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Deposition of gradient-index antireflective coating: An approach based on ultrastructure processing
Journal
262
of Non-Crystalline
Solids 91 (1987) 262-270 North-Holland, Amsterdam
DEPOSITION OF GRADIENT-INDEX ANTIREFLECTIVE COATING: AN APPROACH BASED ON ULTRASTRUCTURE PROCESSING J.C. DEBSIKDAR Botrelle Received
Columhw 2 October
Diuision,
Columbus.
Ohio
43X/-2693.
USA
1986
A new concept. based on ultrastructure processing. has been developed to deposit graded-index antireflective coatings. According to this concept the substrate is coated, in proper sequence. by a series of solutions (or sois) of identical composition but of differing particle size. The microporous gel coating deposited in this way is characterized by a gradation of porosity due to the deposition of larger particles at the top surface and smaller particles in the substructure. On thermal consolidation, the gel coating forms a single-layer. graded-index coating; thus the chemical leaching/etching steps presently followed for porosity grading could be avoided. In this communication the results of some preliminary experiments which demonstrate the technical feasibility of the concept are reported.
1. Introduction Antireflective coatings on glasses and other materials have the potential for high-energy applications such as laser optics (lenses and windows) as well as for solar-energy applications. The basic objective of depositing an antireflective coating is to eliminate loss of light energy due to back reflection from the surfaces of transmitting optical elements. Since the reflection results from an abrupt change of the refractive index from the transmitting element to the interface, it becomes imperative to modify the index of refraction of the surfaces of the transmitting element(s) to develop the antireflective property. The aim of this surface modification is to develop a graded-index surface so that the reflection losses could be eliminated by optical interferences. The graded-index surface can be produced by coating the transmittive surface with one or more selected transparent materials using both vacuum and non-vacuum techniques. The most commonly used vacuum technique [l-4] has involved deposition of two or more layers of materials, with different refractive index, by physical vapor deposition (PVD). Among the non-vacuum techniques, the processes based on chemical leaching and etching [5-91 of phase-separated alkali-borosilicate/alkali-lime-silica glasses, and those based on the deposition of sol-gel derived porous coating [lo-201, have gained significant interest. The deposition of multi-layer coatings by the PVD method is somewhat complicated; moreover, these coatings have shown low laser-damage resistance 0022-3093/87/$03.50 6 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. C. Debsikdar
/ Gradient-index
antireflective
coating
263
[l&21], perhaps due to the presence of absorbing impurities at the interface between the coating and the substrate [21]. Chemical leaching (and etching) processes have been used successfully to produce graded porosity in phaseseparated glasses that results in very low reflectivity over a broad spectral region. The main limitation of these processes is that the glass composition should be such that on heat treatment it induces phase separation on a microscopic scale. Moreover, process variables such as the heat-treatment schedule (i.e., temperature and time), the composition of the leaching solution, and the leaching parameters (i.e., treatment time and temperature) must be very carefully controlled if reproducible results are to be achieved in terms of pore size and distribution across the thickness of the graded pores, because the refractive index gradation is a result of the smooth density transition from air to the bulk glass. The sol-gel process, on the other hand, is based on depositing a porous coating, generally from a partially hydrolyzed metalorganic precursor solution, followed by thermal treatment to consolidate the coating and remove the volatiles. To produce a porosity gradient in the coating, the thermally treated coating requires chemical etching to tailor the pore size grading across the thickness of the otherwise porous coating. However, pore size tailoring by chemical etching often poses practical problems in terms of the reproducibility of the coating microstructures [15,17,18]. The present work provides an alternative sol-gel approach by which coatings with graded porosity can be deposited without involving the chemical etching step. The approach is based on the concept of successively coating a substrate from a series of solutions (or ~01s)of the same chemical composition
Fig. 1. A model
system
for depositing
sol-gel
derived
broad-band
antireflective
coating.
