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Enabling 3X nm DRAM: Record low leakage 0.4 nm EOT MIM capacitors with novel

stack engineeringM.A. Pawlak, M. Popovici, J. Swerts, K. Tomida, Min-Soo Kim, B. Kaczer, K. Opsomer, M. Schaekers, P. Favia, H. Bender,

C. Vrancken, B. Govoreanu, C. Demeurisse, Wan-Chih Wang*V. V. Afanas'ev*, I. Debusschere, L. Altimime, J.A. Kittl

Imec, *K.U .Leuven, Leuven, Belgium, Phone: +32 16 28 18 41, Fax: + 32 16 28 18 44, E-mail: pawlakm@imec.be

Abstract

We report the lowest leakage achieved to date in sub-0.5 nmEOT MIM capacitors compatible with DRAM flows, showingfor the first time a path enabling scalability to the 3X nm node.A novel stack engineering consisting of: 1) novel controlledultrathin Ru oxidation process, 2) TiOx interface layer, is usedfor the first time to achieve record low Jg-EOT in MIMcapacitors using ALD Sr-rich STO high-k dielectric and thin Rubottom electrode. Record low Jg of 10-6A/cm2 (10-8A/cm2) isachieved for EOT of 0.4 nm (0.5 nm) at 0.8 V. Our data iscompared favorably (> 100Jg reduction at 0.4 nm) to previousbest values in literature for MIMcaps with ALD dielectrics.

IntroductionThe DRAM 3X nm node will require MIMcaps with low

leakage at 0.4 nm, deposited with highly conformalatomic layer deposition (ALD) processes for compatibility with

large aspect ratio structures. Hf- [1] or Zr-based dielectrics withk0.6 nm.The only ALD high-k dielectric films which have shownpromise for scaling below 0.6 nm are SrxTiyOz (STO) [2, 3, 4]and Al-doped Rutile TiOxgrown on Ru-based bottom electrode(BE) [5]. However, in all previously reported data for MIMcapwith ALD grown dielectrics, leakage increased to >10 -4A/cm2

when scaling EOT to 0.4 nm, raising a serious concern on thepossibility of continuing DRAM scaling towards the 3X node ITRS roadmap). This work presents for the first time a viablepath for DRAM scaling to the 3X node, by using a novel stackengineering.

Experiments, Results and DiscussionMIM capacitors were fabricated in a 300 mm line. The flow

sequence and a schematic cross-section of the MIMcap stack areshown in Fig. 1. Ru films (5 nm) were deposited by ALD withexcellent smooth surfaces (< 0.2 nm RMS by AFM) on 10 nmTiN (some wafers were left without Ru, i.e. with TiN BE, forcomparison). A novel controlled oxidation process (Fig. 2) wasapplied to the wafers with Ru BE, resulting in ultrathin,controlled oxidation of the surface forming a 1 nm RuOx layerwith no penalty in roughness (i.e. maintaining < 0.2 nm RMS byAFM). Previously reported oxidation processes result in eitherformation of large RuOx crystals or significant roughening ofthe film surface, both unacceptable for DRAM MIMcapmanufacturing. A thin TiOxinterfacial layer (0.5 or 1 nm), wasthen grown by ALD at 250oC with Ti(OCH3)4and H2O. Somewafers were kept without the TiOx interfacial layer forcomparison. STO films (7 to 9 nm) were then deposited byALD at 250oC using Sr(t-Bu3Cp)2 and Ti(OCH3)4 precursorsand H2O as oxidant [4, 6]. Good composition control over alarge Sr/Ti range is obtained by adjusting the Sr to Ti pulse ratio(Fig. 3). Films were crystallized by RTP annealing in N2 atoC. The properties of crystallized STO films depend onthe composition, as shown in Fig. 4 (for films without TiOxinterface layer), with k-value decreasing and lattice parameterincreasing with increasing Sr (and only small change in bandgap). We had previously determined Sr-rich STO

(Sr/(Sr+Ti)~62 at%) to have better leakage properties than

stoichiometric STO films [4]. This is due to the formation oflarge STO grains (>500 nm) with nano-cracks and star-shapepatterns (seen by SEM) in crystallization of stoichiometricfilms, while small grain size (~ 50 nm) crack-free films areobtained upon crystallization of Sr-rich STO.

