Vitreous silica crucible

Abstract

A vitreous silica crucible includes: a substantially cylindrical straight body portion having an opening on a top end and extending in a vertical direction; a curved bottom portion; and a corner portion connecting the straight body portion with the bottom portion and a curvature of which is greater than that of the bottom portion, wherein an inner surface of the crucible has a concavo-convex structure in which groove-shaped valleys are interposed between ridges, and an average interval of the ridges is 5-100 m.

Claims

1. A vitreous silica crucible comprising; a substantially cylindrical straight body portion having an opening on a top end and extending in a vertical direction; a curved bottom portion; and a corner portion connecting the straight body portion with the bottom portion; and a curvature of which is greater than that of the bottom portion, said vitreous silica crucible characterized in that, an inner surface of the crucible has a concavo-convex structure in which groove-shaped valleys are interposed between ridges, and an average interval of the ridges is 5-100 m.

2. The vitreous silica crucible according to claim 1, wherein the valleys extend substantially in a circumferential direction of the straight body portion.

3. The vitreous silica crucible according to claim 1, wherein a central line average roughness Ra of the inner surface of the crucible is 0.02-0.5 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing a state of an objective lens 10 scanning on an inner surface 11 of a vitreous silica crucible.

(2) FIG. 2 is a cross section view of a vitreous silica crucible 12, and is a schematic view illustrating a scanning direction of the objective lens.

(3) FIG. 3 is a surface photo of an inner surface of a vitreous silica crucible without formation of a concavo-convex structure, obtained by a confocal laser microscope.

(4) FIG. 4 is a surface photo of an inner surface of a vitreous silica crucible with formation of a groove-shaped concavo-convex structure, obtained by a confocal laser microscope.

(5) FIG. 5 is a three-dimensional image of an inner surface of a vitreous silica crucible with formation of a groove-shaped concavo-convex structure, measured and treated using a confocal laser microscope.

(6) FIG. 6 is a graph showing the results measuring the height of the inner surface of the vitreous silica crucible from position A to position B in FIG. 5. The reference point (Z=0) was set so that the height of the inner surface fell within Z=0-2 m.

MODES FOR CARRYING OUT THE INVENTION

Vitreous Silica Crucible

(7) A vitreous silica crucible 12 according to the present invention, for example, as shown in the cross section view of FIG. 2, comprises: a substantially cylindrical straight body portion 15 having an opening on a top end and extending in a vertical direction; a curved bottom portion 16; and a corner portion 17 connecting the straight body portion 15 with the bottom portion 16 and the curvature of which is greater than that of the bottom portion 16.

(8) The vitreous silica is preferred to include a transparent layer 20 on the inside and a bubble layer 14 on the outside thereof. The transparent layer 20 is a layer formed on the inside of the vitreous silica crucible, and is substantially bubble-free. Substantially bubble-free means a bubble content rate and bubble diameter at such a degree that a single-crystal yield does not decrease due to the bubbles. Here, the bubble content rate means the volume of the bubbles occupying a unit volume of the crucible. The image of the crucible inner surface is taken by an optical camera, and the crucible inner surface is divided based on a constant volume as a reference volume W1. A volume W2 occupied by bubbles is determined for the reference volume W1, and calculated by P (%)=(W2/W1)*100. The bubble layer 14, for example, has a content rate of bubbles contained therein of 0.2% or more and 1% or less, and the average diameter of the bubbles is 20 m or more and 200 m or less.

(9) The inner surface of the silica crucible has a concavo-convex structure in which groove-shaped valleys are interposed between ridges. When a fine concavo-convex structure is provided to the inner surface of the crucible, not only is explosive boiling of the silicon melt prevented to suppress melt surface vibration, but also the contact area between the silicon melt and the inner surface of the crucible is increased and the friction resistance therebetween is enhanced, and thus melt surface vibration is suppressed. In addition, when a fine groove-shaped concavo-convex structure is provided at the inner surface of the crucible, even when SiO gas is generated, a small turbulent flow occurs at the concavo-convex portion to attenuate the energy, and thus melt surface vibration is unlikely to occur.