264
J. C. Debsikdar
/ Gradient-index
anrireJlecriue
coating
but with increasing elementary particle (macromolecule) size. On thermal consolidation, the resulting coating is made up of graded particles and hence provides graded porosity across the coating thickness. Consequently, chemical etching used for porosity grading could be avoided. The solution (or sol) preparation parameters (and “aging”, if required) which determine the size of the elementary particles have been reported [22,23]. The present concept is illustrated in fig. 1 using a model coating system where the solutions (or ~01s) A, B, C, and D are compositionally the same, but contain particles of increasing size; the substrate is coated from these solutions (or ~01s) successively in the order of increasing particle size. To test the validity of the concept we have conducted some preliminary experiments on coating deposition using metal-organic-derived silica solutions which had an identical starting composition and preparation history, but were “aged” for different times. The antireflectivity property of these coatings was measured after they were thermally consolidated. In this communication the results of these preliminary experiments are reported.
2. Experimental
procedure
Four (4) batches of polymeric tetramethoxysilane, Si(OCH,), solutions, each containing 5 g equivalent silica, SiOz per 100 ml, were prepared at about 24-h intervals. These polymeric solutions were prepared in plastic beakers by mixing the desired amount of commercially available Si(OCHs), (Tridom/ Fluka Chemical, Inc., New York) in Photrex methanol, CH,OH (J.T. Baker Chemical Company, New Jersey) and then hydrolyzing the solutions each with 2 mol distilled water per mol Si(OCH,), at room temperature. The change of viscosity of a solution of identical composition which could be related to the particle growth in the solution has been reported earlier (ref. [23], fig. 1A). The four solutions (or ~01s) were aged for 17, 18, 19, and 20 days, respectively, before they were used for coating deposition. The approximate viscosity of these solutions is shown in fig. 2 by arrows. The solutions aged for 17, 18, 19, and 20 days are hereafter designated as solutions 1, 2, 3, and 4, respectively. Commercially available 15.24 cm X 15,24 cm (6 in. X 6 in.) Vycor glass plates (Corning 7913) were cut into 7.62 cm X 5.08 cm (3 in. X 2 in.) sizes and used as the substrates. The substrates were cleaned in an ultrasonic bath with “Alconox” solution (supplier: Alconox, Inc., New York), distilled water, and isopropanol (in the same order), dried overnight at 110” C, and cooled in a dessiccator to room temperature. The Vycor substrates were coated using dip coating equipment [17] suitable for controlled linear motion of flat plates at variable angles with respect to the surface of the coating solution. In the present investigation the substrates were withdrawn at 22.5 cm/mm and at an angle of 90” with respect to the solution surface. The time interval between the successive layers of coating was about 5 min to allow drying of the preceding layer at room temperature. The ambient
J. C. Debsikdar
/ Gradienr-index
anrirefleciive
coating
265
100
80
‘” 60 d E ; .Y fj 40 5
20
II
0 10
14
! 18
!
!
22 Time
Fig. 2. Change
of viscosity
!
f
26
i
i
30 (hours
i 34
i
i 38
42
during
aging.
46
!
x 10-l)
of the coating
solution(s)
temperature and relative humidity during coating were 21” C and 59%, respectively. The substrates were coated by a total of four (4) dippings each in three batches as follows: two dippings each from solutions 1 and 2 (hereafter referred to as procedure I); two dippings from solution 1 and one dipping each from solutions 2 and 3 (hereafter referred to as procedure II); and one dipping each from solutions 1, 2, 3, and 4 (hereafter referred to as procedure III). After final dipping, the substrates were dried overnight at 110 o C and then subjected to heat treatment in ambient atmosphere at 600’ C for 2 h. 2. I. Coating characterization The characterization work was limited to examining the particle morphology of solution 4 using a JEOL JEM 100 B (Japan Electra Optic Co.) transmission electron microscope (TEM), examining the coating microstructure on a IS1 Model 100 scanning electron microscope (SEM), and measuring the coating reflectances. For TEM study, solution 4 was diluted with anhydrous ethanol and then sprayed on a carbon-coated grid. For SEM study, a goldcoated specimen was used. The near-normal reflectance of the heat-treated coatings was measured using a tungsten illumination source and a scanning 1.26-m Spex UV-visible, grating spectrometer. An Apple II data acquisition system was used to control scanning of the spectrometer, measure reflectancesignal levels, and subsequently analyze the raw data and plot the results. Thirty data points were measured in each of the two wavelength regions, 300-600 nm and 500-800 nm. A filter for blocking radiation below 450 nm
266
J.C. Debsikdar
/ Gradient-index
antireflective
coating
was inserted in front of the spectrometer during scanning of the upper region to avoid possible second-order interference from radiation in the 300-400 nm regime. Lamp intensity was too weak at wavelengths below 350 nm to give useful results and hence the results were plotted between 350 nm (near UV) and 800 nm (near IR). Since the light source and the spectrometer system efficiency were not uniform with wavelength, a normalization procedure was required to calculate single-surface reflectance from the double-surface reflected intensity data. This normalization was done by measuring the reflectance of the uncoated Vycor substrate. The measured coating reflectances were then ratioed to the substrate reflectance and a constant factor used to convert the result to the effective single-surface percent reflectance.
