Thick Positive Photoresist Research Articles

Experimental Study of Numerical Optimization for 3-D Microstructuring Using DMD-Based Grayscale Lithography

Digital micromirror device (DMD)-based grayscale lithography is a promising tool for 3-D photolithography of thick photoresists, because this technique provides a patterning solution for free-form 3-D microstructures. Among the numerous process parameters in DMD-based grayscale lithography, the exposure dose pattern corresponding to the grayscale mask pattern, the focal position in the photoresist, and the development time most strongly influence the final profile of the 3-D microstructure. However, finding the best combination of the three process parameters is a difficult and time-consuming task. In this paper, we propose a process optimization method for DMD-based grayscale lithography that is based on the 3-D photolithography simulation and sensitivity analysis. This optimization tool provides not only automatic process optimization for all three process parameters, i.e., the exposure dose pattern, the focal position, and the development time, but also a solution to improve fabrication accuracy for critical features by adopting a variable weight factor. Through a series of experiments, using a 20- $\mu \text{m}$ -thick positive photoresist, the validity and effectiveness of the proposed optimization method and the improvement of the fabrication accuracy were successfully demonstrated. [2015-0055]

Fabrication of a circular PDMS microchannel for constructing a three-dimensional endothelial cell layer

We describe a simple and efficient fabrication method for generating microfluidic channels with a circular cross-sectional geometry by exploiting the reflow phenomenon of a thick positive photoresist. Initial rectangular shaped positive photoresist micropatterns on a silicon wafer, which were fabricated by a conventional photolithography process, were converted into a half-circular shape by tuning the temperature to around 105 °C. Through optimization of the reflow conditions, we could obtain a perfect circular micropattern of the positive photoresist, and control the diameter in a range from 100 to 400 μm. The resultant convex half-circular photoresist was used as a template for fabricating a concave polydimethylsiloxane (PDMS) through a replica molding process, and a circular PDMS microchannel was produced by bonding two half-circular PDMS layers. A variety of channel dimensions and patterns can be easily prepared, including straight, S-curve, X-, Y-, and T-shapes to mimic an in vivo vascular network. To form an endothelial cell layer, we cultured primary human umbilical vein endothelial cells inside circular PDMS microchannels, and demonstrated successful cell adhesion, proliferation, and alignment along the channel.

Fabrication of ellipticity-controlled microlens arrays by controlling the parameters of the multiple-exposure two-beam interference technique

We demonstrate a promising method for fabrication of plastic microlens arrays (MLAs) with a controllable ellipticity and structure, by using the combination of multiple-exposure two-beam interference and plastic replication techniques. Multiple exposures of a two-beam interference pattern with a wavelength of 442 nm into a thick positive photoresist (AZ-4620) were used to form different two-dimensional periodic structures. Thanks to the developing effect of the positive photoresist, fabricated structures consisting of hemielliptical- or hemispherical-shaped concave holes were obtained. By controlling the rotation angle between different exposures, both the shape and structure of the holes varied. By adjusting the dosage ratio between different exposures, the shape of the holes was modified while the structure of the holes was unchanged. The photoresist concave microstructures were then transferred to plastic MLAs by employing replication and embossing techniques. The fabricated MLAs were characterized by a scanning electron microscope and atomic force microscope measurements. We show that the ellipticity of the microlenses can be well controlled from 0 (hemispherical) to 0.96 (hemielliptical) by changing the rotation angle or dosage ratio between the two exposures.

A new approach to integrate PLZT thin films with micro-cantilevers

In the present work, we report the preparation of PLZT thin films in pure perovskite phase by RF magnetron sputtering without external substrate heating and their integration with micro-cantilevers. The ‘lift-off’ process for patterning different layers of a micro-cantilever including PLZT, Pt/Ti and Au/Cr was employed. The basic requirement of lift-off process is that the deposition temperature should not exceed 200°C otherwise photoresist will burn out. Therefore, one of the aims of the present work was to prepare PLZT film at lower deposition temperatures, which can be subsequently annealed to form pure perovskite phase. This also strongly favours the incorporation of ‘lift-off’ process for patterning in the complete process flow. As no external substrate heating was required in the deposition of PLZT film, this objective has been successfully accomplished in the present work. The ‘lift-off’ process has been successfully adopted for patterning the composite layers of PLZT/Pt/Ti and Au/Cr using thick positive photo-resist (STR-1045). Different types of cantilever beams incorporating PLZT films have been successfully fabricated using ‘lift-off’ process and bulk micromachining technology. The proposed process can be advantageously applied for the fabrication of various MEMS devices.

