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Abstract
A high-sensitivity porous silicon (PS) powder-based capacitive UV-sensor is developed on the PS-film structure. To the best of our knowledge, this is the first report on PS powder-based capacitive UV-sensors. An anodization etching approach based on a sawtooth waveform electrolytic voltage is adopted to get sufficient PS-powders. The collected PS-powders are embedded on the PS-film and covered with epoxy to improve the UV sensitivity. The results show that the increase of UV sensitivity on capacitive characteristics is almost linearly related to the PS powder mass embedded inside. The proposed method is suitable for fine-tuning and improving the equivalent permittivity of the capacitive UV-sensor without changing the device geometry design, which can be compatibly applied to Si-VLSI technology.
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1. Introduction
Porous silicon (PS) is discovered by Arthur Uhlir in 1956 [1], and its photoluminescence is observed by Canham in 1990 [2]. Due to quantum confinement in nanocrystals [3–9], PS has unique luminescence features with a high density of nano-scaled pores and pillars on the surface [1–10]. Also, it possesses the unique negative differential conductance characteristic [3–7]. Furthermore, due to the high surface-to-volume ratio, PS is used in many sensor applications, such as gas sensors [8], chemical sensors [9], optical sensors [11], pressure sensors [12], etc. Several manufacturing methods [13–16] are invented for PS fabrication, and the anodization etching method is the most commonly used one and adopted in our experiments. In most research, PS-films are focused, and PS-powders are usually only made by chance and removed during the processes since the tiny PS-powders are challenging to produce and handle. Recently, PS-powders has attracted great attention for their applications in the biomedicine field, like medical imaging [16] and bio-sensors [17,18].
In addition, to reduce exposure to UV radiation, the UV-sensors are the necessity in certain environment. On the UV-sensor designs, ZnO like nanowires have attracted attention recently [19,20]; however, with the doping of Cd and Mg, it is more expensive and difficult to be accurately controlled. In this paper, a low-cost UV-sensor design based on PS-powders/films is proposed. PS-powders are originally used for the fine-tuning and improving the permittivity and sensitivity on capacitive UV-sensors since each PS-powder has a very high surface-to-volume ratio and sensitive to UV-light. A special approach based on a sawtooth waveform electrolytic voltage during the anodization etching is adopted to get sufficient PS-powders. In addition, the PS-powders are measured in the scanning electron microscope (SEM) and the laser particle size analyzer (LPSA) for the nanostructure views and particle size distribution analyses, respectively. All processes are compatible with Si-VLSI technology and no heavy metals are required. Such a new UV-sensor shows high performance on UV sensitivity, which is nearly linearly dependent on the magnitude of the mass of the PS-powders embedded in the device.
2. Experimental
The main etching tank is a vertical design, as shown in Fig. 1(a), and made by Teflon for better acid protection. A copper plate is put under the bottom as the anode contact, and an O-ring with anti-acid corrosion property is placed on the wafer to prevent the leakage of the etching solution.
Fig. 1. (a) Illustrative sketch of the vertical electrochemical etching tank. (b) The periodic voltage waveform serves as etching voltage to produce lots of PS-powders. (c) Illustrative sketch of the PS-films with the PS-powders produced on the surface during the electrochemical etching process.
Under the anodization etching, the electrolytic current passes through the p-Si sample and pushes the charges (holes, h+) to react with hydrogen fluoride (HF) to produce lots of nano-scaled and soluble H2SiF6 and then tiny pores on the surface of Si samples, i.e., PS-structures. The illustrative equations of overall chemical reactions to form PS-films and PS-powders are indicated as below [17]. To avoid the perturbation from the gas product H2, the etching tank was designed vertically to eliminate the gas effectively from the surface of Si samples.
Fig. 2. (a) The photo of the PS-powders on the PS-films. (b) The photo of the collected PS-powders in a bottle under UV-illumination with orange light luminescence. (c) The figure of the volume percentage versus the particle diameters of the PS-powders measured in the laser particle size analyzer.
Fig. 3. The pictures measured by SEM in the study, (a) the typical PS-films under 20,000x magnification, (b) under 100,000x magnification, and (c) the PS-powders under 30,000x magnification.
All the PS-powders/PS-films were prepared from/on p-type Si (100). After preliminary experiments, the etching voltage is designed as a periodic sawtooth waveform with a period time of 200 s rising from 5 to 15 V for an overall etching time of 1200 s long, as shown in Fig. 1(c), to produce sufficient PS-powders and high-quality PS-films. In addition, the etching solution of HF and C2H5OH (5:7) is used. The porosity of the PS-films is fixed in a certain range as the controlled variables, for the applicability, which is 70% in this study after some preliminary experiments. It is well known that the conventional definition of the porosity is (m1-m2)/(m1-m3), where m1 is the mass of the chip before etched, m2 is the mass after etched, and m3 is the remained mass after the PS-film is polished [33]. But it is challenging to apply this definition to calculate the PS-powder porosity precisely, which is too small to capture.
