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Sensing properties of palladium-gate MOS (Pd-MOS) hydrogen sensor-based on plasma grown silicon dioxide
Sensors and Actuators B 71 (2000) 161±168
Sensing properties of palladium-gate MOS (Pd-MOS) hydrogen sensor-based on plasma grown silicon dioxide D. Dwivedi, R. Dwivedi, S.K. Srivastava* Department of Electronics Engineering, Centre for Research in Microelectronics, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India Received 18 March 1998; accepted 15 December 1998
Abstract This paper deals with the performance of a palladium-gate MOS hydrogen sensor studied by conductance method. Structure of the device was fabricated on a n-type h100i silicon wafer having resistivity of 1±6 O cm using plasma technology. Sensitivity and response±recovery time of the fabricated sensor have been studied for different concentration (1480±11 840 ppm) of hydrogen with varying signal frequency (500 Hz, 10 and 100 kHz) at room temperature. Hydrogen-induced interface-trapped density (Nit) has been also evaluated as a function of gas concentration using a bias scan conductance method. Obtained results show that device performance is improved (i.e., high sensitivity and low response recovery time) and further it has been concluded that implementation of plasma technology (i.e., dry plasma cleaning of Si surface and in-situ RF anodization of Silicon in oxygen plasma near room temperature) may be a future step towards development of MOS-based sensors and integrated arrays with improved performance at room temperature. # 2000 Elsevier Science B.V. All rights reserved. Keywords: MOS gas sensor; Plasma technology
1. Introduction The SiO2-based palladium-gate MOS hydrogen sensors have widely been studied by numerous researchers [1±3]. In such MOS capacitors, silicon dioxide has been used as insulator with thickness varying from 3 to 100 nm. These devices exhibit change in capacitance and conductance, when exposed to hydrogen. Shivaraman et al. [1] and Ruths et al. [2] have reported these changes in the characteristics of MOS devices due to decrease in the work function of palladium upon exposure to H2. More detailed explanation has been given by Lundstrom [3]. It has been observed that when MOS sensor is subjected to hydrogen gas, hydrogen molecules are ®rst dissociated on the palladium metal surface because of its catalytic behaviour. Subsequently, some of the hydrogen atoms diffuse through the palladium ®lm and are adsorbed at the metal insulator (SiO2) interface. At the SiO2 interface, these atoms polarise and give rise to a dipole layer which in turn changes the work function of the palladium metal. Change in metal work function results shift in ¯at-band voltage of MOS capacitor which is a function of adsorbed gas concentration. *
Corresponding author. Telefax: +91-542-316925.
Formoso and Maclay [4] have reported that in the presence of hydrogen, the interface state densities of ultra thin palladium-gate MOS sensor increases with increasing concentration of test gas. It has been found that hydrogen atoms diffuse through the thin SiO2 (12.5 nm) Si/SiO2 interface and alter the interface state density. Armgrath [5] has reported the hydrogen-induced oxide surface charging in Pd-gate MOS devices, which also causes shift in ¯at-band voltage of MOS capacitor. Kobayashi et al. [6] have reported the voltage shift of the I±V curve for the Pt/SiO2/Si MIS tunnelling diodes following on exposure to H2, which is attributed to: (i) the decrease in the effective Pt work function, (ii) the internal field-dependent movement of hydrogen ions in the SiO2 layer, (iii) the formation of interface states. They have also studied [7,8] the mechanism of the formation of the hydrogen-induced interface states at the Si/ SiO2 interface investigated by conductance measurement. It is often reported [4±8] that interface states seriously affect the electrical characteristics of MOS devices. The above studies made on MOS sensors are based on thermally grown SiO2 layer of desired thickness. However, the requirement of SiO2 (3±100 nm) for MOS gas sensors is very critical and this possess several constraints to
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Nomenclature A Co Cs and Rs Cin Gin Cit Nit Ha Hai o t c1 and d1 GPi DGP C DC
Gate area [cm2] Oxide capacitance [pF] Capacitance and resistance associated with interface traps Input capacitance [pF] Input conductance [mS] Interface trap capacitance [pF] Interface trap density (also known as interface state density) [eV cm2]ÿ1 Hydrogen atom Hydrogen atom at interface Varying signal frequency Interface trap capture time Rate constant Initial conductance peak position [V] Shift in conductance peak position [V] in H2 Initial concentration of H2 gas Change in H2 gas concentration
conventional (thermal) technology for growth of SiO2. The normal temperature for thermal growth of SiO2 lies between 900 and 12008C, and already, the disadvantages of such high temperature processing in submicron technology have been reported [9±16]. Recently Chanana et al. [12,13] have demonstrated the growth of ultra thin (6.3 nm) SiO2 using RF oxygen plasma near room temperature with in-situ dry cleaning of Si surface. These authors found the electrical properties of such silicon dioxide favourable for the development of MOSbased gas sensors. However, to the best of the authors' knowledge, the use of in-situ RF oxygen plasma technology, which is capable to grow silicon dioxide near room temperature on dry plasma precleaned silicon surface, is yet to be implemented for the development of MOS-based gas/ odour sensors. Keeping in view the need for low temperature processing in the present study, 6.9-nm-thick SiO2 layer has been grown by RF anodization of dry plasma precleaned silicon surface, in oxygen plasma near room temperature with in situ. Fabricated MOS sensor has been tested for varying concentration of hydrogen (1480±11 840 ppm), also the sensitivity and response recovery time have been calculated in terms of shift in conductance peak position upon exposure to hydrogen. Obtained results were compared to that reported earlier [4,7,21,24,25], and it has been found that the device performance is improved (i.e., high sensitivity and low response recovery time). 2. Experimental Device was fabricated on a n-type h100i Si wafer having resistivity of 1±6 O cm. A cross-sectional view of
