XE-200/300 - State-of-the-Art Atomic Surface Profiler (ASP) for Industrial Metrology of 200/300 mm Wafers by Park Systems

Topics Covered

About Park Systems
Emergence of Atomic Surface Profiler
High Performance Z-Servo
10 Times Higher Throughput of Sample Measurement
Conclusion

About Park Systems

Park Systems is the Atomic Force Microscope (AFM) technology leader, providing products that address the requirements of all research and industrial nanoscale applications. With a unique scanner design that allows for the True Non-Contact imaging in liquid and air environments, all systems are fully compatible with a lengthy list of innovative and powerful options. All systems are designed with ease-of-use, accuracy and durability in mind, and provide your customers with the ultimate resources for meetiong all present and future needs.

Boasting the longest history in the AFM industry, Park Systems' comprehensive portfolio of products, software, services and expertise is matched only by our commitment to our customers.

Emergence of Atomic Surface Profiler

Atomic Force Microscopy (AFM) is emerging as an essential tool in the semiconductor industry. As aspect ratios become higher, features become smaller, and requirements for planarity tighten, atomic force microscopy has begun to replace profilometry both in development and in production, for topographic measurements such as trench and via depths, step height, and micro-planarity measurements. The main bottleneck of previous AFMs employed as semiconductor metrology tools has been the sampling plan limited by the throughput of the conventional AFM. Park Systems, the original pioneer of AFM technology, has changed the concept of AFM by introducing the XE-200/300 as shown in Figure 1, the completely automated Atomic Surface Profiler (ASP) for industrial metrology of 200 mm/300 mm wafers (See XE-200/300 product datasheets). With its throughput ten times higher than previously available ASP in the market, the represents the state-of-the-art in ASP. Park Systems's mission is to provide industrial customers with advanced ASP products and solutions that extend the critical innovations of the XE to industrial metrology in semiconductor applications such as critical dimensions, chemical mechanical polishing, defect review, surface metrology, and dopant profiling.

With Long Range Profilers (LRP) easily meeting the scan range requirements for semiconductor processing, the XE-200/300 ASP truly represents the state-of-the-art linear translation capability with minimal vertical deviation of less than ±10nm from the reference plane over a 10 mm profiling distance. The LRP is built into the ultra flat vacuum wafer chuck. The scanning mechanism translates the wafer chuck relative to the AFM probe, and causes essentially no degradation of AFM measurement, and has minimal impact on the system's physical configuration.

Figure 1. The XE-200/300 is a completely automated Atomic Surface Profiler (ASP) system for semiconductor metrology of 200/300 mm wafers.

High Performance Z-Servo

The XE scan system is the core feature that gives the XE ASP its competitive edge. XE's innovative scanner design separates the Z-scanner from the XY, as shown in Figure 2, enabling exceptional Z-servo performance and scan accuracy that is unmatched by other ASPs.

The Z-scanner, being separate from the XY-scanner, is designed to have a higher resonant frequency than conventional piezoelectricelectric tube scanners. For this reason, a stacked piezoelectric actuator with a very fast response speed and at least 10 kHz Z-servo bandwidth is used for the Z-scanner. This enables more than 10 times faster scan rates than is possible with a conventional tube type scanner, increasing the speed of the measurements as shown in Figure 3. Since the Z-servo response of the XE scan system is very fast, the probe can precisely follow the steep curvature of a sample without crashing or sticking to the surface, enabling True Non-Contact AFM (See True Non-Contact XE Mode Notes). A majority of sample images by the XE-series AFM are acquired in True Non-Contact AFM. The high performance of the Z-servo not only boosts the speed of a profiling scan, but also protects the tip, resulting in XE's ability to acquire clear, high resolution images for an extended period of time.

Figure 2. The XE Scan System separates the Z-scanner from the XY, enabling exceptional Z-servo performance and scan accuracy. The innovative optical design allows for direct on-axis optical view of a sample from the top.

10 Times Higher Throughput of Sample Measurement

Process control by an ASP has been limited in the past due to its relatively slow throughput and poor reliability. Due to this, the statistical modeling based on the maturity of the process has not been possible. Placing multiple, redundant ASPs in the fab is counter-productive for reasons of cleanroom space and cost-of-ownership.

