HVAC Control in the New Millennium.pdf - HVAC.Amickracing

Advanced HVAC Control, Information Technology and Open SystemsIntegrated Circuit FabricationA sequence of patterned layers is used to build NMOS integratedcircuits and systems. The fabrication of a basic inverter circuit takesplace in a system and all other circuits are simultaneously implementedby the same process.The first mask includes the sources and drains of all transistors inthe circuit, together with the transistor gate areas and any circuit interconnectionpaths. This mask is used for the first step in the process.The next step is to differentiate transistors that are normally on(depletion mode) from those that are normally off (enhancement mode).This is done by an overcoat of the wafer with resist material, exposingthe resist material through openings in another mask, and developing it.The pattern leaves an opening in the resist material over the area to beturned into depletion mode transistors.The actual conversion of the underlying silicon is done by implantingions of arsenic or antimony into the silicon surface. The resist materialacts to prevent the ions from reaching the silicon surface. Ions areonly implanted in the silicon area free of resist which causes a slight n-type conductivity in the underlying silicon.The wafer is then heated while exposed to oxygen, to grow a thinlayer of silicon dioxide over its entire surface. It is then coated with athin layer of polycrystalline silicon. This layer forms the gates of all thetransistors in the circuit and also serves as a second layer for circuitinterconnections.The n-type regions are diffused into the p-type silicon substrate,forming the sources and drains of the transistors and the first level ofinterconnections. This is done by first removing the thin gate oxide inthe areas not covered by the polysilicon.The wafer is then exposed to n-type impurities such as arsenic,antimony, or phosphorous at high temperature which allows these impuritiesto convert the exposed underlying silicon to n-type material.The polysilicon area and the thin oxide under it act to prevent impuritiesfrom diffusing into the underlying silicon.Interconnections are made with a metal layer that contacts both thediffused areas and the polycrystalline areas. Another layer of insulatingoxide is coated over the entire circuit and then removed in places wherecontacts are desired. The wafer surface is usually coated with another layerof oxide. This is called overglassing and provides physical protection for©2001 by The Fairmont Press, Inc. All rights reserved.