Using an interdisciplinary approach to effectively package MEMS accelerometers (Part 1)

summary

Micro-electromechanical systems (MEMS) packaging is growing in importance and plays a key role in the successful commercialization of MEMS products. The packaging system should perform the sensing function of the MEMS and also protect it from the external environment while continuously improving quality to achieve high ppm performance. One of our accelerometers in a SOIC package has low ppm performance with device fracture. This MEMS package is very unique and must maintain a specific resonant frequency to prevent the sensor from sticking or jamming. At the same time, the package must ensure that the sensor is reliable and intact without fracture or output deviation. We used a multidisciplinary research approach to determine the appropriate die-bonding material to completely solve the problem of device fracture, which involved vibration analysis, electrical response measurement, stress analysis, and fracture mechanics.

Introduction

MEMS accelerometers have been used as crash sensors in automotive airbag systems for more than a decade. More recently, MEMS accelerometers have been further applied to consumer electronics such as cell phones, laptops, game consoles, and handheld PDAs. Accelerometer products undergo two distinct manufacturing processes: sensor fabrication and packaging. A surface micromachining process is used to create the movable pointer and quality control for the accelerometer sensor chip. Effective packaging of MEMS devices is often challenging and is one of the main factors hindering the breakthrough growth and adoption of MEMS technology [1]-[2]. MEMS packaging technology is primarily derived from microelectronic packaging technology. In integrated circuits, packaging is primarily used to protect electronic devices from external damage. MEMS packaging, in contrast, must allow the device to actually contact or observe the external environment to perform the sensing function while protecting the device from damage and long-term failure. The challenge of sensor packaging is that in addition to providing a base for the PC board, the stresses caused by the environment or the packaging on the inverter should not affect the performance of the sensor.

A MEMS accelerometer consists of a MEMS device that interfaces with an ASIC that converts the capacitive output of the MEMS device into a voltage that represents the acceleration felt by the MEMS device. The acceleration is summed up to the velocity of the device. In airbag applications, when the velocity is high enough, the airbag will be deployed. Any distortion of the MEMS accelerometer, such as excessive package stress or device vibration, will introduce erroneous signals into the airbag algorithm. The airbag sensor must respond to the forces generated by a car crash while isolating the response from the vibration environment in which the airbag is located. The mechanical characteristics of the package and sensor are very important considerations. For example, in some automotive applications, the frequency of the vibration signal can be as high as 20kHz. If one or more of the natural frequencies of the package are the same or close to the frequency of the high energy input signal, the sensor's package output signal will be distorted and may even cause mechanical damage to the sensor.

The sensor of Freescale MEMS accelerometer is fabricated by surface micromachining on a silicon substrate. The active parts of the sensor are isolated from the external environment by an etched cavity cover wafer, which is then bonded to the substrate using a glass frit. After the bonded sensor wafer is formed, it is bonded to a copper lead frame by die bonding and then wired to the ASIC chip. The sensor is coated with a gel coat and then overmolded after assembly. As a lower cost solution, a 16-lead SOIC package was selected, with the ASIC chip and the sensing element placed side by side. The main causes of signal distortion were found to be related to package resonance and die bond resonance. To address these packaging issues, potting was used and hard die bond adhesive was used. Unfortunately, the ppm performance of the implemented hard die bond material for die fracture was very low. We used a multidisciplinary research approach to evaluate the impact of process and material on die fracture and provide a better solution to completely solve this problem.

Problems and Solutions

Figure 1(a) shows a finished package model of a 16-lead SOIC package and a cross section of its accelerometer. The package consists of a sensor die, detailed in Figure 1(b), bonded to a housing that extends over the substrate near the wire bond pads. The ASIC and sensing unit are placed side by side, and the die is attached to the copper lead frame. Prior to overmolding, the entire sensing unit is covered with an extremely low modulus silicone gel.

In the application, the vibration environment requires the package's natural frequency to be greater than 20kHz to avoid resonance-related issues with the sensor. To meet these requirements, the previous 4-point soft die attach adhesive A was replaced with a high modulus epoxy die attach adhesive D. The low ppm-resistant components did not work properly, and it was later discovered that the substrate die of the sensing unit was cracked. As shown in Figure 2, the crack penetrated the entire active device. The cross-sectional analysis showed that the crack started from the top of the sensing unit substrate and extended along the interface between the substrate, glass frit, and housing. The crack usually started from the cut edge, and some components had only one fracture starting point, while other components may have multiple fracture starting points. Therefore, the next question is what caused the sensing unit die to crack? When did the fracture occur? Was it during processing, assembly, or during solder reflow? Can the package be redesigned to make it more robust and reliable during processing, assembly, and when used in the field at the customer?

Figure 1 (a) Finished package and analysis of 16-lead SOIC package (b) SEM image of the sensor

Finite element analysis was used for failure analysis and package redesign. Two methods were used to evaluate chip stress and fracture risk during assembly and exposure to the external environment. One method was conventional stress analysis and the other was based on fracture mechanics. The stages that needed to be analyzed included the bonding of the cover wafer to the sensing unit substrate wafer, the connection of the sensing unit to the lead frame, wire bonding, overmolding, reflow soldering, and thermal cycling.

Figure 2 SEM image of cracks on the sensing unit substrate