POLYCRYSTALLINE SILICON-BASED BASE PLATE FOR LIQUID JET RECORDING HEAD, ITS MANUFACTURING METHOD, LIQUID JET RECORDING HEAD USING THE BASE PLATE, AND LIQUID JET RECORDING APPARATUS

Abstract: A base plate for a liquid jet recording head, which is provided with electrothermal converter each having a resistor for generating heat energy and a pair of wirings electrically connected to the resistor, is made of a polycrystalline substance. In the surface of the base plate, provided is an oxide layer formed by thermal oxidation and thermal softening. By utilizing such a specific base plate, a long recording head free from warp and bend can be fabricated at a low cost.

Description: FIELD OF THE INVENTION The present invention relates to a polycrystalline silicon-based substrate for use in a liquid jet recording head of conducting recording by discharging a liquid recording medium through discharging outlets utilizing thermal energy, and a process for producing said substrate. The present invention also relates to a liquid jet recording head in which said substrate is used and a liquid jet recording apparatus in which said substrate is used. BACKGROUND OF THE INVENTION There is known a liquid jet recording method for conducting recording by discharging a liquid recording medium such as ink through discharging outlets utilizing thermal energy to sputter said liquid recording medium whereby said liquid recording medium is deposited on a recording member such as papers, plastic sheets, fabrics, or the like The liquid jet recording method is of a so-called non-impact recording method, and it has various advantages in that the noise at the recording can be reduced to a negligible order, there is not a particular restriction for the recording member used, and color recording can be relatively easily attained. And as for the apparatus, that is, the liquid jet recording apparatus, for practicing the above liquid jet recording method, there are advantages in that the structure thereof can be relatively simplified, liquid discharging nozzles can be arranged at a high density, and a high speed recording can be relatively easily attained. In view of this, the liquid jet recording method has recently received the public attention, and various studies have been made thereon. Incidentally, a number of liquid jet recording apparatus have been put on the market. Shown in FIG. 5(A) is a schematic cross-eyed view illustrating the principal part of an example of a recording head used in such liquid jet recording apparatus. FIG. 5(B) is a schematic cross-sectional view taken along the liquid pathway and at the face perpendicular to the substrate of the recording head shown in FIG. 5(A). As apparent from FIG. 5(A) and FIG. 5(B), the recording head is provided with a substrate 8 for liquid jet recording head comprising a plurality of discharging outlets 7 each serving to discharge a liquid recording medium such as ink, liquid pathways 6 each corresponding one of the discharging outlets 7, a liquid chamber 10 serving to supply a liquid recording medium to each of the liquid pathways, heat generating resistors 2a each serving to supply thermal energy to the liquid recording medium, and wirings 3a, 3b for applying an electric signal to the heat generating resistors 2a. The substrate for liquid jet recording head 8 is of the configuration shown in FIG. 5(B) wherein a heat generating resistor layer 2 is disposed on a base member 1, a wiring layer 3 constituted by a material having a good electroconductivity is laminated on said heat generating resistor layer 2, and a portion 2a of the heat generation resistor layer where the wiring layer 3 is not disposed functions as a heat generating resistor. In this configuration, when an electric signal is applied to the heat generating resistor 2a through the wirings 3a, 3b, the heat generating resistor 2a is energized. The substrate for liquid jet recording head 8 may be provided with a protective layer 4 for the purpose of protecting the wirings 3a, 3b and the heat generating resistor 2a. The protective layer 4 serves to prevent occurrence of electric corrosion or/and electric breakdown at the heat generating resistor 2a and the wirings 3a, 3b. As the base member 1 of the substrate for liquid jet recording head 8, there can be mentioned plate-like members of silicon, glass, ceramics, or the like. However, in general, a single crystal silicon plate is used as the base member. The reason for this is due to the following situation. That is, in the case where a glass plate is used as the base member 1, there are disadvantages in that the glass plate is poor in thermal conductivity, and when the energization frequency (the drive pulse in other words) for the heat generating resistor 2a is increased, there is a fear that the heat generated by the heat generating resistor becomes excessively accumulated within the base member 1 and as a result, ink in the liquid jet recording head is heated by virtue of the heat accumulated to cause bubbles, resulting in providing defects in the ink discharging performance. In the case where a ceramic plate is used as the base member 1, there are advantages such that the size of the substrate can be enlarged to a certain extent, and a ceramic plate having a larger thermal conductivity than that of the glass plate can be selectively used. However, even in the case of using such a ceramic plate, there are disadvantages such that the ceramic plate is usually accompanied by surface defects such as pinholes or minute protrusions of some microns to some tens microns in size because it is produced by baking powdery raw materials, and such surface defects are liable to short-circuit or disconnect the wirings, wherein a desirable production yield is hardly attained. There are further disadvantages in this case such that the ceramic plate is usually of a surface roughness of Ra (center line mean roughness) = about 0.15 µm, and because of this, it is difficult to provide a surface roughness optimum for forming a desirable heat generating resistor layer 2 excelling in durability thereon; specifically in the case of preparing a liquid jet recording head using a plate made of alumina ceramics, because of the above reasons, a removal is often occurred between the base member 1 and the heat generating resistor layer 2 or a cavitation is often occurred at a part of the heat generating resistor layer formed on the defective surface of the base member when the bubbles generated are extinguished, resulting in disconnecting the heat generating resistor layer, wherein the performance of the heat generating resistor layer is eventually deteriorated. In order to eliminate these problems in the case of using the ceramic base member 1, there is a proposal of polishing such roughened surface of the ceramic base member to smooth said surface whereby improving the adhesion between the base member 1 and the heat generating resistor layer 2 and preventing occurrence of the premature disconnection of the heat generating resistor layer which will be cased because of cavitations centralized at a part of the heat generating resistor layer. However, this proposal is poor in practicability since the alumina ceramics are of a high hardness and because of this, their surface roughness is hardly adjusted as desired. Other than this proposal, there is another proposal in order to eliminate the above problems in that a glaze layer (a welded glassy component layer) is formed on the surface of such ceramic base member to thereby provide an alumina glaze base member. However, it is almost impossible to form the glaze layer at a thickness of less than a value of 40 to 50 µm by the manner employable in the formation of a glaze layer. As well as in the case of using the glass base member, problems relating to occurrence of excessive accumulation of heat are liable to occur also in this case. Therefore, this proposal is also poor in practicability. In the case of using a single crystal silicon plate as the base member 1, the above described problems relating to occurrence of excessive accumulation of heat are not occurred and the single crystal silicon wafer excels in surface property, and because of this, the foregoing problems relating to disconnection of the wirings and the like are not occurred. For this, for example, Japanese Unexamined Patent Publication No. 125741/1990 describes a substrate for the foregoing liquid jet recording head utilizing thermal energy, in which a single crystal silicon wafer is used. Incidentally, in recent years, in the field of recording using the liquid jet recording method, there has been an increased societal demand for early provision of a recording apparatus capable of obtaining a high quality record image at an improved speed. In order to enable to conduct recording on a wide recording member in reply to such societal demand for high speed recording, various studies have been made of a large-sized recording head, i.e., a so-called full-line recording head having a widened discharging width corresponding to a large-sized recording member. The results of the studies have revealed that the use a single crystal silicon wafer is optimum as the base member as long as the recording head to be prepared is of a relatively small size, but the use of a single crystal silicon wafer in the case of obtaining a large-sized recording head entails such problems as will be described below. Because of this, there are subjects necessary to be solved in order for the single crystal silicon wafer to be usable in a substrate for the large-sized recording head. That is, in the case where a substrate for liquid jet recording head is prepared using a base member comprising a single crystal silicon material, the single crystal base member, i.e., a single crystal silicon wafer is usually obtained by quarrying a single crystal silicon ingot produced by the pull method. The single crystal ingot which can be presently produced by the pull method is a rod-like shaped one of 8 inches in diameter and about 1 m in length at the maximum. Therefore, there is eventually a limit for the size of a single crystal silicon wafer which can be quarried from the single crystal ingot. However, it is possible to quarry a single crystal silicon wafer having an enlarged size from the single crystal ingot. In this case, problems are, however, entailed in that the utilization efficiency is greatly, reduced, resulting in unavoidably raising the cost of the resulting single crystal wafer, and this leads to raising the production cost of a final product. In the substrate for liquid jet recording head, in order to facilitate thermal energy to transmit to the liquid recording medium, there is usually disposed, on the surface of the base member, a heat accumulating layer (a lower layer in other words) capable of attaining a desirable balance between the heat accumulating property and the heat radiating property. In this case, the substrate is obtained in a manner that a single crystal silicon wafer is obtained by quarrying the above described single crystal ingot, the surface of the single crystal silicon wafer obtained is subjected to thermal oxidation to form a SiO 2 layer as the heat accumulating layer, the foregoing heat generating resistor layer and the foregoing wirings are successively formed, and the resultant is cut into a plurality of pieces each capable of serving as a substrate for liquid jet recording head. In the viewpoint of obtaining a large-sized recording head, the present inventor examined these members obtained in the above manner. As a result, there was obtained a finding that some of them, which were quarried from the opposite end portions of the single crystal silicon wafer, are deformed in such a bow-shaped form as shown in FIG. 9(A). And their deformed magnitude (which will be hereinafter called "warp magnitude" or "warp degree") was found to be ranging in the range of 60 to 90 µm. As for these deformed members, it was found that they are apt to break when their deformation is forcibly corrected. And as for some of the base members which are slight in deformation, it was found that there are still problems such that uniform polishing is sometimes hardly attained in the successive polishing step after the quarrying step, precise pattering sometimes cannot be conducted in the step of patterning wirings on the base member, and sometimes, it is difficult to precisely electrically connect the wirings arranged on the base member to an IC or the like. It was also found that in the case where a liquid jet recording head should be obtained using such deformed base member, the liquid jet recording head unavoidably causes a positional deviation of a liquid recording medium to a recording member on which recording is to be performed due to the distortion of the base member, resulting in providing defects such as missing dots or/and uneven dots for an image recorded. It is a matter of course that in the case where the end portions of the single crystal silicon wafer which are apt to cause the foregoing deformation are not used as a base member for a substrate for liquid jet recording head, the production cost for the substrate for liquid jet recording head unavoidably becomes very expensive. The present inventor made studies of the reason why such work in process for a substrate for liquid jet recording head is deformed as above described. As a result, it was found that in the case of the work in process for a substrate for liquid jet recording head not having the foregoing thermal oxide layer as the heat accumulating layer on the base member, such deformation is hardly occurred, and thus, the occurrence of such deformation is due to the thermal oxidation process upon forming the foregoing heat accumulating layer. And there were obtained findings that since after the single crystal silicon wafer having been subjected to thermal oxidization, it is cooled wherein the end portions of the single crystal silicon wafer, particularly four corners thereof, are cooled for the first time, tensile stresses are caused at the periphery in a state as expressed by arrow marks in FIG. 8(A) and those stresses then become distributed into the inside in a state as expressed by (+) marks in FIG. 8(B); that when this single crystal silicon wafer is cut in order to obtain a substrate for liquid jet recording head, part of those stresses is released to make the substrate deformed in such a state as above described; and that when a film for the heat generating resistor and a film for the wirings are successively formed on such single crystal silicon base member, the resulting work in process becomes accompanied by a warpage for which desirable patterning cannot be performed because the focusing position upon exposure is deviated. On the basis of the above findings, it was found that there is an inherent limit for the single crystal silicon wafer to be used as the base member for a substrate for liquid jet recording head in order to attain elongation of the substrate. Therefore, in order to obtain an elongated liquid jet recording head capable of attaining high speed recording, it is necessary to integrate a plurality of relatively short substrates for recording head. However, it is extremely difficult to adjust each of the joint portions among such substrates so that no negative influence is provided for an image recorded. Thus, it is an earnest desire to provide an inexpensive substrate for liquid jet recording head which can be effectively produced without having any restriction for its form depending upon the production process and without occurrence of problems relating to deformation and the like and which enables to easily attain high speed recording. The EP-A-0 570 587, which represents a state of the art in the sense of Art. 54(3) EPC discloses a process for producing a substrate for a liquid jet recording head provided with an electrothermal converting body comprising a heat-generating resistor capable of generating thermal energy and a pair of wirings electrically connected to said heat-generating resistor is formed, in which process a member composed of a polycrystalline silicon-based material as a constituent base member of said substrate is provided, and a step of thermally oxidizing the surface of the polycrystalline silicon-based member is conducted to form a thermal oxide layer on the surface of the polycristalline silicon-based member. SUMMARY OF THE INVENTION The principal object of the present invention is to solve the foregoing problems of the conventional substrate for liquid jet recording head and to provide an elongated substrate comprising a specific material for liquid jet recording head which enables to obtain a large-seized recording head. Another object of the present invention is to provide an elongated substrate for liquid jet recording head in which an elongated base member composed of a polycrystalline silicon material is used. A further object of the present invention is to provide a large-seized liquid jet recording head which can be effectively produced without integrating a plurality of substrates as in the case of using a single crystal silicon wafer and without the foregoing problems relating to the occurrence of a deformation in the work in process for a substrate for liquid jet recording head and the occurrence of a reduction in quality of an image recorded due to said deformation, and the occurrence of defective exposure due to the warpage in the work in process for a substrate for liquid jet recording head, which are found in the case of using a single crystal silicon wafer. A further object of the present invention is to provide a liquid jet recording apparatus provided with the above liquid jet recording head which enables to attain high speed recording of providing a high quality recorded image. A further object of the present invention is to provide a process for producing a substrate for liquid jet recording head, which includes the step of forming a thermal oxide layer having a good surface property on the surface of a base member comprising a polycrystalline silicon material which is used in the above-described substrate for liquid jet recording head. In order to solve the foregoing problems of the conventional substrate for liquid jet recording head and in order to attain the above objects, The present inventor made studies through experiments which will be later described. As a result, the present inventor obtained the following findings. That is, in the case of using a polycrystalline silicon material as the base member for a substrate for liquid jet recording head, (i) the foregoing problems in the case of using a single crystal silicon wafer which are related to the