Abstract:
An etching process yields an optimized formation of via holes through the combination of semiconductor material selection and etchant parameters. Over an interlayer dielectric layer is formed a stop layer having a SiON layer over which is a SiC layer. Selective etching will attack the SiC layer while leaving the SiON layer undisturbed. When etching the via hole, a proportion of about 7:90 O 2 :CO was observed to yield a superior etch.

Description:
FIELD OF THE INVENTION 
     An improved etching process creates a via hole using a 1-step dual damascene method. 
     BACKGROUND OF THE INVENTION 
     Damascene refers to a process in which interconnect metal lines are delineated by isolating dielectrics. Damascening is not performed by lithography and etching, but by chemical-mechanical planarization (CMP). In damascening, an interconnect pattern is first lithographically defined in the layer of dielectric, and then metal is deposited to fill in the resulting trenches. Then excess metal is removed by means of chemical-mechanical polishing (planarization). 
     Chemical-mechanical polishing (CMP), also called chemical-mechanical planarization, refers to a method of removing layers of solid by chemical-mechanical polishing carried out for the purpose of surface planarization and definition of the metal interconnect pattern. 
     Dual damascene is a modified version of the damascene process that is used to form metal interconnect geometry using a CMP process instead of metal etching. In dual damascene, two interlayer dielectric patterning steps and one CMP step create a pattern that would otherwise require two patterning steps and two metal CMP steps when using a conventional damascene process. 
       FIG. 1(   a ) through  FIG. 1(   d ) are drawings explaining a conventional method of manufacturing a semiconductor device having dual damascene structure wiring. 
     In a first conventional method of manufacturing a semiconductor shown in  FIG. 1(   a ), a first silicon nitride (Si 3 N 4 ) film  3 , a first silicon oxide film  4 , a second silicon nitride (Si 3 N 4 ) film  5  and a second silicon oxide film  6  are successively formed over a layer insulation film  2  in which a first wiring layer  1  is embedded. The first wiring layers are formed on a substrate, not depicted in the drawings for the sake of brevity. 
     Next, as is shown in  FIG. 1(   b ), anisotropic dry etching to open via hole  8  is performed using a first photoresist  7  as a mask. This etching is performed until the first nitride film  3  is exposed in the interior of via hole  8 . The via hole  8  is also referred to as a contact hole. The first nitride film  3  acts as a stopper film that stops the progress of etching in this etching process as shown in  FIG. 1(   b ). 
     When the etching to open via hole  8  is finished, the first photoresist  7  is removed from above the second silicon oxide film  6 . Optionally, a second photoresist  9  that has an open portion that corresponds to the wiring slot  10  is formed in its place, as shown in  FIG. 1(   c ). 
     Next, anisotropic dry etching to open wiring slot  10  is performed using the second photoresist  9  as a mask. This etching is performed under the condition that a silicon oxide film can be removed with a significant selection ratio to the silicon nitride film. At this time, the first silicon nitride film  3  and the second nitride film  5  are both used as stopper films that stop the progress of etching. Next, etching for the purpose of removing the second silicon nitride film  5  exposed in the bottom of wiring slot  10  and the first silicon nitride film  3  exposed in the bottom of via hole  8  is performed. If this processing is done properly, via hole  8  which exposes the surface of first wiring layer  1  and wiring slot  10  which leads to via hole  8  are formed as shown in  FIG. 1(   d ). 
     A typical damascene structure according to the conventional art is described by D. Edeletein et al., Proc. IEEE IEDM (1997) and S. Venkatesan et al., Proc. IEEE IEDM (1997). 
     Via hole formation according to the conventional art gives rise to a number of problems as feature size decreases. For example, etch selectivity must be controlled during the etch to prevent photoresist from being prematurely removed to cause damage to the underlying layers. For example, if a plasma etch uses a gas mixture that contains a 1:1 proportion of oxygen to carbon monoxide, then both the photoresist and the material not covered by the photoresist would be etched at the same rate. In order to prevent damage to the semiconductor device, it would then be necessary to use a very thick layer of photoresist, which could then cause poor line, trench and via hole definition. 
