HALLE (SAALE), Germany, September 19-22, 2016

27th EUROPEAN SYMPOSIUM ONRELIABILITY OF ELECTRON DEVICES,FAILURE PHYSICS AND ANALYSIS

ESREF 2016 Halle, Germany

Dielectric charging phenomena in diamond films used in RF MEMS capacitive switches:

The effect of film thickness

M. Koutsourelia, A. Zevgolatisa, S. Saadab, C. Mer-Calfatib, L. Michalasa, G. Papaioannoua and P. Bergonzob

a Solid State Physics Section, Physics Department, University of Athens, Greece.b CEA, LIST, Diamond Sensors Lab., F-91191 Gif-sur-Yvette, France.

Objectives• Dielectric charging effect is one of the most important reliability problem

of RF MEMS capacitive switches.

• Diamond in micro-, nano- or ultra-nanocrystalline form has been used in MEMS capacitive switches and it has shown promising characteristics.

• A correlation of conductivity parameters to the diamond film thickness has not been reported up to now.

What is the effect of NCD film thickness on dielectric charging and on charge transport mechanisms?

2

Goldsmith et.al. IMS 2010, 1246-1249

Outline

3

Introduction

Material Considerations

Experimental Details

Results

Conclusions

Introduction

4

Pull – in Charge injection

Pull – out Charge collection

Up to date reports for diamonds in MEMS:

Satisfactory isolation 2 times faster discharging for

NCD 5-6 orders of magnitude faster

discharging for ultra – NCD Higher conductivity than SiNx

Reduced shift of pull-in voltage after stress

Surface conductivity vs film thickness has been intensively investigated on NCD, but what about conductivity normal to the film?

GREAT interest for RF MEMS switches

Material Considerations

5

NCD is a combination of : Crystalline diamond (Eg = 5.5 eV) Non-Diamond Carbon: Amorphous diamond

like carbon (Eππ* = 2.5 eV) → Conductive

Sp3 bonded carbon → Diamond phase.Sp2 bonded carbon → Non-Diamond Carbon at grain boundaries.

MPCVD Diamond films have a columnar structure → Increase of film thickness results to larger grains

Plano et.al. J. Vac. Sci. Technol. B 22, 1940 (2004)

Material Considerations

6

Conductivity of nanocrystalline diamond films takes place through paths associated with grain boundaries and dislocations.

Hydrogen affects conductivity either by: • interrupt the sp3 network creating dangling

bonds → Increase conductivity.• terminate sp2 bonded carbon → Reduce

conductivity. Correa et.al., JAP 102 113706 (2007)

The increase of film thickness results to:• increase of grain size and decrease of grain boundary

network.• change of the nature of the grain boundaries: graphitic in larger

grains to hydrogen terminated sp3 carbon. (Wiora et al, diam. Rel. Mat. 2009)

• hydrogen content decreases whereas the sp2 carbon content increases.

Experimental Details

7

Samples: MIM capacitors with intrinsic NCD

• Film thickness: 350nm, 600nm, 750nm• Deposition: MPCVD on a TiW/Au/Si substrate.• Columnar structure.• Grain size is increased with film thickness.• MIM capacitors area: 450 x 450 μm2.

Assessments:• Investigate conduction mechanisms → I-V

characteristics from 300K to 400K and for field intensities up to 200 kV/cm.

• Investigate dielectric charging: TSDC method.

350nm

750nm

Experimental Details

8

Assessment - Thermally Stimulated Depolarization Currents (TSDC) The current density produced by the

progressive decrease in polarization in the course of a TSDC experiment is :

Stored charge, measured in external circuit:

Experimental SetupTP = 450K EP= 100 kV/cm

200K < T < 450K Heating rate: β = 2.5K/min

2pS A AD

0 0 A

P T E E1 kTJ T exp exp expkT E kT

2

1

T

TSDC TSDCT

1 J T dT

Results

9

I-V characteristics

The conductivity σ = J/F increases with film thickness

0 50 100 150 20010-4

10-3

10-2

10-1

100

101

102

103

104

105

750nm 600nm

J

[nA

/cm

2 ]

Field [kV/cm]

T = 300K

0 50 100 150 200

100

101

102

103

104

105

750nm 600nm

J

[nA

/cm

2 ]Field [kV/cm]

