# Patent

## Title
Optical die with variable refractive index, adaptive of angle of incidence, and method of fabricating such a die

## Abstract

An optical die, which is intended to be placed in front of an optical sensor of a semiconductor component, has an optically useful zone having an optical axis and exhibiting a variable refractive index. Specifically the refractive index of the die is variable in an annular peripheral zone lying between a radius Ru enveloping the useful zone and a smaller radius Ro. The index is varies as a function of radial distance from a higher value near the smaller radius Ro to a lower value near the radius Ru. The function of the variable refractive index lies between a maximum and minimum profile.

## Background
PRIORITY CLAIM
The present application claims priority from French Application for Patent No. 08 54469 filed Jul. 2, 2008, the disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
Technical Field of the Invention
The present invention relates to the field of optical modules intended to be placed in front of optical sensors of semiconductor components and more particularly the field of optical modules or dies intended to be placed in front of optical sensors of semiconductor components.
SUMMARY OF THE INVENTION
According to one mode, a die may comprise an optically useful zone having an optical axis and exhibiting a variable refractive index, so that it is smaller in its peripheral part than in its just adjacent interior part or in its central part.
In an annular peripheral zone, lying between a radius Ru enveloping the useful zone and a smaller radius Ro, the refractive index may lie between a maximum profile Pmax and a minimum profile Pmin.
The maximum profile Pmax may be such that between the radius Ro and a radius R2 the maximum refractive index is equal to a value n1 and then that between this radius R2 and the radius Ru, the value of the maximum refractive index is defined, as a function of the distance from the center of the die r, by the formula




  n
  =
  
    
      n
      1
    
    -
    
      
        (
        
          
            n
            1
          
          -
          
            n
            3
          
        
        )
      
      ·
      
        
          
            (
            
              r
              -
              
                R
                2
              
            
            )
          
          2
        
        
          
            (
            
              
                R
                u
              
              -
              
                R
                2
              
            
            )
          
          2
        
      
    
  




The minimum profile Pmin may be such that between the radius Ro and a radius R1, the value of the minimum refractive index is defined, as a function of the distance from the center of the die r, by the formula




  n
  =
  
    
      n
      2
    
    -
    
      
        (
        
          
            n
            2
          
          -
          
            n
            0
          
        
        )
      
      ·
      
        
          
            (
            
              r
              -
              
                R
                0
              
            
            )
          
          2
        
        
          
            (
            
              
                R
                
                  1
                  ⁢
                  
                    
                  
                
              
              -
              
                R
                0
              
            
            )
          
          2
        
      
    
  




and than that between this radius R1 and the radius Ru the minimum refractive index is equal to a value n0.
The values of the refractive indices may be such that n1 is greater than n0, n2 lies between n0 and n1, and n3 lies between n0 and n2.
The values of the radii may be such that R1 lies between Ro and Ru and R2 lies between Ro and Ru.
For example, n2 may be equal to n0 plus 8/10 of (n1−n0) and n3 may be equal to 1⁄2 of (n1+n0).
For example, Ro may be equal to 3⁄5 of Ru, R1 may be equal to Ro plus 1⁄3 of (Ru−Ro) and R2 may be equal to Ro plus 7⁄8 of (Ru−Ro).
According to one embodiment, implanted ions may determine the variant of a refractive index.
Moreover, grooves and/or through-holes situated outside the said useful zone may be made in the die.
According to another mode, a method of making a die may comprise the following steps: making at least one mask on the central zone delimited by the radius Ro of at least one face of the die, or at least one mask outside this central part, and immersing the die in an ion bath so as to implant ions contained in this bath in the zones not covered by the said mask.
A method may comprise the following steps: making at least one second mask different from the first mask, and immersing the die in a second ion bath so as to implant ions of this bath through the zones not covered by the said second mask.
A method may comprise a step of rediffusion of the implanted ions.
The implantation may possibly be carried out under the effect of an acceleration electric field.
According to another embodiment, an optical semiconductor device may comprise a semiconductor component exhibiting an optical sensor and, in front of this optical sensor, an optical module comprising a die as defined above.
In an embodiment, an apparatus comprises a transparent optical die having front and rear parallel surfaces and having a rectangular optically useful zone centered on an optical axis and a central circular zone within the optically useful zone and also centered on the optical axis, the transparent optical die having a variable refractive index at least in a region of the transparent optical die located between an outer edge of the central circular zone and an extent of the optically useful zone.



