Characterization of Defects in Glasses and Coatingson Glasses by Microanalytical Techniques

Klaus Bange�, Hartmut M�uller, and Christine Strubel

Schott Glas, Research & Technology-Development, Hattenbergstraûe 10, 55122-Mainz, Germany

Abstract. The most relevant defects in glasses and

thin ®lms on glasses are categorized and investigated

by the appropriate microanalytical techniques. Knots,

which are local glassy inclusions, are described in

greater detail. The combination of EPMA/EDX and

LA-ICP-MS allow the determination of element con-

centrations in the defect down into the low ppm range,

thus ®nally enabling the identi®cation of a special

source of the defect from otherwise non distinguish-

able refractories. The results of analysis of stones and

striae are reported and defect sources are discussed.

Local defects in thin ®lms are characterized which can

be explained by high intrinsic compressive stress in the

®lms. Typical glass and thin ®lm defects are used to

illustrate the problem-solving process in industrial

labs.

Key words: Glass defects; TiO2 ®lms; EPMA; LA-ICP-MS; AFM.

In the production of all kinds of glasses and coatings

on glasses, various types of glass defects may occur.

In general, glass defects are not just a cosmetic fault.

They may endanger the function and usage of an

article, for instance by reducing the mechanical

strength, which may cause problems in tubes or

bottles, or by inducing stress, by which ¯at glasses

may be bent, or by producing optical inhomogeneities

which are not acceptable in TV screens. The defects

are undesired because they are very signi®cant in

economical terms: Products containing such defects

reduce the yield of produced articles and therefore

cause a loss of revenue.

But defects or failures in products appearing during

the application by the customer are undesired as well

because they impair the image of the producer. One of

the outstanding properties of the material glass, its

high transparency, creates problems in that way,

because each inhomogeneity is visible and easily

detectable by the human eye. Especially a certain

illumination makes the eye very sensitive and each

defect in the bulk material or at the surface becomes

visible.

Thin ®lms are deposited usually at the `̀ cold end''

of the glass production line, i.e. the coatings are

generated in general independently from the glass

production. In addition, sometimes the glass sub-

strates are stored for a longer period in unde®ned

atmosphere before coating. Various techniques are

applied for the ®lm preparation. Various physical

vapour deposition (PVD) or chemical vapour deposi-

tion (CVD) techniques and also sol-gel methods are

used. Each deposition method creates characteristic

thin-®lm properties but also very typical defects.

Many products made of glass are based on different

coating properties. Usually oxide ®lms are used to

attain the most important property of glass, transpar-

ency. Typical products are anti-re¯ecting systems, low

emissivity coating, re¯ective devices like mirrors,

cold light re¯ectors for lighting, interference ®lters

and so forth.

Compared to glasses, which have been produced for

over some thousand years and for which a lot of

experience exists, the techniques for depositing thin

®lms are fairly new. Especially over the last two

decades, the coating techniques have been consider-

ably advanced. Many highly sophisticated products

Mikrochim. Acta 132, 493±503 (2000)

� To whom correspondence should be addressed

are based on deposition processes which are non-

stable in thermodynamic aspects and some are not

understood so far. Additional challenges are opti-

mized cleaning procedures for different types of

glasses, where systematic knowledge is also lacking.

Against this background, a multi-method microana-

lytical approach is generally necessary for an ef®cient

characterization of local defects in glasses and thin

®lms which forms the data base for the elimination of

these defects.

In this communication some relevant types of

defects in glasses and coated glasses are summarized

and different methods for ef®cient microdefect

analysis are presented. The problem-solving process

typically used in industrial labs is exempli®ed on

numerous defects in glasses and thin ®lms.

Defects in Glasses and Coatings on Glasses

Glass defects may be divided into the following

categories [1, 2, 3]:

± Knots and striae are glassy inclusions, i.e. glasses

of different composition in the surrounding bulk

glass. Knots and striae are often connected.

± Stones describe compact, larger crystalline inclu-

sions in glass. The crystalline defects may be

caused by the crystallization of the glass itself or

by crystal formation from reactions with refractory

material or contamination, or from undissolved

raw materials and so on.

