Alan GallagherJILA, University of Colorado and National Institute of Standards and Technology

Károly RózsaResearch Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences

Péter Horváth

Radical detection in deposition plasmas by threshold ionization mass spectroscopy

Sun

Motivation:

Territory of Hungary: 93,000 km2

Solar irradiation: ~ 1 kW/m2

Sunny hours/year: ~1000 h

Solar energy could provide 10,000 GW(Total electricity production is 4 GW)

Primary production concerns:1. Price (1/production time)2. Product lifetime (stability)3. Efficiency (10-20%)

Photovoltaics are primarily made ofamorphous or microcrystalline siliconwhich are produced in Plasma Enhanced Chemical Vapor Deposition systems

Other applications:Large-area semiconductor devices:eg. Thin Film Transistor Liquid Crystal Displays(that’s what you are watching right now)

Introduction

It’s a 2 billion dollar industry, growing 25 % a year.

Introduction

Amorphous or microcrystalline?

Amorphous silicon Microcrystalline silicon

Easier and faster productionHigh film growth ratesPure SiH4

Less stable:Light irradiation decreases efficiency

Slower, more expensive productionLow film growth ratesR>15:1 H2-SiH4 mixtures

More stable

Two goals:• Understanding of the amorphous-microcrystalline transition• Increasing the growth rate of the microcrystalline film

How are these devices produced?

Plasma Enhanced Chemical Vapor Deposition

SiH4

Low pressure glow discharge(usually RF)

Substrate

Ions: SixHy+

Electrons: e-

Neutral radicals: SixHy

Higher silanes: SixH2x+2 (publ.)

Hydrogen: H2

Silicon particles (publ).

Sili

con

film

Discharge chemistry:Optical detection methods

Film properties:optical methods,

diffraction methods device testing

Samplingorifice

Massspectrometer

Particle fluxes:Mass spectrometry

Atomic Hydrogen: H

Introduction

H2-SiH4

or

Mass spectrometer:Breakup pattern

Ele

ctro

n be

am

Ion optics

QuadrupoleMass selector

ChanneltronElectron multiplier

Source

Ar Ar+ Ar++, 36Ar+e-beamSiH4

Si+

SiH+

SiH2+

SiH3+

Si++

SiH++

SiH2++

SiH3++

29Si+29SiH+

29SiH2+

29SiH3+

Si2+

Si2H+

Si2H2+

Si2H3+

Si2H4+

Si2H5+

Si2H6+

e-beam

neutral ion

Exp. Method

28Si 92 %29Si 5 %30Si 3 %

Mass spectrometer:What do we actually measure?

Discharge

Ele

ctro

n be

am

Ion optics

QuadrupoleMass selector

ChanneltronElectron multiplier

?30Ion+ SiH2

+

or 29SiH+

SiH4

SiH3

SiH2

SiH2+

29SiH29SiH+

Satellite peak of SiH3+

(depending on MS resolution)

To make things even worse, the radicaldensities are less than 10-4 of silane density!

ionsneutrals

and ions

Exp. Method

Neutral Parents

Si+ SiH+ SiH2+ SiH3

+

Si 8.2

SiH 11.2 9.5

SiH2 10.4 13.1 9.7

SiH3 13.1 11.3 12.5 8.4

SiH4 12.5 15.3 11.9 12.3

Principles of Threshold Ionization Mass SpectrometryThreshold energies (eV) for SiHn ions from neutrals

TIMS requires precise control of electron energy (±0.1 eV)Low temperature, indirectly heated cathode is necessaryto reduce the thermal energy spread of electrons

Exp. Method

The main trick is to tune the energy of the electrons in the ionizer

Mass spectrometer:Improving measurement accuracy

Discharge

Ion deflectors

Ele

ctro

n be

am

Ion optics

QuadrupoleMass selector

ChanneltronElectron multiplier

RF Discharge is modulated

(on/off)

Reactive species can be separated

from stable molecules

Deflectors canbe turned on/off

We can measurethe ion signal orreduce it by a factor of 104

Electron energyand current can

be changed

We can perform athreshold scan or

eliminate the ion currentby measuring at diff.

electron currents

Can operate at2 or 5 MHz

5 MHz is used for0-5 AMU range

Channeltrongain can be setby setting HV

Ampl.

