Tubular composite body made of quartz glass and method for producing and using the same

Abstract

A known method for producing a tubular quartz glass composite body in an outer deposition process comprises providing and rotating a substrate tube about an axis of rotation, depositing SiO.sub.2 particles on the outer jacket surface of the tube forming a composite consisting of the tube and a SiO.sub.2 soot body, and sintering the composite by heating to form the tubular quartz glass composite body, and using a holding device which is suitable for holding the composite body at least temporarily in the heating zone with the longitudinal axis of the substrate tube oriented vertically. To enable the production on this basis of a tubular composite body consisting of quartz glass with a particularly large inner diameter and with a wall with reduced scrap, it is proposed that a holding device is used which comprises a holding element which is produced in a holding region of the substrate tube.

Claims

1. A method for producing a tubular quartz glass composite body in an external deposition method, comprising the following method steps: (a) providing a substrate tube which has a continuous through-opening running coaxially to a longitudinal axis of the substrate tube, a substrate tube outer diameter, a substrate tube inner diameter, a substrate tube outer jacket surface, a substrate tube inner jacket surface, and a substrate tube wall having a wall thickness; (b) rotating the substrate tube about an axis of rotation running coaxially with or parallel to the longitudinal axis of the substrate tube; (c) depositing SiO.sub.2 particles on the outer jacket surface of the substrate tube by means of at least one deposition burner, forming a composite (1/9; 21/9) from the substrate tube and an SiO.sub.2 soot body; and, (d) sintering the composite (1/9; 21/9) by heating at a sintering temperature in a heating zone to form the tubular quartz glass composite body (100; 110) and using a holding device which is suitable for holding the composite body at least temporarily with a vertically oriented longitudinal axis of the substrate tube in the heating zone; wherein a holding device is used which comprises a holding element which is produced in a forming step in a holding area of the substrate tube.

2. The method according to claim 1, wherein the holding element is produced prior to the deposition of the SiO.sub.2 particles according to method step (c), wherein the deposition of the SiO.sub.2 particles according to method step (c) is preferably carried out in such a way that the SiO.sub.2 soot body covers the holding area.

3. The method according to claim 1, wherein the holding element is produced after the deposition of the SiO.sub.2 particles according to method step (c) and before or during the sintering according to method step (d).

4. The method according to claim 1, wherein the holding element is realized as a constriction of the inner diameter of the substrate tube or as an expansion of the outer diameter of the substrate tube.

5. The method according to claim 4, wherein the constriction of the substrate tube inner diameter has a longitudinal extension in the direction of the substrate tube longitudinal axis in the range from 20 to 200 mm, preferably in the range from 30 to 100 mm, wherein the constriction preferably brings about a maximum reduction in the substrate tube inner diameter in the range from 4 mm to 80 mm, preferably in the range from 6 mm to 50 mm.

6. The method according to claim 4, wherein the constriction is designed as a local indentation of the inner jacket surface of the substrate tube and/or as a taper of the substrate tube through-opening in the holding area.

7. The method according to claim 6, wherein the taper of the substrate tube through-opening is produced during sintering according to method step (d) by softening an upper substrate tube end together with a shaped body placed thereon, against which the upper substrate tube end rests, and bending the upper substrate tube end inward under the influence of the weight of the shaped body.

8. The method according to claim 4, wherein the expansion of the outer diameter of the substrate tube has a longitudinal extension in the direction of the longitudinal axis of the substrate tube in the range of 20 to 200 mm, preferably in the range of 30 to 100 mm.

9. The method according to claim 4, wherein the expansion brings about a maximum enlargement of the outer diameter of the substrate tube in the range of 4 mm to 80 mm, preferably in the range of 6 mm to 50 mm.

10. The method according to claim 4, wherein the expansion is preferably produced during sintering according to method step (d) by softening an upper substrate tube end together with an expansion device, which has an expansion body that can move radially outward and rests against the inner wall in the region of the upper substrate tube end, and moving the expansion body radially outward under the influence of the weight of the expansion device, and deforming the substrate tube wall in the area of the upper substrate tube end while forming the bulge.

11. The method according to claim 1, wherein a substrate tube is provided that consists at least partly of quartz glass of a first quartz glass quality, and that the soot body consists of quartz glass of a second quartz glass quality, wherein the first quartz glass quality has a material-specific viscosity at the sintering temperature which is higher than the material-specific viscosity of the second quartz glass quality.

12. The method according to claim 11, wherein at a measurement temperature of 1350 C., the common logarithm of the viscosity of the first quartz glass quality is at least 0.25 log (dPa.Math.s), preferably at least 0.4 log (dPa.Math.s) and particularly preferably at least 0.6 log (dPa.Math.s) higher than that of the quartz glass of the second quartz glass quality.

