METHOD FOR MONITORING FOCUS ON AN INTEGRATED WAFER

• US 20090284722A1
• Filed: 05/19/2008
• Published: 11/19/2009
• Est. Priority Date: 05/19/2008
• Status: Active Grant

• Current Assignee: GlobalFoundries, Inc.
• Original Assignee: International Business Machines Corporation
• Inventors: Li, Wai-Kin, Gabor, Allen H.
• Granted Patent: US 9,046,788 B2
• Time in Patent Office: Days
• US Class Current: 355/53
• CPC Class Codes: G03F 7/70333 Focus drilling, i.e. increase in depth of focus for exposure by modulating focus during exposure [FLEX]
• CPC Class Codes: G03F 7/70508 Data handling in all parts of the microlithographic apparatus, e.g. handling pattern data for addressable masks or data transfer to or from different components within the exposure apparatus
• CPC Class Codes: G03F 7/70641 Focus

Abstract

A method and apparatus are provided for improving the focusing of a substrate such as a wafer during the photolithography imaging procedure of a semiconductor manufacturing process. The invention is particularly useful for step-and-scan system and the CD of two features in each exposure field are measured in fields exposed at varying focus to form at least two Bossung curves. Exposure focus instructions are calculated based on the intersection point of the curves and the wafer is then scanned and imaged based on the calculated exposure focus instructions. In another aspect of the invention, when multiple wafers are being processed operational variances may cause a drift in the focus. The focus drift can be easily corrected by measuring the critical dimension of each of the features and comparing the difference to determine if any focus offset is needed to return the focus to the original calculated focus value.

Claims

1. A method of monitoring focus during photolithography on a wafer comprising the steps of:

loading a wafer and a patterned reticle into a step-and-scan wafer imaging photolithography exposure system;

exposing the reticle using the step-and-scan system at a first focus forming a first exposure field of the reticle pattern on the wafer;

repeating the exposing step at a different focus for a next exposure field until a desired number of exposure fields are formed on the wafer, each repeating exposing step being performed at a different focus;

measuring the critical dimension of a feature in two different regions in each exposure field, the features and regions being the same in each exposure field;

plotting Bossung curves of critical dimension versus focus for the features in the two different regions;

plotting the difference between the Bossung curves for the features in the two different regions;

determining if the difference plot has sufficient sensitivity to be used as a focus monitor and, if not, choosing new features in two different regions and repeating the measuring and plotting steps until sufficient sensitivity for use as a focus monitor is achieved;

determining focus instruction values based on the difference plot focus monitor;

scanning and exposing a new wafer based on the determined focus instruction values.

2. The method of claim 1 further including determining if the exposure system focus instruction value has drifted by exposing a new wafer to create exposure fields at a different focus based on the focus instruction values, again measuring critical dimension of a feature in two different regions in each exposure field to create new Bossung curves, plotting the difference between the new Bossung curves, determining if the difference between the new Bossung curves has changed compared to the difference between the previous Bossung curves, and adjusting if necessary the focus instruction values based on the difference between the new Bossung curves.

3. The method of claim 2 wherein a correlation is made between the difference between the critical dimension for each feature versus focus shift and the focus instruction values based on the correlation are adjusted.

4. The method of claim 3 wherein, for a first feature A in a first region on an exposure field and for a second feature B in a second region on the exposure field, if the critical dimension for feature B is less than the critical dimension of feature A the focus instruction value is increased.

5. The method of claim 3 wherein, for a first feature A in a first region on an exposure field and for a second feature B in a second region on the exposure field, if the critical dimension for feature B is greater than the critical dimension for feature A the focus instruction value is decreased.

6. The method of claim 1 wherein the determined focus instruction value may vary up to ±

10%.

7. The method of claim 1 wherein the determined focus instruction value may vary up to ±

200 nm.

8. An apparatus for making an integrated circuit device such as a wafer wherein photolithography means are used to image the wafer during the making of the integrated circuit device comprising:

a loading apparatus for loading a wafer and a patterned reticle into a step-and-scan wafer imaging photolithography exposure system;

the photolithography exposure system including an exposure apparatus to expose the reticle at a first focus forming a first exposure field reticle pattern on the wafer and repeating the exposure for a next exposure field at a different focus until all the desired reticle patterns are formed on the wafer, with each repeating step being performed at a different focus;

a measuring apparatus to measure the critical dimension of a feature in two different regions in each exposure field, the features and regions being the same in each exposure field;

a plotting apparatus to plot a Bossung curve of critical dimension versus focus for the features in the two different regions;

a plotting apparatus for plotting the difference between the critical dimension for the features in the two different regions;

a focus determination apparatus to determine a focus instruction value for the exposure fields based on the above plots;

a scanning and imaging apparatus for scanning and imaging the wafer based on the determined focus instruction value; and

a release apparatus for releasing the wafer and unloading the wafer from the exposure system.

9. The apparatus of claim 8 wherein the focus determination apparatus varies the determined focus instruction value up to ±

200 nm.

10. The apparatus of claim 8 which is used to monitor the focus of each scan during use of the apparatus to scan multiple wafers further comprising:

an exposing apparatus to expose a new wafer to create exposure fields at different focus based on the focus instruction values;

a measuring apparatus to measure the critical dimension for feature A and feature B;

a calculating apparatus to calculate the difference between the critical dimension of a feature in each exposure field to create new Bossung curves;

a plotting apparatus to plot the difference between the new Bossung curves;

a determining apparatus to determine if the difference between the new Bossung curves has changed compared to the difference between the previous Bossung curves;

an adjusting apparatus to adjust the focus instruction value if necessary based on the difference;

a scanning and exposing apparatus to scan and expose the next wafer based on the adjusted focus instruction values; and

continuing the above steps until the scanning and exposing of each wafer is completed.

11. The apparatus of claim 10 wherein a correlation apparatus is used to form a correlation between the difference between the critical dimension for each feature of the scan position versus focus shift and adjusting the focus instruction value based on the correlation.

12. The apparatus of claim 11 wherein the focus adjustment apparatus varies the determined focus instruction value up to ±

200 nm.

13. The apparatus of claim 10 wherein the adjustment apparatus increases the focus instruction value if the critical dimension for feature B is less than the critical dimension for feature A.

14. The apparatus of claim 10 wherein the adjustment apparatus decreases the focus instruction value if the critical dimension for feature B is greater than the critical dimension of feature A.