Front-side Etching Research Articles

Laser-induced frontside etching of silicon using 1550-nm nanosecond laser pulses

High-quality, low-defect laser machining of dielectric materials remains challenging due to the high energy densities required for inducing the physical processes of laser ablation. Therefore, experiments were performed to demonstrate the laser-induced frontside etching (LIFE) of silicon by nanosecond laser pulses at a wavelength of 1550 nm, utilizing a thin chromium layer as the absorber. Etching was found at a fluence range from 200 to 700 mJ/cm², where the etched surfaces were smooth in the center (5 to 80 nm rms). The etching process was accompanied by incubation as well as saturation effects. At a fluence of 280 mJ/cm², an averaged etching rate of about 0.05 nm/pulse was determined for a high number of laser pulses (N > 1000). At the edges where the fluence was below the etching threshold of about 200 mJ/cm², chromium film melting and the formation of different kinds of laser-induced periodic surface structures (LIPSS) were observed.The LIFE was accompanied by different surface modifications. The optical absorption of silicon likely increased due to the incorporation of chromium into the silicon and the formed defects near the surface. Further, LIPSS were observed at fluences near the threshold of laser etching. In particular, the formation of regular linear ripples with a period of approximately 1.52 µm was observed. The morphological characteristics of the ripples suggest that the mechanism was related to surface scattering at surface features or small ripples and the hydrodynamic instabilities of the molten silicon. In addition, outside the etching track, periodic but nonlinear LIPSS were observed in the chromium film. These hexagonal arranged dot-like patterns had a period of around 440 nm. It is suggested that the hexagonal dot-like patterns resulted from the multi-pulse irradiation and the sequential stress relaxation in the chromium film below the melting point of chromium during spot movement.

Aluminum thin film enhanced IR nanosecond laser-induced frontside etching of transparent materials

Laser processing of glass is of significant commercial interest for microfabrication of precision optical engineering devices. In this work, a laser ablation enhancement mechanism for microstructuring of glass materials is presented. The method consists of depositing a thin film of aluminum on the front surface of the glass material to be etched. The laser beam modifies the glass material by being incident on this front-side. The influence of ablation fluence in the nanosecond regime, in combination with the deposition of the aluminum layer of various thicknesses, is investigated by determining the ablation threshold for different glass materials including soda-lime, borosilicate, fused silica and sapphire. Experiments are performed using single laser pulse per shot in an air environment. The best enhancement in terms of threshold fluence reduction is obtained for a 16nm thick aluminum layer where a reduction of two orders of magnitude in the ablation threshold fluence is observed for all the glass samples investigated in this work.

Structuring of glass fibre surfaces by laser-induced front side etching

The fabrication of sub-μm structures on glass fibre surfaces poses a big challenge for the laser processing. However, the laser-induced front side etching (LIFE) method has a great potential for the fast, nm-precision, and cost-effective production of surface structures. LIFE is a method for laser etching of transparent materials using thin absorber layers with a high absorption coefficient like metal layers. The LIFE process of the front surface of a fused silica wafer as well as of a glass fibre is studied in dependence on the laser parameters. A KrF excimer laser with a wavelength of 248nm and a pulse duration of 25ns was used. The resultant structures were analysed with microscopic methods (white light interferometry, scanning electron microscopy (SEM)). The analysing of the surface structures presented that the LIFE methods allow the fabrication of well-defined periodic sub-μm structures. Furthermore, the structuring process was simulated by a thermodynamic equation including an approach of the laser–plasma interaction. The theoretically predicted results presented a good agreement with the experimental results.

A Three-Axis Single-Proof-Mass CMOS-MEMS Piezoresistive Accelerometer with Frequency Outp

Accelerometers have been used in a wide range of applications such as automobiles, mobile phones, and game controllers. In this paper, we present a monolithic complementary metal-oxide-semiconductor micro-electromechanical systems (CMOSMEMS) accelerometer with frequency output. The output oscillation frequency can be converted to a digital code by using a counter so that the sensor can be easily integrated with digital signal processing units on the same chip. The accelerometer is composed of a single proof mass suspended by four suspension beams. Polysilicon piezoresistors are placed at the ends of the beams to sense the displacement of the proof mass. The piezoresistors are used in RC oscillators whose oscillation frequencies vary owing to the change in resistance upon application of an external acceleration. Acceleration components in three axes can be obtained by the proper combination of signals from the piezoresistors at different locations. The accelerometer was fabricated by standard CMOS processes followed by backside and frontside dry etching postprocessing. The measured mechanical resonant frequency is 464 Hz. The oscillation frequency of the z-axis oscillator is about 70 MHz. The measured absolute sensitivity, relative sensitivity, and resolution along the z-axis are 198 kHz/g, 2.8×10−3 ∆f/f0/g, and 10.9 mG/ Hz , respectively, for a sampling rate of 400 Hz and acceleration of 6 g at 27 Hz.

