SVC Education Guide – Materials Science

Glass as a Substrate Material Reprinted from the Education Guides to Vacuum Coating Donald M. Mattox, Management Plus, Inc., Albuquerque, NM Processing, Society of Vacuum Coaters, pp. 15-16, 2001

n PVD processing, glass is encountered as a Al2O3 = 0.2-1.5 wt%, and are called sodium solidification, where some material is enriched substrate material in many forms. he most silicate or soda-lime glasses. A high-lead at the surface, or it can occur with time. For Icommon are optical components, such as (lead-silicate) glass, such as is used in cut-glass example, in high-lead glasses, lead will dif- lenses and plates, such as those used for mirrors crystal, may have 25 to 50 wt% PbO, which fuse to the surface over a long period of time, and windows. Glass is an amorphous (vitreous) gives a low viscosity at relatively low working forming a lead oxide layer. This enriched sur- solid that has no long-range atomic order but temperatures so that it can be easily molded. face layer was the source of the development of oten has a short-range (near-neighbor) order. The fictive temperature of a glass is the a natural antireflection (AR) coating on “old” In the glass, atoms or groups of atoms form a temperature above which mass movement camera lenses before the use of deposited AR three-dimensional network that doesn’t have a will allow atom rearrangement. The fic- coatings. The armonica is a musical instru- repetitive order (i.e., polymerize). An element, tive temperature is not a set temperature ment that is played by rubbing wet fingers on such as selenium, can form a glass—but more but varies with the cooling rate. Glasses are the lips of rotating glass bowls. Lead poisoning commonly, glasses consist of network-forming metastable materials that are cooled through was a problem to the armonica musician in the oxide molecules such as SiO2, B2O3, GeO2, their fictive temperature before crystallization early 1800s when lead glass was used for the As2O3, or P2O5. Silica (SiO2)-based (silicate) occurs. If cooled slowly, the material may bowls; now fused silica is used for the bowls. glasses are the most common glasses. In the crystallize. For example, the silica tetrahedra, Optical glass is often formed by grinding and silicate glasses, two SiO2 molecules share two which form a fused silica glass, can also crys- polishing. The polishing operation can leave -4 oxygen ions giving two SiO4 tetrahedral ionic tallize into crystalline quartz if cooled slowly. polishing material, such as cerium oxide, units. he tetrahedral shape can be visualized as Many glasses can crystallize (devitrify) over embedded in the glass surface. being formed by three large oxygen ions (radius long periods of time at elevated temperatures. Glass will react with water vapor to form of O-2 = 1.32Å) in triangular coordination on a Glasses can develop a high internal stress a hydrated surface layer that will increase in plane, with the fourth oxygen ion on top form- during fabrication, particularly if the glass has thickness with time. This hydrated surface ing a pyramidal shape and the small silicon ion a high coefficient of thermal expansion. After layer is generally more brittle than is the (radius of Si+4 = 0.4Å) in the hollow formed by fabrication (working), glass items often are re- surface of newly formed glass. The surface the four balls in contact. he tetrahedral units heated to relieve residual stress. The annealing composition of a glass can be changed by then randomly polymerize into a three-dimen- temperature for a glass is defined as being the improper handling. For example, if the fused sional network forming the glass by joining at temperature at which the glass has a viscosity silica glass envelope of a halogen lamp is the apexes of pyramids but not along the sides. of 1013.4 poises. At that temperature the contaminated by sodium from fingerprints Glasses containing both SiO2 and B2O3 are internal stresses will be substantially relieved the sodium will diffuse into the glass at the called borosilicate glasses (BSG), and those con- in 15 minutes, though the amount of stress high operating temperature and the glass may taining both SiO2 and P2O5 are called phospho- relief will depend on the thickness of the “soften” and deform during operation. silicate glasses (PSG). glass. The strain point of the gas is defined as As Griffith showed in 1920, the strength of The network-formers are often combined being the temperature at which the viscosity is glass is not a fixed property but is controlled 14.6 with network-modifiers, such as Na2O, K2O, 10 poises. At that temperature the stresses by flaws such as scratches in the surface. CaO, BaO, MgO, PbO, etc. Some network will be substantially relieved in 4 hours. The Flaws can be introduced into the glass surface modifiers have large ions (radius of Na+1 = softening point of a glass is at a viscosity of by corrosion, abrasion, grinding, or by the 0.98Å), which act as substitutional ions for 107.6 poises, while the working temperature is deposition of an adherant film with a high the silicon and lower the bond strengths. For when the viscosity is less than 107 poises. For tensile stress. Glass can be strengthened by example, the bond strength for Pb-O is 1.6 eV/ example, the working temperature of a typical generating a high compressive stress in the bond, for Na-O it is 0.9 eV/bond, while for Si-O soda-lime glass is above about 750°C, and the surface of the glass. This is generally done by the bond strength is 4.6 eV/bond. In the case annealing temperature is about 500°C. For tempering. In the tempering process the glass of Al2O3, the aluminum ion is small (radius of fused silica, the working temperature is about is heated well above its annealing temperature, Al+3 = 0.6Å) and substitutes for silicon without 1600°C, and the strain point is 990°C. then the surface is rapidly cooled below the lowering the bond strength appreciably, thus The composition of the glass determines fictive temperature while the center is still forming aluminosilicate glasses. The Al-O bond the coefficient of thermal expansion (CTE), hot. As the plate cools down, the material in strength is 4.4 or 3.4 eV/bond depending on the which is an important consideration in many the center would like to contract more than coordination number. applications. Glasses can be formulated to the surface but is constrained by the rigid The network modifiers change the properties meet the CTE required to match that of other near-surface region. This puts the center of of the glass, such as viscosity, index of refrac- materials. For example, borosilicate glasses the plate under tensile stress and the surface tion, optical dispersion, coefficient of thermal have CTEs of about 4.5 x 10-6 cm/cm/°C, and region under compressive stress. When the expansion, etc. The modifiers Na2O and PbO high phosphate glasses can be formulated to maximum compression is obtained without decrease the viscosity of the glass. For example, match the CTE of aluminum, a metal with a internally fracturing the glass, the plate architectural glass and blow-molded glass con- very high CTE (~28 x 10-6 cm/cm/°C). is called “fully tempered.” Less than fully tainers (so-called “soft glasses”) may have the The chemical composition of the surface of tempered glass is called strengthened or composition SiO2 = 72-74 wt%, Na2O = 12-15 a glass can be different from that of the bulk toughened glass. The compressive stress in wt%, CaO = 6-10 wt%, MgO = 3-5 wt% and of the glass. This difference can occur during the surface can also be generated chemically

