Low Level Measurements Handbook

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1.3.6 The SourceMeter ® InstrumentThe SourceMeter instrument is very similar to the source-measure unit inmany ways, including its ability to source and measure both current andvoltage and to perform sweeps. In addition, a SourceMeter instrument candisplay the measurements directly in resistance, as well as voltage andcurrent.The typical SourceMeter instrument doesn’t have as high an inputimpedance or as low a current capability as a source-measure unit. TheSourceMeter instrument is designed for general-purpose, high speed productiontest applications. It can be used as a source for moderate to lowlevel measurements and for research applications.Unlike a DMM, which can make a measurement at only one point, aSourceMeter instrument can be used to generate a family of curves, becauseit has a built-in source. This is especially useful when studying semiconductordevices and making materials measurements.When used as a current source, a SourceMeter instrument can be usedin conjunction with a nanovoltmeter to measure very low resistances byautomatically reversing the polarity of the source to correct for offsets.1.3.7 The <strong>Low</strong> Current PreampSome SMUs and SourceMeter instruments may have a remote low currentpreamp. With this design, the sensitive amplifier circuitry is separate fromthe SMU or SourceMeter instrument. This makes it possible to place themost sensitive part of the instrument very close to the device being tested,thereby eliminating a major source of error, the noise and leakage from thecables themselves.1.3.8 The Micro-ohmmeterA micro-ohmmeter is a special type of ohmmeter designed especially formaking low level resistance measurements. While the techniques used formaking resistance measurements are similar to those used in a DMM, microohmmetercircuits are optimized for making low level measurements. Thetypical micro-ohmmeter can resolve resistances as low as 10µΩ.<strong>Measurements</strong> made using the micro-ohmmeter are always performedusing the four-wire technique in order to minimize errors caused by testleads and connections. The typical micro-ohmmeter also has additional featuressuch as offset compensation and dry circuit testing to optimize lowresistance measurements. Offset compensation is performed by pulsing thetest current to cancel offsets from thermoelectric EMFs. The dry circuit testmode limits the voltage across the unknown resistance to a very small value(typically
1.4 Understanding Instrument SpecificationsKnowing how to interpret instrument specifications properly is an importantaspect of making good low level measurements. Although instrumentaccuracy is probably the most important of these specifications, there areseveral other factors to consider when reviewing specifications, includingnoise, deratings, and speed.1.4.1 Definition of Accuracy TermsThis section defines a number of terms related to instrument accuracy.Some of these terms are further discussed in subsequent paragraphs. Table1-1 summarizes conversion factors for various specifications associated withinstruments.SENSITIVITY - the smallest change in the signal that can be detected.RESOLUTION - the smallest portion of the signal that can be observed.REPEATABILITY - the closeness of agreement between successive measurementscarried out under the same conditions.REPRODUCIBILITY - the closeness of agreement between measurements ofthe same quantity carried out with a stated change in conditions.ABSOLUTE ACCURACY - the closeness of agreement between the result of ameasurement and its true value or accepted standard value. Accuracyis often separated into gain and offset terms.RELATIVE ACCURACY - the extent to which a measurement accuratelyreflects the relationship between an unknown and a reference value.ERROR - the deviation (difference or ratio) of a measurement from its truevalue. Note that true values are by their nature indeterminate.RANDOM ERROR - the mean of a large number of measurements influencedby random error matches the true value.SYSTEMATIC ERROR - the mean of a large number of measurements influencedby systematic error deviates from the true value.UNCERTAINTY - an estimate of the possible error in a measurement, i.e., theestimated possible deviation from its actual value. This is the oppositeof accuracy.“Precision” is a more qualitative term than many of those defined here.It refers to the freedom from uncertainty in the measurement. It’s oftenapplied in the context of repeatability or reproducibility, but it shouldn’t beused in place of “accuracy.”1.4.2 AccuracyOne of the most important considerations in any measurement situation isreading accuracy. For any given test setup, a number of factors can affectaccuracy. The most important factor is the accuracy of the instrument itself,which may be specified in several ways, including a percentage of full scale,1-10 SECTION 1
Page 1 and 2: www.keithley.comLLM6 th EditionLow
Page 4 and 5: TABLE OF C ONTENTSSECTION 1 Low Lev
Page 6 and 7: 3.3 Low Resistance Measurements....
