Abstract:
A process for manufacturing components in a multi-layer wafer, including the steps of: providing a multi-layer wafer comprising a first semiconductor material layer, a second semiconductor material layer (, and a dielectric material layer arranged between the first and the second semiconductor material layer; and removing the first semiconductor material layer initially by mechanically thinning the first semiconductor material layer, so as to form a residual conductive layer, and subsequently by chemically removing the residual conductive layer. In one application, the multi-layer wafer is bonded to a first wafer of semiconductor material, with the second semiconductor material layer facing the first wafer, after micro-electromechanical structures have been formed in the second semiconductor material layer of the multi-layer wafer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention refers to a process for manufacturing components in a semiconductor material wafer with reduction in the starting wafer thickness. 
     2. Description of the Related Art 
     As is known, various processes have been developed for manufacturing micro-electromechanical structures, such as micromotors or microactuators usable for finely controlling the position of heads in hard disk drivers. 
     According to some of these processes, both the micro-electromechanical structures (or microstructures, as referred to hereinafter) and control circuits for controlling the microstructures are made in a same semiconductor material wafer. In a known process, the microstructures are formed according to the following steps: 
     deposition of a sacrificial layer on the substrate of the wafer; 
     growth of an epitaxial layer; 
     definition of rotor regions and stator regions, comprising suspended portions, in the epitaxial layer; and 
     removal of the sacrificial layer to free the suspended portions of the rotor and stator regions. 
     In this way, the microstructures may be formed by processing a single face of the semiconductor wafer. 
     More recently, the use of two distinct semiconductor wafers has been proposed. In a first wafer, the microstructures are formed by deposition of a sacrificial layer, epitaxial growth, and definition of the rotor and stator regions described above, while a second wafer is used as a support for the microstructures. In addition, in the second wafer the control circuits for controlling the microstructures may be formed. 
     Before removing the sacrificial layer, the two wafers are bonded together, so that the face of the first wafer where the microstructures have been formed is set facing the second wafer. Subsequently, the substrate of the first wafer is partially removed using a mechanical process (milling), so that a residual portion of substrate is obtained having a given thickness, normally of approximately 10-100 μm. Next, trenches are formed having a such depth to reach the sacrificial layer, which is finally removed so as to free the suspended portions of the rotor and stator regions. 
     The process described above has, however, certain drawbacks, mainly linked to the step of milling the substrate of the first wafer. In fact, since the final thickness to be achieved is in any case small (10-100 μm), the mechanical stresses generated by the mechanical members, especially at the end of the milling step, may cause cracks in the semiconductor wafer, in particular in the rotor and stator regions, thus rendering the wafer unusable. A somewhat high number of wafers must thus be discarded, and the process, which falls short of optimal yield, is, on the whole, costly. In addition, the milling process does not enable an accurate control of the thickness of the residual portion of substrate to be obtained. 
     The same problem is encountered also in electrical circuits formed in a wafer of semiconductor material which, for some reason, is to be thinned. 
     BRIEF SUMMARY OF THE INVENTION 
     An embodiment of the present invention provides a process for forming components (whether electronic components or micro-electromechanical structures), that enables a reduction in the mechanical stresses acting on the semiconductor material wafer the thickness of which is to be reduced. 
     According to an embodiment of the present invention, there is provided a process for manufacturing components in a multi-layer wafer. The process includes the steps of providing a multi-layer wafer comprising a first semiconductor material layer, a second semiconductor material layer, and a dielectric material layer arranged between the first and the second semiconductor material layer, then removing the first semiconductor material layer, initially by mechanically thinning the first semiconductor material layer, so as to form a residual conductive layer, and subsequently by chemically removing the residual conductive layer. In one application, the multi-layer wafer is bonded to a first wafer of semiconductor material, with the second semiconductor material layer facing the first wafer, after micro-electromechanical structures have been formed in the second semiconductor material layer of the multi-layer wafer. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For a better understanding of the present invention, embodiments thereof are now described, purely to provide non-limiting examples, with reference to the attached drawings, wherein: 
     FIGS. 1 and 2 show cross-sections of two starting wafers used in an embodiment of the process according to the invention; 
     FIGS. 3-5 show cross-sections of the wafer of FIG. 2, in successive processing steps; 
     FIG. 6 shows a top plan view of the wafer of FIG. 5; 
     FIGS. 7-12 show cross-sections of a composite wafer in successive processing steps; 
     FIGS. 13 and 14 show cross-sections of two starting wafers used in a second embodiment of the process according to the invention; 
     FIGS. 15 and 16 show cross-sections of the wafer of FIG. 13, in successive processing steps; 
     FIGS. 17-20 show cross-sections of a composite wafer in successive processing steps; and 
     FIG. 21 shows a top plan view of the wafer of FIG.  19 . 
