Tetramethylammonium Hydroxide Etching Research Articles

Hafnium oxide: A thin film dielectric with controllable etch resistance for semiconductor device fabrication

Thin film dielectrics are ubiquitous in the manufacture of electronic devices and are frequently deposited and etched away at various stages of device fabrication. We demonstrate that hafnium oxide (HfO2) thin films grown via atomic layer deposition on silicon and silicon pre-coated with aluminum oxide (Al2O3) have etch resistance properties, which can be tuned simply by changing the post-deposition annealing temperature. The etching rates of films in hydrofluoric acid (HF) solutions were found to be dependent on annealing temperature, with the etch rate decreasing with increasing temperature. A transition region in the etch rate was identified between 300 and 350 °C, corresponding to the crystallization of the HfO2 films, as identified via x-ray diffraction. HfO2 films deposited directly onto silicon annealed above 350 °C were resistant to 10% HF solutions over the course of several hours. In the case of Si/Al2O3/HfO2 stacks, closer inspection reveals the existence of channels, which reduces the etch resistance of HF acid, as evidenced by tetramethylammonium hydroxide etching of the silicon substrate. Crystallized HfO2 can be used to protect other dielectrics in device processing, and we demonstrate its use in single-sided fabrication of patterned structures of Al2O3, which can control the effective charge-carrier lifetime in silicon wafers for use in modulating THz and mm-wave radiation.

InGaN Laser Diodes with Etched Facets for Photonic Integrated Circuit Applications

The main objective of this work is to demonstrate and validate the feasibility of fabricating (Al, In) GaN laser diodes with etched facets. The facets are fabricated using a two-step dry and wet etching process: inductively coupled plasma-reactive ion etching in chlorine, followed by wet etching in tetramethylammonium hydroxide (TMAH). For the dry etching stage, an optimized procedure was used. For the wet etching step, the TMAH temperature was set to a constant value of 80 °C, and the only variable parameter was time. The time was divided into individual steps, each of 20 min. To validate the results, electro-optical parameters were measured after each step and compared with a cleaved reference, as well as with scanning electron microscope imaging of the front surface. It was determined that the optimal wet etching time was 40 min. For this time, the laser tested achieved a fully comparable threshold current (within 10%) with the cleaved reference. The described technology is an important step for the future manufacturing of photonic integrated circuits with laser diodes integrated on a chip and for ultra-short-cavity lasers.

Study on the kinetics of standalone Si micro-pyramid formation using tetra methyl ammonium hydroxide as etchant

In this work, we consider eight silicon wet etching conditions using 20% or 25% Tetramethyl Ammonium Hydroxide (TMAH), each at 60°C,70°C,80°C or 90°C in order to study the TMAH kinetics and planar dependence on formation of upright pyramids by anisotropic etching on a (100) Si substrate. With the aid of negative square mask patterning, we are able to fabricate upright micro-pyramids having well defined facets with a smooth morphology. A new methodology termed as the ‘Variable Time Index Method’ is employed to understand the etch rate variation as a function of time and identify the etch rates along the (111) family of planes and consequently the planar dependence due to TMAH etch conditions. For each of the etching conditions, analysis using Scanning Electron Microscopy (SEM) is conducted to investigate the etched pyramid morphology and suitably identify the conditions under which deviation from the desired plane direction and surface are obtained. Ideal micro-pyramid morphology, vertex alignment and orientation are obtained for etch conditions of 20% and 25% TMAH, at 80°C.

Interplay of sidewall damage and light extraction efficiency of micro-LEDs.

This Letter describes the impact of shape on micro light-emitting diodes (µLEDs), analyzing 400 µm2 area µLEDs with various mesa shapes (circular, square, and stripes). Appropriate external quantum efficiency (EQE) can yield internal quantum efficiency (IQE) which decreases with increasing peripheral length of the mesas. However, light extraction efficiency (ηe) increased with increasing mesa periphery. We introduce analysis of Jpeak (the current at peak EQE) since it is proportional to the non-radiative recombination. Etching the sidewalls using tetramethylammonium hydroxide (TMAH) increased the peak EQE and decreased the sidewall dependency of Jpeak. Quantitatively, the TMAH etching reduced non-radiative surface recombination by a factor of four. Hence, shrinking µLEDs needs an understanding of the relationship between non-radiative recombination and ηe, where analyzing Jpeak can offer new insights.

