Technique for CD measurement on the basis of area fraction determination
The present invention provides a technique for estimating critical dimensions of highly scaled circuit features on the basis of scanning electron microscopy, wherein area fractions of a scan area are determined. Preferably, the SEM is operated with high electron beam energies to enhance the overall resolution and to reduce edge effects and image artifacts. Thus, fast and statistically significant measurement results may be obtained, thereby allowing enhanced process control.
1. A method of monitoring critical dimensions of features formed above a substrate, the method comprising:
scanning an area of said substrate by a scanning inspection tool to obtain an image of said area, said area including at least a portion of one or more of said features;
determining an area fraction in said image on the basis of at least one image parameter and a predefined threshold for said parameter; and
using said area fraction as a measure of a mean value of said critical dimension in said area.
2. The method of claim 1 , wherein said inspection tool comprises a scanning electron microscope (SEM).
3. The method of claim 2 , wherein scanning said area comprises directing a beam of primary electrons onto a surface comprising said area, said beam defining a beam axis, wherein said beam axis is oriented substantially perpendicular to said surface.
4. The method of claim 2 , wherein scanning said area comprises directing a beam of primary electrons onto a surface comprising said area and wherein an acceleration voltage of said primary electrons is approximately 5 kV or more.
5. The method of claim 4 , wherein said acceleration voltage is approximately 10 kV or more.
6. The method of claim 1 , further comprising:
selecting a second area on said substrate;
scanning said second area by said scanning inspection tool to obtain an image of said second area, said second area including at least a portion of one or more of said features;
determining a second area fraction in said image of the second area on the basis of at least one image parameter and said predefined threshold for said parameter; and
using said second area fraction and said area fraction for evaluating uniformity of said features across said substrate.
7. The method of claim 1 , wherein said image parameter represents an intensity value of pixels of said image.
8. The method of claim 1 , wherein said area is a portion of a test structure in a die region of a semiconductor device.
9. The method of claim 8 , wherein said features represent line-like features that are formed in a common manufacturing process for forming gate electrode structures.
10. An inspection system, comprising:
a measurement section configured to generate an image signal in response to scanning a selected area of a surface portion of a substrate;
an image processing section configured to establish an image of said selected area on the basis of said image signal;
an area calculation section configured to determine an area fraction in said image on the basis of a threshold for a predefined image parameter; and
a comparison unit configured to compare said area fraction with a reference value for estimating critical dimensions of features contained in said area.
11. The inspection tool of claim 10 , wherein said measurement section comprises a scanning electron microscope.