Process for manufacturing mechanical, electromechanical and opto-electromechanical microstructures having suspended regions subject to mechanical stresses during assembly

Abstract: To reduce the risk of breakage of the moving parts of an integrated microstructure during manufacture steps causing mechanical stresses to the moving parts, a temporary immobilization and support structure (20; 30; 40) is formed thereby a moving region (12, 13) of the microstructure is temporarily integral with the fixed region (10, 11). The temporary structure (20; 30; 40) is removed at the end of the assembly operations by non-mechanical removal methods. According to one solution, the temporary structure is formed by a fusible element (20) removed by melting or evaporation, by applying a sufficient quantity of energy thereto. Alternatively, a structural region (30) of polymer material is formed in the trench (15) separating the moving part from the fixed part, or an adhesive material layer (40) sensitive to ultraviolet radiation is applied.

Description: The invention relates to a process for manufacturing mechanical, electromechanical and opto-electromechanical microstructures having suspended regions subject to mechanical stresses during assembly. As known, during the manufacture of microstructures of mechanical, electromechanical and opto-electromechanical type, suspended regions are formed which may be damaged by mechanical stresses induced by the final assembly processes. For example, during the manufacture of integrated hard disk drive units, comprising a microactuator to which a read/write head is glued, the steps of attaching the head to the microactuator and glueing, causing pressure on the rotor of the microactuator, may cause the breakage of the suspension points of the rotor and the collapse thereof. Similar problems arise in opto-microelectromechanical devices where it is necessary, for example, to move an optical fibre with a micromotor. The forces required for this assembly operation, of the order of tens of grams force, may irreparably damage the moving parts of the microstructures. US-A-5 6060 039 teaches a process for manufacturing microstructures according to the preamble of claim 1, including forming metal regions 78 on the surface of the structure to temporarily bond a region intended to form a supported member to a fixed layer. In the alternative, a temporary restrain is achieved by forming a silicon bridge that is laser cut after packaging. Weijie Yun et al "Surface micromachined, digitally force-balanced accelerometer with integrated CMOS detection circuitry" Technical digest, IEEE solid-State Sensor and Actuator workshop (Cat No. 92TH0403-X), Hilton Head Island, SC, USA, 22-25 June 1992, page 128 discloses forming fuses that are severed by sending a current after completion of all processing steps. US-A-5 258 097 discloses forming temporary posts under a diaphragm by filling holes formed in a sacrificial layer. The posts, of polymer, provide a support for a structural layer during etching of the sacrificial layer and are then removed. The object of the present invention is therefore to devise solutions to reduce the risk of breakage of the suspensions of the moving parts of microstructures of the mentioned type. The invention provides a process for manufacturing microstructures, as defined in Claim 1 and an intermediate structure for forming a microstructure, as defined in claim. For an understanding of the invention, one preferred embodiment will now be described, purely by way of non-exhaustive example, with reference to the accompanying drawings wherein: Figure 1 is a perspective view, partially interrupted, of a micromechanical structure during a step of a manufacturing process which is not an embodiment of the present invention; Figure 2 shows a view similar to that of Figure 1, in a subsequent step; Figure 3 is a perspective view, partially interrupted, of a micromechanical structure during a manufacturing process which is not an embodiment of the present invention; Figures 4-8 show cross-sections through a wafer of semiconductor material in successive steps of the manufacturing process according to Figure 3; and Figure 9 shows a perspective view, partially interrupted, of a micromechanical structure during a step of an embodiment of the process. Figure 1 shows a portion of a microactuator 1 which is not an embodiment of the present invention comprising a stator 2 and a rotor 3 used, for example, in a hard disk read/write assembly with micrometric actuation. The microactuator 1 is formed in a semiconductor material wafer 4 comprising a substrate 5 and an epitaxial layer 6 inside which electronic components (not shown), the stator 2 and the rotor 3 are formed. The stator 2 is carried by the substrate 5 and comprises a fixed region 10 connected to a plurality of fixed arms 11, also carried by the substrate 5. The rotor 3 is separated from the stator 2 by a trench 15, it is held by suspension arms, not shown, and comprises a suspended mass 12 and a plurality of moving arms 13, only one of which is visible in Figure 1. An air gap 14 separates the rotor 3 from the substrate 5 and extends underneath the rotor 3. To prevent the rotor 3 from