Abstract:
A method for fabricating a high voltage transistor includes the following steps. Firstly, a substrate is provided. A first sacrificial oxide layer and a hard mask layer are sequentially formed over the substrate. The hard mask layer is removed, thereby exposing the first sacrificial oxide layer. Then, a second sacrificial oxide layer is formed on the first sacrificial oxide layer. Afterwards, an ion-implanting process is performed to introduce a dopant into the substrate through the second sacrificial oxide layer and the first sacrificial oxide layer, thereby producing a high voltage first-type field region of the high voltage transistor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for fabricating a high voltage transistor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Generally, an integrated circuit chip includes a logic function circuit and a power supply circuit. The logic function circuit is implemented by a complementary metal-oxide-semiconductor (CMOS) transistor. The power supply circuit is implemented by a high voltage metal-oxide-semiconductor field-effect transistor. However, the conventional isolated high voltage N-channel metal-oxide-semiconductor field-effect transistor (also referred as an isolated HV-NMOS) has some drawbacks. For example, due to the structural defect, a current leakage problem possibly occurs, and thus the charge pump circuit usually has a failure. Therefore, there is a need of providing an improved method to solve these drawbacks. 
       SUMMARY OF THE INVENTION 
       [0003]    In accordance with an aspect, the present invention provides a method for fabricating a high voltage transistor. The method includes the following steps. Firstly, a substrate is provided. A first sacrificial oxide layer and a hard mask layer are sequentially formed over the substrate. The hard mask layer is removed, thereby exposing the first sacrificial oxide layer. Then, a second sacrificial oxide layer is formed on the first sacrificial oxide layer. Afterwards, an ion-implanting process is performed to introduce a dopant into the substrate through the second sacrificial oxide layer and the first sacrificial oxide layer, thereby producing a high voltage first-type field region of the high voltage transistor. 
         [0004]    In an embodiment, the substrate is a P-type silicon substrate, the first sacrificial oxide layer is a pad oxide layer, and the hard mask layer is a silicon nitride layer. 
         [0005]    In an embodiment, the step of removing the hard mask layer is performed by treating the silicon nitride layer with a hot phosphoric acid solution. 
         [0006]    In an embodiment, the step of forming the second sacrificial oxide layer is performed by a low-temperature deposition process. 
         [0007]    In an embodiment, the low-temperature deposition process is performed by carrying out a low-pressure tetraethylorthosilicate chemical vapor deposition to form a silicon oxide layer. 
         [0008]    In an embodiment, the second sacrificial oxide layer has a thickness in a range from 50 to 200 angstroms. 
         [0009]    In an embodiment, the low-pressure tetraethylorthosilicate chemical vapor deposition is carried out at about 680° C. 
         [0010]    In an embodiment, the step of forming the second sacrificial oxide layer is performed by a spin-on glass process. 
         [0011]    In an embodiment, the second sacrificial oxide layer is made of silicon oxide, silicon oxynitride, silicon carbide, silicon oxycarbide or silicon carbonitride. 
         [0012]    In an embodiment, the dopant is an N-type dopant, and the high voltage first-type field region is a high voltage N-field region. 
         [0013]    In an embodiment, before the step of removing the hard mask layer, the method further includes a step of forming a shallow trench isolation structure in the substrate. 
         [0014]    In an embodiment, after the high voltage first-type field region is formed, the method further includes a step of removing the second sacrificial oxide layer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
           [0016]      FIG. 1  is a schematic cross-sectional view illustrating an isolated high voltage N-channel metal-oxide-semiconductor field-effect transistor; 
           [0017]      FIGS. 2A-2C  are schematic cross-sectional views illustrating a method for fabricating a high voltage metal-oxide-semiconductor field-effect transistor according to an embodiment of the present invention; and 
           [0018]      FIGS. 3A-3E  are schematic cross-sectional views illustrating a method for fabricating a high voltage metal-oxide-semiconductor field-effect transistor according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0019]    The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
         [0020]      FIG. 1  is a schematic cross-sectional view illustrating an isolated high voltage N-channel metal-oxide-semiconductor field-effect transistor. 
