Thin film thickness distribution by alpha absorption

NUCLEAR

INSTRUMENTS

A N D M E T H O D S T (1960) 160--166, N O R T H - H O L L A N D

PUBLISHING

CO.

THIN FILM THICKNESS DISTRIBUTION BY ALPHA ABSORPTION M. D E CRO~S, W. P A R K E R and K. SEVIER, Jr. Institute of physics, Uppsala, Sweden

Received 4 Januari 1960

An i n s t r u m e n t for the determination of t h i n film thickness distribution is described. Thickness distribution curves for films of organic compounds, metals, and aluminium oxide which are used primarily for counter windows and in

connection with radio-active source preparation are presented. The principle employed, where the "gas-equivalent" of the film or foil of material is measured, provides results to a good degree of accuracy.

1. Introduction absorption is where the detector is placed in a When measuring low energy beta emission fixed position and a suitable alpha emitting from isotopes and, in particular, where high isotope, such as Po 210, is attached to a microresolution is required, it is desirable to know not meter screw. The numbers of pulses obtained only the thickness of the supporting backing • over a given time interval are plotted against material but also the uniformity of its thickness. the various micrometer settings at which these Also when using thin films for Geiger counter numbers of pulses are obtained. The measurewindows, knowledge of the non-uniformity and ment is carried out, using air as the gas medium. thickness distribution is desired. Methods are The introduction of a foil or film between the available whereby the mean thickness of such source and counter causes a decrease in the materials m a y be determined but, with the counting rate which m a y be compensated for possible exception of weighing, these involve by adjusting the micrometer screw. Thus the expensive equipment. A technique which is stopping power, or rather the "air-equivalent", becoming more and more frequently used is the can be read directly from the micrometer. It will absorption of alpha and beta particles through be obvious that, using the above arrangement, matter and the literature dealing with this kind only limited accuracy can be expected due to of measurement is fairly extensivel-7). Of course, the fact that the solid angle subtended by the due to its comparatively high penetration ranges, source with respect to the counter is constantly beta radiation is more suitable for absorption being changed. measurements of films and foils on the order of 100 mg/cm~. Since such thicknesses are seldom 2. Apparatus In considering apparatus for the study of used in beta spectroscopy, where the foils or films are usually on the order of #g/cm 2 in alpha particle stopping powers and their thickness, alpha absorption is to be preferred. application to various other studies, two alterThe most common arrangement for alpha natives are to be considered: 1. The pressure of the gas medium is held 1) K. Siegbahn, Thesis. Nobel Institute for Physics, Stockholm, Sweden, 1944. constant while the source-film or the film~) W. C. Barber, Rev. Sci. Instr. 24 (1953) 469. counter distance is changed. s) L. Mandel, Brit. J. Appl. Phys. 5 (1954) 287. 2. The film, source, and counter are placed 4) E. N. Shaw, Radioisotope Conference 2 (1954) 140. 6) O. Te-tchao, J. Phys. Rad. 17, No. 12 (1956) 1019. at a known distance apart while the gas pressure 6) H. A. Enge et al., Rev. Sci. Instr. 28, No. 2 (1957) 145. is changed. ~) K. R a m a v a t e r a m and D. J. Porat, Nuclear InstruThe first of these is the easiest to utilize, but ments 4 (1956) 238. 160

T H I N F I L M T H I C K N E S S D I S T R I B U T I O N BY A L P H A A B S O R P T I O N

approximate measure of the uniformity of the film can be calculated from the difference in slopes of the two curves. The apparatus constructed and used by the authors is based on t h a t of Davison and can be seen in fig. 1. The vacuum chamber measures

the inverse-square effect of the intensity of the alpha beam and the uncertainty of the energy of the alpha particles entering or leaving the film add to the error of the calculations. The second has certain advantages which make it very attractive.

~,CROMETER

161

+.+

"

c++o,

H

V'J

C R O ~ SECTION A'A'

TO PUPlP

B

A

P

GAS INLET

Fig. 1. Construction of the measuring apparatus.

Davison8) has described a method of interest where the source and counter, as well as the film to be measured, are held fixed and the counting rate is plotted as a function of pressure of dry air. Since this arrangement enables fixed geometry to be employed, the inverse-square effect is avoided resulting in a sharper cut-off at the end of the absorption curve. The pressure displacement of the curve obtained when a thin film is inserted into the path of the alpha beam is a measure of the thickness of the film and is called the "air-equivalent" of the film. An s) W. H. V. Davison, J. Sci. Instr. 34 (1957) 418.

