Patent Number: 
Section: description

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, xe2x80x9chigh irradiancexe2x80x9d means irradiance that exceeds the irradiance damage threshold of the tool portion, as described below. With reference to FIG. 2, processing tool 50 comprises, in order along axis A2, 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. 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 xe2x80x9cspills overxe2x80x9d 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, xe2x80x9cradiation 82xe2x80x9d) 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. 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. 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) 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 A2 beyond outer edge 60 of workpiece support member 54. 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 225xc3x97225 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. 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. 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""s irradiance damage threshold, IDS (Joules/cm3). The irradiance (energy density) absorbed in shield 130 is given by: IABS(Joules/cm3)=IR a (1xe2x88x92exp(xe2x88x92at)) (Joules/cm3) less than IDS(Joules/cm3).xe2x80x83xe2x80x83(1)  where a is the absorption coefficient of the shield in units of (cmxe2x88x921) (also known as the inverse of the absorption length), t is the thickness of the absorption shield, and IR is the irradiance (Joules/cm2) of radiation 140 incident the shield at upper surface 134. 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 xcex less than 400 nm applications), and NG series glass (for 400 nm less than xcex less than 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: IR(exp(xe2x88x92at)).xe2x80x83xe2x80x83(2)  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 IABS at upper surface 134 is approximately: IABS(Joules/cm3)=IR a (Joules/cm3).xe2x80x83xe2x80x83(3)  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 IDT of tool portion 66. The irradiance of radiation 146 is given by: IT(Joules/cm2)=IR(Joules/cm2) (exp(xe2x88x92at)) less than IDT(Joules/cm2)xe2x80x83xe2x80x83(4)  In general, a minimum value for (at) is 1, and more practical values range from 2 to 5. Note that the units for surface damage thresholds are given in J/cm2, whereas the units for volumetric damage thresholds are in J/cm3. 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), "THgr". The scattering coefficient, "THgr", is defined such that the energy is scattered into a volume determined by a cone with an angle "THgr", as shown. For a purely diffuse scattering shield, "THgr"=4 n, and the intensity drops off as approximately 1/("THgr"d2) (or 1/(4nd2)). 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.). Scattered light 210 exits lower surface 206 and is incident tool portion 66 located a distance d, away from scattering shield 200. To serve its purpose, scattering shield 200 needs to have a scattering coefficient, "THgr", sufficiently large so as to keep the irradiance of scattered light 210 incident tool portion 66 tool below the tool portion irradiance damage threshold, IDT. The irradiance IET of radiation 210 exiting lower surface 206 and incident tool portion 66 is approximately given by: IT(Joules/cm2)=IR A/("THgr"d2) (Joules/cm2)xe2x80x83xe2x80x83(5)  wherein A is the area illuminated by incident radiation 140. It should be noted that the above equation is an approximation. Irradiance, IT, can be considered an amount of energy emanating from scattering shield 200 as a xe2x80x9cvirtualxe2x80x9dlight 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 r2. Thus, the energy density on the surface of the sphere is given by: ItA/(4nr2).xe2x80x83xe2x80x83(6)  If the source emits into a hemisphere, (i.e., "THgr"=2n), the energy density goes up by a factor of two, 2x. Thus, equation (5) above scales adequately until the limit of "THgr" approaches zero. 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) with 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), "THgr", to form scattered radiation 320, in a manner similar to that described above in connection with the second embodiment of the present invention. 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""s irradiance damage threshold, IDS (Joules/cm3). As set forth above, the irradiance (energy density), IABS, absorbed in volume 302 of shield 300 is given by: IABS(joules/cm3)=IR a (1xe2x88x92exp(xe2x88x92at)) (joules/cm3)xe2x80x83xe2x80x83(7)  where a is the absorption coefficient of the shield, t is the thickness of the shield, and IR 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 IDT. In other words, the combined irradiance of scattered radiation 320 and attenuated radiation 324 needs to be below the tool portion irradiance damage threshold IDT. The combined irradiance of radiation 320 and 324 exiting lower surface 306 and incident tool portion 66 is given by: IT(joules/cm2)=IR (exp(xe2x88x92at)) A/(d2"THgr") (joules/cm2)xe2x80x83xe2x80x83(8)  where d, A and "THgr" 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). 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 practice by empirically determining the best arrangement (position) and appropriate of the shield thickness for each individual application.