Basics

Lithography, in the context of building integrated circuits such as DRAMs and microprocessors, is a highly specialized printing process used to put detailed patterns onto silicon wafers. An image containing the desired pattern is projected onto the wafer, which is coated by a thin layer of photosensitive material called "resist". The bright parts of the image pattern cause chemical reactions which cause the resist material to become soluble, and thus dissolve away in a developer liquid, whereas the dark portions of the image remain insoluble. After development, the resist forms a stenciled pattern across the wafer surface which accurately matches the desired pattern. Finally, the pattern is permanently transferred into the wafer surface, for example by a chemical etchant which etches everywhere that is not protected by resist. ( Hence the term resist for the material which "resists" the etch.)

The basic lithographic process is similar to the photolithographic process used to make plates for offset printing presses. However, building state-of-the-art chips places unusual demands on the lithographic process, including:

• High Resolution - current chips require printing lines with features as small as 350nm, roughly 150 times narrower than the width of a human hair, and smaller than the 500nm wavelength of visible light. Over the next 10 years, this minimum linewidth is likely to shrink by roughly a factor of three. [ The unit nm stands for nano-meter, or one billionth of a meter. ]
• Large, complex pattern - How can the pattern on a chip the size of a thumb nail be considered large? Let us count up the number of pixels in this chip, where we imagine a pixel to be a square with a length of 500nm, the minimum linewidth. We find that a square chip 1 cm on a side contains roughly one billion pixels. This is the equivalent of roughly 1000 high resolution CRT screens of information, contained within each chip. If even one of these billion pixels is incorrect, a so-called killer defect, the chip will be non-functional.
• Tiny Overlay Error - integrated circuits are fabricated by printing a large number of individual pattern layers, typically 15-25. A functional chip requires that each of these layers must be accurately aligned to all the previously patterned layers to within a fraction of the linewidth. The problem of accurately overlaying all of these layers is similar to the problem of color printing where a red image, yellow image, blue image and black image must be accurately superposed.

IBM advances in Optical Lithography

Many types of lithographic processes, with different exposure energy, have been explored for building advanced chips. Optical lithography, where the exposure energy is visible or UltraViolet light, is the most important technique for volume manufacturing. For many years, IBM has been a leader in the drive to reduce the wavelength of light in order to print smaller features. Exposure wavelengths have gone from blue (436nm) to UltraViolet (365nm) to Deep UltraViolet(248nm), and will go to 193 nm over the next decade. IBM researchers have invented new resist processes for these shorter wavelengths, many of which depend on the idea of Chemically Amplified Resists, a powerful concept invented by researchers at IBM's Almaden Research Center. IBM has pioneered the use of step and scan lithographic exposure tools, phase shift mask techniques, anti-reflection coatings and many other innovations which are in use around the world.

A lithographer in an IBM clean room holds a patterned silicon wafer, containing roughly one hundred 64Megabit DRAM chips. The colorful patterns are caused by light diffracted by myriad tiny lines on the wafer.

X-Ray Lithography

IBM is a world leader in the area of X-ray lithography, where the exposure wavelength is reduced to roughly 1nm, a two hundred-fold reduction from Deep UltraViolet. At our Advanced Lithography Facility in Fishkill, NY, a compact synchrotron is used as a powerful, and reliable, source of X-rays.

This Scanning Electron Micrograph shows 130nm wide lines printed by X-ray lithography, a dimension several generations beyond those in production today. The steep sidewalls of this line are considered ideal for subsequent processing steps.

WWW sites related to lithography

• For many examples of advanced integrated circuits built using IBM lithographic technology, visit IBM Microelectronics
• The Semiconductor Subway at MIT connects to many R&D organizations interested in integrated circuit design, fabrication and lithography.
• SVG Lithography, Inc. is one of the manufacturers of the DUV step and scan tools used in advanced lithographic processes.
• A career in lithography is one of the many possibilities of employment at IBM.