- Email: [email protected]
Sodium redistribution between oxide phases
]OURNAI~ OF
ELSEVIER
Journal of Non-Crystalline Solids 197 (1996) 154-163
Sodium redistribution between oxide phases R.J. Araujo *, F.P. Fehlner Coming Incorporated, Sullivan Park Fr-3-1, Corning, NY 14831, USA
Received 22 June 1995;revised 7 November 1995
Abstract Alkali transport between a glass substrate and an oxide over layer occurs by a mechanism of ion exchange between the alkali and protons. Thermodynamics requires that the difference in chemical potential of the alkali and the proton be the same in both phases at equilibrium. The influence of chemical composition of the glass on the chemical potential of alkali ions is discussed. Good agreement between predictions of the direction of alkali transport and observed transport is obtained.
1. Introduction The use of glass substrates for the fabrication of thin film transistors (TFTs) is an important factor which has enabled active matrix liquid crystal displays (AMLCDs) to achieve widespread use in flat display technology. However, most glass compositions contain sodium, if only as impurities at the 1000 parts per million by weight (ppm) level. Since sodium is known to lead to instabilities in TFTs, it is important to prevent sodium from migrating from the glass to the TFT during processing. The potential for migration of the sodium is greatest during the fabrication of the TFT because the system is maintained for a significant time at temperatures as high as 600°C. Introduction of a nominally alkali-free oxide barrier layer such as silica, alumina, or tantala be-
* Corresponding author. Tel.: + 1-607 974 9000; fax: + 1-607 974 3675; e-mail: [email protected]. 1 Presented at the 8th International Conference on the Physics of Non-Crystalline Solids, Turku, Finland, 28 June- 1 July 1995.
tween the TFT and the glass is an obvious means of doing so. Unfortunately, the efficacy of a barrier layer film is limited by two factors. First, the films, themselves, may contain sodium impurities. It is not uncommon for silica films to contain sodium impurities at concentrations between 300 ppm and 1000 ppm. The second factor is illustrated by a phenomenon observed when a soda-lime glass is coated with a silica film and the system is heated. The concentration of the soda in the silica film is observed to increase significantly. Both the soda initially in the film and the additional soda introduced to the film by the glass can contribute to the contamination of the TFT. A family of glasses to be described below strongly extracts sodium ions from a silica layer when the system is heated. Thus, the concentration of sodium in the barrier layer which is available for contaminating the TFT is reduced. Moreover, transport of sodium from the silica film to the glass is observed even if the glass initially contains a concentration of sodium ions considerably in excess of that initially in the silica. The behavior of this family of glasses will
0022-3093/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-3093(95)00632-X
155
R.J. Araujo, F.P. Fehlner/Journal of Non-Crystalline Solids 197 (1996) 154-163
be discussed in terms of the thermodynamics of open systems and in terms of the structure of glasses in this family.
2. Alkali transport Because electrical neutrality must be maintained in both phases, transport of sodium ions between the glass substrate and the silica film requires special conditions. Either a negative charge must move in the same direction as the sodium ion or another positive ion must move in the opposite direction. The authors believe that the latter process is the important one in this study. Both glasses and silica films made under a variety of conditions contain an appreciable amount of water in the form of hydroxyl units, each bonded to a silicon atom. Some of the hydroxyl units are ionized, thereby producing protons. Transport of sodium ions between phases is accompanied by counter-current transport of protons, so that electrical neutrality is maintained in both phases. A flux of particles can be observed only in the presence of a gradient in the chemical potential of those particles. If the energy of a particle does not vary spatially, the gradient in the chemical potential is parallel to the gradient in the concentration of the particles and the flux is in the direction opposite to the concentration gradient. This is not necessarily true if the energy of a particle depends on its position in space. In such a case, it is sometimes possible for particles to diffuse uphill against the concentration gradient. 2 In either case, fluxes vanish when the spatial distribution of all the particles produces thermodynamic equilibrium in the system. When a glass substrate coated with a silica barrier layer is heated, the sodium and proton fluxes observed are those which tend to produce the equilibrium distribution of both species. In a multiphase multicomponent system in which the transport of components between phases is not constrained, the chemical potential of each component must have the same value in every phase at equilibrium. Appendix A indicates how the character of the equilibrium is
2 Diffusion against a concentration gradient is illustrated by the process of phase separation in a multicomponent system.
changed by the requirement of electrical neutrality in a system which contains two mobile ions having the same valence. In such a system, equilibrium requires only that the difference in the chemical potential of the two ions be the same in the two phases. This requirement and the initial concentrations of the components in the two phases determine their equilibrium concentrations. In the present case, the equality ( ]£H -- /£Na)glass = ( /tLH -- /'LNa)film
(1)
must hold at equilibrium. This requirement will be used in conjunction with a simple model. It will be assumed that the free energy of the system is changed during the ion exchange only by the ideal entropy of mixing of the two ions and by the differences in the bonding energies of each of the two ions in the two phases. For reasons to be described below, it is assumed that the bonding energy of the proton is the same in the two phases, whereas that of the alkali ion is not the same in the glass as it is in the film. With these assumptions, the expressions for the chemical potentials may then be derived, and the equilibrium composition of the glass relative to that of the film can be determined. Let • represent the amount by which the bonding energy of the sodium ion in the glass exceeds that in the film. The calculation for a silica film on glass is summarized in the following equations, wherein F represents the Helmholtz free energy: Ffilm = ( i d e a l entropy)film,
(2)
Fgla,s = - [Na + ] × • + (ideal entropy)glass,
(3)
[Na + ]
[Na + ]
•
The last equation suggests several ways in which the concentration of sodium ions in the film can be minimized. Because the sum ([Na+ ] + [H+ ]filrn) is limited by its initial value, the concentration of sodium at equilibrium can be decreased by decreasing the alkali concentration, but also by depletion of the hydrogen species from the barrier layer. The work of Mizuhashi et al. [1] confirms this expectation. They found that exposure of the samples to dry nitrogen or vacuum during the heat treatment process
156
R.J, Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
diminished and even prevented the movement of sodium. Alternatively, one can reduce the sodium in the film at equilibrium by increasing the density of protons initially in the glass. The efficacy of this approach has not yet been demonstrated. A third approach is the main subject of this report. The concentration of sodium ions in the silica film can be limited to a small fraction of the sum of the initial concentrations of protons and sodium ions if • is very large. In principle, • can be derived from a knowledge of the heats of reaction of water and of the alkali oxide of interest with the oxide comprising the film and with the glass substrate. Knowledge of the heat of reaction of a specified glass with either an alkali oxide or with water is often not known. Therefore, it is necessary to provide some means of estimating the dependence of •, or at least the algebraic sign of •, on the glass composition. The kind of bonding characteristic of some simple glasses is understood well enough so that the sign of • can be determined with moderate confidence. The following section summarizes the principles involved.
