Thin-film transistor with a protective layer and method of manufacturing the same

Abstract: Disclosed herein is a thin-film transistor manufactured by a method which comprises the steps of forming a gate electrode (12) on an insulating substrate (11), a gate insulating film (13) covering the gate electrode, and an i-type a-Si layer (14) on the gate insulating film, forming a blocking film (17) made of metal such as Cr or the like on a channel-forming region of the i-type a-Si layer, and forming an n-type a-Si layer (15) covering the i-type a-Si layer and the blocking film, forming a metal layer on the n-type a-Si layer, and etching a predetermined portion of the n-type a-Si layer and a predetermined portion of the metal layer, thereby forming a source electrode (16a) and a drain electrode (16b). That portion of the blocking film which is located below a gap between the source electrode and the drain electrode is removed from the i-type a-Si layer.

Description: The present invention relates to a thin-film transistor having thin semiconductor films formed on an insulating substrate, and also to a method of manufacturing the thin-film transistor. In particular, the present invention relates to a method of manufacturing a thin-film transistor, comprising a first step forming a gate electrode on an insulating substrate, a gate insulating film covering the gate electrode, and an i-type a-Si layer on the gate insulating film; a second step of forming a blocking film made of metal on a channel-forming region of said i-type a-Si layer; a third step of forming an n-type a-Si layer covering said i-type a-Si layer and said blocking film, forming a metal layer on the n-layer a-Si layer, and etching a predetermined portion of the metal layer, thereby forming a source electrode and a drain electrode and a portion of the metal layer disposed between the source and drain electrodes; and a fourth step of removing that portion of said blocking film which is located below a gap between said source electrode and said drain electrode. Thin-film transistors (TFTs), each comprising various thin films of a predetermined shape which are formed, one upon another, on an insulating substrate, are used as active elements in an active matrix liquid-crystal display. The active elements of the liquid-crystal display of this type are reverse stagger type TFTs. As is shown in Fig. 2, the reverse stagger type TFT comprises a gate electrode 2 formed on the insulating substrate made of glass or the like, a gate insulating film 3 made of SiN (silicon nitride) and covering the gate electrode 2, an i-type semiconductor layer 4 made of a-Si (amorphous silicon) and formed on the gate insulating film 3 and opposing the gate electrode 2, an n-type semiconductor layer 5 made of a-Si doped with an n-type impurity and formed on the i-type semiconductor layer 4, and a source electrode 6a and a drain electrode 6b, both formed on the n-type semiconductor layer 5. A part of the n-type semiconductor layer 5 has been etched away, thus exposing the channel-forming region of the i-type semiconductor layer 4 and which is located below the gap between the source electrode 6a and the drain electrode 6b. The thin-film transistor shown in Fig. 2 is manufactured in the following method. First, the gate electrode 2 is formed on the insulating substrate 1. Then, the gate-insulating film 3, the i-type semiconductor layer 4, and the n-type semiconductor layer 5 are formed, one upon another in the order mentioned, on the substrate 1 and the gate electrode 2. The source electrode 6a and the drain electrode 6b are formed, spaced apart from each other, on the n-type semiconductor layer 5. Thereafter, said part of the n-type semiconductor layer 5 has been etched away, thus exposing the channel-forming region of the i-type semiconductor layer 4 which is located below the gap between the electrodes 6a and 6b. The n-type semiconductor layer 5 should not contact the channel-forming region. If the layer 5 contacts the channel-forming region, the surface of the i-type semiconductor layer 4 will inevitably etched and damaged when said part of the layer 5 is etched away. To prevent such damage to the i-type semiconductor layer 4, a blocking film 7 is formed on the channel-forming region of the i-type semiconductor layer 4 as is shown in Fig. 5. Hitherto, the blocking film 7 is made of SiN. The blocking film is formed in the following way. After the i-type semiconductor layer 4 has been formed on the gate insulating film 3 covering the gate electrode 2, an SiN film is deposited on the i-type semiconductor layer 4 by means of plasma CVD method. Next, the SiN film is patterned, forming the blocking film 7 large enough to cover the entire channel-forming region. Then, the n-type semiconductor layer 5, the source electrode 6a, and the drain electrode 6b are formed, and