Photosensitive Polyimide Research Articles

Bonding Pad Over Active Structure for Chip Shrinkage of High-Power AlGaN/GaN HFETs

This paper reports a bonding pad over active (BPOA) structure with photosensitive polyimide (PSPI) as the intermetal dielectric layer to reduce the chip size of high-power enhancement-mode AlGaN/GaN heterojunction FETs (HEFTs) on a 150-mm (6-in) Si substrate. The fabricated AlGaN/GaN HFETs with a BPOA structure exhibited a threshold voltage and a maximum current of 0.6 V and 38.6 A, respectively, at $V_{\textrm {GS}}$ of 6 V. The leakage current was $4.3 \times 10^{-5}$ A at 700 V, which was the same value as that of the AlGaN/GaN HFETs without a BPOA structure. The reliability of the AlGaN/GaN HFETs with the BPOA structure during the packaging process was improved by the PSPI layer and a modified bonding pad structure, which endured the stress during the wedge wire bonding and epoxy molding processes.

Development of Novel Low-temperature Curable Positive-Tone Photosensitive Polyimide with High Elongation

Development of Novel Low-temperature Curable Positive-Tone Photosensitive Polyimide with High Elongation

Polyimide-isolated ridge waveguide InGaN/GaN laser diodes based on back-ward exposure

The manufacture of metal electrode window on the ridge structure of laser diodes (LDs) is a confused issue in the LDs fabrication process. We show that the processing of the self-aligned step and negative photosensitive polyimide (NPP) thermal imidization which can achieve both accurate alignment of p-electrode window on the ridge structure and integrated dielectric layer at once time. This back word exposure technic can effectively simplify the process of accuracy of alignment and avoid etching of isolating layer. The optical and electrical performance of the LD chip measured at room temperature under a pulse current condition (the current duty ratio was 25%) were shown. The output optical power was greater than 20 mW, at 7.7 V operating voltage, and the reversed current is 3.9 × 10−5 mA when the bias voltage is −5 V. A lasing peak wavelength at around 442 nm became dominant at 320 mA. Our results provide an alternative effective approach to fabricate the well behavior LDs.

Band-Separated UV Exposure on a Photosensitive Polyimide Layer With Embedded Reactive Mesogen for a High-Speed Liquid Crystal Display Device

In general, surface anchoring energy is one of the most important parameters that strongly affect electrooptical performance, especially the optical response time of liquid crystal (LC) displays. In this paper, we proposed a photoalignment method for strong surface anchoring energy by applying double ultraviolet (UV) exposure with a separated bandwidth to a photosensitive polyimide layer with embedded reactive mesogens (RMs) that can increase the anchoring energy during polymerization. We first polymerized the embedded RM molecules using long wavelength UV rays (over 340 nm), and then achieved the ordered photosensitive alignment layer using short UV rays (between 254 and 340 nm). We compared the measured surface anchoring energy and the optical response time by the proposed method with the conventional UV exposure using the in-plane switching LC mode for verification. As a result, we confirmed that the anchoring energy increased two times and the optical response time improved by 22% compared with the conventional pure UV exposure method.

Low Temperature Photosensitive Polyimide Based Insulating Layer Formation for Microelectromechanical Systems Applications

Photosensitive polyimide (PSPI) based insulating layer fabrication for microelectromechanical systems (MEMS) application has been systematically investigated in this work. The PSPI was spin coated on a silicon substrate as an insulating layer between two metal lines. In consideration of thermal properties, a low temperature hard bake process was carefully optimized. Finally, the polyimide insulating layer was hardened by exposure to air at 80°C for 120 min + 150°C for 60 min + 180°C for 60 min + 250°C for 60 min + 350°C for 60 min in a dry furnace. Using this optimized hardening process, the outgassing effect in post-fabrication heat treatment can be completely eliminated, which presented an excellent thermal resistance in MEMS fabrications. The reliability test was accomplished through an immersion in organic solvent and acid/base solution in order to verify the corrosion resistance of the PSPI frame for insulating application. The excellent resistance which is on the order of 108 Ω between two metal lines, shows an outstanding insulating property.

