TWI Knowledge Summary

Electrostatic bonding

by Norman Stockham

Process description

Electrostatic bonding (also known as anodic or field assisted bonding) was first reported in 1969 having been developed and patented by P R Mallory and Co Inc (for which reason it is sometimes known as Mallory bonding).

The technique is used to join glass to metals and semiconductors at temperatures well below the softening point of the glass. The components to be joined are polished to a smooth, flat surface finish (e.g. 50µm rms) then heated to a temperature below the softening point of the materials, but sufficiently high for ionic conduction to occur (200-600°C for glass). A d.c. voltage is applied across the components such that the metal (or semiconductor) is at a positive potential with respect to the glass. The voltage applied can vary from a few hundred volts to three thousand volts, for bonding times of 10 seconds to several hours.

A bond is formed as a result of the joint interfaces being brought into intimate contact by the electrostatic forces generated by ion migration in the glass. No external pressure is applied other than that required to hold the components in contact.

A reasonable match of thermal expansion coefficients is desirable to avoid strain in the joint, but joints between unmatched glass/metal combinations have been made successfully when the metal is in the form of thin foil or film.

Current status

Electrostatic bonding is a technique primarily used for joining glass to silicon for pressure transducer fabrication and solar cell encapsulation. However, it has also been investigated for the joining of glass windows in FeNiCo alloy opto-electronic packages. Use of the process is therefore limited and specialised. A brief description of these applications together with the reasons for electrostatic bonding being chosen as the most suitable joining process are given below.

	Fig.1. A typical capacitive transducer structure
	Fig.2. Electrostatic bonded silicone to glass pressure sensor array

Figure 1 shows the configuration of a typical capacitive pressure transducer. A Si die is bonded to an opposing glass plate (Pyrex), the two materials having a fairly good match of thermal expansion coefficient. The essential requirement of the joining process used for these devices is to provide a hermetic seal. A similar approach is being applied to a range of other pressure transducer designs. The glass and silicon are electrostatic bonded as wafers ( Fig.2), then diced into individual devices for subsequent packaging.

Solar cells

Solar cells require a glass cover for protection from the various harsh environments in which they must operate. Adhesives are commonly used but both technical and economic problems are encountered. High stress levels in the cover material and reduction in cell performance are technical disadvantages. Reduced cell output and low cover glass assembly rate are the economic ones.

Together, these have led to the adoption of electrostatic bonding as a suitable joining process for certain solar cells. Bonds have been achieved between 7070 borosilicate glass and either Si or Si coated with an anti-reflective layer (e.g. Ta ₂ O ₅ ), used to improve cell efficiency.

Opto-electronic devices

Light-activated switches and photodiodes are examples of opto-electronic devices. The opto-electronic device is housed in a FeNiCo alloy package and the window (in the lid) is usually glued, or crimped, into position. A hermetic seal between the window and the lid is required which, by the present joining techniques, is difficult to achieve. Furthermore, it is not easy to avoid contamination of the opto-electronic device during bonding. Electrostatic bonding has been shown to be a means of successfully placing a 747 borosilicate glass window into the FeNiCo alloy package, hermetically and without contaminating or damaging the devices.

Other applications

The benefits of being able to directly join materials like glass to silicon and certain metals has led to a range of other applications being investigated, these include:

Silicon chip mounting on glass substrates, including the use of micro-machined cooling channels in the glass.
The attachment of glass fibres to silicon and the fabrication of fibre optic couples.
Low temperature encapsulation of radioactive waste.

Materials combinations used in electrostatic bonding

Materials combinations that have been electrostatically bonded include:

Silicon to glass (e.g. Pyrex, Borosilicate)
FeNiCo alloy to glass (e.g. Pyrex, Borosilicate)
-Al ₂ O ₃ to FeNiCo

Important process issues

Electrostatic bonding is established as a high volume production process for silicon pressure transducers. The market for small silicon and glass devices is increasing rapidly with the development of micro-engineered structures for the automotive, aerospace, telecommunications, medical and chemical industries. Electrostatic bonding can offer these sectors a clean, rapid and hermetic joining technology that can be operated on very small and relatively large components.

To fully exploit the potential of this process, development is required to expand the range of materials and surfaces that can be bonded.

Benefits

The primary benefits of electrostatic bonding are:

Direct joining process for glass to silicon and some metals.
Clean process.
Hermetic joints.
Relatively low bonding temperatures.
No externally applied bonding pressure.
Rapid production process.
Simple equipment ( Fig.3).

Risks and required special care

There is very limited information on the performance of electrostatic bonding outside its main silicon to glass sensor application area.
Component surfaces need to be clean, flat and polished.
Components need to be able to withstand process temperatures of typically 200 - 400°C and high d.c. voltages (typically 200 - 2000V).
To avoid strain in the joint/components a reasonable match of thermal expansion coefficient is desirable.

Fig.3. Electrostatic bonding machine at TWI

Further information

Additional information about electrostatic bonding can be found in the items detailed below:

Joining ceramics - a guide to best practice

Electrostatic bonding - applications and principles (Bulletin Feb.1983) TWI Industrial Members only

Electrostatic bonding for electronic applications (Bulletin April 1985) TWI Industrial Members only

Diffusion bonding - ceramics and ceramic/metal joints

You can use the Weldasearch literature database to supplement what you find in JoinIT.

Contacts

Chris Otter, email: chris.otter@twi.co.uk
N R Stockham, email: norman.stockham@twi.co.uk