Patent Application
20080042145
In order to prevent the silver from migrating, the structure includes a first layer 15 of titanium tungsten (TiW) alloy on the reflecting metal layer 13. A layer of titanium tungsten nitride (TiWN) 16 is on the first titanium tungsten layer 15, and a second layer of titanium tungsten 17 is on the titanium tungsten nitride layer opposite from the first titanium tungsten layer 15. As illustrated in
Although the schematic illustration of
Even though
The titanium tungsten nitride layer 16 provides a favorable barrier against migration of the reflecting metal layer 13. The adhesion properties of the titanium tungsten nitride layer 16 are less favorable, however, than the adhesion properties (to adjacent layers) of titanium tungsten and thus the titanium tungsten layers 15, 17 provide an additional structural advantage as well as forming part of the overall barrier.
The reflecting metal layer 13 is typically silver, but can be selected from any other appropriately reflecting metal, example of which include gold, silver, aluminum, and combinations of these metals.
The barrier layers 15, 16 and 17 have a total thickness that is sufficient to prevent migration or diffusion of the reflecting metal from the reflecting metal layer 13 into the remainder of the diode 10, but less a thickness at which the resulting stress would encourage delamination and related structural problems in the titanium-containing layers 15, 16, and 17. Those familiar with the growth of epitaxial layers of semiconductors and related thin materials will recognize that the barrier layers only need to be thick enough to accomplish the intended purpose. Once the barrier is thick enough to prevent migration, increasing the layer thickness may tend to increase the physical stress within each layer without any added benefit as a barrier.
Generally, successful barriers have been formed with the titanium tungsten layers 15, 17 each being about 1000 angstroms (Å) thick and the titanium tungsten nitride layer being about 2000 Å thick.
In exemplary embodiments, the semiconductor epitaxial layers 11 and 12 are Group III nitrides. Group III nitrides include those compounds of gallium, aluminum, indium and nitrogen that form binary, ternary, and quaternary compounds. The selection of any one or more of these layers for homojunctions, heterojunctions, single or multiple quantum wells, or superlattice structures, is a matter of choice when used in conjunction with the present invention. Thus the present invention can incorporate any number of such compounds or layers. In some embodiments, the epitaxial layers are gallium nitride (GaN), while in others they are aluminum gallium nitride (AlGaN) or indium gallium nitride (InGaN).
Those of skill in the art recognize that these formulas are more precisely expressed as AlxGa1-xN or InxGa1-xN. In particular, because the band gap of InxGa1-xN changes based upon the mole fraction of indium in the compound, InGaN diodes can be produced with output at a desired wavelength by correspondingly selecting the proper mole fraction of indium.
In
As in
As in the previously described embodiment, the reflecting metal layer 26 is most typically selected from the group consisting of gold, silver, aluminum, and combinations thereof. Although not illustrated, because of the relative size of
The diode 24 corresponds in its general structure to the XBRIGHT® series of diodes available from Cree, Inc., the assignee herein. Because these diodes are in the flip chip orientation, their method of manufacture and resulting structure often include a submount structure which
In partial summary, the invention is a layer of titanium tungsten nitride sputter-deposited as a compound between two layers of titanium tungsten alloy. This prevents diffusion of metal or moisture through the layers. The titanium tungsten nitride compound acts as a barrier and prevents metals such as gold, silver, aluminum from diffusing, even during or after heat treatment. As a result, this barrier can replace more elaborate or expensive barriers such as platinum in current barrier layers resulting in large cost savings. Although the bordering layers of titanium tungsten do not by themselves form the barrier to silver migration, they do provide adhesion layers for incorporating the barrier more easily and functionally into device designs.
The invention also includes the method of forming the light emitting diode structure. In particular, the method comprises a first step of depositing a layer of titanium tungsten on the diode precursor structure (i.e.; including the active structure described herein in terms of the epitaxial layers 11 and 12) at a temperature below the temperature that would otherwise interfere with the structure or function of the light emitting diode.
A second step comprises depositing a layer of titanium tungsten nitride on the first titanium tungsten layer and again at a temperature below the temperature that would otherwise interfere with the structure or function of the light emitting diode. A third step comprises depositing a second layer of titanium tungsten on the titanium tungsten nitride layer and again carrying out the depositing step at a temperature below the temperature that would otherwise interfere with the structure or function of the light emitting diode.
In exemplary embodiments, the TiW and TiWN layers are deposited by sputtering. The nature, concept and specific steps of sputter deposition are well understood in this art and will not be described in detail. In general, a relatively high voltage is applied across a low pressure gas, such as argon (Ar) at about 5 milliTorr, to create a plasma. During sputtering, the energized plasma atoms strike a target composed of the desired coating material and cause atoms from that target to be ejected with enough energy to travel to, and bond with, the desired substrate.
