Cm Christian Leewis

Formation of cationic silicon clusters in a remote silane plasma and their contribution to hydrogenated amorphous silicon film growth

The formation of cationic silicon clusters SinHm+ by means of ion–molecule reactions in a remote Ar–H2–SiH4 plasma is studied by a combination of ion mass spectrometry and Langmuir probe measurements. The plasma, used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H), is based on SiH4 dissociation in a downstream region by a thermal plasma source created Ar–H2 plasma. The electron temperature, ion fluence, and most abundant ion emanating from this plasma source are studied as a function of H2 admixture in the source. The electron temperature obtained is in the range of 0.1–0.3 eV and is too low for electron induced ionization. The formation of silicon containing ions is therefore determined by charge transfer reactions between ions emanating from the plasma source and SiH4. While the ion fluence from the source decreases by about a factor of 40 when a considerable flow of H2 is admixed in the source, the flux of cationic silicon clusters towards the substrate depends only slightly...

Reaction-diffusion model for the preparation of polymer gratings by patterned ultraviolet illumination

A model is developed to describe the migration mechanism of monomers during the lithographic preparation of polymer gratings by ultraviolet polymerization. The model is based on the Flory–Huggins theory: a thermodynamic theory that deals with monomer/polymer solutions. During the photoinduced polymerization process, monomer migration is assumed to be driven by a gradient in the chemical potential rather than the concentration. If the chemical potential is used as the driving force, monomer migration is not only driven by a difference in concentration, or volume fraction, but also by other entropic effects such as monomer size and the degree of crosslinking of the polymer network, which is related to the ability of a polymer to swell. Interaction of the monomers with each other or the polymer is an additional energetic term in the chemical potential. The theoretical background of the model is explained and results of simulations are compared with those of nuclear microprobe measurements. A nuclear micropro...

In-situ compositional and structural analysis of plastic solar cells

Bulk-heterojunction photovoltaic cells consisting of a photoactive layer of poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and a C60 derivative, (1-(3-methoxycarbonyl)propyl-1-phenyl-[6,6]-methanofullerene), (PCBM), sandwiched between an indium tin oxide (ITO) anode covered with poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), and an aluminum cathode have been analyzed using transmission electron microscopy (TEM) and cryogenic Rutherford backscattering spectrometry (RBS) to assess the structural and elemental composition of these devices. TEM of cross sections of fully processed photovoltaic cells, prepared using a focused ion beam, provide a clear view of the individual layers and their interfaces. RBS shows that during preparation diffusion of indium into the PEDOT:PSS occurs while the diffusion of aluminum into the polymer layers is negligible. An iodinated C60 derivative (I-PCBM) was used to determine the concentration profile of this derivative in the vertical direction of a 100 nm active layer.

Mass transport phenomena during lithographic polymerization of nematic monomers monitored with interferometry

Photopolymerization of liquid-crystalline diacrylates is a versatile tool to make optical films for liquid-crystal display (LCD) enhancement. The constant drive towards LCD’s having an improved front-of-screen performance demands optical films with properties that can be adjusted on (sub) pixel level. Birefringent films made from liquid-crystalline diacrylates allow for the required adjustment of the optical property on (sub) pixel level. In this paper we report on the composition of the acrylate mixture that results in planarly aligned nematic films usable as optical retarder in transflective LCD’s as well as the mass transport phenomena that take place during heating of a mask-exposed birefringent film of liquid-crystalline diacrylates. The mass transport phenomena are studied by interferometry as a function of temperature and time. Upon heating a pronounced surface corrugation arises from the latent image formed during the mask exposure. The surface profile largely depends on lateral feature sizes. For...

Simulations with a dynamic reaction-diffusion model of the polymer grating preparation by patterned ultraviolet illumination

Simulations of volume fraction profiles formed during the lithographic preparation of polymer gratings are made with a reaction/diffusion model, based on the Flory–Huggins theory. Monomer migration is driven by a gradient in the chemical potential rather than a gradient in the concentration. If the chemical potential is used as the driving force, monomer migration is not only driven by a difference in concentration, or volume fraction, but also by other entropic effects: the differences in monomer length and the degree of crosslinking of a polymer network. The monomer volume fractions are simulated as a function of position for different ultraviolet intensities and various grating pitches. Profound edges of the monomer volume fractions caused by the fact that the reaction rate is high compared to the diffusion rate are both measured and simulated. An excellent agreement with nuclear microprobe measurements on the polymer gratings is obtained.

