Fabrication of microstructures in III–V semiconductors by proton beam writing

While the creation of microstructures in Si and GaAs by irradiation with high energy protons and helium ions and subsequent electrochemical etching has already been reported, the creation of microstructures in InP using this technique is investigated here for the first time. It is demonstrated that proton irradiation of Fe-doped semi-insulating InP(0 0 1) leads to an amplified material removal in the irradiated areas during electrochemical etching in an HF solution. By this way microstructures were produced using a 2.25 MeV proton beam focused down to ～1 μm. The depth of the etched structures depends on the etching procedure. In addition, amplified etching of non-irradiated material encircled by closed irradiated patterns was observed. Furthermore, irradiation with helium ions leads to the formation of thin freestanding InP layers by simultaneous underetching as well as etching from the sample surface of the irradiated regions. In contrast to the semi-insulating material, the irradiation and electrochemical etching of n-type InP results in an amplified formation of porous material which partly leads to a complete removal of the material in the irradiated areas. A comparison between GaAs and InP concerning the dissolution characteristics shows mainly a similar behavior of the semi-insulating types of these semiconductors and of the n-type materials.     

Fabrication of a microreactor by proton beam writing technique

Microreactors are innovative and promising tools in technology nowadays because of their advantages compared to the conventional-scale reactors. These advantages include vast improvements in surface to volume ratio, energy efficiency, reaction speed and yield and increased control of reaction conditions, to name a few examples.The high resolution capability of the micromachining technique utilizing accelerated ion beams in the fabrication technology of microreactors has not yet been taken advantage of. In this work we present the design of a prototype micro-electrochemical cell of 1.5 μL volume (2.5 × 2.5 × 0.240 mm) created with a 3 MeV proton microbeam. The cell can be separated into two half-cells with a suitable membrane applicable to galvanic or fuel cells as well. We deposited gold electrodes on both of the half-cells.The operability of the device was demonstrated by electric current flow between the two electrodes in this micro-electrochemical cell containing a simple electrolyte solution. We used a polycapillary film to separate the two half-cells, hindering the mixing of the anolyte and catholyte solutions. As a result of the minimal mixing caused by the polycapillary film, this cell design can be suitable for electro-synthesis. Due to the high resolution of proton beam writing, it is planned to reduce the dimensions of this kind of microreactor.     

Micro-machining of resists on silicon by proton beam writing

We report micro-machining of resists on silicon by proton beam writing (PBW) at Takasaki Ion Accelerators for Advanced Radiation Application (TIARA), JAEA Takasaki, Japan. We studied the proton beam irradiation effects on typical positive and negative resists such as PMMA and SU-8, respectively, for application of the PBW technique to micro-machining. These resist materials were subjected to the scanning of a focused beam of protons accelerated using the microbeam facility of TIARA. Diameter of the proton beam was focused to about 1 μm. The fluence was varied to examine the irradiation effects on these resists as a function of the beam current and irradiation time. After exposure to proton beam, samples were developed and evaluated by a scanning electron microscope. Attempts to fabricate nickel stamps were also made by electroplating on the structures formed by PBW for application to imprint lithography.     

高能聚焦质子束无掩模刻写方法研究初步

在中国科学院上海应用物理研究所的扫描质子微探针装置上,研制了图形化扫描器,同时研究了适合质子束刻写的光刻胶制备、显影、定影技术.在此基础上,开展高能聚焦质子束无掩模刻写实验,在厚度约11μm的PMMA光刻胶上刻写出高纵横比的任意图案,取得了初步质子刻写结果,为进一步开展质子纳米束刻写奠定基础.     

Si micro-turbine by proton beam writing and porous silicon micromachining

A 3D Si micro-turbine characterized by high aspect ratio vertical walls was formed by the combination of proton beam writing (PBW) and subsequent selective porous Si (PS) etching. Crystal damages generated by the implanted protons result in increased resistivity, thereby limit or even prevent the current to flow through the implanted area during electrochemical etching. Characteristic feature of the proposed process is that the shape of the micro electro-mechanical (MEMS) components is defined by two implantation energies. A higher energy is applied for defining the housing of the device while the lower energy is used to write the moving components. The implantation energies were selected such as to result appropriate difference between the two projected ranges, thereby providing structures with different height after development. The thickness of the walls of the moving component and the isotropic etching profile of the electrochemical PS formation was also taken into consideration. The electrochemical etching is driven until the sacrificial PS layer completely underetches the moving components, but the etch-front does not reach the bottom of the housing. Therefore, the dissolution of PS results in a ready-to-operate device with a released moving component embedded in the cavity of the housing. This work is the first demonstration of a silicon device containing a moving part made by proton beam writing.     

Acoustic mode converters micromachined in silicon by proton beam writing

Proton beam writing is a powerful tool for the production of microstructures for acoustic applications because it allows to create structures inclined to the original sample surface which therefore can act as acoustic mode converters. We report on experiments, finding optimal structure sizes in p-type 12 Ω cm silicon for this purpose. For the creation of the structures the proton beam at the LIPSION laboratory was used. Furthermore, by investigating the micromachined silicon with a phase sensitive acoustic microscope we give evidence that inclined structures such as rods and walls can be used to change the mode of acoustic waves in the crystal.     

