Apparatus for performing heat treatment on semiconductor wafers

Abstract: An apparatus for containing semiconductor wafers (16) and performing heat treatment thereon, comprising heaters (4) disposed in a high temperature furnace (2) and forming at least one heating space therein, and an insertion and retrieval aperture of the heating space disposed at a lower portion of each heating space for allowing passage of wafers therethrough, each of the heating spaces adapted to receive one or two wafers and to achieve simultaneous and uniform heat treatment on the wafers.

Description: The invention relates to a high temperature furnace for performing heat treatment on semiconductor wafers. In a semiconductor device, the so-called "semiconductor function" is exhibited by introducing a minute amount of atoms of boron, phosphorus arsenic, etc. as impurities in a highly pure solid state substrate, such as a silicon wafer. Impurity atoms are introduced into a substrate in an ion implanting apparatus and then the substrate is heated at 800 - 1200°C for a predetermined time period in an atmosphere of nitrogen gas, argon gas, hydrogen gas etc. in a diffusion apparatus to diffuse the impurity atoms by a predetermined depth with tolerable dispersion and to cure the crystal defects caused by the ion implantation. Also, in the case of forming an oxide film on a substrate, e.g. a gate oxide film of a MOS transistor or an inter-element isolation oxide film, a substrate is heated in an atmosphere of oxygen gas or steam in a diffusion apparatus to form a thermal oxide film with a tolerable thickness dispersion. In the heat treatment apparatus used in the conventional diffusion process, a multiplicity of semiconductor wafers are arranged on a quartz boat and are inserted into a horizontally placed cylindrical furnace. In recent years, it is requested to form shallow diffusions and thin oxides uniformly over the whole surface of a semiconductor wafer by a short time heating corresponding to the miniaturization of patterns. As a heat treatment apparatus for satisfying such kind of requests, there is proposed a structure, as disclosed in the specification of JP-A-60-171723, in which a vertical cylindrical high temperature furnace has an opening at a lower end and a horizontally supported wafer, one for each time, is inserted into the high temperature furnace from the lower end opening to heat the wafer. This conventional technique, however, does not pay any attention for simultaneously and uniformly heating two wafers in a short time period. Heating the wafers one by one is accompanied with a poor productivity. If two wafers are to be simultaneously heated, they are placed side by side horizontally. Then, there will be generated very large temperature differences from wafer to wafer and in one wafer. Further, if a wafer is heated for a short time period and rapidly taken out of a high temperature furnace, a very hot wafer will be directly exposed to the external atmosphere and may be contaminated thereby. Yet further, the afore-mentioned technique does not pay any attention for heating a large wafer uniformly which will be required by the recent trend of further scaling down the patterns and employing wafers of a larger diameter. From US-A-4,649,261, a heating apparatus for a semiconductor wafer is known comprising an upper and lower array of lamps disposed adjacent the upper and lower end wall of an integrating light pipe. The semiconductor wafer to be treated is disposed between the upper and lower array of lamps in a target plane that is perpendicular to the longitudinal axis of the light pipe. The apparatus employs CW lamps, pulsed lamps and a combination of the two. EP-A-0 225 501 A2 discloses an apparatus for performing heat treatment on semiconductor wafers comprising a cylindrical heating means surrounding a quartz tube and a pair of flat-plate resistance heaters disposed at both end faces of the cylindrical heating means substantially parallel to the surfaces of the wafers to be heat-treated. The cylindrical heating means comprises a plurality of heating elements which may be controlled separately. The object of this invention is to provide a high temperature furnace capable of successive heating operations of semiconductor wafers uniformly in short time periods, and of performing rapid and uniform cooling of the semiconductor wafers without exposing them to the external atmosphere, thereby performing heat treatment of high quality and high efficiency such as avoiding generation of thermal stress defects in the wafers. This object is solved by the features of claim 1. Into the high temperature furnace according to the invention, one or two semiconductor wafers can be simultaneously inserted and can be taken out therefrom at a lower position, heating of the heater in correspondence to the regions in the wafer surface is possible, prevention of generation of thermal stress defects and uniform short-period heating are made possible by reduction of temperature distribution in the whole face of successively supplied semiconductor wafers. Dependent claims are directed to preferred embodiments of the invention. Fig. 1 is a general schematic perspective view of a diffusion apparatus to which this invention is applied. Fig. 2 is a longitudinal cross-section of a high temperature furnace and a temperature control system. Fig. 3 is a longitudinal cross-section of a high temperature furnace. Fig. 4 is a perspective view of a wafer-inserting holder. Fig. 5 is a longitudinal cross-section of the holder of Fig. 4. Fig. 6 is a longitudinal cross-section of the high temperature furnace along a direction perpendicular to that of Fig. 2 and an associated control system. Fig. 7 is a perspective view of a process tube. Fig. 8 is a perspective view of an example of the high temperature furnace showing a divided heater region arrangement. Fig. 9 is a flow chart of operation processings in the temperature controller of the heat treatment chamber. Fig. 10 is a perspective view of another example of the high temperature furnace showing another divided heater region arrangement. Fig. 11 is a perspective view of main parts of a wafer supply mechanism. Fig. 12 is a perspective view of a loading holder and an unloading holder. Fig. 13 is a perspective view of a linear tube. Fig. 14 is a longitudinal cross-section showing how parts of a lower portion of a high temperature furnace are assembled. Fig. 15 is a graph of experimental results showing change in the effective heat treatment temperature and the heater set temperature for respective numbers of insertion. Fig. 16 is a graph of experimental results showing time dependence of the inner temperature of the heat treatment chamber and the wafer temperature. Fig. 17 is a graph of experimental results on the wafer cooling characteristics. Fig. 18 is a graph showing calculation results on the temperature difference in a wafer surface. Fig. 19 is a graph of experimental results showing the relation of the gas flow rate and the amount of mixing the external atmosphere. Fig. 20 is a graph of experimental results showing the relation between the speed of wafer insertion and the amount of external atmosphere mixed. Fig. 21 is a perspective view of a process tube according to another embodiment of the invention. Fig. 22 is a longitudinal cross-section of a high temperature furnace according to another embodiment of this invention. Fig. 23 is a flow chart of operation processings in a heat treatment temperature controller according to another embodiment of this invention. Fig. 24 is a longitudinal cross-section of a high temperature furnace in a diffusion apparatus according to another embodiment of this invention and an associated control system. Figs. 25 and 26 are flow charts of operation processings in a wafer heat treatment controller according to another embodiment of this invention. Figs. 27 to 31 are longitudinal cross-sections of wafer loading portions of wafer inserting holders according to other embodiments