Patent Application
20080093553
For a better understanding of various embodiments of the invention, reference should be made to the following detailed description that should be read in conjunction with the following figures wherein like numerals represent like parts.
For simplicity and ease of explanation, the invention will be described herein in connection with various embodiments thereof. Those skilled in the art will recognize, however, that the features and advantages of the various embodiments may be implemented in a variety of configurations. It is to be understood, therefore, that the embodiments described herein are presented by way of illustration, not of limitation.
In general, various embodiments of the invention provide an infrared (IR) microbolometer device having a perpendicular conduction structure. The various embodiments may be used, for example, as a detector in thermal cameras.
In particular, various embodiments of the invention may be implemented in an IR imager 20 as shown in
The IR imager 20 in various embodiments is configured as an uncooled IR detector such that no external cooling device is provided. However, it should be noted that the IR imager 20 alternatively may be a cooled IR detector. More particularly, the IR imager 20 may include one or more microbolometer units 28 configured as uncooled thermal sensors. In operation, the IR imager 20 operates to measure incident electromagnetic radiation received and focused by the optical assembly 22 onto the IR sensor 26. Essentially, the one or more microbolometer units 28 each include one or more microbolometers that measure the radiation at one or more frequency ranges, which is detected as a resistance change. The detected resistance changes are measured and processed, which may include filtering the signal in any know manner and/or converting the signal from an analog input to a digital output using the filter/converter 32. The processor 34 then may generate a temperature map based on stored settings in the memory 38 and provide the output as a thermal image on the display 36.
It should be noted that different controls also may be provided to the IR imager 20. For example, biasing and/or reference signals may be provided to control and calibrate the IR imager 20, for example, to receive radiation at different frequency ranges.
The one or more microbolometer units 28 may be configured as a grid 40 of individual microbolometers 42 as shown in
As shown in
In operation, radiation absorbed by the absorber layer 48 raises the temperature above that of the heat sink layer 50 such that the higher the power absorbed, the higher the temperature. The temperature measuring device connected to the absorber layer 48 measures the temperature, from which the absorbed power can be calculated in any known manner. Thus, the grid 40 may be formed from a plurality of, for example, vanadium oxide or amorphous silicon heat sensors on top of a corresponding grid of silicon. Infrared radiation from a specific range of wavelengths may strike the top grid layer and changes the electrical resistance of that layer. The change in resistance is measured and processed into temperatures that may be represented graphically or used to form an image as described herein (e.g., in an IR camera). The grid 40 essentially includes a plurality of sensing elements defining a plurality of pixels of thermally sensitive film with an IR absorbing coating, which upon irradiation of infrared energy, causes the device temperature to rise, resulting in a change of electrical resistance. It should be noted that the change in electrical resistance may be sensed electrically and processed (e.g., translated) into a video signal. Further, each pixel essentially defines a thermistor, the resistance of which changes with temperature. For example, the pixels may be configured in different manners, for example, to detect about one-twentieth of one degree Celsius changes in temperature or generally less than one-tenth of one degree Celsius. However, each pixel may be configured to detect higher or lower temperature changes, such as changes of one or more degrees Celsius.
It should be noted the microbolometers 42 may be formed, for example, by a semiconductor deposition process. For example, an aluminum layer may be deposited on a substrate followed by the deposition of an amorphous silicon on the aluminum layer. Thereafter another aluminum layer may be deposited on the amorphous silicon. Detection areas (e.g., windows) then may be cut into the formed material using a photolithography process. The various steps used to form the microbolometers may be provided in any manner known in the art.
Various embodiments of the invention provide a microbolometer having a perpendicular conduction mode. More particularly, an IR sensitive core material 60 as shown in
When the resistance across the first and second conductor layers 64 and 66 can be neglected as compared to the resistance through the bolometer layer 62, the resistance of the IR sensitive core material 60 may be defined as follows:
R=ρ*t/(L*W) (1)
where ρ represents the resistivity of the bolometer layer 62, t represents the thickness of the bolometer layer 62, L represents the length of the bolometer layer 62 and W represents the width of the bolometer layer 62. It should be noted that the bolometer layer 62 may be a thin film layer, for example, about 0.01 microns in thickness. 100231 In operation, the sensing current flows from the tab 68 of the first conductor layer 64, into the first conductor layer 64, spreading in-plane, through the thickness (t) of the bolometer layer 62 and to the opposite conductor layer, namely the second conductor layer 66. The sensing current then flows in-plane through the second conductor layer 66 and is collected at the tab 68. It should be noted that the tabs 68 may generally define first and second leads of the first and second conducting layers 64 and 66.
