Heterogeneous sensors of pressure sensor and ultraviolet photodetector fabricated by vertical 3D stacking as a multi-functional device

Abstract

Microelectromechanical system (MEMS) piezoresistive pressure sensor and ZnO nanowires (NWs) ultraviolet (UV) photodetector were 3D integrated into a single chip with a vertically stacked structure. The MEMS pressure sensor with a UV transparent SiO₂ diaphragm was stacked on the top of UV photodetector. The fabricated pressure sensor presents good linearity and stability with increasing applied pressure from 0–500 mbar. The measured interval of 0.2 mbar also shows its high sensitivity with a small change in pressure and altitude. For the ZnO NWs UV photodetector, the cutoff wavelength was around 360 nm, and the measured responsivity was 6.2 × 10⁻¹ A W⁻¹ with 5 V applied bias. To discuss the influence of the applied pressure and UV on this 3D stacked device, the pressure sensor was also measured with and without UV illumination, and the UV photodetector was measured at various applied pressures. The results show that UV variation can be ignored for pressure sensor measurement. However, the photoresponses of ZnO NWs decreased, whereas the applied pressure was increased.

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Introduction

Sensors are important devices in our lives. They are used widely in our environment, which could be applied in human life, commercial business, military, biology and medical treatment. They provide a direct message to monitor and control the environment. Among them, UV photodetector and pressure sensor are two of the most useful detectors, which are completely different in properties. MEMS pressure sensor has been widely used in various applications by the advantages of low power consumption, miniaturization, high sensitivity and good excellent linearity.^1,2 In recent years, devices with Global Positioning System (GPS) have been popularized in personal devices. MEMS pressure sensor using piezoresistor is a candidate to provide a small change in atmospheric pressure, which can be used as an altimeter to position the altitude because the atmospheric pressure is related to altitude. Unlike GPS, these MEMS pressure sensors can also be used indoors without any satellite signal, which can position the certain floors in skyscrapers, for example. Such properties with small size and lower power consumption imply that it is suitable for wearable devices.

For UV photodetector, 1-D NWs-based devices have attracted considerable attention because they could provide a much larger surface-to-volume ratio compared to bulk- and film-based devices. With this advantage, NWs-based sensors normally exhibit a larger response, particularly for semiconducting metal oxide sensors. It has been reported that SnO₂,³ TiO₂,⁴ WO₃ (ref. 5) and In₂O₃ (ref. 6) NWs can all be used as photodetectors and gas sensors. Among these metal oxides, ZnO is a thermally stable n-type semiconductor with a large exciton binding energy of 60 meV and a large bandgap energy of 3.37 eV at room temperature.⁷

Electronic products have changed and improved quickly in this period, particularly for consumer electronics or wearable products. The properties of light, compact and small are the trends for future products, and thus, how to reduce the product size and increase its functionalities are important issues in the current stage. Sensors with different properties are usually packaged separately. The packaging efficiency can be improved further if 3D packages can be made practical. In this experiment, a MEMS pressure sensor was vertically stacked on a UV photodetector into a single chip, which performs multi-functionalities of atmospheric pressure and UV. A SiO₂ film was utilized as the diaphragm of upper pressure sensor, which allowed the UV light to pass through the pressure sensor and reach the lower UV photodetector. This unique structure allows these two different sensors to be 3D integrated vertically into one chip, which is potentially useful indoors/outdoors for wearable devices, such as smartphones or smartwatches in future.

