Lecture notes for Microelectronics pdf

microelectronics circuit analysis and design, microelectronics and computer technology corporation and microelectronics advanced research corporation, microelectronics digital and analog circuits and systems
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Prof.SteveBarros,United Kingdom,Teacher
Published Date:28-07-2017
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CHAPTER 1 MICROELECTRONICS LEARNING OBJECTIVES Learning objectives are stated at the beginning of each topic. These learning objectives serve as a preview of the information you are expected to learn in the topic. The comprehensive check questions are based on the objectives. By successfully completing the OCC-ECC, you indicate that you have met the objectives and have learned the information. The learning objectives are listed below. Upon completion of this topic, you will be able to: 1. Outline the progress made in the history of microelectronics. 2. Describe the evolution of microelectronics from point-to-point wiring through high element density state-of-the-art microelectronics. 3. List the advantages and disadvantages of point-to-point wiring and high element density state-of- the-art microelectronics. 4. Identify printed circuit boards, diodes, transistors, and the various types of integrated circuits. Describe the fabrication techniques of these components. 5. Define the terminology used in microelectronic technology including the following terms used by the Naval Systems Commands: a. microelectronics b. microcircuit c. microcircuit module d. miniature electronics e. system packaging f. levels of packaging (0 to IV) g. modular assemblies h. cordwood modules i. micromodules 6. Describe typical packaging levels presently used for microelectronic systems. 7. Describe typical interconnections used in microelectronic systems. 8. Describe environmental considerations for microelectronics. 1-1 INTRODUCTION In NEETS, Module 6, Introduction to Electronic Emission, Tubes, and Power Supplies, you learned that Thomas Edison's discovery of thermionic emission opened the door to electronic technology. Progress was slow in the beginning, but each year brought new and more amazing discoveries. The development of vacuum tubes soon led to the simple radio. Then came more complex systems of communications. Modern systems now allow us to communicate with other parts of the world via satellite. Data is now collected from space by probes without the presence of man because of microelectronic technology. Sophisticated control systems allow us to operate equipment by remote control in hazardous situations, such as the handling of radioactive materials. We can remotely pilot aircraft from takeoff to landing. We can make course corrections to spacecraft millions of miles from Earth. Space flight, computers, and even video games would not be possible except for the advances made in microelectronics. The most significant step in modern electronics was the development of the transistor by Bell Laboratories in 1948. This development was to solid-state electronics what the Edison Effect was to the vacuum tube. The solid-state diode and the transistor opened the door to microelectronics. MICROELECTRONICS is defined as that area of technology associated with and applied to the realization of electronic systems made of extremely small electronic parts or elements. As discussed in topic 2 of NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, the term microelectronics is normally associated with integrated circuits (IC). Microelectronics is often thought to include only integrated circuits. However, many other types of circuits also fall into the microelectronics category. These will be discussed in greater detail under solid-state devices later in this topic. During World War II, the need to reduce the size, weight, and power of military electronic systems became important because of the increased use of these systems. As systems became more complex, their size, weight, and power requirements rapidly increased. The increases finally reached a point that was unacceptable, especially in aircraft and for infantry personnel who carried equipment in combat. These unacceptable factors were the driving force in the development of smaller, lighter, and more efficient electronic circuit components. Such requirements continue to be important factors in the development of new systems, both for military and commercial markets. Military electronic systems, for example, continue to become more highly developed as their capability, reliability, and maintainability is increased. Progress in the development of military systems and steady advances in technology point to an ever- increasing need for increased technical knowledge of microelectronics in your Navy job. Q1. What problems were evident about military electronic systems during World War II? Q2. What discovery opened the door to solid-state electronics? Q3. What is microelectronics? EVOLUTION OF MICROELECTRONICS The earliest electronic circuits were fairly simple. They were composed of a few tubes, transformers, resistors, capacitors, and wiring. As more was learned by designers, they began to increase both the size and complexity of circuits. Component limitations were soon identified as this technology developed. 