RU2841334C1 - Method of saw-tooth type diffraction grating manufacturing - Google Patents

Abstract

FIELD: optical instrument making.

SUBSTANCE: invention relates to optical instrument-making and a method of making a sawtooth diffraction grating. Method involves applying a layer of chromium onto the surface of a silicon substrate. Resist layer is deposited on the chrome, wherein a pattern of alternating parallel strips is illuminated with a laser, light or electron beam and then developed. Further, method comprises performing anisotropic chemical etching or plasma-chemical etching, accompanied by formation of a binary profile of relief, with protrusions. Chromium is sputtered onto the prepared sample of the binary grating in the sample swinging mode in the angle range from normal incidence of 90 degrees relative to sample to 30 degrees for formation of saw-tooth profile of chromium between projections of the binary grating. Further, the second anisotropic plasma-chemical etching is carried out, during which the initial binary grating is removed and a sawtooth grating remains. Then, vertical chromium filaments left after chromium sputtering in swinging mode are removed in cerium etchant solution, and a reflecting layer is sputtered.

EFFECT: improved quality of the grating and high efficiency of inputting radiation into the waveguide using a diffraction grating.

1 cl, 10 dwg

Description

Diffraction gratings are widely used in various optical devices: spectral instruments for obtaining monochromatic light (monochromators, spectrophotometers, etc.), as optical sensors of linear and angular displacements, for polarizers and optical filters, and even in so-called anti-glare coatings. Currently, due to the rapid development of photonics, diffraction gratings have come to be widely used in photonic devices for inputting light into flat waveguides. The efficiency of input into flat waveguides depends heavily on the shape of the diffraction grating. The shape of the grating that provides the maximum diffraction efficiency is sawtooth. Therefore, the task was set to manufacture a sawtooth diffraction grating.

The invention relates to the technology of manufacturing optical devices, and more precisely to a method of manufacturing a diffraction grating of both a reflective type and a transmission grating when copying onto light-transmitting materials.

The patent [1] describes a method for producing a diffraction grating. A layer of metal (for example, aluminum) is applied to a glass substrate in a vacuum, then a microprofile is formed using a dividing machine with a diamond cutter, consisting of strokes of the required frequency with the required angle of inclination of the working face.

In this case, a "piled-up" is formed on the non-working face. After that, to correct the microprofile, the diffraction grating is placed in a vacuum chamber, where the surface microprofile is subjected to the directed action of an ion flow of inert gas (for example, argon), as a result of which the microprofile of the lines acquires a form corresponding to the required theoretical triangular one. In this case, the diffraction efficiency increases, and the level of parasitic interfering radiation decreases.

The method of cut diffraction gratings is used, which consists in forming a surface triangular microprofile of lines with a diamond cutter on a special dividing machine in a layer of metal, most often aluminum, vacuum-sputtered onto a glass substrate. The method allows for the production of a wide range of diffraction gratings with high diffraction efficiency with a spatial frequency of up to 2400 lines per millimeter and with different angles of inclination of the working face (the so-called "shine" angle), which determines the maximum spectral characteristic. The disadvantage of this method is the need to use very complex, unique and expensive equipment - a dividing machine that cuts diffraction gratings with a sawtooth profile with a diamond cutter. Mechanical cutting takes a very long time. In the case of a large grating, the diamond travels up to 10 km or more by the end of the cut. During this time, it wears out, so the shape and depth of the line at the end of the cut are somewhat different from the initial one. This is due to the noticeable focusing effect of some flat gratings, which, however, does not cause significant inconveniences during operation. The diamond cutting the grating on soft metal squeezes it out, creating a rather complex profile of the stroke, which is not always accurately reproduced. Therefore, the instrumental contour and the distribution of energy by orders do not quite exactly coincide with the calculated one.

The fundamental disadvantage of the cutting method on a dividing machine is the difference in the shape of the realized micro profile from the theoretical triangular one due to the inevitable formation of the so-called "heap" on the non-working edge of the line, which is the cause of parasitic scattered radiation and a decrease in diffraction efficiency, worsening the characteristics of spectral devices. In the patent [1], the correction of the triangular micro profile of lines is carried out by removing the "heap" on the non-working edge of the line by the directed action of the ion flow of inert gas on the surface micro profile, a significant improvement in the optical characteristics of diffraction gratings is achieved.

