Abstract:
A method for buffering a chemical mechanical polish chemical slurry is disclosed. Buffering the slurry reduces buildup of local acidic areas at the interface between the polished metal and the polishing pad. Reduction of the local acidic areas improves the uniformity of the polish and an endpoint signal used to determine when to finish the polish operation.

Description:
BACKGROUND 
   1. Field of the Invention 
   The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to chemical additives for improved end point signals for slurries for the chemical mechanical polishing (CMP) of thin films used in semiconductor integrated circuit manufacturing. 
   2. Background 
   Today, integrated circuits can be made up of literally millions of active devices formed in or on a silicon substrate. The active devices which are initially isolated from one another are later connected together to form functional circuits and components. The devices are typically interconnected together through the use of multilevel interconnections. 
   A cross-sectional illustration of a typical multilevel interconnection structure  100  is shown in  FIG. 1 . Interconnection structures typically have a first layer of metallization, an interconnection layer  102  (typically a copper alloy or an aluminum alloy with up to 3% copper), a second level of metallization  104 , and sometimes a third, fourth or even higher level of metallization. Interlevel dielectrics  106  (ILDs), such as doped and undoped silicon dioxide (SiO 2 ), are used to electrically isolate the different levels of metallization in silicon substrate or well  108 . 
   The electrical connections between different interconnection levels are made through the use of metallized vias, such as metallized via  110  formed in ILD  106 . In a similar manner, metal contacts such as metal contact  112 , are used to form electrical connections between interconnection levels and devices formed in or on substrate or well  108 . The metal vias  110  and contacts  112 , hereinafter being collectively referred to as “vias” or “plugs”, are generally filled with tungsten  114  and generally employ an adhesion layer  116  such as titanium nitride (TiN). Adhesion layer  116  acts as an adhesion layer for the tungsten metal layer  114  which is known to adhere poorly to SiO 2 . At the contact level, the adhesion layer also acts as a diffusion barrier to prevent a reaction between tungsten and silicon of the substrate or well  108 . 
   In one process for filling vias which has presently gained wide interest, metallized vias or contacts are formed by a blanket tungsten deposition and a chemical mechanical polish (CMP) process. In a typical process, illustrated in  FIGS. 2–4 , via holes, such as via hole  202 , are etched through an ILD  204  to interconnection lines or substrate  206  formed below. Next, thin adhesion layer  308 , such as TiN, is generally formed over ILD  204  and into via hole  202 , as shown in  FIG. 3 . Next, a conformal tungsten film  310  is blanket deposited over adhesion layer  308  and into via  202 . The deposition is continued until via hole  202  is completely filled with tungsten. Next, the metal films formed on the top surface of ILD  204  are removed by chemical mechanical polishing, thereby forming metal vias or plugs  110  shown in  FIGS. 1 and 4 . 
     FIG. 4  is a side view schematic cross-section illustrating a via after chemical mechanical polish removal of the excess tungsten. All excess tungsten  310  and adhesion layer  308  have been removed and the via  110  is flush with ILD layer  204 . Via  110  fills the gap between layers in ILD  204 . Via  110  may contact the top of an interconnection line or a substrate  206 . 
   In a typical chemical mechanical polishing process, a substrate or wafer is placed face-down on a polishing pad which is fixedly attached to a rotatable table. In this way, the thin film to be polished is placed in direct contact with the polishing pad. A carrier or chuck is used to apply a downward pressure against the backside of the substrate or wafer. During the polishing process, polishing pad, and the table on which the polishing pad is mounted, are rotated. The substrate is also rotated by a motor coupled to carrier. An abrasive and chemically reactive solution, commonly referred to as a “slurry”, is deposited onto the polishing pad during polishing. The slurry initiates the polishing process by chemically reacting with the film being polished. The polishing process is facilitated by the rotational movement of the polishing pad relative to the wafer, and rotation of the wafer on the polishing pad, as slurry is provided to the wafer/pad interface. Polishing is continued in this manner until all of the film on the wafer is removed. 
   Slurry composition is an important factor in providing a manufacturable chemical mechanical polishing process. Several different tungsten slurries have been described in literature. One slurry available is Commercial Tungsten Slurry: Semi Sperse W2000 available from Cabot Corporation/Microelectronics Materials Division of Aurora Ill. It has been found that slurries support a chemical reaction of the material being polished in addition to assisting with the physical removal of the material from a substrate by physical means. Many slurries contain an abrasive such as silica SiO 2  or alumina Al 2 O 3  to remove oxidized material from a substrate. 
   When tungsten is placed in water there is a spontaneous reaction generating an oxidation product. The tungsten and water react to form tungsten oxide, hydrogen ions and free electrons. The reaction may be described as:
 
