Abstract:
A principal feature of the present invention is to clean a surface of a semiconductor substrate without providing a damaged layer to the surface thereof. A native oxide film formed on the surface of a silicon substrate is etched by plasma employing a gas containing fluorine. The surface of the semiconductor substrate is again subjected to plasma etching by employing a gas containing fluorine in order to remove a surface damaged layer and a fluorocarbon layer formed in the above step of plasma etching. The semiconductor substrate surface is irradiated with ultraviolet rays under a low pressure in order to dissociate and remove fluorine atoms chemically adsorbed to the semiconductor substrate surface upon the latter plasma etching.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a method of treating a surface of a semiconductor substrate and, more particularly, to a method of cleaning the surface of the semiconductor substrate without a damaged layer left in the surface thereof. The invention further relates to a method of manufacturing a semiconductor device including such a method of treating the semiconductor substrate surface. 
     2. Description of the Background Art 
     A surface of a semiconductor substrate, e.g., a silicon substrate must be cleaned in order to manufacture a highly reliable semiconductor device. 
     FIGS. 7A-7D are cross-sectional views showing a conventional treating method of a semiconductor substrate surface. (See 1985 Dry Process Symposium.) 
     Referring to FIG. 7A, a native oxide film 2 is usually formed on a surface of a silicon substrate 1. This native oxide film 2 is removed by a reactive ion etching employing a CHF 3  gas. At this time, with reference to FIG. 7B, a fluorocarbon layer (a polymer layer of CF x ) 3 is formed on the surface of silicon substrate 1. Since the surface of silicon substrate 1 is subjected to plasma irradiation, a surface damaged layer 31 is formed in the surface of silicon substrate 1. 
     Referring to FIG. 7C, the surface of silicon substrate 1 is irradiated with ultraviolet rays in a C1 2  gas atmosphere in order to remove fluorocarbon layer 3. Thus, with reference to FIG. 7D, fluorocarbon layer 3 adhering to the surface of silicon substrate 1 is removed. 
     In this method, however, although fluorocarbon layer 3 can be removed without native oxide film 2 left, there is a disadvantage that surface damaged layer 31 formed in the surface of silicon substrate 1 cannot be removed. 
     FIGS. 8A-8D are cross-sectional views showing another conventional example of a treating method of a semiconductor substrate surface. 
     Referring to FIG. 8A, a native oxide film 2 formed on the surface of a silicon substrate 1 is removed by a reactive ion etching employing a CHF 3  gas or a mixed gas of C m  F n , H 2  and the like. 
     At this time, with reference to FIG. 8B, a fluorocarbon layer (a polymer layer of CF x ) 3 is formed on the surface of silicon substrate 1. Since the surface of silicon substrate 1 is subjected to plasma irradiation, a surface damaged layer 31 is formed in the surface of silicon substrate 1. Referring to FIGS. 8C and 8D, the surface of silicon substrate 1 is lightly etched by afterglow discharge employing a mixed gas of CF 4  and O 2  in order to remove fluorocarbon layer 3 and surface damaged layer 31. Conventionally, the surface of the silicon substrate is treated by this method, thereby decreasing an electric resistance of the substrate. 
     In addition, the foregoing method is applied not only to the step of removing native oxide film 2 but also to a single step for manufacturing a transistor, i.e., the step of selectively etching a silicon oxide film covering the surface of the substrate in order to form sidewall spacers on sidewalls of a gate. 
     However, with reference to FIG. 8D, the surface treatment method of the above-described latter conventional example has a disadvantage that fluorine remains in the form of an SiF x  layer 4 in the surface of silicon substrate 1. 
