Vapor Deposition Polymerized Polyimide Thin Films For Miniaturized Electronic Devices

Polyimide(PI) is aromatic heterocyclic polymer compound, which containing imide groups in its molecular structures. The polyimide has been found more than 100 years, but really get large-scale applications started until the mid-twentieth century. With the development of aerospace technology, the demand for structural materials of the heat-resistant, high-strength, light-weight in various sectors get more urgent, which prompting polyimide become the most widely used in the field of polymer materials.Early on, the main application of polyimide is as structural material. In recent years, although the application of polyimide in the field of electronics and optoelectronics are expanding rapidly, such as widely used in the interlayer insulating, device package, devices anti-radiation reinforcement, surface passivation and integrated optical waveguides, etc.. However, the polyimide still acting as auxiliary materials used in these fields, never be a core part of the device. Therefore, as a function material, the depth and breadth of its application is still limited, particularly in the field of microelectronic devices(micro-electronic devices, micro-mechanical devices, etc.), the application potential have not yet to be fully excavated.At present, the main obstacle of the polyimide applying in micro and small devices is preparing and processing techniques for the polyimide film. The conventional "wet" techniques is not only difficult to prepare high quality, thinner polyimide film, but also difficult to achieve the local area covering in micro-devices. In order to expand the application depth of polyimide film in the micro-electronic devices, this paper we attempts to use vacuum technology(referred to as: vapor deposition polymerization) to prepare polyimide film, and further explore the application of polyimide as a core part in thin film bulk acoustic resonator, piezoelectric micromachined ultrasound transducers and GaN high electron mobility field effect transistor. Therefore, we carried out the following works in the dissertation:First, a co-evaporation vacuum equipment has been self-design and builded, which lay the foundation to carry out the "vapor deposition polymization" preparation of polyimide film. The vacuum systems equipped with two thermal evaporation sources furnace and an electron gun thermal evaporation sources. The thermal evaporation sources furnace using Knudsen cell that often configured in molecular beam epitaxy equipment, mainly used for evaporate the monomer to synthesis polyimide film. The electron beam evaporation source adopting an e-type electron gun(e-gun), which mainly used for evaporating metals materials.Base on the self-designed co-evaporation vacuum equipment, the research on preparation process of polyimide film by vapor deposition polymerization was carried out firstly. Pyromellitic dianhydride(PMDA) and 4,4 ’-diaminodiphenyl ether(ODA) are used as monomer to synthesize polyimide evaporated by thermal evaporation sources furnace. In the process of preparing polyimide film, the PMDA and ODA vapor inject on the substrate and react to form polyamide acid(PAA), then in situ heating of substrate been carried out to achieve a polymerization reaction, thereby the polyimide film was prepared. The influence of process parameters, such as PMDA and ODA evaporation temperature, substrate temperature during film deposition, polymerization temperature and post-polymerization time on the microstructure and electrical properties were systematic studied.After master the process of “vapor deposition polymerization” method to prepare polyimide film, the further explore was carry out which is the preparation of composite films of nano-sized metal particles and polyimide. The preparation process is: at the same time of thermal evaporation PMDA and ODA, using electron beam evaporate metal Cr(represented by metal chromium, other metals may also be used), so that the nano-sized metal Cr particles are uniformly dispersed in the polyimide film. The electrical properties of the prepared metal nano-particles / polyimide composite film were systematic studied.Based on the preparation and characterization of vapor deposition polymerization polyimide films, the paper chose three typical miniature electronic devices to carry out the application research of vapor deposition polymerization polyimide films used in micro and small electronic devices.Firstly, this paper explores the vapor deposition polymerization polyimide films used in passive microwave devices. The solid state assembly-type thin film bulk acoustic resonator(FBAR) is selected for the object of study. In the FBAR device, the PI film is used as a low acoustic impedance material instead of the traditional SiO2 film, and metal Mo film as a high acoustic impedance material. Two cycles of Mo/PI multilayers were setting as Bragg reflection, the c-axis oriented AlN piezoelectric thin films were prepared by middle frequency magnetron reactive sputtering. The electromechanical coupling coefficient of as prepared FBAR is 5.4%, which preliminary verifies the application value of vapor deposition polymerization polyimide films as low acoustic impedance material used in FBAR.Secondly, the "vapor deposition" polyimide film used in MEMS devices(MEMS) has been explored. The piezoelectric micromachined ultrasound transducer(pMUT) is selected for the object of study. The polyimide film is used as acoustic impedance matching layer and supporting layer is used in the devices. Piezoelectric AlN film is used as perceive sound wave material. The study results show that: the performance of device is significantly improved, compared with the traditional structure, the detected sound and light signal strength increased by 1 times of the pMUT device use PI as acoustic impedance matching layer and supporting layer. The photoacoustic medical imaging application potential was also demonstrated and achieved good results.Finally, the application of nano metal particles / polyimide composite film in microelectronic devices is been explored. The use of the film in high electron mobility transistors(HMETs) based on GaN is studied. In the device, nano metal particles / polyimide composite film is used as a high dielectric constant passivation layer of GaN HEMTs. The study results showed that: the introduction of metal nano-particles / polyimide composite film not only improve the output characteristics of the device, but also significantly increase the breakdown voltage of the device. Compared with the GaN HEMT passivated with PI film, the breakdown voltage is increased by 60% of the device using PI/Cr as passivation layer. The withstand voltage capability of the device is significantly improved, which reflecting a similar "field plate" effect(hereinafter referred to as "high dielectric field plate "). Since the metal particles / polyimide composite film shows technology compatibility with GaN device and the device technology is simple, therefore, high dielectric constant polyimide film will have important value in the development of GaN power devices.