Post Exposure Bake Research Articles

Embedded Microstructure Fabrication Using Developer-Permeability of Semi-Cross-Linked Negative Resist

This paper reports on a novel and simple 3-D fabrication technique of microstructures embedded in a single-layer negative resist. The proposed technique allows one the fabrication of an embedded microstructure with a single exposure and the subsequent development process. The unique feature of the proposed fabrication technique is the development method which enables the rapid fabrication of polymer-based microfluidic systems with relatively large areas but with micrometer-sized features. For example, features of microchannels, on the order of 100 m in width and 50 mm in length, sufficient for microfluidic systems, were successfully fabricated with a relatively short (<; 20 min) development time. These features are realized by the interesting physical response of the top-membrane to the developer; the developer permeates through the top-membrane region made of semi-cross-linked photoresist, and the permeated developer dissolves the uncross-linked photoresist at the same time. As a step toward the practical use of the proposed development method, process parameter sets (exposure dose, postexposure bake (PEB) time, and temperature) related to the cross-linking reaction of the top-membrane region were investigated by employing the cross-linking reaction model describing the chemical reaction during the UV exposure and the PEB. Through a series of experiments, 1) a criterion of process parameter sets for the fabrication of centimeter-long embedded microchannels was obtained, and 2) the applicability to polymer-based microfluidic systems was successfully demonstrated.

Photonic crystals with defect structures fabricated through a combination of holographic lithography and two-photon lithography

This paper presents the capability of direct laser writing of complex defect structures in holographically formed three-dimensional photonic crystals in dipentaerythritol penta/hexaacrylate (DPHPA) monomers mixed with photoinitiators. The three-dimensional photonic crystal template was fabricated through prism-based holographic lithography. Defect structures are fabricated through the two-photon polymerization excited by a femtosecond laser. The strengths of two optical lithographic techniques are combined with holographic lithography providing a rapid and large area microfabrication and two-photon lithography providing flexibility in fabrication of defect structures. The optical fabrication process is simplified in the negative tone DPHPA without prebake and postexposure bake as is required of SU-8 while maintaining a capability for constructing photonic structures with small features.

Modification of Chemically Amplified Resists by Radical Copolymerization

AbstractModification of a phenolic resist has been carried out by copolymerization with methacrylic monomers. Influence of irradiation source and its intensity has been studied by analyzing sensitive curves of a various resist composition. Inversion of imaging type at varying post exposure bake temperature has been investigated for resists based on copolymers containing methacrylic acid units, which has been related with crosslinking of macromolecules.

Fabrication of desired three-dimensional structures by holographic assembly technique

We demonstrate a novel technique to fabricate desired three-dimensional (3D) periodic structures by holographically assembling multiple one-dimensional (1D) or multiple two-dimensional (2D) structures. Thanks to the high-absorption effect of the used material, we fabricated for each time, by employing a two-beam interference technique, a 1D or a 2D structure with very limited film thickness. By using the same sample and repeating the same fabrication process, i.e., (i) spin coating, (ii) exposure, and (iii) post-exposure bake, we created, layer-by-layer, a 3D structure as desired, without the limitation of the number of layers. This technique allows rapid fabrication of very large and thick 3D photonic crystal template with variable period, flexible design, low cost, and possible introduction of arbitrary defects inside a 3D structure.

Architectural Effects on Acid Reaction-Diffusion Kinetics in Molecular Glass Photoresists

Understanding acid reaction-diffusion kinetics is crucial for controlling the lithographic performance of chemically amplified photoresists. In this work, we study how the molecular architectures of positive-tone chemically amplified molecular glass resists affect the acid reaction-diffusion kinetics during the post-expose bake (PEB) or annealing step. We compare the acid reaction-diffusion kinetics of a common photoacid generator in molecular glass resists with chemical similarity to poly(4-hydroxystyrene), and that are designed with branched and ring architectures. In situ Fourier transform infrared (FTIR) spectroscopy methods are used to measure reaction rate, acid trapping behavior, and acid diffusivity as a function of PEB temperature. We find that the acid reaction-diffusion kinetics in molecular glass resists is correlated to the film molar density that in turn depends on the architecture of the molecular glass molecules. These results allow modeling of the latent image formation in molecular glass...

