Query: Control of Wafer Scanners: Methods and Developments

Candidate papers:

abstract: In this tutorial paper, control design aspects of wafer scanners used in the semiconductor industry will be highlighted. At the same time, challenges for control design development as to meet the ever increasing demands on accuracy and speed are presented. Mechatronic systems that will be discussed are: (a) the light source needed to generate the ultraviolet light that is used for wafer exposure, (b) the optical and metrology systems needed for accurate measurement and imaging, and (c) the reticle and wafer stage systems needed for accurate and fast positioning. The control challenges associated with these systems mainly involve dealing with: (a) rejection of high frequency aliased disturbances, (b) large scale or fast updated (state) reconstruction, (c) vibration control and isolation in view of structural vibrations and disturbances, (d) inherent design tradeoffs like Bode's sensitivity integral and gain phase relationships, (e) multivariable plant identification of (quasi static) deformations and structural dynamics for point of interest control, and (f) thermal modelling, model reduction, and the control of (local) time varying deformation. Results will be discussed using representative examples.
author_names: Marcel Francois Heertjes

Hans Butler

N J Dirkx

Stan van der Meulen

R Ahlawat

K O'Brien

James Simonelli

Kuo-Tai Teng

Yuanyuan Zhao
title: Control of Wafer Scanners: Methods and Developments
venue: 2020 American Control Conference (ACC)
year: 2020

abstract: With decreasing CDOF (Critical Depth Of Focus) for 20/14nm technology and beyond, focus errors are becoming increasingly critical for on product performance. Current on product focus control techniques in high volume manufacturing are limited; It is difficult to define measurable focus error and optimize focus response on product with existing methods due to lack of credible focus measurement methodologies. Next to developments in imaging and focus control capability of scanners and general tool stability maintenance, on product focus control improvements are also required to meet on product imaging specifications. In this paper, we discuss focus monitoring, wafer (edge) fingerprint correction and on product focus budget analysis through diffraction based focus (DBF) measurement methodology. Several examples will be presented showing better focus response and control on product wafers. Also, a method will be discussed for a focus interlock automation system on product for a high volume manufacturing (HVM) environment.
author_names: Young Ki Kim

Yen-Jen Chen

Xueli Hao

Pavan Samudrala

Juan-Manuel Gomez

Mark O Mahoney

Ferhad Kamalizadeh

Justin K Hanson

Shawn Lee

Ye Tian
title: Focus control enhancement and on product focus response analysis methodology
venue: SPIE Advanced Lithography
year: 2016

abstract: The introduction of advanced technology nodes in deep UV (DUV) lithography (litho) involving multiple litho steps, has tightened the wafer on product overlay specifications [1] The industry trend already pushes the overlay requirements to the sub nanometer regime (and so the mask registration requirements tightens as well) In the most general view, wafer on product overlay errors are a combination of intra field (within field) and inter field (field to field) errors. A given litho layer intra field overlay error includes several systematic sources, such as scanner lens to lens residuals and mask writer residuals. Parallel to the nodes and litho techniques advancing, the ability to accurately measure overlay at high lateral resolution have being successfully introduced, both at wafer and mask side. The recent developments in scanner technology improved the ability to control intra field overlay at high order across the exposure field. However, this is still in several millimeter lateral resolution control ability at its best, leaving residual errors in the sub millimeter to few millimeter regime without the ability to further suppress them to the target specification, nevertheless, not to the sub nanometer magnitude. In this work, we have empirically evaluated the ZEISS state of the art mask tuning solution named ForTune ERC (Enhanced Registration Control) This solution is based on laser processing of the mask bulk by the ZEISS ForTune tool. It allows to suppress few nanometer overlay residuals (post the scanner best can do) down to deep sub nanometer, all even at sub millimeter sampling resolution (x1 wafer level) and low to high residuals modulating frequency. For the sake of this study, we have used a dual image mask to form one overlay signature at wafer side. Two wafers have been exposed prior to the laser based tuning of the mask bulk; the wafers overlay error was measured and used as an initial overlay problem to begin with. A second exposure of two additional wafers was performed post the problem solving by the ERC model and the consequent mask laser based tuning. The pre/post wafers were then compared to examine the improvement in overlay at wafer side. CD uniformity (CDU) data has been collected as well, to confirm no degradation in CDU due to the ForTune ERC process. The combination of this advanced method of intra field control with high order correction per exposure (CPE) by the scanner, provides an efficient co optimized solution to tightly control the overlay of existing and future nodes at DUV litho.
author_names: Avi Cohen

