Wire bonding process

Abstract: A robust, reliable and low-cost metal structure and process enabling electrical wire/ribbon connections to the interconnecting copper metallization of integrated circuits. The structure comprises a layer of barrier metal that resists copper diffusion, deposited on the non-oxidized copper surface in a thickness such that the barrier layer reduces the diffusion of copper at 250 °C by more than 80 % compared with the absence of the barrier metal. The structure further comprises an outermost bondable layer which reduces the diffusion of the barrier metal at 250 °C by more than 80 % compared with the absence of the bondable metal. Finally, a metal wire is bonded to the outermost layer for metallurgical connection. The barrier metal is selected from a group consisting of nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. The outermost bondable metal layer is selected from a group consisting of gold, platinum, and silver.

Description: BACKGROUND OF THE INVENTION The present invention is related in general to the field of semiconductor devices and processes and more specifically to the process of wire bonding to bond pads of copper-metallized integrated circuits. DESCRIPTION OF THE RELATED ART In integrated circuits (IC) technology, pure or doped aluminum has been the metallization of choice for interconnection and bond pads for more than four decades. Main advantages of aluminum include easy of deposition and patterning. Further, the technology of bonding wires made of gold, copper, or aluminum to the aluminum bond pads has been developed to a high level of automation, miniaturization, and reliability. Examples of the high technical standard of wire bonding to aluminum can be found in U.S. Patents # 5,455,195, issued on Oct. 3, 1995 (Ramsey et al., "Method for Obtaining Metallurgical Stability in Integrated Circuit Conductive Bonds"); # 5,244,140, issued on Sep. 14, 1993 (Ramsey et al., "Ultrasonic Bonding Process Beyond 125 kHz"); # 5,201,454, issued on Apr. 13, 1993 (Alfaro et al., "Process for Enhanced Intermetallic Growth in IC Interconnections"); and # 5,023,697, issued on Jun. 11, 1991 (Tsumura, "Semiconductor Device with Copper Wire Ball Bonding"). In the continuing trend to miniaturize the ICs, the RC time constant of the interconnection between active circuit elements increasingly dominates the achievable IC speed-power product. Consequently, the relatively high resistivity of the interconnecting aluminum now appears inferior to the lower resistivity of metals such as copper. Further, the pronounced sensitivity of aluminum to electromigration is becoming a serious obstacle. Consequently, there is now a strong drive in the semiconductor industry to employ copper as the preferred interconnecting metal, based on its higher electrical conductivity and lower electromigration sensitivity. From the standpoint of the mature aluminum interconnection technology, however, this shift to copper is a significant technological challenge. Copper has to be shielded from diffusing into the silicon base material of the ICs in order to protect the circuits from the carrier lifetime killing characteristic of copper atoms positioned in the silicon lattice. For bond pads made of copper, the formation of thin copper (I) oxide films during the manufacturing process flow has to be prevented, since these films severely inhibit reliable attachment of bonding wires, especially for conventional gold-wire ball bonding. In contrast to aluminum oxide films overlying metallic aluminum, copper oxide films overlying metallic copper cannot easily be broken by a combination of thermocompression and ultrasonic energy applied in the bonding process. As further difficulty, bare copper bond pads are susceptible to corrosion. In order to overcome these problems, a process has been disclosed to cap the clean copper bond pad with a layer of aluminum and thus re-construct the traditional situation of an aluminum pad to be bonded by conventional gold-wire ball bonding. A suitable bonding process is described in U.S. Patent # 5,785,236, issued on Jul. 28, 1998 (Cheung et al., "Advanced Copper Interconnect System that is Compatible with Existing IC Wire Bonding Technology"). The described approach, however, has several shortcomings. First, the fabrication cost of the aluminum cap is higher than desired, since the process requires additional steps for depositing metal, patterning, etching, and cleaning. Second, the cap must be thick enough to prevent copper from diffusing through the cap metal and possibly poisoning the IC transistors. Third, the aluminum used for the cap is soft and thus gets severely damaged by the markings of the multiprobe contacts in electrical testing. This damage, in turn, becomes so dominant in the ever decreasing size of the bond pads that the subsequent ball bond attachment is no longer reliable. A low-cost structure and method for capping the copper bond pads of copper-metallized ICs has been disclosed in European Patent Application No.01000021.4, filed on 18 Feb. 2001. The present invention is related to that application. An urgent need has arisen for a reliable method of bonding wires to capped bond pads which combines minimum fabrication cost with maximum up-diffusion control of metals potentially capable of impeding subsequent wire bonding. The bonding method should be flexible enough to be applied for different IC product families and a wide spectrum of design and process variations. