High Resolution E-beam Lithography Research Articles

Ultrasensitive Detection of PSA Using Antibodies in Crowding Polyelectrolyte Multilayers on a Silicon Nanowire Field-Effect Transistor.

Immunosensors based on field-effect transistors with nanowire channels (NWFETs) provide fast and real-time detection of a variety of biomarkers without the need for additional labels. The key feature of the developed immunosensor is the coating of silicon NWs with multilayers of polyelectrolytes (polyethylenimine (PEI) and polystyrene sulfonate (PSS)). By causing a macromolecular crowding effect, it ensures the "soft fixation" of the antibodies into the 3-D matrix of the oppositely charged layers. We investigated the interaction of prostate-specific antigen (PSA), a biomarker of prostate cancer, and antibodies adsorbed in the PEI and PSS matrix. In order to visualize the formation of immune complexes between polyelectrolyte layers using SEM and AFM techniques, we employed a second clone of antibodies labeled with gold nanoparticles. PSA was able to penetrate the matrix and concentrate close to the surface layer, which is crucial for its detection on the nanowires. Additionally, this provides the optimal orientation of the antibodies' active centers for interacting with the antigen and improves their mobility. NWFETs were fabricated from SOI material using high-resolution e-beam lithography, thin film vacuum deposition, and reactive-ion etching processes. The immunosensor was characterized by a high sensitivity to pH (71 mV/pH) and an ultra-low limit of detection (LOD) of 0.04 fg/mL for PSA. The response of the immunosensor takes less than a minute, and the measurement is carried out in real time. This approach seems promising for further investigation of its applicability for early screening of prostate cancer and POC systems.

Triangular nanoperforation and band engineering of InGaAs quantum wells: a lithographic route toward Dirac cones in III–V semiconductors

The design of two-dimensional periodic structures at the nanoscale has renewed attention for band structure engineering. Here, we investigate the nanoperforation of InGaAs quantum wells epitaxially grown on InP substrates using high-resolution e-beam lithography and highly plasma based dry etching. We report on the fabrication of a honeycomb structure with an effective lattice constant down to 23 nm by realising triangular antidot lattice with an ultimate periodicity of 40 nm in a 10 nm thick InGaAs quantum well on a p-type InP. The quality of the honeycomb structures is discussed in detail, and calculations show the possibility to measure Dirac physics in these type of samples. Based on the statistical analysis of the fluctuations in pore size and periodicity, calculations of the band structure are performed to assess the robustness of the Dirac cones with respect to distortions of the honeycomb lattice.

Biosensor based on a silicon nanowire field-effect transistor functionalized by gold nanoparticles for the highly sensitive determination of prostate specific antigen

We have demonstrated label-free and real-time detection of prostate specific antigen (PSA) in human serum using silicon nanowire field effect transistors (NW FETs) with Schottky contacts (Si-Ti). The NW FETs were fabricated from SOI material using high-resolution e-beam lithography, thin film vacuum deposition and reactive-ion etching processes eliminating complicated processes of doping and thermal annealing. This allowed substantial simplifying the transistors manufacturing. A new method for covalent immobilization of half-fragments of antibodies on silicon modified by 3-glycidopropyltrimethoxysilane with thiol groups and 5nm gold nanoparticles (GNPs) was established. NW FETs functionalized by GNPs revealed extremely high pH sensitivity of 70mV/pH and enhanced electrical performance in the detection of antigen due to enhanced surface/volume ratio, favorable orientation of antibody active sites and approaching the source of the electric field close to the transistor surface. Si NWFETs were applied for quantitative detection of PSA in a buffer and human serum diluted 1/100. Response time was about 5–10s, and analysis time per sample was 1min. The limit of PSA detection was of 23fg/mL, concentration range of 23fg/mL–500ng/mL (7 orders of magnitude). The PSA concentrations determined by the NW FETs in serum were compared with well-established ELISA method. The results matched well with the correlation coefficient of 0.97.

