Corrosion tolerant bonding pad and method of fabricating same.

0001The present invention relates to bonding pads used to attach lead wires to an integrated circuit.

0002In conventional integrated circuits, as shown in the partial cross-section of Fig. 1, interconnects 12 provide electrical connections between individual devices (not shown) on the integrated circuit and extend to the edges of the integrated circuit where enlarged regions called bonding pads 13 are provided. Metal "lead" wires 14 are attached to the bonding pads 13 in order to apply power, signals, and other electrical interconnections to the integrated circuit. The interconnects 12 are usually formed of 90%-100% pure aluminum and lead wires 14 are usually formed of gold, but may also be formed of aluminum. Lead wires 14 are attached to the bonding pads 13 by thermal compressive bonding which creates a bonded region 15.

0003The interconnects 12 and bonding pads 13 are covered by passivation layers 16 formed of SiO₂, doped SiO₂, Si₃N₄, or a combination of these layers. Apertures 18 of approximately 16-20 mil² are provided through portions of the passivating layer 16 corresponding to the center portions of the bonding pads 13 which are usually 20-25 mil². Bonded region 15, however, covers only 30-70% of the portion of bonding pad 13 exposed by aperture 18. Thus, a portion of each bonding pad 13 remains exposed after the leads 14 are attached thereto.

0004Passivation layer 16 prevents corrosion of interconnect 12, but in the regions of aperture 18 where bonded region 15 does not cover bonding pad 13 the aluminum bonding pad 13 is subject to oxidation, or other corrosion. This oxidation or corrosion transforms aluminum bonding pad 13 into a non-­conducting material, e.g., aluminum oxide or aluminum hydroxide. If the oxidation of bonding pad 13 occurs in region 13b, it will cause increased resistance between the bonding pad 13 and interconnect 12, and in the worst case an open circuit between bonding pad 13 and the remaining portion of interconnect 12.

0005In the case where the passivation layer 16 is doped SiO₂, moisture which enters the integrated circuit package combines with phosphorous, which is present as a dopant in the passivating layers 16, to form phosphoric acid which reacts with the exposed aluminum converting the aluminum to aluminum oxide or aluminum hydroxide. This phenomenon is facilitated by the direct contact between the exposed portion of bonding pad 13, particularly portion 13b, and passivating layer 16.

0006Other corrosion sources may be associated with the packaging material of an integrated circuit. Generally, the packaging is a plastic material which may have components which cause corrosion. In addition, other sources of corrosive materials may exist in the integrated circuit itself or may occur as the result of fabrication or packaging processes.

SUMMARY OF THE INVENTION

0007We will describe a corrosion tolerant bonding pad.

0008We will describe a corrosion resistant barrier layer on the enlarged portion of the interconnect to which a connection is to be made in order to prevent corrosion of the interconnect.

0009We will describe an arrangement to prevent or delay corrosion of the bonding pad structure, particularly the exposed bonding layer.

0010We will describe a physical separation between any exposed aluminum layers and phosphorous-doped passivating layers to reduce or eliminate corrosion by phosphoric acid.

0011In accordance with the present invention a corrosion tolerant bonding pad, comprising: a conductive interconnect; a passivating layer provided on said conductive interconnect, said passivating layer having an aperture which exposes a portion of said conductive interconnect; a conductive barrier layer provided on said conductive interconnect in said aperture; and a bonding layer provided on said barrier layer.

0012The barrier layer may be non-corrosive, conductive material, for example, a compound of titanium and tungsten (TiW -- known as ti-tungsten). To increase the physical separation between the passivating layer and the bonding layer, thereby reducing or eliminating any corrosion of the bonding layer by phosphoric acid formed from the phosphorous dopant in the passivating layer, the area of the bonding layer is made less than the area of the barrier layer.

0013A bonding pad in accordance with the present invention has the advantage that the bonding region of the interconnect is less likely to oxidize or otherwise corrode. The electrical connection between a lead wire and the interconnect is not affected by corrosion of the portions of the bonding layer not protected by the material used to bond the lead wire to the bonding layers, and thus the bonding pad is corrosion tolerant.

BRIEF DESCRIPTION. OF THE DRAWINGS

0014<ul id="ul0001" list-style="none"><li>Fig. 1 is a cross-sectional view of a conventional bonding pad;</li><li>Fig. 2A is a plan view of a corrosion tolerant bonding pad in accordance with the present invention;</li><li>Fig. 2B is a cross-sectional view taken along line B-B′ in Fig. 2A; and</li><li>Fig. 3 is a cross-sectional view of an alternative embodiment of a corrosion tolerant bonding pad in accordance with the present invention.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

0015A corrosion tolerant bonding pad in accordance with the present invention and a method of fabricating same will be described with reference to Figs. 2A and 2B. It is to be understood that the materials, thicknesses, and structures described with respect to the preferred embodiments of the present invention are exemplary and are not intended to limit the scope of the invention. As used herein, the term "bonding pad" refers to any single or multi-layer structure useful in the electrical interconnection of an interconnect and a lead wire.

