DIFFRACTIVE OPTICAL ELEMENT AND METHOD FOR MANUFACTURING THE SAME

Abstract

A diffractive optical element is provided that includes at least two layers with different etching speeds for dry etching process. The diffractive optical element has a substrate of glass and a microstructure layer arranged on the substrate. The ratio of dry etching speed in thickness direction of the substrate to that of the microstructure layer is no more than 1:2 so that the substrate functions as an etching stop layer. The ratio of dry etching speed in horizontal direction of the substrate is substantially equal to that of the microstructure layer. The composition of glass includes, but is not limited to, Al.sub.2O3, alkaline material (M.sub.2O) and alkaline earth material (MO), where the weight percentage of Al.sub.2O.sub.3+M.sub.2O+MO>=5%.

Claims

1. A diffractive optical element, comprising: a substrate of glass, the glass comprising Al.sub.2O.sub.3, alkaline metal oxides M.sub.2O, and alkaline earth metal oxides MO with a sum of content Al.sub.2O.sub.3+M.sub.2O+MO greater than 5 wt %, the substrate of glass exhibiting a refractive index in a range from 1.40 to 2.2; a microstructure layer arranged on the substrate of glass; a ratio of dry etching speed in thickness direction of the substrate of glass to that of the microstructure layer is no more than 1:2 so that the substrate of glass is an etching stop layer; and a ratio of dry etching speed in a horizontal direction of the substrate of glass is substantially equal to that of the microstructure layer.

2. The diffractive optical element of claim 1, wherein the ratio of dry etching speed in thickness direction is no more than 1:20.

3. The diffractive optical element of claim 1, wherein the sum of content Al.sub.2O.sub.3+M.sub.2O+MO is greater than or equal to 10 wt %.

4. The diffractive optical element of claim 1, wherein the microstructure layer has a feature dimension with an etched width less than 2000 m.

5. The diffractive optical element of claim 1, wherein the microstructure layer has a step with an etched depth less than 1000 m.

6. The diffractive optical element of claim 5, further comprising a deviation between the etched depth and a mean etch depth that is less than 30%.

7. The diffractive optical element of claim 1, wherein the microstructure layer has a total thickness (T.sub.max) that equals a largest etching depth (H.sub.max) plus a tolerance ().

8. The diffractive optical element of claim 1, wherein the substrate of glass has a surface roughness of less than 10 nm on an un-etched region.

9. The diffractive optical element of claim 1, wherein the microstructure layer is made of a material selected from a group consisting of silicon oxide, silicon nitride, TiO.sub.2, and combinations thereof.

10. The diffractive optical element of claim 1, wherein the microstructure layer is nonconductive and exhibits an electric resistivity larger than 10.sup.10 .Math.m.

11. The diffractive optical element of claim 1, wherein the substrate of glass has a refractive index substantially equal to that of the microstructure layer.

12. The diffractive optical element of claim 1, wherein the microstructure layer exhibits a refractive index in a range from 1.4 to 2.2.

13. The diffractive optical element of claim 1, wherein the substrate of glass exhibits an internal transmittance of higher than 96% in the wavelength of 400 nm to 1500 nm.

14. The diffractive optical element of claim 1, wherein the substrate of glass exhibits a transmittance of less than 30% when the wavelength is lower than 200 nm.

15. The diffractive optical element of claim 1, wherein the diffractive optical element exhibits an efficiency of higher than 30%.

16. The diffractive optical element of claim 1, further comprising a further layer on a location selected from a group consisting of an upper surface of the microstructure layer, a bottom surface of the substrate of glass, and between the microstructure layer and the substrate of glass.

17. The diffractive optical element of claim 16, wherein the further layer is a layer selected from a group consisting of an electric conduction layer, a protection layer, an electric conductive ITO layer, and an electric conduction metal layer.

18. The diffractive optical element of claim 1, wherein the etching speed in the thickness direction is in a range from 0.01 to 0.8 m/min for an etching gas of C.sub.4F.sub.8 or SF.sub.6 with plasma power 1600 w, a substrate power 200 w; a gas flow of 25 SCCM, and a pressure 0.2 Pa.

