Erasing methods by hot hole injection to carrier trap sites of a nonvolatile memory

[0002] 1. Field of the Invention

[0003] The present invention relates to erasing methods of a nonvolatile memory cell and array using hot holes.

[0004] 2. Description of the Prior Art

[0005] MONOS (Metal-Oxide-Nitride-Oxide-Semiconductor, in which “Metal” is a representative of a conducting gate material) or SONOS (Silicon (as a conductive gate)-Oxide-Nitride-Oxide-Silicon) memories have been proposed for the improvement of MONOS (Metal-Nitride-Oxide-Semiconductor) memory in scaling down the thickness of the nitride film for the device scaling in the lateral dimension and scaling in the programming voltage and also for improvement in reproducibility. In the MONOS or SONOS memory (hereinafter “MONOS memory” will be used to represent both), carrier trap sites, which are located in the nitride film and at the interface between the nitride and the top oxide, are used for capturing and storing carriers as electrical information.

[0006] In an original MONOS paper (Reference 1: E. Suzuki, et al., “A Low-Voltage Alterable EEPROM with metal-oxide-nitride-oxide-semiconductor (MONOS) Structure”, IEEE Transaction on Electron Devices, Vol. ED-30, February. 1983), direct tunneling of electrons and holes is used for the programming; that is, direct-tunnel injection of electrons into the trap sites is used for “write” and direct-tunnel extraction of electrons from the trap sites and/or direct-tunnel injection of holes into the trap sites is used for “erase.”

[0007] On the other hand, injection of hot electrons into the trap sites for write and erasure by the injection of hot holes, which was originally applied to a floating gate device (see Reference 2: Y. Tarui, Y. Hayashi, K. Nagai, “Electrically Reprogrammable Nonvolatile Memory,” IEEE Journal of Solid-State Circuits, Vol.SC-7, No.5, October., 1992, p.369-375), was proposed for programming single gate MONOS memories with thicker bottom oxide for better retention characteristics (see Reference 3: T.Y. Chan, K. K. Young, Chemning Hu, “A True Single-Transistor Oxide-Nitride-Oxide EEPROM Device”, IEEE Electron Device Letters, Vol.EDL-8, No.3, 1987, p.93-95) and for doubling bit density (see Reference 4: B. Eitan, et al., “Can NROM, a 2 Bit, Trapping Storage NVM Cell, Give a Real Challenge to Floating Gate Cells?”, 1999 Conference on Solid State Devices and Materials, Tokyo, Extended Abstracts, p. 522-523, 1999).

[0008] The above single gate MONOS memory is characterized by a single gate bridging over a channel region where the single gate is disposed on ONO (oxide-nitride-oxide) gate insulator on the channel forming semiconductor region and on a part of the drain and source region.

[0009] For the “write,” hot electrons are injected into a part of the ONO gate insulator adjacent to a junction between the drain and the channel forming region and trapped in the trapping sites in the ONO gate insulator. For the “erase,” hot holes are injected into a part of the ONO gate insulator adjacent to the junction. However, erased states by hot holes turned out to be not reproducible and/or not stable with respect to the threshold voltage (V_th.) under certain hole injection bias conditions. In References 3 and 4, above, this instability was masked by stable electrical characteristics of a portion of the channel forming semiconductor region under an un-programmed portion of the ONO film because a portion of the channel forming semiconductor region under an erased portion of the ONO film is connected in series with the above “unprogrammed” portion of the channel forming semiconductor region, where V_this higher than the V_thof the “erased” portion. On the other hand, the instability will be observed if the majority of the un-programmed portion of the channel forming semiconductor region is controlled and turned on by a separate gate. In this case, another gate is laid side by side with and insulated from the separate gate disposed over the programmed portion of the channel forming semiconductor region.

[0010] It is a purpose of the present invention to provide a method of stably erasing a non-volatile memory in a gate insulator in which carrier-trapping sites for carrier storage are furnished.

[0011] It is another object of the present invention to provide a method of uniformly erasing a non-volatile memory array with a gate insulator in which carrier-trapping sites for carrier storage are furnished.

[0012] It is a further object of the invention to provide a method of erasing to realize better endurance of a non-volatile memory.

