KR20070022032A - Structure and method for contact pads having an overcoat-protected bondable metal plug over copper-metallized integrated circuits - Google Patents

Abstract

A metal structure for a contact pad of an integrated circuit (IC) having a copper interconnect metallization. A portion 301 of this metallization is exposed to provide a contact pad for the IC. A conductive barrier layer 330 is disposed over the exposed portion of copper metallization. Above this barrier layer, a plug 350 of bondable metal, preferably aluminum, between about 0.4 and 1.4 μm thick is disposed. A protective overcoat layer 320 surrounds the plug and has a thickness 320b such that the exposed surface 322 of the plug is at or below the exposed surface 320a of the overcoat layer. Optionally, a portion 321 of the overcoat layer between about 0.1 and 0.3 μm wide may overlap the circumference of the plug.

Integrated Circuits, Contact Pads, Interconnecting Metallization, Overcoats

Description

STRUCTURE AND METHOD FOR CONTACT PADS HAVING AN OVERCOAT-PROTECTED BONDABLE METAL PLUG OVER COPPER-METALLIZED INTEGRATED CIRCUITS}

TECHNICAL FIELD The present invention generally relates to the field of electronic systems and semiconductor devices, and more particularly, to bond pad structures and methods of manufacturing copper metallized integrated circuits.

In integrated circuit (IC) technology, pure aluminum or doped aluminum has been the metallization of choice for interconnects and bond pads for over 40 years. Main advantages of aluminum include ease of deposition and patterning. In addition, techniques for bonding wires made of gold, copper, or aluminum to aluminum bond pads have been developed with a high degree of automation, miniaturization, and reliability.

In the continuing trend to miniaturization of ICs, there is an increasing number of cases where the RC time constant of interconnection between active circuit elements dominates the IC speed-power product achievable. As a result, the relatively high resistivity of interconnected aluminum is now shown to be inferior to the lower resistivity of metals such as copper. In addition, the remarkable sensitivity of aluminum to electromigration is a serious obstacle. As a result, there is now a strong trend in the semiconductor industry to use copper as a preferred interconnect metal, based on its higher electrical conductivity and lower electromigration sensitivity. However, in terms of mature aluminum interconnect technology, this transition to copper is a significant technical challenge.

When copper diffuses into the silicon base material of the IC, copper must be prevented from diffusing into the silicon gas to protect the circuit from the carrier lifetime killing characteristics of the copper atoms located in the silicon lattice. Bond pads made of copper should prevent the formation of thin copper (I) oxide films during the manufacturing process flow, because these films are reliable for bonding wires, especially for conventional gold-wire ball bonding. Because it severely interferes with reliable attachment. In contrast to the aluminum oxide film on the metal aluminum, the copper oxide film on the metal copper can be easily broken by a combination of thermocompression and ultrasonic energy applied during the bonding process. In another difficulty, bare copper bond pads are susceptible to corrosion.

To overcome these problems, the semiconductor industry has employed a structure that reproduces the traditional situation of aluminum pads to be bonded by conventional gold-wire ball bonding by capping an aluminum layer on a clean copper bond pad. However, this approach has some disadvantages. Firstly, the manufacturing cost of the aluminum cap requires more steps to deposit, pattern, etch and clean the metal, so the manufacturing cost is more than desired. Secondly, the cap must be thick enough to allow reliable wire bonding and to prevent copper from diffusing through the cap metal and possibly damaging the IC transistors.

Third, the aluminum used in the cap is soft and severely damaged by the markings of the multiprobe contacts in the electrical test. This damage is evident in the size of the bond pad, which is constantly decreasing, so that subsequent ball bond attachments are no longer reliable. Finally, the elevated height of the aluminum layer over the surrounding overcord face increases the risk of metal scratches and smears. At the tight bond pad pitch of many high input and output circuits, aluminum smears exhibit unacceptable shorts between neighboring pads.

