Ni–Al diffusion barrier layer for integrating ferroelectric capacitors on Si
B.T.Liu,a͒
C.S.Cheng,F.Li,L.Ma,Q.X.Zhao,Z.Yan,and
D.Q.Wu
College of Physics Science&Technology,Hebei University,Hebei071002,China
C.R.Li
Institute of Physics,Chinese Academy of Sciences,Beijing100080,China
Y.Wang
Department of Applied Physics,The Hong Kong Polytechnic University,Hong Kong,China
X.H.Li and X.Y.Zhang
Key Laboratory of Metastable Materials Science and Technology,Yanshan University,Hebei066004,China
͑Received1February2006;accepted18May2006;published online21June2006͒
We report on the use of amorphous Ni–Alfilm͑a-Ni–Al͒as conductive diffusion barrier layer to
integrate La0.5Sr0.5CoO3/PbZr0.4Ti0.6O3/La0.5Sr0.5CoO3capacitors on silicon.Cross-sectional
observation by transmission electron microscope reveals clean and sharp interfaces without any
discernible interdiffusion/reaction in the sample.The physical properties of the capacitors are
vertically characterized as the parameters of memory elements.Excellent ferroelectric properties,
<,large remnant polarization ofϳ22␮C/cm2,small coercive voltage ofϳ1.15V,being
fatigue-free,good retention characteristic,imply that amorphous Ni–Al is an ideal candidate for
diffusion barrier for the high-density ferroelectric random access memories integrated with silicon
transistor technology.©2006American Institute of Physics.͓DOI:10.1063/1.2214142͔
Pb͑Zr,Ti͒O3͑PZT͒thinfilms,which possess favorable characteristics such as low operation voltage,large r
emnant polarization,and piezoelectric constant,have been exten-sively investigated due to their potential applications for ferroelectric nonvolatile memory,1–4field effect devices,5,6 microelectromechanical systems͑MEMS͒,7–9and pyroelec-tric sensors.10Currently,great efforts have been intensively made to integrate PZT based ferroelectric capacitors with modern silicon transistor technology to yield one-transistor–one-capacitor͑1T-1C͒based memory architectures2in order to realize high-density ferroelectric random access memo-ries.In such a memory cell,the drain must be electrically in contact with the bottom electrode of ferroelectric capacitor stack.If the PZT ferroelectric capacitor stack is directly in-tegrated on Si,however,the chemical reactions and interdif-fusion between the bottom electrode and silicon may gener-ate a nonconducting layer,resulting in the failure of the memory cell due to the deterioration of the electric contact between the bottom electrode and silicon.To solve this prob-lem,an extra layer,usually called“diffusion barrier layer,”is interposed between PZT capacitor stack and Si wafer to separate them from direct contact.Ideally the diffusion bar-rier should possess a nature of good oxidation resistance, high thermal and chemical stabilities and large electrical con-ductivity͑so as to function as a bridge to conduct electric current between its bordering layers͒.In literature,several intermetallic systems such as Pt–͑Ti,Al͒N and Ir–TiN were investigated for the use of barriers.7,11–13While these mate-rials possess very high electrical and thermal conductivities, there are still some concerns such as the formation of Pt hillock
structure due to the stress release in the processing, the difficulty in developing a reliable reactive ion etching process for the refractory metals,and the inherently high cost.14Tofind alternative material systems is thus an active topic in recent years.Among several candidates,amorphous Ni–Al seems to be a good choice because it is highly resis-tant to oxidation,low cost,and ready to be patterned by conventional etching techniques.
In this letter,we report the integration and characteriza-tion of a ferroelectric heterostructure La0.5Sr0.5CoO3/Pb Zr0.4Ti0.6O3/La0.5Sr0.5CoO3on highly doped silicon using amorphous Ni–Al thinfilm as the conductive diffusion bar-rier layer͑the whole structure is abbreviated as LSCO/PZT/LSCO/a-Ni–Al/c-Si͒,mainly aiming to dem-onstrate the suitability of a-Ni–Al for this application.The heterostructure LSCO/PZT/LSCO has been extensively stud-ied in literature.LSCO has a very good electrical conductiv-ity both in its amorphous and crystalline states and the bot-tom LSCO layer can also serve as a template to achieve fatigue-free PZT capacitors.15,16The preparation of PZT by sol-gel technique has been well established in literature and in our laboratory and the properties of PZT are well known, giving us a good reference for assessing the quality of our samples.
