Abstract:
A method for depositing ohmic contact material in a semiconductor, ridge type waveguide device is provided. Ohmic contact material is deposited on a semiconductor wafer and a ridge is fabricated with the deposited material and a first layer of photoresist material. A dielectric material layer is deposited on the ridge and a second photoresist material layer is deposited on the dielectric material layer. The second photoresist material layer is opened to expose the ohmic contact layer and any extra metal overhang is removed to expose the self-aligned ohmic contact layer on the ridge.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to devices and methods used in fiber optic networks and more particularly, to fabricating semiconductor ridge type waveguide devices.  
           [0003]    2. Background  
           [0004]    Semiconductor lasers and photodetectors are used extensively in fiber optic networks. Waveguides in such devices guide the flow of electromagnetic energy in a direction parallel to the waveguide axis. One type of waveguide used in such devices is the “ridge type waveguide.” FIG. 1A shows a cross-sectional view of a conventional ridge type waveguide  100 . Turning in detail to FIG. 1A, a laminated structure is sequentially formed by a n-type cladding layer  104 , a waveguide core layer  103 , a p-type cladding layer  102  and an ohmic contact layer  101 , on a semiconductor substrate  105 . An electrode (not shown) is mounted on ohmic contact layer  101  and on the back surface of layer  105 .  
           [0005]    In the case of semiconductor lasers, a forward bias is applied between layer  102  and  104  that generates optical energy in layer  103 . In the case of a semiconductor photodetector, a reverse bias is applied between layer  102  and  104  and incident light (not shown) guided to layer  103  is converted into a photoelectric signal.  
           [0006]    Proper ohmic contact minimizes resistive heating in semiconductor lasers resulting from contact resistance, and also reduces the RC time constant for photodetectors. Typically, ohmic contact layer  101  is 1.0 to 2.5 microns wide.  
           [0007]    Conventional process steps for fabricating ohmic contact layer  101  are as follows:  
           [0008]    (i) a ridge shaped waveguide surface is created by chemically etching a semiconductor wafer;  
           [0009]    (ii) a dielectric material layer is deposited on the wafer;  
           [0010]    (iii) a photoresist material layer is added over the dielectric layer;  
           [0011]    (iv) an ohmic contact layer area is exposed using expensive and complicated photolithographic alignment and using UV light energy; and  
           [0012]    (v) thereafter, ohmic contact material is deposited on the ridge.  
           [0013]    [0013]FIGS. 1B through 1D illustrate the foregoing conventional process steps for fabricating ohmic contact layer  101 .  
           [0014]    Turning in detail to FIG. 1B, is a cross-sectional view of a ridge type, waveguide  106  with ohmic contact area  107  covered with photoresist material  108 . When UV light  109  is applied it creates an opening  107 A (using expensive and complicated photolithographic alignment process) which should be equal to ohmic contact area  107 , as illustrated in the cross-sectional view of FIG. 1C. Thereafter, as illustrated in FIG. 1D, ohmic contact material  110  is deposited on top of the exposed ridge  106 A and photoresist material layer  108 . The foregoing conventional techniques for depositing ohmic contact material  110  are based on physical and/or chemical vapor deposition techniques.  
           [0015]    The foregoing process has disadvantages. If opening  107 A (as shown in FIG. 1C) in the photoresist material layer  108 , after UV light  109  is applied, is smaller than ohmic contact layer area  107 , the net ohmic contact area is reduced. This increases contact resistance.  
           [0016]    If opening  107 A in photoresist material layer  108  is greater than ohmic contact area  107 , contact layer  110  may be electrically shorted to waveguide  106 . This may permanently damage the semi conductor wafer.  
           [0017]    Therefore, there is a need for a reliable process for fabricating the ohmic contact layer that does not depend upon expensive and complicated photolithographic alignment.  
