Abstract:
A method is described for fabricating a cloisonné structure, in which a top surface of a metal oxide layer is made coplanar with a top surface of a metallic structure formed on a substrate. A nitride layer is deposited on at least the top surface of the metallic structure, and the metal oxide layer is deposited over the metallic structure and the nitride layer. The metal oxide layer is then polished by a chemical-mechanical polishing (CMP) process using a slurry, to expose the nitride layer on the top surface of the metallic structure. Polishing of the nitride layer causes ammonia to be generated in the slurry. The ammonia is extracted as a gas from the slurry, and a signal is generated in accordance with the ammonia concentration. The CMP process is terminated in accordance with a change in the signal. In a preferred embodiment, the metal oxide is aluminum oxide, the nitride is aluminum nitride, and the nitride layer is deposited as a conformal layer on the substrate and the metallic structure.

Description:
FIELD OF THE INVENTION 
     This invention relates to the fabrication of read/write heads for magnetic storage devices, and more particularly to improvement of process control in chemical-mechanical polishing (CMP) processes used in fabrication of such heads. 
     BACKGROUND OF THE INVENTION 
     In the field of semiconductor processing, chemical-mechanical polishing (CMP) is a widely used technique for planarization of material and controlled removal of a layer of material from a stack of films on a substrate. In a typical CMP process, a film is selectively removed from a semiconductor wafer by rotating the wafer against a polishing pad (or moving the pad against the wafer, or both) with a controlled amount of pressure in the presence of a slurry. FIG. 1 shows a typical CMP apparatus  10  in which a workpiece  100  (such as a silicon wafer with one or more layers deposited thereon) is held face down by a wafer carrier  11  and polished using a polishing pad  12  located on a polishing table  13 ; the workpiece is in contact with slurry  14 . The wafer carrier  11  is rotated by a shaft  15  driven by a motor  16 . The entire surface of the workpiece is polished by the polishing pad in the presence of the slurry. Accordingly, surface irregularities are removed from the films deposited on the substrate, and a high degree of planarization is obtained. CMP has been used to remove and/or planarize a wide variety of materials from a stack of films on silicon substrates, including polysilicon, silicon oxides and silicon nitride. 
     More recently, CMP has been used in a cloisonné process in the fabrication of a magnetic read/write head. This process involves polishing of an aluminum oxide (Al 2 O 3 ) film. As shown in FIG. 2A, a layer  22  of aluminum oxide is deposited over small structures  21  of NiFe; the NiFe structures  21  are disposed on an underlying structure  1 , which may be a substrate or a stack of films. The aluminum oxide layer  22  is then planarized and removed by CMP until the top surface  21 a of each NiFe structure  21  is exposed (see FIG.  2 B). A cloisonné pattern is thus obtained, with the top surface of the Al 2 O 3  layer  22  coplanar with the top surface of the NiFe structures  21 . 
     In CMP processes generally, it is extremely important to stop the process at a desired predetermined location in the film or stack of films (that is, when the endpoint has been reached). Overpolishing (removing too much) of a film renders the workpiece unusable for further processing, thereby resulting in yield loss. Underpolishing (removing too little) of the film requires that the CMP process be repeated, which is tedious and costly. Underpolishing may sometimes go unnoticed, which also results in yield loss. In the above-described cloisonné process, it is particularly important to maintain tight tolerances on the thickness of structures  21 , while assuring that enough of the aluminum oxide layer is removed to expose surface  21   a.    
     In a conventional approach to the CMP endpoint detection problem, the thickness of the layer to be removed and the polishing rate are measured for each workpiece, in order to determine a desired polishing time. The CMP process is simply run for this length of time, and then stopped. Since many different factors influence the polishing rate, and the polishing rate itself can change during a process, this approach is far from satisfactory. In particular, the polishing rate of a film generally changes substantially near an interface; this further compounds the problem of predicting the desired polishing time. 
     Furthermore, as shown in FIG. 2B, the desired cloisonné structure has a very small pattern factor; that is, polishing of the aluminum oxide layer  22  must continue until the NiFe surface  21   a  is exposed, but the total exposed NiFe area is only about 2% of the total Al 2 O 3 /NiFe interface area. 
     Application of CMP processing to head technology therefore requires a CMP endpoint detection technique which is effective with materials such as Al 2 O 3  and NiFe. In addition, it is desirable that the CMP endpoint be detected in situ and in real time; that is, not rely on external measurements such as the current drawn by shaft motor  16  or on extrapolation from previous layer thickness measurements. 
     SUMMARY OF THE INVENTION 
     The present invention enables the use of CMP in head fabrication by providing a real-time, sensitive, high-resolution endpoint detection technique for use in the CMP removal of metal oxide films. 