Amplifier gainis controlled by

computer

106 dynamicrange for ion

detection

Exp. Method

Scanning electron energy below the dissociative ionization threshold. Note that the actual electron energy is 2.6 eV lower than the cathode voltage.

Sample threshold scan on SiH3+ for detecting SiH3 radicals

Exp. Method

9 10 11 12 13 14 15 16 17 181E-4

1E-3

0.01

0.1

1

10

100

1000

10000D

ete

cto

r si

gn

al [

nA

@ 2

30

0 V

]

Cathode voltage [VK]

Discharge On 1.1uA Discharge On 0.1 uA Discharge Off 1.1 uA Discharge Off 0.1 uA

Ions

Radicals

Background Gas

Radical threshold

Dissociative ionizationthreshold

Ionization due to high energy tail of electronenergy distribution

80 counts/s

Experimental ApparatusApparatus

Film growth rate measurement setup

Laser PhotoDiode

RF Gen

Ion deflectors

Magnets

Cathode

Anode

Faraday Cup

Ion lensesQuadrupole

ChanneltronPressuregauge

Electron beam

RF Plasma

Pumpingorifice

Samplingorifice

+

–

Turbo pump 1Turbo pump 2

Turbo pump 3

Sparker

Electro-meter

Gas inlet

0.3 Torr

1.6x10-4

Torr

1.5x10-6

Torr3x10-5

Torr

Film growth rate measurement setup

Laser PhotoDiode

RF Gen

Ion deflectors

Magnets

Cathode

Anode

Faraday Cup

Ion lensesQuadrupole

ChanneltronPressuregauge

Electron beam

RF Plasma

Pumpingorifice

Samplingorifice

+

–

Turbo pump 1Turbo pump 2

Turbo pump 3

Sparker

Electro-meter

Gas inlet

0.3 Torr

1.6x10-4

Torr

1.5x10-6

Torr3x10-5

Torr

• High vacuum (~ 10-8 Torr) to keep the e-gun cathode surface clean• Pressures: 0.3 Torr (pure silane) and up to 2 Torr (H2-SiH4) during operation• Gas flow: 5 sccm in pure silane and up to 100 sccm in hydrogen mixture• Vacuum: < 10-5 Torr during operation

Apparatus

Apparatus

Mass spectrometer and ionizer head

Apparatus

The mass spectrometer head was designed and built to minimize the discharge-ionizer distance in order to maximize the radical density in the ionizer. Effective differential pumping was also important to minimize the background pressure in the ionizer.

Si SiH SiH

2SiH

3Si

2Si

2H Si

2H

2Si

2H

3Si

2H

4Si

2H

5

0

10

20

30

40

50

60

70

80

90

100

110R

el.

De

nsi

ty

Species

5<0.1 <1

100

0.3 Torr SiH4

1 A/s film growth rate

<0.11.4

18

1.66.3

2.5

ResultsRadical densities in pure silane discharge

Measured relative radical densities in pure silane discharge agree with the results of Robertson and Gallagher (J. Appl. Phys. 59 (10) 3402, 1986)

Experimental investigation of radicals so far concentrated mainly on low pressure (<0.1 Torr) SiH4 and Ar-SiH4 discharges. This time, we also investigate the H2-SiH4 discharges.

Si SiH SiH

2SiH

3Si

2Si

2H Si

2H

2Si

2H

3Si

2H

4Si

2H

5

0

10

20

30

40

50

60

70

80

90

100

110

0.1 A/s film growth rate

<6<4<3<3

Re

l. D

en

sity

Species

100

<3

15

<4 <6

1.5 Torr R=40 H2-SiH

4 mixture

<8

ResultsRadical densities in hydrogen-silane discharges

The measured radical densities for 40:1 and20:1 H2-SiH4 discharges have similar behavior: SiH3 is by far the most important radical; Si2H2 is smaller, but comparable, while the rest of the radicals can not be positively identified. (Only R=40 is shown here.)

These results are new and provide importantinformation about the mechanism of film growthin the ever more important hydrogen-silane discharges.

