Claude Montcalm

Multilayer reflective coatings for extreme-ultraviolet lithography

Multilayer mirror coatings which reflect extreme UV (EUV) radiation are a key enabling technology for EUV lithography. So/Si multilayers with reflectances of 67.5 percent at 13.4 nm are now routinely achieved and reflectances of 70.2 percent at 11.4 nm were obtained with Mo/Be multilayers. High reflectance is achieved with careful control of substrate quality, layer thicknesses, multilayer materials, interface quality, and surface termination. Reflectance and film stress were found to be stable relative to the requirements for application to EUV lithography. The run-to- run reproducibility of the reflectance peak position was characterized to be better than 0.2 percent, providing the required wavelength matching among the seven multilayer- coated mirrors used in the present lithography system design. Uniformity of coating was improved to better than 0.5 percent across 150 mm diameter substrates. These improvements in EUV multilayer mirror technology will enable us to meet the stringent specifications for coating the large optical substrates for our next-generation EUV lithography system.

EUV Engineering Test Stand

The Engineering Test Stand (ETS) is an EUV laboratory lithography tool. The purpose of the ETS is to demonstrate EUV full-field imaging and provide data required to support production-tool development. The ETS is configured to separate the imaging system and stages from the illumination system. Environmental conditions can be controlled independently in the two modules to maximize EUV throughput and environmental control. A source of 13.4 nm radiation is provided by a laser plasma source in which a YAG laser beam is focused onto a xenon-cluster target. A condenser system, comprised of multilayer-coated mirrors and grazing-incidence mirrors, collects the EUV radiation and directs it onto a reflecting reticle. A four-mirror, ring-field optical system, having a numerical aperture of 0.1, projects a 4x-reduction image onto the wafer plane. This design corresponds to a resolution of 70 nm at a k1 of 0.52. The ETS is designed to produce full- field images in step-and-scan mode using vacuum-compatible, one-dimension-long-travel magnetically levitated stages for both reticle and wafer. Reticle protection is incorporated into the ETS design. This paper provides a system overview of the ETS design and specifications.

Advances in multilayer reflective coatings for extreme-ultraviolet lithography

Multilayer mirror coatings which reflect extreme UV (EUV) radiation are a key enabling technology for EUV lithography but must meet stringent requirements in terms of film quality, stability, and thickness control across multi optical elements up to 300 nm in diameter. Deposition technology has been dramatically improved to meet those specifications for thickness control and repeatability over large curved optical substrates. Coating uniformity was improved to +/- 0.055 percent peak-to-valley (P-V) on 140- mm flats and +/- 0.1 percent P-V across 160 mm curved substrates. the run-to-run reproducibility of the reflectance peak wavelength was improved to 0.13 percent on flats to enable fabrication of wavelength-matched sets of optics. Multilayers with reflectances of 67.5 percent at 13.42 nm and 70.2 percent at 11.34 nm are typically achieved for Mo/Si and Mo/Be multilayers, respectively. Also, we have recently achieved a reflectance of 70.1 percent at 13.5 nm for a Mo/Si multilayer deposited with a modified process. The reflectance and stress of these multilayers appear to be stable relative to the requirements for application to EUV lithography. These improvements in EUV multilayer mirror technology enable us to meet the stringent specifications for coating the large optical substrates for our next- generation EUV lithography system. The primary remaining issues are improving the run-to-run wavelength repeatability on curved optics to realize the maximum optical throughput, and verifying long-term stability of the multilayers within the environment of a production EUV lithographic system.

System integration and performance of the EUV engineering test stand

The Engineering Test Stand (ETS) is a developmental lithography tool designed to demonstrate full-field EUV imaging and provide data for commercial-tool development. In the first phase of integration, currently in progress, the ETS is configured using a developmental projection system, while fabrication of an improved projection system proceeds in parallel. The optics in the second projection system have been fabricated to tighter specifications for improved resolution and reduced flare. The projection system is a 4-mirror, 4x-reduction, ring-field design having a numeral aperture of 0.1, which supports 70 nm resolution at a k1 of 0.52. The illuminator produces 13.4 nm radiation from a laser-produced plasma, directs the radiation onto an arc-shaped field of view, and provides an effective fill factor at the pupil plane of 0.7. The ETS is designed for full-field images in step-and-scan mode using vacuum-compatible, magnetically levitated, scanning stages. This paper describes system performance observed during the first phase of integration, including static resist images of 100 nm isolated and dense features.