of this invention. Fig. 32 is a perspective view of a wafer-inserting holder according to another embodiment of this invention. Figs. 33 to 35 are longitudinal cross-sections of wafer-loading portions of wafer-inserting holders according to other embodiments of this invention. Fig. 36 is a perspective view of a wafer-inserting holder according to another embodiment of this invention. Fig. 37 is a longitudinal cross-section of the wafer-inserting holder of Fig. 36. Figs. 38 to 40 are longitudinal cross-sections of wafer-inserting holders according to other embodiments of this invention. Figs. 41 to 44 are longitudinal cross-sections of further high temperature furnaces. Now, an embodiment of this invention will be described referring to the drawing. Fig. 1 shows a general view of the structure of a diffusion apparatus to which this invention is applied. Figs. 2 and 3 show longitudinal cross-sections of a high temperature furnace 2 and a control system of the apparatus of Fig. 1. Referring to Figs. 2 and 3, the high temperature furnace 2 has a rectangular shape. A heat insulating member 6 is provided around a pair of right and left plate shaped heaters 4a, 4b and 4c (which may be formed of folded iron-chromium-aluminum alloy (Kanthal) resistance heater sheet embedded in an alumina heat insulator member, etc.), each of which is divided into a plurality of heat generating domains. Inside the heater, there are provided a linear tube 8 (made of silicon carbide etc.) and a process tube 10 (made of quartz glass, etc.), which are supported by a flange 12 (made of stainless steel etc.) and constitute the high temperature furnace 2. From the underside of the high temperature furnace 2, a pair of wafers 16 mounted on a wafer-inserting holder 14 are inserted almost vertically into the inside of a process tube 10. The lower part 18 of the process tube 10 extends below the high temperature furnace 2, and is surrounded by a cooling member 20 (such as a plate in which coolant fluid is allowed to flow). Fig. 4 is a perspective view of a wafer-inserting holder. Fig. 5 is a longitudinal cross-section of the wafer-inserting holder shown in Fig. 4. The wafer-inserting holder 14 comprises a disk 22 to intervene between a pair of wafers, a top end portion 26 having grooves 24 for accommodating the disk 22 and wafers 16, a support pillar 28 of a thin wall pipe shape, and a flange 32 for mounting the holder on an elevating carrier 30. The disk 22 has a greater thickness at a circumferential portion 34 than a central portion 36, except a lower portion, and has a diameter equal to that of the wafers 16. The top end portion 26 is formed of thin plates and shaped in circular rods at the portions having the grooves 24. The support pillar 28 has a gas communication hole 38 at a lower position and a hermetic structure elsewhere. The wafer-insertion holder 14 may be made of quartz glass, polycrystalline silicon, silicon carbide, etc. The disk 22 and the support pillar 28 may be made of different materials. Further, those composite materials which are made by coating a polycrystalline silicon film, a silicon nitride film, etc. on a quartz glass member may also be used. Typical dimensions of the wafer-inserting holder 14 will be shown hereinbelow. In case of heat-treating a pair of wafers each having a diameter of 150 mm and a thickness of 0.6 mm, the disk 22 has an outer diameter of 150 mm, a thickness of 1 mm at the central portion 36 and a thickness of 2 mm at the peripheral portion 34 with a width of 8 mm from the circumference for the thick peripheral portion. The gap between each of the wafers 16 and the thick portion 34 of the disk is designed at 2.5 mm. The top end of the holder having the grooves 24 is formed in circular rods of a diameter of 5 mm. Each of the grooves 24 for accommodating the wafers has a width of 0.7 mm, and a depth of 2.5 mm with rounded corners in the groove. The plate members of the top end portion 26 and the pipe of the support pillar 28 have a thickness of 1.5 mm. The grooves 24 are so designed that a pair of wafers 16 and the disk 22 is slanted by 5 degrees from the vertical position and become parallel to each other when accommodated on the top end portion 26. The reason why the wafers are slanted is to avoid vibrations of wafers in the horizontal direction when the insertion holder 14 is transferred vertically. Since the slant angle is small, there is almost no difference in the heat transfer characteristics between the pair of wafers in heating in the high temperature furnace and in cooling in a cooling zone even when the wafers are slanted. The up/down carrier 30 is mounted on an up/down drive mechanism 40 in which ball screws etc. are installed. Control signals are given from a main controller 42 to the up/down drive mechanism 40. Heat generating portion temperature measuring sensors 44a, 44b and 44c are inserted in the respective divided heater regions 4a, 4b and 4c. The amount of heat generation is controlled by heater temperature controllers 46a, 46b and 46c of PID thyristor control type, and heater power sources 48a, 48b and 48c so that the heat generating portion temperatures at respective heater regions approach given set temperatures. A heat treatment chamber internal temperature sensor 50 is inserted at a wafer insertion position between the process tube 10 and the linear tube 8 and is connected to a heat treatment chamber temperature controller 52. In the heat treatment chamber temperature controller 52, an operation processing as described later is performed to give set temperatures to the heater temperature controllers 46a, 46b and 46c. The heat treatment chamber temperature controller 52 accepts status signals, such as commencement of wafer insertion, from the main controller 42. Fig. 6 shows a longitudinal cross-section of the high temperature furnace along a face perpendicular to that of Fig. 2 and a control system. Fig. 7 is a perspective view of the process tube 10. In the process tube, a gas such as nitrogen, argon, oxygen, water steam selected in dependence on the intended use of the diffusion apparatus, is pre-heated, supplied, and is allowed to flow from the top to the bottom of the process tube 10, i.e. downwards. On the left and right sides of the process tube 10, gas supply tubes 54a and 54b are provided. Gas supplied from a gas source 56 is guided to the gas supply tubes 54a and 54b through either path of a low rate piping system 58 and a control valve 60 or a high rate piping system 62 and a control valve 64, is preheated while rising up in the gas supply tubes outside the process tube 10, and fed into the inside of the process tube 10 at an upper portion of the process tube 10. Control valves 60 and 64 are open/close controlled by the signal from the main controller 42 to control the gas flow rate large or small. Fig. 8 is a perspective view of a high temperature furnace 2 showing division of the heater. The heater comprises two parallel plates, each being divided into five regions 4a to 4j. From the symmetries of front-rear and right-left, four heat generations, in central (4b and 4g), upper (4a and 4f), lower (4c and 4h) and side (4d, 4e, 4i and 4j) portions, are controlled independently. Corresponding to four heat generations of the heater, the heater temperature controllers 46a to 46d, the heater power sources 48a to 48d, and the heat generation portion temperature measuring sensors 44a to 44d are divided into four independent systems. For keeping the symmetries of front-rear or right-left relation, the heater divisions of front-rear and right-left are controlled to always generate the same amount of heat. In the manufacture of the heater, the resistance values of the regions in the symmetry positions may vary to some extent. Adjustment may be made by attaching adjusting resistors to the external wiring system. Fig. 9 is a flow chart of operation processings to be achieved in the heat treatment chamber temperature controller 52. Regarding the heaters 4b and 4g in the central region, let