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Thus, copper combines a low-cost method of manufacturing bond pad structures, complete control of up-diffusion, elimination of smearing or scratching, and reliable methods of bonding wires to these pads. There is a need for a metallurgical bond pad structure suitable for ICs with copper interconnection metallization. This bond pad structure must be flexible enough to accommodate different IC product families and a wide range of design and process variations. Preferably, these innovations must be achieved without the need for expensive additional manufacturing equipment while shortening the production cycle time and increasing throughput.

One embodiment of the present invention is a metal structure for a contact pad of an integrated circuit (IC) having a copper interconnect metallization. A portion of this metallization is exposed to provide a contact pad for the IC. A conductive barrier layer is disposed over the exposed portion of the copper metallization. On this barrier layer, a plug of bondable metal, preferably aluminum, between about 0.4 and 1.4 μm thick is disposed. A protective overcoat layer surrounds the plug and has a thickness such that the exposed surface of the plug is at or below the exposed surface of the overcoat layer. Optionally, a portion of the overcoat layer between about 0.1 and 0.3 μm wide may overlap the circumference of the plug.

Another embodiment of the present invention is a wafer-level method of fabricating a metal structure for contact pads of an integrated circuit, having copper interconnect metallization. The method includes chemically-mechanically polishing a wafer to expose patterned contact pad regions of the copper metallization embedded in an insulating material. A barrier metal layer is then deposited over the wafer containing the exposed copper metallization. Next, a bondable metal layer (preferably aluminum) is deposited over the barrier layer to a thickness sufficient for wire ball bonding. Next, both of the deposited metal layers are patterned so that the layer portions outside the contact pad regions are removed and the layer portions over the contact pad regions remain to form a bondable metal plug over each contact pad. A layer of protective overcoat is then deposited over the wafer, including metal plugs of the patterned layer portions. This overcoat layer has a thickness such that its surface is on or above the surface of the bondable metal layer. Finally, windows are opened in the overcoat layer to expose the bondable metal plugs.

Embodiments of the invention relate to wire-bonded IC assemblies, semiconductor device packages, surface mount, and chip-scale packages. One technical advantage is that the present invention provides a low cost method of reducing the risk of aluminum-smearing or scratching and electrical shorts between contact pads. Therefore, the assembly yield of high input / output devices can be significantly improved. A further technical advantage is that the present invention facilitates the reduction of the pitch of the chip contact pads without the risk of yield loss due to electrical shorts. Another technical benefit includes the opportunity to shrink the assembly to smaller dimensions, supporting the trend of IC miniaturization.

The technical advantages represented by the specific embodiments of the present invention will become apparent when considering the following description of the preferred embodiments of the present invention in conjunction with the novel features set forth in the accompanying drawings and the appended claims.

1 shows a schematic cross section of a contact pad of an integrated circuit (IC) with copper metallization according to the known art. Bondable metal is added as an additional layer which is elevated on the wafer surface.

2 illustrates a schematic cross section of two wire-bonded contact pads of a copper metallization IC in the known art. The protruding bondable metal layers were scratched and smeared to cause an electrical short.

3 is a schematic cross-sectional view of an embodiment of the present invention showing a contact pad of an IC having a copper metallization, having a metal plug to which the contact pad can be bonded.

4 is a cross-sectional view of the bond pad metallization according to the present invention, in which a ball bond is attached to a bondable metal plug.

5 is a block diagram of a device manufacturing process flow in accordance with another embodiment of the present invention.

