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Solid State Ionics 172 (2004) 335–340

Interfacial reaction in anodically bonded joints during receiving

reverse voltage

Makoto Takahashi*, Hiroki Yasuda1, Kenji Ikeuchi1

Department of Welding Mechanism, Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Received 9 November 2003; received in revised form 2 February 2004; accepted 23 February 2004

Abstract

A voltage in the opposite direction to that of anodic bonding was applied to anodically bonded joints of Fe and Kovar alloys to

borosilicate glass at a temperature at which thermal diffusion of the alkali ions in the glass was activated. Disjunction of the bond

interfaces occurred in the Kovar/glass joints. On the other hand, the Fe/glass joints retained cohesion of their bond interface after a long

application of the reverse voltage. In an as-bonded Kovar alloy/glass joint, a reaction layer of a crystalline Fe oxide was found at the bond

interface. The content of Fe in Kovar alloy adjacent to the bond interface decreased. The oxide layer was formed by the reaction between

Fe from the Kovar alloy and O from the alkali ion depletion layer in the glass. The reaction layer in an Fe/glass joint was of amorphous

Fe–Si oxide. This layer was much thicker than the reaction layer in the Kovar/glass joint. After application of the reverse voltage, Fe in

the amorphous oxide layer in Fe/glass was substituted by alkali ions. It was suggested that this process mitigated concentration of alkali

ions at the bond interface and prevented disjunction of Fe/glass joints by the reverse voltage.

D 2004 Elsevier B.V. All rights reserved.

PACS: 61.14.-x; 61.16.Bg; 66.10.Ed; 68.35.-p; 68.35.Ct; 68.35.Fx

Keywords: Anodic bonding; Disjunction of joints; Borosilicate glass; Iron; Kovar alloy; Ionic conduction; Alkali ion; Interfacial reaction

1. Introduction

Anodic bonding is a method for bonding metal or

semiconductor to glass containing alkali ions by applying

a D.C. voltage between them with the metal side being

anode. A glass sheet and the metal part to be joined are

stacked on a conductor (Fig. 1). The materials are heated to

a bonding temperature of 500–600 K to activate the

diffusion of alkali ions in the glass. The metal to be bonded

is connected to the positive terminal of a D.C. power

source, and the other conductor is connected to the negative

terminal. A bonding voltage of 100–1000 V is applied to

them. The alkali ions in the glass drift toward the cathode

side, and an alkali ion depletion layer forms in the glass

near the joint surface. This layer has a strong negative

charge because of the presence of non-bridging oxygen (O)

0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ssi.2004.02.071

* Corresponding author. Tel.: +81-6-6879-8664; fax: +81-6-6879-8689.

E-mail addresses: makotot@jwri.osaka-u.ac.jp (M. Takahashi),

yasuty@jwri.osaka-u.ac.jp (H. Yasuda), ikeuchi@jwri.osaka-u.ac.jp

(K. Ikeuchi).1 Phone: +81-6-6879-8664; fax: +816-6879-8689.

anions that lose their bonds with the alkali ion. An electro-

static force acting between this charge and the charge

appearing on the surface of the anode metal brings the

glass and the metal in intimate contact, and a permanent

bond is achieved by the oxidation of the metal surface by

excess O ions in the alkali depletion layer [1]. The bonding

temperature for anodic bonding is relatively low and

deformation of glass does not occur in this temperature

range [2]. This is the advantage that makes anodic bonding

indispensable for bonding in the manufacturing of micro-

machines and microsensors.

The application of voltage to an anodically bonded joint

in the opposite direction to that for bonding causes

degradations of joint: disjunction of the bond interface,

cracks in the glass, discoloration of the bond interface, and

so on [3–6]. These phenomena have harmful effects on the

reliability of anodically bonded joints in case they receive

reverse voltages in the course of manufacturing or during

use. On the other hand, if we can prevent the degradation

of glass and bring about only the disjunction of joints, this

phenomenon can be applied to the dismantlement of

exhausted products and the recycling of materials. The

phenomena accompanying the application of reverse

Fig. 1. The principle of anodic bonding.