Experimental

Prior pressure sensor process, silicon substrate with (100) orientation was wet cleaned by RCA (Radio Corporation of America) cleaning. A boron-doped 300 nm poly-Si film with a 1 × 10²⁰ cm⁻³ concentration was patterned as the piezoresistor on a 500 nm SiO₂ film. Al was then used as the electrode to connect the piezoresistors, forming a Wheatstone quarter-bridge. Another 1.5 μm SiO₂ was deposited on the top surface to cover the piezoresistor and electrode. To benefit the wire bonding of the pressure sensor, this upper SiO₂ film was patterned to expose the Al electrode for each piezoresistor. A deep Si etching process was then used to remove the backside Si, forming a 900 μm × 900 μm square cavity. SiO₂ film previously deposited on the top surface was used as the etch stopping layer for backside Si etching. Thus, in the cavity, there is only a 2 μm SiO₂ film and the patterned piezoresistor, which serve as the deforming diaphragm while applying different pressure force (Fig. 1a). For the UV photodetector process, a Corning 1737 glass substrate was wet cleaned in acetone and deionized water. A Cr/Au (10/100 nm) film was deposited onto the glass substrate by thermal evaporation. Photolithography was then performed to define the two contact electrodes with a gap distance of 10 μm. ZnO NWs were then grown using a Vapor–Liquid–Solid (VLS) method. The patterned substrate and 99.9% Zn powder were placed on an alumina boat. While growing, the boat was inserted into a quartz tube. Argon and oxygen gases were then introduced into the quartz tube. The positions of the substrate, Zn powder and the alumina boat were controlled carefully so that they were on the same horizontal level and heated to the same temperature. The conditions of growing pressure, growing time, oxygen flow rate and growing temperature were 5 Torr, 1 h, 0.6 sccm, and 600 °C, respectively (Fig. 1b). To integrate these two heterogeneous sensors, a printed circuit board (PCB) with six electrodes was used as the bottom layer (first layer). UV photodetector and pressure sensor were 3D stacked vertically as the second and third layer, respectively. To form an absolute pressure environment, the cavity of the pressure sensor was sealed with epoxy grease on the second layer (photodetector substrate). A wire bond was then used to connect the pressure sensor and UV photodetector to the bottom PCB (Fig. 1c and d).

Fig. 1 Schematic diagram of (a) pressure sensor, (b) ZnO NWs UV photodetector, and (c and d) the 3D stacked structure.

The crystallographic properties and surface morphology of the as-synthesized ZnO NWs were examined by X-ray diffraction (XRD, MAC MXP18) and field-emission scanning electron microscopy (FE-SEM, JEOL JSM-7001F). While measuring, the integrated chip was placed in a sealed chamber with a transparent lid. For the pressure measurement, air with different pressures was applied to the chamber. Different deformations of the piezoresistive film cause different output voltages of the Wheatstone quarter-bridge. For UV measurement, the spectral responsivity was performed using a JOBIN-YVON SPEX system with a 300 W xenon lamp as the light source and a standard synchronous detection scheme.

Results and discussion

Fig. 2a shows the structural layout of the piezoresistive pressure. The green, yellow, blue and purple patterns were the piezoresistors, Al electrodes, wire bonding contact and backside cavity, respectively. The piezoresistors were distributed near the edge of the cavity due to the higher stress. The length and width of the piezoresistor were labelled as L and W, respectively. Fig. 2b shows the output voltages of these Wheatstone quarter-bridges with different L/W ratios in dark environment. It can be observed that there was a higher voltage output with a higher L/W ratio. At the 465 mbar applied pressure, the output voltages were 0.23, 2.39, 3.64 and 4.27 V for the L/W ratio 3, 5, 10 and 15, respectively. In other words, the voltage output with a L/W ratio 15 was approximately 18 times higher than the L/W ratio 3. Such results were due to higher change in resistance for higher L/W ratio. The resistance of L/W 15 was 5 times higher than the L/W ratio 3. It should be noted that the sensor with L/W 15 was much linear than the others. In this experiment, the upper SiO₂ diaphragm was deposited by plasma-enhanced chemical vapor deposition (PECVD). For a L/W ratio of 3, 5 and 10, the depositing parameters of SiH₄, N₂O, pressure and RF power were 25 sccm, 1100 sccm, 1000 mTorr and 100 W, respectively. The parameters of a L/W ratio of 15 were 13 sccm, 1000 sccm, 1000 mTorr, and 50 W for SiH₄, N₂O, pressure and RF power, respectively. The measured film stress of SiO₂ with L/W ratio 3 and 15 were −625 and −80 MPa, respectively. These results also indicate that lower film stress can improve the linearity of the pressure sensor (L/W 10: Y = 0.567 + 0.007X, R² = 0.913; L/W 15: Y = −0.127 + 0.009X, R² = 0.9950). To further discuss the stable performance, the pressure sensor with a L/W ratio of 15 was measured backwards and forwards five times in the pressure range from 0 to 550 mbar. Fig. 3a shows that the results were very stable with small standard deviations. Fig. 3b also shows the measured dynamic response of the pressure sensor. The pressure sensor was measured initially without any applied pressure for 60 seconds, and the applied pressure was then changed to 150 mbar for another 60 seconds controlled by an electronic valve. The sensor was measured dynamically for 50 min. The voltage was rapidly increased to around 1.3 V as 150 mbar pressure was applied, and it decreased rapidly to its initial state as the pressure was turned off. Such results also highlight the stable properties of the pressure sensor.

