KEYWORDS: Semiconducting wafers, Temperature metrology, Process control, Chemical elements, Lithography, Wafer testing, Robotics, Sensors, Data communications, Critical dimension metrology
To run the various steps of the process, multiple robot arm transfers within the Hot and Cold Plate modules which directly influence the critical dimension of the production wafers were performed on the lithography track. Wafer positioning inside these modules was found to be one of the key parameters to obtain the best critical dimensional uniformity across the wafer. With the currently realized track monitoring and conventional Statistical Process Control (SPC), potential process drifts or errors within these modules can only be detected from wafers measured during the post process control of product parameters. To catch all potential non-conformal production wafers directly at the tool, minimize equipment downtime and identify the root cause of maintenance issues, the real-time control of tool and process parameters is required. This paper presents the results of the evaluation of an Advanced Process Control (APC) solution used to detect in real-time mode any wafer positioning issues within the Hot and Cold Plate modules of a lithography track based on the monitoring of the plate temperature profile during wafer processing. After an explanation of the methodology used to collect the data from the tool, an initial phase of analysis of the temperature profile of the different Hot Plate modules was carried out. The monitoring of the temperature range was identified as the key parameter for the detection of wafer positioning issues where the temperature profile depends on the number of resistive heating elements, temperature settings and process conditions of the Hot Plate. The wafer tilt was simulated to compare the temperature profile to standard process conditions and in turn determine the detection capability. For the Cold Plate module, it was necessary to know the time between the end of the hot step and the start of the following cold step in order to detect a real tilt issue.
As device dimensions shrink the number of parameters influencing CD increases (PEB dispersion, development uniformity, resist thickness, BARC thickness, +/- scan focus control, scanner focus control at edge of the wafer...). Separation between all these contributors is not easy using only CD-SEM measurement, and particularly with isolated lines. For high volume manufacturing (where "time is money") and in the case of litho cluster drift, a quick and accurate diagnostic capability is an advantage for minimizing tool unavailability. An important attribute of this diagnostic capability is that its implementation is on standard production wafers. The use of production wafers enables continuous monitoring and also allows a direct correlation between monitoring measurements and the impact on product.
The technology that enables this type of diagnostic capability makes use of a compact dual tone line-end-shortening based target. A key benefit to this technology is that it provides a separation of the dose and focus parameters, which leads to quicker route cause determination.
After building a calibration model and determining minimum dose and focus sensitivity, both short term and long term stability of the model is investigated. The impact of wafer topology on model prediction is also investigated in order to assess on-product monitoring capability. The main error contributors are then identified for both track and scanner and the impact on CD control is evaluated. These cluster error contributors are then varied, first separately, and then combined. Measurement results are compared to the input parameters in order to determine error detection ability, measurement accuracy and separation capability.
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