Erik Novak

Performance advances in interferometric optical profilers for imaging and testing

Interferometric optical profilers deliver non-contact, fast, full-field measurements with vertical resolution down to a fraction of a nanometer. Over the last decade advancements to these instruments have allowed for the analysis of not only static but also dynamic objects, like cantilevers and other microelectromechanical system devices, moving or vibrating at up to 1 MHz frequencies. Special objectives and illumination allow for imaging and testing of objects enclosed in environmental or protective chambers or immersed in liquids—including biological samples. Advanced analysis of the interference signal allows for the measurement not just of the surface profile but also transparent coatings.

Template-based software for accurate MEMS characterization

One of the primary challenges in MEMS metrology is the large variety of shapes, lateral feature sizes, and vertical steps on MEMS devices. This paper describes a software approach by which ideal surface templates are generated for each MEMS device from the design files or prior measurements. These templates may contain multiple sub-regions, or data islands, each of which is generally characterized in a different manner. Surface measurements from a white-light interference microscope are matched with the ideal MEMS template using a variety of techniques and threshold criteria. The template-based technique is tolerant of errors both rotation and translation, allowing accurate characterization of each data island and their relative positions. This paper will explore the concepts used to generate templates, align actual data with the original template, and sources of error and robustness of each technique on several datasets. The effect of measurement and positional errors on both the overall match and on the sub-region analyses will be explored for characterization of datasets.

MEMS metrology techniques

The MEMS industry currently produces over

TSOM Method for Semiconductor Metrology

13 billion in annual revenue, with devices in such diverse applications as blood pressure sensors, projection displays, optical switches, printers, hard drives, and gyroscopes. As production techniques improve, ever more functions may be served by MEMS, and the industry is growing at an annual rate of more than 15%. The large diversity of MEMS leads to many challenges in metrology, as each design has different critical factors which will affect its performance. Unlike traditional semiconductor devices, MEMS require characterization both in their static state and under actuation. Parameters of interest include shape, dimensions, surface roughness, sidewall angles, film thickness, residual stress, feature volumes, response times, thermal properties, resonance frequencies, stiction, environmental immunity and more. This talk will discuss the strengths and weaknesses of a variety of techniques for MEMS surface metrology. Bright- and dark-field microscopy, scanning electron microscopy, contact and non-contact surface profilometry, atomic force microscopy, laser Doppler vibrometry and digital holography are some of the primary techniques used to evaluate MEMS surfaces and motion. While no single technique can fully characterize all MEMS devices, or even one device under all conditions, the utility of each of the different types of instruments is increasing as they are pushed by MEMS and other industries to provide more characterization capability. With a broad understanding of the various metrology techniques available, the one or few critical instruments to measure a given class of devices will hopefully be more easily understood.

Dynamic MEMS measurement using a strobed interferometric system with combined coherence sensing and phase information

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.

Transfer function characterization of laser Fizeau interferometer for high-spatial-frequency phase measurements

To assure proper MEMS performance, accurate metrology of the devices during actuation is required. Stiction, motion versus drive signal, radii of curvature changes, angular repeatability and resonant frequency are all of interest. By obtaining topographic information as these drive conditions are varied, MEMS devices can be fully characterized. Interference microscopes have long been used to obtain MEMS topography. Through the addition of a strobed-illuminator, the motion of a device can be effectively frozen. By varying the phase, frequency and drive signal of the strobe and MEMS device, a complete picture of the motion can be obtained. Topologies and motions of several millimeters may be measured with nanometer-level accuracy. This paper describes the design of the hardware and software for a strobed interference microscope for MEMS metrology. We present data on MEMS devices showing their motion under different conditions and relating that data to critical part parameters.

Application of Ritchey-Common test in large flat measurements

Large, high power laser systems such as that being constructed by Lawrence Livermore National Laboratories for the National Ignition Facility require accurate measurements of spatial frequencies of up to 2.5 lines/mm over a 100mm field of view.In order to ensure accurate measurements of the parts, the test apparatus must be well characterized. The systems transfer function (STF) of the interferometer under development to perform these measurements was calculated by comparing the power spectra of measurements of known phase objects to their theoretical power spectra. Several potential problem areas were identified and studied. Of primary concern was the effect on the STF of the rotating diffuser and incoherent relay system employed in most commercial laser Fizeau interferometers. It was determined that such an arrangement degraded the transfer function beyond acceptability. The other major concern was possible inability to measure certain frequencies due to propagation between the test piece and alignment of the system optics.Use of strictly coherent imaging and small propagation distance between the test piece and return flat, the system transfer function could be kept at acceptable levels within the range of interest.

Effects of source shape on the numerical aperture factor with a geometrical-optics model

The Ritchey-Common test is a well-known method for large flat measurements. This paper describes a straightforward implementation ofthe formulas, to allow accurate surface height calculation using relatively few separate measurements. Both Ritchey-Common test and direct measurement results are presented. In comparison of the two methods, the RitcheyCommon test is in good agreement with the direct measurement.

Application of grazing incidence interferometer to rough surface measurement

We study the effects of an extended light source on the calibration of an interference microscope, also referred to as an optical profiler. Theoretical and experimental numerical aperture (NA) factors for circular and linear light sources along with collimated laser illumination demonstrate that the shape of the light source or effective aperture cone is critical for a correct NA factor calculation. In practice, more-accurate results for the NA factor are obtained when a linear approximation to the filament light source shape is used in a geometric model. We show that previously measured and derived NA factors show some discrepancies because a circular rather than linear approximation to the filament source was used in the modeling.

Surface profiler for fixed through-glass measurement

In practical grazing incidence measurements using a commercial interferometer, repeatability and accuracy suffer from several effects related to part placement. In this paper, the authors analyzed three primary error resources in grazing incidence interferometry: incidence angle error, rotation angle error and included angle. The mathematical foundations of these errors as well as measurement results of several samples are presented showing the effects of each of these errors on. Using the test procedures described in the paper, the errors can be greatly reduced and the repeatability and accuracy increased by 20%.