Constantinos D. Diakoumakos

Photolithographic patterning of proteins with photoresists processable under biocompatible conditions

The microlithographic patterning of proteins on solid substrates using photoresists, which can be processed in the presence of biomolecules without affecting their bioactivity, is reported. Chemically amplified resist materials based on poly(t-butyl acrylate) and a newly synthesized copolymer of t-butyl methacrylate, 2-hydroxyethyl methacrylate, isobornyl methacrylate, and acrylic acid designed for this application, are evaluated regarding capabilities for processing under biocompatible conditions (processing temperatures at about 50 °C or lower and development with dilute aqueous base developers). The photoresist formulations based on the newly synthesized (meth)acrylate copolymer had higher sensitivity and contrast allowing lithographic processing even without postexposure bake. Patterns down to 3.75 μm lines/spaces of two different protein molecules, rabbit Immunoglobulin G and bovine serum albumin, on aminosilane-treated silicon surfaces, were obtained with a photoresist formulation based on the new c...

Roughness characterization in positive and negative resists

Abstract Different roughness parameters, such as the root mean square deviation (rms or σ), the correlation length Lcor, the fractal dimension D and the Fourier spectrum, are presented and compared. The scaling behavior of σ determining the Lcor as well as the dependence of σ and D on the exposure dose for two negative tone (wet- and plasma-developed) and one positive tone resist are investigated. The experimental analysis reveals an interesting interrelation (inverse behavior) between σ and D which is not predicted by theory, and elucidates the dependence of Lcor on the exposure dose.

In situ monitoring of thermal transitions in thin polymeric films via optical interferometry

Abstract A novel, low-cost, rapid, accurate, non-invasive and high throughput method based on the principles of Optical Interferometry (OPTI method) has been developed and applied for the in situ monitoring in one simple run of first (melting) and second (glass transition) order transitions as well as of the thermally induced decomposition of various thin polymeric films spin coated on flat reflective substrates (untreated silicon wafers). The new method has been applied successfully for measuring the glass transition, melting and decomposition temperatures of six commercially available polymers [poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate), (PHEMA), poly(vinyl acetate-co-crotonic acid), (PVACA), poly(vinyl pyrrolidone) (PVP), poly(vinyl chloride-co-vinyl acetate) (PVCVA) and crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVFHP)] of known Tgs or Tms. The recorded interferometric signals were identified and characteristic signal patterns were qualitatively correlated to specific transitions. The monitoring of first and second order transitions in thin polymeric films is based on detectable differentiations of the total energy of a fixed wavelength laser beam incident almost vertically (angle of incidence

Dilute aqueous base developable resists for environmentally friendly and biocompatible processes

Photoresists developable in dilute aqueous bases are introduced for environmentally friendly photoresist processing and biopatterning. The proposed photoresists are of positive or negative tone, chemically amplified, based on synthesized (meth)acrylate copolymers bearing either pendant t-butyl ester group or 2-hydroxylethyl ester group for positive and negative imaging, respectively. Characteristic high-resolution lithographic results (0.13 μm lines) obtained with a positive (meth)acrylate-based chemically amplified resist formulation upon development with 13 × 10 -3 M TMAH solution are presented. On the other hand, a slightly modified resist formulation was used for biomolecule patterning under biocompatible conditions and protein microstructures (<10 μm equally spaced lines) are demonstrated.

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.

Non-destructive method for monitoring glass transitions in thin photoresist films

Abstract To a great extent, the glass transition temperature ( T g ) of a photoresist affects its lithographic processing. In the current work, a non-invasive method based on optical interferometry has been developed and applied for the monitoring of T g film for thin polymeric films in the lithographically useful thickness range. The measurement of T g film is based on the change of film thickness and/or refractive index of the polymeric film during the glass transition, which is reflected as a curvature in the interferograms (interferometric signal vs. temperature). The method has been applied successfully in the case of two methacrylate-based polymers. For these materials the T g film coincides with T g for the polymer for a thickness range from thick films down to ∼200–250 nm. For thinner layers, the T g film increases as resist thickness decreases.

Fractal Roughness of Polymers after Lithographic Processing

The morphology of photoresist polymer surfaces fabricated by lithographic processes is shown to exhibit self-affine behavior for a specific range of scales. The roughness parameters appropriate for the characterization of self-affine surfaces (surface width w, correlation length ξ and roughness exponent α) are found to depend on the exposure dose (i.e. the solubility) involved in the lithography process and to be correlated. A similar dependence and correlation is extracted from a Monte-Carlo simulation of polymer dissolution, thus indicating the pronounced contribution of the dissolution process to the formation of lithographic roughness. The self-affinity and the correlated behavior of the roughness parameters is a general phenomenon for dissolving polymers of varying solubility and not limited to the lithographic process.

Photoresist etch resistance enhancement using novel polycarbocyclic derivatives as additives

In house synthesized mixed derivatives containing at least two carbocycles per molecule from the group of anthracenes, adamantanes and steroids with functionalized carbon chains are used as modifiers of resist properties and especially etch resistance enhancement and absorption characteristics. Etch resistance enhancement is demonstrated in oxygen, fluorine, and chlorine-containing plasmas under reactive ion etching and inductively coupled plasma etching conditions for poly(methyl-methacrylate) and a chemically amplified, positive-tone, methacrylate-based 193 nm photoresist formulation. High resolution experiments show that the proposed additives do not adversely affect the lithographic properties of the photoresists. The additive role is explained with empirical etch parameters such as the Ohnishi number and the ring parameter.

Negative (meth)acrylate resist materials based on novel crosslinking chemistry

Abstract Novel negative-tone resist materials suitable for 193 nm and e-beam lithography, based on newly-synthesized (meth)acrylate copolymers that contain (2-hydroxyethyl methacrylate) as the necessary component for imaging and aqueous base development, are presented. A representative resist of this class is formulated using poly(2-hydroxyethyl methacrylate-co-cyclohexyl methacrylate-co-isobornyl methacrylate-co-acrylic acid) (PHECIMA) and a sulfonium salt photo acid generator. The negative image formation is based on acid induced crosslinking of the hydroxyl groups of the 2-hydroxyethyl methacrylate (HEMA) moieties. The negative resist presented no swelling phenomena in the aqueous base developer and enhanced etch resistance. Dense 0.18 μm and isolated 0.13 μm lines have been obtained upon 193 nm and e-beam lithography establishing PHECIMA’s resist formulations as promising candidates for high-resolution lithography upon a further material and process optimization.