GB2181567A - Infra-red fresnel lenses and methods of fabrication - Google Patents

Abstract

In the prior art, infra-red lenses have been fabricated from germanium and zinc selenide, and these have shown to possess useful optical characteristics whilst also being robust. However, the commercial cost of design and fabrication of such lenses is considerable. To reduce the design and fabrication costs, Fresnel-type lenses of certain polymer materials have been made. Such lenses generally have unacceptable optical characteristics and lack thermal and mechanical stability. To overcome these problems, an infra-red lens having a Fresnel-type structure fabricated from an infra-red transparent elemental or compound semiconductor material such as silicon is proposed. A method of fabrication of such lens includes a step of impressing a desired lens structure into the surface of a wafer of the infra-red transparent material by means of a light controlled etching technique.

Description

SPECIFICATION Improvements in and relating to lenses The present invention relates to lenses and more particularly to a lens structure for an infra-red lens and to its method of fabrication.

Three classes of optical material are commonly employed in windows and lenses for thermal detection and imaging devices. Thus the semiconductors germanium and silicon, Il-IV materials such as MgF2, CaF2, MgO, ZnS and ZnSe, and the various chalcogenide glasses all find some application within the wide spectrum of infra-red system (reference J.M. Lloyd, Thermal Imaging Systems, Plenum Press 1 975r Chapter 6). However to date, out ofthis considerable list of materials, only germanium (in polycystalline orsingle cyrstalform) and zine selenide (chemical vapour deposited) find real commercial exploitation in the fabrication of precision infra-red lenses.Both these materials possess the high refractive indices required to minimise lens curvatures and hence lens thickness, together with low infra-red absorption coefficients and adequate long wavelength cut-offs required of lenses that are typically between about 2 and 1 Omm in thickness. The cost of optical quality germanium is relatively high, and, being a strategic material of rather low natural abundance its price has risen significantly over recent years. This, together with the high costs of optical design and lens fabrication, means that germanium infra-red lenses are of considerable cost.Zinc selenide finds less application than germanium being mechanically less robust, but is employed where its very wide transmission range, low absorption and freedom from thermal runaway (a problem in relatively narrow bandgap semiconductor materials such as germanium) are demanded, for example in high power carbon dioxide infra-red laser work.

Some attention has been paid recently to methods to reducethe cost of infra-red optics, but has generally resulted in an unacceptable optical performance. Thus for example certain polymer materials, such as poly thene,which possess acceptable transmission in the 8-14 lim infra-red band when used at lowthicknesses, have been examined as alternatives to germanium. In order to achieve adequate transmission and yet obtain the relatively high lens curvatures demanded for short focal length lenses in the low index polymer materials, a Fresnel lens structure has been employed.While the polymer material costs are very low and mass production moulding techniques can be employed to produce lenses at low unit cost, the optical performance and thermal and mechanical stability of these polymer Fresnel lenses are generally inadequate forspecialist applications. The low refractive indices of polymer materials demands steep zone angles and largezone numbers in a Fresnel lens structure, thus limiting lens resolution and increasing zone edge losses. Polymer materials also possess very high thermal expansion coefficients, and a large variation of refractive index with temperature thus giving poor stability of optical properties over a range oftemperatures.

An object of the present invention is to provide a lens structure, lens material and methodoflensfabrica- tion which has the potential to produce a very significantly lower cost lens for infra-red applications whilst retaining useful lens performance.

According to the present invention there is provided an infra-red lens having a Fresnel type lens structure fabricated from an infra-red transparent elemental or compound semiconductor material.

According to another aspect of the present invention there is provided a method of fabricating an infra-red lens having a Fresnel type lens structure the method including the step of impressing the desired lens structure into the surface of a wafer of the infra-red transparent elemental or compound semiconductor material by means of a light controlled etching technique. In a preferred method offabrication the light controlled etching technique includes a photoelectrochemical etching process.

