NON-VOLATILE SEMICONDUCTOR MEMORY DEVICE AND MANUFACTURING METHOD THEREFOR

Abstract: A non-volatile semiconductor memory device (50) includes, formed in successive layers on a substrate (10), a silicon oxide layer (13), a silicon oxynitride layer (14), a silicon nitride layer (15), a further silicon oxynitride layer (16) and a silicon gate electrode (19). At least one of the oxynitride layers (14, 16) is of graded oxygen:nitrogen composition. An alternative embodiment includes only a single oxynitride layer (14) formed on the oxide layer (13), the single oxynitride layer (14) being graded. The grading of the oxynitride layers (14, 16) is in incremental steps. The oxynitride layers (14, 16) and the nitride layer (15) are formed by low pressure chemical vapor deposition at the same temperature. The devices have the advantage of high retention.

Description: NON-VOLATILE SEMICONDUCTOR MEMORY DEVICE AND MANUFACTURING METHOD THEREFOR Technical Field This invention relates to non-volatile semi- conductor memory devices of the kind including a semi¬ conductor substrate, a silicon oxide layer provided on said semiconductor substrate, a silicon oxynitride layer provided on said silicon oxide layer, a silicon nitride layer provided on said oxynitride layer, and a silicon gate electrode overlying said silicon nitride layer. The invention also relates to a process for forming a non-volatile semiconductor memory device. Background Art A non-volatile memory device is a device that can retain stored information in the absence of power. One way of accomplishing this is by using a field-effect transistor where the threshold voltage is determined by charges trapped in the gate insulator of the device. Such charge trapping may be accomplished, for example, t>y using a dual dielectric layer for the gate insulator, such as silicon dioxide and silicon nitride in MNOS and SNOS devices. When a suitable voltage is applied to the terminals of an MNOS transistor, charges from the trans¬ istor channel region tunnel through the normally very thin oxide layer and are trapped at the interface between the oxide and the nitride and in the nitride bulk. The nitride is made thick enough to prevent any further tunneling of charges through it. Most of the charges will remain in their trapped location since they need the influence of an electric field of opposite polarity to the one that caused them to tunnel through the thin oxide initially. This phenomenon provides the non¬ volatile character of the structure. Among the most important characteristics of thin memory oxide-nitride non-volatile semiconductor yΪ OMPI merriory devices is long-term charge retention, that is, the ability of the memory device to retain its stored charge subsequent to a write or clear operation. Improving charge retention in MNOS memory devices is always a goal in microelectronics technology. One way of improving charge retention in these devices is to make the first dielectric (oxide) layer relatively thick. However, a thick memory oxide necessitates rather high voltages to write or erase the device and also decreases the device erasure speed. To decrease the write/erase voltage in the thick memory oxide de¬ vices, generally, a thin nitride layer is used. How¬ ever, when a thin nitride layer is used the retention characteristic suffers. In addition, unnecessary information is written during the read mode by hot charge carriers which are generated in small amounts during the read mode, causing incorrect operation of the device. A further desirable feature of non-volatile memory devices is a low operating voltage, thereby re¬ ducing power consumption. A non-volatile semiconductor memory device of the kind specified is known from U.S. Patent Specifica¬ tion No. 4,151,537. Disclosure of the Invention It is an object of the present invention to provide a non-volatile semiconductor memory device of the kind specified capable of providing a high degree of retention. Therefore, according to the present invention, there is provided a non-volatile semiconductor memory device of the kind specified, characterized in that said silicon nitride layer is in contact with a layer of graded silicon oxynitride which may be constituted by said silicon oxynitride layer provided on said silicon oxide layer. f O According to a further feature of the invention, the layer of graded silicon oxynitride is provided as a further layer of silicon oxynitride lying between the silicon nitride layer and the silicon gate electrode. According to another aspect of the invention, there is provided a process for forming a non-volatile semiconductor memory device including the steps of providing a semiconductor substrate, providing a silicon oxide layer on said semiconductor substrate, providing a silicon oxynitride layer on said silicon oxide layer, providing a silicon nitride layer on said oxynitride layer and providing a silicon gate electrode overlying said silicon nitride layer, characterized by the step of providing, in contact with said silicon nitride layer, a layer of graded silicon oxynitride which may be con¬ stituted by said silicon oxynitride layer provided on said silicon oxide layer. A process according to the invention has the advantages of low fabrication cost, a relatively high yield and large throughput. For ease of reference, the following defini¬ tions which are applicable herein, are set forth at this point: "Silicon" includes polycrystalline silicon (polysilicon) and onocrystalline silicon. "SNOS" is silicon-nitride-memory oxide-semi¬ conductor "SNOOS" is silicon-nitride-ungraded oxynitride I-memory oxide-semiconductor "SNO OS" is silicon-nitride-graded oxynitride I-memory oxide-semiconductor "SONOOS" is silicon-ungraded oxynitride II- nitride-ungraded oxynitride I-oxide-semiconductor "SONO OS" is silicon-ungraded oxynitride II- nitride-graded oxynitride I-memory oxide-semiconductor "SO NOOS" is silicon-graded oxynitride II- nitride-ungraded oxynitride I-memory oxide-semiconductor - - "SO NO OS" is silicon-graded oxynitride II- nitride-graded oxynitride I-memory oxide-semiconductor The single oxynitride in SNOOS or SNO OS devices is referred to as oxynitride I or memory oxy- nitride. Finally, while discussing SONOOS, SO NOOS, SONOgOS and SOgNOgOS devices, the oxy *nitride formed over the oxide is referred to as (graded or ungraded, as the case may be) oxynitride I or memory oxynitride (e.g., SONOOS) and the oxynitride formed over the nitride is referred to as (graded or ungraded, as the case may be) oxynitride II or interfacial oxynitride (e.g. SONOOS). In brief summary, in one embodiment of this invention the memory device consists of a silicon-gate oxynitride II-nitride-oxynitride I-oxide-silicon sub- strate structure. The two oxynitride layers may have a graded (varying) or ungraded (uniform) composition of oxygen and nitrogen. In the SO NO OS structure, the composition of- the oxynitride I layer is such that the oxygen:nitrogen ratio varies, in increments, from a maximum at the oxide-oxynitride I interface to a minimum at the oxynitride I-nitride interface and the compo¬ sition of the oxynitride II layer is such that the oxygen:nitrogen ratio varies, in increments, from a minimum at the nitride-oxynitride II interface to a maximum at the oxynitride II-silicon interface. In the SO NOOS structure, the interfacial oxynitride is graded in the same fashion as the corresponding oxynitride layer in the SO NO OS structure. In the SONO OS struc- g g g ture, the memory oxynitride is graded in the same fash- ion as the corresponding oxynitride in the SO NO OS structure. In another embodiment of the present inven¬ tion the memory device consists of an SNO OS structure, where the single oxynitride I is graded in the same fashion as the oxynitride I in the SO NO OS structure discussed above. In brief summary of one embodiment of a pro¬ cess according to the invention, there is first formed a thin silicon oxide layer over a semiconductor substrate. Then, the oxynitride and nitride layers are continuously formed over the thin oxide, all in the same deposition furnace tube, at the same temperature using the low pressure chemical vapor deposition (LPCVD) technique and by controlling the reactant gases used for forming these layers. In the SNO OS structure, the single graded oxynitride I layer is formed by gradually varying.the oxygen:nitrogen ratio from a maximum at the oxide-oxy¬ nitride I interface to a minimum at the oxynitride I- nitride interface. In the SO NO OS structure the oxy- g nitride I layer is formed as described in connection with the SNO OS device and the oxynitride II layer is formed by varying the oxygen:nitrogen ratio gradually from a minimum at the nitride-oxynitride II interface to a maximum at the oxynitride II-silicon interface. In the SO NOOS and SONO OS structure the graded oxynitride II and graded oxynitride I, respectively, are formed in the same fashion as the corresponding graded oxynitride layers in the SO NO OS structure. In the SONOOS struc- g g ture, the two oxynitride layers are formed using a fixed oxygen:nitrogen ratio. Brief Description of the Drawings Fig. 1 is a cross-sectional representation of an embodiment of the non-volatile memory device accord- ing to the present invention. Figs. 1A-1D show in a cross-sectional repre¬ sentation the various steps of forming the memory device of Fig. 1 in accordance with the process of this inven¬ tion. Fig. 2 is a cross-sectional view of a second embodiment of the non-volatile memory device according to the present invention. Fig. 3 is a cross-sectional view of a non¬ volatile trigate memory device formed using this inven- tion. OMPI Fig. 4 is a schematic representation of an LPCVD system sui.table for forming the gate structures in accordance with the process of this invention. Fig. 5 is a graphical illustration comparing the retention of the devices of this invention and prior art devices. Best Mode for Carrying Out the Invention Referring now to Fig. 1 there is shown in this figure a partial sectional view of a portion of an exemplary memory device 50 embodying the principles of the present invention. Fig. 1, in particular, illustrates an n-channel transistor 