With respect to the TCR of the bolometer layer 62, which may be formed, for example, from an amorphous silicon material, a wide resistivity range may be provided by varying the impurity doping levels. When forming the various layers of the IR sensitive core material 60, the first and second conductor layers 64 and 66 are formed as thin layers such that the first and second conduction layers 64 and 66 do not dominate thermal conduction in the plane of the sensitive core material 60. The thermal conduction also may be reduced by other aspects of the optimization performed during the thermal design (e.g., long thermally isolating leads). However, the reduction of the thermal conduction of the first and second conductor layers 64 is described herein an independent factor. Accordingly, the thickness of the first and second conductor layers 64 is determined as follows. It should be noted that because the electrode material may be, for example, a metal such as, but not limited to, platinum and aluminum, among others, with high electrical and thermal conductivity, the thickness of the first and second conductor layers 64 may be substantially thinner than the thickness of the bolometer layer 62. Thus, in embodiments where the in-plane thermal conductivity of the sensitive core material 60 is significant to the thermal design, the following equation defines the relationship between the first and second conductor layers 64 and 66, and the bolometer layer 62:
Gc*tc<Gb*tb (2)
where Gc and Gb are the thermal conductivities of the materials for the first and second conductor layers 64 and 66, and the bolometer layer 62, respectively, and tc and tb are the thicknesses of the first and second conductor layers 64 and 66, and the bolometer layer 62, respectively.
As an example, for amorphous Si, Gb=5 W/m-K near room temperature with similar values for other amorphous semiconducting materials used to form bolometers. For Gc the thermal conductivity of metal increases with the electrical conductivity of the metal and the values may range, for example, from 100 W/m-K to 400 W/m-K. For example, Al has a thermal conductivity of approximately 300 W/m-K near room temperature. Accordingly, the thickness of the first and second conductor layers 64 and 66 are provided such that minimal additional in-plane heat dissipation is provided by the first and second conductor layers 64 and 66.
Further, the first and second conductor layers 64 are provided such that the first and second conductor layers 64 contribute only a small portion of the overall resistance of a pixel element as described in more detail below. Specifically, the following equations are provided for configuring the various layers:
R
total=(ρb*tb/(L*W))+(2*(ρc*L/(W*tc)) (3)
and
(ρb*tb/(L*W))>(2*(ρc*L/(W*tc)) (4)
where (1) ρb and ρc are the electrical resistivity of the bolometer layer 62, and the first and second conductor layers 64 and 66, respectively, (2) tb and tc are the thickness of the bolometer layer 62, and the first and second conductor layers 64 and 66, respectively, and (3) W and L are the width and length of the layers.
Equations 2 and 4 may be combined to derive the following equation:
Accordingly, Equation 5 may be used to determine the thickness for the bolometer layer 62. For example, amorphous silicon and aluminum have resistivities of approximately 103 ohm-cm and 3×10−6 ohm-cm, respectively. Using Equation 5, results in tb>L*6*10−4, or a minimum thickness of the bolometer layer of 18 nanometers (nm) for a 30 micrometer (μm) by 30 μm sensing area. Thus, the thickness of the bolometer layer 62 may be, for example, at or above 50 nm (e.g., factoring in design tolerances). In various IR imager applications the thickness of the bolometer layer 62 may be, for example, between 10 nm-1000 nm with the thicknesses of the first and second conductor layers 64 being 10 nm each. However, the thickness of the bolometer layer 62 may be greater or smaller as desired or needed.
Thus, various embodiments of the invention provide a microbolometer having a perpendicular conduction mode, thereby improving TCR and resulting in improved sensitivity. Accordingly, bolometer materials with higher resistivity may be used to provide a desired or required device total resistance. It should be noted that when using bolometer materials with a negative TCR (e.g., amorphous Si, VOx, and other semiconducting materials) higher TCR values may be obtained with a higher resistivity. Accordingly, a higher TCR coefficient is provided with higher resistivity material.
It should be noted that the various embodiments, including, for example, the various layers described herein may be formed from any suitable material, and based on, for example, the particular application. Further, the size and shape of the various layers also may be modified as desired or needed. Additionally, the various embodiments may be used in connection with a system or device wherein a bolometer or microbolometer is needed or desired.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the various embodiments of the invention can be practiced with modification within the spirit and scope of the claims.
