WO2010106484A1

WO2010106484A1 - Bond frame integrated in electronic circuit

Info

Publication number: WO2010106484A1
Application number: PCT/IB2010/051111
Authority: WO
Inventors: Klaus Reimann; Zidong Liu; Peter Gerard Steeneken; Olaf Wunnicke; Aarnoud Laurens Roest
Original assignee: Nxp B.V.
Priority date: 2009-03-18
Filing date: 2010-03-15
Publication date: 2010-09-23

Abstract

The invention relates to encapsulated integrated RF circuits comprising an integrated bond frame. Many integrated circuits need packaged for protection of the circuit or to connect the fine pitch of IC pins to the coarse pitch of a printed circuit board. It is known that passive components such as coils can be integrated into, e.g., lead-frame packages. Most MEMS (Micro Electro Mechanical Systems) require special packages, i.e., hermetic packages. Prominent examples are capacitive or galvanic MEMS switches for RF. A small and cost-effective package will lead to a wafer-scale package, where the MEMS wafer itself forms one part of the package. The other half can be another wafer that is bonded on the MEMS wafer, e.g., by eutectic bonding. The closed bond frame between the wafers forms an electrically coupled inductive element.

Description

Bond frame integrated in electronic circuit

FIELD OF THE INVENTION

The invention relates to encapsulated integrated RF circuits comprising an integrated bond frame.

BACKGROUND OF THE INVENTION

Many integrated circuits are packaged for protection of the circuit or to connect the fine pitch of IC pins to the coarse pitch of a printed circuit board. It is known that passive components such as coils can be integrated into, e.g., lead-frame packages.

Most MEMS (MicroElectroMechanical Systems) require special packages, i.e., hermetic packages. Prominent examples are capacitive or galvanic MEMS switches for RF. A small and cost-effective package will lead to a wafer-scale package, where the MEMS wafer itself forms one part of the package. The other half can be another wafer that is bonded on the MEMS wafer, e.g., by eutectic bonding onto a bond frame.

Microelectromechanical systems (MEMS) are the technology of the very small, and merge at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines (in Japan), or

Micro Systems Technology - MST (in Europe). MEMS are separate and distinct from the hypothetical vision of Molecular nanotechnology or Molecular Electronics. MEMS are made up of components between 0.01 to 100 μm in size (i.e. 0.00001 to 0.1 mm) and MEMS devices generally range in size from 1 μm to a millimeter. The packaged devices are to be used in circuits. An integrated circuit saves area and is especially attractive at higher frequencies where the passive components such as coils and capacitors become smaller. An alternative is to include the full circuit into the package. Various patent documents disclose encapsulated RF integrated circuits. WO2007/001976 Al describes a RF structure having an antenna formed of seal rings.

It is noted that the above disclosure requires an interrupted conductor. As a consequence no closed seal ring can be formed for a hermetic package.

US2007/290326 Al describes a hermetically sealed assembly were components are fabricated as the packaging so as to reduce size, weight and cost. Also an antenna is formed on the package. The metal seal provided purely seals the cavity. US2005/168306 Al describes a MEMS device with integral packaging. The metal layer from the seal ring is also used for galvanic contact of the MEMS.

US2003/045044 Al describes a RF package with an integrated inductor. It describes a hermetically sealed cavity with therein passive components including inductors and antennas. A special via/interconnect technique is employed to span the distance between the active wafer and the passive components at the bottom of the cavity of the other wafer. A closed metal seal ring is provided.

It is noted that a metallic bond- frame for a hermetic package consumes wafer-space, and causes parasitic capacitances and/or losses due to the wide metallic RF feed-throughs under the bond- frame. In practice this means that a compromise needs to be sought for the width of the RF feed-through, because increasing the width thereof increases the parasitic capacitance, while decreasing its width increases the losses. The manufacturing of these feed-throughs usually requires extra masks and process steps and extra dielectrics are needed to electrically separate the metallic feed- through from the bond- frame. Although the losses of the RF feed-through can be reduced by making it thicker, the presence of the feed-through metal usually causes extra height differences on the wafer which increases the difficulty of manufacturing a hermetically sealed bond-ring. Transmission lines and coils in- and outside the package need to keep spacing to the metallic bond- frame in order to avoid a reduction in performance. The bond- frame should also be grounded to avoid cross talk. Even further, the ground-connections can consume additional space. These features are serious drawbacks of prior art RF circuits.

Thus there still is a need for an improved RF circuit, without the above drawbacks. The present invention is aimed at providing an RF circuit overcoming one or more of the above disadvantages, and at the same time not jeopardizing other characteristics of an RF circuit.

