Practical effects of ion implantation for micro-scale punch tools

Abstract

Micro electro-mechanical system (MEMS) devices fabricated by photo lithography techniques require expensive equipment and resources. MEMS devices such as acceleration sensors, pressure sensors, and optical scanners are only components of more complex devices, and therefore their fabrication must involve inexpensive processes to be cost-effective. We proposed to use the die-forming that is a possible inexpensive process in MEMS manufacturing facilities. Die-forming is a traditional fabrication system and has only been applied to sheet materials. Its advantages include low cost, high throughput, and assembly of parts made using the same die. This technique uses micro-scale punch tools to shear, and currently the lifespan of these tools is too short for use in mass production of MEMS. In this study, to increase the lifespan, ion implantation was applied to a Co–WC micro punch tools used to punch 150-μm holes in a 0.2-mm-thick stainless spring steel. Co–WC was selected as the material due to its hardness and its high resistance to compressive stress, although its resistance to tensile stress is low. This low tensile strength causes a Co–WC punch tool to fracture, and thus determines the lifespan of the tool. The friction between the punch tool and the punched material actually induce tension of the tool. To improve the friction relate to the tensile strength, and therefore the lifespan, of a Co–WC punch tool, in this study this tool was implanted with gold ions accelerated at 75 keV at doses 1 × 10 16 cm − 2 (1 × 10 20 m − 2 ) and then annealed at 700 K for 30 min. Cross-sectional transmission electron microscopy was used to observe the changes in the fabricated tool induced by the implantation and annealing, and a sliding test was also applied to evaluate the friction coefficient of surface modified samples. The heavy gold ions were expected to break the crystal lattice and boundary between Co and WC, and the post annealing to rearrange the disordered lattice and thus improve the toughness of micro punch tools. Gold ions also act as a catalyst to oxidize the Co–WC. In conclusion, gold ion implantation and post annealing process together increased the lifespan of the punch tool by a factor of 16. Such punch tools can therefore be used in mass production of MEMS devices.