IEEE TRANSACTIONS ON MAGNETICS, VOL. 37, NO. 4,JULY 2001 1957 Non-Destructive Defect Detection Scheme Using Kerr-Channel Optical Surface Analyzer Jack W. H. Tsai, Marshall Nathanson, Robert Kimball, Roy Geiss, Tony Logan, and Thao Nguyen Abstract—Magnetic defects on the surface of magnetic disks, un- like normal topological defects, are difficult to detect because they are solely due to magnetic signal loss as seen by the MR element and often do not have any obvious topological features. Defects without any topological features usually render conventional defect finding techniques such as visual inspection, optical microscopy, and SEM useless. In the past, the painstaking way of locating the defect was to index the drive with a strobe light and followed with ferrofluid decoration of the surface to determine the location of the defect. The disadvantage is that the magnetic particles in the decoration technique can often confound the origin of the defect. Further- more, the old technique does not have high enough resolution to lo- cate defect with a size on the order of a micrometer. In this work, we will describe how a magnetic marking technique (MMT) is utilized to circumscribe the magnetic defects. The markers are written in such a way that both the defect and the magnetic markers are easily detected using Kerr-Channel Optical Surface Analyzer and Mag- netic Force Microscope. Typically, the accuracy of locating the de- fect using the MMT is within a track-pitch radially and within one data sector circumferentially. The new scheme provides at least one order of magnitude improvement in detection resolution over the old technique. Index Terms—Magnetic defect, optical surface analyzer, read- back mapping. I. INTRODUCTION M ANY techniques have been explored over the years in an attempt to correlate magnetic errors flagged during drive testing with actual physical defects on magnetic disk sur- faces [1]–[3]. The difficulty has always been that the magnetic defects on the disks as seen by the MR element are not usu- ally the same as observed by most conventional analytical tools. The magnetic defects are usually defined as degradation in the magnetic spacing due to particle erasure, data encoder failure, or offset overwrite. In some of these cases, the magnetic de- fects are not visible beyond the MR sensors, therefore becomes a difficult task to locate them for the analysis. Normally, the de- fects are first detected in the drive during the build process or by reliability testing and are confirmed on the oscilloscope. Af- terwards, the magnetic disk is strobe-indexed to establish the radial and circumferential point of reference from which the de- fect of interest was later located. However, given the precision of indexing is limited to within 0.1 degree, it means that the uncertainty of locating the defect in the circumferential direc- tion is 40 m at best (at ID radius). In addition, manually Manuscript received February 2, 2001. J. W. H. Tsai, R. Kimball, R. Geiss, T. Logan, and T. Nguyen are with Advance Technology, Maxtor Corporation, Milpitas, CA 95035 (e-mail: Jack_Tsai@maxtor.com). M. Nathanson is with MRN Consulting, Thousand Oaks, CA 91360. Publisher Item Identifier S 0018-9464(01)06191-X. placing the cross-hair on the location of interest is normally ac- curate to within 50 m. Based on the current areal density of 10–20 Gb/in , it means one can only be certain of the exact defect location to within one to two data sectors in the circum- ferential direction and 100 tracks in the radial direction. The inaccuracy in the old technique would make it difficult to pre- cisely locate the defective region. During the failure analysis, the magnetic media is re- moved from the drive and an optical microscope is used to inspect defects for topological origins. Typically, the smallest resolvable object is m for the light microscope and 1000 magnification is required to detect them. For sub-micrometer sized defects, they are usually beyond the detection limit of the light microscope. However, with increase of areal density of magnetic media, the defects that are micron sized or smaller have become common and difficult to locate using the light microscope alone. Thus facing with these challenges, a new technique for locating the defect is needed. II. EXPERIMENTAL All the issues mentioned in the introduction were addressed with the Magnetic Marking Technique (MMT), which incorpo- rated 1) mapping of defective areas using an MR element, 2) the magnetic marking of the defect, and 3) magnetic marker de- tection using a Kerr-Channel on an Optical Surface Analyzer (OSA) model TS2100 by Candela Instruments [4]. In the new scheme, an error was first flagged by the MR element in the drive and the extent of the error was mapped using the Magnetic Readback Mapping software [5]. The defect was then marked using a new technique called Magnetic Marking, which applied a Non-Return-To-Zero scheme to directly translate the channel bits into magnetic transitions during the writing of magnetic pat- tern around the defect. Both the magnetic pattern and the defect were detected using the Kerr-Channel” on the Optical Surface Analyzer (OSA) [6]. The magnetic markers were designed so the written area showed a different optical contrast from the non- written area due to Kerr effect. This enabled one to quickly find the defective regions on the disk. After the defect was located, the laser scriber as part of OSA was used to precisely laser-mark the defect for subsequent failure analysis. The advantage of the MMT was that no external contaminants were introduced. Fur- thermore, it allowed one to pinpoint the defect to within 5 m and eliminated a lot of guesswork in locating the defect, which was very crucial for finding the root cause of the failure. For demonstration, a drive with defect on the disk was located using the new method and failure analyzes were conducted to deter- mine the physical nature of the defects. 0018–9464/01$10.00 © 2001 IEEE