Other suitable light controlled etching techniques suitable for performing the fabrication include uv iightor excimer laser controlled photochemical or "photolytic" etching of the material in a reactive gas atmosphere.

In a preferred embodiment ofthe present invention the infra-red transparent material is silicon.

In other embodiments of the present invention the infra-red transparent material is germanium, gallium arsenide, gallium phosphide, indium phosphide and ternary orquaternary composed Ill-V semiconductors.

Preferably the infra-red transparent material is in single crystal form. This enablesthetransferofthedes- ired Fresnel lens structure into the material by an etching process without the potential problems associated with preferential grain boundary etching and etching anisotropy related tothe use of polycrystalline mat- erial.

High purity, single crystal silicon of well controlled impurity and defect content is readily available at reasonable cost, in the form of wafers up to 1 50mm diameter and typically 0.4to 0.6mm thickness, as the starting material forthe silicon integrated circuit industry. The optical,thermal and mechanical properties of silicon are very significantly superiorto those of the polymer materials. In particularthe high refractive index ofsilicon (n= 3.42) means that the shallow zone angles and modest zone numbers would be required in a Fresnel lens structure, thus enhancing the potential lens resolution and minimising zone edge effects.Silicon has not generallyfound application as an infra-red lens material to date because of its high absorption coefficient relative to germanium. However, if silicon were to be used in a Fresnel lens structure, then the low material thickness would ensure adequate infra-red transmission. Provided the silicon purity and resistivity were selected so that the absorption coefficient over the 8 to 14 lm band did not exceed approximately 2cm then absorption losses would be less than 10% for a typical silicon wafer thickness. Silicon materials that exceed this specification are available. Optical components in both silicon and germanium require single or multilayer antireflection/filtercoatingsto counteract the surface reflection losses inherent in the use of a high index material.

The basic physical and optical properties of germanium and silicon, but excluding the infra-red absorption characteristics which can vary considerably depending upon doping and impurity levels, are tabulated below Property Germanium Silicon Atomic No. 32 14 AtomicWt. 72.60 28.09 Densityg/cc 5.323 2.3291 Hardness,Knoop 692-850 1150 ElasticModuli c11 = 128.6 C11=167 GN/m2 c12=48.3 c,2=65 c44=66.8 c44=80 Melting point 938 1410 Expansion coefficient 5.7 at 20"C 2.6 at 20"C ppm/K 6.2 at 127"C 3.2 at 127"C Specific heat 0.314 at 17 C 0.71 at 22 C J/g.K 0.348 at 107 C 0.77 at 100 C Thermal Conductivity 0.67 at 0 C 1.68 at 0 C W/cm.K 0.465 at 100 C 1.08 at 100 C Relative Permittivity 16 11.7 Refractive Index, 4.015-5 Fm 3.4221 - 5,am n at 20 C 4.003-10 Fm 3.4177 - 10 Fm 4.0001-15pm Temp. Coefft. of n 3.96 x 10-4/K 1.5 x 10-4/K dn/dT Temp. Coeffi. ofn 4.13x10-4/K 1.56x10-4/K optical path dn/dT+ a(n-1) Bandgap,eV 0.67 1.11 IR transmission 2.23 1.5-15 range It can be seen in particular that silicon possesses a higher hardness, elastic moduli, melting point, specific heat, thermal conductivity and optical bandgap than germanium.On the other hand the density,thermal expansion coefficient, permittivity, refractive index, temperature coefficient of index and optical path are lower for silicon than for germanium. These property differences can be broadly understood in terms ofthe lower atomic mass of silicon and the stronger atomic bonding in the silicon lattice. The superior mechanical and thermal properties and the higher stability ofthe optical properties of silicon are considered to be advantages in the present context. The infra-red refractive index of silicon, at3.42, is stili sufficiently high to provide low lens curvatures, as discussed later.The lower mass and stronger binding in silicon are also evident in the reduced infra-red transmission range, the long wavelength cut-off being defined by the lattice phonon frequencies, v, which are, in the simplest case, given by an equation oftheform: vcu(F.M)M A.

where F isth atomic force constant and M is the atomic mass.

