INSTRUMENTATION, Vol. 11, No. 4, December 2024
Copyright: © 2024 by the authors. This article is licensed under a Creative Commons Attribution 4.0 International License (CC BY) license(https://creativecommons.org/licenses/by/4.0/).
Introduction
Force parameters play an important role in intelligent robots, biomedical applications, materials science, and dimension measurements[1,2]. Many strategies have been proposed to realize force or pressure sensing in recent years. The operating principle of widely used force sensors is usually based on piezoelectric[3,4], magnetic[5,6], and capacitive[7-9] mechanisms. Piezoelectric force sensors usually have an edge in terms of cost, linear response and sensitivity, and piezoelectric sensors effectively convert dynamic forces to electrical signals by the direct piezoelectric effect. On the other side, a common charge of the neutralized surface makes it face some challenges in measuring static forces.[10] Magnetic force sensors operate with Lorentz force. The sensors commonly have a compact structure, wide measurement range, low energy consumption, high sensitivity and suitable performance. But the operating temperature has a great influence on this type of sensor, so it is necessary to add additional temperature compensation circuitry[6]. Meanwhile, capacitive sensors utilize the change in capacitance caused by external conditions as the input. Similar to magnetic force sensors, capacitive force sensors are also extremely sensitive to conditions such as temperature and humidity; their accuracy is not outstanding because it is more difficult to measure capacitance compared to resistance. All the above-mentioned devices can measure force levels from mN to pN[11]. Compared to the above-mentioned typical force sensors, a force sensing system based on optical methods is more accurate and easier to realize. Optical force sensors usually contain a mechanical unit and a signal acquisition system with a light source and a photodetector or an optical spectrometer. The discrete device structure provides the optical force sensor system great freedom in terms of component selection and parameter optimization, and it is usually more inexpensive. For example, Uzun et al. prepared an optical force sensor that showed an enhanced resolution of 0.03 N from 0 to 20 N with the Fabry-Pérot Interferometry (FPI) method[12]. Leal-Junior et al. presented the development of a bioinspired multifunctional flexible optical sensor (BioMFOS) as an ultrasensitive tool for force (intensity and location) and orientation sensing[13]. This sensor allows the measurement of μN-level forces and the reconstruction of the shape of forces applied on the sensor with sub-millimeter spatial resolution. Although this force sensing system has seen great success, multifunctional devices are urgently needed for achieving miniaturization and integrating sensing units; consequently, they can be used to improve the device performance further.
Recently, InGaN/GaN-based multi-quantum wells (MQWs) have been widely researched for use in light-emitting diodes (LEDs) [14, 15], with applications in lighting, displays, communication devices, etc[16-18]. These LEDs exhibit good thermal stability; they are prone to structural degradation at temperatures higher than 900°C, which affects their normal operation. Therefore, devices based on these LEDs can work normally under non-extreme temperature conditions. Since both the emitter and detector co-exist in MQW-based devices, they also function as bi-functional optoelectronic devices that can simultaneously emit, detect, and modulate light[19-21]. These multi-function devices have also been used in sensing applications. For example, Liu et. al reported a fully integrated patch based on lamellar porous film-assisted GaN optopairs for wireless intelligent respiratory monitoring[22]. Our previous work also presented a chip-integrated optical fiber magnetic field sensing system based on GaN MQWs[23]. More importantly, since the transceiver integration is an inherent property of any GaN LED device with MQW structure, the design of the optical sensor system can also be built with commercialized LED, the cost of the entire system can be compressed to several dollars or less, and it is expected to be applied to disposable replaceable sensing units.
