Fabrication of micro sloping structures of SU-8 by substrate penetration lithography

In this paper, three-dimensional (3D) micro sloping structures were fabricated by ordinary mask pattern and diffraction phenomenon. Especially, we fabricated the structures with SU-8 negative photoresist and substrate penetration lithography. In this method, exposure is performed arranging in order of a mask, a substrate and the SU-8 resist. There is a gap that is equal to the thickness of the substrate between resist and mask. In narrow slit of mask, resist is less exposed than usual because of Fraunhofer diffraction. The amount of exposure depends on slit width so that the height of SU-8 resist can be controlled. A 173 μm height of structure was obtained in the case of 27 μm width slit and 24.2 μm height of structure was obtained in the case of 7.4 μm width slit. By using this method, high aspect ratio 3D SU-8 structures with smooth sloping were fabricated in the length of 100–300 μm and in the height of 50–200 μm with rectangular triangle mask pattern. In the same way, there is influence of Fresnel diffraction on edge of aperture so that micro taper structures were fabricated. A lot of taper structures were fabricated by the method to make the surface repellency. The contact angle was achieved more than 160° in this study.     

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.     

A new UV sensitive positive resist for X-ray masks manufacture

LIGA is a well-established process to fabricate metallic micro parts with high resolution, high precision and very low sidewall roughness by means of X-ray lithography and electroplating. The availability of a precise X-ray mask is a precondition for the final precision of the manufactured micro parts. Typical mask substrate materials, e.g. beryllium, carbon based foils, Si₃N₄ or SiC show different disadvantages such as low X-ray transparency or high toxicity or high prices or low conductivity or high thermal expansion or surface porosity causing X-ray scattering. For the fabrication of X-ray masks, PMMA with its unique features such as high aspect ratio patterns with high precision, exhibits low sensitivity and the layers preparation is not easy. SU-8, an epoxy-based UV and X-ray sensitive, chemically amplified, negative tone photoresist exhibits high aspect ratio patterns with vertical sidewalls. The difficult remove of the resist after the electroplating process significantly hinders the inspection of the fabricated X-ray mask. We present the use and suitability of an UV sensitive, chemically amplified, viscous, aqueous-alkaline developable, and easy removable positive tone photoresist, XP mr-P 15 AV, exhibiting high aspect ratio patterns with vertical sidewalls for the fabrication of X-ray masks by means of UV lithography on vitreous carbon substrates.     

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.

     

Sandwich structure based on back-side etching silicon (100) wafers for flexible electronic technology

An SU-8-silicon(100)-SU-8 flexible composite sandwich structure is studied. Besides preventing corrosion to the thin silicon membrane, SU-8 photoresist coated on the silicon membrane improves its flexibility as shown by an ANSYS finite element simulation. Using a plasma enhanced chemical vapor deposited SiO₂/Si₃N₄ composite film as an etch mask on the polished side, a 4″ (100) silicon wafer was thinned to 26 µm without rupture in a 30 wt% KOH solution. The wafer was coated on both sides with 20 µm of SU-8 photoresist and then bent over cylinders with various diameters, to flex the sandwich in a controlled manner. The determined minimal bending radius of the fabricated thin silicon-based sandwich structure is approximately 3.5 mm. The fabrication of this sandwich structure is compatible with conventional microelectronic fabrication processing. Thus, it allows one to thin fully fabricated devices in a post-fabrication process to make high-performance flexible electronics.

     

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.     

Fabrication of electrostatic MEMS microactuator based on X-ray lithography with Pb-based X-ray mask and dry-film-transfer-to-PCB process

Lithographie Galvanoformung Abformung (LIGA) is a promising approach for fabrication of high aspect ratio 3D microactuator for dual-stage slider in hard disk drive. However, this approach involves practically challenging X-ray lithography and structural transfer processes. In this work, electrostatic MEMS actuator is developed based on a LIGA approach with cost-effective X-ray lithography and dry-film-transfer-to-PCB process. X-ray lithography is performed with X-ray mask based on lift-off sputtered Pb film on mylar substrate and photoresist application using casting-polishing method. High quality and high aspect ratio SU8 microstructures with inverted microactuator pattern have been achieved with the interdigit spacing of ~5 μm, vertical sidewall and a high aspect ratio of 29 by X-ray lithography using the low-cost Pb based X-ray mask. A new dry-film-transfer-to-PCB is employed by using low-cost dry film photoresist to transfer electroplated nickel from surface-treated chromium-coated glass substrate to printed circuit board (PCB) substrate. The dry film is subsequently released everywhere except anchor contacts of the electrostatic actuator structure. The fabricated actuator exhibits good actuation performance with high displacement at moderate operating voltage and suitably high resonance frequency. Therefore, the proposed fabrication process is a promising alternative to realize low-cost MEMS microactuator for industrial applications.     

