Abstract:
The manufacture process includes: forming a first wafer of semiconductor material housing integrated electronic components forming a microactuator control circuit and a signal preamplification circuit; forming microactuators, each including a rotor and a stator, in a surface portion of a second wafer of semiconductor material; attaching the second wafer to the first wafer, with the surface portion of the second wafer facing the first wafer; thinning the second wafer; attaching the second wafer to a third wafer to obtain a composite wafer; thinning the first wafer; cutting the composite wafer into a plurality of dice connected to a protection chip; removing the protection chip; attaching read/write transducers to the dice; and attaching the dice to supporting blocks for hard-disk drivers.

Description:
TECHNICAL FIELD 
     The present invention regards a process for manufacturing a group comprising at least two elements, one whereof includes an encapsulated micro-integrated structure, and a thereby obtained group. In particular, the invention may be advantageously applied for assembling a microactuator, an integrated device including microactuator control circuitry and a head in a hard-disk read/write unit with double micrometric actuation. 
     BACKGROUND OF THE INVENTION 
     As is known, hard disks are the most widely used data-storage medium; consequently, they are produced in very large volumes, and the maximum data-storage density increases from one year to the next. Hard disks are read and written by actuator devices, the general structure of which is shown in FIGS. 1 and 2 and is described hereinafter. 
     In particular, FIG. 1 shows a known actuator device  1  of the rotary type comprising a motor  2  (also called “voice coil motor”) fixed to a support body, generally called “E-block” because of its E-like shape in side view (see FIG.  2 ). The support body  3  has a plurality of arms  4 , each of which carries a suspension  5  including a cantilevered lamina. At its end not fixed to the support body  3 , each suspension  5  carries a R/W transducer  6  for reading/writing, arranged (in an operative condition) facing a surface of a hard disk  7  so as to perform roll and pitch movements and to follow the surface of the hard disk  7 . To this end, the R/W transducer  6  (also referred to as slider) is fixed to a joint, called gimbal or flexure  8 , generally formed from the suspension  5  and comprising, for example, a rectangular plate  8   a  cut on three and a half sides starting from the lamina of the suspension  5 , and having a portion  8   b  connected to the suspension  5  and allowing flexure of the plate  8   a  under the weight of the slider  6  (see FIG. 2A) 
     In order to increase the data storage density, it has already been proposed to use a double actuation stage, with a first, rougher actuation stage including the motor  2  moving the assembly formed by the support body  3 , the suspension  5  and the R/W slider  6  through the hard disk  7  during a coarse search for the track, and a second actuation stage performing a finer control of position of the slider  6  during tracking. According to a known solution, the second actuation stage comprises a microactuator  10  arranged between the gimbal  8  and the slider  6 , as may be seen in FIG. 3, which shows, in exploded view, the end of the suspension  5 , the gimbal  8 , the slider  6 , and the microactuator  10 , in this case, of the rotary type. The microactuator  10  is controlled by a signal supplied by control electronics on the basis of a tracking error. 
     At present, the circuit for pre-amplificating the signal picked up by the slider  6  is arranged on the board, or at most on the end of the support body  3 , while the microactuator controlling circuitry is integrated with the microactuator. This integration is made possible by silicon microprocessing techniques, such as epitaxial microprocessing or metal electroplating. 
     The above-mentioned technologies make it difficult, if not impossible, to obtain a group comprising the microactuator-controlling circuitry, the microactuator, the slider, and the pre-amplifying circuit on a same wafer. 
