Techniques for fabricating and packaging multi-wavelength semiconductor laser array devices (chips) and their applications in system architectures

Abstract

Phase masks which can be used to make both linear and curved gratings of single or multiple submicron pitches, with or without any abrupt quarter-wavelength shifts (or gradually varying finer phase shifts) simultaneously on the wafer/substrate. The phase masks are made using direct write electron or ion-beam lithography of two times the required submicron pitches of linear and curved gratings on commercially available π phase-shifting material on a quartz substrate and wet or dry etching of the π phase-shifting material. The phase masks can be used in connection with making multi-wavelength laser diode chips. The laser diodes have a ridge structure with metal shoulders on either side of the ridge. The laser diode chip, with different wavelength lasers, is bonded and interfaced to a novel microwave substrate that allows for high signal-to-noise ratio and low crosstalk. The substrate is packaged in a low loss rugged housing for WDM applications.

What is claimed is:  
     
         1 . A method of forming a phase mask for use with light of a particular wavelength, the method comprising the steps of: 
 providing a substrate having a layer of phase-shifting material thereon;    coating the layer of phase-shifting material with a resist material;    patterning the resist material using electron beam or ion beam lithography to define a mask grating pattern with submicron pitch;    etching the exposed phase-shifting material to expose substrate material; and    removing the resist material to reveal regions of phase-shifting material alternating with regions of exposed substrate material according to the mask grating pattern.    
     
     
         2 . The method of  claim 1  wherein the layer of phase-shifting material causes light of the particular wavelength passing through the substrate and a given region of phase-shifting material to be 180 degrees out of phase with light of the particular wavelength passing through an adjacent region of exposed substrate.  
     
     
         3 . The method of  claim 1  wherein the resist material is a multi-layer structure.  
     
     
         4 . The method of  claim 1  wherein the mask grating pattern includes a plurality of mask grating pitches.  
     
     
         5 . The method of  claim 4  wherein the plurality of mask grating pitches are in separate regions of the phase mask.  
     
     
         6 . The method of  claim 1  wherein the mask grating pattern includes a phase shift corresponding to half the mask grating pitch.  
     
     
         7 . The method of  claim 1  wherein the mask grating pattern includes at least one curved grating pattern or at least one grating of continuously varying pitch.  
     
     
         8 . The method of  claim 1  wherein the patterning step is carried out using multiple passes at a partial dose to reduce sub-field and field stitching errors.  
     
     
         9 . The method of  claim 1  wherein the etching step includes submicron anisotropic reactive ion (magnetron enhanced) etching of the exposed phase-shifting material utilizing a gas mixture including chlorine and oxygen.  
     
     
         10 . The method of  claim 9  wherein the gas mixture includes 80%-90% chlorine and 10%-20% oxygen.  
     
     
         11 . The method of  claim 1  wherein: 
 the resist material includes germanium; and  
 the patterning step includes removing native germanium oxide with deionized water, followed by an isotropic reactive ion etching of the germanium with carbon tetrafluoride gas.  
 
     
     
         12 . The method of  claim 1  wherein: 
 the resist material includes silicon; and  
 the patterning step includes an isotropic reactive ion etching of the silicon with carbon tetrafluoride gas.  
 
     
     
         13 . The method of  claim 1  wherein the patterning step includes etching the resist material with oxygen gas.  
     
     
         14 . The method of  claim 1  wherein the submicron pitch is less than 400 nm.  
     
     
         15 . A method of forming a phase mask for use with light of a particular wavelength, the method comprising the steps of: 
 providing a substrate;    coating the substrate with a resist material;    patterning the resist material using electron beam. or ion beam lithography to define a mask grating pattern with submicron pitch;    etching the exposed substrate to a specified depth; and    removing the resist material to reveal regions of etched substrate alternating with regions of unetched substrate material according to the mask grating pattern;    wherein the specified depth is such that light of the particular wavelength passing through a given region of etched substrate is 180 degrees out of phase with light of the particular wavelength passing through an adjacent region of unetched substrate.    
     
     
         16 . The method of  claim 15  wherein the resist material is a multi-layer structure.  
     
     
         17 . The method of  claim 15  wherein the mask grating pattern includes a plurality of mask grating pitches.  
     
     
         18 . The method of  claim 17  wherein the plurality of mask grating pitches are in separate regions of the phase mask.  
     
     
         19 . The method of  claim 15  wherein the mask grating pattern includes a phase shift corresponding to half the mask grating pitch.  
     
     
         20 . The method of  claim 15  wherein the mask grating pattern includes at least one curved grating pattern.  
     
     
         21 . The method of  claim 15  wherein the patterning step is carried out using multiple passes at a partial dose to reduce sub-field and field stitching errors.  
     
     
         22 . The method of  claim 15  wherein the etching step includes reactive ion etching with a gas mixture including carbon tetrafluoride and argon.  
     
     
         23 . The method of  claim 15  wherein the gas mixture includes 95%-98% carbon tetrafluoride and 2%-5% argon.  
     
     
         24 . A method of fabricating a desired device grating structure in a semiconductor optical device, the device grating structure having features characterized by one or more desired pitch values, the method comprising the steps of: 
 providing a phase mask having a corresponding mask grating structure with features corresponding to the desired device grating structure but characterized by one or more pitch values, each pitch value of the mask grating structure being twice the corresponding pitch value of the desired device grating structure;    the features of the mask grating structure being defined by alternating regions having alternating first and second optical thicknesses;    providing a substrate having at least portions of the semiconductor device formed therein, said substrate being covered with a photoresist material;    disposing the phase mask proximate or in contact with the substrate;    illuminating the phase mask with normally incident light of a particular wavelength so as to expose the photoresist on the substrate;    the particular wavelength being such that light of the particular wavelength traveling through one of the alternating regions of the phase mask and light traveling through an adjacent one of the alternating regions of the phase mask are 180 degrees out of phase;    whereupon the light encountering the photoresist is characterized by an intensity distribution having pitch values that are half the corresponding pitch values of the mask grating features, which intensity distribution corresponds to the desired device grating structure;    developing the photoresist; and    etching the substrate to impose the desired device grating structure on the substrate.    
     
