Data Storage Systems Center

This project focuses on developing probe addressable phase change vias for reconfigurable electronics. The focus of the work is on via materials, via sizing and on the addressing using probes. Additionally, the project addresses the fabrication and demonstration of the process using conducting atomic force microscopy. Finally, optimal circuits for use with these reconfigurable vias are being examined. Recent results includes measurements, models and designs developed over the past year for a viable via structure for reconfiguring RF inductors over a wide dynamic range. One important specific aspect of these designs, is a connection topology, using dual tip AFM-type probes. This design subdivides a single phase change via into a parallel array of three-terminal sub-vias which are well-suited to addressing with probes. This sub-division reduces required power and current to acceptable levels.

CMOS-MEMS electrothermal probe actuators with nickel metalized probe tips have been designed and characterized. These probes are being designed to access on-chip pads that route electrical current for on-chip on/off programming of phase change vias. The target is to use the probe via technology for reconfigurable RF circuits. The MEMS probes offer a way to supply the reconfiguration current, while contributing no parasitic capacitance when disconnected. Recently, a new “push-up” probe actuator design has been successfully produced. The probes do not touch the chip pads when powered off. The new design allows more probe contact force to be applied as the actuator power is increased. A 10.1 µm vertical upward actuation on unconstrained probes has been achieved with a 14 V, 63.7 mW drive to the embedded polysilicon heater resistors. The thermal cut-off frequency is 153 Hz, the mechanical resonant frequency is 40920 Hz, and the quality factor is 221. The thermal cross-talk at adjacent probes is about 6%. Thermal buckling is not observed at 14 V. The measured contact resistance is between 1 – 10 Ω and decreases with increasing force as expected. Vertical position and force sensors are being developed to stabilize the contact resistance. We are designing capacitive sensors to achieve nm-scale position resolution and piezoresistve sensors to achieve μN-scale force resolution. Yield and reliability of Ni-plating is not so good. A more reliable metallization process is being developed. A future goal is to complete a force servo to provide feedback to stabilize the contact force, and thus the contact resistance.

Metallized CMOS-MEMS electrothermal probe actuators have been designed and characterized. A 23.9 µm vertical actuation has been achieved at 4 V 3.7 mW. The thermal cut-off frequency is 800 Hz, and the mechanical resonant frequency is 19820 Hz. The thermal cross-talk at adjacent probes is about 10%. Thermal buckling is observed at high drive (> 4 V). The contact resistance is characterized to be about 10 – 100 Ω and changes with force as expected. To save power, lower contact resistance is preferred in reconfiguring IC’s.

Dual probe actuators are needed to decrease the area of reconfiguration part in IC chips. To save power, upwards actuation, instead of regular downwards actuation, is preferred. High applied force is necessary to achieve reliable electrical contacts with low contact resistance. Based on these considerations, we are designing these stiff (5 N/m), large upwards stroke (10 µm), high applied force (50 µN) dual probe actuators. Probe tips with different height, such as M3-topped or M4-topped, and different size, such as 0.85 x 0.85, 2 x 2, 2.6 x 2.6, 3 x 3, 4 x 4, and 5 x 5 µm2, are designed to find the minimum feature, such as top metal and contact area to reconfigure the IC’s.

Review Talk

Prior work on MEMS probe-based data storage has explored thermal [1], magnetic [2] and phase change [3] modalities. Probe cards in IC chip testing [4] and RF switches [5] explore electrical contact with relatively large electrodes (> 100 µm2) and mN forces. Contacts with much smaller area are viable, as evidenced by conductive AFMs and RF MEMS switches [6], which contact on surface asperities. In this work we consider MEMS actuated conductive contacts with 1 µm2 scale electrodes. We report on our progress on the development of CMOS-MEMS electrothermal conductive probes for Memory-Intensive Self-Configuring Integrated Circuits (MISCICs). The MISCIC vision is to use MEMS conductive probes to reconfigure circuits by mechanically addressing and passing current through resistance change vias embedded within the chip circuitry. Cantilevered probes are designed with 1, 4, 9, 16 and 25 µm2 areas that are plated with nickel. The vertical range of the electrothermal actuator is 23.9 µm with 3.7 mW heating power, but the range is reduced when in contact with a gold-coated glass slide due to thermal effects. Probe contact to gold varies from 10-50 Ω for multiple make and break cycles with 1 µm2 tips.

www.zurich.ibm.com/st/nanofabrication/microcantilevers.html www.el.utwente.nl/smi/content/probe/uspam/details.html S. Gidon et al., Appl. Phys. Lett., vol. 85, no. 26, pp. 6392-6394, 2004. C. Tsou et al., J. Micromech. Microeng., vol. 16, pp. 2197-2202, 2006. E.R. Brown, IEEE Trans. Microwave Theory Tech., vol. 46, pp. 1868-1880, 1998. S. Majumder et al., IEEE MTT-S Int’l Microwave Symposium Digest 2003, pp. 1935-1938, 2003.

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