Thermosonic bonding is widely used to wire bond silicon integrated circuits into computers. Alexander Coucoulas was named "Father Of Thermosonic Bonding" by George Harman,the world's foremost authority on wire bonding, where he referenced Coucoulas's leading edge publications in his book, Wire Bonding In Microelectronics. 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.
Alexander Coucoulas is an American inventor, research engineer, and author. He was named "Father Of Thermosonic Bonding" by George Harman, the world's foremost authority on wire bonding, where he referenced Coucoulas's leading edge publications in his book, Wire Bonding In Microelectronics. A Thermosonic bond is formed using a set of parameters which include ultrasonic, thermal and mechanical (force) energies. Figure 1 (below) shows a diagram of a Thermosonic Bonding machine which includes a magnetostrictive or piezoelectric-type transducer which is used to convert electrical energy into vibratory motion. 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.
A central processing unit (CPU), also called a central processor or main processor, is the electronic circuitry within a computer that carries out the instructions of a computer program by performing the basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions. The computer industry has used the term "central processing unit" at least since the early 1960s. Traditionally, the term "CPU" refers to a processor, more specifically to its processing unit and control unit (CU), distinguishing these core elements of a computer from external components such as main memory and I/O circuitry.
A thermosonic bond is formed using a set of parameters which include ultrasonic, thermal and mechanical (force) energies. A thermosonic bonding machine 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.
Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure and latent heat. It is derived from the Greek word πιέζειν; piezein, which means to squeeze or press, and ἤλεκτρον ēlektron, which means amber, an ancient source of electric charge. French physicists Jacques and Pierre Curie discovered piezoelectricity in 1880.
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.
Recrystallization is a process by which deformed grains are replaced by a new set of defect-free grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility. Thus, the process may be introduced as a deliberate step in metals processing or may be an undesirable byproduct of another processing step. The most important industrial uses are softening of metals previously hardened or rendered brittle by cold work, and control of the grain structure in the final product.
Hot working process metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is important because recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength and hardness low and ductility high. This contrasts with cold working.
At present, the majority of connections to the silicon integrated circuit chip are made using thermosonic bondingbecause 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.
Thermocompression bonding describes a wafer bonding technique and is also referred to as diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g. gold (Au)-gold (Au), are brought into atomic contact applying force and heat simultaneously. The diffusion requires atomic contact between the surfaces due to the atomic motion. The atoms migrate from one crystal lattice to the other one based on crystal lattice vibration. This atomic interaction sticks the interface together. The diffusion process is described by the following three processes:
Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics and metals, and especially for joining dissimilar materials. In ultrasonic welding, there are no connective bolts, nails, soldering materials, or adhesives necessary to bind the materials together. When applied to metals, a notable characteristic of this method is that the temperature stays well below the melting point of the involved materials.
An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed of discrete electronic components. The IC's mass production capability, reliability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.
Thermosonic bonding is also used in the flip chip process which is an alternate method of electrically connecting silicon integrated circuits.
Flip chip, also known as controlled collapse chip connection or its abbreviation, C4, is a method for interconnecting semiconductor devices, such as IC chips and microelectromechanical systems (MEMS), to external circuitry with solder bumps that have been deposited onto the chip pads. The technique was developed by General Electric's Light Military Electronics Dept., Utica, N.Y. The solder bumps are deposited on the chip pads on the top side of the wafer during the final wafer processing step. In order to mount the chip to external circuitry, it is flipped over so that its top side faces down, and aligned so that its pads align with matching pads on the external circuit, and then the solder is reflowed to complete the interconnect. This is in contrast to wire bonding, in which the chip is mounted upright and wires are used to interconnect the chip pads to external circuitry.
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(DC SQUID).
The Josephson effect is the phenomenon of supercurrent, a current that flows indefinitely long without any voltage applied, across a device known as a Josephson junction (JJ), which consists of two or more superconductors coupled by a weak link. The weak link can consist of a thin insulating barrier, a short section of non-superconducting metal (S-N-S), or a physical constriction that weakens the superconductivity at the point of contact (S-s-S).