[0013] Accordingly, a first method of the present invention is the application of a discharge pulse(s) to a gate after erasure where the discharge pulse(s) discharges unstable holes injected into the gate insulator. Hot holes are injected into a gate insulator while negatively or zero biasing a conductive gate and positively biasing an opposite conductivity region whereby hot holes are generated in a surface of the opposite conductivity region, wherein positively biasing the opposite conductivity region with respect to a first channel forming semiconductor region is larger in value than an electric barrier height against the hot holes wherein the electric barrier is formed at an interface between the gate insulator and the opposite conductivity region. The conductive gate is positively biased to a voltage which generates an average electric field of more than 5 MV/cm in the first gate insulator.

[0014] A second method of the invention is injection of electrons into the trap sites of all the cells in a memory array to be erased before erasure. This makes V_thdistribution across the memory array uniform after erasure. Electrons are injected into a gate insulator wherein the electrons are trapped in carrier trap sites within the gate insulator. Then, hot holes are injected into the gate insulator while applying a negative or zero bias to a conductive gate and applying a positive bias to an opposite conductivity region.

[0015] A third method of the invention is a reduced bias approach to erase stably the electrons stored in the trap sites. Hot holes are injected into a gate insulator while applying a negative or zero bias to a conductive gate and applying a positive bias to an opposite conductivity region wherein the positive bias is larger in value than an electric barrier height against the hot holes, wherein the electric barrier is formed at an interface between the gate insulator and the opposite conductivity region and wherein a sum of absolute values of the positive bias and ⅖ of the negative or zero bias is equal to or smaller than 6V wherein the negative or zero bias ranges from 0V to −5V. This includes not only literally “erase,” but also “annihilate or neutralize” trapped electron charge by hole charge.

[0016] At least one of the above methods is applied to a memory cell or a memory array made from memory cells where the memory cell comprises at least:

[0017] a) a first channel forming semiconductor region in a surface of a substrate,

[0018] b) an opposite conductivity type region adjacent to the first channel forming semiconductor region and in the surface of the substrate,

[0019] c) a gate insulator on the first channel forming semiconductor region,

[0020] d) a conductive gate on the gate insulator and carrier trap sites in the gate insulator,

[0021] e) a second channel forming semiconductor region contacting the first channel forming semiconductor region in the surface of the substrate,

[0022] f) a second gate insulator on the second channel forming semiconductor region, and

[0023] g) a second gate on the second gate insulator where the second conducting gate is separated and insulated from the first conducting gate by an insulator.

[0024] More specifically, the memory cell comprises:

[0025] a) a channel forming semiconductor region of one conductivity type in a surface of a substrate where the substrate is a semiconductor substrate or a silicon-on-insulator (SOI) substrate,

[0026] b) a first opposite conductivity region and a second opposite conductivity region being disposed in the surface of the substrate where the first and second opposite conductivity regions are spaced apart and separated from each other by the channel forming semiconductor region,

[0027] c) the channel forming semiconductor region comprises:

[0028] i) a first channel-forming region laterally contacting the first opposite conductivity region,

[0029] ii) a second channel forming region contacting the second opposite conductivity region, and

[0030] iii) a third channel forming region in contact with the first and second channel forming regions and disposed between the first channel forming region and the second channel forming region,

[0031] d) gate insulators comprising:

[0032] i) a first gate insulator disposed on the first channel forming region and a part of the first opposite conductivity region adjacent to the first channel forming region,

[0033] ii) a second gate insulator disposed on the second channel forming region and a part of the second opposite conductivity region adjacent to the second channel forming region, and

[0034] iii) a third gate insulator disposed on the third channel forming region,

[0035] e) conductive gates comprising:

[0036] i) a first gate electrode on the first gate insulator,

[0037] ii) a second gate electrode on the second gate insulator, and

[0038] iii) a third gate electrode on the third gate insulator

[0039] wherein the gate electrodes are electrically insulated from each other and wherein a part of the first gate overlaps a part of the first opposite conductivity region via the first insulator and a part of the second gate overlaps a part of the second opposite conductivity region via the second insulator, and

[0040] f) carrier trapping sites for carrier storage furnished in the first and second insulators.

[0041] As one example, the carrier trapping sites are embedded in a three layered insulator structure. A typical embodiment is an oxide-nitride-oxide layer, where, in the fabrication process, carrier trapping sites are generated in the silicon nitride and/or in the interface between the silicon nitride and the silicon oxide. The silicon nitride can include a fraction of oxygen and the silicon oxide can be a nitrided oxide in which a part of the oxygen atoms of the oxide are replaced by nitrogen atoms.

[0042] As another example, the carrier trapping sites are realized by small particles of conductor or semiconductor such as silicon buried in the first and second gate insulators.

