Abstract:
The present invention discloses a micro-electro-mechanical system (MEMS) device, comprising: a mass including a main body and two capacitor plates located at the two sides of the main body and connected with the main body, the two capacitor plates being at different elevation levels; an upper electrode located above one of the two capacitor plates, forming one capacitor therewith; and a lower electrode located below the other of the two capacitor plates, forming another capacitor therewith, wherein the upper and lower electrodes are misaligned with each other in a horizontal direction.

Description:
BACKGROUND 
     1. Field of Invention 
     The present invention relates to a micro-electro-mechanical system (MEMS) device, in particular an out-of-plane sensor, and a method for making the MEMS device, which are less affected by the stress caused by the manufacturing process. 
     2. Description of Related Art 
     An out-of-plane sensor senses the capacitance variation resulting from a change of the distance between two electrodes, and generates a corresponding signal. Such out-of-plane sensor may be used in, e.g., an accelerometer. Related art can be found in, e.g., U.S. Pat. Nos. 6,402,968; 6,792,804; 6,845,670; 7,138,694; and 7,258,011. 
     The above-mentioned prior art discloses either single-capacitor structure occupying a large area, or differential capacitor structure made by wafer bonding. The former consumes too much area, while the latter requires a complicated process that is not compatible with standard CMOS process. 
     Therefore, it is desired to provide an out-of-plane sensor with reduced area, which can be manufactured by standard CMOS process and less affected by the stress caused by the manufacturing process. 
     SUMMARY 
     In view of the drawback of the prior art, it is a first objective of the present invention to provide a MEMS device, in particular an out-of-plane sensor, which is less affected by the stress caused by the manufacturing process. 
     It is a second objective of the present invention to provide a method for making a MEMS device such as an in-plane sensor. 
     In accordance with the foregoing and other objectives of the present invention, and from one aspect of the present invention, the present invention discloses a MEMS device, comprising: a mass including a main body and two capacitor plates located at the two sides of the main body and connected with the main body, the two capacitor plates being at different elevation levels; an upper electrode located above one of the two capacitor plates, forming one capacitor therewith; and a lower electrode located below the other of the two capacitor plates, forming another capacitor therewith, wherein the upper and lower electrodes are misaligned with each other in a horizontal direction. 
     In the MEMS device, preferably, the mass further includes two outer mass parts connected respectively with the two capacitor plates at the outer side of each of the two capacitor plates. 
     In another aspect of the present invention, the present invention discloses an out-of-plane sensor, comprising a plurality of MEMS structure units, each of the MEMS structure unit including: a mass including a main body and two capacitor plates located at the two sides of the main body and connected with the main body, the two capacitor plates being at different elevation levels; an upper electrode located above one of the two capacitor plates, forming one capacitor therewith; and a lower electrode located below the other of the two capacitor plates, forming another capacitor therewith, wherein the upper and lower electrodes are misaligned with each other in a first horizontal direction. 
     In the out-of-plane sensor, preferably, the mass further includes two outer mass parts connected respectively with the two capacitor plates at the outer side of each of the two capacitor plates. 
     The plurality of MEMS structure units in the out-of-plane sensor may be arranged in various layouts; in one example, at least two MEMS structure units are orthogonal to each other in a horizontal plane. 
     In the out-of-plane sensor, preferably, a continuous length of the main body in at least one horizontal direction is less than a predetermined length limit, e.g., 60 μm-100 μm. 
     In another aspect of the present invention, the present invention discloses a method for making a MEMS device, comprising: providing a substrate; depositing and patterning at least a contact layer, a metal layer and a via layer, the contact layer, metal layer and via layer as a whole include a to-be-etched region therein; and removing the to-be-etched region to form a MEMS device as described in the above. 
     In the method for making a MEMS device, preferably, the step of removing the to-be-etched region includes: first etching the to-be-etched region by an anisotropic reactive ion etch, and then etching the to-be-etched region by hydrogen fluoride vapor etch or buffered oxide etch. 
     It is to be understood that both the foregoing general description and the following detailed description are provided as examples, for illustration and not for limiting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings. 
         FIG. 1  shows a structure embodiment according to the present invention. 
         FIG. 2  explains how the present invention reduces the impact of the residual stress. 
         FIGS. 3-5  show several other structure embodiments of the present invention. 
         FIG. 6  shows, as an example, how the top view may look like from the A-A section of  FIG. 5 . 
         FIG. 7  schematically shows a top view of the MEMS structure unit  100  according to the present invention. 
         FIGS. 8-10  show three layout embodiments of the present invention. 
