Conductive trace with reduced RF impedance resulting from the skin effect

[00002] The present invention is related to application Ser. No. 10/219,791 for “Dual Damascene Metal Trace with Reduced RF Impedance Resulting from the Skin Effect” by Peter J. Hopper et al., application Ser. No. 10/219,896 for “Metal Trace with Reduced RF Impedance Resulting from the Skin Effect” by Peter J. Hopper et al., and application Ser. No. 10/219,212 for “Etched Metal Trace with Reduced RF Impedace Resulting from the Skin Effect” by Peter J. Hopper et al., all filed on an even date herewith.

[00003] 1. Field of the Invention

[00004] The present invention relates to conductive traces and, more particularly, to a conductive trace with reduced RF impedance resulting from the skin effect.

[00005] 2. Description of the Related Art

[00006] Metal traces are common integrated circuit elements that are used in a multi-level interconnect structure to connect together various elements of a circuit. In addition, a metal trace can be used to form an integrated circuit inductor by forming the trace to have a number of coils or loops. Inductors are common circuit elements in radio frequency (RF) applications, such as digital cellular telephones.

[00007] FIGS. 1A-1D show views that illustrate a prior art integrated circuit inductor 100. FIG. 1A shows a plan view of inductor 100. FIG. 1B shows a cross-sectional view taken along lines 1B—1B of FIG. 1A. FIG. 1C shows a cross-sectional view taken along lines 1C—1C of FIG. 1A. FIG. 1D shows a cross-sectional view taken along lines 1D—1D of FIG. 1A.

[00008] As shown in FIGS. 1A-1D, inductor 100 is formed on top of a fourmetal layer interconnect structure that includes a fourth layer of insulation material I4, and a metal trace 110 that is formed on insulation layer I4 from a fourth metal layer M4. In addition, the metal interconnect structure includes a fifth layer of insulation material I5 that is formed on metal trace 110, and a via 112 that is formed through insulation layer I5 to make an electrical connection with metal trace 110.

[00009] As further shown in FIGS. 1A-1D, inductor 100 includes a metal trace 114 that is formed on top of the fifth layer of insulation material I5 from a fifth metal layer M5. Metal trace 114, which has a width W and a depth D, has a first end 120 that is formed over via 112 to make an electrical connection with via 112, and a second end 122. Metal trace 114, which makes one and a half loops in the same plane, is typically formed on top of the metal interconnect structure to avoid inducing currents in the substrate.

[00010] One important measure of a metal trace is the RF impedance of the trace, which affects the quality factor or Q of an inductor formed from the metal trace. High Q inductors are desirable in a number of RF circuits, such as resonant circuits. The Q of an inductor is a measure of the ratio of magnetic energy stored in the inductor versus the total energy fed into the inductor, and is given by equation (EQ.) 1 as:
Q=ωL/Z,  EQ. 1
where ω is related to the frequency f of the signal applied to the inductor (ω=2(pi)(f)), L represents the inductance of the inductor, and Z represents the RF impedance of the inductor. (impedance is the vector sum of resistance and reactance, and introduces a phase shift.) Thus, as indicated by EQ. 1, the smaller the impedance, the higher the Q of the inductor.

[00013] One problem with metal traces is that when gigahertz-frequency signals are placed on the trace, the skin effect causes current to flow primarily at the surface. This effectively increases the RF impedance of the trace which, in turn, lowers the Q of an inductor formed from the trace.

[00014] One common approach to reducing the impedance of an integrated circuit inductor is to increase the size of the metal trace. However, in integrated circuit applications, there are practical limitations to the size of the metal trace. As a result, there is a need for a metal trace with reduced RF impedance which, in turn, allows a high Q integrated circuit inductor to be realized from the trace.

[00015] The present invention provides a conductive trace that has reduced RF impedance at gigahertz frequencies. When the conductive trace is formed in a spiral configuration with a number of loops, the spiral trace forms an integrated circuit inductor, while the reduced RF impedance increases the Q of the inductor.

[00016] A semiconductor structure in accordance with the present invention includes a layer of insulation material that is formed over a semiconductor substrate. In addition, the semiconductor structure includes a conductive trace that is formed on the layer of insulation material. The conductive trace has a base region and a plurality of spaced-apart fins that extend away from the base region. The conductive trace can be formed in a spiral, having a number of loops, and the loops can be formed to lie substantially in the same plane.

[00017] The present invention also includes a method of forming a semiconductor structure that includes the steps of forming a layer of insulation material over a semiconductor device. The method further includes the steps of forming a layer of conductive material on the layer of insulation material, and etching the layer of conductive material to form a base region of a conductive trace. The base region is then covered with a layer of isolation material, and etched to remove the isolation material above the base region to form a first trench.

[00018] The method further includes the step of depositing a first conductive film on the isolation material and base region so that the first conductive film makes an electrical connection with the base region. The first conductive film forms a second trench that is smaller than the first trench. An insulating layer is then deposited over the first conductive film to fill up the second trench.

[00019] A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.

