Adhesive layer bonding, in particular on a wafer-level, is a simple and robust process featuring the following key advantages: insensitivity to dust particles, surface roughness and non-planarization up to a few microns of the bond interfaces, thereby reducing process costs. Very few limitations exist for the choice of the materials to be bonded;
and most importantly wafer-level packaging benefits from protection of the device to be packaged from dust and moisture ingress prior to dicing [3]. Traditionally these packaging processes are carried out using substrate bonders restricting the use of temperature sensitive materials within the package. In this chapter the combination of the benefits of laser-based joining – truly localised heating – and adhesive layer bonding was demonstrated in a novel localised heating packaging approach using the example of joining glass to silicon with a BCB intermediate layer. Due to the long time constant of the bonding process (tens of seconds) and the high thermal conductivity of the silicon
substrate (149 W m-1 K-1) the heat flow to the sensitive device area in the centre was restricted further by active cooling. This novel bonding procedure was first demonstrated for single chip packaging and then a further development of this process to wafer-level packaging was shown. To assess the quality of the laser-sealed devices the bonding experiments were accompanied by shear force testing.
4.4.1 Chip-level Packaging
Successful chip-level packaging was demonstrated in a laser-based process where the temperature in the centre of the device was kept at least 120°C lower than in the bonding area. Optimisation of the bonding parameters has shown that there is a trade-off between the bonding speed and the quality of the seal. For bonding times down to 8 s, shear forces of 150 N on average with a maximum of 290 N were achieved. This is comparable and some cases even higher than results obtained by other researchers using conventional flip-chip bonding (typically ~200 N) [56] and the results (typically
~170 N) presented in the feasibility study on laser-based adhesive layer bonding here at Heriot-Watt University. The latter bonding techniques, however, both resulted in heating of the entire device during the bonding process unlike the localised heating process presented in this study. The comparison of the shear force test results shows that the selective laser heating and the heat sinking do not lower the mechanical stability of the seal provided that a full cure of the BCB adhesive layer has been achieved.
For further improvement of this bonding process control of the bonding temperature and laser power respectively (see chapter 3.3.1) would be highly desirable. Temperature measurements by pyrometry were identified as the only viable technique to accurately monitor the temperature in the joining area. For the experiments described above, the bonding parameters had to be obtained in experiments by judging the degree of cure of the BCB adhesive layer by its colour change. In some cases it turned out to be rather difficult to evaluate when exactly a full cure was achieved. Therefore, sometimes a range of bonding parameters, in particular for the samples bonded on a wafer-level, has been defined, rather than discrete optimal parameters. An alternative, more accurate method to determine the exact degree of cure is to monitor the change of certain peaks in the absorption spectrum of the BCB using FTIR spectroscopy. It is recommended that this method should be used in any future investigations.
4.4.2 Wafer-level Packaging
The feasibility of wafer-level packaging in laser-based joining was shown with successful bonding of simplified patterns of 5 or 9 samples on a single wafer. For the pattern of 5 successful bonding of all samples on the wafer was achieved for both spacings. The best bonding results were accomplished with the following parameters:
a laser power of 30 W was applied for 50-70 s to each individual ring for the small pattern and
33 W for 57-77 s for the large pattern.
Comparison with devices packaged at chip-level showed that for the pattern of 5 with the large spacing the same quality of the seal with shear forces averaging around 165 N and a maximum of 285 N could be achieved. A reduction of the distance between the neighbouring samples resulted in a slightly reduced bond quality. A process such as this can be used in applications where high seal strengths and wafer-level packaging are required given the benefits of parallel processing out-weigh the waste of material (un-used silicon areas) due to the reduced sample density.
After successful packaging of this basic pattern a more densely packaged pattern of 9 samples on the same wafer was investigated. This pattern resembles full wafer-level bonding. The best bonding results were achieved for the following parameters:
for the small spacing a laser power of 32 W for 50 s and
for the large spacing 37-39 W for 76-90 s.
In some cases successful sealing of all nine samples on the same wafer was achieved proving the feasibility of wafer-level packaging using this localised heating bonding process. In most cases bonding of seven to eight of the nine samples could be achieved with high repeatability, comparable to the yield rate of 87.5% achieved by Mohan et al.
[99] who investigated wafer-level solder bonding in a laser-based process. For the pattern of 9 with the small spacing, shear forces averaging around 60 to 70 N and a maximum up to 95 N were shown and for the large spacing an average of 80 to 90 N and a maximum of up to 150 N. The shear forces were considerably lower than for the devices packaged on a chip-level. They are restricted by the mechanical stability of the cover glass of thickness 200 µm and by the reduced wetting of the silicon interface during bonding due to the higher stress in the cover glass of the more densely packed
pattern. This process, however, is beneficial in applications where the device to be encapsulated needs to be packaged prior to dicing to protect it from dust and moisture ingress during the dicing process.
To improve this process and to achieve successful bonding of all nine samples on the same wafer with high repeatability the setup needs to be refined further with particular focus on applying the bonding force individually for every single device. Once this issue has been resolved this bonding method can easily be extended to larger numbers of individual samples on the same wafer.
5 Hermetic Packaging of LCC Packages Using Glass Frit Layer
This chapter presents an investigation of glass frit material as a sealing layer for hermetic packaging of Leadless Chip Carrier (LCC) devices in laser-based processes.
There are well-documented [43, 44] advantages associated with glass frit packaging for the encapsulation of micro-devices, however, established processes require the entire package to be heated to the joining temperature of typically 300°C to 500°C. In this chapter the development of two laser-based glass frit packaging processes in both air and vacuum is presented. The process development focuses on the restriction of the high process temperatures to the joining area only. The development of a temperature monitoring method of the glass frit packaging process and quality testing of the packaged devices are also included in this chapter.
As described in chapter 2, glass frit packaging is a universal, simple and robust method and provides a very promising solution to the significant challenges in the encapsulation of micro-devices. One of the main advantages of glass frit bonding is the non-stringent requirements for the flatness of the surfaces to be bonded due to the high wetting abilities which even enable bonding of unpolished surfaces. Hermetic seals with high bonding strength can be achieved with most materials commonly used in manufacturing and packaging of micro-devices in high yield (>90%) processes and with good process repeatability. Conventional glass frit packaging processes are mostly furnace-based and hence, such processes do not allow temperature sensitive materials (e.g. polymer and magnetic materials) to be used within the package since the entire device is heated to the bonding temperature of several hundred degrees. These furnace based processes also generate problems in multi-step bonding processes where several bonding cycles need to be carried out in sequence where parts joined in an earlier heating step can disassemble in a later one.
An ideal solution here is to use a high power laser as a highly localised heat source to minimise the heat input into the bulk of the device. In this chapter the clear benefits of combining the merits of both glass frit packaging and localised laser heating to laser-based LCC packaging processes are demonstrated. Due to the long time constants (up to several minutes) of the bonding process, appropriate heat sinking was also introduced to further reduce the lateral heat flow from the bond line to the sensitive device area and thereby minimize the local temperature rise during the bonding process.