1-2 VACUUM-TUBE EQUIPMENT Vacuum tubes were found to have several built-in problems. Although the tubes were lightweight, associated components and chassis were quite heavy. It was not uncommon for such chassis to weigh 40 to 50 pounds. In addition, the tubes generated a lot of heat, required a warm-up time from 1 to 2 minutes, and required hefty power supply voltages of 300 volts dc and more. No two tubes of the same type were exactly alike in output characteristics. Therefore, designers were required to produce circuits that could work with any tube of a particular type. This meant that additional components were often required to tune the circuit to the output characteristics required for the tube used. Figure 1-1 shows a typical vacuum-tube chassis. The actual size of the transformer is approximately 4 × 4 × 3 inches. Capacitors are approximately 1 × 3 inches. The components in the figure are very large when compared to modern microelectronics. Figure 1-1.—Typical vacuum tube circuit. A circuit could be designed either as a complete system or as a functional part of a larger system. In complex systems, such as radar, many separate circuits were needed to accomplish the desired tasks. Multiple-function tubes, such as dual diodes, dual triodes, tetrodes, and others helped considerably to reduce the size of circuits. However, weight, heat, and power consumption continued to be problems that plagued designers. Another major problem with vacuum-tube circuits was the method of wiring components referred to as POINT-TO-POINT WIRING. Figure 1-2 is an excellent example of point-to-point wiring. Not only did this wiring look like a rat's nest, but it often caused unwanted interactions between components. For example, it was not at all unusual to have inductive or capacitive effects between wires. Also, point-to- point wiring posed a safety hazard when troubleshooting was performed on energized circuits because of exposed wiring and test points. Point-to-point wiring was usually repaired with general purpose test equipment and common hand tools. 1-3 Figure 1-2.—Point-to-point wiring. Vacuum-tube circuits proved to be reliable under many conditions. Still, the drawbacks of large size, heavy weight, and significant power consumption made them undesirable in most situations. For example, computer systems using tubes were extremely large and difficult to maintain. ENIAC, a completely electronic computer built in 1945, contained 18,000 tubes. It often required a full day just to locate and replace faulty tubes. In some applications, we are still limited to vacuum tubes. Cathode-ray tubes used in radar, television, and oscilloscopes do not, as yet, have solid-state counterparts. One concept that eased the technician's job was that of MODULAR PACKAGING. Instead of building a system on one large chassis, it was built of MODULES or blocks. Each module performed a necessary function of the system. Modules could easily be removed and replaced during troubleshooting and repair. For instance, a faulty power supply could be exchanged with a good one to keep the system operational. The faulty unit could then be repaired while out of the system. This is an example of how the module concept improved the efficiency of electronic systems. Even with these advantages, vacuum tube modules still had faults. Tubes and point-to-point wiring were still used and excessive size, weight, and power consumption remained as problems to be overcome. Vacuum tubes were the basis for electronic technology for many years and some are still with us. Still, emphasis in vacuum-tube technology is rapidly becoming a thing of the past. The emphasis of technology is in the field of microelectronics. Q4. What discovery proved to be the foundation for the development of the vacuum tube? Q5. Name the components which greatly increase the weight of vacuum-tube circuitry. Q6. What are the disadvantages of point-to-point wiring? Q7. What is a major advantage of modular construction? Q8. When designing vacuum-tube circuits, what characteristics of tubes must be taken into consideration? 