DIELECTRIC MAPPING DEVICE AND METHOD

TECHNICAL FIELD

This invention relates to a device for dielectric mapping of a sheet material and a method of mapping dielectric property of a sheet material, for example, paper and other plastics.

BACKGROUND OF THE INVENTION

In electro-photography, printing on a paper surface occurs when charged toner particles are electro-statically transferred from a photoreceptor to the paper surface under the action of an electric field, and subsequently, the toner is fused at an elevated temperature and pressure. One of the emerging print quality issues associated with electro-photography is severe print mottle in solid images, particularly in colour printing. In the past, paper has often been treated as a uniform structure. In practice, however, paper structure is quite non-uniform and believed to be non-homogeneous in electric properties. Mass density variations, surface roughness, paper thickness variations and moisture variations are all considered to be possible sources of print density mottle in electro-photography. Particularly, mass density, filler distributions and calliper variations have been shown to affect the local dielectric constant of paper [1, 2]. Therefore, it is important to characterise dielectric non-uniformity of paper, which can predict toner transfer variations leading to print mottle.

The average dielectric property of paper has been determined mostly for paper as an insulating material in the electric industry. For example, Delevanti and Hansen introduced an apparatus to determine the dielectric constant of paper by measuring the capacitance and dissipation angles [3]. Calkins described a similar method [4]. De Luca et al utilised a capacitive condenser solution method to obtain the dielectric constant of cellulose [5]. These methods are commonly used to determine the dielectric constant

and dissipation factor from measuring the average capacitance value of a sheet of paper [6] [7] [8] [9].

Knowledge of the capacitance of sheet material is important and of interest in other technologies. In particular, it is of importance in other technologies where a sheet material is printed with an electric circuit. An area of particular interest is that of radio frequency identification (RFID) which typically employs a sheet material, for example, in the form of a tag or label, which comprises a sheet substrate which supports a printed circuit. Such tags or labels receive and respond to radio frequency queries from an RFID transceiver.

Variations in capacitance of such a tag or label are indicative of the variations in quality of the sheet substrate and of the material of the printed circuit. In addition, line uniformity, line connectivity and short circuit between lines printed with conductive inks on a dielectric substrate are also indicative of the quality in the printed electronics.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method of mapping dielectric constant of sheet material such as paper and radio frequency identification devices.

It is another object of this invention to provide a device for dielectric mapping of sheet material such as paper and radio frequency identification devices.

In accordance with one aspect of the invention, there is provided a method of mapping the dielectric constant of a sheet material comprising: supporting a sheet material between first and second electrodes in opposed spaced apart relationship, said first electrode having a probe head facing said sheet material and being of a small dimension relative to an area of the sheet material to be mapped, scanning the area to be mapped, with said probe head, at a plurality of spaced apart sampling points across said area, in each of which the probe head is in facing relationship with a sampling point of the

plurality, determining the dielectric constant of the sheet material at the sampling points, and establishing a map of the dielectric constants.

In accordance with another aspect of the invention, there is provided a device for dielectric mapping of sheet material comprising an assembly of: first and second electrodes in opposed, spaced apart relationship for location of a sheet material therebetween, said first electrode having a probe head of small dimension relative to an area of sheet material to be mapped, support means for supporting the sheet material between said electrodes, means for adjusting the relative positions of the sheet material and the probe head across the area of sheet material to be mapped, so as to establish a plurality of spaced apart sampling points in the area; in combination with: means for determining the dielectric constant at the sample points and establishing a map of dielectric constants for the sheet material.

In particular, the sheet material may be paper sheet.

The invention is particularly described hereinafter, for convenience, by reference to the embodiment in which the sheet material is a paper sheet, but it applies similarly to other sheet materials which may exhibit a variation in capacitance, for example plastic sheet.

