RU2350996C1 - Method for manufacture of litographic mask for liga-technology - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: application: for manufacture of lithographic mask. Resistive mask is formed by means of X-ray lithography on working surface of bearing membrane or processed substrate, which are blanks for lithographic mask and are made of materials with low atomic weight, and processes of electroplating metals with high atomic weight via formed resistive mask, at that at first thick layer of X-ray resist is applied on working surface of bearing membrane or processed substrate, and then thin layer of either photo- or highly sensitive electronic resist, in the latter resistive mask is shaped and then masking coating with its help, which serves as stencil for performance of lithography by X-ray radiation of appropriate spectral range for creation of resistive mask in the layer of X-ray resist.

EFFECT: decrease of geometric deformations of topological X-ray absorbing pattern of lithographic mask.

10 dwg

Description

The present invention relates to LIGA technology, and more specifically to a method for manufacturing a lithographic mask (mask for X-ray lithography, X-ray lithography mask, X-ray template, X-ray template) used to carry out the first stage of LIGA technology - deep X-ray lithography, i.e. for conducting exposure of thick (up to several millimeters) layers of an X-ray resist or X-ray sensitive material.

A typical lithographic mask used in LIGA technology contains a supporting membrane in the form of a film or plate, on the working surface of which a topological X-ray absorbing pattern formed by electroplating methods is held by adhesion forces, mounted on the support ring or, if its mechanical strength allows, directly on the mask holder (support framed). The main requirement for the lithographic mask used in LIGA technology is that the geometric deviations of its topological X-ray absorbing pattern that arise during its manufacture must be within certain tolerances (topological norms), the values of which decrease as the minimum size of the elements decreases topologies.

The first stage of LIGA technology - shadow screen X-ray lithography is characterized by the use of hard exposure radiation (EI) with a wavelength of about λ≈1Å and involves the manufacture of a topological x-ray pattern in the form of a metal film of relatively large (several tens of micrometers) thickness, which is required to provide the necessary contrast values of the lithographic mask. The galvanic growth of a topological pattern of such a large thickness, in turn, involves the formation of a resistive mask, the thickness of which noticeably exceeds the thickness of the X-ray absorbing layer.

The formation of a thick resistive mask having minimal geometric deviations of the topological pattern is a technical problem, the solution of which is the aim of the present invention.

As an analogue method, [described in the work of Artamonov L.D., Gavryushkina N.I., Gashtold V.N., Gluzdakova G.V., Deis G.A., Domakhina AM, Kolomeets AN, Kolomeets T.M., Prokopenko BC, Cherkov G.A. - X-ray patterns for X-ray lithography and LIGA technology. // Report of the Siberian International Center for Synchrotron Radiation for 1991-1992 / Institute of Nuclear Physics. Budker SB RAS. - Novosibirsk, 1993, p.229-231] a method for manufacturing a lithographic mask for LIGA technology, comprising the following steps:

1. make a silicon transition x-ray template containing the required topological x-ray absorbing pattern of the required contrast for exposure using relatively soft x-ray beams (MRI) with a peak intensity of x-ray quanta per energy E≈3 keV, which corresponds to a wavelength of about λ≈4Å;

2. apply adhesive electrically conductive sublayers on a silicon wafer doped with boron from the side of the working surface to a depth of about 3 microns;

3. apply a layer of X-ray resist on the working surface of the silicon wafer and form a resistive mask by shadow screen X-ray lithography using a transition X-ray pattern and MRI beams;

4. conduct galvanic deposition of a gold X-ray absorption pattern (up to 8 μm thick) and remove the resistive mask;

5. form the supporting membrane and carry out the control and elimination of defects.

The silicon transition x-ray pattern used in the above method of manufacturing a lithographic mask has the same design as the lithographic mask, and is made in a similar way, differs only in thickness and the method of formation (for example, by photo or electronic lithography) of the resistive mask required for the galvanic deposition X-ray absorbing pattern, as well as its thickness.

