Name: fabrication of flexible image sensor based on lateral nipin phototransistors
Uploaded: 2018-06-23T14:00:00
Description: Watch this Scientific Journal Video about Fabrication of Flexible Image Sensor Based on Lateral NIPIN Phototransistors at JoVE.com

This research was supported by the Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (NRF-2017M3D1A1039288). Also, this research was supported by the Institute for Information and Communications Technology Promotion (IITP) grant funded by the Korea government (MSIP) (No.2017000709, Integrated approaches of physically unclonable cryptographic primitives using random lasers and optoelectronics).

CAUTION: Some chemicals (i.e., hydrofluoric acid, buffered oxide etchant, isopropyl alcohol, etc.) used in this protocol can be hazardous to health. Please consult all relevant material safety data sheets before any sample preparation takes place. Utilize appropriate personal protective equipment (e.g., lab coats, safety glasses, gloves) and engineering controls (e.g., wet station, fume hood) when handling etchants and solvents.
1. Si Doping and Isolation
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The fabrication technology described here contributes significantly to the progress of advanced electronics and wearable devices. The fundamental concepts of this approach use a thin Si membrane and metal interconnectors capable of stretching. Although Si is a brittle and hard material that can easily be fractured, a very thin Si layer can obtain a flexibility26,27. In the case of the metal interconnector, the wavy shape offers stretchability and flexibility<sup ...

Bio-inspired imaging systems can provide many advantages compared to the conventional imaging systems1,2,3,4,5. Retina or hemispherical ommatidia is a substantial component of biological visual system1,2,6. A curved image sensor, which mimics the critical element of animal eyes, can provide a compact and simple configuration of optical systems with low aberrations7. Diverse advancements of fabrication techniques and materials, for example, the use of intrinsically soft materials such as organic/nanomaterials8,9,10,11,12 and the introduction of deformable structures to semiconductors including silicon (Si) and germanium (Ge)1,2,3,13,14,15,16,17, realize the curved image sensors. Among them, Si-based approaches provide inherent advantages such as an abundance of material, mature technology, stability, and optical/electrical superiority. For this reason, although Si has intrinsic rigidity and brittleness, Si-based flexible electronics have been widely studied for various applications, such as flexible optoelectronics18,19,20 including curved image sensors1,2,3, and even wearable healthcare devices21,22.
In a recent study, we analyzed and improved the electrical performance of a thin Si photodetector array23. In that study, the optimum single unit cell of the curved photodetector array is a phototransistor (PTR) type that consists of a photodiode and blocking diode. The base junction gain amplifies a generated photocurrent, and hence it exhibits a route to improve an electrical performance with a thin film structure. In addition to the single cell, the thin film structure is suitable to suppress a dark current, which is considered as noise in the photodetector. Regarding doping concentration, a concentration larger than 1015 cm-3 is sufficient to achieve an exceptional performance in which the diode characteristics can be maintained with a light intensity over 10-3 W/cm2 23. Moreover, the PTR single cell has a low column noise and optically/electrically stable properties compared to that of the photodiode. Based on these design rules, we fabricated a flexible photodetector array that consists of thin Si PTRs using a silicon-on-insulator (SOI) wafer. In general, an important design rule of flexible image sensors is the neutral mechanical plane concept which defines the position through the thickness of the structure where strains are zero for an arbitrarily small r24. Another crucial point is a serpentine geometry of the electrode because a wavy shape provides fully reversible stretchability to the electrode. Due to these two important design concepts, the photodetector array can be flexible and stretchable. It facilitates the 3D deformation of the photodetector array into a hemispherical shape or a curved shape like the retina of animal eyes2.
In this work, we detail the processes for the fabrication of the curved PTR array using semiconductor fabrication processes (e.g., doping, etching, and deposition) and transfer printing. Also, we characterize a single PTR in terms of an I-V curve. In addition to the fabrication method and individual cell analysis, the electrical feature of the PTR array is analyzed in deformed states.

