Indian Journal of Pure & Applied Physics

Vol. 46, October 2008, pp. 738-743

Development of low cost set up for anodic bonding and its characterization

C C Tripathi+, Shruti Jain

+, Pawan Joshi

+, S C Sood

+ & Dinesh Kumar*

+Ambala College of Engineering. & Applied Research, Devasthli, Ambala +E-mail: jain.shruti15@gmail.com

*Electronic Science Department, Kurukshetra University, Kurukshetra

Received 3 September 2007; revised 3 January 2008; accepted 3 April 2008

A low cost experimental set- up for anodic bonding has been developed indigenously in the college laboratory and glass

silicon bonding parameters characterized. Anodic bonding between silicon and glass substrates has been characterized in

detail. The effects of magnitude of the applied voltage on the time required for complete bonding have also been

investigated. The effect of voltage, point contact, bond strength and electrostatic force in anodic wafer bonding process has

also been analyzed. The glass to silicon bonding at 1150V, 450ºC has been successfully performed. This enables simple, but

highly accurate, alignment of pre-patterned glass and silicon wafers. Fabricated devices have wide benefits like glass

transparency at optical wavelengths

Keywords: Low cost set-up, Silicon glass bonding

1 Introduction

Wafer bonding is one of the key process steps for

the construction and packaging of micro electro

mechanical systems (MEMS). It is also a cost

effective method for zero level MEMS packaging,

and it has increasingly become a key technology for

material integration in various areas of MEMS,

microelectronics and opto-electronics. The

manufacturers of MEMS, require wafer level bonding

of one silicon wafer to another silicon substrate or a

glass wafer. This provides the first level packaging

solution that makes these processes economically

viable. Different wafer bonding approaches are

currently used in MEMS industry: Fusion, adhesive,

eutectic, anodic, solder bonding etc1,2

. Fusion bonding

process requires a high temperature annealing, which

is not always suitable for the devices with aluminium

or copper integrated circuits. Also adhesive bonding

been non-hermetic, more emphasis is being laid on

low temperature bonding, which being not only

reduces process cost and time, but also minimize

bonding induced stress and warp age after cooling.

Anodic bonding is one of the most used wafer level

packaging procedures and the most robust process.

This process is being widely used for bonding glass

substrate to other conductive materials due to its good

bond quality. It can serve as a hermetic and

mechanical connection between glass and metal

substrates or a connection between glass and

semiconductor substrate3,4

. Anodic bonding, which is

also called field assisted thermal bonding or

electrostatic bonding, is commonly used for adhesive

free bonding of glass to silicon. This technique is used

to join glass with metals and semiconductors at

temperature well below the softening point of the

glass5.

In view of the large efforts for the development of

MEMS devices to take advantage of micro fabrication

technique, a new thrust has been generated to initiate

work in the field of MEMS devices. This has led to

the development of low cost set-up for fabrication of

MEMS devices, which are dimensionally not very

critical. In this paper, we present the development

strategy of a low cost anodic bonding set-up using

locally available equipments and facilities. The anodic

bonding process for silicon substrates and glass

materials has been investigated in detail. Firstly, the

time needed for a complete bonding (thereafter as

bonding performance) at different applied voltages

has been evaluated. Effect of point contact, bond

strength and electrostatic force have also been

analyzed. The electrostatic force depends not only on

the applied voltage, but also on other factors such as

the bonding temperature and the sodium content in

the glass wafer. The gap between the two wafers has a

significant effect on the magnitude of electrostatic

TRIPATHI et al.: LOW COST SET UP FOR ANODIC BONDING

739

force, which implies that the bond quality through the

anodic bonding technique heavily relies on the surface

smoothness. This conclusion has been confirmed

experimentally.

2 Experimental Details

2.1 Experimental set-up

The bonding system comprises of a dc power

supply, electrodes and temperature controlled

workstation as shown in Fig. 1.

A low cost set- up using laboratory hot plate (550

W) AC mains and klystron power supply (500 Volts,

40 mA) three numbers are connected in series to get

the required voltage available. Equipments available

in Microwave laboratory of Electronics and

Communication Engineering discipline with slight

modifications in the hot plate and electrode assembly

was used to perform the anodic bonding. The hot

plate was modified with PID type temperature

controller and chrome alumel thermocouple to

monitor the temperature of silicon wafers to be

bonded. Different types of electrodes were designed

and tested before reaching the final plum type cathode

electrodes designed and fabricated in college

mechanical workshop. These enable anodic bonding

using indigenously developed low cost equipment.