Fig. 2 (a) Structural layout of piezoresistive pressure sensor. (b) Measurement of pressure sensors with different L/W ratios.

Fig. 3 (a) Pressure measurements of L/W 15 pressure sensor with repeatable performance. (b) Dynamic responses measured by L/W 15 pressure sensor at 0–150 mbar.

To benefit the use as an altimeter, pressure sensor with high sensitivity is important if it is used to determine the small changes in altitude. Generally, 10 mbar change in atmospheric pressure leads to around 86.2 m change in altitude. Fig. 4 shows the measurement with the interval of 0.2 mbar, which means that the pressure sensor has a resolution of 1.7 m in altitude.

Fig. 4 L/W 15 pressure sensor measured at 0.2 mbar intervals.

For a UV photodetector, Fig. 5a shows the top view SEM image of the surface morphology after growing ZnO NWs. It can be observed clearly that high-density ZnO NWs with uniform length and diameter were grown on the substrate. Fig. 5b shows the SEM image of cross-sectional ZnO NWs, where the average length of ZnO NWs was around 1.2 μm. The inset in Fig. 5b also shows XRD analysis of these ZnO NWs. It can be observed clearly that only ZnO (002) diffraction peak appears in the whole spectrum. Such a result indicates that these ZnO NWs were oriented preferentially in the (002) direction. The full-width-half-maximum (FWHM) of the XRD θ-scan peak was only 0.16°. Such a narrow line width implies that the c-axes of these NWs are well aligned along the growth direction.

Fig. 5 (a) Top-view and (b) cross-sectional SEM images of ZnO NWs. The inset in (b) shows XRD analysis of the as grown ZnO NWs.

Fig. 6a shows the responsivity of the fabricated ZnO NWs UV photodetector under a normal temperature and pressure environment (NTP, 25 °C, 1 atm) with different applied bias (1–5 V). The photoresponses of the fabricated photodetector were flat in the short wavelength region and had a sharp cutoff at around 360 nm. With an incident light wavelength of 360 nm and an applied bias of 1 V, the measured responsivity of the photodetector was 7.2 × 10⁻² A W⁻¹. As the applied bias was increased to 5 V, the measured responsivity increased to 6.2 × 10⁻¹ A W⁻¹. The significant increase in responsivity with the applied bias implies that there exists a photoconductive gain in the photodetector. The quantum efficiency of the UV photodetector was determined from the measured spectra response to be^8,9

where η is the quantum efficiency, R is the measured responsivity, q is the electron charge, λ is the incident light wavelength, h is the Plank constant, and c is the speed of light. Using this equation, the 6.2 × 10⁻¹ A W⁻¹ responsivity at 360 nm corresponds to an efficiency larger than 100%. Such a result again indicates that there is a large photoconductive gain in this photodetector.

Fig. 6 (a) Spectral responsivity of ZnO NWs with various applied biases. (b) Time-dependent photoresponse of ZnO NWs with UV switched on/off under NTP environment (UV: 360 nm, 1.9 mW cm⁻²).