Integrated fabrication of electrostatic microactuator for HDD R/W head positioning

We designed an electrostatic actuator which can work with the voice coil motor in a dual-stage servo control system to accurately position the read/write (R/W) head in hard disk drives. An integrated fabrication process for this microactuator has been proposed, considering total fabrication of R/W head and alumina-titanium carbide (AlTiC) slider. Adhesive bonding technique using photoresist SU-8 was selected for wafer bonding between silicon and AlTiC. A microactuator prototype has been fabricated by the proposed process. The stroke of the microactuator has been measured during the static testing.     

Investigation in the interface adhesion strength between SU-8 photoresist and metal substrate by ultrasonic treatment

During the fabrication process of metal microstructures and microdevices by ultraviolet Lithographie Galvanoformung Abformung technology of SU-8, it is common for interface separation and bind failure due to the poor adhesion strength between SU-8 photoresist and metal substrate. In this paper, ultrasonic wave was introduced into the lithography process to improve the interface adhesion strength. Scratch test method was adopted to measure the interface adhesion strengths between SU-8 photoresist and metal substrate without ultrasonic treatment and with ultrasonic treatment under different ultrasonic conditions. The effects and corresponding mechanisms of ultrasonic time and ultrasonic power on the interface adhesion strength between SU-8 photoresist and metal substrate were investigated. The results showed that due to the input of the ultrasonic vibration, the crosslink network characteristics of SU-8 photoresist were changed. The internal stress in SU-8 photoresist was reduced and the interface adhesion strength was improved. The interface adhesion strength firstly increased and then decreased with the increase of ultrasonic time. The maximum interface adhesion work was 0.427 J/m² at an optimized ultrasonic time of 10 min, which was higher than that of the sample without ultrasonic treatment by 39.5 %. Besides, in the experimental conditions, the interface adhesion strength first increased and then decreased with the increase of the ultrasonic power and the maximum interface adhesion strength attained at the ultrasonic power of 352 W. The ultrasonic treatment method can effectively improve the interface adhesion strength, so as to increase the product yield and performance.     

Design and fabrication of an electrostatically suspended microgyroscope using UV-LIGA technology

A micromachined electrostatically suspended gyroscope, with a wheel-like rotor housed by top stator and bottom stator, using UV-LIGA microfabrication technology, was presented. The designed structure and basic operating principle of the gyroscope are described. The key steps in the fabrication process, such as wet etching of Pyrex glass pits for soldering, and integration of thick nickel structures by removal of SU-8 mold, were considered in detail and well solved. Cr/Pt/photoresist was used as etching mask and the etched pits, in depth of near 30 μm, with aspect ratio (depth to undercutting) of 0.75, were obtained. With metal foundations constructed for consolidation, successful integration of the nickel structures, in thickness of 200 μm, was achieved by successful removal of the SU-8 mold using oleum. After the two stators and the rotor were fabricated separately, they were assembled and soldering bonded to form axial and radial small gaps, hence, the initial prototype of the microgyroscope was realized. The key techniques described in this paper can be applied to fabrication of other micro devices. The metal foundation method, associated with removal of SU-8 mold by oleum, is expected to make SU-8 wider applications in making integrated microstructures with fabricated circuitry on the same chip.     

Reduction of diffraction effect for fabrication of very high aspect ratio microchannels in SU-8 over large area by soft cushion technology

Edge bead due to spin coating has been found to cause an air gap as large as 53 m for a 99-m thick (measured at the wafer center) SU-8 coating over a 4-in. wafer. This caused poor mask width replication and non-uniformity of SU-8 pattern width. We have devised a process using a soft cushion technology to improve mask dimension replication and large-area pattern uniformity. A soft cushion was placed beneath the substrate to produce convex bending of the wafer in order to improve the contact between the mask and photoresist top surface during UV exposure. Dramatic improvements in pattern uniformity, from over 30% variation in SU-8 width across the wafer to less than 10%, and mask width replication, from 54% deviation from mask width to 20%, have been demonstrated. Numerically calculated increases in SU-8 width from the wafer center to the edge bead using the Fresnel diffraction equation match well with observed values. Further, employment of this technique with a narrow (10 m) dark-field mask enabled the fabrication over the entire surface of 4-in. diameter wafers of dense SU-8 gratings separated by microchannels with aspect ratio over 18.     