± Metallic inclusions originate either from metals

used in the melting units (e.g. electrode materials),

from refractory materials, from batch contamina-

tion or from different types of reduction processes

in the glass tanks.

± Bubbles are hollow spaces within the glass ®lled

with gas. They are one of the most common types

of defect. The bubbles contain one or more gases

such as oxygen, carbon dioxide, sulfur dioxide,

argon, carbon monoxide and hydrogen.

The various types of glass defects are generated in

different steps and at different locations in the

production process. They may originate from the

raw materials used, or may be induced during the

melting process in the tank, or during re®ning, or

evoked by cooling and manufacturing processes, but

they can also be created during the application. Many

defects originate in the melting units. Knots and striae

may be caused by the evaporation of glass compo-

nents, which leads to compositional inhomogeneities

in the free surface, by reactions of the hot and

aggressive glass melt with different types of refractory

materials, or by insuf®cient homogenization. Bubbles

are mainly created by the decomposition of raw

material from ®ning or reboil processes, interactions

with refractory materials, or with combustion air, but

also from contaminations. Different types of defects

are produced also during the processing of the glass.

Hot-forming processes like pressing, drawing or

¯oating create characteristic faults in the region near

the surfaces. Inappropriate cooling procedures may

lead to phase separations. The ®nishing process may

also lead to faults in glasses.

Defects in coatings on glasses may be divided

roughly into the following groups [4, 5]:

± Film impurities are fairly common phenomena.

They produce, for example, changes in the optical

properties.

± Film delaminations from the substrate are cata-

strophic defects because the product is destroyed

in general. Delamination problems can be caused

by weak adhesion in the ®lm/glass surface inter-

face or by a too high ®lm stress.

± Local inhomogeneities in ®lms are sometimes

observed. They originate from unfavorable deposi-

tion conditions.

± Undesired crystallization may be caused by high

temperature during the deposition or application or

by high stress [6] and can create optical losses.

The different types of defects in coatings may be

created at different steps of the production process.

They can originate from surface impurities which may

result from an insuf®cient cleaning of the glass

substrate, from the deposition materials used, and

more generally, from the selected deposition condi-

tions. But the unusual physical and chemical proper-

ties of thin ®lms may also be changed during the

application, for example by aggressive environmental

conditions like in dishwashers.

Problem-Oriented Use of Microanalytical

Techniques for Defect Analysis

The quality of analytical and measured information

produced in a lab depends strongly on the quality of

the information which is available from the client.

This is the basis for managing and de®ning the

494 K. Bange et al.

analytical problem, the design of the analytical

strategy and for adjusting its features so that the

results meet the required objectives. In that way, it is

important to have a clear understanding of analytical

properties such as accuracy and sample representa-

tiveness, and of their relationship to quality in the

analytical process and to the results. Since these two

facets of the analytical process are frequently

inconsistently dealt with, some confusion is created

in lab work and in communication with service users.

A hierarchy exists for analytical properties. Their

mutual relationship as well as social and economic

aspects have to be considered for a given analytical

problem which effects the dominance of some

properties over the others [7]. Dealing with analytical

properties in isolation when designing strategies to

address speci®c problems is often inadequate and in

general produces erroneous results.

Different analytical tools are applied for the

investigation of defects (Fig. 1). Geometry and

morphology of glass defects are ®rst determined by

means of light microscopy (LiMi). More detailed

information is obtained by electron probe microana-

lysis (EPMA). EPMA is the method of choice,

especially to analyze small pieces of material at high

magni®cation. The focused beam is either scanned

across an area or is stationary. Topographic informa-

tion is taken from the secondary electrons, while

backscattered electron or characteristic X-rays give

chemical information.

The composition of solid glass defects is in general

determined by EPMA/EDX (energy dispersive X-ray

analysis) and and/or EPMA/ WDX (wave length

dispersive X-ray analysis); the determination of

boron, if necessary, can be done by EPMA/WDX.

