Lithography Mirror Control via Over-Sensing and Modal Suppression
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Solution Overview
Problem
In extreme ultraviolet lithography systems, the mechanical properties of mirrors with limited Young's modulus lead to unavoidable vibrations and displacements, limiting control bandwidth due to close eigen frequencies of bending and torsional modes, which conventional methods like notch filters or increasing mirror thickness cannot effectively address without extensive adjustments.
Innovation Solution
A method involving over-sensing with a plurality of sensor elements to detect movements beyond the degrees of freedom, modifying sensor signals using a modified transformation matrix to reduce eigen modes or resonances, and employing a pseudo inverse and null space contributions to suppress undesired resonances, allowing for increased control bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional notch filters are used to suppress eigen modes, then resonance suppression is achieved, but extensive manual tuning and gauging is required for each mirror
Solution Approach 1:
The system performs self-identification of eigen modes through automated modal analysis. The mirror's eigen modes are automatically detected and characterized without requiring manual gauging or tuning, enabling the control system to adapt to each mirror's specific characteristics automatically.
Solution Approach 2:
The system dynamically adjusts control parameters based on the identified eigen modes. By changing the control strategy to account for the specific modal frequencies and shapes of each mirror, the system achieves effective resonance suppression without manual intervention.
2Reliability
If mirror thickness is increased to raise eigen frequencies, then resonance suppression is achieved, but the mirror cannot be positioned closer to the wafer station
Solution Approach 1:
Instead of changing the physical parameter of mirror thickness, the system changes the control parameters to compensate for the mirror's flexible modes. This allows maintaining the original mirror geometry and positioning while achieving resonance suppression through adaptive control.
Solution Approach 2:
The patent replaces the mechanical solution of increasing mirror thickness with a control-theoretic solution. By using active control systems that identify and compensate for eigen modes, the need for thicker mirrors is eliminated, allowing mirrors to be positioned closer to the wafer station.
3Speed
If bending and torsional modes are close in frequency, then control bandwidth is limited, but increasing control bandwidth is needed for precision
Solution Approach 1:
The control system segments the mirror's dynamic behavior into distinct eigen modes through modal analysis. By identifying and controlling each mode separately, the system can manage the interaction between bending and torsional modes even when their frequencies are close, thereby maintaining control stability while extending bandwidth.
Solution Approach 2:
The system employs dynamic control strategies that adapt to the mirror's actual modal characteristics. By using real-time or pre-characterized modal information, the controller can adjust its response to each mode, allowing stable control across a broader frequency range despite close modal frequencies.
Data Source
AI summary
A method for controlling a vibrating optical element of a lithographic system the optical element having a predetermined number of degrees of freedom comprises: detecting a number of displacements of the optical element, each displacement corresponding to a degree of freedom, wherein the number of detected displacements is larger than the number of degrees of freedom; for each displacement according to a degree of freedom, generating a sensor signal corresponding to a movement in a degree of freedom; wherein the optical element moves as a function of a rigid body transformation matrix, the optical element movement including a first type of movement and a second type of movement; and modifying the sensor signals as a function of a modified transformation matrix, wherein the modified transformation matrix at least partially reduces at least one eigen mode or resonance of one of the first type of movements or the second type of movements.