Elvis Wanga, Damian Chena, Chris Youngb, Kevin Sunb, Jack Yanb, Prasanna Digheb, Avinash Saldanhab, David Feilerb
bKLA-Tencor Corporation, One Technology Drive, Milpitas, CA, USA 95035
ABSTRACT
Detection of resist residue and organic contamination after photo resist strip and wafer clean early in the high K/metal gate (HK/MG) manufacturing process flow is critical as it has been known to significantly impact yield. This residue, when exposed to subsequent thermal process steps, transforms into solid hard spot(s), and can then be detected by a wafer inspection tool, but unfortunately it is too late to take corrective action. A unique process control solution to detect the presence of residues was developed using advanced analysis of an optical scattering inspection of a litho checkerboard pattern. The presence of residue was then validated with film thickness measurements.
1. INTRODUCTION As CMOS scaling continues below 45nm, conventional silicon oxide technologies cannot sustain equivalent oxide thickness (EOT) and leakage current requirements set in the International Technology Roadmap for Semiconductors [1]. Due to the limitation of physical thickness scaling and high tunneling current, next-generation CMOS devices require the introduction of high-K and metal gate electrodes to reduce gate leakage and poly depletion [2, 3]. The manufacturing infrastructure for high-k and metal gate stacks is reasonably mature. On the reliability front, however, much work remains to be done. Dual metal gate CMOS integration steps require multiple wet etch process steps to separate two different metal gates within transistors on the same wafer [4]. Integration schemes as well as wet etch chemistries must be developed to completely remove the first metal gate material without damaging the underlying gate dielectric. Photo resist material specific for high K/metal gate (HK/MG) layers must be chosen carefully and effective resist strip processes have to be developed and extensively characterized as incomplete removal of the photo resist leads to the presence of residue. Detection of such resist residue after Cap2 resist cleaning step is critical as it has been known to impact yield and affect device performance. This residue, when exposed to subsequent thermal process steps, transforms into solid hard spot(s), and can then be detected by an inspection tool, but this is unfortunately too late. A unique and innovative process control solution was developed to detect the presence of residue. This solution uses a simple litho checkerboard wafer layout; the Surfscan SP2 wafer inspection tool, which measures the optical scattering from the wafer’s surface; and, SURFmonitor, a process signature and metrology proxy add-on module for the Surfscan SP2. In this study, the litho checkerboard technique involved creating wafers where some of the regions of the wafer were left un-exposed and some regions were exposed with a line space pattern. These wafers were then inspected on the above mentioned optical scattering tool and the data was analyzed using SURFmonitor Overlay Differential Analysis (SODA) and Regions of Interest (ROI) techniques. These analyses indicated the presence of resist residues on the exposed and un-exposed regions of the wafers. These results were validated with film thickness measurements conducted by an optical film metrology (OFM) tool. OFM can be used to monitor for resist residue after the cleaning step, but the discrete sampling strategy used can miss
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localized process issues on the wafer. The unique process control solution using Surfscan SP2 and SURFmonitor provides superior sensitivity, quicker time to results and significant cost benefits. Although this study was conducted on a set of epitaxial test wafers, this solution can be easily implemented in a typical production process flow for <45nm design rule HK/MG manufacturing process.
A traditional blanket checkerboard wafer layout (Figure 1a) is a useful methodology to check for photo resist (PR) residue after exposure/development, PR strip and wafer clean in the lithography cell. The limitation of blanket checkerboard is that it can only be used for large exposure and non-exposure areas on a blanket wafer. A method called pattern checkerboard (Figure 1b) can be applied to detect resist residue in a small cell of patterned area.
Figure.1a Figure.1b
Figure 1. Schematics of the traditional blanket checkerboard and patterned checkerboard wafer layouts.
2.2 SURFmonitor Overlay Differential Analysis (SODA) and Regions of Interest (ROI)
SURFmonitor uses background haze data from Surfscan SP2 scans to monitor process signatures and detect low contrast defects. SODA and ROI are analysis techniques applied to SURFmonitor data. SODA is a proprietary methodology in which data from two SURFmonitor scans is overlaid to generate a ratio map. The ROI analysis technique determines the average haze values for regions of interest specified by the user. SODA and ROI analyses were applied to the localized haze data on a checkerboard wafer in order to find the presence of PR residue. The regions of interest defined for ROI analysis were set up to match the pattern checkerboard wafer layout, with each region of interest overlaying a pattern exposure square or a non-exposure square.
