RU2721172C1 - Method of manufacturing self-bearing x-ray lithography mask - Google Patents

Abstract

FIELD: medicine.

SUBSTANCE: invention relates to a method for producing X-ray lithography mask. X-ray mask manufacturing method includes processes of formation of topological masking X-ray absorbing layer by perforation of metal foil and its fixation in support ring. On one of the surfaces of the metal foil a protective mask is made of metal having a low etching rate compared to the foil metal in the corresponding chemically active plasma etching foil metal, then foil is placed on cooled table of plasma chemical etching and through holes are etched in foil through protective mask by action of flow of chemically active ions.

EFFECT: as result, X-ray lithography mask produced in this manner does not contain a bearing membrane and therefore is characterized by maximum achievable levels of X-ray lithographic contrast (with given material and thickness of masking layer), as well as relatively high mechanical strength.

5 cl, 5 dwg

Description

Various designs of x-ray templates are known, containing, as a rule, a masking x-ray absorbing layer made of heavy (with a large atomic number) metal, which is fastened by adhesion forces to a thin supporting membrane, which is an organic or inorganic film that is fixed on the support ring. The claimed invention relates to a method for manufacturing a self-supporting, perforated (i.e. through holes) X-ray template that does not contain a supporting membrane, and its masking X-ray absorbing layer, is a mesh metal structure that is attached (fixed) to the support ring.

As an analogue, a method for manufacturing an X-ray template was selected [described in RF patent for the invention No. 2469369 author: Gancelev A.N. - X-ray lithographic template and method of its manufacture. Publ. 12/10/2012, bull. No. 34, S. 430].

An analogous method of manufacturing an x-ray template contains the following operations:

изготовление исходной электропроводящей (или содержащей электропроводящий поверхностный слой) составной подложки, состоящей из двух выполненных из разных материалов деталей: опорного кольца и внутреннего диска;

the manufacture of the initial electrically conductive (or containing an electrically conductive surface layer) composite substrate, consisting of two parts made of different materials: a support ring and an inner disk;

формирования резистивной маски на рабочей поверхности исходной подложки;

forming a resistive mask on the working surface of the original substrate;

электроосаждение через резистивную маску металлического маскирующего рентгенопоглощающего слоя;

electrodeposition through a resistive mask of a metal masking X-ray absorbing layer;

удаления селективным травлением остатков резистивной маски;

removal by selective etching of residues of the resistive mask;

удаление селективным травлением внутреннего диска.

removal by selective etching of the inner disk.

In FIG. 1 is a diagram illustrating an analogous method of manufacturing an X-ray template, where a metal self-supporting masking X-ray absorbing layer 1 is formed on the working surface of a composite metal substrate consisting of an inner disk 2 and a ring 3 made of various metals. The disk 2 is removed at the last stage of the manufacture of the template by selective etching, as a result of which the masking X-ray absorbing layer 1 remains fixed with its edges to the ring 3, which becomes the support ring of the X-ray template.

This design of the X-ray pattern is characterized by a relatively high strength of the galvanically grown masking X-ray absorbing layer, as a result of which it can be in direct contact with the sample or the resistive layer irradiated with X-ray radiation, which can significantly reduce the contribution of the exposure radiation diffraction to the final lithographic resolution.

The disadvantages of the analogue method include the following problem, which means that using this method to produce a high-contrast (with contrast ≥100) X-ray pattern for the spectral range of the exposure radiation with a wavelength of λ≈ = 0.5 ÷ 3 Å is rather problematic due to that electrolytes containing heavy precious metals (such as Pt, Au) or rare metals (Re, Os, Ir) are quite expensive, and electroplating of heavy base metals (such as Hf, W, Ta, etc.) is performed [as shown in : Stern K.N., Stenly T., Gadomsky ST Electrodeposition of tungsten powders from mineral-salt electrolyte // Journal of Electrochemical Society, 1983. V. 130, No. 2. P. 300-305; Konstantinov V.I. Electrolytic production of tantalum, niobium and their alloys. M .: Metallurgy, 1977. 240 pp.] From molten salts at temperatures ≥750 ° C, at which the resistive mask is destroyed. The desired degree of contrast can be achieved by forming a masking X-ray absorbing layer of silver (Ag) or copper (Cu) metals from aqueous solutions of their salts through a thick (thickness ≥70 μm) resistive mask. It is known that such a mask can be formed by screen X-ray or X-ray lithography, however, these are quite complex and time-consuming technologies.