Pre-cleaned silicon and glass substrates were

sandwiched. Then, the aluminium bars and screws

were used to fix the bonding pair including the

electrodes. The assembly was heated at different

temperatures and a dc voltage was applied to the

electrodes, ensuring a positive electrode potential on

the silicon side with respect to the glass.

2.2 Wafer preparation and pre-cleaning

The silicon wafers used in the study were 1”, boron

doped <100> silicon with a resistivity in the

range 2-10 Ω-cm. The glass wafers tried were

ordinary microscope glass slides, optical glass and

commonly used pyrex borosilicate glass. The optical

flatness of the glass surface was of the order of λ, λ/4,

50 λ (λ=0.55 micron) for glass slides, optical glass

and one of the pyrex glass, respectively. The

thicknesses of both silicon wafer and glass wafer are

of the order of 500µm and 1500 micron, respectively.

Prior to alignment, the surfaces of the silicon and

glass wafers were cleaned in the solution of sulphuric

acid and hydrogen peroxide. A solution of hydrogen

peroxide and sulphuric acid in the ratio of 1:3 was

prepared and samples were dipped in for 10 min, then

rinsed in hot distilled water for 5 min and then rinsed

in cold distilled water for 5 min. Next dipped in

methanol and finally, dried before mounting for

bonding. 2.3 Glass to silicon bonding

To bond silicon wafer and glass in an anodic

bonding process, the cleaned silicon and glass were

sandwiched to each other and kept at the hot plate as

shown in Fig. 2. The temperature of the hot plate was

slowly raised to keep it in bonding temperature range

i.e. from 400 to 450°C and then voltage was applied.

The bonding voltage was varied and it was dependent

upon silicon, glass surface quality, surface preparation

and temperature of the hot plate along with sodium

content in the glass selected for bonding. However,

low temperature bonding between 400 to 450°C is

most suitable for all pre-fabricated devices on silicon.

The placing of the silicon and glass to be bonded,

should be such that the silicon wafer is electrically

connected to the anode and the glass to the cathode.

Fig. 1—Schematic drawing of the typical apparatus used in

the anodic bonding

Fig. 2—Experimental set up for anodic bonding

INDIAN J PURE & APPL PHYS, VOL 46, OCTOBER 2008

740

Accordingly, the glass wafer was kept on the top and

the silicon wafer was kept at the bottom. The

temperature was observed by thermocouple attached

to the hot plate, which is controlled by temperature

controller. The hot plate operating temperature was

kept near the glass-softening point, but below its

melting point. The bonding process was observed to

be complete within 10-30 min. 2.4 Bonding mechanism

To understand how silicon wafer and glass stick

together in an anodic bonding process, we must know

the element that makes up silicon and the glass that

are used in the bonding. Experiments showed that the

elements that make up the glass to be bonded (pyrex

borosilicate glass or pyrex) have sodium oxide

(Na2O). It was found that there is a content of 3.5 per

cent of sodium oxide in the pyrex. When the silicon

wafer and the pyrex are put together and placed at the

anodic bonding set-up and heat is added, at a certain

temperature the pyrex is hot enough and becomes

soft. Since the pyrex is softened, an applied voltage

produces an electric field between the silicon wafer

and the pyrex. The electric field exists because the

applied voltage makes the presence of the mobile

metal ions to exploit to the high negative voltage of

the pyrex. The high negative voltage pulls most of the

positive metal sodium ions (Na+) to the top, attracting

them and neutralizing them. As a result, the positive

ions move away to the cathode leaving behind

permanent negative ions.

These permanent negative ions then form a

depletion region between the silicon wafer and the

pyrex as shown in Fig. 3. This depletion region gives

rise to a large electric field, which is between the

silicon wafer and the pyrex. As a result of the electric

field, the silicon wafer and the pyrex are pulled into

contact, the strong electrostatic attraction between the

silicon wafer and glass wafers, fixing them firmly in

place. Increasing the bonding temperatures up to

450°C, can further enhance the mobility of these

positive ions. In addition, the electric field makes

oxygen from the glass to transport to the glass-silicon

interface where it combines with silicon to form SiO2,

creating the permanent bond that bonds the silicon

and glass together. Hence, the following assumption

is made. The oxygen ions carrying negative charges

are assumed to be evenly distributed in the glass

wafer and sodium ions are neutralized at the surface

once they reach the cathode. This assumption is based

on the fact that the oxygen is tightly bonded to the

glass networking structure in the glass wafer and there

is no free oxygen ion available for movement.

3 Results and Disscusion

3.1 Bonding of different types of glass with silicon

Bonding of different types of glass like soda lime

glass, optical glass, and pyrex glass with silicon at

different temperatures and voltages has been done.