The disadvantages of this method include the fact that the production of diffraction gratings, as described in the patent [1], ultimately comes down to correcting the quality of the cut diffraction gratings and has no independent application. Also, this method does not allow changing the geometry of the diamond-cut lines.

A method is known [2]. The authors of this method also improve the method for producing diffraction gratings proposed in the patent [1], according to which a layer of plastic material is applied to a polished substrate made of optical glass and lines are formed in it using a diamond cutter on a dividing machine with an interference control system [3].

As a result, an original matrix grating is obtained, which is used to obtain replicas of diffraction gratings. With this method of manufacturing cut diffraction gratings, the given geometry of the groove profile is determined mainly by the shape of the diamond cutter used for this, as well as the plastic properties of the material in which the grooves are formed. The main disadvantage of the known method is the low diffraction efficiency of replicas of diffraction gratings obtained from the matrix of the echelle grating in the spectral range of short wavelengths, due to the fact that the "shine" angle, which determines the region of maximum diffraction efficiency of the grating, is equal to 63.5° with a groove frequency of typical echelle gratings of 300, 75 and 37.5 lines/mm [3]. In addition, in the process of forming grooves with a diamond cutter, a "pillar" inevitably occurs over the non-working edge of the groove, which is the cause of parasitic scattered radiation. To increase the diffraction efficiency in the specified range of cut diffraction gratings, it is necessary to increase the "shine" angle. However, the practical maximum sharpening angle of a diamond cutter of the correct geometry (the angle of inclination of its working edge is approximately equal to the "shine" angle of the grating) does not exceed 70°. Increasing this angle is a very complex, labor-intensive and not always feasible task. In addition, the shape of the groove profile is not ideally triangular. In practice, the problem of improving the quality of the diffraction gratings proposed in the patent [2] is solved by cutting shallower grooves on a dividing machine and etching the prepared substrate in anisotropic plasma. The increase in the groove depth occurs due to the difference in the etching rates of the material applied to the substrate and the substrate itself, which has a higher etching rate than the material with the applied grooves. So, in this patent, it is also necessary to use a dividing machine that cuts the gratings to a smaller depth and therefore wears out less.

The disadvantages of this method include duration, complexity and labor intensity. Another disadvantage of this method is the use of complex unique technological equipment - a dividing machine. Also, this method does not allow changing the geometry of the diamond-cut strokes.

[4] was chosen as a prototype. This method for forming a diffraction grating does not require a dividing machine, which cuts lines with a diamond cutter. The method for producing a diffraction grating consists of forming a protective mask on the surface of a monocrystalline silicon substrate (111) with a misorientation angle α, through which anisotropic chemical etching is carried out in a potassium hydroxide solution, accompanied by the formation of a profile with an inclined groove plane with an inclination angle α and a protrusion on the ridge of a triangular profile, removing the protective mask, removing the silicon protrusions and applying a reflective coating. To form a protective mask, a resist layer is applied to the surface of the substrate, in which a laser or electron beam is used to expose and then develop a pattern of alternating parallel stripes by treating with an aqueous solution of tetramethylammonium hydroxide or potassium hydroxide; a chromium layer is applied to the entire surface of the substrate; a protective mask is formed by lift-off lithography, removing stripes of photoresist with a layer of chromium applied in an organic solvent, after which anisotropic chemical etching is carried out in a solution of potassium hydroxide, accompanied by the formation of a profile with an inclined plane of a groove with an inclination angle a and a protrusion on the ridge of the profile, the protective mask of chromium is removed by etching in a cerium etchant, the protrusions are removed by anisotropic chemical etching.

The sawtooth tilt angle is created by crystallographic misorientation on silicon substrates by anisotropic etching in a 20% KOH solution. The tilt angle of the sawtooth lattice is less than 1 degree.