(10) In addition, the inner surface of the vitreous silica crucible is dissolved by a reaction between the crucible inner surface and the silicon melt. Therefore, oxygen is supplied to the silicon melt and this oxygen is mixed into the silicon single crystal and used to form a getter site. In the vitreous silica crucible of the present invention, since the contact area between the silicon melt and the crucible inner surface is increased, a reaction between the crucible inner surface and the silicon melt is likely to occur, oxygen can be efficiently supplied to the silicon melt, and thus a problem due to lack of oxygen can be prevented.

(11) The concavo-convex structure is preferably provided to the entire inner surface of the straight body portion of the crucible, from the viewpoint of preventing melt surface vibration. In addition, it is preferably provided to the entire crucible, especially a position lower than the initial melt surface during pulling of silicon single crystal, from the viewpoint of supplying oxygen.

(12) The groove-shaped valleys are preferred to extend substantially in the circumferential direction of the straight body portion of the crucible. When the valleys are formed along the circumferential direction, toward the top end or the bottom end of the straight body portion, the valley may be formed to be slightly inclined, and also may be formed to meander. When the valley extends in the circumferential direction, the contact resistances between the silicon melt and the crucible inner surface are increased especially, and melt surface vibration is effectively suppressed.

(13) The average interval between ridges is 5-100 m, preferably 20-60 m, more preferably 15-50 m. The interval between ridges is a distance from a top point to a top point of the ridges. The average interval is, more specifically, for example, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100 m, and it may be within the range between any two of the numerical values exemplified here. When the average interval is small, the contact area between the silicon melt and the crucible inner surface may be too large and the friction resistance therebetween may be too great to effectively suppress melt surface vibration. On the other hand, when the average interval is large, the contact area between the silicon melt and the crucible inner surface may be too small and the friction resistance therebetween may be too low to effectively suppress melt surface vibration. The intervals between ridges are preferably approximately equal intervals, for example, the interval between ridges is 15-50 m, preferably 20-30 m.

(14) The concavo-convex structure can be measured in a non-contact manner by using an optical detection unit comprising: a light-emitting apparatus for irradiating light on the inner surface of the vitreous silica crucible, and a light-receiving apparatus for receiving the reflection of light irradiated on the inner surface of the vitreous silica crucible. As the irradiation light, for example, any light such as visible light, ultraviolet light, infrared light, and laser light can be used, as long as the concavo-convex structure of the crucible inner surface can be detected.

(15) The light-emitting apparatus may be integrated into the optical detection unit, and in that case, those can be operated rotatably along the inner surface of the vitreous silica crucible are preferable. The light-receiving apparatus can be appropriately selected depending on the type of irradiation light, and, for example, an optical camera with a light-receiving lens and an image pickup unit can be used. For the purpose of detecting the concavo-convex structure of the inner surface, it is preferred that only light converging at the focal point is received by the light-receiving apparatus. For the purpose of receiving only the light converging at the focal point, it is preferred that the light-receiving apparatus includes, for example, a pinhole in front of the light detector.

(16) As for a more particular measurement method, first, as shown in FIG. 1, an objective lens 10 is placed at the inner surface 11 of the crucible 12 in a non-contact manner. Next, by scanning toward scanning direction 13, the concavo-convex structure can be determined. As other scanning modes, for example, sample scanning mode and laser scanning mode are exemplified. Sample scanning mode is a mode of driving a stage carrying the sample in the XY-direction to obtain a two-dimensional image. Laser scanning mode is a mode of applying a laser in the XY-direction to scan on the sample two-dimensionally. Any scanning mode may be employed. As for the scanning direction, for example, vertical direction 18 and horizontal direction 19 of the straight body portion 15 are exemplified. In addition, it is also possible to scan only a part of the inner surface of the crucible. For example, it is possible to focus on scanning an area around the melt surface position contacted with the seed crystal.