     In case of epitaxial microprocessing, there is a dimensional incompatibility between the microactuator and the pre-amplification circuit. This incompatibility is due to the fact that the minimum photolithographic dimension of the microactuator is of the order of 1 μm; instead, because of the high operating frequencies, the pre-amplification circuit requires an extreme photolithographic process not exceeding 0.35 μm. The difference between the minimum photolithographic dimensions thus renders integration of the two devices on the same technological platform not very convenient 
     The metal electroplating technique makes it possible to obtain mechanical structures on the circuitry, but presents certain drawbacks. In fact, it is not possible to effectively protect the micromechanical structures from contamination caused by particles present in the hard-disk driver; assembly of the slider on the microactuator proves difficult; in addition, electrical isolation of the signals supplied by the head from the signals controlling the microactuator is difficult. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention is a process for assembling a micromechanical structure, in particular a microactuator, on a supporting element, in particular an integrated device containing the circuitry, that protects the micromechanical structure at least during assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a clear understanding of the present invention, a preferred embodiment is now described, simply as a non-limiting example, with reference to the attached drawings, wherein: 
     FIG. 1 is a top view of a hard disk actuator of known type; 
     FIG. 2 is an enlarged side view of some parts of the actuator of FIG. 1; 
     FIG. 2A is a further enlarged view of some parts of FIG. 2; 
     FIG. 3 is an exploded view of the micrometric actuation unit in an actuator device equipped with double actuation stage; 
     FIGS. 4 and 5 show cross-sections through two starting wafers used in a process according to the invention; 
     FIG. 6 shows a top view of the wafer of FIG. 5, at a slightly reduced scale; 
     FIG. 7 shows a cross-section of the wafer of FIG. 5, in a subsequent manufacture step; 
     FIG. 8 shows a top view of the wafer of FIG. 7, in the same scale as FIG. 6; 
     FIGS. 9 and 10 show cross-sections of the wafer of FIG. 7 in a subsequent manufacture step; 
     FIG. 11 shows a top view of the wafer of FIG. 10, in the same scale as figure 6; 
     FIG. 12 a  shows a cross-section of the wafer of FIG. 10, in a subsequent manufacture step, taken along line A—A of FIG. 13; 
     FIG. 12 b  shows a cross-section of the wafer of FIG. 10, taken along line B—B of FIG. 13; 
     FIG. 13 shows a top view of the wafer of FIGS. 12A and 12B; 
     FIG. 14 shows a cross-section of the wafer of FIG. 12A, in a subsequent manufacture step; 
     FIG. 15 shows a cross-section of the wafer of FIG. 14, after bonding to the wafer of FIG. 4; 
     FIGS. 16 and 17 show cross-sections of the composite wafer of FIG. 15, in subsequent manufacture steps; 
     FIG. 18 shows a cross-section of the composite wafer in a manufacture step subsequent to FIG. 17, taken along cross-section line XVIII—XVIII of FIG. 19; 
     FIG. 19 shows a top view of the composite wafer of FIG. 18, in the same scale as FIG. 6; 
     FIG. 20 shows a cross-section of the wafer of FIG. 17, after bonding to a third wafer; 
     FIG. 21 shows a cross-section of a die obtained by cutting the composite wafer of FIG. 20; 
     FIG. 22 shows a perspective view of the die of FIG. 21, after final assembly; 
     FIG. 23 shows a cross-section of the die of FIG. 22; 
     FIG. 24 shows a different embodiment of the microactuator in a top view similar to that of FIG. 19; 
     FIG. 25 shows a cross-section of the wafer of FIG. 24, taken along the cross-section line XXV—XXV. 
     FIGS. 26 a - 26   b  show a simplified cross-section of the third wafer of FIG.  20  and of the wafer of FIG. 17 before bonding; 
     FIG. 27 shows a top view of the third wafer of FIG. 20, during a separation phase from the wafer of FIG. 17; 
     FIG. 28 shows a cross-section of the third wafer of FIG. 20, during a separation phase from the wafer of FIG. 17; 
     FIG. 29 shows a perspective view of a different embodiment of the third wafer of FIG. 20; and 
     FIG. 30 shows a cross-section of the wafer of FIG. 17, after bonding to the wafer of FIG.  29 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description, reference will be made to the process for assembling a wafer, integrating encapsulated-type microactuators, on a wafer containing circuits for controlling the microactuator and signal-preamplification circuits, as well as for assembling the dice, obtained after cutting, on respective sliders and suspensions  5 . In practice, with reference to FIG. 3, and as will be explained more clearly hereinbelow, the die containing a microactuator-controlling circuit and a signal-preamplification circuit is arranged between the microactuator  10  and the flexure  8 . 