Proton beam writing of microstructures at the ion nanoprobe LIPSION

Proton beam writing (PBW) is a direct writing technique which allows the creation of three-dimensional structures with a high aspect ratio in the micrometer and nanometer range in photoresist materials without the use of templates. At the high-energy ion nanoprobe LIPSION of the University of Leipzig this technique was established recently. In this article the latest results concerning this topic are presented. PBW was successfully performed with the positive resist polymethyl methacrylate (PMMA) which is now used at LIPSION in addition to the negative resist SU-8. With the new data acquisition system MICRODAS being commissioned, a new scan program was developed and tested which is dedicated to the creation of arbitrarily shaped structures. Squares with dimensions down to 1.2 μm were created in SU-8 at LIPSION. Furthermore, Ni grids were produced by electroplating, using templates written in PMMA resist. In addition, first structures for studying the growing behavior of Bragg reflectors and the optical characterisation of ZnO nanowhiskers were produced by PBW.     

Advanced applications in microphotonics using proton beam writing

Proton beam writing (PBW) is a powerful tool for prototyping microphotonic structures in a wide variety of materials including polymers, insulators, semiconductors and metals. Prototyping is achieved either through direct fabrication with the proton beam, or by the fabrication of a master that can be used for replication. In recent times we have explored the use of PBW for various advanced optical applications including fabrication of subwavelength metallic structures and metamaterials, direct write of silicon waveguides for mid IR applications and integrated waveguides for lab-on-a-chip devices. This paper will review the recent progress made in these areas with particular emphasis on the main advantages of using the PBW technique for these novel applications.     

Fabrication of smooth silicon optical devices using proton beam writing

This work gives a brief review of proton beam writing and electrochemical etching process for the fabrication of smooth optical devices in bulk silicon. Various types of structures such as silicon-on-oxidized porous silicon waveguides, waveguide grating and disk resonators have been produced. Optical characterization has been carried out on the waveguides for both TE and TM polarization using free space coupling at 1.55 μm. Various fabrication and processing parameters have been optimized in order to reduce the propagation loss to approximately 1 dB/cm. A surface smoothening technique based on controlled oxidation has also been used to achieve an RMS roughness better than 3 nm.     

Proton beam writing of microstructures in Agar gel for patterned cell growth

A rather useful prerequisite for many biological and biophysical studies, e.g., for cell-cell communication or neuronal networks, is confined cell growth on micro-structured surfaces. Solidified Agar layers have smooth surfaces which are electrically neutral and thus inhibit receptor binding and cell adhesion. For the first time, Agar microstructures have been manufactured using proton beam writing (PBW). In the irradiated Agar material the polysaccharides are split into oligosaccharides which can easily be washed off leaving Agar-free areas for cell adhesion. The beam diameter of 1 μm allows the fabrication of compartments accommodating single cells which are connected by micrometer-sized channels. Using the external beam the production process is very fast. Up to 50 Petri dishes can be produced per day which makes this technique very suitable for biological investigations which require large throughputs.     

Proton beam writing and electroplating for the fabrication of high aspect ratio Au microstructures

We present an approach to fabricate tall high aspect ratio Au microstructures by means of proton beam direct writing. Combining proton beam direct writing and electroplating, we successfully produced gold structures with sub-micrometer lateral dimensions, structure heights in excess of 11 μm, and aspect ratios over 28. Sidewall quality of the Au structures was improved by lowering the process temperature to 20 °C when developing PMMA patterns with GG developer. The application of such structures as X-ray masks for deep X-ray lithography with synchrotron radiation was demonstrated.     

Enhancement of proton beam writing in PMMA through optimization of the development procedure

The development step of the proton beam writing (PBW) process plays an important role in the performance characteristics that can be achieved with a resist-developer system. A common developer for PBW in PMMA resist is the mixture IPA/water, used in combination with conventional dip development.In this paper, we investigate the use of the GG-developer, much used in the LIGA-process, and show that the GG-developer is able to dip develop proton beam written structures of feature sizes down to 133 nm in a PMMA layer of 2.4 μm thickness. Moreover, both contrast and sensitivity are found to be higher for the GG-developer compared to dip development in 7:3 IPA/water.The development method as well as the type of developer influences resist development. The effect of megasonic agitation (frequency of 1 MHz) on the development of structures in PMMA was investigated for the developer 7:3 IPA/water. Compared to conventional dip development, structures developed with megasonic agitation showed larger feature sizes, indicating that the development rate was increased. However, performance characteristics were not enhanced: both contrast and sensitivity were found to be lower than after dip development in 7:3 IPA/water.     

PDMS patterning by proton beam

In this paper poly-(dimethylsiloxane) (PDMS) is introduced as a resist material for proton beam writing. We were looking for a biocompatible micropatternable polymer in which the chemical structure changes significantly due to proton beam exposure making the polymer suitable for proton beam writing.Up to now PDMS has been used as a casting or replicating material in microfabrication to form microchannels, micromolds, microstamps, etc. PDMS has not been used as a resist material for direct write techniques. In this work we investigated the surface topography of the irradiated regions of PDMS under and without stress (on the cut surface and on the original fluid surface, respectively). In the samples wherein stress was not developed, noticeable compaction was observed. In the case of samples wherein stress was developed, noticeable swelling occurred.     

