US3679471A - Method for the production of electrical resistor bodies,and electrical resistor bodies produced in accordance with said method - Google Patents

Abstract

A NOVEL METHOD FOR THE PRODUCTION OF ELECTRICAL RESISTOR BODIES IS DISCLOSED, AS ARE NOVEL ELECTRICAL RESISTOR BODIES PRODUCED BY SAID METHOD. THE METHOD AND PROCESS OF THE INSTANT INVENTION CONTEMPLATES THE STEPS OF EMBEDDING A POLY-CRYSTALLINE POROUS MIXTURE OF GRAINS IS A COHERENT MATRIX DEPOSITED IN THE PORES OF THE MIXTURE BY A CHEMICAL REACTION OF VAPORS AT HIGH TEMPERATURE, THE SOURCE OF SUCH VAPORS BEING DISPOSED OUTSIDE THE REACTION ZONE AND AT A LOWER TEMPERATURE THAN THAT IN THE REACTION ZONE. THE MATRIX IS DEPOSITED THROUGHOUT THE ENTIRE MIXTURE AND, DURING DEPOSITION, IS DOPED IF DESIRED IN A CONTROLLED MANNER THROUGH THE ADDITION OF KNOWN DOPING AGENTS AND AMOUNTS THEREOF TO THE REACTING VAPORS. WHILE MATRIX IS DEPOSITED THROUGHOUT THE ENTIRE MIXTURE, IT DOES NOT FILL UP THE ENTIRE PORE VOLUME.

IN THE PREFERRED INVENTIVE EMBODIMENT, THE ELECTRICAL RESISTOR BODIES SO PRODUCED ARE HEATED IN AN INERT ATMOSPHERE AND AT A HIGHER TEMPERATURE THAN THAT PREVALENT DURING THE VAPOR DEPOSITION OF THE MATRIX SUCH THE THE DOPANTS ARE ALLOWED TO DIFFUSE TO CERTAIN KNOWN DEPTHS.

Description

Jufly 25, 1972 H. wYss 3,679,471

' METHOD FOR THE PRODUCTION OF ELECTRICAL RESISTOR BODIES, AND ELECTRICAL RESISTOR BODIES PRODUCED IN ACCORDANCE WITH SAID METHOD Filed Dec. 8, 1969 FIG.7

INVENTOR:

HUGO WYSS ATTORNEYS.

United States Patent 'Oflice US. Cl. 117-201 5 Claims ABSTRACT OF THE DISCLOSURE A novel method for the production of electrical resistor bodies is disclosed, as are novel electrical resistor bodies produced by said method. The method and process of the instant invention contemplates the steps of embedding a poly-crystalline porous mixture of grains in a coherent matrix deposited in the pores of the mixture by a chemical reaction of vapors at high temperature, the source of such vapors being disposed outside the reaction zone and at a lower temperature than that in the reaction zone. The matrix is deposited throughout the entire mixture and, during deposition, is doped if desired in a controlled manner through the addition of known doping agents and amounts thereof to the reacting vapors. While matrix is deposited throughout the entire mixture, it does not fill up the entire pore volume.

In the preferred inventive embodiment, the electrical resistor bodies so produced are heated in an inert atmosphere and at a higher temperature than that prevalent during the vapor deposition of the matrix such that the dopants are allowed to diffuse to certain known depths.

BACKGROUND OF THE INVENTION This invention generally relates to electrical resistor bodies and particularly concerns the composition and production of electrical resistor bodies which contain semiconductors.

Resistors of this general type can be divided into three categories or groups, depending upon the function and use. One such group includes linear resistors having characteristics which follow Ohms law and wherein the temperature coeflicient of the resistivity thereof is small. Linear resistors are utilized as heating elements in high temperature furnaces, as ballast resistors in power generators and the like.

A further group of electrical resistors comprises nonlinear resistors which have characteristics which do not follow Ohms law and wherein the temperature coefficient of resistivity thereof is generally large. Non-linear resistors of this type are utilized as voltage limiting devices such as surge arrestors and varistors in high-power and electronic applications, respectively. The third group of resistors comprises thermistors having electrical characteristics generally following Ohms law but wherein the temperature coeflicient of resistivity thereof is large.

