Abstract:
Methods and systems for growing uniform oxide layers include an example method including growing a first layer of oxide on first and second facets of the substrate, with the first facet having a faster oxide growth rate. The oxide is removed from the first facet and a second oxide layer is grown on the first and second facets. Removing the oxide from the first facet includes shielding the second facet and exposing the substrate to a deoxidizing condition. The second facet is then exposed to receive the second oxide layer. Areas having differing oxide thicknesses are also grown by repeatedly growing oxide layers, selectively shielding areas, and removing oxide from exposed areas.

Description:
BACKGROUND OF THE INVENTION 
   Silicon has increasingly been used in optical applications. Currently such optical components as waveguides, beam splitters, detectors, lasers, and the like can all be formed in silicon. Forming such components from silicon enables small high frequency response, lower energy components as well as large scale manufacturing using semiconductor fabrication methods. 
   Recently, thin silicon oxide layers have been grown on silicon components to form antireflective coatings on silicon in order to achieve a desired light path. In order to function properly, antireflective coatings must have a thickness matched to the wavelength of light used in the optical system. Any variation in the thickness of the antireflective coating can introduce unwanted reflections, attenuation, and other irregularities. Inasmuch as the optical spectrum is from 400 to 700 nanometers, achieving a specified antireflective coating requires extremely accurate manufacturing processes. 
   Silicon is a face centered cubic (FCC) crystal structure having 100, 110, and 111 plans, and permutations thereof. In the presence of oxygen, oxide layers will grow on facets parallel to the 100 and 111 planes at different rates. Accordingly, two facets on the same substrate that are parallel to the 100 and 111 planes, respectively, will have oxide layers of different thicknesses after having been exposed to oxygen for the same period of time. As a result, the facets will not bear oxide layers suitable for suppressing reflection of light at the same wavelength. 
   For example,  FIG. 1  illustrates a substrate  10  having a faceted upper surface having facets  12   a - 12   c  parallel to the 100 plane  14  of the silicon substrate and facets  16   a ,  16   b  parallel to the 111 plane  18 . For purposes of the disclosure the 100 plane  14  may mean any of the permutations of the 100 planes of an FCC material including the 100, 010, and 001 planes. Other facets formed on the substrate  10  may be parallel to the 110 plane and the 101 and 011 permutations thereof. 
   The substrate  10  is exposed to an oxidizing environment such that an oxide layer  20  is grown on the silicon substrate  10  will have a thickness  22  on facets  12   a - 12   c  that is less than a thickness  24  on facets  16   a ,  16   b  due to the faster growth rate of the 111 plane  18 . Oxide layers grown on the 110 plane may likewise have a thickness different than the thicknesses  22 ,  24 . 
   In view of the foregoing it would be an advancement in the art to provide a system and method for growing uniform oxide layers over both the 100 and 111 planes. Such a system and method should be capable of use in large scale manufacturing of silicon optical components. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention includes methods and systems for growing uniform oxide layers evenly over a silicon substrate. One method includes growing a first layer of oxide on first and second facets of the substrate, with the first facet having a faster oxide growth rate. The oxide is then removed from the first facet and a second oxide layer is grown on the first and second facets. Removing the oxide from the first facet includes shielding the second facet and exposing the substrate to a condition suitable for removing the oxide layer, such as a wet etching process. The second facet is then exposed to receive and a second oxide layer is grown on the first and second facets. Shielding the second facet includes applying a photoresist to the substrate and removing the photoresist from the first facet. Shielding may also include selectively metallizing the second facet. 
   Growing the first and second oxide layers includes exposing the silicon substrate to an oxidizing environment for first and second periods, respectively. The first period has a duration sufficient to grow oxide having a thickness about equal to S*(1−X/Y), where S is a final thickness of oxide grown on the second facet for a total period about equal to a sum of the first and second periods, X is a first growth rate for the second facet, and Y is a second growth rate of the first facet. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings. 
       FIG. 1  is side cross-section view of a silicon substrate having multiple facets and an oxide layer formed thereon; 
       FIG. 2  is process flow diagram of a method for uniform oxide layer formation, in accordance with an embodiment of the present invention. 
