1. Treinamento: Testes Paramétricos em Semicondutores Setembro 2012
Section 1 – Parametric Measurement Basics
Cyro Hemsi
Engenheiro de Aplicação

2. Agenda
Section #1 - Parametric Measurement Basics
Parametric Measurement Terminology
Triaxial Cabling & Fixturing
4-Wire (Kelvin) Measurements
Understanding the Ground Unit

3. Parametric measurement terminology

4. Where is Parametric Test in the Semiconductor Value Chain?
Final Test
(Functional)
Board Test
Parametric Test
Wafer Processing
Agilent Parametric Test Products
4080 Series
E5250A/B2200A/B2201A
E5260 Series & E5270B
B1500A
B1505A
B2900A Series

5. 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.

6. What Does Parametric Test Involve?
SMU2
SMU 1
SMU 3
SMU 4 Id
MOSFETs have 4 terminals:
4 SMUs  Magic Number!
Semiconductor parametric test involves the measurement of voltage and current very accurately and very quickly. It also involves the measurement of capacitance.

7. Parametric Test is Done Primarily on Wafers
Functional testers test product die
Parametric testers sometimes test special structures in the scribe lane
This minimizes the area of a production wafer

8. Accuracy/ Resolution Reducing noise
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.
Resolution is the lowest resolvable quantity of data that an instrument can accurately measure
Reducing noise
Shielding (EMI)
Guarding (leakage current)
Kelvin probes (eliminate the effect of cable resistances)
Integration time (power line cycle noise)

9. Understanding Accuracy & 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.
High accuracy, low repeatability
Low accuracy, high repeatability

10. Understanding Measurement Resolution
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.
Simplified analog-to-digital converter (ADC) circuit.

11. What is a Half Digit of Resolution?
 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.

12. Triaxial cabling and fixturing

13. Why Use Triaxial Fixturing & Cables? Required For Measurements < 1nA
Shield (Ground)
Guard
Force
Shield (Ground)
Force
Vg Triax Cables
MOSFET Subthreshold Id fA pA nA uA mA
Leakage
Vg Coax Cables
MOSFET Subthreshold Id fA pA nA uA mA
The triax cable is a special low dielectric loss, high impedance cable. This cable may be used down to fA levels when properly used with a guarded probe. The guard voltage tracks the force voltage exactly, so that no voltage drop can exist between guard and force. This eliminates the capacitive loading that would otherwise limit low current measurements.
If low impedance coax cables are used with outer layer at ground potential, two limitations will be immediately apparent. The cable leakage will limit the low current measurement floor. In addition, when the voltage is swept, the sudden change will cause additional cable charging. This distorts the low current portion of a MOS Subthreshold curve as shown.
RULES:
Unguarded coax cable is OK for measurements above 1nA.
Triax cable or coax with outer layer at guard potential should be used for measurements below 1 nA.
Eliminate cable leakage and charging currents.

14. 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.

15. 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
The guard connection is needed for measurement < 1 nA. Below 1 nA a regular coax cable's capacitance dominates over the DUT (device under test) capacitance. What you see is cable charging current. I = C(dv/dt) where dv/dt is the rate of change of SMU voltage from one step to the next of a coax cable with no guard. C is the total capacitance of the cable.
The above diagram shows how the cable capacitance is eliminated with a triaxial cable. The guard is driven at the same voltage as the force center conductor. No current can flow between guard and force when they are held at the same potential. Guard and force are isolated by a buffer amplifier. They can never be shorted together.

16. How Do I Connect Triaxial and Coaxial Connections?
???
Force / Sense
Line
Driven Guard
Ground Shield
Ground Shield
Signal
What do I do with the driven guard?
Does the current I am measuring affect how I connect to a BNC connector?
Where can I get the necessary TRIAXIAL to BNC connectors?

17. 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.

18. 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!

19. Summary of Agilent Connectors
Agilent Part Number
Description
Safe. Not suitable for low-current measurements
Triaxial (F) -BNC(M)
Triaxial (M) - BNC(F)
WARNING!!!! Shock Hazard!!!! Required for low-current measurements.
Triaxial (F) - BNC(M)
Triaxial (F) - BNC(F)

20. 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.

21. 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.

22. 4-Wire (Kelvin) Measurements

23. What is a 4-Wire (Kelvin) Measurement?
IForce
Force Line 2
Rcable
Rcable
Force Line 1
RDUT - +
VSense
Rcable
Rcable
Sense Line 2
Sense Line 1
I=0
I=0
 Eliminate cable resistance from the measurement

24. Non-Kelvin Measurements Can Introduce Significant Error
monitor
Slope = 1/Re
(Kelvin)
Slope = 1/(Re+Rcable)
(Non-Kelvin)
voltmeter
sweeping current I B
Re = 0.55
Rcable = 0.40
In the example above, the device is connected with a SMU on the base sweeping current, a voltmeter on the collector, and the emitter is grounded with a Kelvin SMU. The base SMU does not have to be Kelvin since we are only forcing current and do not care about measuring the cable loss in the base. Also, the collector SMU is being used only as a high impedance voltmeter, so there is no cable loss in this lead.
The emitter on the other hand, must be connected to a Kelvin SMU. Because of this, we can compensate for the 0.40 ohm path through the cable and fixture. From the graph we can see the emitter resistance is 0.55 ohm when compensated using the Kelvin connection. Non-Kelvin resistance is 0.95 ohm, due to the extra 0.40 ohm cable and fixture resistance error.
Kelvin SMU
Cable resistance comparable to resistance being measured

