US20090015830A1 - Methods and devices for measuring a concentrated light beam - Google Patents

Abstract

Methods and devices are provided for profiling a beam of light that includes a wavelength λ. The beam of light is received. Secondary light is generated at a wavelength λ′ different from wavelength λ by fluorescing a material with the received beam of light. The secondary light is separated from the received beam of light. The separated secondary light is optically directed to a sensor.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS
• This application is a continuation of, and claims the benefit of the filing date of, U.S. Nonprovisional Pat. Appl. No. 11/261,439, entitled “METHODS AND DEVICES FOR MEASURING A CONCENTRATED LIGHT BEAM,” filed Oct. 28, 2005, now allowed, which is a nonprovisional of, and claims the benefit of the filing date of, U.S. Prov. Pat. Appl. No. 60/623,720, entitled “METHODS AND DEVICES FOR MEASURING A CONCENTRATED LIGHT BEAM,” filed Oct. 28, 2004 by Timothy N. Thomas, the entire disclosure of which, including the Appendix, is incorporated herein by reference for all purposes. The Appendix to U.S. Prov. Pat. Appl. No. 60/623,720 corresponds to published PCT application WO 03/089,184 and is sometimes referred to herein as “the Thermal Flux Processing application.”
BACKGROUND OF THE INVENTION
• Concentrated light beams, such as are provided by certain lasers, are used in a variety of different applications. One characteristic of such beams that makes them valuable in these varied applications is their ability to deliver a highly concentrated beam of optical power as a collimated beam that provides precision in position, size, and distribution at high intensity levels. The quality of this performance may, however, be impaired by degradation of the quality of the light beam, such as may result from aging of components, vibration and shock, deterioration of a lasing medium, thermal drift, poor optical alignment, and various other sources of component nonlinearity. A change in the intensity profile of the light beam, even if there is no change in the total power output of the beam, may have significant adverse consequences on performance.
• Because of these concerns, it is useful for the light beam to be profiled periodically so that the intensity profile may be evaluated. A challenge in performing such profiling is the intensity of the beam itself since the very high power transfer may damage the profiling device. In particular, many conventional beam-profiling systems face difficulties when beam power density approach values on the order of thousands of watts per square centimeter.
• There is accordingly a general need in the art for methods and devices that permit profiling of concentrated light beams.
BRIEF SUMMARY OF THE INVENTION
• Embodiments of the invention make use of a fluorescent material that radiates light at a different wavelength than the wavelength of the light beam to be profiled, using this radiated light to evaluate the light beam. In some embodiments, a method is thus provided for profiling a beam of light that includes a wavelength λ. The beam of light is received. Secondary light is generated at a wavelength λ′ different from wavelength λ by fluorescing a material with the received beam of light. The secondary light is separated from the received beam of light. The separated secondary light is optically directed to a sensor.
• In some embodiments, the fluorescent material is disposed within a portion of the received beam of light, and the beam of light is moved relative to the fluorescent material. For example, the beam of light may be incident on a cylinder having an axis substantially orthogonal to an incident direction of the beam of light so that the beam of light is moved relative to the fluorescent material by rotating the cylinder about the axis. In another instance, the beam of light is incident on a disk having an axis substantially parallel to an incident direction of the beam of light, with the beam of light being moved relative to the fluorescent material by rotating the disk about the axis; the sensor may also be rotated about the axis of the disk. The directed separated secondary light may be focused onto the sensor. In addition, the directed separated secondary light may be filtered to block light at wavelength λ. In one embodiment, λ is approximately 808 nm and λ′ is approximately 1064 nm. The beam of light may be substantially monochromatic in an embodiment.
• In other embodiments, a device is provided for profiling a beam of light that includes a wavelength λ. The device comprises a body, a fluorescent material, a light sensor, and an optical arrangement. The fluorescent material is disposed proximate a surface of the body oriented to receive the beam of light. The fluorescent material radiates at a wavelength λ′ different from wavelength λ in response to excitation by the beam of light, and the body is substantially transparent to wavelengths λ and λ′. The optical arrangement is adapted to separate the light at wavelength λ′ from the beam of light and to direct the light at wavelength λ′ to the light sensor.
• The fluorescent material may be disposed on the surface of the body, such as in one embodiment where it is comprised by a film deposited over the surface of the body, or it may be disposed within the body under the surface of the body. The optical arrangement may include a surface within the body that substantially transmits light having a wavelength of one of λ and λ′ and substantially reflects light having a wavelength of the other of λ and λ′. In one embodiment, the optical arrangement includes a lens disposed to focus the light directed to the light sensor onto the light sensor. The optical arrangement may further include a filter having transmission characteristics that block transmission of light having wavelength λ disposed to filter the light focused onto the light sensor. In different embodiments, the light sensor may comprise a photodetector or may comprise a camera. In one embodiment, λ<1000 nm and the fluorescent material comprises Nd:YAG.
