JPH0827404B2 - Optical element manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing an optical element,
In particular, the present invention provides a thin film type micro optical element having excellent optical characteristics.

2. Description of the Related Art In recent years, thin film type micro optical elements such as micro Fresnel lenses and micro gratings have been attracting attention as optical elements that are compact and lightweight and have various functions. In order to realize various light distributions and high efficiency, it is necessary to control the cross-sectional shape of the Fresnel lens and the grating. For example,
If the cross-sectional shape is a saw-tooth shape with an appropriate film thickness, a high diffraction efficiency of almost 100% can be realized.

As a conventional method for manufacturing a thin film type micro optical element, first, as shown in FIG. 2a, an electron beam resist 2 is formed on a substrate 1.
And an antistatic film 3 such as Au are applied or deposited, and as shown in b, the number of times of scanning of the electron beam 4 is controlled while the accelerating voltage of the electron beam 4 is constant, and the resist is directly drawn on the resist 2 to form the resist. There has been a method of manufacturing a Fresnel lens and a grating 2A having a sawtooth-shaped cross section by exposing and then developing the antistatic film 3 by etching and developing as shown in c. (Fujita et al .: "Blazed micro Fresnel lens by electron beam drawing fabrication", IEICE Transactions (c), J66-C, 1, pp. 85-91 (Sho 58-1)) Problem In such a conventional method, since writing is performed by using a low-mass electron beam, so-called proximity effect occurs due to scattering in the resist and from the substrate, and further, the sensitivity characteristic of the resist is accurately corrected. Therefore, it is difficult to realize an ideal sawtooth shape, that is, an optical element having good optical characteristics is required. I couldn't get it.

The present invention has been made in view of the above points, and an object thereof is to provide a thin-film optical element having excellent optical characteristics.

Means for Solving the Problems In the present invention, a positive resist is coated on a substrate, and the positive resist is changed by changing the accelerating voltage using an ion beam so as to correspond to the shape of the optical element. Then, the film thickness of the resist is changed by performing a development process by drawing the image on the substrate.

Action The present invention uses an ion beam having a heavier mass than an electron beam, so that scattering in the resist is less likely to occur. Further, since the depth of penetration of ions changes depending on the acceleration voltage, it is possible to precisely control the exposure in the depth direction.

Example 1 FIG. 1 is a manufacturing process diagram of a grating according to an example of the present invention. In the figure, 1 is a substrate, 6 is a positive resist, and 3 is an antistatic film. In this embodiment, glass is used as the substrate 1, PMMA is used as the positive resist 6, and Au film is used as the antistatic film 3. However, the substrate 1 may be any one as long as it has excellent transmittance at the wavelength used by the grating. The Au film 3 is used only to prevent the ion beam 5 from being charged, and is necessary when there is no concern about the ion beam 5 being charged, that is, when the substrate 1 or the resist 2 is conductive. Alternatively, another metal thin film such as Al may be used.

Next, the manufacturing process will be described. First, as shown in FIG. 1a, a positive resist 6 is formed on the substrate 1, for example,
1.3 μm is applied, and the antistatic film 3 is applied on it, for example, 100 Å
It was vacuum deposited. Next, the acceleration voltage of the ion beam 5 is periodically changed from a large value to a small value or a small value so as to correspond to the shape of the grating 2'having a period of 5 μm, which is manufactured as shown in FIG. 1b. Was changed to a large value and focused on the positive type resist 6 and directly drawn. The present inventors have determined that the penetration depth of the ion beam 5 into the resist 6 layer is determined by the acceleration voltage of the ion beam 5, and that the film thickness after development is almost constant even if the exposure amount is slightly exceeded. I have found For example, Be ²⁺ as an ion beam
The acceleration voltage is 50KV, 100KV, 150KV, 200KV
When the exposure is more than proper exposure, the film thickness of the resist 6 peeled off by development is 0.5 μm, 0.8 μm, 1.0 μm and 1.2 μm, respectively.
The exposure voltage is almost independent of the exposure amount and is determined by the acceleration voltage.
Further, since the ion beam 5 is hardly transmitted to the substrate 1 side, the sensitivity of the resist 6 is improved and the drawing time is shortened.

