Characterization of K and Na -Sensitive Membrane ...

Journal of The Electrochemical Society, 158 (4) J91-J95 (2011)

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C The Electrochemical Society 0013-4651/2011/158(4)/J91/5/$28.00 V

Characterization of K1 and Na1-Sensitive Membrane Fabricated by CF4 Plasma Treatment on Hafnium Oxide Thin Films on ISFET Tseng-Fu Lu,a Chia-Ming Yang,c Jer-Chyi Wang,a,* Kuan-I Ho,a Chi-Hung Chin,d Dorota G. Pijanowska,e Bohdan Jaroszewicz,f and Chao-Sung Laia,b,z a

Department of Electronic Engineering, Chang Gung University, Kwei-Shan, Tao-Yuan, Taiwan Biosensor Group, Biomedical Engineering Center, Chang Gung University, Tao-Yuan, Taiwan c Inotera Memories, Inc., Tao-Yuan, Taiwan d Graduate Institute of Electro-Optical Engineering, Chang Gung University, Tao-Yuan, Taiwan e Institute of Biocybernetics and Biomedical Engineering, Polish Academy of Sciences, Warsaw, Poland f Institute of Electron Technology, Warsaw, Poland b

In this paper, we report the effects of fluorine incorporation on the alkali ion sensing properties of a HfO2 dielectric material. The fluorinated-HfO2 film prepared by the postdeposition CF4 plasma treatment was used as a sensing layer of ion sensitive field-effect transistors (ISFET) for Kþ and Naþ ion detection. The fluorinated-HfO2 gate ISFET is sensitive to Kþ (25.86 mV/M) and Naþ (33.77 mV/M) ions for both in the concentration range between 3 and 135 mM. In comparison with the same structure without plasma treatment, the sensitivity was improved fivefold, and lower selectivity coefficients of Kþ and Naþ ions against Hþ ion were obtained. In the stability test, the result of time-dependent drift measurements showed that this low power plasma process could not degrade the sensing performance, and lifetime evaluated based on analysis sensitivity change in time was over 15 months. The chemical states of the HfO2 film after CF4 plasma treatment were examined by x-ray photoelectron spectroscopy. Analysis of the spectra of F 1s, O 1s, and Hf 4f showed that the fluorine atoms are incorporated into the HfO2 film. This fluorinated-HfO2 film fabricated by inorganic CF4 plasma is compatible with advanced complementary metal oxide semiconductor technology and possible biosensor applications. C 2011 The Electrochemical Society. V [DOI: 10.1149/1.3543922] All rights reserved. Manuscript submitted August 12, 2010; revised manuscript received December 28, 2010. Published February 11, 2011.

The concentration of biologically relevant ions is a very important indicator associated with evaluation of patient’s condition. The detection of potassium ion (Kþ) and sodium ion (Naþ) concentrations is of interest to biomedical research. Changes in the Kþ ion concentration in human serum increases the risk of acute cardiac arrhythmia, and changes in the Naþ ion concentration in human blood increases the risk of kidney failure.1,2 Besides the aforementioned applications in the biomedical field, the measurement of the Kþ and Naþ ions concentrations is also important for food and wine quality testing and water pollution monitoring in the industrial and environmental fields.3–5 Ion sensitive field-effect transistors (ISFETs) have attracted interest recently because of their great potential as label-free, realtime, and in vivo sensors.6–9 The concept of ISFETs is derived from metal-insulator-semiconductor field-effect transistors (MISFET) with the metal gate replaced by a reference electrode, an electrolyte, and an ion sensitive membrane. The potential at the interface electrolyte/sensing membrane dependent on concentration of certain ions in the electrolyte causes changes in the channel conductance and the threshold voltage. Compared with conventional ion selective electrodes (ISEs), ISFETs exhibit a number of advantages including small size, fast pH response time, low cost, and rugged solid-state construction as well as low output impedance. To develop specific ion sensors based on ISFETs, much attention has been paid to the research and development of new sensing materials with a good sensitivity and selectivity. In recent years, a single hafnium oxide (HfO2) layer with a high pH sensitivity, low drift, low body effect, and compatible processing with complementary metal oxide semiconductor (CMOS) technology was proposed as a promising sensing material for pH detection.10,11 However, for multiple ion sensing applications in the biomedical field, the unmodified HfO2 sensing membrane exhibits a low capability due to a low sensitivity and selectivity for the detection of Kþ and Naþ ions. In this paper, a new method of Kþ and Naþ sensing membrane fabrication using a low-power carbon tetrafluoride (CF4) plasma treatment based on a fluorinated-HfO2 gate ISFET is presented. An increase in the pK- and pNa-sensitivity and a decrease in the selectivity coefficient over the Hþ ions can be obtained. For stability * Electrochemical Society Student Member. z E-mail: [email protected]

