Comparison of NH₃ and N₂O Plasma Treatments on Bi₂O₃ Sensing Membranes Applied in an Electrolyte–Insulator–Semiconductor Structure

Abstract

In this study, bismuth trioxide (Bi₂O₃) membranes in an electrolyte–insulator–semiconductor (EIS) structure were fabricated with pH sensing capability. To optimize the sensing performance, the membranes were treated with two types of plasma—NH₃ and N₂O. To investigate the material property improvements, multiple material characterizations were conducted. Material analysis results indicate that plasma treatments with appropriate time could enhance the crystallization, remove the silicate and facilitate crystallizations. Owing to the material optimizations, the pH sensing capability could be greatly boosted. NH₃ or N₂O plasma treated-Bi₂O₃ membranes could reach the pH sensitivity around 60 mV/pH and show promise for future biomedical applications.

1. Introduction

Fifty years ago, the first ion-sensitive field-effect transistor (ISFET) was invented by Bergveld in 1970 [1,2]. Following the invention, semiconductor-based ion sensing technology [1,2,3] has been developed since the late 20th century. Among various types of ion sensing semiconductor devices, electrolyte–insulator–semiconductor (EIS) sensors with rapid response, high reliability and simple structure have been intensively studied [4]. Because of low capacitance and poor electrochemical properties, SiO₂ has been replaced by various oxides to improve the membranes properties [5]. Recently, Ta₂O₅ [6], WO₃ [7], and La₂O₃ [8] have emerged as novel membrane materials [9]. However, to further boost the membrane sensing performance, novel materials and new treatments are worthwhile to be explored. Bismuth trioxide (Bi₂O₃) with a bandgap around 2.5 eV has been utilized as photocatalyst [10], super capacitors, and gas sensor materials [11]. However, Bi₂O₃-based pH sensing membranes [12] have not been clearly reported, yet. Furthermore, to enhance the sensing capability, Bi₂O₃ membranes were treated with two types of plasma- NH₃ and N₂O [13,14]. To investigate the improvement of the treatment, multiple material analysis techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and secondary ion mass spectrometry (SIMS) were performed. Material analysis results indicate that Bi₂O₃ membranes treated with NH₃ plasma for 3 min and N₂O plasma for 1 min had strong crystallization, silicate suppression, high grainization, and effective nitrogen passivation. Moreover, the pH sensing measurements [15] indicate that Bi₂O₃ membranes treated in these plasma treatment conditions had high pH sensitivity around 60 mV/pH and high linearity close to 100%. Hysteresis and drift [16] rate evaluation also reveal that the membrane treated in these conditions had the lowest hysteresis voltage and the smallest drift rate. NH₃ and N₂O plasma treatments could incorporate the nitrogen atoms into the deep part of the devices to fix the defects and eliminate the silicates.

According to previous reports [17,18], plasma treatment can eliminate the silicate layer because, because silicates can have chemical reaction and transform from SiOx with dangling bonds to Si–O–Si bonds. The atoms in the plasma such as F or N can facilitate the combination of the silicon or oxygen dangling bonds to form well-crystallized Si–O–Si [19]. Therefore, silicate can be reduced, and near-perfect crystals can replace the silicate. The effective electric field across the membranes can be enhanced so the sensing capability can be improved [20,21]. Moreover, the nitrogen incorporation in the bulks may form NH₂ bond to strengthen the chemical bonds and improve the material properties [22]. On the other hand, plasma treatment of the sensing film could cause an EIS structure sensitive to H⁺ ions [23] because the increase of the metal ions produced on the surface sites to decrease the diffusion capacitance in the solution and enhance the sensitivity. Furthermore, based on the Gouy–Chapman–Stern model [24,25], the sensing parameter β is proportional to the density of surface states, as shown in (1), where Ns is the number of surface sites per unit surface area and C_DL is the double layer capacitance. Therefore, the sensing capability can be enhanced by NH₃ and N₂O plasma treatments.

$$\mathsf{\beta }=\frac{2{\mathrm{q}}^2\mathrm{Ns}\sqrt{{\mathrm{K}}_{\mathrm{a}}{\mathrm{K}}_{\mathrm{b}}}}{{\mathrm{KTC}}_{\mathrm{DL}}}$$

(1)

Owing to high sensitivity, fair linearity, stable response, Bi₂O₃-based EIS membranes [26] with NH₃ or N₂O plasma treatments show promise for future industrial biomedical sensing [27] applications.