BRIEF DESCRIPTION OF THE DRAWINGS
Other advantages and characteristics of the invention will become apparent on examining the detailed description of modes of implementation and embodiments, wholly non-limiting, and the appended drawings in which:
FIG. 1 represents a radial cut through a device comprising a die;
FIG. 2 represents a view from above of a die;
FIG. 3 represents a graph of profiles of maximum and minimum refractive indexes;
FIG. 4 represents a first exemplary embodiment of a refractive index profile;
FIG. 5 represents a second exemplary embodiment of a refractive index profile;
FIG. 6 represents a third exemplary embodiment of a refractive index profile;
FIG. 7 represents a fourth exemplary embodiment of a refractive index profile;
FIG. 8 represents a view from above of a die equipped with a mask;
FIG. 9 represents a radial cut through the die of FIG. 8;
FIG. 10 represents a view from above of the die of FIGS. 8 and 9 after a treatment;
FIG. 11 represents a radial cut through the die of FIG. 10;
FIG. 12 represents a graph of the profile of the refractive index of the die of FIGS. 10 and 11;
FIG. 13 represents a radial cut through a die equipped with a mask;
FIG. 14 represents a cut through the die of FIG. 13 after a treatment;
FIG. 15 represents a graph of the profile of the refractive index of the die of FIG. 14;
FIG. 16 represents a radial cut through a die equipped with a mask;
FIG. 17 represents a cut through the die of FIG. 16 after a treatment;
FIG. 18 represents a graph of the profile of the refractive index of the die of FIG. 17;
FIG. 19 represents a radial cut through a die equipped with a mask;
FIG. 20 represents a cut through the die of FIG. 19 after a treatment; and
FIG. 21 represents a graph of the profile of the refractive index of the die of FIG. 20.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, it may be seen that represented therein is a stack 1 comprising a semiconductor component 2, for example flat and with a square contour, exhibiting at the center of a face 3 an optical sensor 4, for example rectangular, and an optical module 5 placed in front of the face 3, in contact with or a very small distance from this face by virtue of a peripheral spacing ring 3a, and intended to focus the light rays towards the optical sensor 4.
The optical module 5 can comprise a transparent die 6, for example made of glass, with parallel faces and a square contour corresponding to that of the semiconductor component 2. It may possibly furthermore comprise an optical lens 7 placed in front of the die 6, possibly with interposition of a diaphragm 8 and of a peripheral spacing ring 8a. 
As shown in FIG. 2, the die 6 may exhibit an optically useful rectangular zone 9 corresponding to the optical sensor 4 and exhibiting the same optical axis 10 situated at the center.
It may be advantageous for the die 6 to exhibit a refractive index n, considered in relation to any radial plane, that can vary over at least an annular part between its center 10 and the periphery of its useful zone 9, being smaller in its peripheral part than in its just adjacent interior part or in its central part.
In particular, the value of the refractive index n, considered in relation to any radial plane, may be variable over an annular peripheral zone 11 lying approximately between a radius Ru enveloping the useful zone 9 and a smaller radius Ro.
The radius Ru may be approximately equal to half a diagonal of the useful zone 9, whereas the radius Ro may for example be approximately equal to at least half the small side of the useful zone 9. Thus, the variation in the value of the refractive index may be more particularly achieved in the corners of the useful zone 9.
As shown by FIG. 3, in the annular peripheral zone 11, the profile of the refractive index, considered in relation to any radial plane, may lie between a maximum profile Pmax and a minimum profile Pmin that are such as defined hereinbelow.
(a) the maximum profile Pmax may be such that between the radius Ro and a radius R, the value of the maximum refractive index is equal to a value n1 and then that between this radius R2 and the radius Ru, the value of the maximum refractive index is defined, as a function of the distance from the center of the die r, by the formula




  n
  =
  
    
      n
      1
    
    -
    
      
        (
        
          
            n
            1
          
          -
          
            n
            3
          
        
        )
      
