Abstract:
A radiofrequency unit comprising a first dielectric substrate supporting a first conductive antenna layer; a second dielectric substrate supporting circuit elements connected or coupled to ground formed in a second conductive layer, and comprising a radiofrequency antenna line; and a third screen conductive layer arranged between the first and second substrates, provided with a slot to couple the antenna line to the antenna layer, this conductive layer being floating; in which the thickness and the nature of the second substrate are chosen by taking into account the surface of said circuit elements for the screen layer to be coupled to ground by a capacitor forming a short-circuit for radiofrequencies.

Description:
PRIORITY CLAIM  
         [0001]    This application claims priority from French patent application No. 02/14905, filed Nov. 27, 2002, which is incorporated herein by reference.  
         BACKGROUND  
         [0002]    1. Technical Field  
           [0003]    The present invention generally relates to a radiofrequency communication unit, and in particular a radiofrequency communication unit for replacing a cable link between two electronic devices with a radio link when the distance is small between the two devices.  
           [0004]    2. Discussion of the Related Art  
           [0005]    Such a communication unit, of a range of a few meters, exchanges radiofrequency signals (having a frequency ranging between 1.8 and 10 GHz) by means of a small flat antenna, generally called in the art a patch antenna, coupled to a radiofrequency signal processing chip. Input/output pads of the unit enable the chip to exchange so-called “low frequency signals (having a frequency ranging between 10 kHz and 10 MHz) with a device in which the unit is integrated.  
           [0006]    [0006]FIG. 1 schematically shows a cross-sectional side view of a radiofrequency communication unit  2 , comprised of a stratified substrate  4  formed of two dielectric substrates  6  and  8  arranged on either side of a conductive screen layer  10 . A conductive layer  12  forming a patch antenna is printed on the upper surface of substrate  6 . The lower surface of substrate  8  supports a printed radiofrequency antenna line  16  connected to a terminal  18  of a  10  chip  20  intended to transmit or receive radiofrequency signals. Radiofrequency line  16  is coupled to antenna layer  12  by a coupling slot  22  made in the screen layer  10  perpendicularly to line  16 . The lower surface of substrate  8  also supports printed tracks  24  which define a plurality of input/output pads (I/O) of the unit and their connection to terminals  26  (a single one of which is shown) of chip  20 . Each of the input/output pads is formed of a metallized surface where a connection ball (or welding ball) is placed. At least one of the pads is provided to be connected to ground and at least another one is provided to be connected to a supply terminal of the unit; the other pads are provided to transmit low-frequency signals between chip  20  and the outside of the unit. At least one via  28  made in substrate  8  connects screen layer  10  to a grounded pad.  
           [0007]    Coupling slot  22  is made in screen layer  10  vertically above a portion O of antenna line  16 . Upon transmission, the radiation of portion O is captured by the antenna  12  which retransmits it. Upon reception, the unit operates symmetrically.  
           [0008]    Such a unit operates satisfactorily, but a problem results from the fact that the welding balls arranged on the I/O pads, which enable a simple assembly with a low bulk, have a height limited to approximately 0.5 mm. This imposes assembling chip  20  head-to-tail directly on tracks  24  printed under substrate  8 . Now, such an assembly imposes that the chip  20  and the substrate  8  have substantially identical thermal expansion coefficients to avoid occurrence of mechanical constraints likely to result in a tearing of the chip terminals. Thus, in the conventional case of a silicon chip  20 , substrate  8  must preferably be made of glass. A glass substrate being very difficult to bore, the forming of via  28  requires great precautions. Further, glass is poorly wettable and the filling of via  28  with a conductive material is also difficult. All this substantially increases the unit manufacturing cost. It is, however, necessary for the voltage of the screen layer not to be left floating, since screen layer  10  captures the undesirable radiation of line  16  towards antenna  12  and the radiation of antenna  12  towards the inside of the unit. The voltage of screen layer  10 , if it was left floating, would vary under the effect of the captured radiation and screen layer  10  would radiate in the radiofrequency field. Such a radiation would disturb the operation of antenna  12  and that of chip  20 , which is not desirable.  
