Deep ultraviolet (DUV) lithography improvements have been focused on two paths:
further increases in the effective numerical aperture (NA) beyond 1.3, and double
patterning (DP). High-index solutions for increasing the effective NA have not gained
significant momentum due to several technical factors, and have been eclipsed by an
aggressive push to make DP a high-volume manufacturing solution. The challenge is to
develop a cost-effective solution using a process that effectively doubles the lithography
steps required for critical layers, while achieving a higher degree of overlay performance.
As a result, the light source requirements for DP fall into 3 main categories: (a) higher
power to enable higher throughput on the scanner, (b) lower operating costs to offset the
increased number of process steps, and (c) high stability of optical parameters to support
more stringent process requirements. The XLR 600i (6kHz, 90W @15mJ) was
introduced last year to enable DP by leveraging the higher performance and lower
operating costs of the ring architecture XLR 500i (6kHz, 60W @10mJ) platform
currently used for 45nm immersion lithography in production around the world. In
February 2009, the XLR 600ix was introduced as a 60/90W switchable product to
provide flexibility in the transition to higher power requirements as scanner capabilities
are enhanced. The XLR 600ix includes improved optics materials to meet reliability
requirements while operating at higher internal fluences. In this paper we will illustrate
the performance characteristics during extended testing. Examples of performance
include polarization stability, divergence and pointing stability, which enable consistent
pupil fill under extreme illumination conditions, as well as overall thermal stability which
maintains constant beam performance under large changes in laser operating modes.
Furthermore, the unique beam uniformity characteristics that the ring architecture
generates result in lower peak energy densities that are comparable to those of a typical
60W excimer laser. In combination with the XLR's long pulse duration, this allows for
long life scanner optics while operating at 15mJ.
We investigate the effect of finite laser bandwidth on line-space (L/S) imaging, both through simulations and experiment. We will show that the primary effect of laser bandwidth is a change of the optical-proximity behavior of the scanner, i.e., a modification of the critical dimensional pitch [CD(Pitch)] characteristic, and that depth of focus typically remains unaffected. The simulation part of this study resulted in a simple estimator, expressing bandwidth-induced CD changes in terms of the “quadratic focus-sensitivity” (1/2 d2CD/dF2) of the L/S structure, a parameter which we call the “second moment” of the laser spectrum, M2, and the longitudinal chromatic-aberration of the scanner (dF/d) only. The experimental part of this study, in which we measured CD(pitch) curves at different laser-bandwidth settings of the ASML XT:1700i NA=1.20 immersion scanner at Interuniversitary Micro-Electronic Centre (IMEC), confirms the results of the theoretical part, while relating the bandwidth dependency of the CD effects also to the experimentally available E95 metric. We conclude that even though the laser bandwidth of modern scanners is quite low, bandwidth effects do contribute to their proximity behavior and impact proximity stability as well as scanner-to-scanner proximity differences. We present a critical evaluation of current laser-bandwidth metrics and comment on the trade-off between the average laser bandwidth and laser bandwidth stability in order to achieve a required level of proximity control (e.g., between scanners).
Double patterning (DP) lithography is expected to be deployed at the 32nm node to enable the extension of high NA
(≥1.3) scanner systems currently used for 45nm technology. Increasing the light source power is one approach to address
the intrinsically lower throughput that DP imposes. Improved energy stability also provides a means to improve
throughput by enabling fewer pulses per exposure slit window, which in turn enables the use of higher scanner stage
speeds. Current excimer laser light sources for deep UV immersion lithography are operating with powers as high as
60W at 6 kHz repetition rates. In this paper, we describe the introduction of the XLR 600i, a 6 kHz excimer laser that
produces 90W power, based on a recirculating ring technology. Improved energy stability is inherent to the ring
technology. Key to the successful acceptance of such a higher power, or higher energy laser is the ability to reduce
operating costs. For this reason, the recirculating ring technology provides some unique advantages that cannot be
realized with conventional excimer lasers today. Longer intrinsic pulse durations that develop in the multi-pass ring
architecture reduce the peak power that the optics are subjected to, thereby improving lifetime. The ring architecture also
improves beam uniformity that results in a significantly reduced peak energy density, another key factor in preserving
optics lifetime within the laser as well as in the scanner. Furthermore, in a drive to reduce operating costs while
providing advanced technical capability, the XLR 600i includes an advanced gas control management system that
extends the time between gas refills by a factor of ten, offering a significant improvement in productive time. Finally, the
XLR 600i provides a novel bandwidth stability control system that reduces variability to provide better CD control,
which results in higher wafer yields.
