Micromachined ultrasonic transducers based on lead zirconate titanate (PZT) films

The design, fabrication and measuring of piezoelectric micromachined ultrasonic transducers (pMUTs), including the deposition and patterning of PZT films, was investigated. The (100) preferential orientation of PZT film have been deposited on Pt/Ti/SiO₂/Si (100) substrates by modified sol–gel method. PZT film and Pt/Ti electrode were patterned by novel lift-off using ZnO as a sacrificial layer avoiding shortcomings of dry and wet etching methods. pMUT elements have been fabricated by an improved silicon micromachining process and their properties were also characterized. As measured results, the pMUT tends to operate in a standard plate-mode. The receive sensitivity and transmit sensitivity of pMUT element whose active area only has 0.25 mm² are ?218 dB (ref. 1 V/μPa) and 139 dB (ref. 1 μPa/V), respectively.     

Manufacture of microfluidic glass chips by deep plasma etching, femtosecond laser ablation, and anodic bonding

Two dry subtractive techniques for the fabrication of microchannels in borosilicate glass were investigated, plasma etching and laser ablation. Inductively coupled plasma reactive ion etching was carried out in a fluorine plasma (C₄F₈/O₂) using an electroplated Ni mask. Depth up to 100 μm with a profile angle of 83°–88° and a smooth bottom of the etched structure (Ra below 3 nm) were achieved at an etch rate of 0.9 μm/min. An ultrashort pulse Ti:sapphire laser operating at the wavelength of 800 nm and 5 kHz repetition rate was used for micromachining. Channels of 100 μm width and 140 μm height with a profile angle of 80–85° were obtained in 3 min using an average power of 160 mW and a pulse duration of 120 fs. A novel process for glass–glass anodic bonding using a conductive interlayer of Si/Al/Si has been developed to seal microfluidic components with good optical transparency using a relatively low temperature (350°C).     

Micro-mixer device with deep channels in silicon using modified RIE process: fabrication,packaging and characterization

In the present work, silicon based micromixer microfluidic devices have been fabricated in silicon substrates of 2-inch diameter. These devices are of 2-input and 1-output port configuration bearing channel depth in the range 80–280 µm. Conventional reactive ion etching (RIE) process used in integrated circuit fabrication was modified to get reasonably high silicon etch rate (~1.2 µm/min). It was anticipated that devices with channel depth in excess of 150 µm would become weak and susceptible to breakage. For such devices, a bonded pair of silicon having a 0.5 µm SiO₂ at the bonded interface was used as the starting substrate. The processed silicon wafer bearing channels was anodically bonded to a Corning^® 7740 glass plate of identical size for fluid confinement. Through-holes for input/output ports were made either in Si substrate or in glass plate before carrying out anodic bonding. Micro-channels were characterized using stylus and optical profiler. Surface roughness of the channel was observed to increase with increasing channel depth. The devices were packaged in a polycarbonate housing and pressure drop versus flow rate measurements were carried out. Reynolds number and friction factor were calculated for devices with 82 µm deep channels. It was observed that up to 25 sccm of gas and 10 ml/min of liquid, the flow was laminar in nature. It is envisaged that using bonded silicon wafer pair and combination of RIE and wet etching, it is possible to get an etch stop at the SiO₂ layer of the bonded silicon interface with much smaller value of surface roughness rendering smooth channel surface.     

Vertical Si nanowire with ultra-high-aspect-ratio by combined top-down processing technique

Vertical Si nanowires with ultra-high-aspect-ratio were fabricated using a combined process of deep reactive ion etching and sacrificial oxidation. The combined process starts with etching the Si substrates by the Bosch process to form micrometer-scale structures. The etched micrometer-scale structures are shrunk to nanometer-scale by sacrificial oxidation. The fabricated Si nanowires that were aligned vertically to the substrate had a diameter of less than 200 nm and a length greater than 10 μm. One of the fabricated Si nanowires had a diameter of 110 nm and a length of 11 μm. The resulting aspect-ratio reached 100, which is a value that is significantly high for vertical Si nanowires fabricated by using a top-down approach.     

