Effect of RF and microwave oxygen plasma on the performance of Pd gate MOS sensor for hydrogen

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Solid State Sciences 12 (2010) 1540e1546

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Solid State Sciences

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Effect of RF and microwave oxygen plasma on the performance of Pd gate MOSsensor for hydrogen

Preeti Pandey*, J.K. Srivastava, V.N. Mishra, R. DwivediCenter for Research in Microelectronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India

a r t i c l e i n f o

Article history:Received 23 March 2010Received in revised form30 May 2010Accepted 7 June 2010Available online 16 June 2010

Keywords:MOS gas sensorPlasma treatmentSiO2

SensitivityPalladiumHydrogen detection

* Corresponding author.E-mail address: [email protected]

1293-2558/$ e see front matter � 2010 Elsevier Masdoi:10.1016/j.solidstatesciences.2010.06.012

a b s t r a c t

The combined effect of microwave and RF oxygen plasma treatment of SiO2 surface on the hydrogensensitivity of Pd gate MOS sensor has been studied. Nine different samples of thermally grown SiO2

surface have been taken and treated with oxygen plasma of different microwave power (100 W, 150 Wand 200 W respectively) while keeping RF power fixed (20 W) for different durations (5 min, 10 min and15 min). Pd gate MOS sensors with these plasma treated SiO2 surface as dielectric have been fabricatedand tested for different concentrations (500e3500 ppm) of hydrogen at room temperature. It is observedthat the sensitivity of the sensor increases for higher duration of plasma exposure and also withmicrowave power but decreases when the sensor is treated with 200 W microwave power for 10 minand 15 min durations. The sensor treated with oxygen plasma of 200 W microwave power for 5 minduration exhibited the highest hydrogen sensitivity (74.4%). Fixed oxide charge density has also beenevaluated as a function of exposure time for varying microwave power. Surface morphology of plasmatreated SiO2 surfaces was studied by AFM to have the estimation of porosity. The high sensitivity can beattributed to the fact that oxygen plasma treatment provides the availability of higher number ofadsorption sites and modification in the surface state density i.e. surface state density increases forplasma treated sensors.

� 2010 Elsevier Masson SAS. All rights reserved.

1. Introduction

Today hydrogen has many important applications such as its usein the processes of many industries that include chemical, petro-leum, food and semiconductor. Furthermore, the inevitable deple-tion of fossil fuel reserves have lead to renewed interest in cleanenergy technologies, especially those involving hydrogen.Hydrogen sensors would form an integral part of any such systemsincorporating hydrogen as a fuel [1].

Hydrogen-sensitive metal-insulator-semiconductor (MIS)devices have been in existence for more than three decades, and area promising solid-state technology for monitoring hydrogen andhydrogen containing gases in a wide range of commercial appli-cations [2]. Lundström et al. first fabricated and reported the MOScapacitor-type hydrogen sensor in 1975 [3]. Since then, the Si-basedhydrogen sensors have been widely and comprehensively investi-gated [4e7]. The basic principle of operation of these devices hasbeen detailed by Lundstrom and coworkers [8e12]. According totheir models, gas-phase H2 dissociates on the surface of the

(P. Pandey).

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catalytic metal to form H atoms. These H atoms then rapidly diffusethrough the metal film to the metaleinsulator interface and arepreferentially trapped in interfacial adsorption sites. The layer ofinterfacial hydrogen created by this process exists in the form ofdipole layer, creating an additional voltage drop across the MISsensor that can be measured as either a shift in the capaci-tanceevoltage (CeV) curve of a capacitor, or in the currentevoltagecharacteristic of a diode or transistor [2].

Surface and interface states play an important role in deter-mining the sensing performance of the MOS devices. It has beenshown [10,13] that the dominating part of the response of MISsensor is due to the charge trapping on the insulator side of theinterface. Eriksson et al. [14] have attempted to correlate thesensing properties (such as detection limit, sensitivity and satura-tion concentration) of MIS gas sensors with the surface propertiesof the insulator and concluded that surface properties of theinsulator have a dramatic effect on the sensing behavior of sensor.

