Abstract:
In a polishing process (e.g. CMP), the endpoint is declared after (a) detecting that the friction between the polishing tool and the structure being polished is rising, then (b) determining that the friction is falling, then (c) waiting for a predetermined period of time (which can be zero). This algorithm results in reduced over-polishing in some embodiments. Other embodiments are also described.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to endpoint detection in a polishing process, and more particularly to endpoint detection based on friction between the polishing tool and the structure being polished.  
           [0002]    Chemical mechanical polishing (CMP) is widely used in fabrication of integrated circuits. FIGS. 1 and 2 illustrate fabrication of tungsten plugs  120  that provide electrical contact between a layer  130  and another, overlying layer (not shown). Layer  130  can be a metal layer (e.g. tungsten) formed over a monocrystalline silicon substrate  140  and, possibly, over some other layer or layers  150 . Silicon dioxide  160  is formed over layer  130 . Openings  170  are etched in oxide  160  to expose metal  130 . A thin titanium nitride layer  110  is deposited (e.g. sputtered) over the structure to promote adhesion (tungsten  120  does not adhere well to silicon dioxide). Then tungsten  120  is deposited by chemical vapor deposition to fill the openings  170  and cover the structure. The top surface of the structure is polished by CMP until tungsten  120  and titanium nitride  110  are removed from the top surface. The resulting structure is shown in FIG. 2. Another conductive layer (not shown) can be formed on this structure. This layer will electrically contact the layer  130  through the metal plugs  120 / 110  in openings  170 . (For brevity, we will refer to plugs  120 / 110  as tungsten plugs  120 .)  
           [0003]    The CMP process should remove all of the tungsten  120  and titanium nitride  110  from the top surface of oxide  160  in order to avoid electrical shorts and excessive current leakage between the plugs. The CMP endpoint can be determined by monitoring the friction between the wafer and a polishing pad of the CMP tool. FIG. 3 illustrates an example CMP tool available from SpeedFam-IPEC of Chandler, Ariz. Wafer  180 , which incorporates the structure of FIG. 1, is held upside down on a carrier  210  (the wafer&#39;s front side  180 F faces down). A motor (not shown) rotates the carrier  210 , thus causing the wafer to rotate. Another motor  212  rotates a polishing pad  220  which polishes the wafer. (The motor rotates a platen on the pad is positioned). Friction sensor  230  detects the friction between pad  220  and wafer  180  by detecting the current drawn by motor  212 . Controller  240  stops the polishing process based on the friction data from sensor  230 . Suitable controllers  240  and sensors  230  are available from LUXTRON Corporation of Santa Clara, Calif.  
           [0004]    [0004]FIG. 4 is a chart showing the friction data FR produced by sensor  230 . FR is shown as a function of time. Initially, signal FR decreases as tungsten  120  and titanium nitride  110  are being polished. At some time t 1 , signal FR levels off, indicating that the polishing pad has reached the oxide  160 . Software programmable controller  240  (FIG. 3) is programmed to stop the CMP when the signal FR levels off.  
           [0005]    [0005]FIG. 5 shows signal FR for another CMP process. In this example, layer  110  is titanium. At some time t 2 , the friction FR starts to rise. Then FR falls (starting at some time t 3 ), and then levels off at a time t 1  when the oxide  160  is reached. Controller  240  is programmed to perform the following steps, in the order shown:  
           [0006]    1. Detect rising friction.  
           [0007]    2. Detect falling friction.  
           [0008]    3. Detect the friction leveling off, and declare an endpoint to stop the CMP.  
         SUMMARY  
         [0009]    The inventors have observed that the endpoint detection method described above (stopping the CMP when FR levels off) results in excessive over-polishing. Too much of oxide  160  gets polished off. In some embodiments of the present invention, the CMP is stopped before FR levels off. In the example of FIG. 6, the friction signal FR is as in FIG. 5. The CMP is stopped a predetermined time “dt” after the time t 3  (the time when FR starts falling off). Controller  240  is programmed to:  
           [0010]    1. Detect rising friction.  
