Introduction
The M.I.T. Microelectronics WebLab or "WebLab" is a remote microelectronics device characterization laboratory. It enables users to measure the current-voltage characteristics various microelectronics devices at any time, from any physical location, using a Java-enabled web browser.
WebLab is one of the laboratories available under the iLabs project which is dedicated to the proposition that online laboratories - real laboratories accessed through the Internet - can enrich science and engineering education by greatly expanding the range of experiments that students are exposed to in the course of their education. Unlike conventional laboratories, iLabs can be shared across a university or across the world. The iLabs vision is to share expensive equipment and educational materials associated with lab experiments as broadly as possible within higher education and beyond.
The major goal of the WebLab project is to deliver the educational benefits of hands-on experimentation to students anywhere, at any time. This project provides the technology to make microelectronics test equipment available over the Internet. Students are able to enjoy the complete laboratory experience which can be delivered to any conventional Java-enabled web browser. Today, web browsers are ubiquitous; therefore, through WebLab, students can take current-voltage measurements on transistors and other devices in real time from anywhere and at anytime. In addition, expensive equipment and educational materials associated with lab experiments are shared as broadly as possible within higher education and beyond.
Some of the devices available for I-V characterization include pn diodes, nmos/pmos field effect transistors, and npn/pnp bipolar junction transistors. The user is able to change the operating conditions of the experiment such as bias voltages and currents, compliance values, voltage and current sweep ranges, etc using a simple graphical interface. The results of the experiments are immediately displayed as graphs in the Weblab applet console, and can be downloaded as a comma separated file for post processing using matlab or any spread sheet application.
Architecture
The Microelectronics WebLab is based on the iLabs Shared Architecture. iLabs' design separates online labs into three distinct modules connected by a Web service architecture.
• The Lab Server is operated by the lab's owner and deals with the actual operation of the lab hardware. While the semiconductor test equipment performs the testing, the Lab Server component is what enables this testing to be initiated remotely. The Lab Server itself is a PC running Windows 2000 that houses all of the software components of the system. In addition, this machine is configured with an Agilent 82350A PC/GPIB Interface card. This allows the Lab Server to control and receive data from all hardware components that are connected to the GPIB network.
• The Lab Client runs on the end user's computer, and provides the interface to the operation of the lab. Specifically, is a Java applet which serves as the client's interface to the instrumentation hardware. It provides the user with an interface similar to that of the front panel of the 4155B while also incorporating the multiple device access and temperature measurement ability of the E5250A and the 34970A respectively. This is where the end-user decides what device to test, what signals to send to the device as input and what ports to listen for data on.
• The Service Broker mediates exchanges between the Lab Client and the Lab Server and provides storage and administrative services that are generic and can be shared by multiple labs within a single university.
Figure 1: Overview of iLabs Shared Architecture
The Semiconductor Test Component of the WebLab System provides the core functionality of the system. Testing and characterization of microelectronic devices is performed by the handful of devices. First, an Agilent 4155B Semiconductor Parameter Analyzer is used to carry out the actual measurements. This device is what provides the test signals and receives/processes returned signals from a given microelectronic device under test. Specifically, the device under test is placed in an Agilent 16442A Test Fixture, which is connected to the signal outputs of the 4155B. The 4155B is directly controlled by the Web Server through the system's GPIB network. This enables the Web Server to configure the 4155B and initiate the test process.
In order to make eight microelectronic devices available for test to the WebLab System, a multiplexing device must be integrated into this component. The system makes use of the Agilent E5250A Low Leakage Switch Mainframe to perform this function. The E5250A is ideal for this application, primarily, because it is an exceptionally low-loss switch. As such, there is little signal degradation across the switch. Additionally, the E5250A can be controlled via GBIP. Thus, the signal output of the 4155B is connected to the input of the E5250A, which routes it to one of eight sets of output ports. Each of these output ports connects one of the above mentioned 16442A Test Fixtures. As was the case with the 4155B, the E5250A is connected to the system's GPIB network. This allows the Web Server to control which of the eight test devices the 4155B's signals are routed to.
