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Plasma-polymerized films as moisture barriers for alkali halide optics
Thin Solid Films, 84 (1981)427-434 PREPARATIONAND CHARACTERIZATION
427
P L A S M A - P O L Y M E R I Z E D FILMS AS M O I S T U R E B A R R I E R S FOR A L K A L I H A L I D E OPTICS* FREDERICKG. YAMAGISHI,DIANAD. GRANGER,ADELEE. SCHMITZAND LEROYJ. MILLER Hughes Research Laboratories, Malibu, CA 90265 (U.S.A.)
(ReceivedMarch 31, 1981; acceptedJune 1, 1981)
The nature of the alkali halide surface is of paramount importance in enabling plasma-polymerized films to act as moisture barriers and protective coatings. We developed an effective in situ surface passivation method that uses the active species formed in a plasma derived from Freon gases to remove surface hydroxide groups with chlorine or fluorine atoms. A plasma-polymerized ethane protective coating deposited onto a passivated NaC1 window provided protection from damage by water vapor at 98~ relative humidity (RH) for at least 100 h. Furthermore, thin films of a variety of surfactants derived from fatty acids on NaC1 and KBr were also useful passivating agents. Lifetimes of up to 10 days were obtained on NaC1 at 88~ RH. Plasma-polymerized films contained long-lived trapped free radicals which reacted with oxygen when the film was exposed to air. This generated oxygen functionality in the polymer which decreased the hydrophobicity and increased the IR absorption. Methods to eliminate these radicals are discussed.
1. INTRODUCTION Alkali halides are attractive materials for use as window and optical elements in IR detector and laser systems, particularly because of their high transmissivity in the far-IR (8-12 ~tm) region. These compounds, however, are fragile and are sensitive to moisture. Exposing polished surfaces to high humidity causes the optical qualities of the window to deteriorate rapidly, ultimately degrading the entire system. The short lifetimes of alkali halides under humid conditions require that moisture-damaged elements be replaced regularly. Aside from the inconvenience and the diminished reliability, this condition greatly increases the cost of these systems. Consequently, there is a need for a moisture-protective coating that would extend the lifetime of alkali halide components in uncontrolled environments. Previous attempts to prepare moisture-protective coatings for alkali halide windows have met with only partial success. Young I showed that vacuumevaporated As2S 3 protected NaC1 for 7.5 h at 100~o relative humidity (RH). Films of * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6-10, 1981.
0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedin The Netherlands
428
F.G. YAMAGISHIe t
al.
BaF 2 and MgF 2 did not prevent damage because the growth of the coating occurred through the coalescing of crystallites, which promoted porosity through voids 1. Damage to the surface always 6ccurred at scratches on the underlying surface caused by mechanical polishing. Similar results were obtained by Hopkins e t al. 2 who thermally evaporated CaF 2 onto mechanically polished NaC1. This film afforded protection for 24 h at 95~/o RH (27-50 °C), after which the film failed by localized moisture penetration along fine cracks in the film. Organic polymers appear to be excellent candidates for moisture-protective coatings. Several polymers are known to be hydrophobic, but many contain functional groups that absorb in the far-IR. Linear hydrocarbon polymers such as polyethylene have second-order absorption in the far-IR that precludes their being used. Also, thin polymer films are known to be porous 3. Thus Hopkins et al. 2 sputter deposited both polytetrafluoroethylene and fluorinated polyethylenepropylene onto NaC1. These films protected the window from moisture damage in 95~o RH for about 72 h before moisture permeated the film dissolving the underlying surface. However, a judicious choice of a monomer to sustain a glow discharge can lead to a polymer with the enhanced properties required for a moisture-protective coating over chemically prepared materials. This process is also called plasma polymerization 4. The first example of the utility of plasma-polymerized films as moisture barriers for alkali halides was reported by Hollahan et al. 5 Films prepared from the monomers chlorotrifluoroethylene and tetrafluoroethylene (TFE) were deposited onto CsI and NaC1 respectively. Plasma-polymerized T F E protected NaC1 from damage by 88.8~o RH for 117 h, after which the testing was arbitrarily stopped. These films cannot be used in the far-IR since the C - - F bonds absorb strongly at about 8 gm. Plasma-polymerized films derived from saturated hydrocarbons, however, are transparent in the 8-12 gm region. Tibbitt e t al. 6 reported that plasma-polymerized ethane (PPE) showed about 10~o as much absorptance in the 8-12 gm region as did polyethylene prepared by flee-radical polymerization. The film showed none of the absorption bands characteristic of C ~ C double bonds and there was no change in the IR spectrum after a coated NaC1 window was allowed to stand in air for 30 days. Dielectric loss factor measurements suggested that there was a very low uptake of water into the polymer matrix when the film was exposed to high humidity. The polymer was insoluble in organic solvents, stable in acid and base and did not melt or degrade when heated to 300 °C. (Polyethylene melts at 115-135 °C v, depending on the density.) Before P P E can be practically utilized as a moisture barrier on an alkali halide surface, substrate surface factors must be considered. Pastor and Pastor 8 have reported that the major impurity on NaCI and KC1 surfaces is most probably the corresponding hydroxides. A plot of the Mulliken electronegativity and the hydration energy of the halide and hydroxide ions against their ionic radii 9 (Fig. 1) shows that the hydroxide ion falls naturally onto the curve for the halides and can be considered a pseudohalide. Since the alkali metal hydroxides are deliquescent and since pure powdered NaCI and KC1 are not inherently hygroscopic 1° it is reasonable to assume that the hygroscopic nature of NaC1 and KC1 (and KBr) windows is due to the presence of hydroxide ions 8. This creates a polar high energy
429
MOISTURE BARRIERS IN ALKALI HALIDE OPTICS
surface, and hydration of the hydroxide groups would tend to lower the surface energy. This process could provide a driving force to pull water through a plasma° polymerized film matrix to lower the energy difference at the substrate-film interface and ultimately to cause film lift-off. Removing surface hydroxide groups would effectively passivate the surface of alkali halides. 2.5
I
-
2.0 ,<
I-
1.5
I
I
I
1 6
I
"~ "a,
B r-
'I
cl-
I" ,_x
I
l\
"m,., x "-e . . I.~
OH-
F1.0
I 3
I 4
I 5 eV
Fig. 1. Anion size v s . the Mulliken electronegativity (Q) and the hydration energy ( I ) .