the fabricated device is shown in Fig. 1a, while the basic equivalent circuit and equivalent circuit including interface trap effect are shown in Fig. 1b (i), (ii) and (iii), respectively. Steps involved in the fabrication of the device are described below. Dry plasma cleaning: Step 1. Keep the wafer in trichloroethylene (TCE) accompanied by ultrasonic agitation for 5 min. Step 2. Immerse the silicon wafer in acetone for 4 min. Step 3. Rinse the wafer with deionized wafer for 4 min and dry it. Step 4. Keep the wafer in 50% CF4 H2 plasma for 4 min in a barrel etcher at 80 W RF power. The formation of HF is by a reaction of H2 with atomic F formed due to the dissociation of CF4 in the RF plasma. The reaction of atomic F and HF formation is given as: CF4 e ! CF 3 F e; 2F H2 ! 2HF: Etching of the native oxide is given by: 4HF SiO2 ! SiF4 2H2 O: The reaction product SiF4 is known to be volatile and is pumped out of the chamber. The atomic F at 50% H2 level reacts ®rst with H2 to form HF. This fact is corroborated by Chapman [16] which showed that the atomic F intensity drops to zero at 25% H2 addition due to the formation of HF in barrel etcher. This is also the reason why silicon is unable to react with atomic F and thus silicon etching is negligible in our 50% CF4 H2 plasma. Thus, the forth step removes the native oxide which contains contaminants from the ambient. The HF also reduces the heavy metal contaminants. 2.1. In-situ dry plasma oxidation (RF plasma anodization of Si surface near room temperature) After step 4 of the above cleaning method, thin SiO2 (6.9 nm) was grown in situ. The chamber is evacuated to 133±26610ÿ1 Pa, pressure. Dry oxygen ¯owed and after obtaining a dynamic chamber pressure of 133 Pa, 70 W RF power is supplied in CW mode to form dry O2 plasma. The wafer was expose to this plasma for 50 min to give 6.9 nm oxide (SiO2). The oxide thickness was measured with the help of manual ellipsometer. Fig. 2 illustrates a pictorial representation of oxygen plasma anodic oxidation mechanism for silicon [23]. Here, at the top of the illustration, ions and electrons are accelerated across the sheath region to become implanted in to the oxide. Since the sheath potential can be low, in the order of tens of volts, the implantation depth will be very slight, probably about 1 nm deep. This implantation will result in an abnormally large excess of oxygen just below the surface of the oxygen ®lm. The implanted oxygen is then expected to react with electrons to form the naturally occurring oxide ion O= in interstitial
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163
Fig. 1. (a) Structure of the fabricated device; (b) cross-section of a practical MOS capacitor, a simple equivalent circuit (i), including trap effect (ii) and (iii) [19,20].
sites. This ion then diffuses and drifts in an electric ®eld to the silicon, where it rapidly reacts to form a silicon±oxygen bond as shown in Fig. 2 [23]. For gate structure, palladium was deposited by evaporating it from a tungsten ®lament in vacuum of 13310ÿ6 Pa. Ohmic contact to the back side of silicon substrate was made by thermal evaporation of aluminium metal. Post-metallization anneal (PMA) was performed in N2 ambient at 4008C for 15 min. In the basic equivalent circuit of the device
(shown in Fig. 1b (i), (ii) and (iii)), Ci is the insulator capacitance and CD is the semiconductor depletion layer capacitance. CS and RS are the capacitance and resistance associated with the interface traps, and are functions of surface potential. The product CSRS is de®ned as the interface lifetime, which determines the frequency behaviour of the interface traps. The parallel branch of the equivalent circuit in Fig. 1b (i) can be converted into a frequency-dependent capacitance CP in parallel with a
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Fig. 2. A pictorial representation of the RF plasma anodization mechanism for silicon [23].
frequency-dependent conductance GP , as shown in Fig. 1b (iii). Conductance response of the fabricated MOS sensor was studied and data (magnitude of conductance peak value, conductance peak position [V] and time) were recorded with the help of Impedance/Gain Phase analyser (HP-4194A). HP-4194A model has inbuilt HPIB interfaceable capacity. This feature of interfaceability was exploited to control the HP-4194A remotely and gathered all the necessary information required for further processing. The analyser was interfaced to a PC via a GPIB-card, of IEEE-488 standard. An interfacing program has been developed to obtain accurate information from the HP-4194A and stored in the computer. The speed of data transfer is limited by the
capability of the HP-4194A and the type of processing to be perform. On-line printing was made possible for a hardcopy of the obtained data. In order to control the hydrogen gas ¯ow inside the test chamber, a rotameter (MARCH Instruments, US-made GCM-100) was calibrated and used for test gas. 3. Result Fig. 3 shows the shift in conductance peak position upon exposure to hydrogen (1480±11 840 ppm) at frequencies 500 Hz, 10 and 100 kHz. It is evident from this ®gure that this shift (magnitude) increases with gas concentration and
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Fig. 5. Variation of sensitivity with hydrogen gas concentration [ppm].
Fig. 3. Shift in conductance peak position with hydrogen gas concentration [ppm].
becomes almost constant at higher gas concentration (11 840 ppm here), also shift is high (100±900 mV) at lower frequency (500 Hz) in comparison to that (100±400 mV) at higher frequency (100 kHz). Fig. 4 shows the effect of frequency on total shift (for 1480±11840 ppm H2) in conductance peak position which is de®ned as: GPi ÿ GPgas 100; (1) Total Shift % GPi where GPiconductance peak position [V] in the absence of hydrogen (i.e., in air ambient), taken as reference;
Fig. 4. Effect of frequency on shift [%] in conductance peak position.