The disadvantages of this method include binding to the crystallographic misorientation of silicon substrates, a small angle of inclination of the line, and the impossibility of making an angle of inclination that will be necessary in various applications. The angle of inclination for some applications should be greater than 45-60 degrees.

The objective of the invention was to increase the efficiency of radiation input into the waveguide.

A method for producing a sawtooth-type diffraction grating, which consists in applying a chromium layer to the surface of a silicon substrate, applying a resist layer to the chromium, in which a pattern of alternating parallel stripes is exposed by a laser, light or electron beam, and then developed by treatment with an aqueous alkali solution, after which anisotropic chemical etching or plasma-chemical etching is carried out, accompanied by the formation of a binary relief profile with protrusions, then chromium is sprayed onto the prepared sample of the binary grating in the sample swing mode in the range of angles from normal incidence of 90 degrees with respect to the sample to 30 degrees, to form a sawtooth profile of chromium between the protrusions of the binary grating, after which a second anisotropic plasma-chemical etching is carried out, after which a sawtooth profile is formed in the silicon substrate between the protrusions, then vertical chromium threads remaining on the protrusions after chromium spraying in the mode are removed in a cerium etchant solution. rolling, and a reflective layer is sprayed, characterized in that the formation of the relief profile is created as follows:

- chromium is sprayed onto the prepared binary grating sample in the sample swing mode in the range of angles from normal incidence of 90 degrees relative to the sample to 30 degrees, to form a sawtooth profile of chromium between the protrusions of the binary grating, after which a second anisotropic plasma-chemical etching is carried out, during which the initial binary grating with protrusions is removed and a sawtooth remains, after which the vertical chromium threads remaining after chromium spraying in the swing mode are removed in a cerium etchant solution, and a reflective layer is sprayed.

It was necessary to introduce radiation into a flat waveguide. The disadvantage of binary gratings when introducing radiation into a waveguide is that the efficiency of introduction is always less than 50 percent in the ideal case, and the angle of radiation introduction cannot be normal, i.e. 90 degrees, which is necessary when introducing radiation using a vertical-cavity surface-emitting laser (VCSEL). A sawtooth grating allows theoretically introducing 100 percent of radiation in the ideal case at an angle of radiation introduction of 90 degrees, as required by new VCSEL lasers.

The claimed method of a diffraction grating, which can be made not only on a silicon substrate, but on any other material regardless of the crystal structure, was implemented to solve the problem of inputting radiation into flat waveguides for the implementation of photonic information processing circuits. It qualitatively allows in a single technological cycle during the manufacture of a Photonic Integrated Circuit (PIC) to manufacture diffraction gratings together with waveguides for inputting and outputting radiation into the PIC. It also allows the use of new VCSEL lasers for vertical input of radiation into the PIC. Compared to the binary gratings used for inputting radiation, it allows theoretically inputting 100 percent of radiation into the waveguide.

The technological process is described below and shown in Fig. 1-10.

To test numerous process modes in the manufacture of a grating in the AutoCAD 2010 system, a template was developed for outputting 60 grating copies with different periods on one plate. Gratings with periods of 1.6 µm, 3 µm, 6 µm and 25 µm were programmed. See Fig. 1.

An enlarged image of one section is shown in Fig.2.

The photomask was produced using a German ZBA-21 lithograph with a technological resolution mode of 90 nm.

After manufacturing the photomask, a blank was produced to create a sawtooth grating. The blank was created by successively applying chromium and positive resist FP-3515 with a thickness of 1 μm to a silicon wafer (KDB12 silicon, orientation - 100, thickness 460 ± 20 μm).

Using a vacuum deposition setup, 50 nm thick chromium was deposited on a silicon wafer, photoresist was applied, a photomask with gratings was exposed, and a binary grating was made on the chromium layer. After this, vertical etching of the obtained sample was performed in anisotropic plasma. As a result, the binary grating shown in Fig. 3 was obtained. Then, etching was performed in a cerium etchant in order to remove chromium from the sample Fig. 4.

