## 1 Introduction  
New and existing technologies on the market have increased the necessity of highly functional optical elements. High surface quality Diffractive Optical Elements (DOEs) offer an alternative to traditional refractive optics due to their flexibility, reduced thickness, and consequently light weight, as well as relatively low material consumption [1]. DOEs are typically used in equipment for applications in telecommunication, imaging, lithography, biomedical devices, optical sensors, and cameras [2, 3, 4, 5, 6, 7]. Injection moulding (IM) is the main replication method for the mass manufacturing of polymer components. However, when replicating high-aspect-ratio micro- and nanoscale features over large areas, HE is utilised [8, 9]. HE has flexibility whereby it can be used for both small-scale and mass production and has proven to be a cost-effective technology for polymer lens manufacturing [10; 11]. In addition, HE offers several benefits, such as lower manufacturing costs due to the ability to utilise a single-stage mould for mass production. HE also yields excellent results with regards to precision [12; 13]. The process of HE requires that the temperature of a polymer substrate be raised above the polymer glass transition temperature (T\({}_{\mathrm{g}}\)). Then the mould insert moves towards the substrate. During contact between the mould insert and the substrate, an embossing force (F\({}_{\mathrm{e}}\)) is applied [14]. Next, the cooling stage of the process begins at which time the polymer and mould are kept in contact above T\({}_{\mathrm{g}}\), and a slow cooling cycle is applied to reduce internal stresses (a requirement for optical functionality) [15]. The final stage is part demoulding whereby the mould and polymer are separated from each other by opening the tool. Tool and substrate adhesion is a very delicate stage of the process as the demoulding forces can inadvertently damage the replicated features [16]. To enable plastic deformation and material flow, the following process factors are considered: melting point of the polymer (T\({}_{\mathrm{m}}\)), embossing temperature (T\({}_{\mathrm{e}}\)), embossing pressure (P\({}_{\mathrm{e}}\)), and embossing time (E\({}_{\mathrm{t}}\)). For the manufacturing of polymer devices, microfluidic optimum replication results differ. In a study by [PERSON] et al., an optimum process yield was achieved at an embossing temperature of 130 \({}^{\circ}\)C and embossing time of 5 min [17]. [PERSON] he et al. found that optimum replication fidelity was achieved when utilising a T\({}_{\mathrm{e}}\) of 150 \({}^{\circ}\)C, T\({}_{\mathrm{e}}\) of 30 min, and P\({}_{\mathrm{e}}\) of 300 N [18]. [PERSON] et al. concluded that optimum replication results were observed at T\({}_{\mathrm{e}}\) of 115 \({}^{\circ}\)C, P\({}_{\mathrm{e}}\) of 10 KN, and T\({}_{\mathrm{e}}\) of 8 min [19]. In the research by [PERSON] et al., high accuracy was achieved with a T\({}_{\mathrm{e}}\) of 135 \({}^{\circ}\)C, P\({}_{\mathrm{e}}\) of 30 kg/cm\({}^{2}\), and E\({}_{\mathrm{t}}\) of 180 s. In addition, the same study also observed that embossing temperature has a crucial impact on the replication accuracy, and it contributes 70.52% of the overall process accuracy [12]. [PERSON] and [PERSON] [20] found that three common characteristics of the HE method are as follows: (1) The majority of the replicated structures have sub-mm sizes, typically 100-400 \(\upmu\)m channels, dimples, and pyramids [21; 22; 23; 24]. HE at the lower end of the micro-scale (1-10 \(\upmu\)m) is exceptionally rare. (2) The majority of the hot embossed parts have coin-sized areas, e.g., microfluidics [25; 26], micro-lenses [27; 28; 29], and micro-electromechanical systems (MEMS) [30; 31]. Many of the studies have been performed on poly (methyl methacrylate) (PMMA) [31; 32; 33; 34; 35; 36; 37]. For the manufacturing of lenses, the advantages of HE can be witnessed in the low material flow rate, which prevents internal stresses and facilitates the replication of delicate microstructures [38].