GPgasconductance peak position [V] for final concentration of hydrogen, which was introduced inside the test chamber in concentration varying from 1480 to 11 840 ppm. This shift in conductance peak position decreases with increase in frequency as it is 75, 62 and 50% for frequencies 500 Hz, 10 and 100 kHz, respectively. The variational behaviour of sensitivity of the fabricate sensor for hydrogen is shown in Fig. 5. Here sensitivity S is de®ned as: DGP G (2) S Pi ; DC C where GPiinitial conductance peak position [V]; DGPshift in conductance peak position [V] in H2; Cinitial concentration of H2 gas; DCchange in H2 gas concentration. From this ®gure, it is evident that the sensitivity increases with increase in gas concentration and this increasing nature is better at lower frequency (500 Hz here). The interface trap density has been evaluated using conductance method [18] and the obtained results are shown here in Fig. 6. From these ®gures, it is evident that Nit, the interface-trapped density, increases with gas concentration. Response and recovery time for the fabricated MOS sensor has been studied for different concentrations (1480±11 840 ppm) of hydrogen at frequencies 500 Hz, 10 and 100 kHz. To obtain the response time of the fabricated sensor, the shift in the conductance peak position was recorded just after the injection of gas inside the test chamber with time at an interval of 4 s (minimum possible interval with available measurement setup). It has been observed that the conductance peak position shifted towards -ive bias-voltage, and the magnitude of this shift initially increased with time, reached a maximum value and then attained stable state. Now, response time was calculated as 10±90% of this time period (i.e., from injection of gas up to the stable condition of shifting peak). Obtained results are
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4. Discussion 4.1. Sensing mechanism in MOS It is known that the application of MOS capacitors as hydrogen detector is based on the shift in ¯at-band voltage and, accordingly, lateral shift in capacitance±voltage (C±V) and conductance±voltage (G±V) response upon exposure to hydrogen [1±3,17,18]. A pictorial representation of sensing mechanism in Pd-MOS hydrogen sensor is shown in Fig. 8. Dissociation of molecular H2 on Pd-surface may be represented as [18]: c1
H2g , 2Ha :
(3)
d1
Fig. 6. Variation of interface trap density with H2 gas concentration.
shown in Fig. 7 (i) and (ii). These ®gures show that response (8 s) and recovery time (1.5 min) is small at lower frequencies (500 Hz here).
The dissociative adsorption of H2 and Pd surface and the subsequent diffusion through metal (given by Eq. (4)) form a dipole layer at Pd/SiO2 interface. This results change in flat-band voltage of device. It has been observed that change in flat-band voltage increases with gas concentration [1±8,17,18]. c1
Ha , Hai :
(4)
d1
4.2. Hydrogen-induced interface traps Formation of hydrogen-induced interface states in PdMOS hydrogen sensor may occur through the following steps: Step 1: Dissociation of the hydrogen molecules on the Pd-surface. Step 2: Adsorption of dissociated hydrogen atom on metal surface. Step 3: Diffusion of hydrogen atoms to the Pd/SiO2 interface. Step 4: Tunnelling of hydrogen through oxide layer and accumulation at Si/SiO2 interface. Step 5: Formation of interface states due to the reaction of hydrogen atoms with interface traps, as represented below: X H It ;
(5)
where Xreactant at Si/SiO2 interface; H Hydrogen concentration; Itinterface states. The hydrogen-induced interface-trapped density has been calculated by conductance method [18] which is directly related to interface traps, yields more accurate and reliable results [18±20], and is given by: G o max : (6) Nit 0:40 q Fig. 7. Effect of gas concentration and frequency on (i) response and (ii) recovery time of fabricated MOS sensor.
In present study, it has been noted that the improved performance (high sensitivity, low response and recovery
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Fig. 8. (a) Schematic illustration of the principle of SiO2-based MOS hydrogen sensor. (b) Effect of gas exposure on (i) C±V and (ii) G±V response of device.
time, etc.) of the fabricated sensor at room temperature, as compared to earlier reports [4,7,17,18], may be attributed to plasma processing used here, which gives Pd/SiO2/n-Si MOS with ultra thin (6.9 nm) SiO2 with in situ on dry plasma precleaned silicon surface. The remarkable variation in Nit at lower frequency (500 Hz here) is due to balanced communication of interface traps with valence and conduction bands of silicon substrate. The oxygen plasma environment used in the present study may also effect the device performance in the following way. As it is well known that the Oxygen plasma comprises of single and multiple species like Oÿ, O2ÿ, O3ÿ, O4ÿ, O4, O3, O2, O, etc., and eÿ [22,23], now when these species interact with each other, the number of active sites for gas adsorption inside the device also gets modulated. When the
gas (H2 here) molecules interact with these species (single or multiple), they release more number of conduction electron, i.e., an enhanced modulation of sensitivity of device. 5. Conclusion In the present investigation, it is concluded that implementation of plasma technology (i.e., dry plasma cleaning and in-situ RF plasma oxidation near room temperature) may be a future step towards the development of MOS-based sensors and integrated arrays with improve performance (i.e., high sensitivity, low response and recovery time at room temperature, etc.). Further, it has been concluded that the MOS-based sensors developed by the present method could be a potential device for detection of low concentra-