Optically pumped deep-UV multimode lasing in AlGaN double heterostructure grown by molecular beam homoepitaxy

Multimode lasing at sub-300 nm wavelengths is demonstrated by optical pumping in AlGaN heterostructures grown on single-crystal AlN substrates by plasma-assisted molecular beam epitaxy. Edge-emitting ridge-based Fabry–Pérot cavities are fabricated with the epitaxial AlN/AlGaN double heterostructure by a combined inductively coupled plasma reactive ion etch and tetramethylammonium hydroxide etch. The emitters exhibit peak gain at 284 nm and modal linewidths on the order of 0.1 nm at room temperature. The applied growth technique and its chemical and heterostructural design characteristics offer certain unique capabilities toward further development of electrically injected AlGaN laser diodes.

Determination of Defect Concentrations in ²⁸Si Crystals Using EPR for the Realization of the Kilogram

To realize the kilogram using the X-ray crystal density method, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si crystals grown by the floating zone method were employed. Samples cut from two <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si crystals, the AVO28 crystal (Si28-10Pr11 crystal) and the Si28-23Pr11 crystal, were analyzed by means of an electron paramagnetic resonance (EPR) spectroscopy. The samples were prepared by mirror polishing and subsequent etching using tetramethylammonium hydroxide (TMAH) solution. TMAH etching eliminates signals from the mechanically damaged surface layers of amorphous silicon and allows for the observation of small signals from vacancy defects in bulk crystals. In the study, the signal level of the vacancy defects was below the detection limit of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1 \times 10^{12}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-3}$ </tex-math></inline-formula> , and the amount of vacancy correction required to realize the kilogram was estimated to be 0.00(23) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{g}$ </tex-math></inline-formula> for both <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si crystals. Therefore, it was concluded that the 39(21) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{g}$ </tex-math></inline-formula> inconsistency between the kilogram realizations achieved using the two <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">28</sup> Si crystals reported thus far cannot be explained by the existence of vacancy defects that are EPR active in the dark or under illumination.

Effects of crystal planes on topography evolution of silicon surface during nanoscratch-induced selective etching

Diamond machining-induced structural defects can usually degrade the optical and electrical performance of silicon (Si)-based devices. Investigating topography evolution of machined Si surfaces during wet etching is of significance for detecting and eliminating these defects and thereby improving device performance. In this paper, effects of Si crystal planes on surface topography evolution during nanoscratch-induced selective etching in the commonly used etchants, i.e., potassium hydroxide (KOH), tetra-methyl-ammonium hydroxide (TMAH), and a mixture of hydrofluoric acid (HF) and nitric acid (HNO3), were investigated using a single-asperity diamond tip. It is noted that nanoscratch-induced subsurface deformation of Si(100) and Si(110) can resist KOH and TMAH solutions etching, and thereby in situ form protrusive hillocks, which can be ascribed to the amorphous Si from the scratching. Higher etching rate of Si(110) planes appears to be responsible for the formation of higher protrusive hillocks. Nevertheless, nanoscratch-induced selective etching on Si(111) surface in TMAH solution is quite distinct from that in KOH solution. The difference in activated atomic steps on Si(111) surface may cause scratched regions to promote KOH etching but resist TMAH etching. It is also found that in case of HF/HNO3 mixtures etching, nanoscratch-induced subsurface deformation on Si surface can promote the etching and thereby form sunken grooves, regardless of crystal plane orientations. Further analysis indicates that Si wafers with lower hardness and elastic modulus are beneficial for forming more serious subsurface damages during the scratching, yielding deeper sunken grooves through subsequent selective etching. These findings provide a useful reference for defect detection on Si surface and developing the nanofabrication based on scanning probe microscope.

Process optimization of the Si0.7Ge0.3 Fin Formation for the STI first scheme

The process optimization of the Si0.7Ge0.3 Fin formation for the shallow trench isolation first strategy is systematically demonstrated in this paper. The optimized Si Fin etching and recess process are firstly developed to attain an improvement of side wall roughness and profile for the Si0.7Ge0.3 selective epitaxial growth. Moreover, a new developed photoresist etching back process, inserted between Tetramethylammonium hydroxide etching and Si0.7Ge0.3 selective epitaxial growth, is employed to eliminate the surface defects completely. Furthermore, an optimized pre-baking temperature of Si0.7Ge0.3 selective epitaxial growth results in a more rounding V-shaped Si recess profile compared with its V-shaped profile produced by the Tetramethylammonium hydroxide etching. The incoming profile optimization is critical to achieve the structural defects free for the Si0.7Ge0.3 layer selective epitaxial growth. Finally, a high crystalline quality of the Si0.7Ge0.3 Fin formation inside narrow trench is realized by utilizing the SiGe chemical mechanical polishing and Shallow Trench Isolation recess process. This work presents a good technique for high-mobility pMOSFETs in future technology node.

