Abstract:
A method of forming fin structure in integrated circuit comprising the steps of forming a plurality of fin structures on a substrate, covering an insulating layer on said substrate, performing a planarization process to expose mask layers, performing a wet etching process to etch said insulating layer, thereby exposing a part of the sidewall of said mask layer, removing said mask layer, and performing a dry etching process to remove pad layer and a part of said insulating layer, thereby exposing the top surface and a part of sidewall of said fin structures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a method of forming integrated circuit structures. In particular, the present invention relates to a method of forming fin structures in integrated circuits. 
     2. Description of the Prior Art 
     In recent years, as various kinds of consumer electronic products are constantly improved and miniaturized, the size of the semiconductor components have accordingly reduced, in order to meet requirements of high integration, high performance, and low power consumption. 
     With the trend in the industry being towards scaling down the size of the metal oxide semiconductor transistors (MOS), three-dimensional or non-planar transistor technology, such as fin field effect transistor technology (Fin FET) have been developed to replace the planar MOS transistors. The three-dimensional structure of a fin FET increases the overlapping area between the gate and the fin structure of the silicon substrate, the channel region is therefore accordingly more effectively controlled. The drain-induced barrier lowering (DIBL) effect and the short channel effect are therefore reduced. The channel region is also longer for an equivalent gate length, thereby increasing the current between the source and the drain. In addition, the threshold voltage of the fin FET can be controlled by adjusting the work function of the gate. 
     To manufacture non-planar FET device, numerous protruding and parallel fin structures must be formed firstly on semiconductor substrate, and deposition, planarization, and etch back processes are then performed on the substrate to form shallow trench isolations (STI) between the fin structures. The height of the fin structure is also defined in this step. During the formation of the fin structures, the etch back process is commonly used in a conventional approach by using diluted hydrofluoric (DHF) acid to etch the STI structures between the fin structures until a predetermined depth is reached, thereby forming the fin structures. However, some disadvantages are found in the fin structures formed by using the conventional approach. For example, the width difference of top surface and bottom surface of the fin structure is larger (ex. larger than 1 nm), the corner of fin structures and surrounding STI structures will have significant wicking features (ex. the height difference between the top surface of the STI adjacent to the fin structure and the top surface of the STI away from the fin structure may exceed 40 Å), and the top surface of resulting fin structures may not be provided with corner rounding feature, so that additional H 2  annealing process is necessary to obtain a corner rounding feature. The above-mentioned disadvantages may impact the performances of resulting non-planar FET devices, or may increase the production time and costs. 
     Accordingly, the present invention is directed to improve the above-mentioned conventional method in order to obtain better fin structures and simplify the process steps. 
     SUMMARY OF THE INVENTION 
     To improve the above-mentioned drawbacks of the prior art, a new semiconductor process is provided in the present invention. The method of the present invention features the steps of performing an etching process to etch the shallow trench isolations (STI) surrounding the fin structures until a predetermined depth is reached, and performing a SiCoNi dry etching process using HF and NH 3  based process gases to define the height of fin structures. The fin structures formed by the method of the present invention will have better profiles, which may improve the electrical properties of the non-planar field effect transistor (FET) formed in later processes. 
     One object of the present invention is to provide a method of forming fin structure in integrated circuit comprising the steps of forming a plurality of fin structures on a substrate, covering an insulating layer on said substrate, performing a planarization process to expose mask layers, performing a wet etching process to etch said insulating layer, thereby exposing a part of the sidewall of said mask layer, removing said mask layer, and performing a dry etching process to remove pad layer and a part of said insulating layer, thereby exposing the top surface and a part of sidewall of said fin structures. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the embodiments, and are incorporated in and constitute apart of this specification. The drawings illustrate some of the embodiments and, together with the description, serve to explain their principles. In the drawings: 
         FIGS. 1-8  are cross-sectional views illustrating the process flow of forming fin structures in accordance with one embodiment of the present invention. 
     
    
    
     It should be noted that all the figures are diagrammatic. Relative dimensions and proportions of parts of the drawings have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments. 
