EC-25.2 Characterising a Device: What Is Measured, and With What
You can plan a characterisation measurement that extracts a named model parameter, choose instrumentation appropriate to currents spanning many decades, and explain what a test structure on a wafer is for.
Before:PE1-1. VLSI TechnologyEC-8. Measurements, Instrumentation and Experimental MethodEC-21. Hardware Verification, FPGA and SoC Design
A model is only as good as the measurements it was fitted to, and device measurement is its own craft. Currents of interest run from picoamperes to amperes, the device is a few micrometres across, and the probe and cable are part of the circuit. This topic is what is measured, on what structure, with what instrument, and how a parameter is extracted from a curve rather than read off it.
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The measurements that define a transistor: threshold, transconductance, leakage and capacitance
A small set of measured curves determines most of a device model, and each has a standard extraction method with known assumptions. Knowing which assumption each rests on is what makes the numbers comparable between laboratories.
NPTEL: Semiconductor Device Modeling and Simulation (IIT Kharagpur) · CourseExtracting a parameter from a curve rather than reading it off
A threshold voltage is defined by a fitting procedure, and three common procedures give three different answers on the same device. Quoting a parameter without its extraction method is close to meaningless.
NPTEL: Semiconductor Device Modeling and Simulation (IIT Kharagpur) · CourseInstruments for many decades of current, and the guarding that makes them work
Measuring picoamperes needs a driven guard around the signal path, because the leakage of an ordinary cable exceeds the current of interest. This is the practical difference between a benchtop meter and a parameter analyser.
NPTEL: Semiconductor Device Modeling and Simulation (IIT Kharagpur) · CourseTest structures in the scribe line, and what they are for
Dedicated devices, resistors and chains placed between dies are measured on every wafer to monitor the process. They are the earliest signal that something has drifted, long before finished parts are tested.
NPTEL: Basic Overview of Semiconductor Device Processing and IC Fabrication (IIT Kanpur) · CourseProbing a wafer: contact resistance, pad damage and repeatability
The probe must make good contact without destroying the pad, and the contact resistance appears in series with everything measured. Repeatability checks on the same site are the standard way of proving the measurement rather than the device.
Measuring across temperature, and why it is not optional
Almost every parameter of interest changes with temperature, often in opposite directions for different mechanisms. A characterisation at one temperature cannot support a specification over a range.
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