Packaging of MEMS gyroscope: Structural Analysis of Deep Hole Packaging

MEMS gyroscope combines the traditional integrated circuit technology and a variety of micro-machining technology, which has gradually developed and improved in recent years, with the rapid development of MEMS gyro design and preparation, the design and manufacture of MEMS gyro has received more and more attention. In the process of MEMS gyroscope manufacturing, storage and use, multiple external environmental factors may have adverse effects on the reliability of MEMS gyroscopes. For example, vibration and shock cause rupture, creep, adhesion, wear, delamination, particle pollution and fatigue of the internal components of the module, and temperature change causes fracture, creep, electrolyte degradation, delamination and fatigue.

In this paper, a new packaging technology for MEMS sensors, the stepped deep hole packaging technology is proposed.

Problems of LCC encapsulation method

At present, the MEMS gyroscope packaging method is flip chip packaging method (LCC packaging). In order to prevent MEMS gyros from failing due to thermal residual stress caused by poor thermal matching between materials during packaging or use, LCC packages generally use base filler to alleviate the problem of poor thermal matching between chip solder joints and package substrate. However, due to the small size of the sensor model, the process of using the substrate filler will become very complicated, and the production cost will increase to a certain extent, so the traditional packaging scheme can not be applied well. Secondly, the easiest package improvement scheme is to change the geometry of the maximum stress concentration area of the bottom plate. However, because the shape of the quartz bottom plate affects its crystal frequency, easily changing its shape will affect the performance of the sensor and limit the improvement of product accuracy. If the shape of any position of the bottom plate changes, the shape of the entire electrode needs to be adjusted accordingly. Greatly increase the cost of packaging.

In order to solve the above technical problems, a MEMS gyro stepped deep hole packaging process is proposed in this paper. The stepped deep hole packaging structure includes quartz substrate, metal shell, chip, gold wire and stepped deep hole, as shown in Figure 1.

Figure 1 MEMS gyroscope ladder deep hole package structure

Where，

1-Quartz substrate

2-Metal shell

3-Chip

4-Holes in the chip

5-Chip inner pad

6-Stepped holes in quartz substrate

8-Step hole inside pad

9-Golden thread

10-Quartz substrate inside the stepped holes

11-Quartz substrate inner lower step hole

16-Vent hole

Step deep hole packaging process flow

The specific process of using the stepped deep hole packaging process is shown in Figure 2. First, the chip is laid on the quartz substrate, and the holes in the chip are aligned with those in the quartz substrate through the positioning tool. Then, the alignment is tested with the help of high-power lens. Gold wire bonding process is a kind of manufacturing technology that connects gold wire to the surface of electronic devices. This process is widely used in the production of semiconductor chips and other micro-electronic devices. It is one of the necessary processes for the manufacturing and packaging of electronic components. The connection between metal wire and electrode is realized by hot pressing welding. After the gold wire bonding is completed, the stepped deep hole is filled with two-component additive potting glue to realize the solidification of the quartz substrate and chip. The overaddition reaction of the two-component additive potting adhesive produces cross-linking, curing into high-performance elastomer, which can be cured at room temperature or accelerated at high temperature below 80 ℃, and the material has no obvious shrinkage and reaction temperature rise during curing. The cured elastomer has excellent electrical properties, aging resistance, high and low temperature resistance, water and moisture resistance, good deep curing, and does not cause corrosion to the contact material and does not cause pollution to the surrounding environment. Finally, by setting a fixed connection between the metal shell and the quartz substrate, the chip is encapsulated in the metal shell. After the metal shell is fixed, the space between the chip and the metal shell is filled and solidified with a two-component additive pot-sealing adhesive to complete the packaging of the entire chip. In the MEMS gyroscope step deep hole packaging process, the stepped structure formed between the chip and the quartz substrate is convenient for the electrical connection of the pad, and can release the stress better to improve the accuracy of the product.

Figure 2 MEMS gyroscope ladder deep hole packaging process

Conclusion

In this paper, a new packaging method different from LCC is proposed to solve the problems of poor thermal matching and stress concentration between materials during the packaging or use of MEMS gyroscope. The proposed MEMS gyros stepped deep hole packaging technology is to set up stepped holes inside the quartz substrate, align with the holes inside the chip, connect the quartz substrate with the chip by the gold wire bonding process, and use the two-component pot-sealing adhesive to fill and solidified and release pressure. The displacement curve and stress curve of the two packaging methods both fluctuate with the periodic fluctuation of temperature load, and the extreme stress value is constantly amplified. The chip displacement and stress of the traditional LCC packaging process are significantly larger than that of the step deep hole packaging, with the latter displacement being about 19%~27% of the former displacement and the latter stress being about 18%~57% of the former stress. The results show that the stepped deep hole package structure can effectively improve the warping and stress concentration of the internal components of the module.

LCC as a reliable traditional packaging method, there are still excellent MEMS gyroscopes using LCC packaging. For example, ER-MG2-50/100, which is mainly used for ground-based north finding, has zero-bias instability of 0.01 to 0.02°/hr and angular velocity random walks of 0.0025 to 0.005°/√hr. The ER-MG2-300/400 is a MEMS gyro for air and sea navigation with zero-bias instability of 0.03-0.05° deg/hr and angular velocity random walks of 0.01-0.025°/√hr.

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