Improved Magnesium Fluoride Process by Ion-Assisted Deposition
R.R. Willey, Willey Optical, Charlevoix, MI; and
K. Patel and R. Kaneriya, Astro Optics Pvt. Ltd., Mumbai, India
ABSTRACT
When Ion Assisted Deposition (IAD) started to appear on the
scene, there was great hope that MgF2 could be done with
IAD on room temperature substrates and also allow robust
AR coating on plastic substrates. The use of IAD for MgF2
on plastics is almost mandatory to get reasonable adhesion
and hardness. A review of the history of attempts to use IAD
with MgF2 at low temperatures has shown limited success.
MgF2 has been and will continue to be an attractive and well
used low index material. It has appeared that there is still an
opportunity for study and improvement of MgF2 deposition
using IAD with respect to absorption, stress, and scattering.
The goal of this work has been to produce, on a chamber-full
production scale, dense and non-absorbing MgF2 films on glass
and plastic at temperatures below 100°C which are equal to
or better than those deposited at 250-300°C without IAD. The
effort of this study has been to investigate the realm of IAD
variables from 30-100 eV and ion-atom-arrival rates (IAAR)
well below the resputtering rates with argon or nitrogen gas,
to the extent possible with a broad beam ion source of the
End-Hall type.
INTRODUCTION
Magnesium fluoride (MgF2, index 1.32-1.39 near 500 nm) has
been used successfully for well over half a century. It has been
the most common optical coating on glass as a quarter wave
optical thickness (QWOT) layer antireflection coating. It can
be hard and relatively insoluble. It transmits well from about
120 nm in the vacuum UV out to about 7µm the mid-infrared.
Olsen and McBride [1] showed that a 2.75 mm thick single
crystal of MgF2 was clear from at least 200 nm out to about
6 µm and then started to increase in absorption toward longer
wavelengths. At 10 µm, the transmittance fell to about 2%. A
thin layer can be used as a top protective layer on coatings for
the 8 to 12 µm region, although it has too much absorption for
a thick layer in that region. There is currently much interest
in MgF2 as a material in vacuum ultraviolet for lithographic
optics in the semiconductor industry.
The hardness, durability, and density of MgF2 without IAD is
a strong function of the substrate temperature during deposition. At room temperature, the films can usually be wiped
off, have high humidity shifts, have an index of about 1.32
in vacuum and a packing density of about 0.82 according to
an extensive study by Ritter [2]. With a 300°C deposition
temperature, Ritter shows that the packing density is about
0.98 and the index nearly 1.39. The 300°C films will pass an
eraser rub test and have a low humidity shift.
MgF2 is often evaporated from molybdenum or tantalum
resistance boat sources where it melts to form a liquid and
evaporates. The use of an E-gun with this and other fluorides
is common, but can have some problems. Granular MgF2
from an E-gun can easily cause “explosive” scattering of
the material if the E-bean applies too much power density to
the material. It is thought that the rush of the fluoride vapors
from evaporated granules below the surface of the granule
pile “blows” the solid granules above like dust. It is therefore
necessary to use great care and slowly bring up the power on
the E-gun. Also, fusing the top layer all over during pre-melt
helps avoid spatter. It is also found that the material in the
E-gun often becomes almost black. The black condition has
also been seen with a boat when the rate control is set for
low rates such as 0.2 nm/sec. It is our opinion that a boat is
the best approach unless there are extenuating circumstances
forcing the use of an E-gun. Without IAD, magnesium fluoride
films tend to have a columnar structure as shown by Guenther
[3], scattering, and high tensile mechanical stress as shown
by Ennos [4].
HISTORY
McNeil et al. [5] reported early IAD work with a gridded
Kaufman ion source using oxygen gas on SiO2 and TiO2
films where they showed the effects of ion energy (eV) and
ion current on the hardness and absorption of the films. They
did not work with MgF2, but their results are generally applicable to this work.
