ALD processes

Several patents dealing with the use of volatile metal amidinate complexes in MOCVD or ALD processes have appeared in the literature.The use of volatile amidinato complexes of Al, Ga, and In in the chemical vapor deposition of the respective nitrides has been reported. For example, [PhC(NPh)2]2GaMe was prepared in 68% yield from GaMes and N,N -diphenylbenzamidine in toluene. Various samples of this and related complexes could be heated to 600 °C in N2 to give GaN. A series of homoleptic metal amidinates of the general type [MIRCfNROilnl (R = Me, Bu R = Pr, BuO has been prepared for the transition metals Ti, V, Mn, Fe, Co, Ni, Cu, Ag, and La. The types of products are summarized in Scheme 226. The new compounds were found to have properties well-suited for use as precursors for atomic layer deposition (ALD) of thin films. 

It was shown that hafnium silicate thin films could be deposited by ALD using HtCl2[N(SiMe3)2]2 with H2O in the temperature range of 150-40010. As the deposition temperature increased, the Si content in the film increased but growth rate linearly decreased. However at low temperature, the Si content was as low as Si/(Hf+Si)=0.05 at ISOU, which was close to hafnium dioxide [4]. The surface mechanism of this ALD process was analyzed by in-situ FT-IR [6]. 

The low temperature ALD process for hafnium silicate films using HfCl2[N(SiMe3)2]2 and H2O was modified to improve the film properties by two different methods. With hydrogen peroxide, the silicon content in the film increased to Si/(Hf+Si)=0.2 at 2001 and the impurity levels decreased due to its strong oxidation effect. By introducing TBOS as an additional Si precursor, the silicon content in the film increased to Si/(Hf+Si)=0.5 at 200 °C and the hafiiium silicate films became fully oxidized with O/(Hf+Si)=2.0. 

Fig. 2 Scheme of a ALD processing window limited by b precursor condensation, c insufficient reactivity, d precursor decomposition and e precursor desorption. If the deposition rate is dependent on the number of available reactive sites as in f, no actual ALD window is observed... 

Because of the sequential nature of ALD, metal and nonmetal precursors are typically separated from each other. The possibility of selecting oxidising or reducing precursors in conventional ALD processes makes it possible to control the reactivity and reactions of the metal precursor. A special case is that of metal alkoxides, which have sometimes been used as oxygen... 

For chalcogenide thin films it is possible to use elemental S, Se, Te as precursors provided that the other source is a volatile and reactive metal. ZnS deposition using elemental zinc and sulphur was the first ALD process developed [4]. Therefore for precursors other than metals, the reactivity of elemental chalcogens is not sufficient. For other precursor types, including halides, 6-diketonates and organometalHcs, simple hydrides, such as H2S, H2Se and H2Te, have typically been used as a second precursor. 

The first ALD thin films were deposited in the 1970s using elemental zinc and sulphur, reacting to form ZnS at 250-450 °C [44], Although metals as ALD precursors seems to be the most straightforward method of producing compound thin films, this type of process is limited by the generally low vapour pressure of elemental metals. Therefore only zinc and cadmium have been used as metal sources in ALD processes. However, some metalloids, e.g. selenium and tellurium, can be used as well. 

Several metal halide precursors have been applied in ALD processes, together with water as an oxygen source. However, the suitability of a particular metal halide for ALD depositions has been found to depend on the metal. Halide contamination of the film may cause problems at low deposition temperatures [45-47], although halides may be successfully used as ALD precursors. Liberation of HX (X = F, Cl, Br, I) during the deposition process may also cause problems, such as corrosion and etching. 

The first ALD processes that exploited true organometalhc precursors were studied using metal alkyls as precursors. This type of precursor has not been specifically developed for ALD, since both trimethyl and triethyl alkyls were first used in CVD deposition. In ALD, volatile metal alkyls were first used to deposit III-V semiconductors containing Ga, Al, In, As and P but more recently their usefulness for the deposition of metal oxide films has been evaluated. 

Fig. 6 Schematic representation of cyclopentadienyl-type precursors applied in ALD processes. The dashed line represents the approximate suitability level for ALD processing as a function of ionic radius. Below this line decomposition takes place. Compounds which contain both cyclopentadienyl and some other ligands have been omitted (e.g., Cp2Zr(CH3)2 and (C5MeH4)Mn(CO)3)...

The kinetic mechanism for Zr02 film growth in the ALD process reduced from the detailed scheme is shown in Table 9.3. 

Kinetic Monte Carlo and Molecular Dynamics Modeling of ZrCL Film Roughness in an ALD Process... 

A rather simple model of film growth can be used for an investigation of the roughness of a growing film in the ALD process. It is possible to... 

The proposed kinetic mechanism was used to model zirconia film growth in the ALD process in the framework of the KMC-DR method. The structures of the zirconia film after first, second, third, and fourth ALD cycles are... 

The chemical nature of the precursor represents the most critical component of a CVD/ALD process. Generally speaking, the choice of a particular precursor is governed by the relative stabilities of the precursor and substrate, as well as the volatility, cost, and hazards of the precursor. The coordination sphere of ligands surrounding the central metal is extremely important the organic ligands in these precursors may... 

The growth of ZnS him is another often used classical example for illustrating the principles of the ALD process. ZnCl2 and H2S are used as precursors. First, ZnCl2 is chemisorbed on the substrate, then H2S is introduced to react with ZnCl2 to deposit a monolayer of ZnS on the substrate and HCl is released as a byproduct. A wide spectrum of precursor materials and chemical reactions has been studied for the deposition of thin films by ALD. Thin films of various materials including various oxides, nitrides, fluorides, elements, and II-VI, II-VI, and III-V compounds in epitaxial, polycrystalline, and amorphous form deposited by ALD are summarized in Table 20.1. ... 

In the simplest case, ALD process consists of (i) pretreatment of the support... 

Nanoparticle Controllable Assembly, Fig. 4 Schematic illustration of the ALD process (Reproduced with permission from Ref. [14])... 

Following the ALD process, the Vycor tubes were surface modified using APTS. Preliminary permeation experiments were then done for pure and mixed gas (CO2 and N2) at 373 K and 303 kPa to calculate the ALD-APTS-modified membrane separation properties. The mixed gas experiments were performed under the following three conditions to check for any hysteresis due to the chemical nature of CO2 adsorption on amine ... 

---