the set temperature of the heater temperature controller upon the i-th wafer insertion be denoted as Hi, the previous set temperature therebefore be denoted as H i-1 , the set temperature in the steady state without wafer insertion be denoted as H o , the internal temperature of the heat treatment chamber in the steady state be denoted as w 1 , the internal temperature of the heat treatment chamber immediately before the i-th wafer insertion be denoted as w i , and the internal temperature of the heat treatment chamber immediately before the second insertion be denoted as w 2 . In Fig. 9, 1 ○ when the heater is turned on, H o , w 1 and w 2 and the set temperatures for the respective heater temperature controllers are inputted from the main controller, and a signal of the set temperature of central region H i = H o is outputted to the heater temperature controller 46b for the central region. Together with this, set temperatures of other regions are outputted to the heater temperature controllers for the other regions. Here, the values of H o , w 1 , and w 2 and the respective heater set temperatures are preliminarily obtained from experiments for each heat treatment condition and stored in the main controller. When the successive supply of the wafers is started, 5 ○ the internal temperature w i of the heat treatment chamber immediately before the wafer insertion is measured. 6 ○ For the first and the second of the successive insertions, 7 ○ the set temperature H i is outputted to the heater temperature controller for the central region in accordance with the interruption time of the wafer insertion. 8 ○ For the third and on of the successive insertion, the set temperature of the heater for the central region is corrected by Δw = w 2 - w i so that the internal temperature w i of the heat treatment chamber immediately before the wafer insertion approaches the value w 2 immediately before the second insertion. 2 ○ When the wafer insertion is interrupted, 3 ○ the heater set temperature for the central region is set H o +2°C for 3 minutes after the interruption. Here, however, if the set temperature theretofore H i-1 is below H o +2, the set temperature is set at H o . 4 ○ After three minutes have passed from the interruption, the heater set temperature for the central region is made equal to H o , to wait the re-start of the wafer supply. The reason why the heater set temperature for the central region is set to H o +2 for 3 minutes after the interruption of the wafer insertion in 8 ○ of Fig. 9 is to avoid a rapid change of the heater set temperature. Also, the reason why the correction Δw of the heater set temperature in 8 ○ successive insertion is in the range 0 - 1°C is to avoid the correction to become very large when the internal temperature sensor 50 of the heat treatment chamber exhibits an abnormal value by noises. The control of the heater set temperature in Fig. 9 is done only for the central heater regions 4b and 4g, with other heater regions 4a, 4c to 4f, and 4h to 4j kept at constant set temperatures. The reason is that the central heater region is cooled by the insertion of a room temperature wafer or wafers, but other heater regions are not cooled thereby. Fig. 10 shows another example of a high temperature furnace 2 illustrating division of the heater. This example does not form a part of the invention. The heater is divided into an upper heater 4k and a lower heater 4ℓ, both disposed substantially parallel to the wafer plane, side heaters 4m and 4n, and a top plane heater 4o, all of which are controlled independently. This embodiment has a less number of division compared to the embodiment of Fig. 8, and still can perform similarly uniform heat treatment. Fig. 11 is a perspective view of main parts of the wafer supply mechanism. All the parts are shown in Fig. 1. The wafer supply mechanism comprises a cassette 66 for containing pre-treatment wafers, a take-out holder 68, a load holder 70, an insertion holder 14, an unload holder 72, a cooling boat 74, a put-in holder 76, a cassette 78 containing post-treatment wafers, etc. Arrows in Fig. 11 indicate the directions of motion of the respective holders. The unload holder 72 has the same structure as the load holder 70. These holders 72 and 70 are provided on the right and left opposing positions with respect to the insertion holder 14. The put-in holder 76 has the same structure as that of the take-out holder 68. Fig. 12 is an exterior view of a top end portion of the load holder 70 and the unload holder 72. It is comprised of round rods (e.g. made of quartz glass) having grooves 80 for accommodating wafers. For keeping away from or allowing passage of the disk 22 of the insertion holder 14, the two wafers are supported separately. As shown in Fig. 1, the system has a drive section 82 for the take-out holder 68, a drive section 84 for the load holder 70, a drive section 86 for the unload holder 72, and a drive section 88 for the put-in holder 76. The power sources for these drive sections are contained in a box 90 together with the heater power source transformer, etc. The control instrument 92 such as the main controller 42 has a control panel 94 of display panels and switches at the front face. Although there are not shown in the figure, there are provided sensors for sensing whether there is any wafer in the cassettes 66 and 78 and the cooling boat 74, and sensors for sensing whether any wafer is being supported by the respective holders 14, 68, 70, 72 and 76. Also, there are provided position detection sensors for the respective holders 68, 70, 72, and 76. Since wafers are placed on the insertion holder 14 with a slight slant, the cassettes 66 and 78, the cooling boat 74 and the respective holders 68, 70, 72 and 76 are a little slanted from the vertical state. Fig. 13 shows an exterior view of the linear tube 8. The tube 8 comprises a cap 96 and a main body 98. The main body 98 has rounded corners for increasing its strength. Fig. 14 is a cross-section of details of the parts for assembling the process tube 10, the linear tube 8, the heater 4, and the heat-insulating member 6 to the flange 12. The process tube 10 should be frequently detached for the purpose of cleaning. For making the detachment easy, the process tube 10 is fixed to the flange 12 with screws and a fixing metal member 100. The linear tube 8 is also arranged to be detachable from the high temperature furnace 2 for the purpose of maintenance. The lower part of the high temperature furnace 2 is surrounded with scavenger 102. The scavenger 102 also works as a cooling member, and guides the treatment gas and dusts ejecting from the process tube to the exhaust system. A heat-insulating member 106 (e.g. made of ceramics, etc.) is provided between the linear tube 8 and the screws and a fixing metal member 104 to enhance reduction of the heat dissipation. Processes for conducting heat treatment on the wafers using the diffusion apparatus as constructed above will be described hereinbelow. An operator inputs the heat treatment conditions (objective heat treatment temperature time period, kind of gas, etc.) into the main controller 42. For example, let the input information be 1000°C, 3 minutes and nitrogen. The main controller 42 transmits these informations to the heat treatment temperature controller 52. The heat treatment temperature controller 52 gives heater set temperatures to the heater temperature controllers 46a to 46d for the respective heater regions. The set temperatures for the lower heater regions 4c and 4h are determined at higher temperatures compared to the central heater regions 4b and 4g, for cancelling the heat dissipation from the lower inlet of the high temperature furnace 2 and the influences of the insertion holder 14. For example, when the set temperature of the central heater regions 4b and 4g is set at 1000°C, the set temperature for the lower heater regions 4c and 4h will be set at 1060°C. The set temperatures of the upper and the side heater regions 4a, 4d, 