glass may 2013

8/10/2019 Glass May 2013

1/6

Gases&Instrumentation www.gasesmag.com

A R T I C L E

We continued our coverage on specialty glass for display appli-cations in the January/February issue.1This article will discussion-exchange chemical strengthening of cover glass for the new

generations of active-matrix liquid crystal and organic light-emittingdiode displays (AMLCDs and AMOLEDs).

In addition to making larger, thinner, and lighter glass for displaypanels, glass manufacturers have also focused on techniques to offerhigh-quality images on damage-resistant, touch-sensitive screens. Corn-ings chemically strengthened Gorilla Glass has taken the world by storm

Glass Part 3: New generation of specialty glass forLCDs and AMOLEDs

Source: Gases & Instrumentation

Figure 1. The compressive stress developed on the surface decreases with depth until it is zero over the depth of the layer. A tensile stress gener-

ated in the glass interior balances this stress. Theoretically, as long as the sum of the compressive residual stress and the tensile stress from an

external load is less than zero, mechanical failure cannot occur. CS = compressive stress; CT = central tension; DOL = depth of (compressive) layer

As display manufacturers respond to demands for larger screens and higher imagequality, as well as lighter and more portable devices, glass manufacturers have beenoptimizing glass composition, properties, and production techniques to meet the elec-tronic manufacturing requirements. Trough ion-exchange chemical strengthening,display covers are stronger and more damage-resistant, while serving multiple userinterface functions.

BYM AGGIEY.M. LEE

since 2008.2The next generations Gorilla 2 and Gorilla Glass 3 wereintroduced in January 2012 and January 2013, respectively.3,4Competingproducts from other leading glass producers followed in 2011 and 2012including Asahis Dragontrail;5Nippon Electric Glass CX-01, CX-01P, andCX-01T;6and Schotts Xensation Cover, Xensation Cover 3D, and Xensation Cover AG (the last in collaboration with Berliner Glas).7,8,9

Combining fanciful product names like Gorilla, Dragontrail, andXensation with marketing imageries of pensive simians; fire-dodgingdemonic, flying dragons with glowing eyes and sharp claws; and

8/10/2019 Glass May 2013

2/6


A R T I C L E

Source: Gases & Instrumentation

Figure 2. Ion exchange process for chemical strengthening of glass. (Left) Glass containing Na+ions is immersed in a KNO3bath. (Right) As the

bath containing the glass is heated and then rapidly cooled, the larger K+ions from the bath replace the smaller Na+ions in the glass, generating

a compressive stress layer near the glass surface

chiffon-swathed, lithe women leaping in mid-air; or simply adopting the practical alphanu-meric CX-01 with little fanfare, the brands ofnew cover glass materials seem to mirror thedemographic profiles across their users.

Low-temperature ion exchangeIn our September 2012 newsletter article on

glass production,10we discussed ion exchangeas a chemical alternative to thermal temper-ing of glass to create compressive stress onthe surface of thinner glass types, especiallythose used as cover glass in electronics, harddisks, etc. Sodium-containing glass (e.g., soda

lime glass, borosilicate glass, aluminosilicateglass) is placed in a bath of molten potassiumsalt, heated to a temperature below the strainpoint of the glass (400600C), and then rap-idly cooled. The combination of heat and thesalt prompts the larger potassium atoms toexchange places with smaller sodium atomsin the glass. As the surface of the glass getscrowded, it becomes compressed. After cool-ing down, the compressive stress generatedon the glass surface, with a balancing tensilestress in the interior, strengthens the glass sub-

stantially. Figure 1 shows the stress distributionin the glass after ion exchange. Any impactneeds to exceed the built up compressivestress before cracks can take hold.