Figure 1 shows a schematic representation of the transition x-ray pattern used in LIGA technology to create a lithographic mask made in the above manner and containing: silicon support ring 1; a supporting membrane 2 in the form of a thin film of silicon doped with boron; X-ray topological pattern 3 made of gold.

In accordance with the laws of crystallography, the crystal lattice constant of boron-doped silicon has a smaller size than the constant of the original undoped silicon. As a result, an X-ray lithography template manufactured using the previously described technology is characterized by a support membrane that is sufficiently stretched on the support ring (with an internal voltage of about 5 ÷ 7.5 · 10 ⁷ N / m ² ). The formation of such a strongly stretched membrane by thinning the central part of the silicon wafer, even after the topological pattern was formed on it, leads to deformation of the X-ray absorption pattern itself and to the deformation of the support ring, the edges of which begin to protrude beyond the planar surface, as shown schematically in FIG. .one. As a result, there is a deflection of the working surfaces made using this technology of both the transition x-ray template and the lithographic mask for LIGA technology, and their non-flatness increases significantly.

The disadvantages of the analogue method, in addition to those that are also inherent in the prototype method and will be listed in the corresponding section of the application, are that both the transitional X-ray template and the lithographic mask for LIGA technology contain a highly stressed carrier membrane, which is a thin film (thickness ~ 3 μm) boron-doped silicon formed after the creation of a topological X-ray absorption pattern, which entails significant deformations as the shape of the product itself (the edges of the support ring protrude beyond the plane, defined by the working surface of the supporting membrane), and X-ray absorbing topological pattern.

These negative aspects reduce the quality of the manufactured lithographic mask for LIGA technology, because:

- they do not allow a gap between the working surfaces of the transitional X-ray template and the silicon wafer (which is the blank of the lithographic mask being manufactured) to be less than the non-flatness of the transitional X-ray template, which leads to a noticeable manifestation of diffraction effects during shadow screen X-ray lithography using EI of the "soft" x-ray range;

- contribute to the "accumulation" of geometric deformations of its topological pattern, due to the internal stresses of the supporting membranes of both the lithographic mask and the transition x-ray pattern.

As the prototype method selected [described in the work of E.W. Becker, W. Ehrfeld, P. Hagman, A. Maner, D. Munchmeyer. Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography galvanoforming and plastic molding (LIGA-process). // Microelectronic Engineering, 4, 1 (1986) 35-56] a method of manufacturing a beryllium lithographic mask for LIGA technology (a schematic representation of which is shown in figure 2), containing the following steps:

1. make a polyimide transition x-ray template containing the required topological x-ray absorbing pattern of the necessary contrast for exposure using MRI beams in the following way:

- a metal adhesive sublayer is sprayed onto the glass substrate and a resist layer is applied;

- form a resistive mask by photolithography;

- conduct galvanic deposition of the gold x-ray absorption pattern and remove the resistive mask;

- a polyimide layer is applied by centrifugation on a glass substrate over a gold pattern and thermopolymerized;

- in the glass substrate by etching the Central part create an x-ray transparent window;

2. spray on a beryllium foil 25 microns thick on both sides with a 300 Å gold sublayer and fix it on the support ring;

3. apply a layer of X-ray resist more than 20 μm thick on the working surface of the beryllium foil and form a resistive mask using the shadow screen X-ray lithography using a polyimide transition X-ray mask and MRI beams;

4. form an X-ray absorbing pattern (with a total thickness of 18 μm) by galvanic deposition on a beryllium foil through a resistive mask of copper (Cu) 8 μm thick and then gold (Au) 10 μm thick;

5. remove the resistive mask and carry out the control and elimination of defects.

Figure 2 shows a schematic illustration made in accordance with the prototype method and used in the LIGA technology transition x-ray template, which contains: a glass support ring 1; a carrier membrane 2 in the form of a thin film of polyimide; X-ray topological pattern 3 made of gold.

