Abstract:
Methods for sealing or passivating the edges of chips such as vertical cavity surface emitting lasers (VCSEL) is disclosed. One method includes oxidizing the edges of die at the wafer level prior to cutting the wafer into a plurality of die. This may be accomplished by etching a channel along the streets between die, followed by oxidizing the channel walls. The oxidation preferably oxidizes the aluminum bearing layers that are exposed by the channel walls inward for distance. Aluminum bearing layers, including AlAs and AlGaAs, may be oxidized to a stable native oxide that is resistant to further oxidation by the environment. After oxidation, the wafer can be cut along the channels into a number of die, each having a protective oxide layer on the side surfaces.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention is related generally to electronics semi-conductor processing. More specifically, the present invention is related to controllable oxidization of the side edges of die at the wafer level, and in particular the passivation of the side edges of aluminum bearing Group III-V semiconductor layers in electronic or electro-optical devices, such as VCSEL chips.  
         BACKGROUND OF THE INVENTION  
         [0002]    Semiconductor lasers are commonly used in modem technology as a light source in various devices, including communication devices, such as fiber optic transmitters, and compact disc players. A typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap low refractive index layers. The low bandgap layer is termed the “active layer” and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types, and when current is passed through the structure, electrons and holes recombine in the active layer to generate light.  
           [0003]    Surface-emitting, rather than edge-emitting lasers have been developed. One surface-emitting laser is a “vertical cavity surface emitting laser” (VCSEL). Vertical Cavity Surface Emitting Lasers offer numerous performance and potential producibility advantages over conventional edge emitting lasers. These include many benefits associated with their geometry.  
           [0004]    Surface emitting devices can be fabricated in arrays with relative ease while edge emitting devices cannot be as easily fabricated. An array of lasers can be fabricated by growing the desired layers on a substrate and then patterning the layers to form the array. Individual lasers may be separately connected with appropriate contacts. Such arrays are potentially useful in such diverse applications as, for example, image processing, inter-chip communications (i.e., optical interconnects), and so forth. Typical edge-emitter lasers are turned on and off by varying the current flow through the device. This often requires a relatively large change in the current through the device which is undesirable. In comparison, surface-emitting lasers often require lower drive current, and thus the change of current to switch the VCSEL need not be as large.  
           [0005]    High-yield, high performance VCSELs have been demonstrated, and exploited in commercialization. Surface-emitting AlGaAs-based VCSELs are producible in a manner similar to semiconductor integrated circuits, and are amenable to low-cost high-volume manufacture and integration with existing electronics technology platforms. Moreover, VCSEL uniformity and reproducibility have been demonstrated using a standard, unmodified commercially available metal organic vapor phase epitaxy (MOVPE) chamber and molecular beam epitaxy (MBE) chamber giving very high device yields.  
           [0006]    VCSELs typically have an active region with bulk or one or more quantum well layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the high-reflectance mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is typically turned on and off by varying the current through the active region.  
           [0007]    VCSELs may have multiple aluminum bearing Group III-V layers. In particular, the VCSEL may have AlAs layers and AlGaAs layers. The aluminum bearing layers are protected from the environment in a vertical direction by the top surface, which can include one or more surface passivation layers. The aluminum bearing layers typically are exposed to the environment at the edges or side face surfaces, particularly after the wafer has been cut into individual die. The aluminum bearing edges can oxidize when the chip is placed into service in an oxidizing environment. The environmentally induced oxidation can cause unreliable oxidation, from the edge inward, of the aluminum bearing layers. If left unchecked, this lateral oxidization can sometimes reach the VCSEL device itself, thereby reducing performance or even preventing operation altogether. To prevent or inhibit such lateral oxidation of the aluminum bearing layers, the chip is commonly mounted in a hermetically sealed package. Hermetically sealed packages have a number of limitations. First, hermetically sealed packages can be relatively expensive, which increases the cost of the device. Second, hermetically sealed packages can be relatively bulky, which increases the space needed to mount the device on a circuit board or multi-chip package, both of which are undesirable.  
           [0008]    What would be desirable is a method for sealing the edges of chips, particularly VCSEL chips, prior to cutting the chips from the wafer. VCSEL chips having sealed edges may not require a hermetically sealed package.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides methods for passivating the edges or side faces of aluminum bearing Group III-V layers in die such as VCSEL die. In one illustrative method, a semiconductor wafer having multiple VCSEL die separated by streets is provided. The VCSEL die preferably have aluminum bearing layers disposed a depth under the laterally disposed top wafer surface. Channels are cut into the streets to a depth sufficient to expose the side edges of the deepest aluminum bearing lateral layer. The channels are then exposed to an oxidizing environment such as steam, oxygen, or any other suitable oxidizing environment.  
