Abstract:
A spectral filter is manufactured using a process wherein a first rectangular bar is formed within a first layer made of a first material, said first rectangular bar being made of a second material having a different optical index. The process further includes, in a second layer over the first layer, a second rectangular bar made of the second material. The second rectangular bar is positioned in contact with the first rectangular bar. The second layer is also made of the first material.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the priority benefit of French Application for Patent No. 1559267, filed on Sep. 30, 2015, the disclosure of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a method of manufacturing a spectral filter. 
       BACKGROUND 
       [0003]    Image sensors or displays integrated in microelectronic devices currently comprise an array of photodetectors or of photoemitters formed in a semiconductor substrate. Each of the photodetectors or photoemitters is generally topped with a spectral filter intended to only transmit light for one wavelength range. 
         [0004]    A known type of spectral filter comprises a layer made of a first material having at least one pattern made of a second material, with an optical index different from that of the first material, formed therein. The pattern extends through the entire thickness of the layer of the first material and may be periodically repeated therein. In the case of a plasmonic spectral filter, one of the first and second materials is a metal and the other one is a dielectric. 
         [0005]      FIG. 1  is a partial reproduction of FIG. 2 of United States Patent Application Publication No. 2014/0374574 (incorporated by reference). This drawing schematically shows, in top view, a network  1  of spectral filters  3 ,  5 , and  7  arranged above an array of photodetectors, each filter  3 ,  5 , and  7  being arranged above of photodetector of the array. Each filter is formed from a layer of a first material having cruciform patterns made of the second material formed therein. The pattern dimensions are selected according to the range of transmitted wavelengths so that the filter has as high as possible a light transmission rate and as high as possible a light rejection ratio outside of this range. According to the transmitted wavelength range, certain dimensions of the patterns may be smaller than some hundred nanometers. 
         [0006]    In practice, to manufacture a filter of the type of those in  FIG. 1 , the layer of the first material is deposited, after which, for each pattern, a hole having the shape of the pattern is etched through the layer. The second material is then deposited to fill each hole and form the corresponding pattern therein. 
         [0007]    In the case of patterns such as crosses or stars, for example having certain dimensions smaller than some hundred nanometers, such a manufacturing method has various disadvantages, some of which at least are desired to be overcome by the present disclosure. 
       SUMMARY 
       [0008]    An embodiment provides a method of manufacturing a spectral filter comprising the successive steps of: a) forming, in a first layer made of a first material, a first rectangular bar made of a second material having an optical index different from that of the first material; and b) forming, in a second layer or in the second layer and at the same time in a portion at least of the first layer, a second rectangular bar made of the second material and in contact with the first bar, the second layer resting on the first layer and being made of the first material. 
         [0009]    According to an embodiment, the first material and the second material are respectively a metal and a dielectric, or conversely. 
         [0010]    According to an embodiment, step a) comprises the successive steps of: a1) etching through the entire thickness of the first layer a first rectangular cavity; and a2) filling the first cavity with the second material to form the first bar therein. 
         [0011]    According to an embodiment, step b) comprises the successive steps of: b1) depositing the second layer on the first layer; b2) etching, through the entire thickness of the second layer and possibly through at least a portion of the thickness of the first layer, a second rectangular cavity; and b3) filling the second cavity with the second material to form the second bar therein. 
         [0012]    According to an embodiment, step a2) comprises the successive steps of: depositing a layer of the second material on the first layer to fill the first cavity; and removing a portion of the second material by planarizing etching down to the upper surface of the first layer. 
         [0013]    According to an embodiment, the first bar and the second bar extend lengthwise along different directions. 
         [0014]    According to an embodiment, said directions are orthogonal. 
         [0015]    Another embodiment provides a spectral filter comprising a layer made of a first material having a first bar and a second bar made of a second material of optical index different from that of the first material arranged therein, the second bar extending lengthwise along a direction different from that of the first bar and having a portion resting on the first bar. 
