Characterization of thin Gd 2 O 3 magnetron sputtered layers

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1 Characterization of thin Gd 2 O 3 magnetron sputtered layers Jacek Gryglewicz * a, Piotr Firek b, Jakub Jaśiński b, Robert Mroczyński b, Jan Szmidt b a Wroclaw University of Technology, Janiszewskiego 11/17, Wroclaw, ; b Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, Warsaw, ABSTRACT Reactive magnetron sputtering technique using O 2 /Ar gas mixture was used to deposit Gd 2 O 3 layers. Following metallization process of Al allowed to create MIS structures, which electrical parameters (κ, D it, U FB, ρ, etc.) were measured using high frequency C-V equipment. Created layers exhibit high permittivity (κ 12) at 100kHz. I-V measurements point out on maximum electric break down field E br 0.4 MV/cm and maximum break down voltage U br 16V. Layers were morphologically tested using AFM technique (R a 0.5 2nm). Layer thicknesses as well as refractive indexes (RI ) were estimated using ellipsometry measurements. Keywords: magnetron sputtering, Gd 2 O 3 1. INTRODUCTION Continuous scaling of device dimensions in CMOS technology force the replacement of the conventional silicon dioxide layers with higher permittivity (high-κ) materials 1,2 where gate dielectric equivalent thickness is much more promising. In recent years oxides containing rare earth materials are subject to intensive studies. Their main characteristics include good thermodynamic stability on Si and high conduction band offset exceeding 2 ev. Among currently examined high-κ materials (Y 2 O 3 κ=18 [3], Ta 2 O 3 κ= 26, HfO 2 κ=25, ZrO 2 κ=25 [2], etc.) gadolinium oxide has promising features including gate dielectric application. It exhibits relatively high permittivity (ca. 14 [4] ), a wide band gap (E g = 5.4 ev) and high band offsets with respect to silicon (ΔE c =3.2, ΔE v =3.9). Crystalline Gd 2 O 3 thin films usually form cubic structures 1, but there are also bixbyite and monoclinic structures 5. However, magnetron sputtered layers very seldom occurs in that crystalline structures. The most probable dominant crystalline structure for magnetron sputtered layers is amourphous (phase). 2. EXPERIMENT In this work thin gadolinium oxide was reactively magnetron sputtered on p-type (ρ = 6 8 Ωcm) Si <100> substrates. Gases used in the process are mixture of argon and oxygen at room temperature. T processes were planned on the basis of one dimensional optimization method using orthogonal matrix L with four input deposition parameters parameters. The list of process conditions is presented in Tab. 1. In order to investigate the thickness of deposited layers, ellipsometric measurements were performed on test structures. On dielectric 1mm diameter aluminum pads (S = 7, cm 2 ) were deposited using magnetron sputtering at room temperature. Created MIS structures were then measured using high-frequency (100 khz) C-V system. Current-voltage characteristics (I-V) were also measured to investigate brake-down field and voltage. Deposited layers were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM) working in tapping mode. The influence of process parameters under investigation on permittivity, resistivity, optical features and morphology is presented and discussed. 3. RESULTS 3.1 Optical and surface characteristics In order to examine basic layer parameters, the atomic force microscopy images were used to calculate surface roughness parameters. Ellipsometry measurement was performed to measure refractive index of deposited layer and thickness of MIS structures. According to process set presented in Tab. 1, average function of thickness, refractive index and surface roughness (R a ) were calculated. Variation of average functions is presented in Fig. 1. Electron Technology Conference 2013, edited by Pawel Szczepanski, Ryszard Kisiel, Ryszard S. Romaniuk, Proc. of SPIE Vol. 8902, 89022M 2013 SPIE CCC code: X/13/$18 doi: / Proc. of SPIE Vol M-1

