Abstract:
The present invention discloses an adhesive-free method for preparation of micro electro-mechanical structure, comprising forming a micro electro-mechanical structure on a first substrate, forming an enclosing space for immersing liquid on the first or second substrate, and applying pressure to fix the first and second substrate. Before applying the pressure, the assembly including the two substrates is flipped, to make the contact surface immersed by the immersing liquid.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a new method for preparation of micro electro-mechanical structure, especially to a method for a micro electro-mechanical structure containing liquid therein. The present invention also discloses micro electro-mechanical structure prepared according to the invented method. 
     BACKGROUND OF THE INVENTION 
     In some applications of the micro electro-mechanical structure, it is necessary to provide fluid in the structure, in order to utilize the characters of the fluid to provide certain functions. One example of these applications is the liquid capacitive micro inclinometer. 
     Taking the liquid capacitive micro inclinometer as an example, in the preparation of such a micro electro-mechanical structure, the microstructure is first formed on a substrate. The structure may further include relative circuits. Then a second substrate, preferably a glass substrate, is prepared. An enclosing space is formed on the second substrate. Fluid is then added into the enclosing space or in the microstructure. The two substrates are combined and fixed to obtain the desired inclinometer. Such process may also be used to prepare other micro electro-mechanical structures wherein fluid is used. 
     One example of the microstructure so prepared is described in Taiwan patent application No. 101135550, “Liquid capacitive micro inclinometer,” by the assignee of this application. 
     In the process described above, adhesives are used to fix the two substrates, i.e., the microstructure side substrate and the enclosing space substrate. Although there are adhesives of a variety of types usable in this process, the compatibility of the adhesive becomes one technical problem to be solved. This is because in the interface of the two substrates, materials of different physical and chemical characters are included. In addition, the photoresist materials are often used in the enclosing space side. It is difficult to select a particular adhesive that is compatible with all these materials, so to fix the two sides. In nature, micro electro-mechanical structures are structures in very tiny scale. For structures in such small scale, slight incompatibility in the interface of the two side would lead to leakage of the fluid during preparation, storage, shipment and use. 
     In addition, when the microstructure side substrate and the enclosing space side substrate are combined, heat and/or high pressure are used in adhering, setting and annealing. The high temperature or high pressure would gasify the fluid, leading to further leakage of the fluid, due to high gaseous pressure. The leakage does not only increase the cleaning costs but also damage the preciseness and correctness in measurement of the micro electro-mechanical components. In other words, yield rate in the preparation of the micro electro-mechanical components is damaged. 
     Therefore, it is necessary to provide a novel method for the preparation of micro electro-mechanical structure, without the need of adhesives in the combination of the microstructure side assembly and the enclosing space side assembly. 
     It is also necessary to provide a new method for the preparation of micro electro-mechanical structure that prevents leakage of fluid during the process. 
     Objectives of the Invention 
     The objective of this invention is to provide a novel method for the preparation of micro electro-mechanical structure, without the need of adhesives in the combination of the microstructure side assembly and the enclosing space side assembly. 
     Another objective of this invention is to provide an adhesive-free method for the preparation of micro electro-mechanical structure that includes a fluid in its structure. 
     Another objective of this invention is to provide a method for the preparation of micro electro-mechanical structure that prevents leakage of fluid during the process. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a novel method for the preparation of micro electro-mechanical structure is provided. The method includes the steps of: 
     preparing a micro electro-mechanical structure on a first substrate; 
     forming partition walls on the first substrate, so that the partition walls define an enclosing chamber; 
     introducing a fluid in the enclosing chamber; 
     overlapping the first substrate with a second substrate; 
     flipping the assembly of the first and second substrates, so that a contact surface of the partition walls and the second substrate is positioned below the surface of the fluid; 
     applying pressure to the first and second substrate, so to weld the contact surface. 
     In some embodiments, the method further includes the step of annealing the assembly so obtained. 
