Diffusion study of thin film formation by leaching optical glass in an acidic solution

Thin Sohd Fdms, 89 (1982) 277-283

277

ELECTRONICS AND OPTICS

D I F F U S I O N STUDY O F T H I N F I L M F O R M A T I O N BY L E A C H I N G O P T I C A L GLASS IN AN ACIDIC S O L U T I O N * K. H. GUENTHER AND E. HAUSER Balzers A G , FL-9496 Bal-ers ( Ltechtenstem ) R. KRAMER In~tltUt /ur Phystkahsche Chemte der Untversttat Innsbruck, A-6020 lnnsbruck (,4ustrta ) (Received August 24, 1981, accepted September 21, 1981)

When pohshed prisms of Schott BaK4 optical glass were immersed in an acidic cleaning solution (pH, 1.7-1.9) a surface layer was formed on the prisms. Auger depth profiling revealed that the surface layers were completely depleted of the barium content of the bulk glass. The depleted surface layers, which are referred to as leached layers, consisted of only pure SiOz according to Auger depth profiles. Because of the lower refractive index of the S102 layers (nL = 1.45) compared with the index of the bulk glass substrate (ns = 1.57), the reflectance of the prism surfaces was partially reduced by interference. A comparison of measured and calculated spectral reflectances confirmed that the SiO2 layers were homogeneous. The thicknesses of the leached layers were determined from the spectral measurements, and they increased with the square root of the immersion time up to about 1000 s. A significant decrease in the rate of thickness increase was observed for longer immersion durations, indicating dissolution of the glass surface. The dissolution was confirmed by the formation of a step on a prism which was only partially immersed in the solution. Using a diffusion model developed on the basis of the experimental results, the effectwe diffusion coefficient and the dissolution rate of the SiO2 frame were determined.

1. INTRODUCTION Optical glasses are prone to form surface layers when they are in contact with aqueous solutions 1. Usually such surface layers are highly undesirable when they appear as corrosion or staining of the glass surface z, as they can degrade its optical, mechanical and chemical properties considerably. This is particularly true when optical interference coatings are to be applied, as they can enhance the visibdity of even very thin surface layers by severe discoloration or staining 3"4. However, surface layers created by leaching optical glass were also formed purposely many years ago 5-7 to act as antireflection (AR) coatings, and recently some interest arose again in this technique for the production of graded index AR coatings 8 for use in high laser damage threshold applications 9. * Paper presented at the Fifth International Thin Films Congress, Herzha-on-Sea,Israel, September 21-25, 1981 0040-6090/82/0000-0000/$0275

© ElsevierSequoia/Printedm The Netherlands

278

K. H. GUENTHER,E. HAUSER,R. KRAMER

The present study was triggered by a failure in optical thin film production when colour variations of AR coatings were caused by a leached layer on the substrate 3. Of the optical glasses available with various chemical durabihties and staining resistivitles 1° we simply chose Schott BaK4 for a comprehensive study of the leaching mechanism as it was the substrate material employed in this particular case. 2 EXPERIMENTALDETAILS

2 1. General condttmns for the leachmgprocess The two small sides adjacent to the 90 ° edge and the opposite hypotenuse of prisms made of Schott BaK4 optical glass were pohshed; the other two sides perpendicular to the edge were ground. The hypotenuse area was about 30ram x 1 5 m m The leaching liquid was a solution of 2.5% Schallopon 4804 (Aachener Chemische Werke, D-5100 Aachen, F.R.G.) in water, kept at 50+ 1 °C and having an acidity corresponding to a pH of 1.7-1.9. This solution was intended to be used with ultrasomc cleaning procedures; it can be charactertzed as an anionic detergent with an organic acid matrix. The prisms were completely immersed one ptece at a time for vartous immersion times.

2.2. Auger electron spectroscopy depth profihng oj the leached layers Auger electron spectroscopy (AES) was chosen (from various possible methods for the characterization of a layer on a glass substrate 11) to obtam the concentratton profiles within the leached layer and the adjacent bulk glass region. Most surprisingly, the leached layer was found to be free of any alkaline or alkaline earth ions (Fig. 1), consisting of the remaining SIO2 framework only. "Free" in this context of course is any concentration below the detectton limit of the method employed, which In this case may be lower than 1%. The smooth increase m the barium concentration in the AES profile of the layer-bulk interface does not reflect the true sharp step In concentration The broadenmg of the interface profile is due to several amfactual influences of the ion mdllng process employed for the depth profiling analysis, such as collisional mixing, recoil implantation and surface roughemng 11. As the shape of the barium

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[m~n]

Fig 1 AES depth profile of a leached layer (physical thickness d, 156 nm) on Schott BaK4 optical glass

STUDY OF FILM FORMATION BY L E A C H I N G GLASS IN ACID

279

concentration depth profile is similar to the theoretical error function curve for a well-defined interface sharply separating two regions of different contents or concentrations, such a sharp interface can also be assumed in the present case.

