IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013 3305

Flexible High-κ/Metal Gate Metal/Insulator/MetalCapacitors on Silicon (100) Fabric

Jhonathan Prieto Rojas, Student Member, IEEE, Mohamed Tarek Ghoneim, Student Member, IEEE,Chadwin D. Young, Senior Member, IEEE, and Muhammad Mustafa Hussain, Senior Member, IEEE

Abstract— Implementation of memory on bendable substratesis an important step toward a complete and fully developednotion of mechanically flexible computational systems. In thispaper, we have demonstrated a simple fabrication flow to buildmetal–insulator–metal capacitors, key components of dynamicrandom access memory, on a mechanically flexible silicon (100)fabric. We rely on standard microfabrication processes to releasea thin sheet of bendable silicon (area: 18 cm2 and thickness:25 µm) in an inexpensive and reliable way. On such platform,we fabricated and characterized the devices showing mechanicalrobustness (minimum bending radius of 10 mm at an appliedstrain of 83.33% and nominal strain of 0.125%) and consistentelectrical behavior regardless of the applied mechanical stress.Furthermore, and for the first time, we performed a reliabilitystudy suggesting no significant difference in performance andshowing an improvement in lifetime projections.

Index Terms— Bending curvature, flexible, high k, metal gate,metal–insulator–metal capacitors (MIMCAPs), silicon (100).

I. INTRODUCTION

DYNAMIC random access memory (DRAM) is a keycomponent in any modern electronic gadget. Their per-

formance is closely associated to their ability for storing andretaining charges, which is directly related to a high storagecapacitor and low leakage current. That leads to the needof large areas to integrate capacitors for storing the informa-tion. To optimize the real estate usage, metal/insulator/metalcapacitor (MIMCAP) is a great alternative to achieve highercapacitances per unit area without compromising leakagecurrent. The technology roadmap for 65-nm-node DRAM andbelow suggests the introduction of MIM structures and high-κmaterials as a solution to overcome the scaling-trend-relatedchallenges in semiconductor technologies [1].

Simultaneously, to expand the horizon of emerging flexibleelectronics, which potentially will draw increased consumer

Manuscript received April 25, 2013; revised July 19, 2013; acceptedAugust 8, 2013. Date of publication August 26, 2013; date of current versionSeptember 18, 2013. This work was supported in part by King AbdullahUniversity of Science and Technology Office of Competitive Research Fundand in part by the Competitive Research under Grant CRG-1-2012-HUS-008.The review of this paper was arranged by Editor H. Shang.

J. P. Rojas, M. T. Ghoneim, and M. M. Hussain are with the IntegratedNanotechnology Laboratory, Electrical Engineering Program, King AbdullahUniversity of Science and Technology, Thuwal 23955-6900, Saudi Ara-bia (e-mail: jhonathan.rojas@kaust.edu.sa; mohamed.ghoneim@kaust.edu.sa;muhammadmustafa.hussain@kaust.edu.sa).

C. D. Young is with the Department of Materials Science andEngineering, University of Texas, Dallas, TX 75080 USA (e-mail:chadwin.young@utdallas.edu).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2013.2278186

attention, several efforts and different approaches have beenmade to achieve a bendable and reliable memory system. Onone side, polymer-based devices can attain excellent flexibilitybut there is still a lot of room for improvement in theirperformance compared with the state-of-the-art semiconductortechnologies-based memory devices [2]–[4]. In addition, tem-perature stability limitations of organic and polymeric materi-als will restrain the use of certain materials, techniques, andapplications requiring higher thermal budgets. Still, excitingadvancements have been attained through transfer printingtechniques of inorganic semiconductor materials for deviceimplementation onto plastic substrates. Using such techniques,it is possible to take advantage of both components, theoutstanding electrical performance from semiconductors andthe reliable mechanical stability of plastics [5]–[8]. Recently,Kim et al. [9] have demonstrated a memristive-based flex-ible memory array through the aforementioned approach.They achieved functional RAM operations without electricalinterference. Nevertheless, the ultrahigh density and circuitcomplexity needed for high-performance applications; cur-rently achieved through meticulous alignment and patterningtechniques for nanometric dimensions, is yet to be reached bytransfer techniques. Therefore, it is naturally a next step toadvance the present trend in flexible electronics championedby Rogers et al. by expanding it to the most conventionalsilicon (100) substrate, which is also popular as more econom-ical substrate than silicon-on-insulator (SOI) or silicon (111)substrates.