Surface modification of a-Si<Subscript>0.60</Subscript>C<Subscript>0.40</Subscript>:H films by Al-assisted photochemical etching: humidity sensing application

The paper investigates the formation of thin porous amorphous silicon carbide (PASiC) by Al-assisted photochemical etching using HF/AgNO₃ solution under UV illumination at λ = 254 nm. Different etching times varying from 2 to 10 min have been used on thin a-Si_0.60C_0.40:H films, which are elaborated by co-sputtering DC magnetron using a single crystal Si target and who deposited onto 86 of hot pressed polycrystalline 6H-SiC stripes of 12.5 mm³. Because of the high electrical resistivity of the thin a-Si_0.60C_0.40:H film higher than 2 MΩ cm, and in order to facilitate the chemical etching, a thin metallic film of high purity aluminum (Al) has been deposited under vacuum, follow-up of a thin palladium deposited under a grid to reduce attacked surface and reinforced solution etching. The etched surface was characterized by scanning electron microscopy, infrared spectroscopy, spectrophotometer UV, and photoluminescence. Results show that the morphology of etched a-Si_0.60C_0.40:H surface evaluates with etching time and presents a spongy and macroporous layers. Where, the diameter of pore size increases with the increasing etching time. A humidity sensors were fabricated through evaporating coplanar interdigital gold electrodes on PASiC and the humidity sensing properties were tested, it show, that the measured resistance Au-PASiC structure, depends highly on the applied bias voltage. Finally, the sensing performances are attributed to the unique surface structure, morphology of the pore and its size, that provide an effective pathway for vapor transportation and enlarged the sensing area of Au-PASiC.     

Submicron polymer structures with X-ray lithography and hot embossing

This article describes the process chain for replication of submicron structures with varying aspect ratios (AR) up to 6 in polymethylmethacrylate (PMMA) by hot embossing to show the capability of the entire LIGA process to fabricate structures with these dimensions. Therefore a 4.7 μm thick layer of MicroChem 950k PMMA A11 resist was spin-coated on a 2.3 μm Ti/TiO _x membrane. It was patterned with X-ray lithography at the electron storage ring ANKA (2.5 GeV and λ _c ≈ 0.4 nm) at a dose of 4 kJ/cm³ using a Si₃N₄ membrane mask with 2 μm thick gold-absorbers. The samples were developed in GG/BDG and resulted in AR of 6–14. Subsequent nickel plating at 52°C resulted in a 200 μm thick nickel tool of 100 mm diameter, which was used to replicate slit-nozzles and columns in PMMA. Closely packed submicron cavities with AR 6 in the nickel shim were filled to 60% during hot embossing.     

Comparison of etch characteristics of KOH,TMAH and EDP for bulk micromachining of silicon (110)

Bulk micromachining in Si (110) wafer is an essential process for fabricating vertical microstructures by wet chemical etching. We compared the anisotropic etching properties of potassium hydroxide (KOH), tetra-methyl ammonium hydroxide (TMAH) and ethylene di-amine pyro-catechol (EDP) solutions. A series of etching experiments have been carried out using different etchant concentration and temperatures. Etching at elevated temperatures was found to improve the surface quality as well as shorten the etching time in all the etchants. At 120°C, we get a smooth surface (Ra = 21.2 nm) with an etching rate 12.2 μm/min in 40wt% KOH solution. At 125°C, EDP solution (88wt%) was found to produce smoothest surface (Ra = 9.4 nm) with an etch rate of 1.8 μm/min. In TMAH solution (25wt%), the best surface roughness was found to be 35.6 nm (Ra) at 90°C with an etch rate of 1.18 μm/min. The activation energy and pre-exponential factor in Arrhenius relation are also estimated from the corresponding etch rate data.

     

Defect-free wet etching through pyrex glass using Cr/Au mask

This paper reports the highest etch depth of annealed Pyrex glass achieved by wet etching in highly concentrated HF solution, using a low stress chromium–gold with assistance of photoresist as masking layer. The strategies to achieve that are: increasing the etch rate of glass and simultaneously increasing the resistance of Cr/Au mask in the etchant. By annealing the Pyrex glass and using a highly concentrated HF acid, a high etch rate can be obtained. Furthermore, a method to achieve a good resistance of the Cr/Au masking layer in the etching solution is to control the residual stress and to increase the thickness of Au deposition up to 1 μm. In addition, the presence of a hard baked photoresist can improve the etching performance. As a result, a 500-μm thick Pyrex glass wafer was etched through.

