Abstract:
A component of a disc drive has a coating of a predetermined length on its surface, the coating having at least two separate tapered regions applied in independent steps, the at least two separate tapered regions each having a length that is less than the predetermined length of the component surface. When the component is a shaft of a spindle motor, the ends of the shaft are masked before the tapered regions of coating are applied, and the thickness of the masks covering the shaft ends is varied to control a taper of tapered regions.

Description:
BACKGROUND 
     The present invention relates to controlling the amount of taper that occurs during shaft coating processes, for example on spindle motor shafts, and more particularly to using a multi-step coating process to control the amount of coating taper during shaft coating processes. 
     DESCRIPTION OF RELATED ART 
     Groove regions, such as grooved pumping seal regions, have been used in fluid dynamic bearing (FDB) motors. Some FDB motors have a predominantly straight journal bearing formed by opposing inner and outer surfaces of relatively rotating components. For example, a journal bearing may be formed between an inner surface of a bearing sleeve and an outer surface of a shaft. Such journal bearings are designed to maintain a gap between the inner and outer surfaces. Lubricating liquid is commonly disposed in the gap. 
     Grooved regions, such as grooved pumping seal regions, are typically disposed at one or both ends of the relatively rotating components. The grooved regions may be for pumping lubricating liquid away from openings from which lubricating liquid may escape and/or evaporate. The grooved regions may also be for establishing a minimum flow of lubricating liquid within portions of the motor. Grooved regions may tend to evacuate lubricating liquid from a portion of the journal, and therefore there is some danger that the relatively rotating components may contact each other if jolted or jarred during operation. Such contact may cause wear in the components and may increase risk of premature drive failure. 
     Further, hydrodynamic fluid bearings used in FDB motors have tight radial gap tolerances. The dynamic performance of a FDB motor is a function of its gap tolerance. One way to maintain the gap tolerance is to have a suitable pair of relatively rotating components (e.g., shaft and sleeve) that ensures insignificant wear of the components if they contact each other upon being jolted or jarred during operation. This can be achieved by coating the surface of one of the components, and selecting a suitable countersurface for the other component. Sputtered carbon or diamond-like carbon (DLC) is a wear-resistant layer used on high performance spindle motor parts. 
     Coating a shaft using conventional sputtering processes is a challenge due to thickness variation along the length of the shaft. The thickness variation most commonly creates a taper in the shaft&#39;s diameter, with coating thickness being the greatest at a point along a surface that is closest to the sputtering target (source). Coating thickness gradually decreases at points of the surface that are farther from the target, creating the taper. In general, taper increases as the coating length (distance from target) increases and as the coating thickness increases. 
       FIG. 1  illustrates an exemplary magnetic disc drive storage system  10 , including a housing base  12  to which is mounted a spindle motor  14  that rotatably carries storage discs  16 . An armature assembly  18  moves transducers  20  across the surface of the discs  16 . The environment in which discs  16  rotate may be sealed by seal  22  and cover  24 . In operation, discs  16  rotate at high speed while transducers  20  are positioned at any one of a radially differentiated track on the surface of the discs  16 . This allows transducers  20  to read and write magnetically encoded information on the surfaces of discs  16  at selected locations. Discs  16  may rotate at many thousands of RPM. 
     To rotate the discs  16 , spindle motor  14  typically includes at least one rotatable portion that is supported by one or more bearing surfaces providing a low friction interface with a relatively non-rotating surface. In some exemplary motors, a shaft may rotate within a journal of a fixed bearing sleeve while in others the shaft may be stationary and the bearing sleeve may rotate about the shaft. Aspects described herein may be used in a variety of motor types, even where described with reference to only one motor type. 
       FIG. 2  illustrates a cross-section of an exemplary spindle motor  14 , including a bearing sleeve  205  with a journal  210  defined by its interior surface (not separately indicated). As illustrated, journal  210  extends from a top  206  of bearing sleeve  205  to a bottom  207  of the motor&#39;s cross section. Groove regions  215 ,  216  are disposed within the journal  210 . Groove regions  215 ,  216  may be asymmetrical and may function as pumping seals and/or to recirculate lubricating liquid through portions of motor  14 . A shaft  220  is disposed within journal  210 . Shaft  220  includes an outer radial surface  221  (illustrated in  FIG. 3 ) that radially opposes the interior surface of journal  210 , to form a gap (not separately indicated) where a hydrodynamic bearing region provides for low friction rotation of shaft  220  in journal  210 . The gap between the interior surface and the shaft  220  may vary in size and shape among motor designs. 