SUMMARY OF THE INVENTION

The present invention relates to a RF circuit comprising a closed bond frame, wherein the bond frame forms an electrically coupled inductive element in a resonator configuration, use thereof, as well as a device comprising said RF circuit. Integrating the bond frame as part of the integrated circuit solves the above problems. The bond frame is used as an inductive element, forming a parallel coil in a resonator configuration.

The present invention provides a closed seal-ring metal. No metal crossing is needed, whereas at the same time a parallel coil is formed. The present invention uses the bond- frame or equivalently parts of the bond- frame as inductive element in an integrated RF circuit to save space and avoid the bad parasitic influences of the feed-through. A problem is that the bond-frame is necessarily a closed metal path (a closed loop) and cannot be used as a single coil by elements inside the frame. It is therefore used as one or more parallel coils in a resonator configuration. The present invention discloses suited designs and applications so that the bond- frame can be absorbed entirely or partially into the circuit. In some configurations also the need for feed-throughs can be eliminated.

The present invention can be advantageous used in, e.g., switchable RF band-pass filters, band-stop filters or matching networks, or as part of an antenna.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect the invention relates to a RF circuit comprising a closed bond frame, wherein the bond frame forms an electrically coupled inductive element in a resonator configuration. Preferably the electrically coupled inductive element consists of two parallel coil, as such an embodiment is easily integrated in an RF circuit. A so-called distributed coil is included in the term "parallel coil". An alternative term for "bond frame" is "seal ring". These terms should be understood generally. To this invention disclosure it is not relevant how the hermetic cavity is actually formed, e.g., by bonding or other sealing techniques.

The present invention is useful for any integrated circuits that use a metallic seal-ring (bond- frame). Most likely these applications use frequencies above 100 MHz, preferably above 1 GHz, due to the limited value of the inductance that can be achieved by the bond- frame. Typically filters are provided in the GHz region, such as at 2 GHz or higher, though also filters up to 100 GHZ and higher are envisaged. New generation devices using frequencies above e.g. 5 GHz are very favorable, as dimensions of the bond frame and frequencies can be matched specifically good, e.g. in terms of losses. Likely devices in these packages are for example MEMS, such as galvanic MEMS (electrostatically or piezoelectrically actuated), MEMS capacitive switches, and tunable capacitors. Applications involve passive LC networks such as adaptive matching networks for power amplifiers or antennas, phase shifters, mechanical oscillators (BAW, SAW, MEMS) as well as band-pass and band-stop filters. The frame might even be used as an antenna or inductive coupler. It may also be used for spectral sensing.

The present invention also relates to a RF circuit comprising one or more bond frames, such as more than two, more than three, or more than four bond frames. As such, the present invention is not limited to one bond frame. As in shown in Fig. 4b2 a bond frame may consist of several layers, forming a stack. The bond frame as such may be regarded as a "composite" bond frame. The bond frame soldering metals are normally not good conductors. In a further preferred embodiment a bond- frame may be extended to the left and/or right by a good conductor (see Fig. 4d). Preferably a metal layer is used with a conductivity of more than 1 * 10⁷ S/m. Typical metals are Ni, Sn, Al, Au, Cu, Ag and alloys thereof. Preferably the bond- frame contains at least 10% of one or more of the elements Au, Cu, Ag, Ni, Sn and Al.

Such a layer is typically placed below (on the substrate) and/or above (on the bonded part) the bond-frame. In a current technology the layer could be 5 μm Al.

Preferably said metal layer extends the bond-frame horizontally by a width / which is larger than the height h of the bond-frame (see Fig. 4c). The solder is only in the center of this conductor and hence has less effect on the RF properties because the RF current concentrates at the edges of the strip. Then a solder- stop mask could be devised to prevent the solder from spreading out all over the metal, such as Al. All this is not needed if thermo-compression or ultrasonic bonding with pure Au is done. Such an implementation is preferably adapted to the bonding technology which might vary from application to application.

Additional well conducting layers can be added which are isolated from the bond-frame by a dielectric layer for, e.g., electrical feed-through for low frequency signals. Preferably said metal layers follow the path of the bond-frame for more than 50% of the part which forms the inductive element of the present invention, preferably for more than 90% thereof.

Further, the present bond frame could be used for sealing a cavity, such as above a substrate on which a bond frame resides. Such is e.g. the case when a MEMS is present. The present RF circuit typically comprises a tunable capacitive device, an RF input, and an RF output. The tunable capacitive switch is connected to a DC voltage Vb, whereas the bond frame is typically connected to a second stable voltage, such as to earth. Preferably the input or output (or both) RF signal connection is directly connected to the bond- frame. As such no RF feed-through is needed. The tunable capacitive switch is preferably further connected to a second capacitor, as well as to the bond frame. Further, a matching circuit may be present.

