Method of manufacture of a thin-film transistor

Abstract: A thin-film transistor comprising an insulating substrate (1); an opaque metal gate electrode (2) disposed on a portion of said insulating substrate (1); a gate insulating layer (3) disposed on said insulating substrate (1) including said gate electrode (2); an a-Si semiconductor film (4) disposed on the portion of said gate insulating layer (3), said a-Si semiconductor film (4) having been formed to attain self-alignment with respect to said gate electrode (2); a-Si contact films (6, 7) constituting source and drain regions, respectively, with a gap therebetween disposed on said a-Si semiconductor film (4), the outer end of each of said contact films (6, 7) being formed to attain self-alignment with respect to said gate electrode (2); source and drain electrodes (8, 9), respectively, disposed on said source and drain regions (6, 7), the thickness of each of said a-Si semiconductor film (4) and a-Si contact films (6, 7) being 100 Å or more and the total amount of thicknesses thereof being 1,000 Å or less. A protective insulating film (5) can further be positioned between the a-Si semiconductor film (4) and each of said contact films (6,7).

Description: BACKGROUND OF THE INVENTION 1. Field of the invention: This invention relates to a method of manufacture of a thin-film transistor that uses a semiconductor film of amorphous silicon (a-Si). 2. Description of the prior art: In recent years, there has been a good potential market for active-matrix display devices, as large-scale display devices that use liquid crystals, etc., in which thin-film transistors made with the use of a semiconductor film of a-Si are formed in a matrix on an insulating substrate such as glass, etc. Figure 16 shows a conventional thin-film transistor Tr using a semiconductor film of a-Si, wherein an active layer 4a disposed on a gate insulating film 3a is much broader than a gate electrode 2a positioned below the gate insulating film 3a , and moreover either the active layer 4a or an n + -a-Si semiconductor film forming both source and drain regions 6a and 7a is made without consideration of its thickness. In the case where the thin-film transistor Tr constitutes a display device with liquid crystals, a back light is placed at the insulating substrate (glass plate 1a) side. When the thin-film transistor Tr is off (i.e., negative voltage is applied to the gate electrode 2a), carrier generation occurs, due to light from the back light, in the portion of the active layer 4a that is not in alignment with the gate electrode 2a, resulting in a decrease in the resistance of the thin-film transistor Tr at the time when the thin-film transistor Tr is off. Thus, the thin-film transistor Tr does not function as a switching device. In order to solve this problem, the thickness of the active layer 4a of an a-Si semiconductor film can be thinned. For example, when the thickness thereof is set to be 10nm (100 Å) or less, the influence of the back light on the active layer 4a is not observed. However, if the thickness of the active layer 4a is made smaller beyond a fixed value, the resistance of the thin-film transistor Tr at the time when the thin-film transistor Tr is on becomes high. For example, "Patent Abstracts of Japan" 10 No. 343 (E-456) [2349] November 19, 1986 & JP-A-61 145 869 discloses a film film transistor similar to that shown in Figure 16. The active layer is stated to be less than 100nm in thickness, to minimise the effect of light on the OFF-resistance. The occurrence of carrier generation in the active layer 4a can be also prevented by the formation of an optical shield in the thin-film transistor Tr, which causes an increase in the number of production steps, making yields low and increasing costs. Moreover, the thin-film transistor Tr can be designed such that the active layer 4a is positioned at a portion of the gate insulating layer 3a corresponding to the gate electrode 2a and is formed into the same shape and size as the gate electrode 2a by a common mask-alignment technique. However, according to such a technique, alignment errors arise unavoidably and side-etchings must be carried out, which makes the size of the thin-film transistor large, resulting in a decrease in the ratio of the surface area of the picture-element electrode to the surface area of the liquid-crystal display panel and an increase in the load capacity between the gate electrode and the drain electrode. Accordingly, the enlargement of the surface area of the display device cannot be attained. DE-A-3 604 368 discloses a thin-film transistor comprising: an insulating substrate; an opaque gate electrode disposed on a portion of a front surface of said insulating substrate; a gate insulating layer disposed on the entire front surface