The Micralign was a family of aligners introduced in 1973 by Perkin-Elmer. Micralign was the first projection aligner, a concept that dramatically lowered semiconductor fabrication costs. According to the Chip History Center, it "literally made the modern IC [integrated circuit] industry". The Micralign addressed a significant problem in the early integrated circuit (IC) industry: the vast majority of ICs printed contained defects that rendered them useless. On average, about 1 in 10 complex ICs produced would be operational, a 10% yield. The Micralign improved this to over 50%, and as great as 70% in many applications. In doing so, the price of microprocessors and dynamic RAM products fell about 10 times between 1974 and 1978, by which time the Micralign had become practically universal in the high-end market. Initially predicting to sell perhaps 50 units, Perkin-Elmer eventually sold about 2,000, making them the by far largest vendor in the semiconductor fabrication equipment space through the second half of the 1970s and early 1980s. Formed into the Microlithography Division, by 1980 its income was the largest of Perkin-Elmer's divisions and provided the majority of the company's profits. The company was slow to respond to the challenge of the stepper, which replaced the projection aligners in most roles starting in the mid-1980s. Their move to extreme ultraviolet as a response failed, as the technology was not mature. Another attempt, buying a European stepper company, did nothing to reverse their fortunes. In 1990, Perkin-Elmer sold the division to Silicon Valley Group, which is today part of ASML Holding.
Background Integrated circuits (ICs) are produced in a multi-step process known as photolithography. The process begins with thin disks of highly pure silicon being sawn from a crystalline cylinder known as a boule. After initial processing, these disks are known as wafers. The IC consists of one or more layers of lines and areas patterned onto the surface of the wafer. The wafers are coated in a chemical known as photoresist. One layer of the ultimate chip design is printed on a "mask", similar to a stencil. The mask is placed over the wafer and an ultraviolet (UV) lamp, typically a mercury arc lamp, is shone on the mask. Depending on the process, areas of the photoresist that are exposed to the light either harden or soften, and then the softer areas are washed away using a solvent. The result is a duplication of the pattern from the mask onto the surface of the wafer. Chemical processing is then used on the pattern to give it the desired electrical qualities. This entire process is repeated several times to build up the complete IC design. Each step uses a different design on a different mask. The features are measured in micrometres, so any previous design already deposited has to be precisely aligned with the new mask that will be applied. This is the purpose of the aligner, a task that was originally completed manually using a microscope. There is a strong economic argument to use larger wafers, as more individual IC's can be patterned on the surface and produced in a single series of operations, thereby producing more chips during the same period of time. However, larger wafers give rise to significant optical issues; focussing the light over the area while maintaining very high uniformity was a major challenge. By the early 1970s, wafers had been about 2.5 inches in diameter for some time and were just moving to 3 inches, but existing optical systems were having problems with this size. Every time a new wafer size was introduced, the optical systems had to be redesigned from scratch.
Contact aligners In the 1960s, the most common way to hold the mask during the exposure processes was to use a contact aligner. As the name implies, the purpose of this device was to precisely align the mask between each patterning step, and once aligned, hold the mask directly on the surface of the wafer. The reason for holding the mask on the wafer was that at the scale of the lines being drawn, diffraction of the light around the edges of the lines on the mask would blur the image if there was any distance between the mask and the wafer. There were significant problems with the contact-mask concept. One of the most annoying was that any dust that reached the aligner's interior might stick to the mask and would be imaged on subsequent wafers as if it were part of the pattern. Equally annoying was that uncured photoresist would stick to the mask, and when the mask was lifted, it would pull off the top surface from the wafer, destroying that wafer and once again adding spurious images on the mask. Any one error might not be an issue because only the ICs in that location will be affected, but eventually, enough errors will be picked up that the mask is no longer useful.
As a result of issues like these, masks generally lasted only a dozen times before having to be replaced. To supply the required number of masks, copies of the original mask were repeatedly printed using conventional silver halide photography on photographic stock, which was then used in the machine. The thermal stability of these masks during exposure to bright light caused distortions, which were not a concern in the early days but became an issue as feature sizes continued to shrink. This forced a move from film to glass masks, further increasing costs. Because any particular wafer could be damaged at any given masking step, the chance that any one wafer would make it through to production without damage was a function of the number of steps. This limited the complexity of the IC designs in spite of the designers being able to make use of many more layers. Microprocessors, in particular, were complex multi-layer designs that had extremely low yield, with perhaps 1 in 10 of the patterns on a wafer delivering a working chip.
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