A SQUID is a very sensitive magnetometer used to measure extremely subtle magnetic fields, based on superconducting loops containing Josephson junctions.
When electrically connecting light-emitting diodes with thermosonic bonding techniques, an improved performance of the device has been shown.
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.
Wire bonding is the method of making interconnections (ATJ) between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one printed circuit board (PCB) to another. Wire bonding is generally considered the most cost-effective and flexible interconnect technology and is used to assemble the vast majority of semiconductor packages. Wire bonding can be used at frequencies above 100 GHz.
Ball bonding is a type of wire bonding, and is the most common way to make the electrical interconnections between a chip and the outside world as part of semiconductor device fabrication.
An intermetallic is a type of metallic alloy that forms a solid-state compound exhibiting defined stoichiometry and ordered crystal structure.
The back end of line (BEOL) is the second portion of IC fabrication where the individual devices get interconnected with wiring on the wafer, the metalization layer. Common metals are copper and aluminum. BEOL generally begins when the first layer of metal is deposited on the wafer. BEOL includes contacts, insulating layers (dielectrics), metal levels, and bonding sites for chip-to-package connections.
Annealing, in metallurgy and materials science, is a heat treatment that alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. It involves heating a material above its recrystallization temperature, maintaining a suitable temperature for a suitable amount of time, and then cooling.
Hot carrier injection (HCI) is a phenomenon in solid-state electronic devices where an electron or a “hole” gains sufficient kinetic energy to overcome a potential barrier necessary to break an interface state. The term "hot" refers to the effective temperature used to model carrier density, not to the overall temperature of the device. Since the charge carriers can become trapped in the gate dielectric of a MOS transistor, the switching characteristics of the transistor can be permanently changed. Hot-carrier injection is one of the mechanisms that adversely affects the reliability of semiconductors of solid-state devices.
Harry Kroger is an American physicist and electrical engineer. He used to be a Bartle professor of electrical engineering at Binghamton University, a part of the State University of New York (SUNY) system. He has been a member of the Institute of Electrical and Electronics Engineers (IEEE) since 1964 and became a Life Fellow of the IEEE in 2001. He has now retired to Florida.
Beam lead technology is a method of fabricating a semiconductor device. Its original application was to high-frequency silicon switching transistors and high-speed integrated circuits. It eliminated the labor-intensive wire-bonding process used for integrated circuits at the time and allowed automated assembly of semiconductor chips onto larger substrates to produce hybrid integrated circuits.
A Scanning SQUID Microscope is a sensitive near-field imaging system for the measurement of weak magnetic fields by moving a Superconducting Quantum Interference Device (SQUID) across an area. The microscope can map out buried current-carrying wires by measuring the magnetic fields produced by the currents, or can be used to image fields produced by magnetic materials. By mapping out the current in an integrated circuit or a package, short circuits can be localized and chip designs can be verified to see that current is flowing where expected.
A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers before being diced into die, tested, and packaged. The package provides a means for connecting the package to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.
The superconducting tunnel junction (STJ) — also known as a superconductor–insulator–superconductor tunnel junction (SIS) — is an electronic device consisting of two superconductors separated by a very thin layer of insulating material. Current passes through the junction via the process of quantum tunneling. The STJ is a type of Josephson junction, though not all the properties of the STJ are described by the Josephson effect.
Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.
Eutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer that can produce a eutectic system. Those eutectic metals are alloys that transform directly from solid to liquid state, or vice versa from liquid to solid state, at a specific composition and temperature without passing a two-phase equilibrium, i.e. liquid and solid state. The fact that the eutectic temperature can be much lower than the melting temperature of the two or more pure elements can be important in eutectic bonding.
Compliant bonding is used to connect gold wires to electrical components such as integrated circuit "chips". It was invented by Alexander Coucoulas in the 1960s. The bond is formed well below the melting point of the mating gold surfaces and is therefore referred to as a solid-state type bond. The compliant bond is formed by transmitting heat and pressure to the bond region through a relatively thick indentable or compliant medium, generally an aluminum tape.