Tutors: Dr. Johan Sidén, MIUN, johan.siden@miun.se MD Mikael Gulliksson, Sensible Solutions, mikael.gulliksson@sensiblesolutions.se
Study programme: Civilingenjör IT inriktning elektronik, 270 p Scope: 12307 word inclusive of appendices
Date: 2009-02-19
M.Sc. Theses within Electrical Engineering D, 30 points
Printed electronics
Implementation of WORM memory in a RF-antitheft system
Krister Hammarling
Abstract
Current printable memory technology are not suited for mass produc‐
tion. With new inexpensive printed memory, it will be possible to manufacture cheap surveillance tags that are capable to tell the user if something has happened within a timeline. This project is within the ITC FrameProgram 7 founded project PriMeBits. The goal is to imple‐
ment a write once read many memory (WORM) onto an RF‐tag together with a sensor that can sense wetness, which can be detected by EAS antitheft systems. Pre researches have been done in the fields printed capacitance, coils and WORMs, all printed with silver ink. Before implementation of a WORM onto a tag simulations and laboratory tests with adjustable resistances were made. Two different circuit models are simulated and tested. When connected to a tag and the WORM is un‐
programmed, the EAS system should not trigger an alarm. But if the WORM is programmed by the sensor, the alarm should trigger. Results show that capacitances and WORMs are printable with this technique but coils are not due to high inner resistance. The simulations show that a tag built as an LCCR‐circuit is the best choice. This is also confirmed with tests done with real resistors. With WORMs connected to a tag the results show that approximately 70% of them work as intended, this is because the WORMs as of now are not completely developed. The conclusion of this project is that it is possible to implement a WORM onto a tag with further research, to make an inexpensive surveillance tag.
Keywords: WORM, Resonance circuit, RF‐tag, EAS, Surveillance
Acknowledgements
I would like to thank all the people that have helped me in this project.
Especially Johan Sidén and Jinlan Gao who had to put up with all the alarms that went of during my tests.
Thank you all
Table of Contents
Abstract ... i
Acknowledgements ...iii
Terminology... vii
1 Introduction ...1
1.1 Background and problem motivation...1
1.2 Overall aim...2
1.3 Scope ...3
1.4 Concrete and verifiable goals ...3
1.5 Outline ...4
1.6 PriMeBits project details ...5
2 Theory ...7
2.1 LC‐circuit...7
2.2 RF‐tag...8
2.3 EAS Radio frequency system ...10
2.3.1 General operating principle of an EAS system 11 2.3.2 Dual antenna systems 14 2.4 Sintering process ...17
2.5 Kapton® ...17
2.6 Silver ink...18
2.7 Material printer ...18
2.8 Antitheft system ...18
3 Pre research ...19
3.1 Printed capacitor ...19
3.1.1 Printed capacitor on photo paper 19 3.1.2 Printed capacitor on Kapton 20 3.2 Printed flat spiral inductor ...20
3.2.1 Coil on photo paper 21 3.2.2 Coil on Kapton 22 3.2.3 Coil calculated from ink specification 22 3.3 Printed WORM(s) ...23
3.3.1 WORM programmed with current 24
3.3.2 WORM programmed with water 26
4 Implementation ...29
4.1 Simulations...30
4.1.1 Simulation of WORM in series of the tag (LCR) 30 4.1.2 Simulation of WORM in series with 2 capacitances of the tag (LCCR) 32 4.1.3 Comparison of LCR‐circuit with WORM in series with the tag and LCCR‐circuit with WORM in series with 2 capacitances of the tag 36 4.2 Laboratory tests with adjustable resistances...37
4.2.1 Test of LCCR‐circuit with adjustable resistor 38 4.2.2 Test of LCR‐circuit with adjustable resistor 42 4.3 Connecting WORM and tag ...43
4.3.1 WORM connected in series on a tag 43 4.3.2 WORM on a double capacitance tag 44 5 Results ...47
5.1 Printed capacitors and coils...47
5.1.1 Printed capacitance 47 5.1.2 Printed coil 47 5.2 Simulation results ...48
5.3 Results with adjustable resistor on the tag...49
5.4 Results from tag with WORM connected...49
6 Conclusions...53
References...57
Appendix A: Test results from manufacturing and programming WORMs for current programming ...59
Appendix B: Test results from manufacturing and programming WORMs for water programming...69
Register ...74
Terminology
Acronyms / Abbreviations
2D Two dimensional
EAS Electronic Article Surveillance EMF ElectroMotiv Force
ID Identification
IEEE Institute of Electrical and Electronics Engineers ITC Information and Communication Technologies LC‐tag Inductor‐capacitor resonant circuit
LCR Inductor‐capacitor‐resistance resonant system
LCCR Inductor‐capacitor‐capacitor‐resistance resonant system Q‐factor The quality factor
Q‐value The quality value
RF Radio Frequency
WORM Wright Once Read Many
1 Introduction
This project is about implementing state of the art printable low voltage non‐volatile memories in RF‐tags. These memories so called Write Once Read Many (WORM) memories is now under development under the ITC FrameProgram 7 founded project PriMeBits. Quote from the PriMe‐
Bit project description:
In the PriMeBits project, a printable electric low voltage nonvolatile memory is developed for printed sensor, media and wireless ID applications. The main strategy is to utilize printed technology where it has a competitive advantage compared to silicon technology. The project builds on basic research of new materials and components and takes the results into prototyping of new applications [1]
Therefore in this project I will use this WORM technology that are under development and combine it with standard technology found in today’s market, to prove that it is possible to implement the new technology with a low cost, and that it may have a purpose in the modern society.
To achieve this I will first study the WORMs and the theory behind an antitheft system. Then simulate different approaches to the problem and last build a real tag with a WORM connected to it, based on the results from pre‐studies, simulations and laboratory tests.
