Hard Bake Research Articles

Novel Method for Development of Nanobubbles with Consistent Size Distribution

Background/Objective:Ultrasound is a versatile non-invasive clinical tool that can be used for monitoring, diagnosis, and management of disease. Contrast-Enhanced Ultrasound (CEUS) takes advantage of microbubbles which are small gas filled bubbles made of lipids to image and evaluate functioning of organs such as the heart, liver, and kidneys. CEUS is better for assessing vascularization of tissues compared to US. However, ultrasound contrast agents have been limited by their size and cannot extravasate blood vessels easily. Current methods also have high polydispersity. We hypothesize that a microfluidic device could deliver nanobubbles with improved size distribution by mixing through micro sized pores.
 Methods:The microfluidic device was fabricated using 3D printing for a proof of concept and photolithography for functional use and was done in a class 1-10-100-1000 cleanroom. SU-8 photoresist was spun onto a 4” silicon wafer at 3000 rpm for 30 seconds. The silicon wafer was then baked and exposed under UV with a photomask designed on AutoCAD. The wafer was then developed, hard baked, and was cross linked with Polydimethylsiloxane (PDMS).
 Results:Dynamic light scattering data of a shaken formulation shows a monodisperse distribution. Further work will have to be done to determine if this can be delivered by microfluidic device and the parameters that affect size and polydispersity.
 Conclusion and Potential Impact:This study will determine the parameters needed for the microfluidic device to efficiently create microbubbles for contrast enhanced ultrasound.

The advantages of methanol-amine solvent mixtures in solution processing of Ge-Sb-S chalcogenide glass thin films

We report on the substantial influence of methanol presence on the dissolution mechanism of Ge20Sb5S75 bulk glass with subsequent superior properties of thin films deposited from such solutions. Raman spectroscopy confirmed significant differences in structural features present in glass solutions prepared from pure amines (n-propylamine and n-butylamine) and their mixtures with methanol. The experiments with dissolved elemental sulfur and Ge25S75 glass analog proved that both antimony and methanol presence induce further splitting of the Ge4S104− cluster structure, which fundamentally affects the properties of deposited thin films. Significant structural and compositional differences were found not only in solutions and as-prepared samples, but also after thin films’ thermal treatment (hard baking up to 210 °C). The as-prepared thin films deposited from amine-methanol mixtures possessed the exact composition of source bulk glass while thin films of other solvent formulations exhibited sulfur deficiency. The annealing up to 210 °C only highlighted this difference. As a result of the different structure of the thin films prepared in this way, the other benefits of methanol addition were found, namely an increase in the refractive index by approx. 0.1 independent of the annealing temperature, or a lower thermally induced thickness contraction (up to 7.5 %).

A novel Bayesian Max-EWMA control chart for jointly monitoring the process mean and variance: an application to hard bake process

In this article, we introduce a novel Bayesian Max-EWMA control chart under various loss functions to concurrently monitor the mean and variance of a normally distributed process. The Bayesian Max-EWMA control chart exhibit strong overall performance in detecting shifts in both mean and dispersion across various magnitudes. To evaluate the performance of the proposed control chart, we employ Monte Carlo simulation methods to compute their run length characteristics. We conduct an extensive comparative analysis, contrasting the run length performance of our proposed charts with that of existing ones. Our findings highlight the heightened sensitivity of Bayesian Max-EWMA control chart to shifts of diverse magnitudes. Finally, to illustrate the efficacy of our Bayesian Max-EWMA control chart using various loss functions, we present a practical case study involving the hard-bake process in semiconductor manufacturing. Our results underscore the superior performance of the Bayesian Max-EWMA control chart in detecting out-of-control signals.

Monitoring the process mean under the Bayesian approach with application to hard bake process

This study introduces the Bayesian adaptive exponentially weighted moving average (AEWMA) control chart within the framework of measurement error, examining two separate loss functions: the squared error loss function and the linex loss function. We conduct an analysis of the posterior and posterior predictive distributions utilizing a conjugate prior. In the presence of measurement error (ME), we employ a linear covariate model to assess the control chart's effectiveness. Additionally, we explore the impacts of measurement error by investigating multiple measurements and a method involving linearly increasing variance. We conduct a Monte Carlo simulation study to assess the control chart's performance under ME, examining its run length profile. Subsequently, we offer a specific numerical instance related to the hard-bake process in semiconductor manufacturing, serving to verify the functionality and practical application of the suggested Bayesian AEWMA control chart when confronted with ME.