Thick single-layer positive photoresist mold and poly(dimethylsiloxane) (PDMS) dry etching for the fabrication of a glass–PDMS–glass microfluidic device

Poly(dimethylsiloxane) (PDMS) is an elastomer widely used in microfluidic devices due to desirable characteristics such as its low cost and moldability. Photoresist patterned to form microfluidic features is often used as a mold, and SU-8 is commonly used. However, negative photoresists like SU-8 are difficult to remove, as compared to positive photoresists. Here, a method of forming microfluidic structures in the PDMS with an AZ9260 thick positive (Novolak-based) photoresist using standard microfabrication technique is reported. The chip consisted of a sandwich structure with glass on the top and bottom and PDMS with microfluidic channels in the middle; these structures have PDMS sidewalls, but are free of PDMS on the bottom and top. A single layer of the positive AZ9260 photoresist was spun to controllable and uniform thicknesses of up to 49 µm and used as a sacrificial mold to create PDMS microfluidic features. Reactive ion etching (RIE) with CF4 and O2 gases and a liftoff technique was used to create these features, and subsequent irreversible bonding to glass was achieved by activation in oxygen plasma by RIE. The final depths of channels were about 20–30 µm. The effect of the gas flow rate ratios on the bonding ability was investigated, and the surfaces of etched PDMS were studied under a scanning electron microscope (SEM), and the bonding strength was measured to be 0.85 MPa. The fabricated microfluidic device was tested and shown to be leak free.

Interdigitated array 3D micromesh electrodes for electrochemical sensors

This paper describes the fabrication method of an interdigitated array 3D micromesh metal structure for electrochemical sensing electrodes. The inverse-micromesh structure formed by double backside exposure of the thick positive photoresist was utilized as a mold for electroplating. Ring-shaped Au patterns laid open by overexposure of the positive photoresist were employed as a seed layer for electroplating. An electrochemical sensor with interdigitated array 3D micromesh electrodes was realized by using the combination of the double-sided Cr pattern and protecting the resist layer to achieve selective electroplating. The diameter of the inclined micropillars was about 5 µm. The height and length of the micromeshes were 20 µm and 480 µm, respectively. The gap between the micromesh electrodes was 28 µm. The number of micromesh electrodes was 68 (34 anode and 34 cathode electrodes). The performance of the interdigitated array 3D micromesh electrochemical sensor was demonstrated using the redox reaction. The current signal measured by the 3D micromesh electrodes was 7.9 times higher than that of conventional planar electrodes.

Fabrication of three-dimensional SiC-based ceramic micropatterns using a sequential micromolding-and-pyrolysis process

A sequential micromolding and pyrolysis process is presented for fabricating three-dimensional (3D) SiC-based ceramic micropatterns with a submicron scale resolution using preceramic resins, which is a promising technique for diverse applications such as tribological micro-stamps of hot embossing. Firstly, a diffuser lithography process (DLP) and a two-photon polymerization (TPP) process have been employed to create master patterns, which are utilized in the fabrication of molds. In the DLP, various hemispheric-concave master shapes were built readily by exposing UV-light onto a thick positive photoresist film through a diffuser, which randomizes the paths of incident UV-light. Alternatively, the TPP process based on two-photon polymerization was used for the creation of real 3D master patterns with a two-photon sensitive resin mixture. Subsequently, the preceramic polymer micropatterns were fabricated via a micromolding process using polydimethylsiloxane (PDMS) molds replicated from the masters. Finally, the UV-cured preceramic micropatterns were transformed into SiC-based ceramic microstructures when pyrolyzed at 800 °C under inert atmosphere.

