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 kind of technology of making multidigit phase diffraction optical element

Technical field

The invention belongs to realm information science and technology field, be specifically related to a kind of technology of making multidigit phase diffraction optical element.

Background technology

Binary optical, or claim binary diffraction optics, abroad at first propose at the end of the eighties by people such as the Veldcamp of Massachusetts Institute of Technology (MIT) (MIT).The control of contraposition phase is to approach ideal value by the method that produces many steps in the binary optical elements.For example a position phase step is 2 π, can be by L=2, and 4,8 ..., 2 ^m, the method for (m is a positive integer) five equilibrium is divided, and this method is equivalent to the curved surface of arbitrary shape on how much can be with many steps plane convergence.In existing manufacture craft, this binary digit phase step is easy to be finished by microelectric technique: the step that produces two grades with a simple black and white mask exposure and etching energy, as using another mask again, repeated exposure and etching, as long as etching depth is last time half, just can obtain the step of four grades, repeating single exposure and etching like this will make the equal progression in position double, continue to repeat simply this technology m time, the phase step that just can put in place is the binary phase part of scale-of-two formula, and its equal progression is 2 ^mFig. 1 represents that the manufacture craft flow process of 4 phase diffraction optical elements (sees Nichilas F.Borrelli.MICROOPTICSTECHNOLOGY:fabrication and application of lens arrays and devices.Marcel Dekker, Inc.NewYork., 1999).In Fig. 1, m piece mask is strict the aligning with the resulting figure of the m-1 time photoetching, otherwise the diffraction efficiency of diffraction element will reduce greatly.For example when alignment error is 0.2 μ m, the element diffraction efficiency of 8 phases descends 20%, the then decline 25% of 16 phases (is seen Yasuyuki Unno.Point-spreadfunction for binary diffractive lenses fabricated with misaligned masks.Applied Optics, 1998,37 (16): 3401～3407).This shows that existing method for making is repeatedly mask and the lithographic technique that adopts in large scale integrated circuit (VLSI) manufacture craft, makes 2 ^mThe binary optical elements of level step needs the aligning alignment m-1 time, and alignment precision requires very high, and the alignment error is bigger to the diffraction efficiency influence.

By the binary optical theory,, can obtain the more diffraction optical element of high-diffraction efficiency by the increase of position phase number of steps.For example when the position number of phases was 8, diffraction efficiency was 95%; When the position number of phases was 32, diffraction efficiency was up to 99.7%.Yet along with the increase of the position number of phases, the corresponding increase of the number of times of photoetching and ion etching, the number of times of aiming in the photoetching also increases thereupon.In the common process, during alignment, m piece mask is strict the aligning with the resulting figure of the m-1 time photoetching, otherwise the diffraction efficiency of diffraction optical element reduces greatly, even causes 2 ^mThe diffraction efficiency of the diffraction optical element of the individual position number of phases is lower than 2 ^M-1The diffraction efficiency of the individual position number of phases.Studies show that influencing diffraction efficiency has three kinds of errors: vertically etching depth mistake, lateral alignment sum of errors live width are made error, and the having the greatest impact of alignment error.In the manufacture craft of existing diffraction optical element, along with the increase of number of steps, the patterning of mask is more and more meticulousr, and the precision of alignment also more and more be cannot say for sure card therefore, when the position number of phases reaches 16, just to be difficult to guarantee alignment precision on the technology.Though the optical element of 16 phases, theoretical diffraction efficiency can reach 99%, and the device efficiency that actual fabrication is come out is difficult to surpass 90%.

Summary of the invention

The object of the present invention is to provide a kind of technology of making multidigit phase diffraction optical element, this technology need not strict overlay alignment, the position of step is determined by initial mask, all the other masks just play self aligned effect, and do not influence the position of step, thereby can obtain the higher diffraction optical element of diffraction efficiency.

For achieving the above object, 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.

We are referred to as " self-registered technology " technology of the present invention.Compare with existing alignment process, this technology has following advantage: 1. need not the strict aligning of alignment, the position of position phase step determines that by first mask all the other masks just play self aligned effect, and does not influence the position of step; 2. to the making precision of mask except first is had relatively high expectations, all the other all require lower, thereby have reduced the cost of manufacture of mask; 3. only need to consider of the influence of the error of vertical etching to diffraction efficiency, and the influence of lateral alignment error need not to consider, by the endpoint monitoring means of reactive ion etching process, can control vertical making error accurately, thereby can obtain the diffraction efficiency higher than existing manufacture craft.

Utilize this invention, actual fabrication the quartzy diffraction microlens array of 8 phases, its diffraction efficiency reaches 90%, and adopts existing technology to make, its diffraction efficiency is significantly less than These parameters.This technology is suitable for making on the material of saturating ultraviolet light such as quartz etc. with the existing technology high-diffraction efficiency multidigit phase optical element that is beyond one's reach.

Description of drawings

Fig. 1 is the manufacture craft process flow diagram of 4 phase diffraction optical elements in the prior art;

Fig. 2 prepares the process chart of 8 phase diffraction microlens arrays for adopting the present invention;

Fig. 3 is 5 designed among embodiment mask layouts;

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

Fig. 5 utilizes the lenticule point spread function of little spot scan technical testing made and the synoptic diagram of diffraction efficiency.

