PERFORMANCE

TECHNICAL FIELD

The present disclosure relates to split-gate memory cells, and more particularly, to systems and methods for improving the read window in a split-gate flash memory cell, e.g., by biasing the control gate terminal during read operations to improve or control the cell read performance.

BACKGROUND

Figure 1 illustrates a side cross-sectional view of a known split-gate flash memory cell 10 including a pair of floating gates 12A and 12B formed over a substrate 14, word lines 16A and 16B extending over floating gates 12 A and 12B, respectively, and a control gate 20 extending over both floating gates 12A and 12B. An oxide region 18A, 18B is formed over each floating gate 12A and 12B. Word lines 16A and 16B may, for example, represent an odd row word line 16A and an even row word line 16B, or vice versa. A doped source region or junction 24 may be formed in substrate 14 below the control gate 20 and extending partially below each floating gate 12A and 12B, and a pair of doped bit line regions or junctions 24A and 24B may be formed in substrate 20 adjacent word lines 16A and 16B.

Memory cell 10 may also include electrically conductive contact regions in contact with word lines 16A and 16B, control gate 20, source region 24, and bit line regions 24 A and 24B, for applying voltages to the various cell components to provide various memory cell functions, e.g., program, erase, and read functions. As shown, these contacts may include word line contacts 30A and 30B, a control gate contact 32, a source contact 34, and bit line contacts 36A and 36B.

In some embodiments, the split-gate flash memory cell 10 is a SuperFlash memory cell (e.g., a SuperFlash ESF1+ cell) by Microchip Technology Inc., having a headquarters at 2355 W Chandler Blvd, Chandler, AZ 85224. Thus, in some embodiments, split-gate flash memory cell 10 may comprise a cell as disclosed in U.S. Patent 8,711,636, or a variant thereof.

Split-gate flash memory cell 10 may be programmed and erased by applying defined voltages to one or more of the following: a selected word line contact 30A or 30B (VWL), the control gate contact 32 (VCG), the source contact 34 (VSL), and/or a selected bit line contact 36A or 36B (VBL) for a defined time to provide either (a) a cell current IrO that corresponds with a programmed state (“off’ state) of the cell or (b) a cell current Irl that corresponds with an erased state (“on” state) of the cell. In addition, the present programmed/erased status of the cell may be read by applying defined voltages to a selected word line contact 30A or 30B (VWL) and the adjacent bit line contact 36A or 36B (VBL).

Figure 2 is a table showing example voltages that may be applied to the various contacts of split-gate flash memory cell 10 to perform program, erase, and read functions, according to a conventional cell operation. As shown, the conventional read function is performed via the word line 16A or 16B and associated bit line 24 A or 24B, by applying a defined VWL and VBL to a selected word line contact 30A or 30B and associated bit line contact 36A or 36B, with no voltage applied to the source contact 34 (VSL=0) or control gate contact 32 (VCG=0).

However, as discussed herein, the inventors have conceived of the idea of applying a selected non-zero voltage (positive or negative) to the control gate contact 32 during read operations, indicated as VCGR, to selectively tune the memory cell for improved or controlled read performance.

SUMMARY

Embodiments of the present disclosure provide systems and methods for improving the read window in a split-gate flash memory cell, e.g., by biasing the control gate terminal with a non-zero (positive or negative) voltage during cell read operations to improve or control the erased state read performance or the programmed state read performance of the cell.

For example, a non-zero negative voltage VCGR may be applied to the control gate to improve the programmed state read performance of the cell, at the possible expense of the erased state read performance. Similarly, a non-zero positive voltage VCGR may be applied to the control gate to improve the erased state read performance of the cell, at the possible expense of the programmed state read performance. In view of these performance trade-offs, the VCGR for any particular memory cell may be set based on the intended usage and/or performance characteristics of that memory cell, e.g., such as endurance, read speed, etc.

For example, for memory cells in which erased state read performance is particularly important, VCGR may be set to a positive value. For example, for memory cells used in high- cycle/high endurance target applications, VCGR may be set to a positive value to improve long- term/high-cycle erase read performance, based on the knowledge that erase performance typically degrades faster or to a greater degree than program performance. Alternatively, for memory cells in which programmed state read performance is particularly important, or for memory cells that may be marginally or poorly programmed, VCGR may be set to a negative value. In addition, different VCGR values may also be used for read operations via the even row WL and read operations via the odd row WL, e.g., to compensate for asymmetry created during the formation of the cell, for example based on the respective overlap (x- or y-direction) between either the word lines 16 A, 16B or the control gate 20 over the floating gates 12A and 12B (see Figure 1 for reference).

Some embodiments provide a device that includes multiple flash memory cells having different measured programmed and/or erased state current values, wherein different VCGR values are set (e.g., stored in respective trim bits) and applied for different flash memory cells in the device.

