Analysis of the effective reflectance of digital micromirror devices and process parameters for maskless photolithography

The evaluation of the effective reflectance of digital micromirror devices (DMD) is used to predict exposure doses in optical maskless photolithography applications. The effective reflectance is determined by multiplying the diffraction efficiency by the reflectance of the DMD in a dynamic mode. The diffraction efficiency is calculated using multiple slit diffraction theory and the two-dimensional grating equation for the wavelengths of the lithography sources, G-line (436 nm), H-line (405 nm), and I-line (365 nm). The diffraction efficiency is also used to determine the optimized dimensions of a DMD’s pitch size. The reflectance of the DMD in a dynamic mode linearly decreases as the frame rate increases, dropping by ∼8% at a frame rate of 16,000 frames per second. The process parameters are analyzed in the terms of spot overlap and blur, which can have an effect on pattern quality. The spot overlap tends to increase with the frame rate of DMD, and it decreases with the stage speed. The spot blur, analyzed as a function of the illumination time and stage speed, also shows proportional behavior with those parameters.     

Broad-band mirror (1.12-1.62 /spl mu/m) using a subwavelength grating

We report the first experimental demonstration of a novel single-layer subwavelength grating (SWG) that has >500-nm-wide reflection spectrum from 1.12-1.62 /spl mu/m and very high reflectivity (>98.5%). This SWG is scalable for different wavelengths by simply changing the grating dimensions, which, thus, facilitates monolithic integration of devices over a wide range of wavelengths.     

N2 effect on GaAs etching at 150 mTorr capacitively-coupled Cl2/N2 plasma

The role of N₂ on GaAs etching at 150 mTorr capacitively-coupled Cl₂/N₂ plasma is reported. A catalytic effect of N₂ was found at 20-25% N₂ composition in the Cl₂/N₂ discharges. The peak intensities of the Cl₂/N₂ plasma were monitored with optical emission spectroscopy (OES). Both atomic Cl (725.66 nm) and atomic N (367.05 nm) were detected during the Cl₂/N₂ plasma etching. With the etch rate and OES results, we developed a simple model in order to explain the etch mechanism of GaAs in the high pressure capacitively-coupled Cl₂/N₂ plasma as a function of N₂ ratio. If the plasma chemistry condition became positive ion-deficient at low % N₂ or reactive chlorine-deficient at high % N₂ in the Cl₂/N₂ plasma, the GaAs etch rate is reduced. However, if the plasma had a more balanced ratio of Cl₂/N₂ (i.e. 20-25% N₂) in the plasma, much higher etch rates (up to 150 nm/min) than that in pure Cl₂ (50 nm/min) were produced due to synergetic effect of neutral chlorine adsorption and reaction, and positive ion bombardment. Pure Cl₂ etching produced 14 nm of RMS surface roughness of GaAs. Introduction of ?20% N₂ gas in Cl₂/N₂ discharges significantly reduced the surface roughness to 2-4 nm. SEM photos showed that the morphology of photoresist mask was strongly degraded. Etch rate of GaAs slightly increased from 10 to 40 nm/min when RIE chuck power changed from 10 to 150 W at 12 sccm Cl₂/8 sccm N₂ plasma condition. The surface roughness of GaAs etched at 12 sccm Cl₂/8 sccm N₂ plasma was 2-3 nm.     

Synthesis and optical characterization of Ag nanoparticles

The results of chemical synthesis and optical behavior of silver nanoparticles with sizes ranging from 2-10 nm which were obtained by reduction of Ag⁺ are reported. The material morphology was examined by transmission electron microscopy (TEM) and physical properties were studied by photoluminescence in the range of 400-550 nm using two different excitation wavelengths (320 and 380 nm). The signature provided by the silver plasmon is readily noticed in the silver nanoparticles. The nonlinear optical properties were obtained using a Z-scan setup and agree with previous results obtained by other methods and preparation of samples. In particular, a positive nonlinear refractive index of 5.0115×10⁻¹⁰ m²/W was obtained for a broad sample of silver nanoparticles between 2 and 10 nm. The agreement between the properties of the thin film sample and the nanoparticles give emphasis to the Z-scan technique for nonlinearity measurements of much more complex metal-dielectric structures.     

