Evelyn L. Hu

Selection of peptides with semiconductor binding specificity for directed nanocrystal assembly.

In biological systems, organic molecules exert a remarkable level of control over the nucleation and mineral phase of inorganic materials such as calcium carbonate and silica, and over the assembly of crystallites and other nanoscale building blocks into complex structures required for biological function. This ability to direct the assembly of nanoscale components into controlled and sophisticated structures has motivated intense efforts to develop assembly methods that mimic or exploit the recognition capabilities and interactions found in biological systems. Of particular value would be methods that could be applied to materials with interesting electronic or optical properties, but natural evolution has not selected for interactions between biomolecules and such materials. However, peptides with limited selectivity for binding to metal surfaces and metal oxide surfaces have been successfully selected. Here we extend this approach and show that combinatorial phage-display libraries can be used to evolve peptides that bind to a range of semiconductor surfaces with high specificity, depending on the crystallographic orientation and composition of the structurally similar materials we have used. As electronic devices contain structurally related materials in close proximity, such peptides may find use for the controlled placement and assembly of a variety of practically important materials, thus broadening the scope for ‘bottom-up’ fabrication approaches.

New Technique For Submicron Linewidth Measurement

We propose a novel technique to accurately measure submi-cron linewidths on photomasks and wafers. We do this by translating a phase-shifting mask across the surface containing the line whose width we wish to measure. With a Fourier-transform lens, we detect the intensity of the zero-order spatial component of the light coming from the surface as a function of the mask position. We show that the detected intensity curve varies dramatically and exhibits sharp changes in direction corresponding to the boundaries of the line. From this informa-tion the linewidth is readily apparent. We present a theoretical analysis and several computer simulations, showing that the technique is relatively independent of variations in the optical reflectance and in the height between the patterned feature and any substrate. Unlike other optical imaging methods for measur-ing linewidths, a high-resolution microscope and precise calibration are not needed. Using a laser, lateral resolution of 0.1 μm , well beyond the limit predicted by the Rayleigh criterion, is theoretically achievable. Preliminary experimental results agree well with the theoretical prediction.

Invited Paper Recent Developments In Reactive Plasma Etching Of III-V Compound Semiconductors

Reactive plasma etching is being increasingly utilized in the fabrication of III-V - based electronic and optoelectronic devices. The high resolution and control afforded by dry etching processes have led to their rapid move from research to manufacturing applications. This paper will review some of those applications, processes, progress and problems associated with reactive plasma etching of III-V materials.

Phase-Shifting And Fourier Transforming For Sub-Micron Linewidth Measurement

By using a translating phase mask, a Fourier-transforming lens and a spatial filter, we can process laser light reflected from a surface in such a way as to avoid the diffraction effects common in conventional imaging. This optical technique is thus inherently capable of measuring minute surface features such as the width of deposited or inscribed lines with better than Rayleigh resolution and should therefore be applicable for metrology of sub-micron features. The novelty of the technique is to transfer linewidth information into the zero-order spatial frequency component of the light reflected from the surface. We present analyses and computer simulations to detail the effects of such features of the system as the sharpness of a step edge in a phase-shifting mask, the magnitude of the phase shift introduced by the mask, the variation in reflectivity and height of various regions of the surface structure, and the effect of instrumental noise on the determination of linewidth. Experimental measurements were performed on specimens with large feature dimensions to verify the inherent capability of the technique. The results agree well with theoretical predictions. It is hard to validate this technique at smaller dimensions because of the necessity for precise lateral translation of the mask with respect to the surface and the sensitivity of the system to the mask-to-surface distance. We discuss modifications in the next-generation experimental set-up that will address both these issues. Current results indicate that this technique will be viable well into the submicron range.