     Shaft  220  may generally be an elongate member with outer radial surface extending from a first end  222  to a second end  224 . In some aspects, shaft  220  may be approximately cylindrical, and first end  222  may have an approximately circular first end surface  225 . Likewise, second end  224  may have an approximately circular second end surface  226 . If desirable, shaft  220  may be crowned or conical (e.g., having a larger diameter at one end) for a journal of a corresponding shape. 
     An exemplary prior art coated shaft having a taper created by a conventional one-step coating process is illustrated in  FIG. 3 . The coating typically covers a predetermined length L of the shaft. 
     SUMMARY 
     The present invention proposes controlling, preferably for decreasing, the taper of a surface coating by utilizing a multi-step coating process. 
     The present invention relates to a method for coating a predetermined length of a shaft having first and second ends. The method comprises covering the first end of the shaft with a mask, applying a first length of coating to the shaft from a first target that is located near one of the first or second ends, the first length of coating being shorter than the predetermined length of the shaft, and applying a second length of coating to the shaft from the first target or a second target that is located near the other of the first or second ends, the second length of coating being shorter than the predetermined length of the shaft. A thickness of the mask is varied to control a taper in a thickness of the first length of coating. 
     The invention also relates to a method for coating a predetermined length of a component of a disc drive. The method comprises applying multiple lengths of coating to the component from at least one target positioned near at least one end of the component, each length of coating being less than the predetermined length of the component. The coating has substantially no taper over the predetermined length of the component. 
     The present invention further relates to a component of a disc drive has a coating of a predetermined length on its surface, the coating having at least two separate tapered regions applied in independent steps, the at least two separate tapered regions each having a length that is less than the predetermined length of the component surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For describing aspects and examples herein, reference is made to the accompanying drawings in the following description. 
         FIG. 1  illustrates a plan view of a conventional disc drive. 
         FIG. 2  illustrates a cross-section of a conventional motor having a shaft relatively rotatable with respect to a journal of a bearing sleeve. 
         FIG. 3  illustrates a vertical cross section of a shaft coated with a conventional one-step method. 
         FIG. 4  illustrates a vertical cross section of a shaft coated in accordance with an embodiment of the present invention. 
         FIG. 5  illustrates a vertical cross section of a shaft coated in accordance with another embodiment of the present invention. 
         FIGS. 6   a - d  illustrate cross sections of various portions of the shaft of  FIG. 4 . 
         FIG. 7  illustrates the effect of mask thickness on coating taper. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable a person of ordinary skill in the art to make and use various aspects of the inventions. Descriptions of specific materials, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the invention. For example, aspects and examples may be employed in a variety of motors, including motors for use in disc storage drives. Motors for disc storage drives may be designed and may operate in a number of ways. Exemplary subject matter provided herein is for illustrating various inventive aspects and is not intended to limit the range of motors and devices in which such subject matter may be applied. 
       FIG. 4  illustrates an exemplary schematic vertical cross-section of shaft  220  that was coated in accordance with the method of the present invention. This exemplary cross-section is not drawn to scale, so that aspects may be better illustrated. As described above, shaft  220  includes first end  222  and second end  224 . Outer radial surface  221  extends from first end  222  to second end  224 . 
     As illustrated in  FIG. 4 , first coating region  235  may be disposed on outer radial surface  221  proximate first end  222 . As illustrated, a portion  302  of outer radial surface  221  remains uncoated, and the coating region  235  is generally thicker near first end  222 , becoming thinner toward second end  224 . Thus, coating  235  may taper monotonically from near first end  222  towards second end  224 . The thickness of coating region  235  near a middle portion  309  of outer radial surface  221  may be negligible. In a preferred embodiment of the invention, a mask M covers first end  222  to prevent a coating from being formed thereon. Additionally, middle portion  309  may be covered with a mask (not shown) to prevent a coating from being formed thereon. When a mask is used, coating region  235  may terminate abruptly near first end  222 , as illustrated by shoulder  305 . 