         [0016]    Another embodiment provides an image sensor comprising: an array of photodetectors arranged inside and/or on top of a semiconductor substrate; and the above-mentioned spectral filter topping at least one photodetector of the array. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, wherein: 
           [0018]      FIG. 1 , previously described, shows a prior art filter structure; 
           [0019]      FIG. 2  is a simplified top view of a spectral filter pattern; 
           [0020]      FIGS. 3A to 3E  are cross-section and perspective views illustrating successive steps of a method of manufacturing a spectral filter; 
           [0021]      FIGS. 4C and 4E  are cross-section and perspective views illustrating a variation of the steps of  FIGS. 3C and 3E ; 
           [0022]      FIGS. 5C and 5E  are cross-section and perspective views illustrating another variation of the steps of  FIGS. 3C and 3E ; and 
           [0023]      FIGS. 6A and 6B  illustrate the variation of the light transmission rate according to the wavelength and for different angles of incidence of light, for various filter types. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. In the following description, terms “top”, “bottom”, “upper”, and “lower” refer to the orientation of the concerned elements in the corresponding drawings. Unless otherwise specified, term “approximately” means to within 10%, preferably to within 5%. 
         [0025]      FIG. 2  is a simplified top view of a pattern of a spectral filter of the type of those in  FIG. 1 . 
         [0026]    A cruciform hole has been etched through a layer  9  made of a first material, after which the hole has been filled with a second material to form a cruciform pattern  11  of the filter therein. 
         [0027]    Due to technological limitations linked to the etch step, the angles of pattern  11  of the filter are not sharp, but rounded. It can be observed that the optical properties of such a filter (rejection ratio outside of the transmitted wavelength range, selectivity, insensitivity to the angle of incidence of light, etc.) are then poorer than expected. This is more particularly true when pattern  11  has dimensions smaller than some hundred nanometers, for example, when arms of the pattern have a first side with a length  13  smaller than 100 nm and a second side with a length  15  smaller than 100 nm. Such a degradation of the optical properties is imputed to the rounded character of the angles of the pattern. 
         [0028]    It is here provided to manufacture a filter equivalent to those of  FIGS. 1 and 2 , by forming each pattern of the filter from bars made of the second material formed one after the others in a layer of the first material. 
         [0029]      FIGS. 3A to 3D  are cross-section and perspective views illustrating successive steps of a method of manufacturing a spectral filter. 
         [0030]      FIG. 3A  shows a structure comprising a support  21  after the deposition of a layer  23  of a first material on the upper surface of support  21 , and the etching of a rectangular cavity  25  through the entire thickness of layer  23 . Cavity  25  extends longitudinally along a first direction. An array of photodetectors are provided in or in conjunction with support  21 . 
         [0031]      FIG. 3B  shows the structure of  FIG. 3A  after the filling of cavity  25  with a second material to form a bar  27  therein. As an example, the cavity is filled by deposition of a layer of the second material on the upper surface of layer  23  and in cavity  25 , and then by removing the excess second material by CMP (“Chemical Mechanical Polishing”), down to the upper surface of layer  23 . The upper surface of bar  27  is then at the level of the upper surface of layer  23 . 
         [0032]      FIG. 3C  shows the structure of  FIG. 3B  after the deposition of a layer  29  of the first material on the upper surface of layer  23  and the etching of a rectangular cavity  31  through the entire thickness of layer  29 . The etching of cavity  31  is performed so that a portion of this cavity is opposite (i.e., crosses over) a portion of bar  27 . Cavity  31  extends longitudinally in a second direction different from the first direction along which bar  27  extends. For example, the first and second directions are orthogonal. 
         [0033]    In this embodiment, as shown in  FIG. 3C , the etching of cavity  31  is stopped on bar  27 , that is, at the level of the upper surface thereof. 
         [0034]      FIGS. 3D and 3E  show the structure of  FIG. 3C  after the filling of cavity  31  with the second material to form therein a bar  33  which rests on bar  27 , support  21  and layers  23  and  29  being absent from  FIG. 3E . The two bars  27  and  33  form, in top view, not shown, a cruciform pattern  35  equivalent to pattern  11  of  FIG. 2 . Pattern  35  is at least partly free of the disadvantages of pattern  11 . The pattern  35  overlies a photodetector of the array. 
