Method and Apparatus for Heating Semiconductor Wafers

The present invention relates to a method for heating semiconductor wafers and to an apparatus for carrying out such a method.

Processes for manufacturing semiconductor devices involve steps in which an impurity is diffused into a wafer, or a coating is formed on at least one surface of a wafer. Generally, such a diffusion of an impurity or a formation of a coating is effected by heating a wafer in an atmosphere within a reaction tube. Heating is achieved by a furnace surrounding the tube. More specifically, an elongated reaction tube is horizontally placed in a furnace surrounding the effective length of the reaction tube, and heated to a predetermined temperature. An appropriate gas, such as N.sub.2, O.sub.2 or H.sub.2, is allowed to pass through the reaction tube to establish a desired atmosphere. Arrays of wafers supported by a holder are then introduced into the reaction tube from one end thereof, allowed to stay there for a predetermined period of time, and then withdrawn from the same end of the tube. By such a heating procedure, an even heating cannot be effected because the integrated heat quantity is different not only from wafer to wafer but, also, from portion to portion in each wafer.

Solutions to the above-mentioned problem are proposed in Japanese Patent Publications No. 43-26809 (claiming a convention priority from a U.S. application Ser. No. 529,288, of Feb. 23, 1966), published on Nov. 18, 1968, No. 48-22540, published on July 6, 1973, and No. 49-26457, published on July 9, 1974. According to these prior art solutions, wafers are passed on wafer holders through an elongated reaction tube heated by a furnace surrounding the tube. Such a prior art approach poses, however, the following problems, which are practically very difficult to overcome. First of all, the wafer holders must be provided with certain mechanical means, such as pinions, for their movement through the heated reaction tube. Such mechanical means must be made of a material resistive to a high temperature at which the wafers are to be processed. If the processing temperature is higher than about 900.degree. C., refractory metal materials can not be used for the construction of such mechanical means, because the intended doping is adversely affected due to impurities from such metallic materials. For processing at high temperatures ranging, for example, from 1100.degree. C. to 1250.degree. C., materials such as, quartz, SiC, polycrystalline silicon or SiN.sub.4, are required. Due to significant thermal distortion at high temperatures and poor fabrication precision of such materials, a smooth movement of the wafers through the high temperature zone is not ensured. Secondly, since the wafers and holders are always moving together with the mechanical means, formation of dust is unavoidable, leading to a reduction in the yield (i.e., an increased production of faulty products). Thirdly, if an endless transporter conveyor system is adopted for the purpose of mass production, it is difficult to protect the wafers and holders from dust when they are moving outside the furnace. For the reasons discussed above, the prior art system in which wafers are moved through an elongated reaction tube, has a limited application and is not applicable to cases wherein a shallow diffusion is desired, as is the case with annealing after ion implantation, or to cases wherein a quick processing at high temperatures (e.g., 1100.degree. C. to 1250.degree. C.) is intended.

An object of the invention is to provide an improved method for evenly heating semiconductor wafers, which has unlimited applications without suffering from the problems discussed above.

Another object of the invention is to provide a novel apparatus for evenly heating semiconductor wafers.

In one aspect of the invention there is provided a method for heating semiconductor wafers wherein semiconductor wafers are externally heated in an atmosphere within an elongated reaction zone, characterized in that a heating means surrounding a part of the length of the reaction zone is moved so that it will pass over each wafer placed in the reaction zone, thereby evenly heating the wafers.

In another aspect of the invention there is provided an apparatus for heating semiconductor wafers comprising an elongated reaction tube, a holder for supporting wafers to be heated, a furnace for externally heating the reaction tube, and means for introducing the holder into the reaction tube and for withdrawing the holder from the reaction tube, characterized in that said furnace surrounds a part of the length of said reaction tube and that means for moving said furnace along said reaction tube is further provided.

FIG. 1 is a schematic axial section of a reaction tube for illustrating a conventional method for heating wafers. The temperature distribution provided by the fixed electric furnace which is used to heat the tube is also shown in FIG. 1.

FIG. 2 is a schematic axial section of a reaction tube for illustrating a method according to the invention. The temperature distribution provided by the movable electric furnace which is used to heat the tube, when it is stationary, is also shown in FIG. 2.

FIG. 3 is a schematic vertical cross-sectional view of an apparatus in accordance with one embodiment of the invention.

FIG. 4 is a schematic vertical cross-sectional view of an apparatus in accordance with another embodiment of the invention.