W+3H 2 O⇄WO 3 +6H + +6 e   − E 0 =0.19 v  ((1))
 
The oxidation potential, E 0 , of equation (1), which is 0.19 v, indicates that this is a spontaneous reaction. However, while this reaction occurs spontaneously, it is not very fast. The generation of the hydrogen ions indicates, however, that this is an acidic reaction. To enhance the reaction rate of the oxidation of the tungsten, hydrogen peroxide is added to the CMP chemical environment. The hydrogen peroxide acts as an oxidizing agent providing a capability to accept electrons. This reaction may be described by the equation:
 
H 2 O 2 +2H + +2 e   − ⇄2H 2 O E 0 =1.77 v  ((2))
 
The balanced reduction oxidation (“redox”) equation is shown below:
 
W+3H 2 O 2 +3H 2 O+6H + ⇄WO 3 +6H + +6H 2 O E 0 =1.96 v  ((3))
 
Removing common terms, which will still somewhat accurately describe the reaction, reduces the equation to:
 
W+3H 2 O 2 ⇄WO 3 +3H 2 O E 0 =1.96 v  ((4))
 
The reaction described in equation (4) has a much higher reaction rate than the reaction described in equation (1).
 
   The oxidation product of equation (4), WO 3 , is unfortunately not soluble to a sufficient degree to allow removal of the oxidation product so fresh metal may be oxidized. To accelerate removal of the reaction product, an abrasive is added to the slurry. The abrasive may be silica or alumina particles. The abrasive particles physically remove the oxidation product from the metal layer clearing the way for additional oxidation of the metal to be removed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
       FIG. 1  is a schematic cross-sectional illustration of a typical multilevel interconnection structure; 
       FIG. 2  is a schematic cross-sectional side view of a via hole through an interlayer dielectric; 
       FIG. 3  is a schematic cross-sectional side view of an adhesion layer and via material on an interlayer dielectric; 
       FIG. 4  is a schematic cross-sectional side view of a via after chemical mechanical polish removal of the excess tungsten; 
       FIG. 5  is a schematic side view illustration of a typical chemical mechanical polishing process; 
       FIG. 6  is an illustration of one embodiment of endpoint comparisons; and 
       FIG. 7  is a flow chart demonstrating one embodiment of the claims. 
   

