Sealing of micromachined cavities using chemical vapor deposition methods: characterization and optimization

This paper presents results of a systematic investigation to characterize the sealing of micromachined cavities using chemical vapor deposition (CVD) methods. We have designed and fabricated a large number and variety of surface-micromachined test structures with different etch-channel dimensions. Each cavity is then subjected to a number of sequential CVD deposition steps with incremental thickness until the cavity is successfully sealed. At etch deposition interval, the sealing status of every test structure is experimentally obtained and the percentage of structures that are sealed is recorded. Four CVD sealing materials have been incorporated in our studies: LPCVD silicon nitride, LPCVD polycrystalline silicon (polysilicon), LPCVD phosphosilicate glass (PSG), and PECVD silicon nitride. The minimum CVD deposition thickness that is required to successfully seal a microstructure is obtained for the first time. For a typical Type-1 test structure that has eight etch channels-each 10 μm long, 4 μm wide, and 0.42 μm tall-the minimum required thickness (normalized with respect to the height of etch channels) is 0.67 for LPCVD silicon nitride, 0.62 for LPCVD polysilicon, 4.5 for LPCVD PSG, and 5.2 for PECVD nitride. LPCVD silicon nitride and polysilicon are the most efficient sealing materials. Sealing results with respect to etch-channel dimensions (length and width) are evaluated (within the range of current design). When LPCVD silicon nitride is used as the sealing material, test structures with the longest (38 μm) and widest (16 μm) etch channels exhibit the highest probability of sealing. Cavities with a reduced number of etch channels seal more easily. For LPCVD PSG sealing, on the other hand, the sealing performance improves with decreasing width but is not affected by length of etch channels

Functional principal component analysis of spatially correlated data

This paper focuses on the analysis of spatially correlated functional data. We propose a parametric model for spatial correlation and the between-curve correlation is modeled by correlating functional principal component scores of the functional data. Additionally, in the sparse observation framework, we propose a novel approach of spatial principal analysis by conditional expectation to explicitly estimate spatial correlations and reconstruct individual curves. Assuming spatial stationarity, empirical spatial correlations are calculated as the ratio of eigenvalues of the smoothed covariance surface Cov (Xi(s),Xi(t))(Xi(s),Xi(t)) and cross-covariance surface Cov (Xi(s),Xj(t))(Xi(s),Xj(t)) at locations indexed by i and j. Then a anisotropy Matérn spatial correlation model is fitted to empirical correlations. Finally, principal component scores are estimated to reconstruct the sparsely observed curves. This framework can naturally accommodate arbitrary covariance structures, but there is an enormous reduction in computation if one can assume the separability of temporal and spatial components. We demonstrate the consistency of our estimates and propose hypothesis tests to examine the separability as well as the isotropy effect of spatial correlation. Using simulation studies, we show that these methods have some clear advantages over existing methods of curve reconstruction and estimation of model parameters

The Gaussian Multiple Access Diamond Channel

In this paper, we study the capacity of the diamond channel. We focus on the special case where the channel between the source node and the two relay nodes are two separate links with finite capacities and the link from the two relay nodes to the destination node is a Gaussian multiple access channel. We call this model the Gaussian multiple access diamond channel. We first propose an upper bound on the capacity. This upper bound is a single-letterization of an $n$-letter upper bound proposed by Traskov and Kramer, and is tighter than the cut-set bound. As for the lower bound, we propose an achievability scheme based on sending correlated codes through the multiple access channel with superposition structure. We then specialize this achievable rate to the Gaussian multiple access diamond channel. Noting the similarity between the upper and lower bounds, we provide sufficient and necessary conditions that a Gaussian multiple access diamond channel has to satisfy such that the proposed upper and lower bounds meet. Thus, for a Gaussian multiple access diamond channel that satisfies these conditions, we have found its capacity.Comment: submitted to IEEE Transactions on Information Theor

Electricity-powered artificial root nodule.

Root nodules are agricultural-important symbiotic plant-microbe composites in which microorganisms receive energy from plants and reduce dinitrogen (N2) into fertilizers. Mimicking root nodules using artificial devices can enable renewable energy-driven fertilizer production. This task is challenging due to the necessity of a microscopic dioxygen (O2) concentration gradient, which reconciles anaerobic N2 fixation with O2-rich atmosphere. Here we report our designed electricity-powered biological|inorganic hybrid system that possesses the function of root nodules. We construct silicon-based microwire array electrodes and replicate the O2 gradient of root nodules in the array. The wire array compatibly accommodates N2-fixing symbiotic bacteria, which receive energy and reducing equivalents from inorganic catalysts on microwires, and fix N2 in the air into biomass and free ammonia. A N2 reduction rate up to 6.5 mg N2 per gram dry biomass per hour is observed in the device, about two orders of magnitude higher than the natural counterparts

Directional excitation of graphene surface plasmons

We propose a scheme to directionally couple light into graphene plasmons by placing a graphene sheet on a magneto-optical substrate. When a magnetic field is applied parallel to the surface, the graphene plasmon dispersion relation becomes asymmetric in the forward and backward directions. It is possible to achieve unidirectional excitation of graphene plasmons with normally incident illumination by applying a grating to the substrate. The directionality can be actively controlled by electrically gating the graphene, or by varying the magnetic bias. This scheme may have applications in graphene-based opto-electronics and sensing

Nanowire Photoelectrochemistry.

Recent applications of photoelectrochemistry at the semiconductor/liquid interface provide a renewable route of mimicking natural photosynthesis and yielding chemicals from sunlight, water, and air. Nanowires, defined as one-dimensional nanostructures, exhibit multiple unique features for photoelectrochemical applications and promise better performance as compared to their bulk counterparts. This article reviews the use of semiconductor nanowires in photoelectrochemistry. After introducing fundamental concepts essential to understanding nanowires and photoelectrochemistry, the review considers answers to the following questions: (1) How can we interface semiconductor nanowires with other building blocks for enhanced photoelectrochemical responses? (2) How are nanowires utilized for photoelectrochemical half reactions? (3) What are the techniques that allow us to obtain fundamental insights of photoelectrochemistry at single-nanowire level? (4) What are the design strategies for an integrated nanosystem that mimics a closed cycle in artificial photosynthesis? This framework should help readers evaluate the salient features of nanowires for photoelectrochemical applications, promoting the sustainable development of solar-powered chemical plants that will benefit our society in the long run