BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to charge-trapping layers in memory devices, such as non-volatile memory devices. In particular, the present invention relates to a charge- trapping layer of a memory cell in a Not-OR (NOR) memory string.

2. Discussion of the Related Art

In conventional NOR-type and NAND-type memory devices, an applied electrical field across a charge-trapping layer adds or removes charge from traps in the charge-trapping layer. Figure 1 shows a schematic cross-section of vertical-channel, polycrystalline thin-film transistor or memory cell 100 with source region 10, drain region 20, channel region 30, tunnel oxide layer 40, charge-trapping layer 50, blocking oxide layer 60, and gate conductor 70. Charge-trapping layer 50 may be, for example, silicon nitride. A current is induced to flow from source region 10 through channel region 30 to drain region 20, when a suitable high voltage — relative to the voltage at source region 10 — is placed on gate conductor 70.

At the same time, electrons are drawn from channel region 30 into charge-trapping layer 50, where their accumulation shifts threshold voltage V _t of memory cell 100 to a higher value. The higher value in threshold voltage V _t of memory cell 100 represents a “programmed” state. By placing a high voltage of the opposite polarity on gate conductor 70, electrons are withdrawn from charge-trapping layer 70, so that threshold voltage V _t of memory cell 100 is shifted to a lower value, which represents an “erased” state.

Fast program and erase of a memory cell are very desirable. Generally, a greater magnitude in the voltage placed on the gate conductor to effectuate programming or erase of a memory cell shortens the program or erase time, as illustrated in Figure 2. As shown in Figure 2, waveform 201a represents the program times for shifting threshold voltage V _t from approximately 1.0 volts to a voltage between 1.0 volts and 6.0 volts using a program voltage of 12 volts. Waveform 201b represents the erase times for shifting threshold voltage V _t from approximately 6.0 volts to a voltage between 1.0 volts and 6.0 volts using an erase voltage of -12.0 volts. Likewise, waveforms 202a and 203a represent the program times for shifting threshold voltage V _t from approximately 1.0 volts to a voltage between 1.0 volts and 6.0 volts using programming voltages of 10.0 volts and 8.0 volts, respectively. Waveforms 202b and 203b represent the erase times for shifting threshold voltage V _t from approximately 6.0 volts to a voltage between 1.0 volts and 6.0 volts using erase voltages of -10.0 volts and -8.0 volts, respectively.

However, a higher voltage at the gate conductor decreases the charge-trapping layer’s endurance. Endurance is the ability of the memory cell to maintain substantially the same “programmed” or “erased” state threshold voltage V _t after many program and erase cycles Figure 3 shows a collection of waveforms each representing threshold voltage V _t of a memory cell after predetermined number of program and erase operations using a predetermined program or erase voltage over 10 ⁸ programming and erase cycles. For example, waveform 301a represents threshold voltage V _t — initially about 1.3 volts — after 10, 10 ² , ..., 10 ⁸ program and erase cycles, using a program voltage of 9.5 volts. Fikewise, waveform 301b represents threshold voltage V _t — initially about 0.0 volts — after 10, 10 ² ,

10 ⁸ program and erase cycles, using a program voltage of -9.5 volts. The memory cell in each waveform in Figure 3 is a thin-film storage transistor with a 1.2-nm tunnel oxide (e.g., S1O2), a 8.5-nm charge-trapping layer (e.g., SiN with a refractive index of 2.20), and a 3.0- nm blocking oxide layer (e.g., SiCF). Each program and erase operation is achieved using a 1.0-microsecond pulse at the specified program or erase voltage.

