Effect of nitrogen containing plasma on interface properties of sputtered ZrO2 thin films on silicon

Materials Science in Semiconductor Processing 19 (2014) 145–149

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Effect of nitrogen containing plasma on interface properties of sputtered ZrO2 thin films on silicon Ashwath Rao n, Anshuman Dwivedi, Manish Goswami, B.R. Singh Indian Institute of Information Technology, Allahabad, Jhalwa, 211012 Allahabad, India

a r t i c l e i n f o

abstract

Available online 1 January 2014

The present paper deals with the electrical characterization of sputtered ZrO2/Si interface deposited in N2 containing plasma. Incorporation of nitrogen in the sputter deposited films was confirmed by glancing angle X-Ray diffraction measurement. MOS C–V and I–V techniques were used for interface characterization. Nitrogen incorporated ZrO2 MOS capacitors exhibited higher breakdown voltage and lower leakage current than structures having ZrO2 dielectric films sputtered in pure argon atmosphere. Different device parameters such as flat band voltage, leakage current, breakdown voltage, charge defects were extracted and compared with and without nitrogen incorporated ZrO2/Si MOS capacitor structures. The effect of post deposition annealing on the electrical behavior of ZrO2/Si interface was also investigated. & 2013 Elsevier Ltd. All rights reserved.

Keywords: High-K Reactive sputtering Nitrogen plasma Zirconium dioxide

1. Introduction In order to meet high speed, low power and high density requirement of CMOS VLSI down scaling of the transistor structures are being continuously attempted [1]. This reduction in size has resulted in different physical effects such as enhanced leakage current, drain induced barrier lowering (DIBL), short channel effects, subthreshold conduction and so on [2–4]. Silicon dioxide has been used as the gate oxide material for several decades. As transistors are scaled down, the thickness of SiO2 gate dielectric is reduced in order to increase the drive current, reduce threshold voltage and to increase device performance [5]. The thinning of the gate dielectric results in higher leakage current and power dissipation. This adversely affects the reliability of the device. Replacing SiO2 with high K gate dielectric materials allows increased gate capacitance and higher physical thickness with less leakage current. High K dielectric materials are the solution to overcome the scaling limit of SiO2 [6–7].

n

Corresponding author. Tel.: þ91 945 156 4084. E-mail address: [email protected] (A. Rao).

1369-8001/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2013.11.039

Amongst the various high K dielectric materials such as Ta2O5, Al2O3, TiO3, HfO2, ZrO2 and their silicates, aluminates, the choice of the alternate gate dielectrics has been narrowed to group IVB oxides namely HfO2 and ZrO2 due to their excellent electrical properties and high thermal stability in contact with Si [8–12]. Both of these oxides are known to have almost same high dielectric constant (25), good thermal stability, high band offset (1.5 eV) and high band gap (5.8 eV) [13–18]. The gate oxides must be thermally and chemically stable with the substrate material. Crystallization of pure HfO2 and ZrO2 occurs at lower temperature. In standard CMOS processes, gate stacks must undergo rapid thermal annealing (RTA) of around 1000 1C and hence they easily crystallize during these processes. The problem of low crystallization temperature is associated with Hf-based and Zr-based oxides. These crystalline structures can increase the gate leakage and provide path for diffusion of dopants and dielectric breakdown. The improvement in crystallization temperature has been attempted by incorporation of elements such as N, Si, Al, Ta and La. Nitrogen incorporation in HfO2 has been studied by many groups [19–21]. Their findings conclude that nitrogen incorporation in HfO2 leads to improved electrical properties as well

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as crystallanity at the cost of decreased bandgap and reduced mobility. Choi et al. observed that addition of nitrogen increases the nucleation temperature which in turn increases the crystallization temperature [22]. The nitrogen incorporation is expected to distort the equilibrium of the lattice and produce disordered state. Further, the nitrogen reduces the diffusion coefficient of oxygen which in turn decreases the crystallization rate thus enhancing the temperature at which crystallization occurs [23–25]. The role of nitrogen incorporation in ZrO2 films which belongs to the same group IVB oxides as HfO2 and has attractive dielectric properties, is not widely explored. In view of this incorporation of nitrogen into ZrO2 oxide layer (hereafter referred as ZrO2:N) has been studied. The fabrication processes leading to improved electrical behavior for material systems has been reported in this paper. Nitrogen incorporation during film preparation has shown considerable improvement in electrical properties and crystallization temperature which may be of considerable technological interest in other dielectric applications such as in MIM capacitors. This paper deals with the properties and reliability of ZrO2:N/Si interface deposited using RF sputtering. For comparison ZrO2 films sputtered in pure argon containing plasma has been used. Capacitance–Voltage (C–V) and Current–Voltage (I–V) techniques have been used to characterize the ZrO2/Si interfaces.

