National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Measurement of Depositing and 

Bombarding Species Involved in 

the Plasma Production of 

Amorphous Silicon and 

Silicon/Germanium Solar Cells 

Annual Technical Report 

1 June 2002 — 31 May 2005 

A. Gallagher, K. Rozsa, P. Horvath, and 

D. Kujundcik 

National Institute of Standards and Technology 

Boulder, Colorado 

Subcontract Report 

NREL/SR-520-40056 

June 2006 
Measurment of Depositing and 

Bombarding Species Involved in 

the Plasma Production of 

Amorphous Silicon and 

Silicon/Germanium Solar Cells 

Annual Technical Report 

1 June 2002 — 31 May 2005 

A. Gallagher, K. Rozsa, P. Horvath, and 

D. Kujundcik 

National Institute of Standards and Technology 

Boulder, Colorado 

NREL Technical Monitor: B. von Roedern 

Prepared under DOE/GO Interagency Agreement No. DE-AC36-02GO10244 

Subcontract Report 

NREL/SR-520-40056 

June 2006 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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iii

Table of Contents 

Abstract............................................................................................................................................1 

Objective..........................................................................................................................................1 

Introduction......................................................................................................................................1 

Apparatus .........................................................................................................................................3 

 1. Overview...............................................................................................................................3 

 2. Electron Beam.......................................................................................................................5 

 3. Ion Collection........................................................................................................................6 

 4. Discharge-Ion Background ...................................................................................................9 

5. Quadruple Mass Spectrometer............................................................................................10 

 6. Gas Handling System..........................................................................................................10 

Results............................................................................................................................................13 

 1. Film growth rates ................................................................................................................13 

 2. Stable Gases........................................................................................................................16 

 3. Radicals...............................................................................................................................17 

 4. Discharge Ions ....................................................................................................................22 

Conclusions....................................................................................................................................28 

References......................................................................................................................................28 

List of Figures 

Figure 1. Diagrammatic representation of the apparatus designed for discharge radical 

detection by threshold ionization mass spectrometry ....................................................4 

Figure 2. Photograph of the ionizer and ion optics mounted on the QMS. The rods of the 

QMS are visible at the right, inside the ~ 2” diameter supporting cylinder ..................6 

Figure 3. Diagram of the ion-collection region of the apparatus, and calculated ion orbits 

for VIon2 = -30 V and VIon3 = -2 V..................................................................................8 

Figure 4. Dependence of the ion signals on ion-lens voltages........................................................9 

Figure 5. Vacuum apparatus, with the mass spectrometer flange at the upper left and the 

rough pumps and pyrolyser at the lower right .............................................................12 

Figure 6. Photograph of the rough pumps and pyrolyser..............................................................13 

Figure 7. Film growth rate (G), versus VRF and PTot, for R = 16 and 36 .......................................14 

Figure 8. Film growth rate with pure silane inlet flow, versus pressure and VRF. The silane 

depletion is 15% at 0.35 nm/s film growth rate ...........................................................15 

Figure 9. Time dependence of densities during discharge............................................................16 

Figure 10. SiH3

+

 signal versus electron energy below the SiH4 dissociative ionization 

threshold. TIMS signals are averaged during and after the switched discharge..........17 

Figure 11. Detection of SiH3 radicals: energy scan below the SiH4 → SiH3

+

 threshold..............19 

Figure 12. Mass 58 signal versus electron-beam cathode voltage, with and without 

discharge ......................................................................................................................19 

Figure 13. Radical densities at the substrate of a 0.3 Torr, 100 C, 200 V, pure-silane 

discharge ......................................................................................................................20 
iv

Figure 14. Radical densities at the substrate of a 100 C, R= 16 silane-hydrogen discharge, 

with VRF = 200 V (p-p) ................................................................................................20 

Figure 15. Radical densities at the substrate of a 100 C, R= 36 silane-hydrogen discharge, 

 with VRF = 200 V (p-p) ...............................................................................................21 

Figure 16. Ions from a 0.3 Torr, 200 V, 100 C pure-silane discharge..........................................23 

Figure 17. Ions from a 1.5 Torr, 200 V, R = 20 silane-hydrogen discharge.................................24 

Figure 18. Ions from a 1.5 Torr, 200 V, R = 40 silane-hydrogen discharge.................................25 

Figure 19. Types of discharge ions at the substrate of a silane/hydrogen RF discharge, 

versus the pressure (inlet flow) ratio of silane to hydrogen.........................................26 

Figure 20. Types of ions at the substrate of an RF discharge in pure silane, versus peak RF 

voltage (V) divided by pressure (Torr). The SixHy

+

 have been summed over y..........27
Abstract 

The properties of an apparatus designed to measure silane and silane/hydrogen discharge 

radicals by threshold ionization mass spectroscopy are described. We report 

measurements of these radicals, as well as discharge ions, at the substrate of an RF (13.6 

MHz) discharge in pure silane and hydrogen/silane mixtures with (inlet flow) density 

ratios R = 16 and 36. The discharge gap, pressure, temperature and power are adjusted to 

fit typical amorphous or microcrystalline silicon device-production conditions. 

Amorphous silicon film deposition rates are measured, to relate the discharge conditions 

to those used in device-production reactors. Radical and ions densities at the substrate 

have been measured for the molecular species SixHn with x = 0,1,2. The primary 

depositing radicals in the pure-silane discharge are SiH3 and Si2H2, while the other SiHn

and Si2Hn become significant in the hydrogen-diluted discharges. Discharge ions are 

dominated by Si- containing ions (SixHy

+

), even in highly hydrogen diluted vapors. 

However the heavier ions (x > 1) are more important as the silane fraction increases, and 

the H content (y value) is lower in pure silane. 

Objective 

The objective of this study is to measure the molecular species that lead to the growth of 

hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si) 

photovoltaic (PV) devices from RF discharges. Neutral radicals produce most of the film 

growth during this PV-device production, and by implication radicals primarily 

determine the device structure and electrical characteristics. The most important feature 

of the present experiment is thus the measurement of neutral-radical fluxes to the 

substrate. Additional depositing species that can influence film properties are positive 

ions and silicon-based particles produced by the discharge; we also measure these 

positive-ion species here. Some studies have already measured some of these radical and 

positive-ion species in silane and silane/argon discharges, but not for discharge 

conditions similar to those used to produce most photovoltaic devices. Our objective is to 

measure all of these species for conditions typically used for device production. In 

particular, we have studied 13.6 MHz-excited discharges in pure silane and 

silane/hydrogen vapors. 

Introduction 

The reactor is a small-scale version of an industrial PV-production reactor, utilizing 

similar electrode gap, temperature, gas pressure, gas mixtures and discharge power 

density. Gas flow is adjusted to produce similar dwell times to industrial reactors, or 

shorter times to clarify the influence of gas depletion and modification by the discharge. 