The disadvantage of the prototype method is the use of a polyimide transition X-ray mask for forming a resistive mask using the shadow screen synchrotron scanning X-ray lithography in the manufacture of a lithographic mask for LIGA technology, since the use of a transition X-ray mask in shadow X-ray lithography inevitably leads to additional geometric deformations of the generated topological X-ray absorption pattern. Additional geometric deformations arise both at the stage of manufacturing the transitional X-ray template due to the occurrence and manifestation of internal stresses, in particular during operations of electrodeposition of the X-ray absorbing metal and thermopolymerization of polyimide, and at the stage of image transfer to the X-ray resist layer due to the presence of a significant gap between the working surfaces of the X-ray transition mask and the X-ray resist layer deposited on the supporting beryllium membrane of the produced lithog aficheskoy mask LIGA-technology, which leads to amplification of diffraction effects, formation penumbral blurring (due to the spatial extent of the EI source) and increase the size of the shadow projection (due to the use of a divergent beam EI). The influence of all these effects is directly proportional to the size of the gap during exposure, which is illustrated by the scheme of shadow screen x-ray lithography shown in figure 3. Thus, the influence of the above undesirable effects will be minimal when implementing a zero gap between the working surface of the transition X-ray template and the layer of the X-ray resist when a latent image is formed in it.

Figure 3 shows the well-known scheme for conducting shadow screen x-ray lithography. X-ray radiation 1 from the source 2, passing through the x-ray transparent sections 3 of the x-ray template 4, containing a masking topological figure 5, the elements of which are made of x-ray absorbing material, exhibits a resist 6 applied to the substrate 7.

The proposed method, in which a resistive mask in the manufacture of a lithographic mask for LIGA technology is formed without using a transitional x-ray template, is free from the above disadvantage.

The aim of the invention is to reduce the geometric deformation of the topological x-ray pattern of a lithographic mask for LIGA technology.

This goal is achieved by applying a two-layer resist to the working surface of the processed substrate, which is a blank of the manufactured lithographic mask for LIGA technology, the layer directly in contact with the substrate is made of X-ray resist and has a thickness exceeding the height of its topological X-ray absorption pattern, and the top layer is made of photo- or highly sensitive electronic resist and has a thickness exceeding the thickness of the formed x-ray absorbing layer, provide th desired amount of contrast during exposure flow MRI. Using a resistive mask formed in the upper layer, a masking coating is created that serves as a stencil for X-ray lithography using MRI beams.

Figure 1 shows a schematic representation of a silicon transition x-ray template manufactured in accordance with an analogous method, comprising: a silicon support ring 1; a supporting membrane 2 in the form of a thin film of silicon doped with boron; X-ray topological pattern 3 made of gold.

Figure 2 shows a schematic illustration made in accordance with the prototype method and used in the LIGA technology transition x-ray template, which contains: a glass support ring 1; a carrier membrane 2 in the form of a thin film of polyimide; X-ray topological pattern 3 made of gold.

Figure 3 shows the well-known scheme for conducting shadow screen x-ray lithography. X-ray radiation 4 from the source 5, passing through the x-ray transparent sections of the x-ray template 6 containing the masking topological figure 3, exhibits a thick layer 8 of the x-ray resistor deposited on the substrate 7.

Figure 4 shows a schematic representation of a lithographic mask for LIGA technology, manufactured without the use of a transitional X-ray template and having passed several initial stages of the technological chain, including the application of a thick layer 8 of a X-ray resist and a thin layer 9 of a photo or highly sensitive electronic onto the working surface of the processed substrate 7 resist and formation in the last resistive mask.

Figure 5 shows a schematic depiction of a lithographic mask for LIGA technology, manufactured without the use of a transitional X-ray pattern and having passed the stage of forming a reverse layer (explosive, lift-off) lithography of a mask layer 10 of material with a large atomic weight on the surface of a thick X-ray resist layer 8.

