Area-Selective ALD of Titanium Dioxide Using Lithographically Defined Poly(methyl methacrylate) Films

Abstract

An approach to area-selective atomic layer deposition techniques based on the use of a lithographically definable polymeric masking layer has been reported. Successful direct patterned deposition of TiO 2 is demonstrated using a poly͑methyl methacrylate͒ masking layer that has been patterned using deep-UV lithography. A number of factors which must be considered in designing patternable polymeric masking materials and processes have been determined and are briefly discussed, including reactivity of the polymer with the atomic layer deposited ͑ALD͒ precursor species, diffusion of ALD precursors through the polymer mask, and remnant precursor content in the masking film during ALD cycling.Atomic layer deposition ͑ALD͒ is gaining significant attention as an alternative technique for depositing high-quality, ultrathin films. 1,2 This method is particularly useful for producing extremely thin, high-quality, conformal films with thicknesses in the 3-10 nm range where other deposition techniques such as chemical vapor deposition ͑CVD͒ have significant limitations. During ALD, film growth depends critically on the chemistry of the surface upon which deposition occurs. Thus, it should be possible to chemically tailor a surface to achieve area-selective deposition. Selective ALD requires that designated areas of the surface be masked or "protected" to prevent the ALD reaction from occurring in those selected areas, thus ensuring that the ALD film nucleates and grows only on the desired unprotected regions. One obvious advantage of such an additive, area-selective deposition process is the ability to perform direct patterned growth, thereby eliminating the need for etching and the associated subsequent cleaning steps. Elimination of these steps can greatly simplify the overall deposition and patterning process, reduce unintended damage to substrates and devices which result from the use of energetic plasma etch processes, and aid in the integration and patterning of new materials which are difficult to etch.The critical challenge in achieving area-selective ALD is devising materials and methods for modifying selected regions of a substrate surface to prevent ALD reactions from occurring, thus preventing film growth. One way to modify a surface is to chemically bond a molecule directly to the surface. Such an approach blocks reactive sites that are present and thereby prevents reactions between precursor molecules and the surface. Alkyl silanes, which contain long hydrocarbon chains terminated with a reactive silane end group, are a well-known surface-modifying agent. In the case of alkyl silanes, the hydrocarbon chains are relatively unreactive and thus provide a good protective or passivating coating for the surface. For example, octadecyltrichlorosilane ͑OTS͒ forms a densely packed, self-assembled monolayer ͑SAM͒ and has been widely investigated as a surface-modifying agent to block nucleation and growth of a variety of inorganic films such as HfO 2 , ZnO, TiO 2 , ZrO 2 . 3-5 Previous studies 3,4 have demonstrated direct ...

Nanopatterning by Area‐Selective Atomic Layer Deposition

“…Reported AS-ALD processes using polymers are summarized in Table 9.2 [39,54,55,[76][77][78][79][80][81][82][83]. In early studies, researchers investigated the blocking ability of polymer layers against various ALD systems.…”

“…In early studies, researchers investigated the blocking ability of polymer layers against various ALD systems. Sinha et al have published several papers on TiO 2 AS-ALD using poly(methyl methacrylate) (PMMA) as a deposition deactivator [76][77][78][79]. They employed titanium tetrachloride (TiCl 4 ) as precursor and water as a reactant [77].…”

“…Sinha et al have published several papers on TiO 2 AS-ALD using poly(methyl methacrylate) (PMMA) as a deposition deactivator [76][77][78][79]. They employed titanium tetrachloride (TiCl 4 ) as precursor and water as a reactant [77]. TiO 2 deposition was observed even on polymer-covered regions of the substrate, and the amount of Ti was dependent on the thickness of the polymer, indicating that precursors penetrated into the PMMA layer, and then reacted at the interface between PMMA and the substrate [77].…”

“…They employed titanium tetrachloride (TiCl 4 ) as precursor and water as a reactant [77]. TiO 2 deposition was observed even on polymer-covered regions of the substrate, and the amount of Ti was dependent on the thickness of the polymer, indicating that precursors penetrated into the PMMA layer, and then reacted at the interface between PMMA and the substrate [77]. In a subsequent paper, they focused on effects of precursor chemistries on the blocking capability of PMMA by using two different Ti precursors, TiCl 4 and titanium isopropoxide ( [78].…”

“…Many early researchers employed this solution-based deposition as summarized in Table 9.3 because it is very simple as well as easy to form high-quality SAMs; in those studies, the SAMs fully coated the entire surface without patterns, allowing the efficacy of deactivation to be investigated [39-42, 45, 51, 52, 54, 60, 61]. For polymer film fabrication, spin coating has typically been used [39,54,55,[76][77][78][79][80][81][82][83]. In spin coating, a polymer is dissolved in an appropriate solvent, and the solution is dropped onto a surface undergoing rotation.…”