Stefan Partel

Growth and alignment of thin film organic single crystals from dewetting patterns.

Studying and understanding the conditions under which organic semiconductors can be engineered to form aligned single crystals in thin films is of primary importance owing to their unique orientation-dependent optoelectronic properties. Efforts to reach this goal by self-assembly from solution-processed films have been rewarded only with limited success. In this article we present a new method to grow single crystalline thin films via solvent annealing. We identify solvate crystal growth in combination with a specific film dewetting morphology as key to successful fabrication of single crystals. Furthermore, these 2D single crystals can align on chemically patterned substrates to minimize their interfacial energy. We explore in situ the conditions for crystal formation and alignment.

Lift-Off Free Fabrication Approach for Periodic Structures with Tunable Nano Gaps for Interdigitated Electrode Arrays.

We report a simple, low-cost and lift-off free fabrication approach for periodic structures with adjustable nanometer gaps for interdigitated electrode arrays (IDAs). It combines an initial structure and two deposition process steps; first a dielectric layer is deposited, followed by a metal evaporation. The initial structure can be realized by lithography or any other structuring technique (e.g., nano imprint, hot embossing or injection molding). This method allows the fabrication of nanometer sized gaps and completely eliminates the need for a lift-off process. Different substrate materials like silicon, Pyrex or polymers can be used. The electrode gap is controlled primarily by sputter deposition of the initial structure, and thus, adjustable gaps in the nanometer range can be realized independently of the mask or stamp pattern. Electrochemical characterizations using redox cycling in ferrocenemethanol (FcMeOH) demonstrate signal amplification factors of more than 110 together with collection factors higher than 99%. Furthermore, the correlation between the gap width and the amplification factor was studied to obtain an electrochemical performance assessment of the nano gap electrodes. The results demonstrate an exponential relationship between amplification factor and gap width.

Dewetting-driven hierarchical self-assembly of small semiconducting molecules

We describe the self-organization of PCBM and a cyanine dye on chemically patterned surfaces during spin coating from solution. On homogeneous surfaces, a transient bilayer forms, which in a later stage decomposes into PCBM droplets in a matrix of the cyanine dye. On the patterned surface also a PCBM droplet phase develops, but the final film structure is greatly determined by contact line pinning of the PCBM domains to the substrate pattern. Three characteristic morphology regimes separated by wetting transitions were observed for different ratios between the natural domain dimensions and the underlying pattern periodicity. We demonstrate that contact line pinning can be an important means to control the film morphology in systems where films are coated from solution. This process can be exploited as a general and versatile method for patterning small semiconducting molecules into 1D and 2D photonic crystals.

In-situ measurement and characterization of photoresists during development

Microfabrication has developed rapidly in the last few decades. As in all areas of engineering, simulation of physical processes is becoming more important. Process optimization through optical lithography simulation, a well-established tool, reduces production costs and time-to-market. To simulate light propagation and photoresist behavior, it is necessary to characterize the resist as it is developed. To obtain this information, the thickness of transparent and semi-transparent films has to be measured rapidly and non-destructively. Much information can be obtained by measuring the thickness reduction of the photoresist over time as it is developed. This information can be calculated from measurements of reflectivity versus wavelength. When measured for several different exposure intensities, the results can also be used to calibrate models of photoresists in software simulations. Groups at different institutions have designed a number of different laboratory setups to analyze dissolution rates of photoresists. Thickness information can be calculated either by measuring reflectance of a single wavelength or a spectrum of wavelengths. Most multi-wavelength approaches are based on least squares fitting of theoretical to measured reflectance, using assumed values for the refractive indices.1–3 A simpler method, based on determining the location of the peaks within the spectrum,4 is advantageous because it would require less sophisticated hardware. Both single and multiwavelength approaches can only provide relative thickness information, and are based on the positions of peaks in the spectrum. For thin resist layers, no peaks are visible, so the resist has to be developed completely before absolute thickness can be calculated. This makes it difficult to characterize photoresist dissolution behavior. Figure 1. Setup of the dissolution rate monitor which shows developer feed, developer cell, light source, spectrometer, and software for data acquisition and data analysis.

Numerical optimization of illumination and mask layout for the enlargement of process windows and for the control of photoresist profiles in proximity printing

Although proximity printing is the oldest and, in view of the basic optical setup, simplest photolithographic technique, it still remains in heavy use in the semiconductor manufacturing industry. The fact that this technique exists for a long time does not mean that there is no more room for improvements or new applications. Lending concepts developed for modern projection scanners and steppers and adapting them for our purposes, we demonstrate how numerical simulation and optimization can help to make the proximity printing process more stable against process variations and to increase the resolution for critical features. For this purpose, we numerically optimize the angular spectrum of the illumination and the mask layout. Furthermore, we couple the optimization of the optical degrees of freedom to the simulation of photoresist development to assess the effects of changes to the illumination and mask on the final photoresist profile.