Referring to FIG. 1, in a conventional method an elongated reaction tube 1 is horizontally placed in a fixed electric furnace (not shown) surrounding the tube and heated to a predetermined temperature. The furnace is such that it provides a heating zone exhibiting a temperature profile having a flat portion D, as shown in FIG. 1. A gas 5 to establish a desired atmosphere is then allowed to pass through the reaction tube 1. A wafer holder 2 carrying three arrays of wafers 3 is then introduced into the reaction tube 1 from a direction indicated by an arrow Q. All the wafers should be placed in those areas of the reaction tube 1 which correspond to the flat portion D of the temperature profile of the furnace. At the end of a predetermined processing time, the wafer holder 2 carrying the arrays of wafers 3 is withdrawn from the heated reaction tube in a reverse direction indicated by an arrow Q'. In this heating procedure, it will be understood that wafers in the left end array receive more heat than those in the right end array. Furthermore, the integrated heat quantity is different from portion to portion in a single wafer. Uneven heating of wafers results in variations in the sheet resistivity, thickness of oxide coating, current amplification factor, cut off frequency and/or other properties of the semiconductor device products.

The method of the invention will now be described with reference to FIG. 2, in which the same numerals designate similar types of parts as those shown in FIG. 1. In a method in accordance with the invention a gas 5 is allowed to pass through a horizontal elongated reaction tube 1. The tube 1 may be at ambient temperature, or may be preheated to a temperature of several hundreds degrees C., at which the intended reaction does not yet take place. A wafer holder 2 carrying arrays of wafers is then introduced into the center of the reaction tube 1 from one end of the tube 1. A movable electric furnace 4 surrounding a part of the whole length of the reaction tube, which is movable along the reaction tube 1 is initially placed at that end of the tube 1 into which the wafers 3 have been introduced and then caused to move in the direction designated by the three arrows in FIG. 2, so that it will pass over each wafer placed in the tube 1 and reach the other end of the tube 1 as shown by the broken lines. By this heating procedure according to the invention, the wafers can be evenly heated. While an electric furnace is preferred, other furnaces such as a high frequency furnace and an infrared furnace can also be used in the practice of the invention. In preferred embodiments, a heating zone formed by the furnace 4, when it is stationary, is such that it exhibits a temperature distribution or profile along the length, as shown in FIG. 2, having a distinct peak at the center of the temperature profile. It is preferable to move the furnace at a constant speed. In these preferred embodiments, properties of the products including, for example, the deepness of diffusion, sheet resistivity, and thickness of oxide coating, may be controlled by appropriately selecting the peak temperature of the furnace and the moving speed of the furnace. In most cases, the peak temperature of the furnace is selected within the range between 1,000.degree. C. and 1,250.degree. C., and the moving speed of the furnace within the range between 10 cm/min and 100 cm/min. In some embodiments, the furnace may be caused to move from one end of the reaction tube to the center of the tube, where it may be stopped for a certain period of time, and thereafter, the furnace may be caused to move from the center of the furnace to the other end of the furnace. For such a heating procedure, the furnace must be sufficiently long to form a heating zone exhibiting a temperature distribution along the length thereof with a flat portion, as shown in FIG. 1. Such a heating procedure is not preferable, although it is not impossible to carry out.

The apparatus for heating wafers shown in FIG. 3, in accordance with one embodiment of the invention, comprises a horizontal elongated cylindrical reaction tube 10. The tube 10 is made of quartz glass and has caps 11 of quartz glass detachably mounted at both ends thereof. The cap 11 is attachable to one end of the tube 10, by a ground glass fitting 12 forming a gas tight connection. When the cap 11 is detached from the end of the tube 10, the tube 10 opens at that end to a space 13 within a clean bench generally designated by the numeral 14. There is a table 15 within the clean bench 14, on which a worker may handle wafers before and after the treatment. Each cap 11 is provided with a gas inlet pipe 16 and a gas outlet pipe 17. The inlet pipe 16 is connected to a pipe 19 by a joint 18, and the pipe 19 is branched into three pipes, each being communicated, via a valve 20, with each gas source (not shown). The outlet pipe 17 is communicated with a gas exhaust system (not shown) via a joint 21 and valve 22. In each clean bench there is provided means for charging and discharging the reaction tube 10 with a boat 23 carrying wafers 24. Such means may comprise a hollow rotating shaft 25 driven by a motor 26 at one end of the shaft via a worm gear (not shown), a threaded nut 27 axially mounted to the shaft 25 at the other end of the shaft, and a screw rod (not shown) engaged with the nut 27. The rotation of the shaft 25 may be converted to the linear movement of the screw rod, which may be transmitted to a quartz rod (not shown) attached to the screw rod in parallel so as to push or pull the boat 23. The apparatus is further provided with an electric furnace 28 surrounding a part of the length of the reaction tube 10 for heating that part of the tube surrounded by the furnace. Endless chains 29 driven by a motor (not shown) may drive the furnace 28 along the reaction tube 10. When the furnace 28 is stationary it may form a heating zone exhibiting a temperature distribution with no flat portions along the length of the mobile, as shown in FIG. 2.

