Thick Photoresist Layers Research Articles

Low-cost fabrication of microlasers based on polymeric micropedestals.

Our current work exploits direct laser writing (DLW) and low one-photon absorption (LOPA) in a low-cost three-dimensional optical fabrication system designed to print micrometric polymeric structures. Micropedestals were obtained by focusing a laser beam on a photoresist layer deposited on a silica glass substrate. Subsequent coating with rhodamine 6G dye allows these pedestals to function as microlasers upon optical excitation at 532 nm. Our microlasers, with a diameter of ∼53µm and a height of ∼40µm, exhibit a broad fluorescence peak in the spectral range 540-600 nm, in addition to narrow lasing peaks, exhibiting quality factors Q exceeding 2000 and a lasing threshold of ∼5µJcm-2. The observed free spectral range associated with the lasing peaks of ∼1.3nm is consistent with simulations, which we include in this paper. In addition, we present simulations for the longitudinal shift of the patterning laser spot, which occurs particularly for relatively thick photoresist layers, coupled with a large index contrast at the photoresist top surface. Such a shift could introduce errors in the resulting microfabricated structures if left unaccounted for. We hope that our work will contribute to the development of microlasers for various photonic applications, particularly if dimensions can be reduced, for on-chip optical communications and data processing.

Modeling of the Formation of Steep Portions of a Piecewise Continuous Profile in a One-Step Technology for Fabricating Diffractive Optical Elements using Oblique Laser Beams

This paper presents the results of modeling the most important component of a one-step technology of fabricating kinoform lenses in thick photoresist layers with increased diffraction efficiency due to the formation of backward slopes of Fresnel zones with a steepness of up to 90° using oblique laser beams. It is shown that this method of fabricating volume structures makes it possible to significantly increase the steepness of backward slopes of the zones in low-contrast photoresists (k = 3) and to form practically vertical slopes in moderate-contrast photoresists (k = 10).

Nearly Aberration-Free Multiphoton Polymerization into Thick Photoresist Layers.

In the era of lab-on-chip (LOC) devices, two-photon polymerization (TPP) is gaining more and more interest due to its capability of producing micrometer-sized 3D structures. With TPP, one may integrate functional structures into microfluidic systems by polymerizing them directly inside microchannels. When the feature of sub-micrometer size is a requirement, it is necessary to use high numerical aperture (NA) oil-immersion objectives that are optimized to work close to the glass substrate-photoresist interface. Further away from the substrate, that is, a few tens of micrometers into the photoresist, the focused beam undergoes focal spot elongation and focal position shift. These effects may eventually reduce the quality of the polymerized structures; therefore, it is desirable to eliminate them. We introduce a method that can highly improve the quality of structures polymerized tens of micrometers away from the substrate-photoresist interface by an oil-immersion, high NA objective. A spatial light-modulator is used to pre-compensate the phase-front distortion introduced by the interfacial refractive index jump on the strongly converging beam.

