In the presented work solvent-free film preparation from molecular glass resists, the evaluation of the patterning performance using thermal scanning probe lithography (tSPL) and an efficient etch transfer process are demonstrated. As the presented materials have a high tendency to crystallize and thus form crystalline films of bad quality when processed by solution casting, two component mixtures prepared by coevaporation were investigated. Stable amorphous films were obtained by selecting compatible material pairs for the coevaporation. One optimized material pair is based on trissubstituted, twisted resist materials with a distinct difference in molecular design. Here a high resolution tSPL prepared pattern of 18 nm half pitch in a 10 nm thick film is demonstrated.
A further optimization is reported for “small” cubic silsequioxane molecules. Again single component films show independent to applied film preparation techniques bad film forming properties due to the high crystallinity of the symmetric cubic silsequioxane molecules. But coevaporation of the phenyl substituted octaphenylsilsequioxane combined with the fully aromatic 2,2',7,7'-tetraphenyl-9,9'-spirobi[fluorene] results in stable amorphous thin films. tSPL investigations demonstrate the patternability by writing high resolution line features of 20 nm half pitch. An important advantage of such a silicon rich resist material is that it can be directly converted to SiO2, yielding to a patterned hardmask of SiO2. This proof of principle is demonstrated and an efficient pattern transfer of 60 nm half pitch line into the underlying HM8006 is reported.
In the presented work solvent-free film preparation from tailored molecular glass resists, their thermal analysis, the characterization of etch resistance for plasma etching transfer processes, and the evaluation of the patterning performance using scanning probe lithography (SPL) tools, in particular electric field and thermal based SPL, are demonstrated. Therefore a series of fully aromatic spiro-based and tris-substituted twisted resist materials were systematically investigated. The materials feature very high glass transition temperatures of up to 173 °C, which allows solvent-free thin film preparation by physical vapor deposition (PVD) due to their high thermal stability. The PVD prepared films offer distinct advantages compared to spin coated films such as no pinholes, defects, or residual solvent domains, which can locally affect the film properties. In addition, PVD prepared films do not need a post apply bake (PAB) and can be precisely prepared in the nanometer range layer thickness. An observed sufficient plasma etching resistance is promising for an efficient pattern transfer even by utilizing only 10 nm thin resist films. Their lithographic resolution potential is demonstrated by a positive and a negative tone patterning using electric field, current controlled scanning probe lithography (EF-CC-SPL) at the Technical University of Ilmenau or thermal scanning probe lithography (tSPL) investigations at the IBM Research - Zurich. High resolution tSPL prepared patterns of 11 nm half pitch and at 4 nm patterning depth are demonstrated.
The presented work deals with molecular glass resist materials based on (i) calix[4]resorcinarene resist systems, (ii) twisted fully aromatic biscarbazole-biphenyl materials, and (iii) fully aromatic spiro resist materials as new promising materials for Scanning Probe Lithography (SPL). Because of the non-chemically amplified resist nature and the absence of corresponding material diffusion, the novel SPL resists have the potential to increase the patterning resolution capabilities at a simultaneous reduction of the edge roughness (LER). In addition, these low molecular weight molecular glasses offer the advantage of solvent-free film preparation by physical vapor deposition (PVD). The PVD prepared films offer a number of advantages compared to spin coated ones such as no more pinholes, defects, or residual solvent domains, which can locally affect the film properties. These high-quality PVD films are ideal candidates for the direct patterning by SPL tools. Presented highlights are the thermal scanning probe lithography (tSPL) investigations at IBM Research - Zurich and the patterning by using electric field, current controlled scanning probe lithography (EF-CC-SPL) at the Technical University of Ilmenau. Further investigations on film forming behavior, etch resistance, and etch transfer are presented. Owing to the high-resolution probe based patterning capability in combination with their improved etch selectivity compared to reference polymeric resists the presented molecular glass resists are highly promising candidates for lithography at the single nanometer digit level.
Star block copolymer synthesis was performed in a controlled fashion by an in-situ core first ATRP route. The obtained
resist materials on the basis of industrial used monomers with tailored star block copolymer architecture were
systematically characterized and patterned. In dissolution investigations an excellent dissolution contrast between
exposed and unexposed state was identified for this new resist material type. Additionally, the materials show an
excellent sensitivity, which surpass the reference linear copolymer by a factor of eight. By a combinatorial resist
optimization realized high resolution features are presented. Finally, preliminary results utilizing a further improved
resist material design are shown.