3. Results and discussion 3.1. Particle morphology of the coating solution The TEM micrograph of the particles of solution 4 is shown in fig. 3. The particles were mostly round in shape, having diameters of the order of 15 to 20 nm which were linked together to form a string-like microstructure. During the TEM observations the material shrank substantially due to the heat generated by the electron beam. The string-like microstructure was formed as a result of gelling. Perhaps, a further dilution of the solution or addition of a few drops of an acid (to modify the particle surface changes) would have revealed the individual particles more clearly.
Fig. 3. TEM
micrograph
of the particles
of solution
4 (1 cm = 0.1 pm).
J.C. Dehsikdar
400
I
/ Gradienr-index
500
I Wavelength
Fig. 4. Percent
reflectance
mrireflecrive
600
coaling
I
700
267
I
800
(nm)
ol heat-treated
procedure
I coating
3.2. Reflectance of heat-treated coatings Figures 4, 5, and 6 show the reflectivity characteristics in the 350-800 nm range of one selected heat-treated sample each produced by procedures I, II and III, respectively. For comparison, the reflectance of the uncoated sample is shown by the dotted line. The coating produced by procedure I (fig. 4) showed antireflectivity over narrow bandwiths, with minima approximately at 360 and 565 nm. In this case, the coating was deposited from solutions 1 and 2 only; the difference in the particle size distribution was perhaps not large enough to produce any signficant porosity gradient across the thickness of the coating. Thus, the conditions for broadband antireflectivity were, evidently, not satisfied. The procedure II sample which was coated from three solutions (1, 2, and 3) had almost zero reflectivity conditions in the 350-410 nm (fig. 5); however, beyond 410 nm the reflectivity gradually increased to some extent up
4a-” ti 5 ; ‘y E n
__________---------------------2-
OWavelength
Fig. 5. Percent
reflectance
(nm)
of heat-treated
procedure
II coating.
J.C. Debsikdar
268
-
I
400
I
Fig. 6. Percent
/ Gradient-index
500
reflectance
I
antireflective
600 Wavelength (nm)
of heat-treated
I
procedure
coating
700
I *do
III coating.
to 570 nm and then decreased marginally. However, in general, the reflectivity characteristics of the procedure II sample indicate that this coating had some degree of porosity gradient in it. The reflectivity characteristic of sample III (fig. 6) was significantly different from that of procedure II samples in that it exhibited bettter antireflectivity in the 500-700 nm range; however, in this case, the antireflectivity was poor in the lower wavelength range compared to the procedure II sample. The procedure II sample had two dippings from solution 1, which is expected to contain the smallest particles among the solutions used in the present study; but the procedure III sample received only one dipping from solution 1. It appears that the difference between the procedure II and III samples in terms of thickness of the layer from solution 1 with the smallest particles caused the difference in antireflectivity in the lower wavelength range. Therefore, it is anticipated that a modified sequence of deposition involving two coats from solution 1 and one coat each from solutions 2, 3, and 4 would be necessary to improve antireflectivity of the procedure III sample at lower wavelength ranges. 3.3. Coating microstructure Figure 7 is a SEM micrograph of the surface morphology of the heat-treated procedure III sample whose reflectance is shown in fig. 6. The microstructure of the coating surface consisted of round particles of the order of 30-60 nm diameter. The pore structure consisted of round as well as channel-like pores of nonuniform sizes; the size range of the round-shaped pores was approximately lo-30 nm. A comparison of the particles sizes of the heat-treated coating (fig. 7) with those of solution 4 which formed this layer of particles (fig. 3) shows that during heat treatment the particle sizes increased almost twofold. Therefore, it is quite evident that significant particle coarsening takes place during thermal consolidation of the gel-derived coating and, consequently, its pore structure.
