Bondable glass and low auto-fluorescence article and method of making it

Abstract

The present disclosure relates to glass articles, a method of making the glass articles, and uses of the glass articles. The glass article has a UV-transmittance of more than 90% at 350 nm and at 500 nm and a total amount of Si.sub.2, B.sub.2O.sub.3 and Al.sub.2O.sub.3 of at least 75 mol %. The article is preferably used in the fields of biotechnology, MEMS, CIS, MEMS-like pressure sensor, display, micro array, electronic devices, microfluidics, semiconductor, high precision equipment, camera imaging, display technologies, sensor/semicon, electronic devices, home appliance, diagnostic product, and/or medical device.

Claims

1. A glass article with Li.sup.+-activatable bonding properties, comprising: a UV-transmittance of more than 90% at 350 nm and at 500 nm; a lithium activatable bonding strength to silicon (LABS) of at least 1.1 MPa; a total amount of SiO.sub.2, B.sub.2O.sub.3 and Al.sub.2O.sub.3 of at least 75 mol %; a proportion of Li.sub.2O that is less than 0.01 mol %; a molar proportion x.sub.Si4+ of Si.sup.4+, a molar proportion x.sub.B3+ of B.sup.3+; and a molar proportion x.sub.Al3+ of Al.sup.3+, wherein the molar proportions x.sub.Si4+, x.sub.B3+, and x.sub.Al3+ are present in accordance with the following ratios: $\begin{array}{cc}i.& \frac{x_{\mathrm{Si}4+}}{x_{B3+}+x_{A13+}}\ge 2.,\mathrm{and}\frac{x_{\mathrm{Si}4+}}{x_{B3+}+x_{A13+}}\le 8.;\mathrm{and}\\ \mathrm{ii}.& \frac{x_{B3+}}{x_{A13+}}\ge 0.5.\end{array}$

2. The glass article according to claim 1, wherein the total amount of SiO.sub.2, B.sub.2O.sub.3 and Al.sub.2O.sub.3 is at least 90 mol %, and wherein the molar proportion x.sub.Si4+ of Si.sup.4+, the molar proportion x.sub.B3+ of B.sup.3+ and the molar proportion x.sub.Al3+ of Al.sup.3+ are present in accordance with the following ratios: $\begin{array}{cc}i.& \frac{x_{\mathrm{Si}4+}}{x_{B3+}+x_{A13+}}\ge 3.,\mathrm{and}\frac{x_{\mathrm{Si}4+}}{x_{B3+}+x_{A13+}}\le 8.;\mathrm{and}\\ \mathrm{ii}.& \frac{x_{B3+}}{x_{A13+}}\ge 1.5.\end{array}$

3. The glass article according to claim 1, having an R.sub.a surface roughness of less than 2 nm.

4. The glass article according to claim 1, having at least one fire-polished surface.

5. The glass article according to claim 1, having a total thickness variation of not more than 10 μm.

6. The glass article according to claim 1, having a Warp of not more than 50 μm.

7. The glass article according to claim 1, having a coefficient of thermal expansion (CTE, 20 to 300° C.) of from 1 to 10 ppm/K.

8. The glass article according to claim 1, wherein the glass of the glass article has a working temperature T.sub.4, and an upper crystallization temperature T.sub.OEG, wherein the difference T.sub.4−T.sub.OEG is more than 50 K, wherein working temperature T.sub.4 is from 1000° C. to 1500° C., and the upper crystallization temperature T.sub.OEG is from 900° C. to 1300° C.

9. The glass article according to claim 1, wherein the glass of the glass article has a luminous transmittance T.sub.D65 at 0.5 mm thickness of more than 91% and/or less than 91.7%.

10. The glass article according to claim 1, wherein the glass of the glass article has a UV cutting edge at 1 mm thickness of lower than 300 nm.

11. The glass article according to claim 1, wherein the glass of the glass article has an autofluorescence emission/excitation intensity ratio at 488 nm of less than 1%.

12. The glass article according to claim 1, wherein the glass article is a glass wafer or glass sheet, having a width, a length, and a thickness, wherein width and length are each independently at least 10 mm, and the thickness is not more than 1.5 mm.

