Lithographic apparatus and device manufacturing method

Abstract

A lithographic apparatus and device manufacturing method makes use of a liquid confined in a reservoir between the projection system and the substrate. Bubbles forming in the liquid from dissolved atmospheric gases or from out-gassing from apparatus elements exposed to the liquid are detected and/or removed so that they do not interfere with exposure and lead to printing defects on the substrate. Detection may be carried out by measuring the frequency dependence of ultrasonic attenuation in the liquid and bubble removal may be implemented by degassing and pressurizing the liquid, isolating the liquid from the atmosphere, using liquids of low surface tension, providing a continuous flow of liquid through the imaging field, and/or phase shifting ultrasonic standing-wave node patterns.

1 . A lithographic projection apparatus comprising:
 an illumination system arranged to condition a radiation beam;   a support structure configured to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section;   a substrate table configured to hold a substrate;   a projection system arranged to project the patterned radiation beam onto a target portion of the substrate;   a liquid supply system configured to at least partly fill a space between the projection system and the substrate with a liquid; and   a bubble reduction device configured to reduce a size, a concentration, or both of bubbles in the liquid, the bubble reduction device comprising a bubble detector configured to detect bubbles in the liquid.   
     
     
         2 . An apparatus according to  claim 1 , wherein the bubble detector comprises an ultrasonic transducer configured to measure attenuation of ultrasonic waves in the liquid so as to obtain information about bubbles present in the liquid. 
     
     
         3 . An apparatus according to  claim 2 , wherein the ultrasonic transducer is configured to measure ultrasonic attenuation as a function of frequency. 
     
     
         4 . An apparatus according to  claim 2 , wherein an ultrasonic transducer is arranged in a pulse-echo configuration, the transducer acting both to transmit ultrasonic waves and, after reflection, to receive ultrasonic waves that have been attenuated during propagation along a path through the liquid. 
     
     
         5 . An apparatus according to  claim 2 , wherein the bubble detector comprises two spatially separated ultrasonic transducers, the first arranged to transmit ultrasonic waves, and the second to receive ultrasonic waves that have been attenuated during propagation along a path through the liquid between the two transducers. 
     
     
         6 . An apparatus according to  claim 1 , wherein the bubble reduction device comprises a bubble removal device. 
     
     
         7 . An apparatus according to  claim 6 , wherein the bubble removal device comprises a degassing device, the degassing device comprising an isolation chamber, wherein a space above liquid in the isolation chamber is maintained at a pressure below atmospheric pressure to encourage previously dissolved gases in the liquid to come out of solution and be pumped away. 
     
     
         8 . An apparatus according to  claim 6 , wherein the bubble removal device is configured to provide a continuous flow of liquid over the projection system and the substrate to transport bubbles in the liquid out of the space between the projection system and the substrate. 
     
     
         9 . An apparatus according to  claim 6 , wherein the bubble removal device includes two spatially separated ultrasonic transducers, arranged to produce ultrasonic standing-wave patterns within the liquid which trap bubbles within nodal regions, the bubble removal device being arranged to displace the bubbles through the use of a phase-adjusting device linked with the transducers, the phase-adjusting device causing spatial shift of nodal regions and bubbles trapped therein. 
     
     
         10 . An apparatus according to  claim 6 , wherein the bubble removal device comprises an electric field generator configured to apply an electric field to the liquid, the electric field being capable of dislodging bubbles attached to the substrate. 
     
     
         11 . An apparatus according to  claim 6 , wherein the bubble removal device comprises a heater configured to selectively control the temperature and therefore size of bubbles of a particular composition. 
     
     
         12 . An apparatus according to  claim 11 , wherein the heater comprises a microwave source. 
     
     
         13 . An apparatus according to  claim 6 , wherein the bubble removal device comprises a particle input device configured to introduce particles into the liquid, and a particle removal device configured to remove the particles from the liquid. 
     
     
         14 . An apparatus according to  claim 13 , wherein the particles comprise a surface with characteristics that encourage bubbles to attach thereto. 
     
     
         15 . An apparatus according to  claim 1 , wherein the bubble reduction device comprises a liquid pressurization device configured to pressurize the liquid above atmospheric pressure to minimize the size of bubbles and encourage bubble-forming gases to dissolve into the liquid. 
     
     
         16 . An apparatus according to  claim 1 , wherein the composition of the liquid is chosen to have a lower surface tension than water. 
     
     
         17 . An apparatus according to  claim 1 , wherein the bubble reduction device is configured to treat the liquid before it is supplied to the space between the projection system and the substrate. 
     
     
         18 . An apparatus according to  claim 17 , wherein the treated liquid is kept in a sealed container, excess space in the sealed container comprising one or more of the following: nitrogen gas, argon gas, helium gas or a vacuum. 
     
     
         19 . An apparatus according to  claim 1 , wherein the bubble detector comprises a light source, a light detector and a light comparator, the light source and the light detector being arranged so that light emitted by the source propagates between the source and the detector through a portion of the liquid, the comparator being arranged to detect changes in the proportion of the emitted light that arrives at the detector after propagation through a portion of the liquid. 
     
     
         20 . A lithographic projection apparatus comprising:
 an illumination system arranged to condition a radiation beam;   a support structure configured to support a patterning device, the patterning device being capable of imparting the radiation beam with a pattern in its cross-section;   a substrate table configured to hold a substrate;   a projection system arranged to project the patterned radiation beam onto a target portion of the substrate;   a liquid supply system configured to at least partly fill a space between the projection system and the substrate with a liquid; and   a detection system configured to detect impurities in the liquid, including a light source, a light detector and a light comparator, the light source and the light detector being arranged so that light emitted by the source propagates between the source and the detector through a portion of the liquid, the comparator being arranged to detect changes in the proportion of the emitted light that arrives at the detector after propagation through a portion of the liquid.