Measurement of Laser Drilling Shift for Opaque Build-up Film by Infrared Microscope

Abstract

With the development of smart phones, the developing direction of Application Chip (AP) is to be thinner and lighter. FOPOP (Fan-Out Package On Package) is the mainstream packaging mode to cope with the ever-increasing layout density [1] [2]. In the mobile phone market where a hundred schools of thought contend, to cope with mobile phone APs of different brands and performances, the structure of without memory stacking has gradually become the mainstream of the FOPOP structure. The FOPOP without memory stacking, in order to complete the memory stacking of different brands and the application of mobile phones at high frequencies, there are strict requirements on the material of the connection surface. Among them, Build-up Film, due to its low dielectric constant (Dk), low dielectric loss and low coefficient of thermal expansion (CTE) at high frequencies, can solve problems such as transmission loss and package warpage [3] [4]. In order to achieve memory stacking, we used the laser drilling technology on the Build-up Film to reveal the pattern that connected to the Memory, and fill the solder into the drill hole in advance, which is convenient for the end product to stack the Memory. The shift of the laser drilling will directly affect the difficulty of stacking the Memory, which will eventually lead to the opening of the circuit. Hence, the shift of the laser drilling is one of the necessary inspection items. However, Build-up Film is a non-traditional transparent film, by laser microscope, we cannot know the position of the pattern under the Build-up Film, and cannot measure the shift. The Infrared (IR) Microscope is generally used to detect backside and side wall cracks in silicon die. The purpose of detection is achieved through the characteristics of IR long wavelengths that easily penetrate silicon. Relatively, the red light laser microscope that most commonly used in the production line cannot penetrate the Build-up Film due to the short wavelength of red light. Hence, we use long-wavelength IR to penetrate the Build-up Film, and use a correction sheet to correct the length to ensure the accuracy of the length measurement. The measurement of laser drilling shift is achieved, to ensure the quality of laser drilling. Finally, we confirmed that the error between IR Microscope measurement result and cross-section & SEM is within 2.14μm, so IR Microscope is a feasible measurement tool for laser drilling shift of Build-up Film.