Defects from substrate particles depend on the sputter deposition process
11/01/2000
P.B. Mirkarimi, S.L. Baker, M.A. Wall, P.A. Kearney, Lawrence Livermore National Laboratory, Livermore, California
D.G. Stearns, OS Associates, Mountain View, California
 Figure 1. Schematic diagram illustrating the difference between near-normal and off-normal incidence deposition. |
overview
With successive IC generations, thin-film defects nucleated by small particles become a greater concern. We have observed that the angle of incidence of the deposition flux can have a major effect on the evolution of film defects nucleated by small substrate particles. Using a new technique to deposit small spheres on Si wafers, we observed that a significant reduction in defect size occurs when near-normal-incidence ion-beam sputtering is used to deposit thin films, and a significant increase in defect size occurs when off-normal-incidence ion-beam sputtering or conventional magnetron sputtering is used to deposit the films.
In order to make faster integrated circuits (ICs), it is necessary to increase transistor density and hence shrink feature sizes. Thin-film defects nucleated by small particles therefore become a greater concern [1-3]. One example is the reflective reticle for extreme ultraviolet lithography (EUVL), the leading next-generation lithography technique, where particles as small as approximately 25nm in diameter on the reticle substrates have the potential to result in reticle film defects that could print at the wafer [4, 5]. Other examples of problems created by small particles include absorber thickness and composition variations in attenuated phase shift reticles, interconnection opens or shorts in metallization processes, and bad sectors and disk failures in high-density magnetic storage disks.
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Ion-beam sputtering of Mo/Si multilayer films has been shown to result in films with significantly lower defect densities than magnetron sputtering [6]. This is presumably due to lower particulate emission during the deposition process. This could in turn be due to the particulate generation mechanisms in magnetron sputtering that are not active in ion-beam sputtering. An example is target nodule formation [7, 8]. Another difference between ion-beam sputtering and conventional magnetron sputtering is that the deposition flux is much more directional in ion-beam sputtering. The evolution of isolated film defects nucleated by small particles has to our knowledge not been investigated. We have found that this parameter can have a major impact on defect evolution.
The experimental technique
We have recently developed a technique to deposit Au nanospheres of uniform size directly onto silicon wafers [9], and this has enabled us to investigate the nucleation and growth of thin films on substrate particles of approximately 20-60nm in diameter. Particles of size <60nm are of particular concern, since these particles are undetectable by commercial optical inspection tools. Mo/Si multilayer films were chosen for these investigations for the following reasons: 1) the time evolution of the film growth can be observed more clearly in transmission electron microscopy when a multilayer structure is employed; 2) Mo/Si films are a critical component of the reflective reticles to be used in EUVL [10], and similar film systems like MoSix are being used in attenuated phase shift masks for more conventional optical lithography tools [11]; 3) Mo/Si films have been well studied [12]; and 4) there is little or no reaction between Au spheres and Mo/Si films [9].
We have also deposited a pure amorphous silicon (a-Si) film on gold nanospheres to ensure that similar defect evolution results are observed with homogeneous (i.e., nonmultilayer) films.
Ion-beam-sputtered films
Silicon wafers were prepared with Au spheres approximately 30nm and 60nm in diameter and coated with ion-beam-sputtered films under two sets of conditions of the depositing flux relative to the substrate. These are: 1) near-normal incidence, where the flux is incident at an angle approximately 8° from the substrate normal; and 2) off-normal incidence, where the flux is approximately 48° from the substrate normal. (The angles were chosen because of their suitability with the particular equipment used.) This is illustrated in Fig. 1. The results for the vertical height of the resulting bump at the surface of the multilayer ("defect height") as well as the volume of this bump ("defect volume") are shown in Table 1. The data show that there is a significant reduction in defect height and volume when near-normal ion-beam sputtering is employed, and almost no reduction in height coupled with a significant increase in defect volume when off-normal ion-beam sputtering is used. It is likely that the observed differences are primarily due to shadowing effects, which can occur even though the substrates were spun about their normal during deposition of the films to reduce such effects.
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Cross-sectional transmission electron microscopy (XTEM) was performed on a 60nm Au sphere coated with Mo/Si at off-normal incidence, and relatively clear images were obtained as shown in Figs. 2a and 2b. The images show a significant disturbance of the multilayer structure for off-normal deposition and are consistent with the atomic force microscope (AFM) results at the surface. XTEM of Mo/Si multilayers films deposited at near-normal incidence on 60nm Au spheres indicates significant smoothing, and this is consistent with the AFM results. This is illustrated by the XTEM image in Fig. 3.