           [0011]    2. Detect when the friction starts to fall.  
           [0012]    3. Declare an endpoint the predetermined time dt after the friction starts to fall.  
           [0013]    Step 1 (detect rising friction) assumes that at some point of time the signal FR is rising. In FIG. 4 (the titanium nitride case), the signal FR does not rise. In some embodiments, the titanium nitride deposition parameters are chosen so that the friction signal FR rises at some point (as in FIG. 5).  
           [0014]    Other features of the invention are described below. The invention is defined by the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIGS. 1 and 2 are cross section illustrations of an integrated circuit in the process of fabrication.  
         [0016]    [0016]FIG. 3 is a side view of a CMP tool.  
         [0017]    [0017]FIGS. 4, 5 are charts illustrating prior art CMP endpoint detection.  
         [0018]    [0018]FIGS. 6, 7A,  7 B,  7 C,  8 ,  9 ,  10 ,  11  are charts illustrating endpoint detection in some embodiments of the present invention.  
         [0019]    [0019]FIG. 12 is a side cross-sectional view of an ionized metal plasma deposition chamber. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0020]    In the embodiment of FIG. 6, controller  240  is programmed to perform the following steps, in the order shown:  
                   TABLE 1                           Step 1   Detect rising friction (the rising friction is detected at time t2)       Step 2   Detect falling friction (the falling friction is detected at time t3)       Step 3   Wait for a predetermined time period dt (dt = 15 seconds in one           embodiment). This period expires at a time t4.       Step 4   Declare an endpoint (stop the CMP at t4).                  
 
         [0021]    Controller  240  marks the conclusion of each step by suitable signals, as will be understood by those skilled in the art.  
         [0022]    The appropriate value for the parameter dt can be found through experimentation, and may depend on the materials, the deposition parameters, the polishing technology, and perhaps other factors. Parameter dt is chosen to avoid under-polishing while minimizing the over-polishing. In some embodiments, dt=0. Step 3 may be omitted.  
         [0023]    Detection of the rising and falling friction (Steps 1, 2) is performed with a precision that depends on the particular tool. Absolute precision may be impossible to achieve. Further, the absolute precision may provide a meaningless result due to noise causing the signal FR to oscillate.  
         [0024]    In some embodiments, the rising or falling signal FR is validated for some time before the Step 1 or 2 is completed, i.e. before the rising or falling slope is signaled as detected. In some embodiments, the rising signal is FR detected when the slope of the signal is larger than some small positive value, and the falling slope is detected when the slope is more negative than some small negative value. Small positive and negative slope values are treated as zero.  
         [0025]    In some embodiments, the rising and falling slopes are detected using a software system of type OptiView 9300 available from LUXTRON Corporation. In that system, the slope of the signal FR at any time t is analyzed using a rectangular window  410  (FIG. 7A). The window is defined by two programmable parameters: (1) width Δt, and (2) half-height h/2. The sides of window  410  are parallel to the coordinate axes “Time”, “FR”. The window is positioned so that the signal FR enters the window at the time t at a point P located in the middle of the window&#39;s left boundary. If the signal FR exits the window by piercing the upper boundary (as in FIG. 7A), the OptiView system indicates that the signal FR is rising, i.e. the slope is positive. The window  410  is called an Up window in this case.  
         [0026]    In FIG. 7B, the signal FR exits the window  410  by piercing its lower boundary. The OptiView system indicates that the signal FR is falling. Window  410  is called a Down window.  
         [0027]    In FIG. 7C, the signal FR exits the window  410  by piercing its right boundary. The system indicates that the signal FR is neither rising nor falling. Window  410  is called a Side window.  