     If such an SiF x  layer 4 exists in the surface of silicon substrate 1, the following problems arise. More specifically, with reference to FIG. 9A, there is a problem that fluorine causes abnormal acceleration of an oxidation reaction in the step of forming a gate oxide film 41 for forming, for example, a transistor, on a silicon substrate 1. That is, when gate oxide film 41 having a film thickness d shown in FIG. 9A is formed on silicon substrate 1, there is a problem that a gate oxide film 41 having an extremely large film thickness d 2  (d 2  &gt;d 1 ) is obtained as shown in FIG. 9B. If the film thickness of gate oxide film 41 increases, an interface state of the gate oxide film becomes higher, leading to a decrease in gate junction voltage strength. The decrease in gate junction voltage strength causes a degradation in electrical characteristics, resulting in a lower reliability of the semiconductor device. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to clean a surface of a semiconductor substrate. 
     Another object of the present invention is to remove a native oxide film formed on a surface of a semiconductor substrate. 
     A further object of the present invention is to remove a native oxide film formed on a surface of a silicon substrate. 
     A still further object of the present invention is to remove an SiF x  layer formed on a surface of a silicon substrate. 
     A still further object of the present invention is to remove a surface damaged layer formed on a surface of a semiconductor substrate. 
     A still further object of the present invention is to provide a method of manufacturing a highly reliable semiconductor device including a MOSFET. 
     A still further object of the present invention is to provide a method of manufacturing a highly reliable semiconductor device having a contact hole. 
     A still further object of the present invention is to provide a method of manufacturing a highly reliable semiconductor device having a trench. 
     The present invention is directed to a surface treating method of a semiconductor substrate, on the surface of which an oxide film is formed. The surface of the semiconductor substrate is subjected to plasma etching by employing a gas containing fluorine in order to remove the above oxide film (step (a)). The surface of the semiconductor substrate is again subjected to plasma etching by employing the gas containing fluorine in order to remove a surface damaged layer and a fluorocarbon layer formed in the above step (a) (step (b)). The surface of the semiconductor substrate is then irradiated with ultraviolet rays under a low pressure in order to dissociate and remove fluorine atoms chemically adsorbed to the surface of the semiconductor substrate in the step (b). 
     Preferably, in the present invention, the irradiation with ultraviolet rays is carried out while heating the semiconductor substrate to 100°-300° C. 
     It is preferable that the ultraviolet ray irradiation is carried out with a reducing gas being supplied to the surface of the semiconductor substrate. 
     Further, it is preferable that the ultraviolet ray irradiation is carried out with reducing radicals being supplied to the surface of the semiconductor substrate. 
     In accordance with the present invention, since the surface of the semiconductor substrate is irradiated with ultraviolet rays under a low pressure after the step (b), the fluorine atoms chemically adsorbed to the surface of the semiconductor substrate in the step (b) are dissociated and removed. 
     In this case, if a reducing gas or reducing radicals are supplied to the surface of the semiconductor substrate, then the dissociated fluorine becomes efficiently removed. 
     The foregoing and other objects, features, aspects and advantages 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1F are cross-sectional views showing treatment steps according to one embodiment of the present invention; 
     FIG. 2 is a conceptional view of a photoreaction chamber for use in a treatment of the present invention; 
     FIG. 3 is a diagram showing a result of an X-ray photoemission spectroscopy analysis of a semiconductor substrate subjected to the treatment of the present invention, along with reference data; 
     FIGS. 4A-4H show an example in which a method of surface treatment with light according to the present invention is applied to process steps for manufacturing a semiconductor device including an LDD type MOSFET; 
     FIGS. 5A-5F show an example in which the method of surface treatment with light according to the present invention is applied to process steps for manufacturing a semiconductor device including a contact hole; 
     FIGS. 6A-6E show an example in which the method of surface treatment with light according to the present invention is applied to process steps for manufacturing a semiconductor device having a trench; 
     FIGS. 7A-7D are cross-sectional views showing conventional process steps for treating a semiconductor substrate surface; 
     FIGS. 8A-8D are cross-sectional views showing another conventional example of process steps for treating a semiconductor substrate surface; and 
     FIGS. 9A and 9B are views showing a problem provided when an SiF x  layer is formed on a surface of a semiconductor substrate. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description of the present invention will now be made with reference to the figures. 
