Releasing high aspect ratio SU-8 microstructures using AZ photoresist as a sacrificial layer on metallized Si substrates

This paper presents a successful method for releasing high aspect ratio SU-8 micro-structures by the use of positive photoresist (AZ 4620) as sacrificial layer. The AZ 4620 photoresist sacrificial layer was dissolved by the SU-8 developer (propylene glycol monomethyl ether acetate). Thus, this process reduces the need for complex microfabrication steps and equipments which are otherwise required in traditional methods using metal sacrificial layers. The current method is both cost-effective and time-effective because no additional releasing method or material is needed to remove the fabricated SU-8 structures. Further, the influence of surface energy on the adhesion between Si and SU-8 was demonstrated and metallic thin layer coating on Si was employed to further reduce the lift-off duration. The results obtained showed that the duration for lift-off of SU-8 structures from metal (Al) coated Si substrate is much lower (approximately 90 % time saving) and the surface morphology of the released structures has lesser micropore concentration compared to the process employing bare Si as the substrate. In both processes AZ 4620 was the sacrificial layer whereas the metalized Si substrate could be re-used.     

Micromolding fabrication of microresistors with a composite of carbon nanotubes and SU-8 polymer and the application in Wilkinson power divider

This paper reports a new method to fabricate microresistors for applications in micro-electromechanical systems. The fabrication is based on ultraviolet (UV) lithography and micromolding replication. A master mold was first made using UV lithography of a negative-tune photoresist, SU-8. The SU-8 master mold was then used to produce a polydimethylsiloxane (PDMS) intermediate mold. The PDMS intermediate mold was used to replicate the microresistor using composite mixture of multi-walled carbon nanotubes (MWCNTs) and SU-8 photoresist. The replicated microresistors were thermally cured at 95 °C for 6 h. The performances of the replicated micro-resistors were then tested. The experiment had also been conducted using the micro-resistors as isolation resistance in a Wilkinson power divider to test their functionality. Because passive communication components such as Wilkinson power dividers can potentially be made with lithography/micromolding technologies and using polymer as structural material, microresistors as reported in this paper may potentially be suitable for using in such applications.     

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.     

Aqueous base developable: easy stripping, high aspect ratio negative photoresist for optical and proton beam lithography

A variety of different photo resists are used for the fabrication of polymer and metal high aspect ratio structures. Among them SU-8, a chemically amplified negative tone photoresist is the mostly used. However, after processing the finished resist pattern (SU-8) is hardly removed from the substrate. In the present work the formulation and process optimization of a negative tone chemically amplified photoresist (TADEP) is presented. TADEP resist owns two advantages: the dissolution of the uncrosslinked areas in IC standard aqueous developers and the easy stripping in acetone by the assistance of ultrasonic bath. The TADEP resist is successfully applied for the fabrication of polymer and metal structures, after electroplating and stripping and also in the case of Proton Beam Writing.     

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.     

Polymeric (SU-8) optical microscanner driven by electrostatic actuation

A torsional micromechanical scanner was fabricated using photosensitive polymer (SU-8). The proposed polymer-based optical microscanner with reduced torsional stiffness offers a new approach to increase scanning angles. The scanner consists of two parts; the top layer (micro mirror and electrodes) and the bottom layer (anchors and electrodes). The SU-8 scanner is actuated by electrostatic force generated by gap-closing electrodes. For the fabricated optical scanner with the mirror size of 3 × 3 mm², the experimentally obtained scanning angles were 0.43° for 60 Hz (non-resonant) and 1.54° for 1.13 kHz (resonant) at the input voltage of 160 V. This paper also proposes a simple and new fabrication method, which can effectively control the stiffness of the torsional springs by molding SU-8 photoresist through V-groove on the silicon substrate, thereby increasing the scanning angles.     

Electroplated compliant metal microactuators with small feature sizes using a removable SU-8 mould

In the work presented in this article a fabrication process for high aspect ratio metal microstructures has been developed using photoresist EPON SU-8. A smallest feature size of 2 μm with near vertical sidewalls has been achieved on a routine basis. The SU-8 photoresist has been used as mould for electroplating high aspect ratio metal micro actuators. A removal process has also been developed. The removal of the SU-8 mould after plating is done in a plasma asher using an oxygen or oxygen/fluorine plasma with plasma power as low as 200 W. To demonstrate the fabrication process a 10 μm thick nickel, electro-thermal micro actuator has been fabricated.     

