TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to fabricating a III-nitride based semiconductor; more particularly, relates to directly etching a chip from top without substrate transferring technique or deep-etching a silicon (Si) substrate from back-side for solving line width problem, where the Si substrate does not need to have substrate thinning process for solving the problem of complex fabrication procedure and the severe bowing problem of large-scaled chip. 
     DESCRIPTION OF THE RELATED ARTS 
     Semiconductor devices of gallium nitride (GaN) and other metal nitride (such as: aluminium nitride (AIN), and indium nitride (InN)) have characteristics of high output current density, high withstand voltage and high power output to be widely used in high frequency components and power devices. In recent years, the advance of epitaxy technology enables GaN components showing its excellent characteristics on being grown with sapphire, silicon carbide and Si substrates. As comparing to the other substrate, growing a GaN device on a large-scaled Si substrate has great advantages, such as good thermal dissipation effect and significant manufacturing cost reduction, and opportunity on integrating existing advanced Si manufacture procedures. 
     However, growing a heterojunction nitride device on a Si substrate still has the following problems: 
     1. The withstand voltage has a value far from ideal. According to a theoretical calculation, GaN should be able to bear a breakdown electric field up to about 3.3 MV/cm (where the value for silicon material is about 0.3 MV/cm). With a heterojunction nitride Schottky diode, a linear growth trend of breakdown voltage is found as following the increase of lateral drift length (which has a slope about 100V/μm). But, following the increase of drift length on the Si substrate, the breakdown voltage of the nitride device will show a saturated trend and the breakdown voltage becomes worse than expected. The reason lies in that the heterogeneous junction between a nucleation layer (usually GaN, AlN or AlGaN) and the Si substrate will generate a parasitic channel owing to the band discontinuity, as shown in  FIG. 9 . When an anode (i.e. Schottky junction) has a large reverse bias, the parasitic channel forms a leaky path. Then, the Schottky junction generates electrons running to a cathode (i.e., ohmic contact) along the leaky path, which results in leakage current surge and early collapse at the Si substrate or between the Si substrate and the nucleation layer. Hence, when the nitride diode is operated under a large reverse bias (about several hundred volts) on the Si substrate, electrons may be easily flown from the anode through leaky path in lateral direction and the buffer layer/nucleation layer in vertical direction to the cathode to generate a large amount of vertical leakage current, where the vertical leakage current is the main cause for early collapse and loss. 
     2. For solving the above problems of breakdown voltage and leakage current, IMEC suggested in 2010 to process measurement after the Si substrate is completely etched out. As shown in  FIG. 10 , the silicon is completely etched out and, then, the device is transferred to another insulating substrate, where the breakdown voltage and the leakage current are found to have significant improvement. In  FIG. 11 , the output voltage current curve of the device in a close state (reverse bias at gate, normal bias at drain) is shown. The main reason is that, by etching out the Si substrate, the parasitic channels exist in the interface of nucleation layer and the Si substrate are removed as well. In addition, after the Si substrate is completely removed, a linear relation between the breakdown voltage and the drift area length (length between gate and drain) is found, as shown in  FIG. 12 . (Bin Lu, et al, “Breakdown mechanism in AlGaN/GaN HEMTs on Si substrate”, Device Research Conference (DRC), 2010) However, this method still has a shortcoming: After the substrate is completely removed and the device is transferred to a glass substrate, the thermal dissipation problem will increase the on-state resistance and decrease the on-state output current, as shown in  FIG. 13 . (P. Strivastava, et al, “Silicon substrate removal of GaN DHFETs for enhanced (&gt;1100 V) breakdown voltage”, IEEE, Electron Device Letters, Vol. 31, No. 8, 2010). 118 lane 14 NO. 
