Ken Shirriff has positioned an uncommon chip beneath the microscope for a have a look at simply the way it’s potential to place wireless-capable electronics right into a disposable piece of paper: the Montreal Métro tickets.
“To make use of the Montreal subway (the Métro), you faucet a paper ticket in opposition to the turnstile and it opens,” Shirriff says. “The ticket works via a system referred to as NFC [Near Field Communication], however what’s occurring internally? How does the ticket work with out a battery? How does it talk with the turnstile? And the way can or not it’s so low-cost that you may throw the ticket away after one use? To reply these questions, I opened up a ticket and examined the tiny chip inside.”
Ken Shirriff has turned his eye to a maybe stunning piece of hidden know-how: the NFC chip inside subway tickets. (📷: Ken Shirriff)
Every ticket, Shirriff explains, comes with an embedded chip sandwiched between the paper — a part of a plastic insert that features metallic foil performing as an antenna. Whereas the tickets additionally embrace what seems to be the gold contacts for a smartcard chip at one finish, they’re printed on the paper with non-metallic ink and are completely ornamental. Because of this, there isn’t any have to put the paper ticket right into a smartcard reader: simply faucet it in opposition to a turnstile and also you’re good to go.
The key, after all, is NFC, or Close to-Subject Communication — a regular by which a compact chip is powered utilizing power harvested from a close-by reader’s radio transmissions. How compact? “It’s in regards to the measurement of a grain of salt,” Shirriff writes. “The chip can also be extraordinarily skinny — 75µm or 120µm — so you possibly can’t even really feel the chip contained in the ticket.”
This composite picture exhibits simply how tiny the chip actually is: these are grains of salt subsequent to it. (📷: Ken Shirriff)
That tiny chip is a MIFARE Ultralight EV1, one of many lowest-cost components within the MIFARE household. It lacks superior options accessible in additional upmarket variations, and is tailor-made for single use — changing printed barcodes or QR Codes with one thing tougher to idiot with a photocopier. Not like printed tickets, it not solely transmits info to the reader however can retailer info in flip — updating inside counters that can be utilized to restrict the variety of instances a ticket is legitimate for transit.
To see what’s contained in the tiny half, Shirriff first extracted it from the paper ticket by soaking it in water. The plastic was then eliminated with boiling sulphuric acid, abandoning the silicon die. Boiling phosphoric acid took care of a passivation layer on high of the die, which was then handled with an etchant and hydrochloric acid. Whereas the method requires the cautious dealing with of a variety of boiling acids, Shirriff had one thing else on his thoughts: “My greatest concern whereas processing this chip,” he explains, “was to keep away from shedding it.”
The tiny chip, bought on a wafer of hundreds for cents apiece, is housed in a hidden plastic layer throughout the ticket. (📷: Ken Shirriff)
Inserting the chip beneath a high-powered microscope, Shirriff was in a position to hint out its numerous blocks: analog circuitry, a big chunk of standard-cell digital logic, an EEPROM to retailer information, and a cost pump to spice up the low-voltage harvested power to a stage appropriate for writing to stated EEPROM. “It is exceptional that these NFC chips may be manufactured so cheaply that they’re disposable,” Shirriff concludes.
The total write-up is out there on Shirriff’s web site.