M. Koh

Development of single-ion implantation — controllability of implanted ion number

The current status of developing single-ion implantation (SII) which enables us to implant dopant ions one by one into fine semiconductor regions is summarized. Single-ions extracted from a focused ion beam (FIB) have been implanted into a nuclear track detector CR-39 and the average number of implanted ions by chopping the ion beam has been evaluated by counting the etch pits on the CR-39 formed by each single-ion incidence. The detection efficiency of the single-ion incidence into a target has also been evaluated by comparing the count of secondary electrons and the implanted ion number, and the controllability of implanted ion number by SII is discussed.

New Process for Si Nanopyramid Array (NPA) Fabrication by Ion-Beam Irradiation and Wet Etching.

It has been found that the rate of Si etching by hydrazine (N2H4H2O) is drastically retarded by ion-beam exposure. By utilizing this new phenomenon, a simple process of fabricating nanopyramid arrays (NPAs) on a Si surface is proposed. Two-dimensional arrays of dots and lines are written directly on a Si substrate with 60 keV Si, P and BF2 ion beams at doses of 1013–1015 cm-2. Subsequently, the Si substrate is dipped in hydrazine solution, where unexposed regions are selectively etched by hydrazine. Using this simple process, 130 nm convex NPAs with 200 nm pitch and 40 nm concave NPAs with 150 nm pitch can be fabricated easily. It is shown that the electrical property of the apex of the pyramid can be controlled by dopant ion irradiation. The cause of the retarded etch rate of ion-beam-exposed Si by hydrazine is comprehensively discussed.

Reduction of Fluctuation in Semiconductor Conductivity by One-by-One Ion Implantation of Dopant Atoms

The inherent fluctuation of electrical properties in a fine semiconductor region has been successfully reduced for the first time by implanting a small number of dopant atoms by means of single ion implantation (SII) which enables us to implant dopant ions one by one into a fine semiconductor region until the necessary number is reached. Trimming of the conductance of a fine resistor which corresponds to an active region in semiconductor devices has been tried by using the SII. Firstly the conductance increase per one dopant atom in a sub-µm scale Si resistor was measured to be 18 nS/atom. Secondly very fine test resistors with a size of sub-µm were made by conventional device fabrication technology and the statistical distribution of conductance in the test devices was obtained. Then the number of single ions necessary to trim the conductance value to a certain value in the higher side of the initial distribution was implanted to each test resistor. The initial conductance fluctuation of 63% has been reduced to only 13%.

Simple nanostructuring on silicon surface by means of focused beam patterning and wet etching

Abstract Two simple and easy processes have been demonstrated to fabricate two-dimensional (2-D) nanostructure array on Si surfaces by using only focused beam patterning and wet etching. First, we took advantage of the enhanced etch rate (ER) of electron-beam-exposed SiO2 in HF based solution. A 30-nm thick oxide layer was shot with 30-keV focused-electron beam with spot doses ranging from 20 to 140 pC/dot. After development of SiO2 layer in 1% HF solution, the Si substrate was etched by hydrazine (N2H4H2O) to form pyramidal etch-pits. By using this process, 50-nm concave nanopyramid array (NPA) with 100-nm period can be fabricated successfully. Second, we utilized the newly found retarded ER of ion-beam-exposed Si in hydrazine. 2-D arrays of dots were written directly on the Si substrate with 60-keV Si focused-ion beam (FIB) with a dose of 5×1014 ions/cm2. The Si substrate was then dipped in hydrazine solution, where the unexposed region was selectively etched by hydrazine. By using this process, 100-nm convex NPA with 200-nm period can be fabricated easily. The performance of the proposed processes is compared in terms of pattern size, throughput and process diversity.

Total dose dependence of soft-error hardness in 64 kbit SRAMs evaluated by single-ion microprobe technique

Total dose effect on the soft-error susceptibility of 64 kbit CMOS SRAM has been investigated by using the single ion microprobe technique which enables us to get a map of soft-error sensitivity in a memory cell by hitting a micron-size area with single ions. The effects of the ion dose on the susceptibility of each error-sensitive site have been evaluated. The errors due to the upset of p-MOSFETs have become more susceptible at higher dose while that of the n-MOSFETs less susceptible. One of the origins of the errors are the negative threshold voltage (V/sub th/) shifts of the MOSFETs which are caused by oxide trapped charges. Displacement damage induced by ion irradiation also affects the susceptibility to the soft-error. >

Ion microprobe system at Waseda University for semiconductor analysis

Abstract In accordance with the recent expansion of ion microprobe applications, many efforts have been devoted to increasing the ion current in order to obtain better statistics. Extremely high current density, however, causes a serious problem because of radiation damage in the sample. In order to reduce the beam effect as much as possible, we have designed and fabricated an annular type SSD for RBS. To investigate the mechanism of radiation induced effects, we have added a SEM column to the target chamber.

