Method of accurate thickness measurement of boron carbide coating on copper foil

Abstract

A method is disclosed of measuring the thickness of a thin coating on a substrate comprising dissolving the coating and substrate in a reagent and using the post-dissolution concentration of the coating in the reagent to calculate an effective thickness of the coating. The preferred method includes measuring non-conducting films on flexible and rough substrates, but other kinds of thin films can be measure by matching a reliable film-substrate dissolution technique. One preferred method includes determining the thickness of Boron Carbide films deposited on copper foil. The preferred method uses a standard technique known as inductively coupled plasma optical emission spectroscopy (ICPOES) to measure boron concentration in a liquid sample prepared by dissolving boron carbide films and the Copper substrates, preferably using a chemical etch known as ceric ammonium nitrate (CAN). Measured boron concentration values can then be calculated.

Claims

1. A process for measuring the effective thickness of a boron coating on copper foil comprising the steps of: (1) removing a sample of the boron coated copper foil from a larger length coated substrate; (2) weighing the sample of boron coated copper foil; (3) contacting the sample of boron coated copper foil with an etching solution; (4) allowing the etching solution to dissolve the sample of boron coated copper foil; (5) diluting the etching solution; (6) determining the concentration of the material in the diluted etching solution utilizing inductively coupled plasma optical emission spectroscopy; and (7) calculating the effective thickness of the coating (t.sub.s) using the equation:
t.sub.s=[(1.3C×V.sub.s×d.sub.f×t.sub.cu×d.sub.cu/M.sub.t)×1/(1−1.3C×V.sub.s×d.sub.f/M.sub.t)] where C=measured concentration from determining step d.sub.f=sample dilution factor V.sub.s=volume of diluted etching solution t.sub.cu=thickness of the copper foil dcu—density of the copper foil Mt=mass of the boron coated sample.

2. The process of claim 1, wherein the removing a sample step comprises removing a sample at least about 1 square centimeter in size.

3. The process of claim 1, wherein the removing step comprises removing multiple samples from the substrate, the samples being taken at intervals of between about 50 and about 200 feet along the substrate.

4. The process of claim 1, wherein the boron coating comprises a boron carbide coating.

5. The process of claim 1 wherein the etching solution of the contacting step comprises a ceric ammonium nitrate solution.

6. The process of claim 5, wherein the ceric ammonium nitrate solution comprises between about 2 and about 25 percent by weight ceric ammonium nitrate.

7. The process of claim 5, wherein the ceric ammonium nitrate solution comprises between about 8 and about 10 percent by weight ceric ammonium nitrate.

8. The process of claim 1, wherein the diluting step comprises diluting the boron concentration to between about 2 and about 25 ppm.

9. The process of claim 1, wherein the determining the concentration step comprises determining the boron concentration to within about 0.1 ppm.

10. A process for measuring the effective thickness of a boron carbide coating on copper foil comprising the steps of: (1) removing a sample of the boron carbide coated copper foil from a larger length of boron carbide coated copper foil; (2) weighing the sample of boron carbide coated copper foil; (3) contacting the sample of boron carbide coated copper foil with ceric ammonium nitrate etching solution; (4) allowing the etching solution to dissolve the sample of boron carbide coated copper foil; (5) diluting the etching solution to between about 2 and about 25 ppm boron; (6) determining the concentration of boron carbide in the diluted etching solution using inductively coupled plasma optical emission spectroscopy; and (7) calculating the effective thickness of the boron coating (t.sub.s) using the equation:
t.sub.s=[(1.3C×V.sub.s×d.sub.f×t.sub.cu×d.sub.cu/M.sub.t)×1/(1−1.3C×V.sub.s×d.sub.f/M.sub.t)] where C=measured concentration from determining step d.sub.f=sample dilution factor V.sub.s=volume of liquid sample prepared t.sub.cu=thickness of the copper foil d.sub.cu—density of the copper foil M.sub.t=mass of the sample.

11. The process of claim 10, wherein the removing a sample step comprises removing a sample at least about 1 square centimeter in size.

12. The process of claim 10, wherein the removing step comprises removing multiple samples from the substrate, the samples being taken at intervals of between about 50 and about 200 feet along the substrate.

13. The process of claim 10, wherein the ceric ammonium nitrate solution comprises between about 2 and about 25 percent by weight ceric ammonium nitrate.

14. The process of claim 10, wherein the ceric ammonium nitrate solution comprises between about 8 and about 10 percent by weight ceric ammonium nitrate.

