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UMD ENMA 490 - An Overview of Atomic Layer Deposition and its role in Transistor Gate Dielectrics

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ConclusionAn Overview of Atomic LayerDeposition and its role inTransistor Gate DielectricsENMA465Dr. Gary RubloffMay 14, 2003Nicole HarrisonBryan SadowskiAnne SamuelKunal ThakerTable of Contents0Introduction 2ALD Theory and Chemistry 2Applications of Atomic Layer Deposition 7ALD Equipment 14Comparison of ALD to other deposition methods for the formation of gate dielectrics 19Current and Future Developments 22Conclusion 26References 27IntroductionALD (Atomic layer deposition), previously known by the name ALE 1(Atomic Layer Epitaxy), was originated by T. Suntola in Finland. It was originally developed for the fabrication of polycrystalline luminescent ZnS:Mn and amorphous Al2O3 insulator films for electroluminescent flat panel displays. Due to its complex surface chemistry, no real break-through was achieved in this area since 1985. But decreasing device dimensions and increasing aspect ratios in the IC circuits increased interest towards this technique since the mid 1990's. ALD has a slow deposition rate (this is the draw back of this technique), however, this is becoming less important as the thickness of the films are reducing to the order of a few nanometers [1]. Since this is a layer-by-layer deposition it produces films of uniform thickness and excellent conformality. In this report, we are discussing ALD theory and its' chemistry, ALD equipment, Applications of ALD in gate dielectrics, Comparison of ALD to other processes for the formation of gate dielectrics and finally, current and future developments of ALD. ALD Theory and ChemistryWhat is Atomic Layer Deposition (ALD): ALD previously known by the name ALE (Atomic layer Epitaxy), was originated by T. Suntola in Finland. This is the deposition method by which precursor gases and vapors are alternately pulsed on to the surface of a substrate. Whenprecursor gases are introduced onto the substrate surface, chemi-sorption or surface reactions take place at the surface. The ALD reactor is purged with an inert gas between the precursor pulses [1]. The surface reactions in ALD are all self-limiting. In fact, self-limiting characteristicsof the process steps are the foundation of ALD [2]. Deposition process steps are repeated to growfilms. The self- limiting process in ALD promotes the growth of conformal films with accurate thickness on large areas. The characteristics features of ALD, with their implications on the film growth and practical advantages are given in Table 1. There are two self-limiting process in ALDthat are discussed below. Table 1. Characteristic features of ALD (ALE), implications on film growth and practical advantages [3].2Self-Limiting Mechanisms in ALD: There are two fundamental self-limiting mechanisms in ALD. They are CS-ALD: Chemi-sorption saturation process followed by exchange reaction and RS-ALD: Sequential surface chemical reactions. In the CS-ALD process, the substrate surface is exposed to the first molecular precursor, which is retained on the surface by chemi-sorption. This layer is then exposed to the second precursor, which reacts on the surface of the first precursor. Exchange reactions take place between two precursors and by-products are formed. Exchange reactions continue until the first precursor reacts with the second precursor, so this is a self-limiting step. A layer of desired film is formed and this sequence is repeated to grow films. The chemi-sorption saturation process sequence is given in Figure 1 below.3Figure 1. Showing the chemi-sorption saturation process sequence [2].In the figure above, ML2 represents the first molecular precursor, AN2 represents the second molecular precursor and LN is the by-product formed due to the exchange reaction between ML2and AN2. The final reaction looks like this:ML2 + AN2  MA (film) + 2 LN (1)In contrast to CS-ALD process, the RS-ALD process is promoted by the chemistry between reactive surface and reactive molecular precursor. The film deposition in this process is as a result of the chemical reactions between reactive molecular precursors and the substrate. TheRS-ALD process sequence is given in Figure 2.4Figure 2. Showing sequential surface chemical reaction [2]. The figure above shows that at the beginning, the substrate surface is activated by AN groups. This surface is then exposed to the first metal precursor ML2 where M can be Al, W, Ta, Si etc. and L can be CH3, Cl, F, C4H11 etc. ML2 molecules react with the surface AN reactive species to form AML groups. The reaction sequence is given equation 2,AN + ML2 AML +NL (2)where NL is the by-product and this reaction self saturates when all AN groups are converted to AML groups. Followed by this reaction, the first precursor is removed by inert gas purging prior to the introduction of the second precursor. Once the first precursor is removed, the second precursor, AN2, is introduced to the ML surface. The second precursor is usually non-metallic where A is O, N, S and N can be H2O, NH3 or H2S. AN2 reacts with ML to form:AML + AN2  MAN + NL (3)This reaction self saturates until all ML groups are converted MAN, which cannot further react with the AN2 precursor. The substrate surface looks like the initial surface, with AN groups 5present on it. The sequence is repeated to grow films and this reaction sequence that restores the surface to the initial surface is known as the ALD deposition cycle [2]. In RS-ALD process, deposition reactions are half-reactions (equations 2 and 3). During each half-reaction, the surface functionality changes from one surface species to another [3]. Andthe final half-reaction leads to the restoration of the initial surface. Restoration of initial surface is the factor that differentiates RS-ALD from CS-ALD. Requirements for ALD: The factors that are taken in account during ALD reaction are the volatility and stability of the precursor materials, their pulsing into the reactor and their interaction with the substrate surface and each other [4].Precursor Requirements: Precursor chemistry plays a key role in ALD [1]. Precursors must be volatile and thermally stable in order to ensure its' efficient transportation, so


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UMD ENMA 490 - An Overview of Atomic Layer Deposition and its role in Transistor Gate Dielectrics

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