     For this purpose (FIG.  4 ), in a first wafer  20  comprising a body  21  of monocrystalline silicon having a thickness of, for example, 600-700 μm, the circuits for controlling the microactuators and the circuits for signal preamplification  22  are formed, wherein these circuits are represented by electrical symbols of MOS and bipolar active components and passive components, using known microintegration techniques. The circuits  22  are of a standard type, and consequently they are not represented in detail. On top of the body  21 , after completion of the electronic devices, an isolation layer  24 , for example BPSG is formed; this layer is opened to form the connections of the circuits  22  with the microactuators  10  and the sliders  6 , and, on top of the isolation layer  24  metal regions  25   a - 25   e  are formed—only some of which may be seen in FIG. 4, while others are indicated by a dashed line in FIG. 11, as will be described hereinafter—for example, of palladium or gold, for electrical connection and bonding, using standard techniques of deposition and photolithographic definition of metal material. In particular, in this step there are formed: a bottom frame region  25   a  (the complete shape of which coincides with that of the region designated by  40  in FIG.  11 ); bottom pads (only one of which, designated by  25   f , may be seen in FIG.  18 ); first electric connection lines  25   b  between the bottom pads; eight bottom conductive sectors  25   c  (the shape of which coincides with that of the top conductive sectors  44  of FIG.  11 ); second electric connection lines  25   d  (FIG. 11) which connect the bottom conductive sectors  25   c  two by two; and third interconnection lines  25   e  between the bottom conductive sectors  25   c  and certain bottom pads  25   f  (FIG.  11  and  18 ). 
     With reference to FIG. 5, a second wafer  28  is moreover formed. The second wafer  28  comprises a substrate  29  of monocrystalline silicon having a thickness of, for example, 600-700 μm. The second wafer  28  is designed to house a plurality of adjacent microactuators  10 , and FIG. 5 shows a portion of a single microactuator  10 . 
     The second wafer  28  is etched so as to form a first trench  30   a , a second trench  30   b , and a plurality of isolation trenches  30 c having a width of approximately 1.5-2 μm and a depth of, for instance, 10 μm, the shape of which may be seen in FIG.  6 . In detail, as is shown in FIGS. 5 and 6, the first trench  30   a  has the shape of a circle with a first diameter and externally delimits a central supporting region  29   a  extending next to a surface  28   a  of the substrate  29 . The second trench  30   b  has the shape of a circle with a second diameter greater than the first diameter and is concentric with the first trench  30   a  so as to delimit, laterally, together with the first trench  30   a , an annular supporting region  29   b  arranged next to the surface  28   a  of the substrate  29 . The isolation trenches  30   c , not visible in FIG.  5  and represented each only by a line in FIG. 6, have a square, or in general polygonal, shape, are arranged outside the second trench  30   b , and delimit respective portions of the substrate  29  designed to form through electric connection regions  29   c , also adjacent to the surface  28   a . In particular, the isolation trenches  30   c  are arranged externally with respect to the second trench  30   b.    
     The first and second trench  30   a ,  30   b  have the purpose of mechanically separating movable portions and fixed portions of the substrate  29 , as will be explained more clearly later on. 
     Subsequently (FIG.  7 ), a first sacrificial layer (for example of silicon dioxide) is deposited for a thickness of approximately 1.5 μm; the sacrificial layer fills the trenches  30   a ,  30   b ,  30   c , is then removed from the surface  28   a  of the substrate  29 , and forms a first immobilization region  31   a  (in the first trench  30   a ), a second immobilization region  31   b  (in the second trench  30   b ), and deep electric isolation regions  31   c  (in the isolation trenches  30   c ), the deep isolation regions  31   c  being visible only in FIG.  8 . Next, on the substrate  29   a  second sacrificial layer (for example of silicon dioxide) having a thickness of, for example, of 2 μm is deposited and defined; the second sacrificial layer forms an anchor defining region  32 A and a labyrinth region  32   b , the shape whereof may be seen in the top view of FIG.  8 . The external area of the labyrinth region  32   b  remains free from oxide areas to reduce the mechanical stress induced by the oxide. 
