Patent Publication Number: US-2004041105-A1

Title: Radiation shield device and method

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to radiation shields, and in particular to such shields for protecting a tool from being damaged by high-irradiance radiation when processing a workpiece with the tool.  
       BACKGROUND OF THE INVENTION  
       [0002] In many modern-day manufacturing applications, such as semiconductor manufacturing or materials processing, it is desirable to process an object (“workpiece”), with a beam of high-irradiance radiation to modify the chemical, physical or electrical properties of the workpiece. In semiconductor manufacturing, the substrate is often a silicon wafer, and in materials processing, the substrate can be a metal plate. For most applications, the radiation source is a high-irradiance (units: J/cm 2 ) laser. Efficient manufacturing techniques typically require that a robotically controlled tool deliver this high-irradiance radiation to the workpiece, preferably with little user intervention.  
       [0003] With reference to FIG. 1, prior art processing tool  10  comprises, in order along axis A 1 , a workpiece support member  14  (i.e., a “chuck”) and a tool portion  20 . The latter includes a variety of components such as vacuum lines, electrical cables, mechanical apparatus, metal surfaces, and the like. Workpiece support member  14  supports a workpiece  24  having an upper surface  26 , a lower surface  28  and an outer edge  30 . A source of radiation (not shown) irradiates upper surface  26  of workpiece  24  with high-irradiance radiation  34 .  
       [0004] A problem often encountered in radiation-based processing tools such as processing tool  10  is that a portion  38  of high-irradiance radiation  34  incident workpiece  24  spills over onto tool portion  20 , or even onto workpiece support member  14 . Portion  38  is referred to herein as “spillover radiation.” This happens most often when a region on surface  26  near the edge of workpiece  24  is being processed. Since spillover radiation  38  has sufficiently high irradiance to modify the surface properties of workpiece  24 , it also typically has sufficient irradiance to modify the surface properties of tool portion  20 . In time, spillover radiation  38  can damage tool portion  20 , can cause unwanted heating problems within processing tool  10 , and can result in potentially harmful reflections.  
       [0005] Unfortunately, it is not possible to simply shield the tool from spillover radiation using conventional shields made of metal or plastic, because the high irradiance beam would damage such a shield. For instance, simply extending workpiece support member  14  to capture spillover radiation is not a practical solution because the workpiece support member typically needs to be made of a light, machinable metal such as aluminum, which is susceptible to damage from high-irradiance radiation.  
       [0006] Ideally, it is desirable to intercept spillover radiation  38  with a shield capable of rendering the radiation harmless before it irradiates the tool. If, for example, a metallic shield is placed between workpiece  24  and tool portion  20  to intercept spillover radiation  38 , the metallic shield will, in all likelihood, ablate or be damaged by the radiation. Further, a metallic shield may reflect radiation onto other portions of processing tool  10 . Generally, speaking, any shielding material that relies primarily upon surface absorption of radiation will probably be damaged.  
       [0007] There are several prior art shields designed to intercept radiation and render it harmless. For example, U.S. Pat. No. 5,153,425, entitled “Broadband Optical Limiter with Sacrificial Mirror to Prevent Irradiation of a Sensor System by High Intensity Laser Radiation,” describes a broadband optical limiter for use in combination with a sensor system operative to prevent irradiation of the sensor system by laser radiation of unknown wavelengths having intensity levels sufficient to damage or disable the sensor system. The broadband optical limiter is further operative to throughput, with minimal optical distortion at wide-angle fields of view, electromagnetic radiation in the operating spectral band(s) of the sensor system. The broadband optical limiter includes a flat or spherically shaped sacrificial mirror that is operative to reflect electromagnetic radiation in the operating spectral band(s) of the sensor system and to be optically machined, i.e., vaporized, by focused laser radiation of unknown wavelengths having intensity levels sufficient to damage or disable the sensor system to create a reflective dead spot. The reflective dead spot prevents the focused laser radiation from being throughputted to the sensor system. The broadband optical limiter further includes optical components to focus incident electromagnetic and laser radiation onto the sacrificial mirror, to turn incident electromagnetic and laser radiation out of the field of view of the sensor system, and to turn electromagnetic radiation reflected by the sacrificial mirror back into the field of view of the sensor system. A major disadvantage of this type of shield, however, is that it is sacrificial, and changes due to the irradiation. Such shields tend to need to be replaced frequently.  
