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As the result of joining forces within the HiTeK project to combine the advantages of silicon-based CMOS circuits from the IHP–Leibniz-Institut für innovative Mikroelektronik in Frankfurt/Oder with those of indium-phosphide circuits from the Ferdinand-Braun-Institut (FBH), Berlin, the partners have accomplished an important step by successfully integrating both circuits on a semiconductor wafer, with measurement results demonstrating their high performance.
Consequently the heads of the two Leibniz institutes in Germany, Wolfgang Heinrich (FBH) and Bernd Tillack (IHP) believe they hold the key to faster and more powerful terahertz chips. FBH is one of the leading institutes in developing III-V semiconductors, while IHP specializes in silicon-based systems and circuits.
With the integration on one chip, new ambitious applications in the THz range are within reach, such as high-resolution imaging systems for medical and security technology as well as ultra-broadband mobile communication applications. For such applications high output powers along with faster computer processors are needed, offering enhanced computer operation per second. In order to achieve this, circuits on the chips have to become smaller.
If the frequency range around 100 GHz and beyond is to be covered, however, the breakdown voltage in the CMOS switching circuits decreases significantly. Accordingly, the available output power of the chips declines, which implies that the capability to generate sufficiently strong signals to establish a radio link and to detect material defects becomes insufficient. To find a solution for this problem, IHP conducts research on bipolar CMOS based on silicon-germanium, enhancing the breakdown voltages at high speed compared to pure CMOS. By combining a standard CMOS circuit with a second indium-phosphide circuit promises further improvement.
Both circuits are realized ‘sandwich-like’ on top of each other. Where the traditional silicon-based CMOS technology reaches its limits, this novel material combination delivers the desired properties: high output powers at high frequencies. The sandwich chips are able to keep benefiting from the high level of production routine and integration of CMOS circuits – particularly regarding the fact that 95 percent of all digital and analogue-digital circuits are based on this technology.
“It was particularly challenging to make both technologies compatible at the interfaces”, underlined Wolfgang Heinrich. To achieve this, the whole development environment of both processes as, for example, the software for the circuit layout had to be merged in a first step. Subsequently, both layers had to be dimensioned so that they reach the essential good transmission properties for frequencies around 200 GHz.
Precision work was also demanded in order to adjust the circuits precisely to each other with an accuracy of less than 10 micrometers. Heinrich is especially proud of the friction-less cooperation: “We managed to align both technology worlds so smoothly that the circuits deliver fully the specified high-frequency performance. This also demonstrates what added value can be created by bundling the competencies of two institutes like IHP and FBH”.
The next steps are to further stabilize the process and to optimize the circuits. A follow-up project has already been granted. This way, the potential of the hybrid chips can be exploited fully to reach the borders of what is feasible – thus setting the stage for the novel sandwich circuits to be integrated in sophisticated applications soon.