Se Stephen Potts

Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges

Plasma-assisted atomic layer deposition (ALD) is an energy-enhanced method for the synthesis of ultra-thin films with A-level resolution in which a plasma is employed during one step of the cyclic deposition process. The use of plasma species as reactants allows for more freedom in processing conditions and for a wider range of material properties compared with the conventional thermally-driven ALD method. Due to the continuous miniaturization in the microelectronics industry and the increasing relevance of ultra-thin films in many other applications, the deposition method has rapidly gained popularity in recent years, as is apparent from the increased number of articles published on the topic and plasma-assisted ALD reactors installed. To address the main differences between plasma-assisted ALD and thermal ALD, some basic aspects related to processing plasmas are presented in this review article. The plasma species and their role in the surface chemistry are addressed and different equipment configuratio...

Low Temperature Plasma-Enhanced Atomic Layer Deposition of Metal Oxide Thin Films

Many reported atomic layer deposition (ALD) processes are carried out at elevated temperatures (>150°C), which can be problematic for temperature-sensitive substrates. Plasma-enhanced ALD routes may provide a solution, as the ALD temperature window can, in theory, be extended to lower deposition temperatures due to the reactive nature of the plasma. As such, the plasma-enhanced ALD of Al 2 O 3 , TiO 2 , and Ta 2 O 5 has been investigated at 25-400°C using [Al(CH 3 ) 3 ], [Ti(O i Pr) 4 ], [Ti(Cp Me )(O i Pr) 3 ], [TiCp*(OMe) 3 ], and [Ta(NMe 2 ) 5 ] as precursors. An O 2 plasma was employed as the oxygen source in each case. We have demonstrated metal oxide thin-film deposition at temperatures as low as room temperature and compared the results with corresponding thermal ALD routes to the same materials. The composition of the films was determined by Rutherford backscattering spectroscopy. Analysis of the growth per cycle data and the metal atoms deposited per cycle revealed that the growth per cycle is strongly dependent on the film density at low deposition temperatures. Comparison of these data for Al 2 O 3 ALD processes in particular, showed that the number of Al atoms deposited per cycle was consistently high down to room temperature for the plasma-enhanced process but dropped for the thermal process at substrate temperatures lower than 250°C.

Electrical transport and Al doping efficiency in nanoscale ZnO films prepared by atomic layer deposition

In this work, the structural, electrical, and optical properties as well as chemical bonding state of Al-doped ZnO films deposited by atomic layer deposition have been investigated to obtain insight into the doping and electrical transport mechanisms in the films. The range in doping levels from 0% to 16.4% Al was accomplished by tuning the ratio of ZnO and Al 2O3 ALD cycles. With X-ray photoelectron spectroscopy depth profiling and transmission electron microscopy, we could distinguish the individual ZnO and AlOx layers in the films. For films with a thickness of 40 nm, the resistivity improved from 9.8 mΩ cm for intrinsic ZnO to an optimum of 2.4 mΩ cm at 6.9 at. % Al. The binding energy of Zn 2p3/2 increased by 0.44 eV from the intrinsic ZnO to the highest Al-doped ZnO. This shift can be ascribed to an increase of the Fermi level. Ex-situ spectroscopic ellipsometry and Fourier transform infrared spectroscopy were used to measure the optical properties from which the carrier concentration and intra-grain mobility were extracted. The results showed that with increasing Al content, the grain boundary mobility increased at first due to an increased Fermi level, and then decreased mainly due to the scattering at AlOx/ZnO interfaces. For the same reasons, the doping efficiency of Al for highly Al-doped ZnO dropped monotonically with increasing Al. Furthermore, a blue shift of the optical band-gap ΔEg up to 0.48 eV was observed, consistent with the shifts of the Fermi level and the binding energy of the Zn 2p3/2 state.

Plasma-enhanced and thermal atomic layer deposition of Al2O3 using dimethylaluminum isopropoxide, [Al(CH3)2(μ-OiPr)]2, as an alternative aluminum precursor

The authors have been investigating the use of [Al(CH3)2(μ-OiPr)]2 (DMAI) as an alternative Al precursor to [Al(CH3)3] (TMA) for remote plasma-enhanced and thermal ALD over wide temperature ranges of 25–400 and 100–400 °C, respectively. The growth per cycle (GPC) obtained using in situ spectroscopic ellipsometry for plasma-enhanced ALD was 0.7–0.9 A/cycle, generally lower than the >0.9 A/cycle afforded by TMA. In contrast, the thermal process gave a higher GPC than TMA above 250 °C, but below this temperature, the GPC decreased rapidly with decreasing temperature. Quadrupole mass spectrometry data confirmed that both CH4 and HOiPr were formed during the DMAI dose for both the plasma-enhanced and thermal processes. CH4 and HOiPr were also formed during the H2O dose but combustion-like products (CO2 and H2O) were observed during the O2 plasma dose. Rutherford backscattering spectrometry showed that, for temperatures >100 °C and >200 °C for plasma-enhanced and thermal ALD, respectively, films from DMAI had a...

Atomic Layer Deposition of Silicon Nitride from Bis(tert-butylamino)silane and N2 Plasma

Atomic layer deposition (ALD) of silicon nitride (SiNx) is deemed essential for a variety of applications in nanoelectronics, such as gate spacer layers in transistors. In this work an ALD process using bis(tert-butylamino)silane (BTBAS) and N2 plasma was developed and studied. The process exhibited a wide temperature window starting from room temperature up to 500 °C. The material properties and wet-etch rates were investigated as a function of plasma exposure time, plasma pressure, and substrate table temperature. Table temperatures of 300-500 °C yielded a high material quality and a composition close to Si3N4 was obtained at 500 °C (N/Si=1.4±0.1, mass density=2.9±0.1 g/cm3, refractive index=1.96±0.03). Low wet-etch rates of ∼1 nm/min were obtained for films deposited at table temperatures of 400 °C and higher, similar to that achieved in the literature using low-pressure chemical vapor deposition of SiNx at >700 °C. For novel applications requiring significantly lower temperatures, the temperature window from room temperature to 200 °C can be a solution, where relatively high material quality was obtained when operating at low plasma pressures or long plasma exposure times.

Plasma-enhanced ALD of TiO2 using a novel cyclopentadienyl alkylamido precursor [Ti(CPMe)(NMe2)3] and O2 plasma

Titanium oxide thin films of both amorphous and anatase morphologies have been deposited using remote plasma ALD over a wide temperature range (100-350 °C), using a novel heteroleptic alkylamido precursor Ti(CpMe)(NMe2)3. A high growth per cycle (GPC) of 0.07-0.08 nm (50 % higher than the GPC obtained with most other organometallic precursors). Films obtained were stoichiometric and of high compositional purity.

Atomic layer deposition

Atomic layer deposition (ALD), also referred to historically as atomic layer epitaxy, is a vapor-phase deposition technique for preparing ultra-thin films with precise growth control. ALD is currently rapidly evolving, mostly driven by the continuous trend in the miniaturization of electronic devices. In addition, many other innovative technologies are increasingly benefitting from the high-quality thin films. This chapter describes—on an elementary level—the key features of ALD. A standard ALD process scheme is used to discuss the relevant concepts of the technique. Materials that can be deposited by ALD are discussed, including typical precursors and co-reactants that can be used. Several example chemistries, specifically for ALD of Al2O3, HfO2, TiN, and Pt, are presented to illustrate the variety in surface chemistry. ALD reactor types are described and, finally, some cases are addressed to illustrate the virtues and practicalities of ALD that are important to advancing present day and emerging thin-film applications.