Institute of Nanotechnology, National Chiao Tung University, Taiwan (ROC)
Fully flexible metal-insulator-metal (MIM) capacitors fabricated on 25 �m thin polyimide (PI) sub-strates via the surface sol–gel process using 10-nm-thick zirconium-silicate (ZrSixOy � and hafnium-silicate (HfSimOn� films as gate dielectrics. The surface morphology of the ZrSixOy and HfSimOn
films were investigated using atomic force microscopy and scanning electron microscopy, whichconfirmed that continuous and crack-free surface growth had occurred on the PI. Both the filmstreated with oxygen (O2� plasma and annealing (ca. 250 �C) consisted of amorphous phase; con-firmed by X-ray diffraction. We employed X-ray photoelectron spectroscopy (XPS) at high resolutionto examine the chemical composition of the films subjected to various treatment conditions. Theshift of the XPS peaks towards higher binding energy revealed the O2 plasma-pretreatment followedby annealing was the most effective process to the surface oxidation at relatively low-temperature,for further passivate the grease traps and making dielectric films thermally stable. The ZrSixOy andHfSimOn films in sandwich-like MIM configuration on the PI substrates exhibited the low leakage cur-rent densities of 7.1×10−9and 8�4×10−9 A/cm2 at applied electric field of 10 MV/cm and maximumcapacitance densities of 7.5 and 5.3 fF/�m2 at 1 MHz, respectively. In addition, the ZrSixOy andHfSimOn films in MIM capacitors showed the estimated dielectric constants of 8.2 and 6.0, respec-tively. Prior to use of flexible MIM capacitors in advanced flexible electronic devices; the reliabilitytest was studied by applying day-dependent leakage current density measurements up to 30 days.These films of silicate-surfactant mesostructured materials have special interest to be used as gatedielectrics in future for flexible metal-oxide-semiconductor devices.
Faster flexible electronics technology is moving by leapsand bounds over the last several years. This has cre-ated new opportunities and challenges in the field offlexible electronics. Many efforts are ongoing toward thereplacement of the traditional rigid silicon and glass sub-strates with plastic substrates. Flexible electronics is fast-flowing towards main-stream applications where low-cost,ruggedness, light weight, unconventional form factors andease of manufacturability are just some of the impor-tant advantages over their conventional rigid-substratecounterparts.1–3 Organic flexible substrates, including thicklight-transparent polyethylene terephthalate (PET) plastic,poly-ether-sulfone (PES), elastomeric poly dimethylsilox-ane (PDMS) and polyimide (PI) have attractive growing
∗Author to whom correspondence should be addressed.
attention for the future flexible electronic devices. Thedevices fabricated over such plastic substrates are com-patible with continuous, high-speed reel-to-reel fabrica-tion techniques, transparent to visible/UV and allowingthe board to conform to a desired shape, or to flex dur-ing its uses.4–6 However, the additional steps and high-temperature processing in achieving high performancedevice limits the implementation to fabricate semiconduc-tor devices on these substrates.To solve the problem and partly the issue of decreas-
ing the transistor size, the thickness of the silicon dioxide,(SiO2� for a gate dielectric has been reduced in propor-tion to the shrinkage of the gate length. In some instances,a metal-oxide-semiconductor field effect transistor (MOS-FET) might be use a 1.5 �m thick SiO2 gate dielectriclayer for a gate length of 70 nm.7 Therefore, the futuredevices may require a gate dielectric layer of SiO2 about5 Å or less. But, such a small thickness requirement for
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a SiO2 layer creates additional problems such as cur-rent leakage across this thin layer. Thus, to enhance thedevice properties of such low thickness dielectrics, needfor advanced materials are being progressively focusedon composite systems. The embedded system of ternaryand binary materials can not only replace the SiO2 butsuppress the leakage current with very thin film. Theprimary advantage of these materials over SiO2 is thatthey offer significantly improved capacitance densities andsimultaneously low leakage currents, as these parame-ters are the key factors in determining the performanceand functionality of metal-oxide-semiconductor (MOS)devices. Moreover, enable to overcome problems relatedto the use of pure oxides, such as high cost and low spe-cific surface area. In particular, hafnium and zirconiumoxides based compounds are practical high-k dielectricsoffer as an alternative gate dielectric layer to replace theSiO2.
