Prabhat K. Dwivedi

Large Area IR Microlens Arrays of Chalcogenide Glass Photoresists by Grayscale Maskless Lithography

The ability to use chalcogenide glass thin films as photoresists for one-step maskless grayscale lithographic patterning is demonstrated. It is shown that the chalcogenide photoresists can be used to fabricate grayscale patterns with smooth and continuous profiles such as arrays of cylindrical and spherical microlenses, which are useful as optical structures for IR applications. The etching and exposure parameters are optimized to obtain smooth reproducible lens arrays of 150 μm periodicity and up to ∼170 nm height on large areas (∼1 cm(2)). The roughness is found to increase as a function of the exposure dose and is attributed to the selective dissolution of the As-Se, As-As, and Se-Se bonds present in the nanodistributed phases and the presence of the oxide phase. Thus, a minimum exposure dose produces optimally patterned lens arrays. The focal length calculated for the smooth microlens array is ∼9.3 mm, indicating the suitability of the lens arrays for focusing applications in the IR region.

Raman study of ion irradiated GeSe films

Abstract Films of a-Ge 20 Se 80 prepared by vacuum evaporation, are exposed to visible light and irradiated with 1 MeV Krypton ions. The changes in bonding configurations are studied by Raman spectroscopy. Three bands are observed in the virgin films and are identified as corresponding to the stretching mode of GeSe 4/2 corner-sharing tetrahedra (201 cm −1 ) and to the vibrations of Se atoms in four member rings composed of two edge sharing tetrahedra (215 cm −1 ) and Se–Se chains (263 cm −1 ). Upon ion irradiation the band positions remain unchanged and no new band appears. In contrast, light soaking produces an additional band at 257 cm −1 . This bond has been identified as arising from the stretching vibrations of Se atoms in helical chain-like and ring-like arrangements. A study of the relative areas under the peaks shows that ion-irradiation causes the Se–Se bridges linked with GeSe 4/2 units to convert into Se–Se chains and the number of GeSe 4/2 bonds remain unchanged. Upon light soaking, the density of corner-sharing GeSe 4/2 bonds increases, no significant variation is observed in the Se bonds. In addition, the width of the peak at 201 cm −1 decreases after ion irradiation but increases upon light soaking. These data show that ion irradiation induces bond changes in Ge 20 Se 80 , which are different from those, created by light soaking.

PECVD based silicon oxynitride thin films for nano photonic on chip interconnects applications

Thin silicon oxynitride (SiO(x)N(y)) films were deposited by low temperature (~300°C) plasma enhanced chemical vapour deposition (PECVD), using SiH(4), N(2)O, NH(3) precursor of the flow rate 25, 100, 30 sccm and subjected to the post deposition annealing (PDA) treatment at 400°C and 600°C for nano optical/photonics on chip interconnects applications. AFM result reveals the variation of roughness from 60.9 Å to 23.4 Å after PDA treatment with respect to the as-deposited films, favourable surface topography for integrated waveguide applications. A model of decrease in island height with the effect of PDA treatment is proposed in support of AFM results. Raman spectroscopy and FTIR measurements are performed in order to define the change in crystallite and chemical bonding of as-deposited as well as PDA treated samples. These outcomes endorsed to the densification of SiO(x)N(y) thin films, due to decrease in Si-N and Si-O bonds strain, as well the O-H, N-H bonds with in oxynitride network. The increase in refractive index and PL intensity of as deposited SiO(x)N(y) thin films to the PDA treated films at 400°C and 600°C are observed. The significant shift of PL spectra peak positions indicate the change in cluster size as the result of PDA treatment, which influence the optical properties of thin films. It might be due to out diffusion of hydrogen containing species from silicon oxynitride films after PDA treatment. In this way, the structural and optical, feasibility of SiO(x)N(y) films are demonstrated in order to obtain high quality thin films for nano optical/photonics on chip interconnects applications.

Humidity Sensing Investigations on Nanostructured Zinc Stannate Synthesized via Chemical Precipitation Method

ABSTRACT In this paper, nanoparticles of zinc stannate in different compositions were synthesized via chemical precipitation method. The as-synthesized material was characterized using differential scanning calorimetry. For the formation of crystalline zinc stannate, the synthesized material was annealed at 600°C. Surface morphologies of the samples were analyzed using scanning electron microscopy. The XRD pattern indicates the formation of nanocrystalline zinc stannate and it has a perovskite phase with an orthorhombic structure having a minimum crystallite size 4 nm. Pellets of the sensing materials were made by hydraulic press machine under a pressure of 616 MPa. Furthermore, the humidity sensing investigations of these sensing pellets were completed. Our results illustrate that zinc stannate in 1:4 weight ratio was most sensitive to humidity in comparison to other compositions under same conditions. Maximum sensitivity of the sensor was 3 GΩ/%RH. The results were reproducible up to 72% after 2 mon of ...

Chalcogenide Glass Photoresists for Grayscale Patterning

Ability to use chalcogenide glass thin films as photoresists for one step gray scale lithography is demonstrated. It is shown that the chalcogenide photoresists can be used to fabricate gray scale patterns such as array of microlens, which can find their application in IR optics. The direct application of the patterned resisits reduces the cost and time of fabrication as well.