Multi-layered [Au/Si3N4/SiO2] thermal actuator realization using metal passivated TMAH micro-machining

This paper presents the design and fabrication of thermally actuated multi-layered microactuator for out of plane actuation application. Multi-layered actuators are fabricated using ammonium persulfate based TMAH (Tetra-Methyl-Ammonium-Hydroxide) etching with metal passivation. Multi-layered [Au/Si3N4/SiO2] microactuators with straight and cross-heaters are characterized using the I–V measurement tool. Temperature across actuator was similar to simulated results. Gold as an upper layer of a microactuator fully protected during the release process. Fundamental mode or resonant frequency of fabricated multi-layered microactuator measured by laser Doppler vibrometer (LDV) which observed around 20 KHz.

Revealing the Role of Sidewall Orientation in Wet Chemical Etching of GaN-Based Ultraviolet Light-Emitting Diodes.

We demonstrated that the tetramethylammonium hydroxide (TMAH) solution possesses different etching abilities to the chip sidewalls with different orientations because the orientation of chip sidewall determines the exposed crystallographic plane of gallium nitride (GaN) and these crystallographic planes are with different chemical stability to the TMAH solution. After TMAH etching treatment, trigonal prisms were observed on sidewalls where m-plane GaN was exposed. For the investigated two types of light-emitting diodes (LEDs) with orthogonal arrangements, the LEDs with their larger sidewalls orientated along the [11–20] direction exhibited an additional 10% improvement in light output power after TMAH etching treatment compared to the LEDs with larger sidewalls orientated along the [1–100] direction.

Enhanced Light Extraction of Flip-Chip Mini-LEDs with Prism-Structured Sidewall.

Current solutions for improving the light extraction efficiency of flip-chip light-emitting diodes (LEDs) mainly focus on relieving the total internal reflection at sapphire/air interface, but such methods hardly affect the epilayer mode photons. We demonstrated that the prism-structured sidewall based on tetramethylammonium hydroxide (TMAH) etching is a cost-effective solution for promoting light extraction efficiency of flip-chip mini-LEDs. The anisotropic TMAH etching created hierarchical prism structure on sidewall of mini-LEDs for coupling out photons into air without deteriorating the electrical property. Prism-structured sidewall effectively improved light output power of mini-LEDs by 10.3%, owing to the scattering out of waveguided light trapped in the gallium nitride (GaN) epilayer.

Comparison of KOH and TMAH Etching on Sinw Arrays Fabricated via AFM Lithography

Silicon nanowire (SiNW) arrays were fabricated by using top-down fabrication approach via atomic force microcopy (AFM) lithography on silicon-on-insulator (SOI) (100) substrate. Local anodic oxidation (LAO) technique carried out by AFM lithography to draw oxide pattern on top of the SOI substrate. The next steps to fabricate silicon nanowires involve chemical etching of the unmasked silicon by wet etching with the addition of organic additive. In this research, isopropyl alcohol (IPA) was used to improve the surface smoothness of silicon nanowires. Two types of etchant, potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) with and without admixture of IPA at different etching time (10s, 20s, and 30s) were studied. After etching, the local oxide pattern was removed using dilute hydrofluoric acid (HF). The relationships between etching time with wall angle and surface roughness of the produced SiNWs were characterized in detail by using AFM and SEM. The result revealed that the wall angle for SiNW arrays that etched using TMAH was higher compare to KOH. Increasing the etching time produced large wall angle (θ) for both etchants. It was also found that the surface roughness for SiNW arrays was decreased with increasing time. SiNW arrays etched using KOH have higher surface roughness compared to TMAH. The obtained results also show that the etching depths increased with increasing etching time.

