Abstract:
A centrifugal wafer drying apparatus comprises a rotor rotatably disposed in a container, a cradle mounted on the rotor, and a carrier mounted in the cradle and having grooves for receiving wafers. The carrier holds the wafers at an oblique angle with respect to a plane normal to the axis of rotation of the rotor, at least during rotation of the rotor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a centrifugal wafer drying apparatus used for drying after washing a semiconductor wafer such as a silicon wafer. 
     FIGS. 1A and 1B show a prior art centrifugal wafer drying apparatus having carriers 2 each receiving silicon wafers in a stack. Each of the carriers 2 has grooves 3 for accomodating wafers 1. Each of the grooves 3 extends along substantially half a periphery of a wafer 1, and has clearance with respect to the thickness of the wafer, e.g., it has a thickness of two to three times the thickness of the wafer to facilitate loading (insertion) and unloading (removal) of the wafer. The apparatus further comprises a cylindrical container 4, and cradles 5 made of stainless-steel disposed in the container 4 to hold the carriers 2. The cradles 5 are mounted on a rotor 6, which is fixed to a rotor shaft 7. The rotor shaft 7 is driven by a motor 8, so that the rotor 6 rotates together with the cradles mounted thereon. Water remaining on the wafers disperse due to the centrifugal force and the wafers 1 are thereby dried. 
     FIG. 2A and FIG. 2B are a plan view and a partial side view of the rotor 6. The rotor 6, disposed in the container 4 has an opening 6a, through which clean air, which has passed a filter (not shown) provided in a lid 4a  of the container 4, is supplied into the rotor 6 and air streams 11 (FIG. 3) are created along the surfaces of the wafers 1, being limited by walls 6b acting as guides. The air streams also serve to disperse water from the surfaces of the wafers 1. 
     Each of the cradles 5 is pivotably mounted on a pivot pin 5a extending at right angles with the rotor shaft 7 and with the direction of a radius from the axis of rotation RX which is an extension of the axis of the rotor shaft 7. Each of the cradles 5 is pivotable between the positions A and B, about 90°, as indicated by an arrow C. At the position A, the rotor is at rest and carriers 2 are loaded in or unloaded from the cradle 5. For this purpose, the cradles 5 each have an opening 5b which faces upward for loading and unloading of the carrier. Each of the carriers 2 has an opening 2a for loading and unloading of wafers in and out of the grooves 3. The opening 2a of the carrier 2 also faces upward when the cradle is at the position A. The cradle 5 is at the position B while the rotor 6 is rotating. 
     When the cradle 5 is at the position B, the opening 2a of the carrier 2 faces the axis of rotation RX of the rotor. At the position B, half the periphery of each wafer that is covered by the groove is positioned on the radially outer side with respect to the axis RX of rotation of the rotor 6. At the position B, the cradle 5 is engaged with a cradle stopper 9. The stopper 9 is provided to hold the cradle 5 in such a position that the surfaces of the wafers 1 are substantially orthogonal to the axis of rotation of the rotor 6. 
     Examples of prior art references showing this type of drying apparatus are Japanese Patent Application Laid-open No. 36930/1984, Japanese Patent Application Laid-open No. 160130/1982 and Japanese Patent Application Laid-open No. 8823/1981. 
     The above-described apparatus however has a disadvantage. This is explained with reference to FIG. 3, which is a section view taken along the periphery of wafers as received by the grooves. When the rotor 6 is at rest, the wafers 1 are at positions D where the lower surfaces 1a of the wafers 1 abut upwardly-facing edges 3a of the grooves 3. This is due to gravity. When the rotor 6 is rotating, the centrifugal force acts in the direction F, substantially parallel with the surfaces of the wafers 1, so that the wafers 1 vibrate between the position D, and the position E where the upper surfaces 1b abut the downwardly-facing edges 3b of the grooves 3. The actual condition which the individual wafers assume is affected by the air stream or wind caused by the rotation of the rotor. Thus, the resultant vibration of the wafers generates dust from the parts of the wafers 1 and the grooves 3 where they collide with each other. The vibration of the wafer also causes the air stream 11 to be unstable, and the effect of the water removal by the air stream 11 is reduced. The result is an unsatisfactory drying. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to prevent vibration of the wafers, thereby to reduce generation of dust and to improve the effect of removing water. 
     According to the invention, grooves holding the wafers are inclined with respect to a plane normal to the axis of rotation so that during rotation, each of the wafers is held against one of the edges, for instance downwardly-facing edge of the groove. The grooves can be inclined by inserting a spacer between the cradle stopper and the cradle. The angle of inclination is preferably within the range of 2.5° to 30°. 
     With the above structure, the wafers are stably held in the grooves abutting one of the edges by the action of the centrifugal force. Accordingly, vibration of the wafers is prevented. As a result, the generation of dust is reduced. Moreover, the air streams around the wafers are stabilized and the effect of removing water is improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a perspective view, partially broken away, showing the general construction of a centrifugal wafer drying apparatus. 
     FIG. 1B is a perspective view showing a cradle for accommodating wafers; 
     FIG. 2A is a plan view showing a rotor on which cradles are mounted; 
     FIG. 2B is a partial side view of the rotor; 
     FIG. 3 is a schematic sectional view showing how the wafers vibrate within the grooves; 
     FIG. 4 is a side view showing an embodiment of the invention; 
     FIG. 5 is a schematic sectional view, similar to FIG. 3, showing how the vibration of the wafers can be eliminated; and 
     FIG. 6 and FIG. 7 are histograms showing the performance of drying of the conventional arrangement and a centrifugal wafer drying apparatus according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 4 and FIG. 5 show a centrifugal wafer drying apparatus of an embodiment of the invention. 