Finite element method based absolute pressure sensitivity optimized membrane type double cavity vacuum sealed piezoresistive sensor

Purpose – The purpose of this paper is to use finite element method for optimizing the membrane type double cavity vacuum sealed structure for the best achievable sensitivity in a piezoresistive absolute pressure sensor and its validation using a standard complementary metal oxide semiconductor (CMOS) process. Design/methodology/approach – A double cavity vacuum sealed piezoresistive absolute pressure sensor has been simulated and optimized for its performance and an analytical model describing the behaviour of the sensor has been described. The 1×1 mm sensor chip has two membrane type 100×30×1.7 μm diaphragms consisting of composite layers of plasma enhanced chemical vapour deposition (PECVD) of silicon nitride (Si3N4) and silicon dioxide (SiO2) each hanging over 21 μm deep rectangular cavity. Potassium hydroxide (KOH) based anisotropic etching of single crystal silicon using front side lateral etching technology is used for the fabrication of the sensor. The electrical readout circuitry uses 318 Ω boron diffused low pressure vapour chemical vapour deposition (LPCVD) of polysilicon resistors arranged in the Wheatstone half bridge configuration. The sensing structure is simulated and optimized using COMSOL Multiphysics. Findings – Front-side lateral etching technology has been successfully used for the fabrication of double cavity absolute pressure sensor. A good agreement with the fabricated device for the chosen location of the piezoresistors through simulation has been predicted. The measured pressure sensitivity of two tested pressure sensors is 12.63 and 12.46 mV/MPa, and simulated pressure sensitivity is found to be 12.9 mV/MPa for pressure range of 0 to 0.5 MPa. The location of the piezoresistor has also been optimized using the simulation tools for enhancing the sensor sensitivity to 62.14 mV/MPa. The pressure sensitivity is further enhanced to 92 mV/MPa by increasing the width of the diaphragm to 35 μm. Originality/value – The simulated and measured pressure sensitivities of the double cavity pressure sensor are in close agreement. Sevenfold enhancement in the pressure sensitivity of the optimized sensing structure has been observed. The proposed front-side lateral etching technology can be adopted for making membrane type diaphragms hanging over vacuum sealed micro-cavities for high sensitivity pressure sensing applications.

Laser-induced fabrication of randomly distributed nanostructures in fused silica surfaces

The nanostructuring of dielectrics is a big challenge for laser patterning methods. In this study a novel laser structuring method for the fabrication of randomly distributed nanostructures, called laser-induced front side etching using in situ pre-structured metal layers (IPSM-LIFE), is presented. The pulsed laser irradiation of a thin metal film deposited onto a dielectric substrate with fluences below the ablation threshold results in the formation of randomly distributed metal structures by self-assembly processes. Further pulsed laser irradiation of these metal structures with higher or equal laser fluences causes the formation of complex patterns at the surface of the dielectric due to localized ablation and melting processes of the dielectric surface induced by the absorption of the laser energy by the metal structures and the local energy transfer into the dielectric surface.

Laser-induced front side etching of CaF2 crystals with KrF excimer laser

The laser-induced front side etching (LIFE) of amorphous materials like fused silica was manifold studied and the LIFE process was sufficient optimized for the fabrication of well-defined etching trenches with a very low surface roughness. The LIFE process is an indirect laser-induced ablation process, the – for the used laser wavelength – transparent substrate was covered by a highly absorbing material and the absorbing process causes a transfer of the laser energy into the substrate and, finally, to an ablation process of the substrate surface.However, the structuring of crystalline materials like CaF2 is a great challenge for the LIFE process. The properties of CaF2(111) and CaF2(001) surfaces etched by KrF excimer laser pulses (pulse duration Δtp=25ns, wavelength λ=248nm) were analysed by white light interferometry (WLI) as well as scanning electron microscopy (SEM). The surface morphologies of laser etched CaF2 surfaces depend on the laser parameters and on the crystal orientation and are frequently characterized by microcracks and flake spallation. The most probable reasons therefore are laser-induced thermal stress or laser-induced shock waves.

