GB2274658A - Optical arrangement for recording, storing and reading of microstructure information - Google Patents

Description

2274658 Optical Store The invention relates to thin layer arrangements f

or recording, storing and reading information, suchas alpha-numeric and graphic information, which is coded in the f orm of lateral microstructures. The invention is also applicable in the manufacture of optical systems such as microscales, microraster scales, microtemplates and microstructure elements of integrated optical systems and other constructional elements whose operational principles are based on lateral microstructures.

Devices for recording, storing and reading information in the form of microstructures of differing two-dimensional is extent are known in numerous embodiments. Their basic principle of operation is based on the optical irradiation of absorbent layers using radiation pencil beams having sections corresponding to the structure breadths or diameters, whereby the irradiation produces, by way of photothermal (and/or photochemical) energy conversion processes, lateral inhomogeneities. These, when read, can be detected optically and registered as deviations in the optical parameters with respect to the non-irradiated regions. The deviations might for example take the form of holes, small bubbles or zones having otherwise modified characteristics.

Optical information processing by recording, storing and reading of such microstructures is preferably effected by optical-sequential radiation techniques with monochromatic radiation, particularly laser radiation.

Photothermally or photochemically active layers are applied together with further optically active layers and protective coatings to a rigid or flexible disc, and the 2 registration of the microstructures on the rotating disc is effected by intermittent irradiation.

Microstructure storage plates or discs-of this type can store information at very high density (for example, an optical SY4 inch storage disc) has the storage capacity for an amount of alpha-numeric information contained on 500,000 A4 pages in the "DRAW" mode, where (DRAW = "Direct Read After Write", that is to say: having the ability to play back without technological intermediate steps). Such densities have already been achieved in the "Write-Once" modification, (allowing single writing by the user): this modification is of great 'Significance, particularly for high value archiving requiring rapid access, in science and technology, in political and manufacturing economics, in sanitation and in numerous other fields.

Previously known technical solutions have serious disadvantages in that the system characteristics which are crucial for a high operational effectiveness - namely a high optical conversion efficiency, high optical signal contrast between the microstructure information and the surroundings and absence of wear of the structure information during reading and long-term stability of the stored microstructures - cannot overall be realised in an optimum manner. In the past, the optimisation of each of these important system characteristics has been achieved only by abandoning the optimum level of one of the other characteristics.

In particular, it is not possible with previously proposed solutions to obtain the desired high optical conversion efficiency and also the desired high optical 3 contrast when resonance absorber materials are used, and at the same time to maintain a high chemical and mechanical stability and resistance to wear under radiation loading. This has a particularly disadvantageous effect because the optical conversion efficiency is determines the radiation intensity that can be used and thus the rate of production of the microstructures. The conversion efficiency in turn affects the constructional expense of the storage device.

The optical signal contrast determines the information flow rate and the bit.error rate when reading.

The object of the invention is to provide thin layer arrangements for recording, storing and reading is microstructure information which improve the optical conversion efficiency for the photothermal (and/or photochemical) energy conversion process, and at the same time maintain an optical signal contrast near to 100-.

with recording layer materials which are stable and mechanically and chemically resistant.

The arrangement in accordance with the invention preferably comprises at least one radiation- absorbing, photothermally (and/or photochemical ly) reactive layer which is enclosed between a first layer structure and a second layer structure. The first layer structure in front of the absorbing layer, allowing substantially unimpeded penetration of radiation into the absorbing layer. The second layer structure, behind the absorbing layer, blocks radiation from passing from the absorbing layer into an underlying substrate. The absorbing layer (the resonance absorber layer) comprises any desired absorbing substance, preferably of a material which is mechanically and chemically resistant and whose optical 4 properties remain stable both in the inoperative and in the operative state.

The entire sequence is preferably arranged as a thin layer resonance absorption system, whereby the absorbent, photothermally (and/or photochemical ly) reactive layer acts as a resonance absorber, the first layer acts as the resonator front wall in accordance with the known principles of resonance absorption, and the second layer acts as the resonator rear wall. The result is that substantially all the.optical radiation impinging on the system penetrates into the absorbing layer, remains concentrated in it and is completely absorbed by it.

is The energy conversion of optical radiation into heat, which takes place due to the absorption, and the temperature increase associated therewith produces differing structural and/or chemical changes in the absorbing layer, depending upon its particular properties. Changes might include for example local area erosion (ablation), material accumulations, small bubbles, changes in texture, discoloration, oxidation or other material phase and modification changes. These changes represent the stored microstructure information and can be optically read as micro-inhomogeneaties in a known manner.