Description

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

(1) A better understanding of the invention can be obtained when the detailed description set forth below is reviewed in conjunction with the accompanying drawings, in which:

(2) FIG. 1 depicts examples of reels of copper foil coated with boron carbide such as are known in the art;

(3) FIG. 2 depiction a boron carbide coated sample such as may be utilized in a preferred method;

(4) FIG. 3 depicts the dissolution of boron carbide and Cu foil using ceric ammonium nitrate such as may be accomplished using a preferred method of the invention;

(5) FIG. 4 depicts a standard ICPOES measurement set up such as may be utilized in a preferred method of the invention for boron concentration measurement from the liquid samples:

(6) FIG. 5 depicts an example of the preferred method of the invention in which more than 10 samples are utilized on ICPOES stage for boron concentration measurement; and

(7) FIG. 6 is plot showing the ratio of effective thickness to desired thickness of boron carbide coating results from tests using a preferred embodiment of the method of the invention for four different sputtering boron carbide tile sets on standard 25 μm thick Copper foil.

(8) FIG. 7 is a process flow diagram of steps of a preferred version of the method of the present invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

(9) This present invention is a method of measuring the thickness of a thin coating on a substrate comprising dissolving the coating and substrate in a reagent and using the post-dissolution concentration of the coating in the reagent to calculate an effective thickness of the coating. The preferred method is especially useful for non-conducting films on flexible and rough substrates, even though it is equally useful for other kinds of thin films provided a reliable film-substrate dissolution technique is found. A preferred method includes determining the thickness of Boron Carbide films deposited on copper foil (for example 25 um thick flexible cold rolled Copper foil). Effective thickness of the boron carbide thin film can be defined as the thickness of a pure boron carbide film which contains the same numbers of boron atoms as in the real boron carbide film. For a film with quite rough surface like that of boron carbide on Cu foil, very locally measured thicknesses has very little significance and can be easily misleading, rather an average thickness from a large region can accurately provide the number of atoms of interest within the sample. The preferred method uses a standard technique known as inductively coupled plasma optical emission spectroscopy (ICPOES) to measure boron concentration in a liquid sample prepared by dissolving boron carbide films and the Copper substrates, preferably using a chemical etch known as ceric ammonium nitrate (CAN). Measured boron concentration values can be converted to effective thickness (t.sub.eff) using the equations shown below.

(10) Copper foil can be coated with boron carbide (10B4C) using a number of methods including the process disclosed in Applicant's pending application, U.S. application Ser. No. 14/060,015. FIG. 1 shows examples of reels of Copper foils coated with boron carbide. This coated product is used in fabricating neutron detectors for different purposes as described in our previous patents and publications.

(11) Easy and reliable measurement of the boron carbide film thicknesses at different locations in the reel is extremely important to assess the coating thickness uniformity.

(12) In one embodiment of the method of the present invention, samples of coated coil as shown in FIG. 2 is taken from a various locations and accurately weighed, preferably up to four decimal places, in grams. Preferably, the method is used for measuring average effective thickness of boron carbide films from samples of larger than 1 cm.sup.2 in size. Preferably, a sample size is at least about 1 cm by 1 cm. Preferably, such samples are taken every 50 to 200 feet of foil.

(13) As shown in FIG. 3, the next step in a preferred embodiment of the method of the present invention involves dissolving the coating samples in CAN solution at ambient conditions. Standard ceric ammonium nitrate solution is commercially available or it can be locally prepared with a desired concentration from ceric ammonium nitrate powder dissolved in DI water. As can be seen in FIG. 3, the CAN is a yellowish solution prior to addition of the sample (Stage 1). During the dissolution process (Stage 2), the solution turns a brownish color and ultimately a greenish color indicating complete dissolution (Stage 3). No gas is evolved during the dissolution process so that the samples can be dissolved in a closed container. The concentration of CAN can be varied to optimize the dissolution time. Preferably, the concentration of CAN is in the range of 2.5 to 20%, and more preferably in the range of 8 to 10%.

(14) Prepared solution is appropriately diluted to prepare the final measurement solution so that the reduced boron concentration value lies within the appropriate range for the ICPOES equipment. Preferably, the final boron concentration is reduced down to about 2-25 PPM. If the concentration is too high the optical detector can get saturated. When an external liquid sample is sent through a plasma (See FIG. 4), the constituent atoms of the sample are excited and optical emission is occurred when the previously excited atoms are de-excited. Intensities for a given emission wavelength increases linearly with the concentrations. Emitted wavelength's positions and intensities from the unknown sample are compared with a calibration straight line derived using standards with different concentrations and a given standard to measure the concentration from the unknown samples.

