Sungwoo Chun

A flexible graphene touch sensor in the general human touch range

We present a transparent touch sensor based on single layers of graphene that works under a gentle touch. Using the flexible characteristics of graphene, a touching event and a vertical force are measured by a change in the channel conductance. In contrast to the previous graphene gauge sensors, this is an alternative scheme that responds to a vertical force using the contacting properties of two isolated and patterned single graphene layers. This sensor responded to pressures ranging from 1 to 14 kPa, corresponding to the lowest human perception. In addition, we outline the processing methods for handling single layers of graphene for the integration of devices on transparent and flexible substrates.

Multi-step ion beam etching of sub-30 nm magnetic tunnel junctions for reducing leakage and MgO barrier damage

We have demonstrated the fabrication of sub 30 nm magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy. The multi-step ion beam etching (IBE) process performed for 18 min between 45° and 30°, at 500 V combined ion supply voltage, resulted in a 55 nm tall MTJ with 28 nm diameter. We used a negative tone electron beam resist as the hard mask, which maintained its lateral dimension during the IBE, allowing almost vertical pillar side profiles. The measurement results showed a tunnel magneto-resistance ratio of 13% at 1 kΩ junction resistance. With further optimization in IBE energy and multi-step etching process, it will be possible to fabricate perpendicularly oriented MTJs for future sub 30 nm non-volatile magnetic memory applications.

Negative electron-beam resist hard mask ion beam etching process for the fabrication of nanoscale magnetic tunnel junctions

The authors have demonstrated fabrication of 30 nm diameter perpendicular anisotropy magnetic tunnel junctions (MTJs) using negative electron-beam resist (NER) as the ion beam etching (IBE) hard mask. The NER pillar of 30 nm diameter and 105 nm thickness was fabricated by electron-beam lithography. The redeposition of the MTJ etching debris generated during the IBE on the outer surface of the NER pillar increased the lateral etch resistance of the resist polymer, allowing the edge profile to remain constant for the duration of the MTJ etching, resulting in a vertical MTJ sidewall profile. A multistep IBE (repetition of 45° primary etching and 30° secondary etching) was conducted to reduce the MTJ sidewall redeposition while reducing mechanical damage. The measurement results showed a tunnel magneto-resistance ratio of 22% at 30 nm junction diameter.

Enhancing the quality of transferred single-layer graphene with poly(4-vinylphenol) interlayer on flexible substrates

We report the use of poly(4-vinylphenol) (PVP) as a promising contact surface of transferred graphene, capable of sustaining the original performance found in as-grown graphene. Enhancement of surface tension obtained by O2 plasma treatment of the PVP surface also increases transferred graphene quality. With an easy coating method, PVP can be applied to any flexible substrate as the interlayer to increase transferred graphene quality. Owing to the mechanical flexibility and chemical inertness of PVP, the introduction of a PVP interlayer provides a general method for graphene soft electronics to be integrated into any flexible substrate.

High-Density Physical Random Number Generator Using Spin Signals in Multidomain Ferromagnetic Layer

A high-density random number generator (RNG) based on spin signals in a multidomain ferromagnetic layer in a magnetic tunnel junction (MTJ) is proposed and fabricated. Unlike conventional spin-based RNGs, the proposed method does not require one to control an applied current, leading to a time delay in the system. RNG demonstrations are performed at room temperature. The randomness of the bit sequences generated by the proposed RNG is verified using the FIPS 140-2 statistical test suite provided by the NIST. The test results validate the effectiveness of the proposed RNGs. Our results suggest that we can obtain high-density, ultrafast RNGs if we can achieve high integration on the chip.

Fabrication of on-chip fluidic channels incorporating nanopores using self-aligned double layer resist processing technique

The authors report on the development of a self-aligned double layer resist processing technique that allows incorporation of ion channel nanopores into on-chip microfluidic channels. The patterned positive/negative electron-beam resist double layer acts as a sacrificial template for the fabrication of on-chip fluidic channels and the nanopores. By controlling the resist dimensions, it was possible to tailor the shape of the on-chip fluidic channel and the nanopore dimensions. Using this technique, the authors demonstrated the fabrication of sub-10 nm nanopore arrays on 2 μm wide and 800 nm high on-chip fluidic channels. With further developments, it will be possible to have controllable on-chip nanopores with integrated nanofluidics.