Tomohiko Iijima

Conduction Tuning of Graphene Based on Defect-Induced Localization

The conduction properties of graphene were tuned by tailoring the lattice by using an accelerated helium ion beam to embed low-density defects in the lattice. The density of the embedded defects was estimated to be 2-3 orders of magnitude lower than that of carbon atoms, and they functionalized a graphene sheet in a more stable manner than chemical surface modifications can do. Current modulation through back gate biasing was demonstrated at room temperature with a current on-off ratio of 2 orders of magnitude, and the activation energy of the thermally activated transport regime was evaluated. The exponential dependence of the current on the length of the functionalized region in graphene suggested that conduction tuning is possible through strong localization of carriers at sites induced by a sparsely distributed random potential modulation.

Gate-Controlled P–I–N Junction Switching Device with Graphene Nanoribbon

A graphene P–I–N junction switching device with a nanoribbon is proposed, which was aimed at finding an optimized operation scheme for graphene transistors. The device has two bulk graphene regions where the carrier type is electrostatically controlled by a top gate, and these two regions are separated by a nanoribbon that works as an insulator, resulting in a junction configuration of (P or N)–I–(P or N). It is demonstrated that the drain current modulation strongly depends on the junction configuration, while the nanoribbon is not directly top-gated, and that the device with a P–I–N or N–I–P junction can exhibit better switching properties.

Electrostatically-reversible polarity of dual-gated graphene transistors with He ion irradiated channel: Toward reconfigurable CMOS applications

We found that a transistor with a graphene channel irradiated with He ion beams can have a transport gap of up to 380 meV. We made novel dual-gated transistors using such a channel and obtained an on-off ratio up to 103 at 200 K. This novel device has a channel region between dual gates, and the polarity of the transistor (n- or p-type) can be electrostatically reversed by simply flipping the bias polarity of one of the dual gates.

Tungsten-based pillar deposition by helium ion microscope and beam-induced substrate damage

The authors use a helium ion microscope (HIM) equipped with a tungsten hexacarbonyl gas injection system (GIS) to form tungsten-based pillars on carbon and silicon substrates by helium ion beam-induced deposition. Tungsten-based pillars with a width of ∼40 nm and height of ∼2 μm (aspect ratio of ∼50) are successfully fabricated using the HIM-GIS method. The pillars consist of face-centered cubic WC1−x and/or W2(C, O) grains. Columnar voids with a width of 1–15 nm form in the center of the pillars, suggesting that the pillars are continuously sputter-etched by the incident helium ion beam during deposition. In addition, the authors observe beam irradiation damage in the form of blistering of the Si substrate at the interface between the pillar and Si substrate. The columnar void width and Si blister height decreases as the volumetric growth rate of the pillars increases regardless of the deposition parameters. The authors consider that at least three phenomena compete during pillar formation, namely pillar...

High-precision alignment of electron tomography tilt series using markers formed in helium-ion microscope

Tungsten nanodots formed in a helium-ion microscope (HIM) provide a practical means of aligning markers of electron tomography tilt series with a high degree of precision. The nanodots were formed using a HIM equipped with a W(CO)6 gas injection system, enabling the precise placement of the nanodots at desired locations of a sample. Template matching was applied to the markers formed in the HIM to detect the positions automatically. The relation between the positions of the markers and the accuracy of the alignment was also determined in order to achieve precise alignment. The method was applied to the markers in order to reconstruct three-dimensional (3D) images of a rod-shaped specimen that contained a 65-nm-diameter via structure in a Cu/Low-k interconnect.

Electrostatically Reversible Polarity of Dual-Gated Graphene Transistors

We developed dual-gated graphene transistors in which the transistor polarity (n-type or p-type) is electrostatically reversible by the gate bias of one of the top gates. In this device, a channel is defined as the region between a pair of top gates, where graphene is irradiated by an accelerated helium ion beam to form a defect-induced transport gap. This device features not only a large current ON-OFF ratio of four orders of magnitude but also unipolarity of transistors, which would otherwise be ambipolar. We also show how these polarity-reversible transistors can be used in logic circuits.

