Method of making a bolometric radiation detector

A bolometric radiation detector wherein the electrically-conductive measuring layer is applied to a carrier film which is supported by a frame. To produce a bolometer of this type the carrier film is produced as a layer on a base material, preferably by anodic or thermal oxidation of the base material. The measuring layer is applied to the carrier film. After providing the base material and the measuring layer with a photoresist layer, the conductors are etched out and a window is made from the reverse side. The carrier film with the measuring layer structure then remains. The parts of the base material that have not been removed serve as a frame for the carrier film.

BACKGROUND OF THIS INVENTION 
1. Field of this Invention 
This invention relates to a bolometric radiation detector, which consists 
essentially of a thin, temperature-sensitive, electrically-conductive 
measuring layer and to a method of producing the detector. 
2. Prior Art 
Thermal radiation detectors for the infrared range are indispensable for 
many applications in spite of the great advances that have been achieved 
in the field of cooled quantum detectors. The advantage of thermal 
radiation detectors is the independence of their radiation sensitivity 
from the wavelength and the ambient temperature. 
Various types of radiation detectors are known. Those that utilize the 
pyroelectric effect of certain crystals can only be successfully used when 
on the one hand the ambient temperature is sufficiently below the Curie 
temperature of the crystal and on the other hand the variations in the 
ambient temperature do not exceed a specific value of about 0.5 K/minute. 
Due to their simple and robust design, thermocouples and bolometers are 
preferred. The bolometer has the further advantages of constant 
sensitivity over the whole detector surface, the possibility of higher 
impedance and the reduction of the settling time by means of a rise in the 
working temperature. 
The bolometer is normally a blackened, tape-like conductor made of a metal 
with a temperature-coefficient of electrical resistance that should be as 
high as possible. The change in resistance as a result of heating up by 
radiation is measured using a bridge circuit. Gold is a favorable 
conductor material since it is easy to work. Pure metal conductors have 
the significant advantage that the sensitivity of the detector 
(signal-to-noise ratio) is limited only by the Johnson noise of the 
bolometer resistance. All other parameters remaining equal, the 
signal-to-noise ratio is independent of the ohmic resistance of the 
bolometer. However, the signal level is dependent on the radiation 
concentration, i.e., on the square of the image scale of the light source 
onto the bolometer. In the manufacture of bolometers according to 
conventional methods, there are mechanical limits which have to be 
respected in the required reduction in the size of the bolometers. As a 
result, with even moderately small dimensions the internal resistance 
becomes too small, the heat capacity and thus the settling time becomes 
too large, and the heat dissipation via the mounting of the bolometer 
strip becomes much too high. 
BROAD DESCRIPTION OF THIS INVENTION 
An object of this invention is to create a bolometer that does not have the 
disadvantages of the known designs. A method of producing highly 
sensitive, extremely small and rapid bolometers is also an object of this 
invention. In particular the method according to this invention should 
permit a short settling time combined with high sensitivity to be achieved 
by reducing the thermal mass or the thermal capacity. The method of this 
invention should also permit the ohmic resistance to be increased by means 
of a suitable design of the temperature-sensitive conductor so that even 
in a simple version of the bolometer the known material-specific maximum 
sensitivity values can be exceeded. 
Increasing the bolometer resistance is associated with a major advantage in 
that the resonant transformer for power and noise matching of the 
bolometer to the input amplifier can be dispensed with. When the favorable 
resistance is achieved, matching to low-resistance and noise-compensating 
preamplifiers is almost perfectly successful. This makes it possible in 
the case of photometric device to abandon a fixed low light alternating 
frequency and to utilize the full thermodynamically-limited band with of 
the bolometer. Devices for frequency analysis and band width limitation 
can then follow the amplifier chain. 
Further, the production method according to this invention has to permit 
application-specific configurations of the bolometer surface and its size 
down to dimensions which correspond to the resolution capacity of optical 
arrangements in the medium infrared range. At the same time the production 
method has to be suitable for large-scale mechanization in order to 
rationalize the work steps in production and to keep the variation range 
of the bolometer data below. A further requirement is that the production 
process be designed in such a way as to produce a stable mounting for the 
bolometer or micro-bolometer, so that the mounting can be located in the 
optical path of the rays where it does not cause disturbance and at the 
same time saves space. 
It has been found that these requirements can be fulfilled by a bolometric 
radiation detector of the type referred to earlier if the measuring layer 
is applied to an electrically-insulating carrier film which is supported 
by a frame and if the thickness of the measuring layer is less than 100 
nm. 
