TWI416014B - Pumped - Google Patents

Abstract

In a pump, a main shaft (16) is rotatably supported by ball bearings (14, 15) in a casing (13) having a suction opening (11) and a discharge opening (12), an impeller (1) is connected to an end of the main shaft (16), and a canned motor (18) rotatably drives the impeller (17) through the main shaft (16). A front shroud (44) is provided at the front side in the axial center direction of the impeller (17), and a rear shroud (45) is provided at the rear side in the axial center direction. A predetermined gap (47) facing the axial direction is formed between the casing (13) and the front shroud (44), and sealing units (48, 49) facing the radial direction are provided between the casing (13) and the front shroud (44). This allows for extended life of the pump.

Description

Pump

The present invention relates to pumps for carrying, for example, supercritical CO ₂ fluids or liquid CO ₂ .

For example, a pump for carrying a supercritical CO ₂ fluid or a liquid CO ₂ has a circulating pump for semiconductor cleaning. With the recent high integration of semiconductor devices, the processing line width of wafers has also been required to be miniaturized. For the current 0.18 μm mainstream, it is estimated to be 0.10 μm or less in the future. However, in the conventional method of using a liquid such as ultrapure water for semiconductor cleaning, the capillary force caused by the interfacial tension of the gas and the liquid during the drying of the wafer sometimes causes the resist formed on the wafer to be spoiled. (The resist is broken).

In order to solve the above disadvantages, a semiconductor cleaning device using a supercritical fluid has been developed to replace a liquid such as ultrapure water. Supercritical fluids have a very high permeability compared to liquids and are capable of saturating any fine structure. Further, since there is no interface between the gas and the liquid, there is a feature that no capillary force is generated during drying.

Supercritical fluids, mainly using carbon dioxide (CO ₂ ). Carbon dioxide is a relatively stable condition compared to other liquid solvents, that is, at a critical temperature of 31.2 ° C and a critical pressure of 7.38 MPa, a critical density of 468 kg / m ³ is reached. In addition, since carbon dioxide is a gas at normal temperature and normal pressure, carbon dioxide vaporizes when it is returned to normal temperature and normal pressure, and it is possible to easily separate the washed matter and the contaminant, so that it is washed after washing. The material does not need to be dried, etc., and the cleaning process can be simplified and the cost can be reduced.

In the above semiconductor cleaning apparatus using a supercritical CO ₂ fluid, the supercritical CO ₂ fluid is usually pressurized to about 20 MPa, so that it can be recycled as a circulating pump for wafer cleaning, which is capable of withstanding high pressure so-called unsealed. Type shielded electric pump in the form of a circulating pump. In addition, the bearing uses a ball bearing which is used in the liquid lift (supercritical CO ₂ ) of the semiconductor detergent.

The ball bearing is subjected to the radial load and axial load of the rotor. Further, the bearing preloading spring provided on the opposite side of the impeller side on the shaft end side bearing controls the preloading load, whereby the revolving sliding (lateral sliding) of the ball bearing can be prevented. In addition, the radial rigidity (spring constant) of the ball bearing is controlled by the bearing preload load, and the number of natural rotor vibrations is also adjusted.

The pump described above has the pump described in Patent Document 1 below.

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2007-231958

However, in the above-mentioned prior pump, the ball bearing is used in a super-viscous supercritical CO ₂ fluid (or liquid CO ₂ ), so that the lubrication of the lift liquid cannot be expected. Therefore, the ball bearing is worn to move the spindle and the impeller in the direction of the rotation axis. The impeller is pressurized by the rotation of the suction port to be sucked out from the discharge port, and the axial gap between the casing and the impeller is set to be extremely narrow in order to prevent the fluid from flowing back from the discharge port to the suction port. However, when the wear of the ball bearing causes the main shaft and the impeller to move in the direction of the rotation axis, interference occurs between the impeller and the casing, resulting in a problem of a reduced service life.

The present invention has been made in order to solve the above problems, and an object of the invention is to provide a pump capable of extending the service life.

