# Manufacturing of Micro-Lens Array Using Contactless Micro-Embossing with an EDM-Mold
## 1 Introduction  
The micro-lens array (MLA), called a Fly-eye lens, is widely used in optical communication, lighting displays, optical sensors, and illumination because of its unique structures and function. There are many methods that can fabricate micro-lenses, including plasma etching [1], laser ablation [2], micro-milling [3], photolithography [4], micro-injection [5], glass reflow [6] and micro hot embossing [7; 8; 9; 10; 11; 12]. Wherein, micro hot embossing is regarded as one of the most promising processes for mass production.  
Micro hot embossing is a flexible and low-cost technique, consisting of heating, embossing, and cooling. In recent years, many types of hot embossing processes have been reported, including PDMS (polydimethylsiloxane) soft mold imprinting [13; 14; 15; 16; 17], Deep reactive ion etching (DRIE) Si template duplicating [18], LIGA (Acronym of German words: Lithografie, Galvanik, Abformung) Ni mold copying [19], focused ion beam (FIB) mold embossing [20; 21], and roll-to-roll embossing [22; 23]. The quality of the embossed micro-lens depends on the precision of the mold insert. For morphologic accuracy of mold fabrication, the mold with the micro-hole array is an effective tool to form the micro-lens array. [PERSON] et al. [24; 25] first proposed the contactless embossing technology to fabricate a micro-lens array (CEM). CEM is a potential method for the achievement of mass production. [PERSON] et al. [26] fabricated a polymer refractive micro-lens array on a stainlesssteel through-holes template by contactless hot embossing. [PERSON] et al. [27] investigated the effects of processing parameters in the polymer substrates contactless embossing process. The method is a reliable way to produce a micro-lens. In recent years, micro-EDM was widely investigated [28; 29]. However, it was first used to fabricate the micro-hole array in this study, which was then used for the manufacturing of the micro-lens array.  
The friction coefficient between polymethyl methacrylate (PMMA) and the mold interface plays an important role in the process, affecting the molding force and demolding force [30; 31]. The friction at the interface of mold/PMMA affects the wear of the mold surface (the mold life and the quality of the embossed lens) [32]. [PERSON] and [PERSON] [33] used a ring compression test to characterize the friction behavior, indicating the universal friction calibration curves. The ring compression test is a reliable way to determine the friction behaviors between PMMA and the mold [34; 35; 36].  
The fabrication of a micro-lens array was investigated using contactless micro-embossing with an EDM-mold. To verify the friction behaviors between PMMA and the mold, ring compression tests were carried out on a Shenzhen University's precision glass molding machine. Then, the micro-hole of the SKD-11 mold was fabricated by electric discharge machining. After that, we obtained a series of micro-lens arrays by contactless embossing at different processing parameters. ABAQUS software was used for the finite element simulation of the PMMA embossing process. Finally, the micro-optical performance of the micro-lens array was measured using optical experiment apparatus.