13. The glass article according to claim 1, further comprising, after lithium enrichment, at least one surface that is a lithium-enriched surface layer, the glass article having a lithium proportion Li.sub.surface in the at least one lithium enriched surface layer, and a bulk lithium proportion Li.sub.bulk, wherein $\frac{{\mathrm{Li}}_{\mathrm{surface}}\left[\mathrm{mol}\%\right]}{{\mathrm{Li}}_{\mathrm{bulk}}\left[\mathrm{mol}\%\right]}\ge 5,$ wherein the lithium-enriched layer has a depth of layer (DoL) of 0.01 to 200 μm.

14. The glass article according to claim 1, wherein the glass article is bonded to a semiconductor wafer with a bonding strength of at least 1 MPa.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the test set up for testing bonding strength.

(2) FIG. 2 is a general illustration of anodic bonding

DETAILED DESCRIPTION OF THE DISCLOSURE

(3) Article

(4) The glass article can have a LABS of at least 1.5 MPa, at least 2 MPa, at least 5 MPa, at least 10 MPa, or at least 15 MPa. The higher the LABS, the higher is the articles susceptibility to the effects associated with lithium enrichment treatment. Bonding strength can be improved by increasing the DoL of the lithium enriched surface layer. A higher DoL can be achieved by increased ion-exchange time, higher process temperature during ion exchange and/or higher concentrations of lithium in the salt bath.

(5) The glass article can have very small thickness. Thinner articles need very little space and add only minor weight to the final product. However, very thin articles may not be stable enough, and may easily break during use or during the bonding strength test. Thus, the glass article can have a thickness of up to 1.5 mm, up to 1 mm, up to 0.9 mm, up to 0.8 mm, up to 0.7 mm, up to 0.6 mm, up to 0.5 mm, up to 0.4 mm, up to 0.3 mm, up to 0.2 mm, up to 0.1 mm, up to 90 μm, up to 80 μm, up to 70 μm, up to 60 m, up to 50 μm, up to 40 μm, up to 30 μm, or up to 20 μm. The article can have a thickness of at least 1 μm, at least 2 μm, or at least 3 μm. Such small and very small thicknesses may be achieved using processes like down draw, or overflow fusion, or re-draw. These processes are superior to other processes like the float process or pressing in that these processes may yield very smooth and flat surfaces. Smooth and flat surfaces are desirable for bondable glasses because bonding strengths will be higher with very flat and smooth surfaces. Thus, the glass article preferably has an R.sub.a surface roughness of less than 2 nm, less than 1 nm, or less than 0.5 nm. It is desirable for the glass article to have at least one surface that has this low roughness, preferably the two major surfaces of the article have such roughness properties. These very smooth surfaces obtainable by the processes mentioned above are also called “fire-polished” surfaces. The glass article can have at least one, or at least two fire-polished surfaces, preferably the two major surfaces of the article have fire-polished properties.

(6) The glass article of this disclosure can have a total thickness variation (TTV) of not more than 10 μm. A low TTV is desirable for good bondability of a glass article. Particularly, low TTV is achieved with processes like down draw, or overflow fusion, or re-draw, if the glass has the relevant viscosity characteristics. The glass used for the purposes of this disclosure has shown to exhibit the viscosity behavior, i.e. the susceptibility of the viscosity to temperature change is such that glass articles with excellent surface and flatness properties can be obtained in economical ways using these processes. The glass article can have a warp of not more than 50 μm. Much like a low TTV, low warp is desirable for good bondability of a glass article. Likewise, low warp may be achieved with processes like down draw, or overflow fusion, or re-draw, if the glass has the relevant viscosity characteristics, which is the case for the glass used for the glass article herein.

(7) One of the aspects that makes the glass suitable for the above-mentioned flat glass processes is the difference between working temperature and upper crystallization temperature of the glass forming the glass article. The greater the difference between these temperatures is, the less susceptible is the glass to temperature changes during production, and the less is the risk of unintended devitrification during production. The glass of the glass article can have a working temperature T.sub.4, and an upper crystallization temperature T.sub.OEG, wherein the difference T.sub.4−T.sub.OEG can be more than 50 K, more than 100 K, more than 150 K, more than 200 K, or more than 250 K. The glass of the glass article can have a working temperature T.sub.4 of from 1000° C. to 1500° C., or from 1100° C. to 1400° C., or from 1200° C. to 1375° C., or from 1225° C. to 1350° C. The upper crystallization temperature T.sub.OEG can range from 900° C. to 1300° C., from 950° C. to 1250° C., from 1000° C. to 1200° C., or from 1050° C. to 1150° C.