Results

160 170 180 190 200 210 220 230 240 2500.00

0.05

0.10

0.15

0.20

0.25R

ad

ica

l sig

na

l [n

A @

23

00

V]

Discharge voltage [Vpp]

SiH3

Si2H

2 x10

1.5 Torr, R=20 H2-SiH

4 discharge

Major radical densities vs. discharge voltage

27 28 29 30 31 32 330

2

4

6

8

10

12

14

16

Sig

na

l [n

A @

13

00

V]

Mass [amu]

Silane Ions from the discharge

54 55 56 57 58 59 60 61 62 63 640

2

4

6

8

10

12

14 Disilane Ions from the discharge

Sig

na

l [n

A @

13

00

V]

Mass [amu]

27 28 29 30 31 32 330.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5Silane Ions from the discharge

Sig

na

l [n

A @

13

00

V]

Mass [amu]54 55 56 57 58 59 60 61 62 63 64

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45 Disilane Ions from the dischargeS

ign

al [

nA

@ 1

30

0 V

]

Mass [amu]

ResultsMass spectra of discharge ions

Pure silane:mostly stripped ions (less hydrogen)

Hydrogen-silane(R=40):hydrogenated ions

SiH+

SiH3+

Si2H2+

Si2H2+

SiH+

SiH3+

Si2+

Si2H5+

Si2H5+

ResultsConclusions

We measured the mono- and disilicon radical fluxes in R=20 and 40 hydrogen-silane discharges and compared the results to the previously known pure silane results. All of the above cases show similar behavior, SiH3 and Si2H2 being the major radicals contributing to film growth. The similar radical chemistry does not explain the difference in resulting film quality and growth rate.

The measured ion spectra is shifted towards the hydrogenated ions in the highly diluted case. The different ion bombardment can result in a different film structure, explaining the amorphous-microcrystalline transition.

Growth rate measurements in highly diluted (R>80) case show negative growth rates. (Not shown here.) This observation indicates a possible atomic hydrogen and/or hydrogen-ion etching of the film. This can also explain the transition between amorphous and microcrystalline film.

Articles:

P. Horvath, K. Rozsa, A. Gallagher: Production of higher silanes in radiofrequency SiH4 and H2-SiH4 plasmas. J. Appl. Phys. Vol. 96, 7660 (2004)

G. Bano, P. Horvath, K. Rozsa and A. Gallagher: The role of higher silanes in silane-discharge particle growth J. Appl. Phys. Vol. 98, 013304 (2005)

P. Horvath and A. Gallagher: Threshold ionization mass spectroscopy of radicals in RF SiH4 and H2-SiH4 plasmas (in preparation)

Publications in this topicPublications

Conference Contributions:

P. Horvath, K. Rozsa, A. Gallagher: Polysilane production in RF SiH4 and H2-SiH4 plasmas XXVIth International Conference on Phenomena in Ionized Gases, Greifswald 15-20 July, 2003

P. Horvath, A. Gallagher: Threshold ionization mass spectroscopy of radicals in a radiofrequency SiH4 plasma, XXVIIth International Conference on Phenomena in Ionized Gases, Eindhoven 17-22 July, 2005

Acknowledgements

I would like to acknowledge the help of my supervisors, Alan Gallagher and Károly Rózsa. I would like to thank my collagues, Wengang Zheng and Damir Kujundzič for valuable discussions. I also thank the JILA instrument shop personel for the construction of the apparatus.

Thank you for your attention!

Film growth rates

Film growth rates are measured by laser inter-ferometry on the grounded electrode. A 780 nmdiode laser was used in a perpendicular setup.

Film growth rate is one of the most importantparameters in industrial systems, and helps usto relate our results to other experiments.

Apparatus

780 nm

diode laser

PD

Polishedelectrode

Growing film

Ground electrode with ~100 nm amorphous silicon film

time

ResultsFilm growth rates

100 150 200 250 300 350 400 450 5000.00

0.05

0.10

0.15

0.20

0.25

0.30

Film

Gro

wth

Rat

e [n

m/s

]

RF peak-to-peak voltage [V]

1.0 Torr 36:1 1.5 Torr 36:1 2.0 Torr 36:1 1.0 Torr 16:1 1.5 Torr 16:1 2.0 Torr 16:1

Film Growth Rates in H2-SiH

4 plasmas

100 150 200 250 300 350 400 450 5000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9Film Growth Rates in SiH

4 plasmas

Film

Gro

wth

Ra

te [n

m/s

]

RF peak-to-peak voltage [V]

230 mTorr 250 mTorr 272 mTorr 300 mTorr 323 mTorr 337 mTorr 355 mTorr

Film growth rates are measured at different pressures, hydrogen dilution ratios and discharge voltages.

