Abstract:
A plasmonic optical filter, including: a periodic repetition of metal slabs above a metal surface; dielectric spacers arranged between the slabs and the metal surface so that there exists an empty space between each slab and the metal surface; and an opening between each of said empty spaces and the outside.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the priority to French patent application number 15/60912, filed Nov. 13, 2015, which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
       BACKGROUND 
       [0002]    The present disclosure relates to optical filters, and more particularly to an optical filter using plasmonic resonators of MIM (metal-oxide-metal) type to selectively transmit or absorb an optical radiation. 
       DISCUSSION OF THE RELATED ART 
       [0003]    Optical filters with plasmonic resonators, or plasmonic filters, are used to selectively transmit or absorb an optical radiation having a selected wavelength. A plasmonic filter may for example be used in a bolometer to selectively absorb an infrared radiation.  FIG. 1  is a perspective view of a plasmonic resonator arranged at the surface of a bolometer membrane  1 , corresponding to  FIG. 1  of “Multispectral microbolometers for the midinfrared”, T. Maier et Al., Optics Letters Vol. 35 N° 22, Nov. 15, 2010. The resonator comprises a metal layer  3  having a silicon nitride dielectric layer  5  covered with a square metal slab extending thereon. Dimension d of the slab sides is equal to λ/2n, λ designating the wavelength, and n designating an effective index of the plasmonic mode, close to the refraction index of layer  5 . 
         [0004]    The quality of the filtering is all the greater as the shape of the slab, which may be of submicron size, is accurately formed. Now, slabs of small size obtained by the available manufacturing techniques in reality have rounded angles and do not exactly have the desired dimensions. The quality of the obtained filtering is then altered. 
       SUMMARY 
       [0005]    Thus, an embodiment provides a plasmonic optical filter comprising a periodic repetition of metal slabs above a metal surface; dielectric spacers arranged between the slabs and the metal surface so that there exists an empty space between each slab and the metal surface; and an opening between each of said empty spaces and the outside. 
         [0006]    According to an embodiment, the metal slabs are arranged in an array and have the shape of squares with sides having a dimension in the range from 0.3 μm to 3 μm. 
         [0007]    According to an embodiment, the metal slabs have a thickness in the range from 30 nm to 100 nm and the dielectric spacers have a thickness in the range from 30 nm to 300 nm. 
         [0008]    According to an embodiment, the dielectric spacers form a grid delimiting said empty spaces, the entire periphery of each slab being arranged on the grid, and said opening being formed in each slab. 
         [0009]    According to an embodiment, the openings have diameters in the range from 10 to 40 nm. 
         [0010]    According to an embodiment, the grid delimits square empty spaces. 
         [0011]    According to an embodiment, the grid delimits circular empty spaces. 
         [0012]    According to an embodiment, the dielectric spacers are pads arranged in an array, each slab having four corners arranged on four neighboring pads, the openings being spaces between the slabs. 
         [0013]    According to an embodiment, the dielectric spacers are bar-shaped, each slab having two edges arranged on two neighboring bars, the openings being spaces between the slabs. 
         [0014]    An embodiment provides a method of forming a plasmonic optical filter on a metal surface, comprising the steps of: 
         [0015]    a) depositing a dielectric layer on the metal surface; 
         [0016]    b) forming, on the dielectric layer, a periodic repetition of separate metal slabs, each of which is provided with an opening; and 
         [0017]    c) removing a portion of the dielectric layer by selective isotropic etching from the openings, to form empty spaces under each metal slab. 
         [0018]    According to an embodiment, the method comprises, between step b) and step c), a step of masking the portions of the dielectric layer accessible between the metal slabs. 
         [0019]    According to an embodiment, the dielectric layer is made of silicon oxide. 