8�9 Ultra thin high-k dielectric films using zirconium-silicate (ZrSixOy� and hafnium-silicate (HfSimOn� providean increasing dielectric constant (k ∼ 10) comparing toSiO2 (k = 3�8) that influences the electrical performanceof the electronic devices. However, ZrSixOy and HfSimOn
have lower dielectric constants compared to its respectivepure oxides but acceptable as long as the resulting leakagecurrents are low enough.There are several existing approaches to developed
metal oxide film, including dc sputtering,8 atomiclayer deposition (ALD),10 plasma-enhanced chemicalvapor deposition (PECVD),9 and metal organic chemicalvapor deposition (MOCVD).11 Sol–gel spin-coating is avery efficient approach toward smooth, crack-free filmsexhibiting excellent surface conformity and uniformityover large area.12�13 The sol–gel method can mix variouscolloidal solvents and precursor compounds when metalhalides are hydrolyzed under controlled conditions. Thethin films are produced on a substrate by spin-coating ordip-coating; i.e., a small puddle of the fluid resin is placedat the center of a substrate, which is then spun at highspeed. However, the dielectric films deposited at low tem-perature performs poorer properties and large current leak-age due to numerous traps present inside the film. Thus, achallenge remains to develop a promising method to over-come these processing limitations. Reportedly, the oxygen(O2� plasma treatment affects the performance of thin filmsdeposited at low temperature via the sol–gel process.13�14
The electrical properties of such films can improve consid-erably after O2 plasma exposure, with enhanced remnantpolarization and decreased leakage current density.In the current study, we have developed a low-
temperature (ca. 250 �C) O2 plasma enhanced method forpreparing ZrSixOy and HfSimOn thin films-based MIMcapacitors fabricated on a flexible organic PI substrateusing sol–gel spin processing and insulating propertieshave been evaluated. The insulating properties of MIMdevices prepared employing ZrSixOy and HfSimOn films
as a dielectric layers exhibited low leakage current den-sities and maximum capacitance densities. The reliabilitytest was also studied by analysis day dependent leakagecurrent density measurements for 30 days. We believe thatZrSixOy and HfSimOn films would be leading candidatesfor use in future flexible MOS devices as a stable gatedielectric.
2. EXPERIMENTAL DETAILS
2.1. Materials and Device Fabrication
Prior to fabrication process, plastic 25 �m-thick DuPontKapton® PI films were used as flexible substrates. Theywere polished and cleaned ultrasonically with ethanol(Fluka; water content: < 0�1%) for 30 min and deionizedwater and then high-pressure N2 gas was used to removethe water and any remaining surface particles from the PI.Next, Cr (thickness: 10 nm; adhesion layer) and Au (thick-ness: 20 nm) were deposited for the gate electrode over thePI substrate using a thermal coater. To deposit ZrSixOy thinfilm, a sol–gel solution was prepared by dissolving ZrCl4(98%, Aldrich, USA) and SiCl4 (99.5%) in ethanol as thesolvent to yield a molar ratio of 1:1:1000, and to depositHfSimOn film dissolving HfCl4 (98%, Aldrich, USA) andSiCl4 (99.5%) in ethanol as the solvent to yield a samemolar ratio of 1:1:1000; after adding a magnetic stirrer,the solutions were heated at 40 �C under reflux while stir-ring for 30 min to mixed-up the solutions properly. Thefilms were grown by spin-coating the sol–gel solution overPI at 3000 rpm for 30 s at room temperature using aClean Track Model-MK8 (TEL, Japan) spin coater. Theas-prepared samples were carried out with O2 plasma treat-ment (OPT) for 2 min in oxygen plasma reactor (HarrickScientific Corp.), which supplied a plasma power of 30 Wand then subsequent annealing (A) in the presence of O2
at 250 �C for 12 h (refer, OPT/A). Finally, 200 nm-thickAl films were pattered as the top electrodes using shadowmask and a thermal coater.