     DETAILED DESCRIPTION 
     In the following detailed description of the invention, reference is made to the accompanying drawings which form a part thereof, and in which are shown specific embodiments in which the invention may be practiced by way of illustration. These embodiments are described in sufficient details to allow those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. 
     The embodiments will now be explained with reference to the accompanying drawings to provide a better understanding of the process of the present invention, wherein  FIGS. 1-8  are cross-sectional views illustrating the flow of forming fin structures in integrated circuits in accordance to one embodiment of the present invention. 
     First, please refer to  FIG. 1 , wherein a semiconductor substrate  10  is provided. In an embodiment, the semiconductor substrate  10  includes silicon. Other commonly used materials, such as carbon, germanium, gallium, arsenic, nitrogen, indium, and/or phosphorus, and the like, may also be included in the semiconductor substrate  10 . The semiconductor substrate  10  may be a bulk substrate. 
     A pad layer  12  and a mask layer  14  may be formed on the semiconductor substrate  10 . The pad layer  12  may be a thin film comprising silicon oxide formed through a thermal oxidation process for example. The pad layer  12  may act as an adhesive layer between the semiconductor substrate  10  and the mask layer  14 . The pad layer  12  may also act as an etch stop layer for the etching mask layer  14 . In an embodiment, the mask layer  14  is made of silicon nitride and formed through a low-pressure chemical vapor deposition (LPCVD) process for example. In other embodiments, the mask layer  14  is formed by thermal nitridation of silicon, plasma enhanced chemical vapor deposition (PECVD), or plasma anodic nitridation. The mask layer  14  is used as a hard mask during subsequent photolithography processes. A photo resist  16  is formed on the mask layer  14  and is then patterned to form openings  18  and to expose the underlying mask layer  14 . 
     Please refer to  FIG. 2 , wherein the mask layer  14  and the pad layer  12  are patterned by etching through openings  18 , exposing the underlying semiconductor substrate  10 . The exposed semiconductor substrate  10  is then etched by using patterned mask layer  14  as hard mask to form trenches  22 . The portions of the semiconductor substrate  10  between the trenches  22  form semiconductor strips  20  parallel to each other (in the top view). The photo resist  16  is then removed after the etching process. Then, a cleaning process may be performed to remove a native oxide of semiconductor substrate  10 . The cleaning process may be performed by using diluted hydrofluoric (DHF) acid. 
     In an embodiment of present invention, a depth D of the trenches  22  may be between about 2100 Å and about 2500 Å, while a width W is between about 300 Å and about 1500 Å. In an exemplary embodiment, the aspect ratio (D/W) of the trenches  22  is greater than 7.0. The width S of the semiconductor strips  20  may be smaller than about 30 nm. One skilled in the art will however realize that the dimensions and the values recited throughout the descriptions are merely examples, and may be changed to fit different scales of integrated circuits. 
     A liner layer  24  is then formed in the trenches  22 , as shown in  FIG. 3 . In an embodiment, the liner layer  24  may be a thermal oxide having a thickness between about 20 Å to about 500 Å. In other embodiments, the liner layer  24  may be formed using in-situ steam generation (ISSG). In yet other embodiments, the liner layer  24  may be formed by using a deposition technique that can form conformal oxide layers, such as selective area chemical vapor deposition (SACVD) processes and the likes. Besides, the liner layer  24  may be a single layer or a composite layer including Si—O compound and/or Si—N compound. The formation of the liner oxide  24  rounds the corners of the trenches  22 , which reduces the electrical fields, and hence improves the performances of the resulting integrated circuits. 
     Please refer to  FIG. 4 , wherein the trenches  22  are filled with a dielectric material  26 . The dielectric material  26  (also referred as an insulating layer) may include silicon oxide, although other dielectric materials, such as SiN, SiC, or the likes, may also be used. In an embodiment, the dielectric material  26  is formed using a high aspect-ratio process (HARP), wherein process gases may include tetraethylorthosilicate (TEOS) and O 3 . In an embodiment, as shown in  FIG. 4 , the dielectric material  26  may cover the whole semiconductor substrate  10 , including the semiconductor strips  20 . The dielectric material  26  may have a non-planar surface higher than the top surface of the mask layer  14 . 