Specifically, there is provided a novel instrumentation referred to herein as a paper dielectric mapping device (DMD) for determining dielectric non-uniformity of paper by measuring point-to-point variations of bulk capacitance of paper. The method is based on the A.S.T.M (Dl 50-98) [6], but is modified to scan a large surface area (1 cm ² minimum) of paper using a small probe head relative to the surface area which is scanned under an AC field. The DMD is preferably housed in a Faraday cage to maximize signal to noise ratio and to improve accuracy of the local capacitance measurements under a controlled relative humidity environment in which the test paper is conditioned.

The DMD characterizes paper dielectric uniformity. Dielectric non-uniformity is considered to be one of the most critical factors affecting toner transfer force variations from the photo-receptor to the paper under an electric field leading to print mottle in electro-photography.

The DMD also provides a measure of the dielectric uniformity of other sheet materials, including printed sheet materials such as employed in radio frequency identification devices.

The DMD was tested to characterize the dielectric non-uniformity of various materials including Mylar film, coated and uncoated papers to be printed in electro-photography. The DMD showed good repeatability and reliability in capacitance and dissipation factor measurements with great sensitivity. The DMD could also be used for characterization of the printed line quality for printed electronic circuits. It is particularly useful to be able to detect change in local dielectric constant of paper with changing moisture content.

The practice of the invention also permits mapping of a paper in an on-line paper manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. l is a schematic view of a DMD assembly of the invention;

Fig. 2 is a schematic view of the DMD in a Faraday cage'

Fig. 3 illustrates schematically how the DMD instrumentation is visible through the Faraday cage window.

Fig. 4 illustrates schematically, measurement of capacitance of a sheet;

Fig. 5 is a plot showing repeatability of the average capacitance values over a scanned area;

Figs. 6a and 6b illustrate graphically the change in capacitance with electrode gap distance for Mylar and coated paper, respectively;

Fig. 7 illustrates graphically change in capacitance as a function of gap distance for difference papers;

Figs. 8a, 8b, 8c and 8d show local capacitance variations for different substrates;

Fig. 9 demonstrates standard deviation of local capacitance for different substrates;

Fig. 10 demonstrates the dissipation factor for various substrates;

Fig. 11 shows mapping in accordance with the invention and a photomicrograph of an RFID antenna consisting of silver ink printed on the coated paper side by side; and

Fig. 12 shows dielectric mapping and a photomicrograph images of a RFID antenna consisting of gold printed on Mylar film side by side.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

With reference to Fig. 1, a dielectric mapping assembly 20 comprises a dielectric mapping device 22, an LCR meter 24, a controller 26 and a computer 28.

Assembly 20 includes a Faraday cage 30 which houses the dielectric mapping device

22.

With reference to Fig. 2, there is illustrated the interior of the Faraday cage 30, revealing device 22. Device 22 comprises a top electrode 32 in opposed relationship with a bottom electrode 34. Electrode 32 has a probe head 36 facing electrode34. A probe gap setting system comprises a digital micrometer 38 which sets the gap between the probe head and electrode 34.

A support table 40 supports a paper sheet 42 in the gap between electrodes 32 and34. Paper holder 44 holds paper sheet 42 remote from the electrodes 32 and 34, and has an

DETAILED DESCRIPTION OF THE INVENTION AND BEST MODE Principle

Paper can be considered as a capacitor and the dielectric constant of paper can be determined from capacitance measurement under an electric field. Paper's dielectric property is characterized as a combination of two parameters: dielectric constant and dissipation factor. Dielectric constant is a measure of polarizability of the material compositions in the paper, and dissipation factor is a measure of the charge drainage, since most materials are not perfect insulators. The dielectric constant of vacuum is defined as 1. Capacitance measurements can be made under either AC or DC fields. For paper, in the present application, there is suitably employed AC with an applied voltage of 1 volt and a frequency of 1 MHz for better measurement stability.

Capacitance C is a function of permittivity constant ε and geometric factor, surface area A and thickness of the material d, as illustrated in Fig. 4. Since the capacitor system consists of paper and air, the total capacitance is the equivalent of two capacitors (layers of paper and air) in series.