In contrast with known layer arrangements for producing, storing and reading microstructure information, the arrangement in accordance with the invention realises a substantially complete energy conversion process in any desired absorbing layer and at any desired small layer thickness D,. This thickness has only to satisfy the requirement that it is not less than the known "threshold layer thickness" for reproducible physical characteristics. The arrangement in accordance with the invention may be distinguished from the prior art by the fact that a high optical conversion efficiency can be achieved even with layer materials of the highest resistance and stability. The photothermal and/or photochemical efficiency is desirably made as high as possible. The initiation of a photothermal effect requires a temperature increase AT which is equal to the difference between the ambient temperature and the melting temperature T. of the absorbing layer material.

This is proportional to the amount of heat AQ supplied and inversely proportional to the mass m of material to be photothermally converted and to the specific heat c of the absorbing layer material. The connection between the quantities is:

AT,AQ/(c.m) with m = -6.S.drI, where 6 = density of the absorbing layer material, S radiation beam cross-section (which determines the structure breadth) and dr = absorbing layer thickness.

Due to the proportionality of AQ and the absorption A in the interior of the absorbing layer at a predetermined microstructure breadth, and with T.T. for the photothermal conversion effectiveness of the store, it follows that Ek (A/d,) / (c. 6 - T.).

The conversion effectiveness of the store for the absorbing layer material in question and thus for the fixed material parameters c, 6 and T. is therefore linearly dependent only on the "optical conversion efficiency" A/d,. d., whereby d. is the threshold layer thickness for the absorbing layer material in question.

6 The essence of the invention may alternatively be defined by the fact that almost 100-0. absorption can be obtained even when using an absorbing layer of the smallest thickness which is physically and - technologically possible, and even when that layer is of a substance having a high stability.

Thus the relevant characteristics are on the one hand the ,absorption density" A/d, and on the other hand the physical and chemical parameters of the absorbing layer material. Preferred materials are those which have high long-term stability and whose threshold layer thickness d. is as small as possible.

is In order to reduce the expense, the material to be used for the absorbing layer should preferably have an absorption coefficient k, greater than 0.1, and more preferably greater than 1, in the spectral range which contains the resonance wavelength X,. It is further convenient for reasons of high conversion effectiveness to choose an absorbing material which has as low as possible a specific heat and density, and as low as possible a melting temperature.

After the material and the thickness d., of the absorbing layer have been determined, the pseudo refractive index Y1 = Re Y1 + j Im Y1 in the front surface of the absorbing layer is calculated with the aid of the known formula Y1 = (K21 + K22-y2U(K11 + K12-Y2), which links the pseudo refractive index in one plane of a layer system with the pseudo refractive index in a second plane. The first plane is referenced with the subscript 1, and the second with the subscript 2. The coefficients Kik signify in known manner the elements of the layer matrix for the 7 region between the two planes, when the absorbing layer has an imaginary substrate of refractive index Y2 = 0 placed behind it.

The second layer structure, mentioned above,which is arranged behind the absorbing layer and which def ines the rear resonator wall, comprises an impermeable layer (preferably a metal layer) to which at least one non absorbent interference layer is applied. Alternatively, it may consist of a plurality of non-absorbent interference layers with alternating high and low refractive indexes. The thicknesses may be calculated with the aid of the pseudo refractive index formula, given above. The second layer structure has, at its is front surface adjoining the absorbing layer, a pseudo refractive index Yh whose real portion Re Yh is small in comparison to 1 (and is preferably smaller than 0.1) and whose imaginary portion is Im Yh = -Im Y,, whereby Y, is calculated from the above formula. With Re Yh small and Im Yh - - Im Y:, the radiation f lux through the rear surf ace of the absorbing layer disappears. The pseudo refractive index Y, at the front surface of the absorbing layer becomes approximately real (Im Y, - 0), thus providing substantially reflection-free matching to the first layer structure.

The invention may be carried into practice in a number of ways, and three specific embodiments will now be described by way of example, with reference to the drawings, in which:

Figure 1 shows the complex Y plane with reference to a fictional example in accordance with an embodiment of the invention. It is shown that the arrangement is not 8 sensitive to slight deviations; Figs. 2, 3 and 4 show the pseudo refractive index during the development of the layer system construction f or a first embodiment, Fig. 5 shows the spectral variation of reflection, transmission and absorption for the complete first exemplary embodiment, Fig. 6 shows the spectral variation of reflection, transmission and absorption for a second exemplary embodiment, is Fig. 7 shows the pseudo refractive index for a layer system construction corresponding to a third exemplary embodiment, Fig. 8 shows the spectral variation of reflection, 20 transmission and absorption for the third exemplary embodiment. Ist Exemplary Embodiment In this exemplary embodiment a thin layer resonance absorption system is described which is constructed, for example, for a resonance wavelength X, = 632.8 nm. Aluminium was used as the absorbing layer material. It is known that a thin closed-pore oxide skin forms on an aluminium surface exposed to the atmosphere. This skin 30 reliably protects the metal from further chemical influences in the normal atmosphere despite its extremely small (optically-insignificant) thickness. Furthermore, aluminium is very suitable for the microstructuring process at the desired low irradiation intensities: it 9 has relatively low density and relatively low melting temperature, both of which are favourable.

