Stepper

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.

Aligners imaged the entire surface of a wafer at the same time, producing many chips in a single operation.

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.

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.

[1] 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.

[1] 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.

[2] 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.

[4]: 2–3 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 [nl] in Holland, who have founded the ASML in the mid of 1960s.

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.

During the 1970s, aligners generally worked at a one-to-one magnification, which limited the amount of detail on the wafer to about whatever could be produced on the mask.

However, the relentless drive of Moore's law pushed the industry to the point where even the maximum magnifications possible in the projection system were not enough to continue shrinking the feature sizes.

The chamber also contains other systems that support the process, such as air conditioning, power supplies, control boards for the various electrical components, and others.

Once this fine alignment is completed, the shot is exposed by light from the stepper's illumination system that passes through the reticle, through a reduction lens, and on to the surface of the wafer.

The greatest limitation on the ability to produce increasingly finer lines on the surface of the wafer has been the wavelength of the light used in the exposure system.

As the desired line widths approached and eventually became narrower than the wavelength of the light used to create them, a variety of resolution enhancement techniques were developed to make this possible, such as phase shifting reticles and various techniques for manipulating the angles of the exposure light in order to maximize the resolving power of the lens.

Eventually however, the desired line widths became narrower than what was possible using mercury lamps, and near the middle of the 2000s, the semiconductor industry moved towards steppers that employed krypton-fluoride (KrF) excimer lasers producing 248 nm light.

Although fluoride (F2) lasers are available that produce 157 nm light, they are not practical because of their low power and because they quickly degrade photoresist and other materials used in the stepper.

Since practical light sources with wavelengths narrower than these lasers have not been available, manufacturers have sought to improve resolution by reducing the process coefficient

However, these techniques are approaching their practical limit, and line widths in the 45 nm range appear to be near the best that can be achieved with conventional design.

However, in order to delay as long as possible the vast expense and difficulty of adopting a whole new type of illumination technology, manufacturers have turned to a technique, previously used in microscopes, for increasing the numerical aperture of the lens by allowing the light to pass through water instead of air.

Successful scanning requires extremely precise synchronization between the moving reticle and wafer stages during the exposure.