The present invention thus relates to using a part of the bond frame electrically, as such not requiring a closed bond frame. In principle it is not required for an electrical system that a bond frame, forming part of a package, is completely closed However, preferably the package is hermitically sealed. The present invention provides advantages specifically if a closed seal ring is present.

The invention thus relates to the use of the bond frame as for example, an antenna application, an inductor and a transmission line. It is noted that an inductor could be considered as antenna matching and would then be an integral or external part of the antenna. Such an antenna typically comprises an input and an output. As a consequence, the seal ring typically forms a complete, non- intermitted, electrical conductive path, from an input to an output thereof.

The present RF circuit typically comprises a bond frame of a so-called low temperature soldering. Further, typically a cap is provided, sealing the RF circuit, such as a MEMS. Such a cap could be made of silicon, a glass, AI2O3, a ceramic, a polymer, etc. For RF properties it is beneficial if the cap is an insulating or high resistivity material.

In a preferred embodiment according to the present invention relates to the RF circuit further comprising one or more of a MEMS tunable or switchable capacitor, a galvanic MEMS switch, a capacitive MEMS, a varactor diode, a MOS varactor, a capacitor, a capacitor bank, an acoustic resonator, an acoustic wave guide, and a semiconductor switch within the bond frame.

Preferred embodiments are for example a MEMS tunable capacitor, a galvanic/ohmic MEMS switch, a MEMS resonator or a BAW or a SAW device. This is preferred since MEMS switches offer the best RF properties and because they often require the type of package, which is the subject of this invention. Further preferred is a rectangular shape of the ring to utilize the full die area. Even further preferred in terms of RF performance are rounded shapes, wherein the bond frame is substantially circular, such as rectangular, square, hexagonal, octagonal, multigonal, oval, ellipse shaped, circular and combinations thereof, such as a rectangular with rounded corners, preferably circular. In terms of losses, coil performance, etc. such shapes are preferred. Also somewhat irregular forms, having e.g. one or more intrusions as shown in Fig. 4e are also envisaged.. Intrusions offer advantages in terms of e.g. connection lines into a package. From an RF point of view round structures have advantages. However, space is lost on a typical die, as it normally is rectangular. A good compromise in this respect might be an almost rectangular shape with rounded or mitered corners.

In a preferred embodiment according to the present invention relates to the RF circuit, wherein the bond frame has a thickness of 1-20 μm, such as 10 μm, and a width of 10-200 μm, such as 50 μm. Width and thickness can be modeled to specific requirements of the RF circuit, e.g. to the frequency used, the specific use of the RF circuit etc.

In a preferred embodiment according to the present invention relates to the RF circuit, for tuning impedance, and/or for forming a band matching network. It is noted that typically the inductance is only tuned indirectly. A packaged component normally is a tunable or switchable capacitor, e.g., an RF MEMS. By designing the bond-frame such that its impedance overweighs the capacitive response of the tunable capacitor, an tunable "effective" inductance can be realized.

In a preferred embodiment according to the present invention relates to the RF circuit, wherein the bond frame is grounded at at least one position and/or wherein the bond frame is extended.

In a second aspect the invention relates to a device, such as band-pass filter, harmonic suppression notch, phase shifter, LC circuit, voltage controlled oscillator, comprising an RF circuit according to the invention.

In a third aspect the invention relates to RF module, such as a MEMS, LC network, BAW, SAW, tunable filter, reconfigurable matching network, RF fixed or tunable oscillator, adaptive RF network, and combinations thereof, comprising an RF circuit according to the invention.

In a fourth aspect the invention relates to a use of a bond frame, such as in an RF circuit according to any of claims 1-7, for an antenna, for a tunable antenna, for an adaptive antenna, for a coil, for a resonant slot antenna, a matching circuit, a hermetic seal, a wireless electrical, magnetic or electromagnetic transducer, for switching operation between bands or adapting to changing an RF source or load, or a combination thereof.

In a fifth aspect the invention relates to a mobile communication device, data transfer device, RF-ID, RADAR, comprising an RF circuit according to the invention, and/or an RF module according to the invention.

The present invention is further elucidated by the following Figs, and examples, which are not intended to limit the scope of the invention. The person skilled in the art will understand that various embodiments may be combined.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an example of a prior art design of a RF circuit. Fig. 2 shows a resonator layout according to the invention. Fig. 3 shows a resonator layout according to the invention.