The infra-red absorption properties of silicon and germanium in the infra-red transparent range, are significantly affected by free carrier absorption. The infra-red absorption properties of germanium have been discussed in detail (Wood et al, GEC Journal ofScience and Technology, Vol.48, No.3,1982, pp 141 to 151 ).The intrinsic carrier concentration, nj, in germanium at room temperature is realtively high at 2.4 x 1013cm-3, as a consequence ofthe relatively narrow bandgap (0.67eV). This is equivalentto a resistivity of about 50Q.cm, and an intrinsic infra-red absorption coefficient at 10 Fm of 0.03cm-1.The intrinsic absorption coefficient is a sensitive function oftemperature. The addition of, for example, germanium or antimony to dope the germ- anium either p or type, can be used to control and stabilise the absorption coefficient. The infra-red adsorption crosssection for electrons is considerably smallerthan that for holes 10,um, and this meansthatdeliberate n-type doping can be used to suppress the hole population and actually reduce the absorption coefficient, while at the same time providing a fixed carrier concentration (and hence a fixed absorption coefficient) over a range oftemperature. This temperature range of stable absorption, known as the 'exhaus- tion range', increases with increasing dopant level, and a useful balance between absorption coefficient and exhaustion range is obtained at a resistivity of a bout 3fl.cm. This resistivity provides an absorption coefficient of 0.02cm-1, remaining below 0.1 cm-1 attemperatures up to 77"C (350K).

The infra-red absorption properties of silicon have been widely studied, since the infra-red absorption spectrum is used to monitor doping and impurity levels in silicon wafers for the semiconductor industry.

Commercially single crystal silicon for semiconductor use is grown by the Czokralski (CZ) or by the Float Zone (FZ) methods (T. Ravi, Imperfections and Impurities in Semiconductor Silicon, John Wiley & ons, 1981). The former process provides silicon with resistivities up to about 25fl.cm (carrier concentrations of 0.3 to 1 x 1015cm3), and which also contains oxygen and carbon impurities. The inherent free carrierabsorp- tion coefficient for n-type CZ silicon at 10 ;;im, for a carrier concentration of 1 x 1015cm -3, is slig ht below 0.1cm-1,withthe phonon-related absorption coefficient contribution of about 0.4cm-1. Interstitial oxygen in CZ silicon is of particular note since this gives rise to an absorption band at 9.1 Fm. The strength of thins absorption band increases linearly with oxygen content, reaching a value of a = 4cm- at the oxygen saturation content of silicon (1.8 x 1 013cm-3). Atypical CZ ingot will contain 6to 8 x 1017cm-3 of oxygen, equivalent to an absorption coefficient of 1 .6cm-1.This value is below the a=2cm-l limit considered to be accept able for a thin (0.5mm) silicon Fresnel lens. Thus, taking into account both the intrinsic phonon absorption and free carrier absorption characteristics over the 8-14 pUm band, lightly doped CZ silicon material proves acceptable for the proposed application.

As with germanium, the infra-red absorption cross-section in silicon for electrons is smallerthan thatfor holes and thus n-type silicon is preferred for infra-red applications. This is fortunate, since n-type material is required to be able to conduct wafer surface profiling by the photoelectrochemical (PEC) or photolytic etching processes referred to later.Indeed at low carrier concentrations it has been found that the free carrier absorption coefficient for n-type CZ silicon decreases from 1 to 0.1 cm- as the carrier concentration falls from 10'6to 1 015cm-3, whereasthe coefficient for p-type CZ material plateaus at 1cm- for carrier levels below 1016cm-3. An infra-red transmission spectrum for a 4.8 Q.cm n-type CZ silicon wafer double polished to a thickness of 0.34mm is presented in Figure A.1. The average transmission level over the 8-14 Fm band is about 95% of the theoretical figure, allowing for reflection losses. A shallow absorption bond at 9.1 ,am can be seen.The 9.1 m oxygen absorption band can be suppressed by heat trnatments which precipitate SiO2 and hence reduce the interstitial oxygen level, although this is atthe expense of producing dislocation loop defectsthat mayaffectthe PEC etching process.