The report of Kennemore and Gibson [6] points to the likelihood that energies lower than the 125-150 eV that they had
used may be optimal for MgF2. Absorption inducing damage
is thought by Tsou and Ho [7] and others to be preferential
sputtering of the fluorine from the films. Figure 1 (in a form
after Figure 18a in Cuomo, Rossnagel, and Kaufman [8])
shows that the MgF2 conditions reported by Kennemore and
Gibson might have been too far toward the “resputtering”
realm. They reported 0.5 nm/sec., 150 eV, and 33 µA/cm2. On
this figure, the region of low ion energy and low IAAR are
© 2010 Society of Vacuum Coaters 505/856-7188
53rd Annual Technical Conference Proceedings, Orlando, FL April 17–22, 2010 ISSN 0737-5921
313
found to have no effect, while the region of too high values
for these variables will sputter away all of the film. The region
desired for this work is where the values are high enough to
densify the film but not to sputter it.
concern for those working in the visible spectrum. However,
the MgO is more soluble than the MgF2 and is therefore not as
environmentally stable and satisfactory. The MgO also raises
the index while decreasing the UV transmittance.
Targrove and Macleod [12] verified that momentum transfer
is the dominant densifying mechanism by using Ne, Ar, and
Xe for IAD. Note also that the atomic weight of oxygen is
only 16, so it has less than half the momentum of argon when
used for IAD. However, O2+ (32) has been shown by McNeil
et al. [5] to be the dominant species over O+ by about 3:1, so
that the most common bombarding O2+ ion is more nearly
like Ar (40).
Allen et al. [13] obtained good results with dual ion beam
sputtering (DIBS) of MgF2 by adding a background gas of
Freon (CF4) at about 1x10-4 torr, which they describe as refluorinating the sputter target while the fluorine was being
preferentially sputtered. They also found the least absorption was obtained when nitrogen was used as the sputtering
gas, which they attribute to its lower atomic weight (14) as
compared to Ar (40).
Figure 1: Ion/atom arrival ratio versus eV for Kennemore and
Gibson reports.
Martin and Netterfield [9] reported on the use of IAD at 700eV
with Ar, O2, and H2O at ambient temperature. They reported
high absorptance at 550 nm with Ar IAD, but none with O2,
and H2O. The implication here is that the IAD preferentially
sputtered away the fluorine, but that oxygen filled the fluorine
vacancies to make MgO where needed.
Gibson and Kennemore [10] subsequently did more work
where they used Freon 116 (C2F4) to replace some of the lost
fluorine. Their results were “less successful than hoped for.”
They also did some work at 80 eV with Ar and with Freon. In
Figure. 1, the small dots on the 80 eV line show the IAAR for
some of their tests. They did conclude that “low bombarding
energies may prove a useful and less absorbing combination.”
They also describe how the MgO content causes the films to
dissolve in a salt water bath after 48 hours. They review the
availability of oxygen in the chamber from water vapor, and
describe baking out the chamber before deposition to reduce
H2O and oxygen so that the films have less opportunity to
acquire oxygen.
Martin et al. [11] did an extensive study with argon and
oxygen IAD. They showed increased absorption in the VUV
from 100-200 nm for IAD films as compared to conventional
evaporation at 440°C. Their IAD was done at 350-700 eV. They
showed that the additional absorption in the UV between 120
and 180 nm was probably due to MgO. This might be of no
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Kolbe et al. [14] used fluorine gas as the reactive species
for several metal fluorides with good results. However, they
did mention the corrosive effects of the fluorine on the ion
source. Their work seems to be consistent with the concepts
that low energy IAD is important, and that oxygen in the
process will create a deposit which has some metal oxide
and/or oxyfluoride that causes higher index and absorption
at the short wavelength end of the spectrum.
Robic et al. [15], in their Figure 11, show the decrease in
extinction coefficient with decreasing anode voltage from
170eV to 60eV for the IAD of YF3 using a Mark II ion source.
This seems to be further evidence for the case of using lower
ion energies for IAD with metal fluoride coatings.
Single IBS bombards the target to be sputtered with high energy
atoms (>1000eV). The pressures can generally be lower than
with IAD. However, there is significant risk of high energy
atoms reaching the substrate and disassociating the fluorine
from the magnesium. Yoshida et al. [16] report on such work
where fluorine gas is used to replenish the lost fluorine atoms
in the metal fluoride coating and to avoid oxygen and water
vapor which would cause MgO and absorption.
Tsou and Ho [7] used an Advanced Plasma Source (APS) for
IAD of MgF2 with argon. They reported that absorption in
the visible region occurred with IAD at discharge voltages
exceeding 75 V, and that absorption was found to increase
with increasing discharge current and voltage of the plasma.
This would imply moving upward and to the right in Figure
1 where resputtering and preferential sputtering of fluorine
would occur.
