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Routine mass-thickness and nonuniformity determinations of thin films
NUCLEAR INSTRUMENTS AND METHODS 32 (1965) 201-,203 ; (D NORTH-HOLLAND P11BLISHINC: CO .
ROUTINE NMSS-1H[CKNESS AND NONUNIFORMITY DETERMINATION'S OF THIN FILMS K. F. WYLIE and G. R. HAGES Monsanto Research Corporation . Mound Laborarory*, Mian:%sburg, Ohio
Received 31 July 1964 A routine technique to rapidly determine the thickness and uniformity of thin plastic films and of thin plastic films coated with beryllium was developed. The thicknesses of Formvar films (approximately 20 to 100 pglcm2) and beryllium coated films (approximately 20 to 70 pg/cm2 of beryllium) were determined
using alpha particle attenuation techniques. Film thicknm were determined by both alpha counting and destructive %vighing. The difference between the two techniques was within tlhc experimental error of 7.5 t.
1. 1nftVd=fon A number of investigators have used alpha particle attenuation techniques to measure the mass-thickness, _ density and/or stopping power of various medial s). Some investigations required special equipment and were conducted in a vacuum . Others were open room studies . In this report, a simple alpha, particle attenuation method is described which can be used for routine, thin film thickness and nonuniformity determinations. This technique possesses the advantages that: 1. all equipment is commercially available ; 2. measurements are conducted at atmospheric pressure in the open room.
ture diameters ranging from 1 .59 to 7.94 mm were constructed . Various distances between source, collimator and detector, as well as various collimator apertures were used and reproduced with this arrangement. The films were prepared by dissolving Formvar plaster in chloroform and spreading the solution on glass using a G.ardner Automatic blade applicator for wet films. The dried films were mounted on three-inch square frames and vacuum coated on bout sides with beryllium, which was vaporized from a heated tantalum boat).
A thick alpha source was employed to obtain a greater spread of the alpha spectrum near the end of the range'-'). Thus, the relative particle intensity at the face of the detectox varied as different thicknesses of film were inserted between the source and detector . Films differing in thickness by 1 #2g/cm2 in a sample with a thickness between 30 and 80 ~cg/cm2 resulted in a 5 % change in the counting rate. 2. Equipment The thickness gage system, shown in fig. l, consisted of a scaler, detector, radioactive source, sample shelf assembly and collimator . The scaler was a Radiation Instruments Development Laboratory (RIDL) Scaler, Model 49-50. The detector was a Nuclear-Ohio, Inc., Thin Window Gras Flow Proportional Counter, Model 1 . The source was a one millicurie Po21° source, which was prepared at Mound Laboratory by precipitating the radioisotope to yield a uniform dispersion throughout the solid. The, source and collimator were located in a hole and groove, respectively, in the source shelfto provide a constant counting geome+xy . Collimators having aper* Mound Laboratoxy is operated by Monsanto Research Corporation for the LT . S. Atomic Energy Commission under Contract No . AT-33-1-GEN-53 .
Fig. 1 . Diagram of thickness gage system .
3. System calibration A number of factors affect the counting rate of tyre system . These factors include : 1 . alpha count rate fluctuations as a function of atmospheric pressure and humidity, 2. the change in alpha attr:nuation `kith fln thickness. A series of plastic films of known averag : -all-nes-., as determiined by weighing, were used as standards. Thr uncoated films varied from 10 to 100 mg!cm2. Beryllium-coated Formvar films contained from 10 to 70 Itg/cm2 of beryllium deposited on Formvar films of approximately 5012g/cm 2, as determined by weighing .
K. F. WYLIE AND G. R. HAGEE
iic k-,w
for each of these films as The d'fFeretwe in the maximurn c'--unti .-T ra Tc-ated- that the
-,ir'z4_4 Tr, :t
rTrJi-,d ;xNie trianner with
0/0
rature and effect on effor. ntensity was
thicknm; and latter was !zIrl z
f~17
TABLE I
A comparison of the alpha counting technique and destructive weighing to determine film thickness.