     In the CMP endpoint detection method of the present invention, an aluminum nitride (AlN) film is embedded as an endpoint-indicating tag in the aluminum oxide where endpoint is desired (e.g. on top of the NiFe structure  21  shown in FIG.  2 A). Reaction of the AlN with the slurry produces a distinct product, namely ammonia (NH 3 ), which is then extracted from the slurry and used as a tag to signal the endpoint of the CMP process. 
     According to a first aspect of the invention, a method of fabricating a cloisonné structure (in which a top surface of a metal oxide layer is made coplanar with a top surface of a metallic structure formed on a substrate) is provided. A nitride layer is deposited on at least the top surface of the metallic structure, and the metal oxide layer is deposited over the metallic structure and the nitride layer. The metal oxide layer is then polished by a chemical-mechanical polishing (CMP) process using a slurry and a suitable polishing pad (optimizing removal rate and polishing uniformity while minimizing scratches). The metal oxide over the nitride layer is removed, and the nitride layer on the top surface of the metallic structure is thus exposed. At this point the metal oxide layer and the nitride layer are polished together by the CMP process; polishing of the nitride layer causes ammonia to be generated in the slurry. The ammonia is then extracted as a gas from the slurry, and a signal is generated in accordance with the concentration of the extracted ammonia. The CMP process is terminated in accordance with a change in the signal. In a preferred embodiment of the invention, the metal oxide is aluminum oxide, the nitride is aluminum nitride, and the nitride layer (the endpoint-indicating tag) is deposited as a conformal layer on the substrate and the metallic structure. 
     The CMP process may be terminated in accordance with a change pattern in the ammonia concentration signal. An initial increase in the signal indicates that the nitride layer is being polished, and a subsequent decrease indicates the eventual disappearance of the nitride layer as it is removed from the top surface of the metallic structure. 
     According to another aspect of the invention, a method is provided for detecting the endpoint for removal by CMP of a metal oxide film overlying a metallic structure. An endpoint-indicating film is first provided between the metal oxide film and the metallic structure, on a top surface of the metallic structure. During CMP of the endpoint-indicating film, a chemical reaction product between the slurry and the endpoint-indicating indicating film is generated in the slurry. The chemical reaction product is extracted as a gas from the slurry, and a signal is generated in accordance with the concentration of the extracted reaction product. A change in the signal indicates the metal oxide CMP endpoint. 
     At the endpoint of the CMP process, a top surface of the metal oxide film is coplanar with the top surface of the metallic structure, so that the CMP process causes formation of a cloisonné structure. The CMP process may be terminated in accordance with the change in the signal. Specifically, the CMP process may be terminated in accordance with a decrease in the signal, since the decrease indicates nearly complete removal of the endpoint-indicating layer from the top surface of the metallic structure. 
     It should be noted that the endpoint-indicating layer may be located anywhere within the oxide film, so that the CMP process may be terminated at a predetermined location in the film or at a predetermined film thickness. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a typical CMP process apparatus. 
     FIGS. 2A and 2B schematically illustrate a CMP film removal process for an aluminum oxide layer. 
     FIGS. 3A-3C schematically illustrate a CMP film removal process for aluminum oxide and aluminum nitride layers, where polishing of the aluminum nitride layer provides a CMP endpoint signal in accordance with the present invention. 
     FIG. 4 shows a cross-section of an ammonia scrubber for reducing the pre-polish ammonia concentration in the slurry. 
     FIG. 5 shows details of an ammonia extraction unit for extracting ammonia from the slurry during polishing. 
     FIG. 6 shows details of an another ammonia extraction unit for extracting ammonia from the slurry during polishing. 
     FIG. 7 is a block diagram of an endpoint detection system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It has previously been found that when CMP is performed on a substrate with a target film of silicon oxide (SiO 2 ) over a stopping film of silicon nitride (Si 3 N 4 ) using a slurry containing water and fumed silica and having a pH of about 10.5, a chemical reaction occurs when the oxide/nitride interface is reached, resulting in the production of ammonia (NH 3 ). Specifically, when silicon nitride is polished with a slurry which is a mixture of fumed silica, water and potassium hydroxide (KOH), the slurry having a pH of about 10.5, the following reaction occurs: 
     
       
         Si 3 N 4 +6KOH+3H 2 O→3K 2 SiO 3 +4NH 3   
       
     
     Ammonia is therefore generated in the slurry as a gaseous reaction product when the oxide/nitride interface is reached. 
     In the present cloisonné CMP process, the same slurry is used; in particular, the pH has the same high value (about 10.5) as in the above-described CMP of silicon nitride. 
     In accordance with the present invention, the NiFe structures  21  and underlying films/substrate  1  are coated with a film  31  of aluminum nitride (AlN). The AlN film  31  is then overcoated with the Al 2 O 3  film  22 , as shown in FIG.  3 A. As is understood by those skilled in the art, the Al 2 O 3  deposition process may be modified to add an AlN film by adding a nitrogen-bearing gas in reactive sputtering of Al 2 O 3 . 
