Abstract:
A method for monitoring the depth of at least one via ( 11 ) in a wafer comprising the steps of arranging the via ( 11 ) as a capacitive plate ( 21 ), providing a corresponding capacitive plate ( 23 ), applying an electrical potential difference to the via ( 11 ) and the corresponding capacitive plate ( 23 ), measuring the resultant capacitance between the via ( 11 ) and a corresponding capacitive plate ( 23 ) and determining the depth of the at least one via ( 11 ) by the capacitance.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to via etching in semiconductor wafers. In particular, the present invention relates to a method of monitoring the depth of the vias etched in a wafer.  
       BACKGROUND OF THE INVENTION  
       [0002]     A via is a vertical microscopic tunnel that penetrates selected inter-metal dielectric layers (IMDs) on the surface of a semi-conductor wafer and is filled with a conductive filler to provide an electrical flow path. Typically, the via is connected to a conductive layer at both its ends.  
         [0003]     Vias are etched into a dielectric layer by exposing selected areas on the surface of the dielectric layer to etching processes. Where vias are not to be formed, the surface is covered with an etch-resistant material during etching, which is removed after the vias are etched. How deeply a via is etched into one or more dielectric layers depends on factors such as etch method, etch rate and etch time.  
         [0004]     When the etch time is insufficient, the via does not penetrate sufficiently through the dielectric layer, or layers, into contact with an underlying conductive layer or device element. Therefore, vias are sometimes slightly over-etched to ensure that the vias are cleared of all dielectric material.  
         [0005]     Several methods may be used to monitor sufficient via depth, such as profilometry, X-ray Scanning Electron Microscopy (X-SEM), Atomic Force Microscopy (AFM) and via resistance measurement. However, profilometry has limited accuracy in profiling surface features having dimensions as small as that of a via. SEM techniques are sample destructive and slow, since the IMD has to be cut to reveal the via cross-section, and are therefore unsuitable as quick means of quality control. AFM requires tedious changing of cantilever tips and is too troublesome to be incorporated into a manufacturing process for quality control. Via-chain resistance measurement is the most commonly used quick-detection technique for monitoring good via connections, which would mean that the vias are not under-etched. However, if the via connections are bad, i.e. have high resistance, via-chain resistance measurement cannot distinguish whether the bad connection is due to under-etching, or via-misalignment leading to non-contact with the underlying conductive layer. Furthermore, via-chain resistance measurement cannot tell us how much via depth is short in the event of under-etching.  
         [0006]     It is, therefore, desirable to provide a method that is sensitive and quick in response for selectively detecting under-etching or via misalignment. It is preferable if the method is also able to indicate by how much the via depth is short of reaching the target depth.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention relates to a method for monitoring via depth in inter-metal dielectric layers (IMDs) on a semi-conductor wafer. It is an object of the present invention to provide an improved method for of monitoring via depth.  
         [0008]     The invention proposes in one aspect the use of capacitance to monitor via depth or placement.  
         [0009]     The invention proposes in another aspect a method for determining a property of a via in a wafer comprising the steps of using the via as a first capacitive member, providing a second capacitive member, applying an electrical potential difference across the via and the corresponding capacitive member, measuring the resultant capacitance between the via and the second capacitive member and determining the property of the via from the capacitance.  
         [0010]     In one specific embodiment, capacitance is obtained between vias separated into two groups, each group representing one of two capacitive plates. In another embodiment, all the vias are charged with the same charge, in bias as a capacitive plate against a conductor having the opposite charge being the corresponding capacitive plate.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which  
         [0012]      FIG. 1  is a schematic diagram of a dielectric layer in a wafer having vias.  
         [0013]      FIG. 2  is an isometric view of an embodiment of the present invention.  
         [0014]      FIG. 3  is a schematic side view of the embodiment of  FIG. 2 .  
         [0015]      FIG. 4  is a schematic plan view of the embodiment of  FIG. 2 .  
         [0016]      FIG. 5  is a further isometric view of the embodiment of  FIG. 2 .  
         [0017]      FIG. 5   a  shows a plot of via-Critical Dimension against capacitance according to the embodiment of  FIG. 2 .  
         [0018]      FIG. 6  is a schematic side view of another embodiment of the present invention.  
         [0019]      FIG. 7  is a further schematic view of the embodiment of  FIG. 6 .  
         [0020]      FIG. 8  is an isometric view of the embodiment of  FIG. 6 .  
         [0021]      FIG. 9  shows a plot of via-Critical Dimension against capacitance according to the embodiment of  FIG. 6 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]      FIG. 1  illustrates two inter-metal dielectric layers  13 ,  19  (IMD) of a semi-conductor wafer  10  having vias  11  etched through a first dielectric layer  13  into contact with a conductive layer  15  laid on the second dielectric layer  19 . In a downstream process, the vias would be filled with conductive fillers before a further conductive layer (not shown) is laid on the first dielectric layer  13 . It is important that a via thoroughly penetrates the dielectric layer  13  in order to contact conductive layers  15  on both ends.  
         [0023]      FIG. 2  is a schematic view of an embodiment  20  of the invention, which uses inter-via capacitance to monitor via depth. Three dielectric layers  13 ,  13   a ,  19  on a wafer is shown, the top two layers  13 ,  13   a  having vias  11   a ,  11   b  etched therethrough. The vias  11  are filled with the conductive fillers after etching to become conductive leads to the conductive layer  15  in the bottom-most dielectric layer  19 . However, the vias  11   a ,  11   b  are stop-etched, i.e. under-etched, as they are not completely etched through the second layer  13   a  into contact with the conductive layer  15 . To determine the depth of the vias  11   a ,  11   b , two metallic electrical contacts, one being a positive connection  21  and the other the corresponding negative connection  23 , are put into contact with the vias  11   a ,  11   b . A voltage is applied to the electrical contacts  21 ,  23  and a resultant potential difference exists between the non-contacting vias  11   a ,  11   b . The vias are thus electrically charged, the longitudinal surface of each via being charged with opposite charges. Therefore the longitudinal sides of one of the vias behaves as a positive plate  11   a  of a capacitor and that of the other via as the corresponding negative plate  11   b.    
         [0024]     Capacitance between two oppositely charged plates is defined as:  
             C   =       ɛ   ⁢           ⁢   A     d             (   1   )             
 