1-4 SOLID-STATE DEVICES Now would be a good time for you to review the first few pages of NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies, as a refresher for solid-state devices. The transition from vacuum tubes to solid-state devices took place rapidly. As new types of transistors and diodes were created, they were adapted to circuits. The reductions in size, weight, and power use were impressive. Circuits that earlier weighed as much as 50 pounds were reduced in weight to just a few ounces by replacing bulky components with the much lighter solid-state devices. The earliest solid-state circuits still relied on point-to-point wiring which caused many of the disadvantages mentioned earlier. A metal chassis, similar to the type used with tubes, was required to provide physical support for the components. The solid-state chassis was still considerably smaller and lighter than the older, tube chassis. Still greater improvements in component mounting methods were yet to come. One of the most significant developments in circuit packaging has been the PRINTED CIRCUIT BOARD (pcb), as shown in figure 1-3. The pcb is usually an epoxy board on which the circuit leads have been added by the PHOTOETCHING process. This process is similar to photography in that copper-clad boards are exposed to controlled light in the desired circuit pattern and then etched to remove the unwanted copper. This process leaves copper strips (LANDS) that are used to connect the components. In general, printed circuit boards eliminate both the heavy, metal chassis and the point-to-point wiring. Figure 1-3.—Printed circuit board (pcb). Although printed circuit boards represent a major improvement over tube technology, they are not without fault. For example, the number of components on each board is limited by the sizes and shapes of components. Also, while vacuum tubes are easily removed for testing or replacement, pcb components are soldered into place and are not as easily removed. Normally, each pcb contains a single circuit or a subassembly of a system. All printed circuit boards within the system are routinely interconnected through CABLING HARNESSES (groups of wiring or ribbons of wiring). You may be confronted with problems in faulty harness connections that affect system reliability. Such problems are often caused by wiring errors, because of the large numbers of wires in a harness, and by damage to those wires and connectors. 1-5 Another mounting form that has been used to increase the number of components in a given space is the CORDWOOD MODULE, shown in figure 1-4. You can see that the components are placed perpendicular to the end plates. The components are packed very closely together, appearing to be stacked like cordwood for a fireplace. The end plates are usually small printed circuit boards, but may be insulators and solid wire, as shown in the figure. Cordwood modules may or may not be ENCAPSULATED (totally imbedded in solid material) but in either case they are difficult to repair. Figure 1-4.—Cordwood module. Q9. List the major advantages of printed circuit boards. Q10. What is the major disadvantage of printed circuit boards? Q11. The ability to place more components in a given space is an advantage of the _______. INTEGRATED CIRCUITS Many advertisements for electronic equipment refer to integrated circuits or solid-state technology. You know the meaning of the term solid-state, but what is an INTEGRATED CIRCUIT? The accepted Navy definition for an integrated circuit is that it consists of elements inseparably associated and formed on or within a single SUBSTRATE (mounting surface). In other words, the circuit components and all interconnections are formed as a unit. You will be concerned with three types of integrated circuits: MONOLITHIC, FILM, and HYBRID. MONOLITHIC INTEGRATED CIRCUITS are those that are formed completely within a semiconductor substrate. These integrated circuits are commonly referred to as SILICON CHIPS. 1-6 FILM INTEGRATED CIRCUITS are broken down into two categories, THIN FILM and THICK FILM. Film components are made of either conductive or nonconductive material that is deposited in desired patterns on a ceramic or glass substrate. Film can only be used as passive circuit components, such as resistors and capacitors. Transistors and/or diodes are added to the substrate to complete the circuit. Differences in thin and thick film will be discussed later in this topic. HYBRID INTEGRATED CIRCUITS combine two or more integrated circuit types or combine one or more integrated circuit types and DISCRETE (separate) components. Figure 1-5 is an example of a hybrid integrated circuit consisting of silicon chips and film circuitry. The two small squares are chips and the irregularly shaped gray areas are film components. Figure 1-5.