of said substrate including the gate electrode; an a-Si layer disposed on a portion of the gate insulating layer and in alignment with said gate electrode; a-Si contact films constituting source and drain regions disposed on said a-Si layer, said source and drain regions having a gap therebetween, and the outer ends of each of said contact films being aligned with said gate electrode; and source and drain electrodes disposed respectively on said source and drain regions. However, in order to make the device disclosed in DE-A-3 604 368, the gate insulating layer is applied over the entire substrate, and is subsequently etched to the required size. Furthermore, it is necessary to apply the gate insulating layer twice in order to make the transistor disclosed in DE-A-3 604 368: once before the gate is etched to size, and once after the gate has been etched. EP-A-0 139 585 discloses a method for fabricating a thin-film transistor. The gate of the transistor is used as a mask for the active layer. The active layer is formed by exposing a resist to light from the back face of the substrate, and then etching using the remaining resist as a mask. This is a more complex method than the present invention, in which the a-Si active layer and the n + a-Si layer are etched in the same step. SUMMARY OF THE INVENTION According to the present invention there is provided a method of manufacturing a thin-film transistor comprising: the steps of disposing an opaque metallic gate electrode on a portion of a front surface of an insulating substrate; disposing a gate insulating layer on the entire front surface of said substrate including the gate electrode; disposing an a-Si layer on the gate insulating layer; disposing an n+ a-Si layer on the a-Si layer, said n+ a-Si layer being in contact with said a-Si layer; disposing a positive photoresist layer on the n+ a-Si layer; exposing a back surface of the insulating substrate to light, so as to remove the photoresist layer except for a portion of the photoresist layer which corresponds to the opaque gate electrode; etching the n+ a-Si layer and the a-Si layer using the portion of the resist layer as a mask, so as to form an n+ a-Si layer and an active a-Si layer which are aligned with the gate electrode; removing the portion of the resist layer; disposing a metallic layer over an entire surface of the wafer on the n+ a-Si layer side; disposing at least two photoresists on the metal layer except for a portion of the metal layer corresponding to a centre area of the n+ a-Si layer; etching the metal layer and the n+ a-Si layer using the photoresists as masks, so as to produce a source region, a drain region, a source electrode and a drain electrode; and removing the photoresists; wherein the thickness of the a-Si layer, of the source regions and of the drain region are each 10nm (100A) or more; and wherein the total thickness of the a-Si layer and of the source region, or of the a-Si layer and of the drain region, is 100nm (1000A) or less. In a preferred embodiment, the method further comprises the step of forming a protective insulating layer on the a-Si layer before the n+ a-Si layer is formed. Thus, the invention described herein makes possible the objectives of (1) providing a thin-film transistor that prevents the occurrence of carrier in the active layer even when negative voltage is applied to the gate electrode (i.e., the thin-film transistor is off), thereby maintaining the off-resistance at a fixed level, and moreover that maintains the on-resistance at a low level when the thin-film transistor is on; (2) providing a thin-film transistor that is simply produced at a low cost; and (3) providing a miniaturized thin-film transistor that attains the enlargement of the surface area of a display device using the thin-film transistor. BRIEF DESCRIPTION OF THE DRAWINGS This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a side sectional view showing a thin-film transistor produced by a method of this invention. Figures 2 to 7 are diagrams showing a production process of the thin-film transistor shown in Figure 1 . Figure 8 is of characteristic curves showing the relationships between the gate-drain voltage V GD and the current Id flowing to the drain region with regard to the thin-film transistor shown in Figure 1 and a conventional thin-film transistor. Figure 9 is a side sectional view showing another thin-film transistor produced by a method of this invention. Figures 10 to 14 are diagrams showing a production process of the thin-film transistor shown in Figure 9 . Figure 15 is of characteristic curves showing the relationships between the gate-drain voltage V GD and the current Id flowing to the drain region with regard to the thin-film transistor shown in Figure 9 and a conventional thin-film transistor. Figure 