     In detail, the anchor defining region  32 A has a generally annular shape, delimited internally by a central opening  34  and having radial notches  35 . The central opening  34  comprises a central area  34   a , which is circular, and of four expansions or “fins”  34   b  extending radially outwards starting from the central area  34   a  and arranged at 90° increments with respect to each other. The fins  34   a  delimit between each other portions of the anchor defining region  32 A, hereinafter referred to as stator insulating regions  33 , which have the function of anchoring and electrically insulating a stator of the microactuator  10 , as will be explained in greater detail hereinafter. The radial notches  35  extend from the outer circumference of the anchor defining region  32 A towards the fins  34   b  and define, as the fins  34   b , areas of the substrate  29  where a rotor of the microactuator  10  is to be anchored, as will be clarified hereinafter. As an alternative, the portions of the anchor defining region  32 A between the notches  35  may not be continuous, but reproduce the shape of the arms of the micromotor, as explained below. 
     The labyrinth region  32   b  has an annular shape and surround at a distance the anchor defining region  32 A. The labyrinth region  32   b  has an inner diameter equal to or smaller than the second immobilization region  31   b , and an outer diameter greater than the outer diameter of the second immobilization region  31   b , as may be clearly seen in FIG.  7 . 
     Subsequently (FIG.  9 ), a seed polycrystalline silicon layer is deposited (for a thickness of approximately 300-500 nm), and then a polycrystalline epitaxial layer  38  is grown, for a thickness of approximately 30 μm. The epitaxial layer  38  grows directly in contact with the substrate  29 , in an area corresponding to the opening  34 , the notches  35 , between the anchor defining region  32 A and the labyrinth region  32   b , as well as outside the labyrinth region  32   b  itself; furthermore, it grows on top of the anchor defining region  32 A and labyrinth region  2   b . The epitaxial layer  38  thus has a first face  36 , which is free, and a second face  37 , which is facing the substrate  29  and is opposite to the first face  36 . 
     Next, a polishing treatment is carried out, using the CMP technique, to reduce the roughness of the epitaxial layer  38 . 
     Subsequently (FIGS.  10  and  11 ), on the first face  36  of the epitaxial layer  38  an electrical connection and bonding material layer, for example, palladium, is deposited and defined so as to form: an upper frame region  40 , surrounding, in top view (FIG.  11 ), the area where the microactuator is to be formed and, as has already been said, having the same shape as the bottom frame region  25   a ; first top pads  42  which are vertically aligned with respective through electric connection regions  29   c  and may be superimposed on first bottom pads  25   f  (FIG.  18 ); four pairs of top conductive sectors  44   a ,  44   b ,  44   c ,  44   d , which may be superimposed on the bottom conductive sectors  25   c  and are vertically aligned with the fins  34   b  and with the stator insulating regions  33  (namely, the pairs of top conductive sectors  44   a  and  44   c  are aligned with the stator insulating regions  33 , and the pairs of top conductive sectors  44   b  and  44   d  are aligned with the fins  34   b ); and finally, a ring-shaped region  46 , which has a greater diameter than the second immobilization region  31   b  and is arranged internally to the top frame region  40 . 
     The epitaxial layer  38  is deep etched using the Reactive Ion Etching (RIE) technique, the etching stopping on the anchor defining region  32 A and on the labyrinth region  32   b . In this phase, as is shown in the sections of FIGS. 12A and 12B (the latter figure being taken at an angle of 45° with respect to the former figure) and in the (simplified) top view of FIG. 13, third trenches  49  are formed that delimit and separate a cylindrical region  50 , a stator  51 , and a rotor  52  from one another, and a fourth trench  58  which laterally separates the rotor  52  from an outer portion  38   a  of the epitaxial layer  38 . In addition, third through contact regions  70  are formed which are aligned with the through electric connection region  29   c.    
     In particular, the cylindrical region  50  is concentric with and electrically connected to the central supporting region  29   a.    
     The stator  51  comprises four stator regions  54 , only one of which is shown completely and schematically in FIG.  13 . Each stator region  54  is completely insulated from the substrate  29  by the anchor defining region  32 A, and comprises a stator anchoring portion  54   a , having a substantially trapezoidal shape and extending underneath one of the top conductive sectors  44   a  and  44   c ; an annular sector portion  54   b , which is provided with holes and is contiguous with and radially external to the stator anchoring portion  54   a ; and a plurality of fixed arms  54   c  extending radially outwards from the annular sector portion  54   b.    