T = 400K

Results

10

I-V characteristics

Fields < 130kV/cm:

Thermally activated conductivity with EA = 0.6 eV – 0.8 eV → Grain boundaries

EA increases with film thickness

Conduction through:

• Hopping (T<300K) • Band conduction (T>300K).2,4 2,6 2,8 3,0 3,2

10-14

10-13

10-12

10-11

10-10

10-9

750nm 0.82eV 600nm 0.73eV 350nm 0.66eV

[S/c

m]

1000/T [K-1]

100 kV/cm

(Visser et al, Phys. Cond. Mat.1992)

Results

11

I-V characteristics

F>130kV/cm & T > 320K:The conductivity of all samples obeys the Frenkel-Poole conduction as modified by Hill:

𝐽=2𝑒𝑁𝑠 (𝑘𝑇 )2𝑣𝑒𝑥𝑝(− 𝐸𝑝

𝑘𝑇 ) h𝑠𝑖𝑛 ( 𝑒𝐹𝑠2𝑘𝑇 )Barrier:

The energy levels Ei can be correlated to conduction paths at grain boundaries.

Increase of NCD film thickness result to:• increase of energy level Ei • decrease of mean separation distance s• increase of density N of the centers

responsible for the conduction: N≈1/s3

𝐸 𝐴=𝐸𝑃−𝑒 ∙ 𝑠 ∙𝐹2

Results

12

TSDC method

TSDC spectra have an envelope that exhibits Arrhenius-like behavior.

Activation energy: 0.8 – 0.9 eV in all samples. Thickness increase → dielectric charging enhancement. The injected charges are trapped through material’s volume.

300 400 500 600 700 8004

5

6

7

8

TSDC

[C

/cm

2 ]

d [nm]2,0 2,5 3,0 3,5 4,0 4,5 5,0

10-12

10-11

10-10

10-9

10-8

200 250 300 350 400 45010-12

10-11

10-10

10-9

10-8

J TSDC

[

A/cm

2 ]

T [K]

J TSDC

[

A/cm

2 ]

1000/T [K-1]

EA=0.87 eV

350nm

Collective Results

13

NCD film thickness

F < 130 kV/cmThermal

activation

F> 130 kV/cmHill conduction

TSDC

EA [eV] s [nm] N [cm-3] Ei [eV]σTSDC

[C/cm2]350 nm 0.66 52.3 7.0x1015 0.84 4.71x10-6

600 nm 0.73 33.6 2.6x1016 0.91 5.43x10-6

750 nm 0.82 28.8 4.2x1016 0.95 7.94x10-6

Increase of NCD thickness results to:• Larger conductivity.• Larger activation energy associated to thermal activation. • Larger defect density and deeper defect states associated to Hill conduction.• Enhanced dielectric charging.

Conclusions

14

The conduction processes of NCD films that can be used in RF MEMS switches and the effect of film thickness on the material’s electrical properties have been investigated.

The films’ conductivity has been found to be thermally activated for low electric field intensities while Frenkel-Poole conduction modified by Hill dominates the conduction process when the electric field becomes stronger than 130 kV/cm.

The increase of film thickness results to an increase of conductivity and an increase of defect density.

TSDC assessment revealed that the injected charges are trapped through material’s volume, thus dielectric charging is enhanced as the NCD film thickness increases.

Taking into account that in order to increase the reliability of a capacitive RF MEMS switch it is important to mitigate dielectric charging effects, the above mentioned conclusions indicate that thinner NCD films seem to be more promising for these devices → The use of thinner NCD films will also improve the switch RF performance, since it will cause an increase of the down-state capacitance which results to higher isolation.

Acknowledgements

15

The authors would like to acknowledge that the present work has been supported by the FP7 ENIAC/ESPA-GR project “Microsystem Based on Wide Band Gap Materials Miniaturized and Nanostructured RF-MEMS” NANOCOM under GA: 270701-2, ENIAC call 3.

The first author, Matroni Koutsoureli, would like to thank “IKY Fellowships of Excellence for Postgraduate Studies in Greece – Siemens Program” that takes place in the framework of the Hellenic Republic – Siemens Settlement Agreement.

ENIAC Project