25. Guard and Kelvin Connection Simplified 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
The only difference between the 4155 and 4156 cable configuration is the addition of the sense line. In this case, sensing is done at the DUT, eliminating the fraction of an ohm of cable resistance. The internal sensing resistor Rs is the only feedback path in the 4155.
Note that the 4156 operates just fine without the sense cable. Then it operates just like the This is important to know because in general you do not need the sensing Kelvin connection. Most MOS measurements are high impedance and the residual cable loss is insignificant.

26. Two Kelvin SMUs Can Make a Kelvin Measurementx1
Buffer
Force V Rs
Sense

27. Do Not Use SMU Sense Output by Itself!
Buffer
Iout x1
Sense Rs
DUT
Force V

28. Both force and sense lines are held rigidly in the same Teflon cable
Kelvin Triaxial Cable Ideal for both low current and low impedance applications.
Ground
Guard
Force
The 4155 uses the same triax cables as the 4142 and These cables are good for low current measurements. However, two cables are necessary for low resistance Kelvin measurements.
Agilent Technologies designed a special Kelvin triax cable for the This cable is optimized for both low current and low resistance measurement. Both force and sense lines are held rigidly in the same Teflon cable. Friction is reduced and the cable is less sensitive to noise caused by moving the cable.
Kelvin triax cable assemblies are available with two connector options:
16434A compatible on one end; compatible on the other end
16493K 4156 compatible on both ends (standard option)
Sense
Both force and sense lines are held rigidly in the same Teflon cable

29. Wafer Prober Kelvin Cable Connections Optimized For Measurement Accuracy
To Kelvin Probe
Photo of SMU cable connection to a Cascade Microtech Summit probe station.
4156's Kelvin triaxial cables mate directly to up to six probes top side and a guarded Kelvin chuck (substrate) connection. There is even a provision for mating the 1.6 A ground unit (GNDU) of the expander box.
This station uses the Micro Chamber (TM) design for a small volume shielded box enclosing only the probes and wafer; not the entire probe station. The rigid mechanical design with guarded chuck provides an ideal environment for fA current, fF capacitance, and uV voltage measurements.
To Guarded Chuck ( to measure substrate current)

30. Nifty Trick: Use the Sense Line as a High Impedance Scope Probe!
Buffer
Measure Gate Voltage versus Time Accurately
Triax to Coax Adapter
(guard floating) To
Scope x1
Guard
Sense
Source
Gate
Drain
Substrate
Force
SMU
The sense line need not be used only for Kelvin connections.
It is ideal for monitoring the voltage on your device with an oscilloscope.
The sense line tracks the force line within 1mv.
All you need is a floating guard coax adapter attached to the sense line at the back of the Then use any BNC cable to direct connect the SMU sense line to the oscilloscope input.
The adapter shown is the Trompeter Electronics AD-BJ20-E2-PL75. V

31. Understanding the ground unit

32. What is the Ground Unit (GNDU) Configuration?
Standard Triaxial Connection:
Ground Unit Connection:
Force / Sense
Line
Driven Guard
Ground Shield
Sense Line
Force Line
Ground Shield

33. Why is the GNDU Configuration the Way It Is?
???
Shield (Ground)
Force
Sense
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.

34. What Happens if I Connect the GNDU to a Standard Triaxial Connection?
Buffer
Isink x1
Sense
Ground Unit Input Rs
Force V
Connecting a standard triaxial connector to the GNDU without an adapter is equivalent to connecting up to the SMU Sense output !

35. 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.
GNDU
Sense Output
Force Output
The Agilent N1254A-100 Ground Unit to Kelvin Adapter will split the Force and Sense lines into the proper Kelvin configuration.

36. 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.
Isense (0 Amps)
GNDU
Sense Output
Force Output
Isink (Up to 4.2 Amps*)
*B1500A/B1505A

37. Benchtop SMUs Typically Have Banana Jack Outputs:
When making a basic 2-wire measurement you should use the Force outputs.
Sense
Force
Guard
High
Low
± 250V = Max
± 210V = Max
Make sure you know the maximum allowable voltage between the SMU high and low inputs.
By default the SMU low outputs are tied to chassis ground, but they can be floated above or below chassis ground.

38. Agilent Can Supply Kelvin and non-Kelvin Banana Jack to Triaxial Adapters
Kelvin Adapter
Non-Kelvin Adapter
N1259A-001 Banana–Triaxial Adapter for 2-wire connection (non-Kelvin)
N1259A-002 Banana–Triaxial Adapter for 4-wire connection (Kelvin)

39. END Of section 1