Abstract:
A VCSEL includes a gap in a mirror stack and a protective layer sealing an end of the gap. The gap defines a boundary of the aperture of the VCSEL without introducing the stresses that oxide regions in oxide VCSELs can cause, and the protective layer, which can be a thin dielectric layer, shields the mirror stack from environmental damage. The VCSEL can thus achieve high reliability. A fabrication process for the VCSEL forms an oxidation hole, oxidizes a portion of an aluminum-rich layer in a mirror stack of the VCSEL exposed in the hole, and then removes all or some of the resulting oxide to form the desired gap. The protective layer can then be deposited to seal an end of the gap.

Description:
BACKGROUND  
       [0001]     Vertical cavity surface emitting lasers (VCSELs) are well-known optoelectronic devices that can be manufactured using semiconductor processing techniques.  FIG. 1 , for example, shows a cross-sectional view of a conventional oxide VCSEL  100  that includes a cavity layer  120  sandwiched between a partially reflective mirror stack  110  and a highly reflective mirror stack  130 . Cavity layer  120  generally contains a lasing material such as gallium arsenide that emits light where an electrical current passes through cavity layer  120 . Mirror stacks  110  and  120  normally have reflectivities and separations selected to achieve a desired gain for the operating light wavelength in VCSEL  100  and are preferably conductive and in contact with the electrical terminals (not shown) of VCSEL  100 .  
         [0002]     An insulating oxide region  112  in mirror stack  110  defines the boundaries of an aperture through which the light beam from VCSEL  100  emerges. To confine the light beam, oxide region  112  channels the current flow into cavity layer  120  to the area where light emissions are desired. Oxide region  112  may also change the reflectivity/refractive index of mirror stack  110  outside the area of aperture  140  so that the optimal gain is limited to the area of aperture  140 .  
         [0003]     Before being sold as a commercial product, VCSELs such as oxide VCSEL  100  generally must pass a reliability test that attempts to identify devices that may have short useful lives or that may fail in some working environments. One such test, commonly known as the 85/85 stress test or Wet High Temperature Operating Life test (WHTOL), is used industry-wide to assess the reliability of VCSELs as well as others optoelectronic devices. Typically, oxide VCSELs rapidly fail the 85/85 stress test.  
         [0004]     Structures and processing techniques that can improve the yield of VCSELs capable of passing the required reliability tests are thus desired.  
       SUMMARY  
       [0005]     In accordance with an aspect of the invention, a VCSEL uses a void or gap in a mirror stack to define a light aperture and a thin protective layer to cover the gap. With the protective layer, the VCSEL can pass the 85/85 stress tests and provide high reliability. Further, the manufacturing process for the thin layer avoids problems associated with forming thick protective layers.  
         [0006]     One specific embodiment of the invention is a device such as a VCSEL that includes a first mirror stack, a second mirror stack, a cavity layer, and a protective layer. The cavity layer is between the first mirror stack and the second mirror stack. A hole extends through the first mirror stack, and a gap extends from a sidewall of the hole into the first mirror stack to define boundaries of an aperture of the device. The protective layer seals an end of the gap at the sidewall of the hole in the first mirror layer.  
         [0007]     Another specific embodiment of the invention is a fabrication process for a device such as a VCSEL. The process generally includes: forming a first mirror stack, a cavity layer, and a second mirror stack on a substrate; etching a hole in the first mirror stack; removing a portion of a layer in the first mirror stack to form a gap extending from a sidewall of the hole into the first mirror stack; and depositing a protective layer that seals an end of the gap at the sidewall of the hole. Forming the gap can include oxidizing the layer in the first mirror stack to form an oxide region and then etching away at least a portion of the oxide region. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  shows a conventional oxide VCSEL.  
         [0009]      FIG. 2  shows a VCSEL in accordance with an embodiment of the invention including a thin layer that seals a gap used to define an aperture of the VCSEL.  
         [0010]      FIGS. 3A, 3B ,  3 C,  3 D, and  3 E illustrate a process for forming the VCSEL of  FIG. 2 .  
         [0011]      FIG. 4  shows a top view of a VCSEL in accordance with an embodiment of the invention. 
     
    
       [0012]     Use of the same reference symbols in different figures indicates similar or identical items.  
       DETAILED DESCRIPTION  
       [0013]     In accordance with an aspect of the invention, a vertical cavity surface emitting laser (VCSEL) having a gap defining the boundaries of an aperture and a thin protective layer protecting the gap provides high reliability. Manufacturing techniques for such VCSELs provide a high yield of devices that pass industry standard reliability tests such as the 85/85 stress test.  
