Abstract:
The present disclosure pertains to the field of carbon nanotube technologies, and provides a carbon nanotube semiconductor device and a manufacturing method thereof. The manufacturing method of a carbon nanotube semiconductor device provided in the present disclosure comprises: forming a carbon nanotube layer with a carbon nanotube solution; and treating the carbon nanotube layer with an acidic solution. The carbon nanotube semiconductor device manufactured by the method of the present disclosure has good performance uniformity.

Description:
FIELD 
       [0001]    The present disclosure pertains to the field of carbon nanotube (CNT) technologies, and relates to the manufacture of a CNT semiconductor device. Specifically, the present disclosure relates to a manufacturing method of a CNT semiconductor device in which CNT layers are treated with an acidic solution. 
       BACKGROUND ART 
       [0002]    CNT thin films have been the focus of nanotechnologies in recent years, wherein carbon nanotubes (CNT) having semiconductor properties can be applied in various semiconductor devices such as diodes, field effect transistors (FET) and so on. As an example, in a carbon nanotube field effect transistor (CNT-FET), carbon nanotubes form conductive channels having semiconductor properties. Thereby, CNT layers can be used for manufacturing the simplest and most fundamental electronic device, i.e., a MOSFET. Moreover, CNT thin films can also be further used for manufacturing a thin film transistor (TFT) formed on the basis of CNT. The channel layer of a carbon nanotube thin film transistor (CNT-TFT) is a CNT thin film layer, and CNT-TFT may be applied as a drive transistor in a display. 
         [0003]    Although CNT thin films are characterized by merits such as flexibility, transparency, high mobility, low cost and large scale, CNT-FETs or CNT-TFTs formed on the basis of CNT thin films still have problems in terms of performance uniformity. For example, multiple CNT-TFTs manufactured under a same process condition may have quite different transfer characteristics. In other words, the performances of CNT-FETs or CNT-TFTs manufactured in mass quantities using the existing method are not uniform. The lack of uniformity also greatly restricts the large scale application of CNT-FETs or CNT-TFTs in an industrial field. 
         [0004]    Similar problems also exist in other CNT semiconductor devices formed on the basis of CNT layers. 
       SUMMARY 
       [0005]    The present disclosure aims to improve the performance uniformity of a CNT semiconductor device. 
         [0006]    To achieve the above goal or other goals, the present disclosure provides technical solutions as follows. 
         [0007]    According to a first aspect of the present disclosure, a manufacturing method of a CNT semiconductor device is provided. The method comprises: forming a CNT layer with a CNT solution; and treating the CNT layer with an acidic solution. 
         [0008]    In the manufacturing method according to an embodiment of the present disclosure, the semiconductor device is a carbon nanotube field effect transistor (CNT-FET) or a carbon nanotube thin film transistor (CNT-TFT), and the CNT layer is used for forming a semiconductor layer between a source and a drain of the CNT-FET or the CNT-TFT. 
         [0009]    Optionally, the manufacturing method according to an embodiment of the present disclosure further comprises: prior to forming a CNT layer with a CNT solution, performing decentralized processing on the CNT solution with an organic solvent. 
         [0010]    Optionally, the organic solvent can be toluene, xylene, chloroform or o-xylene. 
         [0011]    The manufacturing method according to an embodiment of the present disclosure further comprises forming the source and the drain through a patterning process, after treating the CNT layer with an acidic solution. 
         [0012]    The manufacturing method according to an embodiment of the present disclosure further comprises forming the source and the drain through a patterning process, before treating the CNT layer with an acidic solution. 
         [0013]    In another embodiment, optionally, the step of forming the source and the drain through a patterning process comprises: forming a photoresist layer through a patterning process to cover a region other than the one in which the source and the drain are to be formed; depositing a metal thin film layer; and removing the photoresist layer with the metal thin film layer on the photoresist layer being removed at the same time. 
         [0014]    The manufacturing method according to an embodiment of the present disclosure further comprises after forming the source and the drain through a patterning process, forming a protective layer at least on the source and the drain through a patterning process to expose the semiconductor layer between the source and the drain; and removing the protective layer, after treating the CNT layer with an acidic solution. 
