GB2380475A - Chalcogenide glass - Google Patents

Abstract

Chalcogenide glass forming materials have the formula Ge (x-a) As a Se (100-x-b) Te b where 25 < x & 55; 10 & a & 25; 40 < b & 70, and (100-x-b) > 0. They are low melting, sputter in composition, and can be used to provide thin coatings for covering or joining other optical components, including those useful in the infra-red.

Description

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Materials Having a Glassv Phase The present invention relates to materials having a glassy phase. The materials have compositions comprising germanium, arsenic, selenium and tellurium, and may be regarded as chalcogenide glass compositions.

Glass compositions comprising just these four elements are known, and are discussed in a number of articles, including the following: J A Savage et al, Infrared Physics, 20, pp 313-320 (1980) V Q Nguyen et al, J Am Ceramic Soc, 83 (4), pp 855-9 (April 2000) Guorong Cheng et al, J Am Ceramic Soc, 82 (10), pp 2934-6 (October 1999) V Q Nguyen et al, J Non-Crystalline Solids, 248 (2-3), pp 103-114 (June 1999) A F Maged et al, J Eng and App Science, 45 (3), pp 441-52, (June 1998) G R Chen, Glass Tech, 39 (5), pp 179-82 (October 1998) Jian Xu et al, J Non-Crystalline Solids, 184, pp 302-308 (May 1995) S A Fayek et al, Solid State Comms, 93 (3), pp 213-217 (January 1995) Some, but not all, of the above articles refer to the optical properties of the glasses.

The present invention provides a material having the formula Ge (x-a) AsaSe (1oo-x-b) Teb

where 25 < x : S 55 ; 10 < a < 25 ; 40 < b :'S 70, and (100-x-b) > 0. Materials according to the invention are all relatively rich in Te and/or relatively poor in As compared to the glass compositions disclosed in the above articles.

Our copending UK Patent Applications No. GB (ref : P21440GB) ; GB (ref : P21441GB); and GB (Ref : GB 21442) relate to optical constructions in which elements are bonded together using thin optical glass layers, and to methods of making such constructions. As described in these applications, a thin layer of the glass material is deposited on the joining surface of one or preferably both of the elements to be bonded, and the elements are placed together and subjected to heat and pressure to bond them. Our copending UK Patent Application No. GB (Ref : P21358GB)

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relates to the provision of a glass layer on an optical element for subsequent shaping such as by embossing under heat and pressure.

A glassy material according to the invention has been found to be particularly useful in many such constructions. The choice of optical glass will be determined by a number of considerations, including compatibility with the intended use of the element (for example transmissive in the appropriate wavelength range, including the visible and infrared, close index matching), and the need to process it to bond the optical components together. The latter requirement is reflected in preferred thermal characteristics of the glass, as will be discussed below.

Materials according to the invention can be deposited as thin layers by any known method. Sputtering is a particular method which has been adopted, and it has been found that the materials"sputter in composition", i. e. the composition of the material to be sputtered, and the composition of the material which has been deposited by sputtering are substantially the same. When sputtering onto optical substrates, including crystalline substrates such as non-linear crystals, it has been found that the sputtered materials wet the substrate, are mechanically resilient and are thermally resilient in that they withstand thermal cycling up to 200 C or more.

Furthermore, it is normally possible to be able to select a glass material according to the invention for which the first glass transition point Tg is sufficiently low that the substrate is unaffected by the further processing necessary for joining to another substrate, or for embossing. Above Tg, the material is normally sufficiently soft that it (a) facilitates good bonding or embossing at suitably low pressures; and (b) flows into or round any defects on the substrate surface (s), to provide an excellent optical interface.

Preferably, in the forgoing formula, 30 < x < 45, and preferably 30 < x < 40.

Preferably 50 : : ; b < 70. These further restraints, particularly when used in combination, provide glasses with good thermal characteristics and a good refractive index match to materials such as GaAs or silicon (this will of course depend inter alia on the wavelength involved).

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The invention will be further described with reference to the accompanying drawings, in which: Figure I is a composition map illustrating some materials according to the invention, and selected properties thereof ; and Figures 2 to 5 show differential thermal analysis plots for three different materials according to the invention.

Although much of the following description is made in relation to criteria for the use of the materials in the bonding of optical elements, it should be understood that the invention is not limited to such criteria, not to any such use. Chalcogenide glasses have many other interesting and useful properties, including electrical and thermal properties.

In addition to a glass devitrification temperature Tc, corresponding to a change from a glassy phase to a melt phase or a crystalline phase or to decomposition, many glasses have at least one glass transition temperature Tg where a glassy phase is retained but with somewhat different properties. In particular, heating the glass through a temperature Tg to obtain a higher temperature glassy phase (the reader will appreciate that the phase change may require other conditions, and in particular the phase change may take a significant time) can provide a phase which is appreciably softer or more mobile.

