3.1.1. Hot Embossing of Glass
Glass is a material commonly used in microsystem technology. For the manufacturing of microstructures, serial (e.g., laser structuring) and parallel manufacturing processes like etching are available. Also, cutting processes like grinding, milling, and drilling are common. These structuring methods allow the fabrication of structures in the range down to 10 μm [18]. Nevertheless, the structuring methods are limited regarding the surface quality and the damaging of structures during manufacturing. The molding, also hot embossing, of glass is therefore an alternative structuring method for the fabrication of microstructures in glass.
3.1.1.1. Materials
Glass is an amorphous material and fulfills the requirements described above. Nevertheless, compared to polymer, the material properties are different. Suitable glasses for molding are standard glasses like Pyrex (Pyrex 7740) or Borosilicate glass (e.g., D263, Schott).
Pyrex glass is characterized by a glass-transition temperature of 560°C and a softening temperature of 821°C. The typical thermal expansion coefficient is in a range of 3.25 × 10−6, significantly below the typical expansion coefficient of polymers, which are characterized by values in the range of 1 × 10−5 up to 1 × 10−4. The thermal conductivity is typically in a range of 0.59–1.19 W/m K [21], in a comparable range to the thermal conductivity of polymers (0.1–0.8 W/m K [11]).
Borosilicate glass (e.g., Schott D263) is characterized by a glass-transition temperature of 557°C and a softening temperature of 736°C, approximately 80 degrees below the softening temperature of Pyrex glass. D263 is more pliable than Pyrex [26].
3.1.1.2. Molding Parameters
The molding parameters for hot embossing of glass depend, like the molding of polymers, on the design of the mold insert, the aspect ratio of the structures, the cross-section of the cavities, and the molded area.
Schubert et al. [18] used for the molding of Pyrex (7740) glass a molding window between 700–760°C. The viscosity in this temperature range was determined between 109.5 and 108.5 dPa. The replication refers to a silicon mold with smallest structures in the range of 10 μm and a height of 50 μm.
Takahashi et al. [21] also replicated structures with a height of 50 μm in Pyrex glass. Aspect ratios between 1 and 1.66 were investigated. The systematic replication experiments referred to a molding temperature range between 640°C and 650°C, a pressure of 2.83 MPa, and holding times between 300 seconds and 1,200 seconds. Microstructures with 1 μm line and space patterns with a height of 1 μm were replicated at 595°C, a pressure of 0.45 MPa, and a holding time of 180 seconds [22].
Yasui et al. [26] replicated micro- and submicrostructures into borosilicate glass D263. Patterns on an area of 20 μm2 with a depth of 6.5 μm were replicated successfully in a temperature range of 590–600°C and a pressure range of 0.22–0.66 MPa. If the structure size decreased to a line and space pattern of 0.4 μm, the molding temperature increased to 650°C at a pressure of 6.37 MPa.
3.1.1.3. Mold Inserts
Regarding the high temperatures during molding of glass, the requirements compared to the requirements of molding polymers expands to the issues of high temperature resistance and thermochemical stability.
Suitable are high temperature–firm metals like nickel-based alloys or molybdenum alloys. Nickel-Wolfram alloy Incoloy [26] has a thermal expansion similar to borosilicate glass. But also ceramic material, like silicon carbide or aluminum nitride, can be used. Silicon is also a suitable material. If a coating is desired, titanium nitride, for example, can be deposited.
An alternative is glassy carbon [21,22] structured in the micron range, for example, by focused ion beam–machining (FIB) or dicing. Glassy carbon is suitable up to a high temperature of 1,400°C, comparable to the transition temperature of quartz glass. The operation limit is in a range of 2,000°C and the thermal conductivity is in a range of 9 W/m K, significantly below metals.