2.3 Fluorescence Map Theory and Methodology
The KLA-Tencor Surfscan SP2 is widely used for unpatterned wafer defect inspection in the semiconductor industry. Haze, the background signal of scattering, is important for detecting the surface roughness, damage and contamination on a wafer. A new, advanced analysis technique called fluorescence map testing, implemented on the Surfscan SP2, is used for detecting contaminants which are organic in nature like PR residue [5]. The Surfscan SP2 uses a UV laser that can cause some materials (usually organic in nature) to fluoresce. For this reason, the Surfscan SP2 was designed with a fluorescence filter (F1) that can be placed in front of the detector to reduce the background scatter from a fluorescent material, thus increasing the sensitivity to smaller defects on certain layers. Using this option to scan a wafer with and without the F1 filter, and with the help of SURFmonitor to produce a pixel-by-pixel ratio map of the two scans, one can create a fluorescence map. This fluorescence map shows exactly where fluorescent material exists on the wafer. Moreover, the fact that materials like resist residue are organic in nature means that they are
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more likely to fluoresce, and thus resist left after strip processes can be monitored using Surfscan fluorescence maps. Figure 2 shows the theory behind creating a fluorescence map. When a wafer is scanned without the F1 filter, the background scattering (haze) is due to a combination of surface roughness, thickness (only for transparent films), and fluorescence. However, when the same wafer is scanned with the F1 filter, the background scattering is due to a combination of only surface roughness and thickness. Also, the F1 filter attenuates the UV light by an amount known as its attenuation factor, A. Using SURFmonitor, a map can be generated by taking the haze values from the scan without the F1 filter and dividing them by the haze values from the scan with the F1 filter on a pixel by pixel basis. If the wafer scanned does not fluoresce, like clean bare silicon for example, the resulting ratio will be equal to the attenuation factor (A) of the F1 filter. On the other hand, if the wafer scanned does fluoresce, then the resulting ratio will be greater than the attenuation factor of the F1 filter (A). This resultant ratio map is referred to as a fluorescence map, where any value greater than the attenuation factor means that fluorescent material is present on the surface of the wafer in that area.
Figure 2. Theory behind creating a fluorescence map.
2.4 Optical Film Thickness Metrology
KLA-Tencor’s Aleris optical film measurement (OFM) tool, which uses spectroscopic ellipsometry (SE) to measure the thickness of ultra thin films, was used to determine if any PR residue was left after the clean process. Because OFM is a proven technique for determining the presence of PR residue, the OFM data was used to validate the results produced by the Surfscan methodology.
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3. EXPERIMENT PROCEDURE A preliminary evaluation was conducted using four different types of HK/MG films which were deposited on an EPI wafer (test vehicle) with an interlayer oxide on the top and a spin-coated photo resist applied, exposed and developed as pattern checkerboard. A PR Strip was then carried out followed by a wafer clean as shown in Figure 3. A Surfscan SP2 scan was done at two stages: first, after HK/MG film deposition; and second, after wafer clean. We used SODA, ROI analysis, and fluorescence map techniques to determine if any PR residue remained following the wafer clean process.
Figure 3. Design of experiment -- simplified process flow.
4. RESULTS AND DISCUSSION 4.1 Preliminary evaluation on HK/MG film
For each of the four HK/MG films, SURFmonitor scans were used to determine the average haze value across the wafer following film deposition and again after wafer clean. Table 1 shows the average wafer haze values and the average overlay haze ratio. All of the HK/MG films show a haze ratio value greater than 1, indicating the presence of PR residue on the wafer. The Cap2 film wafer has the lowest haze ratio value (1.27), and therefore has the least amount of PR residue. This Cap2 layer was used to collect additional data using SODA, ROI analysis and fluorescence maps. Table 1. Average wafer haze values after film deposition and wafer clean for four HK/MG films. An overlay haze value greater than 1 indicates that PR residue is present on the wafer.
Film Film deposition
(Avg. PPM)
Wafer clean
(Avg. PPM)
Haze Overlay Value
(wafer clean/film deposition)
Cap1 0.368 2.22 6.03
Metal 0.341 0.662 1.94
Cap2 0.376 0.477 1.27
HK 0.37 0.844 2.28
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Using SODA and ROI analysis, the presence of PR residue in a localized area of the wafer could be identified. Figure 4 shows the results of ROI analysis of SURFmonitor data from the Cap2 layer. The colored boxes indicate the defined regions of interest, set to mimic the checkerboard pattern on the wafer. Figure 4a is ROI analysis map of SURFmonitor data after film deposition, with the scale indicating the actual haze value on the wafer. Figure 4b is the ROI analysis map of an overlay of SURFmonitor data collected after film deposition, vs. after patterning and wafer clean, generated using SODA. The white ROI areas on the map in Figure 4b are patterned areas, clearly indicating that there is some material present in these areas after PR strip and wafer clean.
Figure 4a. ROI analysis after film deposition. Figure 4b. ROI analysis on overlay image (wafer clean/film deposition).