restriction for the size of a substrate for liquid jet recording head and to the occurrence of deformation of the substrate can be effectively solved, and a liquid jet recording head capable of providing a high quality recorded image at a high speed can be effectively produced at a reduced production cost; and (ii) in the case of forming a thermal oxide layer on the polycrystalline silicon base member, when the thermal oxide layer is firstly formed by way of thermal oxidation and the thermal oxide layer is followed by subjecting to thermally softening treatment at a temperature region at which the thermal oxide layer is softened, the thermal oxide later becomes to have a smooth and continuous surface with no surface step wherein a thermal oxide layer having an excellent surface property is provided. The present invention has been accomplished based on the findings obtained through the experiments by the present inventor. The present invention includes a substrate for liquid jet recording head of the configuration which will be described below, a liquid jet recording head in which said substrate is used, a liquid jet recording apparatus in which said substrate is used, and a process for producing said substrate. The present invention provides a substrate for liquid jet recording head including an electrothermal converting body comprising a heat generating resistor capable of generating thermal energy and a pair of wirings electrically connected to said heat generating resistor, wherein said substrate includes a base member, composed of a polycrystalline silicon material. The substrate for liquid jet recording head according to the present invention have various advantages in that even if the substrate is of a greatly prolonged length, it can be effectively produced at a lower production cost in comparison with the foregoing case wherein a single crystal silicon wafer is used; no deformation is occurred not only in the case where the substrate is in the form of a normal shape but also in the case where the substrate is in the form of an elongated shape; and highly precise wire-patterning can be easily attained. The present invention provides a liquid jet recording head including: a liquid discharging outlet; a substrate for liquid jet recording head including an electrothermal converting body comprising a heat generating resistor capable of generating thermal energy for discharging liquid from said discharging outlet and a pair of wirings electrically connected to said heat generating resistor, said pair of wirings being capable of supplying an electric signal for generating said thermal energy to said heat generating resistor; and a liquid supplying pathway disposed in the vicinity of said electrothermal converting body of said substrate, wherein said substrate includes a base member composed of a polycrystalline silicon material. The liquid jet recording head according to the present invention is markedly advantageous in that a desired elongation therefor can be easily attained. Particularly, the elongation of a liquid jet recording head in the case of using a single crystal silicon wafer can be attained for the first time by integrating a plurality of substrates for liquid jet recording head. However, in the present invention, such integration process is not necessary to be carried out. Thus, the elongated liquid jet recording head according to the present invention is free of the problems relating to occurrence of unevenness as for images recorded which are caused due to the integration of a plurality of substrates for liquid jet recording head in the case of an elongated liquid jet recording head in which a single crystal silicon wafer is used. Other than this advantage, the liquid jet recording head according to the present invention has further advantages. That is, since the substrate excels in surface property and the head work in process is free of warpage, the liquid jet recording head can be produced at a high yield, and since the positional precision for a liquid recording medium discharged from the discharging outlets to be deposited on a recording member is always insured, there is stably and continuously provided a high quality recorded image. The present invention provides a liquid jet recording apparatus comprising: a liquid jet recording head including a liquid discharging outlet; a substrate for liquid jet recording head including an electrothermal converting body comprising a heat generating resistor capable of generating thermal energy for discharging liquid from said discharging outlet and a pair of wirings electrically connected to said heat generating resistor, said pair of wirings being capable of supplying an electric signal for generating said thermal energy to said heat generating resistor; a liquid supplying pathway disposed in the vicinity of said electrothermal converting body of said substrate; and an electric signal supplying means capable of supplying an electric signal to said heat generating resistor of said recording head, wherein said substrate includes a base member composed of a polycrystalline silicon material. The liquid jet recording head apparatus according to the present invention enables to conduct high speed recording wherein a high quality recorded image is stably and repeatedly provided. The present invention provides a process for producing a substrate for liquid jet recording head in which an electrothermal converting body comprising a heat generating resistor and a pair of wirings electrically connected to said heat generating resistor is disposed on an oxide layer as the heat accumulating layer formed on a base member, said process is characterized by including the step of forming a thermal oxide layer (which will be hereinafter referred to as thermal oxide layer, SiO 2 film or SiO 2 layer according to the situation) having a smoothly flat surface as said heat accumulating layer on the surface of said polycrystalline silicon member. The step of forming the thermal oxide layer in the process for producing a substrate for liquid jet recording head according to the present invention is conducted in a manner which will be described in the following (i) or (ii). That is, the manner (i) is that a given polycrystalline silicon base member is provided, the surface of the polycrystalline silicon base member is subjected to thermal oxidation treatment to form a thermal oxide layer (that is, a SiO 2 layer), and the thermal oxide layer is subjected to thermally softening treatment to thereby form a thermal oxide layer having a smoothly flat surface (that is, a heat accumulating layer) on the polycrystalline silicon base member. The manner (ii) is that a given polycrystalline silicon base member is provided, and the surface of the polycrystalline silicon base member is subjected to thermal oxidation treatment and thermally softening treatment substantially at the same time to thereby form a thermal oxide layer having a smoothly flat surface (that is, a heat accumulating layer) on the polycrystalline silicon base member. According to the process for producing a substrate for liquid jet recording head of the present invention, although a polycrystalline silicon material inherently having an irregular surface is used as the base member, a desirable thermal oxide layer while providing an excellent surface flatness for the layer formed. Thus, it is possible to form, on a polycrystalline silicon base member, a heat accumulating layer which is equivalent to the foregoiing heat accumulating layer formed on a single crystal silicon base member. The heat accumulating layer thus formed has a smoothly flat surface and excels in durability, and because of this, wirings and the like can be formed on the heat accumulating layer in a desirable state in that problems relating to occurrence of a breakdown or the like are hardly occurred therefor. Experiments In the field of solar cell, a plate-like polycrystalline member has been used. However, in the case of using such polycrystalline silicon member in a substrate for liquid jet recording head, it is required to have a flat surface in a desirable state for the reason that precise wirings and the like are disposed thereon. However, The polycrystalline silicon member, being different from a single crystal member, contains various crystals with a different orientation, and because of this, it has an irregular surface. In view of this, it is a common recognition in the field of liquid jet recording head that a desirable flatness which is required for the base member for a substrate for liquid jet recording head is hardly attained for the surface of the polycrystalline silicon member even by means of the polishing technique capable of providing a mirror-ground surface. Hence, a polycrystalline silicon member has never tried to use as the base member in the field of liquid jet recording head. Disregarding this common recognition, the present inventor tried to use a polycrystalline silicon material as the base member for a substrate for liquid jet recording head as described in the following experiments. As described in the following, based on the findings obtained in the experiments, there was obtained a finding that a polycrystalline silicon material can be effectively used as the base member for a substrate for liquid jet recording head. Description will be made of the experiments conducted by the present inventor. Experiment A In the case of producing a semiconductor device using a conventional single crystal wafer, the mechanochemical polishing technique is employed in order to minimize work defect zones present on the single crystal wafer. In the mechanochemical polishing technique, an abrasive material comprising a colloidal silica added with various alkalies such as NaOH, KOH, organic amines, and the like is used in the primary polishing, and an abrasive material comprising a colloidal silica added with ammonia is used in the secondary polishing. However, when the surface of a polycrystalline silicon member is processed by the above polishing technique, steps are usually occurred at the surface. The present inventor presumed that this occurrence would be caused due to the difference in the amount of the silicon material to be etched by the alkali component of the abrasive material depending upon the crystal orientation. The following experiment was conducted based on this presumption. Firstly, there were prepared a plurality of single crystal base member samples in the following manner. That is, a single crystal silicon ingot (8 inch x 110 cm) of a boron dopant p-type was prepared by pulverizing a high purity polycrystal rod with a residual impurity content of less than 1 ppb obtained by way of the precipitation reaction through hydrogen reduction and pyrolysis of SiHCl 3 , fusing the resultant, and pulling the fused material toward the (111) direction by a conventional CZ method. The single crystal ingot obtained was then formed into a prismatic shape by means of a grinder. The resultant was quarried by means of a multi-wire saw, to thereby obtain a plurality of plate members. Each of the plate members obtained was subjected to lapping treatment to remove an about 30 µm thick surface portion whereby obtaining a plate member with a flat surface. Separately, there were prepared a plurality of polycrystalline silicon base member samples in the following manner. That is, there was provided a high purity polycrystalline silicon material, obtained in accordance with the same precipitation reaction through hydrogen reduction and pyrolysis as in the above case of obtaining the foregoing single crystal silicon material. The material obtained was then pulverized, the resultant was fused in a quartz crucible at 1420 °C, the fused material was poured into a casting mold made of graphite, followed by cooling, whereby an ingot of 40 cm in square size was obtained. The ingot obtained was quarried by means of a multi-wire saw to thereby obtain a plurality of plate members. Each of the plate members obtained was subjected to lapping treatment to remove an, about 30 µm thick surface portion whereby obtaining a plate member having a flat surface. In this way, as for each of the single crystal material and the polycrystalline silicon material, there were obtained a plurality of samples each having a size of 300 (mm) x 150 (mm) x 1.1 (mm) (for the simplification purpose, this will be abbreviated as "300 x 150 x 1.1 (mm)") as shown in Table 1. In the following, there was used a single side polishing machine, produced by Speedfarm Kabushiki Kaisha, in the polishing processing. For each sample, the primary polishing and the secondary polishing were separately conducted under the below-described respective conditions. The surface finishing efficiency in relation to the presence or absence of alkali upon the polishing was evaluated. The evaluated results obtained are collectively shown in Table 1. The conditions in the primary polishing: abrasive fabric : polyurethane-impregnated polyester non-woven fabric; abrasive material : colloidal silica (0.06 um in particle size); polishing pressure : 250 g/cm 2 ; polishing temperature : 42 °C; processing speed : 0.7 um/min. The conditions in the secondary polishing: abrasive fabric : suede type urethane foam; abrasive material : silica fine powder (0.01 um in particle size); polishing pressure : 175 g/cm 2 ; polishing temperature : 32 °C; processing speed : 0.2 um/min. From the results shown in Table 1, it was found that even in the case of a polycrystalline silicon base member, it is possible to attain a surface flatness similar to that obtained in the case of a single crystal silicon member by omitting the addition of alkali upon the polishing, and a polycrystalline silicon member can be used as the base member for a substrate for liquid jet recording head. Experiment B In this experiment, discussion was made of a difference between the magnitude of a single crystal silicon base member to be deformed and that of a polycrystalline silicon base member to be deformed. The single crystal silicon base member sample was prepared in the following manner. That is, a single crystal ingot (8 inch x 110 cm) of a boron dopant p-type was prepared by pulverizing a high purity polycrystal rod with a residual impurity content of less than 1 ppb obtained by way of the precipitation reaction through hydrogen reduction and pyrolysis of SiHCl 3 , fusing the resultant, and pulling the fused material toward the (111) direction by a conventional CZ method. The single crystal ingot was formed into a prismatic shape by means of a grinder. The resultant was quarried by means of a multi-wire saw to obtain a plate member. The plate member obtained was subjected to lapping treatment to remove an about 30 um thick surface portion whereby obtaining a plate member having a flat surface. The end portions of the resultant were chanferred by means of a beveling machine, followed by finishing by way of the polish processing, to thereby obtain a mirror-ground member with a surface roughness of Rmax 150 Å. Then, the surface of the mirror-ground member was subjected to thermal oxidation by way of the pyrogenic oxidation method (the hydrogen burning oxidation method) shown in FIG. 7. The thermal oxidation in this case is conducted, for example, in the following manner. That is, hydrogen gas and oxygen gas are separately introduced into a quartz tube 73, wherein these gases are reacted with each other to produce H 2 O, and the unreacted residuals are burned. The mirror-ground member as an object 71 to be treated is arranged in the quartz tube 73, and the object is heated to a desired temperature by an electric furnace 74. The thermal oxidation of the surface of the mirror-ground member using the oxidation apparatus was conducted under the conditions of 1 atm for the gas pressure, 1150 °C for the treating temperature, and 14 hours for the treating period of time, while introducing hydrogen gas and oxygen gas into the quartz tube, whereby a 3 µm thick thermal oxide layer was formed on said member. In this way, there were prepared six single crystal silicon base member samples each having a different size as shown in Table 2. Separately, there were prepared a plurality of polycrystalline silicon base member samples in the following manner. That is, there was firstly provided a high purity polycrystalline silicon material, obtained in accordance with the same precipitation reaction through hydrogen reduction and pyrolysis as in the above case of obtaining the foregoing single crystal silicon material. The material obtained was then pulverized, the resultant was fused in a quartz crucible at 1420 °C, the fused material was poured into a casting mold made of graphite, followed by cooling, whereby an ingot of 120 cm in square size was obtained. In this case, the higher the cooling speed is, the smaller the crystal grain size is, and because of this, the crystal grain size in the vicinity of the center becomes greater. In view of this, the portion of the ingot obtained having a mean grain size of 2 mm was quarried by means of a multi-wire saw to obtain a polycrystalline silicon plate member. The plate member obtained was subjected to lapping treatment to remove an about 30 µm thick surface portion whereby obtaining a plate member having a flat surface. The end portions of the resultant were chanferred by means of a beveling machine, followed by finishing by way of the polish processing, to thereby obtain a mirror-ground member with a surface roughness of Rmax 150 Å. Then, the surface of the mirror-ground member was subjected to thermal oxidation by way of the above described pyrogenic oxidation method under the same conditions employed in the above case, whereby a 3 um thick thermal oxide layer was formed on said member. In this way, there were prepared six polycrystalline silicon base member samples each having a different size as shown in Table 2. As for each of the resultant single crystal silicon base member samples and the resultant polycrystalline base member samples, on the surface thereof, there were laminated an aluminum layer (4500 Å thick) as the wirings, a HfB 2 layer (1500 Å thick) as the heat generating resistor, a Ti later (50 Å) as the layer serving to improve the contact with the protective layer to be formed above, a SiO 2 layer (1.5 µm thick) as the protective layer, a Ta layer (5000 Å thick), and a polyimide film (3 µm thick). Thus, there were obtained six substrates for liquid jet recording head in each case. Now, in the production of a liquid jet recording head using a substrate for liquid jet recording head, an about 20 µm thick negative dry film is formed on the substrate, followed by subjecting to exposure for the