MDZ Flatness and Nanotopography

With the implementation of CMP planarization processes for Shallow Trench Isolation (1,) the nanotopography of the silicon wafer is becoming a more significant factor to consider. Nanotopography is defined as "the deviation of a surface within a spatial wavelength of around 0.2 to 20 mm."(2) This is a parameter that measures the front-surface, free-state topology of an area which can range in size from fractions of a millimeter to tens of millimeters. In this sense, nanotopography differs from front-referenced site flatness in that for nanotopography the wafer is measured in a free state, while for flatness it is referenced to a flat chuck. A wafer may have perfect flatness (in the classical definition of flatness), yet still have nanotopography. If a wafer has surface irregularities on the front and backside of the wafer, but front and back surfaces are parallel, the wafer has perfect flatness. However, the same wafer will exhibit nanotopography (Figure 1). Nanotopography bridges the gap between roughness and flatness in the topology map of wafer surface irregularities in spatial frequency (Figure 2). As linewidths shrink, with non-uniform pattern density and with the use of hard pads for CMP, nanotopography may significantly degrade in dielectric film uniformity (3,4.)

Figure 1

Figure 2

Measurement

Nanotopography is measured by two techniques: light scattering and interferometery. Light scattering tools typically employed for particle and surface-defect characterization can be used to measure the local slope change over the entire surface of the wafer. The local slope change may be integrated to yield height or topography information. Since the beam size can be on the order of fractions of a micron, nanotopography can be measured. Optical interference measurement is straightforward: a beam is split into two components; one component is reflected from the wafer surface, while the second is reflected from a reference mirror; the interference of the combination of the two beams is a measure of the topology of the wafer surface. With both techniques, signal filtering is used to separate the low-wavelength features (e.g. warp) so that only the high-wavelength/low-frequency information, i.e. the true surface nanotopography is measured.

Role in CMP

The interaction between nanotopography upon film removal uniformity in CMP has been under extensive investigation by Boning and co-workers(5,6,7,8) and Tamura et al.(9) The primary effect of oxide uniformity removal is due to the hardness of the CMP pad. The fundamental concept is very simple: soft polishing pads conform to local topology variations (i.e. nanotopography) while hard pads do not. Figure 3, adapted from Boning et al.(6), illustrates this principle. Typically, a wafer has a characteristic nanotopography length (NL, shown in the top illustration of Fig. 3). The soft pad will conform over the nanotopography and maintain a uniform film. The hard pad will not conform to the nanotopography and produce a non-uniform film with high spots on the wafer surface having a thinner film and low spots having a thicker film. Traditionally soft pads have been used for film removal CMP. However, with the need for better planarization, because of more layers, smaller CD and for multi-function logic devices which have several different areas of varying pattern densities(10), stiff pads are required. To some extent, the effect of nanotopography can be minimized by using polishing additives, such as ceria particles(11). Nonetheless wafer nanotopography becomes increasingly important.

Figure 3

To understand the influence of nanotopography in CMP film removal uniformity, the concept of planarization length should be considered. The planarization length (PL) is the spatial length at which polishing cannot reduce the step height of a feature in the film thickness. This is illustrated in Figure 4 (below). The important aspect to consider is when PL is less than NL the film uniformity is maintained. When PL is more than NL one finds non-uniform film removal. Two typical examples are shown in Figure 3. The CMP process and the film uniformity specifications may be considered to determine the level of nanotopography required.

Figure 4

Impact of the Wafer

Nanotopography of the silicon wafer is dictated to a large extent by the polishing process. For single-sided polished (SSP) wafers, the polishing process has been optimized to minimize nanotopography. In this process, to achieve good flatness, the wafer must be mounted or chucked against a flat reference block. Since the wafer backside is etched (not polished smooth), it has surface topology. Because of the fixturing process used to mount the wafers (e.g. wax mounting or vacuum chucking), the topology of the backside of the wafer and the fixturing surface and or adhesive/wax are transmitted to the front side and causes nanotopography. The other technique of mounting a wafer (the one that is normally used in CMP), viz. free mounting, does not cause nanotopography formation, but also does not guarantee the wafer is made flat. Since flatness can also be critical for device manufacture (lithography), it is difficult to produce good, low nanotopography and good, low flatness with conventional SSP.

Summary & References

Nanotopography can cause local film removal non-uniformity if a hard pad CMP process is used to planarize a wafer for STI. Ideally, all other factors held constant, one should consider the use of DSP wafers for critical applications where flatness and nanotopography must be satisfied. This will become more important as device CDs shrink and both CMP processing and lithography process steps become yield limiting.

References

  1. J. Schlueter, "Trench Warfare: CMP and Shallow Trench Isolation," Semiconductor International, October 1999.
  2. SEMI DRAFT Document 3089: Guide for Reporting Wafer Nanotopography.
  3. K. V. Ravi "Wafer Flatness Requirements for Future Technologies," Future Fab International, Issue 7, 207.
  4. C. Shan Xu, E. Zhao R. Jairath and W. Krusell, Electrochemical and Solid-State Letters, 1 (4) 181 (1998).
  5. B. Lee, D. Boning, W. Baylies, N. Poduje, P. Hester, Y. Xia, J. Valley, C. Koliopoulus, D. Hetherington, H. Sun, M. Lacy, "Wafer Nanotopography Effects on CMP: Experimental Validation of Modeling Methods," Materials Research Society (MRS) Spring Meeting, San Francisco, CA, April 2001.
  6. D. Boning, B. Lee, W. Baylies, N. Poduje, P. Hester, J. Valley, C. Koliopoulos and D. Hetherington, "Characterization and Modeling of Nanotopography Effects on CMP," International CMP Symposium 2000, Tokyo, Japan, Dec. 4, 2000.
  7. C. Oji, B. Lee, D. Ouma, T. Smith, J. Yoon, J. Chung, and D. Boning, "Wafer Scale Variation of Planarization Length in Chemical Mechanical Polishing," J. Electrochem. Soc. 147 (11) 4307, Nov. 2000.
  8. B. Lee, T. Gan, D. Boning, P. Hester, N. Poduje, and W. Baylies, "Nanotopography Effects on Chemical Mechanical Polishing for Shallow Trench Isolation," Advanced Semiconductor Manufacturing Conference, Boston, MA, Sept. 2000.
  9. N. Tamura, H. Niwa, M. Hatanaka, M. Kase, and T. Fukuda (Fujitsu Limited), "The Influence of Wafer Nanotopology on Residual Film Thickness Variation after Chemical Mechanical Planarization," 197th ECS Meeting, Toronto, Ontario, Canada, May 2000.
  10. T. Tugbawa, T. Park, B. Lee, D. Boning, P. Lefevre, and L. Camilletti, "Modeling of Pattern Dependencies for Multi-Level Copper Chemical-Mechanical Polishing Processes," Materials Research Society (MRS) Spring Meeting, San Francisco, CA, April 2001.
  11. B. Lee, D. Boning, L. Economikos, "A Fixed Abrasive CMP Model," Chemical Mechanical Polish for ULSI Multilevel Interconnection Conference (CMP-MIC 2001), pp. 395-402, Santa Clara, March 2001.