Figs. 4a-4e show schematic cross sections of a prior art RF circuit and a present RF circuit.

Fig. 5 shows simulation set-up of the present RF circuit.

Fig. 6 shows results of the simulation of the present RF circuit. Fig. 7 envisages a design with a bond- frame.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows an example of prior art design, if the inductive elements were to be integrated on-chip or on an additional carrier (such as PCB, glass, interposer, etc.). The negative impacts of the bond-frame crossings such as loss (resistance), and reduced tuning range (parasitic capacitance) are depicted. Less visible is the negative impact of the bond-frame on the size. Especially the inductive elements should keep a minimum distance from the bond- frame and the bond- frame can be fairly broad (80um), so that performance is reduced and/or extra space is consumed. The bond- frame crossings 102 cause extra resistances 120 and parasitic capacitances 121. Additionally the bond- frame can use considerable space and can be fairly large dependent on the technology constraints. Fig. 2 shows an example implementation where the bond- frame (201) is used as inductive element in a resonator. The bond- frame provides two parallel coils that are tapped at the resonator connections 211 and 212. The parallel configuration allows a closed bond- frame. The tuned capacitor value Cl (230) can be realized, e.g., by capacitive MEMS, capacitor banks switched by galvanic MEMS, varactor diodes, MOS varactors, ferroelectric capacitors or capacitor banks switched by semiconductor switches. It is noted that no feed-throughs beneath the bond- frame are needed. Parasitic couplings are thus avoided. The metals that realize the coils are anyhow wide and thick to realize a good quality factor. The bond- frame is therefore fully embedded into the package. It relates to one of many possibilities how the present integrated resonator could be constructed. The layout with the packaged

MEMS is shown on the left panel. The right panel contains an equivalent circuit The MEMS circuit configuration shown here makes use of an advantageous bias circuitry. 211 and 212 are possible connections to the resonator. Further a second capacitor 231, a resistant Rb (222) and a voltage Vb (260) are shown. Typically prior art integrated resonators do not use packaged MEMS. Prior art means here that the packaged

MEMS and discrete circuits using these MEMS are known. Integrated resonators with unpackaged MEMS are known from literature. Variations of the present invention are possible. One example is that the bond- frame does not need to be rectangular, but only needs to be closed. That can be used to optimally design electrical properties.

Fig. 3 shows such an example where a slot is provided as matching circuit to adapt the inductance of the bond-frame, e.g., for an antenna-matching circuit. Another version could use meandered bond-frame to increase the inductance.

The bond- frame could even be used as an antenna for the circuit inside.

Another example is that multilayer metals can be used below/above the bond frame. These metals are often already there for the bond- frame crossings. Using these metals can lower the resistance even further. This is important if the sealing metal alloy is not well conducting. Thick metal (e.g. Al) can be used beside the bond- frame to back-up conductivity. For electrical performance at RF it is not needed that there is a galvanic contact all over the length of the bond-frame. The (parasitic) capacitive coupling of the crossing metal can be advantageously used to guide the RF signal along the bond- frame.

Other options are using the metals below the bond- frame as coupled lines to tune the inductance or to form wide-band matching.

Another example is a partial use of the bond- frame. Therein two points, grounded to earth, are applied. The coil in between said two points does as a consequence no longer form part of a circuit. Simulations have been done and show the functionality hereof. Also designs that still include bond-frame crossings together with a partial use of the bond-frame are possible and can be provided on request.

To further illustrate the present invention Figs. 4 show schematic cross sections of a prior art RF circuit and a present RF circuit, respectively. In Fig. 4a and 4b, on top of a substrate (400), typically a silicon substrate, a MEMS (410) is provided. Such a MEMS is electrically connected by a conducting track (412) (present invention) and (440) (prior art). In the prior art said connection crosses a bond frame (450) through a dielectric layer (430), typically a silicon oxide layer. The present invention further comprises a cap (420), typically of silicon, glass, AI2O3, etc. Thus the present invention is less complex than the prior art, as no extra dielectric layer is present, nor two extra vias and a buried connection (440), and further the above mentioned disadvantages are absent. It is noted that in Fig. 4b the MEMS switch further comprises a second capacitor C2, as e.g. in Fig. 2, in order to function properly. Said second capacitor is not shown in Fig 4b. Fig. 4b2 shows an embodiment of the present invention, wherein the bond frame consists of several layers. Preferably a further metal layer is present therein, having a good conductivity, as described above. Such further metal layer is even more preferably present at the top section and/or at the bottom section of the bond frame. Even further, more than two metal layers may be present, albeit being complicated to manufacture. Also other layers, e.g. dielectric layers, may be present. Extra metal layers provide a further advantage in terms of behavior of the coil, e.g. with respect to losses.