that part of the layer 5 which is located on the blocking film 7 is etched away, in the method specified above. Since the blocking film 7, thus formed, covers the channel-forming region of the i-type semiconductor layer 5, it protects the channel-forming region from damages which may be done during the etching of the n-type semiconductor layer 5. The conventional method of manufacturing a thin-film transistor is disadvantageous, however, because the blocking film 7 is made of SiN. When the SiN film is patterned to form the blocking film 7, defects such as pinholes may be formed in the gate insulating film 4 which is located below the i-type semiconductor layer 5. The etchant used to pattern the SiN film flows through pinholes, if any in the i-type semiconductor layer 4, reaching the gate insulating film 3. The etchant inevitably etches the gate insulating film 3, forming detects, such as pinholes, also in the gate insulating film 3. Pinholes, if made in the gate insulating film 3, cause interlayer short-circuiting between the gate electrode 2, on the one hand, and the source electrode 6a and the drain electrode 6b, on the other. This is because the n-type semiconductor layer 5 and the electrodes 6a and 6b are formed after the blocking film 7 has been formed by patterning the SiN film. Interlayer short-circuiting may take places, not only between the gate electrode 2 and the electrodes 6a and 6b, but also between the gate line (not shown) formed integral with the gate electrode 2 and the data line (not shown) formed integral with the source electrode 6a and or the data line integral with the drain electrode 6b, or between the data lines integral with the electrodes 6a and 6b. Therefore, it is quite probable that the thin-film transistor can be manufactured by the conventional method, but at a low yield. The object of the present invention is to provide a method of manufacturing a thin-film transistor, in which the blocking film for protecting the channel-forming region against damages in order to provide the thin-film transistor at a sufficiently high yield. To achieve the object, a method of manufacturing a thin-film transistor of the above-mentioned type is characterised in that said second step includes steps of: oxidising the surface of said i-type a-si layer, thereby forming a silicon oxide film thereon; forming a metal layer on said silicon oxide film, etching the metal layer to form said blocking film through a part of the silicon oxide film; and removing an exposed portion of said silicon oxide film. The blocking film is made of metal which can be etched with an etchant which has so small an etching power that it cannot etch the gate insulating film. The etchant flows to the gate insulating film through pinholes, if any in the i-type a-Si thin film, while the blocking film is being patterned, it does not etch the gate insulating film, forming no defects in the gate insulating film. As a result, the gate electrode is not short-circuited to the source electrode or the drain electrode. Hence, the thin-film transistor can be manufactured at a high yield. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A to 1F are cross-sectional views, explaining the steps of a method according to an embodiment of the invention; and Fig. 2 is a cross-sectional view showing a conventional thin-film transistor. An embodiment of the invention, which is a method of manufacturing a thin-film transistor, will be described, with reference to Figs. 1A to 1F. In this method, no metal silicide is formed at the interface between an i-type semiconductor layer and a metal film for a blocking film, as the metal film is formed on the i-type semiconductor layer. [Step 1] As is shown in Fig. 1A, a gate electrode 32 is formed on an insulating substrate 31 made of glass or the like, in the same way as in the conventional method. A gate insulating film 33 made of SiN is formed on the substrate 31 and the gate electrode 32. An i-type semiconductor layer 34 made of a-Si is formed on the gate insulating film 33. The gate insulating film 33 and the i-type semiconductor layer 34 are sequentially formed by a plasma CVD apparatus. [Step 2] As is shown in Fig. 1B, the surface of the i-type semiconductor layer 34 is oxidized with oxygen plasma, thereby forming a silicon oxide (SiO 2 ) film 39 having a thickness of about 10 nm. [Step 3] Next, as is shown in Fig. 1C, a metall film 37 made of chrominum (Cr), aluminum (Aℓ), an aluminum alloy, or the like is formed on the silicon oxide film 39 by a sputtering apparatus. Since the metal film 27 is formed after the silicon oxide film 39 has been formed on the surface of the i-type semiconductor layer 34, no metal silicide layer is formed on the layer 34, unlike in the case where a metal layer is formed directly on the layer 34 made of amorphous silicon. [Step 4] As is shown in Fig. 