Quartz glass heat treatment apparatus for semiconductor wafers and method for producing same

Abstract: A reaction tube 10 without occurrence of crack or breakdown or a wafer boat 20 without generation of particles is provided, such various faults being caused by the difference between the thermal expansion coefficients of a thin film and one of those quartz glass jigs. The present invention is applied to a quartz glass jig used in a thermal CVD apparatus, and is characterized in that the jig is made of transparent quartz glass material, and that the portion or portions exposed to the heat treatment space or the portions contacting with wafers 30 are sand-blasted. The reason why transparent quartz glass is limited as the material is that, in case of transparent quartz glass, the surface hardness is more uniform in comparison to bubble-mixed or opaque quartz glass and thus a predetermined surface roughness is more effectively achieved by sand-blasting, so that the faults as mentioned above due to the stress occurring in the treatment between a CVD film and one of the quartz glass jigs are effectively prevented.

Description: The present invention relates to a heat treatment apparatus of the type comprising a quartz glass reaction tube (hereinafter referred to as a reaction tube), a quartz glass wafer boat (hereinafter referred to as a wafer boat) for supporting wafers during heat treatment in the reaction tube, and a heater for heating the wafers mounted in the apparatus. The invention is particularly concerned with heat-treatment apparatus for semiconductor wafers, such as a CVD apparatus. More particularly, the invention is concerned with a thermal CVD apparatus. The invention also relates to a method of producing such a heat treatment apparatus. Conventionally a CVD apparatus is well known in which a chemical reaction in an activated space is used to form a thin film on a wafer or wafers. There are, for example, a thermal CVD apparatus, a plasma CVD apparatus and a photo-assisted CVD apparatus which respectively use heat, plasma and ultraviolet light or laser as activation energy sources. In a thermal CVD apparatus, vapor phase chemical reaction (for example, thermal decomposition, hydrogen reduction, oxidation or substitution reaction) at a high temperature of a volatile metal halogenide, a volatile metal organic compound or the like is conducted to form an epitaxial film made of semiconductor such as silicon or a metal having a high melting point, or an insulating film or a protection film made of an oxide of semiconductor such as silicon on a wafer or wafers. A reaction tube and a wafer boat used in such CVD apparatuses are generally made of quartz glass having regard to requirements for chemical stability and heat resistance. In the vapor phase growth, since the surface or surfaces of a quartz glass reaction tube and wafer boat are exposed to a vapor phase growth space in the apparatus, a thin film or films adhere on the surface or surfaces of the reaction tube and wafer boat as well as the surface of a wafer or wafers. A reaction temperature of the vapor phase chemical reaction required for thermal decomposition of SiH 4 , which is used for formation of a Si thin film in a thermal CVD method, is in the range of 500°C to 1100°C. A reaction temperature of the vapor phase chemical reaction for reduction of SiCl 4 is up to 1200°C. Furthermore, because the thermal expansion coefficients of a Si film and quartz glass, which is the material of a reaction tube, are different from each other by one figure or so, there is a danger of the film separating due to stress caused by the difference between the expansion coefficients, when a CVD film is thin. On the other hand, when the CVD film is thick and the stress affecting the quartz glass of the reaction tube is larger, there is a danger of breakage of the reaction tube. In the case of a wafer boat in the reaction tube where wafers are disposed in a stacking manner at respective predetermined positions, boat marks (which means poor exterior appearance observed on the surface or surfaces of a wafer caused by contacting with the boat and sometimes accompanied by chipping along the periphery of a wafer) often occur at a contacting portion or portions between the periphery of each of the wafers and the boat, because the peripheries of the wafers and the boat are in the contacting conditions of planes or lines and move over each other with friction due to the difference between the thermal expansion coefficients. Attention is also drawn to the disclosure of Patent Abstracts of Japan, vol. 15, No. 394 (E1119); and JP-A-03-159128. However, this prior art reference relates to sand-blasting a wafer, and not a wafer boat or reaction tube. The present invention was made to overcome the disadvantages of the prior art. It is an object of the present invention to provide a heat-treatment apparatus for semiconductor wafers, in particular a CVD apparatus, and a method for producing the same, that can overcome various disadvantages caused by the difference between the thermal expansion coefficients of the aforementioned thin film and the quartz glass reaction tube and wafer boat. It is another object of the present invention to provide a reaction tube to be