Due to their interesting physical properties, myriad operational regimes, small size, and industrial fabrication maturity, magnetic tunnel junctions are uniquely suited for unlocking novel computing schemes for in-hardware neuromorphic computing. In this paper, we focus on the stochastic response of magnetic tunnel junctions, illustrating three different ways in which the probabilistic response of a device can be used to achieve useful neuromorphic computing power.
Magnetic tunnel junctions (MTJs) provide an attractive platform for implementing neural networks because of their simplicity, nonvolatility and scalability. In a hardware realization, however, device variations, write errors, and parasitic resistance will generally degrade performance. To quantify such effects, we perform experiments on a 2-layer perceptron constructed from a 15 × 15 passive array of MTJs, examining classification accuracy and write fidelity. Despite imperfections, we achieve accuracy of up to 95.3 % with proper tuning of network parameters. The success of this tuning process shows that new metrics are needed to characterize and optimize networks reproduced in mixed signal hardware.
Directed self-assembly (DSA) of block copolymer thin films remains a promising alternative to achieve the resolution gains needed to enable dense patterning with sub-10 nm critical dimensions (CD). Yet, some significant challenges remain. Among others, two challenges stand out: one relating to the thermodynamic and kinetic conditions that lead to finite defect densities while the second relates to a scalability challenge to harness simultaneous gains in both resolution metrics: minimum line width and minimum pitch. Here we present a self-registered self-assembly process that employs a two-step DSA to address both the energetics of defect formation and the scalability limitations to achieve simultaneous gains in both pitch and line width when compared to the guiding patterns.
A block copolymer-directed self-assembly was combined with nanoimprint lithography to generate templates with rectangular patterns through an original double imprint process. A rotary e-beam tool was used to separately expose circumferential and radial line/space chemical contrast patterns with periodicities commensurate to the natural period of two lamellae-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymers. Line patterns are formed by directed self-assembly of PS-b-PMMA on chemical patterns on two separate submaster templates, one with circumferential lines to define concentric tracks, and a second template on which the block copolymer is used to form radial lines at constant angular pitch. The patterns are subsequently transferred to their underlying Si substrates to form submaster templates. Using two sequential nanoimprinting steps, the radial and circumferential submaster line patterns were combined into a final quartz master template with rectangular bits on circular tracks.
Directed self-assembly is emerging as a promising technology to define sub-20nm features. However, a straightforward
path to scale block copolymer lithography to single-digit fabrication remains challenging given the diverse material
properties found in the wide spectrum of self-assembling materials. A vast amount of block copolymer research for
industrial applications has been dedicated to polystyrene-b-methyl methacrylate (PS-b-PMMA), a model system that
displays multiple properties making it ideal for lithography, but that is limited by a weak interaction parameter that
prevents it from scaling to single-digit lithography. Other block copolymer materials have shown scalability to much
smaller dimensions, but at the expense of other material properties that could delay their insertion into industrial
lithographic processes. We report on a line doubling process applied to block copolymer patterns to double the
frequency of PS-b-PMMA line/space features, demonstrating the potential of this technique to reach single-digit
lithography.
We demonstrate a line-doubling process that starts with directed self-assembly of PS-b-PMMA to define line/space
features. This pattern is transferred into an underlying sacrificial hard-mask layer followed by a growth of self-aligned
spacers which subsequently serve as hard-masks for transferring the 2x frequency doubled pattern to the underlying
substrate. We applied this process to two different block copolymer materials to demonstrate line-space patterns with a
half pitch of 11nm and 7nm underscoring the potential to reach single-digit critical dimensions. A subsequent patterning
step with perpendicular lines can be used to cut the fine line patterns into a 2-D array of islands suitable for bit patterned
media. Several integration challenges such as line width control and line roughness are addressed.
We combine block copolymer directed self-assembly with nanoimprint lithography to generate templates with rectangular patterns through an original double imprint process. We use a rotary e-beam tool to separately expose circumferential and radial line/space chemical contrast patterns with periodicities commensurate to the natural period of two lamellae-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymers. Line patterns are formed by
directed self-assembly of PS-b-PMMA on chemical patterns on two separate submaster templates, one with circumferential lines to define concentric tracks, and a second template on which the block
copolymer is used to form radial lines at constant angular pitch. The patterns are subsequently transferred to their underlying Si substrates to form submaster templates. Using two sequential
nanoimprinting steps, we combine the radial and circumferential submaster line patterns into a final quartz master template with rectangular bits on circular tracks.
Bit patterned media (BPM) for magnetic recording has emerged as a promising technology to deliver thermally stable
magnetic storage at densities beyond 1Tb/in2. Insertion of BPM into hard disk drives will require the introduction of
nanoimprint lithography and other nanofabrication processes for the first time. In this work, we focus on nanoimprint
and nanofabrication challenges that are being overcome in order to produce patterned media.
Patterned media has created the need for new tools and processes, such as an advanced rotary e-beam lithography tool
and block copolymer integration. The integration of block copolymer is through the use of a chemical contrast pattern on
the substrate which guides the alignment of di-block copolymers.
Most of the work on directed self assembly for patterned media applications has, until recently, concentrated on the
formation of circular dot patterns in a hexagonal close packed lattice. However, interactions between the read head and
media favor a bit aspect ratio (BAR) greater than one. This design constraint has motivated new approaches for using
self-assembly to create suitable high-BAR master patterns and has implications for template fabrication.
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