Characterization of the Photoacid Diffusion Length and Reaction Kinetics in EUV Photoresists with IR Spectroscopy

A soft-contact film transfer method was developed to prepare multilayer photoresist thin films that enable high-resolution spectroscopic and reflectivity measurements for determining the reaction−diffusion kinetic parameters and photoacid diffusion length. Infrared reflectance absorption spectroscopy was applied to follow, quantitatively, the reaction−diffusion kinetics during the post-exposure bake (PEB) step; the time evolution of the average deprotection level across a bilayer film with model photoresists is described by a kinetics model with three parameters: a reaction rate constant (kP), the phenomenological photoacid trapping constant (kT), and the photoacid diffusion constant (DH). A polymeric and molecular resist for next-generation extreme ultraviolet (EUV) lithography with chemically analogous structure was studied with this methodology. The three kinetic parameters follow an Arrhenius dependence but show quantitative differences between these two photoresists at a given PEB temperature. Further it was demonstrated that the photoacid diffusion length is not a simple function of diffusion coefficient; instead, it is dictated by all three kinetics parameters jointly in addition to the deprotection level at which the resist becomes soluble in an aqueous developer solution. These observations qualitatively explain the experimentally observed shorter photoacid diffusion length of the molecular resist in comparison to its polymeric counterpart.

Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU‐8

By exploiting the thermoplastic and photosensitive nature of SU-8 photoresists, different types of hierarchical pillar arrays with variable aspect ratios are fabricated through capillary force lithography (CFL), followed by photopatterning. The thermoplastic nature of SU-8 enables the imprinting of micropillar arrays with variable aspect ratios by CFL using a single poly(dimethylsiloxane) mold, simply by tuning the initial film thickness of SU-8 on a substrate. The pillar array is subsequently photopatterned through a photomask, followed by post-exposure baking above the glass transition temperature (T(g)) of SU-8. The pillars in the exposed region become highly crosslinked and, therefore, neither soluble nor able to reflow above T(g), whereas the pillars in the unexposed regions can reflow and flatten out. Two developing strategies are investigated after UV exposure of the SU-8 pillar arrays including i) solvent development and drying and ii) thermal reflow to create bilevel hierarchical structures with short pillars and single-level, dual-scaled, high-aspect-ratio (up to 7.7) pillars in a microdot array, respectively.

Lithography: Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU‐8 Small 6/2010

The cover image illustrates an array of high-aspect-ratio (up to 7.7) polymer pillars from SU-8 photoresists imprinted by capillary force lithography (CFL), followed by photopatterning and a thermal reflow process. Because of the thermoplastic nature of SU-8, pillars with variable aspect ratios are created through CFL using a single poly(dimethylsiloxane) membrane as the mold, simply by tuning the initial film thickness of SU-8 on a substrate. The pillars are subsequently photopatterned through a photomask with a square dot array, followed by post-exposure bake above the glass-transition temperature (Tg) of SU-8. The pillars in the exposed region become highly crosslinked, and thus neither soluble nor reflowable, whereas the pillars in the unexposed regions can reflow and flatten out above Tg. In this way, a single-level, dual-scaled, high-aspect-ratio SU-8 pillar array is “developed” in the exposed region without the use of solvent. Such high-aspect-ratio pillar arrays may find applications in biomimetic superhydrophobic surfaces and dry adhesion, microfluidics, diagnosis, tissue engineering, force sensing, and actuation. For more information, please read the Full Paper “Fabrication of Hierarchical Pillar Arrays from Thermoplastic and Photosensitive SU-8” by S. Yang et al., beginning on page 768.