     FIGS. 1A-1F are cross-sectional views showing the steps of treating a semiconductor substrate surface according to one embodiment of the present invention. FIG. 2 is a conceptional view showing a detailed structure of a photoreaction chamber for use in the present invention. 
     A description will first be made on the detailed structure of the photoreaction chamber with reference to FIG. 2 before a description of the treatment steps of FIGS. 1A-1F. 
     A photoreaction chamber 17 includes a treatment chamber 29 for treating a surface of a semiconductor substrate 23 with light in high vacuum. Treatment chamber 29 includes therein a sample board 24 for securing the semiconductor substrate thereon. A plasma generation chamber 28 for generating plasma by microwave discharge or the like is connected to treatment chamber 29. Plasma generation chamber 28 has a gas inlet 28a for introducing a gas into plasma generation chamber 28. 
     Treatment chamber 29 has a gas outlet 27 for putting the interior of treatment chamber 29 in a high vacuum state. An ultraviolet light source 20 (a low-pressure mercury lamp) for emitting ultraviolet rays is disposed at a position which is outside the treatment chamber and opposing to sample board 24. Ultraviolet light source 20 is horizontally movable. Treatment chamber 29 has a window 21 through which light emitted from ultraviolet light source 20 enters in treatment chamber 29. An infrared lamp 25 serving as a light source for heating a sample is disposed on the top of treatment chamber 29. Treatment chamber 29 has a window 26 for introducing infrared rays emitted from infrared lamp 25 into treatment chamber 29. 
     The treatment steps according to one embodiment of the present invention will now be described. 
     Referring to FIG. 1A, a native oxide film 2 formed on a surface of a silicon substrate 1 is removed by a reactive ion etching employing a CHF 3  gas or a mixed gas of C m  F n , H 2  and the like. At this time, with reference to FIG. 1B, a fluorocarbon layer (a polymer layer of CF x ) 3 is formed on the surface of silicon substrate 1. At the same time, since the silicon surface is subjected to plasma irradiation, a surface damaged layer 31 is formed in the surface of silicon substrate 1. 
     Referring to FIG. 1C, soft etching (a surface treatment by afterglow discharge) is carried out by employing a mixed gas of CF 4  and O 2  or an NF 3  gas in order to remove fluorocarbon layer 3 and surface damaged layer 31. At that time, with reference to FIG. 1D, fluorine is left in the form of an SiF x  layer 4 in the surface of silicon substrate 1. 
     Then, with reference to FIG. 1E, a treatment with ultraviolet ray irradiation, which is a feature of the present invention, is performed. 
     Referring to FIG. 2, semiconductor substrate 23 subjected to a soft etching treatment is fixed to sample board 24. The atmosphere in photoreaction chamber 29 is exhausted through outlet 27 by a turbo molecular pump until the pressure in photoreaction chamber 29 becomes 1×10 -5  to 10 -6  Torr. Infrared lamp 25 is turned on, so that infrared rays are directed to sample board 24 through 20 window 26, to heat semiconductor substrate 23 up to 250° C. A highly reducing gas such as an H 2  gas or an NH 3  gas is then introduced into plasma generation chamber 28 through gas inlet 28a at a flow rate of 200 SCCM. Hydrogen radicals are produced in this plasma generation chamber 28 by microwave afterglow discharge and then introduced into treatment chamber 29. With treatment chamber 29 kept in such an atmosphere, low-pressure mercury lamp 20 is turned on, so that ultraviolet rays of a wavelength 184.9 nm are introduced into treatment chamber 29. The irradiance of the ultraviolet rays is 100 mW/cm 2 . A gas pressure in treatment chamber 29 was kept at 0.3 Torr. The surface treatment was carried out for ten minutes. The following reaction occurs in this treatment. 
