NBS

PUBLICATIONS

NBSIR 86-3306

Procedure for Calibration of

Ferrite Gaps in Magnetic Tape

Heads Traceable to

NBS AR-Chromium Optical

Linewidth SRMs

A 1 1 1 2 S11112

NATL INST OF STANDARDS & TECH R. C

All 10251 11 12

Nyyssonen, Diana/Procedure for callbratl QC100 .1)56 NO. 86-3306 V1986 C.2 NBS-PUBU.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Electronics Division

Gaithersburg, MD 20899

February 1986

U.S. DEPARTMENT OF COMMERCE

AL BUREAU OF STANDARDS

100

• U56

86-3306

1986

C. 2

MSS RESEARCH IMFORMATION

CENTER uc

u s y

NBSIR 86-3306

* # 5

PROCEDURE FOR CALIBRATION OF

FERRITE GAPS IN MAGNETIC TAPE

HEADS TRACEABLE TO

NBS AR-CHROMIUM OPTICAL

LINEWIDTH SRMs

D. Nyyssonen

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Electronics and Electrical Engineering

Semiconductor Electronics Division

Gaithersburg, MD 20899

February 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

Procedure for Calibration

of

Ferrite Gaps in Magnetic Tape Heads

Traceable to

NBS AR-Chromium Optical Linewidth SRMs

Diana Nyyssonen

Semiconductor Electronics Division

National Bureau of Standards

Gaithersburg, MD 20899

Abstract

Accurate calibration of micrometer and submicrometer optical linewidth measuring systems requires that the calibration standard

match the properties of the line to be measured. The NBS photomask

linewidth standards have been designed for use by the integrated

circuit community and are not directly suitable for use in other

applications. A method of calibrating systems for measuring the

width of ferrite gaps in magnetic tape heads has been developed

that involves a two-step calibration using the NBS antireflectingchromium photomask as the primary reference standard. This primary

standard is used in transmitted green light to calibrate the linewidths on a secondary black-chromium photomask. This is a valid

procedure because these lines are patterned in black-chromium

layers that are thin compared to the wavelength of the green light

and have negligible transmission. Since this secondary blackchromium photomask has similar optical properties to ferrite gaps

when viewed in reflected green light, it can then be used to calibrate the linewidth measuring system to be used for ferrite gaps.

The results of a preliminary study of this method shows that the

errors associated with this two-step process are below 0.1 pm. The

study also indicated that without this two-step process, the errors

could be 0.3 ym or more.

Key words: dimensional metrology; gapwidth; linewidth; NBS photomask standards; magnetic tape heads; tape-head gaps
2

Accurate calibration and transfer of micrometer and submicrometer linewidth

measurements optically using a standard reference material ( SRM) required

that the optical parameters of normalized transmittance (or reflectance) and

phase at the line edge as shown in figure 1 for both the SRM and the part to

be measured match. When they do match, the optical edge image profiles or

waveforms used by the system to make the measurements will be the same, and

accurate transfer of dimensions is possible.

Using the method to be described here, either the edge detection threshold

used by the system is set to some value using the SRM, or when a fixed

threshold or other edge detection criterion is used, the measurement data are corrected based on a calibration curve derived from the SRM measurement data.

If the optical parameters do not match, the adjustment of the edge detection

threshold or the correction applied to the data will be incorrect for the

part being measured, and errors will result.

It is because of this source of error that the use of the NBS anti-reflective

(AR) chromium optical linewidth SRM will result in measurement errors if used

in any other mode than that for which it was intended, i.e., measurement of

opaque photomask materials viewed in transmitted light at a mean wavelength

of approximately 530 nm.

However, for the case of measurement of the width of the ferrite gaps in

magnetic tape heads, a two-step calibration method has been devised which

appears to yield a reasonably accurate calibration and transfer of linewidth

measurements. This method uses a secondary standard, a black-chromium photomask consisting of a single opaque chromium-oxide etched layer on a glass

substrate which has the following properties: 1) it is opaque when viewed in

transmission and therefore matches the transmission optical parameters of the

NBS AR-chromium photomask SRMs, and 2) when viewed in reflection, its optical

parameters are very close to the optical parameters of the ferrite gap in

magnetic tape heads (fig. 2).

