Abstract:
The specification describes an integrated passive device (IPD) designed to allow implementation of cellular RF and Wi-Fi RF in a single hand held device. To address the problem of RF interference a thin film RF high rejection bandpass filter is formed in an IPD implementation. The IPD implementation preferably uses silicon as the substrate material. This allows the thin film RF high rejection bandpass filter to be made using silicon processing technology, and thus produce low cost filters that still meet stringent performance requirements demanded due to the co-existing RF units. In preferred embodiments of the invention, wafer level processing using silicon substrates adds to the cost effective manufacture of the highly functional IPDs.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to mobile communications devices that contain both cellular and Wi-Fi functionality.  
       BACKGROUND OF THE INVENTION  
       [0002]     (Portions of the technical material contained in this section may not be prior art.)  
         [0003]     Cellular phones interact with a wireless network, and contain a radio frequency (RF) transceiver and antenna to implement that interaction. The RF circuits are designed to operate on carrier frequencies assigned by regulatory authorities. Accordingly, cellular phones are equipped with RF filter devices to restrict RF operation to the assigned bands.  
         [0004]     Advances in mobile communications technology have created a new demand for cellular phones that have Wi-Fi capability. To implement this requires two RF units within a single hand held device. When the two RF units are operated simultaneously, a conflict arises between RF signals interacting with the wireless network and signals for the Wi-Fi unit.  
         [0005]     The cellular phone market continues to demand smaller hand held devices. Thus miniaturization of RF circuits is an important goal of RF technology. Of particular concern are the passive devices in the RF circuits. State of the art radio frequency (RF) electrical circuits use large quantities of passive devices. When new Wi-Fi RF circuits are added to implement the new service features, the problem of miniaturization becomes even more challenging.  
         [0006]     Significant advances in miniaturization have been achieved. In may cases these involve surface mount technology (SMT). Small substrates containing large numbers of passive components are routinely produced using surface mount technology.  
         [0007]     More recent advances involve integrated passive devices (IPDs) where some of the RF components are built on a single platform. See for example U.S. patent application Ser. No. 10/835,338, which describes and claims IPD devices with very efficient integration. This application is incorporated herein in its entirety. In spite of advances in IPD technology, RF devices continue to use a hybrid integration circuit approach where IC chips, IPDs, and discrete RF components are individually assembled to produce the RF unit.  
       SUMMARY OF THE INVENTION  
       [0008]     We have designed IPDs for RF devices that allow simultaneous operation of cellular RF and Wi-Fi RF units in a single hand held device. To address the problem of RF interference a thin film RF high rejection bandpass filter is formed in an IPD implementation. The IPD preferably uses silicon as the substrate material. This allows the thin film RF high rejection bandpass filter to be made using silicon processing technology, and thus produce low cost filters that still meet the stringent performance requirements demanded of the cellphone due to the co-existing RF units. In preferred embodiments of the invention, wafer level processing using silicon substrate adds to the cost effective manufacture of these highly functional IPDs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]      FIG. 1  is a schematic representation of a generic IPD showing the three main types of circuit components integrated on a single substrate; and  
         [0010]      FIG. 2  is a circuit diagram for a high rejection RF bandpass filter;  
         [0011]      FIG. 3  is a layout design for the circuit of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION  
       [0012]      FIG. 1  shows the three major passive components integrated on a single substrate  11 . The substrate  11  may be any insulating substrate, but, for reasons that will be described below, is preferably silicon or polysilicon. A significant advantage of IPD technology is that the IPD can be manufactured at the wafer level. This is considerably more efficient and cost effective than the conventional approach of assembling many passive components on a printed circuit board (PCB). However, wafer level manufacture for IPDs is a relatively recent development. This is partly due to the fact that passive devices are relatively large, and have inherent limitations on the degree to which they can be minimized. Therefore, whereas using wafer level fabrication of integrated passive device circuits may not be attractive commercially using 3″ or 4″ silicon wafers, the arrival in the commercial arena of 8″ or 12″ silicon wafers makes wafer level manufacture of IPDs not only viable, but commercially attractive. A key to the success of this new technology is the use of silicon as the IPD substrate material.  