2. Experimental

To prepare the EIS sensor with Bi₂O₃ sensing film, 3.95 g of bismuth nitrate (Bi(NO₃)₃·5H₂O) was dissolved in 20 mL of nitric acid with the solution concentration of 1 M. The sol-gel solution was dropped onto the cleansed p-Si substrate. Then, a Bi₂O₃ film formed on it, and then the plasma treatments were performed. The samples were treated with NH₃ and N₂O plasma at 100 W RF power and 500 mTorr processing pressure for 1 min, 3 min, and 6 min, respectively. Then, an Al film with a thickness of 300 nm was deposited on the back of the silicon wafer and the silicone glue was used to define the sensing window, and the device was glued to be fixed on a PCB board. Finally, AB glue was used for packaging to prevent oxidation. The device structure is illustrated in Figure 1.

3. Results and Discussion

To examine the crystalline structures of the membranes, XRD was used. Figure 2a shows the X-ray diffraction patterns of Bi₂O₃ film after NH₃ plasma treatments for various times. The two diffraction peaks BiO (012) and Bi₂O₃ (200) are located at 27.8° and 32.6°, respectively. The as-deposited sample shows a peak of BiO. After NH₃ plasma treatment for 3 min, the sample shows the strongest peak of Bi₂O₃ (200) among all the samples.

By contrast, Figure 2b shows the X-ray diffraction analysis of the Bi₂O₃ film after N₂O plasma treatments for various times. The diffraction peaks BiO (012) and Bi₂O₃ (200) are located at 27.8° and 32.6°, respectively. The as-deposited sample also shows a peak of BiO. After N₂O plasma treatment for 1 min, the sample shows the strongest Bi₂O₃ (200) peak among all the samples. With the increase of the plasma treatment time, the intensity of the Bi₂O₃ (200) peak gradually decreased.

Furthermore, XPS analysis was used to study the chemical bonding state of the Bi₂O₃ sensor film after NH₃ and N₂O plasma treatments. The O1s spectra of the samples after NH₃ plasma treatments is shown in Figure 3a. The as-deposited and annealed samples have 4 peak fitting curves, namely SiO₂ (531.8 eV), silicate (531.4 eV), oxygen defect (530.3 eV), and Bi-O (529 eV). After the NH₃ plasma treatment, the oxygen defects and slicates were significantly reduced. Because NH₃ plasma treatment could dope N into the film to improve dangling bonds and strain bonds, the sensing properties were improved.

The O1s spectra of the samples after N₂O plasma treatment is shown in Figure 3b. The as-deposited and annealed samples have three peak fitting curves, namely, silicate (531.4 eV), oxygen defect (530.3 eV), and Bi-O (529 eV). After N₂O plasma treatment, the oxygen defects and silicates were significantly suppressed. Since N₂O plasma treatment could incorporate N into the film, mitigate dangling bonds and strain bonds, and strengthen the film structure, the sensing behaviors could be improved. Results indicate that N atoms in the plasma could transform SiO_x with dangling bonds to form well-crystallized Si–O–Si.

Figure 4a–d shows the atomic force microscope (AFM) images of the Bi₂O₃ film after NH₃ plasma treatments for various times, and Figure 4e–h shows the atomic force microscope (AFM) images of the Bi₂O₃ film after N₂O plasma treatments for various times. The root mean square (Rms) roughness of the sample without plasma treatment and of the samples after NH₃ plasma treatment for 1, 3, and 6 min were 1.31, 5.32, 15.56, and 11.33 nm, respectively. The root mean square (Rms) roughness of the sample without plasma treatment and of the samples after N₂O plasma treatment for 1, 3, and 6 min were 1.31, 3.9, 3.83, and 3.21 nm, respectively. After 3 min of NH₃ plasma treatment of 1 min of N₂O plasma treatment, the Bi₂O₃ film has the largest Rms value. The incorporation of N can passivate the defects improve the crystalline structure, and strengthen the grainization, thereby increasing the surface sites and improving the sensing characteristics.

