Semiconductor device and manufacturing method thereof

Abstract: A semiconductor device includes a gate electrode (2), and a semiconductor layer (7) formed on the main surface of the gate electrode (2) with a gate oxide film (6) therebetween. The semiconductor layer (7) has a channel region (7a) opposing the main surface of gate electrode (2) and source/drain regions (7b, 7c) having the channel region therebetween, and is formed so that the bent angle in the vicinity of the boundaries of the channel region (7a) and the source/drain regions (7b, 7c) is beyond 90°. Thus, the semiconductor layer formed in a thin film transistor has no orthogonal bent, and, therefore the concentration of electric fields is suppressed, improving the performance of the thin film transistor.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to semiconductor devices and manufacturing methods thereof, and more specifically, to a structure of a thin film transistor and a manufacturing method thereof. Description of the Background Art In recent years, for meeting a demand for shrinking the size of a semiconductor device, development of a thin film transistor (hereinafter referred to as TFT) to be formed as a field effect transistor by forming a -semiconductor thin film on an insulator substrate and providing a channel in the semiconductor thin film has been under way. A description of a structure of the TFT follows in conjunction with the accompanying drawings. Referring to Fig. 38, a gate electrode 2 of polysilicon is formed on an interlayer insulating film 1 formed on a bulk transistor (not shown). A gate oxide film 6 is formed along the top flat portion and sidewalls of the gate electrode 2. A semiconductor layer 7 is formed along the gate oxide film 6. Formed at a position of the semiconductor layer 7 opposite to the gate electrode 2 is a channel region 7a. Source/drain regions 7b, 7c are formed at positions having the channel region 7a therebetween. A description of a manufacturing process of a TFT having the above-stated structure follows in conjunction with Figs. 39-45. Referring to Fig. 39, an interlayer insulating film 1 is formed as thick as 1500nm on a bulk transistor (not shown) by thermal chemical vapor deposition. A polysilicon layer 2 to be the gate electrode 2 of the TFT is deposited as thick as 200nm on the interlayer insulating film 1 by means of thermal chemical vapor deposition. Referring to Fig. 40, a resist film 4 having a prescribed form is formed on the surface of the polysilicon layer 2 by a photolithography technique. Referring to Fig. 41, using the resist film 4 as mask, the polysilicon layer 2 is subjected to anisotropic reactive ion etching to form the gate electrode 2. Referring to Fig. 42, the resist film 4 is etched away. Referring to Fig. 43, an oxide film 6 for forming a gate oxide film having a thickness of 50nm is formed on the surfaces of the gate electrode 2 and the interlayer insulating film 1 by thermal CVD. A polysilicon layer 7 to form the semiconductor layer of the TFT is deposited as thick as 50nm on the oxide film 6 by thermal chemical vapor deposition. Referring to Fig. 44, a resist film 8 is formed on the surface of the polysilicon layer 7. The resist film 8 is patterned into a prescribed form by means of a photolithography technique or the like. Using the resist film 8 as mask, an impurity, for example, 1 x 10 15 (cm -2 ) of boron for forming source/drain regions is implanted into the polysilicon layer 7. Referring to Fig. 45, the resist film 8 is etched away. Thus formed is the TFT having a channel region 7a in the region of the polysilicon layer 7 opposite to the gate electrode 2 and the source/drain regions 7b, 7c at the positions having the channel region 7a therebetween. The TFT having the structure described above, however, suffers from the following disadvantage. Referring to Fig. 45, the gate electrode 2 is formed by subjecting the polysilicon layer 7 to anisotropic reactive ion etching. The gate electrode 2 therefore takes a substantially rectangular form. Thus, with the gate electrode 2 having a rectangular form, substantially orthogonal edges 9, 9 are produced in the oxide film 6 the and polysilicon layer 7 formed along the gate electrode 2. Electric fields are likely to concentrate on these edges 9, 9, resulting in hot carriers or the like. The hot carriers enter into the gate oxide film 6, thus deteriorating the insulation of the gate oxide film 6. The article "256 kbit CMOS EPROM HN27C256" from "Hitachi Review", vol. 34, no. 6, December 1985, pp. 295-298, discloses a 256 kbit CMOS EPROM adopting the poly-Si-PMOS technology in order to prevent latch-up. An undoped region between the source and drain regions is used as a channel of the transistor. The channel conductivity is controlled by a voltage applied to a well which is positioned underneath the poly-silicon channel and isolated therefrom