Height Uniformity of Micro-Bumps Electroplated on Thin Cu Seed Layers

The emerging 3-dimensional integration relies on components such as through-silicon vias (TSVs) and micro-bumps for vertical stacking [1, 2]. In a typical micro-bump fabrication process, a thin Cu seed layer is coated with a thick layer of photoresist. After exposure and development, cylindrical openings are formed in the photoresist layer. Depending on the application, bumps with single or multiple metal layers, one of them typically being Cu, are deposited. The removal of photoresist is followed by wet etch of the Cu seed, revealing individual micro-bumps. While Cu in the ‘body’ of the micro-bump is deposited electrochemically, the Cu seed layer is typically fabricated using physical vapor deposition (PVD). As the plated Cu etches up to 30% faster than PVD Cu [3], the Cu part of the bumps may suffer from significant radial etch before the seed is removed completely. Figure 1 shows the post-etch SEM image of a micro-bump with Cu-Ni-Sn metal stack. For 4 µm micro-bumps plated on a 200 nm Cu seed, the contact area with the pad will be reduced by about 25 % after seed etch. In order to minimize the ‘under-etch’ of the micro-bumps, the thickness of the Cu seed layer should be reduced. However, the reduction of Cu seed layer thickness leads to the well-known ‘terminal effect’ on the wafer scale [4]. Figure 2 illustrates the cross-section of a plating cell. Due to the potential drop along the radial direction, the plating current density decreases from the wafer edge to the wafer center, resulting in the micro-bump height decreasing from the wafer edge to the wafer center. In this study a quick and cost-effective approach was developed to evaluate the micro-bump height uniformity on wafer level. We studied dependency of micro-bump height uniformity on Cu seed thickness using 300 mm wafers and demonstrated the impact of terminal effect on height variation along the radial direction. By using only the experimentally measured i-ηcurve for the kinetics, numerical simulation of the wafer-scale plating uniformity was significantly simplified. As the proposed methodology required no explicit knowledge of the plating bath composition and only involved coupon-scale experiments, it provided a fast and cost-effective way to evaluate the effectiveness of proprietary plating chemistries with respect to wafer-scale height uniformity of micro-bumps. The usefulness of the proposed approach was supported by the good match between simulation results and the corresponding experimental measurement. Although the simulation capability was only demonstrated for Cu, the same approach can be applied to other layers deposited for micro-bumps. The overall uniformity can be readily calculated through summation of all the layers involved. The simulation program can also be modified to address different profile control approaches such as additional current collectors or anodes with modified geometry.

Three Dimensional Sculpturing of Vertical Nanowire Arrays by Conventional Photolithography.

Ordered nanoarchitectures have attracted an intense research interest recently because of their promising device applications. They are always fabricated by self-assembling building blocks such as nanowires, nanodots. This kind of bottom up approaches is limited in poor control over height, lateral resolution, aspect ratio, and patterning. Here, we break these limits and realize 3D sculpturing of vertical ZnO nanowire arrays (NAs) based on the conventional photolithography approach. These are achieved by immersing nanowire NAs in thick photoresist (PR) layers, which enable the cutting and patterning of ZnO NAs as well as the tailoring of NAs. Our strategy of 3D sculpturing of NAs promisingly paves the way for designing novel NAs-based nanoarchitectures.

Silicon/SU8 multi-electrode micro-needle for in vivo neurochemical monitoring.

Simultaneous monitoring of glucose and lactate is an important challenge for understanding brain energetics in physiological or pathological states. We demonstrate here a versatile method based on a minimally invasive single implantation in the rat brain. A silicon/SU8-polymer multi-sensing needle-shaped biosensor, was fabricated and tested. The multi-electrode array design comprises three platinum planar microelectrodes with a surface area of 40 × 200 µm(2) and a spacing of 200 µm, which were micromachined on a single 3mm long micro-needle having a 100 × 50 µm(2) cross-section for reduced tissue damage during implantation. Platinum micro-electrodes were aligned at the bottom of micro-wells obtained by photolithography on a SU8 photoresist layer. After clean room processing, each micro-electrode was functionalized inside the micro-wells by means of a micro-dispensing device, either with glucose oxidase or with lactate oxidase, which were cross-linked on the platinum electrodes. The third electrode covered with Bovine Serum Albumin (BSA) was used for the control of non-specific currents. The thick SU8 photoresist layer has revealed excellent electrical insulation of the micro-electrodes and between interconnection lines, and ensured a precise localization and packaging of the sensing enzymes on platinum micro-electrodes. During in vitro calibration with concentrations of analytes in the mM range, the micro-wells patterned in the SU8 photoresist proved to be highly effective in eliminating cross-talk signals, caused by H2O2 diffusion from closely spaced micro-electrodes. Moreover, our biosensor was successfully assayed in the rat cortex for simultaneous monitoring of both glucose and lactate during insulin and glucose administration.

