The present invention relates to manufacturing processes, and more specifically to methods for providing run to run process control using a dynamic tuner.
Run to run process control has been widely developed and utilized in different manufacturing industries where the processing conditions of next run are adjusted based on prior run results. A simple exponentially weighted moving average (EWMA) tuner is often adopted in the run to run process control system to estimate the deviation of model parameters. The simple EWMA tuner can be expressed using the formula:
Pn+1=λ×Rn+(1−λ)×Pn
where:
The EWMA weighing factor, lambda, is usually carefully selected for a process in order to get adequate process tuning. One such approach involves determining an optimum/fixed lambda value based on process capability index values and is described in U.S. Pat. No. 7,809,459 to Morisawa et. al. The simple EWMA tuner is usually quite effective to bring a process with moderate drifts under control, but may not be able to tune the process quick enough to address severe process shifts. Using a large fixed lambda value with the EWMA tuner as might be used in the Morisawa system could accelerate process tuning, but it does not allow for adjustment of the lambda value on the fly and thus introduces a high risk of process over-tuning.
A predictor corrector control scheme, which can also be referred to as a double EWMA controller, is proposed in an article by S. Butler and J. Stefani entitled, “Supervisory Run-to-Run Control of Polysilicon Gate Etch Using In Situ Ellipsometry,” IEEE Trans. Semiconduct. Manufact., vol. 7, no. 2, pp. 193-201, 1994. In this predictor control scheme, a second EWMA tuner is used to compensate for the error incurred from the simple EWMA tuner, so extra tuning can be applied to the model control parameter when large process drifts occur. However, while this approach may be capable of adapting to small changes in process output, it is not well suited to adapting to high magnitude or short time interval type changes in process output.
Another approach that deals with both process drifts and process shifts at the same time has been proposed by R. Guo, A. Chen, and J. Chen, in a portion of a book entitled, “Chapter 19 An Enhanced EWMA Controller for Processes Subject to Random Disturbances,” Run-to-Run Control in Semiconductor Manufacturing, edited by J. Moyne, E. del Castillo, and A. M. Hurwitz, CRC Press LLC, 2001. In the enhanced EWMA controller proposed by Guo, a baseline lambda is utilized to compensate for smaller process drifts. Two EWMA control charts are used as the detection tools for large and medium process shifts. If there is an out-of-control signal on the charts, a dynamic tuning loop is triggered and the lambda value is reset to a higher value. The lambda value is then decreased gradually over the next few runs, and eventually back to the baseline lambda. However, while this approach may be capable of some dynamic tuning using the control charts to tune to particular shifts, it is not well suited for rapid fluctuations in process output as the approach does not tune particularly fast.
Accordingly, an improved run to run control system that can address these shortcomings is needed.
Aspects of the invention relate to methods for providing run to run process control using a dynamic tuner. In one embodiment, the invention relates to a method for providing run to run process control of a process control parameter using a dynamic tuner, the method including receiving a data point for a process output parameter, determining whether the data point is within a desired range for the process output parameter, setting, when the data point is within the desired range, a dynamic lambda value equal to a preselected base lambda value, setting, when the data point is not within the desired range, the dynamic lambda value equal to a value based on the preselected base lambda value, a degree of difference between the data point and a target for the process output parameter, and a scale factor, calculating an exponentially weighted moving average using the dynamic lambda value, and adjusting the process control parameter in accordance with the exponentially weighted moving average.
Referring now to the drawings, embodiments of processes for providing run to run process control of a process control parameter are illustrated. The processes apply an exponentially weighted moving average (EWMA) formula with a dynamic lambda to control a process parameter (e.g., process control parameter or process model parameter). When the process output parameter extends beyond a preselected/desired range, the dynamic lambda can have an aggressive value determined in part by a preselected scale factor and a degree to which the process output parameter extends beyond a target range. On the other hand, when the process output parameter is within the preselected/desired range, the dynamic lambda can have a less aggressive value (e.g., base lambda value) such as a preselected constant lambda value. The EWMA is then calculated using the dynamic lambda value and a process control parameter is adjusted in accordance with the calculated EWMA. The process can then repeat with a new data value, thereby enabling dynamic tuning.