50 having a silicon-oxynitride II-nitride-oxynitride I-oxide-silicon or SONOOS .gate structure 40. The device 50 of Fig. 1 comprises a substrate 10 of one conductivity type, illustratively, p-type silicon. The n-channel transistor 50 includes a pair of n-type surface-adjacent source and drain impurity regions 20 and 21, respectively, which are implanted or diffused into the substrate, and a pair of isolation oxide regions 11—11. The gate structure 40 which over¬ lies the channel region between the source and drain regions includes, in order, a thin layer of gate oxide 13, a first silicon oxynitride (hereafter oxynitride I or memory oxynitride) layer 14, a silicon nitride layer 15, a second silicon oxynitride (hereafter oxynitride II or interfacial oxynitride) layer 16 and a silicon gate electrode 19. Gate electrode 19 is doped with n-type impurities to provide a highly conductive gate electrode for the memory device. In one embodiment both oxynitride layers 14 and 16 (Fig. 1) consist of a variable composition of oxygen and nitrogen resulting in graded oxynitride layers and obtaining an SO NO OS structure. The grading in oxynitride I (14, Fig. 1) is such that the oxygen to nitrogen ratio in this layer varies from a maximum at the oxide (13)-oxynitride I (14) interface to a minimum at the oxynitride I (14)-nitride (15) interface. The grading in oxynitride II (16, Fig. 1) is such that the oxygen to nitrogen ratio in this layer varies from a minimum at the nitride (15)-oxynitride II (16) interface to a maximum at the oxynitride II (16)-silicon 19 inter¬ face. Other embodiments of this invention include the SONO OS and SO NOOS structures. The grading of the g g y respective- oxynitride layers in the just-mentioned structures is analogous to the grading in the corres- ponding oxynitride layers of the embodiment discussed above. The memory device of Fig. 1 is provided with a thick insulating layer 30 made of, for example, phospho- silicate glass, which is appropriately patterned to cover the transistor structure. Insulating layer 30 electrically isolates the polysilicon gate 19 from the overlying metal conductors 31 and 32. Electrical con¬ ductors 31 and 32, made of, for example, aluminum, couple source 20 and drain 21, respectively, to associ- ated electrical circuitry (not shown) such as a voltage supply. Electrical conductor 33 is connected to the gate. Referring now to Fig. 2, there is shown another exemplary embodiment in accordance with the principles of this invention. This embodiment differs from the embodiment shown in Fig. 1 in that the gate structure 60 (Fig. 2) consists of a single oxynitride I layer instead of two oxynitride layers resulting in an SNO OS structure. The grading in the single oxynitride layer of this device is analogous to that in the graded oxynitride I layer (14) of Fig. 1 discussed fully above. The LPCVD system used to form the oxide, oxynitride, nitride and silicon layers is shown in a schematic representation in Fig 4. The System 100 consists of an LPCVD furnace including a furnace tube 111, typically made of fused quartz. Inert and reactant gases are admitted to the tube 111 via manifolds 112 and 113. That is, nitrous oxide (N..O) , ammonia (NH_) and dichlorosilane (SiH„Cl 2 , also referred to herein as DCS) are admitted to manifold 112 via ON-OFF valves 114, 115 and 116, respectively, and their flow rates are con- trolled by conventional electronic mass flow controllers 117, 118 and 119. Similarly, oxygen (O ) and nitrogen (N ) are admitted to manifold 113 via ON-OFF valves 120 and 121 and their flow rates are controlled by mass flow controllers 122 and 123. A pressure sensor 124 is connected to the interior of the tube 111 for providing readings of the internal pressure. In the exemplary arrangement shown in Fig. 4, vent 125 is opened and closed by ON-OFF valve 126 to allow the exit of oxygen and nitrogen from the tube 111 during the wafer loading and oxidation and unloading cycles. The manifold 113 has an ON-OFF valve 127 for controlling the connection of the tube 111 to a vacuum pump (not shown). A power supply and temperature cσn- troller 128 connect to resistance heating coil 129 for heating the tube 111. As shown in Fig. 4, the silicon wafers 130, from which the memory devices of this invention are fabricated, are supported in boat 131. Reference is now made to Figs . 