Experimental Characterization of ALD Grown Al 2 O 3 Film for Microelectronic Applications

The study of high dielectric materials has received great attention lately as a key passive component for the application of metal-insulator-metal (MIM) capacitors. In this paper, 50 nm thick Al 2 O 3 thin films have been prepared by atomic layer deposition technique on indium tin oxide (ITO) pre-coated glass substrates and titanium nitride (TiN) coated Si substrates with typical MIM capacitor structure. Photolithography and metal lift-off technique were used for processing of the MIM capacitors. Semiconductor Analyzer with probe station was used to perform capacitance-voltage (C-V) characterization with low-medium frequency range. Current-voltage (I-V) characteristics of MIM capacitors were measured on precision source/measurement system. The performance of Al 2 O 3 films of MIM capacitors on glass was examined in the voltage range from −5 to 5 V with a frequency range from 10 kHz to 5 MHz. Au/Al 2 O 3 /ITO/Glass MIM capacitors demonstrate a capacitance density of 1.6 fF/µm 2 at 100 kHz, a loss tangent ~0.005 at 100 kHz and a leakage current of 1.79 × 10 −8 A/cm 2 at 1 MV/cm (5 V) at room temperature. Au/Al 2 O 3 /TiN/Si MIM capacitors demonstrate a capacitance density of 1.5 fF/µm 2 at 100 kHz, a loss tangent ~0.007 at 100 kHz and a lower leakage current of 2.93 × 10 −10 A/cm 2 at 1 MV/cm (5 V) at room temperature. The obtained electrical properties could indicate a promising application of MIM Capacitors.


Introduction
Following the "More than Moore" paradigm, we have witnessed extensive integration of passive components, i.e. capacitors and inductors, for further downscaling of electronics in thin film transistors (TFT), thin film capacitors (TFC) and radio frequency (RF) signal integrated circuit (IC) applications [1]. Among those, metal-insulator-metal (MIM) capacitors have been widely used for their low parasitic capacitance and simple structure [2]. In RF and analog ICs applications, MIM capacitors are required to exhibit desirable electrical characteristics including a high capacitance density, low leakage current, and acceptable voltage linearity [3]. However, most of these works focused on thin film dielectrics, with particular emphasis on SiO 2 (k~3.9), which has a relatively low capacitance density, large leakage current and high interface trap when the width is being minimized in order to increase the capacitance. Various high-k dielectrics to replace SiO 2 have been extensively investigated, such as Si 3 N 4 , Al 2 O 3 , HfO 2 , TiO 2 , La 2 O 3 and ZrO 2 for past decades [4]- [9]. Al 2 O 3 stands out for its good electrical insulation performance, very large bandgap (E g ~8.8 eV), desirable thermodynamic stability, excellent chemical stability, high mechanical strength, high temperature resistance (~1600˚C), good biocompatibility [10] and high conduction band offset to semiconductor substrates (Si, SiC, GaN) with relatively low dielectric constant (k~9). Compared to HfO 2 , TiO 2 and ZrO 2 , Al 2 O 3 has a much higher theoretical ultimate breakdown strength E bd at 13.8 MV/cm [11]. Previous work on Al 2 O 3 has reported a capacitance density of 1 -3 fF/µm 2 and a leakage current of 10 −8 A/cm 2 [12] [13]. To fabricate the ultra-thin high-k dielectric, Atomic Layer Deposition (ALD) is used due to its conformity, control over thickness and composition [3].
Atomic layer deposition (ALD) is a chemical thin film deposition technique based on the sequential use of self-terminating and cyclic gas-surface reactions, where the precursor vapors are dosed over the growth surface one at a time. The cyclic nature of the ALD processes provides excellent control over the film thickness, which is often demanded for today's microelectronic devices with dimensions downscaled to the nanometer level [14]. In addition, the self-limited surface reactions guarantee that the films deposited by ALD grow atomic layer-by-layer which enables a precise control of thickness and a conformal deposition of thin films even on high-aspect-ratio nanostructures. These attractive characteristics have led to the apparent increased industrial interest towards ALD in general, and an explosive growth in the number of scientific publications during the last ten years with ALD technique.

