Nanometrics Inc.

Roger R Lowe-Webb

Weidong Yang

John D Heaton

James Matthew Holden

Richard A Yarussi

Pablo I Rovira

William A McGahan

Silvio J Rabello

Guoguang Li

Blaine R Spady

Guonguang Li

A diffraction based overlay metrology system produces the overlay error independent of effects caused by local process variations. Generally, overlay patterns include process variations that provide spectral contributions, along with the overlay shift, to the measured overlay error. The contributions from process variations are removed from the determined overlay error. In one embodiment, the local process variations are removed by measuring the overlay pattern before and after the top diffraction gratings are formed. A plurality of differential spectra from the measurement locations of the completed overlay pattern can then be used with a plurality of ratios of differential spectra from measurement locations of the incomplete overlay pattern can then be used to determine the overlay error by either direct calculation or by fitting techniques. In another embodiment, the local process variations are removed with no premeasurement but with careful construction of the overlay patterns.

Local process variation correction for overlay measurement

A normal incidence reflectometer includes a rotatable analyzer/polarizer for measurement of a diffracting structure. Relative rotation of the analyzer/polarizer with respect to the diffracting structure permits analysis of the diffracted radiation at multiple polarity orientations. A spectograph detects the intensity of the spectral components at different polarity orientations. Because the normal incidence reflectometer uses normally incident radiation and an analyzer/polarizer that rotates relative to the diffracting structure, or vice-versa, the orientation of the diffracting structure does not affect the accuracy of the measurement. Thus, the sample holding stage may use X, Y, and Z, as well as r-&#x3b8; type movement and there is no requirement that the polarization orientation of the incident light be aligned with the grating of the diffraction structure. A non-linear multivariate regression process is used to adjust the parameters of an optical model, such as rigorous coupled-wave analysis, to provide a match with the measured data.

Spectrometer measurement of diffracting structures

An alignment target includes periodic patterns on two elements. The periodic patterns are aligned when the two elements are properly aligned. By measuring the two periodic patterns at multiple polarization states and comparing the resulting intensities of the polarization states, it can be determined if the two elements are aligned. A reference measurement location may be used that includes third periodic pattern on the first element and a fourth periodic pattern on the second element, which have a designed in offset, i.e., an offset when there is an offset of a known magnitude when the first and second element are properly aligned. The reference measurement location is measured at two polarization states. The difference in the intensities of the polarization states at reference measurement location and is used to determine the amount of the alignment error.

Alignment target to be measured with multiple polarization states

An alignment target includes periodic patterns on two elements. The alignment target includes two locations, at least one of which has a designed in offset. In one embodiment, both measurement locations have a designed in offset of the same magnitude but opposite directions. For example, two separate overlay patterns that are mirror images of each other may be used. Alternatively, the magnitudes and/or directions may vary between the measurement locations. The radiation that interacts with the measurement locations is compared. The calculated difference is extremely sensitive to any alignment error. If the difference between the patterns is approximately zero, the elements are properly aligned. When an alignment error is introduced, however, calculated difference can be used to determine the error. In one embodiment, the alignment target is modeled to determine the alignment error. In another embodiment, additional overlay patterns with additional reference offsets are used to determine the alignment error.

Alignment target with designed in offset

A normal incidence reflectometer includes a rotatable analyzer/polarizer, which permits measurement of a diffracting structure. Relative rotation of the analyzer/polarizer with respect to the diffracting structure permits analysis of the diffracted radiation at multiple polarity orientations. A spectograph detects the intensity of the spectral components at different polarity orientations. Because the normal incidence reflectometer uses normally incident radiation and an analyzer/polarizer that rotates relative to the diffracting structure, or vice-versa, the orientation of the diffracting structure does not affect the accuracy of the measurement. Thus, the sample holding stage may use X, Y, and Z, as well as r-&#x3b8; type movement and there is no requirement that the polarization orientation of the incident light be aligned with the grating of the diffraction structure. A non-linear multivariate regression process is used to adjust the parameters of an optical model, such as rigorous coupled-wave analysis, to provide a match with the measured data.

