Vacuum Bonding Technology Creates Long-term Stable IMUs
Until recently, micro-systems technology resonant-mode inertial measurement units (IMUs) have had limited lifetime. However, scientists have developed an eutectic AuSi wafer-bonding process featuring vacuum tightness, high mechanical stress tolerance, and temperature stability that guarantees a 15-year lifetime for micro-sensors, even under extreme stress and harsh environmental conditions.
By Wolfgang Reinert, Fraunhofer Institute for Silicon Technology
Nobody wants a luxury car with failing sensors, especially when it comes to safety. Earlier microsystems technology resonant-mode inertial measurement units (IMUs) were often limited in their lifetime. Scientists* recently made a crucial breakthrough in long-term vacuum stability. They developed an eutectic AuSi wafer-bonding process featuring outstanding vacuum tightness, high mechanical stress tolerance, and temperature stability. By means of an additionally integrated thin-film getter, 15 years of lifetime can be guaranteed for those micro-sensors, even under extreme stress and harsh environmental conditions.
Modern automobiles are equipped with complete micro-sensor systems, which take care of data acquisition, signal processing, and data communication. These components are faced with steadily increasing requirements, including the combination of a maximized number of functions in minimal space, cost-effective manufacturing processes, and extreme robustness and reliability. These requirements can only be fulfilled by an innovative packaging technology, which protects the hermetically sealed microelectromechanical sensors (MEMS) from harmful environmental constraints. Some types of MEMS, such as acceleration and absolute pressure sensors, also need a defined gas or pressure environment to ensure proper function.
For 6 years, scientists have been developing a novel encapsulation technology for resonant-mode IMUs. For this purpose, the advanced packaging research team is relying on a eutectic-gold-silicon wafer-bonding technology with an integrated thin-film getter, which allows for cost-effective production of micro-sensor vacuum packages with long-term stability. Scientists are convinced that a stable vacuum over the entire micro-sensor lifetime can only be achieved by a surface micro-mechanical production process.
The first micro-sensor fabricated in the new wafer-bonding process is a MEMS-type IMU, which measures rotation speed around a space axis. Areas of application include navigation and stabilization systems indicating space-relative movement of a car or a camera. In automotive applications, these sensors serve for active driving dynamics control, or for improvement of GPS coordinates’ precision.
The IMU consists of a micro-mechanical surface, for which finger-shaped structures had to be dry-etched from a 10-µm thin poly-silicon base material. These finger-shaped structures are electrically conductive, and represent both electrodes of a capacitor. They move with an applied alternating voltage in resonating oscillation at an appropriately set frequency, leading to extended amplitude. This sensor layer is sensitive to the Coriolis force, which is responsible for an orthogonal deflection to the motion; thus, the signal measured at the electrodes is proportional to the rotation speed.
Figure 1. The fine flexible comb structures of the rotation rate sensors are only a few micrometers wide.
The delicate sensitivity of the sensor structures, and the necessity for protection through special packages, is shown in Figure 1. The goal of the effort was to package the sensor in a way that it could be handled like a normal micro chip, and processed within a cost-effective, standard semiconductor production line. The single chip had to be rugged enough to endure the plastic transfer molding process in a 90-bar pressure environment. After calibration, this type of sensor must withstand the most demanding, accelerated stress profiles, such as 1000 hours at 85°C temperature and 85% humidity, or a 2000-g shock test.
A special, wafer-level, build-up technology was developed to batch-process a maximum number of sensors on a single silicon wafer (Figure 2). For that purpose, a so-called cap wafer is placed on top of the device wafer, etched, and bonded to each other at a temperature between 380° and 400°C. The device wafer requires a solderable silicon layer.
Figure 2. Build-up of the MEMS system.
Since there is no additional metal frame, the requisite gaseous hydrofluoric acid, which attacks most metals, can be used for release etching the movable poly-silicon structure. Both the cap wafer and the device wafer consist of silicon, which maintains a perfect matching of thermal expansion, and minimizes thermal drift over an entire operating temperature range from -40° to 125°C. Beyond that, silicon is an inexpensive material with no contamination side effects. On the cap wafer, a matrix pattern of cavities (pockets) with a complete metallization is applied (Figure 3).
Figure 3. The cap wafer showing a matrix pattern of cavities (pockets).
Seal frames for conventional glass-frit bonding with glass-ceramic compounds are approximately 500-µm wide. To increase sensor yield-per-wafer, seal frames should be as slim as possible. That is why a metallic gold-bonding process was applied, using silicon for a eutectic alloy. The advantage of this process lies in the fact that frames can be as slim as 60 to 100 µm, while delivering extreme bond strength. Fewer stress problems, and subsequently, fewer early failures, also occur during the AuSi-bonding process (Figure 4).
Figure 4. Encapsulated wafer with rotation rate sensors.
To achieve an exactly defined vacuum atmosphere within the micro-sensor package, experts in the industry go even further. They integrate a structured, thin-film getter of zirconium-based alloy inside the cavities prior to sealing. The broad-range operating getter transforms into a chemical compound with a variety of substances. Not only does it absorb water on its surface - which is given a columnar structure to increase efficiency - but it also absorbs air compounds such as hydrogen, oxygen, nitrogen, and carbon monoxide and dioxide. However, the getter is not able to bind inert gases such as helium, neon, or argon. The getter material needs to be selected for compatibility of its activation temperature with the bonding process.
This bonding technology makes it possible to determine pressure values according to the sensor type, ranging from 10 to 4 and up to 3000 mbar. The operating pressure for the resonant structures of an IMU is in the 0.1-mbar order of magnitude, whereas acceleration sensors operate at 700-mbar pressure. It is important to secure the application-specific value in the field or car. As a rule, the pressure for rotation rate sensors, for example, is allowed to triple, ranging from 0.1 up to 0.3 mbar, over a time frame of 15 years.
A quick assertion of process reliability, particularly sealing tightness, can be achieved by means of an ultra-fine leakage test that measures the effective neon leakage rate of the finished micro-sensors, and then computes the standard air leak rate and vacuum lifetime, including the getter saturation. With this non-destructive, 100% test method, it is possible to select the failure candidates at wafer-level, even before their delivery to customers.
The vacuum bonding process was already tested with different sensor designs on variable wafers. The sealing of the micro-sensors is humidity-resistant and 10 to 16 mbar I/s - proven for air leakage tightness. In the future, these MEMS sensors will be manufactured not only for the automotive industry, but also for applications in avionic, medical, and consumer electronics.
* Fraunhofer Institute for Silicon Technology
WOLFGANG REINERT, research scientist, may be contacted at the Fraunhofer Institute for Silicon Technology, Fraunhoferstrasse 1 D-25524, Itzehoe, Germany; +49/4821 174 617; E-mail: firstname.lastname@example.org.
Eutectic AuSi Wafer-bonding Benefits
- Mechanical ruggedness
- High temperature cycle and wide operating range stability
- Vacuum tightness
- Low gassing during vacuum bonding
- Small bond frame structure width
- Small particle or bond frame scratch tolerance
- Leveling of surface topography due to melting effects
- No static cover charging effects
- Integration with standard production processes
- High component yield
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