     

Fabrication of high precision X-ray mask for X-ray grating of X-ray Talbot interferometer

X-ray imaging is used in many applications such as medical diagnosis and non-destructive inspection, and has become an essential technologies in these areas. In one image technique, X-ray phase information is obtained using X-ray Talbot interferometer, for which X-ray diffraction gratings are required; however, the manufacture of fine, highly accurate, and high aspect ratio gratings is very difficult. X-ray lithography could be used to fabricate structures with high precision since it uses highly directive syncrotron radiation. Therefore, we decided to fabricate X-ray gratings using X-ray lithography technique. The accuracy of the fabricated structure depends largely on the accuracy of the X-ray mask used. In our research, we combined deep silicon dry etching technology with ultraviolet lithography in order to fabricate untapered and high precision X-ray masks containing rectangular patterns. We succeeded in fabricating an X-ray mask with a pitch of 5.3 μm. The thickness of the Au absorber was about 5 μm, and the effective area was 60  × 60 mm², which is a sufficient size for phase tomography imaging. We demonstrated the utility of the Si dry etching process for making high precision X-ray masks.     

Monolithic integration of Pb(Zr,Ti)O3 thin film based resonators using a complete dry microfabrication process

Monolithic fabrication of lead zirconate titanate [Pb(Zr,Ti)O₃ or PZT] based thin film resonant devices such as microcantilevers, Lamb wave and bulk acoustic wave resonators are demonstrated. High-performance PZT thin films with a thickness of 2.6 μm are prepared on a silicon on insulator wafer by a sputtering deposition process. A highly selective reactive ion etching process is employed for micro-patterning of PZT, platinum electrodes, and SiO₂ insulation layer. Self-actuation of the PZT microcantilevers is demonstrated and the frequency response is characterized using a laser Doppler vibrometer. The frequency response of the Lamb wave resonator is evaluated by measuring its transmission characteristic using a network analyzer. For a Lamb wave resonator with a length of 240 μm and an interdigital period of 80 μm, the 1st order and 2nd resonance frequencies are 15.3 and 41.8 MHz, respectively.     

A polymer-metal hybrid flexible mould and application for large area hot roller embossing

In this paper, we report a novel approach to fabricate a low cost, large area and flexible mould and its applications in large area roller embossing. A liquid crystal polymer (LCP) film, which had a high glass transition temperature of 280°C, was clad with copper foils on both sides, was used as a starting material for mould fabrication. The LCP film and the copper foils were 50 and 36 μm thick, respectively. The LCP-Cu flexible mould was obtained through photolithographic patterning and wet etching of the copper foil on top surface of the LCP film. Using this proposed method, a polymer-metal hybrid flexible mould with an area of 150 mm × 150 mm was fabricated. The fabricated mould has a minimum feature size of 25 μm, and has been successfully used to demonstrate large area micro roller embossing. Micro channels, micro dots and micro mixers were embossed on polymeric as well as ceramic green substrates.     

Low temperature carbon nanotube and hexagonal diamond deposition with photo-enhanced chemical vapor deposition

Deposition of carbon nanotube and hexagonal diamond thin films at low substrate temperature with photo-enhanced chemical vapor deposition is described here. Extensive experimentation is conducted to optimize the catalyst layer utilized for deposition by varying Al/Ni/Al metal layer thicknesses on SiO₂ coated Si substrates. The coated substrates are annealed to transform the thin metal layers into nanoparticles. Suitable catalyst layer thicknesses obtained are 3/2/3, 5/1/5 and 5/3/5 nm for Al/Ni/Al sandwich metal layers. Suitable annealing conditions are in the range of 350–450 °C for substrate temperature and from 0.22 to 10 Torr for chamber pressure in ammonia ambient for 25 min. Carbon tetrachloride (CCl₄) is used as a carbon precursor in this work. Argon to CCl₄ flow ratio is varied in 1.5–19 range, chamber pressure is varied in 3–10 Torr range, and the substrate temperature is varied in 350–450 °C range. Carbon nanotubes (CNT) growth is observed at lower chamber pressure, lower partial pressure of CCl₄, lower substrate temperature and for thin Ni catalyst layer. The optimal CNT deposition condition observed is 5 Torr total chamber pressure, 9:1 partial pressure ratio of Ar to CCl₄, 400 °C substrate temperature and 5/1/5 nm thick Al/Ni/Al catalyst layers. The hexagonal diamond deposition is observed at a higher chamber pressure, higher partial pressure of CCl₄, higher substrate temperature and for a thicker Ni catalyst layer. The optimal condition for hexagonal diamond deposition observed is 10 Torr total chamber pressure, 7:3 partial pressure ratio of Ar to CCl₄, 450 °C substrate temperature and 5/3/5 nm thick Al/Ni/Al catalyst sandwich layers.     