In addition, the rate of galvanic deposition of metals through a resistive mask substantially depends on the aspect ratio of the deposited element (i.e., on the ratio of the thickness of the resistive mask to the smallest window size in it) and the deposition rate is lower, the higher the aspect ratio it is characterized by (t .e. in the highest aspect areas there will be the lowest deposition rate). As a result, the thickness of the deposited metal in narrow "grooves" will be less than in areas with a larger width. The presence of this dependence will lead to a “thickness difference” of the deposited masking layer of the template, and, consequently, to different contrast values of its various sections, which may undesirably affect the quality of X-ray lithography performed using this template.

As a prototype, a method for manufacturing an X-ray template [described in the works: Kuznetsov S.A., Gentselev A.N., Baev S.G. The implementation of high-frequency filters of the subterahertz range using high-aspect polymer structures // Avtometriya, 2017, No. 1, P. 107-116 .; Gentselev A.N., Kuznetsov S.A., Baev S.G., Goldenberg B.G., Lonshakov E.A. Creation of quasi-optical selective elements of the terahertz range in the form of pseudo-metal structures by means of deep x-ray lithography // Surface. X-ray., Synchrotron, and neutron, Issled., 2017, No. 7, P. 32-42], in which there are no disadvantages inherent in the analogue method. In the prototype method, it is proposed to produce an X-ray pattern by mechanically fixing an industrially produced foil between two metal rings (frames), and to form an X-ray absorbing structure in it (in the foil) by perforation (creating through holes) by laser cutting. This method greatly simplifies the manufacturing technology of the x-ray template (reduces the number of operations) and provides an equal thickness of all sections of the masking x-ray absorbing layer.

The process of perforating a foil by laser cutting (i.e., creating through holes of an appropriate topology in it using a laser) is illustrated by the simplified diagram shown in FIG. 2, where a perforated metal foil 4 mechanically rigidly fixed in a (consisting of two metal rings) support ring 3 is shown (which acts as a masking layer in the final product) and is mounted on a six-axis mounting table 5. Radiation from a high-power short-pulse laser 6 transmitted through the control shutter 7, it is focused by the optical system 8 into the working plane of the laser cutting installation. Monitoring the process of forming through holes is carried out using a video surveillance system 9, which includes a device for measuring the geometry of a focused laser spot. All processes are fully automated and computer controlled. Information from the device for measuring the geometry of the focused laser spot is analyzed by a computer (not shown in the diagram), which generates control signals for the shutter 7, the shifts of the table 5 and the shifts that move the elements of the focusing optical system 8. To ensure clarity, the proportions of the sizes of the main elements are not observed.

Thus, the prototype method of manufacturing an X-ray template is characterized by simplicity and is as follows: an industrial-releasable metal foil is fixed in a special support ring and the topology of the X-ray absorbing structure is formed by laser cutting.

Using the prototype method, fairly high-quality x-ray templates are obtained from foil with a thickness of not more than 50 μm with a relatively low roughness of the cutting edge (at the level of ± 2 ÷ 3 μm), but the minimum width of the elements of the masking X-ray absorbing layer is about 50 μm. Reducing the width of the elements leads in some cases to their melting, and this is especially evident when cutting foils of refractory metals, the use of which can significantly increase the contrast of X-ray patterns. A possible solution to this problem may be the transition to the use of lasers with a femtosecond pulse duration, but this is a separate and rather non-trivial task waiting to be solved in the future.

In order to ensure the manufacture of heavy metal foils - tantalum (Ta) or tungsten (W), a self-supporting, perforated X-ray template, a new method for its formation is proposed, consisting of the following sequence of operations:

1. The formation by means of known lithographic techniques on the surface of the foil of a protective mask of metal with a significantly lower etching rate than the metal of the foil (in the corresponding plasma), for example, of aluminum, while the thickness of the mask should provide a through plasma-chemical etching of the foil through it;

2. Fixation of the foil by means of a resist on a metal washer with plane-parallel polished surfaces;

3. Conducting through plasma-chemical etching of the foil through a protective mask.

4. The release of perforated foil by dissolving the resist;

5. Fixation of the foil in the support ring, for example, using for its straightening two parts with polished surfaces: lining and cargo.

An example of a specific implementation.

