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Physical sensors drive MEMS consumerization wave (Part 1)

Posted: 18 Oct 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Microelectromechanical-systems? MEMS? silicon micromachining? accelerometer?

By Benedetto Vigna

Microelectromechanical-systems (MEMS) are three-dimensional structures manufactured through silicon micromachining technologies. They made their first appearance in semiconductor fabs in the 1960s and, among many applications, they can be used to sense acceleration, angular rate, pressure and sound pressure.

Our daily life is full of micromachined physical sensors. In the car all active and passive safety systems, like vehicle dynamic control and air bags, are using acceleration and yaw rate sensors to protect our lives. Car gasoline consumption is also very low thanks to the usage of pressure sensors in engine manifolds and fuel lines. But since few years our unconscious interaction with micromachined products doesn't stop here.

The portable PC we use has a three-axis accelerometer to protect data stored in hard disk drive in case of accidental fall. Several mobile phones from different brands exploit the sensing ability of tiny accelerometers to simplify the interface between the man and the equipment. Last, but not least if we play with Nintendo Wii or Sony PS3, we can really appreciate the playing experience thanks to remote controller motion sensing feature. ?Sense and Simple?, that's the value proposition enabled by MEMS in the consumer market in line with Naoto Fukusawasan's dream to brake all the barriers between man and electronic devices.

Tapping MEMS
MEMS are manufactured in semiconductor fabs like the CMOS transistors we find in any electronic chip, but in this case not only electrons are moving. Silicon springs, electrodes, membranes and cantilevers are also moving. Silicon micromachined products compete quite often with quartz and piezoelectric based products in price, size and performances. Most of the time, they represent the best technical and economical solution for the consumer market with a clear roadmap towards miniaturization and integration. And this explains why in the last few years we saw the raise of the 'MEMS Consumerization Wave'.

Motion sensors, like accelerometers and gyroscopes, are bringing the movement detection dimension inside the silicon. Their penetration will continue to increase in the automotive market, also driven by regulations, but their presence in the consumer market will happen at definitely higher rate. In the best case, the speed of growth of automotive MEMS suppliers has been six times smaller than STMicroelectronics' growth in the consumer market. In fact, multi-axis accelerometers, once used only for active and passive safety systems in the car, are finding their space in laptops, HDDs, mobile phones and game controllers.

Besides vehicle dynamic control systems, yaw rate sensors are being used to improve image stabilization in camcorders and digital still cameras. Moreover motion sensors and geo-magnetometers are expected to cluster together in motion measurement units to enable personal navigation in portable devices, thus fostering the deployment of location-based services by telecommunication operators.

Tiny pressure sensors today are widely used in the automotive and the medical markets. Their penetration will rapidly increase in automotive thanks to the tire pressure monitoring application. But recently developed thin, small and inexpensive pressure sensors will appeal also the big consumer market enabling new applications and bringing new sources of revenues to the wireless operators.

Capacitive silicon microphones are also competing adequately with non-surface mountable electro-condenser microphones in mobile phone and laptop.

Clustering of several sensors, like accelerometers, gyroscopes and pressure sensors, in one single module will definitely happen. And thus, MEMS suppliers must be ready to listen to customer requests developing a technology platform enabling multiple sensor fusion.

To date, STMicroelectronics has Jisseki for two and three axis accelerometers worldwide and it has developed two technology platforms, THELMA and VENSENS, for sensor integration. Today, it's developing multiple axis gyroscopes, pressure sensors and microphones and it's also open to partnerships with companies to develop any potential sensor requested by customer, like magnetometers. Production and development activities are all running together in the 8-inch MEMS fab to accommodate fast time-to-market.

Micromachining technologies
Physical sensors interact with the physical world. They are manufactured via a process called micromachining, which shares the same processing steps derived from basic IC techniques. The end result, however, is typically a 3-D mechanical structure, most often on a silicon substrate.

Nonetheless, other materials can be micromachined or microformed. Among these materials are quartz, glass, plastic and ceramic. Quartz and ceramic are used for crystal resonators and for Coriolis-based gyroscopes. Still, silicon is becoming increasingly popular because of its electrical, mechanical and thermal properties.

Aside from excellent physical properties, silicon is extremely attractive because manufacturers can realize thousands of micromachined components at a time on silicon wafers using proven manufacturing techniques developed for silicon chip production. Throughout its history, the microelectronics industry has cumulatively invested trillions of dollars in building up an industrial infrastructure devoted to designing and manufacturing silicon-based microelectronic devices in high volumes while continuously reducing the dimensions of transistors. Hence, MEMS can benefit from the same economies of scale that made microelectronics a success. Moreover, since the components are made side-by-side on wafers and with an extremely well-controlled process, the devices can be made much more precisely and repeatably than similar products manufactured in different ways.

Although the micron-scale of current MEMS devices mean that they can be made in older 6-inch wafer fabs, in the next years many companies must switch to 8-inch line to sustain the fast growing demand and price pressure of consumer market applications. On the technical side, the scope and use of MEMS is primarily due to extremely small size, reliability and low power consumption, which, often allows MEMS to perform faster and more precise operations than their macroscopic equivalents. But the cost advantage for the customer, especially in the consumer world, cannot be ignored.