19. The diffractive optical element of claim 1, wherein the substrate of glass exhibits a TTV of less than 30 m.

20. The diffractive optical element of claim 1, wherein the substrate of glass exhibits a warp of less than 400 m.

21. The diffractive optical element of claim 1, further comprising a second microstructure layer arranged on an opposite side of the substrate of glass from the microstructure layer.

22. The diffractive optical element of claim 1, further comprising: a second substrate of glass; a second microstructure layer arranged on the second substrate of glass; a ratio of dry etching speed in thickness direction of the second substrate of glass to that of the second microstructure layer is no more than 1:2 so that the second substrate of glass is an etching stop layer; and a ratio of dry etching speed in a horizontal direction of the second substrate of glass is substantially equal to that of the second microstructure layer.

23. The diffractive optical element of claim 22, wherein the substrate and second of glass are arranged so that the microstructure layer and the second microstructure layer contact each other.

24. The diffractive optical element of claim 1, wherein the microstructure layer includes a plurality of steps of microstructure.

25. The diffractive optical element of claim 1, wherein the substrate of glass has a CTE of at most 15 ppm/K.

26. The diffractive optical element of claim 1, further comprising a ratio of CTE of the substrate of glass to that of the microstructure layer of at least 0.1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The invention will now be described in more detail by way of exemplary embodiments and with reference to the accompanying drawings. In the drawings, the same reference numerals designate the same or corresponding elements. In the drawings,

[0049] FIG. 1 illustrates a DOE with a glass substrate and one layer of microstructures according to the present invention;

[0050] FIGS. 2a-2d illustrates a method according to the present invention for manufacturing the DOE shown in FIG. 1;

[0051] FIGS. 3a-3c illustrates a further embodiments of the layer arrangement of DOE; and

[0052] FIG. 4 illustrates a structure with three steps of one microstructure layer according to the present invention.

DETAILED DESCRIPTION

[0053] FIG. 1 illustrates a DOE, according to the present invention. The DOE 1, for example, diffuser, includes a microstructure layer 10 and a substrate of glass 12. In one embodiment, the glass is available in the market with the brand D263, AF32, B270, or BF33 from Schott AG. In one embodiment, the DOE structure is SiO.sub.2 layer or other coating materials, such as silicon nitride, which is available for DOE structure. In FIG. 1, the DOE structure is a microstructure of diffractive grating. The diffractive grating could be obtained by dry etching process on SiO.sub.2 layer or the other coating materials. The glass has a feature of much lower dry etching speed compared to SiO.sub.2 layer or the other coating materials and thus, the glass is a good stop-layer so that the etching depth in the SiO.sub.2 layer or the other coating can be controlled well and etching tolerance can be small. One of example is the etching depth is usually controlled by the thickness of coating material. Furthermore, as compared to fused silica used in the art, the glass is cheap in view of cost, but it has similar temperature shift as fused silica.

[0054] Additionally, the refractive index n.sub.d of SiO.sub.2 layer is of 1.5 to 1.6 and the refractive index nd of silicon nitride is less than 2.1. compared to the this, the glass has similar values in a range of 1.40 to 2.2, preferably in a range from 1.45 to 1.8, for example, the glass available in the market from Schott AG, such as AF32eco 1.51, D263Teco 1.52, BF33 1.47. Since the substrate of glass 12 has similar refractive index as the microstructure layer 10 made of SiO.sub.2, the matched refractive index will reduce the light loss on the boundary between the substrate of glass 12 and the microstructure layer 10.

[0055] In the present invention, the substrate of glass 12 has an internal transmittance of higher than 96%, preferably higher than 98%, and most preferably higher than 99% for visible light to infrared light, e.g., at a wavelength in a range from 300 nm to 1200 nm, preferably at 810 nm, 830 nm, 850 nm or 940 nm. Transmission equal 100% minus reflection and absorption. The higher internal transmittance, the substrate of glass 12 has, the less of reflection, the higher efficiency, the DOE can achieve. According to different designs of the diffractive grating features, the DOE 1 can achieve an efficiency of higher than 30%, preferably higher than 50% or more preferably higher than 70%.

