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Evolution of Semiconductor Parameter Analyzers for Three Critical Types of Semiconductor Measurement – Part III

One of the greatest challenges associated with integrating DC I-V, capacitance-voltage (C-V), and ultra-fast I-V measurement capabilities into a single parametric test chassis is that the cabling required for each measurement type is fundamentally different. Although the cabling from the instrument to the semiconductor probe station bulkhead and feed-through is fairly straightforward, the cabling from the bulkhead to the probe tips can be confusing and difficult.

Different Cabling for Different Measurements. DC I-V measurements are made using four triaxial cables. Guarding is necessary to achieve low current I-V measurements, which makes the use of triaxial cables necessary for these measurements. The measurement signal is carried on the center conductor of the cable, the inner shield is driven as a guard for the signal, and the outer shield is used for safety to shield the user from high voltages that may be applied to the guard and signal conductors. Four cables are necessary in order to achieve a remote sense, or Kelvin, connection to allow the instrument to sense the voltage at the device accurately.

Guarding is a technique that reduces leakage errors and decreases measurement response time. Guarding consists of a conductor (shield) surrounding the lead of a high impedance signal and driven by a low impedance source. The guard voltage is kept at or near the potential of the signal voltage.

C-V measurements are made using four coaxial cables. The outer shells are connected together to control the characteristic impedance the signals see. All four cables' outer shells must be inter-connected near the DUT. Typical cabling requirements for different types of measurements are listed below.

DC I-V Measurements

Triaxial cables
Kelvin connections
Isolated, driven grounds

LCR/C-V Measurements

Coaxial cables
Kelvin connections
Shields connected at the probe tips

Ultra-fast I-V Measurements

Coaxial cables
Non-Kelvin connections
Shields connected at the probe tips
Shields optionally connected to a probe tip

Ultra-fast I-V measurements require the highest bandwidth of the three measurement types, so the cable must have characteristic impedance that matches the source impedance to prevent reflections off the DUT from reflecting off the source. Ultrafast I-V testing does not use a remote sense cable and is the only one of the three measurement types that connects the DUT to the outer shield of the cable.

To address the challenges created by different cabling requirements for different measurement types, Keithley developed a high-performance multi-measurement cabling system. These cables support I-V, C-V and ultra-fast I-V measurements. Their use reduces the burden on a test system operator, who would otherwise be forced to go through the laborious process of re-cabling connections from the instrumentation to the prober every time a new measurement type is required.

A good cabling kit maximizes signal fidelity by eliminating measurement errors that often result from poor cabling practices. When combined with a versatile parameter analyzer system, the user will be able to make the three principal types of measurement required for semiconductor devices. High-performance multi-measurement cabling is crucial for connecting various elements of a parameter analyzer to the probe manipulators on a wafer prober, especially when you need to integrate accurate ultra-fast I-V, C-V, and precision DC I-V measurements for a high throughput test system

Conclusions

Ultra-fast I-V sourcing and measurement are the latest capabilities to be added to integrated parameter analyzer systems. Modular architectures in these systems represent a cost-effective way to address new testing needs and techniques as they emerge. Multi-measurement cabling with a broad signal bandwidth is crucial for high measurement accuracy and throughput in these systems. Getting all these features and capabilities in one test system that adapts readily to the industry's changing test needs makes a semiconductor manufacturer's capital investment stretch further and improves its ROI.

About the Author

Lee Stauffer is the Senior Staff Technologist for Keithley Instruments' Semiconductor Measurements Group, based in Cleveland, Ohio. Prior to joining Keithley, his career included designing satellite communication systems, as well as equipment and product engineering in semiconductor fabs. Keithley designs, develops, manufactures and markets complex electronic instruments and systems geared to the specialized needs of electronics manufacturers for high-performance production testing, process monitoring, product development and research.