(17) By scanning as above, the focal point is scanned to obtain a two-dimensional image of the inner surface of the crucible. In addition, an image of a three-dimensional fine concavo-convex structure can be obtained by scanning in the wall thickness direction of the crucible (refer to FIG. 5). From the acquired image, it is possible to identify the direction of the groove-shaped valley. Also, the pitch (the average interval) between ridges may be quantified by scanning the focal point two-dimensionally, and measuring the ridges based on the brightness of the reflection. Further, for the sample, while scanning the focal point in the XY-direction, the Z-position information when focused is recorded and quantified, thus the height information of the sample can be obtained (refer to FIG. 6). These methods are preferable in that the scanning time can be shortened.

(18) The average interval between ridges is a value obtained by dividing the sum of the values of intervals between the ridges by the number of intervals between the ridges. The average interval can be obtained by, for example, processing the image of the fine concavo-convex structure obtained by the measurement method as above with software.

(19) The inner surface of the silica crucible has a concavo-convex structure, and the central line average roughness Ra is preferably 0.02-0.5 m, more preferably 0.05-0.4 m, and even more preferably 0.2-0.4 m. The central line average roughness Ra is, specifically, for example, 0.02, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5 m, and it may be within the range between any two of the numerical values exemplified here.

(20) The measured roughness curve is folded from the central line, and the area obtained by the roughness curve and the central line is divided by the length L to obtain a value from which the central line average roughness Ra can be calculated. The roughness curve and the like, can be measured in the same manner as the groove-shaped concavo-convex structure, and calculated by processing with software.

(21) The silica crucible of the present invention has a groove-shaped concavo-convex structure, as described above, even though having a fine concavo-convex structure with a central line average roughness Ra in a predetermined range, it can effectively suppress vibration of the melt surface.

Method of Manufacturing Vitreous Silica Crucible

(22) Next, an embodiment of the method for manufacturing a vitreous silica crucible according to the present invention will be explained.

(23) Silica powder used for manufacturing a vitreous silica crucible includes crystallized natural silica powder and amorphous synthetic silica powder manufactured by chemical synthesis. Natural silica powder is silica powder manufactured by pulverizing natural mineral mainly consisting of -quartz. The synthetic silica powder can be manufactured by means of chemical synthesis such as gas phase oxidation of silicon tetrachloride (SiCl.sub.4) (dry synthesis method), or hydrolysis of silicon alkoxide (Si (OR.sub.4) (sol-gel method).

(24) First, a natural silica powder is supplied to a mold used for a vitreous silica crucible. The natural silica powder can be manufactured by pulverizing natural mineral mainly consisting of -quartz. Next, a vitreous silica crucible comprising an inner face layer (synthetic layer) vitrified from synthetic silica powder and an outer face layer (natural layer) vitrified from natural silica powder, is manufactured by supplying a synthetic silica powder on the natural silica powder, and melting the silica powder by Joule heat of arc discharge followed by cooling. In the initial stage of the arc melting process, bubbles are removed by subjecting the silica powder layer to a strong depressurization, and thus a transparent vitreous silica layer (transparent layer) is formed, and subsequently, a vitreous silica layer (bubble layer) containing bubbles left by weakening the depressurization is formed. Here, the inner face layer formed from the synthetic silica powder is not necessarily the same as the transparent layer. Moreover, the outer face layer formed from the natural silica powder is not necessarily the same as the bubble layer.

(25) The melting of the silica powder is preferably performed so that the maximum temperature of the inner surface of the rotating mold is up to 2000-2600 deg. C. When the maximum temperature is lower than 2000 deg. C., the gas remaining as bubbles during the manufacture of the vitreous silica or in the vitreous silica cannot be removed completely, and the crucible may expand markedly during the pulling of silicon single crystal. In addition, when the maximum temperature is higher than 2600 deg. C., the viscosity of the vitreous silica is reduced and shape collapse may occur.