     In other embodiments of this invention, the enclosing space is formed on the second substrate. In such embodiments, the method comprises the steps of: 
     preparing a micro electro-mechanical structure on a first substrate; 
     forming partition walls on a second substrate to define an enclosing chamber; 
     introducing a fluid in the micro electro-mechanical structure or the enclosing chamber; 
     overlapping the first substrate with the second substrate, so that the micro electro-mechanical structure is enclosed in the enclosing chamber; 
     optionally flipping the assembly of the first and second substrates, so that a contact surface of the partition walls and the second substrate is positioned below the surface of the fluid; 
     applying pressure to the first and second substrate, so to weld the contact surface. 
     In some embodiments, the method further includes the step of annealing the assembly so obtained. 
     In some particular embodiments, the method further comprises the step of forming a circuit structure on the first substrate, wherein the circuit structure is in connection with the micro electro-mechanical structure. 
     In the preferred embodiments of this invention, the first substrate may include any material suited in the semiconductor preparation process, such as the silicon substrate. The partition walls may be made of any material that is able to maintain the gaseous pressure inside the enclosing chamber. A preferred material for the partition walls is the photoresist material. In addition, the second substrate may be a silicon, glass, metal, metal oxide, plastic, rubber, resin material or their combinations. In the enclosing chamber, a lubrication layer may be formed on the surface of the chamber. Material for the lubrication layer may be any surfactant, such as Teflon. Fluid applicable in this invention includes any fluid suited for the preparation of the micro electro-mechanical structure. The fluid may be conductive or non-conductive. In the preferred embodiments of this invention, silicone oil is used as the fluid. Pressure applied to the two substrates in the combination step may be determined according to type and compositions of the fluid and characters of the partition walls and the material at the contact surface. 
     These and other objectives and advantages of this invention may be clearly understood from the following detailed description by referring to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates the cross-sectional view of a liquid capacitive micro inclinometer, prepared in accordance with the invented method. 
         FIG. 2  shows the flowchart of the method for preparation of micro electro-mechanical structure according to one embodiment of this invention. 
         FIGS. 3A to 3E  respectively show the main steps of the method for preparation of micro electro-mechanical structure according to this invention. 
         FIG. 4  shows the flowchart of the method for preparation of micro electro-mechanical structure according to another embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the followings detailed description of the invented method for preparation of micro electro-mechanical structure will be given by its preferred embodiments. It is appreciated that description to the preferred embodiments serves to illustrate examples of the present invention, without limitation to its scope of protection. 
     The present invention may be used to fabricate any micro electro-mechanical structure containing a fluid in the structure. The micro electro-mechanical structure may be a detector and the fluid may be a liquid. An example of the micro electro-mechanical structure that may be fabricated in accordance with the present invention is the liquid capacitive micro inclinometer. For this and other reasons, in the following descriptions the liquid capacitive micro inclinometer will be taken as example in describing the invented method for preparation of micro electro-mechanical structure. 
       FIG. 1  illustrates the cross-sectional view of a liquid capacitive micro inclinometer, prepared in accordance with the invented method. As shown in this figure, the liquid capacitive micro inclinometer  200  is prepared on a first substrate  10 . The first substrate  10  shown in  FIG. 1  is a substrate commonly used in the standard CMOS process, i.e., the silicon substrate. On the first substrate  10 , a plurality of dielectric layers  11   a ,  11   b ,  11   c ,  11   d , a plurality of metal layers  12   a ,  12   b ,  12   c ,  12   d  and a plurality of vias  13   a ,  13   b ,  13   c  are formed by the standard CMOS manufacturing process. 
     The inclinometer  100  has a pair of differential electrodes and a common electrode. In the inclinometer shown in  FIG. 1 , the 3 electrodes (not shown) are formed in the same metal layer, i.e. the third metal layer  12   c . Support structure  24  including a plurality of dielectric layers, a plurality of metal layers and a plurality of vias surrounds the 3 electrodes. Partition walls  25  are formed on the support structure  24  and are covered by a second substrate  26 , to define an enclosing chamber  27 . Immersing liquid  28  is sealed in the enclosing chamber  27 . 
     In the preferred embodiments of this invention, the partition walls  25  are prepared with a photoresist material and material for the second substrate  26  is glass. Of course, this is not any technical limitation. Other materials may also be used to prepare the partition walls  25  and the second substrate  26 . 