       [0008] U.S. Pat. No 4,114,985, entitled “Shield for High Power Infrared Laser Beam,” describes shielding from and the termination of high power infrared laser beams by interception of the beam by one of two spaced, juxtaposed, ceramic (i.e., clay-based) sheet members. The beam-intercepting member has a thickness to beam power density relationship that allows opaque to translucent conversion of the portion thereof illuminated by the beam. The translucent portion subsequently diffuses the beam. The second ceramic sheet member then absorbs the diffused beam. A major shortcoming of this shield, however, is that it requires two clay-based, opaque sheet members, with the first sheet having to be of sufficient strength to cause a transformation from opaque to translucent by virtue of the incident radiation.  
       [0009] U.S. Pat. No. 4,575,610, entitled “Laser Shielding Device,” describes a laser-shielding device having two spaced-apart layers of shielding material defining a sealed chamber between the two layers. At least one layer degrades in the presence of an impinging laser beam, creating a hole through the layer. A pressure change in the chamber is sensed and signaled to a machine controller to stop the lasing operation. Unfortunately, this shield device is not well-suited for protecting a tool from spillover radiation, since the shield is sacrificial and thus would need to be replaced often. Further, the shield is pressurized, which adds to its complexity.  
       SUMMARY OF THE INVENTION  
       [0010] The present invention relates to radiation shields, and in particular such shields for protecting a tool from being damaged by high-irradiance radiation when processing a workpiece with the tool using high-irradiance radiation.  
       [0011] A first aspect of the invention is a shield for protecting a tool portion having an irradiance damage threshold from high-irradiance radiation from a light source when irradiating a workpiece, said shield arranged between the light source and the tool portion and having an irradiance damage threshold, an absorption coefficient, a volume and a thickness, and designed to absorb in said volume, a portion of said high-irradiance radiation that would otherwise be incident the tool portion, wherein the shield maintains said absorbed high-irradiance radiation below said shield irradiance damage threshold, and wherein radiation exiting the shield and incident the tool portion and incident the tool portion has an irradiance below the irradiance damage threshold of the tool portion.  
       [0012] A second aspect of the invention is a shield for protecting a tool portion having an irradiance damage threshold from high-irradiance radiation from a light source when irradiating a workpiece, said shield arranged between the light source and the tool portion and having an irradiance damage threshold, a scattering coefficient, a volume and a thickness, and designed to scatter in said volume a portion of said high-irradiance radiation that would otherwise be incident said tool portion, wherein said shield maintains said scattered high-irradiance radiation below said shield irradiance damage threshold, and wherein radiation exiting the shield and incident the tool portion has an irradiance below the irradiance damage threshold of the tool portion.  
       [0013] A third aspect of the invention is a shield for protecting a tool portion having an irradiance damage threshold from high-irradiance radiation from a light source when irradiating a workpiece, said shield arranged between the light source and the tool portion and having an irradiance damage threshold, an absorption coefficient, a volume, a scattering coefficient and a thickness, wherein said shield is designed to absorb and scatter in said volume a portion of the high-irradiance irradiation, wherein the shield maintains said absorbed high-irradiance radiation below said shield irradiance damage threshold, and wherein radiation exiting the shield and incident the tool portion has an irradiance below the irradiance damage threshold of the tool portion.  
       [0014] A fourth aspect of the invention is an apparatus that prevents a tool portion having an irradiance damage threshold from being irradiated by high-irradiance radiation from a light source while irradiating a workpiece. The apparatus comprises a workpiece support member for supporting a workpiece, with one of the shields described immediately above, arranged between the light source and the tool portion.  
       [0015] A fifth aspect of the invention is a method of processing a workpiece with high irradiance radiation from a light source using a workpiece processing tool having a tool portion with an irradiance damage threshold. The method comprises the steps of first, supporting the workpiece on a workpiece support member, then arranging one of the shields described immediately above between the light source and workpiece so as to intercept any of the high-irradiance radiation that would otherwise be incident the tool portion, and then irradiating the workpiece. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a cross-sectional schematic diagram of a prior art tool for processing a workpiece with radiation, illustrating how spillover radiation irradiates a tool portion;  
     [0017]FIG. 2 is a cross-sectional schematic diagram of a tool for processing a workpiece with radiation, the tool including the radiation shield device of the present invention, and illustrating two preferred positions in the range of positions of the radiation shield device between the light source and the tool portion;  
     [0018]FIG. 3 is a cross-section schematic diagram of a section of the processing tool of FIG. 2, further including a first embodiment of the shield device of the present invention comprising an absorbing shield;  
     [0019]FIG. 4 is a cross-section schematic diagram of a section of the processing tool of FIG. 2, further including a second embodiment of the shield device according to the present invention comprising a scattering shield; and  
     [0020]FIG. 5 is a cross-section schematic diagram of a section of a processing tool of FIG. 2, further including a third embodiment of the shield device according the present invention comprising a shield that is both scattering and absorbing; 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0021] The present invention is an apparatus for, and method of, preventing high-irradiance radiation damage to a tool during processing of a workpiece, which includes positioning a radiation shield device capable of absorbing and/or scattering high irradiance radiation between the workpiece and a tool portion. Here, “high irradiance” means irradiance that exceeds the irradiance damage threshold of the tool portion, as described below.  