(8) The glass of the glass article can have a coefficient of thermal expansion (CTE, α.sub.20-300° C.), which closely resembles the CTE of silicon. It is beneficial for the glass to have a similar CTE as silicon in order to get a most reliable bond between glass and silicon articles. CTE of the glass can be from 1 to 10 ppm/K, less than 6.5 ppm/K, less than 6.0 ppm/K, less than 5.5 ppm/K, less than 5.0 ppm/K, less than 4.5 ppm/K, less than 4.0 ppm/K, or from 2.5 to 3.5 ppm/K. CTE is influenced by the glass composition, in particular by the amount of alkali metal oxides, but also by the process of production, in particular by the cooling rates applied during glass forming. Notably, a glass with a relatively low CTE is easier to be formed into a particularly flat glass article in terms of low TTV and low warp. Also, the glass article can have a transition temperature Tg of more than 470° C., or more than 490° C. A high transition temperature is particularly advantageous to usage in high temperature. Glass articles having such high transition temperature Tg usually also have an advantageously low CTE.

(9) The article of this disclosure has excellent transmission properties, which can include a luminous transmittance T.sub.D65 at 0.5 mm thickness of more than 91% and/or less than 91.7%, or less than 91.5%. High luminous transmittance T.sub.D65 is advantageous, in particular a good signal can be obtained in various applications. For the article to have a desirable transmission it is desired that the UV cutting edge is as low as possible. The glass of the glass article can have a UV cutting edge at 1 mm thickness of lower than 300 nm, lower than 270 nm, or even lower than 250 nm.

(10) The glass of the glass article can have a refractive index nd of less than 1.55, less than 1.50, less than 1.49, or less than 1.48.

(11) Importantly, the glass of the glass article can have a very low autofluorescence intensity. Autofluorescence intensity is influenced by optical basicity. Low optical basicity leads to reduced fluorescence, which is good for biotechnology applications; Cy3 and Cy5 dyes are often used in the biotechnology field. These dyes fluoresce at 570 nm and 670 nm, where many glasses show auto-fluorescence. Auto-fluorescence leads to bad signal-to-noise ratio in optical signal detection processes. The glass of the glass article can have an autofluorescence emission/excitation intensity ratio at 488 nm of less than 1%. The glass of the glass article can have an optical basicity A less than 0.6, or less than 0.55, or less than 0.53, or less than 0.52, or less than 0.51.

(12) In an embodiment, the glass of the glass article comprises the following proportions of oxides:

(13) TABLE-US-00001 SiO.sub.2 >60 to 90 mol % Al.sub.2O.sub.3 >0 to 15 mol % B.sub.2O.sub.3 >4 to 25 mol % R.sub.2O >0 to <20 mol % RO 0 to <20 mol %

(14) wherein R.sub.2O is the sum of the amounts of the alkali metal oxides Li.sub.2O, Na.sub.2O and K.sub.2O, and RO is the sum of the amounts of ZnO and the alkaline earth metal oxides MgO, CaO, SrO, and BaO.

(15) In an embodiment, the glass of the glass article comprises the following proportions of oxides:

(16) TABLE-US-00002 SiO.sub.2 >60 to 90 mol % Al.sub.2O.sub.3 >0 to 15 mol % B.sub.2O.sub.3 >5 to 25 mol % R.sub.2O >0 to <20 mol % RO 0 to <20 mol %

(17) wherein R.sub.2O is the sum of the amounts of the alkali metal oxides Li.sub.2O, Na.sub.2O and K.sub.2O, and RO is the sum of the amounts of ZnO and the alkaline earth metal oxides MgO, CaO, SrO, and BaO.