purpose of patterning liquid pathways. In this patterning process, if the substrate is accompanied by a warp, the focusing position is often deviated to cause a defective exposure. In this viewpoint, as for each substrate, the magnitude of the warp was evaluated. The evaluation of the warp was conducted by placing the sample on a measuring table and measuring its maximum displacement magnitude by means of a dial gauge of 1 µm in minimum scale. The results obtained are collectively shown in Table 2. The values shown in Table 2 are values relative to the maximum warp magnitude of the polycrystalline silicon substrate sample of 300 x 150 x 1.1 (mm) in size, which was set at 1. Based on the results shown in Table 2, the followings are understood. That is, the respective warp magnitudes of the polycrystalline silicon substrate samples examined are slight and substantially the same, but as for the warp magnitude of each of the single crystal silicon substrate samples examined, it starts increasing from the single crystal silicon substrate sample of 500 x 150 x 1.1 (mm) in size, and the single crystal silicon substrate sample of 800 x 150 x 1.1 (mm) in size is great as much as 3 in terms of relative value; in the case of the single crystal silicon substrate sample of 2 in warp magnitude relative value, the focusing position in the exposure process is liable to deviate to cause a defective exposure, and in the case of the single crystal silicon substrate sample of 3 in warp magnitude relative value, the focusing position in the exposure process is definitely deviated to cause a defective exposure; and the single crystal silicon substrate sample of 500 x 150 x 1.1 (mm) in size is the usable limit for producing a liquid jet recording head. Experiment C In this experiment, as for each of a single crystal silicon base member and a polycrystalline silicon base member, studies were made of the interrelation between the crystal grain size and the occurrence of a deformation at the base member due to warpage. There were prepared 10 mirror-ground single crystal silicon base member samples each having a size of 300 x 150 x 1.1 (mm) (Sample No. 1) in the same manner as in Experiment B. Separately, there were prepared a plurality of mirror-ground polycrystalline silicon base members each having a size of 300 x 150 x 1.1 (mm) in the same manner as in Experiment B. Incidentally, the polycrystalline silicon ingot obtained is of a varied crystal grain size which is gradually increased from the casting mold side toward the center. In view of this, appropriate portions of the polycrystalline silicon ingot were selected upon the quarrying, to thereby obtain seven polycrystalline silicon plates (Sample Nos. 2 to 8) each having a different mean crystal grain size as shown in the columns Sample No. 2 to Sample No. 8 of Table 3. As for each of these seven plates, there were obtained 10 base member samples. In this case, the mean crystal grain size was measured by a crystal grain size measuring method based on the cutting method described in the description of the ferrite crystal grain size examining method in the JIS G 0552. As for each of the single crystal silicon base member sample (Sample No. 1) and the polycrystalline silicon base member samples, a 3 µm thick thermal oxide layer was formed in accordance with the pyrogenic oxidation method described in Experiment B. Now, an elongated integral liquid jet recording head is obtained by cutting the substrate for liquid jet recording head into a plurality of strip forms each being dedicated for a head. In this case, there is a problem in that only the heads cut from the opposite sides of the substrate are always bow-shaped. The situation wherein these bow-shaped heads are caused is shown in FIG. 9(A). Incidentally, if the face to be polished is warped upon conducting the polishing processing, a problem is entailed in that since the distance between the heat generating resistor and the discharging outlet face is not uniform, a defect is liable to provide for an image recorded. In view of this, for the purpose of examining the process yield in the polishing process, each of the opposite side portions of the base member sample was cut by means of a slicer to thereby obtain two strip-shaped test samples of 10 mm in width. Thus, there were obtained 20 test samples as for each of the samples described in Table 3. As for each sample, the maximum deformation magnitude was measured by placing it on a precision XY-table. The measuring manner in this case is shown in FIGs. 9(B) to 9(D). In the manner shown in FIG. 9(D), the measurement of the maximum deformation magnitude was conducted by setting the points a and b to the X axis of the XY-table and measuring a deformation magnitude in the Y direction. As for the results obtained, the sample which was beyond a given allowable deformation magnitude in the polishing process was made to be unfitness, and the fitness proportion was obtained as for each sample. The evaluated results are collectively shown in Table 3, in which the values shown are values relative to the fitness proportion of Sample No. 8 of 0.001 mm in mean crystal grain size, which was set at 1. Based on the results shown in Table 3, there was obtained a finding that in general, a polycrystalline silicon base member is superior to a single crystal silicon base member in terms of deformation magnitude due to warpage. Particularly, as for the polycrystalline silicon base member samples of a mean crystal grain size exceeding 8 um, their superiority to the single crystal silicon base member is not significant; as for the polycrystalline silicon base member samples of a mean crystal grain size in the range of 2 um to 8 um, their superiority to the single crystal silicon base member is significant, but they are inferior to the polycrystalline silicon base member samples of a mean crystal grain size of 2 um or less. From this situation, it is understood that in order for the polycrystalline silicon member base member to be effectively usable, it is desired to be preferably of a mean crystal grain size of 8 µm or less, more preferably of a mean crystal grain size of 2 µm or less. Experiment D As for the base member for a substrate for liquid jet recording head, since wirings are disposed thereon, it is required to have a flat surface in a desirable state. Therefore, even in the case where a polycrystalline silicon material is used as the base member, it is required to meet this requirement. By the way, it is known to use a polycrystalline silicon material as a substrate in the field of solar cell. In this case, as for the surface state of the polycrystalline silicon substrate, there is not such a severer requirement with regard to surface flatness as in the case of the base member for a substrate for liquid jet recording head. In fact, polycrystalline silicon substrates used in the field of solar cell usually contain certain contaminants. A polycrystalline silicon ingot used for obtaining a polycrystalline silicon substrate for a solar cell is prepared by fusing a silicon material in a quartz crucible and cooling the fused silicon material to solidify. The fused silicon material in this case is very chemically reactive and it unavoidably chemically reacts with the constituent quartz of the crucible in a way expressed by the chemical formula SiO 2 + Si → 2SiO. As a result, upon cooling and solidifying the fused silicon material, the silicon is firmly adhered to the inner wall face of the crucible. An when a strain due to the difference between the coefficient of thermal expansion of the silicon material and that of the quartz is provided therein, a crack is liable to occur at the crucible. In order that the ingot formed can be easily taken out from the crucible, a powdery release agent is coated to the inner wall face of the crucible. Therefore, such release agent is unavoidably contaminated into the ingot. The presence of such contaminant in the ingot is not problematic in the case of the substrate for a solar cell. However, in the case of disposing wirings on the surface of a polycrystalline member obtained in accordance with this manner, when the surface of the polycrystalline silicon member is subjected to polishing treatment in order to provide a mirror-ground surface, the contaminants present in the polycrystalline silicon member cause defects at the resulting mirror-ground surface wherein the contaminants are remained at said surface while providing pits or/and protrusions of some tens microns in size. The presence of such defects entails a problem in that when the wirings are patterned by means of a photolithography technique, there are often occurred portions for which a resist is hardly applied or other portions where a resist is accumulated, resulting in causing disconnection, shortcircuit or the like for the wirings. Further, in the case where such defects are present at the position where a heat generating resistor is arranged, there is a fear that cavitation damages are centralized to cause early disconnection for the wirings at the time when bubbles are generated for discharging ink. In this experiment, in view of this situation, studies were made of the influence of a contaminant contained in a polycrystalline material upon using the polycrystalline silicon material as the base member for a substrate for liquid jet recording head. Firstly, from a single crystal silicon material obtained in accordance with the manner described in Experiment B, a single crystal plate of 330 x 150 x 1.1 (mm) in size was quarried, and it was subjected to lapping treatment and polishing treatment, to thereby obtain a mirror-ground single crystal silicon base member having a surface with a surface roughness of Rmax 150 Å. This base member was made to be Sample No. 1. At this stage, the surface state of this base member (Sample No. 1) was observed using a binary image processing by CCD line sensor system (trademark name : SCANTEC, produced by Nagase Sangyo Kabushiki Kaisha). As a result, it was found that the number of defects per unit area is less than 1/cm 2 at every measured point in the detectable range with a diameter of more than 1 um, since no release agent was used in this case. The observed result is shown in Table 4. Separately, a polycrystalline silicon material was fused in a quartz crucible with no application of a release agent to the inner wall face of said quartz crucible, and a polycrystalline silicon ingot of 50 cm in square size was obtained. From this ingot, there was quarried a polycrystalline silicon plate of the same size as the above single crystal silicon plate, and it was subjected to lapping treatment and polishing treatment, to thereby obtain a mirror-ground polycrystalline silicon base member having a surface with a surface roughness of Rmax 150 Å. This base member was made to be Sample No. 2. The surface state of this base member was observed in the same manner as in the case of the above single crystal silicon base member. As a result, it was found that the number of defects per unit area is less than 1/cm 2 at every measured point in the detectable range with a diameter of more than 1 µm, since no release agent was used in this case. The observed result is shown in Table 4. Then, there were prepared a plurality of base members (Sample Nos. 3 to 6) in the same manner as in the case of preparing Sample No. 2, except for using a release agent. The amount of the release agent used was made different in each case. As for each of the resultant base members (Sample Nos. 3 to 6), the surface state was observed in the same manner as in the case of the above single crystal silicon member (Sample No. 1). As a result, it was found that the base members of Sample Nos. 3 to 6 are respectively of less than 5/cm 2 , less than 10/cm 2 , less than 50/cm 2 , and less than 100/cm 2 in terms of the number of defects. Then, as for each of the above base members (Sample Nos. 1 to 6), the surface thereof was subjected to thermal oxidation treatment in the same manner as in Experiment B, to thereby form a 3 µm thick thermal oxide layer. In order to examine the situation of causing disconnection or shortcircuit due to the foregoing contaminant, on the thermal oxide layer of each sample, a return wiring pattern of 20 µm in line width and 10 µm in line interval as a test wiring pattern was arranged by way of forming a 4500 Å thick Al film by a conventional magnetron sputtering technique. In this case, considering the wiring pattern of a liquid jet recording head, as for the return wirings for each sample, there was employed a pattern of 8 mm for the wiring length and 4736 for the number of the wirings. And this pattern was made as a test pattern as for each sample. 15 this patterns were arranged in each sample. Then, as for each sample, continuity check was conducted by connecting a probe-pin to each wiring terminal. The evaluation of the continuity check was conducted based on the criteria in which the case where neither disconnection nor shortcircuit is present is made to be fitness. The evaluated result was expressed by the number of the patterns with neither disconnection nor shortcircuit among the 15 patterns, specifically, the number of the patterns having been judged as being fitness/the 15 patterns. The results obtained are collectively shown in Table 4. Based on the results shown in Table 4, the following findings were obtained. That is, (i) the process yield in the case of a polycrystalline silicon member with no release agent is substantially the same as that in the case of a single crystal silicon base member; (ii) the process yield in the case of a polycrystalline silicon member with a release agent and which is of 5/cm 2 or less in therms of the number of defects of more than 1 µm in diameter is substantially the same as that in the case of a single crystal silicon base member; (iii) the process yield in the case of a polycrystalline silicon member with a release agent and which is of 10/cm 2 or less in therms of the number of defects of more than 1 µm in diameter is slightly inferior to that in the case of a polycrystalline silicon member with a release agent and which is of 5/cm 2 or less in therms of the number of defects of more than 1 um in diameter; and (iv) the process yield in the case of a polycrystalline silicon member with a release agent and which is of 50/cm 2 or less in therms of the number of defects of more than 1 µm in diameter is markedly inferior, and such polycrystal silicon base member is practically unacceptable. In addition, the polycrystalline silicon member with a release agent and which is of 100/cm 2 or less in therms of the number of defects of more than 1 µm in diameter is practically unacceptable also. Based on these findings, there was obtained the following knowledge. That is, in order for a polycrystalline silicon material to be usable as the base member for a substrate for liquid jet recording head, it is required to have a surface with a surface flatness (a surface smooth state) preferably of 10/cm 2 or less, more preferably of 5/cm 2 in therms of the number of defects of more than 1 µm in diameter. Experiment E In this experiment, studies were made in the viewpoint of eliminating the occurrence of surface steps at the surface of a polycrystalline silicon member in the case of using said polycrystalline silicon member as the base member for a substrate for liquid jet recording head. As previously described, in the case of using a single crystal silicon material as the base member for a substrate for liquid jet recording head, a heat accumulating layer is usually formed on the surface of the single crystal silicon base member for the purpose of attaining a desirable balance between the heat radiating property and the heat accumulating property so that the resulting liquid jet recording head exhibits good characteristics. As the heat accumulating layer in this case, there is usually employed a SiO 2 layer formed by thermally oxidizing the surface of the single crystal silicon base member. In this experiment, using a polycrystalline silicon member instead of the above single crystal silicon base member, a SiO 2 layer as the heat accumulating layer was formed by thermally oxidizing the surface of the polycrystalline silicon member, and the surface state of the resultant SiO 2 layer was examined. As a result, it was found that steps of some thousands angstroms in terms of maximum degree are present among the crystal grains at the surface of the SiO 2 layer. In the case where such steps are present at the surface of the base member for a substrate for liquid jet recording head, damages are forced to centralize in the vicinity of such step by virtue of a thermal shock caused upon the heating and cooling operations or/and a cavitation caused upon discharging a recording liquid. And if the heat generating resistor having being formed on such step, a problem entails in that the reliability is reduced particularly in terms of durability. Especially, in the case where recording liquid discharging is repeated at a high speed, the cavitation is centralized in the vicinity of such step and as a result, a rupture is occurred at the heat generating resistor at a relatively earlier stage. As a mean in order to solve these problems, there is considered a manner of forming the above SiO 2 layer and flattening the surface of the SiO 2 layer by the polishing technique. But, the above problems cannot be satisfactorily solved by this manner. That is, the SiO 2 layer, which is accompanied by such surface steps of some thousands angstroms as above described, is desired to be of a thickness of some microns, and therefore, it is difficult to desirably solve the above problems without hindering the function of the SiO 2 layer. In order to solve the above problems, there is considered another manner of making the SiO 2 layer thickened to a remarkable extent and polishing the surface thereof to a certain extent. However, this manner is practically unacceptable also, since the SiO 2 layer having an excessive thickness does not function as the heat accumulating layer, and in addition, the formation of such excessively thick SiO 2 layer is not economical. Independently, the formation of the heat accumulating layer (that is, the SiO 2 layer) was conducted by means of each of sputtering, thermal-induced CVD, plasma CVD, and ion beam evaporation techniques. In any case, there were found problems such that the film thickness is uneven, the film-forming period is relatively long, or foreign matters generated during the film formation are contaminated into a film to result in providing protrusions having a size of some microns in diameter, which will eventually become