Process research of high aspect ratio microstructure using SU-8 resist

SU-8 is a negative, epoxy type, near-UV photoresist. This resist has been specifically developed for ultrathick, high-aspect-ratio MEMS-type applications using standard lithography equipment. However, in practice, SU-8 has shown to be very sensitive to process parameter variation. The orthogonal array was used in our experiments in order to improve the lithography quality and analyze the interaction among the parameters. The analyses show that the interaction between the exposure dose and post exposure bake has played an important role in adhesion between SU-8 resist and the substrate. The proposed process conditions are given. The output structure has straight sidewall profile, fine line and good space resolution. The aspect ratio is larger than 20. Moreover, several metallic films are used as substrates. The Ti film with oxidation treatment was found to have the strongest adhesion to the resist. The result will help to open possibilities for low-cost LIGA-type process for MEMS applications.     

Angle effect of ultrasonic agitation on the development of thick JSR THB-430N negative UV photoresist

 A novel method by using adjusted ultrasonic agitation to improve the developing depth, developing time, surface roughness, and undercut problem of thick JSR-430N negative UV photoresist is proposed. This method has been successfully employed to fabricate ultra-thick microstructures of thickness more than 1.4 mm and aspect ratio at least of 5 by JSR THB-430N negative UV photoresist. With the improved ultrasonic developing procedure, the resist can potentially be a replacement of SU-8 resist for the application of high aspect ratio plating mold, due to its good stripping property [1]. The power and angle of ultrasonic agitation have also been studied and characterized. The development of thick JSR-430N resist under different ultrasonic agitation angles has been verified to have different effects on developing time, resist sidewall profile, and surface roughness. Received: 10 August 2001/Accepted: 24 September 2001     

Process variability in surface roughening of SU-8 by oxygen plasma

This study investigates variability in the topography of SU-8 photoresist subject to surface roughening by oxygen plasma treatment. Surface roughness (expressed as root mean square deviation from the mean) under the range of experimental conditions varied from 8 nm for an untreated baseline to as high as 472 nm. At 200 W RF-power and 200 mTorr chamber pressure, the mean surface roughness was 295 nm with standard deviation less than 10 nm across the specimen and 15 nm across the plasma chamber. The standard deviation in surface roughness at higher power and pressure combinations including 500 W and 800 mTorr was as high as 80 nm, with mean surface roughness less than 200 nm. Replicate runs under identical conditions revealed that run-to-run repeatability can be compromised by chamber conditions, evidenced by second runs having higher standard deviation by nearly 20 % over first runs without intermediate chamber cleaning.     

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 chamber for piezo inkjet printhead by SU-8 photolithography technology and bonding method

The chamber is an important part of the inkjet printhead. However, the present fabrication methods of chamber suffer from a low alignment resolution between nozzle plates and piezoelectric structure and residual SU-8 removing problems during chamber fabricating process. In this paper, a SU-8 chamber was fabricated by using ultraviolet (UV) photolithography and SU-8 thermal bonding method. By this method, the infilling problem of the chamber during thermal bonding process was solved, and low alignment resolution problem of conventional UV exposure system during assembly process was avoided. The thickness of the SU-8 nozzle plate was optimized, and the influence of bonding parameters on the deformation of chamber was analyzed. The simulation results show that the optimal thickness of the SU-8 nozzle plate is 40 μm and the optimal bonding parameters are bonding temperature of 50 °C, bonding pressure of 160 kPa and bonding time of 6 min. The tensile test results show the bonding strength of the SU-8 chamber is 2.1 MPa by using the optimized bonding parameter.     

A wireless powered fully integrated SU-8-based implantable LC transponder

In this work, we report a SU-8-based fully integrated miniaturized inductively powered LC transponder for generic implantable wireless sensor applications. It consists of a 1 mm diameter octagonal spiral inductor and a micro fabricated MIM (metal insulator metal) capacitor. Polyvinylidene Fluoride–Trifluoroethylene (PVDF–TrFE) copolymer is applied as a dielectric material for the capacitor fabrication due to its high dielectric constant. The 1 mm diameter, 154 nH spiral inductor is built on top of the capacitor. The capacitor and the inductor are in parallel connection through SU-8 via holes. SU-8 is used as a packaging material due to its biocompatibility, and also it serves as an insulator between the capacitor and the spiral inductor. The operating frequencies of the LC tanks are decided by the sizes of the capacitors (45 × 45, 55 × 55, 95 × 95 and 100 × 100 μm), and measured operating frequency range is from 385 to 485 MHz. The fabricated LC tanks are held to the power transmitting coil coaxially at distances of 2, 5, 7 and 10 mm, and rectified induced voltage at the LC tank is 8.5 V with 29 dBm input power at a 5 mm distance.     