• Different structures for the body may be accommodated. In one embodiment, the body comprises a hollow cylinder having an axis substantially orthogonal to an incident direction of the beam of light. The optical arrangement includes a surface within a hollow portion of the hollow cylinder that substantially transmits light having a wavelength of one of λ and λ′ and substantially reflects light having a wavelength of the other of λ and λ′. A motor coupled with the body may rotate the hollow cylinder about the axis. In another embodiment, the body comprises a disk having an axis substantially parallel to an incident direction of the beam of light. A motor coupled with the body may rotate the disk and the optical arrangement about the axis.
• A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a flow diagram summarizing methods of the invention in certain embodiments;
• FIG. 2 is a schematic illustration of a device for measuring a concentrated light beam in one embodiment;
• FIG. 3 is a schematic illustration of a device for measuring a concentrated light beam in another embodiment;
• FIGS. 4A-4C are schematic side, end, and isomorphic-projection views of a device for measuring a concentrated light beam in a further embodiment;
• FIGS. 5A and 5B are schematic side and isomorphic-projection views of a device for measuring a concentrated light beam in still another embodiment; and
• FIG. 6 is a top view of a device for measuring a concentrated light beam in an additional embodiment.
DETAILED DESCRIPTION OF THE INVENTION
• Embodiments of the invention make use of a material that fluoresces in response to the energy flow provided by the concentrated beam of light. The invention was developed by the inventors during the course of their work on a thermal processing system like the one described in detail in the Thermal Flux Processing application, but the invention is not limited to such applications and may be used more generally in profiling concentrated light beams that are used in other applications as well. In the thermal processing system described in the Thermal Flux Processing application, the concentrated light beam is provided by a continuous-wave radiation source, and is collimated and focused by an optical arrangement into a line of radiation extending across a substrate surface as part of a process for semiconductor-device manufacture. Heat generated at the surface of the target by the concentrated light raises the temperature to high values useful for annealing in a time frame short enough to prevent diffusion. While embodiments of the invention are suitable for use with continuous-wave light sources like the one described in the Thermal Flux Processing application, other embodiments may be used to profile concentrated light provided as bursts, pulses, or flashes. Furthermore, while the optical arrangement described in the Thermal Flux Processing application is used to focus the light into a line, different embodiments of the invention may be used to profile other geometric configurations of concentrated light.
• In many embodiments, the concentrated beam of light is substantially monochromatic. For a specific application described in the Thermal Flux Processing application in which a silicon substrate is used, the concentrated light has a wavelength between about 190 nm and 950 nm, with a specific example of light having a wavelength of 808 nm being described. In some of the discussion below, this example is also discussed for purposes of illustration, but the invention is not limited to any particular wavelength for the concentrated beam of light. Furthermore, the invention is not limited to profiling of monochromatic beams of light and embodiments may be applied to other beams that have stable spectra.
• The fluorescent light generated by the interaction of the concentrated beam of light with the fluorescent material has generally the same intensity profile as the concentrated beam of light, but at a significantly reduced overall intensity and generally at a different wavelength. For example, in some embodiments, the fluorescent material comprises neodymium:(yttrium aluminum garnet) (“Nd:YAG”), which responds to the 808-nm light beam by fluorescing at a wavelength of 1064 nm. An optical arrangement is used to separate the resulting combination of 808-nm and 1064-nm light, directing the high-intensity 808-nm light so that it is lost and directing the low-intensity 1064-nm light to a sensor. The low-intensity 1064-nm light is profiled with the results obtained from the detector and used as an indicator of the profile of the high-intensity 808-nm light. Because the profiling is performed with light of a lower intensity there is insignificant risk of heating the measuring sensor to the point that it melts, evaporates, or is otherwise damaged by the intensity of the light.
• FIG. 1 provides a generalized overview of different embodiments of the invention. Atblock104, a concentrated, a light beam that includes a wavelength λ is directed towards a profiling device. The light beam is used atblock108 to fluoresce secondary light from a fluorescent source at a different wavelength λ′. The combined light at wavelengths λ and λ′ is directed towards a spectral separator atblock112 to separate the light into its individual wavelength components. The optical structure of the profiling device causes the light at wavelength λ to be lost, as indicated atblock116, and causes the light at wavelength λ′ to be directed to the sensor for measurement.
