The data storage industry seeks data densities of several terabits per square inch, corresponding to dot pitch <15 nm. Cost constraints prohibit using commercial microscopes designed for accurate measurements. However, we show it is possible to routinely make accurate pitch measurements with commercial general purpose atomic force microscopes and scanning electron microscopes by using a calibrated grating as a transfer standard. This provides short-term traceable calibration. Our accuracy was validated in two collaborative projects with three national metrology institutes. In the first project, we measured the pitch of a 144-nm two-dimensional grating. Our mean value agreed within 0.033 nm with that obtained at the PTB (Germany) optical diffraction lab. In the second project, described in detail here, we measured the pitch of a 70-nm one-dimensional grating. Because we used two measurement runs, the statistical treatment is more elaborate. We use basic statistical methods, such as analysis of variance, correlation, and tests of statistical significance, to draw key conclusions and enable us to combine the results of both runs to get an improved mean pitch value with some reduction in uncertainty. Our mean value agreed within 0.025 nm of the values found using the calibrated AFMs at National Institutes of Standard and Technology (NIST, USA) and National Metrology Centre (NMC, Singapore); furthermore, our uncertainty matched that of the other labs. The relative standard deviation (SD/mean) of individual pitch measurements is a figure of merit for the measurement system consisting of grating plus microscope.If the relative SD can be held below 0.5%, we have a clear roadmap to provide useful traceability for pitch standards down to 5 nm.
The National Institute of Standards and Technology (NIST), Advanced Surface Microscopy (ASM), and the National Metrology Centre (NMC) of the Agency for Science, Technology, and Research (A*STAR) in Singapore have completed a three-way interlaboratory comparison of traceable pitch measurements using atomic force microscopy (AFM). The specimen being used for this comparison is provided by ASM and consists of SiO2 lines having a 70-nm pitch patterned on a silicon substrate. For this comparison, NIST used its calibrated atomic force microscope (C-AFM), an AFM with incorporated displacement interferometry, to participate in this comparison. ASM used a commercially available AFM with an open-loop scanner, calibrated with a 144-nm pitch transfer standard. NMC/A*STAR used a large scanning range metrological atomic force microscope with He-Ne laser displacement interferometry incorporated. The three participants have independently established traceability to the SI (International System of Units) meter. The results obtained by the three organizations are in agreement within their expanded uncertainties and at the level of a few parts in 104.
The National Institute of Standards and Technology (NIST), Advanced Surface Microscopy (ASM), and the National
Metrology Centre (NMC) of the Agency for Science, Technology, and Research (A*STAR) in Singapore have
completed a three-way interlaboratory comparison of traceable pitch measurements using atomic force microscopy
(AFM). The specimen being used for this comparison is provided by ASM and consists of SiO2 lines having a 70 nm
pitch patterned on a silicon substrate.
NIST has a multifaceted program in atomic force microscope (AFM) dimensional metrology. One component of this
effort is a custom in-house metrology AFM, called the calibrated AFM (C-AFM). The NIST C-AFM has displacement
metrology for all three axes traceable to the 633 nm wavelength of the iodine-stabilized He-Ne laser - a recommended
wavelength for realization of the SI (Système International d'Unités, or International System of Units) meter. NIST
used the C-AFM to participate in this comparison.
ASM used a commercially available AFM with an open-loop scanner, calibrated by a 144 nm pitch transfer standard. In
a prior collaboration with Physikalisch-Technische Bundesanstalt (PTB), the German national metrology institute,
ASM's transfer standard was calibrated using PTB's traceable optical diffractometry instrument. Thus, ASM's
measurements are also traceable to the SI meter.
NMC/A*STAR used a large scanning range metrological atomic force microscope (LRM-AFM). The LRM-AFM
integrates an AFM scanning head into a nano-stage equipped with three built-in He-Ne laser interferometers so that its
measurement related to the motion on all three axes is directly traceable to the SI meter.