           [0010]    The oxidizing atmosphere oxidizes the aluminum bearing layers into aluminum oxide layers, forming a stable native oxide passivating layer which is resistant to further oxidation by the intended working atmosphere. Some of the non-aluminum bearing layers of a VCSEL, including layers formed from GaAs, InGaAs, InGaAsN, GaAsN, GaAsP, InGaAsP, etc., may also oxidize, but at a reduced rate. In one embodiment, the aluminum bearing layers are oxidized about 10 to 15 microns into the channel wall. After the aluminum bearing layers are sufficiently oxidized, the wafer is removed from the oxidizing atmosphere. The heat (−440° C.) results in almost immediate drying.  
           [0011]    The wafer can then be cut into discrete die with a blade using methods well known to those skilled in the art. The wafer cuts are preferably made along the channels without destroying the intentionally oxidized layers. Preferably, the channel walls are oxidized prior to cutting the wafer into individual die. Alternatively, however, it is contemplated that the wafer may be cut into individual die before the side walls are oxidized. In this latter embodiment, no channels need be formed.  
           [0012]    The present invention also includes discrete VCSEL chip having passivated aluminum bearing layers around the chip periphery. In particular, the present invention includes VCSEL chips having AlAs and AlGaAs layers where the layer regions near the chips side edges or side faces are oxidized to form a stable native oxide. The native oxide layers are believed to include anhydrous forms of aluminum hydroxides and aluminum oxide hydroxides such as alpha-Al 2 O 3 , gamma-Al 2 O 3 , diaspore, and boehmite.  
           [0013]    Chips made according to the present invention have side regions that are resistant to further oxidation by the environment. In particular, VCSEL chips made according to the present invention are believed suitable for direct mounting to boards without hermetically sealed packaging. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a perspective cutaway view of a VCSEL device;  
         [0015]    [0015]FIG. 2 is a cross-sectional side view of a wafer having multiple VCSEL die thereon;  
         [0016]    [0016]FIG. 3 is a cross-sectional side view of the wafer of FIG. 2 after formation of a channel in the wafer;  
         [0017]    [0017]FIG. 4 is a cross-sectional side view of the wafer of FIG. 3 after the channels are exposed to an oxidizing environment; and  
         [0018]    [0018]FIG. 5 is a cross-sectional side view of the wafer of FIG. 4 after cutting through the channel to form discrete chips. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    [0019]FIG. 1 illustrates a typical VCSEL device, shown in a perspective, cutaway view. FIG. 1 is an illustration of a planar, current-guided, GaAs/AlGaAs top vertical cavity surface emitting laser (VCSEL)  10  with a top mirror region  26 . In one embodiment, the n-doped mirror layers  16  are grown by metal organic vapor phase epitaxy (MOVPE) on an n-doped GaAs substrate  14 . The n-type mirror stack  16  can be a 30.5 period n-doped (Te, 1×10 18  cm −3 , nominal) Al 0.16 Ga 0.84  As/AlAs bottom quarter wave stack, wherein each period contains a 200 Angstrom thick graded region. Spacer  18  has a bottom confinement layer  20  and a top confinement layer  24 , wherein each of the confinement layers is formed from Al 0.6 Ga 0.4  As. The thickness of each confinement layer  20  and  24  can be chosen to make the resulting spacer  18  preferably one wavelength thick. The active region  22  can be a three 70 Angstrom thick GaAs quantum-well. The p-type mirror stack  26  can be a 22 period p-doped (Zn, 1×10 18  cm −3 , nominal) Al 0.16 Ga 0.84  As/AlAs DBR, wherein each period contains a 200 Angstrom thick graded region. Numerous device sizes, types and arrays may be simultaneously batch-fabricated, exploiting the flexibility of this technology platform.  
         [0020]    In the embodiment illustrated, layers  16 ,  18 , and  26  include aluminum bearing layers, containing layers of either AlAs or AlGaAs in this example. The aluminum bearing layers are susceptible to oxidation when exposed to an oxidizing atmosphere. Non-aluminum layers, such as substrate  14  and active region  22 , are less susceptible to oxidation when the edges are exposed to an oxygen containing atmosphere, but may form a native oxide to some degree.  
         [0021]    Referring now to FIG. 2, a wafer  50  is illustrated having a first die  52  and a second die  54  separated by a street  56 . Wafer  50  includes a top passivation layer  60 , a top mirror region  62  having a p-type mirror stack, a top confinement region  64 , an active region  66 , a bottom confinement region  68 , a bottom mirror region  70  having an n-type mirror stack, and a substrate  72 . While compositions may vary, top and bottom confinement regions  64  and  68  can be formed of layers including AlGaAs and AlAs, and active region  66  can be formed of GaAs. Top mirror region  62  can be formed of layers including aluminum bearing material such as layers of AlAs, as can bottom mirror region  70 .  