The advantages provided by the present invention can be best understood by comparing embodiments of the present invention to conventional methods of wire-bonding contact pads of integrated circuit (IC) chips, using copper as interconnect metal. An example of a conventional structure is shown in FIG. In the schematic cross-sectional view of an IC contact pad, generally designated 100, 101 is an interlevel dielectric (which may be made of silicon dioxide, a low-k dielectric, or any other suitable insulator customarily used in ICs). intra-level dielectric). 102 represents the upper level IC copper metallization (thickness typically between 200 and 500 nm), with barrier layers 103a and 103b (typically tantalum nitride, thickness to prevent diffusion into other IC materials). Typically 10-30 nm). Copper metal to establish contact within an essentially moisture-impermeable overcoat layer 104 (typically between 500 and 1000 nm of silicon nitride, silicon oxynitride, or silicon dioxide, monolayer or multilayer). There is a contact window 110, generally between 40 and 70 μm wide, that exposes the cargo 102. Barrier layer 103b overlaps overcoat 104 around the window circumference to produce metallization width 111, so that metallization width is greater than window 110 (typically about 45-75 μm diameter). This same width 111 is also applied to the bondable metal layer 120, which is aluminum or a copper-aluminum alloy. For reliable wire bonding, layer 120 typically has a thickness between 700 nm and 1000 nm.

This considerable height 121 of the patterned aluminum layer 120 represents a significant risk for accidental scratching or smearing of aluminum. A typical assembly process flow after aluminum patterning has a number of wafer and chip handling steps. The most important steps are back-grinding; Conveying the wafer from the wafer fabrication fab to an assembly facility; Placing on a tape to saw the wafer; Sawing and rinsing the wafer; Attaching each chip onto a leadframe; Wire bonding; Encapsulating the bonded chip into a molding compound. In each of these process steps, and between the process steps, accidental scratching or smearing may occur.

An example is schematically indicated in FIG. 2, which is a sectional view through two bonding pads 201 and 202 in close proximity (distance 230). The aluminum layer 210 of the pad 201 and the aluminum layer 220 of the pad 202 are scratched so that the aluminum is smeared together at 240. As a result, the bond pads 250 and 251 form an electrical short.

One embodiment of the present invention is shown in FIG. 3, which illustrates a schematic cross-section of a portion 300 of a semiconductor wafer. The interlevel insulating material 310 is made of, for example, a low-k dielectric material, silicon dioxide, or a stack of dielectric materials. 3 further shows portions of the patterned top layer of IC interconnect metallization made of copper or copper alloy, embedded in insulator 310. Particularly illustrated are portions 311 of the copper layer intended to provide contact pads, and portions 312 intended to anchor scribe streets. The thickness of the copper layer is preferably in the range of 0.2 to 0.5 mu m. Copper metallization is surrounded by barrier layers 313a and 313b, respectively, to prevent diffusion into insulator 310 or other integrated circuit materials; Barrier layers 313a and 313b are preferably made of tantalum nitride and have a thickness of about 10 to 30 nm. The bond pad copper layer 311 has a width 301 (typically in the range of 30 to 60 μm).

As indicated in FIG. 3, the exposed surface (top surface) 311a of the copper layer 311, and the exposed surface (top surface) 312a of the scribe street metallization are the top surface of the dielectric material 310. It is at the same level as 310a. The reason for this uniformity is a manufacturing method comprising a chemical-mechanical polishing step (see below).

In order to establish a low-resistance ohmic contact for copper, one or more conductive barrier layers 330 are deposited over copper, as indicated in FIG. In the case of a single layer, tantalum nitride is the preferred choice. In the case of two layers, the first barrier layer is preferably selected from titanium, tantalum, tungsten, molybdenum, chromium and alloys thereof; The layer is deposited over the exposed copper 311 for the purpose of establishing a good ohmic contact to the copper by "gettering" oxygen from the copper. A second barrier layer, generally nickel vanadium, is deposited to prevent outdiffusion of copper. This barrier layer preferably has a thickness in the range of 0.02 to 0.03 μm. In FIG. 3, barrier layer 330 is shown to have the same width 301 as copper metallization 311. Although this is a preferred structure, there may be device designs where the barrier width is somewhat smaller or larger.

Above the barrier layer 330 is a layer 350 of bondable metal having a thickness suitable for wire ball bonding. Preferred thicknesses range from about 0.4 to 1.4 μm. Because of this minor thickness, layer 350 is often called a plug. This bondable metal is preferably an aluminum alloy such as aluminum or an aluminum-copper alloy. In FIG. 3, the exposed surface of this plug is indicated 350a. An aluminum layer 351 of the same thickness is shown over the scribe street metal 312 in FIG. 3.

