Standard Photolithography Research Articles

A novel hybrid III–V/silicon deformed micro-disk single-mode laser

A novel hybrid III–V/silicon deformed micro-disk single-mode laser connecting to a Si output waveguide is designed, and fabricated through BCB bonding technology and standard i-line photolithography. Compared to a traditional circular micro-disk in multi-longitudinal-mode operation, unidirectional emission and single longitudinal-mode output from a Si waveguide are realized. In the experiments, an output power of 0.31 mW and a side-mode suppression ratio of 27 dB in the continuous-wave regime are obtained.

Hydrodynamics of diamond-shaped gradient nanopillar arrays for effective DNA translocation into nanochannels.

Effective DNA translocation into nanochannels is critical for advancing genome mapping and future single-molecule DNA sequencing technologies. We present the design and hydrodynamic study of a diamond-shaped gradient pillar array connected to nanochannels for enhancing the success of DNA translocation events. Single-molecule fluorescence imaging is utilized to interrogate the hydrodynamic interactions of the DNA with this unique structure, evaluate key DNA translocation parameters, including speed, extension, and translocation time, and provide a detailed mapping of the translocation events in nanopillar arrays coupled with 10 and 50 μm long channels. Our analysis reveals the important roles of diamond-shaped nanopillars in guiding DNA into as small as 30 nm channels with minimized clogging, stretching DNA to nearly 100% of their dyed contour length, inducing location-specific straddling of DNA at nanopillar interfaces, and modulating DNA speeds by pillar geometries. Importantly, all critical features down to 30 nm wide nanochannels are defined using standard photolithography and fabrication processes, a feat aligned with the requirement of high-volume, low-cost production.

Schottky diodes and high electron mobility transistors of 2DEG AlGaN/GaN structures on sapphire substrate

We report Schottky diodes (SDs) and High Electron Mobility Transistors (HEMTs) fabricated of 2DEG AlGaN/GaN structures grown by Metalorganic Chemical Vapour Phase Epitaxy (MOVPE) on sapphire substrate. The SDs and HEMTs were designed intentionally without surface passivation and were successfully fabricated at the Center for Physical Sciences and Technology (CPST), using standard UV photolithography procedures. The performance of Ohmic contacts formed of quaternary Ti/Al/Ni/Au stack was optimized varying the temperature of rapid thermal annealing process. Deposited on the semiconductor metal Ni/Au stack was used to form 0.75 eV height Schottky barriers. The fabricated SDs demonstrated low reverse current and high electric current switching ratio while the HEMTs showed high transconductance and drain saturation currents performance with good transistor channel closing. This work paves a way to develop advanced AlGaN/GaN based HEMT structures as well as new electronic components for operation at high powers and high frequencies.

Direct transfer of magnetic sensor devices to elastomeric supports for stretchable electronics.

A novel fabrication method for stretchable magnetoresistive sensors is introduced, which allows the transfer of a complex microsensor systems prepared on common rigid donor substrates to prestretched elastomeric membranes in a single step. This direct transfer printing method boosts the fabrication potential of stretchable magnetoelectronics in terms of miniaturization and level of complexity, and provides strain-invariant sensors up to 30% tensile deformation.

Intrinsic Josephson properties in Pb1-ySr2Y1-xCaxCu2+yO7+δ epitaxial films

We report the first observation of intrinsic Josephson junction (IJJ) characteristics in the Pb1-ySr2Y1-xCaxCu2+yO7+δ (Pb1212) epitaxial film. Pb1212 epitaxial film has been grown on SrTiO3 (100) substrates by a two-step growth technique. A small mesa structure (S = 4μm2) has been fabricated on a film surface using a standard photolithography and an Ar ion milling technique. The superconducting transition temperature of the IJJ is 43 K. At 4.2 K, the value of Jc (the critical current density) is 2.2 kA/cm2. The depth of the hysteresis defined as α = (Jc+Jr)/ Jc is 0.89, where Jr is the return current density. The temperature dependence of Jc shows good agreement with Ambegaokar-Baratoff theory.

