JP2008283001A - Method of forming oxide film on polycrystalline silicon thin film, and semiconductor device comprising the oxide film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of forming an oxide film on a polycrystalline silicon thin film having an irregular surface, and to provide a MOS semiconductor device comprising the oxide film. <P>SOLUTION: A polycrystalline silicon thin film 12 at least partly having a maximum surface roughness exceeding 30 nanometers is immersed into an oxidizing solution 22 at a temperature higher than 100°C, is subjected to spraying of the solution, or is exposed to vapor of the solution to form a silicon oxide film. This causes the surface of the polycrystalline silicon thin film to have a roughness reduced to 30 nanometers or less, and even when the silicon oxide film formed on the polycrystalline silicon thin film is made thin, it has a uniform thickness. Consequently, the oxide film can sufficiently function as a TFT gate insulating film. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a silicon oxide film used for a thin film transistor (TFT) used in a liquid crystal display device or the like, and a MOS semiconductor device provided with the oxide film.

  One configuration of a thin film transistor (TFT) used in a liquid crystal display device is a polycrystalline silicon thin film and a silicon oxide film as a gate insulating film formed on the surface by a chemical vapor deposition (CVD) method. And a gate electrode on the oxide film.

  Here, the roughness of the surface of the polycrystalline silicon thin film formed by polycrystallizing the amorphous silicon layer or microcrystalline silicon layer as a starting material by a laser annealing method is 30 nm or more, and in some cases 100 nm or more. Become. This polycrystalline silicon thin film with large irregularities on the surface is a factor that reduces the manufacturing yield even in the case of a conventional TFT in which a relatively thick gate insulating film of about 50 to 100 nm is formed thereon. In the case where the film thickness is less than 100 nm, this unevenness becomes an extremely large obstacle that hinders improvement in productivity.

  Specifically, when the thickness of the gate insulating film in the TFT is reduced to 100 nm or less, particularly 50 nm or less, this unevenness causes a further decrease in the insulating property of the gate insulating film, and causes variations in the threshold characteristics of the TFT. It will be very big. However, at present, no effective technique has yet been found that can solve the above-described problems and form a highly flat polycrystalline silicon surface. Therefore, there is a strong demand in the industry for a technique for eliminating the unevenness on the surface of the polycrystalline silicon thin film.

By the way, at least a part of the inventors of the present application has already contacted an oxidizing solution or a gas thereof with a semiconductor substrate containing silicon to form a SiO ₂ film on the surface of the semiconductor substrate. A novel oxidation technology has been developed (see, for example, Patent Document 1).
JP 2007-27453 A (published February 1, 2007) JP 2006-13530 A (published January 12, 2006) Japanese Patent Laying-Open No. 2005-313102 (released on November 4, 2005) Japanese Patent Laying-Open No. 2005-3131303 (published on November 4, 2005) Japanese Patent Laying-Open No. 2005-313152 (published on November 4, 2005) JP 2004-47935 A (published February 12, 2002) JP 2002-289612 A (published on October 4, 2002) JP 2002-64093 A (published February 28, 2002) Japanese Patent Laid-Open No. 11-214386 (released on August 6, 1999) Japanese Patent Laid-Open No. 10-050701 (published February 20, 1998) JP 09-045679 A (published February 14, 1997) Nagayama and 2 others, "Low-temperature formation and spectroscopic observation of SiO2 / Si structure by chemical method", Abstracts of lectures of the Physical Society of Japan, The Physical Society of Japan, August 15, 2003, Vol. 58, No. 2, p. 771

  As described above, since the flatness of the surface of the polycrystalline silicon thin film significantly affects the stability of the electrical characteristics of the semiconductor device to be manufactured, a technique for improving the flatness is desired. The object of the present invention is not particularly limited to the contents described below. For example, one object is to improve the flatness of the surface of a polycrystalline silicon thin film having large irregularities. If the flatness of the surface of the polycrystalline silicon thin film increases, even if an extremely thin silicon oxide film is formed on the layer, it can function sufficiently as a gate insulating film of a MOS transistor. Furthermore, variation in electrical characteristics between the transistors is reduced, and the yield of thin film transistor (TFT) manufacturing is improved.