P Leray

Eren Canga

Vladimir Dmitriev

Kujan Gorhad

Yael Sufrin
title: Enhanced wafer overlay residuals control; deep sub nanometer at sub millimeter lateral resolution
venue: Advanced Lithography
year: 2019

abstract: The introduction of advanced technology nodes in deep UV (DUV) lithography (litho) involving multiple litho steps, has tightened the wafer on product overlay specifications [1] The industry trend already pushes the overlay requirements to the sub nanometer regime (and so the mask registration requirements tightens as well) In the most general view, wafer on product overlay errors are a combination of intra field (within field) and inter field (field to field) errors. A given litho layer intra field overlay error includes several systematic sources, such as scanner lens to lens residuals and mask writer residuals. Parallel to the nodes and litho techniques advancing, the ability to accurately measure overlay at high lateral resolution have being successfully introduced, both at wafer and mask side. The recent developments in scanner technology improved the ability to control intra field overlay at high order across the exposure field. However, this is still in several millimeter lateral resolution control ability at its best, leaving residual errors in the sub millimeter to few millimeter regime without the ability to further suppress them to the target specification, nevertheless, not to the sub nanometer magnitude. In this work, we have empirically evaluated the ZEISS state of the art mask tuning solution named ForTune ERC (Enhanced Registration Control) This solution is based on laser processing of the mask bulk by the ZEISS ForTune tool. It allows to suppress few nanometer overlay residuals (post the scanner best can do) down to deep sub nanometer, all even at sub millimeter sampling resolution (x1 wafer level) and low to high residuals modulating frequency. For the sake of this study, we have used a dual image mask to form one overlay signature at wafer side. Two wafers have been exposed prior to the laser based tuning of the mask bulk; the wafers overlay error was measured and used as an initial overlay problem to begin with. A second exposure of two additional wafers was performed post the problem solving by the ERC model and the consequent mask laser based tuning. The pre/post wafers were then compared to examine the improvement in overlay at wafer side. CD uniformity (CDU) data has been collected as well, to confirm no degradation in CDU due to the ForTune ERC process. The combination of this advanced method of intra field control with high order correction per exposure (CPE) by the scanner, provides an efficient co optimized solution to tightly control the overlay of existing and future nodes at DUV litho.
author_names: Yael Sufrin

P Leray

Eren Canga

Avi Cohen

Vladimir Dmitriev

Kujan Gorhad
title: Enhanced wafer overlay residuals control: deep sub nanometer at sub millimeter lateral resolution
venue: European Mask and Lithography Conference
year: 2019

