Aug 21, 2014

A HEMT Cool-Down

posted by Bryon Moyer

Heat has got to be one of the most annoying side-effects of doing useful electrical work. The more work we do, the more things heat up, changing the characteristics of the circuitry and, if we’re not careful, leading to early end-of-life or outright failure.

It’s heat that’s part of why we’ve gone to multicore instead of simply ratcheting up microprocessor clock frequencies forever. Greater dissipation is one reason we end up with power transistors that are larger than they need to be for electrical reasons. And when 3D ICs were first trotted out as an idea some years back, one of the immediate questions was how heat would be removed from the center of the stack.

We do lots of things to mitigate heat: elaborate cooling systems, heat spreaders in packages, and modified silicon designs to reduce thermal density. All of which add cost in one way or another.

Well, for one application, a different solution has been proposed. Gallium nitride (GaN) is a wide-bandgap material used for high-electron-mobility transistors (HEMTs) in high-power RF applications – radar, cellular base station radios, satellite radios, and the like. The GaN typically sits over a silicon substrate, with a transition layer to ease stresses due to mismatches in the crystal lattice spacing of the two materials.

These circuits have localized hot spots that have to be carefully managed (with heat flux that Element 6 says rivals that of the sun). Metal is typically used to wick away heat, and we all know that copper is a good conductor of heat, topping out at about 400 W/mK. But we have looked at one material that is a far better heat conductor than copper: diamond. Diamond can conduct heat in the range of 1000-2000 W/mK.

Unlike copper, which uses electrons to conduct the heat away, diamond does so through vibrations of the crystal lattice – so-called phonons (a fictitious particle for analysis of crystal vibrations and their properties and propagation). So higher-quality crystals will spread heat better than high-defect crystals or polycrystalline depositions.

Element 6 does sell diamond heat spreaders that can be included under standard GaN/silicon or GaN/SiC (silicon carbide) circuits, and they’ll help, but they place the diamond material some hundreds of microns away from the transistor gate, where the heat originates.

A better solution, they say, is to have a transistor consisting of GaN on a diamond substrate rather than a silicon substrate. The standard transition layer between silicon and GaN is also a barrier to a conductive path from gate to substrate, so they’ve eliminated that as well, replacing it with their own “secret sauce” of a transition layer.

By doing this, you’ve now got the transistor gate about 1 micron away, roughly tripling the heat dissipation.

GaN_on_Diamond_-_combined_2.png

Upper image courtesy Element 6; graph credit Professor Martin Kuball, Bristol University

Their actual production process leverages GaN/Si layers already in production. They put a handle wafer on top, flip them over, remove the silicon substrate and the transition layer, and then add their own transition layer and grow a polycrystalline diamond substrate. That substrate is strong, but it’s not thick enough for fab handling, so they temporarily affix another diamond wafer, which is eventually removed and re-used up to 10 times. (They’re working on a cheaper handle wafer solution for this last bit.)

GaN on Diamond allowed Triquint to achieve a three-fold improvement in power density as compared to GaN/SiC, allowing them to meet a challenge set by DARPA.

You can read more in their announcement.

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Aug 19, 2014

In-Situ Real-Time Process Checks

posted by Bryon Moyer

Yesterday we looked at number of different ways of inspecting wafers. Such inspections can be an important part of a process that turns out high yields of high-quality chips. They serve a couple of roles in this regard.

The most obvious is that you catch faulty material early. If rework is possible, you can then rework it; if not, well, you don’t throw good processing money after bad.

But the other reason is probably more important: by looking at wafers at various monitoring points, you get a sense of how the equipment is working. The wafer results act as a proxy for machine monitoring.

So… what if you could measure the machine directly?

That’s what CyberOptics is doing using an in-situ approach that they say is complementary to wafer inspection. They create “fake” wafers outfitted with sensors and feed them into the equipment. The equipment thinks they’re normal wafers and processes them; the sensors measure selected aspects of the setup and report back wirelessly in real time.

And they claim to be the only ones that have this real-time capability. They say other approaches require manual “timestamping” of data that’s downloaded and analyzed after the processing is over. The Bluetooth connection to a nearby rolling host computer allows the data to be transmitted as its captured.

They have setups for measuring air particles; for leveling; gap measurement (used with thin-film deposition, sputtering, etc.); vibration measurement; and a “teaching” system that improves alignment.

Most recently they’ve announced new air particulate measurement platforms: a reticle version, which replaces not the wafer but the reticle in a lithography tool, and a smaller wafer version – 150 mm (6”, roughly). That last one might seem odd, since they say they’ve already got a 450-mm version, and bigger ones usually come later. But in this case, they had to reduce the size of the sensing and electronics to fit the smaller form factor.

aps_-_combined.png

Images courtesy CyberOptics

You can read more in their announcement.

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Aug 14, 2014

A New IoT Protocol

posted by Bryon Moyer

We’ve got a number of ways of getting our devices to talk to each other. Some time back, I opined that Bluetooth Low Energy and WiFi seemed to have the edge, largely influenced by the burgeoning Internet of Things (IoT). Zigbee, meanwhile, seems to have more sway in the Smart Grid.

Well, some folks still aren’t happy with these options. There are three capabilities that are desirable, and yet none of the above standards can do all three:

  • Low power (of course)
  • Native IP6 support
  • The ability to mesh

WiFi is the only one that handles IP-based traffic, but it loses on the power front; Bluetooth can’t mesh natively (although a mesh product has been announced overlaying Bluetooth); and Zigbee doesn’t do IP natively.

Hence the Thread protocol. It’s built over 802.15.4, the low-cost, low-power physical layer and media access control layer that underlie Zigbee and some other protocols. It handles IP6 via 6LoWPAN.

image002.jpg

It appears to have originated out of Nest Labs (now Google), and they’ve assembled a group of other companies to promote the protocol. Most of the other names are familiar electronics guys – ARM, Freescale, Samsung, and Silicon Labs – but they also have a couple ThingMakers: Big Ass Fans (seriously) and Yale (think door locks).

Note that this isn’t about setting a standard: “promote” really is the right verb, since Thread is already shipping in Nest products. They’re going about this by putting together a certification program to ensure that all devices carrying the Thread designation pass muster. The certification program should be in place by the end of the year, with full availability early next year.

And what are the targets for Thread? Their site says, “… all sorts of products for the home.” They list specifically:

  • Appliances
  • Access control
  • Climate control
  • Energy management
  • Lighting
  • Safety
  • Security

Given that this is intended for non-technical consumers connecting Things in the home, they’ve also focused on ease-of-setup, via phone or computer or tablet.

You can find out more (and even participate) via their announcement.

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