Then, chromium was sprayed with the sample rocking. The change in the direction of the sprayed chromium flow during rotation of the workpiece with a binary relief is shown in Fig. 5. Here it is seen that at an inclination angle of 45 degrees, the chromium flow is shadowed by the adjacent protrusions, and there remains a zone near the protrusion on the right side, where chromium does not get. The height of the binary grating is selected in accordance with the period of the diffraction grating T and can be selected with a coefficient of 1-0.25 from the required period of the diffraction grating. It is desirable to take the ratio of the grating period to the width of the protrusion as large as possible, as far as the resolution of the lithograph allows, since the width of the protrusion will limit the length of the sawtooth section along the zone and will ultimately reduce the diffraction efficiency of the grating. Then this grating is fixed on a device that mechanically rotates the grating in the angular range from 90 to 20 degrees in one direction with a return to the initial position and is placed in the thermal spraying unit. The swing angle when turning from the normal in one direction and back is determined as the arctangent of the ratio of the lattice height and the period - arctg (h/T). At h=T the rotation angle will be equal to 45 degrees, at h=0.25T the rotation angle will be equal to 70 degrees. The rotation angle should be chosen slightly greater than arctg (h/T) so that there is no uncoated chrome section of the zone between the protrusions of the binary lattice.

In Fig. 5 it can be seen that if the sample is rotated at an angle from 70 to 90 degrees with the ratio of the period and the height of the steps of the initially manufactured grating selected in the figure, then the flow of sprayed chromium will be deposited only on the side surface of the protrusions, and nothing will fall on the gap between the protrusions - the base of the grating, where the profile is created, therefore the rotation angle is taken from arctg (h/T) to 90 degrees, when all the evaporated substance - chromium falls on the base of the future sawtooth grating.

During the entire spraying process, the grating continuously rotates in the forward and reverse directions. In relation to the falling flow of the evaporated substance, the binary grating is positioned in such a way that the flow vector of the sprayed material changes from normal incidence to the selected one when the grating plane rotates.

The chromium that does not fall to the bottom between the projections is retained by the adjacent zone and settles on the vertical wall on the opposite side of the adjacent zone. Subsequently, at the final stage, this vertical column of chromium is removed with a cerium etchant. When rotating the workpiece with a binary relief from the normal angle of incidence of chromium to the extreme opposite direction with an angle depending on the relief height on the sample (we took an angle of about 20 degrees), a smooth change in the thickness of the sprayed chromium occurs. Chrome will get into the shaded area on the right side of the zone only at a normal angle of incidence of 90 degrees. The maximum amount of chromium will be deposited on the left side of the zone, where the spraying time during rotation will be maximum. As a result of such uneven spraying in thickness, which with uniform rotation back and forth, a sawtooth profile is obtained. If the rotation with the help of a programmable stepper motor is made uneven, with a change in the rotation speed during the rotation period from 20 to 90 degrees and back, then the sputtering profile will also change in accordance with the speed change. So the proposed method allows producing diffraction gratings with any desired profile. The grating rotation period was chosen to be 20 seconds. The rotation period time was chosen based on the fact that it was more than an order of magnitude less than the total sputtering time. The sputtering time, depending on the process modes, changes from 5 to 20 minutes and is controlled using pilot samples. As a result of rocking the sample with a binary grating, a chromium layer with a thickness change according to a sawtooth law is formed between the protrusions, and a vertical chromium layer is formed on the vertical walls of the rectangular protrusions.

The scanning electron microscope image after chromium deposition in the rocking mode is shown in Fig. 6.

After this, the second anisotropic etching was carried out Fig. 7.

The initial binary relief obtained on silicon was completely etched away, leaving only a sawtooth relief formed under the sawtooth film of chromium with a relief height proportional to the etching rates of chromium and silicon. The ratio of etching rates of chromium and silicon can be changed by 30 times, respectively, for chromium thicknesses of 50 nm, the height of the sawtooth profile can be obtained up to 1.5 μm. After the second anisotropic etching, the remaining vertical chromium threads are removed in a cerium etchant solution.