High aspect ratio micromolds for the electroplating of micro electro discharge machining tools

A new approach using micro systems technology for the fabrication of micro electro discharge machining tool electrodes using a combination of positive and negative photoresist fabricated with near UV-lithography as high aspect ratio micromolds was developed. Micromolds with heights varying from 200 to 650 μm and an aspect ratio of up to 26:1 were fabricated using SU-8?, a negative photoresist, as a micromold for the electrodeposition of the micro tool electrodes. Besides serving as a micromold, SU-8? is also used as an insulator to protect the sidewalls of the tool electrodes from corrosions. The fabrication of the SU-8? micromold was optimized to avoid cracks and delaminations. In the experiments, electrodes made of Cu, WCu, and CoFe were deposited into these High Aspect Ratio Micro Structure Technology (HARMST) micromolds. Furthermore, the process technique for electroplating in deep micromolds is discussed in this paper.     

Low cost method for hot embossing of microstructures on PMMA by SU-8 masters

Polymer based microfabrication technologies are used extremely in Bio-MEMS, especially in Microfluidic devices in recent years. In this paper, a novel method for fabrication of microstructures on a polymeric material using hot embossing lithography process is presented. The proposed method involves usage of low cost materials and procedure with respect to previous methods and can be processed in a short time. The master is made from SU-8 on an inexpensive glass substrate which is patterned by standard lithography. The embossing pressure can be increased in our master as the glass substrate used in this paper is more robust than Silicon. Master robustness and SU-8 to glass adhesion is optimized by some substrate pretreatments and SU-8 baking time and temperatures. Microchannels are replicated on a Polymethylmetacrylate (PMMA) stamp which is a plexiglass sheet with thickness of 1 mm. Significant embossing parameters including temperature, pressure and time are discussed and optimum values are determined. Microchannels are imprinted by depth of 50 μm and minimum width of 15 μm and aspect ratio more than 3. The microchannels are sealed by a PMMA cap using thermal annealing bonding.     

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.

     

Indirect removal of SU-8 photoresist using PDMS technique

SU-8 photoresist shows superior images for thick film lithography and has been utilized as an electroplating mold. However, crosslinked SU-8 is difficult to remove reliably from high-aspect-ratio microstructures (HARMs) without damage or alteration to the electroplated metal. In this paper, an indirect SU-8 removal method is proposed. Instead of directly using SU-8 microstructure as the electroplating mold, a polydimethysiloxane (PDMS) replica is employed. The metallic micromold insert obtained through this method can be easily peeled off from the PDMS replica, meanwhile with high resolution and smooth surfaces.     

SU-8胶微结构的尺寸公差研究

对SU-8胶微结构的尺寸及其公差进行了定量研究.在考虑了SU-8的吸收系数和折射系数对紫外光刻尺寸精度影响的基础上,根据菲涅耳衍射理论建立了紫外曝光改进模型和尺寸公差模型,对SU-8微结构的尺寸及其公差进行数值模拟.以硅为基底,进行了SU-8胶紫外光刻的实验研究.实验中掩模的特征宽度分别取50 μm、100μm、200μm和400 μm,SU-8胶表面的曝光剂量分别取400mJ/cm2和800mJ/cm2,测量了SU-8胶微结构的顶部线宽、底部线宽和SU-8胶的厚度,数值模拟结果与实验结果基本吻合.可以用本文的模型来预测SU-8微结构的尺寸及其公差.     

Micro liquid rotor operated by surface-acoustic-wave

A micro liquid rotor operated by surface acoustic waves (SAWs) was proposed and fabricated. The liquid rotor has two interdigital transducers (IDTs) that generate SAWs, in order to increase efficiency of rotating the liquid. The IDTs were fabricated by patterning Al/Cr on a LiNbO₃ substrate. Moreover, the liquid rotor has a cylindrical liquid pool of 4 mm diameter and 200 μm depth. A sidewall of the liquid pool was made of epoxy-based negative photoresist on the substrate. As characteristics of the liquid rotor, we investigated relationships between electric power applied to the IDTs and angular velocity of the liquid or its temperature. Through experimentation, it was shown that the angular velocity and the temperature increase with increasing the electric power. When the fabricated liquid rotor was applied electric power of 2 W, the liquid of 5 μL rotated at 330 rad/s (3,100 rpm) and its temperature reached 65°C.     

Fabrication of polymer micro-tip by optical lensing technique

Fabrication of polymer micro-tips using SU-8 negative photoresist for bio-applications is reported. The SU-8 processing technology and isotropic glass etching process have been developed and utilized to fabricate micro-tips on glass substrate by applying optical lensing effect during photolithography. Experimentally, micro-tips of 25?μm base diameter, ~1?μm tip diameter, and ~250?μm height, have been demonstrated.