      ·
      
        
          
            (
            
              r
              -
              
                R
                2
              
            
            )
          
          2
        
        
          
            (
            
              
                R
                u
              
              -
              
                R
                2
              
            
            )
          
          2
        
      
    
  




(b) the minimum profile Pmin may be such that between the radius Ro and a radius R1, the value of the minimum refractive index is defined, as a function of the distance from the center of the die r, by the formula




  n
  =
  
    
      n
      2
    
    -
    
      
        (
        
          
            n
            2
          
          -
          
            n
            0
          
        
        )
      
      ·
      
        
          
            (
            
              r
              -
              
                R
                0
              
            
            )
          
          2
        
        
          
            (
            
              
                R
                1
              
              -
              
                R
                0
              
            
            )
          
          2
        
      
    
  




and then that between this radius R1 and the radius Ru the value of the minimum refractive index is equal to a value n0,
moreover, the above refractive indices may be defined with respect to one another in the following manner:
n1 may be greater than n0,
n2 may lie between n0 and n1,
n3 may lie between n0 and n2,
Moreover, the above radii may be defined with respect to one another in the following manner:
R1 may lie between Ro and Ru,
R2 may lie between Ro and Ru.
Various relative values could be adopted. Nevertheless, the following relative values are proposed:
n2 may be equal to n0 plus 8/10 of (n1−n0),
n3 may be equal to 1⁄2 of (n1+n0),
Ro may be equal to 3⁄5 of Ru,
R1 may be equal to Ro plus 1⁄3 of (Ru−Ro),
R2 may be equal to Ro plus 7⁄8 of (Ru−Ro).
Moreover, in practice, n0 may lie between 85 and 99% of n1.
Considering the above relative values, examples of index gradients obtained in annular zones 11 of dies 6 and lying between the profiles Pmax and Pmin will now be given.
Referring to FIG. 4, it may be seen that represented therein is a refractive index profile 12 for a die 6 treated in such a way that this refractive index 12 is at the value n1 between the radius Ro and a radius Ra lying between the radii R1 and R2, and then, starting off tangentially, inflects while decreasing down to the value n0 at the radius Ru.
Referring to FIG. 5, it may be seen that represented therein is a refractive index profile 13 for a die 6 treated in such a way that this refractive index 13 is at the value n1 between the radius Ro and a radius Ra, lying between the radii Ro and R1, is at the value n0 between a radius Rb, lying between the radius R1 and the radius R2, and the radius Ru and forms an inverted S joining them between Ra and Rb, meeting up with them tangentially.
Referring to FIG. 6, it may be seen that represented therein is a refractive index profile 14 for a die 6 treated in such a way that this refractive index profile 14 is approximately identical to that of FIG. 5 between the radii Ra and Ru but is distinguished therefrom by the fact that between Ra and Ro, it inflects, decreasing down to a value n4 lying between n1 and n2, starting off tangentially to n1 at the radius Ra.
Referring to FIG. 7, it may be seen that represented therein is a refractive index profile 15 for a die 6 treated in such a way that this refractive index profile 15 is approximately identical to that of FIG. 5 between the radii Ro and Rb but is distinguished therefrom by the fact that between Rb and Ru it inflects, increasing up to a value n5 lying between n0 and n3, starting off tangentially to n0 at the radius Rb.
Referring to FIGS. 8 to 21, the way in which a die 6 may be treated in order to obtain for example any one of the refractive index gradients hereinabove will now be described.
To obtain the refractive index gradient 12 of FIG. 4, it is possible to proceed as follows.
As shown by FIGS. 8 and 9, it is possible to start from a die 6.0 exhibiting a refractive index n0, for example uniformly doped with sodium ions.
Masks 16 covering the die and exhibiting a central orifice in the form of a disc of radius greater than or equal to Ra, that is to say not covering the zone inside the radius Ra, are made on the opposite faces thereof, for example by photolithography.