For these techniques the defects are prepared in such a

way that the object to be analyzed is placed in the

sample surface region. This type of preparation can be

very dif®cult and needs a lot of experience. In general

the composition of the defects is compared with the

defect-free glass composition. Since the equipment is

fairly expensive and the investigations are sometimes

very time-consuming other methods are being tested

at the moment to obtain comparable and/or additional

information. Especially laser techniques which allow

the detection in small spots by focusing seem to have

a high potential for determining compositions in the

low elemental concentration range. In particular trace

elements can be analyzed by laser ablation induc-

tively-coupled plasma mass spectroscopy (LA-ICP-

MS) [8 ± 10] or by laser-induced breakdown spectro-

scopy (LIBS) [15 ± 17].

The composition of gases in bubbles can be

determined by gas chromatography (GC) or mass

spectrometry (MS). For this type of destructive

analysis the bubbles are prepared in such a way that

they can be broken in a chamber with carrier gas or in

vacuum. To obtain statistics often up to 50 bubbles

have to be analyzed in one day. A non-destructive

analysis is Raman spectroscopy. This method is not

used for routine investigations because the measure-

ments are very time-consuming.

Various analytical tools are used to investigate

defects in thin ®lms on glass. The acronyms of the

Fig. 1. Detection and analysis of glassdefects with different sizes and con-centrations

Characterization of Defects in Glasses and Coatings on Glasses by Microanalytical Techniques 495

most relevant techniques are summarized in Table 1.

Details of the methods are described in ref. 4 (and

references therein). Powerful characterization meth-

ods in general use photons, electrons, and ions as

probes, but also forces, for example in atomic force

microscopy (AFM). The different probes interact with

a ®xed volume of the samples and create emitting

photons, electrons, or ions, which are sampled by a

detector. Photons are used as probe in optical

spectroscopy to determine the transmittance (T), the

re¯ectance (R), the absorbance (A), in infrared

spectroscopy (IR), Raman spectroscopy, angle

resolved scattering (ARS), total integrated scattering

(TIS), photothermal de¯ection techniques (PTD),

laser calorimetry (LC), grazing incidence X-ray

re¯ectivity (GIXR), small angle X-ray scattering

(SAXS), grazing incidence X-ray ¯uorescence analy-

sis (GIXF), X-ray diffraction (XRD), extended X-ray

absorption ®ne structure (EXAFS), X-ray absorption

near-edge ®ne structure (XANES) and in electron

spectroscopy for chemical analysis (ESCA). Electrons

are the probe in transmission electron microscopy

(TEM), EDX, WDX, electron diffraction (ED) and

Auger electron spectroscopy (AES). Ions are applied

as primary particles in secondary-ion mass spectro-

scopy (SIMS) or time-of-¯ight (TOF) SIMS, second-

ary-neutral mass spectroscopy (SNMS), ion beam

spectrochemical analysis (IBSCA), nuclear reaction

analysis (NRA), and Rutherford backscattering (RBS).

The different analytical tools can support also the

development of products, the optimization of pro-

cesses or the control of production but they are most

important for trouble-shooting. For this, a special

approach, the problem-solving process has to be

applied, because the instruments used in general are

fairly expensive and the analysis is highly complex

and time-consuming [11]. Therefore the ®rst step, the

de®nition of the problem, forms the basis for

successful defect analysis. Only well-de®ned pro-

blems and samples allow an ef®cient application of

the different techniques. A selection of samples,

classi®ed according to their signi®cance, is the next

point. Moreover, a critical assessment of the potentials

of the available methods with regard to the de®ned

working hypothesis and the problems to be investi-

gated is indispensable for a problem-oriented analysis,

i.e. the selection of the most powerful instrument is

another key question to be answered. This approach,

which is typical for industrial labs, is exempli®ed in

Table 1. For thin ®lm problems some characteristic

®lm properties are connected with methods that give

the desired information in principle [11]. Obviously,

different methods can be applied to investigate a

certain problem. The ®lm composition, for example,

can be analyzed by SIMS, IBSCA, SNMS, RBS,

NRA, ESCA, and AES, but other more sophisticated

techniques can be used as well.

Each method for defect analysis has its merits, but

also its shortcomings. Both should be known in detail

to select the appropriate method for a particular object

of investigation. The demands on the methods are

fairly high. For a suf®cient characterization of defects

the techniques should detect all elements with high

lateral resolution, well-de®ned and preferably low

information depth, and high element sensitivity. In

addition, they should give information about the state

of oxidation and the binding state of elements, and

sometimes they should work non-destructively. Often

a quantitative determination of elements and com-

pounds is desired, i.e. the matrix effect should be

negligible or corrigible. Owing to the high complex-

ity, a multi-method approach is often necessary for a

powerful description of defects [11].