         [0021]    In the isolated HV-NMOS of  FIG. 1 , a high voltage N-field (HVNF) region  16 , an isolated high voltage P-well (ISO_HVPW) region  17  and an isolated deep N-well (ISO_DNW) region  18  are formed by using different ion-implanting processes to dope the substrate with different dopants. It is important to precisely control the depth of the HVNF region  16 . If the HVNF region  16  is deeper than the desired position, the HVNF region  16  is very close to the ISO_DNW region  18 . Under this circumstance, a current leakage problem possibly occurs, and thus the charge pump circuit usually has a failure. For controlling the depth of the high voltage N-field region, the present invention provides a method for fabricating a high voltage metal-oxide-semiconductor field-effect transistor. 
         [0022]      FIGS. 2A-2C  are schematic cross-sectional views illustrating a method for fabricating a high voltage metal-oxide-semiconductor field-effect transistor according to an embodiment of the present invention. 
         [0023]    Firstly, as shown in  FIG. 2A , a pad oxide layer  20  and a silicon nitride layer  21  are sequentially formed on a P-type silicon substrate  2 . A photolithography and etching process is performed to form a shallow trench  22  in the P-type silicon substrate  2 . Then, a shallow trench isolation material  23  is filled into the shallow trench  22 , thereby forming a shallow trench isolation structure  223 . 
         [0024]    The above structure is then immersed in a hot phosphoric acid (H 3 PO 4 ) solution for about 3600 seconds to remove the silicon nitride layer  21 . After the silicon nitride layer  21  is removed, the remaining pad oxide layer  20  is used as a sacrificial oxide layer (see  FIG. 2B ). 
         [0025]    Then, a patterned photoresist layer  28  is formed on the pad oxide layer  20 . Through the openings  280  of the patterned photoresist layer  28 , an ion-implanting process is performed to dope the P-type silicon substrate  2 . Consequently, a high voltage N-field (HVNF) region  24  is formed in the P-type silicon substrate  2  (see  FIG. 2C ). 
         [0026]    Since silicon is a crystalline material with a good crystal lattice structure, the ion-implanting process may result in a channeling effect. Due to the channeling effect, the doped region is formed at a position deeper than the desired position. In the above embodiment of the present invention, the use of the sacrificial oxide layer can prevent generation of the channeling effect during the ion-implanting process of forming the HVNF region  24 . Since the possibility of causing the HVNF region to be close to the isolated deep N-well region is minimized, the drawbacks encountered from the prior art will be obviated. 
         [0027]    However, the above method for forming the HVNF region still has some drawbacks. For example, in the step of removing the silicon nitride layer  21 , a number of wafers in the same batch (e.g. several tens of wafers) are sequentially immersed in the hot phosphoric acid solution, which is stored in the same container. Due the concentration difference between the fresher solution and the older solution, the sacrificial oxide layers of the same batch of wafers usually have obvious thickness variation. For example, prior to the treatment of the hot phosphoric acid solution, the thickness of the pad oxide layer  20  of any wafer is about 110 angstroms. In a case that the wafer is treated with the fresh hot phosphoric acid solution, the thickness of the remaining pad oxide layer  20  is about 70angstroms. Whereas, since the last ones of the same-batch wafers are treated with the older hot phosphoric acid solution, the thickness of the remaining pad oxide layer  20  is about 100 angstroms. That is, the sacrificial oxide layers of the same-batch wafers have obvious thickness variation. Due to the thickness variation, the subsequent process of forming the HVNF region may result in depth variation. Experiments show that if the thickness of the remaining pad oxide layer  20  is smaller than 80 angstroms, the current leakage problem becomes more serious. Therefore, the above method needs to be further improved. 