through a thin SiO 2 layer. The thickness of the SiO 2 layer is lower at the position under the gate electrode so that the transistor structure comprises a recess at the position of the channel region. It is an object of the present invention to provide a semiconductor device having a higher integration density, and a manufacturing method thereof. According to the invention this object is accomplished by a semiconductor device comprising: a semiconductor substrate; semiconductor means provided at a prescribed distance apart from each other on said semiconductor substrate, said semiconductor means comprising gate electrodes formed on said semiconductor substrate; a further gate electrode formed between said gate electrodes of said semiconductor means with an insulating layer between said further gate electrode and each of said gate electrodes, respectively, said further gate electrode having a V-shape; a gate insulating layer on the upper surfaces of said further gate electrode and said insulating layer; and a semiconductor layer on said gate insulating layer, said semiconductor layer having a channel region of a first conductivity type opposing said further gate electrode and source/drain regions of a second conductivity type at the respective ends of said semiconductor layer; said channel region of said semiconductor layer essentially following said V-shape of said further gate electrode. According to the invention this object is also accomplished by a method of manufacturing a semiconductor device, comprising the steps of: forming gate electrodes on the surface of a semiconductor substrate; forming an insulating layer on the surface of said gate electrodes; forming a resist film at a prescribed position on the surface of said insulating layer and etching said insulating layer using said resist film as a mask; removing said resist film; forming a conductor layer on the surface of said insulating layer; forming a further resist film on the surface of said conductor layer; performing thermal oxidation using said further resist film as a mask and forming a further gate electrode having a V-shape; forming a gate insulating layer on the surfaces of said further gate electrode and said insulating layer; forming a semiconductor layer of a first conductivity type on the surface of said gate insulating layer; forming a mask on the surface of said semiconductor layer at a position opposing said further gate electrode, and forming a channel region of said first conductivity type at said position opposing said further gate electrode of said semiconductor layer of said first conductivity type by implanting an impurity of a second conductivity type, and source/drain regions of said second conductivity type at positions with said channel region therebetween. According to the present invention, there is no channel layer of an orthogonal bent formed in the semiconductor layer formed on the gate electrodes with the gate insulating layer therebetween. This suppresses the concentration of electric fields as generated at the orthogonal bents of the conventional semiconductor device described with reference to Fig. 38. Furthermore, hot carriers can be prevented from being implanted into the gate insulating layer in the semiconductor device. The present invention will be described hereinbelow by way of example and with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view showing a structure of a thin film transistor in accordance with a first embodiment of the present invention; Figs. 2 to 10 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the first embodiment of the present invention; Fig. 11 is a cross sectional view showing a structure of a thin film transistor in accordance with a second embodiment of the present invention; Figs. 12 to 14 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the second embodiment of the present invention; Fig. 15 is a cross sectional view showing a structure of a thin film transistor in accordance with a third embodiment of the present invention; Figs. 16 to 22 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the third embodiment of the present invention; Fig. 23 is a cross sectional view showing a structure of a thin film transistor in accordance with a fourth embodiment of the present invention; Figs. 24 to 29 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the fourth embodiment of the present invention; Fig. 30 is a cross sectional view showing a structure of a thin film transistor in accordance with a fifth embodiment of the present invention; Figs. 31 to 36 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the fifth embodiment of the present invention; Fig. 37 is a cross sectional view showing a structure of a thin film transistor in accordance with a third embodiment of the present invention, being used in an SRAM memory cell; Figs. 38 is a cross sectional view showing a structure