EPE441 Micro and Nano Manufacturing Eng. Individual Assignment 1:Micro And Nano Devices Product ‘MEMS Inertial Sensor (MEMS Accelerometer)’

INTRODUCTION One of the many MEMS inertial sensor devices is the MEMS accelerometer device product, which is a micro-electro-mechanical system that measures the static (gravitational pull, g) or dynamic acceleration force, a (m/s²). The dynamic acceleration force occurs when there is vibration or movement applied to the accelerometer. The capacitance, produced by a sensing mechanism, is then served as the system’s raw output data. The MEMS device acts as a simple parallel plate capacitor, as in the equation . Where C is the capacitance of a region, which has a linear relationship to the air dielectric constant, ε, times the area of the overlapped sensing areas, A, and divided by the distance which separates the two sensing areas, d.

MATERIALS The material of the MEMS accelerometer is conductive doped silicon. Due to the material’s properties it is used since there are regions that need to be isolated from each other to avoid short circuits during electrical measurements and testing. Other material used in the fabrication steps includes metal (chrome, gold and platinum), glass and other etching chemicals such as hydrofluoric acid.

FABRICATION METHODS 1. Silicon on Glass (SOG) process. The process is an etching process step which requires the usage of hydrofluoric acid. Starting with the glass wafer, it is etched by the acid at certain rate which creates the tapered slope from the recess’s bottom to the original planar surface of the glass wafer. In summary, the SOG process forms suspended silicon over the glass recess with an air gap of 3µm. The deposited electrode extends from the glass recess to the glass wafer’s original surface. Next, a bare 4” glass wafer of 500µm thickness is used as the substrate. A chrome layer is deposited to serve as the mask layer. 2. Photolithography. The photolithography is to pattern the photoresist in the glass recess pattern. After the development of the photoresist, the exposed areas of chrome are etched away, leaving the original glass surface in a pattern consistent with the glass recess. 3. Etching Process. The hydrofluoric acid etches the exposed glass areas. The photoresist and chrome are then stripped to reveal the original glass wafer with recesses.

4. Photolithography. The photoresist is developed to pattern the bottom electrode metal. Next, using the e-beam evaporation technique, a chrome and platinum layer is deposited across the entire wafer. The acetone bath strips the photoresist and eliminates the metal on the top surface. The product then becomes a bare glass wafer with the bottom electrode patterned in the recesses of the glass extending outward. More photoresist is spun onto the entire sample. 5. Predicing is next, which allows easy separation of the individual device from the wafer after the fabrication process. The dicing tool cuts halfway through the glass wafer, 250µm and the photoresist is stripped and cleaned. Next, the silicon wafer fused to the glass wafer using the anodic bonding process. 6. Deposition of chrome and gold. Deposition of the chrome and gold metal pads on top of the device is performed since the 4” silicon wafer is bonded to the glass wafer. 7. Photolithography is performed to pattern the metal pads. The gold and chrome are etched which leaves the patterned metal pads on top of the silicon wafer. The photoresist is then stripped and photolithography is performed again to pattern the DRIE pattern that will define the silicon structures. 8. DRIE (Deep Reactive Ion Etching). After patterning the photoresist for the final processing step, the sample is placed within the DRIE chamber. A DRIE instrument etches the silicon and remaining the photoresist to reveal the final structures. The figure below shows an example of the final structure of MEMS accelerometer.

Figure 1: The final structure of MEMS accelerometer

APPLICATIONS The MEMS accelerometer’s application includes the functionality such as screen orientation of mobile phone or electronic gadget, interactive games’ gesture recognition, hard disk protection, and power management’s activity monitoring of a device. The MEMS accelerometer is also applied for stabilization of image in camcorders and anti-blur capturing for still camera. The MEMS accelerometer is also used in laptops to protect the hard drives from damages. For example if the laptop were accidentally dropped, the accelerometer detects the sudden freefall, and switches the hard drive off so the heads do not crash on the platters. The high ‘g’ MEMS accelerometer is used to detect the potential of a car crash and deploy airbags of the car at the correct time. The MEMS device detects the rapid negative acceleration of the vehicle.

The MEMS accelerometer is also applied in real-time applications in controlling and monitoring military and aerospace systems. For example, smart weapon systems such as aviation-launched and ship-launched missiles, rockets, projectiles and sub munitions.

TECHNICAL AND ECONOMICAL IMPACTS TO THE CONSUMER SOCIETY For the impacts to the consumer society, economically, since the MEMS accelerometers fabrication uses batch process to attain more benefit from the same economy of larger scale components, thus it has been successful in reducing the manufacturing cost of the MEMS accelerometers. From the technical point of view, due to the trend of miniaturization of existing accelerometers, thus there is an increase in the development of new devices based on principles that do not work at larger scale and development of new tools to interact with the micro scale world. Plus, economically, miniaturization of accelerometer devices also reduces the cost due to the decrease in material consumption. It also increases the applicability of accelerometer because of the reduction in mass and size allowing the MEMS accelerometer to be placed in places where a larger traditional system doesn’t fit. For example, the MEMS accelerometer is developed to replace larger traditional airbag triggering sensor. Thus due to the widespread use of accelerometers in the automotive industry, the application of MEMS accelerometer helps to lower down the cost dramatically.

CONCLUSION MEMS accelerometer is a micro and nano devices product which measures the static force, g or dynamic acceleration force, a (m/s²). MEMS accelerometer is made of conductive doped silicon with other materials used along the fabrication process including metal (chrome, gold and platinum), glass and hydrofluoric acid. The fabrication process to create the MEMS accelerometer encompasses Silicon on Glass (SOG) process, photolithography, etching process, predicing, metal deposition and DRIE (Deep Reactive Ion Etching). The MEMS accelerometer device product not only makes the accelerometer smaller but also makes them to attain a greater performance even at a lower cost.

REFERENCE 1. S. Chen, H.T. Chien, J.Y. Lin and Y.W. Hsu. (2008). A method of fabricating MEMS accelerometers. IEEE 2. Qifang. Hu, Chengchen. Gao, Yilong. Hao, Dacheng. Zhang, Guizhen. Yan, Yangxi. Zhang (2010). Design And Fabrication Of A Mems Capacitive Accelerometer Based On Double-Device-Layers Soi Wafer. IEEE International Conference on Nano/Micro Engineered and Molecular Systems.