Highly featured amorphous silicon nanorod arrays for high-performance lithium-ion batteries

High aspect-ratio vertical structures of amorphous silicon have been realized using hydrogen-assisted low-density plasma reactive ion etching. Amorphous silicon layers with the thicknesses ranging from 0.5 to 10 μm were deposited using radio frequency plasma enhanced chemical vapor deposition technique. Standard photolithography and nanosphere colloidal lithography were employed to realize ultra-small features of the amorphous silicon. The performance of the patterned amorphous silicon structures as a lithium-ion battery electrode was investigated using galvanostatic charge-discharge tests. The patterned structures showed a superior Li-ion battery performance compared to planar amorphous silicon. Such structures are suitable for high current Li-ion battery applications such as electric vehicles.

Electropreconcentration of nanoparticles using a radial micro-nanofluidic device

We have developed a radial silicon micro-nanofluidic device in order to investigate strong nanoparticles electropreconcentration. The device is called “ring like” device and exhibits a circular micro-nanojunction. Hundred-nanometer-deep radial nanochannels were fabricated using standard photolithography and etching techniques. Ion permselectivity is one of the major proprieties of nanofluidic devices. Within the influence of an electric field through an ion-selective nanochannel, nanoparticle repulsion and concentration appear at the anodic and cathodic side, respectively. Here, the cathodic preconcentration is exploited to enriched 50-nm nanoparticles samples. Up to 800, enrichment factor is reached in 1 h of experiment. This scheme could be useful for the enrichment of bionanoparticles (such as viruses or exosomes for instance) which can be critical for several biomedical applications.

Magnetic shielding performance of superconducting YBCO thin film in a multilayer device structure

Magnetic shielding performance of superconducting YBaCu2O7−x (YBCO) thin film on an YBCO microbridge was analyzed in a multilayer structure. A sandwich type multilayer structure was fabricated onto a single crystal (100) SrTiO3 (STO) substrate in the form of YBCO/STO/YBCO by depositing a thin STO interlayer in between two YBCO layers. The top YBCO was patterned as 20μm width meander-type microbridges and the bottom layer YBCO was used as magnetic shield. YBCO and STO thin films were deposited by dc and rf magnetron sputtering respectively, and the patterning was performed by using standard photolithography and wet etching. In order to enhance long-term stability of the final device, an additional STO thin film was deposited onto the device as an encapsulation layer. Electrical and magnetic characterizations of the YBCO thin film layers were carried out by means of ac magnetic susceptibility (χ–T) and resistance vs. temperature (R–T) measurements. The current–voltage (I–V) measurements were performed on the microbridges at 77K by observing the shielding performance of the bottom YBCO layer under various applied magnetic fields. The results were compared with that of a same-type single layer YBCO device without a shielding layer. The zero field critical current value of the single layer 20μm wide YBCO device was measured as 30mA and decreased down to 20mA as the field increased up to 100mT. The same measurements on the multilayer device showed that the critical current values remained almost constant around 27mA as the applied field increased.

Low dark current and high speed ZnO metal–semiconductor–metal photodetector on SiO2/Si substrate

ZnO thin films are deposited by radio-frequency magnetron sputtering on thermally grown SiO2 on Si substrates. Pt/Au contacts are fabricated by standard photolithography and lift-off in order to form a metal-semiconductor-metal (MSM) photodetector. The dark current of the photodetector is measured as 1 pA at 100 V bias, corresponding to 100 pA/cm2 current density. Spectral photoresponse measurement showed the usual spectral behavior and 0.35 A/W responsivity at a 100 V bias. The rise and fall times for the photocurrent are measured as 22 ps and 8 ns, respectively, which are the lowest values to date. Scanning electron microscope image shows high aspect ratio and dense grains indicating high surface area. Low dark current density and high speed response are attributed to high number of recombination centers due to film morphology, deducing from photoluminescence measurements. These results show that as deposited ZnO thin film MSM photodetectors can be used for the applications needed for low light level detection and fast operation.