4e, 4f, 4i and 4j are so selected to make the heat treatment of the wafer uniform. For example, the set temperature of the upper heater regions 4a and 4f is set at 990°C, and the set temperature of the side heater regions 4d, 4e, 4i and 4j is set at 980°C. The reason of selecting the set temperatures for the upper and the side heater regions a little lower than that of the central heater region is that when a pair of wafers are simultaneously inserted into the heat treatment chamber, the peripheral portion of the wafer is more heated by the heat from the side portion than the central portion of the wafer and hence the temperatures of the upper and the side regions should be held a little lower than the central region for heat-treating the wafer uniformly. Under these heater set temperatures, wafer supply is commenced after the internal temperature of the heat treatment chamber becomes steady. Here, until the wafer supply is started, the insertion holder 14 is kept in the heat treatment chamber and is pre-heated. When an operator sets a cassette 66 carrying wafers and an empty cassette 78 in the equipment and inputs the signal of starting the heat treatment from the control panel 94 to the main controller 42, signals from the main controller 42 make the take-out holder 68 to take out the wafers one by one from the cassette 66 and to carry a pair of wafers to the load holder 70. Then, signals from the main controller 42 go to the up/down drive mechanism 40 to move the insertion holder 14 below the high temperature furnace 2. The load holder 70 is actuated to load a pair of wafers on the insertion holder 14. After loading the wafers on the insertion holder 14, the load holder 70 opens its top ends, returns to the original position, receives the next pair of wafers which are transferred by the take-out holder 68, and waits for the next action. The insertion holder 14 moves upwards and loads the wafers 16 in the heat treatment chamber (see Fig. 2). For preventing occurrence of a temperature distribution in the wafers upon insertion, the insertion speed is selected at a high speed, e.g. at 150 mm/sec or more. For preventing any excess external atmosphere from being brought into the heat treatment chamber in accompany with the insertion of the wafers, signals are sent from the main controller 42 to the control valves 60 and 64 to increase the flow rate of the gas. The wafers 16 are contained in the heat treatment chamber and are heated for a predetermined time period. The gas flow during the heat treatment may be small and hence signals from the main controller 42 work on the control valves 60 and 64 to make the flow rate small. Here, depending on the heat treatment conditions, the species of gas may be changed at the time of insertion and retrieval and at the time of heat treatment. For example, nitrogen gas may be used upon insertion and retrieval while oxygen gas may be used upon heat treatment. When the heat treatment is finished, signals from the main controller 42 make the insertion holder 14 go downwards and shift the wafers between cooling members 20 below the high temperature furnace (see Fig. 3). After the wafers are cooled for a predetermined time period between the cooling members 20, the insertion holder 14 further moves down to take out the wafers. The transfer or moving speed of the wafers is selected at a similarly high speed as the insertion speed for preventing occurrence of a temperature distribution in the wafer plane when the holder is moved downwards. For preventing any excess external atmosphere from mixing into the heat treatment chamber, the flow rate of the gas is kept at the low value, as will be described later. The wafers taken out just below the high temperature furnace are cooled for a predetermined time period on the insertion holder, then are taken out by the unload holder 72, carried to the cooling boat 74 by the put-in holder 76, and cooled thereat. During these actions, a new set of wafers are put on the insertion holder 14 by the load holder 70 and repeat the above-mentioned operations. The wafers sufficiently cooled down in the cooling boat 74 are put in the cassette again by the put-in holder 76. Regarding the cooling time period, for preventing the insertion holder 14 from excessively cooled upon unload and load operations, the cooling time period of wafers between the cooling members 20 is selected for example at 10 seconds, the cooling time of wafers on the insertion holder just below the high temperature furnace is selected for example at 10 seconds, and the unload and load times may be, for example, 20 seconds. Each time a new set of wafers are inserted, the set temperature of the central heater region is changed and the heat treatment of the wafers is achieved uniformly along the flow chart of Fig. 9. When the wafer supply is interrupted, a sensor or sensors (not shown) sense the absence of any new wafer on the load holder, and then the insertion holder without any wafer disposed thereon is kept in the heat treatment chamber and waits the next action. Fig. 15 shows the experimental results on the variation of the effective heat treatment temperature and the set temperature Hi of the central heater region for the respective number of insertions when the control of the heater set temperature is conducted along the flow chart shown in Fig. 9. The effective heat treatment temperature is derived by measuring the transient changes in the wafer temperature (by using radiation thermometer as described later as a measuring instrument), and integrating the wafer temperature with a weight of the diffusion speed of impurities in the silicon substrate (in case of Fig. 15, inpurity being arsenic) to obtain an averaged wafer temperature during the heating period. Fig. 15 also shows in a broken line the change of the effective heat treatment temperature when the control of the heater set temperature is not performed and the heater set temperatures are kept at constant values. When the heater set temperatures are kept constant, the internal temperature of the heat treatment chamber will decrease at each wafer insertion and the effective heat treatment temperature of the wafer will also decrease. By controlling the heater set temperatures, the effective heat treatment temperature of the wafer can be kept substantially constant for the respective numbers of wafer insertion. Fig. 16 shows the experimental results on the time dependence of the internal temperature w of the heat treatment chamber and the wafer temperature U. The heat treatment chamber temperature once decreases upon insertion of low temperature wafers, and then gradually increases. It can be seen that the wafer temperature does not become steady within 3 minutes or so. Fig. 17 shows the experimental results on the cooling characteristics of the wafer. The temperature was measured by mounting thermo-couples on the wafers. The wafer temperature becomes about 700°C by cooling the wafer in the cooling zone below the reaction tube for about 10 seconds, and then becomes about 100°C by cooling the wafer in the cooling boat for about 3 minutes. According to this embodiment, since each of the pair of wafer has its one face facing to the plate-shaped cooling member 20, the two wafers can be cooled rapidly at the same cooling speed. Further, since the width of the furnace entrance of the high temperature furnace 2 is small, heat dissipation from inside of the high temperature furnace to the outside can be made small. The insertion holder 14 changes its temperature along with the in/out motion at the high temperature furnace 2. Since the top end portion 26 and the support pillar 28 of the holder 14 have thin thickness structure, the heat capacity thereof is small and give only a little influence to the temperature distribution in the wafer under heating. When two wafers 16 and the disk 22 are aligned with respective gaps therebetween and inserted into the high temperature furnace 2, the outer faces of the two wafers 16 are almost uniformly