Failure Analysis and Research on Shielded-Gate Trench MOSFET

Shielded-Gate Trench MOSFET (SGT MOSFET) complies with two-dimensional charge balance principle. SGT MOSFETs improve the performance of medium and low voltage power MOSFET. There is a shielded gate in the drift region, which provides SGT MOSFET a significant advantage in terms of specific on-resistance RON(SP) and figure of merit (FOM= Ron*Qg). SGT MOSFET has the advantages of low specific on resistance, fast switching speed, low switching loss and simple driving
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.The process of SGT MOSFET is so complex, including more than six times of photoetchings, and several times of insulating oxide layer growth. This leads to thin dielectric layers and complex structures.At the end of process of SFG MOSFET, wire bonding of source is on the aluminum layer above the oxide layers and Gate, which is thin and has low mechanical strength. And it is also the key technology to realize electronic interconnection. Therefore, wire bonding is one of the most difficult and key processes in the packaging process. Failure caused by wire bonding false accounts for about 25% of the whole chip failure. What’s worse is that the chips failed caused by poor wire bonding is often in batches, which will cause great loss to the user and relevant parties. With the development of chip miniaturization, there are also circuit module and functional structures under the bonding PAD. There are oxidizing materials between bonding PAD and functional structure, to realize circuit insulation. The oxidizing material is often too thin to preserve adequate strength, which puts engineers into a dilemma, because it is a big challenge for bonding process control. If the bonding process parameter is not selected properly, the chip may work abnormally. Insufficient bonding strength may lead to poor electrical connection, such as excessive contact resistance. Excess bonding force may cause damage and fracture. The oxidizing material under bonding PAD may be damaged, or chip structure may be damaged, which will cause slight internal leakage, and function of the appliance may be lost. Unfortunately, these defects are unstable and occasional, and they can’t be detected by the packaging factory one hundred percent immediately. Then great loss may be caused to users by these defects
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.However, at present, few cases of failure caused by process defects are published. In this paper, a failed SGT MOSFET was analyzed. Slight leakage current was found between Gate and Source, and function of the SGT MOSFET was lost. No obvious defects or abnormality were found in the SGT MOSFET after it was de-capped. Optical Beam Induced Resistance Change (OBIRCH) was used to check out the possible defects. Something abnormal was found under the bonding area. And then Focused Ion beam (FIB) was used to make micro-section to show the suspicious point. Cracks and holes in oxidizing material were found. There were cracks in the Source metal layer of the cell, and some cracks extend to the Gate of MOSFET, resulting in leakage between Gate and Source poles.SGT MOSFET failed because of poor process quality. Thus, the way to improve quality of SGT MOSFET is to improve oxide layers and reduce bonding pressure. Field oxide layer growth process should be Improved to avoid cracks and holes. Parameter setting and consistency of bonding wire process is also a crucial factor.