Patent Publication Number: US-2007115086-A1

Title: Multilayer circuit with variable inductor, and method of manufacturing it

Description:
FIELD OF THE INVENTION  
      The invention relates to the field of variable inductors in microelectronic circuits, such as silicon based integrated circuits having a plurality of metal layers separated by dielectric layers.  
     STATE OF THE ART  
      Variable passive devices are frequently used in electronic circuits; for example, in many analog and radio frequency (RF) circuits, there is a need to use some reactive components such as capacitors and inductors. Some examples of circuits in which such devices are used are filters, Voltage Controlled Oscillators (VCO), impedance matching networks, and others. In order to provide such circuits with the possibility of some degree of tuning, it is known to use variable capacitors and inductors, that is, capacitors and inductors the values of which can be varied to a certain extent. One very well known example of such a variable component is the so-called varicap diode, which allows a certain variability of the capacitance value by means of applying a variable continuous voltage. However, few examples of variable inductors are known in the field of integrated inductors, that is, in the field of inductors forming part of integrated circuits (IC).  
      However, such inductors can be very useful. For example, in wireless communication, as the number of available standards is increasing (at present, there are, for example, the GSM standard (operating around 900 MHz), WLAN (2.4 GHz and 5.23 GHz), GPS (1.5 GHz), etc.), there is an increasing interest in providing multifunction circuits, that is, circuits that can be easily adapted to different operating frequencies. For example, in order to provide wireless connectivity for different operating standards, instead of using a plurality of specific RFICs (Radio Frequency Integrated Circuits), one for each operating frequency band and each RFIC working exclusively at one of said bands, it could be possible to use one single RFIC, adapting it to a selected operating frequency band by “tuning” it, for example, by modifying the inductance of an inductor in the RFIC (for example, an inductor in an impedance matching network of the RFIC). It is clear that, at least in many cases, such a solution may be more cost-effective than the one involving the use of different building blocks (for example, RFICs), one for each frequency band.  
      Some examples of electronic variable inductors integrated in microelectronic circuits make use of the MEMS (micro-electromechanical system) technology. Obtaining these MEMS based variable inductors involves post-processing of the circuits obtained during the initial microelectronic process (such as a CMOS, BiCMOS or Bipolar process) in order to provide mechanical movement to static parts.  
      One example of this kind of variable inductors is disclosed in EP-A-1463070. Here, two inductors are provided, namely, one that comprises a shorted spiral inductor formed on a low-profile sliding dielectric sheet, and another one comprising a spiral inductor on a substrate. Varying degrees of overlap between the two inductors causes varying inductance values.  
      However, a problem involved with this kind of variable inductors based on MEMS involving moveable elements (like the above-mentioned dielectric sheet) is that their manufacture can be rather complex and expensive, and that it may be difficult to integrate them in integrated circuits, using standard IC processes (such as CMOS processes) for their manufacture.  
      Another variable inductance arrangement, that can be applicable to integrated circuits, and that does not require the use of MEMS, is disclosed in US-A-2004/0066236. Here, a first inductor constitutes an impedance element, and a control circuit is provided that includes a second inductor magnetically coupled to the first inductor. The inductance value of the first inductor can be changed by changing a control current flowing through the second inductor. However, a problem involved with this kind of arrangements is that an additional power-consuming control circuit is needed, which not only implies an extra amount of circuitry, but also additional power consumption, which can be inconvenient and even critical when it comes to the design of low power consumption integrated circuits.  
      Another example of a prior art variable inductor arrangement is disclosed in US-A-2004/0140528. Here, the variable inductor is provided in a multilayer circuit comprising a plurality of conductive (metal) layers separated by dielectric layers, of the type normally used in integrated circuits. Several inductors are provided in different metal layers (at least in two of the metal layers), whereby the inductors are connected in series, between two ports. Further, at least one MOSFET switch is also provided that can shunt one or more of said inductors, whereby the inductance value obtained between the two ports will selectively correspond to one of said inductors or to the series connection of more than one of said inductors, thus providing a variable inductor that can be varied according to a plurality of different inductance values.  