200 mm diameter by 50 mm deep. There are three inlets, one connected to a vacuum pump, the other two for introduction of the gas measuring medium during and air after a measurement. A glass/metal seal is provided for the high voltage connection to the counter. Mounted on the chamber's base plate are the Po 9'10 source, collimator, and counter. The foil or film to be measured is fitted in a traversable carriage intersecting the alpha particle path at right angles. The carriage m a y be moved across the alpha beam by turning a micrometer screw coupled to a rotating vacuum seal. The film to

162

M. DE CROES, W. P A R K E R AND K. S E V I E R , JR.

be measured is mounted on a ring which fits into the carriage. Hence, different points of the f i l l can be brought into the alpha particle beam and measured for thickness. The chamber top is fitted with an "o"-ring and covered by a lucite plate. Collimation is obtained by use of two alternative collimators having diameters of 1 mm and 3 mm. Two 1 mc Po ~'10 sources had been plated over an area of 3 m m diameter onto gold discs and coveredwith 0.6 mg/cm 2mica?. The detector is a halogen filled Geiger counter. The gas medium used during measurements is nitrogen. 3. Calculations When counting rate is plotted against the pressure of the gas in the chamber, there is a sharp drop-off signifying the limit of the range of the alpha-particles in the gas. When there is a uniform film or foil placed between the source and the counter and the counting rate is again plotted against the gas pressure, there will be found a sharp drop-off similar and parallelto the first but displaced to a lower pressure value by a certain amount, Ap. It is immediately obvious that the "gas-equivalent" of the film, signified by Ap, given in units of mass per unit area, will be expressed by: D.Ap 1 x= --p (1) 760 A where D is the source-counter distance; 760 m m Hg represents standard pressure; and A is the mass stopping power of the f i l l material relative to that of the gas; and p is the density of the gas. Hence, the precision of the deterruination of the thickness of the film is dependent upon the accuracy of both the measurement of the gas pressure and the temperature dependent density of the gas. The mass stopping power of the film material can be determined by calculations using (1) and the value of A obtained from the "gasequivalent" of films of known thickness. The mass stopping power of a material is dependent upon the energy of the alpha particles stopped by it. If E1 is the energy of the alpha ? Both Po 2x° sources were prepared by the Radio Chemical Centre, Amersham.

particle entering the film and E2 is the energy of the particle leaving the film, then a good approximation of the best value of the energy at which the stopping power is measured is (El + E2) •

(2)

Since the amount of gas between the source and the film and also the fill and counter vary with pressure, the values of El and E2 will also change resulting in the difficulty of arriving at the best energy value for a given material's mass stopping power as calculated from absorption curve data. The value of (2) has been calculated for each of the measurements and the average of these was evaluated to be 3.4 MeV. Ap was measured for the same energy of alpha particle each time, i.e., Ap was measured always at ½ of the maximum counting rate which is obtained at highest vacuum without a film interposed. Thus the difference in energy between the alpha particle leaving the source and that being detected is constant making (2) dependent upon the range of the particles as a function of particle energyS). For future reference a very good improvement of accuracy of the energy value at which mass stopping powers or thicknesses are measured is made possible by constructing the measuring apparatus so that the source-film distance is equal to the film-counter distance. Here, then, the difference between the energy of the particle emitted by the source and E1 is very nearly equal to the difference between E2 and the energy of the particle being detected the correction of energy difference is a result of the nonlinearity of the relationship of the range vs. energy of alpha particles. Thus, even though E1 and E2 change, (2) remains very nearly constant. The deviation from this constant is small since the difference between the energies of emitted a n d detected alpha particles was found to be about 2 MeV, dependent upon the thickness of the materials covering the source and used as counter window, and over this energy range the range of the particles increases nearly linearly with the energyO). 9) H. A. Bethe, Revs. Mod. Phys. 22, No. 2 (1950) 213.

T H I N F I L M T H I C K N E S S D I S T R I B U T I O N BY A L P H A A B S O R P T I O N

For thickness distribution determination, the pressure at which the absorption curve of the film intersects the count number where the dp is read is reproduced in the chamber and a count is taken over a certain duration of time for various points of the film. This is made possible b y the traversible carriage. The accuracy is primarily limited by the statistical error.

163

4. Results Firstly, the approximate alpha mass stopping powers of various materials relative to that of nitrogen were calculated, A. The value of the thickness is mass per unit area was obtained by weighing a certain known small area of a film or foil of the material. In some cases, thin films of a metal were evaporated upon a thin foil, for

10.4 COUNTS/15 s e c +

4

3

2.

1.

\

0

2bo

250 mm Hg

Fig. 2. Absorption curves for evaporated gold.

The number of counts corresponding to a unit change in thickness ~Nu Nu' at a certain count number, Nu, can be calculated to be, with the help of (1) :

~SNu 1 ¢SNa 1 ~x Nu-- Na -~ Nax/AP N u =

(a) _ Nu6Na I 760A ~-~ Na where

NaJ Op

Na

is the slope of the absorption curve at the count number, Na, at which Ap was measured.

which Ap is already known, over a certain known area. The uniformity of thickness of such evaporated metal films is very good10). The vapor source used was a V-shaped boat of tungsten with a dihedral angle of 90 ° and the distance of the foil from the boat was 9 cm. The area covered by the evaporated metal was of a diameter of 20 mm. The foils were weighed before and after the deposition and the thickness of the evaporated film accessed. The absorption curves for evaporated layers of gold are shown in fig. 2, while those of different thicknesses of nickel foil are shown in fig. 3. For thin films of aluminium oxide, zapon, mylar, and formvar, a certain known area of the absorption measured film was weighed and the thickness accessed. The absorption curves of various materials are shown in fig. 4. The value of the relative stopping 10) L. E. Preuss, J. Appl. Phys. 24, No. 11 (1953) 1401.