Wet Chemical Processing with Megasonics Assist for the Removal of Bumping Process Photomasks

A new generation of negative tone resists by TOK, JSR, and Dow Chemical is gaining momentum in advanced packaging applications. Resist thickness requirements are actually increasing to the 50-100 um range as Cu pillars are adopted to accommodate the tighter pitches required in advanced packaging. In order to form pillars the resist must be thicker to contain the entire bump structure. Thick, negative tone resists are more transparent to exposure light wavelengths than positive tone resists and can be exposed by the lithography process much faster (~1 x 104 cross-linking chemical events are driven by 1 photochemical event vs. 100 – 1000 for positive tone resist).1 Therefore exposure times can be shorter, post-exposure bake steps can be shorter, and delay before or after exposure is not necessary, saving photolithography time and CoO. And, due to more complete cross-linking throughout the resist, the resist mask profiles are truer (from Flack et.al, “A Comparison of New Thick Photoresists for Solder Bumping”, SPIE 2005).1 The major drawback to negative tone resists is that the solvent strip times of the highly cross-linked resist masks are much longer than for positive tone. Flack et.al. noted resist strip times of 5 minutes for two positive resists used in their experiments vs. 50 minutes for the AZ-100nXT negative tone resist, using AZ400T at 80°C (most likely in an immersion tool). Long strip times or special stripping requirements are noted on the data sheets of the other majority suppliers of negative resists as well. Akrion engineers have developed a novel, single-wafer, negative PR strip processes using organic solvents plus Goldfinger megasonics, to provide 30 - 70% reductions in process times and the associated chemical consumption when compared to processes that do not utilize a megasonics assist. The multi-step process can be accomplished in a single process chamber, with each step optimized for time and temperature based on the resist thickness and solvent stripping chemistry used. In the Akrion process, a period of solvent exposure at low wafer spin speed is used initially to begin swelling and dissolving the thick photoresist layer. This is followed by a second solvent exposure step using aggressive frontside megasonic energy to promote polymer chain scission throughout the bulk of the photoresist layer. Following this step A DI water rinse and spin dry step are sufficient to completely remove the solvent, dissolved photoresist and any polymer residues. This paper reviews the development of the Akrion process and several examples of improvement in stripping time and results from work with customers.

Fabrication of nanopatterned metal sheet using photolithography and electroplating

This paper describes a technique for fabricating submicron nickel (Ni) structures on flexible metal substrate. Conventional photolithography and electroplating techniques are used for simple and cost-effective fabrication of size-tunable Ni nanostructures. In the photolithography process, to separate the substrate and metal layer, a residual layer was intentionally preserved when patterning the thick photoresist layer. Then, the Ni seed layer was deposited on the photoresist pattern. By applying pulse electroplating to the seed layer, a Ni metal layer was formed on the seed layer. As a result, a nanopatterned Ni metal sheet was easily obtained using the photoresist strip process. Then, the Ni nanopattern was transferred onto a flexible polycarbonate substrate. In addition, various energy-controlled exposures of the resist were demonstrated for fabricating size-tunable Ni nanostructures, by simply removing the intentionally induced residual layer of the resist pattern. This research is adaptable for the fabrication of various sensor applications and energy storage devices.

Rotary microelectromechanical system comb-drive actuator with smooth sidewall for photonic applications

Rotary comb-drive electrostatic actuators with smooth vertical sidewalls were designed and fabricated for photonic applications. Silicon on insulator wafers with device layer thicknesses of up to 100 μm and oxide layers of 1 to 2 μm were used. Structures were fabricated through bulk micromachining of the handle layer, followed by removal of the buried oxide. Deep reactive ion etching (DRIE) of the device layer defined the comb structure with simultaneous release of the movable arm. Thick photoresist and oxide masking layers were used for the high aspect ratio deep etches. Notching effects or depth variations of device layers were not observed. Etch/passivation cycles in DRIE were optimized to obtain smooth sidewalls of less than 5 nm roughness. Traces of etch scallops were confined only to the first few microns of the top and bottom edges of the reflecting sidewall. A movable arm length of 2 mm with a maximum rotation of 2.8 deg was achieved. The edge deflection was over 100 μm at 100 V. The fabricated structure provides a long reflecting sidewall, that is accessible for hybrid integration, in three dimensional space. The fabricated structures can be reliably operated in the kilohertz range.