      This arrangement can easily be integrated into integrated circuits using standard IC processes (such as CMOS, etc.), but it may not be optimal when it comes to the issues of parasitic capacitance and quality factor. The stacked inductors can give rise to an additional capacitance between them, which can cause, for example, a reduction of the self-resonant frequency of the inductor, making it unsuitable for certain high frequency applications. Also, the disclosed arrangement may not be optimal in what concerns the quality factor; it may provide a rather low quality factor, especially in IC arrangements optimized for radio frequency (RF) applications. In this kind of IC arrangements comprising multi-layer devices with a plurality of metal layers, the upper metal layer is normally thicker than the other metal layers, in order to provide low Ohmic losses (which can be essential in RF applications). Now, when a multi-layer inductor arrangement is provided based on a multi-layer structure comprising several metal layers, the upper one being thicker than the other ones, the quality factor for the top inductor, embodied in the thicker metal layer, can be very good, but when it is combined with the other inductors, embodied in the lower and thinner metal layers, the total quality factor will be lower.  
     DESCRIPTION OF THE INVENTION  
      A first aspect of the invention relates to a multilayer structure or circuit comprising a plurality of electrically conductive layers (such as metal layers) separated from each other by respective insulating or dielectric layers, wherein said multilayer circuit comprises at least one variable inductor. According to the invention, the variable inductor comprises  
      a conductive coil structure following a coil path in a single one of said conductive layers,  
      two ports connected (or “connectable”, through corresponding switches) to said coil structure, and  
      a switch arrangement, comprising at least one switch, for selectively connecting at least one of said ports to one of a plurality of specific positions of said coil structure along said coil path, thus providing for a corresponding selective inductance value of the variable inductor, between said two ports.  
      According to the invention, the coil structure is arranged in the thickest one of said conductive layers.  
      Thus, the entire inductor coil can be embodied in one single layer, whereby there will be no additional capacitance due to stacked inductor coils. Further, with the arrangement of the invention, a good quality factor can be provided, as the entire inductor coil arrangement is implemented in the most appropriate metal layer, in this case, in the thickest metal layer.  
      The coil structure can thus be arranged in a top conductive layer of said multilayer circuit, which can thus be a thicker conductive layer than each of a plurality of lower conductive layers.  
      At least one switch of the switch arrangement can be arranged so as to connect the at least one of said ports to one of said plurality of specific positions of said coil structure, through a conductive bridge embodied in another one of said conductive layers. For example, the coil structure can be embodied in a top conductive layer of said multilayer circuit, and said bridge can be embodied in the conductive layer next to said top conductive layer. The bridge can be connected to the coil structure by one or more vias in a dielectric layer separating the respective conductive layers. In the same way, the bridge can be connected to the output ports (which can be embodied in, for example, the same conductive layer as the coil structure) and/or to the switches, through respective vias.  
      Each switch arrangement can comprise N switches, each one of said switches being arranged between one of the ports and one of said specific positions of said coil structure, along said coil path, so as to selectively connect said port to said specific position or disconnect said port from said specific position, in accordance with an input signal applied to said switch. N can be a number equal to 1 or more; for example, N can be 2 or more, for example, 4 or more.  
      Each switch can comprise a transistor, such as a MOSFET transistor, which can easily be implemented by CMOS processes, for example. The invention as very suitable for implementation based on CMOS construction processes.  
      Each switch can be arranged in a layer different from the layer comprising the coil structure, such as in a transistor layer embodied on a doped silicon layer, which thus makes the invention compatible with CMOS processes.  
      Each switch can thus be connected to the coil structure and to the corresponding port through at least one via through one or more dielectric layers separating the switch from the coil structure and from the port, respectively. Of course, the switches can also be implemented so as to separate different parts of the coil structure, so that one part of the coil structure can be connected to another part of the coil structure, and thus to a corresponding port, through a switch.  
      The conductive layers can be metal layers, such as copper or aluminium layers, or layers made up of an alloy based on copper and/or aluminium. These kinds of metal layers are frequently used in circuits obtained by CMOS processes.  