Preferably the measuring layer is formed as a meander-shaped printed 
conductor. Preferably the measuring layer is made of a noble metal, most 
preferably gold, and is between 10 and 100 nm thick, most preferably 25 
nm. Also, preferably the carrier film consists of oxides of the elements 
berrylium, silicon and aluminum and is less than 100 nm thick. When using 
the radiation detector of this invention, in order to determine the 
position of a radiation signal, preferably several separate measuring 
layers are located adjacent to each other along an axis on a single 
carrier film so as to form a linear array. When using the radiation 
detector of this invention, in order to obtain an image of extended 
radiation signals, preferably several separate measuring layers are 
located on a single carrier film covering the whole surface in the x and y 
directions so as to form an array. 
The bolometer according to this invention is fabricated by producing the 
carrier film on a base material, applying to the carrier film a thin 
conductive layer made of the material of the measuring layer, providing 
the base material for the carrier film and the surface of the measuring 
layer material with a photoresist layer, etching the printed conductor out 
of the conductive layer using a photolithographic method and producing a 
window from the reverse side in such a way that the carrier film with the 
measuring layer structure remains. Such is supported by the frame that is 
formed from the remaining substance. 
Preferably, in order to etch out the printed conductors, a mask prepared 
using a photolithographic method is laid on the parts of the conductive 
layer that are to be maintained; the parts of the conductive layer that 
are not required are removed by a dry physical method by sputtering or 
sputter etching within an electric, preferably high-frequency gas 
discharage burning at a reduced gas pressure. The measuring layer material 
is preferably etched in the form of a meander-shaped printed conductor. 
The carrier film is preferably formed from the base material by anodic or 
thermal oxidation. Preferably the carrier film is applied to a different 
type of base material by the thin-layer technique. The measuring layer is 
preferably formed by evaporation in a high vacuum. Also, preferably the 
measuring layer is formed by sputtering. Preferably the finished 
bolometric radiation detector is fixed to a socket by means of conductive 
adhesives in an "upside-down" process and obtains its electrical contacts 
at the same time. 
Any useful or suitable photolithographic method can be used to etch the 
printed conductors out of the conductive layer and to prepare the mask (or 
photoresists) laid on the parts of the conductive layer that are to be 
maintained. There are many such photolithographic methods, and such 
photolithographic methods are well known in the art and to one ordinarily 
skilled in the art. Some typical references are: Khambata, Adi J., 
"Introduction To Integrated Semiconductor Circuits", John Wiley and Sons, 
Inc., (1963) pp. 24 to 26; Tickle, Andrew C., "Thin-Film Transistors", 
John Wiley and Sons, Inc., (1969), pp. 74 to 75; Broyde, Barret, "Exposure 
Of Photoresists II. Electron and Light Exposure Of A Positive 
Photoresist", J. Electrochem. Soc., Solid State Science, Vol. 117, No. 12, 
(Dec. 1970), pp. 1555 and 1556; Magill, P. J., et al., "Photometallic 
Etching Of Holograms", J. Electrochem. Soc., Solid State Science, Vol. 
118, No. 9, (Sept. 1971), pp. 1514 to 1516; and Blakemore, J. S., et al., 
"Shaping Of Bulk Semiconductor Samples By Photolithography And Chemical 
Etching", J. Electrochem. Soc., Solid-State Science And Technology, Vol. 
128, No. 11 (Nov. 1981), pp. 2410 to 2415.In photolithographic methods, 
photosensitive material called photoresists (patterns or masks) are used 
during the production of the semiconductor devices (such as the invention 
bolometric radiation detector) to protect selected areas of the 
semiconductors and other surfaces against chemical attack. The desired 
pattern in the photoresist is obtained by exposing the photoresist-coated 
surface to light through a master mask or a pattern-controlled electron 
beam. After exposure to light, electrons, etc., the pattern is developed 
by rinsing the photoresist with a solvent. Negative resists are those that 
are insolubilized upon exposure, and positive resists are those that are 
solubilized. 
In the multistage production method according to this invention the 
conductor material of the bolometer is applied as a thin layer to an 
electrically insulating carrier that is supported by a frame. The 
conductive layer then is divided by a photolithographic technique so as to 
produce a long winding printed conductor whose internal resistance can be 
freely adjusted within broad limits by the selection of its geometric 
dimensions. The carrier film is produced on the front of a substrate and 
then exposed by dissolving the substrate from the reverse side. The 
dimensions of the bolometer surface on the carrier film are between about 
30 .mu.m and a few millimeters. The thickness of the layers can be between 
20 and 100 nm. 
The bolometer according to this invention is particularly suitable for the 
infrared range. 
Other objects and advantages of this invention are set out herein or are 
obvious herefrom to one ordinarily skilled in the art. The objects and 
advantages of this invention are achieved by the device and method of this 
invention.

DETAILED DESCRIPTION OF THIS INVENTION 
The carrier film can consist, for example, of aluminum, berrylium or 
silicon oxides. In this embodiment according to this invention an aluminum 
foil, which is not too thick, is used as a basis and serves as a substrate 
for the whole production process. From this the finished bolometer unit is 
finally separated. A ring-shaped part of it serves as a frame for the 
carrier film. 
