IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 3, MARCH 2008 259 Accurate Estimation of Low (< 10 −8 Ω · cm 2 ) Values of Specific Contact Resistivity Madhu Bhaskaran, Student Member, IEEE, Sharath Sriram, Student Member, IEEE, and Anthony S. Holland Abstract—Advancements in nanotechnology have created the need for efficient means of communication of electrical signals to nanostructures, which can be addressed using low resistance contacts. In order to study and estimate the resistance of such con- tacts or the resistance posed by the interface(s) in such contacts, accurate test structures and evaluation techniques need to be used. The resistance posed by an interface is quantified using its specific contact resistivity (SCR), and although multiple techniques have been utilized, inaccuracies of such techniques in measuring values of SCR lesser than 10 −8 Ω · cm 2 have been reported. In this letter, an approach for estimating very low values of SCR (lower than the previously limiting 10 −8 Ω · cm 2 ) using a cross Kelvin resistor test structure is demonstrated using aluminum to titanium silicide ohmic contacts, with a minimum estimated SCR value of 6.0 × 10 −10 Ω · cm 2 . Index Terms—Contact resistance, cross Kelvin resistor (CKR), specific contact resistivity (SCR). I. I NTRODUCTION A DVANCEMENTS in nanotechnology have created the need for efficient means of communication of electrical signals to nanostructures [1]. Electrical contacts made to such nanodevices need to pose minimum possible contact resistance. In order to study and estimate the resistance of such contacts or the resistance posed by the interface(s) in such contacts, accurate test structures and evaluation techniques need to be used. These will pave the way to the identification of new materials and/or contact architectures to develop nanoscale low resistance contacts. The resistance posed by an interface is quantified using its specific contact resistivity (SCR), which is denoted using ρ c (in ohm square centimeters) [2], and multiple techniques have been utilized in measuring SCR values. Cross Kelvin resistor (CKR) test structures were shown to be suitable for the measurement of low values of SCR, but the use of cumbersome error correction curves to estimate the value of SCR and inherent inaccuracies in the technique served as deterrents from the widespread use of this estimation technique [3]–[6]. Using a combination of analytical calculations and finite element modeling, a simplified approach to this problem of SCR estimation using the CKR test structures with varying contact sizes has been developed by the authors of this letter [7]. This letter demonstrates the accuracy Manuscript received October 29, 2007; revised December 4, 2007. The review of this letter was arranged by Editor K. De Meyer. The authors are with the Microelectronics and Materials Technology Centre, School of Electrical and Computer Engineering, RMIT University, Melbourne, Vic. 3001, Australia (e-mail: madhu.bhaskaran@gmail.com). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2007.915378 Fig. 1. Schematic of a CKR test structure. Notations used in the manuscript are denoted. of this technique in estimating SCR lower than the previously limiting values of 10 −8 to 10 −7 Ω · cm 2 . The accurate evaluation of low values of SCR (related to low values of contact resistance) will be demonstrated using two- layer ohmic contacts with aluminum and titanium silicide thin films. Although these ohmic contacts have been characterized before [8], they remain one of the very few ohmic contacts for which very low (< 10 −8 Ω · cm 2 ) values of SCR have been reported. II. CKR TEST STRUCTURE The CKR test structure [9]–[11], as shown in Fig. 1, consists of two “L”-shaped regions (of width w), consisting of the two materials of interest, which, in this case, are aluminum (upper “L”) and titanium silicide (lower “L”). For contact resistance estimation, the CKR test structures using contacts of defined areas (circular or square contacts with diameter or side d, respectively) are used; achieved by defining the contact in an intermediate insulator layer, as shown in Fig. 1. The value of resistance measured from the two voltage taps of the CKR is the Kelvin resistance R K , which is a combination of the contact resistance R c and other parasitic resistances (due to sheet resistance of surrounding material). R c is inversely proportional to the area from the standard resistance–area relationship. This implies that as the area de- creases, by decreasing the diameter d, R c increases, and beyond 0741-3106/$25.00 © 2008 IEEE Authorized licensed use limited to: RMIT University. Downloaded on June 12, 2009 at 02:19 from IEEE Xplore. Restrictions apply.