Table 1. Problem-oriented thin ®lm analysis

Film properties Methods

MacroscopicDensity GIXR, RBSStress BendingAdhesion Micro indenterOptical quantities (n, k) T, R, AScattering TIS, ARS, PTDHardness milling, scratching, etc.Thickness R, T, A, TEM, GIXR, stylusThermal conductivity PTD, LC

MicroscopicComposition (TOF-) SIMS, IBSCA, SNMS, RBS,

NRA, ESCA, AES, EDX, WDX, GIXF-surface composition: ESCA, AES, GIXF, TOF-SIMS-®lm impurities: SIMS, IBSCA, SNMS, TOF-SIMS-element depth pro®les: SIMS, IBSCA, SNMS, ESCA, AES-®lm composition: EDX, WDX, RBS, NRA-interface composition: TEM/EDX, depth-pro®ling

State of oxidation ESCA, AES, SIMS, XANESStructure XRD, ED, Raman, IR, SAXS, EXAFSRoughness of surfaces AFM, TEM, GIXR, microscopy

(Normarski, Fizeau, Mirau, with lightinterference)

Formation of interface TEM, GIXR

496 K. Bange et al.

As described above, in principle a broad spectrum

of analytical tools is available to obtain the different

types of information. But especially oxidic materials

exhibit fairly unusual characteristics which are a

challenge for the methods because special problems

arise in the application to oxides. While an ideal

object for the methods of analysis is electrically

conductive, possesses a smooth and plain surface in

suf®ciently large dimension, and consists of a single

homogeneous phase, defects in oxides exhibit nearly

opposite properties. Oxides are, in general, very good

insulators with occupied valence bands and empty

conduction bands. Most band gaps of oxides have a

width of some electron volts, whereby the materials

are transparent in the visible spectrum of light. As

described, most methods use charged particles (ions,

electrons) as a probe (primary beam), but also the

secondary beam can consist of charged species. Both

can induce charging effects on non-conductive

materials, which often leads to errors in the measured

data and to misinterpretations, especially in electron

spectroscopy [12].

Results and Discussion

Glass defects are detected in general by means of

optical techniques. The example of a knot is

illustrated in the following by a defect in TV screen

glass. Knots always distort the TV image, even if they

are very small and barely visible with the naked eye.

Results are described and discussed in the following

for optical microscopy (Fig. 2), for EPMA (Figs. 3±5)

and LA-ICP-MS investigations (Fig. 6) on a polished

cross section.

In the image from the optical microscope the

inhomogeneity in the glass is easily recognized. The

differences in contrast suggest variation in density, i.e.

the glassy inclusion possesses a composition different

from that of the surrounding material. The BSE image

of the defect is shown in Fig. 3. Similar information on

the structure of the knot can be obtained as from Fig. 2.

The line running through the EPMA image

designates the location of a quantitative linescan

across the knot. This is performed with an analysis

®eld of 20� 20mm2, accelerating voltage 20 kV, beam

current 1.3 nA and an analysis time of 100 s per

analyzed ®eld. The stage was stepping the sample

underneath the beam scanning the 20� 20mm2 ®eld.

Quanti®cation was carried out against standards,

using the ZAF interelement correction procedure.

Figure 4 exhibits a linescan across the knot cross

section. Compared to the surrounding glass matrix,

the concentration of Al2O3 is increased while the glass

component BaO is decreased. The difference pro®le

(element composition of defect minus defect-free

glass) depicted in Fig. 5 makes this effect more

obvious. Some glass elements show an increase in

concentration in the defect area (Al2O3, K2O) while

for others the content is decreased (SiO2, BaO).