Laser-induced front side etching of fused silica with femtosecond laser radiation using thin metal layers

Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation (λ=780nm, Δtp=150fs, f=1kHz). The etching process of fused silica is studied in dependence on the laser fluence, the laser pulse numbers as well as metal absorber layer thicknesses. Especially the influence of the varied parameters on the surface morphology, the etching depth, and the surface modification is presented and discussed. The etched trenches were analyzed with white light interferometry. A fluence window from 0.5 to 2.5J/cm2 for the LIFE process of fused silica with a single laser pulse using a metal layer with a thickness from 5 to 50nm was found. The measured maximum etching depth of approximately 100nm suggests that the LIFE process is appropriated for precision machining.The attempt to simulate the fs laser LIFE process by a thermodynamic model was moderate successfully and exhibits significant differences within the experimental results.

Laser-induced front side and back side etching of fused silica with KrF and XeF excimer lasers using metallic absorber layers: A comparison

Laser-induced front side (LIFE) and back side etching (LIBDE) are methods for nanometer-precision laser etching of transparent materials using thin absorber layers. The etching behaviour of fused silica at a laser wavelength of 248nm (KrF) and 351nm (XeF excimer laser) with a pulse duration of 25ns using a chromium absorber layer was analysed and compared for front and back side etching geometry. For both wavelengths as well as for both processes the etching depth d increases almost linearly in dependence on the laser fluence (it is: d≈δ*(Φ−Φth)). The etching depth at the same laser fluence is higher for 248nm compared to 351nm as well as for back side etching compared to the front side etching process (LIFE: δ(248nm)=20nm/(J/cm2), δ(351nm)=15nm/(J/cm2), LIBDE: δ(248nm)=38nm/(J/cm2), δ(351nm)=8nm/(J/cm2) with Φth,m from 0.3 to 2.65J/cm2). Furthermore, the measured depths were evaluated with the estimated etching depth calculated by a thermal model. The simple thermodynamic model allows a good qualitative description of the etching depth behaviour; however, the model does not allow the quantitative calculation of the etching depth.

Uncooled Thermoelectric Infrared Sensor With Advanced Micromachining

A simple mass producible uncooled thermoelectric infrared microsensor has been designed and fabricated. To improve the cost-efficiency, an advanced micromachining process, which combines wet anisotropic pre-etching and XeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dry isotropic post-etching, is adopted for the sensor fabrication. The wet anisotropic pre-etching removes bulk silicon from back-side and forms a thin silicon membrane for device fabrication, the XeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> dry isotropic post-etching undercuts silicon membrane and releases the microstructure. Experimental results show that the sensor with advanced micromachining exhibits a two times higher responsivity and detectivity than the sensor with only XeF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> front-side etching. In air at room temperature, the sensor with advanced micromachining has a responsivity of 71.57 V W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , noise equivalent power of 0.64 nW Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1/2</sup> , detectivity of 6.21×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> cm Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and a time constant of 13.2 ms. The effect of back-side etch window size on sensor performance is also characterized by finite-element method simulation.

Laser‐induced front side etching of fused silica with KrF excimer laser using thin chromium layers

AbstractLaser‐induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers. This approach is a straightforward advancement of the back side etching techniques. Within this study the etching of fused silica with thin chromium layers as radiation absorbers is presented using nanosecond KrF excimer laser with a wavelength of 248 nm. The laser fluence up to 10 J/cm2, the number of pulses up to 10 as well as the layer thickness from 2 to 250 nm is varied. The treated fused silica was analysed with microscopic (white light interferometry, WLI, scanning electron microscopy, SEM) and spectroscopic methods (energy‐dispersive X‐ray spectroscopy (EDX)).magnified image Schematic illustration of the LIFE process.The LIFE method allows the nm‐precision etching (rms ≤ 8 nm) of fused silica at moderate laser fluences (2–6 J/cm2).