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tion of hydrogen/hydrocarbons using a low frequency conductance response at room temperature, which also (low frequency G±V method) provides more accurate information about the hydrogen-induced interface traps in such devices. Acknowledgements D. Dwivedi is very much thankful to CSIR, the government of India, for providing ®nancial support during the course of this work. References [1] M.S. Shivaraman, I. Lundstrom, C. Svensson, H. Hammarsten, Hydrogen sensitivity of palladium±thin-oxide±silicon Schottky barriers, Electron. Lett. 12 (1976) 484±485. [2] P.F. Ruths, S. Ashok, S.J. Fonash, J.M. Ruths, A study of MlS Schotkky-barrier diode detectorIEEE Trans. Electron Devices ED 28 (1981) 1003±1009. [3] I. Lundstrom, Hydrogen sensitive MOS structure: Part 1. Principles and applications, Sensor and Actuators (1981) 403±426.. [4] M.A. Formoso, G.J. Maclay, The effect of hydrogen and carbon monoxide on the interface state density in MOS gas sensors with ultra thin palladium gates, Sensors and Actuators B 2 (1990) 11±12. [5] M. Armgarth, Hydrogen induced oxide surface charging in Pd-gate MOS devices, J. Appl. Phys. 56 (1984). [6] H. Kobayashi, H. Iwadate, Y. Nakato, Role and mechanism of the formation of hydrogen-induced interface states for platinum/silicon oxide/silicon MOS tunnelling diodes, Sensor and Actuators B 24±25 (1995) 815±818. [7] H. Kobayashi, H. Iwadate, Y. Kogetsu, Y. Nakato, Mechanism of the formation of hydrogen induced interface states for Pt/silicon oxide/Si metal-oxide-semiconductor tunnelling diodes, J. Appl. Phys. 78 (1995) 6554±6561. [8] H. Kobayashi, K. Kishimoto, Y. Nakato, Reaction of hydrogen at the interface of palladium±titanium dioxide Schottky diodes as hydrogen sensors, studied by work function and electrical characteristic measurements, Surface Science 306 (1994) 393±405. [9] B.E. Deal, M.A. McNeilly, D.B. Kao, J.M. deLarios, Solid State Technology 33 (1990) 73. [10] R.J. Archer, Journal of Electrochemical Society 104 (1957) 609. [11] J.M. deLarios, D.B. Kao, B.E. Deal, C.R. Helms, Applied Physics Letter 54 (1989) 715. [12] R.K. Chanana, R. Dwivedi, S.K. Srivastava, Silicon wafer cleaning with CF4/H2 plasma and its effect on the properties of dry thermally grown oxide, Solid State Electronics 35 (1992) 1417±1421. [13] R.K. Chanana, R. Dwivedi, S.K. Srivastava, Study of electrical properties of SiO2 grown over plasma cleaned silicon surfaces, Solid State Electronics 34 (1991) 1463±1465. [14] D.J.D. Thomas, Physica Status Solidi 2 (1963) 2261. [15] D.A. Baglee, P.L. Shah, VLSI electronics, Microstructure Science 7 (1983) 165±196 Chap. 4. [16] B.N. Chapman, Glow Discharge Processes, Wiley-Interscience, 1980, pp. 312.. [17] L. Yadava, R. Dwivedi, S.K. Srivastava, A titanium dioxide based MOS hydrogen sensor, Solid State Electronics 33 (1990) 1229±1234.
[18] D. Dwivedi, R. Dwivedi, S.K. Srivastava, The effect of hydrogen induced interface traps on Titanium dioxide based palladium gate MOS capacitor: a conductance study, Microelectronics Journal 29 (7) (1998) 445±450. [19] E.H. Nicollian, J.R. Brews, MOS Physics and Technology, Wiley, New York, pp. 285±319.. [20] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, pp. 379±386.. [21] M.C. Steele, J.W. Hile, B.A. MacIver, Hydrogen-sensitive palladium gate MOS capacitors, J. Appl. Phys. 47 (6) (1976) 2537±2538. [22] A.T. Bell, Abstract: fundamentals of plasma chemistry, J. Vac. Sci. Technol. 16 (2) (1996) 418±419. [23] J.R. Ligenza, M. Kuhn, DC arc anodic plasma oxidationÐa new vacuum process for solid state device fabrication, Solid State Technology 13 (12) (1970) 33±38. [24] G. Jordan Maclay, MOS hydrogen sensors with ultrathin layers of palladium, IEEE Trans. Electron Devices ED 32 (7) (1985) 1158± 1166. [25] K.W. Jelley, G.J. Maclay, A dual mechanism solid-state carbonmonoxide hydrogen sensor utilizing an ultrathin layer of palladium, IEEE Trans. Electron Devices ED 34 (10) (1987) 2086±2098.
Biographies D. Dwivedi was born on 15 August 1973 in India. He received his postgraduate degree in Physics (specialisation Electronics) from Avadh University, Faizabad, India. Currently, he is pursuing his Doctoral studies in Electronics Engineering at the Centre for Research in Microelectronics, Institute of Technology, Banaras Hindu University, India. He is also a recipient of Junior Research Fellowship (NET) of Council of Scientific and Industrial Research, India. He is working in the field of MOS-based gas/ odours sensor and integrated arrays. His other interests include VLSI CAD and MOS ICs. R. Dwivedi was born in 1952. He obtained his PhD degree in Electronics Engineering in 1978 from Banaras Hindu University, India. He worked as a lecturer in the Department of Electronics Engineering for about 6 years, and has been a reader in the same department since April 1986. Dr. Dwivedi has more than 20 years of research experience in the area of microelectronics and solid-state device technology. He has over 124 research papers published in various international/national journals and in the proceedings of symposia. He is currently working in the areas of sensors based on silicon and thick-film technologies, MOS devices, photovoltaics and LSI/VLSI. S.K. Srivastava was born in 1938. He obtained his PhD degree from Banaras Hindu University. He worked as a lecturer for 11 years up to 1972 and as reader for 7 years. Since March 1979, he has been a professor in the Department of Electronics Engineering. He in now engaged in research in the areas of solid-state device technology, photovoltaics, microsensors, gas sensor arrays, thick-film dielectric pastes and MOS devices. He has over 150 research papers published in various journals in India and abroad. Prof. Srivastava is currently the coordinator of the Centre of Advanced Study and Centre for Research in Microelectronics and the Head of the Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, India.
