The Multi-Channel Measurements for pH-Sensitivity and Drift Coefficient of Thin Hafnium Oxide with CF 4 Plasma Treatment Chao-Sung Lai, Ti-Chuan Wang, Chia-Ming Yang and Tseng-Fu Lu Department of Electronic Engineering, Chang Gung University 259 Wen-Hwa 1st Road, Tao-Yuan, Taiwan Phone: +886-3-2118800 ext. 5786 E-mail:cslai@mail.cgu.edu.tw 1. Introduction Ion sensitive field effect-transistor (ISFET) was in- vented by P. Bergveld in 1970. [1] Compare with conven- tional ion-selective electrode, ISFET has many advantages, such as small size, fast response, low output impedance, small sample requirement, high durability and low cost. [2] The first pH-sensitive membrane for ISFET application was silicon dioxide (SiO 2 ) layer. [3] Then many kinds of insu- lators such as Si 3 N 4 [4], Al 2 O 3 [5] and Ta 2 O 5 [6] were veri- fied as the superior sensitive membrane for ISFET com- mercial products. Recently, hafnium oxide (HfO 2 ) directly deposited on silicon substrate without buffering SiO 2 layer was studied as a new pH-sensitive material, which had high sensitivity and low drift coefficient. [7] However, the ap- plicable pH range of HfO 2 layer was decreased to from pH 2-12 to pH 4-12 when the thickness of HfO 2 layer was de- creased from 30 nm to 8 nm. In addition, the serious drift effect will degrade the measurement accuracy of ISFET. To improve the pH-sensing performance of 8 nm thick HfO 2 layer, a CF 4 plasma treatment was performed. To effi- ciently check the drift coefficient of all samples, a program based on LabVIEW was developed for multi-channel automatically long term drift measurement. 2. Experimental 2.1 Processes flow of HfO 2 -EIS structure To investigate pH-sensitive properties of HfO 2 layer, the EIS structures were fabricated. After standard initial RCA cleaning, the HfO 2 thin film was deposited on silicon substrate by reactive radio frequency (RF) sputtering. The hafnium target with purity of 99.9% was used. The RF power was controlled at 150 watt. The flow rate ratio of O 2 /Ar gas mixture was 5/20 sccm. Afterwards, the CF 4 plasma treatments on HfO 2 surface were performed with the RF power at 30 watt and the process pressure of 0.5 torr. The process time period for plasma treatment was con- trolled at 1, 5, and 10 min. 2.2 Multi-channel automatically controlling program The connection of measurement system including E5250A, HP4284A and LabVIEW is shown in Fig. 1. To perform series capacitance-voltage (C-V) measurements for three samples, a program based on LabVIEW was used to control HPE5250 low leakage switch mainframe and HP4284A high precision LCR meter. The control panel of this program is presented in Fig. 2. To obtain the long term response of EIS structures from C-V curves, the measure- ment parameters such as signal frequency, sweep voltage, time period and channel number can be adjusted. 2.3 pH-sensing measurements For the extraction of pH sensitivity, C-V curves were measured in standard buffer solutions from pH 2 to 12. Sensitivity and linearity can be calculated with the corre- sponding pH of buffer solution by this program after fin- ished all measurements. In the drift measurement, response voltages were extracted from the measured C-V curves every five minutes in pH 7 buffer solution. The drift coeffi- cient was calculated by linearly fitting with the response voltages (V REF ) in the range of 5 to 12 hours immersion. 3. Results and discussions Figure 3 shows the C-V curves of three HfO 2 -EIS structures collected by this controlling program. For the extraction of pH sensitivity, C-V curves were measured in various buffer solutions as shown in Fig. 4 (a). V REF for all samples were automatically calculated with 0.6 C max . [8] Then pH sensitivity and linearity can be obtained by the linear fitting of V REF and the corresponding pH of buffer solution as shown in Fig. 4 (b). The sensitivity distribution of 8 nm thick HfO 2 -EIS structure with CF 4 plasma for various time periods was shown in Fig. 5. Sensitivity can be increased with CF 4 plasma time. A typical time dependent distribution of response volt- ages in pH 7 buffer solution for 8 nm thick HfO 2 -EIS struc- ture with CF 4 plasma surface treatment for 10 min was shown in the Fig. 6. The distribution of drift coefficient for HfO 2 -EIS structures with CF 4 plasma treatment for various time periods was shown in Fig. 7. The drift coefficient in- creased with CF 4 plasma time. The comparison of all per- formances of 8 nm thick HfO 2 -EIS structure with various CF 4 plasma time was shown in Table I. 4. Conclusion A high efficient automatically multi-channel measure- ment system was built for the long term stability monitor- ing of EIS structures. For high sensitivity and low drift co- efficient, an optimized process time of CF 4 plasma treat- ment is 5 min. References [1] P. Bergveld, IEEE Trans. Biomed. Eng. 17 (1970) 70. [2] T. Mastsuo et al., Sens. Actuator B, 1 (1981) 77-96. [3] P. Bergveld, IEEE Trans. Biomed. Eng. 19 (1972) 342. [4] D.L. Harame et al., IEEE TED-34 (1987) 1700. [5] L. Bousse, et al., Sens. Actuator B, 2 (1990) 103 [6] A.S. Poghossian, Sens. Actuator B, 7 (1992) 367 [7] C.S. Lai, et al Jpn. J. Appl. Phy. 45, (2006), 3807–3810 [8] M.J. Schoning, et al., Sens. Actuators B, 35, 228, 1996.