         [0035]    As an example, similarly to the step described in relation with  FIG. 3B , cavity  31  is filled by deposition of a layer of the second material on the upper surface of layer  29  and into cavity  31 , and then by removing the excess second material by planarizing etching. The planarizing etching may be stopped on the upper surface of layer  29 , as shown herein, or above the upper surface of layer  29  so that there remains a layer of the second material coating the upper surface of layer  29 . 
         [0036]      FIGS. 4C and 4E , similar to  FIGS. 3C and 3E , illustrate an alternative embodiment of the steps described in relation with  FIGS. 3C and 3E . 
         [0037]      FIG. 4C  shows the structure of  FIG. 3B  after the deposition of layer  29  and the etching of a rectangular cavity  41  in layer  29 . Cavity  41  is similar to cavity  31  but for the fact that the etching of cavity  41  is stopped to form a notch region in bar  27  and layer  23 . The bottom of cavity  41  is then at a level lower than that of the upper surface of bar  27 . 
         [0038]      FIG. 4E  shows the structure of  FIG. 4C  after the filling of cavity  41  with the second material to form a bar  43  therein. Similarly to bars  27  and  33 , bars  27  and  43  form a cruciform pattern  45  equivalent to pattern  11  of  FIG. 2 , but for the fact that bar  43  penetrates across a portion of the thickness of bar  27 . Pattern  45  is at least partly free of the disadvantages of pattern  11 . 
         [0039]      FIGS. 5C and 5E , similar to  FIGS. 3C and 3E , illustrate another alternative embodiment of the steps described in relation with  FIGS. 3C and 3E . 
         [0040]      FIG. 5C  shows the structure of  FIG. 3B  after the deposition of layer  29  and the etching of a cavity  51  in layer  29 . The etching of cavity  51  is similar to that of cavity  31 , but for the fact that cavity  51  is etched through the entire thickness of bar  27  (to form a notch region) and layer  23 . The bottom of cavity  51  is then at the same level as the lower surface of layer  23 . 
         [0041]      FIG. 5E  shows the structure of  FIG. 5C  after the filling of cavity  51  with the second material to form a bar  53  therein. Similarly to bars  27  and  33 , bars  27  and  53  form a cruciform pattern  55  equivalent to pattern  11  of  FIG. 2 , but for the fact that bar  53  penetrates across the entire thickness of bar  27 . Pattern  55  is at least partly free of the disadvantages of pattern  11 . 
         [0042]    Due to the fact that, in the manufacturing methods described in relation with  FIGS. 3A to 5E , the bars forming a pattern of a spectral filter are formed one after the others, the angles of the pattern are sharp, conversely to the case where the pattern is obtained according to the method described in relation with  FIG. 2 . At least certain optical properties of a filter comprising a pattern of the type of patterns  35 ,  45 , or  55  are then better than those of a filter comprising patterns of the type of pattern  11 . 
         [0043]    As previously indicated, the first and second materials and the different dimensions of patterns  35 ,  45 , or  55  are selected according to the range of wavelengths that the filter should transmit. These materials and these dimensions may be determined by means of a simulation tool such as Comsol, Lumerica, or Rsoft. 
         [0044]    Due to the fact that, conversely to a pattern  11 , each bar of a pattern  35 ,  45 , or  55  may have a different thickness and may penetrate more or less deeply into another bar of the pattern, additional parameters are available for those skilled in the art to adapt the optical properties of a filter comprising one or a plurality of patterns  35 ,  45 , or  55 . 
         [0045]    Further, the fact of forming the bars of a pattern one after the others enables to manufacture a pattern where the angle between the longitudinal directions of two bars of the pattern is smaller than 45°, for example, 20°, which cannot be achieved with the manufacturing method described in relation with  FIG. 2  when the dimensions of the bars become smaller than some hundred nanometers. 
         [0046]    As an example, the first material is a metal selected from the group comprising aluminum, copper, tungsten, platinum, silver, gold, or an alloy of at least two of these metals. The second material is for example a dielectric material selected from the group comprising silicon nitride, silicon oxynitride, silicon oxide, titanium dioxide, magnesium fluoride, hafnium oxide, or silicon carbide. The first and second materials may be exchanged. The first and second materials may also be dielectric materials having different optical indexes. 