       FIG. 3  is a side cross-section view of the silicon substrate having portions of the oxide layer selectively removed from faster-growth facets, in accordance with an embodiment of the present invention 
       FIG. 4  is a side cross-section view of the silicon substrate having a second oxide layer formed thereon, in accordance with an embodiment of the present invention; 
       FIG. 5  is a chart of values for a parabolic rate constant B used to calculate oxide growth times, in accordance with an embodiment of the present invention; 
       FIG. 6  is a chart of values for a linear rate constant B/A used to calculate oxide growth times, in accordance with an embodiment of the present invention; 
       FIG. 7  is a process flow diagram of an alternative method for uniform oxide layer formation, in accordance with an embodiment of the present invention; 
       FIG. 8  is a side cross-section view of the silicon substrate having shielded slower-growth portions, in accordance with an embodiment of the present invention; 
       FIG. 9  is a process flow diagram of an alternative method for uniform oxide layer formation, in accordance with an embodiment of the present invention; 
       FIG. 10  is a side cross-section view of the silicon substrate having a photoresist layer being irradiated through a mask exposing faster-growth facets, in accordance with an embodiment of the present invention; 
       FIG. 11  is a process flow diagram of an alternative method for uniform oxide layer formation, in accordance with an embodiment of the invention; 
       FIG. 12  is a side cross-section view of the silicon substrate being metallized through a mask exposing slower-growth facets, in accordance with an embodiment of the present invention; 
       FIG. 13  is process flow diagram of a method for growing multiple-thickness oxide layers, in accordance with an embodiment of the present invention; 
       FIG. 14A-14D  are side cross-section views of the silicon substrate undergoing the method of  FIG. 13 , in accordance with an embodiment of the present invention; 
       FIG. 15  is a plot illustrating a means for timing shielding application in order to achieve a specific oxide layer thickness, in accordance with an embodiment of the present invention; and 
       FIG. 16  is a process flow diagram illustrating an alternative method for growing multiple-thickness oxide layers, in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 2 through 4  illustrate a method  26  for growing a uniform layer on a substrate notwithstanding the differences in growth rates. At block  28  a first layer, such as an oxide layer  20 , is grown on the substrate  10 . At block  30 , the layer is removed from faster-growth facets, such as facets  16   a ,  16   b , as shown in  FIG. 3 . Removing the layer from the faster-growth facets  16   a ,  16   b  may include removing the first silicon oxide layer  20  by means of a wet-etching process, or other such process. 
   At block  32 , a second oxide layer  34  is grown in addition to the first oxide layer  16 , as shown in  FIG. 4 . Oxidation in silicon proceeds inwardly into the bulk of the substrate. Accordingly, the second oxide layer  34  formed at block  32  is below the first oxide layer  20 . In other applications of the present invention, layers are deposited on the substrate  10  which are deposited at different rates over different planes. In such embodiments, the oxide layer  34  will be above the layer  16 . 
   The method  26  illustrated in  FIGS. 2 through 4  provides a means for equalizing growth rates across the slower-growth facets  12   a - 12   c  and the faster growth facets  16   a ,  16   b . In one embodiment, a uniform layer thickness across multiple planes is achieved by adjusting the thickness of the first oxide layer  16 . The first oxide layer has a thickness T 1  equal to the equation S*(1−X/Y), where S is the combined thickness of the first oxide layer  20  and the second oxide layer  34 , X is the slower growth rate, such as on the 100 plane  14 , and Y is the faster growth rate, such as on the 111 plane  18 . The second oxide layer  34  has a thickness T 2  equal to S−T 1 . 
   The thicknesses of the first oxide layer  20  and second oxide layer  34  may be controlled by adjusting the length of time that the substrate  10  is subject to an oxidizing environment. Accordingly, the first oxide layer  20  may be formed by exposing the substrate  10  to the oxidizing environment for a first period t 1  whereas the second oxide layer  34  is formed by exposing the substrate  10  to the oxidizing environment for a second period t 2 . The ratio of the second period relative to the sum of the first and second period (t 2 /(t 1 +t 2 )) will be equal to X/Y in order to achieve a uniform thickness. 
   For oxide layers having large thickness, such as above 5,000 Angstroms, the rate of oxide layer growth becomes significantly non-linear. Accordingly, the first and second periods may be adjusted to accommodate this nonlinearity. Equation 1 accommodates nonlinearity in oxide growth rates. A description of nonlinearity in silicon-oxide growth rates may be found in “Semiconductor Materials and Process Technology Handbook for Very Large Scale Integration and Ultra Large Scale Integration” (ed. Gary E. McGuire, Noyes Publications, Park Ridge, N.J.). 
   