Chanana et al. [15,16] have obtained the ultra thin (6.3 nm) layerof SiO2 using RF oxygen plasma near room temperature with in-situdry cleaning of Si surface. He found that electrical properties of suchsilicon dioxide are favorable for the development of MOS based gassensors. Utilizing the findings of these studies [15,16], Dwivedi et al.[17] have fabricated the MOS hydrogen gas sensor by growing 6.9-

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Gas in

AIRVENT

THROTTLE VALVE

TURBO PUMP

ROTARY PUMP

Microwave Power

RF PowerProcess

Fig. 1. Schematic illustration of PECVD system.

Fig. 3. CeV response of oxygen plasma treated (with 100 W microwave and 20 W RFpower for 10 min) sensor to various concentrations (500e3500 ppm) of hydrogen inair at room temperature.

P. Pandey et al. / Solid State Sciences 12 (2010) 1540e1546 1541

nm-thick SiO2 layer through RF anodization of dry plasma pre-cleaned silicon surface, in oxygen plasma near room temperatureand found the drastic improvement in sensing performance interms of high sensitivity and low response time of the sensor.

Surface treatment by soft plasma [O2, H2, He, Ar, NH3] is anotherwell established technique to modify the electrical and the struc-tural properties of SiO2 and its interface with Si [18e23] and hencethe sensitivity of the MOS gas sensor. RF plasma has been exten-sively used by most of the previous workers [17,24,25] for thispurpose. The use of microwave plasma for film growth and depo-sition has become a promising technique due to some of its uniquefeatures, namely high plasma density, a wide range of pressure forplasma sustenance and reduced radiation damage due to low self-bias [26,27].

Therefore, considering the advantage of microwave plasma insilicon technology and a dramatic improvement of sensor responsedue to RF plasma, in the present work, combined effect of RF andmicrowave oxygen plasma on the hydrogen sensitivity of Pd gate

Fig. 2. CeV response of oxygen plasma treated (with 100 W microwave and 20 W RFpower for 5 min) sensor to various concentrations (500e3500 ppm) of hydrogen in airat room temperature.

MOS sensor has been investigated. For this purpose, thermallygrown SiO2 layers of Pd gate MOS sensors were treated with RF andmicrowave oxygen plasma of different microwave power (100 W,150 W and 200 W) for different durations (5 min,10 min and




P. Pandey et al. / Solid State Sciences 12 (2010) 1540e15461542

15 min). The fabricated sensors were tested for detection of variousconcentrations (500e3500 ppm) of hydrogen and sensitivity wascalculated in terms of change in capacitance value at fixed bias. Thesensitivity is found to be higher (74.4%) than the previous works[17,24,25] where only RF plasma has been exclusively used.

2. Experimental

In the present work, nine samples of Pd gate MOS capacitorswere fabricated on p-type h100i3" Si substrate and its schematicillustration can be found elsewhere [28]. Initially, all the sampleswere thoroughly cleaned using standard technological cleaningprocedures used in silicon technology. SiO2 layer (approx 110 Å)was grown by dry thermal oxidation of siliconwafer at 900 �C in theoxidation furnace for 12 min.

The samples were divided into three sets (each containing threesamples). Each set was exposed to oxygen plasma with different


microwave power (100 W, 150 W and 200 W respectively) and allthree sample of each set were exposed to plasma for differentdurations (5 min, 10 min and 15 min respectively).

Oxygen plasma was generated at low pressure by using dualfrequency PECVD (plasma enhanced chemical vapor deposition)system (made in National Physical Laboratory (NPL), New Delhi).The schematic diagram of PECVD system is shown in Fig. 1. Thepressure in the chamber was maintained at 0.06 Torr and oxygenflow rate was kept at 99 sccm/min. The surface of the samples wasbombarded by oxygen plasma. The top electrode was driven byvarying microwave power (100 W, 150 W and 200 W) operated at2.45 GHz while the bottom electrode at which the substrate waskept was driven by fixed RF power (20 W) operated at 13.56 MHz.

