Wide Process Window Research Articles

Water states and thermal processability of boric acid modified poly(vinyl alcohol)

ABSTRACTTo further improve the processability of water plasticized poly(vinyl alcohol) (PVA), boric acid (BA), which can rapidly form reversible crosslinked structure with the hydroxyl groups of PVA, was adopted as a modifier, and the water states, thermal performance, and rheological properties of modified PVA were investigated. The results showed that ascribing to the formation of the crosslinked structure between PVA and BA, the content of nonfreezing water in system increased, indicating that the bondage of PVA matrix on water enhanced, thus retarding the tempestuous evaporation of water in system during melt process and making more water remained in system to play the role of plasticizer. Meanwhile, this crosslinked structure shielded part hydroxyl groups in PVA chains, leading to the further weakening of the self‐hydrogen bonding of PVA, and guaranteeing a lower melting point and higher decomposition temperature, thus obtaining a quite wide thermal processing window, i.e., ≥179°C. The melt viscosity of BA modified PVA slightly increased, but still satisfied the requirements for thermal processing, thus reinforcing the flow stability of the melt at high shearing rate. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43246.

Gate patterning in 14 nm and beyond nodes: from planar devices to three dimensional Finfet devices

In this work, we investigated the challenges encountered in 14nm node Finfet gate patterning. The patterning process was originated from a 22nm planar device, in which a SiO2/Si3N4/SiO2 (ONO) multilayer was used as an etch mask. To accommodate with the 3D nature of Finfet structures in 14nm node, the thickness of Si3N4 has been increased in the investigated process. We found out that the standard ONO mask etch process was no longer effective for gate patterning in 3D Finfet devices. It was observed that the etched mask sidewall was significantly more tapered than that in planar devices, resulting in the final CDs of both mask and dummy gate far wider than those of the planar devices. In order to achieve a desirable gate CD, the formation mechanism causing a severely tapered mask profile was first investigated. Our results suggested that redeposition effect of the etch products on the sidewall played a significant role in controlling etched mask sidewall angle. Then, we proposed a two-step etch process which can improve the anisotropy of ONO mask etch and obtain a steep etch profile with a desirable CD. Using this process, a gate CD of 20nm was successfully achieved with a desirable profile and a smooth sidewall. Our results have demonstrated that the newly developed etch process is very robust and has a wide process window.

Initial evaluation and comparison of plasma damage to atomic layer carbon materials using conventional and low Te plasma sources

The ability to achieve atomic layer precision is the utmost goal in the implementation of atomic layer etch technology. Carbon-based materials such as carbon nanotubes (CNTs) and graphene are single atomic layers of carbon with unique properties and, as such, represent the ultimate candidates to study the ability to process with atomic layer precision and assess impact of plasma damage to atomic layer materials. In this work, the authors use these materials to evaluate the atomic layer processing capabilities of electron beam generated plasmas. First, the authors evaluate damage to semiconducting CNTs when exposed to beam-generated plasmas and compare these results against the results using typical plasma used in semiconductor processing. The authors find that the beam generated plasma resulted in significantly lower current degradation in comparison to typical plasmas. Next, the authors evaluated the use of electron beam generated plasmas to process graphene-based devices by functionalizing graphene with fluorine, nitrogen, or oxygen to facilitate atomic layer deposition (ALD). The authors found that all adsorbed species resulted in successful ALD with varying impact on the transconductance of the graphene. Furthermore, the authors compare the ability of both beam generated plasma as well as a conventional low ion energy inductively coupled plasma (ICP) to remove silicon nitride (SiN) deposited on top of the graphene films. Our results indicate that, while both systems can remove SiN, an increase in the D/G ratio from 0.08 for unprocessed graphene to 0.22 to 0.26 for the beam generated plasma, while the ICP yielded values from 0.52 to 1.78. Generally, while some plasma-induced damage was seen for both plasma sources, a much wider process window as well as far less damage to CNTs and graphene was observed when using electron beam generated plasmas.

