Rf Bias Research Articles

Relative intensity noise reduction for noise-like pulses based on synchronous intensity modulation

Extra-cavity synchronous intensity modulation is a straightforward and effective approach to suppress the full-bandwidth relative intensity noise (RIN) for noise-like pulse (NLP) fiber lasers. In this work, the picosecond NLPs are produced by a homemade Er/Yb co-doped mode-locked fiber laser and subsequent optimized by the synchronous intensity modulation device. The photodetector (PD) and electro-optical modulator (EOM) allow the real-time measurement and modulation for each optical pulse. A detailed description of key parameters for components is presented. The impact of DC bias voltage and RF signal amplitude on resulting RIN is experimentally investigated. We demonstrate a noteworthy reduction in RIN from 5.62% before synchronous intensity modulation to 1.91% afterward. Numerical investigations suggest that this method is potential to suppress RIN to below 1% under ideal conditions. The key factor in RIN reduction is correct operating region and high photoelectric-conversion linearity. The synchronous intensity modulation is an effective method to obtain low-RIN NLP laser sources, valuable for some practical applications.

Cryogenic DRIE processes for high-precision silicon etching in MEMS applications

Cryogenic deep reactive ion etching (Cryo DRIE) of silicon has become an enticing but challenging process utilized in front-end fabrication for the semiconductor industry. This method, compared to the Bosch process, yields vertical etch profiles with smoother sidewalls not subjected to scalloping, which are desired for many microelectromechanical systems (MEMS) applications. Smoother sidewalls enhance electrical contact by ensuring more conformal and uniform sidewall coverage, thereby increasing the effective contact area without altering contact dimensions. The versatility of the Cryo DRIE process allows for customization of the etch profiles by adjusting key process parameters such as table temperature, O2 percentage of the total gas flow rate (O2 + SF6), RF bias power and process pressure. In this work, we undertake a comprehensive study of the effects of Cryo DRIE process parameters on the trench profiles in the structures used to define cantilevers in MEMS devices. Experiments were performed with an Oxford PlasmaPro 100 Estrelas ICP-RIE system using positive photoresist SPR-955 as a mask material. Our findings demonstrate significant influences on the sidewall angle, etch rate and trench shape due to these parameter modifications. Varying the table temperature between −80 °C and −120 °C under a constant process pressure of 10 mTorr changes the etch rate from 3 to 4 μm min−1, while sidewall angle changes by ∼2°, from positive (<90° relative to the Si surface) to negative (>90° relative to the Si surface) tapering. Altering the O2 flow rate with constant SF6 flow results in a notable 10° shift in sidewall tapering. Furthermore, SPR-955 photoresist masks provide selectivity of 46:1 with respect to Si and facilitates the fabrication of MEMS devices with precise dimension control ranging from 1 to 100 μm for etching depths up to 42 μm using Cryo DRIE. Understanding the influence of each parameter is crucial for optimizing MEMS device fabrication.

Ion energy distribution measurement device using a capillary plate with high-aspect ratio

An ion energy measurement method using a capillary plate with a high-aspect ratio is proposed to obtain ion energy distribution (IED). When a capillary plate is used, an electron-shading (ES) effect is introduced in which a potential difference occurs between the top and bottom of the capillary plate because of a difference in the mobility of ions and electrons. Electrons are repelled by the potential difference, and only ions reach the bottom of the capillary plate. Additionally, the measured IED at the bottom of the capillary plate is similar to the value at the bottom of the contact hole in the etch process. This means that the actual ion energy reaching the inside of the contact hole can be predicted in the etch process. In our experiment, the IED was obtained using the ES effect at various discharge conditions, such as RF bias power, pressure, and aspect ratio (L/D), of the capillary plate. The ES effect was verified by a particle trajectory simulation, and the ion current measured at the collector was compared with the increasing aspect ratio. The obtained IEDs were in good agreement with previous works.

Direct density modulation of photo-assisted field emission from an RF cold cathode

This paper explores direct density modulation of high-current electron beam emission from an RF cold cathode using optical excitation. We theoretically study the photo-assisted field emission of periodically bunched electron beams of various pulse shapes under the combined excitation of an RF field and an optical field, using an exact quantum model. Both continuous-wave (CW) and pulsed optical fields are considered. The emission current pulse amplitude, pulse width, electron number density per pulse, as well as pulse shape and its harmonic contents are investigated in detail. For CW photon sources in the UV to NIR range (i.e., 200–1200 nm), increasing the optical intensity under an RF bias tends to change the current pulse from a Gaussian to sinusoidal-like shape, thus offering strong flexibility to control the frequency components in beam current emission. Pulsed photon sources combined with an RF field can produce sharp, high-current electron bunches with pulse duration comparable with or even less than that of the optical pulse. A contour map of the density modulation depth is constructed for different combinations of RF and laser fields. The results provide insight into unlocking new opportunities to achieve direct density modulation during electron current emission by optical means.

