Lowell P. Howard

Absolute interferometry with a 670-nm external cavity diode laser

In the past few years there has been much interest in use of tunable diode lasers for absolute interferometry. Here we report on use of an external cavity diode laser operating in the visible (lambda approximately 670 nm) for absolute distance measurements. Under laboratory conditions we achieve better than 1-microm standard uncertainty in distance measurements over a range of 5 m, but significantly larger uncertainties will probably be more typical of shop-floor measurements where conditions are far from ideal. We analyze the primary sources of uncertainty limiting the performance of wavelength-sweeping methods for absolute interferometry, and we discuss how errors can be minimized. Many errors are greatly magnified when the wavelength sweeping technique is used; sources of error that are normally relevant only at the nanometer level when standard interferometric techniques are used may be significant here for measurements at the micrometer level.

A simple technique for observing periodic nonlinearities in Michelson interferometers

Abstract We describe a simple, convenient method for measuring nonlinearities in displacement-measuring Michelson interferometers. Nonlinearities with a spatial periodicity of one optical fringe are a well-known source of error in precision interferometry. Our experimental technique for observing these errors is most immediately applicable to commercial interferometer systems for which the cube-corner retroreflectors can be attached directly to the faces of a beamsplitter cube, creating a monolithic interferometer optic with excellent noise immunity. The optical path difference in this bolted-together interferometer can be changed slightly by rotating the interferometer relative to an external laser. It should be noted that the basic principle described here—generating small path differences through a rotation of the optics relative to the laser—may itself be a source of significant errors in certain length measurements. The validity of our method has been demonstrated by measuring optical mixing errors of calculable magnitude. We describe a matrix method suitable for modeling optical mixing errors in both single-pass and double-pass (plane mirror) interferometers. Also, we report experimental measurements of periodic nonlinearities for two representative interferometers and conclude that, in the majority of engineering metrology applications, these errors are of only minor importance.

Real-time displacement measurements with a Fabry-Perot cavity and a diode laser

We present the basic operating principles of a traceable measurement system suitable for use with atomic force microscopes (AFMs) and nanometer-resolution displacement sensors. Our method is based upon a tunable external-cavity diode laser system which is servo-locked via a phase-modulated heterodyne locking technique to a Fabry-Perot interferometer cavity. We discuss mechanical considerations for the use of this cavity as a displacement metrology system and we describe methods for making real-time (sub 10 ms sampling period) measurements of the optical heterodyne signals. Our interferometer system produces a root-mean-squared (RMS) displacement measurement resolution of 20 pm. Two applications of the system are described. First, the system was used to examine known optical mixing errors in a heterodyne Michelson interferometer. Second, the Fabry-Perot interferometer was integrated into the Z axis of a commercial AFM scanning stage and used to produce interferometer-based images of a 17 nm step height specimen. We also demonstrate atomic resolution interferometer-based images of a 0.3 nm silicon single atomic step-terrace specimen.

A six-degree-of-freedom precision motion stage

This article presents the design and performance evaluation of a six-degree-of-freedom piezoelectrically actuated fine motion stage that will be used for three dimensional error compensation of a long-range translation mechanism. Development of a single element, piezoelectric linear displacement actuator capable of translations of 1.67 μm with 900 V potential across the electrodes and under a 27.4 N axial load and 0.5 mm lateral distortion is presented. Finite element methods have been developed and used to evaluate resonant frequencies of the stage platform and the complete assembly with and without a platform payload. In general, an error of approximately 10.0% between the finite element results and the experimentally measured values were observed. The complete fine motion stage provided approximately ±0.93 μm of translation and ±38.0 μrad of rotation in all three planes of motion using an excitation range of 1000 V. An impulse response indicating a fundamental mode resonance at 162 Hz was measured with...

High-resolution optical metrology

Recent advances in optical imaging techniques have unveiled new possibilities for optical metrology and optical-based process control measurements of features in the 65 nm node and beyond. In this paper we discuss methods and applications that combine illumination engineering and structured targets which enable sensitivity to nanometer scale changes using optical imaging methods. These methods have been investigated using simulation tools and experimental laboratory apparatus. The simulation results have demonstrated substantial sensitivity to nanometer changes in feature geometry. Similar results have now been observed in the laboratory. In this paper we will show simulation data to motivate the use of low numerical aperture and structured illumination optical configurations. We will also present the basic elements and methods which we are now using in the design of an optical tool specifically designed for these types of measurements. Target configurations which enhance the scattered electromagnetic fields will be shown along with experimental verification of the methodology. The simulation and experimental apparatus is used to explore and optimize target geometry, optical configurations, and illumination structure for applications in both critical dimension and overlay metrology.

