Articles you may be interested inDesign and optimization of wafer bonding packaged microelectromechanical systems thermoelectric power generators with heat dissipation path

We present an experimental and theoretical study of nonresonant detection of subterahertz radiation in GaAs/AlGaAs and GaN/AlGaN heterostructure field effect transistors. The experiments were performed in a wide range of temperatures (8–300 K) and for frequencies ranging from 100 to 600 GHz. The photoresponse measured as a function of the gate voltage exhibited a maximum near the threshold voltage. The results were interpreted using a theoretical model that shows that the maximum in photoresponse can be explained by the combined effect of exponential decrease of the electron density and the gate leakage current.

The resonant detection of subterahertz radiation by two-dimensional electron plasma confined in a submicron gate GaAs/AlGaAs field-effect transistor is demonstrated. The results show that the critical parameter that governs the sensitivity of the resonant detection is ωτ, where ω is the radiation frequency and τ is the momentum scattering time. By lowering the temperature and hence increasing τ and increasing the detection frequency ω, we reached ωτ∼1 and observed resonant detection of 600 GHz radiation in a 0.15 μm gate length GaAs field-effect transistor. The evolution of the observed photoresponse signal with temperature and frequency is reproduced well within the framework of a theoretical model.

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Wafer-level three-dimensional integration ͑3D͒ is an emerging technology to increase the performance and functionality of integrated circuits ͑ICs͒, with adhesive wafer bonding a key step in one of the attractive technology platforms. In such an application, the dielectric adhesive layer needs to be very uniform, and precise wafer-to-wafer alignment accuracy ͑ϳ1 m͒ of the bonded wafers is required. In this paper we present a new adhesive wafer bonding process that involves partially curing ͑cross-linking͒ of the benzocyclobutene ͑BCB͒ coatings prior to bonding. The partially cured BCB layer essentially does not reflow during bonding, minimizing the impact of inhomogeneities in BCB reflow under compression and/or any shear forces at the bonding interface. The resultant nonuniformity of the BCB layer thickness after wafer bonding is less than 1% of the average layer thickness, and the wafers shift relative to each other during the wafer bonding process less than 1 m ͑average͒ for 200 mm diameter wafers. When bonding two silicon wafers using partially cured BCB, the critical adhesion energy is sufficiently high ͑ജ14 J/m 2 ͒ for subsequent IC processing.

The effects of thermal cycling on critical adhesion energy and residual stress at the interface between benzocyclobutene ͑BCB͒ and silicon dioxide ͑SiO 2 ͒ coated silicon wafers were evaluated by four-point bending and wafer curvature techniques. Wafers were bonded using BCB in an established ͑baseline͒ process, and the SiO 2 films were deposited by plasma-enhanced chemical vapor deposition ͑PECVD͒. Thermal cycling was done between room temperature and a peak temperature. In thermal cycling performed with 350 and 400°C peak temperatures, the critical adhesion energy increased significantly during the first thermal cycle. The increase in critical adhesion energy is attributed to relaxation of residual stress in PECVD SiO 2 , which in turn is attributed to condensation reactions in those films. Thermal cycling also cures the BCB beyond the ϳ88% achieved in the baseline process, and the residual stress in the BCB is reset at a glass transition temperature corresponding to the increased BCB cure conversion. As more thermal cycles are performed, stress hysteresis in the BCB decreases as the cure stabilizes at 94-95%.

We report on a regime of operation of high-electron-mobility-transistor (HEMT) terahertz detectors, in which we apply a constant drain bias. The drain bias dependence of the gate-to-source and gate-to-drain capacitances results in a much greater asymmetry in the boundary conditions for plasma waves and greatly enhances the HEMT detector responsivity. The measured responsivity increases with the drain current by more than an order of magnitude and saturates at a saturation drain current for a given gate bias. These results confirm our model linking the responsivity increase to the drain bias dependence of the HEMT capacitances.

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