In stage (a) of the process shown in FIG. 1, the bright rolled aluminum 
foil is polished, cleaned and finally anodically oxidized preferably using 
diammonium hydrogen tartrate. The voltage applied during anodic oxidation 
is 20 V to 60 V; the process lasts approximately one hour and results in a 
dense oxide skin which later forms the carrier film. The surface produced 
on the aluminum by the anodic oxidation has the correct properties to 
anchor the conductor layer firmly without applying an intermediate layer 
of an adhesive agent such as chromium. In addition, the surface quality of 
the oxide layer produced in this way reduces the mobility of the conductor 
layer atoms to be condensed from the vapor phase, e.g., gold, so that the 
island effect which is damaging for a high temperature coefficient does 
not occur during the evaporation process. The conductor layer, e.g., gold 
is evaporated onto the oxide layer in a high vacuum and then tempered to 
achieve almost the electrical characteristic values of the compact metal. 
In stage (b) the bolometer structure, e.g., meander, on the conductor layer 
and the window on the reverse side are etched using adjusted photomasks 
made from the photoresist layer that had previously been applied to both 
sides. At the same time a ring-shaped zone is also exposed from the 
reverse side of the substrate which makes it possible at a later stage to 
etch from the aluminum substrate the frame on which the oxide skin will be 
supported and to which the skin will be firmly fixed. This framework 
structure also serves for fitting the bolometer unit into its casing and 
can withstand high vibration at accelerations. The high mechanical 
stability thus ensured is the reason for the insensitivity of the 
bolometer to the microphonic effect. Using the photoresist structure 
produced, the meander-shaped conductor and the connection panels for the 
electrical contacts are etched out of the conductor layer. 
The thickness of the conductor layer produced is preferably 20 to 25 nm in 
order to give the bolometer a resistance that is as high as possible, a 
low thermal inertia and at the same time little variation between 
individual models. 
In stage (c) the freshly etched conductor is provided with a protective 
lacquer for the duration of the subsequent stages, after which the oxide 
skin still present on the reverse of the substrate at the location of the 
future window is removed using hydrofluoric acid. 
In stage (d) the oxide film with the conductor on the front of the aluminum 
substrate remains after etching the window into the aluminum foil and at 
the same time the ring referred to earlier is etched. This is performed 
using concentrated hydrochloric acid to which a small quantity of copper 
chloride is added in order to accelerate the dissolving of the aluminum. 
Etching is performed from the reverse side, at the points that were 
exposed in stage (b) and freed of the oxide layer in stage (c). By 
dissolving the previously supporting aluminum substrate, the bolometer 
disk becomes free and can be removed as a complete unit. Finally the 
residual protective lacquer and photoresist coating are fully removed. The 
bolometer element is blackened preferably using camphor black, which keeps 
the thermally insert mass of the bolometer low. 
FIG. 2 shows a radiation detector produced according to the method 
described above. It consists of meander-shaped conductor 1 with connection 
panels 2 which are supported by aluminum oxide film 3. Absorber 4 is 
fitted on top of the meander structure. At the point where the bolometer 
structure is to rest on oxide carrier film 3, the base material is 
dissolved to form a window so that carrier film 3 is left supported by 
remaining base material 5 which forms a frame. 
The finished bolometer element, i.e., the base material with the window, 
insulator film 3 and bolometer structure 1, can be fixed in socket 6 in a 
particularly simple way by the "upside-down" process, as shown in FIG. 3. 
Conductive adhesives are used. Since contact panels 2 come in contact with 
contact pins 8, the element receives its electrical contacts at the same 
time. In this case radiation-absorbing black layer 4 is applied to the 
reverse side of the bolometer element. 
The size of the window and thus the size of carrier film 3 in the mounting 
has a major influence on the sensitivity and the response time of the 
bolometer. If the window is large and the bolometer is assembled in a 
vacuum, high sensitivity is achieved. If the window area is reduced for 
the same bolometer area, the heat dissipation toward the frame rises and 
the bolometer becomes more rapid. Thus the two usual bolometer types 
(radiation and heat dissipation cooled) as well as all intermediate forms 
can be produced according to this invention using the same method of 
production. 
FIGS. 4 and 5 show that individual separate measuring layers 1 may be 
arranged in a linear manner or so as to cover the whole surface area of 
single carrier film 3 which is represented in the diagrams by a broken 
line. The linear array shown in FIG. 4 permits the position of a radiation 
signal to be detected along an axis. The plane array as shown in FIG. 5 
makes it possible to obtain an image of extended radiation signals. These 
embodiments can be produced simply according to the method described 
above; the carrier film and the frame made from the base material (not 
shown in these diagrams) are common to all measuring layers 1. The 
contacts of the measuring layers can be arranged jointly or separately.