In this work, we designed and fabricated a force-sensing unit by combining an Ag-decorated polydimethylsiloxane (PDMS) mechanical module with an optical channel formed by two ideal InGaN/GaN LEDs. In our design, one LED acts as the emitter and the other as the receiver, thereby forming a sensing path. Meanwhile, the PDMS film functions as a receiver, transforms the pressure into displacement, and eventually generates the photocurrent in the LED. The separation design of the electrical signal unit and the mechanical sensing unit gives the force sensor device significant optimization freedom. The mechanical properties of the PDMS film have been studied using experiments and finite element simulation. The photocurrent properties of the proposed devices are optimized by introducing Ag film, varying the distance between the emitter and receiver, and altering the distance between the LED and PDMS film. Finally, the force-sensing properties of the system are studied.
Experiments section
Fabrication of the InGaN/GaN LED Chip
Similar to our previous work[24], the sample in this study was fabricated using a standardized device process, which involves photolithography and inductively coupled plasma (ICP) etching on a four-inch and 6.2 μm thick sapphire-based GaN film. From top to bottom, the epitaxial film contains Mg-doped p-GaN, InGaN/GaN MQW, Si-doped n-GaN, and unintentionally doped GaN (u-GaN), and their thicknesses are 0.4 μm, 0.35 μm, 2.8 μm and 1.5 μm, respectively. For the device fabrication process, a mesa region was defined via photolithography and ICP etching to expose the n-GaN surface. A deep ICP etching was then performed to obliterate epitaxial films for device isolation (Steps a-b). Then, a transparent indium tin oxide (ITO) current spreading layer with a thickness of 230 nm was deposited via sputtering, which was followed by rapid thermal annealing at 530oC in N2 atmosphere (Steps b-c). Subsequently, the ITO layer was patterned and etched with a mixture of HCl/FeCl3 solution to expose n-GaN. Then, the surface electrode was subjected to a metal lift-off process and metal stacks subjected to rapid thermal annealing (850°C, 30 s, and N2 atmosphere) were deposited on the n-GaN and reflector (DBR) containing 36 pairs of SiO2/TiO2 on the bottom sapphire surface of the LED (Steps d-e). Finally, a square LED with a size of 5×4 mm was prepared (Figure 1f). Subsequently, a comb-like surface electrode was introduced into the device to improve current uniformity. The generated current allows the entire LED to be lit (Figure 1g).
Fig.1. (a–e) Schematic of the sample preparation process of InGaN/GaN MQW devices. Optical images of the actual sample (f), luminous photograph (g), and physical view of the sensing module (h,i). Schematic depicting the principle of the sensing unit (j,k), the inset shows the materials and energy band structure of the LEDs.
Fabrication of the PDMS/Ag film
The PDMS/Ag multi-film used in our work was fabricated via electron beam evaporation. In this process, a 100 nm Ag film was deposited on a commercial PDMS film (10×10 cm in size and 50 μm in thickness) at a rate of 0.1 nm/s. The top-view optical images of PDMS before and after Ag deposition are shown in Figure S1. The pure PDMS film is transparent, and the Ag particle can be seen clearly in the PDMS/Ag film. This particle blocks the light transport properties and augments the reflected signal in sensing applications.
Design of the force sensing unit
Figure 1(h, i) shows that the sensing module is built by placing the PDMS/Ag film in a designed holder as the mechanical sensing element, while two separated InGaN/GaN photonic chips are made to bond with a printed circuit board (PCB) substrate and function as the signal transform unit. The discrete device structure ensures flexible distance control. The luminescence and detection structures in LED1 and LED2 can help the device switch between the emitter and receiver. Overall, the above-mentioned features will benefit further optimization of the sensing properties. The principle of the signal transform unit is shown in Figure 1(j,k). The emitter and receiver are placed behind the PDMS/Ag film. In the absence of pressure (Figure 1(j)), the light produced by LED1 is reflected by the PDMS/Ag film, received by LED2, and transformed into a stable photocurrent. If the PDMS/Ag film is subjected to any form of pressure, a strain is produced and the distance between the PDMS/Ag film and the two LEDs increases. Consequently, the photocurrent of LED2 changes. The working principle of the mechanical module is shown in Figure 2(a); within the elastic deformation range, the device can be used as a strain gauge-based force sensor[25]. The force or pressure imposed on the device tends to change the shape of the PDMS film. To further demonstrate the working principle of the mechanical module, a simulation was performed based on finite element methods. Figure 2 (a, b) shows that when the PDMS/Ag multi-film is subjected to pressure, it undergoes a change from state a to state b. According to the classic Hooke's law, the stress can be expressed as follows[1]:
Fig.2. (a) Simplified mechanical model for the mechanical sensing elements, (b) physical parameters of the mechanical sensing elements under force, and (c) force analysis. Simulation results of (d) tensile stress and (e) z-axis displacement distribution in the polymer membrane under 0.5 N, and (f) relationship between force and normalized z-axis displacement.