2 If the paper does not have the margins shown in Table 1, it will not upload properly. Table 1. Detailed process parameters in L orthogonal matrix Run power [W] pressure O 2 /Ar [mtorr] [n.a.] time [min] , , , , , ,5 2 a) c) d) rc 0a Ê tó E 0.9 ó á c x 1.50 co 9 1.] E E 500 ú J E 475 E E Ê E power (W) pressure (mtorr) O1 /Ar gas ratio [n.a] 25 Figure 1. Average parameter functions of thickness, refractive index and surface roughness (R a ) calculated for power (a), pressure (b), O 2 /Ar gas ratio (c) and processing time (d) Thin layer deposited on Si substrates are characterized by very low surface roughness parameters. Roughness average (R a ), root mean square (R ms ) and median value are treated as reference in comparison. Roughness average and root mean square change its values in similar range R a,r ms nm. Median value changed in range nm. Generally very smooth surface is obtained for highest observed power and high amount of oxygen in O 2 /Ar gas ratio. For higher pressures decrease in average function is also observed (Fig.1.b). The example set of AFM images of substrates with layers deposited using power P = 1300W was depicted in figure 2. Proc. of SPIE Vol M-2

3 a) b) c) eonm nm 13 0 nm GO Figure 2. AFM images of surface morphologies measured for substrates processed in runs: 7 (a), 8 (b) and 9 (c). The influence of operating power on RI stands out from other examined process parameters. For an increasing power we observe an increased value of refractive index. Minimum value of average function is observed for pressure p = 6 mtorr. The optimal oxygen amount in examined parameter range is 25 sccm or 30 sccm in total 50 sccm O 2 /Ar gas flow. Refractive indexes calculated using ellipsometric measurement varied in range of RI Refractive indexes of thin Gd 2 O 3 from literature survey using different deposition / growth techniques are presented in Tab. 2. Table 2. Refractive indexes for various deposition / growth techniques. 650nm technique Ref electron beam evaporation [6] electron beam codeposition [7] epitaxial grow [8] atomic layer deposition [9] reactive magnetron sputtering this work The most predictable characteristics are shown in Figures 1a and 1b where thickness increases with power and processing time. More oxygen in plasma means decreasing amount of Ar which is responsible for sputtering of Gd target, thus deposition rate is low for high amount of oxygen in O 2 /Ar gas. For selected process runs the average deposition rate is higher than 9 nm/min and doesn t exceed 17 nm/min. 3.2 Electrical features MIS structures with aluminum contact (S = 7, cm 2 ) were measured using high-frequency (10kHz, 100kHz, 1MHz) C-V system. Capacitance-voltage characteristics were used to determine permittivity, resistivity and other useful dielectric parameters. Current-voltage characteristics (I-V) were also measured to investigate brake-down field and voltage. The influence of power, pressure and oxygen amount in O 2 /Ar gas on electrical parameters is presented in Fig. 3. Proc. of SPIE Vol M-3

4 I_ ' '\ ' l ' ' _ U" or c n - f a - 50n n ^É008 o al 00n a p 100n 26G 25G ÿ 24G E 23G 0.22G 21G 20G 30G ^28G ÿ 26G tei24g (31'22G 20G 400T 300T - I a E Ç 3001 n power [W] pressure [mtorr] /Af [^/o] Figure 3. Influence of power (a), pressure (b) and oxygen/argon gas ratio (c) on electrical features of Gd 2 O 3 The highest permittivity of examined structures was obtained for 8 th process (see Tab. 1.) and values varied in range of 2 12 at 100 khz. The highest resistivity was observed for power P = 300W, pressure p = 6mTorr and 60% of oxygen in O 2 /Ar gas. Flat-band voltage (U FB ) values systematically decreases while power and pressure was increasing. Minimal value of U FB is observed for 60% of oxygen in O 2 /Ar gas. The influences of power, pressure and oxygen gas ratio on other useful characterization parameters such as density of surface states (D it ), effective charge (Q eff ) and resistivity (ρ) are also depicted in Fig. 3. The variation of permittivity values for all performed process runs is depicted in Fig. 3. It is noticeable that depending on the requirements, it is possible to adjust permittivity from low to relatively high operating frequencies. Proc. of SPIE Vol M-4