1.1 Background and problem motivation
Current printable memory technologies are not suited for mass produc‐
tion. They have several properties and other areas where improvements can be achieved. Properties like i) need for high voltage and current for programming ii) short lifetime iii) poor temperature stability iv) chemi‐
cally reactive materials needing encapsulation and/or v) time consuming temperature annealing steps in fabrication. Therefore the need for new better printed memories is mandatory, if they are to compete with silicon technology. [1]
In comparison to silicon technology a cheap and durable printed mem‐
ory has the advantage in several areas. Such areas include disposable
sensor probes, large area sensor functions and combined printed read only and read‐and‐write memory structures. [1]
With new inexpensive, stable and durable printed memory, it is possible to for example manufacture surveillance tags that are capable to tell the user if a “one time trigger event” has happened, since the last reading of the tag. It can be used in for example shipping in combination with different sensors. Where each container/box may have its own sen‐
sor/sensors. Then it will be possible to pick out one specific con‐
tainer/box, for which a “trigger event” has happened.
This technology can then be used as a complement to today’s normal quality checks in several areas. These areas can be but not exclusive to:
transport, food, packaging and built in sensors (for ex. houses, boats, cars etc.). As better and more accurate quality check may be done, it can save resources, time and money in many areas. If these printed memo‐
ries are implemented and used in the right way, then these printed memories can help us save global resources, help industries to become more effective and the consumer will know that the product bought holds a certain standard.
As a step to achieve this I will in this project combine a standard anti‐
theft system tag normally used in supermarkets, with a printed WORM.
This will in it self prove that there are potential markets for these types of sensors, especially if this technique can be used with already devel‐
oped techniques. Which will greatly reduce developing costs and time to market.
1.2 Overall aim
The goal for this project is to build small scale prototype packaging tag.
With the use of radiofrequency‐tags (RF‐tags) with printed WORM‐
memory and a sensor. This new tag should then be able to detect if some packages have been “submitted” to something that it should not have, for example water. It can then be seen as an automatic detector, for example for frozen food boxes, where the food has un‐freezed, even if during the freight time it has been refrozen.
In the future the tag is not limited to sensing wetness, this was an example of one among several sensors that could be attached to the tag.
It is up to the market and engineers to come up with different uses for a system that uses this “tag with WORM” technique.
How the tag is programmed whether it is electrically, chemically or heat/cold etc, is depending on which area(s) it is going to work and what type of sensor that is connected. Here I will study if it is possible to build such a tag that can have a WORM memory connected to it and a sensor that can sense wetness. And thus be able to tell the user if some‐
thing has happened.
1.3 Scope
This study has a focus on if it is possible to print a complete electronic circuit with WORM, capacitor and inductor, so that the circuit works like an antitheft system tag (commonly used in superstores), but is programmable with the WORM. Else if not possible, print and combine a WORM with a standard antitheft tag. The study will not result in a commercial device, rather it will state if it’s possible with further re‐
search to make a commercial product. This will be achieved by making a prototype with a WORM, sensor and tag combined that can sense if it has been wet. And if it is wet or has been wet and dried, it will trigger an alarm when read by an antitheft system normally used in supermar‐
kets.
Long term effects on the WORM are not studied in this project. Neither is the WORM characterized.
1.4 Concrete and verifiable goals
The surveyʹs objective is to connect a commercial antitheft tag with a WORM. This WORM should be able to be programmed in such a way that before programming the antitheft system will not trigger the alarm when reading the tag, but after programming has been done the alarm system will trigger.
Two different circuit models are to be examined. First LCR‐circuit model (Figure 1a), where the WORM (R) will be the resistance in series with the coil and capacitor. Second an LCCR‐circuit model (Figure 1b), where there are two capacitances parallel connected to each other via the WORM (R). Both these systems are to be resonant circuits with a
resonant frequency at 8.2 MHz, to match the antitheft system used in this project.
Figure 1. Fig 1.a shows an LCR circuit model, fig 1.b shows an LCCR‐circuit model,
where the WORM is denoted as R
Problem 1. Is it possible to connect a WORM to a tag and get it to work, using the two circuit models s stated above in Figure 1.
Problem 2. Is it possible to print the entire tag with silver ink, coil capacitor and WORM.
1.5 Outline
In chapter two one can read about the theory behind the different parts of the tag with WORM that I will build, and how an antitheft system works. Material used for printing is described here.
Chapter three includes pre research that had to be done prior to the development of the tag.
In chapter four you can find simulations over two different approaches, LCR‐ and LCCR‐circuit system. After simulations, tests with real resistors that will work as a “WORM”, are tested towards the antitheft system. Last in chapter three one can find tests done with WORMs connected to tags.
Chapter five is the result chapter where chapter four is compiled.
Chapter six is the conclusions of this project.
1.6 PriMeBits project details
A short resume over the ITC FrameProgram 7 founded project PriMe‐
Bits is stated below. [1]
Project Acronym: PRIMEBITS Project Reference: 215132 Start Date: 2008‐01‐01 Duration: 36 months
Project Cost: 4.1 million euro
Contract Type: Collaborative project (generic) End Date: 2010‐12‐31
Project Status: Execution
Project Funding: 2.9 million euro Participants:
MOTOROLA GMBH GERMANY
ECOLE POLYTECHNIQUE FEDERALE DE
LAUSANNE. SWITZERLAND
MITTUNIVERSITETET SWEDEN
LEIBNIZ‐INSTITUT FUER NEUE MATERIALIEN
GEMEINNUETZIGE GMBH GERMANY
UPC KONSULTOINTI OY FINLAND
STORA ENSO OYJ FINLAND
SENSIBLE SOLUTIONS SWEDEN AB SWEDEN
ARDACO A.S. SLOVAKIA
EVONIK DEGUSSA GMBH GERMANY
Table 1. Participants in the ITC FrameProgram 7 PriMeBit project
2 Theory
In this theory chapter I will present the different parts that is later included for my system. The theory behind reading/detecting a tag in an antitheft system, commonly used in superstores is also explained.
