Thermosonic bonding

Thermosonic Bonding is widely used to permanently wire bond integrated circuits into computers. It was introduced by Alexander Coucoulas in the 1960s.[1][2][3] Owing to the well proven reliability of thermosonic bonds, it is extensively used to connect the central processing units (CPUs), which are encapsulated silicon integrated circuits that serve as the "brains" of today's computers. The bond is formed using a set of parameters which include ultrasonic, thermal and mechanical (force) energies.

Figure 1. Wires bonded to a silicon integrated circuit using thermosonic bonding

Thermosonic bonding is a akin to friction welding, since the introduction of ultrasonic energy (via a bonding tool vertically attached to an ultrasonic transformer or horn) simultaneously delivers a force and vibratory or scrubbing motion to the interfacial contact points between a pre-heated deforming lead-wire and the metallized pads of a silicon integrated circuit (Figure 1). In addition to the delivery of thermal energy, the transmission of ultrasonic vibratory energy creates an ultrasonic softening effect by interacting at the atomic lattice level of the preheated lead wire. These two softening effects dramatically facilitates the lead wire deformation by forming the desirable contact area using relatively low temperatures and forces. As a result of the frictional action and ultrasonic softening induced in the preheated lead wire during the bonding cycle, thermosonic bonding can be used to reliably bond high melting point lead wires (such as gold and lower cost aluminum and copper) using relatively low bonding parameters. This ensures that the fragile and costly silicon integrated circuit chip is not exposed to potentially damaging conditions by having to use higher bonding parameters (ultrasonic energy, temperatures or mechanical forces) to deform the lead wire in forming the required contact area during the bonding process.

History

Earlier wire bonding methods were thermocompression bonding,[4] which used heat and pressure, and ultrasonic bonding,[5] which used vibratory energy and pressure. Thermosonic bonding improved upon the reliability of the earlier processes by preheating the lead wire and/or metallized silicon chip prior to introducing the ultrasonic cycle.[6] In addition to thermal softening the lead wire, the delivery of ultrasonic energy produces further softening by interacting at the atomic lattice level of the wire (known as ultrasonic softening [7] ). These two independent softening mechanisms (heating and delivery of ultrasonic energy to the atomic lattice level) eliminates the incidences of cracking the fragile and costly silicon chip. The improvement occurs because pre-heating the lead-wire and ultrasonic softening of the lead-wire dramatically facilitates its deformation in forming the required contact area using a relatively low set of bonding parameters. Depending on the temperature level and material properties of the lead wire, the onset of recrystallization (metallurgy) or hot working of the deforming wire can occur while it is forming the required contact area. Recrystallization takes place in the strain hardening region of the lead wire where it aids in the softening effect. If the wire was ultrasonically deformed at room temperature, it would tend to extensively strain hardened (cold working) and therefore tend to transmit damaging mechanical stresses to the silicon chip. Thermosonic bonding, initially referred to as Hot Work Ultrasonic Bonding by Alexander Coucoulas,[8][2][3] was found to bond a wide range of conductive metals such as aluminum and copper wires to tantalum and palladium thin films deposited on aluminum oxide and glass substrates all of which simulated the metallized silicon chip.

Applications

At present, the majority of connections to the silicon integrated circuit chip are made using thermosonic bonding[9] because it employs lower bonding temperatures, forces and dwell times than thermocompression bonding, as well as lower vibratory energy levels and forces than ultrasonic bonding to form the required bond area. Therefore the use of thermosonic bonding eliminates damaging the relatively fragile silicon integrated circuit chip 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.

Thermosonic bonding is also used in the flip chip process which is an alternate method of electrically connecting silicon integrated circuits.

Josephson effect and superconducting interference (DC SQUID) devices use the thermosonic bonding process as well. In this case, other bonding methods would degrade or even destroy YBaCuO₇ microstructures, such as microbridges, Josephson junctions and superconducting interference devices[10] (DC SQUID).

When electrically connecting light-emitting diodes with thermosonic bonding techniques, an improved performance of the device has been shown.[11]

See also

References

  1. ↑ Harman, G., Wire Bonding In Microelectronics], McGraw-Hill, Chapt. 2, pg.36
  2. 1 2 Coucoulas, A., Trans. Metallurgical Society Of AIME, “Ultrasonic Welding of Aluminum Leads to Tantalum Thin Films”, 1966, pp. 587–589. abstract https://sites.google.com/site/coucoulasthermosonicbondalta
  3. 1 2 Coucoulas, A., “Hot Work Ultrasonic Bonding – A Method Of Facilitating Metal Flow By Restoration Processes”, Proc. 20th IEEE Electronic Components Conf. Washington, D.C., May 1970, pp. 549–556.https://sites.google.com/site/hotworkultrasonicbonding
  4. ↑ Anderson, O. L.; Christensen, H.; Andreatch, P. (1957). "Technique for Connecting Electrical Leads to Semiconductors". Journal of Applied Physics. 28: 923. doi:10.1063/1.1722893.
  5. ↑ Carlin, B. (1960) Ultrasonics. McGraw-Hill Book Co.
  6. ↑ Coucoulas, A., “Hot Work Ultrasonic Bonding – A Method Of Facilitating Metal Flow By Restoration Processes”, Proc. 20th IEEE Electronic Components Conf. Washington, D.C., May 1970, pp. 549–556.https://sites.google.com/site/hotworkultrasonicbonding
  7. ↑ F. Blaha, B. Langenecker. Acta Metallurgica, 7 (1957).
  8. ↑ Harman, G., Wire Bonding In Microelectronics], McGraw-Hill, Chapt. 2, pg.36
  9. ↑ Harman, G., Wire Bonding In Microelectronics, McGraw-Hill, Ch. 2, p. 36
  10. ↑ Burmeister, L.; Reimer, D.; Schilling, M. (1994). "Thermosonic bond contacts with gold wire to YBa2Cu3O7 microstructures". Superconductor Science and Technology. 7 (8): 569. doi:10.1088/0953-2048/7/8/006.
  11. ↑ Seck-Hoe Wong et al. (2006) "Packaging Of Power LEDs Using Thermosonic Bonding Of Au-Au Interconnects", Surface Mount Technology Association International Conference.
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