M. Takahashi et al. / Solid State Ionics 172 (2004) 335–340336

voltage change with the materials that are joined to glass

[5]. Therefore, the knowledge that leads to the control of

these phenomena can be acquired through investigations of

the phenomena in the joints of combinations of materials

that have different susceptibilities to a reverse voltage. It

was found that the disjunction of anodically bonded joints

of the Kovar alloy with borosilicate glass was caused by

the application of a reverse voltage [4]. Kovar alloy is

composed mainly of Fe, Ni, and Co, and the role of each

component on the disjunction phenomenon has not been

clarified. In this study, a reverse voltage was applied to

anodically bonded joints of iron, the most major compo-

nent of Kovar alloy, to borosilicate glass, and the effects

on the bond interface were investigated. The reverse

voltage causes a migration of alkali ions towards the bond

interface. Reactions between the joint structure and the

alkali ions that accumulate on the bond interface are

important factors in the phenomena that are caused by

the reverse voltage. Therefore, microstructures around the

bond interface in the joint receiving the reverse voltage

were thoroughly investigated by transmission electron

microscopy (TEM) and energy dispersive X-ray spectros-

copy (EDS).

Table 1

Chemical composition of Kovar alloy

C Co Fe Mn Ni Si

Composition (mass%) 0.003 16.2 53.65 0.45 29.6 0.1

2. Experimental

The experimental materials were Kovar alloy and bo-

rosilicate glass. The chemical composition of the Kovar

alloy is shown in Table 1. The borosilicate glass, Matsu-

nami #0700, contained alkali ions of Na, K, and a small

dose of Li. The expansion coefficient of this glass was

similar to that of Kovar alloy in a temperature range of

room temperature to f 800 K. The glass was supplied as

disks of 1 mm in thickness and 25 mm diameter. The

surfaces of the glass disk were polished. The Kovar alloy

was cut into disks of 2 mm in thickness and 20 mm in

diameter. One side of the disk was given a glossy finish by

grinding with metallographic paper and buffing with 0.25

Am diamond paste. In order to produce an Fe/glass

anodically bonded interface, the buffed surface of the

Kovar disk was coated with a pure Fe layer by RF

sputtering at 150 W of RF power for 5400 s in a 2 Pa

Ar atmosphere, and this coated surface was bonded to the

glass. The coated surface was buffed with diamond paste

for a short time to remove contamination and roughness of

the Fe layer just before the bonding. The roughness of the

surface of an as-deposited Fe layer was f 30 nm (Rmax),

and it was reduced to f 7 nm after buffing. The joint-

ability of the Fe layer was considerably improved with this

treatment. When the glass disk received the voltage for

bonding and the application of reverse voltage, the rear

surface of the disk was coated with conductive carbon

paint to make the electric potential on this surface uniform.

The atmosphere of the anodic bonding was a vacuum of

f 10� 4 Pa. The bonding temperature (Tb) was 613 K, the

bonding voltage (Vb) was 500 V, and the bonding time (tb)

was 300 s. Joints of the Kovar alloy, without Fe coating to

the glass were produced for comparison. After bonding,

joint surfaces of all the joints were checked by visual

examination through the glass.

A reverse voltage (Vr) of 500 V was applied to the joints

at 613 K (Tr) in a vacuum of f 10� 4 Pa. The application

time of the voltage (tr) was changed from 120 to 2400 s.

After that, visual examinations of the bond interfaces were

carried out.

The as-bonded interfaces of the Fe/glass joint and the

Kovar alloy/glass joint were observed by TEM. These

joints were cut in a direction perpendicular to the bond

interfaces, and the cross sections were ground with metal-

lographic paper and buffed. A thin foil for TEM was cut

out from the bond interface region on the cross section,

with HITACHI FB-2000S focused ion beam (FIB) pro-

cessing system, equipped with a micro-sampling system.

With this system, the TEM specimens can be produced

from the surface of bulk specimens in a perfectly dry

process.