A prevalent and common prior-art method of manufacturing such resistors contemplates a high-temperature sintering of a mixture of fine semi-conductor crystallites or grains with an electrical insulating ceramic or glassy material acting as a binding agent. Specifically, a mixture of semi-conductor and binder grains is consolidated by tamping or extrusion at room temperature, and the thus obtained bodies are introduced into a sintering furnace oftentimes under a controlled atmosphere. Generally, the binder utilized has a much higher resistivity than does the semi-conductor such that the greater majority of the 3,679,471 Patented July 25, 1972 current is carried by the semi-conductor. Accordingly, the apparent macroscopic resistivity of the resistor body is higher than is the resistivity of the bulk semi-conductor. Oftentimes, this effect is desired because such effect enables a matching of the bodys resistance value to the impedance value of the external current feeding circuit.

Yet, because of the utilization of different materials in the production of such bodies, a mismatch of the thermal expansion coefficient of the bodys components occurs and, accordingly, the bodys resistance to thermal shock is not very great. In view of this, some resistors have simply been manufactured in the prior-art by sintering only semi-conductor grains without binder. While resistance to thermal shock is thus improved, quite high temperatures are required with consequent losses of control over the distribution of electrically active impurities in the semi-conductor grains. Linear resistors have been produced in this fashion although the apparent resistivity thereof cannot be widely varied and generally has unpractically low values. On the other hand, this particular technique cannot be utilized at all for the production of non-linear resistors.

SUMMARY OF THE INVENTION In view of the above general background, it should be apparent that a need exists in the art for an improved method for the production of electrical resistor bodies, and improved electrical resistor bodies made from such method which resistor bodies do not exhibit the mechanical and electrical disadvantages of the prior-art. It is the primary object of the instant invention to provide such an improvel method and an improved electrical resistor body made therefrom.

Further, more specific, yet equally important objects of the instant invention concern the provision of a novel method for the production of resistor bodies containing only a semi-conductor, wherein reproduceable control of the distribution of electrically active impurities in the semi-conductor can be maintained through sintering such resistor bodies at a temperature which is low enough to achieve the desired result.

An additional specific object of the instant invention concerns the provision of a novel method for the production of resistor bodies which contain two or more components, at least one of which is a semi-conductor, which method effects improved electrical and mechanical properties of the resistor bodies, allows for control over the distribution of electrically active impurities in the semiconductors, and affords the possibility of totally new combinations of components.

These objects as well as other objects which will become apparent as the description proceeds, are implemented by the instant invention which is characterized by a novel method for the production of electrical resistor bodies, and novel electrical resistor bodies produced therefrom. In the contemplated preferred method, a poly-crystalline porous material of grains is embedded in a coherent matrix deposited in the pores of the mixture by gaseous pyrolysis or chemical vapor deposition so that a good mechanical strength and advantageous electrical properties of the resulting body is achieved.

The matrix is preferably deposited in the form of whiskers or poly-crystalline layers and does not completely fill the space of the pores. Either the poly-crystalline mixture or the matrix are contemplated to contain a semiconductor.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and advantages and features other than those mentioned above will be appreciated from the following description of preferred inventive embodiment such description referring to the DETAILED DESCRIPTION OF A PREFERRED INVENTIVE EMBODIMENT Initially, it should be understood that the novel invention is based upon the application of a chemical vapor deposition process to the production of a matrix which contemplates a starting mixture and which performs the functions of holding the mixture together, reducing or increasing the current carrying capability of the original mixture, and acting as a source of electrical active impurities or dopants which, at high temperatures, diffuse into the grains of the mixture. Inversely, if desired, the matrix itself can be doped by impurities which diffuse from the grains of the mixture to thereby change the electrical properties of the matrix. The concentration of the electrically active impurities in the matrix or the concentration of doping thereof is controlled during the deposition process by means of the addition of known amounts of impurities to the vapors from which the matrix is deposited.

Now, attention is particularly directed to FIG. 1 of the drawings wherein a highly enlarged sectional view of a resistor body is depicted, such resistor body being seen to contain a poly-crystalline porous mixture A of crystallites of one or more substances. These crystallites are embedded in a matrix B, which matrix does not fill up the entire pore volume as is shown by the empty spaces C. Matrix B forms a coherent, continuous texture throughout the entire resistor body.

In accordance with the instant invention, the same chemical composition can be used for both the mixture A and the matrix B. Yet, the sources of the vapors from which the matrix B is deposited, are chemically different from all of the substances which may be present either in the mixture A or in the matrix B. Such vapor sources are contemplated to be placed outside the reaction furnace utilized, this location having a lower temperature as compared with the temperature in the reaction zone itself. This feature of the invention differentiates the novel process from a conventional sintering technique in which the possibility exists that material transport through the vapor phase takes place from a warmer point to a colder point of the same body.

