Patent Grant
7153182
The present invention relates to the field of semiconductor wafer processing. More specifically, this invention relates to determining or maintaining, in situ, a desired smoothness of a polishing pad used to planarize semiconductor wafers.
Semiconductor wafers are typically fabricated with multiple copies of a desired integrated circuit design that will later be separated and made into individual chips. A common technique for forming the circuitry on a semiconductor is photolithography. Part of the photolithography process requires that a special camera focus on the wafer to project an image of the circuit on the wafer. The ability of the camera to focus on the surface of the wafer is often adversely affected by inconsistencies or unevenness in the wafer surface. This sensitivity is accentuated with the current drive toward smaller, more highly integrated circuit designs. Semiconductor wafers are also commonly constructed in layers, where a portion of a circuit is created on a first level and conductive vias are made to connect up to the next level of the circuit. After each layer of the circuit is etched on the wafer, an oxide layer is put down allowing the vias to pass through but covering the rest of the previous circuit level. Each layer of the circuit can create or add unevenness to the wafer that is preferably smoothed out before generating the next circuit layer.
One of the methods for achieving planarization of the surface is chemical mechanical polishing (CMP). CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer, in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to planarize the exposed surface of the wafer (typically, a layer formed on the wafer), when the wafer is pressed onto the pad. Available CMP systems, commonly called wafer polishers, often use a rotating wafer holder that brings the wafer into contact with a rotary polishing pad moving in the plane of the wafer surface to be planarized. The polishing fluid, such as a chemical polishing agent or slurry containing microabrasives, is applied to the polishing pad to polish the wafer. The wafer holder then presses and rotates the wafer against the rotating polishing pad to polish and planarize the wafer.
Another system used for performing CMP to obtain an effective polishing rate involves linear planarization technology. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the planarization uniformity is improved over CMP tools using rotating pads, partly due to the elimination of unequal radial velocities. One example of such a linear polisher is described in U.S. Pat. No. 5,692,947. Unlike the hardened table top of a rotating polisher, linear planarizing tools use linearly moving belts that are integrated with polishing pad material or upon which the pad is disposed. The ability for the belt to flex can cause a change in the pad pressure being exerted on the wafer. When the pressure of the wafer-pad engagement can be controlled, it provides a mechanism for adjusting the planarization rate and/or the polishing profile across the surface of the wafer. A support, such as a fluid platen, can be placed under the belt for use in adjusting the pad pressure being exerted on the wafer. An example of a fluid platen is disclosed in U.S. Pat. No. 5,558,568.
When CMP is employed, it is generally advantageous to monitor the effects of the planarizing process to determine if the process is being performed according to desired specifications. One significant challenge in CMP processing is the ability to process each wafer of a particular type in the same way as all other wafers of that type. In other words, it is a goal of CMP to characterize and maintain a polishing environment for each wafer so that there is substantially no variation in planarization characteristics from one wafer to the next.
In CMP there are several methodologies for determining in situ removal rate and, in some cases, in situ uniformity. There are difficulties, however, in measuring pattern wafer metrics, such as dishing or erosion, in situ. These process performance metrics are generally dependent on the consumables used in the CMP process and their characteristics. Accordingly there is a need for an improved method and system for determining CMP pattern wafer performance.
In order to address the need described above, a method and system for in situ characterization and maintenance of polishing pad smoothness is described below. The system includes at least one feedback line carrying in situ linear polisher performance data from the linear polisher and a controller in communication with the at least one feedback line and operative to determine a pad smoothness of the polishing pad based on the performance data on the at least one feedback line.
In different embodiments, the linear polisher may include a belt movably mounted on at least one roller, wherein a roller motor rotatably drives the at least one roller. A platen is disposed underneath the belt that is configured to dispense a fluid bearing between the platen and the belt, where the belt includes a polishing pad positioned on a side of the belt facing away from the platen. A flow meter may be used to monitor a flow rate of fluid to the platen and a sensor in communication with the fluid bearing may measure a pressure of the fluid while a material is polished. The controller may be in communication with one or more pressure sensors adjacent to the fluid bearing, the flow meter and the roller motor and configured to determine the polishing pad smoothness from pad smoothness information, where the pad smoothness information includes at least one of roller motor current sensed at the roller motor, fluid flow rate sensed at the flow meter or pressure sensed at the fluid bearing.