Treatment of NaC1 and KC1 with aqueous hydrochloric acid has been shown ~~ to repair small scratches resulting from mechanical polishing while apparently exchanging chloride ions for surface hydroxides. Additionally, KC1 was passivated by careful processing in a CC14 atmosphere (reactive atmosphere processing (RAp)8), Passivation by this method reduced the surface energy to such an extent that water did not wet the surface except at local high energy sites, such as unannealed defect points. 2. EXPERIMENTAL DETAILS 2.1. Materials
The gases ethane and ethylene obtained from Matheson Gas Products for use in this study were chemically pure grade. The various Freon gases (Matheson Gas Products) were at least 99.0~ pure. All the gases were used without further purification. Unpolished NaC1 and KBr windows were purchased from Harshaw and were mechanically polished in house. 2.2. Plasma reactor The in situ surface passivation experiments and the polymerizations were done
in a Tegal PR- 100 capacitively coupled parallel plate reactor enclosed in a Pyrex belljar. The electrode gap was 1 in and the sample was placed on the grounded electrode. Monomer gas was admitted into the upper electrode and directed through small holes in the bottom of the electrode into the plasma region. The plasma was initiated and sustained with a Tega1300P r.f. generator at 13.56 M H z through an impedancematching network. Pressure was measured with a Barocel from Datametrics Incorporated and the pressure was regulated with a Datametrics type 1404 pressure controller. The system was evacuated with a Welch two-stage pump.
430
F.G. YAMAGISHIet al.
2.3. Analytical instrumentation IR spectra were recorded on a Beckman model IR-12 grating spectrophotometer. Thickness measurements were made with a Sloan Technology Corporation Dektak F L M surface-profile-measuring system at a step where the film was scratched off a coated glass slide. 2.4. Environmental testing The moisture resistance of P P E on alkali halide substrates was tested in constant humidity atmospheres over saturated salt solutions (CuSO4-5H20 at 20 °C, 98% RH; KCrO4 at 20 °C, 88% RH) or in a Blue M controlled temperature and humidity chamber at 25 °C. 2.5. Deposition procedure An alkali halide window was mechanically polished to a surface roughness of less than 1 gm by standard methods for the material. The surface was chemically etched with aqueous hydrochloric 11 (or hydrobromic 12) acid, rinsed in 2-propanol and dried in warm air. The window was placed in the reactor which was evacuated to less than 50 mTorr, after which the system was flushed with the proper Freon gas for 10 min. The flow was then adjusted to the equivalent of 11 ml m i n - 1 of air at STP (Matheson Gas Products model 7632 Rotameter). The plasma was initiated at 100 W at a pressure of 300 mTorr and was continued for a total of 5 min. The gas flow was shut off, the discharge terminated and the system evacuated. The reactor was then flushed with ethane for 10 min. High quality films were deposited under the following conditions: a flow rate of 10 ml min-1 at STp, a reaction pressure of 1.0 Torr and a power level of 200 W. The deposition rate was found to be nearly linear so that desired thicknesses could be obtained by depositing for a given amount of time. After the deposition the pale yellow film was stored under 12-15 Torr of ethylene for 15 h before exposing it to air. 3.