In order to achieve the above object, the pump according to the first aspect of the invention includes: a casing having a suction port and a discharge port; and a spindle that is rotatably supported by the ball bearing in the casing; and is coupled to the spindle An impeller at the end of the shaft; and a shield motor capable of driving the impeller through the main shaft, wherein the fluid that is sucked by the suction port is pressurized and discharged from the discharge port by rotation of the impeller, and is characterized in that the shaft of the impeller A front edge is provided on the front side of the core direction, and a rear edge is provided on the rear side of the axial direction of the impeller, and a predetermined gap in the axial direction is provided between the casing and the front edge. A sealing portion facing in the radial direction is provided between the casing and the front edge.

In the pump according to the second aspect of the invention, the sealing portion is provided in a plurality of rows arranged in the axial direction of the impeller.

The pump according to the third aspect of the invention is characterized in that, in the casing, a fluid outlet from the impeller is communicated to the discharge port through a diffuser and a volute, and a shrunken portion is provided in the diffuser. .

The pump according to the fourth aspect of the invention is characterized in that the shape of the neck portion is set based on a fluid outflow angle of the diffuser outlet and a passage area ratio of the diffuser outlet and the scroll chamber.

The pump according to the fifth aspect of the invention is characterized in that the pump is provided with a preloading spring that applies a preload to the ball bearing, and the preloading spring is formed by stacking a plurality of annular wave plates.

The pump according to the sixth aspect of the invention is characterized in that the impeller is capable of transporting a supercritical CO ₂ fluid or liquid CO ₂ by rotational driving, and can be used as a cyclic pump for semiconductor cleaning.

According to the pump of the first aspect of the invention, in the casing having the suction port and the discharge port, the main shaft is rotatably supported by a ball bearing, and the impeller is coupled to the shaft end of the main shaft, and the shield motor is used. The rotating impeller can be driven through the main shaft, and the front edge is provided on the front side of the impeller in the axial direction, and the rear edge is provided on the rear side in the axial direction, and is disposed between the casing and the front edge. At the same time as the specified gap in the direction of the axial direction, a sealing portion facing in the radial direction is provided between the casing and the front edge. Therefore, the sealing portion can prevent the fluid from flowing back from the discharge port to the suction port. Further, even if the wear of the ball bearing causes the main shaft and the impeller to move in the direction of the rotation axis, since the specified gap is provided between the impeller and the casing, The two do not interfere with each other, thus prolonging the pump life.

According to the pumping method of the second aspect of the invention, since the sealing portion is arranged in a plurality of axial directions of the impeller, the fluid leakage between the impeller and the casing can be reduced by using a plurality of sealing portions. Prevent fluid from flowing back from the spout to the suction port.

According to the pump of the third aspect of the invention, since the fluid outlet from the impeller is communicated to the discharge port through the diffuser and the volute, and the diffuser is provided in the diffuser, the diffuser outlet can be made. The fluid outflow angle is formed large, whereby the loss of the diffuser outlet to the diffuser inlet can be reduced, while the movement of the impeller can be allowed to prevent interference.

According to the pump of the fourth invention of the patent application, the shape of the neck portion is set according to the fluid outflow angle of the diffuser outlet and the passage area ratio of the diffuser outlet and the swirl chamber, so that the neck can be formed The portion is set to an appropriate shape, and the total velocity of the radial velocity and the circumferential velocity of the fluid flowing out from the diffuser outlet to the vortex chamber is decelerated, whereby the diffuser effect can be sufficiently ensured, that is, the velocity energy of the fluid can be sufficiently ensured (moving) The pressure is converted into pressure energy (static pressure), which can improve the pumping efficiency.

According to the pump of the fifth aspect of the invention, since the pre-pressure spring for applying a preload to the ball bearing is provided, the annular wave plate is laminated in plural to form the preload spring, so that the ball can be used by the preload spring. Preloading is applied to the bearing, thereby suppressing the revolution of the ball bearing and prolonging the service life, and by stacking the wave plates in multiple layers to form a preload spring, the amount of load change relative to the amount of change in the spring pressure can be reduced, and the load can be appropriately applied. Preloading.

According to the pump of the sixth aspect of the invention, the supercritical CO ₂ fluid or the liquid CO ₂ can be transported by rotationally driving the impeller, and can be used as a circulating pump for semiconductor cleaning, so that it is washed after washing. The material does not need to be dried, so the cleaning process can be simplified and the cost can be reduced.

[Best Embodiment of the Invention]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred pumping embodiment related to the present invention will be described in detail with reference to the accompanying drawings. In addition, the invention is not limited to the embodiment.