TCT Reliability of Organic Passivation Layer for WLCSP

Wafer level chip scale packages (WLCSP) have been increasingly used in portable electronic products such as mobile phones. Solder bumps with redistribution layer (RDL) are typical interconnect technology for WLCSP applications. One of the major concerns in joint reliability is the failure by temperature cyclic stresses. In addition, in terms of heat tolerance or device yields, process temperature of RDL dielectric is limited around 200deg.C in some packaging applications. According to our board level reliability test for temperature cycle test (TCT), photosensitive polyimide (PI) which is 200deg.C curable material has lower fail rate than polybenzoxazole (PBO) by TCT. In this study, we compared the actual board level test and Finite Element Analysis (FEA) during temperature cycle test, and correlated the mechanical and fatigue properties of passivation layer material with TCT reliability.

Dielectric properties of photocrosslinkable polyimide/functional graphene oxide composites

Functional graphene oxide (FGO) with photosensitivity was incorporated into the photosensitive polyimide (PSPI) matrix via solution blending, to prepare the photocrosslinkable nano-composite films. The crosslinked composite films were obtained through UV irradiation. FGO was well dispersed in PSPI matrix and effectively covalently bonded to PSPI main chains through photo-chemical reaction. Due to the confine of electronic polarization caused by the crosslinked structure, the dielectric constant (k) and dielectric loss tangent (tanδ) of the crosslinked PSPI/FGO films decreased effectively, which could be tuned by varying both the UV irradiation time and FGO loading. Among all the composite films, the film with 0.5wt% FGO content exposed under UV light for 900s had the lowest k (2.58) and tanδ (0.026) values at 10MHz. These findings suggest that PSPI/FGO composite films are promising application in the field of photoresist with low dielectric constant.

Study on the Etching Technique of Non-Photosensitive Polyimide

Polyimide is often used as a sacrificial layer material to make floating structure. Polyimide is also divided into photosensitive and non-photosensitive type; photosensitive polyimide currently has more negative photoresist and poor performance in many ways. Compared with photosensitive polyimide, the non-photosensitive type has low stress, stable performance and other advantages, so non-photosensitive polyimide has been chosen as a sacrificial layer material. To achieve the graphical function and release sacrificial layer, A deeply research was made in this dissertation makes on wet etching and dry etching. By controlling the wet etching process of prebake condition, exposure and developing time, and oxygen dry etching process of etching power, bias voltage and other key process parameters, a good sacrificial layer graph and etching effect have been got. Finally, it can be concluded that when the prebake conditions for 105°C, 8min and times of exposure and developing were 11s and 29s, the non-photosensitive polyimide wet etching effect is the best; when the etching power is 1000w, an oxygen flux rate is 50sccm, the reaction pressure is 30mTorr, the bias voltage is 140v, oxygen dry etching has a good effect.

Effect of Anisotropic Photo-crosslinking on Liquid Crystal Alignment with Duplicated Surface-relief-gratings†

Surface-relief-gratings with further photo-crosslinkable property were prepared by softlithography using photo-sensitive polyimide containing cinnamate groups as "ink ". The effect of anisotropic photocrosslinking on the thermos-stability of gratings and the alignment of liquid crystal( LC) on gratings was studied. The thermos-stability of the photo-cured gratings was significantly increased. No shape deformation was observed for the gratings after treated at a temperature 31 ℃ above the glass transition temperature of the uncured polyimide. Both the un-exposed and exposed gratings induced uniform LC alignment on their surfaces.By irradiation with linear polarized UV light perpendicular to the gratings grooves,the LC alignment was enhanced. By changing the polarized direction of the UV light and controlling the depths of gratings,the LC alignment direction on the gratings was adjusted. This study result shows that the photo-crosslinkable gratings can achieve better thermo-stability,better LC alignment and adjustable aligned direction to achieve multi-domain alignment. The above research can lead to further understanding of the LC alignment mechanism.