         [0015]    In a further embodiment, optionally, the step of forming the source and the drain through a patterning process comprises: depositing a metal thin film layer; forming a photoresist layer over the metal thin film layer through a patterning process to cover the region in which the source and the drain are to be formed; and performing etching by using the photoresist layer as a mask until the CNT layer is exposed, with the photoresist layer being retained. Specifically, the photoresist layer is removed after treatment of the CNT layer with the acidic solution. 
         [0016]    In a further embodiment, optionally, the source and the drain are made of a metal material resistant to corrosion of acidic solutions. 
         [0017]    In the manufacturing method according to a further embodiment, the CNT layer is treated with an acidic solution for 1 to 2 minutes. 
         [0018]    In the manufacturing method according to a further embodiment, carbon nanotubes in the CNT layer are single-wall carbon nanotubes, dual-wall carbon nanotubes, multi-wall carbon nanotubes, or a combination thereof. Alternatively, carbon nanotubes in the CNT layer are carbon nanotube bundles. 
         [0019]    In the manufacturing method according to a further embodiment, the acidic solution is a mixed solution of phosphoric acid, nitric acid, and acetic acid. 
         [0020]    In the manufacturing method according to a further embodiment, the phosphoric acid, the nitric acid, and the acetic acid in the mixed solution are respectively 75%-85%, 3%-7% and 3%-7% by mass. 
         [0021]    According to another aspect of the present disclosure, a CNT semiconductor device manufactured by any of the above manufacturing methods is provided. 
         [0022]    According to the CNT semiconductor device and the manufacturing method thereof provided in the embodiments of the present disclosure, chemical impurities in a CNT layer can be effectively removed by treatment with an acidic solution, which greatly improves the performance uniformity of a CNT-TFT semiconductor device. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]    The above and other goals and advantages of the present disclosure will be rendered clearer and more complete in combination with detailed explanations of the drawings below. In the drawings, same or similar elements are indicated by same reference signs. 
           [0024]      FIG. 1  is a schematic flow diagram of a method for manufacturing a CNT-TFT according to an embodiment of the present disclosure; 
           [0025]      FIG. 2  is a schematic flow diagram of a method for manufacturing a CNT-TFT according to a further embodiment of the present disclosure; 
           [0026]      FIG. 3  is a schematic view for a basic structure of a CNT-TFT manufactured according to the method for manufacturing a CNT-TFT provided in the present disclosure; and 
           [0027]      FIG. 4  is a schematic comparison view for transfer characteristics of CNT-TFTs obtained without treatment by an acidic solution and with treatment by an acidic solution respectively. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0028]    What will be introduced below is some of many possible embodiments of the present disclosure for the purpose of providing a basis understanding of this disclosure, instead of confirming key or crucial elements of the present disclosure or limiting the scope sought for protection. It should be understood that, according to the technical solution of the present disclosure, a person having ordinary skills in the art can propose other interchangeable implementations without changing the essential spirits of the present disclosure. Therefore, the following embodiments and drawings are only exemplary explanations for the technical solution of the present disclosure, and should not be regarded as the entire disclosure or a limitation or restriction to the technical solution of the present disclosure. 
         [0029]      FIG. 3  is a schematic view for a basic structure of a CNT-TFT manufactured according to the method for manufacturing a CNT-TFT provided in the present disclosure. In  FIG. 3 , various thicknesses of layers and regions are exaggerated for clarity. Moreover, proportional relationships between dimensions of each component in  FIG. 3  are only intended to be schematic, rather than reflecting the true proportional relationships between the dimensions of each component. The CNT-TFT shown in  FIG. 3  can be manufactured according to an embodiment of a manufacturing method disclosed below. 
         [0030]    It should be noted that the semiconductor device in an embodiment of the present disclosure is a carbon nanotube field effect transistor 
         [0031]    (CNT-FET) or a carbon nanotube thin film transistor (CNT-TFT). Besides, the CNT layer is at least partially used for forming a semiconductor layer between a source and a drain of the CNT-FET or the CNT-TFT. The embodiment of  FIG. 3  is illustrated with the semiconductor device being a CNT-TFT as an example, but it is not limited thereto. 
