FF18_Musgrave_wp_05.qxd 12/22/04 10:24 AM Page 2 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 126 | FUTURE FAB International | Issue 18 FF18_Musgrave_wp_05.qxd 12/22/04 10:25 AM Page 3 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 >> FUTURE FAB International | Issue 18 | 127 FF18_Musgrave_wp_05.qxd 12/22/04 10:26 AM Page 4 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.