Figure 4. ROI analysis map of SURFimage on the Cap2 film wafer. The ROI overlay image data (wafer clean / film deposition) from Figure 4b was further analyzed using proprietary offline software. The patterned and non-exposure areas are separated, and a ROI analysis data map is generated for each type of area (figure 5). For each individual region of interest, the average overlay ratio value is calculated. These values are shown as the black dots on the data maps in figure 5. A ROI overlay ratio value greater than 1 indicates the presence of PR residue. Figure 5a shows the ROI wafer signature map for patterned areas. It can be clearly seen that the wafer edge has the worst PR residue condition, displaying a ROI overlay ratio value greater than 2.1 (orange color). The mean value of the ROI overlay ratio for the patterned areas across the entire wafer is 1.53. In contrast, the non-exposed areas of the wafer (figure 5b) show little indication of PR residue – the ROI ratio value is around 1.1 across the wafer (dark blue color). These data indicate the presence of PR residue in the patterned areas, and little to no PR residue in the non-exposure areas.
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Figure 5a. ROI analysis data map for patterned areas. Figure 5b. ROI analysis data map for non-exposure areas.
Figure 5. ROI analysis data maps for the Cap2 film wafer.
To confirm presence of residue, the wafer underwent SEM review and Energy Dispersive X-ray (EDX) analysis. The SEM image shows a thin and laminate image at the pattern region as shown in Figure 6. EDX unfortunately is not sensitive enough to analyze this kind of PR residue.
Figure 6. SEM Picture showing thin and laminate PR residue area Figure 7 shows the fluorescence map (figure 7a) and corresponding haze percentage histogram (figure 7b) for the Cap2 film wafer after the clean process step, generated using the methodology described in section 2.3. Using a bare epi wafer, the attenuation factor, A, of the F1 filter was determined. Overlay values greater than A will indicate fluorescence, and thus, the presence of PR residue. The fluorescence map clearly indicates patterned areas and non-exposure areas that have an overlay value greater than A. These data confirm the presence of PR residue in the patterned areas, and indicate that there is some PR residue present in the non-exposure areas of the wafer.
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Figure 7a. Fluorescence map of the Cap2 film wafer. Figure 7b. Histogram (haze percentage) of the fluorescence map.
Figure 7. Fluorescence map used to determine the presence of PR Residue on the Cap2 film wafer.
4.2 Reproducing the experiment on Cap2 layer
The experiment was performed on a second Cap2 film wafer as it had the least amount of residue present among the four HK/MG films (Table 1). Adjustments were made within the lithography cell to try to reduce the amount of PR residue present on the wafer. In addition to collecting and analyzing SURFmonitor data, an optical film thickness measurement using an Aleris tool was performed to validate the optical scattering results. If any PR residue remained on the wafer after the clean process, the film thickness tool would be able to measure it. Table 2 shows the average wafer haze values of the Cap2 film wafer after film deposition and after wafer clean, and the corresponding overlay haze ratio. The overlay ratio value is around 1, indicating that there is no PR residue on the wafer. Table 2. Average wafer haze values after film deposition and wafer clean for a second Cap2 film wafer. The overlay haze ratio value of 1 indicated that there is no PR residue on the wafer.
Film Film deposition
(Avg. ppm)
Wafer clean
(Avg. ppm)
Haze Overlay Value
(wafer clean/film deposition)
Cap2 0.364 0.364 1
ROI data analysis wafer maps were generated for the patterned (figure 8a) and non-exposure areas (figure 8b) of the second Cap2 wafer. The ROI overlay ratio values for both the patterned areas and non-exposure areas are approximately 1 (pink color). This further confirms that there is no PR residue present on the wafer.
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Figure 8a. ROI analysis data map for patterned areas. Figure 8b. ROI analysis data map for non-exposure areas.
Figure 8. ROI analysis data maps of the second Cap2 wafer. Figure 9 shows the fluorescence map (figure 9a) and corresponding haze percentage histogram (figure 9b) for the second Cap2 film wafer after the clean process step. The map and histogram clearly show that all the patterned and non-exposure areas have an overlay ratio value around A, the value of the attenuation factor of the F1 filter used to determine fluorescence. Since the overlay ratio values are below A, there is no fluorescence on the wafer, and thus, no PR residue present on the wafer.
Figure 9a. Fluorescence map of the second Cap2 film wafer. Figure 9b. Histogram (haze percentage) of fluorescence map.
Figure 9. Fluorescence map used to determine whether or not PR residue is present on the Cap2 film wafer.
The Aleris optical film measurement (OFM) tool is used to measure the thickness of ultra thin films in the HK/MG process. Figure 10 shows the 49pt line scan of film thickness measurements on the Cap2 wafer at non-exposure areas. The thickness difference between the film deposition and wafer clean steps is less than 1Å, indicating no significant thickness increase after the PR strip and wafer clean processes. This validates the fact that there is no PR residue on the wafer.