     During polishing in the presence of the high-pH water solution, the AlN film reacts to produce ammonia. The reaction is believed to be as follows: 
     
       
         2AlN+3H 2 O→Al 2 O 3 +2NH 3   
       
     
     When the Al 2 O 3  film  22  is planarized and partially removed to expose the top surface  31   a  of the AlN film  31  where the film overlies the NiFe structure (FIG.  3 B), polishing of the AlN film begins. The exposed A lN promptly reacts with water in the slurry to produce ammonia. The ammonia produced is dissolved in the slurry, and because of the relatively high pH it exists primarily in the form of NH 3  rather than NH 4   + . Accordingly, a change in the level of ammonia in the slurry indicates a change in the degree of exposure of the AlN film  31 . 
     When the AlN film is cleared from the top of the NiFe structure  21 , the amount of exposed AlN abruptly decreases; only a small portion  31   b  is then exposed (FIG.  3 C). The level of ammonia in the slurry therefore also decreases, signaling the desired endpoint of the CMP process. 
     It should be noted that the AlN layer  31  is conformal with the NiFe structures  21 ; the Al 2 O 3 /AlN interface is therefore first reached during the CMP process as the Al 2 O 3  removal process exposes the AlN surface  31   a  on the top surface  21   a  of NiFe. Stated another way, the ammonia-generating film is self-aligned to the surface on which endpoint is desired. 
     More generally, the endpoint for removal of any non-AlN-containing film overlying an AlN-containing film can be detected by monitoring the level of ammonia in the slurry. 
     It should also be noted that the endpoint for removal of a film overlying another film can be detected even if the films are the same type, provided that a reaction with the slurry yields a chemical reaction product with different concentrations from the two films. For example, a nitride contaminant in an oxide layer will react with the slurry to form ammonia. If two oxide layers have different nitride concentrations, the concentrations of ammonia reaction product will be different. This difference in concentration may be detected and used to determine the endpoint for removal of one oxide layer overlying the other oxide layer. 
     Furthermore, it will be appreciated that an AlN film can serve as an endpoint indicator wherever it is embedded in the Al 2 O 3  layer. 
     In order to implement ammonia production in the slurry as part of an endpoint detection scheme suitable for use in manufacturing, real-time and in-situ slurry collection and sampling are required. Preferably, the collection and sampling provide a rapid response with high sensitivity to ammonia, and minimize the effect of interference from other substances in the slurry and in the surrounding air. 
     The slurry described above generally contains ammonia prior to being used for polishing. The pre-polish ammonia concentration may be as high as 5.0×10 −5  M. Polishing at a Al 2 O 3 /AlN interface with a low pattern factor (that is, a small area of exposed AlN compared to the total area of AlN and Al 2 O 3 ) may generate an ammonia concentration on the order of 1.0×10 −5  M. The change in ammonia concentration due to process endpoint may therefore not be distinguishable from fluctuations in the pre-polish ammonia concentration in the slurry. Accordingly, the pre-polish concentration of ammonia in the slurry must be reduced to obtain the desired sensitivity. This may be done by using an ammonia scrubber  400 , shown in FIG.  4 . The main component of the scrubber is a Liqui-Cel membrane contactor  401  (model 4×28 from Hoechst-Celanese), containing Celgard™ microporous polypropylene fibers  402 . These fibers are hydrophobic and do not allow water-based solutions to penetrate through the fiber membranes, but do allow gas exchange. Slurry from a reservoir (not shown) enters contactor  401  at  404  and flows through contactor  401  on the outside of the fibers, allowing ammonia to permeate to the inside of the fibers before exiting at  406  and recirculating back to  404 . To facilitate removal of the ammonia in the slurry, an aqueous HCl solution from another reservoir (not shown) with a pH of approximately 3 is circulated on the inside of the fibers, entering at  408  and exiting at  410 , then recirculating back to  408 . Ammonia gas from the slurry crossing into this HCl stream is immediately converted to NH 4   + due to the low pH, effectively preventing buildup of NH 3  in the interior of the scrubber. The recirculating HCl stream and reservoir can be a sink for a large amount of ammonia from the slurry. The pre-polish ammonia concentration in the slurry may thus be reduced to approximately 2.5×10 −6  M. 