The reactor contains pure silane or mixed silane and hydrogen, under conditions typically 

used to produce hydrogenated amorphous (a-Si:H) and microcrystalline (μc-Si) silicon 

solar cells. Radio frequency (RF) discharges are studied, as these are most commonly 

used for industrial production of solar cells. Our primary interest is in measuring radicals 

that produce the devices in RF discharge, H2/SiH4 mixtures, as these are most frequently 

used for device production. However, to improve understanding of the deposition 

1
chemistry, it is also useful to compare the radicals from various H2/SiH4 mixtures to 

those from a pure silane vapor. We utilize threshold ionization mass spectroscopy 

(TIMS) to detect the radicals that pass through a small orifice in the substrate. None of 

these radical fluxes to the substrate have previously been measured for the pressure range 

and deposition-rate conditions typically used for device production. However, quite a few 

measurements have been made in low pressure, high power density discharges, as 

described next. 

Some of the first discharge-radical measurements in silane discharges were done at this 

laboratory, using TIMS.1,2 These observations demonstrated important radical chemistry 

and the dominance of SiH3 deposition for conditions used to produce good quality a-Si:H 

films. However, they used a DC discharge at low (< 0.1 Torr) silane pressure, with 

varying Ar dilution. Thus, they did not provide radical data for hydrogen dilution or for 

RF discharges, as is now used for device production. For the last 20 years the laboratory 

of Toshio Goto has used optical methods to detect the radicals Si, SiH, SiH2 and SiH3. 

These observations have yielded valuable radical-silane reaction rate constants, and 

confirmed the importance of SiH3.

3,4,5 However, they have been carried out only in low 

pressure gases with very high power loading, yielding a mixture of radicals that is 

expected to be very different than under the high pressure, low power conditions 

normally used to produce photovoltaics. (Microcrystalline Si can be produced with some 

discharges studied.) Moreover, most of the measurements utilize a DC hollow cathode or 

ECR discharge. Jerome Perrin and collaborators have also studied radicals in RF silane 

discharges for many years, using TIMS.6,7 They have measured the very important 

surface reaction probabilities of several radicals on a-Si:H and uc-Si. However, most of 

their measurements utilize very low silane pressure (0.06 Torr) and high power density, 

hence the observed radical densities do not apply to device production conditions. The 

exception is a 1 Torr, SiH4/H2 = 1/1 discharge6

 that produced 1.2 nm/s film growth; 

conditions that cause major particle growth. This yielded large SiH3 and Si2H5 densities, 

with most of film growth produced by the SiH3. In contrast to these prior studies, the 

present research utilizes 0.3 Torr pure silane or 1.5 Torr SiH4/H2 mixtures with ratios 

(1/R) of 1/20 and 1/40. The film growth rate, which characterizes the discharge power 

density, is also normally a low (~0.1 nm/s) value, as used to produce the best a-Si:H 

films. 

This project has been staffed by three scientists, in addition to the PI, Alan Gallagher. Dr. 

Karoly Rozsa, a senior scientist from Budapest, Hungary, with many years of discharge 

experience initiated the design and construction phases. After about a year he was joined 

by two graduate students: Peter Horvath, a Physics Department student of Budapest 

University, and Damir Kujundzic, a EE student at the University of Colorado. Dr. Rozsa 

had other responsiblities at his Budapest laboratory, and left about halfway through the 

program. Damir Kujundzic also left about two years into the program, and essentially all 

measurements were carried out by Peter Horvath. He is currently analyzing these results 

and incorporating them into his thesis at Budapest University. We will publish them after 

his graduation and our analysis is completed. The data presented in this report is thus 

somewhat preliminary, and may be corrected or further analyzed in the publications. 

2
Apparatus 

1. Overview 

 

The reactor is a small-scale version of an industrial PV-production reactor, utilizing a 

similar electrode gap (2 cm), temperature (~100 °C), gas pressures (0.3-2 Torr), gas 

mixtures and discharge power density. The latter is typically adjusted to produce 0.1-0.3 

nm/s film growth rates, again as is commonly produced in industrial reactors. But in 

contrast with industrial reactors, this experimental reactor is about 8cm diameter and will 

normally be isothermal. Gas flow is adjusted to produce similar dwell times to industrial 

reactors, or shorter times to clarify the influence of gas depletion and modification by the 

discharge. The reactor contains pure silane or mixed silane and hydrogen, under 

conditions typically used to produce hydrogenated amorphous (a-Si:H) and 

microcrystalline (μc-Si) silicon solar cells. Radio frequency (RF) discharges are studied, 

as these are most commonly used for industrial production of solar cells. 

As mentioned in the Introduction, the neutral radicals have been measured using 

threshold ionization mass spectrometry (TIMS). In this technique, an electron beam of 

near-threshold energy ionizes the radicals, and the ions are collected by a quadrupole 

mass spectrometer (QMS). This discriminates against the same ions produced by 

fragmentation of stable gases, since the threshold energy for the dissociative process is 

higher than for radical ionization without dissociation, usually by a few eV. As an 

example, the threshold for producing SiH3

+

 from the SiH3 radical is 8.1 eV, and from 

SiH4 is 12.3 eV. To confirm that the radical is actually being observed in the presence of 

much higher densities of parent gases, ions are collected while the electron energy is 

scanned through the threshold energy region. 

This project required developing a new experiment, requiring apparatus design, 

construction and testing before data could be obtained. However, the TIMS method had 

previously been used in this laboratory and other laboratories. The TIMS method can 

detect very small radical fractions within their parent molecular gas, as is needed for 

diagnosis of these discharges. However, the ionization efficiency is much smaller than in 

conventional mass spectrometry, and good electron-energy control and ion collection 

efficiency is required. Thus, a unique apparatus design, geared to this application, is 

necessary, and is shown diagrammatically in Fig.1. The quasi-neutral plasma (glowing) 

portion of the discharge is shown diagrammatically; other dimensions are only 

representative. Gas flows mostly over the top of the RF electrode and across it to a 

pumping orifice at the bottom. (The orifice size is adjusted to yield the desired gas flow.) 

A 0.4 or 1 mm diameter orifice (#1) in the grounded (substrate) electrode allows radicals 

and ions, accompanied by the discharge gases, to exit toward the QMS. Since the QMS 

and electron multiplier (EM) must operate at pressures below 10-5 Torr, while the 

discharge operates at 0.2-2 Torr, several stages of differential pumping are required to 

prevent most discharge gases exiting orifice #1 from reaching the QMS. The first stage is 

provided by Pump 1, which also handles the full gas load. The next stage, which is 

separated from the first by a 1 mm x 4 mm slot-aperture (#2), includes the ionizer region 

and is evacuated by pump 2. The final stage is separated from the ionizer region by a 2 

3
mm diameter QMS entrance aperture (#3), and is evacuated by pump 3. Pump 1 is a 300 

liter/s turbomolecular pump, while pumps 2 and 3 are 50 liter/s turbomolecular pumps. 