Hot deformation behavior and dynamic recrystallization of a β-solidifying TiAl alloy

Hot deformation behavior and dynamic recrystallization (DRX) of a β-solidifying TiAl alloy Ti–43Al–2Cr–2Mn–0.2Y (at%) were investigated by means of isothermal compression tests in a temperature range of 1100–1225°C and strain rate range of 0.01–0.05s−1. A processing map at a true strain 0.8 was developed based on the dynamic materials model. The current alloy possesses a wide processing window, and the optimum deformation condition is 1175–1225°C/0.05s−1. The main softening mechanism is DRX of γ grains. The DRX proceeded preferentially at triangle boundaries, and subsequently in the lamellae. Both deformation temperature and strain rate have an obvious impact on the microstructural evolution. The partial DRX occurred when the alloy deformed at low temperature and high strain rate. Some remnant lamellar structures existed due to the plastic anisotropy of lamellar colony. With the increase of temperature and /or the decrease of strain rate, the lamellae were effectively broken down into fine grains, and a relatively complete DRX occurred. Dynamic recovery is the main deformation mechanism for β0 phase, which promotes the compatible deformation between γ and α2 phases and prevents premature cracks. Additionally, the DRX mechanism of γ phase was investigated through TEM observations. The DRX of γ grains begins with the formation of sub-boundaries, followed by a process of rearrangement of sub-boundaries to form new grains.

Synthesis and characterization of phenylethynyl-terminated polyimide oligomers derived from 2,3,3′,4′-diphenyl ether tetracarboxylic acid dianhydride and 3,4′-oxydianiline

With the goal of improving processability of imide oligomers and achieving high toughness of thermosetting polyimides, a series of 4-phenylethynylphthalic anhydride (PEPA)-terminated imide oligomers prepared by the reaction of 2,3,3′,4′-diphenyl ether tetracarboxylic acid dianhydride (a-ODPA) and 3,4′-oxydianiline (3,4′-ODA) with different molecular weights (degree of polymerization: n = 1–9) were formed. The resultant oligomers with different molecular weights were characterized for their chemical architecture, cure behavior, thermal properties, solubility in organic solvents and rheological characteristics. Besides, the thermal properties and tensile test of cured polyimide films were also evaluated. The imide oligomer (degree of polymerization: n = 1) has some somewhat crystalline phase, and imide oligomers (degree of polymerization: n = 2–9) showed excellent solubility (40 wt%) in N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc) at room temperature. Furthermore, the rheological properties of imide oligomers showed very low melt viscosity and wider processing window. The cured films exhibited good thermal properties with the glass transition temperatures of 282–373 °C and 5 wt% thermal decomposition temperatures higher than 551 °C in nitrogen atmosphere. The elongation at break of the prepared films was found to be high (almost > 9.3%).

Synthesis and characterization of multifunctional propenyl-endcapped aromatic co-monomers and their use as bismaleimide modifiers

A series of novel modifiers for bismaleimide, bearing propenyl and phenoxy functional groups has been synthesized. Structural information of the monomers was obtained through Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) spectroscopy and elemental analysis. Polymerization characteristics demonstrate that all four systems prepared have a cure temperature below 210°C. This remarkably lower cure temperature compared to that of other polymerization reactions involving diallyl bisphenol A and bismaleimide (DBMI) originates from propenyl groups being present in the structures as well as their larger free volume. The rheological behaviors leading to low melt viscosities and the wide process window of the prepolymer are particularly suitable characteristics for the production of performance resin-based composite materials via resin transfer molding processes. The dynamic mechanical analysis of the materials reveals glass transition temperatures in a range between 260°C and 293°C. Thermal stabilities show a 5% weight loss at temperatures ranging from 363°C to 428°C with the production of char ranging from 38.5% to 57.6% at 800°C under nitrogen. The latter is a clear indication for the excellent thermal stabilities featured by the cured resins. Furthermore, the dielectric properties exhibit a significantly lower dielectric constant and dissipation factors of the propenyl-modified cured systems compared to those of DBMI resins at 10 GHz. Copyright © 2015 John Wiley & Sons, Ltd.

