Stacked Metal Finger Capacitors Providing High Capacitance Density and High Quality Factors (03-Nov-2009)

Stacked Metal Finger Capacitors Providing High Capacitance Density and High Quality Factors

Stacked metal finger capacitors (also called Vertical Natural Capacitors or VNCAPs for short) are a "cost-free" or "mask free" design of capacitors [1]. Vertically stacked BEOL metal fingers are used to form capacitors, and capacitance values are basically voltage independent. Besides capacitance value or capacitance density, the quality factor Q is also an important feature in the application of metal finger capacitors. The quality factor Q of a VNCAP device is typically related to its capacitance value C and resistance value R by the relation

Q = 1/(2 π f R

tot

), (1)

where f is the frequency of an AC signal. Typically, the resistance R of a VNCAP can be decreased (and thus to increase the quality factor of the VNCAP device) by using wider fingers. But this decreases VNCAP's capacitance density.

New design:

Finger width varies from very narrow (at min. design width of a given metal level in a semiconductor technology) to somewhat wider than the min. design width (see Figs. 1 to 3). Compared with a typical metal finger capacitor layout in which the finger width is a constant when moving along the finger length direction and the finger width is typically larger than min. wire width at the given metal level, the new layout of the varying finger width can give the same capacitance density but a higher quality factor Q.

C

tot

Fig. 1.

1

Fig. 2. A side view along the left cut line in Fig. 1.

Fig. 3. The same side view as Fig. 2, but at a different cut line -- along the right cut line in Fig. 1.

Figures 4 to 6 show a few embodiments of varying finger width shown in Fig. 1 under the Manhattan layout restriction: The direction of a line edge must be either horizontal or vertical.

Fig. 4. An embodiment of varying finger width in Manhattan-type of layout. Each finger is divided into 4 segments, i.e., moving along the finger, the finger width has 4 different values.

2

Fig. 5. Another embodiment of varying finger width in Manhattan-type of layout: Each finger is divided into 3 segments, i.e., moving along the finger, the finger width has 3 different values, from a maximum finger width to a minimum finger width.

Fig. 6. A third embodiment of varying finger width in Manhattan-type of layout. Here, each finger is divided into two segments, i.e., moving along the finger, the finger width has two different values.

An enlarged view of...