heated. For the inner faces, however, heating through the gaps becomes larger as it approaches the outer periphery. Here, the peripheral portion 34 of the disk 22 has an increased thickness and hence an increased heat capacity compared to the central portion 36 of the disk 22, and hence is hard to change its temperature. As a result, the peripheral portion of the wafers 16 has substantially similar temperature increase as in the central portion of the wafer due to both the actions of heating through the gaps and the heat capacity of the disk. Therefore, the wafer surface attains a uniform temperature. The reason why there is no thick portion in the lower portion of the disk 22 is that there are the top end portion 26 and the support pillar 28 of the insertion holder on the lower portion which can perform a similar function as a thick portion. Transient temperature changes when room temperature wafers are loaded on an insertion holder of 500°C and inserted into a high temperature furnace are calculated by numerical computation. Fig. 18 shows the calculation results of the temperature difference (between the peripheral portion and the central portion) in the wafer surface for both cases when the wafers sandwich a disk of said typical dimensions and when there is no disk between the wafers. The abscissa represents the temperature of the peripheral portion of the wafer in the transient state. Such approximations are adapted in the calculation that the disk is made of opaque quartz, that the high temperature furnace has a uniform temperature field of 1000°C, and that the influence of the support pillar of the wafer insertion holder are neglected. It can be seen that the existence of the disk reduces the temperature difference in the wafer surface in the transient state to about a half. When the thickness ratio between the central portion 36 and the peripheral portion 34 of the disk 22 is further increased, the maximum temperature difference around the wafer temperature of 700°C decreases, but the temperature difference around 1000°C increases. In this embodiment, the bottom portion of the process tube 10 is kept always open. Since the interior gas is of a high temperature and flows downwards, the exterior atmosphere cannot invade into the process tube 10 by convection of diffusion in the steady state. Fig. 19 shows the experimental results illustrating the relation of the amount of gas flow and the amount of the exterior atmosphere allowed to mix into the process tube 10 upon insertion and take-out of the wafers. High purity nitrogen gas was used as the gas and the oxygen content at the center of the heat treatment chamber was measured. The temperature of the heat treatment chamber was set at 1000°C, and the wafer transfer speed upon insertion and take-out was set at 200 mm/sec. The instantaneous maximum increment of the oxygen content upon insertion is shown by a solid curve and the instantaneous maximum increment upon retrieval is shown by a broken curve. The amount of mixing exterior atmosphere upon insertion can be reduced when the gas flow rate is made 20 Nℓ/minute or more, while the amount of mixing exterior atmosphere upon retrieval can be made small when the gas flow rate is made 20 Nℓ/minute or less. The reason why the amount of mixing external atmosphere becomes smaller as the gas flow rate is made larger upon insertion is that the function of blowing off the in-coming external atmosphere accompanied with the wafers becomes larger with the increase of the gas flow rate. The reason why the amount of mixing external atmosphere becomes larger as the gas flow rate is made larger upon retrieval is as follows. As the gas flow rate is high, the pre-heating of the gas becomes insufficient. Then, the gas temperature of the gas flowing into the process tube does not reach the temperature of the heat treatment chamber. Thus, there occurs a convection in the heat treatment chamber. When the wafers are taken out in such a state, external atmosphere of such amount is sucked in from the furnace entrance that is equal to the volume of the wafers and the insertion holder which have gone out of the heat treatment chamber. Such in-coming atmosphere can go deep into the heat treatment chamber by the convection in the heat treatment chamber. From the above, it can be seen that the amount of mixing external atmosphere can be made small by increasing the gas flow rate upon insertion and decreasing the gas flow rate during the heat treatment and upon retrieval. In the example of Fig. 19, the amount of mixing external atmosphere can be always be made small even when the as flow rate is kept constant at 20 Nℓ/minute. When the gas flow rate is further reduced during the heat treatment and upon retrieval, there is obtained a merit of reducing the gas consumption. When the heat treatment chamber is at a lower temperature than in the case of Fig. 19, the limit above which the amount of mixing external atmosphere upon wafer retrieval shifts to a lower value than that for Fig. 19 case (i.e. the broken line in Fig. 19 shifts leftwards). Then, the amount of mixing external atmosphere cannot be made small simply by keeping the gas flow rate constant throughout insertion and retrieval. Fig. 20 illustrates the relation between the wafer transfer speed and the amount of mixing external atmosphere upon wafer insertion. When the wafer transfer speed is 150 mm/sec or below, the amount of mixing external atmosphere upon insertion becomes large. In the above description, the heater temperature controllers 46a, 46b and 46c were assumed to be of PID control type, but they may also be of other types such as feed-forward control type. Also, although two control valves 60 and 64 are on/off controlled to high/low change the flow rate of the treatment gas, one flow rate controller can be used instead. It may also be adopted that experiments may preliminarily be made for all the conditions to obtain data of set temperatures of the heater temperature controllers with respect to time and those data base may be stored in the main controller and heater set temperatures may be outputted according to the operation conditions without performing operation processings. It may also be adopted that without preliminarily storing data w 1 and w 2 in the main controller, the internal temperature w 1 of the heat treatment chamber in the steady state and the internal temperature w 2 of the heat treatment chamber immediately before the second insertion may be measured for each successive insertion to use them in the operation processings of the heater set temperatures upon the third insertion and the following insertions. A second gas supply tube may also be provided separately from the gas supply tube 54 and a large amount of gas may be flowed from the second gas supply tube upon the wafer insertion. When the temperature sensor 50 inside the heat treatment chamber is pluralized and the temperature control inside the heat treatment chamber is made fine, the uniformity can further be improved. In the preceding embodiment, the maintenance is made easy because the disk 22 and the support pillar 28 are detachable. The disk 22 and the pillar 28, however, may be integrated. Also, since the wafer diameter and the disk diameter are equal, there is obtained a merit that the dimensions of the high temperature furnace can be made substantially the same as those for the case of loading only the wafers. The disk diameter, however, can be made a little larger than the wafer diameter. In such a case, also, the inner diameter of the thick ring portion 34 of the disk may be selected a little smaller than the wafer diameter. Although the cooling members 20 are used in the preceding embodiment, the wafers may be cooled by directly radiating to the external space without the cooling members. Further, as shown in Fig. 21, it is economical to wind the piping 54 of the treatment gas dense around the lower part 18 of the process tube 10, so as to form a cooling zone with the treatment gas and to pre-heat