In addition to lithography process and equipment induced variations, processes like etching, annealing, film deposition and planarization exhibit variations, each having their own intrinsic characteristics and leaving an effect, a ‘fingerprint’, on the wafers. With ever tighter requirements for CD and overlay, controlling these process induced variations is both increasingly important and increasingly challenging in advanced integrated circuit (IC) manufacturing. For example, the on-product overlay (OPO) requirement for future nodes is approaching <3nm, requiring the allowable budget for process induced variance to become extremely small. Process variance control is seen as an bottleneck to further shrink which drives the need for more sophisticated process control strategies. In this context we developed a novel ‘computational process control strategy’ which provides the capability of proactive control of each individual wafer with aim to maximize the yield, without introducing a significant impact on metrology requirements, cycle time or productivity. The complexity of the wafer process is approached by characterizing the full wafer stack building a fingerprint library containing key patterning performance parameters like Overlay, Focus, etc. Historical wafer metrology is decomposed into dominant fingerprints using Principal Component Analysis. By associating observed fingerprints with their origin e.g. process steps, tools and variables, we can give an inline assessment of the strength and origin of the fingerprints on every wafer. Once the fingerprint library is established, a wafer specific fingerprint correction recipes can be determined based on its processing history. Data science techniques are used in real-time to ensure that the library is adaptive. To realize this concept, ASML TWINSCAN scanners play a vital role with their on-board full wafer detection and exposure correction capabilities. High density metrology data is created by the scanner for each wafer and on every layer during the lithography steps. This metrology data will be used to obtain the process fingerprints. Also, the per exposure and per wafer correction potential of the scanners will be utilized for improved patterning control. Additionally, the fingerprint library will provide early detection of excursions for inline root cause analysis and process optimization guidance.
Improving wafer On Product Overlay (OPO) is becoming a major challenge in lithography, especially for multipatterning techniques like N-repetitive Litho-Etch steps (LEN, N ≥ 2). When using different scanner settings and litho processes between inter-layer overlays, intra-field overlay control becomes more complicated. In addition to the Image Placement Error (IPE) contribution, the TWINSCANTM lens fingerprint in combination with the exposure settings is playing a significant role as well. Furthermore the scanner needs to deal with dynamic fingerprints caused by for instance lens and/or reticle heating.
This paper will demonstrate the complementary RegC® and TWINSCANTM solution for improving the OPO by cooptimizing the correction capabilities of the individual tools, respectively. As a consequence, the systematic intra-field fingerprints can be decreased along with the overlay (OVL) error at wafer level. Furthermore, the application could be utilized for extending some of the scanner actuators ranges by inducing a pre-determined signatures. These solutions perfectly fit into the ASML Litho InSight (LIS) product in which feedforward and feedback corrections based on YieldStar overlay and other measurements are used to improve the OPO. While the TWINSCANTM scanner corrects for global distortions (up to third order) - scanner Correctable Errors ( CE), the RegC® application can correct for the None Correctable Errors (NCE) by making the high frequency NCE into a CE with low frequency nature. The RegC® induces predictable deformation elements inside the quartz (Qz) material of the reticle, and by doing so it can induce a desired pre-defined signature into the reticle. The deformation introduced by the RegC® is optimized for the actual wafer print taking into account the scale and ortho compensation by the scanner, to correct for the systematic fingerprints and the wafer overlay. These two applications might be very powerful and could contribute to achieve a better OPO performance.
The understanding and control of the intra-field overlay budget becomes crucial particularly after the introduction of multi-patterning applications. The intra-field overlay budget is built-up out of many contributors, each with its own characteristic. Some of them are (semi-)static like the reticle writing error (RWE) fingerprint, the scanner lens fingerprint, or the intra-field processing signature. Others are more dynamic. Examples are reticle heating and lens heating due to the absorption of a small portion of the exposure light. Ideally, all overlay contributors that are understood and known could be taken out of the feed-back control loop and send as feed-forward corrections to the scanner. As a consequence, only non-correctable overlay residuals are measured on the wafer.
In the current work, we have studied the possibility to characterize the reticle writing error fingerprint by an off-line position measurement tool and use this information to send feed-forward corrections to the ASML TWINSCANTM exposure tool. The current work is an extension of the work we published earlier. To this end, we have selected a reticle pair out of 50 production reticles that are used to manufacture a 28-nm technology device. These two reticles are special in the sense that the delta fingerprint contains a significant higher order RWE signature. While previously only the linear parameters were sent as feed-forward corrections to the ASML TWINSCANTM exposure tool, this time we additionally demonstrate the capability to correct for the non-linear terms as well. Since the concept heavily relies on the quality of the off-line mask registration measurements, a state-of-the-art reticle registration tool was chosen. Special care was taken to eliminate any effects of the tool induced shifts that may affect the quality of the measurements. The on-wafer overlay verification measurements were performed on an ASML YieldStar metrology tool as well as on a different vendor tool.
