Treinamento: Testes Paramétricos em Semicondutores Setembro 2012 Cyro Hemsi Engenheiro de...

Treinamento:Testes Paramétricos em

Semicondutores

Setembro 2012

Cyro HemsiEngenheiro de Aplicação

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

PARAMETRIC MEASUREMENT TERMINOLOGY

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

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.

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!

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

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.

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.

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.

TRIAXIAL CABLING AND FIXTURING

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

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.

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

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?

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.

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!

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)

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

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

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

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

Two Kelvin SMUs Can Make a Kelvin Measurement

x1

Buffer

Force

V

Rs

Sense

x1

Buffer

Force

V

Rs

Sense

Do Not Use SMU Sense Output by Itself!

DUT

x1

Buffer

Force

V

Rs

Sense

Iout

Kelvin Triaxial CableIdeal for both low current and low impedance applications.

Guard

Force

Ground

Sense

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

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!

UNDERSTANDING THE GROUND UNIT

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:

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.

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

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

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

Benchtop SMUs Typically Have Banana Jack Outputs:

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

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

Treinamento: Testes Paramétricos em Semicondutores Setembro 2012 Cyro Hemsi Engenheiro de...

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