Automated SEM Offset Using Programmed Defects

Oliver D. Patterson, Andrew Stamper IBM Semiconductor Research and Development

Center 2070 Route 52, Mail Stop: 46H

Hopewell Junction, NY 12533 USA

Roland Hahn KLA-Tencor

20 Corporate Park Drive, Suite C Hopewell Junction, NY 12533 USA

Abstract - Defect inspection plays a large role in the development and manufacture of semiconductor technologies. Defects detected in today’s inspections tools are generally a fraction of a micron and require SEM review to analyze and justify corrective measures. It is very important that the review SEM drives to the exact location of the defects as a FoV (Field of View) of 2µm is necessary to provide the resolution needed for defect redetection without the inefficiencies associated with repeated ‘zooming’ of the image. A methodology which allows quick and accurate alignment of the review SEM to the defects in the results file is presented. This methodology uses a special structure containing programmed defects. The methodology is illustrated using the challenging example of PWQ wafers.

Keywords- SEM Alignment, defect offset, review SEM, deskew

I. INTRODUCTION

Optical defect inspection plays a large role in the development and manufacture of semiconductor technologies. Tens of optical inspections are strategically interlaced throughout the process sequence in order to detect, quantify and classify defectivity affecting the wafer. Because of the small feature size of today’s technologies, and in turn the small size of critical defects, SEM review is almost always necessary to classify the defects. Redetection of defects by review SEM has become particularly challenging in recent years, again because of the small size of the typical defect. Robust wafer alignment and a common die corner are two necessary factors for successful defect review. Despite excellent review SEM stage accuracy, a small offset between defects across the wafer still exists. This is because of variability in the calibration wafers, temperature, identification of the center of a defect and other factors. Therefore a third parameter, the defect deskew, is also necessary. The process of calculating the defect deskew, also termed ‘defect deskew’, may be performed automatically or manually. In addition to correcting offset within a wafer, defect deskew also compensates for a systematic offset between different inspection tools and modes. For example, the coordinate accuracy of darkfield inspection tools gets worse with larger

spot size. Also, the coordinate accuracy of bare wafer inspection degrades with higher throughput. The following offsets can be corrected by using an deskew: translation, scaling, rotation and non-orthogonality. To perform an efficient defect deskew, a set of reference defects needs to be selected, relocated and marked on the wafer. Defects detected by the inspection tool are sometimes not visible to the review SEM. When they are, the visible ones are not always well distributed across the wafer as required for an ideal deskew. Deskew is especially difficult for Focus Exposure Matrix (FEM), Process Window Qualification (PWQ) and Process Window Centering (PWC) inspections [1]. The nature of these inspections results in very high defect density and large defects in the higher modulations, making it difficult to reliably locate a suitable set of defects for deskew. In this paper, we propose the use of programmed defects (PD) to assist in the deskew process. This methodology is described in Section II. Application of this methodology to a number of PWQ wafers for comparison with current methods is discussed in Section III.

II. METHODOLOGY

Traditionally, PDs have been used for calibrating the sensitivity of inspection techniques such as e-beam and brightfield inspection [2,3]. This paper introduces a special structure, called the SEM Alignment Structure, which contains PDs at key levels throughout the process. These include active, deep trench, gate-stack, contact and all the metal and via levels. A small area of the structure layout around the PD at the active and contact levels is shown in Fig. 1. A small area of the metal 1 structure layout around the PD and a corresponding wafer image are shown in Fig. 2. This structure is 58um x 58um so that it can easily fit within the scribe line. The structure must have a repetitive pattern so that it can be inspected in array mode. A random mode inspection will not work for 1x1 reticles, which are common in development, because the PD appears on the same location in each reticle field. The PDs for each level are stacked on top of each other so that only a single PD is detected at each level. Since brightfield can sometimes

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Figure 1: SEM Alignment Structure design showing the programmed defects at the active (yellow) and contact (pink) levels

Figure 2: Metal 1 SEM Alignment Structure design (left) and corresponding wafer image (right)

detect defects at prior levels, this is necessary. To use the methodology, the inspection must include an array mode test to capture the PDs. All die may be inspected, but it is sufficient to just inspect the die that will be used for deskew. A special class code is assigned to these defects so they may be easily be identified during SEM review for deskew. Currently for the KLA-Tencor eDR-5210 review SEM used for this work, relocation of the PDs and then deskew must be done manually, but a software patch to allow this to be done automatically will soon be available.

