Abstract:
An inductor formed on a semiconductor substrate, comprising a coil formed with at least a single metal layer having a plurality of slots and an insulator layer filled in the plurality of slots, wherein the insulator layer is encompassed in the single metal layer and the insulator layer does not cover the top surface of the single metal layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a divisional of an application Ser. No. 11/468,789, filed on Aug. 31, 2006, now allowed for issuance as a patent. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of Invention 
         [0003]    The present invention relates to an inductor fabricated on a semiconductor substrate. More particularly, the present invention relates to an inductor formed with insulator slots inside. 
         [0004]    2. Description of Related Art 
         [0005]    Inductor is widely used for radio frequency (RF) application. The application covers a very broad spectrum. Some applications, such as personal handy-phone system (PHS), require an inductor to work with frequencies below 1 GHz. Accordingly, the maximum quality factor (abbreviated as Q hereinafter) of the inductor has to move into the sub-GHz range. 
         [0006]    For an inductor to have acceptable inductance and maximum quality factor below 1 GHz, a conventional solution is stacking metal layers in parallel. However, this solution increases sidewall capacitance coupling and consequently degrades the broadband performance of the quality factor. Another solution is widening the coil of the inductor, but an over-wide coil tends to violate chemical-mechanical polishing (CMP) rules. In addition, although widening the coil lowers the resistance and improves the maximum quality factor, it also significantly accentuates the variation in the fabrication process. 
         [0007]    The maximum quality factor of an inductor should be as high as possible. Therefore it is desirable to have an inductor which works properly with a frequency less than 1 GHz and is without the drawbacks of conventional solutions. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is to provide an inductor formed on a semiconductor substrate. The inductor provided by the present invention can be widened without violating CMP rules. In addition, the inductor provided by the present invention features acceptable inductance and maximum quality factor comparable to that of conventional inductors under a lower operation frequency (for example, a sub-GHz frequency). This inductor also alleviates the variation in the fabrication process. 
         [0009]    For the purposes mentioned above, an inductor formed on a semiconductor substrate is provided in an embodiment of the present invention. The inductor comprises a single metal layer and an insulator layer. The metal layer constitutes the coil of the inductor. The insulator layer comprises at least one insulator slot, and each insulator slot is encompassed in the metal layer. 
         [0010]    In this embodiment, the insulator slot may be formed using silicon oxide. The insulator layer may comprise a row of insulator slots arranged along the coil of the inductor. 
         [0011]    For the purposes mentioned above, an inductor formed on a semiconductor substrate is provided in another embodiment of the present invention. The inductor comprises a plurality of metal layers and an insulator layer. The metal layers constitute the coil of the inductor. The insulator layer comprises at least one insulator slot, and each insulator slot is encompassed in one the metal layers. 
         [0012]    In this embodiment, the insulator slot may be formed using silicon oxide. The insulator layer may comprise a row of insulator slots arranged along the coil of the inductor. 
         [0013]    In this embodiment, each insulator slot may be encompassed in the same one of the metal layers. Alternatively, the insulator layer may comprise a plurality of insulator slots, and the insulator slots may be distributed in each one of the metal layers. 
         [0014]    In this embodiment, the metal layers may be connected in parallel to constitute the coil of the inductor. Alternatively, the metal layers may be connected in series to constitute the coil of the inductor. 
         [0015]    For the purposes mentioned above, an inductor formed on a semiconductor substrate is provided in another embodiment of the present invention. The inductor comprises a coil formed with at least a single metal layer having a plurality of slots and an insulator layer filled in the plurality of slots, wherein the insulator layer is encompassed in the single metal layer and the insulator layer does not cover the top surface of the single metal layer. 
         [0016]    For the purposes mentioned above, an inductor formed on a semiconductor substrate is provided in another embodiment of the present invention. The inductor comprises a coil formed with at least a single metal layer having a plurality of slots and an insulator layer filled in the plurality of slots, wherein the insulator layer is encompassed in the single metal layer and the insulator layer does not cover the top and the bottom surface of the single metal layer. 