Capacitance Measurements and Calculation of Apparent Dielectric Constant

The DMD 22 in Figs. 1 and 2 includes a probe head 36, an x-y sample moving stage in the form of the X-Y linear adjuster 46, the probe gap setting system 38, and the sensitive LCR meter 24. The probe head 36 and related assembly and paper holder assembly 44 are housed in the Faraday cage 30 to minimize electro-magnetic noise and at the same time to provide a tightly controlled humidity environment. Relative humidity can be varied ranging from 6.7 % to 97 % using saturated solutions of specific salts as shown in Table 1. The principle of determining the dielectric constant of paper is based on the capacitance measurement using a parallel capacitor model. The probe assembly consists of the two electrodes 32, 34, electrode 34 having a small top pin probe head 36 which may be made of brass and of small dimension, for example 900 μm diameter; and a flat bottom brass plate electrode 34, which may be, for example

X-Y stage in the form of an X-Y linear adjuster 46 which moves the paper holder 44 and hence paper sheet 42, incrementally in an X or Y direction, relative to the probe head 36.

The support table 40 and X-Y linear adjuster 46 are mounted on an insulating base 48 which in turn is supported on a perforated support plate 50, the whole being housed in the Faraday cage 30. A salt pan 52 in cage 30 below plate 50 serves to establish a desired relative humidity in the cage 30. A BNC (bayonet Neill-Concelman) connector adapter 54 serves to minimize 'noise'. The LCR meter 24 for measuring capacitance is connected to the device 22 at BNC adapter 54 and open short adapter 56.

With further reference to Fig. 1, the DMD 22 of the invention is housed in the Faraday cage 30, illustrated in Fig. 2. The Faraday cage 30, is typically a large rectangular box, with the LCR meter 24 supported on the roof of the Faraday cage 30, and the controller 26 on the roof of the LCR meter 24, the computer 28 enters commands for the controller 26 and receives and records results. As seen in Fig. 1, the Faraday cage 30 has a central rectangular plate of aluminium 60, cage 30, suitably has openings or ports 66, 68, which are closed by disc-like covers 70, 72 at either side of the plate , these ports or openings 66, 68 permitting access to the interior of the cage 30. The paper sheet 42 to be scanned is manipulated by the X-Y linear adjuster 46 of the DMD 22, which adjuster 46 is motorized and has an arm which is controlled by computer 28, and capacitance measurements are made at sampling points with the sensitive LCR meter 24.

In Fig. 3, the DMD 22 is visible through the Faraday cage window 62 , the central aluminium plate 60 having been removed.

Fig. 4 illustrates schematically the measurement of capacitance of an insulator such as paper, at a sample point, and the dielectric constant at such sampling point is calculated from the measured capacitance.

of 4.8 mm diameter. The probe head 36 of electrode 32 can be replaceable by various pin sizes typically ranging from 1,130 μm to 270 μm in diameter, while the bottom electrode 34 is fixed. The distance between the top electrode 32 and bottom electrode 34 can be varied depending on the paper thickness using a digital micrometer 38 with 1 micron resolution. The very sensitive LCR meter 24 may be employed to measure the capacitance and dissipation factor, and dielectric constant value can be calculated generally from the capacitance value by using the following equation:

1 "icutψle _| d _aW

C AK E ₀ A e ₀

dsampie :Paper thickness d _air : Air gap

C: Capacitance

K: Dielectric constant

Go: Permittivity constant (0.0885 pf/cm for vacuum)

A: Probe Surface Area

Since paper consists of many materials such as fibres, fillers and air, "equivalent dielectric cpnstants" can be determined using the following modified equation:

K=CTVlOOO/ (5.62-C (H - d _sample )/1000) (1)

C: Capacitance (fF) dsampie: Thickness of sample (μm)

H: Gap between two electrodes (μm)

Scanning area: 10x10 mm ²

Probe size: 0.56 mm ²

Scanning time: 30 minutes

Sampling Points: 400

In general, the area of the paper sheet being scanned is at least 100 mm ² , and within such an area there will typically be at least 100, more generally 100 to 10,000, and preferably about 400 sampling points.