III. APPLICATION

A. Case 1: Comparison to manual offset using a metal 1 PWQ wafer Application of the SEM Offset Methodology to a metal 1 PWQ wafer is described here to demonstrate the usefulness of this structure. The wafer was inspected with a KLA-Tencor 2825 brightfield inspection tool using KLA-Tencor PWQ methodology. An additional array mode test was added to capture the PDs in the SEM Alignment Structure. The result file was sent to the review SEM. First, manual deskew was performed as accurately as possible using the defects on the wafer other than the programmed defects. Images of all defects were taken. A

special program was used to determine the offset of each defect relative to the center of the FoV of the SEM image. These are plotted in Fig. 3. The defect scatter is 3µm. This inaccuracy is caused by the difficulty of selecting the correct defects within an image for deskew. Figure 4 shows the defect map for this wafer. The wafer can be divided into three zones. In Zone 1, all the defects are non-visual. In Zone 3, each FoV is swamped with defects, so reliable selection of the correct defect is impossible. Only Zone 2 contains discrete defects which are useful for SEM deskew. Unfortunately, this area is a small fraction of the entire wafer and so the deskew is poor. Figure 5 shows a case from Zone 1. The difference image shows three differences between reference and defect. Unfortunately, a real defect is not visible under the reviewSEM. This defect cannot be used for deskew. SEM non-visual defects can occur on any wafer, not just PWQ wafers.

Figure 3: Defect offsets for all SEM visible defects on the metal 1 PWQ wafer

Figure 4: On PWQ wafers, only defects in Zone 2 are generally useful for

deskew.

Zone 3

Zone 3

2 2 2

2

1 1

Figure 5: Zone 1: Bottom: optical defect, reference and difference images. Top: review image. The defects in the difference image are just not visible

in the SEM image.

Zone 3 Zone 2 Zone 1

Figure 6: Defects from the logic area of a metal wafer. Zone 3 is so defective, the review SEM cannot know which defect to chose.

Figure 6 shows images for the same site from the logic area of a metal wafer. Multiple defects appear in the Zone 3 (the higher modulation die) review image. It is impossible to reliably select the correct defect for a proper deskew. Figure 7 further illustrates the type of defects seen in the different zones. These images are from within the SRAM for a gate-level PWQ wafer. Next the programmed defects, at the ideally spaced locations show in Fig. 8, were used to deskew the wafer. After deskew using the SEM Offset Methodology, images of all the defects were taken, and the offsets for these defects were measured again. The results are shown in Fig. 9. The defect scatter is now 0.2um, a very substantial improvement.

Zone 3 Zone 2 Zone 1

Figure 7: Defects from a gate-level PWQ wafer. Zone 3 is too defective. Zone 2 is good as the defects are discrete. Zone 1 is bad because of no

defects or just non-visual defects.

Figure 8: PDs selected for deskew of the M1 PWQ wafer.

Figure 9: Defect offsets when the SEM Offset Methodology is used

B. Extension to other tools and levels with a deskew file Once the deskew has been calculated for a particular recipe and inspection tool, it is saved in the review recipe in the form of a “deskew cache file”. This file corrects for the systematic offset between the review SEM and the particular inspection tool. It may also be used for different recipes, different modes and even different inspection tools within a device/product. Figure 10 shows the defect scatter when applying the deskew file obtained using the SEM Offset Methodology across multiple recipes from the same inspection tool. The scatter is 1.5um. Figure 11 shows the defect scatter when applying the deskew file obtained using the SEM Offset Methodology across multiple recipes and multiple inspection tools of the same type. The scatter is 2um. While the scatter is better than in Fig. 3, it is not nearly as good as in Fig. 9. Use of the SEM Offset Methodology for each new wafer, would substantially improve the SEM alignment accuracy down to 0.2um error. An advantage of this would be to be able to take higher resolution images, 1um rather 2um FoV, of the defects.

