Improved Magnesium Fluoride Process by Ion-Assisted

2024年3月1日发(作者：野外工作)

Improved Magnesium Fluoride Process by Ion-Assisted DepositionR.R. Willey, Willey Optical, Charlevoix, MI; and

K. Patel and R. Kaneriya, Astro Optics Pvt. Ltd., Mumbai, IndiaABSTRACTWhen 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 u of IAD for MgF2

on plastics is almost mandatory to get reasonable adhesion

and hardness. A review of the history of attempts to u IAD

with MgF2 at low temperatures has shown limited success.

MgF2 has been and will continue to be an attractive and well

ud 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, den and non-absorbing MgF2 films on glass

and plastic at temperatures below 100°C which are equal to

or better than tho 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 UCTIONMagnesium fluoride (MgF2, index 1.32-1.39 near 500 nm) has

been ud 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.

Oln 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 increa in absorption toward longer

wavelengths. At 10 µm, the transmittance fell to about 2%. A

thin layer can be ud 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 miconductor hardness, durability, and density of MgF2 without IAD is

a strong function of the substrate temperature during deposi-tion. 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

erar 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 u of an E-gun with this and other fluorides

is common, but can have some problems. Granular MgF2

from an E-gun can easily cau “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 u 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 en with a boat when the rate control is t for

low rates such as 0.2 nm/c. It is our opinion that a boat is

the best approach unless there are extenuating circumstances

forcing the u 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].

HISTORYMcNeil 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 ap-plicable to this report of Kennemore and Gibson [6] points to the likeli-hood that energies lower than the 125-150 eV that they had

ud 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/c., 150 eV, and 33 µA/cm2. On

this figure, the region of low ion energy and low IAAR are

313© 2010 Society of Vacuum Coaters 505/856-7188

53rd Annual Technical Conference Proceedings, Orlando, FL April 17–22, 2010 ISSN 0737-5921

found to have no effect, while the region of too high values

for the 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.

Figure

Gibson reports.

1: Ion/atom arrival ratio versus eV for Kennemore and

Martin

with Ar, Oand Netterfield [9] reported on the u of IAD at 700eV

high absorptance at 550 nm with Ar IAD, but none with O2, and H2O at ambient temperature. They reported

and H2,

sputtered away the fluorine, but that oxygen filled the fluorine

2O. The implication here is that the IAD preferentially

vacancies to make MgO where and Kennemore [10] subquently did more work

where they ud Freon 116 (Cfluorine. Their results were “less successful than hoped for.”

2F4) to replace some of the lost

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 uful and less absorbing combination.”

They also describe how the MgO content caus 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

Hacquire oxygen.2O and oxygen so that the films have less opportunity to

Martin

oxygen IAD. They showed incread absorption in the VUV

et al. [11] did an extensive study with argon and

from 100-200 nm for IAD films as compared to conventional

evaporation

showed that the additional absorption in the UV between 120

at 440°C. Their IAD was done at 350-700 eV. They

and 180 nm was probably due to MgO. This might be of no

314concern for tho working in the visible spectrum. However,

the MgO is more soluble than the MgFenvironmentally stable and satisfactory. The MgO also rais

2 and is therefore not as

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

ud for IAD. However, O2+et al. [5] to be the dominant species over O (32) has been shown by McNeil

+that the most common bombarding O by about 3:1, so

like Ar (40).

2+ ion is more nearly

Allen et al. [13] obtained good results with dual ion beam

sputtering (DIBS) of MgFFreon (CF2 by adding a background gas of

fluorinating the sputter target while the fluorine was being

4) at about 1x10-4 torr, which they describe as re-preferentially sputtered. They also found the least absorption was obtained when nitrogen was ud as the sputtering

-gas, which they attribute to its lower atomic weight (14) as

compared to Ar (40).Kolbe et al. [14] ud fluorine gas as the reactive species

for veral metal fluorides with good results. However, they

did mention the corrosive effects of the fluorine on the ion

source. Their work ems 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 caus higher index and absorption

at the short wavelength end of the et al. [15], in their Figure 11, show the decrea in

extinction

170eV to 60eV for the IAD of

coefficient with decreasing

This ems to be further evidence for the ca of using lower

YFanode voltage from

3 using a Mark II ion source.

ion energies for IAD with metal fluoride

atoms (>1000eV). The pressures can generally be lower than

IBS bombards the target to be sputtered with high energy

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 ud to replenish the lost fluorine atoms

in the metal fluoride coating and to avoid oxygen and water

vapor which would cau MgO and absorption.

Tsou and Ho [7] ud an Advanced Plasma Source (APS) for

IAD of MgFthe visible region occurred with IAD at discharge voltages

2 with argon. They reported that absorption in

exceeding 75 Vwith increasing discharge current and voltage of the plasma.

, and that absorption was found to increa

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 MgFobtained robust films on plastic at low temperatures.2 without absorption. They

Ristau et al. [18] compared the results of IBS with both boat

and

found the index at 248 nm to be 1.40-1.41 for the IBS films

E-beam Physical Vapor Deposition (PVD) for MgF2. They

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 stress were compressive for IBS and tensile

for at 100-150 eV by

lar damage results, but none as good as non-IAD at 250-Alvisi et al. [19] produced reasonable

300°C. The evidence ems 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 et al. [20] ud argon IAD and E-beam evaporated

MgFwith films deposited with no IAD. They modeled the effects

2 to compare UV absorption in the 200-400 nm range

of lost fluorine with oxygen and hydroxyl ion replacement.

