SILICON HEATING ELEMENT

Abstract: A silicon heating element comprises an electrically isolated support (A) of high thermal conductivity - preferably made of beryllium oxide or of aluminium nitride - bearing a silicon layer (B) and electric contacts (C). The silicon layer (B) consists of polycrystalline or amorphus silicon which is doped by semiconducting dopants of the p- or n-type, preferably in a concentration higher than 10<18>/ccm. The contacts are highly temperature resistant and alloyed to the silicon layer (B). Alternatively the contacts are composed of a contact layer (C') of refractory metal being in contact with the electrically isolated support (A) and of a second temperature resistant material (C'') alloyed onto the contact layer (C'). The silicon heating element is produced by laying a layer (B) of polycristalline or amorphous silicon onto an electrically isolated support (A) of high thermal conductivity. It is deposited as a thin layer (B) and is doped with semiconducting dopants of the p- or n-type either during or after this deposition. Electric contacts (C) are then connected to the silicon layer (B). The silicon heating element can also be used as a temperature sensor.

Description: SILICON HEATING ELEMENT The invention relates to a silicon heating element as described in YU patent application No. 1097/85 dated June 28, 1985 and to a method of its production. The conventional electric heater with wire heating element has a low efficiency particularly in open constructions (where is no forced transfer from source to load) and at low and medium heating temperatures (up to 400°C) . The essential cause is a high heater temperature (about 800°C) in relation to the desired working temperature: a great part of heat en¬ ergy is lost as the high temperature of the heater radiates the heat all around and in consequence only a part of it rea¬ ches up to the useful location. Conventional heating elements are made of alloy Cr-Ni-Fe-Al with a resistance of about 0.04 Ohm/cm. A relatively low spe- cific electric resistance and a poor thermal conductivity of such heating material determine the manner of its use: the heater is made in form of wire coil that must be electrically isolated with robust ceramics or an equivalent isolation material, which in turn has also a low thermal conductivity. Finally the element is protected by a metal covering. Such an element has a great mass and because of the poor thermal con¬ ductivity it has to be heated to about 800°C to enable the heat to reach the heating location as quick as possible. The high temperature of the heating element induces the isotropic expansion of the heat by radiation and results in a low de¬ gree of efficiency. It particularly relates to smaller hea¬ ting units e.g. household heating appliances. The essential problem of impossiblity how to obtain a higher degree of efficiency in conventional heating elements is the manner of heat transfer from the heating - source - up to the working area - the load. So far this transfer is mainly limi¬ ted to radiation, the efficiency must consequently be low and energy loss rather high. A good degree of efficiency can be obtained if the following conditions are met: a) the thermal resistance of the material itself for the active part of the heating element as low as possible; b) the mass of the heating element as small as possible; c) thermal resistance from the source up to the load also as low as possible; d) the difference between the working temperature and the heater temperature as low as possible. Conventional heating elements cannot satisfy these require¬ ments. The conventional wire heater can be easily constructed by using simple technologies (e.g. winding a wire into coil). During the past some 60 years, the conventional heating ele¬ ment has been widely used, the energy saving has not been a motivation for a more intensive development of a new, energy saving type of heatig element. Only in the past few years the energy saving has become imperative to the modern technology and a more significant energy saving can no more be achieved by use of a conventional heater. Now, when the energy saving becomes increasingly important, the work on development of a heating element in the world that will allow a more economi¬ cal energy consumption may be expected to take a more inten¬ sive move. The invention proposes a new heating element made of doped polycrystalline or amorphous silicon which is heated up to a temperature of 400°C. Polycrystalline or amorphous silicon have a thermal conductivity about 5 times that of the wire material of a conventional heater and therefore the heating energy can be directed to the consumption place at minimal SUBSTITUTE SHEET losses. Such new type of heating element is proposed in 6 ba¬ sic constructions: 1. An element which consists of an electrically isolating support with a high thermal conductivity spread with an active layer of doped polycrystalline or amorphous sili¬ con and surface electrodes to which the working electric supply is connected. 2. An element in which the surface heating' layer has been contoured in meander in order to optimize the electric resistance. 