C. Case 2: Comparison to use of a current deskew file with a gate-stack PWQ wafer The SEM Offset Methodology was also compared to existing methodology, where an existing deskew file is used. For this second study, a gate-stack PWQ wafer was studied. Rather than manual deskew, in this case the existing deskew file loaded in the inspection was used. This deskew file was created using a different level and possibly a different inspection tool. Review images of all defects were captured. Again a special program was used to determine the offset of each defect relative to the center of the FoV of the SEM image. Figure 12 shows the defect scatter, which is 1um. The population is offset -0.5um so the greatest offset is also 1um. The SEM Offset Methodology was then applied to this same wafer. Figure 13 shows the defect scatter. The scatter is now 0.4um and perfectly centered. D. Case 3: Review of Voltage Contrast Defects Redetection of voltage contrast (VC) defects from an e-beam inspection (EBI) tool can be difficult. E-beam inspection tools use high beam currents of 25nA or more for VC inspection. Review tools have maximum beam currents of about 1nA. Review of VC defects can be useful as they may be caused by much smaller physical defects only visible with a well centered, high magnification SEM image. Figure 14 shows an example at the gate level for a 28nm bulk technology. The EBI image shows a bright gate line. This defect type can be caused by a variety of issues, some visible from the surface and some not. The low magnification review SEM image shows the bright gate line is barely visible to the eye. Rather than detecting the brighter gate line, the review SEM centers on a small unrelated physical defect, merging of spacer. Even if the physical cause for the VC signal is visible, it would be missed in the high resolution image because it is not in the FoV.

Figure 10: Using the same deskew across multiple layers on the same tool

Figure 11: Using the same deskew file across multiple layers and tools

A special beam condition can be used on the review SEM to enhance the VC signal. The problem is this condition will not be nearly as good at imaging physical defects. Therefore, SEM review of EBI defects such as this is an excellent application for the SEM Offset Methodology.

IV. FUTURE PLANS AND SUMMARY An array mode redetection algorithm is being implemented for the KLA-Tencor eDR review SEM platform. With this improvement, automatic deskew will be possible for every wafer with a SEM Alignment Macro.

Figure 12: Defect offset using a stored deskew file

Figure 13: Defect offset using the SEM Offset Methodology In this paper, a methodology for fast, accurate alignment of the review SEM to the inspection defect map is presented. This methodology utilizes a special macro containing programmed defects. The methodology was demonstrated using several PWQ wafer. PWQ wafers are one good application, but this methodology can be used for any wafer. The key benefits are 1) the time searching for good defects for SEM offset will be eliminated and 2) defects will be centered with an accuracy of better than 0.2um enabling a 1um FoV image for a better image of the defect.

ACKNOWLEDGMENT

This work was performed at the IBM Microelectronics, Semiconductor Research & Development Center, Hopewell Junction, NY 12533. Thanks to Kourosh Nafisi for his help in testing these structures.

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[3] H. Xiao, L. Ma, F. Wang, Y. Zhao, J. Jau, K. Selinidis, E. Thompson, S.V. Sreenivasan, D. J. Resnick, “Inspection of 32nm imprinted patterns with an advanced e-beam inspection system,” BACUS, 2009.

Figure 14: Top left: EBI image showing a bright gate line. Top right: Review SEM image. The bright PC line is barely visible with a standard

condition. Bottom: The automatic high magnification review SEM image. If the high magnification image is not centered on the VC defect as in this

case, then any physical cause visible at the surface will be missed.