Baldwin et al. [17] report on work at 36eV to 72eV (60-120
anode drive volts) argon IAD which supports the hypothesis
that low energy IAD is key MgF2 without absorption. They
obtained robust films on plastic at low temperatures.
Ristau et al. [18] compared the results of IBS with both boat
and E-beam Physical Vapor Deposition (PVD) for MgF2. They
found the index at 248 nm to be 1.40-1.41 for the IBS films
and higher by about .03 for the PVD films. It is conjectured
that this is due to MgO and other contaminants in the PVD
films. The IBS film showed an index of about 1.387 at 500
nm. Coating stresses were compressive for IBS and tensile
for PVD.
Work at 100-150 eV by Alvisi et al. [19] produced reasonable
laser damage results, but none as good as non-IAD at 250300°C. The evidence seems to point to IAD with energies
between 30-100 eV (50-167 drive volts) should produce the
desired results at low temperatures, but even there, it will be
necessary to keep the IAAR below the resputtering rates to
avoid absorbing films.
Dumas et al. [20] used argon IAD and E-beam evaporated
MgF2 to compare UV absorption in the 200-400 nm range
with films deposited with no IAD. They modeled the effects
of lost fluorine with oxygen and hydroxyl ion replacement.
They used normalized momentum per the work of Targrove
and Macleod [12] as the most salient variable of the process.
It was shown that the stoichiometry dropped from over 1.95
F/Mg at low momentum to 1.7 above a certain threshold
momentum. This again seems to confirm that the differential
sputtering of fluorine is the major cause of absorption in IAD
films of MgF2.
EXPERIMENTS
For ease of production of MgF2 films for the visible spectrum
in a typical 0.5 to 1.0 meter box coater, it is desirable to have a
process that does not require fluorine replacement. Hard coatings without MgO are needed for environmental durability.
A low process temperature (<100°C) is needed for the ability
to coat plastic (and other temperature sensitive objects) and
for reduced process time. It was the goal of this work to find
the process parameters which meet these requirements using
the available broad beam IAD source.
The equipment setup used for this work was a Leybold L560
box coater with a Leybold Turbovac 1000 turbomolecular
pump of 1000 liter/sec capacity. The starting vacuum was
1x10-5 torr. The ion source was a Kaufmann Robinson, Inc.
(KRI™) model EH400 (end-Hall). The distance from the
KRI to the substrates was approximately 43 cm. The MgF2
was evaporated from a tantalum boat at rates of 0.2-1.6 nm/
sec. Argon gas was supplied at 4-8 sccm in the argon-only
experiments which gave 2.9-8.6x10-4 torr deposition pressure. For nitrogen-only experiments, gas at 6-15 sccm gave
2.0-5.4x10-4 torr.
An issue with most broad beam IAD sources is that large gas
flows are needed to obtain low discharge voltages (50-100V).
This increases the chamber pressure to the point that most
of the ions and energetic atoms are nearly “thermalized” to
the energy of the gas in the chamber before they reach the
substrate, and therefore they have little energetic effect. At
these higher pressures, the gas atoms are also competing with
the depositing atoms at the surface which tends to cause the
films to be porous and weak. It appears that, even at the lowest pressures and ion currents, very few IAD atoms reach the
substrates without having multiple collisions with other vapor
molecules/atoms. The mean free path (MFP) in centimeters
is 5x10-3 divided by the pressure in torr. Therefore, at 1x10-4
torr, the MFP would be 50 cm., or approximately the source
to substrate distance here. The percentage of ions which have
not had a collision while travelling a distance X is:
This means that, at a pressure of 1x10-4 torr, only 37% of
the ions have reached the substrate 50 cm distant without a
collision, and only 0.67% at a pressure of 5x10-4 torr. This
implies that the ion flux reaching the substrate has had many
collisions and lost much of its energy and momentum. The
actual eV of the ions reaching the substrate is probably an order
of magnitude below the drive voltage of the ion source, and
therefore the observed effects are only relative with respect to
what is shown in Figure 1. However, the concern of this work
is with what control parameters produce the desired results.
Therefore, knowledge of the actual details at the surface are
of interest but not critical to the goals of this work.
It is seen, from the history of the foregoing investigations,
that: 1) the ion energies (eV) may need to be kept below some
threshold, 2) the IAAR (momentum per depositing atom) need
to be adequate but not too much, and 3) oxygen needs to be
avoided in the deposited MgF2 films.
