Field of the Invention
The present invention relates to methods and apparatus for determining focus of a lithographic apparatus usable, for example, with pupil-plane detection or dark-field scatterometry in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
Background Art
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., including 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 lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes, which are often used to measure critical dimension (CD), and specialized tools to measure overlay (the accuracy of alignment of two layers in a device) and defocus of the lithographic apparatus. Recently, various forms of scatterometers have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target structure, for example a grating, and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength, intensity at one or more wavelengths as a function of reflected angle, or polarization as a function of reflected angle—to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods, library searches, and principal component analysis.
The targets used by conventional scatterometers are relatively large, e.g., 40 μm by 40 μm, gratings and the measurement beam generates a spot that is smaller than the grating (i.e., the grating is underfilled). This simplifies mathematical reconstruction of the target as it can be regarded as infinite. However, in order to reduce the size of the targets, e.g., to 10 μm by 10 μm or less, e.g., so they can be positioned in amongst product features, rather than in the scribe lane, metrology has been proposed in which the grating is made smaller than the measurement spot (i.e., the grating is overfilled). Typically such targets are measured using dark-field scatterometry in which the zeroth order of diffraction (corresponding to a specular reflection) is blocked, and only higher orders processed.
Diffraction-based overlay using dark-field detection of the diffraction orders enables overlay measurements on smaller target structures. These targets can be smaller than the illumination spot and may be surrounded by product structures on a wafer. Multiple targets can be measured in one image.
In the known metrology technique, overlay measurement results are obtained by measuring the target structure, such as a grating, twice under certain conditions, while either rotating the target or changing the illumination mode or imaging mode to obtain separately the −1st and the +1st scatterometer signals, in this example diffraction order intensities. Comparing these intensities for a given grating provides a measurement of asymmetry in the grating.
Asymmetry in a pair of stacked grating structures can be used as an indicator of overlay error. Similarly, asymmetry in a focus-sensitive grating can be used as an indicator of defocus. A focus-sensitive target structure produced by a lithographic apparatus may have at least one feature which has a profile that has an asymmetry that depends on the focus of the lithographic apparatus on the substrate. Focus measurement results may be obtained using the same measurement of asymmetry in the target as used for obtaining overlay measurement results. The asymmetry thus obtained is related to defocus. The relationship between measured asymmetry and defocus can be determined by experimentation.
However, any effect in the scatterometer pupil that leads to an asymmetry change in the scatterometer pupil will be attributed to scanner defocus.