Self-aligned, fluxless flip-chip bonding technology for photonic devices

Abstract

The self-aligned flip-chip (FC) bonding technique is a very attractive means for the assembly of photonic devices containing multiple optical as well as electrical waveguide interconnects. In this article we propose a fluxless FC-bonding technology which nevertheless ensures efficient solder oxide reduction by applying molecular hydrogen (H2) under vacuum conditions. Bonding experiments were carried out in a newly developed FC-bonder of which some interesting details are reported. Reproducable bonding accuracies below the required tolerances for fiber to tapered waveguide coupling of <or=3 mu m have been achieved. Using eutectic tin-lead (SnPb 60/40) solder and platinum (Pt) as a wettable pad metallization these bonding results have been obtained at moderate temperatures (250 degrees C) and heating durations (2 min). An investigation on the oxidation kinetics of molten SnPb 60/40 solder confirms the ability of H2 to reduce solder oxides when thermodynamic boundary conditions are met. The leach resistance of the Pt thin-film metallization (300 nm) has been prooved by SIMS depth profiles for the required bonding temperatures and durations

Oxidation and Reduction of Liquid SnPb (60/40) under Ambient and Vacuum Conditions

“…Sn + 0, # SnO2 [1] Po, (250°C) = ioPa [2] Equation 1 describes the oxidation of Sn to stannic oxide (SnO,). For equilibrium conditions (zXG = 0), the reaction Table I.…”

New reflow process for indium bump

“…It is known that molecular H2 gas is efficient in removing tin oxide [4]. Oxygen in tin oxide comes out from the surface if the vapor pressure of the oxygen is higher than the environment pressure and it reacts with H2 molecule.…”

“…Bumps made by the lift-off process will be gathered to form higher bump during the reflow. If the bump is reformed as half sphere shape, the bump height will be approximately represented as h=(3V/87t)"3 (4) h: bump height reformed by the reflow V: Volume of indium bump before the lift-off…”

Numerical Simulation of Dynamic Wetting Behavior in the Wetting Balance Method

“…• C from the manufacturer are: kinematic viscosity of 0.50 cm 2 /s (µ = 0.48 g/cm s), density of 0.963 g/cm 3 , and surface tension of 0.00208 J/m −2 . Experiments were made on 99.99% pure Cu plate.…”

“…This interconnection process, however, has several problems: toxicity and radioactivity of lead as a solder alloy element, corrosion of substrate by using flux cleaning agents and excessive growth of Sn-Cu intermetallic compound. [1][2][3] Thus, new solder materials and soldering process are required to address the above problems and the solderability becomes a critical property for both developing new solder materials and soldering processes.…”