PN Device Fabrication-The Photolithographic Process

The method most widely used for integrated-circuit fabrication is now also used to produce discrete devices. The process to which we refer, called batch production using photo lithographic methods, is a truly remarkable blend of ingenuity and precision, and is responsible for many dramatic achievements in the lido of micro-miniature electronics. The manufacturing procedure is called belch processing because it permits the simultaneous production of many identical chips from a single wafer, This procedure has been made possible by technological advances in a number of diverse fields, including chemical process control, photochemistry. and computer-aided design and manufacturing (CAD/CAM). The essence of the photolithographic process is the use of photographic methods to Hiler the characteristics of a special coating applied to the surface of a wafer. The coating is altered in an intricate pattern so that tmv regions of the crystal will become P- or N-type material when the wafer is later subjected to certain doping treatments. One such doping treatment is called impurity diffusion, whereby donor  or acceptor impurities are allowed to diffuse through the surface pattern and enter the crystals at the desired loc”ions. We will discuss some details of this important process and then return to the details of photolithography. Recall that carrier diffusion is the migration of holes or electrons from a region where there is a surplus of one or the otherto a region where there is a corresponding scarcity. The diffusion process used in batch production is a similar phenomenon, except that it is impurity atoms such as boron or phosphorus that are allowed to Integrated Circuits 221 diffuse into semiconductor material to make it either P- or N-type. To enable atoms to diffuse into silicon, the material must be raised to a high temperature, on the order of IOOO”C. At that temperature. silicon atoms leave the crystal structure. thus making it possible for migratmg impurity atoms to OCCUI1V the vacancies. Recall that boron atoms entering the crystal structure create hok:. and therefore produce Ptype  material. while phosphorus atoms supply excess electrons to create N material. The first step in the photolithographic processing is to create a thin layer of  silicon dioxide S;02) on the surface of a silicon wafer. This is accomplished by placing the crystalline wafer in a furnace containing oxygen gas (02), which then reacts chemically with the Si to produce Si02• Impurity atoms cannot diffuse through the SiOJ layer and into the Si crystal. so the SiOz is used to prevent the creation of P- ami N-tYf.e regions where they are not desired. To create P and N regions where they arc desired, “windows” must be made in the Si02 layer; that is, the SiO! must be removed at selected locations to permit impurity atoms to diffuse into the crystal. We say that windows are “opened” where the Si02 is removed. The next step in the process is to deposit a coating of photoresist (PR) material on top of the Si02 layer. The photoresist is a photosensitive organic material that changes its composition (becomes polymerized) wherever it is exposed to ultraviolet light. The PR coating is exposed to ultraviolet light through a glass mask that pieuents exposure (is opaque) at any location where a window is d”.;in!d. Thus the ma ‘k creates a pattern of unexposed PR corresponding to regions where Impurity
diffusion will be allowed to OC~Ui. The unexposed PR material is removed and hydroelectric acid (HF) is applied to the surface. The acid does not affect the regions  of exposed PR but etches away the Si Oz where it is not prerecorded by the PR, thus creating the windows. Once the windows in the Si have been opened by the HF
acid treatment. the exposed PR material is removed and the structure is ready for the impurity diffusion step. The wafer is placed in an oven am impurity atoms  diffuse through the windows into the silicon crystal. The depth to which the diffused P or N layer penetrates the crystal. and its concentration (doping density). arc
determined by closely controlling the temperature of the process and the length of time it is allowed to occur.Because diffusion occurs only at those locations where windows have been opened. the process is often called selective diffusion.