  The present invention solves the above technical problem, improves the performance of the gate insulating film of the MOS type semiconductor device formed using the polycrystalline silicon thin film, and the performance of the MOS type semiconductor device using the gate insulating film. It contributes to the improvement. The inventors found that protrusions called ridges that occur when an amorphous silicon layer or microcrystalline layer is polycrystallized by a laser annealing method or the like greatly affects the characteristics of the silicon oxide film that is the subsequent gate insulating film. Recognized. Accordingly, the inventors have found that reduction of protrusions on the surface of the polycrystalline silicon thin film and formation of a uniform oxide film can be achieved simultaneously by low-temperature chemical oxidation using nitric acid or the like, and the present invention has been completed.

  One method of forming a silicon oxide film according to the present invention is to form a polycrystalline silicon thin film having a maximum surface roughness (in other words, a maximum ridge) exceeding 30 nanometers at least in part at an oxidizing solution exceeding 100 ° C. Soaking or exposing to the vapor of the solution.

  By this method, the surface roughness of the polycrystalline silicon thin film exceeding 30 nanometers is initially reduced to 30 nanometers or less. According to this method, even if the initial polycrystalline silicon thin film has a surface roughness of 50 nanometers or more, this is reduced to 30 nanometers or less. In addition to the improvement in flatness, the formed silicon oxide film itself has sufficient electrical characteristics as a gate insulating film of a MOS device, despite a thin film of about 10 nm.

  Another method of forming a silicon oxide film according to the present invention is to immerse a polycrystalline silicon thin film having a maximum surface roughness of more than 30 nanometers at least in part in an oxidizing solution of more than 100 ° C. or a solution thereof. A step of forming a silicon oxide film by exposing to the vapor, a step of etching the silicon oxide film, and a step of forming a silicon oxide film again after the etching step.

  Also by this method, the surface roughness of the polycrystalline silicon thin film, which was initially over 30 nanometers, is reduced to 30 nanometers or less. In this method, even if the initial polycrystalline silicon thin film has a surface roughness of 50 nanometers or more, this is reduced to 30 nanometers or less. In addition to the improvement in flatness, the formed silicon oxide film itself has sufficient electrical characteristics as a gate insulating film of a MOS device regardless of the film formation method.

  In addition, one MOS type semiconductor device of the present invention includes a silicon oxide film formed by immersing a polycrystalline silicon thin film in an oxidizing solution of over 100 ° C. or exposing to a vapor of the solution, and the polycrystalline silicon thin film. The maximum roughness of the interface with the thin film is 30 nanometers or less, and the silicon oxide film is a gate insulating film.

  According to this MOS type semiconductor device, in a polycrystalline silicon thin film whose flatness has been improved by immersion in an oxidizing solution or exposure of the solution to vapor, a dense silicon oxide film is at least in contact with the polycrystalline silicon thin film. Since it is formed at the interface, the formed silicon oxide film is excellent in electrical characteristics as a gate insulating film.

  Furthermore, another MOS type semiconductor device according to the present invention comprises forming a silicon oxide film by immersing a polycrystalline silicon thin film in an oxidizing solution exceeding 100 ° C. or exposing to a vapor of the solution, and then forming the silicon oxide film. The silicon oxide film formed on the surface of the polycrystalline silicon after etching the oxide film is a gate insulating film.

  According to this MOS type semiconductor device, since the flatness of the polycrystalline silicon thin film is improved by immersion in an oxidizing solution or exposure of the solution to vapor, the silicon oxide film formed on the layer is In particular, it has excellent electrical characteristics as a gate insulating film regardless of the film formation method.

