Research activity

Micromachining

Properties of resist materials

Contact: István Rajta, rajta (at) atomki.hu

A large part of our activities is about investigation of new types of resist materials. Previously only PMMA and SU-8 have been used for proton beam writing (PBW). We first introduced polyallyl diglycol carbonate (PADC, also known as CR-39) as a potential candidate as resist material in PBW, as this polymer is widely used as a solid state nuclear track detector [64]. Furthermore, we have also shown in this paper the first demonstration of proton beam written microstructures in Foturan™ which is a photosensitive glass.

We have investigated a new type of negative resist material, developed in the Institute of Microelectronics in Greece, and shown that this material is suitable for production of 3D microstructures. The polymer mix is called TADEP, and it consists of 78% (w/w) PHS with 12% degree of hydrogenation and 22% (w/w) EP and 1-(4-hydroxy-3-methylphenyl) tetrahydrothiophenium triflate (o-CS-triflate) 3% (w/w). In contrast to SU-8, this polymer is developable in aqueous base chemicals and thus not too harmful for the environment [65].

The third type of material that we investigated is poly-(dimethylsiloxane) (PDMS), a commonly used silicon-based organic polymer, optically clear, generally considered to be inert, non-toxic and biocompatible. PDMS has been used as a resist material for direct write techniques only in very few cases [66]. PDMS is usually used as a replicating material. Our structures (e.g. optical gratings, Fresnel lenses, etc.) were irradiated directly into the polymer.

We have also explored the changes of the materials properties due to the irradiation. We measured the refractive index change in poly(methyl-methacrylate) (PMMA) as a function of the ions penetration depth [67] using ellipsometry method. This information is of crucial importance when one needs to produce optical waveguides in the polymer. Experiments on other physical and chemical properties are in progress.


Devices produced by proton beam writing

A 3D Si micro-turbine [68] characterized by high aspect ratio vertical walls was formed by the combination of PBW and subsequent selective porous Si (PS) etching. Characteristic feature of the proposed process is that the shape of the micro electromechanical (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. This work is the first demonstration of a silicon device containing a moving part made by proton beam writing. The video can be seen here: http://iba.atomki.hu/video/

An array of microcapillaries has been produced by PBW (see figure below). A PMMA (poly(methyl-metacrylate)) film of 50 µm thickness was used, and circular holes of 10 µm diameter were fabricated. These structures can be used in various fields in atomic physics (guiding of highly charged ions), and medical applications (filtering).


The polycapillary film applied in the microreactor and in biomedicine.

Due to the direct write way of PBW, the array of the capillaries allows a high ratio of open holes vs. substrate area. At the same time the holes have a circular shape, and unlike in the case of the conventional filters made by a broad beam of heavy ions, they are not overlapped.

A microreactor or microstructured reactor is a device in which chemical reactions take place in typical lateral dimensions below 1 mm. Microreactors are innovative and promising tools in technology nowadays because of their advantages compared to the conventional scale ones, including vast improvements in surface to volume ratio, energy efficiency, reaction speed and yield, increased control of reaction conditions, etc.

The most typical forms of microreactors are microchannels or microfluidic reactors [69]. These kinds of reactors are extremely important tools in technology today, because a large number of synthetic reactions have been carried out since the first successful synthesis of azo dyes. The microfluidic chip reactors require less space, reagents and energy and provide higher yields and improved reaction selectivity in short reaction time and nevertheless produce less wastes because of its short diffusion distance, large specific surface area and increased thermal transfer. The figure below shows one of our microfluidic reactor designs.


A microfluidic reactor created by 2 MeV ion beam. The channel diameter is 30 µm, channel length is about 10 cm.
Applicable in organic synthesis.

References:

[64] I. Rajta, I. Gómez Morilla, M.H. Abraham, Á.Z. Kiss: Proton beam micromachining on PMMA, Foturan and CR-39 materials, Nucl. Instr. and Meth. B 210 (2003) 260

[65] I. Rajta, E. Baradács, M. Chatzichristidi, E.S. Valamontes, I. Uzonyi, I. Raptis: Proton beam micromachining on strippable aqueous base developable negative resist, Nucl. Instr. and Meth. B 231 (2005) 423

[66] S.Z. Szilasi, R. Huszánk, C. Cserháti, A. Csik, I. Rajta: PDMS patterning by proton beam, ATOMKI Annual Report 2008, Paper 8.7

[67] I. Rajta, S.Z. Szilasi, J. Budai, Z. Tóth, P. Petrik, E. Baradács: Refractive index depth profile in PMMA due to proton irradiation, Nucl. Instr. and Meth. B 260 (2007) 400

[68] I. Rajta, S.Z. Szilasi, P. Fürjes, Z. Fekete, Cs. Dücso: Si Micro-turbine by Proton Beam Writing and Porous Silicon Micromachining, ATOMKI Annual Report 2008, Paper 8.9

[69] R. Huszánk, S.Z. Szilasi, K. Vad, I. Rajta: First Experiments on a Microreactor Created by Proton Microbeam, ATOMKI Annual Report 2008, Paper 8.8