Technology - Lithography
Figure 3 - Field data from a high volume fab showing stable operation over the course of one year while running at 90W power
power (Fig. 3). Such high stability enables the very good CD uniformity and overlay performance that is required for sub 32nm node devices.
Furthermore, one of the elements that enables higher scanner throughput is the ability to increase scanning speed, which, in turn, results in fewer pulses per scan window (since the laser repetition rate is fixed at 6kHz). High pulse-to-pulse energy stability at high pulse energies enables the use of fewer pulses per scan window, reducing the reliance of pulse averaging that a large number of pulses would provide.
High uptime 32
as long optics life, as proven through accelerated lifetime testing (Fig. 2). Thus, both high performance and high availability can be achieved simultaneously using these new technologies.
Figure 4 - Constant electrode gap concept introduced to improve discharge chamber performance and reduce frequency of service events
Optical performance stability In addition to a stable system design, key light source optical parameters such as energy, wavelength and bandwidth require sophisticated control systems to ensure consistentcy within wafer and within die CD uniformity. For a typical die, about 300 – 500 pulses of light are used, so the control systems in the light source have to ensure high pulse-to-pulse repeatability. The control elements include high accuracy and high speed metrology to measure wavelength, bandwidth and energy for every pulse and fast processors and algorithms to drive actuators for closed-loop control. This combination results in very tight dose stability (<0.1%), low bandwidth variability (<30fm) and low wavelength variation (<20fm) under typical operating conditions and high, 90W output
While these performance advances improve the system capabilities, it is important to further improve overall system uptime and maximize the wafer productivity to keep the cost per wafer down. To this end, advances in key light source components that reduce the service frequency are equally important. Cymer has developed breakthrough technologies in the laser discharge chamber design that extend the time between service events. One of the key aging mechanisms of a laser discharge chamber is the erosion of the discharge electrodes that lead to a gradual increase in the gap between these electrodes.
Over time, this increasing electrode gap drives the need for a higher voltage to drive a discharge, until the voltage reaches an operating limit. The solution that Cymer implemented involves introducing movable electrodes to maintain a constant electrode gap (Fig. 4). While simple in concept, this solution has been elusive for many years: introducing movable parts in a discharge chamber without impacting other performance properties or system reliability. Maintaining a constant electrode gap has improved performance stability and resulted in fewer service events. This new chamber design has been implemented and field datahas confirmed an increase in system availability (Fig. 5).
Another common service event for excimer lasers is the periodic need to refresh the discharge chamber gas as it slowly degrades over time. Historically, one countermeasure to maintain a stable discharge over time has been to periodically feed new gas into the chamber in small quantities while the laser is firing so as not to interrupt production. This countermeasure has
www.euroasiasemiconductor.com Issue IV 2011
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