causes of causing the foregoing rupture by virtue of a cavitation. It was also found that such protrusion occurred permits an electric current to leak therethrough, resulting in causing a shortcircuit. Based on these findings, there was obtained a knowledge that any of the above-mentioned vacuum film-forming methods is not suitable for the formation of the foregoing heat accumulating layer (that is, the SiO 2 layer). Then, the formation of the heat accumulating layer (that is, the SiO 2 layer) was formed by means of each of the spin-on-glass method and the dipping method. As a result, it was found that any of the SiO 2 films formed by these methods is poor in film quality, any of these methods is difficult to attain a desired film quality, contamination of foreign particles into a film formed is often occurred in any of these methods, and therefore, any of these methods is not suitable for the formation of the foregoing heat accumulating layer. By the way, in the case of producing a semiconductor device, there is usually employed the so-called flattening process as a means of eliminating the problem relating to the occurrence of a breakdown at a step portion of a multi-layered wiring. As a typical example of the flattening process, there can be mentioned a PSG film-reflowing technique which is often employed in the case of preparing a MOSLSI. To flatten steps of a PSG film as the interlayer insulating film by this technique is conducted, for example, in a manner that a few mole % of P 2 O 5 is incorporated into a SiO 2 film formed, for example, by means of the CVD technique to thereby reduce the softening point of the PSG film, followed by subjecting to thermal treatment (reflow treatment). The reflow temperature in this case is made to be in the range of about 800 to 1000 °C with a due care about occurrence of a negative influence to the wirings and the like formed. However, the above flattening process is not effective to eliminate the foregoing surface steps at the thermal oxide layer formed on the polycrystalline silicon base member for a substrate for liquid jet recording head. That is, in the case of a liquid jet recording head which is apparently different from the semiconductor device in the viewpoints of constitution, function, performance and use purpose, the substrate of the liquid jet recording head is required to be sufficiently durable against a temperature of about 1100 °C since the heat generating resistor disposed on said substrate is energized to said temperature for generating thermal energy for discharging liquid recording medium upon conducting recording using the liquid jet recording head. The constituent material of the substrate is, therefore, is essential to meet this requirement. Now, in the case where a base member constituting the above substrate is composed of a polycrystalline silicon material which is apparently different from the PSG film used in the semiconductor device and a thermal oxide layer is formed on the polycrystalline silicon base member by way of thermal oxidation treatment, a step is unavoidably occurred at the surface of the thermal oxide layer as above described. The present inventor employed the above-described step-eliminating method in the semiconductor device in order to eliminate this step, but the object could not be accomplished. This situation is apparent with reference to the results obtained through the following experiments. That is, in summary, the problem relating to the step at the surface of the thermal oxide layer could not be eliminated even by conducting the reflow treatment using about 1100 °C, which is beyond the maximum reflow treatment temperature of about 1000 °C employed upon the step elimination in the semiconductor device. Thus, it was found that any conventional technique is not effective in eliminating the problem relating to occurrence of a step (a surface step in other words) at the thermal oxide layer formed on the polycrystalline silicon base member for a substrate for liquid jet recording head. In view of this, the present inventor made a trial of eliminating the problem relating to the occurrence of a surface step at the thermal oxide layer by employing a so-called thermally softening treatment through the following experiments. In the experiments, there were employed two manners; a manner (i) in which a thermal oxide layer is formed on a polycrystalline silicon base member by thermally oxidizing the surface of the polycrystalline silicon base member, and the thermal oxide layer is subjected to thermally softening treatment; and a manner (ii) in which the thermal oxidation treatment and thermally softening treatment are concurrently conducted for the surface of a polycrystalline silicon base member. In the following, with reference to FIG. 4(A) to FIG. 4(C), description will be made of (a) the reason why a thermal oxide layer (a SiO 2 layer) formed on a polycrystalline silicon base member by thermally oxidizing the surface of the polycrystalline silicon base member becomes to have a surface step at the surface thereof and also of (b) a finding obtained by the present inventor through the experiments in that a SiO 2 layer free of a surface step can be formed on a polycrystalline silicon base member in the case where a thermal oxide layer with a surface step formed on the polycrystalline silicon base member by thermally oxidizing the surface of the polycrystalline silicon base member is subjected to thermally softening treatment at a temperature at which the thermal oxide layer is softened. That is, when a polycrystalline base member 11 as such shown in FIG. 4(A) itself is thermally oxidized, its volume is increased upon conducting the thermal oxidation and the constituent crystal grains 12 are individually oxidized at a different oxidation speed because these crystal grains are different one from the other in terms of crystal orientation, and because of this, as shown in FIG. 4(B), the thickness of the resulting thermal oxide film 13 becomes different depending on each of the crystal grains 12, resulting in causing steps at the surface. The line a in FIG. 4(B) indicates the surface position of the polycrystalline silicon base member 11 prior to the thermal oxidation. Particularly, for instance, when an about 3 µm thick thermal oxide film 13 (that is, a SiO 2 layer) is formed on the surface of the polycrystalline silicon base member 11, steps caused at the surface of the thermal oxide film are of about 1000 Å. In the case of a liquid jet recording head prepared using a substrate for liquid jet recording head comprising a polycrystalline silicon base member having a thermal oxide layer with such surface steps formed thereon, cavitation damages caused when bubbles are extinguished above the heat generating resistor of the substrate are centralized at step portions upon conducting recording while driving the liquid jet recording head, resulting in making the heat generating resistor damaged at very early stage. Herein, description will be made of the thermal oxidation process of the surface of a polycrystalline silicon base member. At the very beginning stage of the forming of the thermal oxide layer by thermally oxidizing the surface of the polycrystalline silicon base member, a linear relationship is established between the thickness of the thermal oxide film 13 and the oxidation speed. That is, the reaction of oxygen gas (O 2 ) at the interface between the polycrystalline silicon (Si) and the silicon oxide (SiO 2 ) constituting the thermal oxide layer becomes a rate-limiting factor. In this case, the oxidation speed of the oxygen gas is different depending on the crystal orientation. On the other hand, after the thermal oxide layer 13 having been formed to a certain extent, the process of the oxygen gas to be diffused in this thermal oxide layer 13 becomes a rate-limiting factor. It is considered that the diffusing speed of the oxygen gas in the thermal oxide layer 13 is not governed by the crystal orientation of the silicon crystal grain 12. In this connection, it is presumed that a surface step at the surface of the thermal oxide layer 13 (that is, the thermal oxide film) formed as for each of the crystal grains 12 of the polycrystalline silicon base member 11 will be occurred at the very beginning stage of the thermal oxidation process and after the formation of the thermal oxide layer 13 having proceeded to a certain extent, the steps are not grown further. When heat treatment (that is, thermally softening treatment) is conducted for said steps at an elevated temperature (a softening temperature) at which the polycrystalline material is not fused, the thermal oxide layer gradually becomes showing a flowability, eventually resulting in providing a smoothly flat surface as shown in FIG. 4(C). Particularly, to apply thermal energy makes the surface state of the thermal oxide layer deformed and flattened such that the surface steps are averaged, and this leads to prevent occurrence of the problem of centralizing cavitation damages at the heat generating resistor formed on the thermal oxide layer, resulting in providing an improvement in the durability of the heat generating resistor. Being different from the case of forming a multi-layered wiring in the process of producing a LSI wherein the interlayer insulating film on the wiring is flattened, the present invention is aimed at flattening the surface steps of the thermal oxide layer formed on the polycrystalline silicon base member, and therefore, the purpose can be attained by providing a certain flowability for the steps. The above thermally softening treatment can be conducted after the thermal oxidation treatment (the formation of the thermal oxide layer) or it can be conducted concurrently together with the thermal oxidation treatment. In any case, the polycrystalline silicon base member may be incorporated with a given impurity and the polycrystalline silicon member. In this case, the softening temperature of the thermal oxide layer is lowered and as a result, an improvement is provided for the treating efficiency. Particularly, the thermally softening treatment can be conducted at a relatively low temperature and the period of time for the thermally softening treatment can be shortened. However, in the case where the thermally softening treatment is conducted at a relatively high temperature, the softening of the thermal oxide layer effectively proceeds and as a result, the flattening of the steps can be more effectively conducted. By proceeding the softening state of the thermal oxide layer in this way, an improvement can be attained for the close contact between the thermal oxide later formed on the polycrystalline silicon base member and a heat generating resistor formed on the thermal oxide layer. In order to confirm the effects provided by conducting the thermally softening treatment, the following experiments were conducted by preparing a substrate for liquid jet recording head. Experiment E-1 In this experiment, studies were made of the effects of a polycrystalline silicon base member having a thermal oxide layer formed by conducting the foregoing thermally softening treatment following the thermal oxidation treatment by preparing a substrate for liquid jet recording head using said base member. Firstly, a polycrystalline silicon ingot with a mean crystal grain size of about 2 mm was produced by the foregoing casting technique. The resultant ingot was quarried to obtain five rectangular plates. Each of the plates obtained was subjected to lapping treatment and polishing treatment, to thereby obtain a polycrystalline silicon base member of 300 x 150 x 1.1 (mm) in size and having a mirror-ground surface with a surface roughness of Rmax 150 Å. On the surface of each of the polycrystalline silicon base members, there was formed a thermal oxide layer by thermally oxidizing said surface in the manner and under the same conditions employed in Experiment B, except in that the quartz tube (see, 73 in FIG. 7) was replaced by a quartz tube made of SiC. Each of the resultant five polycrystalline silicon base members each having a thermal oxide layer thereon was introduced into a thermal oxidation furnace, wherein the thermal oxide layer was subjected to thermally softening treatment in an atmosphere maintained at a different temperature of 1380 °C, 1330 °C, 1280 °C, 1230 °C or 1180 °C for an hour. Thus, there were obtained five polycrystalline silicon base member samples as Sample Nos. 1 to 5. As a result of having conducted the above thermally softening treatment, each of the polycrystalline silicon base members became to have a heat accumulating layer comprising the thermal oxide layer (that is, the SiO 2 layer) thereon. The thickness of the heat accumulating layer (that is, the SiO 2 layer) in each case was found to be 3.0 µm. As for the heat accumulating layer of each polycrystalline silicon base member sample, evaluation was made of its surface step state while measuring it by means of a conventional surface profiler by stylus. The conditions for the measurement and the criteria for the evaluation were made as follows. The measurement conditions: the stylus scanning distance : 10 mm, the number of the positions measured : 15 positions as for each sample, and the position measured : 15 intersections of the three linear lines by which the short side of 150 mm in width is divided into four equal zones and the five linear lines by which the long side of 300 mm in length is divided into six equal zones as for each sample. The evaluation criteria: o ○ : the case where the maximum step height among the 15 measured positions is between 0 µm and less than 0.05 µm, ○ : the case where the maximum step height among the 15 measured positions is between 0.05 µm and less than 0. 1 µm, and X : the case where the maximum step height among the 15 measured positions is more than 0.1 µm. The evaluated results revealed that the surface step state of each of Sample Nos. 1 and 2 is o ○, the surface step state of each of Sample Nos. 3 and 4 is ○, and the surface step state of Sample No. 5 is X. As for each of the polycrystalline silicon base member samples obtained in the above, on the surface of the heat accumulating layer, there were formed a plurality of heat generating resistor each comprising HfB 2 (size : 20 µm x 100 µm, thickness : 0.16 µm, wiring density : 16 Pel (that is, 16/mm)) and a plurality of Al electrodes (width : 20 µm, thickness : 0.6 µm) each being connected to one of the heat generating resistors using the photolithography technique. Then, a protective layer comprising SiO 2 /Ta was formed above each portion where the heat generating resistor and electrode were formed by means of a conventional sputtering technique. Thus, there was obtained five substrates for liquid jet recording head each being of the configuration shown in FIGs. 1(A) and 1(B). In the above, Sample No. 1 was found to be accompanied by a deformation which was caused at the time of the thermally softening treatment wherein an excessively high softening temperature was employed. And in the process of preparing a substrate for liquid jet recording head using this sample, a crack was occurred at the base member, and because of this, a substrate for liquid jet recording head could not be prepared. As for each of the resultant four substrates for liquid jet recording head of Sample Nos. 2 to 5, a plurality of liquid pathways and a liquid chamber were formed using a dry film, followed by cutting with the use of a slicer to form a plurality of discharging outlets, whereby a liquid jet recording head of the configuration shown FIGs. 5(A) and 5(B) was obtained. As for each of the resultant four liquid jet recording heads, the discharging durability test was conducted by repeatedly applying 1.1 Vth (Vth : discharging threshold voltage) and a driving pulse (a printing signal) with a pulse width of 10 µs to each of the heat generating resistors to thereby discharge ink from each of the discharging outlets. The evaluation of the durability of each of the liquid jet recording heads was conducted by obtaining a survival rate of the heat generating resistors, specifically, the number of the heat generating resistors not disconnected versus the total number of the heat generating resistors, when the integrated value of the driving pulses became each of 1 x 10 7 , 1 x 10 8 and 3 x 10 8 . The evaluated results are shown in each of the columns of Sample Nos. 2 to 4 of Table 5-1. From the evaluated results, it is understood that in the case of each of the four recording heads based on Sample Nos. 2 to 4, no cavitation disconnection is occurred and the survival rate is 100 % even after 3 x 10 8 times repetition of the driving pulse, but in the case of the recording head based on Sample No. 5, a cavitation disconnection is occurred at an early stage, and the survival rate is markedly low. Based on these facts, it was recognized that by forming a thermal oxide layer on the surface of a polycrystalline silicon base member by thermally oxidizing the surface of the polycrystalline silicon base member and subjecting the thermal oxide layer to thermally softening treatment at a temperature in the range of 1230 °C to 1330 °C, there can be formed a desirable heat accumulating layer with a desirable surface wherein steps are smoothed in a desirable state, and there can be obtained a desirable liquid jet recording head which provides superior results in the discharging durability test. Experiment E-2 In this experiment, studies were made of the effects of a polycrystalline silicon base member having a thermal oxide layer (a heat accumulating layer) formed by concurrently conducting the foregoing thermal oxidation treatment and thermally softening treatment. Following the manner employed in Experiment E-1, there were obtained five polycrystalline silicon base member samples (Sample Nos. 6 to 10) each being of 300 x 150 x 1.1 (mm) in size and having a mirror-ground surface with a surface roughness of Rmax 150 Å. On the surface of each of the polycrystalline silicon base members, using the same apparatus used in Experiment E-1, there was formed a thermal oxide layer by concurrently conducting the thermally oxidation treatment and thermally softening treatment for the surface of the polycrystalline silicon base member. Particularly, each of the five polycrystalline silicon base member samples was introduced into a thermal oxidation furnace, oxygen gas was supplied therein by way of the pyrogenic technique, and the inside of the thermal oxidation furnace was maintained a given temperature, whereby the surface of the polycrystalline silicon base member sample was thermally oxidized and thermally softened at the same