Fabrication of SU-8 photoresist micro–nanofluidic chips by thermal imprinting and thermal bonding

Micro–nanofluidic chips have been widely applied in biological and medical fields. In this paper, a simple and low-cost fabrication method for micro–nano fluidic chips is proposed. The nano-channels are fabricated by thermal nano-imprinting on an SU-8 photoresist layer followed by thermal bonding with a second SU-8 photoresist layer. The micro-channels are produced on the second layer by UV exposure and then thermal bonded by a third layer of SU-8 photoresist. The final micro–nano fluidic chip consists of micro-channels (width of 200.0 ± 0.1 μm and, depth of 8.0 ± 0.1 μm) connected by nano-channels (width of 533 ± 6 nm and, depth of 372 ± 6 nm), which has great potential in molecular filtering and detection.

     

Rapid soda-lime glass etching process for producing microfluidic channels with higher aspect ratio

This paper presents a simple method to produce microfluidic channels in soda-lime glasses with the aspect ratio >0.5 utilizing a modified wet etching protocol. A low-cost positive photoresist (PR) layer is used as the etching mask for the wet etching process. Prior to the PR and primer coating procedure, a UV activation process is adopted for enhancing the binding strength of the hexamethyldisilazane primer layer and the glass substrate, resulting in an better adhesion for the PR layer. A fast etching recipe is also developed by increasing the acidity and the temperature of the buffered oxide (BOE) etchant. Since the photoresist etching mask does not peel during the etching process shortly, the structure of the etching mask forms a barrier and results in a different diffusion rate for the etchant inside the etched trench structure. A slower etching rate for the glass is observed at the undercut region such that the proposed anisotropic etching pattern can be achieved. Results show that the etching rate of the modified glass etching process is as high as 7.7 μm/min which is much faster than that of pure BOE etchant (0.96 μm/min). Sealed microfluidic channel with the aspect ratio of around 0.62 is produced with the developed method. The method developed in the present study provides a rapid and efficient way to produce microfluidic channels with higher aspect ratio.     

A new removable resist for high aspect ratio applications

A variety of different photo resists are used for fabrication of MEMS. Presently good results were reported for SU-8, a chemically amplified negative tone photoresist. But SU-8 has a disadvantage for some applications in LIGA technique, especially in the X-ray mask fabrication. After processing the finished resist pattern are hardly soluble from the substrate. This paper will briefly describe the current status of the development of the new negative tone photoresist CAR 44 whose big advantage is the easy removableness of the cross linked pattern. This work widely uses the contents of the presentation “A New Removable Resist for High Aspect Ratio Applications” to the High Aspect Ratio Micro Structure Technology workshop HARMST 2005 held in Gyeongyu (Republic of Korea), June 10–13, 2005.     

Dual layer photoresist complimentary lithography applied on sapphire substrate for producing submicron patterns

In this study, the combined technologies of dual-layer photoresist complimentary lithography (DPCL), inductively coupled plasma-reactive ion etching and laser direct-write lithography are applied to produce the submicron patterns on sapphire substrates. The inorganic photoresist has almost no resistance for chlorine containing plasma and aqueous acid etching solution. However, the organic photoresist has high resistance for chlorine containing plasma and aqueous acid etching solution. Moreover, the inorganic photoresist is less etched by oxygen plasma etching process. The organic and inorganic photoresist deposit sequentially into a composite photoresist on a substrate. The DPCL takes advantages of the complementary chemical properties of organic and inorganic photoresist. We fabricated two structures with platform and non-platform structure. The non-platform structure featured structural openings, the top and bottom diameters and the depth are approximately 780, 500 and 233 nm, respectively. The platform structure featured structural openings, the top and bottom diameters and the depth are approximately 487, 288 and 203 nm, respectively. The precision submicron or nanoscale patterns of large etched area and patterns with high aspect ratio can be quickly produced by this technique. This technology features a low cost but high yield production technology. It has the potential applications in fabrication of micro-/nanostructures and devices for the optoelectronic industry, semiconductor industry and energy industry.     