Rapid fabrication of 3D terahertz split ring resonator arrays by novel single-shot direct write focused proximity field nanopatterning

For the next generation of phoXonic, plasmonic, opto-mechanical and microfluidic devices, the capability to create 3D microstructures is highly desirable. Fabrication of such structures by conventional top-down techniques generally requires multiple time-consuming steps and is limited in the ability to define features spanning multiple layers at prescribed angles. 3D direct write lithography (3DDW) has the capability to draw nearly arbitrary structures, but is an inherently slow serial writing process. Here we present a method, denoted focused proximity field nanopatterning (FPnP), that combines 3DDW with single or multiphoton interference lithography (IL). By exposing a thick photoresist layer having a phase mask pattern imprinted on its surface with a tightly focused laser beam, we produce locally unique complex structures. The morphology can be varied based on beam and mask parameters. Patterns may be written rapidly in a single shot mode with arbitrary positions defined by the direct write, thus exploiting the control of 3DDW with the enhanced speed of phase mask IL. Here we show the ability for this technique to rapidly produce arrays of "stand-up" far IR resonators.

The Fabrication of THz Emitting Mesas by Reactive Ion-Beam Etching of Superconducting Bi2212 with Multilayer Masks

Generation of powerful THz radiation from intrinsic Josephson Junctions (IJJs) of Bi2Sr2CaCu2O8+δ (Bi2212) may require mesas with large lateral dimensions. However, there are difficulties in fabrication of perfect rectangular mesas. The lateral angles of mesas should be close to 90 degrees to obtain IJJs with same planar dimensions for synchronization of IJJs. We patterned Ta/photoresist and photoresist/Ta/photoresist masks on Bi2212 and used selective ion etching to overcome the thick photoresist layer shading on the lateral dimension of mesa during the ion-beam etching. The reactive ion-beam etchings have been done with ion beams of Ar, N2 and O2, and we have obtained mesas about 1 μm with lateral angle of approximately 50 to 75°, which is better than the mesas fabricated with photoresist mask.

Fabrication of Photonic/Microfluidic Integrated Devices Using an Epoxy Photoresist

AbstractUsing a single layer of SU‐8 photoresist to fabricate optical waveguide cores and microfluidic channels on Pyrex glass is an ideal way to achieve photonic/microfluidic integration on a single chip. To address the problem of poor bonding, a thin nanoscale intermediate polymer layer was applied to reduce the stress generated from the material processing while maintaining strong adhesion between the patterning polymer layer and Pyrex. It was found that a 186–600 nm thick intermediate layer of a specialty epoxy photoresist effectively served the purpose without deteriorating the optical performance of the involved waveguides. Quality photonic/microfluidic integrated devices with satisfied optical performance were fabricated.magnified image

Sandwiched long-period fiber grating filter based on periodic SU8-thick photoresist technique

A loss-tunable sandwiched long-period fiber grating (SLPFG) filter is proposed in this Letter. This SLPFG utilized thick SU8 photoresist layers to induce refractive index modulations, and ultrahigh period precision was hence achieved. The external force required to tune this SLPFG filter is about 0.3942 N with the deepest loss attenuation 31.29 dB at 1534 nm wavelength. This process not only improves the overall performance of traditional long period fiber grating but also cuts down the facility cost by 50%. The proposed SLPFG has great potential for low-cost and compact force transducer applications.

Photolithography of thick photoresist coating for electrically controlled liquid crystal photonic bandgap fibre devices

Thick photoresist coating for electrode patterning in an anisotropically etched V-groove is investigated for electrically controlled liquid crystal photonic bandgap fibre devices. The photoresist step coverage at the convex corners is compared with and without soft baking after photoresist spin coating. Two-step UV exposure is applied to achieve a complete exposure for the thick photoresist layer at the bottom of the V-groove, and minimise the reduction in resolution and image distortion. The resolution reduction of the different open window width for electrode pattern transfer is also experimentally found.