(18) The glass can comprise the following proportions of oxides:

(19) TABLE-US-00003 SiO.sub.2 >80 to <85 mol % Al.sub.2O.sub.3 >0.5 to <3 mol % B.sub.2O.sub.3 >8 to 15 mol % R.sub.2O >0 to <5 mol % RO 0 to <5 mol %

(20) wherein R.sub.2O is the sum of the amounts of the alkali metal oxides Li.sub.2O, Na.sub.2O and K.sub.2O, and RO is the sum of the amounts of ZnO and the alkaline earth metal oxides MgO, CaO, SrO, and BaO.

(21) In an embodiment

(22) $\frac{x_{B3+}}{x_{A13+}}\ge 7.5,\mathrm{or}\frac{x_{B3+}}{x_{A13+}}\ge 10.$

(23) In certain embodiments, the glass can be characterized by

(24) $\frac{x_{B3+}}{x_{A13+}}\le 25,\frac{x_{B3+}}{x_{A13+}}\le 20,\mathrm{or}\frac{x_{B3+}}{x_{A13+}}\le 15.$

(25) Certain components have a negative influence on UV transmittance. The proportions of such components are desirably limited in the glass. The glass of the glass article can comprise SnO.sub.2, Sb.sub.2O.sub.3, Ce.sub.2, TiO.sub.2 and/or Fe.sub.2O.sub.3 in individual amounts of 0 to 0.5 mol %, or of less than 0.01 mol %.

(26) The glass article can be a glass wafer or glass sheet, having a width, a length, and a thickness. The width and length are each independently at least 10 mm, and the thickness is not more than 1.5 mm.

(27) After option lithium enrichment, the article can have on at least one surface a lithium-enriched surface layer. Lithium enrichment can be done on one or both of the major surfaces, on surfaces to be bonded, and/or on all surfaces of the article. In certain embodiments, the article can have a lithium proportion Li.sub.surface in at least one lithium enriched surface layer, and a bulk lithium proportion Li.sub.bulk, wherein

(28) 0$\frac{{\mathrm{Li}}_{\mathrm{surface}}\left[\mathrm{mol}\%\right]}{{\mathrm{Li}}_{\mathrm{bulk}}\left[\mathrm{mol}\%\right]}\ge 5.$
For the purposes of the present disclosure, a “surface layer” is a proportion of the glass which is at the glass/air interface. The glass forming the surface will here be referred to as “surface layer”; the remaining glass located further in the interior will here be referred to as “bulk”. A precise demarcation between surface and bulk is difficult; therefore, it is specified for the purposes of the present disclosure that the surface layer is present in a depth of about 6 nm. The properties of the surface layer are consequently determined at a depth of about 6 nm. The properties of the bulk glass are determined by calculation since the glass composition at a greater depth does not experience any change as a result of production. Bulk glass is in any case present at a depth of 500 nm. The surface can be advantageously influenced by particular measures during glass production. The properties of the surface layer are critical for particular properties of the glass which are measured on the surface. These include, in particular, the base resistance and the hydrolytic resistance. The composition of the surface glass at a depth of about 6 nm can be measured by means of Cs-TOF-SIMS at 1000 eV.

(29) Lithium enrichment enriches one or more surfaces with Li.sup.+. The lithium-enriched layer can have a depth of layer (DoL) of 0.01 to 200 μm, of 0.1 to 100 μm, of 0.5 to 50 μm, or of more than 1 μm. A higher DoL increases the effect of lithium enrichment so that higher bonding strengths and/or milder bonding conditions may be achieved. The Li.sup.+ content in the lithium-enriched layer can be greater than 1 ppm (m/m), or greater than 10 ppm, or greater than 20 ppm, or greater than 30 ppm. The higher the lithium content the higher the effect of lithium enrichment so that higher bonding strengths and/or milder bonding conditions can be achieved.

(30) The glass article can be bonded to one or more semiconductor articles, such as wafers. Bonding can have a bonding strength of at least 1 MPa, at least 2 MPa, at least 3 MPa, at least 4 MPa, at least 5 MPa, at least 8 MPa, at least 10 MPa, or at least 15 MPa. The semiconductor wafer can be a silicon wafer.

(31) Method of Making

(32) The present disclosure includes a method of making an article as set out herein. The method of making can include the steps of: providing a composition of glass raw materials in accordance with the desired glass composition, melting the composition, preparing the glass article.

