Measurement of Depositing and Bombarding Species Involved in the Plasma
Production of Amorphous Silicon and Silicon/Germanium Solar Cells

Annual Technical Report

June 1, 2002 -- Aug. 31, 2003

DOE Interagency Agreement De-AC36-02G010244

Principal investigator: Alan Gallagher

Contributors: Karoly Rozsa, Peter Horvath, Damir Kujundcik

DOE Program Officer: Ms. Lizana Pierce

NREL Technical monitor: Dr. Bolko Von Roedern

The technical approach to this project is to utilize mass spectrometry
to measure radical and ion species that arrive at the substrate of a
plasma-enhanced-chemical-vapor-deposition (PECVD) reactor. A small-scale
reactor mimics those used to produce hydrogenated amorphous (a-Si:H) and
microcrystalline (μc-Si) silicon and silicon/germanium (a-Si:Ge:H) solar
cells. Radio frequency (RF) and high frequency (HF) discharges will
normally be studied, and the reactor utilizes a similar electrode gap
(2-3 cm), substrate temperature (20-250 °C), gas pressures (0.1-5 Torr),
gas mixtures and discharge power density to that used in an industrial
reactor.

All of the apparatus has now been constructed, assembled and all leaks
have been sealed. In addition to improvements in the welded stainless
steel high vacuum apparatus, this required some work on the rough lines
due to the silane pyrolysis method used. This pyrolysis method utilizes
a 950 °C, stainless steel tube placed in the roughing line to break up
the silane (and/or germane) into hydrogen gas and solid. 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.

We are utilizing a Dycor/Amatec quadrupole mass spectromerter (QMS) for
the radical detection, with the ionizer and ion collection portions
modified for use in threshold ionization mass spectrometry (TIMS). The
discharge and TIMS regions are connected by two small orifices and three
stages of differential pumping, an arrangement shown diagrammatically
and explained in the previous quarterly reports. We find the QMS
electronics very unreliable, obscure and ill suited for our need to
average weak ion signals during ionizing electron energy (E~el~) scans
under computer control. Thus, we are now engaged in producing 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 (*V~DC~*) must
reach hundreds of Volts (RMS) and be independently controlled, as this
determines the transmission and mass resolution. The correlated *A* and
*V~DC~* are scanned under computer control to scan mass, or fixed during
an E~el~ scan. A simple, inexpensive and reliable set of electronics now
provides these voltages under computer control, providing for computer
control that can be reproduced for other experiments. However, the QMS
operation has not yet to been tested using this new electronics. The QMS
RF frequency varies from 2-5 MHz, as needed for various mass ranges, and
using a similar method to produce a well controlled 13.5 MHz discharge
voltage is attractive and being evaluated.

Continuing studies of the TIMS apparatus, being carried out in a
separate vacuum chamber, indicate that the ion collection efficiency of
the ion-optics and QMS may be as little as a few percent of total
radical ionization. This is attributed primarily to a very small
diameter of the QMS inlet that achieves ion transmission. This makes
data collection slower than expected, but does not preclude the desired
radical observations. Nonetheless, we are continuing to study this, and
to try various schemes to improve this ion collection efficiency

The next step in the experiment is to test the efficiency of the
differential pumping through the several chambers used to separate
background gas while transmitting radical flux. The next step is to test
discharge operation, diagnosing optical emission through small side
windows and mass spectrometer signals through an on-axis orifice, using
the normal stable-molecule detection mode. The former observations
diagnose discharge uniformity and purity, while the latter establishes
the silane depletion fraction, and thereby the power dissipation within
the discharge. Then we will evaluate the TIMS-QMS system for its ability
to detect neutral radicals from the discharge.

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
Photograph 1, and the discharge chamber is to the right. The discharge
region \~ 6 cm diameter, with an adjustable gap currently at \~ 2.5 cm.
Photograph 2 shows the 6′′ flange to which the discharge chamber is
attached, including a capacitance manometer that measures the discharge
pressure, an RF connector and gas inlet valves. Photograph 3 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.

![](media/image1.jpeg){width="5.998611111111111in"
height="4.498611111111111in"}

Photograph 1. Complete 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.

![](media/image2.jpeg){width="5.996527777777778in"
height="7.995138888888889in"}

Photograph 2. The discharge-chamber side of the apparatus. The inlet
flange of the 300 L/s turbo pump that handles the primary gas flow rests
on the table surface, directly below the discharge chamber. The 50 L/s
turbo pump seen to the left exhausts the first differential-pumping
chamber between the discharge and the mass spectrometer. The capacitance
manometers measure the pressure in these two chambers.

![](media/image3.jpeg){width="5.998611111111111in"
height="4.498611111111111in"}

Photograph 3. Photograph of the rough pumps and pyrolyser. The larger,
middle rough pump, center, exhausts the discharge-chamber turbo, and the
smaller pump, right, exhausts the differential-pumping and
mass-spectrometer turbos. This separation is necessary to avoid hydrogen
backstreaming into the mass spectrometer chamber, since turbo pumps have
low compression ratios for hydrogen. The third rough pump, left,
exhausts the pyrolyser, seen at the center top of the photo.