Magazine autumn00 why reticle inspection

Lithography S

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Why Reticle Inspection Tools are Required in both Photomask Shops and Wafer Fabs

by Brian J. Grenon, Grenon Consulting, Inc.

The increased demand on mask fabricators to produce photomasks with tighter tolerances and with faster turnaround times has created a greater opportunity for undetected reticle anomalies to find their way into wafer fabs. Most recently, it has been reported that critical dimension (CD) errors and contamination under pellicles have been detected on reticles in the wafer fabs1, 2. For this reason, it is becoming more critical for the mask maker and mask user to have the same reticle characterization tools so potential yield-detracting anomalies can be detected. This approach will help optimize both fab yields and revenues. In order to meet the challenges provided by 130 nm fabrication, “Best-of-Class” metrology and inspection tools are required.

With the advent of 130 nm design rules comes one certainty: lithographers will have to deal with phase shifting masks (PSM), optical proximity correction (OPC), sub-wavelength low k1 lithography, and its by-product, the mask error enhancement factor (MEEF).

As a result of these challenges, robust mask and wafer characterization is mandatory.

More importantly, mask and wafer characterization need to be clearly understood and correlated. The type of data taken from the mask and wafer—and how it was taken— are critical to accomplishing this task.

Additionally, “Best of Class” characterization systems can guarantee higher yields and improved dialogue between the mask maker and mask user.

“Best of Class” systems can be defined as those systems which provide accurate and true answers to the most challenging mask and wafer design rules. A “Best of Class” inspection or metrology system must have the following characteristics:

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accuracy

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repeatability Autumn 2000

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reliability

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ease of use

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correlation to both mask and wafer metrology and inspection systems

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integration with the complete metrology and inspection scheme

As lithography challenges increase and k1 values become lower, stand-alone mask and wafer inspection and metrology systems add little value to the complete picture of wafer fab yields and profitability. The complete lithography scheme, which includes mask and wafer lithography, requires a seamless metrology and characterization approach. This is best accomplished by having identical metrology and inspection systems in the mask shop and fab. This approach becomes more critical as fabs transition to the 130 nm technology node. To better understand the challenges, it is essential to review the mask requirements for this node.

130 nm technology node mask requirements

Prior to the advent of 130 nm design-rule technology, mask specifications provided some measure of quality.

The parameters measured on the mask, however, often provided poor correlation of the mask contribution to

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Parameter (nm) Target CD

Ultra-Critical Layer 360 nm

Critical Layer 360 nm

Sub-Critical Non-Critical Layer Layer > 800 nm > 1200 nm

CD Tolerance

10

15

CD Uniformity

15

20

45

85

X/Y Delta

5

7

10

15

Iso/Dense Feature Bias

5

7

10

15

35

70

Linearity

5

7

10

15

CD Tool

CD SEM

CD SEM

CD SEM

CD SEM

Contact Layers CD Tool

CD SEM

CD SEM

CD SEM

CD SEM

Residual Registration

30

35

50

80

Butting/Stitching Errors

<5

<7

< 10

< 15

Registration Tool

Optical

Optical

Optical

Optical

Pattern Defect Size

100

130

180

200

Contam. Defect Size

100

130

180

200

Inspection Wavelength

< 365 nm

< 365 nm

< 365 nm

< 365 nm

Repair Trans. Loss

< 3%

< 5%

< 5%

< 5%

Verify Tool

AIMS or STARL i g h t

Table 1. Mask specification for 130 nm technology mask set.

final fab yield and chip speed sorts. The fundamental reason for this lack of correlation is essentially a result of failure to measure the right parameters and obtain the right quantity of measurements.

Table 1 provides an overview of the key parameters for various level 130 nm masks. Many of the parameters represent a new approach to defining mask quality.

While there are other parameters that can be considered, the parameters in the table provide the key elements of a quality photomask. There are basically three different elements to a high quality mask: global/ localized CD uniformity, global/localized placement accuracy, and transmission integrity across the mask exposure field.

