Abstract:
Means for trimming an inductor on a chip for use in RFID tags. The inductor coil ( 4 ) is manufactured on a chip ( 1 ) and covered with a dielectric layer ( 3 ). Upon the dielectric layer ( 3 ) strips ( 5 ) are made above the inductor ( 4 ) thus forming a capacitor in parallel with the inductor. The strips ( 5 ) may be of different widths and one end of each may be electrically connected ( 6 ) to the inductor. Selective removal of strips ( 5 ) allows the capacitance in parallel to the inductor to be adjusted and so the effective inductance as seen by an external circuit is adjusted.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to an on-chip inductor with trimmable inductance and a method for making such an inductor, which can be used for high frequency integrated circuits, and more particularly as an antenna used in RFID tags. The invention relates also to a method for adjusting the impedance of the inductor. 
         [0003]    2. Description of the Related Art 
         [0004]    On-chip antennas of RFID tags are normally conductive coils, the inductance of which need to be matched with the associated RFID tag. Particularly, the inductance of the antenna should have a value with which a resonance condition can be reached together with the capacitance constituted by the circuit to which it is connected, whereby optimum performance can be attained. 
         [0005]    On-chip inductors used in RF integrated circuits (RFIC) also need accurate values for applications, e.g. in RF matching circuits and filter circuits. Since mass production cannot ensure exact inductance values, the required inductance value should be adjusted. Adjusting is generally provided by a gradual trimming of predetermined shunt paths provided in the inductive path. Such an adjustment has become very important in RF applications, particularly in high GHz frequency circuits. 
         [0006]    Circuits of RFID tags with on-chip antennas and RFICs with on-chip inductors are widely fabricated with foundries&#39; standard CMOS technology to achieve low cost. Therefore, to find a trimming method associated with the-state-of-art foundries&#39; fabrication environment will be much helpful to design and manufacture RFID tags with on-chip antennas and RFICs. Such trimming should be carried out in a short time and with low costs. 
         [0007]    In state-of-the-art solutions the inductors or inductive antennas are trimmed by changing the effective length of the inductive path of the device. U.S. Pat. No. 6,480,110 discloses a solution particularly for antennas used in RFID tags working at a relatively low frequency, e.g., 13.56 MHz, in which each antenna is designed and made on an insulating substrate, being generally a card, with several centimeters in size. The RFID tags&#39; circuit dies are attached to the substrate and connected separately with the ports of the associated antenna. So, they cannot be regarded as RFID tags with on-chip antennas. If the method disclosed in the cited U.S. Pat. No. 6,480,110 would be applied to on-chip inductive antennas or on-chip inductors, it would be connected with certain technical problems. For example, as discussed in another prior art specification, namely in U.S. Pat. No. 6,369,684, the tunable metal wires in the centre region of the coil adversely influence the magnetic field generated by the coil. As a result, the losses increase and the quality coefficient Q of the inductance gets greatly decreased. 
         [0008]    As disclosed in U.S. Pat. No. 6,369,684 an extra surface area is needed for the layout of tunable wire lines. The increased size increases costs when applied in chip-level implementations. If it is applied for on-chip inductors, the relevant compact model is complicated, which is not friendly to a circuit design environment. 
         [0009]    To overcome the aforementioned drawbacks a trimming method for on-chip inductors is disclosed in US 2004/0063039, which is based on the principle of tuning the intrinsic capacitor between winding lines of a coil that forms the inductance. Here a method for trimming high frequency inductance of a passive component is disclosed, which includes the steps of forming a first dielectric layer on an insulation substrate, forming an inductance pattern (coil) on the upper surface of the first dielectric layer, forming a second dielectric layer on the inductance pattern, and forming a spaced metal layer pattern on the upper surface of the second dielectric layer. In such a design the adjustment of the inductance value can be easily performed by laser trimming of the upper metal layer pattern. This trimming method has proven not sufficiently efficient and it can ensure a comparatively low adjustment range only. The design occupies a large surface area on the tuned inductors. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, it is an object of the present invention to provide an on-chip inductor with trimmable inductance that can be applied to high frequency integrated passive devices, wherein the trimming can be carried out efficiently in a broad adjustment range. 
         [0011]    Another object of the present invention is to provide a method to trim on-chip inductors and on-chip RFID tags&#39; inductive antennas with in an effective way, which can also be carried out in CMOS interconnect process or CMOS compatible process or relevant chip packaging process. 
         [0012]    A further object of the invention is to provide an efficient adjustment method by which the desired inductance can be set in a fast, accurate and easy way. 
         [0013]    To achieve the above-mentioned objects it has been found that the range of adjustment will be noticeably higher if the spaced capacitive metal layer surfaces arranged above and partially covering (overlapping) the underlying adjacent winding lines are electrically connected at one of their end zones with one of the coil lines underneath. This connection will not decrease the quality coefficient of the inductance. 
         [0014]    According to the invention an on-chip inductor with trimmable inductance has been provided that comprises:
       at least one electrically conductive path constituting the inductor and acting as a RF antenna provided on a substrate, the path defining a plurality of spaced conductive lines;   a dielectric layer on the conductive path;   at least one strip provided on the dielectric layer spaced from and partially covering at least two lines of the conductive path underneath, to represent a capacitance;   respective connecting pads associated with and electrically connected to a respective one of the strips, each connecting pad extends through the dielectric layer and connects the strip with only one of the lines;
 