         [0022]    Referring now to FIG. 3, wafer  50  is illustrated after a channel  74  has been etched or otherwise cut into wafer  50 , preferably along the street  56 . Channel  74  is preferably etched using photolithography and chemical etching techniques well known to those skilled in the art. Other patterning techniques are also contemplated, such as plasma etching and ion milling. Channel  74  has a width indicated at  76 , a depth indicated at  78 , and side walls  80 . In a preferred embodiment, channel  74  has a depth that is deeper than the deepest layer that is intended to be oxidized, which is often between about 5 and 10 microns. In FIG. 3, channel depth  78  is preferably sufficiently deep to cut into the lowest or deepest aluminum bearing layer, which is layer  70  in this illustration. In many wafers, channel  74  may be cut at least into bottom mirror layer  70 , if not into substrate  72 . Cutting channel  74  to this depth can enable oxidizing all the aluminum bearing layers. Channel width  76  is preferably wider than the width of the final cut made through the wafer to form the discrete chips to prevent damage to the oxidized side walls.  
         [0023]    After forming channel  74 , the channel can be oxidized using an oxidizing atmosphere or fluid. A preferred oxidizing atmosphere includes hot, moist air or steam. One such suitable oxidization process is described in U.S. Pat. No. 5,696,023 to Holonyak, Jr. et al., which is incorporated herein by reference. Other oxidizing environments may include, for example, oxygen, or oxygen radicals flowing in a carrier gas such as nitrogen.  
         [0024]    Referring now to FIG. 4, channel  74  is illustrated after oxidation. The aluminum bearing layers are illustrated having oxidized portions extending inward from channel walls  80 . Layers  62 ,  64 ,  68 , and  70  may be seen to have oxidized portions at  63 ,  65 ,  69 , and  71  respectively. The lateral extent of the oxidation may vary depending on the composition of the layer. For example, bottom mirror layer  70  has a lateral oxidation extent indicated at  84 , which is different than the lateral oxidation extent of bottom confinement region  68  indicated at  86 . In one embodiment, the lateral extent of the native oxidation is between about 10 and 15 microns. The extend of lateral oxidation can be controlled by controlling the time that the wafer is exposed to the oxidizing environment. Temperature, steam pressure, and other factors also influence the lateral oxidation rate, and thus are also preferably controlled.  
         [0025]    [0025]FIG. 4 illustrates layers  62 ,  64 ,  68 , and  70  as monolithic layers for ease of illustration. Those skilled in the art will recognize that such layers are often not monolithic but formed of alternating sub-layers, some of which sub-layers may not form a native oxide or may form a native oxide at a smaller rate. Thus, the oxidized layers illustrated in FIG. 4 may actually be formed of alternating sublayers where one sublayer is oxidized and the adjacent sublayer is not oxidized or oxidized to a lesser degree. Applicants do not believe this to be a problem as the non-aluminum bearing layers typically do not require passivation prior to exposure to adverse environments.  
         [0026]    The oxidized aluminum bearing layers are believed to include aluminum oxide, where the aluminum oxides are in an anhydrous form such as alpha-Al 2 O 3  and gamma-Al 2 O 3 . The oxidized aluminum bearing layers are also believed to include anhydrous forms of aluminum oxide hydroxides such as AlO(OH), diaspore and boehmite. These forms of oxidized aluminum are believed to be more stable, and discourage the formation of other oxygen rich compounds, such as aluminum oxide hydrates and aluminum suboxides.  
         [0027]    After oxidation is complete, and as shown in FIG. 5, wafer  50  can be cut using conventional techniques well known to those skilled in the art. Wafer  50  can be cut or cleaved along channel  74  using a conventional blade or other cutting or cleaving instrument. The first die  52  is then separated from the second die  54  along the cut or cleave. The width of the cut is preferably less than the channel width to preserve the lateral width or penetration into the oxidized aluminum bearing layers. Because the width of the cut is preferably less than the channel width, a shoulder  85  is often provided. The shoulder  85  corresponds to the portion of the bottom surface of the channel  74  that is not removed during the cutting process.  
         [0028]    Preferably, the oxidation is performed before the chips are cut into discrete chips. This has the advantage of maintaining the wafer in a single piece and eases handling. In another method, however, the wafer can be cut into individual die prior to oxidation. In this method, the discrete chips have the entire edge exposed to the oxidizing fluid and require no further cutting after oxidation. In this latter method, no channels need to be formed before the wafer is cut into discrete die.  
         [0029]    Because of the protective oxidization layer, the present invention may allow VCSELs and other devices to be directly mounted on circuit boards using conventional methods such as wire bonding or surface mounting. In addition, it is contemplated that the present invention may allow VCSELs or other devices to be mounted in smaller, non-hermetic plastic packages. Finally, it is contemplated that the present invention may reduce chip sizes by allowing devices to be placed closer to the chip edge, which may increase yield and reduce the cost of such devices.  
         [0030]    While the invention is described above primarily with reference to a VCSEL device, it is contemplated that the present invention may be applied to any electronic device that includes aluminum bearing Group III-V semi-conductor layers. Such device may include, for example, various linear or digital circuits, various opto-electonic devices, etc.  
         [0031]    Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.