Toward efficient Cu(In,Ga)Se2 solar cells prepared by reactive magnetron co‐sputtering from metallic targets in an Ar:H2Se atmosphere

AbstractIn this paper, we show that a reactive co‐sputtering process using metallic CuGa and In targets; an Ar:H2Se atmosphere is well suited for the deposition of photoactive Cu(In,Ga)Se2 (CIGSe) absorber layers for thin‐film solar cells in a single process step. The achievement of single‐phase and well‐crystallized layers is thereby no major problem if a sufficiently high H2Se content and substrate temperatures in the range of 400–500 °C are used. However, in order to achieve the desired Cu‐poor film stoichiometry, which is crucial for the device performance, it has to be considered already that, at moderate substrate temperatures in the range of 400–500 °C, indium has a strong tendency to re‐evaporate from the film surface if the film composition is Cu‐poor. If excess indium is supplied, this effect can lead to a self‐adjustment of the film composition. This allows a very wide process window in a one‐stage process concerning the supply ratio from the two targets of [Cu]/([In] + [Ga])supply ≈ 0.35–0.8. However, the maximum efficiencies achievable with such a process are limited to 11.7% because an adequate Cu‐poor composition can only be achieved with significant Cu‐poor conditions, which allow only a low material quality. By using an improved process with an intermediate Cu‐rich composition and a final Cu‐poor stage, the absorber quality could be significantly improved; efficiencies of up to 14.3% have been achieved with CIGSe films prepared on Na‐doped Mo back contacts. Copyright © 2015 John Wiley & Sons, Ltd.

Surface prediction model for thermocapillary regime pulsed laser micro polishing of metals

The objective of this work is to develop a surface prediction model for thermocapillary regime pulsed laser micro polishing (PLμP). Two distinct polishing regimes have been discovered: capillary and thermocapillary. The difference between the two regimes is the melt pool flow mechanisms. A surface prediction model for capillary regime PLμP has been developed with results within 10% of experimental measurements. However, this model's predictions deviate from measured outcomes for longer pulse durations due to the presence of thermocapillary flow, which is not included in the capillary regime model. Previous work has also shown that this thermocapillary flow can be well predicted by multiphysics modeling [5] and that a simpler analytical relation can be derived to predict the extent of thermocapillary flow in the form of a normalized average displacement (NAD) of the liquid metal during laser polishing [5]. The current work incorporates the NAD analytical prediction for thermocapillary flow into the capillary regime surface prediction method to create a model that is valid for both the capillary and thermocapillary polishing regimes. The proposed thermocapillary flow model was tested for area polishing of titanium alloy Ti-6Al-4V and S7 tool steel. Conditions were chosen to include both the capillary and thermocapillary regimes. In all cases, the predicted average surface roughnesses were within 15% of the measured values. The results indicate that the proposed prediction model adequately captures the process physics and can provide a guide for parameter selection and process optimization of PLμP across a wide processing window.

Analysis of surface-reaction layers formed by etching Si3N4 with pulsed-microwave plasma

The mechanism of highly selective etching of Si3N4 by a pulsed-microwave electron-cyclotron-resonance plasma was investigated by analyzing surface-reaction layers formed by etching using a CH3F/O2/Ar gas chemistry. A hydrofluorocarbon (HFC) layer formed not only on nonpatterned materials (Si3N4 and others) but also at the bottoms of line-and-space patterns were analyzed by X-ray photoelectron spectroscopy. Thermal reactivity between the HFC layer and the Si3N4 layer was also investigated by thermal-desorption spectroscopy. The investigation results show that nitrogen contained in the Si3N4 layer thermally reacted with the HFC layer to form NH3 or HCN, and silicon contained in Si3N4 had high reactivity with fluorine contained in the HFC layer. Owing to the high reactivity between the fluorine-rich HFC layer and the Si3N4 layer in the pulsed-microwave plasma, the HFC layer became thin, even at a low wafer bias, and thus promoted ion-assisted etching. A wide process window was provided by the formation of the fluorine-rich thin HFC layer using the pulsed-microwave plasma.