     3. For solving the dissipation problem, IMEC suggested partial-etching silicon trench around drain in the meeting of IEDM, 2011. Only the drain below the Si substrate is etched, where the remaining Si substrate helps solving the dissipation problem (P. Srivastava, et al, “Si trench around drain STAD technology of GaN-DHFETs on Si substrate for boosting power performance” IEEE, International Electron Devices Meeting (IEDM), 2011). The IMEC method comprises the following steps: 
     (1) A Si substrate is polished and/or etched to be made thin to 50˜100 μm. 
     (2) GaN on the Si substrate is transferred to another substrate, such as a glass substrate, through direct bonding. 
     (3) Exposed area is defined at the back-side of chip for deep etching to a depth of 50˜100 μm of the Si substrate. 
     However, this method still has the following disadvantages: 
     1. The Si substrate at bottom of the chip is made thin by being polished and/or etched to about 50˜100 μm for deep-etching the Si substrate. Yet, after this process, the chip is usually bowing. Serious bowing state will easily destroy epitaxial structure during process. Moreover, the production yield may be greatly lowered, especially for the large-scaled chips. 
     2. For transferring the substrate, direct bonding or flip chip is required on re-bonding. 
     3. Line width is not easy to be shrunken on deep-etching the Si substrate in the future. 
     Hence, the prior arts do not fulfill all users&#39; requests on actual use. 
     SUMMARY OF THE INVENTION 
     The main purpose of the present invention is to directly etch a chip from top without substrate transferring technique or deep-etching a silicon (Si) substrate from back-side of chip. 
     Another purpose of the present invention is to solve the problem of complex fabrication procedure and the bowing problem of large-scaled chip without Si substrate thin-down process. 
     Another purpose of the present invention is to simplify fabrication procedure; to reduce production cost; to be compatible with modern procedures; and to be suitable for producing large-scaled chips with enhanced breakdown voltages, where the Si substrate does not need to be made thin and, therefore, the on-state output current is not lowered and the thermal dissipation problem becomes small. 
     To achieve the above purposes, the present invention is a method of fabricating a III-nitride based semiconductor on a partial isolated Si substrate, comprising steps of: (a) obtaining a diode device, comprising steps of: (a1)) obtaining a Si substrate and forming a nucleation layer on the Si substrate; a buffer layer on the nucleation layer; an active area on the buffer layer; and a channel layer located in the active area on the buffer layer, where the active area is isolated by an isolating part; (a2) forming a barrier layer on the channel layer; and (a3) forming an anode and a cathode on the barrier layer or the channel layer; and obtaining a drift area in the Si substrate between the anode and the cathode; (b) defining an etching area of the diode device and directly etching the diode device from top to etch out the barrier layer, the channel layer, the buffer layer, the nucleation layer and a part of the Si substrate, where the etching area is defined inside or outside the active area; and (c) processing an isotropic/non-isotropic lateral etching to the Si substrate until the drift area of the diode device. Accordingly, a novel method of fabricating a III-nitride based semiconductor on a partial isolated Si substrate is obtained. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
       The present invention will be better understood from the following detailed descriptions of the preferred embodiments according to the present invention, taken in conjunction with the accompanying drawings, in which 
         FIG. 1A  is the sectional view showing the initial state of the diode device according to the present invention; 
         FIG. 1B  is the sectional view showing the perpendicular etching of the first preferred embodiment according to the present invention; 
         FIG. 1C  is the sectional view showing the lateral etching of the first preferred embodiment; 
         FIG. 2A  is the sectional view showing the perpendicular etching of the second preferred embodiment; 
         FIG. 2B  is the sectional view showing the lateral etching of the second preferred embodiment; 
         FIG. 3A  is the sectional view showing the perpendicular etching of the third preferred embodiment; 
         FIG. 3B  is the sectional view showing the lateral etching of the third preferred embodiment; 
         FIG. 4  is the view showing the continuous etching; 
         FIG. 5  is the view showing the discrete etching; 
         FIG. 6  is the sectional view showing the fourth preferred embodiment; 
         FIG. 7  is the sectional view showing the fifth preferred embodiment; 
         FIG. 8  is the sectional view showing the sixth preferred embodiment; 
         FIG. 9  is the sectional view of the prior art; 
         FIG. 10  is the view of the etching process of the prior art; 
         FIG. 11  is the view of the prior output voltage and current at the off state; 
         FIG. 12  is the view of the prior linear relation between the breakdown voltage and the drift area length; and 
         FIG. 13  is the view of the prior output voltage and current at the on state. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following descriptions of the preferred embodiments are provided to understand the features and the structures of the present invention. 
