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Passivation Effect for the Reduction of 1/ f Noise in Poly(3,4-

ethylenedioxythiophene):Poly(styrene sulfonate) Thin Films Based on Uncooled Type

Microbolometer Applications

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2009 Appl. Phys. Express 2 041501

(http://iopscience.iop.org/1882-0786/2/4/041501)

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Passivation Effect for the Reduction of 1=f Noise

in Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate)

Thin Films Based on Uncooled Type Microbolometer Applications

Hyeok Jun Son�, Il Woong Kwon, and Hee Chul Lee

School of Electrical Engineering and Computer Sciences, Korea Advanced Institute of Science and Technology,

373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea

Received January 14, 2009; accepted March 11, 2009; published online April 3, 2009

In this study, resistance fluctuation or 1=f noise was measured in thin films of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)

(PEDOT:PSS). The 1=f noise of thin films is much larger than what is typical uncooled type microbolometer sensing material such as

vanadium oxide films and amorphous silicon films. To use PEDOT:PSS thin films for a bolometric application, the 1=f noise should be

reduced. This study discusses the effect of passivation for reducing the 1=f noise. # 2009 The Japan Society of Applied Physics

DOI: 10.1143/APEX.2.041501

Most resistors through which current is flowingexhibit voltage fluctuations with a power spectraldensity inversely proportional to the frequency, or

nearly so.1) This 1=f noise has not been well understood.Generally, the mechanism of this noise involves the randomemission and capture of free carriers from deep defects in thebulk and recombination of traps in surface.2) Such a randomprocess induces free carrier fluctuation and results inresistance fluctuation. This fluctuation follows the activationbarrier distribution as shape of a 1=f power spectrum.Consequently, the 1=f noise signal is stronger at a lowfrequency, which is used for the realization of the thermalinfrared image3) (video frame). In particular, uncooled typebolometers based on semiconducting materials are known tohave higher 1=f noise compared to metal film bolometers.3)

According to Hooge,3,4) the 1=f noise is strongly dependenton the material itself followed by the noise power,Sð f Þ1= f ¼ K� ðVbiasÞ2=f , where K is a 1=f noise factorthat strongly depends on the bolometric materials, and Vbias

is the bias voltage. And the 1=f noise factor-K has auniversal relationship of K ¼ �H=ðn�Þ ¼ ��=�, where �H isan empirical constant depending on the resistor materials, nis the charge carrier density of the material, � is the totalvolume of the resistor, � is a parameter related to materialpreparation, and � is the electrical resistivity of the material.It is notable that the 1=f noise is very strong function of theresistivity, especially in inhomogeneous systems.2) Apossible means of lowering a 1=f noise of resistors is theintroduction of low resistivity.3) This, however, indeed leadsto decrease temperature coefficient of resistance (TCR)[TCR ¼ 1=R� ðdR=dT Þ � 100] of the resistor becauseit is widely known that the TCR is also proportional tothe resistivity.3) Because the performance of bolometer isproportional to the TCR and inversely proportional tothe noise,3) it is very important to reduce the 1=f noisewithout sacrificing the TCR value or decreasing theresistivity. In this study, poly(3,4-thylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is studied as abolometric material. The PEDOT:PSS is one of the mostattractive conducting polymers owing to its excellentenvironmental stability, high electrical conductivity, andlow cost.5) Furthermore, Aleshin et al. reported that a high

pH of PEDOT:PSS dispersion leads to films that havestrong temperature-dependent conductivity.6) Therefore,these properties are feasible for use as a bolometric material.However, until now, it has not been studied the 1=f noiseof PEDOT:PSS thin films intensively which is used forthe figure of merit for bolometric materials.