wherein trimming of the strips can be performed by selectively separating the strips in two respective parts, preferably just above the spacing between the two lines underneath. The separation is often referred to as “chopping”.
       
 
         [0019]    In an embodiment of the invention, when the underlying structure is a CMOS structure, the on-chip inductor is formed in the top metal layer of a CMOS interconnect. In this case, the manufacturing costs can be reduced if the strip(s) and the connecting pad(s) are formed along with a CMOS pad process. 
         [0020]    According to a further embodiment of the invention, the adjustment, that is the trimming of the inductance, can be carried out in an even easier and more accurate way if at least some of the strip(s) have different widths thus representing respective different capacitance values. 
         [0021]    A possible field of application lies in the connection of the path to an RFID tag to form the antenna thereof. 
         [0022]    According to the invention a method has also been suggested for providing a variable inductance of an on-chip inductor by trimming, that comprises the steps of: 
         [0000]    forming a long electrically conductive path having several spaced lines on a substrate with a predetermined inductance to constitute an inductor and to act as a RF antenna;
 
forming a dielectric layer on the electrically conductive path;
 
forming at least one metal layer strip on the dielectric layer spaced from and partially covering at least two of the lines of the electrically conductive path underneath;
 
electrically connecting only one end of the strip with one of the lines underneath by a connecting pad through the dielectric layer; and
 
varying the inductance by reducing the strip capacitance by separating the strip in two parts.
 
         [0023]    According to an exemplary embodiment of the invention, a plurality of strips is provided in a spaced apart arrangement during the metal layer strip forming step, wherein the strips can have different widths. 
         [0024]    In such cases during the inductance-varying step the strips with different widths are selectively chopped, whereby a coarse and fine adjustment can be realized. 
         [0025]    According to a further aspect of the invention a method has been provided for adjusting the inductance of an on-chip inductor to a required value that comprises the steps of:
       forming a long electrically conductive path that has several spaced lines on a substrate with a predetermined inductance to constitute the inductor and to act as a RF antenna;   forming a dielectric layer on the electrically conductive path;   forming a plurality of discrete metal strips with differing width on the dielectric layer spaced from and partially covering at least two adjacent lines underneath, each of the strips represent a transformed capacitance value proportional to the actual width thereof; and   trimming the strips by separating them in a stepwise manner to achieve to the required inductance value.       
 