Facile Photo‐Crosslinking of Azide‐Containing Hole‐Transporting Polymers for Highly Efficient, Solution‐Processed, Multilayer Organic Light Emitting Devices

A novel framework of azide containing photo‐crosslinkable, conducting copolymer, that is, poly(azido‐styrene)‐random‐poly(triphenylamine) (X‐PTPA), is reported as a hole‐transporting material for efficient solution‐processed, multi‐layer, organic light emitting diodes (OLEDs). A facile and energy‐efficient crosslinking process is demonstrated with UV irradiation (254 nm, 2 mW/cm2) at a short exposure time (5 min). By careful design of X‐PTPA, in which 5 mol% of the photo‐crosslinkable poly(azido‐styrene) is copolymerized with hole‐transporting poly(triphenylamine) (X‐PTPA‐5), the adverse effect of the crosslinking of azide moieties is prevented to maximize the performances of X‐PTPA‐5. Since the photo‐crosslinking chemistry of azide molecules does not involve any photo‐initiators, superior hole‐transporting ability is achieved, producing efficient devices. To evaluate the performances of X‐PTPA‐5 as a hole‐transporting/electron‐blocking layer, Ir(ppy)3‐based, solution‐processable OLEDs are fabricated. The results show high EQE (11.8%), luminous efficiency (43.7 cd/A), and power efficiency (10.4 lm/W), which represent about twofold enhancement over the control device without X‐PTPA‐5 film. Furthermore, micro‐patterned OLEDs with the photo‐crosslinkable X‐PTPA‐5 can be fabricated through standard photolithography. The versatility of this approach is also demonstrated by introducing the same azide moiety into other hole‐transporting materials such as poly(carbazole) (X‐PBC).

Spatial shaping for generating arbitrary optical dipole traps for ultracold degenerate gases.

We present two spatial-shaping approaches - phase and amplitude - for creating two-dimensional optical dipole potentials for ultracold neutral atoms. When combined with an attractive or repulsive Gaussian sheet formed by an astigmatically focused beam, atoms are trapped in three dimensions resulting in planar confinement with an arbitrary network of potentials - a free-space atom chip. The first approach utilizes an adaptation of the generalized phase-contrast technique to convert a phase structure embedded in a beam after traversing a phase mask, to an identical intensity profile in the image plane. Phase masks, and a requisite phase-contrast filter, can be chemically etched into optical material (e.g., fused silica) or implemented with spatial light modulators; etching provides the highest quality while spatial light modulators enable prototyping and realtime structure modification. This approach was demonstrated on an ensemble of thermal atoms. Amplitude shaping is possible when the potential structure is made as an opaque mask in the path of a dipole trap beam, followed by imaging the shadow onto the plane of the atoms. While much more lossy, this very simple and inexpensive approach can produce dipole potentials suitable for containing degenerate gases. High-quality amplitude masks can be produced with standard photolithography techniques. Amplitude shaping was demonstrated on a Bose-Einstein condensate.

DNA immobilization on high aspect ratio glassy carbon (GC-MEMS) microelectrodes for bionanoelectronics applications

We present the immobilization of DNA on a high aspect-ratio glassy carbon microelectrodes patterned using standard photolithography techniques and fabricated from pyrolyzed negative-tone photosensitive polymer, and their subsequent electrical characterization as part of a new bionanoelectronics platform. The 3D glassy carbon microelectrode architecture introduced here consists of mainly high aspect-ratio microelectrode structures (>20 μm height) with amide bond between the carboxylated glassy carbon electrode and aminated ?-DNA bridge suspended between two electrodes. The use of these glassy carbon electrodes patterned through lithography and derived from the pyrolysis of negative-tone resist polymers (hence called GC-MEMS) as opposed to the commonly used metal electrodes offers advantages of an inert carbon electrode surface that is resistant to non-specific binding of salts--an issue that has plagued most ?-DNA attachment experiments reported in the literature. The results demonstrate the ability to immobilize DNA on the surface or generate a DNA bridge between to GC-MEMS electrodes using oxygen plasma functionalization and carbodiimide crosslinking with aminated oligonucleotides. We also discuss in detail (1) the effect of concentration of reactants, (2) pH of the crosslinking reaction, and (3) the use of electrokinetic mixing in promoting DNA attachment on this GC-MEMS platform.