Room temperature Coulomb blockade effects in Au nanocluster/pentacene single electron transistors

Single-electron transistors incorporating single ∼1 nm gold nanocluster (AuNCs) and pentacene as a complex charge transport system have been used to study the quantum Coulomb blockade and its single electron tunnelling behaviour at room temperature (RT) (300 K). Monodisperse ultra-small (0.86 ± 0.30 nm) AuNCs were deposited by the tilted-target sputtering technique into 12 nm nanogaps fabricated by high-resolution e-beam lithography. Tunnelling resistance was modulated to ∼109 Ω by addition of a pentacene layer, allowing clear observation of quantum staircases and Coulomb oscillations with on/off current modulation ratio of ∼100 in RT current–voltage measurements. The electron addition energy and average quantized energy level spacing were found to be 282 and 80.4 meV, respectively, which are significantly larger than the thermal energy at 300 K (25.9 meV).

New insights into subsurface imaging of carbon nanotubes in polymer composites via scanning electron microscopy

Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanning electron microscopy (SEM), significant controversy exists concerning the imaging depth and contrast mechanisms. We studied CNT-polyimide composites and, by three-dimensional reconstructions of captured stereo-pair images, determined that the maximum SEM imaging depth was typically hundreds of nanometers. The contrast mechanisms were investigated over a broad range of beam accelerating voltages from 0.3 to 30 kV, and ascribed to modulation by embedded CNTs of the effective secondary electron (SE) emission yield at the polymer surface. This modulation of the SE yield is due to non-uniform surface potential distribution resulting from current flows due to leakage and electron beam induced current. The importance of an external electric field on SEM subsurface imaging was also demonstrated. The insights gained from this study can be generally applied to SEM nondestructive subsurface imaging of conducting nanostructures embedded in dielectric matrices such as graphene-polymer composites, silicon-based single electron transistors, high resolution SEM overlay metrology or e-beam lithography, and have significant implications in nanotechnology.

Dry-etching damage to magnetic anisotropy of Co-Pt dot arrays characterized using anomalous Hall effect

Using high-resolution e-beam lithography and ion beam etching, dot arrays were formed from Co–Pt alloy films with perpendicular magnetization. Dry-etching damage to uniaxial magnetic anisotropy Ku of the fabricated dot arrays was examined. Hcp–Co75Pt25 films and L11-type Co50Pt50-ordered alloy films with film thickness δ of 3, 5, and 10 nm were used for this study. Dot diameter D was varied from 10 nm to 400 nm. The effective magnetic anisotropy of the dots, Kueff, was measured using the generalized Sucksmith–Thompson method with anomalous Hall effect (AHE) and an averaged AHE signal over 100–71,000 dots. By compensating the dot shape anisotropy, Ku was calculated from Kueff. The Ku for a dot array of D = 400 nm was almost equal to that for an original continuous film, but Ku decreased gradually for D < 30 nm. The circumferential area (periphery) of dots with width WD was assumed to be damaged by etching. The damaged area was inferred as Ku = 0 with original saturation magnetization Ms. The WD values, as estimated using experimental data, were 1.6–1.8 nm for hcp–Co75Pt25 dot arrays independent of δ (δ = 3–10 nm) and 1.8–1.9 nm for L11–Co50Pt50 dot arrays (δ = 3–5 nm). For all dot arrays, Ku at D = 10 nm were nearly half those for the original continuous films, suggesting that films having Ku of twice that for original films were necessary to overcome Ku reduction by etching damage.

Fabrication of large scale arrays of metallic nanodots by means of high resolution e-beam lithography

We present the fabrication of large nanodot arrays with very small single dot sizes made of different metals and on different substrates using e-beam lithography and lift-off techniques. Nanodot arrays have a high potential for bioanalytical applications aiming towards single molecule detection. In addition, they can be used as well defined models of heterogeneous catalysts. For this purpose an independent control of the dot size and the distance between the dots is necessary. This is a limitation for many fabrication techniques, which can be overcome by e-beam lithography. However, aiming for the dot size of few tens of nanometers, we observed a strong influence of the beam focusing and astigmatism on the quality of the fabricated nanodot arrays. A significant improvement, i.e. reduction of defect density as well as better control of the dot size, was achieved by modification of the fine focus and stigmation system of the used e-beam writer. Using lift-off technique, we successfully fabricated platinum dot arrays on glassy carbon with target dot diameter ranging from 25 to 35nm. By applying additional annealing step, we could fabricate nanodot arrays made of gold on SiO2 with very small dot sizes down to 6nm and pitch of 100nm. Furthermore, the generation of large area arrays of nanosized pillars was demonstrated using the same exposure strategy in a negative tone HSQ resist.