Figure 3. Comparison of Z-scanner design used in XE-series AFM and conventional piezoelectric tube-based AFM. The XE Z-scanner is a flexure guided high force Z-scanner with Z-servo bandwidth of more than 10 kHz.

Consequently, there are two things to consider when one compares the performance of different ASPs: scan speed and reliability. The expectation is that an ASP accurately measures feature sizes below 0.1 mm, and so it would be unacceptable to sacrifice or compromise its resolution and accuracy for the sake of other attributes. Given the minimum feature size required by the semiconductor industry, the scan speed becomes a critical factor in defining the throughput of an ASP. It is the Z-servo bandwidth that determines the maximum scan speed of an ASP. The scan speed is the operating Z-servo bandwidth multiplied by the minimum feature size as shown in Figure 4. This means that it is the Z-servo bandwidth that limits how fast one can scan in line profiling. In other words, the Z-servo bandwidth has to be high enough so that a cantilever can properly trace the vertical profile in Z while a sample moves in XY. If the Z-Servo bandwidth is not high enough, then sample movement must slow down or the Z-scanner would not have enough time to trace the sample features accurately. This decreases the throughput of line profiling. Therefore, the higher the Z-servo bandwidth, the faster one can scan along the line scan.

The scan speed comparison between XE ASP and a conventional piezoelectric tube scanner based ASP are given in the table below.

 

XE ASP

Piezoelectric Tube Scanner
Based ASP

Practical Scan Speed*

200 µm/sec

20 µm/sec

Time to Scan 3 mm

15 sec

150 sec

Time to Scan 300 mm

1,500 sec (25 min)

15,000 sec (about 4 hours)

* Used one fifth of the maximum Z-servo bandwith possible for each product

So let's compare the Z-servo bandwidth of XE ASP with that of other conventional systems. The Z-servo bandwidth of the XE-200/300 system is at least 10 kHz while that of the piezoelectricelectric tube based systems are at most 1 kHz. It is very unlikely that one can use the full maximum Z-servo bandwidth in practicality, especially for the minimum features as exemplified in Figure 4. One fifth of the full maximum Z-servo bandwidth is a practical bandwidth level during scanning by ASP. Assuming the minimum feature sizes of 0.1 mm as shown in Figure 4, one can calculate the practical scan speed, and subsequent scan time for a fixed length as shown in the table.

As one can observe from the table, it can be seen that one XE ASP can have a throughput equal to ten conventional ASPs. This is a direct consequence of having ten times higher Z-servo bandwidth. As a result, it will take only 15 seconds to scan a 3 mm length with the XE ASP while other conventional ASP systems would require 150 seconds. It would take more than four hours for a conventional ASP to scan a 300 mm line across a wafer. One must wonder what usefulness having a 300 mm line profiling capability serves when the scan time of 300 mm line profiling is so impractical. Besides, it is obvious that optical profiler should be used for measuring global flatness of the whole 200 mm/300 mm wafer. ASPs should be used for local metrology. The XE ASP was designed to profile up to 10 mm, not the entire 300 mm wafer in order to improve the structural rigidity and system stability during local metrology of critical features.

The high performance of the Z-servo bandwidth in the XE ASP came from the revolutionary design of the XE scan system that adopts a separate scanner for the XY plane and Z axis (See Technology Behind the XE-series). Also, the XE's Z-scanner is a flexure guided scanner actuated by a high force stacked piezoelectric; it is no wonder that the Z-servo bandwidth of the XE ASP is ten times higher than the conventional system that uses the now outdated piezoelectric-tube scanner design.

Figure 4. Practical scan speed of an ASP is determined by Z-servo bandwidth multiplied by minimum feature size.

Conclusion

The XE-200/300 represents the state of the art ASP with ten times faster scan speed than conventional ASPs. The revolutionary design of the XE AFM technology provides unprecedented performance in Z-servo speed and zero background curvature. With its throughput ten times higher than previously available ASPs in the market, the XE-200/300 provides the opportunity for additional engineering measurements to improve processing yield.

Source: Park Systems

For more information on this source please visit Park Systems.

Date Added: Feb 12, 2010 | Updated: Sep 19, 2013
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