  According to the method for forming a silicon oxide film of the present invention, the surface roughness of the polycrystalline silicon thin film, which initially exceeded 30 nanometers, is reduced to 30 nanometers or less. Further, according to the MOS type semiconductor device of the present invention, in the polycrystalline silicon thin film whose flatness is improved by immersion in an oxidizing solution or exposure of the solution to vapor, oxidation of silicon formed on the layer is performed. The film is excellent in electrical characteristics as a gate insulating film.

  Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this description, common parts are denoted by common reference symbols throughout the drawings. In the drawings, the elements of the embodiments are not necessarily shown to scale.

<First Embodiment>
FIG. 1 is a schematic cross-sectional view showing a configuration of a MOS type semiconductor device of this embodiment (a MOS capacitor in this embodiment). The MOS type semiconductor device 100 of this embodiment includes a glass substrate 10 (hereinafter also simply referred to as “substrate”) as a support substrate, a polycrystalline silicon thin film 12 formed on the glass substrate 10, A silicon oxide film 14 which is a gate insulating film formed on the crystalline silicon thin film 12 and a metal layer 16 such as aluminum (Al) formed on the silicon oxide film 14. Yes.

  Next, a method for manufacturing the MOS semiconductor device of this embodiment will be described.

First, an amorphous silicon layer or a microcrystalline silicon layer is formed on the glass substrate 10 by a PECVD (Plasma-Enhanced Chemical Vapor Deposition) method or the like using a known gas species such as silane (SiH ₄ ). When this amorphous silicon layer or microcrystalline silicon layer is heated to be polycrystallized by laser irradiation or the like, a polycrystalline silicon thin film 12 is formed.

  Here, the surface roughness of the polycrystalline silicon thin film was confirmed by a cross-sectional TEM photograph. FIG. 2 is a cross-sectional view of the surface portion immediately after the formation of the polycrystalline silicon thin film 12 in the present embodiment. As shown in FIG. 2, it was found that protrusions having a height of about 100 nm periodically exist on the surface of the polycrystalline silicon thin film. This kind of protrusion is caused by the undulation of the crystal grain growth boundary (grain boundary) as a result of accelerating crystallization by instantaneously locally heating an amorphous silicon layer or a fine polycrystalline silicon thin film by laser heating or the like. it is conceivable that.

  Next, as a pretreatment for forming a silicon oxide film on the polycrystalline silicon thin film, an immersion treatment in a dilute hydrofluoric acid solution having a concentration of 0.5 vol% is performed for about 5 minutes. Thereafter, rinse treatment (cleaning) was further performed with ultrapure water.

  After this cleaning process, the glass substrate 10 on which the polycrystalline silicon thin film 12 is formed is immersed in an oxidizing solution using a processing apparatus 200 as shown in FIG. 3A. Specifically, the substrate 10 held by a known substrate holding tool (not shown for convenience) has a concentration of 40 wt% or less (40 wt% in this embodiment) at room temperature (about 25 ° C.) filled in the processing tank 24. In concentrated nitric acid (aqueous solution) 22 (referred to as “first oxidizing solution” for convenience). In this state, the substrate 10 and the polycrystalline silicon thin film 12 are heated to a boiling state (boiling temperature is about 108 ° C.) by the heater 26 (for example, an acid-resistant quartz heater for liquid heating). The In the present embodiment, this state continues for 10 minutes.

As a result, a silicon oxide film was formed on the polycrystalline silicon thin film 12. As a result of subsequent analysis, this oxide film (for convenience, referred to as “first oxide film”) is porous and has a relatively low atomic density, silicon having an atomic density of 2.18 × 10 ²² atoms / cm ³ . It was found to be an oxide film. At this time, the silicon oxide film thickness was 1.1 nm.

  By continuing this boiling state while the glass substrate 10 and the polycrystalline silicon thin film 12 on which the silicon oxide film is formed are immersed in the concentrated nitric acid 22, the nitric acid is in an azeotropic state (concentration 68 wt%, boiling point 120). .7 ° C.). In the present embodiment, the polycrystalline silicon thin film 12 was immersed in an azeotropic aqueous nitric acid solution (referred to as “second oxidizing solution” for convenience) for 2 hours.