     In this example, the 3 electrodes are formed in a single metal layer, while in other embodiments, the 3 electrodes are formed in a plurality of metal layers, i.e., a plurality of metal layers with dielectric layer(s) sandwiched therein, if the structure is prepared according to the standard CMOS process. 
     In order to prevent or reduce the immersing liquid  28  from adhering to the surface of the 4 3 electrodes, due to the capillary force, selected portions or the full surface of the 3 electrodes are applied with a lubrication layer (not shown). Suited materials for the lubrication layer are known to those having ordinary skills in the art. In some embodiments, the lubrication layer is Teflon. 
     The inclinometer with the structure described above may be fabricated in accordance with the standard CMOS process. Therefore, the detection circuits may be prepared on the same substrate, simultaneously when the micro electro-mechanical structure is prepared. This would simplify the fabrication process of the inclinometer and provide a solution to the integration of the detector and the detection circuits. 
     In the followings, examples for the invented method for preparation of micro electro-mechanical structure will be described.  FIG. 2  shows the flowchart of the method for preparation of micro electro-mechanical structure according to the first embodiment of this invention.  FIGS. 3A to 3E  respectively show the main steps of the method shown in  FIG. 2 . As shown in  FIG. 2 , when the method of this embodiment is used to prepare the micro electro-mechanical structure, a first substrate  10  is first prepared in step  201 . Material for the first substrate  10  is not limited to any particular material. As a general case, the first substrate  10  may be made of a material commonly used in the standard CMOS process, i.e., the silicon substrate. The silicon substrate is advantageous, because the micro electro-mechanical structure thus may be fabricated in the standard CMOS process. Of course, it is possible to use any other rigid material or another material usable in the standard CMOS process, to obtain identical or similar effects. 
     In the following, at step  202  a material stack is formed on the first substrate  10 . The stack may include: a dielectric layer  11   a  on the substrate  10 , several metal layers  12   a ,  12   b ,  12   c ,  12   d  and several dielectric layers  11   b ,  11   c ,  11   d , alternatively formed on the dielectric layer  11   a , and a plurality of vias  13   a ,  13   b ,  13   c  formed in the stack. The stack includes patters of an inclinometer (detector)  100  and a detection circuit  30 . Such a stack of material may be prepared in accordance with any commercially available process that produces circuit structure and/or microstructure. Among the commercially available processes, the standard CMOS process is preferred. The detection circuit  30  may be any circuit structure designed using the commercially available design tools, such as the multilayered circuit structure prepared in the standard CMOS process. The detection circuit  30  detects variations in capacitance and converts the detection results into signals representing tilt angles. Detection circuits having these functions are available in the market and anyone having ordinary skills in the art may use any commercially available design tools to design the circuit and the commercially available process to form the circuit on the first substrate  10 . Details thereof are thus omitted. 
     As to the detector  100 , it includes a pair of differential electrodes and a common electrode that may be formed in one metal layer, such as the third metal layer  12   c , as in this particular example. The preparation of the detector  100  may include etching the metal layer  12   c  to form patterns of the desired electrodes. Any method in forming patterns of electrodes may be used in forming the electrodes. In addition, it is possible to form more than one pair of differential electrodes in one plane or in substantially the same plane of the metal layers, by using the conventional art. Those having ordinary skills in the art may easily complete the preparation of the electrodes, after having read the disclosure of this patent specification and its attached drawings. Details thereof are also omitted. 
     The stack material may include a support structure  24  including a plurality of metal layers, a plurality of dielectric layers and a plurality of vias. In the support structure  24  the vias respectively extend through a plurality of metal layers and a plurality of dielectric layers, so to strengthen the support structure  24 . The support structure  24  is designed to support an enclosing chamber inside it. Similarly, the support structure  24  may be prepared using the standard CMOS process, at the same time when the detection circuit and the electrodes are formed. Since such technologies are known in the art, details thereof are thus omitted. The structure so obtained is shown in  FIG. 3A . 