To examine the surface morphologies, Figure 5a–f shows the field emission scanning electron microscope (FESEM) images of the deposited Bi₂O₃ film and the Bi₂O₃ film after NH₃ and N₂O plasma treatments. After 1 min of NH₃ plasma treatment, irregular crystals with uneven distribution were produced on the surface. After 3 min of plasma treatment, the crystals became denser. After 6 min of plasma treatment, the crystals became sparsely distributed again. Therefore, the NH₃ plasma treatment in 3 min had the best material properties. (FESEM) images of the deposited Bi₂O₃ film and the Bi₂O₃ film before and after N₂O plasma treatment for 1 min are shown in Figure 5e,f. After N₂O plasma treatments for 1 min, the crystallization of the film became obvious. Due to the incorporation of N, the dangling bonds and strain bonds in the film can be fixed, and the crystalline structure could be strengthened, so the sensing could be improved.

In addition, the images of the two types of plasma treatments are compared. It can be found that the uniformity of the film after N₂O plasma treatments is more uniform than that of the Bi₂O₃ film after NH₃ plasma treatment. Therefore, Bi₂O₃ film maybe more stable after N₂O plasma treatments than NH₃ plasma treatments.

Figure 6a,b shows the SIMS of the samples with NH₃ and N₂O plasma treatment for various time. It can be seen that as the plasma treatment time increased, the thickness of the sensing film decreased. As the film after plasma treatment would be etched with the increase of the plasma treatment time, the sensing characteristics were slightly reduced. On the other hand, nitrogen atoms can be introduced into the Bi₂O₃/Si interface by NH₃ and N₂O plasma treatment as shown in the two SIMS profiles. These accumulated nitrogen atoms can passivate the defects of the interface. Since the incorporation of N can improve the dangling bonds and strain bonds of the film, the sensing performance is improved.

As the two plasma treatments are compared, it can be found that the amount of nitrogen incorporated after the N₂O plasma treatment was relatively stable, and the nitrogen content only slightly decreased with the increase of the plasma time. Therefore, the N₂O plasma treatment had relatively stable sensing characteristics.

To assess the pH sensing behaviors, C–V curves of Bi₂O₃ after different NH₃ and N₂O plasma treatment conditions were measured. Figure 7a–d shows C–V curves of Bi₂O₃ after NH₃ plasma for various times. The sensitivity value without NH₃ plasma treatment was 42.66 mV/pH, and the linearity was 94.483%. After NH₃ plasma treatment for 1 min, 3 min and 6 min, the sensitivity values became 30.87, 59.84 and 40.83 mV/pH, respectively, and the linearity values became 84.45%, 99.25% and 97.17%. As for the sensitivity among the Bi₂O₃ samples with different NH₃ plasma time, it can be found that the Bi₂O₃ sensor film had the highest sensitivity after NH₃ plasma treatment for the 3 min sample. Consistent with the FESEM images, the film treated in this condition produced densely arranged and layered crystals, which produced a larger contact area and increased sensitivity. After 3 min of NH₃ plasma treatment, there were small and dense Bi₂O crystals, thereby increasing the sensitivity of the sensing film.

Figure 7e–h shows the C–V curve of Bi₂O₃ after different N₂O plasma treatment time. After N₂O plasma treatment for 1 min, 3 min and 6 min, the sensitivity were 60.43, 59.91 and 59.8 mV/pH, respectively. The linearity was 99.82%, 98.97% and 99.64%. As for the sensitivity of Bi₂O₃ under different N₂O plasma time, it can be found that the sensitivity of the Bi₂O₃ film after N₂O plasma for various times and the samples in all the conditions were improved, consistent with FESEM images of the uniform distributed crystals under various plasma treatment conditions. After N₂O plasma treatment, surface defects could be passivated, and dangling bonds and strain bonds can be fixed. Therefore, plasma treatments could improve the material properties and enhance the crystalline structure and grainization effect, and thereby increasing sensitivity.