Multilevel electroforming for the components of a microdroplet ejector by UV LIGA technology

A microdroplet ejector actuated by the shear mode PZT actuator is proposed. It consists of three electroforming components including a vibration plate, a chamber plate and a nozzle plate. The three components are fabricated by nickel electroforming technology and assembled by aligning apparatus using epoxy adhesive. The AZ-type thick positive photoresist is used as the electroforming mould, which is patterned by UV lithography. The repeat steps including the photoresist patterning and nickel electroforming make the component form a quasi 3D (or 2.5D) multilevel microstructure. The vibration plate has the feature of two different sized bulges piled on the diaphragm. Those nozzles in the nozzle plate are formed by the nickel lateral deposition when depositing over columnar photoresist moulds.

Shape-controlled, high fill-factor microlens arrays fabricated by a 3D diffuser lithography and plastic replication method

This paper describes a simple and effective method to fabricate a plastic microlens array with controllable shape and high fill-factor, which utilizes the conventional lithography and plastic replication. The only difference from conventional lithography is the insertion of a diffuser that randomizes paths of the incident ultraviolet (UV) light to form lens-like 3D latent image in a thick positive photoresist. After replication of the developed concave microlens mold onto the polydimethylsiloxane (PDMS), the focal length of the fabricated hemispherical microlens was observed to be 13-88 microm depending on the UV exposure dose. Two PDMS curing conditions were tested, where the elevated temperature of 85 masculineC resulted in smoother surface roughness of 2.6 nm in RMS value in the microlens mold. The proposed method can be extensively applied for microlens fabrication with other plastic materials due to its simplicity and versatility.

Polymer embossing tools for rapid prototyping of plastic microfluidic devices

In this paper, we describe a simple and rapid method of fabricating hot embossing tools using polydimethylsiloxane (PDMS), which are then used to rapidly fabricate microchannels in polymethylmethacrylate (PMMA). A negative photoepoxy SU-8 or thick positive photoresist AZ4620 on silicon was used for molding during PDMS casting. Fabrication time of these PDMS tools was considerably less than those using conventional techniques and remains the same regardless of the aspect ratio of features. The described approach was used to emboss microchannels in PMMA of aspect ratios up to 2, with depths from 5 to 250 µm and widths over 40 µm. The technique was also applied to fabricating orthogonal 3D microchannels using multiple lithography steps. The use of a soft tool material increased cycle time and limited the tool lifetime to approximately 20 cycles. Our successful demonstration of PDMS embossing tools presents an alternative approach for rapidly prototyping microfluidic biochips when fast processing and low cost are important and the number of samples is relatively low.

Photoresist parabolas for curved micromirrors

Microscopic parabolic surfaces have been fabricated in thick positive photoresist by exposing it using a suitably designed mask on an X5 stepper projection reduction system. The resulting cylindrical parabolic surfaces were coated with reflective material to form parabolic micromirrors. Measurement results of focal length are in agreement with the expected theoretical values. The devices form the basis for the fabrication of compound parabolic microreflectors which are very suitable for thin solar cells.

A dedicated micromachining technology for high-aspect-ratio millimetre-wave circuits

This paper presents a new technological process to implement efficient millimetre-wave passive circuits on a silicon substrate. This process associates a thick positive photoresist acting as a mould for the realization of thick conductors (several microns) with an ultra-thin dielectric membrane using only two layers (SiO 2/Si 2N 3.8) in order to reduce both dielectric and ohmic losses in the coplanar millimetre-wave circuits. Coplanar transmission lines and a band-pass filter in the 30 GHz range, featuring respectively transmission losses lower than 0.2 and 1 dB, illustrate some potentialities of this process.

A simple optical system to optimize a high depth to width aspect ratio applied to a positive photoresist lithography process

The fabrication of micro electro mechanical systems by electrodeposition inside resist moulds has provided much interest in recent years. In this paper, we propose a method to optimize the lithography process of a thick positive photoresist. This technique is based on the variation of transparency of the photoresist during exposure. During exposure the absorption of the light-sensitive compound decreases due to its conversion into indene carboxylic acid. A very good aspect ratio (height:width) of up to 10:1 and high edge steepness has been obtained from one coat and one UV exposure.

Printing Isolated Features with k 1= 0.2 Using Multiple-Pupil Exposure

Recently, we have investigated the effects of radially non-symmetric pupil filters and multiple-pupil exposure sequences in optical lithography [to appear in: Proc. SPIE 2726 (1996)]. Based on numerical simulations we predicted that extremely narrow lines could be delineated using the superposition of exposures with symmetric and anti-symmetric pupil functions. Here, we present experimental results verifying this prediction. Using conventional masks and a pupil filtering i-line stepper with a numerical aperture of 0.4, we were able to achieve linewidths of 0.18 µm in 1.0 µm thick positive photoresist, with a depth of focus of 1 µm.