As mentioned above, since surfaces 310a and 311a are at a common level, the combined thickness of barrier layer 330 and bondable plug 350 geometrically sticks out above this common level; As shown in FIG. 3, this combined height above the level is indicated at 360. In order to prevent any accidental scratching or smearing, a protective overcoat layer 320 is deposited (see below for details). Preferred overcoat materials are practically moisture impermeable or moisture retaining, mechanically robust; Eg, one or more layers of silicon nitride, silicon oxynitride, silicon carbide, or a stack of insulating materials comprising polyimide. The overcoat has a thickness 320b in the range of 0.5 to 1.5 μm, preferably 1.0 μm. In FIG. 3, the exposed surface of overcoat layer 320 is indicated 320a.

According to the present invention, the deposited protective overcoat layer 320 has a thickness 320a such that the exposed surface 350a of the plug 350 is at or below the exposed surface 320a of the overcoat layer 320. It surrounds the plug 350. A window of width 322 is opened in the overcoat 320 to expose the surface 350a of the plug 350. Preferably, the width 322 is narrower than the width 301 of the plug 350; Thus, a portion of the overcode 320 (indicated by 321 in FIG. 3) overlaps the circumference of the plug 350. Similar statements apply to the overcoat layer 320 for the aluminum layer 351. The plug surface 350a and the layer surface 351a do not protrude relative to the overcoat surface 320a; Thus, plug 350 and layer 351 are each protected against accidental scratches, providing an unobstructed plug metal for reliable ball bonding.

The cross-sectional view of FIG. 4 schematically illustrates the contact pad of FIG. 3 after the chip is singulated from the wafer and the ball bond is attached in the sawing process (scribe street indicated by 410). The free air ball 401 (preferably gold) of the metal wire 402 (preferably gold) is an unobstructed surface 403a of the plug 403 (preferably aluminum or aluminum alloy). Pressure-bonded). In the bonding process, intermetallic compounds 404 are formed in the contact region of the ball and plug.

Another embodiment of the present invention is a wafer-level method of fabricating a metal structure for contact pads of an integrated circuit, having copper interconnect metallization. The process flow is shown in the schematic block diagram of FIG. The method begins with step 501 and chemically mechanically polishes the wafer in step 502 to expose patterned contact pad regions of copper metallization embedded in an insulating material.

In a next process step 503, a barrier metal layer is deposited over the wafer, including the exposed copper metallization. Preferred barrier metal species include tantalum or tantalum nitride, and nickel vanadium; Preferred barrier layer thicknesses are between about 20 and about 30 nm. In step 504, a bondable metal layer over the barrier layer is deposited to a thickness sufficient for wire ball sealing. Preferred bondable metal species include aluminum or aluminum alloys, and the preferred bondable metal layer thickness is between about 0.4 and 1.4 μm.

In a next process step 505, both of the deposited metal layers are patterned such that the layer portions outside the contact pad regions are removed and the layer portions above the contact pad regions remove a bondable metal plug over each of the contact pads. Left to form.

In a next process step 506, a layer of protective overcoat is deposited over the wafer, including metal plugs of the patterned layer portions formed in step 505. This overcoat preferably comprises one or more layers of silicon nitride, silicon oxynitride, silicon dioxide, silicon carbide, or other moisturizing compounds. This overcoat layer has a thickness such that its exposed surface is on or above the exposed surface of the bondable metal layer. Preferred overcoat thicknesses range from about 0.6 to 1.5 μm.

In process step 507, windows are opened in the overcoat layer to expose the bondable metal plugs. These windows can be sized such that an overcoat frame having a width between about 0.1 and 0.3 μm is left around the circumference of the bond pad area to provide the plug with additional protection against accidental scratches. This method ends the process at step 508.