[Examples]

Fig. 1 is a cross-sectional view showing a pump for charging a semiconductor cleaning apparatus according to an embodiment of the present invention, and Fig. 2 is an enlarged view showing a main portion of a circulating pump for a semiconductor cleaning apparatus according to the present embodiment.

The circulator pump for the semiconductor cleaning apparatus of the present embodiment includes a casing 13 having a suction port 11 and a discharge port 12 as shown in Figs. 1 and 2, and a ball bearing 14 is used in the casing 13. a spindle 16 that is rotatably supported; an impeller 17 coupled to the shaft end of the spindle 16; and a shield motor 18 that can drive the rotary impeller 17 through the spindle 16 so that the suction port 11 can be sucked by the rotation of the impeller 17 The fluid pressure is discharged from the discharge port 12.

The casing 13 is configured such that the annular discharge and suction side casing 21 and the flushing side casing 22 are disposed so as to sandwich the cylindrical outer casing 23, and are connected by a joint bolt 24, and are discharged to the suction side machine. The manifold 25 is fixed to the outer side of the case 21, and is coupled by a connecting bolt 26. Next, the manifold 25 is a suction port 11 in which a liquid is formed on the axial extension axis of the main shaft 16, and a discharge port 12 is formed on the outer peripheral side of the suction port 11.

The ball bearings 14, 15 are bevel ball bearings which are mounted on the discharge and suction side casing 21 and the flush side casing 22, and support the main shaft 16 so as to be rotatable. Next, the impeller 17 is fitted to the shaft end of the main shaft 16, and is fixed by a joint bolt 27.

Further, in the present embodiment, the ball bearings 14, 15 are made of a ceramic material for the inner and outer wheels and the balls in order to improve the wear resistance and corrosion resistance and to reduce the centrifugal load at the time of high-speed rotation (for example, tantalum nitride Si ₃ N ₄ , aluminum Al ₂ O ₃ , tantalum carbide SiC, etc.). As described above, the bearing material is entirely ceramic, and it is possible to improve the wear resistance of the bearing particles with respect to the foreign particles. In addition, the retainer is designed to reduce drag loss (rotational resistance). As a result, it is possible to prevent the revolution from sliding and reduce the preload load (thrust shaft load), and it is possible to achieve a longer life of the ball bearings 14 and 15. The material of the retainer is a PEEK material (polyether ketone ether) from the viewpoint of its corrosion resistance to the detergent, abrasion resistance, and strength for ensuring high-speed rotation. Alternatively, stainless steel or fiber composites can be used in place of PEEK.

The shield motor 18 is composed of a stator 28 that is fixed to the inner circumference of the outer cylinder 23, and a rotor 29 that is disposed on the outer periphery of the main shaft 16 so as to face the stator 28.

On the other hand, in the flushing side casing 22, a flushing port 30 capable of discharging a part of the sucked liquid is formed on the axial extension axis of the main shaft 16. Further, a preload spring 31 is interposed between the flushing side casing 22 and the bevel ball bearing 15. The preload spring 31 is formed by laminating a plurality of annular wave plate springs at a position around the other end of the main shaft 16, and applies a preload in the axial direction to the ball bearing 15 by a constant pressure spring.

Therefore, when the shield motor 18 is energized, the rotor 29 will rotate about the stator 28, and the main shaft 16 will rotate together with the rotor 29, and the rotation will cause the impeller 17 to rotate. As a result, the liquid is sucked from the suction port 11, and is boosted by the centrifugal force of the impeller 17, and is guided to the outside of the discharge port 12 side. Further, a part of the liquid suctioned from the suction port 11 passes through the ball bearings 14, 15 and the shield motor 18, and is cooled and discharged from the flushing port 30 as a flushing flow.

In the circulatory pump of the above configuration, in the present embodiment, the impeller 17 is of a closed type, and the main shaft 16 and the impeller 17 are supported relative to the casing 13 so as to move freely by a predetermined amount in the direction of the axial center simultaneously with the casing 13 and the impeller. The radial direction of 17 forms a seal.

