Abstract:
A laser produced plasma device comprises a shutter assembly for mitigating the contaminating effects of debris generated by the plasma. In one embodiment, the shutter assembly includes a rotatable shutter having at least one aperture that provides a line-of-sight between a radiation source and an exit of the device during a first period of rotation of the shutter, and obstructs the line-of-sight between the radiation source and the exit during a second period of rotation. The shutter assembly in this embodiment also includes a motor configured to rotate the shutter to permit passage of the X-rays through the at least one aperture during the first period of rotation, and to thereafter rotate the shutter to obstruct passage of the debris through the at least one aperture during the second period of rotation.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application Ser. No. 60/591,410, filed on Jul. 27, 2004 entitled “Razor Array Shutter for LPP Debris Mitigation” commonly assigned with the present application and incorporated herein by reference in its entirety for all purposes. 
     
    
     TECHNICAL FIELD  
       [0002]     This disclosure generally relates to laser-produced plasma (LPP) devices, and more particularly to devices and methods for obstructing the passage of debris from an LPP device through the use of a rotating debris shutter during a radiation generating event.  
       BACKGROUND  
       [0003]     Laser-produced plasma (LPP) devices are an attractive source of X-rays or short-wavelength radiation due to their relative small size, high brightness and high spatial stability. Two established applications for LPP are microscopy and lithography. However, conventional LPP devices utilize solid targets that produce debris that may easily contaminate, coat, or destroy sensitive X-ray components, such as optics or zone plates, that are positioned close to the plasma. Unfortunately, increasing the distance or introducing filters in order to protect the components typically reduces the amount of radiation that can be captured or utilized.  
         [0004]     For convenience, solid targets have often been used for LPP soft X-ray sources. Examples of solid target LPP systems are described in U.S. Pat. Nos. 5,539,764; 6,016,324; 6,307,913; and 6,707,101, all of which are hereby incorporated by reference herein in their entirely for all purposes. In general, targets formed from materials having low molecular weights yield emission spectra that are very narrow, while targets formed from materials having high molecular weights yield emission spectra having continuum radiation due to Brehmsstrahlung emission. Thus, low molecular weight targets are desirable for LPP applications. Unfortunately, with low molecular weight targets, significant amounts of debris, e.g., hot ions and larger particles, are created. In addition, such debris often follows the generated X-rays out of the laser ablation chamber of the LPP device, which can contaminate or damage components outside of the chamber as well.  
         [0005]     Several methods have been developed to reduce the effect of debris, such as using a small back pressure of helium or other gas, or strategically locating a relay mirror for the protection of sensitive components. Additionally, thin film tape targets, which are becoming more commonplace, help reduce the amount of debris by avoiding shock wave ejection or delayed evaporation. Unfortunately, significant amounts of debris particles are produced, presumably from cooler zones illuminated by the noncentral parts of the laser beam. Gas-phase targets have been another low-debris alternative; however, such low density results in low X-ray intensity.  
         [0006]     Other approaches to debris reduction have included waterjet devices and liquid droplets used for the laser target. However, even these approaches still result in the creation of some amount of debris when the X-rays are generated, and thus the potential for debris escaping the vacuum chamber and potentially contaminating and/or damaging outside components still exists. Accordingly, since the production of debris within a typical LPP device is difficult to eliminate, devices and methods are needed to prevent the LPP-generated debris from exiting the vacuum chamber and damaging delicate components of the LPP device.  
       SUMMARY  
       [0007]     Disclosed are devices and methods for mitigating the exit of debris from a laser-produced plasma (LPP) device during a laser ablation process. In one embodiment, an LPP device comprises a laser source for generating a laser used for irradiation of a target, and a radiation source (sometimes called a “point source”) that generates short-wavelength radiation (e.g., X-rays) and debris by irradiating the target with the laser so as to generate a plasma. In these embodiments, the LPP device also includes a shutter assembly for mitigating the damaging effects of the ablated debris, where the shutter assembly includes a rotatable shutter having at least one aperture that provides a line-of-sight between the radiation source and an exit of the device during a first period of rotation of the shutter, and obstructs the line-of-sight between the radiation source and the exit during a second period of rotation. The shutter assembly in this embodiment also includes a motor configured to rotate the shutter to permit passage of the X-rays through the at least one aperture and to the exit during the first period of rotation, and to thereafter rotate the shutter to obstruct the passage of the debris through the at least one aperture during the second period of rotation.  