Although the present invention has been described with reference to exemplary embodiments, this description should not be interpreted in a limiting sense. Various modifications and combinations of the exemplary embodiments, as well as other embodiments of the present invention, will be apparent to those skilled in the art with reference to this description. Accordingly, the appended claims are intended to cover any such modifications and embodiments.

Claims (19)

An integrated circuit having copper interconnecting metallization, wherein a portion of the metallization is exposed to provide a contact pad for the integrated circuit; One or more layers of conductive barrier metals disposed on the exposed portion of the copper metallization; A bondable metal layer disposed on the barrier layer, the bondable layer having a thickness suitable for wire bonding and an exposed surface; And A protective overcoat layer surrounding the bondable layer, such that the exposed surface of the bondable layer is at or below the exposed surface of the overcoat layer Integrated circuit having a copper interconnect metallization comprising. A metal structure for an integrated circuit having a copper interconnect metallization, wherein a portion of the metallization is exposed to provide a contact pad for the integrated circuit; A conductive barrier layer disposed on the exposed surface of the copper metallization; A plug of bondable metal disposed on the barrier layer; And A protective overcoat layer surrounding the plug, such that the exposed surface of the plug is at or below the exposed surface of the overcoat layer Metal structure for an integrated circuit having a copper interconnect metallization comprising. The metal structure of claim 2, wherein the overcoat thickness is in the range of about 0.6 to 1.5 μm. The metal structure of claim 2, wherein the overcoat layer overlaps between about 0.1 and 0.3 μm over the plug circumference. The metal structure of claim 2, wherein the overcoat comprises one or more layers of silicon nitride, silicon oxynitride, silicon dioxide, silicon carbide, or other moisture-retaining compounds. The metal structure of claim 2, wherein the bondable metal plug is aluminum or an aluminum alloy. The metal structure of claim 2, wherein the plug has a thickness between about 0.4 and 1.4 μm. The metal structure of claim 2, further comprising a ball bond attached to the plug. The metal structure of claim 2, wherein the barrier layer comprises tantalum nitride. 3. The barrier layer of claim 2 wherein the barrier layer is selected from the group consisting of tantalum, titanium, tungsten, molybdenum, chromium, vanadium, alloys thereof, stacks thereof, and chemical compounds thereof. Metal structure. The metal structure of claim 2, wherein the barrier layer has a thickness between about 0.02 and 0.03 μm. The metal structure of claim 2, wherein the barrier layer is patterned into the same area as the contact pad portion of the metallization. 3. The metal structure of claim 2 wherein said bondable metal plug is patterned into the same area as said contact pad portion of said metallization. The metal structure of claim 2, wherein a portion of the overcoat layer overlaps a circumference of the plug. A wafer-level method of manufacturing a metal structure for a contact pad of an integrated circuit having a copper interconnect metallization, comprising: Chemically-mechanically polishing a wafer to expose patterned contact pad regions of the copper metallization embedded in an insulating material; Depositing a barrier metal layer on the wafer comprising the exposed copper metallization; Depositing a bondable metal layer over the barrier layer to a thickness sufficient for wire ball bonding; Patterning both the deposited metal layers so that layer portions outside the contact pad regions are removed and the layer portions over the contact pad regions remain to form a bondable metal plug over each of the contact pads. Doing; Depositing a layer of protective overcoat on the wafer comprising the metal plugs of the patterned layer portions, the overcoat layer being thick such that its exposed surface is on or above the exposed surface of the bondable metal layer. Has-; Opening a window in the overcoat layer to expose the bondable metal plugs. Wafer-level manufacturing method of a metal structure comprising a. The method of claim 15, wherein depositing a bondable metal layer comprises aluminum in a thickness range of about 0.4 to 1.4 μm. The method of claim 15, wherein the overcoat has a thickness in the range of about 0.6 to 1.5 μm. The method of claim 15, wherein the overcoat frame has a width between about 0.1 and 0.3 μm. 16. The method of claim 15, wherein the opening step in the overcoat layer leaves a frame of overcoat around the perimeter of each plug.

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