Microelectronics Journal 42 (2011) 21–27
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System of nano-silver inkjet printed memory cards and PC card reader and programmer Henrik Andersson a,n, Alexandru Rusu b, Anatoliy Manuilskiy a, Stefan Haller a, ¨ b, Hans-Erik Nilsson a Suat Ayoz a b
Mid-Sweden University, Department of Information Technology and Media, SE-851 70 Sundsvall, Sweden EPFL-STI-IEL-NANOLAB, Bat ELB342, Station 11, CH-1015 Lausanne, Switzerland
a r t i c l e in fo
abstract
Article history: Received 28 May 2010 Received in revised form 25 August 2010 Accepted 1 September 2010 Available online 18 September 2010
This work describes the development of inkjet printed, low-cost memory cards, and complementary pair of memory card reader and card reader/programmer for PCs. This constitutes a complete system that can be used for various applications. The memory cards are manufactured by inkjet printing nanosilver ink on photo paper substrate. The printed memory structures have an initial high resistance that can later be programmed to specific values representing data on the cards, the so called Write Once Read Many (WORM) memories. The memory card reader measures the resistance values of the memory cells and reads it back to the computer by USB connection. Using multiple resistance levels that represent different states it is possible to have a larger number of selectable combinations with fewer physical bits compared to binary coding. This somewhat counters one of the limitations of resistive memory technology that basically each cell needs one physical contact. The number of possible states is related to the resolution of the reader and the stability of the WORM memory. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Printed memory Silvernano ink Inkjet Printed electronics
1. Introduction This work shows the development of very low cost memory cards that are inkjet printed on photo paper, and a complementary pair of memory card reader and card reader/programmer for PCs. The sintering of the memory cell is based on the Rapid Electrical Sintering (RES) technique invented by VTT [1–3]. The reader and printed memory cards constitutes a complete system that can be used for various applications. For example data on cards can be connected to personalized information on the internet so that when a memory card is inserted in the reader, a webpage is automatically displayed. It can also be used for advertising, for personalized vouchers or tickets or similar applications where the printed memory will be able to add value to a printed graphic product [3]. When used as an ePIN it could replace traditional paper coupons for WLAN access in hotels or conferences [3]. The solution could also be used to enhance security for internet logins [3]. Because of its low cost it is possible to distribute the reader free of charge to the user, who will then have the possibility of using memory cards distributed with magazines, brochures, on boxes, etc. The cards can be both personalized, where each card has a unique content for the recipient, or more generalized, where a number of cards will contain the same code. Different applica-
n
Corresponding author. Tel.: +46 703622245, fax: + 46 60 148456. E-mail address:
[email protected] (H. Andersson).
0026-2692/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2010.09.008
tions can be realized by designing different software. It is also possible to reprogram the memory cells to a lower resistance value after the first programming. If the programmer is used, a certain memory cell can be changed when the information has been accessed or certain choices have been made, thereby dynamically updating the information [3]. In some applications both read and write might be desirable and in others read only is chosen due to simplicity and lower cost of the reader hardware. The printed memory structures have an initial high resistance that can later be programmed to specific values representing the data on the cards, the so called Write Once Read Many (WORM) memories. The programming of the memory devices is based on the Rapid Electrical Sintering (RES) technique that have been developed and described by VTT [1–3]. Also, other types of printed memory types on both flexible and rigid substrates have been described earlier [4–8].
2. Inkjet printed memory cards Inkjet technology has been used for some time to produce printed electronics on flexible substrates. The benefits of using inkjet among others are that it is an additive process, depositing material only where it is needed, and the large flexibility in changing print patterns. A variety of different electrical components manufactured by inkjet printing have previously been described [9–11].