     
       
         
           
             
               
                 t 
                 = 
                 
                   
                     
                       
                         x 
                         o 
                         2 
                       
                       - 
                       
                         x 
                         i 
                         2 
                       
                     
                     B 
                   
                   + 
                   
                     
                       
                         x 
                         o 
                       
                       - 
                       
                         x 
                         i 
                       
                     
                     
                       B 
                       A 
                     
                   
                 
               
             
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
                 : 
               
             
           
         
       
     
   
   Where t is the time required to form an oxide layer of thickness x O  over a silicon substrate bearing an oxide layer having a thickness x i . B is a parabaolic rate constant and B/A is a linear rate constant used to describe oxide growth. B and B/A are calculated according to Equations 2 and 3 or Equations 4 and 5. Alternatively, B and B/A may be determined by referencing measured values illustrated in  FIGS. 5 and 6 . 
   
     
       
         
           
             
               
                 A 
                 = 
                 
                   2 
                   ⁢ 
                   
                     
                       D 
                       eff 
                     
                     ⁡ 
                     
                       ( 
                       
                         
                           1 
                           k 
                         
                         + 
                         
                           1 
                           h 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
                 : 
               
             
           
           
             
               
                 B 
                 = 
                 
                   2 
                   ⁢ 
                   
                     D 
                     eff 
                   
                   ⁢ 
                   
                     
                       C 
                       k 
                     
                     
                       N 
                       1 
                     
                   
                 
               
             
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
                 : 
               
             
           
         
       
     
   
   Where D eff  is the effective oxidant diffusion constant in oxide, k and h are rate constants at the Si—SiO 2  and gas-oxide interfaces, C k  is an equilibrium concentration of the oxide species in the oxide, N 1  is the number of oxidant molecules in the oxide unit volume. 
   
     
       
         
           
             
               
                 B 
                 = 
                 
                   
                     C 
                     1 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       
                         E 
                         1 
                       
                       kT 
                     
                   
                 
               
             
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
                 : 
               
             
           
           
             
               
                 
                   B 
                   A 
                 
                 = 
                 
                   
                     C 
                     2 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     ⅇ 
                     
                       
                         E 
                         2 
                       
                       kT 
                     
                   
                 
               
             
             
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
                 : 
               
             
           
         
       
     
   
   Where T is the temperature at which the oxidation takes place expressed in degrees Kelvin and k is a rate constant at the Si—SiO 2  and gas oxide interfaces. 
   For the 111 plane of silicon under dry oxidation conditions 
   
     
       
         
           
             
               C 
               1 
             
             = 
             
               7.72 
               × 
               
                 10 
                 2 
               
               ⁢ 
               
                 
                   μ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     m 
                     2 
                   
                 
                 hr 
               
             
           
           , 
           
             
               C 
               2 
             
             = 
             
               6.23 
               × 
               
                 10 
                 6 
               
               ⁢ 
               
                 
                   μ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     m 
                     2 
                   
                 
                 hr 
               
             
           
           , 
           
             
               E 
               1 
             
             = 
             
               1.23 
               ⁢ 
               eV 
             
           
           , 
           
             
               E 
               2 
             
             = 
             
               2.0 
               ⁢ 
               
                   
               
               ⁢ 
               
                 eV 
                 . 
               
             
           
         
       
     
   
   For the 111 plane of silicon under wet oxidation conditions 
   
     
       
         
           
             
               C 
               1 
             
             = 
             
               3.68 
               × 
               
                 10 
                 2 
               
               ⁢ 
               
                 
                   μ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     m 
                     2 
                   
                 
                 hr 
               
             
           
           , 
           
             
               C 
               2 
             
             = 
             
               1.63 
               × 
               
                 10 
                 8 
               
               ⁢ 
               
                 
                   μ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   
                     m 
                     2 
                   
                 
                 hr 
               
             
           
           , 
           
             
               E 
               1 
             
             = 
             
               0.78 
               ⁢ 
               
                   
               
               ⁢ 
               eV 
             
           
           , 
           
             
               E 
               2 
             
             = 
             
               2.05 
               ⁢ 
               
                   
               
               ⁢ 
               
                 eV 
                 . 
               
