SEMICONDUCTOR MEMORY DEVICE HAVING A CHARGE BARRIER LAYER AND METHOD FOR PRODUCING THE SAME

Abstract: A semiconductor memory device comprising a semiconductor substrate (11) having source and drain regions (16, 17) each having a conduction type opposite to that of the semiconductor substrate (11); an insulation film (13) formed on a main surface of the semiconductor substrate (11) having first and second contact windows (30, 31); and a gate electrode (14₁) formed on the insulation film (13) so as to be located between the source and drain regions (16, 17). The semiconductor substrate (11) has a charge barrier layer (15) which has the same conduction type as the semiconductor substrate (11) and has an impurity concentration higher than that of the semiconductor substrate (11). The charge barrier layer (15) is formed so that a depth (d₁) of the charge barrier layer (15) located under the gate electrode (14₁) measured from the main surface of the semiconductor substrate (11) is smaller than a depth (d₂) of the charge barrier layer (15) located under the source and drain regions (16, 17) measured from the main surface of the semiconductor substrate (11).

Description: The present invention generally relates to a semiconductor memory device having a charge barrier layer and a method for producing the same, and more particularly, to improvements in the charge barrier layer. Recently, there has been considerable activity in the development of a dynamic random access memory (DRAM) device having an increased integration density. It is required particularly to reduce the size of an area of a memory cell which occupies a semiconductor chip. Therefore, there is a tendency for a memory capacitor to be reduced. It is known that α-particles-induced soft errors increase with a decrease in size of the memory capacitor. Soft errors refer to a random failure not related to a physically defective device. When α-particles entires the semiconductor substrate, electron-hole pairs are generated. Holes move toward an electrode on the semiconductor substrate. On the other hand, electrons are diffused while some electrons are recombined with holes, and are collected in a depletion region under a gate electrode, a source region or a drain region. Further, electrons are collected in the source or drain region. The electrons captured in the depletion region or the source or drain region cause an erroneous operation of DRAM devices. The influence of α-particles is also a problem in a static random access memory device (SRAM device). In order to suppress the occurrence of soft errors, various proposals have been reported. A proposal teaches an increase of the impurity concentration in a silicon semiconductor substrate. Another proposal teaches a p⁺-type impurity diffusion region formed in a semiconductor substrate so as to be positioned below n⁺-type diffusion regions such as source and drain regions formed in the semiconductor substrate. The p⁺-type impurity diffusion region functions as a charge barrier layer for preventing electrons arising from the movement of α-particles in the semiconductor substrate, from being collected in the depletion regions, and source and drain regions. The p⁺-type charge barrier layer is described in the Japanese Laid-Open Patent Application No. 58-107667, which corresponds to US-A-4 506 436. Further prior art devices of the same type are described in INTERNATIONAL ELECTRON DEVICES MEETING TECHNICAL DIGEST, Washington, D.C., 7th-9th Dec. 1981, pages 40-43, IEEE, N.y., US and in IBM TECHNICAL DISCLOSURE BULLETIN, vol. 26, no. 5, Oct. 1983, pages 2569-2573, N.y., US. However, the prior art disclosed in JP-A-58-107667 has disadvantages as follows. The charge barrier layer is formed at the same depth from various points on the main surface of the substrate. Therefore, a vertical width of a depletion region formed under the impurity diffusion region such as source and drain regions is not identical to a vertical width of a depletion region of a channel under the gate electrode located between the source and drain regions. As a result, a portion of the charge barrier layer with respect to the depletion regions formed under the source and drain regions is different in charge collecting ability from a portion of the charge barrier layer with respect to the depletion region under the gate electrode. Normally, the charge barrier layer is formed at