Two manufacturing methods of a self-supporting X-ray template are described, the masking X-ray absorption layer of which is a perforated metal foil fixed in a support ring made of tungsten (W) or tantalum (Ta). The difference of options in the plasma-chemical etching modes of the corresponding metal. The following is an already optimized (with preliminary forced straightening of the foil and its gluing on a resist to a metal washer, with a cyclic etching mode, as well as forced straightening of the foil when it is fixed in the support ring), the sequence of operations for manufacturing such a template:

1. A circle of the desired diameter is cut out of a metal foil with a thickness of ~ 30 μm, its working surface is cleaned and, after preliminary heating to a temperature of ~ 500 ° C, magnetron sputtering of a thin layer of aluminum with a thickness of ~ 1 ÷ 1.5 μm is performed in a vacuum chamber.

2. Form a protective mask of aluminum with the appropriate topology, for example, by liquid etching a layer of aluminum through a resistive mask formed by photolithography, using known lithographic methods on one of the surfaces of the foil.

3. Fix the foil to improve heat dissipation (to the cooled table) by means of a resist on a metal washer (with plane-parallel polished surfaces and through holes for the outflow of excess resist, made of metal with high thermal conductivity, for example, copper or aluminum, see Fig. 3), since the initial foil, as a rule, is not flat, but has a curved surface, which is due to the technology of its manufacture (winding the foil on the rolls).

4. Produce through plasma-chemical etching of the foil through the protective mask formed on its working surface (see Fig. 4).

5. Release the perforated foil by dissolving the binder resist from the metal washer;

6. Fix the perforated foil in the support ring, for example, pinching it between two polished halves of the support ring (see Fig. 5), using two parts with polished surfaces: lining and load.

Plasma-chemical etching operations are performed, for example, on a Plasmalab 80 Plus installation with an inductively coupled plasma (ICP) source, using a cyclic mode with alternating etching and cooling operations so that the temperature of the stage at the beginning of each successive cycle is ~ 5 ° C.

The etching mode of tungsten foil: pressure p = 8 mTorr, gas feed rates: BCl ₃ - 10 cm ³ / min, Ar - 20 cm ³ / min; input powers: RF = 100 W, ICP = 600 W, tungsten etching rate ~ 0.35 μm / cycle (cycle: etching - 1 min, cooling - 2 min).

Tantalum foil etching mode: p = 10 mTorr, gas feed rates: NF ₃ - 30 cm ³ / min, Ar - 10 cm ³ / min; input powers: RF = 100 W, ICP = 600 W, tantalum etching rate ~ 2 μm / cycle (cycle: etching - 1 min, cooling - 3 min).

Since considerable power (~ 700 W) is supplied to the sample during etching, it heats up and the temperature of the table, controlled by the sensor, rises accordingly. At the cooling stage, it decreases to ~ T = 5 ° C and this temperature is the starting point for the start of a new cycle. Heat removal from the sample (i.e., its cooling) occurs in two ways: through the heat sink to the cooled stage (with which the sample has mechanical contact) and through heat transfer processes in the argon gas medium (at the cooling stage, the active gas flow is blocked and the argon flow increases significantly up to 50 cm ³ / min).

It should be noted that prior to plasma-chemical etching, the foil must be straightened and fixed on a metal washer made of metal with good thermal conductivity. Otherwise, inhomogeneity of its etching will occur due to the inhomogeneous temperature distribution on its surface facing the plasma, due to the difference in the local conditions for its cooling, which will occur if the initially non-planar foil is placed on the cooled stage of the installation. Pressing the foil, for example, with a metal ring around the periphery, does not always lead to the desired effect, since the ion beam incident on the foil diaphragms, due to which different parts of the foil are in different thermodynamic conditions, which leads to its warping.

Plasma-chemical etching can also be carried out in a continuous continuous mode with a significant decrease in the electric power used to create and maintain plasma combustion, and, therefore, supplied and “planted” on the processed foil, however, this can, in general, lead to instability of plasma burning as well as inhomogeneous etching of the foil, since if different sections of the foil, due to its warping, are in significantly different thermodynamic conditions, then as the exposure of the beam of chemically active ions to the foil lasts, these differences do not level out, but rather increase (poorly cooled areas are heated even more). From general considerations, it is clear that if the beam size is smaller than the size of the foil, then it will be warmer more in the place of the beam, which will lead to its additional warpage due to temperature deformations, and this in turn leads to the loss of direct mechanical contact with the cooled table in some areas foil, which is the reason for their even greater heating.