In conclusion, we have extended and proven the concept of using off-line reticle registration measurements to enable higher order feed-forward corrections the ASML TWINSCANTM scanner. This capability has been verified by on-wafer overlay measurements. It is demonstrated that the RWE contribution in the overlay budget can be taken out of the feedback control loop and sent as feed-forward corrections instead. This concept can easily be extended when more scanner corrections become available.
The on product overlay specification and Advanced Process Control (APC) is getting extremely challenging particularly after the introduction of multi-patterning applications like Spacer Assisted Double Patterning (SADP) and multipatterning techniques like N-repetitive Litho-Etch steps (LEN, N ≥ 2). When the latter is considered, most of the intrafield overlay contributors drop out of the overlay budget. This is a direct consequence of the fact that the scanner settings (like dose, illumination settings, etc.) as well as the subsequent processing steps can be made very similar for two consecutive Litho-Etch layers. The major overlay contributor that may require additional attention is the Image Placement Error (IPE). When the inter-layer overlay is considered, controlling the intra-field overlay contribution gets more complicated. In addition to the IPE contribution, the TWINSCANTM lens fingerprint in combination with the exposure settings is going to play a role as well. Generally speaking, two subsequent functional layers have different exposure settings. This results in a (non-reticle) additional overlay contribution.
In this paper, we have studied the wafer overlay correction capability by RegC® in addition to the TWINSCANTM intrafield corrections to improve the on product overlay performance. RegC® is a reticle intra-volume laser writing technique that causes a predictable deformation element (RegC® deformation element) inside the quartz (Qz) material of a reticle. This technique enables to post-process an existing reticle to correct for instance for IPE. Alternatively, a pre-determined intra-field fingerprint can be added to the reticle such that it results in a straight field after exposure. This second application might be very powerful to correct for instance for (cold) lens fingerprints that cannot be corrected by the scanner itself. Another possible application is the intra-field processing fingerprint. One should realize that a RegC® treatment of a reticle generally results in global distortion of the reticle. This is not a problem as long as these global distortions can be corrected by the TWINSCANTM system (currently up to the third order). It is anticipated that the combination of the RegC® and the TWINSCANTM corrections act as complementary solutions. These solutions perfectly fit into the ASML Litho InSight (LIS) product in which feedforward and feedback corrections based on YieldStar overlay measurements are used to improve the on product overlay.
The on-product overlay specification and Advanced Process Control (APC) are getting extremely challenging
particularly after the introduction of multi-patterning applications like Litho-Etch-Litho-Etch (LELE). While the Reticle
Writing Error (RWE) contribution could be marginalized for quite some time in the layer-to-layer overlay budget, it will
become one of the dominating overlay contributors when the intra-layer overlay budget is considered. While most of the
overlay contributors like wafer processing, scanner status, reticle transmission, dose, illumination conditions drop out of
the intra-layer overlay budget, this is certainly not the case for reticle to reticle writing differences.
In this work, we have studied the impact of the RWE on the on-product overlay performance. We show that the RWE
can be characterized by an off-line mask registration tool and the modelled results can be sent as feed-forward
corrections to the ASML TWINSCANTM. By doing so, the overlay control complexity (e.g. send-ahead wafers, APC
settling time) can be reduced significantly. Off-line characterization enables that all reticles virtually become equal after
correction (at least to the level of correction capability of the scanner). This means that all higher order RWE
contributions (currently up to a third order polynomial) can be removed from the fingerprint. We show that out of 50
production reticles (FEOL, 28-nm technology), 30% can be improved on residual level when non-linear feed-forward
corrections are considered as well. The additional benefit of feeding forward linear corrections to the scanner is even
higher: it is anticipated that a large portion of the APC variation might find its origin in the RWE contribution.
In order to send feed-forward corrections to the scanner, we obviously rely on the quality of the off-line RWE
measurements. These measurements are usually provided by a registration tool at the mask shop. To secure the quality,
an independent experimental verification test was developed to check if off-line RWE measurements can be used as
feed-forward corrections to the scanner. The test has been executed on an ASML NXT: 1950i scanner and was designed
such to isolate the reticle writing error contribution. The match between the off-line measurements and the experiment is