Lecture 20: Introduction to semiconductor manufacturing Contents 1 Introduction 1 2 Integrated circuits 5 3 Device miniaturization 7 4 Challenges in IC manufacturing 11 5 IC manufacturing stages 17 1 Introduction There are a wide variety of electronic devices starting with the simple pn junction diodes, transistors, and extending into opto-electronic devices like LEDs, lasers, and solar cells. These are made from a variety of semiconduc- tor materials though silicon is the dominant material in the micro electronics industry. Other semiconductors are used, especially for optical devices, since silicon is an indirect band gap material. How these devices are manufactured and assembled to form useful devices, like computers, tablets, cell phones, and a host of other microelectronic devices is a critical part of the industry. This is especially important, since, with increased miniaturization, devices are becoming smaller and have greater functionality. Other form factors like battery life, operating power, heat generation and dissipation, also become critical, especially for mobile computing. Understanding the various steps behind fabrication of these devices is important to understand the challenges facing the semiconductor industry. The first electronic device invented was the vacuum tube, by Lee Deforest in 1906. This was the triode, called audion, and the schematic of the device 1 MM5017: Electronic materials, devices, and fabrication Figure 1: Schematic of the vacuum tube (a) triode and (b) diode. Sources http://en.wikipedia.org/wiki/Triode and http://en.wikipedia.org/wiki/Diode is shown in figure 1. Before the invention of the triode, the two terminal vacuum tube diode was postulated by Thomas Edison. The schematic of the diode is shown in figure 1. In a diode, the central cathode is heated to give electrons, a process called thermionic emission. The electrons that are generated, are accelerated to the anode and produce current. Current in the reverse direction, from an- ode to cathode, is not possible due to the biasing of the device. The triode improves upon this arrangement by using a third electrode, grid, which can independently control the current from the cathode to the anode. This en- ables the vacuum tube to perform two functions, switching and amplification (forerunner to the modern solid state transistors). The drawbacks of vacuum tubes are that they are huge and bulky. They are also not energy efficient since the glass tubes can lose vacuum and also consume a lot of power. The invention of the vacuum tube started the modern electronics indus- try. It made possible commercial devices like the radio and television. The world's first electronic computer, ENIAC, was also made using vacuum tubes. ENIAC expands as Electronic N umeric I ntegrator And C alculator. It was first demonstrated in the Moore school of Pennsylvania in 1947. The ENIAC was a huge computer compared to modern systems, as seen in figure 2. Some of its statistics are shown in table 1. It was a massive machine occupying a large area of 1500 sq feet, with around 18000 vacuum tubes. It also con- 2 MM5017: Electronic materials, devices, and fabrication Figure 2: Two programmers operating the ENIAC. Typical statistics of the computer are listed in table 1. Source http://en.wikipedia.org/wiki/ENIAC Table 1: Some typical statistics of the ENIAC. Compared to the modern computer, it was a massive machine. SourceMicrochip fabrication - Peter van Zant. Size, ft 30 × 50 Weight, tons 30 Vacuum tubes, nos. 18,000 Resistors, nos 70,000 Capacitors, nos 10,000 Switches, nos 6000 Power requirement, W 150,000 Cost (in 1940) $ 400,000 3 MM5017: Electronic materials, devices, and fabrication Figure 3: Schematic of the first transistor developed in Bell labs. Adapted from Microchip fabrication - Peter van Zant. sumed a large amount of power and consequently generated a lot of heat. This made it highly unreliable with the longest operating period, without any vacuum tube failure, of 5 days (around 116 hours). The large size and poor performance of the ENIAC was due to the presence of vacuum tubes, which had to individually wired to achieve the desired performance. For any size reduction the triode size had to be reduced. This was made possible by the development of the modern solid state transistor, which started the revolution in micro electronics. The first solid state based triode i.e. the transistor was invented in Bell Labs in 1947. It was invented by John Bardeen, William Schokley, and Walter Brattain. The device was an electrical amplifier based on germanium, shown in figure 3. The device functioned similar to the vacuum tube triode, but was smaller, lighter, and had a much lower power requirement. A replica of the first transistor is shown in figure 4. The inventors of the solid state transistor won the Nobel prize in Physics for their work in 1956. John Bardeen then moved to University of Illinois at Urbana-Champaign where he won a sec- ond Nobel prize in Physics in 1972 for his work with Leon Cooper and John Schrieffer on a theory of superconductivity (BCS theory). Thus, he became the only person to win two Nobel prizes in Physics. The invention of the transistor started the era of solid state devices. Discrete electrical components like transistors, diodes, capacitors, and resistors can be fabricated and then joined to form the required device. While these were still smaller than vacuum tube devices, true miniaturization could only be achieved by integrating the various devices in one wafer. 