         [0013]     Recognizing that the silicon in the IPD implementation of the invention is passive, i.e. it does not contain active transistor elements as in a conventional silicon integrated circuit, a variety of new choices are available for the substrate material. One of these is to use a so-called junk wafer. In a wafer production facility, after sawing and polishing the wafers, each wafer is subjected to quality control, where the wafer is measured for conformity to rigid standards for physical size and electrical properties. Typically wafers with chips or scratches will be rejected. Wafers that have excessive or non-uniform conductivity are also rejected. In many cases the rejected wafers are scrapped, and sometimes referred to as “junk wafers”. In this description, a “refuse” wafer includes wafers that are cut from a boule, measured by one or more physical or electrical tests, and rejected for failing a test. Refuse wafers have relatively low commercial value. A refuse wafer may be expected to have a value of less than 50%, and more typically, less than 10%, of the value of an acceptable wafer. To obtain a relatively high resistivity surface for the IPD, a thick silicon dioxide layer, or other suitable insulating layer, may be formed on the silicon wafer surface.  
         [0014]     Another option for the silicon substrate is to use a polysilicon substrate. The polysilicon may be deposited on another wafer, e.g. a refuse wafer, as described in the patent application referenced above. Polysilicon can have relatively high resistivity, a property desired for the PD substrate. Polysilicon is characterized by a grain structure, wherein the layer or body is comprised of many grains of silicon, separated by grain boundaries. The grain boundaries behave electrically as recombination centers, drastically reducing the lifetime of free carriers in the body. In terms of electrical behavior, this characteristic separates polysilicon from single crystal silicon. While single crystal silicon is a semiconductor, the large number of grain boundaries in polysilicon make it, in an undoped or intrinsic state, an insulator. The resistivity of polysilicon is a partly a function of the number of grain boundaries, or the fineness of the grain structure. Thus very fine grain polysilicon may have very high resistivity. Polysilicon may easily be produced with a resistivity of more than 10 KOhm-cm. In the context of the invention, resistivity values of more than 0.1 KOhm-cm, and preferably more than 1 KOhm-cm are desired. The polysilicon may be a layer produced, for example, by CVD (as described in the referenced application). Other approaches may be found useful. For example, methods are known for e-beam evaporation of polysilicon. Any suitable alternative for forming a thick, wide area, low resistivity, polysilicon substrate or substrate layer is suitable for these embodiments of the invention. In processing the polysilicon wafer for an IPD substrate, it is preferred that a layer of silicon dioxide be grown or deposited on the surface of the polysilicon wafer.  
         [0015]     Yet another option is to use conventional single crystal silicon wafers. In this case also, a thick layer of silicon dioxide is first grown or deposited on the surface of the substrate. Advances in silicon technology show promise of single crystal wafers that have inherently high resistivity. These may be especially suitable as substrates for IPDs.  
         [0016]     In this description, the use of the term silicon as an IPD substrate material should be interpreted as generic to all of the options just described.  
         [0017]     Although silicon wafers are physically thin (e.g. 200-500 microns), they are relatively robust physically, and can be handled and processed. They are very flat over a large area, and typically have a highly polished uniformly smooth surface. And, importantly, they are compatible with silicon wafer fabrication processes and tools.  
         [0018]     As mentioned above, the preferred IPD production approach is wafer scale fabrication. In this approach, a large number of finished, or nearly finished, devices are produced on the silicon wafer. After fabrication is essentially complete, the wafer is diced into IPD devices. As mentioned earlier, as the size of wafers increases wafer level fabrication becomes more attractive. For example, a twelve-inch wafer is capable of providing more than 500 device sites. Each site is approximately a centimeter square, easily large enough to accommodate an IPD.  
         [0019]     The effectiveness of wafer scale fabrication can be multiplied using thin film fabrication approaches for forming the passive devices. A common prior art approach, even at the wafer level, is to mount and attach discrete passive elements to the wafer substrate. Typically this is done using surface mount technology (SMT). The circuit may be a hybrid circuit having an active portion and a passive portion. However, to implement the IPD of the invention, with an integrated, high performance, high rejection filter, thin-film fabrication techniques are preferred.  
         [0020]     Referring again to  FIG. 1 , integrated passive elements or combinations of interconnected passive elements are generically represented. The substrate  11  is provided with grown oxide layer  12 . The resistor comprises resistor body  13 , and contacts  14  and  15 . The capacitor comprises lower capacitor plate  16 , upper capacitor plate  18  and lower capacitor plate contact  17 . The inductor comprises spaced apart metal runners  19 .  
         [0021]     A feature of IPD technology is that elements of more than one component may be formed from the same metal level. For example, resistor contacts  14 ,  15 , and capacitor plate contact  17  may be formed using the same processing steps. Upper capacitor plate  18 , and one of the inductor spirals  19  (contacts not shown), may formed using the same steps. The structure is protected with polyimide layer  20 .  
         [0022]     The thin film passive elements may be formed by a variety of thin film techniques. These techniques are well developed and the specifics need not be reiterated here. See for example U.S. Pat. No. 6,075,691, issued Jun. 13, 2000, and U.S. Pat. No. 6,005,197, issued Dec. 21, 1999. The latter patent describes a multi-layer structure for PCBs, which could easily be adapted for the application described here. A convenient way of defining a thin film passive device is a passive device that is formed on a substrate using one or more layers, typically a plurality of layers, deposited on the substrate. In addition, as mentioned above, one or more elements of two or more components may be formed using the same processing steps.  
