# Patent

## Title
Differential method for layer-to-layer registration

## Abstract

A system for precisely measuring layer-to-layer mis-registration is provided. The system includes a new type of mark and a comparison system, which compare the right and left signals from the mark to eliminate non-alignment noise, to enlarge the alignment information hundreds times then the actual shiftiness between two layers and to measure the mis-registration.

## Background
FIELD OF THE INVENTION
The invention related to the precisely alignment between layers. The application which exist in semiconductor wafer manufacturing layer to layer alignment, or any other place need precise alignment.
BACKGROUND
When need precisely to align one object to another object, usual way is to put align mark to each object, then to measure the shift between these two marks to decide the shiftiness of the two object, then to align them. Unfortunately, when the precision requirement reaches nm level, it could not be accurately measured optically because optical wavelets limitation. It can use light diffraction such as applying grating structure, but because the line thickness also affect the light diffraction, to use grating diffracting to measure layer to layer shiftiness still has problems.
SUMMARY OF THE INVENTION
The invention is to use different pitch grating for two layers. By compare left and right reflected/diffracted or any combined light signal, to decide the mis-aligned information. By this method, all unrelated things such as grating thickness or line width signals will be canceled each other, and only the shiftiness signal will be enlarged dozens or hundreds times, even the stiffness is sub-nm, the final signal from this invention will be hundred nm large, and can be actually measured optically.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIG. 1 exemplifies, in accordance with principles of the present invention, a grating 100 having a pitch of n, a grating 101 having a pitch of n+1, and a grating 103 in which the grating 100 overlaps the grating 101;
FIG. 2 exemplifies a system that, in accordance with principles of the present invention, catches the reflecting, diffracting, and/or interference light from overlapped gratings, and compares the left and right deference; and
FIG. 3 exemplifies an overlapped grating having about 500 nm shiftiness observation, while the actual shiftiness between two gratings is only about 10 nm.



DETAILED DESCRIPTION OF PREFERED EMBODIMENTS
Referring now to the drawing, and in particular to FIG. 1, grating 100 has a pitch n, for example, 50 um, grating 101 has a pitch n+1, in this example is 51 um. 102 are overlapped grating.
Referring now to FIG. 2, a system catch the reflecting/diffracting/interference light from the overlapped gratings, to compare left and right deference.
Let d1 be line pitch of previous layer mark, d2 be line pitch of current layer mark, c is a constant, Δs be actual shift between layers, the overlapped grating image intensity meets the following equation




  
    
      
        
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      (
      
        
          d
          2
        
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          d
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      )
    
  




The FIG. 2 system 103 will see the shift Δx=xp−x0:




  
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This means that the overlaid image distribution shift Δx (compare left and right) is d1/(d2−d1) times large then actual lay shift Δs. Use d1=0.5 um, d2=0.51 um value here, the enlargement here is 50 times, as shown in FIG. 3. The picture shows a 10 nm shift between two layers, the microscopy observed 500 nm shiftiness, which is very easy to measure.

## Claims

1. A method for aligning two selected layers of two or more layers of a semiconductor wafer, a first layer of said two selected layers defining a first mark of at least one mark of said first layer, said first mark having a pitch grating d1, a second layer of said two selected layers defining a second mark of at least one mark of said second layer, said second mark having a pitch grating d2, said first mark and said second mark overlapping each other, the method comprising steps of:
directing light from at least one light source to at least one first portion and at least one second portion of each of said first and second marks;
receiving at least one first signal from said light directed at said at least one first portion of each of said first and second marks;
receiving at least one second signal from said light directed at said at least one second portion of said first and second marks;
comparing said at least one first signal and said at least one second signal to determine an overlaid image distribution shift Δx;
determining an actual lay shift Δs =Δx(d2−d1)/d1; and
adjusting said two selected layers of said semiconductor wafer by said Δs relative to each other to thereby align said two selected layers of said semiconductor wafer with each other.

2. The method of claim 1, wherein the step of comparing further comprises comparing said at least one first signal and said at least one second signal to determine by means of optical measurements an overlaid image distribution shift Δx.

3. The method of claim 1:
wherein said light directed at said at least one first portion of said first and second marks is at least one of reflected and diffracted from said at least one first portion of said first and second marks;
wherein the step of receiving said at least one first signal further comprises receiving said at least one first signal from said light directed at, and at least one of reflected and diffracted from, said at least one first portion of said first and second marks;
wherein said light directed at said at least one second portion of said first and second marks is at least one of reflected and diffracted from said at least one second portion of said first and second marks; and
wherein the step of receiving said at least one second signal further comprises receiving said at least one second signal from said light directed at, and at least one of reflected and diffracted from, said at least one second portion of said first and second marks.

4. An apparatus for aligning two selected layers of two or more layers of a semiconductor wafer, a first layer of said two selected layers defining a first mark of at least one mark of said first layer, said first mark having a pitch grating d1, a second layer of said two selected layers defining a second mark of at least one mark of said second layer, said second mark having a pitch grating d2, said first mark and said second mark overlapping each other, the method comprising steps of:
at least one light source configured for directing light to at least one first portion and at least one second portion of each of said first and second marks;
at least one first receiver configured and positioned relative to said at least one light source and said semiconductor wafer for receiving at least one first signal from said light directed at said at least one first portion of each of said first and second marks;
at least one second receiver configured and positioned relative to said at least one light source and said semiconductor wafer for receiving at least one second signal from said light directed at said at least one second portion of said first and second marks;
a comparator coupled to said at least one first receiver and said at least one second receiver for receiving and comparing said at least one first signal and said at least one second signal to determine an overlaid image distribution shift Δx
a determiner coupled to said comparator for determining an actual lay shift Δs =Δx(d2−d1)/d1; and
an adjuster coupled to said determiner for receiving said Δs and for adjusting said two selected layers of said semiconductor wafer by said Δs relative to each other to thereby align said two selected layers of said semiconductor wafer with each other.

5. The apparatus of claim 4, wherein said comparator is further configured for receiving and comparing said at least one first signal and said at least one second signal to determine by means of optical measurements an overlaid image distribution shift Δx.

6. The apparatus of claim 4:
wherein said light directed at said at least one first portion of said first and second marks is at least one of reflected and diffracted from said at least one first portion of said first and second marks;
wherein said at least one first receiver is further configured for receiving said at least one first signal from said light directed at, and at least one of reflected and diffracted from, said at least one first portion of said first and second marks;
wherein said light directed at said at least one second portion of said first and second marks is at least one of reflected and diffracted from said at least one second portion of said first and second marks; and
wherein said at least one second receiver is further configured for receiving said at least one second signal from said light directed at, and at least one of reflected and diffracted from, said at least one second portion of said first and second marks.