where E is Young's modulus of the material, A0 is the cross-section area, ϵ is the strain, L is the length of the material, and ∆L represents the changes in material length. The PDMS film which has a size of 90 mm× 90 mm was used in our experiment and the diameter of the block is 18 mm. During the stress analysis, the length in the z-direction is 1 mm; thus, a = 36 mm, d = 18 mm, and b ranges between 0 and 1 mm. The change in the length of the PDMS film can be calculated as follows:
When pressure is applied to the PDMS film with the experimental device shown in Figure S2, the angle change induced in its shape is defined as follows:
Based on the mechanical analysis in Figure 2(b), the equivalent vertical force F2 is expressed in Figure 2(c):
Based on Eq. 1-4 and the parameters shown in Figure 2(b), the relationship between the pressure or force and the change in the PDMS film can be obtained. Besides, the force can hold a linear relationship with the sharp changes of PDMS film in a small θ degree, and the relationship will change for a big θ degree. Normally, the sharp changes will be the function of Young's modulus and Poisson's ratio. A simulation is performed by selecting the Young's modulus as 750 kPa and Poisson's ratio as 0.49 for the PDMS film, the Ag film thickness as 100 nm, and the PDMS film thickness as 50 μm. The simulation results in Figure 2(d, e) imply that strain is mainly distributed on the central portion of the PDMS film, which causes a displacement in the z-direction. As expected, with increasing pressure, normalized displacement increases linearly in the force region of 0–1 N (Figure 2(f)). By monitoring the reduced changes in photocurrent, the force-sensing properties of our device can be obtained. Moreover, since the LEDs have the same structure as that of InGaN/GaN multiple quantum wells (MQWs), the coexistence of light emission and detection in this design can ensure that each LED serves as both the emitter and photodetector (PD)[26]. The LED can receive light from the other LED nearby and once the sensing channel is formed, the receiver and emitter can be interchanged.
Results and discussion
Luminescence and detection characteristics of the InGaN/GaN LED
Electroluminescence (EL) and photodetector properties of the InGaN/GaN LED are essential for preparing sensing devices. Figure 3(a) shows that the current-voltage (I-V) properties of the individual InGaN/GaN LED have been measured using an enhanced Keithley source meter (B2901 of 100 fA). The turn-on voltage is ~2.33 V and the resistance is 106 Ω (Figure 3(a)). EL spectra of the sample were measured with a spectrograph (Acton SP2500i). The broad spectra of the InGaN/GaN LED peaks at 469 nm, while the full width at half maximum (FWHM) is 19.5 nm and the chromaticity coordinates are (0.1788, 0.157) in the CIE 1931 chromaticity diagram (Figure 3(b, c)). The EL intensity increases after a driven current is added (Figure 3(a) inset).
Fig.3. EL properties of InGaN/GaN LED as an emitter: (a) Current-voltage (IV) and impedance characteristics; inset shows the driven-current related EL intensity. (b) Coordinates of the CIE 1931 chromaticity diagram. (c) Driven current-related EL spectra and normalized spectral sensitivity. (d) PD properties of InGaN/GaN LED as the receiver: (d) IV curves and (e) photocurrent variation with different LED currents. (f) Dynamic photocurrent response of the MQW-PD with emitter and receiver.
