M. David Henry

Tunable Visible and Near-IR Emission from Sub-10 nm Etched Single-Crystal Si Nanopillars

Visible and near-IR photoluminescence (PL) is reported from sub-10 nm silicon nanopillars. Pillars were plasma etched from single crystal Si wafers and thinned by utilizing strain-induced, self-terminating oxidation of cylindrical structures. PL, lifetime, and transmission electron microscopy were performed to measure the dimensions and emission characteristics of the pillars. The peak PL energy was found to blue shift with narrowing pillar diameter in accordance with a quantum confinement effect. The blue shift was quantified using a tight binding method simulation that incorporated the strain induced by the thermal oxidation process. These pillars show promise as possible complementary metal oxide semiconductor compatible silicon devices in the form of light-emitting diode or laser structures.

Techniques of Cryogenic Reactive Ion Etching in Silicon for Fabrication of Sensors

Cryogenic etching of silicon, using an inductively coupled plasma reactive ion etcher (ICP-RIE), has extraordinary properties which can lead to unique structures difficult to achieve using other etching methods. In this work, the authors demonstrate the application of ICP-RIE techniques which capitalize on the cryogenic properties to create different sensors geometries: optical, electrical, magnetic, and mechanical. The three techniques demonstrated are (1) single step deep etches with controllable sidewall profiles. Demonstrating this, silicon pillars with over 70 µm depth and less than 250 nm sidewall roughness were etched using only 1.6 µm of photoresist for use as solar cells. (2) Using the cryogenic etch for thick metallization and liftoff with a thin photoresist mask. Demonstrating this second technique, a magnetic shim was created by deposition of 6.5 µm of iron into 20 µm deep etched trenches, using the remaining 1.5 µm photoresist etch mask as the liftoff mask. Using the same technique, 15 µm of copper was lifted off leaving a 20 µm deep plasma enhanced chemical vapor deposition silicon oxide coated, silicon channel with copper. (3) Use of a two step cryogenic etch for deep etching with reduced sidewall undercutting. This was demonstrated by fabrication of deep and anisotropic microelectromechanical systems structures; a mechanical resonator was etched 183 µm deep into silicon with less than 3 µm of undercutting. This work also describes the etch parameters and etch controls for each of these sensors.

Size Tunable Visible and Near-Infrared Photoluminescence from Vertically Etched Silicon Quantum Dots

Corrugated etching techniques were used to fabricate size-tunable silicon quantum dots that luminesce under photoexcitation, tunable over the visible and near infrared. By using the fidelity of lithographic patterning and strain limited, self-terminating oxidation, uniform arrays of pillar containing stacked quantum dots as small as 2 nm were patterned. Furthermore, an array of pillars, with multiple similar sized quantum dots on each pillar, was fabricated and tested. The photoluminescence displayed a multiple, closely peaked emission spectra corresponding to quantum dots with a narrow size distribution. Similar structures can provide quantum confinement effects for future nanophotonic and nanoelectronic devices.

Advanced Plasma Processing: Etching, Deposition, and Wafer Bonding Techniques for Semiconductor Applications

Plasma processing techniques are one of the cornerstones of modern semiconductor fabrication. Low pressure plasmas in particular can achieve high radical density, high selectivity, and anisotropic etch profiles at low temperatures and mild voltages. This gentle processing environment prevents unwanted diffusion and degradation of materials due to heat and lattice damage from ion bombardment. Plasma treatments have a minimal effect on existing wafer structure, which is a key requirement for large scale integration schemes such as CMOS. In addition, recent progress in plasma-assisted wafer bonding has demonstrated low temperature, low pressure recipes utilizing O_2 plasma surface treatment for joining dissimilar semiconductor materials, such as silicon (Si) and indium phosphide (InP) (Fang et al., 2006).

