Analysis of isolated and aperiodic structures with simultaneous multiple angle of incidence measurements
A method is disclosed for evaluating isolated and aperiodic structure on a semiconductor sample. A probe beam from a coherent laser source is focused onto the structure in a manner to create a spread of angles incidence. The reflected light is monitored with an array detector. The intensity or polarization state of the reflected beam as a function of radial position within the beam is measured. Each measurement includes both specularly reflected light as well as light that has been scattered from the aperiodic structure into that detection position. The resulting output is evaluated using an aperiodic analysis to determine the geometry of the structure.
1. A method for monitoring the process of fabricating an aperiodic structure on semiconductor wafers comprising the steps of:
generating a probe beam of radiation;
focusing the probe beam substantially normal onto the aperiodic structure to create a spread of angles of incidence that extends from the normal to at least sixty degrees;
monitoring the light from the probe beam diffracted from the structure with an array of photodetecting elements and simultaneously generating a plurality of independent output signals corresponding to a range of angles of incidence from the normal of sixty degrees; and
comparing the output signals with a set of expected signals corresponding to the desired geometry of the aperiodic structure to determine if the process is falling within specified tolerances.
2. A method as recited in claim 1 , wherein the structure is an isolated feature.
3. A method as recited in claim 1 , wherein the structure is an isolated feature selected from the group consisting of a line, a hole, a via, a trench and a overlay registration marking.
4. A method as recited in claim 1 , wherein the probe beam is focused to spot less than five microns in diameter.
5. A method as recited in claim 1 , wherein the probe beam is focused to spot less than two microns in diameter.
6. A method as recited in claim 1 , wherein the intensity of the probe beam light is monitored.
7. A method as recited in claim 1 , wherein the change in polarization state of the probe beam light is monitored.
8. A method as recited in claim 1 , wherein probe beam light focused on the sample is coherent.
9. A method as recited in claim 1 , wherein the probe beam is scanned with respect to the structure being evaluated.
10. A method as recited in claim 1 , further including the steps of:
illuminating the structure with an incoherent polychromatic probe beam;
monitoring the reflected polychromatic probe beam;
generating second output signals corresponding to a plurality of wavelengths;
comparing the second output signals with a second set of expected signals corresponding to the desired geometry of the aperiodic structure to determine if the process is falling within specified tolerances.
11. A method as recited in claim 10 , wherein changes in intensity of the polychromatic probe beam are monitored.
12. A method as recited in claim 11 , wherein changes in polarization state of the polychromatic probe beam are monitored.
13. A method as recited in claim 1 , wherein the probe beam is monitored along two orthogonal axes.
14. A method as recited in claim 1 , wherein the probe beam is monitored using a two dimensional array of detectors.