Optical response spectra of the InGaN/GaN device as a PD are shown in Figure 3(c). The device exhibits a light response in the 350 to 480 nm region, which decreases as the receiver is measured under light irradiation provided by the nearby LED. Without light irradiation, the dark gradually for incident light. The overlap between EL and response spectra (~40 nm) is key to realizing a sensor with a transceiver integrated chip[27, 28]. IV curves of a single device photocurrent are in the nA range (Figure 3(d)). The photocurrent increases with a rise in the driven current of LEDs nearby. The photocurrent increases to μA levels under light illumination. When the driven current ranges from 0 to 35 mA, photocurrent increases linearly at a bias voltage of 0 V (Figure 3(e)). Thus, we used a bias voltage of 0 V for further experiments. The dynamic photocurrent response of the proposed LED and PD channel is shown in Figure 3(f). Given that LED1 and LED2 exhibit the same device structure, the signals they produce are nearly identical with regard to response speed and signal strength. The reproducibility and stability of the device were evaluated via cyclic testing (Figure 4(a)). The output photocurrent signals remain remarkably stable for 200 s, which is equivalent to over 100 cycles. Subsequently, we further extended the testing time and tested the stability and reliability of the signal which lasted for about 30 minutes. The error range obtained was within ±1%, as shown in Figure S3. The transient photo-response in Figure 4(b) shows the recovery time, which is defined as the time required for the increase in photocurrent to drop from 90% to 10%; the rise time is ~68.4 ms and downtime is ~61.0 ms. Although the recovery time is slower than other sensors, it is fast enough for steady-state sensing applications[29].
Fig.4. (a) Dynamic photocurrent response of the MQW-PD over 100 cycles. (b) Instantaneous response of the device.
Parameter optimization for the signal transform unit
Issues such as the driven current of the LED, the space between the LEDs and PDs, and the gap between the mechanical module and electrical module may influence the sensing performance of the device. To figure out it, Figure 5 shows how different variables affect the sensor's detection performance, which ultimately includes the distance between the two LEDs, the distance between the detection structure and the physical structure, and whether a silver-modified PDMS film is used. Define D as the gap between the emitter and receiver and H as the height between the PDMS film and LEDs pairs. For samples with Ag in Figure 5(a), the photocurrent decreases with the addition of H values. For a fixed driven current of 10 mA for the emitter, photocurrents of the receiver at different D values and H values are shown in Figure 5(b, c). The sample without Ag film exhibits a photocurrent value of 415 nA to 10.5 nA when its thickness ranges from 1.5 to 6 mm and the spacing region ranges between 0 and 30 mm (Figure 5(b)), current decreases with the addition of D values but no obvious regular can be seen for the changes of H values. The photocurrent ranges from 1.74 μA to 40 nA for samples with a Ag film when its thickness ranges from 1.5 to 6 mm and the spacing region ranges between 0 and 30 mm (Figure 5(c)). The photocurrent decreased with the addition of H and D values. Besides, the photocurrent is enhanced because the Ag film augments the light reflected by the PDMS film and the signal is more stable. To further demonstrate this, we simulated the electromagnetic field via FDTD simulation (see details of the simulation methods in the supporting information). Figure 5(d) shows that the PDMS film with an Ag film has a higher electromagnetic field. All these results indicate that the sample with an Ag film, which is placed near the emitter and receiver and between the PDMS film and LEDs, will have a higher sensing signal. Thus, we chose D as 0 mm and H as 1.5 mm for subsequent force sensing experiments.
Fig.5. (a) Photocurrent of PD at different heights between the LED and PDMS-Ag film. Photocurrents vary with the space between the LED and PD: (b) without Ag film, (c) with Ag film. (d) Electromagnetic field distribution of the PDMS film with or without Ag.