an optimum depth determined based on a vertical width of the depletion region under the source or drain region. In this case, since the depletion region under the gate electrode is unnecessarily wide in the vertical (depth) direction of the substrate, the p⁺-type diffusion region cannot provide a sufficient function of preventing charges (electrons) due to α-particles from being collected in the depletion region under the gate electrode and the source and drain regions. It is noted that an increased vertical width of the depletion region has the increased ability of capturing electrons due to α-particles. Further, in the above case, the charge barrier layer hardly functions with respect to electrons generated due to oblique incidence of α-particles. On the other hand, if the charge barrier layer is formed at an optimum depth determined based on a vertical width of the depletion region under the gate electrode, the vertical width of the depletion region under the source or drain region exceeds a peak of an impurity concentration of the p⁺-type charge barrier layer. SUMMARY OF THE INVENTION It is therefore a general object of the present invention to provide a novel and useful semiconductor device having a charge barrier layer and a method for producing the same in which the aforementioned disadvantages are eliminated. A more specific object of the present invention is to provide a semiconductor device having a charge barrier layer capable of effectively preventing electrons induced by α-particles from being collected in depletion regions and highly doped regions such as source and drain regions. The above objects of the present invention can be achieved by a semiconductor device having a charge barrier layer which includes a semiconductor substrate having source and drain regions each having a conduction type opposite to that of the semiconductor substrate, an insulation film formed on a main surface of the semiconductor substrate having first and second contact windows, and a gate electrode formed on the insulation film so as to be located between the source and drain regions. The semiconductor substrate has a charge barrier layer which has the same conduction type as the semiconductor substrate and which has an impurity concentration higher than that of the semiconductor substrate. The charge barrier layer is formed so that a depth (d₁) of the charge barrier layer located under the gate electrode measured from the main surface of the semiconductor substrate is smaller than a depth (d₂) of the charge barrier layer located under the source and drain regions measured from the main surface of the semiconductor substrate. Another object of the present invention is to provide a method for producing the above semiconductor device. The above another object of the present invention can be achieved by a method which includes the steps of forming an insulation film on a main surface of a semiconductor substrate; forming a gate electrode on the insulation film, the gate electrode having the conduction type opposite to that of the semiconductor substrate; implanting first impurity ions into the semiconductor substrate through the entire main surface thereof, the first impurity ions having a conduction type identical to that of the semiconductor substrate and having an impurity concentration higher than that of the semiconductor substrate, whereby the charge barrier layer is formed so that a depth (d₁) of the charge barrier layer located under the gate electrode measured from the main surface of the semiconductor substrate is smaller than a depth (d₂) of the charge barrier layer located under source and drain regions measured from the main surface of the semiconductor substrate; and implanting second impurity ions into the semiconductor substrate so as to form the source and drain regions located on both sides of the gate electrode. The second impurity ions have the conduction type opposite to that of the first impurity ions. Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS.1 and 2 are cross sectional views at different steps of a conventional DRAM manufacturing process; FIGS.3A and 3B are graphs which illustrate disadvantages of a conventional DRAM device manufactured by the steps of FIGS.1 and 2; FIG.4 through 11 are cross sectional views observed at different steps of a method of a preferred embodiment of the present invention. DETAILED DESCRIPTION Before describing the embodiment of the present invention, a description is given of a conventional DRAM semiconductor device having a charge barrier layer and a method for producing the same, with reference to FIGS.1, 2 and FIGS.3A and 3B. The following description with respect to FIGS.1 through 3 is described in the Japanese Laid-Open Patent Application No. 58-107667, which corresponds to US-A-4 506 436. Referring to FIG.1, a field insulation film 2 of silicon dioxide is formed on a main surface of a p-type silicon semiconductor substrate 1 by a selective thermal oxidation process in which an oxidation-resistant mask film (not shown) such as a silicon nitride film is used. Next, the silicon nitride film is removed in its entirety so as to expose the surface of the silicon substrate 1, or a surface of an active region. Then a p⁺-type charge barrier layer 3 is formed under the active region by ion implantation. Referring to FIG.2, a gate insulation film 4 of silicon dioxide is formed on the surface of the active region by thermal oxidation. Then a polysilicon film is grown on the entire surface of the silicon substrate 1 by chemical vapor deposition. Then the polysilicon film is patterned by the conventional photolithography technology so as to form a gate electrode (or a word line) 5. Thereafter, an impurity such as an arsenic (As) ion is implanted by the ion implantation process, in which the gate electrode is used as a mask film. Thereby, an n⁺-type source region 6 and an n⁺-type drain region 7 are formed in the silicon substrate 1. After that, formed are a memory capacitor constituted by a stacked capacitor which is in contact with the n⁺-type drain region 7, and a bit line which is in contact with the n⁺-type source region 6. In the semiconductor memory device thus produced, a radiation such as the α-ray penetrates the silicon substrate 1 and produces electron-hole pairs. Electrons produced in a substrate portion under the p⁺-type charge barrier layer 3 are prevented from being collected in the depletion regions and n⁺-type source and drain regions 6 and 7. Therefore, the occurrence of soft errors can be reduced. However, the conventional semiconductor memory device described above has the disadvantages described below. Normally, the n⁺-type source and drain regions 6 and 7 are approximately 0.2 [µm] in depth. The vertical width of the depletion region formed under each of the source and drain regions 6 and 7 is smaller than the width of a depletion region of a channel formed under the gate electrode 5. FIG.3A shows the impurity profile with respect to a silicon substrate portion including the source or drain region 6 or 7. FIG.3B shows the impurity profile with respect to a silicon substrate portion under the gate electrode 5. In FIGS.3A and 3B, the horizontal axes denote vertical depth from the surface of the silicon substrate 1, and the vertical axes denote impurity concentration. Xd is the width of the depletion region formed under the source or drain region 6 or 7, and Xc is the width of the depletion region of the channel region formed under the gate electrode 5. As can be seen from FIGS.3A and 3B, the width Xc of the depletion region is larger than the width Xd of the depletion region because of the presence of the source and drain regions 6 and 7. If the p⁺-type charge barrier layer 3 is formed so as to have an optimum position determined based on the width Xd of the depletion regions under the source and drain regions 6 and 7, it is inevitable that the width Xc of the depletion region under the gate electrode 5 increases unnecessarily. It is noted that as the width Xc of the depletion region increases, more electrons are collected in the depletion region. Therefore, erroneous operation increases. As a result, the charge barrier layer 3 decreases in the charge preventing function. On the other hand, in a case where the position of the p⁺-type charge barrier layer 3 is determined based on the width Xc of the depletion region, the width Xd of the depletion region under the source or drain region 6 or 7 exceeds a peak of the impurity concentration curve with respect to the p⁺-type charge barrier layer 3. From this region, the charge preventing function of the p⁺-type charge barrier layer 3 deteriorates. The present invention intends to overcome the aforementioned disadvantages. A description is given of a preferred embodiment