Fabrication of thin SU-8 cantilevers: initial bending, release and time stability

SU-8 cantilevers with a thickness of 2 µm were fabricated using a dry release method and two steps of SU-8 photolithography. The processing of the thin SU-8 film defining the cantilevers was experimentally optimized to achieve low initial bending due to residual stress gradients. In parallel, the rotational deformation at the clamping point allowed a qualitative assessment of the device release from the fluorocarbon-coated substrate. The change of these parameters during several months of storage at ambient temperature was investigated in detail. The introduction of a long hard bake in an oven after development of the thin SU-8 film resulted in reduced cantilever bending due to removal of residual stress gradients. Further, improved time-stability of the devices was achieved due to the enhanced cross-linking of the polymer. A post-exposure bake at a temperature TPEB = 50 °C followed by a hard bake at THB = 90 °C proved to be optimal to ensure low cantilever bending and low rotational deformation due to excellent device release and low change of these properties with time. With the optimized process, the reproducible fabrication of arrays with 2 µm thick cantilevers with a length of 500 µm and an initial bending of less than 20 µm was possible. The theoretical spring constant of these cantilevers is k = 4.8 ± 2.5 mN m−1, which is comparable to the value for Si cantilevers with identical dimensions and a thickness of 500 nm.

Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development

In recent years SU-8 resist attracted a high interest for fabrication of structures with high topography or high aspect ratio structures. The reasons for SU-8 popularity can be found in its unique properties - high chemical and mechanical stability, biological compatibility, optical transparency, high aspect ratio capability and low cost of fabrication. SU-8 can be used as an alternative molding material to LIGA process; in comparison to the standard LIGA process, relatively thick (1 mm range) SU-8 layers can be processed with UV lithography and do not require expensive X-ray light source. Nevertheless, processing of thick layers of SU-8 is not without challenges - coating high viscosity material, critical soft bakes in order of several hours, exposure requiring good contact, critical post exposure bake and extremely long development times. One aspect of the SU-8 processing, which is explored in this paper, is improvement of the development process time - by using single wafer megasonic-enhanced development. In this paper, the experimental results from manufacturing of SU-8 structures with aspect ratio 1:23 by using UV lithography and megasonic-enhanced development will be presented. Significant development time reduction from 240 min down to 10 min was achieved.

Characterizing and smoothing of striated sidewall morphology on UV-exposed thick SU-8 structures for micromachining millimeter wave circuits

Sidewall roughness is a critical parameter when a thick SU-8 process is employed for fabricating millimeter wave circuits, as it determines the conduction loss of the circuit. A striated sidewall morphology was detected in our UV-exposed thick SU-8 structure, and this rough morphology was clearly illustrated and quantitatively characterized by a confocal laser scanning microscope (CLSM) for the first time. The rms line roughness of the striated sidewall is up to ∼1 µm, several-fold higher than the skin depth of copper at 220 GHz. We also tried to discuss the possible causes of sidewall striations. A proper counter measure, i.e. higher temperature and longer baking time of post-exposure bake, was effectively employed to flatten the sidewall striations. It was found that the rms line roughness on the sidewall detected by the CLSM is reduced to ∼72 nm, for the sample post-exposure-baked at 107 °C.

Potential and limitations of a T-NIL/UVL hybrid process

A T-NIL/UVL hybrid process offers the potential to combine the advantages of thermal nanoimprint (T-NIL) – to define patterns in the nanometre range in parallel over large areas, with the advantages of photolithography in the ultraviolet range (UVL) – to define large patterns easily. It is attractive for the preparation of e.g. sensors, where typically nm-scaled electrodes are connected to macroscopic contacts. We address the critical issues of such a process that until now have hampered its application in praxis. Major aspects result from the required compatibility of both processes, asking for resists that are imprintable at temperatures well below any deterioration of the photoactive components. Furthermore, the lithography step has to be performed over a pre-patterned surface, which may require overexposure; the pre-patterned surface is also crucial during an eventually required post-exposure bake, where pattern loss may result from unintended viscous flow during crosslinking. Examples with SU-8 as a chemically amplified negative resist demonstrate that the hybrid process works, if some processing parameters are well tuned.

