Theses 2015

Conclusions: 

• During this research work, a microfluidic phantom is prepared that aims to calibrate existing angiograhpic imaging devices.
• Turbiding nature and optical properties, i.e. μa and μs‘ of human cutis are successfully mimicked in this artificial phantom achieved.
• Multi layers of device mimic the dermis and epidermis of human cutis. Thickness of both layers is same as of layers of skin tissue i.e. 200 μm for epidermis and 400μm for upper part of dermis.
• Capillary network at the dermo-epidermal of human skin is mimicked using copper and by the process of embedding-etching a hollow pattern for fluid flow is created in between two layers.
• Diameter of micro channel is 50 μm that is in the range of human capillaries in the skin.
• COMSOL, CFD simulation is done to predict the velocity and pressure changes of the fluid flowing through hollow structure.
• SIMDOS diaphragm pump is assigned to perfuse intra lipid solution at the flow rate of 1 ml/min.
• D-OCT sutdy is performed as an application case of calibrating the OCT device. As a result, it is deduced that OCt device can detect the slowest velocity of 0.085 cm/sec of a fluid flowing through the microfluidic device.

Abstract: The optical path length is required to calculate the regional change in hemoglobin concentration from the change of measured light absorbance. The partial path length factor (PPF) is generally used for path length estimation in diffuse reflectance imaging measurements. To noninvasively determine absolute concentrations of hemoglobin (Hb) in skin tissue by means of regular near infrared (NIR) light diffuse reflectance measurements, the modified Beer Lambert law is applied. The method was developed to inversely calculate the concentration of oxygenated hemoglobin (Hbo), deoxygenated hemoglobin (Hb) as well as an effective partial path length, which NIR light passes through in the tissue between source and detector. Applying four different skin equivalent phantoms that mimic superficial skin tissue, it is shown that the optical density is in quadratic relation with both oxygenated and deoxygenated hemoglobin concentrations, thereby validating the modified Beer Lambert law in the optical density range of 0.02 and 0.08. Based on this, the partial path length factor is found to be linearly proportional with the concentration of hemoglobin. The variation of the partial path length factor for both the oxygenated and deoxygenated hemoglobin is between 0.3 and 0.6. The results also showed that in the NIR mainly in the range (700nm–1200nm) the absorbance of the oxygenated hemoglobin is more stable than that of the deoxygenated hemoglobin. It is in agreement with the prediction made based on the optical density.

Chemically Amplified Resist Modeling for Proximity Printing

Abstract: Lithography is being pushed beyond its limits with the help of lithography simulation. A lot of researches are being carried out to minimize the feature sizes of lithography. It is impossible to achieve this goal without lithography simulation due to time and cost that has to be invested on manufacturing. Lithography simulation can predict the impact of different parameters and the final outputs of a lithography process with small cost and time.

In this thesis, a simplified and fast chemically amplified resist model for proximity lithography is developed and implemented. The model computes and predicts features in small computing time without loss of accuracy and small number of process parameters. Contrary to other full modeling techniques, the model assumes separate and sequential process steps in the post-exposure bake. The PEB step is separated into neutralization, diffusion and deprotection process steps. Each process is modeled individually and implemented under the assumption that the processes occur in sequences rather occurring at the same time. The order of the processes is determined by their reaction rate constants. In addition, a Fickian-diffusion is assumed for diffusion modeling of acid and quencher concentration. The model is also calibrated with the simulation results of full-resist model of Dr.LiTHO.

The impact of the parameters on the resulting CD values and profile shapes was investigated with the developed model. The model is able to predict and simulate in a good conformity with the experimental data.

Abstract: CT-like 3D imaging with angiographic C-arm devices has been proved to be a valuable tool to assist angiographic therapeutic procedure for planning, guidance and assessment. The field-of-view (FOV) is determined by the given detector size. In some cases the FOV might not cover the region-of-interest requested by clinical applications such as interventions of spine, descending aorta, peripheral arteries, etc..

Various source trajectories for extended acial FOV could be implemented on a multii-axis angiographic C-arm system. The ellipse-line-ellipse (ELE) source trajcectory is complete, allowing exact and stable image reconstruction, and has the potential of achieving a large axial FOV per rotation.

