Abstract:
A method for planarizing the surface of an isolating layer that is deposited on a semiconductor body is described. Zones where the isolating layer has a low level are covered with a block mask in order to be able to selectively etch zones of the isolating layer with a higher level.

Description:
BACKGROUND OF THE INVENTION  
       Field of the Invention  
         [0001]    The present invention relates to a method for planarizing a surface of an isolating layer that is deposited on a semiconductor body of an integrated semiconductor circuit configuration and whose surface lies on a higher level relative to the semiconductor body in first regions of the semiconductor circuit configuration than in second regions of the semiconductor circuit configuration.  
           [0002]    In the fabrication of integrated circuits such as semiconductor memories, their surfaces must be planarized after certain processing steps before further processing steps are initiated. The planarizing usually occurs in a chemical mechanical polishing (CMP) step. An example of this is the masking of the active region (active area (AA) mask) with the subsequent isolation trench (IS) etching of this region in DRAM fabrication. After the AA masking the inactive regions are filled with silicon dioxide, what is known as a shallow trench isolation (STI) oxide, for electrical isolation and then planarized by a CMP step. It is known that the desired planarity cannot be achieved by a CMP step and specified CMP tools.  
         SUMMARY OF THE INVENTION  
         [0003]    It is accordingly an object of the invention to provide a method for planarizing an isolating layer which overcomes the above-mentioned disadvantages of the prior art methods of this general type, with which it is possible to minimize levels between individual regions of the isolating layer and between the isolating layer and a top edge of silicon.  
           [0004]    With the foregoing and other objects in view there is provided, in accordance with the invention, a method for planarizing a surface of an isolating layer deposited on a semiconductor body of an integrated semiconductor circuit configuration. The surface lies at a higher level in first zones of the integrated semiconductor circuit configuration than in second zones of the integrated semiconductor circuit configuration. The method includes the steps of covering the second zones with a block mask, and etching the isolating layer in the first zones.  
           [0005]    The object is inventively achieved with a method of the above type by first covering the second zones with a block mask, and then subjecting the isolating layer to an etching process in the first zones.  
           [0006]    In an advantageous development of the invention, the isolating layer is an STI layer formed of silicon dioxide. The level of the STI layer can be at least 10 nm higher in the first zones than in the second zones. The first zones can advantageously form a cell field, while a logic configuration is placed in the second zones.  
           [0007]    Dry or wet chemical etching can be applied for the etching process with which the isolating layer in the first zones is etched back. The etching process can be performed before or after the removal of a silicon nitride layer that covers the semiconductor body outside the isolating layer.  
           [0008]    A suitable material can be employed for the block mask, such as a resist, a varnish, etc.  
           [0009]    What is essential to the inventive method is that zones with a lower level, such as a logic area or a thin oxide zone, are first covered with a block mask. The regions with a higher level, such as an active region like a cell field or a thick oxide zone, are then etched back by a dry or wet chemical process. The etching can be performed before or after the removal of a nitride layer or some other isolating layer that may still be present in the active zone or cell field, i.e. before or after a nitride strip. A CHF 3 /CF 4  oxide etching chemistry is preferably utilized for the etching for which a reactive ion etching (RIE) can be utilized.  
           [0010]    With the inventive method, the planarization can be appreciably improved after a CMP step by dry and wet chemical etching with the aid of the block mask. The level in the individual zones can be reduced by the selective etching with the aid of the block mask, so that the planarity comes optimally close to the desired planarity for regions with low coverage densities, such as in the logic area, and for regions with high coverage density, such as in the cell field. If necessary, it is possible to remove the block mask from the second zones after an etching process in the first zones and to perform an additional dry or wet chemical etching procedure in the second zones, potentially together with the first zones.  
           [0011]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0012]    Although the invention is illustrated and described herein as embodied in a method for planarizing an isolating layer, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0013]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIGS. 1 and 2 are diagrammatic, sectional views illustrating a first exemplifying embodiment of a method according to the invention;  
         [0015]    FIGS.  3  to  6  are sectional views illustrating a second exemplifying embodiment of the inventive method;  
         [0016]    [0016]FIGS. 7 and 8 are sectional views illustrating a first method according the prior art; and  
         [0017]    FIGS.  9  to  12  are sectional views illustrating a second method according to the prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 7 thereof, there is shown a result reached at the end of chemical mechanical polishing (CMP) step for a region  1  and a logic area  2  of a semiconductor body  3 ,  4 . The semiconductor body  3 ,  4  contains a chip and is represented in divided form for the purpose of illustrating the difference between the region  1  and the logic area  2 .  
