MEMS are a great fit for real-time clocks

This article explores the significant performance enhancements that MEMS technology enables in accurate real-time clock applications, writes Paul Nunn, senior business manager, Maxim Integrated.

MEMS (microelectromechanical systems) technology has been implemented in accurate real-time clocks (RTCs), making them extremely rugged, highly accurate both over time and temperature, and significantly smaller than clocks built using standard cylindrical crystal technology.

With 47 times less area and 182 times less volume than that of a 32.768kHz tuning-fork cylindrical crystal, the MEMs resonator technology provides a significant advantage in size and packaging options for RTCs today.

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This size differential allows for smaller packaging options, provides significantly enhanced ruggedness in high-vibration and shock environments, and demonstrates little to no aging (< ±1ppm total) over the life of the device.

However, the advantages that MEMS brings to this technology do not end with size. There are four distinct areas of technology where MEMS characteristics deliver enhanced technical advantages. These areas include: process and development, assembly and manufacturing, environmental ruggedness, and in-field product performance and service.

MEMS in CMOS Process and Development

Let’s quickly compare a MEMS process with crystal assembly.

The MEMS resonator technology discussed here was developed in standard CMOS fabs. CMOS fabrication is especially advantageous for meeting targeted frequency responses based on the shape and sizes of device elements established at the photolithographic stage of development.

Since MEMS is a silicon technology, the benefits of repeatability and sustainability apply to the manufacturing of MEMS wafers. The manufacturing temperatures reached while processing MEMS wafers can exceed +700°C, but the MEMS resonator can be subjected to multiple reflow temperatures of+260°C without degrading its performance. (We will talk about this in more detail below.) This durability can be attributed to its material makeup, design, and wafer processing flow.

In contrast (and well understood), crystal assembly is a less robust process and prone to sizeable variations in product-to-product output. Frequency tuning and trimming generally require the deposition or removal of material from the crystal electrode to achieve the desired frequency.

Additionally, a vacuum must be established in the cylindrical carrier for the crystal resonator to vibrate once voltage is applied to the device.

Consequently, to produce high-quality devices, special materials are required for attachment of the crystal to its leads. These materials help the crystal survive high-temperature (~260°C) reflow operations.

There is a caveat, nonetheless. Care must be taken when subjecting crystals to multiple high-temperature reflow cycles. Frequency shifts can be attributed to aging of “crystal-attach” material, quality of the vacuum, and/or imperfections in the crystal blank.

MEMS in Manufacturing and Assembly

In the final assembly and manufacturing flow of an RTC, four important factors give the MEMS-based RTC its advantages over crystal counterparts.

First, MEMS is effectively an IC. Therefore, when MEMS is combined with the control-die/RTC, standard IC packaging technologies apply and can be used. This contrasts markedly with crystal assemblies which require custom manufacturing flows to attach and affix the crystal and RTC die in the same package.

Secondly, wire-bonding operations are used to electrically connect the control die to the MEMs resonator. Crystal assemblies must use either a more complicated and less robust solder attachment or weld the crystal leads to connect the control die to the crystal resonator.

Third, highly efficient wire-bonding operations and standard packaging assembly flows lend themselves well to high-volume, less costly assembly and manufacturing operations.

Fourth, the vast difference in size between MEMS and crystals offers smaller sized packaging options, including chip-scale assemblies that are not possible with crystals. 

For comparable functionality and performance, the DS3231MZ+ RTC is assembled in an 8L 150 mil SO, while the previous generation crystal-based DS3231S RTC is packaged in a 16L 300 mil SO. The 8L SO package is less than half the size of the 16L 300 mil package and smaller packages lower the cost.

MEMS Is Environmentally Rugged

MEMS-based RTCs have proven and demonstrable performance advantages based on environmental criteria and observation.

In reflow operations (3x at +260°C) that replicate customer attachment, MEMS devices demonstrate frequency shifts of less than ± 1ppm (see Figures). Crystal-based products facing the same regiment of reflow temperature exposure demonstrate shifts as high as ± 5 ppm.
 
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Figure: Data for the DS3231M RTC are shown before reflow (top) and after reflow (bottom). The frequency shift is less than ±1ppm.
 
MEMS-based RTCs have been subjected to shock and vibration testing through the AEC-Q100 qualification. They can sustain mechanical shock in excess of 2900g (x5) (JESD22-B104-C Condition-H) and variable frequency vibration in excess of 20g (JESD22-B103B Condition-1). 

Performance data and processing experience prove that a MEMS-based RTC provides distinct advantages over a traditional crystal-based RTC. We spoke of the specific advantages in manufacturing and assembly above. Additionally, frequency accuracy over time (lifetime) will be less than ± 5ppm with the MEMS clock.

Frequency accuracy over temperature and after reflow will still be less than ± 5ppm.

MEMS operates at higher temperatures. It comes in smaller packaging and, finally, lowers cost. It is most certainly hard to argue against designing with MEMS-based, accurate RTC products.

Paul Nunn is a Senior Business Manager for Maxim Integrated.

Tags: clocks, design, Maxim, MEMS

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