Novel Organosilicate Polymer Resists for High Resolution E-Beam Lithography

Novel organosilicate polymer e-beam resists are reported, which show significantly improved e-beam sensitivity and long-term stability over those of conventional hydrogen silsesquioxane (HSQ) while maintaining its high resolution capability and good line-edge roughness. The improvement is made possible by incorporating norbornene groups in the organic moiety of organosilicate polymers, and the responsible mechanism is proposed.

Dot arrays of L11 type Co–Pt ordered alloy perpendicular films

Magnetic properties of dot arrays of L11 type Co–Pt ordered alloy perpendicular films were studied. L11-Co–Pt films with a large uniaxial magnetic anisotropy Ku of the order of 107 erg/cm3 were fabricated at a substrate temperature of 360 °C using ultrahigh vacuum sputter film deposition. Dot patterns with dot diameters of 70–200 nm were made using high resolution e-beam lithography and reactive ion etching (RIE). The values of Ku were measured by the GST method using the Anomalous Hall Effect; we observed the averaged signals of 6000 dots. The values of Ku for dot arrays of 10-nm-thick L11-Co50Pt50 films deposited on MgO(111) substrates (single crystal films) and glass disks (polycrystalline films) were nearly the same as those of the original films independent of D, indicating no significant etching damage by the RIE process. Magnetic force microscopy images revealed that all dots were single domains in the present D region. The coercivity Hc of the dot arrays was 25.0 kOe [MgO(111) substrate, D=70 nm] and 14.3 kOe (glass disks, D=80 nm). The switching field distribution σ/Hc was relatively small, σ/Hc=0.15, even for dot arrays fabricated on glass disks, indicating the homogeneous formation of a L11 type ordered structure in the Co50Pt50 layers.

Magnetic properties of Co–Pt∕Co hard/soft stacked dot arrays

The magnetic properties of Co80Pt20(10nm)∕Co(0,3,5nm) hard/soft stacked dot arrays with dot diameters D of about 50nm were studied. Co–Pt films were deposited on Ru (001) seed layers, resulting in a large uniaxial magnetic anisotropy Ku with the c axis perpendicular to the film plane. Dot patterns were formed using high resolution e-beam lithography and reactive ion etching. Magnetic force microscopy images revealed that all dot arrays showed a single domain state, even after removal of an applied field equal to the coercivity. The remanence coercivity Hr decreased from 9.1to6.9kOe as the Co layer thickness increased from 0to5nm, indicating that the hard/soft stacked structure was effective at reducing the switching field, as theoretically predicted. The applied field angular dependence of Hr for 10-nm-thick Co–Pt dot arrays was in good agreement with calculations based on the Stoner–Wohlfarth model, HS-W indicating coherent rotation of the magnetization during nucleation. Moreover, it was confirmed that the hard/soft stacked structure reduced the angular variation of Hr.