Figure 2: Architecture of Semiconductor Test System
Typical Experiment
A relatively simple experiment which can be run using the M.I.T. Microelectronics WebLab is obtaining the output characteristics of a pn diode. The diagram in Figure 3 shows the layout of the experimental set up. The diode symbol has a triangle pointing into a vertical line. In a diode, current can flow in the direction indicated by this pointy triangle, but not in the other direction; stated simply, a diode is a one way electronic valve. In the diagram of Figure 1, the diode is connected to an instrument that is going to perform the actual measurements. In WebLab, this is an Agilent 4155B Semiconductor Parameter Analyzer. The boxes labeled SMU1 and SMU2 represent the signal measurement units of this instrument to which the diode is connected. Characterizing the electrical behavior of the pn diode requires that these two SMU's be programmed appropriately.
Figure 3: The WebLab 6.1 Client
This experimental setup grounds SMU2 and successively applies a voltage to SMU1 from -1.5 to +1.5 volts in 10 milliVolt increments, and will measure the current flowing through SMU 1 . The voltages and currents on each of the leads of the pn diode are controlled by SMU1 and SMU2. Clicking your mouse on the SMU's would bring up dialog boxes which are used to configure the voltages or currents on each of these leads. The Webab user's manual explains to you how to program the SMU's . The tutorial examples on this web site illustrates how to do this.
Once the configuration of the experiment is completed, you can click on the "Measurement" item on the menu bar. Do this and select the "Run Measurement" option (the only option available on this menu item). This will submit the experiment over the Internet to the WebLab server for execution. You can also click on the "Running Man" icon in the upper right of the WebLab client program to submit an experiment. Once the experiment is complete the results are downloaded to the client. The resulting window should look like Figure 4 below.
Figure 4: Plot of Output from Diode Experiment
The graph shows the voltage across the diode on the x axis and the current through the diode on the y axis. For this simple experiment, the graph is the diode characteristic that is familiar to electrical engineers. Namely, the current remains negligible until the voltage reaches about 0.6 Volts, at which point the current increases rapidly in an exponential way. In our experiment, we limit the current to 100 milliAmps. This limit is imposed by the experimental setup.
This characteristic behavior allows the pn diode to act as a valve for current; if the voltage between the leads is less than about 0.6 Volts (the voltage needed in the diode to allow it to conduct current), no current flows; if it it a bit higher than 0.6 Volts, the current rises quickly to the maximum allowed value.
You can change the way the experimental data is plotted by altering the scale of the axes. For example, try resetting the scale on the y axis from linear to logarithmic. The data should be redisplayed with the new axes.
Policies and constraints
In order to provide a seamless experience, the WebLab team introduced a number of policies and constraints:
• Number of data points in the results generated is limited to a particular value. The Weblab applet checks for this constraint at run time and informs the user if this value is exceeded.
• There is a maximum voltage or current that can be applied to the leads of a device. This is called the default compliance value is automatically set to protect the measuring equipment and devices from damage. Thus, whenever the voltage or current in an experiment exceeds the compliance value, the results obtained saturates to that value.
• Only one applet instance can be opened per internet browser.
Resources
Figure 5: Agilent 4155B - Semiconductor Parameter Analyzer
Agilent 4155B - Semiconductor Parameter Analyzer: Characterizes a given microelectronic device. It performs the required current and voltage measurements on any connected device such as a transistor or diode.
Agilent E5250A - Low Leakage Switching Matrix: Multiplexes the I/O signals from the Agilent 4155B to one of the eight test fixtures connected to the output ports of the matrix. This increases the system's capacity (in terms of devices connected at a given time) from 1 to 8.
Agilent 16442A - Test Fixture: Houses a device or circuit that is connected to the system. Provides a means for the I/O signals from the Agilent 4155B to be broken out of their triaxial lines and connected to individual device terminals.
Agilent 34970A - Data Logger: Used with a digital multimeter module to record the ambient temperature of the lab for the student. This information is useful as input to device models/simulations.
All of this equipment is connected via a common GPIB communication interface to a computer that controls all three of the Agilent devices.
References
"MIT Microelectronics WebLab", J.A. del Alamo, V. Chang, L. Brooks, C. McLean, J.L. Hardison, G. Mishuris, and L. Hui
"An Open-Source Export Package for the MIT Microelectronics WebLab", J.L. Hardison