     [0022] With reference to FIG. 2, processing tool  50  comprises, in order along axis A 2 , a workpiece support member (i.e., a chuck)  54  comprising a body  56  having an upper surface  58  with an outer edge  60 . Processing tool  50  further includes a tool portion  66 , which may include a variety of components making up tool  50 , such as vacuum lines, electrical cables, mechanical apparatus, metal surfaces, and the like (not shown). Workpiece support member  54  supports or holds on upper surface  58  a workpiece  70  having an upper surface  72 , a lower surface  74 , and an outer edge  76 . Processing tool  50  also includes a light source  78  for providing high-irradiance radiation  80  to upper surface  72  of workpiece  70 .  
     [0023] As discussed above, a problem often encountered in radiation-based processing tools such as processing tool  50  is that a portion  82  of high-irradiance radiation  80  that is preferably incident workpiece  70  instead “spills over” workpiece edge  76  and irradiates tool portion  66 , or possibly workpiece support member  54 . This typically occurs when a field or area (not shown) on upper surface  72  near outer edge  76  of workpiece  70  is being processed. Since high-irradiance radiation  80  has sufficiently high irradiance to modify the surface properties of workpiece  70 , spillover high-irradiance radiation portion  82  (hereinafter, “radiation  82 ”) also has sufficient irradiance to modify the surface properties of tool portion  66 . In time, spillover high-irradiance radiation  82  can damage tool portion  66 , can cause unwanted heating problems within processing tool  50 , and can cause potentially harmful reflections.  
     [0024] Accordingly, with continuing reference to FIG. 2, processing tool  50  further includes a radiation shield device  100  of the present invention arranged (positioned) between light source  78  and tool portion  66  so as to intercept a portion of high-irradiance radiation  80 .  
     [0025] In one preferred embodiment, radiation shield device  100  is arranged between light source  78  and workpiece  70 , so that the high-irradiance radiation in high-irradiance radiation  80  that would otherwise form high-irradiance spill over radiation  82  is attenuated below the irradiation damage threshold of tool portion  66  prior to spilling over workpiece edge  76  (see dashed-line radiation shield device  100  in FIG. 2)  
     [0026] In another preferred embodiment, radiation shield device  100  is arranged adjacent to workpiece support member  56  between lower surface  74  of workpiece  70 , and tool portion  66 , so radiation  82  is attenuated below the irradiance damage threshold of tool portion  66  prior to being incident the tool portion. In this embodiment, it is preferable that workpiece outer edge  76  extends outwardly from axis A 2  beyond outer edge  60  of workpiece support member  54 .  
     [0027] In practice, workpiece support member  54  may be modified to have a lip  106  surrounding workpiece support member body  56 , as shown, with the lip having an upper surface  108  upon which radiation shield device  100  may be supported. Also in practice, radiation shield device  100  may be round or square with a round central aperture sized to fit over body  56  so as to rest on upper surface  108  of lip  106 . By way of example, for a laser thermal processing (LTP) tool capable of processing circular workpieces having a diameter of about 200 mm (e.g., 200 mm silicon wafers), an exemplary workpiece support member  54  has a body  56  that is cylindrical with a diameter of about 175 mm, with lip  106  extending outwardly therefrom by about 20 mm (i.e., about 195 mm across), and a radiation shield device  100  that is square with dimensions of about 225×225 mm, the shield including a central aperture sized to fit over cylindrical body  57  so that the shield device can rest on lip upper surface  108 .  
     [0028] To render radiation  82  harmless, radiation shield device  100  of the present invention needs to be made of a material that either absorbs high-irradiance radiation within a large volume (rather than on the surface), or scatters high-irradiance radiation into a sufficiently wide spatial and angular range, or does both, so that the irradiance of the radiation  110  exiting radiation shield device  100  and incident tool portion  66  is insufficient to damage the tool portion. Described below are several different embodiments of radiation shield device  100  of the present invention.  