After surface treatment by oxygen plasma the back side oxide ofall the samples was removed by using photolithography technique.Subsequently, Palladium was deposited on the front face of thewafer by vacuum evaporation method using a standard maskhaving holes of 1 mm diameter to form the gate of MOS capacitor.Similarly, the ohmic contact on the back side of Si substrate wasmade by evaporating Al metal on the back face. The sample was




Fig. 11. Variation of sensitivity of sensors treated at fixed (100W microwave and 20WRF) plasma power for different durations (5, 10 and 15 min) with hydrogenconcentration.

Fig. 12. Variation of sensitivity of sensors treated at fixed (150W microwave and 20WRF) plasma power for different durations (5, 10 and 15 min) with hydrogenconcentration.

Fig. 13. Variation of sensitivity of sensors treated at fixed (100W microwave and 20WRF) plasma power for different durations (5 and 10 min) with hydrogen concentration.


then annealed at 450 �C in nitrogen ambient for 7 min for achievinga proper front and back contact.

CeV analyzer, (model 590 KEITHLEY Instruments, USA) andPrecision LCR meter HP-4284A (having frequency range of 20 Hz to

Fig. 14. Effect of Variation of microwave plasma power on the hydrogen sensitivity ofthe sensor treated for fixed duration (5 min).

Fig. 15. (a) AFM image of SiO2 surface treated with oxygen plasma at 20 W RF and 100 W microwave power for 5 min, (b) AFM image of SiO2 surface treated with oxygen plasma at20 W RF and 100 W microwave power for 10 min, (c) AFM image of SiO2 surface treated with oxygen plasma at 20 W RF and 100 W microwave power for 15 min, (d) AFM image ofSiO2 surface treated with oxygen plasma at 20 W RF and 200 W microwave power for 5 min, (e) AFM image of SiO2 surface treated with oxygen plasma at 20 W RF and 200 Wmicrowave power for 10 min and (f) AFM image of SiO2 surface treated with oxygen plasma at 20 W RF and 200 W microwave power for 15 min.


Fig. 16. Fixed oxide charge density as a function of exposure time for varying micro-wave power (100 W, 150 W and 200 W).


1 MHz) were used for studying the CeV and GeV characteristics ofthe fabricated MOS sensor at different concentrations(500e3500 ppm) of hydrogen. Both the instruments were inter-faced to a PC. Metrics ICS (Interactive characterization software,version 3.7.0) software was used to obtain accurate informationfrom the instruments and stored in a computer. The entire exper-iment was performed in a clean air ambient.

The microstructure analysis was carried out by Atomic forcemicroscopy (NT-MDT model No: solver PRO-47, Russian made).

3. Results and discussion

The variation of capacitance with gate voltage for all the fabri-cated sensors was recorded in air as well as upon exposure tohydrogen in the concentration range of 500e3500 ppm at 100 kHzfrequency and the sensitivity was calculated by:

Sð%Þ ¼ DCC

� 100

where, C e Capacitance in clean airDC e change in capacitance at a certain gas concentrationFigs. 2e10 show the CeV characteristics of all the nine fabricated

sensors on exposure to different concentration (500e3500 ppm) ofhydrogen. It is observed that with increase in the hydrogenconcentration, the entire CeV curve shifts towards the negativeside of the voltage axis.

The sensitivity of sensors treated at 100 W microwave and RFoxygenplasmapower for different durations (5,10 and15min) uponvarying concentration of hydrogen is shown in Fig. 11. It is observedhere that as the duration of plasma exposure increases (5e15 min)the sensitivity of the sensor also increases. Themaximumsensitivityhas been in the case of 15 min duration of plasma exposure. Thesimilar results are obtained for 150 W and 200 W (Figs. 12 and 13).However, in case of 200W, for plasmaexposureof 10minand15mindurations the sensitivity of the sensor decreases and is found to benegligible for 15 min of duration.

Fig. 14 shows the effect of microwave plasma power on thehydrogen sensitivity for fixed duration of exposure (5 min). It isevident from the figure that the sensitivity of the sensor increaseswith increase in microwave plasma power i.e. sensor treated with200 Wmicrowave power for 5 min possesses the higher sensitivitythan sensors treated with lower power. However, for higherexposure time (10 min and 15 min) the sensitivity of sensor treatedwith 150 W microwave plasma power exhibits higher sensitivitythan 200 W plasma power treated sensor. From the results, it isclear that sensor that is treated with 200 W microwave plasmapower for 5 min duration possess the highest sensitivity (74.4%)towards hydrogen.