testing, drift, showing a short-term stability and lifetime expressed as sensitivity change over a long period of time, was investigated. To analyze the influence of the plasma treatment and a chemical composition and bonding structure of samples, x-ray photoelectron spectroscopy (XPS) was performed. Experimental ISFET fabrication.— To investigate the sensing properties of Kþ and Naþ, single HfO2 gate ISFETs with and without CF4 plasma treatment were fabricated. To reduce crosstalk between chips in a sensor array, p-well isolation for the active area was first formed on the n-type h100i-oriented silicon wafer of 0.01–0.001 X cm. Extended source and drain areas achieved a sufficient separation between the pads, and the sensing gate areas were designed for easy encapsulation However, one of the drawbacks is the high series resistance. Thus, a 15-nm amorphous HfO2 film was deposited by reactive radio frequency (rf) sputtering from a pure Hf target (99.99%) in the O2/Ar gas mixture after a standard RCA cleaning. The gas flow of O2 and Ar were set to 5 and 20 sccm, respectively, and the pressure during sputtering was kept at 20 mTorr. The rf power was 150 W, and the deposition rate was around 1 nm/min. After deposition, some wafers were subjected to a post-CF4 plasma treatment in a plasma-enhanced chemical vapor deposition (PECVD) system with an rf power of 30 W and a processing pressure of 500 mTorr for 5 min. Next, the rapid thermal annealing (RTA) was performed in N2 at 700 C for 1 min.12,13 Then, the front side aluminum contact holes were opened on the extended source and drain areas, and the thermally evaporated aluminum was deposited to form contact pads. To increase the density of the aluminum layer for high quality contacts, the sintering process was performed in an N2 atmosphere at 450 C for 10 min. The schematic diagram of the cross-section view of the fluorinated-HfO2 gate ISFET structure is shown in Fig. 1. The dimensions of the ISFET chip are 4.9 4.9 mm2, and the gate dimensions are 16 lm in length and 600 lm in width. Then, wire bonding was used as a connection between the aluminum pads of the ISFET and the Cu line of the printed circuit board (PCB). Finally, a handmade epoxy Sylgard 184 (Sil-more) was used to encapsulate the ISFET samples and Cu line of the print circuit board.

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Figure 1. (Color online) Schematic diagram of the fluorinated-HfO2 gate ISFET structure with CF4 postdeposition treatment in a PECVD system.

Test solutions.— To investigate the Kþ and Naþ sensing characteristics, a 5-mM Tris/HCl buffer solution of pH 8.0 was prepared. The concentrations of Kþ and Naþ in a range between 105 and 101 M were changed by a standard addition method, where 0.1 M NaCl/Tris-HCl (or 0.1 M KCl/Tris-HCl) and 1 M NaCl/Tris-HCl (or 1 M KCl/Tris-HCl) were used as standards.

increase. The linear pK-sensitivity of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET extracted for the concentration range between 3 and 135 mM were 5.24 and 28.57 mV/M, respectively. The linearity of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET were 99.3 and 99.9%, respectively. It indicated that the fluorine incorporation of the fluorinated-HfO2 gate ISFET by the postCF4 plasma surface treatment presented a larger pK-sensitivity than the ISFET without plasma treatment. In Fig. 3b, the responses of the Naþ ions at a concentration higher than 104 M show the same trend as the responses of Kþ ion detection in the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET. The linear pNa-sensitivity of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET extracted from the concentration range between 3 and 135 mM were 8.32 and 38.35 mV/M, respectively. The linearity of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET were 96.9 and 99.9%, respectively. The difference between the pK-sensitivity and pNa-sensitivity is not high enough. In our recent research on fluorinated-HfO2 EIS-electrolyte-