      Another aspect of the invention relates to an integrated circuit comprising a multilayer circuit as described above, further including further circuit components. For example, the integrated circuit can be an integrated circuit for a radio frequency application, and can comprise, for example, at least one low noise amplifier comprising an amplifier circuit; the variable inductor or inductors can thus form part of an impedance matching circuit for selectively adapting the low noise amplifier to one of at least two frequency bands.  
      A further aspect of the invention relates to a method of manufacturing a multilayer circuit as described above. The method comprises the steps of:  
      providing said at least one switch on a base structure;  
      building, on said base structure, a further, multilayer, structure comprising the plurality of electrically conductive layers separated from each other by respective dielectric layers, while embodying the conductive coil structure following a coil path in a single one of said conductive layers, namely, in the thickest one of said conductive layers (for example, a top conductive layer of said multilayer circuit), and connecting said two ports to said coil structure, so that at least one of said ports is connectable to at least one of a plurality of specific positions of said coil structure along said coil path in said one of said conductive layers through said switch arrangement comprising said at least one switch, and through vias through at least one of said dielectric layers separating said at least one switch from the coil structure.  
      Thus, the method can easily be implemented using conventional CMOS processes, for example.  
      What has been said concerning the circuit is also applicable to the method of manufacturing it, mutatis mutandis. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      To complete the description and in order to provide for a better understanding of the invention, a set of drawings is provided. Said drawings form an integral part of the description and illustrate some preferred embodiments of the invention, which should not be interpreted as restricting the scope of the invention, but just as an example of how the invention can be embodied. The drawings comprise the following figures:  
       FIG. 1 : a schematic top view of an arrangement in accordance with an embodiment of the invention.  
       FIG. 2 : a schematic cross section view of the arrangement shown in  FIG. 1 , along the dotted line indicated in  FIG. 1 .  
       FIG. 3 : a schematic top view of an arrangement in accordance with another embodiment of the invention.  
       FIG. 4 : a schematic perspective view of the coil structure and bridges in accordance with another embodiment of the invention.  
       FIG. 5 : a schematic cross section view of the layered structure in accordance with a possible embodiment of the invention, showing the relation between the inductor arrangement and the conductive layers.  
       FIG. 6 : a schematic circuit diagram of a low noise amplifier for an RF application, in accordance with an embodiment of the invention.  
      FIGS.  7 A- 7 D: schematic circuit diagrams, with and without the variable inductors of the invention.  
       FIGS. 8A and 8B  schematically illustrate the behaviour of a Low Noise Amplifier (LNA) implemented using variable inductors in accordance with the invention. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION  
       FIGS. 1 and 2  schematically illustrate a possible embodiment of the invention, wherein the multilayer circuit comprises a plurality of metal layers ( 1 ,  2 ,  3 ,  4 ) (for example, copper and/or aluminium layers) separated by dielectric layers ( 11 ,  12 ,  13 ,  14 ) of silicon dioxide. A coil structure ( 20 ) is provided in the top metal layer  1 , which is the thickest metal layer, as usual in CMOS circuits for many applications. Further, there are two ports ( 30 ,  31 ) to which the coil structure is connected. One of said ports ( 31 ) is connected to a fixed position of the coil structure (namely, to one end of the coil), whereas the other port ( 30 ) is connected to the coil structure through a switch arrangement comprising a plurality of MOSFET transistor switches ( 40 ), arranged so as to selectively connect the port to one of a plurality of specific positions (P) of the coil arrangement. Thus, depending on which one of said switches is in the closed state, the effective length of the coil structure between the two ports ( 30 ,  31 ) will vary, whereby the inductance between the two ports can be set to specific values by means of controlling the switches ( 40 ), so that a selected one of the switches is closed, whereas the other switches are open. Connection between the port ( 30 ) and the coil structure ( 20 ) takes place through a bridge ( 21 ) in the metal layer ( 2 ) immediately below the top metal layer ( 1 ), to which the bridge is connected by respective vias ( 22 ). Further, as shown schematically in  FIGS. 1 and 2 , the connection takes places through the respective switch ( 40 ), which is implemented in a transistor layer below the metal layers, and through the corresponding vias ( 41 ) passing through the plurality of dielectric layers ( 11 ,  12 ,  13 ,  14 ) separating the first metal layer ( 1 ) and the transistor layer.  