High-power broad-area InGaNAs/GaAs quantum-well lasers in the 1200 nm range

High-power broad-area InGaNAs/GaAs quantum-well (QW) edge-emitting lasers on GaAs substrates in the 1200 nm range are reported. The epitaxial layers of the InGaNAs/GaAs QW laser wafers were grown on n⁺-GaAs substrates by using metal-organic chemical vapor deposition (MOCVD). The thickness of the InGaNAs/GaAs QW layers is 70 Å/1200 Å. The indium content (x) of the In_xGa_1−xN_yAs_1−y QW layers is estimated to be 0.35-0.36, while the nitrogen content (y) is estimated to be 0.006-0.009. More indium content (In) and nitrogen content (N) in the InGaNAs QW layer enables the laser emission up to 1300 nm range. The epitaxial layer quality, however, is limited by the strain in the grown layer. The devices were made with different ridge widths from 5 to 50 μm. A very low threshold current density (J_th) of 80 A/cm² has been obtained for the 50 μm × 500 μm LD. A number of InGaNAs/GaAs epi-wafers were made into broad-area LDs. A maximum output power of 95 mW was measured for the broad-area InGaNAs/GaAs QW LDs. The variations in the output powers of the broad-area LDs are mainly due to strain-induced defects the InGaNAs QW layers.     

Effect of pressure on efficiency of UV curing of CVD-derived low-k material at different wavelengths

Low-k dielectrics prepared by CVD in the form of 200 nm thick layers on Si wafers were thermally treated at 410 °C and irradiated using UV lamps emitting photons of different wavelengths around 172 nm, 185 nm, and 222 nm. The treatment was performed in high vacuum and under a nitrogen atmosphere at various pressures ranging from 0.1 mbar up to 700 mbar. Subsequently, the samples were investigated using FTIR transmission spectroscopy, contact angle measurement, X-ray photoelectron spectrometry (XPS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray reflectometry (XRR), surface acoustic wave spectrometry (SAW), and purged UV spectroscopic ellipsometry (PUVSE). It was found that for all UV wavelengths applied for curing the depth profiles of the chemical composition were homogeneous. For all properties evaluated, irradiation at wavelengths below 200 nm resulted in more pronounced changes than at longer wavelengths. Generally, a decrease in residual porogen content, conversion of the Si-O-Si bonds from cage to network/suboxide, degradation of Si-CH₃ bonds, formation of H-SiO bonds, increase in surface energy, changes of element concentrations and of density, increase in Young’s modulus, and changes in dielectric constant were observed. These findings were confirmed by quantum-chemical calculations. With increasing nitrogen pressure the effects were more considerable. An attempt was undertaken to explain the effect of nitrogen pressure in course of the role of nitrogen molecules as collision partners.     

Multi-technique characterisation of MOVPE-grown GaAs on Si

The heterogeneous integration of III-V materials on a Si CMOS platform offers tremendous prospects for future high-speed and low-power logic applications. That said this integration generates immense scientific and technological challenges. In this work multi-technique characterisation is used to investigate properties of GaAs layers grown by Metal-Organic Vapour Phase Epitaxy (MOVPE) on Si substrates - (100) with 4° offset towards <1 1 0> - under various growth conditions. This being a crucial first step towards the production of III-V template layers with a relatively lower density of defects for selective epitaxial overgrowth of device quality material. The optical and structural properties of heteroepitaxial GaAs are first investigated by micro-Raman spectroscopy and photoluminescence and reflectance measurements. High-resolution X-ray diffraction (HR-XRD) is used to investigate structural properties. Advanced XRD techniques, including double-axis diffraction and X-ray crystallographic mapping are used to evaluate degrees of relaxation and distribution of the grain orientations in the epilayers, respectively. Results obtained from the different methodologies are compared in an attempt to understand growth kinetics of the materials system. The GaAs overlayer grown with annealing at 735 °C following As pre-deposition at 500 °C shows the best crystallinity. Close inspection confirms the growth of epitaxial GaAs preferentially oriented along (100) embedded in a highly textured polycrystalline structure.     

The effects of quantum dot coverage in InAs/(In)GaAs nanostructures for long wavelength emission

We present a study on the effects of quantum dot coverage on the properties of InAs dots embedded in GaAs and in metamorphic In_0.15Ga_0.85As confining layers grown by molecular beam epitaxy on GaAs substrates. We show that redshifted emission wavelengths exceeding 1.3 μm at room temperature were obtained by the combined use of InGaAs confining layers and high quantum dot coverage. The use of high InAs coverage, however, leads to detrimental effects on the optical and electrical properties of the structures. We relate such behaviour to the formation of extended structural defects originating from relaxed large-sized quantum dots that nucleate in accordance to thermodynamic equilibrium theories predicting the quantum dot ripening. The effect of the reduced lattice-mismatch of InGaAs metamorphic layers on quantum dot ripening is discussed in comparison with the InAs/GaAs system.     

