1. Field of the Invention
The present invention relates to semiconductor packaging technology generally and, more specifically, to wire bonding of integrated circuit devices to a substrate using copper bond wires.
2. Description of the Related Art
Wire bonding is a widely used technique for electrically interconnecting a semiconductor device or “chip” to conductors on an organic-based substrate, such as a thin (less than one millimeter thick) glass-epoxy board. Traditionally, gold bond wires were used to do the interconnection between a die pad, typically aluminum, on the device and a nickel/gold-plated copper substrate pad on the substrate. However, due to the high cost of gold, copper and palladium-coated copper bond wires have become popular. The copper bond wires are bonded between bond pads on the device and substrate pads on the substrate in a wire bonder machine using conventional ultrasonic bonding techniques.
Because copper is less noble and therefore more reactive than gold, care must be employed to prevent contamination of the copper wire and pads so that a reliable bond can be made. However, even with controlled environments and extensive cleaning techniques to prevent contamination, it is clear that copper wire bonded devices experience a slightly elevated field failure rate relative to gold wire bonded devices. This failure rate might be one reason copper bond wire technology has not been readily adopted in high-reliability applications such as in the automotive industry.
Moreover, the temperature cycling, humidity (with and without an applied bias), thermal exposure, and other stresses can lead to the formation of interface defects including cracks that can eventually cause separation of the copper bond wire and a pad, possibly resulting in a functional failure of the wire bonded device. Thus, it is desirable to understand the mechanism causing the failures and provide a technique to address those failures to increase the reliability of copper bond wire technology and, concomitantly, a more reliable wire bonded device.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
Described embodiments include a wire bonder having a heater block configured to heat a substrate and devices attached to a major surface of the substrate, a clamp configured to press the substrate down onto the heater block and thereby isolating a region of the substrate and devices attached thereto from a remainder of the substrate and devices, a bonder head operable within the isolated region and configured to attach bond wires from bond pads on the devices to substrate pads on the major surface and adjacent to the device being wire bonded, and a gas source configured to flood a portion of the substrate and the devices attached thereto with a purge gas while the substrate and attached devices are being heated by the heater, the portion being apart from the substrate and the devices in the isolated region.
Other embodiments of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings in which like reference numerals identify similar or identical elements. The drawings are not to scale.
Reference herein to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments. The same applies to the term “implementation”.
As used in this application, the word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word exemplary is intended to present concepts in a concrete fashion.
It should be understood that the steps of the exemplary methods set forth herein are not necessarily required to be performed in the order described, and the order of the steps of such methods should be understood to be merely exemplary. Likewise, additional steps might be included in such methods, and certain steps might be omitted or combined, in methods consistent with various embodiments of the present invention.
Also for purposes of this description, the terms “couple”, “coupling”, “coupled”, “connect”, “connecting”, or “connected” refer to any manner known in the art or later developed in which energy is allowed to be transferred between two or more elements, and the interposition of one or more additional elements is contemplated, although not required. Conversely, the terms “directly coupled”, “directly connected”, etc., imply the absence of such additional elements.
The present invention will be described herein in the context of illustrative embodiments of an apparatus and process to wire bond an integrated circuit device to a substrate by wire bonding bond pads on the integrated circuit device to substrate pads on the substrate using copper bond wire. While the device and substrate are heated just prior to the step of wire bonding, an inert or low moisture content purge gas is used to flush the substrate to prevent build up of chlorine or other reactive gases during wire bonding. Here, low moisture content refers to the dew point of the purge gas and is desirably less than 10° C., and preferably less than −20° C.
For simplicity, in the view in
The devices 104 might be formed from silicon, gallium arsenide, indium phosphide, or another semiconductor material suitable for the desired function of the devices 104, or a combination thereof. The substrate 102 might be formed from a glass-epoxy (commonly known as FR-4), polytetrafluoroethylene (PTFE), polyimide, ceramics, silicon, glass, another insulating material suitable as a substrate, or a combination of these materials. Typically, the thickness of the substrate 102 is less than two millimeter and might be as thin as 50 microns (μm). The bond pads 114 are typically made of copper or aluminum and the substrate pads are typically made of copper. The bond wire is a copper or palladium-covered copper wire having a diameter ranging from approximately 10 to 250 μm or so.