Sample Preparation
The deposition of Al 2 O 3 thin film was performed in a hot-wall ALD system (Picosun SUNALE TM R200 Advanced). The indium tin oxide (ITO) pre-coated glass substrates were used as mechanical substrates in order to take advantage of their bulk insulating properties. ITO is known to be a degenerated n-semiconductor and therefore can be treated as a metal [15]. P-type 500 µm thick, one-sided polished Si (100) wafers with a resistivity of 1 -10 Ω·cm were also used as alternative substrates. After the conventional cleaning of the Si wafers without removing the native oxide with a diluted Hydrofluoric acid (HF) solution, 50 nm-thick TiN was deposited on Si as bottom electrode at 400˚C using TiCl 4 and NH 3 plasma gas as the Ti and N sources by Plasma-Enhanced ALD (PEALD). Liquid NH 3 at room temperature was used as the NH 3 plasma source. The plasma power and NH 3 gas flow rate were 2500 W and 150 sccm, respectively. Subsequently these substrates were cleaned using trichloroethylene and acetone within an ul-  Table 1.
For the complete fabrication of the MIM capacitor devices, photolithography and standard lift-off technique were used in order to properly define the array of electrode patterns. Right after Al 2 O 3 deposition, film sample was cleaned for 5 minutes in an ultrasonic bath of acetone, then rinsed in IPA and de-ionized (DI) water, and finally dried with nitrogen to remove atmospheric dust and contamination. Photoresist (Clariant AZ5214E) was first spin-coated on ITO/Glass and TiN/Si substrates to be 1.4 µm thick and then baked on a hot plate at 100˚C for 1 minute. Photolithography was performed using a Maskless Aligner (Heidelberg Instruments MLA 150) system with a dose of 150 mJ/cm 2 and a wavelength An array of circular top electrode with 150 nm thickness Au was fabricated on  Figure 1 with the corresponding thickness for every layer.

Measurement
Al 2 O 3 thin film was characterized using X-ray diffraction (XRD), to investigate its structural quality, on Empyrean multipurpose X-ray diffractometer (Malvern Panalytical) with filtered Cu-K α radiation source with a wavelength of λ = were stored in computer files for data analysis. All electrical characteristics were investigated at room temperature without post annealing of MIM capacitors.

XRD Characterization
The XRD measurements of the 50 nm thick Al 2 O 3 film on ITO/glass and TiN/Si substrates have been performed to evaluate structural and growth quality. Generally, the dielectric properties of high-k materials can be affected by the degree of crystallinity, crystal structure, and crystallographic orientation, in addition to their stoichiometric composition. There have been several reports on the dependence of the dielectric constant upon the crystal structure or crystallographic orientation in high-k materials [16]. However, crystalized phase can be transformed thermodynamically at high temperature (over 700˚C), whereas the amorphous and monoclinic phase appears at room temperature which is preferable for semiconductor process. Since some studies have been performed to avoid crystallization [17] and control over the amorphous structures is highly demanded, this report strives to understand the dielectric properties of amorphous Al 2 O 3 films.
In the XRD θ-2θ scans in Figure 2 (inset), only single broad diffused reflections can be seen, which confirms that the Al 2 O 3 films deposited on ITO/Glass exhibit reasonable amorphous phase. This is also supported by the θ-2θ scans for

Electrical Measurements
The performance of 50 nm Al 2 O 3 film on ITO pre-coated glass substrates with 200 µm cylinder shape in the frequency range from 10 kHz to 5 MHz is shown in     MHz which is in very good agreement with ceramic oxides properties (86% to 99.9% alumina, ε r = 8.5 -10.1 at 1 kHz -1 MHz) [18]. Based on frequency dependent dielectric relaxation, capacitance value decreased from 51 pF to 37 pF as the frequency increases up to 5 MHz. In the range from 10 kHz to 5 MHz, the frequency dispersion of capacitance and dielectric constant were about 26%.
Further, the C-V characteristics can be evaluated using the capacitance densities and dissipation factor shown in Figure 5. It can be found that the capacit-  films and a lower capacitance value [20]. The rougher surface will also affect decreasing capacitance characteristics, which tends to indicate that dielectric loss drastically increased [21]. Figure 7 shows the capacitance densities (varying from 1.2 to 1.5 fF/µm 2 ) at 10 kHz -5 MHz with an applied DC field from −100 to 100 MV/m and the dissipation factor (varying from 0.007 to 0.16) at 10 kHz -1 MHz. These results seem to confirm that the roughness is one of the major property to optimize in order to improve the quality of the dielectric materials.
Leakage current density is also a crucial parameter for MIM capacitors, par-