CVD and ALD Precursors

Product Manager：Nick Wilde

Introduction

Chemical vapor deposition (CVD) is a technique that employs gaseous or vaporized materials to react within the gas phase or at the gas-solid boundary, resulting in the formation of solid coatings. Atomic layer deposition (ALD), a specialized form of CVD, enables precise deposition at the atomic scale through self-terminating surface reactions, typically achieved by sequentially exposing the substrate to distinct precursor gases.

CVD precursors

Gas source precursors offer the most flexibility and precision in control; however, their large-scale production is constrained by safety considerations, as well as challenges related to storage and volume. For instance, methane (CH₄), a flammable gas, can serve as a precursor by reacting with anhydrous ethanol (CH₃CH₂OH). This reaction, when combined with ammonia borane and boron oxide precursors, facilitates the preparation of boron-doped diamond films.

Precursors engineered for specific reactivity, volatility, stoichiometry, and thermal properties are often liquids or solids under standard conditions, making them more suitable for use in CVD processes. For example, pyromellitic dianhydride and 4,4'-diaminodiphenyl ether are utilized as precursors. These substances are heated to sublime, allowing them to fully interact and react within a vacuum chamber. Subsequently, the substrate and the resulting viscous polyamide acid are heated, causing the polyamide acid to dehydrate and form a polyimide film.

ALD precursors

The precursors employed in Atomic Layer Deposition (ALD) must generally satisfy the following criteria:

• Sufficient Vapor Pressure: They should possess a high enough vapor pressure to guarantee complete coverage or saturation of the matrix material's surface.
• Chemical Stability: Good chemical stability is essential to prevent self-decomposition within the reaction's maximum temperature range.
• Non-Toxic and Non-Corrosive: The precursors should be non-toxic, non-corrosive, and the resulting products should be inert to avoid impeding the growth of self-limiting films.
• Strong Reactivity: They must exhibit strong reactivity, enabling rapid adsorption onto material surfaces to achieve saturation or rapid and effective reactions with surface groups of materials.

In certain cases, the ALD process may employ the same precursors as those used in Chemical Vapor Deposition (CVD), but with distinct alternating exposure to the precursors instead of simultaneous exposure. Typical ALD steps involve the reaction of a precursor containing a metal or nonmetal source atom, followed by a chemical reaction such as reduction, oxidation, or nitridation. Each step necessitates the generation of adsorption and reaction sites (e.g., reactive ligands) for subsequent reactions. Common ALD precursors include hydrides (SiH4, Si2H6), halogen-substituted hydrides (e.g., SiCl4), metal halides (e.g., AlCl3, WF6, TaCl5, HfCl4), metal nitrates (e.g., Ti(NO3)4, Hf(NO3)3), and metalorganics containing alkyls (e.g., AlR3), alkoxides (e.g., Si(OR)4,Ti(OR)4), amides (e.g., Ti(NR2)4), or β-ketonates and their derivatives (e.g., Zr(thd)4, Hf(acac)4). Additionally, oxygen (H2O, H2O2, O2, O3) and nitrogen (NH3, N2H4, NHxR3-x, HN3) sources are employed to produce oxides and nitrides, respectively. While gas and liquid precursors are most commonly used, solid source precursors can also be effectively utilized in ALD processes.

Application
As the demands across various application domains continue to escalate, new requirements have emerged for both Chemical Vapor Deposition (CVD) and Atomic Layer Deposition (ALD) precursors. For instance, the integration of CVD technology with chemical fluidized bed technology has found widespread industrial utility, particularly in the realm of advanced nuclear fuel production. Additionally, conventional refractory metal materials can serve as precursors in ALD processes to synthesize the desired films. Currently, numerous advanced nations have achieved large-scale industrial implementation of chemical vapor deposition technology for refractory metal coatings.

The prospects for CVD and ALD applications are exceedingly promising. These techniques not only enhance material lifespan, refine material characteristics, and conserve material resources but also facilitate the synthesis of novel materials.

Reference
1. Hampden‐Smith, Mark J.; KODAS, Toivo T. Chemical vapor deposition of metals: Part 1. An overview of CVD processes. Chemical Vapor Deposition, 1995, 1.1: 8-23.

Aladdin：https://www.aladdinsci.com