Abstract:
A resistor structure for an integrated circuit includes a first set of contacts connected between a semiconductor layer and a first conductive layer; and a second set of plugs connected between the first conductive layer and a second conductive layer, wherein the first set of contacts and the second set of plugs are coupled together as a first resistor segment to provide a predetermined resistance for the integrated circuit.

Description:
BACKGROUND 
       [0001]    The present invention relates generally to an integrated circuit (IC) design, and more particularly to a stack resistor structure for integrated circuits. 
         [0002]    An IC often contains many resistors. For example, an analog-to-digital converter (ADC) may include many groups of resistors for dividing the voltage. Ideally, the resistance of the groups of resistors should match in order to divide the voltage equally. Conventionally, the resistors are fabricated by forming a number of silicided or non-silicided polysilicon layers on a silicon wafer. The resistors can also be fabricated by forming N-type or P-type doped regions in the semiconductor substrate of the wafer. 
         [0003]    One drawback of the conventional resistor is that it requires a large surface area. For example, for a reference voltage resistor-ladder used in an 8-bit ADC, the resistor formed on a polycide layer with sheet resistivity of 10 ohm/sq needs an area of approximate 30 μm 2  to provide 1 ohm resistance. As another example, the resistor formed on a metal layer with sheet resistivity of 40 mohm/sq needs an area of approximate 625 μm 2  to provide 1 ohm resistance. 
         [0004]    Moreover, the conventional resistors have relatively poor resistance-matching uniformity. This creates uneven voltages when the resistors are used as voltage dividers in an ADC. 
       SUMMARY 
       [0005]    The present invention discloses a resistor structure for an IC. In one embodiment of the present invention, the resistor structure includes a first set of contacts connected between a semiconductor layer and a first conductive layer; and a second set of plugs connected between the first conductive layer and a second conductive layer, wherein the first set of contacts and the second set of plugs are coupled together as a first resistor segment to provide a predetermined resistance for the integrated circuit. 
         [0006]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a top view of conventional polycide resistors. 
           [0008]      FIG. 2  illustrates a top view of conventional metal resistors. 
           [0009]      FIG. 3  illustrates a cross-section view of a resistor structure in accordance with one embodiment of the present invention. 
           [0010]      FIG. 4  illustrates a cross-section view of a resistor structure in accordance with another embodiment of the present invention. 
           [0011]      FIG. 5  shows the area efficiency of a 6×6 resistor array in accordance with one embodiment of the present invention. 
           [0012]      FIG. 6  shows the area efficiency of a 4×4 resistor array in accordance with one embodiment of the present invention. 
       
    
    
