Patent Application
20080122567
The embodiments of this invention generally relate to an inductor on a substrate.
Inductors can be created from a spiral trace of a conducting material on a substrate. Inductors can create capacitance between the plurality of traces of the inductor and between the inductor and surrounding materials. The efficiency of an inductor is measured by the quality factor. The capacitance created between the traces of an inductor can lower the quality factor of an inductor. The quality factor for each inductor is determined by the frequency of the signal, the inductance, the capacitance, and the resistance of the traces. An ideal inductor has a quality factor approaching infinity because there is no resistance in the spiral traces.
In an integrated circuit, the spiral traces of an inductor can be in close proximity to each other to reduce the area of the substrate that is used and increase the inductance. To improve reliability, an inductor can be added as a discrete and separate component from the integrated circuit. Adding discrete components can increase the size and cost of a circuit, in some embodiments.
A spiral inductor can be manufactured on a substrate. In one embodiment, the substrate can also include an integrated circuit. Manufacturing inductors on the same substrate as an integrated circuit can in some embodiments, reduce the cost of including an inductor in a circuit.
A spiral inductor can be created from a trace material winding around the center point at an increasing distance from the center point. The spiral inductor can be in a square, a circle, or another geometric shape.
The efficiency of an inductor is measured by the quality factor. A characteristic of determining the quality factor of an inductor on a substrate is the parasitic capacitance. The parasitic capacitance can be formed between the spiral traces of an inductor and adjacent traces of the inductor. In one embodiment, reducing the parasitic capacitance between the spiral traces of an inductor, can improve the quality factor.
The inductance and the quality factor of an inductor can be affected by two spiral traces coming in contact with one another. Supporting the spiral traces of the inductor with a substrate material can, in one embodiment, improve the reliability by reducing the possibility that two adjacent traces of the spiral inductor can come in contact with one another. Creating trenches adjacent to the spiral traces of an inductor can remove the substrate material and may reduce parasitic capacitance, in one embodiment.
Substrate islands can be created between the trenches and the spiral traces can be attached to the islands. The substrate islands can, in one embodiment, support the spiral traces of an inductor.
With reference to the figures,
The spiral trace 105 can be applied to the surface 110 of the substrate 115. Trenches 120 can be formed between adjacent spiral traces 105. Substrate islands 125 can remain between the trenches to support the spiral trace 105. In one embodiment, a ground trace 130 can be applied to the surface of the substrate 110 surrounding the spiral trace 105. A trench 120 can be formed between the spiral trace 105 and the ground trace 130, in some embodiments.
The spiral trace 105 can be a conductor. For example, conductors that may be used include aluminum, copper and gold. The substrate 115 may be a semiconductor, for example, silicon, or gallium arsenide.
Between the islands 125, the substrate material 115 can be removed to create a trench 120. The trenches 120 can be filled with a material having a low dielectric constant. In one embodiment, the dielectric material contained in the trenches 120 can be a gas. Examples of gases with low dielectric constants can be oxygen, nitrogen, and combinations of dielectric gases. Surrounding the spiral traces 105 can be the ground traces 130, in one embodiment. The substrate material 115 can be removed between the spiral traces and the ground traces 130, in one embodiment, to form a trench 120.
A top electrode can be deposited at block 325 onto the insulating layer. In one embodiment, the top electrode can be aluminum. Lithography can be used to create a spiral trace pattern on the top electrode and the pattern can be etched at block 330.
The bottom electrode can be patterned at block 335. Patterning the bottom electrode at block 335 can create traces used to connect the spiral inductor to components of an integrated circuit. Vias can be prepared in the substrate at block 340 to electrically couple the bottom electrode to the top electrode.
A metal mold can be prepared at block 345. The metal can be, for example, gold, copper, or another material with a higher conductivity than aluminum. A material with higher conductivity than aluminum can, in one embodiment, increase the quality factor of an inductor by reducing the resistance in the inductor traces. In one embodiment, the metal may be 6 μm thick on a substrate with a thickness of 250 μm. The thickness of the substrate can be measured from the surface of the substrate at a 90-degree angle to the surface.