(26) Arc melting is performed, for example, by arc discharge of three-phase (R phase, S phase, T phase) alternating current. Therefore, in the case of three-phase alternating current, 3 carbon electrodes are used to generate arc discharge; and thereby the silica powder layer is melted. Arc melting starts the arc discharge at the position where the tip of the carbon electrode is positioned higher than the opening portion of the mold. Thus, the silica powder layer near the opening portion of the mold is melt preferentially. Thereafter, the carbon electrode is lowered to melt the silica powder layer of the straight body portion, the corner portion, and the bottom portion.

(27) When lowering the carbon electrode, by stepwise lowering it, a groove-shaped concavo-convex structure in which groove-shaped valleys are interposed between the ridges can be formed on the inner surface of the crucible. The lowering rate of the carbon electrode can be 10-35 mm/min, specifically, for example, 10, 13, 15, 17, 18, 20, 23, 25, 28, 30, or 35 mm/min, and it may be within the range between any two of the numerical values shown herein. The lowering rate may be an average value. Stepwise lowering means lowering while repeatedly lowering and stopping, for example, and may be pulse drive which repeats the lowering and stopping of the arc electrode. In this case, the pulse width is, for example, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 ms, and it can be in the range between two values of the values exemplified here. For example, when the lowering rate is 10-30 (mm/min), the pulse width may be 50-250 ms. The duty cycle can be, for example, 30-70%, specifically, 30, 40, 45, 50, 55, 60, 70%, and it may be within the range between any two of the numerical values exemplified herein. The duty cycle is preferably 45-55%, more preferably 50%, for the purpose of obtaining a constant interval between ridges.

(28) Moreover, by lowering while vibrating the carbon electrode, a concavo-convex structure in which the central line average roughness Ra is 0.02-0.5 m can be formed.

Example of Use

(29) The vitreous silica crucible according to the present invention, for example, can be used as follows.

(30) In a vitreous silica crucible, polysilicon is melted to produce silicon melt, and a silicon single crystal can be manufactured by pulling up while rotating a seed crystal with the tip of the silicon seed crystal being immersed in the silicon melt. The shape of the silicon single crystal is as follows: a cylindrical silicon seed crystal from the upper side, followed by a conical silicon single crystal, a cylindrical silicon single crystal having the same diameter as the bottom surface of the upper conical silicon single crystal, and a silicon single crystal having a vertex orienting downward.

(31) The pulling of silicon single crystal is usually performed at about 1420 deg. C. Particularly at an initial stage of the pulling, a melt surface vibration is likely to occur. The vitreous silica crucible of the present invention can suppress the occurrence of melt surface vibration since a particular concavo-convex structure is formed in the inner surface.

EXAMPLES

Example 1

(32) A vitreous silica crucible of Example 1 was manufactured on the basis of a rotating mold method. The carbon mold opening diameter was 32 inches (813 mm), the average thickness of silica powder layer deposited on the mold inner surface was 15 mm, arc discharge was performed with three electrodes using three-phase alternating current. The energization time of the arc melting process was 90 minutes, output was 2500 kVA, and the silica powder layer was depressurized 90 kPa from atmospheric pressure for 10 minutes from the start of energization. During arc melting, the carbon electrode stepwisely descended. The average descending rate was 20 mm/min, the pulse width was 100 ms, and the duty cycle was 50%.

Comparative Example 1

(33) The vitreous silica crucible of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the carbon electrode descended continuously (descending rate: 20 mm/min) during arc melting.

Comparative Example 2

(34) The vitreous silica crucible of Comparative Example 2 was manufactured in the same manner as in Example 1, except that the carbon electrode descended gradually with an average descending rate of 35 mm/min, a pulse width of 100 ms, and a duty cycle of 50%, during arc melting.

Comparative Example 3

(35) The vitreous silica crucible of Comparative Example 3 was manufactured in the same manner as in Example 1, except that the carbon electrode descended gradually with an average descending rate of 10 mm/min, a pulse width of 100 ms, and a duty cycle of 50%, during arc melting.

(36) The manufacture conditions of Example 1 and Comparative Examples 1-3 are partially shown in table 1.