    
    
     In the ensuing description, reference will be made to the process for manufacturing a composite wafer obtained by assembling a first semiconductor material wafer incorporating encapsulated microstructures (for example, microactuators) and a second semiconductor material wafer containing control circuits for controlling the microactuators and pre-amplification circuits. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to FIG. 1, a first wafer  1 , comprising a body  2  of semiconductor material, for instance monocrystalline silicon, initially accommodates a control and pre-amplification circuit  3 , of a known type and represented in a schematic and simplified way through active and passive components. The control and pre-amplification circuit  3  is obtained via standard processing steps, which are not shown in detail. 
     Subsequently, an insulating layer  4 , for example BPSG, is deposited on a surface  5  of the body  2  and is excavated, then connections  7  are formed. Then, via standard steps of deposition and photolithographic definition, metal regions  6  are provided on top of the insulating layer  4 . The metal regions  6 , which have functions of electrical connection and bonding, as is explained hereinafter, are electrically connected to the control and pre-amplification circuit  3  and are preferably made using chromium-palladium. 
     With reference to FIG. 2, on a second wafer  8  of semiconductor material, comprising a monocrystalline substrate  9  having a thickness of, for example, 675 μm, a silicon-dioxide layer is grown, intended to form a stop layer  10 . 
     Next, a polycrystalline-silicon germ layer  11  (indicated by a dashed line) is deposited on top of the stop layer  10 , an then a first epitaxial layer  13  is grown, which has a preset thickness of, for example, 10 μm. At the end of the epitaxial growth, a structure is thus obtained which has two conductive regions (the substrate  9  and the first epitaxial layer  13 ) separated by a buried insulating region (stop layer  10 ). In this case, one of the conductive regions is made of monocrystalline silicon (substrate  9 ) and the other of polycrystalline silicon (first epitaxial layer  13 ). The second wafer  8  is then planarized via chemical-mechanical planarization (CMP). 
     Next, using standard trench etching, a first trench  15  and a second trench  16  are formed, which are circular and concentric and extend in depth until they come into contact with the stop layer  10  (FIG. 3; the shape of the trenches  15 ,  16  in plan view is shown in FIG. 6 by a dashed line). The first trench  15 , which has a smaller radius, delimits a first supporting region  17 . A second supporting region  18 , having annular shape, is enclosed between the first trench  15  and the second trench  16 , and is separated from an external portion  13   a  of the first epitaxial layer  13  by the second trench  16 . 
     Subsequently, a sacrificial layer, for example of silicon dioxide, is deposited and fills the trenches  15 ,  16 , forming portions of oxide  19 , and is then selectively removed from the surface of the first epitaxial layer  13  so as to form sacrificial regions  20  and expose portions of the first supporting region  17 , portions of the second supporting region  18 , and portions of the external portion  13   a  of the first epitaxial layer  13 . 
     After depositing a second polycrystalline-silicon germ layer (not shown), a second epitaxial layer  22  is grown (FIG.  4 ), so as to form an epitaxial region  21  including the first and second epitaxial layer  13 ,  22 . The epitaxial region  21  has an overall thickness preferably of between 10 μm and 100 μm (for example, 45 μm). The second wafer  8  is then once again planarized via CMP. 
     Subsequently, a hard mask  23  is formed which covers the second epitaxial layer  22  except for windows  23 ′ overlying the sacrificial regions  20 , and the second epitaxial layer  22  is deeply etched—performing for example an advanced silicon etch (ASE)—which stops on the sacrificial regions  20  (FIG.  5 ). In this processing step are formed a third trench  27 , which separates a stator  29  from a rotor  30 , and a fourth trench  31 , which externally defines the rotor  30  and separates it from an external epitaxial portion  21 ′ of the epitaxial region  21  (FIGS.  5  and  6 ). 