2.1 LC-circuit
A typical LC‐circuit is made of one inductor and one capacitance. An LC‐circuit can be built in several ways but in this report I will use variations of a normal LC‐tank circuit shown in Figure 2 and Figure 3. A normal antitheft tag commonly used in superstores is an ordinary LC‐
tank, which has a specific resonance frequency. [2]
Figure 2. LC‐tank circuit Figure 3. LC‐tank with R as capacitor loss
Resonant frequency of an LC‐tank is [3]
f LC π 2
1
0 = (1)
And zero‐reactance frequency is [4]. Where capacitor loss is denoted as R (Figure 2. LC‐tank circuit Figure 3)
2 2
1 1
2 1
C LC R
fz = −
π (2)
Reactance is the imaginary part of the electrical impedance.
2.2 RF-tag
Photo 1. Antitheft tag
In this project I will use antitheft tags bought from Gunnebo Nordic AB This antitheft tag uses the resonant technique LC‐tank, which is de‐
scribed in 2.1 LC‐circuit. The resonant frequency of the tag is 8.2 MHz.
[5].
Specifications of Gunnebo Nordic AB sticker tag (measured and calcu‐
lated): Table 2
Capacitance 105 pF measured with 34405A from Agilent technologies (capacitance including also parasitic capacitances from the coil) Inductance 3.59 μH calculated below, Eq 4
Resistance over C infinite measured with 34405A from Agilent technologies Resistance through coil = 1 ohm measured with 34405A from Agilent technologies
Thickness dielectric
≈ 36 μm measured with Millitast 1083 from Mahr
Thickness coil + dielectric
≈ 95 μm measured with Millitast 1083 from Mahr
Thickness coil ≈ 59 μm ((thickness coil + dielectric) – dielectric, 95 μm – 36μm = 59 μm from above)
Dielectric constant
5.3 calculated below, Eq 6
Area of capaci‐
tance
≈ 6*10+3*3/2+1*2*8 = 80.5 mm2 measured with ruler
Table 2. Specifications on Gunnebo Nordic AB, 40 mm, 8.2 MHz sticker tag
Inductance calculation [3]
f LC π π ω
2 1 2 =
= (3)
Gives with above numbers that
H 3.59 10
* 3.58776 970
2787255.23 1 4
2 1 1
6 - 2
2 2
π µ
π ⎟⎟⎠ = = = ≈
⎜⎜ ⎞
⎝
⎛
= C f C
L f (4)
Capacitance [3]
a
C=εr 0ε A (5)
Gives with above numbers the dielectric constant of the plastic in the tag as:
5.3
10
* 7.12747
10
* 3.78 10
* 854 . 8
* 10
* 5 . 80
10
* 36
* 10
* 105
16 - -15 12
6
6 12
0
≈
=
=
= −− −−
ε ε Α
Ca
r (6)
It is possible to deactivate the antitheft tag, without removing it from the product bought. This is done by applying a strong magnetic field at the same frequency as the tag has. The strong magnetic field induces a current so strong that literally the capacitor in the tag is burnt, i.e. the capacitor is destroyed. The capacitors are intentionally manufactured with short circuit points, so called dimples, which makes it easier to destroy them. [2]
2.3 EAS Radio frequency system
Figure 4. General EAS RF‐system [6]
An Electronic Article System (EAS) that operates with radio frequencies is based upon a transmitter, receiver and an LC‐circuit (tag), se Figure 4.
Sometimes the transmitter also doubles as receiver. It works as a 1‐bit transponder, when read and the tag returns a signal the “bit” is 1 else the “bit” is 0. That means that the system can only have two states, “tag in interrogation zone” or “tag not in interrogation zone”. These types of system are common in warehouses as antitheft system. [2, 7]
The tags used are essentially an LC‐tank that has a resonance peak anywhere from 1.75 MHz to 9.5 MHz. The most popular frequency is 8.2 MHz. Sensing is achieved by sweeping around the resonant fre‐
quency and detecting the dip. By using frequency sweeps for detection of the tag, two things are achieved i)) The tag can have a larger error where the resonant frequency lies, making them cheaper to build ii)) It is easier to detect the tag if the frequency is swept. [2, 7]
Deactivation for 8.2 MHz label tags is achieved by detuning the circuit by partially destroying the capacitor. This is done by “submitting” the tag to a strong electromagnetic field at the resonant frequency which
will induce voltages exceeding the capacitorʹs breakdown voltage, which is artificially reduced by puncturing the tags. [8]
It is possible to make an EAS system so strong that a tag could be read at a further distance then used today (up to two meter). But to protect against established adverse effect to human health, from exposure by RF electric, magnetic and electromagnetic fields, IEEE has worked out the standard IEEE Std C95.1‐2005, to limit these effects in the range of 3 kHz to 300 GHz. The IEEE Std C95.1‐2005 is a revision of IEEE Std C95.1, 1999 edition [B70] and IEEE Std C95.1b‐2004 edition [B71]. [9, 2]
2.3.1 General operating principle of an EAS system
Figure 5. Operating principle of the EAS radio frequency procedure [2]
In a general EAS system (Figure 5), the transmitter generates an alternat‐
ing magnetic field. And if a resonant circuit is moved into the vicinity of this field, then energy from the alternating magnetic field can be in‐
duced into the LC‐circuit via the coil in the LC‐circuit (Faraday’s Law).
