Seebeck/peltier thermoelectric conversion element with parallel nanowires of conductor or semiconductor material organized in rows and columns through an insulating body and process

Abstract

A novel and effective structure of a stackable element (A 1 ,A 2 ) or more generally adapted to be associated modularly to other similar elements to form a septum of relatively large dimensions for a Seebeck/Peltier thermoelectric conversion device, may be fabricated with common planar processing techniques. The structure basically consists of a stack (A 1 , A 2 ) of alternated layers of a first dielectric material ( 2 ), adapted to be deposited in films of thickness lesser than or equal to about 50 nm, of low heat conductivity and which is etchable by a solution of a specific chemical compound, and of a second dielectric material ( 3 ) of low heat conductivity that is not etched by the solution. For the whole width, the stack is interrupted by parallel trenches (T 1 , T 2 , T 3 ) the width (w) of which may correspond to the minimum linewidth of definition allowed by the resolution of the lithographic process used for defining the parallel trenches, but which may eventually be limited by other parameters, in primis the height of the stack (h) to be subjected to the vertical etch to cut the stack in order to form the parallel trenches.

1 . An element (A 1 , A 2 ) fabricable with planar processing techniques, modularly associable to other similar elements for constituting a septum for a Seebeck/Peltier thermo-electric conversion device, comprising a stack of width (W) corresponding to the thickness of the septum and a length (L) corresponding to a dimension or fraction of a dimension of the surface area of the septum, the element having a population of nanowires ( 4 ) of conductor or semiconductor material of high Seebeck coefficient, parallel and insulated from one another extending across the width (W) of the stack, organized in rows and columns, said stack comprising:
 a) alternated layers of a first dielectric material ( 2 ) depositable in continuous film of thickness lesser than or equal to 50 nm, etchable by a solution of a specific chemical compound, and of a second dielectric material ( 3 ) of low heat conductivity not etchable by said solution;   b) parallel trenches (T 1 , T 2 , T 3 , . . . , Ti . . . ) the lower limit of width (w) of which corresponds to the minimum linewidth of lithographic definition, spaced by at least 1 μm;   c) parallel cavities ( 4   c ) of retreat of the etch front of the layers of said first dielectric material ( 2 ) by said etching solution, along opposite surfaces of each trench;   d) wirelike nanodimensioned residues of a conductor or semiconductor filler material of said parallel nanocavities ( 4   c ) constituting said parallel spaced nanowires ( 4 ) organized in rows and columns.   
     
     
         2 . The element of  claim 1 , wherein said conductor or semiconductor material of high Seebeck coefficient belongs to the group composed by an intrinsic or doped semiconductor, a metal, a metal alloy, an alloy of semimetals, an alloy between a metal and a semimetal and mixtures thereof. 
     
     
         3 . The element of  claim 2 , wherein said conductor or semiconductor material of high Seebeck coefficient belongs to the group composed of monocrystalline or polycrystalline silicon with natural isotopic presence or enhanced presence of  29 Si, intrinsic or doped with donor and/or acceptor atoms, a silicon-germanium alloy with natural isotopic presence or enhanced presence of  29 Si, either intrinsic or doped with donor and/or acceptor atoms, tungsten, titanium and alloys thereof. 
     
     
         4 . The element of  claim 1 , wherein said first dielectric material ( 2 ) is SiO 2  and said second dielectric material ( 3 ) is Si 3 N 4 . 
     
     
         5 . The element of  claim 1 , wherein said first dielectric material ( 2 ) is S 3 N 4  and said second dielectric material ( 3 ) is SiO 2 . 
     
     
         6 . The element of  claim 1 , comprising a substrate ( 1 ) of a low heat conductivity dielectric material resistant to the etching solution of said first dielectric material ( 2 ). 
     
     
         7 . The element of  claim 1 , wherein said nanowires ( 4 ) have disuniform cross section along their longitudinal extension and morphologically rugged outer surface. 
     
     
         8 . The element of  claim 1 , characterized in that the stack has a density of nanowires ( 4 ) of average cross section of about 40 nm×20 nm, per unit of area of side surfaces of heat input/output the stack up to 5×10 10  nanowires/cm 2 . 
     
     
         9 . The element of  claim 1 , wherein the height (h) of the stack and the periodicity or pitch of replication of said parallel trenches are of the same order of magnitude. 
     
     
         10 . A fabrication process for an element (A 1 , A 2 ) modularly associable to other elements for constituting a septum for a Seebek/Peltier thermo-electric conversion device, comprising the steps of:
 a) depositing a first layer of one or the other of a first dielectric material ( 2 ) depositable in continuous film of thickness lesser than or equal to 50 nm, etchable by a solution of a specific chemical compound and of a second dielectric material ( 3 ) of low heat conductivity not etchable by said solution, on a flat substrate ( 1 ) of a low heat conductivity dielectric material resistant to said solution;   b) depositing over said first layer of one of said two different dielectric materials, a layer of the other dielectric material and repeating steps a) and b) for a number of times sufficient to reach a desired height of the stack of layers of step a) alternated to layers of step b);   c) forming or applying a mask over a stack defining parallel etch openings, the minimum limit of width (w) of which corresponds to the minimum linewidth of definition of the lithographic technique used, spaced by at least 1 μm, extending for the whole width (W) of the stack;   d) etching in plasma or in reactive plasma or by sputtering through said mask openings the multilayered stack forming parallel trenches (T 1 , T 2 , T 3 , . . . , Ti, . . . ) deep as far as reaching the surface of said substrate ( 1 );   e) etching with said solution the etch surfaces of the layers of said first dielectric material ( 2 ) as far as backing them in, between adjacent layers of said second dielectric material ( 3 ), by an average distance of 20 nm, forming parallel cavities ( 4   c ) on the opposite etch surfaces of each trench (T 1 , T 2 , T 3 , . . . Ti, . . . );   f) eventually removing from the surface of the stack residues of said mask, chemically depositing from vapor phase conductor or semiconductor material of high intrinsic Seebeck coefficient, filling said parallel cavities ( 4   c ) as far as growing a conformal layer ( 4   m ) free of discontinuities over horizontal and vertical parallel surfaces of the slotted stack;   g) forming or applying anew said mask over the surface of the stack;   h) etching in plasma, in a reactive plasma or by sputtering through the mask openings the conductor or semiconductor material of said conformally deposited layer ( 4   m ) as far as removing it completely from said horizontal and vertical planar surfaces, leaving nanowire-like residues ( 4 ) of said conductor or semiconductor material inside said parallel cavities ( 4   c ).   
     