         [0028]    At Step 1 of Table 1 above, the rising signal may be validated for some predetermined, programmable number of windows before the rising signal is detected. FIG. 8 illustrates an example of a rising signal validated with three Up windows  410 . 2 ,  410 . 3 ,  410 . 4  which follow a Side window  410 . 1 . Each subsequent window begins where the signal FR leaves the previous window. For example, the signal leaves the window  410 . 1  at a point P 1 . The window  410 . 2  is defined so that the middle of its left boundary is at the point P 1 . The signal leaves the window  410 . 2  at a point P 2 . The window  410 . 3  is defined so that the middle of its left boundary is at the point P 2 .  
         [0029]    Similarly, at Step 2 of Table 1, the falling slope may be validated for some predetermined number of Down windows. Step 2 completes when the signal has been validated.  
         [0030]    There are several ways to program controller  240  with the OptiView system to perform the steps of Table 1. In the embodiment of FIG. 8, Step 1 is performed in “Slope Start” mode, i.e. the rising friction is detected at t 2  immediately upon the occurrence of a predetermined number of Up windows. In FIG. 9, Step 1 is performed in “Slope End” mode. The rising slope is detected upon the occurrence of a predetermined, programmable number of Up windows (windows  410 . 1 ,  410 . 2 ,  410 . 3 ) immediately followed by a predetermined, programmable number of Side windows (windows  410 . 4 ,  410 . 5 ). Step 1 is completed at some time t 2 . 0  shortly before t 3 .  
         [0031]    In FIG. 10, Steps 1 and 2 are combined by programming the controller  240  to detect a peak of signal FR. A peak is defined as an Up window ( 410 . 1  in FIG. 10) immediately followed by zero, one or two consecutive side windows (window  410 . 2 ), immediately followed by a Down window (window  410 . 3 ). In other embodiments, more than two consecutive Side windows are required. Also, more than one Up window and more than one Down window may be required.  
         [0032]    In another embodiment, Step 1 is performed by programming the controller  240  to detect a valley (defined as a Down window, immediately followed by zero, one or two consecutive Side windows, immediately followed by an Up window). Step 2 is performed by programming the controller to detect either a falling slope in Slope Start mode or a peak. The invention is not limited to any particular programming. The invention is not limited to the OptiView 9300 system or a system using windows or having any particular programming features. Other systems, known or to be invented, can also be used.  
         [0033]    [0033]FIG. 11 illustrates another signal FR obtained in some embodiments. This signal has two peaks  430 . 1 ,  430 . 2 . This signal FR can be obtained with layer  110  consisting of two titanium layers deposited by different techniques to have different friction characteristics. Layer  120  can be tungsten. Signal FR begins to rise at some time t 2 , when the top titanium layer is reached. Then FR begins to fall at some time t 3 . This provides the peak  430 . 1 . Then FR rises again, starting at some time t 4 , possibly due to the bottom titanium layer. While the above explanation of the shape of signal FR is believed to be true, the invention does not rely the correctness of this explanation.  
         [0034]    In some embodiments of FIG. 11, controller  240  is programmed as follows:  
                   TABLE 2                           Step 1   Detect rising friction (the rising friction is detected at t2)       Step 2   Detect falling friction (the falling friction is detected at t3)       Step 3   Detect rising friction (the rising friction is detected at t4)       Step 4   Detect falling friction (the falling friction is detected at t5)       Step 5   Wait for a predetermined period dt (e.g. 15 seconds). This period           expires at a time t6.       Step 6   Declare endpoint (stop the CMP at t6).                  
 
         [0035]    These steps are performed in the order shown. The time dt may be zero. Step 5 may be omitted.  
         [0036]    The invention is not limited to any number of peaks  430  or titanium layers in layer  110 . Non-titanium layers can also be used. Different sub-layers of layer  110  may have different chemical composition.  