Higher resistivity silicon (up to 200 flcm, carrier concentration approximately 2 1 013cm-3),which has very significantly reduced oxygen and carbon impurity content, may be produced by the FZ process. An n-type FZ silicon material with a carrier concentration of about 1 x 1015cm-3, is probably close to the ideal silicon material. As with germanium, the provision of a fixed dopant level provides a stable carrierconcentration and hence a stable free carrier related absorption coefficient over a range oftemperature. However, in the case ofsilicon, the significantly wider bandgap (1.1eV) provides a substantially greater exhaustion range than is the case for germanium (0.67eV).Thus an n-type FZ silicon material at a carrier concentration of 1x 1015cm-3 should provide a free carrier absorption coefficientof about 0.1 cm- at 10 ism, which remains below 0.2cm-' fortemperatures up to about 250"C. The phonon contribution is about O.4cm- at 10 clam, rising to about O.6cm- at 250"C. This is a very distinct advantage of silicon over germanium.It should be noted that the intrinsic carrier concentration for silicon at room temperature is 1.45 x 1 010cm-3; 3 orders below that ofgermanium. For this reason the initial suppression of the intrinsic absorption coefficient on low level n-type doping of germanium, is not found in the case of silicon, since the lowest practicable doping level in silicon will totally swamp the intrinsic carrier concentration and indeed at low dopant levels the phonon contribution to the absorption coefficient becomes dominant. Should uniformity of dopant level prove crucial to the optical properties or PEC etching behaviour ofthe silicon, then neutron transmutation- doped (auto-doped) silicon could be employed.

The lens structure concerned is that of the Fresnel lens, in which a conventional lens is effectively divided into a series of thin zones and then reconstructed by displacing all the zones into one plane, on an appropriate thickness of supporting material. Several different designs of Fresnel lenses are possible in which the surfaces of each zone are plane orcurved (spherical section and aspherical section curves), and with a variable number of zones. Fresnel zone surfaces may be defined on either or both surfaces of the lens material to produce the Fresnel equivalents of conventional plano-convex, plano-concave, convex, concave and meniscus lenses. The point of particular note in the context ofthe present invention is that such lenses are of low thickness.

The use of a material efficient lens structure, likethe Fresnel iens, allows a thin lens to be achieved and thus allows materials to be employed that have higher infra-red absorption coefficients than would be required for more traditional lens thicknesses. Thus silicon, which can have an infra-red absorption coefficient that is largely below2cm- overthe8-14 band, provided the doping level and impurity content aresuitably controlled, can be employed in a thin lens structure (below about 1 mm thickness) without excessive loss of radiation through absorption. High Purity Germanium, with an infra-red absorption coefficient of about 0.03cm-1, is essential for more conventional lens thicknesses.Thus the use of a Fresnel lens structure permitsthe use of a higher absorption material like silicon. Silicon is widely available in suitablewaferformwith precisely controlled defect dopant and impurity content as the starting material for the semiconductor IC industry. Silicon is also very significantly cheaperthan germanium.