     
         11 . The process of  claim 10 , wherein said conductor or semiconductor material of high intrinsic Seebeck coefficient belongs to the group composed of a semiconductor intrinsic or doped, a metal, a metal alloy, an alloy between semi-metals, an alloy between a metal and semimetal and mixtures thereof. 
     
     
         12 . The process of  claim 11 , wherein said conductor or semiconductor material of high intrinsic Seebeck coefficient belongs to the groups composed of monocrystalline or polycrystalline silicon with natural isotopic presence or enhanced presence of  29 Si, intrinsic or doped with donor and/or acceptor atoms, a silicon-germanium alloy with natural isotopic presence or enhanced presence of  29 Si, either intrinsic or doped with donor and/or acceptor atoms, tungsten, titanium and alloys thereof. 
     
     
         13 . The process of  claim 10 , wherein said first dielectric material ( 2 ) is SiO 2 , said second dielectric material ( 3 ) is Si 3 N 4  and said etching solution is an aqueous solution of hydrofluoric acid. 
     
     
         14 . The process of  claim 10 , wherein said first dielectric material ( 2 ) is Si 3 N 4 , said second dielectric material ( 3 ) is SiO 2  and said etching solution is an aqueous solution or a melt of phosphoric acid. 
     
     
         15 . A septum for a Seebeck/Peltier thermo-electric conversion device composed of any number of associable elements (A 1 , A 2 , . . . ), each comprising a stack of width (W) corresponding to the thickness of the septum and a length (L) corresponding to a dimension or fraction of a dimension of the surface area of the septum, each element (A 1 , A 2 , . . . ) having a population of nanowires ( 4 ) of conductor or semiconductor material of high intrinsic Seebeck coefficient, parallel and insulated from one another extending across the width (W) of the stack, organized in rows and columns, the stack of each element comprising:
 a) alternated layers of a first dielectric material ( 2 ) depositable in continuous film of thickness lesser than or equal to 50 nm, etchable by a solution of a specific chemical compound, and of a second dielectric material ( 3 ) of low heat conductivity not etchable by said solution;   b) parallel trenches (T 1 , T 2 , T 3 , . . . , Ti . . . ) the lower limit of width (w) of which corresponds to the minimum linewidth of lithographic definition, spaced by at least 1 μm;   c) parallel cavities ( 4   c ) of retreat of the etch front of the layers of said first dielectric material ( 2 ) by said etching solution, along opposite surfaces of each trench;   d) wirelike nanodimensioned residues of a conductor or semiconductor filler material of said parallel nanocavities ( 4   c ) constituting said parallel spaced nanowires ( 4 ) organized in rows and columns;   e) metallizations ( 5 ) on side surfaces of heat input/output of the septum, adapted to electrically connect in parallel by groups said parallel nanowires ( 4 );   f) means ( 6 ) of electrical connection in series of said groups of nanowires ( 4 ) in parallel, to two terminals of the whole series-parallel network of nanowires ( 4 ) of the septum.   
     
     
         16 . The septum of  claim 15 , wherein said conductor or semiconductor material of high Seebeck coefficient belongs to the group composed by an intrinsic or doped semiconductor, a metal, a metal alloy, an alloy of semimetals, an alloy between a metal and a semimetal and mixtures thereof. 
     
     
         17 . The septum of  claim 15 , wherein said conductor or semiconductor material of high intrinsic Seebeck coefficient belongs to the groups composed of monocrystalline or polycrystalline silicon with natural isotopic presence or enhanced presence of  29 Si, intrinsic or doped with donor and/or acceptor atoms, a silicon-germanium alloy with natural isotopic presence or enhanced presence of  29 Si, either intrinsic or doped with donor and/or acceptor atoms, tungsten, titanium and alloys thereof. 
     
     
         18 . The septum of  claim 15 , wherein said first dielectric material ( 2 ) is SiO 2  and said second dielectric material ( 3 ) is Si 3 N 4 . 
     
     
         19 . The septum of  claim 15 , wherein said first dielectric material ( 2 ) is Si 3 N 4  and said second dielectric material ( 3 ) is SiO 2 . 
     
     
         20 . The septum of  claim 15 , characterized in that the stack has a density of nanowires ( 4 ) of average cross section of about 40 nm×20 nm, per unit of area of side surfaces of heat input/output of the stack up to 5×10 10  nanowires/cm 2 . 
     
     
         21 . The septum of  claim 15 , characterized in that the height (h) of the stack and periodicity or pitch of replication of said parallel trenches (T 1 , T 2 , T 3 , . . . , Ti . . . ) are of the same order of magnitude.