An electron multiplier after the QMS allows single-ion sensitivity. 

Fig.1. Diagrammatic representation of the apparatus designed for discharge radical 

detection by threshold ionization mass spectrometry. Discharge radicals pass through an 

aperture (#1, farthest left) into the ion deflector region, then through a slot (# 2, middle) 

into the ionizer region, which is evacuated by pump 2. Ions from this region are focused 

through an aperture (#3, farthest right) into the mass spectrometer, which is evacuated by 

pump 3. When biased, the ion-deflector screens prevent discharge ions from entering the 

mass spectrometer, for detecting neutral radicals, and discharge ions are studied by 

removing the bias. 

Devices are normally deposited onto heated substrates, at temperature TS, and this can 

influence discharge operation and chemistry. Thus, it is useful to study depositing species 

over a temperature range of 25-250 °C. While the RF electrode is not directly heated in 

most reactors, its temperature typically rises to 50-90 % of TS. In our small, experimental 

reactor it is not practical to attempt reproducing this temperature gradient between the 

electrodes. We therefore heat the entire discharge region, and in the present design the 

entire apparatus, to the desired TS. 

An additional apparatus feature allows in-situ measurement of film growth rates. A laser 

beam that passes through a small orifice in the RF electrode (lower left in Fig.1) and is 

detected after reflecting off the growing film and substrate. This provides instantaneous 

film growth data, since the reflected intensity undergoes interference fringes that depend 

on film thickness. We and other researchers have frequently found that discharge optical 

4
emission can provide a valuable diagnostic of silane-containing discharges. Thus, we 

utilize a discharge-viewing window to measure discharge emission. 

We utilize full computer control of the experiment and data collection, using National 

Instruments input-output cards and LabView programming. We control the discharge 

power and timing and the QMS settings, and recording several types of data. The data 

includes the discharge pressure, RF voltage, ionizer electron beam current and energy, 

the a-Si:H film growth rate, and the ion signal from the mass spectrometer. 

2. Electron Beam 

A critical component of the apparatus is the ionizer, shown diagrammatically in the 

center of Fig.1. Here a magnetically-confined electron beam, of typically 1 μA, crosses 

the ionization region and is collected in a cup on the opposite side. The cathode is 

operated at the lowest possible temperature that will yield this current, since the highenergy thermal tail of the electron-energy distribution will produce an interfering ion 

signal from the parent gases. The ionization region is enclosed with a screen, to assure 

equipotential surfaces necessary for efficient ion collection, while allowing gases to be 

efficiently pumped away. The electron beam current is controlled by a “anode” 

immediately in front of the cathode, so that it can be held constant while the electron 

energy is scanned in TIMS. A “Faraday cup” collects this current for control and 

analysis. 

The pressure in the ionizer region is considerably higher than in the QMS region. This is 

not expected to influence the electron or ion motion as long as pressures are too low to 

initiate a hot-cathode discharge, but silane reactions with the cathode surface can cause a 

loss of electron emission. The magnetic field (~ 40 Gauss) is sufficient to confine the 

electron beam, but too small to influence the ion motions. In addition, the location and 

size of the magnets is designed to minimize the fringing magnetic field in the RF plasma 

region. In practice, this fringing field will be a few Gauss, which minimally influences 

the discharge since νColl >> νCyl, where νColl is the electron collision frequency and νCyl

the electron cyclotron frequency. The actual ionizer structure, and its mounting to the 

QMS, is shown in Fig.2. 

Electron beam tests yielded the desired 1-5 μA current at 8-12 eV energy from “oxide” 

cathodes at 800-900 °C. This is quite satisfactory, particularly as tests in the presence of 

~ 10-4 Torr silane yielded long operation at relatively stable 1 μA currents. The current 

passes through a 1 mm hole in an “anode” ~ 2 mm from the cathode emitting surface. 

The anode voltage is held constant relative to the cathode, providing a constant current 

while the electron energy (cathode voltage) is scanned. If a much larger current were 

used, the electrons would produce a “space charge depression” of the potential in the 

ionization region, while the accompanying electric fields distort ion motion and diminish 

ion collection efficiency. These fields are proportional to the electron current and 

relatively independent of the electron beam size, so it is necessary to limit the electron 

current in the indicated range. The ~ 1mm electron beam diameter provides a relatively 

compact ion source region for focusing into the QMS. 

5
Fig.2. Photograph of the ionizer and ion optics mounted on the QMS. The rods of the 

QMS are visible at the right, inside the ~ 2” diameter supporting cylinder. The o-ring in 

the center of the figure is mounted on a polyimid disc that separates the ionizer and QMS 

regions. 

3. Ion Collection 

The ion collection efficiency is a crucial element of TIMS, since the small electron 

current and threshold-region ionization cross section yield much smaller ionization rates 

than in conventional mass spectrometry. Thus, in our design a coaxial screen applies a 

relatively strong electric field at the ionization location, to draw out essentially all ions 

and focus them into the QMS orifice. The typical bias of 30 V on this nearby “drawout” 

lens can also overcome fields due to surface potential variations. However, this also 

produces an electron energy spread in the ionization zone, so this field must not be too 

large. 

6
Studies of the TIMS apparatus indicate that the ion collection efficiency of the ion-optics 

and QMS is typically one percent of total radical ionization. This is attributed primarily 

to a very small diameter of the QMS inlet that achieves ion transmission. This slows data 

collection but does not preclude the desired radical observations. 

The electrostatic lenses that focus ions into the mass spectrometer are shown in more 

detail in Fig. 3, where calculated ion orbits are also shown. Due to necessity of detecting 

low radical densities at the substrate, and the small fraction of radicals exiting the 

substrate orifice that are ionized by the electron beam, it is important to have efficient ion 

collection into the MS. The radicals expand thermally from the substrate orifice, and 

when ionized the ions initially have almost the same (vector) velocity, as if they had 

started at the orifice. The ion optics is therefore designed to focus the substrate orifice 

into the MS input orifice. This design, and its performance, will now be presented. 

An electrostatic lens is always positive, with a strength that depends on the ratio (RE) of 

ion kinetic energy entering and exiting the lens. Here the initial kinetic energy is the 

thermal energy, typically ~ 0.04 eV, and the ions are accelerated to ~30 eV in the first 

lens, so RE is very large for this lens, that is between the grounded-screen, “source” 

electrode and the “First Ion Lens” electrode. This large RE has two important advantages. 