Any anomaly that alters the transmission integrity across the field such that lithography ground rules cannot be maintained should be considered a defect. These defects can be discreet opaque or clear defects, semitransparent contaminants, or CD or placement defects.

A closer look at each of the specification elements will help to understand why “Best of Class” characterization systems are required to meet the lithography challenges.

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CD measurement and characterization

Most CD measurements are made with optical measurement systems that have limited resolution and use a NIST linewidth measurement standard with a high degree of uncertainty (~35nm) due to line-edge roughness. This uncertainty is higher than the tolerances requested by the mask user. It is, therefore, more important to have the ability to correlate the errors on the mask with what is found on the wafer. Additionally, the optical measurement limitation is around 0.50 µm, below which the measurements are suspect. Line-edge roughness and corner rounding tend to be “smoothed” by photo-optical measurements.

What is required for meaningful understanding of mask quality is an image that is a true representation of the features on the mask. CD SEM metrology provides imaging and measurements that better represent the true chrome features on the mask because edge anomalies are taken into consideration. This is particularly true when measuring contact-level masks and masks with OPC shapes, such as serifs and assist features.

Historically, CD uniformity was defined by measuring pre-defined images on the mask. These images were either “Ls” or crosses that were not part of the functional design. Measurement of features that are part of the functional design is necessary to assure mask quality at 130 nm technology. CD mean-to-target, uniformity, CD X/Y delta, isolated line-to-dense feature bias, and CD linearity must all be considered as part of the overall CD error budget. Photo-optical measurement systems do not have the capability to measure the small-error values outlined in the mask specification. Additionally, 130 nm mask specifications require more global and localized measurements to assure mask quality.

As shown in Figures 1a and 1b, localized errors can be detected and verified by CD SEM tools. Photo-optical measurement systems do not provide this capability. As reported in Monahan et al., a comprehensive systems approach is required to understand reticle CD errors and their contribution to total lithographic process window.3 CD SEMs can also be used to map cross-field and crosswafer errors, thus creating model-based uniformity maps for generating feedback to the mask and wafer lithographer. Since the reticle and wafer are measured in the same tool, better reticle-to-wafer correlation can be achieved.

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Figure 1a. Single line CD error

Figure 1b. CD SEM measurement

detected by a KLA-Tencor 365UV

of the same CD line error taken

HR inspection system.

with a KLA-Tencor 8100 XP-R,

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reticle CD SEM.

Mask registration and pattern placement errors

The second key element to mask quality is mask registration or image placement accuracy. While typically non-pelliclized masks are measured for mask registration, it has been reported that pellicle-frame-induced distortions often exceed the mask specification. Hence, there is a greater emphasis on measuring mask registration after the pellicle has been mounted on the reticle.

While the degree of placement errors is determined by the measurement of pre-defined crosses or “Ls” on the reticle, small localized placement errors at butting or stitching boundaries often contribute to the overall pattern placement accuracy. Butting/stitching errors can be global or localized. These errors, not generally detected by x/y measurement systems, are detected by defect inspection systems. The magnitude of CD errors can be easily measured using a CD SEM. Mask defect detection and defect metrology

In the past, mask defects have been classified as opaque or clear. As we continue with sub-wavelength lithography (low k1), the “black and white” type of defect represents only a small percentage of the types of defects that need to be detected and controlled in the mask shops and fabs. There are now “shades of gray” or partial transmission-loss defects. These types of defects can be a result of the mask fabrication process or can form after the reticle has been used in the fab.

The types of defects that can be present on a reticle and detected in the mask shop or fab are opaque (unwanted 38

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chrome), clear (missing chrome), particles, partially transparent films, scratches (on the reticle or pellicle), electrostatic discharge damage (ESD), non-uniform transmission through the pellicle, micro-fissures in the quartz, polishing grooves on the quartz, transmission loss from repair and sub-pellicle crystal growth. Many of these defects are a product of the mask materials or mask processes; however, partially transparent films, ESD, scratches, transmission loss through the pellicle, and sub-pellicle crystal growth are the result of mask usage and handling. For these reasons, it is important to continuously assure the quality of a reticle in the fab.