As a result, an oxide film 14 (referred to as “second oxide film” for convenience) thicker than the first oxide film was formed on the polycrystalline silicon thin film 12. As a result of subsequent analysis, the second oxide film 14 was a relatively dense film having an atomic density of 2.22 × 10 ²² atoms / cm ³ . Here, the second oxide film 14 was formed so as to take in the first oxide film, and seemed to be integrated. At this stage, the silicon oxide film 14 has grown to a thickness of about 10 nm.

  FIG. 4 shows a cross-sectional SEM photograph showing the vicinity of the interface between the polycrystalline silicon thin film 12 and the silicon oxide film layer 14 formed thereon at this time. As shown in this figure, it can be seen that a silicon oxide film 14 having a uniform thickness is formed on the polycrystalline silicon thin film 12. Furthermore, it is particularly noteworthy that the unevenness of about 100 nm originally formed on the polycrystalline silicon surface has been reduced to 20 nm or less as the maximum roughness. As a result of the improved flatness of the polycrystalline silicon thin film 12, the interface between the uppermost aluminum layer 16 and the silicon oxide film layer 14 is very smooth.

  Here, the reason why the protrusions on the surface of the polycrystalline silicon thin film are remarkably reduced is not clear. However, by including the step of immersing in an oxidizing aqueous solution (in this embodiment, nitric acid aqueous solution) having a concentration of 40% or less, the surface of the polycrystalline silicon thin film, particularly the crystal grain growth boundary (grain boundary in this layer) It is thought that the main reason is that the surface of the part) is more eroded. As a result, the surface roughness of the polycrystalline silicon thin film, in other words, the reduction in the roughness of the interface between the silicon oxide film and the polycrystalline silicon thin film, and the formation of a uniform silicon oxide film can be achieved simultaneously. Is done.

  Next, an aluminum layer 16 as an electrode is formed on the above-described silicon oxide film 14 by a known sputtering method or the like. The MOS type semiconductor device of this embodiment is manufactured in this way.

  As described above, the MOS type semiconductor device according to the present embodiment has a significantly improved flatness as compared with the surface roughness of the conventional polycrystalline silicon thin film. Therefore, not only as a MOS capacitor but also in a thin film transistor (TFT). The electrical properties and the stability of the properties between products are improved. In addition, the manufacturing yield is improved. More specifically, for example, in the MOS TFT semiconductor device according to the present embodiment, the driving voltage having one electrical characteristic can be reduced by about 50% compared to the conventional case.

  By the way, in the present embodiment, when the first oxide film is formed and when the second oxide film is formed, the oxidizing solution used for immersing the polycrystalline silicon thin film is heated to a boiling state. The temperature may not reach a complete boiling state. Specifically, the effect of the present invention is substantially achieved even near the boiling point (for example, a temperature lower by about 1 to 5 ° C. than the boiling point).

  Further, as in the present embodiment, instead of a solution with a continuous increase in concentration, at least one predetermined concentration heated to or near the boiling point (for example, 20 wt% or 40 wt% nitric acid (aqueous solution)). And at least two solutions of nitric acid in a substantially azeotropic state or azeotropic state may be prepared in advance. In this case, the same effect as that of the present embodiment can be obtained. Here, being near the boiling point is a temperature lower by about 1 to 5 ° C. than the boiling point, and the substantially azeotropic state is a temperature lower by about 1 to 5 ° C. than the temperature of the solution in the azeotropic state.

<Second Embodiment>
This embodiment has the same manufacturing process up to the formation stage of the silicon oxide film (second oxide film) in the first embodiment. Accordingly, the description of the same manufacturing process and the configuration of the MOS type semiconductor device as in the first embodiment will be omitted. 5A to 5C show a manufacturing process flow of the MOS semiconductor device according to this embodiment. 5A to 5C show cross-sectional views of the MOS type semiconductor device. First, FIG. 5A shows a state in which the second oxide film 14 is formed on the polycrystalline silicon thin film. Thereafter, as shown in FIG. 5B, the second oxide film 14 is etched by buffered hydrofluoric acid. Thereafter, as shown in FIG. 5C, a silicon oxide film 18 is again formed on the polycrystalline silicon thin film 12 by a plasma CVD method under known conditions.