     In accordance with the present invention, the middle portion  309  may have a negligible amount of coating, even when the coating portions  235 ,  240  extend to meet each other in the central portion of the shaft. Each coating then covers about a half of the predetermined length of the shaft. In a preferred embodiment of the invention, the entire length of the shaft (with the exception of the masked ends) is covered with some amount of coating. 
     A second coating region  240  may be disposed on outer radial surface  221 , proximate second end  224 . A portion  303  of outer radial surface  221  remains uncoated (preferably by masking it before coating is applied), and the coating region  240  is generally thicker near second end  224 , becoming thinner toward first end  222 . Thus, coating  240  may taper monotonically from near second end  224  towards first end  222 . The thickness of coating region  240  near the middle portion  309  of outer radial surface  221  may be negligible. A predetermined length L from the shoulder  305  of coating region  235  to the shoulder  310  of coating region  240  is generally the same as the length L of the coating accomplished via conventional one-step coating processes as illustrated in  FIG. 3 . 
     In accordance with the present invention, coating the shaft in a two-step process allows coating taper to be controlled so that, for example, it can be reduced to improve radial gap tolerance. In the embodiment illustrated in  FIG. 4 , a sputtering target would be placed adjacent the first end  222  and the second end  224  for coating, either simultaneously or sequentially. In the case of sequential placement, the same target could be used at both ends of the shaft. The ends would preferably be masked, and the middle portion can also be masked If desirable. In a preferred embodiment of the invention utilizing sequential coating, the process involves applying the first coating  235  in a first process while keeping the rest of the shaft masked so that it is not coated, and then coating the masked half of the shaft in a second process (second coating  240 ) while making sure the first coating  235  is not coated again. 
     In the prior art coating process shown in  FIG. 3 , coating from a single source must cover the entire predetermined length L of the shaft. Therefore, a sputtering target located at one end of the shaft must apply enough coating to successfully coat the entire predetermined length L of the shaft. This results in a thicker coating being applied on the unmasked portion of the shaft that is closest to the sputtering target. Thus, the taper increases as coating length increases (i.e., there is a larger difference between the thickness of the coating closest to the source and the thickness of the coating farthest from the source). The present invention allows taper to be reduced by having two shorter coating lengths rather than a single, longer coating length. It is to be understood that the present invention contemplates breaking up the predetermined length L to be coated into any number of coating lengths, which may be applied simultaneously or sequentially. 
       FIG. 5  illustrates an exemplary schematic vertical cross-section of shaft  220  that was coated in accordance with another embodiment of the method of the present invention. In this embodiment, a three-step coating process is utilized. First coating region  235 ′ and second coating region  240 ′ are disposed on outer radial surface  221 ′ proximate the first and second ends  222 ′,  224 ′, respectively. Additionally, at least one additional coating region  245  is located between the first coating region  235 ′ and the second coating region  240 ′. As illustrated, portions  312 ,  314  of outer radial surface  221 ′ may have a negligible amount of coating applied, and may even have no coating, and the coating regions  235 ′  240 ′,  245  are generally thicker near the end at which their sputtering target was located. In a preferred embodiment of the invention, one or more masks (not shown) can be used to cover first end  222 ′, second end  224 ′, and even middle portions  312 ,  314  to prevent a coating from being formed thereon. 
     In accordance with the present invention, coating the shaft in a three-step process allows taper to be reduced by providing three shorter coatings rather than one long coating. In the embodiment illustrated in  FIG. 5 , a sputtering target would be placed adjacent the first end  222 ′ and the second end  224 ′. The sputtering target adjacent the first end  222 ′ would create the first coating region  235 ′ and the additional coating region  245 , and the sputtering target adjacent the second end  224 ′ would create the second coating region  240 ′. The present invention contemplates alternative placement of the sputtering targets. For example, a single target located at either the first end  222 ′ or the second end  224 ′ could be used to create all of the coating regions  235 ′,  240 ′,  245 . As stated above, the ends  222 ′,  224 ′ would preferably be masked. 
     In a preferred embodiment of the invention utilizing sequential coating, as described above for the two-step process illustrated in  FIG. 4 , the process involves applying each coating while keeping the rest of the shaft masked. That way, areas that should not be coated will remain uncoated, and each coating region will only be coated a single time. 
