Micromachining Method Research Articles

A Review on Nanocomposites. Part 2: Micromachining

AbstractMicromachining of nanocomposites is deemed to be a complicated process due to the anisotropic, heterogeneous structure and advanced mechanical properties of these materials associated with the size effects in micromachining. It leads to poorer machinability in terms of high cutting force, low surface quality, and high rate of tool wear. A comprehensive review on mechanical properties of nanocomposites aiming to pointout their effects on micro-machinability has been addressed in part 1. In part 2, the subsequent micro-machining processes are critically discussed based on relevant studies from both experimental and modeling approaches. The main findings and limitations of these micro-machining methods in processing nanocomposites have been highlighted together with future prospects.

Study of improvement in silicon bulk micromachining by metal assisted chemical etching

Ensuring the integrity and uniformity of the fabricated structure is critical for silicon bulk micromachining. Our research demonstrates an efficient and low-cost method for silicon bulk micromachining by metalassisted chemical etching (MACE). The integrity and uniformity of the etched structures are greatly improved by the utilization of ethanol as the solvent instead of water. Our research helps to understand the mechanism of MACE, and can provide new ideas for the further study of morphology control of silicon block micromachining using MACE.

Diamond optical vortex generator processed by ultraviolet femtosecond laser.

We propose a precise diamond micromachining method based on ultraviolet femtosecond laser direct writing and a mixed acid heating chemical treatment. The chemical composition of the attached clusters generated during laser ablation and their effects on morphologies were investigated in experiments. The averaged roughness of pristine and processed regions reduced to 0.64 nm and 9.4 nm from 20.5 nm and 37.4 nm, respectively. With this method, spiral zone plates (SZPs) were inscribed on a high-pressure high-temperature diamond surface as micro-optical vortex generators. The optical performances of the diamond SZPs were characterized in both experiments and simulations, which were very consistent with each other. This chemical auxiliary processing method will contribute greatly to the wide application of integration and miniaturization of diamond surface optical components.

High speed and low roughness micromachining of silicon carbide by plasma etching aided femtosecond laser processing

Silicon carbide has a high hardness and corrosion resistance. Accordingly, high rate machining of this material is a big challenge to obtain good surface quality. In this work, plasma etching aided femtosecond laser micromachining was proposed to address this issue. A series of square and circular blind holes for forming SiC pressure sensor diaphragms were obtained by using a 50fs, 800 nm, 1000Hz Ti:Sapphire laser. Using the control variable method, the effects of laser machining power, scan line spacing and scanning rate on surface topography of the structure were investigated. As demonstrated by experimental results, surface roughness of the structure increases with the laser machining power within a certain range. While the scanning rate has negligible effects on surface quality of the structure. The maximum surface roughness after femtosecond laser micromachining is 2.867 μm measured by laser scanning confocal microscope, and this value is reduced to 2.143 μm after inductively coupled plasma etching. In addition, a Raman spectroscopy shows that this composite micromachining method can eliminate accumulation of amorphous carbon on SiC surface caused by the femtosecond laser processing. The reduction of surface roughness and removal of amorphous carbon can effectively decrease probability of crack failure of the sensor diaphragm and increase its deformation capability, thereby ensuring sensitivity of the sensor. The composite micromachining method fully combines the advantages of high efficiency of femtosecond laser machining and high surface quality of plasma etching, making it has high potential to be applied in precision machining of silicon carbide and fabrication of SiC sensors.

A Simple and Robust Fabrication Process for SU-8 In-Plane MEMS Structures.

In this paper, a simple fabrication process for SU-8 in-plane micro electro-mechanical systems (MEMS) structures, called “border-bulk micromachining”, is introduced. It aims to enhance the potential of SU-8 MEMS structures for applications such as low-cost/disposable microsystems and wearable MEMS. The fabrication process is robust and uses only four processing steps to fabricate SU-8 in-plane MEMS structures, simplifying the fabrication flow in comparison with other reported attempts. The whole fabrication process has been implemented on copper-polyimide composites. A new processing method enables the direct, laser-based micromachining of polyimide in a practical way, bringing in extra processing safety and simplicity. After forming the polymeric in-plane MEMS structures through SU-8 lithography, a copper wet etching masked by the SU-8 structure layers is carried out. After the wet etching, fabricated in-plane MEMS structures are suspended within an open window on the substrate, similar to the final status of in-plane MEMS devices made from industrial silicon micromachining methods (such as SOIMUMPS). The last step of the fabrication flow is a magnetron sputtering of aluminum. The border-bulk micromachining process has been experimentally evaluated through the fabrication and the characterization of simple in-plane electrically actuated MEMS test structures. The characterization results of these simple test structures have verified the following process qualities: controllability, reproducibility, predictability and general robustness.