In Figure 3, waveform 301a indicates that the “programmed” state threshold voltage V _t of the memory cell has shifted to about 3.0 volts after 10 ⁸ program and erase cycles, and the corresponding “erased” state threshold voltage V _t has also moved up to about 2.7 volts over the same time period. These shifts in threshold voltages are believed to be caused by trap sites created in the tunnel oxide layer as a result of the tunneling currents over a long period of programming and erase cycles. The electrons trapped in trap sites in the tunnel oxide layer are not removed in an erase operation on the charge-trapping layer. Figure 3 therefore illustrates degradation in the memory cell’s endurance, as evidenced by the shifts in the “programmed” and “erased” state threshold voltages at higher program and erased voltages as the number of program and erase cycles increase. Thus, there is a trade-off in endurance between fast program and erase times and the magnitude of program and erase voltages at the gate conductor. With the high density of memory cells in a state-of-the-art memory circuit, the threshold voltages of the memory cells are not uniform, resulting in a distribution in threshold voltages amongst the memory cells. Figure 4 shows a distribution of memory cells in “programmed” and “erased” states in one memory system. As shown in Figure 4, memory cells having a threshold voltage less than 2.0 volts (i.e., region 401a), with a mean threshold voltage at about 1.5 volts, are deemed in the “erased” state; and memory cells having a threshold voltage greater than 5.0 volts (i.e., region 401b), with a mean threshold voltage at about 5.5 volts, are deemed in the “programmed” state. The difference in threshold voltages between memory cells arises primarily from a difference in the number of electrons stored in their charge-trapping layers. A greater number of electrons results in a higher threshold voltage.

To allow a determination of whether a given memory cell is in the “programmed” state or in the “erased” state, the memory system requires a minimum difference in threshold voltage V _t (AV _t ) between the memory cells in the “programmed” state and the memory cells in the “erased” state. It is also desirable to minimize variations in threshold voltage within each population (i.e., “programmed” state population or the “erased” state population). For example, suppose the highest thereshold voltage V _t among memory cells in the “erased” state is V _e and the lowest thereshold voltage V _t among memory cells in the “programmed” state is V _p , a memory cell having threshold voltage V _ti < V _e requires a higher program voltage or a longer programming pulse to program the memory cell to threshold voltage V _P than a memory cell with a threshold voltage V _e in the erased state. Similarly, a memory cell with a threshold voltage V _t 2 > V _p in the “programmed” state requires a higher voltage (in magnitude) or a longer erase pulse to erase the memory cell to threshold voltate V _e . In either case, using a minimal program or erase pulse may not succeed in programming or erasing any given memory cell to threshold voltage V _p or V _e , respectively. To ensure a high probability of success in the programming or erase operation using only one program or erase pulse, one must resort to using a high (in magnitude) program or erase voltage. The resulting distriubtions of threshold votlages of memory cells in the “programmed” or “erased” states, however, will become wider. In general, many more charge-trapping sites are available in the charge-trapping layer of a memory cell than the electrons needed to shift the threshold voltage of the memory cell by AV _t , which contributes to a wider distribution in either “programmed” or “erased” state.

In the prior art, a method to reduce the spread of the distributions for memory cells in the “programmed” or “erased” state is to “read” the memory cell at a low voltage (in magnitude) to verify that the cell has achieved the desired “programmed” or “erased” state, and to repeat the programming or erase operation until the desired “programmed” or “erase” state is achieved. This iterative procedure is referred to as a “read-verify” procedure. It is not uncommon to program or erase the memory cell many times during the read-verify procedure to achieve the desired state. While the read-verify procedure is effective in achieving distributions of reduced widths using lower (in magnitude) program or erase voltage, overall program and erase times are increased because of the inevitable repetitive program and erase steps.

SUMMARY The present invention provides an optimized charge-trapping layer in a memory device which allows a memory system incorporating such memory devices to have reduced programming and erase times. The optimized charge-trapping layer may be applicable, for example, to memory cells in a 3-dimensional array of non-volatile NOR-type memory strings. The parameters to be optimized include, for example, the number of charge-trapping sites in the charge-trapping layer, and its physical dimensions (e.g., length, width, and thickness). Furthermore, the electric field across the tunnel oxide layer during the programming or erase operational for the optimized charge-trapping layer is significantly lower than under a conventional programming or erase operation, such that the endurance of the memory device is significantly enhanced.

