Patent Application
20150093706
The present invention relates to the field of recording an image on a medium using photo-lithography, and more particularly to the field of methods for storage of an image.
Storage of an image will here designate the storage of a reproduction of this image for the purposes of reading it later, using transparency reading. In general it involves a reproduction of this image with smaller dimensions, often as a single copy.
Throughout the text, the term “image” means any graphic representation such as a photograph, text, a table, a drawing etc.
The invention also relates to the field of image media made using such methods.
Many methods for recording an image on a medium using photo-lithography are known in the prior art. Here the field of storage of an image will be more specifically described, although the invention that is described below can be extended to the wider field of recording of an image using photo-lithography.
It is usual for images to be stored in digital form on storage media such as computer hard disks and recordable digital optical disks (CDs ROMs, DVDs etc.). Such media, however, have a limited lifetime, typically of the order of tens of years. Moreover, coding formats may become obsolete and prevent the image from being recovered from stored digital data.
Various methods offering long-term storage solutions with small overall dimensions are known in the prior art. These methods involve carrying out a screening (or rasterizing) of the image, then using maskless photo-lithography to etch a layer of opaque material deposited on a transparent substrate to reproduce the screened image in smaller dimensions. A storage medium is thus obtained wherein an image is stored in analogue format. An example of such an image storage method will be found in document EP 2 105 921.
The screening step involves converting an image in grey levels, into a black and white image, made of a matrix of cells of the same dimensions called “screening cells”. The term “grey level” can also mean the gradations of intensity of a colour. For example frequency modulation screening is known wherein each cell is homogeneous: entirely black or entirely white. Amplitude modulation screening is also known wherein each cell contains a shape of variable size recorded in white on black or in black on white.
Maskless photo-lithographic etching is carried out by direct writing using a laser, with no step involving masking of predefined zones. Maskless photo-lithographic etching can involve direct etching: the opaque material is photosensitive and the laser beam is applied directly on the layer of opaque material.
The storage medium can be read using a low-pass filter which eliminates fine variations in the screening structure below the scale of the cell, so as to retain only changes averaged at the scale of a few cells. The storage medium may be read, for example, using simple means of optical enlargement. The storage medium 31 is lit by a light source 32. An optical system 33 images the storage medium 31 on a detector 34, such as a matrix detector. The optical system 33 has an optical transfer function which introduces optical blurring similar to a Gaussian spot with a width (or “waist”) equal to a few pixels, typically two or three. The magnification of the optical system 33 is such that the size of a screening cell corresponds to the size of a pixel of the matrix detector 34. The dark hues coded in the storage medium 31 relate to the opaque layer and the light hues relate to the openings which are etched in the opaque layer and which expose the surface of the transparent substrate. The image 35 is obtained on the matrix detector 34.
If the image is in colour, several reproductions are stored which correspond to the decomposition of the image into three components, for example:
For the opaque layer the existing methods typically use a layer of platinum oxide PtOx with a thickness of about 60 nm, where x is in particular a real number between 1 and 2. Such a platinum oxide layer has a transmission of less than 5% for a minimum opening size which is equivalent to a disk of diameter 1.5 μm.
These methods exhibit limitations in cases where it is desired to store high-definition images. These high-definition images are made, for example, using new generations of digital photo equipment.
For example, a high-definition image is coded as 8 bits: each pixel of the image can take a grey level from amongst 256 (28) available grey levels. In order to code this many grey levels in the storage medium, the known solution is that one pixel of the image is equivalent to large number of screening cells. A drawback of such a solution is that the surface area of the zone of the storage medium corresponding to this-high definition image increases.
One objective of the present invention is to propose a method for recording a source image which does not exhibit at least one of the drawbacks of the prior art.
In particular, one objective of the present invention is to propose a method for recording a source image using photo-lithography, which allows compact writing of source images to be achieved.
In particular, one objective of the present invention is to propose a method for recording a source image using photo-lithography, which allows compact writing of high-definition source images, typically with a definition of at least 8 bits, for example of 16 bits, to be achieved.
Another objective of the present invention is to propose an image medium obtained using such a process.
The present invention is defined by a method for recording a source image wherein a reproduction of the source image is made in the form of a matrix of elementary patterns, the method comprising the following steps:
The method according to the invention moreover includes the following steps:
Throughout the text, a layer of opaque material, or opaque layer, means a layer whose transmission coefficient is less than that of the first substrate. This transmission coefficient may be zero, seeing that it allows the elementary pattern obtained by the superimposed layers to be formed. The transmission coefficient is defined for visible wavelengths, preferably between 400 nm and 800 nm.