(37) TABLE-US-00001 TABLE 1 Descending method of the Average carbon descending rate Pulse width Duty electrode [mm/min] [ms] cycle [%] Example 1 gradually 20 100 50 Comparative continuously 20 Example 1 Comparative gradually 35 100 50 Example 2 Comparative gradually 10 100 50 Example 3

(1) Surface Structure

(38) In the vitreous silica crucibles of Example 1 and Comparative Examples 1-3, the surface of the transparent layer of the straight body portion was observed using a confocal laser microscope. The scanning direction was in the vertical direction from the rim of the vitreous silica crucible. The scanning surface was an area of 3 cm3 cm of a vitreous silica crucible of before use. The results are shown in Table 3 and 4.

(39) FIG. 3 is a surface photo of the inner surface of the vitreous silica crucible according to Comparative Example 1, obtained by a confocal laser microscope. As shown in FIG. 3, at the inner surface of the conventional vitreous silica crucible, a concavo-convex structure was not observed but an uneven distortion structure was observed.

(40) FIG. 4 is a surface photo of the inner surface of the vitreous silica crucible according to Example 1, obtained by a confocal laser microscope. As shown in FIG. 4, at the inner surface of the vitreous silica crucible according to Example 1, a concavo-convex structure in which groove-shaped valleys were interposed between ridges was formed.

(41) At the inner surface of the vitreous silica crucible according to Comparative Example 2, a concavo-convex structure was formed, but the interval between the ridges was wide (not shown).

(42) For more detailed analysis, a three dimensional image of the inner surface of the vitreous silica crucible of Example 1 was obtained using a confocal laser microscope. FIG. 5 is the obtained three-dimensional image. As shown in FIG. 5, a concavo-convex structure was formed such that it was orthogonal to position A to B. It should be noted that position A is at the opening portion side of the crucible, and position B is at the bottom portion side of the crucible.

(43) FIG. 6 was obtained by graphing the height of the inner surface of the vitreous silica crucible from position A to position B. The reference point (Z=0) was set so that the height of the inner surface fell within Z=0-2 m. As a result, a concavo-convex structure in which multiple groove-shaped valleys interposed between ridges were detected. All results of the obtained average intervals are shown in Table 2.

(3) Central Line Average Roughness Ra

(44) For the inner surface of the vitreous silica crucible of Example 1, while scanning the focal point in the XY direction, the Z position information when focused was recorded and quantified, and thus the height of the sample was measured. The central line average roughness Ra was calculated by using numerical processing software to obtain a result of 0.37 m.

(45) In the same manner as Example 1, the central line average roughness Ra was obtained for Comparative Examples 1, 2, and 3. The results were respectively, 0.65 m, 0.78 m, and 0.01 m. All results of the obtained central line average roughness Ra are shown in Table 2.

(4) Melt Surface Vibration

(46) In each of the vitreous silica crucibles of Example 1 and Comparative Examples 1-3, about 500 kg of polysilicon was added, and heated to a temperature of about 1450-1600 deg. C. by using a carbon heater, then the presence of melt surface vibration was confirmed by observation camera while performing silicon single crystal pulling. For the vitreous silica crucibles of Comparative Examples 1, 2, and 3, the occurrence of melt surface vibration was confirmed. On the other hand, for the vitreous silica crucible of Example 1, the suppression of melt surface vibration was confirmed.

(47) TABLE-US-00002 TABLE 2 occurrence Average interval Central line average of melt [m] roughness Ra [m] surface vibration Example 1 25 0.37 no Comparative 0.65 yes Example 1 Comparative 105 0.78 yes Example 2 Comparative 3 0.01 yes Example 3

(48) From the results above, unlike the vitreous silica crucible manufactured by a method which is complex and poorly reproducible as in prior art (for example, Patent Literature 1), the vitreous silica crucible of the present invention can suppress melt surface vibration stably. In addition, not only when immersing the seed crystal in the initial stage of the pulling process of silicon single crystal, but also during the growing of silicon single crystal, melt surface vibration can be suppressed stably.