—Hybrid integrated circuit. STATE-OF-THE-ART MICROELECTRONICS. Microelectronic technology today includes thin film, thick film, hybrid, and integrated circuits and combinations of these. Such circuits are applied in DIGITAL, SWITCHING, and LINEAR (analog) circuits. Because of the current trend of producing a number of circuits on a single chip, you may look for further increases in the packaging density of electronic circuits. At the same time you may expect a reduction in the size, weight, and number of connections in individual systems. Improvements in reliability and system capability are also to be expected. Thus, even as existing capabilities are being improved, new areas of microelectronic use are being explored. To predict where all this use of technology will lead is impossible. However, as the demand for increasingly effective electronic systems continues, improvements will continue to be made in state-of- the-art microelectronics to meet the demands. LARGE-SCALE INTEGRATION (lsi) and VERY LARGE-SCALE INTEGRATION (vlsi) are the results of improvements in microelectronics production technology. Figure 1-6 is representative of lsi. As shown in the figure, the entire SUBSTRATE WAFER (slice of semiconductor or insulator material) is 1-7 used instead of one that has been separated into individual circuits. In lsi and vlsi, a variety of circuits can be implanted on a wafer resulting in further size and weight reduction. ICs in modern computers, such as home computers, may contain the entire memory and processing circuits on a single substrate. Figure 1-6.—Large-scale integration device (lsi). Large-scale integration is generally applied to integrated circuits consisting of from 1,000 to 2,000 logic gates or from 1,000 to 64,000 bits of memory. A logic gate, as you should recall from NEETS, Module 13, Introduction to Number Systems, Boolean Algebra, and Logic Circuits, is an electronic switching network consisting of combinations of transistors, diodes, and resistors. Very large-scale integration is used in integrated circuits containing over 2,000 logic gates or greater than 64,000 bits of memory. Q12. Define integrated circuit. Q13. What are the three major types of integrated circuits? Q14. How do monolithic ICs differ from film ICs? Q15. What is a hybrid IC? Q16. How many logic gates could be contained in lsi? FABRICATION OF MICROELECTRONIC DEVICES The purpose of this section is to give you a simplified overview of the manufacture of microelectronic devices. The process is far more complex than will be described here. Still, you will be able to see that microelectronics is not magic, but a highly developed technology. 1-8 Development of a microelectronic device begins with a demand from industry or as the result of research. A device that is needed by industry may be a simple diode network or a complex circuit consisting of thousands of components. No matter how complex the device, the basic steps of production are similar. Each type of device requires circuit design, component arrangement, preparation of a substrate, and the depositing of proper materials on the substrate. The first consideration in the development of a new device is to determine what the device is to accomplish. Once this has been decided, engineers can design the device. During the design phase, the engineers will determine the numbers and types of components and the interconnections, needed to complete the planned circuit. COMPONENT ARRANGEMENT Planning the component arrangement for a microelectronic device is a very critical phase of production. Care must be taken to ensure the most efficient use of space available. With simple devices, this can be accomplished by hand. In other words, the engineers can prepare drawings of component placement. However, a computer is used to prepare the layout for complex devices. The computer is able to store the characteristics of thousands of components and can provide a printout of the most efficient component placement. Component placement is then transferred to extremely large drawings. During this step, care is taken to maintain the patterns as they will appear on the substrate. Figure 1-7 shows a fairly simple IC MASK PATTERN. If this pattern were being prepared for production, it would be drawn several hundred times the size shown and then photographed. The photo would then be reduced in size until it was the actual desired size. At that time, the pattern would be used to produce several hundred patterns that would be used on one substrate. Figure 1-8 illustrates how the patterns would be distributed to act as a WAFER MASK for manufacturing. Figure 1-7.—IC mask pattern. 1-9 Figure 1-8.—Wafer mask distribution. A wafer mask is a device used to deposit materials on a substrate. It allows material to be deposited in certain areas, but not in others. By changing the pattern of the mask, we can change the component arrangement of the circuit. Several different masks may be used to produce a simple microelectronic device. When used in proper sequence, conductor, semiconductor, or insulator materials may be applied to the substrate to form transistors, resistors, capacitors, and interconnecting leads. SUBSTRATE PRODUCTION As was mentioned earlier in this topic, microelectronic devices are produced on a substrate. This substrate will be of either insulator or semiconductor material, depending on the type of device. Film and hybrid ICs are normally constructed on a glass or ceramic substrate. Ceramic is usually the preferred material because of its durability. Substrates used in monolithic ICs are of semiconductor material, usually silicon. In this type of IC, the