Apparatus and method for the measurement of diffracting structures

An overlay target for spectroscopic measurement includes at least two diffraction gratings, one grating overlying the other. The diffraction gratings may include an asymmetry relative to each other in order to improve resolution of the presence as well as the direction of any mis-registration. For example, the asymmetry between the two diffraction gratings may be a phase offset, a difference in pitch, line width, etc. The overlay target may be spectroscopically measuring, for example, using an optical model and a best fit analysis. Moreover, the overlay target may be optimized by modeling the overlay target and adjusting the variable parameters and calculating the sensitivity of the overlay target to changes in variable parameters.

Spectroscopically measured overlay target

An alignment system for aligning two elements includes an alignment target with periodic patterns on each element. The alignment target includes two locations, at least one of which has a designed in offset. If desired, both locations may have designed in offsets of the same magnitude but in opposite directions. The diffraction patterns produced at the two locations are compared. If the difference between the patterns is at a minimum, the elements are properly aligned. When an alignment error is introduced, however, the calculated difference can be used to determine the error. In another embodiment, bands in the moir&#xe9; fringes from the different locations may be compared to determine the alignment error. The two elements may then be moved relative to each other to minimize the alignment error. Thus, the alignment target may advantageously be used in any alignment system, such as an exposure tool.

Positioning two elements using an alignment target with a designed offset

An alignment target includes periodic patterns on two elements. The periodic patterns are aligned when the two elements are properly aligned. By measuring the two periodic patterns with an incident beam having a single polarization state and detecting the intensity of the resulting polarized light, it can be determined if the two elements are aligned. The same polarization state may be detected as is incident or different polarization states may be used. A reference measurement location may be used that includes a third periodic pattern on the first element and a fourth periodic pattern on the second element, which have a designed in offset, i.e., an offset when there is an offset of a known magnitude when the first and second element are properly aligned. The reference measurement location is similarly measured with a single polarization state.

Measuring an alignment target with a single polarization state

Method and apparatus for using an alignment target with designed in offset

An encoder includes a layer on the scale that has a thickness that varies as a function of position along the length of the scale. The position of the sensor head with respect to the scale may be determined by measuring the thickness of the layer or index of refraction, e.g., using a reflectometer, and converting the thickness to the lateral position. In one embodiment, the thickness of the layer is used to provide a rough position of the sensor head with respect to the scale and an alignment target that includes periodic patterns on both the sensor head and scale is used to provide a refined position.

Encoder measurement based on layer thickness

An encoder uses an alignment target that includes periodic patterns on the movable element and the stationary element. The alignment target may include at least two measurement locations, each location having a different offset between the periodic pattern on the movable element with respect to the periodic pattern on the stationary element. Alternatively, two measurements using different polarization states may be made at one location. When the periodic patterns on the movable element and the stationary element are aligned, the difference between the two measurements will produce a minimum, i.e., approximately a zero value plus noise. By counting the minima, the precise position of the movable element with respect to the stationary element can be determined. The resolution of the encoder may be further increased using reference measurements.

Encoder with an alignment target

Measuring an alignment target with multiple polarization states

The calibration of a metrology tool with a rotatable polarizer separates the angular dependence of the irradiance from the temporal dependence. The angular dependence of the metrology tool is then modeled, e.g. using a Fourier expansion. The Fourier coefficients are parameterized as a function of wavelength. The actual irradiance, e.g., the reference irradiance and/or back reflection irradiance, is then measured for the metrology tool for one angle of the rotatable polarizer. From the measured irradiance and the modeled angular dependence, the total irradiance of the metrology tool can be determined, which is independent of the angle of the rotatable polarizer. The irradiance, e.g., reference and/or back reflection, can then be determined for any desired angle of the rotatable polarizer using the total irradiance and the angular dependence of the metrology tool.

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Sunnyvale, CA, United States of America

Roger R Lowe-Webb