A universal test
Capacitance voltage (CV) testing is widely used to determine semiconductor parameters, especially in MOSCAP and MOSFET structures. However, other types of semiconductor devices and technologies can also be characterized by CV measurements, including bipolar junction transistors (BJTs), JFETs, complex III-V devices, photovoltaic cells, MEM devices, organic TFT displays, photodiodes, carbon nanotubes (CNTs), and many others.
The fundamental nature of these measurements makes them useful in a wide range of applications and disciplines. They are used in university research laboratories and semiconductor manufacturers to evaluate new materials, processes, devices and circuits. CV measurements are extremely important to product improvement and yield engineers who are responsible for improving processes and equipment performance. Reliability engineers use these measurements to qualify material suppliers, monitor process parameters, and analyze failure mechanisms.
With the right methodologies, instruments and software, a multitude of semiconductor and material parameters can be extracted. This information is used throughout the production chain starting with the evaluation of epitaxially grown crystals, including parameters such as average doping concentration, doping profiles and carrier lifetime. In wafer processes, CV production mim can detect oxide thickness, oxide charges, mobile ions (impurity), and interface trap density. These measurements continue to be used after other process steps, such as lithography, etching, cleaning, dielectric and polysilicon depositions, and metallization. After devices are fully fabricated on the wafer, CV is used to characterize threshold voltages and other parameters during device reliability and baseline testing and to model the performance of these devices.
A MOSCAP structure is a fundamental device formed during semiconductor fabrication. Although these devices can be used in actual circuits, they are typically integrated into fabrication processes as a test facility. Since they are simple structures and their fabrication is easy to control, they are a convenient way to evaluate underlying processes.
The metal/polysilicon layer is a capacitor plate, and the silicon dioxide is the insulator. Since the substrate below the insulating layer is a semiconducting material, it is not itself the other plate of the capacitor. In fact, most of the load carriers become the other plate. Physically, the capacitance, C, is determined by the variables in the following equation:
C = A (?/d), where A is the surface area of the capacitor, ? is the dielectric constant of the insulator and d is the separation of the two plates.
Therefore, the larger A and κ, and the thinner the insulator, the higher the capacitance. Typically, semiconductor capacitance values range from nanofarads to picofarads, or smaller.
The procedure for taking CV measurements involves applying DC bias voltages across the capacitor while taking measurements with an AC signal. Typically, AC frequencies from about 10 kHz to 10 MHz are used for these measurements. The bias is applied as a DC voltage sweep that drives the MOSCAP structure from its accumulation region to the depletion region and then to reverse.
A strong DC bias causes most of the carriers in the substrate to accumulate near the insulator interface. Since they cannot pass through the insulating layer, the capacitance is at a maximum in the accumulation region as charges accumulate near that interface (ie, d is at a minimum). One of the fundamental parameters that can be derived from CV accumulation measurements is the thickness of the silicon dioxide, toxi.
As the bias voltage decreases, most of the carriers leave the oxide interface and the depletion region is formed. When the bias voltage is reversed, the charge carriers move the greatest distance from the oxide layer and the capacitance is at a minimum (ie, d is at a maximum). From this capacity of the inversion region, the number of majority carriers can be deduced. The same basic concepts apply to MOSFET transistors, although their physical structure and doping is more complex.
Many other parameters can be derived from the three regions as the bias voltage passes through them. Different frequencies of the AC signal can reveal additional details. Low frequencies reveal what are called quasi-static characteristics, while high frequency testing is more indicative of dynamic performance. Both types of CV testing are often required.
Because CV measurements are actually made at AC frequencies, the capacitance for the device under test (DUT) is calculated as follows:
CDUT = IDUT / 2?fVAC, where IDUT is the magnitude of the AC current through the DUT, f is the test frequency, and VAC is the magnitude and phase angle of the measured AC voltage
In other words, the test measures the AC resistance of the DUT by applying an AC voltage and measuring the resulting AC current, AC voltage, and the phase angle of the resistance between them. These measurements take into account the series and parallel resistance associated with the capacitance, as well as the dissipation (leakage) factor.
Several challenges are associated with this testing. Typically, testing personnel have problems in the following areas:
Low capacitance measurements (picofarad and smaller values)
CV instrument connections (via a probe) to the wafer device
Flow capacitance measurements (high D).
Using hardware and software to retrieve data
Extracting parameters
Overcoming these challenges requires careful attention to the techniques used along with the appropriate hardware and software.
Low capacity measurements. If C is small, the AC response current of the DUT is small and difficult to measure. However, at higher frequencies, the DUT impedance decreases, so the current increases and is easier to measure. Often the semiconductor capacitance is very low (less than 1pF), which is below the capabilities of many LCR meters. Even those that claim to measure these small values of capacitance can have confusing specifications that make it difficult to determine the ultimate accuracy in the measurement. If the accuracy over the full measuring range of the instrument is not explicitly stated, the user should clarify this with the manufacturer.
High D (leaking) capacitors. In addition to having a low C value, a semiconductor capacitor can also have leakage. This is the case when the equivalent R in parallel with C is too low. This results in the resistive impedance dominating the capacitive impedance and the C value is lost in noise. For devices with ultra-thin oxide layers, D values can be greater than five. In general, as D increases, the accuracy of the C measurement degrades rapidly, so high D is a limiting factor in the practical use of a C meter. Again, higher frequencies can help solve the problem. At higher frequencies, the capacitive impedance is lower, resulting in a current C that is higher and easier to measure.
