Through-focus scanning optical microscopy (TSOM) shows promise for patterned defect analysis, but it is important to minimize total system noise. TSOM is a three-dimensional shape metrology method that can achieve sub-nanometer measurement sensitivity by analyzing sets of images acquired through-focus using a conventional optical microscope. Here we present a systematic noise-analysis study for optimizing data collection and data processing parameters for TSOM and then demonstrate how the optimized parameters affect defect analysis. We show that the best balance between signalto- noise performance and acquisition time can be achieved by judicious spatial averaging. Correct background-signal subtraction of the imaging-system inhomogeneities is also critical, as well as careful alignment of the constituent images used in differential TSOM analysis.
KEYWORDS: Silicon, Overlay metrology, Scanning electron microscopy, Optical microscopes, Metrology, 3D acquisition, 3D metrology, Polarization, 3D modeling, Photomasks
Through-focus scanning optical microscopy (TSOM) is a new metrology method that achieves 3D nanoscale
measurement sensitivity using conventional optical microscopes; measurement sensitivities are comparable to what is
typical when using scatterometry, scanning electron microscopy (SEM), and atomic force microscopy (AFM). TSOM
can be used in both reflection and transmission modes and is applicable to a variety of target materials and shapes.
Nanometrology applications that have been demonstrated by experiments or simulations include defect analysis,
inspection and process control; critical dimension, photomask, overlay, nanoparticle, thin film, and 3D interconnect
metrologies; line-edge roughness measurements; and nanoscale movements of parts in MEMS/NEMS. Industries that
could benefit include semiconductor, data storage, photonics, biotechnology, and nanomanufacturing. TSOM is
relatively simple and inexpensive, has a high throughput, and provides nanoscale sensitivity for 3D measurements with
potentially significant savings and yield improvements in manufacturing.
KEYWORDS: Scanning electron microscopy, Helium, Ions, Ion beams, Scanning helium ion microscopy, Nanotechnology, Electron beams, Electron microscopes, Gold, Carbon nanotubes
Helium Ion Microscopy (HIM) is a new, potentially disruptive technology for nanotechnology and nanomanufacturing.
This methodology presents a potentially revolutionary approach to imaging and measurements which has several
potential advantages over the traditional scanning electron microscope (SEM) currently in use in research and
manufacturing facilities across the world. Due to the very high source brightness, and the shorter wavelength of the
helium ions, it is theoretically possible to focus the ion beam into a smaller probe size relative to that of an electron beam
of an SEM. Hence higher resolution is theoretically possible. In an SEM, an electron beam interacts with the sample and
an array of signals are generated, collected and imaged. This interaction zone may be quite large depending upon the
accelerating voltage and materials involved. Conversely, the helium ion beam interacts with the sample, but it does not
have as large an excitation volume and thus the image collected is more surface sensitive and can potentially provide
sharp images on a wide range of materials. The current suite of HIM detectors can provide topographic, material,
crystallographic, and electrical properties of the sample. Compared to an SEM, the secondary electron yield is quite high
- allowing for imaging at extremely low beam currents and the relatively low mass of the helium ion, in contrast to other
ion sources such as gallium results in no discernable damage to the sample. This presentation will report on some of the
preliminary work being done on the HIM as a research and measurement tool for nanotechnology and
nanomanufacturing at NIST.
If nanomechanical testing is to evolve into a tool for process and quality control in semiconductor fabrication, great advances in throughput, repeatability, and accuracy of the associated instruments and measurements will be required. A recent grant awarded by the NIST Advanced Technology Program seeks to address the throughput issue by developing a high-speed AFM-based platform for quantitative nanomechanical measurements. The following paper speaks to the issue of quantitative accuracy by presenting an overview of various standards and techniques under development at NIST and other national metrology institutes (NMIs) that can provide a metrological basis for nanomechanical testing. The infrastructure we describe places firm emphasis on traceability to the International System of Units, paving the way for truly quantitative, rather than qualitative, physical property testing.
At the National Institute of Standards and Technology, we are building a metrology instrument called the Molecular Measuring Machine (M3) with the goal of performing nanometer- accuracy two-dimensional feature placement measurements over a 50 mm by 50 mm area. The instrument uses a scanning tunneling microscope to probe the surface and an interferometer system to measure the lateral probe movement, both having sub-nanometer resolution. The continuous vertical measurement range is 5 micrometer, and up to 2 mm can be covered by stitching overlapping ranges. The instrument includes temperature control with millikelvin stability, an ultra-high vacuum environment with a base pressure below 10-5 Pa, and seismic and acoustic vibration isolation. Pitch measurements were performed on gratings made by holographic exposure of photoresist and on gratings made by laser-focused atomic deposition of Cr. The line pitch for these gratings ranged from 200 nm to 400 nm with an estimated standard uncertainty of the average pitch of 25 X 10-6. This fractional uncertainty is derived from an analysis of the sources of uncertainty for a 1 mm point-to- point measurement, including the effects of alignment, Abbe offset, motion cross-coupling, and temperature variations. These grating pitch measurements are uniquely accomplished on M3 because of the combination of probe resolution and long-range interferometer-controlled stage. This instrument could uniquely address certain dimensional metrology needs in the data storage industry.
KEYWORDS: Interferometers, Metrology, Scanning tunneling microscopy, Motion measurement, Digital signal processing, Control systems, Temperature metrology, Chromium, Actuators, Data acquisition
We at NIST are building a metrology instrument called the Molecular Measuring Machine (MMM) with the goal of performing 2D point-to-point measurements with one nanometer accuracy cover a 50 mm by 50 mm area. The instrument combines a scanning tunneling microscope (STM) to probe the surface and a Michelson interferometer system to measure the probe movement, both with sub-nanometer resolution. The instrument also feature millidegree temperature control at 20 degrees C, an ultra-high vacuum environment with a base pressure below 10-5 Pa, and seismic and acoustic vibration isolation. High-accuracy pitch measurements have been performed on 1D gratings. In one experiment, the MMM STM probe imaged an array of laser-focused, atomically deposited chromium lines over an entire 5 micrometers by 1 mm area. Analysis of the data yielded an average line spacing of 212.69 nm with a 5 pm standard uncertainty. The uncertainty estimate is derived for an analysis of the sources of uncertainty for a 1 mm point-to-point measurement, including the effects of alignment, Abbe offset, motion cross-coupling, and temperature variations. In another measurement, the STM probe continuously tracked a holographically-produced grating surface for 10 mm, counting out 49,996 lines and measuring an average line spacing of 200.011 nm with a 5 pm standard uncertainty.
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