Atomic Force Microscopes or AFMs have been used extensively for research purposes as well as a metrology tool in industry. Height measurement by AFM is one of the key applications in metrology.
The different types of height measurement are:
- Simple step height measurement.
- CMP dishing measurement.
- Pole tip recession (PTR) measurement for hard disk read/write head.
- Trench depth measurement, etc.
AFMs are also used for surface roughness measurements. However, as most AFMs operate in a top-down configuration, the AFM has a restricted access to the sidewall. This is true when the sidewall angle is close to, or greater than 90°. Some AFMs can use the side of a flared tip to image the sidewall. However, the flared tip is quite blunt when compared to the sharp conical tip and the resolution on the sidewall is reduced.
Furthermore, a flared tip is much more difficult to make and its quality is hard to control. A temporary solution has been used to characterize the sidewall roughness by cleaving the sample mechanically and then imaging the sidewall with AFM on tilted samples with a regular conical tip.
Recently, Park Systems has introduced a new Park 3D AFM imaging technology using a tilted Z scanner with a sharp conical tip for imaging. The tilting of the tip, helps in reaching the sidewall of the sample features, even when the sidewall angle is close to, or more than 90°. This new technology is very powerful for sidewall roughness measurements. In this paper, the throughput and repeatability of this new Park 3D AFM technology as a metrology solution for sidewall roughness is discussed.
Park 3D AFM With Tilted Z Scanner
The new Park 3D AFM is based on a decoupled XY and Z scanning system. In this decoupled scanning configuration, the XY scanner is a 2D flexure scanner. The XY scanner moves the sample only in the XY direction and is independent from the Z scanner. The Z scanner is driven by a high force multi stack piezo element that moves the tip only in the Z direction as shown in Figure 1-A. The independent Z scanner allows the tip to be intentionally tilted to easily access the sidewall as shown in Figure 1-B. The tilted scanner design enables CD measurement at the top, middle, and bottom of the lines, as well as roughness measurement along the line sidewall. The method builds upon the standard AFM tip design resulting in a method that a) maintains the same resolution as the conventional AFM, b) can be used with sharpened conical tips for best image resolution and c) does not suffer from corner inaccessibility from limited shape of tips with large radius.
Figure 1. Decoupled XY and Z scanning system (A). 3D AFM using tilting Z scanner (B).
To take a full 3D image, the AFM typically scans at three tilted positions: center, left and right and correspondingly acquires three images. Each of these three images offer unique information. For example, the image acquired at the left tilted position offers comprehensive information on the left sidewall, as well as the top and bottom surface. These three images can be integrated together to form a complete three-dimensional image of the feature as shown in Figure 2. The total image offers detailed 3D information of the feature that is imaged, such as the top CD, middle CD, bottom CD, LER, LWR, sidewall angle, etc.
Figure 2. Imaging with Park 3D AFM.
Sidewall Roughness Measurement
The Park 3D AFM can also use the regular sharp conical tip for AFM imaging. The tip sharpness determines the resolution. When compared with other types of tips such as cylindrical tip or flared tip, the conical tip has high sharpness that enables Park 3D AFM to offer excellent resolution for the sidewall imaging. Figure 3 shows a Park 3D AFM image of a photoresist line. This image was taken by scanning in the direction along the line. The scan resolution is 256 pixels in the X direction and 512 pixels in the Y direction. This image shows high resolution information on both the top and bottom flat surfaces and the sidewall. On the sidewall, the very rough and detailed grainy surface of the photoresist is visible, while the top and bottom surfaces are very smooth.
Figure 3. 3D AFM image of the sidewall of a photoresist line.
The high resolution 3D AFM image allows to extract single line profiles on the top, sidewall and bottom surface from . Figure 4 shows the single line profiles at distinct surfaces of the photoresist. These line profiles also show that the sidewall is rougher than the top and bottom surfaces.
Figure 4. Profiles of a photoresist line at different surfaces.
This Park 3D AFM sidewall image of the photoresist offers comprehensive information about the sidewall surface when compared to conventional metrics (e.g. LER). This information is beneficial for photoresist research and photolithography process development.
Park 3D AFM with sharp conical tip not only offers a high resolution image of the sidewall, it also measures the sidewall roughness with excellent accuracy. Repeatability was tested with five sites and ten repeats. Table 1 shows the testing results. The average sidewall RMS roughness of the tested photoresist sidewall is about 6.1 nm. The standard deviation (STD) of the sidewall roughness measurement is about 0.20 nm or better for each individual site. And for the 5-site wafer average sidewall roughness, the measurement STD is 0.08 nm, which is only about 1.3% of the measured roughness.
Table 1. 5-site, 10-repeat data of sidewall roughness measurement.
Figure 5 shows the 5-site average roughness of all the ten repeats. The plot shows that the roughness is very consistent over the ten repeats and that there is no pattern showing it is increasing or decreasing during the repeated measurements. This means that the tip remains as sharp as it was before the 50 measurements were done. Meanwhile, the soft photoresist was damaged only after ten repeated measurements. This is expected since the AFM is operated in non-contact mode.
Figure 5. 5-site average roughness of 10 repeats.
While conducting the repeatability test, each image was scanned for 32 lines along the sidewall direction, each line has 512 data points. The image scan range is 0.5 ìm across the photoresist line and 3 ìm along the photoresist line. Each measurement took about two minutes. Of these two minutes, the image scanning took about one minute for each image (32 lines at 0.5 Hz). The sample navigation and alignment took another one minute, which included the stage moving, optical pattern recognition, AFM alignment and tip approaching.
This throughput can be enhanced further. 32 lines were scanned over 0.5 ìm across the photoresist line, and about half of the line was on the top or bottom surface. Sixteen lines over 0.3 ìm were enough to cover the sidewall surface with a decrease in the line density on the sidewall. By reducing the scan range along the photoresist line from 3.0 ìm to 1.5 ìm, the tip could scan at 1.0 Hz without compromising the surface tracking at the same settings. Hence, the scanning time for each image could be reduced to 16 seconds. With further optimization of the materials and procedure, the sample navigation and alignment time can be further minimised to about 45 seconds, yielding a throughput of about 1 minute/site.
Park 3D AFM using a tilted Z scanner with sharp conical tip offers high resolution sidewall imaging. It is also non-destructive and provides high throughput measurements. Park 3D AFM can accurately measure the sidewall roughness with measurement STD of 0.08 nm (5 site average) for the sidewall roughness of about 6.1 nm. Park 3D AFM can be used in applications, such as Fin-FET Device Characterization, EUV Photoresist Evaluation, Etching Process Fine Tuning, Photolithography Process Development, OPC Optimization, and OCD Calibration, among others.
This information has been sourced, reviewed and adapted from materials provided by Park Systems Inc.
For more information on this source, please visit Park Systems Inc.