RESULTS AND DISCUSSION
3.1. NaCl We carried out an empirical investigation into the use of P P E as an effective moisture barrier on alkali halide optical windows. We found that the quality of the window surface was of paramount importance in enabling the film to protect against damage caused by exposure to high humidity. Thus, commercially polished NaC1 coated with P P E incurred extensive erosion by water after 10 min at 98% RH. Repolishing increased the damage threshold to 20 min. Treating mechanically polished windows by chemical etching with hydrochloric acid enabled the film to protect against moisture for 8 h. This method creates a smooth surface, rounding off jagged defects caused by mechanical polishing. Chemical etching partially passivates the surface of NaC1 by removal of hydroxide-containing surface layers and replacement of hydroxide groups with chloride. We have developed an in situ method, analogous to the RAP s method, for passivating alkali halide surfaces in the plasma reactor before depositing the P P E film. Thus, a plasma derived from a variety of chlorocarbon and fluorocarbon gases will efficiently remove hydroxide groups. Specifically, dichlorodifluoromethane
431
M O I S T U R E BARRIERS I N A L K A L I H A L I D E O P T I C S
(Freon-12) is expected on the basis of bond energies to form preferentially chlorine atoms and chlorodifluoromethyl radicals, as shown by the analogous reactions CF3--C1 -* C F 3 + C1 C F 3 - - F -* CF3 + F
B o n d strength 85 kcal m o l - 1 129 kcal mo1-1
Replacement of hydroxides can occur by impingement of the active chlorine species on the surface. There is the possibility of concurrent mechanisms in which other active species (e.g. F) may act as replacement agents as well. Our method of in situ passivation for NaC1 permitted a film of P P E 2.4 I.tm thick to protect the surface from damage by water vapor at 98% R H for more than 100 h, after which the testing was arbitrarily stopped. At that time there were no signs of bubbles, cracks or condensed water droplets. These conditions of testing are severe for NaC1, as reported by Youngk Figure 2 shows a very rapid incre/tse in water uptake when polished NaC1 is exposed to different relative humidities.
3
--
< w (,J Z
2 --
<
1 --
0
0
t 10
1 2-0
_ 30 40 50 60 70 RELATIVE HUMIDITY, %
80
90
100
Fig. 2. M a s s increase of a polished N a C I disc of d i a m e t e r 2.5 c m w h e n e x p o s e d to w a t e r v a p o r at 300 K. T h e e x p o s u r e time was 30 m i n for each v a l u e of R H .
Some physical properties were measured on P P E films prepared in a similar manner. The index of refraction was determined to be 1.67 by measuring the optical thickness in the IR on ZnS substrates. Laser calorimetry was used to measure an absolute absorptance of 2.14% at 10.6 ~tm for a film of thickness 5.66 I~m on NaC1. Another film of thickness 2.5 ~tm on KC1 prepared in a different reactor had an absorptance of 1.1%. These values are comparable with those of P P E films prepared by Reis et al. 13 The data, adjusted for thickness, are summarized in Table I. The adhesion of PPE to NaC1 was found to be good. The film passed both the Cellophane tape and eraser tests (performed in accordance with MIL-C-675B).
432
F.G. YAMAGISHIet al.
TABLE 1 ABSORPTANCE OF PLASMA-POLYMERIZEDETHANE ON KCI BY LASERCALORIMETRYAT [0.6 I.tm
Sample
Source
Thickness (gm)
Absorptance (%)
Absorptance per unit thickness (%; p_m- a)
1 2 3 4
HRL" HRL Reis et al. Reis et al.
2.5 5.66 ~0.5 ~0.28
1.1 2.14 0.35 0.11
0.44 0.38 0.70 0.39
"Films prepared at Hughes Research Laboratories.
We made investigations using a surfactant derived from a fatty acid as a passivating agent. This c o m p o u n d should mask or eliminate by chemical reaction any surface hydroxide groups and convert the polar alkali halide surface to a nonpolar surface by the presence of the hydrophobic fatty acid groups of the surfactant which are oriented normal to the surface. This reversal of polarity at the surface would enhance the adhesion of the plasma-polymerized film which is non-polar. We chose the surfactant tetrachloro-~t-hydroxo-g-carboxylatochromium(III) (QuilonC, DuPont) since this c o m p o u n d will chemically bind, through the polar end of the surfactant molecule, with surface hydroxyl groups, eliminating HC1. Thus, 1 ml of a 2-propanol solution of Quilon-C was concentrated under a stream of nitrogen and the residue was dissolved in 200 ml of 2-ethoxyethanol. This solution was spin cast onto the surface of a chemically etched NaC1 window. P P E was deposited (5.5 6.0 gm thick) onto this passivated surface. After 248 h exposure to 98~o RH there were some small striations in the film but no bubbles or cracks were present in the film and there were no visible signs of surface degradation due to water dissolution. It is known that long-lived free radicals are trapped in plasma-polymerized films 14-16. For the film to retain m a x i m u m hydrophobicity, the free radicals must be efficiently quenched so that no polar oxygenated sites can form on the surface or in the bulk by exposure to the atmosphere. Free-radical-quenching methods are compared in Table II. In each case a P P E film was deposited onto NaC1 and treated as described above before the window was exposed to air. The samples were stored in dry air and monitored periodically by IR spectroscopy for the formation of a carbonyl group (about 2 ~ absorption). At present we allow the residual free radicals in P P E to act as initiating agents for the chain-reaction polymerization of ethylene. Although this method works reasonably well it is neither efficient nor completely effective, as expected from the data shown in Table II. The treatment of P P E for 5 min with hydrogen atoms derived from a hydrogen plasma was also effective, but no more so than storage under ethylene. The effort to quench residual free radicals TABLE II A COMPARISON OF FREE-RADICAL-QUENCHINGMETHODSa
Treatment
TimeJbr C = O appearance (days)
Stored under O z Stored under ethylene H 2 plasma and stored under
37 56 52
H 2
The methods were monitored at 5.8 p.m for the formation of a carbonyl group which would arise by reaction of oxygen with the residual free radicals.