Pyrolyzed Photoresist Carbon Electrodes in Aprotic Solvent: Bilirubin Electrochemistry and Interaction with Electrogenerated Superoxide

Pyrolyzed photoresist carbon electrodes (PPCEs) are fabricated by the photopatterning of a negative tone epoxy-based photoresist, SU-8, through optimized standard UV photolithography. The electrochemical characteristics of PPCEs are investigated in dimethyl sulfoxide (DMSO), observing a wider accessible potential window and a smaller capacitance with respect to glassy carbon electrodes. PPCEs are used to study the cyclic voltammetric behavior of bilirubin (BR) in DMSO. Detailed information is obtained on the multiple steps involved both in the electrochemical oxidation and reduction of BR. Interesting points concerning the electrochemical oxidation of BR to biliverdin are clarified, identifying the formation of an intermediate whose fate depends on the time scale of the electrochemical experiment.PPCEs are also used to electrogenerate the superoxide anion O2−• in DMSO for studying possible reactions between BR and O2−•. The results obtained demonstrate that BR is an efficient superoxide scavenger and that a concentration 2mM of BR is high enough to consume all the O2−• generated by oxygen reduction at the PPCE/DMSO interface.

Standard AZ 5214E photoresist in laser interference and EBDW lithographies

In this paper examples of the applicability of the standard AZ 5214E photoresist are shown. The resist is besides its sensitivity to UV radiation sensitive also to e-beam exposure. The arrays of patterns (holes and columns) were exposed in this photoresist by the experimentally produced Laser Interference Lithography method as well as by the Electron-Beam Direct Write Lithography. With both methods comparable results have been achieved with less than half micron spacing of the patterns, obtaining thus dimensions smaller than usually achieved by standard optical photolithography with the AZ 5214E photoresist.

Released micromachined beams utilizing laterally uniform porosity porous silicon.

Suspended micromachined porous silicon beams with laterally uniform porosity are reported, which have been fabricated using standard photolithography processes designed for compatibility with complementary metal-oxide-semiconductor (CMOS) processes. Anodization, annealing, reactive ion etching, repeated photolithography, lift off and electropolishing processes were used to release patterned porous silicon microbeams on a Si substrate. This is the first time that micromachined, suspended PS microbeams have been demonstrated with laterally uniform porosity, well-defined anchors and flat surfaces.PACS81.16.-c; 81.16.Nd; 81.16.Rf

Enhanced cell adhesion and alignment on micro-wavy patterned surfaces.

Various micropatterns have been fabricated and used to regulate cell adhesion, morphology and function. Micropatterns created by standard photolithography process are usually rectangular channels with sharp corners (microgrooves) which provide limited control over cells and are not favorable for cell-cell interaction and communication. This paper proposes a new micropattern with smooth wavy surfaces (micro-waves) to control the position and orientation of cells. To characterize cell growth and responses on the micro-patterned substrates, bovine aortic endothelial cells were seeded onto surfaces with micro-grooves and micro-waves for 24 h. As a result, the cells on the micro-wavy pattern appeared to have a lower death rate and better alignment compared to those on the micro-grooved pattern. In addition, flow-induced shear stress was applied to examine the adhesion strength of cells on the micro-wavy pattern. Results showed that cells adhered to the wavy surface displayed both improved alignment and adhesion strength compared to those on the flat surface. The combination of increased alignment, lower death rate and enhanced adhesion strength of cells on the micro-wavy patterns will offer advantages in potential applications for cell phenotype, proliferation and tissue engineering.