J. C. Debsikdar
Fig.
7. SEM
micrograph
of heat-treated
/ Gradient-index
procedure
antireflective
III
coating
coating
surface
269
(1 cm = 0.2 pm)
A recent study by Debsikdar [24] has shown that during thermal treatment the net change of pore structure of gels depends upon the initial pore structure, and the rate of pore closing and pore enlargement was found to be closely related to burnout of the residual organics and/or loss of chemisorbed water. The presence of residual organics in the metal-organic-derived gel depends upon the chemistry of the solution (or sol) preparation procedure. It is widely believed that the factors which influence the rate of hydrolysis of an alkoxide (factors such as the molar ratio of water to the alkoxide, the presence of an acid catalyst, and the temperature) also influence the amount of residual organics in the gel structure. Thus, the solution (or sol) preparation chemistry (and “aging” of the solution or sol, if necessary) is important not only for controlling the microstructure (i.e., particle and pore morphologies) of the gel coating but also for controlling the microstructure during thermal consolidation of the gel coating. The results of the present study have demonstrated that it is technically feasible to produce a broad-band antireflective coating when the substrate is coated successively from metal-organic-derived solutions (or ~01s) containing particles of. increasing size ranges, followed by thermal consolidation of the coating. However, further systematic studies on the particle growth phenomena in the coating solutions (or sols), the selection of coating deposition sequence, and optimization of the thermal treatment procedure would be necessary to fully establish the present approach.
270
J.C. Debsikdar
/ Gradient-index
anrirefective
coating
4. Conclusion A sol-gel-derived gradient-index antireflective coating can be produced on a substrate by depositing successive layers of solutions (or ~01s) containing particles of increasing size ranges, followed by an appropriate thermal treatment. This approach has the potential to eliminate the chemical leaching/etching step(s) presently followed for pore size tailoring (to produce the desired index gradient across the coating thickness). The author wishes to acknowledge the laboratory assistance of L. Myers for solution preparation and deposition of coatings. TEM and SEM micrographs were taken by C. Minton and A. Skidmore, respectively. Reflectance measurements were made by D. Grieser of the Optical Science Group, Electronic Systems Department, Battehe Columbus Division. Financial support for this work was provided by the Battelle Columbus Division.
References [l] E. Ritter, Appl. Opt. 15 (10) (1976) 2318. [2] J.A. Debrowolski, in: Handbook of Optics, eds. W.G. Dricoil and W. Vaughn (McGraw-Hill, New York, 1978) Ch. 8. [3] T.M. Christmas and D. Richmond, Opt. Laser Technol. 9 (6) (1977) 109. [4] J. Elbert, Proc. Sot. Photo-Opt. Inst. Eng. (1983) 325. [5] M.J. Minot. J. Opt. Sot. Am. 66 (6) (1976) 515. [6] T.H. Elmer, and F.W. Martin, Am. Ceram. Sot. BuU. 58 (11) (1979) 1092. [7] L.M. Cook, W.H. Lowdermilk, D. Milarn and J.E. Swain, Appl. Opt. 21 (8) (1982) 1482. [8] A. Iqbal, S.C. Danforth and J.S. Haggerty, J. Am. Ceram. Sot. 66 (4) (1983) 302. [9] V.T. Rohe, S.C. Danforth and J.S. Haggerty, J. Am. Ceram. Sot. 67 (2) (1984) 142. [lo] H. Schroeder, in: Physics of Thin Films, Vol. 5, eds. G. Hass and R.E. Thun (Academic Press, New York, 1969) p. 87. [ll] H.J. Hovel, J. Electrochem. Sot. 125 (6) (1978) 983. [12] B.E. Yoldas, and T.W. O’Keeffe, Appl. Opt. 18 (18) (1979) 3133. [13] B.E. Yoldas, Appl. Opt. 19 (9) (1980) 1425. [14] H. Dislich and E. Hussmann, Thin Solid Films 77 (l-3) (1981) 129. [15] C.J. Brinker and M.S. Harrington, Solar Energy Mater. 5 (1981) 159. [16] B.E. Yoldas. Appl. Opt. 21 (16) (1982) 2960. [17] S.P. Mukhejee, J.C. Debsikdar, J.F. Cordaro, K.J. Wurm, and G.T. Ruck, Final Report (Contract No. 908-007). Lawrence Livermore National Laboratories (January 1983). [18] B.E. Yoldas, and D.P. Partlow, Appl. Opt. 23 (9) (1984) 1418. [19] I. Strawbridge, J. Phalippou, and P.F. James, Phys. Chem. Glasses 25 (5) (1984) 134. [20] B.E. Yoldas, Appl. Opt. 23 (20) (1984) 3638. [21] W.H. Lowdermilk, and D. Milam, J. Quant. Electron 17 (9) (1981) 1888. [22] S.P. Mukhejee and J.C. Debsikdar, Am. Ceram. Sot. Bull. 62 (3) (1983) 413 (abstract only). [23] J.C. Debsikdar, Adv. Ceram. Mat. 1 (1) (1986) 93. [24] J.C. Debsikdar, J. Mater. Sci., to be published.
262
of Non-Crystalline
Solids 91 (1987) 262-270 North-Holland, Amsterdam