Through metallic mask electrochemical micromachining of micro-groove with a porous cathode

This paper proposes a method of electrochemical micromachining through a metallic mask to reduce the undercutting of micro grooves and improve machining localization. In this method, a metallic mask with through-slit was directly placed on the workpiece instead of using the traditional insulated masks. This decreases the undercutting by reducing the electric field intensity at the edge of micro grooves. In addition, a porous cathode with jet flow mode was employed to allow firm contact of the mask with the workpiece so as to enhance the mass transfer in the machining area. Simulated results show that the metallic mask reduces the current density at the edge of micro groove effectively, and so the undercutting of the profile is evidently reduced. Experimental results indicate that for the same depth, the etch factor (EF) for micro grooves reaches 5.6 by using a metallic mask, and the machining localization is improved significantly. The pulse duty cycle has an important effect on the machining localization and with a pulse duty cycle of 20% a micro groove with better machining localization is obtained. Although a metallic mask improves machining localization of micro grooves, however a low current efficiency is observed with this process as compared to traditional electrochemical micromachining.

A New Method of Roughness Construction and Analysis of Construct Parameters

In micro-manufacturing, roughness is unavoidable due to the tolerance of micro-machining methods. Roughness in microchannel could have a significant influence on flow and heat transfer since the size of microchannel is very small. In our work, roughness is modeled as a superposition of waves. A simple Fourier series method is proposed to construct the rough surface. With this method, roughness is constructed on the bottom of the rectangular microchannel which has a hydraulic diameter of 0.5 mm. Two important parameters during roughness construction, triangulate size and correlation length are studied under the same relative roughness 1%. Results show that flow and heat transfer characteristics are not sensitive to triangulate size. While triangulate size is changing from 0.1 mm to 0.05 mm, the variations of pressure drop and average Nusselt number are less than 1%. Correlation length could influence the topography of roughness surface a lot, smaller correlation length will lead to more pressure drop and lower Nusselt number.

Manufacturing of Al Alloy Microrods by Micro Cutting in a Micromachining Center.

This paper presents the micromanufacturing of aluminum (Al) alloy microrods using micro turning as a competing process to other nontraditional micromachining methods. In that regard, the challenges in such manufacturing have been identified and overcome. The strategies of step-by-step cutting have also been delineated. In addition, the influence of step size and step length on the cutting and thrust forces were investigated. The chip morphology for micromachining was examined using scanning electron microscopic imagery. The safe dimension of the microrod was calculated and, subsequently, used to fabricate microrod, conical tip rod, and grooved rod from 3 mm long and 1.5 mm diameter rod using an appropriately coded computer numerical control (CNC) micromachining center. Our results showed that the thrust force was responsible for part deflection, emphasizing the necessity for computing safe dimensions. At shallow step sizes, the thrust force was more dominant, causing plastic deformation associated with rubbing and burnishing. The chips produced were irregular and sliced in nature. Conversely, at high step sizes, the cutting force superseded the thrust force, resulting in chips that were spread more along the width as opposed to the depth. The chips also had a smoother interacting surface. Finally, micro turning was successfully implemented to manufacture milli-scale structures (i.e., 3 mm long) with micro features (150 to 230 μm diameter) on aluminum alloy materials.

Rapid prototyping of microfluidic chip with burr-free PMMA microchannel fabricated by revolving tip-based micro-cutting