What is required to complete this concept is a method of impressing the desired lens structure into the surface of the silicon wafer. Photoelectrochemical (PEC) etching, in which the depth of material removed by the etching process in any given location is controlled by the local visible or UV light intensity incident atthat location, is a suitable technique. The PEC technique is applicable to n-type doped semiconductors, including Si, Ge, GaAs, InP and ternary and quaternary Ill-V compound semiconductors. The semiconductor,for ex- ample silicon, is immersed in an electrolyte and biased to a potential where the etch rate is proportional to the light intensity. The image of a photomask is projected onto the surface ofthe wafer to produce a spatial variation of light intensity to etch the desired shape.Such a mask may comprisefor exampleveryfine bars and spaces ofvaried mark-space ratios. The fine scale ofthe mask (submicron), combined with demagnification, slight defocus of the mask during projection onto the wafer orthe deliberate exploitation of chromatic aberration with a white light source, effectively provides a smoothly varying light intensity at the wafer.

The combination of the use of low cost silicon wafers, with a Fresnel lens structure and the PEC etching technique for defining the lenses provides a novel method for fabricating low cost infra-red lenses with a useful performance.

Lenses for infra-red applications in the 8 to 14 ism are band are produced in single and multielementform, using both spherical and aspherical surfaces. Aspheric and multielement lenses tend to be found where aperture ratios greater than about F/2 are required. Single element focussing lenses, of the meniscus or plano-convexform, are used for aperture ratios up to F/2. Popular lens diameters range from about 15 upto 50mm, with focal lengths from 25 up to 250mm.The Fresnel lens equivalents of all these single element lens types, meniscus, plano-convex and aspheric, can, in principal, be produced by PEC or photolytic etching, providing sufficient control of the illumination profile and the PEC process can be achieved.However a simple worked example, for a plano-convex lens, is sufficientto illustrate the principles involved.

To take for example a plano-convex lens which is required to give a focal length of 50mm x 2%, with a lens diameter of 50mm. Five such lenses could be obtained from a 1 50mm diameter silicon wafer, such as wafer of 10 Q.cm double polished FZ material.

Using the standard thin lens formula 1/f= (n 1) ( ) B.1 r1 r2 wheref is the focal length, n is the refractive index, and r1 and r2 are the radii or ofcurvature of the lens surfaces, we have n = 3.4177 for silicon at 10 Fm f=50mm r2 = - which gives r1 = 170.9mm forthe radius of the convex surface. We now convert this plano-convex lens (minimum central thickness = 1 .83mm) to a Fresnel lens, by dividing the lens into a series of slices ofthickness t, and displacing these slices into a single plan on a supporting thickness of material, as illustrated in figure B.1.It may be shown that, for small values oft, the lens curvature, etched deptht andthe outer radius of the nth zone xn, are related by the simple expression x, -(2nt.R)% B.2 we may calculate the Fresnel lens geometry for different etch depths, as tabulated below Etched depth, Fresnel Fm 20 50 100 lens geometry Total no. ofzones in 50mm 91 36 18 diameter lstzone diameter, mm 2.61 4.13 5.85 Spacing ofoutermostzones, 0.137 0.342 0.684 mm.

Published work on the PEC etching of silicon has shown that etch depths of over 20 CLm are possible with a smooth surface, free from pitting. A 20 m etch depth provides a reasonable number of zones, 91, with the narrowest zones still morethan 10 wavelengths in width. the small step at each zonaedge (2 2 wavelengths) will minimise zone edge losses and obscura- tion for off-axis objects. With PEC etching the etch depth does not need to remain constant across the lens. Variants where, for example, the depth increases towards the lens perimeter would be possibleandwould reduce zone numbers. thefinal PEC Fresnel lens thickness is now determined solely bythe requirements for mechanical integrity.Thus, comparing the example lens, with a waferthickness of say 0.6mm,with the original plano-convex lens (thickness 1 .83mm), a material saving of at least a factor of 3 is obtained.

One particular advantage of using a high index material in a Fresnel lens structure arises from the (n- 1)factor in equation B.1.Thisfactor is 3 for germanium and 2.4forsilicon, compared with 0.5 for a typical polymer material. For a given Fresnel zone thickness or depth,t, a polymer lenswouldtherefore require 5to 6timesthe numberofzones of a silicon or germanium lens. Conversely, for a given number of zones, the polymer material would require a similar increase on zone depth. In either case a significant increase in zone edge losses and obscuration results forthe polymer material.