Azobenzene-containing diblock copolymers are a promising material class for holographic data storage. They have many
advantages, yet, a problem are their long writing times. Low-molecular-weight glasses containing azobenzene moieties
are also photo-addressable and develop holographic gratings much faster. By blending molecular glass formers with
diblock copolymers, materials for holographic data storage with distinctly improved properties can be obtained. The
writing times of holographic gratings in the blends decrease with increasing content of the molecular glass former;
nevertheless, the gratings are still stable.
As the semiconductor industry moves forward, resolution limits are being pushed to the sub-30 nm regime. In order to
meet these demands, radical new resist design and processes must be explored. We have developed a molecular glass
system for all-dry processing conditions. Physical vapor deposition (PVD) has been used for film formation onto silicon
wafers. PVD deposits a uniform film of controlled thickness free from impurities that are often introduced by casting
solvents used in traditional spin coating methods. Thermal development is used as an alternative to processing in
solvents in order to prevent resist swelling and pattern collapse by capillary forces. The deposited molecule is designed
to crosslink upon E-beam irradiation without additives, and therefore form a homogeneous, single component film.
PAG-attached molecular glasses have been synthesized in order to promote film homogeneity as well. By tethering PAG
directly to the molecular glass core, issues such as PAG aggregation can be remedied. Acid migration, which increases
blur and LER, can also be hindered.
Linear and star-shaped ArF photoresists were prepared and preliminary lithographic comparison was performed
using electron-beam exposure. An oligo-initiator based on saccharose forms the core of the star shaped photoresist
from which three standard ArF photoresist monomers, α-gamma butyrolactone methacrylate (GBLMA), methyl
adamantyl methacrylate (MAMA) and hydroxyl adamantyl methacrylate (HAMA) were polymerized. Conditions
were adjusted to obtain a low polydispersity, 6 kg/mol star polymer with a degree of polymerization of approximately five mers per arm. For comparison, a linear photoresist control was prepared using the same scheme. The star resist architecture was found to improve roughness without reducing sensitivity or resolution.
In recent years the developments in computer technology have drastically accelerated. In order to handle the ever
growing amount of data to be stored, an increase in the storage capacities of mass storage media is necessary. A
promising approach is holography, where the storage capacity is greatly increased by using the entire volume instead of
only the surface of the medium.
Write-once media, which are mainly based on photopolymer systems, seem to be fairly advanced. For a rewriteable
media, only few systems can meet the strict requirements for holographic data storage materials. Photoaddressable
azobenzene-based polymer systems are the most promising candidates, however the photo-physical sensitivity of these
materials has to be further increased.
Low molecular-weight organic glasses with azobenzene moieties can also be used for reversible inscription of
holographic volume gratings. They exhibit a faster response time than a comparable photoaddressable polymer due to a
lack of chain entanglements. A new photochromic molecular glass with optimized physical and photo-physical
properties is investigated as blending materials to improve the photochromic response of photoaddressable polymers. By
doping this photochromic molecular glass into photoaddressable polymers we are able to combine the high stability of
polymer systems with the fast response of molecular glasses, thus creating a system which has the advantages of both
material classes.
This paper reports on a new series of photochromic azobenzene-containing molecular glasses which are investigated as
blending materials to improve the photochromic response of photoaddressable polymers. In order to identify potential
candidates for blending, a variety of molecular glasses are synthesized and screened with respect to their photochromic
response. Various end groups at the azobenzene chromophore and different core compounds are employed to obtain high
thermal stability and good glass-forming properties as well as high sensitivity and refractive-index modulation. The best
combination of structural variations is chosen to tailor a photoaddressable material with optimized physical and
photophysical properties. By doping this photochromic molecular glass former into a photoaddressable polymer we are
able to combine the high stability of polymer systems with the faster response of molecular glasses, thus creating a
system which has the advantages of both material classes. With this approach we achieve an increase of the low
photochromic sensitivity of the polymer, which is the biggest problem of these materials. In order to obtain a better
understanding of the influence of the photochromic molecular glass former on the photochromic response in a polymer
system we conduct holographic experiments on a concentration series of the glass former in an inert polymer matrix. By
inscribing the holographic gratings at elevated temperatures we are able to increase the sensitivity of our molecular glass
further.