the treatment gas. Further, as shown in Fig. 22, a gas flow ports 108 may be provided at a lower part of the high temperature furnace to blow a lower part 18 of the process tube 10 with a coolant gas to cool it. Further, when the wafers are to be cooled at a lower part 18 of the process tube 10 after the heat treatment, the wafers may be transferred from the insertion holder to a separate cooling holder (not shown) and a next set of wafers may be loaded on the insertion holder 14 and inserted into the inside of the high temperature furnace 2, so as to reduce the cycle of the heat treatment and to improve the productivity. Also, without extending the lower part of the process tube, another structure, separate from the reaction tube, may be used to shield the cooling domain from the external air, with similar technical effect. Also, two or more temperature zones may be formed in the high temperature furnace and the wafers may be pre-heated before the heat treatment. In this case, the temperature zone of a higher temperature is better be positioned higher for preventing gas convection. When the wafers under heat treatment are rotated in the high temperature furnace, it becomes possible to heat the wafers more uniformly. In this case, a wafer or wafers may be attached to a disk and the wafers may be rotated together with the disk. Also, two semiconductor wafers may be brought into close contact to each other and may be subjected to the heat treatment. In this case, there is no need to provide an intervention of a disk between the wafers, but some mechanism may be provided to bring a pair of wafers in close contact to each other and to separate the contacting wafers. The high temperature furnace, wafers and all the other mechanisms may be slanted even to a large degree (for example 45 degrees from the vertical direction). Fig. 23 shows a flow chart of operation processings to be done in the heat treatment temperature controller 52 according to an alternative operating process. In this process, 8 ○ from the second insertion and on of the successive insertion, the heater set temperature for the central region is corrected so that the heat treatment chamber internal temperature w i immediately before the wafer insertion approaches the value w 1 in the steady state. Fig. 24 shows a longitudinal cross-section of a high temperature furnace 2 of a diffusion apparatus and a control system according to another embodiment of this invention. Between a linear tube 8 and a process tube 10, a prism 110 (made of quartz glass, etc.) is inserted at the position of wafer insertion and a mirror 112 is provided just below the prism 110 outside the high temperature furnace, and further a radiation thermometer (e.g. contactless type) 114 is provided. The radiation energy emitted from the wafers 16 are guided to the radiation thermometer 114 through the prism 110, and the mirror 112, to enable measurement of the temperature of the wafer under heat treatment. The data of the wafer temperature from the radiation thermometer 114 are sent (via an electrical connection) to a wafer heat treatment controller 116, where the operation processings as described later are conducted to supply set temperatures to the respective heater temperature controllers 46a, 46b and 46c. Fig. 25 shows a flow chart of the operation processings in the wafer heat treatment controller 116. In this figure, 5 ○ from the second and on of the successive retrieval, the heater set temperature for the central region is corrected such that the wafer temperature U i-1 immediately before the wafer retrieval approaches the value U 1 immediately before the first retrieval of the wafer. According to this embodiment, since the wafer temperature is directly measured and the heater temperature is controlled thereby, the wafer heat treatment can be done at a high precision and uniformly. Fig. 26 shows a flow chart of the operation processings to be performed in the wafer heat treatment controller 116 according to an alternative operating process. In this figure, 2 ○ after the wafers are inserted, 4 ○ the wafer temperature U i is measured continuously by the radiation thermometer 114, 5 ○ and the calculation of the amount of heat treatment X is done, 6 ○ when the target heat treatment X e is achieved, 7 ○wafer retrieval is instructed to the main controller. In Fig. 26, the calculation formula for the heat treatment quantity X is exemplified by the formula of diffusion distance of arsenic atoms in a silicon wafer. It is desirable to change the calculation formula depending on the conditions of heat treatment. In this embodiment, the heater set temperatures for all the heaters are not changed at every number of insertion. According to this embodiment, the heat treatment of wafers can be made uniform even when the temperature of the heat treatment chamber is varied. Figs. 27 to 31 show longitudinal cross-sections of the wafer-loading portion of alternative wafer insertion holders to be used in this invention. Fig. 27 shows a case where a thick portion 34 is provided also at the lower portion of the disk 22. Fig. 28 shows a case where a ring plate 118 intervenes a pair of wafers 16. For reducing the temperature distribution in the wafer surface upon insertion, the thickness of the ring plate 118 may better be selected at similar value as the thickness of the wafer 16. Fig. 29 shows a case where the disk member is changed into a laminate structure of three layers wherein a pair of disks 22 are attached on the both sides of a ring plate 118. Fig. 30 shows a case where the disk member is changed into a laminate structure of two layers wherein a ring plate 118 and a disk 22 are superposed. Fig. 31 shows a case where the thickness of the disk 22 is continuously varied along the radial direction. An exterior view of another alternative wafer insertion holder to be used in this invention is shown in Fig. 32. The holder has a ring 120 which is located around the periphery of the wafer with a little larger diameter than the wafer diameter, and a top end portion 26 disposed inside the ring 120 and formed of round rods with grooves 24 and thin plates. The distance between the inner wall of the ring 120 and the outer edge of the wafers is preferably selected 10 mm or below and the width of the ring 120 is preferably about three times as wide as the gap between the wafers. According to this embodiment, when the insertion holder carrying two wafers is being inserted into the inside of the high temperature furnace 2, heat otherwise radiating through the gap between the two wafers to the inside of the two wafers is shielded by the ring 120 and both the wafers are heated uniformly only by the heating upon the outer surfaces. The effect can be made larger than the surface of the ring 120 is roughened by sand-blast, etc. or is made opaque by coating, etc. Figs. 33 to 35 show longitudinal cross-sections of wafer-loading portions of the wafer insertion holder according to other embodiments of this invention. Fig. 33 shows a case where the ring 120 has an arcuate cross-section to be disposed around the wafers 16. Fig. 34 shows a case where the ring 120 has a V-shaped cross-section to be disposed around the wafers 16. In this embodiment, the ring 120 diverges towards the outside along the radial direction, enhancing loading and unloading of the wafers. Fig. 35 shows the case where the ring 120 has an angled U-shape cross-section to be disposed around the wafers. Fig. 36 shows an exterior view of a wafer insertion holder according to another embodiment of this invention, and Fig. 37 shows a longitudinal cross-section of the holder of Fig. 36. Inside the outer periphery of the two wafers 16, a ring 120 is provided. The cross-section of the ring 20 is circular with a diameter a little smaller than the gap between the wafers. There is provided no ring at the lower part of the wafers so that the wafers can be carried by the load holder and the unload holder as shown in Fig. 12. Fig. 38 shows a case where the