Proceedings of the 6th International Caribbean Conference on Devices, Circuits and Systems, Mexico, Apr. 26-28, 2006

Characterization of Polymethyl Methacrylate (PMMA)Layers for OTFTs Gate Dielectric

Israel Mejia and Magali EstradaDepto. Ing. E1ectrica, (SEES), CINVESTAVAv. IPN, 2508 Apdo. postal 14-740 CP 07360

Mexico D.F.e-mail: joganacaggmail.com

Abstract-Polymethyl Methacrylate (PMMA) is an organicmaterial widely used in electronic photoresists. Recently,pentacene OTFTs were fabricated using this material asdielectric film, which is considered to have a dielectric constantsimilar to SiO2. In this paper we characterize PMMA layers inorder to determine its dielectric constant, current density andinterface states density when used as a gate dielectric in themanufacturing of organic thin film transistors. MIS capacitorswere prepared, with PMMA as a dielectric, chromium as gatemetal and aluminum as the back contact on Si wafers. I-V andC-V characteristics measurements reported a dielectricconstant of 2.6 at 1 MHz. Current density across the dielectricis less than 2x10-8 A/cm2 and interface states density is below2x10'2 cm-2.

I. INTRODUCTION

Polymethyl Methacrylate (PMMA) has been commonlyused as high resolution positive resist in photolithographicprocesses. This resist has been also used as a protective layerthanks to its good mechanical and thermal stability.Recently, this polymeric material was used as gate dielectricin pentacene OTFTs [1] instead of SiO2, since it isconsidered to have dielectric constant similar to SiO2 andhigh resistivity (> 2 x 1015 Q cm) [1]. However, we have notfound a specific characterization of this material concerningits current density and interface properties with thesemiconductor when used as gate dielectric.

In addition, to become a possible substitute of inorganicgate dielectrics, the compatibility of these layers withphotolithographic processes has to be solved.

PMMA can be easily deposited by spin-coating withgood control of the layer thickness, depending on the spinspeed and PMMA percent in the solution. PMMA usuallycomes dissolved in chlorobenzene or anisole. Anotheradvantage of organic materials, in general, is that they needlow processing temperature and can be deposited on largearea substrates. This low-cost process for depositing thepolymeric materials suggests that they can be competitive ina few years for substituting inorganic amorphous materialsfor applications such as LCDs, sensors and switching

transistors. In the case of PMMA, its melting temperature isaround 2300 C, so the complete device processing must bekept at temperatures below this.

The development of OTFTs with polymeric active layerand inorganic dielectric is not a complete low-cost processdue to the necessity of using, either a high temperaturefurnace or a complex CVD equipment, in order to grow ordeposit the inorganic dielectric. Because of this, it isconvenient to find dielectric materials which can becompatible with the low-cost process and substituteinorganic active layers used at present in OTFTs.

In this paper we present an analysis of dielectric constant,current densities, interface states density and manufacturingprocess ofPMMA in order to better know its possibilities, ifused as a gate dielectric.

II. EXPERIMENTAL PARTMIS devices of different areas were fabricated with the

structure shown in Fig. 1. This structure consists in a P-typesilicon substrate with an aluminum layer evaporated at thebackside of the structure.

PMMA was purchased in Aldrich [2] with molecularweight of 950 K diluted in chlorobenzene (6%). With thisPMMA solution, uniform layers in the range of 900 to 600nm thicknesses were prepared. To prepare thinner layers, inthe order or 150 to 300 nm, PMMA was further diluted inchlorobenzene, to obtain a total 300 dilution.

Chromium gate contacts

Figure 1. MIS structure with PMMA as a dielectric.

This work was supported by CONACYT projects 39708 and 45689

1-4244-0041-4/06/$20.00 ©2006 IEEE. 375

Samples were prepared spinning previous indicatedPMMA solution at 5000 RPM for 45 seconds to obtainPMMA layers of 275 nm. The refractive index obtained was1.49. These values were obtained by means of variable angleellipsometric measurements, using a Gaertner L 1 17ellipsometer. The wafers were baked at low temperature in aconventional oven for 20 minutes.

Once deposited the dielectric, a chromium contact on top,was deposited by sublimation, followed by a

photolithographic process to define different capacitors area,ranging from 1.6x10-5 cm2 to 2.9x10-3 cm2.

For this photolithography, it was first determinedpossible chemical solutions in which metals, as Cr and Aucan be removed, without damaging the PMMA layer. It wasdetermined that Cr can be etched in a mixture of ammoniumnitrate, acetic acid and water without affecting the PMMAlayer. Similarly Au can be etched in a mixture of potassiumiodide, iodine and water. However, the standard etchingsolution of for Al containing orthophosphoric acid andmethanol etches PMMA, so it can not be used.

PMMA, however, does present a problem duringphotolithography. Since it is removed in acetone, and thepositive photoresist used in the photolithography is alsoremoved in acetone, an alternative treatment had to be doneto overcome this situation. After normal photolithography ofCr, the remaining photoresist that has to be eliminated was

exposed to the UV light. This UV exposure polymerizes theremainder positive photoresist so it can be removed from thewafer by means of normal KOH based developer solution.This technique has proven to give good results.