         SUMMARY OF THE INVENTION  
         [0018]    There is provided in accordance with one aspect of the present invention, a method for depositing ohmic contact material in a ridge waveguide device that addresses the foregoing deficiencies. Ohmic contact material is deposited on a semiconductor wafer and a ridge is fabricated using a first layer of photoresist material. A dielectric material layer is deposited on the ridge and a second photoresist material layer is deposited on the dielectric material layer. The second photoresist material layer is opened to expose a self aligned ohmic contact layer.  
           [0019]    In accordance with another aspect of the present invention, there is provided a method wherein expensive and tedious photolithographic alignment is not required since ohmic contact material is deposited before the ridge is created, and the ohmic contact area is self-aligned with respect to the ridge.  
           [0020]    This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description of the preferred embodiments thereof in connection with the attached drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1A described above, is an illustration of a typical ridge type waveguide structure.  
         [0022]    [0022]FIGS. 1B through 1D as described above, illustrate conventional process steps for depositing ohmic contact material on a ridge of a ridge type waveguide device.  
         [0023]    [0023]FIG. 2 is a flow diagram showing process steps for depositing ohmic contact material in an embodiment according to the present invention.  
         [0024]    [0024]FIG. 3 is a cross-sectional view of a semiconductor starting wafer showing ohmic contact material deposited on the wafer, according to an embodiment of the present invention.  
         [0025]    [0025]FIG. 4 is a cross-sectional view of the wafer in FIG. 3 showing photoresist material deposited on the photodetector wafer.  
         [0026]    [0026]FIG. 5 is a cross-sectional view of the wafer in FIG. 4 showing an etched ridge on the wafer.  
         [0027]    [0027]FIG. 6 is a cross-sectional view of the wafer in FIG. 5 showing a dielectric layer deposited on the etched wafer surface.  
         [0028]    [0028]FIG. 7 is a cross-sectional view of the wafer in FIG. 6 showing photoresist material deposited on the dielectric layer with an opening.  
         [0029]    [0029]FIG. 8 is a cross-sectional view of the wafer in FIG. 7 showing metal overhang after the photoresist layer is removed.  
         [0030]    [0030]FIG. 9 is a cross-sectional view of the wafer in FIG. 8 showing a self aligned ohmic contact layer deposited over the ridge. 
     
    
       [0031]    Features appearing in multiple figures with the same reference numeral are the same unless otherwise indicated.  
       DETAILED DESCRIPTION  
       [0032]    In one aspect of the present invention a process is provided such that expensive and tedious photolithographic alignment is not required and a self-aligned ohmic contact layer is created. Referring to the process flow diagram of FIG. 2 for depositing ohmic contact material, according to one aspect of the present invention comprising the steps of; depositing ohmic contact material on a semiconductor wafer, forming a ridge on the semiconductor by a first layer of photoresist material; creating an opening in the dielectric material and removing any extra photoresist material; and removing any extra metal overhang, leaving a self aligned ohmic contact layer on the ridge created on the semiconductor wafer.  
         [0033]    Turning in detail to FIG. 2, in step S 201 , ohmic contact material is deposited on a semiconductor wafer (“wafer”). FIG. 3 shows ohmic contact material  301  deposited on wafer  300  on area  302  (“Ohmic area  302 ”). Ohmic area  302  is greater than the ohmic contact area  305  (FIG. 5). Typically, ohmic area  302  width, ranges approximately between 3.5 to 4.5 microns. It is noteworthy that the present invention is not limited to any particular range of ohmic area  302  width, and may be used without limitation, for any ohmic area  302  width.  
         [0034]    Various techniques may be used to deposit ohmic contact material in step S 201 . Some of the techniques, without limitation, are: sputtering or vacuum evaporation (physical deposition techniques) or chemical vapor deposition techniques. It is noteworthy that the invention is not limited to foregoing processes, other processes may be used to deposit ohmic contact material in step S 201 .  
         [0035]    Sputtering involves the use of plasma to introduce a source material into a vapor state. The plasma consists of high density gaseous ions. When the plasma strikes the surface of the source material, it has enough energy to erode particles of the source material into a vapor or gaseous phase. The vapor phase can then be deposited.  
