Infrared radiation absorption device

The invention relates to an infrared radiation absorption device, which can be unrestrictedly produced from CMOS technology methods and materials. The absorber structure according to the invention comprises a lower layer (1) with a low transmission coefficient, a central layer (2) with a high absorption coefficient and an upper, absorbing component (3) with a low reflection coefficient for the radiation to be absorbed and which is applied from above. The upper component can e.g. comprise depressions in the central layer, whose walls are coated with metal. The absorber structure is used in the inexpensive manufacture of integrated, thermal infrared detectors.

FIELD OF THE INVENTION 
The invention relates to a device for the absorption of infrared radiation, 
which can be produced with CMOS technology materials and methods. Thermal 
infrared detectors based on thermopiles are manufactured and used to a 
significant extent as inexpensive detectors. The conversion of radiant 
energy of the infrared radiation into thermal energy takes place in 
so-called infrared absorbers. 
BACKGROUND OF THE INVENTION 
The most widely used infrared absorbers are black metal layers (e.g. Au, 
Ag, Pt), produced by deposition under a given foreign gas pressure. The 
very high and uniform infrared absorption is obtained through the porosity 
of the layers. The disadvantage of these absorber layers is their 
sensitivity to mechanical and chemical influences, so that the production 
of a clearly defined absorber geometry using CMOS technology processes is 
only possible by deposition via a resist mask, which leads to limitations 
in the technology. Another process for producing metal layers is 
electrodeposition. The problems of this process are inter alia ionic 
impurities of the electrolyte, the stressing of the disk-like support of 
the metal layers (e.g. silicon wafer) during electrolysis and that 
auxiliary planes are required for the electrodeposition of films. At 
present this is not a CMOS technology standard. 
The literature refers to the possibility of infrared absorption by thin 
metal layers and .lambda./4 layers (e.g. A. Hadni and X. Gerbaux, Infrared 
and Millimeter Wave Absorber Structure for Thermal Detectors, Infrared 
Phys., vol. 30, No. 6, 1990, pp 465-478). By means of thin metal films it 
is possible to produce a uniform absorption for wavelengths above 1 .mu.m, 
but which amounts to max 50%. The reflection losses on the absorber 
surfaces can be reduced with antireflection layers (.lambda./4 layers). 
However, the absorptivity of the thin metal layers is very dependent on 
the layer thickness, so that a precise layer thickness control is 
necessary for the production thereof. 
Another group of infrared absorbers consists of organic layers such as 
hydrocarbon blacks and so-called black varnishes. At present, there are no 
CMOS-compatible technologies for the deposition and structuring of these 
layers. 
H. Saha et al, Influence of Surface Texturization on the Light Trapping and 
Spectral Response of Silicon Solar Cells, IEEE Transactions on Electron 
Devices, vol. 39, No. 5, 1992, pp 1100-1106 describes an absorber 
structure, whose surface has pyramidal grooves or depressions formed by 
anisotropic etching in monocrystalline silicon. The disadvantage of this 
structure is that, firstly, a monocrystalline silicon layer must be 
applied for the production thereof, which is a process step requiring 
additional technological expenditure. 
U.S. Pat. No. 4,620,364 discloses solar cells, which have a central 
monocrystalline layer (e.g. silicon crystal), as well as an upper and a 
lower, non-absorbing layer, both of which can be structured. By multiple 
reflection of the infrared radiation between the upper and lower layers, 
the optical path length of the radiation in the absorbing central layer is 
increased and, consequently, the solar cell efficiency is improved. 
Such an arrangement cannot be appropriately applied to a support body (e.g. 
a membrane), as would be necessary for use as a thermal detector. As 
stated hereinbefore, increased technical expenditure is involved in the 
application of a monocrystalline layer, e.g. of silicon. 
SUMMARY OF THE INVENTION 
The problem of the present invention is consequently to provide a device 
for converting infrared radiation into thermal energy for thermal 
detectors, which has a large bandwidth and high absorption and which can 
be simply produced with known technologies. 
