Electrical/Thermal Properties And Crystallization Kinetics Of RAS Glass-ceramics For Anodic Bonding

In recent years,there has been an increasing interest in glass-ceramic applications in anodic bonding areas.Pyrex glass,SD-2 glass and AF-45 glass are commonly used as anodic bonding glass,but there are some problems,which are mainly the package temperature up to 450-500℃and package voltage nearly 1000V.In addition,accompanied by the high temperature during packaging,a large thermal stress may be produced in the substrates,which affects the accuracy and sensitivity of devices,or even leads to serious damage to the devices.All these problems have brought more difficulties for the applications of MEMS(Micro electronic mechanical system). Therefore,it is extremely urgent to seek new bonding materials.Compared with those glasses mentioned above,glass-ceramic has a lot of advantages such as high mechanical strength and hardness,good chemical and thermal stability,good insulation,small dielectric loss,steady dielectric constant and so on.The glass-ceramics have the characteristics of glass and ceramics, the most outstanding property of glass-ceramic is the thermal expansion coefficient which can be adjusted in a wide range.Thus with a wide variety of materials suitable for thermal bonding,and keep the match close to the bonding material in the physical and chemical properties.So glassceramics is a key material which is expected to replace anodic bonding glass and to improve the quality and the overall level of the MEMS package due to its excellent performances.On the basis of our preliminary work,the experiments used R₂O-Al₂O₃-SiO₂(R₂O means Li₂O, Na₂O and K₂O) system glass ceramics as a cathode substrate materials and used stainless steel as an anodic bonding materials instead of traditional anodic bonding glass and silicon.The two basic glasses were prepared by adopting conventional melting-quenching technology and the crystallization kinetics of RAS glass has been analyzed by differential scanning calorimetry(DSC). According to DTA curves,the temperature of nucleation and crystallization are determined,then the basic glasses were heated according to different heat-treatment schedule.The main crystal phase and microstructure were investigated(or examined) by XRD and SEM.The effect of basic glass composition and heat-treatment schedules on the electrical properties,such as electrical conductivityσ,dielectric constantε_r,dielectric loss D,and thermal properties,such as thermal expansion coefficient,of glass-ceramics was studied.The kinetic parameters such as the glass crystallization activation energy(Ea),the Avrami index(n) and the glass transition activation energy(Et) were calculated by Ozawa and Kissinger method.By the optimization of adjustment of the composition and heat-treatment schedules,the RAS glass ceramics which have good electrical properties and thermal properties used as anodic bonding materials could be produced.The results have been got as follows:1.As to the RAS(R₂O-Al₂O₃-SiO₂) system basic glass,the phases of glass-ceramics(composition A) are Li₂Si₂O₅ crystal and Li₂SiO₃ crystal identical with the two exothermic peaks on DSC curves,while the phases of glass-ceramics(composition B) is Li₂SiO₃ crystal after heat treatment.The dielectric constant and dielectric loss of the RAS system glass-ceramics is lower than that of the basic glass.All the samples have good stability under very high frequency(30MHz-300MHz) at room temperature,the dielectric constant of the samples are about 4.3 to 8.2 and the dielectric loss are about 0.02 to 0.075,the test frequency has a little influence on the dielectric properties.The dielectric constant and dielectric loss increase gradually with the rise of temperature at 1 MHz,The dielectric constant of the samples are about 10 to 20 and the dielectric loss are about 0 to 0.9.2.The resistivity of RAS glass-ceramic samples were 1～9×10⁸Ω·m,and the RAS basic glass 1～10×10⁹Ω·m at room temperature.The electrical conductivity curves of samples displayed turning points at 90℃and showed a trend of increase after decreasing with increasing test temperature.When the temperature is below 90℃,the electrical conductivity of RAS glass ceramics was higher than that of basic glass,but we got the opposite results as the temperature is higher than 150℃.The RAS glass ceramics have good insulation at high-temperature.Compared to composition B,composition A has a higher degree of crystallization and more crystals in contents,leading to a higher resistivity.3.RAS glass-ceramic samples have different thermal expansion coefficient because the main crystalline phase changed.According to the selected heat treatment schedules,the thermal expansion coefficient for composition A was 119～140×10^-7/℃at 450℃,and composition B was 150～156×10^-7/℃at 450℃,which could perfectly match that of stainless steel(No.430^#).4.The values of crystallization kinetic parameters(such as Ea,n and Et) of RAS glass calculated by Ozawa and Kissinger method,respectively,agrees very well with each other.The crystallization kinetic parameters of two groups of basis glass were calculated by Kissinger equation,The basis glass(composition A) has two crystallization peaks corresponding to Eal(A)=151.4KJ/mol and Ea2(A)=623.1KJ/mol,respectively,and the crystallization peak of basis glass(composition B) corresponding to crystallization activation energy Ea(B)=50.7KJ/mol.The glass transition activation energy(Et) of the two groups of basis glass corresponds to Et(A)=202.8KJ/mol and Et(B)=220.4KJ/mol,respectively.The Avrami index n of the basis glass(composition A) was 1.70,and the dimensional value of crystal growth deduced from the formula n=m+1 was about 1,indicating that the composition A basic glass was surface-crystallization,while the values of n and m for the basis glass(composition B) were 3.89 and 2.89 respectively,indicating that this basic glass was bulk-crystallization.5.To sum up,the RAS glass-ceramic of composition B has a lower dielectric constant and dielectric loss,possess good insulation at high temperatures and good stability at high frequencies,and its coefficient of thermal expansion matches that of stainless steel(No.430^#) perfectly,so it can be used as substrate materials at high frequencies for chip packaging. Which the heat treatment schedules for the 500℃/3h-700℃/3h(B5),500℃/3h-800℃/3h (B7) were the best samples.The project was supported by the National Natural Science Foundation of China(50472039) and the Hubei Provincial Natural Science Foundation(2005ABA011).