MOISTURE BARRIERS IN ALKALI HALIDE OPTICS
433
completely in plasma-polymerized films is continuing and will be reported at a later date. 3.2. K B r
P P E was found to protect KBr windows if the surface was passivated before the deposition of the film. KBr which was chemically etched with HBr was protected by P P E from damage by 88~o RH for no more than 3 h before water penetrated the film and caused dissolution of the surface. Several methods for passivating the surface were investigated. Of the Freon gases CF3Br, CC12F 2 and CC1F 3, CF3Br was the most effective, allowing a P P E film 7 ~tm thick to protect the window for 95 h at 83~ RH. Surfactants were not as effective on KBr as on NaC1. Quilon-C did not wet the surface well and led to lifetimes of about 50 h at 88~o RH. We also investigated the use of long-chain amines as passivating agents 17. Octadecylamine was melted onto a KBr window and the excess was removed by light rubbing with a tissue. The contact angle of CH2I 2 on this coated surface was 65 °. The contact angle of CH2I 2 on paraffin is 67 ° 18 and on bare KBr is 31 °. Thus a KBr window coated with octadecylamine mimics a hydrocarbon surface. Furthermore, there is no significant IR absorption in the 8-12 ~tm region if the amine is applied as a very thin film. P P E deposited onto an octadecylamine-treated KBr window protected the surface from damage by 88~o RH for 48 h with slow formation of pinholes over the next 5 days. As in the case of Quilon-C, this result is due to uneven application of octadecylamine on the'surface. Recently Wydeven and Johnson ~9 reported that plasma-polymerized films derived from chlorotrifluoroethylene, tetrafluoroethylene and ethylene (denoted as P P E by them) on chemically etched but non-passivated KBr windows were all visibly damaged within 24 h at a temperature of 24 °C and RHs above 80~o. 4.
SUMMARIZING REMARKS
Plasma-polymerized ethane films have been shown to be effective moistureprotective coatings for passivated alkali halide windows. For NaC1, passivation could be achieved by treatment of the surface with the active species derived from a Freon plasma or by application of a surfactant such as Quilon-C. Extended lifetimes for KBr windows were achieved by similar passivation methods. ACKNOWLEDGMENTS
This work was supported in part by the Defense Advanced Research Projects Agency under Contract MDA 903-80C-0096. The authors wish to thank Professor Arthur W. Adamson for many helpful discussions. REFERENCES 1
P . A . Young, Thin Solid Films, 6 (1970) 423.
2
R.H. Hopkins, R . A . HoffmanandW. E. Kramer, Appl. Opt.,14(1975)2631. R . K . Traeger, in Proc. 26th Annu. Electronic Components Conf., San Francisco, CA, April 26-28, 1976, 1976, p. 361.
3
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4 5 6
7 8 9 10 11 12
13 14 15 16 17 18 19
F.G. YAMAGISHI et al.
M. Millard, in J. R. Hollahan and A. T. Bell (eds.), Techniques and Applications o f Plasma Chemistry, Wiley, New York, 1974, Chap. 5. J.R. Hollahan, T. Wydeven and C. C. Johnson, Appl. Opt., 13 (1974) 1844. J.M. Tibbitt, A. T. Bell and M. Shen, in C. R. Andrews and C. L. Strecker (eds.), Proc. 5th ConJ] on Infrared Laser Window Materials, in USAFML Spec. Rep., 1976, p. 206 (U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH). F.W. Billmeyer, Jr., Textbook of Polymer Science, Interscience, New York, 1962, Chap. 13. R.C. Pastor and A. C. Pastor, Mater. Res. Bull., 10 (1975) 117. R.C. Pastor and M. Robinson, Mater. Res. Bull., 9 (1974) 569. H. Remy, Treatise on Inorganic Chemistry, Vol. I, Elsevier, New York, 1956, p. 188. J.W. Davisson, in Proc. Conf. on High Power IR Laser Window Materials, October 30-November 1, 1972, Vol. 2, Air Force Cambridge Research Laboratories, 1973, p. 525. J . W . Davisson, in C. R. Andrews and C. L. Strecker (eds.), Proc. 5th Conf. on Infrared Laser Window Materials, in USAFML Spec. Rep., 1976, p. 114 (U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH). T.A. Reis, H. Hiratsuka, A. T. Bell and M. Shen, in Proc. Conf. on Laser Induced Damage in Optical Materials, July 13 15, 1976, Boulder, CO, in Natl. Bur. Stand. ( U.S.), Spec. Publ. 462, 1976, p. 230. A.R. Denaro, P. A. Owens and A. Cranshaw, Eur. Polym. J., 4 (1968) 93. S. Morita, T. Mizutani and M. Ieda, Jpn. J. Appl. Phys., 10 (1971) 1275. H. Kobayashi, M. Shen and A. T. Bell, J. Macromol. Sci. A, 8 (1974) 373. A.W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 3rd edn., 1976, p. 471. A.W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 3rd edn., 1976, p. 352. T. Wydeven and C. C. Johnson, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 21 (1980) 62.