substrate can be an active part of the IC. Glass or ceramic substrates are used only to provide support for the components. Semiconductor substrates are produced by ARTIFICIALLY GROWING cylindrical CRYSTALS of pure silicon or germanium. Crystals are "grown" on a SEED CRYSTAL from molten material by slowly lifting and cooling the material repeatedly. This process takes place under rigidly controlled atmospheric and temperature conditions. Figure 1-9 shows a typical CRYSTAL FURNACE. The seed crystal is lowered until it comes in contact with the molten material-silicon in this case. It is then rotated and raised very slowly. The seed crystal is at a lower temperature than the molten material. When the molten material is in contact with the seed, it solidifies around the seed as the seed is lifted. This process continues until the grown crystal is of the desired length. A typical crystal is about 2 inches in diameter and 10 to 12 inches long. Larger diameter crystals can be grown to meet the needs of the industry. The purity of the material is strictly controlled to maintain specific semiconductor properties. Depending on the need, n or p impurities are added to produce the desired characteristics. Several other methods of growing crystals exist, but the basic concept of crystal production is the same. 1-10 Figure 1-9.—Crystal furnace. The cylinder of semiconductor material that is grown is sliced into thicknesses of .010 to .020 inch in the first step of preparation, as shown in figure 1-10. These wafers are ground and polished to remove any irregularities and to provide the smoothest surface possible. Although both sides are polished, only the side that will receive the components must have a perfect finish. Figure 1-10.—Silicon crystal and wafers. Q17. What are the basic steps in manufacturing an IC? Q18. Computer-aided layout is used to prepare _______ devices. Q19. What purpose do masks serve? Q20. What type of substrates are used for film and hybrid ICs? 1-11 Q21. Describe the preparation of a silicon substrate. FABRICATION OF IC DEVICES Fabrication of monolithic ICs is the most complex aspect of microelectronic devices we will discuss. Therefore, in this introductory module, we will try to simplify this process as much as possible. Even though the discussion is very basic, the intent is still to increase your appreciation of the progress in microelectronics. You should, as a result of this discussion, come to realize that advances in manufacturing techniques are so rapid that staying abreast of them is extremely difficult. Monolithic Fabrication. Two types of monolithic fabrication will be discussed. These are the DIFFUSION METHOD and the EPITAXIAL METHOD. DIFFUSION METHOD.—The DIFFUSION process begins with the highly polished silicon wafer being placed in an oven (figure 1-11). The oven contains a concentration impurity made up of impurity atoms which yield the desired electrical characteristics. The concentration of impurity atoms is diffused into the wafer and is controlled by controlling the temperature of the oven and the time that the silicon wafer is allowed to remain in the oven. This is called DOPING. When the wafer has been uniformly doped, the fabrication of semiconductor devices may begin. Several hundred circuits are produced simultaneously on the wafer. Figure 1-11.—Wafers in a diffusion oven. The steps in the fabrication process described here, and illustrated in figure 1-12, would produce an npn, planar-diffused transistor. But, with slight variations, the technique may also be applied to the production of a complete circuit, including diodes, resistors, and capacitors. The steps are performed in the following order: 1-12 Figure 1-12.—Planar-diffused transistor. 1. An oxide coating is thermally grown over the n-type silicon starting material. 2. By means of the photolithographic process, a window is opened through the oxide layer. This is done through the use of masks, as discussed earlier. 3. The base of the transistor is formed by placing the wafer in a diffusion furnace containing a p- type impurity, such as boron. By controlling the temperature of the oven and the length of time that the wafer is in the oven, you can control the amount of boron diffused through the window (the boron will actually spread slightly beyond the window opening). A new oxide layer is then allowed to form over the area exposed by the window. 4. A new window, using a different mask much smaller than the first, is opened through the new oxide layer. 5. An n-type impurity, such as phosphorous, is diffused through the new window to form the emitter portion of the transistor. Again, the diffused material will spread slightly beyond the window opening. Still another oxide layer is then allowed to form over the window. 6. By means of precision-masking techniques, very small windows (about 0.005 inch in diameter) are opened in both the base and emitter regions of the transistor to provide access for electrical currents. 