Silicon is stronger than steel and only a third of the weight; it is also brittle and not subject to plastic deformations: this is the principle of the micromachining technology. Once combined with ICs, electrical signals generated by the moving structures (diaphragms and cantilevers) give perception and control capabilities to create sensors for many applications.

Many micromachining processing steps are derived from basic IC manufacturing: photolithography, material deposition, reactive ion and chemical etching. However, while the CMOS roadmap aims to pack more and more devices in plane and thickness, micromachined devices usually have dimensions in the tenths of a millimeter and thickness of several tens of microns. Wet etching, grown or electroplated thick films, stacks of two/three bonded wafers, through silicon vias/holes and high aspect-ratio dry etches are common steps for micromachining technology. Finally, MEMS devices use materials like gold or Glass Frit, that are completely forbidden in a CMOS process.

Over time, MEMS suppliers have been developing their own product-dedicated micromachining processes by exploiting both the processing steps they mastered and the available manufacturing assets. Although each company uses a specific micromachining process, all of the processes can be classified into two broad classes:

A. Bulk micromachining - it is a subtractive process because a large portion of the substrate is removed to form whatever structure is desired. This technique requires less precision than surface micromachining. Thicker structures are easier to fabricate because the substrate thickness can be chosen quite freely, but the shape of the micromachined structure is quite limited by the crystal planes of the silicon substrate. This technology is quite old and it's reaching the end of life.

B. Surface micromachining - it is an additive process requiring the building up of various layers of materials that are selectively left behind or removed by subsequent processing. The bulk of the substrate remains essentially untouched. This technique was initially limited to thin devices (~2?m), since only thin films could be deposited or grown on the substrate. However, the use of thicker films, as well as new wafer bonding techniques can help to create thicker devices. By exploiting all the tricks offered by photolithography, the manufacturing of very complex and innovative mechanical structures is fairly simple. This class of processes has longer roadmap and it's the most used for motion sensors.

ST's micromachining processes
STMicroelectronics currently runs in production two different micromachining processes: THELMA or THick Epitaxial Layer for Microgyroscopes and Accelerometers, and VENSENS or VENice process for SENSor.

The first process is designed to manufacture high-performances and low-cost motion sensors, like accelerometers, gyroscopes and microphones, while VENSENSTM enables the manufacturing of extremely small pressure sensors. Both micromachining processes are a proprietary combination of manufacturing steps of Bulk and Surface Micromachining technologies (Table 1).

Table 1: Comparison among CMOS, Bulk, Surface, THELMATM and VENSENTM micromachining processes.

The THELMA process begins with a standard silicon wafer onto which a layer of first oxide (~2?m) is grown for electrical isolation. A thin polysilicon layer used for interconnections and a second sacrificial oxide (~2?m) are then deposited. Into this layer, holes are etched at the points corresponding to the supports for fixed elements and anchors for moving elements. A thicker polysilicon epitaxial layer (~15?m) is grown on top of this, and into this third layer the structures for the moving and fixed elements of the device are etched with a single mask. Finally, the sacrificial oxide layer beneath the structures is removed by an isotropic etching operation to free the moving parts.

The open space around the structures is filled with a gas, usually dry nitrogen, to reduce or eliminate effects caused by humidity or variations in gas density, which would affect the resonant frequencies of the device. A second wafer is then bonded to the first one to protect the tiny structures during an injection molding process during which high pressures are applied (Figure 1). Fixed fingers with their anchor points are clearly visible in the figure. Moving fingers are interlocked between two fixed fingers to form variable capacitors. Wafer level capping prevents plastic to lock fingers.

Figure 1. SEM pictures of a capacitive micro-machined structure manufactured with THELMA process.

The VENSENS process begins with a standard silicon wafer. A proprietary combination of wet and dry silicon etching steps enables the formation of a sacrificial layer on top of which a monocrystal silicon layer is grown. The thickness of the sacrificial layer is less than 3?m and the thickness of the structural layer can reach 20?m. The end result is very similar to what it's possible to get with bulk micromachining wafer to wafer bonding. But with the big advantage to have thinner, smaller and mechanically more robust chips (Figure 2). Moreover, the sealing of the cavity doesn't require any wafer-to-wafer bonding and thus the reliability of the sealing joint is definitely higher.

Figure 2. (Left) A SEM picture of a cross section of VENSENS micromachined membrane. (Right) An optical picture of the top view of the pressure sensor. The square in the middle is the suspended membrane. The four piezoresistors of the Wheatstone bridge are clearly visibile at the four edges of the diaphragm.

Thanks to good electrical properties of the monocrystal silicon, good and stable resistors can be integrated in the structural layer through implantation or diffusion process. Then these resistors are connected with an aluminum metal layer to realize the four branches of a Wheatstone bridge. The bridge is sensitive to pressure changes thanks to the excellent piezoresitive properties of the monocrystal silicon layer.

The metal layer is then covered with a standard dielectric, like silicon-oxy-nitride, to provide the required protection against the external corrosive agents.

View Physical sensors drive MEMS consumerization (Part 2) here.

About the author
Benedetto Vigna
is general manager of the MEMS product division of STMicroelectronics.

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