Abstract:
A metallization layer for a semiconductor device includes a first layer made of Pt and having a thickness greater than or equal to 15 Å and less than or equal to 50 Å, and a second layer formed on the first layer and made of a plurality of metallic sub-layers such as Ti/Pt/Au. A semiconductor device fabricated from the metallization layer includes a semiconductor substrate having a top layer and mesa structure and corresponding surface for securing an insulating layer and a corresponding exposed surface, and wherein the metallization layer is deposited over the insulating layer and exposed surface. Methods for forming the metallization layer are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Generally, the field of the present invention is adhesion bonding with semiconductors. More particularly, the present invention relates to the adhesion of metallic layers to p-type III-V compound semiconductors. 
     2. Background Art 
     In order to enhance the performance of semiconductors, advances have been made in types of materials used and methods for forming those materials. One such area of performance pertains to the formation and structure of metal and semiconductor contact. For semiconductor lasers, this contact should be ohmic, that is, the contact should exhibit linear I-V characteristics, and a low contact resistance is required. U.S. Pat. No. 5,429,986 describes a process for forming a low resistance ohmic contact electrode that has a layer of Pt interposed between a p-type GaAs layer and a Ti/Pt/Au layer wherein the interposed Pt layer has a thickness greater than 50 Å and less than 400 Å. The &#39;986 also extrapolates that for thicknesses less than 50 Å, unsuitable contact resistances are obtained. 
     However, in addition to contact resistance, other characteristics are desirable for contacts formed on p-type semiconductors. For example, the time required to complete the formation of a contact through annealing can impact the overall cost of manufacturing devices utilizing the contacts. Additionally, the adhesion strength between contact metals and underlying semiconductor and insulating layers can allow subsequent fabrication steps without metal peeling and can determine the infant mortality rate and useful life of devices incorporating the contacts. Thus, reliability remains important and concomitant attributes such as robustness and versatility can extend the scope of use of products fabricated with ohmic contacts. 
     Thus, despite the considerable efforts that have been exerted for many years, there remains a long felt need for a p-metal that provides superior strength, reliability, and processing time without any attendant drawbacks. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an adhesive layer joining opposing device layers of a semiconductor device includes a thin layer of Pt having a thickness greater than or equal to 15 Å and less than or equal to 50 Å. 
     According to another aspect of the present invention, a metallization layer for a semiconductor device includes a first layer made of Pt and having a thickness greater than or equal to 15 Å and less than or equal to 50 Å, and a second layer formed on the first layer and made of a plurality of metallic sub-layers, such as Ti/Pt/Au. 
     According to another aspect of the present invention, a semiconductor device includes a semiconductor substrate including a top layer having a surface, an insulating layer formed on a first portion of the surface and not formed on a second portion of the surface, a metallization layer deposited on the insulating layer and the second portion of the surface, wherein the metallization layer includes a first layer made of Pt and having a thickness greater than or equal to 15 Å and less than or equal to 50 Å and a second layer made of a plurality of metallic sub-layers. 
     According to another aspect of the present invention, a method includes forming a p-type semiconductor layer, and forming a metallization layer on the p-type semiconductor layer wherein the metallization layer includes a thin Pt layer having a thickness less than or equal to 50 Å and greater than or equal to 15 Å and a plurality of metallic layers on the thin Pt layer. 
     The foregoing and other objects, features, and advantages will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a typical p-metal deposited on a p-type III-V compound semiconductor and forming an electrode. 
         FIG. 2  is a cross-sectional view of an exemplary embodiment of the present invention showing an additional Pt layer at the p-type semiconductor interface, said additional Pt layer being shown in greater detail in an expanded bubble. 
         FIG. 3  is a flow-chart diagram showing typical processing steps for making a device using exemplary methods of the present invention. 
         FIG. 4  is a chart showing the superior strength of an exemplary embodiment of the present invention. 
         FIG. 5  is a plan view image of an embodiment of the present invention after destructive testing. 
         FIG. 6  is a plan view image of the remains of typical p-metal deposited on a p-type semiconductor after destructive testing. 
         FIG. 7  is a plan view image of semiconductor devices after destructive testing. 
         FIG. 8  is a chart of the L-I-V curves of both a typical p-metal layer and a layer utilizing an embodiment of the present invention. 
