16 The materials science of semiconductors Si in normal microprocessors? There are many reasons but the primary ones are the lack of a good insulator and good contacts, and the relative fragility of GaAs. These problems have never been solved, while all of the major problems facing application of Si, except its inability to emit light, have been overcome. Thus, when one considers the performance of an electronic material, one must consider that material in the context of a given application and include all aspects of performance in the analysis
16 Si in normal microprocessors? There are many reasons but the primary ones are the lack of a good insulator and good contacts, and the relative fragility of GaAs. These problems have never been solved, while all of the major problems facing application of Si, except its inability to emit light, have been overcome. Thus, when one considers the performance of an electronic material, one must consider that material in the context of a given application and include all aspects of performance in the analysis. The Materials Science of Semiconductors
An Environment of Challenges 1.7 SUMMARY POINTS Microelectronics traces its roots to vacuum tube electronics of the early 20th Practical transistors, invented in the late 1940,s and early 1950s form the basis of current circuit technologies. Significant problems arise as device dimensions shrink concerning the current density carried in a device or in interconnects, the scale of dielectric layers in ate capacitors, and in the number of dopant atoms present in the control volume of a device Microelectronic devices have contained increasing numbers of devices over time following moore’sLaw Electronic materials are defined primarily by their purity and performance rather than by the elements that make them up Raw materials cost is generally a small part of the price of electronic-grade materials due to the high cost of purification Impurity concentrations of lower than parts per billion are sometimes required in The performance of a material in a semiconductor device includes its optoelectro- nic, mechanical, and chemical properties. Compatibility with surrounding materials is essential
An Environment of Challenges 17 1.7 SUMMARY POINTS • Microelectronics traces its roots to vacuum tube electronics of the early 20th century. • Practical transistors, invented in the late 1940’s and early 1950’s form the basis of current circuit technologies. • Significant problems arise as device dimensions shrink concerning the current density carried in a device or in interconnects, the scale of dielectric layers in gate capacitors, and in the number of dopant atoms present in the control volume of a device. • Microelectronic devices have contained increasing numbers of devices over time following Moore’s Law. • Electronic materials are defined primarily by their purity and performance rather than by the elements that make them up. • Raw materials cost is generally a small part of the price of electronic-grade materials due to the high cost of purification. • Impurity concentrations of lower than parts per billion are sometimes required in semiconductors. • nic, mechanical, and chemical properties. Compatibility with surrounding materials is essential. The performance of a material in a semiconductor device includes its optoelectro-
18 The materials science of semiconductors 1. 8 HOMEWORK PROBLEMS 1. Using resources at a local library, write a one-paragraph summary of an nvention, material, or process used in current microelectronic technolog A single atomic layer of oxygen atoms(-5x10 atoms cm2)collects on the surface of a l cm wafer of silicon the silicon is then melted and all of the oxygen dissolves in it. Upon cooling silicon freezes into a single crystal again. Assuming a uniform final oxygen concentration in the silicon of 0. 1 parts per million, how thick would the original silicon wafer have to have been? The atomic density of silicon is 5x10-atoms cm 3. Suppose that all circuits shown in Figure 1. 2. for Moore's Law correspond to I cm chip areas. Thus, 10devices would correspond to a device area of microns x 10 microns. Assuming moore's Law continues at its current rate of change of circuit density with time, in approximately what year will the average device area reach 1 nm x I nm(about 16 atoms area)? 4. What is the lowest doping density in units of cmi that can be obtained in the I nm x I nm device if the thickness of the device is 4 atomic layers(I nm)as 5. Assume that electronic grade Pt can be purchased for $3000 g" and given that the atomic weight of Pt is 195 g mol and its density is 21. 4 g cm". Estimate the cost of a 30 nm layer of Pt used in a I cm microprocessor assuming that it coats the entire area of the device
18 1.8 HOMEWORK PROBLEMS 1. Using resources at a local library, write a one-paragraph summary of an invention, material, or process used in current microelectronic technology. 2. A single atomic layer of oxygen atoms (~5x1014 atoms cm-2) collects on the surface of a 1 cm2 wafer of silicon. The silicon is then melted and all of the oxygen dissolves in it. Upon cooling silicon freezes into a single crystal again. Assuming a uniform final oxygen concentration in the silicon of 0.1 parts per million, how thick would the original silicon wafer have to have been? The atomic density of silicon is 5x1022 atoms cm-3. 3. 2 6 microns x 10 microns. Assuming Moore’s Law continues at its current rate of change of circuit density with time, in approximately what year will the average device area reach 1 nm x 1 nm (about 16 atoms area)? 4. -3 well? 5. Assume that electronic grade Pt can be purchased for $3000 g-1 and given that the atomic weight of Pt is 195 g mol-1 and its density is 21.4 g cm-3. Estimate the cost of a 30 nm layer of Pt used in a 1 cm2 microprocessor assuming that it coats the entire area of the device. 1 cm chip areas. Thus, 10 devices would correspond to a device area of 10 Suppose that all circuits shown in Figure 1.2. for Moore’s Law correspond to 1 nm x 1 nm device if the thickness of the device is 4 atomic layers (1 nm) as What is the lowest doping density in units of cm that can be obtained in the The Materials Science of Semiconductors