Self-aligned n-channel GaAs metal-oxide-semiconductor field-effect transistors (MOSFETs) using HfO2 and silicon interface passivation layer: Post-metal annealing optimization

In this work, using Si interface passivation layer (IPL), we demonstrate n-MOSFET on p-type GaAs by varying physical-vapor-deposition (PVD) Si IPL thickness, S/D ion implantation condition, and different substrate doping concentration and post-metal annealing (PMA) condition. Using the optimized process, TaN/HfO₂/GaAs n-MOSFETs made on p-GaAs substrates exhibit good electrical characteristics, equivalent oxide thickness (EOT) (∼3.7 nm), frequency dispersion (∼8%) and high maximum mobility (420 cm²/V s) with high temperature PMA (950 °C, 1 min) and good inversion.     

InGaAs submonolayer quantum-dot photonic-crystal LEDs for fiber-optic communications

An InGaAs submonolayer (SML) quantum-dot photonic-crystal light-emitting diode (QD PhC-LED) with for fiber-optic applications is reported. The active region of the device contains three InGaAs SML QD layers. Each of the InGaAs SML QD layers is formed by alternate depositions of InAs (<1 ML) and GaAs. A maximum CW output power of 0.34 mW at 20 mA has been obtained in the 980 nm range.     

Preparation and properties of Zn delta-doped GaAs/AlGaAs heterojunction phototransistor

We have prepared, characterized and discussed the performance of AlGaAs/GaAs heterojunction bipolar phototransistor (HPT) including Zn delta-doped base. Due to the existence of δ-doped sheets located in the middle of undoped GaAs base the δ-doped HPT devices exhibit low dark current, nearly zero offset voltage, saturation voltage ∼0.4 V, and rise and fall times in ns range at wavelength of 850 nm up to 6 V of applied voltage. Due to avalanche multiplication behavior at the collector junction, an increased optical gain G>10 can be reached for applied voltages in the range of 6-12 V. For voltages higher than the device breakdown voltage (∼12 V) switching and negative differential resistance (NDR) effect is measurable in the inverted mode of operation.     

AP-MOVPE of InGaAs on GaAs (0 0 1): Analysis of in situ reflectivity response

We have investigated in situ monitoring of growth rate and refractive index by laser reflectometry during InGaAs on GaAs (0 0 1) substrate growth in atmospheric pressure metalorganic vapour-phase epitaxy (AP-MOVPE). The indium solid composition (x_In^s) was varied by changing the substrate temperature or the indium vapour composition (x_In^v). The refractive index of InGaAs alloys as a function of temperature and composition was quantified and compared which that of GaAs for 632.8 nm wavelength by simulation of experimental reflectivity responses. Composition analyses were carried out by high-resolution X-ray diffraction (HRXRD) and optical absorption (OA). The layers thicknesses were estimated by scanning electron microscopy (SEM) observations. The temperature dependence of InGaAs growth rate has been investigated in the temperature range 420-680 °C using trimethylgallium (TMGa), trimethylindium (TMIn) and arsine (AsH₃) sources. It shows Arrhenius-type behaviour with an apparent activation energy E_a of 0.62 eV (14.26 kcal/mol). This value is close to that determinate in the AP-MOVPE of GaAs.     

Well-width dependence of the phase coherent photorefractive effect in ZnSe quantum wells

We report on the well-width dependence of the phase coherent photorefractive (PCP) effect in ZnSe/ZnMgSe single quantum wells (QWs) using 90 fs light pulses. The experiments are performed in a four-wave-mixing configuration at temperatures between 25 and 65 K. The ZnMgSe/ZnSe QWs with 10, 5 and 3 nm well width were grown on GaAs substrate using molecular beam epitaxy. With decreasing QW thickness we observe a reduction of the PCP diffraction efficiency. This is attributed to the higher electron energy in small QWs due to the quantum size effect, which leads to a reduced equilibrium density of captured electrons in the QW. With increasing temperature the PCP signal further decreases which is attributed to thermal activation of the QW electrons back to the GaAs substrate.     

