Patent Publication Number: US-6661325-B2

Title: Spiral inductor having parallel-branch structure

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inductor used in a semiconductor integrated circuit (IC), and more particularly, to a spiral inductor having a parallel-branch structure. 
     2. Description of the Related Art 
     FIG. 1 is a perspective view showing an example of a conventional spiral inductor and FIG. 2 is a plan view of the conventional spiral inductor shown in FIG.  1 . 
     Referring to FIGS. 1 and 2, the spiral inductor  100  includes a first metal line  110  and a second metal line  120 . Although not shown, the first and second metal lines  110  and  120  are vertically spaced apart from each other by an insulating layer (not shown) and are connected to each other by a via contact  130  passing through the insulating layer. The second metal line  120  disposed over the insulating layer spirally turns inward from the outer periphery to the center. 
     Since there is no inductance between the first and second metal lines  110  and  120  in the above-described spiral inductor  100 , the number, shape and size of the second metal line  120  must be changed in order to increase the overall inductance. In this case, however, an increase in the size of the inductor is resulted, reducing the overall integration level. Also, when the inductor has a predetermined area or greater, the overall inductance is not increased any longer due to an increase in the parasitic capacitance between the inductor and the underlying substrate. Also, the quality (Q) factor of the inductor is sharply decreased due to parasitic capacitance components with respect to the substrate of the first and second metal lines  110  and  120 , which makes it impossible for the inductor to function properly. Further, the maximum Q factor of the inductor is not generated at a desired frequency but is generated at a predetermined frequency. 
     FIG. 3 is a perspective view showing another example of a conventional spiral inductor and FIG. 4 is a plan view of the conventional spiral inductor shown in FIG.  3 . 
     Referring to FIGS. 3 and 4, a spiral inductor  200  includes a first metal line  210  and a second metal line  220  vertically spaced apart from each other by an insulating layer (not shown). The first and second metal lines  210  and  220  are connected to each other through a via contact  230 . Here, at least two first metal lines  210  connected to the via contact  230  are disposed to be parallel. Thus, in addition to the inductance due to the second metal line  220 , mutual conductance between the parallel first metal lines  210  is also generated, thereby increasing the overall inductance. Also, a decrease in the overall area of the first metal lines  210  reduces a parasitic capacitance between the inductor and the underlying substrate, leading to an increase in Q-factor. In addition, symmetric arrangement of metal lines facilitates an architecture work of a circuit. 
     In this case, however, although the overall capacitance is rather increased, the increment in capacitance is negligible. Also, the maximum Q factor is still exhibited at a specific frequency rather than a desired frequency. 
     Further, various methods of increasing the cross-sectional areas of metal lines have been proposed, including, for example, making a metal line thicker by further providing the plating step, making a three-dimensional shape using bonding wires, forming multiple-layer metal lines of 3 or more layers to then connect the second and third metal lines through many via contacts, and so on. These methods have several manufacturing disadvantages, for example, a lack in reproducibility, a lack in compatibility with silicon based semiconductor processes, an increase in manufacturing cost, a prolonged manufacturing time and so on. 
     SUMMARY OF THE INVENTION 
     To solve the above-described problems, it is an object of the present invention to provide a spiral inductor having a parallel-branch structure which can be controlled to generate the maximum Q-factor at a desired frequency while increasing the overall inductance and Q-factor without increasing the area occupied by metal lines. 
     To accomplish the above object, there is provided a spiral inductor having a lower metal line and an upper metal line with an insulating layer interposed therebetween, the lower and upper metal lines being connected to each other through a via contact passing through the insulating layer, wherein the upper metal line spirally turns inward from the periphery to the center, and the lower metal line includes a first lower metal line crossing the upper metal line and disposed to be parallel with another adjacent first lower metal line, and a second lower metal line disposed to be parallel with the upper metal line. 
     Preferably, the first lower metal line is relatively shorter than the second lower metal line. 
     The upper and lower metal lines may be electrically parallel connected to each other through the via contact. 
     The area of the lower metal line is preferably determined by a predetermined frequency at which the maximum Q-factor is exhibited. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above object and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which: 
     FIG. 1 is a perspective view of a conventional spiral inductor; 
     FIG. 2 is a plan view of the conventional spiral inductor shown in FIG. 1; 
     FIG. 3 is a perspective view of another conventional spiral inductor; 
     FIG. 4 is a plan view of the conventional spiral inductor shown in FIG. 3; 
     FIG. 5 is a perspective view of a spiral inductor having a parallel-branch structure according to the present invention; and 
     FIG. 6 is a plan view of the spiral inductor shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiment set forth herein. 
     FIG. 5 is a perspective view of a spiral inductor having a parallel-branch structure according to the present invention, and FIG. 6 is a plan view of the spiral inductor shown in FIG.  5 . 
     Referring to FIGS. 5 and 6, a spiral inductor  500  according to the present invention includes a lower metal line  510  and an upper metal line  520 . The lower and upper metal lines  510  and  520  are disposed so as to be vertically spaced apart from each other by an insulating layer (not shown) and to be electrically connected to each other through a via contact  530 . Here, the lower metal line  510  and the upper metal  520  are electrically parallel connected to each other. 
     The upper metal line  520  is spirally wound inward from the periphery to the center. The spiral upper metal line  520  may have various shapes such as rectangle, circle or other polygons. 
     The lower metal line  510  includes a first lower metal line  511  and a second lower metal line  512 . The first lower metal line  511  crossing the upper metal line  520  is disposed to be parallel with another adjacent first lower metal line  511 , and the second lower metal line  512  is disposed to be parallel with the upper metal line  520 . The second lower metal line  512  is not perfectly parallel with the upper metal line  520  and may be disposed so that a current flow direction is at an acute angle of less than 90° with respect to the upper metal line  520 . The first lower metal line  511  is shorter than the second lower metal line  512 . 
     The overall inductance of the above-described spiral inductor is the sum of a self inductance of the upper metal line  520 , a mutual inductance between adjacent first lower metal lines  511  and a mutual inductance between the upper metal line  520  and the second lower metal line  512  disposed in parallel. Thus, according to the preset invention, the Q-factor increasing in proportion to the overall inductance increases, in contrast with the conventional case. Since the upper metal line  520  and the lower metal line  510  are electrically parallel connected, metal line resistance is greatly reduced at a parallel-branch portion, thereby compensating for a parasitic capacitance between the lower metal line  510  and a substrate (not shown) and a reduction in Q-factor. Also, the parasitic capacitance caused by the lower metal line  510  can be adjusted by adjusting the area where the second lower metal line  512  and the upper metal line  520  are parallel to each other. Thus, the frequency band at which the maximum Q-factor, which is inversely proportional to the resistance and capacitance, is exhibited, can be adjusted to a desired frequency band. In some cases, the frequency band can be adjusted by adjusting the line width, length and interval of the lower metal line  510  instead of the area. 
     As described above, in the spiral inductor having a parallel-branch structure according to the present invention, some lower metal lines are disposed to be parallel to each other and the other lower metal lines are disposed to be parallel to an upper metal line to generate a mutual inductance between the lower metal lines and a mutual inductance between the lower metal lines and the upper metal line, thereby increasing the overall inductance, leading to an increase in the Q-factor. Also, a frequency band at which the maximum Q-factor is exhibited can be arbitrarily determined adjusted by adjusting the area occupied by the lower metal lines and the upper metal line which are disposed parallel to each other.