METHOD OF MANUFACTURING A THIN FILM TRANSISTOR

Abstract: A method of manufacturing a thin film transistor comprises the steps of forming a gate electrode (12) on one surface of a transparent substrate (10), forming on the substrate an insulating layer (14) and a semiconductor layer (16, 18) in the named order to cover the gate electrode, and depositing a positive photoresist layer (22) on the semiconductor layer. Thereafter, the photoresist layer is exposed by irradiating (24) from the other surface of the substrate so as to use the gate electrode as a mask. Therefore, if the positive photoresist layer is developed, the unexposed portion remains on the semiconductor layer to correspond to the gate electrode. Then, the semiconductor layer is etched using the remaining photoresist as a mask so as to form a semiconductor island on the insulating layer, and source and drain electrodes are formed on the semiconductor island.

Description: The present invention relates to thin film transistors which can be used as driving elements for panel displays such as liquid crystal displays. More specifically, the present invention relates to a method of manufacturing a thin film transistor, which can realize formation or so-called islands and planar structure in a self-alignment manner. Recently, thin film transistors have been widely incorporated as driving elements in liquid crystal display panels. The thin film transistors are generally formed of amorphous silicon, and mainly divided into two types, the coplanar type and the stagger type. The stagger type is also divided into a normal stagger type and an inverted stagger type. Heretofore, the inverted stagger type thin film transistors have been manufactured in the following manner: a gate electrode metal is deposited on a substrate and then etched to a given pattern of gate electrodes. A gate insulator layer is formed on the substrate to cover the gate electrodes, and an i-type amorphous silicon layer providing an active layer and an n⁺-type amorphous silicon layer for ohmic contact are formed on the gate insulator layer in the named order. Thereafter, the amorphous silicon layers are selectively removed excepting portions to be left for constituting transistors. Namely, semiconductor islands are formed. In the prior art, an independent mask is prepared for formation of islands, and the islands are formed by photolithography. In addition, drain and source electrode wirings are provided to lie on step portions formed by the islands. However, the conventional island forming method is disadvantageous in that the large area patterning of the liquid crystal display is greatly influenced by precision of the mask used, a slight defect of the mask used, and precision of mask alignment. Further, since the drain and source electrode wirings lie on the steps formed by the gate electrodes and the islands, the electrode wirings are often broken at the step coverage portion and because of a so-called side etching when the electrode metal is etched to provide a given pattern of electrodes and wirings. EP-A-139 585 describes a method of manufacturing a thin film transistor comprising the steps of forming a gate electrode on one surface of a transparent substrate, forming on the substrate a first insulating layer and a semiconductor layer in the named order to cover the gate electrode. Furtheron, a positive photoresist layer is deposited on the semiconductor layer, the positive photoresist layer is exposed by irradiating from the other surface of the substrate so as to use the gate electrode as a mask, the positive photoresist layer is developed so that the unexposed portion remains on the semiconductor layer to correspond to the gate electrode, the semiconductor layer is etched using the remaining photoresist as a mask so as to form a semiconductor island on the first insulating layer. Then a n⁺ type amorphous silicon layer is deposited on the remaining photoresist, the semiconductor island and said insulating layer. This n⁺ type amorphous silicon layer is covered by a conductive layer. Thereafter the conductive cover layer, the n⁺ type amorphous silicon layer and the remaining positve photoresist layer are removed, so that the surface of the semiconductor layer is exposed. Then the source and drain electrodes are formed. Accordingly, it is an object of the present invention to provide a method of manufacturing a thin film transistor, which can form so-called islands at a very high positional and dimensional precision. Another object of the present invention is to provide a method of manufacturing a thin film transistor which can form the islands in a self-alignment manner without using