formed by thermal evaporation at 10  6 Torr. A metallic shadow mask was used for electrode area definition. Backside metallization was carried out for bulk contacts. The C–V and I–V measurements were carried out using the Keithley 4200 Semiconductor Characterization System. The reproducibility of the results was checked at least three times on number of samples ranging from four to five. Each sample had around 40–50 MOS capacitors. 3. Result and discussion The foremost thing important to us was to verify the presence of nitrogen in the ZrO2 films RF sputtered in nitrogen containing plasma. The presence of nitrogen in the sample was investigated by glancing angle X-ray diffraction (XRD). XRD pattern of samples grown in nitrogen ambient and pure argon ambient is shown in Fig. 1(a) and (b). For the samples with pure argon we have observed two dominant peaks at 2θ of 28.51 and 58.91 ( Fig. 1b) which is identified as monoclinic (111) and monoclinic (222) according to JCPDF no. 899066. Earlier XRD studies carried on ZrO2 films showed three ternary phases i.e. monoclinic, tetragonal and cubic depending on

2. Experimental

10000

Intensity

ZrO2 MOS capacitors were fabricated using a single crystal silicon wafer having a /100S orientation with a resistivity of 1–10 Ω-cm. The silicon wafers were subjected to standard RCA cleaning in order to remove organic and inorganic contaminants. The interface oxide was etched in dilute HF (1:10) for 30 s, followed by a thorough rinse in De-Ionised (DI) water. The wafers were dried in a nitrogen environment before loading the wafers into the sputtering system. The sputtering chamber was pumped down to a pressure of 10  6 Torr. 4″ ZrO2 sputtering targets (purity 99.99%) were obtained from Testbourne (Basingstoke, UK). The deposition of ZrO2 was carried out using the RF magnetron sputtering system in the presence of nitrogen gas at a flow rate varying from 10 to 30 sccm and high purity argon gas ambient at a pressure of 10  3 mbar to form nitrogen incorporated ZrO2 MOS capacitor. Thin film of ZrO2 was deposited on the silicon substrate at RF power of 150 W for 15 min, maintaining the film thickness of around 45–55 nm corresponding to an effective oxide thickness of 6–8 nm of SiO2. The film thickness was measured using stylus profilometer (model XP-100, Ambios Inc.). The substrate was not heated during sputtering. For comparison ZrO2 MOS capacitors were also fabricated in which ZrO2 was sputtered in pure argon atmosphere. The crystal structure and nitrogen incorporation in the sputtered film was investigated using glancing angle X-ray diffraction (XRD). Post deposition thermal annealing has been carried out in the temperature range of 650–950 1C in N2 ambient to observe the annealing effect on interface property. Al metal gate electrodes were

5000

0

20

40

60

80

2 theta Fig. 1. (a) XRD spectra of ZrO2 MOS capacitors grown in nitrogen ambient. (b) XRD spectra of ZrO2 MOS capacitors grown in pure argon ambient.

A. Rao et al. / Materials Science in Semiconductor Processing 19 (2014) 145–149

the annealing temperature [26–28]. XRD studies on samples with different nitrogen flows determine that the deposited films are crystalline and change to cubic zirconium oxy nitride from their monoclinic ZrO2 crystal structure for nitrogen flows up to 30 sccm and above this nitrogen flow (30 sccm) on the contrary, cubic zirconium nitride (ZrN) has been formed. Two dominant peaks can be seen at 2θ of 331 and 691 from Fig. 1a which is identified as cubic (111) and cubic (311) in addition to peaks of ZrO2 according to JCPDF no. 893839. This indicates the presence of nitrogen in the sample. Our observation is consistent with the experimental findings by Venkataraj et al. [29] for the films reactively sputtered using zirconium target. It is apparent from the data obtained with different flow rates of nitrogen that the

