Abstract:
A micro-rotating device has a rotor  200  (of diameter not more than 1.5 cm), a shaft  202  threaded through the rotor  200 , and a shaft holder  207  for holding the shaft. The shaft holders are formed by etching an Si substrate  501  to form multiple shaft receiving openings  508 . The rotors and shafts too are formed from respective Si substrates  301, 401 . The rotors  200  are located over the shaft holders  207 , and the shafts threaded through the rotors  200  into the openings  509  and attached there by a wafer bonding process. Then the substrate  501  is partitioned to give individual motor elements. Protrusions  206  extend from the rotor in the direction towards the stator to space the rotor from the shaft holder.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to MEMS (micro-electromechanical systems) devices, and in particular to MEMS devices which are axial type electromagnetic motors. The application is related to Singapore patent application no. 200304382.5, having the same priority date as the present application.  
       BACKGROUND OF INVENTION  
       [0002]     Motors are widely used as sources to actuate mechanical components. Unfortunately, most motors are quite big, as they have many parts which are produced by conventional machining and assembly techniques. For that reason, there has recently been much research to develop MEMS micro-rotating devices, that is mechanical devices having dimensions of sub millimeters (for example, in case of a motor, having a maximum diameter no more than say 15 millimeters). Examples of such micro-rotating devices include micro-pumps or motors. However, reducing the size of the motor by reducing the size of its parts increases the difficulty in handling them, and significantly increases production cost.  
         [0003]     Known micro-rotating devices have a rotor element which includes a sleeve and which rotates around a spindle shaft threaded through the sleeve. A typical micro-rotating device is illustrated in cross-section in  FIG. 1 . It comprises a rotor (rotating disc)  101  and a stator  105 . The stator  105  is formed with multiple coil windings (not shown). The spindle shaft  102  extends from the stator  105 , and passes through a central aperture  106  in the rotor  101 . The inner surface of the aperture  106  constitutes the sleeve for the shaft  102 . The rotor includes a trench on its surface facing the stator  105 . The trench encircles the aperture  106  and is filled with a permanent magnet  104  and a yoke  103 . The rotor  101  rotates about the spindle shaft  102  when there is a continuous flow of electrical current into the coil windings on the stator  105 . The micro-rotating device of  FIG. 1  is constructed by placing the sleeve of the rotor disc  101  onto the shaft  102 .  
         [0004]     In contrast to conventional motors, the known MEMS micro-rotating device has a reduced the number of parts because these are fabricated from substrates such as Si and Glass. However, there are still several parts which are required to be put together in the assembly process. For example, it is necessary to combine the shaft with the rotor when a motor is assembled. It is difficult to establish reliable processes for such assembly, because size of each part is very small.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention aims to provide a new and useful method for producing micro-rotating devices.  
         [0006]     In general terms, the present invention proposes that multiple shaft receiving openings are formed on substrate, that shafts are inserted into the openings and attached there, and that the substrate is then partitioned to give individual motor elements.  
         [0007]     Preferably, the rotor elements are attached before the substrate is partitioned. For example, during the process of inserting the shafts into the openings, the shafts may be threaded through the apertures in the rotor elements.  
         [0008]     The rotor elements and/or shaft elements can also be formed within respective substrates.  
         [0009]     Specifically, a first expression of the invention is a method for forming a plurality of micro-rotating devices, each having a shaft, a rotor having a central aperture for receiving the shaft, and a shaft holder, the method comprising the steps of forming a plurality of openings on a substrate to receive respective shafts, inserting respective shafts into the openings, and partitioning the substrate to form individual shaft holders.  
