MEMS oscillator

MEMS oscillators are electronic oscillator circuits that contain a polysilicon resonator as an essential frequency-determining element . MEMS is the abbreviation for English microelectromechanical system , a microsystem .

In contrast to the quartz oscillators , which are comparable in the field of application , the MEMS resonator is manufactured together with other electronic circuits on a semiconductor die, which enables smaller dimensions. The development of the first MEMS oscillators, then known as resonators , go back to work by Raymond J. Wilfinger at IBM at the end of the 1960s. But it was not until the 2000s that it became possible to mass-produce MEMS oscillators suitable for the market.

construction

MEMS oscillator chip with an output frequency of 88.9 MHz in a 7 mm × 5 mm plastic housing on a printed circuit board. Above it is a chip for clock distribution

In the case of a MEMS oscillator, a MEMS resonator made of polysilicon is used instead of the quartz oscillator , which in contrast to quartz is not piezoelectric . For the operation of a MEMS resonator, a direct voltage is required, among other things , in order to mechanically preload the MEMS. The impedance of a MEMS is significantly higher than that of an oscillating crystal. This requires an oscillator circuit specially adapted to the MEMS. In contrast to a quartz oscillator, the MEMS resonator has a strong temperature dependency in the range of −25  ppm / K. For an oscillator that is frequency-stable over the intended temperature range, the temperature deviation in the resonator must be compensated for using its own temperature sensors and electronic circuit parts.

In addition to the oscillator circuit and temperature compensation, commercially available MEMS oscillators also contain a special phase-locked loop (PLL), usually a fractional N-PLL , which generates a configurable output frequency that can be set as desired within certain limits from the resonator frequency , which is fixed by the mechanical variable. In the case of MEMS oscillators, the necessary division ratios of the PLL are stored together with the coefficients for temperature compensation in a read-only memory in the semiconductor chip. The desired output frequency is set by programming this memory once. The actual MEMS resonator has mechanical lengths in the range of a few 100 µm, the additional electronic circuit parts have approximately similar dimensions, which means that the entire MEMS oscillator can be implemented on a base area of ​​less than 1 mm ² .

MEMS oscillators have comparatively high phase noise or jitter , also due to the setting behavior of the electronic temperature compensation. Compared to quartz oscillators, this is offset by cost advantages in production, insensitivity to mechanical vibrations and the possibility of better miniaturizing MEMS oscillators. The deviations in frequency stability in 2012 were in the range of ± 10 ppm to over ± 100 ppm. Specific values ​​vary depending on the oscillator type and quality.

Investigations have shown that an atmosphere with already two percent helium has a destructive effect on unencapsulated MEMS oscillators. Due to the on pure silicon -based The diffuse helium atoms into the structure and adversely affecting the vibration characteristics. Electronic circuits with open MEMS oscillators are also used in smartphones and can render them inoperable under the influence of helium. The problem occurs, among other places, in hospitals in the vicinity of helium-cooled MRT machines.

literature

• Bernd Neubig: MEMS oscillators - opportunities and limits . In: AXTAL Consulting company publication . Lobbach 2008 ( Online [PDF]).

Web links

Electronics practice: MEMS oscillators - advantages and disadvantages under the microscope, article from Sep 18. 2012

Individual evidence

1. ↑ Patent US3614677 : Electromechanical monolithic resonator. Filed April 29, 1966 , published October 1, 1971 . ‌
2. ^ Raymond J. Wilfinger, PH Bardell, DS Chhabra: The resonistor a frequency selective device utilizing the mechanical resonance of a substrate . In: IBM Journal Issue 12 . 1968, p. 113-118 .
3. ↑ Datasheet DSC110x, Low-Jitter Precision CMOS Oscillator. (PDF; 488 kB) (No longer available online.) Discera, archived from the original on September 11, 2012 ; Retrieved February 1, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
4. ↑ iPhones are Allergic to Helium . In: iFixit . ( ifixit.org [accessed November 22, 2018]).