The photoconduction of ZnO NWs is governed mainly by the adsorption and desorption of oxygen.¹⁰ Oxygen molecules are adsorbed on the ZnO NWs surface by capturing free electrons from the metal oxide, creating a low-conductivity depletion layer near the surface of the ZnO NWs [O₂(gas) + e⁻ → O₂⁻(ads.)]. Upon UV illumination, the photogenerated holes migrate to the surface and discharge the adsorbed oxygen ions [O₂⁻(ads.) + h⁺ → O₂(gas)]. This phenomenon decreases the width of the depletion region and increases the conductivity of the n-type material. These oxygen-related hole-trap states on the NWs surface can prevent charge-carrier recombination and prolong the photocarrier lifetime. Therefore, high internal photoconductivity gain can be achieved in ZnO NWs devices.⁹ To understand the UV-to-visible rejection ratio, this ratio was defined as the responsivity measured at 350 nm divided by the responsivity measured at 450 nm. With such a relation, we achieved a UV-to-visible rejection ratio of 57, while 5 V bias was applied.

Fig. 6b shows the transient response of the fabricated ZnO photodetector when 360 nm UV excitation was switched on and off, in intervals of 120 s. The measured current increased rapidly when the UV light was turned on and decreased sharply when the UV light was turned off. It has been reported that the rise and decay time of an n-type metal oxide is associated with the adsorption and desorption of oxygen.^11–13 To discuss the response time of the photodetector, the curve in Fig. 6b was fitted by the following equation:^14,15

I = I₀ + Ae^−t/τ₁ + Be^−t/τ₂ (2)

where I₀ is the steady-state photocurrent, A and B are constants, t is the time, and τ₁ and τ₂ are relaxation time constants. The constant τ₁ is related to the rapid change in the carrier concentration when the UV light is turned on/off, and τ₂ is related to carrier trapping and release due to the oxygen vacancy defects in the thin film.¹⁶ τ_r and τ_d in Fig. 6b are the time constants for the rise section and decay section, respectively, showing the good fit of photoresponse. For the rise section, the rise time constant τ_r1 and τ_r2 are estimated to be 1.22 s and 27.41 s, respectively. For the decay section, the current decay for this device is steep with a τ_d of 2.84 s. These results imply that the ZnO NWs photodetector presents a fast response speed to UV light. It should be noted that the UV responses measured in this experiment are all at 25 °C because the performance of the photoresponse can be affected by the surrounding temperature. Li et al. reported that the ZnO resistivity increased with increasing temperature due to the decreased mobility when temperature is below 100 °C. However, when the temperature is higher than 100 °C, the thermal generation becomes more influential than a decrease in mobility. Therefore, the resistivity decreases with increasing temperature. The response shows a decreasing tendency with increasing temperature, which is mainly caused by the decrease in photocurrent due to bandgap shrinkage and lattice scattering.^17,18

To improve the practicability of such vertical stacking sensors, the effects of the applied pressure and UV illumination should be discussed. Therefore, the influence of UV illumination on the pressure sensor and the influences of pressure on the photodetector were measured. After packaging these two sensors, the pressure sensor was stacked on the photodetector. Fig. 7 shows the measured results of the upper pressure sensor under different light illuminations. In a dark environment, the dynamitic responses presented stable and reproducible performance for different applied pressures 50, 100, 200 and 300 mbar. Under 245 nm and 360 nm UV light illumination, it can also be observed that the responses did not change, which almost correspond to the results in the dark environment. In this experiment, the piezoresistors were designed symmetrically and the resistance of each piezoresistor was equivalent (i.e. R_a = R_b = R_c = R_d). While pressure is applied to the diaphragm, R_a and R_c decrease due to the wider deformation, and R_b and R_d increase due to the longer deformation. The resistance under applied pressure can be expressed as follows:¹⁹

where R_s is the sheet resistance of poly-Si, and L/W and L′/W′ are the length/width before and after applying pressure, respectively. For the Wheatstone quarter-bridge symmetric pressure sensor, the V_out/V_in ratio was approximately ΔR/R. From the above equations, the influence of light illumination on the resistance was only R_s. The sheet resistance under UV illumination is the actual R_s multiplied by a factor. Thus, the results under UV illumination almost correspond to the results in the dark with ΔR/R relationship.

Fig. 7 Dynamic responses of the L/W 15 pressure sensor measured with/without UV illumination.