     In a per se known manner, the stator  29  and the rotor  30 , connected together via spring regions  32 , have stator arms  29   a  and, respectively, rotor arms  30   a , comb-fingered (FIG.  6 ). In addition, the stator  29  is anchored to first supporting region  17 , and the rotor  30  is anchored to the second supporting region  18 . 
     The sacrificial regions  20  are then removed through a selective etch having a preset duration, which does not remove the oxide portions  19  inside the first trench  15  and the second trench  16 . During etching, the stator arms  29   a  and the rotor arms  30   a  are freed, thus remaining suspended. 
     Subsequently (FIG.  7 ), the second wafer  8  is turned upside down, aligned and welded to the first wafer  1  (in which the control and pre-amplification circuits  3  are made) so that the stator  29  and the rotor  30  are facing the first wafer  1 . A composite wafer  35  is thus formed. In particular, the metal regions  6  made on the first wafer  1  are welded to surface portions of the stator  29  and of the external epitaxial portion  21 ′. 
     The substrate  9  of the wafer  8  is then removed via a process comprising at least two steps. Initially, the substrate  9  is thinned out by mechanical milling, which, according to the invention, is interrupted to leave a residual portion  9 ′ having a preset thickness D, preferably of approximately 50 μm (FIG.  8 ). The thickness D of the residual portion  9 ′ is such as to prevent the vibrations caused by the milling operation from producing cracks in the stator  19  and in the rotor  30 , in particular in the stator arms  29   a  and rotor arms  30   a , which are the parts more easily subject to damage. 
     Subsequently (FIG.  9 ), the residual portion  9 ′ is removed via chemical etching, for example a wet etch or a plasma etch that automatically stops on the stop layer  10  (of silicon dioxide), which is exposed and protects the underlying regions (external epitaxial region  21 ′ and first and second supporting regions  17  and  18 ). 
     Next, through oxide etching, the stop layer  10  and the oxide portion  19  are removed. Thereby, the first supporting region  17  and second supporting region  18  are freed and rendered movable with respect to one another. Consequently, also the stator  29  (which is integral with the first supporting region  17 ) and the rotor  30  (which is integral with the second supporting region  18 ) are movable with respect to one another. 
     The process is then completed with known processing steps. In particular (FIG.  11 ), suspended connection lines  36   a  and contact regions  36   b  are formed; the body  2  of the wafer  1  is thinned by milling; and the composite wafer  35  is welded to a service wafer, for example of glass, and then cut, employing usual cutting techniques, to obtain a plurality of dice  35 ′, each of which comprises a microactuator  37  connected to a respective protection chip  38 . Finally (FIG.  12 ), the protection chip  38  is removed, and the microactuator  37  is assembled to a member that can be moved  39   a  (for example, a write/read head of a hard disk) and to a supporting member  39   b  (for example a suspension or gimbal). 
     According to a different embodiment of the invention, the process is used for obtaining a micromotor provided with a translating platform. 
     As shown in FIG. 13, initially a supporting wafer  40  is formed, basically as already illustrated with reference to FIG.  1 . In particular, the supporting wafer  40  comprises a semiconductor material body  41 , accommodating control circuits  42  (represented only schematically through active and passive electrical components) and an insulating layer  43 , which is etched to form contact regions  44  (shown only schematically) on top of first actuation control regions  48 , which are shorter in height than the contact regions  44 . 
     With reference to FIGS. 14-21, on a wafer  46  (having a thickness of between 600 μm and 700 μm, for example 675 μm) a silicon dioxide layer is deposited to form a stop layer  47 , and then an epitaxial layer  49  is grown having a thickness of, for instance, 100 μm. 
     Subsequently (FIG.  14 ), via a trench etch, circular trenches  50  are formed having a depth such as to come into contact with the stop layer  47  (the circular trenches  50  are shown in plan view in FIG.  21 ). In detail, each of the circular trenches  50  delimits a respective cylindrical region  51 ; the cylindrical trenches  50  are arranged at equal distances and are made along the perimeter of a square designed to house the rotor element of a linear-type micromotor the side of which measures, for example, 3 mm. 
     Via a thermal-oxidation step, an insulating layer  52  is then formed which covers the entire wafer  45  and, in particular, the walls of the circular trenches  50  (FIG.  15 ). Next, a conductive layer  53  is deposited, preferably of doped polycrystalline silicon, which fills the circular trenches  50 . The conductive layer  53  and the insulating layer  52  are then dry-etched, so as to be removed from a surface  54  of the epitaxial layer  49 , and subsequently wet-etched, so as to be removed from a bottom face (not shown) of the wafer  45  (FIG.  16 ). 
