The present invention relates generally to semiconductor devices, and more particularly, to high frequency signal isolation in a semiconductor device.
To reduce cost in integrated circuit design, it is desirable to include as much functionality as possible on a single integrated circuit. For example, in a low cost wireless communication system, it is desirable to include the RF (radio frequency) circuits on the same integrated circuit as the digital logic circuits. However, noise generated by the digital logic circuits can be injected into sensitive RF circuit blocks such as phase locked loops (PLL) and low noise amplifier circuits. Conceptually, an ideal Faraday cage prohibits external electromagnetic interference and provides perfect signal isolation. In an integrated circuit, implanted wells are used to reduce the effect of noise and to provide signal isolation. In a CMOS twin well process with a p-type substrate, the pn junction between n-well and p-type substrate provides some signal isolation for PMOS. The NMOS signal isolation is accomplished using a deep n+ implant (DNW) with an n-well to create an isolated p-well (IPW) pocket, and is sometimes referred to as a triple well process. Implanted wells as used to approximate a Faraday cage in integrated circuits, reduce the effect of noise. However, the use of implanted wells fails to provide adequate signal isolation at higher RF frequencies.
FIG. 1 illustrates a top view of a prior art semiconductor device 10. FIG. 2 illustrates a cross-sectional view of the prior art semiconductor device 10 of FIG. 1. Semiconductor device 10 has a p substrate 18. A deep n-well implant 16 together with a n-well ring 15 creates an-isolated p-well pocket 12. A plurality of p+ well ties 14 is implanted in the surface of the isolated p-well 12. Electronic circuits are built into the surface of the isolated p-well (not shown). The isolated p-well 12 functions to isolate the circuits implemented in the well from circuits that are implemented outside of the well. However, the deep n-well implant 16 has a relatively high resistance that is undesirable for signal isolation in the RF frequency range.