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The memory cards are manufactured by inkjet printing silver nano-particle ink by a Dimatix 2831 piezoelectric materials printer. The cartridge type used was 10pL Dimatix 11610. The ink used is Silverjet DGP-40LT-15C manufactured by ANP, which has a solid content of about 40–45% silver, a viscosity of about 16 cP and is diluted in a polar solvent. The substrate used is Canon PT-101 advanced photo paper. The ink is of nano-particles where the silver nano-particles have a diameter of about 30 nm and are surrounded by a polymer shell. Because of the small size of the particles they will have a lower melting temperature than the corresponding bulk material. To get good conductivity the silver particles have to be sintered together after printing, which can be done by thermal heating in an oven, electric sintering or other methods such as microwave sintering [12–14]. To achieve the desired functionality of the memory cells, quite a high resistivity is required. Therefore, the silver ink is just partially sintered for this application. The contact layout of the memory card used for the reader is same as that of the Secure Digital (SD) type flash memory cards. This is because of the convenience of using a commercially available contact and the form factor with quite a large spacing between the contacts. The 9 pin SD layout limits the number of memory cells to 8 and 1 ground pin. The second layout is a 30 pin Zero Insertion Force (ZIF) contact with a total of 30 pins where 20 pins are used for memory cells, 8 are used for alignment verification of the card and 2 are ground pins. Different layouts with more or less contacts could be used if the application demands it. The limit on the number of contacts that can be used is basically by the size and distance between contact pads to achieve an adequately secure alignment. Fig. 1 shows the layout used to print the contacts and memory cells of the SD card design, top, and the ZIF design, bottom. The actual memory cells are the thin lines close to the bottom of the contacts. Fig. 2 shows a photograph of a finished printed SD type memory card. The card can be seen to be laminated with more layers of photo paper to get a thickness comparable to that of an SD memory card. Fig. 3 displays magnification of a memory cell, where the thin line between the contacts are the actual WORM memory cell. This has a length of about 125 mm and a width of 30 mm. The thickness of the WORM silver layer after printing has been measured by a Scanning Electron Microscope (SEM) to be about 450 nm. The resistance of a WORM memory cell after printing is
Fig. 2. Photo of a printed memory card.
Fig. 3. Image of memory cell where the thin line between the contacts are the actual memory. The length of this line is 125 mm and the width 30 mm.
Fig. 1. Layout of the printed memory cards showing contacts and memory cell locations (thin lines) of the SD card type, top, and ZIF type, bottom.
usually in the range of several MO. To pre-program the cells to the desired resistances used in the reader the card is first heat-treated in an oven at 90 1C to evaporate any remaining solvent. The resistance of the WORM memory cell is then still fairly high in the range of a few hundred kO up to about 1 MO. To pre-program the memory cells to their starting values a computer controlled electric sintering setup is used. This setup consists of a Keithley 2400 source meter that measures the resistance continuously and supplies the voltage and current and also a multi-channel switch that in turn switches between the memory cells. The sintering setup is completely automatic and adjusts the output voltage and limits the current to appropriate levels and stops when the desired resistance has been reached and then switching to the next memory cell, repeating the procedure for the whole card. The sintering is usually started at about 15 V and subsequently stepped down to a few volts when the resistance gets close to the target value. The sintering setup
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1.6 Resistance (Ω) Current (mA) Voltage (V*0.1)
106
1.2 1 0.8
105
0.6
Resistance (Ω)
Voltage (V*0.1), Current (mA)
1.4
0.4 0.2 104
0 0
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Time (S) Fig. 4. Typical pre-sintering of memory cell showing voltage (solid line), current (dashed line) and resistance (circles) during the sintering process to a final resistance value of 9 kO. Note that the voltage scale is multiplied by a factor 0.1.
