Patent Application
20050159084
The present invention relates to manufacture of semiconductor integrated circuits and, more particularly to a method for polishing of conductive layers for planarization and removal.
Chemical mechanical polishing (CMP) of materials has important and broad application in the semiconductor industry. CMP is a widely used technique for planarizing metals and dielectrics as well as other types of layers on semiconductor wafers. CMP is often used to flatten/polish the profiles that build up in multilevel metal interconnect fabrication schemes.
In a typical CMP process, a substrate such as a semiconductor wafer is mounted on a substrate carrier, often called a head. The wafer surface to be polished is pressed against a polishing pad surface and the pad and the head are moved with respect to each other. This is typically done by rotating the wafer, moving the pad or both. The polishing pad may be a conventional polishing pad or a fixed abrasive polishing pad. Conventional or polymeric polishing pads are usually employed along with polishing slurries including abrasive particles and chemically reactive agents. The surface of a fixed abrasive polishing pad typically includes abrasive particles embedded in a matrix material. During processing, imbedded abrasive particles perform polishing with the help of a polishing chemistry, which may or may not contain abrasive particles.
As the brief review above shows, a need exists for a chemical mechanical polishing (CMP) system, which can provide accurate, stable and controllable polishing rates on various parts of a wafer.
The present invention employs a flow assembly with a non-planar surface profile to apply fluid flow to a backside of a polishing pad to cause a polishing side of the polishing pad be forced against a workpiece surface during the chemical mechanical polishing of the workpiece. The fluid flow is applied to the polishing pad using a plurality of fluid zones placed in the non-planar flow assembly surface. The fluid flow zones may be arranged into any configuration or array in the flow assembly surface such as concentric or linear. Spaces or regions provided in between the zones may be used to substantially isolate the zones from the neighboring zones and may establish ventilation regions or drains for the fluid leaving the individual zones of the flow assembly.
The flow assembly surface may have any profile or topography, such as a raised profile or a recessed profile, which vary the gap between the flow assembly surface and the pad or the workpiece surface at selected locations. The gap between the backside of the polishing pad and the flow assembly surface of the present invention is defined as a variable gap. The varying profile of the surface of the flow assembly and the resulting variable gap between the backside of the pad and the surface of flow assembly provide a well-defined fluid distribution and pressure profile for each zone. Such well-defined fluid distribution and pressure profiles establish well defined polishing rates on the workpiece surface as the polishing pad polishes the workpiece surface.
In one embodiment of the present invention, material removal rate from the workpiece surface can be controlled by actively varying the flow assembly surface profile or topography by moving the fluid flow zones with respect to the pad or the workpiece surface, which adjusts the variable gap. In another embodiment, the material removal rate can be controlled by using a flow assembly with a fixed surface profile or topography, which keeps a fixed variable gap which is shaped by the fixed non-planar flow assembly surface and the polishing pad.
Accordingly in one aspect of the present invention, an apparatus for polishing a surface of a workpiece includes a carrier configured to hold the workpiece, a showerhead, having a non-planar surface, providing a variable gap between the non-planar surface and the surface of the workpiece and a polishing pad with a polishing side and a back side positioned within the variable gap. The polishing pad is configured to polish the surface of the workpiece with the polishing side when a fluid flow is applied from the non-planar surface to the backside. The fluid flow is applied from a plurality of fluid flow zones formed in the non-planar surface and the fluid flow zones are configured to move to cause a change in the topography of the non-planar surface. A feed back circuit induces a change in the topography of the non-planar surface in response to a change in a removal profile to yield a pre-determined removal profile.
In another aspect of the present invention, a method of controlling material removal rate from a workpiece surface is provided. The method includes the steps of holding the workpiece with a carrier, placing the polishing pad into the variable gap provided between a non-planar surface of a showerhead and the workpiece surface; emitting fluid from the non-planar surface of the showerhead onto backside of the pad to establish pressure; establishing relative motion between the pad and the workpiece surface and removing material from the workpiece surface with polishing side of the pad.
These and other features and advantages of the present invention will be described below with reference to the associated drawings.
CMP system of the present invention applies fluid flow from a flow assembly separated from a backside of a polishing pad with a variable gap to cause a polishing or processing side of the polishing pad be forced against a workpiece surface during the chemical mechanical polishing of the workpiece. The fluid flow may be applied to the polishing pad using a flow assembly that has a plurality of fluid flow zones placed in a flow assembly surface. The fluid flow zones may be arranged into any configuration or array in the flow assembly surface such as concentric or linear. Spaces or regions provided in between the fluid flow zones may be used to substantially isolate the fluid flow zones from the neighboring zones and may establish ventilation regions or drains for the fluid leaving the individual zones of the flow assembly.