3. Structure of borosilicate and aluminosilieate glasses Silica glass is composed of silicon atoms, each of which is bonded to four oxygen atoms by bonds which exhibit significant covalent character. The spatial arrangement of the four oxygen atoms around the silicon to which they are bonded exhibits tetrahedral symmetry. Each of the oxygen atoms is bonded to two silicon atoms by bonds having significant covalent character. Any sample is, in effect, a single gigantic three-dimensional polymeric molecule. When an alkali oxide is added to silica and the system melted to form a glass, some of the oxygen atoms are covalently bonded to only a single silicon atom. Such an oxygen atom exhibits a negative charge. It must, therefore, be compensated by a vicinal positively charged alkali ion. Because such an oxygen atom disrupts the three-dimensional polymeric structure, it is called a non-bridging oxygen atom. If water reacts with silica, the situation is similar to the above case in that some oxygen atoms are not bonded to two silicon atoms. In this case,
however, a proton is bonded to the non-bridging oxygen. This bond has considerable covalent character. 3 In alkali-aluminosilicate glasses, non-bridging oxygen atoms bonded to silicon atoms do not form unless the number of alkali atoms exceeds the number of aluminum atoms. Instead, an aluminum atom forms covalent bonds to four oxygen atoms, 4 and a negative charge is associated with that whole group of atoms. The negative charge must be compensated by a vicinal alkali ion. The term 'tetrahedral aluminum' refers to those aluminum atoms so bonded because all four neighboring oxygen atoms are equivalent and they exhibit tetrahedral symmetry. The complete absence of non-bridging oxygen atoms in such a glass is a clear demonstration of the fact that the formation of tetrahedral aluminum is energetically favored over non-bridging oxygen atoms bonded to silicon. If the alkali exceeds the alumina, then for each excess alkali atom, a non-bridging oxygen bonded to a single silicon atom is formed. It is generally believed that, if water is added to an aluminosilicate glass, hydroxyls are bonded only to silicon atoms. Non-bridging oxygen atoms bonded to aluminum have not been observed and are, therefore, presumed to be less stable than those bonded to silicon. Like silica glasses, boric oxide glasses form gigantic three-dimensional polymeric molecules. Unlike silicon, each boron atom is bonded to only three oxygen atoms whose positions manifest trigonal symmetry. For our purposes, the most important respect in which boric oxide and silica glasses differ is the manner in which b o r o n - o x y g e n bonding changes when an alkali oxide is added. When the ratio of added alkaline oxide to boric oxide is lower
3 In the remainder of this discussion, bonds having significant covalent character will be referred to simply as covalent bonds. 4 The four oxygen atoms to which the aluminum bonds are each bonded to a silicon atom. The four oxygen atoms covalently bonded to one silicon and to one aluminum atom are also called bridging oxygen atoms because they maintain the polymeric structure of the glass. Because alumina added to an alkali silicate tends to restore the polymeric nature of the glass instead of further disrupting it by forming non-bridging oxygen atoms, it is often referred to as a network former even though it does not, by itself, form a glass.
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
than some critical value, 5 non-bridging oxygen atoms are not formed. Instead, some of the boron atoms become bonded to four oxygen atoms in a manner completely analogous to tetrahedral aluminum. If alkali is added beyond the critical ratio, however, the excess gives rise to non-bridging oxygen atoms bonded to boron. In borosilicate glasses, the ratio of silica to boric oxide exerts an influence on the amount of tetrahedral boron and the number of non-bridging oxygen atoms bonded to boron. For example, whereas the critical ratio in a binary alkali borate is 0.43, it is approximately 0.6 when the ratio of silicon atoms to boron atoms is 3.0 [2]. It should be emphasized that, unless the number of alkali atoms exceeds the number of boron atoms, the addition of alkali influences the bonding of only boron atoms, and no non-bridging oxygen atoms are bonded to silicon atoms. Thus, it is clear that non-bridging oxygen atoms bonded to silicon atoms and compensated by vicinal alkali ions are less stable than any of the other structures discussed thus far. Protons are bonded into the structure of borate or silicate glasses primarily as hydroxyls. They cannot compensate the negative charge associated with the tetrahedral alumina or a tetrahedral boron. 6 Therefore, if a silica film which contains low concentrations of protons and alkali ions is placed in contact with an aluminoborosilicate glass which also contains protons, protons will tend to accumulate in the silica film and alkali ions will tend to accumulate in the glass. This is because the energy of a proton is essentially the same in either phase, whereas the energy of an alkali ion is lower in the glass if it
5 In the low temperature limit, introductionof alkali to a boric oxide glass causes the exclusive formation of tetrahedral boron atoms if the ratio of alkali to boron is below the critical value of 0.43. At sufficientlyhigh temperature, some non-bridgingoxygen atoms bonded to boron atoms are formed even when the ratio is below this critical value. 6 Because the four oxygen atoms comprising the tetrahedral aluminumor tetrahedral boron structuralunit each already participate in two covalent bonds, no one of them possesses sufficient electron density to participate in another covalent bond with a proton. The coulombicenergy between a monatomicpositive ion and the somewhat delocalized negative charge of a tetrahedral aluminum unit is much weaker than the energy of the protonoxygen bond in a hydroxyl group.
157
causes the formation of a tetrahedral aluminum or a tetrahedral boron than it is in the silica film, where it can do nothing but give rise to a non-bridging oxygen. Equilibrium is achieved when the decrease in energy achieved by enriching the glass with alkali is balanced by the decrease in entropy caused by the segregation. High field strength ions other than boron and aluminum can give rise to a site of low alkali energy by forming covalent bonds to what would otherwise be non-bridging oxygen atoms. Niobium and tantalum are examples. Magnesium, beryllium, zinc, and to some extent, calcium exhibit analogous behavior. The bonding can be understood in the following 7 way. Imagine that the introduction of an oxygen ion associated with the addition of a magnesium oxide molecule causes the formation of two nonbridging oxygen atoms. The divalent magnesium ion has such a high field strength that it forms partially covalent bonds, not only to the two non-bridging oxygen atoms created by the introduction of the magnesium oxide, but also to another non-bridging oxygen atom created by the introduction of sodium oxide. The formation of covalent bonds to the magnesium, in effect, converts the three non-bridging oxygen atoms to bridging oxygens. The negative charges on the three converted oxygen atoms are compensated by the two positive charges on the magnesium and the positive charge on the sodium. 8
7 This description is based on an unpublishedinterpretationof viscosity data on alkali silicatescontainingcalcium,magnesiumor zinc by R.J. Araujo and G.B. Hares, Coming Incorporated. It applies only in the low temperatureregime. To the extent that the introduction of magnesium oxide decreases the total number of non-bridgingoxygen atoms, magnesium, like aluminum,is said to be a network former. The strength of the magnesium-oxygen bond probably is not sufficient to produce the short bond lengths which would stabilize trigonal coordination of the magnesiumby oxygen atoms. Therefore, in addition to the three oxygen atoms having the strongest bonds, oxygen atoms bridgingtwo silicon atoms probablyare includedin the magnesium'soxygen coordinationshell. It is sometimes speculated that one of the bridgingoxygen atoms is almost as close as the three most stronglybonded oxygen atoms and that the magnesium environmenthas the symmetryof a distorted tetrahedron. As a result of this speculation, the phrase 'tetrahedral atom' is sometimes used loosely to indicatethis type of bondingregardless of the exact symmetryof the oxygencoordinationshell aroundthe small high field strength ions.
158
R.J. Araujo, F.P. Fehlner / Journal of Non-Crystalline Solids 197 (1996) 154-163
The alkali ion at such a site cannot be replaced by a proton because the electron density in each of the three most strongly bonded oxygen atoms has already been so strongly attracted by the magnesium ion that hydroxyl bonds are not stable, In contrast to magnesium, the large alkaline earths such as barium and strontium apparently do give rise to non-bridging oxygen atoms to which a proton can bond. 9 In the present work, it is tentatively assumed that barium and strontium produce two non-bridging oxygens per ion and that magnesium removes one. The behavior of calcium is intermediate. It is, therefore, assumed, in keeping with the ideas of Stevels [3], that an equilibrium exists between those calcium ions that act like the heavier alkaline earths and those that act like the very small ones. It is arbitrarily assumed, with no supporting evidence, that the fraction of calcium ions which each produce two nonbridging oxygens and the fraction of calcium ions that each remove one non-bridging oxygen atom are both equal to one half. The use of the assumption has not been shown to lead to any results that are inconsistent with experiment.
4. Direction of sodium transport Consider a glass that contains a level of alumina, boric oxide, and oxides of other high field strength ions, such as tantalum or magnesium, that exceeds the number of non-bridging oxygen atoms that are produced by the totality of low field strength ions, such as the alkalies and the large alkaline earths. Considerations discussed in the previous section indicate that energetics must favor the replacement of protons in the glass by additional alkali ions when the glass is placed in contact with a silica film containing alkali impurities. The value of E in Eq. (4) varies monotonically and asymptotically with the magnitude of that excess. See Appendix B. Thus, it should be possible to predict with a high degree of reliability the direction of sodium transport that occurs when a silica film is placed in contact with a
9 This statement is based on an unpublished interpretation of the influence of barium on the precipitation of silver halide in photochromic glasses by R.J. Araujo, Coming Incorporated.
