3D NAND memories

Abstract

The thesis investigates the possible space application of a commercial 3D NAND memory as a radiation monitor. Literature has indicated that both traditional 2D and 3D flash memories are sensitive to ionizing radiation. Where the track of particle could be seen passing through the 3D architecture. A flash memory cell stores data with a collection of charge of which some is lost after a particle event. This loss can be measured as a shift in the threshold voltage. A relationship exists between the energy of a particle and the threshold voltage shift. A methodology was developed to measure the threshold voltage of memory cells in a Micron 64 layer 3D NAND memory. By modifying the threshold voltage of boundary separating two logic states, a bit flip can be introduced. A page of memory cells will be reread multiple times, each time with a slightly increased or decreased voltage offset. It is determined that a measurement could be made with a theoretical error as low as 7.5 mV. Based on literature on 3D NAND memory architectures and die and SEM images for the Micron device, the corresponding physical size of a memory cell, page and block were estimated. A memory block measures 7360 ¹m (depth) x 3.37 ¹m (height) x 9.5 ¹m (width). A set of SPENVIS simulations estimate that a single block may see less than one GCR particle event every ten days. If it is possible to detect protons, 104 events may be seen in an ISS orbit, up to 109 events for a MEO orbit. In order to validate the proposed measurement methodology, a test setup was developed with which instructions can be executed on a memory. Requirements for this development were based on the ONFI specification, which standardizes the software and hardware interfaces with memories. The setup features a ZedBoard FPGA, where a VHDL memory controller generates the requested control signals. The interface between the FPGA and memory is provided through a custom designed PCB, which also powers the memory. A memory is placed in a socket such that different devices can be tested. Python scripts generate the correct sequence of commands to execute voltage threshold measurements. A Micron SSD was acquired, from which the 64 layer 3D NAND memory packages were unsoldered. These memories were then used for testing. A first set of tests prove that it is possible to perform voltage threshold shift measurements. The general measurements show the expected voltage distributions. Interesting observations were made when investigating the measurements for a single cell: they do not always transition immediately from one logic state to another. A second interesting observation is that the same cell returned different voltage readings for consequent read cycles. These behaviors should be investigating in further research, as they will impact the errors in the measurement. Concluding, a methodology is proposed and validated to measure the threshold voltage. A test setup was developed and proves the in house capability of making these measurements. It is possible to advance with the study of the radiation effects on the memories. Based on these results, further developments could lead to a future space radiation monitoring payload based on 3D NAND memories.