2 3. Thickness determination of the leachedlavers The refractive index ns of the leached layers can be assumed to be about 1.45 as there is mainly S1Oz present. The influence of the remaining constituents can be neglected in a first approximation as their concentration is certainly below 1%. Hence, as the refractive index ns of the bulk glass is about 1.57, the reflectance of the surface wdl be reduced because of interference effects. By measuring the spectral reflectance of such a glass surface with a leached surface layer by a method described earlier 3 we were able to calculate both the optical thickness and the physical thickness of the layer. Figure 2 compares for four different immersion durations the measured reflectance curve and the fitted curve obtained w~th the indicated optical thickness 4nd. The good agreement confirms the assumption of a homogeneous refractive index for the leached layers. In Fig. 3 the physical thickness d of the leached layer is plotted against the square root of the immersion time t. To examine the mechamsm t~[%] 6'

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280

K H. GUENTHER, E HAUSER, R. KRAMER

of the glass leaching process in more detail, BaC12 was added to the acidic solution and the resulting leached layer thickness for constant immersion time and varying amount of BaC1 z added was again determined by employing the aforementioned spectroreflectance method.

2.4. Dtssolution oj the SzO2framework To confirm the supposed dissolution of the surface of the SiOz framework a prism was immersed in the solution to only about one-half its hypotenuse area. A step was formed on the surface at the solution-air interface. However, probably because of surface tension effects this step was found not to be sharp enough for its height to be measured with sufficient accuracy by means of a stylus-type profilometer. The dissolution could also be estabhshed qualitatively by determining the weight loss of prisms for various (increasing) immersion intervals. 3

RESULTS AND DISCUSSION

The formation of a surface layer on glass m an aqueous or acidic solution has been investigated previously by several workers L2-16. The mechanism found is a diffusion-controlled attack of the glass surfaces by aqueous solutions, which is mainly determined by the lnterdiffusion of hydrogen and alkaline ions 12-14. In every case a square root dependence of the layer thickness on time was observed, which changed into a linear dependence for longer interaction times 15,16. The influence of surface potential and of an associated space charge layer in the surface region of the glass has also been considered, assuming the involvement of field-enhanced diffusion 17. Comprehensive treatises on diffusion processes in glass surfaces have been given by Engelke and Schaeffer 18 and by Doremus ~9. Whereas the references cited nearly exclusively consider the diffusion of alkaline ions, we are here concerned mainly with the diffusion of barium ions. Although BaK4 glass also contains some K 2 0 and Z n O (together about 15 wt.)o), the determining mechanism for the formation of the leached layer is certainly barium diffusion, as the BaO content is as high as about 30 wt.%. The sharp concentration step for Ba 2 ÷ at the inner interface of the leached layer indicates that a distinct phase transition exists between the bulk glass and the S102 framework of the layer from which the dissolution of Ba 2 + occurs. At the very beginning of the leaching process the solution of Ba 2 ÷ in the aqueous medium may be rate limiting, but after this initial period the diffusion of Ba 2 ÷ across the growing leached layer will become the rate-determining step. The Ba 2+ equilibrium concentration C s is then established at the inner interface whereas the Ba 2 ÷ concentration Is held at zero at the outer interface because of the large volume of the leaching solution. The electrical balance is assumed to be brought about by H + diffusion into the leached layer which is much faster than the barium transport because of the high acidity of the solution and the high diffusivity of H +. The rate of increase in thickness of the leached layer with leaching time t as a result of the diffusion mechanism can be written as

dd

DPCs/C b -

dt

D e tf -

d

(1)

d

S T U D Y O F F I L M F O R M A T I O N BY L E A C H I N G G L A S S IN A C I D

281

where D is the diffusion coefficient of Ba 2 + in the aqueous solution, P is the pore factor of the leached layer and Cb is the bulk concentration of Ba 2÷ (6 x 1 0 - 3 mol c m - 3). Equation (1) has the simple solution (2)

d = ( 2 D e f f t ) U~

Indeed the plot ofd versus t 1/2 in Fig. 3 shows a linear part after the very initial period when the dissolution of Ba 2 + is rate limiting. At long leaching times the thickness increase becomes slower and finally a stationary thickness of the leached layer is reached, This indicates that dissolution of the SIO 2 framework by the cleanmg solution diminishes the thickness of the leached layer simultaneously with the diffusion of Ba 2÷ out of the bulk glass. This leads to the extended differential equation dd dt