Exposure parameters in proton beam writing for KMPR and EPO Core negative tone photoresists

In spite of its recent establishment, proton beam writing (PBW) has already demonstrated to be a highly competitive lithographic technique. PBW is a fast direct-write technique capable of producing high-aspect-ratio micro- and nano-structures in resist material. Typical applications can be found in nanoimprinting, biomedical research, photonics, and optics, among other fields. The progress of PBW is linked to the successful introduction of new resist materials. In this paper, KMPR and EPO Core, negative tone photoresists are tested on their compatibility with PBW. KMPR resist has similar chemical and process properties compared to SU-8. Employing UV lithography on KMPR resist, details of 30 μm have been obtained in Ni, indicating a possible advantage compared to SU-8 for optical lithography [1]. In this study, the sensitivity to MeV proton exposure and sub-micron feature sizes are presented in KMPR. PBW has been also combined with Ni electroplating in order to determine the suitability of KMPR and EPO Core resist to fabricate 3D metallic moulds and stamps.     

Micro-fluidic target chamber machined by proton beam writing for the in situ analysis of gas absorption in synthetic crystals

Many synthetic crystals used for chemical and industrial applications have special internal structures, e.g. nano-pores, which allow separating different gases and fluids. Ion beam analytical methods can be used to study the gas diffusion and absorption in these materials in situ and to visualize their inner surfaces which affect these processes. For this purpose, a small target chamber was constructed in PMMA (polymethyl methacrylate) using proton beam writing (PBW). This micro-fluidic structure enables the establishment of a defined atmosphere around a crystal and allows the simultaneous ion beam analysis. In order to confine the gas from the high vacuum in the measurement chamber Si₃N₄-windows of 200 nm thickness were thermally bonded to the structured PMMA block yielding a closed target chamber with the possibility to accomplish PIXE and RBS measurements. In addition, two capillaries were connected to the chamber for gas inlet and evacuation. First tests showed that the construction is leak-proof and allows to establish a defined atmosphere. After that, the Argon gas diffusion into Zn(tbip)-crystals was studied. These measurements have shown unexpectedly high nickel concentrations in the host crystal which reduces the Argon density in these areas after absorption, because the Ni atoms decrease the pore size by replacing Zn-atoms in the Zn(tbip)-lattice. It could be demonstrated that gas diffusion and absorption in organic crystals can be studied in situ with high lateral resolution using ion beam analysis in a dedicated target chamber machined by PBW.     

Luminescent nanoclusters array fabricated by pulsed laser beam irradiation

Ordered luminescent nanoclusters array in the form of grating structures are fabricated on silicon (1 0 0) surface by Q-switched Nd:YAG laser beam irradiation of second harmonic wavelength (532 nm) in vacuum. Blue-green photoluminescence (PL) spectrum from the ordered nanoclusters array exhibits two asymmetrical peaks at 2.58 eV and 2.88 eV in the blue-green region corresponding to the bimodal distribution of nano size clusters. The size of the nanoclusters is estimated from the three dimensional quantum-confined model incorporating Gaussian size distribution. When subjected to rapid thermal annealing at 710 °C for 10 min in N₂ atmosphere there is an enhancement of the PL intensity without any change in the peak emission energy and broadening suggesting that the origin of PL is related to quantum confinement effect in Si nanocrystallite. The surface morphology of the irradiated surface varies considerable with the number of laser shots, laser fluence and ambient conditions.     

Correlation between ion beam parameters and physical characteristics of nanostructures fabricated by focused ion beam

We report a study of the physical characteristics of the pillars of C, Pt and W grown by 10-30 keV Ga focused ion beam (FIB) as a function of Ga ion flux, and present a quantitative analysis of the elements using energy-dispersive analysis of X-rays (EDAX). All the FIB grown pillars exhibit a rough morphology with whisker like protrusions on the cylindrical surface and broadening of the base as compared to the nominal size. For a constant fluence, the height of the pillar initially increases and then reduces after going through a maximum as a function of ion flux in all the cases. The compositional analysis shows good metallic quality for Pt structures but reveals significant contamination of Ga in C and Ga and C in W structures at higher ion fluxes. Explanation to all these observations has been sought in the light of secondary ion and electron effects and the different processes involved which lead to the FIB induced deposition.     

Observation of the evanescent wave excited by proton beam

An exit angle dependence of the intensity of carbon K line during proton bombardment was measured to observe the evanescent wave. A Soller type spectrometer was used to measure the intensity of X-ray lines. A mirror polished Si (1 1 1) wafer was used as a substrate and carbon was deposited onto this surface. As a result a curious exit angle dependence which is similar to the evanescent wave of X-rays was found . The calculated transmission coefficient of X-ray evanescent wave is compared with measured exit angle dependence of X-ray emission. The experimental and theoretical values are in good agreement. Utilization of this phenomenon permits to enhance the surface sensitivity of the PIXE analysis method.