A facile strategy to integrate robust porous aluminum foil into microfluidic chip for sorting particles

High efficiency integration of functional microdevices into microchips is crucial for broad microfluidic applications. Here, a device-insertion and pressure sealing method was proposed to integrate robust porous aluminum foil into a microchannel for microchip functionalization which demonstrate the advantage of high efficient foil microfabrication and facile integration into the microfluidic chip. The porous aluminum foil with large area (10 × 10 mm²) was realized by one-step femtosecond laser perforating technique within few minutes and its pores size could be precisely controlled from 3 μm to millimeter scale by adjusting the laser pulse energy and pulse number. To verify the versatility and flexibility of this method, two kinds of different microchips were designed and fabricated. The vertical-sieve 3D microfluidic chip can separate silicon dioxide (SiO₂) microspheres of two different sizes (20 and 5 μm), whereas the complex stacking multilayered structures (sandwich-like) microfluidic chip can be used to sort three different kinds of SiO₂ particles (20, 10 and 5 μm) with ultrahigh separation efficiency of more than 92%. Furthermore, these robust filters can be reused via cleaning by backflow (mild clogging) or disassembling (heavy clogging).     

Cross-flow microfilters with large-diameter sacrificially etched cross-sections

Cross-flow microfilters were constructed on silicon substrates using photolithography, chemical vapor deposition, and sacrificial etching. These devices consist of an array of channels with arch-shaped cross-sections approximately 50 μm tall and 140 μm wide. Pores, 5 μm in diameter, were etched through channel walls with a high packing density. The microfilters were analyzed by imaging permeate and retentate solutions down the length of the channels to determine percentages of fluorescent microbeads (diameters of 2.2 and 15.5 μm) filtered per length. A simple model using principles of Brownian motion and Monte Carlo simulation closely predicts filtration performance.     

Patterning of burnishing head for hard disk platters by synchrotron radiation lithography

This paper discusses the patterning of the burnishing head for hard disk platters on the AlTiC substrate, performed by X-ray exposure through X-ray mask to identify areas still left on the substrate after the photoresist development. The high energy and the low divergence of the synchrotron light provided the high-aspect-ratio microstructures with high accuracy patterns of burnishing head specifications. After the X-ray lithography was processed, the AlTiC substrate coated with the SU-8 hard mask was dry-etched with CF₄ of the RIE machine and run under the various conditions of the industrial process. An appropriate condition based on a total of 10 experimental conditions, which were based on the CCD technique, was investigated at Minitab using the data collected for analysis and compared with the standard specification. Based on the standard specification of the critical dimension of 8.02 µm and the etched depth of 30.0 µm, the best condition for the experiment, calculated by the desirability approach, was the 250 µm-thick SU-8 photoresist with an exposure dose of 23,010 mJ/cm³ and an RIE etching time of about 20 h. Mathematically, this condition offers a critical dimension and an etched depth of 7.02 and 30.11 µm, respectively. When the actual experiment was conducted to confirm the results, and we found that the critical dimension and the etched depth showed values of 7.03 and 30.02 µm, respectively.     