Laser-Induced front Side Etching: An Easy and Fast Method for Sub-μm Structuring of Dielectrics

Abstract Laser-induced front side etching (LIFE) is a method for the nanometer-precision structuring of dielectrics, e.g. fused silica, using thin metallic as well as organic absorber layer attached to the laser-irradiated front side of the sample. As laser source an excimer laser with a wavelength of 248 nm and an pulse duration of 25 ns was used. For sub-μm patterning a phase mask illuminated by the top hat laser beam was projected by a Schwarzschild objective. The LIFE process allows the fabrication of well-defined and smooth surface structures with sub-μm lateral etching regions (Δx 350 nm) and vertical etching depths from 1 nm to sub-mm.

Laser-induced front side etching of fused silica with XeF excimer laser using thin metal layers

Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers, e.g., for precision engineering or even optical applications.Aluminium, chromium, molybdenum, silver as well as titanium with various layer thicknesses (5–100nm) were applied as absorber for etching trenches in fused silica with nanosecond XeF excimer laser radiation. The sample surfaces were processed at laser fluences up to 10J/cm2 and laser pulse numbers from 1 to 10 pulses.A linear growth of the etching depth at rising laser fluence was found. The film thickness dependency is more complex and mostly influenced by the optical properties of the thin metal films.The influence of the laser fluence, the number of pulses, the absorber material as well as the absorber layer thickness on the etching process, the etching depth, and the surface modification were presented and discussed.A simple model is given that allows the discussion of the etching depth in dependency on the laser fluence and the metal film thickness.The measurements represented a good agreement with the calculated results by a thermal model.The LIFE method allows nm-precision etching of fused silica with etching depths up to 150nm.

Organic vapor sensors based on functionalized macroporous Si using single and double-side electrochemical etching

Organic sensors for ethanol vapor detection were developed by means of electrochemical etching of crystalline silicon as initial substrate following two procedures: front side etching (FSE) and front- and backside (double) surface etching (DSE). The first consists on the attack of the crystalline silicon surface by hydrofluoric acid (HF) in presence of a constant current flow to form a layer of vertically oriented pattern of pores with diameters in the 2-5µm range and a thickness of about 50µm. The second procedure is similar to the first one but allowing a similar etching to the opposite side of the silicon wafer. The etching times for both surfaces were adjusted in such a way to produce a free standing porous layer of around 300µm in thickness. The samples produced by FSE and DSE were submitted to a functionalization methodology using 4-aminopyridine to cover the porous surface. The obtained specimens were mounted in a holder with two soldered contacts and placed in a testing chamber where different flows of ethanol vapors were injected. Conductance responses of both kinds of sensors were recorded and compared between them and with respect to nontreated crystalline silicon samples in terms of sensing capability, reproducibility, and response times. The results indicate that both FSE and DSE sensors have improved sensing properties compared to the nontreated surfaces.

Double-Step Plasma Etching for SiO&lt;sub&gt;2&lt;/sub&gt; Microcantilever Release

In this paper, an isotropic dry plasma etching was used to release the suspended SiO2 microcantilever from the substrate of SOI wafer. Employing the plasma dry etching technique, the frontside etching for the SiO2 microcantilever release is done using the Oxford Plasmalab System 100. To obtain the optimum condition for the microcantilever release using the plasma etcher, the etching parameters involved are 100 sccm of SF6 flow, 2000 W of capacitively coupled plasma (CCP) power, 3 W of inductively coupled plasma (ICP) power, 20°C of etching temperature and 30 mTorr chamber pressure. The optimum parameters yield lateral etch rate of about 5 μm/min and vertical etch rate of about 8 μm/min. Two etching methods have been considered in this study. The first method employs only the isotropic etching to realize the microcantilever release while the second method utilizes both the anisotropic etching and the isotropic etching. For the second method, the process starts with the anisotropic etching from the deep reactive ion etching (DRIE) system which is then followed by the isotropic etching to complete the microcantilever releasing process. The purpose of the anisotropic etching is to create an etching window for the subsequent isotropic etching process. By using double-step etching method which combines both isotropic and anisotropic plasma etching for the microcantilever release process, the releasing process of suspended microcantilever is significantly improved.

