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 Page 4
Parametric Test Measures 4 Basics Device Types: Transistors DiodesResistors Capacitors All measurements are either current versus voltage (I-V) or capacitance versus voltage (C-V) measurements. Page 5
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! Page 6
Parametric Test is Done Primarily on Wafers Functional testers test product die Parametric testers sometimes test special structures in the scribe lane Page 7 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 noise Shielding (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 repeatabilityLow 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. Page 9
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 2 20 or 1,048,576. Page 10
What is a Half Digit of Resolution? Page 11 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.
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 Page 13
Why Are Triaxial Cables Needed for Low-Current? BNC (Coaxial) Cable:Triaxial Cable: Leakage Current: Triaxial cable reduces leakage current by a factor of 100,000,000. Page 14
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 Page 15
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 / Sense Line Driven Guard Ground Shield Signal Where can I get the necessary TRIAXIAL to BNC connectors? Page 16
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. Page 17
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! Page 18
Summary of Agilent Connectors Agilent Part Number Description Safe. Not suitable for low-current measurements 1250-2652Triaxial (F) -BNC(M) 1250-2653Triaxial (M) - BNC(F) Agilent Part Number Description WARNING!!!! Shock Hazard!!!! Required for low-current measurements. 1250-2650Triaxial (M) - BNC(F) 1250-2651Triaxial (F) - BNC(M) 1250-1830Triaxial (F) - BNC(F) Page 19
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.
What is a 4-Wire (Kelvin) Measurement? R cable I Force + - V Sense I=0 Force Line 1Force Line 2 Sense Line 1Sense Line 2 R DUT Eliminate cable resistance from the measurement Page 23
Non-Kelvin Measurements Can Introduce Significant Error I B Re = 0.55 Rcable = 0.40 V C monitor Cable resistance comparable to resistance being measured Page 24 voltmeter sweeping current Kelvin SMU
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 Page 25
Two Kelvin SMUs Can Make a Kelvin Measurement Page 26
Do Not Use SMU Sense Output by Itself! DUT x1 Buffer Force V RsRs Sense I out Page 27
Kelvin Triaxial Cable Ideal for both low current and low impedance applications. Guard Force Ground Sense Page 28 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 Connections Optimized For Measurement Accuracy Page 29
x1 Buffer SMU Guard Sense Force V Source Gate Drain Substrate Measure Gate Voltage versus Time Accurately Triax to Coax Adapter (guard floating) To Scope Nifty Trick: Use the Sense Line as a High Impedance Scope Probe! Page 30
What is the Ground Unit (GNDU) Configuration? Sense Line Force Line Ground Shield Force / Sense Line Driven Guard Ground Shield Standard Triaxial Connection:Ground Unit Connection: Page 32
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. Page 33
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 RsRs Sense I sink Ground Unit Input Page 34
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 Page 35
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 I sink (Up to 4.2 Amps*) I sense (0 Amps) *B1500A/B1505A Page 36
Benchtop SMUs Typically Have Banana Jack Outputs: Page 37 SenseForceGuard 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 AdapterNon-Kelvin Adapter Page 38 N1259A-001 Banana–Triaxial Adapter for 2-wire connection (non-Kelvin) N1259A-002 Banana–Triaxial Adapter for 4-wire connection (Kelvin)