Precursor Vapor Pressure Estimation and Precise QCM Measurement for ALD Process Development with Ideal ALD Characteristics
Authors: Shimogaki Yukihiro , Sato Noboru , Wu Yuxuan , Yamaguchi Jun , Tsukune Atsuhiro , Momose Takeshi 
Published: December 2024

Atomic Layer Deposition (ALD) produces thin films by alternately supplying precursors and reaction gases. In this process, a film with one atomic layer or fewer is formed in a single cycle. By properly controlling the number of cycles, the film thickness can be precisely controlled and reproducible film formation is possible. This technology is widely used in the production of semiconductor integrated circuits (ULSIs). The amount of growth per cycle is called GPC, and it is well known that GPC remains constant over a temperature range known as the ALD Window, within which ideal ALD film-formation characteristics can be obtained. Therefore, a wide ALD window is important for constructing an easy-to-use ALD process. The GPC is constant in the ALD Window because the coverage factor of the chemisorbed precursor is nearly unity. Therefore, it is necessary to consider the conditions under which saturated chemisorption occurs during the adsorption process of the precursor. The surface coverage factor depends on the adsorption constant K and precursor concentration C according to the "Langmuir-Hinshelwood equation," as shown below.
θ=KC/(1+KC)
If the product of the adsorption equilibrium constant K and the precursor concentration C is large (e.g., 100 or more), the coverage factor θ is almost constant at unity, even at different substrate surface temperatures; that is, even if the values of K differ slightly, or even if the concentration C differs slightly between the center and the edge of the wafer. This is the main reason for the good thickness reproducibility and uniformity of the ALD process.
We developed a method to precisely estimate the precursor concentration, that is, the saturated vapor pressure, using quantum chemical calculations (the Modified COSMO-SAC method). This allowed us to identify compounds that could be supplied at appropriate concentrations as ALD precursors. This vapor pressure estimation is also effective for constructing the Atomic Layer Etching (ALE) process.
The adsorption equilibrium constant K of the precursor can be determined by quantum chemical calculations; however, this is time-consuming and computationally expensive. For the synthesized precursor compounds, the adsorption equilibrium constant K can be easily determined experimentally using precise Quartz Crystal Microbalance (QCM) measurements. As QCM is very sensitive to the temperature change, we have specially made “AT cut” Quartz Crystal which is insensitive to the temperature fluctuation at the desired temperature. The combination of a custom-made oscillation circuit for the above AT cut crystal and a frequency counter enables measurement of weight change with a sensitivity of 0.3 ng/cm2 or less and measurement of weight change over time due to adsorption or reaction with high time resolution of 20 points/sec. We combined these calculations with experiments to develop ALD systems that exhibit ideal ALD properties.