Method and apparatus for aligning mask and wafer

Claim 2

In a method for aligning a microscopic mask pattern with a microscopic semiconductor wafer pattern comprising the steps of observing the mask and wafer through a microscope objective having a depth of field smaller than the separation of the mask and wafer, and moving the relative orientation of the mask with respect to the wafer to register the patterns, the improvement comprising the steps of: illuminating the mask and wafer with light including light of a first polarization and light of a second polarization; and directing said illuminating light from the mask and wafer through the microscope objective and a bifocus element, said microscopic objective and bifocus element comprising means for imaging light of the first polarization from the mask at a first image plane and light of the second polarization from the wafer at a second image plane, the first and second image planes being substantially coincident, thereby permitting the mask and the wafer to be observed simultaneously.

Claim 3

The improvement of claim 1 wherein: the bifocus element comprises at least one lens made of birefringent material having its crystal optic axis extending in a direction perpendicular to the central axis of the microscope objective.

Claim 4

The improvement of claim 1 wherein: the bifocus element comprises a positive lens having an optic axis extending in a first direction, and a negative lens having an optic axis extending in a second direction perpendicular to the first direction, the microscopic objective having a central axis perpendicular to both the first and second directions.

Claim 5

The improvement of claim 3 wherein: the positive and negative lenses of the bifocus element have powers which are approximately equal in magnitude but opposite in sign.

Claim 6

The improvement of claim 4 wherein: the bifocus element is located aprroximately at the back focal plane of the microscope objective.

Claim 7

In a method for aligning a microscopic mask pattern with a microscope semiconductor wafer pattern comprising the steps of observing the mask and wafer through a microscopic system having a depth of field smaller than the separation of the mask and wafer and moving the relative orientation of the mask with respect to the wafer to register the patterns, the improvement comprisng the steps of: forming a bifocus element having a first focal length in response to light of a first polarization and a second focal length in response to light of a second polarization, where the first and second polarizations are mutually orthogonal; inserting the bifocus element in the microscope system such that the microscope system and bifocus element comprise means for imaging light of the first polarization from the mask and light of the second polarization from the wafer at a common image plane; and illuminating the mask and wafer with light including light of both the first and second polarizations, thereby permitting the mask and wafer to be observed simultaneously.

Claim 8

In a method for aligning a microscopic mask pattern with a microscope semiconductor wafer pattern comprising the steps of observing the mask and wafer through a microscopic system having a depth of field smaller than the separation of the mask and wafer and moving the relative orientation of the mask with respect to the wafer to register the patterns, the improvement comprisng the steps of: forming a bifocus element having a first focal length in response to light of a first polarization and a second focal length in response to light of a second polarization, where the first and second polarizations are mutually orthogonal; inserting the bifocus element in the microscope system such that the microscope system and bifocus element comprise means for imaging light of the first polarization from the mask and light of the second polarization from the wafer at a common image plane; and illuminating the mask and wafer with light including light of both the first and second polarizations, thereby permitting the mask and wafer to be observed simultaneously.

Claim 9

The method of claim 6 wherein: the step of forming the bifocus element comprises the steps of forming a negative lens of birefringent material having an optic axis perpendicular to the central axis of the microscopic system and forming a positive lens of birefringent material having an optic axis perpendiclar to the optic axis of the negative lens and the central axis of the microscopic system.

Claim 10

The improvement of claim 7 wherein: the positive and negative lenses respectively have powers which are approximately equal in magnitude but of opposite sign.

Claim 11

The improvement of claim 8 further comprising the step of separating the mask and wafer a distance approximately given by the relation dp = 2f.sup.2 |P.sub.x | = 2f.sup.2 |P.sub.y | where f is the focal length of the microscope objective; |P x | is the magnitude of the power of the bifocus element with respect to light of the first polarization; and |P y | is the magnitude of the power of the bifocus element with respect to light of the second polarization.

Claim 12

The improvement of claim 8 wherein: the bifocus element is inserted approximately at the back focal plane of the microscope objective.