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It is theoretically found that by adding a thin silicon layer (35 nm) on top of our previously proposed surface plasmon interference (SPI) biosensor in silicon on insulator (SOI), a sensitivity enhancement of up to 2500 nm/refractive index units (RIUs) for short sensors can be obtained. At the same time, the corresponding figure of merit (FOM) is as high as 237 (RIU^-1). This improvement is caused by the reduction in group index difference between the two interfering modes.