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US6775000_TOT - U5006775000B2(12 United States Patent(10...

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Unformatted text preview: U5006775000B2 (12) United States Patent (10) Patent N0.: US 6,775,000 B2 Harrison et al. (45) Date of Patent: Aug. 10,2004 (54) METHOD AND APPARATUS FOR WAFER- (56) References Cited TESTING OF SEMICONDUCTOR U'S' PATENT DOCUMENTS 5,260,772 A * 11/1993 Pollak et al. ............. .. 356/417 (75) Inventors: James Harrison, Morgan Hill, CA 5,422,498 A * 6/1995 Nikawa et al. ............. .. 257/48 (US); David Leslie Heald, Solvang, CA 5,498,973 A 3/1996 Cavaliere et al. ......... .. 324/765 (Us) 6,057,918 A 5/2000 Geary et al. .............. .. 356/218 6,137,305 A 10/2000 Freund et a1. ............ .. 324/767 - . 6,255,707 B1 7/2001 Bylsma et al. ............ .. 257/414 (73) ASSlgnee‘ Novalux’ Inc" sunnyvale’ CA (Us) 6,264,852 B1 7/2001 Herchen et al. ............ .. 216/60 ( *) Notice: Subject to any disclaimer, the term of this . 6,448,805 B1 9/2002 Heald et al. .............. .. 324/767 patent is extended or adjusted under 35 * Clted by exammer U.S.C. 154(b) by 397 days. Primary Examiner—Hoa Q. Pham (74) Attorney, Agent, or Firm—Fay Kaplun & Marcin, LLP (21) Appl. No.: 10/033,975 (57) ABSTRACT (22) Filed: Dec' 19! 2001 A system and method for manufacturing and wafer-level (65) Pm“. Publication Data testing properties of a wafer comprises a chuck receiving a wafer to be tested and a pump light source directing an US 2003/0164707 A1 56p. 4, 2003 output beam toward selected locations on a wafer received (51) Int Cl 7 G01N 21/00 on the chuck in combination With a laser light detector (57) US' Ci iiiiiiiiiiiiiiiiiiiii 5. 324/767 detecting light emitted from the wafer and a pump beam (5g) Fi'el'd 01', """" " ’ 356/282 440 aiming mechanism selectively varying a position at Which 356/237. 765; 372/43: the pump light source output beam enters the wafer. 45, 50, 46 31 Claims, 3 Drawing Sheets 11:; J 1" 114 t i a { ~ 126 C:: 122 \ ‘1 1 \ 99 —\\ 130 \ 1:— W I _ f H ~ “ _ 1 102 ¥ 100 120 US. Patent Aug. 10, 2004 Sheet 1 0f3 US 6,775,000 B2 Figure 1 US. Patent Aug. 10, 2004 Sheet 2 0f3 US 6,775,000 B2 100 Figure 2 108 106 US. Patent Aug. 10, 2004 Sheet 3 0f3 US 6,775,000 B2 Grow epitaxial layers on wafer 300 in epitaxial reactor . , 302 Map reflectlwty spectrum Scan surface for surface 304 roughness characterization Optically-pumped wafer 305 probing of "as-grown" wafer usin test laser 310 Dispose of wafer 308 ls wafer of N0 sufficeint quality to process? Yes 312 Process wafer electrically probe individual 31 4 devices. mark bad die 316 separate devices from wafer 320 318 separate good from bad die package good die Figure 3 US 6,775,000 B2 1 METHOD AND APPARATUS FOR WAFER- LEVEL TESTING OF SEMICONDUCTOR LASER FIELD OF THE INVENTION The present invention relates to a method and apparatus for testing semiconductor lasers, and more specifically relates to a method and apparatus for wafer-level testing of vertical cavity surface emitting lasers prior to their complete assembly. BACKGROUND OF THE INVENTION Semiconductor lasers in use today can generally be clas- sified as edge-emitting diode lasers and vertical cavity surface emitting lasers (“VCSELs”). In an edge-emitting laser, a semiconductor gain medium, for example a quantum-well semiconductor structure, is formed as a region deposited on a semiconductor substrate, or wafer. Many devices are typically formed from one single wafer, and once an individual device has been detached from the wafer, nirrors are formed or otherwise positioned on opposite edges of the gain medium, perpendicular to the substrate surfaces. The assembly forms a resonant cavity within which he gain medium is located. Electrical or optical pumping of he gain medium generates a laser beam which propagates in a direction along the plane of the substrate. Edge-emitting asers thus generate a beam in a direction along the plane of a substrate forming the laser, exiting the device at an edge where the devices are separated into individual units. It is hus not practical to test these devices prior to separating hem into individual units, thereby exposing the edges from which the beams are output. VCSELis, in contrast, generate output beams in a direc- ion perpendicular to the plane of a substrate on which they are formed. Thus the orientation of individual VCSELs on a wafer substrate, prior to being separated from one another, is potentially suitable for testing before carrying out the manufacturing steps that lead to separation. Conventional testing methods used on VCSELs involve electrically prob- ing the optical aperture side of a wafer, and detecting light emitted from that side while shorting the opposite side of the wafer to ground. Conventional testing methods and devices do not provide a way to screen and map optical character- istics of epitaxial wafers prior to processing the wafer beyond epitaxial growth. SUMMARY OF THE INVENTION The present invention is directed