         [0010]     Preferably the rotor is formed with protrusions from it extending in the direction towards the stator. In fact, a motor having such protrusions on the rotor constitutes a second, independent aspect of the invention. This aspect may be expressed as a micro-rotating device a rotor having a central aperture, a shaft threaded through the aperture, and a shaft holder holding one end of the shaft, the rotor being provided on a surface facing the shaft holder with protrusions extending in the direction of the shaft holder for maintaining a spacing between the surface of the rotor and the shaft holder. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]     Preferred features of the invention will now be described, for the sake of illustration only, with reference to the following figures in which:  
         [0012]      FIG. 1  is a cross-sectional view of a prior art micro-motor;  
         [0013]      FIG. 2  is a cross-sectional view of a micro-motor which is an embodiment of the invention;  
         [0014]      FIG. 3 , which is composed of FIGS.  3 ( a ) to  3 ( f ), is cross-sectional views of a procedure for fabricating the rotor disc of the micro-motor of  FIG. 2 ;  
         [0015]      FIG. 4 , which is composed of FIGS.  4 ( a ) to  4   f ), is a cross-sectional views of a procedure for fabricating the shaft of the micro-motor of  FIG. 2 ;  
         [0016]      FIG. 5 , which is composed of FIGS.  5 ( a ) to  5 ( h ), is cross-sectional views of a procedure for fabricating the shaft holder of the micro-motor of  FIG. 2 ;  
         [0017]      FIG. 6 , which is composed of FIGS.  6 ( a ) to  6 ( e ), is cross-sectional views of the assembly procedure for the micro-motor of  FIG. 2 ;  
         [0018]      FIG. 7  is a cross-sectional view of the procedure for assembling the yoke and magnet of the rotor of  FIG. 2  into the disks produced by the procedure of  FIG. 3 ;  
         [0019]      FIG. 8  is a cross-sectional view of the process for assembling for the stator of the micro-rotor of  FIG. 2 ; and  
         [0020]      FIG. 9 , which is composed of FIGS.  9 ( a ) to  9 ( c ), shows is a view of the lower surface of the rotor disk of  FIG. 2 ( c ) in several different variants of the protrusions within the scope of the invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0021]     Referring to  FIG. 2 , a micro-rotating device which is an embodiment of the invention is shown in cross-section. The device includes a rotor  200  and stator  208 . The rotor  200  includes a silicon body  201  having a central aperture  214 . A shaft  202  projects from the stator  208 , and the two are connected by a shaft holder element  207 .  
         [0022]     The shaft  202  is rotationally symmetric about an axis  211 . The shaft includes a first portion  210  of uniform diameter at all positions along the axis  211 , and a head portion  209  further from the stator  208  and which is of also of uniform diameter at all positions along the axis  211 . The central aperture  214  of the rotor  200  is defined by a profile having a step, and thus including two cylindrical sleeve surfaces  212 ,  213 . The cylindrical sleeve surface  212  is of smaller diameter than the cylindrical sleeve surface  213 .  
         [0023]     Of course, the figure is not to scale. Typically, the maximum diameter of the rotor device (in the sideways direction of  FIG. 2 ) is at most 3 cm, and more preferably less than 2 cm, or less than 1.5 cm. The thickness of the rotor  200  is preferably in the range less than or equal to 600 μm.  
         [0024]     The ‘T’-shaped shaft  202  constitutes a means to avoid tilting of the rotor  200 , and keeps the rotor  200  in position constantly throughout the rotation. Thus, the shaft  202  provides an advantage compared to the regular pin-type shaft  102  of  FIG. 1 .  
         [0025]     As in shown  FIG. 1 , the silicon body  201  includes a trench in its surface which faces the stator  208 , and the trench is substantially filled with a magnetic material  204  (e.g. an alloy) and a yoke layer  203 . These materials induce a rotation motion when appropriate magnetic fields are generated by a coil (not shown) fixed to the stator  208 .  
         [0026]     As a means to enhance the driving mechanism, small protrusions  206  are formed on the surface of the rotor disk  201  facing the stator. Possible configurations for the protrusions are illustrated in  FIG. 9 ( a ) to  9 ( c ). The protrusions  206  are intended to minimize the contact resistance in the spinning motion by creating a separation between the rotor and the stator. Preferably multiple, round protrusions  206  are provided, symmetrically spaced around the aperture  214  at the bottom surface of the rotor disc  201 , to help stabilize the spin motion at the contact interface. As shown in  FIG. 9 ( c ), the protrusions may be in groups. An etching process is preferred for forming the protrusions  206 , since this can achieve the required specifications without imposing other defects on the disc  201  since the Si material is relatively hard.  
         [0027]     To reduce the total thickness of the motor assembly, the thin rotor structure  200  is fabricated using a semiconductor process to selectively create the required profile, and to accurately control the required geometry of the intended rotor  200 . Furthermore, the process allows an array of multiple rotor discs  201  to be made on single substrate concurrently, thereby cutting down on the construction cost and time. In this regard, micro-fabrication techniques, such as plasma etching, are used for producing the step profile, of micrometers in height, on the disc  201 , which is not attainable from conventional machining technology.  
         [0028]     The details for the etching sequence in the fabrication of the Si disc  201  will be explained as follows, with reference to  FIG. 3 .  
         [0029]     As shown in  FIG. 3 ( a ), a thin piece of double-side polished Si substrate  301  is first loaded into a furnace of high thermal heat to develop thin Silicon oxide (SiO 2 ) layer  302  on both its surfaces. This oxide layer functions as a protective mask in later steps of the method in cases when the surface is not coated with another protective material such as spin-coated photo resist. Considering the etching depth requirements, the thickness of SiO 2  layer  302  used is about 2 μm which is sufficient to stop the etching for a low-depth structure.  