abstract: Advances in photolithography are one of the key driving factors in the continuing expansion in capacity and decrease in cost of semiconductors. Extending this trend into the future necessitates the development of next generation lithography technologies in order to overcome the fundamental challenges of improving critical dimension and overlay control, and lowering the total cost of ownership. As feature sizes become smaller and smaller, performance requirements for wafer scanner machines will become more stringent; with regards to motion control, requirements for the wafer stage include sub nanometer positioning precision under high scan velocities and accelerations. Advanced control algorithms are needed to meet these requirements in the face of disturbances such as vibrations, noise, force ripple and friction, as well as model uncertainty.This dissertation focuses on using the repetitiveness of the stage's motion in the photolithography process to improve control precision. Similar to many manufacturing processes, the step and scan motion used to expose a wafer is very repetitive, on a die to die and also wafer to wafer level. By using data gathered from past runs, the control effort for future runs may be improved, thereby exploiting the repetitiveness of the process to increase control precision.In this research, \textit{iterative learning control} (ILC) and \textit{iterative feedback tuning} (IFT) were applied to reduce tracking error of the wafer stage. In ILC, a feedforward control signal for the system is incrementally adjusted to achieve better tracking performance using error signals from previous runs. ILC is an attractive method for high precision control because of its simplicity and data based nature. In this research, ILC algorithm design specifically for attenuating high frequency vibrations is investigated. Through careful design of the ILC update law, fast learning convergence and small final error is achieved. One drawback of ILC is that a feedforward signal learned through ILC is only applicable to the training trajectory; learning must be restarted when the trajectory is changed. A method is presented for making ILC results applicable to any trajectory within a class of scan trajectories; this is accomplished by using ILC as a training method for feedforward signal patterns. In IFT, controller parameters are fine tuned incrementally using only data collected in experimental runs. IFT is applied to tune fixed structure feedforward, feedback, and force ripple compensator controllers. The performance of IFT is also compared with ILC in the context of iterative methods for designing feedforward control. All results are verified through computer simulations and experiments done on a wafer stage testbed system.
author_names: Hoday Stearns
title: Iterative Methods for High Precision Motion Control with Application to a Wafer Scanner System
year: 2011

abstract: This thesis presents the design and control techniques of a device for managing the inertial loads on photoreticle of lithography scanners. Reticle slip, resulting from large inertial loads, is a factor limiting the throughput and accuracy of the lithography scanners. Our reticle assist device completely eliminates reticle slip by carrying 96% of the inertial loads. The primary contributions of this thesis include the design and implementation of a practical high force density reticle assist device, the development of a novel charge controlled power amplifier with DC hysteresis compensation, and the development of a sensorless control method. A lithography scanner exposes a wafer by sweeping a slit of light passing through a reticle. The scanner controls the motion of the reticle and the wafer. The reticlestage moves the photoreticle. To avoid deforming the reticle, it is held using a vacuum clamp. Each line scan consists of acceleration at the ends of the line and a constantspeed motion in the middle of the line, where exposure occurs. If the reticle's inertial force approaches or exceeds the clamp's limit, nanometer level pre sliding slip or sliding slip will occur. The assist device carries the inertial load by exerting a feedforward force on the reticle's edge. The device retracts back during the sensitive exposure interval to avoid disturbing the reticle. The reticle is at the heart of the scanner, where disturbances directly affect the printing accuracy. Our reticle assist device consists of an approach mechanism and a piezoelectric stack actuator. The approach mechanism positions the actuator 1 mm from the reticle edge. The actuator, with 15 mm range, extends to push on the reticle. We have developed control techniques to enable high precision high bandwidth force compensation without using any sensors. We have also developed a novel charge controlled amplifier with a more robust feedback circuit and a method for hysteresis compensation at DC. These technologies were key to achieving high bandwidth high precision sensorless force control. When tested with a trapezoidal force profile with 6400 N/s rate and 60 N peak force, the device canceled 96% of the inertial force. 3 Thesis Supervisor: David L. Trumper Title: Professor of Mechanical Engineering
author_names: Darya Amin-Shahidi
title: Design and control of a self sensing piezoelectric reticle assist device
year: 2013

