Tseng-Fu Lu

pH Sensitivity Improvement on 8 nm Thick Hafnium Oxide by Post Deposition Annealing

A thin hafnium oxide (HfO 2 ) layer (8 nm), for hydrogen ion sensors, was deposited directly on a silicon substrate without the buffer oxides. Post deposition annealing (PDA) was performed to improve the sensitivity. The as-deposited HfO 2 sensing dielectric functions from pH 4 to pH 12, and the sensitivity is 46.2 mV/pH. For the samples with 900°C PDA, the sensitivity is increased to 58.3 mV/pH from pH 2 to pH 12. From atomic force microscope analysis, the improvement is related to the surface morphology. A physical model was proposed to explain PDA effects by the surface site density (surface area) and dissociation constants.

Drift and Hysteresis Effects Improved by RTA Treatment on Hafnium Oxide in pH-Sensitive Applications

Rapid thermal annealing (RTA) treatment in N 2 ambient is applied to optimize the drift and hysteresis effects of HfO 2 -based sensing membrane. The drift coefficient of the 8 nm thick HfO 2 -electrolyte-insulator-semiconductor (EIS) structure was 1.4 mV/h. After being treated with RTA at 900°C, the drift coefficient and hysteresis width of the 8 nm thick HfO 2 layer was reduced to 0.65 mV/h and 1.7 mV, respectively. For the HfO 2 -EIS structure treated with RTA at 500°C, the drift coefficient was increased to 3.25 mV/h because of the crystallization of the HfO 2 layer and was improved by 900°C annealing due to the densification of the HfO 2 layer. Therefore, the drift coefficient of the HfO 2 -EIS structure was reduced to 0.65 mV/h by RTA treatment at 900°C. With excellent pH sensitivity and stability, an HfO 2 layer with proper RTA processing can be used as pH sensing material in ion-sensitive field-effect transistor-type sensors. Moreover, to easily control capacitance-voltage measurements and automatically calculate parameters such as pH sensitivity, drift coefficient, and hysteresis width of an HfO 2 -EIS structure, a program based on LabVIEW was developed.

Thickness Effects on pH Response of HfO2 Sensing Dielectric Improved by Rapid Thermal Annealing

Hafnium oxide (HfO2) was directly deposited on silicon as a sensing dielectric by RF sputtering. This combination was proposed to replace the HfO2/SiO2 stacked structure. The characterizations of HfO2 sensing dielectrics of various thicknesses were widely investigated using post-rapid thermal annealing (RTA) treatments at various temperatures and process times. The pH sensitivity of a 30-nm-thick HfO2 sensing dielectric was 49.2 mV/pH, almost the same as the traditional HfO2/SiO2 stacked electrolyte–insulator–semiconductor (EIS) structure. The verified minimum thickness for a HfO2 sensing dielectric was 4 nm. The ion sensing performance of HfO2-EIS, pH sensitivity, and drift voltage were all improved by RTA treatment. With 900 °C RTA, pH sensitivities can be improved and approach 59.6 mV/pH for all thicknesses of HfO2 sensing dielectric. HfO2 sensing dielectrics with 900 °C RTA are verified as good hydrogen ion sensing materials.

Immobilization of enzyme and antibody on ALD-HfO2-EIS structure by NH3 plasma treatment

Thin hafnium oxide layers deposited by an atomic layer deposition system were investigated as the sensing membrane of the electrolyte-insulator-semiconductor structure. Moreover, a post-remote NH3 plasma treatment was proposed to replace the complicated silanization procedure for enzyme immobilization. Compared to conventional methods using chemical procedures, remote NH3 plasma treatment reduces the processing steps and time. The results exhibited that urea and antigen can be successfully detected, which indicated that the immobilization process is correct.

Effects of CF4 Plasma Treatment on pH and pNa Sensing Properties of Light-Addressable Potentiometric Sensor with a 2-nm-Thick Sensitive HfO2 Layer Grown by Atomic Layer Deposition

We investigated the effect of the carbon tetrafluoride (CF4) plasma treatment on pH and pNa sensing characteristics of a light-addressable potentiometric sensor (LAPS) with a 2-nm-thick HfO2 film grown by atomic layer deposition (ALD). An inorganic CF4 plasma treatment with different times was performed using plasma enhance chemical vapor deposition (PECVD). For pH detection, the pH sensitivity slightly decreased with increasing CF4 plasma time. For pNa detection, the proposed fluorinated HfO2 film on a LAPS device is sensitive to Na+ ions. The linear relationship between pNa sensitivity and plasma treatment time was observed and the highest pNa sensitivity of 33.9 mV/pNa measured from pNa 1 to pNa 3 was achieved. Compared with that of the same structure without plasma treatment, the sensitivity was improved by twofold. The response mechanism of the fluorinated HfO2 LAPS is discussed according to the chemical states determined by X-ray photoelectron spectroscopy (XPS) analysis. The analysis of F 1s, Hf 4f, and O 1s spectra gives evidence that the enhancement of pNa sensitivity is due to the high concentration of incorporated fluorine in HfO2 films by CF4 plasma surface treatment.