13. A tubular composite body consisting of quartz glass, with a length of at least 1000 mm, a tube wall with a wall thickness of at least 25 mm and with an inner diameter of at least 250 mm, wherein the tube wall has an inner wall region and an outer wall region, wherein the inner wall region comprises a holding element which is designed as a constriction of the substrate tube inner diameter or as an expansion of the substrate tube outer diameter.

14. The tubular composite body consisting of quartz glass according to claim 13, wherein the inner wall region consists at least partly of quartz glass of a first quartz glass quality, and the outer wall region consists of quartz glass of a second quartz glass quality, wherein at a measuring temperature of 1350 C., the viscosity of the first quartz glass quality is higher than the viscosity of the second quartz glass quality.

15. A use of the tubular composite body according to claim 13 for producing etching rings for semiconductor manufacturing or a pressure vessel, wherein a quartz glass hollow cylinder is produced by removing the inner wall region, and this cylinder is processed to form the etching rings or the pressure vessel.

16. The method according to claim 5, wherein the constriction is designed as a local indentation of the inner jacket surface of the substrate tube and/or as a taper of the substrate tube through-opening in the holding area.

17. A use of the tubular composite body according to claim 14 for producing etching rings for semiconductor manufacturing or a pressure vessel, wherein a quartz glass hollow cylinder is produced by removing the inner wall region, and this cylinder is processed to form the etching rings or the pressure vessel.

Description

BRIEF DESCRIPTION

[0091] The invention is explained in more detail below with reference to exemplary embodiments and a patent drawing. In detail, in a schematic representation:

[0092] FIG. 1 shows a device for producing a composite body consisting of a substrate tube and soot body in a first embodiment of a substrate tube holder, in a longitudinal section,

[0093] FIG. 2 shows a device for producing a composite body consisting of substrate tube and soot body in a second embodiment of a substrate tube holder in a longitudinal section, partly as a detail, and

[0094] FIG. 3 shows a detail of the substrate tube holder of FIG. 2 in an enlarged view,

[0095] FIG. 4 shows components of a zone sintering furnace as used for vitrification of a composite body,

[0096] FIG. 5 shows the vitrification of a standing composite body using the zone sintering furnace,

[0097] FIG. 6 shows a method step for producing a first embodiment of a retaining edge on the substrate tube,

[0098] FIG. 7 shows a further method step for producing the retaining edge on the substrate tube,

[0099] FIG. 8 shows the vitrification of a partially suspended composite body by means of the first embodiment of the retaining edge on the substrate tube in the zone sintering oven,

[0100] FIG. 9 shows a method step for producing a second embodiment of a retaining edge on the substrate tube,

[0101] FIG. 10 shows a further method step for producing the retaining edge on the substrate tube,

[0102] FIG. 11 shows an embodiment of a substrate tube with a retaining edge produced before the outer deposition process in the form of a taper of its inner diameter,

[0103] FIG. 12 shows the substrate tube of FIG. 11 after completion of the external deposition process during vitrification of the composite body in the zone sintering furnace of FIG. 4,

[0104] FIG. 13 shows a first embodiment of a tubular quartz glass composite body according to the invention in a longitudinal section, and

[0105] FIG. 14 shows a second embodiment of a tubular quartz glass composite body according to the invention in a longitudinal section.

DETAIL DESCRIPTION

[0106] In the embodiments explained below, different substrate tubes are used, of which some properties are summarized in Table 1.

TABLE-US-00001 TABLE 1 Substrate tubes Viscosity Outside Inside [log(dPa .Math. s) at diameter diameter Length No. Material 1350 C.] [mm] [mm] [mm] Mold 1 Natural quartz glass 11.28 280 270 2000 Cylinder (electro-fused) 2 Natural quartz glass 11.28 280 270 1500 Cylinder with (electro-fused) constriction 3 Synthetic quartz glass 10.77 280 270 1500 Cylinder with (thermally dried) constriction 4 Synthetic quartz glass 10.64 280 270 1500 Cylinder with (dried with chlorine) constriction

[0107] Natural quartz glass is melted from naturally occurring SiO.sub.2 raw material, preferably in an electrically heated melting furnace. The substrate tube consisting of natural quartz glass is produced particularly cost-effectively using a vertical crucible drawing method. This quartz glass typically contains aluminum oxide in a concentration in the range of between 6 ppm by weight and 18 ppm by weight, and hydroxyl groups in a concentration of less than 50 ppm by weight.

[0108] Synthetic quartz glass is obtained for example by flame hydrolysis or oxidation of synthetically produced silicon compounds, by polycondensation of organic silicon compounds according to what is referred to as the sol-gel method, or by hydrolysis and precipitation of inorganic silicon compounds in a liquid. The viscosity of synthetic quartz glass depends on its composition, which can vary over a wide range. However, it can very generally be said that synthetic quartz glass typically has a significantly lower viscosity than natural quartz glass.