Position-controlled marker formation by helium ion microscope for aligning a TEM tomographic tilt series

We formed nano-dots using a helium ion microscope (HIM) equipped with a gas injection system. Because of position controllability, the nano-dot markers could be placed efficiently on a specimen using the HIM. The sizes of the dots were controlled by changing the beam radiation time. We tried for the first time to form dots on a rod-shaped specimen to use them as markers for aligning a transmission electron microscope tomographic tilt series before reconstructing 3D images.

Helium ion microscope characterization for Cu / low-k interconnects - SE imaging and focused helium ion beam luminescence detection -

Several novel imaging modes of the recently developed helium ion microscope (HIM) 1) were explored that may make the HIM a tool of particular value to Cu / low-k (dielectric constant) interconnect structures. Mechanism of the “through dielectric” (2) imaging of the Cu interconnects underneath the low-k SiOC film was proposed, and materials contrast in the low-k regions between Cu lines was imaged which might reflect damaged low-k areas. Furthermore possibility of detection of luminescence induced by the focused helium ion beam using the HIM for materials property characterizations was studied for the first time.

Beam-substrate interaction during tungsten deposition by helium ion microscope

We deposited tungsten-based pillars on ~300 nm-thick amorphous carbon and single-crystalline silicon substrates by a helium ion microscope (HIM) using tungsten hexacarbonyl (W(CO)6) as a gaseous precursor. We then investigated beam-induced damage to the substrates correlated with both pillar growth rate and material type of substrates. Faster pillar growth reduced the substrate damage because the pillars shielded the substrates from the incident beam, resulting in a low-damage process. On the other hand, the Si substrate was significantly damaged by the incident beam compared with the carbon substrates. This is because stopping cross-section of 30-ke V helium ion in silicon is ~1.5 times higher than that in carbon. The incident helium ions were considered to induce the substrate damage in the process of losing energy in the substrates.

Luminescence from SiO2 by Helium Ion Microscope (HIM) without any Damage Characterized by TEM-EELS

【背景】ヘリウム(He)イオン顕微鏡(Ion Microscope)(HIM)を用いた低誘電率膜やレジスト パターンに対してダメージ(材料構造変化)の少ない二次電子像観察技術、金属ピラー形成技術 を報告してきた[1-3]。しかしながら He イオンビーム(0.25nm 径)と試料との衝突現象、二次電 子発生機構の理解は不十分であり、He イオンビーム照射条件により試料表面でエッチング、ブリ スタリングが生じる場合もあり、照射条件には注意が必要である。本報告では HIM を用いて SiO2 膜に He イオンビームを標準的な二次電子像観察条件で照射した場合の SiO2膜からのルミネッセ ンス発光現象、およびその照射条件下での SiO2膜のダメージの有無を評価した結果を述べる。 【実験方法】Si 基板上に熱酸化膜を 400nm 形成し、標準的な二次電子像観察条件(加速エネルギ 30kV,イオンドーズ量 1E13~5E14/cm)で He イオンビームを照射し、SiO2膜からのルミネッセン ス、照射後の SiO2試料の TEM EELS 評価[4]を行った。 【結果と考察】ルミネッセンスは SEM CL で観察される波長 672 nm(1.85eV)以外に 281nm(4.41eV), 447nm(2.77eV)の発光が観察された(Fig.1)。447nm は He 原子スペクトルに一致するが、発光ピ ークはブロードであるため SiO2材料に起因するルミネッセンスと考えている。発光強度はドーズ 量と共に増大するが波長に変化は見られないため、この照射条件の範囲ではダメージはないもの と考えられる。TEM観察ではブリスタリングに起因するボイドは一切観察されず、Valence(V)-EELS、 ELNES による評価の結果、ダメージが存在すれば変化する 4~10eV(Fig2.)、100~120eV 領域の スペクトル形状に変化は見られなかった。したがって標準的な観察条件では EELS で検知できるレ ベルのダメージは生じていないと考えられる。ルミネッセンス発光機構に関して当日報告する。 【参考文献】[1] S. Ogawa, et al, Jpn. J. Appl. Phys., 49 (2010) 04DB12, [2] S. Ogawa, et al, Proc. of International Interconnect Technology Conference (2011), [3] K. Kohama et al, 2013 年春季応物 28a-G6-1, [4] Y. Otsuka, et al, Jpn. J. Appl. Phys., 49 111501 (2010)