Yuri Sokolov

Dependence of the Fracture of PowerTrench MOSFET Device on Its Topography in Cu Bonding Process

Dependence of the fracture-resistance of a PowerTrench MOSFET device on its topography in Cu bonding process was investigated. Two different topographies, namely dimple and round, have been tested. A significantly higher cratering rate has been clearly observed on dimple topography. The dimple topography exhibited a cratering rate of 371 k ppm levels compared to 0 ppm in round topographies. Three-dimensional nonlinear finite-element analysis has shown that the largest compressive and shear stresses and their locations were identified, respectively, in borophosphosilicate glass (BPSG)/barrier metal layers of the dimple topography. The round topography had the smallest stress in BPSG/barrier metal layers. The higher compressive stress transferred to silicon in the dimple topography during the bonding process can induce a local crack, consequently causing silicon fracturing during the shearing processes. A significant improvement in the cratering performance was observed when the Al bond pad metal layer was reinforced by adding a barrier layer sandwiched in the Al metal layers. The cratering rate decreased to 1300 ppm levels. Additionally, the change in composition of a BPSG layer caused cratering was briefly discussed and an oxygen rich BPSG film in round topography was confirmed by the energy dispersive spectroscopy (EDS) of a cross-sectional TEM sample. It has been found that the cratering rate on dimple topography significantly increased from 1 k ppm to 100 k ppm levels, when the resulting residual Al pad thickness is less than 0.65 mum for Cu bonding performed with different ultrasonic (US) power and bond forces.

Power Trench MOSFET Devices on Metal Substrates

Power trench MOSFET devices have been accomplished on Cu substrates using a novel silicon-on-metal (SOM) technology. This technology transfers silicon trench MOSFET device layers from SOI wafers to metal substrates. The prototype 30-V (drain-to-source voltage) n-channel SOM device with a pitch of 2.5 mum comprises a 5-mum device layer and an electroplated Cu substrate. These devices are the first of their kind exhibiting a negligible substrate drain contribution to their specific Rdson and have a specific Rdson of 0.198 mOmegaldrcm2 at a gate voltage of 10 V. This specific Rdson is 38% smaller than that of the same device on the silicon substrate. The dc-dc converter with SOM devices shows 11% reduction in device power loss and an energy efficiency of about 2% higher than with the same Si-based devices. The operating temperature of the SOM die in the converter is also 9degC lower than the Si-based die. The ldquocoolerrdquo SOM device is due to primarily improved energy efficiency. The transient thermal resistance of the SOM device is 20degC/W, which is less than half of 57.5degC/W for the Si-based device at a pulse duration of 10 s.

Nucleation of Boron Phosphate in As-Deposited Borophosphosilicate Glass Films

Borophosphosilicate glassy (BPSG) films, used as a premetal dielectric in metal-oxide-semiconductor field-effect transistor devices, can be a serious constraint in the device technology due to the nucleation and growth of boron-phosphate phase (BPO 4 ), especially in BPSG films grown with a high boron-to-phosphorous ratio. It has been commonly believed that BPO 4 forms during the film flow/annealing process. This investigation focused on the mechanism of possible BPO 4 nucleation in the films, deposited by means of plasma-enhanced chemical vapor deposition (PECVD) before high-temperature annealing and its evolution during heating. Our study indicates that the dopant-related precipitates in the as-deposited films are a key factor for the ultimate manifestation of BPO 4 during high-temperature annealing. B 2 O 3 and P 2 O 3 /P 2 O 5 compounds, in conjunction with H 2 O, are essential for BPO 4 materialization in SiO 2 matrix. The impacts of each of the above reactants on the film instability are discussed. The optimization of the PECVD BPSG film structure to improve the film reliability is proposed.

Factors Delineating Stability of the As-Deposited PECVD BPSG Film

This study aims to define a better condition to enhance the stability of the as-deposited plasma-enhanced chemical vapor deposition borophosphosilicate glass (PECVD BPSG) film. Stability is discussed in terms of optimized film composition SiO 2 -B 2 O 3 -P 2 O 3 /P 2 O 5 . Stable oxygen-rich films show better flow ability, lower dopant fluctuation through the growing oxide, reduced film stress, and lesser nitrogen and hydrogen incorporation into the growing film. Optimization of the B/P ratio, along with a lower amount of hydrogen-related species, especially H 2 O, which is a catalyst for BPO 4 defects, makes the film less vulnerable to defect formation. The film structure can be modulated by the deposition parameters to get optimized ratios of N 2 0/SiH 4 , P 2 O 3 /P 2 O 5 , and SiO 2 /P 2 O 5 . Special attention is paid to the BPSG films with low SiH 4 -precursor gas flow. The limited condition for SiH 4 flow is also discussed.