In other words, the suction and discharge side casing 21 for constituting the casing 13 has a receiving hole 41 that can communicate with the suction port 11 at its center portion, and a peripheral ring 42 and a peripheral ring are fixed to the inner peripheral surface of the receiving hole 41. 43. On the other hand, the impeller 17 is configured such that it is disposed between the annular front edge flange 44 provided on the front side in the axial direction and the disk-shaped rear edge flange 45 provided on the rear side in the axial direction. A plurality of vanes 46 are provided at intervals.

In this case, the front flange 44 has a disk portion 44a horizontal to the radial direction of the impeller 17, and a cylindrical portion 44b horizontal to the axial direction. Next, between the one end portion of the outer ring 43 (the casing 13) and the surface portion of the disc portion 44a of the front flange 44, a predetermined gap 47 that faces in the axial direction is provided. Further, between the inner peripheral portion of the outer ring 43 (the casing 13) and the outer peripheral portion of the tubular portion 44b of the front flange 44, sealing portions 48, 49 facing each other in the radial direction are provided. The seal portions 48 and 49 are arranged away from the radial direction of the impeller 17, and are arranged in the axial direction of the impeller 17. Further, the sealing portions 48 and 49 are preferably provided in plural numbers, and are not limited to two, and may be provided in three or more.

Further, in the casing 13, the fluid outlet 17a from the impeller 17 communicates with the discharge port 12 through the diffuser 51, the volute 52, and the connecting passage 53. In other words, the joint portion between the suction and discharge side casing 21 and the manifold 25 is formed with a fluid outlet 17a from the impeller 17, and a manifold 25 is formed in communication with the fluid outlet 17a. The diffuser 51 converts the velocity energy (dynamic pressure) of the fluid into pressure energy (static pressure). The vortex chamber 52 is formed in a spiral shape at the joint portion between the suction and discharge side casing 21 and the manifold 25, and the one end portion communicates with the diffuser 51, and the other end portion communicates with the connecting passage 53.

Next, the diffuser 51 is provided with a necking portion. That is, the diffuser 51 is a width W ₂ (flow path area) toward to the side of the scroll chamber 52 of the flow path outlet portion with respect to the flow path inlet portion 17 side fluid outlet from the impeller 17, the width W ₁ (the flow path area The formation is relatively narrow (small), thereby forming a necking portion. That is, the inlet portion of the diffuser 51 is formed larger than the outlet portion, thereby allowing the specified gap 47 to cause the impeller 17 to move in the axial direction, and the pressure fluid can be appropriately introduced into the diffuser 51.

The shape of the neck portion of the diffuser 51, that is, the angle of inclination, is set based on the fluid outflow angle α from the outlet portion of the diffuser 51 and the area ratio Y of the area of the outlet portion of the diffuser 51 to the area of the scroll chamber 52. The fluid flowing from the diffuser 51 to the vortex chamber 52 has a fluid outflow angle α to the wiring of the diffuser 51, and its velocity V is divided into a circumferential direction velocity Vθ and a radial direction velocity Vm. The area ratio Y of the vortex chamber 52 is a ratio (Y = Ad / Av) of the outlet portion passage area Ad of the diffuser 51 and the area Av of the vortex chamber 52.

Generally, the fluid outflow angle α is the loss due to the friction loss between the outlet portion of the diffuser 51 and the inlet portion of the vortex chamber 52, and the speed difference between the discharge speed of the diffuser 51 and the inflow velocity of the volute 52. The 15° level ensures maximum pump efficiency. Further, the tongue end loss is generated when the difference between the tongue end mounting angle of the volute chamber 52 and the fluid outflow angle is too large. The reason for this is that if the thickness of the tongue end of the volute chamber 52 is too thick, the tongue end mounting angle of the volute chamber 52 is difficult to be several degrees (small angle).

As a result, in the present embodiment, the diffuser 51 is provided with the neck portion, so that the fluid outflow angle α at the outlet portion of the diffuser 51 is formed to be large, thereby reducing the loss. Further, the width of the inlet portion of the diffuser 51 is made large, and the axial direction of the impeller 17 can be allowed to move. In this case, when the diffuser 51 is shrunk, the radial direction velocity Vm rises, but the circumferential direction velocity Vθ is stored as a deceleration (free vortex) via the angular movement amount. Therefore, the diffuser 51 is formed into a neck shape in which the total speed of the radial direction velocity Vm and the circumferential direction velocity Vθ is reduced to a deceleration degree.