427
P L A S M A - P O L Y M E R I Z E D FILMS AS M O I S T U R E B A R R I E R S FOR A L K A L I H A L I D E OPTICS* FREDERICKG. YAMAGISHI,DIANAD. GRANGER,ADELEE. SCHMITZAND LEROYJ. MILLER Hughes Research Laboratories, Malibu, CA 90265 (U.S.A.)
(ReceivedMarch 31, 1981; acceptedJune 1, 1981)
The nature of the alkali halide surface is of paramount importance in enabling plasma-polymerized films to act as moisture barriers and protective coatings. We developed an effective in situ surface passivation method that uses the active species formed in a plasma derived from Freon gases to remove surface hydroxide groups with chlorine or fluorine atoms. A plasma-polymerized ethane protective coating deposited onto a passivated NaC1 window provided protection from damage by water vapor at 98~ relative humidity (RH) for at least 100 h. Furthermore, thin films of a variety of surfactants derived from fatty acids on NaC1 and KBr were also useful passivating agents. Lifetimes of up to 10 days were obtained on NaC1 at 88~ RH. Plasma-polymerized films contained long-lived trapped free radicals which reacted with oxygen when the film was exposed to air. This generated oxygen functionality in the polymer which decreased the hydrophobicity and increased the IR absorption. Methods to eliminate these radicals are discussed.
1. INTRODUCTION Alkali halides are attractive materials for use as window and optical elements in IR detector and laser systems, particularly because of their high transmissivity in the far-IR (8-12 ~tm) region. These compounds, however, are fragile and are sensitive to moisture. Exposing polished surfaces to high humidity causes the optical qualities of the window to deteriorate rapidly, ultimately degrading the entire system. The short lifetimes of alkali halides under humid conditions require that moisture-damaged elements be replaced regularly. Aside from the inconvenience and the diminished reliability, this condition greatly increases the cost of these systems. Consequently, there is a need for a moisture-protective coating that would extend the lifetime of alkali halide components in uncontrolled environments. Previous attempts to prepare moisture-protective coatings for alkali halide windows have met with only partial success. Young I showed that vacuumevaporated As2S 3 protected NaC1 for 7.5 h at 100~o relative humidity (RH). Films of * Paper presented at the International Conference on Metallurgical Coatings, San Francisco, CA, U.S.A., April 6-10, 1981.
0040-6090/81/0000-0000/$02.50
© ElsevierSequoia/Printedin The Netherlands
428
F.G. YAMAGISHIe t
al.
BaF 2 and MgF 2 did not prevent damage because the growth of the coating occurred through the coalescing of crystallites, which promoted porosity through voids 1. Damage to the surface always 6ccurred at scratches on the underlying surface caused by mechanical polishing. Similar results were obtained by Hopkins e t al. 2 who thermally evaporated CaF 2 onto mechanically polished NaC1. This film afforded protection for 24 h at 95~/o RH (27-50 °C), after which the film failed by localized moisture penetration along fine cracks in the film. Organic polymers appear to be excellent candidates for moisture-protective coatings. Several polymers are known to be hydrophobic, but many contain functional groups that absorb in the far-IR. Linear hydrocarbon polymers such as polyethylene have second-order absorption in the far-IR that precludes their being used. Also, thin polymer films are known to be porous 3. Thus Hopkins et al. 2 sputter deposited both polytetrafluoroethylene and fluorinated polyethylenepropylene onto NaC1. These films protected the window from moisture damage in 95~o RH for about 72 h before moisture permeated the film dissolving the underlying surface. However, a judicious choice of a monomer to sustain a glow discharge can lead to a polymer with the enhanced properties required for a moisture-protective coating over chemically prepared materials. This process is also called plasma polymerization 4. The first example of the utility of plasma-polymerized films as moisture barriers for alkali halides was reported by Hollahan et al. 5 Films prepared from the monomers chlorotrifluoroethylene and tetrafluoroethylene (TFE) were deposited onto CsI and NaC1 respectively. Plasma-polymerized T F E protected NaC1 from damage by 88.8~o RH for 117 h, after which the testing was arbitrarily stopped. These films cannot be used in the far-IR since the C - - F bonds absorb strongly at about 8 gm. Plasma-polymerized films derived from saturated hydrocarbons, however, are transparent in the 8-12 gm region. Tibbitt e t al. 6 reported that plasma-polymerized ethane (PPE) showed about 10~o as much absorptance in the 8-12 gm region as did polyethylene prepared by flee-radical polymerization. The film showed none of the absorption bands characteristic of C ~ C double bonds and there was no change in the IR spectrum after a coated NaC1 window was allowed to stand in air for 30 days. Dielectric loss factor measurements suggested that there was a very low uptake of water into the polymer matrix when the film was exposed to high humidity. The polymer was insoluble in organic solvents, stable in acid and base and did not melt or degrade when heated to 300 °C. (Polyethylene melts at 115-135 °C v, depending on the density.) Before P P E can be practically utilized as a moisture barrier on an alkali halide surface, substrate surface factors must be considered. Pastor and Pastor 8 have reported that the major impurity on NaCI and KC1 surfaces is most probably the corresponding hydroxides. A plot of the Mulliken electronegativity and the hydration energy of the halide and hydroxide ions against their ionic radii 9 (Fig. 1) shows that the hydroxide ion falls naturally onto the curve for the halides and can be considered a pseudohalide. Since the alkali metal hydroxides are deliquescent and since pure powdered NaCI and KC1 are not inherently hygroscopic 1° it is reasonable to assume that the hygroscopic nature of NaC1 and KC1 (and KBr) windows is due to the presence of hydroxide ions 8. This creates a polar high energy
429
MOISTURE BARRIERS IN ALKALI HALIDE OPTICS
surface, and hydration of the hydroxide groups would tend to lower the surface energy. This process could provide a driving force to pull water through a plasma° polymerized film matrix to lower the energy difference at the substrate-film interface and ultimately to cause film lift-off. Removing surface hydroxide groups would effectively passivate the surface of alkali halides. 2.5
I
-
2.0 ,<
I-
1.5
I
I
I
1 6
I
"~ "a,
B r-
'I
cl-
I" ,_x
I
l\
"m,., x "-e . . I.~
OH-
F1.0
I 3
I 4
I 5 eV
Fig. 1. Anion size v s . the Mulliken electronegativity (Q) and the hydration energy ( I ) .