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput, and without the need of expensive additional manufacturing equipment. The present invention provides structures and methods as set forth in the claims. Further examples of bondable structures may be found in European Patent Application EP 0949672 and GB Application GB2184288. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be further described, by way of example, with reference to the preferred and exemplary embodiments illustrated in the figures of the accompanying drawings in which: FIG. 1A and 1B illustrate schematic cross sections of the preferred embodiment of the invention. FIG. 1A shows a bondable cap of stacked layers over a bond pad of an integrated circuit having copper metallization. FIG. 1B shows the bond pad of FIG. 1A including a ball-bonded wire. FIG. 2 is a more detailed yet still schematic cross section of a preferred embodiment of the invention. FIG. 3 is a more detailed yet still schematic cross section of a preferred embodiment of the invention. FIG. 4 illustrates a block diagram of the process f low for fabricating the bond pad cap according to the invention. APPENDIX: The Table is listing the calculated thicknesses of barrier metal layers which are required to reduce the up-diffusion of the underlying metal by more than 80 % compared with the absence of the barrier metal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1A shows a schematic cross section of the preferred embodiment of the invention, generally designated 100. An integrated circuit (IC) has copper interconnecting metallization and is covered by a moisture-impenetrable protective overcoat 101. This overcoat is usually made of silicon nitride, commonly 0.5 to 1.0 µm thick. A window 102 is opened in the overcoat in order to expose portion of the copper metallization 103. Not shown in FIG. 1A is the underlayer embedding the copper and preventing its diffusion into parts of the IC (usually made of tantalum nitride, tantalum silicon nitride, tungsten nitride, tungsten silicon nitride, titanium, titanium nitride, or titanium tungsten; see FIG. 3). In FIG. 1A, the dielectric IC portions 104 are only summarily indicated. These electrically insulating portions may include not only the traditional plasma-enhanced chemical vapor deposited dielectrics such as silicon dioxide, but also newer dielectric materials having lower dielectric constants, such as silicon-containing hydrogen silsesquioxane, organic polyimides, aerogels, and parylenes, or stacks of dielectric layers including plasma-generated or ozone tetraethylorthosilicate oxide. Since these materials are less dense and mechanically weaker than the previous standard insulators, the dielectric under the copper is often reinforced. Examples can be found in U.S. Patent applications # 09/312,385, filed on 14/05/1999 (Saran et al., "Fine Pitch System and Method for Reinforcing Bond Pads in Semiconductors"), and # 09/347,212, filed 02/07/1999 (Saran, "System and Method for Bonding over Active Integrated Circuits"). Since copper is susceptible to corrosion and even thin copper(I)oxide films are difficult to bond to, the present invention provides structures and processes of a cap formed over the exposed copper, as illustrated in FIGs. 1, 2 and 3. According to the invention, the cap consists of a stack of metals having coordinated thicknesses such that the stack satisfies three requirements: The cap acts as a barrier against the up-diffusion of copper to the surface of the cap where the copper might impede the subsequent wire bonding operation. Specifically, the cap the metal selections and thicknesses are coordinated such that the cap reduces the up-diffusion of copper at 250 °C by more than 80 % compared with the absence of the barrier metal. The cap is fabricated by a technique, which avoids expensive photolithographic steps. Specifically, an electroless process is used to deposit the cap metal layers. The cap has an outermost metal surface which is bondable. Specifically, conventional ball and wedge bonding techniques can be used to connect metal wires and other coupling members metallurgically to the bond pad. As indicated in FIGs. 1B and 2, wire ball bonding is the preferred