     The rotor  52  comprises an outer annular region  55   a , having an external diameter slightly greater than the diameter of the second immobilization region  31   b  and separated from the external portion  38   a  of the epitaxial layer  38  by the fourth trench  58 , which has a greater diameter and thus is not aligned to the second trench  30   b ; four supporting arms  55   b , formed in areas corresponding to the notches  35 , between pairs of adjacent stator regions  54 ; a plurality of movable arms  55   c  (FIG. 12A) extending radially inwards from the outer annular region  55   a  and alternated with the fixed arms  54   c ; spring elements  55   d  extending from the supporting arms  55   b  inwards between adjacent pairs of annular sector portions  55   b ; and four movable anchoring sectors  55   e  having a substantially trapezoidal shape, each of which extends between a pair of stator anchoring portions  54   a , beneath the top conductive sectors  44   b ,  44   d . The outer annular region  55   a  is in direct contact with the annular supporting regions  29   b  via first rotor anchoring portions  39 . The supporting arms  55   b  are in contact with the annular supporting region  29   b  via second rotor anchoring portions  56  (FIG.  12 B). The movable arms  55   c  can be formed directly on top of and in contact with the annular supporting region  29   b , or else, as shown in the illustrated embodiment, may be separated from the annular supporting region  29   b  by portions of the anchor defining region  32 A. The spring elements  55   d  are isolated with respect to the substrate  29  by the anchor defining region  32 A (FIG.  12 B), and the movable anchoring sectors  55   e  are in direct electrical contact with the central supporting region  29   a  through third rotor anchoring regions  57  formed in areas corresponding to the fins  34   b  (FIG.  8 ). In addition, the spring elements  55   d  have a thin cross-section if compared to the supporting arms  55   b , so as to have the necessary elasticity during movement of the rotor  52 . In particular, the spring elements  55   d  are rigid in the vertical direction (perpendicular to the microactuator plane) and are compliant to rotation. 
     Next (FIG.  14 ), the sacrificial oxide is etched using hydrofluoric-acid (HF) for a time sufficient for removing the labyrinth region  32   b  and the anchor defining region  32 A beneath the fixed arms  54   c , the movable arms  55   c , and the annular sector portions  55   b  (thanks to the presence of holes in the latter), with the exception of the stator insulating regions  33 . Consequently, the stator regions  54  remain anchored to the central supporting region  29   a  of the substrate  29  only at the stator insulating regions  33 , and the rotor  52  remains anchored to the annular supporting region  29   b  of the substrate  29  at the first rotor anchoring portions  39  and the second rotor anchoring portions  56 , and to the central supporting region  29   a  (through the spring elements  55   d ), at the third rotor anchoring portions  57 . 
     Subsequently (FIG.  15 ), the second wafer  28  is turned upside down, aligned and welded to the first wafer  20  (wherein the microactuator-control and signal-preamplification circuits  22  are formed). In this phase, all the metal connection lines present on the first wafer  20  are exploited. In particular, the bottom frame region  25   a  are welded to the top frame region  40 ; the first bottom pads  25   f ,  25   a  are welded to the top pads  42 ; and the bottom conductive sectors  25   c ,  25   a  are welded to the top conductive sectors  44   a - 44   d . A double wafer  60  is thus obtained. 
     The second wafer  28  is then lapped (lapping phase—FIG. 16) until a final depth is obtained equal to the immobilization regions  31   a ,  31   b  (approximately 10 μm). Consequently, the second wafer  28  now has a free surface  61  where the immobilization regions  31   a ,  31   b  end. Furthermore, the central supporting region  29   a , annular supporting region  29   b  and through electric connection regions  29   c  (the latter not being visible in FIG. 16) are now isolated from each other and from the outer portion  29   d  of the second wafer  28 . 