     A planarization process, such as chemical mechanical polish (CMP), is then performed to remove parts of the dielectric material  26  after the covering of dielectric material  26 . The planarization process will expose the underlying mask layer  14 , and the resulting structure is shown in  FIG. 5 . The top surfaces of mask layer  14  and dielectric material  26  form a substantially planar surface. The remaining portions of the dielectric material  26  and liner layer  24  in the trenches  22  are referred to as shallow trench isolation (STI)  28  between the semiconductor strips. 
     Please refer to  FIG. 6 , wherein a first etching process is performed to etch the STI  28  after the exposure of the mask layer  14 . In an embodiment of present invention, the first etching process may be a wet etching process using diluted HF acid if the material of the STI  28  is silicon oxide. This etching process may etch parts of the STI  28  until the top surface of the STI  28  is higher than the bottom surface of the mask layer  14  and lower than the top surface of the mask layer  14 . This may also expose parts of sidewalls of the mask layer  14 . Preferably, the top surface of the STI  28  is at the same level as half of the thickness of the mask layer  14 . 
     Please refer to  FIG. 7 , wherein the exposed mask layer  14  is removed after completing the first etching process. In an embodiment of the present invention, the mask layer  14  may be removed by a wet etching process using hot H 3 PO 4  if the material of the mask layer  14  is silicon nitride. Since the mask layer  14  at this stage is protruding higher than the STI  28 , a recess  30  may be defined by the STI  28  and the pad layer  12  in between after the removal of the mask layer  14 . At this stage, the top surface of the pad layer  12  is lower than the top surface of the STI  28 . 
     Please refer to  FIG. 8 , wherein a second etching process is performed to remove the pad layer  12  above the semiconductor strips  20  and parts of the STI  28  after the removal of the mask layer  14 . The second etching process will etch the STI  28  until a predetermined depth is reached. The semiconductor strips  20  may then protrude higher than the STI  28 . The fin structure  20   a  with a predetermined height H is now defined. This etching process may also etch the liner layer  24  on the sidewalls of the fin structures  20   a . In an embodiment of the present invention, since the thickness of the pad layer  12  above the semiconductor strip  20  is much thinner than the thickness of the STI  28  removed by the second etching process, the pad layer  12  will be completely removed during the second etching process, and even parts of the semiconductor strip  20  will be etched. This way a fin structure  20   a  with top corner rounding features can be obtained. For example, the height difference between the top surface of the STI adjacent to the fin structure and the top surface of the STI away from the fin structure may be controlled at the value lower than 40 Å. The SiCoNi process primarily includes the step of reacting the fluorine-containing gas with the silicon oxide to synthesize diammonium fluosilicate ((NH 4 ) 2 SiF 6 ). In this way, the silicon oxide can be removed optionally. The aforesaid fluorine-containing gas can be hydrogen fluoride (HF) or nitrogen trifluoride (NF 3 ). 
     In an embodiment of present invention, the second etching process may be a dry etching process to etch the STI  28  until a predetermined depth is reached. Preferably, the process gas used in this dry etching process includes H atoms, for example, using hydrofluoric (HF) and ammonia (NH 3 ) based process gases to etch the substrate. Therefore, in an embodiment of present invention, a selective material removing technology (named as SiCoNi™ process) developed by Applied Materials may be utilized in present invention. This process may effectively remove the dielectric material  26  in the present invention and may also better control the height H of fin structures  20   a . Also, the wicking effect at the corner of fin structure  20   a  and the surrounding STI  28  may be significantly improved. 
     Furthermore, thanks to the approach of keeping the top surface of the STI  28  higher than the pad layer  12  before the second etching process in the present invention, the width difference between the top surface and the bottom surface of the fin structure  20   a  will be smaller (ex. lower than 1 nm), and the top surface of the fin structure  20   a  will be provided with a corner rounding feature. No additional H 2  annealing process is required in the present invention. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.