Abstract:
Provided are a structure for detecting a vertical displacement and its manufacturing method. The structure for detecting a vertical displacement includes a body, an inertial mass floated over the body, a plurality of support beams extending from the inertial mass so as to suspend the inertial mass over the body, movable electrodes integrally formed with the inertial mass, and fixed electrodes floated over the body, each being positioned between the neighboring movable electrodes, wherein a vertical length of the movable electrode is different from a vertical length of the fixed electrode. Therefore, the structure and the electrodes can be simultaneously manufactured, thereby making the fabrication process simple. Also, it is possible to manufacture a three-axis accelerometer and a three-axis gyroscope on a single wafer by the same process, to be integrated as a six-axis inertial sensor.

Description:
Priority is claimed to Patent Application Numbers 1) 2001-22675 filed in Rep. of Korea on Apr. 26, 2001 and 2) 2001-66023 filed in Rep. of Korea on Oct. 25, 2001, herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a structure for a vertical displacement and a method for fabricating the same, and more particularly to a MEMS silicon structure. 
     2. Description of the Related Art 
     Generally, a structure for the vertical displacement provides an upper electrode and a bottom electrode horizontally disposed and detects a change in a capacitance therebetween due to a vertical displacement. 
     For the fabrication of the structure, a plurality of patterning procedures must be conducted because the structure member and the electrodes cannot be manufactured at the same time. Also, to maintain a predetermined interval between the electrodes, there may be required a sacrificial layer or a layer attachment, which makes the fabrication complicated. Furthermore, to precisely detect the vertical displacement, the interval between electrodes must be small, which results in a stiction therebetween. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a structure for detecting a vertical displacement wherein the structure member and the electrodes are manufactured simultaneously, thereby simplifying the fabrication procedures and also the electrodes are disposed laterally, thereby being free from a stiction phenomenon. 
     The other object of the invention is to provide a fabrication method for the same. 
     To achieve the above objects of the invention, there is provided a structure for detecting a vertical displacement comprising a body, an inertial mass floated over the body, a plurality of support beams extending from the inertial mass so as to suspend the inertial mass over the body, movable electrodes integrally formed with the inertial mass, and fixed electrodes floated over the body, each being positioned between the neighboring movable electrodes, wherein a vertical length of the movable electrode is different from a vertical length of the fixed electrode. 
     In the case of using the support beams as torsional members, the inertial mass has different density between a first side portion thereof and a second side portion thereof so as to make a rotation centered about the support beams. Preferably, the vertical lengths of the movable electrodes are shorter than the vertical lengths of the fixed electrodes or vice versa. 
     In the case of using the support beams as bending members, the vertical lengths of the movable electrodes located at a first side portion of the inertial mass are longer than the vertical lengths of neighboring fixed electrodes, and the vertical lengths of the movable electrodes located at a second side portion of the inertial mass are shorter than the vertical lengths of neighboring fixed electrodes, the first and second side portions being oppositely positioned centered about the bending members. 
     Within the various embodiments, the body is made of a single crystal silicon wafer or of a SOI wafer including a silicon substrate, an insulating layer and a silicon layer, the inertial mass is fabricated by etching the silicon layer. When using a SOI wafer, the silicon substrate is made of single crystal silicon or epitaxial growth polysilicon. 
     Alternatively, the body can be made of a SOG wafer including glass and silicon layer, the inertial mass is fabricated by etching the silicon layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is a perspective view of a MEMS structure for a vertical displacement in accordance with a first embodiment of the invention; 
     FIG. 2 is a plan view of the MEMS structure; 
     FIG. 3 is a cross-sectional view of a cut surface through III-III′ shown in FIG. 1; 
     FIGS. 4 a - 4   c  are graphs illustrating a detection method for a vertical displacement of the MEMS structure; 
     FIG. 5 shows a second embodiment of the structure shown in FIG. 3; 
     FIG. 6 shows a third embodiment of the structure shown in FIG. 3; 
     FIG. 7 is a plan view of a MEMS structure in accordance with a fourth embodiment of the invention; 
     FIG. 8 is a cross-sectional view of the structure shown in FIG. 7 taken through line VIII-VIII′; 
     FIG. 9 is a SEM picture of an accelerometer using the structure of the invention; 
     FIG. 10 is a picture showing an enlarged view of fixed and moving comb electrodes; 
     FIG. 11 is a graph showing an acceleration signal using the accelerometer of FIG. 9; and 
     FIGS. 12 a - 12   f  are cross-sectional view illustrating a fabrication method for the MEMS structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a structure for a vertical displacement and a fabrication method therefor of the present invention in accordance with preferred embodiments will be described with the attached drawings. 