1D, the metal film 37 is patterned by photolithography, thereby forming a blocking film 37a, which covers the channel-forming region of the i-type semiconductor layer 34. Thereafter, the silicon oxide film 39 is removed, except for that portion covered by the blocking film 37a and not exposed even after the patterning of the metal film 37. As a result, the entire surface of the i-type semiconductor layer 34, except that of the channel-forming region, is cleaned and exposed. The patterning of the metal film 37 is effected by means of wet etching, using an etchant which can not etch the gate insulating film 33 which is made of SiN. Hence, the gate insulating film 33 will not be damaged even if this etchant flows to the film 33 through pinholes made in the i-type semiconductor layer 34. The silicon oxide film 39 is etched with a weak etchant, such as a hydrofluoric acid [(NH 4 )F]-based etchant, which cannot readily etch the gate insulating film 33 made of SiN. Further, since the silicon oxide film 39 is so thin (about 10 nm thick) that it can be etched within a short time, which the etchant can hardly flow to the film 33 through pinholes made in the i-type semiconductor layer 34. Therefore, the gate insulating film 33 will scarcely be damaged with the etchant used in etching the silicon oxide film 39. [Step 5] Then, as is shown in Fig. 1E, an n-type semiconductor layer 35 made of a-Si doped with an n-type impurity is formed on the i-type semiconductor layer 34 and the blocking film 37a. Further, a source electrode 36a and a drain electrode 36b are formed on the n-type semiconductor layer 35. That portion of the n-type semiconductor layer 35 which is located on the blocking film 37a is etched away, thereby exposing the blocking film 37a. In Step 5, the n-type semiconductor layer 35 is formed by a plasma CVD apparatus, a metal layer for forming the source and drain electrodes is formed on the layer 35 by a sputtering apparatus, and the layer 35 and the layer 34 located beneath the layer 35 are patterned into an element region by means of photolithography. Thereafter, that portion of the metal layer which is located above the channel-forming region of the layer 34 is etched away, thus forming the source electrode 36a and the drain electrode 36b. That portion of the n-type semiconductor layer 35 which is exposed through the gap between the electrodes 36a and 36b is then etched away. Since the blocking film 37a is located beneath this portion of the n-type semiconductor layer 35, it protects the channel-forming region of the layer 34 from the etchant used in etching the layer 35. Hence, the channel-forming region is not damaged at all. [Step 6] Next, as is shown in Fig. 1F, that portion of the blocking film 37a, now exposed after the n-type semiconductor layer 35 divided into two parts, is removed, thereby exposing the channel-forming region of the i-type semiconductor layer 34. Then, the silicon oxide film 39 remaining on the channel-forming region is removed. As a result, a thin-film transistor is made. In Step 6, the etchant used to pattern the metal film 37 in Step 4 is utilized to remove that portion of the blocking film 37a which has been exposed through the gap between the two parts of the n-type semiconductor layer 35. Hence, the etchant does not damage the i-type semiconductor layer 34. Since the etchant can hardly etch the gate insulating film 33 made of SiN, it will not damage the film 33 even if it flows to the film 33 through pinholes made in the i-type semiconductor layer 34. Once the blocking film 37a has been removed, the source electrode 36a and the drain electrode 36b are no longer short-circuited; they are electrically isolated. The end portions of the film 37a remain intact as is shown in Fig. 3F since they are covered with the two separated parts of the n-type semiconductor layer 35. Nonetheless, the remaining portions of the film 37a impose no problems at all, though they are made of metal. They are electrically isolated from each other, because the center part of the film 37a has been removed. As has been indicated, a weak, hydrofluoric acid-based etchant is applied to remove the remaining silicon oxide film 39 in Step 6, as has been used in Step 4 to the silicon oxide film 39. Hence, in Step 6, too, the etchant can hardly damage the gate insulating film 33 made of SiN even if it flows to the film 33 through pinholes made in the i-type semiconductor layer 34. The blocking film 37a protecting the channel-forming region of the i-type semiconductor layer 34 is a metal film formed by patterning the metal film 37 with an etchant which cannot etch the gate insulating film 33 made of SiN. Thus, the film 33 is not damaged at all even if this etchant flows to the film 33 through pinholes formed in the i-type semiconductor layer 34. Further, since the center portion of the blocking