Glass phase transitions may be detected by differential thermal analysis, wherein heat is supplied at a controlled rate to a sample and the temperature of the sample is plotted over time. During differential thermal analysis the temperature initially follows a generally linear plot, and phase transitions are indicated by deviations from linearity. In particular a glass transition temperature Tg may be identified by a discontinuity in the plot, generally in the form of a knee. Further transition points may be identified at higher temperatures, and at least one of these may correspond to the devitrification temperature. The latter may be identified since upon performing the reverse measurement by cooling the sample the corresponding knee is absent or at least does not occur at the same temperature.

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Figure 2 shows a differential thermal analysis plot for the material GelsAs1SSe29 Te41, using the following cycle : 1. Hold at 20. 00 C for 1. 0 minutes 2. Heat from 20. 00 C to 450. 00 C at 10. 00 C/minute 3. Hold for 10.0 minutes at 450. 00 C 4. Cool from 450. 00 C to 20. 00oC at 10. 00 C/minute The formula falls within the wider of the two formula above, but outside the narrowed formula, since b is less than 50.

Two inflection points on the rising part of the curve at 120 C and 240 C are respective first and second glass (glass/glass) transition temperatures Tgl and Tg2.

Steeper transitions Tel and Tc2 at 290 C and 380 C are transitions associated with crystal phases, and the lower of these temperatures, Tel, will be the devitrification temperature since at that point the material ceases to be in a glassy phase. In Figure 2 it will be observed that the curve is not retraced upon cooling, shows no (reverse) glass/glass transition points corresponding to Tgl and Tg2, and does not return to the starting point. Thus any thermal processing of this material is likely to be associated with marked changes in the properties of the material, and these changes may be dependent on a number of factors (e. g. times, temperatures, heating rates, atmospheres) so that any change may well be difficult to reproduce reliably.

As used herein, "first glass transition temperature"refers to the lowest glass transition 0 temperature above ambient.

Figure 3 shows a differential thermal analysis plot for the material GeisAsisSeTe using the following cycle: 1. Hold for 1.0 minute at 20. 00oC 2. Heat from 20. 00 C to 440. 00 C at 10. 00 C/minute 3. Cool from 440. 00 C to 80. 00 C at 10. 00 C/minute 4. Hold for 20.0 minutes at 80. 00oC 5. Heat from 80. 00 C to 440. 00 C at 10. 00 C/minute 6. Cool from 440. 00 C to 80. 00OC at 10. 00 C/minute

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7. Hold for 20.0 minutes at 80. 00 C 8. Heat from 80. 00 C to 440. 00 C at 10. 00 C/minute 9. Cool from 440. 00 C to 20. 00 C at 10. 00 C/minute 10. Hold for 60.0 minutes at 20. 00 C The formula falls within both the wide and narrow formulae above.

Compared with Figure 2 this plot is a relatively simple trace involving first and second glass (glass/glass) transition temperatures Tgl and Tg2 at 145 C and 260 C, and a single devitrification temperature Tc I 330oC. On cooling, while the curve is not retraced, reverse glass transition points Tgl and Tg2 at 275 C and 160 C are exhibited. The trace is repeatable, as evidenced by measurements over three cycles with heating to 330 C.

Stacks of GaAs wafers have been bonded together at low temperature using the glass of Figure 3, and the resulting structures have exhibited low optical loss. Bulk samples of the glass of Figure 3 have also been successfully moulded and embossed at low temperatures and pressures.

Figure 4 shows a differential thermal analysis plot for the material Ge19As11Se17Te53, using the following cycle : 1. Hold at 20. 00 C for 1. 0 minutes 2. Heat from 20. 00 C to 500. 00 C at 10. 00 C/minute 3. Hold for 10. 0 minutes at 500. 00 C 4. Cool from 500. 00 C to 20. 00oC at 10. 00oC/minute 5. Hold for 60.0 minutes at 20. 00 C The formula falls within both the wide and narrow formulae above.

This plot is even simpler than that of Figure 3, showing just a single glass/glass transition temperature Tg at 170 C, and no devitrification point up to a temperature in excess of 470 C. There is a large temperature interval between Tg and the highest temperature investigated.

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Figure 5 shows a differential thermal analysis plot for the material Gel5Asl5SesTe65 There is a single glass/glass transition temperature Tg at I60 C, and two devitrification temperatures Tcl and Tc2 at 330 C and 380 C respectively. Below Tg the changes appear to be reversible.

