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1 DETERMINATION OF A PROPERTY OF AN EXPOSURE LIGHT BEAM TECHNICAL FIELD [0001] This disclosure relates to determining a property of an exposure light beam. The exposure light beam may be generated based on an initial light beam that is emitted from a deep ultraviolet 5 (DUV) optical source. BACKGROUND [0002] Photolithography is the process by which semiconductor circuitry is patterned on a substrate such as a silicon wafer. An optical source generates deep ultraviolet (DUV) light used to expose a 10 photoresist on the wafer. DUV light may include wavelengths from, for example, about 100 nanometers (nm) to about 400 nm. Often, the optical source is a laser source (for example, an excimer laser) and the DUV light is a pulsed laser beam. The DUV light from the optical source interacts with a projection optical system, which projects the beam through a mask onto the photoresist on the silicon wafer. In this way, a layer of chip design is patterned onto the photoresist. 15 The photoresist and wafer are subsequently etched and cleaned. If needed, the photolithography process is repeated with a fresh photoresist. SUMMARY [0003] In one aspect, an apparatus includes: an estimation system configured to: determine a set of 20 values related to an initial light beam based on sensed wavefronts of the initial light beam, the set of values including a first value and a second value. The estimation system is also configured to determine an estimate of a property of an exposure light beam based on a non-linear relationship that includes the first value and the second value. The exposure light beam is formed by interacting the initial light beam with an optical system. The apparatus also includes a communications module 25 coupled to the estimation system and configured to output the estimate of the property of the exposure light beam. [0004] Implementations may include one or more of the following features. The property of the exposure light beam may be a convolved bandwidth metric, the convolved bandwidth metric representing a width of a portion of an optical spectrum of the exposure light beam at a wafer that is 30 irradiated by the exposure light beam; and the optical spectrum of the exposure light beam includes intensity of the exposure light beam as a function of wavelength. [0005] The sensed wavefronts of the initial light beam may include a fringe pattern produced from the initial light beam; the fringe pattern may include a plurality of fringes; the first value may include a first width of a first one of the plurality of fringes; and the second value may include a second width 35 of a second one of the plurality of fringes. The first one of the plurality of fringes and the second one of the plurality of fringes may be the same one fringe. The first width may be a width of the one 2 fringe at a first percentage of a peak intensity of the one fringe; and the second width may be a width of the one fringe at a second percentage of the peak intensity of the one fringe. The first percentage and the second percentage may be different percentages. The plurality of fringes may be concentric rings of light centered around a center point and separated by regions of no light; and the one fringe 5 may be the fringe closest to the center point. The apparatus also may include an etalon configured to produce the fringe pattern. [0006] The non-linear relationship may be a second-order relationship. One of the first value and the second value may be squared. [0007] The non-linear relationship also may include a plurality of calibration parameters. The 10 estimation system also may be configured to: access a reference value of the property of the exposure light beam; and determine values for each of the calibration parameters by minimizing a difference between the estimate of the property and the reference value of the property. The reference value of the property may be obtained by a spectrometer. [0008] The apparatus also may include the optical system. 15 [0009] The optical system may include a projection lens and a reticle. [0010] The apparatus also may include a detector configured to sense the wavefronts and to provide data related to the sensed wavefronts to the estimation system. [0011] In another aspect, a system includes: a light source configured to emit a light beam that includes deep ultraviolet (DUV) light; an optical measurement system configured to produce a fringe 20 pattern based on the light beam; a projection optical system configured emit an exposure light beam based on the light beam; and an estimation system configured to: determine a first value and a second value from the fringe pattern; and determine an estimate of a property of the exposure light beam based on the first value and the second value. [0012] Implementations may include one or more of the following features. 25 [0013] The projection optical system may include a projection lens and a reticle. [0014] The estimation system may be configured to determine the estimate of the property based on a non-linear relationship; and the non-linear relationship may include the first value, the second value, and a plurality of calibration constants. The estimation system also may be configured to: determine a value for each of the plurality of calibration constants based on minimizing a difference between the 30 estimate of the property and a reference value of the property. [0015] The optical measurement system may be an etalon. [0016] The light source may include a master oscillator configured to emit a seed light beam, and a power amplifier configured to amplify the seed light beam to produce the light beam that includes DUV light. 35 [0017] In another aspect, a method includes: sensing wavefronts of an initial light beam; determining a set of values of an initial light beam based on the sensed wavefronts; determining a relationship that includes at least two of values in the set of values; and determining an estimate of a property of an 3 exposure light beam based on the relationship. The exposure light beam is produced by interacting the initial light beam with an optical system. [0018] The relationship may be a non-linear relationship. [0019] Implementations of any of the techniques described above may include a system, a method, a 5 process, a device, or an apparatus. The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings. DRAWING DESCRIPTION 10 [0020] FIG. 1A is a block diagram of an example of a system. [0021] FIG. 1B shows an example of an interference pattern. [0022] FIG. 1C is a block diagram of an image plane and an output lens of the system of FIG. 1A. [0023] FIG. 2A is a block diagram of an example of an optical measurement apparatus. [0024] FIGS. 2B and 2C relate to another example of an interference pattern. 15 [0025] FIG. 3 is a flow chart of an example of a process to determine an estimate of a property of the exposure light beam. [0026] FIG. 4 is an example of experimental data showing convolved bandwidth (CBW) estimation error. [0027] FIG. 5 is an example of experimental data showing error in CBW estimated from a second- 20 order relationship as a function of a measured reference CBW. [0028] FIG. 6 is an example of experimental data showing error in CBW estimated from a lin...