Post on 30-Mar-2015
Treinamento:Testes Paramétricos em
Semicondutores
Setembro 2012
Cyro HemsiEngenheiro de Aplicação
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Section 1 – Parametric Measurement Basics
Agenda
Section #1 - Parametric Measurement Basics
• Parametric Measurement Terminology
• Triaxial Cabling & Fixturing
• 4-Wire (Kelvin) Measurements
• Understanding the Ground Unit
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PARAMETRIC MEASUREMENT TERMINOLOGY
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Where is Parametric Test in the Semiconductor Value Chain?
Agilent Parametric Test Products• 4080 Series•
E5250A/B2200A/B2201A• E5260 Series & E5270B• B1500A• B1505A• B2900A Series
Final Test(Functional) Board Test
Parametric Test
Wafer Processing
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Parametric Test Measures 4 Basics Device Types:
Transistors Diodes Resistors Capacitors
All measurements are either current versus voltage (I-V) or capacitance versus voltage (C-V) measurements.
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What Does Parametric Test Involve?
Semiconductor parametric test involves the measurement of voltage and current very accurately and very quickly. It also involves the measurement of capacitance.
SMU2
SMU 1
SMU 3
SMU 4
Id
MOSFETs have 4 terminals:
4 SMUs Magic Number!
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Parametric Test is Done Primarily on Wafers
Functional testers test product die
Parametric testers sometimes test special structures in the scribe lane
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This minimizes the area of a production wafer
Accuracy/ Resolution
• Accuracy is the degree of conformity of a measured or calculated quantity to its actual (true) value.
• Repeatability(also known as precision) is the degree to which repeated measurements or calculations show the same or similar results.
•For parametric test, resolution is what allows us to gauge accuracy and repeatability.
Reducing noiseShielding (EMI)
Guarding (leakage current)
Kelvin probes (eliminate the effect of cable resistances)
Integration time (power line cycle noise)
• Resolution is the lowest resolvable quantity of data that an instrument can accurately measure
Understanding Accuracy & Repeatability
High accuracy, low repeatability Low accuracy, high repeatability
Accuracy – The degree of conformity of a measured or calculated quantity to its actual (true) value.Repeatability (aka precision) – The degree to which repeated measurements or calculations show the same or similar results.
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Understanding Measurement Resolution
Simplified analog-to-digital converter (ADC) circuit.
Resolution – The lowest resolvable quantity of data that an instrument can accurately measure.
The number of bits available to the digital-to-analog converter (DAC) determines the fineness of the measurement detail.
Example: 20 bits of resolution represents the ability to distinguish one part in 220 or 1,048,576.
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What is a Half Digit of Resolution?
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Be careful! This can mean different things for different instruments
Question: What is the ½ digit here?
Answer: It is the 7. In this case the least significant digit only has ½ the accuracy of the other digits.
TRIAXIAL CABLING AND FIXTURING
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Shield (Ground)Guard
Force
Shield (Ground)
Force
Vg Coax Cables
MOSFET Subthreshold
Id
fA
pA
nA
uA
mA
Eliminate cable leakage and charging currents.Vg Triax Cables
MOSFET Subthreshold
Id
fA
pA
nA
uA
mA
Leakage
Why Use Triaxial Fixturing & Cables?Required For Measurements < 1nA
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Why Are Triaxial Cables Needed for Low-Current?
BNC (Coaxial) Cable: Triaxial Cable:
Leakage Current: Leakage Current:
Triaxial cable reduces leakage current by a factor of 100,000,000.
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Triaxial Guard Connection Simplified Diagram
The guard voltage tracks the force voltage exactly.
Cable charging current and noise is eliminated..
x1
Buffer
Simplified SMU Output
Force
V
Guard
Rs
Do not ever short the guard to the force line or shield line
Shield
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How Do I Connect Triaxial and Coaxial Connections?
What do I do with the driven guard?
???
• Does the current I am measuring affect how I connect to a BNC connector?
Force / SenseLine
Driven Guard
Ground Shield Ground Shield
Signal
• Where can I get the necessary TRIAXIAL to BNC connectors?
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Triaxial to Coaxial Adapters: Measuring Currents > 1 nano-Amp
In this case it is OK to float the guard connection, since current leakage between the center conductor and the outer ground shield does not significantly impact the measurement.
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Triaxial to Coaxial Adapters: Measuring Currents < 1 nano-Amp
The only way to maintain low-current measurement accuracy in a coaxial environment is to connect the driven guard to the outer shield of the coaxial connector. This presents a potential safety hazard and must be done with great care.
Warning! Shock Hazard!
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Summary of Agilent Connectors
Agilent Part Number
DescriptionSafe.Not suitable for low-current measurements1250-2652 Triaxial (F) -BNC(M)
1250-2653 Triaxial (M) - BNC(F)
Agilent Part Number
DescriptionWARNING!!!!Shock Hazard!!!!Required for low-current measurements.