Effect of Graphene Passivation on Ge Pin Photodiode

We characterized the electrical and optical properties of graphene layer deposited photodiode with n-Ge/i-Ge/p-Si hetero-junction structure which were grown on boron doped p-type Si (100) wafers with 5~15 Ω-cm resistivity using the rapid thermal chemical vapor deposition (RTCVD) method. Furthermore, a reference p-i-n hetero-junction photodiode also fabricated and its comparison results explained by corresponding band diagrams and Ge surface passivation.Fig. 1(a) shows the cross-sectional SEM image of the n-Ge/i-Ge layer. The p-i-n structure consists of a ~ 0.3 µm thick heavily phosphorus doped n-Ge layer and ~ 2.5 µm thick intrinsic Ge layer grown on p-Si substrate. Crystalline homogeneity and the microscopic strain state of the n-Ge/i-Ge epilayer analyze was performed high resolution X-ray diffraction (HR-XRD). The (400) peak of the n-Ge/i-Ge layer was observed at 5522 arcsec separated with respect to Si peak, as shown in the Fig 1(b). The HR-XRD rocking curve shape demonstrates the superior quality n-Ge/i-Ge layer and absence of tensile strain components because of two peaks are clearly observed.In order to investigate the number of layers and quality of the CVD grown graphene after deposit on hetero-structure, the room-temperature Raman spectra were obtained using a 487.55 nm Ar-ion laser in the range of 1.000 to 3.000 cm-1 as shown in the Fig 2. The laser power applied to the sample was about 1.011mW. Raman spectrum of the graphene film is found to consist of two sharp peaks: i.e., 2D-band peak at ~ 2698 cm-1 and G-band peak at ~ 1581 cm-1, suggest that the transferred graphene was a monolayer graphene.The p-i-n photodiode with round mesa structure as shown in schematic form in Fig.3. Diameter of mesa were 250 µm and mesa were defined by standard photolithography and CF4 reactive ion etching of the n-Ge/i-Ge layer to p-Si substrate by using inductively coupled plasma (ICP) etching.Fig. 4 shows the typical forward and reverse biased J–V curves under dark and photo illuminated graphene/n-Ge/i-Ge/p-Si and n-Ge/i-Ge/p-Si hetero-junction photodiodes. We obtained reduced dark J from proposed graphene/n-Ge/i-Ge/p-Si photo diode. Dark current density and photocurrent density values were 0.49 A/cm2 and 3.01 A/cm2, respectively at reverse 1 V. Also, these values obtained to be 1.43 A/cm2 and 3.819 A/cm2 from n-Ge/i-Ge/p-Si photodiode at same bias. Ideality factor n was found to be 1.3 in the low voltage range. This implies that the dark current is dominated by diffusion rather than recombination, which indicates the high quality of the material.The spectral photo-responsivity of the two p-i-n photodiodes measured at reverse 2 V for wavelength in the range from 1520-1630 nm. Detailed comparison results and discussions will be present in the full manuscript.

Detection of Effect of Strain on the Valence Band Structure of SiGe by HXPES with High Spatial Resolution