   DETAILED DESCRIPTION 
   Reference will now be made to drawings wherein like structures will be provided with like reference designations. In order to show the structures of the claims most clearly, the drawings included herein are diagrammatic representations of integrated circuit structures. Thus, the actual appearance of the fabricated structures, for example, in a photomicrograph, may appear different while still incorporating the essential structures of the claims. Moreover, the drawings only show the structures necessary to understand the claims. Additional structures known in the art have not been included to maintain the clarity of the drawings. 
   In a typical chemical mechanical polishing process, as shown in  FIG. 5 , the substrate or wafer  500  is placed face-down on a polishing pad  512  which is fixedly attached to a rotatable table  514 . In this way, the thin film to be polished (i.e., tungsten film  310  shown in  FIG. 3 ) is placed in direct contact with pad  512 . A carrier  516  is used to apply a downward pressure F 1  against the backside of substrate  500 . During the polishing process, pad  512  and table  514  are rotated. Substrate  500  is also rotated by a motor  520  coupled to carrier  516 . An abrasive and chemically reactive solution, commonly referred to as “slurry”  522 , is deposited onto pad  512  during polishing. The slurry initiates the polishing process by chemically reacting with the film being polished. The polishing process is facilitated by the rotational movement of pad  512  relative to wafer  500 , and rotation of wafer  500  on pad  512 , as slurry is provided to the wafer/pad interface. Polishing is continued in this manner until all of the film  310  and  308  on insulator  204  is removed 
   In one embodiment, shown in  FIG. 5 , motor  520  is attached to carrier  516  and rotates the carrier relative to polishing pad  512 . Meter  521  attached to motor  520  may record the current required to rotate wafer  300 . The amount of current required to rotate wafer  500  on pad  512  is a function of among other things the coefficient of friction between the surface being polished and polishing pad  512 . Where the coefficient of friction between the interlayer dielectric and polishing pad  512  is greater than that between the metal and polishing pad  512 , a rise in the current required to rotate wafer  500  will be apparent when blanket tungsten  310  has been removed and the majority of the surface being polished is ILD  204 . 
   In one embodiment, motor  520  may be coupled to carrier  516  through a direct drive shaft. In another embodiment, motor  520  may be coupled to carrier  516  through gear wheels and a chain. Carrier  516  may, in one embodiment, grasp the sides of wafer  500  to hold it to polishing pad  512 . In another embodiment, carrier  516  may adhere to a backside of wafer  500  by an adhesive, for example a wax. Slurry  522  may dribble down on to the center of polishing pad  512  and spiral out towards the edge of the pad as pad and table  514  rotate. In another embodiment, slurry  522  may saturate up through polishing pad  512  from a slurry source within rotatable table  514 . Depending on the embodiment, motor current meter  521  may be an analog current meter, digital current meter, chart strip recorder or a computer readable medium. 
   The rise in the motor current may be related to an endpoint indicating the CMP of blanket tungsten layer  310  is complete. The period between when polishing pad  512  first contacts ILD  204  and when the last of sacrificial blanket tungsten  310  and blanket adhesion layer  308  have been removed is called the transition period or clearing time. 
   In one embodiment, in a well-calibrated system, polishing is performed until the middle of the transition period has been reached plus an additional time period, for example, 30 seconds. The characteristics of this transition period or clearing time are dependent on the uniformity of the polish process. Where the polishing process has been performed uniformly across a wafer, there will appear a noticeable change in the current required to rotate substrate  500 . Where however, polishing has not been uniform, the transition in the current required to rotate wafer  500  may involve a smaller change in current that is more gradual and harder to detect. 
   Equation (4) is descriptive of the oxidation reaction taking place at the interface between blanket tungsten layer  310  and polishing pad  512 . However it is believed that equation (3) highlights the roots of a failure mode for CMP of blanket tungsten layer  310  by polishing pad  512 . Equation (3) shows the generation and eventual consumption of large quantities of hydrogen ions. It is believed these hydrogen ions create localized changes in the pH of the slurry solution at the interface of blanket tungsten layer  310  and polishing pad  512 . These hydrogen ions form localized highly acidic regions that cause nonuniformities in the polishing rate of the CMP. The effect of these localized acidic regions is especially critical near and during the transition region from polishing away blanket tungsten layer  310  and polishing ILD  204 . 
   It is believed that the localized acidic regions in the proposed CMP may be reduced if not eliminated by buffering the slurry. A buffer, in this context, is a compound that may absorb or release hydrogen ions without large changes in pH of the compound to which they have been added. Buffers are generally weak acid/salt pairs. In one embodiment, organic acids may act as a buffer. The functional group of an organic acid is the carboxyl group comprising a carbon-oxygen double bond and a hydroxyl group along with the positive (H + ) ion in aqueous solutions. 