     Additional difficulties in conventional etch processes arise when silicon is etched using a fluorocarbon gas such as CF 4 . The etch process converts the silicon to SiF 4 , but a polymeric carbon residue remains on the semiconductor, frequently appearing as a “fence” around the via hole. 
     As has been shown, the conventional art technologies have disadvantages arising from inefficient etching. These disadvantages give rise to manufacturing inefficiency, semiconductor damage and such phenomena as the creation of fences from residual materials around the via holes. These fences give rise to such unwanted effects such as parasitic capacitance and reduced conductance by the metal component. Accordingly there is a need to provide new etch technologies that minimize these disadvantageous phenomena. 
     SUMMARY OF THE INVENTION 
     The invention, in part, pertains to a method for forming a via hole on a dual damascene structure that overcomes the disadvantages of the conventional art. 
     The invention, in part, pertains to a process for manufacturing a semiconductor device that includes forming a first stopper layer over an insulation layer in which a wiring layer is embedded, forming a first interlayer dielectric over the first stopper layer, forming a second stopper film over the first interlayer dielectric, forming a second interlayer dielectric over the second stopper film, forming a third stopper film over the second dielectric, and etching a via hole in the semiconductor device, wherein the etching comprises at least one etch using an etchant gas that has a proportion of oxygen to carbon monoxide of about 7:90. In preferred embodiments of the invention, the first stopper layer comprises SiN, the second stopper layer comprises SiC, and the third stopper layer comprises a layer of SiON over the second dielectric and a layer of SiC over the layer of SiON. The wiring layer preferably comprises copper. 
     The invention, in in a preferred embodiment pertains to a series of sequential etch steps that includes (1) etching using a gas mixture comprising Ar:C 4 F 8 :C 2 F 6 :O 2  in a proportion of about 200:19:30:30, (2) etching using a gas mixture comprising Ar:O 2 :C 4 F 8 :CO in a proportion of about 60:7:12:90, (3) etching using a gas mixture comprising Ar:CHF 3 :C 2 F 6 :N 2  in a proportion of about 125:10:40:20, (4) etching using a gas mixture comprising Ar:O 2 :C 4 F 8 :CO in a proportion of about 60:4:12:90, and (5) etching using a gas mixture comprising Ar:N 2 :C 4 F 8 :CO in a proportion of about 60:8:12:100. In a preferred embodiment of the invention, in part, the method further comprises ashing using not more than 8% oxygen. Step (2) has a high selectivity to SiC. Step (5) does not require a high selectivity to SiC as it is a SiC etch step. The use of oxygen in the etch gas of step (5) is avoided to suppress CuO formation. Accordingly, the etch gas of step (5) should contain zero mol percent oxygen to minimize CuO formation. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention. The drawings illustrate embodiments of the invention and together with the description serve to explain the principles of the embodiments of the invention. 
         FIG. 1(   a ) shows a method of manufacturing a semiconductor device according to the conventional art. 
         FIG. 1(   b ) shows the step of forming a via hole in a semiconductor device according to the conventional art. 
         FIG. 1(   c ) shows the step of forming a wiring slot structure in a semiconductor device according to the conventional art. 
         FIG. 1(   d ) shows the step of etching the wiring slot in a semiconductor device according to the conventional art. 
         FIG. 2  shows a method from manufacturing a semiconductor device according to a preferred embodiment of the invention. 
         FIG. 3  shows trench formation according to a preferred embodiment of the invention. 
         FIG. 4  shows the patterning of via holes according to a preferred embodiment of the invention. 
         FIG. 5  shows the via hole BARC etch sequence according to a preferred embodiment of the invention. 
         FIG. 6  shows the via hole upper ILD ( 110 ) etch sequence according to a preferred embodiment of the invention. 