Atomic-resolution measurements with a new tunable diode laser-based interferometer

We develop a new implementation of a Michelson interferometer designed to make measurements with an uncertainty of less than 20 pm. This new method uses a tunable diode laser as the light source, with the diode laser wavelength continuously tuned to fix the number of fringes in the measured optical path. The diode laser frequency is measured by beating against a reference laser. High-speed, accurate frequency measurements of the beat frequency signal enables the diode laser wavelength to be measured with nominally 20-pm accuracy for the measurements described. The new interferometer design is lightweight and is mounted directly on an ultra-high vacuum scanning tunneling microscope capable of atomic resolution. We report the simultaneous acquisition of an atomic resolution image, while the relative lateral displacement of the tip along the sample distance is measured with the new tunable diode laser Michelson interferometer.

Zero-order imaging of device-sized overlay targets using scatterfield microscopy

Patterns of lines and trenches with nominal linewidths of 50 nm have been proposed for use as an overlay target appropriate for placement inside the patterned wafer die. The National Institute of Standards and Technology (NIST) Scatterfield Targets feature groupings of eight lines and/or trenches which are not resolvable using visible-wavelength bright-field microscopy. Such repetitive patterns yield zero-order images superimposed by interference effects from these finite gratings. Zero-order imaging is defined as the collection of specular reflection from periodic structures without the collection of any possible diffracted beams. As our lines and trenches are formed in different photolithographic steps, the overlay offset can be derived from the relative displacement of these zero-order responses. Modeling this phenomenon will require a thorough characterization of the transmission of light through all points in the optical path as a function of position, angle, and polarization. Linear polarization parallel and perpendicular to these lines and trenches is investigated as a possible enhancer of overlay offset measurement repeatability. In our particular case, nominally unpolarized light proved most repeatable.

Köhler illumination for high-resolution optical metrology

Accurate preparation of illumination is critical for high-resolution optical metrology applications such as linewidth and overlay measurements. To improve the detailed evaluation and alignment of the illumination optics, we have separated Koehler illumination into three components. The three Koehler illumination components are defined as full field spatial intensity variation (Koehler factor 1), angular intensity homogeneity (Koehler factor 2), and wavefront phase/intensity homogeneity (Koehler factor 3). We have also proposed a field aperture pattern transfer method to analyze the illumination properties with respect to systematic variations, such as the shape of the source, the intensity distribution at the back focal plane, and the displacements of elements along and off the optical axis. These factors were investigated in both ideal and practical illumination systems. In particular, any angular asymmetry in the illumination proves to have a detrimental effect upon the distribution of light that illuminates the target. Wavefront asymmetry is also studied in the context of an optical system with a coherent or partially coherent light source.

Analysis of Köhler illumination for 193 nm scatterfield microscope

A scatterfield microscope using 193 nm laser light was developed that utilizes angle-resolved illumination for high resolution optical metrology. An angle scan module was implemented that scans the illumination beam in angle space at the sample by linearly scanning a fiber aperture at a conjugate back focal plane. The illumination light is delivered directly from a source laser via an optical fiber in order to achieve homogeneous angular illumination. A unique design element is that the conjugate back focal plane (CBFP) is telecentric allowing the optical axis of the fiber to be scanned linearly. Initial results from full field and angle-resolved illumination are presented and potential applications in semiconductor metrology are described.

Zero‐Order and Super‐Resolved Imaging of Arrayed Nanoscale Lines using Scatterfield Microscopy

Super‐resolution in optical microscopy has been pursued historically through different methods, e.g., dark‐field, phase contrast, and oblique illumination, etc. For each type of microscopy, the ultimate resolution available is dependent upon the amount of a priori knowledge the observer has about the object. The metrology of photolithographically patterned structures on silicon wafers is aided by a foreknowledge of the target design. By manipulating the angular degrees of freedom of the illumination solid angle, our resolution for these patterned wafers is enhanced through what we have termed scatterfield microscopy. Examples of using oblique illumination to control the illumination angle are provided as we image the zero‐order and first‐order diffraction from sub‐resolved overlay targets. Lines with critical dimensions as narrow as λ/8 are resolved using this technique.