• One specific embodiment for the profiling device is illustrated schematically inFIG. 2. The profiling device comprises ablock204 of material, such as glass, that is generally transparent to light at wavelengths λ and λ′. Theblock204 may comprise anintermediate surface208 that is substantially reflective at one of the two wavelengths and substantially transmissive at the other of the two wavelengths. For instance, theoptical block204 may comprise halves of a rectangular prism that have been joined along their respective hypotenuses after one of the surfaces has been coated with an optical coating having the desired transmission/reflection characteristics. In the specific illustration, the optical coating is transmissive at wavelength λ, which is the wavelength of thelight beam228 incident on the profiling device, and is reflective at wavelength λ′, which is the wavelength of the secondary light generated by thefluorescent source224. In the embodiment ofFIG. 2, thefluorescent source224 is provided as a dot-like structure, although a variety of different geometric patterns may be used in alternative embodiments, including a line structure, a plurality of separated dot-like structures, a combination of line and dot-like structures, or other geometric patterns. Thefluorescent source224 is positioned proximate a surface of thebody204 such that theincident light beam228 encounters thefluorescent source224 to generate the secondary light. Thefluorescent source224 may be disposed on the surface of thebody204 or may be implanted below the surface of the body.
• Thesecondary light236 is focused onto asensor220 by another part of the optical arrangement. The specific embodiment shown inFIG. 2 uses alens216 to focus the light, but any arrangement of lenses and/or reflective surfaces such as mirrors may be used to accomplish the focusing. In some embodiments, an optical component designed to increase collection of incoming rays, such as a Winston cone, may be used. The optical arrangement may also conveniently comprise afilter212 that is transmissive at wavelength λ′ but opaque at wavelength λ along the path of thesecondary light236 to ensure that no stray light from theinitial beam228 is directed onto thesensor220. While thefilter212 is shown disposed to encounter thesecondary light236 prior to an encounter with thelens216, the order of encounters may be reversed, with thesecondary light236 encountering the lens before it encounters thefilter212. Thesensor220 may comprise a photodetector of the type known in the art. By moving the beam or the optical arrangement, such as by moving theprism block204, thelens216, thefilter212, and thesensor220 in concert, the output of thesensor220 is representative of the light intensity profile of theincident beam228.
• An alternative embodiment is illustrated inFIG. 3, and also uses ablock304 of material that is substantially to light at wavelengths λ and λ′, and includes anintermediate surface308 substantially reflective to one of the wavelengths and substantially transmissive to the other of the wavelengths. Instead of providing discrete sources or another geometric pattern of fluorescent material, theblock304 is coated with afilm320 of fluorescent material or is implanted with the fluorescent material. The profiling device otherwise functions similarly to the description provided in connection withFIG. 2, with an incident beam oflight324 interacting with thefilm320 of fluorescent material to generatesecondary light332. The original light at wavelength λ and the secondary light at wavelength λ′ are directed in different directions by interacting with theintermediate surface308. As before, it does not matter which wavelength is reflected and which wavelength is transmitted, althoughFIG. 3 illustrates the specific case where λ′ reflected and λ is transmitted. The secondary light is directed to a sensor for imaging.
• FIG. 3 also illustrates a further variant in the form of the sensor, which may be provided as acamera316 capable of detecting light imaged over the surface of theblock304. In this way, thefull incident beam324 may be profiled without moving thebeam324 or optical arrangement. Afilter312 that transmits light at wavelength λ′ but that is substantially opaque at wavelength λ may conveniently be positioned along the path of the secondary light to prevent stray light at wavelength λ from reaching thecamera316. Thecamera316 may be a charge-coupled device or other type of camera in different embodiments.
• Alternative arrangements for accomplishing the motion used to profile the entire incident beam are illustrated inFIGS. 4A-5B, withFIGS. 4A-4C illustrating one embodiment andFIGS. 5A-5B illustrating an alternative embodiment. In each of these embodiments, the same general structure is illustrated for the portion of the optical arrangement that focuses the secondary light onto the detector. The structure is shown as comprising a filter that transmits light of wavelength λ′ but not light of wavelength λ, and a focusing lens, but may include additional or alternative optical components such as a Winston cone to increase collection of light rays.