Bayesian AEWMA control chart under ranked set sampling with application to reliability engineering

The article introduces a novel Bayesian AEWMA Control Chart that integrates different loss functions (LFs) like the square error loss function and Linex loss function under an informative prior for posterior and posterior predictive distributions, implemented across diverse ranked set sampling (RSS) designs. The main objective is to detect small to moderate shifts in the process mean, with the average run length and standard deviation of run length serving as performance measures. The study employs a hard bake process in semiconductor production to demonstrate the effectiveness of the proposed chart, comparing it with existing control charts through Monte Carlo simulations. The results underscore the superiority of the proposed approach, particularly under RSS designs compared to simple random sampling (SRS), in identifying out-of-control signals. Overall, this study contributes a comprehensive method integrating various LFs and RSS schemes, offering a more precise and efficient approach for detecting shifts in the process mean. Real-world applications highlight the heightened sensitivity of the suggested chart in identifying out-of-control signals compared to existing Bayesian charts using SRS.

Wet etching of gold on graphene for high-quality resist-free graphene surfaces

Wet etching of gold on graphene is challenging due to the weak adhesion of the resist mask to graphene. We report an operating procedure for alkali ion-free wet etching of gold on graphene using a mixture of hydrochloric and nitric acids (aqua regia) with a high lateral resolution down to 100 nm. We investigate the role of positive and negative resists, electron beam lithography (EBL) dose, hard-bake, oxygen etching, aging, and sensitivity to the etch parameters, such as the freshness of dilute aqua regia, etch time, and the order of etched samples. The negative-tone resist provides the best results. The over-dosed EBL exposure can enhance the resist adhesion, as hard-bake below the glass-transition temperature and well-defined wet etch of the resist-residua-free gold surface. We also present a cleaning procedure to avoid bubble formation after the hard bake. Our results demonstrate that wet etching of gold on graphene using aqua regia is a viable method for achieving high-quality resist-free graphene surfaces. This method has potential applications in graphene nanoelectronics and nanophotonics, where high-quality graphene surfaces are essential for device performance.

Adaptive EWMA control chart using Bayesian approach under ranked set sampling schemes with application to Hard Bake process

The memory-type control charts, such as cumulative sum (CUSUM) and exponentially weighted moving average control chart, are more desirable for detecting a small or moderate shift in the production process of a location parameter. In this article, a novel Bayesian adaptive EWMA (AEWMA) control chat utilizing ranked set sampling (RSS) designs is proposed under two different loss functions, i.e., square error loss function (SELF) and linex loss function (LLF), and with informative prior distribution to monitor the mean shift of the normally distributed process. The extensive Monte Carlo simulation method is used to check the performance of the suggested Bayesian-AEWMA control chart using RSS schemes. The effectiveness of the proposed AEWMA control chart is evaluated through the average run length (ARL) and standard deviation of run length (SDRL). The results indicate that the proposed Bayesian control chart applying RSS schemes is more sensitive in detecting mean shifts than the existing Bayesian AEWAM control chart based on simple random sampling (SRS). Finally, to demonstrate the effectiveness of the proposed Bayesian-AEWMA control chart under different RSS schemes, we present a numerical example involving the hard-bake process in semiconductor fabrication. Our results show that the Bayesian-AEWMA control chart using RSS schemes outperforms the EWMA and AEWMA control charts utilizing the Bayesian approach under simple random sampling in detecting out-of-control signals.