         [0032]      FIG. 1  is a schematic flow diagram of a method for manufacturing a CNT-TFT according to an embodiment of the present disclosure. The manufacturing method of a CNT-TFT in an embodiment of the present disclosure will be explained in detail as follows with reference to  FIG. 1  and  FIG. 3 . 
         [0033]    Firstly, in step S 110 , a substrate which has been cleaned is provided. 
         [0034]    In this step, the provided substrate can be but not limited to silicon wafer, and it corresponds to a silicon substrate  310  shown in  FIG. 3  and a gate dielectric layer  320  formed thereon. The gate dielectric layer  320  can specifically be silicon dioxide, which is formed by thermal oxidation of the silicon substrate, or by other methods such as film deposition. 
         [0035]    Furthermore, in step S 120 , a CNT thin film layer is formed on the substrate with a CNT solution. 
         [0036]    In this step, the CNT thin film layer can be obtained on the basis of a CNT solution by a soaking method. Specifically, the substrate can be soaked into the CNT solution. After a period of time, the substrate is taken out, then washed and dried. Obviously, the CNT thin film layer can be formed on the basis of a CNT solution by solution spraying, chemical assembling (for example, LB thin film method) and so on. A method for forming a CNT thin film layer on the basis of a CNT solution can be called “a solution process”, the specific implementation of which is not limited to those specific embodiments of the present disclosure. Besides, the thickness of the formed CNT thin film layer can be determined according to various structural parameters of the CNT-TFT to be formed. 
         [0037]    Specifically, in other preferred embodiments, prior to formation of a CNT layer, decentralized processing is performed on the CNT solution with an organic solvent so as to obtain a CNT solution which is comparatively uniform and stable. The specific organic solvent in use can be but not limited to toluene, xylene, chloroform or o-xylene. It should be understood that the procedure of dispersing carbon nanotubes to form a solution is complicated in itself. It is usually necessary to use a surfactant and other chemical substances such that powdered carbon nanotubes can be easily dissolved in the solvent, thereby obtaining a stable CNT solution. It should also be understood that since the CNT powder for forming a CNT solution may contain both a metal type CNT and a semiconductor type CNT, a chemical agent (for instance a surfactant) may be needed to remove the metal type CNT, thereby obtaining a stable semiconductor type CNT solution with a comparatively high purity. As an example, the CNT solution used in the above embodiments is a semiconductor type CNT solution with the metal type CNT being removed. 
         [0038]    The CNT thin film layer formed by the above steps comprises a semiconductor type CNT, and can be used to form a semiconductor layer between a source and a drain of the CNT-TFT, i.e., a channel layer between the source and the drain. CNTs in the CNT thin film layer can be single-wall carbon nanotubes, dual-wall carbon nanotubes, multi-wall carbon nanotubes, or a combination thereof. Alternatively, CNTs in the CNT layer are carbon nanotube bundles. 
         [0039]    It should be noted that, optionally, a patterning process can be performed upon actual needs on the CNT thin film layer formed through the above steps. For example, the CNT thin film layer is patterned by etching to form a semiconductor layer between the source and the drain, and specific patterns for the patterning are not limited. 
         [0040]    Furthermore, in step S 130 , a source and a drain are formed on the substrate through a patterning process. 
         [0041]    In this step, photolithography can be adopted. A source  330  and a drain  340  shown in  FIG. 3  are formed by photolithographic patterning. The source  330  and the drain  340  can be formed by a metal thin film layer, and they are connected via the CNT thin film layer. 
         [0042]    It should be noted that the step of forming the source and the drain through a patterning process can be carried out either before treatment of the CNT layer with an acidic solution or after treatment of the CNT layer with an acidic solution. In the present application, explanations are mainly made by taking the latter as an example. 
         [0043]    Furthermore, the formation of the source and the drain through a patterning process is implemented in two ways as follows. 
         [0044]    The first option: a region other than the one in which the source and the drain are to be formed is covered by a photoresist layer. Specifically, the region in which the source and the drain are to be formed is not covered by the photoresist layer (i.e., the semiconductor layer between the source and the drain is exposed). Then a metal thin film layer is deposited, and furthermore the photoresist layer is removed. In this case, the metal thin film layer on the photoresist layer is removed at the same time. In this way, the metal thin film layer in the region in which the source and the drain are to be formed is retained, thereby forming the source and the drain. 