This thesis focuses on implementing differentiated backprojection reconstruction method followed by inverse Hilbert transform (DBP-HT) using ELE trajectory for long-object cone-beam imaging. To be specified, we implement reconstruction method based on M-lines for cylindrical simulated phantom. Emphasis is placed on accuracy. Phantom is designed to satisfy long-object scanning requirement. Matlab is used to simulate the entire process from forward cone-beam projection acquisition to image reconstruction.

Abstract: Three dimensional Terahertz (3D-Thz) imaging offers great potential in non-destructive testing because of the transparency and high penetration depth through a wide range of plastic materials. Reflection based imaging offers the most practical configuration for distance measurement and opaque objects.

However, number of limitation facts strongly complicates the 3D imaging of investigated objects. First, in the traditional imaging approach the minimal achievable resolution is limited by the relation of the wavelength and dimension of the implemented imaging optics. Unwanted scattering effects can also often lead to a lack of spectral resolution. By increasing the number of sensors and size of apertures a higher resolution can be achieved. However, this makes the measurement system more expensive and unpractical for some case when a small system size is required.

Measurement system consists from electronic terahertz source and detector both mounted on translational stage which can perform movement in 2D plane. The developed solution includes an automated measurement software which allows it to work in both, focused and unfocused THz beam configuration. In order to improve the penetration depth an optical system for unfocused mode was simulated and manufactured. Second part of software solution includes an analysis toolkit based on synthetic aperture focusing imaging reconstruction technique (SAFT) adopted from radar systems. Reasons of possible imaging artifacts were investigated and procedures to consider them were included into the developed SAFT algorithm. These improvements allow reconstructing qualitatively 3 D objects from data recorded with an unfocused beam. Furthermore, using such algorithm improves measurement speed since lower number of points needed for reconstruction. Beam parameters can easily be modified depending on a particular sample by using auxiliary optics. Automated calibration procedure gives a possibility to adjust input algorithm parameters for improvement in final image quality. First results show significant improvement of imaging results and measurement speed.

Summary and conclusion: The manufacturing of structure in the μm regime with lithography tools can be realized very fast with different techniques in these days, but the analysis of the structures (CD-SEM) is very time consuming. Lithographic modeling is used to simulate the lithographic process and to calibrate different models (like photoresist models) what saves much time and costs.

So this work was concerned with assembling an easy to use calibration progrm for DNQ type photoresits calibration using exposures by a stepper. This was achieved with the use of Python which is utilized as scripting language for the Dr.Litho lithographic modeling tool from Fraunhofer IISB.

The exposure experiments were done with the Ultratech Sapphire 100E stepper in the clean room of the Fraunhofer IISB as well as the CD measurements with a SEM. The AZ5214E photoresist was used for coating the wafers. This so called image reversal resist can be used for both, application of a positive tone and negative tone photoresist due to the processing of it. So first it was applied as a positive photoresist and exposure was made with a dose variation in range of 130mJ/cm2 – 200mJ/cm2 for mask linewidth feature widths of 1ηm and 2 ηm. The beavior of the measured CDs was not distinct enough to apply a calibration so the calibration procedure explained in chapter 6.2.1 was done by using measurement data from a previous work. Nevertheless the expected behavior of the feature width, namely the decrease with rising dose was verified.

For another measurement the photoresist was applied as image reversal resist (simplified negative tone) and an exposure in a dose range of 15mJ/cm2 – 55mJ/cm2 was investigated. Here the behavior of the increase of the feature width with rising dose could also be confirmed. Nevertheless the top view measurements are not sufficient so further profile measurements were done by breaking the wafer into small pieces and inspect those in side view under the SEM. Here the behaviour is the same but i becomses obvious that the small feature width structures are not fully developed to the bottom of the substrate. The structures with a dose of 25mJ/cm2 exhibits the best quality so an additional series of measurements with this dose value was performed for varying focus positions. It became apparent that for the small structures only the focus range of -2.2ηm evaluable. So the measurement dta was used for the calibration of the image reversal photoresist applied as negative tone resist. Here it is to say, that by using dense line structures the DOF is relative large. Further experiments could be significant and informativ regarding CD and profile behavior for the focus variation by using isolated structures.