Post micromachining of MPW based CMOS–MEMS comb resonator and its mechanical and thermal characterization

In recent years micro electro mechanical system (MEMS) based micro resonant sensors have been given a lot of attention due to their potential as a platform for the development of many novel physical, chemical, and biological sensors. That is why this paper covers post processing of the structures fabricated through Multi-Project-Wafer using 0.35 µm MIMOS CMOS technology with particular focus on dry etching of Si and SiO₂ from the front side of CMOS–MEMS chip that is optimized using aluminum coated carrier wafer and achieved results are debris free as compared to photoresist coated carrier wafer. The device is etched through from the front side to avoid parasitic capacitances and squeeze film damping by keeping minimum size of the die. The etching of SiO₂ as well as deep Si etch-through using the same plasma etcher (SS110A Tegal) is successfully demonstrated in this work. Finally, after the successful post CMOS micromachining of the device, resonance frequency i.e. 8164 Hz and quality factor i.e. 51.34, is determined. The joule heating effect due to the passing of current through the central shuttle of the device is characterized. The maximum temperature close to the anchors of the comb resonator where the piezoresistors are located is determined through temperature coefficient of resistance measurement using PE-4RF type probe station and it is found to be 37.62 °C.     

X-ray zone plates with 25 aspect ratio using a 2-μm-thick ultrananocrystalline diamond mold

Hard X-ray phase zone plates are focusing optics used for X-ray microscopes at synchrotron radiation facilities. The resolution is determined by the outer-most zone width (OZW) and modern lithographic techniques are capable of patterning OZW less than 100 nm. Efficiency of a phase zone plate will peak when the zones have a thickness that provides a π-phase shift to the X-rays. Thus, a hard X-ray zone plate with ideal efficiency and sub-100-nm resolution requires fabricating high-aspect-ratio, dense-packed structures in materials suitable for exposure to synchrotron radiation. The fabrication method implemented involves an electroforming mold process where a top resist layer is lithographically patterned and used for pattern transfer into a bottom layer which acts as the electroform mold. The resulting mold is filled with Au by electroplating, and afterwards the mold is not removed but remains in place for mechanical support. Ultrananocrystalline diamond (UNCD) was used as the mold layer. UNCD is deposited by hot-filament chemical vapor deposition with well-controlled stress and thickness up to 2 μm. The top resist layer is hydrogen silsesquioxane, which is a high-contrast electron beam lithography resist and resistant to the oxygen reactive ion etching required for UNCD pattern transfer. Using this fabrication method, we successfully produced zone plates with OZW down to 80 nm and an aspect ratio up to 25 for a thickness of 2 μm. The efficiency of several fabricated zone plates were measured, demonstrating their functionality.     

High aspect ratio electrostatic micro actuators using LIGA process

High-power electrostatic microactuators using LIGA process have been fabricated. Comb drive type actuators and a wobble motor were designed and fabricated. A basic structure of the microactuators was composed of movable and fixed electrodes of Ni, a sacrificial layer of SiO₂ and a Si substrate, and carried out by one mask process. As design rules, a minimum resist width of 2 μm, resist height of 120 μm, maximum width of movable parts of 10 μm, minimum width of fixed parts of 40 μm and driving voltage of about 100 V, were decided. A 120 μm-thick PMMA resist was formed on a Si substrate by a casting method. The PMMA was exposed using a compact SR source “AURORA”, using an X-ray mask with 7 μm-thick Au absorber on a 2 μm-thick poly-Si membrane. The exposed PMMA was developed by a developer. Ni microstructures with 100 μm-height, 2 μm-minimum width, 2 μm-minimum gap, and then maximum aspect ratio of 50, were made by electroforming. Ni microstructures used for movable electrodes were separated from the substrate by lateral etching of SiO₂. After lateral etching of SiO₂, Au wires were bonded to electrodes. Actuation of the comb drive type actuator and rotation of the wobble motor were confirmed. The applied voltage to the comb drive actuator and the wobble motor were 65 and 125 V.     

Defect-free wet etching through pyrex glass using Cr/Au mask

This paper reports the highest etch depth of annealed Pyrex glass achieved by wet etching in highly concentrated HF solution, using a low stress chromium–gold with assistance of photoresist as masking layer. The strategies to achieve that are: increasing the etch rate of glass and simultaneously increasing the resistance of Cr/Au mask in the etchant. By annealing the Pyrex glass and using a highly concentrated HF acid, a high etch rate can be obtained. Furthermore, a method to achieve a good resistance of the Cr/Au masking layer in the etching solution is to control the residual stress and to increase the thickness of Au deposition up to 1 μm. In addition, the presence of a hard baked photoresist can improve the etching performance. As a result, a 500-μm thick Pyrex glass wafer was etched through.