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by substituting large ions such as K+1 (radius = called stones, and bubbles of gas called seeds. source material, quasi-reactive deposition of 1.33Å) for smaller ions such as Na+1 (radius = Stones of NiS are notorious for causing glass source material in a plasma of a reactive 0.98Å) in the surface region by diffusion into spontaneous fracture when fabricating fully gas, or reactive deposition by vaporization of the surface from a molten salt at a high tem- tempered glass. Glass can also contain metallic the condensable materials in a reactive plasma. perature. When fully tempered or strengthened ions that lend color to the glass. Examples are: The coefficient of thermal expansion of PVD- glass is fractured, it breaks into small pieces Cr+ for green, Mn+ for purple, Fe+ for yellow deposited glass can be an important property, called shards and not into large jagged frag- and Co+ for blue. Iron is often an impurity in since differences in CTE between the film and ments, as does unstrengthened glass. For this soda-lime glass. the substrate materials, as well as the nature of reason tempered or strengthened glass is used Glass surfaces can be corroded by the the film growth, can give high residual stresses in such applications as shower doors. environment. Alkaline water, in particular, in the high-modulus glass films. Generally the The glass for optical lenses is often chosen is a strong corrosive agent. Glass should be deposited glass films should be annealed after for its index of refraction and optical disper- stored in such a manner that water does not deposition, but this can be a problem unless the sion, which is the sensitivity of optical proper- condense and remain on the glass surface, CTE of the film matches that of the substrate ties to wavelength. For example, crown glasses, since the condensed water leaches alkaline material. The CTE is also important in the PVD which are typically soda-lime glasses, have a low material from the glass and becomes progres- processing itself. For example, pure silica (SiO2) index of refraction and low dispersion, and flint sively more corrosive. The corrosion products glass has a very low CTE (~0.6 x 10-6 cm/ glasses, which typically have a high PbO con- are typically calcium and sodium carbonates cm/°C) and is not susceptible to thermal shock. tent, have a high index of refraction and high that can be removed by acid washing. The best It is used in the RF sputter deposition of SiO2 dispersion. The spectral transmittance of glasses material for separating glass sheets in storage from an SiO2 target. Glasses that have a high can vary widely in the infrared. Soda-lime glass is PMMA beads loaded with an adipic acid, CTE are difficult to sputter at a high rate, since strongly adsorbs wavelengths longer than 4 though styrofoam beads, paper, sawdust and thermal gradients cause the sputtering target microns, which is the most intense radiation many other materials are often used. to fracture when it is heated unevenly or is from a black-body at 500°C, while fused silica Glass can be formed into shapes by cutting, stressed by the holding fixture. In the semicon- (often mistakenly called fused quartz) transmits grinding, rolling, pressing, slumping (sagging) ductor industry phosphosilicate glasses (PSG) well into the infrared and is used for envelopes into or over forms, and blow-molding into for encapsulation are deposited by plasma in tungsten-filament radiant heaters. This is forms. Glasses are also used for coatings by enhanced chemical vapor deposition (PECVD). an important factor in heating glass substrates flowing molten glass over the surface of metals using radiant energy in PVD processing. (enameling) or ceramics (glazing). Reference Until 1959 plate glass was generally formed Glass films can be deposited by PVD Chapter 1, “Glasses,” in Handbook of Materials and Techniques for Vacuum Devices by Walter H. Kohl, by rolling soft glass, often followed by polishing processes by evaporation or sputtering glass Reinhold Publishing, 1967—available as an AVS reprint. to improve the optical properties. In 1959 Pilkington patented the float glass process, which is now the most widely used technique for forming large-sized optical-quality glass plate. In the float glass process, molten glass is fed onto the surface of molten tin and spreads out, giving smooth surfaces on both sides of the plate as it cools. In a typical float-glass plant, glass plate 0.25 inches thick is continu- ally formed at a rate of about 3000 ft/hour and a width up to about 10 feet. In a typical soda-lime float-glass plant, the raw materials are melted at about 1600°C, poured onto the molten tin at about 1100°C, and passed onto rollers at about 600°C. This results in an atomi- cally smooth surface. Glass formed by the float-glass process has had one side in contact with the molten tin (tin side), and tin has diffused into the surface, giving a surface layer containing a high tin oxide content that can extend several microns into the surface. This side can be identified using UV radiation since the tin oxide fluoresces under the UV. The top surface

(fire side), which sees an inert or reducing ______atmosphere, is depleted in alkali and enriched in silica. In some cases, deposited PVD films will nucleate differently on the two sides of the float glass because of their different composi- tions. Common defects in glass include regions of nonuniform composition, solid inclusions

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