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SOI MEMS: A Winning Combination

Greater innovation, reliability, cost-efficiency and size reduction – these are just a few of the excellent reasons to build MEMS devices in SOI.

 

Tiny Pump, Big Promise

By Laurent-Dominique Piveteau, PhD,
Business Development Manager,
Debiotech

 

 

 

Debiotech has developed a new generation of disposable drug delivery pumps based on SOI-MEMS. Diabetes patients will be the first beneficiaries.

 

The first application of the tiny SOI-MEMS Nanopump™ will be for treating diabetes. (Courtesy: Debiotech)

An SOI-based MEMS solution developed by Debiotech and manufactured by STMicroelectronics is poised to make a major impact on the lives of people suffering from diabetes and other long-term illnesses.

At the heart of the system is the Nanopump™, a volumetric membrane pump. The pump consists of a membrane micromachined in an SOI wafer, which is in turn sandwiched between two Pyrex™ plates with throughholes. A piezoelectric actuator moves the membrane to compress and decompress the fluid in the pumping chamber.

1^st app: insulin

The first application of the Nanopump is the Insulin Nanopump™ for people with diabetes.

The world’s 12 million type-1 or insulin-dependent diabetics have to check their blood repeatedly throughout the day, then self-inject appropriate levels of insulin to adjust for variations in glucose levels caused by meals.

While most diabetics use syringes, a growing number are turning to electronic “insulin pumps” offered by leading medical technology companies. The current generation of pumps is improving, but there are still drawbacks, notably upfront cost and bulkiness.

A typical, classic pumping system consists of a cannula that stays in place in the skin for several days, and an electronic insulin-pumping unit, that is attached to the cannula via a catheter of about 18” (45cm). The attached pumping unit – typically about the size of a pack of playing cards – must be belted in place or carried in a pocket or pouch day and night.

By replacing the heart of the pumping system with a microfluidic MEMS chip, Debiotech has reduced the Insulin Nanopump to about a quarter the size of existing systems. Because it’s so small, the unit can be attached directly to the skin patch, eliminating the nuisance (and potential danger) of the loops of catheter. For the user, it’s also far more visually discrete – a much appreciated advantage.

SOI for precision, size, savings

Beyond the aesthetics and convenience, the use of SOI wafers for fabricating the Nanopump MEMS device has significant medical and economic advantages.

The SOI-based structure allows for the highest reliability in the smallest possible package, enabling very tight control and precision of the pumping mechanism. The flow rate is steady, and it is insensitive to pressure, temperature, viscosity and aging.

It also offers extreme dosing precision. Each pump actuation delivers just 200 nanoliters of insulin – approaching nearly continuous delivery.

From a manufacturing standpoint, the Nanopump is designed to be part of the disposable supply pack, so cost is critical. SOI enabled a drastic reduction in die size, making if far cheaper to produce than a bulk silicon equivalent. In turn, the tiny chip enables a far smaller – and less expensive – pumping unit.

STM’s SOI MEMS expertise

For volume manufacturing, Debiotech turned to STMicroelectronics for its expertise in large-scale production of microfluidic MEMS devices. As Dr. Frédéric Neftel, President and CEO of Debiotech noted, “The collaboration with ST has proven to be extremely efficient in terms of time and achievements.”

Because Debiotech is strictly an R&D company, the Insulin Nanopump will be licensed exclusively to a major medical technology company.

The Nanopump itself, however, could be used and is available for licence in other drug delivery devices requiring high precision over a wide range of conditions.

As such, it heralds a new era in continuous drug delivery, combining the highest level of reliability and performance with previously unmet levels of convenience and safety for the patient.

“Working with our key partner Debiotech, we are readying a tiny piece of silicon that can make a huge difference to millions of diabetes patients around the world.”

Benedetto Vigna,
Group Vice President and General Manager,
STMicroelectronics MEMS and Healthcare, RF and Sensors Division.

SOI MEMS Resonators for Timing Application

By Markus Lutz,
CTO,
SiTime

SiTime has developed a range of ultra-slim, SOI-MEMS based timing devices aimed at replacing bulky crystal quartz products in high-volume portable electronics.

By using an SOI-MEMS solution, SiTime’s products are a fraction of the size of existing quartz crystal based timing products. They are also 10 times better in terms of reliability and shock resistance, while consuming a third of the power. (Courtesy: SiTime)

The timing market can be divided into three segments: passive quartz resonators, active oscillators and timing devices. The annual volume exceeds 15B products sold per year with annual revenue of greater than $5B.