Table 1 Methods of film preparation C - APCVD S - RF magnetron sputtered from oxide S' - Reactively RF magnetron sputtered from metal P - PECVD E - Electron beam evaporated
glass of known composition. It is convenient to define a symbol to represent the aforementioned excess: P ' = [M +] + 2 × ([Sr +2] + [Ba +2] + ½[Ca+2]) - [Al+3],
P = P ' - 0 . 6 1 8 + 3 1 - ( [ Z n + 2 1 + [Be +21
+[Mg +2 ] + ½[Ca+2]), where all concentrations are expressed in cation percent. The reason for defining P' separately will be discussed in Section 6.2. The concentration of boron is multiplied by 0.6 because that is the value of the critical ratio of alkali to boron, above which additional alkali produces non-bridging oxygen atoms for glasses in which the silica exceeds the boron by a factor of three or more. For glasses richer in boron, a somewhat smaller value probably should be used. See the previous section. The reason for taking half the concentration of calcium was also discussed in the previous section. When P is negative, energetics favors a larger ratio of alkali to protons in the glass than in the film. For the levels of alkali impurity and protons commonly observed in silica films, a negative value of P should indicate stronger transport of sodium from the film into the glass than would be expected from the concentration gradient alone. The degree to which sodium is extracted from the film by the glass increases asymptotically with the magnitude of the negative value.
5. Experimental Thin films of silica were deposited by atmospheric pressure chemical vapor deposition (APCVD), plasma enhanced chemical vapor deposition (PECVD), or by reactive sputtering on carefully
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
159
Table 2 Glass composition in weight percent Oxide
SiO2 AI203 B203 BaO S~ CaO MgO ZnO K20 Na20 Li20 As203
Sample ID 0215
7740
7059
1724
AX
BDC
BDD
BDE
1735
72.1 1.8 0.0 0.0 0.0 7.3 3.8 0.0 0.2 14.0 0.0 0.0
0.8 2.3 12.5 0.0 0.0 0.0 0.0 0.0 0.0 4.0 0.0 0.0
49.5 10.5 14.6 25.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.5
57.2 16.5 4.9 8.0 0.0 7.8 5.6 0.0 0.0 0.1 0.0 0.0
72.1 4.6 9.8 3.4 0.0 0.5 0.0 2.7 0.6 5.8 0.0 0.0
56.3 2.8 30,7 0,0 0.0 0.0 0.0 0.0 0.0 10.2 0.0 0.0
70.3 2.6 17.7 0.0 0.0 0.0 0.0 0.0 0.0 9.5 0.0 0.0
62.0 10.5 17.9 0.0 0.0 0.0 0.0 0.0 0.0 9.6 0.0 0.0
56.9 14.5 4,7 12,4 0.2 11,1 0.0 0.0 0.0 0.1 0.0 0.8
BDD
BDE
Table 3 Glass composition in cation percent Oxide
Sample ID 0215
SiO2 A1203 B203 Na20 K20 MgO CaO BaO ZnO P' P
7740
7059
1724
1735
AX
BDC
6Z6 1.8 0.0 23.6 0.2 4.9 6.8 0.0 0.0
71.6 2.4 19.1 6.9 0.0 0.0 0.0 0.0 0.0
50.8 12.7 25.9 0.2 0.0 0.0 0.0 10.1 0.0
54.5 18.5 8.0 0.2 0.0 7.9 7.9 3.0 0.0
57.1 17.1 8.1 0.2 0.0 0.0 11.9 4.8 0.0
65.2 4.9 15.3 10.2 0.7 0.0 0.5 1.2 1.8
42.5 2.5 40.0 14.9 0.0 0.0 0.0 0.0 0.0
57.5 2.5 25.0 15.1 0.0 0.0 0.0 0.0 0.0
50.0 10.0 24.9 15.0 0.0 0.0 0.0 0.0 0.0
+25.4 + 17.1
+4.5 -7.0
+7.6 -7.9
-4.3 -21.1
+5.1 -5.8
+7.5 -3.8
+ 12.5 - 11.5
+ 12.5 -2.5
+5.0 - 10.0
Table 4 Results for silica films Substrate glass code
Barrier film/(nm)
Control ppm Na20
Time/Temp. (h)/(°C)
ppm Na20
- A[Na20] (ppm)
Ratio HT/ control
1735 1735 7740 0215 AX 7740 BDC BDD BDE 7059 7059 7059
S / ~ 200 C/100 St/100 S/100 S/100 S/100 C/100 C/100 C/100 C/80 S'/100 C/100
569 527 1670 2130 365 321 13 100 43 400 32 600 2570 380 453
25/650 25/650 25/650 25/350 25/350 25/350 25/500 25/500 25/500 25/650 25/650 1000/650
207 121 2770 7380 395 653 8760 27 000 8560 1300 130 153
362 406 - 1100 - 5250 - 30 - 332 4340 16 400 24 000 1270 250 300
0.36 0.23 1.66 3.46 1.08 2.03 0.67 0.62 0.26 0.51 0.34 0.34
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
160 Table 5 Results for alumina films Substrate glass code
Barrier film/(nm)
Control ppm N a 2 0
Time/Temp. (h)/(°C)
ppm Na20
- A[Na20] (ppm)
Ratio H T / control
7059 7059 1735 1724
S/100 S/140 S/100 S/IO0
9.4 10.1 7.7 15.3
25/650 25/650 25/650 25/650
17.9 18.3 12.3 8.9
- 8.5 - 8.2 - 4.6 6.4
1.90 1.81 1.60 0.58
cleaned glass substrates varying moderately widely in composition. Symbols representing the various techniques for depositing the films are defined in Table 1. The compositions of several of the glasses are listed in Tables 2 and 3. Most of the depositions were performed in a class 1000 cleanroom. 10 The samples were heat treated in a 2.5" diameter silica tube mounted in a Varian Marshall tube furnace. The heat treatment conditions employed in specific experiments are indicated in Tables 4 and 5. During the heat treatment, samples were stacked so that films faced other films of the same composition in order to minimize sodium loss or gain from the furnace atmosphere. The relative humidity of the air was maintained between 98% and 100% at room temperature as measured by an Omega RH-200 meter. The profile of the sodium concentration, as well as that of the silicon concentration in the film and in the glass, was determined before and after heat treatment by SIMS measurements. Signals were converted to sodium concentrations by use of known standards. The accuracy of the measurements is within twenty percent of the recorded values. The value of the sodium concentration listed in Tables 2 and 3 is obtained from the ratio of sodium to silicon averaged over the thickness of the film. Similar experiments were performed on samples made by depositing alumina films on various glass substrates. CVD and sputtering techniques were both employed to deposit alumina films. Interpretation of
l0 Some samples were coated at the Watkins-Johnson Corporation of Scotts Valley, CA, USA. In these cases, deposition was not performed in a clean room.
the significance of the results is discussed in Section 6.2
6. Results
6.1. Silica films The amount of sodium measured in the silica films before the samples were heat treated varied considerably, depending on the deposition technique and varied with the specific apparatus even for the same technique. The sodium concentration changed as a result of the heat treatment in every sample. In some cases, the concentration of sodium in the film decreased, and in other cases it increased. The second to the last column in each of Tables 4 and 5 exhibits the decrease in sodium concentration; i.e., a negative value indicates an increase. In general, the results bear out the predictions based on the value of P. In the case of glass 0215, for which P is positive, a large increase in sodium concentration is observed. In all of the glasses for which P was negative, except AX and 7740, a decrease in sodium is observed in the silica film. In the case of glass AX, the value of P is only slightly negative, and the amount of sodium transport is very slight. The behavior of this glass is not considered to be inconsistent with the model. Only the system of silica on glass 7740 seems inconsistent with the predictions of this model. Sodium is found to move from the glass into the silica film despite the negative value of P. However, it is known that glass 7740 can phase separate to form a silica-rich phase and a sodium borate phase
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
[4]. It is not known if the sodium borate further separates into two phases, but it is not forbidden to do so by thermodynamics [5]. If it does, then one of the phases would surely be characterized by a ratio of alkali to boron associated with non-bridging oxygens. The alkali from this phase may be responsible for the observed alkali enrichment of the silica film. One of the samples, not shown in the table, in which silica was deposited on glass 7059 by reactive sputtering, also exhibited a significant increase in the concentration of sodium in the film. The reason for this behavior is not understood. The behavior of thirteen of the samples in which silica was deposited on glass 7059 was in agreement with the model.
