Electroless Gold Plating on Aluminum Patterned Chips for CMOS-Based Sensor Applications

Abstract

We presented an approach for the activation of aluminum ͑Al͒ alloy using palladium ͑Pd͒ and the subsequent gold ͑Au͒ electroless plating ͑ELP͒ for complementary metal oxide semiconductor ͑CMOS͒-based sensor applications. In this study, CMOS process compatible Al patterned chips were used as substrates for easy incorporation with existing CMOS circuits. To improve the contact resistance that arose from the Schottky barrier between the metal electrodes and the single-walled carbon nanotubes ͑SWCNTs͒, electroless deposition of gold that has a higher work function than Al was adopted because the SWCNTs has p-type semiconductor properties. Each step of the Au ELP procedure was studied under various bath temperatures, immersion times, and chemical concentrations. Fine Pd particles were homogeneously distributed on the Al surface by the Pd activation process at room temperature. Au ELP allowed selective deposition of the Au film on the activated Al surface only. The SWCNT networks formed on the Au plated chip by a dip-coating method showed improved contact resistance and resistance variation between the Au electrode and SWCNTs. We also tried SWCNT decoration with the Au particle using the upper Au ELP method, which was expected to be applied in various areas including field-effect transistors and sensor devices. Recently, the carbon nanotube ͑CNT͒ has been considered a promising candidate for a sensor material due to its high conductivity, small size, high mechanical strength, and stability in a harsh chemical environment.1-7 For these sensor materials to be a part of the commodity technology in the future, the integration of the nanomaterial with the complementary metal oxide semiconductor ͑CMOS͒ integrated circuit platform is indispensable for lower cost, reduced power consumption, and miniaturization. However, big technical challenges in the integration to CMOS chips are to acquire a reliable method of mounting the CNTs on required positions, and obtaining small contact resistance with aluminum ͑Al͒, which is used as a final metal pad for the conventional CMOS process. So, metal plating such as with gold ͑Au͒ is needed on top of the Al pad which forms the ohmic contact with CNTs.On the other hand, electroless plating ͑ELP͒ of noble metals on Al has been used in a variety of potential applications including flip-chip bonding of the CMOS chips on the printed circuit board, and protective coatings. [8][9][10][11][12][13] Traditionally, Au ELP 14 on Al is preceded by the zincate process ͑Al → Zn → Ni → Au͒, which activates the Al surface. [15][16][17][18][19] The process involves the immersion of the Al substrate in a strong alkaline zincate solution. This process consists of dissolution of a native Al oxide and the galvanic displacement reaction between zinc ͑Zn͒ ion and Al, and the formed Zn acts as a catalytic layer of the Au electroless deposition.The selective formation of Pd particles directly onto patterned Al substrates is a potential alternative in zincating processes used as intermediate layers in electrochemi...

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