The bond interface areas in anodically bonded joints

receiving reverse voltage were rich in alkali ions. The

alkali ions have high reactivity with water. Hence, it was

difficult to prepare TEM specimens from those areas by

usual metallographic techniques in which specimens were

exposed to water. From those areas, TEM specimens were

prepared in the following way. Pure Fe was deposited on

the glass by RF sputtering at 150 W of RF power for 5400

s. The Fe layer and the glass substrate were anodically

bonded and the reverse voltage was applied to the joint.

After that, thin foil specimens traversing the interface

between the Fe layer and the glass were cut out with the

micro-sampling FIB system.

Fig. 2. Anodically bonded Kovar alloy/borosilicate glass joints as-bonded (a), receiving the reverse voltage for 300 (b), 420 (c), and 600 s (d).

M. Takahashi et al. / Solid State Ionics 172 (2004) 335–340 337

The TEM observations and EDS analyses were carried

out with a JEOL JEM-2010 transmission electron micro-

scope operating at 200 kV.

3. Results and discussion

The macroscopic changes in bond interfaces of anodi-

cally bonded Kovar alloy/glass joints with the application of

the reverse voltage are shown in Fig. 2. In the as-bonded

joint (Fig. 2a), intimate contact was achieved on the whole

joint interface. In the joint receiving the reverse voltage for

300 s (Fig. 2b), obvious disjunction could not be detected by

visual inspection. After application of the reverse voltage

for 420 s (Fig. 2c), many small exfoliations appeared on the

joint interface. In the joint receiving the reverse voltage for

600 s (Fig. 2d), the exfoliations aggregated and large

disjunction areas were formed in the central part of the

joint interface. No crack in the glass was found in these

joints.

The appearances of the Fe/glass joints receiving the

reverse voltage are shown in Fig. 3. Good bonding was

achieved on the whole joint interface in the as-bonded joint

(Fig. 3a). Even in the joint receiving the reverse voltage for

2400 s (Fig. 3d), no disjunction was detected. The joint

faults that were found in the joints receiving the reverse

voltage for 600 and 2400 s (Fig. 3b and d) had already

existed before the application of the reverse voltage.

The application of the reverse voltage to the Kovar alloy/

borosilicate glass joints caused disjunction of bond inter-

Fig. 3. Anodically bonded Fe/borosilicate glass joints as-bonded (a), re

faces. By contrast, the Fe/borosilicate glass joints were

retained with no exfoliations after a long application of

the reverse voltage.

The electric currents that flowed through the joints

during the anodic bonding process and the application of

the reverse voltage, and the charges that the electric currents

transferred were shown in Fig. 4 as functions of the voltage

application time for a Kovar alloy/glass joint and an Fe/

glass joint. A significant difference was not observed in the

profiles of the currents that flowed through the Kovar alloy/

glass joint and the Fe/glass joint during bonding (Fig. 4a). A

decrease in the currents with time was caused by the growth

of the alkali ion depletion layers. The alkali ion depletion

layer was poor in current carrier (i.e. alkali ions), and its

specific resistance was higher than that of the rest of the

glass. In the application of the reverse voltage, the current

that flowed through the Fe/glass joint was large at first, and

it decreased more rapidly than that flowing through the

Kovar alloy/glass joint (Fig. 4b). After all, the difference in

the transferred charges of the two joints was not large in the

application of the reverse voltage for 0–1200 s. The

transferred charge represented the amount of alkali ions

drifted in the glass. Therefore, the difference in the behav-

iors of the Kovar alloy/glass joints and the Fe/glass joints in

the application of the reverse voltage could not be explained

by the difference in amount of the alkali ions that drifted in

the joints.

In Fig. 5, the microstructure around the bond interface in

an as-bonded Kovar/glass joint observed by TEM and the

results from the EDS point analyses are shown. At the bond

ceiving the reverse voltage for 600 (b), 1200 (c), and 2400 s (d).

Fig. 4. The currents that flowed through an Fe/glass joint and a Kovar alloy/glass joint, and the charges that the currents transferred during bonding (a) and the

application of reverse voltage (b) as functions of voltage application time.