(33) The step of preparing the glass article can include forming the glass article in accordance with a flat glass process selected from the group consisting of down-draw (such as slot down-draw), overflow fusion, floating, rolling, pressing, and re-draw. From those processes mentioned down draw, overflow fusion and re-draw are the most preferred processes. These processes are desirable because they may yield glasses with excellent flatness (e.g. TTV, Warp) and surface properties (R.sub.a, fire-polished). It is thus desired to use glasses that combine the properties of being bondable with high bonding strength in mild conditions, have low auto-fluorescence, excellent light transmission properties, and the viscosity-temperature behavior needed to process the glasses in the preferred flat glass processes mentioned before. The glass of the glass article above has these properties.

(34) The method of making the glass article can include cooling the glass from a temperature T.sub.1 that corresponds to a viscosity of 10.sup.10 dPas to a temperature T.sub.2 that corresponds to a viscosity of 10.sup.15 dPas with an average cooling rate of >5 K/s and/or <200 K/s. Preferably, cooling takes place in air. Very fast cooling will lead to decreased flatness, such as TTV and warp. Cooling speed has an influence on CTE and on the susceptibility of the glass to ion exchange treatment such as lithium enrichment and chemical toughening. The cooling rate can be limited to less than 150 K/s, less than 100 K/s, or less than 50 K/s.

(35) The method may optionally include a step of enriching at least one surface layer of the glass article with Li.sup.+. As mentioned above, lithium enrichment may improve bonding ability of the glass article and may facilitate bonding at milder conditions. Lithium enrichment may be done by subjecting the glass article to a lithium-containing salt, or a lithium-containing mixture of salts, e.g. in a respective salt bath, at a temperature of at least 350° C. for more than 1 minute.

(36) The amount of Li.sup.+ in the salt or mixture of salts can be at least 1 ppm (m/m), at least 10 ppm, at least 50 ppm, at least 100 ppm, or at least 150 ppm. A higher amount of lithium will allow for shorter enrichment treatment times and/or deeper penetration of the glass article. However, at very high levels there may be no additional effect. In an embodiment the amount of Li.sup.+ in the salt or mixture of salts is up to 1,000 ppm (m/m), up to 800 ppm, up to 600 ppm, up to 400 ppm, or up to 300 ppm.

(37) The temperature at which lithium enrichment is done can be limited to 500° C., or less than 480° C., or less than 420° C. A lower temperature makes the process more economical because of reduced energy consumption in a given time, but it also slows down the process so that the temperature should not be too low either. The glass article can be subjected to the salt or mixture of salts for a time of at least 5 minutes, at least 10 minutes, at least 20 minutes, at least 30 minutes, at least 45 minutes, at least 60 minutes, or at least 90 minutes. A sufficient amount of time should be chosen in order to achieve a sufficient DoL of lithium in the article.

(38) However, there is no additional benefit of increasing the time over a certain threshold. The glass article can be subjected to the salt or mixture of salts for a time of up to 5 hours, up to 4 hours, up to 3 hours, or up to 2 hours.

(39) In an embodiment the step of subjecting the glass article to a lithium-containing salt, or a lithium-containing mixture of salts can include: immersing the glass article in a salt bath.

(40) The lithium-containing salt, or a lithium-containing mixture of salts can include LiNO.sub.3. The mixture of salts can include LiNO.sub.3, and optionally at least one of NaNO.sub.3, KNO.sub.3, CsNO.sub.3 and AgNO.sub.3

(41) Before or after enriching at least one surface layer of the glass article with Li.sup.+, the glass article can be chemically toughened. The chemical toughening can also be performed on a glass article of this disclosure that has not been subjected to lithium enrichment. The glass article can be chemically toughened using a K.sup.+ containing salt. The other steps of chemical toughening can be the same as in the lithium enrichment treatment with the difference that the surface will be enriched with potassium during chemical toughening.

(42) The method can include a step of bonding the glass article to another article, including articles of glass or semiconductors. The other article can be a semiconductor wafer, such as a silicon wafer. Bonding can include anodic bonding and/or thermal bonding. Anodic bonding can comprise the following steps: cleaning the glass article, cleaning the other article, anodically bonding the two cleaned articles.