Application of TiO2 nanoparticle and polyimide blend alignment layer in liquid crystal lens

Suspension of colloidal nanoparticles (NPs) in liquid crystals (LCs) is a common technology to modify the electro-optical characteristics of a LC device. Unfortunately, NP aggregation spontaneously proceeds over time through the dipolar attractive interaction between NPs and results in the light scattering and distortion of the nematic order, which degrades the performance of the LC device. Here, rutile titanium dioxide (TiO2) NPs were suspended in the alignment layer rather than LCs to prevent the proceeding NP aggregation. During the deposition of the alignment layer, the NPs were hardened in the alignment layer with a hard bake process. The blend alignment layer was utilized to develop a hole-patterned liquid crystal (HLC) lens. This layer enhances the polar anchoring of LCs because of the intermolecular interaction between NP and LC and reduces the ion density and rotational viscosity of the LC mixture through harvested effect of free-ions. It also assists in spreading the fringing electric field toward lens center by TiO2 NP contribution. These results speed up the dynamic response of the HLC lens. The introduction of blend alignment layer decreases the focus and defocus times of the HLC lens by 15% and 34%, respectively. Moreover, the operation voltage of the HLC lens for achieving the shortest focal length is reduced by ∼40%. The decreased switching time is verified by calculating the transient phase conversions of HLC lenses with various viscosities. The calculated results correspond to the experimental results. The blend alignment layer did not degrade the lens and focusing qualities of the HLC lens. Long-term observation revealed that the NP aggregation state remained similar for the LC sample with NPs suspended in the alignment layer. By contrast, NP aggregation spontaneously proceeded and connected networks appeared with time for the LC sample with NPs suspended in LCs. The blend alignment layer can improve the operation voltage and dynamic response of a LC device and prevent device deterioration over time.

Fabrication of ultra-thin laser induced graphene electrodes over negative photoresist on glass for various electronic applications

Photoresists play a pivotal role in pattern transfer applications to achieve microstructures with high aspect ratios and vertical sidewalls for MEMS and semiconductor industries. Negative photoresists (NPR) have been leading the photoresist industry ever since their inception with excellent chemical resistance and high thermal and mechanical stability. However, owing to its low conductivity, NPR is restricted only to structural and sacrificial applications. For this first time, this work showcases the utilization of NPR to obtain ultra-thin and highly conductive laser Induced graphene (LIG) over glass substrates through laser ablation. Different combinations of NPR samples were considered at different spin speeds (2000 rpm to 4000 rpm) to obtain various thickness values to comprehend the effect of parameters, such as UV exposure, baking conditions, and photoresist thickness over obtained conductivity of the LIG. It was observed that samples which were coated with NPR at 4000 rpm followed by hard baking without UV exposure exhibited a conductivity of 529.7 S/m upon repeated ablation of laser. The functionality of the fabricated thin film electrodes was tested for biofuel cell and electrochemical applications. The novel method to create conductive patterns over glass substrates not only pave a possible way for realizing glass-based electronic devices but also open up new avenues for microelectronic industry.

AFM Characterization of Temperature Effect on the SU-8 Mechanical and Tribological Properties.

This study presents the effect of temperature on the mechanical and tribological properties of SU-8 polymer. The temperature of investigated samples increasing during testing and the variation of mechanical and tribological properties were monitored. The samples for tests were SU-8 hard baked at different temperatures. The hard bake temperature changes the mechanical and tribological properties of polymers. The aim of this research work is the reliability design improvement of SU-8 microstructures from electro-thermally actuated devices where a thermal gradient produces the softening and modification of SU-8 behavior. As a function of the hard baked temperature, different mechanical and tribological properties were experimentally determined using the atomic force microscopy (AFM) technique. The mechanical properties of interest are the modulus of elasticity and hardness. The investigated tribological properties involve the adhesion and friction forces. The modulus of elasticity and hardness decrease if the operating temperature increases based on the thermal relaxation of material and their viscoelastic behavior. The adhesion force between AFM tip and investigated samples increases if the operating temperature increases, respectively. The same evolution was experimentally observed in the case of friction force. Moreover, for the same testing temperature, the modulus of elasticity and hardness increase, and the adhesion and friction forces decrease if the SU-8 is hard baked at high temperature.