Investigation of Wide Process Temperature Window for Amorphous Silicon Suboxide Thin-Film Passivation Deposited by Inductively Coupled PECVD

Hydrogenated intrinsic amorphous silicon suboxide thin films deposited onto c -Si wafers by decomposing hydrogen, silane, and carbon dioxide in an industrial remote inductively coupled plasma tool are studied. Compared with intrinsic amorphous silicon deposited in the same tool, this material displays an improved process temperature window and excellent surface passivation quality, which is important for industrialization. The wide process window of over 200 °C (100 to 350 °C) mainly results from the slow depletion of H atoms at elevated temperature due to a suppressed epitaxial growth, whereas the excellent passivation quality is due to a much higher H content in the film compared with amorphous silicon. The temperature stability is further supported by a study using a high-resolution transmission electron microscopy. Under the optimal condition, the amorphous silicon suboxide demonstrates an effective minority carrier lifetime of over 4.7 ms on planar n-type 1-Ω · cm Czochralski silicon wafers, which is equivalent to an effective surface recombination velocity of less than 1.7 cm/s, and an implied open-circuit voltage of 741 mV.

Process Optimization of CoSi2 Formation on P-Doped Poly-Si by Hot Wall-Based Rapid Thermal Annealing

Low resistivity junctions and low resistivity contacts are essential for fabricating high performance devices with advanced device modes. Significant effort has been expended to develop low resistivity junctions, industry wide. However, low resistivity contact formation has not received significant attention, other than changing the silicide of choice, depending on required minimum line width or device node. Once the candidate silicide material is chosen for the device node, relatively small improvement in resistivity has been made in recent years, despite silicidation process optimization which include modifications of process steps and integration schemes. In this study, we have investigated the effect of annealing temperature and time on the resulting resistivity of Co2Si contacts with P-doped poly-Si using a single wafer furnace-based (hot wall) rapid thermal annealing (RTA) system. The hot wall RTA resulted in significantly (>20%) lower Rs, down to 19.6 ohm/sq. from equivalent RTA process using conventional tungsten halogen lamp-based (cold wall) RTA systems over a very wide process window (RTA temperature and time). Secondary ion mass spectroscopy (SIMS) dopant (P) depth profiling results revealed that the P atoms in the CoSi2 film and at the CoSi2/P-doped poly-Si interface, redistribute very differently under different RTA conditions. The same Rs value can be realized by different P profiles resulting from different RTA conditions. To enable contact resistance reduction, P pile-up near the CoSi2/P-doped poly-Si interface for and to suppress P depletion from P-doped poly-Si layer for resistivity reduction, lower temperature RTA for longer times in the hot wall RTA system was found to be beneficial. Wafers with almost identical P depth profiles after CoSi2 formation RTA in the cold wall RTA and the hot wall RTA systems, still showed an Rs difference of ~20%. The wafers annealed in the hot wall RTA system always showed lower Rs values indicating higher P dopant activation in the P-doped poly-Si layer. The difference in wafer heating mechanisms between the cold wall (lamp-based) RTA and the hot wall (furnace-based) RTA may have played an important role in the dopant activation and Rs differences. Hot wall RTA is very promising for realizing the low resistivity contacts required in advanced high performance devices. The simple change of annealing technique may extend the life cycle of CoSi2/P-doped poly-Si contacts. The Rs response surface (as a function of RTA temperature and time) and SIMS depth profiles of P atoms will be presented at the meeting. Figure 1

Green flame retarding bismaleimide resin with simultaneously good processing characteristics, high toughness and outstanding thermal stability based on a multi-functional organic boron compound