If the frequency fg of the alternating field corresponds with the resonant frequency fr of the resonant circuit, then the resonant circuit produces a sympathetic oscillation. If then the frequency is swept from frequency A to C where resonant frequency B are between frequency A and C. The energy that is provided to the resonant circuit can be detected as brief changes in current or voltage. This brief increase in coil current (or voltage drop) in the generator, is known as a dip (Figure 6). The relative magnitude of this dip is dependent of the distance in between the coils and the quality factor (Q‐factor) of the induced resonant circuit. [2]
Figure 6. Graph over a typical impedance dip at the generator coil generated by a RF‐tag in the field, when the generator frequency is swept between two cut‐off frequencies [2]
Whenever the swept generator frequency exactly corresponds with the resonant frequency of the resonant circuit (the tag), it starts to oscillate.
This oscillation generates a dip, which is detectable by the receiver (or in the sender). In swept system the dip is dependent of the rate of fre‐
quency change, faster frequency change rate generates a clearer dip. The sweep frequency rate can be adjusted to optimize the system, to get an optimal recognition rate. [2]
Figure 7. The real and imaginary portions of the impedance spectrum of the sensor after the background subtraction. The resonant frequency f0 is defined as the maximum of the real impedance, while the zero‐reactance frequency fz is when the imaginary impedance is zero [10]
If the oscillating circuit is measured during a frequency sweep, and a background subtraction is made. It is then clearly seen what happens in the resonant circuit with regards to the impedance. At resonant fre‐
quency the real part of the impedance is at its highest point and the imaginary part is 0 (Figure 7). [10]
In alternating current system the overall impedance, phase and effective power is denoted as [3]:
jX R
Z = + (7)
⎟⎠
⎜ ⎞
⎝
= ⎛
R arctan X
ϕ (8)
( )
ϕ cos VI*P= (9)
Due to the fact that in an alternating current system the power is de‐
pendent on the phase ϕ Eq 9, and if the imaginary part is zero, then ϕ is zero Æ cos(0) = 1, which will give the highest power in the system. This means that the oscillating system will radiate with the highest power
when fz = 0 (gives resonant frequency f0). In the receiver the largest dip will be detected.
2.3.2 Dual antenna systems
If only one antenna is used for reading the resonant circuit, then the reading antenna itself will have background noises. These noises can bee so high that it is hard to read out the small signals that an LC‐circuit produces. By using a two antenna approach for monitoring the LC‐
circuit, and the receiving antennas has two loops that is winded in opposite directions, these noises will be cancelled out. Instead of meas‐
uring the impedance directly, the two antenna approach measures the induced voltage over the receiving antennas terminals. The transmitting antenna should be winded as two parallel connected loops (Figure 8). [4]
Figure 8. Set‐up for two‐antenna monitoring approach. Where receiving antenna loops are twisted and sending antenna loops parallel. [4]
Wireless monitoring of an LC‐circuit is achieved through the mutual inductance coupling between the inductor on the LC‐circuit and the loop antenna(s). An electromotiv force (EMF) is induced in the LC‐
circuit, which in return generates a back‐EMF that can be read by the same antenna that sent the EMF or by another antenna. Interactions between the antennas can be described using circuit models. A circuit model of one antenna approach is shown in Figure 9, and a model of two antenna circuit in Figure 10. [4]
Figure 9. Circuit model of one antenna approach [4]
Figure 10. Circuit model of two antenna approach [4]
In Figure 9 Z1 is the intrinsic impedance of the sending antenna and Z2 is the intrinsic impedance of the LC‐circuit, and the driving voltage is denoted as V1. The mutual inductance coupling M between sender and LC‐circuit is represented by two voltage sources V12 and V21. The total impedance across the terminal of the sending antenna, ZT is given by [4]
2 2 2
1 Z
Z M ZT = +ω
(10)
Where ω is the angular frequency in rad/s (radian per second) and M the mutual inductance. If a back‐ground reduction is used to eliminate the intrinsic impedance of Z1, then all that is left is the net LC‐circuit response. With background reduction, ZT becomes [4]
2 2 2
Z ZT =ω M
(11)
In the circuit model used for two antenna monitoring approach (Figure 10) with two loops on sending and receiving antennas. The loops in transmitting antenna are denoted as 1A and 1B, and for the receiving antenna the loops are denoted 3A and 3B. To simplify the equations the two antennas are identical when it comes to size and intrinsic imped‐
ance, thus called Z1. The intrinsic impedance for the LC‐circuit is de‐
noted Z2. The transmitting antenna is driven by a voltage V1. Between the antennas and the sensor, the inductive coupling will be represented as a voltage source VXY. Where ‘x’ represents the element that receives the inductive coupling and ‘y’ the element that causes the inductive coupling. The mutual coupling is denoted in the same way, MXY. By superposition the total voltage over the receiving antenna VTV, can be expressed as [11]
1
3 3 2 3 1 3 1 3 1 1
3 3 2 3 1 3 1 3 1
2
) (
2
) (
Z
V V V
V Z Z
V V V
V
VTV Z AA AB A A B B A + B B + B + B A + −
+
= + (12)
The transmitting and receiving antennas are as stated above identical, thus it is possible to apply symmetry on some of VXY, V3A1A = V3B1B, V3A1B
= V3B1A and V3A3B = V3B3A, simplifying Eq. 12 to
2 V3A2 3B2
TV
V −V
= (13)
Where V3A2 and V3B2 are determinated as [11]
( )
2 1
1 2 3 21 21
2 2
3 2Z Z
M V M M
V A =−ω A + B A (14)
( )
2 1
1 2 3 21 21
2 2
3 2Z Z
M V M M
V B =−ω A + B B (15)
Substituting Eq. 14 and 15 into Eq. 13, yield the total voltage across the receiving antenna as
( )( )
2 1
2 3 2 3 21 21
1 2
4Z Z
M M
M M
VTV V A + B A − B
=ω
(16)
And when the LC‐circuit is absent, the voltage across the receiving antenna is
1 1 2
4Z VTV ω V
= (17)
When designing the loop antennas care must be taken so that the antennas self‐resonant frequency is higher then f0.