16 is a side sectional view showing a conventional thin-film transistor. DESCRIPTION OF PREFERRED EMBODIMENTS This invention provides a method of manufacture of a thin-film transistor that comprises an active layer and source and drain regions, which are formed in a manner to attain self-alignment with respect to a gate electrode made of opaque metals. Example 1 Figure 1 shows a thin-film transistor which comprises an insulating substrate 1 made of a glass plate having a thickness of about 1 mm, a gate electrode 2 of opaque metals such as Ta, Cr, Mo, Al or W , a gate insulating layer 3 , an active layer 4 made of an a-Si film, a protective insulating film 5 , source and drain regions 6 and 7 made of a phosphorus-doped n + -a-Si film that attains an ohmic contact with source and drain electrode 8 and 9 made of a metal film. This thin-film transistor is produced by a method of this invention as follows: On the insulating substrate 1 of a glass plate, the gate electrode 2 with a desired pattern made of metals such as Ta , etc., is disposed as shown in Figure 2 . Then, as shown in Figure 3 , on the entire surface of the insulating substrate 1 including the gate electrode 2 , the gate insulating layer 3 and the a-Si semiconductor layer 4A having a thickness of 20nm (200 Å) are successively disposed by plasma assisted chemical vapor deposition. Then, the protective insulating film 5 that has a width smaller than that of the gate electrode 2 is disposed on the portion of the said a-Si semiconductor layer 4A corresponding to the gate electrode 2 . Then, as shown in Figure 4 , on the a-Si semiconductor layer 4A including the protective insulating film 5 , the phosphorus-doped n + -a-Si layer 6A having a thickness of 200nm (2,000 Å) and a positive photoresist layer 10A are successively disposed. Thereafter, the wafer is exposed to light from the back face of the insulating substrate 1 . The gate electrode 2 that is opaque functions as a photomask, and the portions of the positive photoresist layer 10A except for the portion of the positive photoresist layer 10A corresponding to the gate electrode 2 are removed, as shown in Figure 5 , resulting in a resist 10 that is in alignment with the gate electrode 2 . When the total amount of thicknesses of the n + -a-Si layer 6A and the a-Si semiconductor layer 4A is 100nm (1,000 Å) or less, light effectively passes through the positive photoresist layer 10A . Then, the n + -a-Si layer 6A and the a-Si semiconductor layer 4A are etched with the use of the resist 10 as a mask, as shown in Figure 6 , resulting in an n + -a-Si layer 6B and the active layer 4 that are in alignment with the gate electrode 2 . The resist 10 is then removed. As shown in Figure 7 , on the entire surface of the wafer at the n + -a-Si layer side, a metal layer 8A is then disposed and photoresists 11 are disposed on the metal layer 8A except for the portion of the metal layer 8A corresponding to the center area of the n + -a-Si layer 6B . Then, the metal layer 8A and the n + -a-Si layer 6B are etched with the use of the photoresists 11 as a mask, as shown in Figure 1 , resulting in the source and drain regions 6 and 7 and the source and drain electrodes 8 and 9 . The photoresists 11 are then removed, resulting in a desired thin-film transistor. The protective insulating film 5 functions to prevent the occurrence of interface charges at the interface between the active layer 4 and the protective insulating film 5 , thereby attaining an improvement of the transistor characteristics of the thin-film transistor in the off-state. Moreover, the protective insulating film 5 prevents the active layer 4 from being etched when the metal layer 8A and the n + -a-Si layer 6B are etched. The thickness of the active layer 4 is set to be in the range of 20 to 30nm (200 to 300 Å). The width of the protective insulating layer 5 disposed on the active layer 4 is smaller than that of the gate electrode 2 . The protective insulating layer 5 is made of Si 3 N 4 or Al 2 O 3 and has a thickness of 100nm-1 µm (1,000 Å-1 µm), preferably 200nm (2,000 Å). The n + -a-Si contact film constituting the source and drain regions 6 and 7 has a thickness of 10-50nm (100-500 Å), preferably 20-30nm (200-300 Å). When the thickness of these layers and films are set to be the above-mentioned experimental values, the resistance of the thin-film transistor when the said transistor is on is maintained at a fixed level and moreover, the positive photoresist 10A is effectively exposed to light from the insulating substrate side so as to form the active layer having a desired pattern. Despite the above-mentioned experimental thickness values, in fact, when the thickness of each of the active layer 4 and the n + -a-Si film 6A is 10nm (100 Å) or more and the total amount of thickness