Method of manufacturing semiconducting devices having floating gates

Abstract: The invention provides a novel method of manufacturing a semiconductor device comprising those sequential steps including the following, formation of a floating gate electrode (25) on a region predetermined for the formation of the first conductive channel across an insulation film (24), followed by superimposition of a control gate electrode (27) on the floating gate electrode (25) across another insulation film (26). After completing the formation of the stacked gate electrode unit, the first conductive impurities are injected into silicone substrate by applying a minimum of 8 degrees of angle against the normal of this substrate under aid of ionic injection, and then forms a region (40b) containing strong density of the first conductive impurities adjacent to the boundary of a layer of diffused second conductive impurities which is at least predetermined to become the drain region (41b) of the transistor incorporating the stacked gate electrode unit. As a result of the provision of the region (40b) containing strong density of impurities injected in the oblique direction, the efficiency in the writing of data into the floating gate electrode (25) is significantly promoted, and at the same time occurrence of "short-channel" effect can securely be suppressed as well.

Description: The present invention relates to a method of manufacturing a semiconductor device. More particularly, the invention is applicable to the formation of a floating-gate type memory cell in the manufacture of a non-volatile semiconductor memory. Recently, as a result of successful materialization of non-volatile memory units incorporating vast memory capacity, contraction of the dimension of memory cell and the data writing time is strongly urged among the concerned. To solve this problem, a proposal has been raised on the "diffusion self-aligned" (hereinafter called "DSA") structure from the standpoint of the structure of the memory cell. Fig. 1 designates a conventional non-volatile memory presented for explaining the DSA structure. The reference numeral 11 designates a P-type silicon substrate, 12 oxidized film for separation, 13a a source region containing N + type impurities, 13b a drain region containing N + type impurities, 14 a floating gate, 15 a control gate, 16 a region containing P type impurities, 17 a channel region, 18 a gate insulation film, and 19 designates an insulation film between gate electrodes, respectively. As shown in Fig. 1, according to the DSA structure, the region 16 containing P type impurities having density stronger than that is present in the center of the channel region 17 is provided by way of surrounding the drain region 13b of an N-channel type cell transistor. The region 16 is hereinafter merely called the "P-pocket" region. In writing data provision of the P-pocket region 16 promotes intensity of electric field of the channel region 17 adjacent to the drain region 13b to allow more amount of current to flow into the floating gate 14. In particular, there is problem inherent in the floating gate type cell transistor in terms of the resistance of the cell transistor against "punch-through" effect. Concretely, when a floating gate type field effect transistor is made available for the cell, on the way of feeding a voltage to the drain region 13b, due to capacitive coupling effect between the floating gate 14 and the drain region 13b, potential of the floating gate 14 is lifted. As a result, unlike any single-gate type transistor, "punch-through" effect can easily be applied to the cell transistor. The shorter the length of the gate of transistor, the easier the occurrence of this phenomenon. Actually, this makes up the most critical problem among the concerned in the effort to contract the size of the cell transistor. On the other hand, provision of the "P-pocket" region 16 suppresses expansion of depletion, thus promoting the resistance of the cell transistor against occurrence of the "punch-through" phenomenon. This in turn advantageously promotes contraction of the size of the cell. Due to the above reason, application of the DSA structure to the formation of the floating type memory cell transistor accelerates the speed of writing data in it, and yet, promotes the resistance of this cell transistor against occurrence of the "punch-through" phenomenon, and therefore, the DSA structure is quite essential for such a large-capacitive non volatile memory today. In order to fully generate proper function of an N-channel type cell transistor incorporating the DSA structure mentioned above, profile of the density of impurities in the P-pocket region 16 adjacent to an edge of the drain region 13b of the floating gate is particularly important. To securely promote the resistance of the cell transistor against the "punch-through" phenomenon and accelerate the data writing speed, the density of impurities in the P-pocket region 16 close to an edge of the drain region 13b must constantly be held stronger than that is present in the channel region 17. Nevertheless, since the P-pocket region 16 is formed by means of P-type impurities aids by injection of ion after completing the formation of stratified gate electrodes, it results in the occurrence of problem described below. The edge of the drain 13b is covered with stacked gate