6.2. Alumina films Alumina films were placed on various glasses to compare their behavior to that of the silica films. In some cases, transport of alkali from the glass to the film was observed in spite of the fact that P was negative. This result suggests that alkalis are more stable relative to protons in alumina films than in silica films. Such a conclusion is consistent with estimated heats of reaction. ~1 However, this fact does not necessarily mean that the alkali is more stable in the alumina film than it is in any possible glass. The fact, that in aluminoborosilicate glasses only the alkali in excess of the alumina alters the bonding of boron, suggests that the formation of tetrahedral aluminum decreases the energy of an alkali more strongly than the formation of the other structural units, referred to loosely as tetrahedral atoms. This suggests that E might possibly be positive for systems comprising an alumina film and a substrate glass containing alumina in amounts in excess of the alkalies and the large alkaline earths. Accordingly,
H The reaction of sodium oxide with silica to form sodium metasilicate is more exothermic by 37 kcal/mol of X 2 0 than the reaction of water with silica. The reaction of sodium oxide with alumina is more exothermic by 55 kcal/mol of X 2 0 than the reaction of water with alumina. Thus, if a film of alumina which contains sodium and proton impurities is placed in contact with a silicate containing hydroxyls and sodium, protons would tend to aggregate in the silicate and alkali ions in the alumina.
161
the function P' was defined in such a way as to include only alumina as a stabilizing tetrahedral atom. A negative value of P' is obtained only for glass 1724. It is only for that glass that transport from the alumina film to the glass is observed. This correlation corroborates the hypothesis that, while alkali is more stable in alumina films than it is at some of the tetrahedral atom sites in silicates, it is not as stable as alkali at the site of a tetrahedral aluminum atom in silicates. It is expected, therefore, that the algebraic sign of P' is a good predictor for systems comprising glass in contact with alumina films, just as the sign of P is a good predictor for systems comprising glass in contact with silica films. Although experiments with other oxide films, such as tantalum oxide, have not been performed, the direction of transport of alkali between such films and the substrate glass is expected to depend on the composition of the glass in a similar manner. Unless the heats of reaction of water and of alkaline oxides with tantalum oxide indicate that, for a tantala film on glass, ~ is less positive than that in the alumina system, a negative value of P' should indicate that alkali will flow from the tantala film into the glass.
7. Conclusions Extraction of alkali from a substrate glass is believed to involve an alkali-proton ion exchange. It is postulated that protons are not very stable in a glass unless they are covalently bonded to an oxygen in the form of a hydroxyl unit. Such a unit is not formed unless the proton is introduced to glass which contains non-bridging oxygen atoms. Consequently, alkali-proton exchange is not an effective mechanism for alkali extraction in a glass which is free of non-bridging oxygen atoms. Simple formulas indicate the density of non-bridging oxygen as a function of glass composition. The same formulas enable one to predict semi-quantitatively the alkali diffusion when a barrier layer is put in contact with a substrate glass. The predictions of the formulas are in good agreement with experimental results when a substrate glass is put into contact with either silica or alumina barrier layer films.
R.J. Araujo, F.P. Fehlner / Journal of Non-Crystalline Solids 197 (1996) 154-163
162
Acknowledgements
constant, it follows that ~N1 = - ~ N 2 and Eq. (A.4) can be written as
The authors are grateful to Paul Sachenik and K.R. Salisbury for assistance in sample preparation and to N.J. Binkowski for SIMS measurements. The authors are also grateful to Bruce Mayer of Watkins-Johnson Corporation for sample preparation.
~F - (/.£1 -/-.t2)~N1 __ 0.
(A.5)
Appendix A. Potential function for constrained open system
Therefore, F - (/.t I -/~2)N1 is the potential function appropriate to this system. If the volume of the glass is very large compared to the volume of the silica film, the glass may be considered a chemical potential reservoir for protons and sodium ions. At equilibrium the difference in the chemical potentials of protons and sodium ions in the film must be the same as the difference in the glass.
The mathematical statement of the combined first and second laws is given by
Appendix B. Asymptotic dependence of • on P
~E <_ T ~ S - P d V +
~., Izi~N i.
(A.1)
i
The above equation assumes that all work is PV work. The appropriate mathematical statement is taken to be the inequality for irreversible processes and taken to be the equality for reversible processes. From the definition of the Helmholtz free energy, F, Eq. (A.1) can be written as
gF < - S g T -
P g V + ~_, p~igNi.
(g.2)
i
When no PV work is performed in an isothermal system, it is clear that Eq. (A.2) reduces to
g r - ~ [/,iN/_~ 0.
(A.3)
i
If the system is in contact with a reservoir in which the chemical potential of each of the species is maintained constant, then F - ~i/-~iN/ is the appropriate potential function. If the system is a condensed system, the same potential function may be used even if the system is in contact with a pressure reservoir because condensed systems are virtually incompressible, and processes therein involve negligible PV work. For the specific case that only protons and sodium ions can cross the boundary between the film and the glass, Eq. (A.3) can be written as g F - NlgN 1 - p,2gN2 _< 0.
(A.4)
Since electrical neutrality demands that N 1 + N 2 --
When the oxide of a low field strength ion, e.g. sodium, is incorporated into the structure of a silicate glass, it can give rise to non-bridging oxygen atoms. If the glass contains high field strength ions, e.g., alumina, the alkali oxide can be incorporated in a different manner. As mentioned in the text, the aluminum ion can form a partially covalent bond with four oxygen atoms (a number exceeding the formal charge on the aluminum ion by one). The presence of a neighboring sodium ion preserves electrical neutrality. The mode of incorporation of the sodium oxide is that one which produces the lowest free energy. Even though the energy clearly favors the formation of the tetrahedral aluminum, the free energy may favor the formation of non-bridging oxygen, if the number of silicon atoms is overwhelmingly larger than the number of aluminum atoms. The value of • used in Eq. (4) is an average value determined by the relative numbers of the two kinds of structures formed. As the concentration of aluminum ions increases, the entropy favoring the formation of non-bridging oxygen atoms upon the addition of an incremental amount of sodium oxide diminishes. At a high enough level of alumina, virtually no non-bridging oxygen atoms are formed, and e attains its asymptotic value. It is stated in the text that • is positive whenever P is negative (if a silica barrier layer is used). However, it is reasonable that when P is too close to zero, the value of • may be too small to cause sodium to diffuse against a concentration gradient. Thus, predictive power based on
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163 the algebraic sign o f P m a y be lost if its absolute v a l u e is too small.
References [1] M. Mizuhashi, Y. Gotoh and K. Adachi, Rep. Res. Lab. Asahi Glass Co. Ltd. 36 (1986) 1.
163
[2] Y.H. Yun and P.J. Bray, J. Non-Cryst. Solids 27 (1978) 363. [3] J.M. Stevels, in: The Structure and Physical Properties of Glass in Encyclopedia of Physics, Vol XIII, ed. S. Flugge (Springer, Berlin, 1962) p. 540. [4] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics, 2nd Ed. (Wiley, New York, 1976) p. 105. [5] R.R. Shaw. and D.R. Uhlmann, J. Am. Ceram. Soc. 51 (1968) 377.