First, the lens has almost the same strength for RE = 1000 as for RE = 50 (ie. for large RE), 

and as a result the large spread of initial thermal energies does not produce a large spread 

of ion orbits. The second advantage of this large “Ion2” voltage is that it produces a 

strong electric field at the ion-source location. As a result, stray electric fields due to 

surface contact-potential differences and electron-beam space charge do not seriously 

perturb the ion orbits. The disadvantages of the large first-lens RE, and the resulting short 

focal length, is that the image (“Image1”) of the substrate aperture occurs only slightly 

beyond the lens plane (the plane between the grounded screen and “Ion2”), and the ions 

have a large angular spread at the image. The MS would not transmit a very large fraction 

of these ions. To overcome this we introduced lens two, between elements “Ion2” and 

“Ion3”, to produce “Image2” at the MS input. This re-imaging lowers the ion angular 

spread entering the MS, allowing for higher MS ion transmission. The ion energy inside 

the MS is typically 7 eV, and “Ion2” is typically set to ~3 Volts, so the second lens 

decelerates the ions from 30 to 3 eV, and there is a third, weak (RE = 2.3) lens at the MS 

orifice. 

7
Electron 

Beam 

Sampling 

orifice 

Ion 

Orbits 

Ion 2 

Lens Element 

Ion 3 

Lens Element 

Quadrupole 

Mass 

Spectrometer 

Fig.3. Diagram of the ion-collection region of the apparatus, and calculated ion orbits for 

VIon2 = -30 V and VIon3 = -2 V. The lens diameters are 1 cm, and the vertical dimension is 

expanded a factor of 3 (relative to horizontal) for clarity. The ionization occurs in a 

grounded-screen chamber, and the mass spectrometer is biased to -7 V. 

8
0.38

0.19

0.56

0.75

0.94

1.1

1.3

1.5

-10 -8 -6 -4 -2 0

-40

-35

-30

-25

-20

-15

-10

-5

0

Ion2 voltage [V]

Ion3 voltage [V]

Values are in [nA/mTorr]

Using ~10 mTorr Argon 

The "low voltage" focusing mode of ion optics

Fig.4. Dependence of the ion signals on ion-lens voltages. 

The measured dependence of ion signal on the voltages of the ion-focussing lenses is 

shown in Fig.4. This compares favorably with ion-orbit calculation using the SIMION 

code, from which an example set is shown in Fig.3. By comparing the voltage 

dependencies of the SIMION orbits to the data in Fig.4, we conclude that the ion 

collection is working approximately as designed. It appears that ions from ~ 0.5 cm of 

electron-beam length, out of 1 cm inside the “source” screen region, are collected with 

good efficiency. To miminize contact potential differences, particularly in the presence of 

silicon deposition, the ion lens elements and the grounded screen are usually coated with 

“Aero Dag”, a sprayed carbon black. 

4. Discharge-Ion Background 

The discharge-ion deflector shown in Fig.1 is a pair of screens biased to deflect discharge 

ions that exit through the sampling orifice. Several volts bias on one screen prevents most 

discharge ions from entering the QMS and interfering with the desired radical-ionization 

signals. This is a small enough voltage to prevent a discharge from developing in this 

region outside the main discharge. The flux of discharge ions at the substrate is typically 

only a few percent of the radical flux, but only ~ 10-7 of the radicals passing through the 

sampling orifice are ionized. Thus, the discharge-ions must be suppressed by a factor of 

9
>105

 to avoid a serious loss of signal/noise in the TIMS data. In practice, we have 

achieved suppression factors of 104-106 for the various ions and discharge conditions. 

5. Quadrupole Mass Spectrometer 

While some commercially available quadrupole mass spectromerters QMS can be 

adapted for TIMS measurements, they are quite expensive and the ionizer region would 

still have to be reconstructed and independently controlled. Thus, we have chosen to 

utilize and modify a QMS left over from another project in our laboratory. This includes 

replacing an unreliable “channel-plate” electron multiplier with a “Channeltron” electron 

multiplier from Burle Corp. This provides large and stable gain, allowing individual ions 

to be easily and reliably detected. 

We utilize a Dycor/Amatec QMS for the radical detection, with the ionizer and ion 

collection portions modified for use in TIMS. We found the QMS electronics very 

unreliable, obscure and ill suited for our need to average weak ion signals during ionizing 

electron energy (Eel) scans under computer control. Thus, we have produced our own 

QMS electronics. This requires generating a fixed RF frequency and precisely controlling 

its amplitude (A) on the QMS rods, since A determines the transmitted mass. The DC rod 

voltages (VDC) must reach hundreds of Volts (RMS) and be independently controlled, as 

this determines the transmission and mass resolution. The correlated A and VDC are 

scanned under computer control to scan mass, or fixed during an Eel scan. A simple, 

inexpensive set of home-built electronics now provides these voltages under computer 

control, and QMS operation using this electronics is reliable and accurate. We have also 

provided for switching the QMS RF frequency between 2 and 5 MHz, where the former 

is best for the mass range corresponding the SixHn

+

 peaks with x = 1-3, and the latter 

allows Hn

+

 (n = 1-3) ions to be collected without major interference from the “zero-mass” 

peak. The final mass spectrometer development was to study the DC and RF voltage 

spaces to obtain optimum ion-collection efficiency and mass resolution for the radicals 

studied. 

Another factor that contributes to the size of the SixHn radical signals is the transmission 

of the mass spectrometer, at the resolution required to fully separate the SixHn

+

 peaks. 

This has been evaluated by comparing the current entering a 2 mm aperture into the 

quadrupole region, versus that leaving this region (see Fig.2). A typical transmission in 

this mass 30 region is 10%. In the mass region of 56-62, corresponding to the disilane 

radicals Si2Hn, this transmission has dropped to ~ 2 %, and for H and H2 it is near 50%. 

Thus, we are most sensitive to H atoms, once H+ is well resolved from H2

+

 and “zero 

mass” by switching the mass spectrometer frequency to 5 MHz. 

6. Gas Handling System 

The gas handling system includes a ~ 950 °C gas pyrolizer between the turbomolecular 

and final rough pump, to break down the silane gas into Si plus hydrogen gas that can be 

vented safely. Similarly, ultra-high vacuum, stainless steel valves control the inlet and 

outlet gas flows. The pyrolizer is placed between the rough pumps that evacuate the 

10
chambers and another rough pump that prevents air from back streaming into the 

pyrolizer. This pyrolizes silane into surface-bonded Si and H2 gas that can be safely 

vented. Similarly, ultra-high vacuum, stainless steel valves control the inlet and outlet gas 

flows. 

The pyrolysis produces a dense, loose layer of Si-metal alloy flakes over a period of a 

year or two, and we must prevent small flakes from backstreaming from the pyrolyser 

into turbo molecular pump bearings when the system is brought up to air. For this reason 

the pyrolyser has been placed after two rough pumps that evacuate the three turbo pumps. 

But one must also prevent air backstreaming into the pyrolyser, so a third (small) rough 

pump evacuates it. Most rough pumps are not designed for vacuum integrity at the outlet 

side, so this required modifying the first two rough pumps to achieve an outlet vacuum. 