Treatment of NaC1 and KC1 with aqueous hydrochloric acid has been shown ~~ to repair small scratches resulting from mechanical polishing while apparently exchanging chloride ions for surface hydroxides. Additionally, KC1 was passivated by careful processing in a CC14 atmosphere (reactive atmosphere processing (RAp)8), Passivation by this method reduced the surface energy to such an extent that water did not wet the surface except at local high energy sites, such as unannealed defect points. 2. EXPERIMENTAL DETAILS 2.1. Materials
The gases ethane and ethylene obtained from Matheson Gas Products for use in this study were chemically pure grade. The various Freon gases (Matheson Gas Products) were at least 99.0~ pure. All the gases were used without further purification. Unpolished NaC1 and KBr windows were purchased from Harshaw and were mechanically polished in house. 2.2. Plasma reactor The in situ surface passivation experiments and the polymerizations were done
in a Tegal PR- 100 capacitively coupled parallel plate reactor enclosed in a Pyrex belljar. The electrode gap was 1 in and the sample was placed on the grounded electrode. Monomer gas was admitted into the upper electrode and directed through small holes in the bottom of the electrode into the plasma region. The plasma was initiated and sustained with a Tega1300P r.f. generator at 13.56 M H z through an impedancematching network. Pressure was measured with a Barocel from Datametrics Incorporated and the pressure was regulated with a Datametrics type 1404 pressure controller. The system was evacuated with a Welch two-stage pump.
430
F.G. YAMAGISHIet al.
2.3. Analytical instrumentation IR spectra were recorded on a Beckman model IR-12 grating spectrophotometer. Thickness measurements were made with a Sloan Technology Corporation Dektak F L M surface-profile-measuring system at a step where the film was scratched off a coated glass slide. 2.4. Environmental testing The moisture resistance of P P E on alkali halide substrates was tested in constant humidity atmospheres over saturated salt solutions (CuSO4-5H20 at 20 °C, 98% RH; KCrO4 at 20 °C, 88% RH) or in a Blue M controlled temperature and humidity chamber at 25 °C. 2.5. Deposition procedure An alkali halide window was mechanically polished to a surface roughness of less than 1 gm by standard methods for the material. The surface was chemically etched with aqueous hydrochloric 11 (or hydrobromic 12) acid, rinsed in 2-propanol and dried in warm air. The window was placed in the reactor which was evacuated to less than 50 mTorr, after which the system was flushed with the proper Freon gas for 10 min. The flow was then adjusted to the equivalent of 11 ml m i n - 1 of air at STP (Matheson Gas Products model 7632 Rotameter). The plasma was initiated at 100 W at a pressure of 300 mTorr and was continued for a total of 5 min. The gas flow was shut off, the discharge terminated and the system evacuated. The reactor was then flushed with ethane for 10 min. High quality films were deposited under the following conditions: a flow rate of 10 ml min-1 at STp, a reaction pressure of 1.0 Torr and a power level of 200 W. The deposition rate was found to be nearly linear so that desired thicknesses could be obtained by depositing for a given amount of time. After the deposition the pale yellow film was stored under 12-15 Torr of ethylene for 15 h before exposing it to air. 3.