     The general construction of the centrifugal wafer drying apparatus of this embodiment is similar to that shown in FIG. 1A, FIG. 1B and FIG. 2A, and the description made with reference to these figures also applies. Moreover, the description made with reference to FIG. 2B also applies in so far as it does not contradict the features shown in FIG. 4 and FIG. 5. According to the features of the embodiment as shown in FIG. 4 and FIG. 5, a spacer 21 is provided for each of the cradles and is fixed to the bottom wall of the cradle 5 to abut the cradle stopper 9 when the cradle is rotated downward. When the spacer 21 abuts the cradle stopper 9, the cradle 5 is inclined, and the carrier 2 accommodated in the cradle 5 and the grooves 3 of the carrier 2 in which the wafers 1 are inserted are inclined with respect to the plane normal to the axis RX of rotation of the rotor. The direction of inclination is such that a radially outer extremity 1e of each wafer positioned radially outward with respect to the axis RX of rotation is at the lowest height. This is advantageous because gravity does not cause the wafer to slide down out of the groove. Moreover, the angle of rotation of the cradle from the position A to the position B&#39; can be reduced. The angle θ of inclination is 11° , for example. In this position B&#39;, the rotor 6 rotates. The wafers 1 are then rotated keeping the angle of inclination, without vibration and held at an unequivocal position. 
     This will be explained with reference to FIG. 5, which shows a cross section along the same line as FIG. 3. As illustrated, when the grooves are inclined in the upward direction as shown in FIG. 5, the centrifugal force F serves to press the wafers 1 against the downwardly-facing edges 3b of the grooves and the wafers 1 are kept in this state. As a result, the vibration of the wafers 1 is prevented, and the generation of dust from those parts of the wafers 1 and the grooves 3 is reduced. Moreover, because the vibration of the wafers 1 is eliminated, the air streams around the wafers 1 are stabilized, and the effect of removing water from the wafers 1 is improved. 
     To prevent the vibration of the wafers, the angle θ of inclination must be large enough to create a greater moment than that of gravity while the rotor 6 is rotating at the designed speed. The moment exerted by the centrifugal force F is clockwise in FIG. 5 and about a point at which the radially outer extremity 1e of the wafer and the extremity 3e of the groove engage. The moment due to the gravity is counterclockwise. For example, when the rotor rotational speed is 1000 rpm, and the distance from the center of gravity of the wafer and the axis of rotation is 190 mm, the angle θ of inclination must be not smaller than 2.5° to prevent vibration of the wafer. Preferably, the angle θ should be not smaller than 11° to completely suppress the vibration. The minimum inclination angles, θ min, to completely suppress the vibration for the respective rotational speed N under the above condition are given below: 
     
         ______________________________________  N (rpm)         θmin (°)______________________________________   600   20   800   17  1000   11  1200    8  1400    5______________________________________ 
    
     The maximum inclination angle θmax, which is about 30° in the embodiment illustrated, is given where the air streams are blocked and the drying of the rear or upper surfaces of the wafers 1 is incomplete. Moreover, with a commonly-employed construction of the drying apparatus, a protruding part of the cradle 5 abuts an inner or lower surface of the lid 4a of the container 4. The inclination angle θ should therefore be within the range of 2.5° to 30°. 
     For evaluating the centrifugal wafer drying apparatus of the above described embodiment, various tests were carried out. The wafer 1 had a diameter of 125 mm. The rotor 6 was rotated at 1000 rpm for 70 seconds. The inclination angle θ was 11°. Similar tests were also carried out with a conventional centrifugal wafer drying apparatus, for the purpose of comparison. The results are as follows: (a) Dust 
     The wafer (test piece) was dipped in water and dried by rotation. The number of dust particles of not smaller than 0.3 micrometer that were attached during the rotation was counted. A wafer surface defect inspector was used for the counting. The same test was carried out 12 times, and the numbers of the dust particles counted were averaged. 
     (a1) With the conventional arrangement, the average number was 31. 
     (a2) According to the invention, the average number was 24.3. 
     (b) Removal of water 
     The wafer (test piece) was dipped in water, dried by rotation, and examined to ascertain whether a water drop remained. The same test was repeated five times for each time length of rotation (for which the rotor was rotated) as is shown at the left end of the histograms of FIG. 6 and FIG. 7. This indicates how many times out of the five trials a water drop remainder after drying by rotation. As seen from the histrograms, the conventional apparatus took 150 sec. to achieve complete drying without failure, while the apparatus according to the invention took only 50 sec. 
     Thus it is clear that inclining the wafers results in better drying and less dust. The invention therefore contributes to improvement in the yield and improvement in the productivity of the wafers. 
     In the embodiment described, the spacer 21 is fixed to the bottom surface of the cradle 5, and is made to abut the cradle stopper 9. Alternatively, the spacer 21 may be fixed to the cradle stopper 9, or the cradle stopper 9 itself may be shifted. It may be so arranged that the position of the cradle stopper 9 is adjustable to make the inclination angle variable. Moreover, the manner of hanging the cradle can be changed. For instance, the position of the pivot pin 5a can be changed to change the angle θ. 
     In the embodiment described the grooves are so formed that the wafers inserted therein are normal to the direction of the stack along which the wafers are stacked. Alternatively, the grooves can be formed to extend obliquely so that the wafers inserted therein are oblique with respect to the direction of the stack. In such a case, the spacer 21 can be eliminated. 
     Various other modifications can be made without departing the scope and the spirit of the invention.