of a thin film transistor in accordance with a conventional technique; and Figs. 39 to 45 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the conventional technique. DESCRIPTION OF THE PREFERRED EMBODIMENTS A description of embodiments of a thin film transistor (TFT) in accordance with the present invention follows in conjunction with the accompanying drawings. A structure of a TFT in accordance with the first embodiment of the present invention will be described in conjunction with Fig. 1. The TFT according to the first embodiment has an interlayer insulating film 1 formed on a bulk transistor (not shown). A gate electrode 2 having a central flat portion and inclined sidewalls descending rightward and leftward from the right and left ends of the central flat portion is formed on interlayer insulating film 1. First insulating layers 5, 5 are formed in contact with and along the right and left ends of the central flat portion of the gate electrode and the inclined sidewalls. A gate oxide film 6 which is a second insulating layer is formed on the surfaces of gate electrode 2 and first insulting layers 5, 5. A semiconductor layer 7 of first type conductivity, for example, p type is formed on the surface of gate oxide film 6. A P type channel region 7a is formed on the p type semiconductor layer at the position opposing the central flat portion. Source/drain regions 7b, 7c of second type conductivity, for example, n type is formed at the positions of p type semiconductor layer 2 having channel region 7a therebetween. A description of a manufacturing process of a TFT having the above-states structure follows. Referring to Fig. 2, interlayer insulating film 1 is formed on the bulk transistor (not shown). A polysilicon layer 2 is deposited as thick as 200nm on interlayer insulating film 1 by means of thermal chemical vapor deposition. Referring to Fig. 3, a nitride film 3 having a thickness of 200nm is formed on polysilicon layer 2 by thermal CVD. Referring to Fig. 4, a resist film is applied onto nitride film 3. Thereafter, the resist film is patterned by a photolithography technique, to form a resist film 4 having a prescribed form. Referring to Fig. 5, using resist film 4 as mask, nitride film 3 is subjected to anisotropic reactive etching to take a prescribed form. Referring to Fig. 6, after removal of resist film 4, first insulating layers 5, 5 are formed on the upper surface of polysilicon layer 2 by thermal oxidation in accordance with LOCOS process. Referring to Fig. 6, gate electrode 2 having the central flat portion and the inclined sidewalls descending rightward and leftward from the right and left ends of the central flat portion is thus formed. Referring to Fig. 7, after removal of nitride film 3, a gate oxide film 6 having a thickness of 50nm is formed on the surfaces of gate electrode 2 and first insulating layers 5, 5 by means of thermal chemical vapor deposition. Referring to Fig. 8, a semiconductor layer, a polysilicon layer 7 is deposited as thick as 50nm on gate oxide film 6 by thermal CVD. Referring to Fig. 9, a resist film 4 having a prescribed form is formed on the surface of semiconductor layer 7 at the position opposite to the central flat portion of gate electrode 2. Using resist film 4 as mask, an impurity of second type conductivity, for example, n type is introduced at a prescribed position of semiconductor layer 7. Referring to Fig. 10, channel region 7a of p type which is the first type conductivity is formed in semiconductor layer 7 at the position opposite to the central flat portion of gate electrode 2. Source/drain regions 7b, 7c of n type which is the second type conductivity are formed at the positions having channel region 7a therebetween. Thus, TFT in accordance with the present embodiment is completed. The bent angle is formed to be beyond 90° in the vicinities of the boundaries of channel region 7a and source/drain regions 7b, 7c. Semiconductor layer 7 is therefore formed with no orthogonal bent produced. This allows the concentration of electric fields in channel region 7a to be suppressed. A description of a structure of a TFT in accordance with a second embodiment in accordance with the present invention follows in conjunction with the accompanying drawings. Referring to Fig. 11, the structure of the TFT in accordance with the second embodiment has a substantially identical structure to the TFT in the aforementioned first embodiment. The gate electrode 2 of the TFT described above has a structure isolated from first insulating layers 5, 5. The TFT in accordance with the second embodiment has polysilicon layer 2 for forming gate electrode 2 deposited thicker than usual. Thus, gate electrode 2 exists on the entire surface of interlayer insulating film 1. Now, a manufacturing process of the TFT in accordance with the second embodiment will be described in conjunction with Figs. 12 to 