JOURNAL OF THE OPTICAL SOCIETY OF AMERICA

An Infrared Reflectometer*t

MAX S. OLDHAMINaval Ordnance Laboratory, White Oak, Maryland

(Received May 14, 1951)

The design and construction of a reflectometer for use in the infrared are described. The properties ofthis reflectometer include the use of essentially parallel radiation and the capability of choosing the angleof incidence at the surface being investigated. The alignment and calibration procedures which were usedare presented. A description of component parts such as special mirror mounts and infrared polarizers aswell as type and reliability of resultant measurements are included. Sample measurements taken on evapo-rated beryllium films of various thicknesses are presented in graphical form.

IN a study of the optical constants of thin evaporatedfilms,' it was found necessary to measure experi-

mentally the reflectance and transmittance of the filmsconsidered. For this purpose, a reflectometer attach-ment to a Perkin-Elmer infrared spectrometer was de-signed and built. This reflectometer, as constructed,had the following properties: (a) a capability for rapidand flexible use, (b) adaptability for the measurementof reflectance at various angles of incidence of anessentially parallel beam of radiation plane polarized atvarious angles with respect to the plane of incidence.A top view of the reflectometer is shown in Fig. 1.Radiation from the source Si is reflected from a 10° off-axis parabolic reflector Ml to the sample at P, throughthe polarizer, reflected from the plane mirror M2 andanother 100 off-axis parabolic mirror M3, and thencefocused on the entrance slit of the spectrometer. Thesource Si and the mirror Ml were mounted on a hori-zontal disk rotatable around a vertical axis at P. Thesample, indicated by the dotted line, was mounted on

S~~~~~~~~S

FIG. 1. Schematic diagram of the IR refiectometer.

* Presented in part at the New York meeting of the OpticalSociety of America, March, 1948.

t A portion of a doctoral dissertation, Iowa State College,Ames, Iowa, 1949.

: Present address: Headquarters, Strategic Air Command,Offutt Air Force Base, Omaha, Nebraska.

I M. S. Oldham, doctoral dissertation, Iowa State College,Ames, Iowa, 1949.

a smaller disk which was free to rotate independentlyaround the same axis. Thus, relative positions could bechosen for the two disks such that the path of theradiation through the polarizer remained unaltered forvarious angles of incidence at the sample. Notches andmechanical stops were installed for rapid and repro-ducible choice of angle of incidence at the sample. Thesource S, was an electrically heated carborundum rod(Globar) furnished as a standard source with the spec-trometer. The two 10° off-axis parabolic mirrors, each-inch-diameter and 5-cm focal length were furnished

by the Ferson Optical Company. The polarizer andmechanical parts were constructed at the Naval Ord-nance Laboratory.

The mirror mounts which carried M, and M3 (Fig. 1)are shown in detail in Fig. 2. The adjusting screws pro-vided for rotation of the mirrors around a point at thecenter of their reflecting surface. This feature greatlyfacilitated the alignment of the system. The polarizers,shown in Fig. 3, utilizing two reflections at the polarizingangle of incidence from selenium plates, permitted thevariation of the plane of polarization without eitherdisplacing or changing direction of a beam of radiationtraversing the polarizer. The sample holder, mounted onthe smaller disk (Fig. 1), had adjusting screws to permitpositioning the sample so that its front surface wascoincident with the axis of rotation of the two rotatabledisks. In Fig. 4 is shown the completely assembledreflectometer attached to the spectrometer. The angle

FIG. 2. Schematicdiagram of the mir-ror mounts.

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MAX S. OLDHAM

FIG. 3. Schematic diagram of the IR polarizer.

of incidence of the radiation beam at the sample in thisphotograph is approximately 300.

Before the radiation from the mirror M3 (Fig. 1)entered the entrance slit of the spectrometer, it waschopped at 15 cycles per second. The chopped radiation,after traversing the spectrum path through a rocksaltprism, was finally brought to focus on a BTL thermistorbolometer.

The following procedure was adopted to achievealignment of the reflectometer. The off-axis parabolicmirror (M3 in Fig. 1) was first mounted at its focaldistance from the entrance slit of the spectrometer.The position in space of the center of this reflector wasadjusted to be coincident with a central beam ofradiation originating at the exit slit and traversing thespectrometer in the reverse direction. This reflector wasthen adjusted by means of its special mount so thatthe incident central beam after reflection made anangle of 200 with its path before reflection. This re-flected central beam was then projected directly overthe axis of rotation of the two disks by adjustment ofthe plane mirror (M2 in Fig. 1). The larger disk wasthen rotated so that the central beam passing over theaxis of rotation P struck the mirror Ml at its center.M1 was then adjusted so that it formed an image of theilluminated exit slit on the surface of the globarsource S,. The source was then turned on and radiationfrom it was allowed to traverse the entire light path tothe detector. Final small adjustments were made tomaximize the signal on the recorder for fixed values of

FIG. 4. IR reflectometer mounted on PE spectrometer.

source intensity, slit widths, amplifier gain, and wave-length setting.

