Crystal Clear

HISTORICAL NOTE Crystal Clear We use clear, transparent glass for The resulting grayish glass, somewhat less land remained dependent on imported many things, from window glass and fine transparent than the colored material, glass for the next century. crystal goblets to thermometer tubes. proved acceptable if the finished article re In 1664 the Worshipful Company of Glassmaking is an ancient art, but colorless mained thin-the loss of transparency was Glass Sellers, a group English glass retail glass is a relatively recent development. less noticeable than the unwanted tint. ers, incorporated to discover ways to over Since the beginning of civilization, peo This cristallo, named because of its resem come foreign domination in their market, ple have known about two types of natu blance to natural rock crystal, proved to be and also to express their dissatisfaction rally occurring glass. When lightning the greatest export product of the Venetian with the quality they received. In 1675 the strikes sand, the heat can fuse the silica glassmaking industry. Worshipful Company commissioned grains into long slender glass tubes that are Cristallo was an extremely ductile mate chemist and experimenter George known as "petrified lightning;' or fulgu rial, which cooled quickly and could be Ravenscroft to investigate new glass rites. The heat and pressure of a volcanic blown very thin. The glassmakers' need to making techniques using local raw materi eruption can fuse sand and rock into a work with great speed and dexterity to als. The company hoped to create a glass black glass called "obsidian:' shape the glass properly influenced the superior to Venetian cristallo, a glass that As early as about 3000 B.C., some arti style and shape of Venetian glass objects. more closely resembled rock crystal in clar sans discovered how to make their own fur the first half of the 16th century, cris ity and transparency, without the gray or forms of glass as glazes on ceramic vessels. tallo glasses remained rather simple, but brown muddiness seen in much of the Ve The art of glass manufacture reached great soon the glass blowers developed great netian product. heights during the Roman empire, but skill and creativity in shaping and decorat Ravenscroft first attempted to substitute many of the techniques were lost during ing their products. They made elaborate calcined flints instead of sand, and potash the Dark Ages. toys and wineglasses with intricate decora instead of soda ash. His "flint glass" was a Simple fused-silica glass can be made by tions; they used tools with diamond points failure at first, developing many fine cracks melting sand alone. This makes fine to engrave designs into the clear surface. as it aged. Ravenscroft added lead oxide to quality glass, but it is very difficult to man the flux, however, and this proved to be a ufacture because the melting point of sand tremendous success. Not only did the lead is 1723°C, a temperature higher than most Glassmaking is an oxide eliminate the formation of fine furnaces can reach. Adding a flux of soda cracks, it produced a high-quality lustrous ash (sodium carbonate), though, reduces ancient art, but colorless glass that was also heavy and durable. the melting point to only 850°C, a temper glass is a relatively "Lead glass" was soft enough to be cut and ature much more easily attainable to early engraved easily. It had a greater refractive societies. Unfortunately the resulting recent development. power than common crown glass and a glasses are relatively water soluble, making somewhat greater light-dispersion proper them impractical for many uses. ties than other types of glass. The addition of a small amount of cal The Worshipful Company of Glass cium carbonate, from limestone, to the Venetian glass was superior to that made in other parts of Europe, but glassmaking, Sellers then set exacting standards for the mixture makes the glass insoluble again production of lead glass as they began to and suitable for many purposes. These like all crafts in Venice, was regulated by strict guilds and considered a state monop export English glass to foreign markets. In "soda-lime" or "crown" glasses are the honor of Ravenscroft's work, their earliest most common types produced even today, oly. The trade secrets were considered so valuable that all the Venice glassworks glassware bears an engraved raven's head. accounting for about 90% of current glass Ravenscroft's glass is what we generally re manufacture. were removed to the island of Murano as early as 1291. Workers were forbidden to fer to today as "crystal" or "lead crystal:' The basic ingredients of glass-sand, The term "flint glass" is also sometimes ash, and lime-were readily available to leave the island so they couldn't sell their knowledge elsewhere. (The move to Mu used, though flint is no longer included as early societies, but many natural impuri a base for its manufacture. ties affected the transparency and color of rano may also have been motivated by the need to protect the city from fires started English lead glass quickly superseded the resulting