Advances in the reduction and compensation of film stress in high-reflectance multilayer coatings for extreme-ultraviolet lithography

Due to the stringent surface figure requirements for the multilayer-coated optics in an extreme UV (EUV) projection lithography system, it is desirable to minimize deformation due to the multilayer film stress. However, the stress must be reduced or compensated without reducing EUV reflectivity, since the reflectivity has a strong impact on the throughput of a EUV lithography tool. In this work we identify and evaluate several leading techniques for stress reduction and compensation as applied to Mo/Si and Mo/Be multilayer films. The measured film stress for Mo/Si films with EUV reflectances near 67.4 percent nm is approximately -420 MPa, while it is approximately +330 MPa for Mo/Be films with EUV reflectances near 69.4 percent at 11.4 nm. Varying the Mo-to-Si ratio can be used to reduce the stress to near zero levels, but at a large loss in EUV reflectance. The technique of varying the base pressure yielded a 10 percent decrease in stress with a 2 percent decrease in reflectance for our multilayers. Post-deposition annealing was performed and it was observed that while the cost in reflectance is relatively high to bring the stress to near zero levels, the stress can be reduced by 75 percent with only a 1.3 percent drop in reflectivity at annealing temperatures near 200 degrees C. A study of annealing during Mo/Si deposition was also performed; however, no practical advantage was observed by heating during deposition. A new non-thermal buffer-layer technique was developed to compensate for the effects of stress. Using this technique with amorphous silicon and Mo/Be buffer-layers it was possible to obtain Mo/Be and Mo/Si multilayer films with near zero net film stress and less than a 1 percent loss in reflectivity. For example a Mo/Be film with 68.7 percent reflectivity at 11.4 nm and a Mo/Si film with 66.5 percent reflectivity at 13.3 nm were produced with net stress values less than 30 MPa.

Molybdenum-strontium multilayer mirrors for the 8-12-nm extreme-ultraviolet wavelength region.

Mo-Sr multilayer mirrors were successfully deposited by dc-magnetron sputtering and characterized in situ with synchrotron radiation. Normal-incidence (3.6 degrees ) reflectance of 23.0% at 8.8 nm, 40.8% at 9.4 nm, and 48.3% at 10.5 nm were measured before the samples were exposed to air. After exposure, as a result of the reactivity of Sr with oxygen and water vapor, the reflectance of these multilayers decreased rapidly. Attempts to use thin layers of C to passivate the surface of these Mo-Sr multilayers were unsuccessful.

EUV scattering and flare of 10X projection cameras

Two new Schwarzschild cameras have been fabricated for the EUV 10x microstepper. The surface topography of the mirrors was characterized over the full range of spatial frequencies both before and after multilayer coating. EUV scattering from the individual mirrors was measured and compared with the surface profilometry. A knife-edge test was used to directly measure the flare of the assembled cameras. The flare measured in this way is in excellent agreement with the contrast of isolated printed lines and with the point spread function of the camera as determined by EUV interferometry. The measured flare of the camera is also in good agreement with the flare calculated from the combined surface profile measurements of the individual mirrors. Consistent with the improvements made in the surface finish of the mirror substrates, a significant reduction in the flare is observed as compared with previously existing cameras.

Performance of normal-incidence molybdenum-yttrium multilayer-coated diffraction grating at a wavelength of 9 nm

The first experimental investigation of a normal-incidence Mo-Y multilayer-coated diffraction grating operating at a 9-nm wavelength is reported. The substrate is a replica of a concave holographic ion-etched blazed grating with 2,400 grooves/mm and a 2-m radius of curvature. The measured peak efficiency in the -3 order is 2.7% at a wavelength of 8.79 nm. To our knowledge, this is the highest normal-incidence grating efficiency ever obtained in this wavelength region.

Multilayer optics for an extreme-ultraviolet lithography tool with 70-nm resolution

One of the most critical tasks in the development of extreme ultraviolet lithography (EUVL) is the accurate deposition of reflective multilayer coatings for the mirrors comprising the EUVL tool. The second set (Set 2) of four imaging optics for an alpha-class EUVL system has been coated successfully. All four mirrors (M1, M2, M3, M4) were Mo/Si- coated during a single-deposition run with a production- scale DC-magnetron sputtering system. Ideally, the multilayer coatings should not degrade the residual wavefront error of the imaging system design. For the present EUVL camera, this requirement is equivalent to depositing multilayer coatings that would add a figure error of less than 0.11 nm rms. In addition, all mirrors should be matched in centroid wavelength, in order to insure maximum throughput of the EUVL tool. In order to meet these constraints, the multilayer deposition process needs to be controlled to atomic precision. EUV measurements of the coated mirrors determined that the added figure errors due to the multilayer coatings are 0.032 nm rms (M1), 0.037 nm rms (M2), 0.040 nm rms (M3) and 0.015 nm rms (M4), well within the aforementioned requirement of 0.11 nm rms. The average wavelength among the four projection mirrors is 13.352 nm, with an optic-to-optic matching of 1(sigma) =0.010 nm. This outstanding level of wavelength matching produces 99.3% of the throughput of an ideally matched four-mirror system. Peak reflectances are 63.8% (M1), 65.2% (M2), 63.8% (M3) and 66.7% (M4). The variation in reflectance values between the four optics is consistent with their high frequency substrate roughness. It is predicted that the multilayer coatings will not introduce any aberrations in the lithographic system performance, for both static and scanned images of 70 nm - dense features.

MoRu–Be multilayer-coated grating with 10.4% normal-incidence efficiency near the 11.4-nm wavelength

A MoRu-Be multilayer coating was applied to a diffraction grating to enhance the gratings normal-incidence efficiency near 11.4 nm wavelength. The holographic grating substrate had a blazed groove profile with 2400 grooves/mm and a microroughness of 0.8 nm. The efficiency in the second diffraction order, measured with synchrotron radiation, was 10.4% at a wavelength of 11.37 nm.