The flow assembly surface may have any profile or topography, such as a raised profile or a recessed profile, which vary the gap between the flow assembly surface and the pad or the workpiece surface at selected locations. Accordingly, when the polishing pad is placed over the flow assembly, the gap between one or more zones and the backside of the polishing pad may be smaller than the gap between the backside of the polishing pad and the rest of the zones. Therefore, the gap between the backside of the polishing pad and the flow assembly surface of the present invention is defined as a variable gap. The variable gap between the backside of the polishing pad and the fluid flow zone forming the highest point on the flow assembly surface may be nearly zero or more than zero. The varying profile of the surface of the flow assembly and the resulting variable gap between the backside of the pad and the surface of flow assembly provide a well-defined fluid distribution and pressure profile for each fluid flow zone. Such well-defined fluid distribution and pressure profiles, in turn, establish well defined polishing rates on the workpiece surface as the polishing pad polishes the workpiece surface.
In one embodiment of the present invention, material removal rate from the workpiece surface can be controlled by actively varying the flow assembly surface profile or topography by moving the fluid flow zones with respect to the pad or the workpiece surface, which adjusts the variable gap. In another embodiment, the material removal rate can be controlled by using a flow assembly with a fixed surface profile or topography, which keeps a fixed variable gap which is shaped by the fixed flow assembly surface and the polishing pad. Therefore, in this application, a gap between the flow assembly surface and the polishing pad is a variable gap which may be an adjustable or fixed variable gap. In either embodiment, in addition to variable gap feature, fluid flow rates at each zone may also be varied, in which case the process window within which removal rates are varied may be further widened.
In other words flow rate control and variable gap control become two important process knobs that can be varied independent from each other to control the profile of removal rate. After pushing the polishing pad toward the workpiece surface, the fluid from the zones exits the assembly, partially through the drain region if drains are used, thus reducing fluid flow effects on the neighboring zones. In this design, passages or drains between the zones for the used fluid may or may not be employed because the variable gap itself introduces differences between the pressures over the various zones with different gap values.
As exemplified with dotted line, zones z1 and z2, may each be moved with respect to the polishing pad 104 using a moving mechanism (not shown) to vary the gap between these zones and the pad or the front surface 106 of the wafer. As illustrated in
During the material removal process, a wafer carrier 110 retains the wafer 108, preferably at a fixed elevation so that only the distance between the back surface of the polishing pad and the fluid flow zones vary. The polishing pad 104 may be any of a fixed-abrasive polishing pad, or a more standard polymeric polishing pad. The polishing pad 104 includes a first surface or a process surface 112 and a second surface or a back surface 114. The polishing pad 104 may preferably be tensioned by a tensioning mechanism (not shown). Process surface 112 of the polishing pad 104 polishes the surface 110 of the wafer during the CMP process. Material removal from the front surface 106 may be performed using a polishing solution or slurry, which may or may not contain abrasive particles. The front surface 106 of the wafer 108 may include a conductive layer such as copper or a dielectric layer that the material removal process of the present invention is applied. The polishing pad 104 may be moved linearly, preferably bi-linearly, using a moving mechanism (not shown). Alternately, the polishing pad may be round and may be rotated like in standard rotary CMP tools. Exemplary CMP systems using bi-linear motion to polish surfaces are exemplified in the following patents. U.S. Pat. No. 6,103,628 entitled Reverse Linear Polisher with Loadable Housing, U.S. Pat. No. 6,464,571 Polishing Apparatus and Method with Belt Drive System Adapted to Extend the Lifetime of a Refreshing Polishing Belt Provided Therein, and U.S. Pat. No. 6,468,139 Chemical Mechanical Polishing Apparatus and Method with Loadable Housing, which are owned by the assignee of the present invention.
During the CMP process of the present invention, an airflow through the showerhead 102 is applied onto the back surface 114 of the polishing pad 104. Application of the airflow to the polishing pad 104 may be performed using a plurality of fluid openings 116 formed through the fluid flow zones z1, z2, z3 and z4. The fluid openings 116 may be arranged into any configuration or array with ventilation openings 118 among them. The ventilation openings 118 vent out the air used to push the pad against the surface of the workpiece, and optionally the ventilation openings may be connected to a vacuum system (not shown) for more efficient ventilation. The fluid openings 116 are formed through the fluid flow zones z1, z2, z3 and z4 to create a fluid flow distribution profile on the showerhead 102. As shown in
In accordance with the principles of the present invention, by varying the distance between the individual fluid flow zones z1-z4 and the polishing pad 104, the profile of material removal rate on corresponding areas of the front surface 106 of the wafer is effectively controlled and sharper removal profiles are obtained.
