Abstract:
A method of reading a 2-bit memory cell having a drain, a source, a control gate, and a floating gate is disclosed. First, a voltage is applied to the source and drain to generate a gate induced drain leakage (GIDL) current. Next, a measurement is taken of a drain GIDL current at said drain and a source GIDL current at said source to determine the data stored in said memory cell.

Description:
FIELD OF THE INVENTION 
     The present invention relates to 2-bit ETOX flash memory, and more particularly, to a method of reading the 2-bit ETOX flash memory using gate induced drain leakage (GIDL) current. 
     BACKGROUND OF THE INVENTION 
     stack-gate ETOX cell, one of the most popular cell structures for flash memories, is widely programmed by channel hot-electron (CHE) and erased by Fowler-Nordheim (FN) tunneling through the source side or the channel area. 
     The n-channel ETOX cell is conventionally fabricated using a triple-well process, as shown in FIG.  1 . The triple-well structure is typically used to protect cells from noises generated outside the deep n-well by reverse-biasing the deep n-well to p-well junction. The n+ source is typically doubly implanted by As 75  (with a high dose of 3E15/cm 2 ˜1E16/cm 2  for the n+ junction) and P 31  (with a lower dose of ˜1E14/cm 2  for the n-junction) so that the source junction can be biased at high voltage (e.g. ˜12 v) during erase operation. The n+ drain is typically implanted by As only with a high dose (˜1E16/cm 2 ) and the drain side does not need the lightly-doped-drain (LDD) implant and spacer structure. 
     The ETOX cell of FIG. 1 is programmed by channel-hot-electrons (CHE). The bias for programming is typically: V d =7 v, V cg =9 to 12 v, and V s =0 v. Under these bias conditions, there is a large channel current (˜1 mA/cell) for hot electron generation near the channel surface of the drain. Hot electrons are injected into the floating-gate when the oxide energy barrier is overcome and when assisted by the positive control gate bias. After programming, the amount of net electrons on the floating-gate increases, which results in an increase of the cell threshold voltage (V T ). The electrons in the floating-gate will remain for a long time (e.g. 10 years at room temperature), unless intentionally erased. 
     The cell is erased by Fowler-Nordheim (F-N) tunneling through the source side. The bias during source side erase is typically: V d ˜0 v or floating, V cg ˜−5 v to 0 v, and V s =+9 to +12 v. This establishes a large electrical field (˜10 Mv/cm) across the tunnel oxide between the floating-gate and source overlap area. Electrons on the floating-gate will tunnel into the source and be removed away. 
     The read biases of the prior art ETOX-cell are typically: V d ˜1 v to 2 v, V cg ˜V cc , V s ˜0 v, V pw ˜0 v, V dnw =Vcc, and V sub ˜0 v. The channel may be inverted or not depending on the net electron charge stored on the floating-gate, and results in the on and off of the cell as measured by the read current I read  representing the digital information of “1” or “0” stored in the cell. 
     One drawback of conventional ETOX cells, as exemplified by FIG. 1, is that the charge on the floating gate may continuously leak away when there are “weak” spots in the tunnel oxide. This is the main limitation that prevents a further decrease in the thickness of the tunnel oxide, which in turn prevents a decrease in the program and erase voltages required. Additionally, the fabrication process for a conventional ETOX cell is more complicated than logic processes, due to the use of high voltage transistors and isolations. Further, the ETOX cell is a single bit memory cell, i.e., only a single bit of data is stored in each ETOX cell. 
     A prior art multi-bit flash memory cell is exemplified by U.S. Pat. No. 6,011,725 to Eitan. In the &#39;725 patent, a 2-bit cell is formed by replacing the ETOX floating-gate with a charge trapping layer, e.g. nitride or silicon rich oxide (SRO). The charge can be stored locally above the channel near the drain and/or source (referred to as right-bit and left-bit respectively). 
     The programming of the 2-bits is based on a 2-step procedure using conventional channel-hot-electron (CHE) injection, i.e. CHE programming the drain side (right-bit) by biasing the drain (to 6-7 v) and control-gate (to 10-15 v), then CHE programming the source-side (left-bit) by biasing the source (6-7 v) and control-gate (10-15 v). The read operation is based on a 2-step read, i.e. read the left-bit first by a read bias on the drain (2 v) and control gate (3-5 v), then read the right-bit by biasing the source (2 v) and control gate (3-5 v). 