In addition to the exchange of Na+from glasscontaining Na2O with K

+in a molten potassiumsalt (KNO3or K2SO4), as shown in Fig. 2, otheralkali salts with an ionic radius larger than thealkali ion contained in the glass can be used.In essence, ion-exchange strengthening canbe used for lithium and sodium aluminosilicateglass compositions based on the exchange oflarger alkali ions (ionic radius: Li+< Na+< K+)for lithium or sodium using nitrate or sulfate

salt baths. The magnitude of the surface com-pressive stress increases with increasing differ-ence in ionic size. For example, Li+in lithiumaluminosilicate (LAS) glass can be replacedwith larger Na+or K+if the glass is immersedin a heated sodium or potassium salt bath(or a mixture of both).11A mixed salt of KNO3and a small amount of NaNO3 results in highcompressive stress, primarily owing to the K+ions, and a low depth of the compressive layer(DOL), primarily owing to the Na+ions, in shortprocess times.12

While Schotts Xensation Cover consists ochemically strengthened, sodium aluminosilicate glass, its Xensation Cover 3D is a chemi-cally strengthened, lithium aluminosilicateglass to accommodate a lower transformation temperature (Tg= 505C vs. Tg= 615C osodium aluminosilicate glass used for Xensation Cover and Cover AG) for more efficientthree-dimensional glass molding and a deepecompressive stress layer.12

Since ion exchange is based on the diffusionof ions, the thickness of the compressive layecan be varied by altering the glass composition, glass thickness, and treatment duration

Different types of exchanges can be combinedin a two-step (or double) ion exchange, foexample, type A ions are first exchanged withtype B ions, followed by a second exchangewhere type B ions are exchanged with typeC ions; in a three-step exchange, type C ionscan be exchanged to re-introduce type Aions into the glass surface layer; and so on. Insome cases, the sodium-containing glass is firstimmersed in a moderately heated sodium saltbath to enrich the glass surface with more Na+

When the glass is then immersed in a heated

8/10/2019 Glass May 2013

3/6


A R T I C L E

Thermal

tempering13

Chemical tempering

Corning24,18 Asahi5,17Nippon

Electric Glass6Schott7,8,9

Gorilla Glass

Gorilla Glass 2

Gorilla Glass 3

Concore (Ion-exchanged full sheet IOX-FS) for touch

screens and one-glass-solution applications

Dragontrail

0.28-mm substrate

for touch screens

CX-01

CX-01P

CX-01T

Xensation

Cover

Xensation

Cover 3D

XensationCover AG

Compressive stress built up in glass

after strengtheningLow (95150 MPa)

High (300900 MPa)

Capable 800 MPa

1000 MPa @40 m DOL

950 MPa @50 m DOL

950 MPa @ 50 m DOL

Capable >550 MPa

Capable >845 MPa

@20 m

750 MPa @45 m

Capable >850 MPa

Capable >900 MPa

Capable >900

MPa

Capable >700

MPa

Capable >900

MPa

Depth of compressive stress layer

(DOL)

Thick (approx.1/6 of

plate thickness)

Thin (10300 m)

Capable 40 m

Capable 50 m

Capable 50 m

Up to 40 m for 0.7-mm thick glass

Capable >90 m Capable >70 m

Capable >75 m

Capable >50 m

Capable >120 m

Capable >50 m

Youngs modulus

High

(approx. 1/2 of the

compressive stress)

Low

71.7 GPa

71.5 GPa

69.3 GPa

71.7 GPa

74 GPa

75 GPa

75 GPa

74 GPa

83 GPa

74 GPa

Time required Short (510 minutes) Long (30 minutes to a few hours; up to one week for special applications)

Tempered glass thickness and

formatLimited

Unlimited except for ultra-thin material

Thickness range: 0.52.0 mm

Sheet size: Available in

GEN 5 (1250 mm 900 mm)



Concore sheet size: up to GEN 6

(up to 1850 mm1500 mm)

Thickness range:

0.5>5.0 mm

Sheet size: 1220 740

mm or 4829

Thickness range: 1.54.2

mm

Sheet size:

300 500 mm


mm

Sheet size:

400500 mm


mm

Sheet size: GEN 5 (1300

1100 mm)

Thickness Range:

0.53.0 mm

Sheet Size:

1150950 mm

575475 mm

Thickness Range:

0.52.0 mm

Sheet Size:

1150950 mm

Thickness Range:

0.53.0 mm

Sheet Size:

950680 mm

575475 mm

Drawing process Any

Overflow down-draw (fusion)




Float

Float

Roll-out (polishing

required)

Float (light polishing

required)

Overflow down-draw

Microfloat

Microfloat

Microfloat

Glass composition Most glasses

Alkali- aluminosilicate




Aluminosilicate

Soda lime

(for touch screens only)

Aluminosilicate

Aluminosilicate

Aluminosilicate

Sodium alumino-

silicate

Lithium alumino-

silicate

Sodium alumino-

silicate

Table 1. Chemical and thermal tempering

8/10/2019 Glass May 2013

4/6


A R T I C L E

potassium salt bath, more Na+are available forexchange with K+to generate higher compres-sive stress after cooling.

