A technique developed several years ago for improving optical microscopes has now been applied to monitoring the next generation of computer chip circuit components, providing a crucial tool for developing 3-D components. For decades, computer chips have resembled city maps in which components are essentially flat. But as designers have strived to pack more components onto chips, they have been forced to build upward. New generations of chips feature 3-D structures that stack components atop one another, but ensuring that these components are all made to the right shapes and sizes requires a new dimension of measurement capability. The through-focus scanning optical microscopy (TSOM) developed by NIST researchers can detect minute differences — as small as 10 nm across, or perhaps smaller — in 3-D shapes of circuit components, which until very recently have been 2-D objects. The technique could address some key industry measurement challenges, the investigators say, including manufacturing process control and maintaining the viability of optical microscopy in electronics manufacturing. “Previously, all we needed to do was show we could accurately measure the width of a line a certain number of nanometers across,” said NIST’s Ravikiran Attota, a key player in the development of the TSOM method. “Now, we will need to measure all sides of a three-dimensional structure that has more nooks and crannies than many modern buildings. And the nature of light makes that difficult.” The 3-D tri-gate (FinFET) transistors shown here are among the 3-D microchip structures that could be measured using NIST’s technique for improving through-focus scanning optical microscopy (TSOM). Courtesy of Intel Corp. Part of the difficulty is that components are getting so small that light beams are not able to get to them. Optical microscopes are limited to features larger than about half the wavelength of the light used. To work around this issue, microscopists line up several identical components at regular distances apart and observe how light scatters off the group, fitting the data with optical models to determine the dimensions. These optical measurements, as currently used in manufacturing, have difficulty measuring newer 3-D structures. Other nonoptical methods of imaging such as scanning probe microscopy are expensive and slow. TSOM uses a conventional optical microscope, but rather than taking a single image, it collects 2-D images at different focal positions to form a 3-D data space. A computer extracts the brightness profile from these multiple out-of-focus images and uses the differences between them to construct the TSOM image. Although somewhat abstract, the differences between the constructed images are still clear enough to infer minute shape differences in the measured structure — bypassing the use of optical models, which introduce complexities that industry must face. “Our simulation studies show that TSOM might measure features as small as 10 nm or smaller, which would be enough for the semiconductor industry for another decade,” Attota said. “And we can look at anything with TSOM, not just circuits. It could become useful to any field where 3-D shape analysis of tiny objects is needed.” The findings were published in Applied Physics Letters (doi: 10.1063/1.4809512). For more information, visit: www.nist.gov