Micro-Bolometer Design and Simulation
From night vision cameras used by the military and law enforcement, to medical imaging technologies like 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.
Figure 1: Uncooled infrared imaging products
The most widely used sensing technology employed in thermal (infrared) cameras is based on uncooled micro-bolometers. They are typically arrayed in a grid so that a thermal image can be formed.
Design Challenges
The micro-bolometer is primarily a thermal sensor, so understanding its electro-thermal characteristics is one of the key challenges faced by designers. The current-voltage (IV), voltage-temperature (VT), and thermal time constant (TC) are the key performance characteristics that need to be simulated and optimized.
CoventorWare provides a complete solution for simulating these characteristics and exploring design variations. The solution is based upon MEMS-specific solid modeling and finite element tools. First, material properties, process information and layout are defined using the CoventorWare’s process-driven design entry. These data are used to build a 3D model, which the user can mesh using MEMS-optimized mesh generators. Simulations can be run in CoventorWare’s MemMech solver and the results can be examined in tabular format, 2D graphs or 3D visualizations.
Figure 2: Microbolometer design flow in CoventorWare (example design from University of Pretoria)
The MemMech solver is used to simulate the electro-thermal and electro-thermo-mechancial behavior of the micro-bolometer. This versatile solver allows users to include voltage and temperature profiles as inputs, to simulate 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 since 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 the device.
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. Figure 4 demonstrates that the predicted time constant is a few millisecons for our sample bolometer. CoventorWare Analyzer can also be 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 achieved, a voltage pulse is applied across the bolometer to measure resistance. This resistance is related to the temperature increase 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 upon the Analyzer simulation results.