The measurements for this interlaboratory comparison have been completed and the results are in agreement within
their expanded uncertainties and at the level of a few parts in 104.
Production of objects with 5 to 25 nm width or pitch requires metrology with picometer-scale accuracy. We imaged a
new 70-nm pitch standard by AFM and made it traceable to the international (SI) meter. We describe data capture and
analysis procedures that produce metrology-quality results from general purpose AFMs and SEMs. We suggest that traceable pitch standards are most useful when the expanded uncertainty (k=2, 95% confidence) is less than ±1.33% for single pitch values and ±0.5% for mean pitch. We show a projected chain of comparisons (roadmap) leading to a 5-nm pitch standard with expanded uncertainty of 52 pm (1.04%) for single values and 16 pm (0.32%) for the mean value, significantly better than the target.
We measured the pitch of a 144-nm pitch, two-dimensional grid in two different laboratories. Optical Diffraction gave
very high accuracy for mean pitch and Atomic Force Microscopy measured individual pitch values, gaining additional
information about local pitch variation. The measurements were made traceable to the international meter. Optical
diffraction gave mean value 143.928 ± 0.015 nm (95% confidence limit, per GUM). AFM gave mean value 143.895 ±
0.079 nm. Individual pitch values had standard deviation 0.55 nm and expanded uncertainty ± 1.1 nm. Mean values
measured by the two methods agreed within 0.033 nm. Because this was less than the uncertainty due to random
variation in the AFM results, it suggests that the AFM measuring and analysis procedures have successfully corrected all
systematic errors of practical significance in microscopy. We also discuss what precision may be expected from the
AFM method when it is applied to measure smaller pitches.
We describe statistical analysis of AFM measurements of bump size, shape and position on DVD stampers. We present statistical concepts that lead to useful measurements of process position and process noise. These physical measurements are compared with key electrical measurements such as asymmetry and jitter.
We have developed a new technique for measuring pit geometry, track pitch, jitter and wobble on compact discs (CD) and digital versatile discs. This method uses direct physical inspection with a Atomic Force Microscope. The images are analyzed by our automated method and yield statistically robust results, so that process windows can be determined. In both types of media we report a variety of statistical parameters including mean and standard deviation and create trend charts and other graphs. In addition to the media previously mentioned we demonstrate imaging the data marks of a written CD-RW using surface potential.
We describe a computerized method to measure the geometry of nanometer-scale data marks from AFM images. By compiling measurements of hundreds offeatures, we obtain statistically robust results, not only for mean values ofstructural parameters, but also for the standard deviations, so that process windows can be determined. On DVDs, we measured the following parameters: track pitch, bump height, bump width and length (at various threshold levels), bump length, and four sidewall slope angles, in each case reporting mean, standard deviation and other statistics. For each 10x10 pm image of a DVD stamper, containing about 100 bumps, we tabulated over 1000 values. In a plot of bump width vs. bump length, we found that width at half height increased from 328 nm for the shortest bumps (440 nm long) to about 385 nm for bumps longer than 800 nm; this matches the increase seen for corresponding optical signals produced when a finished disc is played. Where sidewall angle deviated from the norm, we were able to review the image data to identify the specific nature of the defect. Thus, feature geometry will no longer be a hidden variable in the path between controlling production equipment and observing the good or bad electrical performance of a finished disc.
We describe a computerized method to measure the geometry of regular, nanometer-scale structures. By compiling measurements of hundreds of features, we obtain statistically robust results, not only for mean values of structural parameters, but also for the standard deviations, so that process windows can be determined. On DVDs, the smallest feature are pits or bumps about 400 nm long, 320 nm wide, 120 nm high, with a track pitch of 740 nm. We measured the following parameters: track pitch, bump height, bump width and length, bump length, and four sidewall slope angles, in each case reporting mean, standard deviation and other statistics. For each 10 by 10 micron image of a DVD stamper, containing about 100 bumps, we tabulated about 1000 values. In a plot of bump width versus bump length, we found that width at half height increased from 328 nm for the shortest bumps to about 385 nm for bumps longer than 800 nm; this matches the increase seen for corresponding optical signals produced when a finished disc is played. Where sidewall angle deviated from the norm, we were able to review the image data to identify the specific nature of the defect. Thus, feature geometry will no longer be a hidden variable in the path between controlling production equipment and observing the good or bad electrical performance of a finished disc.