Development of optical‐driven nanoneedles using SU‐8 nanofabrication

AbstractWe developed an optical‐driven nanoneedle that is operated in aqueous solution by two focused laser beams. A nanoneedle tool consists of 2 μm‐sized structures for optical trapping and a nanoneedle having 200 nm width and 20 μm length. Nanoneedle tools are fabricated with single ultraviolet (UV) lithography of SU‐8 photoresist with highly scalable manner. We evaluated the conditions of the lithography processes: rinse, hard bake and post exposure bake (PEB) that determine the shape of the nanoneedles and recovery rate. We demonstrated the trapping of a nanoneedle tool to verify the manipulation of the position and the orientation. Nanoneedle tools have an advantage in the use of closed microchanells, which can lead to the applications including delivery and extraction of substances inside single cells and pinpoint molecular processing of biomolecules. In this paper, we report on the fabrication process of the nanoneedle tools and the demonstration of the manipulation.

Analytical Study of Porous Organosilicate Glass Films Prepared from Mixtures of 1,3,5- and 1,3-Alkoxysilylbenzenes.

Organosilicate glass (OSG)-based porous low dielectric constant (low-k) films with different molar ratios of 1,3,5-tris(triethoxysilyl)benzene to 1,3-bis(triethoxysilyl)benzene bridging organic groups (1:3 and 1:7) were spin-on deposited, followed by a soft bake in air and N2 at 150 °C and hard bake in air and N2 at 400 °C. Non-ionic template (Brij®30) concentrations were varied from 0 to 41 wt% to control the porosity of the films. The chemical composition of the matrix of the films was evaluated and discussed with the shrinkage of the film during the curing, refractive indices, mechanical properties, k-values, porosity and pore structure. The chemical composition of the film cured in both air and N2-containing ambient were evaluated and compared. The benzene bridging groups containing films change their porosity (0 to 43%) but keep the pore size constant and equal to 0.81 nm when porosity is lower than 30%. The k-value decreases with increasing porosity, as expected. The films containing benzene bridge have higher a Young’s modulus than plasma-enhanced chemical vapor deposition (PECVD) methyl-terminated low-k films with the same porosity and show good hydrophobic properties after a hard bake and close to the values reported for 1,4-benzene-bridged films. The fabricated films show good stability after a long time of storage. However, the improvement of mechanical properties was lower than the values predicted by the published literature data. It was concluded that the concentration of 1,3,5-benzene bridges was below the stiffness threshold required for significant improvement of the mechanical properties. The films show UV-induced luminescence with a photon energy of 3.6 to 4.3 eV. The luminescence is related to the presence of oxygen-deficient-type defects or their combination with organic residues. The most intensive luminescence is observed in as-deposited and soft bake samples, then the intensity is reduced after a hard bake. It is assumed that the oxygen-deficient centers form because of the presence of Si–OC2H5 groups in the films and the concentration of these centers reduces when all these groups completely transformed into siloxane (Si–O–Si).

On Designing a Progressive EWMA Structure for an Efficient Monitoring of Silicate Enactment in Hard Bake Processes

Statistical process control provides package of statistical tools applied for improving quality of manufacturing, production and services processes. Cumulative sum, exponentially weighted moving average (EWMA) and progressive mean charts belong to family of memory-type charts which effectively spot small and persistent shifts in the process parameter(s). EWMA chart requires normality and a proper choice of smoothing parameter to perform efficiently. Any deviation from these conditions affects its performance in terms of efficacy and robustness. For the said concerns, progressive exponentially weighted moving average (PEWMA) chart is developed to monitor the shifts in the process location. The performance of the proposed PEWMA chart is evaluated in terms of average run length and some other metrics of run length distribution. The assessment of the proposed chart has been made under standard normal, Student’s t, gamma, Laplace, logistic, exponential, contaminated normal and lognormal distributions. The performance of the proposed PEWMA chart is also compared with some existing competitors including the classical EWMA, classical CUSUM, HWMA, MEC, MCE and DEWMA charts. The analysis reveals that the proposal offers a design structure which has high sensitivity to small and persistent drifts in the process mean and has advantage of robustness under non-normal scenarios. An application from substrates manufacturing process (in which flow width of the resist is the key quality characteristic) is also provided for practical implementations.