One preliminary experiment is done on an epoxy ingot (1.2 g) with PS-powders (4 mg) embedded inside. During the embedding procedure, the mixture of PS-powders and epoxy is stirred for homogeneous dispersion. The photoluminescence property of epoxy ingot samples is studied under UV-light. As shown in Fig. 4(b), the whole epoxy ingot sample emits orange light under the UV-light (wavelength = 365 nm) compared to the sample under normal-light (Fig. 4(a)). It also indicated that the PS-powders is homogeneously dispersed in the whole epoxy ingot since the embedding PS-powder procedure with stirring is applied to all samples in this paper. In addition, the photoluminescence spectrum is shown in Fig. 4(c) for the samples with and without PS-powders, respectively. Epoxy is one kind of vital thermoset plastic widely used as adhesives and coatings, which is formed from the chemical crosslinking reaction of epichlorohydrin (C3H5ClO) and bisphenol A (C15H16O2). In the study, it is used as the protection of PS-powders.
Fig. 4. (a) Naked eye photos of epoxy ingot sample with PS-powders under normal-light. (b) Naked eye photos of epoxy ingot sample with PS-powders under UV-light. (c) The photoluminescence spectrum of epoxy ingot samples with & without PS-powders under UV-light.
The PS-powders are embedded between the epoxy layer and PS-films, and then the samples are deposited with metals (Pt) on both sides to serve as a capacitive UV-sensor. Since the UV-sensing mechanism is based on the absorption of photon energy from UV-illumination, the metal layer is patterned to open some areas as UV-light windows. It is predictable that the more the PS-powders are embedded in the capacitive structure, the higher the sensitivity of capacitance on UV-detecting. It is experimentally proved in the paper.
Although there are minor electrical contact problems on the composite structure, which may increase the resistivity of the device; however, it only matters little in the capacitive UV sensor in the study, not like the resistive UV sensors. The equation for the capacitance of the UV-sensor is expressed as C = ɛ(A/d). In the equation, A is the device area and d is the total thickness between the two metal layers, in which a fixed area (A = 1 cm2) and a fixed thickness (d = 2.525 mm) are used in the study. Moreover, ɛ represents the equivalent absolute permittivity of the overall layer structure, which is stacked by the epoxy layer (2 mm) with PS-powders, PS-film layer and Si-substrate. The ɛ is affected by the UV-light stimulation, especially caused by the quantum effect in the PS-powders and PS-films, which can be fine-tuned by the magnitude of the mass of PS-powders.
The typical sandwich structure based on the PS-powders and PS-films is sketched in Fig. 5(a). Four experimental samples (Cases-I, II, III and IV) with various amounts of PS-powders are produced to compare their UV sensitivity. Among them, the mass of the embedded PS-powders is the main issue to be studied, as shown in Fig. 5.
Fig. 5. (a) Illustrative sketch of the detector structures, and (b) the structures with the PS-powders in Case-I (1 mg), Case-II (2 mg), Case-III (3 mg), and Case-IV (4 mg).
The PS-film is well prepared at first, then the native PS-powders on the surface are swept out and collected separately. The swept-out powders are measured as 1.0 mg weight and stored in a collecting bottle, as shown in Fig. 2(b). After that, the PS-film is protected/covered by a transparent epoxy layer. In Case-I, the native PS-powders (about 1.0 mg) produced during the PS-film anodization process are coated on the sample. And the PS-film and PS-powders are directly covered by epoxy. The same procedure is followed in Case-II, III, and IV, yet by adding extra PS-powders of 1 mg, 2 mg and 3 mg from our collecting bottle into native powders before the coating process of epoxy. Then the resulting PS-powders mass in Case-II, III, and IV are 2 mg, 3 mg and 4 mg, respectively. All samples are deposited with metal layers on both sides to form the capacitive UV-sensors.
3. Results and discussions
The experimental results on the capacitive sensitivities (SC) for the four cases under UV-light compared with those under normal-light and in the dark are shown in Fig. 6. In this study, the 30 W UV lamp with a wavelength of 365 nm (UVA) is used for UV-light emission, and the normal-light is the 20 W cold white visible LED with a wavelength of 390–780 nm is used as the normal-light. The other UV lamp with the wavelength of 254 nm (UVC) has been tried in the preliminary experiment. Similar results and tendencies are obtained in both 365 nm and 254 nm. Due to the lack of availability of UV lamps with different wavelengths and the concern of the safety in our laboratory, the 365 nm UV is adopted in this study. It is also conceivable that the PS-powders is responsive to the normal-light. Hence, the reported PS UV sensor can work under the excitation wavelength from a broadband light source but with different sensitivities. The capacitive sensitivity is defined as the ratio of the variations on capacitance under light (UV-light or normal-light) to that under dark.
