SiC MOSFET FIT leads the impact with grid oxide dependability

SiC MOSFET FIT leads the impact with grid oxide dependability

[introduction] besides function, dependability and consistence are the topic with SiC MOSFET most discussion. We bear consistence definition for parts of an apparatus the ability of specific special pressure event, for example, short circuit ability or pulse electric current handle ability. Dependability points to the stability that parts of an apparatus issues in the normal operating conditions inside target life. The phenomenon related to dependability includes the drift of certain and electric parameter or the stoppage that destroy a gender. To hard breakdown, the form that leads with FIT normally undertakes quantifying. The device that FIT rate says to understand some kind anticipates inside certain period have how many times breakdown. Current, cosmic ray effect basically is restricting the FIT rate of high-power silicon parts of an apparatus.

With respect to SiC character, the issue of dependability of grid oxidation layer that because stress of electric field of bar oxidation layer is caused,still needs a consideration. Following plan institute are shown, the total FIT rate of SiC is the sum of rate of cosmic ray FIT and oxide FIT rate. To cosmic ray failure rate, the kind that can pass a test gets the FIT of some kind of technology is led, combine applied goal according to these results, can implement a products plan that accords with target FIT rate. The electric field design that optimizes drift area can achieve low FIT rate normally. To the FIT rate of oxide, need to apply to choose a process to lower FIT rate, because be compared with silicon photograph, the blemish density of SiC is quite high still. However, although be in our silicon power parts of an apparatus, the filtration of grid oxide remains what assure measure as a kind of quality and use.

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The challenge of the grid oxide dependability of SiC MOS parts of an apparatus is, apply in certain industry given below working requirement, assure the oldest fault rate under 1 FIT, this and today’s IGBT fault rate comparative.

The 2 oxidation silicon that because carborundum is mixed,grows on silicon material (the quality of SiO2) and character are identical almost, the Si MOSFET of because this is theoretic same area and oxidation layer ply and SiC MOSFET can be in stress of roughly identical oxidation layer electric field bears inside same time (same Ben Zhengshou fate) . But, this does not include the foreign matter that concerns with blemish in parts of an apparatus only, be not the ability when imposing limitation originally namely effective. Compare with Si MOSFET photograph, the blame in showing oxide of grid of level SiC MOSFET asks for blemish density to want to be gotten high originally much.

Report is chosen reduced dependability risk

Compare with the photograph of parts of an apparatus that does not have blemish, have be not the parts of an apparatus that imposes limitation originally to appear earlier breakdown. Although flawless parts of an apparatus also is met fatigue invalidation, but life is very long. Normally the circumstance falls, enough thick flawless the collects invalidation time to fall than normal application originally use time of oxidation layer should grow class of a few amounts. Accordingly, inside typical chip life, the FIT rate of oxide is complete by be not this asking for blemish decision.

The challenge that makes sure the grid oxidation layer of carborundum MOSFET has enough reliability is — how will suffer be not the measure of parts of an apparatus that signs blemish influence originally, the high proportion from original working procedure ends (be like 1%) , the acceptability small proportion when reducing product shipment to give a client (be like 10ppm) . A when achieve this one goal accepted method is to use report to choose.

The course is chosen in report in, every parts of an apparatus is in bar to charge stress pattern. The choice way of stress mode is, the parts of an apparatus that has serious flaw invalidation, and the parts of an apparatus that does not have these blemish, or the parts of an apparatus that is not decisive blemish only can pass a test. The parts of an apparatus that did not choose through the sieve will from produce a line to move except. Carry this kind of kind, we are potential dependability risk changeover crop loss.

To make parts of an apparatus can bear particular grid stress, grid oxidation layer needs to have a specific the smallest ply. If the ply of grid oxidation layer is too low, parts of an apparatus appears because of exhaustion to draft be no longer in force originally in the meeting in choosing a course, perhaps appear after choose the circumstance that threshold value voltage and raceway groove mobility drop. On the other hand, thicker grid oxidation layer can increase threshold value tension, be in given raceway groove electrical conductivity is lowered below VGS(on) condition. Next graphs showed the balance between rate of grid oxide FIT and function of parts of an apparatus, this is in in also have discuss.

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We had thrown many time and sample, got the mass data of the bar oxidation dependability of SiC MOSFET. E.g. , we divide 3 groups to the SiC MOSFET that chooses through report, every groups bring to bear on different losing grid stress to slant buy, checked on-state reliability leaves 100 days in 150 ℃ . Every groups of sample have 1000 parts of an apparatus. Next graphs showed the result below condition of technology of different grid combustion, the respect has the technology reliability that final amount produces to be improved apparently.

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Use initiative craft condition, in the double 30V at the proposal grid bias voltage falls, 10 invalidation are less than only in 1000 parts of an apparatus. When improvement executive technology reduces this one word 30V only a breakdown, 25V and – the breakdown when 15V is 0. An invalidation dispute of only is inherent invalidation, however, this is not crucial, because be below use condition of rated grid bias voltage, the time place of invalidation happening can exceed formulary product life far.

Of course, besides the dependability that evaluates on-state oxide, the stress that evaluates off-state oxide is very important also, because of SiC power parts of an apparatus medium electric field condition compares component of silicon power MOS to approach the limit of SiO2 more.

Screen is to guide the balance between electrify block and dependability

Crucial strategy is to pass the proper design of pair of deep P trap to come the oxide area with effectively sensitive screen. The efficiency of screen is to guide the balance between electrify block and dependability. In groove MOSFET, deep P trap forms the structure of similar JFET below the raceway groove area of MOSFET, promote screen effectively. This kind of JFET (transistor of knot field effect) to guide electrify block added an extra weight, basically depend on the distance between the P area of bury and adulteration. The grid oxidation layer that the design characteristic of structure of this kind of screen lays to avoiding to close position degrades or puncture of grid oxidation layer is crucial.

For CoolSiC of test and verify? The off-state dependability of MOSFET, the stress that we issued to undertake 100 days to exceeding the SiC MOSFET of 5000 1200V in the condition of 150 ° C, VGS=-5V and VDS=1000V checks. These conditional correspondence apply at industry already enough astringent. Accept the restriction of the puncture voltage of parts of an apparatus, VDS cannot continue to increase again.

In taller leakage extremely undertake checking meeting making a result lack fidelity below voltage, because of other breakdown mechanism, the trouble that causes like cosmic ray may appear. The result is, in test of this off-state dependability, neither one is malfunctioned by the parts of an apparatus of the test. Because 650V parts of an apparatus follows as same as 1200V parts of an apparatus design standard, accordingly predicting meeting has identical reliability.

Origin: Ying Feiling

Author: Friedrichs Peter, vice President SiC,

Infineon Technologies AG

Interpreter: Zhao Jia

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