1.01
WORM 1 WORM 2 WORM 3 WORM 4 WORM 5 WORM 6 WORM 7, SU−8 WORM 8, SU−8
1 Resistance (Normalized)
has been evaluated and shown to consistently reach the desired resistance values within a few hundred O. If scaled up, such a system can be used to program memory cards in large quantities for production. A typical sintering of a WORM memory cell is shown in Fig. 4. The starting resistance of the memory cell is 1.6 MO and the starting voltage is chosen to be 15 V. It can be seen that the resistance drops fairly quickly in this region, and reaches 400 kO after 3 s (note the resistance log scale). Then the voltage is switched to 10 V to slow down the sintering process, which would otherwise accelerate fast. After a total time of about 15 s a fast drop in resistance can be observed, showing the quite non-linear sintering behavior of the WORM memory, after which the voltage is adjusted to 4.5 V. The final resistance reached is about 9 kO, which is close to the set value of 10 kO. An important factor to consider when designing such a system with printed memory cards is the long term stability of the resistance. Therefore, stability tests of memory cells in room climate have been performed as well as a test to protect the memory cells against changing climate and mechanical wear by coating the printed WORM memory cards. For this purpose an epoxy photoresist, SU-8, was inkjet printed over the memory cells as a cover layer. The SU-8 is manufactured by Microchem and is mainly used for spin coating in the semiconductor industry. The type used, SU-8 2002, has a viscosity of 7.5 cSt and can be used in the Dimatix materials printer with good results. The SU-8 was printed over the WORM memory cell but leaving the contacts uncoated, after which a short bake in a convection oven to 90 1C was done followed by exposure to UV light to harden the epoxy. The surface becomes scratch resistant and protects the WORM memory well from mechanical damage. To evaluate the stability of resistance during storage a long term stability test was performed. In this test some memory cells with and without SU-8 protective cover were continuously measured every 30 min for 14 days. The humidity was varied between 25% RH and over 50% RH and the temperature between 23 and 28 1C. The relative resistance change is shown in Fig. 5. The resistance of the WORMs that were uncoated changed at most about 8% and the ones coated with SU-8 about 4%. It can be concluded that a 4% change during 2 weeks is quite small and will not pose a problem for the function of cards with an adequate separation in resistance between the different states. In a previously reported study on the stability of inkjet printed nano-silver structures it was shown that
0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.92 0
2
4
6 8 Time (days)
10
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Fig. 5. Resistance change of uncoated and coated WORM memory cells during 14 days.
the change was in the same range; however in that study the printed structures were larger and the substrates used were plastic and not paper, so these cases are not completely comparable [15]. Also, a cover layer is an advantage because of its value as a mechanical protection.
3. Memory card reader and programmer The reader and the programmer/reader were constructed with some differences in operation to test the possibilities with the printed memory cards. The reader operation mode is to read the resistance of the WORM memory cells. The cells have several possible resistance steps, where each step represents a different state. Using more than two states the possible combinations for the 8 bits increase from 256, if two states are used, to 6561 with three states and 65,536 with four states and so on. The number of possible states is related to the resolution of the reader and the stability of the memory cell. The reader has a range of up to 10 kO with a resolution of 50 O. This could be extended using dynamic gain in the reader, giving the possibility to use a wider range and
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thereby more memory states. The limiting factor for the number of states that could be used is in practice not the reader but mostly how exactly the memories can be programmed and resistance stability over time. After printing the memories are pre-programmed to 10, 6 or 2 kO, which give a suitable separation of resistance values for the reader. The programmer on the other hand uses three different states, blank, programmed and fused. A blank memory card means that the resistance is above a set threshold value for all memory cells on the card and will be ready to be programmed. A memory card is defined as programmed when at least one cell is below the set threshold and a fused cell means the cell has been burned off and has a near infinite resistance. For the programmer, when the card is inserted in the connector slot of the readout device, its presence will be automatically detected by the computer. The user interface software then verifies whether the card is blank and will prompt the user for the desired action. The programmer has a variable resolution in the range of 5 O at low resistance values and rising up to 1 kO for larger ones (4100 kO). If the card is blank, the