Chirality assignment to carbon nanotubes integrated in MEMS by tilted-view transmission electron microscopy

Front side etching combined with sample tilting – instead of wafer through etching – allows for transmission electron microscopy (TEM) investigations on nanostructures integrated in microelectromechanical systems (MEMS). We present electron diffraction of an individual single-walled carbon nanotube (SWNT) suspended between sharp polycrystalline silicon tips as far as 165 μm away from the MEMS chip edge. This novel approach for transmission-beam characterization avoids complex wafer backside processing and facilitates alignment of the SWNTs to the focal plane using tips defined directly by photolithographic means. The demonstration of chirality assignment to the integrated SWNT paves the way for correlating experimentally measured response of the SWNT sensing element upon stimuli with the response predicted by theory.

Tilted-view transmission electron microscopy-access for chirality assignment to carbon nanotubes integrated in MEMS

Front side etching in combination with sample tilting – instead of wafer through etching – allows for transmission electron microscopy (TEM) investigations on nanostructures integrated in microelectromechanical systems (MEMS). We present electron diffraction (ED) of an individual single-walled carbon nanotube (SWNT) suspended between sharp polysilicon tips on the bulk of a MEMS chip. This novel approach for transmission beam characterization avoids complex wafer backside processing while allowing for chirality assignment to the integrated functional SWNT, as demonstrated here.

Fabrication of thin-film thermopile micro-bridge with XeF 2 etching process

These days, MEMS-based thin-film thermopiles are mainly fabricated anisotropically by wet-etching process at the back of the wafer. Their backside etching process is, however, complex, expensive, and wastes a large amount of silicon real estate. On the other hand, the front-side etching has better reliability of photolithography and also makes it possible to fabricate smaller micro-thermopile, as compared to that obtained from backside etching. In this paper, a thin-film thermopile is fabricated on a micro-bridge structure created by using the front-side etching with XeF 2 gas. The resulting device is about 50% smaller in size than that of the conventional chip. The output voltage of the device is found to increase by 2.13 times and the Seebeck coefficient to enhance by 0.17 μV/°C, due to less heat-flow from hot junction to cold junction and the increase in aluminum etching hole area.

A New High-Filling-Factor CMOS-Compatible Thermopile

To reach a high fill factor, a new CMOS-compatible thermopile was designed and fabricated. The floating membrane of the thermopile that we designed was formed by a T-shape anisotropic etching window with a minimum etching area. The design and fabrication of thermopile sensors are realized by using 1.2-mum CMOS IC technology combined with a subsequent anisotropic front-side etching. The proposed T-shape etching windows are designed at four quadrants of a membrane to form the extended undercut etching area of opened windows of overlap. The floating membrane has a larger area of 1100 times 1100 mum2 and is 2 mum thick. The area of the proposed membrane is increased by about 21.5%, which absorbs more infrared radiation than the conventional design and enhances responsivity very well, as shown in the measurement. A surface morphology measurement of the thermopile is implemented to evaluate the influence of residual stress and practically characterize the geometric shape of the membrane. More careful analysis of the surface morphology shows that the bending of suspension parts has a deviation of responsivity of less than 0.167%. In this paper, the T-shape structure of the thermopile with large absorption area and high performance by using a CMOS-compatible process is proven to be very successful and is easily fabricated.

Micro Fabrication Design of a Planar Methanol Sensor

This study explains a design of the microfabricated planar methanol sensor and conducts a series of methods to achieve a real device. By utilizing the microfabrication technology, it is possible to develop the miniature planar methanol sensor to integrate with direct methanol fuel cells (DMFC). The electrochemically reactive area can be adjusted effectively to obtain adequate strength of the methanol oxidation current. The innovation of the methanol sensor design is on a matrix detecting area with the in-line monitoring functions. Each detecting holes in matrix has been connected together by a serpentine channel to conduct electrochemical reaction at the surface of electrodes. In front side of wafer, the interdigitate electrode design provides a flexible adjustment in the reactive area for modulating the strength of methanol oxidation current. A compatible fabrication of methanol sensor and DMFC has also been proposed in this work. The serpentine channel and detecting holes of methanol sensor are anticipated to be made in opposite side of DMFC fuel channels. Also, the through holes have to be formed by the combination of front-side and backside Deep RIE etching. Both of them require a precise double-side alignment. At the end, a simple planar methanol sensor has been made for verifying electrochemical characteristics and the integration solution with micro DMFC has been discussed to benefit the micro DMFC system development.