Zinc sulfide scintillator
st. f'oT 1 ows :
--jid a114--ow :he relative particle intensity at - ~~ &-wor to vwy significantly as diffefent 'r '-C"ed : 1. the size of the colliarrination of about 10'," of m0c_ -Pmsurement
was related to the reducs 0u chosen resulted the 0.95 confidence e v#w of the order count. The unattenuated 1-1cc .
ZZotaf
,-,cd Tyrior to and immediatel
s, slight
weTe considered in p'na particle Lion mt easurernents dh of the filims, C~` .7 oi ine zotal f6U7 11 ~11 11 MU mn"-asureinf-nts . The r.- 7! 771 7, u t-Z measuremeriTs was - or av erage attenuation a Z'~- cf th two oneI'll Mia:N C_~ he
h
F the film, lut was n.m . lin oF&-r to oj a series of
Alpha Destrictive counting I weighing
End-aindow proportiowl counter
ovimum
uremat comitioas of 1 . thecount rate without i maxim.um *ithout coincideace 1* symm ; 2. the sourceto-detector
Average film thickness (UWCM2)
T~Pe or detwor
1
46.2 41 .2 53 .8 22-8 74.8 91 .5 49 .9 21 .4 25.0 24.0 28.6 26.1 29.4 33.6
DO:== between the two techniques C/O
N .Lonunifonnity by alpha countiAg* (0/0)
-4.1 -3.7 1 .5 2.2 -1 .2 -1 .5 7.5 -7.4 3.0 7.1 5 .9 - 1 .1 -7 .0 1 .2
6.7 10.7 3.9 7.9 4.3 3.7 25.3 17.9 23.2 3,9-9 . liz 26.5 19.8 15.5
48.2 42-8 53.0 22.3 75 .7 92.9 46 .4 23,1 23.8 27-4 27.0 26.4 31 .6 33.2
Non-undbrtnity max. measured thiciatess- min . measured thkkness average measured thicknew
Forrnvar films were determinvA by alpha attenuation and by destructiva weighing of these same films to determine the average, film thicknesses. The results are shown in table 1 .
5. Discussion of results The results obtained with a zinc sulfide scintil.lator, used in the early phases of the investigaition . are also
included for comparison of films cast during the operation . In these eight cases (table 1) the differences in thicknesses were less than ± 7 .5% . In general, the TABLE 2 Comparison of the alpha counting and destructive weighing technique of determining -the total thickness of the beryllium coated films. Film
1 2 3 5 6
Total thickness of Be coated films (Ug/CM2)
89.8 106.8 64.6 69 .8 64.0 104 .0 58 .5
90.3 102 .4 64.7 66 .2 60 .2 104 .6 61 .8
Difference
1
-0.6 4 .3 -0 .2 5 .4 6 .3 -08 -5 .3
MANOTF 203
greatest differences occur in the more nonuniform films. The results ofthe measurements ofseveral berylliumcoated. Formvar films by alpha counting and destruc-
tive weighing are shown in table 2. These Formvar films were obtained from the same sheet which had an average alpha gagethickness of 49.8 jgjem'.The results were comparable with those obtained on Formvar films. The differences of less than 6% in the alpha gaging and destructive weighing techniques indicate that alpha attenuation can be used to determine accurately the thickness ofa metal coat on a plastic film. The thickness of a thin film can be determined by the nondestructive alpha gaging technique.
The authors wish to expresi their thanks to R . A .
