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Method of measurement, an inspection apparatus and a lithographic apparatus

Method of measurement, an inspection apparatus and a lithographic apparatus description/claims

The present invention relates to a method of inspection usable, for example, in the manufacture of devices by a lithographic technique and to a method of manufacturing devices using a lithographic technique.

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.

In order to determine one or more properties of the substrate, such as the overlay between a resist pattern and an underlying processed pattern, a beam is reflected off the surface of the substrate, for example at an alignment target, and an image is created on, for example, a camera of the reflected beam. By comparing one or more properties of the beam before and after it has been reflected off the substrate, a property of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with one or more known substrate properties.

Such a system of illuminating a target and collecting data from the reflected radiation is often used to illuminate a plurality of superimposed patterns, for example a plurality of gratings. The second grating has a predetermined bias compared to the first grating. By analyzing the characteristics of the reflected radiation it is possible to determine the overlay error, OV, between the gratings. This is achieved by introducing a known shift, d, between gratings in different layers. However, the detected overlay accuracy is dependent on both the overlay accuracy itself and asymmetry due to sensor asymmetry resulting from, for example, radiation scattering. A method of reducing the effect of sensor asymmetry includes rotating the substrate but this takes time and may result in a significant loss of throughput. An alternative is to use a reference target but this also may require significant time and space on the substrate since many different targets are required.

It is desirable to, for example, provide an alternative, simple method of reducing the effect of sensor asymmetry.

According to an aspect of the invention, there is provided a method of measuring the overlay error of a substrate, comprising:

projecting a first beam of radiation onto a first target of the substrate, the first target comprising at least two superimposed patterns having a bias of +d between a first of the patterns arranged in or on a first layer and a second of the patterns arranged in or on a second layer, and measuring the asymmetry of that first beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a first signal indicative of the measured asymmetry;

projecting the first beam of radiation onto a second target of the substrate, the second target comprising at least two superimposed patterns having a bias of −d between a first of the patterns arranged in or on a first layer and a second of the patterns arranged in or on a second layer, and measuring the asymmetry of that first beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a second signal indicative of the measured asymmetry;

projecting a second beam of radiation onto the first target and measuring the asymmetry of that second beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a third signal indicative of the measured asymmetry;

projecting the second beam of radiation onto the second target and measuring the asymmetry of that second beam of radiation reflected from the substrate that is indicative of a property of the substrate and generating a fourth signal indicative of the measured asymmetry,

wherein the first and second beams have different polarizations, or different wavelengths, or both, and the overlay error is determined on the basis of the first, second, third and fourth signals.

According to a further aspect of the invention there is provided an inspection apparatus configured to measure a property of a substrate, the apparatus comprising:

a radiation projector configured to project radiation onto the substrate;

a detector configured to measure asymmetry of radiation reflected from the substrate; and

a processor configured to calculate an overlay error on the basis of the asymmetry, measured by the detector, of radiation of a plurality of wavelengths, or a plurality of polarizations, or both, reflected from the substrate.

According to a further aspect of the invention there is provided a lithographic apparatus, comprising:

an illuminator configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beam onto a target portion of the substrate; and

• Provisional Patent
• Utility Patent

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