         [0028]    For solving the above problems, the present invention further provides another method for fabricating the high voltage metal-oxide-semiconductor field-effect transistor.  FIGS. 3A-3E  are schematic cross-sectional views illustrating a method for fabricating a high voltage metal-oxide-semiconductor field-effect transistor according to another embodiment of the present invention. 
         [0029]    Firstly, as shown in  FIG. 3A , a pad oxide layer  30  and a silicon nitride layer  31  are sequentially formed on a P-type silicon substrate  3 . A photolithography and etching process is performed to form a shallow trench  32  in the P-type silicon substrate  3 . Then, a shallow trench isolation material  33  is filled into the shallow trench  32 , thereby forming a shallow trench isolation structure  323 . 
         [0030]    The above structure is then immersed in a hot phosphoric acid (H 3 PO 4 ) solution for about 3600 seconds to remove the silicon nitride layer  31 . After the silicon nitride layer  31  is removed, the remaining pad oxide layer  30  is used as a sacrificial oxide layer (see  FIG. 3B ). 
         [0031]    The pad oxide layer  30  is usually formed by carrying out a thermal oxidation process at a temperature of about 1100° C. Although the increase of the operating time period of the thermal oxidation process is effective to increase the thickness of the pad oxide layer  30 , the device performance will be deteriorated because the thermal budget is improperly increased. For reducing the thermal budget, a low-temperature deposition process is performed to form an auxiliary sacrificial oxide layer  39  on the pad oxide layer  30  (see  FIG. 3C ). In an embodiment, the low-temperature deposition process is performed by carrying out a low-pressure tetraethylorthosilicate chemical vapor deposition (LP-TEOS CVD) process to deposit a silicon oxide layer (i.e. the auxiliary sacrificial oxide layer) on the pad oxide layer  30 . The thickness of the auxiliary sacrificial oxide layer  39  is in the range from 50 to 200 angstroms. In an embodiment, the low-pressure tetraethylorthosilicate chemical vapor deposition process is carried out at a low temperature such as 680° C. In such way, the auxiliary sacrificial oxide layer  39  can be formed without increasing the thermal budget. Alternatively, in some embodiments, the auxiliary sacrificial oxide layer  39  is a silicon oxide layer formed by a spin-on glass (SOG) process. In the above embodiment, the auxiliary sacrificial oxide layer  39  is made of silicon oxide. Alternatively, in some embodiments, the auxiliary sacrificial oxide layer  39  may be made of silicon oxynitride, silicon carbide, silicon oxycarbide or silicon carbonitride. 
         [0032]    Then, as shown in  FIG. 3D , a patterned photoresist layer  38  with openings  380  is formed on the auxiliary sacrificial oxide layer  39 . By using the patterned photoresist layer  38  as an implantation mask, an ion-implanting process is performed to introduce an N-type dopant into the P-type silicon substrate  3  through the auxiliary sacrificial oxide layer  39  and the pad oxide layer  30 . Consequently, a high voltage N-field (HVNF) region  34  is formed in the P-type silicon substrate  3 . 
         [0033]    After the high voltage N-field region  34  is performed, the auxiliary sacrificial oxide layer  39  is removed (see  FIG. 3E ). After the auxiliary sacrificial oxide layer  39  is removed, the subsequent process will be done. For example a series of ion-implanting processes are performed to dope the substrate  3  with different dopants, thereby forming a high voltage P-field region  35  and a deep N-well region  36 . The resulting structure of the high voltage N-channel metal-oxide-semiconductor field-effect transistor with a gate structure  37  is shown in  FIG. 3E . 
         [0034]    From the above description, since the depth of the high voltage N-field region can be precisely controlled, the depth variation will be reduced or eliminated. As a consequently, the performance of the high voltage metal-oxide-semiconductor field-effect transistor will be enhanced. 
         [0035]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.