[0043] FIG. 1 is a cross-sectional representation of an example of a non-volatile memory cell to which the hole injection erasing method of the present invention is applied.

[0044] FIG. 2 is a graphical representation of erased threshold voltages versus erase voltages and threshold voltage drifts after each read operation.

[0045] FIG. 3 is a graphical representation of the erased threshold voltages versus erase voltages with high threshold voltage and low threshold voltage before the erase.

[0046] FIG. 4 is a graphical representation of written threshold voltages and erased threshold voltages versus the write-erase cycle.

[0047] FIG. 1 shows an example of a non-volatile memory cell to which the hole injection methods of the present invention are applied. It will be understood by those skilled in the art that the methods of the present invention can be applied to other suitable non-volatile memory cells than the one shown in FIG. 1 without departing from the spirit and scope of the present invention.

[0048] In the surface of a semiconductor substrate 100, a well region 200 is formed. In the surface of the well region 200, a channel forming semiconductor region 300, a first opposite conductivity region 401, and a second opposite conductivity region 402 are formed. The channel forming semiconductor region 300 is of p-type and the first and second opposite conductivity regions are of n-type.

[0049] The channel forming semiconductor region 300 further comprises:

[0050] a) a first channel forming region 301 contacting the first opposite conductivity region 401,

[0051] b) a second channel forming region 302 contacting the second opposite conductivity region 402, and

[0052] c) a third channel forming region 303 contacting the first and second channel forming regions at both sides.

[0053] A first gate insulator 501 is disposed on the first channel forming region 301 and a part of the first opposite conductivity region 401 adjacent to the first channel forming region. A second gate insulator 502 is disposed on the second channel forming region 302 and a part of the second opposite conductivity region 402 adjacent to the second channel forming region. A third gate insulator 503 is disposed on the third channel forming region 303.

[0054] A first gate electrode 601 is formed on the first gate insulator 501. A second gate electrode 602 is formed on the second gate insulator 502. A third gate electrode 603 is formed on the third gate insulator 503. The first and third gate electrodes and the second and third gate electrodes are electrically insulated from each other by insulators 701 and 702.

[0055] A part of the first gate electrode 601 overlaps a part of the first opposite conductivity region 401 via the first insulator 501 and a part of the second gate electrode 602 overlaps a part of the second opposite conductivity region 402 via the second insulator 502.

[0056] For example, a channel length of the third channel forming region 303 is 0.26 micrometers. Channel lengths of the first and second channel forming regions are each 0.055 micrometers. Impurity concentrations of the first and second channel forming regions are about 1E18 atoms/cm²near the surface. Impurity concentrations of the first and second opposite conductivity regions are between about 1E19 and 1E20 atoms/cm²near the surface under the first and second gates, respectively. The third gate insulator is silicon oxide with a thickness of about 9 nanometers. The first and second gate insulators comprise three layers: 3.6 nm nitrided silicon oxide/5 nm silicon nitride/4.2 nm silicon oxide.

[0057] Two groups of carrier trapping sites 501tand 502tare located in the silicon nitride with the concentration on the order of 1E 19 sites/cm³and at the interface of the silicon nitride and the silicon oxide and the interface of the silicon nitride and the nitrided silicon oxide with a density of about 2E13 sites/cm².

[0058] Now, the stable erasing methods by hot hole injection of the present invention will be described with reference to the memory cell illustrated in FIG. 1.

[0059] Hot hole injection into the first insulator 501 from the first opposite conductivity region 401 is made by applying a positive electrical potential Vbit1r (larger than an electric barrier height against a hole, V_Bh) to the first opposite conductivity region 401 with respect to the first channel forming region 301 and applying a zero to negative potential Vcg1 to the first gate electrode 601 with respect to the first channel forming region 301. Hot hole injection into the second insulator 502 from the second opposite conductivity region 402 is made by applying a positive electrical potential Vbit2r (larger than an electric barrier height against a hole, V_Bh) to the second opposite conductivity region 402 with respect to the second channel forming region 302 and applying a zero to negative potential Vcg2 to the second gate electrode 602 with respect to the second channel forming region 302.

[0060] The holes to be injected are generated by band to band tunneling in the surface of the first or second opposite conductivity type region (401 and 402, respectively). The abovementioned “electric barrier height against a hole V_Bh” means a value (measured in volts) of electric barrier height against a hole formed between the surface of the first or second opposite conductivity region and the first or second insulator. The electric barrier height against a hole, V_Bh, is known as 3.8 electron volt (eV) for a barrier between silicon and SiO₂. The value is smaller for a nitrided silicon oxide.