Preferably, the recording method according to the invention stores a source image in a storage medium comprising a first transparent substrate and a second transparent substrate. According to such a method:
The first and the second opaque layers preferably have respective degrees of opacity which differ from one another. The term degree of opacity means the extinction level or extinction coefficient, that is, the inverse of the transmission level or coefficient. The degree of opacity is preferably defined for visible wavelengths, preferably between 400 nm and 800 nm.
The first and the second opaque layers preferably have different thicknesses and are formed of the same material. Thus first and second opaque layers are made which have respective degrees of opacity which differ from one another.
Alternatively, the first and second opaque layers may each consist of a different material. These materials advantageously exhibit different light transmission properties. Thus first and second opaque layers can be made which have the same thickness and with respective degrees of opacity which differ from one another.
The method according to the invention may comprise an initial step for coding of the source image, using frequency modulation screening or amplitude modulation screening, where the screening uses elementary patterns according to the invention.
According to one advantageous embodiment:
According to one alternative of this embodiment:
The method according to the invention, when it carries out storage as described above, can comprise the following steps:
Alternatively, the method according to the invention, when it carries out storage as described above, can comprise the following steps:
Alternatively, the method according to the invention, when it carries out storage as described above, can comprise the following steps:
Alternatively, the method according to the invention, when it carries out storage as described above, can comprise the following steps:
Thus in each of these alternatives the deposition of the second opaque layer is carried out after etching of the first opaque layer.
The etching of the second opaque layer may be carried out before or after deposition of the second opaque layer on the first etched opaque layer.
The method according to the invention, when it carries out storage as described above, can comprise the following steps:
Preferably, an image to be stored is broken down, or decomposed, using a HSV decomposition to obtain a “hue” component, a “saturation” and a “light intensity” component, the “hue” and “saturation” components are recorded in the high-definition storage zone, and the “light intensity” component is recorded in the low-definition storage zone.
The method according to the invention advantageously comprises a step of transparency reading (or in other words transmission reading) of the reproduction of a source image, comprising the following steps:
The invention also relates to an image medium wherein a reproduction of a source image is stored in the form of a juxtaposition of elementary patterns, said image medium comprising, on a first substrate, a first opaque layer etched over its entire thickness using photo-lithography and forming a matrix of first cells each having one from amongst several predetermined first patterns. The medium according to the invention has the following characteristics:
The image medium according to the invention can form a source image storage medium and:
The first opaque layer and the second opaque layer may have different thicknesses, with each thickness being between 10 nm and 200 nm.
Each first cell and each second cell can be fully etched or not etched.
Alternatively, each first cell and second cell may be fully etched or not etched, or etched so as to form a closed transparent area in a cell with opaque edges or a closed opaque area in a cell with transparent edges, closed areas formed in various first cells having different dimensions, closed areas formed in different second cells having different dimensions.
In one preferred embodiment, the image medium is a storage medium and it comprises:
The invention also relates to a system comprising an image medium according to the invention and means for transparency reading (or in other words transmission reading) of the reproduction of a source image located in this image medium.
The means for transparency reading comprise a light source located on one side of said image medium, and a detector located on the side of the image medium away from that of the light source.
The present invention will be better understood on reading the description of example embodiments given purely as an indication and in no sense restrictively, making reference to the appended illustrations in which:
First of all, with reference to
The specific case wherein the recording method according to the invention forms a storage method has been chosen for description. The image medium made may then be called the “storage medium”.
The recording method according to the invention comprises a first step 401, for the deposition of a first opaque layer 42 on a first transparent substrate 41. At this stage, the first opaque layer 42 forms a matrix 43 of first cells 430 which are all identical and opaque.
During a second step 402, etching of the first opaque layer 42 is carried out using photo-lithography.
The etching using photo-lithography may involve maskless etching, in particular in the context of storage of a single copy of a source image.
Alternatively, etching using photo-lithography may involve etching using a mask. Etching using a mask uses etching of a set of patterns in a source mask. As many copies of the set of patterns as desired are reproduced on substrates by projections through this mask. This alternative is advantageous for the reproduction of several copies of an image. This, for example, will be the case for product marking applications for the purpose of accurate identification of the product's manufacturer.