RESULTS AND DISCUSSION
3.1. NaCl We carried out an empirical investigation into the use of P P E as an effective moisture barrier on alkali halide optical windows. We found that the quality of the window surface was of paramount importance in enabling the film to protect against damage caused by exposure to high humidity. Thus, commercially polished NaC1 coated with P P E incurred extensive erosion by water after 10 min at 98% RH. Repolishing increased the damage threshold to 20 min. Treating mechanically polished windows by chemical etching with hydrochloric acid enabled the film to protect against moisture for 8 h. This method creates a smooth surface, rounding off jagged defects caused by mechanical polishing. Chemical etching partially passivates the surface of NaC1 by removal of hydroxide-containing surface layers and replacement of hydroxide groups with chloride. We have developed an in situ method, analogous to the RAP s method, for passivating alkali halide surfaces in the plasma reactor before depositing the P P E film. Thus, a plasma derived from a variety of chlorocarbon and fluorocarbon gases will efficiently remove hydroxide groups. Specifically, dichlorodifluoromethane
431
M O I S T U R E BARRIERS I N A L K A L I H A L I D E O P T I C S
(Freon-12) is expected on the basis of bond energies to form preferentially chlorine atoms and chlorodifluoromethyl radicals, as shown by the analogous reactions CF3--C1 -* C F 3 + C1 C F 3 - - F -* CF3 + F
B o n d strength 85 kcal m o l - 1 129 kcal mo1-1
Replacement of hydroxides can occur by impingement of the active chlorine species on the surface. There is the possibility of concurrent mechanisms in which other active species (e.g. F) may act as replacement agents as well. Our method of in situ passivation for NaC1 permitted a film of P P E 2.4 I.tm thick to protect the surface from damage by water vapor at 98% R H for more than 100 h, after which the testing was arbitrarily stopped. At that time there were no signs of bubbles, cracks or condensed water droplets. These conditions of testing are severe for NaC1, as reported by Youngk Figure 2 shows a very rapid incre/tse in water uptake when polished NaC1 is exposed to different relative humidities.
3
--
< w (,J Z
2 --
<
1 --
0
0
t 10
1 2-0
_ 30 40 50 60 70 RELATIVE HUMIDITY, %
80
90
100
Fig. 2. M a s s increase of a polished N a C I disc of d i a m e t e r 2.5 c m w h e n e x p o s e d to w a t e r v a p o r at 300 K. T h e e x p o s u r e time was 30 m i n for each v a l u e of R H .
Some physical properties were measured on P P E films prepared in a similar manner. The index of refraction was determined to be 1.67 by measuring the optical thickness in the IR on ZnS substrates. Laser calorimetry was used to measure an absolute absorptance of 2.14% at 10.6 ~tm for a film of thickness 5.66 I~m on NaC1. Another film of thickness 2.5 ~tm on KC1 prepared in a different reactor had an absorptance of 1.1%. These values are comparable with those of P P E films prepared by Reis et al. 13 The data, adjusted for thickness, are summarized in Table I. The adhesion of PPE to NaC1 was found to be good. The film passed both the Cellophane tape and eraser tests (performed in accordance with MIL-C-675B).
432
F.G. YAMAGISHIet al.
TABLE 1 ABSORPTANCE OF PLASMA-POLYMERIZEDETHANE ON KCI BY LASERCALORIMETRYAT [0.6 I.tm
Sample
Source
Thickness (gm)
Absorptance (%)
Absorptance per unit thickness (%; p_m- a)
1 2 3 4
HRL" HRL Reis et al. Reis et al.
2.5 5.66 ~0.5 ~0.28
1.1 2.14 0.35 0.11
0.44 0.38 0.70 0.39
"Films prepared at Hughes Research Laboratories.
We made investigations using a surfactant derived from a fatty acid as a passivating agent. This c o m p o u n d should mask or eliminate by chemical reaction any surface hydroxide groups and convert the polar alkali halide surface to a nonpolar surface by the presence of the hydrophobic fatty acid groups of the surfactant which are oriented normal to the surface. This reversal of polarity at the surface would enhance the adhesion of the plasma-polymerized film which is non-polar. We chose the surfactant tetrachloro-~t-hydroxo-g-carboxylatochromium(III) (QuilonC, DuPont) since this c o m p o u n d will chemically bind, through the polar end of the surfactant molecule, with surface hydroxyl groups, eliminating HC1. Thus, 1 ml of a 2-propanol solution of Quilon-C was concentrated under a stream of nitrogen and the residue was dissolved in 200 ml of 2-ethoxyethanol. This solution was spin cast onto the surface of a chemically etched NaC1 window. P P E was deposited (5.5 6.0 gm thick) onto this passivated surface. After 248 h exposure to 98~o RH there were some small striations in the film but no bubbles or cracks were present in the film and there were no visible signs of surface degradation due to water dissolution. It is known that long-lived free radicals are trapped in plasma-polymerized films 14-16. For the film to retain m a x i m u m hydrophobicity, the free radicals must be efficiently quenched so that no polar oxygenated sites can form on the surface or in the bulk by exposure to the atmosphere. Free-radical-quenching methods are compared in Table II. In each case a P P E film was deposited onto NaC1 and treated as described above before the window was exposed to air. The samples were stored in dry air and monitored periodically by IR spectroscopy for the formation of a carbonyl group (about 2 ~ absorption). At present we allow the residual free radicals in P P E to act as initiating agents for the chain-reaction polymerization of ethylene. Although this method works reasonably well it is neither efficient nor completely effective, as expected from the data shown in Table II. The treatment of P P E for 5 min with hydrogen atoms derived from a hydrogen plasma was also effective, but no more so than storage under ethylene. The effort to quench residual free radicals TABLE II A COMPARISON OF FREE-RADICAL-QUENCHINGMETHODSa
Treatment
TimeJbr C = O appearance (days)
Stored under O z Stored under ethylene H 2 plasma and stored under
37 56 52
H 2
The methods were monitored at 5.8 p.m for the formation of a carbonyl group which would arise by reaction of oxygen with the residual free radicals.