The process then sets (106), when the data point is within the desired range, a dynamic lambda value equal to a preselected base lambda value. In several embodiments, when the data point is within the desired range, the process uses the base lambda value that corresponds to a less aggressive value than the value the process will use when the data point is beyond the desired range. The process then sets (108), when the data point is not within the desired range, the dynamic lambda value equal to a value based on the preselected base lambda value, a degree of difference between the data point and a target for the process output parameter, and a scale factor. The scale factor can be selected based on empirical data and a preselected degree of aggression that is desired for tuning the process control parameter to meet the target for the process output. In several embodiments, a higher scale factor corresponds to a higher degree of aggression. In several embodiments, the scale factor is selected to be greater than 0.
The target value of the process output is usually chosen based on process or device performance requirements. Different processes have different target values. After the target value is set for a process output, process control can be adjusted based on the target value, which is usually not changed unless the process or device performance requirements change.
The process then calculates (110) an exponentially weighted moving average (EWMA) using the dynamic lambda value. In several embodiments, the conventional formula for EWMA is used along with the dynamic lambda value to generate the EWMA output value. The process then adjusts (112) the process control parameter in accordance with the exponentially weighted moving average output value.
In one embodiment, the process control parameter can be a polishing rate, a deposition rate, an etch rate, an angle control for a material removal process, or a chemical mechanical planarization rate. In one embodiment, the process control parameter corresponds to a back pressure for a chemical mechanical planarization process. In other embodiments, the process control parameter can correspond to other control parameters of a manufacturing process. In several embodiments, the process control parameter can correspond to control parameters of a manufacturing process for magnetic transducers used in magnetic storage devices, including, for example sliders for hard disk drives.
In one embodiment, the process can perform the sequence of actions in a different order. In another embodiment, the process can skip one or more of the actions. In other embodiments, one or more of the actions are performed simultaneously. In some embodiments, additional actions can be performed.
In a number of embodiments, the scale factor (SF) is greater than 0. In some embodiments, the scale factor (SF) is greater than 1. In one embodiment, the scale factor (SF) is greater than 1 and less than 10. In other embodiments, other suitable values for the scale factor (SF) can be used. In several embodiments, a higher scale factor (SF) corresponds to a higher degree of aggression for tuning the process control parameter to meet the target of the process output. In one such embodiment, the scale factor (SF) is selected such that the dynamic lambda remains between 0 and 1. In another such embodiment, the scale factor (SF) is increased to compensate for a relatively low data point (e.g., maintain lambda between 0 and 1).
On the other hand, if the new process output parameter data (Yn) is outside of the desired range, then the process sets (314) the deciding factor (DF) equal to 1 and calculates (316) the dynamic lambda (λd) according to the formula equal to the base lambda (λb) plus a value equal to the scale factor (SF) multiplied by the absolute value of the difference between the new process output parameter data (Yn) and a preselected target value for the process output parameter (Ytarget). In such case, the process continues from block 316 and adjusts (312) the process model/control parameter in accordance with the EWMA formula (e.g., Pn+1=λd×Rn+(1−λd)×Pn) using the calculated dynamic lambda value. From block 312, the process uses (318) the process model/control parameter to determine the process conditions for the next run/iteration of the process. The process then executes (320) the next run/iteration of the process by returning to block 302 to obtain new process output parameter data and thereby restarts the process.
In one embodiment, the process can perform the sequence of actions in a different order. In another embodiment, the process can skip one or more of the actions. In other embodiments, one or more of the actions are performed simultaneously. In some embodiments, additional actions can be performed.
Note that the severe process shift occurred at about run number 6, where the key process output parameter became much higher than the target, which is 0. Under the simple EWMA tuner 404 with the fixed lambda value of 0.2, the key process output did not go back to the desired range until about run number 18. However, using the dynamic EWMA tuner 402, where the base lambda was set also to be 0.2 and the dynamic lambda could go up to 0.7, the key process output was tuned back into the desired range at about run number 10, which is much faster than the simple EWMA tuner 404. The dynamic lambda 408 from run number 6 to run number 9 is higher than the base lambda 406 since the key process output is out of the desired range. The dynamic lambda values 408 for those runs also changed on the fly, depending on how far the key process output was away from the target. Once the key process output gets into the desired range, the dynamic tuner 402 uses the base lambda 406 to continue tuning the process closer to the target.
As mentioned above, using a large lambda value in the simple EWMA tuner could accelerate process tuning, but it also introduces a high risk of process over-tuning.
As illustrated in
While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as examples of specific embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.
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