A matrix 44 of first cells 440 is thus formed, each cell having one from amongst several predetermined patterns. The first opaque layer is etched over its entire thickness.
In a third step 403, at least one second opaque layer 45 is deposited, so that this second opaque layer 45 is placed between the first opaque layer 42 and a second transparent substrate 46, and this second opaque layer is etched. One can also speak about the creation of at least one second opaque layer, so that this second opaque layer 45 is between the first opaque layer 42 and the second transparent substrate 46. The second transparent substrate 46 acts as a protective layer for the etched opaque layers. The method according to the invention is not limited to a given order for the steps for deposition of the second opaque layer 45 and etching of the latter, as the following examples show. The second opaque layer 45 is also etched using photo-lithography (with a mask, or by a maskless method). The second opaque layer is etched over its entire thickness. This etching forms a matrix 47 of second cells 470, each cell having one from amongst several pre-determined patterns. The superimposition of the matrix 44 of first cells onto the matrix 47 of second cells forms a matrix 48 of cells 480 each having an elementary pattern formed by the superimposition of a second pattern on a first pattern. At least one elementary pattern is formed by the superimposition of a second pattern on a first pattern, said second pattern being different from the first.
The matrix 48 is a matrix of elementary patterns and is equivalent to a reproduction, in smaller dimensions, of a source image, where this source image has undergone screening.
Thus it can be seen that for a given cell size, the invention allows a greater number of elementary patterns to be obtained which are available to build a reproduction, with smaller dimensions, of the source image. Thus all grey levels of a high-definition image could be coded using a limited number of cells. Thus compact storage of a high-definition source image, typically with a definition of at least 8 bits, can be achieved.
It should be noted that this effect is achieved without it being necessary to have recourse to etching methods or tools which have greater precision than in the prior art. It should also be noted that this effect is achieved without it being necessary to have recourse to etching methods or tools which allow areas of smaller dimensions to be etched than in the prior art.
In
The example of etching of a cell by the creation of a circular opening (or disk) in the cell has been taken. This example is not restrictive, and other examples will be listed with reference to
The etching processes use photo-lithographic methods which offer etching resolutions in an opaque layer of the order of μm.
Multi-level screening or multi-layer screening can also be used to describe the method according to the invention.
Alternatively, the first substrate is not necessarily transparent and the second opaque layer 46 is not covered with a second transparent substrate. This alternative is equivalent to a recording of an image on a medium using photo-lithography. Here also the method according to the invention offers excellent image definition, for a smaller recording area. This variant is advantageously combined with the use of etching using photo-lithography with masking.
The first transparent substrate and the second transparent substrate each consist, for example, of a layer of quartz or of sapphire. The first transparent substrate and the second transparent substrate can also be made of a flexible plastic material or a rigid plastic material such as those used to make optical disks. The thickness of the first and second transparent substrate is of the order of nm, for example less than 1 mm and in particular equal to 0.7 mm.
Each opaque layer consists, for example, of a metallic layer such as a layer of chromium, a layer of titanium nitride or a layer of platinum oxide PtOx. The thickness of an opaque layer is generally less than 200 nm, or even less than 100 nm, for example equal to 60 nm, 40 nm or 30 nm.
It can be seen that the first non-etched cell 61 has a transmission coefficient which is greater than that of the second non-etched cell 62. The transmission coefficient depends on the thickness of the corresponding opaque layer. For example a layer of thickness 40 nm of platinum oxide has a transmission coefficient of close to 5% whereas a 30 nm thick layer of platinum oxide has a transmission coefficient of close to 10%.
Each first cell 61 is either fully etched or not etched. Each second cell 62 is either fully etched or not etched. A set of four elementary patterns 631, 632, 633 and 634 is thus obtained which correspond, respectively, to the superimposition:
These four elementary patterns 631, 632, 633, 634 define four coding patterns available in the context of frequency modulated screening. The grey level in such a coding pattern corresponds to the transmission coefficient of the associated elementary pattern.
An example of frequency modulation screening is Floyd Steinberg type error distribution dithering. This involves comparing, for each pixel in an image, its grey level with the grey level of two coding patterns (black or white). The image is coded by associating one of the coding patterns with each pixel. In order to do this the following are taken into consideration:
This principle is readily adapted to a number N of coding patterns, where N is greater than 2. Known software such as “Gimp” allows Floyd Steinberg type error distribution dithering over N coding patterns to be carried out, where N is greater than 2.