     [0029] Absorbing Shield Embodiment  
     [0030] With reference to FIG. 3, in a first embodiment of the present invention, the radiation shield device  100  of FIG. 2 comprises an absorbing shield  130  having a volume  132 , an upper surface  134 , and a lower surface  136 . Shield  130  comprises a material designed to absorb a portion of a high-irradiance beam  140  (which may be a spillover beam such as spillover beam  82 , or a direct beam such as high-irradiance radiation  80 , depending on the position of absorbing shield  130 ; see FIG. 2) within volume  132 . The goal is to dissipate the absorbed energy in volume  132  of absorbing shield  130  (as indicated by arrows  138 ) and to radiate this energy out into space as heat (as indicate by wavy lines  142 ), rather than to absorb the energy from beam  140  at upper surface  134 , as with conventional shields. To properly effectuate volume absorption shielding using absorbing shield  130  such that the shield need not be replaced often (if at all), the absorption coefficient of the shield material needs to be such that the absorbed energy density is maintained below the shield&#39;s irradiance damage threshold, I DS (Joules/cm 3 ).  
     [0031] The irradiance (energy density) absorbed in shield  130  is given by: 
       I   ABS (Joules/cm 3 )= I   R   a (1−exp(− at ))(Joules/cm 3 )&lt;I DS (Joules/cm 3 ).  (1) 
     [0032] where a is the absorption coefficient of the shield in units of (cm −1 ) (also known as the inverse of the absorption length), t is the thickness of the absorption shield, and I R  is the irradiance (Joules/cm 2 ) of radiation  140  incident the shield at upper surface  134 .  
     [0033] A number of commercially available partially transmitting glasses have an absorption length greater than 0.1 mm. Exemplary materials for absorber shield  130  is one of a variety of partially transmitting glasses produced and sold by Schott Glass Technologies, Inc. (Duryea, Pa.), such as one of the F5-type glasses (for λ&lt;400 nm applications), and NG series glass (for 400 nm&lt;λ&lt;1200 nm applications). Other glasses include neutral density and color filter glass. For these glasses, a thickness t of a few millimeters will absorb a sufficient amount of radiation at the appropriate wavelengths. Depending upon the irradiance of the incident radiation, these glasses can be designed (either analytically or empirically) to have a thickness that will distribute the absorbed energy into a sufficiently large volume such that they will not be damaged. The irradiance of radiation  140  will decrease as it progresses through volume  132  according to the equation: 
     I R (exp(− at )).  (2) 
     [0034] From equation (2), it can be seen that the volumetric absorption will be greatest near upper surface  134  (i.e., at the surface of incidence, where t=0), before the incident radiation has an opportunity to be absorbed by the plate. The volumetric absorption I ABS  at upper surface  134  is approximately: 
       I   ABS (Joules/cm 3 )= I   R   a (Joules/cm 3 ).  (3) 
     [0035] Shield  130  also needs to have a sufficient thickness, t, such that residual radiation transmitted through the shield is incapable of damaging tool portion  66 . In other words, the irradiance of radiation  148  exiting shield  130  from lower surface  136  needs to be attenuated such that it is below the irradiance damage threshold I DT  of tool portion  66 .  
     [0036] The irradiance of radiation  146  is given by: 
       I   T (Joules/cm 2 )= I   R (Joules/cm 2 )(exp(− at ))&lt; I   DT (Joules/cm 2 )  (4) 
     [0037] In general, a minimum value for (at) is 1, and more practical values range from 2 to 5.  
     [0038] Note that the units for surface damage thresholds are given in J/cm 2 , whereas the units for volumetric damage thresholds are in J/cm 3 .  
     [0039] Scattering Shield Embodiment  
     [0040] With reference to FIG. 4, in a second embodiment of the present invention, the radiation shield device  100  of FIG. 2 comprises a scattering shield  200  having a volume  202 , an upper surface  204 , and a lower surface  206 . Shield  200  comprises a material designed to scatter a portion of high-irradiance beam  140  (which may be a spillover beam such as spillover beam  82 , or a direct beam such as high-irradiance radiation  80 , depending on the position of shield  200 ) within volume  202  over a scattering angle (scattering coefficient), Θ. The scattering coefficient, Θ, is defined such that the energy is scattered into a volume determined by a cone with an angle Θ, as shown. For a purely diffuse scattering shield, Θ=4n, and the intensity drops off as approximately 1/(Θd 2 ) (or 1/(4nd 2 )). An exemplary material for a scattering shield is opal glass (including single and double flashed opal), available from Corning, Inc (Corning, N.Y.). Other sources of opal glass are DESAG (Germany), S.A. Bendheim (Oakland, Calif.), Hollander Glass (Santon, Calif.) and Edmund Scientific (Barrington, N.J.). In addition, shield  200  can comprise translucent porcelain or a turbid media, such as milk, sea water, or a solution of water mixed with small particles, such as latex or polystyrene spheres, which can be purchased from Interfacial Dynamics (Portland, Oreg.).  
