Abstract:
A method for oxidizing a nitride film is disclosed, which includes the steps of: providing a nitride film formed on an electrically conductive substrate; irradiating the nitride film with a light beam and getting close to the nitride film with a electrically conductive probe; and exerting a bias between the electrically conductive substrate and the electrically conductive probe. The method can oxidize the nitrides quickly and reduce the cost building a nano-structure in the nitride film. An apparatus for oxidizing a nitride film is also disclosed herewith.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and an apparatus for oxidizing nitrides, and more particularly, to a method and an apparatus for oxidizing nitrides in order to form a structure with nano-pattern in the nitrides. 
   2. Description of Related Art 
   Owing to the excellent dielectric property, nitrides have become important materials in the manufacture process of semiconductor devices. On the other hand, nitrides often act as masks in the process of manufacturing microstructures due to its high resistance against acids and bases. In addition, nitrides have also become one of the important optical materials in recent years because of their excellent optical property. For example, silicon nitride (Si 3 N 4 ) can serve as an insulating layer in semiconductor devices as well as mask layer in a CMOS manufacturing process. On the other hand, aluminum nitride (AlN) is not only used for forming insulating layers of semiconductor devices, but also for forming heat conductive layers of the same devices. Furthermore, aluminum nitride can also be utilized in the manufacturing process of UV sensors. 
   Although a nitride film can be widely applied, the nitride film is not easy to be patterned because of its stable chemical property. Moreover, the oxidation rate of nitrides is very slow so that complex processes are required to oxidize or modify the nitride film for subsequent application. Therefore, it is difficult to carry out the oxidation process of a nitride film. For example, the temperature for transforming silicon nitride into silicon oxide by using conventional wet thermal oxidation technique is required to be as high as 1100° C. (with reference to T. Enomoto, R. Ando, H. Morita, and H. Nakayama, Jpn. J. Appl. Phys. Vol. 17, p. 1049 (1978)). On the other hand, gallium nitride can be transformed into gallium oxide by reacting with the hot air only when the reaction temperature is higher than 900° C. (with reference to S. D. Wolter, et al., Appl. Phys. Lett. Vol. 70, p. 2156 (1997)). However, heat also decomposes nitrides and thereby the film quality deteriorates through heating (with reference to C. B. Vartuli et al., J. Vac. Sci. Technol. B 14, p. 3523 (1996)). Even though there is alternative (e.g. anode electrolysis oxidation) for transforming nitride film into oxide film without heat, however, the oxidation rate of Si 3 N 4  in conventional anode electrolysis oxidation process is too slow (with reference to T. B. Tripp, J. Electrochem. Soc. Vol. 117, p. 157 (1970)). 
   U.S. Pat. No. 6,190,508 discloses a method to enhance the oxidation probability of InAlN, InGaN and GaN with exposure of light. However, since the scope of oxidation is too large, it is hard to build a micro- or nano-structure without cooperating with a photolithography process. Recently, a method to oxidize nitrides on the conductive substrate by using scanning probe lithography is also suggested, which uses a voltage of 5V at ambient temperature [F. S. Chien, et al., Appl. Phys. Lett., Vol. 76, No. 3, p. 360 (2000)]. Through the oxidation illustrated above, the line width of the resulted silicon oxide can be shorter than 100 nanometers. Subsequently, owing to the high selectivity to acids or bases, careful etching can form a structure having a high aspect ratio and a line width shorter than 100 nanometers. In other words, the desired pattern can be “written” directly without using a mask. Consequently, the manufacturing cost can be lowered significantly. Although this method can oxidizes nitrides at an ambient temperature, it is not practical for application due to the shield of electronic field and the retardation of electron flow as the thickness of the oxidized film increases. Therefore, the chemical reaction will stop and the thickness of the oxidized scope will range in a few nanometers only. 
   SUMMARY OF THE INVENTION 
   The object of the present invention is to provide a method for oxidizing nitrides so that parts of the nitrides can be oxidized quickly, the thickness of oxidized nitrides can be increased, and the cost for building a nano-structure in the nitride film can be lowered down. 
   The object of the present invention is to provide an apparatus for oxidizing nitrides so that parts of the nitrides can be oxidized quickly, the thickness of oxidized nitrides can be increased, and the cost for building a nano-structure in the nitride film can be lowered down. 
   To achieve the object above-mentioned, the method for oxidizing nitrides of the present invention comprises the following steps. First, a nitride layer formed on an electrically conductive substrate is provided. Afterwards, the nitride layer is illuminated with a light source and approached by an electrically conductive probe. Finally, a bias is exerted between the electrically conductive substrate and the electrically conductive probe. 
   To achieve the object above-mentioned, the method for oxidizing nitrides of the present invention comprises the following steps. First, a nitride layer formed on an electrically conductive substrate is provided. Afterwards, the nitride layer is approached by an optical fiber, of which the surface is plated with an electrically conductive material. Finally, a bias is exerted between the electrically conductive substrate and the electrically conductive material. 
