Method of forming silicon oxide film containing fluorine

Abstract: By using a vapor containing alkoxyfluorosilane as a main component, a silicon oxide film containing fluorine is formed on a substrate (104) at a temperature of 200°C or less in a reaction chamber (101) having a predetermined temperature and a predetermined pressure. For example, triethoxyfluorosilane may be supplied to the chamber (101) from a bubbler (107) and steam may be supplied from a bubbler (108). In addition, an organic film such as a photoresist film (304) can be used as a mask to selectively form the silicon oxide film. Although the silicon oxide film can be formed at a very low temperature of 30°C, the film has a water content smaller than that of a silicon oxide film formed at a temperture of 250°C by a conventional method. In addition, the film properties of the silicon oxide film are better than those of the silicon oxide film formed by the conventional method. Furthermore, a two-layered aluminum wiring structure can be easily formed by a selective growing method.

Description: Background of the Invention The present invention relates to a method of forming an insulating film and, more particularly, to a method of forming a silicon oxide film containing fluorine. As a conventional method of forming a silicon oxide film of this type, a chemical vapor-phase deposition method is popularly used. For example, the following method is used. That is, tetraethylorthosilicate (chemical formula: Si(OC 2 H 5 ) 4 ) serving as a source gas is evaporated by bubbling and then mixed with ozone in a reaction chamber so as to form a silicon oxide film at a substrate temperature of 300°C to 400°C and at atmospheric pressure. In addition, a method of doping phosphorus and boron by using trimethylphosphate (chemical formula: PO(OCH 3 ) 3 ) and trimethylborate (chemical formula: B(OCH 3 ) 3 ) as doping gases is used ("Ozone/Organic-Source APCVD for Conformal Doped Oxide", Journal of Electronic Materials, Vol. 19, No. 1, 1990). This conventional method of forming a silicon oxide film has the following problems. That is, since the temperature for forming the film is high, i.e., 300°C to 400°C, the silicon oxide film cannot be selectively formed by using an organic film such as a photoresist film as a mask. In addition, when the temperature is decreased, the film properties are considerably degraded, and the film disadvantageously cannot be used in practical applications. Furthermore, since a silicon oxide film formed by the conventional method has a strong tensile stress of about 10 4 N/cm 2 (1 x 10 9 dyn/cm 2 ) as an internal stress, when a film having a thickness of 2µm or more is formed, cracks easily occur in this film For this reason, this film is not easily formed on a substrate such as an aluminium wiring layer having an uneven surface. A paper published in the Journal of Non-Crystalline Solids, Vol. 100, 1988, at pages 269 to 273 describes a method of forming fluorine-doped silicon oxide by a sol-gel method in which triethoxyfluorosilane is added to the sol component. This method produces fluorine-doped silica glasses for manufacturing optical fibres. The sol is cast into moulds to prepare wet gels shaped as rods and tubes as preforms for optical fibres. After gelation, the gel is dried and then sintered at temperatures up to 2000°C. The object of this process is to control the refractive index of the resulting optical fibre. International Patent Application WO91/07774 describes a process for producing an intermediate oxide insulation layer for equalising surface irregularities in an integrated circuit. Polysiloxane is first produced by photo-inductive polymerisation in the gas phase. The polysiloxane is then condensed onto a substrate and subsequently converted into a silicate layer by the application of ultraviolet light. Summary of the Invention The invention provides a method of forming a silicon oxide film containing fluorine as defined in the appendent claims, to which reference should now be made. In a preferred embodiment of the invention the film is selectively formed on a substrate surface having an organic film pattern formed thereon. The film forms at portions of the substrate not covered by said organic film pattern. In the preferred embodiment the substrate temperature is not more than 150°C. Brief Description of the Drawings The invention will now be described in more detail by way of example with reference to the drawings, in which:- Fig. 1 is a schematic view showing an apparatus for forming a silicon oxide film by a method according to a first embodiment of the present invention; Fig. 2 is a graph showing a Fourier transform infrared absorption spectrum of a silicon oxide film formed by a method embodying the present invention; Fig. 3 shows Fourier transform infrared absorption spectra of a conventional silicon oxide film; and Figs. 4A to 4E are sectional views showing the steps in manufacturing a semiconductor device by a method according to a second embodiment of the present invention. Detailed Description of the Preferred Embodiments The present invention will be described below with reference to the accompanying drawings. In the following embodiments, triethoxyfluorosilane (chemical formula: F-Si(OC 2 H 5 ) 3 ) is used as alkoxyfluorosilane. A film forming apparatus is schematically shown in Fig. 1. In Fig. 1, a susceptor 102 is arranged in a reaction chamber 101, the susceptor 102 has a heater 103, and a semiconductor substrate 104 is fixed on the susceptor 102. The susceptor 102 may be arranged on either an upper or lower side of the