   A good example of an organic acid is citric acid. Citric acid has three carboxyl groups making citric acid capable of providing three positive ions. Citrate ion −3  then is capable of absorbing three positive ions. In one embodiment, the buffer used for CMP of this tungsten system may be potassium citrate. The soluble salt, (potassium citrate) provides citrate ions as previously stated, available to buffer the hydrogen ions generated by the oxidation of tungsten. The citric acid thus formed dissociates to a degree of about 8 to 10 percent in water. This low disassociation degree allows citrate to absorb hydrogen ions without influencing the pH of the chemical system much. In another embodiment, acetic acid and potassium acetate, or ascorbic acid and potassium ascorbate, may be used as the buffer. 
   By absorbing the hydrogen ions generated in the oxidation of the tungsten metal in the CMP process, potassium citrate removes the localized acidic regions that cause non-uniformity&#39;s in the polishing of blanket tungsten layer  310 . Controlling the uniformity of the pH, and therefore the polish of the metal system, contributes to a quick and sharp transition region or clearing time in the CMP of Tungsten. 
   A CMP that is sufficiently non-uniform, for example due to localized acidic regions in the slurry, may reduce the change in the motor current to such an extent and prolong the clearing time so long as to allow indicia of the transition to hide in the background noise of the current meter. In such an embodiment, the endpoint is not readily ascertainable, and endpoints may be missed or false positives may be encountered.  FIG. 6  illustrates this principle. 
   The curves in  FIG. 6  are idealized representations of the motor current recorded by current meter  521 . Current meter  521  records the current required by motor  520  to drive carrier  516  which rotates substrate  500  on polishing pad  512 . The curves have had random noise and drift in signal removed to highlight the benefits of the techniques described herein. 
   Curve  640  in  FIG. 6  represents the current required by motor  520  to rotate wafer  500  in an embodiment without a buffer in slurry  522 . Region  610  on curve  640  is representative of the current required by motor  520  while the CMP process is polishing blanket tungsten layer  310 . Region  610  begins at some arbitrary point in the CMP of blanket tungsten layer  310 , and ends as motor current begins to climb because polishing pad  512  begins to encounter ILD  204 . Region  620  represents a transition region from when polishing pad  512  first begins to contact ILD  204  until all of the blanket tungsten  310  has been removed, and the only tungsten metal in contact with polishing pad  512  is the tungsten on the top of vias  110 . Region  630  represents the current required by motor  520  while polishing wafer  500  after blanket tungsten layer  310  has been removed, and the majority of the material being polished is ILD  204 . Current region  630  represents a greater current demand than region  610 , because the coefficient of friction between polish pad  512  and ILD  204  is greater than the coefficient of friction between polishing pad  512  and blanket tungsten layer  310 . This difference in coefficient of friction is the reason more current is required by motor  520  to rotate wafer  500  after all blanket tungsten  204  has been removed. 
   The relative difference in motor current required between region  630  and region  610  in  FIG. 6  may in one embodiment be measured as about a 9 millivolt signal. In addition, a non-uniform surface may extend the time period over which the transition from polishing blanket tungsten layer  310  to polishing ILD  204  takes place. The combination of the height of the motor current change and the extended clearing time may be such that in many embodiments the transition may be lost in the random or white noise of the recording system. 
   Curve  645  in  FIG. 6  represents the current required by motor  520  to rotate wafer  500  with a buffer in slurry  522 . Region  610  on curve  645  is representative of the current required by motor  520  to rotate wafer  500  while the CMP process is polishing blanket tungsten layer  310 . Region  610  of curve  645  has been normalized for easier comparison to curve  640 . Region  625  represents a transition region from when polishing pad  512  first begins to contact ILD  204  until all of the blanket tungsten  310  has been removed, and the only tungsten metal in contact with polishing pad  512  is the tungsten on the top of vias  110 . Region  635  represents the current required by motor  520  while polishing wafer  500  after blanket tungsten layer  310  has been removed, and the majority of the material being polished is ILD  204 . 
   Transition region  625  of curve  645  has a steeper slope and larger differential signal than transition region  620 . The differential signal of curve  645  is sometimes three times, and at least twice, the size of the differential signal of curve  640 . The combination of steeper slope and greater differential signal make transition region  625  easier to detect than region  620 . The buffer placed in slurry  522  has improved the uniformity of the polishing process and increased the endpoint signal of the polishing process. The increased endpoint signal increases the reliability of the polish endpoint process from about 95 percent to greater than about 99.9 percent. This increase in endpoint signal reliability changes the failure rate from one in about 20 wafers to about one in 100,000 wafers. This later figure is acceptable for manufacturing. 
     FIG. 7  is a flow chart showing one method of chemical mechanical polishing. Excess via material from wafer containing an integrated circuit is removed by the chemical mechanical polish technique. The technique uses a slurry and an oxidizing agent between the via metal and the polishing pad as at block  710 . The current required to rotate the wafer on the surface is monitored as a way of measuring the endpoint of the CMP of the excess via material as shown in block  720 . The endpoint signal monitored to determine the endpoint of the CMP is optimized by the addition of a buffer to the slurry as shown in block  730 . 
   In the preceding detailed description, the invention is described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.