Design, fabrication and characterization of solution-based molded chalcogenide optics for infrared application

We report design, fabrication and characterization of molded chalcogenide microlens array for Infrared sensing applications. A master of desired microlens array with high sag value is prepared using ultraviolet lithography and thermal reflow method on a positive photoresist (ma–P1275HV). The negative replica of the master is created using polydimethylsiloxane which serves as a mold for micro-molding. Further, chalcogenide solution is prepared in ethanolamine solvent and spin coated on a substrate to get a uniform film; these films are characterized and are found to have the same optical properties as the parent bulk chalcogenide glass. Finally, the microlens array is fabricated by the micro-molding of chalcogenide film. Fabricated chalcogenide microlenses are characterized for geometrical parameters, which are used to estimate the optical parameters.

Solution processed chalcogenide films and micro-patterns via self-assembly

Chalcogenide (ChG) are the choice materials for IR applications due to their high refractive index, mid IR transparency and high nonlinear optical properties. In this work, we study the characteristics of solution processed Chalcogenide films, As2S3 prepared by various amine solvents, for possible pattern fabrications. Since solution processed ChG films tend to contain solvent related defects, it is important to optimize the process parameters to create defect free films, structurally similar to bulk ChG. We have studied the physical integrity and morphology of solution processed ChG films as a function of annealing conditions and film thickness. Optical and morphological characterizations of these films are carried out in order to fabricate defect free, optically useful micro-structures.

Structure–property correlation of pure and Sn-doped ZnO nanocrystalline materials prepared by co-precipitation

The structural, optical and electrical properties of pure and tin (Sn) doped zinc oxide (ZnO) nanocrystalline materials prepared by co-precipitation method have been studied as a function of Sn doping concentration. The phase identification through powder X-ray diffraction methods confirmed that pure and Sn-doped zinc oxide powder have typical hexagonal wurtzite structure (a = 3.407 Å and c = 4.592 Å) with slight change in lattice parameters. The surface morphological examination with field emission scanning electron microscopy revealed the fact that the grains are closely and densely packed and pores/voids between the grains decrease with increasing the doping concentration of Sn from 0% to 15%. The energy bandgap of pure ZnO was found to be 3.35 eV from optical absorption spectra obtained by ultraviolet–visible (UV–Vis) absorption spectrophotometer. The variation of energy bandgap and electrical resistivity of Sn-doped ZnO were also determined with tin doping. Upon increasing the Sn dopant concentration from 0 to 15 wt%, the optical bandgaps of ZnO increases from 3.35 to 3.42 eV. The electrical resistivity of Sn-doped ZnO has been decreased at least two orders of magnitude, i.e. from 1263.17 to 28.64 Ω cm. This decrement in electrical resistivity may be due to the partial substitution of divalent Zn2+ ions with tetravalent Sn4+ ions, generating more free electrons for conduction.

Infrared microlenses and gratings of chalcogenide: confined self-organization in solution processed thin liquid films

This work demonstrates the fabrication of chalcogenide microstructures such as gratings, lenses and needles using a lithographically directed, evaporative self-organization of chalcogenide thin liquid films for the first time. Using a two-step annealing protocol, excess solvent of freshly coated ChG films is eliminated and then the liquid films are patterned using elastomeric masters with continuous or disconnected features during solvent evaporation. Although microcontact printing or capillary flow lithography has been proven to be useful to create continuous gratings and waveguide like structures in solid films, our method overcomes the limitation of structural continuity of the generated pattern and uses self-organization of solute ChG within the masters confinement to produce isolated microstructures. Fabrication of disjointed arrays of microlenses of various dimensions as well as conical shaped needles in ChG thin films has been demonstrated for relevant optical IR applications. This methodology establishes evaporative self-organization of ChG thin films as a viable alternative to creating microstructures in bulk ChG with hot-embossing, bypassing the need for ultra high temperature processing.

Enhanced electrical conductivity in solution processed carbon nanotubes incorporated As 2 S 3 glass films

The present paper deals with the synthesis and characterization of solution processed pure and multiwalled carbon nanotubes (MWCNTs) incorporated As2S3 glass films. As2S3 glass has been synthesized using melt quenching technique. The solutions of pure and MWCNTs containing As2S3 glass have been prepared in n-butylamine under inert atmosphere. These solutions have been used to obtain pure and MWCNTs incorporated As2S3 glass films on the glass substrates via spin coating. These films have been annealed, subsequently, to get rid of organic solvent. The morphologies of these films have been analyzed using scanning electron microscopy (SEM) which reveals the porous nature of these films. In the SEM micrographs, a very few MWCNTs are seen on the surface of the films and it is argued that most of the MWCNTs are buried in the film. Elemental and crystallographic analyses of these films have been carried out using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) respectively. While EDS studies confirms the elemental composition of these films, XRD patterns depicts the amorphous nature of these films. Raman and Fourier transform spectroscopies have also been carried out and revealed some critical information regarding these films. Raman spectroscopy of these films indicates towards the functionalization of MWCNTs by intermediate active species which formed during annealing. dc-conductivity measurements and its analysis show that the dc-conductivity and carrier concentration of these films increases drastically with the increase in MWCNTs content in these films. The results obtained in this work may be useful for the chalcogenide glasses based electronic and optoelectronic applications such as energy harvesting and storage, photovoltaics and flexible electronics.