The assembled calibration program worked also for the implemented inverted Mack model and was used with the determined CD-SEM measurements values. The behaviour of an increasing feature width with rising dose was confirmed and a best fitness of 0.06ηm was achieved. The comparison of the measurement data with the simulated ones showed that the simulated feature widths were always smaller than the measured ones. The 3D images from the simulations showed a more distinct undercut form of the features than the experimental ones. This explained the samler CD values due to the fact that for the CD-SEM measurements the top CD values had to be used while the simulation CD’s were bottom CD values.

At last it is to say that the simulation has to be further developed to improve the prediction of the profile of the structures. The theoretical undercut profiles are not like the real measurements show. The footing effects have also to be taken better into account. Those are apparent for the AZ5214E image reversal photo resist as well as for special negative tone resist for lif-off processes like the AZnLOF2070 photo resist.

Abstract: Photonic Crystal Fibres (PCF) as novel waveguides, have found various applications. In this thesis, two of them are investigated, namely spectral phase characterization using a solid-core PCF and stimulated Raman scattering using a hollow-core PCF.

Spectral phase characterization is an important feature for appropriate interpretations and simulations in experiments where femtosecond pulses are used. Here, a reference pulse is used and analyzed through linear spectra interferometry, which offers a better sensitivity and reliability. Both, the reference pulse and the pulse to be measured, will propagate in a solid-core PCF: For the reference pulse a fibre with minimum dispersion effect is used. As a consequence of self-phase modulation, the pulse in the time domain will remain the same, while its spectrum wil broaden. This is the mean feature used, as a benefit of solid-core PCF. Afterwards both pulses are superposed and the interference spectral fringes will reveal the spectral phases.

The second application is regarding stimulated Raman scattering. Since discovered it become a broad topic for research. This inelastic scattering phenomenon has shown orders of magnitude enhancement when performed using a hollow-core PCF, as a consequence of long interaction lengths between low-density media such as gases and the laser light. In order to lower the threshold of the pump power required for its generation, additionally a seed laser is used. Here, this laser is modulated using an acousto-optic modulator. This configuration aims to observe the effect of the modulated seed on the generation of the Stokes pulses and the pump depletion.

Summary and outlook: In our experiment we have measured keyhole length and width for alloy CuZn37 welding with a high-speed camera. According to the ration between length and width, we can quantify the elongation of a keyhole. The bigger a ration is, the more elongated a keyhole is. The high speed video can also confirm with the variation of ratios. It is also interesting to note that keyhole tail and surface wave around a keyhole rim varies with different laser power, welding speed and defocusing combination.

From the welding workpiece point of view, we have analyzed the welding samples from seam surfaces and cross sections. According to the analysis of the initial, middle and end seam surface, we can estimate how much surface waves affect HAZ. Moreover, we can find how much laser penetrates into the smaple with the analysis of top seam width, middle seam width, and root width for full penetration and penetrating depth for partial penetration. We also experimentally confirm the relation between the keyhole elongation and welding seam measurements.

In this thesis, our research into defocusing influences on keyhole dimension and alloy sheet is not complete. There are many other factors which can affect laser welding on CuzN alloy sheet. Shielding gas is one of them that we have not taken into account in this thesis yet. When shielding as is delivered, the pressure of the gas can be applied to balance the closing effects from surface tension pressure especially when the recoil pressure is not big enough to keep a keyhole open in the low welding speed. In addition, variation of a keyhole size is dependent on different percents of Cu and Zn elements in alloys and thickness of alloy sheet. This can also be researched for the future prospect.

Abstract: The abdominal aortic aneurysm is one of the most common aortic diseases. The aneurysm is a dilation of the aorta that may result in rupture. Today, most of the abdominal aortic aneurysms are repaired endovascularly. The physician places a stent graft at the location of the aneurysm to reduce the pressure on the aortic wall and prevent further growth of the aneurysm. The accuracy of the stent placement is critical, not to close the main branching arteries of the aorta, such as the renal arteries. The novel stent placement procedure is guided by intraoperative C-arm images overlayed with a segmented preoperative computed tomography (CT). However, the inserted devices may distort the anatomy and reduce the accuracy of the overlay. In this work the aorta and the iliac artery meshes are deformed accordingly, in order to have a correct overlay and minimize contrast agent usage. The implemented method, the skeleton-based as-rigid-as-possible (ARAP) mesh deformation, with an appropriate control point selection shows visually, as well as quantitatively promising results. The initial mean Euclidean distance between an ostium and the corresponding landmark of 19.81 mm (variance: 293.92 mm²) was reduced to 4.56 (variance: 7.88 mm²).