     Referring to FIGS. 1 through 3, there is provided a rectangular frame  32  floated above a main body  10  made of single crystal silicon wafer. At opposing sides of the frame  32 , torsional springs  20  are formed therefrom and extend outward so as to be connected to respective inner walls of the main body  10 , thereby supporting the frame  32 . The frame  32  is formed with an inertial mass  34  inwardly extending and movable electrodes  30 . Between neighboring movable electrodes  30 , fixed electrodes  40  are disposed and each formed with a trench  44  with a narrow opening. As shown in the figures, the movable and fixed electrodes  30  and  40  can have a shape of a comb. The fixed electrodes  40  are connected to a fixing anchor  42 , via element  46 , so as to be floated from the bottom of the main body  10 . That is, the frame  32 , the moving electrodes  30  and the inertial mass  34  are supported at the main body  10  through the torsional springs  20  while the fixed electrodes  40  being supported at the main body  10  through the anchor  42 . The vertical length of the fixed electrode  40  is shorter than that of the movable electrode  30 . 
     In accordance with the present invention, the vertical displacement can be detected when the movable electrodes  30  move slidably against the fixed electrodes  40  resulting in a change in a capacitance formed between the electrodes  30  and  40 . Therefore, when the frame  32  rotates counter-clockwisely about the torsional spring  20 , referring to FIGS. 4 a  and  4   b , the capacitance C left  being formed in left section decreases along with the vertical displacement as shown in plus Z direction of the graphs while the capacitance C right  in the right section is maintained during a predetermined displacement distance. On the other hand, when the frame  32  turns clockwisely, referring to the minus Z direction of FIGS. 4 a  and  4   b , the capacitance C right  of the right section decreases along with the vertical displacement while the lefthand capacitance C left  is maintained during a predetermined displacement distance. Accordingly as shown in FIG. 4 c , the difference in capacitance C right −C left  varies linearly with the vertical displacement, thereby enabling the detection of the vertical displacement. 
     In above embodiment, the fixed electrode  40  is shorter than the movable electrode  30  in their vertical length, it is possible to make the fixed electrode  40  is longer than the movable electrode  30  in vertical length. 
     It is an advantage of the present invention that since the electrodes are aligned laterally, when the movable electrodes  30  move vertically relative to the fixed electrodes  40 , there is no stiction phenomena therebetween. 
     FIG. 5 shows another embodiment of the invention. An SOI (silicon on insulator) is used, and the substrate of the SOI preferably includes single-crystal silicon or epitaxially grown poly-silicon. 
     Referring to FIG. 5, the SOI wafer includes a substrate  13 , an insulating layer  14  and a silicon layer  15  sequentially. The fixed and movable electrodes  40  and  30  and the inertial mass  34  are floated over the insulating layer  14 , and the fixing anchor  42  is fixed onto the insulating layer  14 . 
     Referring to FIG. 6, a third embodiment of the invention is described, showing a case of using an SOG (silicon on glass) wafer. Elements having the same functions with the previous embodiments are given same numerals as well. A silicon layer  17  is anodic bonded onto a glass  16 . As vertical moving structure, the fixed electrode  40 , the movable electrode  30  and the inertial sensor  34  are floated over the glass  16 , and the fixing anchor is fixed to the glass  16 . 
     Accordingly with the second and third embodiments where SOI and SOG wafers are used, the structure for vertical displacement is insulated by the insulating layer  14  and the glass  16 , so that the latter procedure for electrically insulating the fixed and movable electrodes  40  and  30  can be deleted. 
     FIG. 7 shows a fourth embodiment of the invention and FIG. 8 is a cross-sectional view of FIG. 7 taken along the line VIII-VIII′. Elements having the same functions as those in the previous embodiments are given the same numerals. 