         FIG. 9  is a chart of the L-I curves of catastrophic optical damage testing of both a typical p-metal layer and a layer utilizing an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIG. 1 , a semiconductor device structure  10  is shown that includes a semiconductor crystal  12  epitaxially grown and processed so as to form a mesa structure  14  from p-type semiconductor epi layers  16 . The semiconductor  12  typically includes several p-type semiconductor layers  16  having varying compositions, including a surface layer  20  made of, for example, GaAs. Other surface layers may be used depending on the device made. For example, InGaAs surface layers can be used for InP based semiconductor devices. The various epi layers  16  are typically grown on an n-type substrate  18 . The mesa structure  14  may be trapezoidal as shown in  FIGS. 1 and 2  or it may have other shapes as suitable for different applications. A dielectric layer  22 , such as silicon oxide or silicon nitride, is deposited on top semiconductor surface  24  and is selectively removed above a portion  26  of the surface  24  of top layer  20  on the mesa  14 . A p-metal metallization layer  28  is applied to bare surface  26  of p-type layer  20  and surface  30  of silicon oxide layer  22 . Typically, the p-metal  28  has three layers  32 ,  34 ,  36  comprised of Ti, Pt, and Au, respectively; however other p-metal layer combinations can also be used. The metal/semiconductor structure  10  is annealed in order to form a secure mechanical and electrical connection between the metallization layer  28  and the semiconductor surface and silicon oxide layers  20 ,  22 . The structure  10  may undergo additional processing steps and may then become suitable for use in various applications, such as semiconductor lasers. 
     Referring to  FIG. 2 , in an exemplary embodiment of the present invention, a metal/semiconductor semiconductor device structure  38  is shown. Similar to structure  10 , structure  38  includes a multi-layer p-metal  40 , such as three layers  32 ,  34 , and  36  made from Ti, Pt, and Au, respectively, a dielectric layer  22  such as silicon oxide or silicon nitride, as well as multiple semiconductor layers  16  epitaxially grown on an n-type substrate  18 . The structure  38  also includes an additional thin Pt layer  42  formed between p-metal  40  and surfaces  26 , of p-type semiconductor and silicon oxide layers  20 ,  22 . Structure  38  may also take the form of a planar structure or depressed structure (not shown) as well. The layer  42  has a deposited thickness of 50 Å or less and causes the structure  38  to exhibit several superior characteristics over structures having layers of Pt with thicknesses larger than 50 Å or having no additional Pt layer ( FIG. 1 ). More particularly, Pt layers  42  having a thickness of about 15-25 Å exhibit highly desirable characteristics overall. 
     Referring now to  FIG. 3 , typical processing steps for metal/semiconductor structures  10 ,  38  are shown including steps for methods of the present invention. Using various semiconductor growth techniques, such as metalorganic chemical vapor deposition (MOCVD), several semiconductor layers  16  are epitaxially grown, as per step  50 , on a semiconductor substrate  18 . The surface of the semiconductor is etched, as per step  52 , through one or more layers  16  using suitable processing techniques such as lithography and acid-etching in order to form mesa structures  14 . Silicon oxide  22  is deposited, as per step  54 , on the surface  24  of the semiconductor to form an insulating barrier and then a contact portion  26  is exposed by removing the silicon oxide using lithography and acid etching or other suitable techniques. A metallization layer  40  is deposited, as per step  56 , on surfaces  26 ,  30  of p-type and silicon oxide layers  20 ,  22  using conventional techniques, such as electron beam evaporation. However, as opposed to using a series of three layers Ti/Pt/Au or other suitable combination, a thin layer of Pt  42  is deposited first and other p-metal layers, such as Ti/Pt/Au, are deposited subsequently. 
     After depositing the Pt/Ti/Pt/Au p-type metallization layer  40 , the metal/semiconductor structures are annealed, as per step  58 , in order to form a low resistance ohmic contact and secure the layer  40  to the exposed contact surface  26  and the surface  30  of the insulating silicon oxide layer  22 . In contradistinction to more commonly applied metallization layers requiring annealing times of approximately 20 minutes at a temperature of 400° C., the Pt/Ti/Pt/Au metallization layer  40  achieves superior adhesion and low resistance results when annealed at between 375 and 425° C. for only 1 minute. Thus, when warm-up and cool-down times of approximately five minutes each are included, the Pt/Ti/Pt/Au metallization layer  40  requires approximately two thirds less time in the annealing process than conventional Ti/Pt/Au metallization layers  28 . The substantial reduction in annealing time results in significantly improved manufacturing throughput. 
     One result of the annealing process step  58  is the formation of an ohmic contact at exposed mesa surface  26 . In order for the contact to be ohmic, the contact resistance must be low enough so as not to have a significant effect on the operation of the device and the I-V characteristics across the contact should be as linear and symmetric (positive and negative bias) as possible. After annealing the Pt/Ti/Pt/Au metallization layer, an ohmic contact is formed that exhibits linear and symmetric I-V characteristics and that exhibits contact resistances in a range between 0.7 and 2.7 μΩ-cm 2 . Such contact resistances are similar to those achieved for structures which do not have Pt layer  42 , and more importantly, the contact resistances achieved are much smaller compared to the resistances associated with other portions of the package, such as across the p-n junction or substrate. 