apparatus, and the semiconductor substrate surface may face either the upper or lower direction. Triethoxyfluorosilane is present in a bubbler 107 kept at a predetermined temperature, is bubbled by nitrogen gas having a predetermined flow rate controlled by a flow rate controller 105, and is supplied to the reaction chamber 101. Steam used for hydrolysis reaction between triethoxyfluorosilane and containing distilled water is bubbled by nitrogen gas having a predetermined flow rate controlled by a flow rate controller 106 and is supplied from a bubbler 108 kept at a predetermined temperature to the reaction chamber 101. The reaction chamber 101 is always evacuated to be kept at a predetermined pressure. The temperatures of gas supply tubes 111 and 112 respectively extending from the bubblers 107 and 108 to the reaction chamber are kept to be constant by heaters 109 and 110. A silicon oxide film was formed on a silicon substrate having a diameter of 150 mm (6 inches) in the above apparatus. The temperature of the bubbler 107 for triethoxyfluorosilane and the bubbler 108 for distilled water were kept to be 40°C and 60°C, respectively, the flow rate of nitrogen gas used for bubbling was set to be 1 SLM, a substrate temperature was fixed to 30°C, and a pressure in the reaction chamber 101 was set to be about 93300 Pa (700 Torr). In Figs. 2 and 3, Fourier transform infrared absorption spectra of this embodiment and the prior art were compared with each other. According to this embodiment, when a film deposition was performed for 60 minutes to form a silicon oxide film containing fluorine and having a thickness of about 0.5 µm, the fourier transform infrared absorption spectrum (FT-IR) indicated by X in Fig. 2 could be obtained. When silicon oxide films each having a thickness of about 0.5 µm were formed at temperatures of 250°C and 350°C in a method according to the prior art, Fourier transform infrared absorption spectra (FT-IR) indicated by Y and Z in Fig. 3 could be obtained. In Fig. 2, absorption peaks corresponding to hydroxyl radicals, Si-O bonds, and Si-OH bonds are observed at wave numbers of around 3,300 cm -1 , around 1,070 cm -1 , and around 930 cm -1 , respectively, and it is understood that a film having Si-O as a main component is formed. As a result, although the film of this embodiment is formed at a very low temperature of 30°C, the absorption peak intensity corresponding to the hydroxyl radicals in Fig. 2 is smaller than that (indicated by Y in Fig. 3) corresponding to the hydroxyl radicals of the silicon oxide film formed at the temperature of 250°C in the conventional method. This means that the silicon oxide film according to the present embodiment has a water content less than that of the silicon oxide film formed at a relatively low temperature of 250°C in the conventional method. In addition, when the silicon oxide film according to the present embodiment is compared with a silicon oxide film formed at a temperature of 350°C (indicated by Z in Fig. 3) in the conventional method, the absorption peaks corresponding to the hydroxyl radicals of both the silicon oxide films are equal to each other. That is, the water content of the silicon oxide film formed at a very low temperature of 30°C on the basis of the present embodiment is equal to the water content of the silicon oxide film formed at the temperature of 350°C in the conventional method. The refractive index of the silicon oxide film formed in this embodiment was 1.42 which was lower than the refractive index of 1.45 of a conventional silicon oxide film. In addition, when a fluorine content in the silicon oxide film of this embodiment was measured by an X-ray photoemission spectroscopy, a fluorine atom concentration was about 7 atom%. According to this embodiment, a film stress of 2.10 3 N/cm 2 (2 × 10 8 dyn/cm 2 ) which was smaller than the film stress (10 4 N/cm 2 ) (1 × 10 9 dyn/cm 2 ) of the silicon oxide film formed by the conventional method can be obtained. An aluminum electrode having an area of about 4 mm 2 was formed on the silicon oxide film formed in this embodiment, and its electric characteristics were examined. As a result, a leakage current density of the electrode during an application of 5 V was about 5 × 10 -11 A/cm 2 , and the value was smaller than the leakage current density of about 2 × 10 -10 A/cm 2 of a silicon oxide film having the same thickness as that of the above silicon oxide film and formed at the temperature of 350°C in the conventional method. In addition, a dielectric breakdown strength of 5 MV/cm which was greater than a dielectric breakdown strength of 4 MV/cm of the silicon oxide film formed in the conventional method could be obtained. A relative dielectric constant calculated from capacitance-voltage characteristics at the measurement frequency of 1 MHz was 3.7, and the value was smaller than the relative dielectric constant of 4.0 of the silicon oxide film formed in the conventional method. Film forming conditions are not limited to those in this embodiment. According to this embodiment, a substrate temperature may be set to be 200°C or less, and the temperature is set such that projections called hillocks are not formed when a silicon oxide film is formed on an aluminum wiring layer. In contrast to this, projections called hillocks were formed at a temperature above 250°C. In addition, a pressure in the reaction chamber, a gas flow rate, and the like can be changed. The second embodiment of the present invention will be described below with reference to Figs. 4A to 4E. In this embodiment, the same film forming