     In order to detect the endpoint of a CMP process by monitoring the concentration of ammonia produced during polishing, the ammonia must be extracted from the slurry. This may be accomplished using an ammonia extraction unit  200 , as shown in FIG.  5 . The extraction unit  200  has a body  210  (for example, a plastic block) with a channel  201  formed therein. The channel  201  is connected to a source (not shown) of a dry, clean (that is, free of ammonia, nitrogen oxides and amines) carrier gas (e.g. conditioned air) through an inlet connector  202 . The channel  201  is also connected through an outlet connector  203  to a detection unit (not shown), which in turn is connected to a vacuum pump (not shown). The bottom of the unit is covered by a hydrophobic, gas permeable membrane  220 . The channel  201  thus comprises a gas flow path with a gas-permeable membrane running along a portion of its length. The carrier gas pressure in channel  201  is maintained below atmospheric pressure. The extraction unit is placed in contact with the slurry  14  on top of the polishing pad  12 . Ammonia dissolved in the slurry penetrates the gas-permeable membrane  220  and enters the gas stream due to the pressure difference across the membrane. The contact between the extraction unit and the slurry, and between the slurry and the surface of the polishing pad, serves to prevent atmospheric air from entering the gas stream. An additional membrane  230  is placed in the gas stream to prevent slurry from reaching the detection unit in the event the membrane  220  is damaged and slurry thereby enters the channel  201 . The dry carrier gas, with the ammonia reaction product entrained therein, is pumped out through a flow restriction device  240 . 
     An alternate arrangement for an ammonia extraction unit is shown in FIG.  6 . The extraction unit  300  may be constructed from polypropylene microporous hollow fibers  302  obtained from a dismantled Liqui-Cel contactor (Hoechst Celanese, model 2.5×8). Fibers  302  allow gas but not liquid to pass from the outside to the inside of the fibers. Slurry is pumped into the extraction unit through inlet  304 , on the outside of fibers  302 , and exits the extraction unit through outlet  306 . A clean, dry carrier gas (e.g. conditioned air) is pumped into the extraction unit through inlet  308  through the inside of the fibers and exits through outlet  310 , carrying the ammonia gas molecules. Alternatively, the slurry and carrier gas may be pumped along the outside and inside, respectively, of a hydrophobic gas-permeable membrane. The carrier gas may be pumped through a drier (not shown) containing an ammonia filter before entering extraction unit  300 . The carrier gas is pumped at a reduced pressure of approximately 30 Torr to facilitate ammonia transport from the slurry through the fibers and into the gas stream. The reduced pressure also increases the overall flow velocity, thereby reducing the response time for measuring the change in ammonia concentration. 
     The ammonia-containing gas sample stream is then fed into an analyzer unit  500  which monitors the gas for the presence of ammonia. The analyzer unit outputs a signal  501  indicating the presence of ammonia in the gas stream, thereby indicating the endpoint of the film removal process. 
     A variety of methods may be used to detect the presence of ammonia in a gas stream with high sensitivity and fast response. One such method, which has been successfully used by the present inventors, involves detection of a chemiluminescence signal. As detailed by Li et al., “Indirect endpoint detection by chemical reaction and chemiluminescence,” U.S. patent application No. 09/073,604, the ammonia may be converted to a different product such as nitric oxide, which then emits chemiluminescence upon reacting with ozone. Specifically, a catalytic reaction is used to convert the ammonia, 
     NH 3 +O 2 →NO+H 2 O 
     and a chemiluminescent species is produced by reacting the NO product with ozone: 
     NO+O 3 →NO 2 * +O 2    
     NO 2 * →NO 2 +hν 
     The emitted light is detected by a photomultiplier tube. This technique has been found to detect ammonia at sub-parts-per-billion levels while avoiding interference from other chemicals. 
     FIG. 7 is a block diagram showing the various units described above, when used to detect the endpoint of an aluminum oxide CMP process with an embedded AIN endpoint indicating layer. The controller  600 , which accepts as input the signal  501  from the analyzer unit  500 , may advantageously include a computer executing a control program to monitor the CMP process and determine the process endpoint. When the endpoint of the CMP process is reached, the computer may send a control signal  601  to the polishing apparatus  10  to terminate the film removal process. The controller  600  also receives a start signal from the polishing apparatus  10 , which triggers the program to begin monitoring the endpoint signal automatically. A signal conditioning unit  610  may be used to ensure that signals reaching the polishing apparatus  10  and controller  600  are at appropriate voltage levels. The use of an endpoint signal for closed-loop process control has been disclosed elsewhere and need not be discussed in detail here. 
     The actual intensity of the signal  501  depends on several factors, and in general will not be uniform from one workpiece to the next. In particular, the amount of ammonia generated depends critically on the pattern factor (the relative area of exposed AlN as the Al 2 O 3 /AlN interface is reached). Accordingly, the endpoint control signal  601  should not depend directly on the ammonia concentration, but instead on a change in the ammonia concentration. As noted above, an increase in the ammonia concentration indicates that the Al 2 O 3 /AlN interface has been reached (see FIG.  3 B), and a subsequent decrease in the ammonia concentration indicates that the exposed surface  31   a  of the AlN has been cleared, thereby exposing the NiFe structure  21  (see FIG.  3 C). 
     While the invention has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the invention is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the invention and the following claims.