While the industry is making progress to offer EUV lithography schemes to attain ultimate critical dimensions down to 20 nm half pitch, an interim optical lithography solution to address an immediate need for resolution is offered by various integration schemes using advanced PSM (Phase Shift Mask) materials including thin e-beam resist and hard mask. Using the 193nm wavelength to produce 10nm or 7nm patterns requires a range of optimization techniques, including immersion and multiple patterning, which place a heavy demand on photomask technologies. Mask schemes with hard mask certainly help attain better selectivity and hence better resolution but pose integration challenges and defectivity issues. This paper presents a new photomask etch solution for attenuated phase shift masks that offers high selectivity (Cr:Resist > 1.5:1), tighter control on the CD uniformity with a 3sigma value approaching 1 nm and controllable CD bias (5-20 nm) with excellent CD linearity performance (<5 nm) down to the finer resolution. The new system has successfully demonstrated capability to meet the 10 nm node photomask CD requirements without the use of more complicated hard mask phase shift blanks. Significant improvement in post wet clean recovery performance was demonstrated by the use of advanced chamber materials. Examples of CD uniformity, linearity, and minimum feature size, and etch bias performance on 10 nm test site and production mask designs will be shown.
Mask defectivity is often highlighted as one of the barriers to a manufacturable EUV solution. As EUV lithography
matures, other components of mask making also emerge as key focus areas in the industry: critical dimension (CD)
control, film variability, selectivity, and profile tolerance. Mask materials and specifications continue to evolve to meet
the unique challenges of EUV lithography, creating the need for etch capabilities that can keep pace with the latest
developments. In this study, the performance of a new EUV mask etch system will be evaluated using a variety of mask
blanks to determine the relative performance of each blank type. Etch contributions to mean to target (MTT), CDU,
linearity, selectivity, capping layer uniformity, line edge roughness (LER), and profile quality will be characterized to
determine tool performance. The new system will also be used to demonstrate multilayer etching capabilities, important
for opaque frame and alternating phase shift applications. A comprehensive summary of the etch performance of various
EUV films and the readiness for manufacturing applications will be provided.
A method is described to monitor etch selectivity real time in Applied Materials' advanced Tetra<sup>TM</sup> mask etcher module.
With the built-in Transmission Endpoint (TEP) capability, the transmission information for a wide range of spectra is
collected. As resist thickness continues to be reduced during photomask etching process, interference fringes can be
observed at selected wavelengths on the TEP spectrum. Based on known value from <i>n & k</i> simulation, the peak/valley
positions of interference fringes can be defined. With the help from an algorithm developed to determine the
corresponding time for each peak/valley position, the average resist etch rate can be obtained. In addition, the starting
and ending resist thickness on the plate being etched can be calculated, so the incoming resist quality can be verified and
being monitored. Combined with Cr etch rate derived from the endpoint time with plasma emission spectra, the Cr to
photoresist etch selectivity can be monitored for each production plate automatically.
Optical emission represents the bulk property of plasma, which in turn can be correlated to the
chamber surface condition and can be exploited for monitoring and characterizing chamber
condition. This presentation demonstrates the approach of utilizing plasma optical emission spectra
(OES) for the application on Applied Materials' Tetra<sup>TM</sup> etcher chamber condition monitor. Time-resolved
plasma optical emission spectra are collected with a spectrometry unit built in to the
TetraTM photomask etch module. Studies on OES analysis show that information related to chamber
surface condition can be correlated to the changes in emission spectrum of plasma. The effectiveness
of this methodology can be verified by Cr etch rates. Results can lead to procedure development for
chamber monitoring, chamber recovery and chamber seasoning applications.
As technology advances with feature size shrinking for the
state-of-the-art integrated circuit (IC) fabrication, the degree
of reduction in critical dimension (CD) features on a photomask shrinks at a faster pace, thanks to the ever aggressive
optical proximity correction (OPC) design. In addition to stringent CD requirement, defect control has also become one
of the most difficult challenges for advanced photomask manufacturing as a result of reduction in printable defect size.
Therefore, keeping a photomask etching chamber at an optimal condition becomes very critical for controlling in both
defectivity and CD fidelity.
In the present study, analyses on optical emission spectrum (OES) collected in an Applied Materials' Tetra<sup>TM</sup> chrome
etch module have been performed to understand (1) the impact of Cr etching on the chamber condition, and (2) the
effectiveness of in-situ chamber dry clean for chamber condition control and potential particle reduction. Results showed
that, with the right selection of chamber materials (to be compatible with process chemistry and etching condition), the
main impact of Cr etching on chamber condition and particle performance is from resist etch-by-products. Various
plasma dry clean chemistries have been explored to address the effectiveness for the removal of such etch-by-products.
As a result, an in-situ chamber clean (ICC) procedure is developed and has been validated to be production-worthy for
desired particle control and chamber stability control.
Requirements to meet the 45nm technology node place many challenges on photomask makers. Resolution Enhancement Techniques (RET), employed to extend optical lithography in order to resolve sub-resolution features have burdened mask processes margins. Also, yield compromises rise with every nanometer of error incurred on the photomask (and device) platforms.
As photomask costs rise, strict performance control is required for all photomask varieties utilized in the mask shop. Mask etching for future technology nodes, requires a system-level data and diagnostics strategy. This necessity stems from the need to control the performance of the mask etcher at increasingly stringent and diverse requirements of the photomask production environment.
From etch applications perspective, alternating phase-shift masks (APSMs) and OPC masks pose key challenges. Specifically, the etcher needs to provide highly uniform CD performance across the entire active area of the photomask - for various feature sizes and load distributions, with no degradation to profiles. It is challenging to strike this balance, yet maintain adequate process window. Future etch systems require sensitive controls and knobs to provide this high precision and repeatable performance. Additionally, incoming variation in plate characteristics and quality necessitate tuning knobs capable of targeting the optimum performance across a diversity of applications.
A robust photomask etching process was studied and developed for 65 nm node photomask production with zero CD process bias. The fabrication process, including pattern generation and transfer do not use data sizing, saving photomask delivery time, improving yield, and reducing fabrication costs. The photomask patterns, without using data sizing cover chrome loads from about 1 percent to 80 percent. For 65 nm critical layer EAPSM, the CD bias of Cr and MoSi etching together is equal to or less than about 20 nm for high and low load photomasks. The etch process and dose adjustment on the 50 keV e-beam writer allow for zero CD process bias, i.e. the data sizing becomes unnecessary in the 65 nm node photomask fabrication. The SMIF pot utilization in both pattern generation and transfer processes significantly improved the defectivity control. Cr and MoSi etch endpoints of 1% load photomasks were clearly detected. Point-to-point CD etch contributions for dark and clear features are 5 nm (3 sigma) or less and final CD value ranges are 8 nm or less. CD etch linearity and other etch properties on SRAF and serif are also discussed. An equation was proposed for calculating phase angle non-uniformity distribution, and phase angle range can be controlled in the range of 1.4±0.3 degree. "Self-mask", i.e. using AR sub-layer as hard mask for beneath chromium sub-layer etch was also discussed.