Multi-function and green are two keywords of developing new flame retarding thermosetting resins, however, to achieve this target is still a big challenging today. Herein, a new kind of flame retarding bismaleimide resins with simultaneously good processing characteristics, high toughness and outstanding thermal stability were prepared by copolymerizing 4,4′-bismaleimidodiphenylmethane (BDM) with allyl triphenylborate (ATPB). The structure and integrated performances of BDM/ATPB resins were systematically studied and compared with the BDM/o,o′-diallylbisphenol A resin (coded as BD) that is almost known to be the best modified bismaleimide resin available. Results show that BDM/ATPB resins are solids with low softening points; they can be dissolved in acetone and have wide processing window, completely overcoming the poor processing characteristics of BDM. The properties of the BDM/ATPB system are dependent on the molar ratio of imide and allyl groups, and BDM/ATPB3 resin of which the molar ratio of imide to allyl groups (1:0.85) is the same as that of BD resin not only has significantly improved flame retardancy, reflected by obviously longer time to ignition, 1.5–3.0 times higher fire performance index, and greatly decreased heat releases, but also has about 10 °C higher initial decomposition temperature in both air and nitrogen atmospheres as well as about 1.2–1.3 times higher impact and flexural strengths, clearly demonstrating that ATPB is a multi-functional and green modifier for bismaleimide. The origin behind these attractive results of BDM/ATPB resins was intensively discussed.

Process Optimization of CoSi2 Formation on P-Doped Poly-Si by Hot Wall-Based Rapid Thermal Annealing

To achieve low resistivity contacts, the effect of annealing temperature and time on the resulting resistivity of CoSi2 contacts with P-doped poly-Si was investigated using a single wafer furnace-based (hot wall) rapid thermal annealing (RTA) system. The hot wall RTA resulted in significantly (>20%) lower sheet resistance (Rs) from the equivalent RTA process using conventional tungsten halogen lamp-based (cold wall) RTA systems over a very wide process window. Dopant (P) depth profiling results by secondary ion mass spectroscopy (SIMS) revealed that the P atoms in the CoSi2 film and at the CoSi2/P-doped poly-Si interface, redistribute very differently under different RTA conditions. The CoSi2 formation process was optimized utilizing characteristics of P pile-up near the CoSi2/P-doped poly-Si interface to suppress P depletion from the P-doped poly-Si layer.

Phenylethynyl- and naphthylethynyl-terminated hyperbranched polyimides with low melt viscosity

In the past decades, 4-phenylethynyl phthalic anhydride has been the most commonly used endcapper for polyimide (PI). A series of different groups of terminated hyperbranched PIs (HBPIs) were synthesized from 1,3,5-tris(2-trifluoromethyl-4-aminophenoxy)benzene, bis[4-(3,4-dicarboxyphenoxy)phenyl]ether dianhydride, and 4-phenylethynylphthalic anhydride or 4-(1-naphthylethynyl)phthalic anhydride. The glass transition temperatures ( Tgs) of the polymers varied from 166°C to 193°C. The 5% weight loss temperatures of polymers were in the range of 548 to 571°C and those of the endcapped HBPIs increased to the range of 600 to 601°C after curing. The PIs exhibited rather low melt viscosity at processing temperature and the complex viscosity increased continuously with the temperature increase. Because there is a large temperature difference between the Tg and cross-linking exothermic temperature, HBPIs provided a wide processing window.

Effect of mid-annealing process on the device characteristics of the TFT using Al-doped ZnO active channels prepared by atomic layer deposition

A specified mid-annealing process, which is a thermal treatment in oxygen ambient right after an active layer deposition, was proposed for obtaining a sufficiently wide process window for the atomic layer deposition in order to realize a high performance Al-doped ZnO (AZO) thin-film transistors (TFTs). While the crystalline phases of the AZO thin films were not changed after the mid-annealing process, the electrical conductivities of the films experienced drastic changes owing to the significant reduction of oxygen vacancies during the mid-annealing process. The decrease in the electrical conductivity was more markedly observed for the AZO films prepared at higher deposition temperature. Top-gate-structured TFTs using the mid-annealed AZO active channel layers were fabricated and characterized. Sound on/off switching behaviors of TFTs were obtained at a wider range of deposition temperature. Additionally, the improvements in carrier mobility and negative bias stress stability were successfully confirmed.