In this study, we examine the effect of passivation on thedecrease of the 1=f noise of PEDOT:PSS thin films andshow the feasibility of bolometric applications. To testthis effect, we fabricated a simple bar pattern. An aqueousdispersion of PEDOT:PSS known commercially asClevious� P (standard grade) was used in this study. First,a solution of 45% KOH (potassium hydroxide) was added tothe PEDOT:PSS dispersion to change the pH value andobtain a high TCR value;6) this was filtered with a 0.45 �msyringe filter and then sonicated. Thin films of a thickness ofapproximately 60 nm were spin-coated in ambient air at2000 rpm onto a SiO2 substrate with a patterned goldelectrode (30 nm). To improve the adhesion of the gold onthe substrate, a thin layer (10 nm) of chromium was initiallydeposited. Prior to the spin-coating of PEDOT:PSS, thesubstrates were thoroughly cleaned using a metal cleaningmethod (dipping in boiling organic solvent, acetone, andmethanol for 15min). They were then treated with O2

plasma to form a hydrophilic surface. The thin films weresubsequently thermal-treated on a hot plate at 200 �C for30min under nitrogen ambient to evaporate the water. Toobtain the noise and TCR values comparable to those oftypical materials, the PEDOT:PSS thin films were patternedin the form of a typical bolometer size,3) i.e., 50� 50 �m2 asshown in Fig. 1. To pattern the films, silicon nitride wasdeposited as a protection layer using low-temperatureplasma-enhanced chemical vapor deposition (PECVD) at150 �C. Finally, the films were patterned using conventionalphotolithography and dry etching technology. Samples werecharacterized by using electrical measurements. Noiseexperiments were performed using a specific noise measure-ment system7) (a SR570 low-noise amplifier with an AgilentSR780 dynamic signal analyzer), and TCR experimentswere performed using R–T measurement with a thermo-electric cooler to induce a stable temperature under air.

We first examined the 1=f noise and TCR of the patternedPEDOT:PSS thin films. Figure 2 shows that they behave asnormal semiconductors’ 1=f noise as follows: First, the 1=f�E-mail address: ok.junni@gmail.com

Applied Physics Express 2 (2009) 041501

041501-1 # 2009 The Japan Society of Applied Physics

noise of PEDOT:PSS thin films strongly depends on theresistivity which is changed by the pH of the dispersion.Second, the 1=f noise follows the Hooge theorem3,4)

including bias dependency and shape of spectrum. Thisresult also shows that the 1=f noise factor-K is larger thanthat of the typical bolometric materials such as vanadiumoxide3) and amorphous silicon films.8) This high 1=f noisemay originate from the amorphous structure. Although theyhave high TCR that exceeds �4%/�C as a result of the highpH of PEDOT:PSS dispersion, the high 1=f noise prevents ahigh performance from being obtained. Therefore, it isnecessary for the 1=f noise to be reduced further.

The source of 1=f noise in semiconductor materials suchas PEDOT:PSS is known to be caused not only by the bulk,but also by the surface or interface state.9) In case of thinfilms, in particular, the interface states between the surfaceand the surrounding ambient can be the predominant sourceof noise. The surface of PEDOT:PSS films is covered with aPSS thin layer, which is especially hygroscopic and maycause H2O molecules to form a thin layer above the PSSchains.10) In other words, water uptake implies thatconductivity of films can be influenced by moisture. And itmay act as source of resistance fluctuation. Therefore thisunstable surface state of PEDOT:PSS films can contribute to1=f noise. In the case of PEDOT:PSS thin films, thethickness is thin of a few nm; that is, the surface exposed toair is almost top side. In the test pattern used in themeasurement, this top side was passivated or protectedbecause the post dry etch process to pattern the PEDOT:PSSfilms requires a protection layer, as shown in Fig. 1.Therefore, the exposed surface is actually a lateral face,which occupies only 4.5% of the total surface area.Moreover, because this lateral face is etched by a dry etchprocess, some area of the lateral face is formed with aPEDOT-rich cluster and by not PSS layers. Consequently,due to the small portion of the connection or interactionbetween the PSS layer and air, the test device did not sufferfrom abrupt changes of electrical resistivity under air.However, it remains unclear whether slight resistancefluctuation in the noise level exists or does not exist.Therefore, if all unstable surface factors are diminished, the1=f noise would be lowered by reducing the 1=f noisecaused by the surface state.