2.4 Sintering process
Sintering is a method for making object from a powder. By heating the powder (below its melting point – solid state sintering) the particles adhere to each other, making a solid object. Sintering is sometimes also called ‘baking’. [12]
In Figure 11 one can see how powdered copper goes from state powder to a more solid state. And that the heat has an effect on how solid the object becomes.
Figure 11. 2D reconstructions (virtual slices) perpendicular to the cylindrical axis showing Cu particles at different stages of the sintering process: (a) before sintering, (b) after sintering at 1000°C, and (c) after sintering at 1050°C. Identical regions (inside the rectangle of (a)) are shown at a higher magnification below [13]
2.5 Kapton®
In some parts of this project Kapton HN is used. Kapton HN is a poly‐
imide film with a wide temperature range. Kapton HN is a general purpose polyimide film that can be used for ex. Printed electronics, electrical insulation, etching and several other areas. [14]
The Kapton HN used has a thickness of 75 μm, a dielectric constant of 3.5, and has a temperature range from ‐2690 to 4000 C. [14]
A complete datasheet of Kapton HN can be downloaded at:
http://www2.dupont.com/Kapton/en_US/assets/downloads/pdf/HN_dat asheet.pdf [14]
When printing on Kapton HN it is very important to clean the surface before printing. In this project the surface of the Kapton HN is cleaned with Isopropanol (C3H8O) before printing. See Photo 2 where silver ink is printed on Kapton HN that has not been thoroughly cleaned.
Photo 2. Silver ink on Kapton HN when the surface is not correctly cleaned
2.6 Silver ink
In this project I will use silver ink from Advanced Nano Products co LTD (ANP). The silver ink used is: Silverjet DGP‐40LT‐15C, with a solid content of 40%. [15]
2.7 Material printer
For printing the various parts I used a material printer from FUJIFILM Dimatix inc, and the printer name is Dimatix Material Printer DMP‐
2800. In the printer it is possible to install printer cartridges with either 10 pl or 1 pl nozzles. It also has a built in camera, with which one can look at the printed results and save pictures (see Photo 2 for ex.). [16]
2.8 Antitheft system
An 8.2 MHz RF antitheft system “202 Crome” from Gunnebo Nordic AB is used for testing if manufactured tags works as supposed. Also 8.2
3 Pre research
This chapter describe the different parts in the tag, coil, capacitor and WORM. The parts have been printed with silver ink on photo paper and Kapton HN. The results from this chapter are the base from which I later will build my tag with WORM system.
The photo paper “HP Advanced Photo Paper” have been tested in an oven. This to see how high temperature one can have, before the paper is destroyed, in such a way that the printed structures can be damaged.
The temperature found that for certain is not destroying the structures, was 800 C.
3.1 Printed capacitor
Laboratory tests of two different printed parallel‐plate capacitors. One capacitor on “HP Advanced Photo Paper” with tape as dielectric and one on Kapton HN.
3.1.1 Printed capacitor on photo paper
Figure 12. Capacitance schematic model
The capacitor is printed on photo paper “HP Advanced Photo Paper”
with tape as dielectric. The tape used as dielectric is a writeable tape from scotch, with a thickness of 54 μm.
Printed capacitor with the sides 15x15 mm gives the capacitance:
C = 169 pF (measured with 34405A from Agilent technologies)
Capacitance: [3]
)
0 (in SI units d
C =εrε A (18)
Gives with above numbers the dielectric constant of the tape to 58
. 4 10
* 854 . 8
* ) 10
* 15 (
10
* 54
* 10
* 169
12 2
3
6 12
0
=
=
= − − −−
ε ε A
Cd
r (19)
If C is divided with the area we get the C/mm2 (capacitance per square millimetre)
2 2
2 0.751 /
225
/ 169 pF mm
mm mm pF
C = = (20)
3.1.2 Printed capacitor on Kapton
A capacitor on Kapton HN (75 μm thick), was built by printing 2 layers of ink directly on one side of the Kapton HN. Thereafter it was dried 20 min in 800 C, before the other side of the capacitor was printed. The whole capacitor was then sintered 20 min at 2500 C.
A capacitor of size 25*18 mm ended up with a capacitance of 170 pF. If calculated (Eq. 21) for C/mm2, calculations give a capacitance of 0.38 pF/mm2
2
2 0.38 /
18
* 25
/mm 170 pF mm
C = = (21)
Gives with above numbers the dielectric constant for Kapton HN to 2
. 10 3
* 854 . 8
* ) 10
* 18
* 10
* 25 (
10
* 75
* 10
* 170
12 3
3
6 12
0
=
=
= − − − − −
ε ε A
Cd
r (22)
Compared to the datasheet of Kapton HN 75 μm which state a dielectric constant of 3.5 (tested at 1 kHz).
3.2 Printed flat spiral inductor
Laboratory tests have been done to print coils on “HP Advanced Photo Paper” and Kapton HN.
3.2.1 Coil on photo paper
The coil is printed on photo paper “HP Advanced Photo Paper”.
Figure 13. Flat spiral inductor
The pattern (Figure 13) is a copy of a real tag bought from “Gunnebo Nordic AB”. But it’s scaled up from 40x40 mm to 42x42 mm, this due to the bad surface tension between the ink and the Kapton HN film. This up‐scaling lessen the possibility that the space between tracks are filled with ink. The coil have 10 turns for a total length of 119 cm wire. The printed pattern has the outermost turn 1 mm wide and the rest is 0.75 mm wide. And the distance in‐between each turn is 0.5 mm. When printed one get approximately 0.85 mm wide tracks and 0.4 mm in‐
between. A coil printed with 3 layers of ink, ends up with
approximately 130 ohm resistance, when sintered 20 min at 800 C. That gives a surface resistance (SR) (measured in ohm/square) value of (Eq.