Etching mechanism of high-aspect-ratio array structure

Through-Silicon-Via (TSV) is an significant technology that is being widely employed in micro-electro-mechanical system (MEMS) manufactories. However, it is remain challenging to obtain a deep silicon etching with vertical-angle and smooth-sidewall characterizes by silicon-etching anisotropy through TSV technology. Herein, we use inductively coupled plasma (ICP) to obtain the deep silicon etching anisotropy. Chamber pressure, Bosch process cycles, bias powers, and the size of the etching pattern, which are the critical controllable process parameters, have been investigated to achieve the silicon-etching anisotropy. The results reveal that the sidewall angle of the array structure becomes increasingly slanted while raising chamber pressure from 2 to 15 Pa in the SF6 etching process of the first step of the cycle in the Bosch process. As the cycles increase from 60 to 240 cycles at the bias power of 100 w, the anisotropy decreases from 0.80 to 0.5. With RF bias powers rise from 200 to 400 w, the anisotropy enhances from 0.80 to 0.94. Due to the collision of active ions with each other, the sidewall angle and anisotropy of a high aspect-ratio array structure severely depend on the incidence angle of the active ions. The “Si etching balance model” has been established for the fabrication of TSV.

Second-harmonic currents in rf-biased, inductively coupled discharges

Capacitively-coupled plasmas generate strong current or voltage signals at harmonics of their driving frequencies. Inductively coupled plasma (icp) systems generally do not, unless they are equipped with capacitively-coupled rf bias, which generates strong signals at harmonics of its driving frequency. Recently, however, at an asymmetric, rf-biased electrode, a current component was detected at the second harmonic of the inductive source frequency, not the rf-bias frequency. The origin of this current is here investigated (in argon discharges at 1.3 Pa) by comparison with measurements made at a symmetric electrode and predictions made by two numerical models. The first simulates the sheath at the rf-biased electrode; the second models the plasma. Because capacitive coupling from the inductive source was minimized by a Faraday shield, the nonlinearity of the sheath contributes negligible second-harmonic current. Modulation of the photon flux in the plasma, however, produces a second-harmonic current photoemitted from the rf-biased electrode. The external circuitry and nonlinear inductive coupling produce a second-harmonic sheath voltage, which in turn generates second-harmonic current both directly and through a transit-time effect. The second model simulates how electrons emitted from the electrode—and then reflected at the quartz dielectric window of the inductive source—are deflected by the electric and magnetic fields in the plasma. It also gives predictions for the transit-time effect. Magnetic deflections and the transit-time effect usually dominate the electric deflection. Together these three mechanisms produce a second-harmonic current that has a Fourier amplitude approximately half the current that is elastically reflected at the icp window. These results suggest it may be possible to use the second-harmonic current to determine the elastic reflection coefficient at the window.

Ultrahigh-linearity dual-drive scheme using a single silicon modulator.

Here, a high-linearity dual-drive scheme using a single silicon dual-drive Mach-Zehnder modulator is presented. The bias voltages and RF amplitudes of the two driving arms are adjusted such that the nonlinearity of the transfer function of the Mach-Zehnder interferometer cancels out the nonlinear response of the arms. Using the proposed scheme, the spurious-free dynamic range of the third-order intermodulation distortion is 123.4 dB Hz6/7, which is believed to be a record-breaking value for silicon modulators. In comparison, the result obtained using a conventional single-drive scheme is 102.6 dB·Hz2/3. The proposed scheme could simplify the design of modulators and promote high-performance microwave photonic links.