The tolerance on focal length in the example lens, at + 2%, is equivalentto a similar percentage tolerance on etch depth, i.e. +0.4 pWm in 20 pwm. It is suggested that optical inter ference techniques, using, for example, a laser interferometer, could well be used to provide in-situ monitoring ofthe development ofthe Fresnel lens profile. The PEC process is, in effect, a 'non-contact optical machining' process which would allow such monitoring, through a beam splitter, by appropriate periodic interruption of the etch process. Control to better than one half wavelength of visible light (i.e. < 0.3 im), should be possible.

For a PEC etch that is simply linearly related to light intensity we may showthatthe illum- ination profile in the nth zone should follow an equation of the form 09 x2 T(x)=(0.1 + [--(n-1)t]] B.3 t 2R wherex lies between xn 1 and x, (i.e. in the nth zone),t, n and Rare as defined earlier, the maximumtransmission is taken as unity, and the minimum masktransmission is taken as 0.1 The spherical aberration of the example lens is poor, but this is dominated by the choice of a plano-convex lens of large aperture. This aberration is largely related to lens form and aperture, and is relatively insensitive to material optical properties.A memiscus lens, with R2 = 1.5 R1, would give a more acceptable spherical aberration and coma figure for apertures up to F/2. The chromatic aberrntions on the other hand arecontrolled by the material's optical properties. Thus a lens fabricated in silicon will show chromatic aberration of a factor of 3 lowerthan an equivalent lens in germanium, since the refractive index of silicon varies more slowlywith wavelengths overthe 8-11 am band.

Lenses with curvature on both faces can in principal be fabricated by the PEC technique, provided a suitable front-to-back alignment technique can be established. A potentially suitable alignment technique would involve the etching of alignment holes rightthrough the silicon slice prior to PEC etching of the two surfaces, using anisotropic etching processes based on KOH or ethylene diamine-pyrocatechol-water (EDP) etchant solutions. These selective, anisotropic etching techniques, and appropriate masking techniques, are well established. Again the PEC process should also be suitable for defining aspheric Fresnel lens surfaces, with attendant gains in aberration control, providing the mask definition and etching processes are sufficiently controllable.

The general principles of PEC etching of n-type semiconductors have been outlined above. The purpose of the description below is to briefly review some of the relevant work on the PEC etching of silicon and germanium, and to indicate other relevant literature.

The Anodic dissolution of n-type silicon under illumination has been studied by Sharpe and Lilley (J.

Electrochem, Soc. Vol. 127 No.9, September1980, p. 1918). Theirworkwas essentially concerned with developing a method for measuring carrier concentration profiles of eptaxial silicon structures by a combination of anodic dissolution and in-situ C-V measurements. This involved establishing an electrolyte and suitable operating potentials for (1) smooth and flat dissolution to depths in excess of 20 Fm to allow controlled profiling and (ii) interface conditions approximating a Schottky barrierso that meaningful C-V measurements could be made. In the present context the former criterion is of particular interest. The electrolyte system studies by Sharpe and Lilley was the NaF: H2SO4: H2O system.The anodic V-I characteristics showed a saturation currentfor both p and n-type silicon, the saturation current increasing with increasing NaF concentration and with illumination level for the n-type material. Currents below the saturation level gave a pitted etch, together with traces of a darkfilm. However samples etched in the saturation region were fond to be con siderably smoother, particuarly under strong illumination, with electrolytes with the higher acid content giving the smoothest etches with few pits or spike on the surfaces. It was also apparent that electrolytes with a high sodium fluoride content gave a flatter etched surface in terms of gross uniformity, and an electrolyte comprising 1 M NaF: 0.05M H2SO4was selected as a reasonable comprise. Uniform, deep etches (210 im) were possible, provided the electrolyte was pulsated during the etching process.

