This opened up plenty of possibilities for new ALD chemistries and processes. A large number of ALD processes have been demonstrated and published to date, and the list keeps growing (Below Table).
概述
Table：ALD沉积材料举例
ALD前驱体概述
General ALD precursor requirements differ from other chemical gas-phase methods since all gas-phase reactions should be excluded and reactions take place only at the surface. Although chemical vapor deposition (CVD) precursors can sometimes be used for ALD, nowadays specific precursors have been synthesized for ALD because this deposition technique allows the use of significantly more reactive precursors than CVD. In the ALD method, in order to avoid uncontrolled reactions, sufficient thermal stability of the precursors is needed in the gas phase as well as on the substrate surface within the deposition temperature range, which is typically 150–500◦C
perhaps the most widely used reducing agent, but metallic
zinc vapor, silanes, and B2H6 have also been successfully applied. Molecular hydrogen is quite inert towards typical metal precursors and therefore quite high deposition temperatures are needed to maintain ALD reactions.
and organometallics, simple hydrides, such as H2S, H2Se,
and H2Te, have typically been used as a second precursor, although their toxicity must be carefully addressed. Recently, novel Se and Te precursors have been utilized for ALD, enabling the deposition of selenides and tellurides.
NANO-MASTER
ALD 原子层沉积 前驱体、工艺和材料
吴运祥
2017年7月25日
概述
New ALD processes and materials developed at several Universities. are being actively
ALD was demonstrated for the TFEL application for the first time by ZnS deposition using elemental zinc and sulfur in the mid-1970s, followed by the use of ZnCl2 and H2S. After the initial period, several other inorganic precursor types have emerged. During the second half of the 1980s metal organic precursor chemicals were adapted for ALD material research.
非金属前驱体
Because of the sequential nature of ALD, metal and nonmetal precursors are typically separated from each other. The ability to select an oxidizing or reducing precursor in conventional ALD processes makes it possible to control reactivity and reactions of the metal precursor. In a non-oxidizing regime, reducing the precursor is also necessary for depositing elemental metal films. Hydrogen is
compounds, such as (CH3)NNH2,tBuNH2,and CH2CHCH2NH, have also been studied.
非金属前驱体
For chalcogenide thin films it is possible to use elemental S, Se, and Te as precursors provided that the other source is a volatile and reactive metal. The first ALD process to be developed was ZnS deposition using elemental zinc and sulfur. For other precursor types, including halides, β-diketonates,
ALD前驱体概述
Since ALD relies on self-limiting reactions, a sufficient amount of the precursor is only required during one pulse to cover the adsorption sites on the surface and the excess will be purged by the inert gas between the reactive precursors. Because ALD is a gas-phase process, solid the case of ALD-processed oxide films, precursors attached to the surface can be oxidized with H2O, H2O2,N2O4,N2O,O2,orO3, the choice dependent on the metal precursor selected. Water has frequently been used as an oxygen source and indeed it readily reacts with many metal halides, alkyls, and alkoxides.
金属前驱体
The most used volatile metal-containing ALD precursors can be classified into five different main categories, namely halides, β-diketonate complexes, N-coordinated compounds (amides, amidinates), alkoxides, and true organometallics, i.e. metal alkyls and cyclopentadienyl-type compounds. Other compounds have occasionally been used as ALD precursors for thin films, for example metal nitrates, carboxylates, and isocyanates. Several metal halide precursors have been applied in ALD processes, usually with water as an oxygen source. They have enough high deposition rates and the price for industrial use is reasonable. However, for delicate applications halide contamination of the film may cause problems at low deposition temperatures. In addition, HX (X= F, Cl, Br, or I) evolution during the deposition process may cause problems such as corrosion and etching of the film.
For metal β-diketonate-type compounds, only ozone or
oxygen plasma can be used owing to the higher thermal stability of the precursor. The use of a strong oxidizer guarantees that only a small amount of carbon is left in the film, as well as ensuring better interface quality.