Sensing properties of palladium-gate MOS (Pd-MOS) hydrogen sensor-based on plasma grown silicon dioxide D. Dwivedi, R. Dwivedi, S.K. Srivastava* Department of Electronics Engineering, Centre for Research in Microelectronics, Institute of Technology, Banaras Hindu University, Varanasi 221 005, India Received 18 March 1998; accepted 15 December 1998
Abstract This paper deals with the performance of a palladium-gate MOS hydrogen sensor studied by conductance method. Structure of the device was fabricated on a n-type h100i silicon wafer having resistivity of 1±6 O cm using plasma technology. Sensitivity and response±recovery time of the fabricated sensor have been studied for different concentration (1480±11 840 ppm) of hydrogen with varying signal frequency (500 Hz, 10 and 100 kHz) at room temperature. Hydrogen-induced interface-trapped density (Nit) has been also evaluated as a function of gas concentration using a bias scan conductance method. Obtained results show that device performance is improved (i.e., high sensitivity and low response recovery time) and further it has been concluded that implementation of plasma technology (i.e., dry plasma cleaning of Si surface and in-situ RF anodization of Silicon in oxygen plasma near room temperature) may be a future step towards development of MOS-based sensors and integrated arrays with improved performance at room temperature. # 2000 Elsevier Science B.V. All rights reserved. Keywords: MOS gas sensor; Plasma technology
1. Introduction The SiO2-based palladium-gate MOS hydrogen sensors have widely been studied by numerous researchers [1±3]. In such MOS capacitors, silicon dioxide has been used as insulator with thickness varying from 3 to 100 nm. These devices exhibit change in capacitance and conductance, when exposed to hydrogen. Shivaraman et al. [1] and Ruths et al. [2] have reported these changes in the characteristics of MOS devices due to decrease in the work function of palladium upon exposure to H2. More detailed explanation has been given by Lundstrom [3]. It has been observed that when MOS sensor is subjected to hydrogen gas, hydrogen molecules are ®rst dissociated on the palladium metal surface because of its catalytic behaviour. Subsequently, some of the hydrogen atoms diffuse through the palladium ®lm and are adsorbed at the metal insulator (SiO2) interface. At the SiO2 interface, these atoms polarise and give rise to a dipole layer which in turn changes the work function of the palladium metal. Change in metal work function results shift in ¯at-band voltage of MOS capacitor which is a function of adsorbed gas concentration. *
Corresponding author. Telefax: +91-542-316925.
Formoso and Maclay [4] have reported that in the presence of hydrogen, the interface state densities of ultra thin palladium-gate MOS sensor increases with increasing concentration of test gas. It has been found that hydrogen atoms diffuse through the thin SiO2 (12.5 nm) Si/SiO2 interface and alter the interface state density. Armgrath [5] has reported the hydrogen-induced oxide surface charging in Pd-gate MOS devices, which also causes shift in ¯at-band voltage of MOS capacitor. Kobayashi et al. [6] have reported the voltage shift of the I±V curve for the Pt/SiO2/Si MIS tunnelling diodes following on exposure to H2, which is attributed to: (i) the decrease in the effective Pt work function, (ii) the internal field-dependent movement of hydrogen ions in the SiO2 layer, (iii) the formation of interface states. They have also studied [7,8] the mechanism of the formation of the hydrogen-induced interface states at the Si/ SiO2 interface investigated by conductance measurement. It is often reported [4±8] that interface states seriously affect the electrical characteristics of MOS devices. The above studies made on MOS sensors are based on thermally grown SiO2 layer of desired thickness. However, the requirement of SiO2 (3±100 nm) for MOS gas sensors is very critical and this possess several constraints to
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Nomenclature A Co Cs and Rs Cin Gin Cit Nit Ha Hai o t c1 and d1 GPi DGP C DC
Gate area [cm2] Oxide capacitance [pF] Capacitance and resistance associated with interface traps Input capacitance [pF] Input conductance [mS] Interface trap capacitance [pF] Interface trap density (also known as interface state density) [eV cm2]ÿ1 Hydrogen atom Hydrogen atom at interface Varying signal frequency Interface trap capture time Rate constant Initial conductance peak position [V] Shift in conductance peak position [V] in H2 Initial concentration of H2 gas Change in H2 gas concentration
conventional (thermal) technology for growth of SiO2. The normal temperature for thermal growth of SiO2 lies between 900 and 12008C, and already, the disadvantages of such high temperature processing in submicron technology have been reported [9±16]. Recently Chanana et al. [12,13] have demonstrated the growth of ultra thin (6.3 nm) SiO2 using RF oxygen plasma near room temperature with in-situ dry cleaning of Si surface. These authors found the electrical properties of such silicon dioxide favourable for the development of MOSbased gas sensors. However, to the best of the authors' knowledge, the use of in-situ RF oxygen plasma technology, which is capable to grow silicon dioxide near room temperature on dry plasma precleaned silicon surface, is yet to be implemented for the development of MOS-based gas/ odour sensors. Keeping in view the need for low temperature processing in the present study, 6.9-nm-thick SiO2 layer has been grown by RF anodization of dry plasma precleaned silicon surface, in oxygen plasma near room temperature with in situ. Fabricated MOS sensor has been tested for varying concentration of hydrogen (1480±11 840 ppm), also the sensitivity and response recovery time have been calculated in terms of shift in conductance peak position upon exposure to hydrogen. Obtained results were compared to that reported earlier [4,7,21,24,25], and it has been found that the device performance is improved (i.e., high sensitivity and low response recovery time). 2. Experimental Device was fabricated on a n-type h100i Si wafer having resistivity of 1±6 O cm. A cross-sectional view of