The samples were prepared by a multistep procedure.͑Step1͒Amorphous Ni–Al thinfilm͑5–50nm thick͒w
as deposited on highly doped silicon͓with resistivity of͑1–3͒ϫ10−3⍀cm͔by means of radio frequency͑rf͒magnetron sputtering.As crystallinity of sputtered Ni–Alfilm greatly depends on the deposition conditions͑e.g.,deposition power density and deposition pressure͒,the deposition parameters were carefully optimized in order to avoid the crystallization of Ni–Alfilm.We found that the sputtering,using power of 7W,at room temperature and in an atmosphere of high-purity argon with a pressure of3Pa could yield high quality amorphous Ni–Alfilms.͑Step2͒LSCO thinfilms͑70nm͒were deposited on a-Ni–Al/Si substrates by magnetron sputtering under the following conditions:temperature=ϳ25°C,Ar:O2=3:1,and power=50W.Postannealing was conducted at500–550°C in a tube furnace withflowing
a͒Electronic mail:btliu@mail.hbu.edu
APPLIED PHYSICS LETTERS88,252903͑2006͒
0003-6951/2006/88͑25͒/252903/3/$23.00©2006American Institute of Physics
88,252903-1
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oxygen.͑Step 3͒PZT thin films were prepared by the spin coating technique using a modified PZT sol-gel solution fol-lowed by annealing at the same temperatures as for LSCO films.͑Step 4͒On PZT,another LSCO layer was prepared as mentioned in step 2.͑Step 5͒Pt circular pads with an area of 1.16ϫ10−3cm 2were defined as the top electrodes for LSCO/PZT/LSCO capacitors through standard photolithog-raphy,Pt deposition,and wet chemical etching techniques.͑Step 6͒The back side of the silicon wafer was cleaned,on which a 100nm thick aluminum film was deposited to facili-tate Ohmic contact for vertical measurements.The phase and crystallinity of the samples were characterized by x-ray dif-fraction ͑XRD ͒,and the interfaces were studied using trans-mission electron microscopy ͑TEM ͒and high-resolution TEM ͑HRTEM ͒.The ferroelectric properties of the samples were vertically determined using a precision LC unit from Radiant Technologies.
Figure 1shows a typical XRD spectrum of the LSCO/PZT/LSCO/a -Ni–Al/c -Si heterostructure.PZT is found to have been well crystallized with a random crystal-lographic orientation.No evident peaks of impurities are found,indicating that no reaction occurred between the Ni–Al layer and its adjacent layers.The TEM image of the cross-sectional LSCO/PZT/LSCO/a -Ni–Al/c -Si hetero-structure ͓Fig.2͑a ͔͒shows that the interfaces related to Ni–Al layer are clear,sharp,and free of inter-reaction and diffusion.HRTEM images of the cross-sectional LSCO/a -Ni–Al/Si shown in Figs.2͑b ͒–2͑d ͒were further employed
to characterize the interfaces and crystallinity.Notice that Ni–Al film is ϳ5nm and still amorphous after high tempera-ture process.The absence of grain boundaries makes amor-phous diffusion barriers suitable to overcome this type of failure mechanism.The interfaces between Ni–Al and its ad-jacent ,Si/LSCO,are sharp,indicating no reaction at the interfaces.Also,stripes can be found in LSCO image from Fig.2͑d ͒,implying that LSCO is crystallized.The crys-tallinity of each layer of the heterostructure can also be con-firmed by the selected-area electron diffraction ͑SAED ͒pat-terns as shown in the inset of Fig.2͑a ͒.These results are analogous to the recent report of Aggarwal LSCO/Nb-PZT/LSCO/a -Ti–Al heterostructure integrated on polycrystalline-Si/Si substrates,14suggesting that a -Ni–Al is chemically stable under the thermal treatments de-scribed above.The oxidation processes and correlation of crystallinity with deposition parameters for a -Ti–Al film have been well studied;17a -Ni–Al film needs to be further investigated.
