(722d) Investigation of High χ Block Copolymers for Directed Self-Assembly: Selective Block Removal of PS-b-Phost Patterns Via Selective ALD and Etch

Directed self assembly (DSA) of block copolymers (BCP) could enable pitch multiplication resulting in dense features with pitch less than 20 nm.  According to theory, microphase separation of block copolymers can only occur when the critical condition that χN>10.5 is met, where χ is the Flory Huggins interaction parameter and N is the total degree of polymerization for the block copolymer. In order to generate smaller DSA pattern pitches, the degree of polymerization of the block copolymer is reduced since this reduces the characteristic length scale for the polymer (e.g. radius of gyration). Thus, as N is reduced, the effect of this reduction on χN must be balanced by increasing χ to maintain a given level of phase separation. Currently, most DSA work has focused on the use of poly(styrene)-b-poly(methyl methacrylate) diblock copolymers (PS-b-PMMA) whose low χ value (i.e. ~0.04) limits the practical DSA pitch using such materials to approximately 20nm.  Poly(styrene)-block-poly(hydroxystyrene) diblock copolymers (PS-b-PHOST) are one promising alternative to PS-b-PMMA since it provides an estimated χ which is one order of magnitude larger than PS-b-PMMA.  However, this material lacks the etch contrast necessary for effective block removal using simple plasma etch processes directly, thus preventing pattern transfer into the underlying substrate.  This same limitation of poor etch contrast between the two blocks in diblock copolymers is common to many high χ polymer systems one might like to consider.  Therefore, part of our work has focused on developing post-phase separation processes that can be used to allow for selective block removal in such polymers.  In this paper, a novel process that utilizes area selective atomic layer deposition techniques (ASALDT) to selectively grow an etch barrier directly on the surface of one of the polymer blocks to enhance its etch resistance and permit relief pattern formation via plasma etching is discussed.  In particular, ALD has been used to directly grow titania (TiO₂) onto the surface of PHOST domains in PS-b-PHOST thin film phase separated patterns and will be discussed.  The high etch contrast between this TiO₂ layer and the PS domains that do not grow titania coatings allows selective removal of the PS domain with simple plasma etches (e.g. oxygen plasmas).  This selective ALD technique is first demonstrated and characterized on PHOST and PS homopolymer films with tetrakis(dimethylamido) titanium(IV)  (Ti-TDMA), and water precursors, which demonstrate linear controlled growth of TiO₂ (0.035 nm/cycle) on PHOST surfaces while the PS films exhibit no TiO₂ growth.  It will be shown that titania layers as thin as 2 nm provide sufficient etch resistance against oxygen plasmas to allow for highly anisotopic block removal of the PS block in DSA patterns formed using PS-b-PHOST.  An analysis of the line edge roughness of such grown titania films and the final etched relied patterns made using these titania patterns will be provided and compared to the roughness characteristics of the parent block copolymer microphase separated patterns that generate them.