Fig. 2. Knot in a glass (optical microscope, transmitted light)

Fig. 3. BSE image of a knot

Characterization of Defects in Glasses and Coatings on Glasses by Microanalytical Techniques 497

Many defects can be explained on the basis of this

information, but some defects need additional data to

be traced back to the origin. Especially the impurities

and trace elements in the knots are sometimes very

helpful. These element concentrations can be

detected, e.g. by LA-ICP-MS. Figure 6 exhibits a

longitudinal section of the knot, shown in Fig. 2, after

the laser shooting during the LA-ICP-MS investiga-

tions. The laser beam with a wavelength of 248 nm

has been focussed into the center of the knot. Figure 6

shows that the knot has not been crossed by the laser

beam and only material of the defect volume is

analyzed.

Fig. 4. Quantitative linescan across the knot cross section of Fig. 3

Fig. 5. Quantitative differences of knot glass to matrix glass ofFig. 4

Fig. 6. Longitudinal section of knot after LA-ICP-MS investigation

498 K. Bange et al.

The crater size is 200 mm in diameter. The amount

of laser shots which can be directed to the sample

surface correlates with the measuring time of the ICP-

MS system. In this case 1000 single shots with a

frequency of 10 Hz have been emitted.

Table 2 depicts results of the LA-ICP-MS measure-

ment. As seen from the linescan of EPMA the LA-

ICP-MS underpins the examination and detects an

enrichment of Al2O3 and K2O. The content of SiO2

and BaO is decreased. Moreover, the method is

sensitive enough to analyze rare-earth elements in the

ppm-range. Elements like Ga, Y, Gd, Dy, Er, Yb, Lu,

Th, U are increased compared to the surrounding glass

matrix, but La and Nd are depleted.

The formation of knots or striae be due to many

causes, such as:

± Incomplete dissolution of batch materials, mostly

quartz.

± Reactions with refractories in direct contact with

the glass.

± Reaction products which have accumulated on the

bottom of the tank.

± Volatilization of components from the glass melt,

leaving a highly viscous glass layer on the melt

surface.

± Reaction of these volatilized components with

refractories above the glass melt, resulting in

melting and run-offs or drips.

± Contaminations in batch material with highly

refractive properties and others.

The depicted EDX concentration pro®les are

typical for knots stemming from corrosion of

alumina-zirconia-silicate (AZS) material. This is

indicated by the very high alumina content in the

defect and in addition by the enrichment of K2O.

ZrO2, which is present in the matrix glass, shows

depletion in one part of the knot, enrichment in

another part (at approx. 800 mm). All other glass

components are depleted across the whole cross

section. The dip in the ZrO2-concentration (below

the level of the matrix glass, although AZS contains

large amounts of ZrO2), can be explained by the very

high alumina content in this part of the knot, since

with rising Al2O3-content the solubility of ZrO2

decreases.

In general, different types of refractory materials

are used in different parts of glass melting tanks. The

main components of the refractory materials are

identical while differences exist in minor species.

Additional investigations on several refractory mate-

rials with laser techniques like LA-ICP-MS or LIBS

give information e.g. on the presence of lanthanides

and actinides. These elements can be used to

differentiate between the location where the knots

are created. For example the element Y is present only

in one type of AZS material. In that way the reactions

of the refractories in direct contact with the glass

combined with the knowledge of traces in a knot can

help to identify the initiator material. This leads to the

conclusion that the investigated knot is due to one

special AZS material.

The schematic representation in Fig. 7 shows a

different defect type i.e. the striae in a drinking glass,

which appear as distortions of the surface. This defect

is a cosmetic fault but customers will object. Since the

defect is located in the bulk glass for analysis the

defect has to prepared to the surface. Cutting this

glass perpendicular to the surface in the defect area at

the indicated level reveals strong striae in the volume

of the glass which is depicted in Fig. 8. The

inhomogeneity is visible in the optical microscope.

Fig. 7. Schematic of drinking glass with striae

Table 2. LA-ICP-MS results of a knot (enrichment or depletioncompared to the defect-free bulk glass) of Figs. 2, 3, 4, 5 and 6

Enrichment: Al2O3, K2O, Ga, Y, Gd, Dy, Er, Yb, Lu,Th, U

Depletion: SiO2, BaO, La, Nd

Characterization of Defects in Glasses and Coatings on Glasses by Microanalytical Techniques 499

Fig. 8. Striae in drinking glass (optical microscope, transmitted light)

Fig. 9. BSE-image of striae in drinking glass

Fig. 10. EDX difference spectrum ofstriae (knot glass minus matrix glass)

500 K. Bange et al.

The same micro-area can be investigated with EPMA.