         [0030]     Next, the Si wafer is spin-coated on one side with a photo-resist masking layer  303  of about 7 μm thickness of on one side above the SiO 2  layer  302 , and the layer  303  is subsequently patterned with the required profile by means of conventional lithography. Chemical plasma etching (using CF 4  and oxygen plasma) is then carried out, removing the unwanted portion of the SiO 2  layer  302 , as shown in  FIG. 3 ( b ),  
         [0031]     A further photo-resist masking layer  304  is applied to the same surface of the water  301 , covering the remaining portions of the 2 μm oxide layer. The layer  304  is patterned, and then chemical etching is again performed, as illustrated in  FIG. 3 ( c ), to create another profile for succeeding etching steps.  
         [0032]     Owing to the fact that ions travel straight towards the substrate during the plasma etching process, deep perpendicular walls can be formed subsequently, with polymer passivation, as in the Bosch process, that shields the walls from the bombarding ions. In the most etched portions of the Si substrate  301  about one-third of the total thickness remains, as shown in  FIG. 3 ( c ).  
         [0033]     Following that, the residual photo-resist layer is removed using chemical solvent, such as dipping in acetone. A further DRIE (deep reactive ion etching) plasma etching is carried out to take out about 20 μm from areas of the Si body  301  not covered by the remaining SiO 2    302 . This forms a set of uniformly distributed protrusions  206  dedicated for the air-bearing function, illustrated in  FIG. 3 ( d ). Subsequently, the SiO 2  left behind is removed by dipping the Si body  301  in Hydrofluoric (HF) acid.  
         [0034]     Following that, the Si body  301  is inverted. Etching methods are again employed using another patterned masking layer  305  to make through-holes  306 ,  307  through the Si substrate  201  as shown in  FIG. 3 ( f ) and  FIG. 3 ( g ), in line with those earlier made cavities. Through hole  306  becomes the aperture of the disk  201 , and through holes  307  singulate individual disks  201  from the substrate  301 . To ensure that the etching of the holes  306 ,  307  is aligned on both sides of the substrate  201 , an optical aligning system is used, along with the appropriate markings, to position the Si substrate when it is inverted in the etching chamber.  
         [0035]     The photo-resist layer  305  is also used to reduce the thickness of the substrate  301  in the region around the central aperture  306  before the discs  201  are detached from the wafer substrate  301 . The dimension of the sleeve on the rotor disc  201  has a stepped profile, intended to accommodate the different dimensions of the T-shaped shaft  202  as mentioned earlier.  
         [0036]     At this point, the fabrication of the rotor disc  201  is completed. It has a shape of a flat round plate, with a through hole  306  at the centre. One side of the disc has a large, circular trench removed, used in the later assembly process. When each of the rotor discs  201  is detached from the substrate, it carries a residual layer of polymer resulting from the Bosch polymer passivation during the etching. It is then desirable to remove this thin coat entirely from the surface by dipping into ultra-sonic mixture bath containing Hydrosulfuric acid (H 2 SO 4 ).  
         [0037]     For further improvement to reduce friction, an addition thin layer of conforming coating film  309  is deposited on all surfaces of the disc  201 , including the sleeve hole  306  on the etched disc  201 . The thin layer  205 , which is preferably Diamond-Like-Carbon (DLC) film, provides a lubricating effect due to its high hardness material property. One possible deposition technique comprises sputtering or chemical vaporization as well as, in this proposal, Filtered Cathodic Vapor Arc (FCVA) methods where low-temperature conformal film can be achieved with uncomplicated control for the required film thickness.  
         [0038]     Turning to  FIG. 4 , the process is shown for fabricating multiple shafts on a single substrate. The intended material for shaft is Si, which has good rigidity and hardness characteristics. As shown in  FIG. 4 ( a ), a Si wafer  401  similar to wafer  301  of  FIG. 3 ( a ) is first spin-coated with approximately 2 mm of Cytop polymer adhesive  402 . The polymer  402  is patterned into the required profile by lithography, and then RIE etching performed using a photoresist mask  403 , as shown in  FIG. 4 ( b ). The polymer adhesive  402  offers the advantages of low temperature bonding and chemical resistance characteristics.  
