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ASML AND ITS MAIN BUSINESS, LITHOGRAPHY
ASML is the world's leading provider of lithography systems for the semiconductor industry. We manufacture complex systems critical to the production of integrated circuits or chips. ASML's lithography systems transfer circuit patterns onto silicon wafers to make every kind of chip we use today, as well as those for tomorrow. Manufacturers of mobile telephones, consumer electronics, personal computers and communication and information technology equipment continue to push the industry to new limits. With each new generation of chips, products become smaller, lighter, faster, more powerful, more precise, more reliable and easier to use. Therefore, chipmakers must be able to produce chips with ever-finer circuit lines on silicon wafers. Lithography, or imaging as it is sometimes called, is the critical technology that shrinks the width of circuit lines, allowing chip manufacturers to continually design and produce more chips per wafer, more powerful chips, or both. Finer line widths allow electricity to flow around the chip faster, boosting its performance and improving its functionality. For chip manufacturers, such technological advancements mean increased manufacturing productivity and improved profitability. How small is small? A typical advanced chip, the size of a thumbnail or smaller, can contain more than 25 million transistors or storage for more than 256 million data bits. This incredible density is only possible because the circuit features on these chips are so small. Today’s chips are getting smaller and smaller, as are the features on them, as chipmakers seek to squeeze more circuitry onto one tiny chip. Early in 2000, the most advanced chips in production had minimum feature sizes of 180 nanometers, or 180 billionths of a meter. Industry people sometimes talk in terms of microns. This is 1000 nanometers, or one-millionth of a meter. This was the standard for many years, but it is being replaced by nanometers, as dimensions shrink to small fractions of a micron. A 180-nanometer feature is 0.18 micron. By 2001, chips with 150-nanometer features were in production, and 130-nanometer devices followed soon after. Today, ASML's latest range of advanced lithography systems is capable of imaging features as small as 70 nanometers. It’s difficult to relate a dimension like 70 nanometers to the real world, as there is nothing familiar that is anywhere near that small. One commonly used reference is the typical human hair, which is somewhere between 60,000 and 100,000 nanometers across, or approximately 1000 times as wide as a 70-nanometer feature. The clean room experience In this industry of minute tolerances, a particle of dust, dirt, or other material one-quarter the thickness of your hair can, if it lands in the wrong place, become a killer defect and ruin a chip. For this reason, chip making takes place in wafer fab clean rooms. This is a closed environment in which air is filtered, machines are specially designed to not shed particles, and people are wrapped from head to toe in so-called bunny suits. Visitors must usually watch through windows from an outside corridor, so let us describe the experience. The yellow lights of the room cast an eerie glow that makes everything neither bright nor dull. However, unlike the real world, the light inside the cleanroom never varies. The natural rhythms of sunrise and sunset go unnoticed. Working in a cleanroom is not easy. Before you enter, you must put on special boots, a coverall garment (bunny suit), a hair net, hood, gloves and often a facemask and safety glasses. You then pass through an air shower; a special corridor lined with air nozzles that blow any lingering dust or dirt off your bunny suit. Eating, drinking, and smoking are strictly prohibited. All writing must be done on special cleanroom paper that will not generate particles. There are reasons for this slavish attention to cleanliness. To get a sense of the challenges in running a cleanroom, consider that a cubic foot (0.03 m3) of regular outdoor air contains about 35 million particles of 100 nanometers or larger. Each one of those has the potential to become a killer defect. A person sitting or standing generates 100,000 particles per cubic foot, while someone walking generates 10 million particles per cubic foot. All of this is invisible to the naked eye. Cleanliness is also a primary consideration for the tools that go into the fab such as the ASML lithography tools. Moving parts in machines must be made of specialized metals and plastics to minimize particulate generation, and engineers minimize situations where parts rub against each other. Gaskets, washers, and other small parts are fabricated from exotic (and expensive) engineered plastics that will not flake or give off gases during high-temperature processing. Any