Page 8 and 9: SECTION 1Low Level DCMeasuringInstr
Page 10 and 11: 1.1 IntroductionDC voltage, DC curr
Page 12 and 13: FIGURE 1-3: Typical Digital Multime
Page 14 and 15: Coulombmeter FunctionCurrent integr
Page 18 and 19: TABLE 1-1: Specification Conversion
Page 20 and 21: Transfer StabilityA special case of
Page 22 and 23: FIGURE 1-6: Common Mode NoiseMeasur
Page 24 and 25: FIGURE 1-8: Voltage Amplifier+-V 2A
Page 26 and 27: Picoammeter amplifier gain can be c
Page 28 and 29: Using a small-signal transistor in
Page 30 and 31: FIGURE 1-17: High Resistance Measur
Page 32 and 33: FIGURE 1-20: High Resistance Measur
Page 34 and 35: Micro-ohmmeterThe micro-ohmmeter al
Page 36 and 37: FIGURE 1-26: Typical Digital Electr
Page 38 and 39: In order to cancel internal offsets
Page 40: Sense circuitry is used to monitor
Page 43 and 44: 2.1 IntroductionAs described in Sec
Page 45 and 46: cause significant loading errors. I
Page 47 and 48: Cable leakage resistance is a commo
Page 49 and 50: FIGURE 2-6: Guarding Leakage Resist
Page 51 and 52: Example: Assume R S = 10GΩ and C S
Page 53 and 54: MaterialTABLE 2-2: Properties of Va
Page 55 and 56: QuartzQuartz has properties similar
Page 57 and 58: FIGURE 2-11: Guarding as Applied to
Page 59 and 60: FIGURE 2-13: Guarding the Leakage R
Page 61 and 62: FIGURE 2-15: Simplified Model of a
Page 63 and 64: time and/or temperature. Zero offse
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to equalize charges and minimize ch
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If insulators become contaminated,
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The input resistance of a feedback
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ence. However, in some cases, shiel
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FIGURE 2-27: Feedback Coulombmeter
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Advantages of Using a Coulombmeter
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FIGURE 2-30: Constant-Voltage Metho
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In addition to the voltage drop lim
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FIGURE 2-34a: Effects of Cable Resi
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Such devices require extreme care i
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charge will be lost through the zer
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FIGURE 2-40: Proper ConnectionCurre
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Figure 2-43 shows an example of AC
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fering voltage or current. A guard
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esponse is the rise time of the ins
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FIGURE 2-47: Shunt Capacitance Effe
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Resistance Measurements (Constant-C
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FIGURE 2-52: Noise Voltage vs. Band
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Source ResistanceAfter the bandwidt
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a. ConfigurationShieldCenterconduct
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• Shielding and Guarding: The fix
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floating circuits, a second grounde
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3.1 IntroductionLow voltage and low
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Thermoelectric EMFsThermoelectric v
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Reversing Sources to Cancel Thermoe
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quency spectrum of these interferen
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ment is almost entirely determined
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FIGURE 3-9: Low Voltages Generated
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FIGURE 3-11a: Multiple Grounds (Gro
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Common-Mode Reversal ErrorsReversin
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FIGURE 3-14: Two-Wire Resistance Me
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FIGURE 3-16: Canceling Thermoelectr
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Both V A and V B are affected by th
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If using a micro-ohmmeter or DMM to
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FIGURE 3-20: Dry Circuit Testing Us
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SECTION 4Applications
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For timing and integrating applicat
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FIGURE 4-2:Using an Electrometer to
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If current flows, the electrodes wi
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4.3 Low Current Measurement Applica
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DC. The Model 6517A can also be use
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voltage (V DS ) and measures the re
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The Keithley Model 248 High Voltage
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FIGURE 4-16: Ion Collector with Gro
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Figure 4-19 shows a Model 6430 conn
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FIGURE 4-21: SIR Test System to Mea
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FIGURE 4-22: Volume ResistivityElec
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these problems, the Alternating Pol
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FIGURE 4-25: Four-Point Collinear P
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Using two electrometers eliminates
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A plot of this function is shown in
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FIGURE 4-31: van der Pauw Measureme
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Through interactive programming, th
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objects. As discussed in Section 2.
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FIGURE 4-36: Faraday CupOutside Ele
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FIGURE 4-38: Connections for Standa
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FIGURE 4-40: Microcalorimeter with
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Measurement MethodFigure 4-42 illus
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FIGURE 4-43: Using a Nanovoltmeter
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exceed the critical current of the
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equation for V/I. Most materials ha
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FIGURE 4-49: van der Pauw Connectio
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5.1 IntroductionChoosing a specific
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TABLE 5-1a: Low Current/High Resist
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Table 5-1b: Source-Measure Instrume
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Table 5-2: High Speed Power Supplie
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Table 5-3: Connectors, Adapters, an
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Table 5-4: CablesTERMINATIONS LENGT
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Table 5-5: Test Leads and ProbesMOD
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Table 5-6: Switching Cards for the
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APPENDIX ALow LevelMeasurementTroub
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Measurement Type andTypical Applica
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Proper cable and connector assembly
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APPENDIX CGlossary
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channels. For matrix cards, a chann
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R. Buckminster Fuller. Sometimes ca
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NORMAL-MODE REJECTION RATIO (NMRR).
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SOURCEMETER. A SourceMeter instrume
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APPENDIX DSafetyConsiderations
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The types of product users are:Resp
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are the same. Other components that
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1/f noise, 3-7 to 3-83dB point, 2-5
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Normal mode rejection ratio (NMRR),
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Specifications are subject to chang
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Specifications are subject to chang
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Low Level Measurements Handbook

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