MOISTURE BARRIERS IN ALKALI HALIDE OPTICS
433
completely in plasma-polymerized films is continuing and will be reported at a later date. 3.2. K B r
P P E was found to protect KBr windows if the surface was passivated before the deposition of the film. KBr which was chemically etched with HBr was protected by P P E from damage by 88~o RH for no more than 3 h before water penetrated the film and caused dissolution of the surface. Several methods for passivating the surface were investigated. Of the Freon gases CF3Br, CC12F 2 and CC1F 3, CF3Br was the most effective, allowing a P P E film 7 ~tm thick to protect the window for 95 h at 83~ RH. Surfactants were not as effective on KBr as on NaC1. Quilon-C did not wet the surface well and led to lifetimes of about 50 h at 88~o RH. We also investigated the use of long-chain amines as passivating agents 17. Octadecylamine was melted onto a KBr window and the excess was removed by light rubbing with a tissue. The contact angle of CH2I 2 on this coated surface was 65 °. The contact angle of CH2I 2 on paraffin is 67 ° 18 and on bare KBr is 31 °. Thus a KBr window coated with octadecylamine mimics a hydrocarbon surface. Furthermore, there is no significant IR absorption in the 8-12 ~tm region if the amine is applied as a very thin film. P P E deposited onto an octadecylamine-treated KBr window protected the surface from damage by 88~o RH for 48 h with slow formation of pinholes over the next 5 days. As in the case of Quilon-C, this result is due to uneven application of octadecylamine on the'surface. Recently Wydeven and Johnson ~9 reported that plasma-polymerized films derived from chlorotrifluoroethylene, tetrafluoroethylene and ethylene (denoted as P P E by them) on chemically etched but non-passivated KBr windows were all visibly damaged within 24 h at a temperature of 24 °C and RHs above 80~o. 4.
SUMMARIZING REMARKS
Plasma-polymerized ethane films have been shown to be effective moistureprotective coatings for passivated alkali halide windows. For NaC1, passivation could be achieved by treatment of the surface with the active species derived from a Freon plasma or by application of a surfactant such as Quilon-C. Extended lifetimes for KBr windows were achieved by similar passivation methods. ACKNOWLEDGMENTS
This work was supported in part by the Defense Advanced Research Projects Agency under Contract MDA 903-80C-0096. The authors wish to thank Professor Arthur W. Adamson for many helpful discussions. REFERENCES 1
P . A . Young, Thin Solid Films, 6 (1970) 423.
2
R.H. Hopkins, R . A . HoffmanandW. E. Kramer, Appl. Opt.,14(1975)2631. R . K . Traeger, in Proc. 26th Annu. Electronic Components Conf., San Francisco, CA, April 26-28, 1976, 1976, p. 361.
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7 8 9 10 11 12
13 14 15 16 17 18 19
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M. Millard, in J. R. Hollahan and A. T. Bell (eds.), Techniques and Applications o f Plasma Chemistry, Wiley, New York, 1974, Chap. 5. J.R. Hollahan, T. Wydeven and C. C. Johnson, Appl. Opt., 13 (1974) 1844. J.M. Tibbitt, A. T. Bell and M. Shen, in C. R. Andrews and C. L. Strecker (eds.), Proc. 5th ConJ] on Infrared Laser Window Materials, in USAFML Spec. Rep., 1976, p. 206 (U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH). F.W. Billmeyer, Jr., Textbook of Polymer Science, Interscience, New York, 1962, Chap. 13. R.C. Pastor and A. C. Pastor, Mater. Res. Bull., 10 (1975) 117. R.C. Pastor and M. Robinson, Mater. Res. Bull., 9 (1974) 569. H. Remy, Treatise on Inorganic Chemistry, Vol. I, Elsevier, New York, 1956, p. 188. J.W. Davisson, in Proc. Conf. on High Power IR Laser Window Materials, October 30-November 1, 1972, Vol. 2, Air Force Cambridge Research Laboratories, 1973, p. 525. J . W . Davisson, in C. R. Andrews and C. L. Strecker (eds.), Proc. 5th Conf. on Infrared Laser Window Materials, in USAFML Spec. Rep., 1976, p. 114 (U.S. Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH). T.A. Reis, H. Hiratsuka, A. T. Bell and M. Shen, in Proc. Conf. on Laser Induced Damage in Optical Materials, July 13 15, 1976, Boulder, CO, in Natl. Bur. Stand. ( U.S.), Spec. Publ. 462, 1976, p. 230. A.R. Denaro, P. A. Owens and A. Cranshaw, Eur. Polym. J., 4 (1968) 93. S. Morita, T. Mizutani and M. Ieda, Jpn. J. Appl. Phys., 10 (1971) 1275. H. Kobayashi, M. Shen and A. T. Bell, J. Macromol. Sci. A, 8 (1974) 373. A.W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 3rd edn., 1976, p. 471. A.W. Adamson, Physical Chemistry of Surfaces, Wiley, New York, 3rd edn., 1976, p. 352. T. Wydeven and C. C. Johnson, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 21 (1980) 62.