Low Temperature Thermal ALD of Silicon Nitride Utilizing a Novel High Purity Hydrazine Source
Authors: Alvarez Daniel , Andachi Keisuke , Lucero Antonio Tomas , Kondusamy Aswin , Hwang Su Min , Meng Xin , Kim Harrison , Kim Jiyoung , Spiegelman Jeffrey 
Published: February 2020

The demand for faster, smaller and more energy efficient logic devices as well as higher density, higher speed, and increased reliability for advanced memory devices has led to numerous challenges in Semiconductor device manufacturing. Novel metal materials, 3D architecture and increasing High-aspect-ratio (HAR) structures are being used to address these challenges, however this has placed additional constraints on film deposition methods. CVD and ALD of SiN are used in several applications including, gates, spacers, etch stops, liners, encapsulation layers as well as passivation layers.1 Recently PEALD of SiN is taking on an increasingly important role due to new temperature constraints of <400°C. However, several challenges remain on HAR and 3D structures in applications where plasma approaches may not meet conformality requirements. In addition, thermal ALD with NH3 may not be feasible due to the high temperature requirement (500°C-700°C) of these reactions.2

Our approach involves the development and use of a novel hydrazine delivery system in order to develop methods for thermal ALD of SiN at <400°C. The resulting films must meet requirements of high growth rate, high density, and low wet etch rate similar to materials grown by thermal ammonia ALD at 600°C, or Ammonia PEALD at 400°C or less. In addition, the methods developed must show promise for highly uniform growth on 3D and HAR structures where PEALD methods currently have difficulties.
A hydrazine delivery system was developed to provide a stable flow of ultra-dry hydrazine gas from a liquid source in a sealed vaporizer. The liquid source combines anhydrous hydrazine and a proprietary solvent that acts as a stabilizer. The solvent is highly non-volatile, where high purity hydrazine gas may be generated in-situ and delivered to the deposition chamber while the solvent remains in the vaporizer. Testing confirms that hydrazine vapor pressure is maintained at levels viable for ALD (12-14 torr) even in the presence of the solvent used to dilute the liquid source to safe handling levels. Previous studies with hydrazine were plagued by oxygen contamination.3 This study demonstrates high purity hydrazine delivery at <800ppb water contamination in the gas phase. Delivery system safety and optimization versus conventional hydrazine will be addressed.
A study of silicon nitride deposition was conducted using hexacholorodisilane (HCDS) and hydrazine on a Si-H substrate. A custom made thermal ALD reactor was used to deposit silicon nitride films from 250-400°C. Film growth per cycle (GPC) with hydrazine was 0.4-0.5 Å/cycle at 400°C with refractive index of 1.813. Film stoichiometry was confirmed with X-ray photoelectron spectroscopy. SiN films with low impurities were achieved for oxygen (<2%) and chlorine (<1%). Highly uniform films were obtained across a 4 inch wafer for 200 as well as 400 cycles. Results were similar to films deposited using PEALD at 360°C with HCDS and NH3. Film growth and resulting properties at 350°C closely resemble those grown thermally at 400°C for hexachlorodisilane and hydrazine. Some discrete differences in film density and composition start to become apparent at a growth temperature of 325°C. Specifically, film density decreases and wet etch rate increases. The presentation will compare growth rates, film density, refractive index and wet etch rate results at different temperatures for hydrogen terminated silicon. Initial growth studies with the use of Hydrazine and Organosilicon Amide precursors will also be discussed.
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