     Subsequently (FIG.  17 ), a silicon-dioxide layer  62  with a thickness of approximately 2-3 μm is deposited on the surface  61 . Vias  63  are formed through the silicon-oxide layer  62  aligned with the through electric connection regions  29   c , as shown by dashed lines in FIG.  17 . Using a resist mask (not shown), metal contact regions with a thickness of approximately 5 μm are grown galvanically (see FIG.  19 ), for example made of nickel (approximately 4 μm) coated with gold (for a thickness of approximately 1 μm). In detail, the following regions are formed: an annular metal region  64   a  extending on the annular supporting region  29   b ; four head connection regions  64   b  arranged at 90° to each other, externally to the annular metal region  64   a  and separate from each other; second contact pads  64   c  aligned vertically with the first top pads  42  (as may be noted from a comparison between FIGS.  19  and  11 ); and four electric connection lines  64   d  connecting the head connection regions  64   d  to as many second contact pads  64   c . The second contact pads  64   c  extend also in the vias  63 , thus guaranteeing electrical contact with the through electric connection regions  29   c , as is shown only in part in FIG.  18 . 
     The silicon-dioxide layer  62  is then etched without a mask and removed everywhere, except where it is shielded by the annular metal region  64   a  and by the four head-connection regions  64   b . By appropriately selecting the width of the electric connection lines  64   d  and the etching time, the silicon-dioxide layer  62  is not, instead, shielded by the electric connection lines  64   d  (which thus remain free). In this phase, also the oxide present in the immobilization regions  31   a ,  31   b  is etched, so freeing the first trench  30   a  and the second trench  30   b . The structure shown in FIG. 18 is thus obtained, in which, for a more complete representation of the three-dimensional structure of the double wafer  60 , the section line is not straight but is as shown in FIG.  19 . 
     In particular, FIG. 18 on the left shows, aligned with each other: a first bottom pad  25   f , a first top pad  43 , a second through connection region  70  formed in the outer portion  38   a  of the epitaxial layer  38 , a through electric connection region  29   c  formed in the substrate  29 , and a second contact pad  64   c  a portion whereof is formed where previously a via  63  was present. FIG. 18, on the right, shows the portion of the silicondioxide layer  62  remaining underneath a head-connection region  64   b , fixed arms  54   c  and movable arms  55   c . Furthermore, FIG. 18 shows the labyrinth structure comprising the second trench  30   b  and the fourth trench  58 , mutually disaligned and connected together by a labyrinth path  68 , where the labyrinth region  32   b  has been removed. 
     Next (FIG.  20 ), the double wafer  60  is glued to a third wafer  75  with the free surface  61  (where the contact metal regions  64   a - 64   d  are formed) facing the third wafer  75 ; in this way, a composite wafer  78  is obtained. The third wafer  75  has a service function and is made, for example, of glass or silicon. For the bonding operation, adhesive rings  76  are made on the face of the third wafer  75  facing the double wafer  60 . The adhesive rings  76  form closed lines, each surrounding an area of the second wafer  28  where a respective microactuator is formed, and are of a material enabling subsequent separation of the third wafer  75  from the double wafer  60 . For example, an indium or lead-tin layer may be deposited by sputtering or screen printing and then defined. Alternatively, on the free surface  61  a double layer of chromium-gold  102  (approximately 500 Å of chromium and 1 μm of gold) may be deposited (FIG. 26A) whereas on the face of the third wafer  75  facing the double wafer  60  a nickel layer  100  (approximately 2000 Å) may be deposited by sputtering and then a lead-tin layer  101  (approximately 3 μm) may be grown on the nickel layer  100  by electrodeposition (FIG.  26 B). 
     Advantageously, the separation of the third wafer  75  from the double wafer  60  can be carried out by using hot air jets  103  arranged in front of the four sides of the third wafer  75 , as shown in FIG. 27, so as to heat only the periphery ( 110 ) of the third wafer  75  and not the double wafer  60 . The direction of the hot jet airs  103  (FIG. 28) is such as to easily remove the third wafer  75 . In particular, there is no need to have a temperature control of the third wafer  75 , because, as soon as the lead-tin portion is enough melted, the third wafer  75  is pushed out naturally by the pressure of the hot air jets  103 . Moreover, as the third wafer  75  has a higher temperature than the double wafer  60 , the main volume of the lead-tin portion remains on the periphery ( 110 ) of the third wafer  75 , attracted by the higher temperature of the latter. Furthermore, as the air pressure generates a very small extracting force, the lead-tin portion is not “bracken”, but it is well melted when the third wafer  75  is separated from the double wafer  60 , making the remaining lead-tin portion on the double wafer  60  periphery a smooth reflowed shape, without particles. 