[00024] FIGS. 2A-2B show views that illustrate an example of an integrated circuit structure 200 in accordance with the present invention. FIG. 2A shows a plan view of structure 200, while FIG. 2B shows a cross-sectional view taken along line 2B—2B of FIG. 2A. As described in greater detail below, structure 200 utilizes a conductive trace that has been formed to have an increased surface area. The increased surface area, in turn, reduces the impedance of the trace at gigahertz frequency levels.

[00025] As shown in FIGS. 2A-2B, structure 200 includes a first circuit 210 that operates on a gigahertz frequency signal, and a second circuit 212 that operates on a gigahertz frequency signal. In addition, structure 200 includes a metal trace 214 that is electrically connected to circuits 210 and 212. Metal trace 214, which is formed on a layer of insulation material 216, passes gigahertz frequency signals between circuits 210 and 212.

[00026] As further shown in FIG. 2B, metal trace 214 has a width W (of approximately four microns) and a depth D (of approximately four microns). Metal trace 214 also has a base region 220 with a top side 220A and a bottom side 220B, and a number of spaced-apart fins 222 that extend away from top side 220A. Fins 222, in turn, substantially increase the surface area of metal trace 214 when compared to a conventional metal trace that has the same width W and depth D, such as metal trace 114.

[00027] In operation, when a signal in the gigahertz frequency range is placed on metal trace 214 by circuit 210 or circuit 212, current flows primarily at the surface of metal trace 214 due to the skin effect. Thus, in accordance with the present invention, since current flows primarily at the surface and fins 222 substantially increase the surface area of metal trace 214, fins 222 allow more current to flow. As a result, fins 222 effectively reduce the RF impedance of metal trace 214.

[00028] Thus, the present invention reduces the RF impedance of a metal trace that interconnects two gigahertz frequency devices. (The metal trace connecting together two gigahertz frequency devices can be formed from any one of the layers of metal used to form the metal interconnect structure, such as the first layer of metal, or a combination of metal layers and vias.)

[00029] FIGS. 3A-3D show views that illustrate an example of an integrated circuit inductor 300 in accordance with the present invention. FIG. 3A shows a plan view of inductor 300, while FIG. 3B shows a cross-sectional view taken along lines 3B—3B of FIG. 3A. FIG. 3C shows a cross-sectional view taken along lines 3C—3C of FIG. 3A. FIG. 3D shows a cross-sectional view taken along lines 3D—3D of FIG. 3A.

[00030] As described in greater detail below, inductor 300 is formed from a metal trace that has been formed to have an increased surface area. The increased surface area, in turn, reduces the RF impedance of the metal trace when gigahertz-frequency signals are placed on the trace. As a result, the metal trace of the present invention can be used to form integrated circuit inductors with an increased Q.

[00031] In the example shown in FIGS. 3A-3D, like inductor 100, inductor 300 is formed on top of a four metal layer interconnect structure. The interconnect structure includes a fourth layer of insulation material I4, and a metal trace 310 that is formed on insulation layer I4 from a fourth metal layer M4. In addition, the metal interconnect structure includes a fifth layer of insulation material I5 that is formed on metal trace 310, and a via 312 that is formed through insulation layer I5 to make an electrical connection with metal trace 310.

[00032] As further shown in FIGS. 3A-3D, inductor 300 includes a metal trace 314 that is formed on top of the fifth layer of insulation material I5 from a fifth metal layer M5. (Metal trace 314 can be formed from any metal layer, including the first metal layer. The fifth metal layer of the present example is but one possibility. By forming inductor 300 on top of a metal interconnect structure, however, induced substrate currents are minimized).

[00033] In addition, metal trace 314 has a first end 320 that is formed over via 312 to make an electrical connection with via 312, and a second end 322. (In this example, second end 322 can be connected to a via connected to a metal-4 trace, or a via connected to a pad or another overlying metal trace.)

[00034] Metal trace 314 also has a width W (of approximately four microns) and a depth D (of approximately four microns). Further, metal trace 314 makes one and a half loops in the same plane. (Trace 314 is not limited to one and a half loops, but can be formed with a different number of loops.)

[00035] As further shown in FIG. 3B, metal trace 314 has a base region 324 with a top side 324A and a bottom side 324B, and a number of spaced-apart fins 326 that extend away from topside 324A. Fins 326, in turn, substantially increase the surface area of metal trace 314 when compared to a conventional metal trace that has the same width W and depth D, such as metal trace 114.

[00036] In operation, when a signal in the gigahertz frequency range is input to inductor 300, current flows primarily at the surface of metal trace 314 due to the skin effect. Thus, in accordance with the present invention, since current flows primarily at the surface and fins 326 substantially increase the surface area of metal trace 314, fins 326 allow more current to flow.

[00037] As a result, fins 326 effectively reduce the RF impedance of metal trace 314, thereby increasing the Q of inductor 300. In addition, as illustrated by FIG. 3C, metal trace 310 can be formed as metal trace 214, thereby providing a low RF impedance pathway from inductor 300 (a first gigahertz frequency device) to another gigahertz frequency device.