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fabrication of flexible image sensor based on lateral nipin phototransistors

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, but several challenging points remain, such as a low absorption efficiency due to a thin active layer and low flexibility. We present an advanced method to fabricate a flexible phototransistor array with an improved electrical performance. The outstanding electrical performance is driven by a low dark current owing to deep impurity doping. Stretchable and flexible metal interconnectors simultaneously offer electrical and mechanical stabilities in a highly deformed state. The protocol explicitly describes the fabrication process of the phototransistor using a thin silicon membrane. By measuring I-V characteristics of the completed device in deformed states, we demonstrate that this approach improves the mechanical and electrical stabilities of the phototransistor array. We expect that this approach to a flexible phototransistor can be widely used for the applications of not only next-generation imaging systems/optoelectronics but also wearable devices such as tactile/pressure/temperature sensors and health monitors.

Figure 3a and 3b show the designed and fabricated structure of NIPIN PTR considering previous studies2,23. The inset in Figure 3a exhibits a basic I-V characteristic of PTR. The detailed structural parameters of PTR are shown in Figure 3b. The doping process for a Si layer on an SOI wafer was conducted using the ion implantat...

Watch this Scientific Journal Video about Fabrication of Flexible Image Sensor Based on Lateral NIPIN Phototransistors at JoVE.com

Fabrication of Flexible Image Sensor Based on Lateral NIPIN Phototransistors

We present a detailed method to fabricate a deformable lateral NIPIN phototransistor array for curved image sensors. The phototransistor array with an open mesh form, which is composed of thin silicon islands and stretchable metal interconnectors, provides flexibility and stretchability. The parameter analyzer characterizes the electrical property of the fabricated phototransistor.

Flexible photodetectors have been intensely studied for the use of curved image sensors, which are a crucial component in bio-inspired imaging systems, ...