Therefore, when the impeller 17 rotates, the liquid is sucked from the suction port 11, and the centrifugal force is increased by the centrifugal force of the impeller 17. The boosted liquid is passed through the diffuser 51 from the liquid outflow port 17a, whereby the velocity energy of the fluid is converted into pressure energy, and then flows to the volute chamber 52, and is discharged from the discharge portion 12 to the outside through the connecting passage 53. At this time, since the casing 13 and the impeller 17 are sealed in the radial direction by the sealing portions 48 and 49, the liquid which is pressurized by the impeller 17 does not leak into the suction port, and can appropriately flow into the diffuser 51 from the fluid outlet 17a. .

Further, even if the main shaft 16 and the impeller 17 are moved in the axial direction by the predetermined gap 47, the sealing portions 48 and 49 can change the positional relationship between the outer ring 43 and the front flange 44, so that the liquid does not leak into the suction. The port 11 can be appropriately flowed into the diffuser 51.

As described above, the pumping of the present embodiment is such that the main shaft 16 is rotatably supported by the casing 13 having the suction port 11 and the discharge port 12 by the ball bearings 14, 15, and the impeller 17 is coupled to the shaft end portion of the main shaft 16. The rotary impeller 17 can be driven by the shield motor 18 through the main shaft 16, and the front flange 44 is provided on the front side in the axial direction of the impeller 17, and the rear flange 45 is provided on the rear side in the axial direction. A predetermined gap 47 in the axial direction is provided between the casing 13 and the front flange 44, and sealing portions 48 and 49 facing each other in the radial direction are provided between the casing 13 and the front flange 44.

Therefore, the sealing portions 48 and 49 can prevent the fluid from flowing back from the discharge port 12 to the suction port 11, and the main shaft 16 and the impeller 17 move in the direction of the rotation axis even if the wear of the ball bearings 14, 15 and the like causes the impeller. A predetermined gap 47 is provided between the 17 and the casing 13, so that the two do not interfere with each other, so that the pump life can be extended.

Further, the pumping of the present embodiment is provided in a plurality of positions in which the sealing portions 48 and 49 are arranged in the axial direction of the impeller 17. Therefore, fluid leakage between the impeller 17 and the casing 13 can be reduced by the plurality of sealing portions 48 and 49, and the backflow of the fluid from the discharge port 12 to the suction port 11 can be appropriately prevented.

Further, in the pumping of the present embodiment, the fluid outlet 17a from the impeller 17 is communicated with the discharge port 12 through the diffuser 51 and the volute 52, and the diffuser 51 is provided with a neck portion. Therefore, the total velocity of the fluid flowing out from the fluid outlet 17a of the impeller 17 to the diffuser 51 in the circumferential direction and the velocity in the radial direction are decelerated, whereby the velocity energy (dynamic pressure) of the fluid can be appropriately converted in the diffuser 51. Pressure energy (static pressure).

At this time, since the neck portion 51 is provided with the neck portion, the fluid outflow angle α at the outlet portion of the diffuser 51 can be made large, whereby the loss can be reduced. Further, the width of the inlet portion of the diffuser 51 is made large, whereby the loss of the diffuser inlet of the impeller outlet caused by the axial direction movement of the impeller 17 can be reduced.

Further, the shape of the neck portion of the diffuser 51 is set in accordance with the fluid outflow angle from the outlet portion of the diffuser 51, and the passage area ratio of the outlet portion of the diffuser 51 to the inlet portion of the scroll chamber 52. Therefore, by setting the neck portion to an appropriate shape, the radial speed of the diffuser 51 can be accelerated, but the speed in the circumferential direction is decelerated, and the speed is decelerated based on the total speed. As a result, the speed energy of the fluid can be decelerated by the diffuser 51. Properly converted to pressure energy, and in addition, the fluid velocity of the vortex chamber 52 is at an appropriate speed to improve pump efficiency.

Further, in the pumping of the present embodiment, a preloading spring 31 for applying a preload to the ball bearings 14 and 15 is provided, and the preloading spring 31 is formed by laminating a plurality of annular wave plates. Therefore, by applying the preload to the ball bearings 14 and 15 by the preload spring 31, it is possible to suppress the revolution of the ball bearings 14, 15 and to extend the service life, and to reduce the spring by stacking the wave plates in plural to form the preload spring 31. The amount of change in the load relative to the amount of change in the pressure can be applied with an appropriate preload.