Spectroscopic second harmonic generation measured on plasma-deposited hydrogenated amorphous silicon thin films

Optical second harmonic generation (SHG) has been measured for plasma-deposited thin films of hydrogenated amorphous silicon (a‐Si:H) at different polarization states for pump photon energies between 1.0 and 1.7eV. Distinct resonance peaks are observed in this energy range and it is shown that the SH signal originates from an isotropic contribution at both the film-surface and substrate-interface region. The possibility that the SH signal originates from surface and interface dangling bond states of a‐Si:H is discussed.

Formation of large positive silicon-cluster ions in a remote silane plasma

The formation of hydrogen poor cationic silicon clusters SinHm+ with up to ten silicon atoms in an expanding argon–hydrogen–silane plasma has been studied by mass spectrometry and Langmuir probe measurements. Sequential clustering reactions with silane, initiated by silane ions, cause their size to depend on the product of silane density and geometrical path length having possible implications for a-Si:H films deposited by remote plasmas. Reaction rates, estimated by a one-dimensional model, show no strong dependence on the number of silicon and hydrogen atoms present in the ions in contrast with rates determined by ion-cyclotron resonance mass spectrometry studies. Possible causes of the discrepancy are discussed as well as the hydrogen poverty of the clusters. The maximum contribution of the cationic clusters to the growth flux is about 6% for the conditions investigated.

Monomer diffusion assisted preparation of polymer gratings: A nuclear microprobe study

Abstract Polymers with an ordered molecular structure can be applied in optical systems for e.g. data transport, data storage and displays. Patterned UV photo-polymerization is used to prepare polymer gratings from a mixture of two acrylate monomers. A 3 MeV proton microprobe is used to study these gratings, prepared from two different monomers, each containing a different easily detectable label element, e.g. Cl, Si or F. During the preparation process, the difference in reactivity and mobility of these two monomers in combination with polymer–monomer interaction results in diffusion of monomers. Since this diffusion process takes place on length scales of micrometers, a scanning ion microprobe is a powerful tool for the quantitative analysis of the polymer films, obtained after complete polymerization. The microprobe is equipped with PIXE, PIGE and RBS, to quantify both the label elements and C and O. This makes it possible to determine the concentration of monomer units as a function of position and thus to study the diffusion process. Two combinations of different monomers are studied. In the case of a 0.5:1 mixture of a monofunctional and a difunctional monomer, both monomers migrate to the illuminated areas and large thickness variations are observed. When a 1:1 mixture of two difunctional monomers is used, opposite migration of the two monomers is observed, while the film shows no variation in thickness.

PIXE monitoring of diffusion during photo-polymerization

Patterned UV photo-polymerization is used to prepare polymer gratings from a mixture of mono- and di-functional acrylate monomers. The difference in reactivity and mobility of these two monomers induces a concentration gradient during the photo-polymerization process. Uniform illumination afterwards fixes the grating. Monomers with different easily detectable elements, e.g., Cl, Br, Si, F, enable the observation of lateral variations of concentration in these gratings with PIXE and PIGE using a scanning ion probe of 3 MeV protons. In addition, backscattering spectrometry is applied to correct for lateral thickness variations.

The mutual diffusion coefficient for (meth)acrylate monomers as determined with a nuclear microprobe

The value of the mutual diffusion coefficient DV of two acrylic monomers is determined with nuclear microprobe measurements on a set of polymer films. These films have been prepared by allowing the monomers to diffuse into each other for a certain time and subsequently applying fast ultraviolet photo-polymerization, which freezes the concentration profile. The monomer diffusion profiles are studied with a scanning 2.1 MeV proton microprobe. Each monomer contains a marker element, e.g., Cl and Si, which are easily detected with proton induced x-ray emission. From the diffusion profiles, it is possible to determine the mutual diffusion coefficient. The mutual diffusion coefficient is dependent of concentration, which is concluded from the asymmetry in the Cl- and Si-profiles. A linear dependence of the mutual diffusion coefficient on the composition is used as a first order approximation. The best fits are obtained for a value of b=(0.38+/-0.15), which is the ratio of the diffusion coefficient of 1,3-bis(3-methacryloxypropyl)-1, 1,3,3-tetramethyldisiloxane in pure 2-chloroethyl acrylate and the diffusion coefficient of 2-chloroethyl acrylate in pure 1,3-bis(3-methacryloxypropyl)-1,1,3,3-tetramethyldisiloxane. Under the assumption of a linear dependence of the mutual diffusion coefficient DV on monomer composition, it follows that DV = (2.9+/-0.6)10(-10) m(2)/s at a 1:1 monomer ratio. With Flory-Huggins expressions for the monomer chemical potentials, one can derive approximate values for the individual monomer diffusion coefficients.