electrodes. When injecting ion by applying a conventional ion injection method, in order to prevent occurrence of "channeling effect", in other words, in order to prevent ion from deeply being injected beyond path which allows easy passage of ion, only a maximum of 7 degrees of angle is applied to the injection of ion against the normal of the silicon substrate 11, and thus, the P-type impurities cannot fully be injected into the region close to the drain region 13b below the floating gate 14. Because of this, actually, any conventional art injects ion needed for the formation of the P-pocket region 16, and then executes an annealing process to diffuse the P-type impurities into the region further than the edge of the drain region 13b below the channel region 17 before eventually injecting ion needed for the formation of the source and drain regions 13a and 13b. In this case, an additional round of annealing process is needed for the formation of the P-pocket region 16 containing the DSA structure. Because of this additional annealing process, the conventional art cannot simultaneously execute the double ion injection processes, and yet, an additional round of patterning process must also be executed. When providing the P-pocket region 16 by diffusing the P-type impurities as mentioned above, profile of the density of impurities cannot perfectly be controlled, and yet, enough density of the P-type impurities cannot be generated in such region close to the drain region 13b. Furthermore, the annealing process needed for the formation of the P-pocket region 16 gravely affects the profile of the density of the P-type impurities. Furthermore, when providing the P-pocket region 16, in order to thermally diffuse the P-type impurities in the lateral direction to full extent and maintain enough density of the impurities in the diffused region, substantial dose of ion must be injected. This in turn expands capacity at junctions and weakens the junction breakdown voltage. In particular, the junction breakdown voltage is determined by the breakdown voltage of a high P-type impurity region formed of the overlapping portions the P-pocket region and a channel stopper region located below an isolation region, and by the breakdown voltage of the pn junction formed between the N + regions of the regions 13a and 13b. Because of this, the junction breakdown voltage is largely dependent on the distribution of the density of impurities present in those adjacent regions. EP-A-333 583 discloses a method of producing a semiconductor device comprising the steps of: preparing a semiconductor substrate, forming a gate insulating layer on the semiconductor substrate, forming a gate electrode on the gate insulating layer, forming a source/drain region in the semiconductor substrate, forming a cover layer on the entire exposed surface, forming a mask on the cover layer having an opening over the gate electrode, implanting one conductivity type impurity ions into the semiconductor substrate through the cover layer, the gate electrode and the gate insulating layer as the first ion implanting process, implanting opposite conductivity impurity ions into the semiconductor substrate therethrough as the second ion implanting process, at an implanting angle larger than that used in the first ion implanting process with respect to the normal plane of the semiconductor substrate and to substantially the same depth as the first ion implanting process, and at a dosage smaller than that in the first ion implanting process, whereby the one conductivity type impurity ions are compensated at laterally spread portions. Patent abstracts of JP-A-57-130475 discloses a semiconductor memory and its manufacture, wherein the reproducibility of the efficiency of writing and the characteristics of an EPROM can be improved by laminating and forming a floating gate and a control gate, introducing an impurity having the conduction type as a substrate to a channel region from the end section of the both gates and shaping source and drain regions. The publication T. Hori: JEE/Journal of Electronic Engineering 26, no. 267, (1989), pages 128-131, which is considered to represent the most relevant prior art, discloses a field effect transistor using Large-Tilt-Angle implantation in order to form n + -punch-through stoppers. Especially, an angle of 25° or less than 60° is suggested. Further, this publication discloses that the density of impurities in the punch-through stopper region is higher than in the center of the channel region and weaker than that in the drain region. The object of the present invention is achieved by a method as described in claim 1. The subclaims 2 to 5 show further advantageous developments of the method according to the present invention. To realize the object, according to the present invention, there are provided portions of first impurity ion-activated regions, which are in the vicinity of the channel region of the EPROM transistor and adjacent to the surface of the semiconductor substrate. In order to realize the above-mentioned portions, claim 1 includes the step of injecting ions of an impurity having a first conductivity type into the semiconductor substrate at an angle of 8° and a maximum of 60° against the normal of this substrate. This is because, like any conventional art, if ion were injected into the substrate at a maximum of 7 degrees of angle against the normal of the substrate, then the effect of strengthening the density of impurities will be minimized in the P-pocket region adjacent to the edge of the drain region below the floating gate electrode. Injection of ion at a minimum of 8 degrees of angle initiates promotion of density of impurities beyond the conventionally available level. Conversely, if the ion injection angle exceeds 60 degrees, then difficulty is present in the effect of fully injecting ion into the substrate. Since the system embodied by the invention injects ion at a minimum of 8 degrees against the normal of the substrate, the above-cited channel effect arises. Nevertheless, since ion is deeply injected, no critical problem arises. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 designates a sectional view of a conventional non-volatile memory cell; Figs. 2A through 2F are respectively explanatory of sequential steps of manufacturing the semiconductor device according to an embodiment of the invention; Fig. 3 designates sectional views of those regions adjacent to the drain region; Fig. 4 graphically designates profile of the density of impurities present in the P-pocket region shown in Fig. 3; Figs. 5A and 5B respectively designate sectional views of the essential components of another embodiment of the invention; and Fig. 6 is characteristic chart of the resistance of the cell transistor against occurrence of the "punch-through" phenomenon. Referring now to the N-channel type semiconductor device shown in Figs. 2A through 2F, the method of manufacturing an EPROM cell transistor according to an embodiment of the invention is described below. First, as is conventionally performed, the system embodied by the invention forms a field insulation film 22 and a channel stopper region 23 on the surface of a P-type silicone substrate 21, that separates elements. Next, by applying a thermal oxidation process, the P-type silicone substrate 21 is superficially provided with a gate insulation film 24 having about 20 nm of thickness. In order to properly adjust threshold voltage of the cell transistor, conventionally, only after forming up the gate insulation film 24, ionized P-type impurities are injected into the channel region. On the other hand, since it is possible for the art embodied by the invention to properly adjust the threshold voltage of the cell transistor based on specific conditions applicable to the injection of ion into the P-pocket region (where these conditions include the injection angle, dosed amount, and accelerated voltage), actually, the art embodied by the invention forms the channel region merely by applying the P-type impurities held in the P-type silicon substrate 21 without injecting the ionized P-type impurities therein for strengthening the density. Next, the system forms phosphor-doped polycrystalline silicon layer 25 on the gate insulation film 24, where this layer 24 is available for composing a floating gate electrode. Next, thermally oxidized film 26 having about 20 nm of thickness is formed on the polycrystalline silicone layer 25 by applying a thermal oxidation process. Then, the system forms the secondary phosphor-doped polycrystalline silicon layer 27 on the thermally oxidized film 26 (see Fig. 2A), where this layer 27 is available for composing a control gate electrode. Next, patterning processes are sequentially executed against the secondary phosphor-doped polycrystalline silicon layer 27, the thermally oxidized film 26, and the first phosphor-doped polycrystalline silicon layer 25, respectively. As a result, stacked gate electrode unit composed of the control gate electrode 27, the second gate insulation film 26, and the floating gate electrode 25, is thus completed. Then, an oxide film 31 is formed on the stacked gate electrode unit and the surface of the P-type silicon substrate 21, respectively (see Fig. 2B). Next, the system injects the P-type impurities into the P-type silicon substrate 21 in conjunction with ion by way of self-alignment the stacked gate electrode unit serving as mask, and then forms up ion-injected P-type layers 32a and 32b adjoining those regions predetermined to become source and drain regions. Ion is injected into the P-type silicon substrate 21 while continuously rotating it more than 1 round per minute by injecting more than 5 × 10 12 cm -2 of a dosed amount of boron by applying 10 ≦  ≦ 45 degrees of angle against the normal 33 of the silicone substrate 21 (see Fig. 2C and 2D). In this case, it is suggested that care should be taken to inject ionized impurities by properly adjusting the density of impurities to be stronger than that one being present in the center of the channel region and weaker than that one being present in the drain region of this cell transistor. In this case, the process of ion injection can be implemented in accordance with the method of manufacturing semiconductor device based on the condition specified by the expression shown below. X p ·tan ≧ X jl where X p designates the mean projected range of the P-type conductive impurities injected into the P-pocket region in conjunction with ion from the surface of the P-type silicone substrate;  designates the angle at which ion is injected against the normal of the P-type silicon substrate; and X jl designates the extended distance of the edge of the drain or source region