Determination of Micro Rhodium Film Thickness and of Gold Plating Thickness on Printed Circuits by Beta Radiatio n Backscatter Measurements V. L. EGGEBRAATEN, L. J. WALKER, and E. W. STROBELT Aero Space Division, Boeing Airplane Co., Seattle, Wash.
Beta radiation backscatter techniques were applied to thickness measurements of gold plating on printed circuit cards and of very thin rhodium films vaporized on a sapphire crystalline base. Gold thicknesses between 100 to 500 pcm. were determined with an accuracy of 18 pcm. Rhodium film thickness was measured from 0.5 to 5 pcm. with an accuracy of 0.5 pcm. The method of analysis described here is nondestructive and is designed to give sensitive and reliable results.
T
having a general thickness distribution from 0.5 to 500 pcm. have experienced a rapid increase in their use for industrial applications. Some of these areas include printed circuitry, infrared sensing devices, and metal bonding processes in which the thin metallic coating is a part of the final product. This has resulted in engineering and quality control requirements demanding more accurate thickness measuring techniques for these items. Solving the problem of microthickness determinations has become increasingly complex since very sensitive, nondestructive, and reliable methods of testing are desired. Two principal techniques have been used for the thickness measurements of metallic coatings which approach the range given above ( 5 ) . The first method, which is commonly used for the calibration of thickness measuring procedures, involves weighing the sample on a microbalance before and after the coating is applied. Samples may usually be weighed with a sensitivity of 2.5 pg. which is well within the accuracy requirements of normal thickness determinations. The second method also involves a measurement for the quantity of metallic film per unit area and may be accomplished through x-ray spectroscopy, activation analysis, radioactive tracing, or other sensitive analytical techniques. Two fallacies encountered in these procedures, however, are that the sample size must HIN METALLIC COATINGS
usually be small, especially in the use of a microbalance, and the area which is coated must be fairly easy to calculate. This latter problem arises in the case of printed circuitry where the patterns are complex. The measurement of scattered beta radiation from coated materials when applied to thickness determinations has been found to possess several advantages. First there is a large assortment of beta-emitting isotopes, representing a wide spectrum of beta energies, which may easily be used with this technique. Employing the proper radioactive source for each application enables the beta backscattering method to be an extremely sensitive measuring device for thin films. The method is also nondestructive, does not require a calculable area, does not limit the sample to any definite size, and is a relatively simple and straightforward technique, thus leading to a more versatile and reliable testing procedure than the previous methods described. This technique has been applicable in solving two types of thickness measuring problems. The first concerns sensitivity in which the thickness of vaporized rhodium films on a sapphire base is determined. These films which are used as a part of infrared detecting equipment, range from 0.5 to 5 pcm. and require a thickness control of 0.5 pcm. accuracy. The second problem is a geometrical one in which the method of scattering and measuring beta radiation from a 40-mil wide circuit is described. The range of gold thickness is between 100 to 500 pcm. with an accuracy of 18 pcm. being achieved. THEORY
I t i s known that the interaction of beta radiation with matter is such that the scattering of beta particles is dependent on the atomic number of the scattering material(1-4, 6). The statistical probability and thus the intensity of this scatter is closely proportional to atomic number and represents the basis for the measurement of thin films. Since the atomic numbers of
rhodium and gold are greater than their respective base elements, the scatter of beta radiation from these materials will increase with the thickness of the metal coating. A proportional relationship will be observed up to a point approaching the saturation thickness of the metals which is dependent upon the density of the metal and the energy of the radiation. PROCEDURE FOR RHODIUM FILMS
Apparatus. The backscatter gage used for these tests is a cubic box made of '/4-inch Plexiglas and measures 1 foot on each side. The cover of this gage holds a changeable radioactive source in the center as shown in Figure 1. The Geiger tube is connected a t right angles to a preamplifier which in turn feeds signals to a scaler. Once the sample is in position as shown, the scattered radiation intensity to the Geiger tube is easily recorded. The radioactive source used for this determination consists of 0.1 mc. of carbon-14 in the form of barium carbonate. The source is located in the bottom of a glass cup with a a/,,inch inside diameter opening, and is covered a i t h a thin Mylar film. The carbon14 was purchased from the Oak Ridge National Laboratory and has a specific activity of 0.44 to 2.2 curies per gram of carbon. Carbon-14 was chosen for this application due to its low radiation energy which is a maximum of 0.155 m.e.v. A low energy is desired because the probability of scatter a t the surface of a substancg increases as the energy of the radiation decreases and thus results in a much higher degree of sensitivity. Although the efficiency for counting carbon-14 is low due to a large amount of absorption, the efficiency of scatter by a very thin film is extremely high. Calibration. Sapphire crystals inch thick and I inch in diameter were used to calibrate this procedure. These crystals were cleaned well and coated with thin vaporized films of metallic rhodium, varying the thickness for each sample. Three separate counting rates were required to measure the amount of radiation scatter from these films. A background count was first taken with the source in position but with no sample over the hole. A VOL. 33,
