05-10-2012, 11:08 AM
Integrated Circuit Fabrication Process
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Do you ever wonder how the processor in your computer was actually fabricated? How is it that engineers can put
hundreds of millions of transistors into one device that measures only a few centimeters on a side (and with so few
errors) so the devices actually function as expected?
Devices such as modern computer processors and semiconductor memories fall into a class known as integrated
circuits (IC). They are so named because all of the components in the circuit (and their “wires”) are fabricated
simultaneously onto a circuit during the manufacturing process. This is in contrast to circuits where each component
is fabricated separately and then soldered or wired together onto a common board (such as those you probably build
in your lab classes). Integrated circuits were first demonstrated independently by Jack Kilby at Texas Instruments and
Robert Noyce at Fairchild Semiconductor in the late 1950s. Once developed, the ability to manufacture components
and their connections in parallel with good quality control meant that circuits with thousands (then millions, then billions)
of components could be designed and built reliably.
Semiconductor Processing Basics
All mainstream semiconductor integrated-circuit processes start with a thin slice of silicon, known as a substrate or
wafer. This wafer is circular and ranges from 4 to 18 inches in diameter and is approximately 1 mm thick (hence its
name). Each wafer is cut from a single crystal of the element silicon and polished to its final thickness with atomic
smoothness (Fig. TF7-1). Most circuit designs (like your processor) fit into a few square centimeters of silicon area;
each self-contained area is known as a die. After fabrication, the wafer is cut to produce independent, rectangular dies
often known as chips, which are then packaged to produce the final component you buy at the store.
Fabricating a Diode
In Section 2-7, we discussed the functional performance of the diode as a circuit component. Here, we will examine
briefly how a diode is fabricated. Similar but more complex multi-step processes are used to make transistors and
integrated circuits. Conceptually, the simplest diode is made from two slabs of silicon—each implanted with different
atoms—pressed together such that they share a boundary (Fig. TF7-5).
have been implanted with atoms (known as impurities) that increase or decrease the number of electrons capable of
flowing freely through the silicon. This changes the semiconducting properties of the silicon and creates an electrically
active boundary (called a junction) between the n and the p areas of silicon. If both the n and p pieces of silicon are
connected to metal wires, this two-terminal device exhibits the diode i–v curve shown in Fig. 2-35©.
Figure TF7-6 shows the process for making a single diode. Only one step needs further definition: oxidation. During
oxidation, the silicon wafer is heated to > 1000◦C in an oxygen atmosphere. At this temperature, the oxygen atoms
and the silicon react and form a layer of SiO2 on the surface (this layer is often called an oxide layer). SiO2 is a type
of glass and is used as an insulator.