Advanced On-Wafer DeviceCharacterization Using theSummit 10500Today’s emerging technologies require a higher degree ofdevice characterization than ever before. This applicationnote provides information on how the Summit 10500Parametric Probe Station enables state-of-the-art DC/CV,LCZ, and high-frequency measurements at fA-, fF-, andGHz-levels. The Summit 10500 is the first integratedon-wafer measurement system for fundamentalcharacterization tasks.

A CASCADE MICROTECHTM

Advanced On- Wafer Device Characterizationusing the Summit 10500

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ContentsSummary . . . . . . . . . . . . . . . .I

Fundamental Device CharacterizationRequirements . . , . . . . . . . . . . . . 1

Description of the Summit 10500 System . . . 3

Connection Diagrams . . . . . . . . . . . . 6

Example Applications . . . . . . . . . . . 9

Low-Current Measurement Fundamentals . . 13

High-Frequency Measurement Fundamentals 17

References . . . . . . . . . . . . . . . 18

Suggested Equipment and Accessories . . 18

SummaryThis application note provides information on advanced,on-wafer device characterization techniques using theCascade Microtech Summit 10500 Parametric ProbeStation. Fundamental Device CharacterizationRequirements are outlined, leading to a discussion ofthe probe station features needed for various devicecharacterization measurements at fA-, fF-, andGHz-levels. The Summit 10500 is a state-of-the-art,on-wafer characterization solution for low-current,low-capacitance, and high-frequency measurements.

Next, Connection Diagrams between the probe stationand instruments are outlined for various measurements.The Example Applications illustrate the Summit 10500’sability to make precision measurements for a number ofneeds. Low-Current and High-Frequency MeasurementFundamentals are then discussed, including topics suchas guarding, shielding, ground loops, noise currents,Kelvin probes, and bipolar device measurements. Finally,lists of References and Suggested Equipment andAccessories are provided.

Fundamental DeviceCharacterization RequirementsBefore current and next-generation integrated circuits(ICs) can be designed, fabricated, and tested, individualdevices must be characterized and modeled. All otheractivities related to IC design, fabrication, and testingdepend on these highly-accurate device models.Consequently, all leading IC manufacturers provide themost accurate and precise equipment to their devicecharacterization teams.

Characterization measurements include the followingfundamental requirements:

DC tests Forcing voltages and measuringcurrents, including very low values in the10 to100 fA range. The device under test (DUT)must be shielded from light and noise (EMI, staticcharges, etc).

LCZ tests Forcing low-frequency AC voltages,and measuring AC currents. Capacitance,inductance, and impedance are calculated fromthese measurements. Capacitance values of1 to 10 fF are often measured. Again, the DUTmust be isolated from light and stray capacitance.

High-frequency tests Measuring high-frequencyand high-speed performance using vector networkanalysis or time domain reflectometry (TDR).Direct measurement of cutoff frequencies (ft) andother high-frequency parameters is now widelydone. As IC speeds increase, additional elementssuch as diodes, vias, crossovers, power busses,and interconnects need to be characterized athigh frequencies.

Because the devices to be characterized are so small,they are generally left together as fabricated on thewafer and electrically contacted on a wafer prober. Thisapproach saves the time and cost of dicing the waferand packaging the devices for testing within a fixture.

Probe stations are traditionally designed for only onefunction: either for simple DC testing or forhigh-frequency characterization. However, stations forDC testing are not designed for advanced, low-levelDC/CV or LCZ testing. The stringent requirements forlow-level current measurements necessitate a shieldedand dark environment for guarded and noise-free

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measurements. In addition, air purging around thewafer is often needed to control the temperature andhumidity of the test environment.

with bandwidths up to 65 GHz should becompatible. Standard DC needle probes are alsorequired. High-impedance or active probes forunobtrusive node probing may also be used.

Until recently, neither isolation, shielding, nor purginghas been available as part of an integrated devicecharacterization probe station (although some fixturescan provide some shielding and isolation). Presenton-wafer solutions use large, unwieldy dark/shieldboxes to enclose the entire probe station. A lid isopened to operate the station and to view the deviceunder test (DUT) through the microscope. But thispartial solution does not enable low-level current andcapacitance measurements.

Dark box The station must incorporate lightshielding, since many devices are light sensitive.Ideally, the probe positioners and station controlsshould be accessible without disturbing thelight-controlled environment.

EMI shielding Measurements as low as 1 O-l 00fA require integrated EMI shielding to minimizenoise.

Ideally, these characterization requirements should becombined into one unit providing DC, LCZ, and GHzmeasurement capability with integrated shielding,isolation, and dark. For rapidly-developing technologies,the following probe station functionalities are requiredfor accurate and throrough model development:

Isolation Ground loops must be prevented.

l Electrical performance The station should makefA-level DC measurements, which are possible ifthe wafer chuck is biased and isolated. Thestation must also provide 1 fF LCZmeasurements. For GHz characterization, thestation should easily measure deviceS-parameters to 65GHz or TDR

Environment purging The station must providea controlled environment to reduce the effects ofchanging humidity, temperature, and particlecontamination. Ideally, the station should providethe ability to purge with nitrogen or other inert gasto minimize humidity damage to sensitive waferelements.

