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Laser joining of Microsystems (2004 2010)

Norbert Lorenz, Robert Maier, Duncan Hand

Overview

In industry, a wide range of actual micro-packages (see Figure 1) are sealed using a laser-based process. As direct bonding techniques suffer from several limitations, investigations of intermediate layers bonding of micro-devices is carried out by local laser heating of:

  • Polymer bonding;

  • Solder bonding;

  • Glass frit bonding.

Figure 1: Laser bonded micro-devices

A rather large part of this work was focused on developing packaging processes with a glass frit intermediate layer, which is an ideal solution for applications where strong and hermetic seals with long-term stability are required.

Packaging processes with all these 3 intermediate layers are already well established in the market, however, they are all based on processes where the entire package is heated up to the required bonding temperature and thereby restricting the use temperature sensitive materials within the package.

 

Figure 2: Schematic of the local heating experimental setup (left), and chip packages (right).

The key issue of this work is the thermal management during the bonding process. We want to keep the temperature in the centre of the device as low as possible and only heat the joining area to the bonding temperature. This was investigated for 2 particular packages. One is the silicon to glass package with the polymer bonding layer. A sketch of the bonding process and a picture of the actual setup are shown here. My work did not only involve the bonding process development but also the design of the entire bonding setup: see Figure 3.

 

Figure 3: Bonding setup and scan head.

 

Most of the setup cant actual be seen because it is integrated into this bonding chamber which also allows packaging in controlled atmospheres like nitrogen or vacuum. For bonding the samples are placed onto a heat sink and a focused laser beam is scanned at high speed along the bonding track of the sample. At the same time as the sample is heated from the top the access heat is drawn away through the bottom of the substrate. Thereby the temperature in the centre is kept considerably cooler than the joining area and enabling the packaging of temperature sensitive materials.

 

Figure 4: Schematic of glass frit packaging (left); Temeperature plot of laser bonding process (right).

For glass frit packaging precise temperature control is required hence a temperature monitoring method during the bonding process was developed. Three thermocouples were integrated into the package as shown in the sketches 2 along the joining layer and 1 in the centre of the package. As before the laser beam is scanned at high speed across the sample.

Figure 4 shows a typical temperature profile of such a bonding process. By active cooling of the substrate during the bonding process the temperature in the centre of the device can be kept below 140C despite a temperature of 375C in the joining region thereby enabling the use of temperature sensitive materials in the package.

 

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