Currently, and in the method of the invention, a favored sputtering technique uses pulsed direct current (DC) power. The use of pulsed DC power (as opposed to continuous DC power or RF power) for thin-film deposition in semiconductor manufacturing is generally well understood in this art. Helpful discussions can be found in numerous sources including, Belkin et al., Single-Megatron Approach Reactive Sputtering of Dielectrics, Vacuum Technology & Coating, September 2000, or from magnetron and power supply manufactures such as Advanced Energy Industries, Inc. of Fort Collins, Colo. 80525 USA (www.advanced-energy.com) or Angstrom Sciences, Inc. Duquesne Pa. 15110 USA (www.angstromsciences.com).
As described in these sources and as understood in this art, pulsed DC sputtering techniques can be carried out as cold-momentum transfer processes and thus avoid the effects of high temperature on the substrate or the coating, which high temperatures tend to be produced by other forms of sputtering. Additionally, pulsed DC sputtering can be used to apply either conductive or insulating materials to a wide variety of substrates including metals, semiconductors, ceramics, and even heat-sensitive polymers.
In further detail, the titanium tungsten nitride (TiWN) layer is produced by reactive ion sputtering using the pulsed DC technique. Reactive ion sputtering includes a deposition source material in the plasma gas. Thus, the titanium tungsten nitride layer is formed by sputtering titanium and tungsten from respective solid sources in the presence of both argon and nitrogen gas.
In particular, the respective deposition steps are carried out below the dissociation temperature of the semiconductors that form the epitaxial layers. Furthermore, the deposition steps should be carried out below temperatures that would encourage unwanted side effects such as dopant migration within the active layers, or activation of elements, states, or defects within the epitaxial layers, all of which can affect the electronic behavior of the active structure or can physically interfere with the emission of light from the resulting diode.
Because gallium nitride tends to dissociate above temperatures of about 600° C. (depending upon ambient conditions) the deposition steps should be carried out below this temperature and preferably below about 500° C.
The adjustment of the sputter deposition process to meet these requirements is generally well understood in the art. Some of the relevant parameters include the target power density, the current applied to the electromagnets in the deposition system, the flow and partial pressure of argon (and where appropriate nitrogen), the deposition temperature, and the substrate rotation. Those of skill in this art will recognize that the exact adjustment of each of these parameters can and will differ from system to system, but that the deposition can be carried out without undue experimentation.
The sputter deposition is typically carried out using a titanium tungsten alloy target and, for the titanium tungsten nitride layer, nitrogen in the argon atmosphere. The composition of the resulting coatings can be expressed as TixWy or as TixWyNz. For the TiW layers, X is between about 0.6 and 0.7 (60 and 70 mole percent) with Y as the remainder. For titanium tungsten nitride, X is between about 0.3 and 0.45, Y is between about 0.3 and 0.4, and Z is between about 0.25 and 0.3.
The quality of the resulting layers expressed in terms of non-migration of the silver, can be identified using the following procedures.
The titanium tungsten nitride layers were characterized in the following manner. Two 3-inch liftoff monitors were placed in two rows on a pallet of SEGI. Two thermally oxidized 3-inch wafers were placed in two rows on the pallet of SEGI. Two double-side polished thin 3-inch silicon wafers were placed in two rows on a pallet of SEGI. The inner wafer edge of all wafers was 0.5 inches from the inner edge of the pallet. The titanium tungsten nitride alloy was sputter deposited using pulsed DC in ten experiments as indicated in Table 1. The thickness was measured from the liftoff monitor using P10. Sheet resistance was measured using a four-point probe on thermal oxide monitors. Stress was calculated from pre- and post-bow measurements on opposite sides of the film on the thin silicon wafer. Bulk resistance was calculated from thickness and sheet resistance measurements.
Table 2 provides ellipsometer measurements used to evaluate the resulting structures. The angle measurement was taken with a Gaertner Ellipsometer (Gaertner Scientific, Skokie, Ill. 60076, USA) and proved that the TiWN layer is a solid barrier for Au/Ag diffusion. As Table 2 illustrates, Ψ and Δ remained substantially identical after heat treatment. Wafers were then put in vacuum oven at 350° C. and Au was ellipsometric spectra monitored.
No interaction between the TiWN and Au was observed for any of the wafers.
In the drawings and specification there has been set forth a preferred embodiment of the invention, and although specific terms have been employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.
