Glass frit packaging is a simple and robust process where strong and hermetic seals can be achieved in high yield processes. Conventional, furnace-based glass frit packaging requires the entire package to be heated to the bonding temperature of several hundred degrees. In this chapter the development of two laser-based glass frit packaging processes in both air and vacuum has been investigated. Due to the localised heat input of the laser beam and active cooling of the substrate of the device the heat is restricted to the joining area. This enables the use of temperature sensitive materials within the package. The combination of the key advantages of glass frit packaging and localised heating using a laser as the heat source was demonstrated. Temperature monitoring experiments for an in-depth understanding of the temperature profile during the bonding procedure and process optimisation and quality testing of the packaged devices were also presented in this chapter.
5.3.1 Packaging in Air
A laser-based glass frit packaging process of LCC devices in air has been demonstrated using the glass frit material DM2700P/H848 from Diemat. Temperature monitoring experiments have shown that for packaging of the standard LCC substrate and the bare LCC substrate laser powers of 70 W and 72 W respectively, are required for 75 s to achieve successful bonding. Furthermore, it was demonstrated that by active cooling of the substrate the temperature in the centre of the device is kept below 140°C despite a temperature of 375°C in the joining region. Through a combination of localised laser heating and thermal management using heat sinking a localised heating glass frit packaging process has been created which enables packaging of temperature sensitive materials whilst still benefiting from the well-known advantages of glass frit as sealant material. Extensive testing of the packaged devices to assess the quality of seal according to MIL-STD-883G was accomplished. Leak testing showed that hermetic seals were achieved in a high yield process (>90%). Shear forces as high as 855 N were required to split apart these samples, which were bonded using localised heating; even
exceeding the strength of devices (610 N) where the lateral heat flow during bonding was not restricted [114]. Additional temperature cycling and acceleration testing showed that the LCC packages even pass a major part of the qualification process required in military and space grade products. In summary, a localised heating glass frit packaging process in air was developed which shows all the benefits of glass frit packaging without any negative effect of the thermal management on the quality of the seal.
5.3.2 Packaging in Vacuum
For the development of a laser-based glass frit packaging process of LCC devices in vacuum the paste 5115HT1 from AGC was used which is specially developed for vacuum applications. Initial temperature monitoring experiments in vacuum showed that a PTFE sheet of thickness 0.26 mm must be placed as an interface material between the bottom of the LCC substrate and the copper block to ensure a constant, repeatable thermal transfer required for successful bonding. Hermeticity testing confirmed that a very reliable joining process (yield rate ~95%) had been developed. The temperature in the centre of the device was kept below 250°C despite a temperature of 440°C in the joining region. Using a special cooling sandwich structure the heat flow was tailored additionally to reduce the temperature in the centre of the device below 200°C.
Hermeticity and shear force testing demonstrated that the overall process time for a single bonding cycle was reduced from 50 minutes to 7 minutes without any adverse effect on the quality of the seal. Shear force testing underlined the excellent of properties of glass frit as sealant; the packages withstood shear forces in excess of 1 kN (limit of the test machine). Residual gas analysis has shown that a moderate vacuum of around 5 mbar was achieved inside the vacuum packaged LCC devices. Miniature pressure gauges were packaged successfully to emphasise the main advantage of this laser-based localised heating glass frit packaging process in vacuum. The sensors can only withstand a temperature of maximum 300°C and would be destroyed if heated to the bonding temperature (440°C) of the glass frit material. Using this laser-based packaging process in combination with active cooling the temperature in the centre of the LCC substrate was kept below 250°C enabling packaging of temperature sensitive materials.
6 Conclusions and Future Work
The project, on which this thesis has been based, set out to develop a technology to provide a solution to a problem of critical importance in the area of packaging of temperature sensitive micro-devices. The successful development of a range of solutions which address this need has been demonstrated. Two novel laser-based packaging processes for applications in device manufacture in microsystems technologies are presented in this PhD thesis.