The formation of phase and surface relief gratings in low-molecular-weight organic glasses containing azobenzene
moieties has been studied with holographic methods. Advantages of this class of materials are the simple synthesis, the
perfectly amorphous phase, and the possibility of blending them with polymers. Surface relief gratings are formed very
efficiently in molecular glasses, and this process can be explained by the gradient force model. Heights up to 610 nm
were measured; the temporal evolution of the diffraction efficiency could be reproduced in computer simulations. For
technical applications, the surface relief gratings can easily be duplicated by replica molding. Since surface gratings are
detrimental to holographic data storage at high densities, it is also possible to suppress their formation by using proper
polarizations of the writing beams. Reorientation of the azobenzene groups in the bulk of the glasses and angular
multiplexing was demonstrated and the thermal stability of the corresponding phase gratings was studied. Different
combinations of molecular cores and substituents at the azobenzene moieties were investigated to find the best systems
which yield a high sensitivity and fast grating build-up.
The aim of the paper is the development of an all-dry photolithographic process in which the film preparation step as
well as the development step is performed without the use of solvent. To implement an all-dry photoresist system we
focused on coumarin derivatives, as this class can be photodimerized in the solid state and features sufficient high
thermal stability. The dimerized product exhibits sufficiently different physical properties. The monomer can be
evaporated at elevated temperatures whereas the dimerized product remains non-volatile under these conditions. With a
tailored glass forming coumarin derivative we demonstrated the capability to develop clear patterns. A combinatorial
approach, i.e. producing a compositional library in combination with the variation of exposure dose was utilized to
efficiently optimize the all-dry photoresist system.
The optically induced birefringence in different block copolymers containing azobenzene and mesogen side groups has been studied with holographic methods. In the materials, the light-sensitive blocks carrying the azobenzene moieties are embedded in a matrix of polystyrene. This has the advantage that the reorientation of the chromophores does not lead to the formation of surface relief gratings. In addition, it is possible to reduce the macroscopic chromophore concentration to any desired value while, at the same time, maintaining the stabilization effect due to cooperative reorientation of the chromophores inside the photo-addressable phases. These short-range interactions give rise to long-term stability of the inscribed gratings. Angular multiplexing of holographic plane-wave gratings as well as of two-dimensional images has been demonstrated in samples with thicknesses up to 1.1 mm. The achievable refractive-index modulation, the photo-sensitivity, and the stability of the inscribed gratings were compared for different materials. In contrast to photopolymers, our materials do not exhibit shrinkage upon illumination. Instead, a weak light-induced volume expansion was detected and studied in detail.
The optically induced birefringence in different azobenzene and mesogen containing block copolymers has been studied with holographic methods as well as with measurements between crossed polarizers. The influence of various parameters for possible applications as holographic data storage materials was investigated. In the block copolymers, the functionalized blocks are confined in a matrix of polystyrene. This has the advantage that no surface relief
gratings appear when the chromophores are reoriented. Also liquid-crystalline stabilization due to cooperative reorientation occurs inside the functionalized blocks. These short-distance interactions in the blocks give rise to long-term stability of the inscribed gratings. So far injection-molded blends with azobenzene containing block copolymers were capable to store up to 200 holograms on one spot. On such a sample we could also demonstrate a fundamental physical effect which generally plays a role for light diffraction on thick gratings. According to the Kramers-Kronig relations, an attenuation of light is always coupled with a phase shift. In the case of a thick holographic grating, the intensity of the transmitted wave (of diffraction order zero) is a function of the angle of incidence, because part the light is diffracted into the first order when the Bragg condition is fulfilled. We measured the phase shift of the transmitted wave as function of the Bragg mismatch angle and found good agreement with analytical calculations.
We synthesized lithium quinolate complexes, 8- hydroxyquinolinolato lithium (Liq) and 2-methyl-8- hydroxyquinolinolato lithium (LiMeq) as emitter and electron injection/transport materials to be used in conventional two layer organic light emitting diodes in combination with N,N'-bis(p-methoxyphenyl)-N,N'-diphenyl benzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8- hydroxyquinolinolato Al(III) as emitter material. The device efficiency using these complexes were optimized using combinatorial methods. We also compared the electron injection, transport and emission properties of Li-complexes with the well known emitter Alq3 in the same experiment taking advantage of the combinatorial approach. The Li- quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq3 devices. Contrary to the Alq3 emission, the Li-quinolates exhibit bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence for exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li-quinolate and DMeOTPD with the observed EL spectrum. However, the lithium complexes increase the efficiency of an optimized ITO/DMeOTPD/Alq3/Al device considerably when used as a thin interface layer between Alq3 and aluminum. The improvement of device characteristics using lithium quinolates is similar to that obtained using LiF salt. The mechanism of improvement of efficiency using a lithium complex interface layer seems to be of chemical doping nature.