      In the embodiment illustrated in  FIG. 1 , there are four switches, whereby the inductor can be selectively set to four different values, in accordance with the input instructions delivered to the switches, setting their states to open and closed, respectively.  
      In the following table, examples of different inductance values obtainable with this kind of circuit, as well as the corresponding quality factors, are shown.  
                                                  Inductance (nH)   Quality Factor                                 Number of turns   2.5 GHz   5 GHz   2.5 GHz   5 GHz                                         1   3.74   9.9   4.31   6.3       2   2.54   9.1   2.80   8.4       3   1.33   8.8   1.39   10.7       4   0.55   9.6   0.55   13.8                  
 
 The Inductance (nH) has been calculated as L(nH)=Im(1/Y11)/(2*pi*f) and the quality factor, Q=Im(Y11)/Re(Y11), where Y is the admittance matrix describing the inductor model (so that Y11 is the admittance that is presented at one of the two ports of the coil) and f is the frequency of the signal, in accordance with a simulation of a possible embodiment of the circuit. 
 
       FIG. 3  illustrates a similar arrangement, but with a coil structure ( 20 A) featuring a larger number of turns, and with a larger number of switches ( 4 ), namely, seven switches, thus providing for seven different inductance values.  
       FIG. 4  shows a slightly different coil structure ( 20 B), and the way two MOSFET transistor switches ( 40 ) are provided for selectively connecting the specific positions (P) of said coil structure to the port ( 30 ). One of the specific positions is an end position that can be directly connected to the port through vias ( 41 ) and the switch ( 40 ), while another of said specific position further requires a bridge ( 21 ) in a metal layer different from the metal layer of the coil structure ( 20 B); the bridge ( 21 ) is connected to the coil structure and to the port ( 30 ) by respective vias ( 22 ).  
       FIG. 5  shows a cross section of the multilayer circuit according to a possible embodiment of the invention, based on a CMOS structure in which a p-doped silicon substrate ( 6 ) is provided, in correspondence with which a so-called transistor layer is embodied, comprising a plurality of MOSFET transistors ( 40 ), one of which is shown in  FIG. 5  (this structure is a conventional CMOS structure), whereby each transistor comprises its corresponding source and drain, and also a gate to which the control signal for controlling the transistor switch ( 40 ) is applied (in a manner evident to the skilled person, wherefore no further discussion on this matter is needed here).  
      Above the silicon substrate  6  and the transistor layer there is a plurality of metal layers (1, 2, . . . , n) separated by the respective dielectric layers (11, . . . ). As mentioned above, the transistor switches ( 40 ) are, in this embodiment, connected to the top metal layer ( 1 ) by vias ( 41 ) through the respective dielectric layers; thus, the switch ( 40 ) can connect the relevant part of the coil structure ( 20 ,  20 A,  20 B) to the port ( 30 ).  
      On top of the last metal layer there is one or more dielectric layers and/or coating layers ( 8 ) provided for protecting the circuit (in accordance with conventional CMOS circuit layout). A metallic contact ( 9 ) can be provided so as to allow a port ( 30 ) to be connected to an external circuit.  
      According to one embodiment of the invention, the multilayer circuit can be or include a Low Noise Amplifier (LNA) for radio frequency applications, whereby the variable inductor or inductors ( 100 ) can be used to optimize the performance of the amplifier in two different frequency bands.  
      LNAs are used in radio devices to capture the signal coming from the antenna and to amplify it so that it can be used by the rest of the receiver part of the radio. The signal at the antenna terminals is very weak, that is, it has a low amplitude. That means that the LNA that is used to amplify the signal (LNA) must introduce very little noise, in order to maintain a good sensibility and, thus, be able to correctly sense and amplify the low level signals received from the antenna (this is why the amplifier is called a Low Noise Amplifier). In order to interface the LNA with the antenna, a matching network is needed, that adapts the impedances between the amplifier part and the antenna part, so as to provide maximum power transfer from antenna to amplifier at the lowest possible noise level. This matching network is implemented with passive devices such as inductors and capacitors. Especially, high quality factor inductors are needed to design such matching networks, so that they do not introduce too much noise.  