This extra effort is the disadvantage of this approach, but the advantage of this pyrolysis 

method is that it is reliable, inexpensive and effective; it is obvious if a rough pump is not 

working, and no silane fires or explosion problems have occurred in connection with its 

use for > 20 years. However, some turbo-pump bearing have failed due to the 

aforementioned flake problem, and this new design should overcome this problem. This 

new pyrolysis system is now completed and working, and the inlet gas-handling system 

has also been constructed, assembled and evacuated, and is connected to silane, hydrogen 

and argon gases. 

The following photographs show the completed vacuum apparatus, that encompasses the 

discharge chamber, differential pumping chambers, mass spectrometer, three turbo 

pumps, three rough pumps, inlet control valves and the pyrolyser. The QMS is mounted 

inside the left flange in Fig.5, and the discharge chamber is to the right. Fig. 6 shows the 

pyrolyser and rough pumps in detail. The pyrolyser is a ~ 2′ long, 2′′ diameter stainless 

tube, with three smaller stainless tubes pressed inside to assure gas contact with a hot 

surfaces and to strengthen the outer tube, which weakens as it alloys with silicon. This is 

externally heated to 900-950 °C, by heaters enclosed in insulating alumina wool and then 

aluminum foil. The tube is water cooled at the ends to protect the vacuum flanges. 

For measurements of discharges in silane/hydrogen gas mixtures, a much larger H2 gas 

flow must be handled by the pumps. To achieve this, a Roots pump is added between the 

discharge-chamber turbo pump and the rough pump seen here. 

11
Fig.5. Vacuum apparatus, with the mass spectrometer flange at the upper left and the 

rough pumps and pyrolyser at the lower right. The hydrogen gas that exits the pyrolyser 

is vented out the top of the building through the flexible aluminum tube in the middle of 

the photograph. 

12
Fig.6. Photograph of the rough pumps and pyrolyser. The (larger) middle rough pump 

exhausts the discharge-chamber turbo, and the (smaller) right pump exhausts the 

differential-pumping and mass-spectrometer turbos. The outlet of the right pump must be 

vacuum tight, to avoid silane release into the atmosphere. This pump prevents hydrogen 

backstreaming into the mass spectrometer chamber, since turbo pumps have 

comparatively low compression ratios for hydrogen. The third rough pump, left, exhausts 

the pyrolyser, seen at the center top of the photo. This prevents air from entering the 

pyrolyser. 

Results 

1. Film Growth Rates

The power delivered to a RF discharge is typically only 10-20% of the power to the 

matching network, which is normally measured in device reactors. Thus, it appears best 

to compare “discharge power” conditions using film growth rates. We therefore measure 

film-growth rate (G) versus silane pressure, hydrogen pressure and discharge RF voltage, 

then measure the discharge radicals and ion versus these same parameters, effectively 

relating them to film deposition rate. A vacuum window and a small hole through the 

cathode allows film-thickness measurement using 780 nm laser reflection fringes in the aSi:H film. Film thickness changes as small as ~0.01 nm can be measured, allowing also 

observations of the influence of starting transients and film surface changes following 

13
changes in growth conditions. This does not provide the detailed film-surface information 

achievable with ellipsometry, but it is sensitive to film properties and readily shows any 

lack of reproducibility. 

Most industrial and experimental reactors measure the ratio (R) of H2 versus SiH4 flows, 

the reactor chamber pressure (PCh). The RF voltage (VRF) applied to the discharge is easy 

to measure, and is also monotonically related to the discharge power, so we measure G 

versus R, PCh and VRF to relate our conditions to those of device makers. In essence, 

similar conditions exist in different reactors when the values of G, R and PCh are the 

same. There are actually several additional variables that these parameters do not provide. 

One is the silane depletion in the reactor. The actual R value of the gas mixture depends 

on this depletion, and can become much higher than the inlet H2/SiH4 flow ratio. 

Furthermore, disilane builds up in the chamber, and can be important. Sometimes it is 

possible to estimate this depletion from G, if the gas flows are well confined to the 

reactor volume. Another variable is the electrode gap; we utilize a 2 cm gap, which is in 

the middle of the range used for in many reactors. Two additional variables are the 

temperatures of the substrate (TSub) and the RF electrode (TRF). While TSub is normally 

measured and reported, TSub -TRF can also be important and is normally not known. 

100 150 200 250 300 350 400 450 500

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Film Growth Rate [nm/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

Fig.7. Film growth rate (G), versus VRF and PTot, for R = 16 and 36. 

14
The film-growth rate data, for a range of conditions, is shown in Figs. 7 and 8. These 

measurements were carried out with the entire discharge chamber at 300K. Primarily the 

vapor densities influence the discharge behavior, so for an isothermal chamber at the 

typical operating temperature of 500 K, the pressures should be multiplied by 5/3. 

100 150 200 250 300 350 400 450 500

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 Film Growth Rates in SiH4

 plasmas

Film Growth Rate [nm/s]

RF peak-to-peak voltage [V]

 230 mTorr

 250 mTorr

 272 mTorr

 300 mTorr

 323 mTorr

 337 mTorr

 355 mTorr

Fig.8. Film growth rate with pure silane inlet flow, versus pressure and VRF. The silane 

depletion is 15% at 0.35 nm/s film growth rate. 

Comparing Figs. 7 and 8, one can see that diluting the silane to R = 16 does not have a 

major influence on G, at least for PTot = 1.5 and 2.0 Torr. The silane pressure is ~0.1 Torr 

for these cases, which are “on the edge” of producing microcrystalline films. However, 

diluting further to R = 36 reduces G by a factor of 5-10. Here the hydrogen etching is 

seriously competing with silicon deposition, and microcrystalline films are being 

produced. This reduction in G can be partly compensated by increasing VRF, but this is 

accompanied by an increase in ion bombardment energies. 

15
2. Stable Gases 

 

As silane is depleted in the discharge, the silicon atoms go into the a-Si:H film or higher 

silanes, primarily disilane (Si2H6). The disilane is a minor fraction of the gas, but it plays 

a major role in the plasma chemistry and an even larger role is silicon particle growth. 

We also have to know the disilane density in the discharge to calibrate the mass 

spectrometer measurements of Si2Hn radicals that contribute to film growth. Thus, it is 

desirable to measure how much of the depleted silane yields disilane and higher silanes, 

versus film. This branching has been measured for pure silane discharges, also in this 

laboratory, but most device deposition is now done using hydrogen-diluted silane. We 

have therefore measured this branching for the type of silane/hydrogen mixtures now 

used in device production, and versus temperature. Our full results are available in the 

publication “Production of higher silanes in radio frequency SiH4 and H2-SiH4 plasmas”.8

 

This knowledge of the amount of stable gases disilane and trisilane that are produced in 

H2-SiH4 discharges has been used to calibrate the mass spectrometer signals from the 

discharge for these higher-silane species. However, the substrate temperature is 300 K, 

not the typical 500 K. The pressure is 0.3 Torr, of which 95% is SiH4 and the remainder 

is H2 from (~ 10%) decomposed SiH4. The electrode gap is 2 cm and the film growth rate 

is near 0.1 nm/s. 

