Updates of Iowa State University

S. Dumpala, S. Broderick and K. Rajan

Oct-2, 2013

Summary

• Preliminary analysis of silicon oxidation results in comparison with ReaxFF simulations (Adri’s papers)

• Comparison of plasma oxidation and thermal oxidation with hyperthermal oxidation

• Literature review on surface damage studies using Field Ion Microscopy (FIM)

Oxidation of Silicon

ReasFF Simulations APT- Experimental

• Hyper thermal oxidation (atomic and molecular oxygen beam source)

• Dynamic study

• Smaller time scales (3pc)

• Monolayer detection

• Plasma oxidation (ambient oxygen)

• Static study (post deposition study)

• Longer time scales (minutes)

• Sub nano scale detection

Effect of Temperature on Growth Mechanism

0

5

10

15

20

25

-5 -4 -3 -2 -1 0 1 2 3 4 5

Si2O %

SiO2

SiO

0

2

4

6

8

10

12

14

16

18

20

-5 -4 -3 -2 -1 0 1 2 3 4 5

Si2O %

SiO

SiO2

Conc

entr

ation

(at%

)

Distance (nm)

0

2

4

6

8

10

12

14

Region I Region II Region III

Si2O

SiO

SiO2

0

50

100

150

200

250

300

350

400

450

500

Region I Region II Region III

Si2O

SiO

SiO2

Ave

rage

Con

cent

rati

on (a

t%)

0

10

20

30

40

50

60

70

80

90

100

-5 -4 -3 -2 -1 0 1 2 3 4 5

Conc

entr

ation

(at%

)

Distance (nm)

2 nm

Region IRegion III Region II

APT - 383 K

0

10

20

30

40

50

60

70

80

90

100

-5 -4 -3 -2 -1 0 1 2 3 4 5

Conc

entr

ation

(at%

)

Distance (nm)

2.5 nm

Region III Region IRegion II

0

2

4

6

8

10

12

14

16

18

20

-5 -4 -3 -2 -1 0 1 2 3 4 5

Si2O %

SiO

SiO2

Conc

entr

ation

(at%

)

Distance (nm)

0

2

4

6

8

10

12

14

16

Region I Region II Region III

Si2O

SiO

SiO2

0

50

100

150

200

250

300

350

400

450

500

Region I Region II Region III

Si2O

SiO

SiO2

Ave

rage

Con

cent

rati

on (a

t%)

0

2

4

6

8

10

12

14

16

18

20

-5 -4 -3 -2 -1 0 1 2 3 4 5

Si2O %

SiO

SiO2

0

2

4

6

8

10

12

14

16

18

20

-5 -4 -3 -2 -1 0 1 2 3 4 5

Si2O %

SiO

SiO2

Conc

entr

ation

(at%

)

Distance (nm)

APT - 548 K

ReaxFF

• Plasma oxidation results showed similar trend in growth mechanism compared with hyperthermal (ReaxFF) at two different temperatures.

Thermal Oxidation Vs Plasma Oxidation

Plasma2.5 nm

2.1 nm

Thermal

1. Both experiments at same temperature of 270 C

2. Lower interfacial width in the case of thermal oxidation

SiO

SiO

Region III Region IRegion II

Plasma Oxidation

Thermal Oxidation1. The concentrations of Si+1 (Si2O), Si+2 (SiO) in

the interfacial region II and concentration of Si4+

(SiO2) in the bulk region III for the plasma oxidation match with hyperthermal one.

2. In the thermal case, the concentration of Si4+

(SiO2) in region III is much lower.

3. “In thermal oxidation regime, the oxide thickness remains constant and is equal to native-oxide thickness- U. Khalilov et.al.” - The observed trend is in agreement with no significant increase of Si4+

ReaxFF Simulation

Region III Region IRegion IISi+4

Hyper thermal, Plasma and Thermal Oxidation

APT of Si Tips from Maryland : Anode Voltage: 250 V of Argon exposure

50 monolayers 150 monolayers Bare Si Tip

• Laser APT – 1nj (laser power)

• Higher Ar content in beam exposed tips compared to bare Si tip

Mass Spectra

1. Small peak of Ar

2. Check other condition tips (Different beam currents)

3. Different deposition thickness of monolayers

4. FIM studies

50 monolayers

150 monolayers

Bare Si Tip

• Further increasing the electric field at the surface of the tip induces the ionizationand desorption of atoms from the specimen itself via a process known as fieldevaporation. Successively removing layers of atoms from the surface of specimenenables the visualization of the sub-surface structure of the specimen by using FIM.

• FIM, provides atomic-resolution imaging of the surface of a specimen.

• The image is formed by the impact of image gas ions onto phosphor screen giving the local topography of the tip

Field Ion Microscopy (FIM)

Schematic view of a field ion microscope (not to scale). (a) Only the atoms from the very last shell (in green) of the tip surface can be imaged, as seen in the example of a pure W field ion micrograph (right). (b) Equivalent model of stacked spheres arranged on a bcc-lattice. (c) Similar view, with the most protruding atoms highlighted in green - Gault, B., Moody, M.P., Cairney, J.M., Ringer, S.P. 2012, XXIII, Springer Series in Materials Science, Vol. 160

Field-Ion Microscopy of Tungsten Bombarded by Low-Energy Argon Ions

150 eV Ar+ for 45 sec at 63 K, Ion current – 10-9 A

• Extra 7 bright spots and 25 vacancies have been found (encircled areas)• Brightness of at least 10 atoms has changed considerably• Mechanisms for interstitial diffusion and vacancy migration B. Gregov et. al. Can. J. Phys. 50, 1972

300 eV Ar+ for 2 min at 63 K, Ion current – 1.2x10-9 A

• 28 new bright spots and 26 vacancies have been found • Square areas are changes that appeared immediately after bombardment and encircled

areas are the ones appeared in the heating process B. Gregov et. al. Can. J. Phys. 50, 1972

450 eV Ar+ for 2 min at 63 K, Ion current – 1x10-9 A

• Extensive damage has been induced across the whole areas • Along with numerous interstitials and vacancies, a large number of vacancy clusters have

been created indicated by arrows (Eg - cluster of 10 vacancies extends to (122) plane) B. Gregov et. al. Can. J. Phys. 50, 1972

• Point defects have been observed with 150 -300 eV Ar+• Clusters of vacancies and interstitials found at 400 -450 Ev Ar+• Degree of damage increased with higher energy B. Gregov et. al. Can. J. Phys. 50, 1972

• Low energy ion bombardment results in surface and near-surface defects• The heavier ions produced more damage per unit energy• As the incident energy increased, the depth and extent of damage increased D.J. Farnum, et. al. Phys. Stat. Sol. 80, 287, 1983

W with 300 keV Ni+

• Vacancy and vacancy clusters were observed• Depleted zones (size distribution of the damage) detected• Observed vacancy clusters shape and size distribution matched computer

calculations using TRIM (transport of ions in matter code) S. Sato, et. al. Appl. Surf. Sci. 76/77, 285, 1994

W with Ion erergies and Plasmas of H, De, Ne, Ar

W with 150-450 keV Ar+

Literature on FIM

Future

• Ongoing : Finalizing the silicon oxidation results draft

• In-situ environmental cell reactions of Si oxidation with varying growth parameters

• E-cell experiments with Al-oxidation varying temperature for similar analysis of Si-oxidation in relation with ongoing ReasFF work