possible actions are either to write a code into it or leave it unaltered. When the card is not blank, the following options are available to the user: read the code, modify several bits (the blank bits can be programmed or fused and the programmed ones can be fused) or destroy the card (fuse all bits). Certain bits on the card are initially open circuit or shortened in order to verify correct alignment of the inserted card. The memory card reader and reader/programmer both measure the resistance values of the WORM memory cells and read it back to the computer by a USB connection. Both devices are powered from the USB port, which can provide 5 V and 500 mA. In order to read the memory bits with the programmer, 5 V is applied on each WORM while the current is limited to 1 mA to prevent sintering by an appropriate resistor placed in series. To sinter a WORM with the programmer, 5 V is applied and the current is limited to 20 mA by means of a resistor placed in series with the voltage source and the cell to be sintered. Using these settings the time required for programming is approximately 2 ms but for safety the microcontroller command is sustained for a period of 5 ms. This programmes the WORM cell to a ‘‘low’’ state. To fuse a WORM, a voltage higher than 10 V should be applied. In this case the WORM will be first programmed, if not already, and then fused using heat. The maximum current that can be delivered for fusing is 1 A. In order to obtain the desired levels of voltage and current, a DC–DC converter was used. At 10 V, the time required for fusing is less than 50 ms. A 15 V DC–DC convertor was constructed for reducing the fusing time even further. The voltage levels are read by the A/D converter integrated in the microcontroller and then sent by serial communication to the computer. The read-only device is also designed to connect to a USB port. It features a boot loader for firmware updates directly via USB. This allows the reader to be upgraded and adopted to other applications by just supplying an upgrade tool to the user. The PC connection is established with the help of an FT232R chipset from FTDI, which operates as a virtual serial port. Therefore any VT100 compatible terminal program can be used to communicate with the reader. The reader is designed around an ATMEL ATxmega64A1 /ATxmega128A1 controller. This controller series features two 2Msps Analog to Digital Converters (ADCs) with 8 inputs each. It handles all the functions of communication and resistance reading so the number of extra components is kept to a minimum. The internal analog gain stage can be used to amplify the signal in steps of 2n up to 64 times, which makes it possible to measure
resistance values from approximately 100 O to 100 kO. To increase accuracy the influence of moving ground potential is minimized using differential inputs. Each memory cell is connected to a 2.5 V source that is also used as the Analog to Digital Converter voltage reference. A 100 kO series resistor between source and memory cells limits the current. The source must supply sufficient energy to also be able to read low resistance values properly. The worst case is when all cells have low resistance, for example if each cell has 100 O. The maximal current at which all cells could sink can be calculated . Imax ¼
Vref n Rref þRW
ð1Þ
where Vref is the reference voltage, n the number of memory cells, Rref the reference resistance, set constant in firmware, and RW the resistance value of the WORM memory cell. In the case where Vref ¼2.5 V, n¼ 8, Rref ¼100 kO and RW ¼ 100 O this gives 200 mA. There is no need for a drift stable voltage source. The following equation shows that the source voltage has no influence on the measurement result: RWM ¼ ¼
Rref Vmeas Rref Vref ADC=2047 ¼ Vref Vref ADC=2047 Vref Vmeas Rref Vref ADC Rref ADC ¼ Vref ð2047ADCÞ 2047ADC
ð2Þ
where RWM is the calculated resistance value, Vmeas the measured voltage drop over the memory cell, and ADC the value read from the Analog to Digital Converter. The resistance value of each memory cell, RWM, is calculated by the controller firmware. The ADC value is read out and RWM is calculated as shown in (2). Fig. 6 shows a block diagram of one of the memory cells in series with the 100 kO series resistor and the ADC measurement points over the memory cell. When reading the resistance value, the ATxmega port connects the circuit to ground. When reading a memory cell the power allowed to be dissipated over the structure must be small enough to be sure that it does not start to sinter and lowers the resistance. This is done by limiting the maximal current through the WORM to IWMax, as given by . IWMax ¼
Vref Rref
ð3Þ
For Vref ¼2.5 V and Rref ¼100 kO this gives 25 mA. A time slot of 16 ms is used for reading when the memory cell is powered. During this time 20 measurements are done on each cell, which give an averaged value of the resistance. See Fig. 7 for an oscilloscope readout showing the time when the memory is powered and the 20 ADC readings, shown in a group centered around 2 ms. The ADC readings show only the timing of the readings; it is a digital signal shown to be ‘‘on’’ or ‘‘off’’ and does
Fig. 6. Diagram showing one out of eight identical reading circuits for the memory cells.