(Invited) Electrochemical Formation and Application of Porous Gallium Nitride

Gallium nitride (GaN), which is a III-V compound semiconductor, has wide direct bandgap (3.4 eV), high breakdown electric field (3.3x106 V/cm) and high saturation electron velocity (2.7 x107 cm/sec). On the basis of excellent optical and electrical properties, significant progress has been achieved in the GaN-based devices, such as ultraviolet laser diodes, white light-emitting diodes, high electron mobility transistors (HEMTs), and high-power FETs. In addition, GaN and its alloys are getting much attention as building block materials for electrochemical (EC) energy conversion systems such as chemical sensors, water splitting, and artificial photosynthesis due to their superior chemical stability, direct transition, and widely tunable bandgap by alloying. Among the various techniques for improving their conversion efficiency, porosification utilizing EC reactions is one of the most powerful because a high-density array of pores exhibits high specific surface area, low reflectance, and high absorptance properties. In addition, this technique is performed at room temperature and does not require any complicated process such as lithography, indicating lower damage and higher productivity than other nanostructure fabrication techniques such as reactive ion etching and selective-area growth. In this presentation, we review our recent work on the fabrication of porous GaN utilizing various EC techniques and their applications [1-3]. One example is the two-step process combining EC etching process with conventional chemical etching process. EC etching was first performed in the dark condition using an aqueous electrolyte consisting of H2SO4 and H3PO4. Wet chemical etching was subsequently conducted in 25 % tetramethylammonium hydroxide (TMAH) at 90°C. The porous structure formed by the first EC etching has straight pores oriented perpendicular to the top surface. Pore depth linearly increased with EC etching time, indicating that superior depth control was achieved. This behavior can be explained as resulting from anisotropy of the EC etching. Namely, the etching reaction proceeded only to the direction belong to the high-electric field induced by the anodic voltage. On the other hand, the pore diameter kept a constant value throughout the EC etching and could not be controlled by the EC etching time. However, the pore diameter could be changed by wet chemical etching utilizing TMAH as subsequent treatment after the EC etching. By applying TMAH etching, pore shape changed from a round to a hexagonal shape surrounded with 6 facets of {1−100}. In addition, the pore diameter estimated from SEM observation increased linearly with the TMAH etching time, whereas the pore depth was unchanged. These results indicate that TMAH etching proceeds anisotropically in the horizontal direction, but not in the vertical direction of the substrate. The optical and photoelectrochemical properties of porous GaN were very sensitive to the structural properties. From the photoluminescence (PL) measurements, the intensity of yellow luminescence (YL) related to high-density point defects was drastically decreased by the formation of porous structures, suggesting that gallium vacancy (V Ga) related defects were preferentially removed by EC etching. Photoreflectance measurement revealed that porous sample had an effective refractive index that could be controlled by TMAH etching time. In photoelectrochemical measurement, the incident-photon-to-current conversion efficiency (IPCE) was dramatically enhanced by the formation of porous structures. A series of experimental results were consistently explained by the change of thickness of pore wall with a width of space charge region. These results suggest that precise structural controlling is crucially important to obtain superior capability for EC energy conversion systems, and the two-step process utilizing anisotropic EC etching and TMAH etching is very promising as a nanostructure fabrication technique. Acknowledgement: This work was supported by JSPS KAKENHI - JP15K13937，JP16H06421，JP17H03224. A. Watanabe et al., ECS Electrochem. Lett., 4, H11 (2015).Y. Kumazaki et al., J. Electrochem. Soc., 164, H477 (2017).M. Toguchi et al., 233rd ECS Meeting, Z01-2550, Seattle, WA, 15 May (2018).

Heterojunction Hybrid Solar Cells by Formation of Conformal Contacts between PEDOT:PSS and Periodic Silicon Nanopyramid Arrays.

Surface nanotexturing with excellent light-trapping property is expected to significantly increase the conversion efficiency of solar cells. However, limited by the serious surface recombination arising from the greatly enlarged surface area, the silicon (Si) nanotexturing-based solar cells cannot yet achieve satisfactory high efficiency, which is more prominent in organic/Si hybrid solar cells (HSCs) where a uniform polymer layer can rarely be conformably coated on nanotextured substrate. Here, the HSCs featuring advanced surface texture of periodic upright nanopyramid (UNP) arrays and hole-conductive conjugated polymers, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), are investigated. The tetramethylammonium hydroxide etching is used to smooth the surface morphologies of the Si-UNPs, leading to reduced surface defect states. The uniform Si-UNPs together with silane chemical-incorporated PEDOT:PSS solution enable the simultaneous realization of excellent broadband light absorption as well as enhanced electrical contact between the textured Si and the conducting polymer. The resulting PEDOT:PSS/Si HSCs textured with UNP arrays show a promising power conversion efficiency of 13.8%, significantly higher than 12.1% of the cells based on the-state-of-the-art surface texture with random pyramids. These results provide a viable route toward shape-controlled nanotexturing-based high-performance organic/Si HSCs.