2.2. Characterization
The surface morphology of the ZrSixOy and HfSimOn
films over Cr/PI was evaluated using scanning electronmicroscopy (FE-SEM, JOEL JSM-5410, operated at 5 kV)and atomic force microscopy (AFM, Digital InstrumentsNanoscope, D-5000) at a scan size of 2 �m and a scanrate of 1 Hz. We used ellipsometery techniques to mea-sure the thickness of the ZrSixOy and HfSimOnfilms. Thecrystal structures were characterized by X-ray photoelec-tron X-ray difractometry (XRD). The influence of oxygenplasma treatment on surface properties of Samples wasanalyzed by X-ray photoelectron spectroscopy (XPS). Tocharacterize the leakage currents and capacitances of theseinsulator films, we prepared them in metal-insulator-metal
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(MIM) configuration as represented in Figure 1(a). Fur-ther, Figure 1(b) shows an atomic model of several chainportions of the crystalline structure for zirconium-silicate(or hafnium-silicate) as described by Wilk.15 Which indi-cated that each Zr (or Hf) atom also shares bonds withother O atoms in neighboring chains (only two of the fourbonds to neighboring chains are shown for each Zr (or Hf)atom, for clarity), providing a three-dimensional stabilityto the material. Chemical analysis of homogeneous sili-cate films is therefore expected to show Zr–O (or Hf–O)and Si–O nearest-neighbor bonding. Furthermore, the leak-age current density and electric fields (J–E) measurementswere performed using an Agilent-4156 probe station; thecapacitance was measured using an HP-4284A C–V ana-lyzer. Figure 1(c) displays one of a photograph of a sampleon fully flexible PI substrate under a large surface strain,indicating many capacitor devices but without any cracksappearing on the surface.
3. RESULTS AND DISCUSSION
3.1. Surface Morphology of ZrSixOy and HfSimOn
Films
The ZrSixOy and HfSimOnfilms were prepared, when thesol–gel mother solutions of both the materials were spun-coated over the Cr coated flexible PI substrates. One of
(a)
(b) (c)
Fig. 1. (a) Schematic representation of an MIM capacitor featuring a ZrSixOy or HfSimOn thin film on a PI substrate, (b) Atomic model of severalchain portions of the crystalline structure for zirconium-silicate and hafnium-silicate as described by Wilk ∼see Ref. [14] (refer M==Zr or Hf). Dashedlines represent bonds between Zr or Hf atoms of a given chain and O atoms of neighboring chains, which provide a three-dimensional stability to thestructure, (c) Photograph of our MIM capacitor on the flexible ultra-thin PI substrate.
the major reliability issues for electronic devices is interfa-cial debonding driven by stresses in multilayer structures.These include both thermal stresses induced during oper-ation and/or cooling after processing and residual intrin-sic stresses created during deposition. Thus, Cr was usedto make firm adhesion between PI16 and sol–gel derivedsilicate films. Figures 2(a and b) present top view SEMimages of the as-deposited ZrSixOy and HfSimOn filmsafter baking at 80 �C. Many trap states are present overboth film surfaces, suggesting that they would directlyaffect the electrical performance of the capacitor devices.When O2 plasma treatment was employed for 2 min onthe as-grown sol–gel films and then annealed in presenceof O2 at 250 �C for 12 h (i.e., OPT/A), the clean surfaceswere generated. Figures 2(c and d) displays the top viewSEM images; which indicate the well-ordered, smooth andcrack-free surfaces of ZrSixOy and HfSimOn films over thePI substrate. Thus, the ZrSixOy and HfSimOn films over thePI substrates had occurred uniform morphological surfacesafter OPT/A-treatment condition. The surface roughness ofthe insulator layer is another important factor affecting theperformance of the MOS devices. Here, we used tapping-mod AFM on a length scale of 2 �m× 2 �m to deter-mine the surface roughness of the ZrSixOy and HfSimOn
films. Figures 3(a and b) indicates the surface roughness ofthe ZrSixOy and HfSimOnfilms’ surfaces were of 1.82 and
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Fig. 2. Top-view SEM images of sol–gel deposited ZrSixOy and HfSimOn films on Cr/Au/PI substrates after as-deposited then baking (a, b) and(c, d) after the OPT/A-treatment condition.