           [0009]    A solution consists of replacing via  28  through substrate  8  by an external conductive track located on an edge of the substrate. However, the manufacturing of an external track remains difficult and expensive.  
         SUMMARY  
         [0010]    One aspect of the present invention is to provide a radiofrequency unit which is inexpensive to manufacture  
           [0011]    Another aspect of the present invention is to provide such a radiofrequency unit which is robust.  
           [0012]    Another aspect of the present invention provides a radiofrequency unit comprising: a first dielectric substrate on the upper substrate of which is arranged a first conductive antenna layer; a second dielectric substrate on the lower surface of which are arranged circuit elements comprising a chip connected to input/output pads of the unit by portions of a second conductive layer, and comprising a radiofrequency antenna line connected to the chip; and a third screen conductive layer arranged between the first and second substrates, provided with a slot to couple the antenna line to the antenna layer, this conductive layer being floating; in which the areas of the lower surface of the second dielectric substrate on which are not arranged the circuit elements are covered with grounded portions of the second conductive layer, one at least of the pads being grounded and each of the other pads being connected to ground by a capacitor forming a short-circuit for radiofrequencies; the thickness and the nature of the second substrate being chosen by taking into account the surface of said portions and of said pads for the screen layer to be coupled to ground by a capacitor forming a short-circuit for radiofrequencies.  
           [0013]    According to an embodiment of the present invention, one of the circuit elements is an inductance formed in the second conductive layer.  
           [0014]    According to an embodiment of the present invention, one of the circuit elements is a capacitor formed of two interleaved comb-shaped conductive surfaces formed in the second conductive layer.  
           [0015]    According to an embodiment of the present invention, welding balls are arranged on the input/output pads.  
           [0016]    An aspect of the present invention goes against the prevailing idea according to which the screen layer must be physically connected to ground so that its voltage is not left floating in the radiofrequency field. One aspect of the present invention provides a radiofrequency unit having its screen layer connected to ground only by means forming a short-circuit for radiofrequencies.  
           [0017]    The foregoing, features and advantages 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  
       [0018]    [0018]FIG. 1, previously described, schematically shows a side cross-section view of a conventional radiofrequency unit;  
         [0019]    [0019]FIG. 2 shows a side cross-section view of a radiofrequency unit according to an embodiment of the present invention; and  
         [0020]    [0020]FIG. 3 shows a bottom cross-section view of the radiofrequency unit of FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0021]    The following discussion is presented to enable a person skilled in the art to make and use the invention. Various modifications to the embodiments will be readily apparent to those skilled in the art, and the generic principles herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.  
         [0022]    Same reference numerals designate same elements in FIG. 1 and in the following drawings. Only those elements necessary to the understanding of the present invention have been shown hereafter.  
         [0023]    [0023]FIGS. 2 and 3 schematically respectively show a cross-section side view along an axis A-A and a cross-section bottom view along an axis B-B of a radiofrequency communication unit  2 ′ according to an embodiment of the present invention. Unit  2 ′ comprises the same elements as unit  2  of FIG. 1, excluding via  28 . As an illustration, unit  2 ′ comprises eight pads I/O 1  to I/ 08 . Pads I/O 1 , I/O 2 , I/O 4 , and I/O 8  are directly connected to a terminal of chip  20  by a track  24 ; pad I/O 5  is connected to a pad of chip  20  via a capacitor C formed of two interleaved comb-shaped surfaces; and pad I/O 7  is connected to a pad of chip  20  via an inductance L formed by a conductive line of predetermined length printed in zigzag. Pads I/O 3  and I/O 6 , connected to an external ground not shown, are connected to a ground terminal of chip  20  by a ground conductive plane  30 . Ground plane  30  is further arranged on substantially the entire lower surface of substrate  8  left free by tracks  24  and line  16 .  
         [0024]    According to an embodiment of the present invention, screen layer  10  is not physically connected to any conductive element of unit  2 ′. The present invention however provides connecting screen layer  10  to ground in the radiofrequency field by a plurality of capacitors formed between the screen layer and conductive surfaces arranged on the lower surface of the unit.  
         [0025]    Ground plane  30 , separated from screen layer  10  by dielectric substrate  8 , forms therewith a coupling capacitor, the value of which depends on the surface area of plane  30 , on the thickness of substrate  8 , and on the dielectric constant of substrate  8  (for example, of glass).  