an independent mask. Still another object of the present invention is to provide a method of manufacturing a thin film transistor, which can realize a planar structure so that a drain and source electrode wiring can be formed without a fear of break due to step coverage and side etching. The above and other objects of the present invention are achieved by a method of manufacturing a thin film transistor comprising the steps of claim 1. The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. Figures 1A to 1G are partial sectional views illustrating a process of a thin film transistor manufacturing method; and Figure 2 is a partial sectional view showing one step of an embodiment of the thin film transistor manufacturing method in accordance with the present invention. Figure 1A shows a glass substrate 10 having a chromium gate electrode 12 of a predetermined pattern, a gate insulator layer 14, an i-type amorphous silicon layer 16, an n⁺-type amorphous silicon layer 18, an anti-oxidation protection coating 20 and a positive photoresist layer 22 which are formed on a top surface of the substrate 10 in the named order. This layer structure can be easily formed in conventional manner. For example, the gate electrode 12 can be formed by evaporating a thin film of chromium on the top surface of the substrate 10 and etching the chromium film to a predetermined gate electrode pattern. The gate insulator layer 14 can be formed of Si₃N₄ at a thickness of 300 nm. Further, the i-type and the n⁺-type amorphous silicon layers 16 and 18 are formed at thickness of 100 nm and 20 nm, respectively. These layers 14, 16 and 18 can be formed by a plasma CVD method. The protection coating 20 is formed of an ITO (indium tin oxide) layer of 20 nm thickness, and can be deposited by a sputtering. In this condition, the positive photoresist layer 22 is exposed by irradiating an exposure radiation 24 from the rear surface of the substrate 10, as shown in Figure 1A. In the exposure process, the gate electrode 12 blocks the incident radiation. Namely, the gate electrode 12 functions as an exposure mark. Thereafter, the photoresist layer 22 is developed, and an unexposed photoresist pattern 26 remains just above the gate electrode 12, as shown in Figure 1B. Next, by using the resist pattern 26 as an etching mask, the ITO protection coating 20 is patterned by a wet etching, and the two amorphous silicon layers 16 and 18 are also patterned by a dry etching. Further, the remaining photoresist pattern 26 is removed. As a result, a semiconductor island 28, which is composed of the i-type and n⁺-type amorphous silicon layers 16 and 18 and the ITO protection coating 20, is formed on the Si₃N₄ layer 14, as shown in Figure 1C. This island 28 is perfectly in alignment to the gate electrode 12. The structure shown in Figure 2 is realized by the steps of, after formation of the ITO layer 20, forming on the ITO layer 20 a second insulating layer 40 at a thickness of 600 nm, depositing the positive photoresist layer 22 on the SiN x layer 40, exposing the photoresist layer 22 by irradiating from the rear surface of the substrate 10, etching the second insulating layer 40 and the underlying layers 16, 18 and 20 using the remaining photoresist layer 22 as the mask, and depositing a third insulating layer 30 on the gate insulator layer 14 and the island. In this embodiment, the second insulating layer 40, deposited on the ITO layer 20, is a SiN x layer of a high hydrogen content which is formed to have a thickness of about 600 nm by means of plasma CVD method, and the third insulating layer 30 for planarization is formed of a sputtered SiO₂ layer. This insulating layer 30 is thicker than the semiconductor island 28, and may be formed by a Si₃N₄ layer having a thickness of 200 nm, which can deposited by for example plasma CVD. Thereafter, the buffered HF-H₂O is applied to the structure shown in Figure 2. The etch rate of the buffered HF-H₂O to the SiN x is greatly larger than that of SiO₂. Further, the SiO₂ thickness of the side wall of the island is considerably smaller than that of the SiO₂ layer 30 on the horizontal portion because the island has a step difference of about 800 nm which is greatly larger than the thickness (200 nm) of the SiO₂ horizontal portion. Because of these two reasons, the SiN x layer 40 in the island is quickly removed as compared to the horizontal portion of the SiO₂ layer 30. Thus, the planarization can be realized by utilizing the thickness difference between the vertical and horizontal portions of the SiO₂ layer and the etch rate difference between SiN x and SiO₂, without the patterning process for planarization as shown in Figures 