1A-1D wherein the successive steps of the fabrication process of the present invention are illustrated giving numerous specific details in order to provide a thorough under¬ standing of the invention. The description that follows relates to a non-volatile memory device having a SO NO OS structure. The following description also relates to the fabrication of n-channel devices but it should be understood that one having ordinary skill in the art can easily adapt the process to the fabri- cation of p-channel devices, or a combination of both, i.e., CMOS. It will be obvious to one skilled in the microelectronics art that the specific processes for implementing the various steps may be carried out in a number of different ways. The drawings herein are not to scale. Dimen¬ sions have been expanded where needed in order to clear- ly show the structure. Fig. 1A shows a portion of a p-type semicon¬ ductor substrate 10 having thick silicon oxide regions 11 formed therein adjacent to its surface. The thick oxide regions 11 are grown using a conventional local- ized oxidation of silicon (LOCOS) process at an elevated temperature (typically in the range of 900-1200 β C.) for 10-20 hours. The thick oxide regions 11 electrically isolate adjacent devices and also define the device active regions. One such active region is shown in Fig. 1A where it is designated by the numeral 12. The thickness range of region 11 is typically 10,000-20,000 Angstroms (1-2 microns). After growing the thick oxide regions a thin, uniform, memory quality silicon dioxide layer 13 is formed in the active region 12 by thermal oxidation of the silicon substrate 10. An exemplary procedure for forming the oxide layer 13 is to introduce the semiconductor structure into the LPCVD system fur¬ nace tube 111 (Fig. 4), maintained at a temperature of about 700-900°C. and a pressure of about 1 atmosphere and having a gaseous oxygen flow therethrough, and oxidize the structure. The thickness of the thin oxide layer 13 is typically in the range of 15-30 Angstroms (0.0015-0.0030 microns). Ne t as shown in Fig. IB, the first silicon oxynitride I layer 14, the silicon nitride layer 15 and the second silicon oxynitride -II layer 16 are contin¬ uously formed by low pressure chemical vapor deposition without interruption in the same LPCVD system at the same temperature. To form the oxynitride I layer 14, without changing the temperature of the furnace tube the reac- tant gases, ammonia, dichlorosilane and nitrous oxide are introduced into the system such that the proportion of ammonia:dichlorosilane:nitrous oxide is about 3.5:1:2 at the beginning of forming the oxynitride I layer and this proportion is gradually reduced in a step sequence to about 3.5:1:0 (N„0 flow cut off) at the completion of forming the oxynitride I layer. In other words, the oxynitride I layer is formed such that the oxygen:nitrogen content is gradually decreased from a maximum at the memory oxide-oxynitride I interface to a minimum at the oxynitride I-silicon nitride interface, the latter to be formed in the immediately following step. During the oxynitride I deposition, the pressure inside the furnace tube is in the range of (0.3-1) torr and typically about 0.5 torr. The thickness of the oxynitride I layer 14 so formed is in the range of 20-80 Angstroms (0.002-0.008 microns). Next, without changing the temperature the silicon nitride layer 15 is formed over the oxynitride I layer 14 by maintaining the mass flow of ammonia and dichlorosilane such that the ammonia:dichlorosilane ratio is about 3.5:1. During this deposition step, the internal tube pressure is maintained in the range (0.3-1) torr, preferably at the same pressure at which oxyni¬ tride I was deposited. The thickness of the nitride layer 15 formed in this manner is in the range of 200- 400 Angstroms (0.02-0.04 microns). After forming the nitride layer 15 of the desired thickness, without changing the temperature gaseous N O is * gradually reintroduced into the LPCVD system reaction chamber to form the oxynitride II layer 16. The reactant gas ratio is varied such that the proportion of ammonia:dichlorosilane:nitrous oxide is 3.5:1:0 at the beginning of forming the oxynitride II layer and this proportion is 3.5:1:2 at the completion of forming the oxynitride II layer. In other words, the oxygen: itrogen ratio in the oxynitride II layer is a minimum at the nitride-oxynitride II interface and this ratio increases stepwise to a- maximum at the oxynitride II-silicon interface, the latter to be formed in the next step. During this deposition step, the internal tube pressure is maintained at a fixed level in the range of (0.3-1) torr, preferably at the same pressure at which the oxynitride I layer 14 and nitride 15 were deposited. The thickness of the oxynitride II layer 16 is typically in the range 60-200 Angstroms (0.006-0.02 microns) . The two oxynitride layers 14 and 16 formed in the manner described above thus have varying oxygen and nitrogen composition. In the first oxynitride layer 14, the oxygen: nitrogen ratio varies in increments from a maximum at the memory oxide 13-oxynitride I 14 interface to a minimum at the oxynitride I 14-nitride 15 interface. In the second oxynitride layer 16, the oxygen-nitrogen ratio varies from a minimum at the nitride 15-oxynitride II 16 interface to a maximum at the oxynitride II 16-sili- con 17 interface. After forming the oxynitride II layer 16, as shown in Fig. IB, a layer 17, typically, of silicon, is formed over the entire structure by using conventional techniques such as LPCVD or atmospheric pressure CVD over the temperature range 600-700°C. using silane (SiH. ) or dichlorosilane (SiH^Cl-) arnbients respectively to a thickness of about 3,000-5,000 Angstroms (0.3-0.5 microns) . Subsequent to the silicon deposition, using conventional