         [0047]    Support  21  is for example a layer of the second material. In this case, at the steps of  FIGS. 3E, 4E, and 5E , when a layer of the second material is left in place on the upper surface of the first material, pattern  35 ,  45 , or  55  then extends between two layers of the second material. Support  21  may also be the upper surface of a stack of layers coating a semiconductor substrate having one or a plurality of photodetectors or photoemitters formed therein. Support  21  may also be a window glass, for example, to block ultraviolet and/or infrared rays. 
         [0048]    As a comparison, a first filter and a second filter having their bandwidth centered on the 420-nm wavelength, that is, filters transparent to visible violet light, are considered. The first filter comprises a first cruciform pattern on a single level and the second filter comprises a second cruciform pattern of the type of that in  FIG. 3E , on two levels and equivalent to the first pattern. Each arm of the first pattern has a 40-nm thickness, a first side having a length  13  of 24 nm, and a second side having a length  15  of 72 nm. Each bar  27  and  33  of the second pattern has a 20-nm thickness, a 120-nm length, and a 72-nm width. Each pattern is made of silicon oxide and is periodically repeated in an aluminum layer according to a 150-nm period. 
         [0049]      FIGS. 6A and 6B  illustrate, respectively for the first filter and the second filter defined in the comparative example, the variation of transmission rate T (in %) according to wavelength λ (in nm). The curves are obtained by simulation in the ideal case where the filter patterns are perfect, that is, where their angles are not rounded. 
         [0050]      FIG. 6A  comprises four curves  71 A,  72 A,  73 A, and  74 A of the variation of transmission rate T according to wavelength k, respectively for angles of incidence of light of 0, 15, 30, and 60°. 
         [0051]      FIG. 6B  comprises four curves  71 B,  72 B,  73 B, and  74 B of the variation of transmission rate T according to wavelength λ, respectively for angles of incidence of light of 0, 15, 30, and 60°. 
         [0052]    As shown in the drawings, for a wavelength of approximately 420 nm, the distances between curves  71 B,  72 B,  73 B, and  74 B taken two by two are smaller than the corresponding distances between curves  71 A,  72 A,  73 A, and  74 A. In other words, the second filter comprising pattern  35  on two levels is less sensitive to the angle of incidence of light than the first filter comprising pattern  11  on a single level. 
         [0053]    For wavelengths distant from the bandwidth, the values of the transmission rates of curves  71 B,  72 B,  73 B, and  74 B are smaller than the corresponding transmission rate values of curves  71 A,  72 A,  73 A, and  74 A, respectively. In other words, the rejection ratio of the second filter, outside of the bandwidth, is better than that of the first filter. 
         [0054]    It can also be observed that the bandwidth of the second filter is narrower than that of the first filter. In other words, the second filter is more selective than the first one. 
         [0055]    Thus, the second filter has optical properties, some of which are better than those of the first filter. 
         [0056]    The advantages of the second filter as compared with the first filter still more clearly appear in the case where the first filter is a real filter having rounded cruciform patterns such as shown in  FIG. 2 , and the second filter is a real filter obtained according to the method of  FIGS. 3A to 3E . The inventors have observed that, for filters having the above-defined dimensions, the maximum value of the transmission rate of the first filter becomes lower than 40%, instead of 65% in the ideal case, while the maximum value of the transmission rate of the second filter remains equal to 45%. 
         [0057]    The advantages of the second filter over the first filter remain true when the pattern of the first filter is obtained according to the method described in relation with  FIGS. 4C and 4E  or with  FIGS. 5C and 5E . 
         [0058]    Methods of manufacturing a filter comprising a pattern capable of being divided into two bars have been previously described. It should be understood that such manufacturing methods may be adapted to manufacture filters with pattern(s) capable of being divided into more than two bars. For example, a pattern which may be divided into three bars may be obtained from a structure of the type obtained at the steps illustrated in  FIGS. 3E, 4E, and 5E  by depositing an additional layer of the first material on layer  29 ; by then etching another rectangular cavity through this layer all the way to one and/or the other of bars  27  and  33 ,  43 , or  53 ; and by filling this other cavity with the second material to form therein a third bar of the pattern. 
         [0059]    Various embodiments with different variations have been described hereabove. It should be noted that those skilled in the art may combine various elements of these various embodiments and variations without showing any inventive step. 
         [0060]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.