The spiral inductor traces can be patterned and etched at block 350. The patterning and etching of the spiral inductor traces at block 350 can be used to restore the traces of the spiral inductor created at block 330. A mask can be applied to the traces. The substrate between the traces of the spiral inductor can be etched at block 355. In one embodiment, the substrate can be etched 100 μm from the surface of the substrate, but can be etched other amounts based on the characteristics of the inductor and substrate.
Etching the substrate material with dry isotropic etching can remove the substrate material not covered by a mask, for example a photo resist. Dry etching can remove substrate material not masked by exposing the material to a bombardment of ions. In some embodiments, a dry etching process can create trenches with substantially parallel side walls with less removal of substrate areas that are covered by a photo resist than a wet etching process.
The volume of a trench with substantially parallel walls can be larger than a trench with walls at an angle to the surface of the substrate. In some embodiments, increasing the volume of the trench can increase the amount of the material with a low dielectric constant in the trench and reduce the parasitic capacitance of the inductor.
A parasitic capacitance 420 represents the capacitance between the inductor traces. The parasitic capacitance 420 is coupled between the input 135 and the ground 435. The resistance 425 of the trace material of the spiral inductor is in series with the spiral inductance 430. The resistance 425 and the spiral inductance 430 are in parallel with the parasitic capacitance 420.
Removing materials of high dielectric constant such as silicon dioxide and silicon from areas surrounding a spiral inductor can reduce the parasitic capacitance between inductor turns, in some embodiments. A reduction in the parasitic capacitance can lead to higher resonance frequencies of the inductor and an enhancement in the quality factor. The quality factor is represented by
and where Ls represents the spiral inductance, Rs the metal series resistance, Cs the parasitic capacitance between the inductor turns, Cox the shunt capacitance of the oxide layer, Rsub the shunt resistance due to substrate losses, and Csub the total shunt capacitance of the substrate. The substrate etching in the substrate surrounding the inductor traces can be partially replaced by a material with a low dielectric constant such as a dielectric gas instead of other materials of high dielectric constant such as silicon dioxide and silicon. Replacing materials having a high dielectric constant with materials having a low dielectric constant can reduce Cs leading to an inductor with a higher resonance frequency and an improved quality factor.
In one hypothetical example an inductor has a spiral inductance of 1.36 (nH), a shunt resistance due to substrate losses of 2530 (ohms), a shunt capacitance of the oxide layer of 165.1 (fF), a total shunt capacitance of the substrate of 290 (fF). Etching the substrate between the inductor traces can reduce the metal resistance to 0.66 (ohms) from 0.73 (ohms) and reduce the parasitic capacitance from 85.2 (fF) to 50.1 (fF). The quality factor of the hypothetical inductor increases from 38.6 to 45.3 at a resonance frequency of 5.2 GHz, a 17.4% increase in the quality factor, when the inductor substrate is etched between the traces.
In one embodiment, the trenching of the substrate material adjacent to the spiral traces of the inductor increases the quality factor of a resonator by 2.5 dB.
The cellular telephone 500 can include an input/output (I/O) interface 526 that establishes electrical connection with the connector 544. In this manner, the I/O interface 526 may receive code from the connector 544, and the code can be sent from the processor 505 to the controller 507 to store on the non-volatile memory 510.
Among the other features of the cellular telephone 500, a key pad 534 may be used to enter telephone numbers and may be interfaced between the bus 508 via a keypad interface 530. Furthermore, the processor 505 may drive a display 542 through a display interface 540 that is coupled between the display 542 and the bus 508. The cellular telephone 500 also includes a battery 550 that is coupled to conductive traces, or lines 554, to supply power to the components of the cellular telephone and is coupled to conductive traces, or lines 552, that extend to and are accessible through the connector 544. The lines 552 may be used for purposes of charging the battery 550.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.