[0061] Hot electron injection into the first insulator 501 from a first channel in the surface of the first channel forming semiconductor region 301 is made by applying a positive electric potential Vbit1 w (larger than an electric barrier height against an electron, V_Be) to the first opposite conductivity type region 401 with respect to the first channel forming semiconductor region 301 and by applying a positive potential Vcg1 to the first gate electrode 601 with respect to the first channel forming semiconductor region 301.

[0062] The abovementioned “electric barrier height against an electron, V_Be” means a value (measured in volts) of electric barrier height against an electron formed between the surface of the first or second channel forming semiconductor region and the first or second insulator. The electric barrier height against an electron, V_Be, is known as 3.2 electron volt (eV) for a barrier between silicon and SiO₂. The value is smaller for a nitrided silicon oxide.

[0063] Electrons to be injected are supplied to the first channel from the second opposite conductivity type region 402 through a second channel induced in the surface of the second channel forming region 302 and a third channel induced in the surface of the third channel forming region 303. The first, second, and third channels are electrically induced in the respective surfaces of the first, second, and third channel forming semiconductor regions by applying each bias larger than each gate threshold voltage to the first, second, and third gates.

[0064] The dimension, impurity concentration, and combination of the material and thicknesses of the gate insulators of the memory cell are not limited to the above example

[0065] The following Example is given to illustrate the important features of the invention and to aid in the understanding thereof. Variations may be made by one skilled in the art without departing from the spirit and scope of the invention.

[0066] A memory cell with the following dimensions and materials is used for one of the embodiments. The peak impurity concentration of the first and second channel forming silicon region is between about 5E17 and 4E18 atoms/cm³. The channel length of the first and second channel forming silicon regions is between about 30 and 80 nm. The impurity concentration of the third channel forming silicon region is between about 5E17 and 1E18 atoms/cm³. The channel length of the third channel forming silicon region is between about 60 and 260 nm. The first and second insulators comprises three layers of ONO; e.g. a bottom layer of nitrided silicon oxide 3.6 to 7 nm thick, a middle layer of silicon nitride 3-9 nm thick, and a top layer of silicon oxide 4-8 nm thick. Alternatively, the ONO layer could comprise a bottom layer of silicon oxide 2.5 to 6 nm thick, a middle layer of silicon nitride 3 to 9 nm thick, and a top layer of silicon oxide 4 to 8 nm thick. The impurity concentration of the first and second opposite conductivity type regions is between about 1E19 to 5E20 atoms/cm³at each portion under the first and second gates. The insulator between the third gate and the first or second gate at least on each sidewall of the third gate is silicon oxide or nitrided silicon oxide/silicon nitride/silicon oxide.

[0067] By using the above described device, hole injection into the ONO of the second (or first) gate insulator was done with various bias conditions (combinations of voltages and time duration) and threshold voltages were measured as gate threshold voltages (V_th) of the second gate.

[0068] FIG. 2 shows results of the hole injection (for an injection time of 100 milliseconds). In the figure, voltage of the second gate is designated Vcg2 and voltage of the second opposite conductivity type region is designated as Vbit2 and represented by the lines, as follows. Line 21 indicates a Vbit2 of 0 volts, line 22, a Vbit2 of 3 volts, line 23, 4 volts, line 24, 5 volts, and line 25, 6 volts.

[0069] For Vbit2 equal to or larger than 4 volts, (lines 23, 24, and 25), V_thmeasured as that of the gate threshold voltage of the second gate changes toward the negative direction. This means holes were injected into trap sites in the ONO of the second gate insulator. With larger (Vbit2−Vcg2), the larger value of the change in Vth is obtained. However, too large (Vbit2−Vcg2), more than 10 volts, resulted in a short term instability.

[0070] The erased V_thwith (Vbit2−Vcg2) more than 10 volts showed drift (V_th=˜0.5 V) in values after repetitive measurement for V_thdetection as shown in FIG. 2, where the drift in erased V_this shown as the change in V_thvalues for (Vbit2−Vcg2)=11 V. To reduce this drift in V_th, as one of the erasing method of the present invention, application of a stabilizing positive bias, or discharge pulse, after erasure was adopted to the second gate (or to the first gate) with bit line voltage Vbit2 (or Vbit1) of 0V or of less than the electric barrier height against an electron V_Be,. The stabilizing positive bias, (e.g. 5 V which corresponds to an electric field of 5 Mv/cm in the second gate insulator) is larger in the value than a voltage (e.g. 4V) applied to the second gate (or the first gate) when read and is longer than 1 millisecond in duration. The drift in V_thwhen read was reduced less than 0.1 V by this method.