Masks for high aspect ratio x-ray lithography

The requirements for deep x-ray lithography (DXRL) masks are reviewed and a recently developed cost effective mask fabrication process is described. The review includes a summary of tabulated properties for materials used in the fabrication of DXRL masks. X-ray transparency and mask contrast are calculated for material combinations using simulations of exposure at the Advanced Light Source (ALS) at Berkeley, and compared to the requirements for standard x-ray lithography (XRL) mask technology. Guided by the requirements, a cost-effective fabrication process for manufacturing high contrast masks for DXRL has been developed. Thick absorber patterns () on a thin silicon wafer (m) were made using contact printing in thick positive (Hoechst 4620) and negative (OCG 7020) photoresist and subsequent gold electrodeposition. Gold was deposited using a commercially available gold sulphite bath with low current density and good agitation. The resultant gold films were fine-grained and stress-free. Replication of such masks into thick acrylic sheets was performed at the ALS.

Thick film positive photoresist: Development and resolution enhancement technique

The development of a positive, thick film photoresist consisting of a diazonaphthoquinone sensitizer and a novolac resin is described which has the capability of meeting a wide range of thick film requirements necessary for magnetic recording head fabrication. This photoresist system can coat 4–15-μm-thick films over various types of topography on a variety of different metal and dielectric surfaces (copper, Permalloy, and alumina). Moreover, this resist is compatible with copper, permalloy, and gold plating baths as well as different metal etchants. The photoresist can be used to produce images with wall profiles greater than 80° with a resolution capability approaching 2 μm lines and spaces in a 7.5-μm-thick film. Process latitude is demonstrated over a wide range of exposure doses and focus positions. Enhancement of thick film photoresist resolution has been achieved using a method which involves the treatment of photoresist films with dilute surfactant solutions. A number of factors have been found to effect the outcome of this method including the surfactant structure and concentration, the method and period of application, and the order of processing steps. Under proper conditions significant enhancement of photoresist contrast is observed which is accompanied by improvement of photoresist image profile and resolution. The method is particularly useful in electroplating applications.

Selectively released microstructures electroplated into thick positive photoresist

A fabrication process based on sputtering of a thin film plating base, on conventional UV depth lithography, and on electrochemical deposition of gold, makes microstructures of arbitrary shape in the third dimension and of considerable height (up to 80 mu m) and resolution (aspect ratio: about 10) possible. Applications for the fabrication of microstructures are discussed.

High depth to width aspect ratios in thick positive photoresist layers using near UV lithography

Conventional lithography using liquid positive photoresist leads to mask structures of considerable thickness and resolution. The process is sketched and its present limits are documented. Applications for the fabrication of microstructures are stated.

Shape of Thick Positive Photoresist Patterns

The shape and the sidewall taper angle of positive photoresist patterns 2–5 μm in width and 1.5–9.3 μm in thickness were investigated to achieve a steep sidewall profile. Both experimental and calculated results showed that the taper angle increases with increased coated photoresist thickness, because the exposure dose is low at the bottom of a rather thick photoresist. The sidewall taper angle of a photoresist pattern 5 μm wide and 9.3 μm thick reached 84° when using inorganic alkaline developer. The taper angles simulated by sample agreed with the experimental results within one degree.

A pillar-shaped via structure in a Cu-polyimide multilayer substrate

A pillar-shaped via structure in a Cu-polyimide multilayer substrate and its novel fabrication process are described. The process requires that a fine rectangular via conductor be formed by pattern electroplating using a thick positive photoresist. In addition, a flat polyimide dielectric is formed only by the photolithographic process using a photosensitive polyimide precursor. The resulting pillar-shaped via conductor is 30- mu m square and 25- mu m thick per layer. The area occupied by this via conductor is 25% smaller than that occupied by a conventional via not filled with copper conductor. Its signal line density is twice as high, and the thermal resistance is about 50% less than that of a conventional via. This structure is suitable for high-speed signal transmission in a Cu-polyimide multilayer substrate. >