     Exemplary cross-sections of the shaft  220  of  FIG. 4  with an applied coating are illustrated in  FIGS. 6   a - d .  FIGS. 6   a - d  are not drawn to scale, but instead are drawn for illustrating various aspects discussed below.  FIG. 6   a  illustrates a portion of shaft  220  proximate first end  222  and substantially without coating. The coating regions  235 ,  240  may be disposed to make shaft  220  approximately symmetric about a center of the shaft; that is, thicknesses of each coating region may be approximately circumferentially equal at equivalent distances from respective ends of the shaft. 
       FIG. 6   b  illustrates a thicker portion of coating region  235 , designated as shoulder  305  in  FIG. 4 . The thicker portion of coating region  240 , at shoulder  310  in  FIG. 4 , preferably looks substantially the same. In  FIG. 6   b , the approximately annular shape of the cross-section of coating region  235  is evident upon recognizing shaft  220 , about which coating region  235  is disposed. 
       FIG. 6   c  illustrates a central portion of coating region  235 , designated at  307  in  FIG. 4 . As illustrated, the coating thickness at  307  is thinner than the thickness at  305  (illustrated in  FIG. 6   b ).  FIG. 6   d  illustrates a thinner portion of coating region  235  near the shaft middle  309  (approximately designated). As illustrated, a thickness of coating region  235  at  309  begins to be negligible compared with the diameter of shaft  220 . As discussed above, middle portion  309  may be shielded during coating to keep middle portion  309  substantially free from coating. Portions of coating region  240  preferably look substantially the same. 
     Coating region  235  and coating region  240  preferably comprise a suitable coating material deposited on the outer radial surface  221  of the shaft  220 . The present invention contemplates utilizing a physical vapor deposition (PVD) process (sputtering process), and other deposition processes, to coat the shaft. 
     Coating material may include any variety of suitable material, including diamond-like coating materials and ceramic-type materials. The present invention contemplates a single coating for each coating region, multiple coatings of the same material for each coating region, or multiple separate coatings for each coating region, where each separate coating includes a different material. By example, a first layer of a coating region may be designed to improve adhesion of a later disposed carbon-rich layer. Coating material may also be disposed in numerous coatings, depending on a desired coating thickness and devices used in forming the coating (e.g., some machines may be limited in growth rate per time, or the shaft  220  may be examined during coating deposition). 
     In one exemplary embodiment of the invention, the coating regions are approximately 0.5-3.0 μm thick at their thickest points and taper uniformly toward a central portion of the shaft. In exemplary aspects, near shaft middle  309  in  FIG. 4 , coating region  235  and/or coating region  240  are of negligible thickness, for example, less than 0.5 μm. For other background relating to coatings, refer to U.S. Pat. No. 6,664,685, entitled, “HIGH ADHESION, WEAR RESISTANT COATINGS FOR SPINDLE MOTORS IN DISK DRIVE/STORAGE APPLICATIONS,” filed on Dec. 13, 2001, which is incorporated in its entirety by reference. 
     As described briefly above, to establish coating region  235  in  FIG. 4 , coating material is preferably provided from near first end  222 , and to establish coating region  240  in  FIG. 4 , coating material is preferably provided from near second end  224 . By providing the coating material from near the shaft ends in the present invention, a differential in coating thickness may be established through diffusion along outer radial surface  221  from a source of coating material. Thus, in the present invention, the outer radial surface  221  lies substantially parallel to a source direction of coating material (e.g., the general direction of travel of the coating material from the target). Providing coating material from near the shaft ends more easily establishes a desirable taper shape for the coating regions. 
     The present invention also contemplates controlling coating taper by varying the thickness of the mask M placed over the ends  222 ,  224  of the shaft  220 .  FIG. 7  illustrates a shaft  220  having a mask M placed over each end  222 ,  224  prior to applying coatings  235 ,  240 . The masks M have a thickness as indicated by the arrows. Various distances along the length of the shaft  220  are indicated. Measurement of coating thickness at these distances is discussed below. Studying the effect of two mask thicknesses (100 microns and 250 microns) on resulting coating thickness and taper has shown that a thinner mask applied to an end of the shaft achieves a desired coating thickness at a shorter distance from the mask (and therefore at a shorter distance from the shaft end). In addition, a thicker mask provides a reduced coating taper. Thus, varying the thickness of a mask applied at the ends of the shaft allow the thickness and taper of the adjacent coatings to be controlled. 
     For example, using a known sputtering target near the shaft end and a mask on each shaft end  222 ,  224  with a 250 micron thickness, the first coating  235 ″ has the following thicknesses and standard deviations: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance 
                 Average Coating Thickness 
                 Standard Deviation 
               
