M. Chatzichristidi

Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics.

An acid-sensitive semiperfluoroalkyl resorcinarene was synthesized, and its lithographic properties were evaluated. Its solubility in segregated hydrofluoroether solvents enables the patterning of delicate organic electronic materials.

Orthogonal processing: A new strategy for organic electronics

The concept of chemical orthogonality has long been practiced in the field of inorganic semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage organic materials, preventing the use of photolithography to pattern organic electronic devices. In this article we show that new photoresist materials that are orthogonal to organics allow the fabrication of complex devices, such as hybrid organic/inorganic circuitry and full-colour organic displays. The examples demonstrate that properly designed photoresists enable the fabrication of organic electronic devices using existing infrastructure.

Electrochemical biosensor microarray functionalized by means of biomolecule friendly photolithography.

Microfabrication permits the incorporation of dense electrode arrays in microsystems and small volume diagnostic devices. However, the specific functionalization of arbitrary shape electrodes with different biomolecules remains a challenging issue. In the present work, the problem of fabricating closely spaced microelectrodes (20 microm sensor diameter and 20 microm-spaced interdigitated electrodes array) that can be modified selectively in order to create multi-analyte sensor arrays is addressed by employing a biomolecule friendly photolithographic procedure for the sequential immobilization of different biomolecules onto separated electrodes of the same array. The concept was demonstrated with selective detection of oligonucleotides for breast cancer gene mutation detection, the hormone T4 detected with specific antibodies and sarcosine and glucose detected with specific enzymes immobilized in two-analyte arrays in order to assure that the method is compatible with all the types of biorecognition molecules used in biosensors. Electrochemical techniques were used in this array, because of the low cost, high sensitivity and easy miniaturization of these transducers. Although the array was composed of only two sets of electrodes, the results demonstrate that the method proposed is generic and could be used for patterning of electrochemical multi-analyte biosensors at even higher resolution.

Cross-linkable molecular glasses: low dielectric constant materials patternable in hydrofluoroethers.

We report a new approach to solution-processable low-dielectric-constant (low-k) materials including photolithographic patterning of these materials in chemically benign and environmentally friendly solvents. A series of semiperfluorinated molecular glasses with styrenic substituents were successfully synthesized. These small molecular materials were thermally stable up to 400 degrees C and also exhibited an amorphous nature, which is essential to forming uniform films. Differential scanning calorimetry studies revealed that a cross-linking reaction occurred in the presence of acid, resulting in the formation of robust polymeric films. Atomic force microscopy images of the cross-linked films showed uniform and pinhole-free surface properties. Dielectric constants determined by a capacitance measurement were 2.6-2.8 (100 kHz) at ambient conditions, which are comparable to other polymeric low-k materials. The incorporation of semiperfluorinated substituents was effective in decreasing the dielectric constant; in particular, the fluorinated alkyl ether structure proved best. In addition, the fluorinated substituents contributed to good solubility in hydrofluoroether (HFE) solvents, which enabled the successful photolithographic patterning of those materials in HFEs down to a submicrometer scale.

Orthogonal patterning of multiple biomolecules using an organic fluorinated resist and imprint lithography.

The ability to spatially deposit multiple biomolecules onto a single surface with high-resolution while retaining biomolecule stability and integrity is critical to the development of micro- and nanoscale biodevices. While conventional lithographic patterning methods are attractive for this application, they typically require the use of UV exposure and/or harsh solvents and imaging materials, which may be damaging to fragile biomolecules. Here, we report the development of a new patterning process based on a fluorinated patterning material that is soluble in hydrofluoroether solvents, which we show to be benign to biomolecules, including proteins and DNA. We demonstrate the implementation of these materials into an orthogonal processing system for patterning multibiomolecule arrays by imprint lithography at room temperature. We further showcase this methods capacity for fabricating patterns of receptor-specific ligands for fundamental cell studies.

Acid-diffusion behaviour in organic thin films and its effect on patterning

Acid and its formation and placement is one of the most important aspects in the chemically amplified photolithographic process. The choice of photoacid generator (PAG) in the photolithographic patterning of acidic substrates, such as PEDOT:PSS, has consequences for the resolution and overall quality of the patterned image. In this study, an acid exchange and diffusion mechanism is proposed for the undesired decomposition of the unexposed photoresist layer containing ionic PAGs. The use of non-ionic PAGs has been shown to be a solution to this decomposition problem. In addition, the acidic nature of the PEDOT:PSS substrate is employed to produce patterned images of a cross-linkable light-emitting polymer. With further optimization and development, this is potentially a fast and simple method to introduce patterns in various organic electronic devices.

Strippable aqueous base developable negative photoresist for high aspect ratio micromachining

The patterning of high aspect ratio structures in ultra thick photoresist films is extremely challenging. Photoresists used should provide low absorption in order to achieve vertical sidewalls and easy stripping after the pattern transfer process. In the current work, a novel resist platform is presented and applied for lithographic evaluation in film thickness up to 60 μm. The resist formulation consists of a mixture of an epoxy novolac oligomer, partially hydrogenated poly(4-hydroxystyrene) and onium salt photoacid generators. Since the resist is rich in hydroxyl groups the unexposed regions can be developed in standard aqueous base solutions (TMAH, 0.26 N). Additionally since the required crosslink density for solubility change is limited, the crosslinked regions can be easily removed by commercial wet photoresist strippers. First experimental data revealed promising lithographic performance with resolution down to 20 μm with an aspect ratio 3:1.

Partially hydrogenated poly(vinyl phenol) based photoresist for near UV, high aspect ratio micromachining

A negative resist platform based on a low Mw epoxy novolac polymer, partially hydrogenated poly(vinyl phenol)s and suitable onium salt photoacid generators is presented and evaluated for high aspect ratio micromachining. The high content of hydroxyl groups in this resist system allows development in standard aqueous base solutions (TMAH 0.26 N). The effects of material parameters such as the degree of poly(vinyl phenol) hydrogenation and the onium salt hydrophobicity on the development process are examined. The cross-linked areas of this resist can be easily removed by commercial wet photoresist strippers after processing. Experimental data demonstrate a promising lithographic performance with an aspect ratio of 7:1 for 5 μm closely spaced lines under exposure at 365 nm.

Orthogonal lithography for organic electronics

Organic electronics has recently gained attention as a new field promising cheaper, flexible, and large-scale devices. Although photolithography has proven to be a high-resolution and high-throughput patterning method with excellent registration capabilities, the emerging field of organic electronics has been largely unsuccessful in adapting this well-established method as a viable approach to patterning. Chemical compatibility issues between organic materials and the processing solvents and chemicals required by photolithography have been the main problem. This challenge has led us to identify a set of non-damaging processing solvents and to develop alternative imaging materials in order to extend photolithographic patterning methods to organic electronics. We have identified supercritical carbon dioxide and hydrofluoroether (HFE) solvents as chemically benign to organic electronic materials and which are also suitable as processing solvents. We refer to these solvents as orthogonal in that they do not substantially interact with traditional aqueous and organic solvents. Multi-layered devices are easily realized by exploiting this orthogonality property; subsequent layers are deposited and patterned without damaging or otherwise adversely affecting previously deposited underlying layers. We have designed and synthesized novel photoresists, which are processible in these benign solvents.

Evaluation of a chemocapacitive sensor array for the detection of vapor analytes and their mixtures

The performance of a four -component IDC sensor array to four analytes of varying polarity and varying chemical affinity to the sensing polymer layers has been studied and discussed in relation to (i) the pure analyte vapor concentration, (ii) exposure to binary analyte mixtures and (iii) PCA analysis of the results.