ELSEVIER
Journal of Non-Crystalline Solids 197 (1996) 154-163
Sodium redistribution between oxide phases R.J. Araujo *, F.P. Fehlner Coming Incorporated, Sullivan Park Fr-3-1, Corning, NY 14831, USA
Received 22 June 1995;revised 7 November 1995
Abstract Alkali transport between a glass substrate and an oxide over layer occurs by a mechanism of ion exchange between the alkali and protons. Thermodynamics requires that the difference in chemical potential of the alkali and the proton be the same in both phases at equilibrium. The influence of chemical composition of the glass on the chemical potential of alkali ions is discussed. Good agreement between predictions of the direction of alkali transport and observed transport is obtained.
1. Introduction The use of glass substrates for the fabrication of thin film transistors (TFTs) is an important factor which has enabled active matrix liquid crystal displays (AMLCDs) to achieve widespread use in flat display technology. However, most glass compositions contain sodium, if only as impurities at the 1000 parts per million by weight (ppm) level. Since sodium is known to lead to instabilities in TFTs, it is important to prevent sodium from migrating from the glass to the TFT during processing. The potential for migration of the sodium is greatest during the fabrication of the TFT because the system is maintained for a significant time at temperatures as high as 600°C. Introduction of a nominally alkali-free oxide barrier layer such as silica, alumina, or tantala be-
* Corresponding author. Tel.: + 1-607 974 9000; fax: + 1-607 974 3675; e-mail: [email protected]. 1 Presented at the 8th International Conference on the Physics of Non-Crystalline Solids, Turku, Finland, 28 June- 1 July 1995.
tween the TFT and the glass is an obvious means of doing so. Unfortunately, the efficacy of a barrier layer film is limited by two factors. First, the films, themselves, may contain sodium impurities. It is not uncommon for silica films to contain sodium impurities at concentrations between 300 ppm and 1000 ppm. The second factor is illustrated by a phenomenon observed when a soda-lime glass is coated with a silica film and the system is heated. The concentration of the soda in the silica film is observed to increase significantly. Both the soda initially in the film and the additional soda introduced to the film by the glass can contribute to the contamination of the TFT. A family of glasses to be described below strongly extracts sodium ions from a silica layer when the system is heated. Thus, the concentration of sodium in the barrier layer which is available for contaminating the TFT is reduced. Moreover, transport of sodium from the silica film to the glass is observed even if the glass initially contains a concentration of sodium ions considerably in excess of that initially in the silica. The behavior of this family of glasses will
0022-3093/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-3093(95)00632-X
155
R.J. Araujo, F.P. Fehlner/Journal of Non-Crystalline Solids 197 (1996) 154-163
be discussed in terms of the thermodynamics of open systems and in terms of the structure of glasses in this family.
2. Alkali transport Because electrical neutrality must be maintained in both phases, transport of sodium ions between the glass substrate and the silica film requires special conditions. Either a negative charge must move in the same direction as the sodium ion or another positive ion must move in the opposite direction. The authors believe that the latter process is the important one in this study. Both glasses and silica films made under a variety of conditions contain an appreciable amount of water in the form of hydroxyl units, each bonded to a silicon atom. Some of the hydroxyl units are ionized, thereby producing protons. Transport of sodium ions between phases is accompanied by counter-current transport of protons, so that electrical neutrality is maintained in both phases. A flux of particles can be observed only in the presence of a gradient in the chemical potential of those particles. If the energy of a particle does not vary spatially, the gradient in the chemical potential is parallel to the gradient in the concentration of the particles and the flux is in the direction opposite to the concentration gradient. This is not necessarily true if the energy of a particle depends on its position in space. In such a case, it is sometimes possible for particles to diffuse uphill against the concentration gradient. 2 In either case, fluxes vanish when the spatial distribution of all the particles produces thermodynamic equilibrium in the system. When a glass substrate coated with a silica barrier layer is heated, the sodium and proton fluxes observed are those which tend to produce the equilibrium distribution of both species. In a multiphase multicomponent system in which the transport of components between phases is not constrained, the chemical potential of each component must have the same value in every phase at equilibrium. Appendix A indicates how the character of the equilibrium is
2 Diffusion against a concentration gradient is illustrated by the process of phase separation in a multicomponent system.
changed by the requirement of electrical neutrality in a system which contains two mobile ions having the same valence. In such a system, equilibrium requires only that the difference in the chemical potential of the two ions be the same in the two phases. This requirement and the initial concentrations of the components in the two phases determine their equilibrium concentrations. In the present case, the equality ( ]£H -- /£Na)glass = ( /tLH -- /'LNa)film
(1)
must hold at equilibrium. This requirement will be used in conjunction with a simple model. It will be assumed that the free energy of the system is changed during the ion exchange only by the ideal entropy of mixing of the two ions and by the differences in the bonding energies of each of the two ions in the two phases. For reasons to be described below, it is assumed that the bonding energy of the proton is the same in the two phases, whereas that of the alkali ion is not the same in the glass as it is in the film. With these assumptions, the expressions for the chemical potentials may then be derived, and the equilibrium composition of the glass relative to that of the film can be determined. Let • represent the amount by which the bonding energy of the sodium ion in the glass exceeds that in the film. The calculation for a silica film on glass is summarized in the following equations, wherein F represents the Helmholtz free energy: Ffilm = ( i d e a l entropy)film,
(2)
Fgla,s = - [Na + ] × • + (ideal entropy)glass,
(3)
[Na + ]
[Na + ]
•
The last equation suggests several ways in which the concentration of sodium ions in the film can be minimized. Because the sum ([Na+ ] + [H+ ]filrn) is limited by its initial value, the concentration of sodium at equilibrium can be decreased by decreasing the alkali concentration, but also by depletion of the hydrogen species from the barrier layer. The work of Mizuhashi et al. [1] confirms this expectation. They found that exposure of the samples to dry nitrogen or vacuum during the heat treatment process
156
R.J, Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
diminished and even prevented the movement of sodium. Alternatively, one can reduce the sodium in the film at equilibrium by increasing the density of protons initially in the glass. The efficacy of this approach has not yet been demonstrated. A third approach is the main subject of this report. The concentration of sodium ions in the silica film can be limited to a small fraction of the sum of the initial concentrations of protons and sodium ions if • is very large. In principle, • can be derived from a knowledge of the heats of reaction of water and of the alkali oxide of interest with the oxide comprising the film and with the glass substrate. Knowledge of the heat of reaction of a specified glass with either an alkali oxide or with water is often not known. Therefore, it is necessary to provide some means of estimating the dependence of •, or at least the algebraic sign of •, on the glass composition. The kind of bonding characteristic of some simple glasses is understood well enough so that the sign of • can be determined with moderate confidence. The following section summarizes the principles involved.