0.0 0.5 1.0 1.5 2.0 2.4

0

1

2

3

4

Densities [arb. norm. units]

Time [s]

Silane

Silyl radical (SiH3

)

Disilane

Molecular hydrogen

Atomic hydrogen

Discharge

Fig. 9 Time dependence of densities during discharge. Signals are arbitrarily normalized 

to show on the same scale; backgrounds are subtracted. The electron energy is 25 eV for 

the stable gases, and different for the radicals as explained in the text. These signals are 

shown with arbitrary size: the data described above provides absolute scales for the H2

and Si2H6 densities. 

16
In order to avoid excessive particle production in the reactor and thus clogging the 

pumping orifices, the discharge is pulsed: typically turned on for 0.5 s, then off for 2 s. 

The gas flow then carries silicon particles out of the discharge region while there are 

small enough (<10 nm dia. [3]) to avoid clogging of the apertures and turbo pump. A 

second advantage of the discharge pulsing is that it can be used to verify the detection of 

radicals. These appear only during the discharge, whereas the densities of stable gases 

change slowly with discharge switching. The time dependence of the stable gas and 

radical densities, detected by the mass spectrometer during the pulsed discharge, is shown 

in Fig. 9. Note the rise and decay of stable species and step changes in radical and atomic 

hydrogen densities. 

3. Radicals 

In order to detect radicals, we measure the TIMS signals versus electron energy, using 

electron energy below the stable-gas, dissociative-ionization threshold. Two different 

electron currents are used for each electron energy, to allow subtraction of the signal 

from the discharge ions that leak through the deflector grids. The result of such a 

measurements for mass 31 (SiH3

+

) is shown in Fig. 10. 

6 7 8 9 10 11 12

1E-4

1E-3

0.01

0.1

1

10

100

1000

Ion Signal [nA at multiplier output]

Electron Energy [eV]

 Discharge, 0.1 uA

 Discharge, 1.1 uA

 Background, 0.1 uA

 Background, 1.1 uA

Signal from ion leakage

current from discharge

Fig. 10. SiH3

+

 signal versus electron energy below the SiH4 dissociative ionization 

threshold. TIMS signals are averaged during and after the switched discharge. 

17
Ions escaping from the discharge and leaking through the deflector grids yield the 0.2 nA 

signal at low Eel. This parasitic signal does not depend on the ionizer electron current or 

energy. (This leakage current is 10-5 of the ion current before applying voltage to the 

deflector grids.) One can also note that, due to finite electron temperature, we detect 

some signal from the dissociative ionization of silane even below its 12.3 eV threshold. 

After the ion background is subtracted, the signals due to ionization by the electron beam 

(during and after the discharge) are obtained, as in Fig. 11. The radicals arriving from the 

discharge are clearly identified by their specific ionization threshold. 

The electron energy scale and MS sensitivity for SiHn radicals are calibrated with a 

threshold scan of SiH4→SiH2

+

 from a known silane pressure in the discharge chamber 

(without discharge). After a minor correction for background silane gas in the ionizer 

region, versus only a molecular beam for the radicals, this provides a calibration of the 

radical density at the substrate. (We assume a similar cross section for direct ionization of 

all the SiHn in the threshold-energy region where only one ion species occurs.) 

Measurements of disilane radicals (Si2Hn, n = 0-5) have also been made. At the required 

mass resolution, the mass spectrometer is much less sensitive for these disilane radicals 

versus the SiHn radicals. The mass spectrometer signals for the Si2Hn radicals are 

calibrated in terms of density by comparing to direct near-threshold ionization of a 

known density of Si2H6 in the discharge region. This Si2H6 is actually produced by the 

SiH4 discharge, with the amount known from our prior quantitative measurements of the 

discharge production and destruction of Si2H6.

8

 The Si2H2

+

 signal versus cathode voltage 

is shown in Fig. 12. The other Si2Hn radical signals are considerably smaller than this 

Si2H2 signal, and will not be shown. The electron energy is typically 2.6 eV lower than 

the cathode voltage, mostly due to contact potential differences between the Ba cathode 

surface and the carbon ionization-chamber surfaces. Thus, the “discharge on” signal in 

Fig.12 is consistent with an 8.0 eV threshold, about as expected for Si2H2 → Si2H2

+

. 

We observe that, for a typical 0.3 Torr pure silane discharge operated at 200 Vpp RF 

voltage, which yields a deposition rate of 0.1 nm/s, the density of SiH3 is 2.2x1011

 cm-3, 

which is 2.2x10-5 that of SiH4. Similar measurements with atomic hydrogen versus H2

yield a density of H at the substrate that is ~ 4 % of SiH3 density under these conditions. 

However, the background-gas correction is much larger and more uncertain for H2, so 

this result is uncertain by a factor of 2. The densities of the other SiHn radicals are quite 

small; the measured density of SiH is 5% of the SiH3 density, while we have set upper 

limits of 0.1 and 1 % of SiH3 for the Si and SiH2 radicals. The radical densities observed 

from a 0.3 Torr, 100 C, 200 V pure silane discharge are shown in Fig. 13. 

Only the radicals SiH3 and Si2H2 have been observed from R= 16 and 36 silane-hydrogen 

discharges. This data, and upper limits on the other species is shown in Figs. 14 and 15. 

18
6 7 8 910 11

0

1

2

3

4

5

12

Threshold of SiH4 -> SiH+

3

: 12.3 eV

SiH+

3 Ion Signal [nA at multiplier output]

Electron Energy [eV]

 Discharge

 Background

Threshold of SiH3 -> SiH+

3

: 8.4 eV

Fig. 11. Detection of SiH3 radicals: energy scan below the SiH4 → SiH3

+

 threshold. The 

signals at two different electron currents are subtracted, thus eliminating the signal from 

ion leakage. 

9 10 11 12 13 14

0.0

0.2

0.4

0.6

0.8

1.0

Ion Signal [nA @ 2300 V]

Cathode Voltage [V]

 Discharge 1 uA

 Background, 1 uA

Y = A + B * X

A -0.77459 0.15877

B 0.07274 0.01353

Threshold: 10.65

Fig. 12. Mass 58 signal versus electron-beam cathode voltage, with and without 

discharge. The ion background has already been subtracted from both signals. 

19
Si SiH SiH2 SiH3 Si2 Si2

H Si2H2 Si2H3 Si2H4 Si2H5

0

10

20

30

40

50

60

70

80

90

100

110

Rel. Density

Species

5

<0.1 <1

100

0.3 Torr SiH4

1.2 A/s film growth rate

<0.1

1.4

18

1.6

6.3

2.5

Fig. 13. Radical densities at the substrate of a 0.3 Torr, 100 C, 200 V, pure-silane 

discharge. The Si, SiH2 and Si2 densities are upper limits. 

