Mid-infrared resonant ablation of PMMA

Laser ablation proved to be a reliable micro-fabrication technique for patterning and structuring of both thin film and bulk polymer materials. In most of the industrial applications ultra-violet (UV) laser sources are employed, however they have limitations such as maintenance costs and practical issues. As an alternative and promising approach, mid-infrared resonant laser ablation (RIA) has been introduced, in which the laser wavelength is tuned to one of the molecular vibrational transi-tions of the polymer to be ablated. Consequently, the technique is selective in respect of processing a diversity of polymers which usually have different infrared absorption bands. In this paper, we present mid-infrared resonant ablation of PolyMethyl MethAcrylate (PMMA), employing nanosec-ond laser pulses tunable between 3 and 4 microns. This RIA nanosecond laser set-up is based on a commercial laser at 1064 nm pumping a singly resonant Optical Parametric Oscillator (OPO) built around a Periodically-Poled Lithium Niobate (PPLN) crystal with several Quasi-Phase Matching (QPM) periods. RIA has been successfully demonstrated for structuring bulk PMMA, and selective patterning of PMMA thin films on a glass substrate has been implemented

Accelerated hermeticity testing of biocompatible moisture barriers used for the encapsulation of implantable medical devices

Barrier layers for the long-term encapsulation of implantable medical devices play a crucial role in the devices’ performance and reliability. Typically, to understand the stability and predict the lifetime of barriers (therefore, the implantable devices), the device is subjected to accelerated testing at higher temperatures compared to its service parameters. Nevertheless, at high temperatures, reaction and degradation mechanisms might be different, resulting in false accelerated test results. In this study, the maximum valid temperatures for the accelerated testing of two barrier layers were investigated: atomic layer deposited (ALD) Al2O3 and stacked ALD HfO2/Al2O3/HfO2, hereinafter referred to as ALD-3. The in-house developed standard barrier performance test is based on continuous electrical resistance monitoring and microscopic inspection of Cu patterns covered with the barrier and immersed in phosphate buffered saline (PBS) at temperatures up to 95 °C. The results demonstrate the valid temperature window to perform temperature acceleration tests. In addition, the optimized ALD layer in combination with polyimide (polyimide/ALD-3/polyimide) works as effective barrier at 60 °C for 1215 days, suggesting the potential applicability to the encapsulation of long-term implants

Ultra-long-term reliable encapsulation using an atomic layer deposited Hfo2/Al2o3/Hfo2 triple-interlayer for biomedical implants

Long-term packaging of miniaturized, flexible implantable medical devices is essential for the next generation of medical devices. Polymer materials that are biocompatible and flexible have attracted extensive interest for the packaging of implantable medical devices, however realizing these devices with long-term hermeticity up to several years remains a great challenge. Here, polyimide (PI) based hermetic encapsulation was greatly improved by atomic layer deposition (ALD) of a nanoscale-thin, biocompatible sandwich stack of HfO2/Al2O3/HfO2 (ALD-3) between two polyimide layers. A thin copper film covered with a PI/ALD-3/PI barrier maintained excellent electrochemical performance over 1028 days (2.8 years) during acceleration tests at 60 °C in phosphate buffered saline solution (PBS). This stability is equivalent to approximately 14 years at 37 °C. The coatings were monitored in situ through electrochemical impedance spectroscopy (EIS), were inspected by microscope, and were further analyzed using equivalent circuit modeling. The failure mode of ALD Al2O3, ALD-3, and PI soaking in PBS is discussed. Encapsulation using ultrathin ALD-3 combined with PI for the packaging of implantable medical devices is robust at the acceleration temperature condition for more than 2.8 years, showing that it has great potential as reliable packaging for long-term implantable devices

Mid-infrared resonant ablation for selective patterning of thin organic films

The fast growing market of organic electronics, including organic photovoltaics (OPV), stimulates the development of versatile technologies for structuring thin-film materials. Ultraviolet lasers have proven their full potential for patterning single organic layers, but in a multilayer organic device the obtained layer selectivity is limited as all organic layers show high UV absorption. In this paper, we introduce mid-infrared (IR) resonant ablation as an alternative approach, in which a short pulse mid-infrared laser can be wavelength tuned to one of the molecular vibrational transitions of the organic material to be ablated. As a result, the technique is selective in respect of processing a diversity of organics, which usually have different infrared absorption bands. Mid-IR resonant ablation is demonstrated for a variety of organic thin films, employing both nanosecond (15 ns) and picosecond (250 ps) laser pulses tunable between 3 and 4 microns. The nanosecond experimental set-up is based on a commercial laser at 1064 nm pumping a singly resonant Optical Parametric Oscillator (OPO) built around a Periodically-Poled Lithium Niobate (PPLN) crystal with several Quasi-Phase Matching (QPM) periods, delivering more than 0.3 W of mid-IR power, corresponding to 15 mu J pulses. The picosecond laser set-up is based on Optical Parametric Amplification (OPA) in a similar crystal, allowing for a comparison between both pulse length regimes. The wavelength of the mid-infrared laser can be tuned to one of the molecular vibrational transitions of the organic material to be ablated. For that reason, the IR absorption spectra of the organic materials used in a typical OPV device were characterized in the wavelength region that can be reached by the laser setups. Focus was on OPV substrate materials, transparent conductive materials, hole transport materials, and absorber materials. The process has been successfully demonstrated for selective thin film patterning, and the influence of the various laser parameters is discussed

