Abstract:
A method for forming a variable capacitor including a conductive strip covering the inside of a cavity, and a flexible conductive membrane placed above the cavity, the cavity being formed according to the steps of: forming a recess in the substrate; placing a malleable material in the recess; having a stamp bear against the substrate at the level of the recess to give the upper part of the malleable material a desired shape; hardening the malleable material; and removing the stamp.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a method for forming a variable capacitor, and more specifically a variable capacitor such as that described in French patent application No. 00350911 of the applicant.  
         [0003]     2. Discussion of the Related Art  
         [0004]     The variable capacitor described in the above-mentioned patent application comprises a conductive layer covering the inside of a cavity formed at the surface of a substrate and a flexible conductive membrane placed above the cavity. According to an aspect of this variable capacitor, the cavity portion covered by the conductive layer has the shape of a groove such that its depth increases continuously from one of the groove edges to the groove bottom. Further, the conductive layer covers the inside of this groove portion at least up to one of its two edges that it may cover.  
         [0005]     As described in the above-mentioned patent application, the method for forming such a variable capacitor comprises the forming in a substrate of a partly groove-shaped cavity, of covering the inside of the groove with a first conductive layer, of filling the cavity with a sacrificial portion, of forming on the sacrificial portion a bridge-shaped conductive strip bearing on the lateral groove edges, and finally of eliminating the sacrificial portion. The conductive strip forms a flexible conductive membrane that can deform to come closer or draw away from the conductive layer covering the inside of the cavity.  
         [0006]     The forming in a substrate of an at least partly groove-shaped cavity may be performed in various ways such as described in the above-mentioned French patent application. Except for the method for forming a cavity by means of insulating spacers, the other described cavity-forming methods are relatively complex to implement and require a large number of operations. Further, each of these methods provides a cavity having, in cross-section, a groove shape with a well-determined profile without for it to be possible to obtain an “ideal” profile. Further, the shapes of the cavities obtained according to such methods are not homogeneous and depend, among others, on the density of cavities formed on the substrate.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the present invention is to provide a method for manufacturing a variable capacitor comprising a conductive layer placed in a cavity having a desired shape.  
         [0008]     Another object of the present invention is to provide such a method which is simple to implement.  
         [0009]     The present invention provides a method for forming a variable capacitor comprising a conductive strip covering the inside of a cavity, and a flexible conductive membrane placed above the cavity, the cavity being formed according to the steps of: forming a recess in a substrate; placing a malleable material in the recess; having a stamp bear against the substrate at the recess level to give the upper part of the malleable material a desired shape; hardening the malleable material; and removing the stamp.  
         [0010]     According to an embodiment of the above-described method for forming a variable capacitor, the hardening step comprises a heating step.  
         [0011]     According to an embodiment of the above-described method for forming a variable capacitor, the malleable material is non-reticulated resin.  
         [0012]     According to an embodiment of the above-described method for forming a variable capacitor, the stamp is obtained according to the method of: forming a cavity of desired shape in the upper portion of a substrate; depositing on the substrate a hardenable material; and separating the substrate and the hardenable material, the latter then forming a stamp.  
         [0013]     According to an embodiment of the above-described method for forming a variable capacitor, a portion at least of the cavity has the shape of a groove, the upper substrate portions located close to the groove forming two edges, and the groove depth continuously increasing from one of the edges to the groove center.  
         [0014]     According to an embodiment of the above-described method for forming a variable capacitor, the method further comprises the steps of: forming a first conductive strip covering the inside of the groove-shaped portion of the cavity, the first conductive strip extending at least up to one of the two edges of the groove that it may cover; forming a sacrificial portion in the cavity; forming an insulating strip on the sacrificial portion, substantially above the first conductive strip; forming a second conductive strip on the insulating layer; and eliminating the sacrificial portion.  
         [0015]     According to an embodiment of the above-described method for forming a variable capacitor, the method further comprises, prior to the cavity forming, the forming of two conductive tracks in the upper portion of the substrate, said recess being placed between the two conductive tracks, and the first conductive strip extending on the substrate to cover one of the conductive tracks, and the second conductive strip extending on the substrate beyond the insulating strip to cover the other one of the conductive strips.  
         [0016]     The foregoing object, features, and advantages, as well as others, of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIGS. 1A  to  1 J are cross-section views and  FIG. 1J  is a perspective view of structures obtained in successive steps of a variable capacitor manufacturing method according to the present invention;  
         [0018]      FIGS. 2A  to  2 C are cross-section views of structures obtained on manufacturing of a stamp used in a variable capacitor manufacturing method according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0019]     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale.  
         [0020]     The method for manufacturing a variable capacitor according to the present invention comprises a step of forming of a cavity according to a nanoimprint method using a previously-manufactured stamp. This nanoimprint method comprises the filling of a recess of a substrate with a malleable material, followed by the placing on the substrate of a stamp at the recess level and finally the hardening of the malleable material to set its shape. The material thus hardened comprises in its upper portion a cavity having a predefined shape corresponding to that of the stamp.  
         [0021]     An example of a method for manufacturing a variable capacitor according to the present invention is described hereafter in relation with  FIGS. 1A  to  1 J.  