M. Takahashi et al. / Solid State Ionics 172 (2004) 335–340338

interface, a reaction layer 5–10 nm thick was found. By

EDS analysis, it was found that this layer contained Fe of

high concentration (point ‘B’ in Fig. 5a). This layer

contained much O also (in the table in Fig. 5 the content

of O is omitted because quantification of O by EDS analysis

was difficult; but in the X-ray spectrum from the layer, a

large peak of the characteristic X-rays of O appeared). In the

selected area electron diffraction (SAD) pattern from the

area around the interface, the reflections from the layer were

found. These results showed that this layer consisted of a

crystalline Fe oxide. Analyses of the Kovar alloy showed

that in the area in the Kovar alloy adjacent to the bond

interface (point ‘C’ in Fig. 5a), the content of Fe was lower

than that in the inside area (point ‘D’ in Fig. 5a). It was

thought that Fe in the area near the interface was oxidized

preferentially by excess O ions from the alkali depletion

layer in the glass and formed the Fe oxide layer.

The results from TEM observations of the microstructure

around the bond interface in an as-bonded Fe/glass joint and

EDS point analyses are shown in Fig. 6. At the bond

interface, a reaction layer was found. This layer appeared

as if the interface was lined with particles 20–30 nm in size.

By EDS, it was found that this layer contained Fe, Si, and O

of high concentration. In the SAD pattern from the area

around the interface, no reflection from the layer was found,

and in the TEM image this layer did not have diffraction

contrast. These results showed that this layer consisted of an

amorphous Fe–Si complex oxide. It was thought that the Fe

atoms from the Fe layer penetrated into the glass, and

reacted with excess O ions in the alkali depletion layer to

form the complex oxide layer.

The microstructure around the bond interface in an

anodically bonded joint of the Fe layer deposited on the

glass with the glass substrate and its change with the

application of the reverse voltage are shown in Fig. 7. The

TEM micrograph of the interfacial area in an as-bonded

joint (Tb = 613 K, the Vb = 500 V, and tb = 300 s) is shown in

Fig. 7a. A reaction layer f 40 nm thick was found between

Fig. 5. The microstructure around the bond interface in an anodically

bonded Kovar alloy/borosilicate glass joint as bonded (a) and the chemical

compositions derived by EDS at points indicated in a (b). From points ‘A’

and ‘B’, strong characteristic X-rays of oxygen were detected by EDS other

than the elements listed in b.

Fig. 6. The microstructure around the bond interface in an anodically

bonded Fe/borosilicate glass joint as bonded (a) and the chemical

compositions derived by EDS at points indicated in a (b). From points

‘A’ and ‘B’, strong characteristic X-rays of oxygen were detected by EDS

other than the elements listed in b.

M. Takahashi et al. / Solid State Ionics 172 (2004) 335–340 339

the Fe layer and the glass. By EDS, Fe, Si, and O of high

concentration were detected in this layer, and the SAD

pattern taken from this layer was a halo pattern. This layer

consisted of an amorphous Fe–Si complex oxide. The

difference in morphology between this layer and the layer

shown in Fig. 6 might reflect the difference in the closeness

of Fe to glass before the bonding process.

The TEM micrograph shown in Fig. 7b shows the

changes in microstructure around the bond interface with

the application of the reverse voltage of 500 V at 613 K for

1200 s. The thickness of the reaction layer increased to ~80

nm, and particles of 10–30 nm in size were precipitated in

the layer. An SAD pattern from the central area in Fig. 7b is

shown in Fig. 7c. The spots indicated by white arrows were

from the precipitate indicated by a black arrow in Fig. 7b.

These spots constituted a 011 pole figure of an a-Fe crystal.

The EDS spectrum from the position of the same precipitate

is shown in Fig. 7d. An EDS spectrum from a position other

than the precipitates is shown in Fig. 7e. The content of Fe

at the position of the precipitate was higher than that at the

other position. (Strong characteristic X-rays of Si and O

were observed in the EDS spectrum in Fig. 7d because the

precipitate was buried in the reaction layer. The composition

observed by EDS did not reflect only the composition of the

precipitate.) These results show that the precipitates con-

sisted of metal Fe. A high concentration of Na and K in the

layer was found from the EDS results. It was believed that

Fe atoms in the Fe–Si complex oxide were substituted by

alkali ions that migrated toward the bond interface with

application of the reverse voltage. Then the Fe precipitated

as metal Fe in the reaction layer.