     Referring to FIGS. 7 and 8, a rectangular frame  32  is floated above a bottom  12  of a main body and is formed with a couple of bending springs  22  at both opposing sides thereof extending outward so as to be connected to inner walls of the main body. The bending springs  22  support the frame  32 . The frame  32  is formed with an inertial mass  34  inwardly extending and movable electrodes  30 . Between the movable electrodes  30  of the left-hand side (in FIGS.  7  and  8 ), trenches  44  are formed with narrow openings. As shown clearly in FIG. 8, the vertical length of the movable electrode  30  is shorter than that of the fixed electrode  40  disposed nearby. On the other hand, the fixed electrodes  40  at the right-hand side are formed with trenches  44  having narrow openings and are shorter than the movable electrodes in the vertical length thereof. The structure except the fixing anchor  42  is floated over the bottom  12  of the main body. 
     In accordance with the present invention, the vertical displacement can be detected when the movable electrodes  30  move slidably and vertically against the fixed electrodes  40  resulting in a change in a capacitance formed between the electrodes  30  and  40 . Therefore, when the frame  32  upward moves in plus Z direction, referring to FIG. 8, the capacitance C left  being formed in left section decreases along with the vertical displacement while the capacitance C right  in the right section is maintained during a predetermined displacement distance. On the other hand, when the frame  32  downward moves in minus Z direction, the capacitance C right  of the right section decreases along with the vertical displacement while the lefthand capacitance C left  is maintained during a predetermined displacement distance. Accordingly as shown in FIG. 4 c , the difference in capacitance C right −C left  varies linearly with the vertical displacement, thereby enabling the detection of the vertical displacement. 
     According to the present invention, the structure for vertical displacement detection can be implemented on SOI or SOG wafer. 
     FIG. 9 is a SEM photograph of an accelerometer manufactured by using the micro structure for a vertical movement of the invention, FIG. 10 is an enlarged view of movable and fixed electrodes, and FIG. 11 is a graph showing a result of an acceleration signal detection. 
     The detection experiment is taken under an acceleration input of 1G peak-to-peak sinusoidal wave. Using 15 kHz of carrier signal, the performance of the accelerometer achieves a signal-to-noise ratio above 100:1 and 10 mG of noise equivalent acceleration. 
     As shown in FIG. 12 a , masking is performed on a single crystal silicon wafer by using photoresist (PR). At this time, to make an electrode having a shorter depth, a narrow space  45  unmasked is formed thereon. 
     Next, to make a length difference between electrodes, RIE (Reactive Ion Etching) is conducted by using a high aspect ratio silicon etcher (not shown) utilizing Bosch process so as to make a narrow trench  47  and a wide trench  48  as shown in FIG. 12 b . This result is due to RIE lag, the deeper etching depth on a wider trench. 
     Next, the bottom of the trenches is released as shown in FIG. 12 c  so as to make the electrodes floated over the bottom  12 . To release the underlying layer, SBM (Surface Bulk Machining) or SCREAM (Single Crystal Reactive Etching and Metallization) techniques can be used. The releasing starts from the bottom of the trench toward sideward so that an electrode  40  formed by a shallow trench is shorter than one  30  formed by a wider trench in their vertical length. The shorter electrodes pair  40  are connected each other by a predetermined distance and supported by an anchor  42 . 
     Next, to make insulation between electrodes, an insulating oxide film  54  is deposited onto the outer surface of the body and inside thereof and thereafter a poly-crystal silicon electrode  56  is deposited onto the oxide film as shown in FIG. 12 e . Then, the bottom of the body is etched so as to he separated as shown in FIG. 12 f . A metal electrode  58  is formed on the poly-crystal silicon electrode  56  at the surface of the body for wire bonding as shown in FIG. 12 f.    
     As described above, there is no need to conduct a further patterning to form the electrodes so that the structure for a vertical displacement can be fabricated by a one time photo-etch process. 
     On the other hand, instead of a single crystal silicon wafer, SOI or SOG wafers can be used. In these cases, the bottom surface of the body is an insulating layer or glass so that the aforementioned additional insulating process is not required. 
     This makes the fabrication process simple and when used together with the conventional silicon fabrication process, it is possible to manufacture a structure for detecting lateral and vertical displacements within single wafer and furthermore to integrate a three-axis accelerometer and a three-axis gyroscope on a single wafer. 
     While the present invention has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as defined by the appended claims.