Silicon Clock Challenges Atomic Timekeeping Norms

For decades, atomic clock provided the most consistent means of timekeeping. They measure time Vibrating in time with the resonant frequency of atoms, the method is so precise that it serves as the basis for determining the second.

Now new challenger appeared in the timing arena. Recently, researchers have developed tiny MEMS-based watches that use silicon doping to achieve record-breaking stability. After running for 8 hours, the watch deviated by only 102 nanoseconds, getting closer to the standard. atomic clock yet both require less physical space and less energy to operate. This has not been easy in the past due to the havoc that even slight temperature fluctuations can wreak on timekeeping.

Group presented their new watch in 71st Annual IEEE International Conference on Electronic Devices last week.

Save space and energy

MEMS The watch consists of several tightly connected parts, combined into a chip smaller than the side of a sugar cube. In the center is a silicon wafer topped piezoelectric The film vibrates at its own frequencies, and a nearby electronic circuit measures these vibrations. A tiny built-in heater carefully maintains the optimal temperature of the entire structure. Because resonatorWith the electronics and heater located close together, they can operate as a coordinated system: the resonator creates a timing signal, the electronics monitor and regulate it, and the heater prevents temperature fluctuations that cause drift.

“This clock is unique in many ways,” explains the project consultant and MEMS engineer at the University of Michigan. Ruzbeh Tabrizyan. First, the resonator is “extremely stable under environmental changes,” he says. “You can actually change the temperature from -40°C to 85°C and you will essentially see no change in frequency.”

The resonator is so stable because the silicon from which it is made has been alloy with phosphorus, says Tabrizian. When a material is doped, impurities are added to it, usually to change its conductive properties. However, here the group used doping specifically for stabilization. mechanical characteristics. “We control the mechanics very tightly so that the elasticity of the material does not change with temperature,” he says.

Some other materials such as the commonly used timing crystal. quartzcan also be alloyed for stability. But 'you can't miniaturize [quartz] and you have a lot of restrictions in terms of packaging,” explains Tabrizian. “Semiconductor manufacturing benefits from size miniaturization,” so it's an obvious choice for the next generation. watch.

Doping also allows the electronics to actively suppress any small frequency deviations over long periods of time. This attribute is “the most distinctive aspect of our device's physics compared to previous MEMS clocks,” Tabrizian says. By making the silicon conductive, doping allows the electronics to fine-tune the force of the device's mechanical drive, which counteracts slow changes in frequency.

This system is also unique in that it includes autonomous temperature measurement and control. Banafshe Jabbarigraduate student at the University of Michigan who led the project. “This clock resonator operates in two modes [or resonant frequencies]essentially. The main watch mode is very stable and we use it as [time] link. The second is a temperature sensor. The latter acts as an internal thermometerhelping the electronics automatically detect temperature shifts and adjust both the heater and the main timing mode itself. This built-in self-correction feature helps the watch maintain stable time even when the environment changes.

These features mean this is the first MEMS clock to last 8 hours and deviate by just 102 billionths of a second. When scaling linearly to a week of operation, this corresponds to a drift of just over 2 microseconds. This is several orders of magnitude worse than the best laboratory atomic clocks, but in terms of stability they are not inferior to miniature atomic clocks.

Moreover, MEMS clocks have significant space and energy saving advantages over their atomic competitors. The more isolated from the environment and the more energy they use, the more accurately atomic clocks can measure the vibrations of atoms, Tabrizian explains, which is why they are typically the size of a cabinet and consume a lot of energy. Even chip-sized atomic clocks are 10 to 100 times larger than MEMS clocks, he said. And, more importantly, these new clocks require 1/10 to 1/20 the power of a mini-atomic clock.

Timing for next generation technologies

Jabbari's work arose from DARPA project with the goal of creating a clock that could run for a week and only deviate by 1 µs, so there is still a lot to be done. One of the challenges the team faces is how the doped silicon will behave over longer periods of use, such as a week. “You see some diffusion and some changes in the material,” Tabrizian says, but only time will tell how well the silicon holds up.

It is important for both researchers to continue their efforts as they foresee broad applications for small, energy-efficient MEMS-based clocks. “Essentially all the modern technology we have needs some kind of synchronization,” says Jabbari, and she believes clocks could fill gaps in time synchronization that currently exist.

In situations where technology has reliable access to GPS satellites“There are no problems to solve,” she says. But in more extreme scenarios such as space exploration and underwater missions, navigation technology is forced to rely on internal timing, which must be extremely cumbersome and power-hungry to be accurate. MEMS clocks could be a smaller, less power-hungry replacement.

There are other everyday applications, Tabrizian said. In the future, when more information needs to be delivered faster to every phone (or whatever device we use in 50 years), precise timing will become a critical factor in the delivery of data packets. “And of course you can't put a big atomic clock in your phone. You can't use that much power,” he says, so MEMS clocks could be the answer.

Even with promising applications, the path for this project may be difficult due to existing competition. SeaTimethe company is already producing MEMS watcheven now integration their chips to Apple and Nvidia devices.

But Tabrizian is confident in his team's capabilities. “Companies like SiTime put a lot of emphasis on system design,” thereby increasing system complexity, he says. “Our solution, on the other hand, is completely physics-based and looks at the very complex, very fundamental physics of the semiconductor. We're trying to bypass the need for a complex system by making the resonator 100 times more precise than the SiTime resonator.”