Our approach can help advance progress in the field of flexor semiconductor device fabrication. The main advantage of our fabrication procedure is it improves the electric current by current stabilate of the devices under deformation. Prepare a DUB silicon on insulator sample.This sample is ready for silicon isolation in subsequent steps. This is a schematic representation of its layer structure. Gray represents silicon, green, silicon dioxide.Red and blue are N-plus and P-plus doped regions, respectively. Spin coat a positive photoresist layer on the sample at 4, 000 rpm for 40 seconds. Next, take the sample to a hot plate to soft bake it at 90 degrees Celsius for 90 seconds.After the soft bake, take the sample to an ultraviolet photolithography set-up. There, apply a mask to the sample for 10 seconds. Retrieve the sample to develop it.Do this by immersing it in developer for one minute. Then the clean the sample in deionized water, and dry it with the nitrogen blow-gun. Next, hard bake the sample at 110 degrees Celsius for five minutes.This diagram represents the sample's layers at this point. Move on to dry-edge the sample with an inductively-coupled, plasma-reactive ion etcher for six minutes. When done, the sample has this structure.The next step is to remove the buried oxide layer. To do this, prepare a 49%solution of hydrofluoric acid. Dip the samples in it for two minutes to remove the buried oxide layer.After the acid bath, followed by cleaning and drying, the silicon isolation is complete. The next step involves plasma-enhanced chemical vapor deposition. Over two minutes, deposit a 130 nanometer silicon dioxide sacrificial layer on the sample.Spin-coat positive photoresist on the sample, followed by a soft bake. Employ a photolithography mask and expose the sample to UV light for 10 seconds. This is a representation of the sample after it is developed, and hard baked at 110 degrees Celsius for five minutes.Prepare a container of buffered oxide etch, and immerse the sample in it for 30 seconds. After cleaning and drying, the result is the partial removal of the CVD oxide layer. Deposit polyimide by spin coating it on the sample.Transfer it to a hotplate to begin annealing it at 110 degrees Celsius for three minutes, then at 150 degrees Celsius for 10 minutes. Then, move the sample to an oven with a nitrogen atmosphere, and continue to anneal it at 230 degrees Celsius for 60 minutes. Polyimide layer covers the sample, and provides a surface for a silicon dioxide sacrificial layer.Continue by depositing a 130 nanometer silicon dioxide layer with plasma-enhanced chemical vapor deposition for two minutes. Use positive photoresist to apply a mask, with 10 seconds of UV light, then develop. Perform a soft and hard bake.Form a hard mask by immersing the sample in buffered oxide etch for 30 seconds, followed by cleaning and drying it. Next, dry edge the polyimide using reaction ion etch for 20 minutes. Remove the oxide layer with buffered oxide etch, then clean and dry the sample.Now, take the sample to a sputterer to deposit chromium and gold. After that, spin coat positive photoresist before applying a photolithography mask. This sample has been developed for one minute, and hard baked at 110 degrees Celsius for five minutes.Etch the chromium and gold layers with a wet etchant for 20 seconds, to bring the sample to this state. After chromium and gold wet etching, take care when removing the photoresist, otherwise the gold may peel off. The second polyimide layer deposition and the second metallization steps proceed exactly as the first.Spin coat and anneal the second layer of polyimide to produce layers such as in this diagram. Deposit and pattern a silicon oxide layer as a hard mask layer for dry etching. Next, sputter coat 20 nanometers of chromium and 200 nanometers of gold before patterning it.Begin by adding another spin coated layer of polyimide and annealing it. Use plasma-enhanced chemical vapor deposition to deposit a 650 nanometer silicon dioxide layer over eight minutes. After spin coating positive photoresist on the sample, apply a mask with UV photolithography.Develop the sample, hard bake it, and pattern the silicon dioxide by immersing it in buffered oxide etch. Use reactive ion etching to dry etch the polyimide for 75 minutes, then dry etch the silicon for six minutes with inductively-coupled, plasma-reactive ion etching. It is now time to etch the oxide sacrificial layer.Immerse the sample in 49%hydrofluoric acid for 20 minutes. Retrieve, rinse, and dry the sample before proceeding. When you are etching the sacrificial layer with hydrofluoric acid, it is important to monitor the etching progress.Ensure your course, the device may peel off. Now, hold the sample with carbon tape applied to the substrate. Then attach water soluble tape to the patterned side.Next, strip off the water soluble tape in an instant. This prevents the device from remaining on the substrate. Get degassed PDMS in a 10-to-1 mixture prepolymer to curing agent.Also, obtain PET film sufficient to cover the sample. Spin coat the film with the PDMS at 1, 000 rpm for 30 seconds. Follow this by baking the film on a hotplate at 110 degrees Celsius for 10 minutes.When this is done, get the sample on the water soluble tape and expose it to UV light for 30 seconds. Now, work with both the sample and the film. Attach the water soluble tape with the sample to the PDMS coated PET film.With a pipette, carefully drop water on the water soluble tape to remove it from the film. Employ a slow flow of water to sweep away the water soluble tape. Finally, hold the sample with forceps, and dry it with a nitrogen blow gun.This is the phototransistor array before being transferred to a PET film. It consists of repeating phototransistor cells depicted in this image. The polyimide encapsulated serpentine electrodes allow the device to stretch.Here's the device after transfer onto a PDMS coated PET film. To measure the IV characteristics under different conditions, use a set-up similar to this. Note the definition of the radius of curvature.These are measurements of the IV characteristics of the phototransistor array, with different radii of curvature. The electrical features of the array are independent of the radius of curvature. This plot is of the ratio of the photocurrent to the dark current, as a function of voltage, for different radii of curvature.The dynamic range is about 600 or more above a bias voltage of two volts. The inset has plots of the dark current at the tested curvature extremes. Its low value causes the high dynamic range.Once mastered, this technique can be done in 48 hours, if it is performed properly. While attempting this procedure, it is important to avoid any acts that can peel the electrodes off of its substrate. Following this procedure, other methods, like polymer etching with other gases, can be performed to task, if they can be used under different conditions than those used in dry etching.After its development, this technique paved the way for researchers in the field of flexible devices to explore bio-inspired image attempts. After watching this video, you should have a good understanding of how to manufacture a flexible semi-conductor device. Don't forget that working with the necessary reagents can be extremely hazardous, and precautions, such as using masks, hoods, and headgear at your station, should always be taken while performing this procedure.

Research

JoVE Journal

Engineering

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