as a result of elongation in the direction of the channel region below the stacked gate electrode unit by effect of diffusion until reaching the final production step after completing the ion injection process (see Fig. 3). More particularly, the density of impurities in the P-pocket region at the edge of the drain region can sufficiently be strengthened only when the product of tan  and the distance X p designating the mean projected range of the P-type impurities injected into the P-pocket region together with ion from the surface of the P-type silicone substrate is greater than the distance X jl designating the distance of the edge of the drain region diffused into the portion below the floating gate electrode. This allows the P-pocket region to fully exert own functional effect. In this case, if the depth X p of the injected ion coincides with the depth X j of the junction of the diffused drain layer and if the width were X j /2 ≦ X p ≦ 2X j , then, it is possible for the embodiment of the invention to effectively strengthen the density of the P-type impurities at the largest-curvature spot of the edge of the drain region so that ideal effect can be provided for the "punch-through" of the cell transistor. This in turn effectively suppresses occurrence of "short channel" effect. On the other hand, if the P-type impurities were not deeply injected, then it will cause the P-type impurities to be absorbed into the superficial oxidized film generated by the following thermal treatment including oxidation, thus resulting in the ineffective application of the P-type impurities. Fig. 6 designates the dependence of the "punch through" characteristic under a specific condition on the angle  applied to the injection of P-type impurities, where the condition includes X j = 0.2 µm, X jl ≃ 0.16, and X p = 0.30 µm, respectively. In this case, based on the expression shown above, it is presumed that the P-pocket region is effective when the value of  is more than 28 degrees. As it is clear from Fig. 6, when  = 0° and  = 15°, in other words, when the P-type impurities are injected into the substrate at an angle less than 28 degrees, resistance against the "punch through" voltage sharply declines if the floating gate electrode had less than 0.6 mm of length. On the other hand, when the P-type impurities are injected into the substrate at an angle  = 30° wider than 28°, then the resistance against the "punch through" effect is significantly improved. Sufficient effect is maintained until the length of the floating gate electrode reaches 0.5 µm. As mentioned above, the embodiment of the invention allows the P-type impurities to effectively be injected into the P-pocket region by applying a minimum of 8 degrees and a maximum of 60 degrees as well. The reference numeral 34 shown in Figs. 2C and 2D designates beam of ion containing P-type impurities. The reference numeral 36a and 36b designate ionized P-type impurities injected into field edge. The reference numeral 21 designates the rotating P-type silicon substrate. In the next step, the P-type impurities contained in ion is injected into the substrate by way of self-aligned the stacked gate electrode unit functioning as mask, and then N-type ion-injected layers 39a and 39b are respectively formed. In this case, ion is injected into the P-type silicone substrate 21 so that the injection can be oriented in parallel with the lateral surface of the stacked gate electrode unit or at an angle of 7° to the normal to the surface of the silicon substrate 21. For example, 5 × 10 15 cm -2 of a dosed amount of ionized arsenic is injected into the silicon substrate 21. The reference numeral 38 shown in Fig. 2E designates beam of ionized N-type impurities. In the following step, a thermal process such as an annealing process is executed at 900°C in order to activate ionized impurities and recover the oxidized film 24, 31 from damage incurred from the said two injections of ion. At the same time, the ion-injected P-type layers 32a and 32b, respectively, form P-pocket regions 40a and 40b. On the other hand, the ion-injected N-type layers 39a and 39b, respectively, form source and drain regions 41a and 41b. In the next step, based on the conventional method of manufacturing a MOS integrated circuit, an insulation film 42 above the source and drain regions 41a and 41b is perforated to provide a contact hole 43. Next, an aluminium electrode 44 is formed, thus completing the production of an EPROM cell (see Fig. 2F). The reference numeral 45 shown in Fig. 2F designates the portion where the P-pocket region and the channel stopper region overlap each other. The reference numeral 46 designates the channel region. According to the method of manufacturing the EPROM cell transistor mentioned above, the density of P-type impurities in the P-pocket region adjacent to the edge of the drain region can securely be strengthened, where the density of P-type impurities makes up the decisive factor of the DSA structure. As a result, the efficiency of writing data into the cell memory is promoted. Furthermore, by virtue of the contracted dimension of the cell, "short-channel" effect is improved, thus easily allowing materialization of finer size of the cell and integration of densely built circuits as well. Fig. 3 designates the sectional views of those regions adjacent to the drain edge of the cell embodied