time, resulting in forming a heat accumulating layer (a thermal oxide layer, that is, a SiO 2 layer) on the polycrystalline silicon base member sample. The inside of the thermal oxidation furnace was maintained at a different temperature of 1380 °C, 1330 °C, 1280 °C, 1230 °C or 1180 °C in each case. In order to make the thickness of the heat accumulating layer (the thermal oxide layer or the SiO 2 layer) to be 3 µm in each case, the heat treatment period was made to be 5 hours, 7 hours, 8 hours, 11 hours, or 14 hours. Thus, there were obtained five polycrystalline silicon base member samples based on Sample Nos. 6 to 10. As a result of concurrently having conducted the above thermal oxidation treatment and thermally softening treatment, each of the polycrystalline silicon base members became to have a heat accumulating layer comprising the thermal oxide layer (that is, the SiO 2 layer) thereon. The thickness of the heat accumulating layer (that is, the SiO 2 layer) in each case was found to be 3.0 µm. As for the heat accumulating layer of each of the polycrystalline silicon base member samples, evaluation was made of its surface step state while measuring it by means of the surface profiler by stylus in the same manner, as in Experiment E-1. The evaluated results revealed that the surface step state of each of Sample Nos. 6 and 7 is o ○, the surface step state of each of Sample Nos. 8 and 9 is ○, and the surface step state of Sample No. 10 is X. As for each of the polycrystalline silicon base member samples obtained in the above, on the surface of the heat accumulating layer, there were formed a plurality of heat generating resistor each comprising HfB 2 (size : 20 µm x 100 µm, thickness : 0.16 µm, wiring density : 16 Pel (that is, 16/mm)) and a plurality of Al electrodes (width : 20 µm, thickness : 0.6 µm) each being connected to one of the heat generating resistors using the photolithography technique. Then, a protective layer comprising SiO 2 /Ta was formed above each portion where the heat generating resistor and electrode were formed by means of a conventional sputtering technique. Thus, there was obtained five substrates for liquid jet recording head each being of the configuration shown in FIGs. 1(A) and 1(B). In the above, Sample No. 6 was found to be accompanied by a deformation which was caused at the time of the thermally softening treatment wherein an excessively high softening temperature was employed. And in the process of preparing a substrate for liquid jet recording head using this sample, a crack was occurred at the base member, and because of this, no practically acceptable substrate for liquid jet recording head could be obtained. As for each of the resultant four substrates for liquid jet recording head of Sample Nos. 7 to 10, a plurality of liquid pathways and a liquid chamber were formed using a dry film, followed by cutting with the use of a slicer to form a plurality of discharging outlets, whereby a liquid jet recording head of the configuration shown FIGs. 5(A) and 5(B) was obtained. As for each of the resultant four liquid jet recording heads, the discharging durability test was conducted by repeatedly applying 1.1 Vth (Vth : discharging threshold voltage) and a driving pulse (a printing signal) with a pulse width of 10 us to each of the heat generating resistors to thereby discharge ink from each of the discharging outlets. The evaluation of the durability of each of the liquid jet recording heads was conducted by obtaining a survival rate of the heat generating resistors, specifically, the number of the heat generating resistors not disconnected versus the total number of the heat generating resistors, when the integrated value of the driving pulses became each of 1 x 10 7 , 1 x 10 8 and 3 x 10 8 . The evaluated results are shown in each of the columns of Sample Nos. 7 to 10 of Table 5-2. From the evaluated results, it is understood that in the case of each of the four recording heads based on Sample Nos. 7 to 9, no cavitation disconnection is occurred and the survival rate is 100 % even after 3 x 10 8 times repetition of the driving pulse, but in the case of the recording head based on Sample No. 10, a cavitation disconnection is occurred at an early stage, and the survival rate is markedly low. Based on these facts, it was recognized that by forming a thermal oxide layer on the surface of a polycrystalline silicon base member by concurrently conducting the thermally oxidation treatment and thermally softening treatment for the surface of the polycrystalline silicon base member at a temperature in the range of 1230 °C to 1330 °C, there can be formed a desirable heat accumulating layer with a desirable surface wherein steps are smoothed in a desirable state, and there can be obtained a desirable liquid jet recording head which provides superior results in the discharging durability test. Experiment E-3 In this experiment, studies were made of the effects of a polycrystalline silicon base member having a thermal oxide layer (a heat accumulating layer) formed in the same manner as in Experiment E-1 wherein the surface of a polycrystalline silicon base member is thermally oxidized to form a thermal oxide layer and the thermal oxide layer is then thermally softened, except that the thermal oxide layer formed by way of the thermal oxidation is doped with an impurity and the impurity-doped thermal oxide layer is subjected to the thermally softening treatment. Following the manner employed in Experiment B, there were obtained fifteen polycrystalline silicon base member samples (Sample Nos. 11 to 25) each being of 300 x 150 x 1.1 (mm) in size and having a mirror-ground surface with a surface roughness of Rmax 150 Å. On the surface of each of the polycrystalline silicon base members, there was formed a thermal oxide layer by thermally oxidizing the surface of the polycrystalline silicon base member sample in the same manner as in Experiment E-1. The resultant thermal oxide layer was doped with an impurity in the following manner. That is, the impurity-doping for the thermal oxide layer (the SiO 2 layer) was conducted using a conventional CVD technique. As the dopant-imparting source, there was used POCl 3 as a liquid source, and N 2 gas as a carrier gas was introduced in a reaction chamber containing said liquid source to generate a gaseous atmosphere in a saturated state where the polycrystalline silicon base member sample having the thermal oxide layer thereon was placed. The period of time for diffusing the dopant into the sample was made to be 30 minutes in each case. The dopant-diffusing temperature was made to be 1050 °C as for each of the samples of Sample Nos. 11 to 15, 1000 °C as for each of the samples of Sample Nos. 16 to 20, and 950 °C as for each of the samples of Sample Nos. 21 to 25. As for each of the resultants, the phosphorous content at the surface was measured by means of a secondary ion mass spectrometer (trademark name : IMS-3F, produced by CAMECA Company)(hereinafter referred to as SIMS). The measured results revealed that the phosphorous content at the surface is 5 x 10 21 atoms/cm 3 as for each of the samples of Sample Nos. 11 to 15 for which the dopant diffusion was conducted at 1050 °C; 1 x 10 21 atoms/cm 3 as for each of the samples of Sample Nos. 16 to 20 for which the dopant diffusion was conducted at 1000 °C; and 1 x 10 20 atoms/cm 3 as for each of the samples of Sample Nos. 21 to 25 for which the dopant diffusion was conducted at 950 °C. As for each of the fifteen resultants each having the thermal oxide layer doped with the impurity, the thermal oxide later thereof was thermally softened in the same manner as in Experiment E-1. The thermally softening treatment in each case was conducted for a fixed period of time of an hour at a different temperature of 1230 °C, 1230 °C, 1180 °C, 1130 °C or 1080 °C. The softening temperature employed in each case is shown in Table 5-3. Thus, there were obtained fifteen polycrystalline silicon base member samples based on Sample Nos. 11 to 25. As a result of having conducted the above treatments, each of the polycrystalline silicon base member samples became to have a heat accumulating layer comprising the thermal oxide layer (that is, the SiO 2 layer) thereon. The thickness of the heat accumulating layer (that is, the SiO 2 layer) in each case was found to be 3.0 µm. As for the heat accumulating layer of each of the polycrystalline silicon base member samples, evaluation was made of its surface step state while measuring it by means of the surface profiler by stylus in the same manner as in Experiment E-1. The evaluated results revealed that the surface step state of each of Sample Nos. 19 and 23 is ○, the surface step state of each of Sample Nos. 20, 24 and 25 is X, and the surface step state of each of the remaining samples is o ○. As for each of the polycrystalline silicon base member samples obtained in the above, on the surface of the heat accumulating layer, there were formed a plurality of heat generating resistor each comprising HfB 2 (size : 20 µm x 100 µm, thickness : 0.16 µm, wiring density : 16 Pel (that is, 16/mm)) and a plurality of Al electrodes (width : 20 µm, thickness : 0.6 µm) each being connected to one of the heat generating resistors using the photolithography technique. Then, a protective layer comprising SiO 2 /Ta was formed above each portion where the heat generating resistor and electrode were formed by means of a conventional sputtering technique. Thus, there was obtained fifteen substrates for liquid jet recording head each being of the configuration shown in FIGs. 1(A) and 1(B). As for each of the resultant substrates for liquid jet recording head based on Sample Nos. 11 to 25, a plurality of liquid pathways and a liquid chamber were formed using a dry film, followed by cutting with the use of a slicer to form a plurality of discharging outlets, whereby a liquid jet recording head of the configuration shown FIGs. 5(A) and 5(B) was obtained. As for each of the resultant fifteen liquid jet recording heads, the discharging durability test was conducted by repeatedly applying 1.1 Vth (Vth : discharging threshold voltage) and a driving pulse (a printing signal) with a pulse width of 10 us to each of the heat generating resistors to thereby discharge ink from each of the discharging outlets. The evaluation of the durability of each of the liquid jet recording heads was conducted by obtaining a survival rate of the heat generating resistors, specifically, the number of the heat generating resistors not disconnected versus the total number of the heat generating resistors, when the integrated value of the driving pulses became each of 1 x 10 7 , 1 x 10 8 and 3 x 10 8 . The evaluated results are shown in each of the columns of Sample Nos. 11 to 25 of Table 5-3. As for each of the liquid jet recording heads based on Sample Nos. 11 to 15, numerous disconnections were occurred at the heat generating resistors even at the stage wherein the integrated value of the driving pulses was small. As a result of observing such disconnected portions using a scanning electron microscope, it was found that peelings are present between the thermal oxide SiO 2 layer and the heat generating resistors. In the case of each of the liquid jet recording heads based on Sample Nos. 11 to 15, it is considered that the temperature for the heat accumulating layer (the thermal oxide SiO 2 layer), on which the heat generating resistors are to be disposed, upon conducting the thermally softening treatment was lowered probably due to the high dopant content at the surface and a deformation was occurred at the base member when the heat generating resistors were energized to 1100 °C in terms of maximum temperature. From the evaluated results as ford each of the recording heads based on Sample Nos. 16 to 25, it is understood that in the case of each of the recording heads based on Sample Nos. 20, 24 and 25, a cavitation disconnection is occurred at an early stage, and the survival rate is markedly low, but in the case of each of the remaining recording heads based on Sample Nos. 16-19, and 21-23, no cavitation disconnection is occurred and the survival rate is 100 % even after 3 x 10 8 times repetition of the driving pulse. Based on these facts, it was recognized that pronounced advantages are provided in the case where the thermal oxide layer formed on the polycrystalline silicon member is doped with an impurity in such an amount that the softening temperature thereof is lowered to a relatively low temperature of 1130 °C or above, such that effective elimination of the problems relating to occurrence of surface steps can be attained at a temperature which is lower by more than 100 °C in comparison with the case where no impurity doping is conducted, the operation temperature of the treatment furnace used can be relatively lowered wherein the lifetime of the treatment furnace is eventually extended, and as a result, a product can be provided at a reduced cost. It was also found that the maximum temperature for conducting the thermally softening treatment is desired to be lower than 1330 °C wherein negative influences are not occurred due to a deformation at the polycrystalline silicon base member, as well as in the case of each of Experiment E-1 and Experiment E-2. There was obtained a further finding that in the case where the same conditions relating to the temperature and treating period of time for the thermally softening treatment employed in the case where no impurity-doping treatment is conducted are employed, the surface softening of the thermal oxide later is facilitated to provide a more desirable step-free surface state for the thermal oxide layer. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(A) is a schematic plan view illustrating the principal part of an example of a substrate for liquid jet recording head according to the present invention. FIG. 1(B) is a schematic cross-sectional view, taken along the X-Y line in FIG. 1(A). FIG. 2 is a schematic cross-sectional view illustrating an example of a base member for a substrate for liquid jet recording head according to the present invention. FIG. 3 is a schematic cross-sectional view for explaining the manufacturing process of producing a liquid jet recording head in the present invention. FIGs. 4(A) through 4(D) are schematic explanatory view for the steps of forming a thermal oxide layer on the surface of a polycrystalline silicon base member in the present invention. FIG. 5(A) is a schematic cross-eyed view illustrating the principal part of an example of a liquid jet recording head. FIG. 5(B) is a schematic cross-sectional view, taken along the liquid pathway and at the face perpendicular to the substrate of the above recording head. FIG. 6 is a schematic view illustrating an embodiment of a recording apparatus provided with a liquid jet recording head according to the present invention. FIG. 7 is a schematic explanatory view of an example of a thermal oxidation apparatus used for thermally oxidizing the surface of a base member for a substrate for liquid jet recording head in the present invention. FIGs. 8(A) and 8(B) are schematic views for explaining the mechanism of causing a bowed portion at a base member. FIGs. 9(A) through 9(C) are schematic views for explaining the situation of causing a bowed portion at the time of cutting a base member. FIG. 9(D) is a schematic explanatory view of the manner of measuring the magnitude of a bowed portion occurred at a base member. DESCRIPTION OF THE PREFERRED EMBODIMENTS The principal feature of the present invention lies in a substrate for liquid jet recording head. The substrate is characterized by comprising a polycrystalline silicon base member provided with a heat accumulating layer (a thermal oxide layer) with a smoothly flat surface. The heat accumulating layer is formed by subjecting the surface of a polycrystalline silicon member to thermal oxidation treatment and thermally softening treatment. The surface of the polycrystalline silicon member is not flat due to its constituent crystal grains as described in the foregoing experiments, and because of this, a thermal oxide layer formed thereon unavoidable becomes to have a surface accompanied by surface steps. The present invention has been accomplished based on the findings obtained especially in the foregoing Experiment E which was conducted by the present inventor in order to eliminate the problems relating to such surface step. The polycrystalline silicon base member having a heat accumulating layer with a smoothly flat surface according to the present invention can be realized by providing a base member comprising a polycrystalline silicon material and subjecting the surface of the base member to thermal oxidation treatment and thermally softening treatment to thereby form a heat accumulating layer with a smoothly flat surface. In the present invention, since the polycrystalline silicon base member having such a heat accumulating layer is used as a constituent of the substrate for liquid jet recording head, if an internal stress should be occurred in the substrate due to an uneven shrinkage caused upon repetition of heating and cooling, no problematic deformation is occurred at the substrate. The above thermally softening treatment can be conducted after a thermal oxide layer has been formed by thermally oxidizing the surface of a polycrystalline silicon base member or it can be conducted concurrently together with the thermal oxidation treatment. In the case where the thermal oxidation treatment and thermally softening treatment are concurrently conducted, the period of time required for the formation of the heat accumulating layer on the polycrystalline silicon base member is markedly shortened in comparison with that in the case of forming the heat accumulating layer by individually conducting the thermal oxidation treatment and thermally softening treatment. In the case where the thermally softening treatment is independently conducted, it can be conducted by way of lamp heating using halogen lamp or xenon lamp, or by way of continuous wave heating or pulse wave heating with laser of CO 2 , YAG or Ar, or by way of continuous wave heating or pulse wave heating with electron beam, or by way of high frequency heating. In this case, it is possible for the thermally softening treatment to be carried out only for given portions of the surface of the polycrystalline silicon base member, for instance, only for the surface portions on which heat generating resistors are to be disposed. It is important that the thermally softening treatment is conducted at a temperature which is lower than the fusing point of a polycrystalline silicon