They ud 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

momentum. This again ems to confirm that the differential

at low momentum to 1.7 above a certain threshold

sputtering of fluorine is the major cau of absorption in IAD

films of MENTSFor ea of production of MgFin a typical 0.5 to 1.0 meter box coater, it is desirable to have a

2 films for the visible spectrum

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 nsitive objects) and

for reduced process time. It was the goal of this work to find

the process parameters which meet the requirements using

the available broad beam IAD equipment tup ud for this work was a Leybold L560

box coater with a Leybold Turbovac 1000 turbomolecular

pump of 1000 liter/c capacity. The starting vacuum was

1x10-5(KRI™)

torr. The ion source was a Kaufmann Robinson, Inc.

KRI to the substrates was approximately 43 cm. The MgFmodel EH400 (end-Hall). The distance from the

was evaporated from a tantalum boat at rates of 0.2-1.6 nm/2

c. Argon gas was supplied at 4-8 sccm in the argon-only

experiments which gave 2.9-8.6x10-4sure. For nitrogen-only experiments, gas at 6-15 sccm gave

torr deposition pres-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 increas 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

the higher pressures, the gas atoms are also competing with

the depositing atoms at the surface which tends to cau 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

molecules/atoms. The mean free path (MFP) in centimeters

without having multiple collisions with other vapor

is 5x10-3

torr, the MFP would be 50 cm., or approximately the source

divided by the pressure in torr. Therefore, at 1x10-4

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 1x10-4the ions have reached the substrate 50 cm distant without a

torr, only 37% of

collision, and only 0.67% at a pressure of 5x10-4implies that the ion flux reaching the substrate has had many

torr. This

collisions and lost much of its energy and momentum. The

actual

of magnitude below the drive voltage of the ion source, and

eV of the ions reaching the substrate is probably an order

therefore the obrved 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 is en, from the history of the foregoing investigations,

that: 1) the ion energies (eV) may need to be kept below some

threshold,

to be adequate but not too much, and 3) oxygen needs to be

2) the IAAR (momentum per depositing atom) need

avoided in the deposited MgF2 ion energies depend on the gas flow and drive current

of

MgFthe KRI, and to some extent the deposition rate of the

the IAAR, the atomic mass of the IAD gas atoms, and the

2. The momentum imparted to the coating depends on

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 en in Figure

1, the ultimate parameters of importance are IAAR and the

energy of the arriving ions; the are derived from the results

of controlling flow, current, and rate.

315

Nitrogen, becau of its lower AMU of 14, is the most promising for the IAD gas. All but one ca cited in the history

-above ud argon with an AMU of 40, and/or oxygen with

and effective AMU [5] more nearly 32. The momentum for

oxygen (OAr. Nitrogen is an inexpensive alternative to Ar and much

2+) is 90% that of Ar, and nitrogen is 60% that of

less costly than neon which has an AMU of 20. Reports [13]

indicate that nitrogen does not em to become incorporated

into the MgFnot inert like Ar and Ne. Therefore, it is has been desirable

2 the way that oxygen does, even though it is

to investigate the relative merits of Ar and Nlow eV energy ranges.2 IAD over the

DESIGN OF EXPERIMENTSThe

detailed in Schmidt and Launsby [21], and DOEKISS Softmethodology for Design of Experiments (DOE), as

ware [22] were ud 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 Nthe film durability and absorption. Drive voltage and thereby

2. The key performance parameters measured were

drive

rate allow the computation of a relative IAAR. Peak process

power were recorded. Drive current and deposition

temperature and process speed were also 1 shows the data and results for the argon IAD tests,

where

and rest are measured results and derived data. Table 2 shows

the first four columns contain the independent variables,

the data and results for the nitrogen IAD preliminary results with the argon DOE did not em

promising; therefore, the emphasis was shifted to the nitrogen

DOE.

results were ud to guide additional tests to arch for a hard

After the normal 15 DOE experiments for nitrogen, the

and absorption free SNormally the results, such as absorption and hardness, would

be

and drive current as shown in Figure 2. The checkered area

plotted versus the independent variables of rate, sccm,

here shows the region where less than 1% absorption can be

expected when the deposition rate is 0.5 nm/cond. The

plots

of the DOE. Historical findings make it more beneficial to

could also be made for other rates between 2 and 8

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 erar-rub test, pass is

indicated by an “X” and fail by an “O” where they occur in

Figures 3 and 4.316Figure 2: DOE plot of absorption at a deposition rate of 0.5 nm/S

versus sccm and drive 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, bad 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 cau 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 tho of the tests, and will have approximately ¼

the 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

process are approximately: 0.3 nm/S, 10 sccm of nitrogen,

figure. The parameters which might provide a usable

1.36 drive amps. The 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 2: Nitrogen IAD experimental details and results.317

Figure 3: Contour plot of % Absorption versus drive voltage and

ion/atom arrival rate for argon IAD.

Figure 4: Contour plot of %Absorption versus drive voltage and

ion/atom arrival rate for nitrogen SIONSIt does not appear that there are fully abrasion resistant and

absorption-free solutions for the IAD of MgFnitrogen at “room temperature.” It appears that IAD condi2 with argon or

tions which provide sufficiently densified films to be hard

-enough will also lectively sputter some fluorine and cau

some

practical “room temperature” solutions which will be hard

absorption. However, it does appear that there are

enough and will have low enough absorption to be tolerable

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