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AA5.02 - Characterization of Highly Resistive Nanoscale RRAM Contacts 
April 8, 2015   8:15am - 8:30am

Resistive random access memory (RRAM) is a promising candidate for next generation data storage due to its potential for performance, scalability and compatibility with CMOS processing. The switching of metal oxide films (HfOx, TiOx, AlOx) has been studied [1] and several mechanisms proposed for the formation of low resistance pathways of such films [2,3]. However, the performance of the memory device is also dependent on the contact resistance, especially at sub-5 nm dimensions [4]. Measuring the contact resistance in highly resistive films is challenging due to low current levels, variability and interface ionic migration. Similar studies exist for phase change materials such as Ge2Sb2Te5 (GST) [5,6] but a comprehensive study for the more resistive metal oxide based RRAM materials is still lacking. In this study, we introduce a methodology to measure highly resistive thin films and their interfacial electrical resistances. We carry out low current four-probe measurements to measure contact resistance with a Cross Bridge Kelvin [6] structure. The overlap parameter for the structure is ~100 nm to enhance the signal-to-noise ratio for the low-current level typically used in these resistive devices. This allows accurate extractions of contact resistance and film resistivity for structures with specific contact resistance values greater than 100 ??�cm2 and underlying film resistivity value above 10 ?�cm. Circular transfer length measurements (TLM) are carried out with a Ti/Pt contact stack on 20 nm HfO2 films prepared via atomic layer deposition (ALD) and sputtered GST films � yielding specific contact resistivity values of 7.5 ?�cm2 and 3.2 x 10-4 ?�cm2 at room temperature, respectively. For RRAM films which involve ion migration in the bulk and at the metal oxide interfaces, we utilize a Greek Cross [7] structure for sheet resistance measurement to avoid the injection effect at the interface. We find transfer lengths for contacts to HfO2 to be in the range of 200 nm and for amorphous GST in the range of 4 ?m at room temperature. Linear TLM structures with metal line widths below 200 nm are also fabricated and measured for stoichiometric ALD HfO2 and sputtered HfOx films, to extract the carrier mobility under the metal contact [8]. This measurement platform can be used to precisely measure the interfacial and bulk characteristics for highly resistive films. It is also a useful tool in exploring the switching mechanism, as a single electrode-memory bit interface can be measured accurately, with low current and low noise measurement capability. [1] H.-S.P. Wong et al, Proc. IEEE (2012) [2] R. Waser et al, Adv. Mater. (2009) [3] D. Ielmini, IEEE-TED (2011) [4] C.-L. Tsai, E. Pop, et al ACS Nano (2013) [5] S. Savransky, I. Karpov, MRS Proceedings (2008) [6] D. Roy et al. IEEE-EDL(2010) [7] S. Enderling et al, IEEE-TSM (2006) [8] D. Schroeder, 3rd ed. (2006)

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