Towards Contact Resistance Minimization through CoSi2 Formation Process Optimization on P-Doped Poly-Si by Hot Wall-Based Rapid Thermal Annealing

The effect of annealing temperature and time on the resulting resistivity of CoSi2 contacts with P-doped poly-Si was investigated using a single wafer furnace-based (hot wall) rapid thermal annealing (RTA) system. To achieve low resistivity contacts, CoSi2 formation process optimization was done by detailed design of experiment (DOE). Sheet resistance (Rs) response surface plots as a function of annealing temperature and time for 1st step and 2nd step RTA processes showed a very wide process window. The hot wall RTA resulted in significantly (>20%) lower Rs, over a very wide process window, compared to the equivalent RTA process using conventional tungsten halogen lamp-based (cold wall) RTA systems. Dopant (P) depth profiling results by secondary ion mass spectroscopy (SIMS) revealed that the P atoms in the CoSi2 film and at the CoSi2/P-doped poly-Si interface, redistribute very differently under different RTA conditions. The CoSi2 formation process was optimized utilizing characteristics of P pile-up near the CoSi2/P-doped poly-Si interface to suppress P depletion from the P-doped poly-Si layer for contact resistance minimization.

Growth behavior and structural characteristics of TiO2thin films using (CpN)Ti(NMe2)2and oxygen remote plasma

This study examined the effect of a novel precursor on the growth behavior and structure of titanium dioxide (TiO2) thin films. TiO2 thin films were deposited by remote plasma atomic layer deposition (RPALD). Dimethylamin-cyclopentadienyl titanium, (CpN)Ti(NMe2)2, was used as a Ti precursor and oxygen plasma was used as a reactant. (CpN)Ti(NMe2)2 is a newly synthesized precursor for TiO2 deposition. Characteristics of TiO2 thin films deposited by (CpN)Ti(NMe2)2 were compared with those of films deposited using a titanium tetrakis isopropoxide (TTIP) precursor. A wide processing window from 170 to 400 °C was observed, and the growth rate was around 0.44 Å/cycle at 200 °C. X-ray photoelectron spectroscopy (XPS) measurements revealed that nearly stoichiometric TiO2 films were deposited. The residual impurity concentrations inside the films were almost the same for (CpN)Ti(NMe2)2 and TTIP. TiO2 films deposited on ruthenium showed a pure rutile phase at a low deposition temperature and low power pre-treatment. Therefore, this TiO2 film is very promising for metal–insulator–metal (MIM) capacitor dielectrics for dynamic random access memory (DRAM) applications.

Surface passivation and antireflectance performances for atomic-layer-deposited Al2O3 films

Al2O3 films were deposited on single crystalline silicon wafers by atomic layer deposition. Both passivation and antireflectance performances are studied in detail. 30 nm Al2O3 passivated n-type and p-type Si shows a maximum effective minority carrier lifetime (τeff) of ∼5.2 ms and ∼4.7 ms, corresponding to a low surface recombination velocity of ∼3.8 cm s−1 and ∼4.2 cm s−1, respectively. By drawing a contour map of post-deposition annealing (PDA) temperature, PDA time and τeff, a wide PDA window is obtained for obtaining good passivation performances. The excellent passivation performances are related to the large, negative fixed charge density within Al2O3 films and the formation of interfacial SiO2 layer at the Al2O3/Si interface. Antireflectance performances are also studied in detail for Al2O3 and Al2O3/SiNx double-layer on textured Si. For obtaining a low average reflectance between 2.6% and 3%, a wide film-thickness window of ∼30 nm for Al2O3 and SiNx layer is observed for Al2O3/SiNx double-layer on textured Si. Our results indicate that Al2O3 films have excellent surface passivation and antireflectance performances with a wide processing window, which is favorable for c-Si solar cell applications.