3-D Robotic Positioning Utilizing Holographic Interferometry

We propose a new technique for robotic positioning and calibration which utilizes holographic imaging and interferometry to avoid the disadvantages associated with conventional techniques. This technique allows both gross and fine positioning of robotic devices in space and is useful in situations where it is necessary to repeatably position a moving device to a specific location. Gross positioning is accomplished by matching the robotic device to a holographic virtual image, using a computer vision system to overlap key features of the robotic device on their holographically imaged counterparts. The use of holographic images is an advantage because the robot cannot obscure them from view or collide with them. Fine positioning is achieved to better than one-micron accuracy by utilizing the interference fringes that result when the robotic device and the holographic image are aligned to within about 50 microns of each other. Elimination of the fringes indicates exact coincidence between the robot and its holographic counterpart. A computer vision system was utilized to automate the entire positioning procedure. Theory, algorithms, and experimental implementation are described.

High-Efficiency Nitride-Base Photonic Crystal Light Sources

The research activities performed in the framework of this project represent a major breakthrough in the demonstration of Photonic Crystals (PhC) as a competitive technology for LEDs with high light extraction efficiency. The goals of the project were to explore the viable approaches to manufacturability of PhC LEDS through proven standard industrial processes, establish the limits of light extraction by various concepts of PhC LEDs, and determine the possible advantages of PhC LEDs over current and forthcoming LED extraction concepts. We have developed three very different geometries for PhC light extraction in LEDs. In addition, we have demonstrated reliable methods for their in-depth analysis allowing the extraction of important parameters such as light extraction efficiency, modal extraction length, directionality, internal and external quantum efficiency. The information gained allows better understanding of the physical processes and the effect of the design parameters on the light directionality and extraction efficiency. As a result, we produced LEDs with controllable emission directionality and a state of the art extraction efficiency that goes up to 94%. Those devices are based on embedded air-gap PhC - a novel technology concept developed in the framework of this project. They rely on a simple and planar fabrication process that is very interesting for industrial implementation due to its robustness and scalability. In fact, besides the additional patterning and regrowth steps, the process is identical as that for standard industrially used p-side-up LEDs. The final devices exhibit the same good electrical characteristics and high process yield as a series of test standard LEDs obtained in comparable conditions. Finally, the technology of embedded air-gap patterns (PhC) has significant potential in other related fields such as: increasing the optical mode interaction with the active region in semiconductor lasers; increasing the coupling of the incident light into the active region of solar cells; increasing the efficiency of the phosphorous light conversion in white light LEDs etc. In addition to the technology of embedded PhC LEDs, we demonstrate a technique for improvement of the light extraction and emission directionality for existing flip-chip microcavity (thin) LEDs by introducing PhC grating into the top n-contact. Although, the performances of these devices in terms of increase of the extraction efficiency are not significantly superior compared to those obtained by other techniques like surface roughening, the use of PhC offers some significant advantages such as improved and controllable emission directionality and a process that is directly applicable to any material system. The PhC microcavity LEDs have also potential for industrial implementation as the fabrication process has only minor differences to that already used for flip-chip thin LEDs. Finally, we have demonstrated that achieving good electrical properties and high fabrication yield for these devices is straightforward.

Optical phase-shifting technique for submicron linewidth measurement: simulations and measurements

We have proposed a novel phase-shifting and Fourier transform technique for linewidth measurement. The novelty of the technique is the transfer of linewidth information into the zero-order spatial frequency component of the image. The method we proposed incorporates phase-cancellation and spatial Fourier transform to achieve resolution beyond the Rayleigh diffraction limit and should be applicable for metrology of sub-micron features. Theoretical analyses have shown that this technique will be viable well into the submicron range. Experimental measurements have been made on different features having relatively large dimensions (~ 100 μm) to verify the theory. Experimental results have demonstrated the validity of the concept and given the directions of practical limitations and requirements of the approach in order to obtain true submicron linewidth measurement. Such limitations include consideration of the precision of lateral translation of phase mask with respect to substrate, and of the mask to substrate distance. Further, extensive simulations of the robustness of this technique with respect to material composition, contrast and feature morphology (slope and height of the linewidth features) have been carried out for both individual and periodic features.