         [0026]    Further, each I/O pad not directly grounded is connected to ground by a discrete capacitor D adapted to forming, in practice, a short-circuit for radiofrequencies. On the other hand, the metal surface S of each I/O pad, which is separated from screen layer  10  by dielectric substrate  8 , forms a capacitor coupling screen layer  10  to the pad. The value of pad/screen capacitance  10  depends on surface area S of the pad and on the thickness and on the dielectric constant of substrate  8 . Screen layer  10  thus is, at the level of each pad, also coupled to ground by the series connection of capacitor D with the pad/screen capacitor. In practice, the value of capacitor D may easily be higher than the value of pad/screen capacitor  10  and the series connection of these two capacitors substantially corresponds to a coupling of screen layer  10  to ground by a capacitor having the value of the pad/screen capacitor. Such a coupling is formed in parallel at the level of each of the I/O pads of the unit not connected to ground. These couplings add up and are equivalent to a coupling of layer  10  to ground by a capacitor having n times the value of a pad/screen capacitor, where n is the number of I/O pads of the unit not connected to ground. This capacitor adds to the ground plane/previous screen capacitor.  
         [0027]    This embodiment of the present invention provides choosing the thickness of substrate  8 , the surface area of ground plane  30 , as well as the surface area of the I/O pads so that the ground plane/screen capacitor and the pad/screen capacitors have values such that these capacitors form a short-circuit in the radiofrequency field.  
         [0028]    No account has been taken in the foregoing description of capacitors formed between the low-frequency passive electronic components printed on the lower surface of substrate  8  and the screen layer (for example, capacitor C or inductance L of FIG. 3), but such capacitors advantageously cooperate to the coupling of the screen layer to ground in the radiofrequency field according to this embodiment of the present invention.  
         [0029]    As an example if the surface area of each I/O pad is 0.5 mm by 0.5 mm and if substrate  8  has a 0.2-mm thickness and a 4.10-11-F/m dielectric constant, each pad/screen capacitor has a value of 50 fF. If each coupling capacitor D between pad and ground has a 100-pF value, the assembling in series of the 50-fF capacitor and of the 100-pF capacitor corresponds approximately to the connection of a 50-fF capacitor between the screen layer and the ground. If the radiofrequency unit comprises  20  pads not connected to ground, the screen layer is connected to ground by  20  capacitors of 50 fF connected in parallel, which amounts to the connection of a capacitor of approximately 1 pF between the screen layer and the ground. The ground plane surface area being generally at least equal to that of all pads together, the value of the capacitance between the screen plane and the ground is in practice at least twice the above-mentioned value.  
         [0030]    Due to the ground coupling of screen layer  10  of radiofrequency unit  2 ′, the voltage of screen layer  10  does not vary under the influence of undesirable radiofrequency radiations of line  16  or of the radiation of antenna  12  towards the inside of the unit. As a result, screen layer  10  scarcely radiates in the radiofrequency field although it is not physically grounded.  
         [0031]    A radiofrequency unit according to this embodiment of the present invention, requiring no forming of a via or of a conductive track between the screen layer and another portion of the unit, is particularly inexpensive to manufacture and robust.  
         [0032]    An embodiment of the present invention has been described in relation with a radiofrequency unit comprising for clarity a restricted number of circuit elements, but those skilled in the art will easily adapt the present invention to any unit comprising a larger number of circuit elements, for example, two chips or two antenna lines for two different radio frequencies.  
         [0033]    The radiofrequency unit  2 ′ may be contained in a varity of different types of electronic systems utilizing wireless communications, such as a computer system or personal digital assistant.  
         [0034]    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, the present invention has been described in relation with a specific radiofrequency unit type, but those skilled in the art will easily adapt the present invention to other radiofrequency or ultrahigh frequency unit types in which it may be advantageous to suppress a physical connection between the ground and a screen layer.  
         [0035]    The present invention has been described in relation with a unit using glass substrates supporting a silicon chip, but those skilled in the art will easily adapt the present invention to other types of substrates supporting one or several chips made of another material.  
         [0036]    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.