Process Development for CMOS-MEMS Sensors With Robust Electrically Isolated Bulk Silicon Microstructures

This paper presents a deep reactive-ion etching (DRIE)-based post-CMOS micromachining process that provides robust electrically isolated single-crystal silicon (SCS) microstructures for integrated inertial sensors. Several process issues arise from previously reported three-axis CMOS microelectromechanical system (MEMS) accelerometers, including sidewall contaminations of SCS microstructures in plasma etch and a severe silicon undercut caused by overheating of suspended microstructures. Solutions to these issues have been found and are discussed in detail in this paper. In particular, a lumped-element model is developed to estimate the temperature rise on suspended microstructures in a silicon DRIE process. Based on the thermal modeling and experiments, a thick photoresist layer has been used as a thermal path to avoid the severe silicon undercut. The sidewall contamination problem is also eliminated using the modified CMOS-MEMS process. A three-axis accelerometer with a low-noise, low-power on-chip amplifier has been successfully fabricated using the new process. Footing effect was observed on the backside of the sensor microstructure, but it has little effect on the structural integrity and sensitivity of the sensor.

Infiltration and Inversion of Holographically Defined Polymer Photonic Crystal Templates by Atomic Layer Deposition

The demonstration of a practical technology for 3D optical microfabrication is a vital step in the development of photonic-crystal-based optical signal processing. [1] However, the extension of the optical methods that dominate integrated electronic circuit fabrication to three dimensions is a formidable materials-processing challenge: such a process must be capable not only of sub-micrometer pattern definition in three dimensions, but also of the transfer of this pattern into a homogeneous dielectric with an appropriately high refractive index. In a companion paper, [2] we show that two optical methods, holographic lithography [3] and direct two-photon laser writing, [4–6] can be combined to create a rapid and flexible method for the definition of photonic crystal device structures in photoresist. In this communication, we report a further essential step towards the creation of devices operating within a full photonic bandgap: we have used atomic layer deposition (ALD), itself an established semiconductor processing technique, to create high-index TiO2 inverted replicas of holographically defined photonic crystals, followed by removal of the polymeric template by plasma etching. A range of techniques for 3D optical lithography has been demonstrated. A 3D photonic crystal structure can be written by holographic lithography, [3] which makes use of a periodic interference pattern generated by a multiple-beam interferometer to expose a thick layer of photoresist. 3D microstructures, both periodic and aperiodic, can also be generated by point-by-point exposure of the resist by two-photon absorption at a laser focus. [4–7] Two-photon laser writing is a serial process; point-by-point fabrication of a 3D photonic crystal is necessarily slower than holographic lithography, which is capable of defining the entire periodic structure in a single laser pulse. [3] The two techniques are complementary: two-photon laser writing can be used to modify a holographic exposure. [8] We have shown that, by imaging the distribution of photochemical change induced by holographic exposure, it is possible to align a subsequent two-photon exposure with the 3D photonic crystal lattice to achieve the precise registration that is required of a device structure embedded in a 3D photonic crystal. [2] This hybrid technique is rapid and flexible, but the polymeric resists used for 3D microfabrication have refractive indices n in the range 1.4–1.6, which is too low for most device applications. Devices based on waveguides and microcavities embedded within a photonic crystal [1] are designed to operate at frequencies within a complete (omnidirectional) photonic bandgap in order to suppress radiative loss; [9] to create a complete photonic bandgap, even in an optimized air-dielectric structure, a refractive contrast of at least 1.9 is necessary. [10–12]

Microfabricated microneedle with porous tip for drug delivery

This paper presents a novel approach to fabrication of a silicon microneedle array with porous tips. Dry etching technology with SF6/O2 gas by STS's inductively coupled plasma (ICP) etch tool was used to achieve the pyramidal needle structure. A thin silicon nitride layer was deposited after a thick photoresist layer was coated and reflowed at 120 °C. The silicon nitride layer and residual photoresist on the tips of the pyramidal structures were removed using reactive ion etching (RIE). Electrochemical etching in MeCN/HF was carried out to generate porous silicon on the tips of the microneedles. The fabricated microneedle array has potential applications in drug delivery, since the porous tips can be loaded with a high molecular weight drug. Analytic solutions to the critical loadings of the fabricated microneedle structure are also presented. The variations of the square cross-section were expressed as a function of the axial coordinate to analyze the bending normal stress and critical buckling loading. This analytic method can also be used for other microneedle structures with different cross-sections.

