Integrated Sustainability Analysis of Atomic Layer Deposition for Microelectronics Manufacturing Chris Y. Yuan Atomic layer deposition (ALD) is a promising nanotechnology for wide applications in e-mail: [email protected] microelectronics manufacturing due to its ability to control layer growth at atomic scale. Sustainability of ALD technology needs to be quantitatively investigated in this early David A. Dornfeld development stage to improve its economic and environmental performance. In this pa- e-mail: [email protected] per, we present an integrated sustainability analysis of ALD technology through material and energy flow analyses. The study is performed on the ALD of Al2O3 high-␬ dielectric Department of Mechanical Engineering, film through trimethylaluminum and water binary reactions. The precursor utilizations, University of California, Berkeley, methane emissions, and nanowaste generations from the ALD process are all quantita- 5100A Etcheverry Hall, tively studied. Energy flow analysis demonstrates that the ALD process energy consump- Berkeley, CA 94720-1740 tion is mainly determined by the ALD cycle time rather than the process temperature. Scale-up performance of the ALD technology is also studied for both emission genera- tions and energy consumptions. Strategies and methods for improving the sustainability performance of the ALD technology are suggested based on the analysis. ͓DOI: 10.1115/1.4001686͔ Keywords: atomic layer deposition, sustainable manufacturing, material flow analysis, energy flow analysis, Al2O3 dielectric film 1 Introduction as the oxidant. Deposition mechanism of Al2O3 by ALD is based ͑ ͒ ͑ ͒ on the chemical vapor deposition CVD reaction: 2Al CH3 3 As the miniaturization trend continues in the semiconductor → manufacturing industry, atomic layer deposition ͑ALD͒ has re- +3H2O Al2O3 +6CH4. In the ALD process, this CVD reaction ceived increasing attention in recent years due to its ability to is split into the following two half reactions: obtain atomic layer control of film growth ͓1–3͔. ALD can deposit Al – OH + Al͑CH ͒ → Al–O–Al͑CH ͒ +CH ͑1͒ highly uniform and conformal thin films on extremely complex 3 3 3 2 4 surfaces ͓4͔, and accordingly, has potential applications on a wide → ͑ ͒ variety of electronic products including complementary metal- Al – CH3 +H2O Al – OH + CH4 2 ͑ ͒ oxide-semiconductor CMOS chips, flat panel display, optical fil- This ALD process depends on the surface reactions between ters, etc. ͓5͔. TMA, water, and hydroxyls to form the Al2O3 layers. In the ALD ALD operates by alternating the exposure of a surface to vapors operations, the growth rate is an important indicator of the depo- of two or more chemical reactants to deposit an atomic layer film sition efficiency. Generally, higher concentrations of precursor on the surface. The chemical reactions are separated by complete materials lead to higher growth rates in ALD of Al2O3 from the purging in between. In ALD operations, the surface reaction is TMA and water binary reactions ͓18͔. In order to obtain a suffi- self-limited, and the film thickness could be accurately controlled cient surface reaction and a higher growth rate, excessive amount in atomic scale. As needed, the deposition process can be repeated of precursor materials are usually supplied into the ALD system to obtain a film layer for a specific thickness. ͓18͔. The excessive material consumed, however, is not only a A wide variety of solid materials could be deposited through cost issue but also an environmental concern in the production of ALD, based on the needs of specific applications ͓6–11͔. The Al O dielectric gate in the semiconductor manufacturing indus- ␬ 2 3 typical application of ALD is on deposition of Al2O3 high- di- try. electric films to replace conventional SiO2 dielectric gate in the The precursor material TMA, Chemical Abstract Service ͑CAS͒ metal oxide semiconductor field effect transistors ͑MOSFETs͒ to No. 75-24-1, is a flammable and toxic chemical. When released support the miniaturization of the microtransistors ͓12–15͔. into the atmosphere, TMA is pyrophoric and can readily react with In atomic layer deposition, the film thickness increment and hydroxyls and water in the air, and form Al2O3 nanoparticles, surface roughness profiles depend on the chemical reaction pro- which could be harmful for both occupational and public health if cess on the surface of the silicon wafer. Reaction sufficiency relies in relatively high concentrations. on the concentrations of the reactants in the ALD reactor and the Besides, the principal byproducts of the ALD process is meth- process temperature. As demonstrated through ALD experiments, ane, which is a major greenhouse gas and has a global warming the accuracy of Al2O3 film thickness can be successfully con- potential 25 times that of carbon dioxide ͓19͔. As dielectric mate- trolled at 0.1Ϯ0.01 nm ͓16͔, while the