Threshold ionization mass spectroscopy ofradicals in RF silane discharge

Peter Horvath

5/19/2004

Progress of experiments

Overview

• Experiment is fully assembled• Electronics and computer control work• Electron gun seems to work• Mass spectrometer works (most of the time )• RF generator can operate discharge both in argon and silane• Film growth rate measurement setup is built by Alan, experiment is

ready to start• Discharge and mass spectrometer do not work together (yet)

Schematics of TIMS apparatus

Mass spectrometer and ionizer

Mass spectrometer assemblyIonizer assembly

Electronics and computer control

Mass spectrometer unit and step-up transformer head

Electronics and computer control

Laser PD

RF Gen

Ion deflectors

Magnets

Cathode Anode

F-Cup

Ion lenses

Quadrupole

Channeltron

45 V amplifier Mass spec.Control unit

Electrometer

Rect.

Pressure

Labview

Problems with the electron gun

The current arriving to theFaraday cup is very small,Only 1% of the total emission

Possible reasons:• misalignment (--)• magnetized SS (?)• weak B field (+)• inhom. E field (?)

The total emission is well below space charge limit.

• reflecting anode (+)

Limited cathode lifetime• contamination• operating temp. too high

MagnetScreenF-Cup Cathode

AnodeMagnet

7V offset in the emission vs. voltage curve • the anode may be covered by an insulating layer

Improvements on electron gun

• moly anode and screen• bigger hole on anode• double magnets• F-cup is painted black• anode is painted black• plate in front of the cathode

Improvements on Electron gun:

• system is baked• lower cathode temp

Performance of electron gun

0 2 4 6 8 100

10

20

30

40

50

Anode_scan_051804.OPJ

Em

issi

on c

urre

nt [

A]

Anode-Cathode voltage [V]

10V 9V 8V 7V

Emission current

0 2 4 6 8 100

2

4

6

8

Anode_scan_051804.OPJ

Far

aday

Cup

cur

rent

[A

]

Anode-Cathode voltage [V]

10V 9V 8V 7V

F-Cup current

1 A

-8 -6 -4 -2 0 2 4 6 80

1

2

3

4

5

6

7

16 %

13 %

F-C

up c

urre

nt [

A]

F-Cup bias voltage [V]

12 eV 11 eV 10 eV 9 eV 8 eV 7 eV

13 %

18 %

24 %

31 %

4.6 eV

3.6 eV

3.5 eV

3.4 eV

3.3 eV3.7 eV

Change of currentbw/ 0 & 9 V bias

With of stepbw/ 10 & 90%

Effect of biasing on F-Cup current

Test_e_gun_F_Cup_biased_04-28-04.OPJ

Typical operating conditions:• cathode voltage: -10 V• anode-cathode voltage: 2.4 V• emission current: 4.4 uA• anode current: 0.9 uA• screen current: 2.5 uA• F-cup current: 1.0 uA

Mass spectrometer: mass spectrum

10 15 20 25 30 35 40 45

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

SiH++

2

SiH++

Si+

Ion

sig

na

l [n

A]

Mass [amu]

SiH+

2

SiH+

3

O+

2

Ar+

CO+

2 x20

SiH+

Si++

H2O+

OH+

H3O+

O+

3.9 mTorr SiH4

3.1 mTorr Ar

Residual water x20

Sample mass spectrum

Sample_mass_scan_051604.OPJ

Mass spec. works:• good resolution• reliable mass setting (no drift)• huge dynamic range: 1pA - 1uA• works well with high silane pressure

10 12 14 16 18 200.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

15.23

Data: Thresh042804_B

y=A*exp((x-x0)/t

0)

A 0.10656 ±0.00475x

015.23 ±0

t0

0.41661 ±0.02772

Ion

sig

na

l [n

A]

Electron energy [eV]

Threshold scan with water(ionization energy: 12.61 eV)

Mass spectrometer: threshold scan

Mass spectrometer: optimization of ion optics

-6 -5 -4 -3 -2 -1 0-40

-35

-30

-25

-20

-15

-10

-5

0

Ion3 voltage [V]

Ion

2 v

olta

ge

[V]

-0.005000

0.1887

0.3825

0.5762

0.7700

0.9637

1.157

1.351

1.545

-6 -5 -4 -3 -2 -1 0-40

-35

-30

-25

-20

-15

-10

-5

0

Ion3 voltage [V]

Ion2

vol

tag

e [V

]

0

3.000

6.000

9.000

12.00

15.00

18.00

21.00

24.00

EntranceE-beam

Screen

Ion2

Ion3 (exit)

Silane (beam)

Water (background)

25 26 27 28 29 30 31 32 33 34 35

1E-4

1E-3

0.01

0.1

1

10

100Io

n s

ign

all

[nA

]

Mass [amu]

0 min 3 min 7 min 10 min 16 min 22 min

Discharge_n_mass_spec_051804

Evolution of mass spectrum during discharge

Test of mass spectrometer with discharge

Black curve is the initial spectrum (without discharge)

Observations in silane discharge

Discharge was operated for about an hour on 5/17/04.Pressure: 250-300 mTorr silaneFlow: 5 sccm.Peak-to-Peak RF voltage: 170 VVacuum in the ionizer chamber: 3x10-5 Torr (2x10-6 in the mass. spec chamber)Silicon deposition on windows: ~500 nm (estimation from interference rings)

Mass spectrometer signal is completely disappearedI was not able to record any signal ever since

Observations:

Channeltron may becoated with silicon?

Pressure started to increase at constant flow;Flow is reduced when I adjusted the pressure(flow is measured by foreline pressure)

Vacuum in ionizer region improved to 7x10-6

while the pressure in the discharge chamberwas kept constant

Pumping speed from the discharge chamberis reduced to 1/5 ofthe original

A big chunk of material can be observed at thesampling orifice inside the discharge chamber

The pumping orificesare probably blockedby deposited silicon

Conclusions

• Electronics (including mass spec.) work reliably• Electron gun is ok.• Hopefully the cathode lifetime can be increased by baking the system• The system works well without discharge• Silicon deposition from the discharge will cause a lot of problems…

Plans

• Make the channeltron work again• Deal with the silicon deposition• Film growth rate measurements• Deal with ions from the discharge• Radical measurements• Start Labview in the morning and go hiking…