Bonding pads aluminum

The dominant metal layer used in conventional silicon microelectronics technology is aluminum (Al), usually doped with silicon to reduce spiking and with copper to increase resistance to electromigration. Aluminum is used for electrical connections, for contacting the sensing elements, and as the standard material for bonding pads, which are needed to assure electrical connection to the outside world . 

In automotive sensors, aluminum bonding pads are the obvious choice they can be used for bonding both aluminum and gold wires, and the bonding quality is sufficiently good for the lifetime of the device. However, for surface-microma-chined structures, special care has to be taken regarding the bonding pads to prevent corrosion when the sacrificial oxide is etched. 

Commercial cleaning solutions are used in the pre-clean step. From optical microscopy and AFM results, both steps do not adversely affect the surface of the aluminum bond pads for all 3 types of substrates. Cleaning is done at 60°C for 5 min. 

Thin gold or aluminum wire, with a diameter of 0.025-0.075 mm, is used to make a connection between the bonding pad and the sensor. This wire bonding is performed using standard microelectronic techniques such as thermal compression or ultrasound bonding. The external lead wire is then bonded onto the bonding pad. Because of the thinness of the metallic film of the pad, the external lead wire connection is usually made by thermal compression. Similar to the connection of the thick-film sensor, the conductive epoxy is first applied to the connecting joint and then covered with insulation epoxy or silicone. 

Erratic and high-resistance changes have also been reported when silver-filled epoxies are used to attach devices to thm-film-aluminum metallization and exposed to temperature-humidity environments. In practice, however, aluminum-bonding pads and aluminum conductors on the top surface of an interconnect substrate are protected with barrier coatings of titanium-tungsten or chromium followed by plated gold as the top bonding surface. 

Corrosion of conductor metallizations, devices, wire, wire bonds and other connections, and platings occur by direct or indirect-chemical reactions. The metals most susceptible to chemical attack are the non-noble metals among which are thin-film aluminum used for conductors or bonding pads, thin-film nichrome used for precision resistors, tin-lead solders used for... 

Protective Chip Pad Layer. As with virtually all flip chip processes, the A1 bond pads must be protected to eliminate the formation of nonconductive aluminum oxide. This ensures a low and stable resistance at bond-bond pad interface. The PFC process utilizes an electroless plating technique, using Ni/Au or Pd, to cover the A1 bond pads prior to polymer bumping. The typical metal thickness is 0.5-1.0 pm for Pd and 3.0-5.0 pm for Ni/Au. 

Flip-chip technology, as shown in Fig. 11.14, is similar to TAB technology in that successive metal layers are deposited on the wafer, ending up with solder-plated bumps over the device contacts. One possible configuration utilizes an alloy of nickel and aluminum as an interface to the aluminum bonding pads. A thin film of pure nickel is plated over the Ni/Al, followed by copper and solder. The copper is plated to a thickness of about 0.0005 in., and the solder is plated to about 0.003 in. The solder is then reflowed to form a hemispherical bump. The devices are then mounted to the substrate face down by reflow solder methods. During reflow, the face of the device is prevented from contacting the substrate metallization by the copper bump. This process is sometimes referred to as the controlled collapse process. 

In summary for anodic corrosion, water molecules permeate through the encapsulation material to reach the chip surface. Delamination between the lead frame and encapsulation material must accelerate moisture penetration into devices. Delamination between chips and encapsulation material allows water film formation. If halides, such as chloride ion are present in the water film, the corrosion of aluminum is accelerated. Corrosion takes place mainly on aluminum of the bonding pad, since the chip is usually coated with passivation film. But when passivation defects exist, moisture penetrates through them and aluminum corrosion of the wiring conductor occurs. Then improvements in adhesion, and lowering molding stress and chloride content are important. 

The failure due to the cathodic corrosion of aluminum is also accelo ted ionic impurities existing in the water film on the bonding pad area. Ion chromatographic analysis showed that the extracts from molding compound into water were hydrolyzed products such as R—COO" and inorganic ions from polymer. leakage current... 

Recently, a new failure mode has been found in the accelerated testing of high temperature storage It shows an increase in the resistance of the bonding pad, or in severe cases disconnection, when a device is stored at high temperatures around 200 C. It is due to the formation of an aluminum-gold intermetallic compound Its formation seems to be accelerated by free bromine or chlorine it may be... 

Contact between dissimilar metals occurs frequently in electronic devices, components, and systems. Among the many metallization schemes found in thin film and integrated circuits are Ti/TiN/Al/TiN (see Section 9.4.1) and Ti/Pd/Au. Gold wires bonded to aluminum pads or to alumi-num/copper interfaces in devices are also commonplace. In equipment cabinets, copper ground planes may be connected to the steel frame. Dissimilar metals in electrical contact in the presence of an electrolyte solution may lead to galvanic corrosion in which the more active metal will corrode, while the more noble metal is protected from corrosion. Ranking of metals in terms of their relative activity is known as the galvanic series (Jones, 1992). 

It has been shown that some latent hardeners of the epoxy resins may corrode the aluminum wires or bonding pads. This corrosivity has been demonstrated for the complex of boron trifluoride and monoethyl-amine. In the specification NSA 77-25A, small dots of adhesive are applied to the aluminized side of a Mylar polyester film (www.dupont.com) and are allowed to stand in flie room ambient without cure. After 48 h, the dots are removed from the film by washing with acetone and the requirement is that there are no changes in the light transmission of the film. 

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