Abstract:
A method for protecting selected first surfaces on a semiconductor substrate during application of a second oxide layer to said substrate comprising applying an oxidation barrier layer as a mask over said selected first surfaces, prior to patterning said semiconductor substrate with a resist mask and applying said second oxide layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to processes for the fabrication of integrated circuit devices on semiconductor substrates. In particular, the present invention relates to processes by which oxide dielectric layers featuring variable thicknesses are fabricated across a semiconductor substrate surface.  
           [0002]    In producing DRAM integrated circuits, varying oxide thicknesses are required to be fabricated across the regions (i.e., array/regions vs. supports regions) of the semiconductor chip in order to be compatible with the different voltage requirements. Previously practiced processes for fabricating these different gate oxides on the same wafer typically employ methods that involve the removing the thick oxide from regions where a thin oxide is required. In the prior art, the thick oxide is masked by photoresist to protect it from the etch process. However, these procedures typically expose the thick gate oxide area to photoresist which results in contaminating the gate oxide surface with the inherent impurities commonly included in photoresist compositions.  
         SUMMARY OF THE INVENTION  
         [0003]    Now, according to the present invention, an improved method has been developed to protect selected chip features during oxidation processes. An oxidation barrier is used to mask chip features which require protection during oxidation processes employed to form enhanced oxide thicknesses in areas of a semiconductor substrate where a thicker oxide layer is desirable.  
           [0004]    The presently invented process is designed for producing a first, thin layer of oxide upon a first surface region of a semiconductor substrate and for forming a second, thick oxide layer, of greater thickness than the first layer of oxide upon a second surface region of the semiconductor substrate.  
           [0005]    In one embodiment, a first oxide (for example, SiO 2 , Al 2 O 3 , HfO 2 , TiO 2 ; a preferred oxide is silicon oxide) layer is formed upon a first and second semiconductor substrate region. Preferably, this SiO 2  layer is about 0.5 to about 10 nm thick; most preferably, the SiO 2  layer is about 1 to about 2 nm. The SiO 2  typically is grown by thermal oxidation, or deposited by low pressure chemical vapor deposition (LPCVD) or by plasma-enhanced chemical vapor deposition (PECVD). Then, an oxidation barrier (for example, a nitride such as SiN, TiN, HfN, ZrN; a preferred nitride is silicon nitride) or, alternatively, a metal (such as Ti, HfH, or Zr) layer is formed over the initial oxide layer. Preferably, a SiN layer is formed to a thickness of about 5 to about 100 nm, most preferably about 10 to about 20 nm. Typically, the SiN is deposited by low pressure chemical vapor deposition (LPCVD) or by plasma-enhanced chemical vapor deposition (PECVD). Using lithographic processes, the silicon oxide and silicon nitride layers are patterned to expose the layers over the second substrate region on which a second thicker oxide layer will be formed. The exposed silicon nitride and first oxide layers over the second semiconductor substrate region are selectively removed. Typically, the SiN layer is etched using a F-based dry etch technique; the SiO 2  layer typically is etched using a HF-based wet etch technique. The resist mask then is stripped, and the semiconductor substrate wafer is cleaned to remove resist contaminants. The wafer then is subjected to a thermal oxidation procedure, which results in a second thicker (preferably about 3 to about 50 nm) oxide being grown in the second semiconductor substrate region where bare Si is exposed, and a thinner (at least about 1 to about 5 nm thinner than the thicker SiO 2  layer) layer of silicon oxynitride (SiON) in the first substrate region where the SiN layer mask is present.  
           [0006]    In a second embodiment, a thin nitride (such as SiN, TiN, HfN, or ZrN; SiN is preferred) layer (preferably, about 1 to about 3 nm) first is applied over first and second semiconductor substrate regions. Typically, a SiN is deposited by low pressure chemical vapor deposition (LPCVD) or by plasma-enhanced chemical vapor deposition (PECVD). This is followed by the deposition of a layer (preferably, about 5 to about 20 nm) of a sacrificial oxide film, typically by LPCVD or PECVD techniques. Using lithographic techniques, the sacrificial oxide is patterned to expose the layers over the second region of substrate on which a second thicker oxide layer will be formed. The exposed sacrificial oxide and silicon nitride are selectively removed. Typically, the SiN layer is etched using a H 3 PO 4 -based wet etch technique; the SiO 2  layer typically is etched using a F-based dry etch technique. The resist mask then is stripped and the sacrificial oxide is removed (typically using a HF-based wet etch). The wafer then is subjected to a thermal oxidation procedure, which results in a second thicker (preferably about 3 to about 50 nm) oxide being grown in the second semiconductor substrate region where bare Si is exposed, and a thinner (about 1 to about 5 nm thinner than the thicker SiO 2  layer) layer of oxynitride (e.g., SiON, TiON, ZRON, or HfON) in the first substrate region where the nitride layer mask is present.  