photolithographic techniques, the silicon layer 17, the oxynitride II layer 16, the nitride layer 15, the oxynitride I layer 14 and the memory oxide layer 13 are etched to define the silicon gate structure 18 shown in Fig. 1C. In this figure, 19 designates the silicon gate electrode. The various dielectric layers 13-16 of the gate structure 18 are in precise alignment with the silicon gate 19. Referring now to Fig. ID, the n-type source 20 and the drain 21 are then formed, for example, by n-type ion implantation or by a -standard high temperature (900°C.) diffusion of an n-type impurity such as phos¬ phorus from an n-doped silicon dioxide glass layer (not shown) formed on the entire structure. In accomplishing this doping step, the gate structure 18 can be used as a mask which serves to align the transistor's source 20 and drain 21 with the gate electrode 19. A self-aligned silicon gate, as is well-known, improves performance of the device. Also in accomplishing this doping step, the silicon gate 19 can be doped n thereby making it highly conductive, to reduce power dissipation and increase the speed of operation of the device. As shown in Fig. 1, the resulting structure is covered with an insulating layer 30, made of, for example, phospho-silicate glass for electrically isolating the gate structure from the next-to-be formed source and drain metal contacts 31 and 32. As shown in Fig. 1, the structure is then coated with a contact metal such as aluminum which is then etched to create metal contacts 31, 32 and 33 in the conventional manner. In addition, protective or passivating layers over the device can be formed as desired. However, this type of layer is not shown and described herein inasmuch as it is a standard well-known technique. The following Table I provides a summary of suitable parameters for forming a SO NO OS structure in accordance with the teaching of this invention. TABLE I Process Parameters Memory silicon oxide (dry thermal silicon oxidation) Pressure atmosphere Temperature 700-850°C. Ambient dry oxygen Silicon oxynitride I (LPCVD) Pressure 0.3-1.0 torr Temperature 700-950°C. Reactants ratio NH :SiH p Cl-:N_0 varies from 3.5:1:2 at the beginning to 3.5:1:0 at the end. Silicon nitride (LPCVD) Pressure 0.3-1.0 torr Temperature 700-950°C. Reactants ratio (3-4) :1 NH :SiH 2 Cl 2 Silicon oxynitride II (LPCVD) Pressure 0.3-1.0 torr Temperature 700-950°C. Reactants ratio NH_:SiH 2 Cl ? :N~0 varies from 3.5:1:0 at the beginning to 3.5:1:2 at the end. Example [SO NO OS] — g g In one example of forming a SO NO OS structure g g according to the process of this invention, first, as discussed above, the memory oxide layer 13 (Fig. 1) of thickness of approximately 20 Angstroms was formed by dry thermal oxidation, in the presence of pure, dry oxygen flow of 4,000 cc. per minute on a silicon sub¬ strate 10 supported in the LPCVD system furnace tube 111 which was maintained at atmospheric pressure at a tem¬ perature of about 750°C. After forming the oxide layer 13, the oxygen flow was cut-off and any residual oxygen was pumped out of the furnace tube. Next, maintaining the furnace pressure at about 0.5 torr using a reactant gas mixture of ammonia/dichlorosilane/nitrous oxide and without changing the temperature the graded oxynitride I 14, the nitride 15 and the graded oxynitride II 16 layers were continuously formed without interruption by programming the mass flow controls of the LPCVD system. The graded oxynitride I layer 14 was formed using a deposition sequence which was divided into four segments as explained below. First, ammonia at a flow rate of 70 seem was caused to flow through the chamber for about 10 minutes to purge the LPCVD system. The ammonia flow rate was maintained at this rate throughout the oxynitride I deposition sequence. In the next step, O, at a flow rate of 40 seem was introduced into the reaction chamber for a period of about 2 minutes to ensure that the chamber pressure and temperature remain stable at 0.5 torr and 750 β C, respectively, prior to introduction of DCS. Then, the first segment of the graded oxynitride I deposition was initiated by intro¬ ducing into the LPCVD chamber DCS at a flow rate of 20 seem for about 1 minute. During this segment, the proportion of NH_:DCS:N 9 0 was 3.5:1:2, providing the maximum oxygen:nitrogen ratio. Next, the DCS flow was cut off and N„0 flow was decreased to 30 seem. After stabilizing the system by maintaining the N_0 and NH_ flow rates, the second segment of graded oxynitride I deposition was formed by introducing DCS at the same flow rate of 20 seem as before for about 1 minute. During this segment the NH :DCS:N 9 0 proportion was 3.5:1:1.5. The third and fourth segments of the graded oxynitride I were accomplished in the same manner as the previous two segments, by using N O flow rate of 20 seem and 10 seem, respectively. In other words, the proportion of NH :DCS:N 2 0 for the third and fourth segments were, respectively, 3.5:1:1 and 3.5:1:0.5. The oxynitride I so formed was of a thickness of approxi¬ mately 40 Angstroms. After forming the graded oxynitride