*3.1.1 LIGA*

*Advances in Microfluidics and Nanofluids*

**Technology Feature** 

Deep reactive ion etching

**Table 1.**

**size (**μ**m)**

**38**

**Figure 5.**

*Micro milling and die-sinking EDM fabricated mold insert: graphite electrode (a) and stainless tools (b) [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

Electroforming 0.3 <10 <10 Copper/nickel/alloy

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

**Surface roughness (**μ**m)**

Micro milling 25 ~ 100 0.2 ~ 5 10 Brass, COC, steel *μ*EDM 10 ~ 25 0.05 ~ 1 50 ~ 100 Conductive materials ECM 10 0.02 NA Conductive materials

Laser machining 1 ~ 5 0.4 ~ 1 <50 Any X-ray lithography 0.5 0.02 100 Photoresist UV lithography 0.7 ~ 1.5 NA 22 Photoresist

**Aspect ratio**

2 NA 10 ~ 20 Silicon

**Material**

*Copyright (2015) with permission from IOP publishing, LTD.*

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 technologies assisted by electroforming.

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

#### **Figure 6.**

*(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.*

#### **Figure 7.**

*Basic schemes of manufacturing of mold insert.: (a) metal substrate, lithography, and electroforming; (b) a silicon substrate, lithography, and electroforming; (c) deep reactive ion etching of silicon (d) micro-machining of non-silicon substrates [9]. Copyright (2020) with permission from IOP publishing, LTD.*

feature size of several micrometers to hundreds of micrometers, although the reachable aspect ratio of this technique is limited in the range of ~20 [34]. Considering the specific microfluidic applications, UV-LIGA is sufficient to fabricate the desired micro structures. Additionally, some LIGA-like processes, such as EUV-LIGA, EBL-LIGA, IB-LIGA, are also developed towards nanoscale micro structuring [44]. However, these LIGA-like techniques still have some shortcomings for mold insert fabrication. First, the mold materials usually are nickel and nickel alloy, the tool steel is not included due to the restriction from the electroforming process. Besides, the integration of draft angle onto mold insert is not easy, but it is critical for demoulding of the polymeric part from mold insert to reduce the potential demoulding deformation and damages of micro structures [45]. The research shows that the draft angle can be achieved by properly adjusting the UV/X-rays exposure dose.