   To achieve the object above-mentioned, the apparatus for oxidizing nitrides of the present invention comprises a nitride layer formed on an electrically conductive substrate; a light source adjacent to the electrically conductive substrate for providing the energy that excites the nitrides; an electrically conductive probe close to the surface of said nitride layer for controlling the oxidation scope of the nitride layer; and a bias exerted between the electrically conductive substrate and the electrically conductive probe. 
   To achieve the object above-mentioned, the apparatus for oxidizing nitrides of the present invention comprises a nitride layer formed on an electrically conductive substrate; an optical fiber plated with an electrically conductive material and being close to the surface of the nitride layer for providing the energy that excites the nitrides and controlling the oxidation scope of the nitride layer; and a bias exerted between the electrically conductive substrate and the electrically conductive probe. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view showing a preferred embodiment of the apparatus for oxidizing nitrides of the present invention; 
       FIG. 2  is a schematic view showing another preferred embodiment of the apparatus for oxidizing nitrides of the present invention; and 
       FIG. 3  is a schematic view showing the other preferred embodiment of the apparatus for oxidizing nitrides of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The material of the nitride layer of the method and apparatus for oxidizing nitrides of the present invention can be any conventional nitrides. Preferably, the nitride layer is composed of Si 3 N 4 , oxynitride, Ge 3 N 4 , TiN, BN, AlN, GaN, InN, InGaN, InAlN, or AlInGaN. The electrically conductive substrate of the present invention can be made of any conventional conductive material used for making a substrate. Preferably, the electrically conductive substrate is made of a p-type or n-type silicon wafer, Ge, SiGe, InN, GaN, GaAs, InP, GaP, AlP, InAs, AlAs, AlGaAs, InGaAs, ZnSe, In 2 O 3 :Sn (Tin-doped Indium oxide, ITO), ZnO:F, ZnO:B, SnO 2 :F, ZnSnO 3 , Zn 2 SnO 4 , TiN, Cd 2 SnO4, ZnO:Al, ZnO:Ga, ZnO:In, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Au, Zr, Nb, Mo, Rh, Ag, In, Se, Hf, Ta, W, Ir, Pt, Au or the combinations thereof. The light source of the present invention can be any kind of conventional light sources. Preferably, the light source is a Halogen lamp (254 nm), Nd-YAG (1064 nm, 1320 nm, 532 nm, 354 nm, 66 nm), XeCl (308 nm), XeF (351 nm), KrCl (222 nm), KrF (248 nm), ArF (193 nm), F 2  (157 nm), HeCd (325–441 nm), N 2  (337 nm, 428 nm), Ar (514.5 nm), H 2 (110–162 nm), dye laser (400–800 nm), GaAs/GaAlAs (708–905 nm), HeNe (632 nm), high-pressure mercury lamp, deuterium lamp, or xenon lamp. The electrically conductive probe of the present invention can be any conventional electrically conductive probe. Preferably, the electrically conductive probe is a heavily doped silicon probe, a diamond-like probe, a tungsten probe, or a probe that is plated with a conductive metal. In the present invention, the way that the electrically conductive probe approaches the nitride layer is preferably in a contact mode, an intermittent contact mode, or a tapping mode. The way that the electrically conductive probe approaches the nitride layer is not restricted. Preferably, the electrically conductive probe is driven by a micro actuator to approach the nitride layer. The method and apparatus for oxidizing nitrides of the present invention can further be applied to the oxidation of phosphides, arsenides or metals. Preferably, the phosphides are InP, GaP, AlP, or BP; the arsenides are InAs, GaAs, or AlAs; and the metals are Al, Ti, or Zr. The electrically conductive material plated on the optical fiber of the present invention can be any conventional electrically conductive material. Preferably, the electrically conductive material is the conductive metal, the doped diamond, WC 2 , or doped nitrides. 
   The following detailed description are given by way of example and not intended to limit the invention solely to the embodiments described herein. 
