The influence of process parameters and pulse ratio of precursors on the characteristics of La1 − x Al x O3 films deposited by atomic layer deposition
© Fei et al.; licensee Springer. 2015
Received: 15 November 2014
Accepted: 26 March 2015
Published: 14 April 2015
The influence of processing parameters of aluminum oxide (Al2O3) and lanthanum oxide (La2O3) gate dielectric is investigated. Trimethylaluminum (TMA) and tris(isopropylcyclopentadienyl) lanthanum [La(iPrCp)3] were used as precursors separately, and H2O was used as oxidant. The ultra-thin La1 − x Al x O3 gate dielectric films are deposited on p-type silicon substrates by atom layer deposition (ALD) for different pulse ratios of precursors. Effects of different La/Al precursor pulse ratios on the physical properties and electrical characteristics of La1 − x Al x O3 films are studied. The preliminary testing results indicate that the increase of La precursor pulse can improve the characteristics of film, which has significant effects on the dielectric constant, equivalent oxide thickness (EOT), electrical properties, and stability of film.
It is well known that the key element enabling the successful scaling of Si-based metal-oxide-semiconductor field-effect transistors (MOSFETs) over the last several decades is the traditional gate dielectric, silicon dioxide (SiO2), with superior material properties. The miniaturization of MOSFET technology has pushed the conventional SiO2 gate dielectric approach to its physical limit . As the thickness of SiO2 gate dielectric decreases, it becomes more difficult to grow the superior quality oxides, because excessive tunneling and reliability of ultra-thin SiO2 will cause unacceptably high leakage current and degrade the device performance and reliability seriously. Therefore, in order to solve these problems, high-k gate dielectrics are being widely considered to replace SiO2 . For the past years, people have been attracted by many promising candidates, such as aluminum oxide (Al2O3), hafnium oxide (HfO2), zirconium oxide (ZrO2), lanthanum oxide (La2O3), yttrium oxide (Y2O3), and their pseudobinary [3-10], and some exhilarating achievements have been obtained. As a compound of La2O3 and Al2O3, due to its high dielectric constant (approximately 25), wide energy band gap (5 ~ 6 eV), and thermal stability up to 2,100°C, LaAlO3 is considered as a most promising candidate for SiO2 replacement [11,12].
LaAlO3 films have been deposited on Si substrate using metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), evaporation technique, and pulsed laser deposition (PLD) [12,13]. In contrast to these techniques, atom layer deposition (ALD) is based on self-limiting film growth via alternate saturative surface reaction. Therefore, the films deposited by ALD have high purity, density and accurate thickness. Furthermore, compared to traditional techniques, the deposition temperature can be set between 200°C and 400°C by ALD; this conforms to the future trend of semiconductor industry [14-16]. Due to these incomparable superiorities, LaAlO3 films deposited by ALD are widely used in recent years and some marked achievements have been obtained. It is known that the processing parameters determine the quality of films. However, the methods about optimizing each processing parameter have rarely been discussed. In this work, we focus on the influence of processing parameters on the growth of La2O3 gate dielectric, each processing parameter has been optimized, and the optimal processing parameters are provided for La2O3 film. Moreover, the La2O3 and different pulse ratios of La1 − x Al x O3 films were deposited, and the dielectric constant, equivalent oxide thickness (EOT), and electrical properties of the films are analyzed.
La2O3 and La1 − x Al x O3 gate stacks were deposited on p-type Si (8 to 12 Ω⋅cm) B-doped (100) wafers using an atomic layer deposition reactor (Picosun R-200, Espoo, Finland). Before the deposition, silicon wafers were cleaned in 80°C solution (NH4OH:H2O2:H2O = 1:1:5) for 15 min and followed by a 10 min rinse in deionized water . The thickness of native SiO2 layer on Si substrates was determined by ellipsometry as 1.5 nm. For ALD film deposition, trimethylaluminum (TMA) and tris(isopropylcyclopentadienyl) lanthanum [La(iPrCp)3] were used as precursors, H2O was used as oxidant, and ultra-high purity nitrogen (N2, 99.999%) was employed as carrier and purge gas. The container of the aluminum precursor is at room temperature, corresponding to a vapor pressure of 10 to 15 hPa. The pulse time, purge time, heating temperature and deposition temperature of La precursor and pulse time, and purge time of oxidant will be determined by the following experiments. The Al-injection, purge, oxidant injection, and purge times were 0.1, 3, 0.1, and 4 s. For each experiment, 100 cycles of La2O3 and Al2O3 films are deposited by ALD. Film thicknesses were measured by Woollam M2000D spectroscopic ellipsometry. The capacitance-voltage (C-V) measurements were carried out using a Keithley 590 C-V analyzer (Keithley Instruments Inc., Cleveland, OH, USA) at 1 MHz, and the diameter of mercury probe is 859 μm. The bonding structures of the films were examined by X-ray photoelectron spectroscopy (XPS). The atomic force microscopy (AFM) of the films was measured by Seiko SPI3800-SPA-400 scanning probe microscope (Seiko Instruments Inc., Chiba, Japan).