Novel Process for High-Density Buried Nanopyramid Array Fabrication by Means of Dopant Ion Implantation and Wet Etching

A simple and high-throughput process to fabricate a high-density buried nanopyramid array (BNPA) on a Si surface has been developed by means of dopant ion implantation and wet etching. In this process, the combination of two interesting etching phenomena was utilized to form the BNPA. One is the enhanced etching of ion-exposed SiO2 in HF. The other is the newly found retarded etching of ion-exposed Si in hydrazine (N2H4). A p-type Si(100) substrate with 27-nm-thick SiO2 was exposed to 50-keV phosphorus ions with a dotted pattern. Then, the ion-exposed SiO2 was selectively etched away by dipping in HF. Finally, the BNPA was formed under the patterned SiO2 layer by dipping in hydrazine. By using this simple process, the BNPA with 250 nm pitch was successfully fabricated. The electrical property of the fabricated nanopyramid was also investigated using scanning Maxwell-stress microscopy (SMM).

Ion microprobe system combined with scanning electron microscope for high precision aiming

Abstract An ion microprobe system with high precision for target alignment has been realized by combining a compact scanning electron microscope (SEM) column with a target chamber. The target alignment is done first by imaging sample features with a scanning electron microscope (SEM) and then fine alignment of the sample position with respect to the incident ion beam is accomplished by comparing the secondary electron images of a Si test sample produced by the SEM beam and the ion microbeam. A Si test sample, with a specially designed relief pattern, has been used for both measuring the beam size and for determining the exact incident site of the ion beam on the target. Precision of aiming at present is about 5 μm which is limited mainly by the mechanical precision of the SEM sample holder. A beam spot size of 1.9 × 1.7 μm (FWHM) for 3 MeV He + beam has been achieved. Use of a high precision sample manipulator affords the prospect of an ion alignment precision of about 2 μm.

High-Density Nanoetchpit-Array Fabrication on Si Surface Using Ultrathin SiO2 Mask

Nanoetchpit arrays (NEPAs) with a density of 1.56 T(1012)pit/in2 were artificially fabricated on Si surfaces. The key to the success of high-density-NEPA fabrication is through utilization of ultrathin SiO2 layer as an Si etching mask and N2H4 (hydrazine) solution as an Si etchant. The NEPAs were fabricated in only three steps: (1) focused electron-beam (EB) irradiation onto SiO2 mask, (2) SiO2 mask development using an HF-based solution, and (3) Si etching using hydrazine. The enhanced etching phenomenon of EB-exposed SiO2 in an HF-based solution was applied to pattern the SiO2 mask. Thin SiO2 layers with a thickness of 8 nm at the initial stage (4 nm after the development) were used as Si etching mask to suppress both spreading of the EB-exposed region by the forward scattering of the electrons and lateral extension of the SiO2 etching region during the development. Si substrates were etched through 4-nm-thick SiO2 mask by dipping in a hydrazine solution with an extremely high etching selectivity for Si/SiO2. By using this simple process, 8-nm-diameter NEPAs with a 20 nm pitch were successfully arranged on Si surfaces.

Quantitative investigation of localized ion irradiation effects in n‐channel metal‐oxide‐semiconductor field‐effect transistors using single ion microprobe

Localized ion irradiation effects in n‐channel metal‐oxide‐semiconductor field‐effect transistor (n‐ch MOSFET) have been investigated quantitatively by means of both the single ion microprobe (SIMP) and single ion beam induced charge (SIBIC) imaging. An extremely large leakage current in the subthreshold gate voltage Vg–drain current Id characteristics (Vg< threshold voltage Vth) have been induced by exposing a small fraction of the MOSFET gate area to MeV He single ions, while the turn‐on Vg–Id characteristics (Vg≳Vth) have scarcely been affected. The causes of the large leakage current in the subthreshold region induced by the localized ion irradiation have been discussed.