William B. Penzes

Toward nanometer accuracy measurements

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.

Hybrid optical-electrical overlay test structure

This paper describes the exploratory use of electrical test structures to enable the calibration of optical overlay instruments of the type used to monitor semiconductor-device fabrication processes. Such optical instruments are known to be vulnerable to hard-to-determine systematic process- and instrument-specific errors known as shifts. However, these shift errors generally do not affect electrical test-structure measurements extracted from the same features. Thus the opportunity exists to configure physical standards having overlay targets that can be certified by electrical means, thereby enabling estimates of the shifts prevailing in a particular application. In this work, a new hybrid test structure, meaning one from which overlay measurements can be extracted electrically, as well as by optical instruments, has been designed and fabricated with built-in overlay values ranging from -60 to +60 nm. A selection of structures constituting a test chip has been patterned in a single conducting film with CD (critical dimension) design rules ranging from 1.0 /spl mu/m to 2.0 /spl mu/m and fabricated and tested. Electrical overlay parameters, derived from multiple step-and-repeat die-site measurements, generally match the corresponding optical measurements to within several nanometers, subject to limitations of the pattern-replication process. This paper focuses on the extraction of overlay from the electrical measurements, the dependence of the measurements on CD design rules, and their comparison with the corresponding measurements made both by a commercial optical-overlay instrument and by a coordinate-measurement system having measurements traceable to absolute dimensional standards. It is presented as a first step toward the use of electrical measurements for advancing shift management in optical overlay of features patterned in separate lithographic processes.

Test structure for the in-plane locations of project features with nanometer-level accuracy traceable to a coordinate measurement system

A new test structure is reported. It is designed to measure the positions of the images of an array of features projected from a mask into a resist film on substrate with accuracy better than 10 nm. The resist film on the substrate covers a nominally matching array of partially formed versions of the test structure prepatterned in a conducting film. Instances of the finished structure are formed on the substrate by further selective removal of conducting material from the partially formed test structures where they are overlaid by images of the fiducial marks on the mask. At each array point, the feature of the completed test structure that is defined by the overlay of the image of the fiducial marks on the mask is called the pointer. The part of the partially formed test structure that is unaffected by the overlay of the images of the fiducial marks on the mask serves as a ruler. Electrical testing accurately provides the precise location of the pointer relative to the ruler within each test structure. The locations of the rulers prepatterned on the substrate are determined with a coordinate measurement system (CMS) called the NIST (National Institute of Standards and Technology) Molecular Measuring Machine (M-Cubed).<<ETX>>

Grating Pitch Measurements With the Molecular Measuring Machine

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.

Two-dimensional calibration artifact and measurement methodology

In this paper we describe our design and the manufacturing of a 2D grid artifact of chrome on quartz on a 150 mm X 150 mm X 6.35 mm plate. The design has been agreed upon by a number of Semiconductor Equipment Manufacturers International participants working on a 2D grids calibration task force within the microlithography committee of SEMI. We present the measurement procedures and describe the algorithms used in the measurement process. We have procured a prototype artifact which is expected to be developed into a National Institute of Standards and Technology (NIST) distributed standard reference material once the final design has been agree upon. We will present measurements made at leading industrial sites and develop a traceability chain based on these measurements in combination with NIST based measurements. The artifact has been measured on the NIST linescale interferometer as well as other NIST metrology tools. We will also present the status of the comparisons between these measurements and those performed by the industrial collaborators.

A new method to measure the distance between graduation lines on graduated scales

Line scales are used throughout industry for a variety of applications. The most common is the stage micrometer, a small, graduated glass scale for the calibration of optical instruments such as microscopes. However, stage micrometers are generally not calibrated, except for critical applications, due to the time and cost of optical calibration techniques. A method for calibrating line scales is presented which uses electrical test structure metrology. A description of the technique as well as examples of results from this technique are presented.

Metrology standards for advanced semiconductor lithography referenced to atomic spacings and geometry

It is shown how the needs for calibrating the positional accuracy of features of an X-ray mask membrane or an optical reticle can be addressed by application of a high-accuracy coordinate metrology system known as the Molecular Measuring Machine (M-Cubed). Based on scanning tunneling microscopy and state-of-the-art heterodyne optical interferometry, the measurements of M-Cubed are referenced to fundamental standards of length and angle and with the atomic-resolution of its scanning tunneling microscope probe are validated against the interatomic spacings and geometry of single crystal surfaces. Through the use of a stable reference grid, serving as an intermediate calibration artifact, the positional accuracy of features on an X-ray mask membrane or an optical reticle can be referenced to fundamental standards of length and angle by means of the metrology system of M-Cubed.<<ETX>>

Low noise broadband modulated preamplifiers for a variety of transducers

Abstract Often, increases in signal-to-noise ratio and basic sensitivity are gained by the use of carrier modulation. Further advantage can be taken of this process by the use of varactors. These are suitably biased solid-state diodes and transistors, usually used to provide capacitance reactances. Their capacitance is changed by applying a voltage. The change in capacitance available by applying to their terminals the electrical output of a transducer—such as many of the sensors for light, heat, sound, mechanical stress and deformation—can be used in suitable modulating circuits. The magnitudes of reactance available from varactors fall conveniently into the range of values suitable for the carrier frequencies now accessible by means of another stable, mass produced solid-state device, the crystal clock oscillator. In general, the complexity of the carrier circuit to be chosen depends upon the sensitivity and noise floor requirements. This paper describes circuits for applying the modulation process, and discusses the hierarchy of choices for the methods used. The authors have built circuits with an input noise level equivalent to 0·3 μV for signals over the entire audiofrequency range, having a gain in excess of 50 dB.

Electrical Test Structure for Overlay Metrology Referenced to Absolute Length Standards

This test structure is based on the voltage-dividing potentiometer principle and was originally replicated in a single lithography cycle to evaluate feature placement by a primary pattern generator. A new test structure has now been developed from the single-cycle version and has been used for measuring the overlay of features defined by two different exposures with a stepping projection aligner. The as-measured overlay values are processed by an algorithm that minimizes the effects of nominal random pattern imperfections. The algorithm further partitions measurements of overlay into contributions that derive, respectively, from misregistration of the image fields projected by the two masks and from the drawn misplacement of features on the masks. The numerical estimates of these contributions so obtained from the electrical measurements were compared with those extracted from the same features by the NIST line scale interferometer, providing traceability to absolute length standards. The two sets of measurements were found to agree to within the several-nanometer uncertainty cited for the line scale interferometers readings alone.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

New test structure for nanometer-level overlay and feature-placement metrology

A new electrical test structure for overlay measurement has been evaluated by replicating arrays of its complementary components from two different photomasks into a conducting film on a quartz substrate. The features resulting from images projected from the first mask were used as a reference grid which was calibrated by the NIST line-scale interferometer. A first subset of the relative placements of the images projected from the second mask, which were derived from the electrical overlay measurements and the reference grid, agreed to within 13 nm with corresponding measurements made directly by the line-scale interferometer over distances up to 13.5 mm. A second comparison made at another substrate location indicated that gradients of projected feature linewidths across the exposure site may need to be measured, and corrected for, in the electrical extraction of overlay. >