H. Andersson et al. / Microelectronics Journal 42 (2011) 21–27
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3.5 Voltage Drop Over WORM ADC Readings
Voltage Drop Over WORM (V)
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Time (ms) Fig. 7. Oscilloscope reading showing the voltage drop over the WORM cell, (dotted line), and the 20 ADC readings, (continuous line). The ADC readings are digital, just showing the timing of the readings and not any actual voltage.
not have any relevance to the actual voltage readout. Totally, this gives a power while reading the WORM as given by Pread ¼
Vref 2 t Rref þRW read
ð4Þ
where tread is the time for which the WORM memory cell is powered. With Vref ¼ 2.5 V, Rref ¼100 kO, tread ¼16 ms and RW is 2 kO this gives Pread ¼980 nW s. This shows the power dissipated for each reading cycle using a 2 kO WORM memory cell. This has been determined to be small enough so as to not start sintering the WORMs in any detectable way. If desired, a faster or slower reading speed is possible by adjusting the firmware and no hardware changes are needed. The memory cell is powered only while it is read; in the idle state the ground pin is held at tri-state to avoid current flow through the WORM. Before the ADC reading is started, the pin is driven to a logical low and then the ADC reads the voltage drop over the memory cell and switches the controller pin back to tri-state. Fig. 8 shows a photograph of one fully assembled and functioning memory card reader. Fig. 9 shows one assembled main circuit board of the reader/programmer. Note the inserted printed memory card at the top. To test the different possibilities of the printed memory card some demonstration programs have been written. In one demonstration program the values of each memory cell are read out when a card is inserted and a certain webpage is loaded and displayed depending on the values of the memory cells. Different web pages can easily be linked to different unique memory cards or a series of memory cards. A simple program for Windows has also been developed to demonstrate the functionality of the reader/programmer. If one cell is selected, the program checks if the serial port is opened and sends the selected command for read, sinter or fuse followed by the cell number. After the command is sent, a character array that represents the return value of the ADC is expected. Using this value and setting the appropriate threshold values, values of each cell can be determined. The bits are arranged intuitively; the matrix of 5 4 bits are the memory bits that the user can read/ sinter/fuse and the upper row bits are the control bits that are
Fig. 8. Photograph showing one fully assembled and functioning SD format reader with a printed memory card inserted.
used to verify the proper insertion of the card. Of these, four are short circuited and the next four are open circuited. A screenshot of the reader/programmer user interface is shown in Fig. 10. More complex software is most likely needed for commercial applications; these programs were just done for demo purposes.
4. Discussion During the development of the reader a voltage regulator was used to evaluate different readout strategies. The current design allows to further cut the cost of the reader using a voltage divider or a Zener-diode instead of the voltage regulator. The voltage should be kept in a certain range of 2.4–2.5 V. A lower reference voltage results in a lower voltage drop over the memory cell, which lowers the reading accuracy. A higher reference voltage is not recommended by the microcontrollers internal ADC.
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Fig. 9. Photograph showing the main circuit board of the ZIF format reader/programmer.
Both the physical size of the memory cards as well as the number of bits can be easily scaled for various needs. One of the limitations of using resistance based memories is that it is difficult to multiplex; basically each memory cell needs one physical contact. This limitation is somewhat decreased if the multiresistance level scheme is used where it is possible to have a larger number of selectable combinations with fewer physical bits compared to binary coding. Because the memory cards are manufactured by inkjet printing they are very inexpensive to produce. This makes it possible to use them in many new very cost-sensitive applications where a device with a limited data capacity is needed. The developed reader and reader/programmer together with the printed memory cards form a complete system to deliver information in a new way.
Acknowledgement
Fig. 10. Screenshot of the computer–user interface for the reader/programmer device.
The first ATxmega controller ATMEL could deliver was the ATxmega128A1. The controller resides in a 100-pin case. Now ATxmega controllers in a smaller case with 44 pins are available, for example ATxmega128A4. This will further reduce the cost by saving circuit board space and also the price of the 128A4 controller is approximately 50% lower. Also less external components are needed, which further reduces the size and cost.
5. Conclusions In this article a printed memory card/memory card reader/ programmer system has been developed and shown to work for different applications.
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