The ion energies depend on the gas flow and drive current
of the KRI, and to some extent the deposition rate of the
MgF2. The momentum imparted to the coating depends on
the IAAR, the atomic mass of the IAD gas atoms, and the
ion energy. The gas flow (pressure), drive current, and deposition rate are the independent variables of this process that
need to be investigated with respect to the ability to get the
desired densification and low absorption. As seen in Figure
1, the ultimate parameters of importance are IAAR and the
energy of the arriving ions; these are derived from the results
of controlling flow, current, and rate.
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Nitrogen, because of its lower AMU of 14, is the most promising for the IAD gas. All but one case cited in the history
above used argon with an AMU of 40, and/or oxygen with
and effective AMU [5] more nearly 32. The momentum for
oxygen (O2+) is 90% that of Ar, and nitrogen is 60% that of
Ar. Nitrogen is an inexpensive alternative to Ar and much
less costly than neon which has an AMU of 20. Reports [13]
indicate that nitrogen does not seem to become incorporated
into the MgF2 the way that oxygen does, even though it is
not inert like Ar and Ne. Therefore, it is has been desirable
to investigate the relative merits of Ar and N2 IAD over the
low eV energy ranges.
DESIGN OF EXPERIMENTS
The methodology for Design of Experiments (DOE), as
detailed in Schmidt and Launsby [21], and DOEKISS Software [22] were used here for data processing. The choice of
experimental parameters for flow rate, deposition rate, and
drive current have been found by DOE methodology for both
Ar and N2. The key performance parameters measured were
the film durability and absorption. Drive voltage and thereby
drive power were recorded. Drive current and deposition
rate allow the computation of a relative IAAR. Peak process
temperature and process speed were also measured.
Table 1 shows the data and results for the argon IAD tests,
where the first four columns contain the independent variables,
and rest are measured results and derived data. Table 2 shows
the data and results for the nitrogen IAD tests.
The preliminary results with the argon DOE did not seem
promising; therefore, the emphasis was shifted to the nitrogen
DOE. After the normal 15 DOE experiments for nitrogen, the
results were used to guide additional tests to search for a hard
and absorption free solution.
RESULTS
Normally the results, such as absorption and hardness, would
be plotted versus the independent variables of rate, sccm,
and drive current as shown in Figure 2. The checkered area
here shows the region where less than 1% absorption can be
expected when the deposition rate is 0.5 nm/second. These
plots could also be made for other rates between 2 and 8
of the DOE. Historical findings make it more beneficial to
plot the results in the derived variables of log-drive-voltage
(Vd) (where the peak of eV is approximately 60% of Vd))
versus log-ion/atom arrival rate to be compared with Figure
1. The absorption (which is continuous with the variables)
is plotted in Figures 3 and 4 on a contour plot versus Vd and
IAAR. Since the hardness is not a continuous variable, but
more of a pass/fail of the 50 strokes eraser-rub test, pass is
indicated by an “X” and fail by an “O” where they occur in
Figures 3 and 4.
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Figure 2: DOE plot of absorption at a deposition rate of 0.5 nm/S
versus sccm and drive amps.
Figure 3 shows the results of the absorption at 400 nm (rest of
visible has less absorption) from the argon experiments with
the hardness marks. It shows that the absorption is the least
at low values of Vd and IAAR and is the most at high values.
The shape of the curves are consistent with shape in Figure 1.
This combination predicts that the best results which combine
low absorption and high hardness, based on the experiments
executed, would not be expected to have zero absorption, but
would be likely to lie near the checkered area. It appears that
the same process and levels of IAD which harden (densify)
the films also sputter the fluorine and cause absorption. This
still should provide a usable process for practical work since
a QWOT for a visible antireflection coating would be ¼ as
thick as those of these tests, and will have approximately ¼
the absorption.
Figure 4 is similar to Figure 3 but for the nitrogen experiments. The region expected to yield less than 2% absorption
and 40 or more strokes of hardness is upper middle part of
the figure. The parameters which might provide a usable
process are approximately: 0.3 nm/S, 10 sccm of nitrogen,
1.36 drive amps. These should result in about: 150 Vd (2.17
log Vd), 0.43 relative IAAR (-0.36 log IAAR), and a pressure
of 3.3x10-4 torr.
Table 1: Argon IAD experimental details and results.
Table 2: Nitrogen IAD experimental details and results.
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CONCLUSIONS
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