abstract: In high tech industry (sub)nanometre precision motion control is essential. For example wafer scanners, used for production of integrated circuits, have to provide (sub)nanometre precision positioning whilst satisfying challenging requirements on speed at the same time. It is in demanding cases like this that different requirements begin to conflict with each other. A fundamental trade off between robustness and performance exists as an inevitable result of the waterbed effect in linear control. PID controllers, which have been an industry standard for many years, do not satisfy the ever increasing demands. In this MSc thesis a novel reset control synthesis method is proposed: CRONE reset control, which combines a robust fractional CRONE controller (Commande Robuste d'Ordre Non Entier) with non linear reset control to overcome waterbed effect. In CRONE control, robustness is achieved against gain deviations by creation of constant phase behaviour around bandwidth with the use of fractional operators. The use of fractional operators also introduces more freedom in shaping the open loop frequency response, allowing fractional factors in Bode's gain phase relation. However, waterbed effect remains, which motivates introduction of non linear reset control in the CRONE design. In reset control, controller states are reset when the error between output and reference signal is zero. In frequency domain, using describing function analysis it is predicted that reset filters generate less phase lag for similar gain behaviour. For instance, a reset integrator gives a phase lag of about 38 degrees, which is 52 degrees less compared to the linear integrator. This allows for relief from Bode's gain phase relation, breaking aforementioned trade offs. In the new controller design, reset phase advantage is approximated using describing function analysis and used to achieve better open loop shape. New design rules for CRONE reset control have been developed in this thesis. Three different reset control strategies have been investigated: integrator reset, lag reset and lead lag/lag lead reset. For these controllers, a two degree of freedom non linearity tuning CRONE reset control structure has been developed. This control structure has been implemented digitally within a MATLAB/Simulink environment on a Lorentz actuated (dual) precision stage via a real time dSPACE DS1103 controller interface. For the implemented controllers sufficient quadratic stability has been shown using the Hb condition. It has been shown that simulated frequency responses using describing function correspond well to experimental identified frequency responses. Moreover, frequency domain measurements have shown that better performance for CRONE reset control can be achieved for same phase margin compared to linear CRONE. Relief from both waterbed effect and Bode's gain phase relation has been accomplished. Furthermore, for the same bandwidth, average noise reduction between 1.79dB and 3.93dB has been attained at a number of distinct frequencies above bandwidth. In the fine stage separately and also in dual stage configuration, tracking of a fourth order input shaped reference signal (with second order and fourth order feedforward respectively) showed improvement in CRONE reset control compared to linear CRONE. In the dual stage configuration, after decoupling fine stage and coarse stage, tracking performance of a linear CRONE controller has been compared to a CRONE reset controller. In both cases the same linear CRONE 2 controller with a bandwidth of 80 Hz and phase margin of 50deg was applied to the coarse stage. On the fine stage a CRONE 1 lag lead controller was applied with a bandwidth of 150 Hz and a phase margin of 55deg. RMS error for a triangular scanning wave with maximal acceleration of 0.25 m/s2 and maximal velocity of 75 mm/s over a stroke of 2 cm, was reduced from 929.8 nm to 443.7 nm.
author_names: Linda Chen
title: Development of CRONE reset control
year: 2017

abstract: Embedded control is a key product technology differentiator for many high tech industries, including ASML. The strong increase in complexity of embedded control systems, combined with the occurrence of late changes in control requirements, results in many timing performance problems showing up only during the integration phase. The fallout of this is extremely costly design iterations, severely threatening the time to market and time to quality constraints. This paper reports on the industrial application at ASML of the Y chart method to attack this problem. Through the largely automated construction of executable models of a wafer scanner's mechatronics control application and platform, ASML was able to obtain high level overview early on in the development process. The system wide insight in timing bottlenecks gained this way resulted in more than a dozen improvement proposals yielding significant performance gains. These insights also led to a new development roadmap of the mechatronics control execution platform.
author_names: Jeroen Voeten

Teun Hendriks

Bart D Theelen

J Schuddemat

W Tabingh Suermondt

J Gemei

Kees C Kotterink

C van Huet
title: Predicting Timing Performance of Advanced Mechatronics Control Systems
venue: 2011 IEEE 35th Annual Computer Software and Applications Conference Workshops
year: 2011

