Matthias Müllenborn

Oxidation of hydrogen-passivated silicon surfaces by scanning near-field optical lithography using uncoated and aluminum-coated fiber probes

Optically induced oxidation of hydrogen-passivated siliconsurfaces using a scanning near-field optical microscope was achieved with both uncoated and aluminum-coated fiber probes. Line scans on amorphous silicon using uncoated fiber probes display a three-peak profile after etching in potassium hydroxide. Numerical simulations of the electromagnetic field around the probe–sample interaction region are used to explain the experimental observations. With an aluminum-coated fiber probe, lines of 35 nm in width were transferred into the amorphous silicon layer.

The first low voltage, low noise differential silicon microphone, technology development and measurement results

The first differential silicon microphone is presented. This capacitive working device consists of two backplates with a membrane in between. Due to the balanced arrangement the air gap can be minimized. Thus, a higher electrical field and sensitivity can be achieved for low voltages. A dedicated process sequence has been developed in order to get the optimum mechanical and electrical properties for all structural layers. Furthermore, a sandwich structure has been developed to achieve a reproducible, very sensitive microphone membrane with a thickness of only 0.5 /spl mu/m and a stress of 45 MPa. The total sensitivity for a bias of 1.5 V was measured to be 13 mV/Pa and the A-weighted equivalent input noise was measured to be 22.5 dB SPLA. The upper limit of the dynamic range has been determined to be 118 dB SPL and the total harmonic distortion at 80 dB SPL is below 0.26%.

Fabrication and characterization of truly 3-D diffuser/nozzle microstructures in silicon

We present microfabrication and characterization of truly three-dimensional (3-D) diffuser/nozzle structures in silicon. Chemical vapor deposition (CVD), reactive ion etching (RIE), and laser-assisted etching are used to etch flow chambers and diffuser/nozzle elements. The flow behavior of the fabricated elements and the dependence of diffuser/nozzle efficiency on structure geometry has been investigated. The large freedom of 3-D micromachining combined with rapid prototyping allows one to characterize and optimize diffuser/nozzle structures.

Optical near‐field lithography on hydrogen‐passivated silicon surfaces

We report on a novel lithography technique for patterning of hydrogen‐passivated amorphous silicon surfaces. A reflection mode scanning near‐field optical microscope with uncoated fiber probes has been used to locally oxidize a thin amorphous silicon layer. Lines of 110 nm in width, induced by the optical near field, were observed after etching in potassium hydroxide. The uncoated fibers can also induce oxidation without light exposure, in a manner similar to an atomic force microscope, and linewidths of 50 nm have been achieved this way.

Silicon nanostructures produced by laser direct etching

A laser direct‐write process has been applied to structure silicon on a nanometer scale. In this process, a silicon substrate, placed in a chlorine ambience, is locally heated above its melting point by a continuous‐wave laser and translated by high‐resolution direct‐current motor stages. Only the molten silicon reacts spontaneously with the molecular chlorine, resulting in trenches with the width of the laser‐generated melt. Trenches have been etched with a width of less than 70 nm. To explain the functional dependence of the melt size on absorbed power, the calculations based on a two‐phase steady state heat model are presented, taking the temperature‐dependent thermal conductivities and optical parameters into account.

Fast three-dimensional laser micromachining of silicon for microsystems

Abstract The application of laser direct etching of silicon in developing and processing devices for microelectromechanical systems is discussed. The direct write system presented in this paper features high writing speed, high resolution and large access range. Direct micromachining of truly three-dimensional structures, rapid prototyping, processing on non-planar substrates, deep structuring such as drilling and cutting, and complementary processing and post-processing through laser direct etching are demonstrated. Examples include prototypes of micromachined diffuser/nozzle elements, illustrating the potential for microfluidic devices, lens arrays and structures cut out of a membrane, showing the high flexibility of this process.

Three-dimensional nanostructures by direct laser etching of Si

Abstract Nanostructures have been machined into Si by a high-resolution laser direct write system. The Si substrate is locally heated above its melting point by a continuous-wave laser and rapidly etched by dry Cl2 gas. If the solid-to-liquid transition is adjusted to occur only at the peak of the temperature profile, the melt size is significantly smaller than the diffraction-limited spot size. This can translate to extremely small etched features because of the high selectivity of the etching process for liquid Si compared to crystalline Si. By using objectives with a high numerical aperture, 488 nm as well as 351 nm light from an Ar ion laser, and X/Y/Z translation stages for moving the substrate instead of steering the beam, we have achieved line widths below 200 nm combined with very high scanning accuracy and speed. The resolution limit for Si machining is determined by the selectivity of the chemical reaction rather than the laser spot size. Interfacing to computer-aided design (CAD) software allows us to remove layer by layer of a truly three-dimensional nanostructure.

Laser direct writing of oxide structures on hydrogen-passivated silicon surfaces

A focused laser beam has been used to induce oxidation of hydrogen‐passivated silicon. The scanning laser beam removes the hydrogen passivation locally from the silicon surface, which immediately oxidizes in air. The process has been studied as a function of power density and excitation wavelength on amorphous and crystalline silicon surfaces in order to determine the depassivation mechanism. The minimum linewidth achieved is about 450 nm using writing speeds of up to 100 mm/s. The process is fully compatible with local oxidation of silicon by scanning probe lithography. Wafer‐scale patterns can be generated by laser direct oxidation and complemented with nanometer resolution by scanning probe techniques. The combined micro‐ and nanoscale pattern can be transferred to the silicon in a single etching step by either wet or dry etching techniques.

Fast 3d Laser Micromachining Of Silicon For Micromechanical And Microfluidic Applications

The application of laser direct etching of silicon in processing devices for microelectromechanical systems is discussed. The direct write system presented in this paper features high writing speed, high resolution, and large access range. The possibilities of micromachining truly three-dimensional structures, rapid prototyping, processing on non-planar substrates, complementary processing and post-processing through laser direct etching are demonstrated. Examples include prototypes of micromachined diffuser/nozzle elements illustrating the potential for microfluidic devices and structures cut out of a membrane, which both show the high flexibility of this process.

Laser direct etching of silicon on oxide for rapid prototyping

Structures have been etched in poly-silicon and amorphous silicon deposited on silicon oxide by laser direct writing. These patterns can be written with a high resolution and transferred to the underlying material via reactive ion etching. Three-dimensional structures can be obtained by multiple exposure of the silicon mask. Due to the fast turnaround time of direct writing processes, this technique can be applied for rapid prototyping of large-scale structures.