The variation of CD with pitch, or Optical Proximity Effect (OPE), in an imaging system shows a behavior that is characteristic of the imaging and process conditions and is sensitive to variations in those conditions. Maintaining stable process conditions can improve the effectiveness of mask Optical Proximity Correction (OPC). One of the factors which affects the OPE is the spectral bandwidth of the light source. To date, passive bandwidth stabilization techniques have been effective in meeting OPE control requirements. However, future tighter OPE specifications will require advanced bandwidth control techniques. This paper describes developments in active stabilization of bandwidth in Cymer XLA and 7010 lasers. State of the art on board metrology, used to accurately measure E95 bandwidth, has enabled a new array of active control solutions to be deployed. Advanced spectral engineering techniques, including sophisticated control algorithms, are used to stabilize and regulate the bandwidth of the light source while maintaining other key performance specifications.
The XLA 300 is Cymer's fourth-generation MOPA-based Argon Fluoride light source built on the production-proven XLA platform. The system is designed to support very high numerical aperture dioptric and catadioptric lens immersion lithography scanners targeted for volume production of semiconductor devices at the 45nm node and beyond. The light source delivers up to 90 W of power with ultra-line narrowed bandwidth as low as 0.12 pm FWHM and 0.25 pm 95% energy integral. The high output power is achieved by advancements in pulse power technology, which allow a 50% increase in repetition rate to 6 kHz. The increased repetition rate, along with pulse stretching, minimizes damage to the scanner system optics at this high power level. New developments in the laser optical systems maintain industry-leading performance for bandwidth stability and high level of polarization despite the increased thermal load generated at the higher repetition rate. The system also features state-of-the-art on-board E95% bandwidth metrology and improved bandwidth stability to provide enhanced CD control. The E95% metrology will move bandwidth monitoring from a quality safeguard flag to a tool that can be used for system feedback and optimization. The proven high power optics technology extends the lifetime of key laser optics modules including the line-narrowing module, and the cost of consumables (CoC) is further reduced by longer chamber lifetimes.
The first generation MOPA-based ArF laser XLA-100 was introduced in January 2003 in response to the needs of the high NA ArF scanners for higher power and narrower spectral bandwidth. The second generation product XLA-105 was introduced in early 2004. This paper presents our third generation MOPA-based ArF laser product XLA-200 that is designed and engineered to meet the light source requirements of the ArF immersion lithography. It is expected to be used for 65-nm and 45-nm volume production of semiconductor devices. The XLA-200 is capable of producing a 60W of ultra-line-narrowed 193nm light with the FWHM bandwidth of less than 0.15pm and the E95% integral bandwidth of less than 0.35pm. It features state-of-the-art on-board bandwidth metrology tool that measures E95% bandwidth as well as FWHM. Real-time accurate bandwidth information can be utilized for lithography exposure tool feedback control. The improved dual-chamber laser gas control ensures excellent bandwidth stability, which enables tighter CD control. Together with a lower cost of ooperation, the XLA-200 sets a new performance level for the dual chamber 193nm light source for microlithography.
Since the announcement in March 2002 of plans to develop an advanced light source to meet the future spectral power and cost requirements of photolithography, we have made significant progress in the development and productization of the core technology for an ultra line-narrowed, excimer light source based on a master oscillator-power amplifier (MOPA) approach. In this paper, we will focus on the architecture and performance of the first generation of production-ready, MOPA-based ArF light sources developed at Cymer, Inc. This first generation of MOPA-based ArF light sources is referred to as the XLA 100 product series.