         [0037]    In FIG. 4, layer  110  is titanium nitride. The friction FR does not rise. FR can be made to rise by a suitable choice of the titanium nitride deposition process. In some embodiments, the titanium nitride is deposited by an ionized metal plasma process (IMP) also known as ionized physical vapor deposition (ionized PVD). FIG. 12 illustrates a suitable deposition chamber  610 . Chamber  610  is a magnetron IMP chamber of type Vectra available as part of a system of type ENDURA from Applied Materials of Santa Clara, Calif. Titanium target  620  is mounted at the top of chamber  610 . Target  620  is connected to a negative DC bias source  630 . Wafer  180  is placed on a pedestal  640  whose top surface is made of a dielectric material. RF (radio frequency) bias source  650  biases the pedestal with an AC current of a frequency 13.56 MHz. Argon is flown into the chamber. Bias source  630  helps ionize the argon. Coil  660  generates an RF electromagnetic field to densify the argon plasma, making the plasma high density. The argon ions dislodge titanium atoms from target  620 . Nitrogen flown into the chamber reacts with the titanium atoms to form titanium nitride. Some of the titanium nitride molecules become ionized by the high density plasma. The titanium nitride atoms and ions are deposited on wafer  180 . See “Handbook of Semiconductor Manufacturing Technology” (edited by Yoshio Nishi et al., 2000), pages 395-413, incorporated herein by reference.  
         [0038]    In some embodiments, the titanium nitride deposition parameters are:  
                   TABLE 3                           Base pressure in chamber 610   5 × 10 −7  torr.       (the pressure before the nitrogen       flow is turned on)       Nitrogen flow   28 sccm (standard cubic centimeters per           minute).       Argon flow   25 sccm.       DC power (source 630)   4000 W.       RF power (coil 660)   between 2000 W and 2500 W inclusive           (to provide a high TiN density).       Wafer pedestal bias (source 650)   greater than 150 W (500 W in some           embodiments).       Deposition temperature   200° C.                  
 
         [0039]    TiN layer  110  is deposited to a thickness of 8 nm or more.  
         [0040]    Then tungsten  120  is deposited by chemical vapor deposition (CVD) as described, for example, in U.S. patent application Ser. No. 09/881,607 filed Jun. 13, 2001 by V. Fortin, entitled “Thin Titanium Nitride Layers Used in Conjunction with Tungsten”, incorporated herein by reference. See also S. Wolf, “Silicon Processing for the VLSI Era”, Volume 2—Process Integration (1990), pages 245-247, incorporated herein by reference. The tungsten thickness is at least 350 nm in some embodiments.  
         [0041]    Then the CMP is performed. The friction signal FR is shaped as in FIG. 6. (FIG. 6 does not show an initial signal stabilization period which can be programmed to be a few seconds, e.g. 30 seconds.) The time t 2  is believed to correspond to the polishing tool reaching the titanium nitride. The time t 3  may be the time when most or all of the titanium nitride has been polished off. The invention does not depend on the correctness of this explanation for the times t 2 , t 3 .  
         [0042]    Controller  240  can be programmed as in Table 1. In one experiment using an OptiView 9300 system, the controller  240  was programmed as follows:  
                           TABLE 4                                       Step 1   Detect rising friction in Slope Start mode.           Step 2   Detect falling friction in Slope Start mode.           Step 3   Wait for dt = 15 seconds.           Step 4   Declare an endpoint when dt expires.                      
 
         [0043]    These steps were performed with windows  410  having the following dimensions (see FIGS. 7A, 7B,  7 C):  
         [0044]    Width Δt=1.5 seconds.  
         [0045]    Half-height h/2=5% of the full signal amplitude. The full signal amplitude was about 200.  
         [0046]    The CMP equipment was as described above for FIG. 3. Polishing pad  220  was a stacked pad of type IC1000/SubaIV available from Rodel, Inc.. The polishing slurry was Semi-Sperse® W2585 available from Cabot Microelectronics Corporation, Aurora, Ill.  
         [0047]    At Step 1, the rising friction was validated for three windows. At Step 2, the falling friction was validated for three windows.  
