5 - CVD, Oxidation, and Diffusion Himanshu J. Sant...

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Fundamentals of Microfabrication CVD, Oxidation, and Diffusion Himanshu J. Sant Fundamentals of Microfabrication Fundamentals of Microfabrication Learning Objectives • To learn different types of deposition methods • To compare and contrast these methods • To identify governing parameters for chemical vapor deposition techniques and their effects on thin film deposition
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Fundamentals of Microfabrication Outline Available methods for thin film deposition and important parameters Oxidation – Dry – Wet Chemical vapor deposition – Methods • General operating conditions – Reaction mechanisms and rate equations – Step coverage and film quality – Chemistries for different materials LPCVD PECVD APCVD – Epitaxy Doping – Diffusion – Ion Implantation Fundamentals of Microfabrication Thin-Film Deposition • Spin-on Films – Polyimide (PI), photoresist (PR) – Spin-on glass (SOG) • Physical Vapor Deposition (PVD) – Evaporation – Sputtering • Chemical Vapor Deposition (CVD) – Oxidation – LPCVD – PECVD
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Fundamentals of Microfabrication Critical Parameters Thin film quality – Composition and defects Stresses Uniformity Step coverage Time Processing variables – Temperature – Pressure – Gas Flow and Composition – RF power and frequency Application – e.g. etch mask Thin film material – Chemistry Ideal deposition method? Fundamentals of Microfabrication Thermal Oxidation of Silicon • Formation of the oxide of silicon on the silicon surface is known as oxidation • Thermal Oxidation is characterized by high temperatures (900 - 1200 C) • Two main processes : – Dry Oxidation • Si(s) + O 2 --> SiO 2 1 atm , 1000 C – Wet Oxidation • Si (s) + 2H 2 O --->SiO 2 + 2H 2 – Dry oxidation produces a better (more dense) oxide as compared to wet oxidation. • Why?
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Fundamentals of Microfabrication Dry Oxidation Limitations • The ratio of Si thickness converted (Xs), to resulting oxide thickness (Xox) ~ density diff. – Xs=0.46 Xox – Volume mismatch, thermal expansion difference • Large compressive stress • Limited thickness • Wet oxidation relaxes and has faster growth rate – Bad against alkali diffusion, not for ICs Fundamentals of Microfabrication Growth Rates
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Fundamentals of Microfabrication Chemical Vapor Deposition (CVD) • CVD = formation of non-volatile solid film on substrate by reaction of vapor phase chemicals – Diffusive-convective transport of depositing species to a substrate with many intermolecular collisions-driven by a concentration gradient • Complex heat and mass transfer processes – Molecular phenomena at surface • Sticking coefficient • Adsorption • Diffusion • Reaction • Desorption • Film growth Fundamentals of Microfabrication Low Temperature Oxidation of Silicon • (LTO) SiO 2 is formed using three types of CVD Processes. – APCVD ( Most commonly used method ), LPCVD and PECVD
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5 - CVD, Oxidation, and Diffusion Himanshu J. Sant...

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