23)
SR = R*w/l = 130*0.085/119 ≈ 0.09 Ω/sq (23)
Where R = resistance, w = width and l = length of electrical track.
That is to be compared with the bought coil from Gunnebo Nordic AB that has an inner resistance of 1 ohm and surface resistance = 1*0.06/100
= 0.0006 Ω/sq.
The coil is not measurable with an RLC‐meter due to high resistance, it will only be seen as a resistor, therefore have I measured the coil with a network analyzer.
The coil value is measured to: 153+j213 ohm, 4.1 μF at 8.2 MHz (the network analyzer sees the coil as a “capacitor” therefore it has the value
4.1 μF). To be compared with the coil from Gunnebo Nordic AB which values are: 19.5+j263.5 ohm, 5.1 μH.
3.2.2 Coil on Kapton
For the coil on Kapton HN the same pattern is used as in 4.2.1. and printed with 3 layers of ink.
As Kapton HN can withstand higher temperature then photo paper, the coil is sintered for 20 min at 2500 C, and the resistance of the coil will be reduced compared to a coil on photo paper, due to better sintering. The resistance for a coil on Kapton HN is around 80 ohm. Which gives a surface resistance SR ≈ 0.04 Ω/sq.
This coil is not measurable with an RLC‐meter due to high resistance, therefore have I measured the coil with a network analyzer.
The coil value is measured to: 69+j164.3 3.2 μH at 8.2 MHz. To be compared with the coil from Gunnebo Nordic AB which values are:
19.5+j263.5 ohm, 5.1 μH.
3.2.3 Coil calculated from ink specification
If perfect sintering was made and calculated for inner resistance and Ω/sq, calculations gives (Eq. 24 to Eq. 27):
Calculated resistance for 1 cm length (l), track width (w) 1 mm and thickness (h) 0.0001 (according to specifications of the ink)
ρ(rho) = 3 μohm*cm, l(length) = 1 cm, A(area w*h) = 0.1*0.0001 cm = 0.00001 cm2
Ω
=
=
= − 0.3
00001 . 0
1
* 10
*
3 6
A R ρl
(24)
0.3 Ω and 119 cm length would give 35.7 ohm.
The surface resistance (SR) becomes l sq
w
SR = R = 0.03 Ω 119
1 . 0
* 7 . 35
* (25)
Calculated resistance for 1 cm length (l), track width (w) 1 mm and thickness (h) 0.0003 (according to specifications of the ink)
ρ(rho) = 3 μohm*cm, l(length) = 1 cm, A(area) = 0.1*0.0003 cm = 0.00003 cm2
Ω
=
=
= − 0.1
00003 , 0
1
* 10
*
3 6
A R ρl
(26)
0,1 Ω and 119 cm length would give 11.9 ohm.
The surface resistance (SR) becomes l sq
w
SR R 0.01 /
119 1 . 0
* 9 . 11
* = = Ω
= (27)
3.3 Printed WORM(s)
WORMs is a 1 bit memory. It can have two stages on and off (WORM‐
bit = 1 and 0). WORMs are not like normal bits that have a distinctive on and off. A WORM has two stages, high resistance (bit = 0) and lower resistance (bit = 1). And depending on application these can be in different ranges. There are two types of printed WORMs that I have worked with. One that is printed to be programmed with an electrical current and another which is programmed with water. I have tested to print WORMs on photo paper, Kapton film and directly on a tag. On photo paper it is relatively easy to print a WORM. But on Kapton film and directly on a tag, there is a problem with that the ink floats out (Photo 3), due to bad surface tension and low viscosity. Consequently it is difficult to print these small structures that a WORM is built of (μm range), therefore in this project only WORMs on photo paper is used (Photo 4).
According to Advanced Nano Products co LTD (ANP) product specifi‐
cation over there silver ink “Silverjet DGP‐40LT‐15C”, the viscosity and surface tension can be modified upon customerʹs request. [15]
Photo 3. WORM printed on tag
Photo 4. WORM printed on photo paper
A WORM has two stages, before and after sintering. Henceforward a not programmed WORM (high resistance) will be denoted un‐
programmed or not programmed, and a sintered WORM (low resis‐
tance) programmed. A not programmed WORM will have a higher resistance then after programmed WORM. High resistance is the same as that the WORM‐bit = 0 (un‐programmed), and low resistance is then WORM‐bit = 1 (programmed).
3.3.1 WORM programmed with current
WORMs that are to be programmed with current have a basic structure as seen in Figure 14.
Figure 14. Basic structure of a WORM for current programming
The thin line in the centre of Figure 14 is the WORMs most important structure. It is this thin line that will give the WORM its characteristics.
A thin long line will give a high starting resistance (un‐programmed bit
= 0). But will also have a higher end resistance when programmed (bit = 1), then a shorter and wider centre line. Therefore by changing the length and width of the centreline, it is possible to build WORMs for different applications (resistant ranges).
As for this project there is a need for a WORM with very low end resistance, in the order < 5 ohm or < 100 ohm, depending where in the tag structure the WORM is connected, serial (<5 ohm) or parallel (<100 ohm). Laboratory tests have then been done to find the best WORM structure for my application. A complete list of test results can be found in Appendix A. Long term effects have not been studied on this type of worms.
For a WORM that has to be below 5 ohm when programmed, a structure where the centre line is ≈ 110 μm long and ≈ 45 μm wide was found to be good (Photo 5).
