METHOD AND APPARATUS FOR MINIMIZING OVERLAY ERRORS IN LITHOGRAPHY

• US 20120227014A1
• Filed: 02/29/2012
• Published: 09/06/2012
• Est. Priority Date: 03/02/2011
• Status: Active Grant

• Current Assignee: Carl Zeiss SMS Ltd. (Carl-Zeiss-Stiftung)
• Original Assignee: Carl Zeiss SMS Ltd. (Carl-Zeiss-Stiftung)
• Inventors: Pforr, Rainer
• Granted Patent: US 8,539,394 B2
• Time in Patent Office: Days
• US Class Current: 716/52
• CPC Class Codes: G03F 1/70 Adapting basic layout or design of masks to lithographic process requirements, e.g., second iteration correction of mask patterns for imaging
• CPC Class Codes: G03F 7/70633 Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching

Abstract

The invention relates to a method for minimizing errors of a plurality of photolithographic masks, the plurality of photolithographic masks serving for successively processing a substrate, the method comprises determining a reference displacement vector field, the reference displacement vector field correlates displacement vectors of the errors of the plurality of photolithographic masks, determining for each of the photolithographic mask a difference displacement vector field as a difference between the reference displacement vector field and the displacement vectors of the errors of the respective photolithographic mask, and correcting the errors for each of the photolithographic masks using the respective difference displacement vector field.

Claims

1. A method for minimizing errors of a plurality of photolithographic masks, the plurality of photolithographic masks serving for successively processing a substrate, the method comprising:

a. determining a reference displacement vector field, the reference displacement vector field correlates displacement vectors of the errors of the plurality of photolithographic masks;

b. determining for each of the photolithographic masks a difference displacement vector field as a difference between the reference displacement vector field and the displacement vectors of the errors of the respective photolithographic mask; and

c. correcting the errors for each of the photolithographic masks using the respective difference displacement vector field or the reference displacement vector field.

2. The method according to claim 1, wherein overlay errors are minimized by correlating displacement vectors of pattern placement errors and correcting pattern placement errors.

3. The method according to claim 1, wherein correlating the displacement vectors comprises averaging the displacement vectors of the plurality of photolithographic masks.

4. The method according to claim 1, wherein correlating the displacement vectors comprises:

a. determining difference displacement vectors depending on a position at the photolithographic mask as a difference of the displacement vectors of two different masks;

b. determining an average displacement value by averaging the norms of the difference displacement vectors across an area of the photolithographic mask;

c. determining a maximum displacement value as the maximum of the norms of the difference displacement vectors across the surface of the photolithographic masks;

d. defining a weighted average comprising the average displacement value, the maximum displacement value and weighting parameters;

e. determining a weighted averaged displacement value for each photolithographic mask by averaging the weighted average with respect to the plurality of photolithographic masks; and

f. determining the reference displacement vector field as the displacement vector field of the photolithographic masks having the lowest weighted averaged displacement value of the plurality of photolithographic masks.

5. The method according to claims 2, wherein correcting the pattern placement errors comprises using a femtosecond laser source in order to locally vary a mass density of a substrate of the photolithographic mask.

6. The method according to claim 1, wherein a subset of the plurality of photolithographic masks is used for minimizing overlay errors.

7. The method according to claim 1, wherein for each position x_i,y_i a reference displacement vector D^Mean(x_i,y_i) is calculated according to:

8. The method according to claim 1, wherein the reference displacement vector field D^Ref(x_i,y_i) is determined by:

a. calculating a difference displacement vector Δ

D_mn(x_i,y_i) at the position x_i,y_i as a difference of the displacement vectors D_m(x_i,y_i) of the photolithographic mask m and D_n(x_i,y_i) of the photolithographic mask n according to;

Δ

D_mn(x_i,y_i)=D_m(x_i,y_i)−

D_n(x_i,y_i);

b. calculating an average displacement value of the norms of the difference displacement vectors Δ

