'Ruler' Promises Nano-Precise Litho Calibration

Metrology calibration has always been composed of two parts: precision and accuracy. As defined by Sematech , statistical process control accuracy is the compliance of the measured or observed value to the true value or accepted reference value, while precision is the measure of natural variation of repeated measurements. Obviously, the precision component has always had greater importance than accuracy for calibration purposes, particularly in the accurate matching of tools to ensure that there is as small a variation as is physically possible between the measurements made by various platforms. Therefore, it is not surprising that inspection as well as process equipment OEMs have continuously worked over the years to gradually improve precision. This has been particularly observable in the area of CD-SEM tools. Meanwhile, for advanced sub-100 nm technology nodes, accuracy is playing an increasingly larger part in what is generally referred to as "total measurement uncertainty," which is usually defined as a combination of the tool and the calibration standard uncertainty, making it critical for the standard to be of very low uncertainty.

There are not many low-uncertainty traceable accuracy standards currently available, although they are critically necessary for accurately calibrating and matching metrology tools, particularly in the lithography arena. It takes no great effort of the imagination to see that, if a fab has a tool that differs from another by 5 nm and the process tolerance is 3 nm, it is fairly obvious that it will be difficult to control that process day in and day out.

Until now, available traceable accuracy calibration standards for these types of applications have consisted of a single linewidth on a chip. Although a single linewidth is valuable for calibrating a tool's accuracy for a single measurement point, there is a need to calibrate the linearity of a tool across multiple linewidths. Today, one would need to procure a number of single-linewidth wafer standards of differing values to provide linearity calibration. This multiple-wafer approach to linearity is both costly and time-consuming.

VLSI Standards (San Jose) has developed a way to produce a multiple-linewidth standard on a chip with very low uncertainty. Its NanoRuler is a fully traceable standard to assist in controlling the lithography process. It is capable of providing an accurate reference all the way down to 15 nm. The standard's low uncertainty (1.0 nm 3σ ) will enable tool diagnostics and linearity matching to ensure that all measurements are identical. The NanoRuler is the result of a patented methodology to lay down low-uncertainty single lines. This technique does not use lithography and plasma etch-based processes, which even today still have uncertainties, typically in the 2-3 nm or higher range. The lines in the new standard are produced through a combined thin-film polysilicon and SiO₂ deposition process. This allows each line to be fabricated atom by atom, and the calibration technique used to ensure their accuracy calls on fundamental limits of physics by using a TEM to calibrate the lines by taking cross-sections and counting the silicon crystal lattice. This has resulted in a calibration standard that is capable of serving needs across a range of values, not just a single point.

The NanoRuler standard prototype illustrates multiple lines for SEM calibration. Its final version will provide six lines, 15-140 nm wide. (Source: VLSI Standards)

Multiple-linewidth calibration points are critical to establish accuracy linearity calibration and matching. In a 45 nm node process, for example, the after-etch gate dimension may be 25 nm, but there are also other differently sized structures on other layers that must be calibrated as well. The NanoRuler is composed of two sets of six lines of varying widths — 15, 30, 50, 75, 105 and 140 nm — that will enable it to be used for today's production processes, as well as processes in R&D. Because of the identical low uncertainty of each standard, accuracy matching between tools in production and development fabs for today and future technology nodes will be possible.

A feature of importance in the new standard is that the lines that make up the ruler are longer than those of past, single-line versions, which were certified to 3 mm in length. Certified line length is important because, as a CD-SEM measures a line, the width of the line inevitably begins to alter because of charging and deposition of organics in the air by the electron beam that exposes the line. Thus, having a long certified line with many measurement points becomes important. The lines on the new standard will be certified over a very long line length, up to 8 mm in length. This could allow as many as 8000 measurement sites on each of the 12 lines, extending the standard's lifetime and ROI beyond that of previous ones. The new standard will enable the industry to have a low-uncertainty standard to meet the challenge of lowering total measurement uncertainty, something necessary to control today's, as well as future, lithography processes.

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