abstract: The integration of on product diffraction based focus (DBF) capability into the majority of immersion lithography layers in leading edge logic manufacturing has enabled new applications targeted towards improving cycle time and yield. A CD based detection method is the process of record (POR) for excursion detection. The drawback of this method is increased cycle time and limited sampling due to CD SEM metrology capacity constraints. The DBFbased method allows the addition of focus metrology samples to the existing overlay measurements on the integrated metrology (IM) system. The result enables the addition of measured focus to the SPC system, allowing a faster excursion detection method. For focus targeting, the current method involves using a dedicated focus exposure matrix (FEM) on all scanners, resulting in lengthy analysis times and uncertainty in the best focus. The DBF method allows the measurement to occur on the IM system, on a regular production wafer, and at the same time as the exposure. This results in a cycle time gain as well as a less subjective determination of best focus. A third application aims to use the novel onproduct focus metrology data in order to apply per exposure focus corrections to the scanner. These corrections are particularly effective at the edge of the wafer, where systematic layer dependent effects can be removed using DBFbased scanner feedback. This paper will discuss the development of a methodology to accomplish each of these applications in a high volume production environment. The new focus metrology method, sampling schemes, feedback mechanisms and analysis methods lead to improved focus control, as well as earlier detection of failures.
author_names: Ben F Noyes

Babak Mokaberi

D H Bolton

Chen Li

Ashwin Palande

Kevin Park

M J Noot

Marc Kea
title: Applications of on product diffraction based focus metrology in logic high volume manufacturing
venue: SPIE Advanced Lithography
year: 2016

abstract: BACKGROUND Randomised trials have shown that aspirin reduces the short term risk of recurrent colorectal adenomas in patients with a history of adenomas or cancer, but large trials have shown no effect in primary prevention of colorectal cancer during 10 years' follow up. However, the delay from the early development of adenoma to presentation with cancer is at least 10 years. We aimed to assess the longer term effect of aspirin on the incidence of cancers. METHODS We studied the effect of aspirin in two large randomised trials with reliable post trial follow up for more than 20 years: the British Doctors Aspirin Trial (N=5139, two thirds allocated 500 mg aspirin for 5 years, a third to open control) and UK TIA Aspirin Trial (N=2449, two thirds allocated 300 mg or 1200 mg aspirin for 1 7 years, a third placebo control) We also did a systematic review of all relevant observational studies to establish whether associations were consistent with the results of the randomised trials and, if so, what could be concluded about the likely effects of dose and regularity of aspirin use, other non steroidal anti inflammatory drugs (NSAID) and the effect of patient characteristics. RESULTS In the randomised trials, allocation to aspirin reduced the incidence of colorectal cancer (pooled HR 0.74, 95% CI 0.56 0.97, p=0.02 overall; 0.63, 0.47 0.85, p=0.002 if allocated aspirin for 5 years or more) However, this effect was only seen after a latency of 10 years (years 0 9: 0.92, 0.56 1.49, p=0.73; years 10 19: 0.60, 0.42 0.87, p=0.007) was dependent on duration of scheduled trial treatment and compliance, and was greatest 10 14 years after randomisation in patients who had had scheduled trial treatment of 5 years or more (0.37, 0.20 0.70, p=0.002; 0.26, 0.12 0.56, p=0.0002, if compliant) No significant effect on incidence of non colorectal cancers was recorded (1.01, 0.88 1.16, p=0.87) In 19 case control studies (20 815 cases) and 11 cohort studies (1 136 110 individuals) regular use of aspirin or NSAID was consistently associated with a reduced risk of colorectal cancer, especially after use for 10 years or more, with no difference between aspirin and other NSAIDs, or in relation to age, sex, race, or family history, site or aggressiveness of cancer, or any reduction in apparent effect with use for 20 years or more. However, a consistent association was only seen with use of 300 mg or more of aspirin a day, with diminished and inconsistent results for lower or less frequent doses. INTERPRETATION Use of 300 mg or more of aspirin a day for about 5 years is effective in primary prevention of colorectal cancer in randomised controlled trials, with a latency of about 10 years, which is consistent with findings from observational studies. Long term follow up is required from other randomised trials to establish the effects of lower or less frequent doses of aspirin.
author_names: Enrico Flossmann

Peter M Rothwell
title: Effect of aspirin on long term risk of colorectal cancer: consistent evidence from randomised and observational studies
venue: The Lancet
year: 2007