• The embodiment illustrated inFIGS. 4A-4C is shown with a side view inFIG. 4A, an end view inFIG. 4B, and an isometric-projection view inFIG. 4C. This embodiment uses acylinder404 transparent at wavelengths λ and λ′, with at least a portion of the optical arrangement disposed within thetransparent cylinder404. Positioning of the portion of the optical arrangement disposed within thecylinder404 is simplified when thecylinder404 is provided as a hollow cylinder so that optical components may be placed within the hollow portion. Thecylinder404 is coupled with a motor (not shown) configured to rotate the cylinder about anaxis440. Such rotation permitsfluorescent material408 disposed in a geometric pattern on a surface of thecylinder404 to be moved into different positions, with the rotational position being synchronized with the location of the fluorescent material. A beam oflight428 at wavelength λ incident on thecylinder404 causes the fluorescent material to emit secondary light436 at wavelength λ′. The combined light is separated with anoptical component424 that directs the secondary light to be directed towards thesensor420, being focused with alens416 and perhaps additionally filtered by afilter412 as described above in connection with other embodiments. The separation of the combined light may be accomplished by using a structure similar to that described above. In particular,optical component424 may comprise a block of material that is transparent at wavelength λ, covered by a coating that reflects light at wavelength λ′ and transmits light at wavelength λ. In alternative embodiments, the reflective and transmissive properties may be reversed so that the secondary light at wavelength λ′ is transmitted and then directed to a portion of the optical arrangement that focuses and senses the light.
• A further variant is illustrated inFIGS. 5A and 5B, which respectively provide a side and isomorphic-projection view of an embodiment that uses arotating disk504. Thedisk504 is formed of a material that is transparent to light at wavelengths λ and λ′ and includesfluorescent material508 disposed on a surface of thedisk504 in a geometric pattern like those described above. The optical arrangement is similar to that described in connection withFIGS. 4A-4C, with anoptical component512 being provided to reflect light of one of the wavelengths λ and λ′ and to transmit light of the other wavelength. A beam oflight528 incident on thedisk504 causes thefluorescent material508 to generate secondary light536 that is thereby focused onto asensor524 with alens520, perhaps after being filtered by afilter516 to prevent stray light from reaching thesensor524. Thedisk404 and optical arrangement are coupled with a motor (not shown) that causes rotation aboutaxis540 so that different portions of theincident beam528 may successively be imaged by thesensor524.
• A further alternative is illustrated withFIG. 6, which provides a top view of another embodiment that uses arotating disk604. A plurality ofdots612 of fluorescent material are distributed about the disk in a spiral pattern so that as thedisk604 rotates aboutaxis616, different ones of thedots612 are exposed to thebeam608 at different radial distances from theaxis616. In some embodiments, thedisk604 is rotated rapidly so that each of thedots612 is exposed repeatedly to thebeam608 but for a relatively short period of time. In such instances, a single detector may be used to collect the secondary light, and cross talk between pixels may be minimized. An additional advantage that results from rapidly spinning the disk is that any heat adsorbed from the beam is distributed over a relatively large volume, which is related to the duty cycle of the dots, thereby increasing the maximum usable power density. For purposes of illustration, the inventors have calculated that a disk having a diameter of 12 cm may yield about a 4-μm resolution in the slow axis and dot-size limited in the fast axis.
• The specific embodiments described above are intended to be illustrative of different aspects of the invention, and there are a number of alternatives that may be used, particularly in separating the combined λ and λ′ light and in directing the secondary light to the sensor. For example, the separation of the light has been described in each of the embodiments with use of a coating that transmits light at one of the wavelengths and reflects light at the other of the wavelengths. Such a structure has the advantage that substantially the entire strength of the secondary light is retained and directed to the sensor, particularly when the optical arrangement also comprises a component designed to improve light collection such as a Winston cone. In other embodiments, however, other techniques for separating substantially dichromatic light may be used, even if such techniques result in some loss in intensity of the secondary light. For example, an arrangement in which the dichromatic light is initially focused and directed to a splitter, with one output of the splitter being further directed to a filter that passes only light of wavelength λ′ could be used to collect the secondary light. This and other similar arrangements may be combined with the structures otherwise described in connection withFIGS. 2-6.
• Furthermore, in other alternative embodiments, the use of a fluorescent material may be avoided by providing scattering features that act to scatter light at the wavelength λ of the incident beam, thereby significantly reducing its intensity so that it may be sampled by the sensor. Such scattering features may be placed on the surface of, or embedded with, thetransparent structures204,304,404, and504 described above. The detection of light may then be performed without including a filter that blocks transmission at the wavelength λ of the incident light.
• Having described several embodiments, it will be recognized by those of skill in the art that further modifications, alternative constructions, and equivalents may be used without departing from the spirit of the invention. Accordingly, the above description should not be taken as limiting the scope of the invention, which is defined in the following claims.