the fabricated device is shown in Fig. 1a, while the basic equivalent circuit and equivalent circuit including interface trap effect are shown in Fig. 1b (i), (ii) and (iii), respectively. Steps involved in the fabrication of the device are described below. Dry plasma cleaning: Step 1. Keep the wafer in trichloroethylene (TCE) accompanied by ultrasonic agitation for 5 min. Step 2. Immerse the silicon wafer in acetone for 4 min. Step 3. Rinse the wafer with deionized wafer for 4 min and dry it. Step 4. Keep the wafer in 50% CF4 H2 plasma for 4 min in a barrel etcher at 80 W RF power. The formation of HF is by a reaction of H2 with atomic F formed due to the dissociation of CF4 in the RF plasma. The reaction of atomic F and HF formation is given as: CF4 e ! CF 3 F e; 2F H2 ! 2HF: Etching of the native oxide is given by: 4HF SiO2 ! SiF4 2H2 O: The reaction product SiF4 is known to be volatile and is pumped out of the chamber. The atomic F at 50% H2 level reacts ®rst with H2 to form HF. This fact is corroborated by Chapman [16] which showed that the atomic F intensity drops to zero at 25% H2 addition due to the formation of HF in barrel etcher. This is also the reason why silicon is unable to react with atomic F and thus silicon etching is negligible in our 50% CF4 H2 plasma. Thus, the forth step removes the native oxide which contains contaminants from the ambient. The HF also reduces the heavy metal contaminants. 2.1. In-situ dry plasma oxidation (RF plasma anodization of Si surface near room temperature) After step 4 of the above cleaning method, thin SiO2 (6.9 nm) was grown in situ. The chamber is evacuated to 133±26610ÿ1 Pa, pressure. Dry oxygen ¯owed and after obtaining a dynamic chamber pressure of 133 Pa, 70 W RF power is supplied in CW mode to form dry O2 plasma. The wafer was expose to this plasma for 50 min to give 6.9 nm oxide (SiO2). The oxide thickness was measured with the help of manual ellipsometer. Fig. 2 illustrates a pictorial representation of oxygen plasma anodic oxidation mechanism for silicon [23]. Here, at the top of the illustration, ions and electrons are accelerated across the sheath region to become implanted in to the oxide. Since the sheath potential can be low, in the order of tens of volts, the implantation depth will be very slight, probably about 1 nm deep. This implantation will result in an abnormally large excess of oxygen just below the surface of the oxygen ®lm. The implanted oxygen is then expected to react with electrons to form the naturally occurring oxide ion O= in interstitial
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Fig. 1. (a) Structure of the fabricated device; (b) cross-section of a practical MOS capacitor, a simple equivalent circuit (i), including trap effect (ii) and (iii) [19,20].
sites. This ion then diffuses and drifts in an electric ®eld to the silicon, where it rapidly reacts to form a silicon±oxygen bond as shown in Fig. 2 [23]. For gate structure, palladium was deposited by evaporating it from a tungsten ®lament in vacuum of 13310ÿ6 Pa. Ohmic contact to the back side of silicon substrate was made by thermal evaporation of aluminium metal. Post-metallization anneal (PMA) was performed in N2 ambient at 4008C for 15 min. In the basic equivalent circuit of the device
(shown in Fig. 1b (i), (ii) and (iii)), Ci is the insulator capacitance and CD is the semiconductor depletion layer capacitance. CS and RS are the capacitance and resistance associated with the interface traps, and are functions of surface potential. The product CSRS is de®ned as the interface lifetime, which determines the frequency behaviour of the interface traps. The parallel branch of the equivalent circuit in Fig. 1b (i) can be converted into a frequency-dependent capacitance CP in parallel with a
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Fig. 2. A pictorial representation of the RF plasma anodization mechanism for silicon [23].
frequency-dependent conductance GP , as shown in Fig. 1b (iii). Conductance response of the fabricated MOS sensor was studied and data (magnitude of conductance peak value, conductance peak position [V] and time) were recorded with the help of Impedance/Gain Phase analyser (HP-4194A). HP-4194A model has inbuilt HPIB interfaceable capacity. This feature of interfaceability was exploited to control the HP-4194A remotely and gathered all the necessary information required for further processing. The analyser was interfaced to a PC via a GPIB-card, of IEEE-488 standard. An interfacing program has been developed to obtain accurate information from the HP-4194A and stored in the computer. The speed of data transfer is limited by the
capability of the HP-4194A and the type of processing to be perform. On-line printing was made possible for a hardcopy of the obtained data. In order to control the hydrogen gas ¯ow inside the test chamber, a rotameter (MARCH Instruments, US-made GCM-100) was calibrated and used for test gas. 3. Result Fig. 3 shows the shift in conductance peak position upon exposure to hydrogen (1480±11 840 ppm) at frequencies 500 Hz, 10 and 100 kHz. It is evident from this ®gure that this shift (magnitude) increases with gas concentration and
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Fig. 5. Variation of sensitivity with hydrogen gas concentration [ppm].
Fig. 3. Shift in conductance peak position with hydrogen gas concentration [ppm].
becomes almost constant at higher gas concentration (11 840 ppm here), also shift is high (100±900 mV) at lower frequency (500 Hz) in comparison to that (100±400 mV) at higher frequency (100 kHz). Fig. 4 shows the effect of frequency on total shift (for 1480±11840 ppm H2) in conductance peak position which is de®ned as: GPi ÿ GPgas 100; (1) Total Shift % GPi where GPiconductance peak position [V] in the absence of hydrogen (i.e., in air ambient), taken as reference;
Fig. 4. Effect of frequency on shift [%] in conductance peak position.