         FIGS. 11-14  show a process embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The drawings as referred to throughout the description of the present invention are for illustration only, but not drawn according to actual scale. 
     Referring to  FIG. 1 , in one embodiment of the present invention, the MEMS structure  100  of an out-of-plane sensor includes a mass  10 , a lower electrode  20  and an upper electrode  30 . In the x-direction of the figure, the lower electrode  20  and the upper electrode  30  are misaligned with respect to each other. One differential capacitor is formed between the mass  10  and the lower electrode  20 , and another differential capacitor is formed between the mass  10  and the upper electrode  30 . When the mass  10  moves along the z-direction of the figure, the sensitivity of the sensor is double, because the variation of the capacitance is twice of the variation of the vertical distance. For convenience in capacitance calculation, the vertical distance between the mass  10  and the lower electrode  20 , and the vertical distance between the mass  10  and the upper electrode  30  should preferably be the same, but not necessarily so. In the latter case, the capacitance calculation should take the difference between the distances into consideration. 
     One merit of the structure as shown is that it can reduce the impact of stress. Referring to  FIG. 2 , the mass  10  may be divided into five parts, i.e., the main body  13 , two outer mass parts  11  and  15 , and two structure parts  12  and  14  acting like capacitor plates. The two capacitor plates  12  and  14  are located at different elevation levels (in z-direction), forming two capacitors with the upper and lower electrodes  30  and  20 , respectively. If the structure part  12  is only connected with the main body  13 , but not with the outer mass part  11  (that is, if the outer mass part  11  does not exist), then bending may occur as shown by the dash-line block due to stress generated in the manufacturing process. Likely, if the structure part  14  is only connected with the main body  13 , but not with the outer mass part  15  (that is, if the outer mass part  15  does not exist), then bending may also occur as shown by the dash-line block. However, although both the structure parts  12  and  14  bend upwardly, their relationships with the corresponding upper and lower electrodes  30  and  20  are changed asymmetrically. In other words, the process may impact the structure, and leads to inaccuracy in capacitance calculation. In the present invention, since there are outer mass parts  11  and  15  provided to connect the outer sides of the structure parts  12  and  14 , the likelihood of bending can be greatly reduce. Nevertheless, such outer mass parts  11  and  15  are only preferred, but not necessary to the present invention. That is, the mass  10  can include just the main body  13  and two structure parts  12  and  14 , without one or both of the outer mass parts  11  and  15 . In this case the likelihood of bending should be solved by other means. 
     Under the same spirit of the present invention, the structure of  FIG. 1  may be modified in various ways, and the number of layers may be different. The key features are that (1) the upper and lower electrodes  30  and  20  are misaligned in x-direction, and (2) at least one side of each of the capacitor plates is connected with a relatively larger mass body; preferably, both sides of the capacitor plates are connected with relatively larger mass bodies.  FIGS. 3-5  show several more examples. Note that as shown in  FIG. 4 , the upper and lower electrodes  30  and  20  do not require to have the same cross sectional structure, because what effectively produce the capacitance are the lower surface of the upper electrode  30 , the upper surface of the lower electrode  20 , and the distances between the mass  10  and them. Moreover, as shown in  FIG. 5 , the two capacitor plates  12  and  14  may be connected with the other parts  11 ,  13  and  15  only at one or several connection points, instead of integrated as a whole body. For example, as shown by the dash lines in  FIG. 5 , they may be connected with one another in only one of the interconnection layers, from cross-sectional view. The dash lines in  FIG. 5  imply that the connection points are not at the same cross section as the rest of the figure. Or, they may be connected with one another only at one or several connection points, from top view. For example, the top view taken from the A-A line of  FIG. 5  may be as shown in  FIG. 6 . 
     The MEMS structure  100  shown in  FIGS. 1-5  may be taken as one MEMS unit, having a top view as shown in  FIG. 7 . The lower electrode  20  is shown by dash lines to imply that it is located beneath the mass  10 . Multiple such MEMS units may be grouped together, with properly arranged layout, to increase the accuracy of the overall device.  FIG. 8  shows an example, wherein multiple MEMS units are arranged by the layout as shown. Note that the upper-right and lower-left MEMS units are rotated 90°, to cancel the imbalance cause by the stress in x and y directions, so that the capacitance measured by the overall device is more accurate.  FIG. 8  also shows that the mass  10  includes openings  19 . Such openings are provided so that it is easier to etch the material beneath the mass  10 , as will be understood more clearly with reference to the manufacturing process to be described later. The mass  10  is connected with anchors  50  via springs  40 . 