to an apparatus for testing properties of a wafer having disposed thereon layers that form all or part of a lasing cavity, the apparatus comprising a chuck receiving a wafer to be tested and a pump light source directing an output beam toward selected locations on a wafer received on the chuck in combination with a laser light detector detecting light emitted from the wafer and a pump light source aiming mechanism selec- tively varying a position at which the pump light output beam enters the wafer. In another aspect, the invention is directed to a method of wafer-level testing of semiconductor laser devices, compris- ing the steps of positioning a wafer to be tested in a predetermined position relative to a pump light source, optically pumping preselected regions of the wafer with the pump light source, and analyzing light emitted from each of the preselected regions to determine the laser-related char- acteristics of the preselected regions of the wafer. 10 15 20 25 30 35 40 45 50 55 60 65 2 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view showing an embodiment of the wafer-level testing apparatus for semiconductor lasers according to the invention; FIG. 2 is a side view showing a second embodiment of the wafer-level testing apparatus for semiconductor lasers according to the invention; and FIG. 3 is a flow chart showing exemplary manufacturing steps for forming semiconductor lasers devices on a wafer, including testing according to an embodiment of the inven- tion. DETAILED DESCRIPTION The present invention is a method and apparatus for testing semiconductor lasers at the wafer-level. The inven- tion allows testing a wafer’s laser characteristics before the wafer has been separated into the individual laser devices, and before additional processing steps are carried out to add electrodes and insulators to the laser devices. During manu- facturing of the laser devices, a plurality of devices are formed from a single wafer. For example, 4000 individual devices may be formed on a 4 in. diameter wafer. In the context of this application, the term wafer is used to refer to the assembly of several layers that may include a substrate, reflective structures and an active gain region. The active region is a region in a laser device where light is generated, and may contain quantum wells, quantum dots, or other light-generating structures, and may include other structures (for example barrier layers, which absorb energy and trans- fer it to the light-generating structures). According to the present invention, although the com- pleted devices may be electrically pumped lasers, the active regions of the devices are pumped optically in order to test their optical characteristics, for example laser characteristics such as wavelength, power output, efficiency, etc. or laser- related characteristics such as gain. That is, the active regions of the devices may be excited by either electric or optical energy, but optical pumping may be accomplished at an earlier manufacturing stage than electrical pumping, since electrical pumping requires electrodes and insulators that are typically added to the wafer in later stages of manufacturing. A typical VCSEL manufacturing process begins with the growth of epitaxial layers on a wafer substrate, including for example a p-side mirror, an active region that may contain quantum wells, and a n-side mirror. At this point, the epitaxial layers are relatively uniform, except for non- uniformities caused by the epitaxial growth process itself. Thereafter, the wafer may be further processed to add electrodes, insulators and various metal components to each device, and other structures that individualize or otherwise define the devices are also included. After epitaxial growth, the wafer may be cleaved along device boundaries to separate the devices from one another, and any required final assemblage is performed to complete the individual devices or, for example lines or arrays of devices. The testing according to the present invention may take place at any time after the growth of the epitaxial layers. FIG. 1 shows a cross section of an embodiment of a VCSEL testing apparatus according to the present invention. A semiconductor wafer 100 is positioned in the device for testing after the epitaxial layers of the wafer have been grown. Those of skill in the art will understand that the testing of the wafer 100 may be performed at any time after the epitaxial layers have been formed. However, it is pref- US 6,775,000 B2 3 erable to perform this testing before any additional steps in the VCSEL manufacturing process have been performed as unsuitable wafers, or unsuitable wafer portions, or important laser characteristics (e.g., laser wavelength) may be identi- fied before additional expenditures of time and effort are made. The wafer 100 described as being tested in regard to the apparatus according to this embodiment of the invention includes an active region 102 epitaxially grown on a sub- strate 99, and two mirror regions 120, 122 