         [0023]     The mobile device of the invention has both cellular radio and Wi-Fi functional capability, and contains both cellular RF and Wi-Fi RF circuits. These circuits are part of the transceiver module. Simultaneous operation of both RF units in the same location imposes an RF compatibility issue between the two circuits. The standard band pass filter, used in conventional cellular phones to limit the transmission band of the carrier frequencies, is not adequate to prevent RF interactions between the cellular RF and the Wi-Fi RF signals. Thus according to one aspect of the invention the band pass filter has unusually high rejection characteristics, and specifications that are more demanding than for the band pass filter in a conventional cellular device. To meet these demands, the use of thin film processing for the filter elements is especially effective.  
         [0024]     An example of a suitable filter circuit is shown in  FIG. 2 . The basic building block in this filter circuit is shown in box  39 . It comprises capacitors (C)  27  and  31   a , and inductor (L)  28  connected to ground at  29 . In  FIG. 2  there are two repeating units shown, the other LC unit comprising capacitors  36  and  31   b , and inductor  33 . This LC combination may be used in repeating units to narrow the band pass and increase the out-of-band RF rejection of the filter.  
         [0025]      FIG. 3  shows a plan view of the high rejection filter schematically illustrated in  FIG. 2 . It shows a portion  21  of a larger IPD, as suggested by  FIG. 1 . Other passive components and units that are part of, for example, the transceiver, may be integrated on other parts of the same IPD. In the embodiment described by  FIG. 3, 22  is a ground plane formed at the first level of the thin film fabrication process. Openings in the ground plane are shown in white, at  23  for example. The LC components  24 ,  27 ,  28 ,  31   a ,  31   b , and  33  appear in  FIG. 3 , interconnected as shown in  FIG. 2 , but physically organized as shown in  FIG. 3 . Capacitors  31   a  and  31   b  are electrically equivalent to a single capacitor, which is a suitable alternative to the circuit and layout shown. Via connections from an upper level metal to the ground plane are shown at  25 ,  29 ,  34  and  38 . Crossovers (or alternatively crossunders) are indicated at  30  and  35 . The inductors in  FIG. 3  are shown as spirals with two inductor arms ( 19  in  FIG. 1 ) that are separated by insulating material.  
         [0026]     A variety of materials suitable for thin film processing are well known. As an example, aluminum metallization technology for electrodes, interconnections, and inductor spirals, is well developed. Copper is currently widely used for at least one of the metallization levels, typically the last level that inteconnects to the “outside”. Newer technologies use copper at all levels. TaSi is a useful thin film resistor body material. Silicon nitride is a useful thin film capacitor dielectric. Polyimide is widely used as the passivating material. For flip-chip bonding of an RF IC chip to the IPD, solder bonding to a suitable under bump metallization is useful. All of these choices are well known, and the techniques for employing them are well developed.  
         [0027]     It will be understood that  FIG. 3  shows but one suitable layout. Also, the thin film filter of  FIGS. 2 and 3  is only one of many possible thin film filter designs. The vitalizing aspect shown by  FIGS. 2 and 3  is the integration of a high rejection band pass filter on an IPD with at least the filter element of the IPD, and preferably the passive components associated with the filter element, comprising thin film devices. As mentioned above, when the LC components of the filter comprise thin film elements, they can be manufactured at relatively low cost to meet demanding specifications and tolerances.  
         [0028]     While this description focuses on passive components in the IPD it should be understood that the IPD may carry, in addition to the passive components and the band pass filter, integrated circuit chips with active elements. These may be mounted using any suitable interconnection method, for example, flip-chip bonding or surface mounting. In the usual case these may comprise an RF IC chip, or an RF IC chip and a digital IC chip, e.g. a baseband digital processor.  
         [0029]     It will be appreciated that silicon processing tools may be used to produce the IPD. When the metal layers that comprise the passive elements and the interconnections are photodefined, or defined by other suitable thin film techniques that produce precise dimension control of the patterned layers, the electrical characteristics and performance of the resulting devices will be superior to those manufactured using conventional thick film and discrete hybrid technology. Photodefined elements are most typically formed using substractive techniques, i.e. blanket layer deposition followed by selective removal of unwanted portions of the blanket layer. The unwanted portions are typically removed by forming a mask using lithography, and etching the blanket layer using the mask to define the pattern. This approach produces the precision patterns desired for high performance filter elements:  
         [0030]     The filter specifications in the following table illustrate the capabilities of the invention in meeting demanding requirements of a high rejection bandpass filter suitable for cellular Wi-Fi combination devices.  
                                                                                                     CONVENTIONAL REJECTION FILTER                Frequency Band                GSM850   GSM900   KPCS1700   GSM1800   PCS1900   UMTS2100                        Rejection   48   44   15   12   8   4       (dB)                  
 
