Polarization interferometer for flying height testing

Proc. IDEMA Future Dimensions in Storage Symposium, pp. 89-94 (San Diego, 1997).

Polarization interferometer for flying height testing

P. de Groot, J. Biegen, L. Deck, A. Dergevorkian, T. Erickson, J. Morace, R. Pavlat and J. Soobitsky , Laurel Brook Road, Middlefield, CT 06455

How to measure flying height? An optical height measurement is a preferred method for quality testing of read-write sliders in production, as well as for the development of new slider geometries that reduce flying height and increase storage density. Flying-height testers employ a glass substrate in place of the real magnetic disk, providing direct interferometric measurement of the sub-micron air gap between the glass and the slider.

Direct optical inspection provides important data on slider flight

characteristics and lowest height. Until very recently, flying-height test equipment employed a simple

Laser

intensity measurement at normal incidence.1,2,3,4

Receiver s

p

In 1995, the Data

Storage Industry began working Rotating glass disk

with polarization-based testing to improve the quality and reliability of

Slider Positioner

the measurement for near-contact read-write sliders.5,6,7 As Figure 1

Figure 1:

Geometry of a polarization-based flying height

tester for read-write sliders.

shows,

polarized

light

passes

through the rotating disk, reflects

The receiver measures the

intensity and polarization state of the reflected light.


from the slider-glass interface and passes back through the glass to an interferometric receiver. The receiver analyzes the intensity and polarization state of the reflected beam, thereby providing data for calculating the narrow air gap between the glass and the slider.8,9 Polarization-based testing enjoys several distinct advantages. measurement

is

intrinsically

LASER

The

CCD CAMERAS

more

sensitive at very low flying heights LED

( < 25 nm ) than a simple intensity measurement alone.

The technique

BEAM-SHAPING OPTICS

250 kHz INTERFEROMETRIC RECEIVER

does not require multiple colors, so the cumbersome arc lamp of traditional

DISK

test equipment may be replaced with a long-life,

30,000-hr

laser

SLIDER

diode. Figure 2: Optical design for polarization interferometry.

Further, analysis of the polarization state of the reflected light makes it possible to measure the optical constants n & k of the slider material in situ, obviating the need for a separate metrology step with an ellipsometer. 6,8

Optical design Polarization-based flying height sensing requires a sophisticated optical system. The design shown in Figure 2 achieves a precisely-focused, 25-nm diameter measurement spot at a 50 angle of incidence. Additional optics direct the reflected beam to a high-speed interferometric receiver that measures the relative phase of the s and p polarization states as well as their absolute intensities.10 Inspection cameras at normal and oblique viewing angles facilitate spot and slider positioning. Lateral positioning is assured by precision mechanical stages.


Accuracy, repeatability, reproducibility… The true test of a new measurement technology is how it performs. There are several ways to quantify the measurement quality. For example, a good test is to load a slider, measure the average flying height over a few disk rotations, unload, and repeat. The standard deviation for a series of such trials is a meaningful quantifier of instrument repeatability. The data in Figure 3 show a repeatability of 0.04 inch ( =1 nm, including in situ calculation of n & k ).

10 HEIGHT (nm)

HEIGHT (nm)

50

40

30

0

5

10

TRIAL

Figure 3: Repeatability with retract & load.

0

-10

2

3 SPOT POSITION (mm)

4

Figure 4: Resolution at contact.

Another important indicator of measurement quality is resolution. Qualitatively, resolution refers to the ability of an instrument to distinguish between neighboring values, and is closely related to the signal to noise level.

Resolution is particularly important near contact

(