[00038] FIGS. 4A-4O show cross-sectional drawings that illustrate an example of a method of forming a conductive trace in accordance with the present invention. As shown in FIG. 4A, the method utilizes a layer of insulation material 410 that has been formed over a semiconductor integrated circuit device. Insulation layer 410, in turn, has a number of contacts or vias that have been formed through insulation layer 410.

[00039] In addition, insulation layer 410 can be formed on the surface of the substrate of the device, or on top of a metal trace that is formed from any of the layers of metal that are used to form the metal interconnect structure of the device. For example, when a semiconductor integrated circuit device is fabricated with a five layer metal process, insulation layer 410 can be formed on the metal-4 layer.

[00040] As further shown in FIG. 4A, the method of the present invention begins by forming a layer of conductive material 412, such as copper or aluminum, on insulation layer 410. After this, a layer of masking material 414 is formed and patterned on conductive layer 412.

[00041] Next, as shown in FIG. 4B, the exposed regions of conductive layer 412 are etched to form a base region 416 of the to-be-formed conductive trace on insulation layer 410. Region 416, which has a top surface 416A, can be formed in a spiral configuration having a number of loops that lie substantially in the same plane, thereby forming an inductor.

[00042] In addition, base region 416 can be connected to a contact or a via formed through insulation material 410. (Region 416 can contact more than one contact or via, or no vias if an overlying metal layer and vias are used to make an electrical connection). Once base region 416 has been formed, mask 414 is removed.

[00043] After this, as shown in FIG. 4C, base region 416 is covered with a layer of isolation material 418. Isolation layer 418 is formed to a depth D, which is equal to the overall depth of the to-be-formed conductive trace. Following this, as shown in FIG. 4D, a layer of masking material 420 is formed on isolation layer 418. Masking material 420, which can be implemented with a soft mask material (photoresist) or a hard mask material (nitride), is then patterned to expose regions on the surface of isolation layer 418.

[00044] Next, as shown on FIG. 4E, isolation layer 418 is anisotropically etched to remove isolation layer 418 from the exposed region of top surface 416A of base region 416. The anisotropic etch forms a trench 422 that has a width WD, which is approximately equal to the cross-sectional width W of traces 214 and 314.

[00045] Following this, as shown on FIG. 4F, a first conductive film 424 is deposited on the surface of isolation layer 418 and the exposed region of top surface 416A so that first conductive film 424 makes an electrical connection with base region 416. The deposition of film 424 forms a trench 426 that is smaller than trench 422.

[00046] Next, as shown in FIG. 4G, the method continues by forming a first insulating layer 428 on first conductive film 424 to fill up trench 426. Once layer 428 has been formed, as shown in FIG. 4H, layer 428 is anisotropically etched to remove insulating layer 428 from the top surface of conductive film 424 over isolation layer 418. The anisotropic etching process leaves insulating layer 428 on the sidewalls of trench 426, forming insulating sidewall spacers 428S.

[00047] Next, as shown on FIG. 4I, a second conductive film 430 is deposited on base region 416, first conductive film 424, and spacers 428S to form a narrower, shallower trench 432. Second conductive film 430 makes an electrical connection with first conductive film 424.

[00048] As shown on FIG. 4J, the method continues by depositing a second insulating layer 434 on second conductive film 430 to fill up trench 432. Then, as shown on FIG. 4K, second insulating layer 434 is anisotropically etched to remove layer 434 from the top surface of film 430 over isolation layer 418. The etch forms insulating sidewall spacers 436.

[00049] A third conductive film 440 is then deposited over the second conductive film 430 and the insulating sidewall spacers 436, as shown on FIG. 4L. Third conductive film 440 makes an electrical connection with second conductive film 430, and creates a narrow trench 442.

[00050] As shown on FIG. 4M, an insulating layer 444 is then deposited on conductive film layer 440. Insulating layer 444 is formed in a depth sufficient to fill trench 442 created by the previous conductive film layer. Then, as shown on FIG. 4N, insulating layer 444, conductive films 424, 430, and 440, and spacers 428S and 436 are planarized using, for example, a chemical mechanical polishing process, until the top surface of isolation layer 418 is exposed. The planarization results in formation of a conductive trace 450 with a plurality of fins 452 extending upward from base region 416, in accordance with the present invention.

[00051] Next, as shown on FIG. 4O, the method continues with the formation of insulating material 454 to electrically isolate conductive trace 450 from other components in the semiconductor structure. Following this, the method continues with conventional semiconductor fabrication steps.

[00052] In the present invention, the conductive film layers can be formed from the same or different materials. Similarly, the insulating and isolation layers can be formed from the same or different materials. Further, although the present examples have been described in terms of using three metal depositions to form the fins, by varying the dimensions of the trace and the depth of the deposited layers it is possible to create a trace with a greater or lesser number of fins in accordance with the present invention.

[00053] It should be understood that the above descriptions are examples of the present invention, and that various additional alternatives of the invention described herein may be employed in practicing the invention. Thus, it is intended that the following claims define the scope of the invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.