DEPOSITION OF GRADIENT-INDEX ANTIREFLECTIVE COATING: AN APPROACH BASED ON ULTRASTRUCTURE PROCESSING J.C. DEBSIKDAR Botrelle Received
Columhw 2 October
Diuision,
Columbus.
Ohio
43X/-2693.
USA
1986
A new concept. based on ultrastructure processing. has been developed to deposit graded-index antireflective coatings. According to this concept the substrate is coated, in proper sequence. by a series of solutions (or sois) of identical composition but of differing particle size. The microporous gel coating deposited in this way is characterized by a gradation of porosity due to the deposition of larger particles at the top surface and smaller particles in the substructure. On thermal consolidation, the gel coating forms a single-layer. graded-index coating; thus the chemical leaching/etching steps presently followed for porosity grading could be avoided. In this communication the results of some preliminary experiments which demonstrate the technical feasibility of the concept are reported.
1. Introduction Antireflective coatings on glasses and other materials have the potential for high-energy applications such as laser optics (lenses and windows) as well as for solar-energy applications. The basic objective of depositing an antireflective coating is to eliminate loss of light energy due to back reflection from the surfaces of transmitting optical elements. Since the reflection results from an abrupt change of the refractive index from the transmitting element to the interface, it becomes imperative to modify the index of refraction of the surfaces of the transmitting element(s) to develop the antireflective property. The aim of this surface modification is to develop a graded-index surface so that the reflection losses could be eliminated by optical interferences. The graded-index surface can be produced by coating the transmittive surface with one or more selected transparent materials using both vacuum and non-vacuum techniques. The most commonly used vacuum technique [l-4] has involved deposition of two or more layers of materials, with different refractive index, by physical vapor deposition (PVD). Among the non-vacuum techniques, the processes based on chemical leaching and etching [5-91 of phase-separated alkali-borosilicate/alkali-lime-silica glasses, and those based on the deposition of sol-gel derived porous coating [lo-201, have gained significant interest. The deposition of multi-layer coatings by the PVD method is somewhat complicated; moreover, these coatings have shown low laser-damage resistance 0022-3093/87/$03.50 6 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
J. C. Debsikdar
/ Gradient-index
antireflective
coating
263
[l&21], perhaps due to the presence of absorbing impurities at the interface between the coating and the substrate [21]. Chemical leaching (and etching) processes have been used successfully to produce graded porosity in phaseseparated glasses that results in very low reflectivity over a broad spectral region. The main limitation of these processes is that the glass composition should be such that on heat treatment it induces phase separation on a microscopic scale. Moreover, process variables such as the heat-treatment schedule (i.e., temperature and time), the composition of the leaching solution, and the leaching parameters (i.e., treatment time and temperature) must be very carefully controlled if reproducible results are to be achieved in terms of pore size and distribution across the thickness of the graded pores, because the refractive index gradation is a result of the smooth density transition from air to the bulk glass. The sol-gel process, on the other hand, is based on depositing a porous coating, generally from a partially hydrolyzed metalorganic precursor solution, followed by thermal treatment to consolidate the coating and remove the volatiles. To produce a porosity gradient in the coating, the thermally treated coating requires chemical etching to tailor the pore size grading across the thickness of the otherwise porous coating. However, pore size tailoring by chemical etching often poses practical problems in terms of the reproducibility of the coating microstructures [15,17,18]. The present work provides an alternative sol-gel approach by which coatings with graded porosity can be deposited without involving the chemical etching step. The approach is based on the concept of successively coating a substrate from a series of solutions (or ~01s)of the same chemical composition
Fig. 1. A model
system
for depositing
sol-gel
derived
broad-band
antireflective
coating.