conditions as described in the first embodiment are used. Figs. 4A to 4E are cross-sectional views showing the steps in manufacturing a semiconductor device when a selective growing method of a silicon oxide film according to the second embodiment of the present invention is applied to an insulating interlayer of a two-layered aluminum wiring structure. In Fig. 4A, a PSG (Phosphosilicate Glass) film 302 having a thickness of about 0.5 µm and a first aluminum wiring layer 303 having a thickness of about 1 µm are sequentially formed on a semiconductor substrate 301. At this time, a photoresist film 304 serving as an organic film pattern and formed when the aluminum wiring layer is patterned is left. Note that a Teflon (RTM) or PTFE film or a polyimide film may be used as the organic film pattern. As shown in Fig. 4B, according to the present embodiment a silicon oxide film 305 containing fluorine and having a thickness of about 1 µm is selectively formed on only the PSG film portion having no photoresist film 304. After the photoresist film 304 is ashed in an oxygen plasma, according to the present embodiment as shown in Fig. 4C, a silicon oxide film 306 containing fluorine and having a thickness of about 0.7 µm is formed on the entire surface of the resultant structure again. As shown in Fig. 4D, openings 307 are formed in the silicon oxide film 306 in correspondence with the first aluminum wiring layer by the well-known photoetching method. Finally, as shown in Fig. 4E, a second aluminum wiring layer 308 is formed, such that a two-layer aluminum wiring structure having a perfectly flat dielectric film surface can be obtained. In the two-layer aluminum wiring structure having the first and second aluminum wiring layers 303 and 308 and formed in the above steps, the upper second aluminum wiring layer 308 was free from disconnections and short circuits, and a high yield could be obtained. In addition, no cracks in the interlayer dielectric film were observed. Film forming conditions are not limited to those in this embodiment. A substrate temperature may be set to be lower than a maximum temperature of 150°C at which the photoresist film is not deteriorated. In this embodiment, although aluminum is used as a wiring material, the wiring layer may be made of a material selected from the group consisting of polysilicon, aluminum alloys, gold, tungsten, molybdenum, and titanium alloys. An insulating film formed by another method can be used in place of the silicon oxide film containing fluorine which is formed after the photoresist film is ashed. In addition, a photoresist film is not left when an aluminum wiring layer is patterned, and a silicon oxide film containing fluorine can be formed on the entire surface of the resultant structure and used as an insulating interlayer. In this embodiment, although the method is applied to the production of a multi-layered wiring interlayer, the method can be applied to other applications as a method of selectively forming a silicon oxide film. As described above, according to the embodiments illustrated since a vapor containing alkoxyfluorosilane as a main component is used, a silicon oxide film can be advantageously formed at a low temperature of 200°C or less, more particularly, even at room temperature. Film properties of a silicon oxide film formed on the basis of the present method are better than those of the silicon oxide film formed in a conventional method, and the silicon oxide film according to the present method can be practically used as an insulating film used in a semiconductor device. In addition, according to the present method, the following advantages can be obtained. That is, since a silicon oxide film can be formed at a temperature of 150°C or less, a silicon oxide film can be selectively formed using an organic film such as a photoresist film as a mask, and an uneven surface formed in the manufacturing steps of a semiconductor device can be easily flattened. The present method, therefore, provides many effects to a method of manufacturing a semiconductor device. Furthermore, the present method can be sufficiently applied to other electric devices.

Claims: A method of forming a silicon oxide film containing fluorine, characterised in that the film is formed on a substrate surface exposed to a vapour containing alkoxyfluorosilane (chemical formula: F n -Si(OR) 4-n , R= an alkyl group, n = an integer between 1 and 3) as a source gas and water vapour in a reaction chamber having a predetermined temperature and a predetermined pressure, the substrate being at a temperature of not more than 200°C. A method according to claim 1, wherein the alkoxyfluorosilane is triethoxyfluorosilane (chemical formula: F-Si(OC 2 H 5 ) 3 ). A method according to claim 1 or 2, characterised in that the surface of the substrate comprises an organic film pattern, the fluorine-containing silicon oxide film being selectively formed on portions of the surface not covered by the organic film pattern and the substrate temperature being not more than 150°C. A method according to claim 3, wherein said organic film pattern is formed by any one of a photoresist film, a polytetrafluoroethylene film, and a polyimide film. A method according to claim 3 or 4, wherein said organic film pattern is formed as a wiring pattern on the substrate surface (301), and the fluorine-containing silicon oxide film (306) forms an insulating interlayer on said wiring pattern.

Inventors: HOMMA TETSUYA, HOMMA, TETSUYA, Homma, Tetsuya, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H01L21
IPC codes: H01L21, H01L23, C23C16