1.90 nm, respectively. The thicknesses of the ZrSixOy andHfSimOn films subjected to OPT/A-treatment were mea-sured by ellipsometery techniques to be 10 and 10.5 nm,and the refractive indexes were 1.84 and 1.62 respectively.Low amount of the zirconium and hafnium contents andaddition of silicon decreased the refractive indexes thantheir native oxides.
3.2. XRD Data
Figure 4 displays the XRD patterns of the ZrSixOy andHfSimOn films subjected to OPT/A-treatment condition.
Fig. 3. Tapping-mode AFM image of ZrSixOy (a) and HfSimOn (b) films on Cr/Au/PI substrates subjected to OPT/A-treatment condition.
Crystalline peaks originated from both the films werehardly observed, indicating that the dominant phase of thefilms were amorphous. This was also supported by the topview SEM image and the plane view AFM micrographs ofthe films. These results suggest that the samples have highthermal stability and that O2 plasma treatment for 2 minand then annealing at 250 �C under O2 atmosphere doesnot result in the crystallization of as-deposited ZrSixOy
and HfSimOn films. In general, it is found that most of thegate dielectrics to date are either polycrystalline or sin-gle crystal films, but it is desirable to select a materialwhich remains in glassy phase (amorphous) throughout
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20 30 40 50 60
2θ (degree)
HfSimOn
ZrSixOy
XRD-analysis
Inte
nsity
(a.
u.)
70 80 90
Fig. 4. XRD plots for ZrSixOy (a) and HfSimOn (b) films subject toOPT/A-treatment.
the necessary processing treatments. Polycrystalline gatedielectrics may be problematic because grain boundariesserve as high leakage paths, and this may lead to theneed for an amorphous interfacial layer to reduce leak-age current. In addition, grain size and orientation changesthrough out a polycrystalline film can cause significantvariations in k, leading to irreproducible properties. Thesesilicate films grown by sol–gel method can avoid grainboundaries while providing a good dielectric surface at lowtemperature processing, but, which may only be obtainedafter plasma treatment. Here, the given concerns regard-ing polycrystalline and single crystal dielectric films, itappears that an amorphous film structure is the ideal onefor flexible electronics as the stable gate dielectric, likeZrSixOy and HfSimOn.
3.3. XPS Analysis
The influence of O2 plasma treatment on surface prop-erties of the ZrSixOy and HfSimOn films was analysizedby XPS measurement. Considering, the as-deposited filmexisted in the wet-state, presumably with a –HO–MSi–O–MSi–OR– structure based surface, (refer, M=Zr or Hf).It appeared that a homogeneous network of –O–MSi–O–MSi–O– bonds had not developed on the film surface.Here, R group included the ethyl group (i.e., –C2H5–)and which is expected the combined organic impurities.13
Therefore, O2 plasma treatment is mainly caused to oxi-dize the as deposited films and then etch the carbon impu-rities from the surfaces. There would be a direct interactionof active atoms, ions and molecules in the plasma (O+,O− O, O2, free electrons, ions, etc.) with organic speciesavailable on the film surface.13 The imposing plasma grad-ually oxidized the as deposited thin films, the film surfaceswere near-complete oxidation to and the organic parts werecompletely removed. Figure 5(a) displays high resolution
(a)
(b)
Fig. 5. High-resolution XPS spectra of the (a) Zr 3d energy levels ofzirconium-silicate for as-deposited then baking and OPT/A-treatment,(b) Hf 4f energy levels of hafnium-silicate for as-deposited then bakingand OPT/A-treatment condition.