GPgasconductance peak position [V] for final concentration of hydrogen, which was introduced inside the test chamber in concentration varying from 1480 to 11 840 ppm. This shift in conductance peak position decreases with increase in frequency as it is 75, 62 and 50% for frequencies 500 Hz, 10 and 100 kHz, respectively. The variational behaviour of sensitivity of the fabricate sensor for hydrogen is shown in Fig. 5. Here sensitivity S is de®ned as: DGP G (2) S Pi ; DC C where GPiinitial conductance peak position [V]; DGPshift in conductance peak position [V] in H2; Cinitial concentration of H2 gas; DCchange in H2 gas concentration. From this ®gure, it is evident that the sensitivity increases with increase in gas concentration and this increasing nature is better at lower frequency (500 Hz here). The interface trap density has been evaluated using conductance method [18] and the obtained results are shown here in Fig. 6. From these ®gures, it is evident that Nit, the interface-trapped density, increases with gas concentration. Response and recovery time for the fabricated MOS sensor has been studied for different concentrations (1480±11 840 ppm) of hydrogen at frequencies 500 Hz, 10 and 100 kHz. To obtain the response time of the fabricated sensor, the shift in the conductance peak position was recorded just after the injection of gas inside the test chamber with time at an interval of 4 s (minimum possible interval with available measurement setup). It has been observed that the conductance peak position shifted towards -ive bias-voltage, and the magnitude of this shift initially increased with time, reached a maximum value and then attained stable state. Now, response time was calculated as 10±90% of this time period (i.e., from injection of gas up to the stable condition of shifting peak). Obtained results are
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4. Discussion 4.1. Sensing mechanism in MOS It is known that the application of MOS capacitors as hydrogen detector is based on the shift in ¯at-band voltage and, accordingly, lateral shift in capacitance±voltage (C±V) and conductance±voltage (G±V) response upon exposure to hydrogen [1±3,17,18]. A pictorial representation of sensing mechanism in Pd-MOS hydrogen sensor is shown in Fig. 8. Dissociation of molecular H2 on Pd-surface may be represented as [18]: c1
H2g , 2Ha :
(3)
d1
Fig. 6. Variation of interface trap density with H2 gas concentration.
shown in Fig. 7 (i) and (ii). These ®gures show that response (8 s) and recovery time (1.5 min) is small at lower frequencies (500 Hz here).
The dissociative adsorption of H2 and Pd surface and the subsequent diffusion through metal (given by Eq. (4)) form a dipole layer at Pd/SiO2 interface. This results change in flat-band voltage of device. It has been observed that change in flat-band voltage increases with gas concentration [1±8,17,18]. c1
Ha , Hai :
(4)
d1
4.2. Hydrogen-induced interface traps Formation of hydrogen-induced interface states in PdMOS hydrogen sensor may occur through the following steps: Step 1: Dissociation of the hydrogen molecules on the Pd-surface. Step 2: Adsorption of dissociated hydrogen atom on metal surface. Step 3: Diffusion of hydrogen atoms to the Pd/SiO2 interface. Step 4: Tunnelling of hydrogen through oxide layer and accumulation at Si/SiO2 interface. Step 5: Formation of interface states due to the reaction of hydrogen atoms with interface traps, as represented below: X H It ;
(5)
where Xreactant at Si/SiO2 interface; H Hydrogen concentration; Itinterface states. The hydrogen-induced interface-trapped density has been calculated by conductance method [18] which is directly related to interface traps, yields more accurate and reliable results [18±20], and is given by: G o max : (6) Nit 0:40 q Fig. 7. Effect of gas concentration and frequency on (i) response and (ii) recovery time of fabricated MOS sensor.
In present study, it has been noted that the improved performance (high sensitivity, low response and recovery
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Fig. 8. (a) Schematic illustration of the principle of SiO2-based MOS hydrogen sensor. (b) Effect of gas exposure on (i) C±V and (ii) G±V response of device.
time, etc.) of the fabricated sensor at room temperature, as compared to earlier reports [4,7,17,18], may be attributed to plasma processing used here, which gives Pd/SiO2/n-Si MOS with ultra thin (6.9 nm) SiO2 with in situ on dry plasma precleaned silicon surface. The remarkable variation in Nit at lower frequency (500 Hz here) is due to balanced communication of interface traps with valence and conduction bands of silicon substrate. The oxygen plasma environment used in the present study may also effect the device performance in the following way. As it is well known that the Oxygen plasma comprises of single and multiple species like Oÿ, O2ÿ, O3ÿ, O4ÿ, O4, O3, O2, O, etc., and eÿ [22,23], now when these species interact with each other, the number of active sites for gas adsorption inside the device also gets modulated. When the
gas (H2 here) molecules interact with these species (single or multiple), they release more number of conduction electron, i.e., an enhanced modulation of sensitivity of device. 5. Conclusion In the present investigation, it is concluded that implementation of plasma technology (i.e., dry plasma cleaning and in-situ RF plasma oxidation near room temperature) may be a future step towards the development of MOS-based sensors and integrated arrays with improve performance (i.e., high sensitivity, low response and recovery time at room temperature, etc.). Further, it has been concluded that the MOS-based sensors developed by the present method could be a potential device for detection of low concentra-