Silicon to glass joining, a typical MEMS application, with a Benzocyclobutene (BCB) adhesive layer and hermetic glass frit packaging of Leadless Chip Carrier (LCC) devices, a standard off-the-shelf electronic package, were investigated. These packaging techniques are already established in industry for their robustness and simplicity. However, conventional techniques in general require the entire package to be heated to the bonding temperature thereby limiting the use of temperature sensitive materials within such devices. Here a high power laser is an ideal solution. Due to its high focusability and very flexible, remote positioning of the focal spot the laser can be used to heat the joining area highly localised. To additionally confine the high temperatures to the bonding area only, active cooling is applied to the substrate of the device. The lateral heat flow to the centre of the package, the sensitive device area, is restricted. In comparison to the joining area the temperature in the centre is reduced considerably. It was demonstrated that it is possible to use thermosetting polymers (BCB) and glass frit materials for packaging of micro-devices whilst maintaining low temperatures at the centre of packages during these processes. This allows for the first time the use of highly temperature sensitive devices in packaged devices using these sealing materials. By combining the advantages of intermediate layer bonding and localised laser heating, simple, robust and highly versatile packaging processes have been accomplished.
A multi-purpose bonding setup has been developed to fulfil the numerous requirements of these bonding processes. Bonding experiments have shown that the application of the bonding force is a critical process step. It must be applied uniformly and spread evenly to ensure sufficient contact of the bond interfaces along the entire bond line. The design considerations were focused on this issue in particular to ensure the high process yields required in commercial applications. The bonding processes under investigation
have been developed to such extend that they can be applied quite readily to packaging in device manufacture in industry. The industrial collaborators, who have partly driven the research interest of the work of this PhD thesis, are looking at the exploitation of this technology for their purposes.
In the following the key findings of the three main experimental research areas are discussed and recommendations for future investigations are highlighted.
The three areas are:
development of the bonding setup,
silicon to glass joining with a BCB adhesive layer, and
hermetic glass frit packaging of LCC devices.
6.1 Bonding Setup
In microsystems technologies packaging is often application specific and hence accounts for a major part of the overall production costs of such a device. In order to achieve a considerable reduction of the packaging costs a generic packaging process is required. To provide for such a process a multi-purpose bonding setup has been developed which can be used for a range of intermediate layer bonding processes with only minimal adaptations required to change between the processes, as described in chapter 3.
Typical requirements (section 3.1) of the bonding process and the setup, respectively, are:
packaging in a vacuum or inert gas atmosphere,
application of a bonding force
suitability for wafer-level packaging
active cooling of the device substrate during bonding to keep the thermal load on the bulk of the package as low as possible is an additional requirement for this particular setup.
A design (see section 3.2) has been chosen where all required components are integrated inside a bonding chamber to provide for a controlled atmosphere during the bonding process since often a vacuum or an inert gas environment are required for bonding. A
water-cooled copper block is used as heat sink. The required bonding force is provided by a pneumatic cylinder. Since these components are required to work inside the bonding chamber, the feed-lines and couplings had to be designed carefully to avoid leakage.
Any adaptations of the bonding setup to a particular process were restricted to modules which could be exchanged within minutes without the need for re-alignment of the entire bonding setup afterwards. A camera was integrated into the setup to allow for precise alignment of the beam path of the laser, i.e. area of localised heat input, to the bonding track of the sample.
For the silicon to glass joining process there are two major challenges; especially if this process is expanded to a wafer-level:
the required bonding force needs to be applied individually for every single device on the wafer (section 3.2.4) and
localised heat sinking of the centre of the substrate only to ensure sufficient high process temperatures in the bonding area (section 3.2.5).
Special, exchangeable slide-on cooling platforms, which correspond to the design of pattern to be bonded, were designed to introduce localised cooling. A method to apply the bonding force individually, by means of a single contact point for every sample was developed using precision glass spheres.
As for most bonding processes, temperature control is one of the most important process parameters for glass frit packaging. A temperature monitoring method was developed, as described in section 3.3.2. Using thermocouples the temperature within the bonding layer can be controlled to within ± 10°C of the required process temperature. Furthermore, a technique was developed to monitor the temperature in the centre of the package without significant influence on the natural heat flow within the sample.
The bonding setup has shown its great flexibility and capabilities throughout the experimental work of this PhD. However, three possible upgrades have been identified:
(i) For example, at present the samples are aligned by hand which is undesirable for
industrial applications. The integration of a fully automated pick and place robot should be considered.
(ii) Online process control of the laser power by monitoring the bonding temperature using a pyrometer integrated into the beam path of the laser has been shown to improve the process yield in other laser-based joining techniques. Integration of such a system is highly desirable as it would overcome the shortcoming that currently the bonding temperature for the silicon to glass joining process cannot be measured; however, considerable costs are required for such an upgrade.
(iii) The process speed of the wafer-level packaging process is limited by the fact that each sample needs to be bonded individually and in sequence. Introduction of a multiple-beam approach would result in parallel processing and a considerable reduction of the overall process time.