Briefly and as well understood in the art, during the actual wire bonding the substrate is held into position using the walled clamp 106 that presses the substrate down onto the heater block 206. Due to different coefficients of thermal expansion (CTE) of the substrate and devices, the heater block 206 elevates the temperature of the substrate and devices to a known temperature so that the bond pads and the substrate pads are in predetermined positions for the bonder head 220 to contact. The bonder head 220 operates within the walls of the clamp 106 (region 108) to wire bond the bond pads 114 on the devices 104 to substrate pads 112 on the substrate 102. The bonder wire 222 protruding from the bonder head has a ball 232 formed on the end of the wire by using an electrical spark to melt the end of the bond wire. Then a bonder head is positioned over a device 104 and the ball 232 on the end of the bond wire is brought into contact with a bond pad 114 on the surface of the device. The head vibrates ultrasonically to form a ball bond that attaches the ball 232 on the bond wire to the bond pad. Then head 220 moves over to a respective substrate pad 112 on the substrate 102 and the bond wire is similarly attached to the respective substrate pad and then the wire is cut to complete the wire bond. Once a device as been wire bonded to the substrate, the head 220 moves to the next device within the region 108 and the wire bonding process begins again. Once all the bond pads on the devices in region 108 is wire bonded, the clamp 106 is lifted, the substrate 102 is indexed to the right to move the next set of devices to be wire bonded into region 108, the clamp 106 lowered, and the above process to wire bond the devices repeats until all the devices on the substrate are wire bonded.
An analysis of a failed ball bond formed using the above-described process with the bonder shown in
While not wanting to be held to any particular theory, it is believed that halogens, such as chlorine, and other reactive species evaporate from substrate, die attach adhesive, chamber walls, tooling, etc. contaminate the exposed bond pads and substrate pads while waiting for wire bonding. This is contamination is especially pernicious while the substrate 102 and devices 104 are heated by heater block 206. It was discovered that those devices waiting the longest for bonding in the bonder 100 had the highest likelihood of ball bond failure. Because the ball 232 is formed via arc melting the copper bond wire to a temperature greater than the melting temperature of copper (˜1085° C.), it is believed that the gas stream from nozzle 230 prevents any gas contaminants from interacting with the copper bond wire during ball formation. However, when the copper ball 232 cools from the melting temperature to room temperature as the head 220 is translated toward a bond pad, if the gas stream does not fully flush the region 108 of the contaminant gases, then at temperatures greater than approximately 150-200° C. the copper ball and contaminants might react form Cu3Cl3 gas phases which might tend to stay in and around the ball 232. Even if the region 108 is fully purged, contaminants on the surface of the bond pads might cause the Cu3Cl3 for form during bonding to the bond pads. Moreover, when ball temperature falls below 150-200° C., then the Cu3Cl3 gas decomposes into solid copper, CuCl (a solid), CuCl2 (a solid), and gaseous chlorine. It is believed that the solid phases (CuClx) redeposit on ball to create small copper or CuClx particles that are in contact with or close to the copper ball, part of the porous copper film referred to above, and will be part of the ball bond as will any trapped chlorine gas. The film and particles will tend to drive the formation of a less than ideal copper ball-bond pad interface, thus having regions were no reaction between the copper ball and the aluminum bond pad take place, referred to as voided interfacial regions. These voided regions are weak and susceptible to degradation over time. One such degradation mechanism is believed to be related to chlorine trapped during wire bonding, another is chlorine contamination from the substrate in a post wire bond clean step. If the interface voids are present such that they have channels that are open to the external ambient, then external gases can get into the ball-bond pad interface. One such gas is chlorine. After wire bonding, the device is subjected to a plasma clean step. Chlorine can be generated during this step by, for example, outgassing from the substrate or the die attach adhesive. If there are interface channels then the chlorine gas can get into the interface and react with copper or the reaction product. Subsequently the device is overmolded with a polymer protective coating onto the device and bond wires. This coating will trap any gasses that are present in the ball bond-pad interface. These can then further react and eventually cause device failure.
One mechanism that might explain how the degradation occurs involves copper-chlorine reactions. It is known that copper and chlorine gas can react with two competing reactions: growing of CuClx solid compounds (Cu+Cl/Cl2→CuClx) is dominant when the temperature is less than approximately 150° C., and etching (CuClx→Cu3Cl3+Cu+Cl2) is dominant when the temperature is greater than approximately 150° C. During normal operation of the packaged device, the device might repeatedly experience temperatures in the range of 150-200° C., causing multiple etching/growing cycles and possibly leading to failure of the ball bond. Moreover, any cleaning of the device and substrate after bonding, such as by plasma cleaning using argon, might enhance the etching/growing reaction rates discussed above.
To address the issue of chlorine contamination and other potential contaminates that could lead to less than ideal bond wire-die pad interface formation, a purge gas is directed onto the portion of the substrate 102 having devices 104 thereon waiting for wire bonding. Referring to
While the embodiments described here use a ball bond to attach a bond wire to a bond pad, other types of bonds might be used, such as a wedge bond.
Although the elements in the following method claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence.
It is understood that various changes in the details, materials, and arrangements of the parts which have been described and illustrated in order to explain the nature of this invention might be made by those skilled in the art without departing from the scope of the invention as expressed in the following claims.
This application claims the benefit of the filing date of U.S. provisional patent application No. 61/944,663 filed 26 Feb. 2014 as attorney docket no. L13-14124US1, the teachings of which are incorporated herein by reference.
Number | Date | Country | |
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61944663 | Feb 2014 | US |