Jeroen Haneveld, business manager, Micronit Microtechnologies
In medical and industrial applications, the increasing complexity of miniaturised devices is a sign of maturation of both techniques and tools. Diagnostic platforms consist more and more of highly integrated systems. Manufacturers face the challenge of continuously having to come up with intelligent and reliable techniques to further support this process.
Heterointegration
Emerging trends in MEMS and microfluidics demand more intelligent and therefore more complex devices. The complexity of devices is determined by the number of components that are incorporated. On the one hand, this upscaling of devices can lead to more intensified use of specific features (for example, in high throughput analysis), but on the other, there is the trend of applications becoming more and more individualised.
The aforementioned factors call for the integration of several techniques and components (or units) into one package. These components can consist of an individual die, MEMS device, sensor, actuator, passive component, etc. All of these choices and possibilities can be captured under one term, namely heterointegration.
Heterointegration is multi-chip packaging: the integration of various substrate materials into one package. A key process in heterointegration is bonding, a technology with a rapidly growing relevance in wafer-level packaging (WLP).
An example of heterointegration: a microfluidic well plate fused with a high-density multi-electrode array (MEA) chip. The plate was developed for the InForMed project, the objective of which is to establish an integrated pilot line for medical devices (http://informed-project.eu/).
Wafer bonding and temperature
For many applications, bonding of two or more (structured) substrates is required. Often, the bonding process involves different types of materials. These hybrid material bonding processes bring their own challenges. Various solutions are available for hybrid bonding and for the integration of specific elements into the devices, such as porous membranes, flexible films and (bio)sensors.
Most bonding processes require a typical working temperature. To be able to perform complex hybrid bonding and accommodate the specific properties of various materials, it is necessary to have a range of bonding techniques to choose from.
Micronit provides five different bonding techniques, namely:
- direct/fusion bonding;
- anodic bonding;
- eutectic bonding;
- (patternable) adhesive bonding; and
- laser-assisted room temperature bonding.
These techniques afford numerous hybrid bonding possibilities for many materials and applications.
Bonding techniques
Direct/fusion bonding creates a direct chemical bond between two surfaces by annealing them at elevated temperatures. No additional intermediate layers are required. The bonding process typically comprises three steps, namely:
- wafer pre-processing/cleaning;
- establishing a pre-bond at room temperature; and
- annealing at higher temperatures.
The prerequisites for direct/fusion bonding are: that the wafer material is sufficiently clean, flat and smooth; and that there is no significant coefficient of thermal expansion (CTE) mismatch for the two materials to be joined. It is therefore mainly used in establishing glass-glass, glass-silicon and polymer compounds.
Direct/fusion bonding
Technical details:
- No intermediate layer
- Integration of electrodes possible
- High bond strength
- Aligned bonding
- Multi-stacks
Applications:
- Microfluidics (microreactors/micromixers)
- Cover of microfluidic channels
- MEMS structures
- MEMS sensors
- Packaging
Anodic bonding seals glass to either silicon or metal at lower temperatures than required for direct/fusion bonding. This is achieved by using a sufficiently powerful electrostatic field to generate a bond between the two substrates. There is no need for an intermediate layer.
The requirements for anodic bonding are: that wafer surfaces are clean and even; that borosilicate glass containing a high concentration of alkali ions is used to ensure bonding with the silicon or metal, since the process depends on the migration of these ions due to the applied electrical field; and that the processed glass has a CTE that is equal to that of the bonding partner. This technique can also be used for triple stack bonding (glass-silicon-glass).
Anodic bonding
Technical details:
- Aligned bonding
- Vacuum bonding possible
- Integration of electrodes and biosensors possible
Applications:
- MEMS structures
- MEMS sensors
- Packaging
- Lab-on-a-chip or point-of-care
Eutectic bonding is based on the ability of silicon and certain metals to form a eutectic system. A eutectic system is an alloy of two or more substances (for example, silicon and gold) that, as a homogeneous mixture, melts at a temperature that is lower than the melting point of either of the individual substances. The most commonly used eutectic formations are silicon and gold or silicon and aluminium.
The eutectic bonding process takes place on a silicon or glass wafer that is coated with a (patterned) gold or aluminium layer. A silicon substrate is placed on top and heat as well as pressure are applied until the silicon forms a eutectic system with the alloy on its counterpart. This technique delivers hermetic bonds at lower process temperatures.
Eutectic bonding
Technical details:
- Metal intermediate layer with silicon
- Aligned bonding
Applications:
- MEMS packaging
- 3D Integration
- Microfluidics/Lab-on-a-chip
- Through-connection (via)
(Patternable) adhesive bonding connects the substrates of different material types by an intermediate adhesive layer. The adhesive layer can be organic or inorganic, possibly in the form of a glue or, for instance, a photo patternable dry film resist/adhesive. Certain adhesives are especially suited for MEMS and/or other substrates with electronics. The adhesive layer can be deposited on one or both substrate surfaces. Bonding temperatures depend on the type of intermediate layer and the material of the bonding partners. When using a UV curable glue, the entire process can be undertaken at room temperature. Other techniques may require a thermocompression bond step, but typically the temperatures involved are lower than in other bonding techniques.
The (patternable) adhesive bonding process also allows for bonding of different substrates, for example, silicon, glass, metals and other semiconductor materials. Bonds can be hermetic or not, depending on the type of adhesive used.
Adhesive bonding
Technical details:
- Controlled heating and cooling-off
- Integration of electrodes
- Integration of sensors
- Integration of functional layers
Applications:
- Microfluidics/Lab-on-a-chip
- IC devices
- Isolating layers
- MEMS packaging
Laser-assisted room temperature bonding is a Micronit patented technique that generates hermetic bonds while only locally applying heat. The initial step is to produce a prebond using a light-absorbing (metal) interlayer. The prebond is then fortified by laser processing. A key advantage of the process is that due to the prebond, there is no need to apply external pressure during the laser bonding phase. Overall, less laser power is needed to accomplish the final bond, which allows more precisely localised heating. This means that temperature-sensitive components, such as biomaterials or liquids, can be part of the bonded stack. Usually, the substrates are glass or silicon, or a hybrid combination of both. The metal interlayer can be any metal, as long as a prebond can be achieved with the counterwafer. Commonly used metals are aluminium, chromium, tantalum and titanium.
Laser-assisted room temperature bonding
Technical details:
- Low temperatures
- No external pressure during laser bonding
- Integration of electrodes
- Integration of biomaterials or liquids
Applications:
- Lab-on-a-chip
- Biomedical devices
- IC devices
Functionalities in device design
All of these bonding techniques provide plenty of opportunities to meet the need for heterointegration. There are solid high-temperature bonding options, but at the same time, there is an increasing focus on low-temperature bonding options for applications that involve temperature-sensitive components, such as biomaterials, functional coatings or complementary metal oxide semiconductor (CMOS) processed wafers. Low-temperature bonding is also used to manage large differences in thermal properties between the bonded substrates.
For microfluidic devices, these techniques mean that platforms with active valves, pumps and sensors can be produced. In fact, with any element necessary to create a device that offers total integrated fluid management.
For microelectronics, they mean an ongoing increase in possibilities for MEMS packaging, also with integrated elements. Today, multi-stacks of up to 21 layers of glass are possible, but these numbers will soon increase as the demand for smaller, better and more continues.
A laser bonded chip containing a cavity filled with water. A residual air bubble can be observed due to incomplete filling.
Micronit Microtechnologies