After crystallization and top electrode (TE) processing (TiNpatterned by RIE), films were characterized electrically. EOTvalues were extracted from CV measurements (Fig. 5). Welbehaved CVs and excellent area scaling was found. EOTincreases with increasing deposited STO thickness (Fig. 6a)Note that the apparent k-value extracted from the slope of thisplot (for TiOx=0.5 nm), k~ 85, is too high for Sr-rich STO(k~65 at Sr/(Sr+Ti)=62 at.%). A more important observationhowever, is that EOT was found to decrease with increasingdeposited interfacial TiOx thickness for same STO thickness

(Fig. 6b). TEM characterization of MIM stacks (aftercrystallization anneal, and TiN TE processing) with and withou0.5 nm TiOx interfacial layer showed no observable differencebetween them (Fig. 7a). In both cases, grain size remains small SEM analysis between stacks with and without 0.5 nminterfacial TiOx after crystallization anneal (Fig. 7b). XRDanalysis, however, revealed STO peak shifts correlated to thedeposited interfacial TiOxthickness (Fig. 8). This indicates thathe composition of the STO films after crystallization changeswith TiOx thickness, and can be understood in terms of theintermixing of TiOx and STO layers during crystallizationanneal (Fig. 9). Due to the intermixing, the resulting crystallizedfilms have higher Ti content than the deposited STO films, andin consequence a higher k-value and lower EOT (Figs. 9 and 6)However, the films keep the favorable microstructure of the Sr-rich films. IV characteristics showed excellent area scaling (Fig10). Typical leakage density vs V plots are shown in Fig. 11. AsEOT decreases, leakage becomes more asymmetric, with lowerleakage obtained in positive polarity corresponding to injectionfrom RuOx. This is attributed to the higher WF of RuOcompared to the TiN TE. Jg-EOT plots are shown in Fig. 12,where data from this work is compared to best literature data forbenchmarking. It is observed that the addition of TiOx layeresults in a large decrease in EOT without much penalty inleakage. Our data achieves lower leakage-EOT than all previousreported results for ALD grown high-k dielectrics.

ConclusionsRecord low JG=10

-6 (10-8) A/cm2 at 0.4 (0.5) nm EOT aredemonstrated in MIMcaps using a nove

TiN/Ru/RuOx/TiOx/STO/TiN stack fabricated in a 300 mm linewith DRAM compatible processes (including ALD STO and oC in N2). This is the first demonstration olow leakage at 0.4 nm EOT with ALD high-k, showing for thefirst time a path enabling DRAM scalability to the 3X nm node.References:1) N. Mise et al., p. 267, IEDM 2009.2) O. S. Kwon et al., J. Electrochem. Soc. 154, G127 (2007)3) S. W. Lee et al., Appl. Phys. Lett. 92, 222903 (2008).4) N. Menou et. al., p. 929, IEDM 2008.5) S. K. Kim et al., Advanced Materials 20, p. 1429 (2008)6) M. Popovici et al., J. Electrochem. Soc. 157, G1 (2010)

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TiN deposition

ALD Ru deposition

Ru oxidation

ALD TiOx deposition

ALD Sr-rich STO deposition

Crystallization anneal

TiN deposition

TiN patterning

TiN

ALD-Ru

ALD-Sr-rich STO

TiN

Ru: 0nm o r 5nm

TiOx: 0nm, 0.5nm or 1nm

RuOx: 0nm o r 1nm

600C, N2, 1min PDA

0

10

20

30

40

50

60

1 10 100 1000 10000

Thickness(A)

Oxidation time (s)

no oxidation

Bulk Ru

RuOx on Ru

0.3

0.4

0.5

0.6

0.7

0.8

0.4 0.5 0.6 0.7 0.8 0.9

Sr/(Sr+Ti)atomicra

tio(RBS)

