Alignment system and alignment marks for use therewith

1. A method for aligning a substrate or reticle, comprising:

receiving a raw alignment signal produced by illumination of an alignment mark on the substrate or reticle, wherein a first portion of the alignment mark has a first periodicity and a second portion of the alignment mark has a second periodicity that is different from the first periodicity;

performing a transformation of the raw alignment signal based at least in part on a Hilbert transform to form a first signal having a first amplitude and a first phase, and a second signal having a second amplitude and a second phase, wherein the first signal and the second signal are associated with the first periodicity and the second periodicity respectively;

selecting candidate alignment positions based on local minima in a difference between zero crossings of the first phase of the first signal and zero crossings of the second phase of the second signal;

determining an alignment position of the substrate or reticle based on the selected candidate alignment position having a highest weighted combination of the first amplitude and the second amplitude; and

aligning the substrate or reticle based on the determined alignment position.

2. The method of claim 1 , wherein the weighted combination is formed by taking the square root of the product of the first amplitude and the second amplitude.

3. The method of claim 1 , further comprising:

prior to performing a transformation, performing a background signal removal transformation on the raw alignment signal.

4. The method of claim 3 , wherein the performing a background signal removal transformation includes combining the raw alignment signal with derived versions of the raw alignment signal that include a phase advanced version of the raw alignment signal and a phase retarded version of the raw alignment signal.

5. The method of claim 4 , wherein the phase-advanced version includes an advancement of pi radians and the phase-retarded version includes a retardation of pi radians.

6. An alignment system for aligning a substrate or a reticle comprising:

a radiation source configured to illuminate during use an alignment mark on the substrate or on the reticle, wherein a first portion of the alignment mark has a first periodicity and a second portion of the alignment mark has a second periodicity that is different from the first periodicity;

a detection system configured to detect a raw alignment signal produced from the alignment mark; and

a processor configured to determine an alignment position from the raw alignment signal by:

performing a transformation of the raw alignment signal based at least in part on a Hilbert transform to form a first signal having a first amplitude and a first phase, and a second signal having a second amplitude and a second phase, wherein the first signal and the second signal are associated with the first periodicity and the second periodicity respectively;

selecting candidate alignment positions based on local minima in a difference between zero crossings of the first phase of the first signal and zero crossings of the second phase of the second signal;

determining an alignment position of the substrate or reticle based on the selected candidate alignment position having a highest weighted combination of the first amplitude and the second amplitude; and

aligning the substrate or reticle based on the determined alignment position.

7. The alignment system of claim 6 , wherein the weighted combination is formed by taking the square root of the product of the first amplitude and the second amplitude.

8. The alignment system of claim 6 , wherein the processor is further configured to:

prior to performing a transformation, perform a background signal removal transformation on the raw alignment signal.

9. The alignment system of claim 8 , wherein the performing a background signal removal transformation includes combining the raw alignment signal with derived versions of the raw alignment signal that include a phase advanced version of the raw alignment signal and a phase retarded version of the raw alignment signal.

10. The alignment system of claim 9 , wherein the phase-advanced version includes an advancement of pi radians and the phase-retarded version includes a retardation of pi radians.

11. A lithographic projection apparatus, comprising:

an illumination system configured to condition a radiation beam;

a support constructed to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterning radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beam onto a target portion of the substrate;

a radiation source configured to illuminate an alignment mark on the substrate or on the patterning device, wherein a first portion of the alignment mark has a first periodicity and a second portion of the alignment mark has a second periodicity that is different from the first periodicity;

a detection system configured to detect a raw alignment signal produced from the alignment mark; and

a processor configured to determine an alignment position from the raw alignment signal by:

performing a transformation of the raw alignment signal based at least in part on a Hilbert transform to form a first signal having a first amplitude and a first phase, and a second signal having a second amplitude and a second phase, wherein the first signal and the second signal are associated with the first periodicity and the second periodicity respectively;

selecting candidate alignment positions based on local minima in a difference between zero crossings of the first phase of the first signal and zero crossings of the second phase of the second signal;

determining an alignment position of the substrate or reticle based on the selected candidate alignment position having a highest weighted combination of the first amplitude and the second amplitude; and

aligning the substrate or reticle based on the determined alignment position.

12. The lithographic projection apparatus of claim 11 , wherein the weighted combination is formed by taking the square root of the product of the first amplitude and the second amplitude.

13. The lithographic projection apparatus of claim 11 , wherein the processor is further configured to:

prior to performing a transformation, perform a background signal removal transformation on the raw alignment signal.

14. The lithographic projection apparatus of claim 11 , wherein the performing a background signal removal transformation includes combining the raw alignment signal with derived versions of the raw alignment signal that include a phase advanced version of the raw alignment signal and a phase retarded version of the raw alignment signal.

15. The lithographic projection apparatus of claim 14 , wherein the phase-advanced version includes an advancement of pi radians and the phase-retarded version includes a retardation of pi radians.

16. A method for aligning a substrate or reticle, comprising:

receiving a raw alignment signal produced by illumination of an alignment mark on the substrate or reticle, wherein a first portion of the alignment mark has a first periodicity and a second portion of the alignment mark has a second periodicity that is different from the first periodicity; and

performing a background signal removal transformation on the raw alignment signal to form an output alignment signal, wherein the performing a background signal removal transformation includes combining the raw alignment signal with derived versions of the raw alignment signal that include a phase advanced version of the raw alignment signal and a phase retarded version of the raw alignment signal; and

aligning the substrate or reticle based on the output alignment signal.

17. The method of claim 16 , wherein the phase-advanced version includes an advancement of pi radians and the phase-retarded version includes a retardation of pi radians.