film 37a is removed after the n-type semiconductor layer 35 has been divided into two parts, the source electrode 36a and the drain electrode 36b are electrically isolated. As as been described, the metal film 37 is formed after the surface of the i-type semiconductor layer 34 has been oxidized, forming the silicon oxide film 39 on the layer 34. Hence, no metal silicide film is formed on the i-type semiconductor layer 34 as the metal layer 37 is formed on the layer 34. Since the etchant used to etch the silicon oxide film 39 formed on the layer 34 can hardly etch silicon nitride (SiN), i.e., the material of the gate insulating film 33, it will scarcely damage the film 33 even if it flows to the film 33 through pinholes formed in the i-type semiconductor layer 34 while the film 39 is being removed. Similarly, since the etchant used to remove the silicon oxide film 39 from the channel-forming region of the layer 34 after the blocking film 37a has been removed can hardly etch SiN, this etchant will scarcely damage the gate insulating film 33 even if it flows to the film 33. Hence, the method described with reference to Figs. 1A to 1F is advantageous in that the blocking film 37a protecting the channel-forming region of the i-type semiconductor layer 34 is formed without damaging the gate insulating film 33, whereby no short-circuiting occurs between the gate electrode 32, on the one hand, and the source electrode 36a and the drain electrode 36b, on the other hand, thus enhancing the yield of the thin-film transistors. In addition, the method can provide a thin-film transistor having good characteristics, which has a good ohmic contact between the i-type semiconductor layer 34 and the n-type semiconductor layer 35 and which is free of a source-drain leakage. The surface of the i-type semiconductor layer 34 is oxidized with an oxidizing agent such as nitric acid, instead of oxygen plasma. Alternatively, it can be accomplished by ozone oxidation effected by applying ultraviolet rays. In this embodiment, the silicon oxide film 39 is removed from the channel-forming region of the i-type semiconductor layer 34 after the blocking film 37a has been removed. However, the silicon oxide film 39 need not be removed from the channel-forming region since it is an insulating film. The embodiment described above is a method of manufacturing a thin-film transistor which is to be used as an active element of an active matrix liquid-crystal display. Nevertheless, the present invention may be applied to the manufacture of a thin-film transistor which can be incorporated into various types of electronic circuits. If incorporated in a circuit, the transistor has its source and drain electrodes connected to conductive lines. More specifically, two contact holes are formed in the protective insulating film covering the source and drain electrodes, thus exposing portions of these electrodes, and two data lines extend through the contact holes and, hence, are connected to the source and drain electrodes, respectively, and the surface of each data line is oxidized at the same time the blocking film is oxidized.

Claims: A method of manufacturing a thin-film transistor, comprising a first step forming a gate electrode (32) on an insulating substrate (31), a gate insulating film (33) covering the gate electrode, and an i-type a-Si layer (34) on the gate insulating film; a second step of forming a blocking film (37a) made of metal on a channel-forming region of said i-type a-Si layer; a third step of forming an n-type a-Si layer (35) covering said i-type a-Si layer and said blocking film, forming a metal layer on the n-type a-Si layer, and etching a predetermined portion of the metal layer, thereby forming a source electrode (36a) and a drain electrode (36b) and a portion of the metal layer disposed between the source and drain electrodes; and a fourth step of removing that portion of said blocking film which is located below a gap between said source electrode and said drain electrode, characterized in that said second step includes steps of: oxidising the surface of said i-type a-Si layer, thereby forming a silicon oxide film (39) thereon; forming a metal layer on said silicon oxide film, etching the metal layer to form said blocking film (37a) through a part of the silicon oxide film; and removing an exposed portion of said silicon oxide film. The method according to claim 1, characterized in that in said second step, said blocking film is made of metal having an etching rate which is greatly different from that of said gate insulating film and also from that of said n-type a-Si layer.

Inventors: ISHII HIROMITSU, SASAKI KAZUHIRO, SASAKI MAKOTO, ISHII, HIROMITSU, SASAKI, KAZUHIRO, SASAKI, MAKOTO
Applicants: CASIO COMPUTER CO LTD, CASIO COMPUTER COMPANY LIMITED
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, H01L21