release of foreign matter can result in a wafer, a batch of wafers, or even thousands of wafers being damaged before the problem is noticed. Understanding lithography A wafer fab takes very pure silicon; modifies it with phenomenal dexterity, precision, and knowledge; and produces integrated circuits. Doing this requires leading-edge capabilities in optics, chemistry, mechanics, measurement and many other fields. It demands huge amounts of capital, both financial (new fabs cost more than $1 billion) and intellectual. The complex process constantly redefines its own limitations, and somehow manages to move ahead at a blinding rate, setting the pace for businesses that use chips in their products. Elements of a Chip Chips are made on wafers of silicon, gallium arsenide or other alternatives. These wafers are disc-shaped, anywhere from 3 inches (about 75 mm) to 12 inches (300 mm) across and, as the name implies, very thin. A single wafer can hold anywhere from a hundred to several thousand individual chips, depending on how big each chip is. Wafers are almost perfectly flat, with variations of up to only 130 nm (130 billionths of a meter). If you imagine the wafer expanded to the size of North America, this would mean a total flatness variation of about two meters in 5000 kilometers (or seven feet in 3000 miles), the distance between New York and San Francisco or the distance between the Ural Mountains in Russia and the Costa Brava in Spain. Virtually every chip in the world comprises three basic elements: Steps in the fabrication process In almost all cases, the active components are the first things to be made. After they are in place, they are covered with an insulating layer. Holes are etched into this layer to provide access to the active components below, and metal conductors are deposited. In the old days (before about 1990) that would have been the basic process, but most modern chips have so many components to link up that several layers of interconnection are required. Thus, the process of laying down insulation followed by metal is repeated as many as five or six additional times. Producing the layered structure on a silicon wafer requires several hundred separate steps, which generally take a month or more to complete (although special so-called hot lots can be run through in two or three weeks when conditions warrant). Many of these steps are for cleaning; after many operations, the wafer is run through either a sequence of acid solutions or ultra pure water to remove any particles or impurities that may have formed. These processes use between two million and five million gallons (between 7.5 million and 19 million liters) of water per day. Four processes - lithography, implant, deposition and etch - form the fundamental wafer-fab operations. Lithography is the most glamorous process, as it involves the actual definition of the features being printed. It takes place about 20 to 35 times during the production process, and sets the stage for the other operations. Implant alters the electrical characteristics of the silicon during transistor formation. Deposition lays down conducting or insulating materials and etch selectively removes some of the deposited materials. Lithography In a typical wafer fab lithography area, you will find a (couple of) dozen wafer steppers or scanners. These are multimillion-dollar cameras used to transfer circuit design elements from 6-inch (152-mm) square, glass plates called photomasks (or reticles) onto wafers. A wafer stepper is very similar in function to the enlargers used in photographic darkrooms, with one exception. Instead of enlarging a small negative to make a larger print, it reduces the image on the reticle to one-quarter or one-fifth of its original size to create the image on the wafer, which is coated with a light-sensitive chemical called photoresist (similar to the emulsion on photographic film). Each chip requires between 20-35 reticles. A wafer stepper gets its name because it does not print an image on the entire wafer at once. Instead, it exposes a small portion of the wafer, called a die. Its area is referred to as die size and this area can contain one or several chips. The wafer stepper then steps the wafer die per die, an inch or so to one side before the next exposure. In less than a minute, a wafer stepper can cover the entire surface of a 200-mm (8-inch) wafer. The wafer stepper is something close to miraculous. It contains a massive multimillion-dollar lens, with which it can define features of 150 nanometers or less. The only competitive optics are found in spy satellites. Generating the image, however, is only half the battle. For the layered integrated circuit to function, each of the multiple layers of the chip must be in near-perfect alignment with the layer below. The alignment, or overlay specification, is between one-quarter and one-third of