Measurement and control of edge profiles and edge angles is increasingly important in advanced lithography. Especially
for critical dimension metrology a sophisticated multi-dimensional shape metrology is highly beneficial. Different types
of dimensional metrology instrumentation are in use today for edge profile and edge angle measurement. While
destructive cross section SEM measurements often serve as reference, AFM and optical scatterometry systems are
commonly used for day-to-day or in-line control. Due to the limitations of these metrology systems (AFM: slow,
scatterometry: only integral measurements of periodic structures), the evaluation and modelling of top down SEM
images is increasingly considered, too.
At the PTB both SEM and AFM as well as optical scatterometry are applied for edge angle and/or edge profile
metrology, supported by optical transmission microscopy. At the PTB we have realised a new DUV hybrid scatterometer
for measurements over the full range of 6025 format masks which combines essential elements of a reflectometer, an
ellipsometer, and a diffractometer. In addition to scatterometric measurements this set-up allows to measure the complete
Müller-matrix including transmission, polarisation and depolarisation. This new set-up will be presented in detail.
Finally we study the possibilities of evaluating high resolution top down SEM images to determine edge angles. The
potential of edge angle evaluation using these new analysis procedures will be discussed. We present an overview of the
PTB measurement capabilities with an emphasis on newly developed metrology methods and systems.
A new algorithm for SEM CD evaluation of trapezoidal line structures is presented. It is based on the physical modeling
of SEM image formation and allows the assignment of top and bottom structural edge positions to the SEM signal. The
SEM image profile is described by a set of piecewise continuous functions which is convoluted with the electron probe
intensity profile. The resulting function is fitted to the measured signal profile by a least squares algorithm. The fit
returns both top and bottom edge positions as well as the electron probe diameter. The algorithm is verified against three
different Monte Carlo simulation programs using different physical models of elastic and inelastic electron scattering and
secondary electron generation and transport. The effect of the physical modeling on the evaluated critical dimension is
discussed and the absolute CD deviation of the algorithm is determined for different sets of specimen and tool
parameters like edge slope angle, beam energy, and electron probe diameter.
The PTB has developed a new 6025 photomask standard for calibration of CD metrology tools in a joint project with partners from mask industry in Germany . We report on the design of the standard, its calibration procedures and the results of recent round robin measurements on this standard in which different CD metrology tools of the project partners were involved.
The layout of the CD photomask standard (COG and 193 nm halftone PSM) contains isolated as well as dense features in both tones with nominal CD down to 100 nm. Calibration of the standards was performed at PTB by UV microscopy and LV-SEM, supported by additional AFM measurements for edge slope characterization. For analysis of UV microscopy as well as SEM images appropriate signal modeling was applied, which allowed to extract the feature widths at the top of the structures as well as the widths at 50% height of the structures.
In this contribution we will also discuss results of a recent round robin comparison measurement performed on up-to-date metrology tools available for CD metrology today by means of one of the newly developed CD standards. We used PTB calibrated CD standards in order to provide a set of CD references for the different tools which then should be used in a "blind" comparison to calibrate an unknown CD mask of the same design as the standards. Different type of CD metrology instrumentation, namely standard UV and DUV optical as well as DUV water immersion CD microscopes, a new UV dark field optical microscope, CD-SEM, and AFM/SFM were applied. The outcome of this comparison on cur-rently applied metrology instrumentation provides a valuable source of information for cross calibration issues which are discussed in mask industry today.
The need for fast and accurate inspection of small sample features is eminent considering the developments in micro and nano technology. The scanning probe microscope (SPM) offers extreme resolution and even accuracy when properly calibrated but the principle of operation result in inherently slow acquisition of the measurement data. Therefore scanning probe microscopes are rarely deployed in industrial in-line inspection and quality control where the time aspect usually is critical. We propose an alternative mode of operation that can considerably speed up SPM measurements. In this mode only the areas of interest are probe with maximum accuracy while the rest of the imaging is ignored. The measuring mode is best suited when a-priori information of the surface is available, like in an industrial production line.
We will report on the progress of our project to realize a traceable Scanning Probe Microscope at the Van Swinden Laboratorium of the Nederlands Meetinstituut in the Netherlands. The traceable Atomic Force Microscope (AFM) is constructed from a separate AFM head, a 3D translation stage and an accurate 3D laser interferometer system. Nanometer uncertainty can be maintained in the entire scanning volume of 100 μm × 100 μm × 20 μm. Apart from providing direct traceability to the SI unit of length, the coordinates provided by the laser interferometer are also used in a closed loop position feedback controller to realize accurate positioning at arbitrary locations within the volume provided by the translation stage. In this paper we will emphasize the development of the control system.