Wave-guides are important elements in microwave circuit applications. These devices provide the interconnection between active and passive devices of microwave circuits. A wave-guide is a type of transmission line widely utilized in monolithic microwave integrated circuit (MMIC) applications.
For MMIC applications, wave-guides are often formed as coplanar wave-guides, wherein the ground lines and the signal lines of the same wave-guide are formed in a same plane, often parallel to the plane of the underlying semiconductor substrate. The manufacturing processes of the coplanar wave-guides may be compatible with the existing manufacturing process of the integrated circuits. Further, being able to be formed on the same substrate as CMOS circuits, the wave-guides are readily integrated with the CMOS circuits.
FIG. 1 illustrates a conventional coplanar wave-guide 2, which includes signal line 4, and ground lines 6 on opposite sides of signal line 4. Signal line 4 and ground lines 6 are in a same horizontal plane. Wave-guide 2 is formed over a high-k dielectric layer 10, which is further formed on passivation layer 12. Inter-metal dielectrics (IMDs) 14 underlie coplanar wave-guide 2, wherein IMDs 14 are used for forming metal lines therein. Substrate 16 underlies IMDs 14.
Being formed in the top layer, the conventional wave-guide 2 as shown in FIG. 1 is relatively far away from substrate 16, and hence the energy loss in substrate 16 is expected to be less than forming wave-guide 2 in any layer underlying high-k dielectric layers. However, the wavelength of the microwave that may be carried is typically much greater than the vertical distance between wave-guide 2 and substrate 16. For example, the electro-magnetic wavelength in SiO2 dielectric material is about 3000 μm at 50 GHz. For lower frequencies, the wavelength will be even greater. The wavelength far exceeds the total thickness of layers 10, 12, 14, and the like. Therefore, the distance that can be increased by forming wave-guide 2 in the top layer is very small compared to the wavelength of the microwave signal, and hence the effect of reducing energy loss by increasing the vertical distance is limited.
The conventional wave-guide 2 as shown in FIG. 1 also suffers from other drawbacks. The thickness T of ground lines 6 is determined by the process for manufacturing the respective chip, and hence has little room for modification. This puts a limitation on the adjustment of the characteristic impedance of wave-guide 2. Accordingly, what is needed in the art is a structure and methods for forming wave-guides without incurring the above-discussed problems.