Investigation of the impact of amorphous silicon layers deposited by PECVD and HDP-CVD on oxide precipitation in silicon

The effect of deposited a-Si layers with different layer stress on oxide precipitation was investigated in order to find out if intrinsic point defects affecting oxide precipitation are generated at the interface a-Si/Si and if possibly hydrogen affects the oxide precipitation. A thermal cycle of nucleation at 650 °C for 4 h or 8 h followed by stabilization at 780 °C for 3 h, and growth at 1000 °C for 16 h was applied. It was found that there are no signs for the injection of intrinsic point defects from the interface a-Si/Si into the Si substrate during the applied thermal treatment. However if a-Si is deposited on 1000 nm silicon oxide, deposited previously from TEOS in a plasma process, silicon self-interstitials seem to be injected from the interface silicon oxide/Si into the silicon substrate retarding oxide precipitation in the initial stage of nucleation annealing at 650 °C. There are also no signs of any impact of the layer stress on oxide precipitation or self-interstitial injection. The concentration of hydrogen in the layers can be controlled via the RF bias power. The hydrogen concentration is reduced markedly already during annealing at 650 °C. Part of the hydrogen diffuses into the silicon substrate and enhances oxide precipitation if its initial concentration in the layers is higher than 1.5 × 1022 cm−3. For a-Si deposited on 1000 nm silicon oxide, the enhancement effect appears for hydrogen concentrations in the layer higher than approximately 2.8 × 1022 cm−3.

Effect of time-modulation bias on polysilicon gate etching

The etching characteristics were studied via time-modulation bias (bias pulsing) by varying the pulsing parameters. The etch profiles were verified using polysilicon gate structures with dense and isolated patterns. Ion energy was defined as the peak-to-peak voltage (V pp) controlled by the RF bias power. The durations of the on period and off period (off time) of bias pulsing were adjusted by the pulse frequency and duty cycle. Profile evolution was observed in the variations in V pp and off time. Increasing the ion energy induced vertical profiles of dense patterns and the tapered profiles of isolated patterns. Extending the off time of bias pulsing induced tapered profiles of dense patterns and vertical profiles of isolated patterns. These results indicated that increasing the ion energy and pulse off time simultaneously was the direction to achieve anisotropic etch profiles for both the isolated and dense patterns.

Real-time monitoring of atomic layer etching in Cl2/Ar pulsed gas, pulsed power plasmas by optical emission spectroscopy

Optical emission spectroscopy was used as a real-time monitor of the atomic layer etching (ALE) of Si in an Ar inductively coupled plasma (ICP). Pulses of Cl2 gas were repetitively injected into a continuous flow of Ar, followed by the ignition of the ICP and the application of substrate rf bias power (either continuous or rapidly modulated). Optical emissions from Si, SiCl, SiCl2, Ar, and Cl were monitored along a line parallel and close to the substrate surface as a function of time during the bias period, as well as in the ICP without bias power. From an analysis of the time dependencies of the decays of emissions during the modulated bias periods, it is argued that emissions at high Ar carrier gas flow rates are mainly from the primary products sputtered by the energetic Ar ions. Products decay with different, non-exponential time signatures. Cl and SiCl2 emissions decay to nearly undetectable levels toward the end of the bias period. SiCl emission follows a decay profile between that of Si and SiCl2. The time-integrated SiCl2 emission intensity scales linearly with time and correlates with etching rates measured by laser interferometry. The amount of Si etched per ALE cycle and the degree of self-limiting evolution of etching products is a sensitive function of the timing between the initiation and termination of Cl2 flow into the reactor and the application of ICP power. Spatially resolved optical emission is shown to be a useful in situ diagnosis, providing mechanistic insights, as well as process optimization for plasma-assisted atomic layer etching. It is also shown that the emission bands between 360 and 400 nm that are commonly observed during Si etching in a chlorine-containing plasma and are often ascribed to SiCl3 or SiCl3+ are instead most likely the A2Σ → X2Πr system of SiCl.

Multiband dual- and cross-LFM waveform generation using a dual-drive Mach–Zehnder​ modulator

Linear frequency modulation (LFM) signals are widely used in modern radar systems owing to their intrinsic pulse compression capability but are restricted due to a limited range-Doppler resolution. On the other hand, dual- or cross-LFM signals have complementary LFM waveforms at the same time instant that reduces the range-Doppler coupling and thus improving range-Doppler resolution. We propose a simple microwave-photonic-based approach to generate multiband dual- and cross-LFM waveforms using only a single dual-drive Mach–Zehnder modulator (DDMZM). A theoretical analysis is carried out, which is modeled and verified through experiments. Dual- and cross-LFM waveforms at a carrier frequency of 6 GHz and 18 GHz with a bandwidth of 2 GHz is generated by controlling the DC and RF bias of the DDMZM. The tunability of the carrier frequency from 3 GHz to 9 GHz is also verified. A range resolution of 6.3 cm for dual- and cross-LFM waveforms, is obtained with a time-bandwidth-product (TBWP) of 20,000. The proposed technique is a promising solution for dual- and cross-LFM waveform generation in two-band multipurpose radar systems.