[0071] FIG. 3 shows differences in erased V_thbetween erased V_therased from a high V_th(for Vbit2=3V, line 31 and for Vbit2=4V, line 32) and erased V_therased from a low V_th(for Vbit2=0V, line 34, 3V, line 35, and 4V, line 36). As one of the erasing methods of the present invention, the trap sites which locate under the second gate (or the first gate) and are either in high or low V_thstates, are programmed as a high V_thstate by electron injection before erasure. A more uniform erased V_thdistribution across a memory array comprising the present memory cells is obtained.

[0072] As described above, the drift in erased V_this pronounced for a larger value of (Vbit2−Vcg2). Endurance, which is defined as how many write-erase cycles a memory cell can stand, is also small for a larger value of (Vbit2−Vcg2). The larger value of (Vbit2−Vcg2) may result in excess holes injected into the trap sites and/or holes unstably captured in trap sites. Furthermore, a larger value of Vbit2 results in holes with a higher energy and brings earlier degradation (i.e. increased leakage in the second gate insulator and poor retention), and finally, loss of memory function of the second gate insulator. On the other hand, a larger value of Vcg2 results in electron injection from the second gate into the second gate insulator through the top oxide. This prevents the erasure by the hot hole injection. The larger value of Vcg2 also induces leakage current through the top oxide, in case the top oxide is deposited by a low temperature chemical vapor deposition (CVD) method or by using an organo-silane as a source gas.

[0073] As one of the erasing methods of the present invention, under limited bias conditions characterized as (Vbit2+(⅖)×(−Vcg2)) equal to or smaller than 6V with −Vcg2 equal to or smaller than 5V, more than 1000 cycles of write-erase were obtained. These bias conditions were determined by the abovementioned physics and experiments.

[0074] Examples of experimental results on endurance with erase and write conditions are shown in the following Table 1.

[0075] FIG. 4 shows written V_thand erased V_thas a function of write-erase cycles. During 30,000-cycle operation, a memory window was kept in a readable width under a constant “write” and “erase” (voltage and time) condition. Line 41 shows the V_thfor write at 100 μsec and line 42 shows V_than for erase at 100 μsec. The memory cell could be further written and erased.

[0076] For erasing trap sites under the first gate, the abovementioned methods can be applied by reading Vbit2 and Vcg2 as Vbit1 and Vcg1, respectively.

[0077] The methods of erasing of the present invention comprise:

[0078] 1) injecting hot holes while negative or zero biasing the conductive gate and positively biasing the opposite conductivity region whereby hot holes are generated in the surface of the opposite conductivity region. The positive biasing of the opposite conductivity region with respect to the channel forming semiconductor region is larger in value than the electric barrier height against the holes where the electric barrier is formed at the interface between the gate insulator and the opposite conductivity region. Then, the conductive gate is positively biased to a voltage which generates an average electric field of more than 5 MV/cm in the gate insulator. Positively biasing the conductive gate is achieved by the application of a discharge pulse to the gate after erasure. The discharge pulse discharges unstable holes injected into the gate insulator.

[0079] 2)“write before erase” —injecting electrons into the gate insulator where the electrons are trapped in carrier trap sites within the gate insulator and injecting hot holes while negatively or zero biasing the gate electrode and positively biasing the opposite conductivity type region.

[0080] 3) injecting hot holes while negatively or zero biasing the gate electrode and positively biasing the opposite conductivity type region and generating hot holes in the surface of the opposite conductivity type region by the combination of applying negative or zero bias to the gate electrode and positive bias to the opposite conductivity type region. The positive bias to the opposite conductivity type region with reference to the channel forming semiconductor region is larger in value than an electric barrier height against the holes where the electric barrier is formed at an interface between the gate insulator and the opposite conductivity region. The sum of the absolute values of the positive bias to the opposite conductivity region and ⅖ of the gate bias is equal to or smaller than 6V for the gate bias ranging from 0 V to −5V.

[0081] The process of the present invention provides the following advantages:

[0082] 1) V_thdrift after erasure is improved by applying the stabilizing positive bias to the second (or first) gate after the erasure.

[0083] 2) Uniform distribution of the erased V_thin an array is obtained by “write before erase” according to the present invention.

[0084] 3) Long endurance is obtained by “limited voltage value erasure” according to the present invention.

[0085] While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.