Integral PC Design of Keithley Model 4200-SCS is at the
Leading Edge of a New Instrumentation Trend

By Bill Merkel
Keithley Semiconductor Business Unit

For several years, PCs have been used to automate and increase the efficiency of data acquisition and measurement applications. With the former, PCs house data acquisition boards in their internal data bus slots, where they provide interface and data manipulation functions. In the latter case, PCs can tie together several instruments on an external bus and act as a controller to collect measurement data. Also, in specialized automatic test equipment (ATE), PCs are often embedded on a board and installed in a backplane with the appropriate data bus, instead of being in a separate enclosure. In ATE applications, embedded PCs perform data acquisition, test and measurement functions similar to their desktop and laptop brethren.

PCs present both opportunities and problems in test and measurement applications. On the plus side, they offer tremendous computational power in data collection and manipulation. On the negative side, they create electrically noisy environments that are not conducive to sensitive, low-level measurements. With PCs, there also are potential problems in the management of "overhead" to ensure a reliable system for real-time computations.

Tapping Into PC Power
Managing these conflicting characteristics is a challenge to measurement system designers. Keithley engineers found ways of doing this in the design of its new Model 4200-SCS Semiconductor Characterization System, which performs sub-femtoamp (<10-15 A) level measurements on semiconductor devices. Along with high resolution measurements and speedier throughput, use of an integral PC allows the Model 4200-SCS to perform wafer device characterization with real-time plotting and analysis. The built-in software also delivers superior performance in other areas important to users: setup, data collection, analysis and data storage.

The PC operating system is Microsoft's Windows NT®, which provides a stable environment for all instrument functions and offers many advantages to the user. For example, the familiar Windows interface minimizes the instrument learning curve, which is important in a semiconductor plant where test equipment is shared by several users. Many of these users have infrequent access to the system and need an intuitive interface that simplifies set-up and operation. Also, the PC is running Keithley's Interactive Test Environment, which allows users to quickly set up tests, perform test sequences without programming, export data and plots in PC formats and add drivers for external CV meters, switching matrixes, and related test equipment.

By using a Microsoft programming environment for the measurement software, designers gain easy access to all Windows resources, such as disk functions, video, networking and other I/O operations. A large capacity fixed disk provides data storage while an Ethernet interface allows data to be shared wherever it is needed in a semiconductor plant. A security management interface insures the integrity of data and basic system performance.

The computational power of an integral PC allows a large array of measurement functions. For example, the Model 4200-SCS allows simultaneous measurement of up to eight different measurement channels (compared to only one channel for its closest competitor), and software that requires only one mouse click to move between tests. Data can be saved in either ASCII or Excel format, and spreadsheet analysis functions are included in the software set

The open system architecture also allows new hardware to be easily added, and user-customized test routines will be fully compatible with instrument hardware and software upgrades. Furthermore, the underlying instrument algorithms can be easily updated and moved to new platforms.

To resolve electrical noise problems typically found in PC hardware, Keithley engineers created an instrument grade PC chassis with a high-performance backplane design. To this they added a low-noise power supply with voltage outputs much "cleaner" than those typically found in a standard desktop PC. A PC motherboard mounts in the backplane, along with additional instrument boards providing functions specific to the Model 4200-SCS.

A New Trend Develops
Because an integral PC adds so much power and capability to an instrument, the use of this feature is likely to accelerate. Currently, only a few test systems use them, such as digital storage oscilloscopes and custom built ATE. This is due to the added cost, which is more easily supported by measurement equipment requiring a large number of complex computations. As the cost of computational power, memory and related semiconductor functions continue to fall, integral PCs probably will be used in less exotic instruments. This can only be good news for users. They should find the added functions and familiar GUIs of an integral PC to be a great advantage in getting their jobs done, while the added information supplied by these systems help improve technical decisions.