Controllable deformation of silicon nanowires with strain up to 24

Fabricated silicon nanostructures demonstrate mechanical properties unlike their macroscopic counterparts. Here we use a force mediating polymer to controllably and reversibly deform silicon nanowires. This technique is demonstrated on multiple nanowire configurations, which undergo deformation without noticeable macroscopic damage after the polymer is removed. Calculations estimate a maximum of nearly 24% strain induced in 30 nm diameter pillars. The use of an electron activated polymer allows retention of the strained configuration without any external input. As a further illustration of this technique, we demonstrate nanoscale tweezing by capturing 300 nm alumina beads using circular arrays of these silicon nanowires.

Optofluidic Circular Grating Distributed Feedback Dye Laser

We demonstrate an optically pumped surface emitting optofluidic dye laser using a second-order circular grating distributed feedback resonator. We present a composite bilayer soft lithography technique specifically developed for the fabrication of our dye laser and investigate a hybrid polymer material system [poly(dimethylsiloxane)/perfluoropolyether] to construct high-resolution Bragg gratings. Our lasers emit single frequency light at low lasing thresholds of 6 μJ/mm2. These optofluidic dye lasers can serve as low-cost and compact coherent light sources that are fully integrated within microfluidic analysis chips and provide an efficient approach to construct compact spectroscopy systems.

Ga+ beam lithography for suspended lateral beams and nanowires

The authors demonstrate the fabrication of suspended nanowires and doubly clamped beams by using a focused ion beam implanted Ga etch mask followed by an inductively coupled plasma reactive ion etching of silicon. This method will demonstrate how a two-step, completely dry fabrication sequence can be tuned to generate nanomechanical structures on either silicon substrates or silicon on insulator (SOI). This method was used to generate lateral nanowires suspended between 2 µm scaled structures with lengths up to 16 µm and widths down to 40 nm on a silicon substrate. The authors also fabricate 10 µm long doubly clamped beams on SOIs that are 20 nm thick and a minimum of 150 nm wide. In situ electrical measurements of the beams demonstrate a reduction of resistivity from > 37.5 Ω cm down to 0.25 Ω cm. Transmission electron microscopy for quantifying both surface roughness and crystallinity of the suspended nanowires was performed. Finally, a dose array for repeatable fabrication of a desired beam width was also experimentally determined.

Hermetic wafer-level packaging for RF MEMs: Effects on resonator performance

The work presented here details the wafer-level fabrication and integration of aluminum nitride (AlN) micro resonators into hermetic micro environments. By etching cavities into the lid wafer and then bonding the lid wafer to a wafer of AlN micro resonators, a hermetic micro environment is created. After bonding, the lid wafer is thinned by plasma etching to expose individual die. This sequence presents the opportunity to perform resonator release on a wafer level while providing protection from dicing and other fabrication steps. We present here, fabrication and integration specifics on the wafer-level-packaging (WLP). Further we detail challenges encountered during the integration process including: elimination of micro voids created during eutectic wafer bonding, the use of plasma etching of lid wafers as a replacement to polish based wafer thinning, techniques to confirm hermetic environments, and significant failure mechanisms of the process limiting yield. Finally, we quantify improvements of the AlN micro resonators by correlating quality factors and integrated Pirani gauges.

Oblique patterned etching of vertical silicon sidewalls

A method for patterning on vertical silicon surfaces in high aspect ratio silicontopography is presented. A Faraday cage is used to direct energetic reactive ions obliquely through a patterned suspended membrane positioned over the topography. The technique is capable of forming high-fidelity pattern (100 nm) features, adding an additional fabrication capability to standard top-down fabrication approaches.

Degradation of Superconducting Nb/NbN Films by Atmospheric Oxidation

Niobium and niobium nitride thin films are transitioning from fundamental research toward wafer scale manufacturing with technology drivers that include superconducting circuits and electronics, optical single photon detectors, logic, and memory. Successful microfabrication requires precise control over the properties of sputtered superconducting films, including oxidation. Previous work has demonstrated the mechanism in oxidation of Nb and how film structure could have deleterious effects upon the superconducting properties. This study provides an examination of atmospheric oxidation of NbN films. By examination of the room temperature sheet resistance of NbN bulk oxidation was identified and confirmed by secondary ion mass spectrometry. Meissner magnetic measurements confirmed the bulk oxidation not observed with simple cryogenic resistivity measurements.