     There are several drawbacks of this 2-bit memory cell and its operation. First, the electron trapping layer is still leaky. This is because the charge in the nitride or SRO layer leaks by direct tunneling among traps and/or microscopic silicon islands in the SRO. Note that the SRO is actually oxide with very tiny Si-rich islands (approx. 10 angstroms), which are significantly more conducting than the rest of the oxide. Second, it is difficult to control the trapping density and/or characteristics of the microscopic Si-islands. Third, the 2-step read operation is slow. 
     In another prior art multi-bit cell, polysilicon spacers are placed on the sides of an n-channel transistor. The oxide underneath the polysilicon spacers is thin and serves as a tunnel oxide for program/erase by F-N tunneling mechanism. The gate oxide is thicker along the channel area. The electron charge is stored in the polysilicon spacers representing 2-bits of digital information. The charge stored in the polysilicon spacer will modify the source/drain resistance measurement relative to a reference resistance. However, this read procedure is slow and complicated. 
     SUMMARY OF THE INVENTION 
     A method of reading a 2-bit memory cell having a drain, a source, a control gate, and a floating gate is disclosed. The method comprises: applying a voltage to said source and said drain to generate a gate induced drain leakage (GIDL) current; and measuring a drain GIDL current at said drain and a source GIDL current at said source to determine the data stored in said memory cell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram of a prior art ETOX-cell formed by a triple-well process; 
     FIGS. 2-13 are cross-sectional views of a method for forming an ETOX cell in accordance with the present invention; 
     FIGS. 14-15 are diagrams of the ETOX cell of FIG. 13 during an erase operation; 
     FIG. 16 is a diagram of the ETOX cell of FIG. 13 during a read operation; and 
     FIGS. 17-20 are diagrams of the ETOX cell of FIG. 13 during a program operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 2-13 illustrate the manufacturing process for forming an ETOX cell in accordance with the present invention. Referring to FIG. 2, a silicon substrate  201  is provided. First, a source  203  and a drain  205  are formed within the silicon substrate  201 . The source  203  and drain  205  are formed using conventional masking and ion implantation techniques. Next, a tunnel oxide  207  is formed on top of the silicon substrate  201 . Preferably, the tunnel oxide  207  is deposited using a high temperature oxide (HTO) process. Alternatively, the tunnel oxide  207  can be grown using a thermal process, resulting in a “thermal oxide”. Preferably, the thickness of the tunnel oxide  207  is approximately 70 angstroms, though it may be less. 
     Turning to FIG. 3, after the tunnel oxide  207  is formed, a layer of N-type doped amorphous silicon  301  is deposited. Preferably, the doped amorphous silicon layer  301  is between 50 to 200 angstroms thick and is deposited at a temperature of less than 530° Celsius. Turning to FIG. 4, silicon nuclei  401  are formed on top of the amorphous silicon layer  301 . The silicon nuclei  401  are formed by direct decomposition of SiH 4  and Si 2 H 6  gas. The formation of the silicon nuclei  401  is a conventional step used in the “seeding method” for forming hemispherical grain (HSG) silicon. 
     Next, turning to FIG. 5, a thermal annealing step at between 550-580° Celsius is preformed to facilitate growth of HSG silicon grains  501 . The growth of HSG silicon grains  501  will consume the doped amorphous layer  301 . Preferably, the growth process of the HSG silicon grains  501  is controlled so that the average grain size of the HSG silicon grains  501  is less than 100 angstroms. However, because of difficulty in controlling process variations, the HSG silicon grains  501  in FIG. 5 may be formed with an average grain size of greater than 100 angstroms. 
     In such a case, then the average size of the HSG silicon grains  501  can be reduced by, for example, a mild wet etch or chemical dry etch until the HSG silicon grains  501  have an average grain size of less than 100 angstroms. It is important to have a relatively small grain size to prevent contact between grains so that charge can be stored on individual grains. Thus, as shown in FIG. 6, after the etching, the HSG silicon grains  501  are made smaller. The etching can be done, for example, by a wet etch solution of NH 4 OH\H 2 O 2 \H 2 O. 
     Next, turning to FIG. 7, a first dielectric layer  701  is deposited over the HSG silicon grains  501 . Preferably, the first dielectric layer  701  is a high temperature oxide layer having a thickness of less than or equal to 40 angstroms. 