used in a CVD apparatus in which neither cracking nor breakage occurs, even when a thin film adheres on the internal wall or walls of the reaction tube. It is a further object of the present invention to provide a wafer boat to be used in a CVD apparatus in which neither boat marks on nor chippings along the periphery of a wafer or wafers occur. The present invention in one aspect provides a heat treatment apparatus according to claim 1. The invention in another aspect provides a method of producing a heat treatment apparatus as defined in claim 3. The reason why the material of the reaction tube and the wafer boat is limited to transparent quartz glass is that transparent quartz glass has a more uniform surface hardness in comparison with bubble-mixed or opaque quartz glass and a desired surface roughness is effectively achieved in sand-blasting, so that the problems resulting from stress occurring between the CVD film and a treated surface or surfaces of the reaction tube and the wafer boat are effectively prevented. The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a schematic view of a CVD apparatus to which the present invention is applied; FIG. 2(a) is a schematic front view of a vertical wafer boat to which the present invention is applied; FIG. 2(b) is a cross-sectional view taken on line X - X of FIG. 2(a); FIG.3(a) is an enlarged cross-sectional view of a portion of a reaction tube where a thin film is adhered, and FIG. 3(b) is an illustrative view of a technology adopting sand-blasting according to the present invention; and FIG.4(a) is an enlarged cross-sectional view showing a structural detail of a contacting state between a wafer and a holding groove which supports the wafer, and FIG.4(b) is a illustrative view of a technology adopting sand-blasting according to the present invention. In the present invention, in the case of a reaction tube 10 used in a thermal CVD apparatus in FIG. 1, at least the portion or portions which are heated by a heater 2 of the internal wall 10a are sand-blasted and in the case of a wafer boat 20 used in a thermal CVD apparatus in FIG.2, at least the portions of the wafer boat 20 that contact with each wafer 30 are sand-blasted. According to the present invention, the roughness average ( Ra ) of the surface after sand-blasting is in the range of 1 µm to 20 µm, more preferably 2 µm to 10 µm, still more preferably 3 µm to 5 µm. It is still more preferable that the maximum height ( Rmax ) is in the range of 10 µm to 30 µm, while the roughness average (Ra) is in the range of 3 µm to 5 µm. Abrasive material used in sand-blasting is made of a material or a mixture of materials that are harder than quartz glass and which do not become a contamination source or sources, for example crystallized quartz powders or SiC powders, and the grit size distribution is preferably such that the weight in the range of 100 µm to 400 µm in grit size is 90 % of the total weight. Such a function of the present invention will be explained with reference to FIG.3 (a) and FIG.3 (b)below: For example, a case that a polysilicon film 40 adheres on the internal wall surface10a of a reaction tube 10 in a CVD method is here considered referring to FIG.3 (a). Since the linear expansion coefficient of Si is 4.7 x 10 -6 /°C to 4.9 x 10 -6 /°C and that of SiO 2 is 5.5 10 -7 /°C, thermal contraction of the polysilicon thin film 40 occurs, but that of quartz occurs almost not at all, when the temperature is lowered from anywhere in the range of 550°C to 770°C to room temperature of 20°C during CVD growth. In this drcumstance, the polysilicon thin film 40 causes a compressive stress A in the quartz glass 10 and on the other hand the quartz glass 10 causes a tensile stress B in the polysilicon thin film 40. The thermal stress A, which the polysilicon thin film 40 gives to the quartz glass 10, accordingly is obtained from the following equation: A = E (α 1 - α 2 ) Δ T = ( 1.9 ~ 2.4) x 10 8 dyne/cm 2 ,where E: Young's modulus of polysilicon thin film 40 ( 0.9 x 10 11 dyne/cm 2 ) α 1 : linear expansion coefficient of polysilicon thin film 40 ( 4.5 x 10 -6 /°C) α2: linear expansion coefficient of quartz ( 5.5 x 10 -7 /°C) ΔT: ( 550 ~ 700) - 20 = 530 ~ 680°C If the thermal stress A is ( 1.9 ~ 2.4 ) x 10 8 dyne/cm 2 , this value exceeds the design stress for a reaction tube and breakage thereof thus becomes a real risk, in the case that the dimensions of the reaction tube are respectively 350 mm in diameter and 4 mm ~ 5 mm in wall thickness. When the surface of the quartz glass 10 is sand-blasted, the surface is roughened to have a level of irregularity as shown in FIG.3(b). If a CVD treatment is carried out in this state, a polysilicon thin film 40 is formed on the surface in conformity with the roughened surface profile. Such a polysilicon thin film 40 forms a compression stress B in the surface of the quartz glass which has a serrate contour in correspondence to the irregularity of the original surface of the quartz 10. This causes divergence of the stress in the quartz glass. On the side of the quartz glass 10, the peaks in the surface irregularity of the quartz glass 10 do not have enough strength and, therefore, with the stress A occurring on the side of the quartz glass 10, only