Electrical measurements.— To determine the sensing properties, a constant voltage–constant current (CVCC) circuit was used to collect the sensing voltages from the source terminal of the ISFET in the electrolyte with various concentrations.14 VDS ¼ 0.5 V and IDS ¼ 100 lA were set in the CVCC circuit to obtain the output voltage. A commercial combined pH Ag/AgCl electrode S120C (Sensorex) was used to control pH and its half cell as a reference electrode. To avoid the deviation caused by the pH variation, the apparent output signal was corrected for pH changes measured by commercial pH-electrode during the experiment. To obtain stable pNa and pK responses, all ISFETs were immersed in the 5-mM Tris/HCl for 12 h before measurement.15 In the long-term stability test, the ISFET response was monitored by the CVCC circuit every minute in 0.01 M KCl and 0.01 M NaCl solutions for 12 h. The drift coefficient was calculated by the linear fitting of the output signal in time range from 5 to 12 h.16 To prevent light interference, the measurements were carried out in a dark box at room temperature. XPS.— The surface properties of the HfO2 films with or without the plasma treatment were characterized by XPS (XPS, V. G. Scientific, Co., Microlab-350) using a standard Mg Ka (1253.6 eV) x-ray source. XPS spectra were collected with a pass energy of 30 eV. Results and Discussions The responses of the Kþ and Naþ ions were investigated at different concentrations of KCl and NaCl solutions of pH around 8.0. The pH-sensitivity of the HfO2 ISFET and the fluorinated-HfO2 ISFET are 52.92 mV/pH with linearity of 99% and 59.02 mV/pH with linearity of 99%, respectively. Figures 2a and 3a show the realtime Kþ and Naþ detection of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET by post-CF4 plasma surface treatment in KCl and NaCl solutions, respectively. The output signals were collected from the source terminal of the ISFET in the CVCC circuit every minute and monitored continuously for 10 min during each titration step. The concentration of Kþ and Naþ ions for each addition step were as follows: 1.67 105 M, 3.33 105 M, 9.99 105 M, 3.32 104 M, 9.9 104 M, 3.23 103 M, 9.1 103 M, 3.25 102 M, 8.66 102, and 1.35 101 M. A positive shift of the output signal implies the sensitivity of the ISFETs to the examined ions. The insets in Figs. 2a and 3a show time response of the fluorinated-HfO2 gate ISFET to Kþ and Naþ ions steps from 3.32 104 M to 9.1 103 M. Within a short response time (1 min), about 90% of the total response was achieved. Figure 2b shows the entire calibration curve corresponding to the dynamic response. In Figure 2b, the response of Kþ ions in the HfO2 gate ISFET change only slightly with a concentration higher than 104 M. However, with the post-CF4 plasma treatment, the response of the Kþ ions in the fluorinated-HfO2 gate ISFET shows an obvious

Figure 2. (Color online) (a) Time responses of Kþ ions in addition sequence. (b) pK response of the single-layer HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET. Inset shows the response time of the fluorinatedHfO2 gate ISFET for various Kþ ion concentrations corresponding to addition steps from IV to VIII.



V ¼Cþ

RT pot ln ax þ KX;H aH F

aX pot log KX;H ¼ log aH

Figure 3. (Color online) (a) Responses of Naþ ions in the titration sequence. (b) Sensing voltage to pNa response of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET. The inset figure shows the response time of the fluorinated-HfO2 gate ISFET for various Naþ ion concentrations corresponding to addition steps from IV to VIII.

insulator-semiconductor devices (thermal CF4 plasma at 300 C), the pNa-sensitivity increased quickly with the plasma time within 1 min but the pK-sensitivity increased slowly with the plasma time. Therefore, the optimum difference (21 mV/M) between pK- and pNa-sensitivity could be achieved at the plasma time range between 20 and 40 s (the data not shown here). However, the difference is still not good enough for distinguishing the influence between Kþ and Naþ. Hence, the further research will concern improvement of the selectivity. For practical in vivo analysis of the Kþ and the Naþ ion detection, Hþ ions are the most important interfering ions present in real human body fluids. Therefore, the selectivity over interfering Hþ ions is a significant parameter for sensors. The Kþ ion selectivity or Naþ ion selectivity over interfering Hþ ions can be quantified by means of the selectivity coefficient in the Nicolsky–Eisenman equation, as shown in the following equations:17

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[1]

