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SECTION 7 PROCESS GASES, CHEMICALS AND MATERIALS

Precursors For Atomic Layer Deposition Of High-k Dielectrics

Charles B. Musgrave Roy G. Gordon Stanford University Harvard University

Insulators with high dielectric con- transistor gates. Developing acceptable layer or less. Several excellent reviews[1- stants (k) play several critical roles in higher-k replacements for these materials 4] provide an overview of ALD while this modern semiconductor devices, including is a complex challenge. The leading article aims to summarize the principles capacitors that store memory bits in candidates under investigation are metal that determine precursor properties to aid DRAMs, decoupling filter capacitors oxides based on HfO2 and include precursor selection. protecting microcircuits from undesired ternaries, such as hafnium aluminates, To achieve ALD’s unique characteristics noise, and insulating gates from channels silicates and oxynitrides, as well as and to be suitable as a practical vapor in transistors. Scaling of semiconductor quaternaries such as Hf aluminates and deposition process, ALD precursors must devices to follow Moore’s Law past the silicates containing nitrogen. Other metal have specific properties. They must be 65 nm node will require higher-k oxides being considered include those sufficiently volatile (at least about 0.1 Torr insulators to maintain capacitance on based on zirconium, lanthanum and equilibrium vapor pressure at a smaller devices. At the same time, com- praseodymium. temperature at which they do not decom- plex structures, such as deep trenches A successful dielectric deposition pose thermally). Furthermore, they should and rough pedestals, have been method must deposit contamination-free vaporize rapidly and at a reproducible rate, introduced to keep the available capacitor films of these new materials with precise, conditions that are usually met for liquid areas from shrinking too rapidly. Thus uniform thicknesses over large areas and precursors, but not for solids. For self- any future higher-k dielectrics must aggressive topologies while remaining terminating surface reactions, precursors be produced uniformly must not self-react, includ- within the increasingly ing decomposing on the narrow spaces in these To achieve ALD’s unique characteristics and to be suitable as a surface or in the gas phase. structures. Furthermore, practical vapor deposition process, ALD precursors must have Precursors must also be higher-k dielectrics must specific properties. highly reactive toward the meet stringent require- other precursor previously ments for low leakage attached to the surface, currents and stability during processing cost effective. ALD has already resulting in relatively fast kinetics and thus and in use. All these objectives must be demonstrated these capabilities for lower ALD temperatures and cycle times. met by a reliable, production-worthy and deposition of Al2O3 in current mass- Furthermore, the byproducts must be cost-effective process that produces production of DRAMs. ALD is a vapor volatile and thus easily purged in order to uniform results over 300 mm wafers. deposition process based on sequential prepare for the subsequent half-cycle. Finally, the material and process must self-terminating surface reactions where Moreover, byproducts should not be integrate seamlessly into conventional the precursors are injected separately in corrosive to prevent non-uniformities due semiconductor process flows. pulses added to a flowing carrier gas to film etching and corrosion of the tool. Until recently, silicon dioxide was separated by a purge of excess precursor Precursors having exothermic reactions almost universally used as the dielectric vapor. Each pulse and purge sequence with their complementary precursor tend in microelectronics. Despite its relatively constitutes an ALD half-cycle. Ideally, to produce pure films because the ligands low dielectric constant (k~4), SiO2 has each half-cycle results in one additional are completely removed. A large thermo- served remarkably well as the MOSFET atomic monolayer of material and then dynamic driving force also usually allows gate dielectric due to both its unique the reaction stops even if more precursor low deposition temperatures, resulting in interface properties with Si and simple vapor arrives at the surface. This self- the smooth, amorphous films needed for processing. SiO2 has already been terminating character results in ALD’s gate dielectrics. replaced by aluminum oxide (k~8) in uniformity, conformality and precise ALD precursors for metal oxides are DRAMs, tantalum oxide (k~20) in decou- thickness control. In practice, the deposi- generally classified as metal precursors pling filters and silicon oxynitride (k~6) in tion per cycle is usually half of a mono- and oxygen sources. Metal precursors

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PROCESS GASES, CHEMICALS AND MATERIALS SECTION 7 Precursors For Atomic Layer Deposition Of High-k Dielectrics

