High Q inductor and its forming method

[0002] The present invention relates to an inductor, and in particular, to a high Q inductor and its forming method which first provides a trench on the semiconductor substrate by a basic process such as etching, then fills the trench with a high resistivity material, for example by depositing an epitaxy layer. Consequently, the substrate loss between the inductor winding and the semiconductor substrate due to the substrate resistance can be reduced by several orders of magnitude, greatly improving the quality factor of the formed inductor.

[0003] Given of the rapid growth of wireless and RF communication markets, silicon has become as a crucial material to meet the demands of low-cost, high density integration technologies. For silicon wireless and RF IC applications, the realization of high quality factor (Q) inductors is an important task that calls for immediate attention. Generally speaking, the ratio of an inductor's reactance to its series resistance is used as a measure of the quality of the inductor. The quality factor is referred to as the Q of the inductor.

[0004] Q=ω₀*L/R

[0005] FIG. 1 (Prior Art) is a cross sectional view of a conventional inductor. In FIG. 1, a semiconductor substrate 10, such as a P type silicon substrate or an N type silicon substrate, is provided with an oxide layer 12, such as a thin silicon oxide layer, on its surface. The inductor winding formed of intermediate layer 14, such as another silicon oxide layer, and metal windings 16a, 16b on the intermediate layer 14, is provided on the oxide layer 12. The inductor winding is formed of multilevel interconnect with a protective layer (not shown) thereon to protect the formed inductor from being damaged.

[0006] FIG. 2 (Prior Art) is the equivalent circuit diagram of the conventional inductor as shown in FIG. 1. As shown in FIG. 2, the reactance and capacitance of the metal windings 16a, 16b are respectively represented as an inductor L and a resistor R. The couple effect between the metal windings 16a, 16b is represented as a capacitor C_f. The substrate effects between the metal windings 16a, 16b (input/output terminal) and the semiconductor substrate 10 (ground) are representively represented as a capacitor C1 and a resistor R1 as well as a capacitor C2 and a resistor R2.

[0007] In this example, to get high Q inductors on silicon wafers, the series resistance R is reduced and the inductance L is increased. In addition, the inductor losses should also be controlled to some extent. Since the substrate losses are an important factor in inductor losses and are inversely proportional to the substrate effect between the metal windings and the semiconductor substrate, the substrate resistors Rl, R2 are increased to reduce the substrate losses.

[0008] Therefore, an object of the present invention is to provide a high Q inductor and its forming method. According to the the present invention, a trench is first defined on the semiconductor substrate by dry etching to form an inductor thereon, then a high resistivity material is deposited to fill the trench. Therefore, the substrate resistance (R1, R2 in FIG. 2) between the metal windings and the semiconductor substrate can be increased by several orders of magnitude, greatly reducing the substrate losses to an equal extent.

[0009] To achieve the above-indicated object and others, the present invention provides a method for forming a high Q inductor. The forming method includes the steps of: defining a trench on the semiconductor substrate by dry etching, and filling the trench with a high resistivity material. The resitivity of the material should be higher than that of the semiconductor substrate, such as an epitaxy layer. The trench is filled with the epitaxy layer by first depositing an epitaxy layer on the semiconductor substrate and the trench, then etching back the epitaxy layer to expose the surface of the semiconductor substrate. Thereafter, a dielectric layer is deposited on the semiconductor substrate (including the trench) and a metal winding is deposited on the dielectric layer above the trench. In this method, the dopant concentration of the epitaxy layer is lighter than that of the semiconductor substrate by several orders of magnitude, so the resistivity thereof is far higher than that of the semiconductor substrate.

[0010] The present invention also provides a high Q inductor. The high Q inductor includes a semiconductor substrate, a dielectric layer and an inductor winding. In this inductor, the semiconductor substrate is provided with a trench formed thereon. In the trench is filled with a material having a higher resistivity than that of the semiconductor substrate. The dielectric layer is formed on the semiconductor substrate and the trench. And the inductor winding is formed on the dielectric layer above the trench to obtain the high Q inductor.

