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; 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.
Lee Stauffer is a Senior Marketer with Keithley Instruments in Cleveland, Ohio, USA, where he is responsible for developing and supporting products for the semiconductor manufacturing and research markets. His formal education in electrical engineering and semiconductor device physics is complemented by more than 20 years experience in semiconductor process and product engineering, device characterization and instrumentation design. He can be reached at 440-248-0400, or by e-mail at lstauffer@keithley.com.
