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Stage flatness monitor

IP.com Disclosure Number: IPCOM000004836D
Publication Date: 2001-Jul-03
Document File: 4 page(s) / 241K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method to monitor changes in the flatness of a scanner stage. Benefits include efficiency and accuracy in locating height changes, improved sensitivity, and results are easily interpreted.

This text was extracted from a WORD97 document.
This is the abbreviated version, containing approximately 100% of the total text.

Stage flatness monitor

Disclosed is a method to monitor changes in the flatness of a scanner stage. Benefits include efficiency and accuracy in locating height changes, improved sensitivity, and results are easily interpreted.

The key elements are:

1. A reference wafer

2. Pre-measurement of the reference wafer to create a baseline.

3. Subtraction of the baseline from future measurements of the reference wafer.

The disclosed method addresses the problem of detecting local changes of stage flatness. One cause is particulate contamination on the stage.

Conventionally, stage flatness is monitored by two methods:

A wafer, selected for its uniform flatness, is loaded onto the scanner stage. The topography of the wafer surface is then measured, using a laser system, and inspected for local changes in height. Particles smaller than a quarter-micron are difficult to discern, due to the intrinsic non-flatness of the wafer and stage (see Figure 1).

A scanner and track are used to print a grating on a wafer. The grating is then inspected for defects. Stage issues (such as illumination and uniformity) and wafer issues (such as cleanliness and flatness) limit the sensitivity of the technique.

The disclosed method provides the following benefits:

The invention is efficient and accurate in locating local height changes of 0.15 ┬Ám.

The sensitivity can be increased, as needed, by taking more measurements per surface location.

The invention does not require a skilled observer for interpretation of the results.

Wafers, used by the invention, do not need to be uniformly flat.

The disclosed method includes the following procedure:

1. When the scanner stage is in a POR condition, a wafer is placed on the stage, and the topography of the wafer surface is measured with a laser system.

2. The wafer is stored as a reference, and the measurement file kept as a baseline for future measurements.

3. The reference wafer is again placed on the stage and measured. The recorded baseline is subtracted from the measurement. Signals above a noise threshold can be attributed to real changes in local stage flatness (see Figure 2).

Variations of this method may use different types of wafers or reference objects. Marks can also be printed onto the reference wafers/objects to ensure/check wafer alignment.

Fig. 1

Fig. 2

Disclosed anonymously