Integrated circuit

An integrated circuit (IC), also known as a chip, microchip, or microelectronic circuit, is a semiconductor wafer (usually silicon) on which thousands or millions of resistors, capacitors, diodes, and transistors are fabricated. ICs are the building block for modern electronic devices. Each component fabricated on an IC is usually microscopic, with the resulting circuit, a monolithic chip that is also small, often only a few square millimeters or centimeters. An IC can provide many functions including an amplifier, oscillator, timer, counter, logic gate, computer memory, microcontroller, or microprocessor.

Early transistors came in a separate plastic package, and each circuit consisted of discrete transistors, capacitors, and resistors. Due to the large size of these components, early ICs could only hold a few of them wired together on the circuit board. With advances in solid-state electronics, the development of solid-state electronics made it easier to reduce the size of components. In the late 1950s, inventors Jack Kilby of Texas Instruments, Inc. and Robert Noyce of Fairchild Semiconductor Corporation developed techniques to lay thin paths of metal on devices and have them function as wires. These solutions made it possible to connect small electrical devices leading to modern ICs.

ICs have undergone several generations of advancements and developments according to their design assembly, such as the following:

• Small scale integration (SSI)—ten to hundreds of transistors per chip

• Medium scale integration (MSI)—hundreds to thousands of transistors per chip
• Large scale integration (LSI)—thousands to several hundred thousand transistors per chip
• Very large scale integration (VLSI)—up to 1 million transistors per chip
• Ultra large scale integration (ULSI)—this represents a modern IC with millions and billions of transistors per chip

ICs are manufactured in bulk using a single thin slice of silicon and then cut apart into individual IC chips. Manufacturing takes place in a tightly controlled environment, such as a clean room. Workers wear lint-free garments, gloves, and coverings for their heads and feet.

To prepare the silicon wafer, it is held in a vacuum chamber and heated to its melting point of about 2550°F (1400°C). As the heated region melts, impurities in the silicon become mobile. The heating coil slowly moves down the length of the wafer, carrying the impurities along with the melted region. The bottom is then sliced off, leaving a cylindrical ingot of purified silicon. A thin, round wafer of silicon is cut off the ingot using a precise cutting machine called a wafer slicer. The surface on which the integrated circuits are to be formed is polished and then coated with a layer of silicon dioxide to form an insulating base and to prevent any oxidation of the silicon which would cause impurities. The silicon dioxide is formed by subjecting the wafer to superheated steam under several atmospheres of pressure to allow the oxygen in the water vapor to react with the silicon. Controlling the temperature and length of exposure controls the thickness of the silicon dioxide layer.

The complex and interconnected designs required for ICs are prepared in a process similar to that used to make printed circuit boards. However, the dimensions are much smaller, and there are many layers superimposed on top of each other. The design of each layer is prepared using software, and the image is made into a mask which is optically reduced and transferred to the surface of the wafer. The mask is opaque in certain areas and clear in others. It has the images for all of the several hundred integrated circuits to be formed on the wafer. Photoresist material is placed in the center of the silicon wafer, and the wafer is spun rapidly to distribute the photoresist over the entire surface. The photoresist is then baked to remove the solvent. The coated wafer is then placed under the first layer mask and irradiated with light. Because the spaces between circuits and components are so small, ultraviolet light is used. Beams of electrons or x-rays are also sometimes used to irradiate the photoresist. The mask is removed and portions of the photoresist are dissolved. The uncovered areas are then either chemically etched to open up a layer or are subjected to chemical doping to create a layer of P or N regions.

Atomic diffusion is a method of adding dopants to create a layer of P or N regions is atomic diffusion. Wafer batches are placed in an oven made of a quartz tube surrounded by a heating element. The wafers are heated to an operating temperature of about 1500-2200°F (816-1205°C), and the dopant chemical is carried in on an inert gas. As the dopant and gas pass over the wafers, the dopant is deposited on the hot surfaces left exposed by the masking process. This method is good for doping relatively large areas but is not accurate enough for smaller areas.

Another method to add dopants is ion implantation. A dopant gas, like phosphine or boron trichloride, is ionized to provide a beam of high-energy dopant ions that are fired at specific regions of the wafer. The ions penetrate the wafer and remain implanted. The depth of penetration can be controlled by altering the beam energy, and the amount of dopant can be controlled by altering the beam current and time of exposure. This method is more precise and does not require masking. However, it is significantly slower than the atomic diffusion process.

ICs can be linear (analog), digital, or a combination of the two to suit the intended application. Analog or linear ICs have continuously variable outputs that depend on the input signal level. With this IC type, the output signal level is a linear function of the input signal level. Analog ICs usually use only a few components and are quite simple. Linear ICs are used as audio-frequency (AF) and radio-frequency (RF) amplifiers. The operational amplifier (op amp) is a commonly used device for these applications. Another common application of an analog IC is as a temperature sensor. Linear ICs can be programmed to turn various devices on or off once a signal reaches a particular value. Applications include the following:

• air conditioners

• heaters
• ovens

Unlike analog ICs, digital ICs don't operate over continuous signal amplitudes. Instead, the only operate at a few defined (discrete) levels or states. The fundamental building blocks of digital ICs are logic gates, which work with binary data. Digital ICs are now used in an increasing number of applications:

• computers
• enterprise networks
• modems
• frequency counters

Logic gates can be packaged into their own integrated circuit. Some logic gate ICs might contain a handful of gates in one package. They can be connected inside an IC to create timers, counters, latches, shift registers, and other basic logic circuitry.

Microcontrollers, microprocessors, and FPGAs contain thousands, millions, or even billions of transistors. These components exist in a wide range of functionality, complexity, and size. Typically they are the largest IC in a circuit.

Many modern digital sensors, such as temperature sensors, accelerometers, and gyroscopes, come as ICs. These ICs are usually smaller than the microcontrollers, or other ICs on a circuit board.

IC packages contain the circuitry die and allow the device to more easily connect to other components. Each outer connection on the die is connected via a tiny piece of gold wire to a pad or pin on the package. There are many different types of packages, with varying dimensions, mounting types, and/or pin counts. Every IC pin is unique in terms of both location and function. Therefore, the package allows users to distinguish the various pins, typically using either a notch or a dot to indicate which pin is the first pin.

Range of IC packages.