           [0007]    In yet another embodiment, a thin nitride (e.g., SiN, TiN, ZrN, or HfN) layer (preferably, about 1 to about 3 nm) first is applied over a first and second semiconductor substrate regions. Typically, the nitride is deposited by low pressure chemical vapor deposition (LPCVD) or by plasma-enhanced chemical vapor deposition (PECVD). Using lithographic techniques, the SiN layer is patterned to expose the second region of substrate on which a second thicker oxide layer will be formed. The exposed SiN is selectively removed, typically by using a CF 4 /O 2  down-stream plasma etch, and then the resist mask is stripped. The wafer then is subjected to a thermal oxidation procedure, which results in a second thicker (preferably about 3 to about 50 nm) oxide being grown in the second semiconductor substrate region where bare Si is exposed, and a thinner (about 1 to about 5 nm thinner than the thicker SiO 2  layer) layer of oxynitride (e.g., SiON) dielectric in the first substrate region where the SiN layer mask is present. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    FIGS.  1 - 4  are sectional views illustrating the process steps in accordance with one embodiment of the present invention.  
         [0009]    FIGS.  5 - 9  are sectional views depicting the process steps in accordance with a second embodiment of the present invention.  
         [0010]    FIGS.  10 - 16  are sectional views representing the process steps used in accordance with a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0011]    Referring now to FIGS. 1 through 4, these figures are sectional views illustrating a sequence of process steps utilized for the formation of varying oxide thicknesses across the features of a semiconductor chip.  
         [0012]    In FIG. 1, a first oxide layer  10  is deposited or grown upon a silicon semiconductor substrate  12  using conventional methods. The oxide layer  10  is formed with a thickness of about 1 nm.  
         [0013]    As shown in FIG. 2, a layer of silicon nitride  14  then is deposited over oxide layer  10 , to a thickness of about 1 nm. Employing conventional lithographic processes, silicon nitride layer  14  is patterned with resist pattern  16  which serves to protect a selected first region of the semiconductor chip while leaving a second region exposed. The exposed silicon nitride  14  and oxide  10  above the second substrate region  20  are selectively removed, as depicted in FIG. 3, using standard etching techniques. The resist masking layer  16  then is stripped from the silicon nitride layer  14  over the first substrate region  18 , and a second thicker oxide layer  22  is grown over the second substrate region  20 , resulting in the structure shown in FIG. 4, featuring a semiconductor substrate  12  with oxide layers  20  and  22  of two different thicknesses.  
         [0014]    Referring to FIGS. 5 through 9, sectional views are shown representing a sequence of process steps utilized in a second embodiment of the present invention for the formation of varying oxide thicknesses across the features of a semiconductor chip. As depicted in FIG. 5, a layer of silicon nitride  30  first is applied over the surface of semiconductor substrate  32 . Then, as seen in FIG. 6, a sacrificial oxide layer  34  is deposited over the silicon nitride layer  30  to a thickness of about 5 to about 20 nm.  
         [0015]    Employing conventional lithographic processes, the sacrificial oxide  34  is patterned with a resist layer  36 , as shown in FIG. 7, and then unprotected areas of the sacrificial oxide layer  34  and the silicon nitride layer  30  are selectively removed, using standard etching techniques to expose second substrate region  42  while not disturbing first substrate region  44 .  
         [0016]    The resist mask  36  then is stripped from the sacrificial oxide layer above first substrate region  44 , and sacrificial oxide layer  34  is removed as well, resulting in the arrangement as illustrated in FIG. 8. Thermal oxidation then is performed to produce a second, thicker oxide  40  in the area above second substrate region  42  and convert the SiN layer to SiON in the area above the first substrate region  44 , as depicted in FIG. 9, featuring a semiconductor substrate  32  with oxide layers  38  and  40  of two different thicknesses.  
         [0017]    In a third embodiment, illustrated by FIGS.  10 - 13 , a thin SiN layer  50  is deposited over the surface of a semiconductor substrate  52 , to result in the structure of FIG. 10. Using standard lithographic techniques, the SiN layer  50  is patterned with resist  54  to protect a first substrate region and expose the SiN layer over a second substrate region. The exposed SiN is etched by a plasma etching technique, and then resist  54  is stripped to form the structure of FIG. 11. A thermal oxidation then is conducted to produce a second, thicker oxide  56  in the area above second substrate region and a thin SiON layer  58  over the first substrate region to produce a structure, as in FIG. 13.  
         [0018]    While preferred embodiments have been shown and described, various modifications and substitutions maybe made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the scope of the claims.