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devices grown in nitrogen containing plasma at a flow rate of 30 sccm showed better electrical characteristics. Fig. 2(a) and (b) shows the I–V characteristics of pure ZrO2/Si and ZrO2 deposited in N2 Plasma/Si MOS capacitors. It can be seen from the figure that the leakage current measured at5 V is two orders in magnitude lower in ZrO2: N films. Lower value of leakage current is also observed after annealing (Fig. 2(a)) which indicates improved ZrO2:N/Si interface behaviour. This is also reflected from C–V characteristics (Fig. 3(a) and (b)) measured at different frequencies (10 KHz to 1 MHz). Larger shift can be seen in MOS structures containing pure ZrO2 films as compared to ZrO2:N film. In fact ideally, there should not be any shift in flat band with measurement frequency. The observed shift at different

Fig. 2. (a) I-V characteristic of ZrO2 MOS capacitor annealed at different temperatures. (b) I-V characteristic of ZrO2 deposited in N2 plasma/Si MOS capacitors.

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Fig. 3. (a) Flat band shift in CV curve with varying frequency in ZrO2 MOS capacitor. (b) Flat band shift in CV curve with varying frequency in nitrogen incorporated ZrO2 MOS capacitor.

frequencies is indicative of the presence of increase in the number of traps with varying time constants at the interface. The breakdown characteristic of MOS capacitors having differently deposited ZrO2 as dielectric layer has been used

to assess the reliability of gate oxide at higher fields. The breakdown voltage was measured using the ramped voltage method. The stress voltage was ramped with constant ramp rate of 0.05 V/s and current was measured

A. Rao et al. / Materials Science in Semiconductor Processing 19 (2014) 145–149

Table 1 Breakdown voltage of ZrO2 and nitrogen incorporated ZrO2 MOS capacitor. Annealing condition

Breakdown Voltage in ZrO2 capacitor (V)

Non annealed 650 1C 800 1C 950 1C

7.50 7.7 10.5 11

Breakdown Voltage in Nitrogen incorporated ZrO2 capacitor (V) 15 16.3 17 18.5

0.5

Flat Band Shift (V)

ZrO2 + N2 Pure ZrO2

0.4

0.3

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voltage stress on shift in flat band in annealed ZrO2:N capacitor is much smaller than in pure ZrO2 capacitors. 4. Conclusion The electrical characteristics of ZrO2:N dielectric is studied in this paper. Most of the electrical parameters like flat band voltage, leakage current and hysteresis evaluated using MOS C–V and I–V characteristics clearly demonstrate that nitrogen incorporation helps to improve the electrical properties. A comparison on the breakdown characteristics, dielectric strength and device stability also demonstrates that ZrO2:N films have better dielectric properties. However better understanding of the charge trapping mechanism and control of interfacial layer to reduce leakage current and power dissipation is needed for integrating these high K materials into semiconductor processing.

Acknowledgements The authors would like to express their sincere thanks to Dr. M.D. Tiwari, Director and Prof. M. Radhakrishna for their constant support and encouragement. Thanks are also due to Mr. Upendra Joshi and Mr. Pramod Tripathi, Technical staff for their assistance.

0.2

0.1

0.0

References 1

2

3

4

5

Time (min) Fig. 4. Flat band shift Vs time graph for voltage stress of -5V on nitrogen incorporated and pure ZrO2 MOS capacitor.

until the breakdown occurs. The breakdown voltage measured in MOS capacitor having pure ZrO2 and ZrO2:N films is compared in Table 1. It can be seen that ZrO2:N films have a higher breakdown voltage. It is also consistent with the leakage current characteristics of ZrO2:N MOS capacitor. The thickness of the ZrO2 oxide layer was around 45 nm. The maximum breakdown field calculated using JESD35-A [30] comes out to be approximately 2.4 MV/cm for ZrO2 capacitors and around 4.1 MV/cm for ZrO2:N capacitors. This is due to the fact that interfacial layer of ZrO2:N contains nitrogen at the interface which acts as a barrier to oxygen diffusion and hence increases the breakdown voltage [24]. The voltage stress analysis to describe the interface property and device stability has been done by applying a constant voltage of  5 V at the gate electrode for time duration of 1–5 min. It can be observed from Fig. 4 that flat band shift increases with increase in stress time for  5 V voltage stress. The effect of voltage stress on flat band is minimum in annealed capacitors as compared to non annealed samples. It is because of better interface property and less stress induced defect generation in annealed devices. On application of  5 V stress at the gate electrode the charge causes the C–V curve to shift to more negative values with respect to the ideal C–V curve. It can also be seen from Fig. 4 that the effect of

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