 Where, 
    C=capacitance (fF).     A=total overlap area between a positive and a negative plate (μm 2 ).     ε=dielectric permitivity of vacuum between capacitive plates, e.g. 8.854×10 −3  (fF/μm) for vacuum.     d=distance between the overlapping areas (μm).    
 
         [0029]     Equation (1) shows that the larger the area of overlap between two capacitive plates of opposite charges, the larger the resultant capacitance. Therefore, the depth of the vias  11   a ,  11   b , which is directly proportional to the area of overlap at the sides of the vias  11   a ,  11   b , directly correlate to the resultant capacitance.  
         [0030]      FIG. 3  is a plan view of an embodiment which corresponds to the embodiment of  FIG. 2 , showing how, instead of just two vias  11   a ,  11   b , two groups of several vias  11   a ,  11   b  are used as capacitive plates. The electrodes  21 ,  23  are comb-shaped and are distributed parallel to a dielectric layer  13  underneath. Each electrode  21 ,  23  has fingers  31  interspersed with the fingers  31  of the other electrode  21 ,  23 . Each finger  31  is also configured to contact a plurality of vias such that when a voltage is applied to the electrodes  21 ,  23 , the vias are separated into two groups having opposite charges.  
         [0031]      FIG. 4  shows an isometric view of the embodiment  20  of  FIGS. 2 and 3 . In practice, however, the total capacitance exerted between the electrodes and vias is a sum of both inter-via capacitance and inter-electrode capacitance. Thus, the capacitance between only the vias  11   a ,  11   b  is obtainable by subtracting the inter-electrode capacitance from the total capacitance.  
         [0032]     Referring to  FIG. 5  which represents the embodiment of  FIG. 2  schematically and which corresponds to  FIG. 3  when viewed from the direction indicated by the arrow ‘A’, the inter-electrode capacitance is:  
               C   1     =       K   ⁢           ⁢   ɛ   ⁢           ⁢     lt   1         d   1               (   2   )             
 
