Abstract:
The present invention relates an etch process of a dielectric film deposited on a substrate, characterized by using a chamber of PECVD type having an upper electrode coupled with a RF source and a lower electrode connected to ground on which a silicon substrate is placed. The etch of the dielectric film is obtained by means of a plurality of active gases mixed with at least one conveyance gas so as to obtain a low rate of etch and an uniform etch of said deposited dielectric film.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an etch process of the type with low rate of removal in the manufacture of semiconductor integrated devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the field of semiconductor integrated devices after a first operation of deposition of dielectric films on a silicon substrate, by means of a chamber adapted for the deposition of said films, for example PECVD (Plasma Enhanced Chemical Vapor Deposition), APCVD (Atmospheric Pressure CVD), LPCVD (Low Pressure CVD), HDPCVD (High Density PCVD), there is a second etch operation at said dielectric films made by another chamber called RIE and/or MERIE (Reactive Ion Etch; Magnetically Enhanced RIE).  
           [0003]    The etch operation refers to a selective removal of undesired material from the substrate surface and it is obtained by masks and photoresist.  
           [0004]    There are two types of etch: 1) wet etch, usually identified as chemical etch, using liquid chemical agents for the etch of the substrate surface; and 2) dry etch, usually identified as ionic etch, using plasma, ionic beams and sputter etch for the etch of the substrate surface.  
           [0005]    For the etch operation of thin dielectric films, having a thickness of some tenth of angstroms, such as gate oxide, thermal oxide, tunnel or interpoly oxide, oxides obtained by a reoxidation process, antireflecting dielectrics, classically the wet etch technology is used. Such a process foresees the use of hydrofluoric acid (HF) for silicon oxides or phosphoric acid (H 2 PO 4 ) for silicon oxynitride and nitride because it is possible to obtain a controlled and repeatable process. That is, it is possible to have an etch with a low rate of removal of dielectric films, called etch rate, and to repeat said operation in the time with the same effectiveness.  
           [0006]    Typical values of etch rate in case HF acid is used are about 5 angstroms per minute for the etch of the thermal oxide, while in the case H 2 PO 4  acid is used for the etch of the silicon oxynitride typical values are about 100 angstroms per minute.  
           [0007]    We deduce, therefore, that an advantage of the wet etch is in the precision of the thickness of the removed dielectric.  
           [0008]    The principal disadvantage of the wet etch is that the effectiveness of the acid solution is variable in function of how many etches the solution has made, of the value of the dust quantity (checking of the particles present in the solution) and of the contamination value of the oxide layer (for example contamination of phosphor atoms in the case of a solution based on H 2 PO 4  acid).  
           [0009]    The traditional apparatuses of etch to the oxide layers, called RIE (Reactive Ion Etch) or MERIE (Magnetically Enhanced RIE), use the ionic etch technology. These apparatuses necessitate the excitement of a gas compound by means of an electric field produced by an RF source, that is a radio—frequency source, so that a plasma envelope is produced, adapted to produce ions. These ions collide with the silicon substrate, causing a removal of a determined thickness of the dielectric film.  
           [0010]    Typical values which said chambers can reach as etch rate are about 500 angstroms per minute.  
           [0011]    The principal disadvantage of said apparatuses is that it isn&#39;t possible to have an acceptable etch uniformity since they show values greater than 8% at 1 σ (σ is standard deviation).  
           [0012]    We are in a situation in which the lowest etch rate values, and therefore the best precision, are obtained by an etch of wet type while the technology provides chambers for the etch of dielectric films having an etch of dry type (ionic etch).  
           [0013]    In view of the state of the art described, it is an object of the present invention to provide a process to obtain a low rate of etch, typical of wet etches, and to maintain the advantages of the etches of ionic type with respect to the wet etches.  
         SUMMARY OF THE INVENTION  
         [0014]    According to the present invention, such object is achieved by an etch process of a dielectric film deposited on a substrate, characterized by using a chamber of PECVD type, comprising an upper electrode coupled with a RF source and a lower electrode connected to ground on which a silicon substrate is placed, wherein the etch of the dielectric film is obtained by means of a plurality of active gases mixed with at least one conveyance gas so as to obtain a low rate of etch and an uniform etch of said deposited dielectric film.  