The difference in susceptibility to disjunction by the

application of reverse voltage between the anodically bond-

ed Fe/glass joints and the Kovar alloy/glass joints may be

explained as follows.

The cause of the disjunction of anodically bonded

Kovar alloy/glass joints is thought to be the degradation

of glass near the bond interface with the accumulation of

Na during the application of reverse voltage [5]. The

accumulation of Na in the glass causes formation of a

brittle Na–Si crystalline complex oxide phase, and rupture

of joints goes through this brittle phase. An anodically

bonded Fe/glass joint has a thicker reaction layer at the

bond interface than that in the Kovar alloy/glass joint.

When the reverse voltage is applied to an anodically

bonded Fe/glass joint, alkali ions reaching the bond

interface will substitute Fe in the reaction layer and will

be distributed in the range of the layer. With this mech-

anism, concentration of alkali ions enough to cause the

disjunction may be prevented. The reaction layer in the Fe/

glass joint was amorphous. In an amorphous phase,

diffusion of atoms is more active than that in the crystal-

line phase. Therefore, substitution of Fe with Na and

precipitation of metal Fe phase would proceed rapidly in

this layer.

Fig. 7. The changes in microstructure around the bond interface in an anodically bonded Fe/borosilicate glass joint with the application of reverse voltage: TEM

micrographs of bond interfaces as-bonded (a) and receiving the reverse voltage (b), SAD pattern from the complex oxide layer in b (c), EDS spectra from the

precipitate indicated in b (d) and a position in the complex oxide layer in b other than the precipitates (e). (Signals from Cu and Ga in EDS spectra were from

the Cu mesh sheet for TEM and contamination from Ga FIB process. The specimen materials contained none of these elements.)

M. Takahashi et al. / Solid State Ionics 172 (2004) 335–340340

There is another possible mechanism. The Fe content in

the Kovar alloy adjacent to the bond interface in an

anodically bonded Kovar alloy/glass joints was lower than

that in as-received Kovar alloy. The contents of Ni and Co

become higher accordingly. Therefore, the bond interface

in a Kovar alloy/glass joint becomes similar to an Ni–Co

alloy/glass interface in a measure. If anodically bonded

Ni/glass or Co/glass joints are more susceptible to the

disjunction than Fe/glass joints by the application of

reverse voltage, Kovar/glass joints may also be susceptible.

In order to prove this hypothesis, the application of the

reverse voltage to Ni/glass joints and Co/glass joint must

be carried out.

4. Conclusions

Reverse voltage was applied to Kovar alloy/glass joints

and Fe/glass joints, and changes in the bond interfaces were

observed macroscopically and nanoscopically. The acquired

results are as follows:

(1) The application of the reverse voltage to anodically

bonded Kovar alloy/glass joints causes the disjunction

of the bond interface. On the other hand, the disjunction

of the Fe/glass joints did not occur.

(2) At the bond interface in the Kovar alloy/glass joint, a

reaction layer of crystalline Fe oxide was formed. The

reaction layer in the Fe/glass joint consisted of an

amorphous Fe–Si complex oxide. The reaction layer in

the Fe/glass joint was much thicker than that in the Kovar

alloy/glass joint. These layers were formed with the

reaction between the Fe supplied from metal side and O

supplied from the alkali ion depletion layer in the glass.

(3) In the reaction layer at the bond interface in the Fe/glass

joint receiving the reverse voltage, metal Fe particles

were precipitated. It was caused by the substitution of Fe

in the reaction layer of the Fe–Si complex oxide by

alkali ions accumulating at the bond interface, with the

application of the reverse voltage.

(4) The thick reaction layer in the Fe/glass joints and the

change in the composition of Kovar alloy adjacent to the

bond interfaces might cause a difference in the

susceptibility to disjunction by the reverse voltage

between Kovar alloy/glass joints and Fe/glass joints.

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