Abstract:
At high frequencies, signal losses may occur in circuit designs employing magnetic isolators. Eddy current losses in the magnetic isolator substrate material are at least partially responsible for this signal loss. As the Eddy current losses may depend on the properties of the substrate, the type of substrate chosen for fabricating a magnetic isolator may be critical for reducing these losses. By fabricating the magnetic isolator on a high performance substrate, the Eddy current losses are reduced and the magnetic isolator provides better output signals at high frequencies.

Description:
FIELD  
         [0001]    The present invention relates generally to substrates, and more particularly, relates to a high-performance substrate for use with magnetic isolators.  
         BACKGROUND  
         [0002]    Many electronic applications require some form of signal isolation. Signal isolation enables digital or analog signals to be transmitted without a galvanic connection between the transmitting and receiving side of the circuit. Signal isolation may prevent unwanted current and ground loops, damage to equipment, and injury to humans.  
           [0003]    Opto-couplers and transformers are commonly used to provide signal isolation. Opto-couplers use light to couple two electrically isolated circuits. Opto-couplers may require custom package manufacturing, which may increase the cost of producing these devices. Additionally, while the use of opto-couplers for signal conditioning generally works well in digital signal isolation applications, the same does not hold true for analog signal isolation applications. Analog signals are typically isolated using transformers. However, transformers are bulky and ill suited for many circuit applications.  
           [0004]    Due to problems encountered with these conventional signal isolation devices, the use of magnetic isolators in signal isolation applications has become more common. Magnetic isolators may be less expensive to manufacture than opto-couplers and consume less real estate than transformers. An example of a magnetic isolator can be found in commonly assigned U.S. Pat. No. 6,376,933, which is fully incorporated by reference. Magnetic isolators are typically formed on a bulk silicon substrate.  
           [0005]    Unfortunately, in high frequency isolator applications, there may be a substantial signal loss due to substrate related Eddy currents. Eddy currents are electric currents produced inside a loop when the loop experiences a change in the magnetic flux through its surface or moves through a non-uniform magnetic field. Eddy current losses are energy losses due to eddy currents circulating in a resistive material.  
           [0006]    Therefore, it would be beneficial to reduce the substrate related Eddy current losses in a magnetic isolator so that magnetic isolators may be used in high frequency isolator applications, especially those applications requiring operation in the gigahertz range. A high performance substrate may reduce the substrate related Eddy current losses in a magnetic isolator.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    Presently preferred embodiments are described below in conjunction with the appended drawing figures, wherein like reference numerals refer to like elements in the various figures, and wherein:  
         [0008]    [0008]FIG. 1 is a circuit diagram of a typical magnetic isolator, according to an exemplary embodiment;  
         [0009]    [0009]FIG. 2 is a cross-sectional diagram of a typical magnetic isolator formed on a bulk silicon substrate; and  
         [0010]    [0010]FIG. 3 is a cross-sectional diagram of a typical magnetic isolator formed on a high performance substrate, according to an exemplary embodiment.  
     
    
     DETAILED DESCRIPTION  
       [0011]    A high performance substrate may be used to reduce substrate related Eddy current losses in a magnetic isolator. While a typical magnetic isolator description is provided to describe the high performance substrate, this invention is not limited to any particular magnetic isolator design. It may be useful to describe the function of a typical magnetic isolator in order to describe how Eddy current losses occur and how the high performance substrate may be used to reduce these losses.  
         [0012]    [0012]FIG. 1 is a circuit diagram of a typical magnetic isolator  100 . The magnetic isolator  100  includes an input source signal  102 , a coil  104 , and a magneto-resistive magnetic field sensor  106 . The input signal source  102  supplies an input signal to the coil  104 , which generates an input magnetic field. The magneto-resistive magnetic field sensor  106  senses the input magnetic field and provides an output signal  108  that is proportional to the input signal.  
         [0013]    [0013]FIG. 2 is a cross sectional diagram of a typical magnetic isolator  200 , similar to the one depicted in FIG. 1. The magnetic isolator  200  may be formed on a substrate layer  202 . Typically the substrate layer  202  is a bulk silicon substrate material.  
