Photomask pattern sizes are usually defined by a one-dimensional Critical Dimension (CD). As mask pattern shapes
become more complex, a single CD no longer provides sufficient information to characterize the mask feature. For
simple square contacts, an area measurement is generally accepted as a better choice for determining contact uniformity.
However, the area metric may not adequately characterize complex shapes; it does not lend itself to CD metrology and it
ignores pattern placement. This paper investigates new ways of measuring complex mask shapes with aggressive Optical
Proximity Correction (OPC). An example of more informative metric is center of gravity. This new metric will be
compared to more traditional mask characterization variables like CD mean to target, CD uniformity, and Image
Placement (IP). Wafer simulations of the mask shapes will be used to understand which mask pattern metrics are most
representative of the image transferred to wafer images. The results will be discussed in terms of their potential to
improve mask quality for 32nm technology and beyond.
KEYWORDS: Electron beams, Metrology, Image resolution, Electron microscopes, Scanning electron microscopy, Process control, Photomasks, Critical dimension metrology, Beam controllers, OLE for process control
Measurement of resist critical dimensions (CDs) utilizing a scanning electron microscope (SEM)
based metrology system causes the resist to change due to irradiation effects of the electrons. A new
and novel scanning approach has been developed in an effort to minimize the effects electron
irradiation and exposure during the measurement process. This technique is especially pertinent in
view of the tightening requirements for process control to achieve single digit CD uniformity on
leading edge photo masks being produced today. The measurement of OPC features necessitates
utilization of SEM based metrology due to resolution requirements, but the effects of high
magnification imaging presents unique challenges. By controlling the scanned region of interest
(ROI) it is possible to reduce exposure and irradiation effects. This paper will detail this new
approach as it is utilized on the LWM9045 SEM Metrology system. The LWM9000SEM mask CD
SEM was introduced earlier.
The application of aggressive Optical Proximity Correction (OPC) has permitted the extension of advanced lithographic
technologies. OPC is also the source of challenges for the mask-maker. Small shapes between features and highly-fragmented
edges in the design data are difficult to reproduce on masks and even more difficult to measure exactly with
CD-SEM, which requires not only tool stability but also better measurement methods. To cope with this problem, we
have been focusing on finding better methods for measuring actual mask Critical Dimension (CD) that would show a
good correlation to wafer CD. In BACUS 2006, we presented an effective measurement for closed patterns, which is
"area measurement". In time paper we are introducing new potential solution, which include a reliable method, distance
measurement, for certain types of unclosed patterns.
For instance, we evaluated an unclosed pattern which couldn't be measured with Region of Interest (ROI) that is large
enough, and found a reliable method, Distance ROI. Though the method has a major drawback of image tilt, we also
found an approach to avoid this. Finally we verified that Distance ROI could be new solution for unclosed patterns by
jointly applying tilt monitoring, beam rotation correction, and area scan.