Sr/(Sr+Ti) cycle ratio (n/(n+m))

Sr(tBu3Cp)2 H2OTi(OCH3)4H2O

N2 carrier gas

ALD SrTiO3, wafer T = 250 C

n Sr-cycle m Ti -cycle

STO-cycle

a)

0.0

0.1

0.2

0.3

0

50

100

150

200

250

40 50 60 70

DielectricConstant

Sr/(Sr+Ti) at %

Egap (eV)

a ()

a=3.90

Egap=3.0 eV

k

-1.0 -0.5 0.0 0.5 1.00

2

4

6

8

10

12

14

16

Capac

itance

dens

ity

(uF

/cm

2)

Voltage (V)

RuOx/TiOx/STO/TiN

a)

0 1x10-42x10

-43x10

-44x10

-4

Capacitor area (cm2)

RuOx/TiOx/STO/TiN

b)

4

3

2

1

0Capacitance

[nF]

TiOx(nm)

STO(nm)

EOT(nm)

1 8 0.34

0.5 7 0.36

0.5 8 0.41

X 0.5 9 0.50

0 8 0.66

RuOx/TiOx/STO/TiN

0.0 0.2 0.4 0.6 0.8 1.0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

EOT(nm)

TiOx thickness (nm)

7 8 90.2

0.3

0.4

0.5

0.6

0.7

EOT(nm)

STO thickness (nm)

Calculated assumingintermixing

a) Calculated assumingintermixing

b)

RuOx/0.5 nmTiOx/STO/TiN RuOx/TiOx/8 nm STO/TiN

10nmTiN

TiOx/STO

Ru/RuOx

TiN

TiN

STO

Ru/RuOx

TiNa) b)

RuOx/TiOx/STO RuOx/STO

300nm300nm

10 nm

Fig. 1. Process flow (top); and (bottom) schematic of MIMCapstack fabricated in a 300 mm line, using novel stack engineeringwith controlled Ru oxidation and interfacial TiOxlayer.

Fig. 4. Properties (permittivity, band gap and lattice parameter)of metastable perovskite STO films as function of Sr content.

Fig. 5. a) CV and capacitance vs. area characteristics ofTiN/Ru/RuOx/TiOx/STO/TiN MIMCaps, showing wellbehaved CVs with good area scaling. Layer thicknessesand extracted EOT values are shown in the legend on top.

Fig. 6. a) EOT vs deposited STO thickness for stack with0.5 nm TiOx interfacial layer; b) EOT vs deposited TiOxthickness for stack with 8 nm STO. Symbols correspond tomeasured data and solid line to calculations assumingmixing of the TiOx and STO layers during crystallization(resulting in higher Ti-content STO and higher k-value).

Fig. 2. Controlled Ru oxidation process, showing evolution ofRu and RuOxthicknesses as determined by XRR. Smooth filmsare obtained without large RuOxcrystals or surface roughening.

Fig. 3. (Left) schematic of STO ALD deposition; (right) STOcomposition control by varying the Sr to Ti pulse ratio.

Fig. 7. (Top) cross section TEM images after crystallizationanneal and TiN TE processing of MIMCap stacks; and(bottom) SEM top view images after crystallization anneal fora) TiN/Ru/RuOx/TiOx/STO deposited stack with 0.5 nm TiOxand b) TiN/Ru/RuOx/STO deposited stack (no TiOx). Nodifferences in microstructure are observed between the twostacks despite the significant reduction in EOT obtained withthe TiOxlayer.