 Where 
    C 1 =capacitance between the electrodes  21 ,  23 .     K=relative dielectric permitivity of the dielectric film between the electrodes  21 ,  23 , e.g. air.     ε=dielectric permitivity of vacuum, 8.854×10 −3  fF/μm 2 .     l=the overlapping distance (μm) between two electrodes  31  of opposite charges.     t 1 =height of the electrodes (μm).     d 1 =distance between the positive and negative electrodes  11   a ,  11   b  (μm).    
 
         [0039]     The inter-via capacitance can thus be obtained by subtracting the inter-electrode capacitance from the total capacitance:  
                           C   Total     =     K   ⁡     (         ɛ   ⁢           ⁢     lt   1         d   1       +       ɛ   ⁢           ⁢   n   ⁢           ⁢     m   ⁡     (       t   2     ·   x     )           d   2         )                   =       C   electrode     +     K   ⁢               ⁢     ɛ   ⁢           ⁢   n   ⁢           ⁢     m   ⁡     (       t   2     ·   x     )           d                 ⁢     
     ∴       C   Total     -     C   electrode         =     K   ⁢               ⁢     ɛ   ⁢           ⁢   n   ⁢           ⁢     m   ⁡     (       t   2     ·   x     )           d         ⁢     
     ⁢       C   via     =     K   ⁢                 ⁢     ɛ   ⁢           ⁢   n   ⁢           ⁢     m   ⁡     (       t   2     ·   x     )           d     .                 (   3   )             
 
 where 
    C electode =capacitance between the electrodes  21 ,  23 , as obtained from equation (2)     C Total =total capacitance between the electrodes  21 ,  23  and vias  11   a ,  11   b  as measurable.     C via =capacitance between the vias  11   a ,  11   b.       K=relative dielectric permitivity of the dielectric material between the vias  11   a ,  11   b  and between the electrodes  21 ,  23 . K value is assumed to be the same in this embodiment for both the materials between the vias and between the electrodes, even though air exists between the electrodes while a dielectric material exists between the vias.     ε=dielectric permitivity of vacuum, 8.854×10 −3  fF/μm 2 .     l=the overlapping distance (μm) between two electrodes  31  of opposite charges.     t 2 =etch depth of the vias (μm).     m=the number of metal fingers on each comb  21 ,  23      n=the number of vias  11  on each comb finger, m     x=the average diameter of the via, i.e. Final Inspection Critical Dimension (FICD or via width).     d 1 =distance (μm) between a pair of positive and negative electrodes  11   a ,  11   b,       d2=distance (μm) between two corresponding vias  11   a ,  11   b       d=distance (μm) between two corresponding vias  11   a ,  11   b  (d2) and also distance between the corresponding electrodes  21 ,  23  (d1), assuming d=d1=d2.    
 
         [0053]     The present embodiment  20  therefore allows the depth, t2, of vias to be monitored by inter-via capacitance by re-arranging equation (3)  
               t   2     =         C   via     ·   d       K   ⁢           ⁢   ɛ   ⁢           ⁢   n   ⁢           ⁢   m   ⁢           ⁢   x               (   3.1   )             
 
         [0054]     For a more accurate measurement, a calibration is obtained to correlate via depth, or via-Critical Dimension (CD), to inter-via capacitance.  FIG. 5   a  is a plot of equation (3), where critical dimensions of vias correlates to the inter-via capacitance depending on the via depth.  FIG. 5   a  is obtained by substituting the following example values into equation (2) to get C 1 =1312.29 fF:  
         [0055]     K=4.15,  
         [0056]     ε=0.008854,  
         [0057]     l=100,  
         [0058]     t 1 =100,  
         [0059]     d 1 =0.28. 
 