3. An element the connecting contacts of which are reinforc- ed by a conducting layer of refractory metal (Mo,Ta,W etc. ) . 4. An element installed on a suitable support made of high thermal conductivity metal. An element which after fabrication is installed on a sup¬ port made of highly thermal conductive metal (Al, Cu or similar) that directs the conduction of heat from the heat source to the heat load. 6. An element assembled from two elements as above mentioned in a form of "sandwich" with alloyed connection contacts which allows forming of a series or parallel configura¬ tion for adjustment to the supply voltage and to obtain the necessary power. This variant may be combined with variant 5 that has a high thermal conductivity support closed on three sides and the outlet connection leads on the fourth side. The invention is significant for two essential reasons: TITUTE SHEET 1. Polycrystalline or amorphous silicion are 50 to 100 times cheaper than semiconducting monocrystalline silicon. 2. Polycrystalline or amorphous silicion can be laid on the support in thin films (ranging from 0,1 to 1 μm of film thickness). Using the selective etching technology (as known in the semiconductor technology) such films can be shaped into surface structures of more suitable forms (e.g. meanders) and thus provide a better freedom in the choice of satisfactory resistance of the element. The other characteristics of the polycrystalline or amorphous silicon are very similar to those of the semiconductive mono- crystalline one. This relates especially to the high thermal conductivity and the metallurgical alloying characteristics which are the main reasons why silicon has been selected as a new material. The general and specific characteristics of the silicon that give it precedence over the material used for a conventional heater are stated in the following: 1. While the material for conventional heaters has a defin- ed electric resistivity which is unchangeable, the spec¬ ific silicon resistivity can be changed by several or¬ ders of magnitude either in the course of fabrication or during the thermal treatment taking place afterwards. This allows to obtain the necessary electric resistance of the heater with its minimum mass. 2. The silicon thermal resistance is 5 times lower than the material for the conventional heater wire. The silicon can be perfectly alloyed with metals and consequently connecting leads can be made using other material which is welded or hard soldered to the layer of the active SUBSTITUTE SHEET part of the heater. The welding or alloying characteris¬ tics of silicon can be used for a directed conduction of heat towards the heated media at a minimal loss of en¬ ergy. In conventional wire heaters this is not possible. 3. By dispersion or deposition of silicon in the process of Chemical Vapor Deposition (CVD) or in the Low Pressure Chemical Vapor Deposition (LPCVD) the silicon can be spread on a support in a very thin layer. Said processes are inexpensive, suitable for mass production and re¬ quire small investments. The layers obtained in this way can be doped within the depositing operation to a con¬ trolled level thus programming the specific resistivity of the layer. Doping can also be carried out after the layer has been deposited on the support. This promotes the mobility in production. 4. By using a suitable process the silicon can be covered with a thin dioxide film. This is a layer of quartz glass that protects the silicon layer effectively from a further oxidation and passivates its surface. The dioxide film grows directly from the silicon and the risk of the production of cracks is therefore minimized. 5. The temperature coefficient of the silicon electric resistance is positive: its resistance increases with increasing temperature. This characteristic of the sili¬ con heating element can be used as an inherent overload protection. In a suitable construction, a part of the heater can be used also as a temperature sensor. The layer of polycrystalline or amorphous silicon maintains the positive temperature coefficient of electric resis¬ tance up to the temperature of 600-800°C dependent to the doping level. The above mentioned characteristics of silicon are used to a maximum extent by this invention in order to obtain an opti¬ mized silicon heating element. The heating element uses all stated advantages of such material or, as may be necessary, some of them only. Referring to the accompanying explanatory drawings: Fig.1 shows a schematic illustration of variant 1 of the silicon heating element according to the invention. The layer B of either polycrystalline or amorphous silicon is spread on a ceramic tile A. The layer B is doped during spreading or is laid without dopant and the doping follows afterwards. Con¬ tacts C are placed at the ends of the tile and alloyed on the silicon layer B. The surface has no contour, and the suitable electric resistance is obtained through the shape of the tile A, thickness of the silicon layer B and the doping level of the layer. The silicon heating element of a new, energy saving type as described in Fig.1 allows directing the heat energy from the heat source toward the heat load, noted due to the fact that on