James E. Stevens

Role of nitrogen in the downstream etching of silicon nitride

Chemical downstream etching of silicon nitride (Si3N4) requires the addition of nitrogen to the discharge for obtaining efficient etch rates. A 10% addition of N2 to a CF4/O2 discharge (CF4/O2 = 1.2, 0.525 Torr) causes a factor of 6 increase in the Si3N4 etch rate and a 8% decrease in the silicon dioxide etch rate. The result is selectivities approaching 9:1. Importantly, the conversion of CF4 to F and F‐containing reactive species by the discharge decreases or remains constant as nitrogen is added to the discharge mix, indicating that the etching reaction is not limited by delivery of these species to the substrate. By measuring the amount of NO and NO2 in the etch chamber, it is found that the NO concentration increases by a factor of 6 as N2 is added, while the amount of NO2 remains small and constant. The NO signal is significantly reduced during nitride etching compared to the signal observed during a discharge with an empty etch chamber, implying that the increase in Si3N4 etch rate is related to th...

VHF and UHF mechanically coupled aluminum nitride MEMS filters

This paper reports the development of narrow-bandwidth, post-CMOS compatible aluminum nitride (AlN) MEMS filters operating in the very (VHF) and ultra (UHF) high frequency bands. Percent bandwidths less than 0.1% are achieved utilizing a mechanically coupled filter architecture, where a quarter wavelength beam attached in low velocity coupling locations is used to connect two AlN ring resonators. The filter bandwidth has been successfully varied from 0.09% to 0.2% by moving the attachment of the coupling beam on the ring to locations with different velocity at resonance. Insertion losses of 11 dB are obtained for filters centered at 99.5 MHz with low termination impedances of 200 Omega. Utilizing a passive temperature compensation technique, the temperature coefficient of frequency (TCF) for these filters has been reduced from -21 ppm/C to 2.5 ppm/C. The reduced TCF is critical for narrow bandwidth filters, requiring only 13% of the filter bandwidth to account for military range (-55 to 125 C) temperature variations compared to 100% for uncompensated filters. Filters operating at 557 MHz are realized using overtone operation of the ring resonators and coupling beam where higher insertion losses of 32 dB into 50 Omega are seen due to the finite resonator quality factor and narrow bandwidth design. Overtone operation allows for the implementation of fully differential and balun type filters where the stop-band rejection is as high as 38 dB despite the increased insertion loss.

Optimizing TaOx memristor performance and consistency within the reactive sputtering “forbidden region”

Standard deposition processes for depositing ReRAM oxides utilize mass flow of reactive gas to control stoichiometry and have difficulty depositing a precisely defined sub-stoichiometry within a “forbidden region” where film properties are discontinuous with mass flow. We show that by maintaining partial pressure within this discontinuous “forbidden region,” instead of by maintaining mass flow, we can optimize tantalum oxide device properties and reduce or eliminate the electroforming step. We also show that defining the partial pressure set point as a fraction of the “forbidden region” instead of as an absolute value can be used to improve wafer-to-wafer consistency with minimal recalibration efforts.

Reactive sputtering of substoichiometric Ta2Ox for resistive memory applications

A major class of resistive memory devices is based on transition metal oxides, where mobile oxygen vacancies allow these devices to exhibit multiple resistance states. Ta2O5 based devices in particular have recently demonstrated impressive endurance and forming-free results. Deposition of substoichiometric Ta2Ox (x < 5) films is a critical process in order to produce the required oxygen vacancies in these devices. This paper describes a physical vapor deposition (PVD) reactive sputtering process to deposit substoichiometric Ta2Ox films. The desired film stoichiometry is achieved by feedback control of the oxygen partial pressure in the PVD chamber. A calibration procedure based on Rutherford backscattering spectroscopy is described for locating the optimum oxygen partial pressure.

