Homi Fatemi

Effect of high thermal stability mold material on the gold-aluminum bond reliability in epoxy encapsulated VLSI devices

Cresolic epoxy novolac resins brominated by specially tailored brominating agents to impart a high C-Br bond energy have provided encapsulants with enhanced thermal stability. By isothermal bakes at temperatures from 190 to 250 degrees C, decomposition times of the experimental encapsulants were found to be three to four times longer than of a state-of-the-art commercial material. The presence of halogenated organic residues among some of the experimental materials was found to cause increased gold-aluminum wire-bond failure through degradation of the intermetallic. These residues were byproducts of resin synthesis, which were eliminated by modification of the chemistry and processing. After such modification, the halogen-induced failure time was found to be prolonged by as much as 80% compared to a commercial resin. The apparent activation energy of bond failure was 0.8 eV, which was found to equal that of diffusion of organic halide through the polymer matrix in an aqueous environment, as determined by aqueous ion extraction. High thermal stability of the C-Br bond in the resins as well as purity of the material from halogenated organic residues was found to be crucial for superior reliability of aluminum metallization and gold wire bond in epoxy plastic-encapsulated VLSI devices. >

Aluminum Bond Pad Contamination by Thermal Outgassing of Organic Material from Silver-Filled Epoxy Adhesives

Effects of outgassing from commercial die attach adhesive systems on the mechanical strength of gold aluminum bonds were studied. Bond pad metallizations on unpackaged test dice were exposed to the die attach media under standard cure conditions, and gold bonds were subjected to a steam pressure pot storage. Bond pull strength as well as resistance data were obtained to determine the extent of bond degradation. Bonds exposed to die attach systems containing reactive epoxy diluents were found to cause faster mechanical degradation of the Au/Al bond under steam pressure pot storage as compared with the die attach systems formulated with ester type or other nonreactive solvents. Cleaning the bond pad with N 2 /O 2 plasma prior to wire bonding was found to dramatically improve the mechanical strength of the bonds. It was determined that the epoxy reactive diluents in some of the die attach systems could deposit on the bond pad during the cure and cause growth and reinforcement of the native Al 2 O 3 film on the pad surface with a rigid self-polymerized organic resin. This could prevent complete exposure of the underlying metal during the wire bonding process and leave debris within the bond. Wire bonds made under such a condition would be chemically vulnerable to attack by the moisture and the ionic species present in a plastic package.

Outgassing behavior of spin-on-glass (SOG)

The outgassing behavior and mechanical properties of polysiloxane based and phosphorus doped silicate based films as planarization candidates for device processing were evaluated using various analytical techniques. After curing between 370 °C and 450 °C, a high temperature rebake above 410 °C caused twice the weight loss in polysiloxane based films as in silicate films. This means that further outgassing, which could occur to a greater degree from polysiloxane than from silicate, could lead to a more probable blistering within the interlayer of the sandwiched spin-on-glass (SOG) during subsequent thermal processing. However, a well-cured polysiloxane would be a better candidate for planarization applications because the film was found to absorb less moisture and had lower stress than the silicate. Due to high silanol content and high porosity in silicate, it was found to absorb six times more water than polysiloxane. When water evolved, significantly higher stress levels were observed in silicate than in polysiloxane during thermal cycle tests. Infrared spectroscopic analysis revealed that polysiloxane contained Si–O–CH 3 moiety, which rendered the film flexible, while silicate contained near-stoichiometric SiO 2 bonds, which made for a more rigid and dense structure. This difference in the film structures translated to three times higher stress in silicate than in polysiloxane. During device processing, it was seen that silicate films were more prone to cracking than polysiloxane films. The components of the outgassing materials were either volatile organic species from residual solvents not completely burned out during cure, or carbon dioxide and water vapor as by-products from further cure. Gas chromatography indicated that both types of films contained volatile organic residues when cured at 370 °C. However, at 410 °C, volatile organic species were present in the polysiloxane but not in the silicate. A 30 to 60 min cure at temperature greater than 410 °C was then found to adequately cure polysiloxane. It was concluded that a “well cured” polysiloxane based spin-on-glass (SOG) would be more suitable than a silicate based SOG for planarization application.