(MLn) generally contain one metal atom R M bound to n ligands L and can be R categorized by the types and number of X C CR atoms directly bound to the metal center. M O O (see Figure 1). Selecting precursors with CR all of the desired properties has not been M M O straightforward because relatively little Halides where data is available, and detailed ALD x=Cl, I, Br Alkoxides β-diketonates chemistry has not been well understood. R RN The dominant ALD reactions forming CR R high-k metal oxides appear to be ligand- exchange reactions[5] (see Figure 2). M N R When water is used as the oxygen source, M NR M as is common in ALD of high-k M dielectrics, ligand-exchange involves breaking the metal-ligand bonds of the Alkylamides Amidinates Alkyls Cyclopentadienyls precursor and an O-H bond, and forming Figure 1: Precursor types. The R’s represent alkyl groups consisting of carbon and an M-O bond and a L-H bond. The hydrogen, such as methyl (CH3) or ethyl (C2H5). strengths of the bonds that dissociate and form during the ALD reactions directly determine the thermodynamics of the L L reaction and, less directly, influence L the rates of reaction. Precursors with M strong L-H bonds (again, when H2O is used as the oxygen source) and weak M- OH L O L bonds have strongly exothermic reactions, tending to make pure, amor- + M + HL phous films at low temperatures. L Furthermore, unwanted side reactions L L will be less likely, and the process will usually be faster. However, the metal- Figure 2: Schematic of a ligand exchange reaction for a ML4/H2O ALD process. ligand bond must be sufficiently strong for the precursor to be stable. Another important precursor property ALD precursor for TiO2 and TiN. On the atom, and ß-diketonates (M=(O2C3R3)n, is its size. Ligands are attracted to each other hand, the metal chlorides have such as Zr(thd)4, where each diketonato other through weak van der Waals strong M-Cl bonds, resulting in high ligand is bound to the metal through interactions and thus bulky ligands activation barriers, ALD temperatures two metal-oxygen bonds (the ligand generally lower precursor volatility. and Cl contamination of HfO2, which “chelates” the metal center). Alkoxide However, bulky ligands can also shield causes failure after the required anneal. precursors already possess M-O bonds the metal center from bonding interac- The high ALD temperature may also lead and consequently, ligand exchange tions between precursors, thereby to low observed deposition per cycle due reactions with water maintain the same increasing precursor volatility. Because of to reduction of active OH surface sites. number of M-O and O-H bonds. Thus these competing effects there may be an Furthermore, the metal chlorides produce these reactions have little enthalpic optimum ligand size that maximizes the HCl as a byproduct, which etches the film driving force. The strong dative bonds precursor volatility. Large ligands can leading to non-uniform film thicknesses, between the metal center and surface OH also restrict the packing density of and corrodes reactors. The metal groups and the alkoxo O atom and precursors in the saturated precursor chlorides are the most thoroughly surface metal atoms lead to strongly monolayer. However, more complete investigated high-k precursors. Fluorides bound intermediates. Consequently, ligand exchange reactions during precur- of Zr, Hf and La are not suitable for ALD alkoxide precursors require relatively sor exposure reduce the number of because they have very low volatility, and high ALD temperatures. Alkoxo ligands ligands remaining in the monolayer, HF is a corrosive and hazardous must also be relatively bulky to achieve increasing the monolayer density. byproduct. Transition metal bromides and sufficient volatilities and low melting The metal halides consist of a metal iodides do not offer significant advan- points because the M-O-R structure of the atom directly bonded to halogen atoms tages over the chlorides. alkoxide does not screen the metal center (F, Cl, Br or I), for example HfCl4. Metal precursors with oxygen bonded from interacting with the oxygen atoms of Unfortunately, metal halides are usually to the metal include alkoxides (M-(O-C- nearby precursors. Alkoxides can deposit solids with low volatilities except, for R)n), such as hafnium tert-butoxide, both a metal atom and oxygen atom in a example, TiCl4, which is a liquid at room Hf(OC4H9)4, where each alkoxo ligand is single step when alternated with a temperature and a relatively successful bound to the metal atom through one O second metal precursor – for example, a >>

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SECTION 7 PROCESS GASES, CHEMICALS AND MATERIALS Precursors For Atomic Layer Deposition Of High-k Dielectrics