In accordance with the inventive concepts, at least one of the substances present in the mixture A or in the matrix B comprises a semi-conductor which term is to be construed as including the group of elements or compounds which, at standard conditions and in a very pure state thereof, exhibit a resistivity between and 10 ohms-cm. Accordingly, the term semi-conductor does not include graphite or pyrolytic carbon.

So that the novel inventive method can be better understood and practiced, a particular example will be described of a method for producing resistor bodies which contain silicon carbide.

Silicon carbide has many crystallographic modifications, one of which (ct-SiC or hexagonal) is produced in large quantities with the so-called Acheson process and which has found wide-spread use in the manufacture of resistor fbodies.

The color of SiC depends upon its particular doping. For example, n-type SiC is yellow or green whereas ptype SiC is black or deep blue. If one desires to produce linear resistors, green SiC is utilized due to the low temperature coeificient of its resistivity. On the other hand,

I 4 I I black SiC is utilized for the production of non-linear resistors having electrically blocking layers at the surface thereof and further having a large temperature coefficient of resistivity.

In producing SiC linear resistor bodies "according to the invention, a mixture of n-type SiC grains is used. To these grains, it is desirable, but not essential to add other finer grains of nand/or p-type SiC, whose grit is finer than that of the first grains employed. After the addition of a small amount of temporary binder, the mixture is then consolidated by tamping or extrusion, in order to obtain a body with the desired form. As an alternative, the mixture can be filled into a ceramic or quartz tubing or extruded through it, without having to, add a temporary binder. The mixture is then introduced into the sintering furnace and heated in the presence of vapors, resulting in pyrolytic deposition of SiC.

Vapors which can be employed include, for example, organosilicon compounds such as alkylsilanes (e.g., tetramethyl silane) halogenated alkylsilanes (e.g., trichloromethyl silane), mixtures of hydrocarbons (e.g., methane, propane, toluene and the like) with halogenated silicon compounds, such as trichlorosilane, tetrabromosilane and the like. In addition, mixtures of hydrocarbons and silicon hydrides have also been found useful. These vapors are generally mixed with a carrier gas, such as hydrogen or a noble gas. The vapors together with a carrier gas are introduced into the closed atmosphere of the reaction furnace, so that the vapors pass through the pores of the mixture to be consolidated. Simultaneously, controlled amounts of doping agents in the form of elements (such as nitrogen) or their compounds, such as hydrides and halogenides of the elements of Group III and Group V of the Periodic Table of Elements, can be added to the vapors. It is therefore possible to control the doping of the matrix so that it can be held constant or be varied by regulating the amounts and nature of the dopants to be injected into the gas stream. The crystallographic structure of the resulting matrix is further determined by the reaction temperature, the flow rate and the partial pressures of the vapors.

In FIG. 2, the principal components of an apparatus for the vapor consolidation of SiC resistor bodies are illustrated. The polycrystalline mixture A of SiC grains is introduced into a ceramic or quartz tubing 1, which lies inside the furnace 3. At one end 4 of the tubing 1, the vapors and the carrier gas are introduced. The vapors, for example, toluene and SiCl are produced by passing a part of the carrier gas, for example, hydrogen, through the bubbler bottles 5 and 6, which contain the corresponding liquids, to'which one can add controlled amounts of doping agents if it is so desired. It is also possible to mix the doping agents directly with the carrier gas. If one wishes to obtain a n-type matrix, PCl is added to the silicon tetrachloride, or alternatively PH is added to the carrier gas. For a p-type matrix, BBr is added to the silicon tetrachloride or B H to the carrier gas. The flow rates of the carrier gas through the valves 7, 8 and 9, and the temperature of the bubbler bottles 5 and 6 are controlled by conventional devices not illustrated in this figure.

The temperature of the furnace is controlled by known temperature sensors such as thermocouples or optical pyrometers, and by means of electronic control units which "act on the rate of heating power supplied to the furnace. The flow of vapors through the mixture can be axial such as illustrated in FIG. 2. In this case, it is advantageous to let a hot zone, where the chemical vapor deposition takes place, travel from one end to the other of the tubing 1. This can be accomplished by either moving the furnace or by steadily drawing the tubing 1 dur ing the deposition.

An alternative technique consists of preheating the whole tubing 1 at a temperature 'not sufiicient to cause the reaction and passing an RF coil from one end to the The flow of vapors can also be radial, when an axial hole is provided in the core of the mixture A, into which the vapors are introducedand diir'use through the mixture to the annulus, which has been left between the body and the furnace walls.

