1514 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 37, NO. 12, DECEMBER 2001 Quantum Electronics Letters________________________________________________ An Ion-Assisted Mo–Si Deposition Process for Planarizing Reticle Substrates for Extreme Ultraviolet Lithography Paul B. Mirkarimi, Eberhard A. Spiller, Daniel G. Stearns, Victor Sperry, and Sherry L. Baker Abstract—Substrate particles are a serious concern in the fabrication of reticles for extreme ultraviolet lithography (EUVL) because they nucleate defects in the reflective multilayer films that can print in the lithographic image. We have developed a strategy for planarizing reticle substrates with smoothing-layers and, in this letter, we investigate the smoothing proper- ties of an ion-assisted Mo–Si deposition process. We have observed that ion-assistance can significantly improve the particle-smoothing properties of Mo–Si multilayer films and can do so without a significant increase in the high-spatial frequency roughness of the multilayer film. An ion-assisted Mo–Si smoothing-layer approach to reticle substrate planarization, there- fore, shows significant promise for defect mitigation in EUVL reticles. Index Terms—Ion beam applications, integrated circuit fabrication, masks, photolithography, thin films. Extreme ultraviolet lithography (EUVL) is the leading candidate for producing feature sizes below 70 nm in the high-volume manufacturing of integrated circuits (ICs) for logic and memory devices [1]–[5]. Un- like previous technologies, it employs a reflective reticle. The reticle for EUVL currently consists of a substrate coated with a Mo–Si mul- tilayer film designed to reflect light with a wavelength of 13–14 nm at near-normal incidence; this multilayer-coated substrate is then pat- terned with an absorber material on the surface of the multilayer to form the reticle [6]. Small particles on the substrate that are coated with a Mo–Si multilayer film can nucleate defects in the reflective multilayer [7], [8] that can print in the lithographic image, thereby degrading the device performance and potentially reducing the device yield. We have developed a strategy for mitigating the effect of small substrate contam- inants that relies on depositing Mo–Si multilayer smoothing-layers to planarize the substrates prior to the deposition of the high-reflectance Mo–Si multilayer coating [7], [9]. In this work, we investigate the use of a secondary ion source to etch Mo and Si layers during deposition to enhance smoothing in our Mo–Si buffer-layer deposition process. Mo–Si films were deposited by ion beam sputtering with and without ion assistance on Au spheres on square Si(100) substrates approxi- mately 2.5 cm 2.5 cm in size. The process for depositing Au spheres on Si(100) substrates is described in detail elsewhere [10]. A schematic diagram illustrating the sequential deposition and etching process used to deposit the ion-assisted Mo–Si coatings is shown in Fig. 1. A se- quential deposition and etching process was chosen primarily because previous work suggested that smoothing is enhanced when the deposi- Manuscript received July 5, 2001; revised August 27, 2001. This work was supported by the Extreme Ultraviolet Lithography Limited Liability Corpora- tion under a cooperative research and development agreement, and also by In- ternational SEMATECH. P. B. Mirkarimi, V. Sperry, and S. L. Baker are with Lawrence Livermore National Laboratory, Livermore, CA 94550 USA. E. A. Spiller is with Lawrence Livermore National Laboratory, Livermore, CA 94550 USA, and also with Spiller X-Ray Optics, Mt. Kisco, NY 10549 USA. D. G. Stearns is with Lawrence Livermore National Laboratory, Livermore, CA 94550 USA, and also with OS Associates, Mountain View, CA 94040 USA. Publisher Item Identifier S 0018-9197(01)10037-0. Fig. 1. Illustration of the procedure used to deposit the ion-assisted Mo–Si coatings. tion flux is at normal or near-normal incidence to the film surface [7], [8]. Also, with a sequential process, a wider range of irradiation angles is possible (such as normal incidence irradiation) with our deposition system geometry. The deposition flux was directed at approximately normal incidence to the substrate. A detailed description of this depo- sition system will be provided in a future publication [11]. A bi-layer is defined as one silicon layer plus one Mo layer, and we deposited ten Mo–Si bi-layers with no etching followed by 40.5 Mo–Si bi-layers with etching. The initial ten bi-layers were deposited without etching to en- sure that the Au spheres were not sputtered. For the ion beam-sputter deposition, the primary ion source beam energy was 800 eV, and for the etching the secondary ion source beam energy was 250 eV; Ar was used as the source gas for both ion sources. The target angle, which is the angle between the direction of the flux from the primary ion source and the target normal, was approximately 60 . Assuming that the highly energetic reflected neutrals would tend to be in the specular direction, this target angle would be expected to result in fewer highly energetic reflected neutrals in the portion of the deposition flux that impinges on the substrate, and would enable one to better quantify the effect of ion-assistance from the secondary ion source. The Mo and Si layers were each etched for 6 s, resulting in approximately 0.35 nm of each Si layer and 0.50 nm of each Mo layer being etched away. The final bi-layer thickness for the etched and unetched coatings was 7 nm. The results for the 50.5 bi-layer period coatings are listed in Table I and the profiles of the top surface are plotted in Fig. 2. It can be seen that the 50 nm diameter Au sphere defect is smoothed to a mean defect height of 2.7 nm with an ion-assisted Mo–Si coating, while the defect is smoothed to a mean defect height of 13.5 nm for the coating with no ion-assistance. The mean defect volume in the ion-assisted Mo–Si coating was 18 000 nm as compared to 52 200 nm in the Mo–Si coating deposited with no ion assistance (and versus 73 000 nm for the uncoated sphere). Thus, while an ion beam sputtered Mo–Si coating provides a significant amount of smoothing, this smoothing is further enhanced by the use of an ion-assisted deposition process. 0018–9197/01$10.00 © 2001 IEEE