I layer 14, the LPCVD nitride layer 15 was formed by maintaining the flow rates of ammonia and dichlorosilane at 70 seem and 20 seem respectively (i.e. the ratio of NH.:DCS was 3.5:1) but completely cutting off the N^O flow. The nitride formed in this manner was of thickness of approx¬ imately 280 Angstroms. Next, the graded oxynitride II layer 16 was formed over the nitride 15 by reversing the four-segment deposition sequence used for forming the graded oxyni¬ tride I layer 14. In other words, after purging the system with ammonia, introducing N^O at a flow rate of 10 seem and stabilizing the chamber for about 1 minute, DCS was introduced at a flow rate of 20 seem for about 1 minute. This was the first segment of the four-segment deposition sequence. In the second, third and fourth segments of the oxynitride II deposition sequence (the DCS and ammonia flow rates were maintained at 20 seem and 70 seem respectively) the „0 flow rates used were 20 seem, 30 seem and 40 seem respectively. The oxyni¬ tride II layer 16 so formed was of approximately 80 Angstroms thickness. Example [SONO OS] y To form a silicon gate-ungraded oxynitride II- nitride-graded oxynitride I-oxide-silicon substrate or SONO OS structure, the sequence of steps discussed above in connection with forming the SO NO OS structure is * g g used with the modification that during the oxynitride II deposition, the ammonia:dichlorosilane:nitrous oxide proportion is not varied but fixed at a preselected value in the range 3.5:1:0.5 to 3.5:1:2. Example [SO NOOS] To form a silicon gate-graded oxynitride II- nitride-ungraded oxynitride I-oxide-silicon substrate or SO NOOS structure, the process steps discussed in con¬ nection with forming the SO NO OS structure is used with 3 g g the modification that during the oxynitride I deposition, the ammonia:dichlorosilane:nitrous oxide proportion is not varied but fixed at a preselected value in the range 3.5:1:0.5 to 3.5:1:2. Example CSNO OS] To form the embodiment of this invention (shown in Fig. 2) having a silicon 19-nitride 15-graded oxynitride 14-oxide 13-silicon substrate 10 or SNO OS structure, the process of this invention discussed above in connection with the SO NO OS structure is used with g g the significant modification that the sequence of steps used to form the graded oxynitride II layer 16 (Fig. 1) are omitted. In other words, after forming the nitride layer over graded oxynitride I layer 14, silicon 19 is deposited over the entire structure. To illustrate the improved retention charac¬ teristics of the novel devices of the present invention, several n-channel devices were fabricated, tested and the test results evaluated. These devices include an 0 SNOS device formed in the conventional manner, an SNOOS device having an oxynitride layer of a thickness in the range of 50-200 Angstroms and a nitride layer of a thickness in the range of 300-650 Angstroms, an SNO OS y device (having the basic structure shown in Fig. 2) and 5 SO NO OS and SO NOOS devices (having the basic structure D g g g * shown in Fig. 1), the latter three devices formed in accordance with the teachings of this invention. The test results which focus on the retention characteristics of these devices are summarized in Fig. 5. 0 The procedure used for testing the devices fabricated in the manner described above is well-known. The write and erase curves were generated, for example, by subjecting the devices to various pulse stressing conditions (pulse amplitudes and duration) . The ampli- 5 tude of the pulse was +25 volts, the positive and nega- tive values being applicable to the write and erase data, respectively. The data to determine the charge retention in the devices was obtained by: (1) initializing the devices and determining the initial write and erase threshold voltages; and (2) obtaining retention graphs for the devices by storing the devices at an elevated temperature of 100 β C for a time of up to 10 seconds and determining the threshold voltages at intervals during this time. The initialization procedure (step 1) i.e. obtaining the initial written and erased state threshold voltages, involved applying a +25 volt pulse of ten millisecond duration and a -25 volt pulse of 100 millisecond dura¬ tion, respectively, to the gates of the memory devices. Source 20, drain 21 and substrate 10 (Figs. 1 and 2) were all tied to the ground during this initialization. Using the retention data obtained in the above fashion, the graph of Fig. 5 was constructed. Fig. 5 is a plot of the normalized crossover time versus initial memory window for various devices. Normalized crossover time designates the time of cross¬ over of the write and erase retention curves divided by the initial memory window. This parameter is a measure of the device retention and, generally, the larger the normalized crossover time the better is the retention. In most applications, it is desirable to have a memory device having a larger initial window and a long normal¬ ized crossover time. In Fig. 5 the lines labeled A,B,C,D and E represent (A) the retention data for the conventional