Jeremias D. Romero

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. >

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.

Impact of photo-induced species in O2-containing gases on lithographic patterning at 193-nm wavelength

In order to prevent contaminants and impurities from being deposited on the optical elements of the tool, ArF lithographic patterning of photoresists is currently done in the exposure chamber of a scanner or stepper that is purged with clean dry air. Unfortunately, the 6.4 eV (193 nm) ArF laser photons can dissociate molecular oxygen in air into atomic oxygen, form ozone, form singlet molecular oxygen and atomic oxygen from the photodissociation of ozone, and mediate singlet molecular oxygen formation in polymers by energy transfer mechanisms from impurities or specially added sensitizers (S) (e.g. dyes). Once formed, these species mediate photo-oxidative degradation processes of resist polymers, including cross-linking, chain scission, oxidation, and other secondary reactions by free radical mechanisms, resulting in resist feature erosion, poor resist feature profiles, particularly under bright field illumination in full field scanners and steppers. The occurrence of these photo-oxidative degradation processes has been experimentally verified in photoresist films exposed in commercially available ArF laser scanners and steppers that are purged with clean dry air. Using a custom-built ultra-high vacuum (UHV) chamber equipped with ArF laser beam line, mass spectrometer, and infra-red spectrometer, as well as an ex-situ X-ray photo-electron spectrometer and a Fourier Transform infra-red spectrometer, the effects of different exposure environments (dry air, nitrogen, oxygen, ozone/oxygen mixture, and vacuum) in either contributing to or mitigating these photo-oxidative degradation processes during exposure of photoresist films were studied. The effects of these photo-oxidative degradation processes have been quantified, and it was observed that the processes are initiated at the surface of resist polymers by photo-induced species, and proceed inwards, giving rise to a gradient of deteriorated material across the specimen thickness.

Optical Analyses (SE and ATR) and Other Properties of LPCVD Si3 N 4 Thin Films

Thin silicon nitride films (less than 20 nm) deposited on (100) silicon substrates via low pressure chemical vapor deposition (LPCVD) at three temperatures (730, 760, and 825°C) were analyzed by spectroscopic ellipsometry (SE), attenuated total reflection (ATR), and other tools. Films appeared to have similar optical bandgaps (∼5 eV). and the values decreased slightly with the higher deposition temperature. Second ionic mass spectroscopy results showed that a similar amount of oxygen exists in the interface between silicon and silicon nitride. ATR spectra showed no sign of Si-H bonds and decreasing N-H bonds at higher deposition temperature in the thin films. The electrical properties of the films are also discussed.

Impact of optical absorption on process control for sub-0.15-nm device patterning using 193-nm lithography

The significant optical absorption of most currently available commercial single layer 193 nm resists, even at a coating thickness of 0.4 micrometer, implies increased sensitivity to process control fluctuations of the kind that negatively impact critical dimension (CD) uniformity, process latitude, resist sidewall profile, and line edge roughness. These problems, although less severe on reflective substrates, are particularly acute on wafers with bottom anti-reflection coatings (BARCs), which are useful in CD control. With different intensity of light reaching different levels in the resist film on a BARC, a gradient is thus established in the extent of the chemical amplification reactions on which semiconductor lithography is based. The result is the familiar sloped sidewall profile and poor CD uniformity after the resist is developed. Further, with most of the photoacid generators and the polymer resins in the 193 nm resists having very low quantum efficiencies and significant absorption at 193 nm, respectively, most of the absorbed light in the resist is used up in energy dissipative processes, instead of in generating photoacids which catalyze the chemical amplification chemistry of these resists. One approach to overcome this absorption problem is to use significantly thinner resist films, but etch considerations preclude such option as these materials do not have very good etch stability. The purpose of this paper is to quantitatively assess the impact of absorption on the process control of sub- 0.15 micrometer features patterned on a full field ASML 193 nm scanner, interfaced to a TEL MARK-8 track. Optical properties of different resist films/BARC stack combinations are characterized by UV spectroscopic ellipsometry and broad band spectrometry, and sidewall profiling is done by atomic force microscopy.