It will be seen that a pixel of the source image can be divided into several zones to be associated with one coding pattern.
Each opaque layer is etched so as to create a predetermined checkerboard.
Each first cell 61 is either fully etched or non-etched, or etched so as to draw a transparent closed area in a cell with opaque edges or a closed opaque area in a cell with transparent edges.
In
A set of four elementary patterns 641, 642, 643 and 644 are thus obtained which are equivalent, respectively to the superimposition:
In practice non-circular openings could be envisaged, for example ovals. An opening is considered to have a circular or oval shape without taking edge defects associated with photo-lithographic etching into consideration. Opaque shapes on a transparent background could also be envisaged. Each cell could moreover be etched using openings of different dimensions, for example disks of different diameters. The solutions could be varied between one layer and another. In order to etch an opening in a cell with opaque edges, laser impacts could be carried out by displacement movements of the laser beam starting from the centre of the cell. In order to etch a cell so as to obtain an opaque closed area in a cell with transparent edges, laser impacts could be carried out by displacement movements of the laser beam on the edges of the cell.
The four elementary patterns 641, 642, 643, 644 define four coding patterns available in the context of amplitude modulated screening. The grey level in such a coding pattern is equivalent to the mean transmission coefficient for the associated elementary pattern.
In the context of amplitude modulation screening, each pixel is coded using the coding pattern whose grey level is closest to the grey level of said pixel.
It will be seen that a source image pixel can be divided into several zones to be associated with one coding pattern.
The advantage of amplitude modulation screening over frequency modulation screening is that there is a direct link between a source image pixel and the associated elementary pattern.
According to one advantageous embodiment:
This embodiment will be called “embodiment with cross-polarisations of opaque layers”. An example of such an embodiment is shown in
The same is observed in the case of etching using photo-lithography with a mask.
As stated with reference to
It can be seen that the same reasoning applies in the case of etching of a cell in order to achieve an opaque area in a cell with transparent edges. The cell is etched by etching all around the opaque area and the thickness of the opaque layer is then associated with a maximum size of the opaque area in a cell with transparent edges.
The invention, in that it proposes superimposing at least two opaque layers, provides a clever way of reconciling these two criteria. Using a first thin opaque layer, openings with small dimensions can be etched. As a result of the superimposition of at least two opaque layers, very low transmission coefficients can be achieved for the layer stack, for example of less than 5%. Preferably, different thicknesses are chosen for each of the opaque layers according to the invention.
The explanations above relate more specifically to the case of amplitude modulation screening, but these advantages are also found in a context of frequency modulation screening. According to
For example, take two opaque layers. The first opaque layer exhibits a transmission Ta. The second opaque layer exhibits a transmission Tb. In each cell of the first opaque layer a disk of radius ra is etched, centred on the cell. In each cell of the second opaque layer a disk of radius rb is etched, centred on the cell. Each cell is a square with a side of length Λ. For each cell, the superimposition of the second opaque layer on the first opaque layer will correspond to a grey level Γab (mean transmission coefficient of a cell where the two layers a and b are stacked), then:
The grey level corresponds to cell areas weighted by the corresponding transmission coefficients, with the whole relative to the total surface area of the cell.
The following could be chosen, for example:
The reasoning is similar in the case of the recording of an image on a medium using photo-lithography.
The steps relating to the creation of a library of elementary patterns suitable for a desired screening dynamic range will now be described. In other words, let us take an image wherein each pixel can exhibit one from amongst 256 different grey levels, where the grey levels are regularly spaced from black to white, and it is wished to have available a library of 256 elementary patterns, such that this regular spacing of the corresponding grey levels is achieved, going from white up to black.
Wider openings may be etched using one or more elementary displacement movements of the laser beam relative to the cell 80.
For a given cell, therefore, a set E of possible openings is obtained which correspond to a number Nmsk of possible first patterns:
In the case of a cell with sides of length 2 μm and a method giving a minimum opening which corresponds to a disk of diameter 800 nm, an address matrix with step intervals equal to 100 nm would give access to a maximum of Nmsk=349 first patterns. These data are given as indications only. In practice the number of first patterns available is much smaller than the value given by equation (2), in particular since the etched opening dimensions cannot be accurately incremented in scale with the step intervals δ of the address matrix, and the edge effects on the etching correspondingly reduce the precision of manufacture.