Ferroelectric properties were characterized using polar-ization hysteresis and pulsed polarization measurements.Figure 3shows a typical ferroelectric hysteresis loop mea-sured at 5V for the LSCO/PZT/LSCO capacitor.The rem-nant polarization ͑P r ͒and coercive voltage ͑V c ͒are ϳ22␮C/cm 2and ϳ1.15V,respectively.The dependence of the switchable polarization ͑⌬P ͒on the applied voltage is shown in the inset of Fig.3.18Taking into account that the typical P r values of PZT prepared at highe
r temperatures ͑600–650°C ͒are also around 15–40␮C/cm 2and the mini-mum requirement of P r for ferroelectric memory is ϳ10␮C/cm 2,we can conclude that the ferroelectric polar-ization ͑20–25␮C/cm 2͒of our samples is sufficiently high for memory applications.For comparison,also shown in the inset is the hysteresis loop of the LSCO/PZT/LSCO capaci-tor fabricated on crystalline Ni–Al/c -Si ͑c -Ni–Al/c -Si ͒substrate;we can see that the capacitor is leaky,indicating that crystalline Ni–Al cannot be used as a barrier layer for integrating ferroelectric capacitor stack on Si.Also note that the electrical response of ferroelectric capacitors measured via the underlying silicon substrate is identical to the mea-surements made using conventional capacitive coupling method,indicating the viability of this approach.
Ferroelectric capacitors used for memory applications should be subjected to a large number of read/write cycles in order to retrieve and store information,and thus a good fa-tigue resistance is critically important.Fatigue tests were conducted on our samples.In the test,bipolar-pulsed
cycles
FIG.  1.XRD spectrum of the LSCO/PZT/LSCO/a -Ni–Al/Si
heterostructure.
FIG.2.͑a ͒Cross-sectional structure of the LSCO/PZT/LSCO/a -Ni–Al/c -Si under transmission electron microscope ͑TEM ͒.The corresponding SAED patterns are shown in the inset.͑b ͒–͑d ͒present interfaces from vari-ous layers under high-resolution TEM.No chemical reaction or interdiffu-sion between different layers was
observed.
FIG.3.A typical hysteresis loop of the LSCO/PZT/LSCO capacitors inte-grated on a -Ni–Al/c -Si.The insets show the switchable polarization as a function of applied voltage ͑left,top ͒and hysteresis loop of the LSCO/PZT/LSCO capacitor integrated on c -Ni–Al/c -Si ͑right,bottom ͒.
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of 5V in amplitude and at a frequency of 1.0MHz were applied on the LSCO/PZT/LSCO capacitors.The switched ͑P *͒and nonswitched ͑P ˆ͒polarizations were measured through pulsed polarization tests as a function of fatigue cycles as shown in Fig.4,from which we know that the test capacitors are fatigue-free up to 1011switching cycles.Fig-ure 5demonstrates the retention characteristic,one of the most important concerns for the ferroelectric random access memory.The test capacitors were applied with write pulse of −5V in order to write logic state 1and read pulse of 4V.The difference between switched ͑P *͒and nonswitched ͑P ˆ͒polarizations is maintained at about 22␮C/cm 2throughout the retention measurement time of 105s without any obvious degradation of the polarization.These physical properties further confirm that the ferroelectric behaviors of the LSCO/PZT/LSCO stack were not influenced by the introduction of Ni–Al in the samples.
In summary,amorphous Ni–Al intermetallic film was demonstrated first as a conductive diffusion barrier layer for integrating LSCO/PZT/LSCO ferroelectric capacitors on Si.Robust physical properties of LSCO/PZT/LSCO capacitors,
coupled with the XRD and TEM/HRTEM results,demon-strate that the simple,inexpensive amorphous Ni–Al film can be used as a barrier layer for fabricating high-density ferro-electric random access memories.
This work is partly supported by the NSFC ͑No.50572021͒,SRF for ROCS from both State Education Min-istry and Ministry of Personnel,the NSF of Hebei Province of China ͑No.E2005000130͒,and Foundations of both He-bei Provincial scientific department ͑No.04213579͒and He-bei University.
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The switchable polarization ⌬P ,a parameter that can be directly read on Radiant ferroelectric tester,is defined as the difference between the switched ͑P *͒and nonswitched ͑P Ù͒polarizations.Compared with the conventional P r ,⌬P is regarded to reflect the ferroelectric properties more accurately because the interference coming from the leakage current is removed.The larger the ⌬P value is,the more “ferroelectric”the material
is.
FIG.4.Fatigue characteristic of the LSCO/PZT/LSCO capacitor as a func-tion of switching
cycles.
FIG.5.Polarization of the LSCO/PZT/LSCO capacitor as a function of retention time.
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