BRIEF DESCRIPTION OF THE DRAWINGS

Example aspects of the present disclosure are described below in conjunction with the figures, in which:

Figure 1 illustrates a side cross-sectional view of a split-gate flash memory cell including a pair of floating gates, a word line extending over each floating gate, and a control gate extending over both floating gates.

Figure 2 is a table showing example voltages that may be applied to the various contacts of the split-gate flash memory cell of Figure 1 to perform program, erase, and read operations, according to a conventional cell operation.

Figure 3A is a graph illustrating example effects of applying a control gate voltage Vc when performing a programmed state (off state) read operation on an example split-gate flash memory cell, according to one embodiment.

Figures 3B and 3C are graphs illustrating example effects of applying a control gate voltage Vc when performing an erased state (on state) read operation on an example split-gate flash memory cell, according to one embodiment.

Figure 4A illustrates an example method or algorithm for determining a control gate voltage VCGR to apply to a fabricated memory cell as a function of an initial program current and/or erase current of the cell as fabricated, e.g., as determined in a sort test, according to one embodiment of the invention.

Figure 4B illustrates the application of different control gate voltages VCGR for read operations on different types of memory cells, e.g., program flash memory cells and data flash memory cells, according to one embodiment of the invention. Figure 5 illustrates the ability to compensate for asymmetry in the programmed cell read performance between the even and odd cells (e.g., even and odd word lines) by tuning the control gate read voltage VCGR to different values for reads via the even cell versus reads via the odd cell.

Figure 6 illustrates an example system or device including a plurality of flash memory cells having a control gate, and control electronics configured to apply a non-zero voltage bias to the control gate during read operations of the flash memory cells, according to an example embodiment.

DETAILED DESCRIPTION

Embodiments of the present disclosure provide systems and methods for improving the read window in a split-gate flash memory cell, e.g., by biasing the control gate terminal during read operations to improve or control the cell read performance, e.g., by improving the erased state read performance or programmed state read performance of the cell.

Figure 3 A is a data plot 100 illustrating example effects of biasing the control gate 20 by a control gate voltage VCGR when performing a programmed state (off state) read operation on the example split-gate flash memory cell 10 shown in Figure 1. Plot 100 shows the sensed cell off state current (IrO, in mA) as a function of VCGR applied during a programmed state read operation, for reads at both a well programmed cell (program voltage = 7.5 V) and a marginally programmed cell (program voltage = 6.5V).

As known in the art, programmed state read performance improves with lower sensed off state current (IrO). Thus, Figure 3 shows that as control gate voltage VCGR is decreased and made negative, the sensed off state current IrO decreases, resulting in improved programmed state read operations. In addition, as discussed below with reference to Figure 5, using a negative VCGR may also reduce an asymmetry between even WL row read operations and odd WL row read operations, which is common in split-gate flash memory cells due to manufacturing tolerances or imperfections, especially on marginally programmed cells. Thus, a negative VCGR may be beneficial for improved programmed state read IrO margin. In essence, application of the negative VCGR causes the cell to appear to be well programmed, even when it is not.

Figures 3B and 3C are data plots illustrating example effects of applying a control gate voltage VCGR during an erased state (on state) read operation on an example split-gate flash memory cell, according to one embodiment. In particular, plot 110 shown in Figure 3B shows the sensed on-state current Irl as a function of control gate voltage VCGR and as a function of word line voltage VWL, and plot 120 shown in Figure 3C shows the percentage increase in sensed on-state current Irl as a function of control gate voltage VCGR and as a function of word line voltage VWL.

As known in the art, erased state read performance improves with increased sensed on- state current (Irl). Figures 3B and 3C show that as VCGR is increased (positive value), the sensed Irl increases, thus improving the on-state (erased state) read margin. The Irl increase, and thus on-state read improvement, may be dependent on the coupling ratio between the control gate 20 and the relevant floating gate 12A or 12B, which is typically a function of the inter-poly oxide thickness. Figures 3B and 3C also show that by increasing the VCGR applied during on-state read operations, the word line voltage VWL may be decreased while still providing the same or greater sensed on-state current Irl than provided by a conventional read operation with VCGR = 0.