[0109] The device shown schematically in FIG. 1 is used to produce a large-volume composite consisting of a substrate tube 1 and an SiO.sub.2 soot body 9. It comprises a glass lathe 2 for holding and rotating the substrate tube 1 corresponding to number 1 in Table 1. The substrate tube 1 has a left end face 1a, a right end face 1b, an outer jacket surface 1c, an inner jacket surface 1d, a horizontally-oriented longitudinal axis 1e, and a cylindrical through-hole 1f. Adjacent to the end face 1a is a free substrate tube section 1g on which a reduced deposition of SiO.sub.2 soot particles takes place during the soot deposition process. The free substrate tube section 1g can be formed into a retaining edge during vitrification, which is explained in more detail below with reference to FIGS. 4 and 6 to 10.

[0110] The glass lathe 2 is indicated by two, oppositely-situated chucks 2a, 2b, of which the chuck 2a is spring-loaded, as indicated by the compression spring 2c. The compression spring 2c generates a pressure force F that presses the two chucks 2a, 2b against each other, as indicated by the directional arrows 2d.

[0111] A hollow spindle 3a. 3b made of stainless steel is clamped in each of the chucks 2a, 2b at their proximal ends. In the ideal case, the axes of rotation of the hollow spindles 3a, 3b run coaxially to the substrate tube longitudinal axis 1e. The hollow spindles 3a, 3b have an outer diameter of 90 mm and an inner diameter of 82 mm.

[0112] The distal ends of the hollow spindles 3a, 3b are pivotably connected to an annular pressure plate 4a, 4b made of stainless steel. For this purpose, the distal ends of the hollow spindles 3a, 3b taper conically and, as a result of the force of the spring 2c, press against the corresponding pressure plate 4a, 4b. Here, the conical end protrudes into a center bore of the respective annular pressure plate 4a, 4b and abuts the inner edge of the center bore.

[0113] The pressure plates 4a, 4b each abut buffer disks 5a, 5b made of graphite, which in turn abut the substrate tube end faces 1a and 1b respectively. The buffer disks 5a, 5b have a central bore whose diameter corresponds to that of the pressure plates and which run coaxially to these. The pressure plates 4a. 4b have an outer diameter that is 10 mm smaller than the outer diameter of the substrate tube 1. The buffer disks 5a, 5b have an outer diameter that extends beyond the outer diameter of the substrate tube 1 by 40 mm.

[0114] A tubular centering support 6 made of SiSiC with a total length L.sub.z and an outer diameter of 80 mm extends through the substrate tube through-hole 1f and through the center bores of pressure plates 4a. 4b and buffer disks 5a, 5b. One end 6a of the centering support 6 projects into the hollow spindle 3a over a length La of 500 mm and ends inside it, leaving a variable movement clearance Ba of approximately 6 mm. The other end 6b protrudes into the hollow spindle 3b over a length Lb of 600 mm and ends inside it, leaving a variable movement clearance Bb also of about 6 mm. The entire movement clearance B.sub.z=B.sub.a+B.sub.b for the centering support 6 within the hollow spindles 3a, 3b is thus 12 mm. The outer diameter of the centering support 6 is constant over its length and is adapted with a clearance fit to the inner diameter of the hollow spindles 3a, 3b and telescopically displaceable therein.

[0115] Three centering rings 7a, 7b, 7c made of graphite are placed on the centering support 6. The centering ring 7a is located in the area of the left substrate tube end face 1a, the centering ring 7b is located in the area of the right substrate tube end face 1b, and the centering ring 7c is located approximately in the middle of the substrate tube through-hole 1f. All centering rings 7a, 7b, 7c have an outer diameter that is matched to the inner diameter of the substrate tube with a clearance fit, and they have an inner diameter that is matched to the outer diameter of the centering support with a clearance fit.

[0116] The end centering ring 7a, the buffer disk 5a, and the pressure disk 4a are loosely connected to each other by means of screws 4c. The screws 4c have a thread which adjoins a cylinder portion 4d. The screw thread engages in each case in an internal thread in the pressure disk 4a, so that the cylinder portion 4d lies firmly against the pressure disk 4a in the tightened state. The length of the cylinder portion 4d is greater than the total thickness of the component stack of centering ring 7a and buffer disk Sa, so that the heads of the screws 4c do not rest against the centering ring 7a, but a gap remains between the centering ring 7a and the screw heads. Furthermore, the through-holes for the passage of the cylindrical section 4d in the buffer disk 5a and in the centering ring 7a are greater than the diameter of the cylinder section 4d, so that the screws 4c can also be slightly inclined in the through-holes. This loose connection is therefore suitable both for allowing thermally-induced changes in length between components of the substrate tube holder and for compensating for deviations in the target dimensions, positioning, and alignment of the components. In addition, the screws 4c provide a certain torsional strength between the buffer disk 5a and the pressure disk 4a during the rotational movement of the substrate tube 1, and, in this respect, serve as driving elements for this rotational movement. The same applies to the connection of the centering ring 7b, the buffer disk 5b, and the pressure disk 4b. A gap is provided between the centering ring 7a, 7b and the buffer disk 5a, 5b for the purpose of thermal decoupling (not visible in the figure).

