James E. Potzick

Comparisons of measured linewidths of submicrometer lines using optical, electrical, and SEM metrologies

An investigation is being carried out to determine the ability of three methods of linewidth metrology to measure the dimensions of features of less than 0.5 micrometers . The three methods are transmitted-light optical microscopy, electrical test structure, and scanning electron microscopy (SEM). To permit the inclusion of transmitted-light optical microscopy in this investigation, 100-nm thick Ti films were patterned using normal VLSI processing techniques on a 150-mm diameter quartz wafer. The cross-bridge resistor test structure was used since this structure has been widely used in industry and it allows the results from all three metrological techniques to be compared. The design bridge widths of the test structures range from 0.4 micrometers to 1.0 micrometers . The results of these measurements show systematic and uniform offsets between the different techniques. In this paper we discuss the different techniques and describe the observed results.

TSOM Method for Semiconductor Metrology

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.

Noise averaging and measurement resolution (or “A little noise is a good thing”)

When a continuous quantity is measured with a digital instrument or digitized for further processing, a measurement uncertainty component is incurred from quantization of the continuous variable. This uncertainty can be reduced by oversampling and averaging multiple measurements, but only if there is some noise on the measurand. In this article the optimum noise level is determined, and the subsequent improvement in measurement uncertainty calculated.

Metrology with the ultraviolet scanning transmission microscope

A novel design for an ultraviolet critical dimension measurement transmission microscope utilizing the Stewart platform as the rigid main structure has been implemented. This new design shows improved vibration characteristics and is able to accommodate large specimens. We present alignment procedures and their relevance to obtaining accurate linewidth measurements. Initial measurements with the new system comparing visible and ultraviolet wavelength illumination show expected characteristic dependence of the intensity image as a function of wavelength.

Problem with submicrometer-linewidth standards and a proposed solution

Traceable linewidth measurements of tiny features on photomasks and wafers present interesting challenges. Usually technical solutions exist for the problems encountered, but traceability can be costly in time and labor. A measurement is useful only if its value exceeds its cost. One such problem is that an optical or electron microscope forms a scaled image of the linewidth object, which is measured instead of the object itself. Interpreting this image to identify the objects edges in it can be difficult, because the image depends on instrument and object parameters (such as topography and materials) not directly related to the linewidth. Use of a linewidth standard can provide a traceable measurement if the relevant object parameters are known to match those of the standard. In the majority of cases, however, the object being measured and the linewidth standard will differ in topography and/or materials. Then the instrument images of object and standard will also differ, leading to possible measurement errors. Traceability requires that these measurement errors be quantified--a costly prospect. One can calculate the measurement error resulting from object/standard parameter differences. Then a measurement error tolerance can be chosen, and parameter tolerances can be found corresponding to a parametric measurement uncertainty that is consistent with it. These parameter tolerances define islands of tolerance in parameter space over which the parametric uncertainty will be tolerated. The larger the islands, the greater the measurement uncertainty, but the greater the number of different objects whose parameter differences fall on the island. While the calculations for the islands of tolerance may be complex, a single island can apply to a whole group of different objects, reducing the cost of measuring the parameters and calculating their effects. This is a way to obtain traceable linewidth measurements while balancing measurement cost and measurement uncertainty.

Reevaluation of the accuracy of NIST photmask linewidth standards

Every artifact measurement standard has some uncertainty associated with its calibration, and the NIST Photomask Linewidth Standards are no exception. The methods of estimating measurement uncertainty, however, and the interpretation of its meaning, have been subjects of discussion for many years. The International organization for Standardization (ISO) has recently published a Guide to the Expression of Uncertainty in Measurement, which outlines an operational procedure for estimating and combining the effects of the various contributing factors. A new NIST policy on measurement uncertainty has led to a change in the method of determining the uncertainty of NIST Photomask Linewidth Standards to comply with ISO recommendations. Consequently, their calibration uncertainty has changed. The new method of evaluating systematic components leads to increased systematic uncertainty, and the new method of combining uncertainty components leads to reduced calibration uncertainty. This paper describes the old and new methods and compares their results.

Comparison of Measurement Techniques for Linewidth Metrology on Advanced Photomasks

This paper compares electrical, optical, and atomic force microscope (AFM) measurements of critical dimension (CD) made on a chrome on quartz photomask. Test structures suitable for direct, on-mask electrical probing have been measured using the above three techniques. These include cross-bridge linewidth structures and pairs of Kelvin bridge resistors designed to investigate dimensional mismatch. Overall, the results show very good agreement between the electrical measurements and those made with a calibrated CD-AFM system, while the optical metrology system overestimates the measured width. The uncertainty in each of the measurements has been considered, and for the first time an attempt has been made to describe the levels and sources of uncertainty in the electrical measurement of CD on advanced binary photomasks.

Accuracy and traceability in dimensional measurements

While the importance of the concepts of measurement accuracy and measurement traceability has long been recognized, they have not always been applied in a consistent or rigorous manner. In 1993 the International Organization for Standards (ISO) published two documents which establish consistent definitions of these and other metrology terms and provide an unambiguous way to calculate measurement uncertainty. These documents are widely accepted in the international metrology community. Vendors and buyers of metrology tools and standards will benefit from the improved communication that results from using this standardized metrology vocabulary. Dimensional measurements of importance to microlithography include feature sizes and feature placement on photomasks and wafers, overlay eccentricities, defect and particle sizes on masks and wafers, step heights, and many others. A common element in these measurements is that the object sizes and required measurement uncertainties are often on the order of the wavelength of light or less. This can lead to interesting challenges for certain applications where measurement traceability is desirable. The necessary and sufficient conditions for traceability will be outlined, and some examples will be given.

Accuracy in integrated circuit dimensional measurements

The measurement of critical dimensions of features on integrated circuits and photomasks is modeled as the comparison of the images of the test object and of a standard object in a measuring device. A length measuring instrument is then a comparator. The calibration of the standard and the conditions necessary for a valid comparison are discussed. The principles discussed here apply to many other types of measurement as well.

Automated Calibration Of Optical Photomask Linewidth Standards At The National Institute Of Standards And Technology

An automated system has been developed at the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards, for calibrating optical photo-mask linewidth standards. This system, controlled by a desktop computer, locates each feature to be measured in the field of view of the microscope, centers and focuses the image, scans the image, and calculates the optical linewidth from the scan data. The results are checked for errors and the process repeated until every feature on the photomask has been calibrated. If statistical tests are passed, a calibration certificate is printed.