Towards minimizing read time for NAND Flash Towards minimizing read time for NAND Flash Globecom December 5 th, 2012 Borja Peleato, Rajiv Agarwal, John Cioffi (Stanford University) Minghai Qin, Paul H. Siegel (UCSD)

2 Towards Minimizing Read Time for NAND FLash Outline Introduction –NAND Flash write and read –Memory structure Problem: choosing read thresholds Progressive read algorithm –Estimating Min-BER threshold –Generating soft information Results

3 Towards Minimizing Read Time for NAND FLash N+ Source N+ Drain P-Type Silicon Substrate Floating gate Control gate Dielectric Cell write procedure

4 Towards Minimizing Read Time for NAND FLash N+ Source N+ Drain P-Type Silicon Substrate Floating gate Control gate Dielectric V t = 2V Cell read procedure + _ V t = 1V Exact voltage unknown, read returns 1 if V t < V cell and 0 otherwise

5 Towards Minimizing Read Time for NAND FLash Memory structure Page (~10 5 cells): write/read unit Block (~128 pages): erase unit Main sources of noise: –Over-programming (and write noise) –Leakage –Inter-Cell-Interference (ICI) Noise increases with cell scaling and wear

6 Towards Minimizing Read Time for NAND FLash How do we choose read voltages when noise estimates are not available? Problem

7 Towards Minimizing Read Time for NAND FLash If then hence Solve for read threshold with min BER Estimate Gaussian noise from noisy cdf samples Progressive Read Algorithm t

8 Towards Minimizing Read Time for NAND FLash Min BER threshold Minimum estimation error when reads are close to the means and spread out

9 Towards Minimizing Read Time for NAND FLash Soft information If hard decoding fails, re-use reads for soft decoding –For lower failure rate, reads clustered in uncertainty region… –For best combined noise estimation and decoding, two reads in each region if means and variances are known

10 Towards Minimizing Read Time for NAND FLash S1S1 S2S2 S3S3 | ͡ µ - µ | / µ 1.2 ∙ ∙ | ͡ σ – σ | / σ 7.5 ∙ | ͡ t* - t* | / t* 2.2 ∙ ∙ ∙10 -2 | BER( ͡ t*) - BER(t*) | BER(t*) 1.6 ∙ ∙ ∙10 -2 LDPC fail rate ∙10 -3 Genie LDPC fail rate 103 ∙ S1S1 S2S2 S3S3 | ͡ µ - µ | / µ 1.2 ∙ | ͡ σ – σ | / σ 7.5 ∙ | ͡ t* - t* | / t* 2.2 ∙ ∙ | BER( ͡ t*) - BER(t*) | BER(t*) 1.6 ∙ ∙10 -2 LDPC fail rate Genie LDPC fail rate 10 S1S1 S2S2 S3S3 | ͡ µ - µ | / µ 1.2 ∙ | ͡ σ – σ | / σ 7.5 ∙ | ͡ t* - t* | / t* 2.2 ∙ | BER( ͡ t*) - BER(t*) | BER(t*) 1.6 ∙ LDPC fail rate 1 Genie LDPC fail rate 1 S1S1 S2S2 S3S3 | ͡ µ - µ | / µ | ͡ σ – σ | / σ | ͡ t* - t* | / t* | BER( ͡ t*) - BER(t*) | BER(t*) LDPC fail rate Genie LDPC fail rate Result: ½ read operations 4 reads instead of 8 2x speed

11 Towards Minimizing Read Time for NAND FLash Thank you!