Photo 5. WORM for current programming. Ca 110 * 45 μm centreline
This WORM have an un‐programmed value between 50 and 14k ohm, WORM is then first pre sintered (dried) 1 min in 800 C. And then pro‐
grammed with 1.6V, 44 mA for one minute. The WORM(s) programmed values are between 3.2 to 3.6 ohm (Table 3). The print layout of the
WORMs is as in Figure 15. The test results come from test 9 in Appendix A.
WORM 1 2 3 4 5 6 7 8
Pre sintered (ohm)
14105 175 550 75 285 50 108 52 Programmed
(ohm) 1.6V, 44 mA 1 min
3.3 3.6 3.4 3.4 3.3 3.3 3.2 3.2
Table 3. Table over 110*45 μm WORM before and after programming
Figure 15. WORM layout for WORM nr 1‐4 and 5‐8 for current programming
For WORMs that are to be in a “parallel” circuit, and that should have an end value of less then 100 ohm, no special research has been done for current programming WORM type (Figure 14).
3.3.2 WORM programmed with water
To be able to program a WORM with water a different type of structure is needed. Below in Figure 16 there are samples of different structures that can be used as water programmable WORMs.
Figure 16. 3 different structures of WORMs for water programming
The multi line pattern in these WORMs works as a collector for the water. When water comes on the lines, a reaction starts that dissolves the material between the nano particles, making them come closer to each other and thus lowering the resistance — programming the WORM. The pattern will make the characteristics of the WORM, the more lines that the WORM is built of the lower starting resistance it will have. The perpendicular lines have the same effect and also an effect that will lower the end result further than without. Long term effects have not been studied on this type of worms. The chemistry reaction behind the fact that WORM becomes programmed when subjected to water, have not been studied.
For this project I have found that a multiline WORM with 5 lines suited best (Figure 17, Photo 6). This WORM self dried for 24 hours in room temperature, then measured for start resistance. After that steam was blown over the worms and then measured directly (within 1 min) and measured again after 24 hours has passed, see Table 4. Table 4 is taken from test 11 column 4 in Appendix B.
Figure 17. 5 line WORM for water programming
Photo 6. Photo of a 5 line WORM
WORM 1 2 3 4
Self dried 24 h
1000 2000 2200 1500
Steam 73 94 91 75
24 h after steam
65 81 82 75
Table 4. Multiline WORM, self dried and then steamed ‐> dried
A complete list of test results for water programmed WORMs, can be found in Appendix B.
4 Implementation
Implementation of a WORM on a normal antitheft tag, can be done in several ways. First idea was to print the WORM directly on the tag but as stated in chapter “3.3 Printed WORM(s)”, it is very difficult to print directly on surfaces that have a bad surface tension. Instead a WORM is printed on photo paper and then glued with silver paste onto the tag.
This chapter is divided into three parts. First simulations that have been done with the simulation program Mindy. Second part is laboratory tests with adjustable resistors that act as the WORM. And in the last part, WORMs has been connected to the tags and tested. From the two first parts simulations and laboratory results, data for in which ohm range the WORM(s) needs to be in was extracted and used in part three.
Two different approaches have been studied:
i)) Using serial coupling of the WORM between capacitor and inductor, Figure 18.
ii)) The capacitor in the LC‐circuit is divided into two capacitors with the WORM as a coupling between them, Figure 19.
Figure 18. LCR‐circuit were the WORM is denoted as R
Figure 19. LCCR‐circuit were the WORM is denoted as R
4.1 Simulations
Simulations are made with the simulation program Mindi from Micro‐
chip. [17]
In simulations and constructions one has to consider if there are high speed electro‐magnetism involved. In most cases one do not need to calculate the circuits as high frequency circuits if the Electrical Length is shorter then 1/20. Electrical length is calculated with formula in Eq. 28. [18]
λ Length L
Electrical = (28)
Were L is the length of the wire (antenna) and λ is the wavelength of the signal. Electrical length is unit less.
As the longest part in the circuit is the coil and from “3.2 Printed flat spiral inductor “the length of the inductor is 119 cm and the frequency is 8.2 MHz. The electrical length can be calculated to (Eq. 29) 0.032 which is less then 1/20 = 0.05, and therefore it is not necessary to calculate with high frequencies in the simulations. [18]
05 , 20 0 032 1 , 59 0 , 36
19 ,
1 = < =
=
= λ Lenght L
Electrical (29)
4.1.1 Simulation of WORM in series of the tag (LCR)
The LCR‐circuit as in Figure 18 can be simulated with two inductive couplings. One that simulates sender to tag, and one tag to receiver. In the schematic the coupling between sender‐tag is TX3 and coupling
between tag‐receiver is TX1. The WORM is denoted as R and the capaci‐
tance as C2. The amplitude of the received signal is measured on TX1 secondary side. A 1k ohm resistance (R2) is placed between measuring point and ground as decoupling. Figure 20.
Figure 20. Simulation setup for WORM in series with an LC‐circuit, were the WORM is denoted as R1
As seen in Graph 1 the WORM resistance have a large effect on the resonant peak. The WORMs dampening effect is very high even with low resistances. If one look at the circuits quality value (Q‐value) Eq. 30, one can see that the quality factor Q is directly proportional to the resistance R. This can directly be seen in the Graph 1 as the peak value at 1 ohm is about 3 mV and at 10 ohms ca 300 μV, a dampening factor of 10, just as the Q‐value formula shows. [19]
C L R CR R
Q L
tot
1
1 =
=
= ω
ω (30)
Graph 1. Amplitude/frequency graph from LC circuit (Figure 20) resonant frequency with WORM as resistance in series
4.1.2 Simulation of WORM in series with 2 capacitances of the tag (LCCR)
The LCCR‐circuit as in Figure 19 can be simulated with two inductive couplings. One that simulates sender to tag and one tag to receiver. In the schematic the coupling between sender‐tag is TX3 and coupling between tag‐receiver is TX1. The WORM is denoted as R1 and the capacitances as C1 and C2. The amplitude of the received signal is measured on TX1 secondary side. A 1k ohm resistance (R2) is placed between measuring point and ground as decoupling. Figure 21.