      With the use of high quality variable inductors, such as the one described herein, it is possible to adapt the LNA to different frequency bands, thus making the device useful for operation at different frequencies. This is done by modifying the properties of the matching networks of the device, that is, the input matching network and the output matching network. These networks must be designed very carefully in order to establish an adequate compromise between power gain and noise (two basic variables to consider in connection with LNAs).  
       FIGS. 8A and 8B  schematically illustrate the behaviour of an LNA implemented using a variable inductor in accordance with the invention in the input and output matching networks. In this case the design of the inductors has been made so as to optimise the performance of the LNA at 2.4 GHz (solid line  81  in  FIGS. 8A and 83  in  FIG. 8B ) and also at 4.5 GHz (dashed line  82  in  FIGS. 8A and 84  in  FIG. 8B ).  FIG. 8A  schematically illustrates the noise figure (nf) of the LNA in dB (decibels) (vertical axis) at the input port. The noise figure is a well known concept that describes the noise behaviour of an electric circuit, in terms of its contribution to the noise at an specific point of a circuit. It is calculated according to the IEEE (Institute of Electric and Electronic Engineering) standard with the input port at 290 K, over the frequency range (in GHz) indicated on the horizontal axis and for the 2.4 GHz mode ( 81 ) and the 4.5 GHz mode ( 82 ), respectively.  FIG. 8B  shows a plotting of the gain of the LNA in dB (vertical axis), over the frequency range (in GHz) indicated on the horizontal axis, for the LNA operating in the 2.4 GHz mode (graph  83 ) and in the 4.5 GHz mode (graph  84 ).  
       FIG. 6  shows a schematic circuit diagram of a Low Noise Amplifier ( 200 ) and of its matching networks, both including a capacitor ( 201 ) and a variable inductor ( 100 ) in accordance with the present invention. A radio frequency signal (RFin) is received at the input and a radio frequency signal (RFout) is delivered to an output of the circuit, to which an operating voltage (Vcc) and a bias voltage (Vbias) are applied in a conventional manner. By controlling the state of the switches ( 40 ), the inductance value of the inductor can be changed, so as to selectively adapt the matching network to different frequency bands (one frequency band corresponding to each possible state of the switch arrangement), for example, so as to selectively adapt the device to an operating frequency of 2.4 GHz or 5 GHz, two frequencies in the ISM (Industrial Scientific and Medical) band, for which a large number of products exist in the market.  
      Of course, as suggested above, according to the number of switches used, the inductor can be switched between a corresponding number of inductance values, thus providing for adaptation of the device to a corresponding number of frequency bands.  
       FIGS. 7A and 7B  show examples of prior art receiver/transmitter amplifier ( 200 ) arrangements connected to antennas ( 300 ). Different amplifiers ( 200 ) are provided (each circuit includes two amplifier for the receiving part and two for the transmitting part), and switches are provided so as to use one amplifier or the other in accordance with the respective operating frequency of the device.  
       FIGS. 7C and 7D  show the same types of circuits as those in  7 A and  7 B, respectively, but with only one amplifier for the receiving part and only one amplifier for the transmitting part. Instead, the circuit includes variable inductor ( 100 ) matching networks provided so as to adapt each amplifier to different operating frequencies.  
      Thus, the invention as disclosed provides a convenient way to provide variable inductances in, for example, radio frequency applications. These variable inductances can be integrated in integrated circuits based on, for example, CMOS technology.  
      In this text, the term “comprises” and its derivations (such as “comprising”, etc.) should not be understood in an excluding sense, that is, these terms should not be interpreted as excluding the possibility that what is described and defined may include further elements, steps, etc.  
      On the other hand, the invention is obviously not limited to the specific embodiment(s) described herein, but also encompasses any variations that may be considered by any person skilled in the art (for example, as regards the choice of materials, dimensions, components, configuration, etc.), within the general scope of the invention as defined in the claims.