that are also grown epitaxially. The testing apparatus includes a chuck 108 on which the wafer 100 is positioned and retained in place mechanically by, for example, clamps, a flange or vacuum pressure. As shown in FIG. 1, a pair of C-clamps 106 may be used to retain chuck 108 in place at the desired location. Chuck 108 may be made, for example, of fused silica, sapphire, or any other material that is transparent to the wavelength of a pump light source 110 to be employed in the system, and to the wavelength of an output beam of wafer 100. Alternatively, the chuck 108 may be opaque to either or both of these wavelengths of light. In this case, openings nay be provided to form windows over portions of wafer 100 corresponding to input and output areas of each of the ‘ndividual laser devices or arrays of devices that will be tested, as described in more detail below. Since the pump ight may enter the active region from either the top or the bottom, it may not be necessary to provide a chuck 108 that 's transparent to the wavelength of the pump light. The test apparatus includes a pump light source 110 that generates a beam 112 having a wavelength appropriate to jump energy into the active region 102, to induce the output of laser light. The pump light source may be, for example, a laser such as a semiconductor, gas, or solid state laser device, or another appropriate light source such as a lamp or ight emitting diode (“LED”). Those skilled in the art will inderstand that an appropriate pumping wavelength depends on the characteristics of the materials and structures 'ncluded in the wafer 100. Preferably, the wavelength of the jump light source 110 should be largely absorbed by active egion 102, but other epitaxial layers of the device, such as hose forming mirrors 120, 122 should be relatively trans- aarent to beam 112. In one embodiment, mirrors 120, 122 nay be highly reflective to the wavelength of light generated by the laser device, thus the pump light beam 112 should lave a wavelength different from the output laser wavelength, to avoid being reflected by the mirrors 120, 122. In another embodiment, the pump light wavelength may be closer to the laser wavelength because, for example, a mirror through which it is transmitted is less reflective, the pump light is sufficiently bright to penetrate a mirror, or the structures of the laser device are arranged such that the pump light does not have to travel through a mirror. In this exemplary embodiment, the laser beam from the tested wafer may be discriminated from any reflected pump light using an optical band pass filter or other dispersive optics. In the exemplary case of an output beam 114 having a wavelength of 980 nm, mirrors 120, 122 may preferably be especially reflective within a stop band about 100 nm wide, centered near 980 nm. In this case, the pump light beam 112 should have a wavelength of less than about 930 nm to avoid the mirror stop band. As the pumping light must also have greater energy than the output laser light in order to excite the active region, the wavelength of the pump light must be shorter than that of the output light, as energy per photon increases as the wavelength of light decreases. Thus, although pumping light of more than about 1030 nm would also avoid the stop band of mirrors 120, 122, such light would likely not have enough energy to pump the active region 102. 10 15 20 25 30 35 40 45 50 55 60 65 4 Pump beam 112 may have, in one embodiment, a power level of about 1><105 W/cmz, in a beam diameter of 100 microns. In one embodiment, the pump light source 110 is operated in a pulse mode, with short bursts of light gener- ated. Thus, a pump light source with a low duty factor may be preferred for this application. The “duty factor” of a light source operated in pulsed mode is the ratio of the time “on” to the sum of the time “on” plus the time “off”. For example, a laser operated with 10 microsecond pulses, spaced 90 microseconds apart, will have a duty factor of 0.1 or 10 percent. Operating in the pulse mode makes possible the production of high intensity of pump light, while maintain- ing an average power provided to the wafer 100 over time low enough to avoid excessive heating of the wafer 100. For example, in one embodiment of the invention a duty factor of 10 percent might be used to achieve an average power level of 1 W using a pump light source with a peak power output of 10 W. The pulse widths used in this case might be in the range of 0.1 to 100 microseconds, with corresponding “01f” periods calculated accordingly. Any number of pump light sources could be used in this embodiment of the invention, for example, a solid state Ti doped sapphire laser or a GaAs edge emitter laser may be employed as the pump light source 110 of this embodiment. The testing apparatus shown in FIG. 1 also includes an aiming mechanism with a drive 124 for the laser 110. Drive 124 is designed to accurately move pump light source 110 along wafer 100 to locations on wafer 100 where individual semiconductor