Device and process for ultrasonic thin-die bonding

Abstract

The continual reduction in thickness of semiconductor dies from 200 to 50 μm in recent years has made a great impact on semiconductor packages and their resulting electronic products. These semiconductor thin-dies not only enable ultrathin applications (e.g., smart cards, biological passports, etc.), but also promote ultrahigh-density applications (e.g., memories, CPUs, etc.). Conventional die bonding based on liquid adhesives is unable to accommodate semiconductor dies of such thinness due to unavoidable failures caused by adhesive overflow, spread-out, etc. By contrast, thermocompression bonding of semiconductor thin-dies that are pre-laminated with die attach film (DAF) on their backside after the wafer thinning and stress relief processes but before the wafer dicing process is not subject to the liquid adhesive-induced problems. Today, semiconductor dies of thicknesses less than 75 μm are mostly produced as thin-die-DAF laminates, and thermocompression bonding is regarded as the state-of-the-art technique for bonding these laminates. However, high process temperature (100 160 °C), long process time (>3 s), single die bonding per process cycle, and continual heating of fresh and post-bonded dies and substrates have constrained the bonding process window and throughput of thermocompression bonding. In this research, a novel ultrasonic thin-die bonding technique is proposed to overcome the drawbacks intrinsic in thermocompression bonding. This technique is essentially based on a new generation mechanism of "ultrasonically induced pulse heating (UIPH)" whereby the ultrasonic vibration energy generated from an ultrasonic transducer is absorbed by an ultrasonic-to-thermal apparatus. The absorbed ultrasonic vibration energy is rapidly converted into a spontaneous, highly concentrated, localized, and controllable thermal energy in the form of a heating pulse so as to soften or plasticize the DAFs underneath the thin-dies in a safe, clean (green), and reliable manner. To practically enable the proposed ultrasonic thin-die bonding technique, a bonding device consisting of a 40 kHz piezoelectric ultrasonic transducer and an ultrasonic-to-thermal energy apparatus was designed, fabricated, and characterized. The selection and properties of the constituent materials of the bonding device were described. The evaluation of the bonding device indicated that ultrasonic vibrations instead of ultrasonically-induced heating pulses result when the apparatus is excluded from the bonding device, leading to scratching, breaking or even powdering of the DAF-laminated thin-dies. To acquire an insight into the proposed UIPH mechanism, a series of in situ temperature measurements were performed. The developed bonding device was integrated with a mechatronic test bed formed by an ultrasonic signal generation system, a thermal management system, a pressure control system, a three-axis linear motion system, a device mounting system, and a vision system to form an automated equipment model for the study. Parametric studies on process parameters, including ultrasonic power, process temperature, process pressure, and process time revealed the controllable nature of UIPH by the process parameters, especially for the ultrasonic power. A finite element analysis (FEA) established upon the experimental results was carried out to visualize the transfer of the experimental UIPH through the thin-die to the underneath DAF and substrate. The analysis predicted the effects of implementing UIPH on the bonding of DAF-laminated thin-dies on various substrates and the possibility of bonding several layers of DAF-laminated thin-die simultaneously within a single process cycle. Based on the FEA predictions, a series of process studies were conducted to bond 50 μm thick silicon thin-dies with 10 μm thick DAFs on both glass substrates (i.e., chip-on-glass (COG) bonding) and PCB substrates (i.e., chip-on-board (COB) bonding) using the proposed ultrasonic technique and the state-of-the-art thermocompression technique. An optical microscopy technique and an ultrasonic scanning technique were employed to quantify bondability in term of percentage of voids formed. The results showed that the ultrasonic technique can effectively reduce the process temperature and time as required by the thermocompression technique. Interestingly, ultrasonic bonding at room temperature (23 °C) was achieved with an ultrasonic power of 50 W and a process time of 2 s. More interestingly, ultrasonic bonding could further enable simultaneous stacked die bonding in excess of five layers of thin-die-DAF laminates within a single bonding process cycle of 3 s.