According to the invention, use is made of:
As a result of the superimposition of the two opaque layers a and b, the number of elementary patterns available is:
Nmsk
ab
=Nmsk
a
×Nmsk
b (3)
For example, Nmska=349, for the dimensions Λ, δ and ω cited above. Nmskb=371, for the same dimensions Λ and δ, and for a value of ω which is slightly smaller (thickness of opaque layer b less than that of the opaque layer a). This then gives Nmskab=129,479. Even when allowance is made for the actual precision of the etching, the calculations show that superimposing the two layers allows dynamic range levels reaching 16 bits to be achieved.
Detailed below is how, starting with a number P of elementary patterns that are actually available (allowing for the etching edge effects for example), a library of elementary patterns is organised, suitable for a given screening dynamic range. As seen earlier, the elementary patterns actually available depend on the dimension Λ of a cell, on the step intervals δ of the address matrix and on the radius ω of the minimum opening. They are also dependent on:
An opening in the form of a disk gives, for example:
where:
and
In
In
If the error in the grey level of an elementary pattern exceeds a pre-determined error threshold, this elementary pattern is removed from the library of elementary patterns shown in
It can be seen that, as shown in
It can be seen that in the case of a frequency modulation screening, there is no point in creating a library of elementary patterns. For an image screened using frequency modulation over 2n grey levels, these grey levels are made using superimposition of n opaque layers.
We will now describe, with reference to
In the first embodiment shown in
In parallel or sequentially, the second transparent substrate 46 is covered with the second opaque layer 45. The second opaque layer 45 is etched using photo-lithography (step 116).
Then the assembly formed from the second transparent substrate 46 and the second opaque layer 45 is deposited and bonded onto the first opaque layer 42 so that the second opaque layer 45 is placed between the first opaque layer 42 and the second transparent substrate 46.
In the alternative shown in
In another alternative of this first embodiment, shown in
It should be noted that in this first embodiment, the second opaque layer 45 is turned over in order to bond it onto the first opaque layer. This must be taken into consideration when etching the second opaque layer 45.
Currently available bonding devices offer a precision of alignment for this last bonding step which is of the order of ±1 μm, which means that a square cell with sides of length Λ equals to at least 2 μm can be used (see
According to this second embodiment, the first transparent substrate 41 is covered by the first opaque layer 42. The first opaque layer 42 is etched using photo-lithography (step 120).
Then a layer of transparent material 121 is deposited on the previously etched opaque layer (step 122). This transparent material is, for example, silica SiO2. Thus a flat surface is obtained on the side of the transparent material away from the first layer of opaque material 42.
A new opaque layer 45 is deposited on the transparent material 121. The new opaque layer 45 is etched using photo-lithography (step 123).
The method then comprises a final step 124 of bonding the second transparent substrate 46 onto the previously etched opaque layer 125. As for the first embodiment, this last bonding step can be carried out using resin or by molecular adhesion.
Steps 122 and 123 form a cycle which may be repeated as many times as desired. In other words, this second embodiment of the method according to the invention has at least one cycle comprising the following steps (with the previously etched opaque layer being initially the first opaque layer):
This embodiment exhibits the advantage of allowing more than two opaque layers to be superimposed.
Moreover, the alignment of the opaque layers relative to each other is facilitated by the fact that the etching of an opaque layer is carried out after it is deposited on the lower opaque layer or layers. In effect, the alignment precision of the etching is better than that of the bonding. Precisions of 200 nm are commonly achieved using commercially available laser writing equipment.
Alignment also depends on the coefficient of thermal expansion a of the transparent substrates, such that:
ΔL=L·α·ΔT, where L is the width of the substrate, ΔT the variation in temperature during the operation of the method according to the invention and ΔL the variation in the substrate width resulting from the temperature variation ΔT.
For example, for a storage medium of diameter 200 mm, the expansion of the substrates remains less than 1 μm on condition that the temperature is stabilised at close to 0.7° C. for sapphire, at close to 0.5° C. for glass and at close to 10° C. for quartz. The clean rooms which use the lithography and bonding equipment have a stabilised and controlled temperature. The substrate support plates guarantee thermal uniformity to +/−0.5° C. It can be seen, therefore, that these allowable temperature variations are in accordance with what is technically feasible.
The method shown in
Bonding is carried out, for example, using a resin 133. Alternatively it is carried out using molecular adhesion. The intermediate transparent substrate 131 is, for example, a film of glass with a thickness of 30 μm to 100 μm.