               
                   
                   
               
             
             
               
                   
                 2.10 mm 
                 0.00036 mm 
                 0.00009 mm 
               
               
                   
                 2.46 mm 
                 0.00086 mm 
                 0.00007 mm 
               
               
                   
                 3.02 mm 
                 0.00094 mm 
                 0.00006 mm 
               
               
                   
                 3.57 mm 
                 0.00090 mm 
                 0.00006 mm 
               
               
                   
                   
               
             
          
         
       
     
     Using a mask with a 250 micron thickness, the second coating  240 ″ has the following thicknesses and standard deviations: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance 
                 Average Coating Thickness 
                 Standard Deviation 
               
               
                   
                   
               
             
             
               
                   
                 4.65 mm 
                 0.00081 mm 
                 0.00009 mm 
               
               
                   
                 5.46 mm 
                 0.00087 mm 
                 0.00007 mm 
               
               
                   
                 6.26 mm 
                 0.00076 mm 
                 0.00006 mm 
               
               
                   
                 6.65 mm 
                 0.00023 mm 
                 0.00006 mm 
               
               
                   
                   
               
             
          
         
       
     
     Using a mask with a 100 micron thickness, the first coating  235 ″ has the following thicknesses and standard deviations: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance 
                 Average Coating Thickness 
                 Standard Deviation 
               
               
                   
                   
               
             
             
               
                   
                 2.10 mm 
                 0.00063 mm 
                 0.00012 mm 
               
               
                   
                 2.46 mm 
                 0.00103 mm 
                 0.00008 mm 
               
               
                   
                 3.02 mm 
                 0.00101 mm 
                 0.00007 mm 
               
               
                   
                 3.57 mm 
                 0.00095 mm 
                 0.00007 mm 
               
               
                   
                   
               
             
          
         
       
     
     Using a mask with a 100 micron thickness, the second coating  240 ″ has the following thicknesses and standard deviations: 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Distance 
                 Average Coating Thickness 
                 Standard Deviation 
               
               
                   
                   
               
             
             
               
                   
                 4.65 mm 
                 0.00084 mm 
                 0.00007 mm 
               
               
                   
                 5.46 mm 
                 0.00089 mm 
                 0.00007 mm 
               
               
                   
                 6.26 mm 
                 0.00094 mm 
                 0.00006 mm 
               
               
                   
                 6.65 mm 
                 0.00055 mm 
                 0.00015 mm 
               
               
                   
                   
               
             
          
         
       
     
     Standard deviation is measured as a routine parameter for coating processes. 
     The present invention contemplates other variations to the coating process, including shielding various portions of the shaft  220  for a portion of the predetermined amount of time and exposing those portions for a remaining time. Further variations may include emitting matter for different amounts of time from each source to establish asymmetrical coatings. Still further variations may include measuring coating thickness during deposition and ceasing provision of matter when a desired thickness has been achieved (in addition to or in place of emitting coating material for the predetermined time). Deposition may also be conducted in numerous discrete time intervals, rather than a single predetermined time. Shaft  220  may be disposed on a stationary holder or a conveyor that moves parallel to the first and/or the second matter source. Other modifications and variations may be apparent to one of skill in the art. 
     To summarize certain aspects of the invention, a thickness gradient or taper of coating material may be established from first end  222  and extending towards middle portion  309 . A similar thickness gradient or taper may be established from second end  224  by either a separate source of coating material or by the same source after coating region  235  has been formed (or vice versa if coating region  240  were formed first). By applying the coating in more than one step, as described in detail above, a coating with substantially no taper over the predetermined length L of a workpiece is achieved. 
     Other modifications and variations would also be apparent to those of ordinary skill in the art from the exemplary aspects presented. For example, the method of the invention can be applied to coating other types of workpieces that require a controlled coating taper and have a length of surface requiring coating that would benefit from being coated in a two or more smaller lengths. In addition, various exemplary methods and systems described herein may be used alone or in combination with various fluid dynamic bearing and capillary seal systems and methods. Additionally, particular examples have been discussed and how these examples are thought to address certain disadvantages in related art. This discussion is not meant, however, to restrict the various examples to methods and/or systems that actually address or solve those disadvantages.