Abstract:
A method of treating a carbon layer depositited in or on a substrate, such as a silicon wafer is provided. At least one aliphatic organic compound chosen from the group of an aliphatic alkanes, aliphatic alkenes and aliphatic alkines is utilized as a precursor for plasma treatment of the carbon layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to the plasma treatment of at least one carbon layer in or on a substrate for the production of semiconductor products. More particularly, the present invention relates to the treatment with a plasma generated with at least one precursor from the group of an aliphatic alkane, aliphatic alkene and aliphatic alkine.  
         [0003]     2. Description of the Related Art  
         [0004]     In the production of semiconductor products, especially dynamic random access memory (DRAM) devices, carbon layers are becoming more and more widely used as dielectric material.  
         [0005]     Exemplary methods for the deposition of the carbon layers as such using chemical vapor deposition (CVD) methods are described in U.S. Pat. No. 6,423,384 B1 and WO 01/80309 A2.  
         [0006]     Since the use of carbon layers in the fabrication of semiconductor products is relatively new, the methods for the treatment of such layers are not fully developed.  
         [0007]     It is known that carbon layers can be etched using plasmas with Oxygen or Hydrogen as precursor components. As used herein, the term generally refers to a molecular compound (such as O 2 , H 2 ) which is then used in atomic or ionized form in the plasma. Other precursor materials include, but are not limited to, CO, SO 2  or HBr/O 2 .  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention generally provides a method of treating a carbon layer deposited in or on a substrate, such as a silicon wafer. In particular, at least one aliphatic organic compound chosen from the group of aliphatic alkanes, aliphatic alkenes and aliphatic alkines is utilized as a precursor for the plasma treatment of the carbon layer, not the deposition.  
         [0009]     Those aliphatic compounds are a convenient source of Hydrogen atoms which can be used for the etching of the carbon layer. A passivation reaction would occur through the —CH 2 — radicals.  
         [0010]     In one aspect of the invention, the precursor for the plasma comprises one of the group of CH 4 , C 2 H 4  and C 2 H 2 . These compounds are the simplest aliphatic compounds, CH 4  having the highest Hydrogen to Carbon ratio.  
         [0011]     In another aspect of the invention, the precursors according to the invention are used together with one compound from the group of Oxygen and Nitrogen. In the plasma, molecular Oxygen and Nitrogen form radicals. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     So that the manner in which the above recited features, advantages and objects of the present invention are attained can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by the reference to the embodiments thereof which are illustrated in the appended drawings.  
         [0013]     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0014]      FIGS. 1A , B depict sectional views of a structure in a carbon layer generated by a method known from prior art.  
         [0015]      FIG. 2  depicts a sectional view from a similar structure as in  FIG. 1  generated with a first embodiment of the present invention.  
         [0016]      FIG. 3  depicts a sectional view from a similar structure as in  FIG. 1  generated with a second embodiment of the present invention.  
         [0017]      FIGS. 4A , B depict schematically the principal effects of an excess concentration of CH4 in the method according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     In the following, the reaction mechanism is described by way of example for a process according to the present invention. One skilled in the art will appreciate that this example is not to be understood as limiting the scope of the claimed invention.  
         [0019]     A carbon layer  1  is deposited on a silicon wafer  10  and is to be further processed. In this example, the carbon layer is to be etched, more particular a groove  2  needs to be etched into the carbon layer  1 .  
         [0020]     For one embodiment, CH 4 /N 2 /O 2  are used as precursors to form a plasma which would achieve the desired etching. Suitable plasma etching apparatus are generally known to the person skilled in the art (e.g. from Campbell, The Science and Engineering of Microelectronic Fabrication, Second Edition, Oxford University Press) so that no detailed description of the apparatus is necessary here.  
         [0021]     These precursor materials may be introduced into the plasma chamber with the following flows: 
    40 sccm CH 4       40 sccm O 2       120 sccm N 2      
 
         [0025]     The ratio of CH 4 /O 2  may be approximately 1:1. The plasma chamber may be operated at a pressure of 30 mTorr and may use 500 W high frequency power, 150 W low frequency power.  
         [0026]     In the plasma chamber the substrate  10  with carbon layer  1  is exposed to the described plasma.  
         [0027]     In a simplified form the following reactions may take place: 
 
CH 4 +e − →CH 3 +H+e −   (1) 
 
CH 3 +e − →CH 2 +H+e −   (2) 
 
C (solid) +O→CO   (3) 
 
C (solid) +CH 3 →C x H y (solid)    (4) 
 
C (solid) +CH 2 →C x H y (solid)    (5) 
 
C+N+H→C x H y N z (solid)    (6) 
 
