Amphoteric Aqueous Hafnium Cluster Chemistry

Abstract

Selective dissolution of hafnium-peroxo-sulfate films in aqueous tetramethylammonium hydroxidee nables extreme UV lithographic patterning of sub-10 nm HfO 2 structures.H afnium speciation under these basic conditions (pH > 10), however,i su nknown, as studies of hafnium aqueous chemistry have been limited to acid. Here,w er eport synthesis,crystal growth, and structural characterization of the first polynuclear hydroxo hafnium cluster isolated from base, [TMA] 6 [Hf 6 (m-O 2 ) 6 (m-OH) 6 (OH) 12 ]·38 H 2 O. The solution behavior of the cluster,i ncluding supramolecular assembly via hydrogen bonding is detailed via small-angle X-ray scattering (SAXS) and electrospray ionization mass spectrometry (ESI-MS). The study opens anew chapter in the aqueous chemistry of hafnium, exemplifying the concept of amphoteric clusters and informing ac ritical process in single-digit-nm lithography.Water-soluble molecular oxo-hydroxo metal clusters are recognized building blocks for deposition of functional thinfilm coatings,offering low energy and "green" processing. [1][2][3] Additionally,s elected hafnium-peroxo-sulfate clusters provide unprecedented dimensional control in nanolithography. [4,5] In this lithography,u nexposed oxo-hydroxo Hf regions dissolve in base,m eaning soluble hafnium species exist at high pH.Up to now,h owever,t he aqueous chemistry of hafnium has been documented only for strongly acidic conditions.I n mildly acidic conditions (pH ! 3), gels and precipitates result from rapid hydrolysis and condensation. [6,7] At low pH, oxohydroxo hafnium clusters are commonly stabilized and isolated with capping sulfate ligands,s ometimes linking clusters into 1, 2o r3 -dimensional networks.[ [12,13] Our interest in oxo-hydroxo hafnium speciation in base arises from the development step in lithographic patterning with the water-processed hafnium material known as HafSO x , formulated Hf 4 (OH) 6.4 (O 2 ) 2 (SO 4 ) 2.8 . [4,5,14] Nanolithography allows bulk semiconductors and metals to be shaped into computing devices with sub-10 nm features.[15] New inorganic resists,such as HafSO x ,offer ultra-high-resolution patterns. [6] There are two major steps in the patterning process with HafSOx:1)exposure,which causes dissociation of the peroxo ligand and induces condensation;and 2) development, where unexposed material is selectively dissolved in concentrated aqueous tetramethylammonium hydroxide (TMAH). The nature of the species produced in these basic solutions is unknown. This dissolution, coupled with stability of asimilar peroxo-phosphato-niobium cluster observed in both basic and acidic solutions [16] has led us to propose the concept of amphoteric clusters,w herein simple ligand sets stabilize as ingle cluster or similar clusters at the extreme ends of the pH scale.T his concept broadens the approach to cluster synthesis and study,asmost common cluster types exist across limited pH ranges.[17] Forexample,Group 13 polycations are currently known to exist in the narrow range pH 3-4; [18,19] V, Mo,and Wpolyoxom...

Differentiating Zr/HfIV Aqueous Polyoxocation Chemistry with Peroxide Ligation

“…Differentiating Hf IV and Zr IV complexation and speciation with different ligands is very important for the above-mentioned nuclear applications, as well as materials design and discovery. Of particular interest is Hf/Zr IV sulfate for understanding material self-assembly in water − and Hf/Zr IV sulfate-peroxide for nanolithography. − The sulfate and peroxide ligands exploited in Hf/Zr IV oxide deposition and lithography serve multiple roles in every step of the deposition/patterning/development processes. Specific roles include radiation response (peroxide), solubility (peroxide), and polymerization (sulfate) toward conformal surface coatings.…”

Peroxide-Promoted Disassembly Reassembly of Zr-Polyoxocations

“…The peroxide is converted to oxide upon radiation exposure and provides solubility contrast to create patterned coatings for fabrication of microelectronic devices . From highly alkaline solution, we have isolated a Hf-hexamer ring linked by bridging peroxide, which provided a model for understanding the development chemistry of HfO 2 lithography. Aside from the radiation response of the peroxide in lithography, its coordination structure and chemistry in the acidic deposition solutions and films has not been elucidated, due to lack of relevant structural information.…”