Philipp Häfliger

• Dobroliubov, Oleksandr; Kakati, Abhijit; Stokke, Bjørn Torger; Häfliger, Philipp & Johannessen, Erik Andrew (2026). An Assessment of Drift and Instability of ISFET Sensors as a Function of Geometrical Designs for Implementation With the 0.35-µm CMOS Technology. IEEE Sensors Journal. ISSN 1530-437X. 26(17), p. 25740–25750. doi: 10.1109/JSEN.2026.3714849. Full text in Research Archive Show summary

  Customized devices based on complementary metal oxide semiconductors (CMOS) must comply with the constraints of the chosen technology, often requiring compromises from the ideal design. This is particularly apparent in floating gate (FG) devices, such as the ion-selective field-effect transistor (ISFET), where multiple metal and polysilicon layers displace the oxide/solution interface geometrically away from the transistor through an extended gate design surrounded by thick layers of oxide. This article investigates how FG geometry and parasitic capacitances affect the pH response and threshold voltage ( Vth ) drift in the 0.35- μ m CMOS technology. The sensors responded in a sub-Nernstian manner from 7.4 to 20.3 mV ⋅ pH −1 , whereas the drift in Vth triggered by ±2-V stress pulses from the control gate (CG) ranged from −23.5 to +147.2 mV ⋅ h −1 . Removal of the charge affecting the FG (and thereby the offset drain current) by a wideband ultraviolet (UV) light source (250–384 nm) reduced the drain current ( Id ) drift to 2% ⋅ h −1 –5% ⋅ h −1 . The mismatch between samples was between 10% and 15%, and the light sensitivity subject to a 560-nm green light source ranged from 5% to 70% of the signal obtained per unit pH. The results suggest that sensitivity does not scale proportionally with the FG area and is dependent on the degree of coupling to adjacent parasitic capacitances. Close capacitive coupling to a CG stabilizes Vth drift with time at the expense of a lower pH sensitivity.

• Do, Tonny Nguyen Duc; Johansson, Robert & Häfliger, Philipp (2025). A CMOS Image Sensor Low-Power Counter for Single-Slope ADC at 500MHz Clock Frequency and 28pJ/Conversion-Step Figure-of-Merit. In Sawan, Mohamad (Eds.), Proceedings of the 37th International Conference on Microelectronics (ICM). IEEE. doi: 10.1109/icm66518.2025.11322443. Full text in Research Archive
• Dobroliubov, Oleksandr; Kakati, Abhijit; Stokke, Bjørn Torger; Häfliger, Philipp & Johannessen, Erik Andrew (2025). CMOS microsensor packaging for use in aqueous media. Journal of Micromechanics and Microengineering (JMM). ISSN 0960-1317. 35(7), p. 1–13. doi: 10.1088/1361-6439/ade3e5. Full text in Research Archive
• Nøvik, Steffen; Hirayama-Shoji, Kayoko; Combriat, Thomas; Sakalys, Domantas; Aashamar, Erik & Louch, William E. [Show all 9 contributors for this article] (2025). Bioimpedance and electrophysiology measurements of engineered heart tissues outside the culture medium. Biosensors and Bioelectronics. ISSN 0956-5663. 287, p. 117634–117634. doi: 10.1016/j.bios.2025.117634. Full text in Research Archive
• Dobroliubov, Oleksandr; Kakati, Abhijit; Stokke, Bjørn Torger; Häfliger, Philipp & Johannessen, Erik Andrew (2024). Silicon flow stop frame for encapsulation of CMOS microsensor chips by wet anisotropic etching in KOH. Engineering Research Express. 6(2). doi: 10.1088/2631-8695/ad39c3. Full text in Research Archive Show summary

  Bulk micromachining in silicon is governed by the etching process where anisotropic (wet) etching in KOH can yield complex structures beyond that achievable with isotropic (dry) etching techniques. One example is the miniaturised frame reported herein with an area of 2.9 to 7.5 mm2, walls that are 1/10 mm thick, a height of 525 μm equipped with sloping walls that takes advantage of the 54.7° angle of the (111) planes to the horizontal (100) top surface of the wafer. Convex corners liable to damage are protected by sacrificial bridge structures which are etched thin to a point where the frame can be easily removed from the bulk substrate material. Frames made from isotropic (dry) etching processes have been made for comparison. Although the frame structure has different applications in microfabrication, the intended use is a flow stop barrier preventing liquid resins from entering the active area of a CMOS chemical sensor chip during encapsulation for use in aqueous or gaseous media. Beyond this specific proof-of-concept, the strategy is expected to be of general interest for all who treasures KOH etching and wants to explore new avenues based on this process.

• Farian, Lukasz; Fernandez-Peramo, Pablo; Häfliger, Philipp & Lenero-Bardallo, Juan A. (2022). Centroid estimation method with sub-pixel resolution for event-based sun sensors. 978-1-6654-6700-1. doi: 10.1109/prime55000.2022.9816751. Full text in Research Archive

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• Häfliger, Philipp; Frøland, Mariann Carlsen & Stokke, Anne-Marie Korseberg (2026). Coffee & Chips - PhotoSense: Testing a biomolecular eye implant (UiO). doi: https:/www.forskningsparken.no/event/coffee-chips-fridays. Full text in Research Archive
• Dobroliubov, Oleksandr; Kakati, Abhijit; Stokke, Bjørn Torger; Häfliger, Philipp & Johannessen, Erik Andrew (2025). UV-Assisted Erasure of Trapped Charge in Floating-Gate ISFETs Fabricated in AMS 0.35 µm CMOS. Full text in Research Archive Show summary

