Radiant energy imager using null switching

Abstract

In some aspects, the present invention embodies both the method and apparatus for converting a pattern of irradiation to a visible image. An embodiment of the present invention provides an array of micro-electro-mechanical sensors with each sensor includes a deflectable micro-cantilever, responsive to absorbed incident radiation and to an applied repulsive electrostatic field. In an aspect, the sensor device also includes a null-sensing circuit coupled to a switch contact on or near the substrate, which senses when the micro-cantilever reaches its null location, by electrical connection with an upper switch contact on the micro-cantilever. Other embodiments are also described.

Description

FIELD OF THE INVENTION[0001]This invention relates to radiant energy imaging sensors, and, in particular to such methods, systems, apparatus and devices for sensors as employ a micro-cantilever as a responsive element.BACKGROUND AND PRIOR ART[0002]A variety of prior art irradiance sensors is currently available in the electronics industry. Techniques for converting radiation to visible images are also known in which the absorbed radiant energy is converted to heat to change a measurable property of a sensing element, such as the resistance of a resistor or the position of a bi-material cantilever. The sensed radiation includes ultraviolet, visible, infra-red, or terahertz irradiation, depending on the spectral absorption of the sensor element.[0003]An example of a prior art radiation sensor is the deflectable micro-electromechanical (MEM) cantilever bi-material device formed of a bi-material, anchored on an insulating substrate. The bi-material portion of the micro-cantilever device...

AI Technical Summary

Benefits of technology

[0033]A primary objective of the invention is to provide methods, apparatus and systems for a thermal-type irradiance imaging array capable of lower internal noise and greater dynamic range; requiring neither periodic shuttering nor an internal refrigerator / heater; and measuring irradiance with low noise over many orders of magnitude.

[0037]A fifth objective of the invention is to provide methods, apparatus and systems for a radiant imager using null switching to provide an improved irradiance sensor which can record image radiance in a pseudo-logarithmic scale, such that the measurement scale is compressed for high radiance and expanded for low radiance.

[0040]An eighth objective of the invention is to provide methods, apparatus and systems for a radiant imager using null switching to provide an imager which can provide continual imagery, without the interruption or disturbance of mechanical shuttering.

Problems solved by technology

The prior art bi-material MEM micro-cantilever devices bend, or deflect, when irradiation is absorbed by an absorber element of the micro-cantilever and heats the bi-material section of the micro-cantilever, causing one of the bi-materials to expand at a greater rate than the other bi-material and resulting in deflection or bending of the micro-cantilever.

As a micro-cantilever device bends in response to incident radiation, it approaches a physical limitation to its degree of bending.

For example, if a micro-cantilever device is fabricated to bend freely downward in response to incident radiation, the physical limitation is reached when the micro-cantilever touches the substrate over which it is formed.

For a micro-cantilever device chosen to bend upward in response to incident radiation, this too will reach a physical limitation point past which it can no longer bend as a bi-material.

While a significantly higher dose of radiation forces the micro-cantilever to bend slightly more towards its physical limitation, the degree of bending is not proportional and the device response is not linear in this region.

As a result, the linear range of the device is limited.

Since the intensity of an optical image ultimately produced is based on the degree of bending, prior art devices have a poor dynamic range and are limited linearity, results in producing an image having the same shortcomings.

Moreover they possess a limited range of linear response to irradiative heating.

Aside from its cost and bulk, the disadvantage is that power consumed by this thermoelectric refrigerator / heater is typically the single largest power drain in the imaging system.

Another limit of prior art configurations has been the degree to which the bi-material bends for a given absorbed irradiance.

However, this desirable bending has been limited in prior art by the choices of bi-material materials and bi-material design such as the difference in thermal expansion between the expansive bi-materials, the high thermal conductivity of the thermal isolating leg, and the stiffness of the cantilever support legs.

First, the prior art choice of bi-material materials has been limited to aluminum or gold as the expansive material, and silicon nitride as the relatively non-expansive material.

Second, the bending has been limited by a necessity to achieve rapid thermal stabilization of the temperature of the absorber plate and bi-material within a fraction of a video frame period, so that the bi-material bending can be measured before the end of the frame.

Third, a limit of prior teaching has been the mechanical resonance of the cantilever itself because the support leg, which includes the insulator leg plus bi-material, must be made stiff enough that the freely supported mass of the absorber plate has a resonant frequency higher than that of ambient acoustic fields, typically higher than 20,000 Hz.

Yet another limit of prior art bolometers is their necessity to “blink” frequently with the display periodically going visibly black, using a mechanical shutter to remove the target irradiance from the array and zero out all the various readings of the bolometer sensors as they slowly drift in base temperature.

This is occasioned by the limited irradiance range of the bolometer: essentially all of the dynamic range of the bolometer is required for constructing the final image.

Therefore A / C-coupled systems employ a mechanical chopper, with the attendant issues of weight, power, life time and delicacy as to mechanical shock.

A DC coupled thermal imaging system measures a tiny signal on top of a large DC background signal, which is a primary cause for noise limitation in the minimum discernable signal.

The DC-coupled thermal imaging system must handle the relatively very large offset as well as the signal of interest This complicates the system because the offsets differ from pixel to pixel, and the differences vary slowly with time, increasing spatial noise in the system.

Both A / C and DC-coupled systems perform a similar comparative function by periodically shuttering the system at some time interval from a few seconds to a few minutes, usually resulting in an interruptive image freeze upon shuttering.

(a) Because the temperature of the sensor element must reach a stable level before measurement may be made, the thermal time constant of the micro-cantilever must be made a fraction of the frame time. As a result the thermal conductance of the thermal insulator leg must be large, the temperature rise for a given irradiance is less and the bend of the responsive bi-material is decreased.

(b) Because in the past the use of aluminum or gold for the expansive material and silicon or silicon nitride for the less-expansive material, the difference in thermal expansion has been limited and the bend of the bi-material is decreased.

(c) Because the mechanical resonance of the micro-cantilever must be higher than ambient acoustic frequencies, the bi-material leg must be made stiff, and the bend of the bi-material is in response to irradiance is decreased.

(d) In past micro-cantilever bolometers the measure of irradiance is an analog signal, with the attendant 1 / f, shot, Johnson, and amplifier noise, which together increase the irradiance required to exceed the noise of the sensor.

(e) As a result of the noise and insensitivity, bolometers have required the longest exposure possible to reach a stable temperature for measurement within the limited frame time, and thus have a limited ability to operate at a higher frame rate.

(f) Because of the need for stabilization of the substrate temperature, use of a costly and power-draining thermoelectric temperature regulator has been common.