Abstract:
A monolithic antenna element comprises a microstrip patch antenna and a ground plane, with a substrate between the patch antenna and the ground plane. A feeding via extends from the ground plane layer through the substrate to the patch antenna, connecting to the antenna distal from lateral edges of the antenna. A coplanar waveguide (CPW) feed line is formed in the ground plane layer, and interrupts and is electrically distinct from the ground plane. The CPW extends from a lateral edge of the ground layer to the feeding via. The antenna can be flip chip bonded to a CMOS die, reducing cost of millimeter wave transceivers, e.g. 57-64 GHz. The antenna is fabricated using standard PCB technology and a single substrate for the antenna. Antenna arrays can be fabricated. Appropriately designed antenna feeds, flip chip interconnects and antenna shape provide suitably broad antenna bandwidth, with relatively high efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority from Australian Provisional Patent Application No 2008901258 filed on 14 Mar. 2008, the content of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to integration of a microstrip antenna upon a CMOS die, and in particular relates to a fully integrated CMOS millimeter-wave wireless transceiver comprising such an antenna. 
     BACKGROUND OF THE INVENTION 
     There exists a large allocated bandwidth around the 60 GHz region of the spectrum, offering the appeal of high-speed short distance wireless personal area networks (WPANs), radar applications such as automotive radar, along with other potential industrial, scientific and medical applications. This has motivated research into low cost, efficient and small form factor integrated millimeter-wave devices in order to facilitate their use in consumer electronic (CE) applications. Wireless systems operating at such millimeter-wave frequencies require appropriate antennas. 
     One approach to fabricating antennas having adequate operational bandwidth and efficiency for such applications has been to utilise micromachining technology to construct a post-supported antenna, together with a coplanar waveguide (CPW) antenna feed. Such antennas comprise an air dielectric rather than a silicon substrate dielectric, which increases bandwidth and improves radiation efficiency. However, micromachining is not compatible with standard CMOS technology, increases costs, and may raise doubts as to mechanical stability. A further significant issue in fabrication of such antennas can be transition discontinuity losses at connections between the antenna and other transceiver elements. 
     Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
     Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
     SUMMARY OF THE INVENTION 
     According to a first aspect the present invention provides a monolithic antenna element comprising:
         a microstrip patch antenna formed on a first surface of a substrate;   a ground plane formed on or adjacent to a second surface of the substrate substantially opposite the first surface, such that the substrate is between the patch antenna and the ground plane;   a feeding via extending from the second surface through the substrate to the first surface and being electrically connected to the microstrip patch antenna in a location distal from lateral edges of the microstrip patch antenna; and   a coplanar waveguide feed line formed on or adjacent to the second surface of the substrate and interrupting and being electrically distinct from the ground plane, and extending from proximal to a lateral edge of the second surface to the feeding via.       

     According to a second aspect the present invention provides a monolithic transceiver comprising at least one antenna element in accordance with the first aspect flip chip mounted upon a CMOS die comprising active devices. 
     Embodiments of the second aspect of the invention may comprise a plurality of antenna elements flip chip mounted upon the CMOS die. 
     According to a third aspect the present invention provides a method of fabricating a monolithic antenna element, the method comprising:
         forming on a first surface of a substrate a microstrip patch antenna;   forming on or adjacent to a second surface of the substrate substantially opposite the first surface a ground plane, such that the substrate is between the patch antenna and the ground plane;   forming a feeding via extending from the second surface through the substrate to the first surface and being electrically connected to the microstrip patch antenna in a location distal from lateral edges of the microstrip patch antenna; and   forming on or adjacent to the second surface of the substrate a coplanar waveguide feed line interrupting and being electrically distinct from the ground plane, and extending from proximal to a lateral edge of the second surface to the feeding via.       

     Embodiments of the present invention may thus provide a broad band, efficient antenna for a millimeter-wave wireless transceiver system, giving low cost integration by being compatible with CMOS technology and may also be compatible to other technologies such as SiGe or GaAs, and providing low interconnection losses. Moreover, the use of an under-fill layer with flip chip bonding assists in providing mechanical stability. 
     By selecting flip-chip bonding, this invention provides on chip antenna integration bonding which is compatible with standard CMOS technology, and which avoids the significant transition discontinuity losses suffered by wire bonding and wedge bonding at millimeter wave frequencies. Moreover, flip chip mounting enables a low loss connection between the coplanar waveguide formed on the second surface to a microstrip transmission line or other type of connection as may be formed upon a CMOS die upon which the antenna element is flip chip mounted. 
     Moreover, by feeding the antenna patch by a feeding via, the feed signal may be delivered to the patch at any desired location, and in preferred embodiments the location at which the feeding via is connected to the patch antenna is predetermined in a manner to effect input impedance matching, such as 500 impedance matching. 
