Patent Publication Number: US-2020303613-A1

Title: A method for manufacturing a gradient heat-flux sensor

Description:
TECHNICAL FIELD 
     The disclosure relates generally to gradient heat-flux sensors “GHFS” for measuring thermal energy transfer directly. More particularly, the disclosure relates to a method for manufacturing a gradient heat-flux sensor. 
     BACKGROUND 
     Heat-flux sensors are used in various power-engineering applications where local heat-flux measurements can be more important than temperature measurements. A heat-flux sensor can be based on multiple thermoelectric junctions so that tens, hundreds, or even thousands of thermoelectric junctions are connected in series. For another example, a heat-flux sensor can be based on one or more anisotropic elements where thermal electromotive force is created from a heat-flux by the Seebeck effect. Because of the anisotropy, a temperature gradient has components in two directions: along and across to a heat-flux through the sensor. Electromotive force is generated proportional to the temperature gradient component across to the heat-flux. The heat-flux sensor is called a gradient heat-flux sensor “GHFS” because it generates an electric output signal proportional to the above-mentioned temperature gradient component across to the heat-flux. The anisotropy can be implemented with suitable anisotropic material such as for example single-crystal bismuth. A drawback of gradient heat-flux sensors based on single-crystal bismuth is that they are not suitable for heat-flux measurements in high temperatures because of the low melting point of bismuth. 
     Another option for implementing the anisotropy is a multilayer structure where layers are oblique with respect to a surface of a gradient heat-flux sensor for receiving a heat-flux. The multilayer structure may comprise first layers and second layers so that the second layers are interleaved with the first layers. The first layers can be made of for example semiconductor material, and the second layers can be made of for example metal or metal alloy or of semiconductor material different from the semiconductor material of the first layers. An upper limit of operating temperature of a gradient heat-flux sensor based on a multilayer structure can be significantly higher than that of a heat-flux sensor based on bismuth. Further details of gradient heat-flux sensors based on a multilayer structure can be found from for example the publication: “ Local Heat Flux Measurement in a Permanent Magnet Motor at No Load ”, Hanne K. Jussila, Andrey V. Mityakov, Sergey Z. Sapozhnikov, Vladimir Y. Mityakov and Juha Pyrhönen, Institute of Electrical and Electronics Engineers “IEEE” Transactions on Industrial Electronics, Volume: 60, pp. 4852-4860, 2013. 
     Gradient heat-flux sensors based on a multilayer structure of the kind described above are, however, not free from challenges. One of the challenges is related to manufacturing processes which are typically difficult to adapt for mass-production. Thus, the unit price of gradient heat-flux sensors based on a multilayer structure can be high. The high unit price, in turn, limits the use of gradient heat-flux sensors in mass-produced products. 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention. 
     In this document, the word “geometric” when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric point, a straight or curved geometric line, a geometric plane, a non-planar geometric surface, a geometric space, or any other geometric entity that is zero, one, two, or three dimensional. 
     In accordance with the invention, there is provided a new method for manufacturing a gradient heat-flux sensor based on a multilayer structure where layers are oblique with respect to a surface of the gradient heat-flux sensor for receiving a heat-flux. 
     A method according to the invention comprises:
         depositing a semiconductor layer on a planar surface,   removing material from the semiconductor layer so that mutually parallel semiconductor ridges are formed, the semiconductor ridges being separate from each other and having first sidewalls being slanting with respect to the planar surface and facing towards a first direction and second sidewalls facing towards a second direction different from the first direction, and subsequently   filling gaps between adjacent ones of the semiconductor ridges with one or more materials comprising metal so that i) metal-semiconductor contact junctions are formed on at least the first sidewalls of the semiconductor ridges and ii) gap-fillers constituted by the one or more materials and located in different ones of the gaps are separate from each other so that the metals of the gap-fillers located in different ones of the gaps are free from metallic contacts between each other.       

     The above-mentioned semiconductor ridges and the gap-fillers constitute an anisotropic metal-semiconductor multilayer structure for forming electromotive force in a direction perpendicular to the semiconductor ridges and parallel with the above-mentioned planar surface in response to a heat-flux through the anisotropic structure in a direction perpendicular to the planar surface. The anisotropic metal-semiconductor multilayer structure can be used as also a thermoelement which transfers heat from its first side to its second side in the direction perpendicular to the planar surface when electric current is driven through it in the direction perpendicular to the semiconductor ridges and parallel with the planar surface. 
