SIMPLE TECHNIQUE FOR VERY THIN SiO2 FILM THICKNESS MEASUREMENTSW. A. Pliskin and R. P. Esch Citation: Applied Physics Letters 11, 257 (1967); doi: 10.1063/1.1755124 View online: http://dx.doi.org/10.1063/1.1755124 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/11/8?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Reduction in intermediate layer thickness of CoCrPt – SiO 2 perpendicular recording media by using Ru –SiO 2 J. Appl. Phys. 99, 08E713 (2006); 10.1063/1.2177128 Assessment of UltraThin SiO2 Film Thickness Measurement Precision by Ellipsometry AIP Conf. Proc. 683, 326 (2003); 10.1063/1.1622490 SiO2 thinfilm deposition by excimer laser ablation from SiO target in oxygen atmosphere Appl. Phys. Lett. 57, 664 (1990); 10.1063/1.104253 Method for Vacuum Evaporation of SiO on Thin Plastic Films Rev. Sci. Instrum. 43, 1382 (1972); 10.1063/1.1685935 Spectrophotometric Thickness Measurement for Very Thin SiO2 Films on Si J. Appl. Phys. 41, 787 (1970); 10.1063/1.1658750

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Volume 11, Number 8 APPLIED PHYSICS LETTERS 15 October 1967

When activated with platinum, a number of other oxides have been found to exhibit rapid resistance changes in response to airborne hydrogen. At a temperature of 300°C, these include the oxides of molybdenum, chromium, titanium, iron, niobium, and nickel (chromium oxide shows a resistance in­crease with increasing hydrogen concentrations). Other activators (short-term tested on tungsten oxide) include monolayer amounts of iridium, rhodium, gold, and palladium.

H. R. Phillip, A. E. Newkirk, W. Tantraporn, and J. R. Macintyre have made valuable contri­tions to various portions of this work.

1 T. Seiyama, A. Kato, K. Fujiishi, and M. Nagatani, Anal. Chern. 34, 1502 (1962).

2T. Seiyama and S. Kagawa, Anal. Chern. 38, 1069 (1966). 3S. Sawada,]. Phys. Soc. japan 11, 1237 (1956). ·S. Sawada and G. C. Danielson, Phys. Rev. 113, 803 (1959). 5 E. Gebert and R. J. Ackermann, Inorganic Chern. 5, 136 (1966).

SIMPLE TECHNIQUE FOR VERY THIN Si02 FILM THICKNESS MEASUREMENTS*

W. A. Pliskin and R. P. Esch IBM Components Division

East Fishkill Facility Hopewell Junction, New York

(Received 10 July 1967; in final form 13 September 1967)

The thickness of films «900 A) of silicon dioxide on silicon can be determined by observation near Brewster's angle for silicon with the light polarized in the plane of incidence. Since the intensity of the reflected light in­creases with thickness for these films, the measurement is made by direct comparison of the light reflected by a sample with a calibrated Si02 on silicon "thickness gauge." Film thicknesses can be estimated to an accuracy of about ±30 A for thicknesses less than 150 A and ±50 A for thicknesses between 150 A and 900 A.

The purposes of this Letter are to show that (1) an inexpensive reflection technique for the meas­urement of silicon dioxiqe filmsl on silicon can be used for thickness ranges where other simple techniques2•3 are not operable; (2) thickness values obtained from the peak intensity of the Si-O stretch­ing band (obtained by transmission infrared spec­troscopy) are not as accurate as those obtained from the integr<\ted band intensity; and (3) P-etch rates increase for very thin Si02 films.