NO. 9, AUGUST 1961
1245
-SAMPLE
L I D OF BACKSCATTER CAGE
/
5-
, Figure 1. Schematic diagram showing source, detector, and sample position
counting time of 30 minutes wm s&cient with the counting rate averaging 600 counts per minute. The back or bare side of the crystal was then placed over the source and the scattered radiation was counted over a period of 90 minutes. This count totaled approximately 140,000. The rhodium-coated side was then counted for the same interval.of time. The per cent increase in radiation scatter for the rhodium over the base material is computed from Equation 1. % Increase = d&nt of rhodium - count of base count of base - background x 100
~
The vapor plated crystal was then weighed on an Ainsworth microbalance with an accuracy of 2.5 pg. The sample was scrubbed with a cloth and warm water until the thin coating was removed. After rinsing in alcohol and drying, the sample was weighed again using the difference in weight per unit area as the standardized thickness. The first few samples were scrubbed again and reweighed to check for a loss of weight due to the scrubbing action. However, no difference in weight was noticeable. Figure 2 represents a graph of these calibrated samples showing the increase in radiation scatter us. the film thickness. The determination of an unknown film thickness requires three counting periods as in the calibration procedure. However, one 30-minute background count and one *minute count of the base is sufficient for an entire group of samples provided they are of the same base material. Each individual crystal must be counted for 90 minutes with the rhodium coated side facing the source. These data are formulated in Equation 1 giving the per cent increase in counting rate. The resulting value is plotted on a calibrated graph (Figure 2) to obtain the rhodium thickness. The limiting factor for accuracy in this determination is the statistical error found in the total count. Assuming the scattered radiation from a crystal yields a total count of approximately 140,000, the statistical error at 1246
ANALYTICAL CHEMISTRY
a 95% confidence level will be, 720 counts. This is equivalent to an error of 6 pg./cm.* or approximately 0.5 pcm. A larger total count will reduce the statistical error appreciably and will be useful up to a point approaching the accuracy of the microbalance used for calibration which is 2.5 pg., being equivalent to 1.25 Fg./cm.2 of rhodium. A larger counting rate will be obtained by using a source of higher specific activity so as to partially eliminate self-absorption effects. This will also allow the strength of the source to be increased. PROCEDURE FOR GOLD CIRCUITS
ON PRINTED
The primary obstacle to overcome when measuring the thickness of a 40-mil wide circuit is to scatter the radiation only from the circuit itself. This is essential since the width of the circuit is not precisely constant. These fluctuations of circuit width will result in erratic readings of radiation scatter if the scatter is from both the circuit and surrounding base material. In addition, the geometrical position of the source, sample, and probe must be arranged to efficiently detect the scatter from the circuit without experiencing an abnormally high background count. Apparatus and Source. The only alterations made on the previously described backscatter gage are in the position of the source and the opening in the cover of the gage through which the radiation is allowed to traverse to the sample and back to the detector. The */,-inch diameter opening shown in Figure 1 is now changed to a slit measuring 30 mils in width by a/4 inch in length. This is done by placing two sheets of Plexiglas (6 X 6 X inches) over the center of the opening and securing them 30 mils apart from each other. The 40-mil wide circuit is placed over this slit and aligned to cover it completely by the use of mirrors located a t appropriate angles in the bottom of the gage. Radiation will now scatter from the circuit alone and will be recorded on the Geiger tube mounted below.
t+
,
,
.' 119 / c n 2
I)f
Rhodium
Figure 2. Thickness of rhodium vs. per cent scattered radiation
Pm147 was found to be the most appropriate source for measuring gold thickness in the range of 100- to 500micro cm. This source has a beta energy of 0.23 m.e.v. and a half life of 2.6 years. Five hundred microcuries of the carrier-free isotope in the form of Pm Cls dissolved in 1 ml. of HC1 were used to prepare the source. This solution was diluted to 3 ml. to which 3 mg. of T1 NO3 carrier were added for better electrolytical deposition. A copper inch wire inch in length and in diameter was used for the plating. The top inch of the wire was first pressed to a width of 30 mils. The Pm147 was then plated on this surface a t 3 volts for 30 minutes. A very thin lead foil was wrapped completely around the cylindrical part of the wire except where the wire was to make contact and be supported by the Plexiglas. This was essential for lowering the background count. The top inch of the source, 30 m i l s in width, was wedged between the two Plexiglas supports as they were glued to the cover of the gage, thus forming the 30-mil wide slit. Calibration. Copper coupons 1 inch in diameter were used to calibrate this procedure. These disks were weighed on an Ainsworth microbalance before and after gold plating to an accuracy of 2.5 p g . Three-minute counting intervals were used with the count averaging 11,000 c.p.m. for background and 17,000 to 25,000 c.p.m. with samples in position over the slit. These results are plotted in Figure 3 with the per cent increase in radiation scatter calculated from Equation 1 in which copper is the base material. The accuracy of each determination is again dependent on the statistical error found in the total count. For these particular tests, the count over a 3minute interval is approximately 60,000. For this value, the statistical variat,ion
at a 95% confidence level is 480 counts. This is equivalent t o 18 pcm. over most of the given thickness range. A comparison of a fen thickness readings for circuits rising the beta radiation backscatter and cross-section methods is given in Table I.
