.

2. The balancing circuit shown in Figure 18 balances the volt-age applied to the equivalentcircuit model and the dataitem component. This isaccomplished by applying twosignals that are the same

frequency and signal level but180 degrees out of phase toeach other, and then optimiz-ing the node voltage betweenthe equivalent circuit modeland the data item componentto be close to zero. In orderfor this goal to be accom-plished, before running the

optimization, each of the elements in the equivalent circuit must be defined asvariables that can be optimized.

3. Verify that the optimized node voltage is close to zero. If theoptimized node voltage isgreater than 0.1 V, try chang-ing the equivalent circuitmodel or the parameter rangethat was set prior to the opti-mization. If the extractedparameters are still inaccu-rate, use the tuning functionin ADS to tune the equivalentcircuit’s frequency responseto better match the measureimpedance data.

4. The simulation of the three-parameter model is per-formed in Figure 18, and amodel of further complexityis also possible. By takingadvantage of simulation toolssuch as ADS, a complexequivalent circuit model canbe easily extracted in a shortperiod of time.

9 Conclusion In this product note, we dis-cussed how to evaluate thecapacitance of ultra-thin gateoxides using an impedance analyzer with a probe station.Proper cabling and calibrationenables you to evaluate a deviceaccurately, even it has a largeleakage current. We hope thisproduct note makes it easier foryou to build an ultra-thin gateoxide evaluation system usingthe Agilent 4294A and CascadeMicrotech Probe Stations.

22

References 1. Impedance Measurement Handbook, 2nd Edition,

part number 5950-3000.

2. Agilent E4991A RF Impedance/Material Analyzer, Product Overview literature number 5980-1234E.

3. Achieving Fast Cycle Time Using an Electronic Design Automation(EDA) Tool and the E4991A RF Impedance/Material Analyzer, Agilent E4991A Product Note E4991A-2, literature number 5988-3029EN.

4. Accurate Impedance Measurement with Cascade Microtech Probe System, Agilent E4991A Application Note 1369-3, literature number 5988-3279EN.

5. Agilent 4294A Precision Impedance Analyzer, Product Overview literature number 5968-3808E.

6. New Technologies For Accurate Impedance Measurement, Agilent 4294A Product Note 4294-2, literature number 5968-4506E.

7. Kevin J. Yang and Chenming Hu, “MOS Capacitance Measurementsfor High-Leakage Thin Dielectrics“, IEEE Transactions on ElectronDevices, vol 46, no. 7, July 1999.

8. Akiko Nara, Naoki Yasuda, Hideki Satake, and Akira Toriumi, “A Guideline for Accurate Two-Frequency Capacitance Measurement for Ultra-Thin Gate Oxides,” Extend Abstracts of the2000 International Conference on Solid State Devices and Materials, Sendai, 2000, pp. 452-453.

Related LiteratureImpedance Measurement Handbook, 2nd Edition, part number 5950-3000.

Accurate Impedance Measurement with Cascade Microtech ProbeSystem, Agilent E4991A Application Note 1369-3, literature number5988-3279EN.

23

Glossary

4TP . . . . . . . . . . . . .Four-Terminal Pair configuration ABB . . . . . . . . . . . . .Auto-balancing-bridge ACP . . . . . . . . . . . . .Air coplanar probeADS . . . . . . . . . . . . .Advanced design system ALC . . . . . . . . . . . . .Auto level control BNC . . . . . . . . . . . . .Bayonet Navy connectorBNC-SSMC . . . . . . . .Bayonet Navy connector – small subminiature CC-V . . . . . . . . . . . . . .Capacitance vs. voltageDCP . . . . . . . . . . . . .DC probeDCP-HTR . . . . . . . . .Cascade Microtech productDUT . . . . . . . . . . . . .Device under testFET . . . . . . . . . . . . .Field-effect transistorGPIB . . . . . . . . . . . .General-purpose interface busIBASIC . . . . . . . . . . .Instrument BASICISS . . . . . . . . . . . . . .Impedance standard substrate I-V . . . . . . . . . . . . . .Current-voltage measurementLAN . . . . . . . . . . . . .Local area networkLSI . . . . . . . . . . . . . .Large-scale integrationMOS . . . . . . . . . . . . .Metal-oxide semiconductorSMA . . . . . . . . . . . . .Subminiature A SMD . . . . . . . . . . . . .Surface mount device

For Cascade Microtech products, contact: Cascade Microtech, Inc.2430 NW 206th Avenue,Beaverton, Oregon 97006, USATel: (503) 601-1000Fax: (503)601-1002E-mail: sales@cmicro.comURL: http://www.cascademicrotech.com

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© Agilent Technologies, Inc. 2002, 2003Printed in USA, June 26, 2003

5988-5102EN

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Save up to 20% when you upgradefrom with the 4284A to 4294A with Agilent’s Trade-Up program

• High Frequency C-V measurementcan be performed up to 110 MHz

• Graphic display is available

• Built-in I-BASIC programming function is equipped as standard

• Touchstone format is supported for data transfer to EDA tools

Note: Trade-in credit of the 4284A can be usedtoward purchase of the 4294A. Agilent Trade-Up is not available in all countries. For moredetails of credit amount and program informa-tion, please contact Agilent local sales office.

4294A 110 MHzimpedance analyzer

4284A 1 MHzLCR meter