         [0020]    An embodiment provides a method of forming a plasmonic optical filter on a metal surface, comprising the steps of: 
         [0021]    a) forming a periodic repetition of dielectric spacers on the metal surface; 
         [0022]    b) filling with a sacrificial material the entire volume between the spacers; 
         [0023]    c) forming a periodic repetition of separate metal slabs, each slab mostly resting on the sacrificial material; and 
         [0024]    d) selectively etching the sacrificial material from the openings between the separate metal slabs. 
         [0025]    According to an embodiment, the dielectric spacers are made of silicon oxide, the sacrificial material is silicon nitride, and the selective etching is a RIE etching in a SF 6  and oxygen medium. 
         [0026]    According to an embodiment, the dielectric spacers are made of silicon, the sacrificial material is silicon oxide, and the selective etching is a RIE etching under a CF 4  and oxygen plasma. 
         [0027]    According to an embodiment, the dielectric spacers are made of silicon oxide, the sacrificial material is silicon, and the selective etching is a RIE etching under a BCl 3 , Cl 2  and nitrogen plasma or a dry etching under xenon difluoride (XeF 2 ). 
         [0028]    According to an embodiment, the dielectric spacers are made of aluminum oxide, the sacrificial material is silicon oxide, and the selective etching is a chemical vapor etching with hydrofluoric acid. 
         [0029]    An embodiment provides a bolometer comprising a filter such as hereabove. 
         [0030]    The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of dedicated embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a perspective view of a plasmonic resonator; 
           [0032]      FIGS. 2A to 2D  illustrate steps of a method of forming a plasmonic optical filter; 
           [0033]      FIG. 3  illustrates the absorption according to the wavelength by the optical filter of  FIG. 2D ; 
           [0034]      FIG. 4  is a partial perspective cross-section view of a variation of a plasmonic optical filter; 
           [0035]      FIGS. 5A to 5D  illustrate steps of a method of forming another variation of a plasmonic optical filter; 
           [0036]      FIG. 6  illustrates the absorption according to the wavelength by the optical filter of  FIG. 5D ; and 
           [0037]      FIG. 7  is a partial perspective cross-section view of another variation of a plasmonic optical filter. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    The same elements have been designated with the same reference numerals in the different drawings and, further, the various drawings are not to scale. For clarity, only those elements which are useful to the understanding of the described embodiments have been shown and are detailed. 
         [0039]    In the following description, when reference is made to terms qualifying a relative position, such as term “top”, “bottom”, “upper”, “on”, “under”, reference is made to the orientation of the concerned element in the drawings. Unless otherwise specified, expression “in the order of” means to within 10%, preferably to within 5%. 
         [0040]      FIGS. 2A to 2D  are partial perspective cross-section views illustrating steps of a method of forming a plasmonic optical filter. The plasmonic filter is formed on a support  10  having a metallic upper surface  13 , for example, a bolometer membrane covered with a metal layer  12 . 
         [0041]    At the step illustrated in  FIG. 2A , dielectric pads  14  are formed on metal surface  13  of support  10 . Pads  14  have a square shape and are arranged in an array. Pads  14  are for example formed by lithography. 
         [0042]    At the step illustrated in  FIG. 2B , the space between pads is filled with a sacrificial material  16 , pads  14  being flush with the upper surface of sacrificial material  16 . To perform this filling, a layer of sacrificial material may be deposited on metal surface  13  and on the pads, after which the upper surface of the assembly may be polished at least all the way to the level of pads  14  to obtain a planar surface. 
         [0043]    At the step illustrated in  FIG. 2C , separate metal square slabs  17  are formed on the upper surface of the assembly, for example, by lithography. The corners of each slab are arranged on four neighboring pads  14 , the slab mostly resting on sacrificial material  16 . The slab thus obtained is regular, and the slabs are separated by spacings  18 . 
         [0044]      FIG. 2D  shows the plasmonic optical filter  19  obtained after a step of removing sacrificial material  16  by selective etching. Each slab is held by the pads located on its corners and the most part of each slab is suspended above an empty space  20 . Pads  14  have the function of spacers enabling to keep empty space  20 . 