 and by substituting the following example values into equation  
                 ⁢     k   ⁢           ⁢   ɛ   ⁢           ⁢   n   ⁢           ⁢     m   ⁡     (       t   2     ·   x     )             d   2         
 
 to get 21199 (t 2 .x) fF: 
 
         [0060]     M=500  
         [0061]     n=300  
         [0062]     K=4.15  
         [0063]     ε=0.008854  
         [0064]     d=0.26  
         [0065]     Substituting the values obtained above into the formulae (3) gives the following: 
 
 C= 21199( t   2.   x )+1312  (3.2) 
 
         [0066]     Using equation (3.2) to plot capacitance against via width (x), for every 0.2 um increment in via depth (t 2 ) provides Table 1 and the graph of  FIG. 5   a .  
                                                                                       TABLE 1                           x =           Via       diam-       eter       or   t 2  = etchdepth            width   0.8   1   1.2   1.4   1.6   1.8   2                    0.2   4703.8   5551.8   6399.8   7247.7   8095.7   8943.6   9791.6       0.22   5043.0   5975.8   6908.5   7841.3   8774.0   9706.8   10639.6       0.24   5382.2   6399.8   7417.3   8434.9   9452.4   10470.0   11487.5       0.26   5721.4   6823.7   7926.1   9028.4   10130.8   11233.1   12335.5       0.28   6060.6   7247.7   8434.9   9622.0   10809.2   11996.3   13183.4       0.3   6399.8   7671.7   8943.6   10215.6   11487.5   12759.5   14031.4       0.32   6738.9   8095.7   9452.4   10809.2   12165.9   13522.6   14879.4       0.34   7078.1   8519.7   9961.2   11402.7   12844.3   14285.8   15727.3       0.36   7417.3   8943.6   10470.0   11996.3   13522.6   15049.0   16575.3                  
 
         [0067]     If the vias  11  are thoroughly etched through the dielectric layer  13  into contact with the lower conductive layer  23 , the capacitance would drop as the charge is conducted away. However, if the vias are etched to a sufficient depth but are misaligned, such that one or more vias do not come into contact with the underlying conductive layer  23 , i.e. via misalignment, the capacitance remains high as a potential difference remains between the vias. Therefore, unlike via resistance measurement, the present embodiment indicates whether the cause of a bad connection is due to under-etching or misalignment.  
         [0068]      FIG. 6  shows another embodiment  60  of the present invention, wherein the vias  11  are not separated as two oppositely charged plates to obtain inter-via capacitance. Instead, all the vias  11  are charged with the same charge from one top electrode  61 , which is biased against an opposing electrode  63  underneath the vias  11 . The bottom electrode  63  is a conductive metal layer and has an area that spans underneath the vias  11 . The under-etched vias  11  therefore form several parallel capacitive plates corresponding to the bottom electrode  63 . Typically, the opposing electrode has a thickness of 2×IMD thickness, e.g. about &gt;15000 A.  
         [0069]     Capacitance of parallel plates adds up according to the following relationship: 
 