For (1), the technique utilizes the fact that light is polarized perpendicular to the plane of incidence at Brewster's angle for silicon. If a polarizer is used so that the reflected light is polarized in the plane of incidence, the surface should appear black. An apparatus needed for this technique is shown in Fig. 1. The substrate holder is turned to Brewster's angle and the polarizer is aligned to allow only the parallel component to be transmitted. When an oxiqe or other film is on silicon, reflected light is nb longer polarized perpendicular to the plane of incidence and can be observed. The amount of reflected light increases with film thickness. Thus,

*Presented as "recent news" paper at Electrochemical Society Meeting, Dallas, Texas, May 1967.

the film thickness of the sample can be determined by comparison with a wafer containing calibrated thicknesses. The accuracy is ±30 A for thicknesses less than 150 A and ±50 A for thicknesses between

Ol~

MICROSCOPE

OBJECTIVE

,--....-_ FLUORESCENT BULB

i~ _ L ~ ROTATING SAMPLE

MOUNT

SAMPLE

_~ __ --------MIRROR

Fig. 1. Schematic of Vamfo at Brewster's angle for silicon.

257

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Volume 11, Number 8 APPLIED PHYSICS LETTERS 15 October 1967

ISO A and 900 A. Such accuracies can be obtained by microscopic observation of areas corresponding to a 20 to 30 mil square.

The thicknesses on the calibrated silicon dioxide gauges were determined by using infrared trans­mission spectroscopy through nondegenerate float­zone silicon. Oxide films were grown on the silicon by oxidation at about IOOO°C in oxygen or steam. After each infrared measurement, the film thick­ness was decreased using P-etch; a small region of the wafer was left with the previously measured thickness to serve as a step gauge. In taking each spectrum, a silicon wafer of approximately the same thickness and taken from the same crystal as the sample wafer was placed in the reference beam to compensate for the weak silicon lattice absorption band so that it does not interfere with the Si02 band intensity measurements. The strong 9 JL band, due to oxygen dissolved in silicon, was eliminated by using float-zone or Lopex material instead of pulled crystals.

For (2), the optical density (O.D.) at the band peak can be used as a measure of the film thickness, but the O.D. is not linear with thicknesss. This is because the shape of the absorption band varies with film thickness. The band half-width increases slightly with film thickness and the relative contri­bution of the shoulder near 8 JL increases with film thickness. A more accurate measurement can be ob­tained by integrating the band intensity of a plot of O.D. vs cm- I

. In Fig. 2, curve A is a plot of the transmittance spectrum. The optical density at any frequency (v) is given by

O.D. = LoglO :;'v v

100

90

~ 80 0.00

~ >-

i ~

70 0.05 en z ~ ..J ... 60 0.10 c

~ f 0

50 0,15

O.ZO

1300 IZOO 1100 1000 0.Z5

FREQUENCY (CM-I)

Fig. 2. Transmittance spectrum (A) and optical density (8) VI frequency.

258

where Tv is the transmittance at frequency v and Tov is the background level at the same frequency. The background level is determined by the line drawn more or less tangent to the wings or edges of the absorption band. Curve B is a plot of the opti­cal densities as a function of frequency. By inte­grating curve B over the entire absorption band a better measure of the number of Si-O bonds being measured and consequently the film thickness is obtained, even though minor density and structure changes with film thickness are neglected.

The integrated band intensities were found to be linear with film thickness for films which could be measured by normal Vamf03 measurements as shown in curve B of Fig. 3. Curve A, which shows the relationship between the peak optical density and film thickness, is not linear. The thickness of each thinner film was determined from its inte­grated band intensity using curve B. Then curve A

CM-I

3OO0r-~~~2~0~~~~~~~OO~6~0~7~0~80~~~~I~

~""~ -'~~~"~fh~" 2700

2400

Iii ! 2100

~ 1800

CI>

~ 1500

" ~ 1200

~ 900

600

300

o

/ ///

CURVE B (STRAIGHT LINE) ,,// FILM THICKNESS VS _

II~~%~~W 8AND /

,/ / CURVE A / "-- FILM THICKNESS VS

,/,/ PEAK oPTICAL DENSITY

0.05 0.10 0.15 0.20 0.25 O.~ 0.35 o.~ 0.45

PEAK OPTICAL DENSITY OF 9,3" SiOz BAND

Fig. 3. Integrated band intensity and peak optical density vs film thickness.

i ~ g iii

I zoo

800

600

400

zoo

0 50 100

P-ETCH RATE OF VERY THIN THERMAlLY GROWN (DRY Oz AT 965·C) SiOZ FILM

~ SLOPE REPRESENTATIVE "- OF P-ETCH RATE OF

"'- ..... //"THICKER Si02 FILMS ~ AT 25.5·C, 2.ll/SEC .....