Most suitable temperature and voltage values for

soda-lime glass are 300°C, 480V but cracks are not

controllable, which start appearing during cooling

process at temperature of 96°C. Also even if a small

portion is left unbonded, the bonds start breaking.

Same situation of cracks is there with optical glasses.

In case of pyrex glass, there were no cracks at all. A

strong and non-reversible bonding has been observed

at 1150V, 450°C for optically flat glass to silicon

bonding. It is because the thermal expansion

coefficient (TEC) of the used pyrex glass wafer is

very similar to the TEC of silicon.

3.2 Effect of applied voltage

Fig. 4 shows that the required bonding time drops

significantly as the applied voltage increases from 700

to 1150 V. This may be as follows: With respect to

the bonding mechanism at an elevated temperature,

Na+ ions in the glass become so mobile that they are

attracted towards the cathode as a result of the applied

voltage. This leaves behind relatively immobile

oxygen anions at the glass side of the silicon-glass

interface, at which a space charge region is formed.

This in turn creates an equivalent positive image

charge on the silicon side of the silicon-glass interface

resulting in a high electric field magnitude of 106

V

cm-1

across the silicon-glass interface. Under the high

electric field, oxygen anions drift away from the Na+

depletion region to the silicon surface. As this

Fig. 3—Schematic showing the joining of silicon wafer and

pyrex in anodic bonding

TRIPATHI et al.: LOW COST SET UP FOR ANODIC BONDING

741

happens, oxidation of silicon by the oxygen anions is

presumed to occur and a thin oxide layer is formed at

the interface, which contributes to the migration of the

bonding front. However, at a small applied voltage,

i.e. a reduced electric field, the drift velocity and the

kinetic energy possessed by the oxygen anions cannot

sustain a high oxidation rate at the bonding front of

the silicon-glass interface, thus, a longer bonding time

is required. As the electric field becomes negligible,

reaction is extinguished at the interface and no

bonding can be achieved. We have observed that in

the absence of an electric field, there is no indication

of bonding. A large electric field is applied across the

joint, which causes an extremely strong bond to form

between the two materials.Fig. 5 shows a glass plate

bonded over a cavity etched into a silicon wafer using

anisotropic etching and Fig. 4 shows the plot of

bonding time versus applied voltage for p-type silicon

and pyrex glass.

The bonding current as a function of time is shown

in Fig. 6. The results show that the current rises

rapidly at the beginning of the bonding. It indicates

that the surge of Na+ ions drift to the cathode to

generate a current in the circuit. As the migration of

Na+ continues, the accumulation of positive charge

repels the incoming ions and the current density

decreases to an almost steady value. Bonding was

done at 1150V for 10-15 min. If the bonding voltage

is decreased the bonding time increases. It has been

observed that bonding depends not only upon the

voltage, temperature and time but it also depends

upon surface uniformity. Bonding voltage and time

are considerably reduced in case of optically flat surf

3.3 Effect of point cathode contact

The results indicate that point contact electrode

configuration provides excellent bonding uniformity

as compared to flat contact configuration. Table 1

shows the experimental parameters under which

various material combinations were successfully

bonded. It was found that bonding time depends upon

the applied voltage, temperature and cathode

electrode contact configuration (flat/point). Tabulated

values also show that bonding time strongly depends

on the contact configuration. Bonding, using point

cathode electrode contact6, is much faster as

Fig. 4—Plot of bonding time versus applied voltage for

p-type silicon and pyrex glass

Fig. 5—Silicon and glass bonding

Fig. 6—Plot of current as a function of time

Table 1—Bonding voltage and time

Bonding voltage (V) Bonding time (min)

770 40

1000 35

1050 30

1140 20

1150 10

INDIAN J PURE & APPL PHYS, VOL 46, OCTOBER 2008

742

compared to flat cathode electrode contact. It was

observed that silicon and pyrex glass bonding takes

about one eighth of the time while using point contact

as compared to flat contact. Point cathodes, usually

the end of a stiff wire, are commonly used in anodic

bonding to make electrical contact to the glass. In the

experimental studies of anodic bonding, point cathode

allows researchers to view the glass-silicon interface

through the glass during bonding. A point cathode

also causes anodic bonding to spread radially

outwards from a spot beneath the point cathode. This

radial progression of the bonding front prevents any

air between the glass and metal members from being

trapped at the interface during the bonding process.