According to this fourth embodiment the first transparent substrate 41 is covered by the first opaque layer 42. The first opaque layer 42 is etched using photo-lithography (step 140).
The method then comprises step 142 for deposition of a new opaque layer 45 directly onto the previously etched opaque layer. The new opaque layer 45 is then etched using photo-lithography (step 143).
The method then comprises a final step 144 of bonding the second transparent substrate 46 onto the previously etched opaque layer.
Steps 142 and 143 form a cycle which may be repeated as many times as desired. In other words, this fourth embodiment of the method according to the invention has at least one cycle comprising the following steps (with the previously etched opaque layer being initially the first opaque layer):
In order to simplify the description, reference will be made to a high-definition storage zone 151 and a low-definition storage zone 152, but in reality there are several of these: a low- or high-definition storage zone corresponds to one source image and a given medium holds several reproductions of source images.
Also seen in
Portion 155 represents the main part of the medium, with the thickest opaque layer. Here there are a documents storage zone 1551, a reference zone 1552 (title of the medium, table of contents etc.) and calibration zones and alignment cross-wires 1553.
The reference zone 1552 does not have high-definition information, and therefore it does not need to use the method according to the invention. The document storage zone 1551 may also comprise low-definition data which can be processed by a single opaque layer method according to the prior art. These zones, which do not have any high-definition information, are not found in the second part 156 of the storage medium. The processing of the second part 156 of the storage medium can thus be speeded up.
In the case where a colour image must be stored, this is broken down (or decomposed) into three components, with each component being stored independently. It may be envisaged that some components are stored using a single opaque layer, as in the prior art, and that other components are stored using at least two superimposed opaque layers by a storage process according to the invention.
For example, an image is broken down on the basis of the HSV or LAB decomposition methods. The chromatic component or components determine the dynamic range of the image colour. Thus the choice will be made to store the chromatic component or components using a method according to the invention in the high-definition storage zone 151 of the storage medium. The other components are stored in the low-definition storage zone 152 of the storage medium.
For example an image to be stored is broken down using LAB decomposition to obtain a light intensity component, a first chromatic component and a second chromatic component. The first and second chromatic components are stored in the high-definition storage zone 151 and the light intensity component is stored in the low-definition storage zone 152.
An image to be stored could also be broken down in accordance with HSV decomposition in order to obtain a “hue” component, a “saturation” component and a “light intensity” component. The “hue” and “saturation” components are stored in the high-definition storage zone 151 and the “light intensity” component is stored in the low-definition storage zone 152.
One component could also be favoured by providing a reduced reproduction of this component of the image, using a lower reduction factor.
More accurately, a source image and an image obtained after optical detection of its representation in the form of elementary patterns are compared. The optical detection is that described with reference to
The abscissa axis corresponds to the width wg of the Gaussian relative to the size of the cell.
The ordinate axis corresponds to the mean square error in relation to the source image.
The source image is a grid of grey level tonalities made up of 20×20 zones of different greys randomly distributed in space and within the dynamic range 0 to 255. The zones have an extent of 3×3 pixels; that is, an image to be coded of 60×60 pixels.
Graph 161 corresponds to a storage method according to the prior art which uses a single platinum oxide layer of thickness 45 nm and amplitude modulation screening which uses circular openings in opaque cells. The step interval of the addressing matrix is 0.02 μm. A square cell with sides whose length Λ is equal to 2 μm is used (see
Graph 162 corresponds to a storage method according to the invention which uses two platinum oxide layers of thickness 45 nm and 30 nm and amplitude modulation screening which uses circular openings in opaque cells. Here again the address matrix step interval is 0.02 μm and a square cell, the length Λ of whose sides is equal to 2 μm, is used. The minimum error is about 0.6%, that is about 3 times less than the error obtained with the method according to the prior art under similar conditions.
This figure does not take the detection dynamic range, which is also improved by the method according to the invention, into consideration.
The invention is not limited to the embodiments that have been described. In particular, all forms of openings etched in an opaque layer, other opaque layer thicknesses, and other materials forming the transparent substrates or the opaque layers can also be envisaged.
For some of the figures relating to the invention, the case of superimposition of two opaque layers has been shown. These examples can of course be extended to the case where more than two opaque layers are superimposed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 13 59398 | Sep 2013 | FR | national |