         FIG. 7  shows sequential photoresist ashing. 
         FIG. 8  shows the stopper layer ( 112  and  108 ) etch sequence according to a preferred embodiment of the invention. 
         FIG. 9  shows the via hole lower ILD ( 106 ) etch and trench upper ILD ( 106 ) etch sequence according to a preferred embodiment of the invention. 
         FIG. 10  shows the etch stopper layer ( 112 , 108  and  104 ) etch sequence according to a preferred embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
       FIG. 2  shows a method from manufacturing a semiconductor device according to a preferred embodiment of the invention. A first stopper film  104 , a first interlayer dielectric (ILD)  106 , a second stopper film  108  and a second interlayer dielectric  110  are successively formed over a layer insulation film  102  in which a wiring layer  100  is embedded. The wiring layer  100  can comprise copper, aluminum or an aluminum-copper alloy, but is preferably copper. The first wiring layers are formed on a substrate, not depicted in the drawings for the sake of brevity. The stopper films,  104 ,  108  can comprise silicon nitride (Si 3 N 4 ) or silicon carbide (SiC), but they are not restricted to these materials. In a preferred embodiment of the invention, the first stopper film  104  comprises silicon nitride and the second stopper film  108  comprises silicon carbide. 
     In  FIG. 2 , the ILDs,  106 ,  110  can be formed from any suitable material, including silicon dioxide, BSG (borosilicate glass), BPSG (borophosphosilicate glass) and FSG (fluorosilicate glass). Preferably, the ILDs,  106 ,  110  are formed from FSG. 
     In  FIG. 2 , over the second ILD  110  is formed a layer of SiON  112 , over which is formed a layer of SiC  114 . The layer of SiC  114  is a BARC and etch stopper. The layer of SiON  112  is a BARC layer. Over the SiC layer  114  is formed a layer of photoresist  116  that has been patterned with spaces over the portions corresponding to wiring trenches. 
     The thicknesses of the layers shown in the embodiment of the invention shown in  FIG. 2  are not restricted. Preferred thicknesses of the layers are about 70 nm for the SiC film  114 , about 120 nm for the SiON film  112 , about 400 nm for the second ILD layer  110 , about 50 nm for the second stopper film  108 , about 350–400 nm for the first ILD  106  and about 50 nm for the first stopper film  104 . 
       FIG. 3  shows trench formation according to a preferred embodiment of the invention. The trench is formed using a dry etch such as plasma etch. The preferred etch uses a mixture of gases containing either CF 4  or CHF 3  as the fluorine source. The preferred etch conditions include CHF 3 /CF 4 /O 2 =1.2:15:1, 1800 W, 40 mTorr. The fluorine radical in the gas reacts with the carbon in the SiC layer  114  to generate a carbon-containing gas. However, when the etch reaches the underlying SiON BARC layer  112 , the etch stops because there is no reactive carbon in the SiON BARC layer  112  to react with the fluorine radical. After the etch, the photoresist layer  116  is removed. The photoresist layer can be removed by any conventional method such as ashing. 
       FIG. 4  shows the patterning of via holes according to a preferred embodiment of the invention. Following the removal of the photoresist layer  116 , a second photoresist layer  118  and organic BARC layer  120  are formed that has the pattern of the desired via holes. The photoresist layer  118  and organic BARC layer  120  cover the sides of the SiC layer  114  that forms the sides of the trench. 