     DESCRIPTION 
       [0013]      FIG. 1  illustrates a top-view of a polycide-resistor  100 , which occupies a relatively large layout area. In a conventional ADC, physical dimensions of a reference voltage resistor-ladder need to be wide enough in order to minimize resistance variation due to process fluctuation. This allows large current to flow through the resistor-ladder, thereby minimizing switching noise. Supposing the sheet resistivity of polycide layer is about 10 ohms/sq, the resistor  100  needs to have a width of 15 μm and a length of 1.5 μm in order to provide 1 ohm resistance. Taking the marginal space requirement into account, the length of the resistor  100  minimally requires 2 μm. Thus, for 1 ohm resistance, the resistor  100  requires a minimum area of 30 μm 2  (2 μm*15 μm). 
         [0014]      FIG. 2  illustrates a top-view of a metal-resistor  200 , which also occupies a relatively large layout area. Supposing the sheet resistivity of metal layer is about 40 m ohms/sq, the resistor  200  needs to have a width of 5 μm and a length of 125 μm (with the marginal space requirement factored in) to provide 1 ohm resistance. Thus, for 1 ohm resistance, the resistor  200  requires a minimum area of 625 μm 2  (5 μm*125 μm). 
         [0015]    As the semiconductor processing technology advances, the electronic devices in an IC becomes increasingly small. Thus, the conventional resistors appear too large to be used in the ICs of new generation. 
         [0016]    The present invention discloses a stack resistor structure constructed by a number of conductive plugs and contacts. The stack resistor structure may include several conductive layers and a semiconductor layer, which are separated by insulating material. The conductive plugs are formed between the conductive layers, while the contacts are formed between the bottom conductive layer and the semiconductor layer. Each column of the contact/plug and the conductive/semiconductor layer is defined as a resistor segment. As the semiconductor processing technology advances, the dimensions of the plugs and contacts become smaller, thereby causing their resistance to increase. For example, the contact or plug fabricated by the 0.13 μm generation processing technology has resistance about 1 ohm, while that fabricated by the 90 nm technology has resistance about 16 ohms. As the technology advances into the 1 TMiM process, the resistance of the contact or plug becomes 36 ohms. As these plugs and contacts are small in size, they can be utilized as resistors for ICs made by, for example, the processing technology of 90 nm or more advanced generation. 
         [0017]      FIG. 3  illustrates a cross-section view of a stack resistor structure  300  that includes multiple conductive plugs A and contacts B. Here, the conductive plugs refer to the conductive-material-filled vias disposed between any two vertically adjacent conductive layers, such as M 1 , M 2 , M 3  and M 4 . The contact refers to the conductive-material-filled vias disposed between the underlying poly-silicon layers, such as doped semiconductor regions or silicide layers, and their vertically adjacent conductive layer, such as M 1 . The conductive layers M 1 , M 2 , M 3  and M 4 , are separated by insulating materials (not shown) that have a dielectric constant lower than about 3.0 for reducing the parasitic capacitance between conductive objects. A number of semiconductor layers doped with impurities and/or conductive silicide layers are formed underneath the bottom conductive layer M 1 . Each column of the contact/plug and the conductive/doped-semiconductor layer is defined as a resistor segment. For example, each column of the doped semiconductor layer, the contacts B, the conductive layers M 1 , M 2 , M 3  and M 4 , and the plugs interposed thereamong is defined as a resistor segment. A number of conductive patterns  302  are disposed between two horizontally adjacent conductive layers for serially coupling the neighboring resistor segments. The nodes of the resistors are formed on the upper conductive layer, such as M 4 . 
         [0018]    As discussed above, the plugs and contacts are particularly suitable for being used as resistors when they are made by the semiconductor processing technology of 90 nm generation or beyond. In this embodiment, the plug has a size ranging from 0.01 to 0.3 μm, and the contact has a size ranging from 0.01 to 0.3 μm. Both sizes are preferred to be between about 0.01 to about 0.1 μm in order to obtain better resistor performance. Two neighboring plugs are spaced by a distance ranging from about 0.015 to 2.0 μm. The length of contact and plug, which is determined by the thickness of the intermediate dielectric layer, ranges approximately from 0.1 to 2.0 um. Such small dimension allows the plugs and contacts to provide sufficient resistance, so that they can be used as resistors. The plugs and contacts for the resistor structure  300  can be made of any conductive material, such as W, Al, Cu, silicide or any metal alloy. 
         [0019]    In  FIG. 3 , all of the resistor segments are serially coupled together as a resistor module. A number of such resistor segments can be coupled together in parallel to form a resistor array. In one embodiment, these parallel resistor segments can be used as voltage dividers in an analog-to-digital converter (ADC). One characteristic requirement for the resistor segments to function as the voltage dividers is that their resistances need to be uniform, such that the voltage can be divided equally. The improvement of uniformity is proportional to (N×M) 1/2  where N is the number of the contacts or plugs in each conductive layer, and M is the number of stacked layers. In a real application, the number N×M can be designed to exceed 1000, such that the resistance uniformity can be improved significantly. Thus, the proposed resistor structure  300  is particularly suitable for being used as voltage dividers in the ADC. 
         [0020]      FIG. 4  illustrates a cross-section view of a resistor structure  400  including a number of plugs in several conductive layers, which form a stack structure and are separated by insulating material with a dielectric constant lower than 3.0. The conductive layers include the layers M 1 , M 2 , M 3  and M 4 . The metal plugs A are formed between the vertically adjacent conductive layers. Each column of the plugs and the conductive layers is defined as a resistor segment. For example, each column of the conductive layers M 1 , M 2 , M 3  and M 4 , and the plugs interposed there among is defined as a resistor segment. A number of conductive patterns  402  are disposed between two horizontally adjacent conductive layers for serially coupling the neighboring resistor segments. The nodes of the resistors are formed on the upper conductive layer, such as M 4 . 
         [0021]    Similar to the resistor structure  300  shown in  FIG. 3 , a number of such resistor segments in the resistor structure  400  can be coupled together in parallel to form a resistor array. These parallel resistor segments can be used as voltage dividers in an analog-to-digital converter (ADC). The plugs A also have dimensions and material similar as those of the plugs shown in  FIG. 3 . 
         [0022]    The resistor structure  400  differs from the resistor structure  300  shown in  FIG. 3  in that it does not include the contacts and doped semiconductor layers. Note that while four conductive layers are illustrated in this figure, the resistor structure  400  can have more or less than four conductive layers depending on various design requirements. 
         [0023]      FIG. 5  shows the area efficiency of a 6×6 resistor array layout in accordance with one embodiment of the present invention. The resistor array  500  contains 36 resistor segments arranged in parallel in an area of 2.44 μm 2  (1.56 μm×1.56 μm). If each of the resistor segments is formed by five conductive layers, a doped semiconductor layers, plugs and contacts, the segment can be designed to have a resistance of 20 ohms. A total resistance 1.1 ohm can be obtained by serially connecting two of such resistor arrays wherein the total resistance 1.1 ohm equals to 20×2/36. In this embodiment, 1.1 ohm resistance takes about 5.098 μm 2  (2.44×2+1.56×0.14), whereas in the prior art shown in  FIG. 1 , 1.0 ohm resistance takes about 30 μm 2 , and in the prior art shown in  FIG. 2 , 1.0 ohm resistance takes about 625 μm 2 . It is noted that the square-shaped via/contact shown in this figure is merely an example. Other shapes, such as round, irregularity, ellipse, etc, can also be used to design the via/contact. 
         [0024]      FIG. 6  shows the area efficiency of the 4×4 resistor array in accordance with one embodiment of the present invention. Taking the marginal area into account, each resistor array  600  occupies an area of 1.39 μm 2  [(1.04+0.14)×(1.04+0.14)]. If each of the resistor segments is formed by five conductive layers and plugs, the segment can be designed to have a resistance of 4 ohms. A total resistance 1 ohm can be obtained by serially connecting four of such resistor arrays, each of which contains 16 resistor segments coupled in parallel. Mathematically, 1=4×4/16. Thus, in this embodiment, the area required for 1.0 ohm is 6.007 μm 2  (1.39×4+3×1.04×0.14), whereas in the prior art shown in  FIG. 1 , 1.0 ohm resistance takes about 30 μm 2 , and in the prior art shown in  FIG. 2 , 1.0 ohm resistance takes about 625 μm 2 . 
         [0025]    As discussed above, the proposed resistor structure can provide a desirable resistance in a much smaller area as opposed to the prior art. It also improves the resistance uniformity for a number of resistors. Thus, the proposed resistor structure would be ideally for using as voltage dividers in an ADC. Moreover, the process of forming the proposed resistor structure is compatible with the standard CMOS processing technology. Thus, it would not incur additional manufacturing costs. 
         [0026]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0027]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.