Figure 6 shows a schematic depiction of a lithographic mask for LIGA technology before removing the resistive layer 8 from the working surface of the substrate 7 and having passed the formation step of the topological X-ray absorbing pattern 11 by electroforming.

Figure 7 shows a schematic representation of the finished lithographic mask for LIGA technology after removing the residual resist containing the substrate 7, on the working surface of which the topological X-ray absorbing pattern is held by adhesion forces 11.

Fig. 8 shows a graph of the X-ray transparency of a tungsten layer 0.9 μm thick for X-ray quanta in the energy range E≈1.8 ÷ 3.4 keV, which corresponds to the spectral range λ≈6.8 ÷ 3.6 Å.

Figure 9 shows a graph of the X-ray transparency of the PMMA layer with a thickness of 20 μm for X-ray quanta in the energy range E≈1.8 ÷ 3.4 keV, which corresponds to the spectral range λ≈6.8 ÷ 3.6 Å.

Figure 10 shows a graph of the X-ray transparency of a uranium layer (U ²³⁸ ) with a thickness of 15 μm for X-ray quanta in the energy range E≈1 ÷ 12 keV, which corresponds to the spectral range λ≈12 ÷ 1Å.

The following describes in detail an example of using the proposed method for manufacturing a lithographic mask of a specific design.

In this particular case, the lithographic mask for LIGA technology has a design that is schematically depicted in Fig.7. The lithographic mask contains the initial processed substrate 7 (supporting membrane) made of glassy carbon and representing a plate with a thickness of 500 μm and dimensions of 80 × 80 mm ² , and a topological X-ray absorbing pattern 11 located on its working surface obtained by electrodeposition of a metal with a large atomic weight ( such as gold, platinum, rhenium, tungsten, uranium, etc.). The strength of the processed substrate with the indicated parameters is approximately the same as that of a silicon wafer of a similar thickness, and it (the substrate) is able to withstand mechanical stresses that arise during typical technological operations and during its operation, therefore, it does not need a support ring like this had in the cases described in the description of the analogue method and the prototype method, although this is not a fundamental difference.

In accordance with the proposed method, the manufacture of an X-ray absorbing pattern of such a lithographic mask is carried out in two stages. The result of the first stage is to obtain a masking coating on the surface of a thick X-ray resist layer, the result of the second stage is the electrodeposition of a topological X-ray absorbing pattern on the working surface of the processed substrate through a mask formed in the X-ray resist layer.

We can say that the second stage includes similar operations, which are included both in the analogue method and in the prototype method up to the block of operations designed to form an x-ray transparent window in the initial substrate, which is not required in the proposed method due to the availability X-ray transparent glassy carbon plate in the spectral range used to implement the lithographic process directly in LIGA technology. That is, the main and fundamental differences of the proposed method from the analogue method and the prototype method are enclosed in the first stage.

The sequence of basic technological operations that ensure the manufacture of a topological x-ray absorption pattern located on the working surface of the glassy carbon plate is shown in the order of their production in figures 4, 5, 6 and 7.

First, a thick (about 20 μm) layer 8 of a low-sensitivity PMMA-type X-ray resist is applied to the working surface of the glassy carbon plate, and then a relatively thin layer 9 (~ 1.5 μm thick) of a highly sensitive electronic resist or photoresist is applied over it. In the upper resistive layer, a hidden negative image of the desired topological pattern is created and a resistive mask is formed so that layer 8 does not receive any or very small (for example, ≤1%) exposure dose. Obtained as a result of the above operations, the processed substrate is schematically depicted in figure 4. Then, a reverse (explosive, lift-off) lithography is performed on the formed resistive mask, i.e. a metal layer with a large atomic weight is deposited over a resistive mask, for example, tungsten with a thickness of about 0.9 μm, and then the upper resistive layer is washed off with a suitable solvent. This ends the first stage, and the resulting processed substrate containing a thick layer of X-ray resist, on the surface of which there is a masking coating 10, which is a negative image of the formed topological X-ray absorption pattern, is schematically depicted in Fig. 5.