 C   parallel   =C   1   +C   2   +C   3   + . . . C   n  
 
 where 
 
         [0070]     C parallel  is total capacitance; and  
         [0071]     C 1 , C 2 , C 3  to . . . C n  are capacitors in parallel up to a total number of n capacitors  
         [0072]     Therefore, the capacitance between the vias and the oppositely charged bottom electrode  63  can be treated mathematically as between one combined via and the bottom electrode  63 , as illustrated in  FIG. 7 .  
         [0073]      FIG. 8  shows an isometric view of the embodiment of  FIG. 6 . The vias  11  are etched through a dielectric layer  13 , filled with a conductive filler, and is in contact with an electrode  61  of one charge. An opposing electrode  63  is beneath the vias so that there is a resultant capacitance between the vias  11  and the electrode  63  when a potential is applied thereto,  
         [0074]     Therefore, the depth of the vias  11 , d 3 , relates to the distance, d 2 , between the plates of a capacitor. According to equation (1), capacitance increases as d decreases. The efficiency of the etching process on the depth of the vias can therefore be monitored by the via-electrode capacitance.  
         [0075]     However, the total capacitance in the configuration of this embodiment  60  is a sum of the capacitance between the top electrode  61  and the bottom electrode  63  in areas where there is no via, and the capacitance between the vias  11  and the electrode  63  where the are vias  11 . Therefore, in order to obtain the capacitance between only the vias  11  and the bottom electrode  63 , the capacitance between the top electrode  61  and the bottom electrode  63  has to be subtracted from the total capacitance.  
         [0076]     The capacitance between the electrodes  61 ,  63  without the presence of vias  11  is defined by:  
               C   electrodes     =       K   ⁢           ⁢   ɛ   ⁢           ⁢   A       d   1               (   4   )             
 
 where, 
    C electrode =capacitance between the electrodes  61 ,  63 .     A=area of overlap between the plates, μm 2       ε=dielectric permitivity of vacuum, 8.854×10 −3  fF/μm 2 .     K=relative dielectric permitivity of the dielectric film between the electrodes  21 ,  23 .     d 1 =distance (μm) between the electrodes  61 ,  63     
 
         [0082]     Accordingly, the capacitance between the vias and the bottom electrode  63 , C via , can be found thus:  
                 C   Total     =         ɛ   ⁡     (     A   -     n   ⁢           ⁢     x   2         )         d   1       +       ɛ   ⁢           ⁢     nx   3           d   3     -     d   1             ⁢     
     ⁢       C   Total     =       K   ⁢           ⁢       ɛ   ⁢           ⁢   A       d   1         -           ⁢       ɛ   ⁢           ⁢     nx   2         d   1       +       ɛ   ⁢           ⁢     nx   2           d   3     -     d   1             ⁢     
     ⁢       C   Total     =       K   ⁢           ⁢       ɛ   ⁢           ⁢   A       d   1         -           ⁢     ɛ   ⁢           ⁢     nx   2     ⁢     1     d   1         +     1       d   3     -     d   1             ⁢     
     ⁢       C   Total     =       C   electrode     -     K   ⁢           ⁢   ɛ   ⁢           ⁢     nx   2     ⁢     1     d   1         +     1       d   3     -     d   1             ⁢     
     ⁢       C   Total     =           C   electrode     -     K   ⁢           ⁢   ɛ   ⁢           ⁢     nx   2     ⁢     1       d   3     -     d   1           -     1     d   1         ⁢     
     ∴       C   Total     -     C     electrode   ⁢           ⁢   1           =     K   ⁢           ⁢   ɛ   ⁢           ⁢     nx   2     ⁢     d   3     ⁢     1       (       d   1     -     d   3       )     ⁢     d   1               ⁢     
     ⁢       C   via     =     K   ⁢           ⁢   ɛ   ⁢           ⁢     nx   2     ⁢     d   3     ⁢     1       (       d   1     -     d   3       )     ⁢     d   1                     (   5   )             
 
 where, 
    C electrode =capacitance between the electrodes  61 ,  63 .     C Total =total capacitance between the electrodes and vias  61 ,  11 ,  63 .     C via =capacitance between the vias  11  and the bottom electrode  63 .     K=relative dielectric permitivity of the dielectric film between the electrodes  61 ,  63  and vias  11 .     ε=dielectric permitivity of vacuum, 8.854×10 −3  fF/μm 2 .     d 1 =distance (μm) between the electrodes  61 ,  63      d 2 =distance (μm) between the bottom of the vias  11  and the lower electrode  63 .     d 3 =etch depth of the vias=d 1 −d 2  (μm)     n=the number of vias  11  on each comb finger, m     x=the bottom surface width of the circular via. For simplicity, the area of the bottom of the via is approximated to be x 2  in this embodiment. In practice, the value depends on via dimension (μm 2 ).    
 