       FIGS. 5 ,  6 ,  7 ,  8 ,  9  and  10  show the via hole etch and trench etch sequence according to a preferred embodiment of the invention. A first etch etches through the SiON BARC layer  112 , the SiC film  114  and a portion of the dielectric layer  110 . A second etch step removes an upper portion of the dielectric layer  110  (See  FIG. 6 ). An ashing step to remove the photo resist and organic BARC can be performed between the  110  layer etch step and the  108  layer etch step (See  FIG. 7 ). Then, an etch is performed of the remainder of layer  108  and layer  112  (See  FIG. 8 ). Afterwards, portions of the dielectric layers  110  and  106  are removed (See  FIG. 9 ). Finally, remaining portions of the SiC layer  112  and the SiN layer  104  are removed (See  FIG. 10 ). Removal of the portion of the SiN layer  104  exposes a portion of the metal layer  100 . The conditions and etch gases for each of the etch steps shown in  FIGS. 5 to 10  are not restricted as long as the each etch step etches only the desired portion of the semiconductor stack in the via hole. Also, as discussed above, the presence of oxygen gas in the etch gas of step (5) is preferably completely avoided. Typical preferred etch conditions are given in Example 1. 
     EXAMPLE 1 
     Over a wafer substrate was formed a 100 nm layer of Organic BARC, over which was consecutively formed a 50 nm layer of SiC, a 120 nm layer of SiON, a layer of FSG, a layer of SiN, a layer of FSG and a layer of SiN. The FSG layers have thicknesses of about 500 nm, and the SiN layers have thicknesses of about 50 nm. After forming the trench, a via hole was etched using the etch sequence set forth in Table 1. 
                                                   TABLE 1                   Via hole and trench etch steps                                    Mag.       Step           Etch Gas   Power   Pressure   Field       No.   Etch   Gas   Ratio   (W)   (mTorr)   (G)               1   BARC   Ar:C 4 F 8 :C 2 F 6 :O 2     11:1:2:2   1000   90   20           and           SiC       2   Di-   Ar:O 2 :C 4 F 8 :CO   9:1:2:13   1600   55   20           electric       3   SiC   Ar:CHF 3 :C 2 F 6 :N 2     13:1:4:2    900   120     0       4   Di-   Ar:O 2 :C 4 F 8 :CO   15:1:3:23   1600   55   20           electric       5   SiC   Ar:N 2 :C 4 F 8 :CO   8:1:2:13   1600   55   20               Note: Steps (1)–(5) correspond to FIGS. 5, 6, 8, 9 and 10, respectively.            
Note: Steps (1)–(5) correspond to  FIGS. 5 ,  6 ,  8 ,  9  and  10 , respectively.
 
     In order to maintain a constant depth of all the via holes, an end point detector system was used. Optical emission spectroscopy is the most common method for etch endpoint detection because it is easy to implement with high sensitivity. 
     When etching the via hole, the etch selectivity must be controlled to prevent the photoresist from being prematurely removed. If, for example, in the second etch step (corresponding to No. 2 in Table 1, the ratio of O 2 :CO is 1:1, then the photoresist and the material underlying the via hole portion would both etch at the same rate, which is disadvantageous because there is no selectivity. However, an optimal selectivity was unexpectedly found at a O 2 :CO ratio of about 7:90 (about 1:13) for the second etch step No. 2 in Table 1. 
     After the second etch step, there is an ashing step in the sequence. This ashing step removes excess polymer that forms during the etching, as is explained in comparing equation 1 for SiO 2  and equation 2 for Si.
 
SiO 2 +CF 4 →SiF 4 +CO+O  (1)
 
Si+CF 4 →SiF 4 +C(polymeric)  (2)
 
     In an ashing process according to the conventional art, a high concentration of oxygen is used to ash the photoresist, but the k value (dielectric constant) of the FSG increases. This is not preferable because the capacitance of the embedded lines changes the speed of the transistor. That is, the speed of the transistor decreases as the dielectric constant increases. In the conventional art, the ashing was typically performed using 100 percent oxygen. However, in a preferred embodiment of the invention, the oxygen is diluted with argon to about 8% oxygen or less. Helium and neon can also be used instead of argon as the diluent gas. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of methods and apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. It is to be understood that the foregoing descriptions and specific embodiments shown herein are merely illustrative of the best mode of the invention and the principles thereof, which is therefore understood to be limited only by the scope of the appended claims.