Figure 4A illustrates an example method or algorithm 200 for determining a control gate voltage VCG _R to apply to a fabricated memory cell, by setting a VCG _R trim bit, as a function of a measured program current and/or erase current of the cell, according to one embodiment of the invention. In this embodiment, the initial program current and erase current of the cell may be measured after fabrication, e.g., during a sort test. A fixed current source (I _ref low) and a reference cell current (I _ref high) may define a window (e.g., on-state current on vs. off- state current) for passing cell current distribution. In the example algorithm 200, an initial programmed state current of the cell is measured at 202, and an initial erased state current of the cell is measured at 204. The initial programmed state cell current is compared with a fixed current source ( _low ) 206 using an analog comparator at 210, and the initial erased state cell current is compared with a reference cell current ( _high ) 208 using an analog comparator at 212. The comparators 210 and 212 on both the erased and the programmed side are then used to adjust the VCG _R programmable value at 214, and a corresponding trim bit value is set at 216. For example, the comparator 210 on the programmed state would have an output logic 1 (high) if the initial programed state current 202 is higher than the fixed reference current 206. This output may then drive the downstream comparator 214 to an output state of 1 that would force the VCG _R trim bit to program the VCG _R voltage to a more negative value (e.g., using an iterative approach with incremental/stepped changes to the VCG _R value) until the output of the programmed state comparator 210 flips to a logic 0 (low) state, and this VCG _R value (or the VCG _R value prior to the 1 - 0 flip of comparator 210 output) may be set as the VCG _R trim bit.

Thus, algorithm 200 may calculate or determine a VCG _R value to“shift” the on/off current window to optimize the read performance as desired, and this VCG _R value may be set as a VCG _R trim bit in the flash memory (e.g., using a fuse bit). For example, setting the VCG _R trim bit to a negative value (e.g., -2V) may reduce both the sensed programmed state current IrO and the erased state current Irl . Thus, setting the VCG _R trim bit to a negative value may improve programmed state read performance at the possible expense of erased state read performance. Similarly, setting the VCG _R trim bit to a positive value (e.g., 6V) may increase both the sensed programmed state current IrO and the erased state current Irl . Thus, setting the VCG _R trim bit to a positive value may improve erased state read performance at the possible expense of programmed state read performance.

In view of the trade-offs discussed above, the VCGR for any particular memory cell may be set based on the intended usage and/or performance characteristics of that memory cell, e.g., such as endurance, read speed, etc.

For example, for memory cells in which erased state read performance is particularly important, VCGR may be set to a positive value. For example, for memory cells used in high- cycle/high endurance target applications, VCGR may be set to a positive value to improve long- term/high-cycle erase read performance, based on the knowledge that erase performance typically degrades faster or to a greater degree than program performance.

Alternatively, for memory cells in which programmed state read performance is particularly important, or for memory cells that may be marginally or poorly programmed, VCGR may be set to a negative value.

In one embodiment, a device may include multiple flash memory cells having different measured programmed and/or erased state current values, such that different VCGR values may be set (e.g., stored in respective trim bits) for different flash memory cells in the device.

Figure 4B illustrates the application of different control gate voltages Vc for read operations on different types of memory cells, e.g., program flash memory cells and data flash memory cells, according to one embodiment of the invention. Program flash memory typically has a lower endurance specification than data flash memory. Due to larger size and construction, program flash memory is typically more prone to certain disturb mechanisms such as a column disturb. These disturb mechanisms can be reduced by lowering the off state current (IrO) at the expense of on-state current (Irl). As discussed above, endurance degradation in the cell occurs mostly in the on-state current Irl, so a lower initial (pre endurance) Irl may be tolerated by using a negative VCGR.

In contrast, for the data flash memory, the panel is typically much smaller but has a higher endurance specification. Because the endurance degradation occurs mainly in the on- state current Irl, VCGR may be set to a positive value to obtain a higher sensed Irl, to thereby compensate for the endurance-related erase degradation.

Thus, the VCGR for each flash memory cell may be set based on the intended usage of that cell. In one embodiment, a device may include multiple types of flash memory (e.g., program flash and data flash) having different VCGR settings (e.g., as stored in respective trim bits).

As discussed above, in some embodiments, applying a non-zero VCGR may reduce a cell read current asymmetry between even and odd cells, e.g., even and odd word lines/rows that share a common control gate. This asymmetry is common in split-gate flash memory cells due to manufacturing tolerances or imperfections (e.g., regarding the extent of overlap between the word lines and/or control gate over each floating gate), especially on marginally programmed cells. For example, odd/even row asymmetry may be created during the formation of the cell, based on the respective overlap (x- or y-direction) between either the word lines 16 A, 16B or the control gate 20 over the floating gates 12A and 12B (see Figure 1 for reference). A misalignment during fabrication could cause a large x-direction overlap for the even row as compared with the odd row, or vice versa. This overlap asymmetry may result in a different erase or program performance for the cell as compared to a neighboring cell(s) that share the same source junction. The VCGR for the even and/or odd rows may be tuned to compensate for this asymmetry.