To investigate the reliability of the membranes, Figure 8a shows the hysteresis voltage of the Bi₂O₃ sensing film after NH₃ and N₂O plasma treatment. The Bi₂O₃ sensing film without plasma had a hysteresis voltage of 22.49 mV, and the hysteresis voltage after 1, 3, and 6 min of NH₃ plasma were 24.18, 3.31, and 16.13 mV, respectively. As for the Bi₂O₃ sensing film treated by NH₃ plasma at different times, the NH₃ plasma treatment for 3 min shows the lowest hysteresis voltage. Since the incorporation of N with NH₃ plasma treatment can passivate the defects, thereby inhibiting the diffusion of reactive ions and delaying the reference voltage response. Combined with the XPS analysis, it can be seen that the 3 min NH₃ plasma had the least oxygen vacancies, and the Bi-O bond was the strongest. After calculation, it can be known that the 3 min NH₃ plasma has the lowest Bi²⁺ content, so the sensor film shows low hysteresis voltage.

The hysteresis voltage of the Bi₂O₃ sensing film after N₂O plasma is shown in Figure 8b. The hysteresis voltage of the Bi₂O₃ sensing film without plasma was 22.49 mV, and the hysteresis voltage after 1, 3, and 6 min of N₂O plasma were 2.31, 3.01, and 4.87 mV, respectively. It was observed that after 1 min of N₂O plasma the membrane had a lower hysteresis voltage compared with all the other samples. As the plasma treatment time increased, the hysteresis voltage gradually increased because the crystals gradually became smaller, which caused the hysteresis voltage to rise.

Furthermore, Figure 8c,d shows the drift coefficient of the Bi₂O₃ sensing film after NH₃ and N₂O plasma, respectively. The drift coefficient is an important parameter describing the long-term stability of the sensor. In order to sense the long-term reliability of the film, we placed the Bi₂O₃ sensing film treated with plasma treatments in a pH7 solution for 12 h to obtain the drift rate of the sensing film. The drift rate of the Bi₂O₃ sensing film without plasma was 23.58 mV/hr, and the drift rate of the sensing film with NH₃ plasma after 1, 3, and 6 min were 20.7, 2.57, 15.09 mV/hr. It can be seen that the sensing film after 3 min of NH₃ plasma had the lowest drift rate. This is because NH₃ plasma could effectively passivate the defects, which allow ions to adhere, thereby inhibiting the diffusion of reactive ions and varying the reference voltage response. Therefore, the drift rate was reduced. Figure 8d shows the drift coefficient of the Bi₂O₃ sensing film after N₂O plasma. The drift rate of Bi₂O₃ sensing film without plasma was 23.58 mV/hr, and the sensing film after N₂O plasma for 1, 3, and 6 min were 2.45, 3.44, and 7.57 mV/hr. The sensing film of N₂O plasma had the lowest drift rate in the sample with N₂O plasma for 1 min.

4. Conclusions

Bi₂O₃ EIS sensing membranes in EIS structures were fabricated. To boost the sensing performance, NH₃ and N₂O plasma treatment were performed on the membranes. The results indicated that the sample treated with NH₃ plasma for 3 min and the sample with N₂O plasma treatment for 1 min had higher sensitivity than all the other conditions. Multiple material characterizations confirmed the enhancement of crystallization, and the removal of the defects may cause the improvements of the sensing behaviors owing to nitrogen passivation in the device. The plasma treatments could cause N atoms to incorporate into the bulks and the silicate could be transformed to well-crystallized films. Furthermore, plasma treatment could enhance grainization, which increased the density of the sensing surface sites, thereby boosting the sensing behaviors. Therefore, NH₃ or N₂O plasma treated-Bi₂O₃ membranes could reach the pH sensitivity around 60 mV/pH and show promise for future biomedical applications.