Seagate Technology Characterizes Hard Drive Defects More Rapidly with Solution Based on NI Products

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We used two NI 5112 scope modules to digitize the waveform from the interferometer - one for the beam inspecting the top of the hard-drive platter and another for the beam inspecting the bottom.

Author(s):
Jason Pressesky - Seagate Technology

Developing a Faster Way to Test Hard Drives

At Seagate Technology, one of the world’s largest manufacturers of disk drives, the thin film magnetic media in the hard disk drives typically undergo a series of final tests before incorporating them into the hard drive assembly. The purpose of these tests is to count and characterize the number of defects and detect any roughness on the disk surface that may exceed the head-disk separation, approximately 20 nm. The traditional testing method uses specially calibrated recording heads or transducers. This method requires frequent maintenance, which generates unproductive material costs, excessive labor costs, and equipment downtime. We chose an alternative method known as interferometry, which is a noncontact, optical-inspection technique, because it provides the height information needed to scan a surface for defects under our stringent standards. The system requirements include ease of integration with existing test equipment platforms, a simple command interface between the optical-inspection systems, and the ability to run with the minimum amount of intervention on behalf of the host system.

The system uses an interferometric measuring system from Polytec Optronics, which generates an output signal that is representative of the disk surface topography as it passes beneath the inspection laser beam.

During a test, a precision air-bearing spindle rotates the disk at 12,000 to 15,000 rpm, and the optical probe scans the entire surface of the disk in the radial direction.

The NI 5112 high-speed digitizer captures interferometer waveforms in response to a trigger signal, and the interferometer generates a trigger signal if the displacement signal moves out of the allowed displacement window. We used the integrated PXI backplane to transmit the NI 5112 trigger signal to the NI 6602 counter. The NI 6602 counter registers the spin stand radial and linear encoder counts to latch the exact radial and angular coordinates of the defect. The complete test time is about 50 seconds, much faster than conventional test and inspection techniques for comparable surface coverage.

Integrating NI Hardware for Optimal Performance

PXI gives us the flexibility we need to synchronize the NI 5112 digitizers and the NI 6602 counter, all in the same chassis. We used two NI 5112 scope modules to digitize the waveform from the interferometer – one for the beam inspecting the top of the hard drive platter and another for the beam inspecting the bottom. We chose this module because it is the fastest digitizer from NI and acquires data at a rate of up to 100 MS/s; the automatic restart feature allows us to run out entire test and receive a complete list of defects at the end rather than having to stop after each defect; and it has 16 MB of extended memory per channel that we can take advantage of to shave seconds off each test by logging the data directly onto the module instead of waiting for information to write to the hard drive. To preserve memory, the NI 5112 only saves data containing or immediately surrounding a defect. We set the memory buffer to record 50 percent pretrigger and 50 percent post-trigger to ensure a clean recording of each defect.

To log the exact location of these defects on the hard drive, we used four channels on the NI 6602 counter. On the top and bottom of the hard drive media, we used one channel to measure the distance from the center and another to measure the rotational distance, or the angle of the defect. When the sensor detects a defect, an analog trigger fires and notifies the NI 6602 counter to log the exact location of the defect.

Creating Applications in Weeks Instead of Months with Measurement Studio

We used Measurement Studio to create our application because it complements Microsoft Visual Basic, a programming language that we are comfortable using. We can also perform tasks that would otherwise take too much time and man power to accomplish.

The ActiveX controls for Visual Basic simplify the PXI hardware configuration and synchronization, so we can accomplish more in less time. With the complete set of tools for hardware interfacing, signal analysis, and visualization, we created our application in weeks instead of months.

Author Information:
Jason Pressesky
Seagate Technology
47010 Kato Rd.
Fremont, CA 94538
United States
Tel: 510-353-4926
Fax: 510-651-7916
jason_I_pressesky@notes.seagate.com