             
           
         
       
     
   
   For the 100 plane of silicon 
   
     
       
         
           
             C 
             
               2 
               ⁢ 
               
                 ( 
                 100 
                 ) 
               
             
           
           = 
           
             
               
                 C 
                 
                   2 
                   ⁢ 
                   
                     ( 
                     111 
                     ) 
                   
                 
               
               1.7 
             
             . 
           
         
       
     
   
   In one embodiment of the method  26  the Equation 1 is used to calculate a time t 1  used at block  28  for the duration of oxide growth during formation of the first oxide layer  20 , such that when the second oxide layer  34  is grown thereunder at block  34  for a time t 2  all facets  12   a - 12   c  and facets  16   a ,  16   b  will have substantially uniform thickness. The thickness of the first oxide layer  20  grown on the slower-growth facets  12   a - 12   c  is used as x i  in Equation 1 as applied to the slower growth facets  12   a - 12   c  to determine a time t 2  for oxide growth forming the second oxide layer  34  at block  32 . Equation 1 is used to calculates a time t 2  for growing a second oxide layer at block  32  such that the thickness x 0  on the faster growth planes  16   a ,  16   b  and the thickness x 0  on the slower growth planes  12   a - 12   c  bearing the first oxide layer  20  of thickness x i  are equal to one another at a desired thickness. 
   For example, to achieve a thickness of about 10,150 Angstroms on both the slower growth facets  12   a - 12   c  and the faster growth facets  16   a ,  16   b  for a wet oxidation process carried out at 900° C., t 1  is equal to about 2.8 hours and t 2  is equal to about 10 hours. 
     FIGS. 7 and 8  illustrate a method  36  which is an alternative embodiment of the method  26  for uniform layer formation. In the method  36 , the substrate  10  is oxidized at block  38  by exposing the substrate to an oxidizing environment to yield a first layer, as in  FIG. 1 . The oxidizing environment may include the presence of oxygen at an elevated temperature. At block  40 , the slower growth facets are shielded, such as by a shielding layer  42  shown in  FIG. 8 . The shielding layer  42  may be a layer of photoresist, metal, or the like. The oxide is removed from the faster growth facets  16   a ,  16   b  at block  44 , as shown in  FIG. 3 . The shielding layer  42  is then removed at block  46  to expose the slower-growth facets  12   a - 12   c  and the substrate  10  is again oxidized at block  48  to yield the uniform thickness layer of  FIG. 4 . 
   In order to achieve oxide thicknesses varying locally the first oxide layer  20  is removed in areas where the oxide layer is to be thinner. The steps of oxidizing, shielding, and locally removing oxide, exposing, and oxidizing again may be repeated, selectively shielding different portions each iteration to achieve oxide layers having a broad range of thicknesses on a single substrate  10 . 
     FIGS. 9 and 10  illustrate a method  50  that is an alternative embodiment of the method  36 . In the method  50 , a first oxide layer  20  is grown on the substrate at block  52 . A photoresist layer  54  is then applied at block  56 . The slower-growth facets  12   a - 12   c  are masked at block  58 , such as by a mask  60  interposed between the substrate  10  and a light source. The substrate  10  is then irradiated through the mask  60  at block  62 . For positive photoresist, exposing the photoresist to light weakens the photo resist, making it readily removable. The steps recited for blocks  56 ,  58 ,  62  assume use of a positive photoresist compound that is weakened by exposure to light. Other embodiments may use a negative photoresist compound that remains in a weakened state unless hardened by exposure to light. In such embodiments, the faster-growth facets  16   a ,  16   b  are masked in order to weaken the photoresist coating them. 
   For both types of photoresist, the weakened photoresist is then removed at block  64  to expose the faster-growth facets  16   a ,  16   b . The oxide on the faster-growth facets  16   a ,  16   b  is then removed at block  66 . The hardened photoresist is removed at block  68  and a second oxide layer  34  is grown at block  70 . 