         [0031]    
       
         
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
               
               
                 HIGH REJECTION BANDPASS FILTER 
               
             
          
           
               
                   
                 Frequency Band 
               
             
          
           
               
                   
                 GSM850 
                 GSM900 
                 KPCS1700 
                 GSM1800 
                 PCS1900 
                 UMTS2100 
               
               
                   
                   
               
             
          
           
               
                 Rejection 
                 53 
                 44 
                 43 
                 46 
                 48 
                 41 
               
               
                 (dB) 
               
               
                   
               
             
          
         
       
     
         [0032]     As is evident from these tables, the bandpass filter fabricated for this invention has high rejection in the GSM cellular bands as well as high rejection in the Wi-Fi bands at 1700 and above. A useful specification for the filter of this invention is one that provides at least 30 dB of rejection at at least one frequency band above 1600, and preferably one that provides at least 30 dB of rejection in the KPCS1700 band, the GSM1800 band, the PCS1900 band, and the UMTS2100 band.  
         [0033]     As mentioned earlier, an important aspect of the mobile device with combined cellular and Wi-Fi RF circuits is that the two circuits, basically two radios, are designed to be operated at the same time. The mobile device is therefore equipped with switching elements that enable the transceivers of both radios to operate simultaneously.  
         [0034]     The mobile device of the invention has been described has having simultaneous cellular radio and Wi-Fi radio transceiver operation. However, the mobile device of the invention may comprise other radio combinations, for example, two cellular radios operating simultaneously on different frequency bands. Thus reference to two or more radios will refer to any combination of two or more RF circuits operating simultaneously in a single mobile device, but at different frequencies.  
         [0035]     Additional information about devices made according to the invention may be found at:  
         [0036]     http://www.sychip.com/PUB/IPD.pdf  
         [0037]     http://www.sychip.com/PUB/WLAN6100EB.pdf  
         [0038]     http://www.sychip.com/PUB/WLAN6061EB.pdf  
         [0039]     These are incorporated by reference herein for more details on devices of the invention.  
         [0040]     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.