method of using coupling members to create electrical connections. Another method is ribbon bonding employing wedge bonders. In contrast to wedge bonding, ball bonding operates at elevated temperatures for which the materials and processes of this invention need to be harmonized. The wire bonding process begins by positioning both the IC chip with the bond pads and the object, to which the chip is to be bonded, on a heated pedestal to raise their temperature to between 170 and 300 °C. A wire 110 (in FIGs. 1B), typically of gold, gold-beryllium alloy, other gold alloy, copper, aluminum, or alloys thereof, having a diameter typically ranging from 18 to 33 µm, is strung through a heated capillary where the temperature usually ranges between 200 and 500 °C. At the tip of the wire, a free air ball is created using either a flame or a spark technique. The ball has a typical diameter from about 1.2 to 1.6 wire diameters. The capillary is moved towards the chip bonding pad (102 in FIG. 1A) and the ball is pressed against the metallization of the bonding pad cap (layer 106 in FIGs. 1A and 1B). A combination of compression force and ultrasonic energy creates the formation of a strong metallurgical bond by metal interdiffusion. At time of bonding, the temperature usually ranges from 150 to 270 °C. In FIGs. 1B, schematic form 111 exemplifies the final shape of the attached "ball" in wire ball bonding. It is important for the present invention that recent technical advances in wire bonding now allow the formation of small yet reliable ball contacts and tightly controlled shape of the wire loop. Ball pitches as small as between 75 and 40 µm can be achieved. Such advances can, for instance, be found in the computerized bonder 8020 by Kulicke & Soffa, Willow Grove, PA, U.S.A., or in the ABACUS SA by Texas Instruments, Dallas, TX, U.S.A. Moving the capillary in a predetermined and computer-controlled manner through the air will create a wire looping of exactly defined shape. Finally, the capillary reaches its desired destination and is lowered to touch the contact pad of the object. With an imprint of the capillary, a metallurgical stitch bond is formed, and the wire is flamed off to release the capillary. Stitch contacts are small yet reliable; the lateral dimension of the stitch imprint is about 1.5 to 3 times the wire diameter (its exact shape depends on the shape of the capillary used, such as capillary wall thickness and capillary footprint). It is an advantage of the present invention to provide a metal cap surface of the bond pad hard enough that the fine-tip needles used in electrical multiprobing do not create probe marks. A soft metal surface, torn up by the needle imprint, is particularly difficult to bond to when the area of the bond pad is so small -- a trend in contemporary bond pad shrinking -- that the imprint disturbs most of the available bonding area. According to the invention, the metal cap over the copper 103 is provided by two layers: Layer 105 is positioned over copper 203, sometimes deposited on a seed metal layer (see FIGs. 2 and 3). Examples for layer 105 are nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. These metals are inexpensive and can be deposited by electroless plating; however, they are poorly bondable. In these metals, copper has a diffusion coefficient of less than 1 X 10E-23 cm 2 /s at 250 °C. Consequently, these metals are good copper diffusion barriers. The layer thicknesses required to reduce copper diffusion by more than 80 % compared to the absence of the layers are obtained by diffusion calculations. Generally, a barrier thickness from about 0.5 to 1.5 µm will safely meet the copper reduction criterion. Layer 106 is positioned over layer 105 as the outermost layer of the cap; it is bondable so that it can accept the wire bond 111. Examples for layer 106 are gold, platinum, palladium, and silver. In addition, these metals have a diffusion coefficient for the metals used in barrier 105 (such as nickel) of less than 1 x 10E-14 cm 2 /s at 250 °C. Consequently, these metals are good diffusion barriers for the materials of layer 105. Again, the layer thicknesses required to reduce the up-diffusion of metal used in layer 105 by more than 80 % compared to the absence of layer 106 are obtained from diffusion calculations. Generally an outermost layer thickness of 1.5 µm or somewhat less will safely meet the reduction criterion for metal diffusing from layer 105. Generally, a thickness of outermost layer 106 of about 0.4 to 1.5 µm will safely meet the reduction criterion for metal diffusing from layer 105. The process flow of electroless plating is described in conjunction with FIG. 4. Usually, the plated layers will fit into the size of the bond pad opening (102 in FIG. 1A). For bond pads having protective overcoats of reduced thickness, however, one or more of the