1 Description 2 Background 3 Applications 4 See also 5 References
Description A thermosonic bond is formed using a set of parameters which include ultrasonic, thermal and mechanical (force) energies. The diagram shows a thermosonic bonding machine which includes a magnetostrictive or piezoelectric-type transducer which is used to convert electrical energy into vibratory motion which is known as piezoelectricity. The vibratory motion travels along the coupler system, a portion which is tapered to serve as the velocity transformer. The velocity transformer amplifies the oscilliatory motion and delivers it to a heated bonding tip. It is akin to a friction bond, 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 2). 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. Background
Wires bonded to a silicon integrated circuit using thermosonic bonding
A thermosonic bond falls in the category of a solid state metallic bond which is formed by mating two metal surfaces well below their respective melting points. Coucoulas introduced Thermosonic bonding which significantly improved upon the bond-reliability produced by available commercial solid-state bonding machines where he pre-heated the lead wire (and/or metallized silicon chip) prior to introducing an ultrasonic energy cycle. In addition to thermal softening the lead wire, the subsequent delivery of ultrasonic energy produced further softening by interacting at the atomic lattice level of the heated wire (known as ultrasonic softening). These two independent softening mechanisms (pre-heating lead wire and deliverying the ultrasonic energy at the atomic lattice level) eliminated the incidences of cracking the fragile and costly silicon chip which were observed by Coucoulas when using earlier commercially available solid-state bonding machines. The improvement occurs because pre-heating and ultrasonic softening the lead-wire dramatically facilitated its deformation in forming the required contact area while 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, 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.
At present, the majority of connections to the silicon integrated
circuit chip are made using thermosonic bonding 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.
Flip chip Semiconductor device fabrication Transistor SQUID Light-emitting diodes LED lamps
^ Harman, G., Wire Bonding In Microelectronics, McGraw-Hill, Chapt. 2, pg.36, also search Coucoulas at https://www.amazon.com/WIRE-BONDING-MICROELECTRONICS-3-E/dp/0071476237/ref=sr_1_1?s=books&ie=UTF8&qid=1354948679&sr=1-1&keywords=wire+bonding+in+microelectronics#_ search Coucoulas ^ a b 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 ^ a b c 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 ^ F. Blaha, B. Langenecker. Acta Metallurgica, 7 (1957). ^ Harman, G., Wire Bonding In Microelectronics, McGraw-Hill, Ch. 2, p. 36 ^ 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. ^ Seck-Hoe Wong et al. (2006) "Packaging Of Power LEDs Using Thermosonic Bonding Of Au-Au Interconnects", Surface Mount Technology Association International