Thermal-deformation analysis of a polymer micro-mirror optical device

This paper analyzes the thermal deformation in SU-8 polymer materials when subjected to different fabricating parameters and exposed to a high power light source for long durations for 45° micro-mirror applications. By experimentally optimizing the fabrication process, new fabricating technologies for micro-optical components can be developed. During the fabrication process, a polymer-based material is subjected to different soft bake, post exposure bake (PEB), hard baking temperatures; also, the baking times and exposure dosages were varied. Any of these variables can change the thermal stability of the material’s bonding energy and dynamic molecular behavior. There have not been many studies on the thermal deformation of a micro-mirror structure. A thermal dilatometer DIL-402C was used in this study to measure the thermal stability and determine changes in the material phase of SU-8 material. After optimizing the process parameters and finding that the inclined surface roughness was 49 nm, the thermal deformation was found to be 25 ppm (at 100°C).

Secondary electron contrast modulation in SU-8 photoresist films exposed holographically

We report a Secondary Electron (SE) contrast modulation observed in scanning electron microscope photographs of the cross-section of SU-8 photoresist films exposed holographically. The modulation occurs along the whole depth of the sample and its contrast disappears when the samples are submitted to the post exposure bake (PEB). Diffraction and atomic force microscopy measurements of the samples were performed before and after PEB to investigate this modulation. The results indicate that this SE emission contrast modulation comes from the spatial chemical modulation generated by the photolysis during the exposure of the SU-8 films. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 226–230, 2010

Simulation of shot noise effect on CD and LER of electron-beam lithography in 32 nm designs

Mask fabrication or even maskless lithography operations are performed with electron-beam lithography (EBL). The detailed understanding of the electron scattering mechanisms in resist films and multilayer substrates, as well as on substrates with topography is mandatory. Along the same track the problem of line edge roughness (LER) of created structures is emerging, because it strongly affects the device electrical behavior. Process simulation could be used for the quantification of the effects of electron-beam exposure and resist material on the produced layout. In the current work coupling between a detailed Monte-Carlo e-beam–matter interaction simulator and a stochastic resist film modeling simulator for the materials, postexposure bake, and development allows for simulation of the shot noise, resist material, and process effects on the critical dimension (CD) and LER in the 32 technology patterned by direct EBL. An example of quantification of shot noise and resist contribution to LER is provided.

Fabrication of nanodot array molds by using an inorganic electron-beam resist and a postexposure bake

Nanoimprint lithography (NIL) is regarded as the lithographic technology of the next generation. The photolithography is advancing to perform resolution of 32nm. Although photolithography and high-voltage-acceleration (&amp;gt;50kV) electron-beam-lithography (EBL) techniques are currently used in commercialized lithography, these two methods involve high costs. The authors therefore attempted to fabricate arrays of dots, each with a diameter of 20nm or less, by using low-voltage-acceleration EBL. NIMO-P0701 was used as a positive-type inorganic EB resist in this study and a combination of NIMO-P0701 and low-acceleration-voltage EBL was established. Attempts were then made to use a postexposure bake (PEB) to further reduce the size of the dot array patterns. Finally, the developed dot array mold was transferred by UV-NIL without dry etching. As a result, arrays of dots with a diameter of 20 or 10nm were fabricated by EBL at an acceleration voltage of 4kV and subsequent PEB. The resulting mold was developed with buffered hydrofluoric acid. The developed NIMO-P0701 pattern was used for NIL, and pattern transfer to a UV-curable resin was successfully achieved. In this case, the optimum transfer pressure was 4MPa. Additionally, dot arrays initially produced with diameters of 30 and 18nm were shrunk to sizes of 20 and 10nm, respectively, by means of PEB.