         [0030]    The adjustment will be faster if the method further comprises the steps of measuring the actual inductance value prior to each trimming step to determine the difference relative to the desired value and selecting the next strip to be trimmed depending on the so determined difference, wherein a greater difference in the inductance value is achieved by separating a wider strip and a smaller difference is achieved by separating a narrower strip. 
         [0031]    This adjustment method can be carried out even if the strips on the dielectric layer are not connected electrically with any of the lines underneath, however, such a connection is preferable because it widens the range of adjustment by increasing the transformed capacitance. 
         [0032]    The invention provides a simple solution to the problem to be solved, it perfectly fits to CMOS manufacturing technology and it can be built integrally together with the RFID circuit. 
         [0033]    Features and advantages of the invention will be apparent from the following description of preferred embodiment, given for the purpose of disclosure and taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  is a top-side view of a variable inductor according to an embodiment known from prior art; 
           [0035]      FIG. 2  is a cross-section of the variable inductor of  FIG. 1  along lines A-A; 
           [0036]      FIG. 3  shows a trimming process with the variable inductor of  FIG. 1 ; 
           [0037]      FIG. 4  is a cross-section of the trimmed variable inductor of  FIG. 3  along lines B-B; 
           [0038]      FIG. 5  shows a common circuit model for the arrangements of  FIG. 1  and  FIG. 3 ; 
           [0039]      FIG. 6  is a top-side view of a possible embodiment of the variable inductor according to the invention; 
           [0040]      FIG. 7  is a cross-section of the variable inductor of  FIG. 6  along lines C-C; 
           [0041]      FIG. 8  shows a diagram indicating trimming results achieved by the variable inductor according to the invention; and 
           [0042]      FIG. 9  shows another example of implementation of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    As shown in  FIGS. 1 and 2 , a coil  4 , made of a conductive material, for example aluminum or copper, of an on-chip inductor is provided in a metal layer  2  on an integrated circuit chip. A dielectric layer  3  covers the metal layer  2  and therefore the whole inductor. Tuning metal strips  5  made e.g. of aluminum and constituting trimming members are built on the dielectric layer  3  over adjacent lines  4   a  and  4   b  of the coil  4  to form a capacitor with the two lines  4   a  and  4   b , because the strips  5  cover (overlap) the corresponding areas of the lines  4   a  and  4   b . The capacitance, and thereby the inductance, can be varied by a trimming process shown in  FIGS. 3 and 4 , wherein the strips  5  can be cut or chopped one by one at respective positions between the lines  4   a  and  4   b  to achieve the required value of effective inductance. This separation can be done by using milling technology, e.g. using Focused Ion Beam (FIB) or with laser as well known for a person skilled in the art. The tuning principle is disclosed in said US 2004/0063039 in detail. 
         [0044]    In  FIG. 5 , which is a common substitute circuit network model of the design shown in  FIGS. 1 to 4 , reference numeral  11  designates the intrinsic inductance of the coils;  12  represents the resistance introduced by the coil  4 ;  10  indicates the capacitance between the windings of the coil  4 ;  13   a  and  13   b  show the capacitance introduced by the dielectric layer  3  underneath the strips  5 ;  14   a ,  14   b ,  15   a  and  15   b  designate respective pairs of capacitor and resistor introduced by the substrate of the chip. The strips  5  perform the function of a variable capacitor  20  shown in  FIG. 5 . The effective inductance of the network shown in  FIG. 5  can be expressed as 
         [0000]        L   eff =[image(1/ Y (1,1))]/2 πf    
         [0000]    where, Y(1,1) comes from the Y-parameters of the network, which is a function of the actual values of the components in the network. It can be proven that the value of the inductance L eff  increases when the capacitance of the variable capacitor  20  becomes greater. 