spectra of Zr 3d for zirconium-silicate film, consists of thespin-orbit doublet peaks of Zr 3d5/2 and Zr 3d3/2 at differ-ent binding energies (BEs) of Hf–O bonding in the vicin-ity of Silicate. For the backing-only treated surface, thepeaks for Zr 3d5/2 and Zr 3d3/2 were detected at 182.3 and183.8 eV, respectively. After the OPT/A-treatment, the BEsof Zr 3d5/2 and Zr 3d3/2 were raised to 183.4 and 184.9 eV.The shift towards higher binding energies of ZrSi–O bondsmean that oxygen-plasma introduced some strong polargroups (such as –O– ZrSi –O– ZrSi –O–) to the com-plete oxidation of the Zr-silicate atoms. The literatures15�17
suggest the Zr 4f peak shift towards higher BE for thezirconium-silicate film can be caused by the formation ofZr–O bonds in the vicinity of Silicate. The literature15 alsosuggests the similar Hf 4f peaks for the hafnium-silicatefilm as indicated in Figure 5(b). Specifically, as the filmis treated under the OPA/A-treatment condition, the shiftfor 4 f7/2 peak from 17.4 (as-dep) to 18.0 eV (OPT/A)and for 4 f5/2 from 19.2 (as-dep) to 19.6 eV (OPT/A);the enlargement of peak height are observed. You et al.,12
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reports the peak shift to higher BE for HfSimOn and can becaused by the formation of Hf–O bonding in the vicinityof Silicate. Thus, these relationships of bonding betweenHf–O suggest that Hf atoms are bonded to O atoms asnearest neighbors. Both the BEs of Zr 3d and Hf 4f, shownin Figures 5(a and b) exhibit the shifting to the higherbinding energies upon OPT/A-treatment. This observationsuggests that the oxygen atoms of Zr–O and Hf–O bondsreacted with their nearby Si atoms, forming hafnium andzirconium silicate.
3.4. Electrical Properties
We measured the quantitative leakage current and capac-itance to evaluate the electrical performance of theZrSixOy and HfSimOn films in MIM configuration. The as-deposited samples as shown in Figure 6(a); were bakedat 80 �C did not have sufficiently high thermal budgetsand, thus, their breakdown electric fields were relatively
0 1 2 3 4 51E-9
1E-7
1E-5
1E-3
0.1
10
Electric field (MV/cm)
Leak
age
curr
ent d
ensi
ty (
A/c
m2 )
Leak
age
curr
ent d
ensi
ty (
A/c
m2 )
OPT/A - ZrSixOy
OPT/A - HfSimOn
As-dep - ZrSixOy
As-dep - HfSimOn
1.00.0 0.2 0.4 0.6 0.81E-6
1E-4
0.01
1
100(a) (b)
Electric field (MV/cm)
(c) (d)
Fig. 6. (a) Plots of leakage current density (J) versus electric field (E) under an applied positive voltage for samples prepared using OPT/A-treatmentcondition; (b) plot of J–E for samples under as-deposited then baking-only treatment condition (c) Plot of ln(J) versus the square root of electric field(E1/2� for the OPT/A-treated samples and (d) Plot of ln(J/E) versus (E1/2� for the as-deposited samples. The corresponding schematic energy banddiagram is presented to explain the S-R and P-F emissions.