     In embodiments in which circular polarisation is desired, the patch antenna is preferably substantially square shaped. In such embodiments impedance matching may be effected by locating the feeding via substantially upon one diagonal of the square. 
     The ground layer may be formed on the second surface of the substrate, prior to flip chip mounting of the substrate upon a CMOS die. Alternatively, the ground layer may be formed on a CMOS die and positioned adjacent to the second surface of the substrate by flip chip mounting of the substrate upon the CMOS die. 
     Preferably, a size and position of the metallic bumps upon the CMOS die to effect a flip chip interconnect is selected in order to optimise the antenna characteristics for an intended purpose. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates an antenna element in accordance with one embodiment of the present invention; 
         FIG. 2  is an enlarged view of a portion of the antenna element of  FIG. 1 , showing flip-chip interconnections between coplanar waveguide feed lines on CMOS die and on the antenna element; 
         FIG. 3   a  is an exploded perspective view of the patch antenna structure on CMOS;  FIG. 3   b  gives a cross sectional side view of the antenna construction and  FIG. 3   c  gives a cross sectional top view detail of the flip-chip transition region; 
         FIG. 4  illustrates simulated results of varying the bump and underfill height: (a) bandwidth, (b) gain at 60-GHz frequency; 
         FIG. 5  illustrates simulated results of varying the bump diameter at 60 GHz frequency; 
         FIG. 6  illustrates simulated results of varying the permittivity of underfill substrate on (a) bandwidth, (b) gain and radiation efficiency at 60 GHz frequency. 
         FIG. 7  illustrates measured return loss performance of an integrated antenna with flip-chip interconnect. (Simulation parameters: ∈ r =4.1, bump diameter=60 μm) 
         FIG. 8  illustrates measured gain and radiation efficiency of an integrated microstrip antenna with flip-chip interconnect. (Simulation parameters: h=40 μm, ∈ r =4.1, bump diameter=60 μm) 
         FIGS. 9   a  and  9   b  are separate layer views of a second embodiment of the invention comprising a two element antenna array connected to a signal feed by a split coplanar waveguide flip chip bonded to a CMOS die; and 
         FIG. 10   a  is a perspective view, and  FIG. 10   b  is a cross sectional side view, of a further embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic geometry of a microstrip antenna element  100  in accordance with one embodiment of the invention is shown in  FIG. 1 . The antenna is comprised of a radiating patch  102  printed on the upper surface of the substrate  104 , and a ground plane  106  printed on the underside of the substrate  104 . The substrate  104  is Rogers  5880  which is a low dielectric loss substrate having low dielectric constant. In order to achieve circular polarisation and 50-Ω input impedance matching the shape of the antenna patch  102  is nearly square. The radiating antenna patch  102  on the top surface of the substrate  104  is connected to the signal input on a bottom edge of the substrate  104  by way of a coplanar waveguide (CPW) line  108  extending from the signal input along the underside of the substrate  104  through an interruption in the ground patch/plane  106 . The CPW line  108  leads to a feed via  110  extending through the substrate  104 , with the feed location selected along the diagonal starting at the lower right corner of the square shape of the antenna patch  102 . This type of connection allows the antenna  100  to be fabricated by using standard PCB and flip-chip bonding technology. 
       FIG. 2   a  is an enlarged view of a portion of the antenna element  100  of  FIG. 1 , showing flip-chip interconnections between coplanar waveguide feed lines  118  on CMOS die and  108  on the antenna element  100 . The metallic (gold) bumps  112 , placed on the CMOS die pads through a modification of the “ball bonding” process, provide for flip chip connection of the pads and transmission line of the CMOS die  120  with the antenna input pads. In order to mechanically support the antenna structure  100  an under-fill dielectric layer  114  is placed in the gap between the antenna element  100  and CMOS die  120 . 
       FIG. 3   a  is an exploded view of the embodiment of  FIGS. 1 and 2 . In this geometry of the fabricated antenna, the parameters were: antenna length l=1.65 mm, antenna width w=1.67 mm, substrate ∈ r =2.2, via to edge spacing y′=0.435 mm, and substrate height h s =0.254 mm.  FIG. 3   b  gives a cross sectional side view of the antenna construction and  FIG. 3   c  gives a cross sectional top view detail of the flip-chip transition region. To attach the antenna element  100  to the CMOS die  120  and form flip-chip interconnections, a thermo-sonic flip chip attachment process is used. The thermo-sonic attachment process uses heat, pressure and ultrasonic energy to form a bond between each bump  112  and metallisation surface. The embodiment of  FIGS. 1 to 3  thus employs flip-chip bonding (solder balls) to connect the antenna  100  to the die  120 . This technology uses metallic bumps  112  for device connections which are kept small (less than 100 μm) compared to the length of bond wire and results in better impedance matching and reduces interconnection losses. 