of the present invention with reference to FIGS.4 through 11, which show cross sections of a semiconductor memory device of the preferred embodiment of the present invention observed at different manufacturing steps. The semiconductor memory device described below is a DRAM device which has memory cells each consisting of a memory capacitor constituted by a stacked capacitor, and a transfer gate transistor constituted by a field effect transistor. Referring to FIG.4, a field insulation film 12 of silicon dioxide is formed on a main surface of a silicon semiconductor substrate 11 by the selective thermal oxidation process in which an oxidation-resistant mask film (not shown) such as a silicon nitride film is used. Next, the silicon nitride film is removed in its entirety. A surface portion of the silicon substrate 11 which is exposed by the removal of the silicon nitride film, is subjected to thermal oxidation. Thereby, a gate insulation film 13 of silicon dioxide is formed to a thickness of approximately 200 [Å] (10Å = 1µm). Referring to FIG.5, a polysilicon film is grown to a thickness of approximately 0.2 [µm] by chemical vapor deposition. It is preferable that the polysilicon film is formed so as to have a thickness equal to or nearly equal to a depth of a source or drain region 16 or 17 shown in FIG.7 described later. Thereafter, by a conventional thermal diffusion process in which a source gas of POCl₃ is used , phosphorus (P) ions are introduced so that the polysilicon film can be changed to an n⁺-type polysilicon film. Then a mask film is deposited by the resist process of the conventional photolithography technology, and then the polysilicon film is subjected to the patterning process based on reactive ion etching (RIE). Thereby, gate electrodes (word lines) 14₁ and 14₂ are formed. The gate electrode 14₁ is associated with a transfer gate transistor having source and drain regions 16 and 17 shown in FIG.7. The gate electrode 14₂ is associated with neighboring transfer gate transistor not shown. Referring to FIG.6, by using the ion implantation process, boron (B) ions are implanted into the silicon substrate 11 through the entire surface thereof. Thereby, a p⁺-type charge barrier layer 15 is formed as shown in FIG.6. As is shown in FIG.6, the p⁺-type charge barrier layer 15 has a curved cross section, which is clearly distinct from the cross section of the p⁺-type charge barrier layer 5 shown in FIG.2. A depth d₁ of the charge barrier layer 15 under the gate electrode 14₁ measured from the surface of the silicon substrate 11 is smaller than a depth d₂ of the charge barrier layer 15 measured therefrom. In other words, under the gate electrode 14₁ of FIG.6, the p⁺-type charge barrier layer 15 is formed at a relatively small depth d₁ from the surface of the silicon substrate 11. On the other hand, the p⁺-type charge barrier layer 15 under the source or drain region 16 or 17 is formed at a relatively large depth d₂ from the surface of the silicon substrate 11. It is preferable that the width Xc of the depletion region under the gate electrode 14₁ is approximately identical to the width Xd of depletion regions under the source and drain regions 16 and 17. In other words, as shown in FIG.6, it is preferable that the depth d₁ of the charge barrier layer 15 under the gate electrode 14₁ measured from the surface of the substrate 11 is identical or nearly identical to the depth d₃ of the charge barrier layer 15 from a bottom surface of the source or drain region 16 or 17. The above can be achieved by forming the gate electrode 14₁ so as to have a thickness t₁ almost identical to a depth d₄ (= d₂ - d₃) of the source or drain region 16 or 17. Essential parameters in the above-mentioned boron implantation process are shown below. Acceleration energy: 500 - 1000 [KeV]; Dose amount: 4 x 10¹² [cm⁻²]; Impurity concentration: 5 x 10¹⁷ [cm⁻³]; Depth d₁: ∼ 0.7 [µm]; and Depth d₂: ∼ 0.9 [µm]. It is preferable that the impurity concentration of the p⁺-type charge barrier layer 15 is in the order of 10¹⁷ [cm⁻³] or over. It is further preferable that the dose of boron ions is 1 x 10¹² [cm⁻²] or over. As described previously, the gate electrodes 14₁ and 14₂ are doped with boron ions with an impurity concentration of greater than 1 x 10¹³ [cm⁻³]. For example, a dose of 1 x 10¹³ [cm⁻²] of boron ions corresponds to 5 x 10¹⁷ [cm⁻³] (= 1 x 10¹³/2000 [Å]). As