Further, in the pump of the present embodiment, the supercritical CO ₂ fluid or the liquid CO ₂ can be transported by rotationally driving the impeller 17, and can be used as a circulating pump for semiconductor cleaning, so that the washed laundry is not Drying is required to simplify the cleaning process and reduce costs.

Further, the above embodiment is an example in which the pump of the present invention is circulated for semiconductor cleaning, but the pump of the present invention can also be applied to a normal centrifugal pump.

[Industrial availability]

The pump according to the present invention is provided with a predetermined gap between the casing and the front edge between the casing and the front edge, and a sealing portion facing the radial direction between the casing and the front edge is provided. This configuration is a pump capable of suppressing leakage of a fluid and framing the member and capable of prolonging the service life, and can be applied to any pump.

11. . . suction point

12. . . Spit

13. . . cabinet

14,15. . . Ball bearing

16. . . Spindle

17. . . impeller

18. . . Shield motor

31. . . Preload spring

44. . . Front margin

45. . . Rear margin

46. . . blade

47. . . Specified gap

48, 49. . . Sealing part

51. . . Diffuser

52. . . Vortex chamber

The first circle is a cross-sectional view showing a pump according to an embodiment of the present invention as a cyclic pump for a semiconductor cleaning apparatus.

The second circle is an enlarged view of a main portion of the circulating pump for the semiconductor cleaning apparatus of the present embodiment.

11. . . suction point

12. . . Spit

13. . . cabinet

14,15. . . Ball bearing

16. . . Spindle

17. . . impeller

18. . . Shield motor

twenty one. . . Spit and cum side chassis

twenty two. . . Flush side case

twenty three. . . Outer tube

twenty four. . . Connecting bolt

25. . . Fistula

26. . . Connecting bolt

27. . . Connecting bolt

28. . . stator

29. . . Rotor

30. . . Flushing port

31. . . Preload spring

43. . . Peripheral ring

44. . . Front margin

45. . . Rear margin

46. . . blade

47. . . Specified gap

48, 49. . . Sealing part

51. . . Diffuser

52. . . Vortex chamber

53. . . Connected channel

Claims (8)

a pump having: a casing having a suction port and a discharge port; a spindle that is rotatably supported by a ball bearing in the casing; an impeller coupled to an end of the spindle shaft; and the impeller that can be driven to rotate through the spindle The shield motor is configured to eject the fluid sucked in the suction port from the discharge port by the rotation of the impeller, and is characterized in that a front flange is provided on the front side of the impeller in the axial direction, and the impeller is a rear edge is provided on the rear side of the axial direction, and a predetermined gap facing the axial direction is provided between the casing and the front edge covering, and a gap is provided between the casing and the front edge The seal portion facing in the radial direction connects the fluid outlet from the impeller to the discharge port through the diffuser and the volute chamber in the casing, and the diffuser is provided with a neck portion. The pump according to claim 1, wherein the sealing portion is arranged in a plurality of rows in the axial direction of the impeller. The pump according to claim 2, wherein the fluid outlet from the impeller passes through the diffuser and the volute chamber to communicate with the discharge port, and the diffuser is provided with a neck portion. The pump according to claim 3, wherein the shape of the neck portion is set according to a fluid outflow angle of the diffuser outlet and a passage area ratio between the diffuser outlet and the scroll chamber. The pump described in claim 3, wherein the above The shape of the neck portion is set according to the fluid outflow angle of the diffuser outlet and the passage area ratio of the diffuser outlet to the scroll chamber. The pump according to any one of claims 1 to 5, wherein a preloading spring that applies a preload to the ball bearing is provided, and the preloading spring is laminated by a plurality of annular wave plates. Composition. The pump according to any one of claims 1 to 5, wherein the impeller is capable of transporting a supercritical CO ₂ fluid or a liquid CO ₂ by a rotary drive, and is used as a semiconductor cleaning circulating pump. Pu used. The pump according to claim 6, wherein the impeller is capable of transporting a supercritical CO ₂ fluid or liquid CO ₂ by rotational driving, and is used as a cyclic pump for semiconductor cleaning.