Thermosonic bonding

Thermosonic bonding is widely used to electrically interconnect the all-important metallized silicon integrated circuit (popularly known as the “chip” or "brain") into computers as well as a myriad of other microelectronic devices after it was well investigated and shown to form reliable bonds. It was first introduced by Alexander Coucoulas who is The Father Of Thermosonic Bonding^[2] (as referenced below).

Figure 1. Wires Thermosonic bonded directly to the metallized bonding-pads on the peripheral surface of a silicon integrated circuit "Chip"

Since a Thermosonic bond can be formed well below the melting points of the mating metals, such as gold (with a melting point of 1064°C), it can be used to connect gold wires to the metallized bonding pads of the silicon "Chip" without incurring any thermally related damage. Figure 1 shows gold wires bonded (as low as 300°C) to gold metallized thin film bonding pads which were previously deposited on the peripheral surface of the silicon integrated circuit chip during its manufacturing stage. The bond is formed by transmitting a combination of heat (near 300°C), ultrasonic/vibratory energy and force in order to sufficiently deform the wire and form the necessary contact area against the gold metallized mating surface-pads. Since relatively low Thermosonic bonding parameters were shown to form reliable bonds (see Invention section below), the integrity of the fragile silicon integrated circuit "Chip" is maintained during the bonding cycle and throughout its intended lifetime where it functions as the "brains" in various types of computers. Since reliable Thermosonic bonds can be formed using a relatively low set of bonding parameters (temperature and force) as well as using a range of metal lead-wires (such as gold, copper and aluminum), an evergrowing number of other applications have evolved as described below.

History

Three methods of solid state wire bonding were sequentially developed to form permanent interconnections between the silicon integrated circuits (ICs) to the external circuitry and outside world. In the mid-1950s, solid state wire bonds were made using heat and pressure and was referred to as thermocompression bonding.^[3] The process was generally limited to bonding pre-cleaned gold wires to gold surfaces. In the early 1960s commercial ultrasonic wire bonders were introduced which used dual bonding parameters of vibratory energy and force to form wire bonds.^[4]

Invention

In the mid-1960s, Alexander Coucoulas^[5][6] reported the first thermosonic wire bonds using a combination of heat, ultrasonic vibrations and force. He initially investigated the attachment of aluminum wires to tantalum thin films deposited on glass substrates using bonding pararmeters of ultrasonic energy and force. The metallized glass substrates closely simulated bonding to the fragile metallized silicon integrated circuit or "Chip". He observed that the ultrasonic energy and force levels needed to sufficiently deform a wire and form the required contact area significantly increased the incidences of cracks in the metallized glass substrates. A means of heating the bond region was then added to the ultrasonic bonder. The bond region was then heated during the ultrasonic bonding cycle which virtually eliminated the glass failure mode since the required contact area was achieved with lower ultrasonic energy and force levels. The enhanced deformation of the heated wire during the introduction of the ultrasonic energy was attributed to the transition from cold working (or strain hardening of the wire) to hot working where its softness was largely maintained due to the onset of recrystallization (metallurgy). Christian Hagar^[7] and George Harman^[2] stated that in 1970 Alexander Coucoulas,^[1] reported additional work in forming thermosonic-type bonds which he initially called hot work ultrasonic bonding. In this case, copper wires were bonded to palladium thin films deposited on aluminum oxide substrate. As a result of these earliest reported Thermosonic wire bonds, G.Harman (the world's foremost authority on wire bonding) stated: "as such, Alexander Coucoulas is the Father of Thermosonic Bonding".^[2]

Evergrowing Applications Of Thermosonic Bonding

After Thermosonic Bonding was initially applied to wire bonding the integrated circuit silicon- chip (IC’s) to the outside world, as described above, an evergrowing number of other applications have evolved. This is mainly because a reliable Thermosonic bond can be formed at desirably low bonding parameters (temperature, forces, time and ultrasonic energy) which ensures that: no damage will occur to a given device during the bonding process and; the final bond properties are reliably stable so the bonded device can be used at relatively high temperatures.

