Characterizing a semiconductor device, material, or process thoroughly requires the ability to make three types of measurements. The first two types, DC I-V and AC impedance measurements, are the most familiar to semiconductor manufacturers. Precision DC I-V measurements are typically made with high-precision Source-Measure Units (SMUs) to generate current vs. voltage curves. (SMUs can source and measure both current and voltage.) AC impedance measurements in the semiconductor industry are often made with a capacitance meter. Ultra-fast (transient) I-V is the third type of semiconductor characterization testing, and the one most difficult to achieve. Historically, semiconductor labs have used up to three different test systems to obtain all three types of measurement.
In addition to the expense of purchasing and maintaining multiple test systems and training personnel to use them, the three-test system approach makes it difficult to combine different measurement types in a single application. When measurements are made at different times under varying test conditions with different instruments, accurately correlating results for a specific device or materials research sample becomes problematic.
A growing number of semiconductor devices and materials require testing with multiple measurement types. This makes it highly desirable to have a single test system with the capability to make all the measurements, controlled from a single easy-to-use operator interface.
Multiple-Measurement Applications. One test application requiring multiple measurements is charge pumping (CP), a well-known technique for analyzing the semiconductor/
Similarly, determining the electrical characteristics of photovoltaic (solar) cells often involves measuring the current and capacitance as a function of an applied DC voltage. The measurements are usually done at different light intensities and temperature conditions. Important device parameters can be extracted from the I-V and C-V measurements, such as the output current, conversion efficiency, maximum power output, doping density, resistivity, etc. Electrical characterization is also important to determine losses in the photovoltaic cell in order to learn how to make the cells as efficient as possible with minimal losses.
SMU-based Systems for DC I-V Testing. Early on, semiconductor test system manufacturers recognized the need for tightly integrated instrumentation capable of sensitive measurements, which could be operated in automatic, semi-automatic, and manual modes to accommodate a wide variety of test situations. These early semiconductor parameter analyzers concentrated on DC I-V measurements using multiple SMUs operated under computer control. An operator interface and system software simplified the process by providing ready-to run test routines for commonly used semiconductor tests.
These integrated parameter analyzers have been refined over time to provide more flexibility and sensitivity in DC I-V testing. For example, Keithley’s basic Model 4200-SCS Semiconductor Characterization System includes two medium-power SMUs. These SMUs can source voltage and current up to a 2W output (100mA max.) In addition, a high-power SMU option is also available (1A, 20W); the test system chassis can contain up to nine SMUs in any combination of high- and medium-powered units.
When stimulating a semiconductor device or material with a voltage, the response current can often be quite small, which a conventional SMU may not be able to measure accurately. To handle this situation, the Model 4200-SCS has a Remote PreAmp option that extends the low current measurement range down to 0.1fA (10-15A). In keeping with the desire for simplicity of use, the PreAmp module is fully integrated with the system. To the user, the SMU simply appears to have additional measurement ranges and resolutions available. The Remote PreAmp can also be placed remotely (such as in a light-tight enclosure or on a prober platen) to minimize measurement problems due to long cables.
A Breakthrough in C-V and Pulse Testing. A major advance in parameter analyzers occurred in 2007 when pulse I-V and C-V measurement capabilities were integrated with DC I-V in a single box test system. Keithley accomplished this by adding a plug-in module (Model 4210-CVU) to its Model 4200-SCS chassis for C-V testing. Pulse generator and oscilloscope modules also became available to facilitate pulse testing and response waveform analysis. The modular architecture of the system allows these capabilities to be added to existing units, or specified as options on new systems.
The optional CVU module makes C-V measurements as easy to perform as DC I-V testing. Capacitance from femtofarads (fF) to nanofarads (nF) at frequencies from 1kHz to 10MHz can be measured. This module supports high power C-V measurements up to 400V (200V per device terminal) for testing high power devices (MEMs, LDMOS devices, displays, etc.) and DC currents up to 300mA for measuring capacitance when a transistor is on.
These hardware capabilities are complemented by a broad C-V test and analysis library that becomes embedded in the operating system. This library allows configuring linear or custom C-V and C-f sweeps with up to 4096 data points.
Part II of this 3-part article will explain instrumentation capabilities needed for semiconductor testing using DC I-V, C-V, and ultra-fast pulse I-V measurements. It will then explain how semiconductor parameter analyzers have evolved to make these measurements easier and more precise.
