Knowledge of contact resistance of phase change materials (PCM) to metal electrodes is important for scaling, device modeling and optimization of phase change random access memory (PCRAM) cells. In this article, we report the systematic determination of the speci_c contact resistance (_c) with voltage bias for doped Sb2Te to TiW metal electrodes. These data are reported for both the amorphous and the crystalline state of the PCM

Roy, Deepu

Wolters, Rob

Zandt, Micha in 't

English

in 't Zandt, Micha

Wolters, Robertus A.M.

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Bias dependent specic contact resistance of phase change material to metal contacts

University of Twente Research Information

SAFE WORKSHOP, 2010 1Bias dependent specific contact resistance of phasechange material to metal contactsDeepu Roy, Micha in ’t Zandt, and Rob WoltersAbstract—Knowledge of contact resistance of phase changematerials (PCM) to metal electrodes is important for scaling,device modeling and optimization of phase change random accessmemory (PCRAM) cells. In this article, we report the systematicdetermination of the specific contact resistance (ρc) with voltagebias for doped Sb2Te to TiW metal electrodes. These data arereported for both the amorphous and the crystalline state of thePCM.Index Terms—PCM, specific contact resistance, Cross BridgeKelvin Resistor(CBKR), bias dependence.I. INTRODUCTIONELECTRICAL contacts are an essential part of a device ora circuit for its operation. These contacts are character-ized by the contact resistance, which is expressed in terms ofspecific contact resistance (ρc). ρc is defined as the derivativeof voltage with respect to current density at the contact atzero bias[1]. For a line cell the resistance of the current pathincludes the resistance of the switching part of the line (RL)and the resistance of the crystalline PCM contacts (Rc) [2].In the ovonic type cells the PCM to electrode contact can bein the amorphous or crystalline state. In reality, during theoperation of the device there will be a bias/voltage drop atthe contacts. Depending on the conduction mechanism at thecontact this voltage drop at the contact results in a deviationof ρc compared to the value at zero bias.In this article this bias dependence of ρc is characterizedfor phase change material (PCM) to TiW contacts. This is per-formed by current-voltage (I-V) resistance measurements (Rk)on Cross Bridge Kelvin Resistance (CBKR) test structures asshown in Fig.1.II. EXPERIMENTTo fabricate PCM to TiW CBKR structures, 50 nm TiW isdeposited on a silicon wafer and patterned to form the bottomelectrode layer at the contact. The top layer is 20 nm PCM andthe contact area between the two layers is defined by contactopened in a 50 nm PECVD SiO2 layer. To measure the PCMto metal contact resistance, a current is forced from the metalManuscript received October 11, 2010. This research was carried out underthe project number MC3.07298 in the framework of the Research Program ofthe Materials innovation institute M2i (www.m2i.nl).Deepu Roy and Micha in ’t Zandt are with the NXP-TSMC Research Cen-ter, NXP Semiconductors, High Tech Campus-4, 5656AE, WAGp 02, Eind-hoven, The Netherlands. (phone:0031-4027-26872; fax:0031-4027-43352; e-mail:deepu.roy@nxp.com).Rob Wolters is with the NXP-TSMC Research Center, NXP Semiconduc-tors, High Tech Campus-4, 5656AE, Eindhoven, The Netherlands, and alsowith MESA+ Institute for Nanotechnology, University of Twente, Enschede,The Netherlands.                                                                                 ~~~~~~~~~~~~~~~~WxWyLdMetalPCMContact1 324Fig. 1. Coss Bridge Kelvin Resistor structure showing upper (PCM) layer,lower (metal) layer and the contact structure (δ, L).to PCM (1 to 3) and the voltage is measured orthogonal tothe direction of current flow (2 and 4) using a semiconductorparameter analyzer. This allows measurement of the averagevoltage at the contact[3] and the voltage drop due to currentspreading in the δ. The derivative of this I-V curve gives themeasured resistance at the contact (Rk). From Rk the specificcontact resistance (ρc) of the contact interface only is extractedby using a 2D analytical model excluding the resistance in theδ region [4]:Rk =ρcA+4Rshδ23WxWy[1 +δ2(Wx − δ](1)To be able to measure the contact resistance to amorphousPCM these structures should be processed below the crystal-lization temperature of PCM. The amorphous to crystallinetransition temperature for this doped Sb2Te is 160 ◦C [5][2].So the CBKR structures have been processed with a thermalbudget of maximum 120 ◦C. Contact resistance measurementsare performed on these fabricated structures (PCM amorphous)and after annealing (PCM crystalline) at temperatures of 250◦C for 5 min in N2 ambient. Samples annealed above 250 ◦Cwere protected against oxidation and evaporation by cappingwith a 500 nm PECVD SiO2 layer deposited at 250 ◦C.III. RESULTS AND DISCUSSIONSFig. 