## 1 Introduction
### Epoxy-Based Photoresists  
The negative tone photoresists are based on a resin known as SU-8, where the molecular structure is photosensitive. It was developed initially by IBM, which patented it in 1989; afterward, it was continued to be produced by Shell Chemical (EPON SU-8 epoxy resin) [20; 21]. The molecular structure of the resin present in the photoresists (Figure 2) has three main organic groups in the chemical structure: the epoxy groups (responsible for crosslinking), the aryl rings, and the methylene bridges that bind the polymer chain. The polymer has an excellent ability to interact with its own chains; however, when mixed with other polymeric materials, low adhesion and interaction are observed. It displays good adhesion to metallic surfaces, surfaces pretreated with plasma, and with hydroxyl groups (OH-) bonded to the surface. The photoresist exhibits high optic transparency for wavelengths above 360 nm and high sensitivity to ultraviolet (UV) wavelength (350-400 nm); it is inert to several solvents, bases, and acids and presents thermal-stability after curing (crosslinking reaction) [14; 22].  
The photoresists also contain at least more two components, a solvent and a catalyst to initiate the crosslinking process. The solvent is normally used to tune the viscosity of the polymer; the most applied solvents are cycle ketones or lactones (pentanone or gamma butylolactone). The catalysts, also known as photo acid generators (PAG),are divided into two groups: ionic and non-ionic. The families of triarylsulfonium and diaryliodonium hexafluoride salts are largely applied to initiate the crosslinking reaction (curing process) of epoxy resins, such as the tris(4-methoxy1)sulfonium hexafluoroarsenate [23]. These salts form strong acids to start the crosslinking process, releasing a proton to initiate the reaction in the epoxy groups [23]. The crosslinking reaction can be initiated in acidic or basic conditions; the type of catalyst defines the mechanism. In most cases, the epoxides are polymerized in acidic conditions with the help of high-energy radiation [24], taking as an example the salts mentioned above. The catalysts are present in a range of 1-10% in weight at the composition of the photoresist [25; 26].  
Although the mechanical properties of photoresists and their processing parameters are very well described in the literature [27; 28; 29; 30; 31; 2; 32; 3; 3; 3; 3; 3; 3; 3; 30; 31; 32], less attention has been given to the chemical structure and the effects caused by the catalysts, additives, and solvents in the photoresists' mixtures. The investigation of the chemical properties concerning the quality of the final structures fabricated by DXRL can help to improve the quality of the structures. It also helps to establish a method to evaluate the quality of the photoresist before its use and to estimate its structural quality. This work aims to characterize the negative photoresists used in the standard fabrication of microstructures by DXRL  
Figure 2: Polymer structure for most of the negative tones of photoresists.  
and to evaluate their chemical compositions to optimize the quality of the final structures taking into account the process parameters.  
In this work, two types of negative tone photoresists were evaluated. The first type was named EPR-S1, of which two lots were evaluated (\"a\" and \"b\"). The second negative photoresist used to achieve HAR structures was named EPR-S2, of which eight different lots were evaluated (\"a\" to \"h\").