         [0045]    As previously indicated, dimension d of the slab sides is equal to λ/2n, λ designating the wavelength, and n designating the refraction index of the material located under each slab. Now, n is now equal to 1 under the most part of each slab. Thereby, for a given filtering wavelength, slabs  17  may be up to n times larger than in the case where these slabs rest on a dielectric material. n is for example close to 1.45 for silicon oxide and close to 2 for silicon nitride. For larger slabs, the shapes are formed with a better accuracy, and filter  19  of  FIG. 2D  has a better accuracy as to the position of the obtained filtering peak relative to the one which is desired. 
         [0046]      FIG. 3  shows curves  21  to  27  illustrating absorption A of an optical radiation by embodiments of filters according to the method of  FIGS. 2A to 2D , according to wavelength λ of the radiation. The filters only differ by the dimensions of their pads  14 , slabs  17  being identical with identical spacings  18 . The filter associated with curve  21  has the largest pads  14  and accordingly the smallest empty spaces  20 . The filter associated with curve  27  conversely has the smallest pads  14 , empty spaces  20  being the largest. It can be observed that with slabs of same dimensions, a radiation having a wavelength all the smaller as empty spaces  20  under the pads are large can be filtered. 
         [0047]    As an example, metal layer  12  and metal slabs  17  are made of aluminum. Pads  14  may be made of silicon oxide and the sacrificial material may be silicon nitride, the selective etching of the sacrificial layer can then be performed by reactive ion etching or RIE in a SF 6  and oxygen medium. In a variation, pads  14  are made of polysilicon, the sacrificial material is silicon oxide, and the selective etching is a RIE etching under a CF 4  and oxygen plasma. In another variation, pads  14  are made of silicon oxide, the sacrificial material is polysilicon, and the selective etching is a RIE etching under a BCl 3 , Cl 2  and nitrogen plasma or a dry etching under xenon difluoride (XeF 2 ). In another variation, the pads are made of aluminum oxide, the sacrificial material is silicon oxide, and the selective etching is a chemical vapor etching with hydrofluoric acid. More generally, any combination of two materials to which a selective etch method can be adapted may be selected for the pads and the sacrificial material. 
         [0048]    As an example, dimension d of the sides of the slabs has a length in the range from 0.2 μm to 3 μm, respectively corresponding to a wavelength in the range from 0.4 to 6 μm. 
         [0049]    The slab thickness may be in the range from 20 to 100 nm. The thickness or height of the pads may be in the range from 30 to 300 nm. 
         [0050]      FIG. 4  is a partial perspective cross-section view of a variation of a plasmonic optical filter  30 . Optical filter  30  corresponds to filter  19  of  FIG. 2D  where pads  14  have been replaced with parallel bars  32 . Each slab  17  forms a bridge above the empty space  34  located between two bars. 
         [0051]    Each slab  17  of filter  30  is held by two sides, which provides a better mechanical resistance than that of the slabs of filter  19  of  FIG. 2D  which are only held by their corners. Such a mechanical resistance is advantageous since the device may be submitted to thermal expansions, to pressures, or to vibrations, which may damage the slabs. There however is a sensitivity to biasing in this embodiment, since the structure no longer has the 90° rotational symmetry. 
         [0052]    It should be noted that the empty spaces located under slabs  17  of filter  30 , as well as under slabs  17  of filter  19  of  FIG. 2D , are open towards the outside by spacings  18  between the separate slabs. Thus, in a pressure or temperature variation, the gas present in the empty space under the slabs may freely enter or escape, which avoids adverse mechanical stress. 
         [0053]      FIGS. 5A to 5D  are partial perspective cross-section views illustrating steps of a method of forming another variation of a plasmonic filter on metal surface  13  of a support  10 . 
         [0054]    The step illustrated in  FIG. 5A  corresponds to the step of  FIG. 2A , pads  14  of  FIG. 2A  having been replaced with a grid-shaped structure  36  delimiting square spaces  38 . 
