Electrical characterization of thin film ferroelectric capacitors

Transcription

1 1 Electical chaacteization of thin film feoelectic capacitos M. P. J. Tiggelman, K. Reimann, M. Klee, D. Beelen, W. Keu, J. Schmitz, and R. J. E. Hueting Abstact Tunable capacitos can be used to facilitate the eduction of components in wieless technologies. The tunability of the capacitos is caused by the sensitivity of the elative dielectic constant to a change in polaization with electic field. Thin film feoelectic MIM capacitos on silicon offe a e-use of electonic cicuity, low tuning voltages, a high capacitance density, a low cost, a pesence of bulk acoustic wave esonance(s) and decoupling functionality. The basic opeation and measuement pinciples ae outlined. To assess the pefomance in the micowave fequency ange, MIM test stuctues 1, with a baium stontium titanate dielectic, have been successfully pocessed, and measued. The electical chaacteization of tunable capacitos is demonstated using a 1-Pot Advantest R3767CG VNA in the fequency ange of 10 MHz 8 GHz. Index Tems feoelectic, MIM, tunable capacito, baium stontium titanate, and high-k dielectic. the metal-insulato-metal (MIM) test stuctues electical chaacteization is pefomed. In this wok the electical chaacteization of feoelectic capacitos is successfully demonstated. The basics of the feoelectic mateial, the test stuctues, and typical measuement esults ae discussed fom f = 10 MHz 8 GHz using an Advantest R3767CG vecto netwok analyze (VNA) with a 1-Pot configuation. II. BASIC OPERATION A tunable capacito with a (Ba,S)TiO 3 dielectic can opeate typically in two phase egions. The opeating egion depends on the tempeatue, and the composition of the feoelectic mateial. The effect on the elative dielectic constant is indicated in fig. 1. D I. INTRODUCTION EVELOPMENTS in solid-state feoelectics suited fo micowave applications ae an on-going eseach effot fo moe than 30 yeas [1]. Feoelectic mateial is often integated in capacito stuctues. Integation of these electonic building blocks on silicon has numeous advantages: 1) continuous (low voltage) tuning, 2) low cost, 3) high capacitance density due to the high-κ mateial and thin film technology, and 4) decoupling. The applicability of these types of capacitos has been pointed out in liteatue by typical applications such as phased shiftes [2], impedance matching netwoks [3], delay lines [4], o tunable filtes [5]. The industy has been focusing on diffeent feoelectic mateials. The most fequently used feoelectic is baium stontium titanate (Ba,S)TiO 3. To assess the pefomance of Manuscipt eceived Septembe 27th This wok was suppoted by NXP (fome Philips Semiconductos), and the Univesity of Twente. M. P. J. Tiggelman is a PhD student fom the MESA+ Institue fo Nano Technology woking at NXP in Eindhoven (coesponding autho to povide phone: +31 (0) ; fax: +31 (0) ; nlv14139@natlab.eseach.philips.com). Fig.1 The elative dielectic constant ε of the feoelectic mateial with tempeatue. The Cuie tempeatue T c sepaates the phase egions, and indicates the maximum elative dielectic constant ε at zeo DC voltage with a symmetical stack. The diffeences between the phase egions ae indicated in table 1. Feoelectic phase Paaelectic phase Tempeatue T T < T c T > T c Cystal stuctue Tetagonal Cubic Loss tangent tanδ Highe High Tunability n Highe High Table 1. Basic modes of opeation of a feoelectic (Ba,S)TiO 3 mateial. 439

