The Technology of Micro Adhesive Bonding

Innovation via micro adhesive bonding (gluing):

For the assembly of microparts screws are quite useless, in particular when they are 10 times bigger than the microsystem. Micro adhesive bonding is in most cases the only possible solution. But the properties of adhesives in small dimension can be totally different to those in classical macroscopic dimensions. Adhesives suitable for a similar macroscopic task, can be completely unsuitable on the microscopic scale. In the same way the handling of adhesive materials is different: smallest amounts are necessary, they have to be placed very precisely and may be swallowed by capillarity into the microstructure. The transport of a single drop with defined size must be provided, as well as the fast in-line curing of adhesives with a high precision of the bond or two stage cure processes.

Applications include:

• Conductive joints in dynamically stressed systems such as electronics in automotive components
• Thermal management (high density packing of microelectronics)
• Fiber optic couplers, electro-optic transducers (telecommunications)
• High definition sealing in micro-fluidics (biotechnology, life sciences)

Advantages by micro adhesive bonding:

• Joining of many dissimilar materials
• Multi-functionality of the joint (e.g. mechanical fastening plus conductivity)
• Low heat / cold joining
• Low mechanical stress, uniform stress distribution
• Galvanic isolation of the parts (no contact corrosion)
• New freedom for design
• Innovative technical solutions

Development of an adhesive bonding process:
The development of an adhesive bonding process involves the following aspects:

• Selection and development of the adhesives
• Precise application of minute volumes of the adhesives
• Compatibility with the micro-assembly process on the Micro Production System
• Adhesive curing (e.g. rapid cure, two-stage cure)
• Micro-testing of the joint (quality assurance)

All this special knowledge on micro adhesive bonding in combination with the Micro Production System is provided by our partner, the Fraunhofer institute IFAM.

Micro Adhesive Bonding Modules

as Part of our Micro Production System

Dispenser:
Klocke Nanotechnik learned a lot from the experience of IFAM and developed a special dispenser for spending small amounts of adhesives, see Fig. 6. This dispenser can be used manually and is remote controlled by our Network Controller. It offers shortest pulses as well as continuous dispensing.

Process control:
Dispensers can be programmed like all other axes in our Micro Production System. A typical process like:
1. Find a micropart and move it under the gripper,
2. Lift it up with the gripper,
3. Spend a drop of adhesive onto the target position,
4. Place the micropart at the target position,
5. Cure the adhesive
... is programmed easily by drag-and-drop in a few minutes.

Besides individual commands to the dispenser software modules are available to produces arrays of small adhesive dots (Fig. 1 and 2).

Lines of adhesive and a profile of one line

3D-data set of an area around this line of adhesive. The red marker defines the linescan.

Assembly and alignment in the same process:
The Micro Production System includes Software for the alignment of objects. At first the axes for movement during alignment and the input signal can be selected. Then parameters for a coarse search are entered, to find the first signal for a closed loop. After this signal is reached the program enters a fine positioning loop with own parameters to find the maximum signal intensity in a selectable amount of cycles. Fig. 5 shows an assembled VCSEL-Diode glued into an SMD housing after this active alignment.

Sharpen a hair...
... or bring a tip onto the end of a glass fiber and focus it! This microassembly example of a detection element in Scanning Nearfield Optical Microscopes (SNOM) needs a resolution of better 20 nm². A pyramid-shaped microtip is glued onto the end of a glass fiber. A 150 nm small hole in that tip forms a lens as sub-wavelength light source. The adhesive bonding process requires a very high precision. During the active alignment and the following curing of the adhesive, laser light is coupled through the glass fiber. The light emission of the microtip is used as feedback signal for the alignment. A misalignment of only 30 nm is clearly visible. The glass fiber with such a pinhole at its end then is assembled beside a shear force sensor. This device is used as central detection element in SNOMs. The following SEM picture shows the assembled parts:

a) Cantilever with microtip
b) Glass fiber
c) Shear force sensor
"Gap" = area that has to be filled with adhesive: Have fun!

The amount of adhesive applied between the parts a, b and c in the "Gap"-area is critical: too much adhesive and the shear force sensor is damped. Too little adhesive allows for the cantilever of the microtip to oscillate freely. In both cases the device would be destroyed. The adhesive is applied with one of the Nanorobotics manipulators,

... and it can be even more complex:

Generation of 3-D adhesive structures:
A 3-dimensional path on the surface of a micropart can be generated in the following way:

• The micropart is moved by joystick under a microscope. Each key-click at the joystick stores one position (X,Y, Rotation). This 2-D dataset is used to calculate a 2-D path.
• An FRT Microprof^® sensor attached to the Micro Production System determines the height along this 2-D path and generates the dataset of the third dimension.
• Afterwards the adhesive dispenser moves along this path (with the help of a 4-axis path control electronics and software) and generates 3-D adhesive structures onto the micropart.

Using a conductive adhesive leads to "wires" on microstructures that are placed without heat treatment directly during the assembly process. Fig. 1 shows how small structures of conductive adhesives can be.