material. The contaminants conventional Venetian cristallo, and within varied from region to region, allowing each from the glassmaking furnaces.) By the 16th century, though, European a short time only a few provincial centers local glassmaker to claim a characteristic continued to make the Jess-satisfactory product. Crude manufacturing techniques kings and nobles openly offered hand some rewards for anyone who knew the Venetian-style glass. By the 18th century, also left streaks and bubbles in the glass, lead glass had become the favorite in all further decreasing its clarity. secrets of Venetian glass manufacture. Many Venetian glassmakers escaped from European markets, and England had be Glassmaking underwent a tremendous the island of Murano and fled elsewhere, come the world's leading glass producer. resurgence in Venice in the 13th century, setting up their own shops in other parts of Makers of lead glass produced chande possibly because of revived contact with Europe with substantial support from local liers, bowls, candlesticks, mugs, and cups, the Eastern Roman Empire. Not until later, noblemen. but their primary product was wine though, did Venetian artisans rediscover In 1575 the Venetian refugee Giacomo glasses, whose style evolved and influ how to make transparent and colorless Verzellini was granted a 21-year monopoly enced all other manufacturers of table glass. They discovered how to eliminate in London, provided that he make ware. unwanted colors and contaminants from a Venetian-type cristallo. Despite verzellini's Crown glasses-made from sand, soda glass melt by adding counter-colorizers. monopoly and others like it, however, Eng- ash, and lime-account for most current 52 MRS BULLETIN/MAY 1990 HISTORICAL NOTE glass production. Lead crystal, although more expensive to produce, is easier to melt and fabricate. A high concentration of Workshop on Tungsten and lead oxide in the flux and a relatively low alkaline content imparts desirable electrical Other Advanced Metals properties to the glass. Lead glass has been widely used in radio and television tubes, for ULSI Applications VII in capacitors, and as insulating parts in electric lamps. Lead glass has also been October 22-24, 1990, in Dallas, Texas used for prisms and lenses. Telescope makers found that overlapping a lead glass Announcement and Call for Papers lens with a regular crown glass lens could eliminate most of the chromatic aberration his workshop is the seventh in a series organized to bring to around the telescopic images of bright ob gether active researchers in the field of advanced metallization jects. Lead glass absorbs most ultraviolet T light but little visible light, also an advan for IC applications. tage in telescope lenses. Papers are solicited on: The fine crystal in your house, the wed LPCVD modeling and deposition techniques Grain refinement/roughness control ding gift that gathers dust on the high shelf Selective, planarized horizontal Fundamental surface chemistry and is taken down only for special occa interconnectlprepatterning techniques Film properties (physical, chemical, electrical) sions, has its own long tradition. But apart Contact plug and via fill applications Selectivity enhancers and inhibitors from glamorous tableware, brilliant chan Nucleation and compatibility studies Performance/reliability deliers and art objects, lead glass has Adhesion to thermal and CVD oxides Process control/manufacturability found many other uses in the three centu Refractory metal gate development Film/substrate interaction ries since its creation. The science of Selective cladding of sources, drains, gates, Diffusion barriers, etch barriers glasses has branched out enormously, pro interconnects New device structures Tungsten interconnects Buried layer conductor techniques ducing many new types for specialized ap CVD reactor design enhancement Microsensor and other novel applications plications. In addition to the basic silica Deposition kinetics Backside deposition prevention glasses, we now have oxynitride glasses, Wafer temperature measurement and control Patterning and etching of refractory metals phosphate glasses, chalcogenide glasses, Effect of CVD gas chemistry and impurities Thermal stability/high temperature halide glasses, and others. Materials re on selectivity applications searchers have developed glassy sub CVD precursor development stances for particular uses, depending on thermal expansion requirements, prepara Abstracts are due July 15, 1990 tion and softening temperatures, melt vis Send abstracts (at least 500 words, typed, double-spaced, with an additional page of cosities, and chemical compatibilities. No figures) to Gregory C. Smith, Texas Instruments, Incorporated, P.O. Box 655012, Mail longer are glasses used simply for contain Station 944, Dallas, TX 75265. Include author's name, affiliation, mailing address, and ers, windows, mirrors, and lenses; current phone number on abstract. applications have expanded to include fi For an announcement: ber optics, thin films, and semiconductor and biological uses.

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