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.

Figure 1: Uncooled thermal camera market forecast in millions of units (Yole Development)

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 the designer. 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 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.

  1. 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.

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