Evaluation Engineering - December 2008 - (Page 23)

ply to MOSFETs even though their physical structure and doping are more complex. Many other parameters can be derived from the three regions shown in Figure 2 as the bias voltage is swept through them. Different AC signal frequencies can reveal additional details. Low frequencies uncover what are called quasistatic characteristics where high-frequency testing is more indicative of dynamic performance. Both types of C-V testing often are required. Basic Test Setup Figure 3 is the block diagram of a basic C-V measurement setup. Because C-V measurements actually are made at AC frequencies, the capacitance for the DUT is calculated with the following: Figure 4. Basic Electrical Variables Available From C-V Measurements where: IDUT = magnitude of the AC current through the DUT f = test frequency VAC = magnitude and phase angle of the measured AC voltage In other words, the test measures the AC impedance of the DUT by applying an AC voltage and measuring the resulting AC current, AC voltage, and impedance phase angle between them. These measurements take into account series and parallel resistance associated with the capacitance as well as the dissipation factor. Figure 4 illustrates the basic circuit variables that can be derived from the measurements. nel have problems in the following areas: • Low-capacitance measurements of picofarads and smaller values • C-V instrument connections through a prober to the wafer device • Leaky, high D capacitance measurements • Using hardware and software to acquire the data • Parameter extractions Overcoming these challenges requires careful attention to the techniques used along with appropriate hardware and software. Low-Capacitance Measurements If C is small, the DUT’s AC response current is small and hard to measure. However, at higher frequencies, the DUT impedance is reduced so the current increases and is easier to measure. Often, semiconductor capacitance is less than 1 pF, which is below the capabilities of many LCR meters. Even those claiming to measure these small capacitance values may have confusing speciﬁcations that make it difﬁcult to determine the ﬁnal accuracy in the measurement. If accuracy over the instrument’s full measurement range is not explicitly stated, you need to clarify this with the manufacturer. C-V Measurement Connections In most test environments, the DUT is a test structure on a wafer: It is connected to the C-V instrument through a prober, a probe card adapter, and a switch matrix. Even if no switch is involved, there still is a prober and signiﬁcant cabling. At high frequencies, special corrections and compensation must be applied. Usually, this is achieved with some combination of an open, short, or calibration device. Because of the complexity of the hardware, cabling, and compensation techniques, it is a good idea to confer with C-V test application engineers. They are skilled at working with various probe systems to overcome many types of interconnection problems. High D (Leaky) Capacitors In addition to having a low C value, a semiconductor capacitor also may be leaky. That is the case when the equivalent R in parallel with C is too low. This results in resistive impedance overwhelming the capacitive impedance, and the C value gets lost in the noise. For devices with ultrathin oxide layers, D values can be greater than ﬁve. In general, as D increases, the acContinued on page 24 Challenges of C-V Measurements Although a block diagram of a C-V test setup looks deceptively simple, certain challenges are associated with this testing. Typically, test person- www. ev alua t ion e n gin e e rin g.com December 2008 • EE • 23 http://www.evaluationengineering.com

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Evaluation Engineering - December 2008 Contents Editorial Product Briefing Test Software C-V Measurements Nanoelectronics Test Product Guide Company Guide Machine Vision EMC Test Index of Advertisers

Evaluation Engineering - December 2008

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