Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1997-05-15
2001-11-13
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754120, C324S765010, C324S501000
Reexamination Certificate
active
06316950
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to imaging of semiconductor devices and, in particular, to methods and apparatus for imaging such devices using two-photon absorption.
BACKGROUND OF THE INVENTION
Imaging of structures within semiconductors is of interest not only as a research tool but also as a technique of considerable practical importance in the design, fabrication and testing of semiconductor electronic and optoelectronic devices such as integrated circuits. The operating components of such devices are tiny structures having sub-micron features that can be meaningfully viewed only with microscopic techniques.
Considerable difficulty is encountered in viewing a state-of-the-art semiconductor device because plural layers of connective metallization overlie the operative components on the top and a relatively thick silicon layer underlies the components on the bottom.
One approach to microscopically imaging the components is to use optical beam induced current imaging. A focused beam of light at a frequency suitable for exciting electrons from the semiconductor valence band to the conduction band is scanned over the semiconductor chip and the resulting current is measured. From the current generated and the location of the scanning beam, a computer with image processing software can generate an image representative of the features of the device.
Since the top is usually covered with metal, the device is usually scanned through the bottom. The difficulty with this approach, however, is that the beam encounters absorption in passing through the underlying substrate before it reaches the active layer on the upper surface of the device. This reduces the light available for exciting current at the component-rich active layer and superimposes spurious background effects. The result is limitation on the precision with which components can be imaged. Accordingly there is a need for improved methods and apparatus for imaging semiconductor devices.
SUMMARY OF THE INVENTION
In accordance with the invention, semiconductor devices are imaged using two-photon absorption. The method is similar to conventional optical beam induced imaging except that the light beams used have frequencies (photon energies) insufficient to excite electrons across the semiconductor bandgap. Rather the instantaneous intensity of the lower frequency light is increased, as by using a pulsed laser source, so that electron transitions occur by two-photon absorption predominately in the localized region where the beam is focused. The result is minimal absorption during passage through the substrate and maximal absorption in the component-rich active layer where the beam is focused. This enhances imaging of fine-detail semiconductor devices. Specifically, the quadratic dependence of free carrier generation on the excitation intensity both enhances the resolution and provides a three-dimensional sectioning capability.
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Denk Winfried
Xu Chunhui
Brown Glenn W.
Lucent Technologies - Inc.
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