In the fabrication of integrated circuit structures, active devices such as MOS and bipolar transistors are formed. Passive electronic devices such as capacitors, resistors, inductors, and transformers are also typically formed either in the substrate or in conductive materials formed over the integrated circuit transistors.
Inductors are often used in various microelectronic applications such as voltage control oscillators and power amplifiers. In addition, inductors are often used in radio frequency (RF) circuits such as those used in devices like cellular telephones, wireless modems, and other types of wireless communication equipment.
Transformers are frequently used to pass signals from a first integrated circuit or sub circuit that operates at one voltage to a second integrated circuit or sub circuit that operates with a different voltage.
Inductors and transformers typically require significant area when formed on an integrated circuit. Some system designers have sought to eliminate the use of the relatively large inductors and transformers by simulating those using active circuits. Unfortunately, simulated inductor and transformer circuits tend to exhibit large parasitic effects and often generate more noise than circuits which utilize conventional inductors and transformers.
When unable to eliminate inductors and transformers in their designs, designers have sought ways to reduce the size of the inductors and transformers. For example, inductors and transformers are miniaturized for use in compact communication systems, such as cellular phones and modems. These miniaturized inductors and transformers typically comprise three-dimensional spiral air core inductors and transformers that may be fabricated on the same substrates as the integrated circuits to which they are coupled. Although these three-dimensional spiral air core inductors and transformers may be fabricated using conventional integrated circuit manufacturing techniques, they typically take up a disproportionately large share of the available area.
In addition, spiral air core inductors and transformers have an open magnetic field which may generate interference and/or undesirable magnetic coupling that may cause circuit instability problems. In addition the spiral air core inductors may not be suitable for high frequency applications because their large size presents large resistance and large parasitic capacitance resulting in a self-resonate frequency that is lower than the operation frequency.