0016As shown in Figs. 2A and 2B, a substrate 10 has an interconnect 12 provided thereon. As in conventional semiconductor devices, interconnect 12 may be formed of aluminum or compounds of aluminum and materials such as copper and/or silicon, e.g. aluminum-copper, aluminum-­ silicon, and aluminum-copper-silicon. Alternatively, interconnect 12 may be formed of other materials or compounds which are electrical conductors and useful in fabricating semiconductor devices in accordance with techniques known in the semiconductor industry. Further, the interconnect 12 may be formed directly on substrate 10 or on other layers provided on substrate 10, for example, interconnect 12 may be provided on insulating layers which separate it from other underlying interconnects (not shown) in, for example, a dual metallization structure.

0017A passivating layer 16, formed by conventional techniques, is provided on interconnect 12 and has an aperture (or opening) 18 to expose an enlarged bonding area 26 of interconnect 12. To prevent oxidation or other corrosion of bonding area 26 of interconnect 12, a corrosion resistant, conductive barrier layer 28 is provided on bonding area 26 in aperture 18 of passivating layer 16. Examples of the materials which may be utilized to form barrier layer 28 are tungsten and titanium and compounds of these materials, or other materials which are not subject to significant oxidation or corrosion when exposed to phosphoric acid and other agents present in a packaged integrated circuit. In the preferred embodiment of the present invention the barrier layer is formed of a compound of titanium and tungsten (ti-tungsten or Tiw) and has a thickness of approximately 1000-2000Å.

0018To prevent exposure of bonding area 26 of interconnect 12 to oxidizing or other corrosive environments, barrier layer 28 is provided both in the entirety of aperture 18 and on portion 14a of passivating layer 14 surrounding aperture 18. Barrier layer 28 may be formed be co-sputtering from titanium and tungsten targets or electron beam (E-beam deposition. One important aspect of providing the barrier layer is to insure that an ohmic contact is provided with interconnect 12. Thus, oxidation of interconnect 12 prior to deposition of barrier layer 28 must be avoided and, if any oxidation does occur, the oxide should be removed by, for example, reverse sputtering.

0019A bonding layer 30 formed of, for example, aluminum, is provided on barrier layer 28, and lead wire 14 is attached to bonding layer 30 using conventional bonding techniques, for example, thermal compressive bonding. Bonding layer 30 may also be formed of gold. If the bonding layer 30 is gold, it could be fabricated by standard lift-off techniques. In the preferred embodiment bonding layer 30 is formed of 90%-100% pure aluminum and has a thickness of approximately 1µm. However, thicknesses ranging from 0.25 to 2µm may be used. Bonding layer 30 may be provided with the same techniques used to provide barrier layer 28. Again, an ohmic contact must be provided between barrier layer 28 and bonding layer 30.

0020Another characteristic of the material selected for barrier layer 28 is that it must not be penetrated by the material of bonding layer 30 during subsequent fabrication steps or at temperatures experienced in operation of the integrated circuit.

0021In accordance with the present invention corrosion of bonding area 26 of interconnect 12 is prevented, and even if bonding pad 30 corrodes, the portion of bonding pad 30 underlying bond 18 will be protected from corrosion, thereby preventing or substantially delaying increased resistance and/or open circuits between lead wires 14 and interconnects 12.

0022An alternative embodiment of the present invention will be described with reference to Fig. 3. In the alternative embodiment the bonding layer 30′ has a smaller area than the barrier layer 28. For example, if the barrier layer 26 and bonding layer 30′ are square and have respective dimensions x and y, the area x² of barrier layer 28 is less than the area y² of bonding layer 30′. The purpose of reducing the area of bonding layer 30′ with respect to the area of barrier layer 28 is to increase the physical separation of the aluminum bonding layer 30′ and passivating layer 16, thereby reducing or eliminating the possibility that phosphoric acid formed from the phosphorous dopant ions in the passivating layer will react with the aluminum of bonding layer 30′.

0023The reduced size of bonding layer 30′ may be accomplished by two methods. As with other elements in integrated circuits, barrier layer 28 and bonding layer 30′ are deposited as continuous layers which are patterned by standard photolithographic techniques. If a single mask is to be used to etch the patterns of both barrier layer 28 and bonding layer 30′, the integrated circuit would first be subject to an etchant which etches the aluminum bonding layer 30′, then to an etchant which etches the material of barrier layer 28, and thereafter to a second etching with an etchant for aluminum to cause lateral etching of bonding layer 30′. In this method the etchant for aluminum must be a selective etchant which does not etch the material of the barrier layer 28. The selectivity of the etchant may be provided by selecting appropriate etchants or by selecting materials for barrier layer 25 and bonding layer 30′ which are not affected by the same etchants.

0024If separate masks are used to etch the patterns of barrier layer 28 and bonding layer 30′, then the areas of the mask which are used to protect the barrier layer 28 and the bonding layer 30′ during the etching process are selected to provide the desired difference in the area of barrier layer 30′.

0025The many features and advantages of the corrosion tolerant bonding pad of the present invention will be apparent to those of ordinary skill in the art. Thus, the following claims are intended to cover all modifications and equivalents falling within the scope of the invention.