As mentioned above, airflow towards the back surface 114 of the polishing pad 104 pushes the pad against the front surface 106 of the wafer 108 that is held and rotated by the wafer carrier 110. Accordingly, in this center high configuration, air from the zone z1applies more force per unit area to the polishing pad and the corresponding polished region on the front surface 106 of the wafer 108 when the first zone z1 is elevated and placed at a first elevated position at proximity of the back surface 114 of the polishing pad 104. At the first elevated position, the gap between the back of the polishing pad and top surface of the first fluid flow zone is smallest in comparison to the other fluid flow zones of the shower head 102. The gap between the backside of the polishing pad and top surface of the first fluid flow zone or the highest point on the showerhead 102 may be nearly zero or microscopic. At this position, due to the small gap, the air from the first fluid flow zone z1 is very effective and causes the most material removal from the front surface 106 of the wafer. Since the first fluid flow zone z1is across a center region of the wafer 108, highest material removal rate occurs at the center region of the front surface 106.
The second fluid flow zone z2 is placed in a second elevated position in which the second fluid flow zone z2 applies less force onto the polishing pad 104 than the force applied by the first fluid flow zone z1 at the first position. In the second elevated position, the gap between the surface of the second fluid flow zone is larger then the gap between the surface of the first fluid flow zone z1 and the back surface 114 of the polishing pad 104. The force applied by the air from the second fluid flow zone z2 causes less material removal from the corresponding location on the front surface 106, which surrounds the center region of the front surface 106, due to the larger gap. Similarly, the third and the fourth fluid flow zones z3 and z4 cause less material removal from an edge region of the front surface 106 due to their relatively distant third and fourth elevated positions to the back surface 114 of the polishing pad 104. The step height between the neighboring zones can be adjusted to obtain desired variations of the center high configuration of the showerhead 102 and the resulting material removal profiles.
An alternative center-low configuration of the shower head 102 can be seen in
The second fluid flow zone z2 is placed in a second declined position in which the second fluid flow zone z2 applies more force onto the polishing pad 104 than the force applied by the first fluid flow zone z1 at the first declined position. In the second declined position, the gap between the surface of the second fluid flow zone is smaller then the gap between the surface of the first fluid flow zone z1 and the back surface 114 of the polishing pad 104. The force applied by the air from the second zone z2 causes more material removal from the corresponding location on the front surface 106, which surrounds the center region of the front surface 106, due to the smaller gap. However, the third and the fourth fluid flow zones z3 and z4 cause the highest material removal from an edge region of the front surface 106 due to their smaller gap with the back surface 114 of the polishing pad 104 in the third and fourth elevated positions. The step height between the neighboring zones can be adjusted to obtain desired variations of the center low configuration of the showerhead 102 and the resulting material removal profiles. In this configuration, the step height range between the fluid flow zones z1-z2, z2-z3 and z3-z4 can be between the 0.1 to 10 mils, preferably 0.5 to 2 mils.
In the above embodiments, the position of the zones can be configured using smart systems that can monitor removal profile during the material removal process. Accordingly, by utilizing an electronic feedback mechanism or control system, gaps may be automatically adjusted to get the desired removal profile. Such profile may be changed in-situ during the process or before processing each wafer.
Although in the above embodiments flow assemblies are defined as round with concentric zones, a showerhead 600 may be elongated, for example shaped as a rectangle, as shown in
In the above showerhead embodiments, depending on the vertical position of the fluid flow zones, the gap established between the showerhead and the back surface of the polishing pad varies. As described above, non-planar top surface of the showerhead varies the gap between the top surface of the showerhead and polishing pad. For example, among many others, the gap can be large at the edges but smaller at the center of the showerheads or, alternatively, the gap can be smaller at the edges but large at the center at the showerheads. The gap may be nearly zero between the highest point on the showerhead and the backside of the polishing pad. As opposed to the prior art planar top surface platens, non-planar top surface character of the showerheads and resulting gap variations provide escape passages for the used air. This is not possible with the prior art systems. Therefore, in the above embodiments, the use of ventilation openings is optional and the showerheads can be manufactured without ventilation openings. Although the present invention is described using a CMP example, the above embodiments can be used to perform an electrochemical mechanical polishing (ECMP) process. In ECMP, during the material removal with a polishing pad, an electrical potential difference is applied between the conductive surface and a cathode electrode while an electropolishing solution wets both. The cathode may be the showerhead or a separate electrode.
Accordingly, the present invention provides substantially enhanced control for each zone. The present invention provides distinct fluid flow rate distribution profiles. Such well-defined and uniform fluid distribution, in turn, establishes well-defined polishing rates on the substrate as the polishing pad polishes the workpiece surface.
Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.