[0117] Several deposition burners 8 for generating SiO.sub.2 particles are mounted on a common slide 8a, by means of which they can be moved reversibly and transversely along the outer jacket surface 1c of the substrate tube 1 or along a forming SiO.sub.2 soot body 9, and can be displaced perpendicularly thereto, as indicated by the directional arrows 8b.

[0118] When the same reference numerals are used in FIG. 2 and in FIG. 3 as in FIG. 1, these designate identical or equivalent components or components of the device.

[0119] The device shown schematically in FIG. 2 differs from that of FIG. 1 essentially in terms of the substrate tube holder and the substrate tube 21. When the same reference numbers are used as in FIG. 1, these denote identical or equivalent components or parts of the device explained with reference to FIG. 1.

[0120] Substrate tube 21 corresponds to number 2 in Table 1. In an end region 21a, it has a circumferential constriction 26 of its inner diameter. The constriction 26 is produced before the start of the outer deposition process, for example by local softening of the substrate tube 21 after being clamped in the glass lathe 2. The constriction 26 is located at a distance of approximately 80 mm before the substrate tube end face. The inner diameter is 270 mm, except in the region of the constriction 26, where it is 250 mm.

[0121] The hollow spindles 23a, 23b, which are each clamped in clamping chucks with their proximal end, are made of stainless steel. In the ideal case, the axes of rotation of the hollow spindles 23a, 23b run coaxially to the substrate tube longitudinal axis 1e. The hollow spindles 23a, 23b have an outer diameter of 100 mm. A circumferential extension arm 2c is welded in each case in the region of the distal ends of the hollow spindles 23a, 23b.

[0122] The pivoting connection between the hollow spindles 23a. 23b and the corresponding pressure plates 24a, 24 is designed here as a floating bearing and preferably comprises a cardan ball cone seat. Here, the distal ends of the hollow spindles 23a. 23b each form a convexly-curved seat which has a spherical or radius section on which the pressure plate 24a or the pressure plate 24b is movably mounted by having a concavely-curved spherical or radius section cooperating with the convexly-curved seat.

[0123] FIG. 3 shows an enlarged view of the pivoting connection between the hollow spindles 23a. 23b and the respective pressure plates 24a. 24b. The end centering ring 7a, the buffer washer 5a, and the pressure disk 24a form a stack of components that are loosely connected to each other by means of threaded screws 24c, each of which engages in a thread in the extension arm 23c. The cylindrical portion 24d has a length which is greater than the total thickness of the component stack consisting of centering ring 7a, buffer disk 5a, and pressure disk 24a. In addition, the width of the bore for receiving the threaded screws 24c in the component stack is significantly larger than the diameter of the cylinder portion 24d. This leads to several gaps 25 remaining both between the screw head 24e and the centering ring 7a and between the pressure plate 24 and the extension arm 23c, even with a fixedly-tightened threaded screw 24c, and along the cylinder portion 24d. The gaps 25 ensure that the connection between the hollow spindles 23a, 23b and the corresponding pressure plates 24a. 24b remains pivotable. At the same time, the screws 24c serve as driver elements for the rotational movement of the substrate tube 1.

[0124] Examples for producing a quartz glass composite body are explained below with reference to FIGS. 1 and 4 to 10.

Soot Deposition Process

[0125] Oxygen and hydrogen are supplied to the deposition burners 8 as burner gases, and a gas stream containing SiCl.sub.4 or another silicon-containing starting material is supplied as feed material for the formation of SiO.sub.2 particles. These components are converted into SiO.sub.2 particles in the relevant burner flame, and these SiO.sub.2 particles are deposited on the substrate tube 1 rotating around the longitudinal axis 1e, forming the soot body 9 from porous SiO.sub.2 soot.