of the active layer 4 and the n + -a-Si film 6A is 100nm (1,000 Å) or less, a desired thin-film transistor is obtainable. The thin-film transistor is incorporated with liquid crystals to form a display device, wherein no carrier generation occurs in the active layer 4 even when the thin-film transistor is irradiated with light from the insulating substrate side, and accordingly the resistance of the thin-film transistor in the off-state does not decrease. This is indicated by the following experiments: A display device, which was constructed by the combination of a thin-film transistor (having the active layer 4 with a thickness of 20nm (200 Å) and a width of 10 µm and a length of 12 µm) and liquid crystals, was used. When the voltage V SD between the source electrode and the drain electrode was 10 volts and the thin-film transistor was irradiated with light of 10 4 luxes, the relationship between the gate-drain voltage V GD and the current Id flowing to the drain region 6 is indicated by the Id-V GD characteristic curve C 1 shown in Figure 8 . The Id-V GD characteristic curve C 2 indicates the relationship therebetween in the case where the thin-film transistor was not irradiated with light from the insulating substrate side. The Id-V GD characteristic curve C 3 indicates the relationship therebetween in the case where a conventional thin-film transistor was irradiated with light of 10 4 luxes from the insulating substrate side. Figure 8 indicates that the thin-film transistor of this example attained an improvement of the off-characteristics in the V GD in the range of -20 to -3 volts. Moreover, since the thickness of the active layer 4 is over a fixed value, the resistance of the thin-film transistor of this example at the time when the transistor is on does not rise over a fixed level. The formation of the active layer 4 is carried out using the resist 10 as a mask, which has been aligned with the gate electrode 2 in cooperation with light from the insulating substrate side and the positive photoresist 10A , so that the production of the thin-film transistor can be simplified and the size thereof can be minimized. Example 2 This example provides a thin-film transistor, as shown in Figure 9 , having the same structure as that of Example 1 , except that there is no protective insulating film, wherein the source and drain regions 6 and 7 are directly disposed on the active layer 4 . This thin-film transistor is produced by a method of this invention as follows: On the insulating substrate 1 of a glass plate, the gate electrode 2 with a desired pattern made of Ta is disposed as shown in Figure 10 . Then, as shown in Figure 11, on the entire surface of the insulating substrate 1 including the gate electrode 2 , the gate insulating layer 3 and the a-Si semiconductor layer 4A having a thickness of 20nm (200 Å) are successively disposed by plasma assisted chemical vapor deposition. Then, the phosphorus-doped n + -a-Si layer 6A having a thickness of 20-30nm (200-300 Å) and the positive photoresist layer 10A are successively disposed on the a-Si semiconductor layer 4A . Thereafter, the wafer is exposed to light from the back face of the insulating substrate 1 . The gate electrode 2 that is opaque functions as a photomask, and the portions of the positive photoresist layer 10A except for the portion of the positive photoresist layer 10A corresponding to the gate electrode 2 are removed, as shown in Figure 12 , resulting in a resist 10 that is in alignment with the gate electrode 2 . When the total amount of thicknesses of the n + -a-Si layer 6A and the a-Si semiconductor layer 4A is 100nm (1,000 Å) or less, light effectively passes through the positive photoresist layer 10A . Then, the n + -a-Si layer 6A and the a-Si semiconductor layer 4A are etched with the use of the resist 10 as a mask, as shown in Figure 13 , resulting in an n + -a-Si layer 6B and the active layer 4 that are in alignment with the gate electrode 2 . The resist 10 is then removed. As shown in Figure 14 , on the entire surface of the wafer at the n + -a-Si layer side, a metal layer 8A is then disposed and photoresists 11 are disposed on the metal layer 8A except for the portion of the metal layer 8A corresponding to the center area of the n + -a-Si layer 6B . Then, the metal layer 8A and the n + -a-Si layer 6B are etched with the use of the photoresists 11 as a mask, as shown in Figure 9 , resulting in the source and drain regions 6 and 7 and the source and drain electrodes 8 and 9 . The photoresists 11 are then removed, resulting in a desired thin-film transistor. Since the protective insulating film mentioned in Example 1 is not used here, etching of the n + -a-Si layer 6B is, of course, carried out under conditions where the active layer 4 is not etched but the n + -a-Si