The alignment of the sample plate required that thefilm-bearing front surface be coincident with the axisof rotation P of the two disks. To check this, a sampleplate on which a number of vertical scratches had beenmade was installed on the mount. These scratches wereviewed with a low power microscope mounted hori-zontally, and the position of the plate was adjusteduntil rotation of the disks imparted no visible lateralmotion to the central scratch. Since the adjustable sup-porting points made contact with the front surface ofthe sample plate, it was possible to change plates with-out disturbing the latter adjustment.

To achieve the proper relative positions of the twodisks for the various desired angles of incidence,

100THICKNESS s IN ANGSTROMS

90

70

50

40

30

20 6E

I0

010 20 30 40 50 60 70 80 90

ANGLE OF INCIDENCE (DEGREES)

FIG. 5. Variation of reflectance, R, with angle of incidence forberyllium films of various thicknesses, S, at X = 2.0 microns.

V-shaped notches were cut at the appropriate places inthe disks' peripheries, and mechanical stops, consistingof small spring-loaded balls, were installed. This arrange-ment permitted a rapid and accurate choice of any ofeight different angles of incidence at the sample plate.To achieve correctness of position for the V-notches,a very small cross hair was mounted almost in contactwith the point on the globar source S1 on which thereverse beam of radiation mentioned above was broughtto focus. A plane mirror was then inserted between theaxis of rotation P and the plane mirror M 2 so that theessentially parallel beam of radiation originating at thecross hair was projected onto a screen several metersaway in a direction nearly normal to a line from theaxis P to the mirror M2. The image of the cross hair,being visible on the screen, was marked for vertical and

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INFRARED REFLECTOMETER

P-Z 60

-4'

.-9-

100 200 300FILM THICKNESS (ANGSTROMS)

100

801-

z0:

:

400

FIG. 6. Variation of reflectance at 100 angle of incidence, Rio,transmittance at normal incidence, To, and absorption, A, withfilm thickness, S, for beryllium films at X = 2.0 microns.

horizontal position. A sample was then installed in thesample holder, and upon proper rotation of the twodisks to the appropriate notches for reflection the imageof the cross hair was again visible on the screen. Thecorrectness of every set of notches was checked andadjusted by slight filing where necessary to bring theimage of the cross hair to the previously marked positionon the screen.

The polarizer was aligned separately so as to impartno deviation or displacement to an axial beam and thenplaced between the axis of rotation P and the mirror M2.It was aligned with respect to the parallel portion of thebeam to achieve maximum signal on the recorder.

The apparatus was then complete and aligned. Abso-lute calibration was achieved by testing samples ofknown reflectivity such as aluminum front surfacemirrors, glass plates, etc. Samples could be interchangedon the sample holder with readings reproducible towithin less than 1 percent. The two disks could berotated to the positions for the various angles of inci-dence at the sample plate in any order without affectingthe resulting readings. Over a period of about an hour,the time necessary for a complete set of readings on onesample, the clear beam reading and the zero readingvaried less than 1 percent. It was estimated that the

60

40

20 1

0 I 2 3 4WAVE LENGTH g)

FIG. 7. Variation of reflectance at 100 angle of incidence, Rio, withwavelength for beryllium films of various thicknesses.

over-all experimental error of any reading was less than2 percent.

Measurements were taken of the reflectance atvarious angles of incidence and transmittance at normalincidence of glass-backed metal films of various thick-nesses. The thickness of all films considered was meas-ured by an interferometric method previously described.2

While the present investigation was limited to thewavelength region 1 to 3.5 microns, this limitation beingimposed by the transmission properties of glass backingplate, the equipment was capable of being used out tothe limit of the spectrometer with suitably prepared.samples.

Because the data gathered were so extensive, only asample is given here. In Fig. 5 is shown the reflectance,R. versus angle of incidence for glass-backed berylliumfilms of various thicknesses at X= 2.0 microns. Thevariation of the reflectance at 10° angle of incidence,Rio, the transmittance at normal incidence To, andabsorption A, with thickness S, for this same case isshown in Fig. 6. The percent absorption is hereindefined at 100-R 1 o- To. In Fig. 7 is shown the varia-tion of reflectance at 10° angle of incidence with wave-length for this same case. Similar curves were obtainedfor films of aluminum, copper, silver, gold, and platinum.

2 Oldham, Mundie, Matossi, and Cheydleur, J. Opt. Soc. Am.40, 544 (1950).

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