Typically 0.1-0.3 nm/s growth rates are used for high quality film production. Highergrowth rates result in unstable films.

Microcrystalline silicon films produced in H2-SiH4 has better quality and long-term stability, but the growth rates are limited to < 0.1 nm/s.

IntroductionSilicon based photovoltaic devices

Light

a-Si p

a-Si i

a-Si n

Glass

TCO

Metal

e-

hole

Typical p-i-n structure of a PV device

Photoelectric effect works in every semiconductor material. Silicon is commonly used, because it is easy to dope to both p and n-type.

Amorphous silicon is preferred over single crystals, because it absorbs light 40 times better. → device can be 1 μm thin → cheaper to produce → can be produced on top of flexible materials

Electron-hole recombination rate in pure a-Si is very high due to dangling bonds → + hydrogen → Hydrogenated amorphous silicon (a-Si:H)

a-Si:H devices suffer from the Staebler-Wronski Effect:Device efficiency decreases by ~20% after exposure to sunlight

Microcrystalline silicon (μC-Si:H ) can be produced by adding hydrogen to the feed gas during production → more stable film

• Film growth rate is decreased as dilution ratio (R) is increased• Transition from amorphous (R<10) to microcrystalline film (R>20) is not fully understood

0.0 0.5 1.0 1.5 2.0 2.4

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0Io

n S

ign

als

[arb

. no

rm. u

nits

]

Time [s]

Silane Disilane Molecular hydrogen Atomic hydrogen Silyl radical

Discharge

Time dependence of densities in a pulsed dischargeExtra

27 28 29 30 31 32 330

20

40

60

80

100 Silane spectrum

Sig

nal [

nA @

130

0 V

]

Mass [amu]

54 55 56 57 58 59 60 61 62 63 640.0

0.2

0.4

0.6

0.8

1.0 Disilane spectrum (produced by the discharge)

Sig

na

l [n

A @

13

00

V]

Mass [amu]

Typical mass spectra of stable gases in the discharge

Silane Disilane

Extra

26 27 28 29 30 31 32 33 34 35

1E-4

1E-3

0.01

0.1

1

10

100

1000Io

n S

ign

al [

nA

]

Mass [amu]

DC/RF: 0.1600 0.1610 0.1620 0.1630 0.1640 0.1650 0.1660 0.1670 0.1680 0.1690 0.1700

Silane Peaks vs. DC/RF setting

Mass_spec_resulotion_03-29-05.OPJ

ExtraMass spec. resolution vs. DC/RF setting

34 35 36 37 38 39 40 41 421E-5

1E-4

1E-3

0.01

0.1

1

10S

ign

al [

nA

@ 1

30

0 V

]

Mass [amu]

Properly aligned quadrupole Misaligned quadrupole

Satellite_peaks_03-29-05.OPJ

Extra

Effect of quadrupole misalignment on mass peaks

36Ar

38Ar

40Ar

LG Phillips cathode used for the measurementsExtra

Extra

Apparatus

-40 -30 -20 -10 0 10 20 30 40

-40

-30

-20

-10

0

10

20

30

40

Deflector_optimization_2D_28-13-05.OPJ

H+

3 1400 V

Defl+ [V]

De

fl- [V

]

-3.900

-3.035

-2.170

-1.305

-0.4400

0.4250

1.290

2.155

3.020

Logarithmic scale

Optimizing deflector voltages for maximum ion rejection

Defl+

Defl-

Extra

Design of ion optics:calculation of ion trajectories

Courtesy of Wengang Zheng

Extra

-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

3.000

6.000

9.000

12.00

15.00

18.00

21.00

24.00

Optimizing ion optics to collect the best possible signal

Silane beam

EntranceE-beam

Screen

Ion2

Ion3 (exit)

Extra