Treinamento : Testes Paramétricos em Semicondutores Setembro 2012

1. Treinamento:Testes ParamétricosemSemicondutoresSetembro 2012 Cyro Hemsi Engenheiro de Aplicação Section 1 – Parametric Measurement Basics

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. Wafer Processing Parametric Test Final Test (Functional) Board Test 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

5. Parametric Test Measures 4 Basics Device Types: Capacitors Diodes Resistors Transistors All measurements are either current versus voltage (I-V) or capacitance versus voltage (C-V) measurements.

6. What Does Parametric Test Involve? SMU 1 Id SMU 4 SMU2 SMU 3 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 • Resolution is the lowest resolvable quantity of data that an instrument can accurately measure • 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 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. Low accuracy, high repeatability High accuracy, low 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. Leakage Shield (Ground) Shield (Ground) Guard Force Force MOSFET Subthreshold MOSFET Subthreshold mA mA uA uA Id Id nA nA Vg Triax Cables pA pA Vg Coax Cables fA fA Why Use Triaxial Fixturing & Cables?Required For Measurements < 1nA 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. Do not ever short the guard to the force line or shield line Buffer x1 Guard Rs Force V Shield Simplified SMU Output Triaxial Guard Connection Simplified Diagram The guard voltage tracks the force voltage exactly. Cable charging current and noise is eliminated..

16. Ground Shield Ground Shield ??? Driven Guard Signal Force / Sense Line 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? • Where can I get the necessary TRIAXIAL to BNC connectors?

17. 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. Triaxial to Coaxial Adapters: Measuring Currents > 1 nano-Amp

18. Warning! Shock Hazard! 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. Triaxial to Coaxial Adapters: Measuring Currents < 1 nano-Amp

19. Summary of Agilent Connectors

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. Slope = 1/Re (Kelvin) I B Slope = 1/(Re+Rcable) (Non-Kelvin) Non-Kelvin Measurements Can Introduce Significant Error V monitor C voltmeter sweeping current Re = 0.55 Rcable = 0.40 Kelvin SMU Cable resistance comparable to resistance being measured

25. Buffer Guard x1 Sense Rs Force V Shield Guard and Kelvin ConnectionSimplified Diagram The sense line is added. Cable resistance error is eliminated. Useful if the DUT <50 Ohms.

26. Buffer Buffer x1 x1 Sense Sense Rs Rs Force Force V V Two Kelvin SMUs Can Make a Kelvin Measurement

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

28. Kelvin Triaxial CableIdeal for both low current and low impedance applications. Ground Guard Force Sense Both force and sense lines are held rigidly in the same Teflon cable

29. Wafer Prober Kelvin Cable ConnectionsOptimized For Measurement Accuracy To Kelvin Probe To Guarded Chuck ( to measure substrate current)

30. To Scope Triax to Coax Adapter (guard floating) Drain Substrate Gate Measure Gate Voltage versus Time Accurately Source Nifty Trick: Use the Sense Line as a High Impedance Scope Probe! Buffer x1 Guard Sense Force SMU V

31. Understanding the ground unit

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

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

36. Sense Output GNDU Force Output 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) 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. Make sure you know the maximum allowable voltage between the SMU high and low inputs. Sense Force Guard High ± 250V = Max ± 210V = Max Low 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