For the above reasons, it is preferable to carry out plasma-chemical etching of the foil in a cyclic mode, which ensures good reproducibility of the etching process, since each time the etching cycle starts from the same temperature of the cooled table and, accordingly, of the sample (i.e., the foil adhered to the washer) and thus all etching cycles will occur under substantially the same conditions. In addition, the following conditions must be fulfilled: the size (diameter) of the ion beam must be greater than or equal to the size (diameter) of the foil, and the foil itself must be completely placed on a metal washer, the diameter of which must be equal to the diameter of the cooled table, so that the entire processed foil will be be in relatively identical thermodynamic conditions.

After the end of the etching of the foil, the metal washer with the foil is removed from the installation and placed in a liquid dissolving the resist, the bonding foil and the metal washer. Then, the perforated foil is washed, dried and placed between two polished surfaces of the parts (see Fig. 5): lining 15 (equal in height to the lower ring) and load 16, after which it is fixed in the support ring by tightening two of its halves by screw connections.

In FIG. 1 is a diagram illustrating an analogous method of manufacturing an X-ray template, where a metal self-supporting masking X-ray absorbing layer 1 is formed on the working surface of a composite metal substrate consisting of an internal disk 2 (selectively removed at one of the last stages of manufacturing the template) and ring 3, which are made of various metals.

In FIG. 2 is a diagram illustrating the prototype method, which depicts (performing the role of a masking layer in the final product) perforated metal foil 4 mechanically rigidly fixed in a support ring 3 (consisting of two metal rings) mounted on a six-axis mounting table 5. Radiation from a powerful a short-pulse laser 6, passed through the control shutter 7, is focused by the optical system 8 into the working plane of the laser cutting installation. Monitoring the process of forming through holes is carried out using a video surveillance system 9. All laser cutting processes are automated and controlled by a computer (not shown in the diagram). To ensure clarity, the proportions of the dimensions of the main elements are not observed.

In FIG. 3 is a schematic illustration of the fixation of the foil 4 with a protective metal mask 10 formed on its surface for subsequent plasma-chemical etching. Fixing is carried out by means of a thin layer of resist 11 to a metal washer 12 with plane-parallel polished surfaces containing an array of through holes for the outflow of excess resist.

In FIG. 4 schematically illustrates the process of plasma-chemical etching of the foil in an installation where a metal washer 12 (with foil 4 fixed on it by means of a resist 11) is placed on a cooled table 13, after which the foil 4 is etched by a stream of reactive ions 14 through a protective mask 10.

In FIG. 5 is a diagram illustrating the fixing process in the support ring 3, consisting of two halves, of a perforated foil (shown in black) with its preliminary straightening by clamping between two polished surfaces of the parts: lining 15 (equal in height to the lower half of the ring) and cargo 16.

Claims (5)

1. A method of manufacturing a self-supporting X-ray template, including the processes of forming a topological masking X-ray absorbing layer by perforating a metal foil and fixing it in a support ring, characterized in that a protective mask is made of metal on one of the surfaces of the metal foil, which has a small mask compared to the metal foil the etching rate in the corresponding chemically active plasma etching the metal of the foil, then the foil is placed on a cooled table of the plasma-chemical etching unit and through holes are etched through the protective mask through the action of a stream of chemically active ions. 2. A method of manufacturing a self-supporting X-ray template according to claim 1, characterized in that prior to plasma-chemical etching, the foil is forcibly straightened and fastened by means of a resist to a polished metal plane-parallel washer containing an array of holes for outflow of excess resist, and then a metal washer together with an attached foil Placed on a cooled stage of the plasma-chemical etching installation and through etching of the holes in the foil through a protective mask. 3. A method of manufacturing a self-supporting X-ray template according to claim 1, characterized in that the etching is carried out in a two-stage cyclic mode, where after the etching stage (with the supply of chemically active gases and electric capacities), the cooling stage (characterized by the termination of the supply of chemically active gases and electric capacities) and an increase in the supply of inert gas to ensure improved heat transfer from the processed foil to the cooled table), and the start of a new cycle occurs after a certain time to reach a cooled table at a predetermined temperature. 4. A method of manufacturing a self-supporting X-ray template according to claim 2, characterized in that the metal washer is made of metals or alloys with high values of thermal conductivity, which improves the conditions for heat removal from the foil during its plasma-chemical etching. 5. A method of manufacturing a self-supporting X-ray template according to claim 1, characterized in that the foil before being fixed in the support ring is preliminarily straightened by placing between two parts with polished surfaces: a plane-parallel lining equal in height to the lower half of the support ring and the clamping weight.

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