According to the invention this problem is solved by the infra-red 
radiation absorption device, hereinafter called absorber structure, for 
converting infrared radiation into thermal energy for thermal detectors, 
comprising a support body; a lower layer applied on said support body; a 
central layer located on the lower layer; an upper component located on 
the central layer, said upper component facing incident infrared 
radiation; wherein said lower layer absorbs a portion of the incident 
radiation to be converted which is transmitted by the upper component and 
the central layer, said lower layer reflecting a remaining portion of said 
infrared radiation into the central layer; wherein said central layer has 
a high absorption coefficient for the radiation to be converted and 
wherein the upper component is used for absorbing both the radiation to be 
converted which is applied from above said upper component as well as 
stray radiation received from the lower layer; wherein said upper 
component reflects back into the control layer any stray radiation 
reflected by the lower layer which is not absorbed by said upper 
component; and further wherein said upper component has a low reflectivity 
for the infrared radiation to be converted which is applied from above 
said upper layer. The device according to the invention comprises three 
basic components. The lower component is a layer with a low transmission 
coefficient for the infrared radiation to be absorbed. It absorbs part of 
the incident radiation and reflects the unabsorbed part back into the 
components positioned above it. The central component is formed by a layer 
with a high absorption coefficient and a low reflection coefficient for 
the radiation to be absorbed. It mainly brings about absorption and serves 
as a spacer between the upper and lower components. The upper component is 
used for the absorption of the radiation striking the device from above 
and the stray radiation from the lower components and reflects the 
unabsorbed radiation from the lower absorber components back into the 
absorber structure. The upper component must be designed in such a way 
that the radiation striking said component from above is reflected to the 
minimum extent. Preferred developments of the upper component are further 
described herein. By multiple reflections of the incident infrared 
radiation between the individual components of the device according to the 
invention its total absorptivity is advantageously increased. The device 
according to the invention has absorption over a wide wavelength range. 
The device according to the invention can be produced with CMOS technology 
materials and methods without restriction. As an example of this reference 
is made to the embodiment. Thus, the production process can, in 
advantageous manner and without additional costs, be integrated into a 
CMOS cycle. The continuous, inexpensive production of thermal infrared 
detectors in CMOS technology, e.g. on the basis of thermopiles is 
consequently rendered possible. 
According to a further development of the device according to the invention 
the upper component comprises depressions prepared in the central layer 
and whose walls are coated with metal. These depressions can e.g. be 
arranged in matrix-like manner. In this construction the upper component 
acts on the incident radiation like an array of cavities. Multiple 
reflection of the radiation into the cavities increases absorption, so 
that the absorptivity of the individual cavity is above that of the wall 
material used (metal layer). 
A further embodiment of the upper component of the device according to the 
invention is also described herein. Thus, the upper component is formed by 
an aluminum layer applied to the central layer and which has through 
depressions (through openings), whose depth is only limited by the 
underlying, central layer. The depressions can e.g. be arranged in matrix 
or honeycomb manner. On the aluminum layer is deposited a thin oxide 
protective film, which does not change the surface structure of the upper 
component. 
A preferred development of the device according to the invention is 
obtained by the exclusive use of CMOS technology materials and methods. 
Thus, as described hereinbefore, integrated, thermal infrared detectors 
can be inexpensively manufactured. 
In an advantageous development of the device according to the invention, 
the lower layer is constituted by a 1 .mu.m thick aluminum layer. The 
central layer is an oxide layer with a thickness between 2 and 8 .mu.m. 
The upper component is formed by a matrix of depressions in the central 
layer, whose walls (i.e. also the bottom) are covered with a 500 nm thick 
aluminum layer. The edges of the depressions have rectangular 
cross-sections with an edge length of 2 to 4 .mu.m and are arranged in 
matrix-like manner with a reciprocal spacing of 2 to 10 .mu.m. 