Abstract Cost-effective and rapid prototyping of microfluidic chips has become increasingly possible. However, micromilling, as a popular method for the fabrication of a microfluidic channel, shows no advantage when dealing with a microchannel with a width of less than 50 μm. In this paper, a mimic of the micromilling method, revolving tip-based micro-cutting, was employed to achieve direct writing of a microchannel on polymethylmethacrylate (PMMA) with a width ranging from several micrometres to hundreds of micrometres. A piezoelectric actuator was used to revolve a three-sided nanoindenter instead of a spindle to rotate a mill. Based on the characteristics of revolving cutting, optimization analysis, which focuses on burr formation and surface quality, was performed, and the obtained microchannels exhibited burr-free sides with a bottom surface roughness of Sa = 19 nm. To verify the effectiveness of the proposed micromachining method, two low-cost preparation processes for a microfluidic chip with a fabricated PMMA microchannel were demonstrated. One was directly bonding the fabricated PMMA sheet to another sheet with UV-assisted solvent bonding, where it was concluded that the burr-free sides of the channel played a critical effect on bonding the shallow microchannel. The other was using the fabricated PMMA as a mould to transfer the microchannel to a casted polydimethylsiloxane (PDMS) chip. Finally, a fabricated micro-cross channel was tested to generate a microdroplet, and it was observed that the size and generation rate of microdroplets were controllable.

The Simulation and Research of Etching Function Based on Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has already been employed as a micromachining method for a long time. However, coupling effects of some factors, including the voltage of the tool electrode, the distance between the tool electrode and substrates, tip current, and machining time on the machining process, have not been clearly stated. In this study, based on simulation and experimental results, an etching function between the etching depth and the above influencing factors in the machining process with SECM is proposed. First, the influence of the tool electrode, the distance between the tool electrode and substrates, tip current, and machining time on the etching depth was analyzed by a two-dimensional (2D) axisymmetric finite element model. Second, the etching function between these factors and the etching depth was established. Finally, this relationship was then simplified and verified by etching experiments. In summary, the etching function can be used to guide the etching process to machine 2D and 3D microstructures with SECM.

Influence of bubbles on micro-dimples prepared by electrochemical micromachining

Micro-dimples are the basic surface textures that have played an important role in reducing or increasing friction, anti-wear, decreasing vibration, anti-adhesion, and creeping. Electrochemical micromachining (EMM) is a popular method for preparing micro-dimples. The method of sandwich-like electrochemical micromachining (SLEMM) can be used to enhance machining accuracy of micro-dimples, due to the electrolytic products accumulated on the workpiece surface in the enclosed unit of SLEMM. Therefore, this paper focuses on exploring the influence of bubbles accumulated on the workpiece theoretically and experimentally. The theoretical analysis indicated that 0.2 s machining time is required, and the theoretical maximum depth of micro-dimple is 7.3 μm when the electrochemical dissolution reaction is terminated. The CCD camera was employed to observe the process of SLEMM, and the observations indicated that the changes of gas bubbles are very complex and the electrolytic products accumulated on the workpiece surface have a significant influence on the preparing of micro-dimples. In addition, the experimental results indicate that the dimensions of micro-dimple vary little and the maximum depth of 4 μm for micro-dimples could be prepared with the increase of applied voltage, machining time, and thickness of mask, because of the electrolytic products accumulated on the workpiece during SLEMM.

Multiple slit electrochemical micromachining using a single wire and a constant inter-electrode voltage

Fabrication of micro-slit array in difficult to machine materials is a considerable challenge due to its rigorous surface integrity requirements. This paper proposed a novel method for multiple slit electrochemical micromachining using a single wire and a specific apparatus was developed. The wire is spooled through a specific pulley with multiple grooves to generate a multiple-wire face. Thus, the wire tension and the wire travelling direction would be the same in each wire machining branch. A simplified flow field and electric model illustrated that each machining branch has the same electric supply and the same wire travelling status, as well as the same flow field, which is of great benefits for producing arrayed structures with high consistency. Experiments verified that the inter-electrode voltage as well as the slit width decreased with the increase of electrode amount and the maximum electrode feeding rate was limited. To maintain a constant inter-electrode, the output of the pulse generator was adjusted to a suitable value depending upon the wire amount. Compared with a constant output voltage, the largest electrode feeding rate of each single electrode and the total feeding rate are improved. Finally, a task was designed to compare multi-slit ECMM, HS-WEDM, and LS-WEDM in machining efficiency and surface integrity. It indicated multi-slit ECMM is a feasible alternative for machining arrayed structures requiring good surface integrity in high efficiency.