The dissolution depth, w, in the PEC process is given by

where M and D the molecularweight and density ofthe semiconductor, A is the interface area, N is the dissolution valcence, F is the Faraday and I is the current passed in the interval dt.

Since in the case of a PEC etched Fresnel lens the wafer is uniformly doped throughout its thickness, this equation becomes M M I I C.2 D.N.F. A The dissolution valence for silicon was estimated to be 3.50 -i- 0.3 for standard cell conditions (cell potential 1 .0V, 0.05M H2S04: 1 M NaF). Under suitably intense illumination, current densities of between 1 and 1 0mA/ cm2 can be readily obtained, corresponding to silicon dissolution rates of between 1.3 and 13 Fm/hr. Sincein principal the PEC etching process can be made fully automatic, as have PEC profilers for the assessment of epitaxial layers, this is considered a useful etch rate.The work by Sharpe and Lilley indicates that a PEC etching technologyforfabricating Fresnel lenses in silicon is possible.

Similar work has been reported in the PEC etching of n-type germanium, although without details ofthe etched surface topography (Ashok K. Vijh, Electrochemistry of Metals and Semiconductors, Marcel Dekker Inc. New York, 1973, Chapter, p.64). The material employed was 3flm n-type germanium,which was anodically dissolved in a 0.1 N NaOH electrolyte under 0.4 to 0.7 Fm visible illumination. The characteristic satura- tion current was observed for increasing anodic bias, this current being proportional to illumination level.A maximum current density of 1 OmA/cm2 was achieved.It was reported thatthe dissolution valency of the germanium was a function ofthe electrolyte and varied between 2 and 4, indicating that both holes and electrons are involved in the overall dissolution process.

Some ofthe original and pioneering work on PEC etching of semiconductors was conducted byAmbridge, Elliot and Faktor in 1973 (Journal Applied Electrochem. 3, 1973, pp 14to 15). These authors were concerned with the electrochemical characterisation of n-type GaAs and, in addition to much useful experimental data, published a useful model ofthe PEC phenomena from which the photo-current density can be related to the illumination level, the depletion region width, the minority carrier diffusion length, the surface potential barrier, and the optical absorption coefficient at the visible wavelength of interest. The closest agreement of theory and experiment, the widest range of voltages over which a well defined saturation current plateau was observed, and the best resulting etched surface finish, was obtained atthe lowerst dopant levels. These observations, which should also applyforthe PEC etching of silicon, are encouraging since low dopant levels, corresponding to a low infra-red absorption coefficient, are required for the present application. An example of experimental data from the Ambridge work, showing excellent linearity of saturation currentwith illumination level, is presented in Figure C.1.

Claims (9)

1. An infra-red lens having a Fresnel type lens structurefabricated from an infra-red transparent elemental or compound semiconductor material.

2. An infra-red lens according to claim 1, wherein the infra-red transparent material comprises silicon.

3. An infra-red lens according to claim 1, wherein the infra-red transparent material comprises one or more of the elements/compounds germanium, gallium arsenide, gallium phosphide, indium phosphide and ternary ofquarternary composed Ill-V semiconductors.

4. An infra-red lens according to claim 1, claim 2, or claim 3, wherein at least part of the infra-red trans parent material is in single crystal form.

5. A method offabricating an infra-red lens according to any one of the preceding claims, wherein the method includes impressing a desired lens structure into the surface of a wafer of the infra-red transparent elemental or compound semiconductor material by means of a light controlled etching technique.

6. A method according to claim 5, wherein the light controlled etching technique includes a photoelectrochemical etching process.

7. A method according to claim 5, wherein the light controlled etching technique includes uv lightor excimer laser controlled photochemical or "photolytic" etching of the infra-red transparent material in a reactive gas atmosphere.

8. An infra-red lens substantially as hereinbefore described.

9. A method of fabricating an infra-red lens substantially as hereinbefore described.