cross-section of the ring 120 has a rectangular shape. The inner diameter of the ring 120 is selected approximately equal to the wafer diameter. As shown in Figs. 39 and 40, combination of a disk and a ring (Fig. 39) and a hollow disk and a ring (Fig. 40) may also be employed to finely adjust heat transfer to the wafers. Fig. 41 shows a longitudinal cross-section of a high temperature furnace according to another embodiment of this invention. Inside a high temperature furnace 2 of a vertical cylinder shape, a linear tube 8 of a cylinder shape and a process tube 10 of a rectangular pipe shape are provided. The lower part 18 of the process tube 10 extends below the high temperature furnace 2, and is surrounded by a cooling member 20. In a lower portion of the high temperature furnace 2 between the process tube 10 and the linear tube 8, there are provided radiation shield plates 122. This embodiment employs a cylinder-shaped high temperature furnace 2 and hence is easy to manufacture and has a high strength. Due to the provision of the radiation shield plates 122, heat dissipation from the high temperature space inside the high temperature furnace 2 to the cooling member 20 and to the exterior can be made small. Fig. 42 shows a longitudinal cross-section of a high temperature furnace of a diffusion apparatus according to another embodiment of this invention. Inside a single high temperature furnace 2, two heating spaces are formed and heated by parallel plate shaped heaters 4a, 4b and 4c. In each heating space, a pair of wafers are inserted. A common up/down carrier 30 carries all the wafers. According to this embodiment, four wafers can simultaneously be heat-treated. Fig. 43 shows a longitudinal cross-section of a high temperature furnace. Three high temperature furnaces 2 are connected together, in each of which furnaces a single wafer 16 is heated by a single plate-shaped heater 4a, 4b, or 4c. A common up/down carrier 30 carries and moves all the wafers. According to this embodiment, three wafers can simultaneously be heat-treated. Two or more high temperature furnaces may be connected together. Fig. 44 shows a longitudinal cross-section of a high temperature furnace according to another embodiment of this invention. A high temperature furnace 2 is formed by parallel plate shaped heaters 4a, 4b and 4c. Each of three wafers 16 is carried on an independent insertion holder 14 and moved by an independent up/down carrier 30. Inside the high temperature furnace 2, always two wafers 16 are inserted. The other one insertion holder is taken away from the heater and is subjected to wafer alternation or exchange. As one wafer which has completed the heat treatment is taken out of the heater, the new one wafer is inserted into the heater. According to this embodiment, the high temperature furnace 2 can be effectively utilized to improve the throughput. The functions and the technical advantages of the above-described embodiments may be summarized as follows. First, the functions may be analyzed as follows. (1) In a high temperature furnace, at least one face of each wafer faces against an inner wall of the high temperature furnace and the heater of the high temperature furnace is divided into a plurality of domains in the direction of the wafer face and subjected to the respective heat generation control. Therefore, the whole face of the wafer can be uniformly heat-treated, including the transient state. Further, since two wafers can be heated under equivalent conditions, the heating of the two wafers becomes equivalent. Since the insertion and retrieval of the wafer is done at a high speed, there is allowed almost no difference in the heating time between a portion which is inserted first and a portion which is inserted last. (2) By cooling the high temperature wafer after the heat treatment in a cooling zone at a lower part of the reaction tube, the wafer can be cooled without taking out the wafer from the process tube, preventing direct exposure of the high temperature wafer to the exterior atmosphere. Also, each of the two wafers in the cooling zone has one face facing against the cooling member or the exterior. Thus, the cooling speeds for the two wafers are almost the same and rapid. (3) When two wafers are positioned with a certain gap therebetween, it is easy to load and unload the wafers on and from the insertion holder. Wafers are supported in a small groove or grooves. Thus, the contact area between the wafer and the holder is small and hence the wafer portion which is subjected to non-uniform heat-treatment due to the contact by the holder can be made small. As the two wafers are separated by a certain gap, the peripheral portion of the inside of the wafers are heated from the gap. By intervening between the wafers a disk which has a larger thickness in a pheripheral portion, the speed of temperature rise can be equalized between the peripheral portion and the central portion of the wafer. (4) When the temperature of the heat generating portion of the heater is controlled to keep the internal temperature of the heat treatment chamber constant for each wafer insertion, the heat treatment of the wafer for the respective wafer insertions can be uniformalized. When the wafer supply is interrupted, the interior temperature of the heat treatment chamber can be returned to the steady state value by returning the temperature of the heat generating portion of the heater to a predetermined value. (5) When the gas supply quantity at the wafer insertion is made large, the exterior atmosphere which would otherwise go along with the wafer can be blown off and hence the amount of mixing exterior atmosphere can be made small. When the gas supply quantity at the retrieval of the wafer is suppressed low, convection inside the heat treatment chamber can be suppressed to reduce the mixing of the exterior atmosphere. Second, the technical advantages may be described as follows. (1) When two wafers are simultaneously heat-treated using a high temperature furnace, two wafers can be equally heated and the heat generation of the heater in the directions of the wafer face can be controlled. Thus, it becomes possible to keep the wafer face at a uniform temperature including the transient state and to achieve uniform heat treatment in a short time period. (2) The high temperature wafers which have completed the heat treatment can be cooled uniformly without directly exposing them to the exterior atmosphere. There may be no fear of wafer contamination, and the yield of the heat treatment can be improved. (3) It becomes easy to load and unload two wafers. By intervening a disk, which has a varying thickness along the radial direction, between the wafers, the temperature distribution in the wafer face in the transient state can be reduced. Thus, it becomes possible to achieve uniform heat treatment in a short time period without causing thermal stress defects. In the case as shown in Fig. 18, the temperature difference in the wafer face at insertion can be reduced to about a half and the yield of the heat treatment can be improved. (4) The heat treatment quantity for the wafers over a number of wafer insertions can be uniformalized. For example, in the case as shown in Fig. 15, the effective heat treatment temperature is lowered by 4°C by ten times of wafer insertions without control of the set temperature, whereas it can be suppressed within a temperature dispersion of 2°C with control of the set temperature. (6) The amount of mixing exterior atmosphere can be always made small including the times of insertion and retrieval. Thus, it can be prevented to bring dusts-containing oxygen into the process tube and the yield of the heat treatment can be improved. Employing the furnace of the invention, it is possible to uniformly heat semiconductor wafers in a short period, and to uniformly cool them rapidly without exposing them to the external atmosphere. There is provided a semiconductor heat treatment apparatus of high quality and high efficiency.

