Atomic Layer Deposition ENMA465 14 May 2003 Nicole Harrison Bryan Sadowski Anne Samuel Kunal Thaker
Presentation Outline • ALD Theory and Processes • Applications • Focus on ALD applications in gate dielectrics • Equipment • Comparisons to other Processes • Current and Future Development • Questions
What is ALD? • ALD (Atomic layer deposition) previously known by the name ALE (Atomic layer Epitaxy) was originated by T.Suntola in Finland. • Deposition method by which precursor gases or vapors are alternately pulsed on to the substrate surface. • Precursor gases introduced on to the substrate surface will chemisorb or surface reaction takes place at the surface.
ALD • Surface reactions on ALD are all self-limiting. • Self-limiting characteristics of the process steps is the foundation of ALD. • Two fundamental self-limiting mechanisms in ALD: CS-ALD and RS-ALD. • CS-ALD: Chemisorption Saturation process followed by exchange reaction. • RS-ALD: Sequential surface chemical reaction.
CS-ALD Process • Substrate is exposed to the first molecular precursor. The first molecular precursor is retained on the surface by chemisorption. • Second precursor is introduced on to the surface, which reacts on the surface of the first precursor. • Exchange reaction takes place between the two precursors and by-products are formed. • Exchange reaction continues till the second precursor reacts with first precursor: self-limiting step. • Final reaction is ML2+AN2®MA(film) + 2 LN • Sequence is repeated to grow films.
RS-ALD Process • Promoted by chemistry between reactive surface and reactive molecular precursor. • Deposition process is as a result of the chemical reaction between reactive molecular precursors and substrate. • Substrate surface is made reactive by certain surface groups. And first precursor is introduced to this surface. • The reaction now looks like this AN + ML2® AML +NL(A) reaction self-saturates until AN groups are converted to AML groups.
RS-ALD Process • Followed by this reaction, the first precursor is removed by inert gas purging prior to the introduction of second precursor. • Second precursor is introduced, which reacts with ML surface to form AML + AN2® MAN + NL(B) • Self saturates • The surface now looks like initial surface (AN) and surface is now ready for reaction (A). The repetition of ABAB…sequence will deposit films. • During each half-reaction(A&B), surface functionality changes from one surface species to another.
ALD Precursor Requirements • Must be volatile and thermally stable • Preferrably liquids and gases • Can be solids • no sintering related problems • Should Chemi-sorb on surface or react agressively with surface groups and each other • Short saturation time, good deposition rate, no gas phase reactions • Should not self-decompose • Destorys self-limiting property • Affects thickness, uniformity, and containation • Should not etch, dissolute into film or substrate • Prevents self-limiting film growth • Examples of substrates • Zn, Cd, S, Se, metal halides like AlCl3, TiCl4,TaCl5, Alkyls, b-Diketonates etc [2].
Transistor Gate Dielectrics MEMS Opto-electronics Diffusion Barriers Flat- Panel displays Organic Light Emitting Diodes (OLED) Interconnect Barriers Interconnect seed layer DRAM and MRAM dielectrics Embedded capacitors All thin films (<90 nm) Electromagnetic recording heads Applications
Applications • Transistor gate dielectrics • Smaller transistor sizes • Smaller channel lengths • Thinner gate oxide (limit of ~2.3 nm) • Larger leakage current • Need for high-K dielectrics for transistor gate dielectrics • Will allow for the use of thicker gate dielectrics • Lower leakage current • Lower power loss
Applications • New processes are concurrently being developed as new materials are being tested • ALD provides one of the best alternatives for depositing these new materials • Conformality • Uniformity • Compositional Control • Thickness control
Equipment ● Closed System Chambers - The Reaction chamber walls are such that they effect the transport (trajectory, etc.) of the precursors. ● Open System Chambers - Dimensions of reaction chamber do not interfere with ALD process (very large relative to wafer position). ●Semi-Closed System chambers - A channel is used with the top and bottom surfaces consisting of two separate wafers and the precursor gas is passed through the center. ●Semi-Open System chambers - Similar to a semi-closed system except one side is a wafer and the other side is gas- limited.