         [0014]      FIG. 2  shows a cross-section of a VCSEL  200  in accordance with an embodiment of the invention. VCSEL  200  includes a top mirror stack  210 , a cavity layer  220 , and a bottom mirror stack  230  that are formed on an underlying substrate  240 . A protective layer  250  covers at least selected portions of cavity layer  220  and mirror stacks  210  and  230 , and particularly seals a gap  212  that defines boundaries of an aperture of VCSEL  200 .  
         [0015]     In the illustrated embodiment, cavity layer  220  includes one or more active layers  224  (e.g., one or more quantum wells and/or one or more quantum dots) that are sandwiched between spacer layers  222  and  226 . Alternatively, active layer  224  could be located above or below a single spacer layer. Active layer  224  can be formed from a variety of materials including but not limited to GaAs, InGaAs, AlInGaAs, AlGaAs, InGaAsP, GaAsP, GaP, GaSb, GaAsSb, GaN, GaAsN, InGaAsN, and AlInGaAsP. Other quantum well layer compositions also may be used. Spacer layers  222  and  226  are generally formed from materials chosen based upon the composition of active layer  224 .  
         [0016]     Cavity layer  220  has an overall thickness selected according to the operational wavelength of light emitted from VCSEL  200 . To produce a light beam from VCSEL  200 , a driving circuit (not shown) drives a current through active layer  224 . For connection to a drive circuit, VCSEL  200  has a first electrical contact  252  above mirror stack  210  and a second electrical contact  242  below active layer  220 . However, VCSEL  200  could alternatively employ contacts with other configurations. For example, the second electrical contact could be on top of VCSEL  200  or within bottom mirror stack  230 . In whichever contact configuration used, an operating voltage applied between electrical contacts  242  and  252  preferably produces a current flow in VCSEL  200  through mirror stack  210  and cavity layer  220 , causing lasing in active layer  224 .  
         [0017]     Gap  212  is formed in an aluminum-rich layer  214  of mirror stack  210  to create a confinement region that laterally confines the flow of charge carriers and photons in VCSEL  200 . Layer  214  can be located anywhere in mirror stack  210 , including the top or bottom of mirror stack  210 . In some embodiments, gap  212  circumscribes a central aperture through which current and light preferably flow. Charge carrier confinement results from the relatively high electrical resistivity of gap  212 , which causes electrical current to flow through a centrally located region of VCSEL  200 . Optical confinement results from the low refractive index of gap  212 , which creates a lateral refractive index profile that guides the photons that are generated in cavity layer  220 . The carrier and optical lateral confinement increases the density of carriers and photons within an active region of layer  224  and increases the efficiency of light generation within the active region.  
         [0018]     Mirror stacks  210  and  230  each includes a system of alternating layers of different refractive index that preferably forms a distributed Bragg reflector (DBR) designed for the operating laser wavelength (e.g., a wavelength in the range of 650 nm to 1650 nm). For example, mirror stacks  210  and  230  may include layers of aluminum gallium arsenide (AlGaAs) where the aluminum content of the layers alternates between higher and lower levels. Each layer of mirror stack  210  or  230  in a conventional stack typically has an effective optical thickness (i.e., the layer thickness multiplied by the refractive index of the layer) that is about one-quarter of the operating laser wavelength. One particular layer  214  in mirror stack  210  contains an aluminum-rich material with an aluminum content that is sufficiently high that layer  214  oxides much more quickly than the other layers of mirror stack  210 . In a typical implementation, layer  214  may be about 95 to 98% aluminum, while the alternating layers have aluminum content that typically varies between around 20% and 80%.  
         [0019]     In the illustrative embodiment of  FIG. 2 , mirror stacks  210  and  230  are designed so that VCSEL  200  emits light through mirror stack  210 . In other embodiments of the invention, mirror stacks  210  and  230  may be designed so that the VCSEL emits laser light through mirror stack  230  and substrate  240 .  
         [0020]     Substrate  240 , which provides structural support for VCSEL  200 , can be made of a variety of materials including but not limited to GaAs, InP, sapphire (Al 2 O 3 ), or InGaAs and may be undoped, doped n-type (e.g., with Si) or doped p-type (e.g., with Zn). A buffer layer (not shown) of a material such as GaAs or AlGaAs about 100 angstroms thick can be grown on substrate  240  before other layers of VCSEL  200  to improve bonding to substrate  240 . Substrate  240  is preferably conductive in the illustrated embodiment of VCSEL  200  where electrical contact  242  is on a bottom surface of substrate  240 . Alternatively, substrate  240  can be made of an insulating material, and an electrical contact to cavity layer  220  or bottom mirror stack  230  can overlie substrate  240 .  
         [0021]      FIGS. 3A  to  3 E show cross-sections of intermediate structures created during a fabrication process for VCSEL  200 . For ease of illustration, the underlying support substrate and the contact structure is omitted from  FIGS. 3A  to  3 E. Contact structures for VCSELs are known in the art and can be formed using conventional techniques.  
