Metal-insulator-metal (MIM) capacitors are widely used in hybrid and monolithic electronic circuits. Such capacitors can be vertical, with horizontal metal plates; lateral, with vertical plates; or mixed, employing capacitance between both vertically-separated and horizontally-separated plates. In some applications, special insulator layers are provided for optimized capacitor performance. In others, existing dielectrics are used, such as the inter-metal dielectrics which separate metal interconnections. The capacitors discussed below employ inter-metal dielectrics of this type, and are of mixed orientation.
Interdigitated MIM capacitor structures similar to the capacitor 100 depicted in FIG. 1A have been used extensively in both semiconductor and hybrid processes. They provide reasonably well-controlled capacitance, with acceptable parasitic elements (resistance, inductance) for many applications, while employing only process elements already present for other reasons: metal for interconnects and dielectrics for substrate and/or insulation. Capacitors of this type are often described as being composed of a number of ‘fingers.’ In the capacitor 100 illustrated in FIG. 1A, capacitor terminal 1 is connected to four fingers, and capacitor terminal 2 is connected to three fingers. A cross-section of the capacitor of FIG. 1A along line A-A′ is shown in FIG. 1B, with fingers connected to each of the two terminals identified as 1 and 2 respectively. Capacitance between terminals 1 and 2 in this structure is primarily horizontal, with fringing-field components extending into the vertical dimension.
Terminal 2 of the capacitor 100 has capacitance to terminal 1 along both sides of each of its three fingers, along the length labeled ‘L.’ In addition, finger-ends such as 3, 4, and 5 contribute some capacitance. If this capacitor design is generalized to more or fewer fingers, while maintaining one more finger for terminal 1 than for terminal 2, we can write for the total capacitance between terminals 1 and 2:C=NFLC0+NFC3+2C4+(NF−1)C5  (Equation 1)where NF is the number of terminal-2 fingers; C0 is the capacitance per unit length per finger; C3 is the capacitance per terminal-2 finger-end like 3; C4 is the capacitance per outside corner like 4; and C5 is the capacitance per terminal-1 finger-end like 5. In the example illustrated in FIG. 1, NF=3.
The first term in Equation 1 (NFLC0) is proportional to both the number of fingers NF and the length L, both of which are convenient design parameters. The remaining terms embody the finger-end effects enumerated, and are less subject to control of the designer, but rather depend more on process details. Equation 1 can be simplified by re-combining the second through fourth terms:C=NFLC0+NFC1+C2  (Equation 2)where C0 is again the capacitance per finger per unit length; C1 is a capacitance per finger, independent of finger length; and C2 is a fixed ‘offset’ capacitance, independent of both NF and L. (C2 may be either positive or negative.)