7. Aluminum is then deposited in these windows and alloyed to form the leads of the transistor or the IC. (Note that the pn junctions are covered throughout the fabrication process by an oxide layer that prevents contamination.) 1-13 EPITAXIAL METHOD.—The EPITAXIAL process involves depositing a very thin layer of silicon to form a uniformly doped crystalline region (epitaxial layer) on the substrate. Components are produced by diffusing appropriate materials into the epitaxial layer in the same way as the planar- diffusion method. When planar-diffusion and epitaxial techniques are combined, the component characteristics are improved because of the uniformity of doping in the epitaxial layer. A cross section of a typical planar-epitaxial transistor is shown in figure 1-13. Note that the component parts do not penetrate the substrate as they did in the planar-diffused transistor. Figure 1-13.—Planar-epitaxial transistor. ISOLATION.—Because of the closeness of components in ICs, ISOLATION from each other becomes a very important factor. Isolation is the prevention of unwanted interaction or leakage between components. This leakage could cause improper operation of a circuit. Techniques are being developed to improve isolation. The most prominent is the use of silicon oxide, which is an excellent insulator. Some manufacturers are experimenting with single-crystal silicon grown on an insulating substrate. Other processes are also used which are far too complex to go into here. With progress in isolation techniques, the reliability and efficiency of ICs will increase rapidly. Thin Film Thin film is the term used to describe a technique for depositing passive circuit elements on an insulating substrate with coating to a thickness of 0.0001 centimeter. Many methods of thin-film deposition exist, but two of the most widely used are VACUUM EVAPORATION and CATHODE SPUTTERING. VACUUM EVAPORATION.—Vacuum evaporation is a method used to deposit many types of materials in a highly evacuated chamber in which the material is heated by electricity, as shown in figure 1-14. The material is radiated in straight lines in all directions from the source and is shadowed by any objects in its path. 1-14 Figure 1-14.—Vacuum evaporation oven. The wafers, with appropriate masks (figure 1-15), are placed above and at some distance from the material being evaporated. When the process is completed, the vacuum is released and the masks are removed from the wafers. This process leaves a thin, uniform film of the deposition material on all parts of the wafers exposed by the open portions of the mask. This process is also used to deposit interconnections (leads) between components of an IC. Figure 1-15.—Evaporation mask. 1-15 The vacuum evaporation technique is most suitable for deposition of highly reactive materials, such as aluminum, that are difficult to work with in air. The method is clean and allows a better contact between the layer of deposited material and the surface upon which it has been deposited. In addition, because evaporation beams travel in straight lines, very precise patterns may be produced. CATHODE-SPUTTERING.—A typical cathode-sputtering system is illustrated in figure 1-16. This process is also performed in a vacuum. A potential of 2 to 5 kilovolts is applied between the anode and cathode (source material). This produces a GLOW DISCHARGE in the space between the electrodes. The rate at which atoms are SPUTTERED off the source material depends on the number of ions that strike it and the number of atoms ejected for each ion bombardment. The ejected atoms are deposited uniformly over all objects within the chamber. When the sputtering cycle is completed, the vacuum in the chamber is released and the wafers are removed. The masks are then removed from the wafers, leaving a deposit that forms the passive elements of the circuit, as shown in figure 1-17. Figure 1-16.—Cathode-sputtering system. 1-16 Figure 1-17.—Cathode-sputtering mask. Finely polished glass, glazed ceramic, and oxidized silicon have been used as substrate materials for thin films. A number of materials, including nichrome, a compound of silicon oxide and chromium cermets, tantalum, and titanium, have been used for thin-film resistors. Nichrome is the most widely used. The process for producing thin-film capacitors involves deposition of a bottom electrode, a dielectric, and finally a top electrode. The most commonly used dielectric materials are silicon monoxide and silicon dioxide. Thick Film Thick films are produced by screening patterns of conducting and insulating materials on ceramic substrates. A thick film is a film of material with a thickness that is at least 10 times greater than the mean free path of an electron in that material, or approximately 0.001 centimeter. The technique is used to produce only passive elements, such as resistors and capacitors. PROCEDURES.