Photo-Induced Polarity Change of Photosensitive Polyimide with o-Nitrobenzyl Ester Side Chain

Photo-Induced Polarity Change of Photosensitive Polyimide with o-Nitrobenzyl Ester Side Chain

Photosensitive Polyimide for Packaging Applications

Photosensitive Polyimide for Packaging Applications

Flexible Solution-Gated Graphene Field Effect Transistor for Electrophysiological Recording

Graphene represents a high-performance material for field effect transistor (FET) manufacture. As a monolayer of carbon atoms, graphene offers flexibility, high carrier mobility, and transparency. Solution-gated graphene FETs based on flexible substrates have been developed for various applications. However, none of these FETs offer both flexibility and good insulation from the electrolyte simultaneously. This would restrict their applicability in certain areas, such as implantation. To solve this problem, we used commercially available graphene and polyimide to fabricate a graphene FET. Photosensitive polyimide was used as both substrate and insulator for our device. Two Au electrodes were used separately as source and drain, and the graphene was exposed to an electrolytic solution to form a solution-gated structure. In this paper, we have studied the resistance variation between source and drain during specific fabrication steps to optimize the fabrication process. By analyzing the FET performance after repeated bending tests, our device's performance reliability was also confirmed. When compared with similar published flexible devices, our FET showed higher transconductance and lower noise levels. Also, neural activities were recorded from cortex neurons cultured on graphene FET for the first time, which proved our device's biocompatibility and demonstrated its potential for electrophysiological applications.

Alkaline-developable and Positive-type Photosensitive Polyimide based on Fluorinated Poly(amic acid) from Diamine with High Hydrophobicity and Fluorinated Diazonaphtoquinone

Alkaline-developable and Positive-type Photosensitive Polyimide based on Fluorinated Poly(amic acid) from Diamine with High Hydrophobicity and Fluorinated Diazonaphtoquinone

Self-aligned flat ultra-thin chip package for flexible circuits

Purpose – Ultra-thin chip packaging (UTCP) is one of the flexible assembly technologies, by which thinned dies are encapsulated inside spin-coated dielectric films. For sake of higher density integration and bending stress suppression, two UTCPs can be stacked vertically. The purpose of this paper is to present an improved UTCP process flow to embed thinned chip in a symmetric dielectric sandwich for a flat topography. The UTCP flat top surface is suitable for metallization and further 3D stacking. Design/methodology/approach – In the new process, a central photosensitive polyimide film is introduced, in which a cavity is made for the embedded chip. The cavity is defined by lithography using the chip itself as a photo-mask. In this way, the cavity size and position is self-aligned to the chip. The chip thickness is compensated by the surrounding central layer, and a UTCP with flat topography (flat UTCP) is realized after top dielectric deposition. Findings – A batch of daisy chain test vehicles was produced. The feasibility of the process flow is verified by optical and electrical measurements. The result shows 100 percent yield, which is much better than previous work. A thermal humidity test showed no significant degradation of the flat UTCPs after 1,000 hours. Originality/value – High yield fabrication of flat UTCP is first shown. An innovative self-alignment lithography step is introduced to make a cavity in dielectric for chip thickness compensation by using the chip itself as a photo-mask.

