Silicon Integrated Circuits

Silicon Integrated Circ-uits Much of the early technology of silicon integrated circuits grew out of the physics, chemistry, and metallurgy of discrete transistors, includinga number of key Bell Laboratories developments: oxide-masked diffusion, photolithographic methods, thermocompression bonding, and epitaxial deposition.Important Bell Laboratories innovations for integrated circuits encompassed beam lead sealed junction devices, ion implantation, complementary bipolar integrated circuits, the first metal-oxide-semiconductor (MOS) field-effect transistor, charge-coupled devices, and the self-aligned silicon gateMOS transistor. Also important to the complexity and power of modern integrated circuits areLaboratories Bell advances in the areas of mask making with electron beam exposure systems and computer aids to design. I. INTRODUCTION This chapter traces the development of silicon integrated circuits (SICs) in the Bell System since the mid-1950s. It is really three stories: technology development, innovation in device structures, and applications. We begin with early technology development and have intertwined the stories to show the constant and continuing interdependence of technology, device and systems needs, and opportunity. The developments encompassed Bell Laboratories people from many organizations in research, device devel opment, and systems development, and Western Electric personnel at its Engineering Research Center in Princeton, New Jersey and at the man ufacturing locations in Allentown and Reading, Pennsylvania. The development of integrated circuits (ICs) in the Bell System was aimed not only at improved performance, but at lower cost and higher reliability. The impact of ICs on telecommunications, the semiconductor and electronics industries, and on society as a whole, was profound. It started with the introduction of small-scale and medium-scale integrated (SSI and MSI) circuits during the 1960s. Even more profound changes Principal authors: J.M. Goldey, J. H. Forster, and B. T. Murphy 101 102 Engineering and Science in the Bell System occurred as the full implications unfolded for low-cost, large-scale integrated (LSI) and very large-scale integrated (VLSI) circuits, such as microprocessors, memories, and custom circuits tailored for specific applications. II. EARLY TECHNOLOGY DEVELOPMENTS SIC technology is a complex composite of chemical, electronic., me chanical, metallurgical, and optical technologies. It is, nevertheless, not a new technology unto itself, but it is part of an evolving silicon semiconductor device technology, which initially supported transistors and diodes only. The history of this technology up to the early 1960s, when discrete tran sistors reached a state of maturity, is described in detail in Chapter 1. Since then, SICs have dominated the evolution of the technology,. as covered in this chapter. However, it would be difficult, if not impossible, to describe the history of SIC technology without summarizing certain parts of the preceding technology, since most of the basic innovations used to achieve complex SI Cs were initially conceived as a means for improving transistors. While many aspects of SIC technology are based on Bell Laboratories research and development, this is not to imply that other organizations did not make significant contributions, for indeed they did. Notable among these companies were Texas Instruments and Fairchild Industries, and somewhat later, Intel. It was J. S. Kilby at Texas Instruments who first recognized that silicon technology could produce in one wafer of silicon not only transistors and diodes, but also resistors and capacitors, and that by interconnecting these elements, circuit functions could be realized. 1 R. N. Noyce at Fairchild contributed the idea of using deposited aluminum films on a planar structure to form the interconnections. 2 These early developments were chronicled in greater detail in the IEEE Spectrumin 1976-a truly interesting story. 3 The work led to the first commercial offering of production ICs in 1961 by both Texas Instruments and Fair child. The initial applications were almost exclusively for military purposes; the military funded much of the early IC research and development work. Most Bell Laboratories work, like that of Fairchild, however, was not government funded. In this chapter, we restrict ourselves, with a few exceptions, to a review of Bell Laboratories work, and begin with an overview of certain relevant features of transistor technology. Figure 2-1 shows a cross section of a simple bipolar SIC consisting of a diode, transistor, and diffused resistor, interconnected by thin metal films deposited on an insulating layer over the silicon. Note in the drawing that there are multiple layers of both n-doped and p-doped material, and that interconnection is provided by well-defined paths of metal connected to the silicon through holes defined in the insulator. A key process in SIC technology is the introduction of dopant impuirities into the silicon-the right amount in the right place-to form active and Silicon Integrated Circuits 103 SILICON NITRIDE/ METALIZATION, ' /_..