High-temperature ion exchangeIn a 2009 article, Matsunami Glass in Japan

described a high-temperature process tocreate two layers of ion exchange in a sheet ofcover glass.13The technique involves immers-ing aluminosilicate glass containing Na2O orK2O in lithium salt at a temperature betweenthe softening and transformation tempera-tures (600750C). Ion exchange of Na+or K+in the glass with Li+in the salt occurs beforethe glass is cooled to room temperature. Theglass surface layer that now contains Li+has alower coefficient of thermal expansion (CTE)compared to the internal glass that containsNa++ or K+. The difference in CTEs means that

the interior will try to contract more than thesurface, thus generating compressive stresson the exterior, which strengthens the glass.14

Since the glass contains Al2O3, -spodumene(Li2OAl2O34SiO2) or -eucryptite (Li2OAl2O32SiO2) crystals with very low CTE are alsocreated. This method therefore leads to twolayers of ion exchange, which generates a crys-tallized -spodumene/-eucryptite glass layeron the surface and a second Li-metasilicateglass layer underneath. As the ion exchangeis carried out above the strain point and theglass is then annealed, a very high compressivestress of ~700 MPa forms after cooling and isefficiently combined with the internal tensionto form a very strong glass.13,14

Glass types that are applicable for this high-temperature treatment contain 5766% SiO2,13.522% Al2O3, 811% Na2O, and 1013%Li2O.

13Suitable lithium salts include lithiumhalides (such as LiCl and LiBr) and lithium sul-fate (including Li2SO4, binary Li2SO4Na2SO4,and ternary Li2SO4Na2SO4K2SO4). Alterna-tive Na and Ag salts have also been explored.One drawback of this high-temperature ion-

exchange technique is that the large differencein CTE results in sharp stress gradients at theinterface between the two glass compositions,which can lead to glass deformation.13,14

Using x-ray diffraction analysis of Li-basedion exchange in sodium aluminosilicate glass,UK scientists reported that the surface crystal-lization phase responsible for strength increasewas a low-CTE phase of -spodumene/-eucryptite with a measured crystallite size of235380 . The flexural strength was found tobe proportional to the depth of the crystallized

layer, which in turn was proportional to thesquare root of the duration of ion-exchange. Inaddition, the dependence of strength increasewas shown to be based on the alumina contentof the glass, temperature of treatment, andduration of treatment.15

Although the ion exchange rate is lowerin low-temperature ion exchange comparedto the high-temperature alternative involv-ing surface crystallization, there is minimalglass deformation. It is currently the preferredstrengthening method for cover glass used inmobile electronics.

Alternative ion exchangeIon exchange has been used as a means of

chemical strengthening since the 1940s and1950s. Although the exchange of alkali metalions (e.g., Na+in glass for K+in the medium) is

the most common method, it is not the onlytype of exchange. The reactions of alkalineearth metal ions (e.g., Ca2+ in glass for Pb2+and Zn2+) and other metal ions (e.g., Cu+/Cu2+and Ag+ in copper or silver staining of glass)are also applicable. Dealkalization is based onthe replacement of alkali ions (e.g., Na+) in theglass with H+through surface reactions with anacidgas atmosphere to improve the chemicalresistance.10It is usually carried out throughthe injection of a sulfur- or fluorine/chlorine-containing gas mixture (e.g., SO2, (NH4)2SO4,1,1-difluoroethane mixed with air, HCl) forreactions with sodium from the glass to forma new composition with a lower CTE, as in thecase of high-temperature ion exchange.14

Optimizing the benefits ofchemical strengthening

While chemical strengthening is the currenttechnique of choice for thinner glass, not alltypes of glass are suitable for the process owingto the time and temperature required for ionexchange. Obviously, glasses with low or zeroalkali content are not suitable for ion exchange.