[0056] The size of glass is flexible according to the processing required. Wafer size is preferred for wafer level packing process. The edge of the wafer could be fabricated as frosting surface by etching, coating, printing, sandblasting to offer a non-transparent area for locating in post processing.

[0057] In an additional or alternative aspect, the DOE may include more than two layers. For example, a further layer could be applied between the substrate of glass 12 and the microstructure layer 10, on the bottom surface of the substrate of glass 12 or on the upper surface of the microstructure layer 10. The further layer may fulfill other functions, e.g., electric conduction or protection. Based on the different functions of the further layer, it may include ITO (indium tin oxide), Ag, Au, Cr, Pt, Cu and so on.

[0058] In a further aspect, according to different applications, the DOE may include more than one microstructure layers, e.g., two, three or four microstructure layers.

[0059] Now, referring to FIGS. 2a to 2d, which shows the steps of manufacturing the DOE 1 according to the present invention. Firstly, a substrate of glass 12 is provided a layer of SiO.sub.2 10 is coated on the substrate of glass 12, for example, by CVD or PVD method. Subsequently, a mask layer 14 is coated on the layer of SiO.sub.2 10, e.g., by spin coating, as shown in FIG. 2a. Subsequently, the masking layer 14 is removed on the area, corresponding to the layer of SiO.sub.2 10 to be etched, is removed, by photolithography process, as shown in FIG. 2b.

[0060] Now, a dry etching process is performed on the layer of SiO.sub.2 10 through the via in the pattern of the masking layer 14. During the dry etching process, a high density of plasma is generated by a Radio Frequency (RF) electromagnetic filed created by the external antenna. The layer of SiO.sub.2 10 is etched by chemically reactive gas plasma. Normally, the chemically reactive gas includes perfluoropropane (C.sub.3F.sub.8), perfluoroisobutylene (CF.sub.8), carbon tetrafluoride (CF.sub.4) or sulfur hexafluoride (SF.sub.6). Beside the above reactive gas, oxygen (O.sub.2), Argon (Ar), Nitrogen (N), Helium (He) or chlorine (Cl) could be used together.

[0061] As known in the art, the dry etching method include ion beam etching, ion etching, reactive ion beam etching, reactive ion etching, plasma etching and barrel etching.

[0062] According to one embodiment, the DOE structure is produced by RIE. In a preferable embodiment, the RIE process is performed with the following parameters range as listed in TABLE 1. In a preferable embodiment, the etching process is performed with the parameters: plasma power 1600 w; substrate power 200 w; Gas C.sub.4F.sub.8 with a flow of 25 SCCM; and pressure 0.2 Pa. In this case, the etching time is 15 minutes.

TABLE-US-00007 TABLE 2 Etching speed range of different materials RIE related parameters Etching Plasma Substrate Gas speed power power Reactive flow Pressure Glass (m/min) (W) (W) gas (SCCM) (Pa) D263 0.01-0.3 200-2500 50-500 C.sub.4F.sub.8 or SF.sub.6 5-50 0.1-1 AF32 0.01-0.3 or C.sub.4F.sub.8/Ar B270 0.01-0.3 or SF.sub.6/Ar BF33 0.02-0.8 SiO.sub.2 (fused 0.3-1.3 silica) SiO.sub.2 coating 0.2~1.0 layer

[0063] Here, the process for forming DOE structure is briefly introduced with detail test examples for explaining the etching speed of different glass material as compared to the layer of SiO.sub.2. Besides, the microstructure is not limited to mentioned structure in this invention.

Embodiment 1

[0064] The test has been done via RIE with related parameters of, plasma power 1600 w, substrate power 200 w, gas C.sub.4F.sub.8 with a flow of 25 SCCM, and pressure 0.2 Pa.

