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Accurately identifying disk surface defects

Posted: 01 Aug 2002     Print Version  Bookmark and Share



By Jason Pressesky

Senior Engineering Manager

Seagate Technology

E-mail: jason_l_pressesky

Currently, engineers are facing

the inevitable challenges in de-

veloping a faster acquisition

system to identify defective

sites on disk surfaces. Fortu-

nately, innovative testing solu-

tions keep on surfacing and

National Instruments' Mea-

surement Studio and PXI sys-

tems are some of the tools con-

sidered as such.

The Measurement Studio


two NI 5112 high-speed digitiz-

ers and an NI 6602 counter

timer module to create a solu-

tion that digitizes interferom-

eter waveforms and logs disk

defect locations.

Developing a faster

way to test HDs


film magnetic medium in the

hard disk drives (HDD) typi-

cally undergoes a series of final


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 sur-


disk separation, which is ap-

proximately 20nm. The tradi-

tional testing method uses spe-

cially calibrated recording

heads or transducers. This


tenance, which generates un-

productive material costs, ex-

cessive labor costs and equip-

ment downtime. We chose an


etry, a non-contact, optical in-


this method because it provides

the height information needed

to scan a surface for defects un-




7 Ease of integration with ex-

isting test equipment plat-


7 A simple command interface

between the optical-inspec-

tion systems

7 Ability to run with the mini-

mum amount of intervention

on behalf of the host system

The system uses an inter-

ferometric measuring system

from Polytec Optronics. This

device generates an output sig-

nal that is representative of the

disk surface topography as it

passes beneath the inspection

laser beam.

During testing, a precision

air-bearing spindle rotates the

disk at 12,000rpm to

15,000rpm, while the optical

probe scans the entire surface

of the disk in the radial direc-

tion. The NI 5112 high-speed

digitizer captures interferom-

eter waveforms in response to a

trigger signal. The interferom-

eter generates a trigger signal if

the displacement signal moves



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


in order to latch the exact radial

and angular coordinates of the



ventional test and inspection

techniques for comparable sur-

face coverage.

Integrating hardware for optimal


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 interfer-

ometer--one for the beam in-

specting the top of the hard-

drive platter and another for

the beam inspecting the bot-

tom. We chose this module for

three reasons. First, it is the

company's fastest digitizer, ac-

quiring data at a rate of up to

100MSps. Second, with its au-

tomatic restart feature we can

run our entire test and receive

a complete list of defects at the

end, rather than stopping the

test after each defect. Finally,

the NI 5112 module features

extended memory--16MB of

memory per channel. With

this extended memory, we can

shave seconds off each test by

logging the data directly onto

the module instead of waiting

for information to write to the

Accurately identifying disk surface defects

Figure 1: The NI 5112 high-speed digitizer captures interferometer waveforms in response to

a trigger signal.

Figure 2: In this laser set up, the interferometer generates an output signal that is

representative of the disc surface topography as it passes beneath the inspection laser beam.

hard drive. To preserve

memory, the NI 5112 only

saves data containing or im-

mediately surrounding, a de-

fect. We set the memory buffer

to record 50 percent pre-trig-

ger and 50 percent post-trig-

ger 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 both the

top and bottom of the hard-

drive media, we used a channel

to measure the distance from

the center and another to mea-

sure the rotational distance or

the angle of the defect. When

the interferometer's sensor de-

tects a defect, an analog trigger

fires, telling the NI 6602

counter to log the exact loca-

tion of the defect.

Creating applications in weeks

We chose Measurement Studio

to create this application be-


Visual Basic, a programming

language with which we are al-

ready comfortable. We can also


wise take too much time and

manpower to accomplish. The

ActiveX controls for Visual Ba-

sic simplify the PXI hardware

configuration and synchroniza-


in less time. With the complete

set of tools for hardware inter-


alization, we created our appli-

cation in weeks instead of


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