TEMPERATURE MEASUREMENT OF OPTICAL ELEMENTS IN AN OPTICAL APPARATUS

摘要

According to a first aspect of the disclosure, there is provided an optical apparatus comprising: an optical element having a reflective surface for reflecting incident radiation in a beam path; and at least one sensor configured to sense radiation corresponding to a temperature of a respective portion of a backside surface of the optical element. Advantageously, the at least one sensor is configured to sense radiation, rather than conducted heat as is the case in prior art optical apparatus, as described in more detail below. That is, the at least one sensor is provided in a contactless arrangement with the optical element wherein the at least one sensor, or at least a radiation-sensitive portion of the at least one sensor, is not in physical contact with the respective portion of the backside surface of the optical element. Advantageously, by using such a contacting arrangement, there is no need to adhere the at least one sensor to the backside surface of the optical element, and therefore no SFD effects are induced by thermomechanical stresses or stresses caused by aging of an adhesive. Furthermore, advantageously by using such a contactless arrangement, the optical apparatus may be less likely to be damaged and/or require maintenance, and hence the optical apparatus may have an increased lifetime. A gap may be provided between the at least one sensor and the respective portion of the backside surface of the optical element. The portion of the backside surface may be in a cavity formed in a backside of the optical element. Furthermore, because of the contactless arrangement, such cavities can be formed to extend much closer to the reflective surface of the optical element without incurring an increase in SFD effects than in existing solutions, such as those described below with reference to Figures 3 and 4, wherein sensors are adhered to a backside surface of the optical element. Furthermore, since such a contactless arrangement may lead to a reduction in SFD effects, a greater quantity of cavities and associated sensors may be implemented, thereby enabling a more complete, precise and responsive determination and control of a temperature of the reflective surface of the optical element by a MPHS. The optical apparatus may comprise at least one through-passage connecting a void defined by the gap to an area outside the cavity. The cavity may be formed such that the portion of the backside surface is less than 5 millimeters from the reflective surface. The at least one sensor may comprise a pyrometer configured to sense infrared radiation radiated from the respective portion of the backside surface of the optical element. The optical apparatus may comprise a high-emissivity coating formed on the portion of the backside surface, The optical apparatus may comprise at least one coating formed on the respective portion of the backside surface. The at least one coating may be configured to have temperature-dependent optical properties. The at least one coating may comprise a thermochromic coating. The at least one coating may comprise a plurality of coatings configured RS an interference filter and/or a grating having temperature-dependent optical properties. The grating may be a volume Bragg grating. The optical apparatus may comprise at least one radiation-emitting device configured to emit radiation toward the at least one coating. The at least one sensor may be configured to .sense radiation emitted by the radiation- emitting device and reflected from the at least one coating and/or from the backside surface of the optical element. The optical apparatus may comprise a temperature controller configured to use a signal from the at least one sensor to control a temperature of the optical element. The optical apparatus may comprise a plurality of sensors, wherein the backside surface of the optical element may comprise a plurality of cavities, and each cavity may have an associated sensor configured to sense radiation corresponding to a temperature of a portion of the backside surface of the optical element within the cavity. According to a second aspect of the disclosure, there is provided an optical system comprising at least one optical apparatus according to the first aspect. The optical system may be configured as a projection system for a lithographic apparatus. The optical system may be configured as an illumination system for a lithographic apparatus. According to a third aspect of the disclosure, there is provided a lithographic apparatus comprising the optical system according to the second aspect. According to a fourth aspect of the disclosure, there is provided a method of controlling a temperature of a reflective surface of an optical element in a lithographic apparatus, the method comprising: configuring at least one sensor to sense radiation corresponding to a temperature of a portion of a backside surface of the optical element; and using a signal provided by the sensor to configure a temperat