[2]

where C is a constant value, aX is the activity of the Kþ ion or Naþ pot ion (X ¼ K or Na), aH is the activity of the interfering Hþ ion, KX;H is the selectivity coefficient, R is the gas constant, T is the temperature in Kelvin, and F is the Faraday constant. Selectivity coefficients were evaluated by the fixed interference ion method (FIM) described in Refs. 18 and 19. The primary ions are Kþ and Naþ ions, and the interfering ions are Hþ ions. The activity of the Hþ ions is kept at 108 M. The activity of the Kþ and Naþ ions extracted from the intersection of the extrapolated linear of calibration curve obtained for FIM measurement are 103.42 M and 103.01 M for the HfO2 film without plasma treatment and 103.56 M and 103.66 M for the HfO2 film with plasma treatment, pot respectively. The logarithmic Kþ selectivity coefficient ½log KK;H pot and Naþ selectivity coefficient ½log KNa;H of the single fluorinated-HfO2 gate ISFET are 4.44 and 4.34, respectively, which are lower than the single HfO2 gate ISFET at 4.58 and 4.99. Based on the evaluated ion selective coefficients, it can be stated that the single fluorined-HfO2 gate ISFET with the CF4 plasma treatment was more selective to the Kþ or Naþ ions than to Hþ ions. To check the changes of the surface composition and chemical state of the HfO2 film after the CF4 plasma treatment, XPS spectrum analysis was performed. The F 1s, O 1s, and Hf 4f XPS spectra of the HfO2 film with and without CF4 plasma treatment, which were calibrated from C 1s peak at 284.5 eV, are shown in Fig. 4. The background was subtracted using a tougaard-type shape, and the spectra were fitted with Lorentzian–Gaussian functions. For the F 1s spectra shown in Figs. 4a and 4b, a peak with a high intensity corresponding to the F–Hf bond is observed in Fig. 4a due to the higher F concentration of fluorinated-HfO2 film after the CF4 plasma treatment.20,21 However, in Fig. 4b, a peak with a small intensity is also observed that corresponds to the untreated HfO2 film. It is possibly due to the residual F atoms at the interface between Si and the HfO2 layer coming from the HF dip process before HfO2 film deposition. The Hf 4f spectra of the HfO2 thin films with and without the CF4 plasma treatment are shown in Figs. 4c and 4d, respectively. In Fig. 4d, the binding energies of Hf 4f7/2 and Hf 4f5/2 of the control sample are located at 15.7 and 17.35 eV, respectively. Compared to the HfO2 film with post-CF4 plasma treatment shown in Fig. 4c, higher binding energies of Hf 4f7/2 and Hf 4f5/2, which are shifted positively by 0.25 eV compared to the control sample, were obtained. In numerous publications, the HfO2 film with fluorine incorporation as a gate dielectric demonstrated superior electrical reliability and performance improvement.22–26 The possibly reasons could be the oxygen vacancy passivation and defect passivation by fluorine.27,28 Therefore, the oxygen vacancy and defect of fluorinated-HfO2 film should be less than the HfO2 film. In our plasma treatment, the plasma power - 30 W - is low compared with the plasma power in other publications. The low power plasma should not damage the surface of HfO2 film during the plasma treatment. On the basis of the abovementioned descriptions, we believe that the F–Hf bonding formation could be the reason for the binding energy enhancement of the Hf 4f7/2 and Hf 4f5/2 of the fluorinated-HfO2 film. To confirm the fluorine incorporation, the O 1s spectra with deconvolution were measured as illustrated in Fig. 4e and 4f. In Fig. 4f, two separate peaks can be found at the binding energies of 529.61 and 531.34 eV for the untreated-HfO2 film. The lower one is the chemical bond of HfO2, and the larger one is the contamination. In Fig. 4e, to fit the O 1s spectra well after plasma treatment, a third peak attributed to the HfOxFy bond is applied. The binding energies of these three peaks are at 529.86, 531.43, and 532.93 eV. The peak


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Journal of The Electrochemical Society, 158 (4) J91-J95 (2011) 532.93 eV is considered to be the peak of HfOxFy due to the existence of the F–Hf bonds in the fluorinated-HfO2 film after the plasma treatment.29 The similar formations of Si(OF)x and Al(OF)x after the plasma treatment were observed in other published studies.30,31 From the results of XPS analysis, fluorine incorporation in the HfO2 film after CF4 plasma treatment is evidenced. Based on the XPS analysis, we believe that the electronegativity difference between F and Hf (same as F and O) possibly cause formation of F–Hf and F–O bonds that exhibit polarization properties. The sodium and potassium ions could be absorbed, thanks to the polarized bonds. However, these bonds may also be active for other ions present in an analyzed solution. For practical sensor applications, it is necessary to investigate the stability of the sensing properties, including drift in a short-term time measured at fixed conditions (concentration and temperature)

