Joseph E. Griffith

Dimensional metrology with scanning probe microscopes

We review the application of scanning probe microscopes to dimensional measurement of topographic features. Probe microscopes show great promise as metrology tools because they produce three‐dimensional data over almost all solids in a wide range of ambients. Even though these microscopes readily achieve atomic resolution, there are several aspects of their behavior that can cause them to exhibit large measurement errors. The actuators that drive the probe exhibit hysteresis and creep, so they must be independently monitored. In addition, the geometry of an extended probe moving across the surface makes probe‐sample interaction intrinsically nonlinear. Forces on the probe that cause it to flex are another source of inaccuracy. Probe fabrication and characterization are, consequently, important issues. We describe present understanding of these problems and the techniques being developed to solve them.

Scanning probe tips formed by focused ion beams

Probe tips for scanning tunneling microscopy have been sharpened using focused ion beam milling. Reproducible tips were formed on polycrystalline W and Pt‐Ir shanks, but this technique is not limited to these materials. The tips were found to have cone angles of 12±3° and radii of curvature as sharp as 4 nm. Focused ion beam machining allows precise control of the final shape of the tips which is important in metrology measurements of various nanostructure devices.

A rocking beam electrostatic balance for the measurement of small forces

There exists interest in the measurement of small forces for applications such as microtopography of semiconductor devices and atomic force microscopy. A new method is introduced here in which a small silicon beam, that is acted on by the external force of interest, has its position sensed by an rf phase shift technique. The position information in turn is fed back via electrostatic forces to continuously rebalance the beam about its central support. This force‐feedback approach provides high sensitivity, submillisecond response, inherent force calibration, and electronically controlled stiffness.

Scanning probe metrology

The design of a scanning probe microscope suitable for metrology applications must include solutions to several problems. Actuator errors can be large because of their nonlinear behavior, but this can be solved by independently monitoring the actuators motion. The probe must be shaped properly for a given measurement, and it must be characterized to allow interpretation of the measurement. We have studied the effects of interaction forces and probe shape with emphasis on surface roughness measurements.

Fabrication of optical fiber probes for nanometer‐scale dimensional metrology

The fabrication of cylindrical probes having diameters as small as 50 nm is described in this article. The planar endface (advantageously oriented perpendicular to the axis of the cylindrical probe) and sharp 90° corners of the end portion of the probe enable accurate measurement of a feature being scanned, even at sudden jumps in the surface. Conical and flaired probes can also be fabricated with variations of this technique. The fabrication techniques described in this article are simple and inexpensive; only a Teflon beaker, optical fiber, etching solution, polymer solution, fiber cleaver, and optical microscope are necessary.

Rocking-beam force-balance approach to atomic force microscopy

Abstract We report on the use of a force-balance atomic force microscope to obtain images with nanometer-scale resolution. In addition, force curve measurements were obtained on a mica surface in air with a sensitivity of 10 -8 N.

Probe characterization for scanning probe metrology

Abstract Precision probe metrology requires that the probe be carefully formed and measured. We demonstrate a method to accurately measure the shape of a probe in situ by scanning special measurement structures.

Metrology with scanning probe microscopes

One of the more demanding requirements of sub -micron lithography is dimensional measurement of the patterned features. Probe microscopes can perform this task nondestructively on most solids in a wide range of ambient conditions, though not without careful attention to several sources of measurement error. The most serious problem arises from the finite size of the probe, which has an intrinsically nonlinear interaction with the surface to be measured.

Dimensional Metrology of Phase-Shifting Masks with Scanning Probe Microscopes

Critical dimension metrology of a phase-shifting mask must include depth as well as width measurements because both the phase and the lateral position of the transmitted photons must be controlled. Scanning probe microscopes are well suited to perform these measurements because they achieve high resolution simultaneously in all three dimensions. As with other microscopes, the probe-sample interaction strongly affects critical dimension measurement. The shape of the probe mixes with the measured object in an intrinsically nonlinear manner. We present measurements of phase-shifting masks performed with a scanning force microscope, and we discuss how they illustrate the capabilities and limitations of the technique.

Metrology test reticles for advanced optical lithography

Increased lithographic performance has been the key enabler for the continued reduction of minimum device feature sizes down to 0.25 micrometers and beyond. However, this increase in performance has been accompanied by the added fabrication complexity of the various types of lithographic reticles. In addition to having submicrometer sized features, advanced optical masks are three dimensional in nature with high aspect ratio features of different materials in close proximity to each other. Lithographic process latitude, which determines the ultimate feasibility of that process, is critically dependent upon the level of measurement and control of these mask features. Standard reference materials are needed in order to improve the accuracy of the mask measurements, but do not yet exist. To initiate progress in this area, a set of test reticles has been fabricated to serve as in-house calibration standards and to study various phenomena affecting three-dimensional submicron dimensional metrology for advanced optical masks. The first member of the set, known as the Lateral Resolution Tester (LRT), contains chromium features on unetched quartz (opaque and partially transparent) having linewidths as narrow as 200 nm. The PSM Feature Tester contains many types of phase-shifting mask patterns with varying lateral dimensions. The Herschel Tester contains various phase-shifting apertures of different depths and widths. All of the patterns and concepts used in the set have been brought together in order to produce a new PSM metrology test/calibration mask known as the PSM Round Robin Reticle (RRR). The types of patterns as well as the techniques used to measure them are presented. The RRR will also be used as the test vehicle for a round robin comparison of measurements taken with metrology tools at different mask shops and to determine optimum designs for future PSM metrology calibration standards.