Through-Silicon-Via(TSV) Filling by Electrochemical Deposition with Pulse-Reverse Current

TSV interconnection was widely investigated for next generation integrated-circuit(IC) which has significantly innovated function, excellent performance and high efficiency. Especially, TSV filling process by Cu electrochemical deposition occupied almost 40% of cost in the total TSV process. Therefore, a plenty effort has been put into decrease the processing time and prevent the formation of defects such as void or seam which could cause critical electrical and mechanical problems in the device. In the past, TSV filling studies were mainly focused on the function of chemical additives or bottom-up filling mechanism. In these kinds of researches, TSV filling process window was extremely limited because the applied current in the electrochemical process was a simple direct current or a constant voltage condition. Therefore, TSV filling by electrochemical deposition with pulse current was reported by our group for achieving the extensively wide process window. In this study, TSV filling by electrochemical deposition with pulse-reverse current was carried out for achieving even more extended process window.Electrochemical deposition experiments were carried out in the 1 M CuSO4 electrolyte with suppressor and accelerator. TSV (diameter: 6 μm, depth: 60 μm) coupon sample and Pt electrode was used as working and counter electrode, respectively. Pulse-reverse current was consisted with the peak current (ipeak) and the reverse current (irev) that applied during the on time (ton) and the reverse time (trev) period, respectively. (figure 1) During the on time and reverse time period, deposition and dissolution of Cu was occurred, respectively. The average current (iavg) could be calculated by equation in the figure 1. Peak current density and on time used in this study was fixed to 450 mA/cm2 and 2 μs, respectively. Vertically filled shapes of electrodeposited TSVs were observed by optical microscope (OM). Through achieved images, “bottom-up filling ratio” and “side wall growth ratio” was calculated for analyzing the TSV filling performance.Figure 2a shows the effect of reverse current density. The average current density and processing time was fixed to 2 mA/cm2 and 15 min, respectively. Bottom-up filling ratio was decreased with decreased reverse current. However side wall growth ratio was increased. These results were mainly caused by following reasons. Desorption of suppressor on the side wall during the reverse time was accelerated by increased reverse current. Even the ion depleted layer might be thicker due to decreased anodic current efficiency during the reverse time. Insufficiently relaxed ion depleted layer caused increasing the peak overpotential during the on time period. High overpotential caused accelerating the desorption rate of suppressor. Therefore, inhibition effect on the side wall was decreased, and thicker side wall was formed. Bottom-up filling ratio was naturally decreased because diffusing cupric ions from the top to the bottom of the via was preferentially consumed on the side wall.Figure 2b shows the effect of increased average current density in fixed reverse current density (irev = 5 mA/cm2). Both bottom-up filling ratio and side wall growth ratio was increased as average current density increased due to increased average current density. However, TSV filling mode was suddenly changed to conformal filling at the 5 mA/cm2. This result was mainly caused by following reason. Reverse time was decreased to increase the average current density in fixed reverse current condition. Decreased reverse time caused insufficient ion supplying to the ion depleted layer and ion depleted layer thickness was increased. Therefore, decreased bottom-up filling and increased side wall growth was occurred due to same reason as case of controlling the reverse current.

Direct laser writing of synthetic poly(amino acid) hydrogels and poly(ethylene glycol) diacrylates by two-photon polymerization

The additive manufacturing technique of direct laser writing by two-photon polymerization (2PP-DLW) enables the fabrication of three-dimensional microstructures with superior accuracy and flexibility. When combined with biomimetic hydrogel materials, 2PP-DLW can be used to recreate the microarchitectures of the extracellular matrix. However, there are currently only a limited number of hydrogels applicable for 2PP-DLW. In order to widen the selection of synthetic biodegradable hydrogels, in this work we studied the 2PP-DLW of methacryloylated and acryloylated poly(α-amino acid)s (poly(AA)s). The performance of these materials was compared to widely used poly(ethylene glycol) diacrylates (PEGdas) in terms of polymerization and damage thresholds, voxel size, line width, post-polymerization swelling and deformation. We found that both methacryloylated and acryloylated poly(AA) hydrogels are suitable to 2PP-DLW with a wider processing window than PEGdas. The poly(AA) with the highest degree of acryloylation showed the greatest potential for 3D microfabrication.