5 loo process run 7 8 Figure 4. Variation of Gd 2 O 3 permittivity measured at 10kHz, 100kHz and 1MHz frequencies Corresponding to 8 th run, capacitance-voltage and current-voltage characteristics were presented in Fig. 5. Hysteresis in C-V plots can be attributed to the presence of large number of mobile oxide charges 10. From calculated relative influence factors (IF) arise a fact that density of surface states (D it ) is vulnerable especially for power (IF = 40%) and low oxide amount in O 2 /Ar gas mixture (IF = 22%). Influence factors (IF) are based on comparison of subtract of maximum and minimum (ΔE param ) average parameter values 11 compared to the sum of each ΔE param. In Fig. 2 we see that density of surface states is increased for power P = 1300W and gas O 2 /Ar gas ratio 1:1. Chamber pressure is less influent (IF = 13%), however it also causes an increase of D it which results in generation of charge trapping and hysteresis. Calculation of flat-band voltage difference in forward and backward polarization allowed to estimate hysteresis for 1 MHz, 100 khz and 10 khz values of which are ΔU FB 2.34V, 0.45V, and 0.31V respectively for process 8 th (Fig.4a). Leakage current of capacitors with Gd 2 O 3 layers deposited using power P = 1300W is at level of I leak The break down field is varied in range E br = MV/cm for all examined layers. Maximum break down field was observed for structures numbered 5 (see table 1). Most of the capacitors are characterized by break down voltage on level U br = 7V. Maximum break down voltage was observed for capacitors done in runs 7 and 8 (U br =16V). Proc. of SPIE Vol M-5

6 a) 7n 6n - LL 5n- 0 c 4n - f6 t' 3n - 01 a2nit U in - C ' 1 1 i ' C =6.11nF = 4.70 nf 1U`a=0.45 V LTe,= 0.75 nf AU =2.34V.9 AU Le= 0.31 V 1 ' 1 ' khz --10 khz --1 MHz 03 r» _. :y: _-;,"a--=' b) Voltage [V] 1x10' Voltage [V] Figure 5. C-V hysteresis of magnetron sputtered Gd 2 O 3 (run 8) as a function of operating frequency (a) and I-V characteristics of Gd 2 O 3 capacitors with layers deposited using P = 1300W (b) 4. CONCLUSIONS Reactive magnetron sputtering technique has been successfully used to deposit relatively thin Gd 2 O 3 layers. Through process optimization using L orthogonal matrix, different permittivity (κ) materials exhibiting promising electrical features were created. Based on the results, the highest permittivity material can be obtained for the following process conditions: power P = 800W, pressure p = 6 mtorr and 1:1 O 2 /Ar gas ratio, however process with described conditions is not included in orthogonal array. It`s possible to adjust the desired electrical (κ, D it, U FB, ρ, etc.), optical (RI) and morphological (R a, R ms, etc.) features using presented set of magnetron sputtering parameters. Calculation of relative influence factor (IF) gives an information which parameters is decisive and influence the output (measured) parameter the most. Based on it, further research may be focused on power and O 2 /Ar gas ratio. Performed C-V and I-V measurements proved that MIS structures work in high frequency. However, there is still research area which need further improvement. Acknowledgements This work was co-financed by the European Union within European Regional Development Fund, through grant Innovative Economy (POIG /08-05), National Centre for Science under the grant no. N N , by National Centre for Research and Development through Applied Research Program grant no , by Wroclaw University of Technology statutory grant S20010, B20011 and Slovak-Polish International Cooperation Program no. SK- PL & This work was partially supported by The National Science Centre (Grant no N N ). Fellowship co-financed by European Union within European Social Found. Proc. of SPIE Vol M-6

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