           [0015]    Thanks to the present invention a process is provided by means of which is possible to make a dry etch at a low rate in the manufacture of semiconductor integrated devices by using a classic chamber of PECVD type, designed for the deposition of dielectric films, instead of a chamber of RIE or MERIE type, designed for the etch of dielectric films.  
           [0016]    This process has the advantage of using only one apparatus, the chamber PECVD, adapted to develop the functions of deposition of dielectric films and moreover adapted to make the etch to said dielectric films with remarkable saving of money and time.  
           [0017]    Furthermore the present invention has also the advantage to obtain a repeatable etch thanks that the PECVD chamber has been designed to execute repeatable and uniform depositions of the dielectric films.  
           [0018]    The features and the advantages of the present invention will be made evident by the following detailed description of an embodiment thereof which is illustrated as not limiting example in the annexed drawings, wherein:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 shows a configuration of a chamber of RIE or MERIE type.  
         [0020]    [0020]FIG. 2 shows a configuration of a chamber of PECVD type.  
     
    
     DETAILED DESCRIPTION  
       [0021]    In FIG. 1 a configuration of a chamber of RIE or MERIE type is shown. As shown in such a Figure a RF source  1  is connected with an electrode  2 , called cathode, on which a silicon substrate  3  is placed, on which a dielectric film  26 , for example, of Si 3 N 4  (or SiO 2 , SiO x N y , PSG, BPSG, FSG, SiC, etc.), has previously been deposited by a chamber, for example PECVD (or chambers of APCVD, LPCVD, HDPCVD type), and a second electrode  7 , called anode, connected to ground. On the base of the cathode  2  a hole  10  for the outgoing of exhaust gas (pointed out by an arrow) is foreseen also.  
         [0022]    Moreover a tank  8 , containing active gas, that is connected to the anode  7 , is provided of a diffusion grid  5 , by a valve  9  adapted to regulate the flow of said active gas.  
         [0023]    The voltage difference between cathode and anode is so as to generate a plasma envelope  4  (pointed out with a plurality of arrows), due to the admission and to the excitement of said active gas in the chamber by means of said grid  5 .  
         [0024]    The dielectric film  26 , for example Si 3 N 4 , previously deposited on the substrate  3  by means of a chamber of PECVD type, according to the prior art, therefore, is hit by said plasma flow  4 . In this way the dielectric film Si 3 N 4    26  is undergone to an ionic bombardment, thanks to which it is possible to make the etch.  
         [0025]    The electrodes of cathode  2  and anode  7  with the grid  5  are contained in a chamber (RIE or MERIE) having walls  21  and  22  of insulating material, preferably made by aluminum oxide Al 2 O 3 .  
         [0026]    The reacting gases, therefore, flow through the grid  5  in the chamber wherein they are mixed. The commonly used gases have as common element the fluorine (F) among which there are gases as: NF 3 , CF 4  and SF 6 .  
         [0027]    In chambers of heretofore described type, these gases allow to obtain etch values according to the following table:  
                                             PIE and/or MERIE standard chamber                                    Typical etch rate [nm/min]   Greater than 100           Minimum etch rate [nm/min]   50           Non uniformity   Greater than 8% at 1 σ                      
 
         [0028]    In FIG. 2 a configuration of a chamber of PECVD type is shown. In such a Figure a RF source  11  is connected to an electrode  12 , called anode, at which a plurality of tanks  13 ,  14  and  24  with respective regulation valves  15 ,  16  and  25  are connected. The tank  13  contains the active gas, while the tank  14  contains the conveyance gas and the tank  24  contains the gas adapted for the deposition of the dielectric film  27 . Moreover the cathode electrode  12  is connected to a diffusion grid  20 .  
         [0029]    A silicon substrate  18  on a lower electrode  17 , called anode, having a discharge valve  19  for exhaust gas, is placed. The anode electrode  17  is connected to ground.  
         [0030]    The electrodes of anode  17  and cathode  12  with respective grid  20  are contained in an insulating chamber (PECVD) having walls  21  and  22  of insulating material, preferably made by aluminum oxide Al 2 O 3 .  
         [0031]    These chambers alternate a first deposition step of said dielectric film  27  on said silicon substrate  18 , step for which the PECVD chambers are designed, a second removal step from the chamber of the substrate  18  by means of a mechanic arm (not shown in Figure), a third cleaning step to remove the dielectric film  27  that deposits on the walls  21  and  22  during the deposition operation of said dielectric film  27  by the use of a compound of freon plus oxygen, as hereinafter shown, and a fourth step of admission of a second substrate  18  in the chamber and a new deposition of dielectric film  27  on said substrate  18 .  