     In the preferred embodiment, the process of forming HSG silicon grains having an average grain size of 100 angstroms followed by the deposition of a dielectric layer is repeated. This is shown in FIGS. 8-11, which is simply a repeat of the steps in FIGS. 3-7. In other words, a second amorphous doped silicon layer  801  is deposited over the first dielectric layer  701 . 
     As seen in FIG. 9, silicon nuclei  901  are formed on to the second doped amorphous silicon layer  801 . Next, as seen in FIG. 10, a second HSG silicon layer  1001  is formed through a thermal annealing process. The grains in the HSG silicon layer  1001  are etched, if necessary, as seen in FIG. 11 by reference numeral  1101 , to be less than or equal to 100 angstroms in size. 
     It should be noted that the deposition of a second HSG silicon layer  1001  is optional. In other words, only a single layer of HSG silicon is necessary to implement the present invention. Conversely, multiple layers can be stacked on top of each other to increase the charge storage capacity of the ETOX cell. 
     Where multiple layers are used, it has been found that HSG silicon grains will not grow on top of underlying layers of HSG silicon where there is an overlap. In this case, the number of HSG silicon islands can be increased for charge storage on the floating gate. 
     Turning to FIG. 12, a second dielectric layer  1201  is formed over the HSG silicon grains  1101 . This serves to completely insulate the HSG grains and make the HSG grains form a floating gate. The second dielectric layer  1201  is preferably a composite of oxide/nitride/oxide. These layers can be deposited using conventional means well known in the prior art. Next, a doped polysilicon layer  1203  is formed on top of the second dielectric layer  1201 . The doped polysilicon layer  1203  is deposited using conventional means, for example chemical vapor deposition. Finally, turning to FIG. 13, the multiple layers that have been deposited onto the substrate  201  are patterned and etched to form a stack  1301  between the source  203  and drain  205 . The stack  1301  comprises a tunnel oxide  207 , a floating gate  1303  comprised of the HSG silicon layers, an insulating stack  1305 , and a control gate  1307 . 
     The ETOX cell shown in FIG. 13 has charge stored on the individual HSG silicon grains  501  and  1101 . These grains are also referred to herein as HSG islands. Because the HSG islands are spaced apart by approximately 100 angstroms, the direct tunneling of charge among and between the HSG islands is eliminated. The spacing among the HSG islands can be controlled by varying HSG formation process parameters. 
     While, as in the prior art, the tunnel oxide  207  may include weak points, only those charges that are stored on HSG islands adjacent to the weak spots in the tunnel oxide  207  will be affected. Thus, for maintaining the same performance as prior art ETOX cells, the tunnel oxide  207  may be made thinner, which also results in a lower voltage used for programming and erasing the cell and faster ETOX cell operation. 
     Perhaps more importantly, because of the structure of the present ETOX cell, a 2-bit cell can be implemented by storing charge locally near the drain  205  or source  203  or both. The operation is described below. 
     Turning to FIG. 14, the erase operation of the ETOX cell is shown. Note that in FIG. 14, only a single HSG silicon layer is formed as the floating gate  1303 . The electrons stored on the HSG silicon islands can be erased (i.e. removed) through the drain  205  and/or source  203  by applying a positive drain/source bias of about 5 volts and a negative control gate  1307  bias of −5 to −10 volts. This combination of voltages will drive the electron charge stored on the HSG silicon islands out through the source  203  or drain  205 . 
     In FIG. 15, an alternate method for erasing the ETOX cell is shown. A positive voltage of 10 to 15 volts is applied to the control gate  1307 . The source  203  and drain  205  of the ETOX cell are grounded. This results in electron injection toward the control gate  1307 . Note that because of the curvature of the HSG silicon islands, the electric field is enhanced, aiding in the electron injection. 
     It should be noted that the specific parameters of the ETOX cell may be optimized with respect to the different erase methods. The dielectric layer  1305  is the same as a conventional ETOX cell, i.e. oxide/nitride/oxide with approximately 120 to 180 angstrom equivalent thickness. If F-N erase through the channel is used, then the oxide underneath the HSG silicon islands needs to be as thin (approx. 80-100 angstroms) as tunnel oxide. The gate coupling ratio of the cell needs to be about 0.8 for proper operation. 
     If F-N erase through the control gate  1307  is used (by taking advantage of the field ethancement due to a larger HSG silicon island curvature), then the oxide underneath the HSG islands can be made thicker (e.g., approx. 120 to 160 angstroms) and the cell coupling ratio can be made smaller (e.g., approx. 0.5). In this case, the cell size can be smaller due to less capacitance needed between the control gate  1307  and the HSG silicon islands. 
