## 1 Introduction  
Scratch detection on magnetic disk is an important process of a quality inspection of hard disk drive (HDD) manufacturing because the scratches could possibly affect reading/writing performance of the HDD [1]. Scratches mainly occur due to contamination in the HDD assembly process, or crashing between magnetic recording head and disk [2]. Thus, the scratch inspection is a crucial process to reject latently damaged disks before further processes.  
In order to detect scratches, many tools have been employed, such as scanning electron microscope (SEM) and atomic force microscope (AFM). They were introduced because of their clarified resolution for detecting a scratch and its depth by images [3]. However, they are complicated in terms of utilization procedures and the light microscope with less complication than those proposed with a low resolution gain of about 100\(\times\)[4]. Although these three methods are surface free-contact techniques, they are expensive, destructive, and time-consuming. In order to cover up these disadvantages, the scratch inspection with a satisfied resolution in a quick process was developed using a thermal infrared camera to detect the surface heat and it was found that heat appears only on the surface, not scratch [5]. A magnetic disk, in the real process, rotates at a certain velocity and researchers have proposed various possibilities to detect a scratch dynamically. One showed an inventory of an optical pickup unit for light reflection into the charge coupled device (CCD) with polarizers and Infrared- (IR) cut filters for intensification of the laser beam [6]. However, for better resolution, machine vision inspection methods using various algorithms to examine the 2D surface images have also been studied but they can only be measured in static state [7]. In addition, it is still a vast cost of investment for industry.  
Therefore, a new technique of low cost, easy process, and scratch detectability has been presented by using ordinary laser doppler and CD pick-up head to inspect surface details because of its advantages on the measurement of object roughness and the identification of scratch location, in a compromising process [8; 9]. Moreover, numerous studies have recently shown interesting results of using DVD pick-up head to detect the reflective DNA layers on a DNA chip slide as its potentiality to show the shape of the DNA and count the number of cells [10]. The DVD pick-up head has been developed for better precision by reflecting the beam to photo detector integrated chip (PDIC), as a receiver, and then, the reflected laser light from PDIC is focused and analyzed on four quadrant sensors of photodiode detector [11].  
This study proposes a developed invention of non-destructive detection technique to inspect a 10 + um scale of scratches on a rotating magnetic disk. The current process of scratch inspection in manufacturing randomly chooses disks and it causes failures found in further processes. The assembly processes of latent failure disks lead to the cost of operation. Therefore, this instrument can quickly inspect big scratches of all media disk and a disk with a scratch found in this beginning of the manufacturing process will be thrown away at once. The CD and DVD pick-up heads with 790 and 650 nm wavelengths, respectively, are firstly combined and developed by using PDIC to specify the position and the shape of scratches for a wide range of wavelength selection. This aims to support the invention of its ability to specify the scratch location. The shapes of scratches are also examined by SEM to verify the experimental results. The scratches of 55-332 um size are tested because of the industrial requirement for rejection criteria. In addition, an optimistic rotating speed of magnetic disk to improve performances of CD and DVD pick-up heads has to be investigated.