the minimum feature size that the stepper can print, or approximately 25 or 50 nanometers for today's most advanced devices. That means every time the machine steps the wafer for a new exposure, it must align itself within those parameters. Wafer steppers run 24 hours a day, seven days a week, and are expected to go for weeks at a time without a breakdown. They push the state-of-the-art in many different areas; the optics, the metal work in the stages, the laser alignment systems, and even the climate control systems that ensure that temperature and humidity are within specification. Like a darkroom photo enlarger, wafer steppers use light to transfer the circuit pattern from the photomask to the light-sensitive photoresist on the wafer. As circuit features became smaller, however, a problem arose: there is a direct correlation between the wavelength of light being used, and the feature size it can print. By the late 1970s, the industry had moved to ultraviolet. In the late 1980s, the standard went to 365 nanometer, and then in the 1990s to so-called deep UV, or 248 nanometer. A transition to 193-nanometer light started in 2000, and research work on 157 nanometer began soon after. ASML's role in lithography ASML produces three different types of lithography systems for the semiconductor industry; these include wafer stepper systems (steppers), Step & Scan systems (scanners), and the revolutionary TWINSCAN(TRADEMARK!)system (scanners). Wafer stepper systems Our first generation lithography systems, most of which are still in use today, are the PAS 2500 and PAS 5000 wafer stepper systems. These systems use 365 nm wavelength of ultraviolet light known as I-line. The numerical aperture (NA) of the lenses used in these systems is between 0.40 and 0.48. They are capable of imaging minimum feature sizes between 0.5 microns (500 nm) and 0.7 microns (700 nm) on 3-inch (75-mm) or 6-inch (150-mm) wafers. Step & Scan systems ASML's next generation lithography systems are PAS 5500 Step & Scan systems, known in the industry as scanners. These systems use either the i-line wavelength of ultraviolet (UV) light or deep ultraviolet light (DUV) as sources of illumination. These wavelengths vary between 193 nm for DUV and 365 nm for UV light. Depending on the particular system, the PAS 5500 systems can produce feature sizes from 300 nm down to 90 nm. The maximum size of wafer that these systems can process is 200 mm. TWINSCAN systems These systems are ASML's most advanced Step & Scan systems. They are capable of processing 300-mm wafers in parallel; they have two stages: a moving platform on which the wafer rests. This design allows one stage to be in the measurement position, while the other stage is in the exposure position. In the measurement position, the system measures the wafer and makes a map that the system uses during exposure to keep the wafer in focus. The most important feature of the TWINSCAN is high process speed and high accuracy at the same time; i.e., exposing wafers as quickly and as accurately as possible. TWINSCAN makes it possible for semiconductor manufacturers to produce feature sizes down to 45 nm. The difference between steppers and scanners The wafer stepper (stepper) and the Step & Scan (scanner) systems both expose, somewhat differently, the pattern (image) from the reticle onto the wafer. ASML wafer stepper systems can expose only a relatively small image area, a maximum of 22 mm by 22 mm, at one time. In a wafer stepper system, the wafer and reticle are stationary during exposure. The system exposes one die, at one time. After the exposure, the system moves the wafer, one step, to the location of the subsequent die to be exposed. Because of the increased complexity of ICs, there was a demand for a larger image field, and smaller details in the image. With the wafer stepper system, this is very difficult to achieve. To achieve the higher resolution necessary, while maintaining the low distortion in a larger image field, ASML developed a different system: the Step & Scan system. This system can expose an image area (also called the die size) of 26 mm by 33 mm. The Step & Scan system does not expose the complete image from the reticle onto the wafer at one time. Instead, an elongated beam scans the image on the reticle and exposes that image onto the wafer. This system exposes each die by moving the reticle and the wafer synchronously in different directions. After the scan (effectively the exposure) is complete, the system moves the wafer, in steps, to the subsequent position. This process is typical of a Step & Scan system with a single wafer stage, such as the PAS 5500 systems. ASML took the step-and-scan principle further with the introduction of the TWINSCAN dual-stage systems; with their two wafer stages, these systems increase process speed significantly.
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