3. Structure of borosilicate and aluminosilieate glasses Silica glass is composed of silicon atoms, each of which is bonded to four oxygen atoms by bonds which exhibit significant covalent character. The spatial arrangement of the four oxygen atoms around the silicon to which they are bonded exhibits tetrahedral symmetry. Each of the oxygen atoms is bonded to two silicon atoms by bonds having significant covalent character. Any sample is, in effect, a single gigantic three-dimensional polymeric molecule. When an alkali oxide is added to silica and the system melted to form a glass, some of the oxygen atoms are covalently bonded to only a single silicon atom. Such an oxygen atom exhibits a negative charge. It must, therefore, be compensated by a vicinal positively charged alkali ion. Because such an oxygen atom disrupts the three-dimensional polymeric structure, it is called a non-bridging oxygen atom. If water reacts with silica, the situation is similar to the above case in that some oxygen atoms are not bonded to two silicon atoms. In this case,
however, a proton is bonded to the non-bridging oxygen. This bond has considerable covalent character. 3 In alkali-aluminosilicate glasses, non-bridging oxygen atoms bonded to silicon atoms do not form unless the number of alkali atoms exceeds the number of aluminum atoms. Instead, an aluminum atom forms covalent bonds to four oxygen atoms, 4 and a negative charge is associated with that whole group of atoms. The negative charge must be compensated by a vicinal alkali ion. The term 'tetrahedral aluminum' refers to those aluminum atoms so bonded because all four neighboring oxygen atoms are equivalent and they exhibit tetrahedral symmetry. The complete absence of non-bridging oxygen atoms in such a glass is a clear demonstration of the fact that the formation of tetrahedral aluminum is energetically favored over non-bridging oxygen atoms bonded to silicon. If the alkali exceeds the alumina, then for each excess alkali atom, a non-bridging oxygen bonded to a single silicon atom is formed. It is generally believed that, if water is added to an aluminosilicate glass, hydroxyls are bonded only to silicon atoms. Non-bridging oxygen atoms bonded to aluminum have not been observed and are, therefore, presumed to be less stable than those bonded to silicon. Like silica glasses, boric oxide glasses form gigantic three-dimensional polymeric molecules. Unlike silicon, each boron atom is bonded to only three oxygen atoms whose positions manifest trigonal symmetry. For our purposes, the most important respect in which boric oxide and silica glasses differ is the manner in which b o r o n - o x y g e n bonding changes when an alkali oxide is added. When the ratio of added alkaline oxide to boric oxide is lower
3 In the remainder of this discussion, bonds having significant covalent character will be referred to simply as covalent bonds. 4 The four oxygen atoms to which the aluminum bonds are each bonded to a silicon atom. The four oxygen atoms covalently bonded to one silicon and to one aluminum atom are also called bridging oxygen atoms because they maintain the polymeric structure of the glass. Because alumina added to an alkali silicate tends to restore the polymeric nature of the glass instead of further disrupting it by forming non-bridging oxygen atoms, it is often referred to as a network former even though it does not, by itself, form a glass.
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
than some critical value, 5 non-bridging oxygen atoms are not formed. Instead, some of the boron atoms become bonded to four oxygen atoms in a manner completely analogous to tetrahedral aluminum. If alkali is added beyond the critical ratio, however, the excess gives rise to non-bridging oxygen atoms bonded to boron. In borosilicate glasses, the ratio of silica to boric oxide exerts an influence on the amount of tetrahedral boron and the number of non-bridging oxygen atoms bonded to boron. For example, whereas the critical ratio in a binary alkali borate is 0.43, it is approximately 0.6 when the ratio of silicon atoms to boron atoms is 3.0 [2]. It should be emphasized that, unless the number of alkali atoms exceeds the number of boron atoms, the addition of alkali influences the bonding of only boron atoms, and no non-bridging oxygen atoms are bonded to silicon atoms. Thus, it is clear that non-bridging oxygen atoms bonded to silicon atoms and compensated by vicinal alkali ions are less stable than any of the other structures discussed thus far. Protons are bonded into the structure of borate or silicate glasses primarily as hydroxyls. They cannot compensate the negative charge associated with the tetrahedral alumina or a tetrahedral boron. 6 Therefore, if a silica film which contains low concentrations of protons and alkali ions is placed in contact with an aluminoborosilicate glass which also contains protons, protons will tend to accumulate in the silica film and alkali ions will tend to accumulate in the glass. This is because the energy of a proton is essentially the same in either phase, whereas the energy of an alkali ion is lower in the glass if it
5 In the low temperature limit, introductionof alkali to a boric oxide glass causes the exclusive formation of tetrahedral boron atoms if the ratio of alkali to boron is below the critical value of 0.43. At sufficientlyhigh temperature, some non-bridgingoxygen atoms bonded to boron atoms are formed even when the ratio is below this critical value. 6 Because the four oxygen atoms comprising the tetrahedral aluminumor tetrahedral boron structuralunit each already participate in two covalent bonds, no one of them possesses sufficient electron density to participate in another covalent bond with a proton. The coulombicenergy between a monatomicpositive ion and the somewhat delocalized negative charge of a tetrahedral aluminum unit is much weaker than the energy of the protonoxygen bond in a hydroxyl group.
157
causes the formation of a tetrahedral aluminum or a tetrahedral boron than it is in the silica film, where it can do nothing but give rise to a non-bridging oxygen. Equilibrium is achieved when the decrease in energy achieved by enriching the glass with alkali is balanced by the decrease in entropy caused by the segregation. High field strength ions other than boron and aluminum can give rise to a site of low alkali energy by forming covalent bonds to what would otherwise be non-bridging oxygen atoms. Niobium and tantalum are examples. Magnesium, beryllium, zinc, and to some extent, calcium exhibit analogous behavior. The bonding can be understood in the following 7 way. Imagine that the introduction of an oxygen ion associated with the addition of a magnesium oxide molecule causes the formation of two nonbridging oxygen atoms. The divalent magnesium ion has such a high field strength that it forms partially covalent bonds, not only to the two non-bridging oxygen atoms created by the introduction of the magnesium oxide, but also to another non-bridging oxygen atom created by the introduction of sodium oxide. The formation of covalent bonds to the magnesium, in effect, converts the three non-bridging oxygen atoms to bridging oxygens. The negative charges on the three converted oxygen atoms are compensated by the two positive charges on the magnesium and the positive charge on the sodium. 8
7 This description is based on an unpublishedinterpretationof viscosity data on alkali silicatescontainingcalcium,magnesiumor zinc by R.J. Araujo and G.B. Hares, Coming Incorporated. It applies only in the low temperatureregime. To the extent that the introduction of magnesium oxide decreases the total number of non-bridgingoxygen atoms, magnesium, like aluminum,is said to be a network former. The strength of the magnesium-oxygen bond probably is not sufficient to produce the short bond lengths which would stabilize trigonal coordination of the magnesiumby oxygen atoms. Therefore, in addition to the three oxygen atoms having the strongest bonds, oxygen atoms bridgingtwo silicon atoms probablyare includedin the magnesium'soxygen coordinationshell. It is sometimes speculated that one of the bridgingoxygen atoms is almost as close as the three most stronglybonded oxygen atoms and that the magnesium environmenthas the symmetryof a distorted tetrahedron. As a result of this speculation, the phrase 'tetrahedral atom' is sometimes used loosely to indicatethis type of bondingregardless of the exact symmetryof the oxygencoordinationshell aroundthe small high field strength ions.
158
R.J. Araujo, F.P. Fehlner / Journal of Non-Crystalline Solids 197 (1996) 154-163
The alkali ion at such a site cannot be replaced by a proton because the electron density in each of the three most strongly bonded oxygen atoms has already been so strongly attracted by the magnesium ion that hydroxyl bonds are not stable, In contrast to magnesium, the large alkaline earths such as barium and strontium apparently do give rise to non-bridging oxygen atoms to which a proton can bond. 9 In the present work, it is tentatively assumed that barium and strontium produce two non-bridging oxygens per ion and that magnesium removes one. The behavior of calcium is intermediate. It is, therefore, assumed, in keeping with the ideas of Stevels [3], that an equilibrium exists between those calcium ions that act like the heavier alkaline earths and those that act like the very small ones. It is arbitrarily assumed, with no supporting evidence, that the fraction of calcium ions which each produce two nonbridging oxygens and the fraction of calcium ions that each remove one non-bridging oxygen atom are both equal to one half. The use of the assumption has not been shown to lead to any results that are inconsistent with experiment.
4. Direction of sodium transport Consider a glass that contains a level of alumina, boric oxide, and oxides of other high field strength ions, such as tantalum or magnesium, that exceeds the number of non-bridging oxygen atoms that are produced by the totality of low field strength ions, such as the alkalies and the large alkaline earths. Considerations discussed in the previous section indicate that energetics must favor the replacement of protons in the glass by additional alkali ions when the glass is placed in contact with a silica film containing alkali impurities. The value of E in Eq. (4) varies monotonically and asymptotically with the magnitude of that excess. See Appendix B. Thus, it should be possible to predict with a high degree of reliability the direction of sodium transport that occurs when a silica film is placed in contact with a
9 This statement is based on an unpublished interpretation of the influence of barium on the precipitation of silver halide in photochromic glasses by R.J. Araujo, Coming Incorporated.