The die 6.0 is immersed in an ion bath to obtain ion diffusion in the material of the die 6, which can be accelerated through the effect of an electric field. The conditions of such treatment are suited to the refractive index gradient 12 to be obtained.
For example, ions of thallium or of silver may be implanted.
The masks 16 may then be removed.
As shown by FIGS. 10, 11 and 12, a die 6.1 is obtained whose refractive index is at the value n1 in its central part and up to the radius Ro, and then its refractive index follows the profile of the gradient 12 between the radii Ra and Ru.
According to a variant execution applied to exchanges of different ions, to obtain a corresponding refractive index profile, it would be possible to make a reverse mask, that is to say make a mask on the central part of the die 6.0, perform an implantation from outside and then a diffusion towards the interior.
To obtain the refractive index gradient 13 of FIG. 5, it is possible to start from the die 6.1 treated as hereinabove and to apply thereto, in the presence or absence of the masks 16, a suitable complementary treatment of high temperature ion rediffusion.
In this case, as shown by FIGS. 14 and 15, a die 6.2 is obtained whose refractive index is at the value n1 in its central part and up to the radius Ro, and then its refractive index follows the profile of the gradient 13 up to the radius Rb, and is then at the value n0 up to the radius Ru.
To obtain the refractive index gradient 14 of FIG. 6, it is possible to start from the die 6.1 treated and ridded of the masks 16 and, as shown by FIG. 16, it is possible to make, on the faces of this die 6.1, masks 17 covering the faces of the die and leaving open a ring corresponding approximately to the zones 19 going from the radius Ro to a radius greater than Ra.
The die 6.1 is immersed in an ion bath to obtain ion diffusion in the material of this die, which can be accelerated through the effect of an electric field. The conditions of such a treatment are suited to the refractive index gradient 14 to be obtained.
For example, thallium ions may be implanted.
As shown by FIGS. 17 and 18, a die 6.3 is obtained whose refractive index follows the profile of the gradient 14.
Here again, it would be possible to use a reverse mask.
To obtain the refractive index gradient 15 of FIG. 7, it is possible to start from the die 6.1 treated and ridded of the masks 16 and, as shown by FIG. 19, it is possible to make, on the opposite faces of this die 6.1, masks 20 in the form of discs going approximately up to a radius greater than or equal to Rb.
This die 6.1 is immersed in an ion bath to obtain an ion diffusion in the material of this die, which can be accelerated through the effect of an electric field. The masks 20 are then removed.
For example, it is possible to implant ions of thallium or of silver.
A thermal rediffusion could also be carried out.
As shown by FIGS. 20 and 21, a die 6.4 is obtained whose refractive index follows the profile of the gradient 15.
Here again, a reverse mask could be used.
The various profiles of refractive index gradients just described are certainly not the only ones which could be achieved while remaining within the template defined by the profiles Pmax and Pmin. Likewise, the operating modes could be different and the materials and baths used could be different.
The die 6 may advantageously stem from a wafer in which a large number of dies are made simultaneously.
For this purpose, the aforesaid masks are then formed on the wafer at the locations of the dies to be obtained. The ion implantations and the other treatments, if any, are carried out through the opposite faces of this wafer. Grooves and/or through holes can thus be made in the wafer, outside the optically useful zones 9 of the locations, so as to perform the ion implantations and the other treatments, if any, in particular through the flanks of these grooves and/or through-holes.
The treatments of the wafer having been performed, the said wafer is then sawn along perpendicular lines so as to individualize each die 6 treated.
Starting from an original optical module including a transparent die with substantially constant refractive index, such that the light rays which pass through it exhibit, on the periphery of the sensor, too high an angle of incidence for the image to be captured correctly, the die treatments as described make it possible to decrease this angle of incidence, according to a desired gradient. Such an arrangement may make it possible to reduce the amplitude of an original positive distortion, principally in its periphery, of the captured image.
Although preferred embodiments of the method and apparatus of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.

