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 monitor the lithographic process, it is necessary to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in a lithographic process, including the use of a scanning electron microscope and various specialized tools. One form of specialized inspection tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties 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 known substrate properties. Two main types of scatterometer are known. A spectroscopic scatterometer directs a broadband radiation beam onto the substrate and measures the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. An angularly resolved scatterometer uses a monochromatic radiation beam and measures the intensity of the scattered radiation as a function of angle.
Scatterometry is an active field of research where optical techniques are used to measure subwavelength features of an object. An embodiment of the system of the present invention may be used with apparatus configured to measure the subwavelength features, such as an in-line metrology tool. Such a metrology tool detects a reflected beam that has been reflected from the surface of a substrate and more specifically from a specific target on the substrate, and from the reflected beam and its different diffraction orders, reconstructs the shape of the target on the substrate.
As manufacturing margins for the formation of, for example, integrated circuit devices become smaller and the complexity of lithographic apparatus becomes ever greater, it becomes more significant to monitor the performance of the equipment used in the various parts of the lithography process. Typically this involves executing dedicated tests for each sub-system or performance parameter. However, these tests are costly, time-consuming, do not always address on-product performance criterion and/or are often not very specific in identifying root-causes in errors that are detected. Furthermore, it is often the case that by the time such a test has been completed and the results analyzed, the tested sub-system will have processed many substrates in the whatever condition it is in. Accordingly, a large number of substrates may be processed by a faulty sub-system before the fault is detected, resulting in large numbers of substrates either needing re-work or, in some cases, scrapping. Clearly this may be very costly.