Abstract:
A resonator formed by the steps of defining an active single-crystal silicon layer delimited by a buried insulator layer, depositing a silicon-germanium layer by a selective epitaxy method so that the silicon-germanium layer grows above the active single-crystal silicon area, depositing by a non-selective epitaxy method a silicon layer and etching it according to a desired contour, and removing the silicon-germanium by a selective etching with respect to the silicon and to the insulator.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electromechanical resonator particularly well adapted to applications in the radio-frequency field, for example, in the field of portable phones. 
   The present invention more specifically aims at a manufacturing method of such an electromechanical resonator which is compatible with the manufacturing of an integrated circuit, for example, a MOS-type integrated circuit. 
   2. Discussion of the Related Art 
   Many types of electromechanical resonators made with technologies similar to those of integrated circuits are known, which include a suspended beam connected to a first voltage, under which runs a conductive line at a second voltage, so that, when the electric frequency is close to the mechanical beam vibration frequency, a resonance occurs. 
   When such systems are desired to be formed, it is particularly important to accurately determine the resonance frequency. This frequency especially depends on the beam thickness, on its length and on the mechanical characteristics of the material forming the beam. The sensitivity especially depends on the distance between the beam and the excitation line. 
   SUMMARY OF THE INVENTION 
   The present invention aims at providing such a beam which is of simple manufacturing and compatible with integrated circuit technologies, and which has a well-determined resonance frequency, which is stable in time. 
   Another object of the present invention is to provide such a resonator which can be made in a particularly miniaturized form, to reach very high resonance frequencies that can exceed one GHz. 
   It should be noted that some of these features, and especially the features of stability and reproducibility of the mechanical constant, may exclude the use of polysilicon as the essential element forming the vibratory beam. 
   To achieve these and other objects, the present invention provides an electromechanical resonator formed on an active heavily-doped single-crystal silicon area of a substrate, forming a first electrode delimited by an insulating area, including a single-crystal silicon layer resting on the insulating layer by at least one of its ends and spaced apart from the active area. 
   The present invention provides a method for manufacturing a resonator, including the steps of: 
   defining an active single-crystal silicon area delimited by a buried insulator layer; 
   depositing a silicon-germanium layer by a selective epitaxy method so that the silicon-germanium layer grows above the active single-crystal silicon area and slightly beyond the periphery of this active area; 
   depositing by a non-selective epitaxy method a silicon layer and etching it according to a desired contour; and 
   removing the silicon-germanium by a selective etching with respect to the silicon and to the insulator. 
   According to an embodiment of the present invention, the method further includes, after the silicon-germanium etching, an oxidation step. 
   According to an embodiment of the present invention, the method includes a contact-forming step. 
   According to an embodiment of the present invention, the selective chemical etching of the silicon-germanium is plasma-aided. 
   The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments, in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2  are simplified cross-section views illustrating the successive steps of a manufacturing method according to the present invention; and 
       FIG. 3  is a simplified perspective view illustrating a subsequent manufacturing step of a resonator according to the present invention. 
   

   DETAILED DESCRIPTION 
   As shown in  FIG. 1 , a structure according to the present invention is formed from an active semiconductor area  1  delimited by an insulator area  2  filling a trench formed from the surface of region  1 . 
   Active area  1  may be the upper portion of a semiconductor substrate or may correspond to a thin semiconductor layer formed on an insulating substrate (SOI). In all cases, region  1  will preferably be heavily doped and insulated from an underlying substrate by a dielectric insulation or by a junction insulation, as is well known in the art. Insulating areas  2  are, for example, made of silicon oxide. 
   As illustrated in  FIG. 2 , a silicon-germanium layer  3  is grown by selective epitaxy above active area  1 . Those skilled in the art know various methods to obtain such a selective epitaxy, which methods may be used in the invention. During the epitaxy, a slight lateral increase of the epitaxial layer may occur, above internal periphery  4  of insulating area  2 . Then, a silicon layer  5  (non-selective epitaxy) is uniformly grown. This silicon layer will develop by epitaxy in single-crystal silicon form above silicon-germanium layer  3  and in polysilicon form above insulating layer  2 . The limit between the single-crystal silicon and the polysilicon regions is schematically indicated by reference  6  in FIG.  2 . It should however be noted that this limit is relatively fluctuating and depends on the manufacturing process. 
   Subsequent steps of the method are illustrated in the simplified perspective view of FIG.  3 . In a first step, silicon layer  5  is etched according to a chosen contour. After this, silicon-germanium layer  3  is selectively etched. This etching should be selective, especially with respect to silicon  3  and  1 , and should also be selective with respect to silicon oxide  2 . For this purpose, a plasma-aided chemical etching method, in which the plasma is used to create reactive chemical molecules very close to the structure surface, will, for example, be chosen. This method for example involves NF 3 , H 2 , and N 2 , or CF 4 , H 2 , and N 2  molecules. A wet chemical etching may also be carried out in the presence of HNO 3  mixed with HF and H 2 O 2  or H 2 O. 
   Thus, a single-crystal silicon beam  10 , embedded at its two ends  11  and  12  and separated in a very precise way from substrate  1  by a thickness corresponding to SiGe layer  3  which has been removed by etching, is obtained. 
   After the step of removal by etching of the SiGe layer, it should be noted that the apparent silicon naturally oxidizes and covers up with a very thin native oxide layer (thickness under 1 nm). A voluntary thermal oxidation may also be performed to obtain an oxide layer having a thickness from 1 to a few nm. 
   Contacts  20  and  21  may also be formed, respectively on one end  12  of beam  10  and on active area  1  of the substrate. Other means of contact with active area  1  may be provided. 
   The present invention is likely to have various alterations, modifications, and improvements which will readily occur to those skilled in the art. Thus, the manufacturing of a beam embedded at its two ends and operating as a vibratory rope has been previously described. A beam embedded at a single one of its ends may also be provided by performing a mask etching of thin single-crystal silicon layer  5  to remove embedding  11  on the left-hand side of  FIGS. 2 and 3 . 
   As an example of orders of magnitude, it may be provided to form a resonator such as that in  FIG. 3 , having a length on the order of from 0.1 to 10 μm, a width on the order of from 0.1 to a few μm, having a thickness on the order of from 5 to 100 nm, and having a distance from substrate  1  on the order of from 5 to 100 nm, all single-crystal silicon layers being coated with a thin oxide layer having a thickness from 1 to 5 nm. 
   The present invention has several advantages. Due to the fact that the embedded beam is made of single-crystal silicon, its physical characteristics are well known and thus, in particular, its modulus of elasticity is well defined and its characteristics are not likely to decline over time. Further, as is well shown by the above example, a particularly miniaturized device likely to operate at very high frequencies can be obtained with the present invention. 
   In the case where SiGe epitaxial layer  3  extends laterally above insulator  2 , there is no risk of short-circuit between active silicon area  1  and silicon layer  5  formed by epitaxy. Otherwise, this risk can be eliminated by a slight isotropic etching of the silicon after etching of the silicon-germanium and/or by a thermal oxidation of the silicon. 
   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.