 Figure 3. XTEM image of an ion-beam-sputtered Mo/Si multilayer deposited at near-normal incidence on a 60nm-dia. gold nanosphere. |
Magnetron-sputtered films
Magnetron sputtering is a commonly used technique to deposit thin films for a variety of applications, including metallization, diffusion barriers, and absorber layers on reticles. It is also widely used to deposit thin films for magnetic disk drives. Unless the distance between the magnetron sputter source and substrate is unusually large, it is expected that there will be significant off-normal components in the incoming deposition flux, and based on the above findings, one would expect the defect volume to increase when coated with magnetron-sputtered films. This experiment was performed, and as expected, the defect volume increased substantially when a 0.28mm-thick magnetron-sputtered Mo/Si film was deposited on 50nm-dia. Au spheres. This is shown by the XTEM image in Fig. 4, and the data are shown in Table 2. The observed defect volume is approximately an order of magnitude larger than the initial defect volume, where the initial defect volume is taken as the volume of a gold sphere. The surface topography of films deposited by magnetron sputtering and near-normal-incidence ion-beam sputtering is compared in Fig. 5. Again, it is clear that the magnetron-sputtering process results in significantly less smoothing than the near-normal ion-beam-sputtering process.
 Figure 4. XTEM image of a magnetron-sputtered Mo/Si multilayer film deposited on a 50nm-dia. gold nanosphere. |
There is nothing to suggest that the above findings are particular to multilayer thin films, but just to be sure, a magnetron-sputtered amorphous Si (a-Si) film was also deposited on 50nm-dia. Au spheres. The height and volume of the defect at the surface after coating is similar to that observed for magnetron-sputtered Mo/Si, and both sets of values are listed in Table 2.
Collimated magnetron sputtering has been demonstrated to be very useful in IC fabrication for the coating of high-aspect features such as vias [13, 14]. If the directionality of the deposition flux is causing defect amplification in magnetron-sputtered thin films, then there is the possibility that a technique such as collimated magnetron sputtering would reduce this effect. This may be investigated in the future.
Conclusion
The angle of incidence of the deposition flux can have a major effect on the evolution of film defects nucleated by small substrate particles. A significant reduction in defect size occurs when near-normal-incidence ion-beam sputtering is used to deposit thin films, and a significant increase in defect size occurs when off-normal-incidence ion-beam sputtering or conventional magnetron sputtering is employed to deposit the films. The results are consistent with shadowing effects causing the defects to amplify when the deposition flux becomes off-normal. This work indicates that for applications where small thin-film defects are a concern, near-normal-incidence ion-beam sputtering should be given serious consideration over deposition techniques such as conventional magnetron sputtering or off-normal-incidence ion-beam sputtering.
Acknowledgments
The authors thank F. Grabner for depositing the magnetron-sputtered films, and J. Conner and P. Mangat at Motorola for the XTEM image shown in Fig. 4. This work was performed under the auspices of the US Department of Energy by the University of California Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48. Funding was provided by the Extreme Ultraviolet Lithography Limited Liability Corp. under a Cooperative Research and Development Agreement.
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Paul B. Mirkarimi received his BS in engineering physics from the University of Illinois, Urbana-Champaign, in 1987, and his PhD in materials science and engineering from Northwestern University in 1993. He conducted thin film R&D at Sandia National Laboratories before becoming a scientist at LLNL in 1997. Lawrence Livermore National Laboratory, MS L-395, 7000 East Ave., Livermore, CA 94550; ph 925/423-4848, fax 925/423-1488, e-mail [email protected].
Sherry L. Baker received her AS degree in electron microscopy from San Joaquin Delta College in 1983. She has worked at LLNL since 1984, and her current focus is on the use of atomic force microscopy in materials R&D for EUVL.
Mark A. Wall received his AS degree in electron microscopy from San Joaquin Delta College in 1981. Since 1983, he has worked at LLNL, where his current focus is on the use of transmission electron microscopy in materials R&D.
Patrick A. Kearney received his BS degrees in physics and mathematics from the Rose-Hulman Institute of Technology in 1987, and his PhD degree in optical sciences from the University of Arizona in 1994. At LLNL, he is working on the synthesis of low-defect-density multilayer thin films for reticles in EUVL.
Daniel G. Stearns received his BA degree from the University of Michigan in 1978, and his PhD in applied physics from Stanford University in 1984. From 1984 to 1996, he was a scientist at LLNL. In 1997 he founded OS Associates, which provides consulting services in the optical sciences.