14. Referring to Fig. 12, an interlayer insulating film 1 is formed on a bulk transistor (not shown). A polysilicon layer 2 for forming a gate electrode is formed by thermal CVD on the interlayer insulating film as thick as 400nm which is thicker than the case of the first embodiment. Thereafter, going through the same manufacturing process as the TFT in accordance with the first embodiment, the TFT in accordance with the second embodiment is formed. Also in this structure, as is the case with the TFT in accordance with the first embodiment described above, the bent angle is formed to be beyond 90° in the vicinities of the boundaries of channel region 7a and source/drain regions 7b, 7c, and, therefore, an orthogonal bent is not formed in semiconductor layer 7. This allows the concentration of electric fields to be suppressed in channel region 7a. With gate electrode 2 existing on the entire surface of interlayer insulating film 1, making a contact with gate electrode 2 is possible at an arbitrary position on the substrate. A structure of a TFT in accordance with a third embodiment of the present invention follows. Referring to Fig. 15, the TFT in accordance with a third embodiment is formed between the gate electrode 10a and gate electrode 10b of the bulk transistor with sidewalls formed on a silicon substrate 20. First insulating layers 11, 11 are formed on gate electrode 10a and gate electrode 10b. The gate electrode 13 of the TFT having an approximately V shape is formed on first insulating layers 11, 11. A gate oxide film 14 is formed on the surfaces of first insulating layers 11, 11 and gate electrode 13. A channel layer 15 is formed on the upper surface of gate oxide film 14. A manufacturing process of the TFT in accordance with the third embodiment will be described in conjunction with Figs. 16 to 22. Referring to Fig. 16, formed on semiconductor substrate 20 are bulk transistors 10a and 10b. First insulating layer 11 is deposited as thick as 300nm on bulk transistors 10a and 10b by thermal chemical vapor deposition. Referring to Fig. 17, a resist film patterned into a prescribed form is formed on interlayer insulating film 11 by a photolithography technique. Referring to Fig. 18, first insulating layer 11 between bulk transistors 10a and 10b is removed away by anisotropic reactive ion etching, thereby forming an opening leading to semiconductor substrate 20. Referring to Fig. 19, resist film 12 is removed. Referring to Fig. 20, polysilicon layer 13 for gate electrode is deposited as thick as 300nm on interlayer insulating film 11 by thermal chemical vapor deposition. Thereafter, resist film 12 is applied onto polysilicon layer 13. Resist film 12 is then patterned into a prescribed form by a photolithography technique. Referring to Fig. 21, resist film 12 is removed. Thereafter, TFT gate electrode 13 is formed by anisotropic reactive ion etching. Referring to Fig. 22, gate oxide film 14 having a thickness of 50nm is formed on gate electrode 13 and first insulating layers 11, 11 by thermal CVD. Polysilicon layer 15 for forming the TFT semiconductor layer is deposited as thick as 50nm on gate oxide film 14 by thermal CVD. Thereafter, a resist film (not shown) is formed on the surface of polysilicon layer 15 opposite to gate electrode 13. Using the resist film as mask, an impurity of the second type conductivity is introduced at a prescribed position of polysilicon layer 15. Thus, polysilicon layer 15 is formed with no orthogonal bent formed. Channel region 15a of first type conductivity, for example, p type is formed at the position opposite to gate oxide film 13. Source/drain regions 15b, 15c of second type conductivity, for example, n type are formed at positions having channel region 15a therebetween. According to the above-stated structure, the bent angle in the vicinities of the boundaries of channel region 15a and source/drain regions 15b, 15c is formed to be beyond 90°. The concentration of electric fields can therefore be suppressed by the formation of the semiconductor layer with no orthogonal bent produced in the semiconductor layer. Now, a TFT in accordance with a fourth embodiment of the present invention will be described in conjunction with the accompanying drawings. Referring to Fig. 23, the TFT in accordance with the fourth embodiment is formed immediately above the gate electrode 10 of bulk transistor. A description of a manufacturing process of the TFT having this structure follows in conjunction with Figs. 24 to 29. Referring to Fig. 24, first insulating layer 11 is formed on the gate electrode 10 of the bulk transistor. Referring to Fig. 25, a semiconductor layer 15 formed of polysilicon is deposited on the surface of first insulating layer 11 by thermal CVD. Thereafter, a gate oxide film 14 is formed on the semiconductor layer 15. Gate electrode layer 13 is formed on gate oxide film 14. Referring to