As tin acts as a block to ion exchange, theside of float glass that was directly above themolten tin bath during forming is believed toundergo less ion exchange than the side thatwas in contact with nitrogenhydrogen.16

Some glass types can form significantlythicker layers of surface compressive stressthan others.13Soda lime glass, for example,cannot form the deep compressive stress layer(DOL 25% higher load-to-failure after abrasiontreatment compared to Gorilla Glass of thesame thickness Alternately, Gorilla Glass 2 canbe up to 20% thinner than Gorilla Glass, butjust as tough. While Gorilla Glass 3 has a modified glass composition for enhanced scratchresistance, reduced scratch visibility, and betteretained strength once a scratch occurs, it isstill a chemically strengthened alkali-aluminosilicate (Na2OAl2O3SiO2, K2OAl2O3SiO2or Li2OAl2O3SiO2) glass. The Knoop (microindentation) hardness of the glass is related tothe mole fractions of the primary oxides, but

the introduction of other oxides (e.g., MgOCaO, BaO, ZrO2, SrO) and additives can altethe fracture threshold.

Although Asahis 0.28-mm soda lime floatglass substrates for touch screens can bechemically strengthened,18they are not suitedfor use as cover glass. Aluminosilicate glasshas larger openings on the surface than sodalime glass, which allow the K+ions to penetratedeeper into the glass and create a deepercompression layer. The aluminum contentalso increases the hardness of the glass aftestrengthening. In 2012, Corning introducedfull sheet ion-exchanged (IOX-FS Concoreglasses for integrated-touch or one-glasssolution applications; Concore is a fusiondrawn aluminosilicate glass that is chemicallystrengthened and has indiumtinoxide (ITOsensor patterning underneath.19As shown inTable 1, most displays in the market use chemically tempered covers made of aluminosilicateglass, which are several times more resistant tosurface damage and critical stress than similarly strengthened soda lime glass. In additionthe images have higher visual quality and the

screens offer improved touch sensitivity thansoda lime screens.At the Society for Information Display Inter

national Symposium in June 2012, scientists atAsahi Glass presented their study to developguidelines and parameters for ideal chemicastrengthening. Using glass samples immersedin molten KNO3, NaNO3, and KNO3mixed salat 400450C for 612 h, they determined thatthe samples were optimally strengthenedwhen the compressive stress was maximizedand the depth of the compressive layer was

8/10/2019 Glass May 2013

5/6


A R T I C L E

kept in the 2030 m range (to maintain lowcentral tension in the glass interior low and toprevent strength degradation). Asahis newlydeveloped glass can acquire compressivestress of 1000 MPa at a DOL of over 30 minthe laboratory.20

Laser cutting of chemicallystrengthened glass

Traditionally, glass must be cut to size priorto thermal or chemical tempering because theadded strength vanishes within a small regionof the cut. The cut edge is weakened and theglass sheet can break or shatter easily. Lasercutting and scribing technologies have beendeveloped at Corning and Asahi to cut chemi-cally strengthened glass (e.g., ion-exchangedfull sheets) with a very deep compressive stresslayer.21,22At Corning, a small artificial flaw with

a depth that is larger than the DOL is first intro-duced on the glass surface or at the edge of theglass. Heat from a CO2laser is used to generatea transient tensile stress (~100 MPa on boro-silicate glass, for example) and propagate acrack initiated from the flaw along the cuttingpath, while a water or mist jet; air convection;or a jet of compressed nitrogen, helium, orsimilar gas is used for cooling. The methodenables debris-free separation of the glass withhigh edge strength and without spontaneousshattering.23,24

Manufacturers of touch panels are increas-ingly placing the touch sensor directly onthe chemically-strengthened cover glassinstead of laminating a separate touch panelstructure to the cover glass. Until recently,non-strengthened glass substrates were firstcut and finished to shape before they weretreated with ion exchange. The touch sensorswere then patterned separately onto eachindividual panel component. With progres-sively integrated touch functions and the trendtowards the adoption of the one-glass solu-tion, it is much more economical to pattern

multiple touch sensors onto a single GEN 5or GEN 6 (1300 mm 1100 mm or 1850 mm 1500 mm) sheet of chemically strengthenedsoda lime, borosilicate, aluminoborosilicate, oraluminosilicate glass such as IOX-FS Concore.These ion-exchanged glass sheets can vary inthickness from 0.5 to 1.5 mm and have multipletouch-screen ITO films applied on one side.An ion-exchanged glass sheet with the touch-screen films is then separated into multiplepieces via laser full separation, laser scribe-and-break, mechanical scribe-and-break, or