Analysis on Machining Performance of Nickel-Base Superalloy by Electrochemical Micro-milling with High-Speed Spiral Electrode

As one of the most promising micro-machining methods, electrochemical micro-machining is widely used in the field of metal micro-structures. The electrochemical micro-milling on Nickel-base superalloy by using high-speed spiral electrode was studied in detail. Firstly, the electric field and flow field models of micro-electrochemical milling are established and analyzed by the finite element method. Then, the milling profile was predicted and the effect of high-speed rotation of electrodes on electrolyte promotion and secondary electrolysis prevention were analyzed. Secondly, the influence of the main machining parameters, such as rotating speed, electrical parameters, and feed rate on machining precision and efficiency was analyzed experimentally. Finally, by optimizing the machining parameters, a series of micro-graphic structures with a width of about 150 μm were obtained on Nickel-base superalloy 718 by using the spiral electrode with a diameter of 100 μm. The experimental and simulation results show that the high-speed rotation of electrodes can greatly improve the machining efficiency and stability. It was proved that micro-electrochemical milling with the high-speed rotating electrode technique is an effective method for machining micro-metal parts.

Miniature Optical Fiber Pressure Sensor With Exfoliated Graphene Diaphragm

This paper presents an integrated microcavity optical fiber pressure sensor using exfoliated ultrathin graphene atomic layers as a reflective surface to form a Fabry–Perot (F-P) interferometric structure. The fabrication was based on the focused-ion-beam (FIB) micromachining and dry exfoliated graphene transfer method, where the sensor was fabricated directly on a facet of a standard SMF-28 single-mode fiber with the core diameter of $8~\mu \mathrm {m}$ and the cladding diameter of $125~\mu \text{m}$ . The air leakage from the cavity was negligibly small during the 30-min measurement time. The responsivity of pressure sensors, defined as the spectral shift of F-P fringes in response to the air pressure change, was achieved as large as 1.28 nm/mmHg at 1300-nm wavelength when using a diaphragm diameter of $20~\mu \text{m}$ . Demonstrated responsivity and compactness makes the sensor suitable for medical, environmental, and other applications. An analytical model was presented describing responsivity of an F-P pressure sensor with a thin diaphragm. It has predicted that reducing the diaphragm thickness below some critical value leads to the highest possible responsivity that no longer depends on the elastic properties or thickness of the diaphragm. This upper limit for the responsivity can be increased using the cavity shape other than a simple cylinder, while residual strength of the diaphragm results in reduced responsivity.

An inverse method for the abrasive jet micro-machining of high aspect ratio channels of desired topography – Part I, theory

Control of the cross-sectional shape of micro-channels may be of interest in micro-heat-sinks, microfluidic particle sorting, and micro-machine lubrication applications. It has been previously shown that inverse methods can be used to determine the abrasive jet micromachining (AJM) traverse speed and path required to sculpt desired micro-channel topographies, as long as the aspect ratios (AR, ratio of depth to width) are low (<0.06), such that the channel sidewall slopes are negligible. However, for many applications, higher AR channels with steep sidewalls are required. This paper introduces an iterative inverse method which allows prediction of the machining procedure required to sculpt higher (AR up to 1) micro-channels of prescribed cross-sectional shape using mask-less AJM. The method is developed for AJM in both ductile and brittle erosive systems, its robustness and limitations are discussed, and the iterative methodology is compared to the General Reduced Gradient optimization technique. Part II of the paper will experimentally verify the procedure for a wide variety of desired topographies machined in borosilicate glass and Poly(methyl methacrylate) (PMMA).

An inverse method for the abrasive jet micro-machining of high aspect ratio channels of desired topography – Part II, experiments

Part I of this two-part paper presented an inverse technique to sculpt high aspect ratio (AR) micro-channel cross-sections of desired topography. In this paper, the methodology is experimentally verified for a variety of topographies machined in both brittle and ductile erosive materials. Micro-channels with trapezoidal and semi-circular cross-sections, and semi-circular protrusions were machined in borosilicate glass. Micro-channels with symmetric and asymmetric wedge-shaped cross sections were machined in poly-methyl-methacrylate PMMA. The chosen topographies represent the most challenging cases because they run counter to the natural surface evolution tendency for these two materials. Overall, the average error between the machined and desired profiles was 6.4% in borosilicate glass and 9% in PMMA. The methodology opens up new possibilities for the micro-fabrication of higher efficiency devices containing high-aspect ratio micro-features of virtually any desired shape.