-SI LICON OIOXIDE E;;:::;~ALr •.•... LAYER -i--~-----,..-, n+ SUBSTRATE,.__p-TYPE ___________________ _ Fig. 2-1. Silicon bipolar IC schematic with a diode at left, transistor at the c,enter, and diffused resistor at right. There are multiple layers of both n-doped and p-doped material, and interconnections are provided by well-defined paths of metal connected to the silicon through holes defined in the SiN over SiO 2 insulating film. Electrical isolation between the circuit elements is achieved by p-type diffused regions extending from the surface to the p-type substrate. passive components. Basic to this operation is the oxide-masked diffusion process. The pioneering work on diffusion for silicon devices was carried 4 5 out by C. S. Fuller and J. A. Ditzenberger. ' In this process, impurities, generally in vapor form, surrounding the silicon at high temperature, enter and then move into the semiconductor at a rate fast enough to be practical, yet slow enough to be precisely controllable. The amount of impurity and the distance diffused can be varied over several orders of magnitude by adjusting the time, temperature, or vapor concentration. For example, dif fusion depths are precisely controllable over a range from a small fraction of 1 micrometer (µm) to 20 µm or greater; surface concentra1ions are 3 available from 10 16 to 1020 atoms per cm , with the total number of diffused atoms between 10 11and 10 16 atoms per cm 2 of silicon area. Because of the good control available, it is easy to change the conductivity type of a piece of silicon several times by using increasingly higher concentrations of different impurities and thus fabricate transistors and other components. Oxide masking, first studied by C. J. Frosch and L. Derick, is a process by which the diffusion of impurities is limited to certain portions of the silicon slice, thus forming regions for specific functions, e.g., transistor base, resistor, etc. 6 Where oxide is present, no diffusion occurs, and where it is absent, diffusion of dopants into the silicon proceeds freely. Oxide masking in combination with photolithography offers the opportunity to control areas to very fine tolerances. For example, by 1975, oxide windows as small as 7 .5 µm wide were common, and much smaller windows followed thereafter. It was recognized from the earliest days that since diffusion is a batch process (i.e., many slices, each with many devices, can be diffused si multaneously), it would be inherently a low-cost technology, a key feature for the economic manufacture of ICs. 104 Engineering and Science in the Bell System Initially, control of oxide geometry was achieved by such crude methods as using dots of black wax to protect the oxide from its etchants. The foundations of modern IC photolithographic technology were laid by J. Andrus, who first used photosensitive etch-resistant polymers (photoresists) to define patterns in the masking oxide. 7 Although many advances have been made in the mask-making art (see section VII), and significant im provements in photoresist materials and alignment apparatus have been achieved, the basic photoresist processes used throughout the world in the fabrication of ICs are essentially those of Andrus. The first application of photo-defined oxide-masked diffusion to devices was in the semiconductor stepping device, developed in the mid-1950s by I. M. Ross, L.A. D'Asaro, and H. H . Loar. 8 [Fig. 2-2] A photo of this device is shown in Fig. 2-3(a) and a schematic in Fig. 2-3(b). It consists of four coupled special-purpose p-n-p-n transistors that perform the mem ory and space transfer functions of a four-element stepping circuit, and represents, in the eyes of many, the world's first SIC. Aluminum contacts to silicon are used extensively in SI Cs. As discussed in Chapter 1, aluminum metalization was used early in the development of transistors. While aluminum, which is an acceptor in silicon, is a natural choice for contacting p-type material, S. L. Matlow and E. L. Ralph of Hoffman Electronics Corporation demonstrated that aluminum can also form ohmic contacts to n+ layers on silicon .9 G. E. Moore and Noyce at Fairchild were the first to use aluminum to contact both n and p regions of a diffused silicon transistor. 10 Fig. 2-2. I. M. Ross (left) and L. A D' Asaro (right), who, along with H. H. Loar, developed the semic onductor stepping transistor . Silicon Integ rated Circuits 105 (a) (b) Fig. 2-3. The silicon stepping transistor, the first device to use photo defined oxide-masked diffusion. (a) The light regions are n-type and th e dark are p-type. Both were formed by oxide-masked diffusion. (D ' Asaro, IRE WESCON Conv. Rec., Pt. 3-El ectron Div. (1959): 42] (b) Schemati c of the silicon stepping transistor , consisting of four p -n-p-n diodes in tegrated in a single structure. With electrodes attached, this device operated as a four-stage counter up to a rate of 2 X 106 counts per second.

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