Still another variant to the process consists of extruding the mixture through a tubing, at the mouth of which one places the hot zone of the furnace. The reacting vapors are introduced by the same tubing as the mixture. The body which results from this process consist of n-type SiC grains embedded in. a SiC matrix, whose conduction type and resistivity can be varied within very large limits. Because the product is homogeneous, it has a high resistance to thermal shocks. If the conduction type of the mixture and of the matrix are the same, the body will have a low apparent resistivity, whereas when the conduction type of the mixture and of the matrix are different, the apparent resistivitywill be high, because the density of the current paths in the body is reduced by the blocking properties of the built-in p-n junctions.

Although various processes for producing the electrical resistor bodies of this invention exist, the following examples are illustrative: I g

EXAMPLE I Granular silicon carbide having abrasive qualities and a grit size of 100 mesh, was introduced into a quartz tubing having a 0.4 inch inner diameter. One end of the tubing was sealed by a glass fritted filter, the filter being permeable with respect to the gases but impeding the flow of the silicon carbide grains out of the tubing. The filled tube was then introduced into a horizontal heated furnace of small resistance, 'with the temperature being controlled by a thermocouple, and having a hot zone length of approximately two inches. The temperature of the oven was raised to 1200 C. while subjecting the contents of the tubing with a flow of nitrogen gas. After the temperature had stabilized, the nitrogen gas flow was terminated and then hydrogen gas, containing about 1% trichlorosilane and 0.1% toluene, was passed through the silicon carbide for a period of two (2) hours at the rate of 50 cc./min. No appreciable decrease in the rate of flow of the carrier gas during deposition was observed. The furnace was allowed to cool down and the silicon carbide was again subjected to flowing nitrogen gas. The quartz tubing was then broken by light hammering. A silicon carbide body approximately one (1) inch long was recovered from the middle of the tubing. The body had good mechanical strength and electrical testing showed that it was linear with an apparent resistivity of 1052 cm., which is greater than the values found for self-bonded SiC linear resistors. A microscopic examination showed that the deposited matrix consisted of dark silicon carbide whiskers.

EXAMPLE II Proceeding in the same manner as in Example I, the filled quartz tubing was heated at 1120 C. for a period of three (3) hours and subjected to hydrogen gas saturated with methyltrichlorosilane at 20 C. which was flowing through the silicon carbide at 30 cc./min. After deposition a silicon carbide body one (1) inch long was recovered with a better mechanical strength as in Example I. Electrical testing showed linearity and a resistivity of 1409 cm. The resulting matrix consisted of yellowish silicon carbide layers.

EXAMPLE III The procedure of Example H was followed, except that 5% of nitrogen was added to the flowing hydrogen gas during the deposition step, thus resulting in a lowering of the resistivity of the matrix. The resulting body showed linear behavior and a resistivity of about cm. The matrix consisted of green layers of SiC.

other of tubing 1, inducing a mobile hot reaction zone in Non-linear resistor bodies can also be produced in a similar manner as linear resistor bodies. In producing non-linear resistor bodies, one starts with a mixture of p-type SiC grains to which, if desired although not essential for the invention, one can add other pand/or n-type SiC grains with a finer grit than the mixture of p-type SiC grains. The mixture is then combined according to the inventive process by depositing a matrix with a very high resistivity. By gradually changing the doping, one can further produce a transition from pronounced non-linearity to a substantial linear behavior. The mechanical and electrical properties of the bodies are superior to conventional sintered bodies, allowing for a higher density of energy dissipation when under current load.

Various other types of vapor mixtures and dopants are known and can be used for the chemical vapor deposition of SiC. This enables one to select a vapor mixture, from which SiC is deposited at a temperature which is appropriate for the attainment of specific and desired mechanical or electrical properties. If one desires to maintain a close control over the difiusion of dopants, the vapor deposition can be performed at a temperature which is sufiiciently low in order to practically exclude interdiifusion of the dopants in the mixture and matrix. As an additional step, one can heat the body at a higher temperature Without reacting vapors for a period of time in order to allow the dopants to diffuse to a certain depth. The atmosphere, in which this treatment takes place, can additionally contain still other dopants which would also diffuse into the matrix and influence its electrical properties. Another treatment, which influences the electrical properties of the body, is the partial or total oxidation and/or nitridation of the matrix, which is accomplished by having an oxidizing and/or a nitridizing atmosphere in the furnace wherein the post-deposition treatment takes place.