     Please refer to  FIG. 1A  to  FIG. 1C ;  FIG. 2A  and  FIG. 2B ;  FIG. 3A  and  FIG. 3B ;  FIG. 4 ; and  FIG. 5 , which are a sectional view showing an initial state of a diode device according to the present invention; sectional views showing perpendicular and lateral etchings of a first, a second and a third preferred embodiments; and views showing a continuous etching and a discrete etching. As shown in the figures, the present invention is a method of fabricating a III-nitride based semiconductor on a partial isolated silicon (Si) substrate, comprising the following steps: 
     (a) A diode device  100  is prepared. The diode device  100  is a nitride Schottky diode device, as shown in  FIG. 1A . The diode device  100  is fabricated through the following steps: 
     (a1)) A Si substrate  10  is prepared. A nucleation layer  11  is formed on the Si substrate  10 . A buffer layer  12  is formed on the nucleation layer  11 . An active area  14  is formed on the buffer layer  12 , which is isolated by an isolating part  13 . A channel layer  15  is formed in the active area  14  on the buffer layer  12 . 
     (a2) A barrier layer  16  is formed on the channel layer  15 . 
     (a3) An anode  17  and a cathode  18  are formed on the barrier layer  16  or the channel layer  15 . A drift area  19  is formed in the Si substrate  10  between the anode  17  and the cathode  18 . 
     (b) After defining an etching area of the diode device  100 , the diode device  100  is directly dry-etched or wet-etched from top to etch out the barrier layer  16 , the channel layer  15 , the buffer layer  12 , the nucleation layer  11  and a part of the Si substrate  10 . Therein, the etching area is defined inside or outside the active area  14  at an area near the anode  17 ; at an area near the cathode  18 ; or at both areas near the anode  17  and the cathode  18 . 
     (c) An isotropic/non-isotropic lateral etching is processed to the Si substrate  10  until the drift area  19  of the diode device  100 . 
     In step (a), the channel layer is made of III-nitride, like GaN, InN, AlN or their alloy, like AlGaN or AlInN; and, the barrier layer is made of a III-nitride or a nitride alloy, like AlGaN or AlInN. 
     In step (b), the dry etching is an etching using inductive couple plasma (ICP) or a reactive ion etching (RIE); and, a pattern is formed inside (in  FIG. 4 ) or outside (in  FIG. 5 ) of the active area by continuous etching or discrete etching. 
     In step (c), the lateral etching is a wet etching using a solution of NaOH, KOH, ethylenediamine pyrocatechol (EDP) or ramethyl ammonium hydroxide (TMAH); a plasma of a fluorine(F)-ion-containing gas, like XeF 2  and XeF 4 ; or a vapor of HF. 
     Thus, a novel method of fabricating a III-nitride based semiconductor on a partial isolated Si substrate is obtained. 
     In  FIG. 1A  to  FIG. 1C , the heterojunction nitride Schottky diode device  100  is obtained in step (a) at first to be etched using inductive couple plasma (ICP) or reactive ion etching (RIE). The buffer layer  12 , the nucleation layer  11  and a part of the Si substrate  10  are directly etched at an area outside of the active area  14  near the anode  17 ; and, then, F-ions laterally etch the Si substrate  10  right down the anode  17  of the diode device  100 . 
