A low-cost electroless plating method for producing flip-chip bondable and wire-bondable circuit pads for smart pixel application

Abstract

Introduction: VCSEL based free-space optoelectronics, such as smart pixel arrays, provide excellent lowpower, large-bandwidth, high-density interconnects for high-speed communications and ultra-fast optical computing applications. Flip-chip solder bonding is the most commonly adopted method to integrate the driver circuitry to the VCSELs in a hybrid smart pixel array element [ 11, owing to its superior mechanical, electrical and thermal properties from the packaging point of view. The inherent advantage of VCSELs to be fabricated as a 2-dimensional array makes them perfectly suitable for being flip-chip bonded to CMOS driver chip to realize a compact device footprint with a large number of input/outputs. Using solder bumps for flip-chip has the added advantage of self-alignment, which relaxes the demand on chip-to-substrate placement accuracy [2]. For flip-chip solder bonding, the substrate has to have a matching pattern of solder-wettable bond pads corresponding to the solder bump array on the die. However, the standard metallization of bond pads on chips available ftom CMOS fabrication facilities is an aluminum alloy, which is not solder-wettable by itself. Thus, a reliable re-metallization technique is required to cover the aluminum pads with an oxidation-resistant and solderable layer of metal, typically gold. This can be done with either vacuum deposition (sputtering or evaporation), or conventional electroplating. Both of these methods need mask for selective deposition. An alternative maskless plating technique, which selectively deposits metal from aqueous solutions of metal ions without current being passed through the electrodes, is known as electroless plating [3]. This method is potentially the simplest and most inexpensive means for re-metallizing the aluminum bond pads to a solderable surface. We have developed a complete electroless process cycle, using commercially available reagents, to bump the aluminum bond pads with a 4.0-4.5pm thick nickel/gold layer. We have successfully demonstrated that the deposition is well-adherent to the base

1 Gb/s VCSEL/CMOS flip-chip 2-D-array interconnects and associated diffractive optics

“…We used an electroless plating process to re-metallize the aluminum bonding pads [12] without a need for any lithography or electrode installation on the CMOS circuits. The electroless plating sequences consist of etching the oxide layer on the contacts in a strong acidic etchant, immediately plating with a 0.25-µm layer of zinc to prevent reoxidation of the aluminum, and following this with a 4 um nickel/Au layer.…”