High resolution magnetic imaging: MicroSQUID Force Microscopy

Magnetic imaging at the micrometer scale with high sensitivity is a challenge difficult to be met. Magnetic force microscopy has a very high spatial resolution but is limited in magnetic resolution. Hall probe microscopy is very powerful but sensor fabrication at the one micron scale is difficult and effects due to discreteness of charge appear in the form of significant 1/f noise. SQUID microscopy is very powerful, having high magnetic resolution, but spatial resolution is usually of the order of 10 μm.The difficulties lay mostly in an efficient way to couple flux to the sensor. The only way to improve spatial resolution is to place the probe close to the very edge of the support, thus maximising coupling and spatial resolution. If there has been found a way to bring close the tip, there must be also found a reliable a way to maintain distance during scanning.We want to present recent improvements on scanning microsquid microscopy: Namely the improved fabrication of microSQUID tips using silicon micro machining and the precise positioning of the micrometer diameter microSQUID loop by electron beam lithography. The microSQUID is a microbridge DC SQUID, with two opposite microbridges. The constrictions are patterned by high-resolution e-beam lithography and have a width of 20 nm and a length of about 100 nm.The distance control during scanning is obtained by integrating the microSQUID sensor with a piezoelectric tuning fork acting as a force sensor allowing to control height and even topographic imaging. The detector is placed in a custom built near field microscope and the sample temperature can be varied between 0.1 Kelvin and 10 K.The microscope is used to study magnetic flux structures in unconventional superconductors and will be used to observe thermal domains in superconducting detectors in the voltage state.

Fabrication of Sub-Micrometer SIS Junctions for Radio Astronomy

We present a new fabrication scheme for high quality Nb-AlOx-Nb junctions as used for mm-wave mixers in radio astronomy. The key features of the fabrication process are high-resolution e-beam lithography for junction and contact definition as well as highly selective Niobium and SiO2 inductively coupled plasma etching. Chemical-mechanical polishing, which is difficult to control for low volume production is not required. We further show that a new type of spin-on glass can be used as a reliable replacement for sputtered SiO2. We present yield and parameter scattering of these devices.

Fogging effect correction method in high-resolution electron beam lithography

We report on a fogging effect correction (FEC) method to be used in high-resolution e-beam lithography (EBL). In the new version of the previously presented PROX-In software tool, originally developed to determine the numerical proximity parameters for the proximity effect correction (PEC), was now implemented also the possibility of correcting large-range pattern distortion effects in connection with the modified PROXECCO TM tool from PDF Solutions. This allows a complex exposure optimization by dose modulation of long-range fogging and/or loading effects with the standard PEC method using the same corrector. The presented approach is fast and effective, does not use any special additional technology steps and uses only standard high-resolution measuring techniques. The reviewed method was successfully implemented into mask production at different absorber stacks. It is also used for the determination of FEC input parameters and complex exposure optimization in e-beam direct write and step and flash imprint lithography (SFIL) template manufacturing with sub-50 nm resolution capability.

Fabrication and characterisation of nanoscale programmed defects for EUV lithography

In this paper we have developed and characterised with several measurement techniques a process achieving ultra-smooth nanoscale programmed defects for EUV lithography applications. The originality of this work consists in manufacturing defects of different width (25, 50, 75, and 100 nm) and height (20, 40, and 80 nm), for various densities (1 def/50 μm 2, 1 def/150 μm 2, 1 def/300 μm 2, and 1 def/500 μm 2), all together on the same wafer. Square dots have been manufactured by high resolution e-Beam lithography, using a hydrogen silsesquioxane (HSQ) resist. Three lithographic process iterations are applied to the same wafer using different HSQ solution concentrations. Finally, top-down CD-SEM, light scattering particles counting tools and atomic force microscope measurements were performed in order to check if the programmed defects were correctly printed and to monitor their dimensions. As a result, the fabrication procedure has been shown successful in term of location and size accuracy.

A fabrication process for a silicon tunnel barrier with self-aligned gate

A process for fabricating a device based on tunneling through a very thin vertical silicon membrane is presented. The process has been developed on a 〈1 1 0〉 oriented silicon wafer using high resolution e-beam lithography and KOH anisotropic etching to define the structure. A single evaporation step allows the fabrication of both the source–drain contacts and a control gate self aligned to the top of the silicon membrane. A vertical silicon membrane with a thickness of 15 nm has been obtained.