         [0014]    The magnetic isolator  200  includes a coil layer  204 , a sensor layer  206 , and a plurality of insulating layers. The coil layer  204  may substantially form the coil  104  as shown in FIG. 1. The sensor layer  206  may substantially form the magneto-resistive magnetic field sensor  106  as shown in FIG. 1. The magnetic isolator  200  may also include a first metal layer  208  and a second metal layer  210 .  
         [0015]    The coil layer  204  and the metal layers  208 ,  210  may be composed of a metal. For example, the layers  204 ,  208 ,  210  may be composed of aluminum, gold, copper, or tungsten. Other metals may also be used. The shape of the coil layer  204  may depend on coil configuration. For example, the coil  104  may be in a configuration determined by the number of turns, such as an eight-turn coil, or in a serpentine strip configuration.  
         [0016]    The sensor layer  206  may include a plurality of layers. Some of the layers may be composed of ferromagnetic materials, while other layers may be composed of anti-ferromagnetic materials. The choice of layer materials and the ordering of the layers may depend on the type of magneto-resistive magnetic field sensor used in the magnetic isolator  200 . For example, a giant magneto-resistive (GMR) sensor may include two magnetic layers separated by a non-magnetic conducting layer.  
         [0017]    A first insulating layer  212  may be located substantially between the coil layer  204  and the second metal layer  210 . A second insulating layer  214  may be located substantially between the second metal layer  210  and the sensor layer  206 . A third insulating layer  216  may be located substantially between the sensor layer  206  and the substrate layer  202 .  
         [0018]    The insulating layers  212 ,  214 ,  216  may be composed of silicon nitride or other appropriate insulating material. The thickness of the first insulating layer  212  may determine the breakdown voltage of the magnetic isolator  200 . Typically, a thicker first insulating layer  212  will result in a higher breakdown voltage of the magnetic isolator  200 .  
         [0019]    Eddy currents may develop in the coil layer  204  as the magnetic field in the coil  104  changes. Additionally, the Eddy currents may develop in the second metal layer  210 . For example, if the second metal layer  210  is used for magnetic sensor condition or initialization the layer may be shaped in a coil configuration, which may allow Eddy currents to develop. The Eddy currents flow in a direction opposite to the direction of the magnetic field. The Eddy currents may cause substrate related Eddy current losses. The substrate related Eddy current losses reduce the magnitude of the magnetic fields generated by the coil  104 , which results in signal loss.  
         [0020]    The amount of Eddy current generated is inversely proportional to material resistivity. Therefore, the substrate material chosen for the substrate layer  202  may be critical for reducing Eddy current losses. For example, a substrate material with low conductivity may reduce the Eddy current losses and ultimately, reduce signal losses.  
         [0021]    [0021]FIG. 3 shows the formation of a typical magnetic isolator  300  on a high performance substrate. In this example, a silicon-on-insulator (SOI) substrate is used; however, other semi-insulated or insulated substrates may also be used. The semi-insulated substrates may be a high-rho silicon substrate having a resistivity in the range of 1 K-ohm-cm or higher. The insulated substrates may be a substrate made with ceramic, glass, gallium arsenide (GaAs), or silicon carbon (SiC).  
         [0022]    In this example, the magnetic isolator  300  is fabricated on an SOI substrate. An SOI substrate includes a buried oxide layer  304  over a silicon substrate layer  302 . A top silicon layer  306  is located above the buried oxide layer  304 . The buried oxide layer  304  may provide electrical insulation between the silicon substrate layer  302  and the top silicon layer  306 . The remaining fabrication steps of the magnetic isolator  300  may be unchanged from the fabrication steps of the magnetic isolator  200 .  
         [0023]    By fabricating the magnetic isolator  300  on the SOI, the substrate  302  may be substantially isolated from the source of the Eddy currents, namely the coil  104 . Alternatively, the use of the semi-insulated or insulated substrates may also substantially limit the Eddy currents from penetrating into the substrate layer  302 . The substrate related Eddy current losses may be reduced, if not eliminated, by using the high performance substrate. The high performance substrate may be especially beneficial in high frequency magnetic isolator applications because Eddy current losses increase with frequency. In addition, by using a high performance substrate, power consumption of the magnetic isolator may be reduced.  
         [0024]    It should be understood that the illustrated embodiments are exemplary only and should not be taken as limiting the scope of the present invention. The claims should not be read as limited to the described order or elements unless stated to that effect. Therefore, all embodiments that come within the scope and spirit of the following claims and equivalents thereto are claimed as the invention.