low and their leakage current densities were very highi.e., 5�7×10−5 (ZrSixOy� and 7�0×10−6 A/cm2 (HfSimOn�at electric field of 1 MV/cm. Figure 6(b) displays theleakage current density and electric field (J–E) charac-teristics for the flexible MIM capacitors prepared usingZrSixOy and HfSimOnfilms under various sample treat-ment conditions. Next, the superior low leakage currentdensities for the corresponding films subjected to OPT/A-treatment were found of 7�1×10−9 and 8�4×10−9 A/cm2
at 10 MV/cm, respectively. The baking-only treated filmsexhibited the large leakage currents among these treatedfilms because it had poor dielectric characteristics andnumerous traps present within the films. The leakagecurrent density decreased when the sample was treatedwith OPT/A-treatment condition; indicating that the poorerleakage properties of the other two treated samples arosebecause of the existence of numerous traps over their filmsurfaces. The breakdown electric field (ca. ∼5.0 MV/cm)also increased when the sample was subjected to the
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OPT/A treatment condition. The OPT/A-treated sampleexhibited excellent electrical characteristics on the PI sub-strate because (i) the wet film underwent a high degreeof oxidation under O2 plasma treatment and (ii) subse-quent annealing led to a reduction in the number of traps.The low leakage current for our flexible MIM capacitors iscomparable with that of silicon- and glass-based capacitordevices.15�18
To understand the carrier transport mechanisms ofthe OPT/A-treated ZrSixOy and HfSimOn dielectric films,Figure 6(c) presents a plot of ln(J) with respect to thesquare root of the applied electric field (E1/2�. For stan-dard Schottky–Richardson (SR) emission, the plot of ln(J)versus E1/2 should be linear; can be expressed as19
J = A∗T 2 exp[−q��B −
√qE/4��r�0�
KT
](1)
Where A∗ is the effective Rechardson constant, q�B is theSchottky barrier height, �0 is the permittivity in a vacuum,and and �r is the dynamic dielectric constant. SR emis-sion induced by the thermionic effect is caused by electrontransport across the potential energy barrier, as indicatedin the inset to Figure 6(c); it is independent of traps anddominates the conduction mechanism.20 The conversion ofthe current transport mechanism from trap-assisted tun-neling for the as-deposited then baking-only samples toSR emission for the OPT/A sample demonstrates theo-retically that the sol–gel deposited ZrSixOy and HfSimOn
films were oxidized and traps were completely terminated.Figure 6(d) presents the plots of ln(J/E) versus E1/2 forthe baking only-treated ZrSixOy and HfSimOn films; theinset displays a schematic energy band diagram that eluci-dates the leakage transport mechanisms. It is believed thatthe Poole–Frenkel (PF) emission is due to field-enhancedthermal excitation of trapped electrons in the bulk of theinsulator. The conduction process at higher voltages islikely due to the PF emission, which is described by theequation21
J = CE exp(−q�t
kT
)exp
[1
rkT
√q3
��0 KT
√E
](2)
where C is a constant, q�k� T , and E represent the elec-tronic charge, the Boltzmann constant, the temperature,and the electric field, respectively, �0 denotes the permit-tivity of free space, KT is the high-frequency dielectricconstant (square of the refractive index), and �t is theenergy barrier separating the traps from the conductionband. The coefficient r is introduced in the expression totake into account the influence of the trapping or accep-tor centers (1 ≤ r ≤ 2). As a result, the plot of ln(J/E)as a function of E1/2 in Figure 6(d) reveals that the as-deposited then baking only samples possessed huge num-bers of traps, which decreased the band gap ZrSixOy andHfSimOn films because the thermal budget was insufficient
to form dense and trap-free dielectric layers. For the as-deposited sample, the linear dependence began at a verylow electric field (ca. 0.2 MV/cm). The traps within theZrSixOy and HfSimOn films were not reduced after baking-only treatment; the devices featured the high leakage cur-rents and low breakdown electric fields. Under treatmentwith O2 plasma, the PF emission was gradually restrained.Finally, the current transport mechanism was replaced bySR emission after OPT/A treatments to the as-grown film.These findings confirm that the number of traps was min-imized within low temperature deposited binary ZrSixOy
and HfSimOn films after the plasma treatment.Figure 7(a) displays the capacitance-voltage (C–V)
characteristics of the MIM capacitors under differenttreatment conditions. The maximum capacitance densi-ties for as-deposited films were of 1.4 and 0.4 fF/�m2
at 1 MHz to the ZrSixOy and HfSimOn films, respec-tively. The estimated dielectric constants using capaci-tance and the thickness data for as-deposited films were3.3 and 2.2 respectively. Again, it was found that the
10(a)
(b)
8
6
Cap
acita
nce
dens
ity (
fF/µ
m2 )
4
2
0
–2
–8 –6 –4 –2
Voltage (V)
0 2 4 6
OPT/A - ZrSixOy
OPT/A - HfSimOn
As-dep ZrSixOy
As-dep HfSimOn
10
8
6
Die
lect
ric c
onst
ant (
K)
4
2
0
ZrSixOy HfSimOn
As-dep
OPT/A
Fig. 7. (a) Plot of capacitance density (C) as a function of the appliedvoltage (V) for the OPT/A-treated capacitor device, (b) The dielectricconstant of ZrSixOy and HfSimOn films for as-deposited then baking andOPT/A-treatment conditions as a function of the temperature.
