Patent Publication Number: US-6912267-B2

Title: Erosion reduction for EUV laser produced plasma target sources

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a laser-plasma extreme ultraviolet (EUV) radiation source and, more particularly, to a laser-plasma EUV radiation source that includes a technique for electrically isolating a nozzle of the source from the generated plasma to reduce arcing and nozzle erosion. 
     2. Discussion of the Related Art 
     Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current trend for photolithography light sources is to develop a system that generates light in the extreme ultraviolet (EUV) or soft X-ray wavelengths (13-14 nm). 
     Various devices are known in the art to generate EUV radiation. One of the most popular EUV radiation sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Argon and Krypton, and combinations of gases, are also known for the laser target material. In the known EUV radiation sources based on laser produced plasmas (LPP), the gas is typically cryogenically cooled in a nozzle to a liquid state, and then forced through an orifice or other nozzle opening into a vacuum chamber as a continuous liquid stream or filament. Cryogenically cooled target materials, which are gases at room temperature, are required because they do not condense on the EUV optics, and because they produce minimal by-products that have to be evacuated by the vacuum chamber. In some designs, the nozzle is agitated so that the target material is emitted from the nozzle as a stream of liquid droplets having a certain diameter (30-100 μm) and a predetermined droplet spacing. 
     The low temperature of the liquid target material and the low vapor pressure within the vacuum environment cause the target material to quickly freeze. Some designs employ sheets of frozen cryogenic material on a rotating substrate, but this is impractical for production EUV sources because of debris and repetition rate limitations. 
     The target stream is illuminated by a high-power laser beam, typically from an Nd:YAG laser, that heats the target material to produce a high temperature plasma which emits the EUV radiation. The laser beam is delivered to a target area as laser pulses having a desirable frequency. The laser beam must have a certain intensity at the target area in order to provide enough heat to generate the plasma. 
       FIG. 1  is a plan view of an EUV radiation source  10  of the type discussed above including a nozzle  12  having a target material chamber  14  that stores a suitable target material, such as Xenon, under pressure. The chamber  14  includes a heat exchanger or condenser that cryogenically cools the target material to a liquid state. The liquid target material is forced through a narrowed throat portion  16  of the nozzle  12  to be emitted as a filament or stream  18  into a vacuum chamber towards a target area  20 . The liquid target material will quickly freeze in the vacuum environment to form a solid filament of the target material as it propagates towards the target area  20 . The vacuum environment and vapor pressure within the target material will cause the frozen target material to eventually break up into frozen target fragments, depending on the distance that the stream  18  travels. 
     A laser beam  22  from a laser source  24  is directed towards the target area  20  to vaporize the target material. The heat from the laser beam  22  causes the target material to generate a plasma  30  that radiates EUV radiation  32 . The EUV radiation  32  is collected by collector optics  34  and is directed to the circuit (not shown) being patterned. The collector optics  34  can have any shape suitable for the purposes of collecting and directing the radiation  32 , such as a parabolic shape. In this design, the laser beam  22  propagates through an opening  36  in the collector optics  34 , as shown. Other designs can employ other configurations. 
     In an alternate design, the throat portion  16  can be vibrated by a suitable device, such as a piezoelectric vibrator, to cause the liquid target material being emitted therefrom to form a stream of droplets. The frequency of the agitation determines the size and spacing of the droplets. If the target stream  18  is a series of droplets, the laser beam  22  is pulsed to impinge every droplet, or every certain number of droplets. 
     The target stream  18  provides a certain steady-state pressure of evaporating target material at its location in the vacuum chamber. The pressure within the vacuum chamber decreases the farther away from the target stream  18 . This pressure differential defines lines of constant pressure between the plasma  30  and the throat portion  16 . Within specific pressure ranges that depend on the target material, these lines of constant pressure provide current or arcing paths from the plasma  30  to the nozzle  12 . Electrical discharge arcs are emitted from the plasma  30  to the conductive portions of the nozzle  12  along the lines of constant pressure, and can travel relatively large distances from the plasma  30  to the nozzle  12 . If the pressure is too high or too low, then the electrical discharge arcs cannot be supported. Additionally, fast atoms emitted from the target material and solid pieces of excess, unvaporized target material can impact the nozzle  12 . 
