HIGH Q INTEGRATED INDUCTOR

Abstract: A high value inductor (10) with a high Q factor is formed using integrated circuit techniques to have a plurality of layers (m1-m3), where each layer has formed on it two or more coils. The coils in the various layers are interconnected in series. Although the resulting inductor exhibits a relatively high resistance, the number of coil turns is large. Since inductance increases in proportion to the square of the number of coil turns, the resulting inductor has a very high Q factor.

Description: HIGH Q INTEGRATED INDUCTOR FIELD OF THE INVENTION This invention relates to inductors and, in particular, to an inductor formed using integrated circuit processing techniques. BACKGROUND OF THE INVENTION Inductors are frequently formed in integrated circuits; however, given the inherent limitations of integrated circuit technology, it is difficult to form a high value inductor. An inductor is generally created by forming a conductive coil around a core. The core may be an insulator or a magnetic core. Magnetic cores result in greater inductance values but are impractical to form in many types of integrated circuit. The inductance value is also greatly affected by the number of turns of the coil, where the inductance value is proportional to the square of the number of turns of the coil. Inductance value is also affected to a lesser extent by the radius of the coil and other well known factors. Various methods have been used in an attempt to obtain high inductance values. Two such methods are described in U.S. Patent No. 5,227,659 by Hubbard, and U.S. Patent No. 5,095,357 by Andoh, et al, both patents incorporated herein by reference. In the '375 patent, it is disclosed that a high value inductor may be formed by two substantially flat spirals of metal, either arranged side-by-side or separated by an insulating layer, where an end of one flat spiral is connected to an end of the other flat spiral using an interconnection layer. Such a technique has certain drawbacks. One of the drawbacks is that the substantial length of the flat spirals may result in some destructive interference, due to phase opposition, in high frequency signals through the spiral. Another drawback is that the interconnection layer requires the formation of additional insulating layers and metal layers yet adds little or nothing to the inductance value. In the , 659 patent by Hubbard, a single, multi¬ level coil is described, where one coil turn is provided at each level. Hence, the inductors described in the '659 patent are limited to relatively few coil turns. For most applications of an inductor, such as in a resonant circuit (also known as a tank circuit) , the Q, or quality, factor of the inductor is important. The Q factor is the ratio of the reactance (X) of the inductor at a given frequency (f) to its DC resistance. The reactance of an inductor of value L is equal to 2πfL. Accordingly, it is desirable to improve upon the existing inductors formed using integrated circuit techniques to obtain a high value inductor with a high Q factor. SUMMARY OF THE INVENTION A high value inductor with a high Q factor is formed using integrated circuit techniques to have a plurality of layers, where each layer has formed on it two or more coils. The coils in the various layers are interconnected in series. Although the resulting inductor exhibits a relatively high resistance, the number of coil turns is large. Since inductance increases in proportion to the square of the number of coil turns, the resulting inductor has a very high Q factor. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a simplified perspective view of a three-layer embodiment of the inductor with two coils per layer. Fig. 2 is a cross-section of an integrated circuit structure bisecting the inductor of Fig. 1. Fig. 3 illustrates the inductor of Fig. 1 with the vias shown and input/output leads formed using the first metal layer. Fig. 4 illustrates the inductor of Fig. 1 with the vias shown and input/output leads formed by doped regions in a substrate. Fig. 5 is a simplified top-down view of the structure of Fig. 1 with the coils in the various layers expanded as necessary to illustrate the structure. Figs. 6 and 7 illustrate inductors not in accordance with the present invention but whose performance was compared to that of the inductor of Fig. l. Figs. 8-10 are plots of the actual metal patterns for an inductor having three layers and three coils per layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates a high value inductor 10 formed using three layers of insulation with two metal coils per layer. The coils may have a diameter of anywhere between a few tens of microns to a few thousand microns. The metal may be aluminum or other highly conductive material. The interconnections between the metal coils on different levels are shown as wires for simplicity, but in actuality the interconnections are formed using conductive vias extending through the insulating layers. The formation of vias is well known in the art. The separation between each layer is exaggerated to better illustrate the structure. In the embodiment shown in Fig. 1, a current i is supplied by an input lead 11 to a first end 12 of an inner coil ml. The designation ml connotes a first metal layer. The direction of this current i through each of the metal coils is shown by an arrow, such as arrow 14. It is important to note that the direction of current through all the coils is the same. This increases the inductance value of inductor 10 and avoids destructive interference of the signal as the current flows through the coils. A conductive via 16 connects a second end 18 of inner coil ml to a