a result, even if boron ions are implanted with a dose of approximately 1 x 10¹³ [cm⁻²] after implantation of phosphorus ions with an impurity concentration of greater than 10²⁰ [cm⁻³], a decrease in impurity concentration of the gate electrodes 14₁ and 14₂ resulting from the boron doping is negligible. Thereafter, as shown in FIG.7, arsenic (As) ions are implanted by ion implantation, and thereby the source and drain regions 16 and 17 are formed in the silicon substrate 11. The source and drain regions 16 and 17 are approximately 0.2 [µm] in depth. Referring to FIG.8, an interlayer insulation film 18 of silicon dioxide is formed by chemical vapor deposition. Referring to FIG.9, the interlayer insulation film 18 is selectively etched by the conventional photolithography technology so as to form a storage electrode contact window 30 in the interlayer insulation film 18. Then a polysilicon film is deposited which is used for forming a storage electrode of a memory capacitor, which is paired with an opposed electrode (called cell plate). The deposition of the polysilicon film is carried out by chemical vapor deposition. Subsequently, by ion implantation, arsenic (As) ions are implanted into the polysilicon film to provide the polysilicon film with n⁺-type conductivity. Then the n⁺-type polysilicon film is patterned by the conventional photolithography technology so as to form a storage electrode 19₁. Referring to FIG.10, a thin dielectric film 20₁ of silicon dioxide is formed on the surface of the storage electrode 19₁ by thermal oxidation. The dielectric film 20₁ is approximately 100 [Å] thick. Thereafter, a polysilicon film is deposited which is used for forming an opposed electrode (or a cell plate) of the memory cell. The deposition of the polysilicon film is carried out by chemical vapor deposition. The polysilicon film is approximately 2000 [Å] thick. Then by a conventional thermal diffusion process in which a source gas of POCl₃ is used, phosphorus are introduced into the polysilicon film so that it can be changed to an n⁺-type polysilicon film. Thereafter, the polysilicon film is patterned by the conventional photolithography so that an opposed electrode 21₁ is formed. Then an interlayer insulation film 22 of silicon dioxide is formed by chemical vapor deposition. subsequently, the interlayer insulation film 22 and the gate insulation film 13 are selectively etched by the conventional photolithography technology so as to form a bit line contact window 31 in the interlayer insulation film 22 and the gate electrode insulation film 13. A portion of the surface of the source region 16 is exposed through the bit line contact window 31. Then a polysilicon film and a refractory metal silicide film are formed in this order by chemical vapor deposition and sputtering, respectively. Then the polysilicon film and the refractory metal silicide film are patterned so as to form a bit line 23₁ by the conventional photolithography technology. After that, as shown in FIG.11, a passivation film 24 of phosphosilicate glass (PSG) is formed. Then an aluminum film which improves conductivity of the word lines 14₁ and 14₂ is formed and then patterned so that word lines 25₁ and 25₂ are formed. According to the preferred embodiment of the present invention, the width Xc of the depletion region under the gate electrode 14₁ is made smaller than that of the depletion region under the gate electrode 5 in the conventional DRAM device. In other words, the depth d₁ of the charge barrier layer 15 under the gate electrode 14₁ is smaller than the depth d₂ of the charge barrier layer 15 under the source or drain region 16 or 17. As a result, the p⁺-type charge barrier layer 15 is formed at a suitable depth under both the gate electrode 14₁ and source and drain regions 16 and 17. Therefore, the p⁺-type charge barrier layer 15 has the improved function of preventing electrons from being collected in the depletion regions and the source and drain regions 16 and 17. The above function is enhanced in the case where the depth d₁ of the charge barrier layer 15 under the gate electrode 14₁ is almost identical to the depth d₃ of the charge barrier layer 15 measured from the bottom surface of the source or drain region 16 or 17. The present invention includes SRAM devices. That is, the similar advantageous effects are obtainable by providing the p⁺-type charge barrier layer.