roughness of the growing rials are widely used in semiconductor industry, manufacturing of ͓ ͔ ͑ ͒ surface can be maintained less than 0.3 nm 17 . Al2O3 dielectric gate through Al CH3 3 and H2O binary reactions Typical ALD of Al2O3 dielectric film uses trimethylaluminum could have significant impact on global warming. A rough esti- ͑ ͒ ͑ ͒ TMA ,AlCH3 3, as the metal source, and deionized water, H2O, mate of the global dielectric material demand in the semiconduc- tor industry is in the range of 108 kg per year ͓20,21͔. Based the Al2O3 CVD reaction, deposition of 1 kg Al2O3 would generate Contributed by the Manufacturing Engineering Division of ASME for publication 0.94 kg of CH , which is equivalent to 23.5 kg of CO . in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received 4 2 June 15, 2009; final manuscript received April 16, 2010; published online June 14, Besides the material use and emission generations, energy con- 2010. Assoc. Editor: Curtis Taylor. sumption of ALD is another concern for both economic and envi- Journal of Manufacturing Science and Engineering JUNE 2010, Vol. 132 / 030918-1 Copyright © 2010 by ASME Downloaded 29 Oct 2010 to 129.89.200.16. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm ronmental justifications. ALD has to be operated at the vapor phase of the precursor materials and requires ultrahigh vacuum conditions. The energy consumption of ALD for such process op- erations as heating, pumping, monitoring, controlling, etc., would contribute not only to the cost of the deposited dielectric films but also to the generation of various pollutant emissions from the energy production and supply industry. For sustainable manufacturing of Al2O3 dielectric gate in semi- conductor industry, both material and energy consumptions of the ALD process needs to be well understood to support decision- making during the process improvement and system optimization of ALD technology. In this paper, we present an integrated sus- tainability analysis of the ALD process for Al2O3 dielectric gate in microelectronic manufacturing. The integrated sustainability analysis is conducted on both material and energy use of the ALD process. Material and energy flow analyses are performed to quan- Fig. 1 ALD lattice configurations tify the material and energy consumptions, identify the process emissions, and track the process flows within the ALD reaction system. These results could be helpful in improving both the eco- and environmental performances of the ALD technology. Unfor- nomic and environmental performances of the ALD technology to tunately, such an optimal supply pattern is not established yet; achieve a sustainable application of the ALD technology in mi- excessive amount of precursors are usually loaded into the ALD croelectronics manufacturing. system to ensure a sufficient surface reaction and a high film growth rate ͓18͔. Due to the variation in the ALD system dimen- 2 Material Flow Analysis sions, the common practice to control the supply of precursor materials is to set the pressure of the materials in the ALD system. Material flow analysis is performed here to gain deep insights The effect of process pressure on ALD of Al O has been studied into the ALD system and to obtain a comprehensive understand- 2 3 at various pressure levels ͓22͔. In this analysis, we investigate the ing of the material flows throughout the ALD processes. The ALD precursor material flows of the ALD system under 600 mTorr, 800 material flows are modeled on the process by using the Cambridge mTorr, and 1000 mTorr pressures, by following ͓22͔. The partial NanoTech Savannah S200 ALD system. The reactor is customized pressure of the TMA precursor is set at 15% of the process pres- for 10.16 cm ͑4 in.͒ standard silicon wafers, with 15.24 cm ͑6 in.͒ sure by following ͓13͔, while the partial pressure of supplied wa- inner diameter and half inch inner depth. Nitrogen is used as the ter is decided by considering the correlations between the TMA carrier and purge gas to transport TMA and water precursor va- and H O precursors in the ALD binary reactions. According to the pors to the silicon surface, and purge reaction gases out of the 2 reactor during each half-cycle reaction. The temperature of the Al2O3 deposition mechanism, a complete reaction of the precur- sors in the ALD system needs water molecules to be supplied at reactor and the outlet pipeline are maintained at 200°C and an amount one and a half times of the TMA. But in real opera- 160°C, respectively. An Edwards RV5 rotary vane pump is con- tions, a larger water dose is preferred due to its ability to enhance nected to the end of the outlet pipeline to vacuum the reactor at the Al O layer growth rate ͓23͔. Here we set the amount of water the beginning and pump excessive materials out of the reactor 2 3 supplied as double of the TMA amount, namely, with partial pres- during the experiment.