Claims: A high temperature furnace (2) for performing heat treatment on semiconductor wafers (16) comprising: heater means (4) consisting of a pair of parallel, vertically disposed flat plate resistance heaters positioned within a heat insulating member (6) and defining a vertically extending heating zone therebetween which has an aperture at a lower portion thereof for inserting and retrieving the semiconductor wafers, said flat plate resistance heaters being disposed either side of and substantially parallel to the surfaces of the semiconductor wafers when said wafers are positioned in the furnace for heating, said flat plate resistance heaters further being divided into a plurality of heat generating portions (4a-4e, 4f-4j); controlling means (44a-44d; 46a-46d; 48a-48d) for independently controlling the temperatures of the respective heat generating portions (4a-4e, 4f-4j); and support means for supporting one or two semiconductor wafers (16) and inserting and retrieving the wafers (16) into and from said heating zone. The apparatus according to claim 1, characterized in that the heater means (4) is divided into central (4b, 4g), upper (4a, 4f), lower (4c, 4h) and side (4d, 4e, 4i, 4j) portions. The apparatus according to claim 1 or 2, characterized in that a process tube (10) is disposed in the heating zone having a lower portion (18) projecting downwards from the high temperature furnace (2), said process tube (10) having a cooling zone at a lower portion thereof for cooling semiconductor wafers (16) without direct contact with the exterior atmosphere. The apparatus according to one of the claims 1 or 2, characterized by a process tube (10) disposed in the heating zone and a structure (54, 60, 64) for supplying a pre-heated process gas to flow downwards in the process tube (10). The apparatus according to claim 1, characterized in that the support means (14) is adapted to support two semiconductor wafers (16) substantially vertically and in parallel. The apparatus according to claim 5, characterized in that an intervening plate (22) is adapted to intervene between two semiconductor wafers (16). The apparatus according to claim 6, characterized in that the intervening plate (22) has a disk shape of the same diameter as the wafers to be heated which is thicker at the circumferential region. The apparatus according to claim 1, characterized in that the support means (14) includes a support portion (26) for supporting semiconductor wafers and a support pillar (28) for mounting the support portion (26) on one end thereof, said support pillar (28) has a hollow structure formed of a thin member and an opening (38) bored in said hollow structure at a position which will not be inserted in the heating zone. The apparatus according to claim 5, characterized in that the support means (14) includes a ring (120) having a diameter a little larger than that of the semiconductor wafers (16) to be contained in the heating zone, said ring (120) has a looped strip shape, the strip has a width larger than a gap to be formed between two loaded semiconductor wafers (16), and the strip is disposed at such a position to shield the gap to be formed between two loaded semiconductor wafers. The apparatus according to one of the claims 1 to 9, characterized in that the controlling means (44a-44d, 46a-46d, 48a-48d) for independently controlling the temperatures of the heat generating portions (4a-4e, 4f-4j) includes temperature sensors (44a-44d) for detecting the temperature of a heat generating portion (4a-4j) of the heater means (4), PID thyristor controllers (46a-46d) for adjusting the temperature of the heat generating portions (4a-4j) to a set value based on the detected value, and heater power source means (48a-48d) or supplying a power to the heater means and a correction means (42, 50, 52) for correcting said set values based on the temperature of the heating zone is provider, said correction means (42, 50, 52) includes a heating zone temperature sensor (50) for detecting the temperature of the heating zone, and a heating zone temperature controller (52) for correcting said set value of the temperature of the heat generating portions (4a-4j) of the heater means (4) based on the detected value. The apparatus according to one of the claims 1 to 9, characterized by correction means (42, 114, 116) for correcting the temperature values set by the controlling means (44, 46, 48) based on a surface temperature of the semiconductor wafer (16) to be heat-treated, said correction means (42, 114, 116) includes a contactless type radiation thermometer (114) disposed outside the high temperature furnace (2), a prism (110) and a mirror (112) for a guiding radiation emitted from a semiconductor wafer (16) in the heating zone to the radiation thermometer (114), heat treatment controller (116) and electrical interconnection for supplying the output of the radiation thermometer (114) representing the detected surface temperature of he semiconductor wafer (16) to the heat treatment controller (116). The apparatus according to claim 4, characterized by gas quantity control means (60,64) for controlling the supply quantity of a treatment gas supplied to said heating zone at a high rate at a time of inserting semiconductor wafers (16) and at a reduced rate for other occasions.

Inventors: HIRASAWA SHIGEKI, HONMA KAZUO, KOMATSU TOSHIHIRO, NAGATOMO HIROTO, SAKAI AKIHIKO, TAKAGAKI TETSUYA, TORII TAKUJI, UCHINO TOSHIYUKI, WATANABE TOMOJI, HIRASAWA, SHIGEKI, HONMA, KAZUO, KOMATSU, TOSHIHIRO, NAGATOMO, HIROTO, SAKAI, AKIHIKO, TAKAGAKI, TETSUYA, TORII, TAKUJI, UCHINO, TOSHIYUKI, WATANABE, TOMOJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1988
Main classification: H01L21
IPC codes: H01L21