     The optimized design of the antenna  100  in this embodiment includes interconnections optimisation. The design value for bump diameter is 60 μm, the permittivity of underfill layer is 4.1 (Epotek, U300) and bump height is 20 μm. However, a 20 μm bump height was not achievable in the manufacturing process used (stud bump technology) and the final fabricated bump height was 40 μm (±5 μm tolerance). Parameters studied were the bump height, bump diameter and permittivity of the underfill layer. Other parameters such as the bump pitch and parameters of microstrip line on CMOS are predetermined by the chosen technology and required 50-Ω output impedance of the chip design. 
     It is further noted that fabrication capabilities and tolerances of the particular bump technology may affect antenna performance; reduce the gain and narrow input impedance bandwidth. Also, electrical properties of the underfill material  114  such as permittivity and dielectric loss have an impact on antenna performance. Accordingly, the antenna-chip structure  100  of this embodiment was simulated with the finite element method software HFSSv11 from Ansoft Corporation. The definition of impedance bandwidth that is used for this study is given by 
                     B   ⁢           ⁢   W     =         (       f   H     -     f   L       )       Γ   =       -   10     ⁢           ⁢   db           f   0               (   1   )               
where Γ is the return loss, f 0  is centre frequency, f L  is lower frequency and f H  is the highest frequency at which the return loss is equal to −10 dB. The antenna radiation efficiency is calculated from the ratio between the radiated power and accepted power where the accepted power is a measure of the incident power reduced by the mismatch loss at the antenna input port.
 
     Firstly, the effect of the coplanar bumps  112  on the performance of antenna  100  was investigated by varying the bump height while keeping the bump diameter constant and equal to 60 μm. The thickness of the underfill layer  114  is simultaneously changed in the same increment as the bump height.  FIG. 4  displays the effect of the bump height on the input impedance bandwidth when the heights are altered by 10 μm. The maximum bandwidth is achieved using the shortest metallic bumps. The bandwidth is reduced from 20% to 15% for the height variation from 10 μm to 40 μm. Further increasing the bump height from 40 μm to 60 μm, reduces the bandwidth further from 15% to 13%. The gain and efficiency at 60 GHz increase from 2.7 dBi to 3.7 dBi and from 60.8% to 72%, respectively. A similar trend of gain and radiation efficiency increase occurs through the entire bandwidth. This indicates that the increase of bump height and underfill thickness increases antenna quality factor which is proportional to the radiation efficiency. 
     The relationship of bump diameter to bandwidth was also investigated. The expected range of the bump diameters after fabrication process is from 60-80 μm which is in the order of the pads size on CMOS. The control over bump diameter during the bonding process obtains smooth transition from the CMOS pads to the antenna feed line  108  and achieves lowest return loss.  FIG. 5  displays the effect on bandwidth when the bump diameters are altered from 20 to 140 μm. The height of the bumps  112  and underfill layer  114  are kept constant and equal to 40 μm. The maximum bandwidths are achieved with diameter from 40 to 80 μm. The gain and efficiency at 60 GHz are constant for the whole range of bump diameters from 20 to 140 μm. 
     The relationship of underfill permittivity to the bandwidth and gain was also investigated. The investigation of the effects of underfill layer  114  on the antenna bandwidth and gain is carried out for typical bump height of 40 μm and bump diameter of 80 μm. The known permittivity for most of commercially available underfill materials spans from 3 to 4.4 (e.g. Hysol, Locate).  FIG. 6  displays the effect on the antenna bandwidth when the permittivity constants are altered from 1 to 5. The absence of underfill layer  114  between the patch and chip (i.e. ∈ r =1) decreases the bandwidth since low permittivity increases impedance of the bump interconnection and feed CPW line  118  that causes input impedance mismatch and narrows the bandwidth. The bandwidth increases for 2.5% when the permittivity is altered from 2 to 5. The efficiency and gain at 60 GHz are reduced from 75% to 68% and 3.8 dBi to 3.5 dBi respectively. 
     Measurement results were also obtained from an actual fabricated antenna.  FIG. 7  shows the simulated and measured input return losses of the fabricated antenna with flip-chip interconnection. The return loss was measured on a probe station using a GSG probe. In the frequency band from 60 GHz to 69 GHz, measured return losses lower than −10 dB were achieved (15% bandwidth). Simulation results show that when the bump height increases from 20 to 40 μm the −10 dB bandwidth decreases by 1.5 GHz. This is due to the increased impedance (reactance) of extended bump length which increases input impedance mismatch. To fully cover the 57-66 GHz band this reactance can be compensated by redesign of the antenna  100  with larger patch size. 