         [0017]    For the purposes mentioned above, an inductor formed on a semiconductor substrate is provided in another embodiment of the present invention. The inductor comprises a coil formed with at least one single metal layer having a plurality of slots and an insulator layer filled in the plurality of slots, wherein the insulator layer is encompassed in the single metal layer and the top surface of the insulator layer and the single metal layer is substantially coplanar. 
         [0018]    In order to make the aforementioned and other objects, features and advantages of the present invention comprehensible, a preferred embodiment accompanied with figures is described in detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
           [0020]      FIG. 1  is a schematic diagram showing a conventional inductor formed on a semiconductor substrate. 
           [0021]      FIGS. 2A and 2B  are schematic diagrams showing an inductor according to an embodiment of the present invention. 
           [0022]      FIGS. 3A and 3B  are schematic diagrams showing an inductor according to an embodiment of the present invention. 
           [0023]      FIGS. 4A and 4B  are schematic diagrams showing an inductor according to an embodiment of the present invention. 
           [0024]      FIGS. 5A and 5B  are schematic diagrams showing an inductor according to an embodiment of the present invention. 
           [0025]      FIG. 6 ,  FIG. 7  and  FIG. 8  are schematic diagrams showing inductors according to various embodiments of the present invention. 
           [0026]      FIG. 9  is a plot of inductance versus frequency of a conventional inductor and an inductor according to an embodiment of the present invention. 
           [0027]      FIG. 10  is a plot of quality factor versus frequency of a conventional inductor and an inductor according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
         [0029]      FIG. 1  illustrates a conventional inductor  100  fabricated on a semiconductor substrate (not shown). Inductor  100  is formed purely with metal, without any insulator material. In contrast,  FIG. 2A  illustrates an inductor  200  according to an embodiment of the present invention. Inductor  200  is a metallic inductor with insulator slots embedded inside, which is also formed on a semiconductor substrate (not shown). Inductor  200  comprises a single metal layer and an insulator layer. The metal layer constitutes the coil of the inductor  200 . The insulator layer comprises at least one insulator slot, and each insulator slot is encompassed in the metal layer.  FIG. 2B  is an enlarged view of the lower left corner of inductor  200 . There is a row of insulator slots arranged along the entire length of the coil of inductor  200 . For example, each of insulator slots  201  and  202  is encompassed inside the metal layer. Each insulator slot  201  is a slim rectangular in shape, while insulator slot  202  is L-shaped. The insulator slots may be formed using silicon oxide (for example, SiO 2 ). 
         [0030]    In the scope of the present invention, the insulator slots are not limited to a single-row arrangement. Insulator slots may be arranged into multiple rows. The insulator slots of different rows may be placed in parallel with one another or be placed in an alternate fashion. There is no limitation to the relative positions of the insulator slots. There is also no limitation to the number of rows. For example,  FIG. 3A  illustrates an inductor  300  according to another embodiment of the present invention. There are two rows along the coil of inductor  300 .  FIG. 3B  is en enlarged view of the lower left corner of inductor  300 . As can be seen, one of the rows includes the exemplary insulator slots  301  and the other row includes the exemplary insulator slots  302 . Similarly,  FIGS. 4A and 4B  illustrate an inductor  400  with three rows of insulator slots according to another embodiment of the present invention.  FIGS. 5A and 5B  illustrate an inductor  500  with four rows of insulator slots according to yet another embodiment of the present invention. 
         [0031]    The arrangement of insulator slots is quite flexible in the scope of the present invention. Each insulator slot may be formed in arbitrary size and shape. The number and arrangement of insulator slots are also arbitrary. For example, insulator slots may be distributed either evenly or unevenly in the coil the inductor. In other words, insulator slots may be distributed either symmetrically or asymmetrically in the coil the inductor.  FIG. 6  is an example of uneven distribution. Insulator slots are embedded only in sections  601  and  602  of the coil of inductor  600 . However, it is preferred that insulator slots have an even and symmetrical distribution in the inductor coil. Furthermore, there is no limit regarding the position of an insulator slot relative to the boundary surface of the inductor. An insulator slot may be at arbitrary distance from any boundary surface of the containing metal layer. An insulator slot may be completely embedded in the metal layer, in other words, surrounded in every direction by the metal layer. Alternatively, an insulator slot may be partially embedded in the metal layer. In such a case, an insulator slot may emerge from one or more boundary surfaces of the metal layer. 