Equipment • CVD conversion to ALD • The installation of a module: facilitates fast gas switching • ALD specific equipment • Wafer output • Reaction volume • Factory Automation • Consistency • Conformality • Uniformity
Equipment • Research into more efficient equipment design continues • PEALD/ ALD reactor developed by Genitech Corporation • ● Hf[N(CH3)2]4 as the reactant • species with an O2 plasma • and H2O • ● Temperature in the range of • 200-350C • ● Ar is the carrier and purge • gas • ● Allows for increase in the • choice of reactants • ● Better film quality • ● Increased deposition rates
Comparison of ALD to CVD and PVD • Process description • ALD • atomic layer-by-layer deposition process where precursor is introduced into system, allowed to react with substrate surface until saturation, remaining by-products or unreacted gases are purged, process is repeated to form a monolayer/film • CVD • chemical reaction between gaseous precursors in gas phase; solid product of reaction is deposited on surface of substrate. • PVD • deposition process in gas phase where source material is physically transferred in the vacuum to the substrate without any chemical reactions being involved
Comparison Continued • self-limiting: gas-surface reactions occur till surface is saturated • Precursors are introduced into system separately • reactions occur at the surface • precise thickness control • 100% step coverage even at high aspect ratios • Films are very uniform, smooth, and stoichiometric
Comparison Continued • less contamination • low pressures • typically less than 10-6 torr • Film thickness can be as low as 1nm • produces amorphous films • lower deposition temperatures • as low as 180oC for HfO2
Comparison Continued • slow deposition rates because have to form monolayer in two steps • HfO2: dep. rate of 0.1nm/s @ 300oC • Growth rate • increases, reaches a max, and decreases slightly as temperature is increased • increases with number of cycles Thin Solid Films, 427(1-2), p.147-151.
Current Developments • Materials being used • ZrO2 , Al2O3, HfO2 • All exhibit good qualities • thickness control • good conformity • excellent step coverage
Current Developments • High-k materials • traps electrons between barrier gate and SiO2 • prevents burrowing of electrons through SiO2 • High barrier height • energy required for electrons to pass from gate to SiO2 material formula kox silicon oxide SiO2 3.9 silicon nitride Si3N4 7 Oxynitrides SixNyOz 4 - 7 aluminum oxide Al2O3 9 tantalum pentoxide Ta2O5 25 hafnium oxide HfO2 30 - 40 zirconium oxide ZrO2 25 barium strontium titanate (BST) BaSrTiO3 300
Future Developments • HfO2-Al2O3 laminate • Being Developed by Samsung • combines good qualities of both • Reduced current leakage • due to high-k of HfO2 and high barrier height of Al2O3
Future Developments • SiN/SiO2 stack gate-dielectric • SiO2 experiences soft-breakdown • due to differences in thermal expansion, high electric field, and strained Si-O bonds • low thermal budget (£550oC) • lower leakage current • higher reliability • soft-breakdown free • fewer wasted samples • Reduces Boron penetration in SiO2
Future Developments
Future Developments • Zr(t-OC4H9)4, commonly called ZTB being used to replace ZrCl4 as a precursor when making ZrO2 • Cl supposed to be purged once Zr has been reacted • sometimes reacts with substrate • can cause damage to whole system
References • Leskela Markku, Ritala Mikko. " Atomic Layer Deposition (ALD): From Precursors to Thin Film Structures". Thin Solid films, 2002 Vol 409. PP 138-139. • Ritala Mikko. "Fundamentals of ALD Chemistry" presented at AVS Topical Conference on Atomic layer deposition, May 14-15. 2001 Monterey, California. • Sneh Ofer, Phelps Robert et.al. “ Thin film atomiclayer deposition equipment for semiconductor processing” . Thin solid films, 2001 Vol 402. PP 248-261. • Scott Thompson, Portland Technology Development, Intel Corp. Paul Packan, Technology Computer Aided Design, Intel Corp. Mark Sohr, Portland Technology Development, Intel Corp. Intel Technology Journal. MOS Scaling: Transistor Challenges for the 21st Century. 10 April 2003.<http://www.intel.com/technology/itj/q31998/articles/art_3.htm>. • Ho-Kyu Kang. Samsung Electronics Co., LTD. ALD high k materials for gate and capacitor gate dielectrics. ALD Conference 2002. • ASM. The process of innovation. Pulsar. Single Wafer Atomic Layer CVD. 10 April 20003. <http://www.asm.com/prod_pulsar.asp>. • Moo-Han Corporation. ALD System. ELFRA 7000 Series. 10 April 2003.<http://www.moo-han.com/eng/html/product_ald_2.html>. • Dae-Youn Kim, Kyung-Il Hong, Jeongseok Ha, Cheol-Min Chang, Seung-Woo Choi, Hyung-Sang, Park, Yong-Min Yoo, Wonyong Koh and Choon-Soo Lee.Plasma Enhanced Atomic Layer Deposition of HfO2.Genitech, Inc. ALD Conference 2002. • Ho-Kyu Kang, Samsung Electronics Co., LTD, San#24 Nongseo-Ri, Giheung-Eup, Yongin-City, Gyeonggi-Do, Korea 449-711 • Anri Nakajima and Shin Yokoyama, Research Center for Nanodevices and Systems, Hiroshima University 1-4-2 Kagamiyama, Higashi-, Hiroshima, Hiroshima 739-8527, Japan
Question and Answer
