CN1195240C - Process for mfg. multi-phase diffraction optic element - Google Patents

Abstract

The present invention relates to technology for manufacturing a multi-phase diffraction optic element. Firstly, a layer of chromium film is sputtered on initial optical element substrate, when negative photoresist is exposed from the rear face, masking is formed. The position of each phase step is determined by a first hiding stencil, each latter fabrication step does not relate to the determination of step position, and the subsequent hiding stencil use is only required to align to the phase step position determined by the first hiding stencil in a rough way. When ultraviolet transparent base materials (such as SiO <2>) are etched by reactive ions with large selective etching ratio under negative photoresist masking, parts of the base materials are etched, and the negative photoresist is retained. The requirement of the manufacture precision of the first hiding stencil is higher, the requirement of the rest is lower, and the manufacture cost of the hiding stencil can be lowered. The influence of longitudinal etching error on diffraction efficiency only needs to be considered, and influence of lateral aligning error does not need to be considered. By an end monitoring means of etching technology of reaction ions, longitudinal manufacture error can be controlled in high precision, and thus, the diffraction efficiency higher than the existing manufacture technology can be obtained.

Description

A process for making multi-phase diffractive optical element

technical field

The invention belongs to the field of information science and technology, and specifically relates to a process for manufacturing a multi-phase diffractive optical element.

Background technique

Binary optics, or binary diffractive optics, was first proposed abroad by Veldcamp et al. at the Massachusetts Institute of Technology (MIT) in the late 1980s. The control of the phase in the binary optical element is to approach the ideal value by generating multiple steps. For example, the phase step is 2π, which can be divided according to the method of L=2, 4, 8, ..., 2 ^m , (m is a positive integer). This method is geometrically equivalent to a curved surface of any shape and can be divided The step plane is approaching. In the existing manufacturing process, this binary phase step is easily completed by microelectronics technology: a simple black and white mask can be exposed and etched to produce two levels of steps, such as another mask, repeated exposure And etching, as long as the etching depth is half of the last time, you can get four levels of steps, so repeating exposure and etching will double the number of phase levels, continue to simply repeat this process m times, you can get the phase The step is a binary phase element with a binary phase, and its phase level is 2 ^m . Figure 1 shows the manufacturing process of a 4-phase diffractive optical element (see Nichilas F.Borrelli.MICROOPTICSTECHNOLOGY: fabrication and application of lens arrays and devices.Marcel Dekker , Inc. New York., 1999). In Fig. 1, the pattern obtained by the m-th reticle and the m-1 photolithography is strictly aligned, otherwise, the diffraction efficiency of the diffraction element will be greatly reduced. For example, when the alignment error is 0.2 μm, the diffraction efficiency of the 8-phase element decreases by 20%, and that of the 16-phase element decreases by 25% (see Yasuyuki Unno. Point-spreadfunction for binary diffractive lenses fabricated with misaligned masks. Applied Optics, 1998, 37 (16): 3401~3407). It can be seen that the existing manufacturing method uses multiple masking and etching techniques in the large-scale integrated circuit (VLSI) manufacturing process to manufacture binary optical elements with 2 ^m steps, requiring m-1 alignment overlays , the alignment accuracy is very high, and the overlay error has a great influence on the diffraction efficiency.

According to the binary optics theory, a diffractive optical element with higher diffraction efficiency can be obtained by increasing the number of phase steps. For example, when the number of phases is 8, the diffraction efficiency is 95%; when the number of phases is 32, the diffraction efficiency is as high as 99.7%. However, as the number of phases increases, the times of photolithography and ion etching increase correspondingly, and the times of alignment in photolithography also increase accordingly. In the conventional process, during overlaying, the pattern obtained by the m-th reticle and the m-1th photolithography is strictly aligned, otherwise, the diffraction efficiency of the diffractive optical element will be greatly reduced, and even lead to a gap of 2 ^m phase numbers. The diffraction efficiency of the diffractive optical element is lower than that of 2 ^m-1 phase numbers. Research shows that there are three kinds of errors that affect diffraction efficiency: vertical etching depth errors, lateral alignment errors, and line width manufacturing errors, and alignment errors have the greatest impact. In the existing manufacturing process of diffractive optical elements, as the number of steps increases, the pattern structure of the reticle becomes finer and more difficult to guarantee the accuracy of overlay. Therefore, when the number of phases reaches 16, the process It is difficult to guarantee the accuracy of overlaying. Although the theoretical diffraction efficiency of a 16-phase optical element can reach 99%, it is difficult for the device efficiency to exceed 90%.

Contents of the invention

The purpose of the present invention is to provide a process for manufacturing multi-phase diffractive optical elements, which does not require strict overlay alignment, the position of the step is determined by the initial reticle, and the rest of the reticle is only for self-alignment, not The position of the step is affected, so a diffractive optical element with higher diffraction efficiency can be obtained.