The public health rationale for reducing saturated fat intakes: Is a maximum of 10% energy intake a good recommendation?

AbstractThe role of saturated fat (SFA) in health is frequently debated. Guidelines from different countries are quite comparable, in that they generally advise people to avoid high intakes of SFA and to replace some saturated fats by cis unsaturated fats. A maximum level of intake of 10% energy of SFA (of total energy) is often advised as a guideline for the general population. This paper discusses issues around SFA and health including the pros and cons of randomised controlled trials with hard endpoints, prospective cohort studies and controlled dietary intervention studies with intermediate endpoints and risk markers, concluding that there is not a single kind of research study that will answer the question on whether intake of saturated fat influences health or not. However, taking all the evidence together suggests that replacing SFA by unsaturated fat, preferably polyunsaturated fat (PUFA), to an intake below 10% energy will favourably affect the risk of cardiovascular disease. It is important to take into account that the proposed changes should be implemented in an otherwise healthy and complete diet. To reach an intake of less than 10% energy from SFA without compromising on the quality of the total diet, the intake of fatty meat and baked goods such as cakes and pastries should be reduced and butter, lard and hard baking fats should be replaced by unsaturated oils and fat spreads made from them.

A detailed ellipsometric porosimetry and positron annihilation spectroscopy study of porous organosilicate-glass films with various ratios of methyl terminal and ethylene bridging groups

Abstract Organosilicate-glass films with a varying ratio of terminal methyl and bridging ethylene groups are synthesized using 1,2-bis(trimethoxysilyl)ethane/methyltrimethoxysilane mixtures and sol-gel technology. The films are characterized by Fourier Transform Infrared spectroscopy, Ellipsometric Porosimetry and Positron Annihilation Spectroscopy. The hard bake at 400 °C generates the final pore structure, which depends on the curing environment. It is shown that ethylene bridge is destructed during the hard bake in the air via formation of peroxide radicals that form ≡SiOH during the transformation. Continuous hard bake leads to condensation of silanol groups and form a structure similar to the ordinary silica. The pore size of highly porous materials (>30%) is larger in air cured films. Destruction of the ethylene bridge makes the films matrix soft and micropores collapse due to the capillary forces during the template evaporation. It leads to the film shrinkage, increases the size of internal voids. The air cured samples showed better mechanical properties than N2 cured ones although in the last case ethylene bridging groups were preserved. The reason is that the collapse of micropores increase the internal density and creates more favorable condition for condensation of silanol groups.

Enhanced adhesive strength between SU-8 photoresist and titanium substrate by an improved anodic oxidation method for high aspect-ratio microstructures

This paper presents an anodic oxidation treatment method for improving the adhesive strength between SU-8 photoresist and titanium substrate. The adhesion strengths were measured for the treated titanium substrates by different anodic oxidation techniques, which increased with the enhancement of the surface roughness and thorny structures on the treated substrate. It was found that the adhesion was increased by approximately 86% through anodic oxidation treatment in 3.0 mol l−1 NaOH solution compared to Si wafer substrate. Additionally, the influence of UV dosage and hard bake temperature on the adhesion strength was examined for the anodic oxidation treated substrates. The UV dosage effect, with a range from 0.8 to 2.08 J cm−2, was measured and analyzed for a 300 µm thickness SU-8 cylinder, and the adhesion strength corresponding to the optimal dosage (1.52 J cm−2) was 45.82 ± 3.08 MPa. The adhesion was further strengthened by a hard bake at 110 °C for 60 min and its value was up to 71.64 MPa, which almost approaches the breaking strength of the SU-8 polymer. Using the optimized anodic oxidation and UV lithography conditions, high aspect ratio (24) SU-8 micropillar arrays with 600 µm height were fabricated without any distortion, collapse or delamination. Finally, to evaluate the feasibility of the anodic oxidation treated titanium substrate to serve as a seed layer, the electrochemical properties were measured by polarization curves.