(43) Additionally, or alternatively, bonding can include cleaning the glass article, enriching at least one surface layer of the glass article with Li.sup.+, cleaning the glass article after enriching, cleaning the other article, anodically bonding the two cleaned articles.

(44) Bonding can include anodic bonding with a bonding voltage of less than 1000 V, less than 800 V, less than 600 V, or less than 400 V. A lower bonding voltage is advantageous as it reduces the risk of damaging the articles to be bonded or any circuitry present on these articles. The same is true for bonding temperature. Bonding temperature can be less than 450° C., less than 300° C., or less than 200° C. Bonding results may be improved by pressing together the articles to be bonded. The pressure with which the articles are pressed together can be from 0.1 to 1 MPa, or 0.15 to 0.5 MPa. In order to avoid presence of any gases between the bonded articles, bonding takes place at reduced pressure, such as at a pressure of 10.sup.−4 to 10.sup.−2 Pa.

(45) Bonding can include thermal bonding at a temperature of less than 900° C., less than 800° C., less than 750° C., or less than 700° C.

(46) The glass article of the present disclosure can be processed by laser cutting. For example, a laser may be used for cutting the article into a desired shape. Furthermore, a laser may also be used for drilling fine structures, for example holes, into the glass article. In particular, holes having a diameter of 500 to 800 μm can be drilled by a laser into glass articles having a thickness of 50 to 100 μm. Glass articles provided with such fine structures can preferably be used in microfluidics, biotech, MEMS or comparable applications.

(47) The glass article can also be processed by acid treatment. However, the selectivity of acid treatment is lower, and the precision may be difficult to control. Therefore, processing the glass article with a laser is more preferred.

(48) Use of Glass Article

(49) The present disclosure also relates to a use of the article in the fields of biotechnology, MEMS, CIS, MEMS-like pressure sensor, display, micro array, electronic devices, microfluidics, semiconductor, high precision equipment, camera imaging, display technologies, sensor/semicon, electronic devices, home appliance, diagnostic product, and/or medical device.

(50) High UV transmittance, low UV cutting edge of the articles is particularly advantageous for use in the fields of MEMS, CIS, MEMS-like pressure sensor, display, micro array, electronic devices, microfluidics, semiconductor, high precision equipment, camera imaging, display technologies, sensor/semicon, electronic devices, home appliance.

(51) Low autofluorescence of the articles is particularly advantageous for use in the fields of biotechnology, diagnostic product, and/or medical device.

EXAMPLES

Example 1—Glass Melting

(52) Glass melts were prepared in accordance with the compositions given in the following tables 1a and 1b. Glass raw materials were melted and formed into thin glass sheets of 0.7 mm thickness using the down draw production method. The glass sheets were cut to wafers.

(53) TABLE-US-00004 TABLE la mol % A B C D E F G H I J K SiO.sub.2 82 80 84.2 79 72 70 72.5 75 75 79.4 66.1 Al.sub.2O.sub.3 1.6 4 0.8 2.5 5 4 0.4 1 0.5 2.6 6.5 B.sub.2O.sub.3 12 11 10 12 12 11 19.7 20 18.5 9.6 8 Li.sub.2O 1 5 10 Na.sub.2O 4 3 4.8 5 5.7 2.5 3.9 2 3 4.6 5 K.sub.2O 0.4 2 0.2 0.5 0.3 1.8 2.5 2 2 0.6 0.5 MgO 6.9 CaO 1 1.6 3.3 BaO SrO 1.9 Sb.sub.2O.sub.3 0.2 F 0.7 0.7 ZnO NaCl 0.1 1.6 1.6 TiO.sub.2 R.sub.2O 4.4 5 5 6.5 11 14.3 6.4 4 5 5.1 5.5 RO 0 0 0 0 0 0 0 0 1 1.6 12.2 Density (g/cm.sup.3) 2.2 2.2 2.3 2.2 2.3 2.3 2.2 2.2 2.2 2.3 2.4 CTE (ppm/K) 3.3 3.4 3.2 3.5 3.9 4.7 4.2 3.7 3.8 4.4 5.8 Tg(° C.) 524 520 535 618 610 558 552 560 566 588 619 T.sub.4 (° C.) 1302 1409 1225 1282 1230 1028 1036 1145 1101 1309 1273 T.sub.OEG(° C.) 1137 1243 1077 1116 1078 914 918 1013 973 1140 1138 Littleton 873 942 846 858 836 733 731 783 766 875 906 point(° C.) Young's 64 63 69 64 64 65 62 57 62 65 74 modulus (GPa) Optical 0.52 0.52 0.52 0.52 0.54 0.55 0.51 0.51 0.51 0.53 0.55 basicity Λ Auto no no no no no no no no no no fluorescence at 570 nm Auto no no no no no no no no no no fluorescence at 670 nm