     SiF x  +H Si+HF x   
     While such a case is exemplified that hydrogen radicals are introduced into treatment chamber 29 in this step, the present invention is not limited to this, and the above reducing gas may be introduced as it is into treatment chamber 29. 
     Silicon substrate 1 having a clean surface is thus obtained with reference to FIG. 1F. 
     For evaluation, the silicon substrate thus treated has undergone an X-ray photoemission spectroscopy analysis. 
     FIG. 3 shows the result of the X-ray photoemission spectroscopy analysis on an Si 2p  orbit, wherein the longitudinal axis represents relative values of SiF x  coupling to Si (bulk) coupling. In the figure, the dot denoted with (C) represents data of the semiconductor substrate which has undergone all the steps of FIGS. 1A-1F. The dot denoted with (A) represents data of the semiconductor substrate which has undergone only the step of FIG. 1A (the reactive ion etching only); i.e., data of the semiconductor substrate shown in FIG. 1B. The dot denoted with (B) represents data of the semiconductor substrate which has undergone the steps of FIGS. 1A-1D (the reactive ion etching and the soft etching). As apparent from FIG. 3, it was confirmed that when the ultraviolet-ray treatment shown in FIG. 1E, which is the feature of the present invention, is carried out, the SiF x  coupling decreases and the fluorine is efficiently removed at the surface of the silicon substrate. 
     While such a case is exemplified that the irradiance of ultraviolet rays is 100 mW/cm 2  in the foregoing embodiment, the present invention is not limited to this, and a preferable result can be obtained if the irradiance is higher than or equal to 50 mW/cm 2 . 
     In addition, while such a case is exemplified that the semiconductor substrate is heated up to 250° C. in the foregoing embodiment, the present invention is not limited to this, and a preferable result can be obtained at a temperature in the range of 100° to 300° C. 
     Moreover, while radicals are formed by microwave afterglow discharge in the foregoing embodiment, the present invention is not limited to this, and any methods cable of forming radicals can be employed. 
     While the wavelength of ultraviolet rays is 184.9 nm in the foregoing embodiment, the present invention is not limited to this, and any ultraviolet rays having wavelengths of 200 nm or less than 300 nm can be employed. 
     In addition, while the gas pressure in the treatment chamber is kept at 0.3 Torr in the foregoing embodiment, the present invention is not limited to this, and a preferable result can be obtained at a pressure in the range of 0.3-5 Torr. 
     FIGS. 4A-4H show an example in which the method of surface treatment with light according to the present invention is applied to the steps of manufacturing a semiconductor device including a MOSFET of an LDD structure. 
     Referring to FIG. 4A, a gate oxide film 41 is formed on a silicon substrate 1. A gate electrode 42 is formed on gate oxide film 41. With gate electrode 42 serving as a mask, n -  impurity ions are implanted onto a main surface of silicon substrate 1, thereby to form an n -  impurity layer 43 in the main surface of silicon substrate 1. 
     Referring to FIG. 4B, an SiO 2  film 44 is formed on silicon semiconductor substrate 1 to cover gate electrode 42 by a CVD method. 
     With reference to FIG. 4C, SiO 2  film 44 is subjected to reactive ion etching by employing a CHF 3  gas so that sidewall spacers 45 may be left on sidewalls of gate electrode 42. At that time, a fluorocarbon layer 3 is formed on silicon substrate 1, and a surface damaged layer 31 is formed in the main surface of silicon substrate 1. 
     Referring to FIG. 4D, the surface of silicon substrate 1 is subjected to soft etching (a surface treatment by afterglow discharge) by employing a mixed gas of CF 4  and O 2  in order to remove fluorocarbon layer 3 and surface damaged layer 31. 
     At that time, fluorine remains in the form of an SiF x  layer 4 in the surface of silicon substrate 1, with reference to FIG. 4E. 
     Referring to FIG. 4F, the surface of silicon substrate 1 subjected to the soft etching treatment is irradiated with ultraviolet rays under a low pressure. 