         [0093]      FIG. 9  is a plot of equation (5), where critical dimension (FICD, or width of the Via plug) of the vias has a correlation to via capacitance depending on the via depth. Therefore the depth of the vias  11  can be monitored based on the capacitance between the vias  11  and the bottom electrode  63 .  
         [0094]     Substituting the following example values into equation (4) to obtain C1=174.97 fF:  
         [0095]     n=50000  
         [0096]     K=4.15  
         [0097]     ε=0.008854  
         [0098]     A=100*100  
         [0099]     d=2.1  
         [0100]     Substituting the following example values into equation (5)  
         C   Total     =       K   ⁢           ⁢   ɛ   ⁢           ⁢     nx   2     ⁢     d   3     ⁢     1       (       d   1     -     d   3       )     ⁢     d   1           +     C     electrode   ⁢           ⁢   1             
 
         [0101]     d 1 =IMD total thickness=2  
         [0102]     K.ε.n=50000×0.008854×4.15=1837.205 
 
 gives  
                 C   Total     =       1837   ⁢     x   2     ⁢     d   3     ⁢     0.5     (     2   -     d   3       )         +   174.9       ⁢     
     ⁢   or   ⁢     
     ⁢       C   Total     =       918   ⁢     x   2     ⁢       d   3       (     2   -     d   3       )         +   174.9               (   5.1   )             
 
         [0103]     Using equation (5.1) to plot capacitance against via width (x), for every 0.2 um increment in via depth (d 3 ) provides Table 2 and the graph of  FIG. 9 .  
                                                                               TABLE 2                           X = Via           diameter or   d 3  = etch depth            width   0.8   1   1.2   1.4   1.6   1.8                    0.1   181.0   184.1   188.7   196.3   211.6   257.5       0.2   199.4   211.6   230.0   260.6   321.8   505.4       0.3   230.0   257.5   298.8   367.7   505.4   918.5       0.4   272.8   321.8   395.2   517.6   762.4   1496.8       0.5   327.9   404.4   519.2   710.4   1092.9   2240.4       0.6   395.2   505.4   670.6   946.0   1496.8   3149.2                  
 
         [0104]     A quick and sensitive method of detecting under-etch has been disclosed. In particular, the embodiments provide a method of monitoring via depth using capacitance. As the embodiments monitor via depth in a quick, simple and non-destructive way, they can be used on every wafer during wafer manufacturing for quality control.  
         [0105]     Other than monitoring via depth, the embodiments can be used to monitor depth and alignment of other etched features on an IMD, such as via contacts with the wafer surface (instead of with an underlying metal layer), Dual Damascene vias, Local Interconnects, etc.  
         [0106]     Where the via depth is known, the embodiments can also be used for determining the dielectric constant of the dielectric layer. The embodiments can also be used for comparing microloading effects between alignment mark and via features. The embodiments can also be used to monitor via depth consistency in a situation where the thickness of the dielectric layers on different wafers vary and where performance varies between etch machines. Therefore, recipe setups between machines can be obtained quickly. Furthermore, wafer-wafer or lot-lot comparisons can be made using the embodiments to control consistency in product quality.  
         [0107]     Tables 1 and 2, as well as the graphs of  FIGS. 5   a  and  9  show that the present embodiment is also useable to monitor the diameters of vias, as well depths. The correlation can be used to derive etch depth and, subsequently, calculating the via width. On obtaining the via width and the etch depth, the proportion of the measured capacitance contributed by the via width can be isolated from the capacitance contributed by the etch depth.  
         [0108]     Although only several embodiments are described, it should be understood that the embodiments described herein are but embodiments of underlying concepts of the invention. Alternatives to the embodiments, though not described, are intended to be within the scope of this invention as claimed.