In order to achieve the purpose of the above invention, a process for making a multi-phase diffractive optical element includes the following steps in sequence:

①Use a magnetron sputtering apparatus to sputter a layer of chromium film on the substrate of the optical element to be made, then throw the positive photoresist, and after pre-baking, use the first mask to expose;

②After developing and hardening the film, wet-etch the chromium film not masked by the glue, and use reactive ion beam etching technology to etch the substrate. The depth is d/n, where d is the total etching phase depth, and n is the The number of phase steps;

③Continue to shake the positive photoresist and expose it with the second mask;

④ After development and film hardening, reactive ion beam etching the substrate with an etching depth of 2d/n, and then throwing negative photoresist and exposing from the back;

⑤ After developing and hardening the film, continue to throw a layer of positive resist, and use the third mask to expose;

⑥After developing and hardening the film, corrode the chromium film not protected by the adhesive layer, and etch the substrate with a reactive ion beam to a depth of 2d/n; when n=4, remove both the positive and negative adhesives, and wet-etch the metal film again , the step distribution of 4 phases is obtained, and the preparation process is completed; when n>4, enter step ⑦;

⑦ Keep the adhesive layer, expose it with the next mask, repeat the above steps ② to ⑥, and finally wet-etch the metal film to obtain the step distribution of n phase, where n>4.

The process adopted in the present invention is called "self-alignment process". Compared with the existing overlay process, this process has the following advantages: ① Strict alignment is not required for overlay, and the position of the phase step is determined by the first mask, and the rest of the mask are only for self-alignment. It does not affect the position of the step; ②The manufacturing accuracy of the mask is relatively low except for the first piece, which reduces the manufacturing cost of the mask; ③Just consider the effect of the vertical etching error on the diffraction efficiency However, the influence of lateral alignment error does not need to be considered. Through the endpoint monitoring method of the reactive ion etching process, the longitudinal manufacturing error can be controlled with high precision, so that higher diffraction efficiency can be obtained than the existing manufacturing process.

Utilizing this invention, an 8-phase quartz diffraction microlens array is actually manufactured, and its diffraction efficiency reaches 90%, but the diffraction efficiency is much lower than the above-mentioned index when it is manufactured by the existing technology. This process is suitable for fabricating multi-phase optical elements with high diffraction efficiency that cannot be achieved by existing processes on UV-transmitting materials such as quartz.

Description of drawings

Fig. 1 is the manufacturing process flowchart of 4-phase diffractive optical element in the prior art;

Fig. 2 is the process flow diagram that adopts the present invention to prepare 8 phase diffraction microlens arrays;

Fig. 3 is 5 mask layouts designed in the embodiment;

Fig. 4 is the scanning electron microscope (SEM) photo of the diffraction ultraviolet microlens array that utilizes this invention to make;

FIG. 5 is a schematic diagram of the point spread function and diffraction efficiency of the fabricated microlens tested by the small spot scanning technology.

Detailed ways

Fig. 2 shows the specific process implementation process of making an 8-phase diffractive microlens array by using the self-alignment process of the invention.

①Sputter a 0.8μm thick chromium film on the substrate to be made into a microlens by a magnetron sputtering apparatus, then throw a positive photoresist, and after pre-baking, use the first mask to expose, as shown in Figure 2(a) shown;

②After developing and hardening the film, wet-etch the chromium film not masked by the glue, and use reactive ion beam etching (RIE) technology to etch the substrate with a depth of d/8, where d is the total etching phase depth, such as Figure 2(b) shows. At this time, an initial distribution of 8-phase steps is formed, and the subsequent process does not require strict alignment of the dark and bright boundaries of the mask with the boundaries of these steps.

③Continue to shake the positive photoresist and expose it with the second mask, as shown in Figure 2(c). It can be seen from the figure that the second mask is not strictly aligned with the existing phase step pattern on the substrate. Accurate, this is the biggest difference between the present invention and conventional technology.

④ After developing and hardening the film, the reactive ion beam etches the substrate with an etching depth of d/4. Next, throw the negative photoresist and expose it from the back, as shown in Figure 2(d). In the figure, the metal chromium film is equivalent to the role of the mask plate.

⑤Because of the characteristics of the negative photoresist, the unexposed area can be removed, and the exposed area remains. After development and film hardening, the pattern of the negative resist is just opposite to the pattern of the positive resist, and a layer of positive resist is continued to be thrown and exposed with the third mask, as shown in Figure 2(e).

⑥After developing and hardening the film, corrode the chromium film not protected by the glue layer, and reactive ion beam etching the substrate with a depth of d/4. Then remove the positive and negative glue, and wet-etch the metal film again, forming a 4-phase step distribution.

⑦If the glue layer is kept, repeat the above-mentioned relevant process, as shown in Figure 2, the step distribution of 8 phases can be obtained from (f)-(l).