1D and 1E. Then, a conductor is deposited on the upper surface of the semiconductor island 28 and the SiO₂ layer 30. For example, an ITO layer 38 of 50 nm thickness is deposited by sputtering, and then a patterning is made to provide drain and source electrodes and wirings, as shown in Figure 1G. Namely, the n⁺-type amorphous layer 18, the ITO layer 20 and the ITO layer 38 are divided into two, respectively. Specifically, the n⁺-type amorphous layer 18 is separated to a drain contact region 18A and a source contact region 18B. The ITO layer 20 is divided into a drain electrode 20A and a source electrode 20B. The ITO layer 38 is patterned to provide a drain electrode wiring 38A and a source electrode wiring 38B. As seen from the above, the island can be formed in a self-alignment manner and without using an independent mask, by means of a rear surface exposure using a gate electrode as an exposure mask. Further, a planarization can be realized without using an independent mask, by depositing an planarization insulator layer and a photoresist layer to cover the island, exposing the photoresist layer from the rear surface of the substrate, and selectively etching the planarization insulator layer through the patterned photoresist layer so that the insulator layer remains excepting the top surface of the island. Thus, a mask exposure process can be omitted, and a drain and source wiring can be realized with a small step. Therefore, the possibility of break of wiring due to the step difference can be greatly decreased. The invention has thus been shown and described with reference to the specific embodiments. However, it should be noted that the invention is in no way limited to the details of the illustrated structures but changes and modifications may be made within the scope of the appended claims.

Claims: A method of manufacturing an inverted stagger type of thin film transistor comprising the steps of forming a gate electrode (12) on one surface of a transparent substrate (10), forming on the substrate (10) a first insulating layer (14) and a semiconductor layer (16) in the named order to cover the gate electrode (12), forming on the semiconductor layer (16) a second insulating layer (40) at a thickness greatly larger than that of the semiconductor layer (16), depositing a positive photoresist layer on the second insulating layer (40), exposing the photoresist layer by irradiating from the other surface of the substrate (10) so as to use the gate electrode (12) as the mask, developing the photoresist layer so that the unexposed portion remains on the second insulating layer (40) to correspond to the gate electrode (12), etching the second insulating layer (40) and the semiconductor layer (16) using the remaining photoresist layer as the mask so as to form on the first insulating layer a semiconductor island (16) having the second insulating layer (40) remaining on the top surface thereof, depositing on the substrate (10) a third insulating layer (30) at a thickness larger than the semiconductor layer (16) but sufficiently smaller than the second insulating layer (40) so that the semiconductor island (16), the remaining second insulator (40) and the first insulating layer (14) are covered by the third insulating layer (30), the third insulating layer (30) being formed of a material which can be etched at an etch rate sufficiently lower than that of the second insulating layer (40), and etching the second and third insulating layer (40, 30) with an etchant which etches the second insulating layer (40) at a rate higher than that of the third insulating layer (30), so that there is exposed a substantially flat surface formed by the semiconductor island (16) and the remaining third insulating layer (30) surrounding the semiconductor island (16), and forming source and drain electrodes (38A, 38B) on the semiconductor island (16). A method as claimed in Claim 1 wherein the formation of the semiconductor layer includes the step of forming an amorphous silicon layer on the insulating layer (14). A method as claimed in any of Claims 1 or 2 wherein the formation of the semiconductor layer includes the steps of forming an i-type amorphous silicon layer (16) on the insulating layer (14) and then forming an n⁺-type amorphous silicon layer (18) on the i-type amorphous silicon layer (16). A method as claimed in Claim 3 wherein an anti-oxidation protection coating (20) of a conductive material is provided on the n⁺-type amorphous silicon layer (18) before the deposition of the second insulating layer (40).

Inventors: SUKEGAWA, OSAMU C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H01L21
IPC codes: H01L29, H01L27, H01L21