Fig 4c shows an embodiment wherein an extra metal layer 460 and/or 460 are present. These metal layers offer the above mentioned advantages. More preferably the extension / of the metal layer 460 and 461 is larger than the height h of the bond frame, even more preferably / > 2h. As such, the high frequency signals are better transmitted over the metal lines.

Fig. 4d shows a top view of the metal lines and bond frame. It can be seen that in a preferred embodiment the metal lines extend beyond the outer circumference of the bond frame and/or beyond the inner circumference thereof, even further providing improved characteristics, as mentioned above.

Fig 4e shows a top view of an embodiment, wherein a bond frame has a form with inner extensions (see above). Typically said extensions may extend towards the center for more than 20% of the inner diameter of the bond frame, preferably more than 30%, such as 50%.

The resonator can be used stand-alone as discrete component on a printed circuit board. However, also integrated filters are possible. The following pictures show how the above resonators can be used to construct a band-pass filter. Fig. 5 shows a simulation set-up of the layout (two resonators side-by-side, mirror- symmetrical) and Fig. 6 the simulation results. A final implementation could employ 2D simulations for better accuracy, but this model already shows the feasibility, because state-of-the art transmission line models were employed. In Fig. 5 TLx represent transmission lines, which actually represent the bond-ring, Teex represent T-shaped connections, COMx represent corner shaped sections, Zx represent terminal resistors, and Cx represent capacitors. Typical values used in the simulation are:

Width w of the transmission lines, corners, Tees: 100 μm (TL3,TL8), 300 μm (rest) bond-frame dimensions: 1000x1500 μm² Cl : tunable capacitor 2-5 pF Fig. 6 shows results of simulations. It is clearly visible that depending on values chosen for Cl, various band pass transfer characteristics are obtained with very good performance.

The above description focused on band-pass filters. Other applications such as harmonic suppression notches in adaptive matching networks, phase shifters or other LC circuits could be made.

Fig. 7 envisages a design where the bond- frame is used together with internal capacitors such that it becomes self-resonant and high ohmic at the coupling ports to the bond- frame. Small-band RF devices such as mechanical resonators inside the package could then be connected without the need of bond- frame crossings. This could be used, e.g., in MEMS resonator packaging where the resonance frequency is well defined by the resonator in the package. It allows flat bond-frame metals and ensures a good sealing without planarization steps or thick bond- frame metals.

As already mentioned for the other applications, the bond- frame can also be used as matching circuit for the acoustic resonator.

Claims

CLAIMS:

1. RF circuit comprising a closed bond frame, wherein the bond frame forms an electrically coupled inductive element in a resonator configuration.

2. RF circuit according to claim 1, wherein the electrically coupled inductive element consists of two parallel coils.

3. RF circuit according to claim 1 or claim 2, further comprising one or more of a MEMS tunable or switchable capacitor, a galvanic MEMS switch, a capacitive MEMS, a varactor diode, a MOS varactor, a capacitor, a capacitor bank, an acoustic resonator, an acoustic wave guide, and a semiconductor switch within the bond frame.

4. RF circuit according to any of claims 1-3, wherein the bond frame is substantially circular, such as rectangular, square, hexagonal, octagonal, multigonal, oval, ellipse shaped, circular and combinations thereof, such as a rectangular with rounded corners, preferably circular.

5. RF circuit according to any of claims 1-4, wherein the bond frame has a thickness of 1-100 μm, such as 10 μm, and a width of 10-200 μm, such as 50 μm.

6. RF circuit according to any of claims 1-5, for tuning impedance, or for forming a band matching network.

7. RF circuit according to any of claims 1-6, wherein the bond frame is grounded at at least one position and/or wherein the bond frame is extended.

8. Use of a bond frame, such as in an RF circuit according to any of claims 1-7, for an antenna, for a tunable antenna, for an adaptive antenna, for a coil, for a resonant slot antenna, a matching circuit, or a combination thereof.

9. RF module, such as MEMS, LC network, BAW, SAW, tunable filter, reconfϊgurable matching network, RF fixed or tunable oscillator, adaptive RF network, comprising an RF circuit according to any of claims 1-7.

10. Device, such as band-pass filter, harmonic suppression notch, phase shifter, LC circuit, voltage controlled oscillator, comprising an RF circuit according to any of claims 1-7.

11. Mobile communication device, data transfer device, RF-ID, RADAR, comprising an RF circuit according to any of claims 1-7, and/or an RF module according to claim 9.