ZnO with a wurtzite structure in the hexagonal crystal system is symmetric and has no center of symmetry. Such a structure has favorable piezoelectric properties. Kuo et al. measured the piezoelectric properties by atomic force microscopy (AFM) on bending the ZnO NWs; the longer ZnO NWs generated larger piezoelectric current.²⁰ Wang et al. used the AFM probe to apply stress to ZnO NWs, which made them produce strain, forming a piezoelectric nanogenerator.²¹ However, in this experiment, ZnO NWs were only measured under different atmospheric pressure without any bend in its morphology. Therefore, the piezoelectric effect can be ignored in this experiment when different atmospheric pressures were applied. To further understand the influence of pressure on the ZnO NWs photodetector, the I–V relationships under different applied pressures were examined. Fig. 8 shows the I–V relationships with different applied pressures in the dark environment. At 5 V bias, the current without an applied pressure was 3.09 μA, and the current increased to 2.23 μA under 300 mbar pressure, respectively. This suggests that the resistance of ZnO NWs becomes larger while the pressure increases. When pressure was applied to the diaphragm, the diaphragm deformed downward and the volume of pressure sensor's cavity changed. With increasing pressure, the increasing downward deformation of diaphragm compressed the cavity space, causing an increase in the air pressure in sensor's cavity. At 300 mbar applied pressure, more oxygen molecules are adsorbed on ZnO NWs surface due to the higher air pressure in the cavity. Therefore, more free electrons from the metal oxide were captured, forming a thicker depletion layer near the surface of the ZnO NWs and decreasing the conductivity. According to these results, higher air pressure will cause more adsorption of oxygen, which appears to possess a higher photoresponse because it has more oxygen to be released. However, in Fig. 9, the photoresponses measured under 360 nm UV illumination showed opposite results. Fig. 9b shows the enlarged diagram of the photoresponse with different applied pressures. The change in response was very small; however, it decreased when the pressure was increased. The photoresponse is related to the dark current (I_d) and photo current (I_p). In this experiment, the difference of I_d is small, and thus, it much relies on I_p. Li et al. reported that the photogenerated hole migrates to the surface and the trapped oxygen, forming oxygen molecule, physisorbs on the surface under UV illumination.²² The physisorbed oxygen molecule can leave the NWs surface by thermal activation at room temperature. The ability of physisorption with oxygen was restricted by the increasing ambient pressure. While the trapped oxygen was reduced by the UV generated hole, the high air pressure formed an obstructive force to prevent the physisorption of oxygen gas (Fig. 10). A part of the physisorbed oxygen on the surface may change to chemisorbed oxygen by capturing electrons from the conduction band of ZnO NWs, thereby reducing the response.²² The photoresponses in Fig. 9b also correspond to the results in which the responded currents measured at 0, 50, 100, 200 and 300 mbar were around 13, 12.8, 12.5, 12.2 and 11.9 μA, respectively.

Fig. 8 I–V relationships of ZnO NWs measured under various applied pressures.

Fig. 9 (a) Transient responses of ZnO NWs measured under different pressures with 360 nm and 1.9 mW cm⁻² UV light, and (b) the enlarged diagram of (a).

Fig. 10 Schematics of oxygen molecules on the surface of ZnO NWs at (a) low and (b) high pressure in the dark and under UV illumination.

Conclusions

MEMs piezoresistive pressure sensor was vertically stacked on ZnO NWs photodetector as a 3D structure, forming a single device with multi-functional sensing properties. A pressure sensor with a SiO₂ diaphragm was UV transparent, which allows these two heterogeneous sensors to be measured simultaneously. The fabricated pressure sensor exhibits high sensitivity, linearity and stability. ZnO with a nanowire structure presents UV characteristics, while the device was illuminated under UV illumination. The results also show that the UV influence can be ignored for pressure sensor measurements. However, the photoresponses of the ZnO NWs decreased with increasing applied pressure.

Acknowledgements

This work was supported by the ministry of Science and Technology of Taiwan, under Contract no. MOST 103-2221-E-492-014-MY3.

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