"- ..... ...... .....

" , " -, .,

150 200 250 300 350 400

P-ETCH TIME (SECS)(T·25.5OC)

, 450

Fig. 4. P-etch rates for thin and thick Si02 films.

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Volume 11, Number 8 APPLIED PHYSICS LETTERS 15 October 1967

was plotted from the corresponding peak optical densities. Subsequent thickness measurements were then made from the peak optical density and curve A.

With device wafers one cannot use transmission infrared spectroscopy because of the high doping levels. However, the technique of comparing an unknown sample with a calibrated sample at Brews­ter's angle with the light polarized in the plane of incidence offers a simple and fast technique for reasonable thickness estimates of very thin films.

For (3), a plot of the P-etch rate for very thin

films of Si02 is shown in Fig. 4. This shows that the P-etch rate of Si02 films «1100 A) is faster than for thicker Si02 films. This increased P-etch rate could be attributed to increased strain or to struc­tural differences in the very thin films compared with the thicker fillns.

1 E. A. Corl and H. Wimpfheimer, Solid State Electronics 7, 755 (1964).

'w. A. Pliskin and R. P. Esch,J. Appl. Phys. 36, 2011 (1965). 30ther transparent films for which a standard can be ob­

tained can be estimated by this method.

DIRECT OBSERVATION OF RAPID SELF-DIFFUSION ALONG DISLOCATIONS IN ALUMINUM*

T. E. Volint and R. W. Balluffit Cornell University Ithaca, New York

(Received 17 August 1967; in final form 14 September 1967)

Voids were produced by the precipitation of excess quenched-in vacancies in aluminum. The annealing of these voids in thin foil specimens by a self-diffusion mechanism was studied using transmission electron microscopy. The annealing rates of both isolated voids and voids which were connected to the foil surfaces by dislocations were observed. The voids which were hooked up to the surfaces by dislocations annealed out at appreciably higher rates than the isolated voids. The results indicate that the dislocations acted as rapid self-diffusion pipes for the transport of atoms into the voids from the surfaces.

Under appropriate experimental conditions the excess vacancies quenched into aluminum can be induced to aggregate into voids which are approxi­mately spherical. 1 In thin foil specimens these voids anneal out during subsequent isothermal annealing at measurable rates in the temperature range -100-200°C by a volume diffusion mechanism in which atoms are transported from the foil surfaces to the voids via exchange with vacancies. The writers have previously made a detailed study of the kinetics of this annealing process by means of transmission electron microscopy measurements and have con­cluded that the process is self-diffusion controlled.2

Also, values of the self-diffusion coefficient in the relatively low temperature range 100-200°C were derived from the data. 2

In the present work we plastically deformed a

*This work was supported by the U.S. Atomic Energy Com­mission. Additional support was received from the Advanced Research Projects Agency through the use of the technical facili­ties of the Materials Science Center at Cornell University.

t Department of Materials Science and Engineering, Cornell University, Ithaca, New York.

number of specimens contammg voids and gen­erated dislocations which occasionally became trap­ped at voids as shown schematically in Fig. 1. In such cases the voids were therefore "hooked up" to the adjacent surfaces by dislocations. During sub­sequent annealing the voids connected to disloca­tions in this way annealed out at considerably faster rates than isolated voids which were similar in all other respects. Examples are given in Figs. 2 and 3. The only likely explanation for this effect is that the dislocations acted as rapid diffusion pipes for

I Vlewlno ~ Direction

~~--!::T---- --=--.0 ~~ .......... ~, .....

~B ~ Fig. 1. Void (on left) in thin foil hooked up to surfaces by

dislocation. A and B are intersection points by the dislocation with the free surfaces. An isolated void is shown on the right.

259

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