D_mn(x_i,y_i) according to;

9. The method according to claim 1, further comprising:

a. simulating a corrected displacement vector field for each photolithographic mask comprising displacement errors after application of a correction process, and setting an iteration index to a starting value;

b. determining difference correction displacement vectors as a difference of the corrected displacement vectors of two different photolithographic masks for each combination of photolithographic masks;

c. determining for each photolithographic mask average difference correction displacement vectors by averaging the norms of the difference correction displacement vectors with respect to other photolithographic masks;

d. determining for each photolithographic mask an average correction displacement value by averaging the average difference correction displacement vectors;

e. determining for each photolithographic mask a maximum correction displacement value as the largest numerical value of the average difference correction displacement vectors;

f. defining for each photolithographic mask an optimization parameter as a weighted sum of the average correction displacement value and the maximum correction displacement value, wherein weighting parameters are applied to the average correction displacement value and the maximum correction displacement value, wherein the optimization parameter depends on the iteration index;

g. determining an intermediate correction displacement vector field as the correction displacement vector field of the photolithographic mask having the lowest numerical value of the optimization parameter;

h. if the iteration index has the starting value;

taking the intermediate correction displacement vector field as the reference correction displacement vector field, calculating for each photolithographic mask a difference displacement vector field as the difference between a displacement vector field and the reference correction displacement vector field, comparing the optimization parameter with a predetermined threshold, if the optimization parameter is below a threshold;

correcting the errors for each photolithographic mask using the respective difference displacement vector field, and ending, and if the optimization parameter is above the threshold;

increasing the iteration index by one unit;

i. if the iteration index has not the starting value;

comparing the optimization parameter with the threshold, if the optimization parameter is below the threshold, taking the intermediate correction displacement vector field as the reference correction displacement vector field, calculating for each photolithographic mask a difference displacement vector field as the difference between the displacement vector field and the reference correction displacement vector field, correcting the errors for each photolithographic mask using the respective difference displacement vector field, and ending;

j. if the iteration index has not the starting value and if the optimization parameter is larger than the threshold;

comparing the optimization parameter of the previous iteration step with the optimization parameter of the present iteration step, if the optimization parameter of the present iteration step is larger than the optimization parameter of the previous iteration step, taking the intermediate correction displacement vector field of the previous iteration as the reference correction displacement vector field, calculating for each photolithographic mask a difference displacement vector field as the difference between the displacement vector field and the reference correction displacement vector field, correcting the errors for each photolithographic mask using the respective difference displacement vector field, and ending;

k. if the iteration index has not the starting value and if the optimization parameter of the present iteration step is below the optimization parameter of the previous iteration step;

taking the intermediate correction displacement vector field as the reference correction displacement vector field, calculating for each photolithographic mask a difference displacement vector field as the difference between the displacement vector field and the reference correction displacement vector field, and increasing the iteration index by one unit;

l. simulating the difference displacement vector field for each photolithographic mask to determine a difference correction displacement vector field;

m. determining difference correction displacement vectors as a difference of difference correcting displacement vectors of two different photolithographic masks for each combination of the photolithographic masks; and

n. repeating steps c. to k.

10. The method according to claim 9, wherein for k=1 the difference correction displacement vector Δ

D_mn^Corr(x_i,y_i) at the position x_i,y_i is the difference of the correcting displacement vectors D_m^Corr(x_i,y_i) of the photolithographic mask m and D_n^Corr(x_i,y_i) of the photolithographic mask n according to;

Δ

D_mn^Corr(x_i,y_i)=D_m^Corr(x_i,y_i)−

D_n^Corr(x_i,y_i), and for k≠

1 the difference correction displacement vector Δ

D_mn^Corr(x_i,y_i) is the difference of the difference correcting displacement vectors Δ

D_m^Corr(x_i,y_i) of the photolithographic mask m and Δ

D_n^Corr(x_i,y_i) of the photolithographic mask n according to;