According to another aspect of the invention, a method is disclosed for maintaining a pad smoothness of a polishing pad in a linear polisher, the method includes first determining a target operating range of at least one parameter of the linear polisher having a first polishing pad of a particular pad type, a first pad conditioner of a particular pad conditioner type and used with a first wafer of a particular wafer type, where the target operating range of at least one of the parameters corresponds to a desired polishing pad smoothness. The one or more parameters are then monitored in situ while polishing a second wafer of the particular wafer type on the linear polisher. The first polishing pad is then conditioned with the first conditioner if the monitored parameters are within the target operating range so that the desired polishing pad smoothness is maintained and the dishing on the wafer, associated with the pad smoothness, is kept within desired limits.
In various embodiments, the parameters monitored and responded to may include one or more of fluid flow to a fluid platen, pressure at the fluid platen or motor current to a roller motor of the linear polisher. Additionally, the same target operating range, once determined for a particular pad type, a particular pad conditioner type and a particular wafer type may be used to maintain pad smoothness, and thus dishing performance, for any replacement polishing pad, polishing pad conditioner or wafer of the same type. Other features and advantages of the invention will become apparent to those of ordinary skill in the art upon review of the following drawings, detailed description and claims.
A method and apparatus for characterizing consumables used in chemical mechanical polishing (CMP), and monitoring and maintaining polishing pad performance in is described herein. As used herein, consumables refer to polishing pads, pad conditioners and other materials that are designed to be used up or worn out during the polishing process. In the following description, numerous specific details are set forth, such as specific structures, materials, tools, polishing techniques, and so on, in order to provide a thorough understanding of the present invention.
Referring to
The linear polisher 10 of
The wafer 12 is detachably held by a wafer carrier 24, which is part of a polishing head. The wafer 12 is held in position by a mechanical retaining mechanism, such as a retainer ring, and/or by the use of vacuum. Generally, the wafer 12 is rotated while the belt/pad assembly moves in a linear direction to polish a layer on the wafer 12. A downforce is exerted on the wafer carrier 24 in order to engage the wafer onto the pad with some predetermined force. The linear polisher 10 also dispenses a slurry 26 onto the pad 22. A pad conditioner 28 is typically used in order to recondition the pad surface during use. Techniques for reconditioning the pad 22 are known in the art and often involve scratching the pad with an abrasive-coated puck in order to remove the residue build-up caused by the used slurry and waste material generated in the CMP process. In other embodiments, such as with pads having a fixed abrasive, a non-abrasive conditioner may be used.
A support is disposed on the underside of the belt 14 and opposite from the wafer 12, so that the belt/pad assembly resides between the support and wafer 12. In one embodiment, the support may be a fluid platen 30 that generates a fluid bearing. Alternatively, the support may be a solid platform or may have mechanical bearings or rollers. A primary purpose of fluid platen 30 is to provide a supporting platform on the underside of the belt 14 to ensure that the pad 22 makes sufficient contact with wafer 12 for uniform polishing. When the belt 14 is depressed as the wafer is pressed downward onto the pad 22, the fluid platen 30 provides a counteracting support to this downward force.
In one embodiment, where the support is a fluid platen 30, the fluid flow from the fluid platen 30 can be used to control forces exerted onto the underside of the belt 14 and to reduce friction between the belt and the fluid platen. The fluid is generally air or liquid, although a neutral gas (such as nitrogen) can be used. Using fluid flow control, pressure variations exerted by the pad on the wafer can be adjusted to provide a more uniform polishing profile across the face of the wafer 12. Examples of fluid platens for generating fluid bearings in CMP processes are disclosed in U.S. Pat. Nos. 5,558,568 and 5,916,012, and the entirety of the disclosures of these patents are incorporated herein by reference.
As shown in
A cover plate (or insert) 40, also shown in
In
The leading edge sensor shown in
In one embodiment, the sensors 48a, 48b are pressure sensors positioned to measure a pressure of the fluid between the belt and platen. During a polishing operation, the fluid platen disperses fluid and forms the fluid bearing. Because the belt 14 is within close proximity of the bearing surface, the area between the fluid platen and the underside of the belt 14 is also filled with the fluid bearing. The fluid bearing provides both a counterforce to the wafer downforce and a low friction contact area to allow ease of belt movement. Adequate fluid flow ensures that this space is filled with fluid, so that pressure sensors 48a, 48b will measure the pressure of the dispersed fluid.