As lithography wavelengths become shorter, the potential for pellicle film degradation, crystal growth under the pellicle and the deleterious effects of partially transparent contamination becomes greater. In order to assure a comprehensive defect inspection of a reticle in the mask shop and wafer fabs several types of defect detection systems are required. As for CD measurement and mask registration, “Best of Class” systems are essential.

In the mask shop, there is a need to verify the quality of mask repairs. Transmission loss due to repair around ten percent was tolerable for high k1 lithography. Now that k1 values are consistently below 0.5 at 248 nm, wavelength transmission loss due to mask repair below five percent needs to be maintained.

While defect detection and quality assurance for reticles was the responsibility of the mask shop in the past, the previously mentioned new types of defects require the mask user to routinely re-qualify reticles in the fab.

Figure 2a shows a sub-pellicle crystalline defect that was formed as a result of reticle exposure to DUV illumination and detected by an inspection tool incorporating simultaneously transmitted and reflected light capabilities. This type of defect has been found in many fabs and appears to be ubiquitous. Figure 2b shows an ESD defect that was found during fab re-qualification.

Sub-pellicle crystals and ESD are two of the most commonly found reticle defects in the fab.

Comprehensive integrated reticle/wafer lithography management

As previously mentioned, stand-alone metrology and characterization systems do not provide the best solution to rapid communication and problem solving in the mask and lithography cell. Historically, mask makers and users operated autonomously and, as a result, most fab yield improvement was due to efforts on the part of the wafer fab engineer.

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Figure 2a. Sub-pellicle cr ystal detected by KLA-Tencor’s STARlight UV

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Figure 2b. ESD defect detected by STARlight.

inspection tool.

New software options that provide a critical link in wafer defect analysis by enabling wafer defects to be traced back to their origins on the reticle improve dialogue between the wafer fabs and mask shops, and enable defect data navigation and image review by all key lithography sectors. The capability for reticle-towafer defect coordinate translation allows for better understanding of the impact of reticle anomalies on wafer yield. As a result the mask shop can react more quickly to reticle-induced yield impacts.

2. B. J. Grenon et al., “Formation and Detection of Sub-Pellicle Defects by Exposure to DUV System Illumination”, 19th Annual Symposium on Photomask Technology, SPIE vol. 3873, pp.162-76, 1999.

3. K. Monahan et al., “Collapse of the Deep-UV and 193 nm Lithographic Focus Window”, Proceedings of the 1999 IEEE Symposium on Semiconductor Manufacturing, pp.

115-18, 1999.

KLA-Tencor TeraScan

Figure 3 provides an overview of how “Best of Class” tools can be integrated into a lithography cell to provide an optimized approach to quality management.

Leica LMS IPRO

Summary

KLA-Tencor 8250-R CD-SEM

Every technology node has provided the lithographer with a series of challenges. Many of these challenges have related to identifying, understanding and correcting yield detractors in the dynamic environment of the wafer fab. As we begin to enter the 130 nm technology node, the industry is positioned to take advantage of “Best of Class” systems that will provide the capability to achieve higher yields at an unprecedented rate.

References 1. A. Vacca et al., “Techniques to Detect and Analyze Photomask CD Uniformity Errors”, 19th Annual Symposium on Photomask Technology, SPIE vol. 3873, pp. 209-14, 1999.

Zeiss MSM100/193 AIMS

Mask Shop

KLA-Tencor TeraStar

KLA-Tencor X-LINK

Wafer Fab KLA-Tencor 8250-R CD-SEM

KLA-Tencor TeraStar

Figure 3. Provides an overview of how “Best of Class” characterization and metrology tools integrate into a lithography cell.

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