SNOS device, (B) the SNOOS device, (C) the SNO OS, (D) y SO NOOS and (E) SO NO OS devices in accordance with the g g g teachings of this invention, respectively. In all of these devices, the memory oxide thickness was in the range of (20-30) Angstroms. Also, the oxynitride I and oxynitride II layers in the devices of this invention were of a thickness in the ranges (20-60) Angstroms and (70-90) Angstroms, respectively. Furthermore, the combined thickness of all the gate dielectric layers in all the tested devices was in the range (360-470) Ang¬ stroms. The total dielectric thickness was maintained in this range by varying the nitride layer thickness to accommodate the presence or absence of the oxynitride I and oxynitride II layers. The various thickness measure¬ ments were obtained using the deposition rate and time of forming the layers. The various dielectric layers common to each of the tested devices, including those of the conventional SNOS and SNOOS devices, were formed using the same temperature, pressure and deposition kinetics . The results of Fig. 5 demonstrate, among other benefits of the present invention, that: (1) the SNO OS memory device of this invention has an improved reten¬ tion over both the conventional SNOS and SNOOS devices. In other words, a memory device having a graded oxyni- tride-nitride dual gate dielectric has an improved retention over the conventional devices having, respec¬ tively, a single nitride gate dielectric and a dual ungraded oxynitride-nitride gate dielectric. (2) The SO NOOS memory device of the present invention shows improved retention over not only the conventional SNOS and SNOOS devices, but also the SNO OS device of this g invention discussed in item (1) immediately above. In other words an interfacial graded oxynitride improves the retention still further. (3) The SOgNOgOS memory-j device of this "invention shows a marked improvement in retention over not only the conventional SNOS and SNOOS devices but also the new SNO OS and SO NOOS devices g g discussed in items (1) and (2) above. Measurements were also taken to determine the endurance characteristic of the above novel devices, by subjecting them to program and erase cycling of up to 10 times using pulses of amplitude + 25 volts and wi 10 milliseconds and 100 milliseconds respectively. It was found that although the repetitive programming and erasing somewhat degraded the devices' memories, there was adequate charge retention after such cycling. This demonstrates that the devices are fit for practical use even when they are subjected to normal repetitive program and erase cycles. It is not clear just how . the above multidie- lectric structures of the present invention contribute to the improved retention of the memory devices. One possible explanation is that the novel structures and the unique process parameters * selected to form the structures contribute to the improved retention by enhancing the current density in the multidielectric layer. Another possible explanation is that ' the oxy- nitride layer(s) act as a barrier for charge leakage after the device is written or erased. The above explan¬ ations should, however, be regarded merely as plausi¬ bility arguments rather than as proven fact. Thus, there has been described novel memory devices having novel, graded multilayer gate dielectric structures and processes for forming these structures. These include: (1) SOgNOgOS; (2) SONOgOS; (3) SOgNOOS; and (4) SNO OS device. Each of these devices has a g novel gate dielectric structure and improved retention over the conventional non-volatile silicon memory gate devices. Although the description of the present inven¬ tion so far has been confined to a monogate structure, this process is equally well-suited to split gate and trigate memory structures taught in U.S. Patent No. 3,719,866, issued March 6, 1973, to Naber and Lockwood and assigned to the assignee of the present invention. Referring to Fig. 3, after forming the thick isolation oxide regions 11 a relatively thick silicon dioxide layer 22, of thickness about 500 Angstroms (0.05 microns) is formed over the substrate 10. Then, a central por¬ tion of this oxide layer is etched down to the substrate using conventional photolithographic techniques. There¬ after, a memory oxide 23 (Fig. 3), typically 20 Angstroms thick, is formed in this etched portion. The remainder of the steps of forming the SONOOS trigate structure follows the sequence explained in connection with form¬ ing the monogate structure. The trigate structure 28 (Fig. 3) formed according to the process of this inven¬ tion would thus consist of two relatively thick gate oxide portions 22-22 and a thin memory oxide portion 23. - Overlying- these oxide portions are the oxynitride I layer 24, the nitride layer 25, the oxynitride II layer 26 and the polysilicon gate 27. There are many uses to which the invented device may be applied. These applications include non- volatile electrically alterable memory for very large scale integrations, high speed and low voltage EAROMs and shift registers for bulk storage.