         [0019]    In the region  1  and the logic area  2  in which circuit elements can be realized, individual active regions  5 ,  6 , are separated by STI regions  7 ,  8 , which are formed from silicon dioxide for example.  
         [0020]    After the CMP step, the individual active zones  5  in the region  1 , for instance the memory cell field, are still covered by a silicon nitride layer  9  that functions as a CMP stop layer. The same is true of the logic area  2  with respect to a silicon nitride layer  10  on the active zone  6 . The thickness of the residual silicon nitride layers  9 ,  10  is now dependent on the local coverage density. The layer thickness of the silicon nitride layer  9  in the region  1  is greater than the thickness of the silicon nitride layer  10  in the logic area  2 . Besides this, in the region  1  the STI zones  7  are located between the active zones  5 , with their top side lying at least  10  nm above the top edge of the silicon of the active zones  5 , as can be seen in FIG. 7, left-hand side.  
         [0021]    In other words, the chip containing the integrated semiconductor circuit configuration has level differences on its surface even after the CMP step.  
         [0022]    The next step in processing is the stripping of the silicon nitride layers  9 ,  10 , so that the configuration represented in FIG. 8 remains. The level difference between the top edge of the silicon of the active zones  5 ,  6  and the surface of the STI zones  7  is referenced D, and the level difference between the STI zones  7  and  8  of the active region  1  and the logic area  2 , respectively, is referenced d. The level differences D and d degrade the subsequent processing steps, which may include well implantations and gate oxide formation as well as gate-conductor layer structuring.  
         [0023]    FIGS.  9  to  12  illustrate another existing dual gate oxide (GOX) process for a zone  12  with a thick oxide and a zone  13  with a thin oxide. FIG. 9 represents a chip after a CMP step. FIG. 10 represents the chip after an N 2  ion implantation and a silicon nitride removal in which the silicon nitride layers  9 ,  10  on the active zones  5  were eroded or striped. FIG. 11 represents the chip following a sacrifice oxide (SAC oxide) striping in which the STI zones  7  were thinned. FIG. 12 shows the chip after the depositing of gate oxide layers  14 ,  15  with different layer thicknesses, namely a thick oxide layer  14  in the zone  12  and a thin oxide layer  15  in zone  13 . Different levels are visible for the oxide layers  14 ,  15  to the STI zones  7 . The reason for this is that the silicon dioxide which forms the STI zones  7  is etched differently by wet etching depending on whether or not it has undergone an N 2  ion implantation. Implanted zones are etched more rapidly than non-implanted zones during the sacrifice oxide strip.  
         [0024]    It has been shown that the desired planarity cannot be achieved by a CMP step and specified CMP tools.  
         [0025]    Like FIG. 7, FIG. 1 represents a section through the region  1  and the logic area  2  of the semiconductor body  3  and  4  with the active zones  5  and  6  as well as the STI zones  7  and  8  after a chemical-mechanical polishing process. In order to achieve a reduction of the levels D and d between the top edges of the STI zones  7  in the region  1  and the top edge of the silicon in the active zones  5 ,  6 , and between the top edges of the active zones  7  and  8 , respectively, (FIG. 8), a block mask  11  formed of varnish or a resist material or some other selectively etchable substance is inventively deposited on the logic area  2 . The region  1  is then subjected to dry or wet chemical reactive ion etching (RIE), in order to etch back the top side of the STI zones  7  until they have achieved the same level as the STI zones  8  in the logic area  2 . The RIE etch-back can be performed before or after the removal of the silicon nitride layer  9 .  
         [0026]    As emerges from the schematic representation in FIG. 2, the level d can be brought down practically to zero. Besides this, a substantial reduction of the level D is also possible.  
         [0027]    FIGS.  3  to  6  illustrate how to prevent different levels in the zone  12  with the thick oxide and the zone  13  with the thin oxide with the inventive method. Namely, the block mask  11  is deposited on the thin oxide zone  13  (FIGS. 3 and 9), so that the STI zones  7  formed of silicon-dioxide can be etched back in the thick oxide-zone  12  (“oxide etch-back”). This makes it possible to set levels between the active zones  5  and the STI zones  7  such that the thick oxide layer  14  is planar with the thin oxide layer  15  after the stripping of the sacrifice oxide (FIGS. 6 and 12). Different etch rates in implanted and non-implanted zones can thus be compensated. Of course, this compensation can also be achieved when height (level) differences are produced by other causes. The steps of FIGS.  3  to  6  correspond to the steps of FIGS.  9  to  12 , respectively.