SiTime uses ultra small SOI-MEMS resonators to replace classic quartz crystal based products. Target markets include high-volume portable electronics, such as high capacity SIM cards, smart cards, mobile phone system in package (SIP) modules, digital cameras, LCD TVs, media players, gaming solutions, hard disk drives and USB peripherals.

The advantages of SiTime’s MEMS resonators over quartz are in size, robustness and manufacturability. The large volumes require scalability, reproducibility and reliability from die to die, wafer to wafer, and lot to lot.

SOI’s mechanical advantages

MEMS resonators’ mechanical performance depends strongly on two factors: material properties of the mechanical resonator and cleanliness of the vacuum packaging.

The material requirement is solved by choosing bonded SOI wafers. The mechanical resonators are formed in the upper silicon layer. The SOI layer is made from single crystalline bulk wafers, which guaranties defect free material with well controlled doping levels for conductivity.

From a mechanical perspective the single crystal silicon is perfect: the material properties are the same from lot to lot, and the only impurities are the dopants which account for less than 1ppm of the atoms and therefore do not affect the mechanical parameters.

A high temperature wafer level encapsulation process provides the ultra clean vacuum.

 

 Fig.1 Frequency stability

Performance and yield

The performance enabled by SOI wafer material leads to high manufacturing yield, zero drift, and a performance similar to or better than quartz.

Figure 1 shows the temperature stability of the high-performance, programmable oscillator SiT8102, which is significantly better than the required specification of ±25ppm over the temperature range of -40°C to +85°C.

 

 

Standard packaging

SiTime has introduced 11 product families all based on this resonator technology. The resulting wafer level packaged MEMS resonators can be packaged with any standard chip packaging technology such as plastic molding, flip chip, and SIP.

SiTime’s MEMS First™ Fabrication Technology

Fabrication cross section

SEM picture of a cleaved resonator

 

Using a resonator as an example:
1. A 10µm SOI substrate is patterned with a Bosch deep reactive ion etch (DRIE) into a resonant structure.
2. An oxide is deposited and patterned to cover selected parts of the resonator.
3. A thin epi silicon layer is deposited and patterned with vents.
4. The oxide over and under the structures is removed through the vents.
5. The resonator chamber is sealed in an epi environment with SiTime’s EpiSeal process, giving an extremely clean enclosure.
6. The wafer surface is patterned with contact isolation trenches.
7. Insulation oxide, metal interconnections, and a scratch mask are deposited and patterned.

 

A co-founder of SiTime (a spin-off of the Bosch Research and Technology Center in Palo Alto, CA), Markus Lutz was the initial inventor of the company’s MEMS-First™ integration technology. He started his career at Robert Bosch GmbH, where he invented the world’s first mass-produced silicon-based MEMS gyroscope.

 

Grace Under Pressure

By Giorgio Fagnani,
Project Engineer,
Gefran

With SOI-MEMS, Gefran’s Impact™ pressure sensors can go right into hot spots like the injection nozzles of plastics manufacturing systems.

An SOI-based MEMS device is at the heart of Gefran’s Impact™ I and J series sensor technology for the plastics processing industries. (Courtesy: Gefran)

Sensors are fundamental elements in the control of industrial processes. For plastic transformation processes such as injection, extrusion and blowing, “melt pressure” sensors help ensure plant safety, prevent excessive machine pressure, and increase performance thanks to a stable and optimum flow rate.

Gefran, a worldwide leader in systems and components for automation of industrial processes, is a top designer and manufacturer of melt pressure sensors. We have leveraged SOI-based MEMS technology in our new Impact™ range of sensors, which provides dramatic advantages over other market offerings.

Only on SOI

Because the sensing device sits directly behind the membrane that’s in contact with the molten plastic, sensors for these applications need to withstand up to 3000 bar of pressure and temperatures of up to 350°C (660°F).

The piezoresistor MEMS device is the “sensing element” that converts pressure into an electrical signal. SOI-MEMS is currently the only technology that enables us reach such high temperatures with optimal performance. Bulk silicon can’t go beyond 150°C due to the leakage current at the p-n junction of piezoresistors.

Moreover, with SOI we have good process yields and ease of manufacturing.

Heat & reliability

These SOI-based MEMS sensors are particularly well suited for plastic extrusion systems used in the food, beverage and pharmaceutical industries, where they can replace traditional mercury-filled sensors.

For blow-molding systems used in making plastic bottles for water and other consumables, the MEMS technology allows us to have a process membrane that is up to 35 times stronger and can function continuously for over three million cycles.

With the die-size reduction enabled by SOI, the system is so small that the entire sensing unit can be installed in an injection nozzle or other very narrow spot. That means the measurements can be taken at the right place in real time, for far greater process control and automation precision.

For our customers, this little piece of technology makes an enormous difference.

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