2 2 The latte two points in the table ae known as the pefomance paametes of a tunable capacito. The tunability n, expessed in eq.1, defines the amount of capacitance tuning fom zeo DC voltage until a specific voltage (lowe than the beakdown voltage). ε (0 Vdc ) ε ( V max) Tunability n = (1) ε (0 V ) The loss tangent, expessed in eq. 2, indicates of the intinsic loss in the feoelectic mateial and the electodes. R( Loss tangent tan δ = (2) I( Z ) The combination of these factos can give a quick pefomance assessment of the capacito. Fo micowave applications the geneal equiements ae a low loss tangent and a high tunability. The loss tangent is highe in the feoelectic phase due to hysteesis effects caused by spontaneous polaization. The contibution of feoelectic domains causes exta losses compaed to test stuctues opeating in the paaelectic phase egion [6]. A DC bias satuates the polaization in educing the elative dielectic constant (see figue 9). Since measuements ae pefomed fom 1 khz up to the 1 MHz with an impedance/gain-phase analyze the focus in this aticle will be on the effects of applying a DC bias in a wide fequency ange with a VNA. 11 III. RF MEASUREMENT SETUP AND PARAMETERS The pefomance assessment of the tunable capacitos is pefomed with a typical 1-Pot RF measuement setup as shown in fig. 2. dc paamete output of the VNA. A Matlab pogam convets the s 11 -paametes to impedance Z 11 -paametes. A simple equivalent model, shown in fig.3, epesents the tunable capacito, and can be used fo electical chaacteization. This model can be conveted to Z-paametes. Fom this point fowad the fequency and voltage behavio of the elative dielectic constant ε, the capacitance, the tunability n expessed in eq.1, loss tangent tanδ expessed in eq. 32, and the the seies esistance, can be detemined. Fig.3 The equivalent seies - model of a tunable feoelectic capacito. The elative dielectic constant ε can be expessed as d ε =, (3) ε 0 A wheeby the fee pemittivity of fee space ε 0 = 8.85e-12 F/m, the aea A, and the dielectic thickness d. Fom the ε (V) cuve infomation can be obtained e.g. hysteesis effects, capacito symmety, pope gounding, and the beakdown voltage. The two main pefomance paametes, the tunability and loss tangent, ae expessed in eq.1 and 2. IV. RF TEST STRUCTURES The layout of the test stuctues is based on a technique descibed by Ma et al.[7]. Only the top electode is stuctued fo the capacito design (see fig.4). The RF test stuctues ae designed fo GS o GSG RF pobes. Coss view: Top view Fig.4 The uppe pictue displays the coss view, and the lowe placed figue displays the top view of the RF test stuctues. Fig.2 A 1-Pot S 11 -paamete RF measuement setup. The magnitude and phase infomation of the 1-Pot measuement data ae contained in the insetion loss s 11 - A lage capacitance goes fom the gound top electode to the bottom electode, and a small capacitance fom the signal electode to the bottom electode. The total capacitance esembles two capacitos in seies. The total capacitance equals the small capacitance fom the top signal electode to 440

3 3 the bottom electode. V. MEASUREMENT RESULTS Fig.6 A distibuted RC netwok. The scatte paamete s 11 of a wide-band VNA can be tansfomed to give valuable chaacteistics about the fequency dependency of the tunable MIM capacitos. The measuements ae pefomed fom f = 10 MHz until f = 8 GHz. The peviously mentioned paametes ae investigated futhe. The elative dielectic constant ε In the fequency ange of seveal MHz up to seveal hunded MHz the elative dielectic constant ε is nealy constant (see fig.5) as one would expect. Anothe obsevation is that the ε deceases with a highe field as explained biefly in section II. The seies capacitance The seies capacitance is popotional to the elative dielectic constant, and is expessed in eq. 3. The tunability n The tunability pefomance paamete in fig. 7 follows fom the measuement data in fig. 5. The minimum (-20 V) and maximum beakdown voltage (24 V) ae incopoated in the tunability figue. Fig.5 The elative dielectic constant with fequency of a thin film (dielectic thickness d = 273 nm) capacito with a size of 50 μm x 50 μm at oom tempeatue. At high adio fequencies the seies esistance of the electodes can become the limiting pefomance facto. This will be elaboated futhe in the figue with the eal pat of the impedance R(Z 11 ) with fequency. If the fequency exceeds f 1 GHz fom the elaxation behavio (staight dash-dotted line) then the ε (f) deviates. The lumped capacito becomes distibuted if the fields change consideably by within the dimension of the test stuctue. The chage is concentated aound the pobe-tip educing the effective aea, and theeby the elative dielectic constant. The capacito must then be descibed as a distibuted RC netwok (see fig. 6). At f = 3 GHz the electostictive dielectic mateial [8] causes a bulk acoustic esonance with applied bias. Fig. 7 The fequency dependency of the tunability n shows an excellent tuning ange fo both the minimum and maximum DC voltage. Both cuves show an excellent tuning ange fo both the negative and positive DC voltage. The loss tangent tanδ The second pefomance paamete, the loss tangent tanδ, is the sum of the high-κ mateial loss and the electode seies esistance loss. At low fequencies and high fields the leakage cuent dominates and at highe fequencies the electode esistance detemines the pefomance of the thin film high density (tunable) capacito [9]. The left side of the minimum loss tangent in fig. 8 indicates the dielectic loss, and the loss on the ight side is dominated by the thickness and conductivity of the bottom and top electode. The intinsic loss tangent of the mateial is not visible yet. It should be a plateau between the two othe egimes. The leakage cuent dominates the loss below f = 4 MHz. At 0 V bias the dielectic loss become visible at high voltages. Howeve, the constant 441

