(251s) A Block Copolymer Self-Assembly Approach for Deterministic Doping of Semiconductors

**Note to Chair**The presenting author, Bhooshan C. Popere, is a faculty candidate.

As the dimensions of integrated circuit elements continue to shrink and new device architectures are developed, there is a vast need for new technology to demonstrate reliable nanoscale doping with well-defined and uniformly doped ultrashallow junctions. Specifically, as devices (e.g. MOSFETs) are scaled down to sub-30 nm channel lengths, the stochastic nature of dopant distribution in the channel poses severe limitations on the reproducibility of the electrical characteristics of the devices. Thus, reliable manufacturing of new ultra small-scale devices requires deterministic doping, i.e. control over the exact position of dopant atoms in semiconductor structures to produce the necessary ultrashallow, doped junctions with well-defined insulating areas. The conventional ion implantation process falls short with regards to the scaling needs of the semiconductor industry due not only to poor control over the spatial distribution of the implanted ions, but also due to the inability to achieve an implantation range and abruptness down to the nanometer length scale (limited by lithographic resolution and pattern fidelity). Additionally, the conventional ion implantation process entails severe crystal damage that can be ameliorated by thermal annealing, but only at the cost of non-uniform junctions resulting from uncontrolled thermal diffusion of the implanted ions.

Presently, such deterministic control is achieved only through energy-intensive and small-scale proof-of-concept processes like single-ion implantation and molecular beam epitaxy (MBE). To address this fundamental design limitation, we have developed a modular approach relying on the self-assembly of block copolymers to achieve deterministic doping in semiconductors on a technologically relevant ‘nanoscopic’ length scale. Briefly, an organic dopant precursor small molecule (SM) is encapsulated via hydrogen bonding into the interior of the micellar aggregates of a model block copolymer [polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP)] formed in solution. The hydrogen bonding between SM and P4VP provides the driving force for the incorporation of the dopants in the micelles. This eliminates the need for tailored syntheses for different polymer/dopant compositions, thereby making our approach highly modular. Thin films of these polymer/SM aggregates demonstrate a quasi-hexagonal close packed assembly of spheres wherein, each micelle contains a predetermined concentration of dopant atoms. Consequently, shallow doped regions (5-7 nm deep), ordered in a quasi-hexagonal close packed array (30-70 nm periodicity), are achieved via spike rapid thermal annealing (spike-RTA). During this process, the encapsulated dopant atoms are driven into the underlying substrate while the polymer film burns away. Our experiments reveal that the position of the dopants atoms in the semiconductor and hence, the deterministic nature of the doping process, is a direct consequence of the periodic morphology of the dopant infused block copolymer, which in turn depends on the molecular weight and the composition of the block copolymer.The modular nature of our approach enables precise control over multiple dopant types with concentrations between ~2 x 10¹⁹ atoms/cm³ and 9 x 10¹⁹ atoms/cm³.  We find that a macroscopic substrate property such as resistivity bears a direct correlation to the microscopic property of the block copolymer i.e. domain spacing indicating that the doped regions are indeed discrete and nanoconfined, which is supported by electron micrographs of doped substrates. Finally, the simultaneous control of 3D dopant positions and concentrations enables our approach to lend itself to the fabrication of various device features including the channel and the source/drain contacts for MOSFETs and electrical insulation trenches, among many others.