an electric isolated support A of a suitable surface con¬ tour and a high thermal conductivity a layer of polycrystal- line or amorphous silicon B is spread, which is doped either in the course of spreading or after the completion of spread¬ ing of silicon layer, by semiconducting dopants of the p- or n-type in a concentration higher than 10 /cc on which layer at the end of ground, highly temperature resistant contacts C are alloyed for electric supply. Fig.2 represents a schematic illustration of variant 2 of the proposed heater: unlike variant 1, here a layer C r of high- melting metal (Mo, Ta, W or similar) with contact contour (Fig.2) is first laid on the ends of the support. A layer of silicon B is laid over the entire surface and then etched by SUBSTITUTE SHEET masking according to Fig.2b. Contacts C" linking layers B and layer C 1 and enabling the connection to the electric supply are then alloyed both on the contour of silicon and that of the high-melting metal C' (Fig.2 (Detail K) ) . This variant is suitable for work on high temperatures since the high-melting metal contact tolerates mechanical loads at temperatures as high as 600-800°C. Fig.3 illustrates a variant as in Fig.2, but the contour of the silicon layer B is shaped using the method of photolito- graphy by masking and etching into a meander or a similar suittable contour of desired form in order to obtain a higher electric resistance of the heater when required (e.g. work with higher voltage and small intensity heating current). Fig.4 shows a variant of heater according to Figs.1,2 and 3 with a protective layer of silicon dioxide D over the doped layer B of polycrystalline or amorphous silicon. This layer is obtained by oxidizing the silicon layer B at high tempera- ture (1000°C) in an oxidizing water vapour atmosphere so that the conducting layer is covered by an insulating layer D of silicon dioxide which passivates and protects the active heating layer and thus increases its safety and durability. This technological step takes place simultaneously with dif- fusing the dopant into the silicon layer namely with its dis¬ tribution and homogenisation in the case when doping is ef¬ fected during the layer spreading. Therefore the layer pro¬ tection by oxidation does not require a separate process step. Fig.5 illustrates a variant of a heater installed on a metal support E of high thermal conductivity (e.g. Cu, Al or simi¬ lar) to form a heating unit. This gives a complex heating element where the first step of the thermal link from the heating location towards the heat consumption has been solved within the heating element itself. The support E directs the conduction of heat energy from the heat source to the heat load. In the heater according to Fig.6 there are two heating ele- ments according to Fig.1 to 4 assembled in a "sandwich" I which doubles the heating power in a simple fashion and al¬ lows an "enclosed" construction of heater which realizes the first step in the thermal link from the heater to the load by means of support F bearing in its inside the "sandwich" I of heating elements as per Fig.6. This is the hatched area on the sketch in Fig.6. The two heating elements have been assembled in a "sandwich" type construction by alloying the connection contacts and form a unit of series or parallel configuration in order to adjust the supply voltage and to obtain necessary power. Such "sandwich" construction is installed in a support made of high thermal conducting metal in a closed form. In all above described variants of the new silicon heating element it is emphasized that the silicon film is laid on the support which should be an electric insulator and an excel¬ lent thermal conductor at the same time. Due to high work temperature a ceramic material for the support is suitable. However, ceramic is a poor thermal conductor. Until a few years ago the only ceramic material that, as to the thermal conductivity, stood out among others was beryllium oxide (BeO) . Its conductivity is approximately equal to that of pure aluminium. Unfortunately, BeO is a highly toxic material and its use is therefore limited. During the past 5 years the high technologies development in the world led to the production of a new ceramic material with characteristics very similar to those of BeO but comple- tely non-toxic and noticeably cheaper. This is aluminium ni¬ tride (AlN) . Therefore, for the support of the new silicon SUBSTITUTE SHEET heating element dealt with in the instant invention, we have decided primarily on A1N although there are other supports which can meet the requirements in the cases where a high thermal conductivity of the support is not an imperative requirement. The silicon heating element of the invention represents a brand new and original component and its industrial applica¬ tion can consequently be assumed with a high degree of proba- bility. The application shall certainly take place in two ways: 1. Production of silicon heating elements within an indu¬ stry related to semiconductor industry as a component for production of corresponding appliances and equip¬ ment. 2. Application of the silicon heating element in products may be assumed significant for