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OPT/A-treatment is feasible and easy process in low pro-cessing temperature to enhance the capacitance propertiesof the flexible MIM capacitor devices. To the corre-sponding films subjected to OPT/A-treatment, the max-imum capacitance densities were estimated of 7.5 and5.3 fF/�m2(OPT/A) at 1 MHz. In addition, accordingto the capacitance and the thickness data, we estimatedthe dielectric constants (k� to OPT/A-treated ZrSixOy andHfSimOn films were found to be 8.2 and 6.0, respectively.Those are very close to the original dielectric constantsof zirconium-silicate and hafnium-silicate material films.However, the addition of silicate element actually lowersthe dielectric constant of pure ZrO2 or HfO2 films. But,the addition of metal element, improves the film qual-ity of high-k oxides without increasing their leakage cur-rents. Thus, one approach which has been investigated isthe use of materials with dielectric constants higher thanthat of SiO2 materials as gate dielectric materials, wherebythe ZrSixOy and HfSimOn dielectric materials with dielec-tric constants of about 8.2 and 6.0 or above, replace theconventional SiO2-based low-k dielectric materials withdielectric constants of about 8 or below. An increasedamount of temperature was also found by means of ther-modesorption and results in an increasing dielectric con-stant. The k-value increases with oxygen plasma treatmentare illustrated clearly in Figure 7(b) which can be as highas 8.2 for ZrSixOy and 6 for HfSimOn followed by anneal-ing at 250 �C. For both the films the dielectric constantincreases with OPT/A-treatment. We have compared thedielectric constant for further confirm the leakage currentmechanism. The dielectric constant increases, expectablythat may be due to chain segment mobility increases andpolar groups in silicate material start to move in responseto the applied electrical filed which enhances the dielectricproperties.22
The electrical properties for both films indicate theZrSixOy and HfSimOn materials exhibit superior insulatingproperties on the flexible PI substrate compared with pre-viously reported high-k and SiO2/Al2O3 films fabricatedon silicon substrates (Table I).23–29 For flexible capaci-tive components, which form the basis of many elec-tronic devices, the high temperature processing limits the
Table I. Comparison of electrical properties with high-k dielectric materials and Al2O3–SiO2 films.
Reference Dielectric film Film thickness (nm) Dielectrics constanta Leakage Current Density (A/cm2� Substrate Growth Temperature (�C)
aCalculated from equation Ci = ��kA/d (where k is the dielectric constant, �� the vacuum permittivity, A the capacitor’s surface area and d the insulator thickness),bRoom-temperature.
degree of conventional fabrication; this limit is now beingapproached for the materials currently in use. Therefore,high dielectric constant films, such as ZrO2, HfO2, titanateetc are invariably incorporate chemical elements to currentmicroelectronics fabrication procedures, and must passextensive compatibility tests before they can be used com-mercially. In respect to compatibility, the present silicatefilms prepared by sol–gel technique are considered morepromising (although its dielectric properties are lowestthan ZrO2 or HfO2 films), and it is known to form high-quality thin films in conventional fabrication processes.
