Method of measuring a parameter and apparatus

1. A method of determining a parameter of a patterning process applied to an object comprising two features, the method comprising:

irradiating the two features of the object with a radiation beam;

receiving at least a portion of the radiation beam scattered from the two features of the object, the at least a portion of the radiation beam comprising a first portion comprising at least one diffraction order and a second portion comprising at least one diffraction order that is different from a diffraction order of the first portion;

moderating a phase difference between the first and second portions;

combining the first and second portions such that they interfere to produce a time dependent intensity signal; and

determining the parameter of the patterning process from a contrast of the time dependent intensity signal.

2. The method of claim 1 , wherein each of the two features comprises a plurality of sub-features,

wherein the irradiating the two features of the object with a radiation beam comprises sequentially irradiating each sub-feature of a first one of the features and a corresponding sub-feature of the second one of the features with the radiation beam;

wherein for each such pair of the corresponding sub-features, the first and second portions are combined such that they interfere to produce a time dependent intensity signal; and

wherein the determining the parameter of the patterning process comprises determining the parameter of the patterning process from a contrast of each such time dependent intensity signal.

3. The method of claim 2 , wherein each pair of the corresponding sub-features has a different nominal offset.

4. The method of claim 2 , wherein the determining the parameter of the patterning process from the contrast of each time dependent intensity signal involves fitting a parameterization to the contrast for each pair of the corresponding sub-features of the first and second features as a function of the nominal offset of the pair of the corresponding sub-features and comparing the fit to a nominal curve.

5. The method of claim 1 , wherein the first portion and the second portion are each formed from the interference between a portion of radiation scattered from a first of the two features and a portion of radiation scattered from a second of the two features.

6. An apparatus for determining a parameter of a patterning process applied to an object comprising two features, the apparatus comprising:

a radiation source operable to irradiate the object with a radiation beam;

a measurement system operable to receive at least a portion of the radiation beam scattered from the object and further operable to determine the parameter of the patterning process therefrom, wherein the measurement system comprises optics configured to combine a first portion and a second portion of the at least a portion of the measurement radiation beam scattered from the object and a sensor configured to determine an intensity of the combined first and second portions; and

a phase control mechanism operable to moderate a phase difference between the first and second portions before they are combined such that the intensity of the combined first and second portions is time dependent;

wherein the measurement system is operable to determine the parameter of the patterning process from a contrast of the time dependent intensity of the combined first and second portions.

7. The apparatus of claim 6 , wherein the phase control mechanism comprises:

a support structure configured to support an object; and

a movement mechanism operable to move the support structure relative to the radiation source.

8. The apparatus of claim 7 , wherein:

the movement mechanism is operable to move the support structure such that a plurality regions of the object are successively irradiated by the radiation beam; and

the measurement system is operable to determine a contrast of the time dependent intensity of the combined first and second portions for each such region.

9. The apparatus of claim 6 , wherein the measurement system is further operable to fit a parameterization to the determined contrast for each region of the object as a function of a nominal offset of the region and to compare the fit to a nominal curve so as to determine the parameter of the patterning process.

10. The apparatus of claims 6 , wherein the radiation beam comprises infrared radiation.

11. The apparatus of claim 6 , that is configured to determine a parameter of a patterning process applied to an object comprising two features by:

irradiating the two features of the object with a radiation beam;

receiving at least a portion of the radiation beam scattered from the two features of the object, the at least a portion of the radiation beam comprising: a first portion comprising at least one diffraction order and a second portion comprising at least one diffraction order that is different from a diffraction order of the first portion;

moderating a phase difference between the first and second portions;

combining the first and second portions such that they interfere to produce a time dependent intensity signal; and

determining the parameter of the patterning process from a contrast of the time dependent intensity signal.