RU2556313C2 - Fabrication of amplitude diffraction optical elements and masks for production of phase structures - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: this process relates to optical instrument making, particularly, to production of diffraction optics and masks for making of phase structures. It includes application of molybdenum 35-45nm-deep film on dielectric substrate surface and subjecting it to focused laser radiation. Laser radiation density makes (0.8-1.2)·10⁷ W/cm² to allows a complete removal (ablation) of metal film in the area of effects.

EFFECT: decreased number of process jobs, shorter process cycle.

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Description

The method relates to optical instrumentation and can be used to create diffractive optical elements of the visible and ultraviolet range - Fresnel lenses, focusers, correctors, etc.

A known method of microstructuring by laser recording technology, where the main masking layer is a polymer aromatic acrylate, which is the reaction product of an oligomer and a curing diluent (US patent No. 2009/023154, IPC G02C 7/06, published on September 17, 2009) . As substrates, an aluminum sheet 508 μm thick with a nickel layer galvanically deposited on top was used. To remove (ablate) the material, we used a laser system with a laser wavelength of 248 nm with a frequency of 150 pulses per second.

The disadvantages of this method is the low resolution, as well as the low resistance of polymer acrylate in the case of the formation of microrelief in plasma.

A known method in which photolithography is combined with the fusion of the obtained resist structures, resulting in a decrease in their size and increases the resolution of the method (patent RU 2400790, IPC G03F 7/00, B82B 3/00, publ. 09/27/2010).

The disadvantages of this method are the effect of chemical solutions on the substrates, the use of expensive lithographic equipment, the repeated repetition of the same lithography operations, which leads to a complication of the production process, reduced yield of elements and an increase in the total time of the technological cycle.

Closest to the proposed one is a method of laser-induced oxidation of thin titanium films with a thickness of 3-60 nm, due to which structures of submicron sizes, as well as sizes smaller than the diffraction limit (200-400 nm), were created. In the area of the laser beam, the titanium film is oxidized, which makes it possible to form a mask by subsequent selective etching (Patent DE No. 19544295, IPC D01J 9/02, B23K 26/34, published 05.06.1997).

The main disadvantage of this method is that elements with sizes below the diffraction limit could be obtained only on very thin films (~ 6-8 nm), the stability of deposition and the thickness of which is difficult to control in laboratory conditions, as well as the need for additional operations to form the relief in the mask .

The basis of the invention is the task of direct exposure to focused laser radiation to form a mask on the surface of the substrate, to reduce the number of operations and thereby reduce the time of the technological cycle and the cost of manufacturing DOE.

The problem is solved due to the fact that in the method of manufacturing amplitude diffractive optical elements and masks for the manufacture of phase structures, comprising applying a metal film to the surface of a dielectric substrate with subsequent exposure to focused laser radiation on a film, according to the invention, a molybdenum film is applied with a thickness of 35-45 nm, and the effect of focused laser radiation is carried out with a power density of (0.8-1.2) · 10 ⁷ W / cm ² , providing complete removal (ablation) of metal films in the affected area.

Another advantage of this method along with an increase in resolution is its versatility. The resulting product can be used as an amplitude optical element, lithographic template, as well as a semi-finished product for the subsequent manufacture of a binary optical element.

The proposed method is as follows.

A molybdenum layer 35-45 nm thick is applied on the surface of a dielectric substrate by magnetron sputtering in vacuum. At thicknesses less than 35 nm, the plasma resistance of the masking coating decreases; at more than 45 nm, the resolution of the method decreases due to the propagation of laser radiation energy over the width of the track. Then the film is exposed to focused laser radiation with a power density exceeding the energy of evaporation of the material, due to which there is a local removal of molybdenum.

The energy condition for the beginning of intense evaporation of the metal E ₂ is the excess of the value over a certain value of E ₁ . However, given the continuity of exposure, this excess should be such that the evaporation is explosive. At these values, the process proceeds with the formation of a narrow deep hole and the ejection of part of the molten metal.

As a basis for estimating the critical power density E _{1 of the} transition to intensive evaporation, one can take the characteristic energy storage time τ, which leads to explosive boiling of a certain volume of material. It is difficult to determine the energy storage time, since this requires consideration of the dynamics of the whole process taking into account evaporation. Approximately this time can be determined using the simplest thermophysical estimate by the formula:

where x is the characteristic size of the zone covered by boiling;

a is the coefficient of thermal diffusivity.

The expression for E ₁ has the form:

where ρ is the density of the material;

A is the absorption coefficient of the material;

L _isp - specific heat of vaporization.

For molybdenum, the value of E ₁ = 1.32 · 10 ⁶ W / cm ² [A.G. Grigoryants, I.N. Shiganov. Laser welding of metals. - M.: Higher School, 1988, 207 p.].

As a result, a topological pattern of the element is formed on the surface of the substrate. The time of deposition of a metal molybdenum film of the previously indicated thickness, the power density of the laser beam are determined by both the structural and specific technological parameters of the manufactured product. The resulting product can be used as an amplitude optical element, lithographic template, as well as a semi-finished product for the subsequent manufacture of a binary optical element.

Concrete example

On the optically smooth fused silica substrates (KB grade) of size 50 × 50 × 3 mm, molybdenum films 35-45 nm thick were deposited by the magnetron method on a Carolina D12-A installation. Substrates were then structured at a CLWS-200 circular laser recording station. The wavelength of the laser radiation is 488 nm, the exposure mode is continuous, the diameter of the laser spot is 0.5 μm with a Gaussian distribution of the power density in the cross section. Ring structures with a period of 3 μm were determined on molybdenum films. The outer diameter of the structures is 2 mm. The laser beam power decreased from 100 mW to 0 in increments of 0.5% for each ring. For our studies, the range of variation of the power density of the laser radiation E = (0.05 ... 2) · 10 ⁷ W / cm ² . Studies have shown that the minimum width of the track (element) is achieved when the radiation power density of 1 · 10 ⁷ W / cm ² . Subsequent formation of the relief was carried out on an ion-reactive etching unit in an ICP plasma “Carolina PE15 ″ with an excitation current frequency of 13.56 MHz and a power of 1 kW in CF ₄ medium under the following conditions: CF ₄ consumption - 3 l / h, power supplied to ICP source - 400 W, power supplied to the table with the substrate - 150 W, etching time - 10 min.

In FIG. 1 is a snapshot of the surface of a molybdenum film after laser recording.

In FIG. 2 is a section (profile) of a binary relief formed using a molybdenum mask.

The microrelief parameters were measured on a Solver-pro scanning probe microscope (SPM) and a Karl Zeiss scanning electron microscope (SEM) Supra 25.

As a result, ring structures with a minimum line width of 250-300 nm and a depth exceeding 100 nm were obtained.

The method allows direct exposure to form a mask on the surface of the substrate, reduce the number of operations and thereby reduce the time of the technological cycle and the cost of manufacturing DOE.

Claims (1)

A method of manufacturing amplitude diffractive optical elements and masks for the manufacture of phase structures, comprising applying a metal film to the surface of a dielectric substrate, followed by the action of focused laser radiation on a film, characterized in that the molybdenum film is applied with a thickness of 35-45 nm, and the effect of focused laser radiation is carried out with power density (0.8-1.2) · 10 ⁷ W / cm ² , providing complete removal (ablation) of the metal film in the affected area.

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