  The ion-sensitive field-effect transistor (ISFET), fabricated using standard CMOS technology, offers several key advantages, including miniaturization, scalability, and mechanical robustness. The flexibility of CMOS processes enables the integration of ISFET arrays with microscale spatial resolution, paving the way for advanced sensing platforms. These ISFET microarrays have been successfully utilized in extracellular imaging [1], DNA detection [2], monitoring of pH levels [3] as well as ion concentrations of potassium, sodium, and calcium [4]. The ISFET represents a key element in this project that addresses a major biomedical challenge: developing therapeutic solutions for millions affected by retinal degenerative diseases like Age-related Macular Degeneration and Retinitis Pigmentosa. A promising treatment for late-stage cases involves retinal implants that incorporate thin films of the light-sensitive protein bacteriorhodopsin. This protein mimics photoreceptor activity by converting light into electrical signals for neural stimulation in the retina [5]. The effectiveness of such implants can be evaluated through concurrent measurements of light attenuation via photodiodes and pH variations detected by an ISFET array [6]. While photodiodes are well-characterized devices, ISFET arrays pose certain challenges—most notably, the presence of trapped charge in the floating gate (FG). This issue can lead to large offset potentials as well as threshold voltage shifts during operation. There may also be a heterogeneity between the different ISFET sensors embedded in the array. One established technique for removing trapped charge from floating gates is the exposure to hard ultraviolet (UV) light, commonly used in EPROM erasure, where the gate is made accessible to light through a nanometer-thick quartz window. The objective of this study was to investigate whether a similar UV-based erasure technique could be applied to ISFETs implemented in standard AMS 0.35 µm CMOS technology, which includes a relatively thick (~2 µm) passivation layer of stacked SiO₂ and Si₃N₄ deposited above the floating gate. The impact of layout design, UV wavelength range (250–385 nm), and exposure conditions on the erasure efficiency was systematically investigated.

• Kakati, Abhijit; Dobroliubov, Oleksandr; Abdollahi, Roozbeh; Dixit, Krishna; Johnson, Ashley J. & Greco, Jordan [Show all 12 contributors for this article] (2025). CHARACTERIZING THE PHOTOACTIVE RESPONSE OF BACTERIORHODOPSIN USING A CMOS ISFET MICROSYSTEM. doi: 10.70477/RPAH4863. Full text in Research Archive Show summary

  Bacteriorhodopsin (BR) is a photosensitive transmembrane protein from Halobacterium salinarum, which undergoes a 15 ms photochemical cycle that facilitates transmembrane proton translocation [1,2]. When the protein is integrated into a multilayered thin film, the proton gradient can be used to stimulate intact neuronal cells following the loss of photoreceptors through retinal degeneration. The phototransduction properties can be probed using miniaturized electrode systems based on gold/indium tin oxide (ITO) and ion-sensitive field-effect transistors (ISFET) based on complementary-metal-oxide-semiconductor (CMOS) technology. The BR is immobilised directly on a Si₃N₄ gate insulator, creating a uniform thin film (Figure 1). The temporal increase in protons that is generated will adsorb and create a positive charge on the gate insulator. This positive charge induces an n-channel that increases the drain current of the ISFET which is translated into a voltage signal by a common source amplifier. The pH response is tuned to a dynamic range of pH 6-8 through a control (CG) and tunnelling device (TUN) permitting charge induction or removal from the floating gate (FG). The sensor assembly (Figure 2) includes a chip carrier, fluidic container, flow stop frame and encapsulation system [3]. The ISFET sensors are based on the XFAB 0.18 micrometre technology where each transistor has a gate area of 2 x 0.6 µm² which extends to the top metal layer (MET6) defining the pH sensitive area (70x75 µm²). The chip measures 1.5x1.5 mm2 in extent. The ISFET response to changing pH buffers is shown in Figure 3. The output signal (Vout) exhibits a good linear and temporal response (time constant of 4.4 ms unfiltered) with an accuracy of ± 0.02 pH units (based on maximum deviation over 5 min with a 10 nF load) within the rail to rail (0-3.3 volts) operating range of the amplifier. The photocurrent measurements from exposure to 560 nm green light (Figure 4), shows the response of BR to light pulses ranging from 1 sec to 15 ms. The light-off peak disappeared below 50 ms suggesting an incomplete photocycle, whereas the light-on peak disappeared below 15 ms suggesting that the photocycle was not initiated beyond this time limit. The ISFET sensors should therefore be able to operate within the time domain of the BR and its known photophysical response [1,2]. Given that the retinal flicker fusion frequency in most humans is around 20 ms or 50 Hz [4,5], the comparable time domain of BR combined with its phototransduction properties renders it a viable candidate for the replacement of damaged photoreceptor cells in the retina. In further work, we will integrate the microelectrode system into an array format to investigate the spatial and temporal properties of the charge generation mechanism. This will help identify the signal propagation required for communication with the interfacing neural network from future biohybrid sensing applications designed to restore vision in the eye.

• Kakati, Abhijit; Dobroliubov, Oleksandr; Sylva, Hope; Sylva, Daniel; Johnson, Ashley & Dixit, Krishna [Show all 12 contributors for this article] (2024). FABRICATION OF A CMOS ISFET MICROSYSTEM TO CHARACTERIZE THE PROTON PUMPING OF BACTERIORHODOPSIN. Full text in Research Archive

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