Effects of Thickness Effect and Rapid Thermal Annealing on pH Sensing Characteristics of Thin HfO2 Films Formed by Atomic Layer Deposition

In this article, thin hafnium oxide (HfO2) films deposited by atomic layer deposition (ALD) were investigated as a sensing layer on an electrolyte–insulator–semiconductor (EIS) structure for pH sensor applications. Compared with sputtering, ALD provides the possibility of shrinking the thickness of the HfO2 sensing layer down to 3.5 nm with a low drift coefficient (<0.2 mV/h). To increase the pH response, an additional rapid thermal annealing was performed on a 3.5-nm-thick ALD-HfO2 film. After annealing at 900 °C, the pH sensitivity could be effectively increased to that near the Nernstian response (59.6 mV/pH), and the drift coefficient (<1 mV/h) and hysteresis width (4.26 mV) are still sufficient. On the basis of the results of atomic force microscopy (AFM) analysis, the increase in surface roughness is possibly the reason for the increase in pH response after annealing. For compatibility with advanced complementary metal–oxide–semiconductor (CMOS) technology, the thin ALD-HfO2 film is a promising candidate for pH sensor fabrications.

Sodium and potassium ion sensing properties of EIS and ISFET structures with fluorinated hafnium oxide sensing film

The potassium (K+) and sodium (Na+) ion-sensitive membranes treated by the carbon tetrafluoride (CF4) plasma by using plasma-enhanced chemical vapor deposition (PECVD) on the single HfO2 sensing films based on Electrolyte-Insulator-Semiconductor (EIS) and Ion Sensitive Filed-Effect Transistor (ISFET) were proposed in this work. With CF4 plasma treatment, the fluorinated-EIS exhibits higher pK-sensitivity and pNa-sensitivity, wide sensing concentration range of potassium and sodium ion and high linearity. Before plasma treatment, the same ion sensitivity with 7 mV/pX (X = Na or K) in the concentration between 1 mM and 10 mM of K+ and Na+ was observed. After plasma treatment, the sensitivity of K+ and Na+ were increasing with increasing plasma time. The optimal sensitivity of potassium ion (49.38 mV/pK) and sodium ion (50.47 mV/pNa) was obtained during 5 min plasma treatment in the concentration range between 0.1 mM to 100 mM. For the results of fluorinated-ISEFT, the obvious changes as a function of potassium and sodium ion concentration were observed during titration.

Reference Electrode–Insulator–Nitride–Oxide–Semiconductor Structure with Sm2O3 Sensing Membrane for pH-Sensor Application

We investigate a reference electrode–insulator–nitride–oxide–semiconductor (RINOS) structure with a Sm2O3 sensing membrane and using silicon nitride as the charge trapping layer for pH detection. The proposed RINOS device with the oxide–nitride–oxide (ONO) structure exhibits a high pH sensitivity (larger than the ideal Nernst response, ~59 mV/pH) owing to hydrogen ion adsorption by the trapped electrons within the embedded Si3N4 layer when applying a stress voltage. As the applied voltage and time increase, pH sensitivity increased. The possible sensing mechanism based on charge attraction was demonstrated using schematic band diagrams. To improve the retention of an increased sensitivity, an additional SiO2 layer as a blocking layer between the Sm2O3 and Si3N4 films to form the RIONOS device was proposed. Compared with the conventional electrolyte–insulator–semiconductor (EIS) structure, the proposed RIONOS device can be used to detect ultra small pH variations owing to its high pH-sensing response.

Sensitivity improvements of Hf x W y O z sensing membranes for pK sensors based on electrolyte-insulator-semiconductor structure

In this study, the hydrogen ion and potassium ion sensing properties of mixing hafnium oxide-tungsten oxide (HfxWyOz) membranes on an electrolyte-insulator-semiconductor (EIS) structure by co-sputtering method were investigated. For tungsten oxide, the increasing of pH-sensitivity (35.47 mV/pH to 46.22 mV/pH), linearity (98.11% to 99.87%), measuring range (pH 6-pH 12 to pH 2-pH 12) and the decreasing of hysteresis (17.63 mV to 2.34 mV) were observed with increasing the ratio of hafnium oxide incorporation. For hafnium oxide, an increasing of pK-sensitivity (7.07 mV mV/ pK to 26.84 mV/ pK in the concentration range between 1 mM to 100 mM) was observed with increasing the ratio of tungsten oxide incorporation. For potassium ion detection, the HfxWyOz (HfO2-60%) sensing membrane with good pK sensitivity (26.84 mV/pK) and high linearity (99.67%) in the concentration range between 1 mM and 100 mM and with good selectivity (low pH-sensitivity) over hydrogen ion was chosen as the optimal condition for pK sensor application.

1.2.1 Urea Biosensor Using NH3 Nitrided Amine Groups on Flexible Substrate

In this study, indium tin oxide (ITO) layers were deposited on polyethylene terephthalate (PET) substrates (ITO/PET) as a sensing membrane. In order to generate the amine groups on the surface for urease immobilization, NH3 plasma was used with RF power of 100 W for various times. The pH sensitivities of ITO/PET electrodes treated without and with NH3 plasma for 3 and 6 min were all around 51±4 mV/pH. However, the pH sensitivities of the samples treated with NH3 plasma for 9 min were slightly reduce to 45±5 mV/pH. In addition, the urea sensitivities of ITO/PET EGFET can be increased from 20.7 to 43.3 mV/pCurea by NH3 plasma from 3 to 9 min.