An Environment of Challenges 1. 9 SUGGESTED READINGS REFERENCES Ball, Philip, Made to Measure: new materials for the 21st century, Princeton: Princeton University Press, 1997. Braun, Ernest and Macdonald, Stuart, Revolution miniature. 2nd ed. Cambrid Cambridge University Press, 1982 Cahn, R W, The Coming of Materials Science, Amsterdam: Pergamon, 2001 Dummer, G.w.A., Electronic Inventions and Discoveries: Electronics from its earliest beginnings to the present day, 3rd ed. Oxford: Pergamon, 1983 Seitz, Frederick and Enspruch, Norman G, Electronic Genie: The Tangled History of silicon Urbana: University of Illinois Press, 1998 References. [1 Sheats, J ," Organic electroluminescent devices". Science, 1996, 273: 884-8 2]Forrest, Stephen R, "The road to high efficiency organic light emitting devices Organic Electronics 2003: 4: 45-48 3 D'Andrade, Brian and Brown, Julie J,"White phosphorescent organic light emitting Byrd, James C, Desjardins, Daniel D; Forsythe, Eric W and Girolamo, Henry J., Proc. of the SPIe2006,6225:6225141. [4 Cree Research result reported by Whitaker, Tim on optics. org news for June 22, 2006
An Environment of Challenges 19 1.9 SUGGESTED READINGS & REFERENCES Suggested Readings: Ball, Philip, Made to Measure: new materials for the 21st century, Princeton: Princeton University Press, 1997. Braun, Ernest and Macdonald, Stuart, Revolution in Miniature, 2nd ed. Cambridge: Cambridge University Press, 1982. Cahn, R.W., The Coming of Materials Science, Amsterdam: Pergamon, 2001. Dummer, G.W.A., Electronic Inventions and Discoveries: Electronics from its earliest beginnings to the present day, 3rd ed. Oxford: Pergamon, 1983. Seitz, Frederick and Enspruch, Norman G., Electronic Genie: The Tangled History of Silicon. Urbana: University of Illinois Press, 1998. References: [1] Sheats, J.; “Organic electroluminescent devices”. Science, 1996; 273: 884-8. [2] Forrest, Stephen R.; “The road to high efficiency organic light emitting devices”, Organic Electronics 2003; 4: 45-48. [3] D’Andrade, Brian and Brown, Julie J., “White phosphorescent organic light emitting devices for display applications”, in Defense, Security, Cockpit and Future Displays II, Byrd, James C; Desjardins, Daniel D; Forsythe, Eric W.; and Girolamo, Henry J., Proc. of the SPIE 2006; 6225: 622514-1. [4] Cree Research result reported by Whitaker, Tim on optics.org news for June 22, 2006
Chapter 2 THE PHYSICS OF SOLIDS Before beginning a general discussion of electronic devices and the nore complex aspects of semiconductors and other electronic aterials, it is helpful to have an idea of their physics, especially their electronic structure. This chapter provides a partial review of the physics of solids. The nature of materials is determined by the interaction of their valence electrons with their charged nuclei and core electrons. This determines how elements react with each other what structure the solid prefers, its optoelectronic properties and all other aspects of the material. The following sections describe the general method for understanding and modeling the energies of bands of lectronic states in solids. a more detailed discussion of semiconductor bonding is provided in Chapter 5 2.1 ELECTRONIC BAND STRUCTURES OF SOLIDS here are two approaches hen considering how the weakly bound(valence electrons interact with the positively charged atomic cores(everything about the atom except the valence electrons)and with other valence electrons in a solid. We will consider first the direct approach of solutions to the differential equations that describe the motion of electrons in their simplest form and the consequences of this behavior. This requires many simplifying assumptions but gives a general idea for the least complex problems. The second approach is to follow the electronic orbitals of the atoms as they mix themselves into molecular states and then join to form
Chapter 2 THE PHYSICS OF SOLIDS Before beginning a general discussion of electronic devices and the more complex aspects of semiconductors and other electronic materials, it is helpful to have an idea of their physics, especially their electronic structure. This chapter provides a partial review of the physics of solids. The nature of materials is determined by the interaction of their valence electrons with their charged nuclei and core electrons. This determines how elements react with each other, what structure the solid prefers, its optoelectronic properties and all other aspects of the material. The following sections describe the general method for understanding and modeling the energies of bands of electronic states in solids. A more detailed discussion of semiconductor bonding is provided in Chapter 5. 2.1 ELECTRONIC BAND STRUCTURES OF SOLIDS There are two approaches taken when considering how the weakly bound (valence) electrons interact with the positively charged atomic cores (everything about the atom except the valence electrons) and with other valence electrons in a solid. We will consider first the direct approach of solutions to the differential equations that describe the motion of electrons in their simplest form and the consequences of this behavior. This requires many simplifying assumptions but gives a general idea for the least complex problems. The second approach is to follow the electronic orbitals of the atoms as they mix themselves into molecular states and then join to form