Preparation of a finely patterned La1.89Ce0.11CuO4-x film via chemical modification

In this paper, three La1.89Ce0.11CuO4-x (LCCO) photosensitive sols were prepared with benzoylacetone (BzAcH), acetylacetone (AcAc) or 2,2′-bipyridine (BPY) as the chemical modifiers. UV absorption spectroscopy was employed to analyze the chelation mechanism of three chemical modifiers and metal ions in the sols. It has been found that the copper ions chelated with the chemical modifiers mentioned above and endowed ultraviolet photosensitivity to the sol, whereas the lanthanum ions and cerium ions did not chelate with the chemical modifiers. UV irradiation experiments indicated that the chelate compounds were decomposed and the decomposition products greatly reduced the solubility of LCCO gel films in the organic solvent. The LCCO films with fine structure were fabricated by using self-photosensitive property of the three gel films. The micro-machining process showed that LCCO fine patterns, prepared by the sol of BzAcH as the chemical modifiers, exhibited the highest precision and the minimum size of the prepared patterns reached to micron level. XRD patterns seen that the addition of chemical modifiers had no effect on the structure of LCCO inorganic film. R-T curve tests of fine-patterned LCCO films prepared with the above three sols revealed that the micro-machining method was a simple and cost-effective process, which did not influence the electrical properties of the LCCO films.

Experimental Study of Electrochemical Micro-milling of Silicon

Electrochemical machining (ECM) is a common method of silicon micromachining, but it is difficult to be directly used for three-dimensional processing. Therefore a new electrochemical micro-milling (ECMM) method is proposed. The method combined electrochemical machining and milling processes technology. Micro-removal of materials can be achieved by electrochemical machining technique, while milling processes enable three-dimensional machining. ECMM utilizes the passivation of silicon in sodium hydroxide solution to reduce electrochemical stray corrosion and improve processing accuracy. Experiments showed that the ECMM technology can limit the electrochemical stray corrosion to less than 10 μm. This method can be used for the machining of tiny three-dimensional structures of silicon materials.

Advanced Micro- and Nano-Gas Sensor Technology: A Review.

Micro- and nano-sensors lie at the heart of critical innovation in fields ranging from medical to environmental sciences. In recent years, there has been a significant improvement in sensor design along with the advances in micro- and nano-fabrication technology and the use of newly designed materials, leading to the development of high-performance gas sensors. Advanced micro- and nano-fabrication technology enables miniaturization of these sensors into micro-sized gas sensor arrays while maintaining the sensing performance. These capabilities facilitate the development of miniaturized integrated gas sensor arrays that enhance both sensor sensitivity and selectivity towards various analytes. In the past, several micro- and nano-gas sensors have been proposed and investigated where each type of sensor exhibits various advantages and limitations in sensing resolution, operating power, response, and recovery time. This paper presents an overview of the recent progress made in a wide range of gas-sensing technology. The sensing functionalizing materials, the advanced micro-machining fabrication methods, as well as their constraints on the sensor design, are discussed. The sensors’ working mechanisms and their structures and configurations are reviewed. Finally, the future development outlook and the potential applications made feasible by each category of the sensors are discussed.

Influence of pulse repetition rate on morphology and material removal rate of ultrafast laser ablated metallic surfaces

Ultrafast laser ablation is an efficient method for precise micro-machining. Thanks to the recent development of high repetition rate ultrafast lasers, high speed laser scanning of surfaces is more and more employed to generate micro-/nano- surface structures on metals for a vast variety of applications. Issues associated with these lasers are also identified in micro-machining practice. It is commonly believed that, due to possible shielding effects, the conditions of high fluences and high repetition rates compromise an efficient material removal. However, in this study, based on topography and differential weighing evaluations, we report that the material removal rate holds constant even in sub-MHz regime, up to about 20 J/cm2. The morphology of the post-irradiated surface is found to be determined not only by laser processing conditions but also by the material properties on the other hand. Two trends are experimentally identified in surface laser ablation of Ni, Cu, titanium alloy TA6V and stainless steel 316L: while the former two show a low roughness (Ra below 0.5 µm) at all irradiation conditions, the machining quality of the two later ones degrades rapidly with increasing fluences and repetition rates. In such a scenario, a rugged surface layer of 10–20 µm thickness is formed with the presence of numerous subsurface voids. Microstructural analysis is carried out in order to infer physical transition involved in the micro-machining process. Possible mechanisms accounting for the observation are discussed, especially those related to the electron-phonon coupling, plasma dwelling, and capillary waves. These insights pave the way for tailored, material dependent optimizations of ultrafast laser micro-machining processes.