1. Field of the Invention
The present invention relates to accelerometers and other force-sensing devices, and more particularly to a gun-hard, in-plane MEMS capacitive accelerometer that includes a proof mass formed from a single piece of material positioned above a plurality of electrodes on a substrate. When the substrate accelerates, the proof mass moves in a direction parallel to the upper surfaces of the substrate, changing the capacitance between the proof mass and the substrate. This change in capacitance can be used to measure the displacement and to determine the acceleration of an object to which the substrate is attached.
2. Description of Related Art
Accelerometers are a critical component in the Inertial Measurement Units (IMUs) commonly used in navigation and guidance systems for all types of vehicles. A typical IMU consists of three equal modules, each including a linear accelerometer, a gyroscopic rotational rate sensor, and associated electronics. These three-axis IMUs are used for navigation, guidance, and data-measurement systems in aerospace applications ranging from aircraft and spacecraft, to precision-guided missiles and artillery rounds. In many of these applications, the IMU is exposed to extreme vibrations and shock loads; it must be designed to withstand these harsh conditions.
Inertial Measurement Units capable of surviving harsh shock loads are known as gun-hard IMUs. These high-performance IMUs remain fully functional even when exposed to forces that are thousands of times stronger than the pull of gravity. Use of high-performance accelerometers and other components allow for reliable, consistent, and precise guidance of the vehicle or projectile on which the IMU is installed.
High-performance accelerometers with near micro-gravity resolution, high sensitivity, high linearity, and low bias drift are critical for use in gun-hard IMUs. Traditionally, IMUs included large mechanical accelerometers and conventional spinning mass gyroscopes. However, most current IMUs, and especially gun-hard, high-performance IMUs, are made using microelectromechanical systems (MEMS) fabrication techniques.
MEMS fabrication technology plays a critical role in ensuring that large mass, large capacitance, and small damping are simultaneously obtained in the accelerometer while achieving micro-gravity resolution. Silicon capacitive accelerometers have several advantages that make them very attractive for gun-hard IMUs. Silicon capacitive accelerometers have high sensitivity, good direct current response and noise performance, low drift, low temperature sensitivity, low power dissipation, and a simple structure. It would be beneficial to have a gun-hard, high-performance, three-axis accelerometer built on a single chip using MEMS fabrication techniques. But this requires building both out-of-plane and in-plane accelerometers on the chip using the same fabrication techniques.
Known in-plane accelerometer configurations include MEMS comb-finger accelerometers, where the sensing gaps are formed between side walls and the sensitivity is determined by the size of the sensing gaps. A conventional MEMS comb-finger accelerometer is shown in FIG. 7 and designated generally by numeral 700. Accelerometer 700 includes a proof mass or sensing plate 702 attached by springs to two anchors 704 and having a plurality of movable fingers 706. Movable fingers 706 are interdigitated with a plurality of fixed fingers 708, with a lateral gap formed between the movable and fixed fingers. The minimum size of the lateral gap is limited to between about 1/10th and 1/15th of the plate thickness by the aspect ratio of the Dry Reactive Ion Etching (DRIE) technology used to fabricate in-plane accelerometer 700. This means that the minimum lateral gap for a plate that is 75 μm (microns) thick is between 5.0 and 7.5 μm. Existing DRIE technology is only capable of producing gaps as small as 10 μm; it is not possible to fabricate a MEMS comb-finger, in-plane accelerometer using this technique for a plate that is 75 μm thick.
Other fabrication techniques combining surface micromachining and bulk micromachining can be used to reduce the lateral gap to 1.1 μm. Polysilicon deposition techniques are one example. However, the process flow for these techniques is very complicated—resulting in a low yield. Moreover, the resulting structure is fragile—making the structures unsuitable for high shock applications. These known techniques cannot be used to fabricate gun-hard MEMS accelerometers.
Additionally, the conventional comb-finger accelerometer configuration has inherent nonlinearity issues. To improve linearity, the change in the lateral gap must be limited to a small range, which leads to a small differential capacitance output.
Given these limitations, there is a compelling need for a gun-hard, high-performance, three-axis accelerometer that includes both an in-plane accelerometer and an out-of-plane accelerometer built on a single chip using the same MEMS fabrication techniques. The present invention addresses this need.