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1 EXTREME ULTRAVIOLET LIGHT GENERATION SEQUENCE FOR AN EXTREME ULTRAVIOLET LIGHT SOURCE BACKGROUND 5 [0001] 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 can be a mask or a reticle, can 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 10 substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation- sensitive material (photoresist or simply “resist”) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatuses 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 15 irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”- direction) while synchronously scanning the target portions parallel or anti-parallel to this scanning direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. [0002] A lithographic apparatus typically includes an illumination system that conditions radiation 20 generated by a radiation source before the radiation is incident upon a patterning device. A patterned beam of EUV light can be used to produce extremely small features on a substrate. EUV light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having wavelengths in the range of about 5-100 nm. One particular wavelength of interest for photolithography occurs at 13.5 nm. 25 [0003] Methods to produce EUV light include, but are not necessarily limited to, converting a source material into a plasma state that has a chemical element with an emission line in the EUV range. These elements can include, but are not necessarily limited to, xenon, lithium and tin. In various implementations, these methods are suitable for use in the production of microelectronic circuits and/or semiconductor devices. 30 [0004] In one such method, often termed laser-produced plasma (“LPP”), the desired plasma can be produced by irradiating a source material, for example, in the form of a droplet, stream or wire, with a laser beam. [0005] One technique for producing a laser-produced plasma involves irradiating a source material, often a droplet, with a series of laser pulses. A first pulse from a first laser, often termed a pre-pulse, 35 can expand the droplet into a disc-like shape to form a so called “tin target”, or simply “target”. A second pulse from a second laser, often termed a rarefication pulse or a rarefaction pulse, can change 2 the density of the target, resulting in a “rarefied target”. Lastly, a third pulse from a third laser can irradiate the rarefied target to generate a plasma. [0006] To maximize energy generated per pulse and stability of generated EUV light, each of the three laser beams must be properly aligned with the droplet and target. Additionally, various operating 5 parameters of an EUV light source (e.g., beam characteristics, droplet characteristics, etc.) can be set with countless permutations for generating EUV light. If the three laser beams are misaligned and/or the EUV light source parameters are set improperly, an EUV light source can produce lower and more unstable power, resulting in dose errors at the wafer level. 10 DETAILED DESCRIPTION [0007] The aspects described herein, and references in the specification to “one aspect,” “an aspect,” “an exemplary aspect,” “an example aspect,” etc., indicate that the aspects described can include a particular feature, structure, or characteristic, but every aspect may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same 15 aspect. Further, when a particular feature, structure, or characteristic is described in connection with an aspect, it is understood that it is within the knowledge of those skilled in the art to effect such feature, structure, or characteristic in connection with other aspects whether or not explicitly described. [0008] Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “on,” “upper” and the like, can be used herein for ease of description to describe one element or feature’s relationship to 20 another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein can likewise be interpreted accordingly. [0009] The terms “about,” “approximately,” or the like can be used herein indicates the value of a 25 given quantity that can vary based on a particular technology. Based on the particular technology, the terms “about,” “approximately,” or the like can indicate a value of a given quantity that varies within, for example, 10–30% of the value (e.g., ±10%, ±20%, or ±30% of the value). [0010] Aspects of the present disclosure can be implemented in hardware, firmware, software, or any combination thereof. Aspects of the disclosure can also be implemented as instructions stored on a 30 computer-readable medium, which can be read and executed by one or more processors. A machine- readable medium can include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium can comprise read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g., 35 carrier waves, infrared signals, digital signals, etc.), and others. Furthermore, firmware, software, routines, and/or instructions can be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions result from 3 computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. The term “machine-readable medium” can be interchangeable with similar terms, for example, “computer program product,” “computer-readable medium,” “non-transitory computer- readable medium,” or the like. The term “non-transitory” can be used herein to characterize one or more 5 forms of computer readable media except for a transitory, propagating signal. [0011] Before describing such aspects in more detail, however, it is instructive to present an example environment in which aspects of the present disclosure can be implemented. [0012] Example Lithographic Systems 10 [0013] FIG. 1 shows a lithographic apparatus 100 in which aspects of the present disclosure can be implemented. In some aspects, lithographic apparatus 100 can comprise the following: an illumination system (illuminator) IL configured to condition a radiation beam B (for example, deep ultra violet or extreme ultra violet radiation); a support structure (for example, a mask table) MT configured to support a patterning device (for example, a mask, a reticle, or a dynamic patterning device) MA and connected 15 to a first positioner PM configured to accurately position patterning device MA; and, a substrate table (for example, a wafer table) WT configured to hold a substrate (for example, a resist coated wafer) W and connected to a second positioner PW configured to accurately position substrate W. Lithographic apparatus 100 also comprises a projection system PS configured to ...