264
J. C. Debsikdar
/ Gradient-index
anrireJlecriue
coating
but with increasing elementary particle (macromolecule) size. On thermal consolidation, the resulting coating is made up of graded particles and hence provides graded porosity across the coating thickness. Consequently, chemical etching used for porosity grading could be avoided. The solution (or sol) preparation parameters (and “aging”, if required) which determine the size of the elementary particles have been reported [22,23]. The present concept is illustrated in fig. 1 using a model coating system where the solutions (or ~01s) A, B, C, and D are compositionally the same, but contain particles of increasing size; the substrate is coated from these solutions (or ~01s) successively in the order of increasing particle size. To test the validity of the concept we have conducted some preliminary experiments on coating deposition using metal-organic-derived silica solutions which had an identical starting composition and preparation history, but were “aged” for different times. The antireflectivity property of these coatings was measured after they were thermally consolidated. In this communication the results of these preliminary experiments are reported.
2. Experimental
procedure
Four (4) batches of polymeric tetramethoxysilane, Si(OCH,), solutions, each containing 5 g equivalent silica, SiOz per 100 ml, were prepared at about 24-h intervals. These polymeric solutions were prepared in plastic beakers by mixing the desired amount of commercially available Si(OCHs), (Tridom/ Fluka Chemical, Inc., New York) in Photrex methanol, CH,OH (J.T. Baker Chemical Company, New Jersey) and then hydrolyzing the solutions each with 2 mol distilled water per mol Si(OCH,), at room temperature. The change of viscosity of a solution of identical composition which could be related to the particle growth in the solution has been reported earlier (ref. [23], fig. 1A). The four solutions (or ~01s) were aged for 17, 18, 19, and 20 days, respectively, before they were used for coating deposition. The approximate viscosity of these solutions is shown in fig. 2 by arrows. The solutions aged for 17, 18, 19, and 20 days are hereafter designated as solutions 1, 2, 3, and 4, respectively. Commercially available 15.24 cm X 15,24 cm (6 in. X 6 in.) Vycor glass plates (Corning 7913) were cut into 7.62 cm X 5.08 cm (3 in. X 2 in.) sizes and used as the substrates. The substrates were cleaned in an ultrasonic bath with “Alconox” solution (supplier: Alconox, Inc., New York), distilled water, and isopropanol (in the same order), dried overnight at 110” C, and cooled in a dessiccator to room temperature. The Vycor substrates were coated using dip coating equipment [17] suitable for controlled linear motion of flat plates at variable angles with respect to the surface of the coating solution. In the present investigation the substrates were withdrawn at 22.5 cm/mm and at an angle of 90” with respect to the solution surface. The time interval between the successive layers of coating was about 5 min to allow drying of the preceding layer at room temperature. The ambient
J. C. Debsikdar
/ Gradienr-index
anrirefleciive
coating
265
100
80
‘” 60 d E ; .Y fj 40 5
20
II
0 10
14
! 18
!
!