Claims: CLAIMS : 1. A non-volatile semiconductor memory device including a semiconductor substrate (10), a silicon oxide layer (13) provided on said semiconductor substrate (10), a silicon oxynitride layer (14) provided on said silicon oxide layer (13), a silicon nitride layer (15) provided on said oxynitride layer (14), and a silicon gate electrode (19) overlying said silicon nitride layer (15), characterized in that said silicon nitride layer (15) is in contact with a layer of graded silicon oxynitride (14, 16) which may be constituted by said silicon oxynitride layer (14) provided on said silicon oxide layer (13). 2. A non-volatile semiconductor memory de¬ vice according to claim 1, characterized in that said layer of graded silicon oxynitride is provided as a further layer (16) of silicon oxynitride lying between said silicon nitride layer (15) and said silicon gate electrode (19) . 3. A non-volatile semiconductor memory device according to claim 2, characterized in that both silicon oxynitride layers (14, 16) are formed by layers of graded silicon oxynitride. 4. A non-volatile semiconductor memory device according to claim 2, characterized in that said further layer (16) has a thickness in the range 60-200 Angstroms. 5. A non-volatile semiconductor memory device according to claim 1, characterized in that said silicon oxynitride (14) provided on said silicon oxide layer (13) has a thickness in the range of 20-80 Ang- stroms. OMPI 6. A non-volatile semiconductor memory device according to claim 1, characterized in that said layer of graded silicon oxynitride (14, 16) has an oxygen: nitrogen ratio varying from a minimum at the boundary thereof in contact with said silicon nitride layer to a maximum at the remote boundary thereof. 7. A non-volatile semiconductor memory device according to claim 6, characterized in that said oxygen:. *" — * , - ' *# nitrogen ratio varies in steps. 8. A non-volatile semiconductor memory device according to any one of the preceding claims, character¬ ized in that the combined thickness of all the silicon bearing materials formed between said substrate (10) and said gate electrode (19) is in the range 360-470 Angstroms. 9. A process for forming a non-volatile semiconductor memory device including -the steps of providing a semiconductor substrate (10), providing a silicon oxide layer (13) on said semiconductor substrate (10), providing a silicon oxynitride layer (14) on said silicon oxide layer (13), providing a silicon nitride layer (15) on said oxynitride layer (14) and providing a silicon gate electrode (19) overlying said silicon nitride layer (15), characterized by the step of pro- viding, in contact with said silicon nitride layer (15), a layer of graded silicon oxynitride (14, 16) which may be constituted by said silicon oxynitride layer (14) provided on said silicon oxide layer (13). 10. A process according to claim 9, charac¬ terized by the step of providing a further layer (16) of silicon oxynitride constituting said layer of graded silicon oxynitride lying between said silicon nitride layer (15) and said silicon gate electrode (19). OMPI 11. A process according to claim 10, charac¬ terized by the step of providing said silicon oxynitride layer (14) in the form of a layer of graded silicon oxynitride. 12. A process according to claim 9, character¬ ized in that said step of providing a layer of graded silicon oxynitride (14) is effected by utilizing the reactant gases ammonia, dichlorosilane and nitrous "5 ~ 1 ' ■ ■■ • .55 " '* ' oxide in an ammonia:dichlorosilane:nitrous oxide pro¬ portion varying from about 3.5:1:2 at the beginning to about 3.5:1:0 at the end. • 13. A process according to claim 10, charac¬ terized in that said step of providing a further layer (16) of silicon oxynitride constituting said layer of graded silicon oxynitride is effected by utilizing the 5 reactant gases ammonia, dichlorosilane and nitrous oxide in an ammonia: ichlorosilane:nitrous oxide propor¬ tion varying from about 3.5:1:0 at the beginning to about 3.5:1:2 at the end. 14. A process according to claim 12 or 13, characterized in that said reactant gases are utilized for low pressure chemical vapor deposition at a pressure in the range 0.3-1.0 torr. and at a temperature in the 5 range 700-950°C. 15. " process according to claim 12 or 13, characterized in that the ammonia:dichlorosilane:nitrous oxide proportion is varied in steps. 16. A process according to claim 12 or 13, characterized in that said step of forming said silicon nitride layer (15) is effected utilizing the reactant gases ammonia and dichlorosilane.

Inventors: YEN YUNG-CHAU, YEN, YUNG-CHAU
Applicants: NCR CO, NCR CORPORATION
Application year: 1982
Main classification: H01L29
IPC codes: G11C14, H01L21, H01L29