Fischbein, J . W. Voigt and G . K . Howard far their assistance . ltefe
M. de Croes, W. Parker and K. Sevier, Jr., Nucl. Instr . And Meth. 7 (196ßi 160. 2) H. A. Enge, M. A. Wahlig and 1. Aanderaa, Rev. Sci . Insu. 28 (1957) 145. 3) W. C. Barber, Rev. Sci . Instr. 24 (1953) 469 . 4) W. H. V. Davison, J. Sci. Instr. 34 (1957) 418 . s) A. Shirodkar, Phil. Mag. 15 (1933) 426 . 6) R. P. Gardner and H- H. Ross, Rev. Sci . Insu. 33 (1962) 1227. 7) B. W. Schumacher, Nucleonics 18 (1960) 106. 8) J. W. Voigt, MLM-1194, U. S. Atomic Energy Commission (in publication) . 1)
ROUTINE NMSS-1H[CKNESS AND NONUNIFORMITY DETERMINATION'S OF THIN FILMS K. F. WYLIE and G. R. HAGES Monsanto Research Corporation . Mound Laborarory*, Mian:%sburg, Ohio
Received 31 July 1964 A routine technique to rapidly determine the thickness and uniformity of thin plastic films and of thin plastic films coated with beryllium was developed. The thicknesses of Formvar films (approximately 20 to 100 pglcm2) and beryllium coated films (approximately 20 to 70 pg/cm2 of beryllium) were determined
using alpha particle attenuation techniques. Film thicknm were determined by both alpha counting and destructive %vighing. The difference between the two techniques was within tlhc experimental error of 7.5 t.
1. 1nftVd=fon A number of investigators have used alpha particle attenuation techniques to measure the mass-thickness, _ density and/or stopping power of various medial s). Some investigations required special equipment and were conducted in a vacuum . Others were open room studies . In this report, a simple alpha, particle attenuation method is described which can be used for routine, thin film thickness and nonuniformity determinations. This technique possesses the advantages that: 1. all equipment is commercially available ; 2. measurements are conducted at atmospheric pressure in the open room.
ture diameters ranging from 1 .59 to 7.94 mm were constructed . Various distances between source, collimator and detector, as well as various collimator apertures were used and reproduced with this arrangement. The films were prepared by dissolving Formvar plaster in chloroform and spreading the solution on glass using a G.ardner Automatic blade applicator for wet films. The dried films were mounted on three-inch square frames and vacuum coated on bout sides with beryllium, which was vaporized from a heated tantalum boat).
A thick alpha source was employed to obtain a greater spread of the alpha spectrum near the end of the range'-'). Thus, the relative particle intensity at the face of the detectox varied as different thicknesses of film were inserted between the source and detector . Films differing in thickness by 1 #2g/cm2 in a sample with a thickness between 30 and 80 ~cg/cm2 resulted in a 5 % change in the counting rate. 2. Equipment The thickness gage system, shown in fig. l, consisted of a scaler, detector, radioactive source, sample shelf assembly and collimator . The scaler was a Radiation Instruments Development Laboratory (RIDL) Scaler, Model 49-50. The detector was a Nuclear-Ohio, Inc., Thin Window Gras Flow Proportional Counter, Model 1 . The source was a one millicurie Po21° source, which was prepared at Mound Laboratory by precipitating the radioisotope to yield a uniform dispersion throughout the solid. The, source and collimator were located in a hole and groove, respectively, in the source shelfto provide a constant counting geome+xy . Collimators having aper* Mound Laboratoxy is operated by Monsanto Research Corporation for the LT . S. Atomic Energy Commission under Contract No . AT-33-1-GEN-53 .
Fig. 1 . Diagram of thickness gage system .
3. System calibration A number of factors affect the counting rate of tyre system . These factors include : 1 . alpha count rate fluctuations as a function of atmospheric pressure and humidity, 2. the change in alpha attr:nuation `kith fln thickness. A series of plastic films of known averag : -all-nes-., as determiined by weighing, were used as standards. Thr uncoated films varied from 10 to 100 mg!cm2. Beryllium-coated Formvar films contained from 10 to 70 Itg/cm2 of beryllium deposited on Formvar films of approximately 5012g/cm 2, as determined by weighing .
K. F. WYLIE AND G. R. HAGEE
iic k-,w
for each of these films as The d'fFeretwe in the maximurn c'--unti .-T ra Tc-ated- that the
-,ir'z4_4 Tr, :t
rTrJi-,d ;xNie trianner with
0/0
rature and effect on effor. ntensity was
thicknm; and latter was !zIrl z
f~17
TABLE I
A comparison of the alpha counting technique and destructive weighing to determine film thickness.