22 Time
Fig. 2. Change
of viscosity
!
f
26
i
i
30 (hours
i 34
i
i 38
42
during
aging.
46
!
x 10-l)
of the coating
solution(s)
temperature and relative humidity during coating were 21” C and 59%, respectively. The substrates were coated by a total of four (4) dippings each in three batches as follows: two dippings each from solutions 1 and 2 (hereafter referred to as procedure I); two dippings from solution 1 and one dipping each from solutions 2 and 3 (hereafter referred to as procedure II); and one dipping each from solutions 1, 2, 3, and 4 (hereafter referred to as procedure III). After final dipping, the substrates were dried overnight at 110 o C and then subjected to heat treatment in ambient atmosphere at 600’ C for 2 h. 2. I. Coating characterization The characterization work was limited to examining the particle morphology of solution 4 using a JEOL JEM 100 B (Japan Electra Optic Co.) transmission electron microscope (TEM), examining the coating microstructure on a IS1 Model 100 scanning electron microscope (SEM), and measuring the coating reflectances. For TEM study, solution 4 was diluted with anhydrous ethanol and then sprayed on a carbon-coated grid. For SEM study, a goldcoated specimen was used. The near-normal reflectance of the heat-treated coatings was measured using a tungsten illumination source and a scanning 1.26-m Spex UV-visible, grating spectrometer. An Apple II data acquisition system was used to control scanning of the spectrometer, measure reflectancesignal levels, and subsequently analyze the raw data and plot the results. Thirty data points were measured in each of the two wavelength regions, 300-600 nm and 500-800 nm. A filter for blocking radiation below 450 nm
266
J.C. Debsikdar
/ Gradient-index
antireflective
coating
was inserted in front of the spectrometer during scanning of the upper region to avoid possible second-order interference from radiation in the 300-400 nm regime. Lamp intensity was too weak at wavelengths below 350 nm to give useful results and hence the results were plotted between 350 nm (near UV) and 800 nm (near IR). Since the light source and the spectrometer system efficiency were not uniform with wavelength, a normalization procedure was required to calculate single-surface reflectance from the double-surface reflected intensity data. This normalization was done by measuring the reflectance of the uncoated Vycor substrate. The measured coating reflectances were then ratioed to the substrate reflectance and a constant factor used to convert the result to the effective single-surface percent reflectance.
3. Results and discussion 3.1. Particle morphology of the coating solution The TEM micrograph of the particles of solution 4 is shown in fig. 3. The particles were mostly round in shape, having diameters of the order of 15 to 20 nm which were linked together to form a string-like microstructure. During the TEM observations the material shrank substantially due to the heat generated by the electron beam. The string-like microstructure was formed as a result of gelling. Perhaps, a further dilution of the solution or addition of a few drops of an acid (to modify the particle surface changes) would have revealed the individual particles more clearly.
Fig. 3. TEM
micrograph
of the particles
of solution
4 (1 cm = 0.1 pm).
J.C. Dehsikdar
400
I
/ Gradienr-index
500
I Wavelength
Fig. 4. Percent
reflectance
mrireflecrive
600
coaling
I
700
267
I
800
(nm)
ol heat-treated
procedure
I coating
3.2. Reflectance of heat-treated coatings Figures 4, 5, and 6 show the reflectivity characteristics in the 350-800 nm range of one selected heat-treated sample each produced by procedures I, II and III, respectively. For comparison, the reflectance of the uncoated sample is shown by the dotted line. The coating produced by procedure I (fig. 4) showed antireflectivity over narrow bandwiths, with minima approximately at 360 and 565 nm. In this case, the coating was deposited from solutions 1 and 2 only; the difference in the particle size distribution was perhaps not large enough to produce any signficant porosity gradient across the thickness of the coating. Thus, the conditions for broadband antireflectivity were, evidently, not satisfied. The procedure II sample which was coated from three solutions (1, 2, and 3) had almost zero reflectivity conditions in the 350-410 nm (fig. 5); however, beyond 410 nm the reflectivity gradually increased to some extent up
4a-” ti 5 ; ‘y E n
__________---------------------2-
OWavelength
Fig. 5. Percent
reflectance
(nm)
of heat-treated
procedure
II coating.
