Raluca Tiron

Spin-spin cross relaxation in single-molecule magnets

The one-body tunnel picture of single-molecule magnets (SMMs) is not always sufficient to explain the measured tunnel transitions. An improvement to the picture is proposed by including also two-body tunnel transitions such as spin-spin cross relaxation (SSCR) which are mediated by dipolar and weak superexchange interactions between molecules. A Mn4 SMM is used as a model system. At certain external fields, SSCRs lead to additional quantum resonances which show up in hysteresis loop measurements as well-defined steps. A simple model is used to explain quantitatively all observed transitions.

Spin quantum tunneling via entangled states in a dimer of exchange-coupled single-molecule magnets

A new family of supramolecular, antiferromagnetically exchange-coupled dimers of single-molecule magnets (SMMs) has recently been reported. Each SMM acts as a bias on its neighbor, shifting the quantum tunneling resonances of the individual SMMs. Hysteresis loop measurements on a single crystal of SMM dimers have now established quantum tunneling of the magnetization via entangled states of the dimer. This shows that the dimer really does behave as a quantum mechanically coupled dimer, and also allows the measurement of the longitudinal and transverse superexchange coupling constants.

Quantum tunneling in a three-dimensional network of exchange-coupled single-molecule magnets

A Mn 4 single-molecule magnet is used to show that quantum tunneling of magnetization is not suppressed by moderate three-dimensional exchange coupling between molecules. Instead, it leads to an exchange bias of the quantum resonances which allows precise measurements of the effective exchange coupling that is mainly due to weak intermolecular hydrogen bonds. The magnetization versus applied field was recorded on single crystals of [Mn 4 ] 2 using an array of micro-superconducting quantum interference devices. The step fine structure was studied via minor hysteresis loops.

Optimization of block copolymer self-assembly through graphoepitaxy: A defectivity study

In this paper we report a synoptic methodology to evaluate and optimize the long-range order induced by graphoepitaxy of block copolymer (BCP) self-assembly. The authors focus the study on a BCP that produces hexagonally packed arrays of cylinders oriented perpendicular to the substrate with the copolymer film thickness greater than the trench depth. Prepatterned structures used in the graphoepitaxy approach have been generated by e-beam lithography on a commercial hydrogen silesquioxane resist. A suitable surface modification was accomplished by grafting a random polystyrene-r-poly(methyl methacrylate) copolymer on the prepatterned surfaces. The polystyrene-b-poly(methyl methacrylate) was spin-coated and annealed in order to generate the desired self-assembly. Since the self-assembly process is based on a thermodynamic mechanism, the induced defectivity needs to be reassessed with respect to the standard lithographic process. Using the cylinder center coordinates, a Delaunay triangulation is performed to find the nearest neighbors. This triangulation enables us to easily locate the disclinations which are characterized by having a number of nearest neighbors different from six. Thus, the number of defects can be quantified precisely. Additionally, this methodology affords an accurate evaluation of both the optimum mesa and trench critical dimensions yielding defect-free surfaces and may be extended to monitor the robustness of the BCP directed self-assembly process. Such diagnostics are critical in the implementation of large scale industrial processes.

Scaling-down lithographic dimensions with block-copolymer materials: 10-nm-sized features with poly(styrene)-block-poly(methylmethacrylate)

Abstract. Poly(styrene)-block-poly(methylmethacrylate) (PS-b-PMMA) block-copolymers (BCP) systems synthesized on an industrial scale and satisfying microelectronic’s requirements for metallic contents specifications are studied in terms of integration capabilities for lithographic applications. We demonstrate in particular that this kind of polymer can efficiently achieve periodic features close to 10 nm. These thin films can be transferred in various substrates through dry-etching techniques. The self-assembly optimization for each polymer is first performed on freesurface, leading to interesting properties, and the changes in self-assembly rules for low molecular-weight polymers are investigated and highlighted through different graphoepitaxy approaches. The improvements in self-assembly capabilities toward low periodic polymers, as well as the broad range of achievable feature sizes, make the PS-b-PMMA system very attractive for lithographic CMOS applications. We conclude by showing that high-χ polymer materials developed in Arkema’s laboratories can be efficiently used to reduce the pattern’s size beyond the ones of PS-b-PMMA based BCP’s capabilities.