In some embodiments, a control system may be configured to apply a non-zero VCGR for cell reads via both the odd and even row WLs. For example, the control system may be configured to apply the same non-zero VCGR (either a positive or negative value VCGR) for reads via both the odd and even WLs. As another example, the control system may be configured to apply a first non-zero VCGRI (either a positive or negative value VCGR) for reads via the odd row WL, and a second, different non-zero VCGR2 (with either the same or opposite polarity) for reads via the even row WL. Figure 5 is a plot 300 showing example cell current data for an example memory cell in which VCGR = -2V is applied for reads via the even row WL and no control gate voltage is applied (VCGR = 0V) for reads via the odd row WL, for both erase state and programmed state reads, as indicated in the data clusters labelled“Tuned VCGR, erase read” and“Tuned VCGR, programmed read.” Plot 300 also shows example cell current data for the same memory cell in which no control gate voltage is applied (VCGR = OV) for reads via both the odd and even WL rows, as indicated in the data clusters labelled“Untuned VCGR, erase read” and“Untuned VCGR, programmed read.”

As shown in Figure 5, applying VCGR ₌ -2V for reads via the even row WL may reduce the asymmetry (e.g., difference in average sensed cell current) between the odd and even rows for programmed state reads, as compared with the situation of applying no control gate voltage (VCGR = 0 V).

Figure 6 illustrates an example electronic system or device 400, according to an example embodiment. Electronic system or device 400 may include groups or arrays of flash memory cells 402 (indicated at 402A-402N as discussed below), logic instructions (e.g., embodied as software or firmware), control circuitry 406 for operating flash memory cells 402 (e.g., by performing program, erase, and read operations), and any other suitable hardware, software, or firmware for providing the functionality of the respective electronic system or device 400.

Control circuitry 406 may be configured to apply a non-zero (positive or negative) voltage bias to the control gate, VCGR, during read operations of respective flash memory cells 402, for reads via one or both of the odd and even row WLs, e.g., to improve programmed state read performance or erased state read performance of the respective flash memory cells 402, as discussed above. The control gate read voltage(s), VCGR, applied to respective flash memory cells 402 may be stored in memory as trim bits 410 accessible to memory control circuitry 406.

In some embodiments, the control circuitry 406 may be configured to apply a different VCGR for reads via the odd row WL as compared with the even row WL, e.g., as discussed above regarding Figure 5. For example, control circuitry 406 may determine or control whether each respective read access is being made to an even row or an odd row, and control a voltage source driving the control gate accordingly. For example, a simple register bit that is ANDED with a non-zero VCGR _d rive (e.g., VCGR = -2V) can indicate an even bit if the bit is 1, and 0V when the bit is 0. The control circuity 406 may determine whether each read access is for an even or odd row using any technique known in the art, e.g., by multiplexing with the memory address counter last (least significant or LSB) bit, or other suitable technique.

In the illustrated example, flash memory 402 may include multiple different groups or arrays of flash memory cells 402A - 402N, where each group may include one or more flash memory cell, e.g., split-gate flash memory cells such as shown in Figures 1-2 and discussed above. In one example embodiment, flash memory cells 402A - 402N may include SuperFlash memory cell (e.g., SuperFlash ESF1+ cells or a variation thereof) by Microchip Technology Inc., having a headquarters at 2355 W Chandler Blvd, Chandler, AZ 85224.

In some embodiments, the different groups or arrays of flash memory cells 402A - 402N may have different usage characteristics or requirements (e.g., different endurance, read speed, or other usage requirements), different performance characteristics, different physical specifications, or may represent different batches of memory cells fabricated at different times or using different fabrication parameters, for example. Thus, to compensate for such differences, or to achieve performance characteristics for different memory cell groups 402A

- 402N, control circuity 406 may apply a selected control gate read voltage VCGR (or selected VCGR values specific to odd row reads or even row reads) to each respective memory cell group 402 A - 402N. Thus, as shown in Figure 6, respective control gate read voltage value(s) VCGR_A

- Vc _GR-N may be stored for each respective flash memory cell group 402A - 402N, e.g., as trim bits 410 stored in device memory. Control gate read voltage values VCGR_A - Vc _GR-N may include any number of different voltage values, may include both negative and positive voltage values, and may also include at least one zero value. In other embodiments, a single control gate read voltage value VCGR is used for all flash memory cells 402 in the device.

In some embodiments, device 400 may include control gate voltage management circuity 420 configured to set and/or dynamically adjust the values of one or more control gate read voltage values VCGR_A - VCGR_N. For example, control gate voltage management circuity 420 may be configured to execute method 200 shown in Figure 4A to measure initial program/erase current values for each (or selected) memory cells and set respective value(s) VCGR based on such measurements and/or additional factors (e.g., the usage characteristics or requirements, performance characteristics, physical specifications, etc. of the respective memory cells) for one or more respective memory cell groups 402A - 402N. Further, in one embodiment, control gate voltage management circuity 420 may be configured to dynamically adjust the control gate read voltage value VCGR for at least one memory cell group 402A-402N over time, e.g., based on monitored endurance data for respective memory cells (e.g., using an erase cycle counter), or based on periodic program/erase current measurements taken by circuity 420.