The BSE image is depicted in Fig. 9 at higher

magni®cation exhibiting more details on the structure

of the striae.

The variation in the contrast of the BSE image

already indicates strong deviations in the composition

of the striae from that of the surrounding matrix glass.

This is supported by the EDX spectrum in Fig. 10,

where the spectrum of the matrix glass has been

subtracted from the spectrum of the striae. A strong

enrichment of alumina and Na2O is obtained, while

the other components are depleted. In this case, there

is no ZrO2 present, which leads to the conclusion that

the corrosion of a higly Al2O3-containing refractory

without any ZrO2 is the source for this striae.

Corrosion of high-alumina refractories may not

only lead to striae, but often results in the formation of

stones. The name `̀ stone'' is given to compact,

crystalline inclusions in glass, and while sometimes

the type of stone can already be identi®ed with the

optical microscope, the investigation with EPMA

gives further insight into the mechanism of

formation.

The following series of images (Fig. 11 a ± e) from

a stone found in a technical glass show the appearance

of the stone cross section at the interface stone/glass

(the matrix glass is in the lower left-hand corner of the

images), and also the distribution of elements.

The parts toward the center of the stone (right-hand

side of the images) contain lath-like crystals of

corundum, pure Al2O3, whereas the crystals growing

toward the edge are composed of Al2O3, BaO and also

K2O. The glassy regions between the crystalline

phases consist of SiO2 and Al2O3 with high amounts

of K2O and BaO. Here, an Al2O3-SiO2-refractory was

in®ltrated and attacked by the glass, K2O and BaO

being the mobile and corrosive components, also

resulting in the formation of new, secondary crystal-

line phases (K-Ba-Al-O crystals). The reason for the

occurrence of this type of stone is either the corrosion

of an Al2O3-SiO2-refractory in certain parts of the

glass tank (if such materials are present) or the

contamination of the batch with such material.

Titania (TiO2) ®lms on glasses are used in many

product applications. They are manufactured by a

number of different techniques which create in the

Fig. 11. BSE-image of a polished cross section of a stone (a), X-ray map of Al (b), Si (c), K (d), Ba (e)

Characterization of Defects in Glasses and Coatings on Glasses by Microanalytical Techniques 501

high refractive material very different physical and

chemical properties. Typical products are based on

single ®lms or multilayer systems.

Local defects in such ®lms result immediately in

optical losses of the products, i.e. the product in

question does not ful®ll the quality requirements.

Particularly in the case of high-quality optical parts

such products are outside speci®cation. The surface of

a TiO2 ®lm on glass is shown in Fig. 12. Many small

local defects are recognizable in the AFM image. The

detailed information available not only serves to

identify the faults but also that it has the purpose of

understanding the cause of the defects and helps

removing it. The image shows that in the amorphous

thin ®lm non-regular holes exist. In the three-

dimensional representation and at large magni®cation

a conical shape of the defect is observed. A detailed

investigation of a cross section of the sidewall

material of the defects with electron diffraction

exhibits that this area consists of a rutile phase which

is embedded in an amorphous titania matrix [13].

This type of local defect in TiO2 ®lms is only

observed in ®lms deposited with techniques which are

able to create high ®lm densities. It is reported in the

literature that ®lms with high density create high

compressive stress [14]. This leads to the conclusion

that high compressive stresses initiate a spontaneous

rutile formation with a higher local density than the

amorphous phase which is combined with the creation

of that type of local defect.

Conclusion

It is demonstrated that microanalytical techniques in

combination with the problem-solving process are

important and excellent tools for the detection and

investigation of defects in glasses and coatings on

glasses. The results obtained by these techniques and

the knowledge of processes create the basis for defect

elimination. This approach is illustrated for various

defect types typical in glass industry. Basic mechan-

isms for defect formation are described for glassy

inclusions, i.e. knots and striae, and for crystalline

defects in glasses, i.e. stones local defects in TiO2

®lms are explained by high density and high com-

pressive stress in the ®lm material which can be created

with modern ion-assisted deposition techniques.

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Characterization of Defects in Glasses and Coatings on Glasses by Microanalytical Techniques 503