Since the aggressive scaling of the Si-based metal–oxide–semiconductor field effect transistors (MOSFET) is reaching its limits, the strained-Ge and III–V semiconductors are being studied extensively as one of the alternative channel materials due to their higher carrier mobility [1,2]. Recently, the channel strain engineering has become an essential technology for the future complementary metal oxide semiconductor. The process-induced local strain is currently the mainstream technology [3]. We reported that a selective-ion-implantation technique was developed as a novel method of creating uniaxially strained Si/Ge heterostructures [4]. We investigated the effect of the strain on the binding energy of valence band top, Si 1s and Ge 2p core level by Hard X-ray photoemission spectroscopy (HXPES) with the high spatial resolution.The sample was prepared as follows. A SiO2 film was firstly deposited as a mask on a Si(100) substrate by plasma-enhanced chemical vapor deposition (PECVD), followed by the patterning of stripes with a standard photolithography process. Ar+ ion were selectively implanted into the Si substrate through the SiO2 mask windows at an energy of 25 keV with a dose of 1×1015 cm-2. The line widths of the implanted and unimplanted regions were fixed to be 20 and 2μm, respectively. A SiGe layer with a Ge content of 26% and thickness around 70 nm was pseudomorphically grown on the selectively ion-implanted Si substrate. Subsequently, postgrowth annealing was carried out to promote strain relaxation at 900°C. The hard X-ray (7.94 keV photons) excited Si 1s, Ge 2p, and valence band spectra arising from the sample were measured at photoelectron take-off angle (TOA) of 85 degrees with the energy resolution of 100 meV at undulator beam line (BL47XU at SPring-8 (JASRI, Proposal No. 2012B1269)) using high resolution electron energy analyzer R-4000 with the spatial resolution [11(|)0] direction is about 0.8µm [5]. The Si 1s photoelectron spectra are changing in the position of a sample as shown in Fig. 1 (a). From Fig. 1 (b) and (c), the binding energies of Si 1s and Ge 2p core level are reduced at the position of 8, 30, and 52µm. Here, spectra are shown in figures as a parameter at the position of the [11(|)0] direction. On the other hand, the change of the binding energy of valence band top is less than that of peak of Si 1s and Ge 2p as shown in Fig. 1(d). The comparison of the position of the valence band top in the case in relation to the binding energy of Ge 2p is shown in Fig. 2. Focus on the position of the valence band top, the position of the valence band of the uniaxial strained SiGe is lower than that in the case of the biaxial strained SiGe (50% relaxation rate). The change in the valence band position due to the difference in stress is consistent with the calculated results [6].Acknowledgments We acknowledge Prof. N. Usami and Y. Hoshi for their supports for the MBE growth. This work was partly supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2009-2013, by Grant-in-Aid for Scientific Research (Grant No. 25600079) from MEXT, Japan, and by the Strategic Information and Communications R&D Promotion Programme (SCOPE) from MIC, Japan.[1] ITRS 2012, (http://www.itrs.net)[2] M. Heyns, et al.: MRS Bull. 34 (2009) 485.[3] M. L. Lee, et al.: J. Appl. Phys. 97 (2005) 011101.[4] K. Sawano, et al.: Applied Physics Express 1 (2008) 121401.[5] E. Ikenaga,et al.: ELSPEC-46121 ; No. of Pages 8. [6] Y. Sun, et al.: Appl. Phys. 101 (2007) 104503.

GaN-based light-emitting diodes with hybrid micro/nano-textured indium-tin-oxide layer

Three types of textured indium-tin-oxide (ITO) surface, including nano-texturing and hybrid micro/nano-texturing with micro-holes (concave-hybrid-pattern) or micro-pillars (convex-hybrid-pattern), were applied to GaN-based light-emitting diodes (LEDs). The nano-texturing was realized by maskless wet-etching, and the micro-texturing was achieved by standard photolithography and wet-etching. Compared to LED chips with flat ITO surface, those with nano-pattern, concave-hybrid-pattern, and convex-hybrid-pattern exhibit enhancement of 11.3%, 15.8%, and 17.9%, respectively, for the light-output powers at 20 mA. The electrical performance has no degradation. Moreover, the convex-hybrid-pattern show higher light-output efficiency under small injection current, while the concave-hybrid-pattern exhibit better light-output efficiency at large injection current. The light-extraction efficiency is simulated by use of two-dimensional finite difference time domain method, and the numerical results are consistent with the experiments.

Study of bending reliability and electrical properties of platinum lines on flexible polyimide substrates

We have experimentally studied the variation in electrical resistance of flexible platinum lines patterned on polyimide foil when they are subjected to circular bending constraints. The lines were patterned by means of standard photolithography and sputtering deposition. Two different photolithography masks were used for comparative evaluation: an un-expensive transparency mask and a standard chromium mask. Measurements of the temperature coefficient of resistance (TCR) and time stability of the resistance have been acquired for lines bent down to 1.25mm radius of curvature on a customized bending setup, showing good reliability results. The robustness of the lines has been also assessed by registering their change in resistance while bending at different radii of curvature. The lines showed reliability issues for radii of curvature below 1.25mm, presenting a resistance variation of 19% for transparency mask-fabricated lines and 9% for chromium mask-fabricated lines. The worse reliability performances of transparency mask lines, compared to the chromium mask ones, was found to be due to their imperfect edges, which promoted the formation and propagation of cracks during bending. The results of the experiments in this work permitted to compare the performances of flexible conductive lines with different geometry and fabricated with two different masks, establishing quantitative and qualitative bending limits for their appropriate operation in flexible electronics systems.