Laser Energy vs. Voltage Error Limiter (LEVEL)
Publication Date: 2005-Jan-18
The IP.com Prior Art Database
TITLE: Laser Energy vs Voltage Error Limiter (LEVEL)
Laser Energy vs Voltage Error Limiter (LEVEL)
Purpose and advantage of invention:
Reduction of energy transients and duty cycle effects in a mode similar to external voltage will
greatly improve laser energy performance in a manner which is transparent to the customer
Short description of invention (Include or attach appropriate sketches, and point out novel features):
Some of Cymer’s customers use the laser in “External Voltage” mode. In this mode, the customer directly commands the voltage applied by the pulse power system. The voltage command is computed by the scanner usually based upon energy readings taken from somewhere in the beam path. Figure 1 illustrates this architecture.
Figure 1. External voltage control mode.
This architecture suffers from a major liabilities. Since all voltage control is handled by the scanner, the scanner controller is responsible for handling burst transients, duty cycle effects, drift, and energy vs. voltage sensitivity (dE/dV) changes. This is a problem for our customers since they want the same controller to work not just with our lasers, but with the competition’s lasers as well. The control required to optimally compensate for the above mentioned affects will depend upon the details of the laser design. It is unlikely that an optimal controller for our laser will be the same as that for someone else’s, hence the customer has to custom design controllers depending on laser. In practice, this does not happen. Instead, the scanner manufacturer dictates requirements about the maximum size of burst transients, drift, and dE/dV changes. This often forces us to invent solutions to problems which could be easily solved using active control (e.g. energy transients) using more expensive methods, such as chamber redesign.
A different approach is to let the laser handle energy control. In this case, the laser receives energy data either from an on-board sensor or from a sensor within the scanner. The laser then computes the correct voltage to apply in order to achieve an energy time history which meets certain performance metrics such as moving average (dose), or moving standard deviation (sigma). Figure 2 shows this configuration.
Figure 2. Energy control mode.
This approach also has limitations. First, the scanner must provide an energy target instead of a voltage command. This is a significantly different style of operation and would force the customer to have different controllers for Cymer lasers. Second, in addition to correcting for laser errors (e.g. drift, energy transients, dE/dV changes, duty cycle effects), the laser controller must also tailor the energy time history to maximize performance metrics such as dose or sigma. The problem is that if the customer decides that they have different performance goals for the controller, they must get Cymer to modify the controller to accommodate them. Further, the customer must share mor...