         [0008]     In another embodiment, a method for mitigating debris in an LPP device comprises providing a rotatable shutter having at least one aperture, and rotating the shutter a first period of its rotation to provide a line-of-sight between a radiation source and an exit of the device through the at least one aperture. In this embodiment, the method further includes rotating the shutter a second period of its rotation to provide no line-of-sight between the radiation source and the exit through the at least one aperture. Additionally, in such a method, rotating the shutter during the first period of rotation permits passage of radiation generated at the radiation source through the at least one aperture and to the exit, and thereafter rotating the shutter during the second period of rotation obstructs the passage of debris generated at the radiation source through the at least one aperture and to the exit. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates one embodiment of a shutter assembly constructed according to the disclosed principles for use with a laser-produced plasma (LPP) device;  
         [0010]      FIG. 2  illustrates a plan view of a vacuum chamber of an LPP device, which provides an environment for implementing a rotational mechanical shutter as disclosed herein; and  
         [0011]      FIGS. 3A &amp; 3B  illustrate another embodiment of a shutter assembly according to the disclosed principles and having a shutter with radially extending apertures. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Referring initially to  FIG. 1 , illustrated is one embodiment of a shutter assembly  100  constructed according to the disclosed principles for use with a laser-produced plasma (LPP) device. The assembly  100  is included in the vacuum chamber of the LPP device (see  FIG. 2 ) to reduce or eliminate debris produced by the radiation generating process from exiting the vacuum chamber with the generated X-rays.  
         [0013]     The assembly  100  includes a shutter  105  mounted on a rotating shaft  110 , which is connected to a motor  115 . As discussed in greater detail below, the motor  115  and shaft  110  are used to rotate the shutter  105  (as indicated by arrow A 1 ) at a precise angular velocity selected to allow X-rays or other short-wavelength radiation to pass through the shutter  105 , but block most or all of the unwanted debris from passing through the shutter  105 . The shutter  105  includes a base  120  and a plurality of openings or apertures  125  created between multiple vanes or blades. The blades are held together and precisely spaced using spacers  130  placed between the blades and held together using fasteners, such as bolts  135 . In some embodiments, the vanes may be constructed of metal for durability; however, in other embodiments the vanes may be constructed of plastic or other material. Moreover, construction techniques for the vanes/shutter can include cutting the component from a solid material, such as with an EDM device; however, even manufacturing technique may be employed.  
         [0014]     In accordance with the disclosed principles, the shutter  105  reduces or eliminates the amount of debris created through the laser irradiation process used to generate X-rays in the LPP device by timing the alignment of the apertures  125  with the X-rays to be collected at the output of the vacuum chamber. Specifically, when a laser is used to irradiate a target, such as copper tape, X-rays are generated from the plasma created by the irradiation of the target. In addition, debris from the irradiated target in the form of projectile or ballistic particles is also generated at the radiation source. However, since the X-rays travel much faster than the ballistic debris, the rotation of the shutter  105  is timed so that the X-rays will pass through the apertures  125  in the shutter  105  at the desired time. But the rotation of the shutter  105  is also timed so that after the X-rays pass through the apertures  125 , the shutter  105  turns to obstruct the ballistic debris traveling in the same or similar direction as the collected X-rays. Thus, this debris impacts the blades of the shutter  105  and most if not all of it cannot reach the output of the chamber where the X-rays were collected.  
         [0015]     In the illustrated embodiment, to time the rotation of the shutter  105  with the laser ablation process, a synchronization device having a blade  140  is formed on the rotating base  120 . In this approach, a photodetector  145  receives a light or laser beam  150 , and the blade  140  interrupts the beam  150  at a given point during the rotation of the shutter  105 . Thus, the firing of the laser generator that irradiates the target material to create the X-rays (and the resulting debris) can be timed by when the blade  140  interrupts the beam  150 . Of course, this timing adjustment also takes into account the speed of the X-rays and the debris, as well as the rotational velocity of the rotating shutter  105  and its distance from the radiation source. A more detailed example having such parameters is discussed with reference to  FIG. 2 .  
         [0016]      FIG. 2  illustrates a plan view of a vacuum chamber  200  of an LPP device, which provides an environment for implementing a rotational mechanical shutter as disclosed herein. The embodiment in  FIG. 2  also illustrates a different embodiment of a rotational shutter  205  constructed in accordance with the disclosed principles. In this embodiment, the shutter  205  has a square shape and still includes apertures  210  created by the spacing of a plurality of blades. As before, the shutter  205  is configured to rotate (shown by arrow A 1 ) on a shaft in order to align the apertures  210  with the output  215  of the vacuum chamber  200  at only desired times.  
         [0017]     In operation, as mentioned above, a laser  220  is generated from a source external to the vacuum chamber  200 . That laser  220  is then directed and focused to precisely impact a target (not illustrated) to generate a radiation source  225 . The radiation source  225  created by the irradiation process forms a plasma that generates X-rays  230 , as well as debris  235 . The debris  235  typically consists of particles of the target that have been ablated during the irradiation process. As illustrated, the rotation of the shutter  210  is timed with the irradiation process such that the desired X-rays  230  are allowed to pass through the apertures  210  of the shutter  205  to the output  215  of the chamber  200  to be collected and harnessed as needed.  