Exposure optimization in high-resolution e-beam lithography

In this paper a new method will be described, illustrating how to determine the optimized control point spread function (PSF) for proximity effect corrections (PEC) in e-beam lithography (EBL). A software tool called “PROX-In” was developed in order to help lithographers to determine the numerical inputs for an arbitrary PEC system to satisfy the high critical dimension (CD) control requirements as well as to compensate the shape bias in EBL in connection with the technology steps performed. PROX-In applies new methods allowing highly customized optimum numerical proximity parameter determinations by using a set of extracted experimental data. Compared with other presented methods, this approach is fast and effective. It does not require any additional technology steps and uses only standard measuring techniques. The reviewed method was successfully implemented into mask production for achieving the 90 nm technology node and below at different absorber stacks. It is also used for high-resolution e-beam direct write and SFIL template manufacturing with sub-50 nm resolution.

High resolution e-beam lithography using a thin titanium layer to promote resist adhesion

This paper describes improvements in high resolution large area e-beam lithography when a thin titanium layer is applied to substrates prior to the application of resist. The technique is particularly useful when there is a requirement to pattern long unsupported nanostructures such as lines which are more than a millimetre long but have widths of less than 20 nm. Such feature sizes are now readily achieved using the most advanced e-beam lithography tools which can have patterning writing fields in excess of one millimetre and spot sizes of less than 5 nm. In this paper, we describe a simple technique to improve the adhesion of different resist structures, particularly on III–V materials to enable the rapid evaluation of high resolution e-beam lithography and we show that X-ray photoelectron spectroscopy (XPS) can be useful in understanding substrate-resist interactions.

Probing Electronic Properties of dendritic Ruthenium Complex bound to Single Walled Carbon Nanotubes

ABSTRACTWe have functionalized single walled carbon nanotubes (SWNTs) using ruthenium centered organometallic supramolecular dendritic complexes. These dendrimers have multiple binding sites with chemically specified chirality. Most importantly, they are mechanically rigid. We have fabricated field effect transistors (FET) using these functionalized SWNTs. Devices were processed with standard optical lithography and high resolution e-beam lithography. We are using FET response of these devices to study charge injection into the nanotubes and the resultant effect on the tube's transport properties. Organometallic based molecular adsorbents onto the nanotubes effect the transistor response. These functionalized tubes change their majority carriers from holes to electrons in these FETs. We believe this is due to charge transfer from the metal center through the ligand and finally onto the nanotube. This results in the potential for optically altering the carrier density, and therefore the transport properties of the nanotubes.

Large domain wall magnetoresistance up to room temperature in La0.7Sr0.3MnO3 bridges with nanoconstrictions

To obtain low field magnetoresistance (MR) in manganites, we have introduced a geometrically constrained magnetic domain wall (DW) in La0.7Sr0.3MnO3 micrometric devices. Nanoconstrictions artificially induced using high resolution e-beam lithography are shown to effectively reduce the DW width leading to strongly enhanced DW resistance. Sharp and large resistance switches result from the appearance and annihilation of the DWs. Room temperature sharp resistance switches, with a MR of 16%, are evidenced in a manganite-based device.

A sample scanning system with nanometric accuracy for quantitative SPM measurements

A sample scanning device operating in a working volume of 30×30×18 μm with interferometer and capacitance-based controls of displacements, is described. The xy-stage uses plane mirror linear interferometers and fast phase-meters for control of displacements of precise ball-bearing stages driven by piezo flexure actuators. The stage operates with a full range bandwidth of 200 Hz, and an estimated accuracy ( k=2) of 3 nm+1×10 −3 L, where L is the lateral displacement. A novel z-stage based on a kinematic coupling between two plates, the upper one being moved by three bimorph plates and the distance being measured by three capacitive sensor, is described. The tilt of the z-stage is kept within fractions of a microrad, leading to a full range estimated accuracy of 2 nm+2×10 −3 h, where h is the vertical displacement. The control bandwidth is of about 1 kHz, thus allowing fast and accurate step-height measurements. In order to test the device used in a scanning probe microscope, micrometric patterned surfaces made using high resolution e-beam lithography and precise metal deposition on silicon are imaged. Results of pitch measurements are discussed and compared with those obtained using optical diffractometry.