     Moreover, as shown in FIGS. 29-30, if the third wafer  75  has protruding portions  104 , just the gluing material may be deposited. More specifically, as shown in FIG. 29, the protruding portions  104  define in the third wafer  75  a plurality of cavities  105 , each having square shape and cross-section of substantially trapezoidal form. After the bonding operation, each cavity  105  houses a respective microactuator  10  (FIG. 30) which is free to do little motions in air. If the third wafer  75  is made of silicon, cavities  105  are formed carrying out an anisotropic dry etching (for example, in plasma of sulfur hexafluoride) or an anisotropic wet etching (for example, TMAH). For the bonding operation, a gel  106  is deposited on the protruding portions  104  and on the cavities  105 . As regards the gel  106 , a DGL™ TM film is commonly used for Silicon, GaAs, and InP wafer thinning applications. The film is mounted to the front side surface of the wafer to prevent breakage and provides a superior back side surface finish. The GEL&#39;s unique elastomeric properties uniformly distributed the grinding and lapping loads. For special bumped or inked wafer thinning applications, the thicker 17 mil GEL-PAK film securely holds the wafer and assures no damage to bumps and/or transfer of bump pattern to the wafer back side. The WF film can be held directly to a porous vacuum chuck or mounted to a standard fixture using wax. Placing the film between the wafer front side and wax eliminates the need for costly solvent cleaning steps and reduces wafer breakage. 
     As well, the gel  106  should be fluidtight and releasable to enable subsequent separation of the third wafer  75  from the double wafer  60 . 
     Thus, the microactuator  10  is completely isolated from the outside world, since it is enclosed between the first wafer  20 , the third wafer  75 , the frame regions  25   a ,  40 , and the adhesive rings  76  or the respective cavity  105 . 
     The first wafer  20  is then lapped until it has a thickness of approximately 80-100 μm. 
     Subsequently (FIG.  21 ), the composite wafer  78  is cut using ordinary cutting techniques. In this phase, the microactuator  10  is completely isolated and protected from the outside world, as explained above. Consequently, the suspended structures do not collapse due to the saw cooling water. A plurality of composite dice  77  is then obtained, each including a first die  20 ′ and a second die  28 ′ and being connected to a respective protection chip  75 ′. In this way, the composite dice  77  can be transported easily and with reduced risk of breakage. Alternatively, it is possible to separate the protective chip  75 ′ by heating the composite dice  77  at a low temperature (200-260° C.), without damaging the components, and to transport the individual composite dice  77  on a traditional support using stick foil. 
     Finally (FIGS.  22  and  23 ), the final assembly steps are carried out, including gluing each composite die  77  on a respective gimbal  8 , in a known way (and, in this step, the protection chip  75 ′, if present, protects the composite die  77 ); removing the protection chip  75 ′, if still present; gluing the slider  6  to the head-connection regions  64   b ; and wire-bonding the contact pads  64   c  to corresponding pads  80  provided on the gimbal  8 . 
     As may be seen in FIG. 23, the microactuator  10  has its rotor  52  and stator  51  formed in an operative portion of the second die  28 ′ which is delimited by the first face  36  (arranged toward the first die  20 ′ integrating the circuitry), by the second face  37 , and by the fourth trench  58 . The operative portion of the second die  28 ′ is surrounded, on the second face  37  and on the side delimited by the fourth trench  58 , by an encapsulation structure  81  formed by the central supporting region  29   a , the annular supporting region  29   b , and an external region  82  comprising part of the external portion  29   d  of the substrate  20  and part of the external portion  38   a  of the epitaxial layer  38 . The stator  51  is supported by the central supporting region  29   a  through the stator insulating regions  33 . The rotor  52  is supported both by the central supporting region  29   a , through the third anchor portions  57 , the movable anchoring sectors  55   e  and the spring elements  55   d  (in a manner not visible in the cross-section of FIG.  23 ), and by the annular supporting region  29   b  through the first rotor anchoring portions  39 . In practice, the central supporting region  29   a  and the external region  82  of the encapsulation structure  81  are fixed, and the annular supporting region  29   b  is movable together with the rotor  52 . 