     In  FIG. 2A  and  FIG. 2B , the heterojunction nitride Schottky diode device  100  is obtained to be etched using ICP or RIE. The buffer layer  12 , the nucleation layer  11  and a part of the Si substrate  10  are directly etched at an area outside of the active area  14  near the cathode  18 ; and, then, F-ions laterally etch the Si substrate  10  right down the cathode  18  of the diode device  100 . 
     In  FIG. 3A  and  FIG. 3B , a heterojunction nitride Schottky diode device  100  is obtained to be dry-etched or wet-etched. The buffer layer  12 , the nucleation layer  11  and a part of the Si substrate  10  are etched at both areas outside of the active area  14  near the anode  17  and the cathode  18 ; and, then, the lateral etching is processed to etch the Si substrate  10  right down the anode  17  and the cathode  18 . 
     Please refer to  FIG. 6  to  FIG. 8 , which are sectional views showing a fourth, a fifth and a sixth preferred embodiments. As shown in the figures, etchings are processed inside an active area. 
     In  FIG. 6 , a heterojunction nitride Schottky diode device  100  is obtained to be dry-etched or wet-etched. A barrier layer  16 , a channel layer  15 , a buffer layer  12 , a nucleation layer  11  and a part of a Si substrate  10  are etched at an area inside an active area  14  near an anode  17 ; and, then, an isotropic/non-isotropic lateral etching is processed to etch the Si substrate  10  right down the anode  17 . 
     In  FIG. 7 , the heterojunction nitride Schottky diode device  100  is obtained to be dry-etched or wet-etched. The barrier layer  16 , the channel layer  15 , the buffer layer  12 , the nucleation layer  11  and a part of the Si substrate  10  are etched at an area inside the active area  14  near a cathode  18 ; and, then, the lateral etching is processed to etch the Si substrate  10  right down the cathode  18 . 
     In  FIG. 8 , the eterojunction nitride Schottky diode device  100  is obtained to be dry-etched or wet-etched. The barrier layer  16 , the channel layer  15 , the buffer layer  12 , the nucleation layer  11  and a part of the Si substrate  10  are etched at both areas inside the active area  14  near the anode  17  and the cathode  18 ; and, then, the lateral etching is processed to etch the Si substrate  10  right down both of the anode  17  and the cathode  18 . 
     Thus, the present invention provides a method to etch a chip from top without transferring or deep-etching a silicon (Si) substrate from back-side. As a result, line width problem can be solved; and, the Si substrate does not need to be made thin for solving problems of complex fabrication procedure and bowing large-scaled chip. Hence, the present invention simplifies fabrication procedure, reduces production cost and is compatible with modern procedures. The present invention is suitable for producing large-scaled chips with enhanced breakdown voltages and suppressed leakage current. Moreover, the Si substrate does not need to be made thin and, therefore, the on-state current is not lowered and the thermal dissipation problem becomes small. 
     To sum up, the present invention is a method of fabricating a III-nitride based semiconductor on a partial isolated Si substrate, where a chip is directly etched from top without substrate transferring technique or deep-etching a Si substrate from back-side for solving line width problem; the Si substrate does not need to be made thin for solving the problem of complex fabrication procedure, the dissipation problem and the bowing problem of large-scaled chip without lowering the on-state current; and, thus, the present invention simplifies fabrication procedure, reduces production cost, is compatible with modern procedures and is suitable for producing large-scaled chips with enhanced breakdown voltages 
     The preferred embodiments herein disclosed are not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.
A semiconductor is fabricated on a silicon (Si) substrate. The semiconductor is III-nitride based. The Si substrate is partially isolated. Etching is directly processed from top on a chip for solving wire-width problem. The Si substrate does not need to be made thin. The chip can be large scaled and be prevented from bowing. Thus, the present invention simplifies producing procedure and reduces production cost. Besides, for a large-scaled chip, the breakdown voltage is enhanced; and, without making the Si substrate thin, the on-state current is remained the same and the heat problem is weakened.