Photosensitive polyimide with high pattern stability and low thermal expansion coefficient based on poly (amic acid) containing dual cross-linked groups

Abstract

To obtain negative photocured polyimides with clear photopolymerized patterns and a low thermal expansion coefficient, we synthesized a type of photosensitive polyamic acid (PAA). Using 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA), p-Phenylenediamine (PPD), and 3,5-Diaminobenzoic acid (DABA) containing an active carboxyl group as monomers, we synthesized a polymer chain, then grafted the photo-crosslinking groups 2-(Dimethylamino)ethyl methacrylate (DMAMA) and Cinnamyl alcohol (CA) onto different carboxyl groups in the polymer backbone, with DMAMA formed ionic bonds and CA formed ester bonds, resulting in a photosensitive polyamic acid (PSPAA) with two types of photo-crosslinking groups. After UV curing, this formed a high-density cross-linked network and was converted into polyimide through high-temperature imidization at 330°C. The CA ratio was controlled to synthesize four different PSPAAs (1, 2, 3, 4). The photolithography process parameters of the four PSPAAs and their corresponding PIs were systematically studied. The photocuring properties and thermal properties of PSPAAs and PIs were characterized using Fourier transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (¹H-NMR), scanning electron microscopy (SEM), and other techniques. The tensile strength of PI4 could reach 118 MPa, and the thermal expansion coefficient was as low as 7.8 × 10⁻⁶°C⁻¹. The photolithographic pattern resolution of PSPAA2 was 18 μm, while the line spacing could be as low as 3 μm; PSPAA4 demonstrated good solubility resistance of the exposed area and maintained pattern integrity even after high-temperature processing. This research provides a foundation for the development of new photosensitive polyimides.