The boundary layer thickness of aluminium produced in conventional coating methods under technologically defined conditions is known to be about 3 nm. In the exemplary embodiment the absorbing thickness d, was also 3 nm.

For the purpose of microstructure production by means of optical -sequential irradiation, aluminium layers with thicknesses of the order of 50 to 100 nm have been previously proposed: in the previous state of the art with thin aluminium layers only extremely small optical is conversion efficiencies could be achieved. Due to this high thickness (10 to 20 times higher than the exemplary embodiment proposed here) and the resulting low conversion effectiveness, absorbing layers of aluminium have not previously been much used in the manufacture of optical stores in spite of their apparently favourable characteristics.

As explained above, the second layer structure is arranged after the absorbing layer and acts as the rear resonator wall to block radiation flux; in other words transmission of the radiation flux through the absorbing layer is substantially prevented. In order to optimise the optical characteristics of this second layer structure, the pseudo refractive index yh of the front surface, adjacent the absorbing layer is, as described above, determined so that the real portion Re Y. is as small as possible (and is preferably smaller than 0.1).

In addition, the imaginary portion Im Yh should preferably have a value which results in the pseudo refractive index Y, at the front surface of the absorbing layer being such that matching between the absorbing layer and the first layer structure is optimised. This may be determined by any conventional means. As experience- has shown, this matching process is in general particularly simple and clear if Y, lies in the complex Y plane on the real Y axis or in its vicinity.

The imaginary portion Im Yh is, as described above, calculated using the known formula for the relationship of pseudo refractive numbers. Figure 2 shows the result in the complex Y plane where the 3.5 = thick absorbing layer is initially arranged behind an imaginary zero substrate (Y = 0). The value adopted at the front surface of the absorbing layer for the imaginary portion of the pseudo refractive number Y (Im Y. - 1.356) is transferred with approximately the same absolute value, but with the opposite sign, to the imaginary portion of the pseudo refractive number Yh of the underlying second layer structure: Im Yhs- - Im Y-1.356.

In order to fulfil the radiation flux blocking function of the second layer structure by reducing the real portion of the pseudo refractive number Yh, this structure is initially constructed as an approximately zero substrate (Y-0). This occurs in the described exemplary embodiment in a known manner by means of an interference layer which is applied to a radiation-impermeable aluminium layer having a thickness which compensates for the phase angle shift occurring at the aluminium surface at the resonance wavelength X, In the exemplary embodiment this is realised by a magnesium fluoride layer of thickness d.=98 nm. A further approximation of the pseudo refractive number to the value Re Y=0 in the exemplary embodiment occurs by additionally applying a high refractive index and a low refractive index interference layer having an optical thickness XJ4 (Figure 3).

The transposing of the imaginary portion of the pseudo refractive index Yh from Im Y-O to the calculated value Im Yh occurs in the exemplary embodiment by an additionally applied T'02 layer with a thickness d-23 nm calculated from the above formula for pseudo refractive index relationships.

The top of Figure 4 shows the characteristic of the pseudo refractive index Y, at the front surface of the absorbing layer as this layer is applied with an increasing layer thickness d, to the rear second layer structure having pseudo refractive index Y.. With the absorbing layer thickness d, = 3.5 nm, Y,-0.5. The residual reflection at the first layer structure from the dr (having index 1) is R-11; which is cancelled out by a front MgF2 layer and an S'3M4 layer, having thicknesses X,/4. The transmission of the radiation flux through the rear surface of the absorbing layer is limited in the described exemplary embodiment to a very low value of about 50k. The total residual losses of the arrangement without a first layer structure mounted in front for reflection-free matching to the air, or whatever is in front of the absorbing layer, may be seen from the bottom of Figure 4.

Figure 5 shows the spectral characteristics of reflection, transmission and absorption for the complete exemplary embodiment and shows that at the resonance wavelength X, = 632.8 nm the absorption in the active 12 layer (the absorbing layer) increases to about 0.95 (95%) so that the absorption thickness A/d, of the preferred embodiment reaches the very high value of 0.27/nm.