     FIGS. 11 and 12  illustrate a method  72  that is an alternative embodiment of the method  36 . In the method  72 , the first oxide layer  20  is grown at block  74 . The faster growing facets are then masked at block  76 . The exposed areas of the substrate  10  are then metallized at block  78  to create a metal layer  80  over the slow-growth facets as shown in  FIG. 12 . Metallization at block  78  may be accomplished by E-beam evaporation, sputtering, chemical vapor deposition (CVD), or the like, through a mask  82  exposing the slower-growth facets  12   a - 12   c . Oxide is removed from the unmetalized, faster-growth facets  16   a ,  16   b  at block  84  by plasma etching, wet etching, or the like chosen such that the metal layer  80  is not removed thereby. The metal layer  80  is then removed at block  86 , such as by wet chemical etching, and a second oxide layer is grown at block  88 . 
     FIG. 13  illustrates a method  90  for forming oxide layers having locally varying thicknesses according to a specific criteria, rather than uniform layers. At block  92  a first oxide layer  94  is formed, as illustrated in  FIG. 14A . At block  96  a first area  98  is shielded and the oxide in unshielded areas is removed at block  100 , leaving the substrate  10  as illustrated in  FIG. 14B . At block  102 , shielding is removed and at block  104 , a second oxide layer  106  is grown, as illustrated in  FIG. 14C . At block  108 , the first area  98  an a second area  110  are shielded and at block  112 , the oxide layer is removed from unshielded areas of the substrate  10 . At block  114 , the first and second areas are unshielded, leaving the substrate as shown in  FIG. 14D  having a first area  98 , a second area  108 , and a third area  116  each having a unique oxide layer thickness. The method  90  of  FIG. 13  may use any of the methods  32 ,  50 ,  72  for shielding areas of the substrate during removal of the oxide layer. 
   Referring to  FIG. 15 , the pattern illustrated in FIGS.  13  and  14 A- 14 D may be repeated for any number of oxide layers and areas. For example, a manufacturing process includes multiple iterations of the process including the steps of growing an oxide layer, shielding certain areas, and removing unshielded portions of the oxide layer. The lower axis  118  of  FIG. 15  indicates the number of the iteration and the upper axis  122  indicates the thickness of the oxide layer. In order to achieve an area having a desired oxide layer thickness, one first determines the thickness of the area having the thickest oxide layer and performs sufficient iterations to achieve the desired thickness, 5,500 Angstroms in the illustrated scenario. Areas to have the greatest thickness will be shielded during each iteration. Areas to have a lesser thickness are left unshielded during initial iterations until the number of remaining iterations is sufficient to grow an oxide layer of the appropriate thickness. The lesser-thickness area is then shielded for all remaining iterations. For example, an area may begin to be shielded during oxide layer removal at iteration six in order to achieve an oxide layer thickness of 3000 Angstroms, whereas another area begins to be shielded at iteration seven in order to achieve an oxide layer thickness of 2500 Angstroms. Areas where no oxide layer is to be formed are left unshielded for all iterations. 
   Referring to  FIG. 16 , the thickness of the oxide layer grown during each iteration need not be the same for each iteration. For example, a silicon substrate  10  may have areas A 0  through A N , with A 0  having the thickest oxide layer and A N  having the thinnest oxide layer. An area A i  of the areas A 0  through A N  need not be contiguous. The difference in thickness between each area A i  and A i+1 , is t i , except t N  is simply equal to the thickness of the thinnest layer of oxide. 
   A method  122  for forming a multi-thickness oxide layer includes setting a counter i to zero at block  124 . A first oxide layer having a thickness t i  is grown at block  126 . At block  128 , areas A i  through A 0  are shielded. At block  130  oxide is removed from all exposed areas. At block  132 , the value of i is compared to variable N representing the total number of thicknesses being formed on the substrate  10 . If i is equal to N, the method ends. If i is less than N, than i is incremented at block  134 . Areas where no oxide is to be formed are left unshielded during the entire method  122 . 
   While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the Claims that follow.