## Claims

1. Apparatus, comprising:
an optical module adapted to be placed in front of a semiconductor optical image sensor, the optical module comprising a transparent optical die having front and rear parallel surfaces and having a rectangular optically useful zone centered on an optical axis of the semiconductor optical image sensor and a central circular zone within the optically useful zone and also centered on the optical axis, the transparent optical die having a variable refractive index at least in a region of the transparent optical die located between an outer edge of the central circular zone and an extent of the optically useful zone.

2. The apparatus of claim 1 wherein the variable refractive index in the region varies as a function of radial distance from the optical axis.

3. The apparatus of claim 2 wherein the function for varying the variable refractive index is substantially the same for all radial planes extending from the optical axis.

4. The apparatus of claim 2 wherein the transparent optical die has a substantially constant refractive index within the central circular zone.

5. The apparatus of claim 1 wherein a radius of the central circular zone is approximately one-half a shortest side length of the rectangular optically useful zone.

6. The apparatus of claim 1 wherein the variable refractive index in the region varies from a first, relatively higher, refractive index at the outer edge of the central circular zone towards a second, relatively lower, refractive index for radial distance farther from the optical axis than the outer edge of the central circular zone.

7. The apparatus of claim 1 wherein the variable refractive index decreases with radial distance from the optical axis in the region located between the outer edge of the central circular zone and the extent of the optically useful zone.

8. The apparatus of claim 1 wherein one or more of grooves or through-holes on the die are situated outside the rectangular optically useful zone.

9. The apparatus of claim 1 wherein the variable refractive index is determined by implanted ions.

10. The apparatus of claim 1 wherein the refractive index is smaller in a peripheral part of the rectangular optically useful zone than elsewhere in the rectangular optically useful zone.

11. The apparatus of claim 1 wherein the transparent optical die has a shape corresponding to that of the semiconductor optical image sensor.

12. The apparatus of claim 1 wherein the optical module further comprises a lens positioned adjacent a surface of the transparent optical die.

13. An optical die adapted for use within an imaging optical module to be placed in front of an optical sensor of a semiconductor component, comprising:
an optically useful zone having an optical axis aligned with the optical sensor and exhibiting a variable refractive index which is smaller in its peripheral part than in its just adjacent interior part or in its central part and which, in an annular peripheral zone lying between a radius Ru enveloping the useful zone and a smaller radius Ro, lies between a maximum profile Pmax and a minimum profile Pmin;
(a) the maximum profile Pmax being such that between the radius Ro and a radius R2 the maximum refractive index is equal to a value n1 and then that between this radius R2 and the radius Ru, the value of the maximum refractive index is defined, as a function of the distance from the center of the die r, by the formula





  n
  =
  
    
      n
      1
    
    -
    
      
        (
        
          
            n
            1
          
          -
          
            n
            3
          
        
        )
      
      ·
      
        
          
            (
            
              r
              -
              
                R
                2
              
            
            )
          
          2
        
        
          
            (
            
              
                R
                u
              
              -
              
                R
                2
              
            
            )
          
          2
        
      
    
  



(b) the minimum profile Pmin being such that between the radius Ro and a radius R1, the value of the minimum refractive index is defined, as a function of the distance from the center of the die r, by the formula





  n
  =
  
    
      n
      2
    
    -
    
      
        (
        
          
            n
            2
          
          -
          
            n
            0
          
        
        )
      
      ·
      
        
          
            (
            
              r
              -
              
                R
                0
              
            
            )
          
          2
        
        
          
            (
            
              
                R
                1
              
              -
              
                R
                0
              
            
            )
          
          2
        
      
    
  



and then that between this radius R1 and the radius Ru the minimum refractive index is equal to a value n0;
(c) and in which:
n1 is greater than n0,
n2 lies between n0 and n1,
n3 lies between n0 and n2,
R1 lies between Ro and Ru,
R2 lies between Ro and Ru.

14. The die according to claim 13, in which:
Ro is equal to 3⁄5 of Ru,
R1 is equal to Ro plus 1⁄3 of (Ru−Ro),
R2 is equal to Ro plus 7⁄8 of (Ru−Ro).