Fig. 26, a resist film 12 is applied onto gate electrode layer 13 and the structure is etched into a prescribed form by means of a photolithography technique. Referring to Fig. 27, using resist film 12 as mask, gate electrode layer 13 and gate oxide film 14 are subjected to reactive anisotropic ion etching into prescribed forms. Referring to Fig. 28, resist film 12 is removed away. Thereafter, using gate electrode 13 as mask, an impurity of the second type conductivity is introduced into semiconductor layer 15. Referring to Fig. 29, a p type channel region 15a is formed in semiconductor layer 15 which is the lower portion of the gate electrode 13 of the TFT. N source/drain regions 15b, 15c are formed in the right and left regions of semiconductor layer 15 having p channel region 15a therebetween. Thus, the TFT in accordance with the fourth embodiment is completed. As described above, the formation of the TFT immediately above bulk transistor 10 allows suppression of the concentration of electric fields at a bent, because the bent angle at each of the positions in the vicinities of the boundaries of channel region 15a and source/drain regions 15b, 15c is formed to be beyond 90° and an orthogonal bent is not formed in semiconductor layer 15. Now, a description of a TFT in accordance with the fifth embodiment of the present invention follows. Referring to Fig. 30, in the structure of the TFT in accordance with the fifth embodiment, a gate electrode 2 having a central flat portion and sidewalls formed broaden downwardly at a prescribed inclination with respect to both sides of the central flat portion is formed on an interlayer insulating film 1. A gate oxide film 6 is formed along the central flat portion and sidewalls of gate electrode 2. A semiconductor layer 7 is formed along the surface of gate oxide film 6. A channel region 7a of first type conductivity, for example, p type is formed at the position opposite to the central flat portion of gate electrode 2. Source/drain regions 7b, 7c of second type conductivity, for example, n type are formed at the positions having p type channel region 7a therebetween. A manufacturing process of the TFT having the above-stated structure will be described in conjunction with Figs. 31 to 36. Referring to Fig. 31, a polysilicon layer 2 to form a gate electrode is formed on interlayer insulating film 1. Referring to Fig. 32, a resist film having a central flat portion and sidewalls formed broaden downwardly at a prescribed inclination angle with respect to both sides of the central flat portion. Referring to Fig. 33, using resist film 4 as mask, polysilicon layer 2 is subjected to reactive anisotropic ion etching, thus, polysilicon layer 2 is formed to be gate electrode 2 corresponding to the form of resist film 4 and having a central flat portion and sidewalls formed broaden downwardly having a prescribed inclination with respect to both sides of the central flat portion. Referring to Fig. 34, resist film 4 is removed away. Thereafter, a gate oxide film 6 is formed on gate electrode 2. Semiconductor layer 7 is formed on gate oxide film 6. Referring to Fig. 35, resist film 4 having a prescribed form is formed in the area opposite to the surface of the semiconductor layer 7 of gate electrode 2. Using resist film 4 as mask, an impurity of second type conductivity, for example, n type is introduced into semiconductor layer 7. Thus, channel region 7a of the first type conductivity, for example, p type is formed in the region opposite to the gate electrode 2 of semiconductor layer 7. Source/drain regions 7b, 7c of the second type conductivity, for example, n type are formed at the positions of semiconductor layer 7 having channel region 7a therebetween. Thus, the TFT in accordance with the fifth embodiment is completed. In the TFT having the above-stated structure, as the bent angle in the vicinities of the boundaries of channel region 7a and source/drain regions 7b, 7c is formed to be beyond 90°, no orthogonal bent is formed in semiconductor layer 7, and, therefore, the concentration of electric fields in the orthogonal bent of the conventional semiconductor layer can be suppressed. Referring to Fig. 37, a cross section of the TFT in accordance with the third embodiment used for an SRAM memory cell is shown. As in the foregoing, according to the embodiments of the present invention, the semiconductor layer formed in the thin film transistor is formed in such a manner that no orthogonal bent is produced. Thus, the concentration of electric fields in the orthogonal bent of the semiconductor layer according to the conventional technique can be prevented. Furthermore, hot carriers created by the concentration of electric fields can be suppressed from getting into the insulating film. Consequently, a stable operation of the semiconductor device is secured, and a highly reliable TFT can be provided.

