Browse Prior Art Database

MASK GRATING DUTY FACTOR METER

IP.com Disclosure Number: IPCOM000005415D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 3 page(s) / 130K

Publishing Venue

Motorola

Related People

Paul Fenlon: AUTHOR

Abstract

An accurate and convenient system for measuring the duty factor of periodic space patterns in sur- face acoustic wave devices and photomasks has been conceived and developed. The measurement technique, which reduces a tedious task to a simple light intensity measurement, is based on the energy distribution in the light beams diffracted by a periodic grating. A laboratory version of the system is described.

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m MOlOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980

MASK GRATING DUTY FACTOR METER By Paul Fenlon

ABSTRACT

   An accurate and convenient system for measuring the duty factor of periodic space patterns in sur- face acoustic wave devices and photomasks has been conceived and developed. The measurement technique, which reduces a tedious task to a simple light intensity measurement, is based on the energy distribution in the light beams diffracted by a periodic grating. A laboratory version of the system is described.

INTRODUCTION

  Surface acoustic wave (SAW) devices are built on polished crystal substrates by forming arrays of metal stripes by photo-lithographic techniques. Ultraviolet transparent photomasks are used to define the pattern arrays in photoresist material. Subsequent etching defines the metal line patterns which can extend to 300 to 500 line pairs of alternating metal stripe and crystal surface. The ratio of the metal line width to the periodic pattern spacing is called the duty factor. The measurement of this quantity is the subject of this disclosure.

   Figure 1Ashows the quantities which make up this ratio. It is an experimental fact that several impor- tant electrical and thermal properties of SAW devices depend strongly on the duty factor of the metalliza- tion in transducer and reflector arrays. The measurement of this quantity on photomasks and on finished wafers is an important step in the control of the SAW device manufacturing processes.

   On low frequency devices (f < 50 MHz) the duty factor measurement may be made precisely, but in- conveniently, with a conventional medium power measuring microscope. VHF and UHF devices have line widths of only a few micrometers to direct microscopic measurements, even at high magnification, have little precision. An alternative method, measurements made on a photomicrograph, is time consuming and expensive due to costly photographic materials. Thus, a need exists for a duty factor measurement technique that provides accuracy and precision without excessive expenditure of processing time and resources.

Figure la. Duty Factor of Grating

DIFFRACTION GRATING PROPERTIES

  When the transducer and reflector patterns of SAW masks and devices are examined visually, it is im- mediately apparent that they behave as diffraction gratings, scattering light coherently at angles depen- ding on the line spacing. This occurs because of the periodicity, extent, and fine line geometry of the pat- tern. The grating equation for light of wavelength A, incident on a grating at angle 81 with the surface normal is

sin0, = mh + sin91 (1)

S

@ Motorola. 1°C. 1980 10

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MOTOROLA Technical Disclosure Bulletin Vol. 1 No. 1 August 1980

where em is the angle between the surface normal and the diffracted beam of integral order m, and s is the periodic s...