Proc. SPIE. 3050, Metrology, Inspection, and Process Control for Microlithography XI
KEYWORDS: Microscopes, Statistical analysis, Calibration, Silicon, Physics, Electron microscopes, Atomic force microscopy, Scanning electron microscopy, Scanning probe microscopy, Standards development
Dimensional calibration standards are an important metrology tool for quality control, inspection, and fault analysis. Tools such as atomic force microscopes (AFM), scanning probe microscopes (SPM), or scanning electron microscopes (SEM) require regular calibration to meet the needs of current and projected production processes. Suitable calibration standards have been expensive, difficult to use, and of limited utility. These limitations were, to a large degree, a result of the fabrication process and the accompanying measurement calibration paradigm. Any approach to microscope calibration should make calibration easier, less expensive, and more useful. An improved calibration standard would also be amenable to automation of the calibration process for use in production line instruments. The necessary features include: (1) the ability to calibrate the entire viewing field instead of discrete points; (2) the ability to easily locate and use the calibrated region; (3) the ability to calibrate on the nanometer scale where the most demanding applications push the state of the art; (4) significantly reduced specimen costs. There is an alternative production method for calibration specimens which meets the above criteria. It is based on the concept of physically replicating a light interference pattern to provide the essence of an interferometer in a simple calibration specimen. Modern optics technology has reached the point where large area, very accurate nd regular interference patterns in 1 and 2 dimensions can be produced. The basic physics of the process enables the periodicity of these patterns to be specified and controlled to fractions of a nanometer over these very large areas. This large-area interference pattern can be captured in a physical record suitable for viewing under the microscope. The issues affecting the accuracy and utility of this physical record and its preparation for use as a magnification standard will be discussed. Experience in sue in AFM applications indicates that calibration samples produced by this method can deliver repeatable accuracy of 1.5 nm if properly employed and analyzed. This methodology can be extended to other imaging microscope technologies.
A general purpose SPM can function as a metrology SPM when used with a new type of calibration standard and new data analysis software. The calibration standard is a 288-nm pitch, 1D holographic grating. The holographic exposure process assures uniform feature spacing over the entire specimen area, with an expected accuracy of 0.1 percent. We developed new software for data analysis and used it to diagnose and correct the residual scan nonlinearity of a standard NanoScope SPM. We improved the differential non- linearity of a 10 micron scan from 6.7 percent to 1.1 percent and we improved the integral non-linearity from 0.5 percent to 0.04 percent. We then applied the improved instrument to gauge feature spacing son magnetic disks, integrated circuits, and optical disks.
Photoresist linewidths are presently controlled by inspecting wafers in plan view with a 'metrology SEM'. No standard reference materials exist for checking this measurement. We show the feasibility of creating an in-house quasi-standard for this purpose. Using a modern i-line stepper (0.54 N.A.), we produced three specimens of 0.5 micrometers wide photoresist line test patterns which presented both aberrated and good line shapes. We examined the specimens in cross-section using the AFM and in plan view using the SEM. The AFM images had crisp edges defining the line profile. We measured pitch, linewidth, and slope angles from the AFM images and compared these with the pitches and linewidths reported by the SEM. We found differences for features that had curved sidewalls. We discuss the basis for edge contrast in the AFM and propose a method for mathematical analysis. This work shows that the AFM can directly examine resist line profiles and provide images of useful precision, without adding a conductive coating as is commonly done in high resolution SEM. Any added coating should be avoided on a standard, since it would modify both the physical width of the structure and its electron scattering characteristics.