The sampling points are distributed across the area being scanned, with adjacent sampling points spaced apart 100 μm to 1000 μm , preferably 500 μm,

In accordance with the preferred embodiment of the invention, the scanning across the area being mapped is achieved by moving the paper sheet 42 incrementally in a plurality of X and Y directions, with the X-Y linear adjuster 46, so as to sequentially expose fresh sampling points to the area being mapped, to the probe head 36 of electrode 32. Capacitance of the paper sheet 42 at a sampling point is measured when such sampling point is in opposition to the facing probe head 36, which is of small dimension relative to the area being mapped, and typically has a diameter of 270 μm to 1,300 μm, with 900 μm being preferred, or a probe size of 0.057 to 0.64 mm ² .

In this case, the scanning typically commences from one corner of the area, and proceeds, incrementally alternately in the X then Y direction, to establish the plurality of sampling points over the mapping area of the paper sheet.

The scanning could also be achieved by fixing the paper sheet and moving the electrode 32, provided the electrode 32 remains in opposition to electrode 34, however, there is less disturbance to the capacitance measurements if the electrodes 32 and 34 are fixed in position and other components, such as the linear adjuster 46, are remote from the sampling points, and only the position of the paper sheet is changed.

The paper sheet quality is assessed from the dielectric mapping and variation, or non- uniformity of dielectric constants in the mapping is indicative of expected, corresponding print mottle, if the paper sheet is printed electro-photographically.

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Instrument Structure

Since any moving metal part around the probe 36 will have a large influence on the capacitance measurement as "stray" capacitance, the x-y sample holder 44 suitably is kept at a remote distance away from the probe assembly. In order to eliminate all sources of external and internal electro-magnetic field noises, the probe assembly and x- y sample scanning assembly are housed inside the Faraday cage 30. Also, when the motorized x-y stage of line adjuster 46 is located near the probe electrode 32, the electric field disturbs the capacitance measurement as noise. In order to eliminate the noise, the x-y stage including the linear adjuster 46 is located remote from and typically at least 23 cm from the center of the top electrode 32.

A paper sample is placed on the support table 40, suitably made of phenolic material to eliminate static induction. Phenolic laminated sheets are known to be excellent non- conductive insulators and provide good structural rigidity. A paper strip of 3 cm x 25 cm is attached to the motorized x-y line adjuster 46 to control the scanning positions. To eliminate external electro-magnetic field noise including human, the whole dielectric probe assembly is housed in the Faraday cage 30 and all the supporting parts including the X-Y stage line adjuster 46 are grounded.

In order to eliminate lead and connector capacitance and stray capacitance of the system, an "open and short" switch measures "system capacitance" with or without the lead wires. As the result, the LCR meter 24 compensates the "system stray capacitance" providing correct capacitance values for the test sample. Once the "system capacitance" procedure is done, all the test systems are in the same position, only the paper sample is placed into the capacitor system.

The gap distance between the two electrodes 32 and 34 can be controlled by the digital micrometer 38. A no-rotation spindle is connected to the top electrode 32 through an adapter made of Plexiglas™. Capacitance measurement is made by using the high-

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sensitivity LCR meter 24. Although capacitance will depend on the frequency of the AC applied between the two electrodes 32, 34, it is found that 1 MHz is sufficient to provide the most stable and consistent results.

It is known that the capacitance of paper depends on the environment conditions, such as temperature and humidity. The Faraday cage 30 is designed to provide a tightly controlled relative humidity environment by using various saturated salt solutions. The cage has two chambers, upper and lower, separated by a perforated plate 50 as shown in Fig 2. The DMD 22 is placed in the upper chamber and a bath of a saturated salt solution 52 is placed in the lower chamber. It is recognised that other means of generating humid air could also be used. In order to obtain a stable capacitance measurement, all the tests are made under strict standard conditions in the environmental controlled box in the constant room at 23 ⁰ C and 50% R.H.

Scanning and Data Acquisition

Scanning of paper is made by manipulating the x-y controller i.e. line adjuster 46 with the stepping distance typically varying 500 μm in both directions to preferably acquire a minimum of 400 data points for 1 cm . The scanning area can be extended to cover a much larger area depending on the data memory capacity. User interface "Labview" software and serial interface RS232 may be used to collect the data, however, it will be recognized that various systems may be employed for data collection and are not the subject of this invention.