The unstable surface factor, the PSS layer exposed to air,can be easily diminished using the encapsulation processshown in Fig. 3. Through this encapsulation process, allsurface of PEDOT:PSS thin films were isolated from theexternal ambient, i.e., humidity. Figure 3(b) shows Scanningelectron microscope (SEM) images of the encapsulation ofPEDOT:PSS thin films through an additional passivationlayer using 200 nm of silicon nitride. In case of films withpH value of 3.73 (8��cm), through this process, TCR isalmost maintained at a constant level within roughly 2% aswell as resistivity because of little effect of the postdeposition process such as low temperature PECVD onresistivity. However, interestingly the 1=f noise factor-Kwas further decreased by roughly 30 times as shown inFig. 4. According to Fig. 4, the 1=f noise factor-K ofencapsulated PEDOT:PSS thin films is 1:23� 10�10. Thisvalue is much smaller than that of the pristine bar pattern(top only protection) (3:69� 10�9). From this result, it can

(a)

(b)

Fig. 2. (a) 1=f noise factor-K which indicates the amount of 1=fnoise power [Sð f Þ1=f ¼ K � ðVbiasÞ2=f ] and the TCR vs resistivity.

(b) Noise power at 1Hz with different bias voltages of the PEDOT:PSS

thin films with a pH value of 3.73 (8��cm). Note that the noise power

is proportional to the power of the bias voltage, as in the Hooge

theorem. The insert shows that the slope of the noise power spectrum

is nearly 1 irrespective of bias voltages, following the Hooge theorem.

Au Electrode

Contact Area

Patterned PEDOT:PSS50 ×× 50 µm2

SiO2Cr

AuSiNPEDOT:PSS

(a)

(b)

Fig. 1. (a) SEM image of the bar pattern. (b) Configuration image of

the bar pattern.

H. J. Son et al.Appl. Phys. Express 2 (2009) 041501

041501-2 # 2009 The Japan Society of Applied Physics

be concluded that the passivation of PEDOT:PSS thin filmsis efficient for suppression of the 1=f noise by reducing the1=f noise caused by the surface state effectively.

In conclusion, the results of this study demonstrate theeffect of passivation for reducing 1=f noise in PEDOT:PSSthin films without sacrificing the TCR value. Consequently,this method enhances the performance of the bolometricmaterials. For example, the detectivity3) can be increased by5.51 times through the passivation effect. The optimumperformance of PEDOT:PSS thin films occurs with a TCR of4.5%/�C, and a total noise of 93.9 �V/Hz1=2 at 8��cm. Andthe detectivity was estimated to be 2:7� 109 cm�Hz1=2/W,which is comparable to that of typical bolometric materials3)

(108–109 cm�Hz1=2/W), with a reasonable thermal conduct-

ance of 10�7 W/K. Therefore, they are feasible for use asbolometric materials. Additional studies regarding 1=f noisemechanism in PEDOT:PSS thin films are planned toimprove the bolometric performance further.

Acknowledgments This study was supported by the IT R&D

program of MIC/IITA (2006-S-054-01, Development of CMOS Based

MEMS Processed Multi-functional Sensor for Ubiquitous Environment).

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Reynolds: Adv. Mater. 12 (2000) 481.

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173.

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EDA.

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Fig. 4. Noise power spectrum of pristine PEDOT:PSS films and

encapsulated PEDOT:PSS films with a normalized bias voltage. The

straight line produced by linear fitting is plotted on the graph to extract

the 1=f noise factor-K .

Protection layer (for dry etch of polymer)

Passivation layer(for surface encapsulation)

(b)

Passivation

ProtectionPEDOT:PSS SiN

SiN

PEDOT:PSS

SiO2

(a)

Fig. 3. (a) Configuration of the bar pattern with additional

passivation layers. (b) SEM image of the encapsulated bar pattern,

cross-section view. The insert illustrates the layers in SEM images.

H. J. Son et al.Appl. Phys. Express 2 (2009) 041501

041501-3 # 2009 The Japan Society of Applied Physics