Comment on “The effects of radio-frequency bias on electron density in an inductively coupled plasma reactor” [J. Appl. hys. 102, 113302 (2007)

Sobolewski et al. [J. Appl. Phys. 102, 113302 (2007)] obtained the complex responses of electron density to the long-pulsed RF bias and proposed the gas composition effect as a key underlying factor. In this Comment, the consistent explanations on the complex responses indicate that the ignored slow wall heating should be taken into account.

Simulation of the First Two Microseconds of an Ar CCP Cold Plasma Discharge by the PIC-MCC Method

Most simulations of capacitively coupled radiofrequency cold-plasma discharges (RF-CCP) are focused on the steady state, but the initial discharge time is important for understanding the ignition process and the behavior of pulsed discharges. In this work, the time evolution of an RF-CCP Ar discharge was simulated, considering a pressure of 66.6 Pa, a distance between the electrodes of 20 mm, and RF (13.56 MHz) bias amplitudes in range 100–400 V, and the discharge evolution was observed for the first 2 μs. A 1d3v (1 dimension for particle positions and 3 dimensions for particle velocities) electrostatic particle in cell with montecarlo collisions (PIC-MCC) model was used, with separated particle weights for electrons and ions that varied with the particle density. During the simulations, the time evolution of the electron density, mean electron energy, Debye length, Debye number, and plasma frequency were observed. The spatial distribution of electric potential and the electron energy distribution function were also monitored. A transition between two regimes was observed; the first was characterized by strong oscillation of the mean electron energy and an exponential increase of the mean plasma density with time, while in the second the mean electron energy was lower, and the plasma density increased linearly. The time required for the transition between the two regimes increased as the RF amplitude was raised from 100 to 250 V, then decreased with a further increase of the RF amplitude to 300 and 350 V.

Enhancement-Mode High-Frequency InAlGaN/GaN MIS-HEMT Fabricated by Implementing Oxygen-Based Digital Etching on the Quaternary Layer

A high frequency enhancement mode quaternary InAlGaN/GaN MIS-HEMT with recessed gate (Lg = 150 nm) processed using an oxygen-based digital etching technique is presented. The digital etching was performed by cyclic ICP oxygen treatment to oxidize InAlGaN barrier and HCl wet etching to remove the oxidized layer. In this study, we have demonstrated that the threshold voltage can be adjusted in a wide-range from depletion mode to enhancement mode with a nanometer scale gate for high frequency InAlGaN/GaN MIS-HEMT using the digital etching technique. In addition, the etch rate can be controlled from 0.7 nm/cycle to 3.6 nm cycle−1 with RF bias power changing from 0 W to 40 W with high flexibility in etching rate. The post-etching surface roughness was around 0.12 nm regardless of the ICP oxidation voltage. The enhancement-mode InAlGaN quaternary GaN HEMT with maximum drain current of 955 mA mm−1, gm−1 peak of 440 mS mm−1, Vth of 0.2 V, and ft/fmax of 45/59 GHz were achieved using the digital etching for the gate recess structure.

Experimental study of transition from electron beam to rf-power-controlled plasma in DFCCP in argon with additional ionization by an electron beam

Radio-frequency (rf) plasma with additional ionization by an electron beam (EB) is considered as a possible method for the independent control of plasma density, mean electron energy and mean ion energy. In this study, spatial transition from EB to rf-power-controlled dual-frequency capacitively coupled plasma (DFCCP) was studied using the following movable diagnostics: Langmuir and hairpin probes, a retarding field energy analyzer and optical emission spectroscopy. The beam (1.1–1.4 keV) is generated by a runaway EB module placed near the plasma chamber wall, while the plasma transition is caused by EB degradation with the distance from the EB module. The study was conducted in Ar at 200 and 400 mTorr gas pressures in 81 and 12 MHz DFCCP. When the EB is on, a significant decrease in the mean electron energy is observed, from 6 eV in the rf plasma down to 0.2–0.8 eV in the EB plasma. The EB also changes the shape of the electron energy probability function, from Druyvesteyn-like in rf plasma to Maxwellian-like. When both EB and rf power are applied, the mean electron energy increase and the electron density decrease, with the distance from the EB module, are observed due to the beam degradation. The ion energy distribution at the bottom electrode in rf plasma peaks at 25–30 eV and shifts down to a few eV in EB plasma. As in conventional DFCCP, the ion energy distribution can be fine-tuned by the application of a low-frequency rf bias. However, the use of an EB allows us to reduce the range of ion energies down to a few eV, which cannot be achieved in conventional rf discharges.