     The electrical discharge arcs from the plasma  30  cause the nozzle material to melt or vaporize, creating nozzle damage and excess debris in the chamber. Also, the fast atoms and excess target material erode the nozzle  12 . The generation of this debris also causes damage to the optical elements and other components of the source resulting in increased process costs. Each one of the above-mentioned debris generation mechanisms must be addressed in order to effectively minimize source debris generation. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a laser-plasma EUV radiation source is disclosed that employs one or more approaches for eliminating erosion of and vaporization of material from a nozzle of the source by electrical discharge and arcing generated by the plasma. A first approach includes employing a non-conductive nozzle outlet end, such as a glass capillary tube, that will not conduct the arc. The nozzle outlet end extends beyond all of the conductive surfaces of the nozzle towards the plasma by a suitable distance so that the pressure in the chamber around the closest conductive portion of the nozzle to the plasma is low enough so that it does not support arcing. A second approach includes providing electrical isolation of the conductive portions of the nozzle from the vacuum chamber wall. A third approach includes applying a bias potential to the nozzle to raise the potential of the nozzle to the potential of the arc to inhibit current flow. 
     Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an EUV radiation source; and 
         FIG. 2  is a plan view of a nozzle for the EUV radiation source shown in  FIG. 1 , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to an EUV radiation source including a nozzle that prevents plasma arcing is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. 
       FIG. 2  is a plan view of a nozzle assembly  40  applicable to replace the nozzle  12  in the source  10  discussed above, according to an embodiment of the present invention. The nozzle assembly  40  includes a target material chamber  42  that cryogenically cools the target material to a liquid state and holds it under pressure. The nozzle assembly  40  also includes a nozzle outlet tube  46  that is mounted to the chamber  42  by suitable mounting hardware  44 , where the target material is forced through the tube  46 . The tube  42  extends through the mounting hardware  44  and is in fluid communication with the chamber  42 . A target material filament stream  48  is emitted from the tube  46  and quickly freezes in the chamber. The frozen filament stream  48  is vaporized by the laser beam  22  to generate the EUV radiation  32 , as discussed above. 
     According to the invention, the nozzle outlet tube  46  is made of a non-conductive material so that electrical discharge and arcing from the plasma  30  is not attracted to the tube  46 , and thus does not damage the nozzle assembly  40 . In one embodiment, the tube  16  is a capillary tube made of glass or ceramic. However, this is by way of a non-limiting example in that other non-conductive materials can be employed. Further, other non-conductive nozzle components, such as an orifice plate, can be provided closest to the target area  20  to prevent arcing. 
     The closest conductive portion of the nozzle assembly  40  to the plasma  30  is the mounting hardware  44 . According to the invention, the mounting hardware  44  is set back far enough from the plasma  30  so that it is in a region of the chamber having a pressure that is too low to support electrical discharges from the plasma  30 . In other words, because the arcs from the plasma  30  must travel through a region within the chamber that has sufficient pressure, the arcs will not hit the mounting hardware  44  because the pressure around the mounting hardware  44  is too low. In other designs, the closest conductive portion of the nozzle assembly  40  may not be the mounting hardware  44 , but may be another conductive portion of the nozzle assembly  40  which also would be positioned in a low pressure region of the chamber. 
     In one example, the outlet end of the tube  46  extends beyond all of the conductive surfaces of the nozzle assembly  40  by a sufficient distance, such as 0.1 inch. This distance is set based on the pressure in the vacuum chamber and the type of target material, such as Xenon. In an EUV production chamber, the gas pressure that results from evaporation of the liquid or solid target material will be confined predominantly to the region beyond (downstream of) the opening of the tube  46 . The pressure adjacent to the tube  46  should be insufficient to allow an arc to be established between the plasma  30  and the mounting hardware  44 . 
     According to another embodiment of the present invention, the nozzle assembly  40  includes a non-conductive mounting plate  50  mounted to the chamber wall to electrically isolate the nozzle assembly  40  from the chamber wall, which is typically at ground. Thus, no conductive portion of the nozzle assembly  40  directly contacts the chamber wall. By breaking the current path from the nozzle assembly  40  to the chamber wall, arcing from the plasma  30  will not damage the nozzle assembly  40 . The plate  50  can be any non-conductive isolation member that breaks the electrical continuity between the mounting hardware  44  and the chamber wall. In this design, the tube  46  can be conductive because the mounting plate  50  prevents current from the arcs from traveling through the tube  46 . As will be appreciated by those skilled in the art, the plate  50  can be made of any suitable non-conductive material, such as glass, and can be positioned at any convenient location in the structural configuration of the nozzle assembly  40  to break the conductive path of the current resulting from electrical discharge from the plasma  30 . 
     In yet another embodiment of the invention, a DC bias source  52  is electrically coupled to the mounting hardware  44 , or another conductive portion of the nozzle assembly  40 , to raise the potential of the nozzle assembly  40  to the potential of the arc. By raising the electric potential of the nozzle assembly  40  to the electric potential of the electrical discharge, no current flows into the nozzle assembly  40  from the arcs. In order to be effective, the voltage potential of the arc would have to be known, so the appropriate DC bias potential could be applied to the nozzle assembly  40 . 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.