(15) As shown in FIG. 5, several liquid samples can be prepared and set up on ICPOES stage for boron concentration measurement at the same time. This method is quite fast such that several samples (more than 20) can be measured in just few hours. Preferably, using this embodiment of the preferred method, boron concentration can be easily measured to 1/10th of PPM concentration so that this method is also extremely accurate.

(16) Once the concentrations have been determined using, the next step of the preferred method is to calculate an effective thickness of the boron coating. Preferably, the effective thickness is calculated using the following equations:

(17) Assuming Mt as the total mass of the sample taken, sum of the mass of Copper foil (M.sub.cu) and mass of the boron carbide film (M.sub.s), area of the sample surface (A) can be given by following equation:
A=M.sub.t/(t.sub.s×d.sub.s+t.sub.cu×d.sub.cu)  (1)

(18) Where, t.sub.s=effective thickness of boron carbide film d.sub.s=density of boron carbide t.sub.cu=thickness of Cu foil d.sub.cu=density of Cu foil.

(19) The equation for the area of the sample face can be used to compute the effective thickness of the boron carbide film using the following equation.
t.sub.s=[(1.3C×V.sub.s×d.sub.f×t.sub.cu×d.sub.cu/M.sub.t)×1/(1−1.3C×V.sub.s×d.sub.f/M.sub.t)]  (2)

(20) Where, C=measured boron concentration using ICPOES d.sub.f=sample dilution factor V.sub.s=Volume of the liquid sample prepared
For calculation of the effective film thickness, density of the boron carbide film can be assumed to be same as the bulk density of boron carbide. In pure boron carbide, 4 boron atoms are attached with one carbon atoms so that the effective boron carbide concentration is given by 1.3 times the measured boron concentration by ICPOES.

(21) Using this preferred embodiment of the method several samples taken from different locations of coated product were taken and the effective thickness was measured from the product from several sputtering boron carbide tiles. Results taken from four different tiles are shown in FIG. 6. The measurements indicate a uniform thickness within +/−3% for the entire coated area of the boron-coated tiles. Some of the results were also compared with SEM measurements and were found in very good agreement to each other. The parameters utilized for this testing are as follows:

(22) TABLE-US-00001 M.sub.t t.sub.cu d.sub.cu V.sub.s d.sub.s C t.sub.s (gm) (μm) (gm/cc) (ml) d.sub.f (gm/cc) (mg/l) (μm) 0.05-0.12 25 8.96 10-20 20-30 2.38 3-10 1-2

(23) While the terms used herein are believed to be well-understood by one of ordinary skill in the art, definitions are set forth to facilitate explanation of certain of the presently-disclosed subject matter.

(24) Following long-standing patent law convention, the terms “a”, “an”, and “the” refer to one or more when used in this application, including the claims. Thus, for example, reference to “a window” includes a plurality of such windows, and so forth.

(25) Unless otherwise indicated, all numbers expressing quantities of elements, dimensions such as width and area, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in this specification and claims are approximations that can vary depending upon the desired properties sought to be obtained by the presently-disclosed subject matter.

(26) As used herein, the term “about,” when referring to a value or to an amount of a dimension, area, percentage, etc., is meant to encompass variations of in some embodiments plus or minus 20%, in some embodiments plus or minus 10%, in some embodiments plus or minus 5%, in some embodiments plus or minus 1%, in some embodiments plus or minus 0.5%, and in some embodiments plus or minus 0.1% from the specified amount, as such variations are appropriate to perform the disclosed methods or employ the disclosed compositions.

(27) The term “comprising”, which is synonymous with “including” “containing” or “characterized by” is inclusive or open-ended and does not exclude additional, unrecited elements or method steps. “Comprising” is a term of art used in claim language which means that the named elements are essential, but other elements can be added and still form a construct within the scope of the claim.

(28) As used herein, the phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When the phrase “consists of” appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

(29) As used herein, the phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter. With respect to the terms “comprising”, “consisting of”, and “consisting essentially of”, where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

(30) As used herein, the term “and/or” when used in the context of a listing of entities, refers to the entities being present singly or in combination. Thus, for example, the phrase “A, S, C, and/or O” includes A, S, C, and O individually, but also includes any and all combinations and subcombinations of A, S, C, and O.

(31) It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The foregoing disclosure and description are illustrative and explanatory thereof, and various changes in the details of the illustrated apparatus and construction and method of operation may be made without departing from the spirit in scope of the invention which is described by the following claims.