Characterization of QWIP structures prepared on GaAs-patterned substrates

In this work we investigate the preparation of quantum well infrared photodetectors (QWIP) on planar and patterned GaAs substrates. Mesa ridges with various angles between sidewall and substrate (1 0 0) plane were prepared by wet chemical etching. The QWIP structures were grown at a temperature of 700 °C by use of low-pressure MOVPE. Electrical properties and spectral sensitivity of QWIP structures prepared on tilted sidewalls were measured. Our results showed that mesa ridges confined at the sides by facets tilted at 30° to (1 0 0) were most suitable for the QWIP preparation. Asymmetry in room temperature I-V characteristics and a small photovoltaic effect observed at 77 K was ascribed to asymmetric position of delta doping plane in the quantum well.     

Growth of III-V semiconductor nanowires by molecular beam epitaxy

We present here the growth of GaAs, InAs and InGaAs nanowires by molecular beam epitaxy. The nanowires have been grown on different substrates [GaAs(0 0 1), GaAs(1 1 1), SiO₂ and Si(1 1 1)] using gold as the growth catalyst. We show how the different substrates affect the results in terms of nanowire density and morphology. We also show that the growth temperature for the InGaAs nanowires has to be carefully chosen to obtain homogeneous alloys.     

Photonic quantum ring laser of 3D whispering cave mode

A new whispering cave mode (WCM), a three-dimensional (3D) case of Lord Rayleigh's 2D whispering gallery modes, based upon 3D total internal reflection is presented, where GaAs quantum well (QW) near-vertical microdisk resonators exhibit a strong carrier-photon coupling for the QW carriers located in the Rayleigh band, generating 2D-to-1D carrier phase transitions of photonic quantum ring (PQR). It is a ‘photonic’ quantum corral effect (PQCE), whose simulation work produces a tangled web pattern of imminent recombinant carriers in picoseconds timescale. The PQCE is responsible for the ultralow threshold currents below 300 nA for a single mode GaAs PQR laser, and a prototype GaN PQR laser with a threshold near and below 1 μA.     

Dry etching fin process for SOI finFET manufacturing: Transition from 32 to 22 nm node on a 6T-SRAM cell

This work describes the main challenges encountered for patterning crystalline silicon (c-Si) fins when we scaled down the fin pitch from 124 to 90 nm on a 6T-SRAM cell. The target fins consist of straight structures (40 nm height and 17 nm of critical dimension) patterned on a 22 nm node with 90 nm fin pitch. The patterning stack consists of 70 nm of amorphous carbon as a hard mask with 25 nm of antireflective coating. Scaling down the fin pitch had a direct influence on the fin critical dimension, profile and sidewall roughness. We found out that the fin etching process developed for a 32 nm node with 124 nm fin pitch was no longer functional for patterning fins on a 22 nm node with 90 nm fin pitch, i.e., the critical dimension was wider than the target, the fins sidewalls were isotropically attacked and the profile was sloped. In order to reach 17 nm of critical dimension on 90 nm pitch we had to implement a new hard mask opening step. The c-Si fin sidewall roughness and fin profile were tuned by improving the uniformity across the wafers, optimizing the softlanding etch time and introducing a new overetch step with notch capability.     

Convergent beam electron diffraction for strain determination at the nanoscale

The convergent beam electron diffraction (CBED) technique has been used to quantify the strain in semiconducting GaInAs/GaAs epitaxial layers. Two different systems have been explored, a 100 nm thick GaInAs layer, in the case of very low indium content, and a 10 nm thick GaInAs layer, with 20 at.% In. We show that the direct analysis of the CBED patterns can only be achieved in the first system, provided that specimen and deposited layer thickness are close to each other. In the second system, which is the more frequent in epitaxial systems, we demonstrate that reliable measurements can still be obtained through quantitative fit performed using a CBED pattern simulation software specially developed for analysing non-inhomogeneouly strained specimens.     

High efficient light-emitting diodes with antireflection subwavelength gratings

A two-dimensional subwavelength grating (SWG) has been fabricated on a GaAlAs light-emitting diode (LED). The SWG is patterned by electron beam lithography and etched by fast atom beam with Cl/sub 2/ and SF/sub 6/ gases. The fabricated grating has 200 nm period and the tapered grating shape with aspect ratio of 1.38 to prevent reflection in the spectral region including 850 nm light emission. The emission is increased by 21.6% at the normal emission angle. The total emittance is increased by 60% with the SWG in comparison with that of the flat surface.