     In the microactuator  10  of FIG. 23, the first trench  30   a  is isolated from the external environment by the metal annular region  64   a  which hermetically seals the gap between the slider  6  and the encapsulation structure  81 , so preventing any external contamination both during fabrication and during operation of the hard-disk unit. 
     Any contamination through the second trench  30   b  is limited by the labyrinth conformation defined by the second trench  30   b  itself, by the labyrinth path  68  and by the fourth trench  58 . In fact, any particles that may penetrate into the second trench  30   b  are forced to follow a tortuous path in which they can be entrapped before they reach the rotor  52 . 
     Finally, the gimbal  8  is fixed to an arm  4  of the support body  3  in a per se known, not illustrated manner. 
     The advantages of the described process emerge clearly from the foregoing. In particular, it is emphasized that forming the microactuator  10  in a separate wafer, which is bonded upside down on the wafer accommodating the electronic components, allows the microactuator and the circuitry requiring an extreme lithographic process to be arranged closely, and the latter to be protected during assembly and cutting. In general, the described process enables assembling of any other micro-electromechanical actuation or sensing element that requires a lithographic process with different characteristics as the circuitry controlling and/or processing the signals supplied by the micro-electromechanical element, or when it is desirable to have a micro-electromechanical element formed in a separate wafer with respect to the electronic circuits that are connected to the micro-electromechanical element. 
     In addition, the microactuator  10  is protected both during fabrication, by the encapsulation structure  81 , and during use, by the annular metal region  64   a  and the labyrinth structure formed by the second trench  30   b , the fourth trench  58 , and the labyrinth path  68 , as explained previously. 
     Finally, it is clear that numerous variations and modifications may be made to the process and unit described and illustrated herein, all falling within the scope of the invention, as defined in the attached claims. 
     For example, FIGS. 24 and 25 show a variation of the structure illustrated in FIGS. 18 and 19. In particular (see FIG.  24 ), the second trench  30   b  is not completely circular but is formed in such a way that the annular supporting region  29   b  has intrusions or teeth  90  extending in corresponding compartments of the outer portion  29   d  so as to form mechanical stops to the rotation of the outer portion  29   d  and possibly supply an end-of-travel signal. 
     In addition, stop elements  91 ,  92  are formed to limit the relative movements of the rotor  52  with respect to the fixed parts (stator  51 , encapsulation structure  81 ), movements that are due to assembly operations or to inertial forces acting during operation of the microactuator  10 . In detail, vertical stop elements  91 ,  92  are formed in the same way as the contact metal regions  64   a - 64   d  and are mutually interleaved. Specifically, the vertical stop elements  91  are anchored on the annular supporting region  29   b  and extend beyond the outer portion  29   d ; instead, the vertical stop elements  92  are anchored on the outer portion  29   d  and extend beyond the annular supporting region  29   b . The portions of the vertical stop elements  91 ,  92  that extend in a cantilever way, respectively, above the outer portion  29   d  and above the annular supporting region  29   b  are preferably at least in part perforated to enable removal of the silicon-dioxide layer  62 , as shown schematically only for a few of the vertical stop elements  91 ,  92 . 
     By forming the stop elements  91 ,  92  with an unholed part protruding above the second trench  30   b  and adjacent to each other, a protective cage is obtained for the second trench  30   b.    
     As an alternative to the drawings, the intrusions or teeth  90  and the vertical stop elements  92  may be arranged between the annular supporting region  29   b  and the cylindrical region  50 . 
     Connection between the first and the second wafers  20 ,  28  may be formed also in a way different from what has been shown. For example, the connection and electric contact regions  25   a - 25   e ,  40 ,  44   a - 44   d  and the ring-shaped region  46  may be formed on only one of the two wafers, for example, on the first wafer  20 , using a double metal-level process. 
     Finally, the movable anchoring sectors  55   e  may be joined to the cylindrical region  50 , instead of being separated by the third trench  49 , so that the rotor  52  is anchored also to the cylindrical region  50 .