(54) The glasses melted had excellent CTE and desirable Tg. Processability in the down draw process was excellent.

(55) TABLE-US-00005 TABLE lb mol % L M N O P Q R S T U SiO.sub.2 67.2 67.2 67.2 67.2 67.2 67.2 67.2 67.2 67.2 64.2 Al.sub.2O.sub.3 11.3 11.3 11.3 11.3 11.3 11.3 11.3 11.3 11.3 4.2 B.sub.2O.sub.3 9.9 9.9 9.9 16 14 9.9 7 5 9.9 8.5 Li.sub.2O Na.sub.2O 6.4 K.sub.2O 6.9 MgO 4.6 4.6 11.5 6 9 11.5 14 17 4.6 CaO 5.5 6.9 5.5 BaO 1.4 SrO Sb.sub.2O.sub.3 0.5 0.5 0.5 F ZnO 5.9 NaCl 1.4 TiO.sub.2 3.9 R.sub.2O 0 0 0 0 0 0 0 0 0 13.3 RO 10.1 11.5 11.5 6 9 11.5 14 17 10.1 0 Optical 0.54 0.54 0.53 0.51 0.52 0.53 0.53 0.54 0.53 0.58 basicity Λ Auto no no no no no no no no no fluorescence at 570 nm Auto no no no no no no no no no fluorescence at670 nm

Example 2—Lithium Activation

(56) The glass wafers produced according to example 1 were subjected to a lithium-containing mixture of salts (LiNO.sub.3/NaNO.sub.3) with different lithium proportions ranging from 50 ppm to 400 ppm. Different process times for the ion exchange (ionx) were applied in order to achieve various depths of ion exchanged layers.

(57) The conditions and results are summarized in table 2 below.

Example 3—Bonding Strength Test

(58) After activation by lithium ion exchange treatment, bonding tests were performed in order to test the influence of activation on bonding strengths and conditions. The glass wafers of the previous examples were bonded to silicon wafers.

(59) Anodic bonding was done at different bonding temperatures and voltages. The bonding pressure was 0.2 MPa for all samples. The vacuum was set to 0.001 Pa for all experiments.

(60) The bonding set up is shown in FIG. 2.

(61) For the bonding strength test the bonded pairs were cut into pieces with sizes of 20 mm×20 mm. Aluminum alloy holders were attached to both sides of the bonded pair using epoxy resin. The epoxy resin was solidified, and samples tested on a mechanical strength tester. Tensile force was increased till the glass wafers broke, and the value of maximum force was recorded. Bonding strength is the tensile force per sample surface area. The bonding strength test setup can be seen in FIG. 1.

(62) The results are shown in the following table 2.

(63) TABLE-US-00006 TABLE 2 Example No. 1 2 3 4 5 6 7 8 Melt No. A C A B A C A B Lithium [ppm] 50 50 / 200 200 400 400 400 ionx T [° C.] 350 400 / 300 350 300 350 400 ionx t [min] 60 10 / 60 120 10 60 120 DoL [μm] 5 3 / 7 10 6 12 33 Bonding temperature [° C.] 300 300 250 250 250 200 200 200 Bonding voltage [V] 400 400 360 360 360 300 300 300 Bonding strength [MPa] 10.03 9.81 1.98 10.52 13.66 10.89 15.37 18.23

(64) It can be seen from these tests that the DoL has a remarkable influence on bonding strength. Even very small DoL have a strong effect on bonding strength.

LIST OF REFERENCE SIGNS

(65) 1 Tensile force 2 Aluminum alloy holder 3 Epoxy resin 4 Glass wafer Si wafer 6 Additional pressing force 7 Graphite paper 8 Glass 9 Silicon