Δ

D_mn^Corr(x_i,y_i)=Δ

D_m^Corr(x_i,y_i)−

Δ

D_n^Corr(x_i,y_i);

and/or wherein an average correction displacement vector Δ

D_m^Corr,Avr(x_i,y_i) of the photolithographic mask m with respect to all photolithographic masks n is determined from the norms of the difference correction displacement vectors Δ

D_mn^Corr(x_i,y_i) according to;

11. The method according to claim 9, wherein determining an average correction displacement value Δ

D_m^Corr,Avr for the photolithographic mask m comprises averaging of the average correction displacement vectors Δ

_m^Corr,Avr(x_i,y_i) across an area of the photolithographic mask m according to;

12. The method according to claim 9, wherein the intermediate correction displacement vector field D_int(x_i,y_i) is the correction displacement vector field of the photolithographic mask D_m^Corr(x_i,y_i) having the lowest numerical value of the optimization parameter P_m(k) for all photolithographic masks m according to:

D_int,k(x_i,y_i)=D_m^Corr(x_i,y_i);

and/or wherein if the optimization parameter P_m(k) is below a predetermined threshold T the reference correction displacement vector field is defined by the intermediate correction displacement vector field according to;

D^Corr,Ref(x_i,y_i)=D_int,k(x_i,y_i), and if the optimization parameter of a previous iteration step is smaller than the optimization parameter of the present iteration step the reference correction displacement vector field is;

D^Corr,Ref(x_i,y_i)=D_int,k-1(x_i,y_i).

13. The method according to claim 9, wherein the difference displacement vector field Δ

D_m(x_i,y_i) is the difference of the displacement vector field D_m(x_i,y_i) and the reference correction displacement vector field D^Corr,Ref(x_i,y_i) according to;

Δ

D_m(x_i,y_i)=D_m(x_i,y_i)−

D^Corr,Ref(x_i,y_i);

and/or further comprising simulating the difference displacement vector fields Δ

D_m(x_i,y_i) for each photolithographic mask m to obtain difference correcting displacement vector fields Δ

D_m^Corr(x_i,y_i).

14. The method according to claim 9, wherein if k=1 and if:

a. P_m(1)<

T;

ending; and

b. P_m(1)≧

T;

k=k+1 and performing the next iteration step;

and/or wherein when k≠

1 and if;

c. P_m(k)<

T;

correcting the errors for each photolithographic mask using the respective difference displacement vector field and ending;

d. P_m(k)<

P_m(k−

1);

k=k+1 and performing the next iteration step; and

e. P_m(k)≧

P_m(k−

1);

determining the reference displacement vector field as D^Corr,Ref(x_i,y_i)=D_int,k-1(x_i,y_i), correcting the errors for each photolithographic mask using the respective difference displacement vector field and ending.

16. The apparatus according to claim 15, wherein the apparatus performs a method according to claim 1.

15. An apparatus for minimizing errors of a plurality of photolithographic masks, the plurality of photolithographic masks serving for successively processing a wafer, comprising:

a. at least one computing means adapted for determining a reference displacement vector field, the reference displacement vector field correlates displacement vectors of the errors of the plurality of photolithographic masks, determining for each of the photolithographic masks a difference displacement vector field as a difference between the reference displacement vector field and the displacement field of the pattern placement errors; and

b. at least one laser system adapted for correcting the errors for each of the photolithographic masks using the respective difference displacement vector field or the reference displacement vector field.

17. The apparatus according to claim 15, wherein overlay errors are minimized by correlating displacement vectors of pattern placement errors and correcting pattern placement errors.

18. The apparatus according to claim 15, comprising an inspection system that comprises an ultra-precision stage, at least one laser source and a charge-coupled device camera both operating in the ultraviolet wavelength range.

19. The apparatus according to claim 15, wherein the laser system comprises a femtosecond laser source.

20. The apparatus according to claims 18, wherein the at least one computing means is operable to control the inspection system and/or the laser system.