采用SU—8胶制作模具的工艺研究

主要描述了SU—8胶制造微流体芯片用模具的工艺研究。讨论了各工艺流程主要包括有前烘、中烘、光刻、显影等因素对模具的影响。提出了一个可供参考的模具制作工艺流程,对抗粘层工艺进行了讨论。另外,在模具制造过程中加入反应离子刻蚀(RIE)来提高SU—8与硅基底的粘附性。最终通过上述的工艺研究,成功制作出了应用于流体的模具,并制造成了微流控芯片。     

Fabrication of high aspect ratio comb-drive actuator using deep X-ray lithography at Indus-2

We report microfabrication of high aspect ratio comb-drive using deep X-ray lithography at Indus-2 synchrotron radiation source. Analysis shows that the comb-drive actuator of aspect ratio 32 will produce nearly 2.5 μm displacement when 100 V DC is applied. The displacement increases as the gap between the comb finger decreases. For fabrication of comb-drive, polyimide–gold X-ray mask using UV lithography is made for the first time in India. To pattern on an 800 μm thick X-ray photoresist (PMMA) exposures are performed using our deep X-ray lithography beamline (BL-07) at Indus-2. Metallization on the selective regions of the developed X-ray photoresist with comb-drive pattern was carried out by RF sputtering. Following this the comb-drive actuator of PMMA was fabricated by one-step X-ray lithography. The comb-drive can also be used as a sensor, energy harvester, resonator and filter.     

Fast patterning microstructures using inkjet printing conformal masks

The presented paper describes a novel process using inkjet printing to pattern a conformal (built-on) mask onto photoresist for further microstructure formation. The advantages of using the inkjet printing conformal mask include no Cr photomask required, suitable for non-planar substrate, scalable for large area, and extreme low cost. The ink is ejected from the inkjet print-head controlled by the inkjet system. A CAD pattern from the designer can use this process to place the pattern ink onto the photoresist substrate. A conformal mask (made of ink) was directly built-on the photoresist substrate. The dried ink thickness has to be more than 1.8 μm thick as UV absorber. Following UV exposure, development, and ink removal, the designed microstructure patterns can be realized in photoresist such as microchannels and micro-columns.     

Development of high aspect ratio X-ray parabolic compound refractive lens at Indus-2 using X-ray lithography

A synchrotron beamline dedicated to soft and deep X-ray lithography is operational on Indus-2 synchrotron source and is being used for high aspect ratio microfabrication. This X-ray lithography facility provides the access to X-ray mask fabrication, X-ray exposures and development of micro-nano structures. We report the development of planar parabolic refractive X-ray lenses in SU-8 for energy range 8–20 keV using this facility. The focussing properties of X-ray lenses were studied with synchrotron radiation in the X-ray energy range 8–20 keV on the moderate emittance machine Indus-2. A focal spot of 11 μm at 15.9 keV is obtained with a gain of 18.     

Inexpensive, quickly producable X-ray mask for LIGA

 The capability to produce X-ray masks inexpensively and rapidly is expected to greatly enhance the commercial appeal of the LIGA process. This paper presents a process to fabricate X-ray masks both inexpensively (under $1000) and rapidly (within a few days). The process involves one UV lithography step and eliminates the need for an intermediate X-ray mask. The X-ray mask produced by this process consists of a 125 μm thick graphite membrane that supports a gold-on-nickel absorber pattern. The thickness of the absorber structures is great enough to supply sufficient contrast even when radiation sources with high characteristic photon energies up to 40 keV are utilized and/or when deep exposures are desired. The mask fabrication process is initiated by spin coating 30–50 μm of SU-8 directly on a graphite membrane. The SU-8 is then patterned using a UV mask. Gold-on-nickel absorber structures are electroplated directly onto the SU-8 covered graphite. Once the remaining SU-8 is removed, attaching the graphite membrane to a frame completes the mask. To test the performance of the mask, a nickel mold insert was fabricated. A sheet of PMMA 500 μm in thickness was bonded to a nickel substrate, then exposed to X-rays through the mask, and developed. Electroplating nickel into the patterned PMMA sheet produced a mold insert. SEM pictures taken of the SU-8, the X-ray mask, and the mold insert are shown. This method of rapidly producing an inexpensive X-ray mask for LIGA resulted in a mold insert with smooth, vertical sidewalls whose dimensions were within two micrometers of the UV mask dimensions. Received: 12 December 1998/Accepted: 2 February 1999