According to one embodiment of the present invention, a thin- film memory transistor may include a source region, a drain region, a channel region, a gate electrode, and a charge trapping layer provided between the channel region and the gate electrode and electrically isolated therefrom, wherein the charge-trapping layer has includes a number of charge trapping sites that is 70% occupied or evacuated using a single voltage pulse of a predetermined width of 500 nanoseconds or less and a magnitude of 15.0 volts or less. The charge-trapping layer comprises silicon-rich nitride may have a refractive index of 2.05 or greater or comprises nano-crystals of germanium (Ge), zirconium oxide (ZrCk), or zinc oxide (ZnO). The thin-film memory transistor may be implemented, for example, in a 3- dimensional array of NOR memory strings formed above a planar surface of a semiconductor substrate.

The present invention is better understood upon consideration of the detailed description below in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic cross-section of vertical-channel, polycrystalline thin-film transistor or memory cell 100 with source region 10, drain region 20, channel region 30, tunnel oxide layer 40, charge-trapping layer 50, blocking oxide layer 60, and gate conductor 70.

Figure 2 demonstrates that a greater magnitude in the voltage placed on the gate conductor to effectuate programming or erase of a memory cell shortens the program or erase time.

Figure 3 shows a collection of waveforms each representing threshold voltage V _t of a memory cell after predetermined number of program and erase operations using a predetermined program or erase voltage over 10 ⁸ programming and erase cycles. Figure 4 shows a distribution of memory cells in “programmed” and “erased” states in one memory system.

Figure 5 shows narrow distributions of threshold voltages in memory cells that are in the “programmed” and “erased” states, achieved by limiting the number of charge-trapping sites in their charge-trapping layers.

To facilitate cross-referecnes among the figures, like elements in the figures are assigned like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In this detailed description, process steps described in one embodiment may be used in a different embodiment, even if those steps are not described in the different embodiment. When reference is made herein to a method having two or more defined steps, the defined steps can be carried out in any order or simultaneously (except where context or specific instruction excludes that possibility), and the method can include one or more other steps carried out before any of the defined steps, between two of the defined steps, or after all the defined steps (except where context excludes that possibility).

According to one embodiment of the present invention, the distribution of threshold voltages of memory cells in a “programmed” or an “erased” state may be reduced by limiting the available number of charge-trapping sites needed to obtain a threshold voltage difference of AV _t between memory cells of the “programmed” and “erased” states. Figure 5 shows narrow distributions of threshold voltages in memory cells that are in the “programmed” (i.e., region 501b) and “erased” (i.e., region 501a) states, achieved by limiting the number of charge-trapping sites in their charge-trapping layers. When the number of charge-trapping sites is so small, such that a program pulse supplies sufficient electrons to completely or nearly completely fill the available charge-trapping sites (e.g., greater than 70% filled), the read- verify procedure is obviated for the program operation. Likewise, when the number of charge-trapping sites is so small, such that an erase pulse removes sufficient electrons to completely or nearly completely vacate the available charge-trapping sites, the read- verify procedure is obviated for the erase operation (e.g., greater than 70% evacuated). By eliminating the read-verify procedure, program and erase times for the memory system are much reduced.

Another method to narrow the distributions of threshold voltages in the memory cells in the “programmed” or the “erased” state is to reduce difference AV _t in the threshold voltages between the “programmed” state and the “erased” state in the memory cell. The widths of the distributions of memory cells in the “programmed” and the “erased” states are dependent, in part, on the value of threshold voltage difference AV,. A larger threshold voltage difference AV _t results in wider distributions in memory cells in the “programmed” and the “erased” states. For example, the distributions of memory cells in the “programmed” and the “erased” states are narrower when highest voltage V _e for the “erased” state and lowest threshold voltage V _P for the “programmed” state are set at 1.0 volts and 2.0 volts (i..e, AV _t equals 1.0 volts), respectively, than when they are set at 2.0 volts and 6.0 volts (i..e, AV _t equals 1.0 volts).

