Research On Material And Fabrication Of Microbridge Resistor Arrays For Infrared Scene Projectors

Dynamic infrared scene simulation system has been widely studied and has evolved intoa critical laboratory tool for measurement and evaluation of infrared imagers. The core of thesimulation system is an infrared scene projector. Due to their several advantages such as largespectral band, large dynamic range, high resolution, high frame rates, no flicker, Themicrobridge resistor array infrared projectors have been become the international focus on thedevelopment of the infrared scene projection technology.This dissertation is mainly concerned with the candidate dielectric and metal materials forthe microbridge resistor array and fabrication of160×120arrays. The author’s majorcontributions are outlined as follows:1. The deposition and internal stress of hydrogenated amorphous silicon (α-Si:H Abbre.α-Si) thin films have been studied. α-Si thin films were deposited by plasma enhancedchemical vapor deposition (PECVD),the results show that the main parameter(gas flow rateand RF power) play an important role on the deposition rate and refractive index of the film.The results also indicate that increasing RF power can change the state of the internal stressfrom tension to compression while increasing pressure can change the state of the internalstress from compression to tension. The internal stress in α-Si thin film is strongly dependenton substrate temperature and the internal stress increase with temperature. The total hydrogencontent, SiH and SiH2content increase with RF power. Finally, the optimal processparameters for low stress film were obtained.2. The stress of SiOx and SiNx films and their comparison of properties have beenstudied. SiOx and SiNx films were deposited by PECVD. The stress, Young modulus,hardness, thermal stability, and infrared optical absorption properties of the two types of filmswere investigated and compared. The results show that the SiNx film stress decrease withincreasing the RF power and SiH4/NH3(or SiH4/N2)flow ratio, while increase with thepressure. The H content of the film deposited with N2is less than that of the film depositedwith the NH3. The SiOx film stress becomes more compressive with the N2O/SiH4or RFpower increase. The SiOx film has lower Young modulus and hardness and has a betterthermal stability when compared with the SiNx film. Moreover, the peak wavelength is11.6μm for SiNx film and9.4μm for SiOx film, respectively. The maximum absorptioncoefficient is1.61×104cm-1for SiNx film and2.18×104cm-1for SiOx film, respectively. Theintegral absorption intensity between8μm and12μm of the SiOx and SiNx films isapproximately equal. The results also show that the SiOx and SiNx films can be easily etched by plasma.3. The deposition and properties of silicon oxynitride (SiOxNy) films have been studied.SiOxNy films were deposited by PECVD. The N2O/NH3flow ratio was varied in order toobtain different oxynitride compositions. The compositions, optical constants and infraredoptical absorption properties of silicon oxynitride films were investigated. The results showthat the O atomic content increases and the N atomic content decreases while the Si atomiccontent keeps nearly unchanged as the N2O/NH3flow ratio increases. The silicon oxynitridefilms show a dominant infrared absorption peak due to the Si-O/Si-N bond, with the infraredabsorption peak located between11.6μm (860cm-1) and9.4μm (1063cm-1). The position ofabsorption peak also shifts to a shorter wavelength when increasing the N2O/NH3flow ratio.Meanwhile, the width of absorption peak increases firstly and then decreases with the N2O/NH3flow ratio increasing. Moreover, the H content and the refractive index decrease with theflow ratio increasing. Compared with silicon oxide and silicon nitride films, the siliconoxynitride films with a specific composition have largest width of absorption peak andstrongest intensity between8μm and12μm. The results also show that the stress of SiOxNyfilms is low and can be further changed by N2O/NH3flow ratio. SiOxNy films can be used inoptical coatings as inhomogeneous layers and SiOxNy films have low absorption in thevisible spectrum and low stress at the same time. Finally, the Young modulus and hardness ofthe four kind of dielectric material mentioned above were compared.4. Structural and electrical properties of TiWN thin films have been studied. TiWN thinfilms were deposited by reactive RF sputtering. The nitrogen partial pressure ratio in thegrowth chamber was varied from0to11%. The influence of the nitrogen partial pressure ratioon the properties of TiWN thin films was studied. The electrical resistivity of the TiWN thinfilms vary gradually from117.5μ cm to675μ cm with the increasement of nitrogenpartial pressure ratio from0to6%. However, the resistivity increases sharply if the ratio wasraised further. XPS analysis shows that nitrogen concentration in the films increase with thenitrogen partial pressure ratio and reached36%when the ratio was11%. Binding energyanalysis shows that W is mainly in W2N and Ti is mainly in TiN when the films contain anitrogen concentration of36%. GIXRD analysis confirm that a FCC (Ti,W2)N is present inthe film. Moreover, the temperature coefficient of resistance (TCR) of TiWN thin films wasinvestigated and the typical TCR is-391ppm/℃. The results also show that the formed filmcan duplicates the morphology of the substrate well.5The design, fabrication and thermal properties of microbridge structure have beenstudied. The structure design includes selection of material and layout design. A free-standing structure was fabricated by using micromachining methods. The TiWN film was patterned bymeans of lift-off method. Furthermore, one-level160linear array and two-level160×120array of microbridges were designed and fabricated by using surface micromachining,utilizing polyimide sacrificial layer. The finite element method was used to analyze thethermal properties of microbridge. The temperature distributing analysis indicates a gooduniformity on the surface of microbridge and the thermal deformation is very small. Moreover,the temperature of surface can be increased remarkably when a lower thermal conductivitymaterial is adopted.