  Also in this case, as in the first embodiment, the polycrystalline silicon thin film 12 has a surface roughness of about 1/5 or less than that of the initial film. The flatness of the interface with the crystalline silicon thin film 12 is remarkably improved as compared with the conventional one. As a result, not only the MOS capacitor but also the electrical characteristics of the thin film transistor (TFT) and the stability of the characteristics between products are improved. In addition, the manufacturing yield is improved. More specifically, for example, in the MOS TFT semiconductor device according to the present embodiment, the driving voltage having one electrical characteristic can be reduced by about 30% compared to the conventional case.

<Third Embodiment>
The present embodiment is the same as the manufacturing process in the first embodiment except for the step of forming a silicon oxide film (second oxide film). Accordingly, the description of the same manufacturing process and the configuration of the MOS type semiconductor device as in the first embodiment will be omitted. After the polycrystalline silicon thin film 12 is formed on the glass substrate 10, the surface of the polycrystalline silicon thin film 12 is oxidized using a processing apparatus 300 as shown in FIG. 3B after cleaning. Specifically, the substrate 10 and the oxidizing solution 22 held by a known substrate holding device (not shown) are placed in a predetermined container 34, and the oxidizing solution 22 is evaporated by heating the container 34 with a heater 36. May be. Thereby, since the substrate 10 is exposed to the vapor 32 of the solution 22, the same effect as the first embodiment is exhibited. At this time, the substrate 10 is exposed to fresh vapor 32 by appropriately evacuating with an exhaust pump.

  Further, using a processing apparatus 400 as shown in FIG. 3C, the mist 42 of the oxidizing solution is applied by the sprayer 46 to the substrate 10 that is stored in a predetermined container 44 and held by a known substrate holding device (not shown). You may spray. In this case, the mist 42 may be a vapor of an oxidizing solution. For example, even if the mist 42 is about room temperature and does not reach the boiling point of the oxidizing solution, the mist 42 can be sprayed in the form of a mist. It is effective for.

  After the silicon oxide film 14 is formed on the polycrystalline silicon thin film 12 using the processing apparatus as described above, a MOS type semiconductor device is manufactured as in the first embodiment. Also in this case, as in the first embodiment, the polycrystalline silicon thin film 12 has a surface roughness of about 1/5 or less than that of the initial film. The flatness of the interface with the crystalline silicon thin film 12 is remarkably improved as compared with the conventional one. As a result, not only the MOS capacitor but also the electrical characteristics of the thin film transistor (TFT) and the stability of the characteristics between products are improved. In addition, the manufacturing yield is improved.

  By the way, in each above-mentioned embodiment, nitric acid aqueous solution was used as the oxidizing solution, but instead of this, perchloric acid, sulfuric acid, ozone-dissolved water, hydrogen peroxide solution, and a mixture of hydrochloric acid and hydrogen peroxide solution At least one solution selected from the group consisting of a solution, a mixed solution of sulfuric acid and hydrogen peroxide solution, a mixed solution of ammonia water and hydrogen peroxide solution, a mixed solution of sulfuric acid and nitric acid, and aqua regia, or a solution thereof It is also possible to use the steam. However, the nitric acid aqueous solution used in the above-described embodiments is the most preferable aspect.

  Further, when the azeotropic mixture of each of the above oxidizing solutions and water is used as the second oxidizing solution, the concentration of the solution and vapor (that is, gas) is constant in the boiling state (azeotropic state). Therefore, in the silicon oxide film forming step, the growth of the film can be controlled by time management. In addition, although the above-mentioned azeotrope may be used as the first oxidizing solution, particularly in the case of an aqueous nitric acid solution, it is better to use a solution having a concentration of 40 wt% or less at the beginning for the reasons already described.