Z=l The measured refractive index of this glass is 3.447 0. 005 at 2.128 microns and 3. 357 : ! : 0. 005 at 9.3 microns. The absorption coefficient is estimated to be less than 0.2 cm-'between 2.0 and 12 microns.

As discussed in our copending applications mentioned above, preferably the optical glass is selected such that it undergoes the heating cycle reversibly, e. g. for bonding or embossing purposes, so that its properties at the end of the cycle, i. e. on reverting to ambient conditions, are substantially identical to those at the commencement of the cycle. The glass of Figure 2 does not conform to this criterion and so is not a preferred material. The glasses of Figures 3 to 5 are preferred materials according to this criterion.

Also as discussed, preferably the optical glass has only one glass transition temperature before the devitrification temperature is reached, making the glass of Figures 4 and 5 more preferable than that of Figure 3. In that it has the lower Tg, the glass of Figure 5 is preferred over that of Figure 4.

As further discussed there is preferably an interval of at least 50 C, more preferably at least 100 C, and even more preferably at least 150 C, between the (first) glass transition temperature and any other transition temperature, whether a further glass transition temperature or the devitrification temperature. On this criterion the glass of Figure 4 is preferred over Figure 5, and then Figure 3, although all three glasses conform to the wider criteria.

Infrared transmitting chalcogenide glasses based on the Ge-As-Se-Te system have been prepared with refractive indices in the range n=3.00 to 3.45. These glasses have also been successfully coated onto silicon (n=3. 43) and GaAs (n=3.28) substrates, with layer thicknesses of from 0.1 microns and upwards, using a RF sputtering

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technique, and they can also provide a close index match to ZnGeP2 (n=3. ]), which is useful as a non-linear optical crystal for use in the mid-infrared.

The relatively high refractive index of silicon (Si = 3. 448 at 2.128 microns, 3. 418 at 9. 3 microns) means that it is difficult to find materials, and in particular bonding materials, for index matching thereto. It will be observed that the very good glass of Figure 5 has an almost perfect refractive index match with silicon at 2.128 microns and although it has a slightly higher dispersion, it has a useful index match therewith over the whole of the 2 to 12 micron range, still being within 2% of the value for silicon at 9.3 microns.

The composition map of Figure 1 illustrates a number of material compositions with glassy phases which have been made and investigated. The axis labelled "%As" is calibrated as a percentage of As in (As + Ge), and similarly the axis labelled"% Te" is calibrated as a percentage of Te in (Se + Te). It is important to note that the volume shown in the map does not coincide with all compositions which fall within the scope of the invention since, for example, % Te can be greater than 93.

The attributes of the glassy phases shown in the legend at the right bottom of the map are based on the criteria mentioned with relation to Figures 2 to 5 for the bonding of optical elements. For other purposes, other conclusions might be drawn. The upper of the two numbers associated with each composition is the measured refractive index at 2.1 microns, and the lower number is the index at 9.3 microns.

Preparation of the materials is by techniques known in the art. It should however, be emphasised that it is highly desirable to use the purest of components, and the cleanest of apparatus during the preparation. Furthermore, the thermal-time profile, both during the preparation of the glassy phases, and their subsequent annealing, needs to be carefully controlled on a composition by composition basis if a product having the desired composition and properties is to be obtained. In particular, the annealing process can have a great influence over the measured differential thermal analysis and refractive index, and it is preferably performed so as to remove substantially all strain. The refractive index measurements shown in Figure 1 are

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believed to be the most accurate obtained to date. but they are subject to refinement as the optimum processing steps for manufacture of the glass are determined. z : l It should be understood that the lowermost plane shown in Figure 1 corresponds to x=20, and accordingly the single (very bad") composition shown in that plane, GeloAs, oSeigTe6l, does not fall within the scope of the present invention. The compositions shown in the central plane include those of Figures 2 to 5.

Claims (8)

1. CLAIMS 1. A material having the formula Ge(x-a)ASaSe(100-x-b) Teb where 25 < x :'S 55 ; 10#a#25 ; 40 < b : S 70, and (100-x-b) > 0.
2. 2. A material according to claim 1 wherein 30 < x < 45.
3. 3. A material according to claim 1 wherein 30 < x < 40.
4. 4. A material according to any preceding claim wherein 50 < b < 70.
5. 5. A material according to any preceding claim when in a glassy phase.
6. 6. A material according to claim 5 in a substantially strain-free state.
7. 7. A material as claimed in claim 1 and substantially as hereinbefore described.
8. 8. A method of preparing a material according to any one of claims 1 to 5 by any t : l known technique to obtain it in a glassy phase, and subsequently annealing it to remove substantially all strain therefrom.