RESULTS

Repeatability

Excellent repeatability was found with various test samples including Teflon™, Mylar™ and several paper samples. The test was made to scan the same area at least 3 times, maximum 5 times, to determine the average capacitance and its standard deviations. The numbers in the legend in Fig 5 indicate the standard deviation of the average

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capacitance values over the same scanned area. Repeatability was excellent with a standard deviation ranging from 0.4 % to 1.6 %. Fig. 5 shows results for Teflon™ (± 0.55), Mylar™ (± 0.17) and paper samples: coated free (± 0.35), wood free A (± 0.98), wood free B (± 0.87).

Accuracy

Figures 6a and 6b show change in capacitance as a function of electrode gap distance for Mylar and a coated paper, respectively, from which apparent dielectric constant K was calculated. The dielectric constant value for Mylar was in good agreement with the literature value (K = 2.9). The apparent dielectric constant value for an uncoated free paper (K = 3.2) was found to be reasonable, since the paper was composed of fibres, fillers and pores (air voids). The DMD provided an accurate dielectric constant value.

Apparent dielectric constant for various sample papers

Fig. 7 shows the apparent dielectric constant for various paper samples determined by measuring change in capacitance as a function of electrode gap distances. Apparent dielectric constants were found to be about 1.6 for two newsprints tested, and 3.2 for two uncoated wood free papers tested as well as 2.9 for Mylar film.

Local Dielectric Variations

Fig. 8 shows local capacitance variations for four different samples including a Mylar sheet, a newsprint and two uncoated wood-free papers. The dielectric variation of Mylar was found to be very uniform as expected, while newsprint was reasonably uniform as no fillers are included in the sheet.

Sensitivity

Sensitivity was evaluated using four samples including a Mylar sheet, a newsprint and two uncoated wood-free papers, as shown in Fig. 8. The DMD was found to have good

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sensitivity to differentiate uniform Mylar from non-uniform commercial paper with a wide range of standard variations.

Dissipation Factor

The DMD can also be used to evaluate the dissipation factor. Fig 10 shows the dissipation factor for various sample material, namely Mylar, Teflon, newsprint and coated paper. It is a measure of the charge drainage of the materials. As expected, the Teflon sheet has the lowest dissipation factor compared to Mylar and paper. This indicates that Teflon has a strong charge holding tendency as compared with paper depending on sensitivity to moisture.

Table 1 List of saturated salt solutions for relative humidity control [10]

The relative humidity of the chamber can be varied using the various saturated salt solutions listed in Table 1.

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As indicated above, while the invention has been particularly illustrated by reference to the dielectric mapping of paper which is to be printed by electrophotography, it applies also to other sheet materials such as employed in radio frequency identification.

Substrates that may be employed, but not limited to, as a component of the sheet in

RFID devices include, by way of example, polyester (Mylar™, Tyvek™, Teonex™, Melinex™), polycarbonate, polyetherketone, polyacrylates, polyethersulphone, polystyrene, polyethylene, aromatic fluorene polyester, polyimide (Kapton™, Modflux™), glass, silicon, coated and uncoated paper, coated and uncoated board, aluminum, stainless steel, and other metal surfaces.

Such substrates may be printed with electric circuits by various techniques, for example, screen, offset lithography, flexography, rotogravure, ink jet, laser, web printing or sheet- fed printing, spray coating or spin coating.

The printing on such substrates may be with inorganic or organic inks, conductive polymers, or of semi-conducting or dielectric materials by way of example, the inks may be based on gold, silver, copper,

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aluminum, carbon, other metallic pigments, or organo-metallic pigments.

The conductive polymers may be, for example,

PEDOT:PSS or Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate), polyaniline, polythiophene, polyaniline, polypyrrole, polyacetylene, polyparaphenylene, polyoctylthiophene, polyphenylenevinylene, polyisothianapthene, or polyisothianapthene.