Locking of microwave oscillators on the interharmonics of mode-locked laser signals.

In this paper, the theory of phase-locking of a microwave oscillator on the interharmonics, i.e. non-integer harmonics, of the repetition rate of the optical pulse train of a mode-locked laser (MLL) is developed. A balanced optical microwave phase detector (BOMPD) is implemented using a balanced Mach-Zehnder modulator and is employed to discriminate the phase difference between the envelope of the optical pulses and the microwave oscillator. It is shown mathematically that the inherent nonlinear properties of BOMPD with respect to the microwave excitation amplitude can be used for interharmonic locking. The characteristic functions of the phase detector for interharmonic locking are derived analytically and are compared with the measurement results. An opto-electronic phase-locked loop (OEPLL) is demonstrated whose output frequency locks on interharmonics of the MLL repetition rate when an appropriate modulator bias and sufficient RF amplitude are applied. Thus, for the first time theory and experiment of reliable locking on interharmonics of the repetition rate of a MLL are presented.

High Speed Ashing of Ion Implanted Photoresist by Microwave Excited Water Vapor Plasma with Powered Substrate

This study examined a substrate bias voltage introduced into a microwave excited water vapor plasma to enable fast ashing of photoresist implanted with high-dose ions without occurring popping. The ashing rate at the center of the ashing distribution for the photoresist implanted with boron with an implantation dose of 1×1016 atoms/cm2 was estimated as 1.0 μm/min. Results of plasma emission diagnostics suggest that rf bias application effects generated a plasma rich in excited hydrogen and oxygen atom near the substrate surface. This plasma is presumed to assist the removal of the hardened layer of ion implanted photoresist and to affect hydrogen gas addition. A self-bias voltage during bias application was sufficiently low, approximately －30 V, at a peak-to-peak voltage of 1 kV at 1 MHz.

Effect of RF bias power on discharge mode transition and its hysteresis in inductively coupled plasmas

Hysteresis, one of the interesting characteristics of inductively coupled plasmas, takes place due to the nonlinearities of absorbed power or dissipated power [M. M. Turner et al., Plasma Sources Sci. Technol. 8, 313 (1999)]. In this work, the bias power effect on discharge mode transition and its hysteresis is investigated by measuring the antenna coil currents, time-average substrate voltages, electron densities, and electron energy probability functions (EEPFs). The behavior of hysteresis is comparatively analyzed by introducing a global model that considers the evolution of EEPFs and excitation state species (multi-step ionization). In the absence of bias power, a typical hysteresis appears at a gas pressure of 300 mTorr. The measured EEPF evolves from a Druyvesteyn distribution in the E mode to a Maxwellian distribution in the H mode with growing the antenna power from 25 W to 60 W. Interestingly, when a bias power of 30 W is applied to the substrate, the hysteresis vanishes, and the shape of the EEPF is maintained in each mode. The possible factors are considered the diminished changes in total energy loss (reduced nonlinearity of dissipated power) and in power transfer efficiency between E mode and H mode during the mode transition.

Specific Features of Matching of a Lower Electrode and an RF Bias Generator for Reactive Ion Etching of Bulk Substrates

Specific Features of Matching of a Lower Electrode and an RF Bias Generator for Reactive Ion Etching of Bulk Substrates

Control of V th of the enhancement high-frequency AlGaN/GaN HEMT fabricated by oxygen-based digital etching

We established an oxygen-based digital etching technique to fabricate the recessed gate structure for RF GaN HEMT. The digital etching rates were 2.5 nm cycle–1 and 0.5 nm cycle–1 with 40 W RF bias and 0 W RF bias for ICP oxidation, respectively. The roughness of the surface was around 0.1 nm. The V th of the recessed gate GaN HEMTs can be adjusted from D-mode to E-mode. The GaN HEMT with 110 nm gate length showed a maximum peak gm value of 440 mS mm−1 after 6 cycles of digital etching with V th near 0 V and ft values nearby 42 GHz.