Semiconductor device and manufacturing method thereof

Abstract: A semiconductor device comprises a semiconductor substrate (20), a first gate electrode (10a) of a bulk transistor and a second gate electrode (10b) of a bulk transistor horizontally spaced apart from each other on a surface of said semiconductor substrate with a gate insulating film (11) between said first and second gate electrodes and said semiconductor substrate, a third gate electrode (13) of a thin film transistor having a central portion between said first and second gate electrodes and connected to a portion of said semiconductor substrate between said first and second gate electrodes, a first insulating layer separating said third gate electrode from said first and second gate electrodes, a second insulating layer (14) formed on the surfaces of said third gate electrode and said first insulating layer, and a semiconductor layer (15) of a first conductivity type formed on the surface of said second insulating layer, said semiconductor layer (15a) of the first conductivity type having a channel region of the first conductivity type at a position vertically aligned and above said central portion of said third gate electrode and source/drain regions of a second conductivity type having the channel region therebetween.

Description: The present invention relates generally to semiconductor devices and more specifically, to a structure of a thin film transistor. In recent years, for meeting a demand for shrinking the size of a semiconductor device, development on 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 gate electrode 2. A semiconductor layer 7 is formed along gate oxide film 6. Formed at a position of semiconductor layer 7 opposite to gate electrode 2 is a channel region 7a. Source/drain regions 7b, 7c are formed at positions having 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 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 polysilicon layer 2 by a photolithography technique. Referring to Fig. 41, using resist film 4 as mask, polysilicon layer 2 is subjected to anisotropic reactive ion etching to form gate electrode 2. Referring to Fig. 42, 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 gate electrode 2 and 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 oxide film 6 by thermal chemical vapor deposition. Referring to Fig. 44, a resist film 8 is formed on the surface of polysilicon layer 7. Resist film 8 is patterned into a prescribed form by means of a photolithography technique or the like. Using 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 polysilicon layer 7. Referring to Fig. 45, resist film 8 is etched away. Thus formed is the TFT having channel region 7a in the region of polysilicon layer 7 opposite to the gate electrode 2 and source/drain regions 7b, 7c at the positions having channel region 7a therebetween. The TFT having the structure described above, however, suffers from the following disadvantage. Referring to Fig. 45, gate electrode 2 is formed by subjecting the polysilicon layer to anisotropic reactive iron etching. Gate electrode 2 therefore takes a substantially rectangular form. Thus, with gate electrode 2 having a rectangular form, substantially orthogonal edges 9, 9 are produced in oxide film 6 and polysilicon layer 7 formed along 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 gate oxide film 6, thus deteriorating the insulation of gate oxide film 6. It is an object of the present invention to provide a semiconductor device having a structure allowing formation of a gate oxide film and a channel layer with no orthogonal edge portions created in the gate oxide film and a semiconductor layer covering a gate electrode, in a thin film transistor (TFT). This object is solved according to the appended claim 1. An advantageous modification is set out in the appended claim 2. According to the semiconductor device, a channel layer of orthogonal bent is not formed in the semiconductor layer formed on the gate electrode with the gate oxide film therebetween. This suppresses the concentration of electric fields as generated at the orthogonal bents of the conventional channel layer. Furthermore, the hot carriers can be prevented from being implanted into the gate insulating film in the semiconductor layer. Details of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. Fig. 1 is a cross sectional view showing a structure of a thin film transistor as a first explanatory example Figs. 2 to 10 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the first explanatory example; Fig. 11 is a cross sectional view showing a structure of a thin film transistor as a second explanatory example; Figs. 12 to 14 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the second explanatory example; Fig. 15 is a cross sectional view showing a structure of a thin film transistor in accordance with the present invention; Figs. 16 to 22 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the present invention; Fig. 23 is a cross sectional view showing a structure of a further thin film transistor in accordance with the present invention; Figs. 24 to 29 are cross sectional views showing a manufacturing process of the further thin film transistor in accordance with the present invention; Fig. 30 is a cross sectional view showing a structure of a thin film transistor as a third explanatory example; Figs. 31 to 36 are cross sectional views showing a manufacturing process of the thin film transistor in accordance with the third explanatory example; Fig. 37 is a cross sectional view showing a structure of a thin film transistor in accordance with 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. A description of thin film transistors (TFT) in accordance with the present invention and of explanatory examples of thin film transistors follows in conjunction with the accompanying drawings. A structure of a TFT as a first explanatory example will be described in conjunction with Fig. 1. The TFT according to the first explanatory example 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, the TFT in accordance with the present explanatory example 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 as a second explanatory example follows in conjunction with the accompanying drawings. Referring to Fig. 11, the structure of the TFT in accordance with the second explanatory example has a substantially identical structure to the TFT in the aforementioned first explanatory example. 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 explanatory example 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 explanatory example 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 explanatory example Thereafter, going through the same manufacturing process as the TFT in accordance with the first explanatory example, the TFT in accordance with the second explanatory example is formed. Also in this structure, as is the case with the TFT in accordance with the first explanatory example 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 the present invention follows. Referring to Fig. 15, the TFT in accordance with the present invention 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 present invention 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 further TFT in accordance with the present invention will be described in conjunction with the accompanying drawings. Referring to Fig. 23, the further TFT in accordance with the present invention 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 further TFT in accordance with the present invention 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 a third explanatory example follows. Referring to Fig. 30, in the structure of the TFT in accordance with the third explanatory example, 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 third explanatory example 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 present invention used for an SRAM memory cell is shown. As in the foregoing, according to 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. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims.

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 semiconductor layer (15) formed between said gate electrodes (10a, 10b) of said semiconductor means with an insulating layer (11) between said semiconductor layer (15) and each of said gate electrodes (10a, 10b), respectively, said semiconductor layer (15) having a V-shape; a gate insulating iayer (14) on the upper surfaces of said semiconductor layer (15) and said insulating layer (11); and a further gate electrode (13) on said gate insulating layer (14) essentially following said V-shape of said semiconductor layer (15), 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 semiconductor device us used for an SRAM memory cell.

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