Abstract:
A method for detecting a passivation pinhole includes forming an oxide vertical cavity surface-emitting laser (VCSEL) having an oxidation cavity, forming a passivation layer over a surface of the oxidation cavity, exposing the oxide VCSEL to an etchant vapor, and inspecting the oxide VCSEL for a defect caused by the etchant vapor.

Description:
FIELD OF INVENTION 
     This invention relates to oxide vertical cavity surface-emitting lasers (VCSELs). 
     DESCRIPTION OF RELATED ART 
     Oxide VCSELs are fabricated by oxidizing a layer in the epitaxial stack through cavities etched in the wafer face to define a lasing area. This process leaves an entry path for moisture that causes oxide VCSEL failure in operation. For example, see S. Xie, G. De Brabander, W. Widjaja, U. Koelle, A. N. Cheng, L. Giovane, F. Hu, R. Herrick, M. Keever, and T. Osentowski, “Reliability of Oxide VCSELs in non-Hermetic Environments”, Proceedings of the 15th IEEE Laser and Electro-Optics Society Annual Meeting (LEOS 2002), Glasgow, Scotland, p. 544, paper WW2, Nov. 10–14, 2002; S. Xie, R. Herrick, G. De Brabander, W. Widjaja, U. Koelle, A. N. Cheng, L. Giovane, F. Hu, R. Herrick, M. Keever, T. Osentowski, S. McHugo, M. Mayonte, S. Kim, D. Chamberlin, S. J. Rosner, and G. Girolami, “Reliability and Failure Mechanisms of Oxide VCSELs in non-Hermetic Environments”, Proceedings of SPIE Vol. 4994, Vertical-Cavity Surface-Emitting Lasers VII, paper 4994–21, San Jose, Calif., Jan. 25–31, 2003; and S. Xie, R. Herrick, D. Chamberlin, S. J. Rosner, S. McHugo, G. Girolami, M. Mayonte, S. Kim, and W. Widjaja, “Failure Mode Analysis of Oxide VCSELs in High Humidity and High Temperature”, IEEE/OSA Journal of Lighwave Technology, March 2003 (in press). 
     Thus, oxide VCSELs need to be either packaged in hermetic cans, which is an expensive and cumbersome option for multi-channel arrays, or passivated to prevent moisture from getting into the oxide layer. Passivation films can contain pinholes, which lead to early failure. These pinholes are difficult to screen out so a technique is needed to make them apparent. 
     SUMMARY 
     In one embodiment of the invention, a method for detecting a passivation pinhole includes forming an oxide vertical cavity surface-emitting laser (VCSEL) having an oxidation cavity, forming a passivation layer over a surface of the oxidation cavity, exposing the oxide VCSEL to an etchant vapor, and inspecting the oxide VCSEL for a defect caused by the etchant vapor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  illustrate a cross-section and a top view of a VCSEL in one embodiment of the invention. 
         FIG. 3  is a flowchart of a method for detecting passivation pinholes in one embodiment of the invention. 
         FIG. 4  illustrates a system for detecting passivation pinholes in one embodiment of the invention. 
         FIG. 5  illustrates a top view of a VCSEL with a defect caused by an etchant vapor through passivation pinholes in one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an oxide VCSEL  10  in one embodiment of the invention. Typically, oxide VCSEL  10  includes a VCSEL structure  12  formed by growing a bottom mirror region atop a gallium arsenide wafer, an active region atop of the bottom mirror region, and a top mirror region atop of the active region. Since the construction of VCSELs is well known, the exact structure and process are not described in detail. 
     Typically, the bottom mirror region is an n-doped DBR (distributed Bragg reflector) mirror structure constructed from alternating layers having different refractive indices. The alternating layers can be made of aluminum gallium arsenide (AlGaAs) at two different aluminum mole fractions (e.g., 90% and 15%). 
     Typically, the active region is made of gallium arsenide (GaAs). 
     Typically, the top mirror region is a p-doped DBR mirror structure constructed from alternating layers having different refractive indices. Like the bottom mirror region, the alternating layers can be made of AlGaAs at two different aluminum mole fractions (e.g., 90% and 15%). After growing one or more pairs of the alternating layers in the top mirror region, an oxidation layer  14  is grown. Oxidation layer  14  is made of AlGaAs at the highest aluminum mole fraction (e.g., 95%) in VCSEL structure  12 . Then, the rest of the alternating layers in the top mirror region are grown. 