3.4 Bond strength

The bond strength obtained in this study is

comparable to that (from 5 to 25 MPa) of wafers

bonded using higher bonding temperatures by other

researchers7,8

. The bond strength increases with an

increase in the bonding temperature and voltage. It is

believed that glass is annealed during bonding process

and the fracture strength of the glass improves. It is

observed that the fracture invariably occurs inside the

glass substrate. The crack is initiated and propagates

in the glass. Occasionally, the crack extends into the

silicon wafer through the interface without damaging

the bond. This is a demonstration of the quality of the

bond. The reactive phenomenon at the interface is

largely dependent on the bonding temperature since

temperature is a major driving force of ion mobility

and atomic diffusion. High bonding temperatures

introduce higher energy, drift more oxygen anions to

the interface, enhance the diffusion of oxygen into

silicon and finally promote greater reaction in the

interface. A higher voltage produces a higher electric

field, which increases the drift velocity of the sodium

and potassium ions. Higher voltage also accelerates

the decomposition of the sodium ions from the lattice

matrix and contributes to a larger depletion region of

the alkaline ions. A larger depletion region enhances

the electrostatic force and promoted the bond. This

seems to be the likely reason for the higher bond

strength.

For bonding temperatures higher than 400°C, high

bonding strengths (above pyrex toughness) were

obtained; the pyrex itself broke before it was

debonded. This observation was confirmed by the

blade test. The bondings performed at temperatures

lower than 400ºC exhibited weak mechanical

properties. However, the strength of the samples

bonded at 450ºC increased to a value higher than the

pyrex toughness.

3.5 Influence of electrostatic force

High temperatures generate high ion mobility in the

glass substrate. As the temperature is raised, the

resistivity of glass decreases exponentially. A rapid

build up of the space charge occurs at the interface,

giving rise to electrostatic forces, which pull the two

wafers into intimate contact. Therefore, high

temperatures cause high current and result in good

bond quality. At high temperatures, more ions

decompose from Na2O and K2O to migrate to the

cathode. The equilibrium state is easily obtained and

the transition period is shorter. A higher applied

voltage is expected to increase the mobility of the

Na+ ions. A higher voltage produces a higher electric

field. The higher electric field increases the drift

velocity of the sodium ions. Higher voltage also

accelerates the detachment of the sodium ions from

the lattice matrix and contributes to the concentration

of free sodium ions. This is the likely reason for the

shorter time required to establish the equilibrium

state. Therefore, a higher applied voltage can generate

more free sodium ions and subsequently, contribute to

a larger electrostatic force.

The temperature is a dominant factor, which affects

the electrostatic force, which causes the firm contact

of two wafer surfaces, facilitating the bonding

reaction to occur. The voltage is also significant for

the control of the maximum electrostatic force. The

bonding temperature, applied voltage and bonding

time have an obvious complementary relationship for

a certain electrostatic force needed for chemical

reaction between silicon and glass wafers.

4 Application to Microsystems

Different applications of metal to glass anodic

bonding have been reported recently in the field of

Microsystems technologies9,10

. Metal to glass anodic

bonding has been used to integrate robust fluidic

interconnects to a piezoresistive silicon/pyrex liquid

flow sensor11

. Moreover, in case of silicon

piezoresistive sensors, metal to glass anodic bonding

is foreseen to have applications in the packaging field

of pressure sensors operating in harsh environments.

In the case of titanium, its biocompatible property

combined with its assembling to pyrex materials in

which micro channels can be structured, make it a

good candidate to be used in biocompatible micro

TRIPATHI et al.: LOW COST SET UP FOR ANODIC BONDING

743

fluidic systems. Anodic bonding of pyrex can also be

successfully performed on thin metallic films. An

application of this technology is the encapsulation

under vacuum of quartz resonators. Pyrex caps are

bonded on thin metallic films forming the electrodes

of the resonators. Anodic bonding is gaining

significance in Micro-Total-Analysis Systems (µTAS)

and in miniaturized biological reactors because of its

promising use in sealing silicon and glass based micro

fluidic devices. The bonding technique can be broadly

applied in the fabrication of sensors, actuators,

microstructures, 3D integrated circuits and opto-

electronics devices. The recent developments in the

bonding technique have made the micro mechanical

design more flexible.

5 Conclusion We have successfully bonded silicon with glass at

1150V, 450°C in 10-15 min. It was found that

bonding time depends upon the applied voltage,

temperature and design of the cathode electrode and

the flatness of the surfaces to be bonded. Temperature

plays the major role as the mobility of the ions

increases and bonding takes less time.

Acknowledgement The authors would like to thank Director, Dr. Anil

Joshi, ACE & AR, Devasthali, for his constant

encouragement during the course of this work. The

authors also wish to acknowledge and extend thanks

to Sh Nalini Kant mentor of the college without

whose support and encouragement the work would

not have been possible. We also extend our sincere

thanks to our worthy Chairman, Dr Jaidev for his kind

support and motivation. The authors are also thankful

to Director, CEERI, Pilani, for providing pyrex

wafers used in the experiment.

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