the peaks are broken and the propagation of the stress into the bulk is stopped within the vicinity of the surface, so that breakage and cracking of the quartz glass can be prevented. In the case of a wafer boat installed in the aforementioned reaction tube in which wafers 30 are disposed in a stacking manner at respective predetermined positions, the wafers 30 have each a linear contact relation in a holding groove 21, which is formed in the wafer boat 20 together with a plurality of other similar grooves 21. The difference therebetween in the thermal coefficients causes boat marks on or chippings at the peripheries of the wafers 30. If a wafer boat is made of transparent quartz glass material which has excellent flatness and smoothness, these disadvantages are further increased, because of occurrence of a linear contact relation, which puts a wafer in a closer geometrical condition with the boat. By sand-blasting the internal surfaces of a holding groove 21 mentioned above, which is formed in the wafer boat 20, since the wafer 30 is placed on the peaks of the surface irregularity as illustrated in FIG.4(b), the wafer 30 and the wafer boat 20 are in a point contact relation with each other at a point P, and the wafer 30 does not run a risk of having boat marks or chippings thereon due to divergence of the thermal stress acting in the surface of the wafer 30. A particular embodiment of the present invention will be described in detail below, with reference to the accompanying drawings. It should be understood that the sizes, materials, shapes, relative configuration and the like of the structural parts recited in the embodiment are not intended to limit the scope of the present invention and that they are only exemplified, unless otherwise specified. FIG.1 is a schematic view of a CVD apparatus to which the present invention is applied. A numeral 1 indicates a cylindrical furnace body, which is arranged almost in a vertical position, the top of which is sealed, the lower end of which is open, and on the internal wall of which a heater 2 is provided. A numeral 10 indicates a transparent quartz glass furnace tube, at the top of which a gas outlet pipe 3 is attached to project outwardly of the furnace body 1, the lowest portion of which is extended outwardly of the lower surface of the furnace body 1, and on the extended lowest portion of which a gas inlet pipe 4 is attached. In the reaction tube 10, there is a supporting plate 6 horizontally arranged on a supporting shaft 5, which is shiftable vertically and rotatable by a drive mechanism (not shown), and a wafer boat 20 made of transparent quartz glass, which is held on the supporting plate 6. The wafer boat 20 includes, as shown in FIG.2 (a) and FIG.2 (b), a disk-like bottom plate 201 and a disk-like top plate 202, which are disposed in a relation of one above the other. A pair of side poles 203 and a pair of back poles 204 are disposed between the bottom plate 201 and top plate 202. A plurality of holding grooves 21 are carved in and along the side poles 203 and back poles 204 so that the wafers 30 are horizontally disposed therein in a stacking manner. In such a well known CVD apparatus, the supporting shaft 5 is shifted downwardly to position the supporting plate 6 at a position under the furnace body, the wafer boat 20 loaded with the wafers 30 is then put on the supporting plate 6, and thereafter the supporting shaft 5 is shifted upwardly with the wafers 30 so as to hermetically seal the furnace tube 10 with a sealing plate 8 and at the same time to locate the wafer boat 20 at a predetermined position in the furnace tube 10. In this state, the atmosphere in the reaction tube 10 is replaced with an inert gas and then the pressure is adjusted under vacuum. Thereafter, a predetermined vapor phase growth is performed by supplying a reaction gas, while the temperature is controlled at a predetermined level in the range of 550 °C to 700 °C by the heater 2. In the apparatus, the diameter of the furnace tube was 350 mm and the thickness of the wall was 4 mm to 5 mm, and mono-silane gas as the reaction gas was supplied to conduct a predetermined vapour phase growth at a temperature of 550 °C to 700 °C. The heat-treatment was continued to adhere a polysilicon thin film 40 of 40 µm thickness on the reaction tube 10 and then the temperature was lowered to room temperature. The reaction tubes 10 were inspected in five experiments, and in each case cracking of the reaction tubes were confirmed to occur. The wafers which were treated in the same conditions as in the above experiments ( in batchwise operations each treating 60 wafers ) were inspected to determine whether or not boat marks occurred in each of the batches. The percentages of the wafers on which boat marks were observed in each of the batches were in a range of 40 % to 100 % of each of the 60 wafers. Then, in another group of experiments, the internal surfaces 10a of the reaction tubes 10 and all the surfaces of the wafer boats 20 including the inside of every holding groove were sand-blasted in the following conditions. A heat-treatment was conducted to adhere a polysilicon thin film of 40 µm thickness on the reaction tube 10 in the same conditions as the above experiments and then the