acid etching.With laser and traditional cutting techniques,

the cut edge of an individual glass plate afterseparation was not exposed to ion exchangeand is therefore subject to damage and pos-sible delayed failure (fatigue). Although the

edge can be protected by a coating afterseparation, it does not protect the glass fromdamage incurred by user handling. An addi-tional strengthening process can provide com-pression on the edge or edges of glass platesfor both damage and fatigue resistance afterthey are separated from the ion-exchangedmother glass sheet.25

The laser-cut edges of a plate can bestrengthened locally by ion exchange usingan alkali salt containing alkali metal ions largerthan the exchangeable alkali metal ions in theglass. The plate has two chemically strength-

ened faces and one or more exposed edgesthat are not chemically strengthened, whichprovide fresh material for ion exchange. Anedge of the cut sensor plate is inserted in abath of molten alkali metal salt (e.g., KNO3forexchanging with Na+ or Li+1ions in the glass)at a temperature between 400 and 600C for1060 min.25Alternately, the edges of the indi-vidual plate can be coated with a paste, suchas one consisting of KNO3, clay, and water. Theedge or edges with the K-containing mediumare then locally heated by convection, induc-tion, laser, or microwave radiation at tempera-tures between 330500C for 10 min to 7 h.25

Since touch sensor functions are sensitiveto high temperatures, the ITO touch patternfilms should not be subjected to temperaturesabove 200C. During the edge strengtheningprocess, both planar surfaces of the glass arekept at a temperature of

8/10/2019 Glass May 2013

6/6


A R T I C L E

References

1. M.Y.M. Lee, Glass Part 2: Thin-film Deposition on Glass for Advanced

Displays, Gases & Instrumentation (January/February 2013) pp. 2531.

2. Corning Gorilla Glass Product Information Sheet, Corning Incorporat-

ed, Corning, New York, USA; http://www.corninggorillaglass.com/sites/

all/files/Gorilla%20PI%20Sheet_July%202012_070912.pdf (accessed

April 18, 2013).

3. Corning Gorilla Glass 2 Product Information Sheet, Corning Incorporat-


all/files/GG2%20PI%20Sheet%20Rev%20b_050912.pdf(accessed April

18, 2013).

4. Corning Gorilla Glass 3 Product Information Sheet, Corning Incorporat-


all/files/Gorilla%20Glass%203_2320_PI%20Sheet%20RevD_040513.

pdf(accessed April 18, 2013).

5. Asahi Glass AGC Electronics Dragontrail Catalog; Asahi Glass Co., Tokyo,

Japan; http://dragontrail.agc.com/catalog/AGC_Dragontrail_en.pdf

(accessed April 18, 2013).

6. Nippon Electric Glass Specialty Glass for Chemical Strengthening, Nip-

pon Electric Glass, Shiga, Japan; http://www.neg.co.jp/EN/technology/

focus.html; http://www.neg.co.jp/EN/technology/cx01_e.flv (accessed

April 18, 2013). 7. Schott Xensation Cover Alumino-Silicate Glass for Capacitive Touch

Technologies, Schott Technical Glass Solutions GmbH, Jena, Germany;

http://www.us.schott.com/xensation/english/products/xcover.html


8. Schott Xensation Cover 3D Lithium Alumino-Silicate Glass for Capaci-

tive Touch Technologies, Schott Technical Glass Solutions GmbH, Jena,

Germany; http://www.us.schott.com/xensation/english/products/

cover.html?highlighted_text=xensation+Cover+3D(accessed April 18,

2013).

9. Schott Xensation Cover AG Alumino-Silicate Glass with Anti-Glare

for Capacitive Touch Technology, Schott Technical Glass Solutions

GmbH, Jena, Germany; http://www.us.schott.com/xensation/english/

products/xcover/anti-glare.html(accessed April 18, 2013).

10. M.Y.M. Lee, Gases for Glass Production, Special Treatment, and Coat-

ings, Gases & Instrumentation Newsletter (September 2012); http://www.gasesmag.com/20120910newsletter.php (accessed April 18,

2013).

