Chipworks



Intel Foundries MEMS for Fuel Cell Start-up Nectar

In the last couple of years there have been announcements that Intel will be acting as a foundry for FPGA company Achronix, PLD maker Tabula and programmable network processor provider Netronome, as well as much speculation about making chips for Apple.

All these reports refer to using Intel’s leading-edge 22-nm tri-gate process. However, at CES a couple of weeks ago, my eye was caught by a 200-mm wafer on display at the booth of a little company called Nectar, who were pitching their fuel-cell based USB charging system. They claim that the charger can top up an iPhone battery at least ten times before the fuel pod has to be changed. The whole device can be held in one hand:


Fig. 1 Nectar fuel-cell charger (at right) on display at CES
The cell uses butane fuel in a silicon-based power cell, and by the look of the image below the cells are ~22 mm square.

Fig. 2 Nectar MEMS wafer on display at CES

The press pack given out at the show includes a paper [1] with a description of the technology; a solid oxide fuel cell (SOFC) is used, which is compatible with silicon processing. I’m not a fuel cell expert, so to quote from the paper:

"Fuel cells operate by creating opposing gradients of chemical concentration and electrical potential. When an ion diffuses due to the concentration gradient, the associated charges are transported against the electric field, generating electrical power. In the case of SOFCs, the mobile ion is O2-, and the oxygen gradient is created by providing air on one side (the cathode) and a fuel mixture which consumes any free oxygen on the other side (the anode). Any fuel which burns oxygen will produce power in an SOFC." The schematic below (Fig. 3) illustrates the process.


Fig. 3 Operating principle of solid oxide fuel cell

The butane has to be cracked so that hydrogen is available, which is done in a "fuel processor" within the cell. The following diagram shows the sequence of power generation [1].


Fig. 4 Diagram of fuel cell power generator

The Nectar generator chip contains the fuel processor, fuel cell stack, and catalytic converter. The fuel processor cracks the butane into hydrogen and carbon monoxide by using a lean mixture of air and butane to give incomplete combustion; then O2- ions from the air feed on the other side of the SOFC stack migrate through the stack and combine to give water and carbon dioxide; then the exhaust gases exit through a catalytic converter.

It is here that the MEMS structure comes in – even incomplete combustion of the butane gives temperatures of 600 – 800C, so to integrate this into a package that can be carried around, and also must have conventional silicon for power conditioning has to be a challenge. The fuel processor uses a mechanically suspended reaction zone formed in silicon, with a heat exchanger adjacent to the reaction zone, as shown in Fig. 5 [1, 2]:


Fig. 5 Experimental (top) and later (bottom) MEMS fuel processor

The nitride tubes contain the gas stream, while the silicon bars provide the heat transfer from the exit stream to the input stream. Fig. 6 shows the modeled heat transfer in a pair of tubes (red = hot, blue = cool) [1]. The U-bend at the end is the reaction zone; ignition is started using a platinum heater deposited on the surface, and once started continues autothermally.



Fig. 6 Schematic of modeled heat recovery in reaction loop

The SOFC itself is built of yttrium-stabilized zirconium oxide (YSZ) plates held in a nitride matrix, supported on silicon walls. In order to keep the profile as slim as possible a "planar stack" of plates is formed as shown schematically in Fig. 7(a), with the detail of a single plate in Fig 7(b)[1].


Fig. 7 (a) Schematic of SOFC plates and (b) Cross-section of single cell

Details of the anode and cathode materials are not given, but they clearly have to be porous to allow the gases to diffuse through and react. Similarly nothing is said about the catalytic converter, but that also should be compatible with MEMS manufacturing.

The inherent ability of MEMS processes to provide vacuum-sealed structures helps contain the heat generated within the system, and the chamber is lined with reflective shielding to further reduce heat losses. Even so a new sealing glass had to be developed, since the conventional lead-glass frits used in many MEMS devices was not up to the job.

The whole assembly is packaged in a “tin can” with the gas inlets and exits on the reverse side of the package:


Fig. 8 Assembled and packaged Nectar fuel cell

Of course, smart as the fuel cell manufacturing is, it is only part of a charging system. Fig 9 [1] is a block diagram of the whole system, showing the peripheral components needed to complete the unit and turn it from a concept into a functioning charger. The battery allows power to be drawn instantaneously from the charger while the fuel cell fires up, and also powers the supporting components. 

Fig. 9 Block diagram of Nectar fuel-cell charging system

I started this blog off by talking about Intel, then veered off into a description of the Nectar charger – what was I babbling about? Well, when I was looking at the charger at CES I had a word with Sam Schaevitz of Lilliputian Systems, which developed the Nectar, and asked him who made the MEMS, expecting to hear about of one of the MEMS foundries that are around. (Lilliputian is a spin-off of MIT – Sam is founder and CTO.)

Much to my surprise, he answered "Intel"! As I said at the beginning, there has been quite a bit of comment about Intel moving to the foundry model, but nothing about them being in the MEMS business. It turns out that the work is done at Intel’s fab in Hudson, Mass., which those with long memories will recall was the DEC fab bought by Intel when DEC went under back in 1998.

I had assumed that it would have been closed long ago, but Intel claims to have put $2B into the plant, converting it to 130 nm back in 2001, and it’s now known as Fab 17. It is now Intel’s sole remaining 200 mm facility. In addition they have their Massachusetts Microprocessor Design Center and the Massachusetts Validation Center on the same site, employing ~1700 in total.


Fig. 10 Intel’s Fab 17 in Hudson, MA (source: Intel)

Intel’s Global Manufacturing Fact Sheet states that the fab manufactures “chipsets and other” – the Nectar chip is clearly an “other”! Nectar announced their supply link with Intel back at the end of 2010, but I missed it at the time; Intel Capital also has a stake in Lilliputian.

Aside from the regular processing equipment, Intel must have invested in deep RIE etchers, never mind the deposition gear capable of forming YSZ and the other exotic materials likely used for the anode/cathode and catalytic converter. Presumably Intel’s need for 130-nm chipsets is slowly fading; this looks like a praiseworthy way of keeping the fab going, as well as supporting a local start-up – and one wonders what other foundry work is going on there. If you do have the urge to buy a Nectar mobile power system, it will be available through Brookstone in the summer.

References:

[1] S. Schaevitz, Powering the wireless world with MEMS, Proc. SPIE 8248, Micromachining and Microfabrication Process Technology XVII, 824802 (February 9, 2012)

[2] L. Arana et al., A Microfabricated Suspended-Tube Chemical Reactor for Thermally Efficient Fuel Processing, J. MEMS 12(5) 600-612

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