[0011] The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings, in which:

[0012] FIG. 1 (Prior Art) is a cross sectional view of a conventional inductor;

[0013] FIG. 2 (Prior Art) is an equivalent circuit diagram of the conventional inductor as shown in FIG. 1; and

[0014] FIG. 3A to FIG. 3D are cross sectional views showing the forming method, step by step, of the present invention.

[0015] As shown in FIG. 1 and FIG. 2, in order to reduce the substrate effects of the metal windings 16a, 16b, the present invention employs STI technology to define a trench on the semiconductor substrate. The trench is filled with a high resistivity material, such as an epitaxy layer. Therefore, the resistivity between the metal windings and the semiconductor substrate (the substrate resistance R1, R2) is increased, and the substrate effects are greatly reduced.

[0016] FIG. 3A to FIG. 3D are cross sectional views showing the forming method, step by step, of the present invention. The detailed description follows hereafter.

[0017] First, as shown in FIG. 3A, a semiconductor substrate 20, such as a P type silicon substrate or an N type silicon substrate of a resistivity of about 10 Ω-cm, is provided for the starting wafer. Then a thin oxide layer 22, such as an oxidized or CVD silicon oxide layer of a thickness of about 300˜600 Å is deposited on the surface of the semiconductor substrate 20 to avoid pollution thereon.

[0018] Then, a trench is defined on the semiconductor substrate 20 using basic processes, such as etching and depositing, and the trench is filled with a high resitivity material.

[0019] As shown in FIG. 3B, using a photoresist layer or a dielectric layer (not shown) as a mask, a trench 24 is defined on the semiconductor substrate 20 by dry etching or other methods. The area of the trench 24 is larger than that of the to-be-formed inductor, and the depth of the trench 24 ranges around 3˜5 μm. In addition, the trench 24 is preferably located outside an active region of the semiconductor substrate 20 so as to avoid improper interference generated between the formed inductor and circuits in the active region.

[0020] Thereafter, as shown in FIG. 3C, a high resistivity material 26 is deposited to fill the trench 24. The high resistivity material 26 can be an epitaxy layer or other silicon substrate with a low dopant concentration. The dopant concentration of the silicon substrate or the epitaxy layer can be lower than that of the semiconductor substrate by several orders of magnitude, depending on various situations, so as to increase the resistivity thereof by the same magnitude to several KΩ-cm. In this embodiment, the high resistivity material 26 is filled in the trench 24 by the steps of: first PVD/CVD depositing a low dopant concentration epitaxy layer or a silicon layer on the surface of the semiconductor substrate 20, and simutaneously filling the trench 24, then etching back the deposited material (such as the epitaxy layer 26 or the silicon layer) and the thin oxide layer 22 to expose the surface of the semiconductor substrate 20.

[0021] Afterwards, as shown in FIG. 3D, a dielectric layer 28 of about 10 microns is deposited on the semiconductor substrate 20, as well as the epitaxy layer 26 filling the trench 24. The dielectric layer 28 is planarized using CMP technology. Then, an inductor winding is formed on the dielectric layer 28 to accomplish the acquired inductor. In this step, the inductor winding can be formed of single-level interconnect or multilevel interconnect, such as the intermediate layer 30 and the metal layer 36a, 36b as shown in FIG. 3D. Further, another protective layer (not shown) can also be deposited on the inductor winding so as to protect the acquired inductor from being damaged.

[0022] In summation, the method of the present invention defines a trench filled with a high resistivity material, such as an epitaxy layer, on the semiconductor substrate, therefore the substrate effect (substrate resistance R1, R2) and the quality factor (Q) of the acquired inductor can be greatly improved. Further, the inductor obtained according to the present invention can be formed by using only basic processes, such as depositing and etching, therefore it is not difficult to manufacture such an inductor.

[0023] The foregoing description of a preferred embodiment of the present invention has been provided for the purposes of illustration and description only. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to practitioners skilled in this art. The embodiment was chosen and described to best explain the principles of the present invention and its practical application, thereby enabling those who are skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.