In vitro and in vivo evaluation of a photosensitive polyimide thin-film microelectrode array suitable for epiretinal stimulation

BackgroundEpiretinal implants based on microelectro-mechanical system (MEMS) technology with a polyimide (PI) material are being proposed for application. Many kinds of non-photosensitive PIs have good biocompatibility and stability as typical MEMS materials for implantable electrodes. However, the effects of MEMS microfabrication, sterilization and implantation using a photosensitive polyimide (PSPI) microelectrode array for epiretinal electrical stimulation has not been extensively examined.MethodsA novel PSPI (Durimide 7510) microelectrode array for epiretinal electrical stimulation was designed, fabricated based on MEMS processing and microfabrication techniques. The biocompatibility of our new microelectrode was tested in vitro using an MTT assay and direct contact tests between the microelectrode surface and cells. Electrochemical impedance characteristics were tested based on a three-electrode testing method. The reliability and stability was evaluated by a chronic implantation of a non-functional array within the rabbit eye. Histological examination and SEM were performed to monitor possible damage of the retina and microelectrodes. Electrically evoked potentials (EEPs) were recorded during the acute stimulation of the retina.ResultsThe substrate was made of PSPI and the electrode material was platinum (Pt). The PSPI microelectrode array showed good biocompatibility and appropriate impedance characteristics for epiretinal stimulation. After a 6-month epiretinal implantation in the eyes of rabbits, we found no local retinal toxicity and no mechanical compression caused by the array. The Pt electrodes adhesion to the PSPI remained stable. A response to electrical stimuli was with recording electrodes lying on the visual cortex.ConclusionWe provide a relevant design and fundamental characteristics of a PSPI microelectrode array. Strong evidences on testing indicate that implantation is safe in terms of mechanical pressure and biocompatibility of PSPI microelectrode arrays on the retina. The dual-layer process we used proffers considerable advantages over the more traditional single-layer approach and can accommodate much many electrode sites. This lays the groundwork for a future, high-resolution retinal prosthesis with many more electrode sites based on the flexible PSPI thin film substrate.

Photo-patternable polyimide gate insulator with fluorine groups for improving performance of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiopene (C10-BTBT) thin-film transistors

Surface properties of gate insulators strongly affect the device performance of organic thin-film transistors (OTFTs). To improve the performance of OTFTs, we have developed photo-sensitive polyimide gate insulator with fluorine groups. The polyimide gate insulator film could be easily patterned by selective UV exposure without any photoinitiator. The polyimide gate insulator film, fabricated at 130°C, has a dielectric constant of 2.8 at 10kHz, and leakage current density of <1.6×10−10A/cm2 while biased from 0 to 90V. To investigate the potential of the polyimide with fluorine groups as a gate insulator, we fabricated C10-BTBT TFTs. The field-effect mobility and the on/off current ratio of the TFTs were measured to be 0.76±0.09cm2/Vs and >106, respectively.

Electrochemistry in an Acoustically Levitated Drop

Levitated drops show potential as microreactors, especially when radicals are present as reactants or products. Solid/liquid interfaces are absent or minimized, avoiding adsorption and interfacial reaction of conventional microfluidics. We report amperometric detection in an acoustically levitated drop with simultaneous ballistic addition of reactant. A gold microelectrode sensor was fabricated with a lithographic process; active electrode area was defined by a photosensitive polyimide mask. The microdisk gold working electrode of radius 19 μm was characterized using ferrocenemethanol in aqueous buffer. Using cyclic voltammetry, the electrochemically active surface area was estimated by combining a recessed microdisk electrode model with the Randles-Sevcik equation. Computer-controlled ballistic introduction of reactant droplets into the levitated drop was developed. Chronoamperometric measurements of ferrocyanide added ballistically demonstrate electrochemical monitoring using the microfabricated electrode in a levitated drop. Although concentration increases with time due to drop evaporation, the extent of concentration is predictable with a linear evaporation model. Comparison of diffusion-limited currents in pendant and levitated drops show that convection arising from acoustic levitation causes an enhancement of diffusion-limited current on the order of 16%.