         [0022]      FIG. 3A  shows a cross-section of a structure after formation of bottom mirror stack  230 , cavity layer  220 , and top mirror stack  210 . Conventional epitaxial growth processes, such as metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) can form these layers of VCSEL  200  on the support substrate (not shown). A mask  260  having an opening (or multiple openings) is formed overlying layers  210 ,  220 , and  230 . Mask  260  can be made of photoresist or of another material such as silicon nitride (Si 3 N 4 ) or a metal.  
         [0023]     An etch process using mask  260  creates openings  270  as shown in  FIG. 3B . Openings  270 , which are commonly known as oxidation holes, extend through top mirror stack  210  and cavity layer  220  to a region in the lower mirror stack  230  of VCSEL  200  and therefore expose edges of aluminum-rich layer  214  in top mirror stack  210 . More generally, oxidation holes  270  are not required to extend into lower mirror stack  230  but instead can end within cavity layer  220  or in top mirror stack  210  as long as oxide holes  270  expose aluminum-rich layer  214 . Wet or dry etching process, including reactive ion etching (RIE) and reactive ion beam etching (RIBE), can form openings  270  to the required depth. In one embodiment, openings  270  leave a mesa structure in which VCSEL  200  resides.  
         [0024]     An oxidation process using a steam or dry oxygen environment oxidizes the exposed edge of aluminum-rich layer  214  to form oxide regions  216  as shown in  FIG. 3C . As noted above, the composition of aluminum-rich layer  214  is preferably high such that layer  214  is strongly oxidized while the other layers in mirror stack  210  are more slowly oxidized. For example, layer  214  may be AlGaAs that is about 95% aluminum, while other layers are AlGaAs with typically no more than about 90% aluminum. The high rate of oxidation in aluminum rich layer  214  and the duration of the oxidation process controls the lateral extent of oxide regions  216  and controls the remaining area of layer  214  that defines the aperture of VCSEL  200 . In an exemplary embodiment of the invention, oxide regions  216  extend about 25 μm into layer  214 , leaving an aperture about 10 to 20 μm across. Mask  260  can be removed before or after the oxidation process.  
         [0025]      FIG. 3D  shows the structure after an etching process removes oxide regions  216  leaving gaps  212  in layer  214 . Oxide regions  216  can be removed using a wet etch with a basic solution such as a sodium hydroxide (NaOH) solution. In particular, a basic solution of pH greater than 13 can remove the oxide regions. As an alternative to complete removal of oxide regions  216 , a partial removal of oxide region  216  could leave a portion of oxide region  216 . The removal of all or part of oxide region  216  is believed to improve device reliability by reducing the stress created when oxide regions  216  form.  
         [0026]     A thin protective layer  250  as shown in  FIG. 3E  is deposited over the structure or selectively in regions including oxidation holes  270 . Thin layer  250  can be a silicon nitride layer having a thickness that is less than about 6000 Å, or preferably less than about 2500 Å, and more preferably is about 1100 Å thick. However, other materials such as silicon oxy-nitride (SiON) can be used for protective layer  250 . Alternatively, protective layer  250  may be a composite layer, for example, including silicon nitride (Si 3 N 4 ) layer about 1100 to 1500 Å thick, a silicon oxy-nitride (SiON) layer about 1100 to 1500 Å thick, and a titanium (Ti) layer about 700 to 1000 Å. The deposition process covers the structure/side walls of oxidation holes  270  and seals the exposed end of gap  212 , leaving a seal gap (e.g., a sealed air gap). Good coverage down into holes  270  is important for reliability and can be achieved, for example, with a Plasma Enhanced Chemical Vapor Deposition (PECVD) process. An electrical contact to top mirror stack  210  can be formed before deposition of protective layer  250  or after forming openings (if necessary) where desired in protective layer  250 .  
         [0027]     The VCSEL fabrication process can be completed using conventional techniques, including, for example, backside metal deposition or metal deposition onto or within the lower mirror stack for a lower contact.  
         [0028]      FIG. 4  shows a top view of a VCSEL  400  having a central aperture  410 . An electrical contact/lines  420  include a patterned metal layer surrounding aperture  410  and in contact with the top mirror stack. Four nearly oxidation holes  270  around aperture  410  are separated from aperture  410  by a distance that is equal to or less than the lateral extend of the air gap into the top mirror stack. As a result, the air gap associated with oxidation holes  270  join together to surround aperture  410 . Additionally, electrical contact/lines  420  can include a trace or metal line that runs between the oxidation holes to the area around aperture  410 . Aperture  410  can be further inside the metal lines, forming concentric circles or squares.  
         [0029]     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.