Results and discussion
Optimization of heating temperature of La precursor
The heating temperature of precursor is changed from 120°C to 180°C. The La-injection, purge, oxidant injection, and purge times are 0.5, 6, 0.5, and 10 s, and the deposition temperature of films is 300°C.
Optimization of deposition temperature region for La1 − x Al x O3
Typically, ideal processing parameters of ALD just can operate in a definite range. Under a low deposition temperature, the precursor will stick to the substrate surface when the following precursor reacts to it. The residues of previous substrate exhaust incompletely from the reactor chamber, which causes the increase of the growth rate. On the contrary, in a higher deposition temperature circumstances, the stick precursor can obtain higher energy and can be separated from the surface. The precursor cannot reach saturation adsorption, which will decrease the growth rate of films.
Furthermore, in Figure 3, the growth rate of Al2O3 films reaches the maximum (0.103 nm/cycle) when the deposition temperature is 220°C. The growth rate of Al2O3 films decreases with increasing deposition temperature. Al2O3 films have stable growth rate when the deposition temperature is between 270°C and 320°C. So the best temperature window is between 270°C and 320°C for deposition of La1 − x Al x O3.
Optimization of pulse and purge time for oxidant of La2O3 film
The difference of pulse time and purge time also influences the characteristics of high-k films. The value of pulse time indicates the quantity of reactant per cycle. The longer pulse time lasts, the more precursors enter reactor chamber. The long pulse time wastes precursors and more precursors stick to the surface, which influences the reaction of precursor in the next cycle. On the other hand, the short pulse time causes incomplete reaction between precursor and groups on surface of substrate, which result in the poor quality and uniform of films.
The numerical value of purge time indicates the quantity of inert gases that enter the reactor chamber per cycle. Shorter purge time cannot clean up the by-product completely, which causes massive by-product remained, such as hydrogen ions, carbon group, and so on. In contrast, longer purge time can introduce other impurities such as moisture, which results in poor efficiency for the growth of materials. Therefore, the reasonable pulse time and purge time is very important for ALD process.
Figure 4b shows the thickness of La2O3 film variation with purge time of oxidant. Pulse time of oxidant is 0.3 s; purge time of oxidant is selected as 2, 4, 6, 8, and 10 s, respectively. The thickness of films is almost equal to the thickness of natural growth SiO2 when the purge time of oxidant is less than 5 s. In this situation, the La2O3 films cannot be deposited normally. When the purge time of oxidant is 10 s, the thickness of La2O3 films is unsatisfactory. It is attributed to the moisture and impurity which injected by long-time nitrogen. The moisture and impurity can damage the deposition of films. Growth rate of La2O3 films is ideal when purge time takes 6 and 8 s, respectively.
Optimization of pulse and purge time for La precursor of La2O3 film
Figure 5b shows the thickness of La2O3 film variation with purge time of La precursor. Pulse time of La precursor is 0.3 s; purge time of La precursor is selected as 2, 3, 4, 6, 8, 10, and 12 s, respectively. Because of the time interval between two pulses of precursor is very short, residues of previous precursor and by-product produced by reaction cannot be purged from reactor chamber by N2. The next pulse of precursor is injected into the reactor chamber, which causes the impurities remain on films, and the deposition is unsatisfactory when the purge time of La precursor is less than 4 s. As purge time is increased, growth rate of films grows stable and ideal. Growth rate of films is better when purge time of La precursor is set at 4 s.