         [0045]    The second option: a metal thin film layer can be deposited first, and then a photoresist layer is formed on the metal thin film layer to cover the region in which the source and the drain are to be formed. Furthermore, etching is performed by using the photoresist layer as a mask, until the CNT thin film layer is exposed. Finally, the photoresist layer over the source and the drain is removed. 
         [0046]    Based on the first option, no etching is needed, and the CNT thin film layer, except for the region in which the source and the drain are to be formed, is covered by the photoresist layer, particularly in a region for forming channels. Therefore, the CNT thin film layer can be well protected. 
         [0047]    Furthermore, in step S 140 , a protective layer is formed on the source and the drain by photolithographic patterning. 
         [0048]    In this step, a region other than the channel region is covered by a photoresist, so at least the source and the drain are covered. The photoresist mask is used as a protective layer for the source and the drain. 
         [0049]    The protective layer does not cover the CNT thin film layer for forming channels, such that the CNT thin film layer is exposed and ready for treatment with an acidic solution in the following steps. The protective layer can specifically be but not limited to photoresist S 1813  or electron beam resist PMMA and the like. The protective layer can basically protect the source and the drain against the influence of treatment with an acidic solution in the following step S 150 . 
         [0050]    In another alternative embodiment, if the source and the drain are formed through the second option of the above step S 130 , the photoresist layer over the source and the drain can be retained rather than be removed. In this step, the photoresist layer is used as a protective layer, and thereby the step S 140  can be omitted. 
         [0051]    Furthermore, in step S 150 , at least the CNT thin film layer for forming the channels is soaked into an acidic solution, then taken out and washed with water. 
         [0052]    It is found that when a CNT thin film layer is formed based on a CNT solution, some unnecessary impurities, for example polymeric impurities, may be contained in the solution or on a surface of CNT in the solution. Such impurities will be deposited on the substrate in the manufacture of a CNT thin film layer during the above step S 120 , or even attached to an outer wall surface of part of the carbon nanotubes. When a semiconductor device or a CNT-TFT is manufactured on the basis of such a CNT thin film layer containing impurities, presence of the impurities will lead to more charge traps on an interface between the CNT thin film layer and the dielectric layer in contact with an upper surface and/or a lower surface thereof, thereby influencing the performance uniformity of the CNT-TFT to be manufactured. 
         [0053]    In this step, the CNT thin film layer is soaked into the acidic solution for treatment. Now, the above impurities can be removed by the acidic solution, and thereby a CNT thin film layer which is comparatively clean and uniform can be obtained. That is to say, a CNT thin film layer  350  shown in  FIG. 3  for forming the channels. Such a CNT thin film layer  350  has correspondingly less charge traps, and hence obtains better performance uniformity. In this step, the source  330  and the drain  340  are protected by the protective layer against the acidic solution. 
         [0054]    Specifically, the acidic solution can be a mixed solution of phosphoric acid, nitric acid, and acetic acid. The phosphoric acid, the nitric acid, and the acetic acid in the mixed solution are respectively 75%˜85%, 3%˜7% and 3%˜7% by mass, for example, 80%, 5% and 5% respectively, or 79%, 4% and 4% respectively. The CNT thin film layer can be soaked into the acidic solution for 1 to 2 minutes at a room temperature, then taken out, and washed with water to remove residuals of the acidic solution. 
         [0055]    Furthermore, in step S 160 , the protective layer on the source and the drain is removed. Specifically, the substrate can be soaked in a PG removal solution to remove the protective layer of the photoresist material, and then washed with water. 
         [0056]    So far, the manufacturing method of a CNT-TFT shown in the embodiment of  FIG. 3  is accomplished. It should be understood that a plurality of CNT-TFTs exemplified in  FIG. 3  can be manufactured in mass quantities on the same substrate according to the above manufacturing method. 