Si SiH SiH2 SiH3 Si2 Si2H Si2H2 Si2H3 Si2H4 Si2H5

0

10

20

30

40

50

60

70

80

90

100

110

<6 <5 <4 <4

100

0.9 A/s film growth rate

1.5 Torr R=16 H2

-SiH4 mixture

Rel. Density

Species

<2 <5

19

<4

<15

Fig. 14. Radical densities at the substrate of a 100 C, R= 16 silane-hydrogen discharge, 

with VRF = 200 V (p-p). 

20
Si SiH SiH2 SiH3 Si2 Si2

H Si2H2 Si2H3 Si2H4 Si2H5

0

10

20

30

40

50

60

70

80

90

100

110

0.1 A/s film growth rate

<6 <3 <3 <4

Rel. Density

Species

100

<3

15

<4 <6

1.5 Torr R=36 H2

-SiH4 mixture

<8

Fig. 15. Radical densities at the substrate of a 100 C, R= 36 silane-hydrogen discharge, 

with VRF = 200 V (p-p). 

The flux (Fxm) of Si atoms carried by a radical to the substrate is the most important 

indicator of the contribution of this radical (SixHm) to film growth. Our TIMS signals are 

proportional to the portion (nxm) of the radical density at the substrate that is moving 

toward the surface (where the small aperture allows some to pass through to the ionizer). 

From the kinetic-theory boundary condition, the radical flux that reacts with the substrate 

is FRxm = nxmvxβxm/2, where vx is the mean radical velocity and βxm is the reaction 

probability (per collision) on the substrate; actually the growing silicon-film surface. A 

fraction γxm/βxm of this flux yields deposition of one Si-atom, or Fxm = nxmvxγxm/2, where 

x Si atoms are carried by each radical. Thus, comparing Si2H2 to SiH3, F22/F13 = 

(n22/n13)(v2/v1)(γ22/γ13) = (0.2)(0.7)(γ22/0.09) ≅ 1.6 γ22, using a measured γ13 = 0.09.7

 If 

Si2H2 reacted with high probability, incorporating 2 Si atoms, this would yield γ22 = 2 and 

a Si2H2 contribution to film growth that exceeds that of SiH3 by a factor of 3. But a 

calculation of F13, using the above measured n13 and including several experimental 

uncertainties, yields 0.066±0.03 nm/s film growth rate (G) from SiH3, compared to a 

measured G = 0.1±0.02 nm/s. These numbers agree within their uncertainties, and do not 

allow for a Si2H2 contribution that greatly exceeds that of SiH3. Thus, we conclude that 

γ22 <<1; apparently there is a relatively stable form of Si2H2, similar to acetylene as the 

equivalent hydrocarbon, which has a small surface reactivity. 

The reaction probabilities of the Si, SiH and SiH2 radicals on a-Si:H are not known, but 

can be reasonably estimated as 50% for these very reactive radicals. Thus, in terms of 

reacting flux (F) to the film: F(SiH3) = 30, F(SiH2) < 2, F(SiH) < 2, and F(Si) < 1. It is 

21
clear that, as expected, SiH3 produces most of the film growth. However, these numbers 

allow for a significant fraction from the other SiHn radicals, H reactions may be 

important, and the reactive flux of Si2H2 is not known. Thus, the radical mixture and 

film-growth might have significant dependence on the discharge parameters. 

Consequently, we are refining these preliminary analyses to provide better limits on the 

other radicals, particularly SiH2 and the other Si2Hn. We are also considering possible 

measurements that might yield the very important βxm and γxm factors, to more accurately 

relate the surface density of each radical to its contribution to film growth. These are 

difficult to measure and very dependent on the surface structure. For example, a H 

reaction probability of 0.1 on a-Si:H has been measured on a-Si:H with a thermal 

technique,9 a value of 0.4 has been measured on μc-Si (incorrectly identified as a-Si:H in 

the manuscript) using TIMS,10 and 0.8 has been measured on sputtered Si using H*

emission to identify H.11

4. Discharge Ions 

Ion flux to the substrate is known to be typically a few percent of the radical flux, so ions 

do not have much effect on the film growth rate. However, energetic ions containing Si 

can alter the structure of deep surface layers, below the region where film structure and 

electronic properties are largely frozen in. Hn

+

 ions are not expected to penetrate below 

the top surface layers, nor do they have enough mass to directly alter structure in these 

layers. However, they can etch and rearrange Si-Si bonding in the surface layers, and 

they may be important in the amorphous to crystalline transition. It is therefore of interest 

to establish both the impacting ion energies and species. The ion energies were studied in 

reference 12, and here we measure the species. This is done by grounding the iondeflection screens, thereby allowing ions to pass through to the mass spectrometer. This 

detects ions that are directed towards the substrate at or close to the surface normal, but 

as the more energetic ions have such motion this is of primary interest. The ion species 

Hn

+

, SiHn

+

, Si2Hn

+

, and Si3Hn

+

 have been measured. H3

+

 totally dominates the Hn

+

 ions, 

so a figure is not provided for these ions. Data that shows the n dependence of the silane 

ions for R = 0, 20 and 40 discharges is shown in Figs.16-18. The relative contributions of 

the H3

+

and SixHy

+

 ions (summed over y) in H2/SiH4 mixtures are shown in Figure 19, 

versus the silane to hydrogen ratio. Essentially no Hn

+

 ions are observed from pure silane 

discharges, so only the measured fractions of SixHn

+

 (summed over n) are plotted in 

Figure 20. This data is from room temperature discharges excited by 200-270V RF peak 

voltage, for pressures of 0.3-0.5 Torr, and it is plotted versus RF voltage divided by 

pressure. Short, pulsed discharges are used for all of this data, so the pressures of 

hydrogen and silane vapor are not significantly modified by the discharge. 

22
27 28 29 30 31 32 33

0

2

4

6

8

10

12

14

16

54 55 56 57 58 59 60 61 62 63 64

0

2

4

6

8

10

12

14

83 84 85 86 87 88 89 90 91 92 93 94 95

0

1

2

3

4

5

6

Trisilane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Silane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Disilane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Fig. 16. Ions from a 0.3 Torr, 200 V, 100 C pure-silane discharge. 

23
26 27 28 29 30 31 32 33 34

0

2

4

6

8

10

55 56 57 58 59 60 61 62 63

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

83 84 85 86 87 88 89 90 91 92 93 94 95 96

0.00

0.05

0.10

0.15

0.20

0.25

Silane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Disilane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Trisilane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Fig. 17. Ions from a 1.5 Torr, 200 V, R = 20 silane-hydrogen discharge. 

24
27 28 29 30 31 32 33

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

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

83 84 85 86 87 88 89 90 91 92 93 94 95 96

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

Silane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Disilane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Trisilane Ions from the discharge

Signal [nA @ 1300 V]

Mass [amu]

Fig. 18. Ions from a 1.5 Torr, 200 V, R = 40 silane-hydrogen discharge. 