Other recent implementations based on silicon consist ofthe deposition of a releaser thin film on an already processedwafer, thus creating a stress mismatching, and then peel-off by mechanical force. Static RAM functionality has beendemonstrated through this technique, along with complemen-tary metal–oxide–semiconductor (CMOS) devices [10], [11].However, the use of expensive substrates, such as ultrathinbody SOI, and the fact that the remaining wafer after releaseis not used further, makes it an expensive option. In addi-tion, they have limited bendability, transparency, and potentialdamage probability related to the peel-off process, like otherdemonstrations that depend on mechanical approaches, suchas back grinding.

MIMCAP devices on flexible platform have been previ-ously reported by employing organic and graphene oxide-based materials [12]–[14]. Good bendability (10–20-mm min-imum bending radius) and robust electrical behavior aredemonstrated. However, as mentioned before, incompatibilitywith several processes normally used in industry’s ultrascaled

0018-9383 © 2013 IEEE

3306 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013

electronics hinders their implementation for high-performanceand harsh-environment applications.

We have previously demonstrated the fabrication and char-acterization of metal–oxide–semiconductor capacitors on flex-ible silicon (100) substrate [15], [16]. To extend our study andprepare our platform for memory technologies, we report inthis paper a simple fabrication processing of MIMCAPs withaluminum oxide (Al2O3) as high-κ dielectric and tantalumnitride (TaN) as metal electrodes on flexible and semitranspar-ent silicon (100) fabric. The techniques and set of materialsare completely compatible with industry’s CMOS technology.

The dimensions of the silicon fabric are quite large,6 cm×3 cm×25μm, allowing the allocation of several hun-dreds of capacitors. We compared reliability and electricaldata from devices fabricated on flexible silicon fabric todevices fabricated on conventional silicon and found onlyslight variations. To the best of our knowledge, this is the firsttime reliability analysis is performed on flexible silicon-baseddevices.

Excellent mechanical flexibility, optical semitransparency,and reliable electrical behavior represent the pragmatic steptoward the achievement of high-performance flexible memoryand computation. As of today, the most advanced memorydevices are fabricated on silicon because form factor (F2)is considered as one of the key factors for ultrahigh densitymemory. Ultralarge scale integration is carried out on siliconusing semiconductor industry’s advanced high-resolution litho-graphic tool. Therefore, it is critical to initiate research anddevelopment to transform existing silicon (100)-based devicesand circuits into flexible ones.

II. FABRICATION PROCESS

Capacitors were fabricated on a flexible silicon (100) fabric.The first part of the fabrication involves making the flexiblesilicon (100) fabric. We started with a highly doped 4-in silicon(100) wafer and performed thermal oxidation to produce400 nm of SiO2. A highly doped wafer was chosen becausewe planned to improve back contact during probing. Next,we proceeded with oxide patterning to form etching holes[Fig. 1(a)]. This was followed by DRIE to create verticalchannels for a specific depth that will determine the finalthickness of the flexible substrate. Then, we formed oxidespacer-like protection for the sidewalls of the vertical channelsusing thermal oxide and a very anisotropic RIE [Fig. 1(b)].XeF2 gas was used to isotropically etch the silicon underneaththe channels thus releasing a thin sheet of silicon [Fig. 1(c)].A detailed description of the releasing step of the process canbe found in [17].

