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Siborg Systems develops new tools for optical sensor development


Over the last two years, Waterloo based Siborg Systems teamed up with Sensor Creations from Camarillo, California in development of a practical tool for simulation of the process flow and optical sensor performance.

The companies collaborated in both the semiconductor process and device simulation for optical sensor structures. They have large sizes and require many fabrication steps such as epitaxial growth, implantation, deposition, etching, annealing and oxidation. Due to the large size, use of conventional simulation tools lead to high CPU time. In contrast, MicroTec was able to run a typical process simulation within a few minutes on a regular PC.

MicroTec provides steady-state two-dimensional semiconductor device simulation that is not sufficient for capacitance extraction. A new method was developed allowing to calculate capacitance of a semiconductor structure by solving equation for the total current conservation. The method is equally applicable to 1D, 2D and 3D structures but limited to low frequencies and low-leakage conditions. The most straightforward method is solving the equation of the total current conservation, mutual capacitances may be calculated simply by the formula C=Idt/dV.

Doping profile for 3-junction optical sensor simulated with MicroTec. For 100,000 required CPU time was about 2 minutes on regular PC.

Capacitance extraction using the new MicroTec tool for the 3-junction optical sensor for 4 different ramp speeds. The curves are virtually the same indicating remarkable method stability.

This formula could be improved by using a relation involving resistances as well as capacitances. In order to do that, one more data point is required. Although this expression is more accurate than the first one, it is still not equivalent to the actual compact model of the semiconductor structure because, strictly speaking, it is a set of interconnected transmission lines and therefore any simplification of the equivalent circuit results in a loss of accuracy. The current based method is not very accurate and requires simulation with a properly selected ramp speed. If it is too fast, voltage drop due to Ohm's law distorts the capacitance, and if it is too slow, displacement current becomes too small and is swamped by the numerical noise. Practically this method has a limited application due to high sensitivity to the ramp time.

In contrast to the current method, the charge method provides charges affiliated with the contacts rather than the currents, thus eliminating the problem of result interpretation using equivalent R-C circuit. To calculate the charges we solve the same equation but instead of calculating currents, we use the response to the excitation applied to a contact as a weight function when integrating the charge in the structure. The charges are easily calculated by a convolution of the "affiliation" function with the carrier density. This method appeared very stable and accurate and was successfully used for capacitance calculation in optical sensors.

The picture below shows the capacitance calculated by the charge based method at various ramp speeds. Note that all 4 curves virtually coincide. The method applicability is questionable when significant minority charge is injected as in the case of forward biased junctions. The proposed method has a wider range of applicability but the extent of its accuracy still needs to be studied.

"We used Two-dimensional Semiconductor Process and Device Simulation Software MicroTec from Siborg intensively for the last couple of years. We found it very useful in our practical optical sensor prototype development. It significantly outperforms other available commercial tools by the speed, ease-of-use and robustness. Last, but not least, the license cost is significantly lower as well," says Stefan Lauxtermann from Sensor Creations.

MicroTec is a TCAD tool that has been used by major semiconductor manufacturers such as Hitachi, Texas Instruments, Matasushita, etc. As an educational tool, MicroTec and three-dimensional SibLin are simple and easy to learn.

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