1) Forschungszentrum Karlsruhe, IMT
²⁾ Forschungszentrum Karlsruhe, HVT

The Hot Embossing Technology has become one of the key technologies for the fabrication of demanding polymer microstructures. It is known for the replication of delicate microstructures with high aspect ratios, low internal stress, and small distortions. In vacuum hot embossing a microstructured mold insert is pressed into a thermoplastic material and heated up to its glass transition temperature. After cooling down below its softening temperature, the microstructure is demolded from the mold insert. Small displacements during the compression phase and small material flow are the reasons for minimal internal stress which is an important factor for excellent optical quality of the products. Another application where hot embossing is increasingly used is the replication of large areas. Typical fluidic, homogeneous structures on samples with overall dimensions of about 100 cm² have been replicated. The thickness of the residual layer is smaller than 100 µm.

Numerous applications of microstructured polymer components e.g. in the field of microfluidics and capillary analysis Fehler! Verweisquelle konnte nicht gefunden werden. require micromolding on both sides of the sample. Typical examples are connections through the housings of microvalves and hollow needles.

Double-sided replication of a sample is achieved by mounting two mold inserts opposite to each other into a hot embossing machine. Only minor changes of the hot embossing machine are needed. The two mold inserts must be aligned to each other, because the molded structures on the front and rear mostly are not independent of each other. The precision of the alignment depends on the application and typically is in the range between 10 µm and 100 µm. This precision can be obtained in the hot embossing process with the technology known from aligned molding on prestructured substrates Fehler! Verweisquelle konnte nicht gefunden werden..

Direct contact of the two mold inserts strictly has to be avoided, because the microstructures typically may be damaged easily. Therefore, a residual layer remains between the upper and lower side of the sample. In single-sided micromolding the residual layer is located on the side of the sample distal to the microstructures. Double-sided replication allows to choose a suitable location for the residual layer anywhere over the height of the sample. It is possible e.g. to shift this layer to an intermediate position, which may be advantageous in subsequent process steps.

A suitable concept for demolding in double-sided replication is to design the two mold inserts such that the demolding forces of one mold insert are larger. This can be achieved e.g. by the number, height, and roughness of the microstructures, the slope of side walls, or by a surface treatment of the mold insert. When the hot embossing machine is opened, the sample will remain fixed to the side with the larger demolding forces. Then, demolding from this side can be achieved by the shrinkage of the sample, which takes place during cooling down.