First, we fabricated two MIS structures (PM2 and PM4).We used the same conditions with the difference that PM2was baked only at 900 C for 20 minutes, while PM4 wasbaked first at 900 C for 20 minutes, followed by 20 minutesat 1200 C. Fig. 2 a and b show the I-V characteristic up to 20V. It is seen that the second procedure provides lower currentdensity than the first case, passing from a current densitybelow of 10-7 A/cm2, to a value around 10-8 A/cm2. Thesecurrent densities are higher than those characteristic ofthermal oxides, but are sufficiently low to allow the use ofthis material as a gate dielectric in applications as OTFTs.

Another characterization measurement was C-V curves.Through this measurement we could estimate the dielectricconstant and the interface states density. MIS structures weremade on a 11 Q-cm P-type silicon wafer to obtain C-Vcurves to analyze. P3HR1 sample was made with the same

process used in PM4 samples, since as already indicated,processes with two baking steps on the PMMA layer,showed better current density characteristic.

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Gate Voltage (\M

(a)

lo4-iN

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Gate Voltage (V)

(b)

Figure 2. Current densitiesof (a) PM2 with one step baking ofPMMA at90 °C and (b) PM4 samples with two step baking of the PMMA, one at 90

°C followed by a second at 120 'C..

C-V measurements were made at 1 MHz with a Boontoncapacimeter, model 72B. C-V curves for different devicesare shown in Fig. 3 where we can observe that the curve isdisplaced toward negative voltages around -30 V. Themaximum capacitance is around 4.8 pF at -40 V, and theminimum capacitance is around 3.2 pF at -12 V.

376

PM4

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01U

IV

-36 -34 -32 -30 -28 -26 -24 -22 -20 -18

Voltage (V)

The value obtained was CFB=4.47 pF.

In Fig. 3, it can be seen that the flat band voltage isaround -31.5 V. This gives a value for interface states densityof 1.6x1012 cm-2. This value is quite high for SiO2-Siinterface, and its effect on device characteristics should befurther analyzed, when the PMMA is used on top of asemiconducting organic film, in which case, the situationmay be worst.

The critical field was also estimated and resulted higherthan 2x106 V/cm.

-16 -14

Figure 3. Capacitance curve for high resistivity substrate.

The dielectric constant at 1 MHz was calculated as usual,through equation (1), which give us the maximum relativecapacitance:

maxxo

(1)

where:

so is the permittivity in vacuum

k, is the oxide dielectric constant

x0 is the dielectric thickness

The dielectric constant at this frequency was 2.6. Thevalues of the dielectric constant and permittivity for PMMApreviously reported for thick PMMA films are around 3.5-4.9 at 50 Hz and 2.7-3.2 at 1 MHz, [3]. At higher frequencies(4.74x1014 Hz or 633 nm) the permittivity can be obtainedthrough the optical refractive index according to k = n2 [4].In our case this corresponds to a value of 2.2.

Although driving capability of OTFTs using thisdielectric will be smaller that if SiO2 is used, the material canstill be used.

The flat band voltage was determined after calculatingthe value of the flat band capacitance using:

In table 1 we show the film and calculated parameters.

TABLE I. MIS DEVICE PARAMETERS

Thicknes 275 nm

Current dnity lo-18Acm21 ~~~~12-2ece states desit 1.62 x 10 cm

Refacie inde 1.49

Dieeti constant at M HM 2.6

DMI sat at 474 104H 2.2

Ressity >2 x 1015 Q cm

Critic fied 2 x 106 V/cm

III. CONCLUSIONSResults obtained indicate that PMMA films used as gate

dielectric have as advantages an easy deposition technique.They are resistant to several metal etches used in metalphotolithography. In spite of being soluble in acetone asmust organic materials used as photoresists, we show how tosolve this problem, in order to use normal photolithographicprocedures when this layer is used as gate dielectric.

The current density of the PMMA film was around 2x10-8 A/cm2 and its critical electric field above 2x106 V/cm. Itsdielectric constant at 1 MHz was 2.6, which is lower that thatof SiO2, but still attractive for OTFTs applications. Aninterface states density less than 2x1012 cm-2 can be obtained.This feature has to be further analyzed when the film is usedin conjunction with a semiconductor organic film.

REFERENCES

CFB (2)

where:

ks is the semiconductor dielectric constant

LD is the extrinsic Debye length

[1] J. Puigdollers, C. Voz, A. Orpella, R. Quidant, I. Martin, M. Vetterand R. Alcubilla, "Pentacene thin-film transistors with polymeric gatedielectric" Organic Electronics, vol. 5, pp. 67-71, 2004.

[2] Microlithography Chemical Corp., 1254 Chestnut St., Newton, MA02464.

[3] Tables of physical magnitudes, edited by I.K. Kikoina, Moskva-Atomizdat 1976, (in russian) page 324.

[4] G. R. Fowles, "Introduction to Modern Optics", Holt, Rinehart &Winston, New York, (1968).

377

4.8x1 0-'

4.4x1 0-'

4.0x1 0-'

3.6x1 0-'

3.2x1 0-12

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-40 -38