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Figure 10. Film thickness measurement result using line scan at non-exposure areas across the wafer.
4.3 Summary of results for Cap2 wafers after PR strip and wafer clean processes
Table 3 summarizes the data collected from both Cap2 wafers after the PR strip and wafer clean. The result proves that by using SODA, ROI analysis and fluorescence methods one can easily identify if there is any PR residue remaining on the Cap2 wafer after the PR strip and wafer clean processes. Table 3. Comparison of optical scattering data from the two Cap2 film wafers after the PR strip and wafer clean processes.
Cap2 Wafer
Haze Overlay Value (wafer clean/film dep)
Mean ROI Overlay Value for
pattern areas
Mean ROI Overlay Value for non-exposure areas
Fluorescence
# 1 1.27 1.53 1.1 Detected
# 2 1 1 1 Not Detected
4.4 Flow chart to detect PR residue
Figure 11 shows the flow chart implemented in production to detect PR residue or organic contamination using the process control solutions involving SURFmonitor and pattern checkerboard. OFM is also used to validate the SURFmonitor results. Definition. *Fluorescence Ratio: (No F1/ F1) PR strip and wafer clean / (No F1/ F1) Film deposition, ** Overlay (Post/Pre) ROI Ratio = (PR strip and wafer clean / Film deposition), both pattern and non-patterned area = 1
1
6
11
16
2126
31
1 5 9 13 17 21 25 29 33 37 41 45 49
49 Pt Line Scan
Thic
knes
s (A
)Cap2
PR Wash
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Figure 11. Flow chart to detect PR residue by using the process control solution involving SURFmonitor, pattern checkerboard and OFM.
5. CONCLUSIONS A unique and innovative process control solution was developed to detect the presence of PR residue. This solution used a simple litho checkerboard technique, a Surfscan SP2 wafer inspection tool which can measure the optical scattering from the surface of the wafer, and SURFmonitor, a process signature and metrology proxy add-on module for Surfscan SP2. In this study, these wafers were inspected on the above mentioned optical scattering tool and the data was analyzed using techniques such as SURFimage Overlay Differential Analysis (SODA) and Regions of Interest (ROI) analysis. The result of this analysis indicated the presence of PR residue on the exposed and un-exposed regions of the wafers. Thickness measurements conducted by an optical film metrology tool validated the results. The unique process control solution using the optical scattering tool and analysis methodologies provides superior sensitivity, quicker time to results and significant cost benefits. Although this study was conducted on a set of epitaxial test wafers, this solution can be easily implemented in a typical production process flow for <45nm design rule HK/MG manufacturing processes.
ACKNOWLEDGEMENTS The authors thank Julien Yen of KLA-Tencor Corp. for supporting data collection analysis of optical film metrology data.
REFERENCES [1] International Technology Roadmap for Semiconductors (ITRS), http://member.itrs.net./ [2] B. Maiti, P.J. Tobin, “Metal Gates for Advanced CMOS Technology,” Proceedings of the SPIE, Vol. 3881, pp. 46-
57, 1999C
EPI wafers
HK/MG film deposition
PR + Litho + Strip (checkerboard)
Wafer clean
SP2 measurement with and without F1
Process Release
SP2 measurement with and without F1
SM Ratio and ROI Analysis
(Film deposition; Pre)
SM Ratio and ROI Analysis
(PR Strip and Clean; Post)
Fluorescence Ratio ~1*
Post/Pre ROI Ratio ~1**
Yes
Fine tune PR strip and clean recipe
No
Film thickness measurement (pre)
Film thickness measurement (post)
Pre – Post thickness difference
less than 1Å
Fine tune PR strip and clean recipe
No
EPI wafers
HK/MG film deposition
PR + Litho + Strip (checkerboard)
Wafer clean
SP2 measurement with and without F1
Process Release
SP2 measurement with and without F1
SM Ratio and ROI Analysis
(Film deposition; Pre)
SM Ratio and ROI Analysis
(PR Strip and Clean; Post)
Fluorescence Ratio ~1*
Post/Pre ROI Ratio ~1**
Yes
Fine tune PR strip and clean recipe
No
Film thickness measurement (pre)
Film thickness measurement (post)
Pre – Post thickness difference
less than 1Å
Fine tune PR strip and clean recipe
No
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[3] Toshiyuki Horiuchi, “Logic Device Scaling Trend in ITRS 2007” Proc. of SPIE Vol. 7028, [4] T. Schram et al., “Novel Process To Pattern Selectively Dual Dielectric Capping Layers Using Soft-Mask Only,”
VLSI Tech. Symp. (2008). In press [5] David Feiler, “A proven methodology for detecting photo-resist residue and for qualifying photo-resist material by
measuring fluorescence using SP2 bare wafer inspection and SURFmonitor” Proc. of SPIE Vol. 7520-56
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