Studies On Fabrication Technologies Of The AlN Ceramic Substrates Of High Thermal Conductivity

With the rapid development of micro-electronic information technology,large-scale integrated circuit（LSI）,multi-chip module（MCM）and micro-electro-mechanical system（MENS）technologies,the requirements to the whole electronic machines become higher and higher,aiming at miniaturization,portability and high performance.High-density integration is the most effective solution to achieve the above objects.To realize high-density integration,the heat dissipation problem of devices is firstly solved.One effective solution is used of the high-conductivity substrate.AlN ceramics have high thermal conductivity（up to 320W/（m.K）in theory）,low dielectric constant（about 8.9 at 1MHz）.Its coefficient of thermal expansion（CTE）is close to that of Si(AlN:4.3×10^-6℃^-1,Si:3.4×10^-6℃^-1at 20-400℃),excellent electrical insulation and electrical breakdown resistance,high mechanical properties and safety,non-toxicity,and etc.At the same time,AlN ceramics can overcome the thermal mismatch between substrates and the Si wafer,compared with other ceramic substrates such as Be O(6.3×10^-6℃^-1)and Al₂O₃(7.1×10^-6℃^-1).AlN ceramics act as the most ideal electronic packaging materials for hybrid integrated circuits,microwave power devices,semiconductor devices,large-scale integrated systems now and in near future.And,the use of the AlN substrates turns to be more and more widely.AlN has a strong covalent bond,the sintering temperature of the ceramics is therefore high.So,sintering aids are needed to reduce the sintering temperature and to realize the densification of AlN.The sintering behavior of the AlN ceramics is closely related to the preparation process and raw material characteristics.In this work,the effects of O and impurity content,and morphology,particle size,distribution uniformity and specific surface area of AlN powder on the properties of the AlN ceramic substrates were discussed theoretically.On this basis,AlN ceramic sheets by adding 5 wt%Y₂O₃as sintering aids were prepared by tape casting and dry pressing,respectively After sintering at 1800℃,the density of AlN ceramic sheets prepared by tape casting was 3.314 g/cm³,the flexural strength was 459.8 MPa and the thermal conductivity was 174.2 W/（m.K）,while the density of AlN ceramic sheets prepared by dry pressing was 3.296 g/cm³,the flexural strength was 382.9 MPa and the thermal conductivity was 186.4 W/（m.K）.The densification degrees of AlN ceramics prepared by these two methods are both higher,the mechanical properties of AlN ceramics prepared by tape casting are better,on the contrary,the thermal properties of AlN ceramics prepared by dry pressing are higher.Sintering process is one of the key factors affecting the densification,mechanical and thermal properties of the AlN ceramics,and also the grain size and distribution uniformity,and the composition,content and distribution of the secondary phase in the AlN ceramics.Moreover,the ingredients,moulding and dispensing processes of the AlN ceramic substrates are of equal importance.The choice of the optimal process parameters and processes is necessary for preparing AlN ceramics of high thermal conductivity.Accordingly,the AlN ceramic substrates of a thermal conductivity up to240 W/（m.K）were successfully fabricated.