The second part of the fabrication involves making thecapacitors on the flexible silicon fabric. Initially, the protectiveoxide layers were removed with buffered oxide etchant. Next,we used an atomic layer deposition system, which producesgreat quality films with high thickness accuracy, to deposit20 nm of TaN, as a bottom electrode, followed by 10 nmof Al2O3, as high-κ dielectric, and a second 20 nm ofTaN as a top electrode. A final 200 nm aluminum layerwas deposited by sputtering to form the top metal contact.

Fig. 1. Fabrication process summary of MIMCAPs on flexible silicon (100)fabric. (a) Thermal oxidation and oxide patterning. (b) Deep Reactive IonEtching DRIE and oxide spacers formation. (c) Relase with XeF2 isotropicetch. (d) MIM stack deposition and patterning by photolithography and RIE.

1 cm

a) b)

Fig. 2. (a) Flexed silicon fabric with MIMCAPs showing a minimumbending radius of 1 cm. (b) Sample on the top of a LED screen showingsemitransparency.

Finally, photolithography and RIE were used to define thecapacitors geometries [Fig. 1(d)]. Photoresist (PR) spin coatingis especially delicate due to the wavy nature of the releasedflexible silicon sheet. Nonuniformity in the PR thickness mightlead to uneven etching results during the aluminum metalpatterning.

III. MECHANICAL AND ELECTRICAL CHARACTERIZATION

For mechanical characterization, we bent our sample to aminimum bending radius of 10 mm (maximum curvature of∼133.3 m−1) [Fig. 2(a)] demonstrating high flexibility for aninorganic material, even being as flexible as previous organicdemonstrations [12], [13].

Defining dL as the horizontal displacement needed toachieve a specific bending radius and L as the initial lengthof the sample, we can estimate the applied strain from thefollowing expression [18]:

εAPPLIED = dL/L. (1)

Simultaneously, defining t as the sample’s thickness and Ras the bending radius, we can extract the nominal strain ontop of the sample from [18]

εNOMINAL = t/2R. (2)

Applying (1), we obtain an applied strain of 83.33% andfrom (2), we get a nominal strain of 0.125% for a minimumbending radius of 10 mm.

ROJAS et al.: FLEXIBLE HIGH-κ /METAL GATE MIMCAPs 3307

Fig. 3. (a) Normalized capacitance versus voltage plot. Inset: histogram ofeffective oxide thickness variation over the 18-cm2 sample. (b) Normalizedcurrent density leakage at room temperature showing remarkably low values.

In addition, we showed semitransparency; an effect pro-duced because of the holes used for the release process[Fig. 2(b)]. We have estimated the average light transmittancewith a spectrophotometer in the visible spectrum as 2.24%[Fig. 4(b)]. Even though it is a modest value, the merit goesto the fact that silicon is intrinsically an opaque material unlikeother inorganic materials such as zinc oxide, indium–tin oxide,and so on.

For electrical characterization, we measured thecapacitance–voltage (C–V ) characteristics using a probestation and LCR meter (bottom substrate biasing). Fig. 3(a)shows the C–V characteristics at 100 kHz suggesting goodlinearity and an average dielectric thickness of 11.7 nm [insetin Fig. 3(a)].

Thickness of the high-k dielectric can be easily extractedfrom the parallel-plate capacitor equation and is given by

tHK = (A.εhigh−κ)/C (3)

where tHK is the thickness, A is the area, εhigh−kis the permit-

tivity of the high-k dielectric (we used a relative permittivityof 9.1 for Al2O3), and C is the measured capacitance.

In addition, we have measured the current density leakage[Fig. 3(b)] showing a value of 4 × 10−6 A/cm2 at −3 V,suggesting a good quality of the Al2O3 dielectric film.

Finally, we measured the C–V curves at different bendingradii, results shown in Fig. 4(a). From this figure, we cannotinfer any trending change in electrical behavior but there is a

Fig. 4. (a) Variation of normalized capacitance under different bending radii.Inset: photograph of sample being measured while bent at 5 cm. (b) Lighttransmittance in the visible spectrum.

small deviation in the values of around ±15%, which can beassociated to the accuracy of the measurements itself.