       FIG. 8  shows the measured gain, simulated gain and efficiency. The gain was measured at angle of θ=0° and φ=0° ( FIG. 3   a ). The measurement set-up was calibrated using coaxial calibration kit and V-band horn antennas [ 12 ]. Then, one port is connected to a GSG probe (calibrated at the tips) and a test antenna and the other port to the horn antenna. The fabricated antenna achieves a gain of 2-4 dBi inside the −10 dB impedance bandwidth at θ=0°. This level of gain is low, however, an array can be formed by a number of these antenna elements to enhance the radiation and compensate the path loss for 60-GHz radio. 
     The results of  FIGS. 3 to 8  show that a wide antenna bandwidth can be achieved with very short bumps and thin underfill layer  114  at the expense of decreased efficiency and gain. Bump diameter variations, provided they are within acceptable tolerance, don&#39;t affect the bandwidth, gain or efficiency. The low permittivity of the underfill layer  114  increases gain and efficiency but only slightly reduces the bandwidth. Thus, a standard and inexpensive flip-chip bonding process can be used to achieve antenna-die integration even for a challenging wide bandwidth such as the 57-64 GHz band, provided that careful consideration is given to flip-chip parameters (e.g. bump size and tolerance) during the antenna design stage. 
     This embodiment of the invention thus enables a wireless transmitter system with wide bandwidth and high efficiency that can be achieved with simple structure and low cost standard fabrication processes. In turn, the invention provides for antenna integration with millimeter-wave transceiver circuits using low cost standard printed circuit technology and flip-chip bonding. The flip-chip interconnection allows on-chip mounting of the antenna element  100  and effects a smooth transition from the antenna CPW feeding line  108  to microstrip transmission line  118  or device on the CMOS die  120 . Unlike a wire bonding attachment, the flip chip mounting results in low reflections and wide operational bandwidth at millimeter-wave frequencies. 
     In other embodiments of this invention, an antenna array can be fabricated by connecting two or more antenna elements of this type and integrating them with the CMOS die using the flip-chip bonding technique to connect each antenna.  FIGS. 9   a  and  9   b  show an embodiment comprising an array of two antenna elements  900   a ,  900   b  mounted on the top side of a single substrate  904  and each connected with a coplanar waveguide line  908   a ,  908   b  on the under side of the substrate  904 . The CPW  908   a ,  908   b  extend from a respective via  910   a ,  910   b  connected to each antenna  908   a ,  908   b , from where the CPW  908   a ,  908   b  extend to join each other, and then extend to a single set of metallic bumps  912  for flip chip mounting. A single flip chip connection may then be used to bond the joined CPW to a CMOS die using the metallic bumps  912  and thermo-sonic attachment. Alternative embodiments having multiple antennas may provide a dedicated CPW and set of bumps for each antenna element, for example to permit phased array operation and beam steering. 
       FIG. 10   a  is a perspective view, and  FIG. 10   b  is a cross sectional side view, of an antenna  1000  in accordance with a further embodiment of the invention. In this embodiment the substantially square radiating antenna patch  1002  is once again mounted upon a substrate  1004 , with a via  1010  extending from the patch  1002  through the substrate  1004 . However, in this embodiment, the substrate  1004  does not comprise a ground plane and is flip chip mounted upon a CMOS die with an interposed underfill layer  1014 . In this embodiment the ground layer  1006  is instead formed upon the CMOS die  1020 , and is interrupted by the CMOS coplanar waveguide feed  1018 , which terminates in metallic bump  1012 . The via  1004  is bonded to the bump  1012  by thermo-sonic attachment. 
     This invention thus provides for an on-chip antenna design which allows flip chip bonding to form an integrated RF system (RFIC) in a single package, offering the potential for substantially reduced manufacturing cost of transceivers in the millimeter wave domain, for example the 57-64 GHz band. The digital part of the transceiver can also be integrated on the same chip. The on chip antenna is further fabricated using standard PCB technology and a single substrate for the antenna element, and permits multiple antenna arrays to be fabricated. Appropriately designed antenna feeds and antenna shape provide for a suitably broad bandwidth of the antenna, with high efficiency for such substrates. 
     The present invention notably selects flip chip interconnection. This is in recognition that bonding wire (wire bonding or wedge bonding), while typically employed to connect passive devices such as antennas to chip modules, increases impedance mismatches and power losses at millimeter wave frequencies and requires insertion of compensation networks and accurate electromagnetic modelling of transition discontinuities. The present invention recognises that the parasitic effects of transition discontinuities in millimeter wave systems can be significantly reduced by employing flip-chip bonding to connect the antenna to the die. Testing of the embodiment of  FIGS. 1 and 2  shows that microstrip antenna bandwidths of 15% can be achieved with careful flip-chip interconnection design and fabrication. This is of particular importance for the integration of millimeter wave antennas into the CMOS package which are suitable for operation in the 57-64 GHz band. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.