11. D.W. Rinehart, Lithium containing ion exchange strengthened glass,

US patent US4156755, PPG Industries, Inc., Pittsburgh, Pennsylvania,

USA (May 29, 1979).

12. J. Zimmer, Novel Thin Glass for 3D Shaped Electronics Display Covers,

SID International Symposium, May 47, 2011, 2011 Digest of Technical

Papers, 57.1 (May 2011) pp. 833836.

13. T. Fujita, Cover Glass for Mobile Phone Display, Processing Technol-

ogy and Application of Glass Products (Garasu no Kakogijutu to Seini-

nouyou), Johokiko Co., Ltd, Japan (in Japanese, Jan 2009); http://www

matsunami-glass.co.jp/english/electron/special/writing04.htm


14. S. Karlsson, B. Jonson, C. Stlhandske, The technology of chemica

glass strengtheninga review, Glass Technol.: Eur. J. Glass Sci. Technol

A, Vol. 51, No. 2, (April 2010) pp. 4154.

15. K.K. Mallick, D. Holland, Strengthening of container glasses by ion

exchange dip coating, Journal of Non-Crystalline Solids, Vol. 351

(2005) pp. 25242536.

16. A.K. Varshneya, W.C. LaCourse, Technology of ion exchange strength

ening of glass: a review, Proc. Third Int. Conf. on Advances in Fusion and

Processing of Glass, 29 (1993) 36576.

17. Z, Baharav, Glass Enabling Future Touch Technology, Keynote Speech

IEEE 12th International Symposium on Consumer Electronics (ISCE)

June 46, 2012, Harrisburg Pennsylvania, USA (June 4, 2012); doi

10.1109/ISCE.2012.6241701

18. AGC Unveils Worlds Thinnest Glass Substrate for Touchscreens, Press

Release, April 21, 2011, Asahi Glass Co., Ltd., Tokyo, Japan; http://www

asahi-glass.com/english/news/2011/0421e.pdf (accessed April 18

2013).

19. Corning Concore Glass Product Information Sheet, Corning Incorpo

rated, Corning, New York, USA; http://www.corning.com/WorkArea

showcontent.aspx?id=43401(accessed April 18, 2013).20. K. Hayashi, S. Akiba, T. Sakagmi, S. Sawamura, T. Nakashima, H. Oh

kawa, Cover Glass for Mobile Devices, SID International Symposium

June 58, 2012, 2012 Digest of Technical Papers, (June 2012) pp

15841587P-157L.

21. T. Ikezaki, AGC glass technology solution to highly functional display

needs, SID 2012 International Symposium, June 58, 2012, Exhibito

Forum, June 5, 2012, Boston, Massachusetts, USA.

22. I. Saito, A. Koike, Y. Iwanaga, Y. Kobayashi, T. Iwasaki, Glass plate used in

chemical strengthening prior to laser cutting, WIPO Patent Application

WO/2012/096260, Asahi Glass Co., Ltd., Tokyo, Japan (Jul 19, 2012).

23. X. Li, S. Garner, Corning CO2 laser free-shape cutting of flexible glas

substrates, 31st International Congress on Applications of Lasers and

Electro-Optics, Anaheim, California, September 23-27, 2012.

24. X. Li, B.R. Vaddi, CO2laser scribe of chemically strengthened glass with

high surface compressive stress, Proc. SPIE 7920, Laser Applicationsin Microelectronic and Optoelectronic Manufacturing (LAMOM) XVI

79200U (February 21, 2011); doi:10.1117/12.874925

25. A.A. Abramov, S. Gomez, L.A. Moore, A.M. Streltsov, S. Tsuda, J.E. Walte

Local strengthening of glass by ion exchange, US Patent Application

Publication, US 2012/0216570 A1, Corning Incorporated, Corning, New

York, USA (August 30, 2012).

26. C.-M. Weng, T.-W. Chu, C.-N. Huang, F.-J. Wang, S.-L. Chou, I-W. Lee, C.-H

Cheng, Method of Strengthening glass plate, US Patent US824535 B2

Applied Vacuum Coating Technologies Co., Ltd., Avct Optical Electronic

Co., Ltd., Taiwan, (August 21, 2012).

glass may 2013

Documents