IV. RELIABILITY STUDY

We have studied the reliability of MIMCAP devices builton the released flexible silicon fabric in comparison withMIMCAP devices built on unreleased silicon fabricated in thesame run and on the same Si (100) wafer to minimize processvariations (only a certain area of the wafer was released beforeto MIMCAP formation, therefore in a single wafer, we haveboth unreleased and released capacitors). The major physicaldifference between released and unreleased devices is thepresence of release holes (area loss per run is 16%, however,because we can recycle the remaining wafer by chemicalmechanical polishing up to six times, we reduce the cost andloss). Detailed processes can be found in [19].

Our focus is to study the reliability behavior of flexibledevices including ramped voltage (Vramp), breakdown volt-age (Vbd), and time-dependent dielectric breakdown (TDDB).We are able to compare reliability behavior of the flexibledevices (released) with nonflexible devices (unreleased). Weshow in Fig. 5(a), the ramped breakdown voltage analysis ofseveral released versus unreleased devices (10.4 and 11.6 V,respectively). The plot shows a ∼10% reduction in Vbd, whichcan be attributed to the reduced effective area of the released

3308 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 60, NO. 10, OCTOBER 2013

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Fig. 5. (a) Vramp breakdown voltage analysis of released and unreleasedMIMCAPs (error bars: standard deviation for six devices) showing a 10%decrease in breakdown voltage for released MIMCAPs. (b) Representativedata of TDDB on released and unreleased MIMCAPs at 83% below theramped breakdown voltage (8.65 and 9.66 V, respectively). (c) Lifetimeprojections of released and unreleased MIMCAPs showing an improvement ofthe released devices lifetime over unreleased for operational voltages lowerthan 8.2 V, while both devices exceed the 10-year lifetime at operationalvoltages lower than 7 V. (d) C–V plots show the behavior of the MIMCAPdevices under voltage sweep from −5 to 5 V at different frequencies.

MIMCAP plates because of the presence of release holes(∼16% of area reduction is due to the release of holes inflexible MIMCAPs).This leads to higher current densities thatoccur early on, and allows breakdown to take place at a lowerelectric field. This complies with previous ramping voltageanalysis done on porous materials where our holes would bepores at the microscale. It has been previously shown thatthe porous materials would have a reduced breakdown voltagebecause of defects at the pores (voids or undesired bonds) thatcan be modeled as preexisting defects at zero time [20].

TDDB analysis [Fig. 5(b)] at 83% of Vramp breakdownvoltage (Vbd) shows that the released devices survive longerthan unreleased devices, which explains their longer lifetimeprojections [Fig. 5(c)]. The 83% of Vbd was chosen as theinitial stress condition such that time to breakdown was nottoo short nor too long, thereby allowing a range of voltagesto be selected for lifetime projections. Therefore, it shouldbe noted that the TDDB plot [Fig. 5(b)] is representativefor different samples, and that is why released devices lastlonger because of the lower applied stress voltage (8.65 Vin case of the released devices and 9.66 V for unreleased).Furthermore, released MIMCAPs outperform unreleased onesin terms of lifetime projections, as shown in Fig. 5(c). This canbe deduced from the near three times higher absolute negativeslope of released devices’ lifetime fit. Lifetime of memorydevices is a crucial parameter and has to hit the benchmark of10 years at operational voltage. Evidently, both devices safelypass the benchmark at operational voltage <7 V. Contrary tothe expected lifetime projection decline, our lifetime projectionanalysis shows that the released (flexible) devices pass the10-year benchmark at a higher voltage compared with that