[0126] To rotate the substrate tube 1, the glass lathe 2 transmits a torque to the hollow spindles 2a, 2b. At the same time, a pressure force F acting in the axial direction is generated by means of the compression spring 2c which presses the two hollow spindles 3a, 3b against one another and which, depending upon the deflection of the spring from the spring rest length, is in the range between 0.5 kN and 10 kN. The initially set pressure force F is 1 kN and is applied to the pressure plates 4a, 4b, the buffer disks 5a, 5b, and thus also to the substrate tube end faces 1a, 1b. This pressure force F creates a frictional connection between the buffer disks 5a, 5b, which is sufficient to hold the inherent weight of the substrate tube 1 and the weight of the soot body 9. The centering rings 7a, 7b, 7c are used only to prevent the substrate tube 1 from slipping or bending unexpectedly. A certain amount of axial guidance is provided by the interaction of hollow spindles 3a, 3b and centering support 6, which, due to the mechanical play present, compensates for any radial offsets and angular differences between the axes of rotation of the hollow spindles 3a, 3b, and avoids mechanical stresses.

[0127] The deposition process is terminated as soon as the soot body 9 has reached a predetermined outer diameter which, depending on the density of the soot layer, leads to the predetermined outer diameter of the hollow cylindrical quartz glass composite body, plus an allowance of 1 mm. At a soot density of about 30% (relative to the density of quartz glass) and a target external diameter of the quartz glass composite body of 362 mm, the soot body outer diameter is, for example, approximately 520 mm.

[0128] After the deposition process, the soot body 9 is substantially barrel-shaped and extends up to just before the ends of the substrate tube 1 on both sides. The substrate tube section 1g, which protrudes from the soot body 9 and is only slightly covered by SiO.sub.2 soot, has a length of about 100 mm.

Drying and Vitrification Process

[0129] The substrate tube 1 remains in the soot body 9. The composite (1; 9) consisting of the substrate tube 1 and soot body 9 is subjected to a dehydration treatment in a drying furnace in an inert gas atmosphere, a halogen-containing atmosphere, or under a vacuum. The drying of the soot body 9 here takes place thermally, by heating to a temperature around 1100 C. in a nitrogen atmosphere. By vitrifying the resulting dried SiO.sub.2 soot body under vacuum, a synthetic quartz glass with the following properties is obtained: [0130] Hydroxyl group content: about 200 ppm by weight. [0131] Chlorine content: <0.2 ppm by weight [0132] Viscosity at 1350 C.: 10.77 log (dPa.Math.s).

Vitrification

[0133] The subsequent vitrification of the soot body 9 takes place in a zone sintering furnace with a vertically oriented substrate tube longitudinal axis 1e under a vacuum or in an atmosphere of gases that diffuse quickly in quartz glass, such as helium and hydrogen, and therefore do not cause bubbles. FIG. 4 schematically shows a detail of such a zone sintering furnace 40. The furnace chamber 41 encloses a furnace interior 42 in which an annular heating element 43 and a holding device 44 are located. This device comprises a support rod 45 made of fiber-reinforced carbon, which is connected by its lower end to a pedestal 46 of graphite. The upper end of the support rod 45 is held by means of a movable gripper (not shown in the figure), and can be moved up and down thereby. The support rod 45 extends through the annular opening of the heating element 43 and through a sheath tube 47 of graphite which stands on the pedestal 46. Apart from the passages for the support rod 45, the sheath tube end faces are closed. In comparison to a sheath tube that is open on both sides, the largely closed upper and lower side 47a give the sheath tube 47 a higher dimensional rigidity relative to the pressure acting from the outside.

[0134] During the vitrification, the substrate tube 1 remains in the soot body 9. The holding device 44 is used to hold the composite (1; 9) consisting of the substrate tube 1 and soot body 9, the weight of which over the pedestal 46 is supported by the support rod 45. As shown schematically in FIG. 5, the substrate tube 1 surrounds the sheath tube 47. Its outer diameter is adapted to the inner diameter of the substrate tube 1 in such a way that the annular gap remaining during sintering is as small as possible, taking into account the higher thermal expansion coefficient of the graphite sheath tube 47 compared to the quartz glass substrate tube, and is, for example, in the range of 1 mm to 10 mm, preferably less than 5 mm.

[0135] During vitrification, the heating element 43 is heated to a temperature of around 1400 C., and the support rod 45 is continuously pulled upwards so that the soot body 9 is vitrified from top to bottom. During this, the composite (1; 9) shrinks onto the sheath tube 47 so that its outer diameter defines a lower limit for the inner diameter of the vitrified, tubular quartz glass composite body. The substrate tube 1 consists of natural quartz glass which has a higher viscosity at the sintering temperature than the synthetic quartz glass of the soot body 9. At a measuring temperature of 1350 C., according to Table 1, the viscosity difference corresponding to the difference between the common logarithms of the respective viscosity values is about 0.51 log (dPa.Math.s) [11.28 log (dPa.Math.s)10.77 log (dPa.Math.s)].