Figure 4. F 1s XPS spectra of (a) the fluorinated-HfO2 sensing film and (b) the HfO2 sensing film. The intensity of F 1s peak of the fluorinated-HfO2 sensing film increased substantially after the CF4 surface plasma treatment. Hf 4f XPS spectra of (c) the fluorinated-HfO2 sensing film and (d) the HfO2 sensing film. The higher binding energy of HfO2 double peaks were obtained for the fluorinated-HfO2 sensing film. O 1s XPS spectra of (e) the fluorinated-HfO2 sensing film and (f) the HfO2 sensing film. The HfOxFy peak was formed in the sample with plasma treatment.

with binding energy that is positively shifted by 0.25 eV from the 529.61 eV of the untreated-HfO2 film is attributed to the peak of HfO2. The second peak with the binding energy of 531.43 eV, which is near the binding energy of 531.34 eV of the untreatedHfO2 film, is the contamination peak. The highest binding energy of

Figure 5. Long-term drifts of the HfO2 gate ISFET and the fluorinatedHfO2 gate ISFET in 10 mM KCl=tris buffer solution. The inset shows the extracted drift coefficients of both ISFETs by linear fitting in the range from 5 to 12 h.



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higher pK sensitivity and pNa sensitivity, a wider sensing concentration range for Kþ and Naþ ions for linear response, and a lower selectivity coefficient for Hþ ions than the HfO2 gate ISFET without the CF4 plasma treatment. The reason behind the improvement could be the incorporation of fluorine ions by the CF4 plasma treatment, which was proved with the evidence of XPS data. For stability testing, the results show that this low power plasma process causes no damage on the membrane, and the sensitivity is still stable over 15 months. The proposed fluorinated-HfO2 treated by inorganic CF4 plasma treatment, which is compatible with advanced CMOS technology, could be a candidate for future biosensor applications. However, drift coefficient higher than the expected was possibly due to imperfect handmade encapsulation. Therefore, more investigation on stability testing of the fluorinated-HfO2 membrane should be done. Acknowledgment This work is supported by the National Science Council of the Republic of China under the contract number NSC 97-2221-E-182054. Chang Gung University assisted in meeting the publication costs of this article.

Figure 6. Long-term pNa-sensitivity change of the fluorinated-HfO2 gate ISFET.

as well as long-term measurements for estimation of a lifetime— evaluated here as a change of sensitivity in time. The continuous monitoring of the HfO2 gate ISFET and the fluorinated-HfO2 gate ISFET at the 10 mM KCl solution for 12 h is shown in Fig. 5a. The output signal change during the measurement, DVS, was obtained according to DVS ¼ VT V0

[3]

where VT is threshold voltage dependent on a certain ions concentration and V0 is the t at the initial threshold voltage, i.e., output signal at the beginning of the drift measurement. The drift coefficient was calculated by linear fitting of DVS per time unit in the time span from 5 to 12 h as illustrated in the inset of Fig. 5a. The extracted drift coefficients are shown in Fig. 5b. For sensors measured in 10 mM KCl and NaCl solutions, the drift coefficients for the fluorinated-HfO2 gate ISFET in the region between 5 and 12 h are close to those for the HfO2 gate ISFET. It means that the surface damage created in the plasma treatment can be ignored due to the comparable drift effect. In addition, the larger drift coefficient compared with a commercial ISFET could also be caused by the unstable encapsulation of the ISFET device. The lifetime was evaluated based on sensitivity change for Naþ ion (Fig. 6a). The relative sensitivity is obtained by normalizing sensitivity with the initial sensitivity on the first day. The stable and repeatable responses of the proposed sensor could be used for detection Naþ ions for at least 15 months. The maximum deviation of the pNa-sensitivity was less than 15% during the testing time. To extend lifetime, the plasma treatment method with a low plasma power could be applied for Kþ ion and Naþ ion sensor fabrication. Based on the aforementioned results, this proposed CF4 plasma treatment method is practical to fabricate the Kþ and Naþ ion sensitive layers. Conclusions In this study, a novel method with CF4 plasma treatment by PECVD to improve the sensitivity of Kþ ion and Naþ ion detection on a single-layer HfO2 gate ISFET was proposed. With the CF4 plasma treatment, the fluorinated-HfO2 gate ISFET exhibited a

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