4 MM5017: Electronic materials, devices, and fabrication Figure 4: Replica of the first transistor from Bell Labs. Source http://en.wikipedia.org/wiki/History of the transistor 2 Integrated circuits The first attempt in fabricating integrated circuits (ICs) was made by Jack Kilby from Texas Instruments. In 1959, he integrated transistors, diodes, and capacitors (a total of 5 components) on a single wafer of Ge. Resistors were formed by using the natural resistivity of Ge and the device were connected by external wiring. A schematic of Kilby circuit is shown in figure 5 and a picture of the original Kilby circuit is shown in figure 6. A modification to the Kilby IC was made by Robert Noyce, working in Fairchild Camera. This was based on an earlier design of a solid state device by Jean Horni, also working at Fairchild Camera, that was made using Si. A top down picture of the transistor is shown in figure 7. The advantage of using Si is that it naturally forms an oxide layer, which can help in getting a planar profile. The Horni transistor design also had evaporated aluminum as electrical contacts so that external wiring was not required. Robert Noyce was then able to fabricate the individual devices on a single wafer of Si to form the first monolith IC. The design of the Noyce IC is shown in figure 8. A monolith integrated circuit is defined as a set of electronic circuits that are fabricated on a single chip. Usually, silicon is the material of choice for the chip, but not always. For optoelectronic devices, GaAs is mainly used, as it is a direct band gap semiconductor and can be used as the substrate for growing other materials on top. The advantage of integrating the circuits on 5 MM5017: Electronic materials, devices, and fabrication Figure 5: The design of the Jack Kilby IC. Except for the metal wires, the rest of the IC was fabricated on a single wafer of Ge. Adapted from Microchip fabrication - Peter van Zant. Figure 6: Picture of the first IC. Source http://en.wikipedia.org/wiki/Jack Kilby 6 MM5017: Electronic materials, devices, and fabrication Figure 7: The Horni transistor made using Si with evaporated metal lines for electrical contact. Adapted from Microchip fabrication - Peter van Zant. a single chip is that it is much smaller than joining discrete devices. Also, the small distance that the carriers have to travel from one component to the other increases the speed of the device and reduces electrical losses (less power consumption). Initial ICs that were introduced in 1960s had only a few components but over time the number of components (usually measured as the number of transistors) have rapidly increased and correspondingly the individual transistor size has also reduced. There are essentially two kinds of improvements 1. Process - this refers to fabrication of devices and structures in smaller dimensions. In the simplest form, the original structure is not modified but only the individual components are scaled down. 2. Structure - this refers to newer device designs for greater performance. The new design makes use of the reduced size that allows to pack more components in the same area. 3 Device miniaturization Integrated circuits are characterized by the size of the individual device com- ponents and the density (number per unit area) of components. The feature size for a IC refers to the smallest dimensions in the device. Typical devices now have dimensions of tens of nm. This can be compared to the original device where dimensions were of the order of µm. This reduction in size correlates with a large increase in number of components. In 1965, Gordon Moore (one of the founders of Intel, the other two being Robert Noyce and 7 MM5017: Electronic materials, devices, and fabrication Figure 8: The patent application of the Robert Noyce IC showing the circuit design. A top-down and side view are included. Source http://www.computerhistory.org/semiconductor/timeline/1959-Noyce.html . 8 MM5017: Electronic materials, devices, and fabrication Figure 9: Semi-log plot of transistor count vs. manufacture year. Source http://en.wikipedia.org/wiki/Moore's law Andrew Grove) came up with a prediction that the number of transistors in a IC will roughly double every 18 months (the original prediction was every 2 years).