Claims: A semiconductor memory device comprising a semiconductor substrate (11) having source and drain regions (16, 17) each having a conduction type opposite to that of the semiconductor substrate (11); an insulation film (13) formed on a main surface of the semiconductor substrate (11) having first and second contact windows (30, 31); and a gate electrode (14₁) formed on the insulation film (13) so as to be located between the source and drain regions (16, 17), the semiconductor substrate (11) having a charge barrier layer (15) which has the same conduction type as the semiconductor substrate (11) and has an impurity concentration higher than that of the semiconductor substrate (11), characterized in that the charge barrier layer (15) is formed so that the depth (d₁) of the charge barrier layer (15) located under the gate electrode (14₁) measured from the main surface of the semiconductor substrate (11) is smaller than the depth (d₂) of the charge barrier layer (15) located under the source and drain regions (16, 17) measured from the main surface of the semiconductor substrate (11). A semiconductor memory device as claimed in claim 1, characterized in that the depth (d₁) of the charge barrier layer (15) from the main surface of the semiconductor substrate (11) under the gate electrode (14₁) is identical or nearly identical to a depth (d₃) of the charge barrier layer (15) from a bottom surface of each of the source and drain regions (16, 17). A semiconductor memory device as claimed in claim 1, characterized in that the thickness (t₁) of the gate electrode (14₁) is identical or nearly identical to the depth (d₄) of each of the source and drain regions (16, 17) measured from the main surface of the semiconductor substrate (11). A semiconductor memory device as claimed in claim 1, characterized in that the impurity concentration of the charge barrier layer (15) is on the order of 10¹⁷ [cm⁻³] or more. A semiconductor memory device as claimed in claim 1, characterized in that the depth (d₁) of the charge barrier layer (15) under the gate electrode (14₁) measured from the main surface of the semiconductor substrate (11) is approximately 0.7 [µm]. A semiconductor memory device as claimed in claim 1, characterized in that the depth (d₂) of the charge barrier layer (15) located under the source and drain regions (16, 17) measured from the main surface the semiconductor substrate (11) is approximately 0.9 [µm]. A method of producing a semiconductor memory device, comprising the steps of forming an insulation film (13) on a main surface of a semiconductor substrate (11); forming a gate electrode (14₁) on the insulation film (13), the gate electrode (14₁) having the conduction type opposite to that of the semiconductor substrate (11); implanting first impurity ions into the semiconductor substrate (11) through the entire main surface thereof, the first impurity ions having a conduction type (p⁺-type) identical to that of the semiconductor substrate (p-type) and having an impurity concentration higher than that of the semiconductor substrate (11), whereby the charge barrier layer (15) is formed so that the depth (d₁) of the charge barrier layer (15) located under the gate electrode (14₁) measured from the main surface of the semiconductor substrate (11) is smaller than the depth (d₂) of the charge barrier layer (15) located under source and drain regions (16, 17) measured from the main surface of the semiconductor substrate (11); and implanting second impurity ions into the semiconductor substrate (11) so as to form the source and drain regions (16, 17) located on both sides of the gate electrode (11), the second impurity ions having the conduction type opposite to that of the first impurity ions. A method as claimed in claim 7, characterized in that in the step of forming the gate electrode (14₁), the gate electrode (14₁) is formed to a thickness (t₁) identical or nearly identical to the depth (d₄) of each of the source and drain regions (16, 17) measured from the main surface of the semiconductor substrate (11). A method as claimed in claim 7, characterized in that the first impurity ions are boron ions, and in that in the step of implanting the first impurity ions, the acceleration energy is in a range of 500 to 1000 [KeV], and the dose amount of boron ions is equal to or greater than 4 x 10¹² [cm⁻²]. A method as claimed in claim 7, characterized in that the first impurity ions comprise p⁺-type impurity ions and the second impurity ions comprise n⁺-type impurity ions. A method as claimed in claim 7, characterized in that the step of implanting the first impurity ions is carried out so as to form the charge barrier layer (15) at the depth (d₁) equal to approximately 0.7 [µm]. A method as claimed in claim 8, characterized in that the depth (d₄) of each of the source and drain regions (16, 17) from the main surface of the semiconductor substrate (11) is approximately 0.2 [µm].

Inventors: EMA, TAIJI
Applicants: FUJITSU LIMITED
Application year: 1988
Main classification: H01L29
IPC codes: H01L29, H01L27, H01L21