After manufacturing the gratings (Fig. 9) with a step height of 0.4 μm, the gratings were tested in red laser light. The experiment was organized so that the work of all gratings with different periods could be seen at once by assessing the power of the plus and minus first diffraction orders. For this purpose, a red laser with a wavelength of 650 nm and a beam aperture of 3 mm was used to illuminate all 4 gratings simultaneously. The gratings were arranged in a group. Each grating was 5 mm wide and 10 mm long. Four gratings grouped by width to each other in this way had a size of 2 mm. The gratings were tested for reflection. The laser was shone so that the reflection angle was close to zero. The reflected beam from the silicon substrate mirror had maximum intensity, since the largest area of the beam covered the mirror surface located around the gratings. The energy of the laser beam that hits each grating is distributed into the plus and minus first diffraction orders, so the quality of each grating can be judged by the ratio of energies in the plus and minus first diffraction orders. In an ideal grating, one of the orders tends to 1, and the other to zero.

As a result, sawtooth diffraction gratings with a height of 0.4 µm and periods of 1.5 µm, 3 µm, 6 µm and 25 µm were tested for reflection.

The tests were carried out using reflected light from a red laser with a wavelength of 650 nm.

In the attached photograph Fig. 10, the first orders of diffraction of 4 gratings simultaneously are visible relative to the zero order on both sides.

On the right side (relative to the central bright spot of direct reflection and zero order) 4 bright spots are visible. The last one on the right edge of them belongs to the grating with a period of 1.5 µm (the largest diffraction angle). Then, successively to the center, there are the first orders of diffraction from gratings with a period of 3 µm, 6 µm and 25 µm.

On the left side (relative to the central bright spot of direct reflection and zero order) 4 spots are also visible, but of much lower intensity. These are the minus first diffraction orders of gratings with a period of 3 μm, 6 μm and 25 μm. The minus first diffraction order of the 4th in a row on the left side of the central spot (grating 1.6 μm) is practically absent, which indicates a very good quality of the obtained sawtooth relief. Visually, in intensity for the remaining gratings, the minus first orders are 10 times less than the plus first orders. Between the main diffraction orders, small orders of higher diffraction orders (second, third, etc.) are visible, characteristic of binary gratings or for gratings with a non-ideal profile, different from a sawtooth.

From this we can conclude: the proposed method allows to produce sawtooth diffraction gratings with good quality and with a large and small period. Gratings with a period from 1.6 µm to 25 µm are demonstrated.

Fig. 3-9 show specific examples of the implementation of the proposed idea. When 4 gratings with different periods were made on one photomask, binary gratings were etched, then the samples were swung during sputtering and sawtooth gratings were made using the technology described above. Thus, we have proven the operability of the proposed technology.

Sources of information

1. RF Patent No. 2615020 / 04/03/2017. Guzhov Vasily Yurievich.

2. Patent of the Russian Federation No. 2643220 / 31.01.2018. Lukashevich Ya.K.

3. Gerasimov F.M., Yakovlev E.A. Diffraction gratings // Modern trends in spectroscopy techniques. - Novosibirsk, "Nauka", 1982, pp. 68-71, pp. 76-81.

4. Patent of the Russian Federation No. 2809769 / 18.12.2023. Mokhov D. V. - Prototype.

Claims (1)

A method for producing a sawtooth-type diffraction grating, which consists in applying a chromium layer to the surface of a silicon substrate, applying a resist layer to the chromium, in which a pattern of alternating parallel stripes is exposed by a laser, light or electron beam, and then developed by treatment with an aqueous alkali solution, after which anisotropic chemical etching or plasma-chemical etching is carried out, accompanied by the formation of a binary relief profile with protrusions, then chromium is sprayed onto the prepared sample of the binary grating in the sample swing mode in the range of angles from normal incidence of 90 degrees with respect to the sample to 30 degrees to form a sawtooth profile of chromium between the protrusions of the binary grating, after which a second anisotropic plasma-chemical etching is carried out, during which the initial binary grating with protrusions is removed and a sawtooth remains, after which the vertical chromium threads remaining after the chromium spraying are removed in a cerium etchant solution in the swing mode, and a reflective layer is sprayed.