         [0022]     In an initial step, illustrated in  FIG. 1A , conductive tracks  2  and  3  are formed in an insulating substrate  1 , in openings of the upper portion of substrate  1 .  
         [0023]     At the next step, illustrated in  FIG. 1B , substrate  1  is etched to form a recess  10  in the upper portion of substrate  1  between conductive tracks  2  and  3 . Recess  10  is in this example formed according to an anisotropic etch method so that the recess walls are substantially vertical. However, recess  10  may be formed according to an isotropic etch method.  
         [0024]     At the next step, illustrated in  FIG. 1C , a malleable material  20  that can be subsequently hardened is placed in recess  10 . Such a malleable material, for example, is non-reticulated resin.  
         [0025]     At the next step, illustrated in  FIG. 1D , a stamp  30  is placed on substrate  1  at the level of recess  10 . Stamp  30  is in this example a wafer portion exhibiting a bulged outgrowth  31  embedding in recess  10 . This entire structure is then placed in a heating enclosure to harden material  20 . Once material  20  has been hardened, stamp  30  is removed.  
         [0026]     As visible in  FIG. 1E , a portion of hardened material  20  then has, in cross-section, the shape of a groove  35 . The upper portions of the substrate located between conductive tracks  2 ,  3  and groove  35  form two lateral edges  36  and  37 . It should be noted that the depth of groove  35  continuously increases from one of edges  36 ,  37  to the center of groove  35 .  
         [0027]     At the next step, illustrated in  FIG. 1F , a conformal deposition of a conductive layer on the previously-obtained structure is performed. This conductive layer is then etched to keep a conductive strip  40  covering groove  35  and extending on edges  36 ,  37  to cover one of the conductive tracks, track  3  in this example.  
         [0028]     At the next step, illustrated in  FIG. 1G , a sacrificial layer is deposited on the previously-obtained structure. The portions of the sacrificial layer located above substrate  1 , conductive track  2 , and conductive strip  40 , outside of the previously-formed cavity are then eliminated. A sacrificial portion  50  placed in the previously-formed cavity is thus obtained. This partial elimination of the sacrificial layer may be performed by chem.-mech. polishing of the sacrificial layer to expose substrate  1 , conductive strip  40 , and conductive track  2 , or according to a method of photolithographic etching of the sacrificial layer.  
         [0029]     At the next step, illustrated in  FIG. 1H , an insulating layer is deposited on the previously-obtained structure, then etched to keep an insulating strip  60  on sacrificial portion  50 . Insulating strip  60  is placed substantially above conductive strip  40 , transversely to groove  35  and extends on edges  36  and  37  of groove  35 . The insulating strip extends above edges  36  and  37 . Insulating strip  60  must not cover conductive track  2  but may extend above conductive track  3  on conductive strip  40 .  
         [0030]     At the next step, illustrated in  FIG. 11 , a conductive layer is deposited above the previously-obtained structure and etched to keep a conductive strip  70  covering conductive track  2  and insulating strip  60  up to edge  37  located close to the other conductive track  3 .  
         [0031]     In a last step, illustrated in  FIG. 1J , sacrificial portion  50  is eliminated according to an anisotropic etch method. Conductive strip  70  and insulating strip  60  are then “free” and can deform. Conductive strip  70  then forms a flexible conductive membrane. Conductive tracks  2  and  3  form terminals of application of a voltage between conductive strip  40  and conductive membrane  70  which form two electrodes of the variable capacitor thus formed. According to the applied voltage, conductive membrane  70  draws closer or moves away from conductive strip  40  and the capacitance of the capacitor increases or decreases.  
         [0032]     Other variable-capacitor structures can be obtained by the method of the present invention. The terminals of application of a voltage between the capacitor electrodes may be formed differently, for example via contacts placed on conductive membrane  70  and on conductive strip  40 . Further, insulating strip  60  may be placed on conductive strip  40  and not under conductive membrane  70 .  
         [0033]     The stamp used in a manufacturing method according to the present invention can be obtained according to the following method, described in relation with  FIGS. 2A  to  2 C.  
         [0034]     In an initial step, illustrated in  FIG. 2A , a cavity  100  is formed in the upper portion of a substrate, for example, made of silicon. Cavity  100  may be formed according to one of the methods described in the above-mentioned French patent application.  
         [0035]     At the next step, illustrated in  FIG. 2B , a material  110  such as nickel that can “harden” once deposited and then keep its shape when it is placed in a heated enclosure to heat up the malleable material used in the method according to the present invention, is deposited.  
         [0036]     In a final step, illustrated in  FIG. 2C , material  110  and substrate  100  are separated. Material  110  then forms a stamp that can be used to form the cavity of a variable capacitor manufactured according to the method of the present invention.  
         [0037]     Of course, the present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. In particular, those skilled in the art may devise other methods for hardening the malleable material.  
         [0038]     Further, several variable capacitors may be formed in and above a same substrate according to the method of the present invention. Such capacitors may be of different sizes and shapes. To form the cavities in and above which the capacitors are then formed, a “stamp” plate comprising a set of stamps, identical or not, may be used, each stamp corresponding to bulged outgrowths of the plate.  
         [0039]     Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.