Keibock Lee, president, Park Systems, and Byong Kim, senior director, Technical Services, Park Systems
Low scaling of devices and scale up in production have meant that metrology is receiving more attention than before. Both the need for high functionality and the value of metrology tools continue to increase. To keep up, metrology tools are expected to improve in four major categories, namely precision, repeatability, throughput and cost of ownership1.
In terms of precision, metrology tools should be compatible with the fabrication process for smaller nodes and larger wafers. Therefore, higher resolution is required more than before. However, higher resolution comes at the cost of throughput, which includes the time required for sample preparation. As a result, achieving lower cost of ownership while maintaining measurement repeatability is the challenge for the latest generation of high-resolution metrology tools. This is also true for surface characterisation.
The chemical mechanical planarisation (CMP) process is used to polish the surface of a wafer, specifically a physical pad and chemical active slurry remove the wafer’s topography and control its surface flatness to the sub-nanometre2. CMP plays a significant role in shallow trench isolation (STI) as well as trenched metal interconnection in the damascene process. However, CMP is a blind technique, which makes it harder to know if the desired amount of material is removed. Therefore, metrology tools such as surface profilers and atomic force microscopes are required for monitoring surface characterisation.
Surface profilers have been in use for longer than atomic force microscopes and are capable of both surface profiling and stress measurement. The typical in-plane range of measurement is a few hundred millimetres scale with an out-of-plane range of up to 1 mm. An in-plane resolution of tens of nanometres and out-of-plane resolution at the angstrom level are typical specifications of surface profilers. Therefore, it would be challenging for surface profilers to provide sub-angstrom surface roughness measurement over dielectric or polysilicon materials, perform deep trench measurements, or detect defects of a few nanometres in lateral dimension.
Atomic force microscopes have been used in fabs for over a decade. They have typically been used to measure lateral dimensions of below 70 μm with out-of-plane dimensions of around 10 μm. Their major application has been monitoring surface roughness, step height and critical angle measurements in etch, deposition and CMP processes. A major challenge of using atomic force microscopes has been the noise level in fabs. They traditionally employ piezo tube tip scanning systems, which are associated with a background out-of-plane motion that has to be compensated or filtered from the images. Tapping mode is used as the standard imaging mode, but this results in a shortened tip life and challenges in repeatability.
In order to achieve the required higher resolution of atomic force microscopy (AFM) and larger measurement range of surface profiling, a hybrid tool called the atomic force profiler was introduced. However, the conventional AFP has the associated limitations of conventional tube-based AFM systems3.
This paper introduces the in-line Park NX-Wafer atomic force microscope, which has been developed to address the aforementioned limitations associated with traditional atomic force microscopes. This system is based on decoupled XY and Z scanners, designed to eliminate the cross-talk between the scanners and provide better positioning capabilities, which means it can also be used for automatic defect review (ADR). The decoupled XY scanner allows additional capabilities such as improved optical vision, especially for wafers after the CMP process and unpatterned wafers with small defects. The Park NX-Wafer can perform measurements for 300 mm wafers and extreme ultraviolet light (EUV) reticles.
Profiling with decoupled flexure-guided XY and Z scanners
The Park NX-Wafer uses a decoupled flexure-guided XY scanner to address the limitations in lateral scan range of conventional AFM and the significant out-of-plane background. The XY scanner’s main plate is moved by two pairs of stacked piezo actuators in two in-plane directions. The background out-of-plane motion of the XY scanner is below 2 nm for a 100 μm scan range. The reduced background out-of-plane motion allows for surface profiling of up to 100 μm ranges using only the XY scanner. Figure 1 shows an example of small-scale atomic force profiling (AFP) using the XY scanner.
Figure 1:
An example of small-scale atomic force profiling (AFP) using the XY scanner. The range is 100 µm and the step height is 160 nm. The inset plot shows a magnified section of the same profile.
The Park NX-Wafer uses a decoupled flexure-guided Z scanner to control the Z movement. The Z scanner’s decoupled flexure-guided design and lowered mass allow for increased resonance frequency by about nine-fold over traditional tube scanners. This has multiple advantages. First, the straightness of the Z scanner is preserved as its movement is independent of the XY scanner offset, unlike traditional tube scanners. Second, the smaller dimension of the Z scanner allows for the incorporation of a direct on-axis optical camera, ultimately enabling enhanced optical vision for AFM. Third, the higher resonance frequency of the Z scanner allows a faster response and fast scanning functionality.
Long-range profiling
The Park NX-Wafer features a sliding stage beneath the XY scanner, which enables it to be used for long-range AFP. The sliding stage provides surface profiling using the decoupled Z scanner over millimetre-scale ranges rather than µm-scale ranges like a conventional AFM scanner. The decoupled Z scanner provides an engaging orthogonal angle with less than 0.015 percent deviation. No special software algorithm is required to maintain the orthogonality of the Z scanner, unlike some conventional AFM scanners.
A common long-range AFP application is monitoring dishing and erosion of copper (Cu)-based structures on a patterned wafer, as shown in figure 2. There is a polishing rate difference between Cu and silicon, which leads to disproportionality in the polished amount on various areas on the wafer. AFP can be used to accurately measure dishing and erosion of the structures after the CMP process. AFP exerts minimal force during the measurement compared with other techniques, thus avoiding damage to the sample. It also provides high resolution in both vertical and lateral directions.
Figure 2:
AFP of erosion and dishing of copper (Cu)-based structures on a patterned wafer. The plot on the right shows dishing and erosion measurement of a 9 µm/1 µm structure using a 2 mm line profile. The inset plot on the left shows a zoomed area from the profile on the right, highlighting the high resolution of the collected AFP data.
Another long-range AFP application is monitoring wafer edge, as shown in figure 3. One of the objectives in foundries is to utilise the most area on the wafer, which means the information on the quality of devices at the edge of the wafer become crucial for both process engineers and process tool providers.
Figure 3:
AFP of a wafer edge. The AFP image is 3 x 3 mm. Two line profiles are shown in the middle plots. The plots on the right show magnified height scale profiles.
Conclusion
This article provides an overview of the Park NX-Wafer atomic force microscope, which has been designed around decoupled flexure-guided scanners. The Park NX-Wafer is capable of performing non-contact mode imaging for applications such as surface roughness measurement. The system’s enhanced vision allows it to achieve better positioning on highly polished patterned wafers and be used in automatic defect review. The decoupled XY scanner has minimised out-of-plane background motion and can be used for small-scale surface profiling up to 100 μm. A sliding stage enables long-range profiling, namely in the millimetre-scale ranges. Long range profiling is used for various applications such as monitoring CMP of Cu pads or wafer edge bevel measurement. These examples demonstrate how AFP is a valuable in-line reference metrology tool in today’s fabrication processing.
Park Systems
References
1Smith, G. T. (2002). Industrial Metrology: Surfaces and Roundness. London: Springer.
2Krishnan, M., Nalaskowski, J. and Cook, L. (2010). Chemical Mechanical Planarization: Slurry Chemistry, Materials, and Mechanisms. Chemical Reviews, volume 110, issue 1, pp.178–204.
3Cunningham, T., Serry, F., Ge, L., Gotthard, D. and Dawson, D. (2000). Atomic force profilometry and long scan atomic force microscopy: new techniques for characterisation of surfaces. Surface Engineering, volume 16, issue 4, pp.295–298.