A COMPUTER INTEGRATED SPECTROPHOTOMETER FOR FILM THICKNESS MONITORING

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Submitted on 1 Jan 1983

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A COMPUTER INTEGRATED

SPECTROPHOTOMETER FOR FILM THICKNESS MONITORING

H.-E. Korth

To cite this version:

H.-E. Korth. A COMPUTER INTEGRATED SPECTROPHOTOMETER FOR FILM THICK- NESS MONITORING. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-101-C10-104.

�10.1051/jphyscol:19831021�. �jpa-00223478�

Colloque CIO, supplément au n°12, Tome M, décembre 1983 page C10-101

A COMPUTER INTEGRATED SPECTROPHOTOMETER FOR FILM THICKNESS MONITORING H.-E. Korth

IBM Germany, D-7032 Sindelfingen, F.R.G.

Résume - Nous décrivons un spectrophotomètre à réseau miniaturisé, destiné à l'analyse des couches minces, cons- truit pour s'intégrer dans l'environnement d'un mini- ordinateur. Le spectromètre, l'électronique de contrôle, la mémoire et l'alimentation sont intégrés sur un seul cir- cuit imprimé.

Abstract - A miniaturized grating spectrophotometer for thin film analysis is described that was designed to fit into a minicomputer environment. Spectrometer, control electronics, buffer storage and power supply are integrated on a single print card.

Introduction

Thin films of various kind are essential constituents of computer components. Epitaxial layers, isolation layers and photoresist layers are thin films typically occurring in semiconductor components, while magnetic coatings and air gaps are important in magnetic storage products. For the thickness measurement of such films the evaluation of white light reflectance spectra is an approach that produces fast, accurate and unambiguous results (1-4).

The implementation of this technique in an advanced manufacturing or testing environment requires a simple and compact spectrophotometric sensor. Interfacing the probing site and the spectrometer with a fiber optic cable gives a maximum of freedom in the definition and respec- tive location of tester and evaluation device.

The computer integrated spectrophotometer presented here was developed from the FTP (Film Thickness Probe) spectrometer head; a device that has found wide distribution within IBM.

The Spectrophotometer

To fit on a printed circuit card in a densely packed card cage a maximum thickness of 12 mm was allowed for the spectrophotometer. This requirement led to an in-plane design with a blazed reflection grating

(300 lines/mm) and an achromate lens (f=40 m m ) , that both collimates the light emerging from the multimode fiber and focuses the diffracted light on a linear photodiode array (IPL 4050). A deflection prism is used to separate input and output light beams and to align the spec- trum on the photodiode array.

A field stop in front of the first of the fifty photodiodes allows to use this diode as a reference to compensate the array for temperature drift.

Using a well-centered fiber and a tightly tolerated connector the fiber cable can be removed and reinserted without recalibration of the spectrometer.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831021

CIO-102 JOURNAL DE PHYSIQUE

The spectrometer is aligned using the transmission spectrum of a Schott BG36 glass whose characteristic peaks can be detected automa- tically by software.

The spectral range from 550 nm to 950 nrn matches the sensitivity distribution of the photodiodes, the transmission of the optical system and the spectrurn of the illurnination (halogenide lamp).

The resolution of the spectrometer is limited by the convolution of input and output aperture, i.e. the fiber core and the photodiode size. With a spectral resolution of about 10 nm a wide range of film thickness variation can be covered.

The fiber optic cable

For a fiber in a spectrophotometer set-up, there are quite different requirements in comparison to data transmission link applications. As the overal length of the fiber cable is typically some 20 meters, light absorption in the fiber is not a critical parameter. Dispersion effects can be neglected too.

The fiber core diameter should be as large as possible to maximize light thsoughput, but not much larger than the photodiode size

(100/uml in order to avoid a degradation in spectral resolution.

This means that good performance can be expected for core diameters in the range 5 0 p m to 250 pm. The core diameter together with the relay lens magnification defines the probing spot size on the object under test.