     Thus, fluorine atoms chemically adsorbed to the surface of silicon substrate 1 are dissociated and removed, with reference to FIG. 4G. 
     Referring to FIG. 4H, with sidewall spacers 45 serving as a mask, n +  impurity ions are implanted onto the main surface of silicon substrate 1. Accordingly, an MOSFET of an LDD structure is obtained. Since the SiF x  layer is removed from the main surface of silicon substrate 1 by the surface treatment with light, an MOSFET having excellent semiconductor characteristics is obtained. 
     FIGS. 5A-5F are views showing an example in which the method of surface treatment with light according to the present invention is applied to the steps of manufacturing a semiconductor device including a contact hole. 
     Referring to FIG. 5A, a gate electrode 42 is formed on a silicon substrate 1. An interlayer insulation film 50 is formed on silicon substrate 1 to cover gate electrode 42. A resist 51 having a predetermined pattern is formed on interlayer insulation film 50. With resist 51 serving as a mask, interlayer insulation film 50 is subjected to reactive ion etching, thereby to form a contact hole 52 for exposing a contact surface la of silicon substrate 1 in interlayer insulation film 50. The following steps will be described with reference to an enlarged view of the portion indicated by a reference character A in FIG. 5A. 
     Referring to FIG. 5B, a fluorocarbon layer 3 is formed at contact surface la during plasma etching of interlayer insulation film 50. At that time, since the contact surface is subjected to plasma irradiation, a surface damaged layer 31 is formed in the surface of contact surface 1a. 
     Referring to FIG. 5C, soft etching (a surface treatment by afterglow discharge) is carried out for contact surface la by employing a mixed gas of CF 4  and O 2  or an NF 3  gas in order to remove fluorocarbon layer 3 and surface damaged layer 31. At that time, with reference to FIG. 5D, fluorine remains in the form of SiF x  layer 4 at the surface of contact surface 1a. 
     Referring to FIG. 5E, the contact surface is irradiated with ultraviolet rays under a low pressure. Accordingly, fluorine atoms chemically adsorbed to contact surface 1a are dissociated and removed. A semiconductor device having clean contact surface 1a is thus obtained with reference to FIG. 5F. 
     FIGS. 6A-6E are views showing an example in which the method of surface treatment with light according to the present invention is applied to the steps of manufacturing a semiconductor device having a trench. 
     Referring to FIG. 6A, a silicon oxide film 60 having a predetermined opening 61 is formed on a silicon substrate 1. 
     With silicon oxide film 60 serving as a mask, silicon substrate 1 is subjected to reactive ion etching by employing a gas of HB r , Cl 2  or SF 6 , thereby forming a trench 62 in silicon substrate 1, with reference to FIG. 6B. A deposition film 63 such as of SiB r  O x  or SiClO x  is formed on sidewalls of trench 62. A layer 64 such as of SiBr x  or SiCl x  is formed at an inner wall surface of trench 62. 
     Referring to FIG. 6C, silicon oxide film 60 and deposition film 63 are removed by wet etching using HF. 
     Referring to FIG. 6D, the inner wall surface of trench 62 is irradiated with ultraviolet rays under a low pressure. Thus, halogen atoms chemically adsorbed to the inner wall surface of trench 62 are dissociated and removed. 
     Accordingly, trench 62 having a clean surface is obtained with reference to FIG. 6E. 
     As has been described heretofore, in accordance with the present invention, since the surface of the semiconductor substrate is irradiated with ultraviolet rays under a low pressure after the removal of the surface damaged layer and the fluorocarbon layer formed on the surface of the semiconductor substrate, the fluorine atoms chemically adsorbed to the surface of the semiconductor substrate are dissociated and removed. Consequently, such an effect is provided that the semiconductor substrate having a clean surface is obtained. 
     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 spirit and scope of the present invention being limited only by the terms of the appended claims.