ALD Precursors and Reaction Mechanisms

Abstract

ALD has become accepted as a reliable tool for production of thin films in the microelectronics industry. Dielectrics and metal electrodes for DRAM capacitors are now routinely produced by ALD, which is the only technique capable of uniformly coating inside the narrow structures required by current technology. Transistors will soon be produced as 3D structures, with requirements for conformal coatings over their increasingly complex surfaces. Metal circuits between transistors are also pushing toward narrower and deeper structures, such as through-silicon vias. In order to extend the use of ALD into these new areas, a greater variety of materials will need to be deposited.Successful use of ALD requires suitable chemical precursors used under reaction conditions that are appropriate for them. There are many requirements for ALD precursors: sufficient volatility, thermal stability, and self-limited reactivity with substrates and with the films being deposited. The precursor vapor should not etch or corrode the substrate or deposited film. In addition, it is easier to produce the required vapors if the precursor is liquid at room temperature, or if it is a solid with a melting point below its vaporization temperature, or if it is soluble in an inert solvent with vapor pressure similar to that of the precursor. Ideally, the precursors should be non-flammable, non-corrosive, non-toxic, non-hazardous, and simple and inexpensive to make. In applications of ALD to microelectronics, precursors with high purity may be required.It is not easy to find ideal precursors that satisfy all of these requirements. When the properties of precursors are inadequate, satisfactory ALD results cannot be achieved.

Thermal Chemistry of Metal Organic Compounds Adsorbed on Oxide Surfaces

“…Metal complexes with amidinate ligands have been known for a long time, − and their use for the deposition of solid films was initially reported by Gordon and further tested by several other groups. ,− Related precursors such as iminopyrrolidinates and guanidinates have been used for this purpose as well. ,, Our past studies of the surface chemistry of these compounds ,− have shown that on metals they are highly reactive, typically decomposing at relatively low temperatures. Quantum mechanics (density functional theory, DFT) calculations have indicated that this reactivity may be due to the ease with which the ligands are displaced by adjacent metal surface atoms. , Once bonded to the surface, such ligands usually react via a β-hydride elimination step from the outside alkyl moieties and the scission of the corresponding N–C bond to yield smaller adsorbed fragments. − Some byproducts desorb, as olefins and HCN, for instance, but others dehydrogenate and leave carbon and nitrogen atoms behind.…”

“…In catalysis, it has been a long-term goal to combine the exquisite selectivity attainable in homogeneous systems with the process benefits afforded by heterogeneous counterparts, including the ease of separation of the catalysts from the products and the minimization of solubility problems. For this application, the traditional tethering of whole complexes through one of its ligands − has evolved into alternative approaches where the metal center(s) is­(are) more intimately bonded to the solid to create so-called single sites. , Another application of metal organics is in the chemical deposition of thin solid films of either metallic or metal-containing materials such as oxides, nitrides, and sulfides − to fulfill demands in terms of film conformality when coating topographically elaborate surfaces with nanoscale features, as is currently the case in the microelectronics industry. Traditional chemical vapor deposition (CVD) , has recently been improved by splitting the surface chemistry used to grow the films into two or more complementary and self-limiting half-reactions.…”

Atomic layer deposition of vanadium oxides: process and application review

“…The molecule features a distorted geometry from octahedral toward trigonal prismatic. The precursor features a monomeric structure in the gas phase due to the bulky ligand and higher coordination to vanadium (III), which presents a distorted octahedral geometry, [82]. M. Weimer et al, [83,84]…”

Synthesis of 5,5-Bicyclic Amidines as Ligands for Thermally Stable Vapor Deposition Precursors

“…Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are vapor deposition processes that are capable of depositing highly conformal thin films while affording a great degree of control over film thickness. − They have found many applications in areas such as microelectronic devices, machine tools, and window coatings . The precursors must possess high volatility, high thermal stability, and high reactivity with other precursors, with volatile and noncorrosive reaction byproducts …”