Further, the manufacture of a lithographic mask for LIGA technology follows a well-known technological route. X-ray lithography, i.e. the treated substrate from the side of the masking coating is irradiated with an MRI flux, the intensity of which is concentrated in the spectral range λ≈6.8 ÷ 3.6 Å, i.e., the energy of the exposure X-ray quanta is in the range E≈1.8 ÷ 3.4 keV. Figures 8 and 9 respectively show graphs of X-ray transparency of tungsten layers with a thickness of 0.9 μm and PMMA with a thickness of 20 μm for x-ray quanta in the specified spectral range. The graph shown in Fig. 8 indicates that the masking coating layer is sufficiently contrasted for X-ray lithography in the spectral range λ≈6.8 ÷ 3.6 Å, and the graph in Fig. 9 illustrates that the correct selection of the spectral range of EI it is easy to achieve a situation where the PMMA layer is sufficiently uniformly exposed over the entire thickness (i.e., the ratio of exposure doses on different surfaces of the X-ray resist will not exceed 5). The irradiated X-ray resist is shown and the windows formed in the resist are filled with metal with a high atomic weight (such as gold, platinum, rhenium, tungsten, uranium, etc.) using the electrodeposition method. The thickness of the deposited metal is limited by the thickness of the resistive mask (about 20 μm) and is, for example, equal to 15 μm. Such a thickness of the X-ray absorbing layer is quite sufficient for the lithographic mask produced for LIGA technology to have a significant contrast value (about 100 or more) in the EI spectral range up to a wavelength of λ≈1 Å, which is illustrated in the graph shown in Fig. 10. Then the remnants of the X-ray resist are removed together with the masking coating located on its surface, as a result of which the topological X-ray absorption pattern formed by electrodeposition remains on the glassy carbon plate, which is shown schematically in Fig. 7. This completes the process of manufacturing a lithographic mask for LIGA technology and it does not contain an operation - shadow screen X-ray lithography using a transition X-ray pattern, the use of which, as previously indicated, entails various geometric distortions of the generated topological pattern. And although in the proposed method a resistive mask for electrodeposition of metal is formed using x-ray lithography, however, due to the fact that the masking coating is located directly on the surface of the X-ray resist layer, many distortions, the contributions of which, as is the case in the prototype method, are directly proportional to the value the gap between the working surfaces of the transition x-ray pattern and the processed substrate, are significantly reduced, since in the proposed method the size of the gap between the masking m coating and the resist layer is zero.

The above method can include operations to create thin (100-300 Å thick) adhesive electrically conductive sublayers (for example, from metals Ag, Ni, Cr, Ti or their combinations) by spraying on the working surface of the treated substrate (carrier membrane) removal, using the formed X-ray absorbing pattern as a mask for selective etching, or without removing them. However, the presence or absence of these operations is not fundamental and does not change the essence of the proposed manufacturing method. In addition, a masking pattern located on the surface of a thick X-ray resist layer can be formed not only by reverse (explosive) lithography, but also by a galvanic method through preliminary deposition of a conductive film on the surface of the X-ray resist, but this also does not matter.

Claims (1)

A method of manufacturing a lithographic mask for LIGA technology, including the processes of forming, by X-ray lithography, a resistive mask on the working surface of a carrier membrane or a processed substrate, which are blanks of a lithographic mask made of materials with low atomic weight, and electrodeposition processes through a formed resistive metal mask with large atomic weight, characterized in that on the working surface of the carrier membrane or the processed substrate is first applied th layer x- ray, and then a thin layer of a photo or electron resist high sensitivity, the latter forming the resistive mask and with the help of masking material, which serves for conducting stencil lithography corresponding spectral band X-ray mask to create the resistive layer in an x- ray.

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