[0136] The substrate tube 1 is thus comparatively thermally stable and does not deform or slightly deforms. In this way, it produces a stabilization of the soot body 9 during vitrification. In particular, the risk is counteracted of the soot body 9 collapsing during vitrification and forming circumferential folds, or of the inner diameter expanding which results in scrap.

[0137] After cooling, a tubular composite body consisting of the substrate tube 1 and a glass layer is obtained with a thickness of about 41 mm, which has been obtained by vitrifying the soot body 9. Despite the shrinking of the synthetic quartz glass onto the inner graphite sheath tube 47, said glass can be easily removed after vitrification since graphite has a significantly higher coefficient of thermal expansion than quartz glass and contracts more during cooling.

[0138] The quartz glass of the substrate tube 1 can then be removed by mechanical processing, for example by drilling. After grinding off and smoothing the outer wall, a quartz glass hollow cylinder having an outer diameter of 360 mm and an internal diameter of 290 mm is obtained.

Comparative Example

[0139] As explained above, due to its comparatively high viscosity the substrate tube 1 contributes to the shape stabilization of the soot body 9 during vitrification.

[0140] In order to investigate the effect of the substrate tube 1 on the shape stabilization of the soot body 9, in a first comparative experiment, the substrate tube was removed before vitrification, and only the soot body 9 was vitrified in the zone sintering furnace 40 starting from the top downward. In so doing, the soot body 9 collapses under its own weight, and the vitrified region detaches from the sheath tube 47, which leads to a local widening of the inner diameter. The quartz glass tube produced in this way was unusable.

[0141] In a further comparative experiment, a substrate tube of synthetic quartz glass was used having the same dimensions as the substrate tube 1, as specified under number 3 of Table 1. The viscosity of this quartz glass is lower than the viscosity of the quartz glass obtained by vitrification of the soot body 9. It has been found that when this composite body is used, the sheath tube imparts better adhesion to the vitrified soot body 9 so that detachment over a large area of the vitrified material did not occur. However, the continuously increasing weight of the vitrified upper region of the composite body resulted in compression toward the end of the process, so that the quartz glass tube produced in this way was also ultimately unusable.

[0142] In a further comparative experiment, a substrate tube consisting of synthetic quartz glass with the same dimensions as substrate tube 1 was used, as specified in number 4 of Table 1. The viscosity of this quartz glass corresponds to the viscosity of the quartz glass that is obtained by vitrification of the soot body 9. However, here as well, during sintering of the composite (1; 9), it was shown that there was compression toward the end of the process so that the quartz glass tube produced in this way was also ultimately unusable.

Suspended Vitrification

[0143] To counteract the effect of compression from intrinsic weight, a vitrification method is often used in which the composite (1; 9) does not stand up permanently on the pedestal 46 during vitrification, but is held suspended at least temporarily. This method, which is known for example from EP 0 701 975 B1, is referred to here for short as suspended vitrification.

[0144] It is thereby ensured that the composite (1; 9) is either kept suspended from the beginning in the vitrification furnace, or the holding of the composite (1; 9) during the vitrification process can transition from standing at the beginning of the vitrification to suspended holding as soon as the unavoidable axial sintering shrinkage becomes noticeable. To implement the suspended holding, measures can be taken on the substrate tube even before the outer deposition method is carried out. This can take place for example by shaping a substrate tube, or by the shaping process during the manufacture of the substrate tube, such as by producing a constriction of the inner bore of the substrate tube during a drawing process in which the substrate tube is drawn from a melt, or in which a mother tube is elongated to form the substrate tube. Alternatively or in addition, and equally preferably, these measures are generated in situ during vitrification.

[0145] A suitable measure for realizing suspended vitrification is the formation of a retaining edge on the substrate tube which serves to ensure that the composite (1; 9) is vitrified while suspended vertically (and not exclusively standing) at least some of the time. As a result of the at least temporary suspended holding, in addition to the above-explained effect of the thermally stable substrate tube 1, a compression of the soot body 9 during vitrification is counteracted so that it retains its desired geometry, and scrap is avoided.

[0146] For the suspension of the composite (1; 9), for example the substrate tube section 1g can be reshaped to form a retaining edge during vitrification of the soot body 9. Suitable methods for producing the retaining edge in situ are explained in more detail below with reference to FIGS. 6 to 10.

[0147] FIG. 6 schematically shows a first method and a device for producing the suspension in situ. The composite (1; 9) consisting of the substrate tube 1 and soot body 9 is placed in the vitrification furnace 40 and supported on the platform 46, with the soot body longitudinal axis 1e oriented vertically, by means of a support rod 45 and sheath tube 47. An annular spacer 61 is placed on the upper end face 47a of the sheath tube 47, and a conical body 62 in the form of an inverted cup is placed on the upper end of the substrate tube 9. The spacer 61 and the conical body 62 consist of graphite. The conical body 62 has an inner cone 62a which merges into a flat support surface 62b in which there is a through-opening 62c. In the initial state, the inner cone 62 rests against the outer side of the upper substrate tube section 1g. The support rod 45 extends through the through-opening 62c and through the annular spacer 61.