Demonstration of the force sensing system
Based on the experimental setup of Figure 1(j, k), the force sensing system is built. The applied force was analyzed by transforming the shape change of the PDMS film into the photocurrent of InGaN/GaN LEDs. With the optimized parameters, a real force sensing system was constructed (Figure 6 (a–c)). By adding a weight to the PDMS film, different photocurrents are observed. The photocurrents increase linearly when the force varies from 0 N to 0.7 N at a sensitivity of 1.23 μA/N (Figure 6(d)). Subsequently, we conducted repetitive tests for force-related photocurrent under various lighting conditions to verify the experiment's robustness, and determined that the standard deviation of the system's sensing performance was within 30-49 nA, as shown in the supplementary material (Figure S4). To further demonstrate the sensing properties, the fatigue tests and shape changes induced sensing properties were verified. Based on the schematic shown in Figure 6 (e, f), sharp changes induced photocurrent of the device were performed with a stepper motor that controls the block (see picture of real products in Figure S2) with a move region of 0 to 1 mm. For each position in Figure 6 (g), photocurrent increases linearly from 0 to 0.7 mm and remains stable for the next 0.3 mm. This phenomenon is similar to the observations made after weights were placed on top of the sensor (Figure 6(d)); the results confirm the working principle of our system. Figure 6(h) displays the measurement results of the sensor for cyclic tests; As expected, the photocurrent is increased then stable and then decreased with the times. The photocurrent profiles imply high stability over 150 cycles. As shown by the transient response of the sensor in Figure 6(i), the photocurrent remains stable with increasing time and then rises linearly with time.
Fig.6. (a-c) Pictures of the device with different weights, (d) force-related photocurrent, (e, f) simplified system diagram for fatigue tests,(g) position-related photocurrent, (h) cyclic measurement of the sensor over 150 cycles, and (i) transient response of the sensor.
To demonstrate the advantages of our sensor, we compared its parameters with those of some existing sensors, including sensitivity, measurement range, and response time; the response time was replaced with the rise time. The comparison reveals that our sensors exhibit good sensitivity and response speed. The response time is better than most other sensors in the comparison. However, it has a smaller range than other sensors.
| Sensor type | Sensing medium | Sensitivity (kPa-1) | Range (kPa) | Rise time |
|---|---|---|---|---|
| This work | Ag-modified PDMS | 0.320 | 0~2.23 | 68.4 ms |
| Capacitive flexible sensor[30] | PDMS/BaTiO3/Sr TiO3 dielectric layer | 2.681 | 0~5 | 39 ms |
| Capacitive resistance composite sensor[31] | PPy filter | 0.186 | 0~23 | 1.5 s |
| Iontronic flexible sensor[32] | PBST nanofiber membranes | 902.4 | 0~6 | 147 ms |
|
Resistance sensor[33] |
PDMS-Silicon nanofilms | 0.0023 | 0~200 | 85 ms |
Conclusion
In this study, a force sensor with two separated InGaN/GaN photonic chips function as the signal transform unit and a designed PDMS holder acts as the mechanical sensing element has been fabricated. The LEDs are functioned as both the emitter and receiver. As an emitter, the turn-on voltage, EL peak, and FWHM of the device are 2.33 V, 469 nm, and 19.5 nm, respectively. As a receiver, the photoresponse spectra peaked at 390 nm with a cut-off wavelength of 480 nm and a falling edge of 61.0 ms. The Ag/PDMS multi-film was fabricated by depositing a 100 nm thick Ag film on a commercial PDMS film to function as the mechanical module. The multi-film was attached to a homemade holder to fabricate a force-sensing device. The light produced by the emitter is reflected back to the receiver by the PDMS film, and the signal strength is increased by the Ag film in a linear region of 0-0.7 mm. Within the elastic deformation range, the variations in the force applied to the PDMS film were transformed into the photocurrent and force sensing was achieved. The gap between the emitter and receiver and the distance between the mechanical module and signal transform unit can influence the performance of the device. Our results indicate that the photocurrent is higher when the PDMS film has a height of 1.5 mm and the emitter and receiver are close to each other. Finally, force sensing was demonstrated using weights, and a system resolution of 1.23 μA/N was achieved from 0 to 0.7 N.
Supporting documents
Dates: