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1 SYSTEM FOR AND METHOD OF CALIBRATING MEASUREMENT OF DUV LASER BEAM SPECTRAL PROPERTIES BACKGROUND 5 [0001] Photolithography is a process by which circuitry is patterned on a semiconductor substrate such as a silicon wafer. A photolithography radiation source provides DUV radiation (radiation having wavelengths in a range of about 100 nanometers (nm) to about 400 nm) used to expose a photoresist on the wafer. Often, the radiation source is a laser source and the radiation is a pulsed laser beam. The radiation beam is passed through a beam delivery unit, then through or reflected by a reticle or a mask, 10 and then projected onto a silicon wafer coated with photoresist. In this way, a chip design pattern is formed in the photoresist, and then in the silicon wafer by etching. The photoresist is then removed by cleaning. [0002] In many systems that produce a laser beam (such as a laser generator) or employ a laser beam (such as a photolithography system), there is an optical train that includes one or more optical 15 components (such as mirrors, gratings, prisms, optical switches, filters, etc.). Optical components in the optical train may, wholly or partially, reflect, process, filter, modify, focus, expand, etc. the laser beam to obtain one or more desired laser beam outputs. [0003] In such systems it is generally desired to measure and control various spectral properties of the DUV radiation beam such as its wavelength (e.g., center line wavelength) and its bandwidth (e.g., 20 full width half maximum (FWHM) bandwidth). A spectral analysis module is used to measure spectral properties of the radiation beam, and such measured spectral properties are then used to control aspects of the radiation beam. [0004] It would be advantageous to improve the accuracy of the measurement of the radiation beam spectral properties. It is in this context that the need for the subject matter of the present disclosure 25 arises. DETAILED DESCRIPTION [0005] Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for 30 purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details associated with it below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments. This summary is not an 35 extensive overview of all contemplated embodiments and is not intended to single out as key or critical any elements of any embodiments nor delineate the scope of any or all embodiments. 2 [0006] Systems such as those described herein may render benefits in a wide range of applications and implementations. For the sake of having a specific nonlimiting example to facilitate description, one such application is in semiconductor photolithography. FIG. 1 shows a photolithography system 100 that includes an illumination system 105. As described more fully below, the illumination system 5 105 includes a radiation source that produces a pulsed radiation beam 110 and directs it to a photolithography exposure apparatus 115 such as a scanner that patterns microelectronic features on a wafer 120. The wafer 120 is placed on a wafer table 125 constructed to hold wafer 120 and connected to a positioner 130 configured to accurately position the wafer 120 in accordance with certain parameters. 10 [0007] The pulsed radiation beam 110 may have a wavelength in the DUV range. The scanner 115 includes an optical arrangement 135 having, for example, one or more condenser lenses, a mask, and an objective arrangement. The mask is movable along one or more directions, such as along an optical axis of the pulsed radiation beam 110 or in a plane that is perpendicular to the optical axis. The objective arrangement includes a projection lens and enables an image transfer to occur from the mask to 15 photoresist on the wafer 120. The illumination system 105 adjusts the range of angles for the pulsed radiation beam 110 impinging on the mask. The illumination system 105 also homogenizes (makes uniform) the intensity distribution of the pulsed radiation beam 110 across the mask. [0008] The scanner 115 can include, among other features, a lithography controller 140 that controls how layers are printed on the wafer 120. The lithography controller 140 may include a memory 20 that stores information such as process recipes that determine the parameters of the beam including a length of the exposure on the wafer 120 based on, for example, the mask used, as well as other factors that affect exposure. During lithography, a burst of pulses of the pulsed radiation beam 110 illuminates the same area of the wafer 120 to constitute an illumination dose. [0009] The photolithography system 100 also preferably includes a control system 145. In general, 25 the control system 145 includes one or more of digital electronic circuitry, computer hardware, firmware, and software. The control system 145 can be centralized or be partially or wholly distributed throughout the photolithography system 100. [0010] FIG. 2 shows a pulsed laser source that produces a pulsed laser beam as the radiation beam 110 as an example of an illumination system 105. FIG. 2 shows a two-chamber laser system as a 30 nonlimiting example but it will be understood that the principles explained herein are equally applicable to a single chamber laser system or a laser system having more than two chambers. The gas discharge laser system may include, e.g., a solid state or gas discharge master oscillator (“MO”) seed laser system 200, an amplification stage, e.g., a power ring amplifier (“PRA”) stage 205, relay optics 210, and laser system output subsystem 215. The seed system 200 may include, e.g., an MO chamber 220 which 35 includes a pair of electrodes 222 and 224. [0011] The MO seed laser system 200 may also include a master oscillator output coupler (“MO OC”) 230, which may comprise a partially reflective mirror, forming an MO discharge chamber 220 3 with an oscillator cavity, defined in part by a reflective grating (not shown) in a line narrowing module (“LNM”) 235, that oscillates to generate ...