The reflection-free matching to the overlying or surrounding atmosphere or material is not limited to air, and is not limited to layers with optical thicknesses of XJ4. The matching can instead be carried out in a known manner by a practically unlimited plurality of layer 10 combinations. 2nd Exemplary Embodiment: one of the crucial characteristic features, which distinguishes the invention from the previously known is prior art, is the construction of the second layer structure which is arranged after the absorbing layer. On the one hand this acts as the rear resonator wall to block radiation flux losses through the absorbing layer; on the other it transposes the pseudo refractive number 20 at the front surface of the resonance absorbing layer into a complex refractive number range from which substantially reflection-free matching to the air, or whatever is in front of the absorbing layer, (glass, plastics,...) and/or to thin protective layers is rendered possible. The construction of this second layer structure is not limited to the structure described in the first exemplary embodiment. Thus, for instance, in order to maintain 30 transmission losses as small as desired, further X/4 alternating layers can be provided in front. The transposing of the imaginary portion of the pseudo refractive index into the front surface of the second layer structure can also be achieved by suitable 13 dimensioning of all the interference layers of the second layer structure, with all layers having the same optical thicknesses.

Where the layers are thermally unsuited, thermal losses by condution into the deposited metal can occur which are non-negligible and which can reduce the overall effectiveness of the arrangement. Such losses can be demonstrated both by measurement and calculation. Such losses can be prevented or reduced by the use of one or more interference layers in the second layer structure of a material of low thermal conductivity. It is also thermally and optically favourable to replace the arrangement of the second layer structure of the first is exemplary embodiment by a dielectric interference layer system of layers having alternating low and high refractive indexes; this allows elimination of the impermeable aluminium layer. The second exemplary embodiment realises such a solution. It corresponds substantially to the first exemplary embodiment but includes a metal-free second layer structure which comprises, for example, four layer pairs of M9F2/T'02 having optical thicknesses X,/4 and an additional 23 nm thick T'02 layer which effects the transposition to the calculated Im Yh value.

Figure 6 shows the spectral characteristics of reflection, transmission and absorption for this exemplary embodiment. At the resonance wavelength X, = 632.8 nm absorption of nearly the same amount and thus approximately the same absorption density A/d, as in the first exemplary embodiment are achieved.

14 3rd Exemplary Embodiment This embodiment was also constructed for the resonance wavelength Xr = 632.8 nm. Chrome was used as the material for resonance absorption. Chrome layers are known to be very stable over long periods of time and extremely resistant both mechanically and chemically. A f ew of the material characteristics of chrome are, however, unfavourable for high conversion effectiveness. In particular, the relatively high melting temperature is a reason why chrome was previously not considered for widespread use in connection with microstructuring processes with optically sequential irradiation at low irradiation intensities. Mixed layers of chrome and other materials have previously been proposed but very high layer thicknesses had to be deposited. Since the present embodiment succeeds in achieving high optical conversion efficiencies even with very low chrome layer thicknesses, the disadvantage caused by the high melting temperature can be substantially compensated for by the achievable high absorption thicknesses A/d, The threshold layer thickness for technologically reproducible chrome layers is known to lie under 3 nm.

In the exemplary embodiment the absorbing layer was deposited with a thickness of 3 nm.

Due to the optical parameters of the chrome (real refractive index n absorption coef f icient k), with a system having Im Yh -0 and a small Re Yh (e. g. less than 0.1) the pseudo refractive indexes of thin chrome layers are already in the region of the complex refractive number plane from which substantially reflection-free matching with the air etc. is possible. The second layer structure of this exemplary embodiment was thus assembled with Yh -0.02, as in Figure 3. With the absorbing layer is thickness dr = 3 nm the pseudo refractive index Yr -Re Y, -0.35 is obtained (Figure 7). The spectral characteristics (Figure 8) show an absorption above 0.95 (95-0i;) at the resonance wavelength; thus the very high absorption density of A/dr -0.32 is achieved.

The extremely resistant chrome layers thus also become accessible to microstructure production; they have a conversion effectiveness which is close to that of tolurium layers which are optimally suitable for the structuring process but very unstable.

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Claims (1)

1. 6. An optical store as claimed in any one of Claims 1 to 4 in which the

   second layer structure has a pseudorefractive index Yh at its front surface adjacent the absorption layer, where Re Yh is less than 0.1.

   7. An optical store as claimed in Claim 5 or Claim 6 in which Im Y. is approximately the same as the imaginary part of the pseudo refractive index that would correspond to the front surface of the absorption layer if it were to be positioned immediately behind a material of index zero.

   8. An optical store as claimed in any one of the preceding claims in which first layer structure has a is pseudo- refractive index Y, at its rear surface adjacent the absorbing layer, where Re Y, is approximately equal to Re Y.i and Im Y, is approximately equal to Im Y,, and where Yr is defined as the pseudo refractive index at the front surface of the absorption layer. 20 9. An optical store substantially as specifically described with reference to the first, second or third exemplary embodiments.