In this embodiment, the accelerating voltage of the Be ²⁺ beam 5 is set to, for example, 1 KV for the peak portion of the grating 20 and 220 KV for the valley portion, and changes smoothly from 1 KV to 220 KV to correspond to the shape of the grating 2 '. Let Finally, when the antistatic film 3 is removed by etching and development processing is performed, the first
As shown in FIG. 7C, the higher the acceleration voltage of the drawn ion beam, the more the resist 6 was peeled off, and the film thickness of the resist 6 could be changed, whereby the grating 20 was obtained. The grating 20 is affected by the scattering of the ion beam 5 in the resist 6 or the scattering from the substrate 1. There was no influence of so-called proximity effect, and it was possible to realize a good sawtooth cross-sectional shape without sagging or unevenness as designed. Further, since the penetration depth of the ion beam 5 is almost determined by the acceleration voltage regardless of the exposure amount, it is not necessary to accurately correct the sensitivity characteristic of the resist 6 to adjust the exposure amount as in the conventional example, and the reproducibility is improved. A good and good cross-sectional shape could be easily realized. That is, the grating 20 having excellent optical characteristics was obtained.

The film thickness of the resist 6 is d = 1.3 μm in the present embodiment, but the wavelength of the grating 20 is set to λ = 0.63,28 μm for the He-Ne laser and the refractive index of PMMA of the resist 6 is n = 1.5. Therefore, the first-order diffraction efficiency is d = λ / (n−
This is because the maximum is obtained when 1) = 1.3. Since the refractive index of the positive type resist is about 1.5 to 1.7, it was found that the film thickness of the resist 6 should be 2 = 2λ or less in order to obtain high efficiency.

In this example, the case where Be ²⁺ was used as the ion was described, but the same effect was obtained even when other ions were used. At this time, the heavier the mass of the ion, the shallower the penetration depth. Therefore, it was necessary to appropriately set the acceleration voltage according to the mass of the ion. Also, when considering an accelerating voltage of 250 KV or less as practical, Al ^{2+ with} a mass of 27 is 250 KV and the penetration depth is 0.8 μm, and Si ^{2+ with} a mass of 28 is 250 KV and the penetration depth is 0.
It was 6 μm. Since many optical elements such as the grating 20 have a usable wavelength λ = 0.4 to 1 μm, the maximum film thickness of the resist 6 is about 2λ, that is, 0.8 μm to 2 μm. From the above experiments, using ions of mass 27 or less,
It has been discovered that an optical element having particularly high efficiency can be realized. In this embodiment, the case where PMMA is used as the resist 6 has been described, but other resists have the same effect.

FIG. 2 shows an example of use of the grating 20 after the completion of the production of one embodiment of the present invention. In this example, when He-Ne laser light, for example, is incident as incident light 7 from the substrate 1 side, it is diffracted by the grating 20 and emitted as diffracted light 8 at an angle different from that of the incident light 7.

In this example, since the grating 20 having a period of 5 μm was manufactured, the outgoing angle of the diffracted light 8 was about 7 ° with respect to the incident light 7. In addition, since a good grating 20 without sagging could be manufactured by this example, the ratio of diffracted light was a good value close to 100%. Further, the incident light 7 may be incident from the grating 20 side.

Further, although the present embodiment has described the grating having a saw-tooth cross section, similar effects can be obtained in other thin film type micro optical element manufacturing methods such as gratings having various cross sections and Fresnel lenses. It goes without saying that you can do it.

As described above, the present invention uses a focused ion beam to
By irradiating a positive resist while drawing the accelerating voltage while changing it so as to correspond to the shape of the optical element, performing drawing processing, and performing a development process, by changing the film thickness of the resist, scattering in the resist and from the substrate Since it is less likely to scatter, the optical element having a good saw-toothed cross section is realized, and at the same time, the ion beam stays in the resist with almost no transmission to the substrate side, so that the sensitivity of the resist is improved and the writing time is shortened. By utilizing the fact that the ion penetration depth depends on the acceleration voltage, the film thickness after development is not affected even if the exposure amount is slightly exceeded, and the film in the depth direction is not affected. Since the thickness control can be performed with good reproducibility and precision, it has an excellent effect that an optical element having good optical characteristics can be realized.

[Brief description of drawings]

1 and 2 are respectively a manufacturing process diagram and a configuration diagram of a grating of one embodiment of the present invention, and FIG. 3 is a manufacturing process diagram of a conventional grating. 1 ... Substrate, 20 ... Grating, 5 ... Ion beam, 6 ... Positive resist.

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Claims (5)

[Claims] 1. A positive resist is coated on a substrate, an ion beam is focused on the resist while changing the accelerating voltage so as to correspond to the shape of an optical element, and drawing is performed. A method for manufacturing an optical element, which comprises changing the film thickness. 2. The method for manufacturing an optical element according to claim 1, wherein the mass of ions is 27 or less. 3. The method of manufacturing an optical element according to claim 1, wherein the optical element is a Fresnel lens. 4. The method for manufacturing an optical element according to claim 1, wherein the optical element is a grating. 5. The film thickness of the resist is 2 of the wavelength used in the optical element.
The method for manufacturing an optical element according to claim 1, wherein the number is not more than twice.