Femtoliter to Picoliter Droplet Generation for Organic Polymer Deposition Using Single Reservoir Ejector Arrays

Direct deposition of photoresist and other spin-on materials, such as low-k and high-k dielectrics, has the potential to reduce waste as well as production costs. A new design of acoustically actuated two-dimensional (2-D) micromachined droplet ejector arrays can eject various solvents and other fluids ranging from femtoliter to picoliter droplet volumes. These ejectors do not harm fluids that are heat or pressure sensitive. Moreover, they are chemically compatible with the materials used in integrated circuit manufacturing. Therefore, they can be used for benign deposition of photoresist and other spin-on materials, such as low-k and high-k dielectrics. A vibrating circular Si/sub x/N/sub y/ thin-film membrane with an orifice at the center forms the unit cell of a 2-D ejector array. Initially, one side of the membrane is loaded with the ejection fluid. Then, ultrasonic waves generated by a piezoelectric transducer force the membranes to displace at resonance. As a result of this actuation, droplets are ejected through the membrane orifice. We ejected water at 1.06 MHz, isopropanol at 1.14 MHz, ethyl alcohol at 1.06 MHz, and acetone at 1.04 MHz from a 20/spl times/20 single reservoir 2-D micromachined array with 160 /spl mu/m in diameter Si/sub x/N/sub y/ membranes and 10 /spl mu/m in diameter orifices. The performance of single reservoir flextensional membrane-based ejector arrays was compared to flextensional membrane-based ejector arrays with reservoirs. A 50% decrease in the required power per ejected droplet and a reduced design complexity were demonstrated over the 2-D micromachined arrays with individual reservoirs. In addition, we deposited Shipley SPR 3612 photoresist at 1.12 MHz in a dry lab environment. No spinning was done after deposition. We covered a 2/spl times/2-mm area on a wafer with a 5.5-/spl mu/m thick photoresist layer. The maximum thickness variation over the area was 0.4 /spl mu/m. Moreover, we present a directly written 1.6-/spl mu/m thick 900-/spl mu/m wide and 8-mm long homogeneous photoresist line. The photoresist thickness variation along the line was 0.2 and 0.4 /spl mu/m in vertical and horizontal directions, respectively.

Atmospheric pressure operation of a field emission diode based on self-assembled silicon nanostructures

We report on the field emission from self-assembled silicon nanostructures fabricated on n-type silicon (100) substrates using electron beam annealing. Arrays of nanostructures with an average height of 8nm were formed by substrate annealing at 1100°C for 15s. Simple field emission diode structures were fabricated using these nanostructured silicon substrates as cathodes separated from an aluminium anode by a 1μm thick layer of photoresist. The dielectric leakage current was less than 0.4nA which was found to be a negligible component of the field emitted current flowing through the diode. Following conditioning, the Si nanostructure field emission characteristics become stable and reproducible with Fowler–Nordheim tunneling occurring for fields as low as 3.2Vμm−1. At higher fields, current saturation effects are observed with current-field characteristics typical of space charge limited conduction. The low voltage operation of this device results in suppression of ionizing electron–molecule collisions and the diode has thus been shown to work well at atmospheric pressure.

Simulation of deep UV lithography with SU-8 resist by using 365 nm light source

The deep lithography of thick resist layer is the primary step of LIGA technology. UV Proximity lithography, which is used to fabricate high aspect ratio MEMS normally, is investigated in this paper. The light intensity distribution in the thick photoresist layer mainly depends on the diffraction produced by the gap between mask and wafer in proximity lithography. Fresnel diffraction model is used to simulate lithography process normally, which the thin photoresist layer is used in the lithography process. But it is not accurate in deep lithography process. A correction to the Fresnel diffraction theory is used to simulate the lithography process depending on the scalar diffraction theory of light propagation in this paper. The difference of this two models is given in this paper. The simulation results show that the corrected model can obtain more accurate simulation results than the Fresnel diffraction model. The experiment results and the theory analysis both indicate that: the structure contrast decreases with the increase of the film thickness. The smaller the structure linewidth is, the faster the structure contrast decreases. The linewidth of microstructure is not equivalent from the top to the bottom, broaden in the middle part normally. The theory simulation gives the quantitative analysis.