In accordance with another embodiment of this invention, it can be further advantageous to produce heterogeneous bodies in accordance with the process of this invention. Where heterogeneous bodies are desired, two or more different substances, at least one of them being a semi-conductor and preferably having a melting temperature in excess of 1500 C. and a band gap energy greater than 2 ev., can be used. Thus, a resistor body composed of two or more semiconductors or semiconductors and insulators can be produced. As an example of the first class, one can take a body where the mixture consists of SiC and the matrix of B 0, which has been deposited by the reaction of B H and CH Alternatively, the matrix can consist of vapor deposited SiC and the mixture of B 0 grains. In each of these cases, one can perform the vapor deposition under such conditions in order to control the doping of both semiconductors. As an example of the second class of resistors, a resistor body is used where the mixture consists of SiC grains and the matrix of SiO and/or Si N Silicon dioxide can be deposited by the reaction of SiH4 and CO 'Whereas one obtains silicon nitride by the reaction of SiH and NH for example. In this last case, the mixture, which before the deposition was an insulator or a dielectric, becomes conductive due to the matrix.

Other combinations can also be eflected by utilizing bodies where the mixture consists of semiconductors and dielectrics and/or the matrix consists of semiconductors and dielectrics, which have been deposited at the same tune or in alternate fashion.

In accordance with the foregoing discussion, a further illustrative example is set forth below.

EXAMPLE IV Granular silicon carbide, which is heavily p-doped, and having a grit size of 220 mesh was introduced into a quartz tubing in the same manner as Examples I-III. The tubing was then heated at a temperature of 800 C., while subjecting the contents of the tubing to nitrogen gas saturated with tetraethylorthosilicate atv25 C., flowing at the rate 60 ice/min. The treatment lasted for eighteen (18 hours, resulting in the formation of a silicon carbide body bonded by a silicon dioxide matrix and exhibiting good mechanical strength as well. as very high non-linearity.

In accordance with still another vention, it is also possible to produce resistor bodies having new properties and characteristics based upon known physical properties of semiconductors, such characteristics including change of resistance under the influence of mechanical stresses (piezoresistance),change of resistance by heating or cooling (thermistors), change of resistance by absorption of electromagnetic radiation (photoresistance), change of resistance because of current carrier instabilities (electrical breakdown, mobility instabilities at high electric fields, trapping, memory effects and the like) and emission of electromagnetic radiation by current carrying diodes.

Depending upon the nature of the various semiconductors employed, modifications may be made with respect to the vapors, the doping agents, the carrier gas and the temperature at which the vapor consolidation is to-be carried out withinthe teachings of the invention. The desired electrical, thermal and mechanical properties ofthe resistor body produced by the process of the invention may also be varied within the novel inventive teachings through suitable changes in grit size, resistivity, conduction type and mixing proportions of the grains of the mixture A, resistivity, conduction type, thickness of layers or whiskers and porosity of the matrix B, partial pressures and flow rates of the reacting vapors and of the carrier gas, mechanical pressure before and during the deposi: tion length and temperature, nature of the doping atmosphere, annealing, quenching, alloying and the like.

Having now discussed in considerable detail illustrative and preferred embodiments of the invention, it should be apparent that the objects as set forth at the outset of this specification have been satisfied.

embodiment of the in- Accordingly, what is claimed is: r a

1. Aprocess for the production of electrical resistor bodies which contain semiconductors,except'graphite or pyrolytic carbon, whichlcomprises the steps of:.

(a) providing a mixture of polycrystalline porous grains in a reaction zone of high temperature;

(b) providing reacting vapors outside of said'reaction zone at a lower temperature;

(c) introducing doping agents into said reacting vapors;

and

(d) introducing said reacting vapors into said reaction zone and vapor depositing a coherent matrix intotheporesofsaid mixture of grains, whereby a polycrystalline porous mixture of grains is embedded in said coherent matrix. Y I

2. A process according to claim 1 which further COIIl-r prises the step of subsequently heating the-body in an inert atmosphere and at a higher temperature than during said vapor deposition of the matrix, whereby said dopants are permitted to diffuse to a certain knowndepth.

3. A process according to claim 1 which further comprises the step of subsequently heating said body in an atmosphere containing, doping agents. r

, 4. A process according to claim 1 which further comprises the step of subsequently heating said body in an oxidizing or nitridizing atmosphere,

5. A resistor body produced according to the process ofclaiml.

References Cited UNITED STATES PATENTS 7 Rich et a1. 117-406 C

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