—One procedure used in fabricating a thick film is to produce a series of stencils called SCREENS. The screens are placed on the substrate and appropriate conducting or insulating materials are wiped across the screen. Once the conducting or insulating material has been applied, the screens are removed and the formulations are fired at temperatures above 600 degrees Celsius. This process forms alloys that are permanently bonded to the insulating substrate. To a limited extent, the characteristics of the film can be controlled by the firing temperature and length of firing time. RESISTORS.—Thick-film resistance values can be held to a tolerance of ±10 percent. Closer tolerances are obtained by trimming each resistor after fabrication. Hundreds of different cermet formulations are used to produce a wide range of component parameters. For example, the material used for a 10-ohm-per-square resistor is quite different from that used for a 100-kilohm-per-square resistor. CAPACITORS AND RESISTOR-CAPACITOR NETWORKS.—Capacitors are formed by a sequence of screenings and firings. Capacitors in this case consist of a bottom plate, intraconnections, a dielectric, and a top plate. For resistor-capacitor networks, the next step would be to deposit the resistor material through the screen. The final step is screening and firing of a glass enclosure to seal the unit. 1-17 Hybrid Microcircuit A hybrid microcircuit is one that is fabricated by combining two or more circuit types, such as film and semiconductor circuits, or a combination of one or more circuit types and discrete elements. The primary advantage of hybrid microcircuits is design flexibility; that is, hybrid microcircuits can be designed to provide wide use in specialized applications, such as low-volume and high-frequency circuits. Several elements and circuits are available for hybrid applications. These include discrete components that are electrically and mechanically compatible with ICs. Such components may be used to perform functions that are supplementary to those of ICs. They can be handled, tested, and assembled with essentially the same technology and tools. A hybrid IC showing an enlarged chip is shown in figure 1-18. Figure 1-18.—Hybrid IC showing an enlarged chip. Complete circuits are available in the form of UNCASED CHIPS (UNENCAPSULATED IC DICE). These chips are usually identical to those sold as part of the manufacturer's regular production line. They must be properly packaged and connected by the user if a high-quality final assembly is to be obtained. The circuits are usually sealed in a package to protect them from mechanical and environmental stresses. One-mil (0.001-inch), gold-wire leads are connected to the appropriate pins which are brought out of the package to allow external connections. Q22. Name the two types of monolithic IC construction discussed. 1-18 Q23. How do the two types of monolithic IC construction differ? Q24. What is isolation? Q25. What methods are used to deposit thin-film components on a substrate? Q26. How are thick-film components produced? Q27. What is a hybrid IC? Q28. What is the primary advantage of hybrid circuits? PACKAGING TECHNIQUES Once the IC has been produced, it requires a housing that will protect it from damage. This damage could result from moisture, dirt, heat, radiation, or other sources. The housing protects the device and aids in its handling and connection into the system in which the IC is used. The three most common types of packages are the modified TRANSISTOR-OUTLINE (TO) PACKAGE, the FLAT PACK, and the DUAL INLINE PACKAGE (DIP). Transitor-Outline Package Transistor-Outline Package. The transistor-outline (TO) package was developed from early experience with transistors. It was a reliable package that only required increasing the number of leads to make it useful for ICs. Leads normally number between 2 and 12, with 10 being the most common for IC applications. Figure 1-19 is an exploded view of a TO-5 package. Once the IC has been attached to the header, bonding wires are used to attach the IC to the leads. The cover provides the necessary protection for the device. Figure 1-20 is an enlarged photo of an actual TO-5 with the cover removed. You can easily see that the handling of an IC without packaging would be difficult for a technician. 1-19 Figure 1-19.—Exploded TO-5. Figure 1-20.—TO-5 package. The modified TO-5 package (figure 1-21) can be either plugged into view (A) or embedded in view (B) a board. The embedding method is preferred. Whether the package is plugged in or embedded, the interconnection area of the package leads must have sufficient clearance on both sides of the board. The plug-in method does not provide sufficient clearance between pads to route additional circuitry. When the packages are embedded, sufficient space exists between the pads because of the increased diameter of the interconnection pattern, shown at the right in view (B) for additional conductors. 1-20

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