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tion of hydrogen/hydrocarbons using a low frequency conductance response at room temperature, which also (low frequency G±V method) provides more accurate information about the hydrogen-induced interface traps in such devices. Acknowledgements D. Dwivedi is very much thankful to CSIR, the government of India, for providing ®nancial support during the course of this work. References [1] M.S. Shivaraman, I. Lundstrom, C. Svensson, H. Hammarsten, Hydrogen sensitivity of palladium±thin-oxide±silicon Schottky barriers, Electron. Lett. 12 (1976) 484±485. [2] P.F. Ruths, S. Ashok, S.J. Fonash, J.M. Ruths, A study of MlS Schotkky-barrier diode detectorIEEE Trans. Electron Devices ED 28 (1981) 1003±1009. [3] I. Lundstrom, Hydrogen sensitive MOS structure: Part 1. Principles and applications, Sensor and Actuators (1981) 403±426.. [4] M.A. Formoso, G.J. Maclay, The effect of hydrogen and carbon monoxide on the interface state density in MOS gas sensors with ultra thin palladium gates, Sensors and Actuators B 2 (1990) 11±12. [5] M. Armgarth, Hydrogen induced oxide surface charging in Pd-gate MOS devices, J. Appl. Phys. 56 (1984). [6] H. Kobayashi, H. Iwadate, Y. Nakato, Role and mechanism of the formation of hydrogen-induced interface states for platinum/silicon oxide/silicon MOS tunnelling diodes, Sensor and Actuators B 24±25 (1995) 815±818. [7] H. Kobayashi, H. Iwadate, Y. Kogetsu, Y. Nakato, Mechanism of the formation of hydrogen induced interface states for Pt/silicon oxide/Si metal-oxide-semiconductor tunnelling diodes, J. Appl. Phys. 78 (1995) 6554±6561. [8] H. Kobayashi, K. Kishimoto, Y. Nakato, Reaction of hydrogen at the interface of palladium±titanium dioxide Schottky diodes as hydrogen sensors, studied by work function and electrical characteristic measurements, Surface Science 306 (1994) 393±405. [9] B.E. Deal, M.A. McNeilly, D.B. Kao, J.M. deLarios, Solid State Technology 33 (1990) 73. [10] R.J. Archer, Journal of Electrochemical Society 104 (1957) 609. [11] J.M. deLarios, D.B. Kao, B.E. Deal, C.R. Helms, Applied Physics Letter 54 (1989) 715. [12] R.K. Chanana, R. Dwivedi, S.K. Srivastava, Silicon wafer cleaning with CF4/H2 plasma and its effect on the properties of dry thermally grown oxide, Solid State Electronics 35 (1992) 1417±1421. [13] R.K. Chanana, R. Dwivedi, S.K. Srivastava, Study of electrical properties of SiO2 grown over plasma cleaned silicon surfaces, Solid State Electronics 34 (1991) 1463±1465. [14] D.J.D. Thomas, Physica Status Solidi 2 (1963) 2261. [15] D.A. Baglee, P.L. Shah, VLSI electronics, Microstructure Science 7 (1983) 165±196 Chap. 4. [16] B.N. Chapman, Glow Discharge Processes, Wiley-Interscience, 1980, pp. 312.. [17] L. Yadava, R. Dwivedi, S.K. Srivastava, A titanium dioxide based MOS hydrogen sensor, Solid State Electronics 33 (1990) 1229±1234.
[18] D. Dwivedi, R. Dwivedi, S.K. Srivastava, The effect of hydrogen induced interface traps on Titanium dioxide based palladium gate MOS capacitor: a conductance study, Microelectronics Journal 29 (7) (1998) 445±450. [19] E.H. Nicollian, J.R. Brews, MOS Physics and Technology, Wiley, New York, pp. 285±319.. [20] S.M. Sze, Physics of Semiconductor Devices, Wiley, New York, pp. 379±386.. [21] M.C. Steele, J.W. Hile, B.A. MacIver, Hydrogen-sensitive palladium gate MOS capacitors, J. Appl. Phys. 47 (6) (1976) 2537±2538. [22] A.T. Bell, Abstract: fundamentals of plasma chemistry, J. Vac. Sci. Technol. 16 (2) (1996) 418±419. [23] J.R. Ligenza, M. Kuhn, DC arc anodic plasma oxidationÐa new vacuum process for solid state device fabrication, Solid State Technology 13 (12) (1970) 33±38. [24] G. Jordan Maclay, MOS hydrogen sensors with ultrathin layers of palladium, IEEE Trans. Electron Devices ED 32 (7) (1985) 1158± 1166. [25] K.W. Jelley, G.J. Maclay, A dual mechanism solid-state carbonmonoxide hydrogen sensor utilizing an ultrathin layer of palladium, IEEE Trans. Electron Devices ED 34 (10) (1987) 2086±2098.
Biographies D. Dwivedi was born on 15 August 1973 in India. He received his postgraduate degree in Physics (specialisation Electronics) from Avadh University, Faizabad, India. Currently, he is pursuing his Doctoral studies in Electronics Engineering at the Centre for Research in Microelectronics, Institute of Technology, Banaras Hindu University, India. He is also a recipient of Junior Research Fellowship (NET) of Council of Scientific and Industrial Research, India. He is working in the field of MOS-based gas/ odours sensor and integrated arrays. His other interests include VLSI CAD and MOS ICs. R. Dwivedi was born in 1952. He obtained his PhD degree in Electronics Engineering in 1978 from Banaras Hindu University, India. He worked as a lecturer in the Department of Electronics Engineering for about 6 years, and has been a reader in the same department since April 1986. Dr. Dwivedi has more than 20 years of research experience in the area of microelectronics and solid-state device technology. He has over 124 research papers published in various international/national journals and in the proceedings of symposia. He is currently working in the areas of sensors based on silicon and thick-film technologies, MOS devices, photovoltaics and LSI/VLSI. S.K. Srivastava was born in 1938. He obtained his PhD degree from Banaras Hindu University. He worked as a lecturer for 11 years up to 1972 and as reader for 7 years. Since March 1979, he has been a professor in the Department of Electronics Engineering. He in now engaged in research in the areas of solid-state device technology, photovoltaics, microsensors, gas sensor arrays, thick-film dielectric pastes and MOS devices. He has over 150 research papers published in various journals in India and abroad. Prof. Srivastava is currently the coordinator of the Centre of Advanced Study and Centre for Research in Microelectronics and the Head of the Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, India.