         [0055]    The step illustrated in  FIG. 5B  corresponds to the step of  FIG. 2B . Spaces  38  are filled with a sacrificial material  16 . 
         [0056]    The step illustrated in  FIG. 5C  corresponds to the step of  FIG. 2C . Separate square metal slabs  42  are formed on the upper surface of the assembly and form a regular paving. Each slab  42  covers the sacrificial material located in a space  38  and the periphery of each slab is entirely located on grid  36 . 
         [0057]    Each slab  42  is provided with an opening  44 , for example located at the center of the slab. 
         [0058]      FIG. 5D  shows the optical filter  46  obtained after a step of selectively etching sacrificial material  16  from openings  44 . The most part of each slab is located above an empty space  38 . 
         [0059]    Each empty space  38  communicates with the outside through opening  44  in the slab. As previously indicated, the communication openings enable the filter to mechanically withstand pressure variations. Further, each slab is now held along its entire periphery. This feature advantageously provides mechanical filter  46  with a remarkably increased mechanical resistance. Further, this embodiment keeps the insensitivity to biasing, since the structure keeps the 90° rotational symmetry (if the x and y periods are equal). 
         [0060]    The openings may have any shape. As an example, openings  44  are circular, with diameters in the range from 10 to 40 nm. The inventors have observed that the presence of such openings  44  has a negligible effect on the optical properties of the filter. 
         [0061]    This is shown in  FIG. 6 , which illustrates simulation results. Absorption  47  of an optical radiation by a filter  46  is compared with absorption  48  by an identical filter where the slabs would comprise no openings. The presence in filter  46  of circular openings  44  having diameters reaching 40 nm in slabs having 300-nm sides only increases by less than 2% the interval of absorbed wavelengths. With all the more reason, smaller openings in larger slabs have even lesser effects. 
         [0062]    Filter  46  has an optical quality identical to that of filters  19  and  30  of  FIGS. 2D and 4 , and withstands pressure variations just as well, while being provided with a remarkably increased mechanical resistance. Further, the insensitivity to biasing is kept. 
         [0063]      FIG. 7  is a partial perspective cross-section view of another variation of a plasmonic optical filter  50 . 
         [0064]    Optical filter  50  is formed by forming, on a uniform layer of a dielectric material  52  covering metal surface  13  of a support  10 , a regular paving of separate square metal slabs  42 . Each slab is provided with a central opening  44 . Dielectric material  52  is then selectively etched, isotropically, from openings  44  to form an empty space  54  under the most part of each of slabs  42 . The etching may be performed after a masking intended to protect the portions of dielectric material  52  accessible between the slabs. The obtained filter  50  corresponds to filter  46  of  FIG. 5D , where square empty spaces  38  have been replaced with circular empty spaces  54 . The remaining dielectric material  52  forms a grid-shaped structure delimiting circular spaces. As an example, dielectric material  52  may be silicon oxide. 
         [0065]    Filter  50  has the advantage that it can be formed in a very simple way. 
         [0066]    Specific embodiments have been described. Various alterations, modifications, and improvements will occur to those skilled in the art. In particular, metal surface  13  of the described embodiments is a continuous surface on which the formed plasmonic filters are reflection filters, that is, filters absorbing an optical radiation having a selected wavelength and reflecting the optical radiations of other wavelengths. Variations of plasmonic filters transmitting a radiation of selected wavelength are possible, where the metal support comprises separate metal slabs formed on a transparent support. 
         [0067]    Further, although the slabs of the above-described embodiments are square-shaped, the slabs may have other shapes capable of forming plasmonic resonators. As an example, the slabs may be cross-shaped or round. As a variation, the slabs may have rectangular shapes to favor the filtering of radiations having a selected biasing. 
         [0068]    Further, although in the described embodiments, the slabs are arranged in an array, the slabs may be periodically repeated according to other configurations. For example, the slabs may be arranged in a triangular network. 
         [0069]    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.