         [0048]    This process was compared with another process in which the CMP was stopped at time t 1  (FIG. 6). In both cases, TiN  110  was formed as in Table 3, and tungsten  120  was formed by CVD. In the case of Table 4, the CMP removed 25-30 nm less of silicon dioxide  160  than when the CMP was stopped at time t 1  . Yet the process of Table 4 removed all of TiN  110  from the top of oxide  160 .  
         [0049]    The invention is not limited to any particular TiN thickness values or deposition parameters. In the case of Table 3, the thickness can be 20 nm or some other value. Thicker TiN layers are believed to increase the time interval between t 2  and t 3 . The choice of the deposition parameters needed to obtain a rising friction FR may depend on the polishing tool and, in particular, on the controller  240  endpoint detection mechanism.  
         [0050]    In a variation of the process of Table 3, the titanium nitride deposition with a wafer pedestal bias of 500 W is preceded by a titanium nitride deposition at a lower bias, for example, 150 W or 0 W. For example, a 12 nm layer of TiN is deposited at 0 W, then a TiN layer having a thickness of 8 nm or more of is deposited at 500 W. The initial low-bias deposition is performed to protect silicon dioxide  160  from high energy TiN ions generated during the 500 W deposition. The high energy TiN ions can dislodge the silicon dioxide atoms, and the dislodged atoms can settle in openings  170  and increase the contact resistance. See U.S. patent application attorney docket no. M-11989 US filed by V. Fortin on the same date as the present application, entitled “Forming Conductive Layers On Insulators By Physical Vapor Deposition”, incorporated herein by reference.  
         [0051]    Without limiting the invention to any particular theory, the high pedestal bias is believed to provide a TiN layer with a high surface roughness and a low density compared to a lower bias. The high surface roughness is believed to increase the friction between the TiN layer and the CMP pad. In one experiment, the TiN surface roughness was measured with an AFM (atomic force microscopy) tool. TiN was deposited to a 30 nm thickness in a Vectra chamber of FIG. 12. Oxide  160  had been deposited from TEOS to a 700 nm thickness. The surface roughness RMS (root mean square) value was 1.121 nm for TiN layer deposited with the pedestal bias of 500 W. The RMS was 0.685 nm for the pedestal bias of 150 W.  
         [0052]    The invention is not limited to any particular layer thicknesses, frequency values or other deposition parameters, or to particular equipment. In some embodiments, the low-bias TiN deposition (e.g. at 0 or 150 W) is replaced, or used in conjunction with, deposition of some other layer protecting the silicon dioxide. In some embodiments, the high-bias deposition (e.g. at 500 W) is immediately followed by a lower bias TiN deposition. The RF bias from source  650  can be applied directly to wafer  180 , and can be replaced with a DC bias. Non-silicon dioxide insulators can be used for layer  160 . The invention is not limited to the chamber of FIG. 12 or to PVD. The invention is not limited to any particular materials. For example, layer  120  can be copper, and layer  110  can be tantalum nitride. Other conductive materials for layers  110 ,  120  can be used. Layer  130  can be a non-tungsten layer. The invention is not limited to the contact structures of FIGS. 1, 2. The invention can be used to form damascene interconnect structures and other structures, known or to be invented. Substrate  140  can be a non-silicon substrate.  
         [0053]    Friction data FR can be measured as a current drawn by a motor rotating the carrier  220 . The invention is not limited to the friction data being measured as a current drawn by a motor, or to any other way of getting a signal representative of the friction between the wafer and the CMP tool. In some embodiments, the signal FR is an inverse of the friction. FR falls when the friction rises, and vice versa. Detecting a rising friction is performed by the controller detecting a falling signal FR, and vice versa. In other embodiments, FR is some other function of the friction. The invention is not limited to any particular timing or slope parameters in the CMP endpoint detection, to the tool of FIG. 3, or to software programmable controllers. Non-chemical polishing can be used. Other embodiments and variations are within the scope of the invention, as defined by the appended claims.