It can be seen from the above steps that compared with the conventional process, the self-alignment process has more processes such as sputtering chromium film, wet etching chromium film, negative resist lithography and development, and reactive ion selective etching. Relatively complex. However, it can be seen from Figure 2 that after the position of each phase step is determined by the first reticle, the subsequent steps do not affect the position of the step, that is, strict overlay alignment errors are avoided. In the self-alignment process, reactive ion etching is indispensable, and ion beam etching can not be used like the existing process. The reason is that: as shown in (f) and (k) in Figure 2, in the etching base When SiO ₂ in the film is used, the negative resist cannot be etched away, so only a reactive ion etching process with a large selective etching ratio can be used.

The 8-phase ultraviolet diffractive microlenses were fabricated by using the above-mentioned self-alignment process. It can be seen from the above process that the self-alignment process is more complicated than the conventional process. Conventional manufacturing only needs 3 reticles, while self-alignment process requires 5 reticles, and the design of the reticle is different from the conventional process. For the conventional process, the dark and bright ring radii of the reticle used to make the 8-phase UV diffraction microlens are given by the formula

${\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; =</span>\sqrt{\frac{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&lambda;fl</span>}}{{\mathrm{<span\; class="notranslate">\; 2</span>}}^{\mathrm{<span\; class="notranslate">\; m</span>}}}}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; l</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0,1,2</span>}<span\; class="notranslate">\; ,</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; \&Center\; Dot;</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Decision, where, λ is the design wavelength, f is the design focal length of the microlens. In the self-alignment process, the radii of the dark and bright rings of the mask for the first photolithography are given by

${\mathrm{<span\; class="notranslate">\; r</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; =</span>\sqrt{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; l\&lambda;f</span>}<span\; class="notranslate">\; /</span>\mathrm{<span\; class="notranslate">\; L</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

decision, where L is the number of quantization phases. Subsequent photolithography mask plates are determined by the steps to be etched. As shown in FIG. 2(c), the third and seventh steps are to be etched, and the positions of the corresponding reticle are bright areas. In addition, the substrate material should be UV transparent due to the need to expose the negative photoresist from the back side. The RIE etching machine used is the Nextral 860L type produced by Unaxis, and the experimental parameters are shown in Table 1.

          Table 1 Reactive ion etching parameter list

Tab. The etching parameters of RIE Reactive gas CHF3, O2 working pressure 35mtorr RF power 150watt etch selectivity ratio 11 Etching speed 50nm/min Verticality greater than 86 degrees

The example implemented by the present invention is: 848×640 element quartz diffraction ultraviolet microlens array, the unit size is 50×50μm ² , the substrate refractive index is 1.47, the central wavelength is 0.4μm, the etching depth is 1.7μm, F/# is 3.54. Fig. 3 is the designed five mask layouts, and Fig. 4 is a scanning electron microscope (SEM) photo of the diffractive ultraviolet microlens array made by the invention. We tested the point spread function (PSF) and diffraction efficiency of the fabricated microlens using the small spot scanning technology. As shown in Figure 5, the diffraction efficiency is as high as 90.2%. It can only reach 83%. Experimental results show that the invented technology can obtain high diffraction efficiency.

It is not difficult to see that a 16-phase diffractive optical element can also be prepared by the above method.

To sum up, the present invention proposes a new process—self-alignment process. Compared with the conventional process, this process avoids the strict overlay alignment requirements of multiple photolithography, so it is suitable for making diffractive optical elements with higher diffraction efficiency.

Claims (1)

1, a kind of technology of making multidigit phase diffraction optical element comprises the steps: successively

1. adopt magnetic control sputtering device waiting to make sputter one deck chromium film on the substrate of optical element, get rid of positive photoresist then, after the preceding baking, with first mask exposure;

2. develop, behind the post bake, the chromium film that wet etching is not sheltered by glue, and utilize reactive ion beam etching technique etching substrate, the degree of depth is d/n, d is total etching position phase degree of depth, the position phase number of steps of n for needing to make;

3. continue to get rid of positive photoresist, with second mask exposure;

4. behind development, the post bake, reactive ion beam etching (RIBE) substrate, etching depth are 2d/n, get rid of negative photoresist again, from back-exposure;

5. develop, behind the post bake, and continue to get rid of the positive glue of one deck, with the 3rd mask exposure;

6. behind development, the post bake, corrode the chromium film of not protected by glue-line, the reactive ion beam etching (RIBE) substrate, the degree of depth is 2d/n; When n=4, positive and negative glue is all removed, wet etching metal film once more obtains the stepped profile of 4 phases, and preparation process finishes; When n＞4, enter step 7.;

7. keep glue-line, with next piece mask exposure, repeat above-mentioned steps 2. to 6., last wet etching metal film obtains the stepped profile of n position phase, wherein n＞4.

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