Control of the critical dimensions and line edge roughness with pre-organized block copolymer pixelated photoresists

Sphere-forming polystyrene-block-poly(t-butyl acrylate) (PS-b-PtBA) diblock copolymer with catalytic amounts of photo-acid generator (PAG) formulated a pixelated photoresist. In thin films with single-sphere thickness, hexagonal arrays of spheres (∼20 nm diameter on a 40 nm pitch) of PS within a matrix of PAG segregated in PtBA was obtained through solvent annealing. Upon exposure and post-exposure baking, the soluble PtBA matrix was converted to insoluble poly(acrylic acid), such that a negative pattern could be formed in the chlorobenzene developer. The concept of pixelation was demonstrated by exposing line and space patterns with increasing widths. In contrast to the width of the exposure fields that increased monotonically, the widths of the pixelated resist structures after development were quantized with respect to an integer number of rows of spheres. Furthermore, line edge roughness could be correlated with the size of each pixel (diameter of spherical domain).

Submillisecond post-exposure bake of chemically amplified resists by CO2 laser spike annealing

Pattern formation in a chemically amplified photoresist requires a post-exposure bake (PEB) to catalytically deprotect the polymer. Excessive diffusion of the photogenerated acid results in the loss of line edge definition, blurring of latent images, and changes in the line edge roughness. To optimize the process, the authors have explored submillisecond PEB using a CO2 laser-based scanned annealing system [M. Chandhok (private communication)]. Several polymer and photoacid generator resist systems were studied under 800μs laser spike annealing at estimated temperatures between 200 and 400°C. All the resist systems exhibit remarkable stability in this time/temperature regime, with the maximum useful temperature limited by thermal deprotection and/or decomposition of the polymer backbone. At lower temperatures, high resolution patterns with sub-100-nm features are formed, comparable to hotplate reference samples. Resist sensitivity is improved significantly for several resist systems (dose to clear is lowered), while other systems exhibit little change in photosensitivity.

Study of the contour-based optical proximity correction methodology

As design rule continues to shrink, resolution enhancement techniques (RET) such as optical proximity correction (OPC) become more and more complex to enable design printability. As we know, typically integrated circuit (IC) layouts are simple shapes such as rectangles. However, high spatial frequency components of the mask spectrum that are not captured by the low-pass pupil result in a rounded image. In addition, the diffusion process in the postexposure bake (PEB) step makes the wafer rounding effects worse. This means that it is difficult to get the wafer image to match the design exactly at corners, even with the most aggressive OPC methodology. Therefore, pre-OPC site placement optimization is necessary to achieve high quality wafer images. In this work, a contour-based OPC methodology is proposed to minimize the time consumption in pre-OPC simulation site placement optimization and OPC job running. Rounded target contours that best describe the real intended wafer result are used as the target during OPC correction. By comparing classical OPC recipe-driven target point placement and contour-based OPC methodology, it is found that contour-based OPC methodology can achieve comparable image quality in a shorter turn around time (TAT) with fewer engineer resources.

Extreme Ultraviolet Resist Outgassing Quantification Verification by Resist Film Analysis

Numerous investigations on extreme ultraviolet (EUV) resist outgassing have been performed using the pressure rise and gas chromatography mass spectrometry (GC–MS) method. However, the reliability of the results obtained with these methods has not been clearly discussed. In this paper, to analyze and verify the reliability of the resist outgassing results, de-protection reaction and photo acid generator's (PAG's) decomposing reaction in the resist film, which are viewed as the main source of resist outgassing, are investigated using the Fourier transform infrared spectroscopy (FT-IR). Analysis was performed on a resist film after EUV exposure, before post exposure bake (PEB). Based on the measured amount of de-protection reaction in the resist film, the amounts of resist outgassing released were calculated. It was found that the resist outgassing amount obtained through FT-IR was somewhere between those obtained using the pressure rise and GC–MS methods results. These similarities in the results substantiate the reliability of resist outgassing amounts obtained through these analysis methods.