Table 1 Methods of film preparation C - APCVD S - RF magnetron sputtered from oxide S' - Reactively RF magnetron sputtered from metal P - PECVD E - Electron beam evaporated
glass of known composition. It is convenient to define a symbol to represent the aforementioned excess: P ' = [M +] + 2 × ([Sr +2] + [Ba +2] + ½[Ca+2]) - [Al+3],
P = P ' - 0 . 6 1 8 + 3 1 - ( [ Z n + 2 1 + [Be +21
+[Mg +2 ] + ½[Ca+2]), where all concentrations are expressed in cation percent. The reason for defining P' separately will be discussed in Section 6.2. The concentration of boron is multiplied by 0.6 because that is the value of the critical ratio of alkali to boron, above which additional alkali produces non-bridging oxygen atoms for glasses in which the silica exceeds the boron by a factor of three or more. For glasses richer in boron, a somewhat smaller value probably should be used. See the previous section. The reason for taking half the concentration of calcium was also discussed in the previous section. When P is negative, energetics favors a larger ratio of alkali to protons in the glass than in the film. For the levels of alkali impurity and protons commonly observed in silica films, a negative value of P should indicate stronger transport of sodium from the film into the glass than would be expected from the concentration gradient alone. The degree to which sodium is extracted from the film by the glass increases asymptotically with the magnitude of the negative value.
5. Experimental Thin films of silica were deposited by atmospheric pressure chemical vapor deposition (APCVD), plasma enhanced chemical vapor deposition (PECVD), or by reactive sputtering on carefully
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
159
Table 2 Glass composition in weight percent Oxide
SiO2 AI203 B203 BaO S~ CaO MgO ZnO K20 Na20 Li20 As203
Sample ID 0215
7740
7059
1724
AX
BDC
BDD
BDE
1735
72.1 1.8 0.0 0.0 0.0 7.3 3.8 0.0 0.2 14.0 0.0 0.0
0.8 2.3 12.5 0.0 0.0 0.0 0.0 0.0 0.0 4.0 0.0 0.0
49.5 10.5 14.6 25.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.5
57.2 16.5 4.9 8.0 0.0 7.8 5.6 0.0 0.0 0.1 0.0 0.0
72.1 4.6 9.8 3.4 0.0 0.5 0.0 2.7 0.6 5.8 0.0 0.0
56.3 2.8 30,7 0,0 0.0 0.0 0.0 0.0 0.0 10.2 0.0 0.0
70.3 2.6 17.7 0.0 0.0 0.0 0.0 0.0 0.0 9.5 0.0 0.0
62.0 10.5 17.9 0.0 0.0 0.0 0.0 0.0 0.0 9.6 0.0 0.0
56.9 14.5 4,7 12,4 0.2 11,1 0.0 0.0 0.0 0.1 0.0 0.8
BDD
BDE
Table 3 Glass composition in cation percent Oxide
Sample ID 0215
SiO2 A1203 B203 Na20 K20 MgO CaO BaO ZnO P' P
7740
7059
1724
1735
AX
BDC
6Z6 1.8 0.0 23.6 0.2 4.9 6.8 0.0 0.0
71.6 2.4 19.1 6.9 0.0 0.0 0.0 0.0 0.0
50.8 12.7 25.9 0.2 0.0 0.0 0.0 10.1 0.0
54.5 18.5 8.0 0.2 0.0 7.9 7.9 3.0 0.0
57.1 17.1 8.1 0.2 0.0 0.0 11.9 4.8 0.0
65.2 4.9 15.3 10.2 0.7 0.0 0.5 1.2 1.8
42.5 2.5 40.0 14.9 0.0 0.0 0.0 0.0 0.0
57.5 2.5 25.0 15.1 0.0 0.0 0.0 0.0 0.0
50.0 10.0 24.9 15.0 0.0 0.0 0.0 0.0 0.0
+25.4 + 17.1
+4.5 -7.0
+7.6 -7.9
-4.3 -21.1
+5.1 -5.8
+7.5 -3.8
+ 12.5 - 11.5
+ 12.5 -2.5
+5.0 - 10.0
Table 4 Results for silica films Substrate glass code
Barrier film/(nm)
Control ppm Na20
Time/Temp. (h)/(°C)
ppm Na20
- A[Na20] (ppm)
Ratio HT/ control
1735 1735 7740 0215 AX 7740 BDC BDD BDE 7059 7059 7059
S / ~ 200 C/100 St/100 S/100 S/100 S/100 C/100 C/100 C/100 C/80 S'/100 C/100
569 527 1670 2130 365 321 13 100 43 400 32 600 2570 380 453
25/650 25/650 25/650 25/350 25/350 25/350 25/500 25/500 25/500 25/650 25/650 1000/650
207 121 2770 7380 395 653 8760 27 000 8560 1300 130 153
362 406 - 1100 - 5250 - 30 - 332 4340 16 400 24 000 1270 250 300
0.36 0.23 1.66 3.46 1.08 2.03 0.67 0.62 0.26 0.51 0.34 0.34
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
160 Table 5 Results for alumina films Substrate glass code
Barrier film/(nm)
Control ppm N a 2 0
Time/Temp. (h)/(°C)
ppm Na20
- A[Na20] (ppm)
Ratio H T / control
7059 7059 1735 1724
S/100 S/140 S/100 S/IO0
9.4 10.1 7.7 15.3
25/650 25/650 25/650 25/650
17.9 18.3 12.3 8.9
- 8.5 - 8.2 - 4.6 6.4
1.90 1.81 1.60 0.58
cleaned glass substrates varying moderately widely in composition. Symbols representing the various techniques for depositing the films are defined in Table 1. The compositions of several of the glasses are listed in Tables 2 and 3. Most of the depositions were performed in a class 1000 cleanroom. 10 The samples were heat treated in a 2.5" diameter silica tube mounted in a Varian Marshall tube furnace. The heat treatment conditions employed in specific experiments are indicated in Tables 4 and 5. During the heat treatment, samples were stacked so that films faced other films of the same composition in order to minimize sodium loss or gain from the furnace atmosphere. The relative humidity of the air was maintained between 98% and 100% at room temperature as measured by an Omega RH-200 meter. The profile of the sodium concentration, as well as that of the silicon concentration in the film and in the glass, was determined before and after heat treatment by SIMS measurements. Signals were converted to sodium concentrations by use of known standards. The accuracy of the measurements is within twenty percent of the recorded values. The value of the sodium concentration listed in Tables 2 and 3 is obtained from the ratio of sodium to silicon averaged over the thickness of the film. Similar experiments were performed on samples made by depositing alumina films on various glass substrates. CVD and sputtering techniques were both employed to deposit alumina films. Interpretation of
l0 Some samples were coated at the Watkins-Johnson Corporation of Scotts Valley, CA, USA. In these cases, deposition was not performed in a clean room.
the significance of the results is discussed in Section 6.2
6. Results
6.1. Silica films The amount of sodium measured in the silica films before the samples were heat treated varied considerably, depending on the deposition technique and varied with the specific apparatus even for the same technique. The sodium concentration changed as a result of the heat treatment in every sample. In some cases, the concentration of sodium in the film decreased, and in other cases it increased. The second to the last column in each of Tables 4 and 5 exhibits the decrease in sodium concentration; i.e., a negative value indicates an increase. In general, the results bear out the predictions based on the value of P. In the case of glass 0215, for which P is positive, a large increase in sodium concentration is observed. In all of the glasses for which P was negative, except AX and 7740, a decrease in sodium is observed in the silica film. In the case of glass AX, the value of P is only slightly negative, and the amount of sodium transport is very slight. The behavior of this glass is not considered to be inconsistent with the model. Only the system of silica on glass 7740 seems inconsistent with the predictions of this model. Sodium is found to move from the glass into the silica film despite the negative value of P. However, it is known that glass 7740 can phase separate to form a silica-rich phase and a sodium borate phase
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163
[4]. It is not known if the sodium borate further separates into two phases, but it is not forbidden to do so by thermodynamics [5]. If it does, then one of the phases would surely be characterized by a ratio of alkali to boron associated with non-bridging oxygens. The alkali from this phase may be responsible for the observed alkali enrichment of the silica film. One of the samples, not shown in the table, in which silica was deposited on glass 7059 by reactive sputtering, also exhibited a significant increase in the concentration of sodium in the film. The reason for this behavior is not understood. The behavior of thirteen of the samples in which silica was deposited on glass 7059 was in agreement with the model.