In the operation of the apparatus shown in FIG. 3, the furnace 28, which is placed at one end (the left end, in the illustrated case) of the reaction tube, is caused to start to heat.

In one clean bench remote from the furnace (the right one, in the illustrated case) the cap 11 is detached from the reaction tube 11 by disconnecting the joints 16 and 21. The wafer boat 23 is withdrawn from the tube 10 and, then, loaded with wafers 24 to be processed. The boat 23 is then placed in the center of the tube 10, and the cap 11 is attached again to the tube in the right clean bench. The required gas is caused to flow through the tube, from the left to the right in the illustrated case, by appropriately adjusting the valves 20 and 22. The furance 28 is then caused to move at a predetermined speed along the reaction tube 10 until it reaches the right end of the tube. The cap 11 in the left clean bench is then removed from the tube, and the boat 23 is withdrawn from the tube to the left clean bench in order to remove the treated wafers. In the next run, fresh wafers may be inserted from the left clean bench and the treated wafers may be removed at the right clean bench. In this run, the furnace 28 is cause to travel along the tube 10 from right to left under a gas flow from right to left.

FIG. 4 illustrates an apparatus in accordance with another embodiment of the invention. In this embodiment, the apparatus comprises a reaction tube 10' having a cap 11' detachably mounted to one end thereof. The cap 11' has only a gas outlet pipe 17 communicated with the gas exhaust system in the same manner as hereinabove described with reference to FIG. 3. The reaction tube 10' has a gas inlet pipe 16 at the other end thereof, which inlet pipe 16 is communicated with the gas supply system in the manner as described with reference to FIG. 3. In FIGS. 3 and 4, the same numerals designate corresponding parts.

In the operation of the apparatus shown in FIG. 4, after the tube 10 is provided with wafers from the clean bench 14, which is located at the right side of the tube, and after the establishment of a gaseous atmosphere in the tube by passing the gas through the tube from left to right, the furnace 28 is caused to travel along the tube from left to right and then from right to left for the processing of one lot of wafers. The wafers so processed are then withdrawn from the reaction tube to the clean bench 14.

While the dimensions of the parts of the apparatus are not critical, the furnace 28 can be as short as about 50 cm in length for the reaction tube 10 of a length of about 2 to 3 m.

While the method and apparatus according to the invention are generally useful for evenly heating semiconductor wafers, they are particularly suitable for use in the treatment of wafers at a high temperature for a short period of time, as is the case with annealing after ion implantation and a solid-to-solid diffusion. For example, in annealing after implantation of 1.times.10.sup.14 boron ions/cm.sup.2 into a n-type (111) Si substrate of 1 .OMEGA.m, the junction depth of the order of 3000 .ANG. and the sheet resistivity of 950 .OMEGA./square can be achieved by a conventional method using the annealing conditions of 1000.degree. C., 30 minutes, in N.sub.2. Whereas, in accordance with the invention, the same order of the junction depth and the sheet resistivity of 900 .OMEGA./square can be achieved using the peak temperature of 1180.degree. C. and the moving speed of 30 cm/min. This reveals that better activation of implanted atoms is achievable in accordance with the invention than with the conventional method. In a quick diffusion technique involving rapid heating and rapid cooling of a single wafer, a wafer 4 inches in diameter tends to crack if it is repeatedly subjected to such a quick diffusion processing 3 to 4 times. This is not the case if such a wafer is treated in accordance with the invention. In still another example of an emitter diffusion based on solid-to-solid diffusion of P from phosphrus silicate glass in O.sub.2, the same orders of the current amplification factor and the cut off frequency can be achieved either by a quick diffusion technique with the furnace temperature of 1150.degree. C., the boat loader speed of 300 cm/min and the diffusion time of 90 seconds, or by a method of the invention with the peak temperature of 1180.degree. C. and the moving speed of the furnace of 25 cm/min. The total processing time required for the treatment of 50 wafers is about 90 min. for the quick diffusion technique and about 10 min. for the method of the invention.