Embodiment

Fig. 2 shows the concrete process implementing process of utilizing this invention-self-registered technology to make 8 phase diffraction microlens arrays.

1. by magnetic control sputtering device waiting to make the thick chromium film of sputter 0.8 μ m on the lenticular substrate, get rid of positive photoresist then, after the preceding baking, with first mask exposure, shown in Fig. 2 (a);

2. develop, behind the post bake, the chromium film that wet etching is not sheltered by glue, and utilize reactive ion beam etching (RIBE) (RIE) technology etching substrate, the degree of depth is d/8, d is total etching position phase degree of depth, shown in Fig. 2 (b).At this moment, formed the initial distribution of 8 phase steps, follow-up technological process does not need dark, bright border circle of mask to aim at these step borders are strict.

3. continue to get rid of positive photoresist, with second mask exposure, shown in Fig. 2 (c), as can be seen, second mask be the existing position strict aligning of step cutting pattern mutually not and on the substrate among the figure, and this is the difference place of the present invention and common process maximum.

4. behind development, the post bake, reactive ion beam etching (RIBE) substrate, etching depth are d/4.Then, get rid of negative photoresist, from back-exposure, shown in Fig. 2 (d).Among the figure, the crome metal film is equivalent to the effect of mask.

5. because the characteristic of negative photoresist is that the ground that is not exposed can be removed, the place of exposure keeps.Behind development, the post bake, negative glue pattern is just in time opposite with positive glue pattern, and continues to get rid of the positive glue of one deck, with the 3rd mask exposure, shown in Fig. 2 (e).

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 d/4.Then positive and negative glue is all removed, the wet etching metal film has then formed 4 phase stepped profile once more.

If 7. keep glue-line, repeat above-mentioned relevant process, as among Fig. 2 from (f) just-(l) can obtain the stepped profile of 8 phases.

As can be seen from the above step, with the common process contrast, self-registered technology is many sputter chromium film, wet etching chromium film, negative photoresist photoetching and technological processs such as development, reactive ion selective etch, thereby technological process relative complex.Yet as can be seen from Figure 2, after the position of every phase step was determined by first mask, each step of back did not have influence on the position of step, has promptly avoided strict overlay alignment error.In self-registered technology, reactive ion etching is absolutely necessary, and can not can use ion beam etching as existing technology, and reason is: as (f) among Fig. 2 and (k), and the SiO in the etching substrate ₂The time, negative photoresist can not be etched away, so can only be with having selective etching than very big reactive ion etching process.

Utilize above-mentioned self-registered technology to make the ultraviolet diffraction microlens of 8 phases.By above-mentioned flow process as can be seen, the self-registered technology process is comparatively more complicated than common process.Conventional making only needs 3 masks, and self-registered technology needs five masks, and the design of mask is different from common process.For common process, dark, the bright ring radius of mask that is used to make 8 phase ultraviolet diffraction microlens is by formula

$r_{m,l}=\sqrt{\frac{2\mathrm{\&lambda;fl}}{2^m}},l=\mathrm{0,1,2},\&CenterDot;\&CenterDot;\&CenterDot;---\left(1\right)$

Decision, wherein, λ is a design wavelength, f designs focal length for lenticule.And in the self-registered technology, the mask of photoetching for the first time is dark, the bright ring radius by

$r_l=\sqrt{2\mathrm{l\&lambda;f}/L}---\left(2\right)$

Decision, wherein L is the quantization number of phases.Later each time lay photoetching mask plate is then by the step decision of wanting etching.Shown in Fig. 2 (c), what etching is the 3rd and the 7th step, and then the position of corresponding mask is the clear zone.In addition, owing to need expose to negative photoresist from the back side, so substrate material ultraviolet light thoroughly.The Nextral 860L type that the RIE etching machine that uses is produced as Unaxis company, experiment parameter is as shown in table 1.

Table 1 reactive ion etching parameter list

Tab.The etching parameters of RIE

Reacting gas	CHF3，O2
		Operating air pressure	35mtorr
Radio-frequency power	150watt
		Etching selection ratio	11
Etching speed	50nm/min
		Verticality	Greater than 86 degree

The example that the present invention implemented is: 848 * 640 yuan of quartzy diffraction ultraviolet microlens arrays, unit size are 50 * 50 μ m ², refractive index of substrate is 1.47, and centre wavelength is 0.4 μ m, and etching depth is 1.7 μ m, and F/# is 3.54.Fig. 3 is 5 designed mask layouts, and Fig. 4 is diffraction ultraviolet microlens array scanning electron microscope (SEM) photo that utilizes this invention made.We have tested the lenticule point spread function (PSF) and the diffraction efficiency of made to utilize little spot scan technology, and as shown in Figure 5, diffraction efficiency is up to 90.2%, and 8 phase diffraction microlens array diffraction efficiencies that common process is made can only reach 83% usually.Experimental result shows that this invention technology can obtain high-diffraction efficiency.

Be not difficult to find out, adopt said method can also prepare the diffraction optical element of 16 phases.

Summary is got up, and the present invention proposes a kind of new technology-self-registered technology.With respect to common process, this technology has been avoided the repeatedly strict overlay alignment requirement of photoetching, thereby is fit to make the higher diffraction optical element of 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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