Preparation And Photosensitive Mechanism Of Negative Photosensitive Polyimide

Photosensitive polyimide has become an indispensable and important material in the field of microelectronics technology due to its excellent properties such as high heat resistance,high mechanical strength and low dielectric constant.It has been widely utilized as stress buffer layer and multi-layer interconnection dielectric layer for microelectronic packaging.Recently,photosensitive polyimide is also applied as flat layer and pixel definition layer in the booming field of OLED flexible display.However,it is well-known that our country relies heavily on imported photosensitive polyimide materials,which is mainly controlled by American and Japanese enterprises.This dissertation focuses on the preparation and performance research of negative photosensitive polyimide materials,and tries to solve the following two problems:(1)The molecular weight of polyimide or its precursor has a great impact on the performance of photolithographic pattern.Lower molecular weight of matrix could accelerate the photolithography process,but it will lead to lower thermal and mechanical properties of the corresponding polyimide pattern.So how to balance the contradiction?(2)Most negative photosensitive polyimide materials are developed by organic solvents,which is unfavorable for pattern quality and environmental protection.How to design a negative photosensitive polyimide material system that can be developed by alkaline solution?Aiming at the above two problems,this dissertation exhibits a series of design strategies for negative photosensitive polyimides,studies their photolithographic performances,thermal and mechanical properties,and explores their application in patterned electrode.The main research contents are as follows:(1)Based on different molecular weight of poly(amic acid)adjusted by the addition of low-activity diamine monomer of 3,5-diaminobenzoic acid,a series of ionic negative photosensitive polymers are prepared for comparison.The results show that the molecular weight of the co-poly(amic acid)gets lower with increasing content of carboxyl groups,resulting in higher the photosensitive sensitivity of the corresponding photosensitive system.The photosensitivity of the sample with 75 mol%carboxyl-containing diamine could reach to50 m J/cm~2,but shows a low quality of lithography pattern.The hydrogen bonds between the excess carboxyl groups limit the dissolution of unexposed polymer film in developer,leading to some inhomogeneous holes in the resulting polyimide pattern.(2)In order to balance the contradiction between thermal disadvantages and photolithographic advantages of low-molecular-weight matrix,phenylethynyl-containing reactive end-cappers is introduced to the molecular chain of poly(amic acid)for providing continuous reaction sites in the subsequent imidization process.Thus,the ionic negative photosensitive materials based on these end-capped poly(amic acid)s are prepared for investigating the performance difference.The results suggest that the poly(amic acid)with lower molecular weight shows higher photosensitivity and better edge steepness of photolithographic pattern.Although the thermolysis property of polyimide derived from low-molecular-weight poly(amic acid)is worse,the crosslinking reaction based on phenylethynyl end-groups will improve the heat-resistance during imidization,the heat resistance.The T_gvalue of polyimide containing 10 mol%end-capper is higher than that of the uncapped one.(3)To achieve the development process by alkaline solution,diphenylethynyl group is introduced into the main chain molecule of poly(amic acid).The main purposes are as follows:the alkynyl groups provide the active sites for photoinduced cross-linking reaction to create a bridge between poly(amic acid)molecular chains.Meanwhile,the retained carboxyl groups will achieve the purpose of alkaline solution development.Then,a series of co-poly(amic acid)s with different flexible backbone are synthesized by polycondensation of 4,4'-(acetylene-1,2-diyl)phthalic anhydride with different diamines.The results show that these intrinsic photosensitive polymers containing alkynyl group could obtain clear negative pattern with a resolution of 30?m after photolithographic process,but the sensitivities of these photosensitive poly(amic acid)s are higher than 10 m J/cm~2 due to the rigidity of diphenylethynyl.The increasing content of ether bond in the molecular structure of poly(amic acid)is conducive to accelerating the intermolecular photoreaction,but the T_g value of corresponding polyimide decreases gradually.Moreover,their coefficients of thermal expansion show an increasing trend.However,the performance of thermostability for the obtained polyimides exhibits no significant difference and their decomposition temperatures with 5%weight loss are about 560?.(4)The conjugated rigid structure of diphenylethynyl group is not beneficial for the intrinsic photo-crosslinking reaction.To further improve the photosensitivity,thiol crosslinking agent and photo-initiator are added to the diphenylethynyl-containing poly(amic acid)solution.The linking bonds could be formed through click reaction between the sulfhydryl group of thiol and the alkynyl group of poly(amic acid),raising the rate of photoreaction.The results show that the introduction of click reaction significantly increases the sensitivity to 555 m J/cm~2,which is 28 times higher than that of pure poly(amic acid)with alkynyl groups.However,a large number of by products such as sulfoxide will appear in the processes of pre-baking and UV irradiation,restricting the photoreaction rate.In order to improve the absorption at i-line of the above photosensitive material based on click reaction,butanediynyl groups are further introduced into the main chain of diphenylethynyl-containing poly(amic acid).However,the sensitivity reduces with a rate of about 70%due to its more rigid structure.Also,the thermal stability of corresponding polyimide pattern is improved slightly.(5)Photosensitive polyimide has an edge in terms of the precursor for carbonized materials due to the considerable nitrogen content.In order to further expand the application of photosensitive polyimide in energy storage devices and other fields,polyimide-derived carbon patterns are obtained by pyrolysis of photolithographic pattern.The results show that the pattern resolution decreases moderately after carbonization,from 26?m to 30?m after carbonization at 700?or 800?.However,the lines in the pattern still show a clear boundary.As the carbonization temperature raises to 900?,the pattern has been damaged apparently.In addition,the content of nitrogen reduces gradually,from 6.40%at 700?to 2.95%at 900?,but the degree of graphitization will improve.Besides,the carbon pattern could be simply transferred to a flexible substrate fabricated by a spin-coating process,to achieve the practical application in patterned electrode.