all electrothermal pro- ducts of small and medium power where the saving of electric energy becomes important and the reduction of size, weight and price is required. As the element can be applied at temperatures up to 600°C it is expected its application will extend not only to household heaters (electric radiators), cookers, and all electrothermic appliances of wide use but also to some specific professional appliances. It is important to say that this element allows not only the possibility to save energy due to a higher efficiency but also significant savings of material in the production of se¬ veral appliances due to the distributed application of the new heating element. For small dimensions it can be inserted directly into the heating volume or in its close vicinity i.e. just in the place where heating is necessary and conse- quently eliminates robust insulators and transfer means which represent today a great deal of mass of such appliances. Furthermore, small dimensions, possibility of direct control and the temperature registration shall lead to such appli¬ cations which have not been possible before and which cannot be assumed now either. SUBSTITUTE SHEET

Claims: C L A I M S : 1. A silicon heating element that comprises an electrically isolated support (A) of high thermal conductivity bear- ing a silicon layer (B) and electric contacts (C), characterized in that the silicon layer (B) consists of polycrystalline or amorphous silicon and that the con¬ tacts are highly temperature resistant and alloyed to the silicon layer (B) . 2. A silicon heating element as in claim 1 , characterized in that the electrically isolated support (A) is made of beryllium oxide or of aluminium nitride. 3. A silicon heating element as in claim 1 or 2, charac¬ terized in that the silicon layer (B) is doped by semi¬ conducting dopants of the p- or n-type, preferably in a concentration higher than 10 1 R°/ccm. 4. A silicon heating element as in one of the preceeding claims, characterized in that the electric contacts (C) are composed of a contact layer (C 1 ) of refractory metal being in contact with the electrically isolated support (A) and of a second temperature resistant material (C") alloyed onto the contact layer (C 1 ). 5. A silicon heating element as in one of the preceeding claims, characterized in that the silicon layer (B) is shaped - preferably etched - in a way ensuring a prede- termined electric resistance, preferably in the shape of meanders. 6. A silicon heating element as in one of the preceeding claims, characterized in that the silicon layer (B) is covered by an insulating protective layer (D), pre¬ ferably of silicon dioxide. 7. A silicon heating element as in one of the preceeding claims, characterized in that it has at least one elec¬ tric contact (C) in common with another silicon heating element in series or parallel configuration. 8. A silicon heating element as in one of the preceeding claims, characterized in that it is installed on a metal support (E) of high thermal conductivity. 9. Use of a silicon heating element as per any one of the preceeding claims as a temperature sensor. 10. A method of producing a silicon heating element, whereby a silicon layer (B) is laid onto an electrically iso¬ lated support (A) of high thermal conductivity and whereby electric contacts (C) are connected to the si¬ licon layer (B), characterized in that the silicon layer (B) consists of polycrystalline or amorphous silicon and is deposited as a thin layer (B) - preferably within a range of 0,1 to 1μ - onto the electrically isolated sup¬ port (A) and that either during or after this deposition the layer (B) is doped with semiconducting dopants of the p- or n-type, the preferred doping concentration being higher than 10 8 /ccm. 11. A method as in claim 10, characterized in that the sili¬ con layer (B) is specially shaped - preferably in the shape of meanders - by photoetching or by using a seque- stering agent. 12. A method as in claim 10 or 11, characterized in that the silicon layer (B) is covered with a thin, protective layer (D), preferably of silicon dioxide. 13. A method as in claim 12, characterized in that the sili¬ con layer (B) is oxidized at a temperature of about 1000°C in a water vapour atmosphere, preferably simul¬ taneously with the doping during or after the deposition of the silicon layer (B) onto the isolated support (A) . 14. A method as in one of the claims 10 to 13, characterized in the following steps: - a layer of refractory metal (C 1 ) is alloyed onto the ends of the isolated support (A) before deposition of the silicon layer (B); - the silicon layer (B) is deposited on the isolated sup¬ port (A) and on the layer of refractory metal (C 1 ); - at the location of the layer of refractory metal (C' ) the silicon layer (B) is partially etched, using a mask; - the electric contacts (C") are alloyed to the contoured silicon layer (B) and to the metal layer (C' ) . SUBSTITUTESHEET

Inventors: BENDEKOVIC ZDRAVKO, BILJANOVIC PETAR, ZORICIC BOZIDAR, BENDEKOVIC, ZDRAVKO, BILJANOVIC, PETAR, ZORICIC, BOZIDAR
Applicants: BIRO ESTABLISHMENT, ING BIRO ESTABLISHMENT
Application year: 1990
Main classification: H05B3
IPC codes: H05B3