Micro-Bolometer Design and Simulation
From night vision cameras used by the military and law enforcement to medical imaging technologies such as digital infrared thermal imaging, thermal image sensors are becoming ubiquitous. With the increasing commercial applications of this technology, the thermal camera business has been growing nicely and this growth is expected to accelerate over the next decade.
The most widely used sensing technology employed in thermal (infrared) cameras is based on uncooled micro-bolometers. The traditional bolometer, consisting of a thermistor and a Wheatstone bridge, was invented in 1878. Micro-bolometers were developed in the late 1970s and measure the change in resistance caused by incident infrared radiation. They are typically arrayed in a grid so that a thermal image can be formed.
The micro-bolometer is primarily a thermal sensor, so understanding its electro- thermal characteristics is one of the key challenges faced by the designer. The current-voltage (IV), voltage-temperature (VT), thermal time constant (TC) are the key performance characteristics that need to be simulated and optimized.
Coventor provides a complete design flow for exploring and simulating these characteristics. The flow is based upon MEMS-specific solid modeling and finite element tools. First, material properties, process information and layout are defined using the CoventorWare’s Designer module. Designer uses this data to create a 3D model, which is then automatically meshed for simulation in CoventorWare’s Analyzer module. Results can be visualized in tabular format, 2D graphs or 3D.
The MemMech solver in CoventorWare Analyzer is used to simulate the thermal and electro-thermal behavior of the micro-bolometer. This versatile solver allows engineers to assign voltage and temperature profiles as inputs to simulate the steady-state and transient responses. The effects of heat transfer due to conduction and convection can be easily incorporated in the simulations.
- MEMS Design Exploration To simulate IV and VT curves, temperature boundary conditions are assigned. The operating voltage is varied over a user-defined range and the current flow and associated Joule heating simulated. Understanding the IV curve (Figure 3a), whose slope is the resistance of the micro-bolometer, is important as it determines the sensitivity of the device. The V-T curve (Figure 3b) allows designers to understand the effect of Joule heating on the bolometer and determine the temperature gain of the bolometer at steady state when a given potential is applied across it.
To simulate the thermal TC (Figure 4a), mechanical and temperature boundary conditions are assigned together with a voltage pulse. A time domain electro-thermal simulation is then performed. The thermal TC is the time required for the temperature to reach 66% of its steady-state value. This metric gives designers information about the response time of the sensor and helps them design the appropriate control electronics and readout circuits. The curves below show that the predicted time constant for the example bolometer used here is a few milliseconds.
CoventorWare Analyzer is also used to simulate the operating response with time. A heat flux is first applied to model the incident infrared radiation. When the steady-state temperature is reached, a voltage pulse is input across the bolometer in order to measure the resistance, which is related to temperature rise caused by the incident radiation (Figure 4b).
A Complete Platform
Coventor’s solution offers a MEMS specific design flow to easily simulate, tune, and optimize MEMS micro-bolometers. CoventorWare Analyzer is able to predict the key device performance characteristics using its MEMS-specific, multi-physics solvers and visualization tools. Critical performance parameters such as noise equivalent temperature difference and thermal capacitance can be calculated based on Analyzer’s simulation results.