         [0032]    The periodicity of execution of said third step can be every two or three substrates  18 .  
         [0033]    The Applicant has found that the used chemistry (active gas contained in the tank  13 ) during said third phase, that is during the cleaning step of the walls  21  and  22  of the chamber, opportunely mixed with conveyance gas (contained in the tank  14 ) and undergone to predetermined working conditions, such as power of the plasma  23 , working pressure and distance between the electrodes  12  and  17 , allows to realize the etch to the dielectric film with different working points, with different etch rates and etch uniformity.  
         [0034]    Particularly the operation of dry etch is an etch of blanket type, for example an etch to define the spacers, on the whole substrate  18 .  
         [0035]    The active gases, contained in the tank  13  and regulated by the valve  15 , are gases that have as common element the fluor (F) among which there are gases as: NF 3 , C 2 F 6 , CF 4 , SF 6 , C 3 F 8  and PFC 5 .  
         [0036]    The conveyance gases, also called inert gases, contained in the tank  14  and regulated by the valve  16 , are inert gases and preferably we use nitrogen (N 2 ), helium (He) or argon (Ar).  
         [0037]    The deposition gases, contained in said tank  24  and regulated by said valve  25 , are deposition reactant gases that must give origin to silicon oxide and preferably they are SiH 4  and N 2 O or TEOS and O 2 .  
         [0038]    The voltage difference between the cathode and anode is so as to generate a plasma envelope  23  (pointed out with a plurality of arrows), due to the admission and the excitement of said gas in the chamber by means of said grid  20 .  
         [0039]    The dielectric film  27 , for example Si 3 N 4 , previously deposited on the substrate  3  by means of a said PECVD chamber, therefore, is hit by said plasma flow  23  through said grid  20  connected to said anode  12 . In this way, in a first step of feeding of deposition gas the dielectric film Si 3 N 4    27  is deposited on said silicon substrate  18 , and after the feeding of active gas and conveyance gas provides to the cleaning of the walls  21  and  22  of the PECVD chamber and to the etch of the dielectric film  27 .  
         [0040]    In the chambers, heretofore described, etch values according to the following table are obtained:  
                                             PECVD chamber with inert gas                                    Typical etch rate [nm/min]   Lower than 30           Minimum etch rate [nm/min]   6           Non uniformity   Lower that 6% at 1 σ                      
 
         [0041]    Comparing the scheduled values for PECVD chambers and RIE and/or MERIE chambers with the first type a lower etch rate is obtained so as to remove dielectric films having thickness of few angstroms, that is a higher uniformity, and an etch process more repeatable is obtained in the time.  
         [0042]    The use of a PECVD chamber to execute the etch of the dielectric films, moreover, allows a better regulation of the distance between the electrodes  12  and  17  and this makes easier a process optimization, particularly for the etch uniformity.  
         [0043]    To perform the etch to these dielectric films, as heretofore described, active gas adapted for the cleaning “in situ” of the walls  21  and  22  of the PECVD chamber and also of the electrode chucks  12  of the electrodes  12  and  17  must be combined with mixing and conveyance gas.  
         [0044]    It is obtained for example a situation in which said active gas and said conveyance gas are combined, as for example:  
         CF 4 +N 2 O in He, Ar, N 2    
         C 2 F 6 +O 2  in He, Ar, N 2    
         C 2 F 6 +NF 3 +O 2  in He, Ar, N 2    
         [0045]    The conveyance gas must be inert gas and it is preferred a gas as helium (He) thanks also to its higher thermal conductivity wherein the flow is in a range between 500-4000 sccm (standard cubic centimeters per minutes). Moreover the plasma power can be changed in a range between 200-1000 W, the pressure from 1 to 10 Torr and the distance between the electrodes  12  and  17  from 200 to 500 mils by changing opportunely the value of the source RF  11 . With these values it is possible to obtain different working points giving origin to values of etch rate between 60 angstroms per minute and 450 angstroms per minute, with an etch uniformity lower than 6% at 1 σ.  
         [0046]    Although the invention has been described and illustrated with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example, and that numerous changes in the combination and arrangement of parts can be resorted to by those skilled in the art without departing from the spirit and scope of the invention, as hereinafter claimed.