Claims (20)

1. A method for profiling a beam of light that includes a wavelength λ, the method comprising:

rotating a disk about a central axis of the disk, wherein the disk includes a series of discrete fluorescent dots disposed in a spiral pattern around the disk;

directing a beam of light with a wavelength λ toward the rotating disc, such that the beam of light is incident on the disk along a path substantially parallel with the central axis of the disk;

separating secondary light from the received beam of light, the secondary light having been generated by fluorescing the fluorescent dots with the received beam of light and the secondary light having a wavelength λ′ different from the wavelength λ; and

optically directing the separated secondary light toward a sensor.

2. The method recited inclaim 1 further comprising focusing the directed separated secondary light onto the sensor.

3. The method recited inclaim 1 further comprising filtering the directed separated secondary light to block light at wavelength λ.

4. The method recited inclaim 1 wherein λ is approximately 808 nm and λ′ is approximately 1064 nm.

5. The method recited inclaim 1 wherein the beam of light is substantially monochromatic.

6 . A device for profiling a beam of light that includes a wavelength λ, the device comprising:

a prism;

a film of fluorescent material disposed over a surface of the prism oriented to receive the beam of light, wherein:

the fluorescent material radiates light at a wavelength λ′ different from wavelength λ in response to excitation by the beam of light; and

the prism is substantially transparent to wavelengths λ and λ′;

a light sensor; and

an optical arrangement configured to separate the light at wavelength λ′ from the beam of light and to direct the light at wavelength λ′ to the light sensor.

7. The device recited inclaim 6 wherein the optical arrangement includes a surface within the prism that substantially transmits light having a wavelength of one of λ and λ′ and substantially reflects light having a wavelength of the other of λ and λ′.

8. The device recited inclaim 6 wherein the optical arrangement includes a lens disposed to focus the light directed to the light sensor onto the light sensor.

9. The device recited inclaim 8 wherein the optical arrangement further includes a filter having transmission characteristics that block transmission of light having wavelength λ disposed to filter the light focused onto the light sensor.

10. The device recited inclaim 6 wherein the light sensor comprises a photodetector.

11. The device recited inclaim 6 wherein the light sensor comprises a camera.

12. The device recited inclaim 6 wherein wavelength λ is less than 1064 nm and the fluorescent material comprises Nd:YAG.

13. The device recited inclaim 6 wherein wavelength λ is less than 1064 nm and the fluorescent material comprises Nd ions embedded in a glass.

14. The device recited inclaim 6 wherein the beam of light is substantially monochromatic.

15. A device for profiling a beam of light that includes light with a wavelength λ, the device comprising:

a disk rotatable about a central axis of the disk, the disk being oriented to receive the beam of light;

a plurality of fluorescent dots embedded within the rotatable disk in a spiral pattern around the central axis of the disk, wherein the fluorescent dots radiate light at a wavelength λ′ different from wavelength λ in response to excitation by a beam of light of wavelength λ;

a light detector; and

light directing means for separating light with wavelength λ′ from light with wavelength λ and for directing the light with wavelength λ′ toward the light detector.

16. The device for profiling a beam of light according toclaim 15 wherein the light directing means includes an optical element with a coating that transmits light with wavelength λ and reflects light with wavelength λ′.

17. The device for profiling a beam of light according toclaim 15 wherein the light directing means includes an optical element with a coating that transmits light with wavelength λ′ and reflects light with wavelength λ.

18. The device for profiling a beam of light according toclaim 15 wherein the light directing means includes a Winston cone.

19. The device for profiling a beam of light according toclaim 15 wherein the light directing means includes a filter having transmission characteristics that block transmission of light having wavelength λ.

20. The device for profiling a beam of light according toclaim 15 wherein the fluorescent dots comprise Nd:YAG.

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