         [0057]    It should be noted that, in another alternative embodiment, if the source  330  and the drain  340  are manufactured by some metal materials (such as Pt and Au) which are resistant to corrosion of the acidic solution used in the subsequent step S 150 , the electrodes can be directly put into the acidic solution for treatment without the step of forming a protective layer thereon. Thereby, step S 140  is omitted, and obviously step S 160  can also be omitted. 
         [0058]      FIG. 2  is a schematic flow diagram of a method for manufacturing a CNT-TFT according to a further embodiment of the present disclosure. The manufacturing method of a CNT-TFT in an embodiment of the present disclosure will be explained in detail with reference to  FIG. 2  and  FIG. 3 . 
         [0059]    Specifically, in step S 210 , a substrate which has been cleaned is provided. For the implementation of this step, the procedure described in the above step S 110  can be referred to. 
         [0060]    Furthermore, in step S 220 , a CNT thin film layer is formed on the substrate with a CNT solution. For the implementation of this step, the procedure described in the above step S 120  can be specifically referred to. Furthermore, in step S 230 , at least the CNT thin film layer for forming channels is soaked into an acidic solution, then taken out, and washed with water. For the implementation of this step, the procedure described in the above step S 150  can be specifically referred to. Besides, in this embodiment, since the source  330  and the drain  340  have not been formed when performing treatment with the acidic solution, it is unnecessary to form the protective layer as shown in step S 140  in the embodiment of  FIG. 1 . Based on this, the process procedure is simplified. 
         [0061]    Furthermore, in step S 240 , a source and a drain are formed through a patterning process on the substrate. For the implementation of this step, the procedure described in the above step S 130  can be referred to. 
         [0062]    So far, the CNT-TFT shown in the embodiment of  FIG. 3  can also be manufactured. 
         [0063]    It should be noted that, compared with a CNT thin film layer of a traditional CNT-TFT, chemical impurities are greatly decreased in a CNT thin film layer  350  for forming channels according to an embodiment of the present disclosure, and the performance uniformity of the CNT-TFT device is thus improved. This is because chemical impurities influencing the performance uniformity of the CNT-TFT device are removed through treatment with an acidic solution. 
         [0064]      FIG. 4  shows a schematic comparison view for transfer characteristics of CNT-TFTs obtained without treatment by an acidic solution and with treatment by an acidic solution respectively. Specifically,  FIG. 4( a )  relates to transfer characteristic curves of 28 CNT-TFTs obtained when the CNT thin film layer has not been treated with an acidic solution, and  FIG. 4( b )  relates to transfer characteristic curves of 28 CNT-TFTs obtained when the CNT thin film layer has been treated with an acidic solution. As can be seen from  FIG. 4( a ) , the transfer characteristic curves are spread out, which indicates that the transfer characteristics are not uniform between individual CNT-TFT devices. In contrast, as can be seen from  FIG. 4( b ) , the transfer characteristic curves of 28 CNT-TFTs tend to be similar, which indicates that the transfer characteristic uniformity between each CNT-TFT device are distinctly improved. 
         [0065]    It will be understood that the method of performing acidic solution treatment on the CNT thin film layer of CNT-TFT as shown in the embodiments of  FIG. 1  and  FIG. 2  can be applied similarly to the manufacture of other types of CNT semiconductor devices, especially devices manufactured by a solution process. After treatment with an acidic solution, chemical impurities in the CNT layer for forming channels are removed, and in particular chemical substances on an outer wall of one or more carbon nanotubes for forming conductive channels are removed. This will effectively improve the performance uniformity of the semiconductor device. 
         [0066]    The above examples mainly illustrate the manufacturing method of a CNT semiconductor device and the CNT semiconductor device manufactured thereby in various embodiments of the present disclosure. The CNT semiconductor device can specifically be semiconductor devices such as transistors, integrated circuits, display substrates and display panels. Although only implementations of some embodiments of the present disclosure are described, one having ordinary skills in the art should understand that the present disclosure can be implemented in many other forms without deviating from the spirit and scope thereof. Therefore, the examples and embodiments shown above are regarded as exemplary instead of limiting. The present disclosure may cover various amendments and replacements without deviating from the spirit and scope of the present disclosure defined in the appended claims.