3.5. Day-Dependent Reliability Test
Finally to evaluate the environmental reliability test forboth the silicate films over PI, the day-dependent leakagecurrent density measurements were carried out at roomtemperature. Figures 8(a and b) displays day-dependentleakage current behavior of ZrSixOy and HfSimOn filmsbased MIM capacitors. Day-dependent leakage currentdensity was measured at applied 10 V, for duration of 10,20 and 30 days. As displayed in Figure 8(a), the leak-age current densities of ZrSixOy film after 10, 20 and30 days are 2�4×10−9, 1�7×10−11 and 9�6×10−10 A/cm2,respectively. The leakage current densities, as shown inFigure 8(b) of HfSimOn film after 10, 20 and 30 daysare of 1�6× 10−10, 1�6× 10−9 and 2�9× 10−9 A/cm2,respectively. Thus, this small difference in leakage currentdensities of both the silicate films means that our MIMcapacitors have an excellent reliability in normal envi-ronment after one month and able to use for advancedmicroelectronic devices for a long period of time. Flexi-ble capacitor devices also have to be bendable and wearresistant properties in order to survive service conditions.The mechanically-stretchable properties and the frictionand wear characteristics of the MIM capacitor devices onthe PI substrates were experimentally analysed and com-pared and a link between mechanics and macro-scale wearis being established in our previous study.30 Although, theflexible displays and touch screens are becoming increas-ingly important, there is very little work currently done onmechanical and time-of-day dependent reliable properties,
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0 1 2 3 4 51E-10
1E-8
1E-6
1E-4
0.01
1
100
Electric field (MV/cm)
Leak
age
curr
ent d
ensi
ty (
A/c
m2 )
For ZrSixOy dielctric
10 Days20 Days30 Days
(a)
1E-10
1E-8
1E-6
1E-4
0.01
1
100
Leak
age
curr
ent d
ensi
ty (
A/c
m2 )
(b)
0 1 2 3 4 5 6
Electric field (MV/cm)
For HfSimOn dielctric
10 Days20 Days30 Days
Fig. 8. Plots of day-dependent leakage current densities (J) versus elec-tric field (E) for (a) ZrSixOy and (b) HfSimOn films.
either of substrates or of substrates coated with transparentconducting coatings.
4. CONCLUSION
We have successfully investigated the intrinsic dielectricand surface properties of sol–gel deposited ZrSixOy andHfSimOn films on the fully flexible polyimide substrate.The corresponding films prepared in metal-insulator-metal(MIM) structure as dielectric layer exhibited low leakagecurrent densities of 7�1× 10−9 and 8�4× 10−9 A/cm2 atapplied electric field of 10 MV/cm and maximum capac-itance densities of 7.5 and 5.3 fF/�m2 at 1 MHz, respec-tively. The chemical composition of these correspondingsilicate films were characterized by X-ray photoelectronspectroscopy measurement. In addition, the ZrSixOy andHfSimOn films in MIM configuration showed dielectricconstant of 8.2 and 6.0, respectively. This has been provento showed stable performance over the course of onemonth, making them suitable candidates for use in elec-tronic devices as a stable gate dielectric. The surface prop-erties and electrical performance of these films verified the
effectiveness of applying low temperature plasma process-ing. This could be a useful technique in the fabricationof flexible devices, where the film development is verydifficult at high temperature. It is expected that the com-binatorial ZrSixOy and HfSimOn films would be a lead-ing candidate for use in future flexible MOS devices as agate dielectric layer fabricated through processing at lowtemperature.
Acknowledgments: This investigation was generouslysupported by funds provided by National Science Coun-cil of Taiwan (97-2120-M-009-003 and 97-2218-E-009-002). The authors also would like to thank the NationalNanoDevice Laboratories, Taiwan for their immensesupport.
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