An advantageous embodiment of the device according to the invention, is 
provided in which the depressions in the central layer are filled with a 
planarization oxide (e.g. SiO.sub.2). When using the production variant of 
the means according to the invention described in the first embodiment, 
this embodiment is technologically caused. The presence of the 
planarization oxide in the depressions advantageously leads to additional 
damping and therefore to an increase in the absorption of the incident 
radiation. 
The invention is described hereinafter relative to two embodiments and the 
attached drawings, wherein show:

DETAILED DESCRIPTION OF THE DRAWINGS 
An embodiment for the infrared radiation absorption device according to the 
invention is shown in cross-section in FIG. 1. The lower component is here 
formed by a 1 .mu.m thick aluminum layer 1. The central layer 2 is an 
oxide or nitride layer with a thickness between 2 and 8 .mu.m. The upper 
component 3 comprises depressions (with a depth in the .mu.m range), whose 
walls are covered with a 500 nm thick aluminum layer. The edges of the 
depressions have rectangular cross-sections, as is shown in plan view in 
FIG. 2, with an edge length of 2 to 4 .mu.m and are arranged in a 
matrix-like manner with a reciprocal spacing of 4.8 to 8.8 .mu.m. In the 
depressions, there is a planarization oxide 4, which is for technological 
reasons. FIG. 2 also shows the edge 5 of the lower aluminum layer. 
The technology for producing the absorber structure of this embodiment will 
now briefly be explained. Onto a silicon wafer is sputtered the 1 .mu.m 
thick aluminum layer and on it is deposited by a CVD process the 2 to 8 
.mu.m thick oxide layer (SiO.sub.2). By a plasma etching process 
depressions are etched by means of a resist mask, which corresponds to 
contact window technology. Following residual varnish removal, a 500 nm 
thick aluminum film is sputtered onto the structure. This is followed by 
the deposition of a planarization oxide (e.g. SiO.sub.2), which is etched 
back plasma-chemically, so that the planarization oxide is only present in 
the depressions. The planarization oxide covers the aluminum layer in the 
depressions, whereas it is exposed between the depressions. In this way 
the planarization oxide forms the etching mask for the subsequent aluminum 
etching process, after the performance of which the aluminum of the upper 
component is only present on the walls of the depressions. 
An absorption spectrum of the absorber structure of the embodiment is shown 
in FIG. 3, where the absorptivity of the absorber structure is plotted as 
a function of the wavelength of the incident radiation, It can be seen 
that the production of an infrared absorber with a wide absorption 
bandwidth is possible with the absorber structure according to the 
invention without any CMOS technology restrictions. 
FIG. 4 shows the relationship of the infrared radiant power absorbed by the 
absorber structure of the embodiment to the radiant power absorbed by a 
black body as a function of the temperature of a transmitter with the 
emission coefficient=1. The efficiency of the absorber structure according 
to the invention is consequently largely independent of the transmitter 
temperature, 
FIG. 5a diagrammatically shows another embodiment of the structure 
according to the invention, The different layers of the absorber structure 
are applied to a silicon membrane 6 as the support body. The lower layer 1 
is an aluminum layer with a thickness of approximately 500 nm, the middle 
layer 2 a SiO.sub.2 layer with a thickness of approximately 750 nm and 
which has been applied in the manner described in the first embodiment to 
the silicon membrane 6. The upper component 3 comprises an approximately 2 
.mu.m thick aluminum layer 7 and a superimposed SiO.sub.2 protective film 
(not shown in FIG. 5a) having a thickness of approximately 750 nm. The 
aluminum layer has a honeycomb structure with through depressions 8 
(holes) (not shown in FIG. 5a). The honeycomb structure of the aluminum 
layer 7 is shown in FIG. 5b with the through depressions 8. The 
application of the aluminum layer of the upper component to the middle 
layer takes place by sputtering. For producing the honeycomb structure, 
the layer is masked in a conventional manner and RIE-structured. The 
SiO.sub.2 protective film is deposited by CVD.