The semi-conducting and dielectric materials may be, for example, silicon or polyarylamines.

The mapping method of the invention may be employed to evaluate the suitability of the substrates or the materials of the printed circuit, or the suitability of the materials of the printed circuit for particular substrates, in fabricating RFID devices.

In Fig. 11 and Fig.12, dielectric non-uniformity of the circuit lines are visible on the DMD capacitance map.

In Fig. 11 (a) is seen an RFID print circuit microscope image and in Fig. 11 (b) a RFID print circuit DMD dielectric image. The RFID is of silver ink printed on a coated paper.

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In Fig. 12(a) is seen a photographic image and in Fig. 12(b) a dielectric image, both of an RFID consisting of gold printed on Mylar film.

The operating procedure for dielectric mapping of the invention, in a specific embodiment, may be summarized or itemized as follows:

Operating procedure

1.1 Sample preparation

1.1.1 All paper samples are cut into 3.5 cm x 25 cm, and kept in a Faraday cage environment chamber at 23°C, 50% relative humidity for at least 4 hours.

1.2 Thickness measurement

1.2.1 Turn on the power of a precision micrometer; wait for a 15minute warm up.

1.2.2 Use the standard thickness plate to calibrate the micrometer

1.2.3 Put the paper sample to measure the thickness, take three positions of each side for averaging

1.3 Digital micrometer zeroing

1.3.1 Adjust the height of the bottom electrode to the same level of the supporting table, rotate the digital micrometer spindle to reach the top of the electrode as zeroing. Calibrate.

1.4 Capacitance and dissipation factor measuring

1.4.1 Turn on the LCR meter, warm up for 1 hour minimum.

1.4.2 Adjust the testing gap by rotating the digital micrometer (which depends on the thickness of the paper), usually take the thickness plus typically 30μm as the gap to do the test

1.4.3 Determine "System capacitance" by operating "open-short" switch and compensate "stray capacitance" of the LCR meter.

1.4.4 Set the mapping distance as 100 ~1000 μm depending on the scanning area and resolution.

1.4.5 Put the sample on the supporting table, use the paper holder to fix the sample.

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1.4.6 Set the original position of the test.

1.4.7 Start the test.

1.5 Data collection and calculation

1.5.1 The DMD conveniently uses an RS232 collect capacitance and dissipation factor as one raw data.

1.5.2 Save it automatically in a text file.

1.5.3 Open the text file as excel.

1.5.4 Use the equation to calculate the dielectric constant value.

1.5.5 Use the surface chart type to map the data graph.

DMD calibration

The DMD calibration procedure includes two parts:

1. Use the reference thickness for digital micrometer zeroing calibration.

2. System calibration can be done by using one 90 μm thick, uniform Mylar sheet. The Mylar sheet has good uniformity of thickness and composition, and also good stiffness.

The capacitance variation of Mylar should be under 0.05 as the standard deviation.

3. This should suitably be done once a week.

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REFERENCES

1. Provatas, N., Cassidy, A., and Inoue, M., NIP 18 Conference Proceedings, p 770, San Diego (2002).

2. Provatas, N., Cassidy, A., and Inoue, M., NIP 20 Conference Proceedings, p 958, Salt Lake City (2004).

3. Delevanti, C. Jr., and Hansen, P. B., Paper Trade J.121 No.26: 35-43, (1945).

4. Calkins, C. R., TAPPI Vol. 33, No6:278-285, (1950).

5. De Luca, H.A., Campbell, W. B., and Maass, O., Canadian Journal of Research Vol. 16, Sec B: 273-288, (1938).

6. A.S.T.M Designation: D 150-98 (2004).

7. Baum, G. A., Handbook of Physical Testing of Paper, Vol. 2, 333-355, (2001).

8. Simula, S., Handbook of Physical Testing of Paper, Vol. 2, 363, (2001).

9. Busker, L.H., TAPPI, Vol. 15.No8: 348-352, (1968).

10. Handbook of Chemistry and Physics, David R.Lide, Edition (2004-2005).