     Following that, at step  203  the dielectric layers positioned above the electrodes are removed, until the electrodes are exposed. The resulted structure is shown in  FIG. 3B . At step  204  a lubrication layer  15  is applied to the surface of the electrodes. The lubrication layer  15  may be made of any material that is capable of eliminating or reducing the capillary force of the electrodes. In the preferred embodiments of this invention, the lubrication layer  15  is a Teflon, that is, a polytetrafluoroethylene, layer. Of course, any other material that provides the same or similar functions may be used in the lubrication layer  15 . The lubrication layer  15  may be applied to the surface of the electrodes using any conventional method, while spin casting is preferred. The thickness of the lubrication layer  15  is not limited, as long as preciseness or correctness in measurement is not impacted. The material structure so obtained is shown in  FIG. 3C . 
     At step  205 , partition walls  25  are formed on the stack material, such that an inclosing space  27  enclosing the detector  100  in the upper part of the stack material is formed. The formation of the partition walls  25  starts from forming a layer of partition wall material on the material stack obtained in step  204 . The material of the partition wall is not limited, while in the preferred embodiments photoresist materials are used in forming the partition walls, in consideration of convenience in process. Suited photoresist materials include SU-8 and other similar materials. The partition wall material  25  is formed on the material stack using any applicable method, without any limitation in the thickness, provided that a space/chamber having sufficient volume to enclose the immersing liquid therein is defined by the partition walls  25 . In general cases, the thickness of the partition wall material  25  may be between 100 and 2,000 um, preferably between 200 and 1,000 um. Later an enclosing space  27  is formed in the partition wall material  25 , to enclose the immersing liquid  28 . The enclosing space  27  may be formed by partially removing material from the partition wall material layer, using such as wet etching. Of course, the partition wall material may be removed using other methods, such as dry etching. If necessary, cutting lines (not shown in  FIG. 3 ) may be formed. The structure so obtained includes the first substrate  10 , the detector  100 , the detection circuit  30  and partition walls  25  defining an enclosing space  27 , as shown in  FIG. 3D . 
     In the following step  206 , an immersing liquid  28  is introduced into the enclosing space  27 . The immersing liquid  28  may be a conductive liquid or a dielectric liquid. In case of conductive liquid, it may be one selected from an electrolyte liquid, magnetic liquid, a liquid metal, a liquid containing nano metal particles etc., or their combinations. If it is non-conductive, it may be a material of higher proportion and low viscosity, such as silicone oil. Amount of the immersing liquid  28  being introduced is not limited, while in the preferred embodiment volume of the immersing liquid  28  is approximately half of the volume of the enclosing space  27 . 
     In the following step  207 , the structure so obtained is covered by a second substrate  26 . Material for the second substrate  26  is not limited, provided it is rigid and easy to process. In the preferred embodiments, the second substrate  26  is a glass substrate. Of course, other materials, such as plastic, resin, fiber glass, metal, ceramic materials or their combination may also be used in the preparation of the second substrate  26 . At step  208  the assembly of the first substrate  10  with the structure prepared thereon and the second substrate  26  is flipped, so that the contact surface of the partition walls  25  and the second substrate  26  is positioned below the surface of the immersing liquid  28 . Any suitable tool may be used in this step, to prevent the immersing liquid  28  from leakage through the contact surface of the partition walls  25  and the second substrate  26 . Any method to maintain the immersing liquid  28  in the enclosing space, now enclosing chamber, may be used. Clamping the second substrate  26  tightly against the first substrate  10  during the flipping of the assembly is one good example. 
     At step  209  pressure is applied to the first substrate  10  and the second substrate  26 , in a manner sufficient to melt and weld the contact surface and affix the two substrates. Thereafter, at step  210  the assembly is heated to anneal the welding, if necessary. The temperature, duration and time of the annealing may be determined in accordance with the conditions of the assembly. 
     Although it is not intended to limit the present invention by any theory, it is found that the photoresist materials are in particular suited in the preparation of the partition walls  25  of this invention. It may be because the negative photoresist agents tend to generate crosslinking in its internal molecular structure, when exposed. The crosslinking would strengthen the photoresist structure and the crosslinking structure would be further strengthened, if thermal process follows. In other words, by applying pressure to the photoresist material, the crosslinking photoresist molecular would adhere to surface in contact. Experiment results show that the PerMX dry photoresist in combination with SU-8 photoresist, both supplied by DuPont, provides outstanding effects in affixing the partition walls and the second substrate. 