     Thereby, annular structures  58  are formed which comprise two insulating regions  52 ′, set concentrically, and an intermediate conductive region  57 . The annular regions  58  surround the cylindrical regions  51  (forming vias) and isolate them with respect to the outside world. 
     On top of the epitaxial layer  49 , connection regions  60  and second actuation-control regions  61 , for example of chromium-palladium, are then formed, with connection region  60  being positioned on the cylindrical regions  51 . 
     As shown in FIG. 17, the wafer  45  is then set upside down, aligned and welded to the supporting wafer  40 . In particular, the connection regions  60  are aligned to the contact regions  43 , thus electrically connecting the cylindrical regions  51  to the contact regions  43  and to the control circuits  42 . The first and second control regions are set facing one another, even if they are not aligned, for the reasons explained hereinafter. 
     Subsequently, the substrate  46  is removed. In particular, first a milling step is performed to eliminate one part of the substrate  46  and to leave a residual portion  46 ′ having a thickness D′ of approximately 50 μm (FIG.  18 ). Next, also the residual portion  46 ′ is removed, via chemical etching of the silicon, which is stopped by the stop layer  47  (FIG.  19 ). The etch may be either a wet etch or a plasma etch. 
     Through a photolithographic process, the stop layer  47  is selectively etched to form a mask  47 ′ (FIG.  20 ). Using this mask  47 ′, the epitaxial layer  49  is then etched, and a through trench  65  is formed which has a substantially square or rectangular shape; the mask  47 ′ is then removed. In detail, the through trench  65  has a width L 1  of, for instance, 25 μm, and delimits, within it, a platform  66  which is movable with respect to the epitaxial layer  49 ′ along two directions X, Y, parallel to the drawing sheet plane and orthogonal to one another, as a result of the forces generated by the first and second actuation control regions  48 ,  61  when the latter are appropriately biased (FIG.  21 ). The platform  66 , which preferably has a square shape, with a side length L 2  of approximately 2 mm, is connected to the epitaxial layer  49 ′ via springs  67  and is surrounded at a distance by the annular structures  58 . 
     Finally, standard processing steps are carried out to complete a translating-platform micromotor. 
     The advantages of the method according to the present invention emerge clearly from the foregoing description. In particular, thanks to the presence of the stop region  10 ,  47 , removal of the substrate  9 ,  46  of the wafer  8 ,  45  containing the microstructure (microactuator or micromotor) may be completed via a chemical etching step, thus considerably reducing any risk of cracks. The mechanical removal step (milling step) is in fact interrupted when the residual portion  9 ′,  46 ′ of the substrate  9 ,  46  to be eliminated still has a large thickness and is thus able, together with the stop layer  10 ,  47 , to attenuate the stresses that propagate to the parts more easily subject to cracking. Consequently, the percentage of rejects is considerably reduced and the yield of the process is high. 
     Furthermore, the final thickness of the wafer containing the microstructure can be controlled with very high precision. This thickness is in fact basically determined by the duration of the epitaxial growth which leads to the formation of the layers  13 ,  49  and can be easily controlled using current techniques and machinery. 
     A further advantage lies in the fact that, after removing the stop layer  10 ,  47 , the free surface of the epitaxial region has a low roughness, lower than that obtainable via planarization and polishing processes. 
     In addition, the stop layer  10 ,  47  may be advantageously used to form a silicon-etch mask, whenever this is required. 
     Finally, it is clear that modifications and variations may be made to the method described herein, without departing from the scope of the invention. 
     For example, it is possible to manufacture the microstructure starting from a silicon-on-insulator (SOI) wafer. In this case, the microstructure is made in a monocrystalline-silicon region, which can be advantageously exploited for forming also the signal control and pre-amplification circuitry. The wafer welded to the wafer containing the microstructure performs, instead, solely a supporting function. Using an SOI substrate, the process is simplified. 
     As has been pointed out, the process may be used also in case of an integrated circuit formed in a wafer comprising a substrate and an epitaxial layer separated from each other by an oxide layer, in which either the substrate or the epitaxial layer is removed in a final or in an intermediate step of the process. 
     The bonding regions used for welding the two wafers may be of a non-conductive type; for example, they may be made of glass paste.