  It is also a preferred embodiment of the present invention to manufacture the MOS type semiconductor device disclosed in the second embodiment by using the silicon oxide film forming method employed in the third embodiment.

  The present invention can be applied to a thin film semiconductor device such as a TFT having a silicon oxide film on a polycrystalline silicon thin film.

1 is a schematic cross-sectional view showing a configuration of a MOS semiconductor device according to an embodiment of the present invention. It is sectional drawing of the surface part just after formation of the polycrystalline-silicon thin film in one embodiment of this invention. 1 is an apparatus for manufacturing a silicon oxide film according to an embodiment of the present invention. It is a manufacturing apparatus of the oxide film of silicon in other embodiments of the present invention. It is a manufacturing apparatus of the oxide film of silicon in other embodiments of the present invention. It is sectional drawing which shows the interface part between each layer after manufacture of the MOS type semiconductor device in one embodiment of this invention. It is a part of manufacturing process flow of the MOS type semiconductor device in other embodiments of the present invention. It is a part of manufacturing process flow of the MOS type semiconductor device in other embodiments of the present invention. It is a part of manufacturing process flow of the MOS type semiconductor device in other embodiments of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Glass substrate 12 Polycrystalline silicon thin film 14,18 Silicon oxide film 16 Aluminum layer 22 Oxidizing solution 24 Processing tank 26,36 Heater 34,44 Container 46 Sprayer 100 MOS type semiconductor device 200,300,400 Processing apparatus

Claims (10)

Soaking at least in part a polycrystalline silicon thin film having a maximum surface roughness greater than 30 nanometers in an oxidizing solution above 100 ° C. or spraying said solution or exposing it to the vapor of the solution A method for forming a silicon oxide film. Etching the silicon oxide film;
The method for forming a silicon oxide film according to claim 1, further comprising a step of forming a silicon oxide film again after the etching step. The oxidizing solution includes an aqueous solution having a nitric acid concentration of 40 wt% or less and a boiled first oxidizing solution, and an aqueous solution having a nitric acid concentration exceeding 40 wt% and a boiled second oxidizing solution. A method for forming a silicon oxide film as described. The method for forming a silicon oxide film according to claim 3, wherein the second oxidizing solution is an azeotropic nitric acid aqueous solution. The method for forming a silicon oxide film according to claim 1, wherein the polycrystalline silicon thin film is obtained by polycrystallizing an amorphous silicon layer or a microcrystalline silicon layer as a starting material by a laser annealing method. The maximum of the interface between the polycrystalline silicon thin film and the polycrystalline silicon thin film formed by immersing the polycrystalline silicon thin film in an oxidizing solution above 100 ° C., spraying the solution, or exposing to the vapor of the solution. A MOS type semiconductor device having a roughness of 30 nanometers or less, and wherein the silicon oxide film is a gate insulating film. After forming the silicon oxide film by immersing the polycrystalline silicon thin film in an oxidizing solution of over 100 ° C., spraying the solution, or exposing to the vapor of the solution, etching the silicon oxide film A MOS type semiconductor device, wherein a silicon oxide film newly formed on the surface of the polycrystalline silicon is a gate insulating film. The oxidizing solution includes an aqueous solution having a nitric acid concentration of 40 wt% or less and a boiled first oxidizing solution, and an aqueous solution having a nitric acid concentration exceeding 40 wt% and a boiled second oxidizing solution. The MOS type semiconductor device described. The MOS semiconductor device according to claim 8, wherein the second oxidizing solution is an azeotropic nitric acid aqueous solution. The polycrystalline silicon thin film is obtained by polycrystallizing an amorphous silicon layer or a microcrystalline silicon layer as a starting material by a laser annealing method, and before the oxide film of silicon is formed The MOS type semiconductor device according to claim 6, wherein at least a part of the maximum surface roughness is 30 nanometers or more.