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Improving Yield in TFH Wafer Fabrication by Alan J. Fan, Ph.D., KLA-Tencor Corporation
An automated defect inspection system can provide fast and reliable defect information for thin film head (TFH) wafer fabrication, and eliminate the limited sampling size and inconsistent decision making resulting from an operator’s manual inspection. With the capability of full wafer and full slider inspection, the automated defect inspection system can be employed as an in-line yield and process monitoring tool for TFH wafer fabrication.
Introduction
The ultimate goal of any TFH lab is to produce thin film heads with the highest areal density, while minimizing production costs. This production typically involves heads with small writer and reader track width. Small track width becomes more prone to defects and process variation, so that in-line defect inspections become even more important steps during TFH wafer fabrication. Wafer
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quick and reliable defect information for in-line wafer yield and process tool monitoring. TFH wafer inspection
Defect inspection for TFH applications presents a different requirement compared to defect inspection in integrated circuit (IC) wafer fabrication. The tens of thousands of devices on a TFH wafer are patterned in a two-dimensional array, with some large spacing and a wide range of topography. Only critical defects falling within a vicinity of the active device are real "killer" defects. A typical TFH wafer fabrication involves many inspection steps, especially after resist development, resist strip, chemical mechanical polishing, plating, etc. The purpose of defect inspection is to either accurately catch a defective or misprocessed wafer for rework or scrap, or quickly and reliably monitor for process tool excursions.
F i g u re 1. Device layout of TFH head array on a typical TFH wa fer. The a rea occupied by the active device ca n b e on ly a fraction of the total waf er are a .
The current manual inspection methodology can no longer provide sufficient and reliable defect information for TFH wafers in process. An automated defect inspection system with high sensitivity to critical defects becomes necessary in any TFH fab in order to obtain
In many TFH fabs, the current inspection method is to have operators manually inspect a pre-defined structure at a defect-prone location for a pre-defined sample size – anywhere from 5 to 20 sites per wafer (Figure 1). A typical TFH wafer can have anywhere from 20,000 to 40,000 heads. Therefore, the limited inspection sample size can seriously impact the reliability and accuracy of the defect information. In addition, the lengthy inspection scheme can delay the response for some process tool excursions. Spring 2000
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Defect Inspection
Accurate defect information can help prevent further investment in low-yielding wafers and dramatically reduce scrap further down-stream in the manufacturing process. To obtain accurate TFH defect information within a reasonable amount of time, a fully automated defect inspection system becomes an inevitable requirement. An automated system not only presents the capability of full wafer and full slider inspection with high sensitivity to critical defect types, but also eliminates the uncertainty of the decision making by an operator. Lacking the human equation, operator inattention and inadequate sampling size are no longer issues.1
In-line yield monitoring with automated defect inspection
In one major TFH wafer fab, the yield impact with automated defect inspection and manual inspection is studied at the inspection step after top pole plating.
An automated defect inspection system, such as the KLA-Tencor AITTFH system, can be employed at various inspection steps in a TFH wafer fab, including afterdevelopment, post lift-off chemical mechanical polishing, plating, and cleaning. The sensitivity of the automated inspection system can be optimized according to different layers for certain critical defect types. Defects within the area of interest can be reported in both wafer and slider coordinates. Figures 2-5 show various types of defects found by the automated defection inspection system at inspection after top pole plating, coil photo after development inspection, post CMP of base-coat, and post CMP of shared pole.
F i g u re 3. Examples of defect s f ound at After Development In spection (ADI) o f fir st coil photo-resist pat ter n, where the first ima ge shows a g ood co il s tru c t u re.
F i g u re 2. Examples of defects f ound at top p ole lift-of f inspection; t he
F i g u re 4. Examples of defect s f ound at po st CMP inspection for
top i mage sh ows a good po le stru c t u r e for r e f e re n c e .
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F i g u re 5. Examples of defects found at post CMP inspectio n f or shared pole.
The same percentage limits are used for automated and manual inspections, even though the automated defect inspection system performs full wafer and full slider inspection and manual inspection carries out only ten heads per wafer. Wafers with excessive defective heads are scrapped in both cases. Two groups of wafers are processed through the wafer fab, with one group being inspected with the automated defect inspection system and the other being inspected manually. TFH wafers inspected with the automated defect inspection system after top pole plating show less slider loss due to damaged top poles at the subsequent slider fab, as shown in Figure 6. Integrated region reporting capabilities allow for selective reporting of defective events. Full wafer inspection with the automated system can provide quick feedback for engineering process development, such as the amount of CMP film removal (Figure 7), wafer-cleaning process for plating and post lift-off, and ashing process for stripping plasma-hardened resist. Conclusions
As design rules for TFH wafer fabrication move toward smaller geometries, the current manual inspection
F i g u re 7. CMP defects detected by AIT as a function of CMP film re m o v a l .
methods becomes less reliable. Automated defect inspection can provide quicker and more accurate defect information for in-line TFH wafer process monitoring and process tool monitoring. With the capability of full wafer inspection and full slider inspection, automated defect inspection can provide not only reliable defect data toward a decision for scrap or rework during wafer steps, but also valuable information for the down-stream slider fabrication. 1. Alexander E. Braun, Automation Comes to Lithography Inspection, Yield Management Solutions, Spring, 1999.
Acknowledgements
Special thanks to Rajat Roychoudhury, Frank Wu, Erin Mcknistry, TC Chuang, Steve Schatz and Sherry Hanson. This paper is largely derived from a presentation first given in September 1999 during DISKCON. circle RS#040
F i g u re 6. Yield impact of automat ed defect insp ection vs oper ators’ man ual inspect ion.
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