     [0041] Scattered light  210  exits lower surface  206  and is incident tool portion  66  located a distance, d, away from scattering shield  200 .  
     [0042] To serve its purpose, scattering shield  200  needs to have a scattering coefficient, Θ, sufficiently large so as to keep the irradiance of scattered light  210  incident tool portion  66  tool below the tool portion irradiance damage threshold, I DT . The irradiance I ET  of radiation  210  exiting lower surface  206  and incident tool portion  66  is approximately given by: 
       I   T (Joules/cm 2 )= I   R   A /(Θ d   2 )(Joules/cm 2 )  (5) 
     [0043] wherein A is the area illuminated by incident radiation  140 .  
     [0044] It should be noted that the above equation is an approximation. Irradiance, I T , can be considered an amount of energy emanating from scattering shield  200  as a “virtual” light source. If this virtual light source is an isotropic emitter, the emission fills a sphere of radius r, and the surface area of a sphere is 4n r 2 . Thus, the energy density on the surface of the sphere is given by: 
       I   T   A /(4 nr   2 ).  (6) 
     [0045] If the source emits into a hemisphere, (i.e., Θ=2n), the energy density goes up by a factor of two, 2x. Thus, equation (5) above scales adequately until the limit of Θ approaches zero.  
     [0046] Absorbing and Scattering Shield Embodiment  
     [0047] With reference to FIG. 5, in a third embodiment of the present invention, the radiation shield device  100  of FIG. 2 comprises an absorbing and scattering shield  300  having a volume  302 , an upper surface  304 , and a lower surface  306 . Shield  300  comprises a material designed to both absorb and scatter portions of a high-irradiance beam  140  (which may be a spillover beam such as spillover beam  82 , or a direct beam such as high-irradiance radiation  80 , depending on the position of shield  300 ; see FIG. 2) within volume  302 . Thus, a first goal of shield  300  is to dissipate the absorbed energy in volume  302  (as indicated by arrows  310 ) and to radiate this energy out into space as heat (as indicate by wavy lines  314 ), as described above in connection with the first embodiment of the present invention. In addition, a second goal of shield  300  is to scatter radiation that is not absorbed in volume  302  over a scattering angle (i.e., scattering coefficient), Θ, to form scattered radiation  320 , in a manner similar to that described above in connection with the second embodiment of the present invention.  
     [0048] To properly effectuate volume absorption and volume scattering so that shield  300  need not be replaced often (if at all), the absorption coefficient of the shield material needs to be such that the absorbed energy density is maintained below the shield&#39;s irradiance damage threshold, I DS (Joules/cm 3 ).  
     [0049] As set forth above, the irradiance (energy density), I ABS , absorbed in volume  302  of shield  300  is given by: 
       I   ABS (joules/cm 3 )= I   R   a (1−exp(− at ))(joules/cm 3 )  (7) 
     [0050] where a is the absorption coefficient of the shield, t is the thickness of the shield, and I R  is the irradiance of radiation  140 . Shield  300  needs to have a sufficient thickness, t, so that scattered radiation  320  and attenuated radiation  324  due to absorption exiting lower surface  306  of the shield is incapable of damaging tool portion  66 , which has an irradiance damage threshold I DT . In other words, the combined irradiance of scattered radiation  320  and attenuated radiation  324  needs to be below the tool portion irradiance damage threshold I DT .  
     [0051] The combined irradiance of radiation  320  and  324  exiting lower surface  306  and incident tool portion  66  is given by: 
       I   T (joules/cm 2 )= I   R (exp(− at )) A /( d   2 Θ)(joules/cm 2 )  (8) 
     [0052] where d, A and Θ are defined as above, here with respect to shield  300 . Exemplary materials for shield  300  include pot opal and turbid absorbing media, an example of the latter being small particles such as latex or polystyrene spheres suspending in a liquid that absorbs at the wavelength of interest).  
     [0053] While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims. Further, although analytic expressions have been provided for the various embodiments, it will be understood by one skilled in the art that the present invention may be more conveniently practiced by empirically determining the best arrangement (position) and appropriate of the shield thickness for each individual application.