Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2002-03-20
2004-06-22
Gagliardi, Albert (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S332000, C250S339020, C438S069000, C332S176000
Reexamination Certificate
active
06753526
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the manufacture of radiation detectors comprising a set of individual “microdetectors” arranged in matrices or line arrays. The invention is particularly advantageous in the case where these microdetectors are microbolometers.
BACKGROUND
A bolometer is a device designed to transform the radiation to which it is subjected, typically in the infrared range, into thermal energy. The resulting heating of the bolometer gives rise to the variation in an electrical property, for example the electrical resistance of a conductor connected to a circuit exterior to the bolometer. In the case of a detector comprising an array of microbolometers, said electric circuit, known as a “reading” circuit, manages the array addressing functions and the reading stimuli sent to each microbolometer, and converts the resulting signals into a format exploitable for imaging (for example in the form of a video signal). To obtain the best possible performance, the microbolometers are made to operate under relatively low gas pressure (or under moderate pressure of a gas having low thermal conductivity), in order for the thermal dissipation due to this gas to be negligible in relation to the inherent thermal conductance of the bolometers.
Typical methods of manufacture of detectors of this type comprise initial steps carried out directly on the surface of an electronic circuit, in a so-called monolithic manner (i.e. in a continuous sequence of operations on the same substrate, usually of silicon) or hybrid manner (with transfer onto a substrate of prefabricated elements). These steps involve common techniques of the microelectronics industry, in particular techniques of collective production, typically concerning several tens to several hundreds of detectors deposited onto the same substrate (wafer level). During these steps the components that are actually bolometric (optical absorption, and resistance variable with temperature) are mounted on the surface of a layer that is “sacrificial” in the sense that this layer (usually made of polyimide, of polycrystalline silicon, or of metal such as copper or aluminum) is eliminated at the end of the process (by combustion in an oxygen plasma, for example), so as to leave the structures of the bolometer suspended above the substrate.
Further to these initial steps, an automated quality test and sorting operation are performed, then the assembly is cut up into individual detectors. The process of manufacture ends with a so-called “unitary” set of operations, that is to say carried out on each detector individually.
These unitary operations typically comprise the following steps:
The first step is the “freeing” of the microbolometers, which consists in eliminating at least a part of the sacrificial layers. This operation leads to the structures being extremely vulnerable mechanically. Moreover, the slightest contamination by dust (so-called “particulate” contamination) of size greater than a few micrometers also deteriorates the detector locally, since cleaning cannot be envisaged, in any manner, due to the risk of complete destruction: more particularly, the freed microbolometers can withstand neither blowing, nor wetting nor contact.
Next, each detector is glued or soldered to a ceramic die carrier, this die carrier being itself usually glued or soldered onto a system of thermal regulation (Peltier element), and the detector is connected by wire bonding. This assembly is next mounted into a casing comprising at least two parts: a metallic or ceramic die carrier, and a cover comprising a window transparent to infrared radiation. Before sealing of said cover, the electric inputs and outputs of the casing are connected to the metallic tracks of the ceramic, again by wire bonding. The assembly and the number of operations may be optimized, but, as may be seen, this remains very complex overall.
Finally, for the systems of highest performance, before sealing the cover a “getter” is placed within the casing. A getter is constituted by a material known for its capacity to improve the quality of the vacuum (for example iron, titanium, vanadium, cobalt, aluminum, zirconium or manganese, or an alloy of these metals).
All these unitary operations, associated with the supply cost of the elements and components making up the casing, result in an additional manufacturing cost which is considerably higher than the cost of collective manufacture of the detectors. This end of manufacturing cost (after the collective operations) is particularly high in the case of microbolometer imaging systems (it commonly attains 60 to 70% of the cost of the final component), since the level of residual pressure required for optimal operation of the detector (typically of the order of 10
−2
mbar) requires a high quality of vacuum sealing. This justifies the implementation of unitary techniques developed on specific casings (use of vents or evacuation pipes), and a relatively long cycle (several hours) of degasification and activation of the getter. Furthermore, the operations of testing and sorting of the products in course and/or at the end of manufacture are themselves unitary, and very difficult to automate.
It may thus be seen, in relation to the manufacture of detectors comprising arrays of microbolometers, that conventional techniques suffer from a low rate of production, and a high overall cost.
The object of U.S. Pat. No. 5,895,233 is specifically to improve the state of the art in this field. This document provides a collective technique for the production of covers on substrates of conventional dimensions which will be referred to as “window substrates”, and a technique, also collective, of sealing each window substrate onto a “detector substrate”, the two substrates being held at a certain distance from each other by means of solder beads so as to ensure the mechanical protection of the detector under vacuum.
This technique has several drawbacks. First of all, the soldering of the cover over the detector substrate is difficult to manage. Furthermore, the creation of the vacuum requires the use of getters of large size if an ordinary soldering apparatus is used, or otherwise a high technology vacuum soldering apparatus. Furthermore, this technique requires the use of relatively thick materials (several hundreds of microns) to produce the “window substrate”, which limits the optical transparency, and therefore the performance of the detector. Another drawback is that the solder beads occupy a large surface area of the detector substrate, which means that only detectors of large size can be economically worthwhile; but in the case of large detectors, flexing of the two parts of the casing is observed under the effect of the external atmospheric pressure, which gives rise to geometrical aberration, and can even lead the window to come into contact with the microbolometers, in which case the detector is destroyed; to alleviate this problem, the document proposes to arrange a pillar within the casing, but this means that another part of the detector is made blind.
BRIEF SUMMARY
To solve these problems, the invention provides a method of manufacture of radiation detectors, in which the detectors each comprise an assembly of microdetectors, for example microbolometers, under a window that is transparent to said radiation, said method being remarkable in that said detectors are manufactured collectively on a substrate, and in that it comprises notably the following steps:
the construction of several layers, of which, for each of said detectors, at least one layer is transparent to said radiation and serves as a window, and
the partial elimination of said layers principally under said transparent layer, such that said microdetectors are placed, for each of said detectors, in one or more cavities, which are then placed under vacuum or under low pressure.
Thus, the detectors according to the invention comprise, due to their actual construction, the elements adapted to keeping the microdetectors under vacuum. Advantageously, this construction ac
Brinks Hofer Gilson & Lione
Commissariat a l'Energie Atomique
Gagliardi Albert
LandOfFree
Radiation detectors and methods for manufacturing them does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radiation detectors and methods for manufacturing them, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radiation detectors and methods for manufacturing them will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3365582