         [0039]     Subsequently, by conducting DRIE plasma etching using a patterned photo-resist mark  404 , a portion of the intended shaft is formed as shown in  FIG. 4 ( c ). Without removing the photo-resist  404  left from the etching, the substrate is coated with DLC film  405 , as shown in  FIG. 4 ( d ), using the deposition technique described earlier on all surfaces for reducing friction resistance. Following that, the substrate  408  is inverted and secured onto a new Si substrate  208  by a layer of photo-resist. A photoresist mask layer  406  is deposited for further etching, as shown in  FIG. 4 ( e ). Then the photo-resist  406  is removed, and the shafts  202  are singulated by removing the portions of the DLC  405  between them. Subsequently a DLC film  407  is deposited onto the top surface of the shaft  202 , and the parts of sides of the shaft  202  which are not already covered by the DLC  405 . Then the structure is removed from the support substrate  408 . The resist  404  is removed (carrying away the portion of the layer  405  on it) to complete the fabrication of the shaft, as indicated in  FIG. 4 ( f ).  
         [0040]     Turning to  FIG. 5 , the fabrication process for the shaft holder  207  is shown. It too employs an Si wafer  501 . Oxide layers  502 ,  503  are formed on each of its surfaces in a thermal oxidation step, as shown in  FIG. 5 ( a ). A thin mask  504  is formed by lithographic patterning, and RIE etching is carried out on one side of the substrate, as shown in  FIG. 5 ( b ). Subsequently, the SiO 2  layer  503  is removed, the substrate  501  is inverted, and is spin coated with approximately 2 mm of Cytop polymer adhesive  514  on the reverse surface, and unwanted portions of the polymer adhesive  514  are etched off using a patterned masking layer  505 . A photoresist layer  506  is deposited on selected parts of the upper surface of the substrate  501 , and a DLC film  507  is also coated using FCVA as discussed above on the upper surface of the substrate, which is the intended surface of the rotor contact area as shown in  FIG. 5 ( e ). The layer  506  is lifted off, lifting off the portions of the DLC film  507  above it, as shown in  FIG. 5 ( f ). Another patterned photo-resist mask  508  is then deposited, and RIE etching of the Si substrate carried out, to create cavities  509 , as shown in  FIG. 5 ( g ). Then the photo-resist  508  is removed, as shown in  FIG. 5 ( h ). The cavities  509  are helpful for an alignment process described below. The motor bonding process is illustrated in  FIG. 6 . In this process the alignment of the rotor to the stator is crucial. In a first step, shown in  FIG. 6 ( a ), the substrate  501  containing the shaft holder is aligned and adhered, by polymer adhesive, to the detached rotor discs  201  obtained earlier, as shown in  FIG. 6 ( b ). In this process it is helpful to use as guide elements the portions  603  of the substrate  301  (shown in  FIG. 3 ) which were left after the rotors  201  were removed from it. The aperture  214  of each rotor disc  201  is used as a guide for inserting the respective shaft  202  downwardly into the cavity  509  in the shaft holder  207  which is still not detached from the rest of the substrate  501 , as shown in  FIG. 6 ( c ). Then, the assembly  601  is heated in a chamber to a temperature of at least approximately 160° C. for at least 30 minutes, to bond the shaft  202  permanently to the shaft holder  207 . Note that all assemblies  601  on the substrate  501  have the same structures and intended dimensions. Subsequently, the substrate set  501  is inverted, and the substrate  501  is etched through, using the previously made SiO 2  mask layer  502 , to detach each micro-motor assembly from the substrate  501 , as shown in  FIG. 6 ( d ). This etching creates a cylindrical base a few hundreds of micrometer thickness that functions as a support base  602  for securing the shaft  202  which detains the rotor disc  201  during rotation.  
         [0041]     As shown in  FIG. 7 , a ring-shaped yoke plate  203  with high magnetic saturation property, for instance Nickel-iron (Ni—Fe), of about 100 mm thickness is added to the inner surface of the etched trench  308  on the rotor disc  201 , by adhesive, to intensify the induced electromagnetic force. A ring-shaped bond magnet  204  hundreds of micrometers thick and composed of a magnetic alloy, such as Samarium-Cobalt (Sm—Co), is inserted over the yoke  203 . The thickness of each of the magnet  204  and yoke  203  is appropriately controlled during the fabrication to ensure that they fit entirely into the etched hollow trench  308  on the disc  201 , with good flatness at the base surface.  
         [0042]     The assembly of micro-motor is completed by the insertion of the micro-motor assembly fully into the stator  208 , as shown in  FIG. 8 . This can be done by hand, without the use of optical alignment tool. The stator  208  may consist of a thin Printed Circuit Board (PCB). It has a thickness slightly less than that of the shaft holder  207 , and contains plated copper coil windings, which generate electrical fields to induce the electromagnetic torque needed for the rotation. A uniform thickness is maintained throughout the coils&#39; area to keep a consistent air gap between the rotor  200  and the PCB  208  during the rotation. A check is performed that the shaft  202  is perpendicular to the rotor  200  and stator  208 , and that the rotor  200  is parallel to the stator  208 , to ensure steady rotating behaviour.