The present invention provides numerous ways for providing a charge-trapping layer with an optimal number of charge-trapping sites. For example, to achieve a 1.0-volt threshold difference AV _t for a polysilicon thin-film transistor, a total electron area density of 2.7 x 10 ¹² e /cm ² is required of the charge-trapping layer. The trap-site density for silicon- rich silicon nitride (SRN) with an refractive index of 2.20, for example, has been reported to be 2.5 x 10 ¹⁹ e /cm ³ . (See, e.g., the article “Electron trap density distribution of Si-rich silicon nitride extracted using the modified negative charge density decay model of silicon-oxide- nitride-oxide-silicon structure at elevated temperature,” by T.H. Kim, et al., Applied Physics Letters 89, 063508-063511 (2006).) Therefore, for a 1.0 nm thick SRN charge-trapping layer, the trap-site area density is 2.5 x 10 ¹⁹ e /cm ³ x 1.0 x 10 ⁷ cm) or 2.5 x 10 ¹² e /cm ² . For reference, a 1.0 nm thick SRN film is a much thinner charge-trapping layer than those used in current state-of-the-art SRN films used in multi-level (MLC) NAND memory devices; such SRN films in MLC NAND devices are typically 7.0 nm or more thick. MLC NAND devices typically have multiple “programmed” and “erased” states, and require the read- verify procedure to attain any of the multiple “programmed” and “erased” states. The program and erase times of MLC NAND devices are therefore notoriously long (e.g., tens to hundreds of microseconds).

In one embodiment, a memory cell in a NOR memory string - which is configured in the manner disclosed in the Related Application, incorporated by reference above — has a 1.0 nm thick SRN charge-trapping layer provided between a 1.0 nm thick tunneling oxide layer and a 1.0 nm thick blocking oxide layer. The Related Application teaches forming arrays of such NOR memory strings in a 3-dimensional memory structure above a planar surface of a semiconductor substrate. It is desired to program or erase the memory cell using a single pulse of 500 ns or less wide to effectuate threshold voltage difference AV _t of 1.0 volts between “programmed” or “erased” states. In that memory cell the dominant program or erase mechanism is believed direct tunneling. Under such conditions, the magnitude of the required program or erased voltage to program or erase such a memory cell using a single pulse of 500 ns wide or narrower, preferably 100 ns or narrower, may be empirically determined, but estimated to be about 8.0-15.0 volts. According to another embodiment of the present invention, one method to achieve a advantageously small number of trap-sites in a charge-trapping layer of a memory cell is to reduce the trap-site density in the material of the charge-trapping layer (e.g., SRN). The trap- site density in SRN is dependent in part, for example, on its composition. SRN films may vary in refractive index between about 2.0 and about 2.4, for example, between (See, e.g., the article “Explanation of the charge-trapping properties of silicon nitride storage layers for NVM devices Part I: Experimental evidences from physical and electrical characterization,” by E. Vianello, et al., IEEE Transactions on Electron Devices 58, no. 8, pp. 2483-2489 (2011).) When the trap-site density is reduced to less than 2.5 x 10 ¹⁹ e /cm ³ , a SRN charge trapping layer that is greater than 1.0 nm thick may be used to achieve the 1.0- volt threshold difference AV _t , discussed above.

Alternatively, a charge-trapping layer with desirably small number of trap sites may be achieved using nano-crystals (also known as quantum dots) in the charge-trapping layer; suitable materials that contain nano-crystal include, for example, various charge-transporting layers (CTLs) used in many electronic devcies. (See, e.g., the articles (i) “Charge-trap memory device fabrication by oxidation of Sii- _x ,Ge _x ” by Y.-C. King et ah, IEEE Transactions on Electron Devices 48, no. 4, pp. 696-800 (2001); and (ii) “Nanoislands-based charge trapping memory: A scalability study,” by N. El-Atab, et ak, IEEE Transactions on Nanotechnology 16, no. 6, pp. 1143-1145 (2017).) Nanocrystals of germanium (Ge), zirconium oxide (Zr0 ₂ ), or zinc oxide (ZnO), or any suitable material, can be fabricated in a manner to control the area density in the resulting material, therey allowing a good control of the trap-site area density.

As the electric field across the tunnel oxide layer during the programming or erase operational for a charge-trapping layer of the present invention is significantly lower than under a conventional programming or erase operation, the endurance of the memory device is significantly enhanced.

The above detailed description is provided to illustrate specific embodiments of the present inventiuon and is not intended to be limiting. Numerous modifications and variations within the scope of the present invention are possible. The present invention is set forth in the following claims.