plated layers may grow electrolessly beyond the periphery of the opening. FIG. 2 illustrates schematically an example for this layer growth. The protective overcoat layer 201 has reduced thickness 201a (for instance, 0.5 µm instead of the usual 1.0 µm). While the metal seed layer 208, plated directly over the non-oxydized surface 203a of the copper metallization 203, fits easily within the overcoat opening, the barrier layer 205 and the bondable layer 206 grow beyond the opening periphery. This cap region is designated 205a and 206a in FIG. 2; it does not affect the metallurgical attachment of wire "ball" 211, but could affect the minimum distance to adjacent bond pads. . FIG. 3 summarizes the preferred embodiment of the present invention in more detail; most dimension ranges are quoted in FIGs. 1a and 1B, and the electroless plating and other fabrication process steps are discussed in FIG. 4. The protective overcoat 301 has an opening, which defines the size of the bond pad, and a thickness sufficient to accommodate all of the stacked layers, which cap the bond pad IC copper metallization 303. The copper trace 303 is imbedded in refractory metal shield 302 (for example, tantalum nitride), which is surrounded by dielectric 304 and metal re-enforcements 304a; methods quoted above). Directly facing the cleaned and non-oxidized copper surface 303a is the first layer of the cap, a thin layer 308 of seed metal (for example, palladium, about 5 to 10 nm thick; another choice is tin). Immediately following the seed metal layer is metal layer 305 (for example, nickel) as a barrier against up-diffusing copper. On top of this barrier layer is metal layer 306 (for example, gold, or palladium) as a barrier against up-diffusing barrier metal (such as nickel) and, at the same time, as the outermost layer of the cap which is metallurgically bondable. The electroless process used for fabricating the bond pad cap of FIG. 3 is detailed in FIG. 4. After the bond pads have been opened in the protective overcoat, exposing the copper IC metallization in bond pad areas, the cap deposition process starts at 401; the sequence of process steps is as follows: Step 402: Coating the backside of the silicon IC wafer with resist using a spin-on technique. This coat will prevent accidental metal deposition on the wafer backside. Step 403: Baking the resist, typically at 110 °C for a time period of about 30 to 60 minutes. Step 404: Cleaning of the exposed bond pad copper surface using a plasma ashing process for about 2 minutes. Step 405: Cleaning by immersing the wafer, having the exposed copper of the bond pads, in a solution of sulfuric acid, nitric acids, or any other acid, for about 50 to 60 seconds. Step 406: Rinsing in overflow rinser for about 100 to 180 seconds. Step 407: Immersing the wafer in a catalytic metal chloride solution, such as palladium chloride, for about 40 to 80 seconds "activates" the copper surface, i.e., a thin layer of seed metal (such as palladium) is deposited onto the clean non-oxidized copper surface. Step 408: Rinsing in dump rinser for about 100 to 180 seconds. Step 409: Electroless plating of barrier metal against copper up-diffusion. If nickel is selected, plating between 150 to 180 seconds will deposit about 0.4 to 0.6 µm thick nickel. Step 410: Rinsing in dump rinser for about 100 to 180 seconds. Step 411: Electroless plating of outermost layer, which is bondable and simultaneously provides a barrier against up-diffusion of the underlying barrier metal. If gold or palladium is selected, plating between 150 to 180 seconds will deposit about 0.4 to 0.6 µm thick gold or palladium, respectively. A preferred process uses first an immersion step with self-limiting surface metal replacement. If gold is selected, plating between 400 and 450 seconds will deposit approximately 30 nm thick gold. As a second step for thicker metal layer (0.5 to 1.5 µm thick), the immersion process is followed by an autocatalytic process step. Step 412: Rinsing in dump rinser for about 100 to 180 seconds. Step 413: Stripping wafer backside protection resist for about 8 to 12 minutes. Step 414: Spin rinsing and drying for about 6 to 8 minutes. The bond pad cap fabrication process stops at 415. The subsequent metallurgical connection of metal wires or ribbons by a ball or wedge bonding process is described above. While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the invention can be applied to IC bond pad metallizations other than copper, which are difficult or impossible to bond by conventional ball or wedge bonding techniques, such as alloys of refractory metals and noble metals. As another example, the invention can be extended to batch processing, further reducing fabrication costs. As another example, the invention can be used in hybrid technologies of wire/ribbon bonding and solder interconnections.