TABLE-US-00008 TABLE 3 Etching speed values of different materials Fused SiO.sub.2 Substrate silica D263 AF32 B270 BF33 coating Etching speed 345 56 15 12 146 279 (nm/min)

Embodiment 2

[0065] In this embodiment, the substrate of glass consists of AF32 glass. Firstly, the substrate of glass is cut into size of 50*50 mm. subsequently, the layer of SiO.sub.2 is sputtering on the surface of substrate of glass. The thickness of the layer of SiO.sub.2 is 2.021 m. Then, the mask layer is spin coated on the layer of SiO.sub.2 and vias through the mask layer are produced by photolithography method. The target width of the via is 50 nm. The dry etching process is performed by RIE with related parameters: plasma power of 1600 w, substrate power of 200 w, gas of C.sub.4F.sub.8 with a flow of 25 SCCM and pressure of 0.2 Pa.

TABLE-US-00009 TABLE 4 etching depth Etching time (min) 2 5 8 10 Measured etching depth (nm) 618 1382 2023 2056 (include SiO.sub.2 layer and AF32 substrate)

[0066] From TABLE 4, it can be seen that from 2 mins to 8 mins of etching time, the etching depth increasing fast due to high etching rate of SiO.sub.2 layer. From 8 mins to 10 mins of etching time, the etching depth does not change much due to the low etching speed of AF32 glass. From the above, it can be determined that glass AF32 could be a good stop to control the etching precision.

[0067] After dry etching process, the microstructure layer 10 is obtained, see FIG. 2c. Subsequently, the mask layer 14 is removed and the DOE 1 with one layer of DOE structure is achieved, as shown in FIG. 2d. In an embodiment, the depth of the DOE structure h is from 500 nm to 10 m and the width thereof w is width of the structure is from 10 nm to 500 nm.

[0068] From TABLE 1, it also can be seen that the substrate of glass, e.g., glass D263, AF32, B270, BF33 from Schott AG, exhibits much lower etching speed as compared to the layer of SiO.sub.2 10 or fused silica. Preferably, the ratio of dry etching speed in thickness direction of the substrate of glass 12 to that of the microstructure layer 10 is no more than 1:2, preferably no more than 1:5, more preferably no more than 1:10 or most preferably no more than 1:20. Due to the lower etching speed of the substrate of glass 12 as compared to layer of SiO.sub.2 10, the substrate of glass 12 can be act as etching stop layer and thus, the etching tolerance of DOE structure depth, which is most close to the thickness of the layer of SiO.sub.2 10, is much easier to be controlled. Additionally, the ratio of dry etching speed in horizontal direction of the substrate of glass is close to that of the microstructure layer 10.

[0069] In an embodiment, the grating structure might include one step, two steps, three steps, . . . seven steps. As shown in FIG. 4, it shows a grating structure with three steps, 101, 102, 103. In this case, at least the bottom step 103 can has high precision with dry etch process, which also improve the efficiency as well as performance of the DOE.

[0070] As obvious for a person skilled in the art, the DOE structure can be created with dry etching process. However, the process for generating the DOE structure should not be limited to dry etching process. Any other methods that can generate DOE structures can be used. For example, the DOE structure could also be achieved by direct writing, such as laser direct writing and electron beam exposure.

[0071] After producing the DOE structure 10 on the substrate of glass 12, post process can be performed, e.g., dicing, so as to produce individual pieces of DOEs 1. For this purpose, dicing can include sawing, laser dicing and other dicing process.

[0072] In another aspect, the DOE may include pairs of the substrates of glass 12 and microstructure layers 10. For an example, as shown in FIGS. 3a-3c, the DOE includes two pairs of the substrates of glass 12 and microstructure layers 10. As shown in FIG. 3a, the microstructure layers 10 are arranged in a manner so that they contact with each other. Alternatively, as shown in FIG. 3b, the microstructure layer 10 are arranged on both side of one glass 12. Additionally, the pairs of the microstructure layers 10 and the substrates of glass 12 are arranged in an alternate manner.

[0073] In the present invention, the DOE is produced with a high precision in term of at least the height of the grating structure in a simple and easy manner, which can reduce the cost of the production and also increase the efficiency thereof.