material used as the base member. Specifically, the temperature at which the thermally softening treatment is conducted is desired to be in the range of 1230 °C to 1330 °C, as described in the foregoing Experiment E. In the present invention, in the case where the thermal oxidation treatment and the thermally softening treatment are individually conducted, the object of the present can be desirably attained by doping the thermal oxide layer formed by way of the thermal oxidation treatment with an appropriate impurity and subjecting the resultant to the thermally softening treatment. The thermally softening treatment in this case can be conducted at a temperature which is lower than that in the case where thermally softening treatment is conducted without conducting the impurity-doping treatment. As the above impurity to be incorporated into the thermal oxide layer formed by way of the thermal oxidation treatment, any of the conventional elements which are generally used in the field of semiconductor such as P, B, As or the like can be selectively used. The incorporation of such impurity into the thermal oxide layer may be carried out by a conventional impurity-introducing technique generally employed in the field of semiconductor. The concentration of the impurity to be incorporated into the thermal oxide later is somewhat different depending upon the kind of the impurity used. In general, it is should be properly decided with a due care about its upper limit so that the heat accumulating layer (the thermal oxide layer) is not softened at a temperature to which the heat generating resistors disposed thereon are energized and also with a due care about its lower limit so that the heat accumulating layer can be softened in a desirable state to provide a smoothly flat surface therefor. The thermally softening treatment in the present invention is conducted principally aiming at eliminating surface steps occurred at the surface of the thermal oxide layer and providing a smooth surface state for the thermal oxide layer. The heat generating resistors disposed on the smoothly flat surface of the thermal oxide layer provided as a result of the thermally softening treatment are ensured in terms of close contact with the thermal oxide layer. The present invention includes a substrate for liquid jet recording head in which the foregoing polycrystalline silicon-based base member is used, a liquid jet recording head provided with said substrate for liquid jet recording head, a liquid jet recording apparatus provided with said recording head, and a process for producing said substrate for liquid jet recording head. The substrate for liquid jet recording head to be provided according to the present invention comprises a polycrystalline silicon-based base member and an electrothermal converting body disposed on said base member, said electrothermal converting body comprising a heat generating resistor capable of generating thermal energy and a pair of wirings electrically connected to said heat generating resistor, characterized in that said base member has, on its surface, a thermal oxide layer which is formed by subjecting the surface of said base member to thermal oxidation treatment and thermally softening treatment. The liquid jet recording head to be provided according to the present invention includes a liquid discharging outlet; a substrate for liquid jet recording head including an electrothermal converting body comprising a heat generating resistor capable of generating thermal energy for discharging liquid from said discharging outlet and a pair of wirings electrically connected to said heat generating resistor, said pair of wirings being capable of supplying an electric signal for generating said thermal energy to said heat generating resistor; and a liquid supplying pathway disposed in the vicinity of said electrothermal converting body of said substrate, characterized in that said substrate includes a polycrystalline silicon-based base memer having, on the surface of said base bember, a thermal oxide layer formed by subjecting the surface of said base member to thermal oxidation treatment and thermally softening treatment. The liquid jet recording apparatus to be provided according to the present invention includes (a) a substrate for liquid jet recording head including a liquid discharging outlet, an electrothermal converting body comprising a heat generating resistor capable of generating thermal energy for discharging liquid from said discharging outlet and a pair of wirings electrically connected to said heat generating resistor, said pair of wirings being capable of supplying an electric signal for generating said thermal energy to said heat generating resistor, and (b) a liquid supplying pathway disposed in the vicinity of said electrothermal converting body of said substrate, characterized in that said substrate (a) includes a polycrystalline silicon-based base memer having, on the surface of said base bember, a thermal oxide layer formed by subjecting the surface of said base member to thermal oxidation treatment and thermally softening treatment. The process to be provided according to the present invention is for producing a substrate for liquid jet recording head wherein an electrothermal converting body is disposed on a base member, said electrothermal converting body comprising a heat generating resistor and a pair of wirings electrically connected to said heat generating resistor, which is characterized by including the steps of using a member composed of a polycrystalline silicon material as said base member and subjecting said polycrystalline silicon member to thermal oxidation treatment for forming a thermal oxide layer on the surface of said polycrystalline silicon member and to thermally softening treatment for softening the surface of said thermal oxide layer to provide a smoothly flat surface state for said thermal oxide layer, whereby forming a heat accumulating layer with a smoothly flat surface on said polycrystalline silicon member. A typical example of the base member constituting the substrate for liquid jet recording head in the present invention, there can be mentioned a base member composed of a polycrystalline silicon material (this will be hereinafter referred to as a polycrystalline silicon base member). The polycrystalline silicon base member is rather difficult to be deformed in comparison with a single crystal silicon base member. Because of this, as described in the foregoing experiments, the polycrystalline silicon base member provides a prominent effect in that the elongation of a recording head, which is hardly attained in the case of using a single crystal silicon base member, can be effectively attained. In the present invention, the use of the polycrystalline silicon base member in the substrate for liquid jet recording head provides further advantages such that the substrate can be lengthened to a desired length wherein, as described in the foregoing Experiment B, the warp magnitude is slight and is smaller than that of the single crystal silicon base member, and therefore, an elongated liquid jet recording head which is free of the problems relating to occurrence of warpage can be easily and effectively obtained. The elongated recording head is free of the problems relating to occurrence of defects for an image recorded which are caused in the case of an elongated liquid jet recording head obtained by integrating a plurality of miniature recording heads. Further, the elongated liquid jet recording head according to the present invention can attain a desirable recording apparatus capable of performing high speed recording. The warp magnitude is, as described in the foregoing Experiment C, proportional to the mean crystal grain size of the polycrystalline silicon material constituting the base member. In order to attain a desirable yield in the production of the liquid jet recording head according to the present invention, the polycrystalline silicon material constituting the base member for the substrate for liquid jet recording head is desired to be preferably of 8 um or less, more preferably of 2 um or less in terms of mean crystal grain size. To use a polycrystalline silicon base member having a mean crystal grain size in said range enables to obtain a desirable substrate for liquid jet recording head which is free of occurrence of warpage, and as a result, an elongated liquid jet recording head capable of providing a high quality recorded image at a high recording speed can be easily and effectively attained. On the polycrystalline silicon base member for the substrate for liquid jet recording head, a heat generating resistor layer and wirings are disposed. Therefore, the polycrystalline silicon base member is desired not to have defects such as pits, protrusions, or the like at the surface thereof. In the case where these defects are present at the surface of the base member, such defect is liable to lead to causing a disconnection or shortcircuit for the heat generating resistor layer formed thereon. As described in the foregoing Experiment D, in order to attain a high production yield and in order to attain desirable recording characteristics as for the liquid jet recording head, the polycrystalline silicon base member used for the substrate for liquid jet recording head is desired to be such that the number of such defects of about 1 um in diameter present at the surface thereof is preferably 10/cm 2 or less, more preferably 5/cm 2 or less. In the following, description will be made of an embodiment of the substrate for liquid jet recording head according to the present invention. FIG. 1(A) is a schematic plan view illustrating the principal part of an example of the substrate for liquid jet recording head according to the present invention. FIG. 1(B) is a schematic cross-sectional view, taken along the line X-X' in FIG. 1(A). FIG. 2 is a schematic cross-sectional view illustrating a base member constituting said substrate for liquid jet recording head. A substrate 8 for liquid jet recording head has, on a polycrystalline silicon base member 1, a electrothermal converting body comprising a heat generating resistor 2a capable of generating thermal energy for discharging a liquid recording medium and a pair of wirings 3a and 3b electrically connected to said heat generating resistor 2a. After having laminated a heat generating resistor 2 comprising a material with a relatively large volume resistivity and an electrode layer 3 comprising a material having a good electroconductivity on the polycrystalline silicon base member 1, for example, by a conventional sputtering technique, the heat generating resistor 2a and the wirings 3a and 3b are formed respectively in a given pattern by way of the photolithography process. The heat generating resistor thus formed serves to energize upon applying an electric signal to the heat generating resistor through the wirings 3a and 3b. The material constituting the heat generating resistor layer 2 can include hafnium boride (HfB 2 ), tantalum nitride (Ta 2 N), rubidium oxide (RuO 2 ), Ta-Al alloy, and Ta-Al-Ir alloy, other than these, various metals, alloys, metal compounds, and cermets. The material constituting the electrode layer 3 can include metals having a high electroconductivity such as aluminum, gold and the like. The substrate for liquid jet recording head 8 includes a protective layer 4 which is disposed so as to cover the wirings 3a and 3b and the heat generating resistor 2a. The protective layer 4 is disposed for the purpose of preventing the heat generating resistor 2a and the wirings 3a and 3b from suffering not only from electric corrosion but also from electric breakdown which will be occurred when they are contacted with ink or when ink is permeated thereinto. The protective layer may be formed of an electrically insulative material such as SiO 2 , SiC, Si 3 N 4 , or the like. The protective layer may be of a multilayered structure. In this case, the protective layer may take a stacked structure, for example, comprising a layer formed of said electrically insulative material and a layer formed of Ta or Ta 2 O 5 being stacked on the former layer. The above embodiment of the liquid jet recording head is of the configuration wherein the direction in which a liquid recording medium is discharged from the discharging outlet and the direction in which a liquid recording medium is supplied toward the heat generating resistor are substantially the same, but it can take another configuration wherein the two directions are different from each other (for instance, they are substantially perpendicular to each other). In the following, description will be made of an embodiment of a liquid jet recording head in which the above described substrate is used. The principal configuration of the recording head previously has been explained with reference to FIG. 5(A) and FIG. 5(B). Herein, description again will be made. A liquid pathway 6 for supplying ink is formed in the vicinity of each heat generating resistor 2a by connecting a top plate 5 to the substrate. The ink in the liquid pathway is heated by the heat generating resistor to cause a bubble, wherein the ink is discharged through a discharging outlet 7 by virtue of a pressure caused upon forming the bubble, whereby performing recording. In the configuration shown in FIG. 5(A) and FIG. 5(B), there is shown an arrangement in which one heat generating resistor corresponds to one discharging outlet. However, the recording head of the present invention is not limited to this configuration only. That is, any other configurations including, for instance, a configuration in which a plurality of heat generating resistors correspond to one discharging outlet, can be employed as long as the foregoing substrate can be applied. Further, in the configuration shown in FIG. 5(A) and FIG. 5(B), the substrate surface on which the heat generating resistors are arranged is substantially in parallel to the direction in which the ink is discharged. The recording head of the present invention is not limited to this configuration only, but may take such a configuration that the direction in which the ink is discharged is in a relationship of crossing with the substrate surface. The liquid jet recording head of the present invention may be designed such that it can be mounted in an apparatus capable of being a recording apparatus, for instance, in a detachable state, wherein ink is supplied from a separate ink container through a tube. Other than this, it may be designed such that it can be detachably amounted in an apparatus capable of being a recording apparatus while being detachably connected to a separate ink container. As the liquid recording medium usable in the recording head of the present invention, there can be used various kinds. Examples of such liquid recording medium are liquid recording mediums having an ink composition comprising 0.5 to 20 wt.% of dye, 10 to 80 wt.% of water-soluble organic solvent such as polyhydric alcohol, polyalkylene glycol, or the like, and 10 to 90 wt.% of water. As a specific example of such ink composition, there can be mentioned one comprising 2.3 wt.% of C.I. food black, 25 wt.% of diethylene glycol, 20 wt.% of N-methyl-2-pyrrolidone, and 52 wt.% of water. FIG. 6 is an appearance perspective view illustrating an example of an ink jet recording apparatus IJRA in which the recording head of the present invention is used as an ink jet head cartridge IJC. In FIG. 6, reference numeral 120 indicates the ink jet head cartridge IJC provided with nozzle groups capable of discharging ink to the face of a recording member transported onto a platen 124. Reference numeral 116 indicates a carriage HC which serves to hold the IJC 120. The carriage HC is connected to a part of a driving belt 118 capable of transmitting a driving force such that it can be slidably moved together with two guide shafts 119A and 119B arranged in parallel with each other. By this, the IJC 120 is allowed to move back and forth along the entire of the recording member. Herein, although the ink jet head cartridge as the recording head comprises a miniature recording head, it is a matter of course that the elongated recording head of the present invention, which is designed, for example, to be of a so-cally full line type capable of performing recording for a given recording width of a recording member used, can be used. In the case of using such elongated recording head, there can be attained a recording apparatus in which the foregoing advantages of the elongated recording head, namely, an advantage of being free of warpage, an advantage of being free of the problems of causing defects for an image recorded which are found in the case of using a relatively short recording head, and an advantage of making it possible to conduct high speed recording, are fully effectively used. Reference numeral 126 indicates a head restoring device which is disposed at one end of the moving passage of the IJC 120, specifically at the position opposite the home position. The head restoring device 120 is operated by virtue of a driving force transmitted through a driving mechanism 123 from a motor 122, whereby capping the IJC 120. In relation to the capping for the IJC 120 by a cap member 126A of the head restoring device, the discharge restoration treatment of removing adhesive ink in the nozzles is conducted by way of ink sucking by means of an appropriate sucking means disposed in the head restoring device 126 or by way of ink pressure transportation by means of an appropriate pressurizing means whereby forcibly discharging the ink through the discharging outlets. When the recording is terminated, the IJC is protected by capping it. Reference numeral 130 indicates a cleaning blade comprising a wiping member formed of a silicon rubber which is arranged at a side face of the head restoring device 126. The cleaning blade 130 is supported by a blade supporting member 130A in a cantilever-like state. As well as in the case of the head restoring device 126, the cleaning blade 130 is operated by virtue of a driving force transmitted through the driving mechanism 123 from the motor 122, wherein the cleaning blade is made capable of contacting with the discharging face of the IJC 120. By this, the cleaning blade 130 is projected into the moving passage of the IJC 120 timely with the recording performance of the IJC 120 or after the discharge restoration treatment using the head restoring device having been completed to thereby remove dew drops, wettings, dirts, and the like deposited on the discharging face of the IJC 120. The recording apparatus is also provided with an electric signal