reaction tubes 10 were inspected. In the five experiments, no cracking was observed in any of the reaction tubes used. The heat-treatment was continued in a further experiment until a polysilicon thin film of 80 µm thickness adhered on the reaction tubes 10 and the temperature was then lowered down to room temperature; no cracking was observed in any of the reaction tubes 10 inspected. The wafers treated in the conditions as mentioned above, which were treated in batch wise operations with each batch of 60 wafers, were inspected with regard to boat marks in each batch. No boat marks were observed in any of the batches. The used abrasive material was crystallized quartz powder, in other words rock crystal powder, and the grit size distribution is shown in Table 1. Grit Size Distribution of Crystallized Quartz Powders Grit Size Range µm Weight % ~ 300 5 300 ~ 250 21 250 ~ 212 33 212 ~ 180 6 180 ~ 150 23 150 ~ 106 10 106 ~ 2 The surface roughnesses were respectively measured at 10 points in total each on the internal wall surface 10a of one of the reaction tubes 10 and on the internal surfaces of ten wafer holding grooves 21. The roughness average (Ra) of the reaction tube 10 was 3.5 µm and the maximum height (Rmax) was 20 µm. The roughness average (Ra) of the wafer boat was 4.5 µm and the maximum height ( Rmax ) was 25 µm. The conditions for measurement of the surface roughness are as follows: The cut-off value is set at 0.8 mm. The reference length in determination of a maximum height is set at 2.5 mm. Then, crystallized quartz powders with a grit size distribution of 90 % by weight in the size range of 100 µm to 200 µm were used for sand-blasting for wafer boats 20. In the same CVD treatment as described above, heat-treatments of 4 batches each composed of 60 wafers were carried out. The wafers were inspected for boat marks in each of the batches and at most one wafer per batch showed a boat mark or boat marks thereon, which was a faint one. In this case, Ra was 1.8 µm and Rmax was 15 µm, where the cut-off value was 0.5 mm and the reference length was 2.5 mm. Since a wafer held by a wafer boat 20 is so positioned as to have a peripheral edge of the wafer in line contact with the holding groove 21, there is a risk that particles are produced if the roughness average Ra on the surfaces of the grooves 21 exceeds 20 µm. Ra is more preferably set in the range of 2 µm to 10 µm, still more preferably 3 µm to 5 µm, and Rmax is preferably set in the range of 10 µm to 30 µm. According to the present invention, as described above, various problems caused by the different expansion coefficients between a thin film 40 and a quartz glass reaction tube and wafer boat are overcome and a favourable vapor phase growth can especially be conducted in a CVD apparatus. Further, in the case that the present invention is applied to a reaction tube 10, not only is cracking or breakage prevented, but also separation of a thin film 40 does not occur, even when the thin film 40 adheres on the internal wall surface of a reaction tube 10 in a CVD treatment. Still further, in the case that the present invention is applied to a wafer boat 20, a wafer boat 20 in which neither boat marks nor chipping on wafers 30, nor particles caused by chipping, occur can be provided.

Claims: A heat treatment apparatus (1) comprising: a transparent quartz glass reaction tube (10), a transparent quartz glass wafer boat (20) for supporting wafers (30) during heat treatment in the reaction tube (10), and a heater (2) for heating the wafers (30) mounted in the apparatus, characterized in that at least the surface portions of the internal wall (10a) of the quartz glass reaction tube (10) heated by the heater (2) comprises a sand-blasted surface, and in that at least the surface portions of the wafer boat (20) contacting each wafer (30) comprises a sand-blasted surface, and in that the said sand-blasted surface portions each have an average roughness (Ra) in the range of 1 µm to 20 µm. A heat treatment apparatus (1) as claimed in claim 1, characterized in that the sand-blasted surface portions each have a roughness in the range of 2 µm to 10 µm in roughness average (Ra). A method of producing a heat treatment apparatus (1) comprising a transparent quartz glass reaction tube (10), a transparent quartz glass wafer boat (20) for supporting wafers (30) during heat treatment in the reaction tube (10), and a heater (2) for heating the wafers (30) mounted in the apparatus, characterized by comprising the step of sand-blasting at least the surface portions of the internal wall (10a) of the quartz glass reaction tube (10) heated by the heater (2), and also at least the surface portions of the wafer boat (20) contacting each wafer (30) with an abrasive material which is harder than quartz glass and which does not become a contaminating source, so that the said sand-blasted surface portions each have an average roughness (Ra) in the range of 1 µm to 20 µm. A method as claimed in claim 3, characterized in that the abrasive material comprises one or more of crystallized quartz powder and SiC powder.

Inventors: TAKAKU TOSHIAKI, TAKAKU, TOSHIAKI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1995
Main classification: H01L21
IPC codes: H01L21, C03B20