**3.1 Tooling technology**

There are a variety of manufacturing technologies available to fabricate micro mold inserts, such as ultraprecision micro milling, micro-electrical discharge machining (*μ*EDM), electrochemical machining (ECM), silicon wet etching, deep reactive ion etching, laser machining, and LIGA-based processes (LIGA, UV-LIGA). **Table 1** shows a comparison among the mold insert manufacturing technologies according to the achievable feature size, surface roughness, aspect ratio, and machinable materials.

## *Advances in Microfluidics and Nanofluids*


#### **Table 1.**

*Comparison between various manufacturing techniques for the fabrication of microstructured mold insert [33].*

The selection of mold insert fabrication technology is based on the geometrical complexity, desirable feature size, aspect ratio, surface roughness, and processing cost. Micro milling or EDM generally is used for fabricating micro structure with feature sizes larger than 50 μm with a tolerance of several micrometers. However, it is difficult to achieve a low surface roughness and some sharp corners processing. **Figure 5** shows the microfluidic mold insert fabricated by micro milling and die-sinking EDM process. LIGA, silicon wet etching, and deep reactive ion etching have excellent advantages for sub-micron fabrication [34]. ECM is suitable for structuring 3D features with less sub-surface damage and higher dimensional precision at the nanometric range [35]. The selection of mold insert materials is also critical. When the production yield is less important and the mold insert lifetime is not critical, silicon wafer mold insert fabricated from deep reactive ion etching is favorable to the precision replication of plastic microchip with optical surface finishing [36]. However, in most instances, a long-life metal mold insert having high wear resistance and hardness is desirable for the mass production of microinjection molded microfluidic chips. For example, stainless steel mold insert can be against the feature being worn or surface quality degradation under up to tens of thousands of microinjection molding cycles. In this case, ultraprecision machining, LIGA process, laser machining, and μEDM can be more viable. Also, the LIGA process and μEDM can fabricate the micro structure with a high aspect

#### **Figure 5.**

*Micro milling and die-sinking EDM fabricated mold insert: graphite electrode (a) and stainless tools (b) [33]. Copyright (2015) with permission from IOP publishing, LTD.*

**39**

**Figure 6.**

*Prototyping and Production of Polymeric Microfluidic Chip*

technologies for mold insert fabrication are detailed.

ratio and tight tolerance [37, 38]. **Figure 6** shows the dry-etched silicon wafer mold, UV lithographic photoresist mold, and electroformed nickel mold, which were fabricated by the author' team. In the following sections, the main manufacturing

LIGA technique comprising of lithography, electroforming, and molding is a multi-step replication process to generate micro structure with the desired patterns [39]. It has been a promising technology for industrial-scale commercialization [40, 41]. The typical process methods follow the consequential steps below: 1) the photoresist (AZ or epoxy resin SU-8) is firstly evenly coated on the silicon wafer substrate along with the subsequent baking process; 2) a pre-prepared photomask with the desired patterns is placed on the top surface of the photoresist at a good alignment manner for further irradiation exposure; 3) the exposed areas is removed/ remained chemically using developer (depending on the type of photoresist), where the patterns are transferred to silicon wafer from mask and the dimensional accuracy of patterns can be controlled by lithography parameters (exposure time and exposure dose); 4) seed layers of adhesive layer (Ti/Cr) and conductive layer (Au/Ni) are sputtered onto the structured photoresist surface for metallization; 5) the following step is electroforming for fabricating a microstructured mold insert, where the metallized patterns on silicon wafer serve as a cathode for nickel deposition; after electroforming, a electroformed replica is relived via silicon chemically etching and photoresist is chemically removed; the final replica can be used as a mold insert; 6) such an electroformed mold insert can be used as a master for replication of microfluidic chips by microinjection molding process [42]. **Figure 7** details the specific process steps of mold insert fabrication in various microstructuring tech-

Due to costly x-ray synchrotron for X-ray lithography, the LIGA process is not commonly used in the industrial field. As an alternative, UV lithography has been a favorable technology to prepare master for electroforming [37, 43]. As a result, UV-LIGA has been an acceptable process to fabricate the mold insert with the

*(a) Dry etched silicon wafer; (b) UV lithographic photoresist master; (c) and (d) electroformed nickel mold* 

*insert. Copyright (2020) with permission from IOP publishing, LTD.*

*DOI: http://dx.doi.org/10.5772/intechopen.96355*

nologies assisted by electroforming.

*3.1.1 LIGA*

ratio and tight tolerance [37, 38]. **Figure 6** shows the dry-etched silicon wafer mold, UV lithographic photoresist mold, and electroformed nickel mold, which were fabricated by the author' team. In the following sections, the main manufacturing technologies for mold insert fabrication are detailed.
