Karola Richter

Multicolor generation using silicon nanodisk absorber

A multicolored matrix that spans the visible range was demonstrated by using silicon nanodisk arrays. A nanostructured silicon substrate, which featured periodic silicon nanodisk arrays of various diameters, inter-nanodisk distances, and heights, was fabricated using electron-beam lithography and reactive ion etching. These silicon nanodisks were able to support HE1m leaky modes, which depended on the diameter of the nanodisks, resulting in wavelength-dependent reflection spectra. The resonant wavelength redshifted linearly with the increasing nanodisk diameter. The output color lay in the visible range and was observed to be tunable when varying the diameter, interdistance, and height. The results of finite-difference time-domain simulations exhibited close agreement with the observed optical properties of the periodic silicon nanodisk arrays.

Hard x-ray nanofocusing by refractive lenses of constant thickness

In order to focus light or x rays, the thickness of a refractive lens is typically varied over its aperture. Here, we present a refractive x-ray lens made of lamellae of constant thickness, the refractive lamellar lens. Refractive power is created by a specific bending of the lamellae rather than by a concave lens profile. This very special design has the technological advantage that materials like sapphire or diamond can be used to make lenses by coating techniques. A first lens prototype focused x rays with a photon energy E = 15.25 keV to a lateral beam size of 164 nm × 296 nm full width at half maximum.

Focusing hard x rays beyond the critical angle of total reflection by adiabatically focusing lenses

In response to the conjecture that the numerical aperture of x-ray optics is fundamentally limited by the critical angle of total reflection [Bergemann et al., Phys. Rev. Lett. 91, 204801 (2003)], the concept of adiabatically focusing refractive lenses was proposed to overcome this limit [Schroer and Lengeler, Phys. Rev. Lett. 94, 054802 (2005)]. We present an experimental realization of these optics made of silicon and demonstrate that they indeed focus 20 keV x rays to a 18.4 nm focus with a numerical aperture of 1.73(9) × 10−3 that clearly exceeds the critical angle of total reflection of 1.55 mrad.

Submicrometer photolithography by surface imaging - experiment and simulation

Results obtained by investigation of the silylation and dry develoment process in photolithography will be presented. An improved depth resolution of 0.65/um structures was obtained (exposure wavelength 436 nm, aperture 0.3). The degree of silylating was obtained by RBS measurements. A first way of process simulation will be outlined and results are discussed.

Realization of optical multimode TSV waveguides for Si-Interposer in 3D-chip-stacks

Optical connectivity has the potential to outperform copper-based TSVs in terms of bandwidth at the cost of more complexity due to the required electro-optical and opto-electrical conversion. The continuously increasing demand for higher bandwidth pushes the breakeven point for a profitable operation to shorter distances. To integrate an optical communication network in a 3D-chip-stack optical through-silicon vertical VIAs (TSV) are required. While the necessary effort for the electrical/optical and vice versa conversion makes it hard to envision an on-chip optical interconnect, a chip-to-chip optical link appears practicable. In general, the interposer offers the potential advantage to realize electro-optical transceivers on affordable expense by specific, but not necessarily CMOS technology. We investigated the realization and characterization of optical interconnects as a polymer based waveguide in high aspect ratio (HAR) TSVs proved on waferlevel. To guide the optical field inside a TSV as optical-waveguide or fiber, its core has to have a higher refractive index than the surrounding material. Comparing different material / technology options it turned out that thermal grown silicon dioxide (SiO2) is a perfect candidate for the cladding (nSiO2 = 1.4525 at 850 nm). In combination with SiO2 as the adjacent polymer layer, the negative resist SU-8 is very well suited as waveguide material (nSU-8 = 1.56) for the core. Here, we present the fabrication of an optical polymer based multimode waveguide in TSVs proved on waferlevel using SU-8 as core and SiO2 as cladding. The process resulted in a defect-free filling of waveguide TSVs with SU-8 core and SiO2 cladding up to aspect ratio (AR) 20:1 and losses less than 3 dB.

Towards the realization of optical interconnets on Si interposer

The continuously increasing demand for higher bandwidth makes the application of an optical chip to chip interconnect system conceivable. Based on the assumption of a 3D chip package two essential ingredients of such a system are the availability of optical TSVs and a cost efficient process to fabricate waveguides with an efficient coupling to and from the optical TSV. In this paper, we present results on different kinds of optical TSVs. Our waveguide TSV is based on a SU-8 optical core. We developed a fabrication process for horizontal optical waveguides made as well by SU-8 using special imprint lithography.

A Through Silicon Via concept for sensor applications

The use of Through Silicon Via (TSV) extends to different areas of electronics. Besides its utilization for logic and memory chips, a growing attention has been spent to sensor applications in the last years. Each application has its specific requirements which affect the whole processing scheme. Typically TSVs are generated by filling blind holes with copper and thinning the wafer afterwards, or by coating through holes with copper. Here we present an alternative concept which can be applied for biochemical sensors, so that a blind hole is created stopping on a SiO2 membrane. By this the need for a post wafer thinning is avoided.

Complex micro-patterning in silicon with varied tilt angles realized by advanced plasma etching

The objective of our investigations was to develop a new etching process, which accomplished well defined positive etch profiles in silicon. Using this so-called PPE-process (Positive Profiles Etching process) we realized patterns in silicon with tilt angles of the sidewalls between 60 and 88° and an etch depth up to 200 μm. Etch rates between 3 and 5 μm/min can be achieved and a large number of process parameters enables the variation of the etch profiles in a wide range. Based on these results we could create complex etch profiles by a combination of several silicon etching processes according to application-oriented demands.