Normally-off AlGaN/GaN-based MOS-HEMT with self-terminating TMAH wet recess etching

Normally-off AlGaN/GaN-based MOS-HEMT has been fabricated by utilizing damage-free self-terminating tetramethyl ammonium hydroxide (TMAH) recess etching. The device exhibited a threshold voltage of +2.0 V with good uniformity, extremely small hysteresis of ∼20 mV, and maximum drain current of 210 mA/mm. The device also exhibited excellent off-state performances, such as breakdown voltage of ∼800 V with off-state leakage current as low as ∼10−12 A and high on/off current ratio (Ion/Ioff) of 1010. These excellent device performances are believed to be due to the high quality recessed surface, provided by the simple self-terminating TMAH etching.

Fabrication and testing of silicon-based microchannel heat exchanger inspired by leaf venation pattern for electronic cooling

Microchannel heat sinks (MCHS) provide an efficient means to dissipate large heat flux. The task of fabricating MCHS becomes challenging due to availability of large number of newer materials and processes. In this work, tetra methyl ammonium hydroxide (TMAH) etching and LASER machining methods for fabricating the microchannels are compared with respect to dimensional accuracy, the profile of channel, surface roughness, aspect ratio, material compatibility, cost and thermal performance. TMAH etching results in trapezoidal cavity instead of the rectangular cavity, finest surface finish (8 nm), and is suited for batch fabrication. While, LASER machining results in small taper profile (10o to vertical surface), rough surface (2.83 μm), and better dimensional accuracy and is suited for job production. Experimental results show that both MCHS exhibit similar thermal performance at lower flow rates. At higher flow rates, however, LASER fabricated MCHS have 25% higher thermal performance than TMAH etched channels.

Precise Structural Control of GaN Porous Nanostructures Utilizing Anisotropic Electrochemical and Chemical Etching for the Optical and Photoelectrochemical Applications

A low-damaged wet process utilizing electrochemical (EC) etching and subsequent chemical etching has been developed for the fabrication of GaN porous structures. Superior controllability in depth and diameter could be obtained by achieving anisotropic nature of the vertical direction to the substrate by EC etching and horizontal direction by tetramethylammonium hydroxide (TMAH) etching, respectively. The optical and photoelectrochemical properties of GaN porous structures were very sensitive to the structural properties. Photoreflectance measurement revealed that porous sample had an effective refractive index that could be controlled by TMAH etching time. In photoelectrochemical measurement, the incident-photon-to-current conversion efficiency (IPCE) was dramatically enhanced to as high as 91% by the formation of porous structures. A series of experimental results were consistently explained by the change of thickness of pore wall and width of space charge region.

Effects of etching conditions on surface morphology of periodic inverted trapezoidal patterned Si(100) substrate

In this paper, the anisotropic etching process of Si(100) wafers in tetramethyl ammonium hydroxide (TMAH) solution with isopropyl alcohol (IPA) is investigated in detail. An inverted trapezoidal pattern is developed. A series of experiments are performed by changing TMAH concentration, IPA concentration, etching temperature and etching time. The structure of inverted trapezoidal patterns and roughness of the bottom surface are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that with TMAH concentration increases, the roughness of bottom surface will decrease. The addition of IPA into TMAH solution improves the morphology of the bottom surface significantly. Low temperature is beneficial to get a smooth bottom surface. Furthermore, etching time can change the bottom surface roughness. A model is proposed to explain the etching processes. The hillock area ratio of the bottom surface has the same tendency as the etching area ratio. Finally, smooth silicon inverted trapezoidal patterns are obtained for epitaxial growth of GaN-based light emitting diode (LED) devices.