         [0018]     Since the generated X-rays  230  travel much faster than the generated debris  235 , the rotation of the shutter  205  is also timed so that the apertures  210  no longer provide a line-of-sight between the radiation source  225  and the output  215  of the vacuum chamber  200  by the time that the debris  235  reaches the shutter. As a result, debris  235  with the same or similar trajectory as the X-rays  230  will not be permitted to pass through the shutter  205  to the output  215 . Thus, most or all of this debris  235  will be prevented from exiting the chamber  200  at the point where the X-rays  230  are collected. Moreover, as the shutter  205  continues its rotation to allow the next batch of generated X-rays to pass through the apertures, the vanes or blades of the shutter  205  further work to brush or knock away debris  235  that may be lingering near the shutter  205  after its prior rotation obstructed the line-of-sight to the output  215 .  
         [0019]     By obstructing the path of the debris  235 , equipment and other components of the LPP device proximate to the output  215  of the chamber  200  will receive much less contamination by the debris  235 . Accordingly, cleaning or replacement of these components is reduced or even eliminated. Maintenance time and costs on the LPP device may consequently be reduced by obstructing debris with a shutter constructed and operated according to the disclosed principles.  
         [0020]     In the embodiment illustrated in  FIG. 2 , the shutter  205  is again constructed in the shape of a square and, in this example, has dimensions of approximately 2 cm on all sides. Of course, other sizes and shapes for a shutter constructed as disclosed herein may be utilized. Moreover, in the illustrated embodiment of  FIG. 2 , the shutter  205  is located 200 cm from the radiation source  225 . By taking into account these dimensions, as well as other parameters of the X-ray generation process, the precise rotation of the shutter  205  needed to perform as disclosed herein can be easily calculated. For example, if a source laser  220  and target are selected such that X-rays  230  of about 5 mrad are generated, the X-rays  230  are transmitted at the speed of light (3.0×10 10  cm/sec). In addition, if the debris  235  from this particular ablation process is determined to be traveling at approximately 10 5  cm/sec or slower, then the rotation of this embodiment of the shutter  205  (e.g., having apertures  210  that are approximately 1.2 mm wide and lengths of 2 cm in this example) may be calculated to be about 0.06 radians in 2 ms. Through conversion, in order to travel 0.06 radians in 2 ms, it is determined that the shutter  205  should be rotated at about 5 Hz, or 300 rpm. With these parameters, the shutter  205  should be capable of blocking about 90% of debris  235  traveling at this velocity at the outer edges of the shutter  205 , while blocking about 100% of such debris  235  at the center of the shutter  205 . In other examples, where faster debris is present, the rotational speed of the shutter  205  can be adjusted. For example, for debris traveling at a velocity of 10 6  cm/sec, it is determined that the rotation of the shutter  205  should be about 3000 rpm, rather than the previous 300 rpm. Of course, as the rotation of the shutter  205  is adjusted, so too can the timing of the firing of the source laser beam  220  used in the ablation process be adjusted to work in tandem with the debris shutter  205 .  
         [0021]     Turning finally to  FIGS. 3A and 3B  concurrently, illustrated is another embodiment of a shutter assembly  300  constructed according to the disclosed principles. This embodiment provides a “wheel” type shutter  305  that differs from the embodiments discussed above with reference to  FIGS. 1 and 2 . As before, the shutter  305  is again mounted on a rotating shaft  310  at the center of the shutter  305 . In this embodiment, the shaft  310  is coupled to an encoded motor  315  that is configured to rotate the shutter  305  at the precise, needed velocity. More specifically, rather than including the blade and photodetector used in the embodiment of  FIG. 2  for synchronizing the laser ablation process with the rotation of the shutter, the encoded motor  315  may be employed to provide such alignment and timing.  
         [0022]     Once aligned, a laser  320  is fired at a target to generate a radiation source, as discussed above. The generated X-rays  325  travel from the radiation source to the exit or output  335  of the chamber. As seen from the figures, the shutter  305  is rotated so that the X-rays  325  are permitted to travel through rotating openings or apertures  330  in the shutter  305  to the output  335  of the chamber. This embodiment of the shutter  305  includes apertures  330  that are radially arranged within the shutter  305 , and extend through its center. With this arrangement, as the shutter  305  rotates about its center, the apertures  330  provide a line-of-sight between the radiation source and the output  335  of the vacuum chamber. Moreover, although only three apertures  330  are illustrated extending through the diameter of the shutter  305 , it should be noted that any number of apertures  330  may be included in the shutter  305 . The rotation of the shutter  305 , and consequently the alignment of the apertures  330  with the path of the X-rays and blocking of debris, may then be adjusted to account for the change in the number apertures  330  provided in the shutter  305 .  
         [0023]     It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than the foregoing description, and all changes that come within the meaning and ranges of equivalents thereof are intended to be embraced therein.  
         [0024]     Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. § 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in the claims found herein. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty claimed in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims associated with this disclosure, and the claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of the claims shall be considered on their own merits in light of the specification, but should not be constrained by the headings set forth herein.