     In a method according to an exemplifying and non-limiting embodiment of the invention, the above-mentioned gap-fillers are formed by:
         depositing metal layers on the first sidewalls of the semiconductor ridges whilst leaving the second sidewalls uncovered so that the metal layers are separate from each other, and subsequently   filling the gaps with semiconductor material constituting semiconductor fillers so that the semiconductor fillers located in different ones of gaps are separate from each other.       

     The method phases of the above-described method are similar to for example method phases of existing methods for fabricating integrated circuits “IC”. Thus, the method is suitable for mass-production. 
     The semiconductor ridges can be formed for example with a wet etching technique which produces the slanting sidewalls of the semiconductor ridges. It is, however, also possible to use other methods for removing material from the semiconductor layer so that the slanting sidewalls of the semiconductor ridges are formed. Thus, the above-described method utilizes the inherent property of such material removal techniques, e.g. wet etching, which produce non-vertical, i.e. slanting, sidewalls of grooves and cavities. In this document, the term ‘vertical’ means a direction that is perpendicular to the planar surface on which the semiconductor layer is deposited. 
     Various exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims. 
     Various exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non-limiting embodiments when read in conjunction with the accompanying drawings. 
     The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor require the existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or “an”, i.e. a singular form, throughout this document does not exclude a plurality. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: 
         FIG. 1  shows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for manufacturing a gradient heat-flux sensor, 
         FIGS. 2 a , 2 b , 2 c , 2 d , 2 e  and 2 f    illustrate successive manufacturing stages of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention, 
         FIG. 3  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention, 
         FIG. 4  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention, 
         FIG. 5  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention, 
         FIG. 6  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention, and 
         FIG. 7  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EXEMPLIFYING EMBODIMENTS 
     The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise explicitly stated. 
       FIG. 1  shows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for manufacturing a gradient heat-flux sensor.  FIGS. 2 a , 2 b , 2 c , 2 d , 2 e  and 2 f    illustrate successive manufacturing stages of the gradient heat-flux sensor. In phase  101  shown in  FIG. 1 , the method comprises depositing a semiconductor layer on a planar surface. In  FIG. 2 a   , the semiconductor layer is denoted with a reference  210 . The material of the semiconductor layer  210  can be for example n-doped or p-doped silicon. In this exemplifying case, an electrically insulating layer  214  is deposited on a substrate  215  and thereafter the semiconductor layer  210  is deposited on the electrically insulating layer. Depending on the materials of the substrate  215  and the semiconductor layer  210 , it is also possible that the semiconductor layer  210  is deposited directly on the substrate  215 . 
     In phase  102  shown in  FIG. 1 , the method comprises removing material from the semiconductor layer  210  so that mutually parallel semiconductor ridges are formed. In  FIGS. 2 b -2 f   , the semiconductor ridges are denoted with a reference  211 . As illustrated in  FIG. 2 b   , the semiconductor ridges  211  have first slanting sidewalls facing towards a first direction and second sidewalls facing towards a second direction different from the first direction. In  FIG. 2 b   , the first slanting sidewalls are denoted with a reference  225 . As shown in  FIG. 2 b   , the slanting sidewalls  225  of the semiconductor ridges  211  are oblique with respect to the vertical xz-plane of a coordinate system  299 , i.e. with respect to a vertical geometric plane that is perpendicular to the planar surface of the electrically insulating layer  214  and parallel with the longitudinal direction of the semiconductor ridges  211 . 
     Grooves made on the structure shown in  FIG. 2 a    for forming the above-mentioned semiconductor ridges  211  are so deep that the semiconductor ridges  211  are separate from each other. The semiconductor ridges  211  can be formed using for example a wet etching technique. The wet etching can be for example isotropic wet etching which provides rounded edges of the semiconductor ridges such as illustrated in  FIGS. 2 b -2 f   . The isotropic wet etching can be based on e.g. hydrogen fluoride acid, nitric acid, or acetic acid. In cases where the semiconductor layer  210  has a suitable crystal structure e.g. &lt;100&gt;, the semiconductor ridges  211  can be formed using anisotropic wet etching which provides slanted sidewalls that are more planar than those formed with isotropic wet etching. It is also possible that the semiconductor ridges  211  are formed with an isotropic dry etching directed to provide slanting sidewalls, e.g. a skewed DRIE-process, where DRIE means Deep Reactive-Ion Etching. 