Deff d

k

(3)

where k is the dissolution rate of the leached layer. The integration ofeqn. (3) yields

d Oeff I n ( 1 - - k d l t -- t o =

k

k2

\

(4)

DeffJ

The time delay t o at the beginning of the leaching process due to the then ratelimiting solution of Ba 2 ÷ is about 20 s. The final thickness dr obtained at very long leaching times (when d d / d t becomes zero) was calculated from eqn. (3) to be Deff/k. By fitting a curve as g~ven by eqn. (4) to the data given in Fig. 3, the parameters of the leaching process were determined to be D e ft = 2 6 x 10 - 1 . cmZs -1 and k = 1 . 5 x l 0 - S c m s -1. To establish the real solubility equilibrium at the inner interface, additional leaching experiments were conducted with mcreasing amounts of BaC12 dissolved m the leaching solution. The Ba 2 ÷ concentration Ca built up at the outer interface should lower the concentration gradient and inhibit any diffusion when Ca is equal to the solubility Cs of the barium ions Neglecting the dissolution of the glass (which is only significant at long leaching times) the corresponding &ffusion equation is easdy derived from eqn. (1) m the differential form dd

DP(C~ -- C~) -

dt

C~ - C~ ~c

-

Cbd

-

(5)

d

and in the integrated form d 2 oc ( C s - -

Ca)t

(6)

For constant leaching time t the plot of the square of d versus the added Ba 2 + concentration Ca (Fig. 4) shows the linear relation demanded by the theory. Furthermore the solubility of Ba 2 + at the tuner interface is obtained by extrapolating the straight line to d 2 = 0 , yielding Cs = 1.05×10 _3 m o l c m -s. With an estimated volume fraction of 22~o for the pores in the leached layer that are capable of containing the saturated solution we obtain a m a x i m u m Ba 2 + concentration of 0.23 x 10 - s m o l c m -3 within the leached layer. This means an e s t i m a t e d mass fraction of about 1~o related to the bulk glass and is obviously below the detection limit of our AES equipment.

282

K H G U E N T H E R , E H A U S E R , R. K R A M E R

The m e c h a m s m of the leaching process stud~ed m this work can be summarized as follows. During the very first moments of the leaching process the dissolution of barium ions out of the bulk glass determines the rate of film formation. Then the transport of the &ssolved barium ions through the remaining S102 framework becomes dominant. For even longer leaching hines the &ssolution of this $102 framework competes with the movement of the layer-bulk interface caused by the continuing dissolution of barium ions out of the bulk. This c o m p e t m o n finally reaches an eqmhbrium condition where a leached layer of constant thickness moves at constant speed towards the inside of the bulk glass and the dimensions of the optical part of the glass are reduced (F~g. 5). t=Os

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4

CONCLUSION

Although we restricted our investigations to a parhcular well-defined case, we can conclude that the proposed leaching mechanism applies to other optical glasses as well Two aspects should be noted. (1) Under certain conditions, i.e. with highly acidic orgamc cleaning soluhons such as Schallopon, even the rather heavy barium ions diffuse nearly as fast as alkaline or hydrogen ~ons m water or weak aqueous solunons. (2) It ~s not possible to create a leached layer of any thickness. The ultimate thickness depends on the effective diffusion coefficient of the ions and the dissolution rate of the framework. ACKNOWLEDGMENTS

We should like to thank K. Rolder of D Swarovskx & C o , Wattens, Austria, for supplying us with a considerable number of prisms which enabled us to perform the mvestlganons without any restrictions. The help of G. Hobl with the AES depth

STUDY OF FILM FORMATION BY LEACHING GLASS IN ACID

283

profiling and of S. Voser with the numerous leaching runs and subsequent thickness measurements is gratefully acknowledged. The support of Dr. E. Ritter, Thin Film Division, Balzers AG., in thts work as well as the permission of the management of the company to publish this paper ~s highly appreciated. REFERENCES 1

2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19

L K K K H F F M W G

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