Zinc sulfide scintillator
st. f'oT 1 ows :
--jid a114--ow :he relative particle intensity at - ~~ &-wor to vwy significantly as diffefent 'r '-C"ed : 1. the size of the colliarrination of about 10'," of m0c_ -Pmsurement
was related to the reducs 0u chosen resulted the 0.95 confidence e v#w of the order count. The unattenuated 1-1cc .
ZZotaf
,-,cd Tyrior to and immediatel
s, slight
weTe considered in p'na particle Lion mt easurernents dh of the filims, C~` .7 oi ine zotal f6U7 11 ~11 11 MU mn"-asureinf-nts . The r.- 7! 771 7, u t-Z measuremeriTs was - or av erage attenuation a Z'~- cf th two oneI'll Mia:N C_~ he
h
F the film, lut was n.m . lin oF&-r to oj a series of
Alpha Destrictive counting I weighing
End-aindow proportiowl counter
ovimum
uremat comitioas of 1 . thecount rate without i maxim.um *ithout coincideace 1* symm ; 2. the sourceto-detector
Average film thickness (UWCM2)
T~Pe or detwor
1
46.2 41 .2 53 .8 22-8 74.8 91 .5 49 .9 21 .4 25.0 24.0 28.6 26.1 29.4 33.6
DO:== between the two techniques C/O
N .Lonunifonnity by alpha countiAg* (0/0)
-4.1 -3.7 1 .5 2.2 -1 .2 -1 .5 7.5 -7.4 3.0 7.1 5 .9 - 1 .1 -7 .0 1 .2
6.7 10.7 3.9 7.9 4.3 3.7 25.3 17.9 23.2 3,9-9 . liz 26.5 19.8 15.5
48.2 42-8 53.0 22.3 75 .7 92.9 46 .4 23,1 23.8 27-4 27.0 26.4 31 .6 33.2
Non-undbrtnity max. measured thiciatess- min . measured thkkness average measured thicknew
Forrnvar films were determinvA by alpha attenuation and by destructiva weighing of these same films to determine the average, film thicknesses. The results are shown in table 1 .
5. Discussion of results The results obtained with a zinc sulfide scintil.lator, used in the early phases of the investigaition . are also
included for comparison of films cast during the operation . In these eight cases (table 1) the differences in thicknesses were less than ± 7 .5% . In general, the TABLE 2 Comparison of the alpha counting and destructive weighing technique of determining -the total thickness of the beryllium coated films. Film
1 2 3 5 6
Total thickness of Be coated films (Ug/CM2)
89.8 106.8 64.6 69 .8 64.0 104 .0 58 .5
90.3 102 .4 64.7 66 .2 60 .2 104 .6 61 .8
Difference
1
-0.6 4 .3 -0 .2 5 .4 6 .3 -08 -5 .3
MANOTF 203
greatest differences occur in the more nonuniform films. The results ofthe measurements ofseveral berylliumcoated. Formvar films by alpha counting and destruc-
tive weighing are shown in table 2. These Formvar films were obtained from the same sheet which had an average alpha gagethickness of 49.8 jgjem'.The results were comparable with those obtained on Formvar films. The differences of less than 6% in the alpha gaging and destructive weighing techniques indicate that alpha attenuation can be used to determine accurately the thickness ofa metal coat on a plastic film. The thickness of a thin film can be determined by the nondestructive alpha gaging technique.
The authors wish to expresi their thanks to R . A .
Fischbein, J . W. Voigt and G . K . Howard far their assistance . ltefe
M. de Croes, W. Parker and K. Sevier, Jr., Nucl. Instr . And Meth. 7 (196ßi 160. 2) H. A. Enge, M. A. Wahlig and 1. Aanderaa, Rev. Sci . Insu. 28 (1957) 145. 3) W. C. Barber, Rev. Sci . Instr. 24 (1953) 469 . 4) W. H. V. Davison, J. Sci. Instr. 34 (1957) 418 . s) A. Shirodkar, Phil. Mag. 15 (1933) 426 . 6) R. P. Gardner and H- H. Ross, Rev. Sci . Insu. 33 (1962) 1227. 7) B. W. Schumacher, Nucleonics 18 (1960) 106. 8) J. W. Voigt, MLM-1194, U. S. Atomic Energy Commission (in publication) . 1)