   Embodiment 1 
   With reference to  FIG. 1 , a nitride film  20  with a thickness of 3–15 nanometers is formed on the p-type silicon wafer  10  by the chemical vapor deposition process. Afterwards, the p-type silicon wafer  10  having a nitride film  20  is put on the sample stage, which is electrically connected with the p-type silicon wafer  10 . Moreover, a light source  30 , such as a halogen lamp, emits the UV light with a wavelength of 254 nm onto the nitride film  20 . If the wavelength of the UV light satisfies the following equation:
 
hv&gt;E g ,
 
   wherein h is the Plank constant, ν is the frequency of the emitting light, and E g  is the energy gap of nitride, and then the electron of the nitride is excited. Thereafter, the probability that the electron migrates from the valence band to the conductive band is increased, i.e. the probability for forming hot electrons and holes is increased. Even if the wavelength of the UV light does not satisfy the above equation and the hot electrons and holes are not formed, any other additional energy or lowered energy gap will also increase the probability of electron migration. It is optionally to mount an optical device  40  for adjusting the scope of the nitride film  20  that is illuminated by the UV light. Generally, the luminance of the UV light is kept at 10 mW/cm 2  or more. Subsequently, an electrically conductive probe  50  approaches the nitride film  20 . The electrically conductive probe  50  can be a heavily-doped silicon probe, a diamond-like probe, or a probe that is plated with an electrically conductive metal used in the atomic force microscope (AFM), or a tungsten probe used in the scanning tunneling microscope (STM). The electrically conductive probe can be driven by an atomic force microscope or other micro-actuators to approach the nitride film  20 . Preferably, an atomic force microscope is used to make the probe of the atomic force microscope approach the gallium nitride  20  in contact mode, intermittent contact mode or tapping mode. The contact position of the electrically conductive probe  50  on the nitride film  20  is within the scope  60  of the UV light. At the atmospheric environment, the water is adsorbed on the nitride film  20  to form a water film. When the electrically conductive probe  50  approaches the nitride film  20 , a water bridge is naturally formed therebetween. Afterwards, a bias V is exerted between the electrically conductive probe  50  and the p-type silicon wafer  10 , wherein the p-type silicon wafer  10  is at a high voltage, and the electrically conductive probe  50  is at a low voltage. Preferably, the bias and the wavelength of the UV light satisfy the equation eV+hv&gt;E g . Accordingly, the oxide  21  is formed between the nitride film  20  and the electrically conductive probe  50 . Different voltage will result in oxide film having different height. By another way, if the bias is constant, the height of the oxide can be changed according to the exerting time of the bias. 
   Embodiment 2 
   This embodiment is quite similar to Embodiment 1, except that when the substrate  100  is transparent, the light source  150  can also be located below the substrate  100 . The optical device  110  having a shutter controls the illumination of the light from the light source  150  to the nitride film  120 , and the illumination time thereof is also a controlled variable. Let the bias and wavelength of the illuminating light satisfy the equation eV+hv&gt;E g , and then the oxide  140  can be formed between the nitride film  120  and the probe  130 . Furthermore, the height of the oxide film can be adjusted by changing the wavelength, the illuminating time, or the luminance of the illuminating light. 
   Embodiment 3 
   With reference to  FIG. 3 , a probe made of optical fiber  200  (for example, the probe used in a Scanning Near-field Optical Microscope, SNOM) is plated with an electrically conductive material  210 , such as conductive metals, the doped diamond, WC 2 , or doped nitrides at its tip. Afterwards, let the optical fiber approach the nitride film  220  in a contact mode, an intermittent contact mode, or a tapping mode. Thereafter, a bias V is exerted between the electrically conductive material  210  and the substrate  230 , wherein the substrate  230  is at high voltage, and the electrically conductive material  210  is at low voltage. Particularly, the bias V is less than the minimal critical voltage V 0  that can oxidize the nitride film. After the light coming from the light source  240  passes the optical device  250 , it enters the optical fiber  210 . The optical device  250  has a shutter, which controls whether the light illuminates on the nitride film  220  or nor, and the illuminating time of the light on the nitride film  220 . Also, the optical device  250  can adjust the luminance of the light on the nitride film  220 . Preferably, the bias and the wavelength of the illuminating light satisfy the equation eV+hv&gt;E g . Consequently, the oxide  260  is formed between the nitride film  220  and the optical fiber  210 . Similarly, the height of the oxide film can be adjusted by changing the wavelength, the illuminating time, or the luminance of the illuminating light. 
   In those aforesaid embodiments, an exquisite structure can be obtained by carrying out a selective etching process after the nitride is oxidized. For example, the oxide may be removed by the wet etching process using HF, and then the nitride can serve as a mask. Or, the nitride may be removed by the dry etching process using the plasma, and then the oxide can serve as a mask. On the other hand, the nitride film can also be removed by the wet etching process using H 3 PO 4 , and then the oxide acts as a mask. Or the nitride is removed by the dry etching process using the plasma, and then the oxide acts as a mask. Finally, let the resulted structure serve as a mask in a selective etching process to enhance the aspect ratio. The etchant may be KOH, EDP (Ethylene Diamine Pyrocatechol), or TMAH (Tetra-methyl Ammonium Hydroxide) for the wet etching process, or the plasma for the dry etching process. 
   The present invention uses the light to illuminate and excite the electron of the nitride film, as well as the electrically conductive probe to generate a concentrated electric field above the nitride film. Due to the absorption of the photon energy, the electrons with higher dynamic energy thus migrate from the valence band to the conductive band. Thereafter, the electrons react with hydrogen ions on the probe, and the electric field and the electric holes aid the nitride near the probe to react with the hydroxide ions. Consequently, both the excited electrons and electric holes can accelerate the local oxidation reaction and increase the thickness of the oxide film. 
   While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the spirit and scope of the invention. It is intended that the claims are interpreted to cover those disclosed embodiments, those alternatives which have been discussed above and all equivalents thereto.