Epitaxial Co-Pt thin films electrodepositedon Cu/Si substrates

I. Zana*°, G. Zangari*° and M. Shamsuzzoha°

*MINT Center and °Department of Metallurgical and Materials Engineering

The University of Alabama

This project was funded by DOD grant no. DAAH04-96-1-0316

MINT Spring Review, April 2002.

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Introduction

• Co-rich Co-Pt alloys exhibit hard magnetic properties due to:- suitable microstructure (small, magnetically separated grains);- increased magneto-crystalline anisotropy by alloying with Pt;

• Advantages of electrodeposition: - enables the growth of hard magnetic films up to several microns thickness; - additive fabrication provides for small structures with limited defects, vertical walls and high aspect ratio; - can be easily incorporated in a CMOS flow process for fabrication of microelectromechanical systems (MEMS).

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Experiment

• Substrate:- 100 nm Cu deposited by DC magnetron sputtering on H- terminated

Si(100) or Si(111).- sputtering conditions: PAr=10 mTorr and PAr=0.4 mTorr respectively,

P.D.=7.96 W/cm2, dT-S=7 cm.

• Electrodeposition: - method: galvanostatic (I=const.). - electrolyte: amino citrate based (10 mM Pt and 0.1 M Co) pH=8, T = 65 C, unstirred. - Co sheet used as counter electrode.

• We investigated:- Influence of current density (CD), film thickness and substrate on

structural and magnetic properties

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Decrease ingrain size with current density

Pronounced faceting

75 nm

250 nm

AFM of Co-Pt on Cu(100) thin films

10 mA/cm2 30 mA/cm2 50 mA/cm2

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

• Fig.(a):- Cu grows epitaxially on H- terminated Si(100)[1].- Co-Pt peak (2Θ=43.415°) is FCC(111) and/or HCP(0002).

- increased intensity of Cu(200) due to additionalcontribution from Co-Pt(200).

• Fig.(b):- Current density (CD) does not greatly influence thelattice spacing (in agreement with XPS data: Co1-xPtx,x=20+/- 2%).

- increase in intensity of Co-Pt peak (2Θ= 43.415°)is due to:a. increase in grain size with decreasing CD (see above);b. improved texture.

[1] H. Jiang et al., J. Vac. Sci. Technol. A 16, 3376 (1998).

XRD spectra of Co-Pt thin films on Cu(100) substrate

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

25 nm

Bright field TEM image of electrodeposited Co-Pt/Cu(100)/Si(100).

Selected area diffraction taken with the beam (a) normal to filmsurfaceand (b) parallel to [110] of FCC grains.

(a)(b)

• Bright field micrograph shows:- well-defined grains with size range 20-35 nm.- large number of stacking related faults.

• Selected area diffraction (SAD) - Fig(a) - taken with beam normal to the film surface:- FCC diffraction spots with (100) texture

- spot splitting along Co-Pt due to {111} twins and their associated lattice strain.

• SAD - Fig(b) - taken with beam along [110] of FCC grains:- FCC (C,D,E) and HCP (F,G,H) diffraction spots

- paired spots and streaks due to twinning.

)).020(),002(( BA

)).1021(),1121(),0002(),111(),002(),111(( HGFEDC

TEM of 125 nm Co-Pt deposited on Cu(100) substrate at 50 mA/cm2

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Bright field TEM image of electrodeposited Co-Pt/Cu(100)/Si(100).

Selected area diffraction taken with the beam parallel to [110] of FCC grains.

• Bright field micrograph shows:- well-defined grains with size range 25-50 nm.- lower density of stacking related faults.

• Selected area diffraction taken with beam along [110] of FCC grains:- mainly FCC spots- streaks and satellite spots due to stacking faults- data suggest that the matrix is FCC with asmall fraction of HCP.

)).111(),002(),111(( CBA

TEM of 125 nm Co-Pt deposited on Cu(100) substrate at 10 mA/cm2

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

XRD spectra of Co-Pt thin films on Cu(111) substrate

- Cu grows epitaxially on H- terminated Si(110)[1].- Co-Pt peak (2Θ=43.415°) is either FCC(111) and/or HCP(0002).- increased intensity of Cu(111) due to additional contribution from either Co-Pt(111) or Co-Pt(0002).

[1] H. Jiang et al., J. Vac. Sci. Technol. A 16, 3376 (1998).

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

• Bright field micrograph shows (right hand side):- well-defined grains with size range 15-35 nm.- no stacking related faults.

• Selected area diffraction taken with beam along [0001] of HCP grains (left hand side):- only HCP spots- no streaks and satellite spots due to stacking faults

TEM of 125 nm Co-Pt deposited on Cu(111) substrate at 50 mA/cm2

50 nm

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

TEM of 125 nm Co-Pt deposited on Cu(111) substrate at 10 mA/cm2

60 nm

• Bright field micrograph shows (right hand side):- well-defined grains with size range 25-55 nm.- very low density number of stacking related faults.

• Selected area diffraction taken with beam along [0001] of HCP grains (left hand side):- HCP spots and a number of Co3Pt type superlatice.

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

• Intrinsic coercivity (Fig. a and b.):- increases with thickness up to 3.7 kOe.- HCI(_|_) (Fig.b) increases slightly with CDdue to a higher HCP fraction with c normalto substrate.

• Retentivity (Fig. c. and d.):- for out-of-plane direction (Fig.d) increases

rapidly then reaches a plateau.- for in-plane direction (Fig.c) the maximum can be explained as a result between competing anisotropies - shape and crystalline (HCP).

Influence of CD on magnetic properties of Co-Pt thin films on Cu(100)

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Very low numberStacking faultsNone

Twins and stacking

faults Defects

Cu3Au-typeHCPNoneHCP Other phases

HCPDisordered FCC-likeHCPDisordered FCC-likeMatrix

25-5525-5015-3520-35 Grain size, [nm]

YesYesYesYesEpitaxial

Cu(111)Cu(100)Cu(111)Cu(100)

10 mA/cm250 mA/cm2

Comparison of microstructure of Co-Pt thin films on Cu substrates

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Conclusions / Future work

• Conclusions:-epitaxial Co80Pt20 films on either Cu(100) or Cu(111) have been successfully grown.

- we demonstrated that by employing a suitable substrate, the matrix could be influenced radically (i.e. Co-Pt on Cu(100) is mainly FCC, on Cu(111) is mainly HCP).

- HCP phase with the c-axis perpendicular to the plane (as in our case) is especially important for the case of electrodeposition of cylinders arrays for patterned media, when one can have both shape and crystalline anisotropy favorable.

- Only for the sample deposited on Cu(100) the intrinsic coercivity achieved in the out-of- plane direction was HCI=3.7 kOe, depending slightly on CD. We expect a substantial increase in coercivity for the samples deposited on Cu(111).

• Future work:- fully characterize the magnetic properties of Co80Pt20 films grown on Cu(111)/Si(110)- grow thicker films to asses limitations of the growth method.

Center for Materials for Information TechnologyA NSF Materials Research Science and Engineering Center

Epitaxial Co-Pt thin films electrodepositedon Cu/Si substratesIntroductionExperiment