     There are many variations of the layouts, other than that shown in  FIG. 8 .  FIGS. 9 and 10  show two more layout embodiments. In the embodiment of  FIG. 9 , some of the MEMS units are misaligned in at least one of the x and y directions, in x direction in this embodiment (the MEMS units  100  have been rotated 90° as compared with  FIG. 7 , and therefore to be consistent, we define the x dimension to be the dimension where the upper and lower electrodes  30  and  20  are misaligned with each other). The misalignment of the MEMS units  100  is for the purpose to limit the continuous length of the mass  10  in x, y or both directions (only in x direction in this embodiment). More specifically, as shown in the figure, the maximum length of the mass  10  is no longer x; the x dimension is divided into multiple segments x 1 , x 2  and x 3 , each being less than a predetermined length, e.g., 60 μm-100 μm. The shortened continuous length in a direction helps to avoid bending in that direction. The length limit is applied only to the x dimension in this embodiment, but it can certainly be applied to the y dimension as well, or both x and y dimensions. Note that the mass  10  in this embodiment also includes openings  19 . 
       FIG. 10  shows a layout embodiment which includes both features of “MEMS unit rotation” and “length limit”. The maximum length in y dimension is limited. This embodiment also shows that the MEMS units  10  do not have to be of the same size. The springs and the anchors are not shown in the figure; they may be located upper and lower sides of the mass  10  (in y dimension) or at the left and right sides of the mass  10  (in x dimension). 
     Hereinafter a manufacturing process according to the present invention will be explained with reference to  FIGS. 11-14 , taking the structure shown in  FIG. 5  for example. A six-metal-layer process is shown as an example, but the present invention can certainly be embodied in a process of any other number of metal layers, for making the structures of  FIGS. 1-10  and other structures under the spirit of the present invention. 
     Referring to  FIG. 11 , in this embodiment, a zero-layer wafer substrate  71  is provided, which for example can be a silicon wafer so that the process is compatible with a standard CMOS process. Next, transistor devices can be formed by standard CMOS process steps as required (not shown), followed by deposition, lithography and etch steps to form interconnection layers including a contact layer  72 , metal layers  73 , and via layers  74 . The patterns of the interconnection layers are such that they form the upper and lower electrodes  30  and  20 , and the mass  10  (including the main body  13 , the outer mass parts  11  and  15 , and the two capacitor plates  12  and  14 ); and an oxide region  75  (the “to-be-etched region”) is concurrently formed in the interconnection layers. In order that the etching of the oxide region  75  does not damage the other areas in the MEMS device, preferably, the oxide region  75  is encompassed in a guard ring  60 , The guard ring can be formed concurrently with the patterns of the layers  72 ,  73  and  74 . 
     In the structure as shown, for example, the metal layers can be made of aluminum; the contact layer and the via layers can be made of tungsten; and the oxide region  75  may be made of silicon oxide. The mass  10  and the lower electrode  20  may be completely made by tungsten, or made by oxide  76  enclosed by surrounding tungsten, as shown in the figure. Of course, the latter costs lower. 
     Referring to  FIG. 12 , a mask  77  is formed and patterned to expose the oxide region  75 . The mask  77  for example can be a photoresist layer patterned by lithography, or other materials such as a metal layer or an amorphous silicon layer. 
     Referring to  FIG. 13 , an oxide etch step is performed according to the pattern of the mask  77 , to remove the oxide inside the narrow portions of the oxide region  75 . The etch for example can be anisotropic RIE (reactive ion etch). 
     Next referring to  FIG. 14 , another etch step is performed on the oxide region  75  to remove it. The etch for example can be HF (hydrogen fluoride) vapor etch, or BOE (buffered oxide etch) by immersing the whole wafer in an acid tank. 
     Finally, if the mask  77  is a photoresist layer, it can be removed; or, if the mask  77  is a layer of some other material, it can be kept or removed as desired. A desired MEMS device is thus obtained. 
     Although the present invention has been described in detail with reference to certain preferred embodiments thereof, the description is for illustrative purpose and not for limiting the scope of the invention. For example, the openings in the mass and the layout of the MEMS units may be arranged in various other ways than that shown in  FIGS. 8-10 . As another example, the aluminum (for metal layers), tungsten (for contact and via layers) and silicon dioxide (for the to-be-etched region) may be replaced by copper and low dielectric constant materials. One skilled in this art can readily think of other modifications and variations in light of the teaching by the present invention. In view of the foregoing, it is intended that the present invention cover all such modifications and variations, which should interpreted to fall within the scope of the following claims and their equivalents.