164

M. DE CRO~S, W. P A R K E R AND K. S E V I E R , JR.

powers were corrected for temperature and pressure to 20 ° C and 760 m m Hg. The average energy at which the absorption was measured was calculated to be, as stated before, 3.4 MeV by considering the pressure of nitrogen necessary 104x COUNTS/15

to stop the alpha particles and the mass stopping power of nitrogen for alpha particles. The values of the relative mass stopping powers, with their standard errors, of various materials are listed in table 1.

sec

4

! Ol

0

,

,

50

100

.

,

_

150

I

200

250 mm Hg

Fig. 3. Absorption curves for nickel foils.

10"

9'

8-

7-

o

i~

6-

5-

x

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1. NITROGEN , . , , , ox,

,~

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3. zAPo,.. 61,g/~m ~

4.5~ tVlYLAR:0.BBmg/cm2 (DIFF. PTS.)

4-

ill

3,

iI.I

2,

~,l

11

i.~l

iil

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10

0

5~

160

1~0 -

260

2~0 mm Hg

Fig. 4. Absorption curves for various materials. The curves for mylar are those measured at different points (diff. pts.) on the film.

THIN FILM THICKNESS

DISTRIBUTION

COUNTSl3Osec

~g/cm2

51000

-3

~J

,-2

BY A L P H A A B S O R P T I O N

COUNTS/20sec

~/cm 2

50000.

-12

# #

t

II II III:, t

165

--9

t

-6

.-3 "860

#

t

.+2

,+3"

45000-

'+6

"

,÷3

'+9 ,+12 +15

MYLAR 860pg/cm 2 ALUMINIUM OXIDE 97pg/cm 2

490000

2

4

6

8

10

40.000

12 mm

0

Fig. 5. Thickness d i s t r i b u t i o n across an a l u m i n i u m oxide film.

pg,'cm2

COtlCTS/20sec

--9

1

2

3

--3 61 -+3 -+6

~ t #

#

5

6

7

8

9

10 11 12 13mm

Fig. 8. M y l a r f i l m thickness d i s t r i b u t i o n .

COUNTS/30sec

IJg/cm2

• 5000(

, -15

0

"-6

55000-

4

- -10

4

--5

#

170

4S00(

-+5

;+9

5oooo-

.+10

#

ZAPON 61pg/cm 2

o

40000 45000

. . . . . . . . . . . . . . . 0 1 2 3 4 5 6 7 8 9 10 II t2 13 1,'1.15mm

Fig. 6. T h i n z a p o n film thickness distribution.

pglcm 2

COUNTS/30sec

• -75

60000.

#

# t

,°~0

e 50000.

e

.+15

ALUMINIUM FOIL 170 pg/cm 2

2

4

6

8

10

12

14 mm

Fig. 9. A l u m i n i u m foil thickness distribution.

COUNTS~30sec

~g/cm2

42000-

-8

-6 -4 -2 1500 +2 ,.+4

•-25

225

40000. #

• +25

4000(

-+6

+50

30000.

4

-+8

.+75 NICKEL FOIL 1.50rng/¢m2

ZAPON 225 pg/cm 2

20000, •

i

" i'"

~ " g 'lb"

3800

1"2m~

Fig. 7. Thick z a p o n film thickness distribution.

2

4

6

8

10

12

14 mm

Fig. 10. Nickel foil thickness distribution.

166

M. DE CROES, W. P A R K E R AND K. S E V I E R , JR. TABLE I Material

Gold Silver Nickel Aluminium Aluminium Oxide Zapon Mylar Formvar Nitrogen

Relative mass stopping power, A 0.275 4- 0.010 0.391 4- 0.010 0.567 4- 0.011 0.776 4- 0.039 0.770 -4--0.038 0.872 4- 0.076 1.070 -4- 0.099 O.709 + 0.O45 1.000 4- 0.000

Thickness distribution determination m a y be carried out according to the previously mentioned procedure. Using eq. (3) the resulting distribution calculations of thickness are very satisfactory; the standard error is not enough to seriously affect the thickness distribution calculations. Fig. 5 shows the extreme uniformity of the thickness of an aluminium oxide film. The

scale in m m at the bottom of the figure is for the distance of the intersection of the alpha beam with the film as measured from the film's edge. Figs. 6 and 7 show the thickness distribution of two different zapon films. The thickness distribution of the two films are very different, but it should be noted that the thicker film was made by allowing several drops of zapon laquer to spread a 16cm diameter distilled water surface while the thinner was made with only two drops. More uniform thick films of zapon are usually obtained by putting together several thin films. Figs. 9 and 10 show the thickness distribution of commercial aluminium and nickel foils respectively.

Acknowledgement The authors wish to thank Prof. Kai Siegbahn for his suggesting the investigation and for his interest during the work.