Pattern density multiplication by direct self assembly of block copolymers: toward 300mm CMOS requirements

In this paper we investigate the possibility to reach 300mm CMOS requirements by integrating graphoepitaxy of PS-b-PMMA self-assembly. Different schemes to integrate DSA process by using 193nm dry lithography or e-Beam lithography will be presented. Moreover, several challenges like solvent compatibility, bake kinetics and defectivity will be addressed. Concerning defectivity, we will propose a methodology in order to evaluate and optimize the long range order induced by graphoepitaxy of the block copolymer DSA. This approach affords the monitoring of the overall block copolymer self-assembly process and enables us to easily optimize the parameters required for a long-range order structuration, leading to a near zero-defects block copolymers self-assembled arrays. Transfer capabilities of the PS masks in the bulk silicon substrate by using plasma-etching will be also detailed, both with the film on bare silicon or organized with graphoepitaxy approaches. These results show the high potential of DSA to be integrated directly into the conventional CMOS lithography process in order to achieve high resolution and pattern density multiplication, at a low cost.

Contact hole shrink by directed self-assembly: Process integration and stability monitored on 300 mm pilot line

The semiconductor devices dimensions continue to shrink to keep up with the ITRS roadmap. Due to delay and extensive cost of EUV for 14 nm technology node and beyond, the directed self assembly (DSA) process has great potential for extending optical lithography, and enables to reduce the critical dimension (CD) and pitch of the final feature. After the recent implementation of DSA processes in 300 mm clean room environment, it is now time to move to the forward maturity step and demonstrate process stability through time. This study investigates the potential of DSA for contact hole shrink patterning using poly(styrene-block-methyl methacrylate) (PS-b-PMMA) di-block copolymers to target contact holes CD down to 15 nm. Based on the 300 mm pilot line available at LETI, the DSA manufacturability is considered through different criteria to achieve high resolution and pattern density multiplication, at a low cost in fully 300 mm wafers production line. The DSA process flow performance based on grapho-epitaxy approach is controlled after each step to follow the thicknesses of random and BCP materials supplied by ARKEMA. Moreover, the natural period of block copolymers and CD uniformity on free surface are also measured and defectivity is evaluated after etch transfer by image treatment. The thermal budget of DSA of both random and block copolymers have been evaluated to define optimum conditions. The paper has shown that UV exposure prior to PMMA wet development improves PMMA degradation to enable complete removal by wet development in acetic acid. DSA process for contact hole shrink patterning has shown final contact holes with an average CD of 21 nm and intra-wafer CD uniformity of 1.1 nm with an open yield of more than 99.9%.

Guided self-assembly of block-copolymer for CMOS technology: a comparative study between grapho-epitaxy and surface chemical modification

Recent progress in Block Copolymer lithography has shown that guided self-assembly is a viable alternative for pushing forward the resolution limits of optical lithography. The main two self assembly methods considered so far have been the surface chemical modification, which is based on the chemical modification of a brush grafted to the silicon, and the grapho-epitaxy, which is based on creating topographic patterns on the surface. We have tested these two approaches for the 22 nm node and beyond CMOS technology, using PS-PMMA block copolymers synthesized by RAFT (Reversible Addition-Fragmentation Chain Transfer) polymerization.

Quantum dynamics of exchange biased single-molecule magnets

We present a new family of exchange biased single molecule magnets in which antiferromagnetic coupling between the two components results in quantum behavior different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs.

Hexacyanometalate molecular chemistry: trinuclear CrNi2 complexes; micro-SQUID magnetisation studies of intermolecular interactions

Abstract Three different CrNi 2 complexes were synthesised. They differ from each other by the nature of the terminal ligand and of the counter anion: [Cr(CN) 4 {CN Ni(tetren)} 2 ]Cl, that crystallises in two different crystallographic systems and [Cr(CN) 4 {CN Ni(dienpy2)} 2 ](ClO 4 ). The ground state spin value is 7/2 for the three systems (ferromagnetic interaction between chromium(III) and nickel(II) ions). The magnetisation of the three CrNi 2 complexes was measured using an array of micro-SQUIDs in a temperature range between 0.04 and 7 K, under a magnetic field up to ±1 T. The three samples present a three-dimensional magnetic ordering. The correlation between the structure and the intermolecular coupling is analysed in terms of steric hindrances of the terminal ligand, orientation of the molecules in the unit cell (canted structure) and crystal symmetry.