The Effect of Bake Temperature on SU-8 Gate Insulator of IGZO Thin Film Transistor

SU-8 photoresist was applied to the gate insulator of a-IGZO TFT. The hard bake temperature of SU-8 is important and was varied from 95 to 185 °C. The FTIR showed that hard-bake temperature higher than 125 °C is necessary for complete polymerization. The leakage current and breakdown voltage were improved as increasing hard bake temperatures to 155 °C. However, the crack was generated at 185 °C degrading the electrical characteristics of insulator. The SU-8 insulator was successfully applied to a-IGZO TFT where the on-off ratio was highest for hard-bake temperature of 155 °C.

Low-stress photosensitive polyimide suspended membrane for improved thermal isolation performance

In this paper, we introduce a method of isolating thermal conduction from silicon substrate for accommodating thermal-sensitive micro-devices. This method lies in fabrication of a low-stress photosensitive polyimide (PSPI) suspension structure which has lower thermal conductivity than silicon. First, a PSPI layer was patterned on a silicon wafer and hard baked. Then, a cavity was etched from the backside of the silicon substrate to form a membrane or a bridge-shape PSPI structure. After releasing, a slight deformation of about 20 nm was observed in the suspended structures, suggesting ultralow residual stress which is essential for accommodating micro-devices. In order to investigate the thermal isolation performance of the suspended PSPI structures, micro Pirani vacuum gauges, which are thermal-sensitive, had been fabricated on the PSPI structures. The measurement results illustrated that the Pirani gauges worked as expected in the range from 1– 470 Pa. Moreover, the results of the Pirani gauges based on the membrane and bridge structures were comparable, indicating that the commonly used bridge-shape structure for further reducing thermal conduction was unnecessary. Due to the excellent thermal isolation performance of PSPI, the suspended PSPI membrane is promising to be an outstanding candidate for thermal isolation applications.

Effect of pre-drying treatments on solution-coated organic thin films for active-matrix organic light-emitting diodes

Due to capillary rise, organic thin films fabricated by solution coating exhibit the concave thickness profile. It is found that the thickness and emission uniformities within pixels vary depending sensitively on the pre-drying treatment that has been done before hard bake. We investigate its effect on the film quality by varying the temperature, time, pressure, fluid flow-related solute concentration, and evaporation-related solvent. To this end, we carry out spin coatings of a non-aqueous poly(N-vinylcarbazole) (PVK) for a hole transporting blanket layer. With a low-boiling-point (BP) organic solvent, the pre-drying makes no significant impact on the thickness profiles. With a high-BP organic solvent, the PVK films pre-dried in a vacuum for a sufficient time exhibit very uniform light emission in the central region, but non-emission phenomenon near the perimeter of pixels. It is addressed that such a non-emission phenomenon can be suppressed to some extent by decreasing the vacuum pressure. However, the rapid evaporation by heat conduction during the pre-drying degrades the thickness uniformity due to a rapid microflow of solute from the edge to the center. No further enhancement in the thickness uniformity is obtained by varying the solute concentration and using a mixture of low- and high-BP solvents.

Fabrication of Circuits on Flexible Substrates Using Conductive SU-8 for Sensing Applications.

This article describes a new low-cost rapid microfabrication technology for high-density interconnects and passive devices on flexible substrates for sensing applications. Silver nanoparticles with an average size of 80 nm were used to create a conductive SU-8 mixture with a concentration of wt 25%. The patterned structures after hard baking have a sheet resistance of 11.17 Ω/☐. This conductive SU-8 was used to pattern planar inductors, capacitors and interconnection lines on flexible Kapton film. The conductive SU-8 structures were used as a seed layer for a subsequent electroplating process to increase the conductivity of the devices. Examples of inductors, resistor-capacitor (RC) and inductor-capacitor (LC) circuits, interconnection lines and a near-field communication (NFC) antenna are presented as a demonstration. As an example of high-resolution miniaturization, we fabricated microinductors having line widths of 5 m. Mechanical bending tests were successful down to a 5 mm radius. To the best of the authors’ knowledge, this is the first report of conductive SU-8 used to fabricate such planar devices and the first on flexible substrates. This is a proof of concept that this fabrication approach can be used as an alternative for microfabrication of planar passive devices on flexible substrates.