1250-2650 Triaxial (M) - BNC(F)
1250-2651 Triaxial (F) - BNC(M)
1250-1830 Triaxial (F) - BNC(F)
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Shielding: Maintaining a Low Noise Floor
The purpose of shielding is to prevent electrostatic noise from interfering with a measurement.
Key points: Keep all charged objects and conductors away from the measurement area. Use highly conductive materials instead of insulating materials near the test circuit. Avoid movement and vibration near the measurement area. When measuring currents < 1 pA, shield the measurement area with a conductive (metal) enclosure and connect the enclosure to the test instrument common(shield) and/or to earth ground. Minimize the capacitance between the shielding enclosure and the test circuit.
General rule of thumb
When making measurements below 1 nanoamp you should use guarding;
When making measurements below 1 picoamp you should use both guarding and shielding.
4-WIRE (KELVIN) MEASUREMENTS
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What is a 4-Wire (Kelvin) Measurement?
Rcable
Rcable
Rcable
Rcable
IForce
+-VSense
I=0I=0
Force Line 1Force Line 2
Sense Line 1Sense Line 2
RDUT
Eliminate cable resistance from the measurement
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Non-Kelvin Measurements Can Introduce Significant Error
Slope = 1/Re(Kelvin)
Slope = 1/(Re+Rcable)(Non-Kelvin)
IB
Re = 0.55
Rcable = 0.40
VC monitor
Cable resistance comparable to resistance being measured
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voltmetersweeping current
Kelvin SMU
Guard and Kelvin ConnectionSimplified Diagram The sense line is added.
Cable resistance error is eliminated. Useful if the DUT <50 Ohms.
x1
Buffer
Force
V
Rs
Guard
Sense
Shield
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Two Kelvin SMUs Can Make a Kelvin Measurement
x1
Buffer
Force
V
Rs
Sense
x1
Buffer
Force
V
Rs
Sense
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Do Not Use SMU Sense Output by Itself!
DUT
x1
Buffer
Force
V
Rs
Sense
Iout
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Kelvin Triaxial CableIdeal for both low current and low impedance applications.
Guard
Force
Ground
Sense
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Both force and sense lines are held rigidly in the same Teflon cable
To Kelvin Probe
To Guarded Chuck ( to measure substrate current)
Wafer Prober Kelvin Cable ConnectionsOptimized For Measurement Accuracy
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x1
Buffer
SMU
Guard Sense
Force
VSource
Gate
Drain
Substrate
Measure Gate Voltage versus Time Accurately
Triax to Coax Adapter(guard floating)
ToScope
Nifty Trick: Use the Sense Line as a High Impedance Scope Probe!
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UNDERSTANDING THE GROUND UNIT
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What is the Ground Unit (GNDU) Configuration?
Sense Line
Force Line
Ground Shield
Force / SenseLine
Driven Guard
Ground Shield
Standard Triaxial Connection: Ground Unit Connection:
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Why is the GNDU Configuration the Way It Is?
Shield (Ground)
ForceSense
???
• In standard triaxial connections the middle conductor is a driven guard, which eliminates any cable leakage current by always keeping the driven guard the same potential as the center Force/Sense line.
• In the case of the ground unit the potential of the Force and Sense lines is always at zero volts, so there is no need to shield it from the outer ground shield to prevent leakage currents.
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What Happens if I Connect the GNDU to a Standard Triaxial Connection?
Connecting a standard triaxial connector to the GNDU without an adapter is equivalent to connecting up to the SMU Sense output !
x1
Buffer
Force
V
Rs
Sense
Isink
Ground Unit Input
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Proper GNDU Connection
Unless your equipment is designed to handle the GNDU connection, you must use an adapter that splits out the GNDU Force and Sense lines into standard triaxial configurations.
The Agilent N1254A-100 Ground Unit to Kelvin Adapter will split the Force and Sense lines into the proper Kelvin configuration.
GNDU
Sense Output
Force Output
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Connections to the GNDU Should be Kelvin
Remember! Pumping large currents through cables will cause an Ohmic drop unless this is compensated via a Kelvin measurement configuration. Since assumedly the reason you are using the GNDU is to sink large currents, you should always connect up both the Force and Sense lines.
GNDU
Sense Output
Force Output
Isink (Up to 4.2 Amps*)
Isense (0 Amps)
*B1500A/B1505A
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Benchtop SMUs Typically Have Banana Jack Outputs:
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Sense Force Guard
High
Low
± 250V = Max
± 210V = Max
By default the SMU low outputs are tied to chassis ground, but they can be floated above or below chassis ground.
Make sure you know the maximum allowable voltage between the SMU high and low inputs.
When making a basic 2-wire measurement you should use the Force outputs.
Agilent Can Supply Kelvin and non-Kelvin Banana Jack to Triaxial Adapters
Kelvin Adapter Non-Kelvin Adapter
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N1259A-001 Banana–Triaxial Adapter for 2-wire connection (non-Kelvin)
N1259A-002 Banana–Triaxial Adapter for 4-wire connection (Kelvin)
END OF SECTION 1
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