[0148] The upper substrate tube section 1g is shaped to form a suspension during the vitrification process. The shaping process is shown schematically in FIG. 7. By means of the support rod 45, the upper substrate tube section 1g is moved into the heating element 43, which is heated to vitrification temperature, far enough that it softens. Due to its weight, the cone body 62 presses the soft upper substrate tube section 1g inward in the direction of the substrate tube longitudinal axis 1e. In so doing, the substrate tube section 1g rests against the inner cone 62a and is thereby reshaped to form an outer cone. The reshaping process is terminated as soon as the spacer 61 comes to rest against the support surface 62b of the cone body 62.

[0149] During further vitrification, the composite body (1;9) is heated zone by zone, starting with its upper end. In so doing, the composite (1; 9) collapses successively onto the graphite sheath tube 47 and also shrinks in length. The sintering shrinkage forces are strong enough here for the length of substrate tube 1 to also be shortened. However, the length shortening is small.

[0150] In a first vitrification phase, the composite (1; 9) stands on the platform 46. FIG. 8 schematically shows a second vitrification phase. During this, the former substrate tube section 1g, which has been formed into an outer cone, moves out of the heating area of the heating element 43, cools down, and solidifies. It rests by its inner side on the upper side of the sheath tube 47a and then acts as a suspension 63 for the composite (1; 9). As a result of the length shrinkage, said composite lifts off the platform 46, forming a narrow gap 48, which then enables the further suspended vitrification. This counteracts a collapsing of the soot body 9, wherein the substrate tube 47 additionally stabilizes the shape of the resulting tubular quartz glass composite body due to its higher viscosity.

[0151] FIG. 9 schematically shows a second method and a device for producing the suspension in situ, i.e. in one operation with the vitrification of the composite (1; 9) consisting of the substrate tube 1 and soot body 9. This is placed in the vitrification furnace 40 and supported on the platform 46, with the soot body longitudinal axis 1e oriented vertically, by means of the support rod 45 and sheath tube 47. A circular ring 91 is placed on the upper end face 47a of the sheath tube 47 and is composed of two or more separate circular sector plates 91a, 91b, which adjoin a circular ring center opening 91b and which are mounted so as to be movable in the radial direction. A conical body 92 consisting of graphite, which has a conical shaft 92a and a conical head 92b with a flat underside, projects into the center opening 91b from above. The circular ring 91 and conical body 92 consist of graphite. The outer diameter of the circular ring 91 corresponds approximately to the inner diameter of the substrate tube 1; it lies against the inner wall of the upper substrate tube portion 1g. The diameter profile of the cone shaft 92a is designed so that, in the initial state, it is inserted approximately halfway into the center opening 91b.

[0152] During further vitrification of the composite body (1; 9), the bulge 93 moves into the area above the heating element 43, cools down and solidifies. The composite body (1, 9) is heated zone by zone starting at its upper end, as described above for the first method. In a first vitrification phase, the composite (1; 9) stands on the pedestal 46, and during the second vitrification phase, the bulge 93 acts as a holder for the suspended vitrification. The circular sector plates 91a protrude from the inside into the bulge 93 and are fixed therein in a vertical direction together with the substrate tube 1.

[0153] The length and inner diameter of the quartz glass composite body thus obtained are predefined by the substrate tube 1. The former soot body forms a layer of transparent synthetic quartz glass with a layer thickness of 41 mm. After the removal of the quartz glass material of the substrate tube 1 and grinding off and smoothing the outer wall, a quartz glass tube having an outer diameter of 360 mm and an internal diameter of 290 mm is obtained. From this, etching rings can be cut for single-wafer plasma etching chambers and pressure vessels for use in chemical engineering.

[0154] To produce the etching ring or the pressure vessel, a composite body is preferably used having an outer wall region having an inner diameter which is smaller by at least 1 mm than the target inner diameter. The target inner diameter can be set by mechanical or chemical processing of the composite body inner bore.

[0155] In the following, a further exemplary embodiment for the realization of a retaining edge for the suspended vitrification of the composite body (21, 9) is explained with reference to FIGS. 2 and 3 and FIGS. 11 to 13.

Soot Deposition Process

[0156] The soot deposition process is conducted as explained above with reference to FIG. 1. FIG. 11 shows the substrate tube 21 used in this process in a longitudinal section. The previously produced constriction 26 is provided in the end region 21a. As can be seen in FIG. 2, the soot body 9 is produced in such a way that it completely covers the constriction 26. However, this measure is not absolutely necessary; the constriction can also lie completely outside the soot body 9.