As illustrated in
Variations in the force exerted at a particular location during polishing will cause an increase (or decrease) in the pressure being exerted onto the fluid at that location. If base parameters, such as downforce of the wafer, fluid pressure of the fluid from the fluid bearing and pad velocity remain constant, the fluid pressure will typically remain somewhat constant as well. However, if certain polishing parameters are changed, then forces acting on the pad-wafer interface can cause a pressure difference that will be sensed by the pressure sensors 48a, 48b. Concurrently, a change of polishing parameters will often lead to a change in fluid flow, as measured at the mass flow meter 32, to the fluid platen 30. Also, the roller motor current will vary due to changes in the load on the motor 52 resulting from the changing polishing conditions. Pressure, flow rate and motor current can be used to track process parameters such as pad smoothness and dishing performance. Each of these three parameters can be used on their own, or in combination, to track polisher performance.
This monitoring the fluid pressure can be utilized to identify certain process characteristics. One process characteristic that can be tracked using the absolute measured fluid pressure and comparing it to a previously determined desired baseline pressure is pad smoothness. Another process characteristic that can be tracked, by monitoring fluid pressure changes, is an end point condition. During polishing the pad/wafer interface generates a shear force that is counteracted by a gradient in the fluid bearing pressure within the bearing-belt gap. The pressure gradient is generally greatest at the leading edge region of the wafer, as illustrated in the example of
The shear force at the pad/wafer interface will vary depending on the material being polished and the smoothness of the polishing pad. Because there is a correlation between the smoothness of a polishing pad and shear force at the pad/wafer interface, and because of the correlation between wafer polishing performance and pad smoothness, monitoring the pressure provides a means to determine the pad smoothness. By adjusting the pad smoothness, as for example through pad conditioning, the polishing performance can be monitored in situ and adjusted in situ.
In an embodiment utilizing only pressure to determine pad smoothness, the two pressure sensors 48a, 48b are utilized. The pressure being monitored may be from the leading edge sensor 48a only. Thus, the present invention can be practiced utilizing only one sensor. Although the sensor may be located elsewhere, the preference is to have it at the leading edge. The second sensor 48b is utilized in the example of
In another embodiment, the system and process may monitor fluid flow to the fluid platen 30 to determine in situ pad smoothness. In this embodiment, the controller 50 monitors information from the mass flow meter 32 while a patterned wafer or other items polished so that a flow rate is recorded. As with the pressure embodiment discussed above, the fluid flow embodiment is implemented by first establishing a baseline measurement to find the fluid flow that yields the desired dishing performance. When the process produces a wafer with the desired level of dishing, the flow information may be used both to prepare other polishing pad and pad conditioner sets of the same type for use with the same type of wafer and to maintain the desired pad smoothness during wafer processing.
In general, a parallel configuration of the wafer being polished to the platen will result in a steady state condition where a fixed fluid pressure being applied to the fluid bearing results in a uniform fluid flow. Deviation from this parallel configuration of wafer and platen will require the mass flow meter to increase fluid flow to maintain a pressure. In other words, in order to balance the forces of the downforce of the wafer against the polishing pad, the pressure provided by the fluid bearing, and the friction force of the wafer against the pad during polishing, and the pressure distribution is non-uniform.
In another embodiment, another measurable parameter that may be used to reduce polisher break-in time and dependence on numerous dummy wafers (such as copper slugs) and patterned wafers is motor current. Just as the fluid flow and pressure measurements can be correlated to pad smoothness, and thus dishing performance, measurements of electric motor current at the roller motor have also been found to correlate with pad smoothness/wafer dishing performance. In this embodiment, the roller motor current may be monitored and fed back to the controller 50 for use by the controller in maintaining the proper pad conditioning regimen to maintain the pad smoothness within the preferred operating range. The same type of calibration procedure discussed with respect to the pressure and fluid flow parameters may be used to determine the desired relationship of pad smoothness to motor current.
With reference to any of the pressure, flow, and motor current attributes discussed above, the present system and method takes advantage of one or more of these quantifiable measurements to help reduce costs and time for preparing a pad and a conditioner for optimum planarization performance. Using one or more of the pressure, fluid flow and motor current parameters, a baseline measurement is made on a test wafer, such as a patterned wafer to determine the values of the monitored parameters that give the target dishing performance. The controller is then given instructions to automatically adjust the conditioning parameters in the recipe in order to maintain the desired monitor parameter values. If the monitored parameters stray from the desired values, the controller will then manipulate the pad conditioner to achieve the desired pad smoothness. The controller may accomplish this through application of an algorithm that operates as a function of the monitored feedback parameter(s). The algorithm may cause the controller to automatically manipulate, in one embodiment, the pressure applied by the pad conditioner to the polishing pad. In another embodiment, the algorithm may cause the controller to automatically manipulate the total time the conditioner is applied to the polishing pad.