         [0045]      FIGS. 6 and 7  show an embodiment of the present invention, wherein one end of the strip  5  is electrically connected by a connecting pad  6  made of electrically conductive material, for example aluminum or copper, with one of the lines  4   a  or  4   b , e.g.  4   b , of the coil  4 . The variable capacitance  20  in  FIG. 5  provided by the tunable strip  5  has a wider range of adjustment compared with the possible adjustment of the coil described in US 2004/0063039 mentioned above. The wider range of adjustment will be apparent if the internal capacitances formed between the respective coil lines  4   b  and  4   a  (see enlarged part of  FIG. 7 ) and the metal strip  5  are considered. First, the conditions without the interconnection pad  6  should be considered. Between the outer coil line  4   b  and a portion of the strip  5  overlapping it an internal capacitance C 1  is formed. Similarly, a capacitance C 2  is formed between the inner coil line  4   a  and a portion of the strip  5  overlapping the inner coil line  4   a . The two capacitors C 1  and C 2  are connected in series between the coil lines  4   a  and  4   b , so that the resulting capacitance constitutes a capacitive load for the last turn of the coil. It is known that the resulting capacitance of two capacitors connected in series is always smaller than any of the two component capacitors, and if C 1 =C 2 , the resulting capacitance will be the half of this capacitance. According to the invention, the pad  6  shorts the capacitor C 1  and leaves the capacitor C 2  unchanged. The resulting capacitance between the two coil lines  4   a  and  4   b  is, therefore, about twice as high as if the pad  6  did not exist. A larger transformed capacitance between the coil winding exerts a greater change on the overall reactance of the coil, thus the presence of the pad  6  is very significant. When the strip  5  is cut in the middle, the connection path is broken, and the effective capacitance between the lines  4   b  and  4   a  will be much smaller. In this way the step of interconnecting one of the lines  4   b  or  4   a  with the metal strip  5  above it practically doubles the value of the transformed capacitance, which increases the range of adjustment. 
         [0046]    As a practical example, when the structure underlying the on-chip inductor according to the invention is a CMOS structure, the coil  4  of the on-chip inductor can be realized in the top metal layer of interconnect in standard CMOS back end of line (BEOL). Therefore, the connecting pad  6  extending through the dielectric layer  3  and interconnecting the strip  5  and the coil line  4   b , can be formed in association with the pads&#39; open process in standard CMOS BEOL. Accordingly, the strip  5  can be fabricated along with the CMOS aluminum pads&#39; manufacturing process. Hence, no extra process and mask is required for the implementation of the present invention. 
         [0047]      FIG. 8  shows three diagrams indicating simulated results showing variation of the inductance L eff  as a function of frequency, demonstrating that the invention can greatly improve the trimming efficiency compared to the closest prior art. In higher frequencies the invention is even more effective. In  FIG. 8 , the inductance of the coil  4  alone, i.e. without the tunable strip(s)  5  above it is also given as a reference, and it is designated by the line wherein the marker m 1  is shown. Marker m 2  is associated with the line showing the inductance characteristic achieved according to US 2004/0063039 and the upper line with marker m 3  shows the inductance characteristic to the present invention. 
         [0048]    The number of lines  4   a ,  4   b  of the coil  4  can be more than two, and the strip  5  can cover at least two lines  4   a ,  4   b  but it can also cover more than two. One important feature and advantage of the invention is that each strip  5  used for tuning the capacitance formed between the lines  4   a ,  4   b  of the coil  4  should be connected with one and only one of the lines  4   a  or  4   b  of the inductive coil  4  underneath through the associated connecting pad(s)  6 . 
         [0049]      FIG. 9  shows another exemplary embodiment of the invention, wherein strips  5   a  to  5   d  with different widths are used. In this way a flexible trimming can ensure both coarse and fine adjustment. A small tuning step is obtained if a strip  5  with narrow width is chopped, e.g. strip  5   a  in  FIG. 9 . Conversely, the tuning step will be large when a wide strip  5  is chopped, e.g. strip  5   d . If the inductance is measured and compared with the desired value, the difference can be calculated, and following some experiments, the strip(s) with the optimum widths, which can compensate this difference, can be determined and chopped. 
         [0050]    Furthermore, in the embodiment shown in  FIG. 9  each strip, from  5   a  to  5   d , has one side connected with the associated line  4   a  or  4   b  of the coil  4  underneath. 
         [0051]    While the present invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.