Most photorefractive (PR) materials require plasticizers in order to decrease the glass transition temperature, allowing for orientational enhancement by the chromophores. Introduction of the plasticizer, however, alters not only the viscosity but also the photoconductive properties of the material. This can be shown by comparing two different plasticizers which were introduced into a bifunctional low-molecular-weight PR glass and into a polyfluorene guest-host polymer. The latter reaches response times down to 600 microsecond(s) at a writing intensity of 1 W/cm2. We have recently improved the concept of low-molecular-weight PR glasses. A suitable, photoconducting unit allows the synthesis of a bifunctional system with a glass transition of 22.6 degree(s)C. Therefore, no plasticizer is needed. The material is based on a triphenyldiamine (TPD) moiety to which a nonlinear-optical chromophore is directly attached. The system is the first representative of a whole class of TPD molecules and polymers for photorefractive applications.
A combinatorial approach combining vapor deposition of organic molecules and a mask technique was used to prepare on one substrate a matrix of 49 organic light emitting diodes (OLEDs) with different configuration and layer thickness. A landscape library with two orthogonal, linear gradients of an emitter and a hole blocking electron transport material on top of a hole transport layer of constant thickness was prepared. The aim of this experiment was to investigate the influence of an additional electron transport material on the efficiency. Using a semi-automated measurement set-up, the device parameters for each of the 49 OLEDs were evaluated. The existence of an optimum Alq3 layer thickness for two-layer devices ITO/TPD/Alq3/Al is confirmed and such an optimized two-layer structure could not be improved by adding an additional hole blocking layer to the optimum Alq3 layer. But an improvement of photometric efficiency can be obtained by replacing the optimum Alq3 layer thickness by certain combinations of Alq3/spiro-Quinoxaline layers.
An experimental set-up was used to optimize the layer thicknesses of hole transport materials and electron transport/emitter material in multi-layer light emitting diodes by combinatorial methods. The method is based on a movable mask/shutter technique and simultaneous evaporation of organic molecules resulting in linear gradients of layer thickness. This allows the preparation of different devices in one single experiment under identical conditions. In the first experiment, we studied the influence of the Alq3 layer thickness on photometric and power efficiency in two layer devices using various TPD derivatives as hole transport material at a constant thickness. Some new low molecular weight TPDs and a polymeric TPD were utilized. Both photometric and power efficiencies depend considerably on the thickness of the Alq3 layer. In a second experiment, the efficiency dependence on both the TPD and Alq3 layer thickness was investigated simultaneously by preparing a landscape library with two orthogonal linear gradients of TPD and Alq3. It was observed that the device efficiencies depend on both TPD and Alq3 layer thicknesses and additionally on the total thickness of the organic layer.
We present an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a novel plasticizer and doping with the well known sensitizer C60 we obtained a composite material, which shows extremely fast initial response times of 2.5 ms at writing beam intensities of Iwrite equals 1 W/cm2 and 450 microsecond(s) at Iwrite equals 10.8 W/cm2. Combined with high refractive index modulations of up to (Delta) n equals 6 X 10-3 and sample lifetimes of over 6 months this material exhibits an excellent overall performance.
Uwe Hofmann, Stefan Schloter, Andreas Schreiber, K. Hoechstetter, G. Bauml, Stephan Zilker, Dietrich Haarer, Mukundan Thelakkat, Hans-Werner Schmidt, Kai Ewert, C.-D. Eisenbach
Photorefractive guest-host systems based on photoconducting polymers such as poly(N-vinylcabazol) or polysiloxane generally exhibit large diffraction efficiencies and photorefractive gain coefficients. Their response times however, are limited by the orientation of the nonlinear optical chromophores and by the photoconducting properties of the polymer. Rise times down to 50 ms have been observed until now.
Various fluoro-functionalized aromatic 1,3,5-triazine monomers were prepared. A series low molar mass and poly- (1,3,5-triazine)-ethers were synthesized by a condensation reaction. The polymers as well as the low molar mass compounds have excellent thermal stability and are amorphous. In order to examine the potential to apply these compounds in organic electroluminescent devices, the redox properties were studied by cyclic voltammetry. It was found that the monomers have high electron affinity and reach LUMO values in the range of -2.7 to -3.1 eV. Additionally high oxidation stability with HOMO values lower than -6.4 eV follows hole blocking capabilities. This opens the possibility to utilize 1,3,5-triazine containing materials as electron injection/hole blocking layer in LEDs. First LED results are in agreement to these high electron affinities.