     Turning to FIG. 16, the read operation of the ETOX cell is illustrated. The read operation is based on the fact that the gate induced drain leakage (GIDL) current at the drain  205  and/or source  203  is strongly (exponentially) dependent oil the charge stored in the HSG silicon islands. The GIDL current typically occurs in thin gate oxide MOS devices and is current between the drain and/or source and the substrate. The basis of the GIDL current is band-to-band tunneling that occurs on the surface of the gate-to-drain or gate-to-source overlap region. Additional information on GIDL current may be found in “Design for Suppression of Gate-Induced Drain Leakage in LDD MOSFET&#39;s Using a Quasi-2-Dimensional Analytical Model,” by Parke et al., IEEE Transactions on Electron Devices, Vol. 39, No. 7, July 1992, pp. 694-1702. In that article, the authors explain that an n+ region underneath a gate edge produces a high vertical electrical field that results in hole generation on the surface of an n+ region underneath the gate by band-to-band tunneling in the device. Since the GIDL current is flowing toward the substrate  201 , the GIDL current at the drain  205  and source  203  can be read simultaneously. 
     As seen in FIG. 16, the preferred bias for the one-step read operation is the following: Vcg=−Vcc (−3.3 v); both Vd and Vs at approx. +Vcc (approx. 3.3 v); and Vpsub=0 v. When the HSG silicon islands adjacent to the drain  205  (or source  203 ) are negatively charged (programmed), the magnitude of the field between the floating gate and the drain  205  (or source  203 ) is large enough (&gt;3 Mv/cm) such that a large GIDL current results. When the cell is not programmed (i.e. floating gate not charged or charged positively), the electrical field between the floating gate and the drain is small (&lt;3 Mv/cm), resulting in a small GIDL. The drain (or source) current is therefore strongly modulated by the HSG silicon island charge near the drain (or source) and represents the digital information “1” or “0” stored in the drain side of the cell. By measuring the GIDL current through both the source and the drain, the data stored in the ETOX cell can be determined. For example, if the GIDL current is above a predetermined threshold, this indicates that a negative charge is stored in the floating gate adjacent the source or drain, as the case may be. This one-step procedure for reading 2-bits in a cell is novel. 
     Moreover, although the read operation has been described in connection with the ETOX-cell shown in FIG. 13, the method of the read operation can easily used with any multi-bit flash memory cell, such as those described in above in the Background of the Invention section. 
     Turning to FIG. 17, the programming of the ETOX cell is shown. Using conventional channel hot electron (CHE) injection, the cell may be programmed near the drain side only (representing a logical “01”) by applying a control gate  1307  bias of 10 to 15 volts and a drain  205  bias of 5 to 7 volts. The source  203  is maintained at ground. Under these bias conditions, electrons are stored in those HSG silicon islands that are near the drain  205 . 
     Similarly, turning to FIG. 18, by applying a ground voltage to the drain  205 , and a voltage of 5 to 7 volts on the source  203 , electrons may be stored on those HSG silicon islands that are closest to the source  203  (representing a logical “10”). The control gate  1307  is biased 10 to 15 volts. 
     Turning to FIG. 19, if both the drain  205  and the source  203  are biased to 5 to 7 volts and the control gate  1307  is biased to 10 to 15 volts, then electrons are stored in all of the HSG silicon islands. This corresponds to a logical “11” memory state. Finally, in FIG. 20, if both the drain  205  and the source  203  are not programmed (e.g. grounding both the source and the drain), this results in no charge being stored in the HSG silicon islands. This represents the logical “00” memory state. 
     The 2-bit ETOX-like cells of the present invention presents many advantages. First, although the charge on individual HSG silicon islands may leak through adjacent local weak spots (defect) in the tunnel oxide, the overall effect on the total charge in the floating gate is negligible. Thus, the charge retention performance is significantly better than a conventional ETOX cell. Second, as a trade-off to charge retention, the tunnel oxide may be thinner for lower-voltage cell operation (program/erase) operations. 
     Finally, it should be noted that a corresponding 2-bit p-channel cell does not work. This is because a corresponding channel hot “hole” injection is not only a slow process, but also seriously damages the quality of the tunnel oxide. 
     While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.