layer 6B is etched. This is, for example, attained by the regulation of the etching rate. The thickness of the active layer 4 is set to be in the range of 20-30nm (200 to 300 Å). The n + -a-Si contact film constituting the source and drain regions 6 and 7 has a thickness of 10-50nm (100-500 Å), preferably 20-30nm (200-300 Å). When the thickness of these layers and films are set to be the above-mentioned experimental values, the resistance of the thin-film transistor when the said transistor is on is maintained at a fixed level and moreover, the positive photoresist 10A is effectively exposed to light from the insulating substrate side so as to form the active layer having a desired pattern. Despite the above-mentioned experimental thickness values, in fact, when the thickness of each of the active layer 4 and the n + -a-Si film 6A is 10 nm (100 Å) or more and the total amount of thickness of the active layer 4 and the n + -a-Si film 6A is 100nm (1,000 Å) or less, a desired thin-film transistor is obtainable. With a display device using the above-mentioned thin-film transistor and liquid crystals, the same experiments as in Example 1 were carried out and the results are shown in Figure 15 , wherein the Id-V GD characteristic curves C 11 indicates the relationship between the gate-drain voltage V GD and the current Id flowing to the drain region 6 in the case where the thin-film transistor of this example is irradiated with light under the same conditions as in Example 1 , the Id-V GD characteristic curve C 22 indicates the relationship therebetween in the case where the thin-film transistor of this example is not irradiated with light, and the Id-V GD characteristic curve C 33 is the same as the curve C 3 shown in Figure 8 in Example 1 . It Can be seen from Figure 15 that the thin-film transistor of this example attained an improvement of the off-characteristics in the V GD in the range of -20 to -3 volts. Moreover, the thickness of the active layer 4 is likewise over a fixed value, the resistance of the thin-film transistor of this example in the on-state does not rise over a fixed level. The active layer 4 and the n + -a-Si layer constituting the source and drain regions 6 and 7 are formed by self-alignment with respect to the gate electrode 2 in cooperation with the light from the insulating substrate side and the positive photoresist 10A , and thus the production of the thin-film transistor can be simplified and the size thereof can be minimized. It is understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope of this invention as defined by the appended claims.

Claims: A method of manufacture of a thin-film transistor comprising the steps of: disposing an opaque metallic gate electrode (2) on a portion of a front surface of an insulating substrate (1); disposing a gate insulating layer (3) on the entire front surface of said substrate including the gate electrode (2); disposing an a-Si layer (4A) on the gate insulating layer (3); disposing an n + a-Si layer (6A) on the a-Si layer (4A), said n+ a-Si layer (6A) being in contact with said a-Si layer (4A); disposing a positive photoresist layer (10A) on the n + a-Si layer (6A); exposing a back surface of the insulating substrate (1) to light, so as to remove the photoresist layer (10A) except for a portion of the photoresist layer (10) which corresponds to the opaque gate electrode (2); etching the n + a-Si layer (6) and the a-Si layer (4A) using the portion of the resist layer (10) as a mask, so as to form an n + a-Si layer (6B) and an active a-Si layer (4) which are aligned with the gate electrode (2); removing the portion of the resist layer (10); disposing a metallic layer (8A) over an entire surface of the wafer on the n + a-Si layer side; disposing at least two photoresists (11) on the metal layer (8A) except for a portion of the metal layer (8A) corresponding to a centre area of the n + a-Si layer (6B) etching the metal layer (8A) and the n + a-Si layer (6B) using the photoresists (11) as masks, so as to produce a source region (6), a drain region (7), a source electrode (8) and a drain electrode (9); and removing the photoresists (11); wherein the thickness of the a-Si layer (4), of the source regions (6) and of the drain region (7) are each 10nm (100A) or more; and wherein the total thickness of the a-Si layer (4) and of the source region (6), or of the a-Si layer (4) and of the drain region (7), is 100nm (1000A) or less. A method of manufacture of a thin-film transistor as claimed in claim 1 and further comprising the step of forming a protective insulating layer (5) on the a-Si layer (4A) before the n + a-Si layer (6A) is formed.

Inventors: KODEN MITSUHIRO, KODEN, MITSUHIRO
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1987
Main classification: H01L29
IPC codes: H01L29