Evaluating tantalum oxide stoichiometry and oxidation states for optimal memristor performance

Tantalum oxide has shown promising electrical switching characteristics for memristor devices. Consequently, a number of reports have investigated the electrical behavior of TaOx thin films. Some effort has been made to characterize the composition of the TaOx films and it is known that there must be an optimal stoichiometry of TaOx where forming and switching behavior are optimized. However, many previous reports lack details on the methodology used for identifying the chemistry of the films. X-ray photoelectron spectroscopy has been the most commonly used technique; however, peak fitting routines vary widely among reports and a native surface oxide of Ta2O5 often confounds the analysis. In this report a series of large area TaOx films were deposited via sputtering with controlled O2 partial pressures in the sputtering gas, resulting in tunable oxide compositions. Spectra from numerous samples from each wafer spanning a range of oxide stoichiometries were used to develop a highly constrained peak fitting...

Microresonant impedance transformers

Widely applied to RF filtering, AlN microresonators offer the ability to perform additional functions such as impedance matching and single-ended-to-differential conversion. This paper reports microresonators capable of transforming the characteristic impedance from input to output over a wide range while performing low loss filtering. Microresonant transformer theory of operation and equivalent circuit models are presented and compared with measured 2 and 3-Port devices. Impedance transformation ratios as large as 18:1 are realized with insertion losses less than 5.8 dB, limited by parasitic shunt capacitance. These impedance transformers occupy less than 0.052 mm2, orders of magnitude smaller than competing technologies in the VHF and UHF frequency bands.

In situ wafer temperature monitoring of silicon etching using diffuse reflectance spectroscopy

Real time, in situ temperature measurements during chemical downstream etching of silicon wafers have been performed using a diffuse reflectance spectroscopy based sensor [Weilmeier et al., Can. J. Phys. 69, 422 (1991)]. The spectrometer has a spatial resolution of 1 cm2, updates the temperature every 2 s, and has a temperature resolution of better than 1 °C. The thermal time constant the wafers and the thermally regulated electrostatic chuck (10 °C<T<90 °C) varied between 7 and 30 s depending on clamping and backside gas pressure. The exothermic etch process is accompanied by increases in the silicon wafer temperature consistent with the thermal conductivity conditions and with the etch chemistry. The temperature uniformity across the wafers was better than 2 °C during the entire etch process.

High-resolution submicron retarding field energy analyzer for low-temperature plasma analysis

A retarding potential energy analyzer having 750 nm diameter, self-aligned grid apertures and micron scale grid separation has been fabricated using polycrystalline silicon and silicon dioxide. High-resolution in situ measurements of ion velocity distributions have been demonstrated in inductively coupled argon plasmas. Measurement results agree well with those from a macroscopic analyzer. Important differences are observed in the energies of plasma ions when measured with respect to chamber wall versus those measured with respect to the plasma floating potential. Preliminary measurements under rf bias conditions have also been made and results follow the expected trends.

Spatially resolved electron temperature measurements with a microfabricated retarding field analyzer

Electron temperature (Te) is an important parameter to quantify in the high-density plasmas commonly used in semiconductor manufacturing. Te is characteristic of the electron energy distribution which determines the plasma density and distribution of neutral and ionic species. Through application of theoretical considerations for the presheath and sheath, Te can be estimated from the ion energy distribution to a floating substrate. Utilizing microfabricated retarding field analyzers (RFAs) to measure the local ion energy distribution to a floating surface, spatially resolved Te measurements in an inductively coupled argon plasma have been made. Quantitative agreement between the RFA and Langmuir probe Te measurements was observed and the RFA Te measurements display the expected power, pressure, and spatial dependencies.

Advanced Atom Chips with Two Metal Layers

A design concept, device layout, and monolithic microfabrication processing sequence have been developed for a dual-metal layer atom chip for next-generation positional control of ultracold ensembles of trapped atoms. Atom chips are intriguing systems for precision metrology and quantum information that use ultracold atoms on microfabricated chips. Using magnetic fields generated by current carrying wires, atoms are confined via the Zeeman effect and controllably positioned near optical resonators. Current state-of-the-art atom chips are single-layer or hybrid-integrated multilayer devices with limited flexibility and repeatability. An attractive feature of multi-level metallization is the ability to construct more complicated conductor patterns and thereby realize the complex magnetic potentials necessary for the more precise spatial and temporal control of atoms that is required. Here, we have designed a true, monolithically integrated, planarized, multi-metal-layer atom chip for demonstrating crossed-wire conductor patterns that trap and controllably transport atoms across the chip surface to targets of interest.