6.2. Alumina films Alumina films were placed on various glasses to compare their behavior to that of the silica films. In some cases, transport of alkali from the glass to the film was observed in spite of the fact that P was negative. This result suggests that alkalis are more stable relative to protons in alumina films than in silica films. Such a conclusion is consistent with estimated heats of reaction. ~1 However, this fact does not necessarily mean that the alkali is more stable in the alumina film than it is in any possible glass. The fact, that in aluminoborosilicate glasses only the alkali in excess of the alumina alters the bonding of boron, suggests that the formation of tetrahedral aluminum decreases the energy of an alkali more strongly than the formation of the other structural units, referred to loosely as tetrahedral atoms. This suggests that E might possibly be positive for systems comprising an alumina film and a substrate glass containing alumina in amounts in excess of the alkalies and the large alkaline earths. Accordingly,
H The reaction of sodium oxide with silica to form sodium metasilicate is more exothermic by 37 kcal/mol of X 2 0 than the reaction of water with silica. The reaction of sodium oxide with alumina is more exothermic by 55 kcal/mol of X 2 0 than the reaction of water with alumina. Thus, if a film of alumina which contains sodium and proton impurities is placed in contact with a silicate containing hydroxyls and sodium, protons would tend to aggregate in the silicate and alkali ions in the alumina.
161
the function P' was defined in such a way as to include only alumina as a stabilizing tetrahedral atom. A negative value of P' is obtained only for glass 1724. It is only for that glass that transport from the alumina film to the glass is observed. This correlation corroborates the hypothesis that, while alkali is more stable in alumina films than it is at some of the tetrahedral atom sites in silicates, it is not as stable as alkali at the site of a tetrahedral aluminum atom in silicates. It is expected, therefore, that the algebraic sign of P' is a good predictor for systems comprising glass in contact with alumina films, just as the sign of P is a good predictor for systems comprising glass in contact with silica films. Although experiments with other oxide films, such as tantalum oxide, have not been performed, the direction of transport of alkali between such films and the substrate glass is expected to depend on the composition of the glass in a similar manner. Unless the heats of reaction of water and of alkaline oxides with tantalum oxide indicate that, for a tantala film on glass, ~ is less positive than that in the alumina system, a negative value of P' should indicate that alkali will flow from the tantala film into the glass.
7. Conclusions Extraction of alkali from a substrate glass is believed to involve an alkali-proton ion exchange. It is postulated that protons are not very stable in a glass unless they are covalently bonded to an oxygen in the form of a hydroxyl unit. Such a unit is not formed unless the proton is introduced to glass which contains non-bridging oxygen atoms. Consequently, alkali-proton exchange is not an effective mechanism for alkali extraction in a glass which is free of non-bridging oxygen atoms. Simple formulas indicate the density of non-bridging oxygen as a function of glass composition. The same formulas enable one to predict semi-quantitatively the alkali diffusion when a barrier layer is put in contact with a substrate glass. The predictions of the formulas are in good agreement with experimental results when a substrate glass is put into contact with either silica or alumina barrier layer films.
R.J. Araujo, F.P. Fehlner / Journal of Non-Crystalline Solids 197 (1996) 154-163
162
Acknowledgements
constant, it follows that ~N1 = - ~ N 2 and Eq. (A.4) can be written as
The authors are grateful to Paul Sachenik and K.R. Salisbury for assistance in sample preparation and to N.J. Binkowski for SIMS measurements. The authors are also grateful to Bruce Mayer of Watkins-Johnson Corporation for sample preparation.
~F - (/.£1 -/-.t2)~N1 __ 0.
(A.5)
Appendix A. Potential function for constrained open system
Therefore, F - (/.t I -/~2)N1 is the potential function appropriate to this system. If the volume of the glass is very large compared to the volume of the silica film, the glass may be considered a chemical potential reservoir for protons and sodium ions. At equilibrium the difference in the chemical potentials of protons and sodium ions in the film must be the same as the difference in the glass.
The mathematical statement of the combined first and second laws is given by
Appendix B. Asymptotic dependence of • on P
~E <_ T ~ S - P d V +
~., Izi~N i.
(A.1)
i
The above equation assumes that all work is PV work. The appropriate mathematical statement is taken to be the inequality for irreversible processes and taken to be the equality for reversible processes. From the definition of the Helmholtz free energy, F, Eq. (A.1) can be written as
gF < - S g T -
P g V + ~_, p~igNi.
(g.2)
i
When no PV work is performed in an isothermal system, it is clear that Eq. (A.2) reduces to
g r - ~ [/,iN/_~ 0.
(A.3)
i
If the system is in contact with a reservoir in which the chemical potential of each of the species is maintained constant, then F - ~i/-~iN/ is the appropriate potential function. If the system is a condensed system, the same potential function may be used even if the system is in contact with a pressure reservoir because condensed systems are virtually incompressible, and processes therein involve negligible PV work. For the specific case that only protons and sodium ions can cross the boundary between the film and the glass, Eq. (A.3) can be written as g F - NlgN 1 - p,2gN2 _< 0.
(A.4)
Since electrical neutrality demands that N 1 + N 2 --
When the oxide of a low field strength ion, e.g. sodium, is incorporated into the structure of a silicate glass, it can give rise to non-bridging oxygen atoms. If the glass contains high field strength ions, e.g., alumina, the alkali oxide can be incorporated in a different manner. As mentioned in the text, the aluminum ion can form a partially covalent bond with four oxygen atoms (a number exceeding the formal charge on the aluminum ion by one). The presence of a neighboring sodium ion preserves electrical neutrality. The mode of incorporation of the sodium oxide is that one which produces the lowest free energy. Even though the energy clearly favors the formation of the tetrahedral aluminum, the free energy may favor the formation of non-bridging oxygen, if the number of silicon atoms is overwhelmingly larger than the number of aluminum atoms. The value of • used in Eq. (4) is an average value determined by the relative numbers of the two kinds of structures formed. As the concentration of aluminum ions increases, the entropy favoring the formation of non-bridging oxygen atoms upon the addition of an incremental amount of sodium oxide diminishes. At a high enough level of alumina, virtually no non-bridging oxygen atoms are formed, and e attains its asymptotic value. It is stated in the text that • is positive whenever P is negative (if a silica barrier layer is used). However, it is reasonable that when P is too close to zero, the value of • may be too small to cause sodium to diffuse against a concentration gradient. Thus, predictive power based on
R.J. Araujo, F.P. Fehlner/ Journal of Non-Crystalline Solids 197 (1996) 154-163 the algebraic sign o f P m a y be lost if its absolute v a l u e is too small.
References [1] M. Mizuhashi, Y. Gotoh and K. Adachi, Rep. Res. Lab. Asahi Glass Co. Ltd. 36 (1986) 1.
163
[2] Y.H. Yun and P.J. Bray, J. Non-Cryst. Solids 27 (1978) 363. [3] J.M. Stevels, in: The Structure and Physical Properties of Glass in Encyclopedia of Physics, Vol XIII, ed. S. Flugge (Springer, Berlin, 1962) p. 540. [4] W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to Ceramics, 2nd Ed. (Wiley, New York, 1976) p. 105. [5] R.R. Shaw. and D.R. Uhlmann, J. Am. Ceram. Soc. 51 (1968) 377.