We synthesized low molecular weight triphenyldiamines (TPDs), novel 1,3,5-tris(diarylamino)benzenes (TDABs), polymeric triphenyldiamines and insoluble triphenylamine networks based on tris(4-ethynylphenyl)amine as hole transport materials for electroluminescent displays. The HOMO energy values as determined from cyclic voltammetry measurements for TPDs and TDABs are between -4.97 and -5.16 eV. By using a polymeric TPD as hole transport layer and tris(8-quinolinolato) aluminium as emitter, LEDs with an onset voltage of 3V and a luminance up to 900 cs?m2 were obtained under ambient conditions, using airstable Al-electrode as cathode and ITO as anode.
Electroluminescent devices consisting of triphenylene derivatives as hole transport layer (HTL) and 8- hydroxyquinoline aluminium complex (Alq3) as emitting layer with ITO as anode and aluminium as cathode are presented. Triphenylene compounds were evaporated or spin coated from solution. In addition side chain polymers with triphenylenes are used. Finally we prepared a polymeric HTL by photopolymerization of an acrylate functionalized triphenylene monomer. Some of the devices, based on ITO/Triphenylene/Alq3/Al showed brightness values up to 1000 cd/m2 and low turn on voltages from 4-5 V.
We investigated polymeric materials based on polysiloxane (PSX), polymethylmethacrylate (PMMA), polyurethane (PU), as well as a triphenylamin-based glass (DRDCTA) with respect to their photorefractive properties. Electric-field dependencies of the two-beam coupling gain, diffraction efficiencies, refractive index amplitudes and holographic rise-times could be obtained by means of two-wave mixing and degenerate four-wave mixing measurements. The examined PSX polymer systems were composed of a photoconducting polysiloxane host doped with trinitrofluorenone (TNF) as a sensitizing moiety and various chromophores, namely, an azo derivative, a stilben derivative and a tolan derivative. Due to their comparatively low glass-transition temperatures Tg, an orientational enhancement of the photorefractive properties was observed. Furthermore, the influence of photoisomerisation (based on trans-cis-trans cycles) on the holographical properties could be determined for the different chromophores. In addition, a class of fully functionalized polymers with azo chromophores and carbazole-units covalently attached to PMMA- and PU-backbones was synthesized. These systems show comparatively high glass transition temperatures of more than 80 degrees C. The third type of materials investigated is a glass of triphenylamin with attached carbazole and NLO-chromophore moieties. It has a glass transition temperature of 120 degrees C. For the high-Tg materials, poling procedures--essential for the photorefractive properties--could be monitored in-situ by second-harmonic generation. Absolute values for the nonlinear Pockets coefficients χ(2) (-ω; ω, 0) have been obtained by electro-optical measurements.
We propose a new class of intersubband lasers and amplifiers that achieve net gain without population inversion. The laser scheme is based on a unipolar semiconductor double quantum- well structure where gain occurs at a transition between conduction band subbands. In order to achieve net gain without inversion, we utilize Fano-type interferences. The semiconductor laser scheme that we are considering is analogous to the atomic lambda system that has been extensively analyzed in the context of electromagnetically induced transparency and lasing without inversion. A coherent coupling field however, is not required in the present scheme. The electronic coherence necessary for Fano-type interferences is established by resonant tunneling. For nonlinear optics applications, the asymmetry of the structure allows for (chi) (2) processes and therefore higher conversion efficiency or parametric gain.
Polymethacrylate copolymers, containing non-linear optically active 4'-dialkylamino-4-nitro- azobenzene side groups and crosslinkable 2-butenyl side groups have been studied. Films of these copolymers can be thermally crosslinked at elevated temperatures and photochemically crosslinked by exposure with UV light; both methods allow the control of the crosslinking density. In situ second harmonic generation (SHG) of a Nd-YAG laser (1.064 micrometers ) while poling was used to study films with different degrees of pre-crosslinking. A pre-crosslinking step, prior to the corona poling step, can be applied to optimize alignment and relaxation of the chromophores. Depending on the conditions of pre-crosslinking, the SHG signal intensity and the alignment stability can be maximized.
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