Abstract: GaN/InGaN based nanorod (NR) core-shell structures have become increasingly popular in recent years due to their potential in optoelectronic applications such as solid state lighting however more cost effective ways are still needed for the NR fabrication and overgrowth. I present here a novel approach to form these nano structures by a top down approach which is scalable. The unique properties of the NRs due to their geometry offer great advantage, the core-shell NR structures in which the active layer (shell) is wrapped around a central three dimensional core structure offers a much larger active region compared to a planar structures hence presenting much better quantum efficiency. In this thesis I present our approach toward achieving good quality NR (core) structures through reactive ion etching (RIE) and wet chemical etching from a n-GaN planar layer and then the growth through metal organic vapour phase epitaxy (MOVPE) of the active (shell) InGaN multiple quantum wells (MQW) layer on the polarization free m-plane. These MQW active layers are then optically characterized and we can achieve a broad range of spectrum by changing the morphological growth conditions of the NR and the QW growth parameters. A p-GaN layer is then grown by epitaxy to achieve a p-i-n junction which will be further electrically characterised as part of other research hence a basic working device may be achieved in the near future.

Abstract: GaN/InGaN based nanorod (NR) core-shell structures have become increasingly popular in recent years due to their potential in optoelectronic applications such as solid state lighting however more cost effective ways are still needed for the NR fabrication and overgrowth. I present here a novel approach to form these nano structures by a top down approach which is scalable. The unique properties of the NRs due to their geometry offer great advantage, the core-shell NR structures in which the active layer (shell) is wrapped around a central three dimensional core structure offers a much larger active region compared to a planar structures hence presenting much better quantum efficiency. In this thesis I present our approach toward achieving good quality NR (core) structures through reactive ion etching (RIE) and wet chemical etching from a n-GaN planar layer and then the growth through metal organic vapour phase epitaxy (MOVPE) of the active (shell) InGaN multiple quantum wells (MQW) layer on the polarization free m-plane. These MQW active layers are then optically characterized and we can achieve a broad range of spectrum by changing the morphological growth conditions of the NR and the QW growth parameters. A p-GaN layer is then grown by epitaxy to achieve a p-i-n junction which will be further electrically characterised as part of other research hence a basic working device may be achieved in the near future.

Abstract: Extreme ultraviolet lithography (EUV) is a next-generation lithography technology using 13.5 nm wave-length indicdent light. For EUV system employing numerical aperture (NA) 0.5 and a demagnification of M = 4, the incident EUV light is in a range of 9° ± asin(0.5/4), that is, 9° ± 7.2°. This requires a re-design of EUV masks, to reflect the light over the above-mentioned incident angle range.

By using genetic algorithm (GA) module in Dr.LITHO, a software developed by Fraunhofer IISGB, Erlangen, we develop several bilayer stacks which exhibit relativly high and uniform reflectivity curves. By using mulit-objective genetic algorithm module in Dr.LITHO, we compute and plot the pareto fronts of average reflectivity rate and the reciprocal of standard derivation (SD) of the reflectivity rate, which are obtained from different mask options.

The imaging performance of EUV binary masks is investigated by employing DR.LITHO. For dense line feature, semi-dense line feature, contact hole feature, and line’s end features, the aerial images are plotted, the thresholds, normalized image log-slope (NILS) vaules, Depth of Focus (DOF) range and other lithographic process criteria are computed along both X- and Y-orientations. High reflectivity rate results in high threshold, small SD and large DOF range are correlated.

The imaging performance for EUV systems employing NA 0.5 is compared with that for NA 0.33 systems. Larger NA leads to larger NILS values, but smaller DOF ranges.