[0157] The deposition process is terminated as soon as the soot body 9 has reached a specified outer diameter: at a soot density of 30% (based on the density of quartz glass) and a target outer diameter of the quartz glass composite body of 362 mm, the soot body outer diameter is approximately 520 mm.

[0158] The composite (21; 9) present after the soot deposition process is subjected to a dehydration treatment in a chlorine-containing atmosphere at a temperature of 1200 C. By vitrifying the resulting dried SiO.sub.2 soot body under vacuum, a synthetic quartz glass with the following properties is obtained: [0159] Hydroxyl group content: <0.2 ppm by weight [0160] Chlorine content: about 1,000 ppm by weight [0161] Viscosity at 1350 C.: 10.64 log (dPa.Math.s)

[0162] The composite (21; 9) is then vitrified under a vacuum in the zone sintering furnace 40. FIG. 12 shows the composite (21; 9) placed in the zone sintering furnace 40. During vitrification, the heating element 43 is heated to a vitrification temperature of around 1400 C., and the support rod 45 is continuously pulled upwards so that the soot body 9 is vitrified from top to bottom. During this, the composite (21; 9) shrinks onto the graphite sheath tube 47 so that its outer diameter defines a lower limit for the inner diameter of the vitrified, quartz glass composite body. Since the substrate tube 21 consists of a quartz glass which has a higher viscosity at the vitrification temperature than the quartz glass of the soot body 9, it does not deform or deforms only slightly, and brings about stabilization of the soot body 9 during vitrification. In particular, this counteracts the risk of the soot body 9 collapsing during vitrification and forming circumferential folds, or of expansions of the inner diameter arising.

[0163] At the shown stage of the method, the vitrification process is already advanced, and a certain shrinkage of the soot body 9 has already taken place. This means that the transition from the initial vitrification phase with a standing composite (21; 9) to suspended vitrification is already complete. Due to the shrinkage of the soot body 9, the length of the substrate tube 21 has also shortened a little, and it has already been lifted somewhat from the pedestal 46, forming the gap 98.

[0164] In an alternative method, the composite (21; 9) is held suspended in the zone sintering furnace from the outset by the fact that the constriction 26 rests on the upper edge of the graphite sheath tube 47.

[0165] After cooling, a composite body 100 (FIG. 13) consisting of the substrate tube 21 and a glass layer 9a with a thickness of about 41 mm is obtained, which was obtained as a result of the vitrification of the soot body 9.

[0166] In a comparative experiment, a substrate tube consisting of synthetic quartz glass was used having the same dimensions as the substrate tube 21, as specified under number 3 of Table 1. The viscosity of this quartz glass is higher than the viscosity of the quartz glass that is obtained by vitrification of the soot body 9. More precisely, the common logarithm of the viscosity of this quartz glass at a measuring temperature of 1350 C. is higher by 0.13 log (dPa.Math.s) than the common logarithm of the viscosity of the quartz glass obtained by vitrification of the soot body 9.

[0167] However, it turns out that this difference in viscosity is too small to impart sufficient stability to the large-volume and heavy substrate tube when sintering the composite body.

[0168] FIG. 13 shows a cross-section of the composite body 100 obtained after vitrification in a view along the line A-A drawn in FIG. 2. Shown are the substrate tube 1 with the through-holes 1f, the diameter constriction 26, and the layer 9a of synthetic quartz glass obtained after vitrification.

[0169] After the removal of the quartz glass material of the substrate tube 21 and grinding off and smoothing the outer wall, a quartz glass hollow cylinder having an outer diameter of 360 mm and an internal diameter of 290 mm is obtained.

[0170] Etching rings with the corresponding inner and outer diameter values for use in holding semiconductor wafers in single-wafer plasma etching chambers or pressure vessels for industrial use can be sawn therefrom.

[0171] FIG. 14 schematically shows a longitudinal section of another tubular composite body 110 obtained after cooling. This has an inner wall layer 111 which consists of the former substrate tube 1 and it has a glass layer 112 with a thickness of about 41 mm which was obtained by vitrifying the former soot body 9. The inner wall layer 111 shows a holding element 113 which, in the exemplary embodiment, is realized as a taper of the substrate tube (FIG. 8, reference numeral 1).

[0172] After the removal of the quartz glass material of the substrate tube 1 and grinding off and smoothing the outer wall, a fully synthetic quartz glass hollow cylinder having an outer diameter of 360 mm and an internal diameter of 290 mm is obtained. Etching rings with the corresponding inner and outer diameter values, or with larger inner and smaller outer diameter values, for use in holding semiconductor wafers in single-wafer plasma etching chambers and pressure vessels consisting of quartz glass for use in chemical engineering can be sawn therefrom.