Broadband dielectric spectroscopy of BPDA/ODA polyimide films

Dielectric spectroscopy of a high-temperature photosensitive polyimide was investigated in wide temperature and frequency ranges during heating and cooling cycles (from −150 to 370 °C and from 0.1 to 1 MHz). During the heating phase measurements two sub-glass relaxation processes were observed, noted as γ and β relaxations. The γ relaxation appears at a low temperature (around −60 °C at 1 kHz) with an activation energy of 0.44 eV during the heating phase and disappears during the cooling one, indicating that the peak is initially related to the presence of water in the polyimide films. The β relaxation appears at higher temperatures (around 180 °C at 1 kHz) with a higher activation energy of about 1.5 eV. The β peak location and intensity for low temperatures (between 100 °C and 120 °C) change slightly on comparing the heating and cooling spectra, indicating also the effect of water molecules, which may act as a plasticizer. However, for higher temperatures, the β peak does not show any significant effect of the thermal cycle, and the relaxation is mainly attributed to the non-cooperative relaxation of the polyimide molecules. The ac conductivity (σ′) values show that the electronic hopping process is influenced by the dynamics of the segmental and macromolecular chains of the polyimide in the γ and β relaxation regions. At high temperatures (>250 °C) a plateau region appears in the ac conductivity allowing the extraction of the dc conductivity values, which are not affected between the heating and cooling measurements. This leads us to conclude that there are no significant morphological or chemical changes in the polyimide even for temperatures higher than its glass transition one under N2 for short periods. For temperatures above 300 °C an increase in the values of relative permittivity is observed and referred to the Maxwell–Wagner–Sillars or to the electrode polarization phenomena. In this range the activation energy of the polarization peak frequency, conductivity relaxation peak frequency and the dc conductivity is the same and equal to 2.4 eV, indicating that those three parameters are governed by the same underlying mechanism.

Facile formulation of alkaline-developable positive-type photosensitive polyimide based on fluorinated poly(amic acid), poly(amic acid), and fluorinated diazonaphthoquinone

An alkaline-developable, positive-type, and photosensitive polyimide (PSPI) based on fluorinated poly(amic acid) (FPAA), poly(amic acid) (PAA), and fluorinated diazonaphthoquinone (FDNQ-1) as a photoactive compound has been successfully developed. The solution of FPAA, PAA, and FDNQ-1 was spin-coated on a silicon wafer and prebaked. The two phase-separated layers, a thin top layer of FPAA containing FDNQ-1 and a thick bottom layer of PAA, were formed due to the high polarity difference between PAA and FPAA containing FDNQ-1. This phase separation was directly observed in an SEM image. The upper thin layer acted as a photosensitive layer, and a fine pattern was formed in the bottom layer. The PSPI containing PAA (85 wt%), FPAA (15 wt%), and FDNQ-1 (25 wt% to polymers) showed an excellent sensitivity of 60 mJ cm−2 and a high contrast of 3.3 when it was exposed to 365 nm wavelength light (i-line) and developed with a 0.238 wt% tetramethylammonium hydroxide aqueous solution (TMAHaq) at 25 °C. A fine positive pattern having a 6 μm resolution on the 1.0 μm film was obtained by exposure to 100 mJ cm−2i-line in the contact-printed mode. The resulting polymer film was converted to the corresponding PI cured at 300 °C. This novel positive patterning PI system can be one of the candidates for the next generation microchip fabrication process which provides a facile formulation of PSPI.

Facile photo-patterning of source and drain electrodes with photo-sensitive polyimide for organic thin-film transistors

We report a facile and cost-effective photo-patterning process to obtain patterned source and drain electrodes in organic thin-film transistors (OTFTs). Instead of conventional photoresist, photo-sensitive polyimide (PSPI) was used in the photo-patterning process. PSPI with cinnamate moieties could be crosslinked by UV-light exposure without any photoinitiator. After the photo-patterning process of this work, gold/PSPI bilayer electrodes were easily prepared and they were used as source and drain electrodes of the pentacene OTFT. The field effect carrier mobility, threshold voltage and on/off current ratio of the pentacene OTFT with the gold/PSPI bilayer electrodes were 0.067cm2/Vs, −5.3V and 1.4×104, respectively.