first end 20 of inner coil m2, overlying and insulated from inner coil ml. A second conductive via 22 connects a second end 24 of inner coil m2 to a first end 26 of inner coil m3 , overlying and insulated from inner coil m2. A second end 28 of inner coil m3 is connected by lead 30 to a first end 32 of outer coil m3 , formed on the same level, and at the same time, as inner coil m3. A conductive via 34 connects a second end 36 of outer coil m3 to a first end 38 of outer coil m2, formed on the same level, and at the same time, as inner coil m2. A conductive via 40 connects a second end 42 of outer coil m2 to a first end 44 of outer coil ml, formed on the same level, and at the same time, as inner coil ml. A second end 46 of outer coil ml is connected to an output lead 48. Output lead 48 and input lead 11 are connected to any circuit requiring use of an inductor. As seen, the six coils in Fig. 1 are effectively connected in series, where the serial connections are first made between the inner coils and then made between the outer coils. The inductor 10 of Fig. 1 may also be formed to have input lead 11 and output lead 48 connected to a top level of coils, which is easily visualized by turning Fig. 1 upside down. The structure of Fig. 1 could easily be modified to have each of the coils be rectangular or square, although circular coils generally provide a higher inductance value. The concepts described with respect to Fig. 1 may be applied to an inductor with two or more levels of coils, where the interconnections between the coils repeat for each level. In practical embodiments, these levels may range from 2 to 7 or more. Further, it can easily be seen how three or more coils per level can be serially connected to coils on different levels. For example, for a three coil per level structure, the innermost coils and the middle coils would be serially connected like the inner and outer coils in Fig. 1. The outermost coils would then be serially connected to the lead 48 in Fig. 1. In this case, an input lead would connect to the bottom layer, and the output lead would connect to the top layer. For a four (or any even number) coil per layer structure, the input and output leads would connect to the same level, which is desirable. Fig. 2 is a cross-section of an integrated circuit structure, including a substrate 60, which bisects the inductor 10 of Fig. 1. Circuitry connected to the inductor 10 would also be included on the substrate 60. Substrate 60 may be a semiconductor, such as silicon or gallium arsenide, or may be formed of an insulating material. In one method for forming the structure of Fig. 2, an insulating layer 62 of silicon dioxide or other suitable insulator is deposited on the surface of substrate 60. This step would not be necessary if substrate 60 were sufficiently insulating. A first layer of metal, such as aluminum, is then deposited on the insulating layer 62. Using conventional photolithographic and etching techniques, the metal layer is then patterned to form the inner coil ml and the outer coil ml shown in Fig. 1. The input lead 11 and output lead 48 in Fig. 1 are also formed at this time if such leads are formed on the same level as the coils ml. Such leads 11 and 48 are shown in Fig. 3. Fig. 3 also shows one embodiment of the various vias and other connections between the coils rather than the more abstract wiring of Fig. 1. Similarly numbered elements in Figs. 1 and 3 perform the same function. Note that in Fig. 3 there is no need for a cross-over for input lead 11 to extend beyond the boundary of outer coil ml. In another embodiment, shown in Fig. 4, vias 65 and 66 are formed through the insulating layer 62 to connect the ends of inner coil ml and outer coil ml to highly doped regions 67 and 68 formed in substrate 60 which extend to contact pads or to other circuitry. A silicide layer may be used to lower the resistivity of the regions 67 and 68. Those skilled in the art would understand the various ways which may be used to interconnect the inductor 10 with other circuits. Referring back to Fig. 2, a next insulating layer 70 is deposited over coils ml. The various layers of insulation are shown merged since they are the same material. Vias 16 and 40 (Figs. 3 and 4) are then formed through insulating layer 70 using conventional photolithographic and etching techniques to provide the interconnections between the subsequently formed coils m2 and the coils ml. The conductive material for the vias may be deposited at the same time that the metal for coils m2 is deposited. A second layer of metal is then deposited over the insulating layer 70 and patterned using conventional photolithographic techniques to form coils m2. A third insulating layer 72 is then deposited over coils m2, and vias 22 and 34 (Figs. 3 and 4) are suitably formed to connect the subsequently formed coils m3 to the coils m2. A third metal deposition and patterning step is used to form metal coils m3 , which are thus serially connected to coils ml and m2 in the manner shown in Fig. 1. Fig. 5 is a top-down view of the structure shown in Fig. 1 which has been slightly altered to cause the various coils to not overlap so they may be viewed from above. In Fig. 5, each separate coil is identified. Vias are indicated with dashed lines and are identified with an X or a Y. An X indicates a via between levels one and two, and a Y indicates a via between levels two and three. Connecting the coils of Fig. 1 in series, rather than in parallel, increases the DC resistance of the inductor; however, the series interconnection effectively results in the inductor coil having six turns. Since inductance is related to number of coil turns squared, the