collapsing on the substrate 5 during the operations of attaching the head (not shown) and bonding, according to a solution which is not an embodiment of the present invention, fusible elements are provided, anchoring the suspended regions to the fixed regions. In particular, Figure 1 shows a fusible element 20 anchoring the moving arm 13 to the fixed region 10. The fusible element 20 is formed by a pair of walls 21 extending from two adjacent zones of the fixed region 10 as far as the end of the fixed arm 13, forming a kind of wedge. The thickness of the walls 21 is such as to permit them to melt or evaporate when a predetermined quantity of energy is applied (electrically, with a laser beam, or by other methods); preferably this thickness is less than 5 µm. In the embodiment shown, wherein melting takes place by electrical means, a conductive region 22, highly doped, is formed inside the fixed region 10 and suitably isolated from the rest of the fixed region 10 (in a not shown manner, by junction isolation or a suitable trench for example); the walls 21 extend from the conductive region 22 and are electrically connected thereto. Contact regions 23 shown solely diagrammatically in Figure 1 electrically connect the conductive region 22 to metal tracks 24 and to a current source (not shown). The fusible element 20 is formed when excavating the trench 15 separating the stator 2 from the rotor 3. To this end, during the usual steps for forming the components integrated inside the wafer 4, the conductive region 22 and the portion of the epitaxial layer 6 wherein the fusible elements 20 are to be formed are suitably doped and the mask for defining the trench 15 is suitably shaped so as to form a plurality of fusible elements 20 to ensure sufficient resistance of the rotor 3 to mechanical stresses during the subsequent manufacturing steps, particularly when attaching the read/write head (not shown) to the rotor 3 and bonding the electrical connections. The fusible element 20 is removed when the operations which may cause a stress to the rotor 3 are completed. In particular, a high current is fed through the metal tracks 24 and the conductive region 22 to the walls 21 so as to cause them to melt, as shown in Figure 2. For example, a current equal to 200-300 mA is applied. Figure 3 shows another solution that is not an embodiment of the invention. According to this solution, the suspended structures is temporary locked by structural immobilization elements of a material resistant to the chemical agents used to free the moving structure and such as to connect it to the substrate or other fixed parts. As shown in Figure 3, wherein the parts common with the embodiment of Figure 1 have been denoted by the same reference numerals and are not described again, an immobilization region 30 is formed inside the trench 15, in a position adjacent to and in contact with the moving arm 13 of the rotor 3. The immobilization region 30 is supported on the substrate 5 and is preferably made of photoresist or another polymer which can be removed in gaseous phase. The immobilization region 30 is formed in the manner described below with reference to Figures 4-8. Figure 4 shows an initial structure comprising the substrate 5, the epitaxial layer 6 and a sacrificial region 31 arranged between the substrate 5 and the epitaxial layer 6. The sacrificial region 31 is made, for example, of silicon oxide grown selectively before forming the epitaxial layer 6, in per se known manner. In particular the sacrificial region 31 is present in the zone where the rotor 3 and the trench 15 are to be formed. After the possible manufacture of electronic components (not shown), immediately before the trench 15 is formed, the wafer 4 is masked and chemically etched to selectively remove portions of the epitaxial layer 6 and the sacrificial region 31 in the zones where the immobilization regions 30 are to be formed, forming openings 32. In particular, Figure 5 shows an opening 32 extending completely through the epitaxial layer and the sacrificial region 31 as far as the substrate 5. A layer of structural material (photoresist, polymer) which fills the openings 32 is then deposited. The part of the layer of structural material which projects from the openings 32 is then removed, by chemical-mechanical action for example (CMP - Chemical-Mechanical Polishing). The immobilization regions 30 are thus formed inside the openings 32, as shown in Figure 6. A trench 35 for defining the microactuator 1 is then formed in per se known manner. Through the trench 35, the remaining portions of the sacrificial region 31 are also removed, forming the air gap 14, as shown in Figure 7. The assembly steps are then carried out; for example, according to Figure 7 a read/write head 33 is glued to the suspended mass 12 and the electrical connections are bonded (as shown diagrammatically by the ball-shaped connections 34). During this step, the rotor 3 is immobilized and held by the immobilization structures 30 which absorb any stresses, preventing the suspension arms (not shown) from breaking. Finally, the immobilization structures 30 are removed, e.g. by plasma etching, as shown in Figure 8, and the trench 35, together