         [0032]    In the above embodiments, each inductor comprises a single metal layer. The present invention also provides inductors comprising multiple metal layers. The metal layers constitute the coil of such an inductor.  FIG. 7  illustrates an inductor  700  according to an embodiment of the present invention. As shown in  FIG. 7 , inductor  700  may comprise three, four or five metal layers. The metal layers (for example,  701 ) are stacked in parallel and are connected by vias (for example,  702 ). In contrast,  FIG. 8  illustrates another inductor  800  according to another embodiment of the present invention. Three metal layers  801 - 803  are connected in series by vias  811  and  812  to constitute the coil of inductor  800 . 
         [0033]    For simplicity, insulator slots are not shown in  FIG. 7  and  FIG. 8 . In fact, each of inductors  700  and  800  comprises at least an insulator layer. The insulator layer comprises at least one insulator slot, and each insulator slot is encompassed in one the metal layers. The distribution of insulator slots among the metal layers is quite flexible in the scope of the present invention. For example, each insulator slot may be encompassed in the same metal layer. Besides, the insulator slots may be distributed in each of the metal layers, or be distributed only in part of the metal layers. 
         [0034]    As discussed above, there is no limitation regarding the sizes, shapes, positions, number, arrangement, distribution and other characteristics of the insulator slots. The only restriction is that the inductor has to comply with CMP rules and the maximum quality factor of the inductor has to be in the desired low frequency range (for example, below 1 GHz). 
         [0035]      FIG. 9  and  FIG. 10  illustrate the effect of an embodiment of the present invention.  FIG. 9  is a plot of inductance versus frequency of inductor  100  in  FIG. 1  and inductor  600  in  FIG. 6 . Curve  901  in  FIG. 9  is the plot of inductor  100 , while curve  902  is the plot of inductor  600 .  FIG. 10  is a plot of quality factor versus frequency of inductor  100  and inductor  600 . Curve  1001  in  FIG. 10  is the plot of inductor  100 , while curve  1002  is the plot of inductor  600 . Inductor  100  is formed using metal only, with a coil width of 10 micrometers and an inner diameter of approximately 200 micrometers. Inductor  600  is formed using metal and insulator slots, with a coil width of 20 micrometers and an inner diameter of approximately 200 micrometers. As shown in  FIG. 10 , this embodiment of the present invention moves the maximum quality factor from the conventional operation frequency of 2.488 GHz to the desired lower frequency of 0.981 GHz. Furthermore, the maximum quality factor of inductor  600  does not decay too much from that of inductor  100 . 
         [0036]    In addition to inductor structures as discussed in the embodiments above, the present invention also provides a method for forming an inductor on a semiconductor substrate. In fact, the method is for forming inductors in the embodiments above. The major steps of the method are forming one or more metal layer on the semiconductor substrate, and then forming an insulator layer on the semiconductor substrate. The metal layers constitute the coil of the inductor. The insulator layer comprises at least one insulator slot, and each insulator slot is encompassed in one the metal layers. The technical details of the method are the same as those of the inductor structures discussed in the previous embodiments so they are not repeated here. 
         [0037]    Thanks to the insulator slots, the coil of the inductor provided by the present invention can be widened without violating CMP rules. In addition, the inductor provided by the present invention features acceptable inductance and maximum quality factor comparable to that of conventional inductors under a lower operation frequency (for example, a sub-GHz frequency). 
         [0038]    The present invention has been disclosed above in the preferred embodiments, but is not limited to those. It is known to persons skilled in the art that some modifications and innovations may be made without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be defined by the following claims.