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2.2 Measurement setup
The experimental facilities at the KAI GmbH laboratory include a Süss MicroTec on–wafer probe station for electrical characterization of the devices. The system is mounted on a vibration isolated platform and protected by a shield box. The sample under test is placed on a thermostatic chuck, hold in place by a vacuum pump and it can be contacted with DC probes using a Motic PSM1000 optical microscope (see Fig. 2.2). An ATT P40 temperature control unit allows to heat up or cool down the sample. The user can set the chuck temperature between −60 °C and 200 °C. A dry air flow across the wafer surface ensures that no moisture is retained onto the devices, which would lead to the creation of a frozen water layer at temperatures lower than 0 °C.
Various instruments can perform a range of different measurements: current–voltage (IV), capacitance–voltage (CV), pulse generation and time–domain (transient) measurement. An Agilent B1500A parameter analyzer features ten module slots for source monitor units (SMUs), an impedance bridge Agilent 4294A is dedicated to impedance measurements and an Agilent E5250A switching matrix allows to manage the connections between the instruments and the needles in the probe station through triaxial cables. When coaxial cables must be used, as for impedance measurements, proper adapters are used.
Figure 2.2: Close–up of the interior of the probe station. The electrical contact is made with eight DC probes, which can be moved along the three directions by the manipulators surrounding the window on the wafer under test, and using the optical microscope. The light guide (grey and blue cable) and its mounting is also visible in the background. See Section 2.4 for the description of the optical setup.
During the course of the thesis the software for communicating with the instruments, driving the measurements and storing the data has been developed in LabView. It is based on a producer–consumer architecture, which handles the user interface through the front panel and the other operative tasks in parallel. A screenshot of the main window during the execution of a series of measurements is shown in Fig. 2.3. The communication with the hardware is based on a general purpose interface bus (GPIB), which can send command strings to the instruments and read out measurement data. The core of the program consists in a file parser which loads the measurement instructions contained in a special text file, and writes to another output file the result of the measurement in plain text format. In this way, the user has the possibility to plan a series of consecutive measurements and to control the temperature of the chuck, using a syntax developed ad hoc. The output file is ready to be analyzed in a second stage with software programs like Matlab or Python.
Figure 2.3: A screenshot of the control panel during a measurement with the LabView software developed in the course of this thesis. The input command file is loaded and shown in the upper part together with the current measurement plot. On the bottom part, the chronology of the past measurements is shown and the relative plot can be activated in the window below. A third window shows warning and error messages received from the instruments, or raised during the execution of the program.
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