with the openings 32, forms the final trench 15. Figure 9 shows an embodiment of the invention according to which external structural elements, removable after assembly, are applied to the surface of the wafer 4 at the areas where the moving and fixed parts join. Specifically, in Figure 9 an adhesive material layer 40, such as "stick foil" sensitive to ultraviolet rays, extends on top of the surface 41 of the wafer 4. In particular, the adhesive material layer 40 adheres to part of the fixed region 10, the fixed arms 11 and to the end of the moving arms 13. The adhesive material layer 40, which is transparent, is deposited on the entire surface of the wafer 4 and is then selectively dry removed, for example from the zone of the rotor 3 on which the read/write head (not shown) is to be glued, by known photolithographic methods. The adhesive material layer 40 thus holds the rotor 3 during assembly and is removed at the end of the stress by dry etching in O 2 -based plasma. By virtue of the present process the moving parts of an integrated microstructure are rendered integral with the fixed parts during the manufacturing steps wherein they are subject to mechanical stress and thus ensure the integrity thereof. The described embodiment is completely compatible with manufacturing processes normally used in microelectronics, therefore it does not cause application problems and does not damage any electronic devices formed inside the wafer. The described process ensures high efficiency in the assembly operations, is simple to apply and is of low cost. Finally it is evident that numerous modifications and variants may be introduced to the process described and illustrated herein, all of which come within the scope of the invention, as defined in the accompanying claims. In particular, it should be emphasized that the present process is applicable to all microstructures regardless of their material (silicon, metallic material such as nickel etc.), even if it is advantageously applied in case of microstructures formed in the epitaxial layer (generally in a multicrystal portion thereof, as described in EP-A-0 822 579 for example).

Claims: A process for manufacturing microstructures comprising at least one fixed region (10, 11) and a moving region (12, 13), comprising the steps of forming a temporary immobilization and support structure (40) that renders said moving region (12, 13) temporarily integral with said fixed region (10, 11); applying mechanical stress; and removing said temporary immobilization and support structure (40) through non-mechanical removal methods, characterized in that said step of forming a temporary immobilization and support structure comprises the step of applying an adhesive stick foil (40) to the surface (41) of said microstructure. A process according to Claim 1, characterized by the steps of: forming a semiconductor material wafer (4) comprising a substrate (5), an epitaxial layer (6) overlaying said substrate and sacrificial regions (31) extending between said substrate (5) and said epitaxial layer (6); and forming a trench (15) in said epitaxial layer (6), said trench (15) separating said moving region (12, 13) from said fixed region (10, 11), wherein said step of forming a temporary immobilization structure (40) is carried out after forming the trench. A process according to claim 1 or 2, wherein said temporary immobilization structure is applied on the entire surface of the microstructure, the process further comprising the step of selectively removing portions of said adhesive stick foil (40) through photolithographic methods. A process according to any of Claim 1 to 3, characterized in that said adhesive material layer (40) is sensitive to ultraviolet radiation. An intermediate structure for forming a microstructure comprising at least one fixed region (10, 11) and a moving region (12, 13), comprising a temporary immobilization and support structure (40) rendering said moving region (12, 13) temporarily integral with said fixed region (10, 11); characterized in that said temporary immobilization and support structure comprises an adhesive stick foil (40) extending on the surface (41) of said microstructure and adhering to portions of said fixed region (10, 11) and of said moving region (12, 13). An intermediate structure according to Claim 5, characterized in that it comprises a substrate (5), an epitaxial layer (6) overlaying said substrate, sacrificial regions (31) extending between said substrate and said epitaxial layer, and a trench (15) extending in said epitaxial layer (6) and separating said moving region (12, 13) from said fixed region (10, 11). An intermediate structure according to claim 5 or 6, characterized in that said stick foil (40) is sensitive to ultraviolet radiation. An intermediate structure according to any of claims 5-7, characterized in that said adhesive stick foil (40) covers the entire surface (41) of the wafer (4) accommodating said microstructure, except for a zone carrying a read/write head.

Inventors: MASTROMATTEO UBALDO, VIGNA BENEDETTO, MASTROMATTEO, UBALDO, VIGNA, BENEDETTO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1999
Main classification: F16B1
IPC codes: B23K10, B81C1, B81C99