4 4 sheet esistance of the electodes. The seies capacitance and the seies esistance ae quite constant at RF. The eq.4 of the loss tangent is capable of pedicting the behavio of the loss tangent at RF. Capacitance-voltage cuve A typical -V dc is measued fom zeo DC voltage in steps of 2 V until beakdown at both sides with the VNA and a DC voltage souce. Fig.8 The loss tangent vs. fequency at diffeent voltages until beakdown. seies esistance contibution of the electodes dominates the loss at adio fequencies (see the linea dotted slope at the ight side of the optimum). The loss tangent can be modeled by means of the seies model. The loss tangent can be expessed by R( tan δ = = ω R s (4) I( This fomula indicates that the loss tangent will decease if the seies capacitance becomes less by e.g. applying a bias o by using smalle capacitos. Seies esistance R(Z 11 ) The seies esistance is equal to the eal pat of the impedance (see the seies model in fig. 3). At highe fequencies the eal pat becomes constant (see fig. 9) and is equal to the effective electode esistance. Fig. 10 The -V dc cuve of two test stuctues with identical shape and dimensions at diffeent fequencies. The -V dc cuve in fig. 10 is almost symmetic fo positive and negative voltages. The capacitance and tunability decease with fequency in the RF ange due to distibuted effects as explained ealie. VI. CONCLUSIONS The electical chaacteization of tunable feoelectic capacitos with a VNA has successfully been demonstated. An excellent tuning ange and low intinsic dielectic losses have been obtained. Smalle test stuctues with bette conducting electodes will impove pefomance at RF. ACKNOWLEDGEMENTS The autho would like to thank M. Ruedige of Philips Reseach fo manufactuing pat of the samples. Fig.9 The eal pat of the impedance R(Z 11 ). REFERENCES [1] K. Bethe, Ube das mikowellenvehalten nichlineae dielektika, Philips Reseach Repot, Vol. 25, Suppl.2, 1970 [2] Q. Meng et al., An impedance matched phase shifte using BST thin film, IEEE Micowave and wieless components lettes, vol 16, no. 6, June The seies esistance contibution at RF is popotional to the 442

5 5 [3] L.-Y. Vicki Chen, Roge Fose, D. Chase, and Robet A. Yok, Analog Tunable Matching Netwok Using Integated Thin-Film BST Capacitos,IEEE MTT-S Digest, [4] D. Kuylenstiena, A. Voobiev, P. Linné, and S. Gevogian, Feoelectically Tuneable Delay Lines, 34 th Euopean Micowave Confeence, [5] J. Nath et al, An Electonically Tunable Micostip Bandpass Filte Using Thin-Film Baium Stontium Titanate (BST) Vaactos, IEEE Tansactions on micowave theoy and techniques, vol. 53, no. 9, Septembe [6] A.K. Tagantsev, V.O. Sheman, K.F. Astafiev, J. Venkatesh & N. Sette, Feoelectic Mateials fo Micowave Tunable Applications, Jounal of Electoceamics, vol. 11, pp. 5 66, [7] Z. Ma et al., RF Measuement Technique fo Chaacteizing Thin Dielectic Films, IEEE Tansactions on electon devices, vol. 45, no. 8, August [8] W.P. Mason, Electostictive effect in Baium Titanate Ceamics, Physical Review, vol. 74, no.9, Novembe [9] J. Schmitz, F. N. Cubaynes, R. J. Havens, R. de Kot, A. J. Scholten, and L. F. Tiemeije, RF Capacitance Voltage Chaacteization of MOSFETs With High Leakage Dielectics, IEEE Electon Devices Lettes, vol. 24, no.1, Januay