Table I. Comparison of Thickness Values of Production Circuit Cards
Beta Thickness (pcm.) 234
Cross-Section Thickness (pcm.)
305
400
406
437
CONCLUSIONS
The beta radiatioii backscatter gage has been used by this laboratory over the past three years covering a large number of applications. The wide range of thickness mrnzurements made possible by this equipmrnt is partly responsible for i t s value as an analytical tool. Sources emitting weak beta radiation, less than 0.25 in.e.v., are used for very thin films such as the ones discussed here and in some cases have a sensitivity approaching 0.25 pcm. The measurement of thicker films extending to several mils is determined using sources with beta energies ranging from 0.5 to 1.5 m.e v. For example, the 0.76-m.e.v. radiation from thallium-204 is used for the measurement of copper thickness on a phenolic base. The range of this operation is 1000 to 6000 pcm. with an accuracy of 200
4 3
410 430
452
/
-CL
203 305
i
=c LC
I
. c i
4
5
,
LITERATURE CITED
-
Figure 3. Thickness cent radiation scatter
,ucrn. and requires a determination time of 1 minute. The measurement of gold thickness has also been ideal for this method because of its large atomic number which displays a high efficiency for scattering beta radiation. Gold thickness measurements have been made from 0.25 to 1000 bcm. using different radiation energies which best fit the application.
(1) Carlin, J. R., Rubber Age 66, 173 ( 1949).
(2j Danguy, L., Znteni. J. A p p l . Radiatiorb and Isotopes 1, 276 (1957). (3) Evans, R. D., “The Atomic Nuc’leus,” Chap. 19, hIcGraw-Hili, New I‘ork,
1959:
(4) Grey, P. R., Clnrey, D. H., Beamer, W. H., ANAL. CHEM.32, 582 (1060). (5) Holland, I,;, “Vacuum Deposition of
Thin Films, pp. 220-32 m‘iley, New York, 1956. (6) Muller, H. El.,ANAL.CHEM.29, 969 (1957). RECEIVED for review November 15, 1960. Resubmitted April 4, 1961. Accepted May 9,1961.
Thermal Analysis A Critique PAUL D. GARN Bell Telephone laborafories, Inc., Murray Hill, N. 1.
b Data obtained by thermogravimetry can and should agree with differential thermal analysis data. Each technique may be used to its own best advantage; yet with reasonable care in design, thermal and atmosphere environments may be essentially duplicated. Thermogravimetric sample holders must be designed to permit rapid transfer of heat to the sample. Misleading data can be avoided by routine consideration of thermal and atmospheric environments. The translation of dynamic data to static heating is unreliable. The techniques described are thermodynamically sound and experimentally successful.
A
the problems encountered in differential thermal analysis (DTA) is the c o r r a t i o n of temperature data with other techniques, particularly thermogravimetry. The general conXlONG
cern over the DTA sample holder design, especially the continuing disagreement over the relative merits of cups or blocks, has obscured the major problem of de. fining reaction temperatures for those cases in which a gaseous reactant or product is involved. Observers (1, 9-11, 17), including the present author (6), have each pointed out the rnwits of some particular sample holder design from a heat transfer and heat capacity point of view. Each, including the present author, failed to appreciate the full effect of atmosphere on reactions of the type delineated. This paper comprises a n examination of techniques used in the thermal analysis, consideration of the factors mentioned above, and discussion of some remedies. These remedies specifically include techniques which permit direct correlation of thermogravimetric and differential thermal analysis data and a n extension of the thermogravimetric techniques of Garn and Kessler (7, 8).
DISCUSSION OF CURRENT PRACTICES
Stone (24) made a very sigiiificant contribution by showing the substantial benefits of using a dynamic atmosphere, but his work did little to improve the correlation between differential thermal analysis and thermogravimetric analysis. There must necessarily be a oneto-one correspondence between the techniques, since a sample will respond to its environment whether it is supported from a balance beam or a thermocouple is stuck into it. Several attempts have been made to obtain correspondence of data by simultaneous measurement of both effects. This has been done on the same specimen (20,21) or contiguous specimens (12, I S : . Reisman ($1) uses extremelyslow heating rates, y x r a t ing near thermal and chemical equilibrium at all times; but this solation is not generally useful Kissinger, AicJIurdie, ilnd Si-phori, (IS) have used two bl,echens, tme in a VOL. 33, NO. 9, AUGUST 1961
1247