The divergence of the light emerging from the fiber is of critical importance for the sensitivity of the system, because the aperture of the spectrophotometer is limited due to the compact design. This means that gradient index fibers should not be used. Best performance can be expected if high quality step index fibers are used with a minimum of flaws and scattering that produces mode coupling and hence increases light divergence.

Fig. 1. Spectrometric sensor card.

The spectrophotometer (top right) receives light through a multimode fiber. The A/D converter (center) digitizes the photodiode signals. A buffer (top left) then stores the data. Power supplies and control circuitry (below) complete the card. Via top- connectors the card is attached to the computers DI/DO interface.

with 250/um core diameter and a 100 ,um glas fiber with rounded step index profile. For short cable length (10 m) the PCS fiber showed good performance. For 25m cable length significant losses ( - 10 dB) occur- red due to high output divergence. Here the rounded step fiber per- formed better (loss about 6 dB).

The control electronics

The spectrophotometer is mounted on a standard size print card for direct insertion into a mini-computer (IBM S/1). The card holds

several power supplies for the photodiode array, the clocking circui- try and the analoq/digital conversion of the diode array output

(Fig. 1 ) . Moreover, a 1K-word buffer memory was provided to become independent of the processor timing. This means that the spectrophoto- meter can be started to execute a preselected number of spectrum readings. When the processors workload permits these stored readings will be transferred to the main memory via a standard DI/DO interface.

Fig. 2. Spectrum with envelopes.

Reflectance spectrum from a SiO film (thickness = 1139 nrn) on gi. The data were norma- lized with the known reflec- tance of a silicon reference surf ace (upper envelope)

.

lower envelope was calculated from the refractive indices of film and substrate.

Film Thickness Algorithms

The normalized spectrum (Fig. 2) that is obtained after subtraction of the diode arrays "dark signal" and after comparison with a "reference signal" (e.g. blank substrate) can be evaluated in different ways. The normalized spectrum is a periodic approximately cosine-shaped function of the wavenumber. The density of the periodic fringes is proportional to the product of film thickness and refractive index. The films refractive index (and dispersion) as well as the phase shift at the film and substrate are to be determined separately.

To select an optimum algorithm one has to make a trade off between accuracy, measuring range, error detection/correction, a priori information and processor/memory requirements:

o The straightforward search for extrema of the normalized spectrum is very fast but sensitive to signal noise and limited to spectra with at least one extremum.

o The iterative computation of a spectrum with minimum deviation from the measured spectrum is time consuming but accurate.

0 Phase reconstruction (i.e. table look-up for the inverse of the periodic spectrum function, matching of sign and order number and film thickness computation from averaged slope) is fast and uses all the information contained in the spectrum.

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o Difference ratio table look-up from three consolidated points on the spectrum is limited to low order spectra but fast and accu- rate. This algorithm eliminates any effects from unanticipated spectrum amplitude and offset variations.

Using the phase reconstruction algorithm the following performance data were obtained:

Thickness range solid layers 15

-

-

<

Repeatability up to 3000 nm

-

Spectrum acquisition 6 msec.

Measuring cycle 30

-

The phase reconstruction algorithm was extended for multilayer systems With the recursive "effective substrate" approach (3) the thickness of the top layer can be determined, when the optical constants of the substrate and all layers as well as the thickness of the lower layers are known.

Present Field Applications of FTP o Semiconductor manufacturing.

N

,

Multilayer structures including poly-Si.

o Thin film magnetic head manufacture.

A1 0 and photoresist layers on permalloy. Flight height an3 !?lying angle.

Feasibility was shown for

o Etch and development rate monitoring

o Real time monitoring of photoresist spinning o Coating thickness measurement of magnetic disks.

References

1. CORL E.A., WIMPFHEIMER H., Solid State Electr. (1964) 755 2. KONNERTH K.L., DILL F.H., Sold State Electr. (1972) 371 3. HAUGE P.S., J. Opt. Soc. Am

69

4. FLOWERS D.L., HUGHES H.G., Semicond. Intern., Jan (1981) 79