     In phase  103  shown in  FIG. 1 , the method comprises filling gaps between adjacent ones of the semiconductor ridges so that metal-semiconductor contact junctions are formed on the above-mentioned first sidewalls of the semiconductor ridges. The gaps are filled so that gap-fillers formed into the gaps are separate from each other. In  FIGS. 2 e  and 2 f   , the gap-fillers are denoted with a reference  230 . The gap-fillers  230  are separate from each other because contacts between the gap-fillers  230  would disturb or even prevent the operation of the gradient heat-flux sensor because they would at least partly short-circuit the electromotive forces created from a heat-flux by the Seebeck effect. 
     In the exemplifying case illustrated in  FIG. 1  and in  FIGS. 2 a -2 f   , the phase  103  for filling the gaps comprises sub-phases  103   a  and  103   b . The sub-phase  103   a  comprises depositing metal layers on the above-mentioned first sidewalls of the semiconductor ridges whilst leaving the above-mentioned second sidewalls uncovered. In  FIG. 2 c   , the metal layers are denoted with a reference  212 . As illustrated in  FIG. 2 c   , the metal layers are separate from each other. The metal layers can be made of for example aluminum, copper, molybdenum, constantan, nichrome, or some other suitable metal. The sub-phase  103   b  comprises filling gaps between adjacent ones of the semiconductor ridges with semiconductor material so that the semiconductor material filling the gaps constitutes semiconductor fillers that are separate from each other. In  FIG. 2 e   , the semiconductor fillers are denoted with a reference  213 . The semiconductor fillers  213  are advantageously same material as the semiconductor ridges  211 , or material with properties of electrical conductivity and Seebeck coefficient similar to those of the material of the semiconductor ridges  211 . The material of the semiconductor fillers  213  can be for example n-doped or n-doped silicon. The semiconductor fillers  213  can be made of epitaxial silicon or polycrystalline silicon. In the exemplifying case illustrated in  FIGS. 2 a -2 f   , the semiconductor fillers  213  are formed by depositing a semiconductor layer on the structure comprising the semiconductor ridges  211  and the metal layers  212 , and subsequently removing material from the semiconductor layer. In  FIG. 2 d   , the above-mentioned semiconductor layer is denoted with a reference  216 . The above-mentioned semiconductor ridges  211 , the metal layers  212 , and the semiconductor fillers  213  constitute an anisotropic metal-semiconductor multilayer structure for forming electromotive force in a direction parallel with the y-axis of the coordinate system  299  in response to a heat-flux through the anisotropic structure in a direction parallel with the z-axis of the coordinate system  299 . 
     In a method according to an exemplifying and non-limiting embodiment of the invention, the semiconductor layer  216  and thereby the semiconductor fillers  213  are shorter than the semiconductor ridges  211  in the longitudinal direction of the semiconductor ridges, i.e. in the x-direction of the coordinate system  299 , so that end-regions of the gaps remain unfilled. This approach is advantageous in cases where there would be a risk that adjacent semiconductor fillers  213  would get, due to processual non-idealities, in contact with each other on the end-regions of the semiconductor ridges if the semiconductor layer  216  were deposited to fully cover the structure comprising the semiconductor ridges  211  and the metal layers  212 . Unwanted contacts between the semiconductor fillers  213  would disturb or even prevent the operation of the gradient heat-flux sensor because they would at least partly short-circuit the electromotive forces created from a heat-flux by the Seebeck effect. Other methods for avoiding unwanted contacts, such as trench isolation or structures of insulating material, are also possible. Furthermore, a dedicated mask pattern and a subsequent etching phase can also be implemented to remove excess semiconductor filler material. 
     A method according to an exemplifying and non-limiting embodiment of the invention further comprises depositing an electrically insulating layer on top of a structure comprising the semiconductor ridges  211 , the metal layers  212 , and the semiconductor fillers  213 . In  FIG. 2 f   , the above-mentioned electrically insulating layer is denoted with a reference  217 . 
       FIG. 3  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, each of the metal layers  312  is deposited on one sidewall of a respective one of the semiconductor ridges  311  and on a substantially flat top surface of the semiconductor ridge under consideration. Furthermore, the metal layer  312  of each semiconductor ridge is in contact with a base of a neighboring semiconductor ridge. The gaps between adjacent ones of the semiconductor ridges  311  are filled with semiconductor fillers  313  as illustrated in  FIG. 3 . 
