The demand for more functionality in smaller sizes fuels the development of integration technologies. The classical example is the rapid growth in silicon transistor integration, identified by the semiempirical Moore’s Law, which states that “the number of transistors on a given amount of silicon doubles every two years.” However, silicon can not fulfill all functionality needed for a stand-alone system that consists typically of a sensor (electrical, mechanical, optical, chemical, etc.), a processing and decision circuit (analog, digital, mixed mode) and an actuator (electrical, mechanical, optical, chemical, etc.). The integration of a high level of functionality is not trivial due to non-silicon-based devices such as GaAs and nonplanar devices such as micromechanical structures. Even simpler cases of integration of memory and processors or analog and digital circuits are not straightforward due to fine-tuned processing techniques that are not directly compatible. The efforts toward integration of dissimilar materials and devices lead to the concept of “heterogeneous integration.” A planar version of heterogeneous integration will yield “systems-on-a-chip,” where more complex functionalities can be obtained in a single piece of silicon.
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