J.C. Debsikdar
268
-
I
400
I
Fig. 6. Percent
/ Gradient-index
500
reflectance
I
antireflective
600 Wavelength (nm)
of heat-treated
I
procedure
coating
700
I *do
III coating.
to 570 nm and then decreased marginally. However, in general, the reflectivity characteristics of the procedure II sample indicate that this coating had some degree of porosity gradient in it. The reflectivity characteristic of sample III (fig. 6) was significantly different from that of procedure II samples in that it exhibited bettter antireflectivity in the 500-700 nm range; however, in this case, the antireflectivity was poor in the lower wavelength range compared to the procedure II sample. The procedure II sample had two dippings from solution 1, which is expected to contain the smallest particles among the solutions used in the present study; but the procedure III sample received only one dipping from solution 1. It appears that the difference between the procedure II and III samples in terms of thickness of the layer from solution 1 with the smallest particles caused the difference in antireflectivity in the lower wavelength range. Therefore, it is anticipated that a modified sequence of deposition involving two coats from solution 1 and one coat each from solutions 2, 3, and 4 would be necessary to improve antireflectivity of the procedure III sample at lower wavelength ranges. 3.3. Coating microstructure Figure 7 is a SEM micrograph of the surface morphology of the heat-treated procedure III sample whose reflectance is shown in fig. 6. The microstructure of the coating surface consisted of round particles of the order of 30-60 nm diameter. The pore structure consisted of round as well as channel-like pores of nonuniform sizes; the size range of the round-shaped pores was approximately lo-30 nm. A comparison of the particles sizes of the heat-treated coating (fig. 7) with those of solution 4 which formed this layer of particles (fig. 3) shows that during heat treatment the particle sizes increased almost twofold. Therefore, it is quite evident that significant particle coarsening takes place during thermal consolidation of the gel-derived coating and, consequently, its pore structure.
J. C. Debsikdar
Fig.
7. SEM
micrograph
of heat-treated
/ Gradient-index
procedure
antireflective
III
coating
coating
surface
269
(1 cm = 0.2 pm)
A recent study by Debsikdar [24] has shown that during thermal treatment the net change of pore structure of gels depends upon the initial pore structure, and the rate of pore closing and pore enlargement was found to be closely related to burnout of the residual organics and/or loss of chemisorbed water. The presence of residual organics in the metal-organic-derived gel depends upon the chemistry of the solution (or sol) preparation procedure. It is widely believed that the factors which influence the rate of hydrolysis of an alkoxide (factors such as the molar ratio of water to the alkoxide, the presence of an acid catalyst, and the temperature) also influence the amount of residual organics in the gel structure. Thus, the solution (or sol) preparation chemistry (and “aging” of the solution or sol, if necessary) is important not only for controlling the microstructure (i.e., particle and pore morphologies) of the gel coating but also for controlling the microstructure during thermal consolidation of the gel coating. The results of the present study have demonstrated that it is technically feasible to produce a broad-band antireflective coating when the substrate is coated successively from metal-organic-derived solutions (or ~01s) containing particles of. increasing size ranges, followed by thermal consolidation of the coating. However, further systematic studies on the particle growth phenomena in the coating solutions (or sols), the selection of coating deposition sequence, and optimization of the thermal treatment procedure would be necessary to fully establish the present approach.
270
J.C. Debsikdar
/ Gradient-index
anrirefective
coating
4. Conclusion A sol-gel-derived gradient-index antireflective coating can be produced on a substrate by depositing successive layers of solutions (or ~01s) containing particles of increasing size ranges, followed by an appropriate thermal treatment. This approach has the potential to eliminate the chemical leaching/etching step(s) presently followed for pore size tailoring (to produce the desired index gradient across the coating thickness). The author wishes to acknowledge the laboratory assistance of L. Myers for solution preparation and deposition of coatings. TEM and SEM micrographs were taken by C. Minton and A. Skidmore, respectively. Reflectance measurements were made by D. Grieser of the Optical Science Group, Electronic Systems Department, Battehe Columbus Division. Financial support for this work was provided by the Battelle Columbus Division.
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