       FIG. 4  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. The gradient heat-flux sensor comprises, among others, semiconductor ridges  411 , metal layers  412  covering one slanting sidewall of each semiconductor ridge, and semiconductor fillers  413  between adjacent ones of the semiconductor ridges. The structure of the gradient heat-flux sensor illustrated in  FIG. 4  is otherwise similar to the structure illustrated in  FIG. 2 f    but the semiconductor ridges  411  have a cross-sectional profile that is more round-shaped than the cross-sectional profile of the semiconductor ridges  211  illustrated in  FIG. 2 f   . In gradient heat-flux sensors according to different embodiments of the invention, the cross-sectional profile of the semiconductor ridges can have a shape of a triangle, a trapezoid, a modified trapezoid having curved sides as shown in  FIGS. 2 b -2 f    and  3 , a bell curve as shown in  FIG. 4 , or another shape providing slanting sidewalls and implementable with a suitable material removal process such as e.g. wet etching. 
       FIG. 5  illustrates a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. In this exemplifying case, the method comprises constructing, on a same substrate, many anisotropic multilayer structures  518 ,  519 ,  520 ,  521 , and  522  for forming electromotive forces in response to heat-fluxes through the anisotropic multilayer structures  518 - 522  in a direction parallel with the z-axis of a coordinate system  599 . In this exemplifying case, the method further comprises connecting the anisotropic multilayer structures  518 - 522  electrically in series so that voltage U is indicative of the total heat-flux through the anisotropic multilayer structures  518 - 522 . The anisotropic multilayer structures  518 - 522  are connected in series with the aid of electrical connectors  523 . It is also possible that anisotropic multilayer structures of the kind mentioned above are electrically connected in parallel or in mixed connections where e.g. groups of series connected anisotropic multilayer structures are connected in parallel or groups of parallel connected anisotropic multilayer structures are connected in series. The semiconductor ridges of the anisotropic multilayer structures  518 - 522  can be made by forming mutually parallel first grooves having slanting sidewalls and then by forming mutually parallel second grooves  524  perpendicular to the first grooves so as to separate adjacent ones of the anisotropic multilayer structures from each other. In  FIG. 5 , the first grooves and thereby the semiconductor ridges are parallel with the x-direction of the coordinate system  599  and the second grooves are parallel with the y-direction of the coordinate system  599 . In  FIG. 5 , the cross-hatched areas indicate the locations of the metal layers deposited on the appropriate slanting sidewalls of the semiconductor ridges. 
       FIG. 6  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. The gradient heat-flux sensor comprises, among others, semiconductor ridges  611  and gap-fillers  630  between the semiconductor ridges  611 . In this exemplifying case, the first sidewalls of the semiconductor ridges  611  are slanting but the second sidewalls are substantially perpendicular to the planar surface onto which the semiconductor ridges  611  are formed, i.e. the second sidewalls of the semiconductor ridges  611  are substantially parallel with the xz-plane of a coordinate system  699 . The gap-fillers  630  are formed by filling the gaps between the semiconductor ridges  611  with metal that can be e.g. aluminum, copper, molybdenum, constantan, nichrome, or some other suitable metal. Because of the slanting metal-semiconductor contact junctions on the first sidewalls of the semiconductor ridges, electromotive force is produced in a direction parallel with the y-axis of the coordinate system  699  in response to a heat-flux penetrating the gradient heat-flux sensor in a direction parallel with the z-axis of the coordinate system  699 . As the second sidewalls are substantively vertical, an opposite electromotive force produced at the metal-semiconductor contact junctions on the second sidewalls is small and thus the opposite electromotive force does not cancel the electromotive force produced at the metal-semiconductor contact junctions on the first sidewalls. 
       FIG. 7  illustrates a structure of a gradient heat-flux sensor manufactured by a method according to an exemplifying and non-limiting embodiment of the invention. The gradient heat-flux sensor comprises, among others, semiconductor ridges  711  and gap-fillers  730  between the semiconductor ridges  711 . In this exemplifying case, the gap-fillers  730  are formed by depositing layers  731  of first metal on the first sidewalls of the semiconductor ridges  711  whilst leaving the second sidewalls uncovered and then filling the gaps between the layers of the first metal and the second sidewalls with second metal different from the first metal. The first metal can be e.g. aluminum, copper, molybdenum, constantan, nichrome, or some other suitable metal. The second metal can be e.g. copper, molybdenum, constantan, nichrome, aluminum or some other suitable metal different from the first metal. The material of the semiconductor ridges, the first metal, and the second metal are selected so that a net electromotive force is produced in a direction parallel with the y-axis of a coordinate system  799  in response to a heat-flux penetrating the gradient heat-flux sensor in a direction parallel with the z-axis of the coordinate system  799 . 
     The specific examples provided in the description given above should not be construed as limiting the applicability and/or interpretation of the appended claims. It is to be noted that lists and groups of examples given in this document are non-exhaustive lists and groups unless otherwise explicitly stated.