2 shows the I-V characteristics of CBKR structures withPCM in the crystalline and in the amorphous state. CrystallinePCM contacts show a linear current voltage (I-V) characteristicunlike the amorphous PCM. Rk with voltage bias is calculated- 147 -SAFE WORKSHOP, 2010 2-500.0µ 0.0 500.0µ-30.0m0.030.0mVoltageinVCurrent in A-2.0µ -1.0µ 0.0 1.0µ 2.0µ-1.4-0.70.00.71.4VoltageinVCurrent in A(a) (b)Fig. 2. I-V characteristics for the CBKR structures with (a) PCM in the crystalline state and (b) PCM in the amorphous state.Ω Ω(a) (b)Fig. 3. Change in ρc with applied bias voltage. (a) Crystalline PCM, (b) amorphous PCM.form the derivative of the I-V curve, from which the ρc iscalculated. The extracted ρc for doped Sb2Te to TiW contactsin the amorphous and crystalline state for positive and negativebias voltage across the contact is shown in Fig.3.The crystalline state of the PCM results in a linear I-Vof the contacts and the Rk and ρc remains unchanged withthe applied bias voltage. From Hall measurements we havederived a carrier concentration of approximately 21021cm−3for crystalline doped Sb2Te phase change material. Giventhis carrier concentration this PCM behaves like a metal tosilicon contact showing ohmic behavior with a linear I-V.Here the tunneling process dominates and the value of ρc ishardly dependent on the work function of the metal [1]. In thecrystalline state the ρc value for PCM to metal contacts is atleast two orders of magnitude higher than common metal tometal contacts [6].In the case of amorphous PCM to metal contacts, theI-V characteristics is non-linear and here the extracted ρcdecreases with the voltage bias at the contact (Fig. 3(b)).Thedependence of ρc on the sign and magnitude of the applied biasvoltage is determined to investigate the effect on the switchingcharacteristics of PCRAM cells. This variation in ρc withbias should be taken into account for contact characterizationand device modeling. The bias dependence on reset currentin PCRAM cells has been reported in the literature [7]. Ourmeasurements show a stronger dependence of ρc with negativebias compared to positive bias. A similar behavior in ρcvalues is also reported for metal semiconductors contacts inthe case of thermionic field emission. The ρc values are notsymmetrical with the bias voltage and the resistance peak isnot occurring at zero bias [8].IV. CONCLUSIONIn this article we report the bias dependence of specificcontact resistance for TiW to doped Sb2Te in the amorphousand crystalline state of using dedicated CBKR measurementstructures. The electrical properties of crystalline PCM tometal contacts show similarities with the highly doped siliconto metal contacts. In the amorphous state ρc values show astrong dependence with sign and the magnitude of appliedvoltage bias. The knowledge of ρc is critical for the scaling,device modeling and optimization of the switching character-istics for PCRAM cells.REFERENCES[1] S. M Sze, Physics of semiconductor devices, 2nd ed. Harlow, Wiley-Interscience, New York, 1981,ch. 5.[2] M. H. R. Lankhorst, B. W. S. M. M. Ketelaars, and R. A. M. Wolters,”Low-cost and nanoscale non-volatile memory concept for future siliconchips,” Nature Materials, vol. 4, pp. 347-352, April 2005.- 148 -SAFE WORKSHOP, 2010 3[3] S. J Proctor and L. W Linholm, ”A direct measurement of interfacialcontact resistance,” IEEE Electron Device Lett., vol. 3, no. 10, pp.294-296, Oct. 1982.[4] T. A. Schreyer, K. C. Saraswat, ”A two dimensional analytical model ofthe cross bridge Kelvin resistor”, IEEE Electron Device Lett., vol-7,1986, pp.661.[5] I. Friedrich, V. Weidenhof, N. Njoroge, P. Franz and M. Wuttig,”Structural transformations of Ge2Sb2Te5 films studied by electricalresistance measurements,” J. Appl. Phys., vol. 87, no.9, pp. 4130-4134, May 2000.[6] N. Stavitski, J. H. Klootwijk, H. W. Van Zeijl, A. Y. Kovalgin andR. A. M. Wolters, ”Cross bridge kelvin resistor structure for reliablemeasurement of low contact resistances and contact interface character-ization,” IEEE Trans. Semiconductor manufacturing, vol. 22, no.1,pp. 146-152, Feb 2009.[7] S. Lee, J. Jeong, T. S. Lee, W. M. Kim, and B. Cheong, ”Bias polaritydependence of a Phase change memory with Ge-doped SbTe: A methodfor multilevel programming,” Appl. Phys. Lett.,” 92, 243507 (2008).[8] C. R. Crowell and V. L. Reideout, ”Thermionic field resistance maximain metal semiconductor (schottky) barriers,” Appl. Phys. Lett., vol. 14,no. 3, Feb. 1969.- 149 -

A direct measurement of interfacial contact resistance,

A two dimensional analytical model of the cross bridge Kelvin resistor,

Bias polarity dependence of a Phase change memory with Ge-doped SbTe: A method for multilevel programming,

Cross bridge kelvin resistor structure for reliable measurement of low contact resistances and contact interface characterization,

Low-cost and nanoscale non-volatile memory concept for future silicon chips,

Physics of semiconductor devices, 2nd ed.

Structural transformations of Ge2Sb2Te5 lms studied by electrical resistance measurements,

Thermionic eld resistance maxima in metal semiconductor (schottky) barriers,

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Bias dependent specic contact resistance of phase change material to metal contacts

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