In the study, the capacitances of all samples are measured at 1 kHz, the values of Clight and Cdark in detail are listed in Table 1. The relative capacitive sensitivities are calculated and shown in Fig. 6. In our preliminary experiment, if the UV power is changed from 30 W to 41 W, the capacitive sensitivity will slightly increase because the more photons from UV-light are absorbed, the more particles inside the PS-film and PS-powder of the capacitor will receive the UV radiation energy. More electron and hole pairs are stimulated, resulting in an increase in photoenergy. But there are not many choices of power of the UV-light source as a variable in this lab and this study focuses on the discussion of the variable of PS-powder amount, so it will be a subject of our further research in the future. The study result is similar to the other research [34].
Table 1. The capacitances of the UV sensor in Case I–IV in the dark and under normal-light/UV-light
As shown in Fig. 6, the capacitive sensitivity under UV-light is much higher than that under normal-light and in the dark. In addition, the more PS-powders are added, the higher the sensitivity. The reason for the increment in the capacitive sensitivity under light is because of the increase of the electron-hole pairs from the light source (UV-light/normal-light). The electron-hole pairs are photogenerated towards both the electrodes sequentially because the photon energy is larger than the bandgap of PS-powder and PS-film. Higher sensitivities under UV-light than those under normal-light is due to the high frequency as well as the large photon energy of UV-light. Moreover, the addition of PS-powders provides more electron-hole pairs and charge-transfer states, and more free carriers accumulate onto the top electrode. Therefore, based on the formula, C = Q/V, the increase in quantity of electric charge (Q) on the electrode causes the increase in capacitance under UV-light/normal-light. After removing the light source (in the dark), electron-hole pairs recombine again, resulting in the decrease of sensitivity. Similar results also appeared in other researches [35–38].
This result indicated that the new structure sensor proposed herein could serve not only as the UV-light sensor under normal-light room but also as a normal-light sensor in a dark environment. For the sensitivity enhancement between the 4 cases (PS-powder: 1, 2, 3, and 4 mg), when the average value of the sensitivities in the 4 cases are used as the reference values for eliminating the unavoidable experimental error effect, the enhancement magnification ratio between the values in Case IV to the average values are 1.58 fold under UV-light and 1.51 fold under normal-light, respectively. The slight difference comes from the fact that the PS-powders are a little more sensitive to UV-light than to normal-light because of the quantum effect of the PS-powders with high bandgap.
On the other hand, the high surface-to-volume ratio effect of PS-powders also contributes to the sensitivity because there are more capture centers and hence more photons from the light source can be absorbed by PS-powders. Overall, the equivalent permittivity increases by the embedded PS-powders, which is easy to be regulated by the weight amounts. However, before reaching the saturation point of the curves in Fig. 6, there is a limitation in our trial experiments. As the PS-powders amount is nearly 5 mg, the composite structure begins to split slightly on the interface of PS-film and epoxy layer. It is because of the increased cohesive force among the PS-powders, resulting in the inhomogeneous dispersion of the PS-powders in epoxy, which has a great influence on the layer mismatch. Further study on this research problem is underway in our laboratory. A capacitive UV sensor with adding PS-powder is originally fabricated. The technology has been experimentally proven feasible, and providing a new option for Si-VLSI process in this paper.
4. Conclusions
A new UV-sensor design is proposed for the first time based on the embedding method of PS-powders with the protection of epoxy layer on the surface of PS-film, and then sandwiched between two electrodes as the capacitive UV-sensors. It has been proved that the PS-powders have a very high surface-to-volume ratio to absorb the photon energy, and show a very excellent sensitivity to UV-light and normal-light. With a high UV sensitivity of the PS-powders, the proposed device is available to be fine-tuned on the sensitivity value by the amounts of PS-powders, without changing the device area and thickness as the conventional way.
In our experiments, the sample in Case-IV with the total powders of 4 mg, the UV sensitivity can be up to 1.85 in capacitance. Most of the reported capacitive UV sensors are based on advanced materials to achieve high sensitivity. For example, Qasuria et al. employed an expensive perovskite in 2020 to improve the sensitivity by 3.105 [39]. In addition, Saha et al. adopted polyvinylidene difluoride (PVDF) as the material to form a flexible capacitive UV sensor, in which the sensitivity was about 0.011 [35]. In this study, just only Si is used and the sensitivity reaches 1.85, which is practical in the industry. Although some further research studies are needed, the main contribution of our research is to provide a capacitive UV sensor manufacturing process with pure silicon as the main body, of which the sensitivity can be controlled with the amount of PS-powder without changing the layout and the size of the component, providing a thoroughly compatible option for Si-VLSI technology.
Funding
Ministry of Science and Technology, Taiwan (MOST 110-2221-E-305-013-MY2).
Disclosures
The authors declare no conflict of interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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