         [0028]     Reaction (1) describes the formation of radicals through collisions in the plasma. Reaction (2) describes the further reaction from the product of reaction (1). The reaction product CH 2  might react with C to form C x H y  in a passivation reaction in the gaseous state or on the surface of the substrate.  
         [0029]     Reaction (3) describes the etching of the carbon with oxygen radicals.  
         [0030]     Since this is anisotropic etching, the downwards (Y) direction of the etching reaction C+O═CO is dominant. The passivation CH 2 +C reaction may be responsible for deposition of material on the sidewalls (X-direction). The Oxygen may etch the C x H y  to some minor extent.  
         [0031]     Reactions (4, 5) describe the passivation of the carbon layer by the formation of a solid C x H y  film. Reaction (6) describes the passivation of the carbon layer by the formation of a solid C x H y N z  film.  
         [0032]     The profile tuning may be influenced by pitch and the aspect ratio of the etched recess. High aspect ratio structures can lead to different surface areas of exposed films on the wafer. This may likely lead to the need to alter pressure and/or gas ratio in order to maintain a suitable profile.  
         [0033]     In  FIGS. 1A , B sectional views taken from a SEM (scanning electron microscope) image are given. In both figures a structured carbon layer  1  on an oxide substrate  10  is shown. The structures are a series of long trenches  2  of which sectional views for one of the trenches are shown here in detail. A capping layer  3  is positioned on top of the carbon layer  2 . Typically the capping layer  3  is made from dielectric material.  
         [0034]     The sectional dimensions at the center of the trench  2  in  FIG. 1A  is approximately 330 nm deep and 75 nm wide (at the bottom). Thus, the aspect ratio (depth/width) is approximately 4.4.  
         [0035]     The sectional dimensions at the edge of the trench  2  in  FIG. 1B  is approximately 270 nm deep and 84.46 nm wide (at the bottom). The aspect ratio is approximately 3.2. The trench  2  becomes shallower at the edges compared to the center.  
         [0036]     The trench  2  of which sectional views are given in  FIGS. 1A , B is produced by etching according to a prior art process with a plasma in which o 2 /N 2  were used as precursors. As can be seen from  FIGS. 1A , B the sectional profile of the trench is not optimal since the side walls of the trench are not straight as would be desirable. In effect, the etching produced a bottle shaped trench  2  which is wider at the bottom than on the top.  
         [0037]     This contrasts with a similar structure etched with a first embodiment of the present invention as depicted in  FIG. 2 . The trench  2  structure includes basically the same layers and materials as in  FIG. 1A , B. However, the walls of the trench  2  shown in  FIG. 2  are almost vertical and no sizable line edge roughness is detectable. This may be attributed to using CH 4  as precursor for the etching plasma. For the illustrated example, the etch parameters were as follows. The other precursors used were O 2  and N 2 . The ratio between O 2  and CH 4  was approximately 1:1 (40 sccm O 2  and 40 sccm CH 4 ). The N 2  flow was 120 sccm. The etching was performed at a pressure of 30 mTorr. 500 W high frequency Power and 150 W low frequency power were used. The temperature at the top was 60° C., at the bottom 20° C. The magnetic flux density was 300 Gauss.  
         [0038]     In  FIG. 3  the result of using a second embodiment of the present invention is depicted. The same structure was produced in essentially the same method as described in connection with  FIG. 2 . Only the ratio of O 2  and CH 4  was changed to 1:0.5, i.e. the CH 4  content was halved.  
         [0039]     Even though the sectional profile in  FIG. 3  is considerably better than the profile obtainable through the prior art method described in connection with  FIG. 1 , the side wall of the trench is not as straight as with the application of an O 2 /CH 4  ratio of 1:1 (shown in  FIG. 2 ). This shows the sensitivity to the CH 4  concentration.  
         [0040]     In  FIGS. 4A , B two applications of embodiment of the present invention are shown, that serve to illustrate the ability to taper walls or create straight sidewalls. In  FIG. 4A  a sectional view is presented having straight sidewalls. This is achieved by a lean process, i.e. a process using a low CH 4  flow. The shape of the sectional profile is tuned by changing the CH 4  flow to the plasma, as described in connection with  FIGS. 2 and 3 .  
         [0041]     If the CH 4  flow is in excess of O 2  (greater flow rate means more CH 2 ), the rich process moves into a deposition mode, i.e. carbon is deposited on the side walls as shown in  FIG. 4B . This can be used to protect carbon sidewalls while etching with a known O 2  plasma. Deposition on the etch front direction (Y) is zero since the high energy incident ions will remove deposition under the action of the reactive ion etching (RIE) process. This is understood for RIE applications. The low frequency RF enables this to occur.  
         [0042]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.