Andrey et al. [29] have suggested the better gas sensitivity ofMIS structures containing higher density of surface states ina dielectric layer and at the interface of insulator/silicon. Their maintheme for higher gas sensitivity happens to have higher surfacestate density where large grain boundaries and porosity playdominant role.

In the present work, the AFM analysis of RF and microwaveoxygen plasma treated SiO2 surfaces reveals the granular micro-structure consisting of various grains of size 100e700 nm withlarge number of grain boundaries and also showing the certainporosity (Fig. 15). The porosity of the surface is found to increasewith exposure time and attains maximum value for 15 min dura-tion for a fixed microwave power (100 W). However, for themicrowave plasma power of 200 W, when exposure durationexceeds beyond 5min, the oxide surface gets damaged (Fig. 15e andf). This is evident from the results of sensitivity measurements(Fig. 14) where sensitivity is found to decrease for higher durationof plasma exposure. It is already reported [30] that due to plasmaexposure surface sputtering may take place which is more likely athigh power.

Plasma is electrically energized matter in a gaseous state, con-sisting of electrically neutral gas molecules, charged particles (ionsand electrons) and photons. The parameters that affect the ability ofthe plasma to activate the gas are: plasma density (the number ofcharged particles per unit volume), the electron energy and the ionimpact energy [31].

The characteristics of microwave plasma can be more suitablethan RF plasma for low energy applications such as surface treat-ment as microwave plasma has higher plasma density but lowerion energy [31]. As the power input increases, ion impact energycan also be expected to increase. This may initially be useful interms of crystallization of SiO2 on Si but, if this energy becomes toogreat this energymay start damaging the surfacewhich seems to behappening in the present case as applying the oxygen plasmabeyond 200 W microwave power the SiO2 surface has beenobserved to getting damaged. In case of RF plasma the ion impactenergy has relatively higher values than that of the microwaveplasma.

Aguas et al. [32] have already showed that oxygen plasmatreatment of the surface oxides is an effective method to increasethe oxide barrier by changing the oxide stoichiometry and conse-quent improvement in the electrical properties of the surface.

In the present work the fixed charge density has been evaluatedas function of exposure time for varying microwave plasma power.It is observed that as the plasma power increases (100 We200 W)the fixed charge density also increases (Fig. 16). However, in case of200 W of plasma power, the value of fixed charge density is foundto be maximum for plasma exposure time of 5 min but for higherdurations (10 and 15 min) the fixed oxide charge density decreases.This seems to be due to surface sputtering [30].

Oxygen plasma comprises of single andmultiple species like O�,O2�,O3�, O4�, O4þ, O3þ, O2þ., Oþ. etc., and e� [33,34] now whenthese species interact with each other, the number of adsorption


sites inside the device also gets modulated. On the exposure ofgases like hydrogen the adsorbed gas molecules (in present caseH2) interact with these species (single or multiple) which arepresent at the SiO2 surface due to plasma treatment, and canexchange electrons with Si surface and results in an enhancementof sensitivity of sensor. Due to the use of microwave plasma greaternumber of oxygen species can be produced and hence the highersensitivity can be achieved than the previous results [17] whereonly RF plasma has been used.

4. Conclusion

Employment of microwave in combination with RF oxygenplasma for surface treatment of SiO2 has resulted in dramaticimprovement in the hydrogen sensitivity of the MOS gas sensor.AFM analysis of plasma treated SiO2 surface has demonstrated thatthe RF and microwave oxygen plasma treatment causes the rear-rangement of the surface atoms resulting in high porosity andgreater number of adsorption sites and hence high surface statedensity in the oxide which ultimately reflected in the enhancedsensitivity of the sensor to hydrogen. The sensitivity was found toincrease with increase in exposure time and microwave power.However, the SiO2 surface treated for 200 W microwave power for10 min and 15 min durations start getting damaged as it is evidentfrom AFM study. This happens probably due to ion-sputteringwhich is more likely due to a very high sustained plasma densityover SiO2 surface.

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Effect of RF and microwave oxygen plasma on the performance of Pd gate MOS sensor for hydrogen