Study Of Improving Testing Coverage And Reducing Testing Time Of NAND FLASH CP

The NAND FLASH capacity increases every year,and the cost for each bit decreases correspondingly.So the NAND FLASH market is becoming competitive.The increasing capacity of NAND FLASH memory causes the difficulty of memory's testing and high cost of testing.For NAND FLASH OEMs(Original equipment manufacturers),it is important to decrease testing time and cut down testing cost using effective testing methods.Only in this way,OEMs will produce the specified products for the customer and take an important place in the NAND FLASH market.Moreover,NAND FLASH reliability directly determines the market share of NAND FLASH OEM.The high testing coverage is required.So it is important for OEMs to improve testing coverage and reduce testing time.Aiming to improve testing coverage and reducing testing time,the CP test program is developing in this paper.The details are described as below.In terms of developing CP mass production testing program,all the sub-files of the test program are defined according to the tester and prober card firstly.Then the testing flow is defined by the program control table.This table calls the program control files and pattern files when compiling the test program.We can get the whole testing program package after compiling the program.When executing the package on the tester,we can eventually get the wafer map automatically after finishing the testing.In terms of improving test coverage,this paper introduces the topic of the low yield issue about the back-end testing.It is found that the front-end test program cannot effectively screen out the WL2WELL(Word Line to Well)short.So the testing pattern of the WL2 WELL stress testing needs to be improved.The WL2 WELL split experiment is designed,which has total four splits with different stress condition.The result indicates that the new stress testing is most effective.It can decrease the WL2 WELL failure rate from 2.2% to 0.73%,which improves the WL2 WELL test coverage effectively.In terms of reducing testing time,we firstly solve the data conflict problem when we write the trim code in the chip and optimize the testing program to reduce the test time.Then,by improving the trimming algorism and improving the program control of writing trim code,we effectively reduced testing time of the trimming flow by 169.733 s totally,which decreased 87.4% of trimming testing time and about 30% of total testing time.By improving the trimming testing,we effectively reduce the testing cost.The methods of improving NAND FLASH testing coverage and reducing the test time in this paper has been used in actual mass production flow.It can be used as a guide for enhancing the coverage and decreasing the test time of the next generation of NAND FLASH product.