inductance of inductor 10 of Fig. 1 is relatively high. Table I below compares the qualities of the series-connected inductor 10 in Fig. 1, an inductor 76 (Fig. 6) formed as a single, flat coil, and an inductor 78 (Fig. 7) formed using a parallel coil configuration, where the inner coils are connected in parallel and the outer coils are connected in parallel. The physical size of the structures, the pitches between coils on the same level, the metal thickness and the test frequency are identified below Table I. Inductor type Resistance Inductance L/R Q Single coil 2OK ohms 1.9 mH .095 0.060 Fig. 6 Parallel coil 8.8K ohms .8 H .090 0.057 Fig. 7 Series coil 6OK ohms 53 mH .883 0.555 Fig. 1 (1) each test structure area = 1 mm 2 (2) ml = ιrι2 = m3 pitch = 2 um (3) ml = 6500 A, m2 = 6700 A, m3 = 8800 (4) test frequency = 100 KHz As seen, the resistance of the series-connected coil of Fig. 1 is much greater than the parallel coil and three times greater than the single coil, but the resulting inductance greatly offsets this increase in resistance. The resulting inductance is approximately 28 times that of the single coil. The resulting L/R value and Q factor are almost ten times that of the single coil and the parallel coil. The results of Table 1 are obtained from actual test results. The series-connected inductor described herein may be formed to have an inductance of virtually any value depending upon how many layers and how coils per layer are used. However, it is generally desirable to form the inductor having the same number of layers as the number of layers already used in the integrated circuit process for forming the remainder of the circuitry, such as an oscillator in a voltage controlled oscillator (VCO) circuit. The inductance value will also be limited by the available real estate on the die. The inductance values shown in Table I are for relatively large inductors formed for test purposes, and a typical value of an inductor in an actual integrated circuit, such as a VCO, will be on the order of tens or hundreds of nanohenrys. Figs. 8, 9, and 10 are plots for metal layers Ml, M2, and M3, respectively, of a three layer inductor having three coils per level. The via locations for connections between levels are identified with bars 90. It is to be understood that any coil shape or material used to form an inductor is within the scope of this invention and that the various methods of creating interconnections between the inductor coils and other circuitry on the chip would depend upon the particular application of the inductor. While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.

Claims: CLAIMS What is claimed is: 1. An inductor formed on a substrate using integrated circuit techniques, said inductor comprising: a first conductive coil and a second conductive coil formed on a first insulating surface, said first conductive coil being substantially formed within the boundaries of said second conductive coil; a third conductive coil and a fourth conductive coil formed on a second insulating surface overlying said first conductive coil and said second conductive coil, said third conductive coil being substantially formed within the boundaries of said fourth conductive coil, a first end of said first conductive coil being connected to a first end of said third conductive coil, a second end of said third conductive coil being connected to a first end of said fourth conductive coil, a second end of said fourth conductive coil being connected to a first end of said second conductive coil, so as to create a series combination of said first conductive coil, said third conductive coil, said fourth conductive coil, and said second conductive coil. 2. The inductor of Claim 1 further comprising a first conductive lead connected to a second end of said first conductive coil, and a second conductive lead connected to a second end of said second conductive coil, said first conductive lead and said second conductive lead being connected to terminals of a circuit. 3. The inductor of Claim 1 wherein said first conductive coil, said second conductive coil, said third conductive coil, and said fourth conductive coil are substantially circular. 4. The inductor of Claim 1 wherein said first conductive coil, said second conductive coil, said third conductive coil, and said fourth conductive coil are substantially rectangular. 5. The inductor of Claim 1 further comprising a fifth conductive coil and a sixth conductive coil formed on a third insulating surface, said fifth conductive coil and said sixth conductive coil underlying and being insulated from said first conductive coil and said second conductive coil, said fifth conductive coil being substantially within the boundaries of said sixth conductive coil, a first end of said fifth conductive coil being connected to a second end of said first conductive coil, a first end of said sixth conductive coil being connected to a second end of said second conductive coil. 6. An inductor comprising: a plurality of insulating levels; and a plurality of substantially flat conductive coils on each of said insulating levels, each of said coils having a first end and a second end, said conductive coils being electrically connected in series by conductive vias extending between adjacent ones of said insulating levels such that an inductance value of said inductor is generally proportional to the square of the number of said conductive coils. 7. The inductor of Claim 6 wherein each of said coils is substantially circular.

Inventors: ISSAQ ENAYET, MERRILL RICHARD B, ISSAQ, ENAYET, MERRILL, RICHARD, B.
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1996
Main classification: H01F17
IPC codes: H01F17