Claims: A semiconductor device comprising: a semiconductor substrate (20); semiconductor means provided at a prescribed distance apart from each other on said semiconductor substrate (20), said semiconductor means comprising gate electrodes (10a, 10b) formed on said semiconductor substrate (20); a further gate electrode (13) formed between said gate electrodes (10a, 10b) of said semiconductor means with an insulating layer (11) between said further gate electrode (13) and each of said gate electrodes (10a, 10b), respectively, said further gate electrode (13) having a V-shape; a gate insulating layer (14) on the upper surfaces of said further gate electrode (13) and said insulating layer (11); and a semiconductor layer (15) on said gate insulating layer (14), said semiconductor layer (15) having a channel region of a first conductivity type (15a) opposing said further gate electrode (13) and source/drain regions (15b, 15c) of a second conductivity type at the respective ends of said semiconductor layer (15); said channel region of said semiconductor layer (15) essentially following said V-shape of said further gate electrode (13). A semiconductor device according to claim 1, wherein said gate insulating layer (14) is in contact with the surface of said semiconductor layer (15) in its entire region; and in contact with said further gate electrode (13) in at least that region of said further gate electrode (13) which opposes said channel region (15a). A semiconductor device according to claim 1, wherein said further gate electrode (13), said semiconductor layer (15), and said gate insulating layer (14) form a MOS field effect transistor. A semiconductor device according to claim 1, wherein said further gate electrode (13) is a polysilicon layer doped with an impurity. A semiconductor device according to claim 1, wherein said semiconductor layer (15) is a polysilicon layer doped with an impurity. A semiconductor device according to claim 1, wherein said gate insulating layer (14) is a silicon oxide film. A method of manufacturing a semiconductor device, comprising the steps of: forming gate electrodes (10a, 10b) on the surface of a semiconductor substrate (20); forming an insulating layer (11) on the surface of said gate electrodes (10a, 10b); forming a resist film (12) at a prescribed position on the surface of said insulating layer (11) and etching said insulating layer (11) using said resist film (12) as a mask; removing said resist film (12); forming a conductor layer (13) on the surface of said insulating layer (11); forming a further resist film (12) on the surface of said conductor layer (13); performing thermal oxidation using said further resist film (12) as a mask and forming a further gate electrode (13) having a V-shape; forming a gate insulating layer (14) on the surfaces of said further gate electrode (13) and said insulating layer (11); forming a semiconductor layer (15) of a first conductivity type on the surface of said gate insulating layer (14); forming a mask on the surface of said semiconductor layer (15) at a position opposing said further gate electrode (13), and forming a channel region (15a) of said first conductivity type at said position opposing said further gate electrode (13) of said semiconductor layer (15) of said first conductivity type by implanting an impurity of a second conductivity type, and source/drain regions (15b, 15c) of said second conductivity type at positions with said channel region (15a) therebetween. A method of manufacturing a semiconductor device according to claim 7, wherein said step of forming said conductor layer (13) is conducted by depositing polysilicon by thermal chemical vapor deposition. A method of manufacturing a semiconductor device according to claim 7, wherein said step of forming said insulating layer (11) forms an oxide film by thermal oxidation in accordance with a LOCOS process. A method of manufacturing a semiconductor device according to claim 7, wherein said step of forming said semiconductor layer (15) is conducted by depositing polysilicon by thermal chemical vapor deposition. A method of manufacturing a semiconductor device according to claim 7, wherein said step of forming said gate insulating layer (14) forms an oxide film by thermal oxidation.

Inventors: KOHNO YOSHIO, KURIYAMA HIROTADA, OKADA TERUHIKO, KOHNO, YOSHIO, KURIYAMA, HIROTADA, OKADA, TERUHIKO, Kohno, Yoshio, c/o Mitsubishi Denki K.K. LSI, Kuriyama, Hirotada, c/o Mitsubishi Denki K.K. LSI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, H01L21