Investigation of Warpage Behavior of Silicon Semiconductor on a Silicon - Adhesive - Ceramic Integrated Structure at Cryogenic Temperatures--STUDENT BEST PAPER $1500

Silicon wafer is widely used as a base material for readout integrated circuit (ROIC) of infrared sensors. There is a heterogeneous component assembly with the silicon wafer material. Warpage behavior of silicon readout integrated circuit is dependent on the material properties and geometrical properties of the integrated materials. Warpage behavior of the silicon material directly affects the warpage of the sensor which must be operated at cryogenic temperatures (around 80 K). There exist a great difference between the operation and storage temperatures (~ 300 K) of these devices. When different materials with different thermal expansion coefficients are used such devices, thermal stresses develop on the components and surface deformations named as “warpage” are observed on the materials. The measurement of excessive thermal stress or warpage formation on the sensor is vital for reliability issues. In this study, warpage behavior of silicon material is examined in the temperature range from room temperature down to cryogenic temperatures (80 K) and under vacuum conditions less than 1 mTorr. The silicon ROIC is integrated on an alumina ceramic by applying an adhesive between these two layers. After the application of the adhesive material, the integration of the silicon to ceramic is accomplished using a pick and place equipment. The warpage of silicon wafer is measured by a Fizeau Laser Interferometer which uses a 633 nanometer wavelength He – Ne laser. The warpage of the diced silicon is measured before and after the integration to the ceramic so that the effect of curing process of the adhesive is determined after which, the warpage of the silicon material is measured at atmospheric pressure and also under vacuum conditions at room temperature. The warpage of silicon material on the integrated structure is measured with increments of 10 K for both cooling from room temperature to 80 K and heating from 80 K to room temperature. In order to reach cryogenic temperatures, a liquid nitrogen cooled vacuum envelope is utilized. The envelope has an optical flat (made of BK7 material and 2.35 mm thick) for interferometric measurements. There are a total of five integrated structures for the warpage measurements. At each of these structures, the silicon material thickness is different. Comparison of the warpage behavior of the silicon material for different thicknesses are performed. Thermal cycling between room temperature and 80 K is also performed up to 5 cycles for each of the integrated structures. Thermal cycling effect on silicon warpage is discussed for silicon - alumina - adhesive trimaterial assembly structure.