     The micro electro-mechanical structure so obtained is shown in  FIG. 3E . In another embodiment of this invention, the partition walls  25  are formed on the second substrate  26 . This approach also supports the invented method for preparation of micro electro-mechanical structure.  FIG. 4  shows the flowchart of the method for preparation of micro electro-mechanical structure according to the second embodiment of this invention. 
     As shown in  FIG. 4 , in the preparation of the inclinometer according to the method of the second embodiment, at step  401   a  first substrate  10  is prepared. At step  402  a stack of material is formed on the first substrate  10 . This stack includes a dielectric layer  11   a  on the substrate  10 , several metal layers  12   a ,  12   b ,  12   c ,  12   d  and several dielectric layers  11   b ,  11   c ,  11   d , alternatively formed on the dielectric layer  11   a , and a plurality of vias  13   a ,  13   b ,  13   c  formed in the stack. The stack may include a detector part  100  and a detection circuit  30 . The detector part  100  includes at least one pair of differential electrodes and a common electrode. At step  403  dielectric layer above the electrodes are removed, until the electrodes are exposed. At step  404  a lubrication layer  15  is applied on the surface of the electrodes. 
     In the following, at step  405  a partition wall material layer  25  is formed on a second substrate  26 . The material and thickness of the partition wall material layer is the same as that of the first embodiment. Then at step  406  an enclosing space  27  is formed in the partition wall material layer  25 , to function as enclosing chamber for immersing liquid  28 . Method for forming the enclosing space  27  is the same as that of the first embodiment. If necessary, cutting lines may be form in the partition wall material layer  25 . 
     Thereafter, at step  407  immersing liquid  28  is introduced into the recess formed in the stack on the first substrate  10  after portions of the partition wall material are removed or in the enclosing space  27  on the second substrate  26 . Volume of the immersing liquid  28  is not limited but is preferably approximately half of the volume of the enclosing space  28 . At step  408 , the second substrate  26  is flipped to overlap the material stack of the first substrate  10 , so that the exposed electrodes face the enclosing space  28 , if the immersing liquid was introduced into the recess formed on the first substrate  10  at step  407 . If in step  407  the immersing liquid was introduced into the enclosing space  28 , in step  408  the first substrate  10  is flipped to cover the enclosing space  28  of the second substrate  26 , so that the exposed electrodes face the enclosing space  28 . In the resulted assembly, the material stack on the first substrate  10  and the enclosing space  28  on the second substrate  26  jointly define an enclosing chamber. 
     In the following, if necessary, at step  409  the assembly of the first substrate  10  as well as its material stack and the second substrate  26  is flipped, so that the contact surface of the partition walls  25  and the material stack of the first substrate  10  is positioned below the surface of the immersing liquid  28 . To be precise, if after step  408  the contact surface is already positioned below the surface of the immersing liquid  28 , the assembly is not flipped. In this step  409 , necessary tools may be used to prevent the immersing liquid from leakage via the contact surface. 
     At step  410  pressure is applied to the first substrate  10  and the second substrate  20 , in a manner that the contact surface melts and is welded. The method, reaction conditions, parameters for this step are all the same as that of the first embodiment. If necessary, the assembly so obtain is annealed in step  411 . The micro electro-mechanical structure so obtained is shown in  FIG. 3E . 
     In the above-described embodiments, suited immersing liquid may be any fluid used in the fabrication and application of micro electro-mechanical structures. The liquid may be either a liquid or a sticky substance and may be conductive or non-conductive. In the described embodiments, the immersing liquid is silicone oil. The present invention provides a simplified method for the preparation of micro electro-mechanical structures. No adhesives are needed in the invented method. The invented method does not only reduce the fabrication cost of the micro electro-mechanical structure but also prevent the immersing liquid from leakage during process, storage, shipment and use.