by the invention. The P-pocket region shown in Fig. 3 with a broken line designates the case in which the conventional ion injection method is applied to the formation of the P-pocket region of the above embodiment. The reference character X j shown in Fig. 3 designates the depth of the junction of the drain region 41b, whereas X jl designates the extended distance of the edge of the drain or source region as a result of elongation in the direction of the channel region below the floating gate electrode by effect of diffusion until reaching the final production process after completing the ion injection process. Fig. 4 graphically designates concrete distribution of the density of impurities across the A-A' section shown in Fig. 3. The density of boron shown by means of a broken line designates the case in which the conventional ion injection method is applied to the formation of the P-pocket region of the above embodiment. The method of manufacturing the semiconductor device according to the invention provides those useful advantages described below. (1) The density of impurities in the P-pocket region 40b adjacent to the floating gate electrode 25 can be promoted to such a level stronger than that of the channel region 46. In consequence, electric field in the periphery of the drain region 41b is intensified to result in the generation of a greater amount of hot electrons. This in turn promotes the efficiency of writing data into the cell memory. (2) Owing to those advantages described in the above paragraph (1), the pn junction between the source or drain region and the P-pocket region can be composed of extremely dense impurities to prevent depletion from expanding itself. As a result, the effective length of channel in the cell transistor can fully be secured to suppress occurrence of "short-channel" effect. (3) Owing to those advantages described in the above paragraphs (1) and (2), threshold voltage of the cell transistor can be activated merely by means of the P-pocket region, and therefore, injection of ion conventionally needed for the provision of channel for the cell transistor can be deleted. (4) According to the method embodied by the invention, in connection with the profile of the density of P-type impurities in the P-pocket region, such a region affecting the efficiency of writing data into the memory cell, in other words, the region adjoining the drain edge below the floating gate electrode 25, and such a region affecting the "short-channel" effect, in other words, the portion adjoining the bottom of the drain region below the floating gate electrode 25, can independently be controlled by injecting ion twice in various accelerating voltage or angle for example. Accordingly, the system embodied by the invention can flexibly deal with any variation of the cell structure and variable factors generated in the course of thermal treatment after completing the formation of the cell transistor. (5) The method embodied by the invention dispenses with the thermal process otherwise needed for any conventional ion injection system to form the P-pocket region. (6) Owing to the advantage described in the above paragraph (5), the method embodied by the invention allows a process for injecting ion for forming the P-pocket region of the cell transistor and another process for injecting ion for forming the source and drain regions of this transistor to simultaneously be executed once, thus deleting the patterning process by means of a masking process. (7) According to the method embodied by the invention, due to transmitting or scattering at the field edge of the region 45 overlapped by the channel-stopper region 23 below the isolation region 22 and the P-pocket region 16, ion containing the P-type impurities has graded profile ranging from the surface of the substrate to the innermost region of this substrate. When obliquely injecting ion into the P-pocket region by rotating wafer, the amount of the ionized P-type impurities injected below the field edge is maximized in the direction of the layer 36b in the P-pocket region of the layer 32b, for example, whereas little of the ionized P-type impurities is injected in the direction of the layer 36a, and thus, the density of the P-type impurities at the portion 45 overlapped by the P-pocket region 36b and the channel stopper region 23 is weakened (see Fig. 2C). Because of this, unlike the pn junction made as a result of forming the P-pocket region according to the conventional ion injection method, owing to the effect of weakening the density of the P-type impurities adjoining the pn junction and also owing to the effect of slackened gradient of the density of the P-type impurities, the strength of the pn junction at the portion 45 is effectively reinformed. It should be understood that the scope of the invention is by no means confined to the embodiment thus far described, but the invention is also applicable to a variety of uses. For example, the above embodiment has formed the P-pocket region by way of covering the source and drain regions (see Fig. 2F). However, it is apparent that the P-pocket region shown in Figs. 5A and 5B can also locally be provided via a masking process by causing the P-type silicon substrate 21 to stand still or intermittently rotate itself. Although the description of the above embodiment has solely referred to the case of forming up the P-pocket region on both sides of the drain and source regions, it is quite apparent that the embodiment can also provide the P-pocket region solely on the part of the drain region or the source region via a masking process. Furthermore, although the above description has solely referred to the N-channel type cell transistor, the embodiment of the invention is also applicable to the P-channel type cell transistor as well. Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A method of manufacturing a semiconductor device, comprising the steps of: forming a floating gate electrode (25) of an EPROM transistor on an insulation film (24) formed on a region of a semiconductor substrate (21) of a first conductivity type (P), said floating gate electrode being located above that region of the layer in which a first conductivity type channel is to be formed, and forming a control gate (27) on another insulation film (26) formed on the floating gate electrode so as to form a stacked gate electrode unit; injecting, using by way of self-alignment the stacked gate electrode unit as a mask, ions of an impurity having a first conductivity type into said semiconductor substrate (21) at an angle of a minimum of 8° and a maximum of 60° to the normal to the substrate, thereby to form first impurity ion-implanted regions (32a, 32b) having a predetermined concentration; injecting, using by way of self-alignment the stacked gate electrode unit as a mask, ions of an impurity having a second conductivity type into said semiconductor substrate (21) to form second impurity ion-implanted regions (39a, 39b) having a predetermined concentration of the second conductivity type impurity ions (N + ) in the first impurity ion-implanted regions (32a, 32b) in such a manner that those portions of the first impurity ion-implanted regions (32a, 32b) in the vicinity of the channel region of the EPROM transistor which are adjacent to the surface of the semiconductor substrate are still retained as the first conductivity type portions; and annealing the first and second impurity ion-implanted regions (32a, 32b, 39a, 39b) to activate the impurity ions in the first and second impurity ion-implanted regions (32a, 32b, 39a, 39b) to form first impurity ion-activated regions (40a, 40b) and second impurity ion-activated regions (41a, 41b), the second impurity ion-activated regions (41a, 41b) being a source region (41a) and a drain region (41b) of the EPROM transistor, wherein said predetermined concentration of the first conductivity type impurity ions in said first impurity ion-implanted regions (32a, 32b) is adjusted to be higher than in the center of the channel region and lower than the concentration of the second type impurity ions in said drain and source regions (41b, 41a), said first impurity ion-activated regions (40a, 40b) extend adjacent to the surface of the semiconductor surface further into said channel region than said drain and source regions (41b, 41a), and the portions of said first impurity ion-activated regions (40a, 40b), which are in the vicinity of the channel region of the EPROM transistor and adjacent to the surface of the semiconductor substrate increase the injection efficiency of hot electrons generated in the vicinity of the drain region (41b) into the floating gate electrode of the EPROM transistor. The method of manufacturing a semiconductor device according to claim 1, characterized in that said process for injecting ions into said substrate is executed by continuously or intermittently rotating said substrate. The method of manufacturing a semiconductor device according to claim 1 or 2, characterized in that said process for injecting ions into said substrate executed by applying a minimum of eight degrees and a maximum of sixty degrees of angle against the normal of said substrate is consummated simultaneously with injection of ions executed for the formation of the source and drain regions of said transistor. The method of manufacturing a semiconductor device according to any of the claims 1 to 3, characterized in that said process for injecting ions into said semiconductor substrate is executed in accordance with the condition specified by an expression shown below: Xp . tan ≥ Xjl where Xp designates the mean projected range of the first conductive ion impurities injected into said substrate from the surface of said substrate;  designates the angle at which ions are injected against the normal of said substrate; and Xjl designates the extended distance of the edge of the drain or source region as a result of elongation in the direction of channel region below said stacked gate electrode unit by effect of diffusion until reaching the final production process after completing the ion injection process. The method of manufacturing a semiconductor device according to any of the claims 1 to 4, further comprising a step of ion injection in which Xp satisfies the condition specified by an expression shown below: 1 2 Xj≤Xp≤2Xj where Xp designates the mean projected range of the first conductive ion impurities injected into said semiconductor substrate from the surface of the substrate; and Xj designates the depth of junction of diffused drain layer of said substrate.

Inventors: SAKAGAMI EIJI, SAKAGAMI, EIJI, SAKAGAMI, EIJI, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01L21
IPC codes: H01L27, H01L29, H01L21