applying means for applying an electric signal to the recording head. Further, the recording apparatus includes, other than the above embodiment of conducting recording to a recording member, an embodiment comprising a textile printing apparatus of recording patterns to a fabric or the like. In the case of the textile printing apparatus, it is necessary to conduct recording to a fabric with an extremely wide width, wherein the elongated recording head of the present invention is very effective. Other Embodiments The present invention provides prominent effects in an ink jet recording head and ink jet recording apparatus of the system in which ink is discharged utilizing thermal energy. As for the representative constitution and the principle, it is desired to adopt such fundamental principle as disclosed, for example, in U.S. Patent No. 4,723,129 or U.S. patent No. 4,740,796. While this system is capable of applying either the so-called on-demand type or the continuous type, it is particularly effective in the case of the on-demand type because, by applying at least one driving signal for providing a rapid temperature rise exceeding nucleate boiling in response to recording information to an electrothermal converting body disposed for a sheet on which liquid (ink) is to be held or for a liquid pathway, the electrothermal converting body generates thermal energy to cause film boiling on a heat acting face of the recording head and as a result, a gas bubble can be formed in the liquid (ink) in a one-by-one corresponding relationship to such driving signal. By way of growth and contraction of this gas bubble, the liquid (ink) is discharged trough a discharging outlet to form at least one droplet. It is more desirable to make the driving signal to be of a pulse shape, since in this case, growth and contraction of a gas bubble take place instantly and because of this, there can be attained discharging of the liquid (ink) excelling particularly in responsibility. As the driving signal of pulse shape, such driving signal as disclosed in U.S. Patent No. 4,463,359 or U.S. Patent No. 4,345,262 is suitable. Additionally, in the case where those conditions disclosed in U.S. Patent NO. 4,313, 124, which relates to the invention concerning the rate of temperature rise at the heat acting face, are adopted, further improved recording can be performed. As for the constitution of the recording head, the present invention incudes, other than those constitutions of the discharging outlets, liquid pathways and electrothermal converting bodies in combination (linear liquid flow pathway or perpendicular liquid flow pathway) which are disclosed in each of the above mentioned patent documents, the constitutions using such constitution in which a heat acting portion is disposed in a curved region as disclosed in U.S. Patent No. 4,558,333 or U.S. Patent No. 4,459,600. In addition, the present invention may effectively take a constitution based on the constitution in which a slit common to a plurality of electrothermal converting bodies is used as a discharging portion of the electrothermal converting bodies which is disclosed in Japanese Unexamined Patent Publication No. 123670/1984 or another constitution based on the constitution in which an opening for absorbing a pressure wave of thermal energy is made to be corresponding to a discharging portion which is disclosed in Japanese Unexamined Patent Publication No. 138461/1984. Further, in the case of an ink jet recording apparatus comprising a full-line type recording head having a length corresponding to the width of a maximum recording member onto which recording can be performed, the foregoing effects are more effectively provided. The present invention is effective also in the case where a recording head of the exchangeable chip type wherein electric connection to an apparatus body or supply of ink from the apparatus body is enabled when it is mounted on the apparatus body or other recording head of the cartridge type wherein an ink tank is integrally disposed on the recording head itself is employed. Furthermore, the present invention is extremely effective not only in a recording apparatus which has, as the recording mode, a recording mode of a main color such as black but also in a recording apparatus which includes a plurality of different colors or at least one of full-colors by color mixture, in which a recording head is integrally constituted or a plurality of recording heads are combined. In the above-described embodiments of the present invention, explanation has been made with the use of liquid ink, but it is possible to use such ink that is in a solid state at room temperature or other ink which becomes to be in a softened state at room temperature in the present invention. In the foregoing ink jet apparatus, it is usual to adjust the temperature of ink itself in the range of 30 °C to 70 °C such that the viscosity of ink lies in the range capable of being stably discharged. In view of this, any ink can be used as long as it is in a liquid state upon the application of a use record signal. It is also possible to those inks having a property of being liquefied, for the first time, with thermal energy, such as ink that can be liquefied and discharged in liquid state upon the application of thermal energy depending upon a record signal or other ink that can start its solidification beforehand at the time of its arrival at a recording member in order to prevent the temperature of the head from raising due to thermal energy purposely used as the energy for a state change of ink from solid state to liquid state or in order to prevent ink from being vaporized by solidifying the ink in a state of being allowed to stand. In the case of using these inks, they can be used in such a manner as disclosed in Japanese Unexamined Patent Publication No. 56847/1979 or Japanese Unexamined Patent Publication No. 71260/1985 in which ink is maintained in concaved portions or penetrations of a porous sheet in a liquid state or in a solid state and the porous sheet is arranged to provide a configuration opposite the electrothermal converting body. Examples In the following, the features and advantages of the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not restricted by these examples. Example 1 (preparation of a ploycrystalline silicon base member for a substrate for liquid jet recording head) A polycrystalline silicon ingot as the stating material was prepared in the following manner. That is, there was firstly provided a high purity polycrystalline silicon material obtained in accordance with the conventional precipitation reaction manner through hydrogen reduction and pyrolysis, which is usually employed in the production of a single crystal silicon material. The polycrystalline silicon material was then introduced into a quartz crucible wherein it was fused at 1420 °C. The resultant fused material was poured into a casting mold made of graphite wherein it was cooled, to thereby obtain a polycrystalline silicon ingot of 80 cm in square size. In this case, no release agent was used. The ingot thus obtained was quarried at the position thereof with a mean crystal grain size of 2 mm by means of a milti-wire saw, to obtain twelve plate samples each having a different size shown in one of the columns Sample No. 1 to Sample No. 12 of Table 6. Each of the twelve plate samples was subjected to lapping treatment to remove an about 30 µm thick surface portion to thereby provide a flat surface therefor. The end portions of the resultant were chanferred by means of a beveling machine, followed by subjecting to polishing treatment using a single side polishing machine produced by Speedfarm Kabushiki Kaisha, to thereby obtain a mirror-ground member with a surface roughness of Rmax 150 Å. In this case, the polishing treatment was conducted without using an alkali, in order to prevent a surface step from being formed, which will be occurred due to that the etching by an alkali component contained in the abrasive material has a crystal orientation dependency. Thus, there were obtained twelve mirror-ground polycrystalline silicon plate samples. As for each of the resultant polycrystalline silicon plate samples, namely, the polycrystalline silicon base members, its surface state was examined by the same surface examination manner using the inspection system for substrate surface employed in the foregoing Experiment D. As a result, each of the base members was found to be of less than 1/cm 2 in terms of the number of defects based on irregularities in the maximum detectable range of more than 1 um in diameter at all the measured points. Further, each of the base member samples was examined with respect its surface flatness using a surface profiler by stylus produced by Lasertech Kabushiki Kaisha. As a result, each of the base member samples was found to be free of occurrence of a surface step. Four of the polycrystalline silicon base member samples were chosen, and as for each of them, a SiO 2 film as the heat accumulating layer was formed on the surface thereof by subjecting the surface of the sample to thermal oxidation treatment by way of the pyrogenic method. In this case, the following film-forming conditions were employed: thermal oxidation temperature : 1150 °C, inner pressure of the furnace : 1 atm., and period of the thermal oxidation treatment : 14 hours. Then, as for each of the four resultant base member samples each having the SiO 2 layer thereon, the surface of SiO 2 layer was flattened by subjecting the SiO 2 layer to thermally softening treatment. The thermally softening treatment in this case was conducted under the following conditions: thermally softening temperature : 1330 °C, inner pressure of the furnace : 1 atm., and period of the thermally softening treatment : 1 hour. In this way, there were obtained four polycrystalline silicon work in process samples (Sample No. 1 to Sample No. 4) for a substrate for liquid jet recording head, each having a 3 µm thick thermal oxide layer (a SiO 2 layer) as the heat accumulating layer. In addition, four of the remaining eight polycrystalline silicon base member samples were chosen, and as for each of them, the foregoing thermal oxidation treatment and thermally softening treatment were concurrently conducted under the following conditions, to thereby form a step-free heat accumulating layer on the surface of the base member sample. heat treatment temperature : 1150 °C, inner pressure of the furnace : 1 atm., and heat treatment period : 7 hours. In this way, there were obtained four polycrystalline silicon work in process samples (Sample No. 5 to Sample No. 8) for a substrate for liquid jet recording head, each having a 3 µm thick thermal oxide layer (a SiO 2 layer) as the heat accumulating layer. Finally, gas for each of the remaining four base member samples, a SiO 2 film as the heat accumulating layer was formed on the surface thereof by subjecting the surface of the sample to thermal oxidation treatment by way of the pyrogenic method. Successively, an impurity in gaseous state was diffused into the SiO 2 layer thus formed. The thermal oxidation treatment and the impurity diffusion were conducted under the following respective conditions: the conditions for the thermal oxidation treatment: thermal oxidation temperature : 1150 °C, inner pressure of the furnace : 1 atm., and thermal oxidation period : 14 hours. the conditions for the impurity diffusion: diffusion source : POCl 3 , diffusion manner : low pressure thermal-induced CVD process, and diffusion temperature : 1000 °C. As a result of measuring the content of P diffused at the surface of each polycrystalline silicon base member sample by the SIMS, the p-content was found to be 1 x 10 21 atoms/cm 3 in every case. Then, as for each of the four resultant base member samples each having the SiO 2 layer thereon, the surface of SiO 2 layer was flattened by subjecting the SiO 2 layer to thermally softening treatment. The thermally softening treatment in this case was conducted under the following conditions: thermally softening temperature : 1330 °C, inner pressure of the furnace : 1 atm., and period of the thermally softening treatment : 1 hour. In this way, there were obtained four polycrystalline silicon work in process samples (Sample No. 9 to Sample No. 12) for a substrate for liquid jet recording head, each having a 3 um thick thermal oxide layer (a SiO 2 layer) as the heat accumulating layer. As for the twelve samples of Sample Nos. 1 to 12 thus obtained, evaluation was made of the surface step state of the heat accumulating layer while measuring it by means of a conventional surface profiler by stylus. The condition for the measurement and the criteria for the evaluation were made as follows. The measurement conditions: the stylus scanning distance : 10 mm, the number of the positions measured : 15 positions as for each sample, and the position measured : 15 intersections of the three linear lines by which the short side of 150 mm in width is divided into four equal zones and the five linear lines by which the long side of 600 mm, 500 mm, 400 mm or 300 mm in length is divided into six equal zones as for each sample. The evaluation criteria: o ○ : the case where the maximum step height among the 15 measured positions is between 0 µm and less than 0.05 µm, ○ : the case where the maximum step height among the 15 measured positions is between 0.05 µm and less than 0.1 µm, and X : the case where the maximum step height among the 15 measured positions is more than 0.1 µm. The evaluated results revealed that all the samples of Sample Nos. 1 to 12 are o ○, and each of them has a smoothly flat surface wherein steps are desirably flattened. Then, as for each of the twelve samples of Sample Nos. 1 to 12, using the photolithography technique, there were formed, on the surface thereof, a plurality of heat generating resistors each comprising HfB 2 (size : 20 µm x 100 µm, thickness : 0.16 µm, pitch interval : 63.5 µm) and a plurality of Al electrodes (width : 20 µm, thickness : 0.6 µm) each being connected one of the heat generating resistors. Then, a protective layer comprising SiO 2 /Ta (the thickness of the SiO 2 film : 1.3 µm, the thickness of the Ta film : 0.5 µm) was formed above each portion, where the heat generating resistor and electrode were formed, by means of a conventional sputtering technique. Thus, there were obtained twelve substrates for liquid jet recording head (Sample No. 1 to Sample No. 12) each having the configuration shown in FIGs. 1(A) and 1(B). Successively, as for each of the resultant twelve substrates for liquid jet recording head, a plurality of liquid pathways were formed in accordance with the photolithography technique using a photosensitive dry film wherein exposure is conducted. Herein, in each case, evaluation was conducted by examining of whether those ink pathways could be precisely formed upon the exposure processing and obtaining an exposure fitness proportion. Particularly, as for each substrate sample, 15 patten samples for liquid jet recording head each comprising a plurality of ink pathways for ink discharging were formed, wherein each of the 15 pattern samples for each of Samples Nos. 1, 5 and 9 comprising 8576 ink pathways, each of the 15 pattern samples for each of Samples Nos. 2, 6 and 10 comprising 7244 ink pathways, each of the 15 pattern samples for each of Samples Nos. 3, 7 and 11 comprising 5504 ink pathways, and each of the 15 pattern samples for each of Samples Nos. 4, 8 and 12 comprising 4288 ink pathways. As for each of the resultant samples of Sample No. 1 to Sample No. 12, an exposure fitness proportion was obtained on the basis of the criteria in that the case where a pattern defect was occurred with regard to at least one discharging outlet pattern as a result of the focusing position having been deviated due to a warpage of the base member among the 15 pattern samples is made to be unfitness, and the case where no such pattern defect was occurred is made to be fitness. The results obtained are collectively shown in Table 6. As apparent from the results shown in Table 6, it is understood that all the resultant samples of Sample No. 1 to Sample No. 12 are of 100 % in terms of exposure fitness proportion. It is also understood that since the thermal oxide layer is thermally softened to provide a smoothly flattened surface, the heat generating resistors formed thereon excel in close contact with the thermal oxide layer in every case. Comparative Example 1 (preparation of a single crystal silicon base member for a substrate for liquid jet recording head) There was firstly provided a single crystal silicon ingot as the starting material. Using this single crystal silicon ingot and in accordance with the same manner employed in Example 1, there were obtained four mirror-ground single crystal silicon base member samples each having a different size shown in one of the columns Sample No. 1 to Sample No. 4 of Table 8 and having a surface roughness of Rmax 150 Å (comparative Sample No. 1 to Comparative Sample No. 4). In each case, the polishing treatment was conducted with the addition of alkali. As for each of the resultants, there was formed a 3.0 µm thick thermal oxide heat accumulating layer by thermally oxidizing the surface thereof by the pyrogenic method in the same manner employed in Example 1, except that the thermally softening treatment was not conducted. Thus, there were obtained four work in process samples for a substrate for liquid jet recording head (Comparative Sample No. 1 to Comparative Sample No. 4). Using each of the four resultant samples, there were obtained four comparative substrate samples for liquid jet recording head by repeating the procedures of Example 1 (Comparative Sample No. 1 to Comparative Sample No. 4). As for each of the resultant liquid jet recording head substrate samples of Comparative Sample No. 1 to Comparative Sample No. 4, an exposure fitness proportion was evaluated in the same manner employed in Example 1. The results obtained are collectively shown in Table 8. As apparent from the results shown in Table 8, it is understood that Comparative Sample No. 2 is of a reduced value in terms of exposure fitness proportion, Comparative Sample No. 1 is substantially unfitness, and each of Comparative Samples Nos. 3 and 4 each being relatively short in length is 100% in terms of fitness proportion. Example 2 (preparation of a liquid jet recording head using a polycrystalline silicon substrate) In this example, using each of the twelve liquid jet recording head substrate samples (Sample No. 1 to Sample No. 12) shown in Table 6 which were prepared by repeating the procedures of Example 1, there