         [0036]    Vacuum evaporation may also be used to deposit ohmic contact material in step S 201 . Aluminum and gold are heated to the point of vaporization, and then evaporated to form a thin film covering wafer  300 . All ohmic contact material  301  is deposited under vacuum or very controlled atmosphere  
         [0037]    Chemical vapor deposition is a process by which insulating or conducting films are deposited on a substrate by using reactant gases and an energy source that produces a gas-phase chemical reaction. The energy source may be thermal, optical or plasma in nature. Plasma enhanced, chemical vapor deposition (PECVD) may be used to deposit conducting films, like ohmic contact material  301 .  
         [0038]    In step S 202 , a ridge is formed on wafer  300 . Photoresist material  303  and ohmic contact material  301  are used as masks to etch two channels to form the desired ridge. FIG. 4 shows photoresist material  303  deposited on wafer  300 . FIG. 5 shows ridge  304  with ohmic contact area  305  where ohmic contact area  305  is less than ohmic area  302 ; typically the width of ohmic contact area  305  is 1.0 to 2.5 microns. It is noteworthy that the present invention is not limited to any particular range of ohmic contact area  305  width, and may be used without limitation, for any ohmic contact area  305  width.  
         [0039]    In step S 203 , dielectric material is deposited on ridge  304 . Generally PECVD process is used to deposit the dielectric material. Other methods for depositing dielectric material are vacuum evaporation and sputtering. FIG. 6 shows dielectric material layer  306  deposited on ridge  304 . It is noteworthy that the present invention is not limited to PECVD process, and may be used without limitation, with any other process that may deposit dielectric material layer  306  deposited on ridge  304 .  
         [0040]    In step S 204 , a second layer of photoresist material is deposited on dielectric material layer  306 . PECVD process may be used to deposit the photoresist material. FIG. 7 shows photoresist material layer  307  deposited on dielectric material layer  306  after an opening in the photoresist is made over the ridge. It is noteworthy that the present invention is not limited to the PECVD process, and may be used without limitation, with any other process that may deposit photoresist material layer  307  on dielectric material layer  306 .  
         [0041]    In step S 205 , an opening is made in the dielectric material over the ridge and photoresist material is removed. The width of the opening is defined by the photoresist in step S 204 . FIG. 8 shows metal overhang  308  after photoresist material layer  307  is removed.  
         [0042]    Photoresist material (in step S 205 ) may be removed by dry etching. Typically, dry etching uses gas-phase reactants, inert or active ionic species or a mixture of the foregoing to remove unprotected layers of a substrate by chemical processes, physical processes, or a combination of these, respectively. Dry etching is an anistropic etch process, such that the etch rate may be varied in different directions. Plasma etching is a common dry etch technique that uses a RF plasma to generate chemically active etchants that form volatile etch species with the substrate. Ion etching is another example of dry etching that uses inert species (e.g. Ar ions) either in a beam or with a parallel plate sputtering system. Commercial photoresist stripper or acetone may be used to remove any extra photoresist material left after the foregoing opening is created. It is noteworthy that the invention is not limited to removing photoresist material by dry etching, any other process may be used to remove photoresist material  307 .  
         [0043]    In step S 206 , commercial ultrasonic process or solvent spray is used to remove any extra metal overhang, leaving a self-aligned, ohmic contact layer on ridge  304 . FIG. 9 shows a self-aligned ohmic contact layer  309  over ridge  304 .  
         [0044]    One aspect of the present invention is that expensive photolithographic alignment is not required because ohmic contact material is deposited before the ridge is created, and hence the ohmic contact area is self-aligned with respect to the ridge.  
         [0045]    While the present invention is described above with respect to what is currently considered its preferred embodiments, it is to be understood that the invention is not limited to that described above. To the contrary, the invention is intended to cover various modifications and equivalent arrangements within the spirit and scope of the appended claims.