In other embodiments, specific combinations of two or more of the pressure, fluid flow and motor currents measurements may be combined to optimize the belt smoothness detection. For example, certain mathematical transformations of the three parameters are contemplated. In one embodiment, the parameters of pressure, flow and current are added together to provide a sum that used by the controller to determine changes in the pad conditioning regimen applied in this closed loop process. In other embodiments, it is contemplated that the reference parameter will be the pressure divided by the flow, and in yet other embodiments the motor current may be divided by the pressure multiplied by the flow. Again, the method may be adapted to use only one of the three parameters. Similarly, the system may be configured to only measure and feedback to the controller one of these three linear polisher criteria for use in controlling, in situ, the pad smoothness.
The system and method may be applied to both in situ characterization and control of pad smoothness for a particular set of consumables (i.e. polishing pad and pad conditioner) and to in situ characterization and control of any set of consumables of the same type as the initial set. In other words, once a pad and a conditioner have been characterized in a CMP process for a particular type of wafer, any replacement pad or pad conditioner of the same type (e.g. the same model polishing pad from the same manufacturer) may be introduced into the polisher. The previously determined baseline parameters should result in the same pad smoothness control for the replacement pad and/or pad conditioner. The baseline parameters corresponding to the desired performance level can also be transferred to other polishers of the same type (e.g. same model and manufacturer) as the polisher on which the baseline measurements were made. In this manner, costs savings may be realized through using fewer dummy and patterned wafers on new pad and pad conditioner sets of the same type.
As shown in
Although the embodiments discussed above relate to monitoring CMP processing in situ to obtain the characteristics of the wafer polisher parameters representative of a desired polishing performance, to maintaining the polisher parameters in this desired operating region, and to replicating these polishing parameters on different sets of consumables in polishers applying the same process to the same type of wafer or other material, the pressure, fluid flow and/or motor current parameters may also be used to assist in end-point detection. In order to provide for an end-point detection of an on-going process, the controller 50 of the polisher 10 may be configured to recognize a characteristic change in one or more, or a combination of, these parameters rather than the absolute value of the parameter. When one material is polished away during CMP to reveal an underlying material of different composition, thus indicating the end point of the polishing process, the shear forces change. The change in the shear forces causes a change in the linear polisher parameters. This change may be detected by the controller via the feedback information from the pressure sensor, the mass flow meter and/or the roller motor. Thus, a polishing end point can be detected by calibrating the controller to identify the appropriate change and then to monitor the desired parameter or parameters to identify the change on subsequent wafers. For end-point detection, a relative change in the monitored parameter is significant such that a differential in the parameter measurement may be sufficient to identify an end-point. In contrast, an absolute parameter reading, or range, is monitored for pad smoothness.
A system and method for monitoring the pressure, fluid flow and/or motor current to characterize, monitor and maintain a desired pad smoothness has been described. Although pressure, fluid flow, and electric motor current are specifically noted, it is contemplated that other types of linear polisher parameters may be adapted for measuring the shear force in situ. Additionally, although the embodiments above are described with reference to performing CMP on a semiconductor wafer, the invention can be readily adapted to polish other materials as well, such as glass, metal substrates or other semiconductor substrates, including substrates for use in manufacturing flat panel displays.
An advantage of the system and method discussed herein is the potential for reduction of costs in characterizing a polishing process. In order to initially characterize a polishing process, a patterned wafer and/or wafer blanks may be used on a polisher and polished until a desired result is achieved. Once the desired result is achieved, the parameters of the polishing process, such as fluid flow rate, pressure, and motor current, are recorded so that one or more of the parameters may be monitored by the polisher. When one or more parameters drift away from the ideal parameters, the polisher can then automatically apply a conditioner to bring that parameter into the desired range. The initial parameter measurement on a patterned wafer will depend on the underlying material being used. Once baseline measurement is experimentally obtained, the best parameter values can be utilized in a manufacturing setting to monitor an on-going process to maintain pad smoothness. Additionally, the same parameters can be used on other polishers. Accordingly, in-situ pad smoothness can be characterized and maintained, thereby reducing dishing, through the use of pressure, fluid flow and motor current, alone or in various combinations. Furthermore, the system and method helps to maximize the lifetime of the polishing pad and conditioner while maintaining wafer polishing performance.
It is therefore intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention.
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