     The annealing process step  58  also results in an increased mechanical strength or adhesion between the metallization layer  40  and the layers of silicon oxide  22  and surface p-type semiconductor  20 . As can be seen in  FIG. 4 , the die shear force strength curve  70  achieved by the metallization layer  40  using 20 Å samples is significantly higher, having approximately two times higher average strength, than the strength curve  72  of conventional layers  28 , such as Ti/Pt/Au, which omit thin Pt layer  42 . Additionally, the strength curve  70  for metallization layer  40  has a more Gaussian and symmetric shape, yielding more reliable strength behavior for semiconductor structures using layer  42  as well as devices that include those structures and processes that work with those structures. 
     Referring now to  FIGS. 5-6 , images are shown of p-type GaAs after destructive testing which further indicate the improved adhesion strength characteristics introduced by additional Pt layer  42 .  FIG. 5  shows a plan view of semiconductor mesa structures  14  separated by channels  74 . A line  76  is scribed across the structures  14  that have a p-metal  40  deposited thereon and annealed. Tape is placed over top surface  48  and aggressively peeled away to cause damage to the deposited layers  40  and semiconductor structure. After peeling the tape, dark regions  78  are revealed that are underlying damaged GaAs p-type semiconductor  20 . Thus, instead of the p-metal layer  40  failing at an interface between the p-type semiconductor  20  and the p-metal  40 , portions of the underlying p-type semiconductor  20  are removed. In contrast,  FIG. 6  shows a plan view of similar semiconductor mesa structures  14  with deposited p-metal  28  omitting thin Pt layer  42 . Without layer  42 , some p-metal  28  is stripped away to reveal the interface (gray color) between the p-metal  28  and the underlying p-type semiconductor  20 , i.e., the surface  26  of the p-type semiconductor  20 . Consequently, the more conventional p-metal layer  28  is peeling from the semiconductor surface and failing before the stronger p-type semiconductor lattice  20  to which layer  28  should remain secured. 
     When the thickness of Pt layer  42  is greater than 50 Å, the adhesion between the p-metal  46  and SiO2 is adversely affected. Similar destructive tests were used for such thicker Pt layers and the result is shown in  FIG. 7 . A line  76  was scribed across metalized mesa structures  14  (five shown) and top surface  48  was taped and peeled. Similar dark regions  78  reveal where p-metal was removed along with portions of underlying p-type semiconductor  20  in accordance with a preferred failure. However, also shown are gray regions  80  where the surface  30  of silicon oxide layer  22  was revealed. Hence, the Pt layer  42  thickness is up-limited by the adhesion between the Pt layer and silicon oxide. 
     After annealing processing step  58 , thick gold is coated, such as by plating, as per step  60 , over the metallization layer  40  typically to a thickness of a couple of μm. The structure  38  is lapped and polished, as per step  62 , on the bottom end (not shown) in preparation for subsequent processing steps. An n-metal is typically deposited, as per step  64 , on the underside  44  of the structure  10 ,  38  and the devices are cleaved, and if it is a laser, its facet is usually coated, as per step  66 . The resulting devices may then be attached, as per step  68 , to an additional substrate for further processing. There are various applications for a finished semiconductor device, including as a semiconductor laser. Performance characteristics of laser diodes made from semiconductor structures  38  utilizing the Pt layer  42  in the metallization layer  40  match or are very close to the characteristics of devices without the Pt layer  42 . Shown in  FIG. 8  are L-I-V curves  82  for laser diodes utilizing the Pt layer  42  as well as overlapping L-I-V curves  84  for laser diodes using a conventional Ti/Pt/Au layer  28 . The results of catastrophic optical damage testing are shown in  FIG. 9 . As shown by comparison of curve  86  representing diode laser performance for devices using layer  42  and curve  88  representing diode laser performance for reference devices using a conventional metallization layer, minimal differences are observed. Thus, the overall performance of devices utilizing the Pt layer  42  demonstrate similar threshold current, slope, and burn-in stability as devices without the Pt layer. 
     However, devices such as laser diodes using layer  42  exhibit improved reliability over diodes that do not utilize layer  42  in Mil standard tests such as thermal cycling, shock, and vibration testing. After experiencing one hundred temperature cycles, vibration, and shock tests under Mil-Std-883, none of fifteen devices using Pt layer  42  shows degradation. For devices using conventional p-metal Ti/Pt/Au, one of fifteen failed after 38 temperature cycles, and five parts in fifteen showed increased thermal resistance. Moreover, with the addition of Pt layer  42 , the failure rate experienced during later device fabrication is substantially reduced, such as during cleaving and coating process step  66  and die bonding process step  68 . 
     It is thought that the present invention and many of the attendant advantages thereof will be understood from the foregoing description and it will be apparent that various changes may be made in the parts thereof without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the forms hereinbefore described being merely exemplary embodiments thereof.