Easy to use Mechanical functions should besimilar to current probe stations and shouldoperate intuitively.

response to 5 ps.The station’s probingenvironment shouldeliminate noisecaused by static,EMI, dielectricabsorption, andtriboelectric cableeffects.

l Probe types ForfA- and fF-levelmeasurements, thestation mustaccommodate bothKelvin and non-Kelvin DC-guardedcoaxial probes. Forhigh-frequencycharacterization,coplanar probes Figure 1. Summit 70500 Parametric Probe Station features

Advanced On- Wafer Device Characterizationusing the Summit 10500

RF/DC probepositioners

top hat

;$r loading

guardedchuck

MicroChamber T‘M

Advanced On- Wafer Device Characterizationusing the Summit 10500

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Description of the Summit 10500Parametric Probe SystemThe Cascade Microtech Summit 10500 (Figure 1)provides the necessary features for making fundamentaldevice characterization measurements. The Summit10500's key feature is the MicroChamberTM (patentspending), which provides a complete semiconductormeasurement environment that shields the chuck andprobes from electromagnetic interference (EMI) and light.Standard DC probes, guarded coaxial probes, andhigh-frequency coplanar probes are optimized for use inthis system. The chuck is electrically-guarded to allowaccurate, fA-level substrate current measurements.

MicroChamberThe MicroChamber (Figure 1) surrounds the waferstage, shielding it and the probes from both EMI andlight. Sliding metal plates underneath the chuck thateliminate noise from the x-y motors complete theshielding. The MicroChamber and the platen can beelectrically isolated from the rest of the station and fromearth ground, thus eliminating ground loops. Althoughthe MicroChamber ground safety link is factory-set,connecting the MicroChamber to ground, it can beremoved by the user if desired. A more detaileddiscussion on shielding and ground loops is foundstarting on page 14.

The top hat (Figure 1) completes the EMI/dark shielding,allowing probes to be inserted through the polymerwindows, yet preventing light from entering. Various sizedpolymer/steel rings surround the microscope objective,allowing the microscope both vertical and horizontalmovement without light and EMI leakage. TheMicroChamber enclosure shields the wafer chuck, DUT,and probes from EMI, and prevents light from affecting theDUT, yet allows convenient access to the microscope andprobe positioners. A convenient wafer loading door allowseasy access to the chuck.

The MicroChamber also provides a complete air sealaround the wafer chuck, probes, and microscopeobjective. The system can be purged with nitrogen orinert gas to provide a clean, humidity andtemperature-controlled environment. An intake manifoldat the rear of the station provides convenient air or gashookup. The MicroChamber typically evacuates in lessthan 5 minutes, depending on air flow and pressure.

Guarded chuckGuarded measurement techniques are required foraccurate and rapid current measurements below 1 nA.Because some devices generate low (cl nA) substratecurrents that need to be measured, the Summit 10500provides a guarded chuck assembly, allowing accuratedevice and substrate measurements at the fA-level.Figure 2 shows the basic design. The chuck is astandard wafer chuck, but is isolated from the guard by1 El3 ohms. The guard is also isolated from the shieldby 1 El3 ohms. When the guard is connected in theguarded mode to a source measurement unit (SMU),fA-level currents are accurately measured. The guardedchuck is also optimized to provide low capacitiveloading for DC parametric and LCR test instruments.This enhances the test instrument’s settling time andaccuracy.

Conventional chucks are typically specified with 1 Eg ohmsisolation (Figure 3). If the chuck is assumed to be biasedat -1 volt, a current meter will measure -1 V/l Eg ohms or1 nA current flowing through the chuck isolation resistor.This is in addition to the current flowing from the DUT.Lower level current measurements of semiconductordevices are limited to about 1 nA when biasingconventional chucks.

groundMicroChamber

Figure 2. The Summit 10500’s guarded chuck design andMicroChamber shielding

Current meter

lEgQl+

I I

Figure 3. Conventional unguarded chuck leakage current

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In contrast, the Summit 10500’s giarded chuck typicallyhas leakage currents of 1 mV/1 E ohms or 0.1 fA(Figure 4). Most test and measurement instrumentswith guarded signal lines (like the HP 4142B) have amaximum guard error of 1 mV or less. This means thatthe voltage difference between the chuck and the chuckguard will be within 1 mV. Further discussion ofguarding is found beginning on page 13.

IMicroChamber

Current mater

(unity gain)

Figure 4. Summit 10500 guarded chuck leakage current

single coaxial

front view

gold-platedouter conductor

Teflon insulation

. tungsten needle

Guarded coaxial probesCoaxial probes for measuring low-level currents andcapacitance values are different than conventional DCneedle probes. The center conductor is used for thesignal, while the outer conductor is used as either aguard for guarded current/voltage measurements, or asa shield for capacitance measurements. Figure 5 showsthe DCP-100 Series Coaxial Probes in both non-Kelvinand Kelvin versions.

When measuring high currents (>100 mA) orsmall-value precision resistors (

Advanced On- Wafer Device Characterizationusing the Summit 10500

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(