Figure 21. Simulation setup for an LCCR‐circuit where Ctot is divided into 2 capacitances (C1 & C2) and a resistor (R1) in between
In this circuit the values of C1 and C2 can be changed so that the ratio between the main capacitor C1 and the secondary capacitor C2 is different. Three different simulation ratios for values of Ctot (main + secondary capacitance) have been simulated, 60‐40%, 70‐30% and 90‐
10%. See Graph 2, Graph 3 and Graph 4.
Graph 2. Amplitude/frequency graph from LCCR circuit (Figure 21) resonant frequency with WORM in series with 2 capacitances, ratio C1 = 60% and C2 = 40% of total C
Graph 3. Amplitude/frequency graph from LCCR circuit (Figure 21) resonant frequency with WORM in series with 2 capacitances, ratio C1 = 70% and C2 = 30% of total C
Graph 4. Amplitude/frequency graph from LCCR circuit (Figure 21) resonant frequency with WORM in series with 2 capacitances, ratio C1 = 90% and C2 = 10% of total C
A comparison is made in Graph 5 over the three different ratios, 60‐40%, 70‐30% and 90‐10%. For 10 ohm resistance the resonant frequency is the same for all three ratios and difference is within 2 mV, with 70‐30% as highest value almost 4 mV, and 60‐40% with the lowest value of 2 mV.
At 5k ohms resistance the amplitude is the same for all three ratios ≈ 450 mV. Instead it is the resonant frequency that is shifted. For 60‐40% the resonant frequency becomes 10,5 MHz, 70‐30% has a resonant frequency of 9,75 MHz and for 90‐10% the resonant frequency is 8,5 MHz.
Graph 5. Comparison of different capacitance ratio in an LCCR‐circuit, comparing extreme values 10 and 5000 ohms
By comparing the results in Graph 5, I have found that the best ratio between C1 and C2 is 70‐30%. Ratio 70‐30% has the highest resonant peak and fairly good frequency offset. Which should give a better ohm range (preferably < 100 ohm) for the WORM compared to simulations done in 4.1.1 “Simulation of WORM in series of the tag”.
4.1.3 Comparison of LCR-circuit with WORM in series with the tag and LCCR-circuit with WORM in series with 2 capacitances of the tag In Graph 6 which is a comparison between the LCR‐circuit and the LCCR‐circuit 70‐30%, were WORMs simulated as 1, 10 and 100 ohm has been compared. It is seen that the LCCR‐ circuit 70‐30% has a higher resonant peak at all resistances compared to the LCR‐circuit at the same resistance. The “LCCR 10 ohm” is just a little higher then “LCR 1 ohm”
and “LCCR 100 ohm” is a little higher then “LCR 10 ohm”, from that a conclusion can be made that the LCCR‐circuit has a factor 10 better Q‐
value, except in the lowest range ≈ 1 ohm.
Graph 6. Comparison between LC‐circuit with series resistance (LCR) and LC‐
circuit built with 2 capacitances and a resistance in between (LCCR)
4.2 Laboratory tests with adjustable resistances
For this laboratory test the coil from a real tag is used, together with a printed capacitance and an adjustable resistance. A schematic over the test LCCR‐circuit and LCR‐circuit can be seen in Figure 22. and Figure 23. The adjustable resistor simulates the WORM in both figures. In Figure 22 the circuit is divided into two circuits. One LC‐circuit that will always have a god Q‐value (no resistance), and one parallel circuit with the WORM that has a bad Q‐value (WORM and extra capacitor).
Figure 22. Test setup of LCCR equivalent circuit
Figure 23. Test setup of LCR equivalent circuit
4.2.1 Test of LCCR-circuit with adjustable resistor
First step was to find the best total capacitance Ctot = C1 + C2, for which when the system is built together has a resonant frequency of 8.2 MHz.
From “2.2 RF‐tag” we get that a capacitance of ≈ 105 pF is needed to get the correct resonant frequency. A large capacitance 25*18 mm was printed on Kapton HN (170 pF), the capacitance was then connected to the coil (no resistor). The LC‐circuit was then centred in between the antitheft systems transmitter and receiver, 43.5 cm from both. The capacitance was then cut smaller and smaller until the alarm went off.
The LC‐circuit was measured with a network analyser, and fine tuned to 8.2 MHz. Final size of capacitance is 12*18 mm.
From the result in 3.1.2 “Printed capacitor on Kapton” one can calculate the capacitance (area*(pF/mm2)) to 12*18*0.38 ≈ 82 pF. The rest of the capacitance up to 105 pF, comes from capacitances that are contributions from the coil.
A new double capacitance was printed. The capacitance will share the same bottom layer, but the top of the capacitance is divided into two areas. The two areas are 18*9 mm and 18*3 mm and are separated 1 mm, Figure 24.
Figure 24. Layout of double capacitance, 18*9 mm + 18*3 mm
This new double capacitance together with the coil from a tag and an adjustable resistor, was built together as in Figure 22. The adjustable resistor is a multiturn resistor 0‐200 ohm.
Test 1 start with high resistance (200 ohm) and go down until the antitheft system triggers. Tag is in the centre between sender and receiver (43.5 cm from both), 90 cm up from floor. When the alarm started to sound, I stopped turning the resistor and measured the value of the resistor. See Table 5 and Figure 25.
18 mm
3 mm 1 mm 9 mm Bottom layer
Top layer Capacitance Layout
C2
C1