of the unreleased devices. This can be explained by thepossibility of the trench holes acting as heat mitigationvoids through which the heat dissipated by the device canescape. This in turn would reduce the injection of hotcarriers through the dielectric, i.e., electrons with sufficientkinetic energy to break through the dielectric energy bar-rier. This hypothesis is supported by the empirical obser-vation of an increase in the time to breakdown such thatat voltages below ∼8 V (intersection of the two lines)the released devices survive longer than their unreleasedcounterparts. This subsequently affects the slope of the bestfit and leads to higher acceptable voltages for a projected10-year lifetime. It is worth mentioning that the linear fit is aconservative version of the lifetime projections that was usedto drive the point home that the released devices are as reliable(if not better) than their unreleased counterparts. Other lifetimeprojection methods include nonlinear fits that would interceptthe 10-year benchmark at a much higher voltage comparedwith the used linear fit. It is also to be noted that typicallyfor lifetime projections, the stress voltages used are usuallywithin a very narrow window and the 200 mV step is common[21]–[24]. This is especially true as we can observe fromthe experimental points that the time to breakdown increasessignificantly because of minor increment in voltage. Therefore,the three points suffice for the lifetime projection as anyTDDB test at mildly lower voltages will take unreasonablyextended time to breakdown while any voltages slightly abovethe included range would breakdown almost instantly. Finally,our projection can still be considered as safe as the linearfitting is a conservative E-model (linear on a semilog scale)compared with the power-law model (lifetime α V −n) fitting,which would lead to significantly higher lifetimes for sameoperational voltage [24]. Hence, our preliminary reliabilitystudy demonstrates that the released (flexible) devices likethe unreleased devices would pass the 10-year benchmark atreasonable operational voltages.

Finally, we also show the impact of trenches from theperspective of C–V measurements between released andunreleased MIMCAPs. The C–V plots show the behaviorof the MIMCAP devices under voltage sweep from −5 to5 V at different frequencies [Fig. 5(d)]. Evidently, as thefrequency increases, the normalized capacitance/unit planararea decreases due to the inability of the charges to followthe voltage changes, and therefore inability to contribute tothe measured capacitance. The normalized capacitances aremeasured per unit planar area for fair comparison, i.e., thereleased MIMCAPs were treated as 100 μm × 100 μm platesneglecting the reduced effective area due to the presenceof the release holes, which underestimates their normalizedcapacitance but is necessary for fair comparison as this area islost on the wafer during trench formation process and shouldbe accounted for when comparing using wafer real estate. Thiscan be however mitigated by the fact that the released sampleis only 25-μm thick and that we can recycle the wafer.

V. CONCLUSION

MIM capacitors represent an important component ofDRAM memories and their implementation on a flexible

ROJAS et al.: FLEXIBLE HIGH-κ /METAL GATE MIMCAPs 3309

platform is a key step towards truly high-performance memoryapplications for flexible computation. In this paper, we haveshown a simple batch fabrication process relying on standardmicrofabrication techniques to build MIMCAPs on a substan-tially large flexible and semitransparent silicon (100) fabric(18 cm2). We did not observe any meaningful variation inelectrical performance because of the radius at which the sam-ple is bent, which suggests robustness and consistent behaviorregardless of the applied mechanical strain (up to 83.33%). Inaddition, our reliability study suggests no significant differencein performance of flexible versus nonflexible MIMCAPs andshows an interesting improvement in lifetime projections.

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Jhonathan Prieto Rojas (S’09) is currently pursuing the Ph.D. degree withthe Electrical Engineering Program, King Abdullah University of Science andTechnology, Thuwal, Saudi Arabia.

His current research interests include flexible electronics and self-powereddevices.

Mohamed Tarek Ghoneim (S’10) is currently pursuing the Ph.D. degree withthe Electrical Engineering Program, King Abdullah University of Science andTechnology, Thuwal, Saudi Arabia.

His current research interests include memory devices on flexible substrateand reliability physics.

Chadwin D. Young (S’96–M’01–SM’06) was with SEMATECH and he iscurrently an Assistant Professor with the Material Science and EngineeringDepartment, University of Texas, Dallas, TX, USA.

Muhammad Mustafa Hussain (M’07–SM’10) is an Associate Professorof electrical engineering with the King Abdullah University of Science andTechnology, Thuwal, Saudi Arabia.