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30 32 34 36 38

Intensity(AU)

Two theta ()

STO (110)

0 nm TiOx/9 nm Sr-r ich STO

0.5 nm TiOx/9nm Sr -rich STO

1 nm TiOx/9 nm Sr-r ich STO

3.85

3.90

3.95

4.00

40

50

60

70

Lattice parameter

Intermixing

0 0.5 1

TiOxthickness [nm]

Measured

la

ttice

para

meter[]

Calculated

Sr/(Sr+Ti)

[at%] Calc. from:

Calculated

k-value

0

50

100

150

200

250

Lattice parameterIntermixing

Calc. from:

a)

b)

c)

4.1E-4

1.0E-4

5.3E-5

2.7E-5

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

-1.0 -0.5 0.0 0.5 1.0

Voltage (V)

Jg

(A/cm

2)

4.1 x 10-4 cm2

1.0 x 10-4 cm2

5.3 x 10-5 cm2

2.7 x 10-5 cm2

RuOx/TiOx/STO/TiN

EOT=0.41 nm

TiOx STO EOT[nm] [nm] [nm]0.5 9 0.500.5 8 0.431.0 8 0.34

-1.0 -0.5 0.0 0.5 1.010

-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Jg

(A/cm

2)

Voltage (V)

0.3 0.4 0.5 0.6 0.7 0.8 0.9

1 V

Ru/STO/Pt [2]

Ru/STO/Pt [3]

TiN/HfAlO/TiN

Ru/RuOx/TiOx/Sr-rich STO/TiNe- injection from TiN(this work)

TiN/Sr-rich STO/TiN(this work)

100

10-1

10-2

10-3

10-4

10-5

10-6

10-7

10-8

10-9

CurrentD

ensityat1V[A/cm2]

EOT [nm]

Ru/RuOx/Sr-rich STO/TiNe- injection from RuOx(this work)

Ru/RuOx/TiOx/Sr-rich STO/TiNe- injection from RuOx

(this work)

TiN/Sr-rich STO/TiN

TiN/Ru/RuOx/TiOx/Sr-rich STO/TiN

Our Work:

[nm] [nm]

0 80.5 70.5 80.5 91 8

+1V: e- injection from BE-1V: e- injection from BE

TiN/Sr-rich STO/Pt ]

Ru/STO/Pt [2]

Ru/STO/Pt [3]TiN/HfAlO [1]

(BE/high-k/TE)

Benchmark:

Ru/RuOx/Al-doped TiOx/

Pt [5]

10-3

10-4

10-5

10-6

10-7

10-8

10-9

CurrentDen

sityat0.8

V[A/cm2]

0.3 0.4 0.5 0.6 0.7 0.8

EOT [nm]

0.8 V

Ru/RuOx/TiOx/Sr-rich STO/TiN

Ru/RuOx/Al-dopedTiOx/Pt [5]

Ru/STO/Pt [2]

(this work)

Fig. 8. Theta-two theta X-ray diffraction scan ofTiN/Ru/RuOx/TiOx/Sr-rich STO stacks after crystallizationanneal, for varying TiOxthickness. The peak position of STOshifts with TiOx thickness indicating a change in latticeparameter. The vertical line corresponds to the position of thebulk stoichiometric STO (110) diffraction.

Fig. 10. Leakage/area vs. V for TiN/Ru/RuOx/TiOx/Sr-richSTO/TiN MIMcaps of different areas fabricated with 0.5 nmTiOx. Excellent area scaling is observed.

Fig. 9. a) STO lattice parameter extracted from XRD data in Fig.8 vs. TiOxthickness; b) resulting STO composition as calculated(using correlations in Fig. 4) from the measured lattice parametershift or by assuming intermixing of the TiOx and Sr-rich STOlayers, c) k-values corresponding to the calculated compositions(using correlations in Fig. 4). EOTs estimated using mixingmodel fit well with measured values (Fig. 6).

Fig. 11. Leakage density vs. V for TiN/Ru/RuOx/TiOx/Sr-rich STO/TiN MIMcaps. Positive voltages correspond to e -injection from the bottom RuOx electrode and negativepolarity to e-injection from the top TiN electrode. Leakagebecomes more asymmetric for thinner EOT values.

Fig. 12. Area leakage density vs EOT (top: 1V, bottom: 0.8V), comparing Ru/RuOx/TiOx/STO/TiN stacks (this work) tobest literature data (benchmark) showing significantimprovement.

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