In this method, a transmitted light optical linewidth measurement system is

set up and calibrated following the procedures accompanying the NBS AR-
3

TRANSMITTED

4> = - (t>

y + kd

1 2

<t>

REFLECTED

= <t>2 - <^] + 2kd

n0

4 d

* 1 2 n s substrate

n0

n$ substrate

(a) (b) (c)

Figure 1. Definition of parameters R and <f> used to characterize line object,

(a) Image edge profile showing IM and l 0 where R = Iq/Im* Tc is

the optical intensity threshold corresponding to the edge location. Ihe

optical phase difference <j> is determined from the optical path difference

between rays 1 and 2 for transmitted light (b) and reflected light (c)

.

In

each case, the values of <J>i and <(>2 can be calculated from the given

complex indices of refraction nQ and n s and the thickness d of the etched

layer using the Fresnel equations. <j> 1 and <J> 2 represent the phase change of

the respective rays upon reflection or transmission.

(a) (b)

Figure 2. Comparison of the image profiles (waveforms) for reflected light

measurement of (a) black-chromium photomask, and (b) the ferrite gap in a magnetic tape head. Measurements were made with laser illumination at a wavelength of 514 nm.
4

chromium photomask SRM. A black-chromium secondary standard (such as that

sold by Roger Sherman Co. [1], Los Gatos, CA) is measured. Note that for

measurement of the ferrite core in magnetic tape heads, only Rows A and E on

the NBS SRM need to be used to generate a calibration curve. The blackchromium secondary standard is then used to set up and calibrate a reflected

(incident) light linewidth measurement system which is used to measure the

magnetic tape head parts. The same procedures may be followed for setup and

calibration of the reflected light system as was used for the transmitted

light measurements.

It is also important that the reflected light system be spectrally filtered

for green light. However, the accuracy and reproducibility of this method

depends critically on how well the waveforms match at each step. (Note that

the coherence parameter and spectral bandwidth are not as critical here as for wafers.) It is recommended that an oscilloscope or chart recorder be

used to display and compare the edge image profiles or waveforms at each step

to determine how closely they match.

NBS has tested this method, checked the accuracy of the calibration of the

black-chromium photomask secondary standard, and compared the reflected light

profiles for both the black-chromium photomask and the ferrite gap in the

magnetic- tape head samples as shown in figure 2 using the NBS laser linewidth

measurement system described in references 2 and 3. Based on this brief

study, it appeared that without this two-step process, errors of 0.3 pm or more could result. With the two-step process calibration process, these errors could be reduced below 0.1 pm. In these measurements, the accuracy

and reproducibility appear to be limited by the quality (edge sharpness and

roughness) of the part being measured as well as by the ability to match

waveforms

.
5

References

1. Certain commercial equipment, instruments, or materials are identified in

this rep>ort in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material

or equipment identified is necessarily the best available for the purpose.

2. Nyyssonen, D. , Calibration of Optical Systems for Linewidth Measurements

on Wafers, Opt. Eng. _21_ (5), 882-887 ( September-October 1982).

3. Nyyssonen, D. , Theory of Optical Edge Detection and Imaging of Thick

Layers, J. Opt. Soc. Am. 12_, 1425-1436 (1982).
NBS-114A (REV. 2-8G) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

SHEET (See instructions) NBSIR -36/3306 February 1986

4. TITLE AND SUBTITLE

Procedure for Calibration of Ferrite Gaps in Magnetic Tape Heads Traceable to NBS

AR-Chromium Optical Linewidth SRMs

5. AUTHOR(S)

D. Nyyssonen

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10.

SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here) Accurate calibration of micrometer and submicrometer optical linewidth measuring

systems requires that the calibration standard match the properties of the line to

be measured. The NBS photomask linewidth standards have been designed for use by

the integrated circuit community and are not directly suitable for use in other

applications. A method of calibrating systems for measuring the width of ferrite

gaps in magnetic calibrating systems for measuring the width of ferrite gaps in

magnetic tape heads has been developed that involves a two-step calibration using

the NBS antire fleeting-chromium photomask as the primary reference standard. This

primary standard is used in transmitted green light to calibrate the linewidths on

a secondary black-chromi um photomask. This is a valid procedure because these

lines are patterned in b lack -chromi um layers that are thin compared to the

wavelength of the green light and have negligible transmission. Since this

secondary black -chromi um photomask has similar optical properties to ferrite gaps

when viewed in reflected green light, it can then be used to calibrate the

linewidth measuring system to be used for ferrite gaps. The results of a preliminary study of this method shows that the errors associated with this

two-step process are below 0.1 ym. The study also indicated that without this

two-step process, the errors could be 0.3 pm or more.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

dimensional metrologv; gapwidth; linewidth;

heads; tape-head gaps

13. AVAILABILITY

NBS photomask standards- magnetic-taoe

14. NO. OF

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