     
         25 . The method of  claim 24  wherein the phase mask features are defined by alternating regions of (a) a phase-shifting material on a substrate material, (b) the substrate material without phase-shifting material.  
     
     
         26 . The method of  claim 24  wherein the phase mask features are defined by etched regions of a mask material alternating with unetched regions of the mask material.  
     
     
         27 . The method of  claim 24  wherein each pitch value of the desired device grating structure is less than 200 nm.  
     
     
         28 . The method of  claim 24  wherein the normally incident light is coherent.  
     
     
         29 . The method of  claim 24  wherein the normally incident light is incoherent.  
     
     
         30 . A high power unstable resonator laser including a curved grating manufactured by method of  claim 24  in combination with a laser diode having curved facets.  
     
     
         31 . A vertically focused laser for launching light into a remote optical fiber comprising a curved grating fabricated according to the method of  claim 24  in combination with a laser diode.  
     
     
         32 . In a method of producing a semiconductor laser diode chip having a semiconductor substrate and a multi-layer laser structure formed on the substrate, the improvement comprising producing a low stress, dense, and low hydrogen content silicon nitride layer using a plasma enhanced CVD process.  
     
     
         33 . In a method of producing a semiconductor laser diode chip having a semiconductor substrate and a multi-layer laser structure formed on the substrate, the improvement comprising producing a layer of organic cyclotene as an insulating layer.  
     
     
         34 . A method of establishing reliable metallic content to p-doped semiconductor comprising: 
 sequentially depositing layers of titanium, titanium nitride, platinum, and gold.    
     
     
         35 . A method of establishing reliable metallic content to n-doped semiconductor material comprising sequentially depositing layers of nickel, germanium, gold, nickel, silver, and gold.  
     
     
         36 . A method of establishing reliable metallic content to n-doped semiconductor material comprising sequentially depositing layers of germanium, gold, nickel, tungsten silicide, titanium, and gold.  
     
     
         37 . In the fabrication of a semiconductor laser having at least one light transmitting facet, the improvement comprising removing native oxide on the facet using a low-power, broad-area argon ion beam or using a low-energy, low-pressure electron cyclotron resonance to produce sequential hydrogen, nitrogen, and argon plasmas.  
     
     
         38 . A laser diode chip comprising: 
 a semiconductor substrate;    a multi-layer laser structure formed on said substrate, said laser structure bounded by an upper surface having a first and second spaced trenches defining a ridge waveguide therebetween, said ridge waveguide extending along a direction of light propagation; and    first and second metal shoulders formed on said outer surface at locations proximate said trenches and separated from said ridge waveguide by said trenches, said shoulders extending above said top surface so as to protect said ridge waveguide.    
     
     
         39 . The laser diode chip of  claim 38  wherein said laser structure includes a grating disposed below said ridge waveguide, said grating being disposed in a plane parallel to said upper surface and having grating lines extending in a direction perpendicular to said direction of light propagation.  
     
     
         40 . The laser diode chip of  claim 39  wherein said laser grating structure includes a phase shift region corresponding to half the grating pitch.  
     
     
         41 . The laser diode chip of  claim 39 , and further comprising at an additional laser structure formed on said substrate, said additional laser structure including an additional ridge waveguide and an additional grating disposed below said additional ridge waveguide, said additional grating having a different pitch than said first-mentioned grating.  
     
     
         42 . An antireflection coating for laser diode facets comprising Ta 2 O 5  (tantalum oxide) and Al 2 O 3  (aluminum oxide).  
     
     
         43 . A laser chip module comprising: 
 a housing having a plurality of pins for communicating signals from outside said housing to within said housing;    dielectric substrate mounted in said housing, the substrate having an upper surface and a metallized lower surface;    a laser chip mounted to said upper surface of said substrate;    first and second conductive signal lines on said upper surface of said substrate, said signal lines extending from particular first and second input pins to respective locations at or near said laser chip;    the substrate being formed with metallized via holes electrically connected to said metallized lower surface, said via holes forming a pattern such that each of said signal lines has a plurality of via holes on either side and at least some of said via holes are located between said signal lines; and    metal structures located above said substrate and electrically connected to said metallized lower surface by said via holes.    
     
     
         44 . The laser chip module of  claim 43  wherein said transmission lines are of constant width.  
     
     
         45 . The laser chip module of  claim 43  wherein said metal structures include metallized traces on said substrate and contacting said via holes.  
     
     
         46 . The laser chip module of  claim 44  wherein said metallized traces are of constant width.  
     
     
         47 . The laser chip module of  claim 44  wherein at least one of said metallized traces is tapered.  
     
     
         48 . The laser chip module of  claim 43  wherein: 
 said pattern of via holes include pairs of via holes distributed along each of said signal lines; and  
 said metal structures include wire arches extending from respective via holes on one side of said signal lines to respective via holes on the other side of said signal holes.  
 
     
     
         49 . The laser chip module of  claim 43  wherein the number of laser diodes is eight or higher.  
     
     
         50 . A network including nodes connected by fiber optic links, at least one link being terminated at a node with WDM equipment, the improvement wherein terminal equipment optically coupled to said WDM equipment comprises: 
 a multi-wavelength laser module having a multi-wavelength ridge laser diode array chip for providing independently modulated light at a plurality of wavelengths.