The optimized process parameters of deposition for La 1 − x Al x O 3 film
pulse time (s)
pulse time (s)
purge time (s)
purge time (s)
Deposition and analysis of La2O3 and La1 − x Al x O3 films
Samples with different La/Al precursor pulse ratios
Order and cycles for as-deposited films
The result indicates EOT decrease and dielectric constant increase with increasing the La/Al precursor pulse ratio for samples A, B, and C. For sample A, EOT and dielectric constant are 3.89 nm and 17.06, respectively; for sample B, EOT and dielectric constant are 3.41 nm and 21.94, respectively; for sample C, EOT and dielectric constant are 2.90 nm and 24.78, respectively. The increase of the dielectric constant is attributed to an increase rate proportional to La, and the theoretical value of dielectric constant (approximately 30) of La2O3 film is high; this will increase the dielectric constant for La1 − x Al x O3 films. EOT and dielectric constant for sample D can be calculated in the same way. For sample D, EOT is 8.33 nm and dielectric constant is 5.05. The formation of thicker EOT is caused by generation of the La-silicate which Si atoms from substrate and La atoms from film diffusion. The smaller dielectric constant is caused by the formation of La-hydroxide which is due to the hygroscopicity of La2O3 films.
Figure 8b shows O1s XPS spectra of the four as-deposited nanolaminate films. The O1s spectrum was fitted with three peaks (indicated by dashed line) after the application of a Smart background for samples A, B, and C. The dotted lines indicate the binding energy of La-O-La, La-Si-O, and La-OH peaks with the increasing binding energy [22-24]. La-O-La peak becomes larger with the increasing La/Al precursor pulse ratio. Furthermore, smaller La-OH peaks with red curves are obtained. These phenomena are attributed to more combination of La-Al-O bonds and less La-hydroxide, which could make films with good quality. La-Si-O peaks with blue curves become smaller with the increasing La/Al precursor pulse ratio. The formation of La-Si-O peak is caused by the interdiffusion of atoms in Si substrate and as-deposited nanolaminate films. On the other hand, sample D has large La-OH and La-Si-O peaks, and this is caused by the hygroscopic characteristics of pure La2O3 film and interdiffusion of atoms, respectively.
Figure 8c shows La3d5/2 XPS spectra of the four as-deposited nanolaminate films. The dotted lines indicate the binding energy of La-O-La peak, La-Si-O peak, and La-OH peak with the increasing binding energy [20,22,24]. The La-OH peak and La-Si-O peak become smaller with the increasing La/Al precursor pulse ratio. This indicates that the samples have good quality of IL and large dielectric constant, which attribute to the less formation of La-hydroxide and La-silicate. As to sample D, pure La2O3 films, it has large La-OH peak, and it will cause the reduction of dielectric constant. Moreover, the peak with red curve is observed in sample D, but the binding energy of this peak is smaller than the standard value of La-Si-O peak. We can infer that La-Si-O and La-Al-Si-O were formed in La2O3 film and attributed to interdiffusion of atoms. These chemical compounds will lead to a reduction of dielectric constant and increase of interfacial layer.
Percentage compositions of different atoms in samples
According to the ratio of La to Al analyzed by XPS, we can observe that the content of La element is insufficient in La1 − x Al x O3 films. La1 − x Al x O3 film is preserved in air for a long time after deposition, which prohibit La and Al atoms diffuse from the substrate and form interfacial layer. Moreover, diffusion coefficient of Al element is higher than that of La; diffusion of La element is faster than that of Al. The closer to the Si substrate, the more La atoms lose in the as-deposited films. Therefore, the ratio of La to Al is less than 1. Moreover, the oxygen atoms in films are abundant. The main reason is that the inter gases contain a little moisture, and it will induce the excessive oxygen content.
In this study, we mainly investigated that the influence of process parameters and pulse ratio of precursors on the characteristics of La1 − x Al x O3 films is deposited by ALD. Firstly, main process parameters for ALD which include heating temperature of precursor, deposition temperature window, pulse time, and purge time for precursor and oxidant, respectively, are obtained. The optimized process parameters of La1 − x Al x O3 film are presented. Furthermore, as the increase of the La/Al precursor pulse ratio, more La-Al-O bonds were formed in La1 − x Al x O3 films. When the ratio of La and Al precursor is 3 to 1, the film has small EOT, high dielectric constant, excellent interface layer, and superior character of surface. XPS analysis shows that the La/Al/O ratio of sample with a La/Al precursor pulse ratio of 3/1 fits the stoichiometric LaAlO3 composition (1:1:3) better, and this result is close to the standard value, thereby making them become a suitable insulator in MIM capacitor devices.
This work is supported by the National Natural Science Foundation of China (Grant Nos. 61376099, 11235008) and the Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20130203130002, 20110203110012).
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