     VCSEL structure  12  is then etched to form one or more oxidation cavities  16 . Oxidation cavities  16  extend into the top mirror region and pass oxidation layer  14 . Oxidation cavities  16  are formed around a lasing area of the resulting VCSEL. A nitride mask layer can be used to define where oxidation cavities  16  are etched. Oxidation cavities  16  can be formed by either wet or dry etch. 
     VCSEL structure  12  is next placed in an oxidation oven. The nitride mask layer used for etching oxidation cavities  16  can remain as an oxidation mask to prevent the oxidation of the top surface of VCSEL structure  12 . Steam is introduced through oxidation cavities  16  to oxidation layer  14 , which laterally oxidizes. The oxidized regions, also called oxidation fronts, form insulation regions that limit current flow and establish optical confinement within an aperture or lasing volume  18  of the resulting VCSEL  10 .  FIG. 2  illustrates a top view of VCSEL  10  with overlapping oxidation fronts  19  that define aperture  18 . 
     Referring back to  FIG. 1 , VCSEL structure  12  is next coated with a passivation layer  20 . Notably, passivation layer  20  covers the surface of oxidation cavities  16 . This layer may be made of multiple film stacks. Passivation layer  20  may be made of silicon nitride (SiN) with a thickness of approximately 0.5 micron. Passivation layer  20  can be formed by plasma-enhanced chemical vapor deposition (PECVD). 
     One or more passivation pinholes  24  may form in passivation layer  20 , exposing VCSEL structure  12  to moisture that can cause it to fail prematurely. In accordance with the invention, VCSEL  10  is exposed to an etchant in a chamber to make the presence of the pinholes visible. 
       FIG. 3  is a flowchart of a method  40  to identify passivation pinholes in one embodiment of the invention. Method  40  exposes VCSEL  10  to an etchant vapor in a furnace to make the presence of the pinholes visible. 
     In step  42 , VCSEL  10  ( FIG. 1 ) is formed. In one embodiment, VCSEL  10  is formed as described above. 
     In step  44 , VCSEL  10  is passivated. In one embodiment, passivation is formed as described above. 
     In step  46 , VCSEL  10  is exposed to an etchant vapor such as hydrochloric acid (HCl) vapor. The etchant vapor does not attack the passivation layers but produces a visually detectable defect  26  ( FIG. 1 ) in VCSEL structure  12  ( FIG. 1 ). The etchant vapor is able to penetrate the passivation pinholes the same way as water vapor. Once the etchant vapor penetrates the passivation pinholes, the etchant vapor attacks VCSEL structure  12  to produce visually detectable defect  26 . 
     In one embodiment illustrated in  FIG. 3 , HCl vapor is produced by heating liquid HCl in a bubbler  72  heated by a hotplate  74 . An inert gas such as nitrogen (N2) is fed to bubbler  72  to force the HCl vapor to a horizontal tube furnace  76 . Inside furnace  76 , VCSEL  10  is exposed to HCl vapor at an elevated temperature. In this embodiment, bubbler  72  is heated to 30° C., N2 flow is maintained at 1 slpm (standard liter per minute), furnace  76  is heated to 285° C., and VCSEL  10  is exposed to HCl vapor for 2 hours. 
     In step  48 , VCSEL  10  is inspected for visually detectable defect  26  under infrared light, which allows the oxidation layer to be examined through the optically opaque semiconductor layers. In one embodiment, VCSEL  10  is inspected under an infrared microscope with a bandpass filter selected to maximize the contrast of the attacked layer to an unattacked layer.  FIG. 5  illustrates a top view of VCSEL  10  captured with the infrared microscope after exposure to HCl vapor. 
     In step  50 , VCSEL  10  is yielded out if it has visually detectable defect  26 .  FIG. 5  illustrates that defect  26  appears as a second oxidation front  90  under infrared light. 
     Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention. Although only one VCSEL is shown in the figures, one skilled in the art understands that the process described can be used to test an array of VCSELs on a die. Furthermore, although etchant vapor is used, etchant liquid could also be used to penetrate the passivation pinholes and produce visually detectable defect  26 . Numerous embodiments are encompassed by the following claims.