A stepper or wafer stepper is a device used in the manufacture of integrated circuits (ICs). It is an essential part of the process of photolithography, which creates millions of microscopic circuit elements on the surface of silicon wafers out of which chips are made. It is similar in operation to a slide projector or a photographic enlarger. The ICs that are made form the heart of computer processors, memory chips, and many other electronic devices. Stepper is short for step-and-repeat camera. The stepper emerged in the late 1970s but did not become widespread until the 1980s. This was because it was replacing an earlier technology, the mask aligner. Aligners imaged the entire surface of a wafer at the same time, producing many chips in a single operation. In contrast, the stepper imaged only one chip at a time, and was thus much slower to operate. The stepper eventually displaced the aligner when the relentless forces of Moore's Law demanded that smaller feature sizes be used. Because the stepper imaged only one chip at a time it offered higher resolution and was the first technology to exceed the 1 micron limit. The addition of auto-alignment systems reduced the setup time needed to image multiple ICs, and by the late 1980s, the stepper had almost entirely replaced the aligner in the high-end market. The stepper was itself replaced by the step-and-scan systems (scanners) which offered an additional order of magnitude resolution advance. Step-and-scan systems work by scanning only a small portion of the mask for an individual IC, and thus require much longer operation times than the original steppers. Step-and-scan systems became widespread during the 1990s and essentially universal by the 2000s. Today, step-and-scan systems are so widespread that they are often simply referred to as steppers. An example of a step-and-scan system is the PAS 5500 from ASML.
History
1957: Attempts to miniaturize electronic circuits started back in 1957 when Jay Lathrop and James Nall of the U.S. Army's Diamond Ordnance Fuze Laboratories were granted a US2890395A patent for a photolithographic technique that could be used to deposit thin-film metal strips that in turn used to connect discrete transistors on a ceramic plate. It was also used to etch holes in the silicon dioxide (SiO2) layers to microfabricate diode arrays. Later, in 1959, Lathrop went to Texas Instruments, working for Jack Kilby, and Nall joined Fairchild Semiconductor. 1958: Based on their works, Jay Last and Robert Noyce at Fairchild Semiconductor built one of the first «step-and-repeat» cameras that repeated identical patterns of the transistors on a single wafer using photolithography. 1959: (Or no later 1961); The David W. Mann division of GCA Corporation became the first company to make commercial step and repeat mask reduction devices called photo-repeaters, which were the predecessors of modern day photolithography steppers. The company was later sold to GCA Corporation/Mann and Perkin Elmer. 1970: the Cobilt company was founded by a group of three engineers from Germany and England (from Kasper Instruments), and one salesman Peter Wolken. The company made what would later be called wafer steppers or lithography machines, at the time referred as mask aligners. The throughput of this machine was one 2-Inches wide wafer at a time. The Cobilt, which also traded abroad and had plants in Hong-Kong, in Europe was originally represented by a company called Advanced Semiconductor Materials (ASM) run by Arthur del Prado in Holland, who have founded the ASML in the mid of 1960s. Around 1971 or so the Cobilt was acquired by Computervision, which had greatly automated Cobilt machine. 1973: Perkin-Elmer had introduced Micralign projection aligner. It helped to decrease amount of defective chips that resulted in low yields and greatly boosted IC industry by helping to lower prices on chips. GCA introduced the first successful stepper, the DSW 4800 , in 1975. It could reach critical dimensions of 1 micron, better than any other system at the time.
The stepper's role in photolithography Integrated circuits (ICs) are produced in a process known as photolithography. The process starts with a large highly purified cylindrical crystal of the semiconductor material known as a boule. Thin slices are cut off the boule to form disks, and then undergo initial processing and treatment to create a blank silicon wafer. Elements of the circuit to be created on the IC are reproduced in a pattern of transparent and opaque areas on the surface of a glass or plastic plate called a photomask or reticle. The wafer is coated with a photosensitive material called photoresist. The mask is positioned over the wafer and bright light, normally ultraviolet, is shone through the mask. Exposure to the light causes sections of the resist to either harden or soften, depending on the process. After exposure, the wafer is developed like photographic film, causing the photoresist to dissolve in certain areas according to the amount of light the areas received during exposure. These areas of photoresist and no photoresist reproduce the pattern on the reticle. The developed wafer is then exposed to solvents. The solvent etches away the silicon in the parts of the wafer that are no longer protected by the photoresist coating. Other chemicals are used to change the electrical characteristics of the silicon in the bare areas. The wafer is then cleaned, recoated with photoresist, then passed through the process again in a process that creates the circuit on the silicon, layer by layer. Once the entire process is complete, the wafer is sawn apart into individual chips, tested, and packaged for sale.
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