laser devices will be formed. In this manner it is possible to characterize the laser properties of the epitaxially grown layers at all the locations that may become devices. The movement of pump light source 110 may be optimized for the geometry of wafer 100. For example, if the devices will be formed on wafer 100 in a rectangular array of regularly spaced rows and columns, drive 124 moves the laser 110 relative to the wafer 100 to direct an output beam 112 from the laser 110 sequentially through all the rows and columns of the wafer 100. Each of the selected locations on the wafer 100 corresponds to a selected one of the devices, so that the beam 112 stimulates emission from the selected device so that the properties of the epitaxial layers at that location may be determined. The pump beam 110 is then directed to the next desired location to test the properties of the epitaxial layers at that next location. Ameasuring system may also be included in drive 124, to accurately control the positioning of beam 112 on the wafer 100. In a different embodiment, the pump light source 110 may be stationary, and the chuck 108 with wafer 100 can move relative to pump beam 112. As long as the position of pump beam 112 is known relative to wafer 100, it does not matter which component moves and which is stationary. In this embodiment, the wafer may be accurately positioned and stepped in relation to the pump light beam by an aiming mechanism including a probing station machine such as a Signatone XXC Probe Station with an S-LDC2 Dry/Dark Chamber or CMS 12 Probe Station available from Signatone Corporation, Gilroy, Calif. or a Suss PA 200 Semiautomatic Probe System available from SUSS MicroTec, Garching, Germany. In another embodiment, pump light source 110 and the assembly of chuck 108 and wafer 100 are both stationary. The pump beam 112 is aimed at the appropriate position over wafer 100 by a system of adjustable focusing mirrors, lenses, and/or other optics. In yet another embodiment, an array of pump beams may be used to simultaneously probe several locations on the wafer. The beams may be generated by an array of pump light sources, or by a single pump light source in conjunction US 6,775,000 B2 5 with beam splitting optics. A corresponding array of detec- tors for the emitted beams may also be provided to simul- taneously analyze the emitted beams emitted from the various devices to determine the properties of the epitaxial layers at each location. Although determining the properties of the spatial mode of the emission of the wafer 100 has been discussed above, other modes may be examined as well. For example, dif- ferent spot sizes of the pump beam 112 may be used to stimulate emissions from wafer 100 in different modes. A beam 112 of small size may be used to characterize the spatial mode, while larger size beams may be used to stimulate emission of higher order modes. Particularly, this application might be used in an embodiment employing a curved mirror or lens as part of the laser cavity of the laser being tested. For example, curved lenses may be etched into the wafer substrate or employed as an external mirror to create an external cavity for the laser device. The exemplary testing apparatus shown in FIG. 1 also includes a detector 116 positioned on the opposite side of wafer 100 from the pump light source 110. In this example, detector 116 is aligned collinearly with pump light source 110 to receive the emitted laser beam 114, and to analyze its properties. For example, the energy of emitted beam 114 may be measured and related to the energy of pump beam 112. The wavelength and the modes of the beam 114 may also be measured to build a two dimensional map of the laser properties of the wafer 100. Other qualities of the emitted laser beam 114 may also be measured by detector 116, such as the phase uniformity and spectral coherence of the emitted light. Detector 116 may also be linked to drive 124 or to a separate drive, so that the detector 116 is always aligned with a laser currently being stimulated by the pump light source 110 to receive the beam 114 emitted therefrom. In the embodiment shown in FIG. 1, the wafer 100 is oriented on chuck 108 so that totally reflective mirror 120 (a p-doped mirror in this case) faces chuck 108, while partially reflective mirror 122 (an n-doped mirror in this embodiment) faces detector 116. Emitted beam 114 thus exits the device as shown in the drawing, towards detector 116. Most if not all of the pump beam 112 is absorbed by the active region 102 of wafer 100. However, an optical filter 126 may be placed in the path of emitted beam 114 to remove any portion of pump beam 112 that may otherwise affect the characterization of emitted beam 114. A third mirror 130 may be included in the testing appa- ratus according to the invention. Mirror 130 is part of the testing apparatus, and is not included in the final laser device. However, during testing of the wafer 100, thir...
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