Characterization of gold nanoparticle layer deposited on gold electrode by various techniques for improved sensing abilities

The deposition of gold nanoparticles (AuNPs) on the surface of gold electrode is believed to enhance the electrochemical characteristics of the surface. According to the existing literature, this could be performed in various ways. The purpose of the current study was to compare these results and report the most effective technique. In this regard, the layer-by-layer deposition, self-assembled monolayer technique and electro deposition method were investigated. Our results showed that cyclic voltammetry electrodeposition of AuNPs causes an observable increase in the peak current, causing improved electrode kinetics and a reduction in the oxidation potential (thermodynamically feasible reaction). These modified electrodes also showed several advantages with respect to stability and reproducibility

Polyimide-ald-polyimide layers as hermetic encapsulant for implants

Several requirements exist for medical devices for long term implantation. Firstly, the foreign body reaction and/or inflammation occurring upon implantation should remain mild and short in time. Moreover, the device needs to be biocompatible during the total implantation duration, hence not causing reactions which decrease the patient’s health. Finally, the device needs to work properly and safe during the total period of implantation, not suffering from corrosion or chemical degradation. To meet these requirements, diffusion of body fluids into the package should be avoided as well as diffusion of toxic device materials into the body, hence a hermetic packaging method is an absolute necessity. Here, a flexible hermetic packaging is presented using alternating polyimide and atomic layer deposited (ALD) metal oxides. Good adhesion between the inorganic ALD layers and the polyimide is required to avoid the creation of lateral diffusion pads. To obtain this, surface modifications of both polyimide and ALD layers are optimized, as presented in this paper. The hermeticity is evaluated in terms of water vapor transmission rate measurements of the film stack

Accelerated hermeticity testing of biocompatible moisture barriers used for encapsulation of implantable medical devices

Acceleration protocol plays an important role on barriers reliability evaluation for encapsulation of long-term implantable medical devices. Typically, acceleration is realized by performing tests at elevated temperature: the higher the selected temperature, the higher the acceleration factor. Nevertheless, at high temperatures, reaction mechanisms might be different, resulting in false acceleration test results. Our standard barrier performance test is based on the evaluation of corrosion of copper patterns (resistivity check, Electroscopic Impedance Spectroscopy (EIS), microscopic inspection). The temperature window for accelerated testing has been investigated for our standard barrier tests. The copper patterns, protected by a barrier layer under test, are immersed in PBS (Phosphate Buffered Saline) at temperatures up to 95°C. As barriers the following material/multilayers are selected: (1) Al2O3 ALD, (2) stacked HfO2/Al2O3/HfO2 ALD (further called ALD-3), (3) polyimide, and (4) polyimide/ALD-3/polyimide. In this presentation, the results of the test protocol evaluation will be presented. As expected, the maximum applicable test temperature is dependent on the barrier under test. Furthermore, during the fine-tuning of the accelerated test protocol, we observed for some barriers a clear influence of the shape of the Cu test patterns on the barrier performance. This can be related with processing effects when fabricating the barrier on the copper patterns. This finding stresses the determination of relevant copper patterns -or test structures in general- in order to predict the barrier performance correct for each individual application

The use of ALD layers for hermetic encapsulation in the development of a flexible implantable micro electrode for neural recording and stimulation

The use of electronic microsystems as medical implants gains interests due to the combination of superior device functionality with extreme miniaturization. Electronic devices are not biocompatible and will suffer from corrosion, hence a very good hermetic device encapsulation is of utmost importance. The hermetic sealing of implantable electronics requires extremely good bi-directional barrier properties against diffusion of water, ions and gases. Moreover, extremely long biostability against body fluids and biomolecules is an important requirement for the barrier materials. In this work, an ALD multilayer of AlOx and HfOx in combination with flexible polyimide is used as a flexible hermetic encapsulation of an electronic CMOS chip which serves as an implantable probe (so called hd TIME (active high-density transverse intrafascicular microelectrode) probe) for neural recording and stimulation [1]. The main part consists of a 35μm thin CMOS chips with electrodes on top encapsulated with alternating layers of spin coated polyimide (PI2611) and biocompatible ALD layers. The total encapsulation is developed to provide excellent barrier properties. Each ALD stack (ALD-3) consists of AlOx (20 nm) capped on both sides with HfOx (8 nm) to avoid hydrolysis of AlOx. The ALD deposition temperature is 250°C. Special attention is payed to the adhesion of the ALD layers toward polyimide and vice versa. 3 to 4 PI/ALD-3 dyads are used for the total encapsulation, since long term implantation of the medical device is envisaged. Testing however is done using only a part of the total encapsulation, in order to enable to learn about the barrier properties in a reasonnable timeframe. The WVTR of a PI/ALD-3/PI film reached a value of 2.1 10-5 g/m2day (38°C and 100% RH), the total encapsulation with 3 to 4 dyads will lead to WVTR’s in the order of 10-6g/m2day. The same PI/ALD-3/PI film has been deposited on structured copper meanders and is exposed to PBS at 60°C for 3.5 years (equivalent to 17.5 years at 37°C) [2]. Up till now, no change in Cu resistivity has been observed proving the excellent barrier properties of the PI/ALD-3/PI film. [1] Rik Verplancke et al., 2020 J. Micromech. Microeng., 30, 015010 [2] Changzheng Li et al. 2019 Coatings, 9, 57