Claims: A structure for metallurgical connections between metal wires (110) and bond pads positioned on an integrated circuit having copper interconnecting metallization, comprising; a bond pad surface of non-oxidized copper (103/203); a layer (105/205) of barrier metal that resists copper diffusion deposited on said copper surface; an outermost layer (206/206a) of bondable metal; one of said metal wires bonded to said outermost bondable metal; characterized in that :- said barrier metal and the thickness thereof are coordinated such that said layer reduces the diffusion of copper at 250 °C by more than 80% compared with the absence of said barrier metal; the thickness of the bondable metal is such that said outermost layer reduces the diffusion of said barrier metal at 250 °C by more than 80 % compared with the absence of said bondable metal; and in that it further includes a thin seed metal layer (208) between said non-oxidized copper and said barrier metal layer, wherein said seed metal is palladium or tin. The structure according to Claim 1 wherein said barrier metal layer is selected from a group consisting of nickel, cobalt, chromium, molybdenum, titanium, tungsten, and alloys thereof. The structure according to Claim 1 or Claim 2 wherein said bondable metal layer is selected from a group consisting of gold, platinum, palladium, and silver. The structure according to any preceding Claim wherein said metal wires are selected from a group consisting of gold, copper, aluminum, and alloys thereof. A method for forming metallurgical connections between metal wires and bond pads positioned on integrated circuits having copper interconnecting metallization, comprising the steps of ; depositing a layer of, seed metal of palladium or tin on the surface of said copper metallization of said bond pads; plating a layer of barrier metal of sufficient thickness to resist copper diffusion at 250°C by more than 80 % compared with the absence of said barrier metal, by electroless deposition; plating a layer of a bondable metal of sufficient thickness, by electroless deposition, to reduce the diffusion of barrier metal at 250 °C by more than 80 % compared with the absence of said bondable metal to form the outermost bondable metal layer of said bond pad; and bonding one of said metal wires onto said outermost metal. The method according to Claim 5 wherein said wire bonding step comprises ball bonding or wedge bonding. The method according to Claim 5 or Claim 6 wherein said bond pads are formed by a process comprising; depositing a protective overcoat over the surface of said integrated circuit, including the surface portions having copper metallization; and opening selected areas of said overcoat by photolithographic techniques, exposing the surface of said copper metallization. The method according to Claim 7 further comprising a cleaning step after said opening step, by immersing said exposed copper surface in a solution of sulphuric acid, nitric acid, or any other acid. The method of any of claims 5 to 8 wherein said step of depositing seed metal comprises immersing the bond pads in a catalytic metal chloride solution. The method according to Claim 9 wherein said metal chloride comprises palladium chloride. The method according to any of Claims 5 to 10 wherein said electroless plating of said bondable metal layer is immersion plating. The method according to Claim 11 wherein said electroless plating of said bondable metal layer is immersion plating followed by autocatalytic plating. The method according to any of Claims 5 to 12 further comprising the step of electrically probing said outermost metal of said bond pad before the step of bonding, leaving substantially no probe marks.

Inventors: AMADOR GONZALO, SUBIDO WILLMAR E, TEST HOWARD R, AMADOR, GONZALO, SUBIDO, WILLMAR E., TEST, HOWARD R.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 2001
Main classification: H01L21
IPC codes: H01L21, H01L23