As a result, it is being used in a process referred to as Flip Chip Thermosonic Bonding of IC’s which uses small gold balls in-place of wires to interconnect the metallized pads of the silicon integrated circuit chip to the outside world.

Josephson Effect and superconducting interference (DCSQUID) devices described in Wikipedia use the Thermosonic bonding process as well. In this case L. Burmeister et al. ^[8] found that other bonding methods were found to degrade or even destroy YBaCuO₇ microstructures, such as microbridges, Josephson junctions and superconducting interference devices (DCSQUID).

Light-emitting diodes are thermosonically connected in the manufacture of LED lamps (or LED light bulbs) which offer a long service life compared to incandescent lamps. The use of thermosonic bonding allows the LED lamps to operate at higher temperatures without a decrease in its performance.^[9]

Background of Alexander Coucoulas (Inventor)

Coucoulas retired from AT&T Bell Labs as a Member Of The Technical Staff in 1996 where he pioneered research in the areas of electronic/photonic packaging and optical fibers which resulted in: producing numerous publications and presentations; co-authoring several technical books and; obtaining more than 30 patents. He was twice awarded best paper-publication and presentation at the 20th and 43rd IEEE Electronic Components Conference for “Compliant Bonding” in 1970, Figure 1.^[10] and AlO Bonding in 1993, Figure 2.^[11] both of which were his patented inventions.^[12][13]

His Ionia-Greek immigrant parents were born in the biblical port-city of Smyrna, situated on the western coast of Asia Minor, which was the birthplace of Homer. His father, Demetrios Koukoulas (a maimed Greek Smyrnaean soldier), was rescued in the Aegean sea by a Japanese naval cruiser during the Fire of Smyrna^[14] in September 1922 and brought to Pereaus, Greece.^[15][16] He immigrated to the United States that same year on the SS King Alexander.

His son, Alexander Coucoulas, served in the US Army as a combat engineer in the Far East Command in the early 1950s and was awarded the National Defense Service Medal for the Korean War (1950-1954).^[17] After serving in the US Army, he obtained his undergraduate and graduate degrees in Metallurgical Engineering and Material Science at New York University which was financed by the GI Bill, a graduate scholarship and part-time jobs in the New York Metropolitan area. His graduate thesis, included in a co-authored paper,^[18] was under the tutorage of dr. Kurt Komarek who later became a Rector (President) of the University of Vienna and is presently a professor emeritus.^[19]

Figure 1. Alexander Coucoulas' publication that first introduced thermosonic bonding which is widely used to connect the all important silicon integrated circuit known as "The Chip" and "brains" of the computer.^[1] Click to read full resolution

Figure 2. AlO Bonding: A Method Of Joining Oxide Optical Components To Aluminum Coated Substrates” was awarded for its presentation and publication at the 1993 IEEE Electronic Components Conference double Click squares to read full resolution.

Summary

At present, the majority of connections to silicon integrated circuits are made using thermosonic bonding^[2][1] because it employs lower bonding temperatures, forces and dwell times than thermocompression bonding,^[3] as well as lower vibratory energy levels than ultrasonic bonding^[4] to form the required bond area. Therefore the use of thermosonic bonding largely eliminates damaging the relatively fragile silicon integrated circuit during the bonding cycle. The proven reliability of thermosonic bonding has made it the process of choice, since such potential failure modes could be costly whether they occur during the manufacturing stage or detected later, during an operational field-failure of a chip which had been connected inside a computer or a myriad of other microelectronic devices.

See also

• Ball bonding
• Flip chip
• Integrated circuits
• Semiconductor device fabrication
• Thermocompression bonding
• Transistor
• Ultrasonic welding
• Wire bonding
• Josephson Effect
• SQUID
• Light-emitting diodes
• LED lamps

References

External links

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