Precise Structural Tuning of Porous GaN Using Two-Step Anisotropic Etching for Optical and Photo-Electric Applications

Gallium nitride (GaN) and its alloys are attractive materials for various functional devices, such as light-emitting diodes, laser diodes, solar cells, and so on. One promising approach for improving device performance is forming the nanostructures with unique electrical and optical properties. GaN nanostructures have been commonly formed using selective-area growth or dry etching processes, however, there are severe limitations on increasing the density of nanostructures. And when a dry etching process is involved, the etching damage induced by ion bombardment is not negligible and could significantly degrade the device performance. One alternative approach is an anodic formation of porous structures utilizing low-energy electrochemical (EC) reactions. This process has a big advantage for high-density formation over the conventional lithography process. However, the control of structural dimension such as a pore diameter and depth has not been achieved adequately for the device applications. In this study, we aimed to improve structural controllability of GaN porous structures by combining EC process with conventional chemical etching process. We have developed the two-step etching process for a GaN (0001) substrate. EC etching was first performed in the dark condition using a standard electrochemical cell with three electrodes. Wet chemical etching was subsequently conducted in 25 % tetramethylammonium hydroxide (TMAH) at 90°C. In order to clarify the structural controllability, the observation using scanning electron microscopy (SEM) was conducted on the porous samples formed by changing the applied EC voltage, EC etching time and TMAH etching time. The optical properties were evaluated by photo-reflectance measurement and photo-electrochemical measurement. Figures 1(a) and (b) show the top and cross-sectional SEM images of the GaN porous structure formed by the first EC etching. Straight pores oriented perpendicular to the top surface was observed. Pore depth linearly increased with EC etching time, indicating that superior depth control was achieved. This behavior can be explained as resulting from anisotropy of the EC etching. Namely, the etching reaction proceeded only to [000-1] direction belong to the high-electric field induced by the anodic voltage. On the other hand, the pore diameter kept a constant value throughout the EC etching and could not be controlled by the EC etching time. Figures 1(c) and (d) show the top and cross-sectional SEM images of the GaN porous structure formed by the two-step etching process with a TMAH treatment. The pore shape appeared at the surface drastically changed from circular shape to hexagonal shape. In addition, the pore diameter estimated from SEM observation increased linearly with the TMAH etching time, whereas the pore depth was unchanged. This result denotes that the etching rate for a (1-100) plane is lower than that for other crystal planes. From these results, we can conclude that the pore depth and diameter were independently controlled by the EC etching time and the TMAH etching time, respectively. We investigated the correlation between the structural properties and the optical properties of the porous structures formed by the two-step etching process. The photo-reflectance of GaN surface decreased after the formation of porous structures. In addition, the reflectance spectra showed oscillation behavior as function of light wavelength, where the oscillation periods were changed with TMAH etching time. The oscillation of photo-reflectance can be explained by the interference of two kinds of light that was reflected at air/porous GaN interface and porous/bulk GaN interface. The results obtained here indicate that the refractive index was varied with the structural features such as a pore diameter that were changed by the TMAH etching time. The photo-electrochemical measurements were conducted on two kinds of porous samples formed by the first EC etching and the two-step etching processes. Figure 2 compared the photocurrents of various samples obtained under the UV light with a wavelength of 350 nm and an intensity of 0.1 mW/cm2. The photocurrents of both porous samples increased as compared with those of planar sample, and especially the large increase by 180% was obtained by the two-step etching. From the systematic measurements, it was found that the increase and decrease of photocurrents were sensitively affected by the change of the pore diameter. These strongly suggest that the efficiency of the photo-electric conversion was determined by such factors as the intensity of the electric filed induced at the GaN/electrolyte interface and the width of the depletion layer formed at the pore wall. In conclusion, the two-step etching process is very powerful technique to control the optical properties of GaN porous structures on the basis of the precise structural tuning. Figure 1

SIMULATION STUDY OF CONVEX CORNER UNDERCUTTING IN KOH AND TMAH FOR A MEMS PIEZORESISTIVE ACCELEROMETER

Undercutting is a common problem in wet anisotropic etching. This problem in turn, influences the performance and sensitivity of MEMS devices. This paper investigates the use of corner compensation to prevent convex corner undercutting in a MEMS piezoresistive accelerometer. The Intellisuite CAD simulation software was used for designing the mask with corner compensation and for analysing wet anisotropic etching profiles in potassium hydroxide (KOH) and tetra-methyl-ammonium-hydroxide (TMAH) solutions at different concentrations and temperatures. Perfect 90 degrees corners on the proof mass was successfully etched using a corner compensation design at etching temperature of 63 °C for KOH and 67.7 °C for TMAH with 25 wt% and 10.3 wt% concentration levels, respectively. Etching in TMAH required lower concentration level, thus making the etching process safer. However, TMAH required longer time to etch perfect convex corners compared to KOH. Nevertheless, both KOH and TMAH etchants have been successfully used to etch perfect convex corners by using the designed corner compensation mask.