25
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

0

10

20

30

40

50

60

70

80

90

100

% of total ions

Silane pressure/H2 pressure

 % H+

3

 % SiH+

n

 % Si2H+

n

 % Si3H+

n

Fig. 19. Types of discharge ions at the substrate of a silane/hydrogen RF discharge, 

versus the pressure (inlet flow) ratio of silane to hydrogen. (This ratio is 1/R.) The 

conditions are 230 V (peak) RF voltage and a total pressure of 1 Torr at 300 K. The 

SixHn

+

 ions have been summed over n. 

26
400 500 600 700 800 900

0

10

20

30

40

50

% of total ions

RF Voltage(V)/CapManPressure(Torr)

 % SiH+

n

 % Si2H+

n

 % Si3H+

n

 Fig. 20. Types of ions at the substrate of an RF discharge in pure silane, versus peak RF 

voltage (V) divided by pressure (Torr). The SixHy

+

 have been summed over y. 

These observations yield several interesting results that are important to understanding 

how ion bombardment of the substrate can influence device properties. (1) The flux of 

Hn

+

 ions is negligible under all conditions where devices are grown (R < 100 or SiH4/H2

> 0.01). Thus, these ions are not a cause of the changing film properties, from amorphous 

to micro-crystalline, versus hydrogen dilution (R value). (2) The number of Si atoms in 

the bombarding ions (x in SixHy

+

) is only weakly dependent on hydrogen dilution (R) for 

R <30, but the large-x ions contribute much less for R > 30 and in pure silane. As even 

larger x values than the ones studied here (up to x = 3) doubtless also contribute to the ion 

flux, this is a significant change. The heavier ions are expected to have much lower 

impact energies per Si atom, so they could remain together and introduce a comparatively 

large Si cluster at the growing surface. (3) The H content (y in SixHy

+

) is almost constant 

versus R in the hydrogen-diluted vapor, but it is much smaller in pure silane. We suspect 

that this results from a much smaller center-of-mass ion-molecule collision energy in ionH2 collisions, compared to ion-SiH4 collisions. The influence of this on film properties is 

not known or easily conjectured. 

27
Conclusions 

The present observations of SiHn and Si2Hn radical densities at a-Si:H and μc-Si substrate 

surfaces indicate that SiH3 is the most abundant radical, but significant amounts of Si2H2

also occur. Other smaller density components such as SiH and Si2H4 occur for a pure 

silane discharge, but are below detection limits for the R = 16 and 36 discharges. Since 

SiH3 is relatively inefficient at depositing Si into the film, these other radicals could 

provide a significant (> 10%) contribution to film growth, and thereby also to device 

properties. The radical density that should vary the most with R (hydrogen dilution) is, of 

course, the H atom. Our current data is ambiguous regarding H atom densities, so we do 

not report these preliminary results here. There is a strong indication that H densities at 

the substrate vary with surface condition as well as with R, but this still has to be verified. 

The other radicals that arrives at the growing film surface are the positive ions. Although 

their flux is a small fraction of the radical flux, due to their high energies and clustering 

they can be important. We measure here the ion spectra, but not the energy with which 

they bombard the surface. We find that the mixture of SixHy

+

 radicals is fairly similar for 

R = 15-100 discharges, although the heavier (larger x) species are of decreasing 

importance as R increases. The ions contain fewer H (smaller y), and larger x values are 

more important, in the pure silane, R=0 case. The only significant Hn

+

 ion is H3

+

, and this 

occurs in unimportant quantities in all cases used for device production (R < 100). Since 

ions typically induce 1-5% of film growth, these changes in SixHy

+

 mixtures versus R 

could be significant for device properties. However, it has also been shown that the 

average ion bombardment energy decreases with hydrogen dilution,12 and this may be the 

most important dependence on R. 

References 

1) “Radical species in argon-silane discharges,” R. Robertson, D. Hils, H. Chatham and 

A. Gallagher, Appl. Phys. Lett. 43 (1983) 544-546. 

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and A. Gallagher, J. Appl. Phys. 59 (1986) 3402-3411. 

3) “Study on the absolute density and translational temperature of Si atoms in very high 

frequency capacitively coupled SiH4 plasma with Ar, N 2, and H2 dilution gases” 

T. Ohta, M. Hori, T. Ishida, T. Goto, and S. Kawakami, Japanese Journal of Applied 

Physics 43 (2004) 6405-6412. 

4) “Laser-induced-fluorescence study of the SiH2 density in RF SiH4 plasmas with Xe, 

Ar, He, and H2 dilution gases” A. Kono, N. Koike, H. Nomura, and T. Goto, Japanese 

Journal of Applied Physics 34 (1995) 307-311. 

5) “Measurement of absolute densities of Si, SiH and SiH3 in electron cyclotron 

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Hori, and T. Goto, Japanese Journal of Applied Physics 33 (1994) 4320-4324. 

28
6) “Mass spectrometry detection of radicals in SiH4-CH4-H2 glow discharge plasmas” P. 

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4 (1995) 250-9. 

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probability on a-Si:H”, > Tsuji, T. Akiyama and H. Komiyama, J. Non. Cryst. Solids 

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Mat. Res. Soc. Symp. Proc. 557 (1999) 115. 

29
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June 2006 

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Subcontract Report 

3. DATES COVERED (From - To)

1 June 2002 – 31 May 2005 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

5b. GRANT NUMBER

4. TITLE AND SUBTITLE

Measurement of Depositing and Bombarding Species Involved in 

the Plasma Production of Amorphous Silicon and 

Silicon/Germanium Solar Cells: Final Technical Report, 

1 June 2002–31 May 2005 

5c. PROGRAM ELEMENT NUMBER

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NREL/SR-520-40056 

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PVB65101 

6. AUTHOR(S)

 A. Gallagher, K. Rozsa, P. Horvath, and D. Kujundcik 

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 National Institute of Standards and Technology (NIST) 

 Boulder, Colorado 

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14. ABSTRACT (Maximum 200 Words): The objective of this study is to measure the molecular species that lead 

to the growth of hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si) photovoltaic (PV) 

devices from RF discharges. Neutral radicals produce most of the film growth during this PV-device production, and, 

by implication, radicals primarily determine the device structure and electrical characteristics. The most important 

feature of the present experiment is thus the measurement of neutral-radical fluxes to the substrate. Additional 

depositing species that can influence film properties are positive ions and silicon-based particles produced by the 

discharge; we also measure these positive-ion species here. Some studies have already measured some of these 

radical and positive-ion species in silane and silane/argon discharges, but not for discharge conditions similar to 

those used to produce most photovoltaic devices. Our objective is to measure all of these species for conditions 

typically used for device production. In particular, we have studied 13.6 MHz-excited discharges in pure silane and 

silane/hydrogen vapors. 

15. SUBJECT TERMS

PV; thin film; solar cells; amorphous silicon; microcrystalline silicon; plasma; germanium; 

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