RFIDs power themselves
RFIDs have enabled a significant improvement in many very costly business systems ranging from warehousing to tagging your luggage at the airport. The secure contactless IC variants have found their way into our credit cards and passports, and their future application almost certainly includes the replacement of the checkout counter at the local grocery chain.
Billions upon billions of "socket wins" in a simple package
RFIDs' main claims to fame are their low cost, their ability to function with no external physical connections, and the lack of a dedicated power supply. They transmit and receive data, and acquire power via RF techniques.
Alien Technology is one of the leaders in RFID technology. In 1993 the company sold 500 million units to Gillete alone, at a price of less than 10 cents per tag. Alien has designed some of the most innovative, and cheapest, RFIDs we've seen. This article talks about one of their recent designs, the ALN-9640.
The Alien ALN-9640 Higgs-3 UHF RFID IC is one of the first devices in Alien's new family of Higgs-3 powered UHF RFIDs. While a general purpose RFID, the company claims it is optimized for use in pallet and case tagging applications. The chip is embedded in a popular form factor, the Squiggle antenna, as shown in Figure 1, and operates in the UHF band (840 to 960 MHz). The ALN-9640 delivers EPC Gen 2 (EPCglobal UHF Class 1 Generation 2) capability and is also World Tag compliant, enabling inlays that operate consistently across the diverse frequencies of the Americas, Europe, Middle East, Asia, and Africa (Reference 1).
The Higgs-3 core provides 512 bits of user memory for applications requiring local data storage, and it offers a 96-bit to 480-bit extensible EPC number for alternative industry standards and legacy part numbering schemas. This chip also features a unique 64-bit TID for use in product authentication and serialization applications and read/write password support to prevent unauthorized viewing/modification of the tag's data. The ALN-9640-WR is manufactured in a three-metal, single poly 0.18 μm CMOS process with the addition of a MIM (metal-insulator-metal) capacitor layer, and it is packaged in a wet inlay together with the antenna.
The data receiver must be able to detect a very weak signal induced on the antenna. The transmitter sends data back by backscattering the external reader's carrier wave. But in my opinion the coolest thing about a passive RFID is the lack of pins. Not only does it transfer data wirelessly, it also acquires its power over the air. You might think that would limit the chip to being quite small and simple, but as shown in the die photo of Figure 2, that's not the case. There is a lot of complex circuitry on this part, all powered from an on-chip power supply. So the most obvious thing to do in this column is to take a look at this supply generator.
The ALN-9640 IC actually has two pads that connect to the Squiggle antenna. The antenna captures the RF signal and transfers this modulated waveform to the chip. The chip's DC power supply is generated by means of a charge pump rectifier from the signal induced in the antenna. The rectifier is formed of a cascade of charge-pump rectifier cells and produces the primary power supply voltage VDD. A schematic of one of these cells is shown in Figure 3.
The antenna pad CK can be seen as the input on the left of the schematic. It drives two balanced coupling capacitors, which then drive complementary active circuits. The VL input in the lower right is the low voltage cascaded in from the previous cell, and the VH output in the upper right is the high voltage being cascaded to the next cell. The active network of NMOS and PMOS effectively pump positive charge from the antenna pad to the output node, and pump negative charge from the antenna pad to the VL node.
As can be seen on the schematic, this circuit uses two different types of on-chip capacitors: LMCAP and MTMOSCAP. LMCAP are MIM capacitors built using intermediate metal as the top plate and metal 2 as the bottom plate. MTMOSCAP capacitors are formed using standard MOS transistors in parallel with the LMCAP structure above them. Views of the poly, metal 1, and metal 2 layer of a typical MTMOSCAP are shown in Figure 4.
This cascaded network of charge-pump rectifier cells is very dependant on process, voltage, and temperature conditions. If the worst case VDD is designed to be high enough to supply the chip, then the best case VDD could easily violate breakdown voltage design rules. Therefore, a VDD clamp is implemented on this chip, as shown in Figure 5. In this circuit six diode-connected NMOS devices and the voltage follower P425 are used to limit the highest voltage VDD can attain.
There are quite a few other circuits involved in regulating and supplying this voltage to the chip. One regulator is used for this internal VDD voltage, and another regulator generates a secondary power-supply voltage. There is also a supply-voltage level detector that flags the chip when VDD is at an unacceptable level. It's clear that generating a DC supply from a signal induced on an antenna is not a simple matter.
The growth of RFIDs in the last few years has been massive. More and more applications seem to be popping up for them. Alien Technology has taken advantage of this growth and created some very advanced devices. Impinj is another company on the cutting edge of RFID technology, and they have just released their latest chip, the Monza 3. Here at Chipworks we are just starting the analysis of this, and I'm looking forward to what new techniques they've implemented. It's a very interesting time in RFID design.