>> metal chloride. However, the kinetics of Organometallic precursors have metal opportunities in atomic layer deposition.” Solid State these reactions are relatively slow, and atoms bound directly to carbon, includ- Technology (2003), 46(5), 67-68, 71. some chloride impurity remains in the ing alkyls M(CxHy)n, such as trimethy- 3. Ritala, Mikko. “Atomic layer deposition.” Editor(s): product films. laluminum, Al(CH3)3; and cyclopentadi- Houssa, Michel. High-k Gate Dielectrics (2004), ß-diketonates are common precursors enyls, such as dicyclopentadienyldimethyl- 17-64. Publisher: Institute of Physics Publishing, for CVD and have been investigated hafnium, Hf(C5H5)2(CH3)2 (a mixed ligand Bristol, UK. for ALD of metal oxides. Because they precursor). Alkyls have not been studied 4. L. Niinistö, J. Päiväsaari, J. Niinistö, M. Putkonen and already possess two M-O bonds per extensively for high-k ALD, except for M. Mieminen, “Advanced electronic and ligand, water does not react with these ALD of Al2O3 using Al(CH3)3. Alkyls are optoelectronic materials by Atomic Layer precursors. Strong oxidizers, such as vulnerable to decomposition to metal Deposition: An overview with special emphasis on ozone, are required to break the strong hydrides and metal alkenes making many recent progress in processing high-k dielectrics and carbon-to-oxygen bonds, but carbon con- of them impractical for ALD. Metal other oxide materials,” Phys. Stat. Sol. (a) 201, 1443- tamination remains, leading to increased cyclopentadienyls (M(C5H5)n or M(Cp)n) 1452 (2004). leakage currents. Furthermore, ß-diketo- are being explored for metal ALD, but not 5. Widjaja, Yuniarto; Musgrave, Charles B. "Quantum nates generally have low volatilities and extensively for high-k ALD. Cp ligands are Chemical Study of the Elementary Reactions in slow vaporization kinetics because they good electron donors and metal Zirconium Oxide Atomic Layer Deposition." Applied are solids. cyclopentadienyls bound to good metal Physics Letters (2002), 81(2), 304-306. Precursors with nitrogen bonded to electron acceptors are relatively stable. 6. Hausmann, Dennis M.; Kim, Esther; Becker, Jill; the metal include metal alkylamides So we come to the bottom line. Which Gordon, Roy G. “Atomic Layer Deposition of (M(NR2)n), such as hafnium dimethy- precursor is most suitable for high-k Hafnium and Zirconium Oxides Using Metal Amide lamide, Hf(N(CH3)2)4 and metal amidi- ALD? The answer depends on which Precursors.” Chemistry of Materials (2002), 14(10), nates (M(N2CR3)n), such as lanthanum metal compound you are depositing. 4350-4358. N,N’-diisopropylacetamidinate, where each Aluminum oxide is usually deposited 7. Kukli, Kaupo; Ritala, Mikko; Sajavaara, Timo; amidinate ligand chelates the metal center from trimethylaluminum and water or Keinonen, Juhani; Leskelä, Markku. “Atomic layer through two M-N bonds. Alkylamido ozone, while aluminum nitride is made deposition of hafnium dioxide films from hafnium precursors have relatively weak M-N more easily from aluminum dimethy- tetrakis(ethylmethylamide) and water.” Chemical bonds and strong byproduct N-H bonds, lamide, Al2(N(CH3)2)6, and ammonia. For Vapor Deposition (2002), 8(5), 199-204. lowering the ALD temperature, although ALD of hafnium oxide and hafnium 8. Lim, Booyong S.; Rahtu, Antti; de Rouffignac, sufficiently strong M-N bonds that the oxynitride, hafnium ethylmethylamide, Philippe; Gordon, Roy G. “Atomic layer deposition of precursors are stable at the normal Hf(N(CH3)(C2H5))4, is a good liquid lanthanum aluminum oxide nano-laminates for temperatures at which they are used. precursor, combining high reactivity electrical applications.” Applied Physics Letters Alkylamides are reactive to both water toward water, ozone and ammonia, with (2004), 84(20), 3957-3959. and ammonia, enabling nitrogen incorpo- sufficient volatility and stability.[6,7] ration into oxide films and even growth of Amidinates, such as lanthanum N,N’- metal nitrides without a plasma. diisopropylacetamidinate, show a unique Biographies Furthermore, the ligands of alkylamides high reactivity to non-oxidized silicon are particularly good at preventing surfaces, which allows growth without a Charles Musgrave neighboring precursors from bonding to low-k interlayer on a silicon surface. Charles Musgrave is an assistant the metal center, and so alkylamides have Addition of some aluminum oxide to professor of chemical engineering at high volatilities as well as low melting lanthanum oxide provides excellent Stanford University. He is known for his points, so most are liquids at room environmental stability. The high band use of quantum chemical methods to temperature. offset of lanthanum aluminate has allowed determine the detailed chemical ALD of oxides of lanthanum and other very low leakage currents to be obtained mechanisms important in various lanthanide metals, such as praseodymi- for ALD lanthanum aluminate films on semiconductor processes, including ALD. um, has been difficult because of the low silicon.[8] Roy Gordon volatility and stability of most of their Roy Gordon is professor of chemistry at compounds. Lanthanide amidinates are References Harvard University. His group has the most volatile lanthanum compounds 1. Leskelä, Markku; Ritala, Mikko. “Atomic layer introduced a number of new precursors known, although they are solids. Because deposition chemistry: recent developments and and processes for ALD of metals, metal the amidinato ligand chelates the metal future challenges.” Angewandte Chemie, nitrides and metal oxides that are being center, they are thermally stable, yet have International Edition (2003), 42(45), 5548-5554. developed for applications in the the high and self-limited reactivity to 2. Seidel, Tom; Londergan, Ana; Winkler, Jerald; Liu, semiconductor industry. water vapor needed for ALD. Xinye; Ramanathan, Sasangan. “Progress and

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