were prepared twelve liquid jet recording heads of the configuration shown in FIG. 3 in the following manner. As for each of the liquid jet recording head substrate samples, a plurality of ink pathways were formed thereon in accordance with the photolithography technique using a photosensitive dry film. Using a slicer, the resultant was cut into a plurality of head units while forming a plurality of discharging outlets. Then, the discharging outlet face was polished to remove defects such as chippings caused at the time of the cutting treatment. Thus, as for each of the liquid jet recording head substrate samples, there were obtained 15 liquid jet recording head works in process. As for each of the 15 works in, process obtained in each case, ICs for driving the heat generating resistors were electrically connected to the wirings in accordance with the flip chip bonding technique, to thereby obtain a liquid jet recording head with a discharging outlet pitch interval of 63.5 um. In this way, as for each of the liquid jet recording head substrate samples based on Sample No. 1 to Sample No. 12, there were obtained 15 liquid jet recording head samples (the twelve groups each comprising the 15 liquid jet recording heads based on each of Sample No. 1 to Sample No. 12 will be hereinafter referred to as Sample No. 1', to Sample No. 12', respectively). As a result of conducting evaluation of the production process yield as for each of Sample No. 1' to Sample No. 12', there were obtained the results shown in Table 7, wherein the mark ○ indicates the case wherein the production yield is within a production yield previously estimated based on the number of discharging outlets, and the mark X indicates the case wherein the production yield is inferior to the production yield previously estimated based. From these results, it was found that each of the liquid jet recording head samples of Sample No. 1' to Sample No. 12' is within a normal level in terms of defect occurrence. Then, as for each of Sample No. 1' to Sample No. 12', one liquid jet recording head was randomly chosen, and it was dedicated for discharge durability test. The durability test was conducted by repeatedly applying 1.1 Vth (Vth : discharging threshold voltage) and a driving pulse (a printing signal) with a pulse width of 10 us to each of the heat generating resistors whereby discharging ink from each of the discharging outlets. The evaluation in the durability test was conducted by obtaining a survival rate of the heat generating resistors, specifically, the number of the heat generating resistors not disconnected versus the total number of the heat generating resistors, when the integrated value of the driving pulses became each of 1 x 10 7 , 1 x 10 8 and 3 x 10 8 . The evaluated results are collectively shown in Table 7. As apparent from the results shown in Table 7, it is understood that the survival rate is 100 % even after 3 x 10 8 times repetition of the driving pulse and thus, the durability is satisfactory in every case. Successively, as for each of Sample No. 1' to Sample No. 12', another one liquid jet recording head was randomly chosen, and it was dedicated for evaluation of a printing performance, wherein a precision between the printed dots and appearance of uneven density were evaluated. There was used ink of the following composition: dye : C.I. direct black 19 -- 3 wt.%, diethylene glycol -- 25 wt.%, N-methyl-2-pyrrolidone -- 20 wt.%, and ion-exchanged water -- 52 wt.%. In this evaluation, there was used a paper with a bleeding probability adjusted to be in a given range. The paper was scanned perpendicularly to the discharging direction of the liquid jet recording head while discharging ink from all the nozzles, to thereby obtain a printed sample having four different printed widths in the nozzle arrangement direction and with a printed area of 200 mm in the direction in which the paper was moved. In this case, the paper moving speed was adjusted so that the printing dot interval became 63.5 µm with a discharging frequency of 1 KHz. The head driving conditions were made as follows. voltage applied to the heat generating resistor : 1.1 Vth (Vth : discharging threshold voltage) driving frequency : I KHz (the voltage applying interval to the heat generating resistor) pulse width : 10 µm (the period of applying one pulse to the heat generating resistor) In Table 7, there is shown a printing width as for each of the liquid jet recording head samples. As for each printed sample obtained by each of the liquid jet recording head samples, evaluation was conducted with respect to printing precision and appearance of uneven density in the following manner. Evaluation of printing precision: As for each printed sample, the printed dot interval (the interval between the dot centers) was observed using a micrometer microscope, whereby a variation range was examined. In this case, the observation was conducted at 10 randomly selected positions each having an area of 2 cm in square size on the printed sample, wherein the direction perpendicular to the paper moving direction was made to be X and the paper moving direction was made to be Y, and the case where as for all the 10 positions each being of 2 cm in square size, the dot interval in the X direction and that in the Y direction were within a range of 43.5 µm to 83.5 µm was evaluated as being fitness. As a result, each of Sample No. 1' to Sample No. 12' was found to be fitness. Evaluation of appearance of uneven density: Each printed sample was evaluated with respect to appearance of uneven density using a Macbeth densitometer. In this case, the entire area of the printed sample was read out by the binary image processing by CCD line sensor system, wherein the optical density was measured as for every I cm width in the direction perpendicular to the paper moving direction. In this evaluation, the case where the optical densities of the adjacent regions were within 0.2 was evaluated as being fitness. As a result, each of Sample No. 1' to Sample No. 12' was found to be fitness. Comparative Example 2 (preparation of a liquid jet recording head using a single crystal silicon substrate) In this comparative example, using each of the four comparative liquid jet recording head substrate samples (Comparative Sample No. 1 to Comparative Sample No. 4) shown in Table 8 which were prepared by repeating the procedures of Comparative Example 1, there were prepared four comparative liquid jet recording head samples (Comparative Sample No: 1' to Comparative Sample No. 4') in the same manner employed in Example 2. As for each of the resultant samples of Comparative Sample No. 1' to Comparative Sample No. 4', the production process yield was evaluated in the same manner as in Example 2. The results obtained are shown in Table 9. Shown in the column relating to the production yield of Table 9 are the results of the evaluation conducted based on the following criteria. X : the case wherein no practically acceptable liquid jet recording head is found, ▵ : the case wherein the number of practically acceptable liquid jet recording heads is few, and ○ : the case wherein the production yield is within a value previously estimated based on the number of nozzles. From the results shown in Table 9, the following facts are understood. That is, no practically acceptable liquid jet recording head can be obtained in the case of Comparative Sample No. 1'; the production yield for a practically acceptable liquid jet recording head is extremely low in the case of Comparative Sample No. 2'; and a desirable production yield is provided in the case of each of Comparative Sample No. 3' and Comparative Sample No. 4'. Then, as for each of the comparative liquid jet recording head samples of Comparative Sample No. 2' to Comparative Sample No. 4', evaluation was conducted with respect to discharging durability, and printing precision and appearance of uneven density in terms of printing performance in the same manner as in Example 2. As a result, each of the practically acceptable liquid jet recording head samples of Comparative Samples Nos. 2', 3' and 4' was found to be fitness with regard to each of the evaluation items of discharging durability, and printing precision and appearance of uneven density in terms of printing performance. Comparative Example 3 (preparation of a liquid jet recording head using a single crystal silicon substrate) In this comparative example, two liquid jet recording head samples of Comparative Sample No. 4' shown in Table 9, which were prepared by repeating the procedures of Comparative Example 2, were integrated to obtain a liquid jet recording head unit with 8576 discharging outlets (Comparative Example No. 4'', see Table 10). The head unit was prepared in the following manner. That is, one of the liquid jet recording head samples was was fixed to a face of an aluminum support member, and the remaining liquid jet recording head sample was arranged on and fixed to the other face of the support member such that the discharging outlets of the two liquid jet recording heads were arranged to correspond to each other precisely as much as possible along the entire length of the liquid jet recording head unit. The resultant liquid jet recording head unit was evaluated with respect to discharging durability, and printing precision and appearance of uneven density in terms of printing performance in the same manner as in Example 2. As a result, it was found to be fitness with respect to durability. But it was found to be unfitness with respect to printing precision. The reason for this was found to be due to the influence based on an error in the assembly of the two heads. Further, as for the evaluation with respect to appearance of uneven density, it was found to be unfitness. The reason for this was found to be due to a difference in the Vth (discharging threshold voltage) among the two heads. The results obtained are collectively shown in Table 10. Sample No. Si-base member the presence or absence of alkali at the time of primary polishing surface roughness R max (Å) step at grain boundary 1 single crystal present 150 - 2 single crystal absent 150 - 3 polycrystalline present 150 occurred (maximum 0.2 µm) 4 polycrystalline absent 150 not occurred Sample No. base member size (mm) maximum warp magnitude in terms of relative value single crystal Si polycrystalline Si 1 800 × 150 × 1.1 3 1 2 700 × 150 × 1.1 2.5 1 3 600 × 150 × 1.1 2 1 4 500 × 150 × 1.1 1.2 1 5 400 × 150 × 1.1 1 1 6 300 × 150 × 1.1 1 1 Sample No. crystallinity mean crystal grain size (mm) fitness proportion in terms of relative value 1 Si single crystal - 0.4 2 Si polycrystalline 15 0.45 3 Si polycrystalline 8 0.8 4 Si polycrystalline 5 0.9 5 Si polycrystalline 2 1 6 Si polycrystalline 1 1 7 Si polycrystalline 0.1 1 8 Si polycrystalline 0.01 1 Sample No. Si-base member used pit number (number/cm 2 ) yield (%) 1 single crystal 1 95 2 polycrystalline silicon with no addition of release agent polycrystalline silicon 1 95 3 with addition of release agent 5 95 4 same as in sample 3 10 90 5 same as in sample 3 50 60 6 same as in sample 3 100 30 sample No. thermally softening temprature (°C) surface step state after the thermally softening treatment recording head production possibility survival rate of the heat generating resistor 1 × 10 7 1 × 10 8 3 × 10 8 1 1380 o ○ impossible - - - 2 1330 o ○ possible 100% 100% 100% 3 1280 ○ possible 100% 100% 100% 4 1230 ○ possible 100% 100% 100% 5 1180 X possible 50% 10% 0% sample No. heat treatment temprature (°C ) heat treatment period (hr) surface step state after the heat treatment recording head production possibility survival rate of the heat generating resistor 1 × 10 7 1 × 10 8 3 × 10 8 6 1380 5 o ○ impossible - - - 7 1330 7 o ○ possible 100% 100% 100% 8 1280 8 ○ possible 100% 100% 100% 9 1230 11 ○ possible 100% 100% 100% 10 1180 14 X possible 50% 10% 0% Sample No. crystallinity base member size (mm) surface step state after the thermally softening treatment exposure fitness proprotion 1 Si polycrystalline 600 × 150 × 1.1 o ○ 100% 2 Si polycrystalline 500 × 150 × 1.1 o ○ 100% 3 Si polycrystalline 400 × 150 × 1.1 o ○ 100% 4 Si polycrystalline 300 × 150 × 1.1 o ○ 100% 5 Si polycrystalline 600 × 150 × 1.1 o ○ 100% 6 Si polycrystalline 500 × 150 × 1.1 o ○ 100% 7 Si polycrystalline 400 × 150 × 1.1 o ○ 100% 8 Si polycrystalline 300 × 150 × 1.1 o ○ 100% 9 Si polycrystalline 600 × 150 × 1.1 o ○ 100% 10 Si polycrystalline 500 × 150 × 1.1 o ○ 100% 11 Si polycrystalline 400 × 150 × 1.1 o ○ 100% 12 Si polycrystalline 300 × 150 × 1.1 o ○ 100% Comparative Sample No. crystallinity base member size (mm) exposure fitness proportion 1 Si single crystal 600 × 150 × 1.1 40% 2 Si single crystal 500 × 150 × 1.1 90% 3 Si single crystal 400 × 150 × 1.1 100% 4 Si single crystal 300 × 150 × 1.1 100%

Claims: A process for producing a substrate (8) for liquid jet recording head provided with an electrothermal converting body comprising a heat generating resistor (2a) capable of generating thermal energy and a pair of wirings (3a, 3b) electrically connected to said heat generating resistor (2a) is formed, characterized by comprising: providing a member (1; 11) composed of a polycrystalline silicon-based material as a constituent base member of said substrate, and conducting a step of thermally oxidizing the surface of said polycrystalline silicon-based member (1; 11) and a step of thermally softening the surface of said polycrystalline silicon-based member (1; 11) to thereby form a thermal oxide layer (13) on the surface of said polycrystalline silicon-based member (1; 11). A process for producing a substrate for liquid jet recording head according to claim 1, wherein the thermally softening step is conducted at a temperature in the range of 1230 °C to 1330 °C. A process for producing a substrate for liquid jet recording head according to claim 1, wherein the thermally oxidizing step and the thermally softening step are successively conducted. A process for producing a substrate for liquid jet recording head according to claim 1, wherein the thermally oxidising step and the thermally softening step are concurrently conducted. A process for producing a substrate for liquid jet recording head according to claim 1 which further comprises a step of diffusing an impurity into the polycrystalline silicon-based member (1; 11). A process for producing a substrate for liquid jet recording head according to claim 5, wherein the amount of the impurity incorporated into the polycrystalline silicon-based member (1; 11) is adjusted to be 1 x 10 21 atoms/cm 3 or less and the thermally softening step is conducted at a temperature in the range of 1130 °C to 1330 °C. A substrate (8) for liquid jet recording head including an electrothermal converting body comprising a heat generating resistor (2a) capable of generating thermal energy and a pair of wirings (3a, 3b) electrically connected to said heat generating resistor (2a), characterized in that said substrate (8) includes a base member (1; 11) constituted of a polycrystalline silicon-based material and said polycrystalline silicon-based base member has a thermal oxide layer (13) formed by subjecting the surface of said polycrystalline silicon-based base member (1; 11) to thermal oxidation treatment and thermally softening treatment. A substrate for liquid jet recording head according to claim 7, wherein the substrate is a substrate for full line type recording head which has a length corresponding to the entire width of the recording area of a recording member on which recording is conducted. A substrate (8) for liquid jet recording head according to claim 7, wherein the thermal oxide layer (13) has a smoothly flat surface formed by conducting the thermally softening treatment at a temperature in the range of 1230 °C to 1330 °C. A liquid jet recording head which includes a substrate (8) for liquid jet recording head including an electrothermal converting body comprising a heat generating resistor (2a) capable of generating thermal energy and a pair of wirings (3a, 3b) electrically connected to said heat generating resistor (2a), and a liquid supplying pathway (6) disposed in the vicinity of said electrothermal converting body of said substrate (8), characterized in that said substrate (8) includes a base member (1; 11) constituted of a polycrystalline silicon-based material, and said polycrystalline silicon-based base member has a thermal oxide layer (13) formed by subjecting the surface of said polycrystalline silicon-based base member (1; 11) to thermal oxidation treatment and thermally softening treatment. A liquid jet recording head according to claim 10, wherein the substrate (8) is a substrate for full line type recording head which has a length corresponding to the entire width of the recording area of a recording member on which recording is conducted. A liquid jet recording head according to claim 10, wherein the thermal oxide layer (13) has a smoothly flat surface formed by conducting the thermally softening treatment at a temperature in the range of 1230 °C to 1330 °C. A liquid jet recording apparatus comprising: a liquid jet recording head (120) including a substrate (8) for liquid jet recording head including an electrothermal converting body comprising a heat-generating resistor (2a) capable of generating thermal energy and a pair of wirings (3a, 3b) electrically connected to said heat generating resistor (2a), and a liquid supplying pathway (6) disposed in the vicinity of said electrothermal converting body of said substrate (8), characterised in that said substrate (8) includes a base member (1; 11) constituted of a polycrystalline silicon-based material, said polycrystalline silicon-based base member having a thermal oxide layer (13) formed by subjecting the surface of said polycrystalline silicon-based base member to thermal oxidation treatment and thermally softening treatment; and an electric signal supplying means capable of supplying an electric signal to said heat generating resistor (2a) of said recording head. A liquid jet recording apparatus according to claim 13, wherein the substrate (8) is a substrate for full line type recording head which has a length corresponding to the entire width of the recording area of a recording member on which recording is conducted. A liquid jet recording apparatus according to claim 13, wherein the thermal oxide layer (13) has a smoothly flat surface formed by conducting the thermally softening treatment at a temperature in the range of 1230 °C to 1330 °C.

Inventors: TERAI HARUHIKO, TERAI, HARUHIKO, TERAI, HARUHIKO, C/O CANON KABUSHIKI KAISHA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: B41J2
IPC codes: B41J2