15. The die according to claim 14, in which:
n2 is equal to n0 plus 8/10 of (n1−n0),
n3 is equal to 1⁄2 of (n1+n0).

16. The die according to claim 13, in which implanted ions determine the variation in a refractive index.

17. The die according to claim 13, further comprising grooves and/or through-holes situated outside the said useful zone.

18. The die of claim 13 wherein implanted ions are located on flanks of one or more of grooves or through holes outside the optically useful zone.

19. The die of claim 13 wherein implanted ions decrease angle of incidence of light rays that pass through the die.

20. The die of claim 13 wherein ions are implanted on the die, the ions including one of thallium or silver.

21. The die of claim 13 wherein n0 lie between 85 and 99% of n1.

22. An optical semiconductor device, comprising:
a semiconductor component having a rectangular optical sensor with an optical axis; and
an optical module placed in front of the optical sensor, the optical module comprising:
a transparent optical die having front and rear parallel surfaces and having a rectangular optically useful zone corresponding in size and shape to the rectangular optical sensor and centered on the optical axis and a central circular zone within the optically useful zone and also centered on the optical axis, the transparent optical die having a variable refractive index at least in a region of the transparent optical die located between an outer edge of the central circular zone and an extent of the optically useful zone.

23. The device of claim 22, wherein the optical module further comprises a lens positioned adjacent the front parallel surface of the transparent optical die.

24. The device of claim 22 wherein the transparent optical die has a substantially constant refractive index within the central circular zone.

25. The device of claim 22 wherein the variable refractive index decreases with radial distance from the optical axis in the region located between the outer edge of the central circular zone and the extent of the optically useful zone.

26. An optical die comprising:
an optically useful zone, the optically useful zone including a peripheral zone and a central zone and exhibiting an optical axis situated at its center, and
a refractive index that is smaller in an annular region of the peripheral zone than in an annular region of the central zone, the annular region defined by a radial distance from the center of the die, wherein the refractive index varies over the annular region in the peripheral zone and includes a maximum profile and a minimum profile,
wherein the maximum refractive index profile is a function of a radius R that is between an innermost portion and an outermost portion of the annular region of the peripheral zone and is a first value when from the innermost portion to a radius R some distance away from the innermost portion and is then a lesser value between R and a second radius which is between R and the outermost portion of the annular region of the peripheral zone; and
wherein the minimum refractive index profile includes values less than the maximum refractive index profile and is a function of radii that are less than R.

27. The die of claim 26 wherein implanted ions on the die determine variations in the refractive index.

28. The die of claim 26 further comprising one or more of grooves or through-holes positioned outside the optically useful zone.

29. The die of claim 26 wherein the first value is n1 and the lesser value is defined by a formula




  
    n
    =
    
      
        n
        1
      
      -
      
        
          (
          
            
              n
              1
            
            -
            
              n
              3
            
          
          )
        
        ·
        
          
            
              (
              
                r
                -
                
                  R
                  2
                
              
              )
            
            2
          
          
            
              (
              
                
                  R
                  u
                
                -
                
                  R
                  2
                
              
              )
            
            2
          
        
      
    
  
  ,



wherein Ru is a radius in the outermost portion, R2 is a radius between the innermost portion and the outermost portion and r is the center of the die.

30. The die of claim 26 wherein the minimum refractive index profile is defined by a formula




  
    n
    =
    
      
        n
        2
      
      -
      
        
          (
          
            
              n
              2
            
            -
            
              n
              0
            
          
          )
        
        ·
        
          
            
              (
              
                r
                -
                
                  R
                  0
                
              
              )
            
            2
          
          
            
              (
              
                
                  R
                  1
                
                -
                
                  R
                  0
                
              
              )
            
            2
          
        
      
    
  
  ,



wherein R0 is a radius in the innermost portion, R1 is a radius between the innermost portion and the outermost portion and r is the center of the die.

31. The die of claim 26 wherein the contour of the die corresponds to that of a semiconductor component associated with the die.

32. The die of claim 26 wherein the optically useful zone corresponds with an optical sensor that exhibits the same optical axis at its center.